U.S. GEOLOGICAL SURVEY BULLETIN 2124
Cover. Detail of pseudokarst labyrinth on summit platform of Cerro Roraima, 

Venezuela. The pervasively cross stratified quartzose of the Mataui Formation that forms 

the summit platform has been deeply etched by orographically intensified rainfall 

estimated at about 5 m per year. High-angle-oblique aerial view from a height of about 

300 m. Photograph by John Dohrenwend.
Geology and Mineral Deposits of the 

Venezuelan Guayana Shield

Edited by Gary B. Sidder, Andr6s E. Garcia, and Judith W. Stoeser

U.S. GEOLOGICAL SURVEY BULLETIN 2124

Prepared as part of a cooperative project with the 

Corporacion Venezolana de Guayana, Tecnica Minera, C.A.

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

Published in the Central Region, Denver, Colorado

Manuscript approved for publication January 12, 1995

Edited by Judith Stoeser

Cartography by Wayne Hawkins

Photocomposition by Wayne Hawkins

Cover designed by Carol Quesenberry

Spanish abstracts prepared by Andres E. Garcia G.

For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Geology and mineral deposits of the Venezuelan Guayana Shield / edited by 

Gary B. Sidder, Andres E. Garcia G., and Judith W. Stoeser.

p. cm. (U.S. Geological Survey bulletin ; 2124)

"Prepared as part of a cooperative project with the Corporacion Venezolana de 

Guayana, T6cnica Minera, C.A." 

Includes bibliographical references. 

Supt. of Docs. no. : 119.3:2124

1. Geology Venezuela. 2. Minerals Venezuela. 3. Guayana Shield. 

I. Sidder, Gary B. H. Garcia G., Andres E. IE. Stoeser, J.W. IV. Series. 

QE75.B9 no. 2124 

557.3 s dc20

[558.7] 95-2337

CIP
CONTENTS

[Letters designate chapters]

A. The Cooperative Project Between the U.S. Geological Survey and the Corporation 

Venezolana de Guayana, Tecnica Minera, C.A., in the Venezuelan Guayana Shield, 

Estado Bolivar and Estado Amazonas, Venezuela

By Jeffrey C. Wynn, Gary B. Sidder, Floyd Gray, Norman J Page, and 

Vicente Mendoza S.

B. Geology of the Venezuelan Guayana Shield and its Relation to the Geology of the 

Entire Guayana Shield

By Gary B. Sidder and Vicente Mendoza S.

C. Analysis of Aeromagnetic Data to Improve Geologic Maps of the Bochinche Mining 

District, Estado Bolivar, Venezuela

By Jeffrey C. Wynn, Anne E. McCafferty, and Edixon Salazar

D. Exploration Geofisica en el Area de Bochinchito, Estado Bolivar, Venezuela 

(Geophysical Exploration in the Bochinchito Area, Estado Bolivar, Venezuela)

By All R. Fernandez P.

E. Geology of the Lo Increfble Mining District and U-Pb Age of the Early Proterozoic 

Yuruari Formation of the Pastora Supergroup, 

Guayana Shield, Venezuela

By Warren C. Day, Richard M. Tosdal, E.L. Acosta, J.C. Aruspon, L. Carvajal, E. 

Cedefio, Glenda Lowry, L.F. Martinez, J.A. Noriega, Fernando J. Nufiez, J. Rojas, 

and F. Prieto

F. Geology and Geophysics of the Sierra Verdun-Cerro Piedra del Supamo Area, Estado 

Bolivar, Venezuela

By Floyd Gray, Fernando J. Nunez, Jeffrey C. Wynn, Gary B. Sidder, Freddy Davila, 

and Angel Baez

G. Gold Prospecting in the Cerro Arrendajo Study Area, Estado Bolivar, Venezuela 

By William E. Brooks, Acenk Guerra, and Fernando J. Nunez

H. Gold Resources of the Rio Chivao Study Area, Estado Bolivar, Venezuela 

By William E. Brooks, Acenk Guerra, and Fernando J. Nunez

I. Estratigrafia del Grupo Roraima e Interpretation Petrologica en la Region del Noreste 

del Parque Nacional Canaima, Estado Bolivar, Venezuela 

(Stratigraphy of the Roraima Group and Petrologic Interpretation in the Northeast 

Region of Canaima National Park, Estado Bolivar, Venezuela)

By Margarita Alberdi and Gloria Contreras

J. Geology, Geochemistry, and Geophysics of Gold-Quartz Veins in the Piston de Uroy 

Area, Estado Bolivar, Venezuela

By Jeffrey C. Wynn, Norman J Page, Gloria Contreras, R.J. Quesada, 

Barry C. Moring, and Robert L. Oscarson
K. Cenozoic Landscape Evolution of the Southern Part of the Gran Sabana, Southeastern 

Venezuela Implications for the Occurrence of Gold and Diamond Placers

By John C. Dohrenwend, Galo Yanez P., and Glenda Lowry

L. Gold and Diamond Resources of the Icabani Sur Study Area, Estado Bolivar, 

Venezuela

By William E. Brooks, Richard M. Tosdal, and Fernando J. Nunez

M. Bauxita en Superficie de Planacion de la Guayana Venezolana 

(Bauxite on a Planation Surface in Venezuelan Guayana)

By Galo Yanez P.

N. Geochemistry of Six Gold-Bearing Quartz-Carbonate-Tourmaline Vein Systems, 

Estado Bolivar, Venezuela

By Sherman P. Marsh, Herbert A. Pierce, Henry Sanchez, and Norman J Page

O. Mineral Deposits of the Venezuelan Guayana Shield 

By Gary B. Sidder
The Cooperative Project Between the

U.S. Geological Survey and the

Corporation Venezolana de Guayana, Tecnica Minera, C.A.,

in the Venezuelan Guayana Shield,

Estado Bolivar and Estado Amazonas, Venezuela

By Jeffrey C. Wynn, Gary B. Sidder, Floyd Gray, Norman J Page, and 

Vicente Mendoza S.

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENTS

Abstract........................................................................................................................... Al

Resumen.......................................................................................................................... 1

Introduction..................................................................................................................... 1

Historical Background.................................................................................................... 2

Primary Objective........................................................................................................... 4

Accomplishments............................................................................................................ 4

Scientific Results..................................................................................................... 4

Training................................................................................................................... 5

Technology Transfer............................................................................................... 5

Site Evaluations, Mineral Resource Appraisals, and Future Studies...................... 5

References Cited............................................................................................................. 7

PLATE

[Plate is in pocket]

1. Map showing selected geographic features of the Venezuelan Guayana Shield.

FIGURES

1. Map showing location of Guayana Shield in northern South America....................................................................... A2

2. Map showing geographic features, Venezuelan Guayana Shield, and areas worked jointly by USGS and

TECMIN geologists..................................................................................................................................................... 3

TABLE

1. Area and site evaluations by scientists of the U.S. Geological Survey, 1986-1990................................................... A6
The Cooperative Project Between the

U.S. Geological Survey and the 

Corporacion Venezolana de Guayana, Tecnica Minera, C.A.,

in the Venezuelan Guayana Shield, 

Estado Bolivar and Estado Amazonas, Venezuela

By Jeffrey C. Wynn, 1 Gary B. Sidder,2 Floyd Gray,3 Norman J Page,3 and

Vicente Mendoza S.4

ABSTRACT

The U.S. Geological Survey and the Corporacion Venezolana de Guayana, Tecnica Minera, C.A., conducted a 

cooperative mapping, exploration, and mineral resource estimation study in Venezuela from 1987 to 1992. The work was 

conducted in the Venezuelan Guayana Shield, an Archean to 

Early and Middle Proterozoic craton in southern and eastern 

Venezuela of almost 415,000 km2 that is mostly covered by 

jungle and savanna. Results obtained from this project contribute a new understanding of the geology and evolution of 

the Venezuelan Guayana Shield and the nature of its mineral 

occurrences. Personnel from Tecnica Minera, C.A., were 

trained both in the field and in the office to conduct modern 

mineral exploration and geologic mapping and research programs. This five-year project was supported financially by 

the Corporacion Venezolana de Guayana.

RESUMEN

El Servicio Geologico de Los Estados Unidos conjuntamente con la Corporacion Venezolana de Guayana y su filial 

Tecnica Minera, C.A., llevaron a cabo un estudio de exploration, cartografia geologica y estimation del potencial mineral en Venezuela. El trabajo fue conducido en la portion

l \J.S. Geological Survey, Unit 62101, APO AE 09811-2101, USA.

2U.S. Geological Survey, Denver Federal Center, MS905, Denver, 

Colorado 80225.

3U.S. Geological Survey, Corbett Building, 210 E. 7th Street, Tucson, 

Arizona 85705.

4Corporacion Venezolana de Guayana, Tecnica Minera, C.A., C.C. 

Chilemex, Piso 1, Puerto Ordaz, Venezuela.

Venezolana del Escudo de Guayana el cual esta situado en el 

sureste de Venezuela y que tiene una edad Arqueozoica a 

Proterozoico Temprano a Medio. El area del escudo es 

aproximadamente 415,000 km2 y esta cubierto principalmente por selva tropical y vegetation de sabana. Los resultados obtenidos de este proyecto han contribuido a un nuevo 

entendimiento de la geologfa y evolution del Escudo de 

Guayana en Venezuela asi como tambien sobre la naturaleza 

de sus recursos minerales. El personal de Tecnica Minera, 

C.A., fue entrenado en tecnicas modernas para la exploration, cartografia geologica y programas de investigation. 

El proyecto que tuvo una duration de 5 afios fue apoyado financieramente por la Corporacion Venezolana de Guayana.

INTRODUCTION

In September 1987 the U.S. Geological Survey (USGS) 

established an office in Puerto Ordaz, Venezuela, as part of 

a cooperative project with the Corporacion Venezolana de 

Guayana (CVG), Tecnica Minera, C.A. (TECMIN). The 

goals of this project were to map the relatively unknown and 

unexplored jungle-covered Precambrian Guayana Shield 

and to assess its potential mineral wealth. To accomplish 

these goals, teams of geologists, geochemists, and geophysicists were sent to numerous areas to conduct both detailed 

and reconnaissance geologic mapping and mineral exploration. Many of the results from the project are presented in the 

chapters of this volume.

These joint studies of the Venezuelan Guayana Shield 

have advanced the understanding of the evolution of the 

shield and the nature of its mineral occurrences. Among the 

results included herein are (1) the first recognition of 

platinum-bearing rocks in the Guayana Shield (in the Piston

Al
A2 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

70° 65° 60° 55° 50°

10°

500 1.000 KILOMETERS

_I

Figure 1.

1993).

Location of Guayana Shield (dashed line) in northern South America. Modified from Derry (1980) and Gibbs and Barron (1983,

de Uroy and La Flor-Carapo areas); (2) the relation between 

shear zones and gold deposits, such as in the Lo Increfble 

district; (3) the timing and character of the several magmatic 

and erogenic events that led to the formation of the shield; 

and (4) a catalog of known mineral deposits and prospects in 

the Venezuelan Guayana Shield classified by mineral 

deposit type.

The Guayana Shield is an Archean and Early to Middle 

Proterozoic craton. Figure 1 shows the location of the Guayana Shield of northern South America. Figure 2 illustrates 

the major physical features of the study area and areas 

discussed herein where USGS and TECMIN geoscientists 

worked together. Plate 1 shows numerous geographic features of the Venezuelan Guyana Shield and was originally 

published as plate 2 of U.S. Geological Survey-Corporation 

Venezolana de Guayana, Tecnica Minera, C.A. (1993).

Acknowledgements. Many individuals contributed 

professionally to the work described in this chapter and 

elsewhere in this volume. In addition to the authors of the 

chapters herein, these include, from TECMIN, Nestor 

Angulo, Cruz Briceno, Juan Candelaria, Luis Guzman,

Glenda Lowry, Felix Martinez, Miguel Martinez, Jorge 

Penott, Enot Quintana, Hay dee Rincon, Ivan Rivero, Yasmin 

Estanga de Sanchez, Gustavo Sardi, and Fernando Susach. 

From the USGS, they include William Bagby, Jerry Consul, 

Dennis Cox, Robert Earhart, Darrell Herd, Richard Krushensky, Stephen Ludington, W. David Menzie, Stephen 

Olmore, Paul Schruben, and Gary Seiner. Wayne Hawkins 

created the figures and plates of this book from the 

pen-scratchings of the many authors.

HISTORICAL BACKGROUND

In 1985, representatives of CVG-Ferrominera Orinoco, 

C.A., proposed a cooperative project between CVG and the 

USGS. Representatives of the USGS made an initial visit to 

Venezuela in January 1986, and in February 1987 a formal 

agreement was signed by the Venezuelan Ambassador to the 

United States, Hernandez-Valentin, and the Associate Director of the USGS, Doyle Frederick, in the Venezuelan 

Embassy in Washington, D.C. In September 1987, the USGS 

established an advisory group to work in Venezuela with the
COOPERATIVE PROJECT BETWEEN USGS AND CVG-TECMIN

66° 63°

A3

ESTADO 

\ DELTA AMACURO

Boundary of Venezuelan 

Guayana Shield

Figure 2. Areas in the Venezuelan Guayana Shield worked jointly by geologists of the U.S. Geological Survey and Corporation 

Venezolana de Guayana, Tecnica Minera, C.A. A, Santa Elena de Uairen-Chiricayen-Icabaru; B, La Flor-Carapo; C, numerous sites in 

Rio Caura and Rio Paragua field areas; D, Bochinche concession; E, Marwani area; F, Anacoco area; G, Venamo prospect; H, Las Flores 

prospecting zone; J, Kavanayen; K, Kamarata; L, El Callao-Lo Increible mining districts; M, Mount Roraima; N, Auyantepuy (Angel 

Falls); P, Piston de Uroy-Rio Chivao-Cerro Arrendajo prospecting areas; R, Kilometer 88-Las Cristinas; S, Santa Rosalia area; T, Cerro 

Bolivar-San Isidro mines; V, Sierra Verdun prospecting site; X, numerous heliports and camps in Estado Amazonas (upper Rio Orinoco 

and Rio Ventuari areas); Y, Cacaro dike area. Table 1 lists these areas and the scientists who worked at each and indicates when studies 

were conducted. Double-dashed lines indicate paved roads.
A4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

CVG. The group initially consisted of two resident scientists, 

later three, and then one, headquartered in Puerto Ordaz and 

Ciudad Bolivar, Venezuela; as many as 10 visiting scientists 

per year worked in Venezuela with the CVG during the 

project.

The Venezuelan host agency, TECMIN, was formally 

organized in 1986 as an outgrowth of the original CVGFerrominera Orinoco mineral exploration and development 

group (Yacimientos). CVG is a government-owned corporation (and government ministry) established by presidential 

decree in 1972 and given charge over the regional economic 

development of the southern half of Venezuela. The area of 

responsibility includes Estado Bolivar, Estado Delta Amacuro, and Estado Amazonas. TECMIN is a mineral exploration company wholly owned by the CVG.

The USGS was established in 1879 to conduct scientific 

investigations on the geology and mineral resources of the 

United States. Since the 1940's, the USGS has participated 

in large cooperative-assistance projects in foreign countries 

including Saudi Arabia, Brazil, Liberia, Indonesia, Colombia, Puerto Rico, Costa Rica, and Pakistan. In addition, 

USGS scientists have worked on smaller projects throughout 

the world.

PRIMARY OBJECTIVE

The primary objective of the USGS-TECMIN cooperative project was to gain a better understanding of the geology of the Guayana Shield by using modern scientific 

methods. This integration of field geologic mapping, 

geochemical sampling, and geophysical surveying, in addition to modern laboratory and data analysis, resulted in training of TECMIN staff, transfer of technology, evaluation of 

specific mining concessions held by TECMIN, and mineral 

resource appraisals of these and other areas.

Technical and professional staff of TECMIN were 

trained in methods for regional mapping, mineral resource 

evaluation, and exploration. Regional geologic mapping was 

done in cooperation with the Inventory Group (Grupo de 

Inventario), and mineral exploration was carried out within 

the Exploration Group (Grupo de Prospeccion), the two 

major operational units of TECMIN. Training was conducted by means of formal short courses (six in the first three 

years of the project), each lasting one to three weeks and 

taught in Spanish at several sites in Venezuela. Less formal 

training of individuals and groups was performed by resident 

USGS scientists in the field and in the office.

Technology transfer included purchase of new 

computers, upgrade of existing computer equipment, 

installation of the latest specialized instrumentation and 

software for chemical analysis, mineral exploration, and 

natural-resources inventory, and training in the use of 

instrumentation and software. A sample preparation 

laborastory and a chemical analysis laboratory were

designed by the USGS and built in Tumeremo, Estado Bolivar.

Site-evaluation studies were conducted by USGS scientists at sites selected by TECMIN geologists and management with input by the resident USGS staff. 

Recommendations from these efforts are being used by 

TECMIN to determine which areas warrant additional 

exploration. These site-evaluation studies were in Estado 

Bolivar and Estado Amazonas (fig. 2). Estado Delta Amacuro is mostly covered by Quaternary sediments, and TECMIN conducted only regional mapping and limited 

reconnaissance mineral exploration there.

Mineral resource appraisals were accomplished by 

USGS scientists in cooperation with TECMIN geologists. 

These appraisals are evaluations of large regions, typically 

l°xla ° or larger in size, and include domain maps that identify areas permissive for the occurrence of various mineral 

deposit types. The domain maps are based on descriptive 

mineral deposit models developed by the USGS (Cox and 

Singer, 1986). The number of undiscovered deposits and 

their metal contents were estimated by using mineral deposit 

density and grade and tonnage curves established for each 

deposit type based on data from known deposits worldwide. 

In addition, Monte Carlo-type computer simulations were 

used to estimate the quantity of metal remaining in place 

(Bliss and others, 1987). Wynn and Sidder (1991) and U.S. 

Geological Survey-Corporation Venezolana de Guayana, 

Tecnica Minera, C.A. (1993) are examples of published mineral resource assessments for part and all, respectively, of the 

Venezuelan Guayana Shield.

ACCOMPLISHMENTS

SCIENTIFIC RESULTS

The chapters in this volume describe the entire spectrum of work conducted under the umbrella of the 

CVG-USGS cooperative project, exclusive of the resource 

appraisals described above. Sidder and Mendoza present a 

new, comprehensive overview of the geology of the Venezuelan Guayana Shield and its relation to the entire Guayana 

Shield and the West African craton that incorporates findings of the cooperative project. Wynn and others combine 

geologic mapping and geophysical modeling to produce a 

bedrock geologic map of the Bochinche area, and they identify a number of new targets for gold exploration. Fernandez 

presents an application of geophysical methods to help map 

and interpret the geology in the Bochinchito district, which 

hosts gold deposits that are currently being mined. Day and 

others provide the first complete description of Lo Increible, 

an active gold mining district north of El Callao, and they 

show the importance of shear zones in the formation of the 

gold deposits. Gray and others report on the Sierra 

Verdun-Cerro Piedra del Supamo area, where
COOPERATIVE PROJECT BETWEEN USGS AND CVG-TECMIN A5

greenstone-belt rocks and granitic rocks of the Early Proterozoic Supamo Complex are intruded by a mafic-ultramafic 

complex. Brooks and others studied two areas of greenstone-belt rocks, Cerro Arrendajo and Rio Chivao, that are 

cut by gold-bearing quartz veins. Alberdi and Contreras 

mapped a previously undescribed area of the Early to Middle 

Proterozoic Roraima Group and its underlying rocks in 

Canaima National Park.

Wynn and others combine geophysical, geochemical, 

and geologic mapping to define a large gold-bearing quartz 

vein system as wide as 8 m at Piston de Uroy. This vein system was discovered during mapping of the northern margin 

of the Gran Sabana. Dohrenwend describes the geomorphology of the southern Gran Sabana and its relation to the source 

of diamonds and gold produced in that area. Brooks, Tosdal, 

and Nufiez mapped an area near Icabaru that contains rhyolitic tuff of the Early Proterozoic Cuchivero Group and 

sandstone of the Early to Middle Proterozoic Roraima 

Group. Diamonds are mined from placer deposits in streams 

that drain the area. Yanez describes the geomorphological 

framework underlying the discovery of the Los Pijiguaos 

bauxite deposit in the western part of the shield. Marsh and 

others describe the results of geochemical sampling of six 

gold-bearing quartz-carbonate-tourmaline vein systems in 

the El Callao, Lo Increible, Bochinche, and Marwani areas, 

all of which contain low-sulfide gold-quartz vein-type 

deposits. Sidder's compilation of mineral deposits and prospects in the Venezuelan Guayana Shield includes a comprehensive map of known deposits and prospects in Venezuela 

south of the Rio Orinoco.

TRAINING

The USGS conducted training in the form of formal, 

one- to three-week-long short courses on mineral deposit 

models, mineral resource estimation, digital geologic mapping, data analysis and statistics, field geologic mapping, 

geochemical sampling, and environmental hazard mitigation. In addition, resident and visiting scientists spent many 

man-months on individual and small-group training in computer usage, geologic mapping, laboratory methods, field 

geochemistry, petrography, geophysical methods, hydrothermal alteration, igneous petrology, and economic geology. This training extended beyond the professionals within 

TECMIN to include, by invitation of TECMIN management, 

representatives of other CVG entities (Mineria de Venezuela 

(MINERVEN), Bauxita Venezolana (BAUXIVEN), and 

Ferrominera Orinoco) and other Venezuelan government 

agencies (Ministerio de Energia y Minas and Ministerio de 

Ambiente), as well as geologists from several universities 

(Universidad Central de Venezuela, Universidad Simon 

Bolivar, and Universidad de Oriente). One geologist from 

Guatemala participated in the field geology course.

TECHNOLOGY TRANSFER

The USGS installed sufficient laboratory equipment to 

bring to operational status a modern sample preparation laboratory in Tumeremo. In August 1992, an emission spectrograph and required ancillary equipment was installed in 

the new USGS-TECMIN-designed chemical analysis facility in Tumeremo. Nine computers were delivered and set up 

in Puerto Ordaz and Ciudad Bolivar, together with peripheral devices, software (both commercial and USGS-developed), plotters, and digitizers.

SITE EVALUATIONS, MINERAL RESOURCE 

APPRAISALS, AND FUTURE STUDIES

Sixteen USGS scientists worked with TECMIN geologists at more than 60 mines, concessions, and mineral prospects in 28 areas in Venezuela. These sites were identified 

initially by noting activity of local prospectors, by looking 

for extensions of known mineral deposits, and during geologic mapping by the Inventory Group. The major areas visited are listed in table 1.

The mineral resource appraisal of Venezuela was carried out on several different levels. Each visiting USGS scientist provided an evaluation of the sites visited and studied. 

On a larger scale, a prototype mineral resource assessment 

was carried out for the NB-20-4 quadrangle (fig. 2) in eastern Estado Bolivar (Wynn and Sidder, 1991). This evaluation provides geologic and geophysical information, a 

compilation of mineral production and current mining activity, and a map of areas interpreted to be permissive for different types of mineral deposits. In the case of the NB-20-4 

quadrangle, few geochemical data are available. Nevertheless, sufficient geologic information was available to permit 

the estimation (using Monte Carlo simulation) of undiscovered gold resources. This estimation was made using calculations of the size of the known (mapped) greenstone belts, 

augmented by aeromagnetic data that show additional areas 

probably underlain by greenstone belts. Using a mineral 

deposit density model (Bliss and others, 1987) for the 

low-sulfide gold-quartz vein type of deposit and grade and 

tonnage cumulative-frequency curves (Bliss and Jones, 

1988), it was estimated that between 16 and 27 deposits of 

the low-sulfide gold-quartz vein type are present within the 

quadrangle, of which only two deposits are currently being 

mined. A total of 8-36 metric tons of gold remains in place, 

using the 90-percent confidence level of a Monte Carlo simulation (Wynn and Sidder, 1991). A shieldwide mineral 

resource assessment was conducted at the end of the cooperative project (U.S. Geological Survey-Corporation Venezolana de Guayana, Tecnica Minera, C.A., 1993); 

geographic, geologic, side-looking radar, mineral occurrence, and mineral assessment maps, all at 1:1,000,000 scale, 

are included in the assessment report.
A6 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 1. Area and site evaluations by scientists of the U.S. Geological Survey, 1986-1990. 

[Shown by letter in figure 2]

Site

Alto Rio Caura (C)

Alto Rio Paragua (C)

Anacoco (F)

Auyantepuy (N)

Bajo Rio Caura (C)

Bochinche (D)

Cacaro dike area (Y)

Cerro Arrendajo (P)

Cerro Bolivar-San Isidro (T)

Chiricayen (A)

Icabani-Los Caribes (A)

Kamarata (K)

Kavanayen (J)

Kilometer 88 (R)

La Flor-Carapo (B)

Las Cristinas (R)

Las Flores (H)

Lo Increfble (L)

Scientists

Sidder

Sidder

Day 

Wynn

Day, Wynn

Sidder

Page, Raines, Krushensky 

Page, Bliss, Krushensky, Wynn, Sidder 

Cox, Sidder, Wynn 

Page 

Marsh

Earhart 

Page, Wynn 

Wynn

Page, Olmore

Brooks, Wynn

Cox, Sidder, Wynn

Page, Herd, Sidder, Marsh, Wynn 

Dohrenwend, Wynn

Page 

Brooks 

Dohrenwend, Wynn

Page, Sidder 

Page, Wynn

Page, Wynn

Bagby, Menzie 

Marsh

Sidder 

Sidder, Olmore 

Sidder, Wynn 

Day, Sidder 

Day 

Gray, Marsh

Cox 

Bagby, Menzie 

Marsh

Page, Krushensky, Raines 

Wynn, Gray

Cox, Sidder, Wynn 

Day, Wynn

Date,

March 1988-April 1988

March 1988

April 1988 

June 1989

July 1989

May 1988

January 1986 

July 1987 

November 1987 

February 1988 

March 1988

April 1988 

November 1988 

May 1989

February 1989

March 1988

November 1987

September 1988 

October 1989

March 1988 

April 1988 

October 1989

July 1987 

September 1988

September 1988

January 1988 

April 1988

March 1989-May 1989 

March 1989 

April 1989 

June 1989 

July 1989 

November 1990

November 1987 

January 1988 

April 1988

January 1986 

September 1989

November 1987 

June 1989

The information used to make mineral resource 

appraisals normally includes (1) reliable tectonic and geologic maps, (2) stream-sediment geochemical data, (3) 

regional geophysical data, and (4) locations of mineral

deposits and prospects. In the case of the Guayana Shield, 

few regional geochemical data are available; however, reasonably good quality aeromagnetic data exist for at least half 

of the area. Using these data, available geologic maps, maps
COOPERATIVE PROJECT BETWEEN USGS AND CVG-TECMIN A7

Table 1. Area and site evaluations by scientists of the U.S. Geological Survey Continued.

Site Scientists Date

Marwani (E)

Medio Rio Paragua (C)

Mina Colombia (El Callao district) (L)

Mt. Roraima (M) 

Piston de Uroy (P)

Sidder, Wynn 

Wynn 

Marsh 

Earhart

Sidder

Cox, Sidder, Wynn

Marsh

Page, Gray

Wynn 

Dohrenwend, Wynn

Page

Page, Wynn 

Wynn

July 1987 

October 1987 

February 1988 

April 1988

April 1988

November 1987 

April 1988 

July 1990

September 1988 

October 1989

March 1988 

January 1989 

May 1989

Rio Chivao (P)

Rfo Ventuari (X)

Santa Rosalia (S)

Sierra Verdun (V)

Upper Rio Orinoco (X)

Venamo (G)

Brooks

Olmore 

Olmore, Wynn

Sidder

Gray, Wynn 

Gray

Olmore, Wynn 

Olmore 

Olmore 

Olmore

Ludington

March 1988

April 1989 

February 1990

February 1988

March 1990 

May 1990

February 1990 

April 1990 

May 1990 

October 1990

February 1988

of mines and prospects, and field reconnaissance data collected during the cooperative study, a new tectonic and geologic map series of the Venezuelan Guayana Shield is being 

assembled. By incorporating geophysical data, it has been 

possible to map terrane boundaries in areas covered almost 

completely with alluvial and eluvial deposits and extensive 

jungle. This tectonic and geologic framework map series, 

compiled on 12 l:500,000-scale quadrangles, is the foundation on which future mineral resource appraisals of the 

Venezuelan Guayana Shield will be made.

REFERENCES CITED

Bliss, J.D., and Jones, G.M., 1988, Mineralogical and grade-tonnage information on low-sulfide Au-quartz veins: U.S. Geological Survey Open-File Report 88-229, 99 p.

Bliss, J.D., Menzie, W.D., Orris, G.J., and Page, N.J, 1987, Mineral 

deposit density A useful tool for mineral-resource assessment, in Sachs, J.S., ed., U.S. Geological Survey Research on 

Mineral Resources 1987 Program and Abstracts: U.S. Geological Survey Circular 995, p. 6.

Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: 

U.S. Geological Survey Bulletin 1693, 379 p.

Deny, D.R., 1980, A concise world atlas of geology and mineral 

deposits: New York, John Wiley and Sons, 110 p.

Gibbs, A.K., and Barron, C.N., 1983, The Guiana Shield reviewed: 

Episodes, v. 1983, no. 2, p. 7-14.

 1993, The geology of the Guiana Shield: New York, Oxford 

University Press, Oxford Monographs on Geology and Geophysics 22, 246 p.

U.S. Geological Survey-Corporacion Venezolana de Guayana, 

Tecnica Minera, C.A., 1993, Geology and mineral resource 

assessment of the Venezuelan Guayana Shield: U.S. Geological Survey Bulletin 2062, 121 p.

Wynn, J.C., and Sidder, G.B., 1991, Mineral resource potential of 

the NB-20-4 quadrangle, eastern Guayana Shield, Bolivar 

State, Venezuela: U.S. Geological Survey Bulletin 1960, 16 p.

Geology of the Venezuelan Guayana Shield and

Its Relation to the Geology of the Entire Guayana Shield

By Gary B. Sidder and Vicente Mendoza S.

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-B

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995

CONTENTS

Abstract........................................................................................................................... Bl

Resumen......................................................................................................................... 2

Introduction.................................................................................................................... 5

Irnataca Complex............................................................................................................ 5

Age.......................................................................................................................... 6

Mineral Deposits..................................................................................................... 8

Pre-Trans-Amazonian Tectonomagmatic Event............................................................ 9

Greenstone Belts............................................................................................................. 9

Pastora Supergroup................................................................................................. 10

Botanamo Group..................................................................................................... 11

Real Corona-El Torno Assemblage....................................................................... 12

Age, Chemistry, and Origin.................................................................................... 12

Mineral Deposits..................................................................................................... 13

Supamo Complex........................................................................................................... 14

Trans-Amazonian Orogeny............................................................................................ 15

Cuchivero Group............................................................................................................ 16

Caicara Formation.................................................................................................. 17

Granite of the Cuchivero Group............................................................................. 17

Age and Origin....................................................................................................... 18

Mineral Resources.................................................................................................. 19

Undivided Proterozoic Rocks......................................................................................... 20

Roraima Group............................................................................................................... 21

Environment of Deposition and Age...................................................................... 23

Mineral Resources.................................................................................................. 25

Avanavero Suite............................................................................................................. 26

Parguaza Granite............................................................................................................. 27

Age and Origin....................................................................................................... 28

Mineral Deposits..................................................................................................... 29

Nickerie Orogeny................................................................................................... 30

Mesozoic Diabase Dikes........................................................................................ 30

Mesozoic-Cenozoic Uplift, Erosional Surfaces, and Alluvium ..................................... 31

5>ummary......................................................................................................................... 32

References Cited............................................................................................................. 33

FIGURES

1, 2. Maps showing:

1. Major geographic features, northeastern South America................................................................................... B4

2. Geologic provinces, Venezuelan Guayana Shield.............................................................................................. 7

3-6. Diagrams showing:

3. Rubidium versus yttrium+niobium, Cuchivero Group, Guayana Shield........................................................... 19

4. Potassium-argon dates, Proterozoic diabase, Guayana Shield........................................................................... 25

5. Potassium-argon dates, Phanerozoic diabase, Guayana Shield.......................................................................... 31

6. Chronology of Venezuelan Guayana Shield....................................................................................................... 32
CONTENTS

TABLES

1. Rock units and tectonic events of the Guayana Shield of Venezuela........................................................................ B6

2-5. Rubidium-strontium whole-rock isochron dates for:

2. Volcanic and plutonic rocks of the Cuchivero Group and its equivalents.......................................................... 19

3. Volcanic rocks of the Roraima Group................................................................................................................ 24

4. Diabase of the Avanavero Suite and hornfels formed from its intrusion into the Roraima Group .................... 24

5 Granitic rocks of the Parguaza province and their equivalents.......................................................................... 29
Geology of the Venezuelan Guayana Shield and 

Its Relation to the Geology of the Entire Guayana Shield

By Gary B. Sidder1 and Vicente Mendoza S.2

ABSTRACT

The Guayana Shield in Venezuela is composed of five 

lithotectonic provinces: (1) an Archean amphibolite- to granulite-facies gneiss> terrane, (2) an Early Proterozoic greenstone-granite te:rrane(s), (3) an Early Proterozoic 

metamorphosed volcanic-plutonic complex, (4) Early to 

Middle Proterozoic continental sedimentary rocks, and (5) 

Middle Proterozolc anorogenic rapikivi-type granite. Early 

Proterozoic rocks in Estado Amazonas of western Venezuela 

are undivided, and their relation to other rocks of the Venezuelan Guayana Shield is uncertain. Early to Middle Proterozoic continental-type tholeiitic dikes, sills, and small 

irregular intrusive bodies and Mesozoic dikes emplaced during the opening of the Atlantic Ocean cut all of the lithotectonic provinces. Major mineral deposits of the Venezuelan 

Guayana Shield include gold, iron, bauxite, and diamonds.

The Archean Imataca Complex, the oldest unit of the 

shield, consists of gneiss and granulite and minor dolomite 

and banded iron formation. Large isoclinal folds that have 

been refolded into relatively open folds are common. Metamorphic grade ranges from granulite facies in the northeast 

part of the belt to amphibolite facies in the southwest. 

Deposits of enriched banded iron formation in the Imataca 

Complex contain more than 2 billion metric tons of iron ore. 

During the pre-Trans-Amazonian tectonomagmatic event, 

about 2,800-2,700 Ma, granitic rocks intruded the Imataca 

Complex, and injection gneiss and migmatite were 

developed.

The Early Proterozoic greenstone belts, which formed 

about 2,250-2,100 Ma, consist of a submarine sequence of 

tholeiitic mafic volcanic rocks, a sequence of tholeiitic to 

calc-alkaline basalt to rhyolite, and a sequence of turbiditic 

graywacke, volcaniclastic rocks, and chemical sedimentary

'U.S. Geological Survey, Denver Federal Center, MS905, Denver, 

Colorado 80225.

2Corporacion Venezolana de Guayana, Tecnica Minera, C.A., C.C. 

Chilemex, Piso 1, Puerto Ordaz, Venezuela.

rocks that characterize the basal, middle, and upper parts, 

respectively. Layered mafic complexes also are present in 

the greenstone belts. Metamorphic grade ranges from greenschist to amphibolite facies. Shear zones that cut the greenstone-belt rocks host numerous deposits of low-sulfide 

gold-quartz veins.

Granitic domes of the Supamo Complex intruded the 

greenstone-belt rocks about 2,230-2,050 Ma, dividing the 

greenstone-belt rocks into branching synclinoria between 

intrusions. The Trans-Amazonian orogeny was a period of 

continental accretion, deformation, and magmatism between 

about 2,150 and 1,960 Ma, during which the Imatata and the 

greenstone-granite terranes were deformed and metamorphosed.

Volcanic, subvolcanic, and plutonic rocks of the 

Cuchivero Group represent postcollisional, postTrans-Amazonian magmatism in the Guyanan Shield about 

1,930-1,790 Ma. Silicic rocks (rhyolite and granite to granodiorite) dominate; intermediate to mafic dikes and lava 

flows are less abundant. Mining has not occurred in rocks of 

the Cuchivero Group, although precious-metal, tin, and 

molybdenum prospects are present, and Olympic Dam-type 

iron-copper-uranium-gold-rare earth element deposits are 

permissive. The only known diamond-bearing kimberlite 

deposit in the Guayana Shield is in the Quebrada Grande 

area. This deposit and the carbonatite at Cerro Impacto are 

within the outcrop area of the Cuchivero Group. The kimberlite was emplaced about 1,732±82 Ma; however, the carbonatite is not dated. Possible periods of intrusion are about 1.7 

Ga or during the Mesozoic after the opening of the Atlantic 

Ocean.

Undivided Proterozoic rocks in Estado Amazonas of 

western Venezuela include granitic rocks, gneiss, and migmatite. Metamorphism and magmatism were most intense in 

this area about 1,860-1,730 Ma.

Unmetamorphosed, posttectonic sedimentary rocks 

including quartzaronite, conglomerate, arkose, siltstone, and 

shale of the Roraima Group were deposited in fluvial,

Bl
B2 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

deltaic, shallow-marine, and lacustrine or epicontinental 

environments. Some rocks of the Roraima Group are at least 

1,670 Ma in age, and others are possibly as old as about 

1,900 Ma and as young as about 1,500 Ma or younger. 

Paleoplacer deposits of gold and diamonds in the lower part 

of the Roraima Group are the source for modern placers.

Continental-type tholeiitic dikes, sills, and irregular 

intrusive bodies of the Avanavero Suite cut all older rocks of 

the Guayana Shield. These intrusions are about 1,650 Ma in 

age and may be as old as about 1,850 Ma.

Middle Proterozoic (about 1.55 Ga) undeformed granite having rapikivi texture is characteristic of the Parguaza 

province. Quartz veins, pegmatite, and greisen in the Parguaza Granite have moderate potential for tin deposits. Similarities in age, composition, and tectonic environment 

indicate that Olympic Dam-type iron-copper-uranium-gold-rare earth element deposits may be present in the 

Parguaza Granite and its associated volcanic rocks.

Continental collision in the westernmost part of the 

Guayana Shield during the Nickerie orogeny reset many 

potassium-argon and rubidium-strontium mineral ages of 

Archean and Early Proterozoic rocks in the central and eastern parts of the shield to about 1,200 Ma. Tholeiitic diabase 

dikes intruded the Guayana Shield during the opening of the 

Atlantic Ocean from about 210 to 200 Ma. Variable lithologic resistance to weathering and erosion of a thick 

sequence of flat-lying to very gently dipping sedimentary 

strata in the Roraima Group have produced at least six planation surfaces in the Guayana Shield at distinct elevations 

between about 2,900 and 50 m above sea level.

Tropical weathering of the diverse rock types of the 

Guayana Shield has formed many deposits of bauxite and 

lateritic bauxite. The largest bauxite deposit is Los Pijiguaos, 

which developed on the Parguaza Granite. Deposits of bauxite and enriched banded iron formation formed on the 

Imataca-Nuria erosional surface. Placer diamond and gold 

deposits are mined in modern channels of the major rivers 

and in colluvial-alluvial deposits in low-order drainages.

Rocks, mineral deposits, and tectonic events in the 

Venezuelan Guayana Shield are generally correlative with 

those elsewhere in the Guayana Shield and (or) in the West 

African craton. Although Archean rocks are not known 

elsewhere in the Guayana Shield, Archean rocks similar to 

those in the Imataca Complex and coeval Archean tectonic 

events are present in the West African craton. Rocks in the 

Early Proterozoic greenstone belts in Venezuela are comparable in age and lithology to greenstone-belt rocks throughout the Guayana Shield and in the West African craton. The 

Trans-Amazonian orogeny in northern South America and 

the Eburnean orogeny in West Africa are the major Early 

Proterozoic tectonic event. Differences in the types of late 

Early Proterozoic and Middle Proterozoic rocks in the 

Guayana Shield and the West African craton are significant. For example, granite and rhyolite similar to the

Cuchivero Group are present throughout the Guayana 

Shield, but are only locally present in the West African 

Shield. Also, rocks comparable to the Roraima Group and 

the Parguaza Granite, although widespread in the Guayana 

Shield, are rare or absent in West Africa.

RESUMEN

El Escudo de Guayana en Venezuela esta compuesto 

por 5 provincias litotectonicas: (1) un terrano Arqueano con 

metamorfiamo de la facie de la anfibolita a la granulita, (2) 

un terrano granitico-cinturones de rocas verdes de edad Proterozoio Temprano, (3) un complete volcanico-plutonico sin 

metamorfismo, de edad Proterozoico Temprano, (4) rocas 

continentales de edad Proterozoico Temprano, y (5) granito 

rapakivi anorogenico de edad Proterozoico Medio. Las rocas 

del Proterozoico Temprano en el Estado Amazonas no han 

sido diferenciadas, por consiguiente, su relation con otras 

rocas del Escudo de Guayana en Venezuela no ha sido 

definida. Los diques, sills y cuerpos intrusivos pequenos e 

irregulares, de composition toleitica y origen continental de 

edad Proterozoico Temprano a Medio, y diques de edad 

Mesozoico que fueron emplazados durante la apertura del 

Oceano Atlantico, cortan las rocas de todas las provincias 

litotectonicas. Los principales yacimientos minerales son de 

oro, hierro, bauxita y diamantes.

El Complejo de Imataca de edad Arqueano es la unidad 

mas antigua del escudo, consiste de gneises y granulita con 

cantidades menores de formation bandeada de hierro y dolomita. El estilo estructural consiste de grandes pliegues isoclinales los cuales han sido replegados formando pliegues 

relativamente mas abiertos. El grado metamorfico varia 

desde la facie de la granulita, en la parte noreste del cinturon, 

a la facie de la anfibolita en la parte suroeste. Los depositos 

de hierro en formaciones bandeadas de hierro enriquecidas 

contienen aproximadamente 2 billones de toneladas metricas 

de mineral de hierro. El Complejo de Imataca fue afectado 

durante el evento tectono-magmatico pre-Trans-Amazonico, 

entre 2,800 y 2,700 Ma, cuando se formaron gneis de injection y migmatitas.

Los cinturones de rocas verdes del Proterozoico Temprano se formaron aproximadamente entre 2,250 y 2,100 

Ma. Estos consisten de una secuencia submarina de rocas 

volcanicas maficas toleiticas, una secuencia toleitica a 

calco-alcalina de basalto a riolita y una secuencia de grauwacas turbiditicas, rocas volcaniclasticas y rocas sedimentarias de origen quimico las cuales caracterizan la base, parte 

media y partes superiores respectivamente. Tambien se 

encuentran complejos estratificados maficos-ultramaficos. 

El grado metamdrfico varia entre las facies del esquisto 

verde y anfibolita. Las zonas de cizalla que cortan los cinturones de rocas verdes contienen numerosos depositos de oro 

en vetas de cuarzo con bajo contenido de sulfuros.
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B3

Los cinturones de rocas verdes fueron intrusionados por 

domos graniticos del Complejo de Supamo aproximadamente entre 2,230 y 2,050 Ma. Los domos graniticos dividieron los cinturones de rocas verdes en sinclinorios los cuales, 

en vista en mapa, presentan ramificaciones. La orogenesis 

Trans-Amazonica fue un periodo de colision y crecimiento 

continental, deformacion y magmatismo entre 2,150 y 1,960 

Ma durante el cual las rocas del Complejo de Imataca y la 

provincia granitico-rocas verdes fueron deformados y metamorfizados.

Las rocas volcanicas, subvolcanicas y plutonicas del 

Grupo Cuchivero representan un periodo de magmatismo 

entre 1,930 y 1,790 Ma despues de la colision 

Trans-Amazonica. Predominan rocas silicicas (riolita y 

granito a granodicrita); diques y lavas intermedias a maficas 

son menos abundantes. Aunque existen prospectos de minerales preciosos, estano y molibdeno, y prospectos del tipo 

Olympic Dam cobre-uranio-oro estan permisivo, hasta la 

fecha no hay operaciones mineras en rocas del Grupo 

Cuchivero. La uni.ca kimberlita diamantlfera conocida en el 

Escudo de Guayana esta localizada en el area de la Quebrada 

Grande. Este deposito, y la carbonatita del Cerro Impacto 

estan localizados dentro de la region del Grupo Cuchivero. 

La kimberlita fue intrusionada aproximadamente hace 

1,732±82 Ma. Aunque la edad de la carbonatita no ha sido 

defmida, las edades posibles de intrusion son acerca de 1.7 

Ma o durante el Mesozoico despues de la apertura del 

Oceano Atlantico,

Las rocas de edad Proterozoico en el Estado Amazonas 

no han sido divididas, pero incluyen rocas graniticas, gneis y 

migmatita. En esta area, el magmatismo y metamorfismo 

fueron ma's intensos aproximadamente entre 1,860 y 1,730 

Ma.

Las rocas sedimentarias del Grupo Roraima fueron 

depositadas despues del tectonismo Trans-Amazonico en 

ambientes fluviales, deltaicos, marino somero y lacustrine o 

epicontinentales. Estas rocas no estan metamorfizadas e 

incluyen cuarzo-arenita, conglomerado, arcosa, limolita y 

lutita. Algunas de las rocas del Grupo Roraima tienen una 

edad de al menos 1,670 Ma, otras posiblemente tienen una 

edad de 1,900 Ma y otras pueden ser tan jovenes como 1,500 

Ma. Los depositos de oro y diamantes en paleoplaceres en la 

parte inferior del Grupo Roraima son la fuente para depositos de placer modernos.

La Suite Av.anavero, la cual comprende diques, sills y 

cuerpos intrusivos irregulares de composicion toleitica continental, cortan todas las unidades mas antiguas del Escudo 

de Guayana. Estas intrusiones probablemente se formaron 

aproximadaments hace 1,650 Ma, y quizas hace 1,850 Ma.

El granito rapakivi, no deformado, de edad Proterozoico Medio (aproximadamente 1,545 Ma), conforma la Provincia de Parguaza. El granito de Parguaza presenta vetas de 

cuarzo, y greisen conteniendo un potencial moderado para 

depositos de estano. La similitud en edad, composicion y

ambiente tectonico del granito de Parguaza con las rocas que 

contienen el deposito Olympic Dam de hierro, cobre, uranio, 

oro y elementos tierras raras, indican que este granito y rocas 

volcanicas asociadas tienen potencial para esos minerales.

La colision continental en el extremo occidental del 

Escudo de Guayana, ocurrida durante la orogenesis Nickerie 

aproximadamente hace 1,200 Ma, re-establecio las edades 

isotopicas de los sistemas potasio-argon y rubidio-stroncio 

de las rocas Arquenas y Proterozoico presentes en la parte 

central y oriental del escudo. Durante la apertura del Oceano 

Atlantico, entre 210 y 200 Ma, diques de diabasa toleitica 

intrusionaron el Escudo de Guayana. El levantamiento y 

erosion del los terrenes Prec&nbricos durante las Eras Mesozoica y Cenozoica produjeron al menos seis superficies de 

erosion en el Escudo de Guayana a distintas elevaciones 

entre 2,900 y 50 m.s.n.m.

La meteorizacion en clima tropical, de los distintos 

tipos de rocas del Escudo de Guayana, ha formado muchos 

depositos de bauxita y bauxita lateritica. El deposito mas 

grande es Los Pijiguaos, el cual se formo sobre el Granito de 

Parguaza. Tambien se formaron depositos de bauxita y 

hierro enriquecido a partir de formation bandeada de hierro 

en la superficie de erosion Imataca-Nuria. Diamantes y oro 

en depositos de tipo placer son actualmente minados en 

canales activos de los rios mayores y tambien en depositos 

coluvio-aluviales en drenajes de orden menor.

Las rocas, depositos minerales y eventos tectonicos en 

el Escudo de Guayana en Venezuela son generalmente correlacionables con esos en otras partes del Escudo de Guayana y/o en el Craton de Africa Occidental. Aunque no se 

conocen rocas Arqueanas en otras partes del Escudo de 

Guayana, las rocas y eventos tectonicos en Africa Occidental 

de edad Arqueana son similares a las del Complejo de 

Imataca. Las rocas Proterozoicas de los cinturones de rocas 

verdes "greenstones" en Venezuela son comparables en edad 

y litologia al resto de las rocas en los cinturones de rocas verdes a traves del Escudo de Guayana y en el Craton de Africa 

Occidental. La orogenia Trans-Amazonica ocurrida en la 

parte norte de Suramerica, y la orogenia Eburneana en Africa 

Occidental es el evento tectonico mas importante durante el 

Proterozoico Temprano. En contraste a la similitud entre 

rocas Arqueanas y del Proterozoico Temprano entre 

Suramerica y Africa, existen diferencias significativas entre 

los tipos de rocas del Proterozoico Temprano tardio y Proterozoico Medio entre las mencionadas regiones. Por ejemplo, a traves del Escudo de Guayana hay granites y riolitas 

similares a aquellos del Grupo Cuchivero, mientras que en el 

Escudo de Africa Occidental estas rocas estan presentes solo 

localmente. Lo mismo sucede con las rocas del Grupo 

Roraima y el granito de Parguaza, las cuales afloran a traves 

del Escudo de Guayana pero no asi en Africa Occidental 

donde raramente afloran o estan ausentes.
B4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

69°

ESTADO 

\ DELTA AMACURO

Boundary of Venezuelan 

Guayana Shield

Figure 1. Major geographic features of the Venezuelan Guayana Shield. Smaller map shows outline of Guayana Shield of northeastern 

South America (modified from Gibbs and Barron, 1983).
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B5

INTRODUCTION

The U.S. Geological Survey assisted the Corporation 

Venezolana de Guayana, Tecnica Minera, C.A. (CVG-TEC 

MIN, or TECMIN) between 1987 and 1992 in its assessment 

of and exploration for new mineral deposits in the Precambrian Guayana Shield of Venezuela (Wynn, Sidder, and others, this volume). The Guayana Shield, in the northern part 

of the Amazonian craton of South America, measures about 

1,100 km north to south and 2,100 km east to west, covering 

an area of about 2,310,000 km2 (fig. 1). Shield rocks crop out 

in Colombia, Venezuela, Guyana, Brazil, Suriname, and 

French Guiana. The Guapore or western Central Brazil 

Shield, south of the Amazon River Basin, forms the southern 

part of the Ama2:onian craton (Gibbs and Barron, 1983; 

Teixeira and others, 1989; Goodwin, 1991). Although now 

separated by the Atlantic Ocean, the geologic histories of the 

Guayana Shield and the West African craton indicate that 

Archean and Early Proterozoic rocks of both areas were 

deposited in similar environments and were affected by 

coeval tectonic and metamorphic events.

The Guayana Shield in Venezuela consists of five lithotectonic provinces: (1) Archean amphibolite- to two-pyroxene granulite-facies gneiss terrane, (2) Early Proterozoic 

greenstone-granite terrane(s), (3) Early Proterozoic unmetamorphosed volcanic-plutonic complex, (4) Early to Middle 

Proterozoic continental sedimentary rocks, and (5) Middle 

Proterozoic anorogenic rapakivi-type granite (Gibbs and 

Barron, 1983; Teixeira and others, 1989). Early Proterozoic 

and possibly Archean rocks in Estado Amazonas of western 

Venezuela are undivided, and their relation to other rocks of 

the Venezuelan Guayana Shield is uncertain. Early to Middle 

Proterozoic continental-type tholeiitic dikes, sills, and small 

irregular intrusive bodies and Mesozoic dikes emplaced during the opening of the Atlantic Ocean are present in all of the 

lithotectonic provinces. Table 1 is a simplified stratigraphic 

chart of the rock units and tectonic events of the Guayana 

Shield of Venezuela, and figure 2 is a simplified geologic 

province map. Geologic and geographic maps of the Guayana Shield of Venezuela are presented in U.S. Geological 

Survey and Corporation Venezolana de Guayana, Tecnica 

Minera, C.A. (1993), and mineral deposits mentioned herein 

are shown on plate 1 of Sidder (this volume).

In this paper, we present an overview of the geology of 

the Venezuelan Guayana Shield and discuss its relation to 

the geology of the entire Guayana Shield. The discussion of 

the evolution of the shield includes recently published uranium-lead and samarium-neodymium isotopic data. Geochronological data have been standardized by recalculation 

with one set of constants as recommended by the Subcommission on Geochronology of the International Union of 

Geological Sciences3 (Steiger and Jager, 1977); errors in all 

dates are given at the 1-sigma level. Such standardization 

helps to define more narrowly the ranges of specific magmatic or tectonic events. For example, a single tectonic episode such as the Trans-Amazonian orogeny, which 

previously had been reported to span 400 m.y., is herein constrained to a much shorter interval of 190 m.y. Similarly, the 

formation of the Supamo Complex (granite and gneiss associated with the greenstone-belt rocks), which reportedly 

occurred between 2,700 and 2,100 Ma, is herein restricted to 

2,230-2,050 Ma. These recalculated dates of the major geologic events in the Venezuelan Guayana Shield allow its history to be interpreted more realistically (table 1, fig. 2).

IMATACA COMPLEX

The Archean Imataca Complex is a northeast-trending 

belt of amphibolite- to granulite-facies metasedimentary and 

metaigneous rocks. This belt is at least 510 km long and 

65-130 km wide, and it forms the northernmost margin of 

the Venezuelan Guayana Shield (fig. 2). Rocks of the 

Pliocene and Pleistocene Mesa Formation (not shown in fig. 

2) and alluvium from the floodplain of the Rio Orinoco cover 

the Imataca Complex along its northern margin, and the Guri 

shear zone separates the Imataca Complex from the Early 

Proterozoic greenstone-granite terrane to the south. The 

Imataca Complex abuts against plutonic and volcanic rocks 

of the Early Proterozoic Cuchivero Group along the Rfo 

Caura in the west. The nature of the contact is unknown 

because it is obscured by thick overburden and alluvium 

along the Rio Caura (Kalliokoski, 1965; Ascanio, 1975; 

Mendoza, 1977a).

The Imataca Complex includes more than 80 percent 

quartzofeldspathic orthogneiss, paragneiss, and felsic granulite, 10-15 percent intermediate to mafic orthogneiss, granulite, and charnockite, 1 percent metamorphosed banded iron 

formation, and minor manganiferous metasedimentary 

rocks, dolomitic marble, and anorthosite. The protolith of the 

Imataca Complex consisted of clastic and chemical sedimentary rocks, silicic calc-alkaline subaerial volcanic rocks, and 

lesser plutonic rocks (Kalliokoski, 1965; Dougan, 1977; 

Gibbs and Wirth, 1986).

The grade of metamorphism in the Imataca Complex 

varies from two-pyroxene granulite facies in that part of the 

belt generally northeast of the Lago de Guri area to

3A11 rubidium-strontium isochron dates reported here have been recalculated with the decay constants and isotopic abundances recommended by 

Steiger and Jager (1977): 87Rb decay constant=1.42xlO" 1 yr" 1 ; atomic ratio 

85Rb/87Rb=2.59265; atomic ratio 86Sr/88Sr=0.1194; atomic ratio 

84Sr/86Sr=0.056584. A best-fit line has been calculated by the method of 

York (1969). Dates reported are those from model 3 of York, which assumes that the scatter of data is due to a combination of the assigned analytical error and a normally distributed variation in the initial 87Sr/86Sr. All 

potassium-argon dates have been recalculated (where sufficient data are 

available) using the decay constants and isotopic abundances recommended 

by Steiger and Jager (1977): A^Ke+A^K^O.SSlxlO'^/yr; 

40Kp=4.962xlO' 10yr' 1 ; 40K=0.01167 atomic percent (1.167xlO'4 moymol); 

or by conversion with the critical table for conversion of K-Ar ages from old 

western constants to new IUGS constants (Dalrymple, 1979).
B6 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 1. Rock units and tectonic events of the Guayana

Shield of Venezuela.

[Rock unit designations as used in figure 2]

Alluvium (Qal) Quaternary alluvial sediments____________________

MESOZOIC-CENOZOIC UPLJFT

_______(uplift, tilt, and formation of erosion surfaces)_______ 

Diabase dikes (Mzd) Thin, elongated tholeiitic dikes (about 210-200

Ma)___________________________________ 

_________NICKERIE OROGENY (about 1,200 Ma)__________

Parguaza Granite (Yp) Massive, coarsely crystalline, porphyritic 

granite and biotite granite, commonly with rapakivi (wiborgite-type) 

texture (about 1,550 Ma)

Avanavero Suite (Xa) Continental tholeiitic dikes, sills, inclined 

sheets, and small irregular intrusive bodies (about 1,650 Ma; 

possibly as old as 1,850 Ma)

Roraima Group (YXr) Continental (fluvatile-deltaic and lacustrine) 

quartz sandstone and quartz-pebble conglomerate with lesser 

feldspathic arenite, arkose, siltstone, shale, jasper, chert, and 

interlayered felsic volcanic rocks (possibly as young as about 1,500 

Ma; possibly as old as about 1,900 Ma; dates from Uaimapue 

Formation are older than 1,650 Ma). Includes the Mataui 

Formation, the Uaimapue Formation, the Kukenan Formation, and 

the Uairen Formation

Undivided Proterozoic rocks (Pu) Synkinematic plutonic rocks 

(granite to tonalite and quartz diorite) and medium- to high-grade 

gneiss with both igneous and sedimentary protoliths in the Estado 

Amazonas only (about 1,860-1,730 Ma)_____________________

UNNAMED OROGENY 

__________(Estado Amazonas; about 1,860-1,730 Ma)________

Cuchivero Group (Xc) Thick sequence of unmetamorphosed felsic to 

intermediate subaerial volcanic rocks and their associated granitic 

rocks (about 1,930-1,790 Ma). Includes the granite of Guaniamito, 

the granites of San Pedro and Santa Rosalia (including the granite of 

Las Trincheras), and the Caicara Formation_________________

TRANS-AMAZONIAN OROGENY 

_____________(about 2,150-1,960 Ma)_______________

Supamo Complex (Xs) Gneiss, schist, migmatite, and granitic rocks 

such as trondhjemite (sodic granite), tonalite, granodiorite, and 

quartz monzonite associated with the greenstone-belt terrane (2,230- 

2,050 Ma)

Greenstone-belt rocks (Xg) Sequences as thick as 11,000 m of 

metamorphosed tholeiitic basalt and gabbro with interflow chemical 

sedimentary rocks in the lower part of the sequence; interstratified, 

porphyritic, tholeiitic and calc-alkalic basaltic to rhyolitic lava flows 

and tuffs in the middle part; and tuffaceous, volcaniclastic, 

turbiditic, pelitic, and chemical sedimentary rocks in the upper part. 

About 2,250-2,100 Ma. Includes the Los Caribes and Caballape 

Formations of the Botanamo Group; the Yuruari Formation and the 

Cicapra and El Callao Formations of the Pastora Supergroup; and 

the Real Corona-El Torno assemblage_______________________

PRE-TRANS-AMAZONIAN TECTONOMAGMATIC EVENT 

_____________(about 2,800-2,700 Ma)_______________

Imataca Complex (Ai) Amphibolite- to granulite-facies

quartzofeldspathic orthogneiss, paragneiss, and felsic granulite, 

intermediate to mafic orthogneiss, granulite, and charnockite, 

metamorphosed banded iron formation, and minor manganiferous 

metasedimentary rocks, dolomitic marble, and anorthosite (>2,800 

Ma; protolith possibly 3,700-3,400 Ma)

amphibolite facies southwest of the area. The gneiss is 

commonly migmatitic and consists of quartz-potassium 

feldspar-plagioclase±biotite±hornblende±orthopyroxene ± 

clinopyroxene±garnet± sillimanite±cordierite± muscovite. 

Estimates of peak metamorphic conditions in granulite-facies rocks indicate that temperature was between 

about 750°C and 800°C and pressure was between about 

8.0 and 8.5 kb (Short and Steenken, 1962; Swapp and Onstott, 1989); in the amphibolite-facies rocks, temperature 

was between about 625 °C and 700°C, and pressure was 

between 4 and 7 kb (Dougan, 1974, 1977).

Rocks of the Imataca Complex are strongly deformed. 

The entire stratigraphic sequence of the complex was folded 

into large isoclinal folds that were refolded by relatively 

open folds (Ruckmick, 1963; Onstott and others, 1989). In 

the northern part of the Imataca Complex, the isoclinal fold 

axes strike generally northwest, whereas in the southern part 

they strike east-west. Fold axes are deflected to the northeast 

close to the Guri shear zone (fig. 2), which can be traced in 

the field for a distance of more than 400 km (Onstott and others, 1989). This shear zone, from several hundred meters to 

1 km in width, is marked by alternating bands of mylonite, 

pseudotachylite, and strongly sheared gneiss and amphibolite. Crushed rock is visible in thin sections of samples collected as far away as 2 km on either side of this shear zone 

(Short and Steenken, 1962). The deflection of structural 

trends and mineral lineations adjacent to the Guri fault indicates that major movement on the fault was left-slip, possibly with later vertical displacement. The rocks of the Imataca 

Complex are also cut by north-verging low-angle thrust 

faults that are associated with the Guri shear zone and in part 

caused the folding (Ascanio, 1975). High-grade mylonite 

zones, such as the El Pao and the Rio Claro fault zones, also 

cut the Imataca Complex (Short and Steenken, 1962; Onstott 

and others, 1989; Swapp and Onstott, 1989). These faults 

may have been reactivated one or more times in the Proterozoic and Phanerozoic during the Nickerie and Caribbean 

orogenies (Gibbs and Barron, 1993; Olmore and others, 

1993). For example, faults that cut the Imataca Complex and 

are subparallel to the Guri fault zone show evidence of 

postuplift, Cenozoic, right-lateral and normal displacement 

(Olmore and others, 1993).

AGE

Most of the radiometric dates of rocks in the Imataca 

Complex record regional metamorphic and magmatic 

events. Metasedimentary protoliths for some gneissic rocks 

of the Imataca Complex have been dated at 3,700-3,400 Ma 

by whole-rock rubidium-strontium isochron and lead-lead 

methods (Montgomery, 1979); it is possible that these dates 

reflect an inherited detrital Archean component rather than a 

primary age of deposition (R.M. Tosdal, U.S. Geological 

Survey, oral commun., 1990). Rocks of the Imataca 

Complex were deformed, intruded, and regionally metamorphosed about 2,800-2,700 Ma. During the TransAmazonian orogeny, about 2,150-1,960 Ma, they underwent upper amphibolite- to granulite-facies metamorphism 

and granitic intrusion (Hurley and others, 1976; Onstott and
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD

64° 62°

B7

60°

Alluvium (Quaternary)

Parguaza Granite (Middle Proterozoic)

Avanavero Suite and diabase dikes _

(Early Proterozoic and Mesozoic) 

YXiv| Roraima Group (Early and Middle 

Proterozoic)

Cuchivero Group (Early Proterozoic)

Undivided Proterozoic rocks

Supamo Complex (Early Proterozoic)

Greenstone-belt rocks (Early

Proterozoic) 

Imataca Complex (Archean)

200 KILOMETERS

Figure 2. Simplified geologic provinces of the Venezuelan Guayana Shield. Rock units are described in table 1. Modified from Rodriguez 

and others (1976).

others, 1989). A sensitive high mass-resolution ion microprobe (SHRIMP) study of the uranium-lead ages of zircons 

collected from sand in the Rio Orinoco west of Ciudad Bolivar identified a small, discrete population of zircons that 

have an age of about 2,800 Ma in addition to a larger population having an age of about 2,100-2,000 Ma (Goldstein 

and Arndt, 1988). The scarcity of zircon grains of Archean 

age demonstrates that Archean rocks form only a minor proportion of the Venezuelan Guayana Shield (Goldstein and 

Arndt, 1988).

Rocks correlative in age to the Imataca Complex are not 

known elsewhere in the Guayana Shield; however, amphibolite- to granulite-facies rocks, including banded iron formation, in the Kenema-Man domain of the Leo Shield and in the 

western Reguibat Shield of the West African craton may be 

correlative with rocks of the Imataca Complex (Cahen and
B8 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

others, 1984; Cohen and Gibbs, 1989; Rocci and others, 

1991). In the Archean terrane of West Africa, quartzofeldspathic gneiss, migmatite, and granite are ubiquitous, and layered and massive gabbro and ultramafic rocks, amphibolite, 

banded iron formation, anorthosite, and calc-silicate or marble beds are common (Williams, 1988; Rocci and others, 

1991). The peak temperature of metamorphism for these 

West African shield rocks was about 750°C-850°C, and 

pressure estimates of 7-10 kb indicate a depth of equilibration of about 21-30 km (Williams, 1988; Rocci and others, 

1991). Paleomagnetic reconstructions and geochronological 

data indicate that the Guri shear zone in the Venezuelan 

Guayana Shield is aligned with the Sassandra-Trou Mountain fault zone in the Leo Shield of the West African craton, 

which may also be equivalent to the Zednes fault in the 

Reguibat Shield. The latter two faults also separate Archean 

high-grade metamorphic rocks from Early Proterozoic, 

lower grade greenstone-granite terranes (Onstott and Hargraves, 1981; Caen-Vachette, 1988; Cohen and Gibbs, 1989; 

Rocci and others, 1991; Boher and others, 1992).

Several high-grade metamorphic terranes in the Guayana Shield and the West African craton have been considered to be Archean in age because of their intense 

deformation and high-grade metamorphism. For example, 

granulite and charnockite in the Apiau Complex, Brazil, the 

Kanuku Complex, Guyana, and the Falawatra Group in the 

Bakhuis Mountains, Suriname, are part of the central Guyana granulite belt (Gibbs and Wirth, 1986; Gibbs and Barron, 1993). Although these rocks and the L'lle de Cayenne 

Complex in French Guiana have structural, stratigraphic, 

and petrographic similarities to the Imataca Complex, they 

have apparent protolith ages of about 2,300-2,200 Ma, and 

peak metamorphism in these rocks is related to the 

Trans-Amazonian orogeny at about 2,000 Ma (Priem and 

others, 1978; Ben Othman and others, 1984; Teixeira and 

others, 1984; Gibbs and Wirth, 1986; Rowley and Pindell, 

1989; Teixeira and others, 1989; Gibbs and Barron, 1993). 

Uranium-lead and rubidium-strontium isotopic evidence for 

an age older than 2,400 Ma does not exist for these rocks 

(Priem and others, 1978; Gibbs and Barron, 1993). Two quasicratonic nuclei, the Pakaraima nucleus in Estado Amazonas of Venezuela and the Xingu nucleus of northern Brazil, 

that consist of granitoid gneiss, migmatite, amphibolite, 

quartzite, and schist are assumed to be Archean in age 

because of their upper amphibolite- to granulite-facies metamorphic grade (Cordani and Brito Neves, 1982; Goodwin, 

1991); however, only a few radiometric dates have been 

reported for these areas (Cordani and Brito Neves, 1982; 

Goodwin, 1991), and the age of these granitic and gneissic 

rocks is still unknown. Similar high-grade metaigneous and 

metasedimentary gneisses in the West African craton have 

precise isotopic ages of formation of about 2.19-2.14 Ga, 

and metamorphism occurred about 2.15-2.14 Ga (Boher and 

others, 1992). Tonalitic and trondhjemitic rocks associated 

with granulitic rocks in the Cupixi area, Amapa Federal Territory, northeastern Brazil, may be Late Archean in age (De 

Vletter and Kroonenberg, 1984; Teixeira and others, 1989).

MINERAL DEPOSITS

Iron is the predominant metal produced from the 

Imataca Complex. Several deposits of enriched banded iron 

formation in the Imataca Complex, such as Cerro Bolivar 

and San Isidro (Sidder, this volume, pi. 1), rank among the 

world's largest (Gruss, 1973; Sidder, this volume). Reserves 

of iron ore are greater than 1,855 million metric tons at a 

grade of about 63 percent iron and about 11,700 million metric tons at a grade of about 44 percent (Rodriguez, 1986, 

1987; Doan, 1994). Banded iron formation protore consisted 

of an oxide facies assemblage in which magnetite and hematite were the dominant iron minerals. Enriched banded iron 

formation ore, composed predominantly of goethite and 

limonite, generally is in the limbs and centers of synclines. 

The iron-rich beds are intimately interbedded with layers of 

silica, present as quartz, and iron-bearing metamorphic minerals such as greenalite, grunerite, cummingtonite, crossite-magnesioriebeckite, acmite, and chlorite (Ruckmick, 

1963; Gruss, 1973; Ascanio, 1985; Moreno and Bertani, 

1985a). These deposits are most similar to Superior-type 

banded iron formation, although some Algoma-type banded 

iron formation may also be present (Sidder, this volume). 

The precious-metal content of these deposits is apparently 

low (Engineering and Mining Journal, 1987).

Small deposits and prospects of manganese and bauxite 

are present in the Upata-El Palmar-Guacuripia area (Sidder, 

this volume, pi. 1). Beds of secondarily enriched manganese 

ore are interstratified with gneiss, migmatite, amphibolite, 

and granulite of the Imataca Complex (Drovenik and others, 

1967). These rocks are part of a stratigraphic sequence of 

gondite, quartz-biotite schist, amphibole schist, and dolomitic marble that is less than 500 m thick. The individual 

manganiferous beds are generally less than 10 m thick and 

have strike lengths of as much as 20 km or more (Drovenik 

and others, 1967). Drovenik and others concluded that the 

sedimentary-nonvolcanogenic manganese deposit model 

best represents the protore deposits of manganese in the 

Imataca Complex; however, the association with felsic to 

intermediate volcanic rocks in some areas (Dougan, 1977) 

rather than with a sedimentary protolith suggests that the 

sedimentary-volcanogenic manganese model (Sidder, 1991) 

may also characterize some of the manganese deposits. 

Bauxite in the Upata district is locally associated with weathered gabbro, amphibolite, and possibly granitic gneiss of the 

Imataca Complex (Candiales, 1961). A large-tonnage, 

low-grade high-silica bauxite deposit was recently discovered north of El Palmar on the Palsapa Plateau. Volcanogenie massive sulfide deposits have not been discovered in 

the Imataca Complex or elsewhere in the Guayana Shield 

(Gibbs and Barron, 1983, 1993).
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B9

PRE-TRANS-AMAZONIAN 

TECTONOMAGMATIC EVENT

Intrusion of homogeneous granitic rocks and injection 

gneiss and the development of migmatite characterize the 

pre-Trans-Amazonian tectonomagmatic event about 

2,800-2,700 Ma in the Guayana Shield. The Cerro La Ceiba 

migmatite in the Imataca Complex is representative of rocks 

formed at this time (Hurley and others, 1976).

This event is correlative with the Liberian tectonothermal event in the Leo and Reguibat Shields of the West African craton, during which Archean rocks were 

metamorphosed and intruded by plutonic rocks between 

about 2,780 and 2,750 Ma (Hedge and others, 1975; Rollinson and Cliff, 1982; Tysdal and Thorman, 1983; Cahen and 

others, 1984). This event is also known as the Aroense in 

Venezuela, the Guriense in Guyana, and the Jequie in Brazil 

(Singh, 1974; Schobbenhaus and others, 1984).

GREENSTONE BELTS

Early Proterozoic greenstone-granite terranes in the 

central and eastern parts of the Venezuelan Guayana Shield 

(fig. 2) comprise an area of about 360 km by 250 km. Felsic 

to intermediate v olcanic rocks of the Cuchivero Group and 

clastic sedimentary rocks of the Roraima Group overlie 

rocks of the greenstone-granite terranes to the west and 

south. Rocks of the greenstone-granite terranes are continuous with those identified in Guyana to the east. The contact 

between the greenstone-granite terranes and the Imataca 

Complex to the north is along the Guri shear zone (Gibbs and 

Olszewski, 1982; Onstott and others, 1989).

Rocks of the greenstone belts in Venezuela have a total 

thickness of 11,000 m or more. Named stratigraphic units of 

the Venezuelan greenstone belts include the Pastora Supergroup and the Botanamo Group (table 1). The Pastora Supergroup, which consists of the Carichapo Group (El Callao and 

Cicapra Formations) and the Yuruari Formation, is discordantly overlain by the Botanamo Group, which consists of 

the Caballape and Los Caribes Formations. Granitic plutons 

and domelike batholiths, gneiss, and migmatite of the 

Supamo Complex divide these metasedimentary and metaigneous rocks of the greenstone belts into branching synclinoria (Menendez, 1968,1972; Benaim, 1972,1974; Cox, Gray, 

and others, 1993). Metasedimentary and metavolcanic rocks 

of the Real Corona-El Torno assemblage to the west of the 

Rio Aro (fig. 2) are tentatively correlated with those of the 

Pastora Supergroup (Kalliokoski, 1965). Rocks of the 

Pastora-Botanamo greenstone belts are correlative with 

those of the Barama-Mazaruni Supergroup in Guyana, the 

Marowijne Group in Suriname, the Paramaca Series (Orapu 

and Bonidoro Groups) in French Guiana, and the Vila Nova 

Group in Brazil (Bosma and others, 1983; Gibbs and Barron,

1983, 1993; Schobbenhaus and others, 1984; Teixeira and 

others, 1984, 1989; Gruau and others, 1985). All of these 

metaigneous and metasedimentary rocks form part of the 

Maroni-Itacaiunas province, a so-called mobile belt or 

greenstone belt that makes up a large part of the supracrustal 

rocks of the Amazonian craton (Cordani and Brito Neves, 

1982; Teixeira and others, 1989; Goodwin, 1991).

Early Proterozoic greenstone belts of the Guayana 

Shield are comparable in age and lithology to Birimian 

greenstone belts of the West African craton (Black, 1980; 

Kesse, 1985; Cohen and Gibbs, 1989; Boher and others, 

1992; Sylvester and Attoh, 1992). Rocks of these greenstone 

belts have been correlated on the basis of grossly similar 

lithostratigraphy, chemistry, age, structure, and intensity of 

metamorphism. The rocks of the Guayana Shield and the 

West African craton probably represent several penecontemporaneous greenstone belts, not one continuous belt.

Rocks of the greenstone belts of the Guayana Shield 

were deposited predominantly in a submarine environment. 

Basalt containing pillow structures and chemical and mineralogical alteration characteristic of submarine spilitization 

dominates the lower parts of the greenstone-belt sequence. 

The middle part of the sequence has a higher proportion of 

porphyritic andesite, dacite, and rhyolite submarine and possibly subaerial lava flows and siliceous and tuffaceous interflow sediments. Turbiditic graywacke, pelite, tuff, chemical 

sedimentary rocks, and volcaniclastic rocks are dominant in 

the uppermost part of the greenstone-belt sequence. The 

transition from volcanic to sedimentary rocks is locally conformable (Menendez, 1972; Bosma and others, 1983; Gibbs 

and others, 1984; Gibbs and Wirth, 1986; Gibbs, 1987; Day 

and others, 1989). In the greenstone belt of western Guyana, 

which is continuous into Venezuela, basalt and gabbro 

(some representing slow-cooled interiors of thick flows and 

sills) make up about 75 percent of the igneous rocks, basaltic 

andesite and andesite flow rocks about 17 percent, and rhyolite flow and pyroclastic rocks about 8 percent (Renner and 

Gibbs, 1987). Both tholeiitic and calc-alkaline chemical 

trends are present in the volcanic rocks of the greenstone 

belts (Renner and Gibbs, 1987; Day and others, 1989).

Ultramafic rocks make up about 1-2 percent of the 

igneous rocks in the greenstone belts of the Guayana Shield 

(Gibbs, 1987). Komatiite has been tentatively identified in 

two isolated areas of the Guayana Shield in Venezuela on the 

basis of high magnesian content (22 weight percent MgO or 

more) (Tosiani and Sifontes, 1989); however, reported relic 

spinifex texture (Tosiani and Sifontes, 1989) is actually 

rosettes of amphibole after pyroxene in contact metamorphic 

aureoles of gabbroic intrusive rocks (Gray and others, this 

volume). Spinifex textures have not been reported elsewhere 

in the Guayana Shield (Gibbs, 1987), although peridotitic 

komatiite has been identified in central French Guiana on the 

basis of whole-rock chemistry (Gruau and others, 1985).

Mafic-ultramafic intrusive rocks are present throughout 

the stratigraphic sequence of the greenstone belts of the
BIO GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Guayana Shield. They commonly form layered complexes 

that include cumulate rocks such as pyroxenite4 and 

peridotite associated with gabbro and lesser anorthosite and 

diorite. These intrusive complexes are present both as 

strongly metamorphosed and deformed bodies and as relatively unmetamorphosed and undeformed bodies. In a few 

cases, the lower grade of metamorphism and less intense 

deformation are only in the interior parts of massive gabbroic bodies; the outer parts of these bodies are strongly 

deformed and metamorphosed. Some gabbroic rocks are 

relatively fresh and undeformed and therefore are apparently 

younger than the Trans-Amazonian orogeny (table 1); these 

gabbroic rocks may be associated with the Early to Middle 

Proterozoic Avanavero Suite (Benaim, 1972; Menendez, 

1972, 1974; De Roever and Bosma, 1975; Gibbs, 1986; 

Gibbs and Barron, 1993). Wynn, Page, and others (this 

volume) describe a mafic-ultramafic layered complex at 

Piston de Uroy, and Gray and others (this volume) describe 

similar rocks in the Sierra Verdun-Cerro Piedra del Supamo 

area.

Mafic and ultramafic rocks in the Venezuelan Guayana 

Shield form belts of small bodies of serpentinite and amphibole-talc-serpentine-carbonate rocks. Some of these gabbroic complexes apparently were preferentially intruded into 

the upper part of the El Callao Formation, where they are 

parallel to subparallel with basaltic lava flows of this formation. The maximum thickness of these gabbroic bodies is 

about 500 m (Menendez, 1972). Possibly some gabbro represents the slowly cooled interior of thick lava flows.

PASTORA SUPERGROUP

The Pastora Supergroup consists of the Carichapo 

Group (El Callao and Cicapra Formations) and the Yuruari 

Formation (table 1). The El Callao Formation is the oldest 

unit of the Pastora Supergroup. Its basal contact is everywhere intruded by granitic rocks of the Supamo Complex, 

and its upper contact is transitional with the Cicapra Formation or concordant with the Yuruari Formation. The El Callao Formation was originally described in the El Callao area 

by Korol (1965) and Menendez (1968). Rocks called El Callao Formation in other areas do not necessarily conform to 

the description of the unit from the type section. This is also 

true for other units of the greenstone belts; that is, descriptions outside of the area of the type section may not agree 

with those of the type section. Thus, it is possible, or even 

likely, that more than one greenstone belt is present in the 

Venezuelan Guayana Shield, even though present strati4For simplicity, the metaigneous and metasedimentary rocks in the 

greenstone belts are referred to by their precursor rock type name; that is, 

pyroxenite rather than metapyroxenite, basalt instead of metabasalt, 

graywacke instead of metagraywacke.

graphic nomenclature implies that only one belt exists. 

Recent compilations of the geology of the Venezuelan Guayana Shield (Cox, Gray, and others, 1993; Cox, Wynn, and 

others, 1993) retain formation names only in the El Callao 

area. In other areas, rocks are divided on a lithologic basis 

into units such as mafic to intermediate rocks containing 

abundant chlorite and actinolite, felsic metavolcanic rocks 

containing abundant quartz and white mica, gabbro, 

peridotite, and mica schist and phyllite (Cox, Gray, and others, 1993; Cox, Wynn, and others, 1993).

The El Callao Formation, as thick as 3,000 m, consists 

almost exclusively of metamorphosed low-potassium basaltic to andesitic lava flows that commonly contain pillow 

structures and have amygdular and brecciated flow tops 

(Menendez, 1968, 1972; Benaim, 1972). Minor ferruginous 

quartzite and ferruginous and manganiferous chert (metamorphosed banded iron formation?) and talc schist lenses are 

present in several areas. Rocks of the El Callao Formation 

have been metamorphosed to the greenschist facies and 

locally to the almandine-amphibolite subfacies of the 

amphibolite facies. Greenschist-facies rocks typically are 

biotite-chlorite-albite-epidote±actinolite schist. Close to granitic intrusions of the Supamo Complex, the rocks are 

amphibolite containing blue-green hornblende and plagioclase (albite to andesine). A metamorphic and color zonation 

has been identified in pillow lavas as far as 6 km from the 

intrusive rocks (Menendez, 1968). The light-green greenschist-facies lavas are darker, grayish-green to greenish-black, amphibolite-facies rocks toward the intrusive 

bodies. Amphibolite is also abundant within 30 km of the 

Guri fault (Cox, Wynn, and others, 1993).

Hills having irregular crests typify the topographic 

expression of the El Callao Formation. They are 300-800 m 

in elevation, about 100-500 m above the surrounding terrain. 

In contrast, gabbro in mafic complexes forms slightly higher 

and smoother crested hills. Red soil on the El Callao Formation supports a dense forest (Menendez, 1968).

The Cicapra Formation overlies the El Callao Formation and includes a sequence as thick as 2,000 m of rhythmically bedded submarine andesitic tuff, turbiditic graywacke, 

and silts tone in packets about 10 m thick. Lithic tuff, tuff 

breccia, volcanic agglomerate, and, in the uppermost part of 

the formation, manganiferous hematitic chert are minor 

components of the Cicapra Formation (Menendez, 1972). 

These rocks are greenschist-facies, porphyroblastic, 

quartz-poor actinolite-biotite-epidote-albite schist. Amphibolite developed locally in this formation near granitic rocks 

does not contain biotite or porphyroblasts of amphibole. 

Schistosity, oblique to parallel to stratification, is generally 

poorly developed; in areas near granitic intrusive rocks, it is 

better developed. Rocks of the Cicapra Formation form a 

completely flat topography covered by a clayey soil the color 

of red wine. This unit wedges out and disappears southeast 

of El Callao, at which point the Yuruari Formation rests on 

the El Callao Formation (Menendez, 1968).
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD Bll

The basal contact between the Yuruari and the Cicapra 

Formations is gradational and that between the Yuruari and 

El Callao Formations is both depositional and faulted. The 

Yuruari Formation consists of mica schist, phyllite, and felsic metatuff possibly derived from epiclastic and turbiditic 

rocks; rhythmically bedded packets of feldspathic sandstone, 

siltstone, and black shale are as thick as 50 m. Locally, tuffaceous breccia, manganiferous phyllite, and intercalated 

dacitic to basaltic tuff, breccia, and lava flows, and chert are 

also present (Menendez, 1968, 1972; Benaim, 1972). Some 

phyllite arid schist in the Yuruari Formation classified previously as metasedimentary rocks commonly contain relict 

phenocrysts of subhedral feldspar and rounded, embayed 

quartz, that indicates some phyllite and schist originated as 

felsic volcanic rocks (Cox, Gray, and others, 1993). The 

overall thickness of this formation is about 1,000 m (Menendez, 1968, 1972; Benaim, 1972; Day and others, this volume). Rocks of the Yuruari Formation are typically 

greenschist-facies chlorite-sericitelcalcite schist. Only 

locally are rocks of the Yuruari Formation intruded by granite of the Supamo Complex. In aureoles of granitic intrusions, homblende-hornfels and pyroxene-hornfels facies 

containing biotite, sillimanite or andalusite, chloritoid, tourmaline, and garae:t developed. The upper contact does not 

crop out, but the contact with the overlying Caballape Formation is apparently an angular unconformity and (or) a tectonic disconformily (Menendez, 1968). Low hills and plains 

exhibiting a rectangular drainage pattern and a varicolored 

(light to dark yellow, reddish yellow, and several shades of 

red) clayey residual soil have developed on the Yuruari 

Formation. Savainna-type vegetation is characteristic 

(Benaim, 1972; Menendez, 1972).

Rocks of the .greenstone belts of the Pastora Supergroup 

are strongly defonned and record at least two episodes of 

deformation. Recumbent isoclinal folds with folded axial 

planes are characteristic, and the axial planes are commonly 

parallel to subparallel with the borders of the granitic intrusions of the Suparno Complex (Menendez, 1972). Thus, the 

rocks of the Pastora Supergroup typically form synforms that 

wrap around granitic domes of the Supamo Complex. Foliation is commonly developed in rocks of the greenstone belt 

and is parallel to subparallel with the primary stratification. 

Foliation is best developed close to the contacts with the 

Supamo Complex. Cleavage parallel with the axial plane of 

the folds is also well developed in these rocks. Major shear 

zones as wide as 1 km and as long as 35 km cut the rocks of 

the Pastora Supergroup (Menendez, 1972, 1974).

BOTANAMO GROUP

Rocks in the Caballape and Los Caribes Formations of 

the Botanamo Group discordantly overlie the Pastora Supergroup. The Caballape Formation includes mafic to felsic 

lava and pyroclastic flows and breccia interbedded with epiclastic and turbiditic sedimentary rocks. Menendez (1968) 

estimated that graywacke, conglomerate, and siltstone make 

up 80 percent of the unit in the El Callao-Guasipati area, the 

remainder being andesitic to rhyodacitic pyroclastic tuff and 

breccia. Benaim (1972) noted, however, that only the basal 

part of the formation crops out in this area. Day and others 

(1989) determined that in the Anacoco area the Caballape 

Formation consists of about 80 percent basaltic to dacitic 

volcanic flows (some with pillow lavas) and associated pyroclastic rocks and about 20 percent volcanic breccia and 

graywacke and thin (1-5 cm thick) horizons of shale. Use of 

the name Caballape Formation should be restricted to those 

rocks in the type section area near El Callao. The basal 

contact of the Caballape Formation is discordant to unconformable with the Pastora Supergroup, and the upper contact 

is reportedly concordant with the Los Caribes Formation 

(Benaim, 1972). The minimum thickness of this formation is 

about 5,000 m (Menendez, 1968). Granite of the Supamo 

Complex apparently does not intrude rocks of the Caballape 

Formation, and thus amphibolite is not present in this formation, in contrast to the Pastora Supergroup (Cox, Wynn, and 

others, 1993). The Caballape Formation consists only of 

greenschist-facies schist containing chlorite, epidote, sericite, quartz, calcite, biotite, and opaque oxide minerals; the 

schist is only moderately folded into broad synforms. The 

terrain underlain by this formation is flat; low hills are elongate parallel with the trend of the beds, and the drainage pattern is rectangular or dendritic. The rocks weather to form a 

bleached soil (Benaim, 1972; Menendez, 1972).

The Los Caribes Formation consists of intercalated red 

phyllite and sandstone, polymict conglomerate, and siltstone 

and minor felsic tuff. Some authors excluded this unit from 

the greenstone-belt sequence and referred to it as 

pre-Roraima metasedimentary rocks or pre-Roraima foliated 

sandstone (Ghosh, 1985). It is likely that rocks of both 

pre-Cuchivero and post-Cuchivero age have been included 

within the Los Caribes Formation because some rocks 

mapped as Los Caribes Formation contain fragments of 

unmetamorphosed felsic tuff typical of the Caicara Formation of the Cuchivero Group, not greenschist-facies metavolcanic rocks (Cox, Wynn, and others, 1993). Thus, use of the 

name Los Caribes Formation should be restricted to that area 

near the type section in the Rio Cuyuni where the contact 

between the Caballape and Los Caribes Formations has been 

mapped as concordant and interdigitated (Benaim, 1972).

Sedimentary rocks of the Los Caribes Formation are 

metamorphosed to the greenschist facies, whereas sedimentary rocks of the Roraima Group are unmetamorphosed or 

only weakly thermally metamorphosed as shown by the 

presence of pyrophyllite and andalusite (Ghosh, 1985). Minerals such as chlorite, muscovite, epidote, chloritoid, and 

recrystallized sheared quartz in the Los Caribes Formation 

are representative of the greenschist facies. Also, in contrast 

to rocks of the Roraima Group, folds in conglomerate, sandstone, and phyllitic shale of the Los Caribes Formation are
B12 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

isoclinal or chevron in shape, and the rocks are foliated and 

have a fracture cleavage at a high angle to bedding (Benaim, 

1972; Ghosh, 1985; Lira and others, 1985). Some conglomeratic units are as thick as 60 m; however, the total thickness 

of this formation has not been determined. The Los Caribes 

Formation probably represents environments transitional 

from marine to continental.

Rocks of the Los Caribes Formation are chronologically correlative with those of the Cinaruco Formation in 

Estado Amazonas and with those of the Muruwa, Rosebel, 

and Orapu Formations in Guyana, Suriname, and French 

Guiana, respectively (Ghosh, 1985). These rocks also resemble a gold-bearing sequence known as the Tarkwaian Series 

in the eastern West African craton (Black, 1980; Bonhomme 

and Bertrand-Sarfati, 1982; Kesse, 1985; Cohen and Gibbs, 

1989; Vinchon, 1989; Ledru and others, 1994). The Tarkwaian Series consists of Early Proterozoic clastic metasedimentary rocks about 2,500-6,700 m thick that are folded within 

the Birimian greenstone-belt rocks (Ntiamoah-Agyakwa, 

1979; Black, 1980; Kesse, 1985; Norman and Appiah, 1989; 

Eisenlohr and Hirdes, 1992; Watkins and others, 1993).

REAL CORONA-EL TORNO ASSEMBLAGE

Supracrustal rocks, including basal feldspathic quartzite and conglomerate, tholeiitic basalt, gabbro, and thin beds 

of shale, chert, and ferruginous quartzite, form an east-trending structural basin about 100 km southwest of Ciudad 

Bolivar (fig. 2). The basin, or syncline, is about 45 km long 

and 16 km wide and is underlain both on the north and south 

by a gneissic basement about 2,240 Ma in age (Kalliokoski, 

1965; Sidder, Day, and others, 1991). The basal quartzite is 

in depositional contact with the basement of granitic gneiss. 

The quartzite is as thick as 150 m; thicknesses of the other 

units are not known. These rocks have been penetratively 

deformed; they exhibit well-developed foliation, mineral 

lineation, and mylonitic fabric and are metamorphosed to the 

amphibolite facies. Hills underlain by quartzite and chert are 

as much as 200 m above the generally flat gneissic terrain. 

Vegetation in the area is savannalike.

AGE, CHEMISTRY, AND ORIGIN

Greenstone belts of the Guayana Shield formed during 

the Early Proterozoic and closely resemble in structure, 

lithostratigraphy, and composition of their metavolcanic and 

metasedimentary rocks and in areal extent greenstone belts 

of Early Proterozoic age such as those associated with the 

West African Shield and the Penokean orogen in the Superior province of the Canadian Shield, as well as many greenstone belts of Archean age (Sims and others, 1989; Sylvester 

and Attoh, 1992). Geochronological data, including uranium-lead zircon and whole-rock samarium-neodymium and

rubidium-strontium isochron dates, document that the 

metavolcanic greenstone-belt rocks and associated granitic 

rocks were emplaced throughout the Guayana Shield 

between about 2,250 and 2,100 Ma (Gibbs and Olszewski, 

1982; Gruau and others, 1985). These ages are the same as 

those obtained from the most detailed geochronological 

study of rocks of the greenstone belts of Venezuela, which 

determined a conservative age range of less than 2,300 to 

about 2,050 Ma, and a narrower range of 2,165 to 2,080 Ma, 

for emplacement of the metavolcanic-metasedimentary 

greenstone-belt sequence and crystallization of granite of the 

Supamo Complex (Klar, 1979). A sample of dacitic tuff 

from the Yuruari Formation in the Lo Increible mining district has a uranium-lead zircon age of 2,131±10 Ma (Day and 

others, this volume), which coincides with the published 

ages of rocks of the greenstone belts. Voluminous magmatism in the Birimian greenstone belts of the Reguibat and 

Leo Shields of the West African craton has been dated by 

uranium-lead zircon and samarium-neodymium and 

lead-lead whole-rock dating methods as about 2.18-2.07 Ga; 

the maximum interval of 2.3-2.0 Ga includes scattered, less 

well constrained dates (Cahen and others, 1984; Boher and 

others, 1992; Hirdes and others, 1992; Taylor and others, 

1992).

The chemistry of rocks of the greenstone belts in the 

Guayana Shield has not been systematically studied. Those 

investigators who have conducted geochemical studies on 

these rocks noted that the original chemical composition of 

the igneous rocks has been altered by weathering, hydrothermal alteration (spilitization and potassium metasomatism), 

and greenschist- and amphibolite-facies regional metamorphism. Recent studies by Gibbs (1987) and Renner and 

Gibbs (1987) in Guyana and by Day and others (1989) in 

Venezuela provide data for representative areas of the Guayana Shield. Both tholeiitic and calc-alkaline differentiation 

trends are common in the volcanic rocks. Low-potassium 

subalkaline tholeiitic basalt and basaltic andesite are the 

dominant rock types; lesser subequal proportions of 

calc-alkaline andesite, dacite, and rhyolite are also present 

(Gibbs, 1987). Few rocks in the lower part of the greenstone 

belts from throughout the Guayana Shield have silica contents in the range from 63 to 68 weight percent, and the overall distribution of silica may be considered bimodal (Gibbs, 

1987); however, rocks of the Caballape Formation in the 

Anacoco area of Venezuela have a compositional continuum 

with a systematic variation in the major and trace elements 

that forms a cogenetic mafic to felsic calc-alkaline magmatic 

series (Day and others, 1989). The Birimian greenstone belts 

of the West African craton also include tholeiitic basalt overlain by calc-alkaline intermediate to felsic tuff and lava 

(Boher and others, 1992; Sylvester and Attoh, 1992; Vidal 

and Alric, 1994).

Early Proterozoic magmatism in the Guayana Shield 

(and in its associated West African craton) represents a 

major period of rapid crustal growth from mantle-derived
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B13

melts; however, discriminant diagrams used to define the 

tectonic setting of volcanic rocks are not consistent in specifying the tectonic environment of deposition of the greenstone-belt rocks of the Guayana Shield (Gibbs, 1987). The 

volcanic rocks were erupted predominantly in a submarine 

environment, and they have chemical characteristics of modern ocean floor basalts, island arc rocks, and continental arc 

rocks. Initial 87Sr/86Sr and 143Nd/144Nd ratios of 0.7019 and 

0.51002, respectively, as well as an eNd value of 2.1 (Gruau 

and others, 1985), indicate that the volcanic rocks are derivatives of mantle melts and do not contain any component of 

Archean continental crust. Similarly, the Birimian volcanic 

and granitic rocks contain a negligible component, if any, of 

Archean crust (Rocci and others, 1991; Boher and others, 

1992; Sylvester and Attoh, 1992; Taylor and others, 1992). 

Ambiguous geochemical signatures in the volcanic rocks of 

the Guayana Shield (and West African craton) may reflect an 

evolutionary history from young, immature, oceanic island 

arc to back-arc marginal basin and mature (thickened) island 

arc environments. The Trans-Amazonian orogeny (Eburnean orogeny in West Africa) resulted in the accretion of 

these new crustal components (Choudhuri, 1980; Bosma and 

others, 1983; Gibbs, 1987; Renner and Gibbs, 1987; Teixeira 

and others, 1989; Liegeois and others, 1991; Rocci and others, 1991; Boher and others, 1992).

Supracrustal rocks of the Real Corona-El Torno assemblage were deposited in a submarine environment within or 

marginal to the craton, possibly as a back-arc marginal basin 

inboard from the Pastora island arc or during an early eratonic rifting phase associated with the Trans-Amazonian 

orogeny. Closure of the basin and regional compressive tectonism associated with the later phases of the Trans-Amazonian orogeny produced penetrative deformation and 

metamorphism and variably thrust the basaltic rocks over the 

basal quartzite. The age of the basement gneiss places a maximum age on the rifting of 2,240 Ma (Sidder, Day, and others, 1991).

MINERAL DEPOSITS

Rocks of the greenstone belts of the Guayana Shield 

contain deposits of shear-zone-hosted, low-sulfide 

gold-quartz veins (Berger, 1986). Several deposits in Venezuela are currently being mined, including those in the El 

Callao, Lo Increible, and Botanamo districts (Sidder, this 

volume). Mafic metavolcanic rocks of the El Callao Formation are the most common ore host in the El Callao district, 

which is the largest gold-producing district in Venezuela and 

has total production of about 200 metric tons of gold; however, all rock types in the greenstone belts throughout the 

shield, except those in the Los Caribes Formation, are known 

to host low-sulfide gold-quartz veins (Korol, 1961; Carter 

and Fernandes, 1969; Menendez, 1972; Dahlberg, 1975; 

Blanc and others, 1980; Barnard, 1990). Ore in the greenstone belts is localized by faults and shear zones (Menendez, 

1972; Day and others, this volume). Quartz veins range in 

thickness from 2 cm to more than 10 m. Quartz is milky 

white to gray and locally banded. Native gold, minor to trace 

amounts of pyrite, and lesser amounts of tetrahedrite, chalcopyrite, bornite, molybdenite, scheelite, and sphalerite are 

the most typical metallic minerals in the quartz veins. Carbonate (commonly ankerite) in the quartz veins and carbonate alteration as much as 30 m into the wallrocks are 

common in some districts such as El Callao. In addition to 

carbonate alteration, the wallrocks are intensely silicified, 

sericitized, and propylitized (with epidote and chlorite) as 

much as several tens of meters from the veins, and tourmaline and mariposite (chrome mica) are variably present in the 

alteration assemblage (Macdonald, 1968; Banerjee and 

Moorhead, 1970; Barron, 1973; Menendez, 1974). These 

shear-zone-hosted, low-sulfide gold-quartz vein deposits are 

similar to Early Proterozoic gold deposits in Ghana and elsewhere in West Africa (Milesi and others, 1992; Dzigbodi-Adjimah, 1993; Petersen, 1993). The quartzvein-hosted and other gold deposits in Early Proterozoic 

rocks of the greenstone belts of West Africa, excluding the 

Tarkwaian gold-bearing conglomerates, have produced 

more than 1,000 metric tons of gold at grades between 2 and 

30 grams per metric ton, and reserves of more than 350 metric tons are present (Milesi and others, 1989, 1992; Dzigbodi-Adjimah, 1993).

Vuelvan Caras and Payapal are the only gold-bearing 

vein deposits in the Venezuelan Guayana Shield known to be 

present in granitic rocks (Graterol, 1974; Wynn and Sidder, 

1991). They are similar to gold deposits in Guyana, such as 

those at Omai, Peters' Mine, and Eagle Mountain, that are in 

stockwork veins within and at the periphery of granitic plutons that range in age from about 2,100 to 1,800 Ma as dated 

by whole-rock rubidium-strontium isochrons (Macdonald, 

1968; Barron, 1969; Carter and Fernandes, 1969; Berrange, 

1977; Elliot, 1986; Barnard, 1990; Gibbs and Barron, 1993). 

Although in Venezuela the association of granitic intrusive 

rocks and gold-bearing quartz veins is not well documented, 

several authors have noted that gold deposits in greenstone-belt rocks in Guyana, Suriname, and French Guiana 

are spatially associated with, if not genetically related to, 

granitic intrusive rocks (Carter and Fernandes, 1969; Banerjee and Moorhead, 1970; Dahlberg, 1975, Blanc and others, 

1980; Gibbs and Barron, 1993). Some gold deposits within 

the Birimian greenstone belts of Ghana in West Africa show 

a spatial association with granitic intrusive rocks (Leube and 

others, 1990). Early Proterozoic granitic rocks associated 

with the Birimian greenstone belts throughout West Africa 

contain gold reserves of less than 50 metric tons, which is a 

minor proportion (about 3 percent) of the total gold reserves 

in Early Proterozoic rocks (Milesi and others, 1992).

Metals other than gold have not been produced in any 

significant quantity from rocks of the greenstone belts of the 

Guayana Shield. Small prospects of manganese in Venezu-
B14 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

ela, such as San Cristobal and La Esperanza, and iron prospects in banded iron formation have not been systematically 

worked or evaluated. The manganese prospects in Venezuela 

are similar to sedimentary-volcanogenic deposits at Matthews Ridge in Guyana, Serra do Navio in Amapa Federal 

Territory, Brazil, and Nsuta in Ghana, Tambao in Burkina 

Faso, and Ziemougoula, Ivory Coast, of West Africa. Matthews Ridge produced about 1.3 million metric tons with a 

grade of about 39.3 percent manganese from 1961 to 1968 

and has ore reserves of about 288,500 metric tons of 37 percent manganese (Barron, 1973). Prospects in the San 

Cristobal area may be extensions of manganiferous strata 

from the Matthews Ridge area (Gibbs and Barron, 1993). 

Serro do Navio has measured reserves of 15.7 million metric 

tons at 39.3 percent MnO2, and total reserves of 20.1 million 

metric tons (Nagell, 1962; Damascene, 1982; Lima, 1984; 

Schobbenhaus and others, 1984). Production from Nsuta 

between 1954 and 1983 totalled about 12.8 million metric 

tons of manganese (Kesse, 1985). Reserves are estimated to 

be about 5 million metric tons of high-grade oxide ore (48.9 

percent manganese) and 2 million metric tons of low-grade 

oxide ore (34.3 percent manganese) and about 28 million 

metric tons of carbonate ore (26.9 percent manganese) 

(Kesse, 1985). Some of the manganese horizons in Ghana 

have a spatial relation with gold deposits (Leube and others, 

1990; Milesi and others, 1992). The manganese deposits are 

on the flanks of the volcanic belts and in the transition zone 

from volcanic rocks to basinal sedimentary rocks. These 

transition zones, or breaks, are characterized by chemical 

sedimentary rocks such as chert, iron-magnesium carbonate 

rocks, and carbon-rich argillite and are the foci for the gold 

deposits (Leube and others, 1990; Milesi and others, 1992).

Undiscovered deposits such as those of Algoma-type 

banded iron formation and Homestake-type gold may be 

present in the Early Proterozoic metavolcanic and metasedimentary rocks of the greenstone belts of the Venezuelan 

Guayana Shield. Banded iron formation is generally absent 

in the Guayana and West African Shields; however, manganese-rich exhalative rocks are common (Holtrop, 1965; 

Leube and others, 1990; Milesi and others, 1992). As discussed previously, manganese-rich rocks and other chemical 

sedimentary rocks are important exploration guides for gold 

deposits in the Early Proterozoic rocks of the greenstone 

belts in Ghana (Ntiamoah-Agyakwa, 1979; Leube and others, 1990).

Volcanogenic massive sulfide deposits have not been 

discovered in the Guayana Shield, although the tectonic and 

volcanosedimentary environments are favorable for volcanic-hosted kuroko-type massive sulfide deposits. Prospects 

in Guyana such as Groete Creek, near Aremu, and in the 

middle Puruni River area contain anomalous copper, zinc, 

and gold in metavolcanic and metasedimentary sequences 

(Barron, 1973; Gibbs and Barron, 1993). Fob/metallic volcanogenic massive sulfide deposits are also uncommon in the 

West African craton (Milesi and others, 1992). The only

known Early Proterozoic volcanogenic massive sulfide 

deposit in West Africa is Perkoa in Burkina Faso. It is a 

zinc-silver deposit dated at 2,120141 Ma (Marcoux and others, 1988); reserves are estimated to be 4.5 million metric 

tons at 17 percent zinc and about 60 grams silver per metric 

ton (Milesi and others, 1989, 1992). Although bands or layers of sulfide-rich rocks are rare in the West African Shield, 

disseminated pyrite and arsenopyrite are common in 

gold-bearing carbonate within the sequence of chemical sedimentary rocks in the transition zone from volcanic rocks to 

basinal sedimentary rocks (Leube and others, 1990).

Platinum-group elements, chromium, and nickel-copper deposits may be associated with mafic-ultramafic complexes in the greenstone belts. Anomalous amounts of 

platinum-group elements are present in mafic rocks at Piston 

de Uroy and in the Real Corona-El Torno assemblage (Sidder, this volume; Wynn, Page, and others, this volume). Platinum-group elements are also present in gold lode and placer 

deposits (Dahlberg, 1975; Sidder, this volume).

SUPAMO COMPLEX

The Supamo Complex includes paragneiss, schist, 

migmatite, and granitic rocks such as trondhjemite, tonalite, 

and granodiorite (Moreno and Mendoza, 1975). Quartz 

monzonite and granite that intrude trondhjemite, tonalite, 

and granodiorite and the greenstone-belt rocks of the Pastora 

Supergroup have also been included within the Supamo 

Complex by some authors (Menendez, 1972). Plutonic rocks 

of the Supamo Complex are massive to foliated and generally form domes; metamorphic grade in the country rocks 

increases from greenschist to amphibolite facies within 

about 6 km of the intrusive bodies. The marginal facies of the 

intrusive bodies are generally concordant with the 

supracrustal host rocks. The granitic rocks generally underlie 

a savanna that has small, rounded, isolated hills and a dendritic drainage pattern. The soil is sandy with minor clay and 

has a bleached or whitish color (Menendez, 1972, 1974; 

Benaim, 1974). Rocks of the Supamo Complex are not 

known to host any mineral deposits.

The age of the Supamo Complex is commonly 

described as ranging from 2,700 to 2,100 Ma; the younger 

rocks are said to be "remobilized Supamo." Recent uranium-lead isotopic data for zircons indicate that 

trondhjemite of the Supamo Complex crystallized between 

about 2,200 and 2,050 Ma (Klar, 1979). The age of emplacement of quartz monzonite plutons into the greenstone-granite terrane and the Imataca Complex also ranges from about 

2,200 to 2,050 Ma, on the basis of uranium-lead zircon dates 

(Klar, 1979), and possibly to 1,958 Ma, as indicated by 

rubidium-strontium data for biotite (Onstott, Hargraves, 

York, and Hall, 1984). The reinterpreted age of gneissic 

rocks in the Supamo Complex is about 2,230 Ma, consistent 

with an age of 2,227 Ma for the apparently correlative Bar-
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B15

tica Gneiss in Guyana (Gibbs and Olszewski, 1982). The 

maximum age for gneiss in the Supamo Complex is about 

2,300 Ma (Klar, 1979). Granite and high-grade gneiss associated with the greenstone belts of the West African craton 

also range in age from about 2,220 to about 2,070 Ma (Rocci 

and others, 1991; Boher and others, 1992; Taylor and others, 

1992). Trondhjemitic intrusive rocks and more potassium 

rich granite and granodiorite of the West African craton have 

similar ages (Boher and others, 1992), as is true in the Venezuelan Guayana Shield. Sodic granitoid rocks associated 

with the Nangodi greenstone belt in Ghana have high Sr/Nd 

and Tb/Dy ratios relative to normal mid-ocean ridge basalt 

that indicate the granitic rocks may have formed by melting 

of relatively hot, subducted slab material ("amphibolite") 

(Sylvester and others, 1993). Essentially contemporaneous 

potassic-rich granitic rocks, therefore, may have had less of 

a slab component and incorporated more sialic crustal material. Gneiss and migmatite in Ghana are coeval and cogenetic 

with the granitic rocks (Opare-Addo and others, 1993). Differences in structural and textural features between the metamorphic and granitic rocks, such as foliation, homogeneity, 

and grain size, are due to emplacement at different depths; 

the migmatites were generated at deeper crustal levels than 

the granitic rocks (Opare-Addo and others, 1993).

TRANS-AMAZONIAN OROGENY

The Trans-Amazonian orogeny was a major cycle of 

greenschist- to upper amphibolite- and granulite-facies 

metamorphism, deformation, and magmatic activity in the 

Guayana Shield during the Early Proterozoic. It was a period 

of continental collision and accretion of assorted Archean 

and Early Proterozoic terranes into the Amazonian craton, 

the subsequent common deformation of these accreted terranes, and the first development of continental environments 

on much of the craton. Paleomagnetic and 40Ar/39Ar data 

indicate that the Guayana and Guapore Shields and possibly 

the West African craton were combined as a single tectonic 

plate during the Trans-Amazonian orogeny (Gibbs and 

Wirth, 1986; Renne and others, 1988; Teixeira and others, 

1989; Rocci and others, 1991; Boher and others, 1992). The 

Imataca Complex was thrust over the Pastora-Supamo 

greenstone-granite terrane during the Trans-Amazonian 

orogeny, and oblique compression between these two terranes resulted in left slip along major fault zones such as the 

Guri shear zone (Swapp and Onstott, 1989). The Guri fault 

zone, and its inferred continuation in the West African craton, the Sassandra-Trou Mountain fault zone 

(Caen-Vachette, 1988; Cohen and Gibbs, 1989), is a suture 

between Archean rocks (the Imataca Complex in Venezuela) 

and Early Proterozoic greenstone-belt rocks.

The Trans-Amazonian orogeny in the Guayana Shield 

occurred between about 2,150 and 1,960 Ma and possibly 

continued to about 1,730 Ma. The wide range in ages may be

the result of tectonic activity that was time transgressive 

across the shield from northeast to southwest (Gaudette and 

Olszewski, 1985). Alternatively, the range in ages may span 

two distinct orogenic episodes, a predominantly collisional 

and metamorphic event accompanied by intrusive magmatic 

activity throughout the shield from about 2,150 to 1,960 Ma, 

and a period of intense metamorphism, deformation, and 

intrusion between about 1,860 and 1,730 Ma in the westernmost part of the shield in Estado Amazonas of Venezuela 

(Klar, 1979; Gibbs, 1980; Bosma and others, 1983; Teixeira 

and others, 1984; Gaudette and Olszewski, 1985). Postorogenic and (or) anorogenic magmatic activity from about 

1,930 to about 1,790 Ma that emplaced rocks of the Cuchivero Group and its equivalents and was accompanied by 

minor uplift but little or no deformation or metamorphism is 

not considered here to be part of the Trans-Amazonian orogeny.

Rocks of the Imataca Complex reveal a history of prograde metamorphism and retrograde cooling and uplift associated with the Trans-Amazonian orogeny. Detailed 

rubidium-strontium isotopic studies of the age of granulite-facies metamorphism in the Imataca Complex indicate 

an age of 2,022±67 Ma for the Trans-Amazonian orogeny 

(Montgomery and Hurley, 1978). This age apparently predates the peak pressure associated with the orogeny and 

postdates the peak temperature (Swapp and Onstott, 1989). 

Plateau dates from 40Ar/39Ar analyses of hornblende and 

biotite in the Imataca Complex range from about 2,044 to 

1,760 Ma and are interpreted to record decompression, 

uplift, and cooling following peak metamorphism of the 

Trans-Amazonian orogeny (Onstott and others, 1989; 

Swapp and Onstott, 1989). The Imataca Complex was 

uplifted from a depth of about 32 km to about 16 km during 

southward thrusting of the Imataca terrane over the greenstone belts within about 30 m.y. after initiation of uplift and 

after reaching peak pressure conditions (Swapp and Onstott, 

1989). Extremely slow cooling rates implied by plateau 

dates of biotite, potassium feldspar, and plagioclase from 

granulite-facies rocks in the core of the Imataca Complex 

indicate that uplift of the Imataca Complex had mostly 

ceased by about 1,962 Ma and that rocks in the complex 

cooled isobarically at intermediate crustal levels of about 15 

km until about 1,100 Ma (Onstott and others, 1989; Swapp 

and Onstott, 1989). All argon-bearing mineral systems were 

closed by 1,100 Ma, perhaps as a result of renewed uplift 

associated with the Nickerie or K'Mudku episode (table 1) 

(Onstott and others, 1989). In contrast, metamorphic rocks in 

the Carajas region, Brazil, of the Guapore Shield cooled rapidly and attained stable magnetization by about 1,910 Ma 

(Renne and others, 1988). Paleomagnetic and 40Ar/39Ar data 

preclude the possibility of any regional metamorphism in the 

Guapore Shield to even the lowest greenschist facies after 

the Trans-Amazonian orogeny (Renne and others, 1988).

The Trans-Amazonian orogeny in the Guayana Shield 

is defined here to consist of deformation, metamorphism,
B16 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

and magmatic activity between about 2,150 and 1,960 Ma. 

Rocks of the Pastora Supergroup were deformed in two 

pulses or episodes of tectonic activity within this interval, 

whereas rocks of the Botanamo Group were affected only by 

the second pulse of deformation.

The Eburnean orogeny in the West African craton was 

a period of igneous and metamorphic activity and deformation about 2,200-1,980 Ma, possibly between only 2,112 and 

2,073 Ma (Hedge and others, 1975; Onstott and Dorbor, 

1987; Cohen and Gibbs, 1989; Feybesse and others, 1989; 

Liegeois and others, 1991), coincident with the 

Trans-Amazonian orogeny in the Guayana Shield. Deformation associated with the Eburnean orogeny in the West African craton has been subdivided into two phases; the lower 

group of Birimian rocks (Birimian I) was affected by both 

phases, and the upper group (Birimian II) was folded just 

once (Rocci and others, 1991; Milesi and others, 1992). 

Recent work suggests, however, that the distribution of 

deformation features is spatially controlled by shear zones 

and is not temporally restricted to particular stratigraphic 

units (Mortimer, 1992; Watkins and others, 1993). Hence, all 

Birimian and Tarkwaian units have been affected by all 

phases of the Eburnean orogeny (Mortimer, 1992: Watkins 

and others, 1993). Deformation Dl is represented by thrust 

faults (Feybesse and Milesi, 1994), foliation SI that is subparallel with bedding (SO), lineation LI defined by the intersection of bedding and foliation SI, isoclinal folds PI, and 

low- to medium-grade metamorphism; it is referred to as a 

tangential (SI parallel to SO) deformation event (Feybesse 

and others, 1989; Milesi and others, 1989, 1992; Liegeois 

and others, 1991; Eisenlohr and Hirdes, 1992). Deformation 

Dl is attributed to collision tectonics. Calc-alkaline volcanism and deposition of volcaniclastic and turbiditic material 

in the upper Birimian, as well as deposition of the Tarkwaian 

sediments (equivalent to the Caballape and Los Caribes Formations in the El Callao area and the so-called eugeosynclinal metavolcanic rocks in the Botanamo and Anacoco areas 

[Cox, Wynn, and others, 1993]), followed deformation Dl. 

Deformation D2 was a sinistral transcurrent shear event 

characterized by left-lateral strike-slip faults that are locally 

associated with thrust zones, large, regional upright folds P2, 

a crenulation cleavage S2 that crenulates foliation SI, 

stretching lineation L2, and low- to medium-grade metamorphism (Feybesse and others, 1989; Liegeois and others, 

1991; Eisenlohr and Hirdes, 1992; Milesi and others, 1992; 

Vidal and Alric, 1994). This second deformation was also a 

period of compressive transcurrent tectonics. Uranium-lead, 

samarium-neodymium, and rubidium-strontium dates determined for Dl and D2 in West Africa indicate that these 

deformations associated with the Eburnean orogeny took 

place between about 2,112 and 2,073 Ma (Feybesse and others, 1989; Liegeois and others, 1991). In the eastern part of 

the West African shield, deformation D3 produced dextral 

shear zones, folds P3, foliation and crenulation cleavage S3, 

a stretching lineation L3, and reactivation and local offset of

D2 faults (Milesi and others, 1989, 1992; Liegeois and others, 1991; Watkins and others, 1993). Event D3 accompanied low-grade metamorphism at about 2,073 Ma (Milesi 

and others, 1989). A late or post-Eburnean tectonic shortening event characterized by a crenulation cleavage and a transcurrent shear zone, but unaccompanied by igneous or 

metamorphic activity, may have occurred as many as 90 m.y. 

after deformation D3 at about 1,980 Ma (Milesi and others, 

1989; Liegeois and others, 1991; Vidal and Alric, 1994).

The Cuchivero Group, about 1,930-1,790 Ma, is 

regarded here to be post-Trans-Amazonian in age. Postcollisional magmatism following collision and amalgamation of 

the island-arc greenstone belts with the Imataca Complex 

resulted in the eruption and intrusion of the Cuchivero Group 

and its equivalents. Rocks of the Cuchivero Group are generally unmetamorphosed and relatively undeformed, and 

their character is sufficiently different from that of rocks of 

the greenstone belts that they should be considered part of an 

unnamed thermomagmatic event. This post-Trans-Amazonian, postcollisional magmatic activity continued until about 

1,790 Ma. Similar, but less voluminous, posttectonic magmatism in the Reguibat Shield of the West African craton 

took place between about 1,970 and 1,750 Ma (Cahen and 

others, 1984; Rocci and others, 1991).

Metamorphism, deformation, and magmatic activity 

about 1,860-1,730 Ma in Estado Amazonas must be considered as part of a separate orogenic event (see discussion of 

undivided Proterozoic rocks). Gaudette and Olszewski 

(1985) correlated this younger orogenic episode in Estado 

Amazonas with the older Trans-Amazonian event 

(2,150-1,960 Ma); however, the age of 1,900±200 Ma 

reported for the Trans-Amazonian orogeny (Moreno and 

others, 1977; Gaudette and Olszewski, 1985), which is based 

on potassium-argon and whole-rock rubidium-strontium 

dates for an intrusion in the Imataca Complex and for rocks 

of the Cuchivero Group, is not compatible with the recent 

and more accurate uranium-lead and argon-argon dates for 

metamorphism and deformation of the Trans-Amazonian 

orogeny. As noted in the discussion on undivided Proterozoic rocks in Estado Amazonas, geologic mapping, 

geochemical sampling, and geochronologic dating are insufficient for a rigorous interpretation of the geology in this 

area.

CUCHIVERO GROUP

Early to Middle Proterozoic supracrustal rocks were 

emplaced in and deposited on the older greenstone-granite 

terrane in the southern, central, and western parts of the Venezuelan Guayana Shield (fig. 2). They include a thick pile 

(greater than 3 km thick) of mostly felsic to lesser intermediate and mafic volcanic, subvolcanic, and plutonic rocks and 

associated volcanogenic sedimentary rocks of the Cuchivero 

Group; sandstone and conglomerate and lesser siltstone,
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B17

shale, chert, and interlayered felsic volcanic rocks (jasper) of 

the Roraima Group; continental tholeiitic dikes, sills, 

inclined sheets, and small irregular intrusive bodies of the 

Avanavero Suite; and rapakivi granite of the Parguaza Granite (Rios, 1972; Mendoza and others, 1975; Ghosh, 1985; 

Gibbs, 1986; Sidder and Martinez, 1990).

The Cuchivero Group includes the relatively older volcanic rocks of the Caicara Formation and the younger granites of Guaniamito, San Pedro, and Santa Rosalia (Rios, 

1972; Mendoza and others, 1975). Mesozoic and (or) Cenozoic tilting of the shield and subsequent erosion have 

exposed the deeper level granitic rocks in the northern part 

of the Cuchivero terrane, whereas high-level volcanic rocks 

of the Caicara Formation predominate in the southern part. 

Broad, open folds are common, and structures such as faults 

and lineaments generally strike northwest to north-northwest 

and north-northeast. The El Viejo Formation, the volcanic 

suite of the Parucito Valley, and other relatively unmetamorphosed volcanic and plutonic rocks in Estado Amazonas, the 

Uatuma Supergroup (including the Surumu and Iricoume 

Formations and the granodiorite of Serra do Mel) in northern 

Brazil, the Kuyuwini and Burro-burro Groups in Guyana, 

and the Dalbana Formation in Suriname all correlate with the 

Cuchivero Group (Mendoza and others, 1975; Montalvao, 

1975; Talukdar and Colvee, 1975, 1977; Berrange, 1977; 

Mendoza and others, 1977; Tepedino, 1985; Gibbs, 1987; 

Sidder and Martinez, 1990; Machado and others, 1991; 

Gibbs and Barron, 1993; Dall'Agnol and others, 1994). 

Minor amounts of rocks equivalent to the Cuchivero Group 

are present in French Guiana and the West African craton 

(Cahen and others, 1984; Gibbs, 1987; Rocci and others, 

1991).

CAICARA FORMATION

The Caicara Formation consists of subaerially deposited pyroclastic rocks including variably welded ash-flow 

and air-fall tuff and breccia and minor lava flows and domes 

and intercalated volcaniclastic rocks. The rocks are aphyric 

to porpltyritic, and both crystal-rich and crystal- and 

lithic-rich varieties are present. Vitroclastic and eutaxitic 

textures including devitrified glass shards and collapsed 

pumice fragments are common (Rios, 1972; Mendoza, 

1977b; Sidder and Martinez, 1990). The Caicara Formation 

is made up of rhyolite, subordinate rhyodacite and dacite, 

and minor proportions of andesite, basaltic andesite, and 

basalt. On a total alkali-silica diagram, some rocks of the 

Caicara Formation are classified as trachyte and trachydacite. Silicic rocks are mainly tuff, whereas andesitic and 

basaltic rocks are lava flows and dikes. These rocks are metaluminous to peraluminous, subalkalic to alkalic, and 

together form an apparent comagmatic calc-alkaline series 

(Talukdar and Colvee, 1975, 1977; Mendoza, 1977b; Sidder 

and Martinez, 1990). Felsic to intermediate volcanic and volcaniclastic rocks near Ichun Tepuy (fig. 1) named the Ichun 

Formation are interpreted to be part of the Roraima Group 

(Briceno and others, 1989); however, these rocks are chemically and lithologically similar to rocks of the Caicara Formation to the north and south along the Rio Paragua and 

have the same stratigraphic position. Moreover, the basal 

conglomerate and overlying sandstone of the Roraima 

Group near Ichun Tepuy conformably overlie the volcanic 

rocks (Sidder, unpublished data, 1988). A similar 

conformable contact has been mapped between rocks of the 

Caicara Formation and the overlying Roraima Group in 

Estado Amazonas (Stephen D. Olmore, U.S. Geological 

Survey, oral commun., 1990), and intercalations of volcanic 

tuff of the Surumu Formation are common near the base of 

the Roraima Group in Brazil within 80 km south of the 

Brazil-Venezuela-Guyana junction (Amaral and Halpern, 

1975).

GRANITE OF THE CUCHIVERO GROUP

Granite associated spatially and temporally with the 

volcanic rocks of the Caicara Formation includes the granites of San Pedro, Santa Rosalia (including the granite of Las 

Trincheras), and Guaniamito. These granitic bodies comprise hypabyssal biotite granite, quartz monzonite, and granodiorite (Rios, 1972; Mendoza, 1974; Tepedino, 1985) and 

are in intrusive and fault contact with the volcanic rocks. The 

rocks are generally equigranular to porphyritic and medium 

to coarse grained. The granite of San Pedro is dominantly a 

fine-grained leucocratic granite that has been interpreted as 

a marginal border phase of the coarser grained biotite granite 

of Santa Rosalia (Mendoza, 1974). The granite of Guaniamito is a porphyritic, medium to coarsely crystalline, 

biotite±hornblende granite. All of these granites are generally massive in texture but locally are foliated, especially 

near the intrusive contact of granite with the volcanic rocks 

of the Caicara Formation (Rios, 1972). Primary minerals 

include potassium feldspar (orthoclase and microcline, 

20-60 modal percent), quartz (10-40 percent), plagioclase 

(albite-oligoclase, 5^0 percent), biotite (<1-10 percent), 

and accessory sphene, apatite, zircon, muscovite, hornblende, allanite, and iron-titanium-oxide minerals (magnetite and lesser ilmenite). Secondary alteration minerals are 

epidote, clinozoisite, and white mica in plagioclase and 

potassium feldspar, chlorite after biotite, and hematite after 

magnetite. Aplite dikes and barren quartz veins commonly 

cut the granitic bodies (Rios, 1972; Mendoza, 1974; Tepedino, 1985; Sidder, unpublished data, 1988). Hypabyssal 

biotite granite and leucogranite in Suriname that are associated with and intrude rhyolitic volcanic rocks of the Dalbana 

Formation (De Roever and Bosma, 1975; Bosma and others, 

1983) are considered to be comagmatic equivalents of the 

volcanic rocks (Mendoza, 1977b; Bosma and others, 1983).
B18 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Granitic and volcanic rocks of the Cuchivero Group and 

their equivalents throughout the Guayana Shield are generally unmetamorphosed. Reports of lower greenschist facies 

metamorphism apparently refer to local contact metamorphic aureoles in volcanic and volcaniclastic rocks close to 

intrusions. For example, field and petrographic evidence of 

metamorphism is not present in samples of volcanic rocks 

from the upper Rio Caura or Rio Paragua areas (Sidder, 

unpublished data, 1990; Sidder and Martinez, 1990). Indeed, 

as noted previously, vitroclastic and eutaxitic textures are 

abundant. Minor alteration in the volcanic and granitic rocks, 

such as partial replacement of feldspar by fine-grained sericite and epidote, chlorite alteration of biotite, and thin veinlets of quartz or epidote+chlorite, is indicative of deuteric 

and local hydrothermal alteration. Rios (1972) recognized 

that the volcanic rocks of the Caicara Formation are thermally metamorphosed in restricted zones close to intrusions. 

It is significant to note that Rios called primary flow bands 

developed during the extrusion and emplacement of the volcanic rocks "foliation," or foliation. He did not recognize 

any effects of regional metamorphism. Other authors have, 

however, referred to the Caicara Formation as a sequence of 

metavolcanic rocks (Tepedino, 1985), and still others have 

called the rocks metavolcanic but have noted that the rocks 

have undergone contact metamorphism ("thermal-regional" 

or "plutono-metamorphism") only due to intrusion of the 

granitic batholith of the Cuchivero Group (Mendoza, 

1977b). Similarly, Bosma and others (1983) referred to rhyolitic rocks of the Dalbana Formation in Suriname as 

metavolcanic but noted (p. 247) that "Distinctly recrystallized metavolcanics***form broad marginal zones along the 

granites ***The recrystallization, without significant foliation or folding, is spatially related to granite intrusions and 

probably took place at shallow depth under hornblende-hornfels facies conditions." Thus, the volcanic rocks 

of the Caicara Formation and granitic rocks of the Cuchivero 

Group and similar rocks throughout the Guayana Shield are 

not regionally metamorphosed, but they are contact metamorphosed or hydrothermally altered in proximity to intrusions, faults, dikes, and veins (De Roever and Bosma, 1975; 

Sidder and Martinez, 1990).

AGE AND ORIGIN

Whole-rock rubidium-strontium dates for rocks of the 

Cuchivero Group and its equivalents throughout the Guayana Shield range from 1,930 to 1,640 Ma, and the majority 

are between 1,930 and 1,790 Ma (table 2) (Hurley and others, 1977; Moreno and others, 1977; Bosma and others, 

1983; Schobbenhaus and others, 1984; Teixeira and others, 

1989; Machado and others, 1991). Initial 87Sr/86Sr ratios 

range from 0.698 for the granites of Guaniamito, San Pedro, 

and Santa Rosalia to 0.721 for volcanic rocks of the Surumu 

Formation in Brazil (table 2). The former initial ratio is not

geologically reasonable because basaltic achondrite meteorites have an initial ratio of 0.69897 (Faure, 1986). The latter 

ratio is extremely high due to scatter of the data; the mean 

squares weighted deviation (MSWD) is 60.9, and the 

1-sigma error in the calculated initial ratio is 0.017. Most of 

the initial ratios for rocks of the Cuchivero Group and its 

equivalents are between about 0.705 and 0.707, values 

between initial ratios inferred for Proterozoic crust (>0.708) 

and mantle-derived magmas (<0.7045) (Priem, 1987; 

Riciputi and others, 1990), and are a further indication of 

some contribution of continental crust in the generation of 

the magmas. Those initial ratios as high as 0.712 may reflect 

melts derived directly from crustal material.

Rocks of the Cuchivero Group and their equivalents 

have been considered both as orogenic, related to the 

Trans-Amazonian collision and deformation, and as anorogenic (De Vletter and Kroonenberg, 1984). Supporters of the 

orogenic interpretation suggest that the contact between the 

felsic volcanic rocks and rocks of the underlying greenstone 

belt is conformable, and therefore they include the Cuchivero Group and its equivalents as a second, dominantly magmatic, stage of the Trans-Amazonian orogeny between about 

2,000 and 1,870 Ma (Bosma and others, 1983; De Vletter 

and Kroonenberg, 1984; Teixeira and others, 1984). As 

noted above, however, characteristics of deformation and 

metamorphism in the Cuchivero Group are significantly different from those in rocks of the greenstone belts, and therefore a conformable contact is extremely unlikely. Those who 

suggest that the predominantly felsic igneous rocks of the 

Cuchivero Group and its equivalents are anorogenic have 

noted that the contact with rocks of the greenstone belts is a 

profound angular unconformity and that crustal extension, as 

evidenced by widespread intrusion of mafic dikes and sills of 

the Avanavero Suite, was penecontemporaneous with deposition of the volcanic rocks (Montalvao, 1975; Gibbs, 1980, 

1986; Gibbs and Olszewski, 1982). As discussed following, 

however, the Avanavero Suite (about 1,650 Ma) is younger 

than the Cuchivero Group, and these units are not coeval.

The volcanic and plutonic rocks of the Cuchivero Group are referred to here as postcollisional, 

post-Trans-Amazonian because they do not have a clear 

association with any orogenic belt, they are not regionally 

metamorphosed, and they are weakly deformed and lack a 

pervasive penetrative fabric. Any deformation that they 

underwent may be attributed to younger post-Trans-Amazonian events, and low-grade contact metamorphism in the 

volcanic rocks is related to granitic intrusions. Granites 

formed in postcollisional tectonic settings commonly postdate the collisional event by about 25-75 m.y. (Sylvester, 

1989), which is the approximate amount of time between termination of uplift of the Imataca Complex (collision with 

rocks of the greenstone belts) and magmatism in the Cuchivero Group. Limited geochemical data suggest that the tectonic environment of the granite of Santa Rosalia and the 

volcanic rocks of the Caicara Formation is transitional; the
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B19

Table 2. Rubidium-strontium whole-rock isochron dates for volcanic and plutonic rocks of the Cuchivero Group and its equivalents,

Guayana Shield.

[Number of samples on which age is based is given in parentheses after age. MSWD is mean squares weighted deviation]

Country

Venezuela

Venezuela

Guyana

Suriname

Suriname

Suriname

Brazil

Brazil

Brazil

Brazil

Unit

Caicara Formation

Granites of Santa Rosalia and San Pedro

Kuyuwini Group

Felsic volcanic rocks (Dalbana Formation)

Granitoid rocks

Granitic and volcanic rocks together1

Surumu Formation

Surumu Formation

Surumu Formation2

Granite of Serra do Mel

Age (Ma)

l,700±220(n=3)

l,880±88(n=7)

l,800±420(n=4)

l,930±48(n=18)

l,850±40(n=14)

1,880±31 («=32)

l,800±94(n=6)

1,640±55 (n=6)

l,820±55(n=10)

l,790±62(n=4)

(87Sr/86Sr)0

0.709

0.698

0.705

0.705

0.707

0.706

0.721

0.714

0.712

0.706

MSWD

1.72

24.3

20.9

2.25

2.58

2.64

60.9

18.2

45.7

4.63

Reference

Gaudette and others (1978).

Gaudette and others (1978).

Berrange(1977).

Priem and others (1971).

Priem and others (1971).

Priem and others (1971).

Basei and Teixeira (1975).

Amaral and Halpern (1975).

Basei and Teixeira (1975).

Basei and Teixeira (1975).

As reported by Priem and others(1971). 

Includes four samples analyzed by Amaral and Halpern (1975) as reported by Basei and Teixeira (1975).

1,000 F100 r

10 100 

YTTRIUM + NIOBIUM, IN PARTS PER MILLION

1,000

Figure 3. Rubidium versus yttrium+niobium for granite and 

volcanic rocks of the Early Proterozoic Cuchivero Group of the 

Guayana Shield. Fields of postcollision granite from Pearce and 

others (1984); solid squares, granite; open triangles, volcanic rocks; 

VAG, volcanic-arc granite; ORG, ocean-ridge granite; WPG, 

within-plate granite; syn-COLG, syncollision granite. Data from 

Sidder and Martinez (1990) and Sidder (unpublished data, 1990).

granitic and volcanic rocks plot between within-plate (anorogenic) granite and volcanic-arc granite on several discriminant diagrams and approximately in the field of 

postcollision granite on the rubidium versus yttrium+ 

niobium diagram (fig. 3) (Pearce and others, 1984; 

Sylvester, 1989; Sidder, unpublished data, 1990). The variously indicated tectonic settings may result from transitional 

mantle and crustal sources; postcollision, within-plate 

crustal magmatism may have been followed by renewed subduction and mantle-derived continental arc magmatism. 

Post-Eburnean granitoid rocks in the West African craton are

posttectonic or anorogenic rocks that were intruded at shallow levels and plot in the field of within-plate granite (Rocci 

and others, 1991).

MINERAL RESOURCES

Major mineral deposits have not yet been discovered in 

the Cuchivero Group. Quartz veins containing silver and 

gold are present locally in volcanic rocks of the Caicara 

Formation, and isolated areas of rhyolite contain trace 

amounts of disseminated cassiterite (Sidder and Martinez, 

1990; Sidder, Brooks, and others, 1991; Sidder, this 

volume). Although epithermal and bonanza-type 

precious-metal vein deposits are uncommon in Precambrian 

rocks (Hutchinson, 1987), the felsic to intermediate composition and pyroclastic character of the volcanic rocks of the 

Cuchivero Group, as well as geochemical anomalies of 

silver, bismuth, and molybdenum in some quartz-sulfide 

veins and gold in panned concentrates, are suggestive of epithermal precious-metal deposits (Sidder and Martinez, 1990; 

Sidder, Brooks, and others, 1991). Mineralized epithermal 

systems in volcanic rocks of the Cuchivero Group may have 

been preserved by burial shortly after deposition by sedimentary rocks of the Roraima Group, as exhibited by locally 

conformable contacts.

Field and geochemical evidence indicates that shallow 

porphyritic granitic intrusive rocks in volcanic rocks of the 

Caicara Formation in western Estado Bolivar and Estado 

Amazonas and possibly equivalent rocks in Brazil have 

moderate potential for associated porphyry molybdenum-type deposits. Molybdenite is relatively common in 

contact zones between felsic volcanic rocks of the Caicara 

Formation and biotite granite of the Cuchivero Group in 

Venezuela (Mendoza and others, 1977). In northern Brazil, 

molybdenite is disseminated in biotite granite and in small 

quartz veins at the faulted contact between granite and 

volcanic rocks of the Surumu Formation (Montalvao and
B20 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

others, 1975; Berrange, 1977; Schobbenhaus and others, 

1984).

Cassiterite and wood tin are sparsely disseminated in 

some samples of high-silica rhyolite in the Caicara 

Formation and its equivalent rocks of the Iricoume 

Formation in Brazil (Jones and others, 1986; Sidder, this volume). Cassiterite is also present in panned concentrates such 

as those from creeks that drain into the upper Rio Paragua 

near the Brazil-Venezuela border (Sidder, this volume, pi. 

1). These occurrences of Cassiterite disseminated in rhyolite 

and in panned concentrates are typical of rhyolite-hosted tin 

deposits (Duffield and others, 1990); however, occurrences 

of economic significance are not yet known. In Brazil, 

granitic rocks, such as alaskite, granite, granodiorite, and 

quartz diorite, of the Uatuma Supergroup that are associated 

with volcanic rocks equivalent to the Caicara Formation host 

stockwork veins and disseminations of Cassiterite (Damasceno, 1988). The granitic rocks are commonly greisenized 

to an assemblage of muscovite, fluorite, topaz, and tourmaline (Damasceno, 1988).

The carbonatite at Cerro Impacto (within the Cuchivero 

terrane) (Sidder, this volume, pi. 1) near the intersection of 

large northeast- and northwest-striking fractures is enriched 

in niobium, thorium, barium, cerium, and other metals and 

rare earth elements (Aarden and others, 1978; Premoli and 

Kroonenberg, 1981). The north west-striking fractures may 

be coextensive with those along which kimberlite was 

emplaced in the Quebrada Grande area, and they are parallel 

with large regional fractures that apparently controlled 

emplacement of pegmatitic dikes into the Parguaza Granite. 

These fractures extend throughout the western part of the 

Guayana Shield in Estado Bolivar, Estado Amazonas, and 

into Brazil. Mendoza and others (1977) suggested that the 

carbonatitic complex intruded plutonic rocks of the Cuchivero Group during the Mesozoic between 150 and 80 Ma; 

however, a rubidium-strontium whole-rock isochron date of 

about 1,732±82 Ma for leached kimberlitic rocks in the Quebrada Grande area (Nixon and others, 1992) indicates that 

the carbonatite at Cerro Impacto may be much older than 

Mesozoic. The carbonatite at Cerro Impacto may be Early 

Proterozoic, or about the same age as kimberlite in the Quebrada Grande area. The date for kimberlite in the Quebrada 

Grande area is similar to carbonatite emplaced elsewhere in 

the world between about 1,800 and 1,650 Ma (Meyer, 1988). 

Additional work is required to date more precisely the age of 

intrusion of both carbonatite and kimberlite in the Guayana 

Shield.

The similarity in age, composition, and tectonic environment between the Early Proterozoic Cuchivero Group in 

Venezuela and the granite-rhyolite terranes of the St. Francois Mountains and the Olympic Dam area suggests that 

Olympic Dam-type deposits are a possible exploration target 

in the Venezuelan Guayana Shield (Sidder, this volume). 

Olympic Dam-type iron-copper-uranium-gold-rare earth 

element magmatic-hydrothermal deposits are genetically

related to magmas, especially those of intermediate to mafic 

composition, that formed the host granite-rhyolite terrane 

(Sidder and Day, 1993; Sidder and others, 1993).

UNDIVIDED PROTEROZOIC ROCKS

Many Proterozoic and possibly Archean rocks in 

Estado Amazonas either have not been studied or have been 

examined only in reconnaissance fashion and thus are herein 

included as an undivided group of rocks (fig. 2). These rocks 

are predominantly granitic and associated volcanic rocks, 

mafic and alkaline intrusive rocks, and medium- to 

high-grade gneiss of both igneous and sedimentary protoliths (Mendoza and others, 1977; Gaudette and Olszewski, 

1985; Cox, Wynn, and others, 1993). Cordani and Brito 

Neves (1982) and Goodwin (1991) referred to the granite 

and high-grade gneiss terrane as basement rocks of the 

Pakaraima nucleus and considered this nucleus to be an 

Archean crustal remnant. Mendoza and others (1977) and 

Gaudette and Olszewski (1985) informally named some of 

these rocks after the area where they were mapped, such as 

the Minicia gneiss or migmatite, the Macabana augen gneiss, 

the granites of Atabapo, San Carlos, Sipapo, and so on. 

Barrios and others (1985) grouped these locally named units 

into two provinces, the Ventuari and Casiquiare dominions 

(fig. 2), on the basis of similar structural, petrologic, and 

geochronologic characteristics. The Ventuari dominion, 

primarily north and east of the Rio Orinoco in Estado Amazonas, consists of volcanic and plutonic rocks similar to the 

Cuchivero Group, the Parguaza Granite, sedimentary rocks 

of the Roraima Group, isolated metasedimentary sequences, 

and massive alkaline and mafic intrusions. Topographic 

relief in the Ventuari dominion is high, and elevations are as 

much as 2,000 m above sea level on the tops of some vertically cliffed plateaus or table mountains, which are called 

tepuis. The Casiquiare dominion, generally south of the Rio 

Orinoco in Estado Amazonas (fig. 2), includes granite, 

gneiss, migmatite, and scarce outcrops of the Roraima 

Group; volcanic rocks and alkaline or mafic intrusive bodies 

are not present. Elevations rarely are more than 500 m above 

sea level in this dominion (Barrios and others, 1985). Similar 

granitic and metamorphic rocks in southeastern Colombia 

near the Colombia-Venezuela border were named 

"Complejo migmatitico de Mitu" (Mitu migmatitic complex) (Priem and others, 1982). Notably, all of these authors 

commented on the complexity of the relations between the 

rocks, the poor exposures in the jungle, and the intense 

weathering of the rocks. Most of the mapped and sampled 

outcrops form discontinuous and isolated exposures along 

rivers.

The undivided Proterozoic and possibly Archean intrusive rocks in Estado Amazonas range in composition from 

biotite granite to tonalite and diorite. They are generally 

medium to coarse grained, equigranular to porphyritic, and
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B21

weakly foliated. The plutonic rocks have been moderately 

deformed by small-scale faults and shears, and cataclastic 

textures are common. Aplite dikes of more than one intrusive 

episode cut the plutonic rocks (Mendoza and others, 1977; 

Priem and others, 1982; Gaudette and Olszewski, 1985). 

Granitic rocks of the Ventuari dominion, such as the granite 

of Padamo and unnamed plutonic rocks along the Rios 

Orinoco, Ventuari, and Paru, are similar to the granite of 

Santa Rosalia. Associated volcanic rocks of the El Viejo Formation, the volcanic suite of the Parucito Valley, and similar 

locally named units such as the felsic to intermediate volcanic rocks of Yavi, Asita, Autana, and others in the Ventuari 

dominion are probably equivalent to the Caicara Formation 

or to volcanic rocks associated with the Parguaza Granite 

(Talukdar and Colvee, 1975, 1977; Mendoza and others, 

1977; Gaudette and Olszewski, 1985). Some granitic rocks 

in the Casiquiare dominion are similar to the Parguaza Granite (Barrios and others, 1985; Gaudette and Olszewski, 

1985).

The undivided Proterozoic metamorphic rocks range 

from poorly foliated and mildly tectonized gneiss to 

well-foliated gneiss and migmatite having cataclastic 

texture. Phyllite and quartzite are weakly metamorphosed 

sedimentary rocks. Granitic gneiss, or metamorphosed 

plutonic rocks, ranges in composition from granite to granodiorite, tonalite, and diorite. The intensity of metamorphism 

is as high as the greenschist and amphibolite facies with 

assemblages of chlorite±muscovite±epidote±chloritoid and 

plagioclase-hornblendeigarnet, respectively (Mendoza and 

others, 1977; Gaudette and Olszewski, 1985).

Geochronological data indicate that peak metamorphism arid magmatism of the undivided Proterozoic and possibly Archean rocks in Estado Amazonas occurred between 

about 1,860 and 1,730 Ma. Gaudette and Olszewski (1985) 

correlated this metamorphic and magmatic activity with the 

Trans-Amazonian orogeny, but it is probably part of a 

younger unnamed event, as discussed previously. Low initial 

87Sr/86Sr ratios of about 0.703 for paragneiss imply that the 

protolith of the metasedimentary rocks was not considerably 

older. Strongly deformed and metamorphosed plutonic rocks 

of about 1,860 Ma, in addition to moderately to weakly 

deformed plutonic rocks of about 1,730 Ma,5 indicate that 

plutonisrn and metamorphism may have been synchronous 

from about 1,860 to 1,730 Ma. Unmetamorphosed, undeformed granitic rocks of about 1,600 Ma represent postorogenic plutonic activity in Estado Amazonas, and the 

Parguaza rapakivi granite (fig. 2) typifies magmatic activity 

at about 1,545 Ma. In general, dates for granitic rocks in the

5This date is recalculated from the original published data. The same 

decay constant, atomic ratios, and experimental errors were used. The 

results determined here are 1,730171 Ma, initial ratio=0.705, and 

MSWD=73.3; however, the data in the publication (Gaudette and Olszewski, 1985) were reported as 1,793±79 Ma, initial ratio=0.704, and 

MSWD=18.4.

Ventuari dominion are older than for those for rocks to the 

southwest in the Casiquiare dominion (Barrios and others, 

1985; Gaudette and Olszewski, 1985; Gibbs and Wirth, 

1986).

Geochronological data from the Amazon Territory of 

Brazil and the Amazonas region of southeastern Colombia 

demonstrate ages of metamorphic and magmatic activity 

similar to those for the Casiquiare dominion of Estado Amazonas of Venezuela. Tassinari (1984) utilized whole-rock 

rubidium-strontium and lead-lead dating methods to date 

granitic to granodioritic gneiss and migmatite of the Rio 

Negro-Juruena province that truncates the MaroniItacaiunas belt (and the Ventuari dominion) in Estado 

Amazonas and southeastern Colombia (Cordani and Brito 

Neves, 1982; Teixeira and others, 1989). He determined that 

a magmatic arc and new continental crust formed in this 

province between about 1,750 and 1,600 Ma from magma 

generated in the upper mantle (initial 87Sr/86Sr ratio 0.7030).

Priem and others (1982) used whole-rock rubidium-strontium and uranium-lead zircon data to conclude that 

the granite gneiss basement of the Amazonas region of 

southeastern Colombia has a minimum age of about 1,850 

Ma. A maximum age of about 1,450 Ma was established for 

the high-grade metamorphism and resetting of the isotopic 

systems. The Complejo migmatitico de Mini formed about 

1,560-1,450 Ma by large-scale granitic plutonism during the 

Parguaza episode and metamorphic reconstitution of rocks 

having a minimum age of about 1,850 Ma (Priem and others, 

1982).

Based on the previously discussed radiometric data, 

Gaudette and Olszewski (1985) suggested that a tectonic 

zone or boundary is present along the Rio Orinoco south of 

about lat 4°00' N. This zone marks the contact between the 

Ventuari and Casiquiare dominions and the MaroniItacaiunas and the Rio Negro-Juruena mobile belts (Barrios 

and others, 1985; Teixeira and others, 1989). The evolution 

of a northeast-facing subduction zone between about 1,900 

and 1,450 Ma and a change from compressional horizontal 

tectonics to tensional vertical tectonics at the end of subduction may account for the geologic relations in the undivided 

Proterozoic rocks of the Amazon region of Venezuela, 

Colombia, and Brazil (Priem and others, 1982; Barrios and 

others, 1985; Gaudette and Olszewski, 1985).

RORAIMA GROUP

The Roraima Group is a generally flat lying (dipping 

less than 20°) suite of sedimentary rocks deposited in fluvial, 

deltaic, shallow-marine, and lacustrine or epicontinental 

environments. It originally covered an area of at least 

250,000 km2 , possibly as much as 1,200,000 km2, that 

extended about 1,500 km east to west from Suriname to 

Brazil and Estado Amazonas of Venezuela; rocks equivalent 

to the Roraima Group are apparently not present in French
B22 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Guiana and the West African craton (Gansser, 1954; Ghosh, 

1985; Cahen and others, 1984; Gibbs and Barren, 1993). The 

original northern extent of rocks of the Roraima Group is not 

known. The north-facing frontal scarp of the Roraima Group 

is apparently erosional, although some structural control is 

possible. Quartzarenite is the predominant rock type; feldspathic arenite, conglomerate, quartzite, arkose, argillaceous 

siltstone, shale, jasper, and chert are also present. This suite 

of rocks is generally 700-1,000 m thick and in some areas is 

more than 3,000 m thick (Ghosh, 1985; Dohrenwend and 

others, this volume). The basal contact is not exposed in all 

places; however, the Roraima Group has been reported 

to overlie unconformably the Early Proterozoic greenstonegranite terrane, the Caicara Formation, the Parguaza Granite, 

and undivided Proterozoic rocks in Estado Amazonas (De 

Loczy, 1973; Ghosh, 1985; Gibbs and Barren, 1993). As 

noted previously, rocks of the Roraima Group locally lie 

conformably on volcanic rocks of the Caicara Formation. 

Rocks that overlie the Roraima Group are not common. Fluvial sedimentary rocks about 30 m thick, possibly Miocene 

to Pliocene in age, overlie the Roraima Group in parts of 

Guyana and Brazil (De Loczy, 1973); however, Late Proterozoic or Phanerozoic sedimentary rocks are not known to 

overlie the Roraima Group in Venezuela (Ghosh, 1985; 

George, 1989).

Rocks of the Roraima Group in Venezuela form startling, vertically cliffed tepuis that rise 1,000 m or more above 

the surrounding jungle. The tops of the tepuis are commonly 

saucer shaped and dip about 20°^5° toward the center of the 

tepuis (Gansser, 1974; Ghosh, 1985). This topography has 

been attributed to (1) inverted topography in which the 

tepuis are on the axes of synclines, (2) isostatic depression, 

(3) buckling related to vertical basement tectonics, and (4) 

faults along the hinge zones between the marginal rims and 

the central plateaus that formed central graben and marginal 

horst blocks (Gansser, 1974; Ghosh, 1985). Briceno and 

others (1990) concluded that the tepuis resulted from topographic inversion and are remnants of doubly plunging 

synclines, the low, eroded areas around the tepuis corresponding to the dismantled anticlines. Karst features such as 

large caves, shafts, underground streams, karren, and dolines 

are locally developed on the tops of some tepuis (Szczerban 

and Urbani, 1974), and several species of plants are native 

only to the tops of individual tepuis (George, 1989).

Structural features in the Roraima Group generally are 

broad, open folds and block faults that strike northeast and 

north-northeast to north-northwest throughout Venezuela 

and the Guayana Shield (Gansser, 1954; Ghosh, 1985; 

Briceno and others, 1990). Numerous joint sets cut the 

Roraima Group; however, joint trends measured in outcrop 

do not coincide with those measured from aerial photographs 

or side-looking airborne radar (SLAR) images (Sidder, 

1988, unpublished data; Briceno and others, 1990). Dohrenwend and others (this volume) recognized a deformational 

gradient from south to north in the Gran Sabana area of

southeastern Venezuela: relatively tight folds and conspicuous axial planar foliation are present in the lower part of the 

Roraima Group in the south, and relatively undisturbed, gently dipping to horizontal, unfoliated strata are present in the 

north. The axial planar foliation imposes a conspicuous ridge 

and valley topography in the south, whereas in the north less 

deformed and undeformed rocks underlie the high plateaus 

(Dohrenwend and others, this volume).

Rocks of the Roraima Group generally are unmetamorphosed; most samples do not show any textural or mineralogical evidence of metamorphism (Ghosh, 1985). 

Pyrophyllite and andalusite in some rocks are interpreted as 

metamorphic minerals that formed from very low grade 

burial metamorphism (Urbani, 1977; Briceno and others, 

1990); however, these minerals are also in localized contact 

metamorphic aureoles around diabasic and gabbroic intrusions of the Avanavero Suite or of the Mesozoic dike swarms 

(Gansser, 1974; Urbani, 1977; Ghosh, 1985; Gibbs, 1986). 

Rocks interpreted, on the basis of the presence of pyrophyllite or andalusite, to have been affected by burial metamorphism may in fact have been metamorphosed by an 

unexposed intrusion. Alternatively, an unknown thickness of 

rocks, but presumably at least 3 km or more to cause burial 

metamorphism, may have been completely eroded from the 

top of the Roraima Group. The matrix of some rocks has 

been recrystallized to interstitial quartz, fine-grained white 

mica, and hematite (Gibbs and Barren, 1993).

Reid (1974a) divided the Roraima Group in the Gran 

Sabana area of Venezuela into four formations, in ascending 

order, the Uairen, Kukenan, Uaimapue, and Matuai Formations. Elsewhere in the Guayana Shield, the Roraima Group 

has been subdivided into lower (generally equivalent to the 

Uairen Formation), middle (the Kukenan and Uaimapue Formations), and upper (Matuai Formation) members (Gansser, 

1954; Bateson, 1966; Priem and others, 1973; Ghosh, 1985; 

Gibbs and Barren, 1993). The stratigraphy of the Roraima 

Group is complex, although it may appear simple in some 

areas. Units are not continuous rather they appear to grade 

both laterally and vertically and few marker beds have 

been established for regional correlation in the shield. For 

example, conglomerate, arkose, and jasperoid tuffaceous 

rocks, which are distinctive in the Gran Sabana area of Venezuela, Brazil, and Guyana, are not present in tepuis in 

Estado Amazonas (Ghosh, 1985). Moreover, the stratigraphic sequences in several tepuis of Estado Amazonas do 

not correlate with each other. The rocks in Estado Amazonas 

are included in the Roraima Group because of their abundant 

thick sequences of crossbedded, fine- to medium-grained 

quartzarenite and feldspathic arenite and lesser interbedded 

layers of shale. Possibly these rocks are time transgressive 

with those of the Roraima Group in the Gran Sabana area 

(Ghosh, 1985).

Alberdi and Contreras (this volume) describe a 

128-m-thick sequence of graywacke, siltstone, and shale that 

underlies the basal sandstone and conglomerate of the
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B23

Uairen Formation north of the Gran Sabana area. They suggest that these rocks be included in the Roraima Group as the 

Urico Formation. In northern Brazil and Guyana, the Urico 

Formation is apparently correlative with the Wailan Formation, which is composed of silty to sandy shale and argillaceous sandstone and is transgressed by conglomerate of the 

Roraima Group (Gansser, 1954); however, Gansser (1954) 

considered the Wailan Formation to be equivalent to the 

Haimarakka Formation, which is correlative with the Caicara Formation rather than with part of the Roraima Group.

The Uairen Formation consists of about 800-900 m of 

quartzitic sandstone, conglomerate, and minor shaly siltstone. Thin lenses and beds (from less than 50 cm to about 

10 m in thickness) of quartz-pebble and polymict conglomerate and thin beds and laminae of shaly siltstone are intercalated with the sandstone (Reid, 1974a; Reid and Bisque, 

1975). Dohrenwend and others (this volume) subdivide the 

Uairen Formation into a lower member about 600 m thick 

and an upper member about 100-300 m thick. The former 

consists of well-sorted, coarse- to medium-grained quartz 

sandstone that is cross-stratified with trough and festoon 

crossbeds and is intercalated with conglomerate and shaly 

siltstone; the latter includes medium-grained sandstone that 

contains abundant trough cross-stratification and intercalated channel gravels (Reid, 1974a; Reid and Bisque, 1975; 

Dohrenwend and others, this volume). The lower member is 

moderately bedded to massive and underlies high cliffs and 

extensive dip slopes of several cuestas along the southern 

margin of the Gran Sabana. The upper member forms conspicuously benched scarp slopes and irregular ridges 

(Dohrenwend and others, this volume).

The Kukenan (also spelled Cuquenan) Formation is 

composed of sandstone interbedded with siltstone, claystone, and shale. The sandstone is well bedded to massive 

and fine to medium grained, and the siltstone, claystone, and 

shale are medium to thin bedded, laminated, and variegated. 

The formation has a maximum thickness of about 100 m in 

the Gran Sabana area (Reid, 1974a; Reid and Bisque, 1975; 

Dohrenwend and others, this volume).

The lower part of the Uaimapue Formation is similar to 

the Uairen Formation in that it consists of sandstone and conglomerate and interbedded siltstone and mudstone; however, 

abundant beds of jasper, chert, and arkose characterize the 

upper part of the formation (Gansser, 1954; Reid, 1974a; 

Reid and Bisque, 1975; Dohrenwend and others, this volume). The total thickness of the Uaimapue Formation in the 

Gran Sabana area is about 250 m. Sandstone in the lower part 

of the formation is fine to coarse grained and pervasively 

channelled with trough cross-stratification. Clasts in the conglomerate are predominantly quartz pebbles. Red arkose, 

green and red jasper, and green, red, and gray chert are interbedded in the upper part of the formation (Reid, 1974a; Reid 

and Bisque, 1975). The arkose contains pyroclastic material, 

and the jasper contains distinct shards of devitrified volcanic 

glass (Gansser, 1974; Reid, 1976; Ascanio and others, 1985).

The jasper beds, which are about 20 cm thick and are interbedded with siltstone and sandstone forming sequences 

about 10 m thick, are interpreted as volcaniclastic tuff 

(Ascanio and others, 1985); they are marker beds for correlation throughout the Guayana Shield and have been radiometrically dated.

The Mataui Formation, the youngest unit of the 

Roraima Group, forms the vertical cliffs of some tepuis and 

is dominantly crossbedded, ripple-marked, and massive 

quartz sandstone and quartzite (Reid, 1974a; Reid and 

Bisque, 1975). Sandstone in the uppermost part of the formation, as on the tops of the tepuis, is less well cemented, friable, and thinner bedded and contains sandy shale horizons. 

The total thickness of the unit is 1,000 m or more (Gansser, 

1954; Reid, 1974a; Reid and Bisque, 1975).

The Roraima Group in Estado Amazonas, which is best 

observed in tepuis, includes quartzarenite, feldspathic arenite, and shale and generally is made up of three members; 

however, the stratigraphic sequence cannot be correlated 

from one tepui to another (Ghosh, 1977, 1985). The lower 

member, about 300-500 m thick, consists of thinly graded 

beds of fine- to coarse-grained, cross-stratified, ripple-marked, laminated quartzarenite, minor quartz wacke, 

and thin conglomeratic beds (Ghosh, 1977, 1985; Mendoza 

and others, 1977). The middle member, about 100-200 m 

thick, includes medium-grained, crossbedded quartzarenite, 

feldspathic arenite, and argillaceous quartzarenite. 

Dark-gray to black shale units as thick as 50 m are typical of 

the middle member. The upper member, which forms 

prominent cliffs, is composed of 500-700 m of medium- to 

coarse-grained quartzarenite and lesser feldspathic arenite 

and lenticular clay beds that have thin carbonate-rich 

laminations (Ghosh, 1977, 1985; Mendoza and others, 

1977). Major units of conglomerate, arkose, or jasper have 

not been observed in Estado Amazonas (Ghosh, 1977, 

1985).

ENVIRONMENT OF DEPOSITION AND AGE

Rocks of the Roraima Group were deposited in fluvial, 

deltaic, shallow coastal marine, and lacustrine or epicontinental environments such as low-sinuosity river channels 

and their floodplains, delta distributaries above tranquil 

interdeltaic lakes, coastal lagoons to interdeltaic bays, nonbarred beaches, and intertidal mud flats (Ghosh, 1985; 

Briceno and Schubert, 1990). The lateral and vertical distribution of the lithologic units depicts an alluvial-deltaic complex near a wave-dominated, high-energy coastline that had 

abundant beaches, sandy flats, and subtidal bars and less 

common mudflats and coastal lagoons (Ghosh, 1985). 

Cross-stratification, ripple marks, and pebble orientation 

indicate that the sediments were transported from a source to 

the northeast, east, and southeast (Gansser, 1954; Keats, 

1974; Reid and Bisque, 1975; Ghosh, 1985; Gibbs and Bar-
B24 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 3. Rubidium-strontium whole-rock isochron dates for volcanic rocks of the Roraima Group, Guayana Shield. 

[Number of samples on which age is based is given in parentheses after age. MSWD is mean squares weighted deviation]

Country

Venezuela 

Venezuela 

Suriname

Unit

Canaima 

Santa Elena de Uairen 

Tafelberg

Age (Ma)

l,730±120(n=8)' 

1,570±83 (n=16)2 

1,660±27 (n=14)

(87Sr/86Sr)0 MSWD

0.708 11.2 

0.721 79.8 

0.708 1.84

Reference

Gaudette and Olszewski (1985). 

Pringle and Teggin (1985). 

Priem and others (1973).

These dates are recalculated from the original published data. The same decay constant, atomic ratios, and experimental errors were used; however, 

the data in the publication were reported as 1,747±49 Ma, initial ratio=0.708, MSWD=2.84.

2These dates are recalculated from the original published data. The same decay constant, atomic ratios, and experimental errors were used; however, 

the data in the publication were reported as 1,579+18 Ma, initial ratio=0.720, MSWD=19.95.

Table 4. Rubidium-strontium whole-rock isochron dates for diabase of the Avanavero Suite and hornfels formed from its intrusion

into the Roraima Group, Guayana Shield.

[Number of samples on which age is based is given in parentheses after age. MSWD is mean squares weighted deviation. N.A. indicates not available]

Country Unit Age (Ma) (87Sr/86Sr)0 MSWD Reference

Suriname Dolerite of Avanavero Suite

Guyana Dolerite of Roraima intrusive suite

Brazil Hornfelsed sandstone of the Roraima Group next to a diabase dike

Guyana Hornfelsed shale of the Roraima Group overlying a diabase sill

1,670±18 (n=22) 0.704 4.24

1,640+58 (n=8) 0.704 11.9

1,990+170 (n=3)' 0.696 0.078

1,600+44 (n=2/ 0.856 N.A.

Hebeda and others (1973). 

McDougall and others (1963). 

Basei and Teixeira (1975). 

Snelling and McConnell(1969).

^hese ages are calculated using Model 1 of York (1969), which assumes that the only cause for scatter from a straight line are the assigned errors; 

however, the errors for 87Rb/86Sr and 87Sr/86Sr were not cited in the original references. Values of 2.0 percent and 0.1 percent, respectively, were used 

for the calculations.

ron, 1993). Rocks of the Roraima Group may have been 

deposited in several basins (fault-block basins?) separated 

by basement highs (Ghosh, 1985).

The Roraima Group may be as old as about 1,900 Ma 

and as young as about 1,500 Ma or younger (Ghosh, 1985). 

Correlation of the Roraima Group throughout the shield is 

problematic because radiometric dating of felsic pyroclastic 

rocks interbedded with sandstone of the middle part of the 

Roraima Group (Uaimapue Formation and its equivalents) 

and diabase dikes and sills that intrude the Roraima Group 

yields erratic dates. For example, felsic volcanic rocks have 

whole-rock rubidium-strontium isochron dates of about 

1,730, 1,660, and 1,570 Ma (table 3) near Canaima in the 

northern Gran Sabana area, in Suriname, and near Santa 

Elena de Uairen in the southern Gran Sabana area, respectively (Gaudette and Olszewski, 1985; Priem and others, 

1973; Pringle and Teggin, 1985, respectively). These dates 

are analytically indistinguishable (1,650 Ma), given the error 

and MSWD associated with each date (table 3). Diabase 

dikes that cut rocks of the Roraima Group throughout the 

central and eastern parts of the Guayana Shield have been 

dated by 40Ar/39Ar and whole-rock and mineral rubidium-strontium and potassium-argon methods. Mineral and 

whole-rock rubidium-strontium isochrons for doleritic6 sills 

that intrude the Roraima Group in Guyana and Suriname 

yield dates of about 1,640 Ma and 1,670 Ma, respectively

6Dolerite is a British term for diabase. The two terms are used synonymously herein; the mafic intrusive rocks in Guyana and Suriname are generally referred to as dolerite (the original usage) and those in Venezuela and 

Brazil as diabase.

(table 4). The potassium-argon method does not give accurate ages for the diabase intrusive rocks (fig. 4) (McDougall, 

1968), and 40Ar/39Ar analyses of biotite and plagioclase 

from one sample of diabase gave dates of about 1,800 and 

1,470 Ma, respectively (Onstott, Hargraves, and York,

1984). Direct dating of the Roraima Group yields an age of 

about 1,650 Ma for the middle member (Uaimapue Formation), which rests on as much as 1,200 m of quartzarenite, 

and indirect dating of diabase dikes and sills indicates an age 

not younger than 1,670-1,640 Ma for the lower and middle 

parts of the Roraima Group (McDougall and others, 1963; 

Hebeda and others, 1973).

Both discordant and concordant contacts have been 

mapped between rocks of the Roraima Group and the Caicara Formation, which was deposited between about 1,930 

and 1,790 Ma (table 2) (Reid and Bisque, 1975; Amaral and 

Halpern, 1975; Sidder, unpublished data, 1988). In addition, 

the contact between the Roraima Group and the Parguaza 

Granite (1,545 Ma) has been identified as an angular unconformity (Mendoza, 1974; Mendoza and others, 1977). In 

support of this interpretation, Ghosh (1985, p. 48) noted that 

"contact metamorphic minerals are conspicuous by their 

absence in the Roraimas overlying the Parguaza Rapakivi 

granite." In contrast, other Proterozoic units, such as the 

Cinaruco Formation in Estado Amazonas (equivalent to the 

Los Caribes Formation), contain andalusite in a contact aureole as much as 0.5 km from the intrusive contact (Ghosh,

1985). These geologic relations bracket the age of the 

Roraima Group as being as old as about 1,900 Ma in southeastern Venezuela and as young as about 1,500 Ma or 

younger in Estado Amazonas.
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B25

i i i i i i i i i r 

1,400 1,500 1,600 1,700 1,800 1,900 2,000 2,100 2,200 2,300 2,400 2,500 2,600 2,700 2,800 2,900 3,000 3,100

AGE, IN MEGA-ANNUM

Figure 4. Potassium-argon dates for Proterozoic diabase of the Guayana Shield. Data from McDougall and others (1963), Snelling (1963), 

Snelling and McConnell (1969), Hebeda and others (1973), Frick and Steiger (1974), Basei and Teixeira (1975), and Teggin and others 

(1985).

Sedimentary rocks of the Roraima Group apparently 

record a long period of sedimentation on a relatively stable 

crust. It has been proposed that the present crustal thickness 

of shields was not attained at the time of "stabilization," or 

the cessation of deformation and magmatism (end of the 

Trans-Amazonian orogeny), but rather that continued 

passive undeiplating of the crust (intrusion of part of the 

Cuchivero Group and its equivalents) and subsequent 

subsidence and sedimentation may have extended for several 

hundred million years after initial stabilization (Rogers and 

others, 1984). The sandstones throughout the Guayana 

Shield presently correlated with the Roraima Group 

apparently record a long period of subsidence and 

continental-nearshore sedimentation that perhaps migrated 

successively westward in multiple basins on a stable craton 

possibly from as early as 1,900 to at least 1,545 Ma.

MINERAL RESOURCES

Modern and paleoplacer deposits of gold and diamonds 

have been mined extensively in the Venezuelan Guayana 

Shield (Mendoza, 1985; Sidder, this volume). Recent field 

work (Dohrenwend and others, this volume) in the southern 

Gran Sabana area identifies three types of placer occurrences: (1) diamond placers within modern channels of 

major rivers, (2) gold and diamond placers in colluvialalluvial deposits of low-order drainages, and (3) gold and 

diamond paleoplacers associated with conglomeratic lenses

and beds within the lower 500-600 m of the Uairen Formation. These paleoplacer deposits are the source of gold for 

modern placer and colluvial-alluvial gravel deposits (Reid 

and Bisque, 1975; Mendoza, 1985; Dohrenwend and others, 

this volume). Paleoplacer gold-bearing units in quartzpebble conglomerate and quartzarenite in the lower part of a 

fluvatile-deltaic sequence of conglomerate, arenite, 

quartzite, and intercalated graphitic phyllite in Colombia 

(Rodriguez and Warden, 1993) are similar to those in the 

Roraima Group.

Conglomerates in the lower part of the Roraima Group 

have been proposed as the source of diamonds by many 

investigators (Gansser, 1954; Pollard and others, 1957; De 

Loczy, 1973; Briceno, 1984; Mendoza, 1985). Reid (1974b) 

suggested that kimberlite, possibly in Brazil or even West 

Africa, was the source for the paleoplacer deposits in the 

Roraima Group. Studies at San Salvador de Paul (Briceno, 

1984) and elsewhere along the Rio Carom and in the Gran 

Sabana area document that conglomerates of the Uairen 

Formation are the source of alluvial diamonds (Reid, 1974b; 

Reid and Bisque, 1975; Dohrenwend and others, this 

volume). Briceno (1984) stated that the conglomerates in the 

Uairen Formation are themselves paleoplacers and were the 

source for diamond-bearing gravels deposited about 8,000 

years ago at San Salvador de Paul. These Holocene paleoplacers may be the source of diamonds in some deposits now 

being mined in active drainages (Gansser, 1954; Briceno, 

1984).
B26 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Kimberlite, a common source of diamonds, and its 

indicator minerals such as chrome pyrope and magnesian 

ilmenite have been identified in Venezuela only in the 

Quebrada Grande area (Baptista and Svisero, 1978; Meyer 

and McCallum, 1993). More than a dozen diamond-bearing 

kimberlitic dikes and sills containing chrome pyrope, 

titanium-rich phlogopite, chromite, and yimengite 

(K(Cr,Ti,Fe,Mg)i2Oi9), a rare alteration product of chromite 

previously recognized only in kimberlite from China, have 

been located (Nixon, 1988; Nixon and Condliffe, 1989; 

Nixon and others, 1989, 1992). A rubidium-strontium 

whole-rock isochron date of leached kimberlitic samples 

indicates that the dikes and sills were emplaced at about 

1,732±82 Ma (Nixon and others, 1992). Mendoza and others 

(1977) suggested that the Cerro Impacto carbonatite and possibly associated kimberlite were intruded during the Mesozoic between about 150 and 80 Ma. Although neither 

kimberlite nor its indicator minerals have been identified 

elsewhere in Venezuela, it is possible that other Proterozoic 

diamond-bearing kimberlites were emplaced along the 

north-northwest-striking fractures that extend throughout the 

western part of the Guayana Shield in Estado Bolivar, Estado 

Amazonas, and into Brazil. Sediments of the Roraima Group 

may have incorporated diamonds from these older Proterozoic kimberlites during their transport and deposition. 

Numerous diamond-bearing kimberlite intrusions were 

emplaced in the West African craton during the Mesozoic, 

particularly about 100 Ma (Williams and Williams, 1977; 

Morel, 1979). Proterozoic kimberlite intrusions in West 

Africa are less well documented; however, the Kanangono 

kimberlite in the Leo Shield of the Ivory Coast is dated as 

about 1,480 Ma (Onstott and Dorbor, 1987). Alluvial diamonds mined in Ghana and elsewhere in West Africa are 

presumed to have a source in rocks of the Early Proterozoic 

Birimian greenstone belts such as tuffaceous graywacke or 

associated with ultramafic rocks, such as within the Birim 

diamond field in Ghana (Kesse, 1985; Milesi and others, 

1992; McKitrick and others, 1993). Neither kimberlite nor 

kimberlitic indicator minerals has been identified in these 

rocks.

Radiometric anomalies have been measured in rocks of 

the Roraima Group on the south end of the Gran Sabana 

between Santa Elena de Uairen and Icabaru. The apparent 

similarity of the basal conglomerate of the Roraima Group 

and gold-uranium-bearing conglomerates of the Witwatersrand, South Africa, Jacobina, Brazil, and Blind River, 

Canada (De Loczy, 1973; Bellizzia and others, 1981) and the 

reported occurrence of authigenic pyrite (Gallagher, 1976) 

have led to the proposal that the basal conglomerates of the 

Roraima Group are a viable exploration target for uranium 

deposits, as well as for gold deposits (De Loczy, 1973; Mendoza, 1985; Brooks and Nunez, 1991). Rocks of the Roraima 

Group are significantly younger (about 1,900-1,500 Ma), 

however, than the Early Proterozoic and Archean 

(3,100-2,200 Ma) gold-uranium deposits and were deposited under a more oxygenated atmosphere. The uranium 

potential in the Roraima Group is low.

AVANAVERO SUITE

Unmetamorphosed mafic intrusive rocks of the 

Avanavero Suite (formerly known as the Roraima Intrusive 

Suite) are present throughout the Guayana Shield as dikes, 

sills, inclined sheets, and small irregular intrusive bodies 

such as laccoliths. These rocks are present from western 

Venezuela to Suriname and Brazil; the age of unmetamorphosed mafic dikes in French Guiana that are cut by Mesozoic diabase dikes is not known (Gibbs, 1986; Gibbs and 

Barren, 1993). Intrusion of a dense network of mafic dikes 

at about 1,600 Ma in the Reguibat Shield also indicates a 

period of contemporaneous crustal extension in the West 

African craton (Cahen and others, 1984; Rocci and others, 

1991). Dikes of the Guayana Shield generally strike north, 

within 15°, or almost east; they are as thick as 1,000 m and 

have strike lengths of as much as 150 km (Bosma and others, 

1983; Gibbs, 1986). Sills in the Guayana Shield are 

restricted to the Roraima and Cuchivero Groups, and they 

were fed by dikes and irregular intrusive bodies in the underlying basement rocks (Hawkes, 1966a; Gibbs, 1986). The 

sills are commonly intruded along the unconformity between 

the greenstone-granite terrane and the Roraima Group, as 

well as throughout the lower and middle members of the 

Roraima Group (Hawkes, 1966b). The stratigraphically 

highest sill was previously thought to be at the base of the 

upper member (Mataui Formation) (Bateson, 1966). On 

Mount Roraima, at the junction between Venezuela, Guyana, and Brazil, a large gabbroic body forms the boundary 

between the middle and upper members (Gansser, 1954). 

Briceno and others (1990) reported, however, that tholeiitic 

diabase stocks and sills are intrusive into the Mataui Formation in several tepuis of the Chimanta massif in southeastern 

Venezuela.

Diabasic rocks of the Guayana Shield range from gabbro and norite to granophyre. They are typically medium to 

coarse grained and massive; chilled margins are developed 

locally (Hawkes, 1966a; Sial and others, 1986). Subophitic 

to ophitic textures are common, and the finer grained rocks 

are commonly porphyritic. Cumulate textures and rhythmic 

layering are present in the Tumatumari-Kopinang dike and 

sill complex in Guyana (Hawkes, 1966b). Plagioclase 

(An35~An7o), hypersthene, bronzite, augite, and inverted 

pigeonite are the primary minerals; accessory amounts of 

biotite, magnetite, ilmenite, corona-textured olivine, hornblende, and graphic intergrowths of quartz and potassium 

feldspar (micropegmatite) and trace amounts of apatite, zircon, pyrite, and chalcopyrite are also present (Hawkes, 

1966a, b; Hebeda and others, 1973; Gibbs, 1986; Sial and 

others, 1986; Briceno and others, 1990). The accessory minerals range in abundance from about 2 to as much as 15
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B27

modal percent. Minor amounts of secondary minerals such 

as hornblende and uralite replacing pyroxene, serpentine 

pseudomorphs of olivine, chlorite after biotite and hornblende, and sericitic and saussuritic alteration of plagioclase 

are also present. Evidence of metamorphism has not been 

observed (Hawkes, 1966a, b; Hebeda and others, 1973; Sial 

and others. 1986). Chemical weathering products of diabase 

throughout the shield are similar, even at different elevations 

and on different planation surfaces. Diabase weathers 

directly to an assemblage of goethite and gibbsite, without 

intermediate clayey phases (Briceno and others, 1990).

The chemistry of diabase throughout the Guayana 

Shield is similar to that of continental tholeiite. All samples 

of dikes and irregular bodies in the basement rocks and sills 

in the Roraima Group are tholeiitic, and major element concentrations are similar, although trace element abundances 

of rubidium, barium, yttrium, strontium, and chromium may 

vary (Hawkes, 1966a, b; Teggin and others, 1985). Gabbroic 

rocks contain about 51-54 weight percent silica and granophyric rocks about 58-65 weight percent silica (Hawkes, 

1966a; Teggin and others, 1985). Diabase analyses plot in 

the tholeiitic field on several discriminant diagrams such as 

total alkali-silica, AFM (Na2O+K2O-FeO*-MgO), and 

Jensen (cation percent (FeO+Fe2O3+TiO2)-Al2O3-MgO) 

plots (Teggin and others, 1985; Sial and others, 1986). 

Quartz and hypersthene are common normative minerals 

(Hawkes, 1966b; Teggin and others, 1985). Samples of Proterozoic diabase contain less total iron, titanium, zirconium, 

vanadium, volatile content (water, carbon dioxide, fluorine, 

and chlorine as measured by loss on ignition), and possibly 

copper than samples of the Mesozoic diabase suite (Teggin 

and others, 1985; Sial and others, 1986). It has been suggested, on the basis of these and other chemical differences 

between the Proterozoic and Mesozoic diabases, that the 

Proterozoic rocks represent differentiates of parent magmas 

that underwent pre-intrusive, shallow-level (crustal) differentiation (Choudhuri, 1978) or that were derived from a shallower region in the mantle (Sial and others, 1986). In 

contrast, the Mesozoic dike swarms represent magmas 

derived directly from the mantle, possibly related to a hot 

spot or mantle plume, during the breakup of Gondwanaland 

and the formation of the Atlantic Ocean (Choudhuri, 1978; 

Sial and others, 1986).

Dating rocks of the Avanavero Suite is difficult. As 

noted previously, diabase from throughout the Guayana 

Shield has been dated by argon-argon and whole-rock and 

mineral rubidium-strontium and potassium-argon methods. 

Potassium-argon dates range from about 3,095 to 1,418 Ma 

(fig. 4), whereas two mineral and whole-rock rubidium-strontium isochrons yield dates of about 1,670 and 

1,640 Ma (table 4). The 40Ar/39Ar integrated age dates for 

biotite and plagioclase in a diabase sill in Guyana are 1,798 

and 1,468 Ma, respectively (Onstott, Hargraves, and York, 

1984). The last 10 percent of 39ArK released from plagioclase indicates an age of 1,823 Ma; the last 40 percent of

released from biotite defines a plateau at 1,810 Ma, 

and individual fractions indicate an age of about 1,850 Ma. 

The dates for biotite are, however, suspect because of the 

high percentage of atmospheric argon (>29 percent and as 

much as 91 percent in the fraction at 1,150°C) in 16 of 17 

fractions of the biotite analysis. The 40Ar/39Ar data for plagioclase support the conclusion that younger dates for the 

diabase are caused by argon loss (McDougall, 1968; Onstott, 

Hargraves, and York, 1984).

The lack of a well-defined peak on the histogram of 

potassium-argon dates for 114 analyses of diabase (fig. 4) 

indicates that this method has not adequately distinguished 

the age of the diabasic intrusive rocks of the Guayana Shield. 

Detailed studies of the distribution of radiogenic argon in 

samples of diabase prove that the argon is inhomogeneously 

distributed throughout the rocks and minerals both by excess 

argon contamination (Hebeda and others, 1973) and by loss 

of argon (McDougall, 1968) after crystallization of the diabase. These changes are attributed to a tectonothermal event 

known as the Nickerie or K'Mudku episode at about 1,200 

Ma that affected the entire Guayana Shield (Snelling and 

McConnell, 1969; Hebeda and others, 1973; Bosma and others, 1983). As stated by McDougall (1968, p. 144), "The 

spread in ages determined by the K-Ar method on the 

Roraima dolerites shows that little is to be gained by undertaking further measurements by this technique on these 

rocks. The question as to the age or ages of emplacement of 

the dolerites probably will only be resolved by detailed and 

precise Rb-Sr whole-rock measurements on many samples."

Rubidium-strontium isochrons for samples of hornfelsed sandstone and shale collected adjacent to diabase 

intrusive rocks yield disparate results (table 4). The initial 

87Sr/86Sr ratio determined for these samples (table 4) indicates that these isochrons may not be representative of the 

true age of emplacement. The small number of analyses precludes a rigorous interpretation of these data.

Paleomagnetic analyses of Proterozoic diabase from 

Venezuela, Guyana, and Suriname define at least two groups 

having remanence orientations approximately opposite in 

declination (Hargraves, 1968; Veldkamp and others, 1971; 

Onstott, Hargraves, and York, 1984); however, the radiometric age determinations do not distinguish separate periods of diabase intrusion (table 4, fig. 4) (Gibbs, 1986). At 

present, the age of diabase intrusion in the Guayana Shield 

can only be estimated to be about 1,650 Ma, and it may be as 

old as about 1,850 Ma.

PARGUAZA GRANITE

The Parguaza Granite constitutes a batholith of at least 

10,000 km2 , and possibly as much as 30,000 km2, in Estado 

Amazonas (fig. 2). These granitic rocks were emplaced predominantly in the Ventuari dominion into rocks equivalent 

to those of the Cuchivero Group. Granite of similar compo-
B28 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

sition and age is present to the south and southeast into 

Brazil and westward into Colombia (Mendoza, 1975; 

Kovach and others, 1976; Gaudette and others, 1978; Priem 

and others, 1982); these intrusive bodies include the Agua 

Boa and Madeira plutons, which host the world-class tin 

deposit at Pitinga, as well as the Surucucus, Mucajai, 

Abonari, Velho Guilherme, and other intrusive suites in 

Brazil (Dall'Agnol and others, 1975, 1987, 1994; 

DalTAgnol, 1982; Schobbenhaus and others, 1984; Jones 

and others, 1986; Issler and Lima, 1987; Gibbs and Barren, 

1993). Volcanic rocks such as the rhyodacite of Guayapo are 

associated with the Parguaza Granite (Mendoza and others, 

1977). Some felsic to intermediate tuffs in Estado Amazonas 

identified as correlative with the Caicara Formation may also 

be related to the Parguaza Granite.

Rocks of the Parguaza Granite include massive, 

coarsely crystalline, porphyritic granite and biotite granite, 

commonly having rapakivi (wiborgite-type) texture. They 

contain quartz (5-34 modal percent), potassium feldspar 

(microcline perthite, 25-55 percent), plagioclase 

(oligoclase, 15-31 percent), biotite (3-17 percent), and hornblende (1-24 percent) and accessory clinopyroxene, apatite, 

sphene, zircon, ilmenite, and magnetite (Mendoza, 1974, 

1975; Gaudette and others, 1978). Rapakivi and, less commonly, antirapakivi textures, with ovoids of potassium feldspar mantled by plagioclase and plagioclase mantled by 

microcline, respectively, are characteristic of these granitic 

rocks. The Parguaza Granite is relatively unmetamorphosed 

and unaltered. Epidote is rarely present as a secondary mineral in plagioclase, and chlorite is not present, in contrast to 

the granitic rocks of Santa Rosalia (Mendoza, 1974; Gaudette and others, 1978).

Rocks of the Parguaza Granite are metaluminous to 

slightly peraluminous and have a tholeiitic affinity, as indicated by high FeO/(FeO+MgO) ratios. Silica ranges from 

about 66 to 74 weight percent; amounts of Na2O and MgO 

are low to moderate, 2.9-3.9 weight percent and 0.2-0.7 

weight percent, respectively; and concentrations of total iron 

as Fe2O3, TiO2, and CaO are relatively high, 2.6-7.2, 

0.4-0.9, and 1.0-3.1 weight percent, respectively (Mendoza, 

1975). Major- and trace-element contents of the Parguaza 

Granite are similar to those of other A-type granites such as 

granophyre from the Duluth and Skaergaard Complexes, 

Nigerian charnockite, rapakivi granite in Finland, and Middle Proterozoic (1.48-1.35 Ga) granite in the granite-rhyolite 

terranes of the Midcontinent of the United States (Mendoza, 

1975; Gaudette and others, 1978; Sims and others, 1987).

AGE AND ORIGIN

The Parguaza Granite was intruded about 1.55 Ga in the 

Ventuari dominion of Estado Amazonas (Gaudette and others, 1978). Other granitic rocks in the westernmost part of 

the Guayana Shield and southward beneath alluvial cover of

the upper Amazon Basin are similiar in age and composition 

(table 5). Together, these granites have been interpreted to 

represent widespread anorogenic magmatism at about 1.55 

Ga (Dall'Agnol and others, 1975, 1994; Kovach and others, 

1976; Gaudette and others, 1978; Priem and others, 1982; 

Gibbs and Barren, 1983; Teixeira and others, 1989). Anorogenic granite magmatism to the east and southeast in the 

Central Amazonian Province of Brazil is generally older, 

having a uranium-lead zircon age of about 1.88 Ga and 

rubidium-strontium isochron ages of 1.8-1.6 Ga 

(Dall'Agnol and others, 1994). These older ages suggest that 

anorogenic granite magmatism in the eastern area was a 

separate, independent event from that in the west.

The generation of the Parguaza granitic magmas has 

been characterized as a rift-related event (Gaudette and others, 1978; Gaudette and Olszewski, 1985; Jones and others, 

1986; Dall'Agnol and others, 1994). A tectonic environment 

represented by within-plate crustal extension accompanied 

by a high thermal gradient due to intrusion of mantle-derived 

basaltic magmas may explain the origin of the Parguaza 

Granite. The Parguaza Granite is similar in age, composition, and tectonic setting to the Middle Proterozoic 

(1,480-1,450 Ma) granite-rhyolite terrane of the St. Francois 

Mountains in southeastern Missouri of the United States 

(Kisvarsanyi and Kisvarsanyi, 1989). The formation of rocks 

in the St. Francois terrane has been explained as a failed 

cratonic rift or an anorogenic extensional tectonic setting at 

a passive continental margin (Kisvarsanyi, 1975; Windley, 

1989) or, possibly, as the result of shallow subduction, 

delamination of continental lithosphere, or orogenic-accretionary processes related to the early stages of the adjoining 

Grenville orogen (Patchett and Ruiz, 1989; Van Schmus, 

1993; Storey and others, 1994). The lack of (1) alluvial 

sediment fill, (2) minor to major quantities of basalt, and (3) 

basalt erupted peripherally to the rift indicates that the St. 

Francois granite-rhyolite terrane did not form in a classic rift 

environment (Patchett and Ruiz, 1989). A similar argument 

may be made against a rift setting for the Parguaza Granite. 

Indeed, the Parguaza Granite may be related to shallow subduction or early orogenic-accretionary processes associated 

with the 1,200-Ma Garzon-Santa Marta granulite belt in 

Colombia, which has been correlated with the Grenville 

erogenic belt (Kroonenberg, 1982; Priem and others, 1989).

Ratios such as Na/K, Ba/Sr, and K/Rb indicate that 

fractional crystallization was an important process during 

the formation of the Parguaza rapakivi granite (Mendoza, 

1974, 1975). The low initial ratio of 87Sr/86Sr (0.701, table 

5), neodymium isotopic data ( 143Nd/144Nd=0.51160), and 

high average content of nickel (about 12 ppm, as high as 710 

ppm) all suggest that the granitic magmas may have been 

derived from lower crustal material of trondhjemitic or charnockitic composition and a component of undifferentiated 

mantle material (Mendoza, 1974,1975; Gaudette and others, 

1978; Allegre and Ben Othman, 1980). Initial ratios of 

87Sr/86Sr between 0.704 and 0.716 and uranium-lead inher-
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B29

Table 5. Rubidium-strontium whole-rock isochron dates for granitic rocks of the Parguaza province and their equivalents, Guayana

Shield.

[Number of samples on which age is based is given in parentheses after age. MSWD is mean squares weighted deviation. N. A. indicates not available]

Country

Venezuela 

Venezuela 

Colombia 

Brazil

Brazil 

Brazil

Unit

Parguaza Granite 

Granite of San Carlos de Rio Negro (Casiquiare dominion) 

Granites of the Rio Inirida and Rio Guaviare (Ventuari? dominion) 

Surucucus granite

Granite of the Upper Amazon Basin 

Agua Boa-Madeira plutons, Pitinga area

Age (Ma) (87

1, 490+1 20 (A^) 1 

1,567±25 (n=4) 

1,485±35 (n=8) 

1, 520+1 40 (n=6) 1

1,530±25 (n=3)2 

1, 700+34 (n=9)3

Sr/86Sr)0

0.701 

0.704 

0.706 

0.696

0.706 

0.701

MSWD Reference

2.09 

0.93 

1.6

22.2

0.29 

5.98

Gaudette and others (1978). 

Gaudette and Olszewski (1985). 

Priem and others (1982. 

Basei and Teixeira (1975), 

Dall'Agnol and others (1975). 

Kovach and others (1976). 

Macambira and others (1987).

!The errors for 87Rb/86Sr and 87Sr/86Sr were not cited in the original references. Values of 1.5 and 0.085 percent (H.E. Gaudette, University of New 

Hampshire, oral commun., 1990) and 2.0 and 0.1 percent, respectively, were used for the calculations of the dates of the Parguaza and the Surucucus 

granites, respectively. The Parguaza Granite has a uranium-lead zircon age of 1,545±20 Ma (Gaudette and others, 1978). Dall'Agnol and others (1993) 

reported a reference isochron age of 1,583 Ma at an initial ratio of 0.708 for the Surucucus granite.

2These three samples are several hundred kilometers from one another and may not be comagmatic. Their model ages, assuming an initial ratio of 

0.706, are 1,541, 1,536, and 1,528 Ma.

3This date is recalculated from the original published data. The same decay constant and atomic ratios were used; however, the date in the publication 

was reported as 1,689+19 Ma, initial ratio=0.707, and MSWD=1.48, which is apparently a model 1, not a model 3, date.

itance patterns in zircon in similar anorogenic granite from 

other parts of the shield indicate a crustal source or an important contribution of continental crust to the magmas that 

generated these rocks (Pimentel and others, 1991; 

Dall'Agnol and others, 1994).

MINERAL DEPOSITS

Placer, eluvial, and lode occurrences of tin in Estado 

Amazonas and Estado Bolivar are spatially associated with 

the Parguaza Granite. Cassiterite in lodes is associated with 

quartz veins that cut the granite, and anomalous values of 

tantalum, niobium, zirconium, and titanium, contained in 

tantalum-rich rutile or struverite, tantalum-niobiumiron-manganese-bearing rutile, tantalite-columbite, stanniferous tantalite or ixiolite, and zircon (Aarden and Davidson, 

1977), are present in pegmatite associated with the granite 

(Rodriguez and Perez, 1982; Perez and others, 1985). The 

best known tin prospect is that near Cano Aguamena (Sidder, 

this volume, pi. 1).

The Parguaza Granite is equivalent in composition and 

origin and approximately equal in age (table 5) to granite in 

the Agua Boa and Madeira plutons, which host the Pitinga 

deposit in Brazil. Pitinga is one of the world's largest tin 

deposits and produces about 12 percent of the western 

world's tin (Thorman and Drew, 1988). As of 1984, it had 

measured reserves of about 203,000 metric tons of tin, of 

which about 30 percent was in alluvial deposits (Daoud and 

Antonietto, 1988). Measured, indicated, and inferred 

reserves totaled about 269,000 metric tons of tin; alluvial 

reserves accounted for about 64 percent (Daoud and Antonietto, 1988). Greisenized granite, locally called apogranite, 

hosts the primary tin ore and is the source of the alluvial 

deposits (Jones and others, 1986; Macambira and others, 

1987; Daoud and Antonietto, 1988; Thorman and Drew, 

1988). In the Surucucus area of northernmost Brazil, alkaline

granite having rapakivi texture is about 1.5-1.6 Ga and has 

potential reserves of 20,000 metric tons of tin (Dall'Agnol 

and others, 1975, 1994; Schobbenhaus and others, 1984; 

Jones and others, 1986). Thus, the Parguaza Granite has high 

potential for identification of undiscovered tin deposits.

The similarity in age, composition, and tectonic environment between the Parguaza Granite and the granite-rhyolite terrane of the St. Francois Mountains suggests that 

Olympic Dam-type iron-copper-uranium-gold-rare earth 

element deposits are a favorable exploration target in the 

Parguaza terrane (Sims, 1988; Sidder and Day, 1993; Sidder 

and others, 1993). Aeromagnetic and radiometric data could 

help locate potential prospects. A highly magnetic area and 

a superimposed uranium anomaly detected in aeromagnetic 

and aeroradiometric data over the Parguaza Granite in the 

westernmost part of Estado Bolivar (lat 5° 10' N., long 64°20' 

W.) (Wynn, 1993) is a good area to explore further.

The bauxite deposit at the Los Pijiguaos mine formed 

from the Parguaza rapakivi granite (Moreno and Bertani, 

1985b). The richest ore is at an erosional level (known as the 

Imataca-Nuria erosion surface) at elevations between 620 

and 690 m, and it formed during an intense weathering cycle 

in the Late Cretaceous and early Tertiary (Short and Steenken, 1962; Menendez and Sarmentero, 1985; Schubert and 

others, 1986). Bauxita Venezolana C.A. (BAUXIVEN) produced about 245,157 metric tons of ore in 1987 during its 

first year of operation at Los Pijiguaos, almost 2 million metric tons in 1991, and a reported 1.05 million metric tons in 

1992 (Ensminger, 1992; Doan, 1994). Initial measured and 

indicated reserves of bauxite were 201.8 million metric tons 

with a grade of 48.7 percent A12O3 and 10.9 percent SiO2, 

which included reserves of 70.1 million metric tons with 

51.8 percent A^C^ and 6.4 percent SiO2 (Menendez and 

Sarmentero, 1985). Newman (1989) reported that proven 

(200 million metric tons) and probable (500 million metric 

tons) reserves of 700 million metric tons are present in the 

Los Pijiguaos area. Three new bauxite deposits in the Par-
B30 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

guaza terrane were discovered in 1989 between the Los 

Pijiguaos mine and Puerto Ayacucho on the Rio Orinoco 

(Ensminger, 1992).

NICKERIE OROGENY

Potassium-argon, argon-argon, and rubidium-strontium 

dates of about 1,350-1,100 Ma for mica and feldspar from 

Archean and Early Proterozoic rocks of the Guayana Shield 

are indicative of partial resetting and overprinting due to the 

Nickerie metamorphic episode (Priem and others, 1968; 

Kroonenberg, 1982; Onstott and others, 1989). Loss of argon 

and strontium due to recrystallization resulted in the abnormally young Middle Proterozoic dates for Early Proterozoic 

and Archean rocks, and increase of argon in diabase of the 

Avanavero Suite caused the aberrantly old dates (fig. 4). 

Rocks affected by this episode extend from western Suriname through Guyana to Venezuela, Colombia, and northern 

Brazil. The eastern boundary of reset mica ages is in central 

Suriname (Priem and others, 1971; De Vletter and Kroonenberg, 1984). East of this boundary, Early Proterozoic rocks 

show Trans-Amazonian, not Nickerie, mineral ages. In 

Venezuela the Nickerie orogeny is also called the Orinoquean orogenesis (Mendoza, 1977a; Moreno and others, 

1977); in Guyana it is named the K'Mudku mylonite episode 

(Barron, 1969; Singh, 1974); and in Brazil it is known as the 

Jari-Falsino event (Kroonenberg, 1982). The Nickerie orogeny is equivalent to the Grenville orogeny in North America 

and Antarctica (Moores, 1991; Storey and others, 1994).

Reactivation of east-northeast-striking faults, such as 

the Guri shear zone, and minor uplift throughout the central 

and western parts of the Guayana Shield characterize the 

Nickerie episode. Cataclastic textures and locally mylonite 

zones and pseudotachylite developed along some faults 

(Short and Steenken, 1962; Priem and others, 1968; Barron, 

1969; Gibbs and Barron, 1993). Minor aplite and pegmatite 

dikes may have been emplaced coincident with faulting 

(Mendoza, 1977a). Extremely low grade to medium-grade 

metamorphism due to cataclasis and mylonitization in the 

central Guayana Shield during the Nickerie episode produced minerals such as pumpellyite, prehnite, epidote, 

albite, muscovite, chlorite, biotite, stilpnomelane, sphene, 

actinolite, and garnet (De Roever and Bosnia, 1975). In the 

western part of the shield, high-grade metamorphism generated charnockitic and enderbitic granulite, mafic granulite, 

amphibolite, and augen gneiss (Priem and others, 1989).

The Nickerie metamorphic episode commonly has been 

described as a regional tectonothermal event (Priem and others, 1968; Barron, 1969; De Roever and Bosma, 1975; Mendoza, 1977a), but a cause for the tectonism or increased heat 

flow has not been identified. Recent geochronological studies of the Garzon massif in the Andes of Colombia indicate 

that a quartzofeldspathic, calc-alkaline (continental arc) 

sequence of rocks along the western margin of the Guayana

Shield was metamorphosed to granulite facies about 1,172 

Ma (Priem and others, 1989). This metamorphism and associated deformation are attributed to continental collision 

(Kroonenberg, 1982; Priem and others, 1989; Park, 1992). 

This collision resulted in an early Neoproterozoic supercontinent during the 1.3-1.0-Ga Grenvillian (-Nickerie) orogeny (Dalziel, 1992; Storey, 1993). The reset mineral ages in 

rocks of the Guayana Shield are herein interpreted to be the 

result of thermal and tectonic effects in the hinterland of this 

proposed collision during the Nickerie orogeny.

MESOZOIC DIABASE DIKES

Narrow (<200 m), thin (<50 m), long (as much as 250 

km) unmetamorphosed diabase dikes that trend approximately east-northeast in Venezuela and north-northwest in 

the eastern part of the Guayana Shield are related to the 

opening of the Atlantic Ocean (MacDonald and Opdyke, 

1974; Gibbs, 1986). They have been called the Apatoe dike 

suite in Guyana (Gibbs and Barron, 1993). These dikes are 

generally thinner and straighter than dikes of the Avanavero 

Suite, possibly due to a more rigid crust at the time of their 

emplacement (Gibbs and Barron, 1993); however, because 

the field appearance and chemistry of these diabase suites 

are similar, the Mesozoic dikes are grouped with those of the 

Avanavero Suite (unit Xa) in figure 2. The dike rocks are 

fine to medium grained, have subophitic to ophitic texture 

(Margraves, 1978), and contain plagioclase (commonly 

labradorite) and augite, minor pigeonite, relict olivine cores 

in pyroxene, biotite, green amphibole, apatite, opaque minerals such as titaniferous magnetite, ilmenite, and rare chalcopyrite and pyrrhotite, and minor interstitial granophyric 

intergrowths of quartz and microcline microperthite 

(Hawkes, 1966a; Hargraves, 1978; Choudhuri and others, 

1984). These rocks are quartz-saturated tholeiite that has a 

continental basalt affinity (Choudhuri and others, 1984), and 

they generally contain more titanium, total iron, zirconium, 

vanadium, volatile content (water, carbon dioxide, fluorine, 

and chlorine as measured by loss on ignition), and possibly 

copper than does diabase of the Avanavero Suite 

(Choudhuri, 1978; Teggin and others, 1985). The chemistry 

of the Mesozoic rocks suggests that the dike swarms were 

derived directly from an undepleted mantle source, possibly 

related to a mantle plume and hot spot, during the breakup of 

Gondwanaland and the separation of South America from 

Africa (Choudhuri and others, 1984).

The age of these younger diabase dikes, which are 

present throughout the Guayana Shield, has not been determined precisely. As shown in the histogram in figure 5, 

potassium-argon dates for 59 samples of diabase range from 

about 550 to 130 Ma. The majority of dates are between 

about 230 and 170 Ma, and many are about 210 Ma (fig. 5). 

Dikes that have different potassium-argon dates have similar 

magnetic poles that are poles characteristic of Permian-
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B31

130 140 150 160 170 180 190 200 210 220 230 240 250 260 270

AGE, IN MEGA-ANNUM

280 290 300 350 400 450 500 550

Figure 5. Potassium-argon dates for Phanerozoic diabase of the Guayana Shield. Data from Priem and others (1968), Frick and Steiger 

(1974), MacDonald and Opdyke (1974), Schobbenhaus and others (1984), andTeggin and others (1985).

Triassic rocks (Veldkamp and others, 1971; Margraves, 

1978); however, scatter of the paleomagnetic pole positions 

does not allow the age of the dikes to be distinguished more 

accurately. Dates for diabase dikes in Liberia, West Africa, 

that intruded Proterozoic basement rocks range from about 

1,222 to 177 Ma. These dikes contain large and variable 

amounts of excess 40Ar that resulted in anomalously old 

dates (Dalrymple and others, 1975; Mauche and others, 

1989). Dates for diabase dikes that intruded Paleozoic sandstone range, however, from about 201 to 177 Ma. Dikes that 

cut sandstone may also contain small amounts of extraneous 

40Ar (Dalrymple and others, 1975; Mauche and others, 

1989). Thus, dates for diabase dikes in the Guayana Shield, 

all of which intrude Proterozoic or Archean rocks, may not 

be representative of the time of dike emplacement and crystallization. All of the dikes are probably latest Triassic to earliest Jurassic, about 210-200 Ma, in age. It is not known 

whether emplacement of the dikes marked the initiation of or 

predated the separation of South America and Africa and the 

opening of the Atlantic Ocean.

MESOZOIC-CENOZOIC UPLIFT,

EROSIONAL SURFACES, AND

ALLUVIUM

Uplift and southward tilt of the Imataca Complex and 

other rocks in the northern Guayana Shield of Venezuela

occurred just prior to or during the separation of South 

America from Africa (Onstott and others, 1989). Additional 

uplift and tilt may also have been associated with mid-Cenozoic orogeny in the Caribbean area (Short and Steenken, 

1962; Olmore and Estanga, 1989; Olmore and Garcia-Gerdes, 1990; Olmore and others, 1993). These uplifts caused 

erosion of the Guayana Shield and subsequent deposition in 

a basin north of the Rio Orinoco (Schubert and others, 1986; 

Olmore and Garcia-Gerdes, 1990; Olmore and others, 1993). 

Geomorphic and geologic evidence of Cenozoic tectonism 

has not been observed in the Gran Sabana area; however, 

slow, broad regional upwarping may not have generated a 

recognizable geomorphic expression (Dohrenwend and 

others, this volume).

At least six planar geomorphic surfaces, marked by distinct elevations, have been identified in the Venezuelan 

Guayana Shield. They are, from oldest to youngest, (1) 

Auyantepui (2,000-2,900 m), (2) Wonken or 

Kamarata-Pakaraima (900-1,200 m), (3) Imataca-Nuria 

(600-700 m), (4) Caroni-Aro (200^50 m), (5) Llanos 

(80-150 m), and (6) Orinoco floodplain (0-50 m) (Short and 

Steenken, 1962; Menendez and Sarmentero, 1985; Schubert 

and others, 1986, 1989; Briceno and Schubert, 1990; Gibbs 

and Barron, 1993; Dohrenwend and others, this volume). 

The ages of the oldest two surfaces are not well known; 

Schubert and others (1986) speculated that they are Mesozoic. The other surfaces range in age from early Tertiary to 

Holocene (Schubert and others, 1986; Briceno and Schubert,
B32 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Alluvium

MESOZOIC-CENOZOIC UPLIFT

Mesozoic diabase dikes

NICKERIE OROGENY

Parguaza Granite

Avanavero Suite

Roraima Group

Undivided Proterozoic rocks

UNNAMED OROGENY, ESTADO AMAZONA

Cuchivero Group 

TRANS-AMAZONIAN OROGENY 

Supamo Complex 

Greenstone-belt rocks

PRE-TRANS-AMAZONIAN 

TECTONOMAGMATIC EVENT 

Imataca 7 ______ ? 

Complex '

-? ? -

-TV

4,000 3,500 3,000 2,500 2,000 

TIME, IN MEGA-ANNUM

1,500 1,000 500 200 Present

Figure 6. Chronology of the Venezuelan Guayana Shield. Rock units described in table. 1

1990). The Eocene-Oligocene Imataca-Nuria surface is 

important economically because bauxite and enriched 

deposits of banded iron formation are developed on it 

(Menendez and Sarmentero, 1985; Schubert and others, 

1986). All of the planation surfaces have been correlated 

with similar surfaces in Brazil, Guyana, Suriname, and (or) 

French Guiana, as well as in West Africa and India (Short 

and Steenken, 1962; Prasad, 1983; Schubert and others, 

1986; Briceno and Schubert, 1990; Gibbs and Barren, 1993).

Episodic periods of tectonic uplift, landscape dissection 

in response to uplift, and parallel slope retreat during succeeding intervals of stability are considered to be the cause 

of formation of these planation surfaces. Many of the surfaces coincide, however, with lithologic units, and the steps 

that separate them commonly coincide with lithologic discontinuities (Kroonenberg and Melitz, 1983). Furthermore, 

within the southern part of the Gran Sabana, ferricrete-capped remnants of a middle(?) Tertiary geomorphic 

surface cut across several of the regional planation surface 

levels and extend almost continuously from dissected strath 

terraces lying less than 10m above modern streams to high, 

structurally controlled ridges rising more than 200 m above 

the floors of the larger valleys (Dohrenwend and others, this 

volume). Thus, present-day relief within the Guayana Shield 

is mostly a function of lithologic resistance. At least some of 

these so-called planation surfaces are a manifestation of 

lithologic control in a relatively stable landscape that is 

developed on a thick sequence of flat-lying to very gently 

dipping sedimentary strata.

Tertiary-Quaternary paleoplacer deposits and the lower 

Roraima Group are the source of diamonds and gold in 

Holocene alluvium (Briceno, 1984; Dohrenwend and others, 

this volume).

SUMMARY

The Guayana Shield of Venezuela consists predominantly of Archean and Early to Middle Proterozoic rocks 

(fig. 6). Archean rocks are not known elsewhere in the 

Guayana Shield, but similar Archean rocks and tectonic 

events are present in the West African Shield. Early 

Proterozoic greenstone-belt rocks and erogenic events in the 

rest of the Guayana Shield and in the West African Shield are 

comparable to those in the Venezuelan Guayana Shield. 

Differences in the types of late Early Proterozoic and Middle 

Proterozoic rocks in the two shields are notable. For 

example, granite and rhyolite equivalent to the Cuchivero 

Group are present throughout the Guayana Shield but are 

only locally present in the West African Shield. Also, rocks 

equivalent to the Roraima Group and the Parguaza Granite, 

although widespread in the Guayana Shield, are absent or 

rare in West Africa.

The only confirmed Archean rocks in the Guayana 

Shield are in the Imataca Complex. The Imataca Complex 

includes gneiss, granulite, amphibolite, dolomite, manganiferous rocks, and banded iron formation. Amphibolite- to 

granulite-facies metamorphism and refolded isoclinal folds 

are characteristic of the Imataca Complex.
GEOLOGY OF THE VENEZUELAN GUAYANA SHIELD B33

The Early Proterozoic greenstone belts in Venezuela 

comprise submarine sequences of mafic volcanic rocks at the 

base, a middle section of basalt to rhyolite, and an upper section of turbiditic graywacke, volcaniclastic rocks, and chemical sedimentary rocks. Tholeiitic and calc-alkaline 

differentiation trends are common in the volcanic rocks. 

Layered mafic complexes are also present in the greenstone 

belts. Rocks of the greenstone belts formed between about 

2,250 and 2,100 Ma. They were metamorphosed to the 

greenschist facies and locally the amphibolite facies near 

granitic domes of the Supamo Complex, which intruded the 

greenstone belts between about 2,230 and 2,050 Ma. Major 

deposits of low-sulfide gold-quartz veins are hosted by rocks 

of the greenstone belts in shear zones. The Imataca Complex 

and the greenstone-granite terranes were deformed and 

metamorphosed during the Trans-Amazonian orogeny, 

which represents a period of continental collision between 

about 2,150 and 1,960 Ma. Postcollisional, post-TransAmazonian magmatism between about 1,930 and 1,790 Ma 

produced volcanic and plutonic rocks of the Cuchivero 

Group. Kimberlite in the Quebrada Grande area (and possibly carbonatite in the Cerro Impacto area) was intruded 

about 1,730 Ma. Undivided Proterozoic and possibly 

Archean(?) rocks in Estado Amazonas include granitic 

rocks, gneiss, and migmatite. Peak metamorphism and intrusion of these rocks occurred between about 1,860 and 1,730 

Ma.

Unmetamorphosed rocks of the Roraima Group were 

deposited in fluvial, deltaic, shallow coastal marine, and 

lacustrine or epicontinental environments. The Roraima 

Group is possibly as old as about 1,900 Ma and as young as 

about 1,500 Ma. Conglomeratic lenses and beds in the lower 

500-600 m of the Roraima Group contain paleoplacers, 

some of which are the source of diamonds and gold in modern placer deposits. Mafic dikes, sills, and irregular intrusive 

bodies of the Avanavero Suite cut all earlier rocks of the 

Guayana Shield. These continental tholeiitic intrusions are 

about 1,650 Ma in age and possibly as old as about 1,850 Ma. 

Middle Proterozoic, 1.55-Ga rapakivi granite of the Parguaza terrane hosts occurrences of tin in quartz veins. Major 

deposits of bauxite are developed on the Parguaza Granite. 

Continental collision during the Nickerie orogeny in the 

westernmost part of the Guayana Shield reset mineral ages in 

Archean and Early Proterozoic rocks of the central and eastern parts of the shield to about 1,200 Ma. The Nickerie orogeny is equivalent to the Grenville orogeny in North America.

Diabase dikes intruded the Guayana Shield during the 

opening of the Atlantic Ocean about 210-200 Ma. Kimberlite and carbonatite may have been intruded during the 

Mesozoic. Six planar geomorphic surfaces developed at distinct elevations in the Guayana Shield during the Mesozoic 

and Cenozoic eras. Deposits of bauxite and enriched banded 

iron formation formed on the early Tertiary Imataca-Nuria 

surface. Gold and diamond placers are present in modern

major river channels and in colluvial-alluvial deposits of 

low-order drainages.

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Analysis of Aeromagnetic Data to Improve 

Geologic Maps of the Bochinche Mining District, 

Estado Bolivar, Venezuela

By Jeffrey C. Wynn, Anne E. McCafferty, and Edixon Salazar

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-C

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENTS

Abstract.......................................................................................................................... Cl

Resumen......................................................................................................................... 1

Introduction.................................................................................................................... 1

Geology of Bochinche Area........................................................................................... 1

Aeromagnetic Data Set................................................................................................... 3

Derivative Maps ............................................................................................................. 4

Discussion of Derivative Products ................................................................................. 7

Magnetic Modeling......................................................................................................... 7

Surficial Geology of Bochinche Area ............................................................................ 8

Conclusions.................................................................................................................... 11

References Cited............................................................................................................. 11

Appendix Aeromagnetic Surveys Covering the Venezuelan Guayana Shield............ 13

FIGURES

1. Map showing location of Bochinche 1:50,000-scale quadrangle and aeromagnetic surveys covering the

Venezuelan Guayana Shield........................................................................................................................................ C2

2. Aeromagnetic map for part of Bochinche quadrangle ................................................................................................ 3

3. Shaded relief map of reduced-to-the-pole aeromagnetic data for part of Bochinche quadrangle............................... 4

4. Terrace-magnetization map derived from aeromagnetic data of figure 2................................................................... 6

5. Two-and-one-half-dimensional model of magnetic data from profile crossing Laguna dike complex,

Bochinche mining district........................................................................................................................................... 8

6. Map showing surficial geology of part of Bochinche area.......................................................................................... 9

7. Map showing bedrock geology of Bochinche quadrangle.......................................................................................... 10
Analysis of Aeromagnetic Data to Improve

Geologic Maps of the Bochinche Mining District,

Estado Bolivar, Venezuela

By Jeffrey C. Wynn, 1 Anne E. McCafferty,2 and Edixon Salazar3

ABSTRACT

A l:50,000-scale aeromagnetic contour map for the 

Bochinche area of the northeastern Venezuelan Guayana 

Shield was manually digitized along flight line-contour line 

intersections. Several derivative products including horizontal gradient of pseudogravity, reduced-to-the-pole, and terrace-magnetization maps were developed from the original 

data. These products, and interpretations of them, were combined with available geologic information to create a bedrock geologic map that indicates several new areas for gold 

exploration in the Bochinche area. This type of analysis is 

applicable to many other areas of the Guayana Shield.

RESUMEN

Un mapa de isolineas aeromagneticas a escala 1:50,000 

del area de Bochinche, el cual esta ubicado al noreste del 

Escudo de Guayana en Venezuela, fue digitizado manualmente a lo largo de las intersecciones de las lineas de vuelo 

con las isolineas aeromagneticas. A partir de los datos originales se derivaron y desarrollaron varies productos tales 

como el gradiente horizontal de pseudogravedad, reduction 

al polo y mapas de magnetization en terraza. Estos productos, e interpretaciones fueron, combinados con information 

geologica disponible para crear un mapa del basamento cristalino. En este mapa se indican varias areas para exploration aurifera en el area de Bochinche. Este tipo de analisis 

es aplicable a muchas areas del Escudo de Guayana.

INTRODUCTION

The Venezuelan Guayana Shield encompasses large 

areas of greenstone belts and associated plutonic rocks. The

'U.S. Geological Survey, Unit 62101, APO AE 09811-2101, USA.

2U.S. Geological Survey, Denver Federal Center, MS964, Denver, 

Colorado 80225.

3Corporacion Venezolana de Guayana, Tecnica Minera, C.A., C.C. 

Chilemex, Piso 1, Puerto Ordaz, Venezuela.

area of the greenstone belts has been a gold-producing 

region for at least a century, but it is extremely difficult to 

map geologically because of dense jungle cover and 

intensely weathered soils. Few outcrops are present, and 

these few are not only difficult to find, but the rocks are also 

difficult to classify because of changes due to weathering, 

metamorphism, and deformation. Little geomorphological 

expression of the geologic features exists; relief is generally 

low throughout the region.

The Bochinche 15-minute quadrangle in the northeastern part of the Venezuelan Guayana Shield (fig. 1) provides 

an excellent setting in which to test the feasibility of using 

available aeromagnetic data to map in heavily forested terrane. The intensely weathered, deformed, and metamorphosed Early Proterozoic greenstone-belt rocks that underlie 

this area are characteristic of the region. Although aeromagnetic surveys have been flown over most of the region (fig. 

1, appendix), the Bochinche area presents an unusual opportunity because attempts have been made to map it geologically.

The geology of the area was mapped during two separate field efforts (Salazar, 1988; Salazar and Franco, 1988), 

primarily using inferences from soil colors and information 

from rare outcrops. An aeromagnetic survey (survey C, 

appendix), with flight lines spaced 500 m apart draping the 

topography at about 150 m, also covers the quadrangle. 

Although the original data were acquired in analog form, 

digitization of the published l:50,000-scale aeromagnetic 

map provided sufficient detail to distinguish different geologic units based on their distinctive magnetic signatures.

GEOLOGY OF BOCHINCHE AREA

Most of the rocks in the Bochinche area are Early Proterozoic, the notable exception being a series of diabase dikes 

that may be as young as Mesozoic (Bellizzia and others, 

1976, 1981; Sidder and Mendoza, this volume). Although 

Archean rocks are present north of the Bochinche area, the 

oldest rocks in the Bochinche area are volcanic,

ci
C2

69°

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

66° 63° 60°

ESTADO

\ DELTA AMACURO 

\

Area o 

Bochinche 

quadrangle

c

VENEZUELA

Boundary of Venezuelan 

Guayana Shield

COLOMBIA

Figure 1. Location of Bochinche l:50,000-scale quadrangle, Estado Bolivar, Venezuela. Letters and dotted lines indicate various aeromagnetic surveys conducted over the Venezuelan Guayana Shield described in the appendix.

volcaniclastic, and turbiditic rocks of greenschist fades, 

dated between 2,250 and 2,100 Ma (Wynn and Sidder, 

1991). These are intermediate to mafic lavas and tuff of the 

Carichapo Group and overlying quartzite, pyroclastic, and 

sedimentary rocks of the Yuruari Formation, all of which are

part of the Pastora Supergroup. These greenstone-belt rocks 

were later intruded and deformed by granitic to granodioritic 

and trondhjemitic rocks of the Supamo Complex about 

2,230-2,050 Ma (Klar, 1979). In areas of contact with 

Supamo intrusive rocks, the greenstone-belt rocks are locally
61°00'

ANALYSIS OF AEROMAGNETIC DATA, BOCHINCHE MINING DISTRICT

60°45'

C3

NANOTESLAS

Figure 2. Aeromagnetic map for part of Bochinche 15-minute quadrangle, eastern Estado Bolivar, Venezuela. Total-field aeromagnetic data were acquired in 1961; contour interval 18 nT, flight-line spacing 500 m, flown at 150-m barometric altitude bearing N. 22° 

W. Line of profile A-A' (fig. 5) is shown.

metamorphosed to amphibolite facies (Menendez, 1968; 

Wyrm and Sidder, 1991).

About 2,150-2,000 Ma a major erogenic cycle of metamorphism, deformation, and plutonism, the Trans-Amazonian orogeny, deformed and metamorphosed rocks of the 

Pastera Supergroup and the Supamo Complex. This orogeny 

is recognized throughout cratonic northern South America 

(Gibbs and Barron, 1983, 1993). Mafic to granitic intrusive 

rocks are associated with this orogeny, and several mafic 

bodies in the Bochinche area apparently date from this event.

Numerous elongate tholeiitic dikes and sills, generally 

oriented east-northeast, formed in the Guayana Shield during 

the opening of the Atlantic Ocean about 200 Ma. The 

Laguna dike discussed following may be a combination of 

two different diabase dikes, coincidentally parallel, but 

injected during different events as part of the late Early 

Proterozoic Avanavero Suite and the Mesozoic dike swarms.

Deposition of Tertiary and Quaternary alluvial, colluvial, 

and eluvial sediments in the Bochinche area, together with 

extensive tropical weathering, has left few outcrops to map. 

Structural and lithologic information from the rare outcrops 

was supplemented during recent mapping with data from 

drill holes, trenches, and roadcuts (Salazar, 1988; Salazar 

and Franco, 1988).

AEROMAGNETIC DATA SET

Most of the gold discovered in the Venezuelan Guayana 

Shield is associated with rocks of the greenstone belts. 

Aeromagnetic contour maps from survey C (fig. 1, 

appendix) were examined for a block of data that would be 

amenable to modern potential-field data processing 

methods. Initially, an area of greenstone terrane that had
C4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

61°00' 60°45'

7°3CJ

Figure 3. Reduced-to-the pole magnetic field map for part of Bochinche 15-minute quadrangle, eastern Estado Bolivar, Venezuela, 

calculated from aeromagnetic data of figure 2. In some cases, anomalies (depending on depth to source) may be shifted northward by 

several kilometers.

been mapped in detail was sought. The Bochinche area in 

eastern Estado Bolivar had the most detailed geologic maps 

available, but, because outcrops are rare, the geology had 

been mostly mapped by changes in soil color. The objective 

of the present study was to develop a more detailed and 

precise bedrock geologic map of the Bochinche mining 

district. Due to economic restrictions, this was accomplished 

without acquisition of additional aeromagnetic data.

The Bochinche mining district is within survey C 

(fig. 1), approximately 225 km east-southeast of Puerto 

Ordaz and adjacent to the frontier with Guyana. The aeromagnetic data were available only in hand-contoured map 

form. To digitally process the aeromagnetic data for the 

l:50,000-scale Bochinche 15-minute quadrangle 

(lat 7°30'-7°45' N., long 60°45/-61°00' W.), maps were 

digitized manually along the flight lines at the contour-line

intersections. The data were subsequently processed using 

public-domain software developed by the U.S. Geological 

Survey (1989).

Figure 2 is an aeromagnetic map in image form 

obtained from the digitized data of part of the Bochinche 

l:50,000-scale map. It is presented as a gray-shaded-relief 

map with an illumination direction of N. 30° W. The line of 

profile A-A' was modeled and is discussed in a later section.

DERIVATIVE MAPS

Maps derived by applying analytical techniques to the 

digitized aeromagnetic data provide considerable new 

information. This information is present in the original aeromagnetic data but is generally not apparent to the eye in con-
ANALYSIS OF AEROMAGNETIC DATA, BOCHINCHE MINING DISTRICT C5

tour form. Several analytical filters were applied to the 

aeromagnetic data to extract additional information about 

rock types and structures in the greenstone terrane and to 

determine how these features might relate to the known 

mineral deposits (especially gold) within the study area. The 

derivative products include reduced-to-the-pole, terrace-magnetization (subsequently called "terrace"), and 

horizontal gradient of pseudogravity maps.

The reduced-to-the-pole map (fig. 3) shows the 

magnetic data in a form equivalent to a vertically incident 

magnetic field (Hildenbrand, 1983). In this region near the 

Equator, the Earth's magnetic field has a shallow inclination 

(31°) that produces markedly dipolar magnetic anomalies 

that do not overlie their sources. The reduction program 

recalculates the field using a multiplicative filter in the 

frequency domain and requires the inclination and declination of Earth's field in the area of interest. An inclination 

(31°) and declination (-12°) of the magnetic field at the midpoint of the data set for the year and month that the survey 

was flown were selected. The northward shift of the 

magnetic highs, which can be viewed as replacement of lows 

with highs and vice versa, between figures 2 and 3 is 

substantial but nevertheless typical for Venezuela.

The magnetic field measured over crustal rocks is 

influenced by many physicochemical phenomena. These 

include induction in the Earth's present magnetic field, 

remanent magnetization acquired when igneous rocks 

cooled in an ancient magnetic field, isothermal remanent 

magnetization, and chemical remanent magnetization. The 

modern magnetic field measured over the Venezuelan 

Guayana Shield could be influenced by a combination of 

some or all of the above. Because isothermal remanent 

magnetization is a soft magnetization that tracks the Earth's 

field through time, it can be treated for calculation purposes 

as part of the induced magnetization. Chemical remanent 

magnetization can be combined with other fixed-polarity 

contributions into remanent magnetization for the same 

purposes. For lack of evidence to the contrary (such as 

dipolar anomalies aligned in directions different from the 

local declination direction of N. 11° W.), it was assumed 

that most of the magnetic sources in the Bochinche aeromagnetic data have magnetization vectors parallel with the 

Earth's present magnetic field. This is a reasonable 

assumption because the anomalies in the study area are 

caused by ancient rocks, and any remanent magnetization 

component has likely been attenuated by long-term decay 

and modification over time by subsequent tectonothermal 

events. Application of the reduced-to-the-pole filter results 

in a plot of the magnetic field that centers anomalies over 

their causative sources.

Where the magnetic latitude is very low, as in the 

Bochinche area, uncorrected magnetic anomalies are generally dipolar and centered, with the positive side south and 

the negative side north, over their source bodies. Note that 

at very low latitudes, a reduction-to-the-pole recalculation

can generate artificial anomalies if there is considerable 

noise in the data and can produce unacceptable artifacts in 

the resulting map (see Hansen and Pawlowski, 1989, for a 

study of a seamount at a magnetic latitude of 17°). This 

was a minor problem in the study area and is shown by the 

elongate northwest-trending artifacts. Although some minor 

artifacts remain in the data, the reduction-to-pole filter was 

successful in producing an interpretable map.

A data processing technique called terracing (Cordell 

and McCafferty, 1989) was applied to the gridded aeromagnetic data to produce a map of inferred magnetization 

called a terrace-magnetization, or terrace, map (fig. 4). 

Some of the inferred magnetization domains and their 

edges coincide with known geologic features such as faults, 

contacts, or igneous bodies. Therefore, a terrace-magnetization map is a geologiclike map that, together with field 

geologic ground checks, can aid geologic mapping in areas 

where rocks are scarce in outcrop or buried. The terrace 

technique in this study was used to define magnetization 

domains and (or) geologic structures within the Venezuelan 

Guayana Shield and to infer rock types based on 

magnetization values within domain boundaries.

The terrace technique is relatively new, and the steps 

followed to produce the terrace map are briefly 

summarized. For a complete description of the theory and 

technique see Cordell and McCafferty (1989) and Phillips 

(1992). The first step involves a mathematical calculation 

to remove a second-order polynomial surface from the gridded data (Cordell and others, 1992), thus producing a 

second-order residual magnetic field. Removal of the 

regional field is necessary in order to decrease the dynamic 

range of the data without degrading the short-wavelength 

features. This step improves the resolution of the shallow 

crustal sources represented by the short-wavelength anomalies. The second-order residual magnetic anomalies are then 

transformed to pseudogravity anomalies (Hildenbrand, 

1983). The positions of magnetic anomalies depend on the 

geomagnetic latitude of the source and the dipolar nature of 

the bodies causing the anomalies; the actual positions of the 

anomalies are displaced from their correct locations. The 

pseudogravity transformation, similar to a reduction-to-pole 

filter, corrects for this shift.

The gridded pseudogravity data are then terraced; that 

is, the smoothly varying anomalies are transformed into 

steplike functions in which the vertical segments of the 

steps mark the steepest parts (or the inflection points) of the 

anomalies. At this point in the process, the amplitude range 

of the terrace function is identical to the range of the input 

anomalies. A more complicated aspect of the terracing procedure involves converting units on the terrace map to units 

of magnetization within each domain. This is done by referring the terrace function to a model represented by a slab of 

rock having a constant thickness and flat top. The top of the 

slab for this model is defined to be the topographic surface; 

topographic relief is only about 60 m in the Bochinche
C6 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

< i +*t mf **, * ^..^« . * * v.. *

 .* > --. :

^ X * f f i * } / \ :

 

. 

~-t V 1 V

B* ** 

* X

 v. w-X t

") \" > 

X ''" * ^

.« 

 

\ i

 ' \

Figure 4. Terrace-magnetization map derived from the aeromagnetic data of figure 2. Maxima of the horizontal-gradient-of-pseudogravity (circles) are superimposed on the terrace map and mark probable lithologic boundaries. Color scale 

is in electromagnetic units per cubic centimeter.

area, and therefore a flat surface is used in the forward 

calculation as an average estimate for the top of the slab.

A slab thickness is chosen for the model that yields a 

best fit between the second-order residual magnetic field 

and the forward-calculated field (not shown), as well as 

plausible magnetization values. The thickness of the slab is 

allowed to vary until best-fit conditions are attained. For 

this data set, a slab 3 km thick best served to determine the 

magnetization values of individual domains.

The terrace-magnetization domains can be visualized 

as jigsaw pieces, each 3 km thick, having a flat top and 

bottom and vertical sides. The magnetization within each

domain is constant in the vertical dimension but varies 

laterally from domain to domain.

The values shown on the color scale in figure 4 represent inferred magnetization values in electromagnetic units 

(emu) per cubic centimeter. Determining an acceptable 

background magnetization value against which to contrast 

the calculated values is difficult without measured samples. 

Because such samples were not available, the 

magnetization of the rocks was assumed to be entirely 

induced in the direction of the present-day magnetic field 

and was then contrasted with the values against a magnetization of 0.00 emu/cm3 . This datum is arbitrary, and
ANALYSIS OF AEROMAGNETIC DATA, BOCHINCHE MINING DISTRICT C7

negative magnetization related either to relatively weak 

magnetization or to predominantly remanent magnetization 

is admissible, although unlikely for reasons discussed 

previously.

To emphasize further the magnetization boundaries, 

the locations (marked as circles in figure 4) tracking horizontal gradient maxima of the pseudogravity data are 

superimposed on the terrace image. As discussed by Cordell and Grauch (1985), the maxima delimit abrupt lateral 

changes in the magnetic field and, by inference, are interpreted to be steep or almost vertical lithologic contacts. The 

positions of the maxima are offset downdip if the contact is 

gradational or does not dip steeply. Location of the contact 

may also be inexact if data sampling is insufficient or if a 

number of sources are spatially close together (Grauch and 

Cordell, 1987). Several sources are spatially close together 

in the northeastern part of the Bochinche quadrangle (figs. 

2, 3), just beyond the area of mapped geology.

DISCUSSION OF DERIVATIVE 

PRODUCTS

In the aeromagnetic map of figure 2, beginning at the 

southwesternmost corner and continuing east-northeast 

across most of the map area, the magnetic signature of the 

Laguna dike is evident. The Laguna diabase dike cuts the El 

Callao gold district more than 100 km to the west of the 

Bochinche area and has been mapped geologically almost 

continuously from west-southwest of El Callao eastward to 

Bochinche and beyond. An intense elliptical magnetic anomaly in the southeastern corner of the Bochinche map area is 

caused by ultramafic rocks, the weathered relics of which 

have been observed on the surface in some places.

Several discrete structures are visible in the terrace map 

(fig. 4), and their boundaries are more precise than in the 

gray image of figure 2. The Bochinche area can be divided 

roughly into three magnetic terranes: a strongly magnetic 

western-southwestern part, a less magnetic southeastern 

part, and a weakly magnetic central and northern part. The 

latter two parts are connected by a linear feature (faintly 

visible in fig. 2, more clearly visible on the original 

l:50,000-scale contoured map) that correlates with the 

Laguna dike. The high magnetization in the southeastern and 

southwestern parts of the area is apparently caused by mafic 

volcanic and volcaniclastic rocks. The generally low magnetization in the northern part of the map area is coincident 

with extensive tracts of quartzite of the Yuruari Formation. 

Terrace maps do not provide information on depths of discrete bodies (Cordell and McCafferty, 1989), and thus several individual magnetic anomalies were modeled.

MAGNETIC MODELING

Figure 5 shows a two-dimensional model calculated to 

fit discrete data points from the original aeromagnetic map in

the area of the Laguna dike complex. The modeled profile 

trends N. 22° W. (along the flight-line direction), starting 

from a point at lat 7°30' N., long 60°50'50" W., on the southern margin of the Bochinche quadrangle. In the lower center 

of the map area of figure 2, two discrete linear anomalies are 

coincident with two known manifestations of tholeiitic 

dikes, called here the northern and southern extensions of the 

Laguna dike. In an earlier unpublished map (Edixon 

Salazar), these two dikes were classified differently the 

northern dike was called a gabbro and the southern a 

diabase mainly to distinguish their different grain size (the 

rocks were analyzed chemically only for gold).

The dikes were mapped in two trenches, and their 

approximate widths are thus known near the model profile. 

Using these widths, the model suggests that the Laguna dike 

is actually two tabular bodies having different susceptibilities. The southern body is only about 800 m deep and 

probably dips north, whereas the northern dike has almost 10 

times the magnetic susceptibility of the southern dike, is at 

least 5 km deep, and dips south. Estimates of the dips of the 

dikes are derived from the model because the trenches 

sampled only the tops of the dikes. Both dikes extend almost 

to the surface (within 3 and 30 m, southern dike and northern 

dike, respectively) in the model; effects of weathering 

extend 10-18 m below the surface, as indicated by drill-hole 

data. Louis Guilloux (Bureau de Recherches Geologiques et 

Minieres, oral commun., 1989) inferred from structural 

measurements in the trenches that the two dikes are subvertical and may dip steeply to the north or south. Although the 

model presented in figure 5 is not a unique solution, it is a 

reasonable estimate of the geometry and susceptibility of the 

dikes.

The magnetic susceptibility of the narrower, northern 

extension of the dike is 0.0048 emu/cm3, almost nine times 

that of the wider southern extension (0.000525 emu/cm3). 

This difference in magnetic susceptibility supports the inference that these two features represent different geologic 

units. In comparison, a sample of diabase from the Gran 

Sabana, about 200 km to the south, has a magnetic susceptibility of 0.0016 emu/cm3, and a sample of diabase from the 

Marwani area, about 80 km to the south-southeast of 

Bochinche, has a magnetic susceptibility of 0.000008 

emu/cm3, significantly less than susceptibilities of samples 

from Bochinche. Although the model presented in figure 5 is 

not a unique solution, it is a reasonable estimate of the dike's 

geometry and susceptibility. In the terrace map, the northern 

dike dominates the image because of its much greater magnetic susceptibility, but, because the two features are so close 

together, both contribute to form a single band.

A broad magnetic low about 3.0 km along the profile 

shown in figure 5 was not modeled because it is caused by 

sideswipe; that is, it is a contribution from a source some distance off line. The source, in fact, is the ultramafic body that 

is manifested as a strong anomaly in the southeastern corner 

of the map area of figures 2 and 3.
C8 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

SOUTH-SOUTHEAST NORTH-NORTHWEST

7,000

6,900

z

gg 6,800

III _l

o H 6,700

t~~ ^

LU Z

g | 6,600

.>

6,500

0 A

2.5

5.0

6.0

i \ r

A__A A /Calculated 7 \

"Sideswipe" Observed

A A A A AA/AA ASA A A AA A A A A AAAA A AA A A A A A A A____A A_____A

2.5 5.0 

DISTANCE, IN KILOMETERS

7.5 10.0 11.0

Figure 5. Two-and-one-half-dimensional model of magnetic data from a profile crossing the Laguna dike complex, Bochinche mining 

district. Data are from the aeromagnetic map (fig. 2) and are represented by triangles. The profile extends N. 22° W. from a zero point at 

lat 7°30' N., long 60°50'50" W. The southern dike has an observed minimum depth (in trenches) of about 3 m and a magnetic susceptibility 

of 0.000525 emu/cm3 . The northern dike has a depth of 30 m or more (from nearby drill holes; possibly somewhat shallower) and a modeled 

magnetic susceptibility of 0.0048 emu/cm3 . Sideswipe refers to an effect from an off-line anomaly (see discussion in the text). Line of profile 

is shown in figures 2 and 7.

SURFICIAL GEOLOGY OF 

BOCHINCHE AREA

Figure 6 is a compilation of two geologic maps prepared during separate mapping surveys in 1985-1987 in the 

southern and western parts of the Bochinche quadrangle 

(Salazar, 1988; Salazar and Franco, 1988). Detailed geologic 

mapping was not carried out in the northern part of the quadrangle because of logistical limitations; less than a fraction 

of a percent of this area has outcrop, and most of the mapping 

was done by inference from soil color. The two geologic 

maps were digitized at their original scale of 1:25,000, then 

combined and reduced in size. The stratigraphy of these 

maps follows that described by Menendez (1968) and 

Mendoza (1977) and modified by Wynn and Sidder (1991). 

The lithostratigraphy is described briefly herein; a more 

complete description is given in Sidder and Mendoza (this 

volume). The surficial geologic map (fig. 6) is located only 

approximately because base maps for the area do not have 

geodetic control; the location of the main Bochinche road

and the two reference points extracted from a third informal 

index map used by Corporation Venezolana de Guayana, 

Tecnica Minera, C.A. (CVG-TECMIN) are shown; the original geologic maps used slightly different locations of this 

road for reference.

The map was compiled, and units assigned, before adding data for the quartz veins. When the field-mapped quartz 

veins were plotted, their locations proved to correlate closely 

with the new, geophysically inferred contacts between intrusive rocks and mafic lavas of the Carichapo Group. This correlation is consistent with the deposit model for low-sulfide 

gold-quartz veins (Berger, 1986; Bliss and Jones, 1988; 

Wynn and Sidder, 1991). This deposit model (also see 

Colvine and others, 1988) allows (1) the presence of mafic 

and ultramafic rocks as a source of gold, (2) an intrusive 

event to provide heat for hydrothermal cells, and (3) highly 

fractured rocks such as volcanic rocks and turbidite 

sequences to host the gold. The new bedrock geologic map 

of figure 7 thus provides new target areas for gold exploration, especially in the southeastern quarter of the Bochinche 

quadrangle.
61°00' 60°50'

7°35' -

7°30' -

5 KILOMETERS

EXPLANATION

Q ' ''. . Quaternary alluvium 

Esc Supamo Complex (granite-monzonite) 

lJ!yEMi Yuruari Formation (pyroclastic rocks)

Yuruari Formation (schist) 

/q/J Yuruari Formation (quartzite) 

Bern , Carichapo Group (mafic lavas)

Ecu Carichapo Group (ultramafic rocks) D

 Geologic contact Approximately located

 Fault mapped on the ground

 Quartz vein mapped in subgroup or outcrop 

G Small gabbro body 

d Small tabular diabase body

H

P

M

D

Figure 6. Surficial geology of part of the Bochinche area, Venezuela, compiled from two surficial geologic maps of the Bochinc

Salazar and Franco, 1988).
CIO GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

6Q°45'

EXPLANATION

Big

Bsc

Eys

Byq

Bern

Late-stage granite intrusive rocks 

Supamo Complex (granite-monzonite) 

Yuruari Formation (schist) 

Yuruari Formation (quartzite) 

Carichapo Group (mafic lava)

Ecu Carichapo Group (ultramafic rocks)

Approximate geologic contact

 - - Fault mapped on the ground

CHHH) Laguna diabase dike

« « « Quartz vein
ANALYSIS OF AEROMAGNETIC DATA, BOCHINCHE MINING DISTRICT Cll

Figure 7 (facing page). Bedrock geology of the Bochinche 

15-rninute quadrangle, Venezuela. This map was compiled using 

the surficial geologic map of figure 6, the terrace map of figure 4 

(including lines showing maximum horizontal gradient of 

pseudogravity), additional information from modeling of discrete 

magnetic anomalies (including fig. 5), and limited anecdotal 

information from several geologists who have worked in the area. 

The diagonal-lined areas represent what is thought to be the location of the eastern extension of the Laguna dike. Unit Big (Early 

Proterozoic late granite intrusive rocks) is similar to rocks north and 

west of the Bochinche quadrangle (Wynn and Sidder, 1991) (see 

text for discussion). Line of profile A-A' (fig. 5) is also shown.

On the geologic map, the Laguna dike is indicated by 

the generally narrow, diagonal-lined bodies. The dike splits 

into two entities, but exactly where this split takes place is 

not clear from the airborne magnetic data. Ground magnetic 

data (Fernandez, this volume) also clearly show two extensions of the Laguna dike. These higher resolution, but limited extent, ground data suggest that the southern extension 

is the continuation of the dike from farther to the west and 

that the northern extension (which is more apparent in both 

the ground-magnetic and the aeromagnetic maps) is a different rock unit, probably of a different age because it has been 

described as a different rock in its weathered form (Salazar 

and Franco, 1988). In the east-central part of the map, the 

Laguna dike is apparently offset or folded along a probable 

fault that also splits the southeastern ultramafic bodies into 

discrete parts. It is possible that the mafic-ultramafic bodies 

were intruded at the same time as, or after, emplacement of 

the dikes and caused their displacement.

The Bochinche district was originally selected for 

exploration by CVG-TECMIN because it is along strike of 

the Laguna dike from the well-known and highly productive 

El Callao gold district. Experienced geologists working in 

eastern Venezuela have subsequently concluded that the El 

Callao deposits are not genetically related to the Laguna dike 

and that the spatial correlation is fortuitous (Vicente 

Mendoza, CVG-TECMIN, oral commun., 1987).

Louis Guilloux (oral commun., 1989) asserted that 

most of the gold-bearing quartz veins are related to structures that strike about N. 20° W. Although some of these 

structures are clearly visible in the strike of the quartz veins 

in figures 6 and 7, unfortunately the aeromagnetic survey 

v/as designed to cross the Laguna dike perpendicularly and 

v/as flown on a strike of N. 22°-25° W. This direction minimizes the usefulness of these data in locating gold-bearing 

structures. Despite this fact, the faults that are shown in 

figure 7 are manifested strongly in the magnetic data and are 

logical areas for further gold exploration.

CONCLUSIONS

Digital processing of aeromagnetic data, especially horizontal gradient of pseudogravity and terracing, can prove 

useful to geologists mapping in areas of deeply weathered 

rock covered by dense tropical forest. With the geologic 

control provided by this study in the Bochinche area of the 

Venezuelan Guayana Shield, the aeromagnetic data were 

used to extend and refine our knowledge of the geology in 

the surrounding region at minimal expense. Digital processing of aeromagnetic data to produce a refined bedrock geology map, as described in this study, may also be useful in 

mineral exploration of the Venezuelan Guayana Shield.

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Bellizzia, Alirio, Pimentel, Nelly, and Bajo de Osuna, R., 1976, 

Mapa geologico estructural de Venezuela: Caracas, Ministerio 

de Minas e Hidrocarburos, escala 1:500,000.

Bellizzia G., Alirio, Pimentel de Bellizzia, Nelly, and Munoz M., 

M.I., 1981, Geology and tectonics of northern South America: 

Caracas, Ministerio de Energfa y Minas, Geodynamics 

Bulletin 3, Boletin de Geologia, Publication Especial 9, 

p. 7-88.

Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz 

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Phillips, J.D., 1992, TERRACE A terracing procedure for 

gridded data, with FORTRAN programs and VAX command 

procedure and Unix C-shell implementations: U.S. Geological 

Survey Open-File Report 92-5A-B, 18 p.

Salazar, Edixon, 1988, Mapa geologico estructural de la zona la 

Esperanza, Distrito Roscio, Estado Bolivar, 1:25,000 scale: 

Internal report of the Gerencia de Proyectos Especiales, Corporation Venezolana de Guayana, Tecnica Minera, C.A., Puerto 

Ordaz, Venezuela.

Salazar, Edixon, and Franco, Luis, 1988, Mapa geologico estructural de la zona de Bochinchito, Distrito Roscio, Estado Bolivar, 

1:25,000 scale: Internal report of the Gerencia de Proyectos 

Especiales, Corporation Venezolana de Guayana, Tecnica 

Minera, C.A., Puerto Ordaz, Venezuela.

U.S. Geological Survey, 1989, Potential-field geophysical programs for VAX 7xx computers: U.S. Geological Survey 

Open-File Report 89-115A-D.

Wynn, J.C., McCafferty, A.E., and Salazar, Edixon, 1989, 

Geologic information derived from digital aeromagnetic data: 

Simposio Sudamerica de COGEODATA, 5th, Caracas 1989, 

Resumen de los Trabajos, p. 32-33.

Wynn, J.C., and Sidder, G.B., 1991, Mineral resource potential of 

the NB-20-4 quadrangle, eastern Guayana Shield, Estado 

Bolivar, Venezuela: U.S. Geological Survey Bulletin 1960, 

16 p.
ANALYSIS OF AEROMAGNETIC DATA, BOCHINCHE MINING DISTRICT C13

APPENDIX

AEROMAGNETIC SURVEYS

COVERING THE VENEZUELAN

GUAYANA SHIELD

Five aeromagnetic surveys have been flown over parts 

of the Venezuelan Guayana Shield (text fig. 1, appendix 

table 1). These surveys have flight-line spacings of from 500 

m (considered good to excellent for mineral exploration) to 

6 km (useful for regional mapping only). Data from Estado 

Amazonas, making up as much as 40 percent of the 

aeromagnetic data available for the shield, could be converted to digital form if flight-line records were released by 

the Ministerio de Energia y Minas of Venezuela. The Amazonas geophysical data set also includes radiometric information, and thus it may be the most useful survey carried out 

in the shield. The remaining aeromagnetic data range from 

published contoured maps (without digital data) to analog 

strip-chart records (neither contoured maps nor digital data). 

Much, but not all, of these data have been published in a 

photoreduced form at 1:500,000 scale (Herrero and Navarro, 

1989; Emilio Herrero, Ministerio de Energia y Minas de 

Venezuela, oral commun., 1987-1990).

Survey A is available in digital form; however, it covers 

only a fraction of the northern margin of the Venezuelan 

Shield because it was acquired as part of a study of the 

Cuenca Oriental petroleum province north of the Rio 

Orinoco. The area of survey B, flown in 1959, is bounded by 

the Rio Orinoco on the north, has an irregular southern 

boundary generally between lat 6°30' and 7°00' N., and 

extends as far east as the El Callao mining district at about 

long 62° W. This was the first survey flown in Venezuela; 

unfortunately, it has such severe registration problems (fiducial points picked during data acquisition and compilation 

can be shown to be displaced from true location by as much

as 7 km) that it is unusable and was not included in the 

magnetic map of Herrero and Navarro (1989).

The area of survey C, flown in 1961, is bounded by survey B on the west, with a little overlap in places, by the 

Guyana frontier on the east, and by Estado Delta Amacuro 

on the north. The southern boundary is irregular but in places 

is near lat 6° N. The data are in the form of contour maps 

(some with flight lines indicated) at a scale of 1:50,000.

The western and northern boundaries of survey D are 

not published and are only imperfectly known to us; Brazil 

and Guyana bound the survey area to the south and east. The 

survey was flown in the late 1970's to cover the Gran Sabana 

region of Venezuela and is only in the form of analog 

strip-chart records; it was not compiled into a contour map, 

and it is not included in the map of Herrero and Navarro 

(1989).

The area of survey E was flown in 1972 and covers 

most of Estado Amazonas. This survey makes up as much as 

40 percent of the aeromagnetic data available for the shield 

and could be converted to digital form if flight-line records 

were released for public use. The digital data are not presently usable because the digital tapes use a nonstandard location coding requiring use of the pilot's flight-path strips to 

recover. These flight-path strips are currently held by the 

Ministerio de Energia y Minas (Jeffrey C. Wynn, U.S. 

Geological Survey, written commun., September 1989). 

This survey includes radiometric information (uranium, thorium, potassium, and total-count) and may be the most useful 

survey carried out in the shield. A number of areas of 

topographically higher elevation, generally underlain by 

rocks of the Roraima Group and covered by clouds almost 

the whole year, were not flown.

The area of survey F, in Estado Bolivar south of lat 

6° N. and bounded on the south by the Brazilian border, on 

the east by Estado Amazonas, and on the west by approximately the Rio Caura and Rio Paragua, has apparently not 

been flown (Emilio Herrero, oral commun., 1988).

Appendix table 1. Aeromagnetic surveys flown over Venezuelan Guayana Shield.

A. 

B. 

C. 

D. 

E.

Survey name

Orinoco 

Northwest Shield 

Northeast Shield 

Gran Sabana 

Amazonas

Date flown

1981-82 

1959 

1961 

Late 1970's? 

1972-1975

Area

North of Orinoco River 

Northwest Estado Bolivar 

Northeast Estado Bolivar 

Southeast Estado Bolivar 

Estado Amazonas

Digital; 

Analog; 

Analog; 

Analog; 

Digital;

Type

contour map 

contour map 

contour map 

no maps 

magnetic, and radiometric

Spacing

3,000 m 

500m 

500m 

6,000 m 

1,000m

Height

500m 

150m 

150m 

Not known 

150m, 300m

Direction

North-south 

N. 22° W. 

N. 22° W. 

North-south 

1, 000-2,000 m?

Exploracion Geofisica en el Area de Bochinchito, 

Estado Bolivar, Venezuela

(Geophysical Exploration in the Bochinchito Area, 

Estado Bolivar, Venezuela)

By All R. Fernandez P.

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-D

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENIDOS

Resumen......................................................................................................................... Dl

Abstract........................................................................................................................... 1

Introduccion.................................................................................................................... 1

Marco Geologico............................................................................................................ 1

Metodos Geofisicos........................................................................................................ 2

Interpretation de Resultados .......................................................................................... 4

Conclusiones................................................................................................................... 15

Bibliografia..................................................................................................................... 16

Anexo El Arreglo Gradiente Una Nueva Formula de Calculo del Factor Geometrico,

para la Configuration Usada en Venezuela................................................................ 17

FIGURAS

1, 2. Mapas de:

1. Ubicacion relativa del area de Bochinchito I y II......................................................................................... D2

2. Geologia local de Bochinchito I y II............................................................................................................ 3

3. Diagrama del levantamiento electrico y magnetico, Bochinchito I y II................................................................ 4

4, 5. Mapas de:

4. Contornos del campo magnetico total, Bochinchito I y II........................................................................... 5

5. Perfiles magneticos de la zona Bochinchito I y II........................................................................................ 6

6, 7. Perfiles de resistividad aparente sobre las picas en el area de:

6. Bochinchito I donde se observaron las anomalias mas fuertes.................................................................... 7

7. Bochinchito II donde se observaron anomalias mas fuertes........................................................................ 9

8-10. Mapas de:

8. Contornos de isovalores de resistividad aparente , Bochinchito I y II......................................................... 10

9. Interpretation magnetico-estructural, Bochinchito I y II............................................................................. 11

10. Interpretation electrico estructural, Bochinchito I y II................................................................................ 12

11-13. Modelaje geologico-geofisico del area de la:

11. Pica 12, Bochinchito I.................................................................................................................................. 13

12. Pica 13, Bochinchito I.................................................................................................................................. 14

13. Pica 4 (izquierda) y pica 5 (derecha), Bochinchito II................................................................................... 15

TABLA

1. Resumen de resultados del modelaje magnetico................................................................................................... D8
Exploracion Geoffsica en el Area de Bochinchito, 

Estado Bolivar, Venezuela

(Geophysical Exploration in the Bochinchito Area, 

Estado Bolivar, Venezuela)

By All R. Fernandez P. 1

RESUMEN

La zona de Bochinchito es considerada de interes geoeconomico por la ocurrencia de vetas de cuarzo con posible 

mineralization de oro. Esta area es similar al prospecto de 

Bochinche, Estado Bolivar, estudiada por Corporation 

Venezolana de Guayana-Tecnica Minera, C.A., durante los 

anos 1986 a 1988. Los objetivos de la prospeccion geofisica 

en la zona fueron la delimitation de contactos y estructuras 

principales ademas de la ubicacion de vetas de cuarzo.

Los resultados muestran la ocurrencia de anomalias 

magneticas debido a : (1) cuerpos maficos y ultramaficos de 

rumbo NE a NE-E de forma tabular, y largos (mas de 4.0 

km), poco profundos. Estos intrusivos estan comunmente 

fallados perpendicularmente. Asociados a estos pequenos 

intrusivos existen cuerpos resistivos (mas de 15,000 Qm) de 

rumbo NE-N, compuestos por metasedimentos cuarciferos, 

vetas de cuarzo y(o) diques felsicos, localizados cercanos 

entre si dentro de una zona sinclinal asimetrica. (2) Zonas de 

contacto litologico entre metasedimentos cuarciferos o cuerpos graniticos y lavas andesiticas. Se recomienda que los 

horizontes resistivos, donde se han verificado carbonates, 

sulfuros y oro, sean estudiadas en detalle con metodos de 

polarization inducida y sondeos electricos verticales.

ABSTRACT

The Bochinchito area in eastern Estado Bolivar is considered important economically because of its proximity to 

the Bochinche mining district, an area of known gold mineralization associated with quartz veins. Geophysical studies 

were conducted in an attempt to identify geologic contacts

Corporacion Venezolana de Guayana, Tecnica Minera, C.A., C.C. 

Chilemex, Piso 1, Puerto Ordaz, Venezuela.

and locate quartz veins in an area almost completely devoid 

of outcrop.

The results demonstrate substantial geophysical anomalies due to (1) ultramafic bodies and (2) important lithologic 

contacts. Geophysical evidence indicates that the ultramafic 

bodies are large, tabular, as long as 4 km, generally shallow, 

and east-northeast striking. They are associated with small, 

highly resistive (more than 15,000 Qm) metasedimentary 

rocks, quartz veins, and (or) felsic dikes, all of which are 

within an asymmetric synform. Distinct lithologic contacts 

can be recognized between quartz-rich metasedimentary 

rocks (or granitic bodies) and andesitic flow rocks.

Further work is recommended. Induced polarization 

and vertical electric soundings might delimit the ultramafic 

bodies and fault structures believed to control the 

quartz-vein system and its associated gold deposits. The gold 

deposits have already been verified in isolated outcrops that 

contain secondary carbonate, sulfide minerals, and gold.

INTRODUCTION

Las areas de estudio estan aproximadamente a 65 km al 

este de la poblacion de Tumeremo, y a 14 km al oeste del 

caserio La Esperanza. Las coordenadas geograficas de la 

zona de estudio (fig. 1) son lat 7°32'02" N. a 7°28'35" N. y 

long 60°5T10" O. a 60°54'45" O.

MARCO GEOLOGICO

La zona de Bochinchito es parte de la provincia 

geologica de Pastora (Mene"ndez, 1968,1972), donde afloran 

rocas del Grupo Carichapo (mas antigua) y alia Formation 

Yuruari (mas joven) los cuales componen el Supergrupo 

Pastora (figura 1). Se encuentran cuerpos graniticos del

Dl
D2 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

62° 61°

50 KILOMETROS

Figura 1. Ubicacion relativa del area de Bochinchito I y II.

Complejo de Supamo que intrusionan al Supergrupo 

Pastora.

Existen tres unidades geologicas importantes en la zona 

de Bochinchito (Edixon Salazar, Corporation Venezolana 

de Guayana-Tecnica Minera, C.A. (CVG-TECMIN), 1988, 

comunicacion escrita). Estas son (1) unidad de lavas maficas 

a intermedias (Grupo Carichapo sin diferenciar), (2) asociacion de rocas piroclasticas de composition intermediaacida, esquistos cuarzo sericiticos, y cuarcitas ferruginosas; 

estas rocas est&i mineralizadas con sulfuros y forman parte 

de la Formation Yuruari, y (3) unidad de rocas felsicas 

graniticas del Complejo de Supamo.

Ademfis, existen otros cuerpos intrusivos tabulares, 

zonas aluvionales derivadas de las unidades felsicas, y gran 

cantidad de vetas asociadas. La tendencia estructural 

regional es noroeste con lineamientos y(o) fallas de rumbo 

noreste y noroeste y vetas de orientation preferencial noreste 

y otras pocas de rumbo noroeste. El modelo del posible 

yacimiento aurifero consiste de vetas de cuarzo con bajo 

contenido de sulfuros (Berger, 1986).

METODOS GEOFISICOS

Los metodos geofisicos empleados fueron el magnetico 

y el electrico con el objetivo de ayudar a la ubicacion de 

vetas de cuarzo, establecer relaciones estructurales, y 

verificar la posibilidad de mineralizaciones en estas. La 

figura 3 muestra la cobertura de los metodos empleados 

sobre toda la zona.

El levantamiento magnetico se ejecuto sobre picas separadas cada 250 m con rumbos de 50°-40° noreste para 

Bochinchito I, y de 0°-16° noroeste para Bochinchito II. Se 

tomaron lecturas cada 25 m, cubriendose un area de 32 km2 

y mas de 150 km lineales. Se utilizaron magnetometros 

convencionales de precesion de protones.

El procesamiento de los datos magneticos se dividio en 

dos fases: procesamiento basico y procesamiento analitico. 

El procesamiento basico corrige la deriva temporal diurna, 

crea el banco de datos con ubicaci6n geografica, topografica, 

y del campo magnetico, y elabora un mapa de contornos del 

campo magnetico total (fig. 4) y perfiles magneticos (fig. 5).
7°32'02

67°57'10"

834.000 N 

EXPLORACION GEOFISICA EN EL AREA DE BOCHINCHITO

720.000 E 728.000 E

D3

60°54'45"

7°20'35'

2000

i i i

4000 METROS 

J

LEYENDA

Sector aluvional 

Vetas de cuarzo

Cuerpos pequenos tabulates (sill, dique) diabasa 

Diques felsicos 

Complejo Supamo 

Supergrupo Pastora 

Bpy | Formacion Yuruari

Grupo Carichapo 

Sin diferenciar 

Volcanicas 

Ultramaficas

- Contacto geologico aproximado

Lineacion mineral y orientaction de 

la disposition de bloques

 Fallas interpretadas

Figura 2. Geologfa local de Bochinchito I y II. Tornado de Edixon Salazar (comunicaci6n escrita, 1988).

El procesamiento analitico de los dates magneticos incluye El levantamiento el6ctrico consistio en la medici6n de

filtros espectrales y modelaje matemdtico de anomalias las resistividades aparentes con el arreglo gradiente, con

magneticas. MN=50 y 100 m, entre las picas P7 a P20 de Bochinchito I,
D4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

720.700 E

834.000 N -

729.000 E

829.000 N 

827.000 N 

LEYENDA

Areas para estudios geofisicos _

Magnetometria

P.I. gradiente 

Campamento 

Helipuerto

Figura 3. Diagrama del levantamiento electrico y magnetico, Bochinchito I y II.

y las picas P4 a P13 de Bochinchito II. Estas lineas se escogieron porque cubrian el area de mayor complejidad 

geologica. Se utilize equipo convencional de polarization 

inducida (PI), usandose como receptor, un microvoltimetro; 

no se midieron valores de cargabilidad. El metodo cubrio un 

area de 14 km2 y una longitud de 60.5 km lineales.

El procesamiento de los datos electricos es similar al de 

los datos magneticos. El procesamiento bdsico crea un banco 

de datos de ubicacion y topografia, calcula la resistividad 

aparente (ver anexo), elabora perfiles de resistividad (figs. 6 

y 7) y un mapa de contornos de resistividad aparente (fig. 8). 

El procesamiento analitico utiliza un filtro de frecuencias 

para una longitud de onda de 0.5 en el ancho de la ventana 

para reducir el ruido geologico y obtener el valor de fondo y 

residual de las resistividades. El metodo permite el modelaje

e interpretation de la forma, tamano, y profundidad de los 

cuerpos resistivos.

INTERPRETACION DE RESULTADOS

La interpretacion de los datos magneticos (fig. 4) permite establecer tendencias estructurales regionales noreste-este. Las fallas son de tipo sinestral y presentan rumbo 

noroeste-oeste.

De los mapas de contornos (figs. 4 y 8) del campo magnetico total y de resistividad aparente, se diferencian cuatro 

subzonas magneticas (fig. 9) y cuatro zonas geoelectricas 

(fig. 10) importantes:
EXPLORACION GEOFISICA EN EL AREA DE BOCHINCHITO D5

721.5

833.9 

722.5 723.5 724.5 725.5 726.5 727.5 728.5 729.5

832.9 

831.9

830.9 

829.9 -

828.9 

827.9 -

826.9 

825.9

LINEAS DE ISOVALORES MAGNETICOS 2000 nT

Figura 4. Mapa de contornos del campo magnetico total, Bochinchito I y II. Contornos a intervales de 2,000 nT.

I. Zona de susceptibilidad magnetica y gradiente del 

campo magnetico bajo y de baja resistividad, entre 0-4,000 

Qm. No presenta anomalias magneticas importantes. Probablemente sea una zona granitica, correlacionable con el 

Complejo de Supamo. Esta zona se subdivide en dos subzonas geoelectricas concentricas, una de ellas denominada QA, 

con resistividades menores a 2,000 Qm, otra denominada 

QB, con resistividades entre 2,000-4,000 Qm. Estas coinciden con areas donde se encuentran quebradas de primer o 

segundo orden, y estan asociadas a litologias aluvionales.

Tambien, aparecen en bajos topograficos al sur y sureste del 

area.

II. Zona de alta susceptibilidad magnetica y alta 

resistividad, entre 10,000 y 15,000 Qm. Presenta anomalias 

magneticas fuertes, pero de bajo gradiente. Se ubica al sur 

del area de Bochinchito I, alrededor del eje del sistema de 

picas. Se asocian con rocas ferruginosas como metasedimentos cuarciferos, esquistos y(o) rocas piroclasticas tipicos de 

la Formacion Yuruari, y tambien a intrusivos felsicos. Esta 

zona se relaciona a un alto estructural con rumbo norte-
D6 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

722 723 724

834 

829 

828 

827 

Figura 5. Perfiles magneticos de la zona Bochinchito I y II. Interpretaci6n y numeration referida al texto.

noreste. Es posible la existencia de vetas de cuarzo de interns 

prospective en esta zona.

III. Zona de alto gradiente del campo magnetico y de 

resistividad intermedia, entre 4,000 y 10,000 ftm. Comprende los valores de fondo de resistividad de la zona. Esta 

zona presenta anomalias magneticas intensas, pero de susceptibilidad intermedia a alta. Se asocia a lava mafica a intermedia, gabro, diques de diabasa, y andesita. Se ubica en una 

franja central desde Bochinchito I a Bochinchito II. Probablemente correspondan a eventos volc£nicos del Grupo Carichapo y menos probable can lavas de la Formatidn Yuruari.

IV. Zona de gradiente intermedio y susceptibilidad 

alta. Consiste de cuerpos magneticos anchos pero poco 

extensos de forma elipsoidal. Se relacionan con cuerpos 

ultramaficos del Grupo Carichapo.

Figura 6. Perfiles de resistividad aparente sobre las picas en el 

area de Bochinchito I donde se observaron las anomalias mas 

fuertes. Se representan filtros de frecuencia (LOWESS, W=0.5) 

que determina el "regional" filtro de cinco puntos 

(al=a2=a4=a5=l/8a3=l/2) que produce un registro electrico menos 

ruidoso. El "residual" es producto de la diferencia entre el perfil observado (ROA1) y el filtrado "regional."
EXPLORACION GEOFISICA EN EL AREA DE BOCHINCHITO D7

35,000

30,000 

-5000 

-10,000 

40,000

35,000 -

LEYENDA

D Resistividad ROAl

Regional (LOV -0.5) 

Filtro 5-PTOS

-5000 

-10,000

2000 1750 1500 1250 1000 750 500 250 0

ESTACION <m)

250 500 750 1000
D8 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

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15,000 Qm. Se vinculan a cuerpos pequenos resistivos y

locales, de forma tabular y delgados, posiblemente sean

diques felsicos o vetas de cuarzo. Esta zona es exclusiva del

sistema Bochinchito I (parte centro-suroeste).

El procesamiento espectral de los datos magneticos

indica lo siguiente:

1. Reduccion al polo. La intensidad del campo magnetico que varia hacia el norte-noroeste de la zona es mas

fuerte a los 100 m y mas debil hacia 350 m.

2. Filtrado pasa banda. Se filtraron longitudes de onda

entre 100 y 1,000 m. Las anomalias resultantes presentan

variaciones entre -1,500 y 1,500 gamma las cuales pueden

clasificarse segun su amplitud, de la siguiente forma:

Fuertes Entre -800 y 600 gamma, asociada a

cuerpos maficos tipo dique

Intermedias Entre -500 y 300 gamma relacionadas a

cuerpos ultramaficos

Bajas Entre -200 y 300 gamma correspondendientes a cuarcitas ferruginosas

Debiles Entre -200 y 100 gamma vinculadas a

contactos entre rocas felsicas y lavas de

composicion intermedia

3. Segunda derivada vertical. Se distingue la

ubicacion de los principales contactos, los cuales son descritos por las anomalias magneticas mayores (tabla 1).

4. Continuacion hacia arnba del campo (100 m). La

continuacion logra filtrar casi todas las anomalias residuales

y se deduce la profundidad del tope de los cuerpos anomalos.

El analisis de los perfiles electricos (figs. 6 y 7) indica

que el valor promedio de resistividad de la zona esta sobre

los 4,000 Qm. En el sistema Bochinchito I, destaca la

presencia de una superposicion de tres anomalias de alta

resistividad.

El "residual"2 muestra tres anomalias de

10,000-15,000 Qm con una anchura no mayor a los 50 m

que tiende a superponer, hasta formar una sola anomalia en

la direccion noreste. Estas anomalias parecen estar dentro de

una unidad de resistividad alta, rodeada de otra de resistividad mas baja. En el sistema Bochinchito II, las anomalias

son de menor intensidad (15,000 Qm) estan escalonadas,

dispersas y son mas amplias que las del sistema

Bochinchito I.

La anomalia 1 esta al suroeste de la zona. Se debe a sedimentos ferruginosos con alta susceptibilidad, tiene rumbo

noreste a este, y se divide en dos ramales.

La anomalia 2 esta en el centre de la zona. Se debe a

cuerpos locales delgados (entre 50 y 100 m) tipo dique de

diabasa, extensos (mas de 4.0 km de largo). Presenta dos

apofisis (basalto-diabasa) de rumbo noreste a este. La

2E1 termino "regional" como el terrain "residual" no tiene el mismo

significado usado en m6todos potenciales como la magnetometria. En este

caso solo se usan para describir la idea de fondo promedio de resistividad y

de resistividades de los cuerpos anomalos locales, respectivamente.
EXPLORACION GEOFISICA EN EL AREA DE BOCHINCHITO D9

I 1 I I T I I I 1 I I I I I I I I I I I I I I I 1 I I [T] I t rh I I 1 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I T I TT1

PicalO(NaS)

LEYENDA

a Resistividad ROA1

Regional (LOV -0.5) 

Filtro 5-PTOS 

A Residual ~

I I I I I I I I I I I I I I I I I I I I I I I I I I ll I I I I I I I I I I M I ll I /|\l I I I I I I I 1 I M I I I I I I I I I I I I I I I I I I I I

875 750 625 500 375 250 125 0 125 250 375 500 625 750 875 1000

ESTACION (m)

Figura 7. Perfiles de restividad aparente sobre las picas en el area de Bochinchito II donde se observaron anomalias mas 

fuertes. Se representan los datos de campo y otros perfiles, al igual que los descritos en la leyenda de la figura 6.
D10 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

721.5

833.9 

832.9 

831.9 

830.9 

829.9 _

828.9 _

827.9 _

826.9 _

825.9

722.5 723.5 724.5 725.5 726.5 727.5 728.5 729.5

LINEAS DEISOVALORES DE RESISTIVIDAD 1,000 OHM-METROS

Figura 8. Mapa de contornos de isovalores de resistividad aparente (MN=100 m), Bochinchito I y II.

anomalia esta desplazada por fallamiento o fracturas ortogonal esa su rumbo, reapareciendo entre las picas P4 y P8 del 

sistema Bochinchito II. El mismo cuerpo o uno similar 

aparece al sur entre PI 1 a P22 de Bochinchito II con posible 

continuidad al este, como expresion extrema del dique de 

diabasa Laguna el cual aflora en la zona de Bochinche y que 

se relaciona con las principales anomalias de oro de este 

prospecto.

La anomalia 3 representa el contacto entre rocas de 

susceptibilidad mayor y menor. Su lado sureste se puede 

superponer con la anomalia 2.

La anomalia 4 marca el contacto de rumbo noreste a 

este entre formaciones diferentes, al norte rocas graniticas y 

al sur lavas maficas a intermedias, o posiblemente se trate de 

un cuerpo mafico delgado, tipo dique.

La anomalia 5 esta representada por un dique al noreste 

de la zona, el cual identifica con la anomalia 4 por correlation lateral al oeste.

La anomalia 6 esta compuesta de pequenas e intensas 

anomalias de rocas ultramaTicas elipsoidales con rumbo este.

La anomalia 7 incluye una pequena anomalia 

estructural al noreste, entre las picas P18 a P19 del sistema
EXPLORACION GEOFISICA EN EL AREA DE BOCHINCHITO Dll

722.000E 723.000E 724.000E 725.000E 726.000E 727.000E 728.000E 729.000E

834.000N

833.000N

832.000N

831.000N

830.000N

829.000N

828.000N

827.000N

826.000N

Zona magnetica de baja susceptibilidad.

Gradiente constante o nulo. No hay anomalias

importantes probable zona granitica (Supamo?) 

Lavas maficas intermedias. Anomalias fuertes.

Gradiente alto susceptibilidad intermedia-alta.

d : Diques varies diabasas 

Zona de cuerpos circulares. Gradiente intermedia

pero susceptibilidad alta asociada a cueros

ultramaficos (Carichapo) 

Cuerpos magneticos tabulares y/o circulares

 - - - Bordes de zona

 Lineamientos

 Falla-fractura

Figura 9. Interpretation magnetico-estructural, Bochinchito I y II.

Bochinchito I, probablemente se deba al contacto entre rocas 

felsicas con diferentes susceptibilidades.

A partir de los perfiles, se efectua el modelaje 

matematico de las anomalias magneticas mas importantes 

(figs. 11, 12, y 13). El modelaje magnetico se realize 

mediante el uso de programas de computacion, basados en el 

algoritmo de Taiwan! y Heirtzler (1964). El modelaje 

electrico se realizo mediante un programa de computacion 

de "solution inversa" (Asch, 1989).

El modelaje electrico permitio diferenciar tres unidades 

importantes: una unidad exterior de 5,000-7,000 Qm de 

resistividad (probablemente de granitos o lavas); seguida por 

otra unidad interior de mas baja resistividad, entre 2,000 y

4,000 Qm, probablemente aluvional; y fmalmente el 

contacto intrusivo entre diques de diabasa y rocas mas resistivas (>15,000 Qm) tales como cuarcitas y rocas piroclasticas. La estructura general se asemeja a un cuerpo sinclinal 

asimetrico con el flanco sur menos inclinado que el flanco 

norte y cuyo eje esta centrado en un entorno a la progresiva 

0+500 noroeste del sistema Bochinchito I. Es de esperar, 

fracturas o fallas radiales, desde la superficie al punto por 

donde pasa el eje del sinclinal. Las vetas se ubican en la zona 

central del sinclinal, con buzamiento alto hacia el noroeste. 

La profundidad de los cuerpos resistivos y su extension son 

inciertos en el modelo realizado, pero se asume la superfi-
D12 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

722.000E 723.000E 724.000E 725.000E 726.000E 727.000E 728.000E 729.000E

834.000N

833.000N

832.000N

831.000N

830.000N

829.000N

828.000N

827.000N

826.000N

Zona baja resistividad, 0<p<4,000 ohm-tn

asociado con aluviones 

Zona de resistividad intermedia

4,000<p<10,000 ohm-m asociado a lavas ~

maficas e intermedias 

Zona de alta resistividad, 10,000<p<15,000

ohm-m asociada a Qz (vetas?). Esquistos

pirodasticos, cuarcitas felsicas, diques

(Fm Yuruari?) 

Zona muy alta resistividad, mayor a 15,000 ~

ohm-m cuerpos muy resistivos Hp dique o veta

felsica

2 KILOMETROS 

_\

 Bordes de zona

Lineamientos 

Falla-fractura

Figura 10. Interpretaci<5n electrico-estructural, Bochinchito I y II.

cialidad de estos (menos de 10 m de profundidad), dado el 

fuerte contraste de resistividad entre la parte central del 

perfil y los extremes.

El espesor de la capa de meteorizacion conductora es 

delgada. Los valores de resistividad3 de estos cuerpos son 

variables, pero superiores a 15,000 Qm. Se caracterizan por 

su alta frecuencia en el registro ele"ctrico y se supone que se

3E1 programa ha demostrado baja resoluci6n en la pseudoseccion en 

profundidad, puesto que se necesitan variaciones de 5-10 veces el valor de 

la resistividad de la "roca caja," para producir un registro con una anomalia 

significativa. La profundizacion de un cuerpo de resistividades menores 

puede no ser notada en el registro el^ctrico.

ubican en un entorno de radio de 20 m al pico o mlximo 

local. Su espesor no debe ser mayor a los 20 m. En el sistema 

Bochinchito II, las zonas de alta resistividad asociadas con 

vetas de cuarzo se distribuyen esparcidamente y tienen 

buzamientos ligeramente al. sureste. La geometria sinclinal 

esta preservada y esta relacionada con los altos topograficos 

estructurales a ambos lados del eje del sinclinal.

Las estructuras modeladas (tabla 1) se disponen en tres 

categorias: (1) al norte, son angostas y de buzamiento 

medio-alto (40°-80°) hacia el sur; (2) al sur, son mas amplias 

y de buzamientos altos (80°) al norte; y (3) las estructuras 

centrales son grandes, amplias y extensas lateralmente. 

Todas las anomalias se orientan con rumbo N. 80° O. hasta 

N. 40° E. Las estructuras son superficiales. La mayoria de las
EXPLORACION GEOFISICA EN EL AREA DE BOCHINCHITO D13

OHM-M 

32,000

28,000

24,000

20,000

16,000

12,000

8000

4000

I

fl 

l\ 

I \ 

1 \ 

I \

I \ s Perfil magnetico

500 500 

DISTANCIA, EN METROS

1000

GAMMAS 

35,200

34,800

34,600

34,200

33,800

33,400

33,000

32,600

32,000

NW

LEYENDA

Dique de diabasa 

Complejo de Supamo-Granitos 

Formacion Yuruari-Lavas acidas-intermedias 

Formacion Yuruari-Cuarcitas f erruginosas

Falla o fractura 

Veta de cuarzo 

Quebrada 

Q Aluviones

Figure 11. Modelaje geologico-geofisico del area de la pica 12, Bochinchito I. Perfil electrico MN=50 m.
D14 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

OHM-M 

32,000

28,000

24,000

20,000

16,000

12,000

8000

4000

500 500 

DISTANCIA, EN METROS

1000

GAMMAS 

34,600

34,200

33,800

33,400

33,000

32,600

32,200

31,800

31,400

1500

NW

LEYENDA

Dique de diabasa 

Complejo de Supamo-Granitos 

Formacion Yuruari-Lavas acidas-intermedias 

Formacion Yuruari-Cuarcitas ferruginosas

Falla o fractura 

Veta de cuarzo 

Quebrada 

Carretera 

Q Aluvioncs

Figura 12. Modelaje geo!6gico-geofisico del £rea de la pica 13, Bochinchito I. Perfil el6ctrico MN=50 m.
EXPLORACION GEOFISICA EN EL AREA DE BOCHINCHITO D15

OHM-M 

28,000

24,000

20,000

16,000

12,000

8000

4000

GAMMAS 

34,200 

33,800 

OHM-M

\/

 4000

GAMMAS 

34,600

34,200

33,800

33,400

33,000

32,600

32,200

500 500 500 500

DISTANCIA, EN METROS DISTANCIA, EN METROS

NW SE NW

B

LEYENDA

Diabasa

Metalavas maficas

(Grupo Carichapo) 

Limolitas

Lavas acidas-intermedias 

(Formacion Yuruari)

- Falla o fractura

 Veta de cuarzo 

v Quebrada

Q Aluviones

d Dique de diabasa

Figura 13. Modelaje geologico-geofisico del area de la pica 4 (izquierda) y pica 5 (derecha), Bochinchito II. A, Arreglo gradiente simetrico. 

B, Arreglo gradiente general.

anomalias se hacen mas amplias en la direction este. Las 

susceptibilidades varian hasta 10 veces el valor promedio, lo 

cual indica heterogeneidad de concentraciones de minerales 

magneticos en las rocas de la zona.

CONCLUSIONES

El estudio de las anomalias magneticas determine la 

presencia de cuerpos magneticos con tendencias regionales. 

Su rumbo es noreste a este, y buzamiento es entre 40°-80° S. 

para cuerpos al norte de la zona y aproximadamente 80° N. 

para los cuerpos al sur. Se determine la superficialidad del

tope de los cuerpos magneticos principales, fallas sinestrales 

de rumbo norte-noroeste, y espesor entre 5-140 m en 

cuerpos tipo dique y hasta 500 m en cuerpos tabulares 

mayores. Se identified la tendencia al aumento del espesor 

en la direction este, una extension lateral de varios 

kilometres, y una variacidn de susceptibilidad amplia, que 

indica heterogeneidad de la concentracione de minerales 

magneticos.

El estudio de las anomalias electricas ha permitido 

diferenciar 3 o 4 unidades geoelectricas mayores. La morfologia de las estructuras modeladas muestran una 

configuration geometrica sinclinal en la cual se encuentran 

cuerpos resistivos delgados, poco profundos o aflorando,
D16 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

con buzamiento alto al noroeste. El espesor de las vetas se 

asume menor a 20 m. La correlation lateral de las zonas 

resistivas se extiende en varios cientos de metros aunque la 

continuidad de una misma veta es dudosa debido a fallas o 

fracturas de rumbo NO-N. La ocurrencia de un sistema de 

pequenas vetas de rumbo norte-noreste es mas probable. La 

complejidad estructural, aunado a la asociacion de rocas 

maficas y rocas sedimentarias ferruginosas, muestra las 

condiciones necesarias para la mineralization de oro en las 

vetas de cuarzo, haciendo de la zona de Bochinchito un buen 

prospecto minero.

Agradecimientos. A los tecnicos Henry Brito, Freddy 

Rodriguez y Jhonny Villarroel (CVG-TECMIN) por su 

colaboracion en la adquisicion de datos de campo. Al 

Geofisico Antonio Ferrero (CVG-TECMIN) por su aporte 

en la elaboration del anexo asi como su ayuda en el levantamiento de campo, y al Geofisico Jorge Penott por sus certeros comentarios sobre este trabajo. Al Dr. Jeffrey Wynn 

(U.S. Geological Survey) por su valiosisima ayuda en la 

asesoria tecnica de este trabajo.

BIBLIOGRAFIA

Asch, T., 1989, Users manual for the program "RESIS2D" on the 

IBM AT: U.S. Geological Survey Order 012814-89, 33 p.

Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz 

veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit 

models: U.S. Geological Survey Bulletin 1693, p. 239.

Menendez V. de V., Alfredo, 1968, Revision de la estratigrafia de 

la Provincia de Pastora segiin el estudio de la region de Guasipati, Guayana Venezolana: Boletin de Geologfa, Caracas, v. 

10, no. 19, p. 309-338.

 1972, Geologfa de la region Guasipati, Guayana Venezolana: Congreso Geoldgico Venezolano, 4th, Caracas, 1971, 

Memoria, Publicacidn Especial 5, v. 4, p. 2001-2046.

Talwani, Manik, and Heirtzler, J.R., 1964, Computation of magnetic anomalies caused by two dimensional structures of arbitrary 

shape, in Computers in the mineral industries, part I: Palo Alto, 

Stanford University Publications, Geological Sciences, v. 9, p. 

464-^80.
EXPLORACION GEOFISICA EN EL AREA DE BOCHINCHITO D17

ANEXO EL ARREGLO

GRADIENTE UNA NUEVA FORMULA

DE CALCULO DEL FACTOR

GEOMETRICO, PARA LA

CONFIGURACION USADA EN

VENEZUELA

El arreglo o dispositive gradiente es una configuration 

geometrica de electrodes de potencial y de corriente que permite medir la resistividad aparente en un punto bajo la superficie. En general, para las mediciones de resistividad 

aparente, la validez de la ley de Ohm determina que:

Ro=(V/I) xK

donde Ro es la funcion de resistividad aparente puntual, 

puesto que la constante K no es fija, sino depende del punto 

de ubicacion dentro del arreglo, para una medicion determinada. En un sistema de cuatro electrodos, esta constante K 

depende de cuatro radio vectores entre cada electrodo y el 

punto de medicion, de modo que :

K=2n{ l/(l/AN-l/AM-l/BN+l/BM)} 

donde AN, AM, BN, y BM son los radios vectores mencionados.

El arreglo gradiente tradicionalmente usado por su simplicidad geometrica es aquel cuya simetria especular, permite el calculo de los radio vectores mediante la solution de 

triangulos rectangulos sencillos (fig. A). De este modo, el 

arreglo gradiente traditional se calculaba con :

AJV=V { (L-d-nxc)2+(mxb)2 }

AM=V { (d+nxc)2+(mxb)2 }

BN=^ {(d+nxc+a)2+(mxb)2 }

W=V { (L-d-nxc-a)2+(mxb)2 }

donde n es entero y c es la separation entre estaciones. Este 

arreglo exige que las lineas (o picas) scan paralelas entre si 

y perpendiculares al eje de simetria. Sin embargo, estas 

condiciones no se cumplen o son dificiles de controlar, en la 

etapa de adquisicion de datos de campo. Dadas estas circumstancias, se desea usar un nuevo factor geometrico mas 

general y arbitrario, dentro del contexto del arreglo gradiente, que no exija requerimientos especiales.

La figura B muestra las condiciones generales que 

pueden presentarse en casos de sistemas de picas o de lineas

mas reales. En este caso, los factores geometricos vienen 

dados por:

AN=^l[{L/2-(nxc+a/2+E-Fxcos$)*cosa} 2+{Fxsen$+

(n*c+a/2)*sena} 2] 

AM=V[ { L/2-(nxc-a/2+E-Fxcos$)*cosa} 2

(n*c-a/2)*sena} 2] 

BN=^![ {L/2+(nxc+a/2+E-Fxcos$)*cosa} 2

(n*c-a/2)*sena} 2] 

5M=V[ {Z72+(nxc-a/2+£:-Fxcos|3)*cosa} 2

(n*c+<2/2)*sena} 2]

A pesar de la mayor complejidad de la ecuacion, este 

nuevo factor geometrico permite poder trabajar con sistemas 

de lineas o picas arbitrarias, sin condicionamientos especiales sobre el arreglo. La formula traditional, del arreglo 

gradiente simetrico, es reproducible con esta formula, bajo 

las condiciones siguientes: O=0°, (3=90°, E=0, y F=mxb (m 

entero).

Estaciones^

AM "

A -

 d f

"5T

 x.

m=1

m=2

m=2

m=1- BN

 * * * * i

b

J

L

B

'Sistema 

de picas

A, B 

electrodos 

corriente 

M, N 

electrodos 

. potencial

Figura A. Arreglo gradiente simetrico.

Eje de 

simetria

P6,

L72

etc

Eje principal

xAlfa

Figura B. Arreglo gradiente general.

Geology of the Lo Increible Mining District and 

U-Pb Age of the Early Proterozoic 

Yuruari Formation of the Pastora Supergroup, 

Guayana Shield, Venezuela

By Warren C. Day, Richard M. Tosdal, E.L. Acosta, J.C. Aruspon, 

L. Carvajal, E. Cedeflo, Glenda Lowry, L.F. Martinez, J.A. Noriega, 

Fernando J. Nufiez, J. Rojas, and F. Prieto

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD 

U.S. GEOLOGICAL SURVEY BULLETIN 2024-E

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENTS

Abstract........................................................................................................................... El

Resumen......................................................................................................................... 1

Introduction.................................................................................................................... 2

Geology.......................................................................................................................... 2

Regional Stratigraphic Setting................................................................................ 2

Geology and Geochemistry of the Lo Increlble Mining District............................ 2

El Callao Formation........................................................................................ 2

Yuruari Formation.......................................................................................... 5

Caballape Formation....................................................................................... 5

Gabbro............................................................................................................ 6

U-Pb Zircon Age of the Early Proterozoic Yuruari Formation...................................... 6

Structural Setting of Gold Deposits................................................................................ 10

Structural Observations.......................................................................................... 10

Gold Deposits......................................................................................................... 11

Conclusions.................................................................................................................... 12

References Cited............................................................................................................. 12

FIGURES

1. Map showing location of Lo Increfble study area....................................................................................................... E3

2. Geologic map of Lo Increfble mining district............................................................................................................. 4

3. Ternary plots showing chemical characteristics of samples from Lo Increfble mining district.................................. 6

4. Chondrite-normalized rare earth element diagrams for rocks of Lo Increfble mining district.................................... 8

5 Spider diagrams for rocks of Lo Increfble mining district.......................................................................................... 9

6. U-Pb concordia diagram showing U-Pb isotopic data for zircon from metadacite tuff in Yuruari Formation of

	the Pastora Supergroup, Lo Increfble mining district.................................................................................................. 10

TABLES

1. Major- and trace-element abundances for rocks of the Lo Increfble mining district.................................................. E7

2. U-Pb geochronologic data (zircon) for dacite from the Yuruari Formation of the Pastora Supergroup..................... 10

3. Gold and trace element abundances for rocks of the Lo Increfble mining district...................................................... 11
Geology of the Lo Increible Mining District and

U-Pb Age of the Early Proterozoic

Yuruari Formation of the Pastora Supergroup,

Guayana Shield, Venezuela

By Warren C. Day, 1 Richard M. Tosdal,2 E.L. Acosta,3 J.C. Aruspon,3 L. Carvajal,3 E. Cedeno,4 

Glenda Lowry,3 L.F. Martinez,3 J.A. Noriega,5 Fernando J. Nunez, 3 J. Rojas, 3 and F. Prieto3

ABSTRACT

Results of new mapping combined with geochemical 

and U-Pb isotopic data provide a framework for the geology, 

age, and origin of gold deposits in the Lo Increible mining 

district of the Guayana Shield of eastern Venezuela. Gold 

deposits within the district are shear-zone-hosted lode 

deposits restricted to a major ductile transpressive shear zone 

that has developed a strong S-C mylonitic fabric, sigmoidal 

boudinage, and small-scale folds. Dextral oblique slip thrust 

(or reverse movement) in the shear zone has placed older 

tholeiitic metabasalt of the El Callao Formation over 

younger quartz-biotite-muscovite schist of the Yuruari 

Formation, both of the Early Proterozoic Pastora Supergroup. The gold deposits formed as discontinuous quartz 

veins parallel with the tectonic foliation within the zone.

Zircon mineral separates from the Yuruari Formation, 

which forms the upper unit of the Pastora Supergroup, yield 

a U-Pb age of 2,131±10 Ma. This is the first published U-Pb 

zircon date of the Pastora Supergroup in Venezuela and confirms the Early Proterozoic age for greenstone-belt rocks of 

the Guayana Shield.

Major- and trace-element geochemical data for 

quartz-normative tholeiitic pillow lavas of the Early Protero-

.S. Geological Survey, Denver Federal Center, MS905, Denver, 

Colorado 80225.

2U.S. Geological Survey, 345 Middlefield Road, MS901, Menlo Park, 

California 94025.

3CVG-T6cnica Minera, C.A., C.C. Chilemex, Piso 1, Puerto Ordaz, 

Venezuela.

4Mineros de Venezuela, Mina Colombia, El Callao, Venezuela.

5Ministerio de Energia y Minas, Ciudad San Cristobal, Guatemala.

zoic El Callao Formation have affinity with modern 

low-potassium island arc tholeiite. Calc-alkaline basalt from 

the younger Early Proterozoic Caballape Formation has a 

composition similar to that of low-potassium calc-alkaline 

basalt and basaltic andesite typical of island arc sequences. 

We suggest that the rocks in the region formed in an immature intraoceanic island arc setting that underwent subsequent oblique dextral thrusting along a northwest-oriented 

axis of tectonic compression.

RESUMEN

Los resultados de la cartografia geologica reciente y 

datos geoquimicos isotopicos por el metodo U-Pb proporcionan un marco de referencia para la geologia, edad y origen 

de los depositos minerales en el Distrito Minero Lo Increible 

en el Escudo de Guayana. Los depositos de oro en este distrito son del tipo de veta y diseminaciones encajados en 

zonas de falla restringidas a extensas zonas de cizallas duetiles con movimiento transpresivo el cual ha originado una 

fuerte estructura milonitica del tipo S-C, budinage sigmoidal 

y pequenos pliegues. Corrimientos oblicuos dextrales 

(o movimiento inverse) en la zona de cizalla ha puesto 

basaltos toleiticos (mas viejos) de la Formacion El Callao 

sobre esquistos quarzo-biotitico-muscovitico (mas joven) de 

la Formacion Yuruari. Ambas formaciones pertenecen al 

Supergrupo Pastora de edad Proterozoico Temprano. Los 

depeositos de oro se formaron en vetas discontinuas de 

cuarzo paralelas a la foliacion tectonica de la zona.

Los analisis isotopicos en zircones separados de la Formacion Yuruari, la cual forma la unidad superior del Supergrupo Pastora, arrojan una edad por el metodo U-Pb de

El
E2 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

2,131±10 Ma. Esta es la primera edad isotopica obtenida en 

zircones por el metodo U-Pb para el Supergrupo Pastora en 

Venezuela, la misma confirma la edad Proterozoico 

Temprano para rocas verdes (greenstones) en el Escudo de 

Guayana.

Datos geoquimicos de elementos mayores y elementos 

traza obtenidos de lavas toleiticas almohadilladas con cuarzo 

normative de la Formation El Callao, la cual tiene una edad 

Proterozoica Temprana, tienen afinidad con toleitas bajas en 

potasio originadas en un ambiente de arco de islas. Los 

basaltos calco-alcalinos de la Formation Caballape (mas 

joven) tienen una composition similar a basaltos 

calco-alcalinos bajos en potasio y andesitas basalticas tipicas 

en sequencias de arcos de islas. Las rocas de la region estudiada se formaron probablemente en un arco de islas inmaduro intraoceanico el cual fue deformado por corrimiento 

oblicuo dextral a lo largo de un eje de compresion tectonica 

orientado en sentido noreste.

Menendez (1968, 1972, 1974) established the stratigraphic 

framework for the area. The oldest rocks in the study area 

belong to the Early Proterozoic Pastora Supergroup. 

Menendez recognized that the oldest part of the Pastora 

Supergroup is made up of the Carichapo Group, which 

consists of pillow lavas of the El Callao Formation and 

amphibole-biotite schist of the Cicapra Formation; however, 

rocks of the Cicapra Formation are not present in the study 

area. Sedimentary rocks of the Early Proterozoic Yuruari 

Formation, which forms the youngest unit within the Pastora 

Supergroup, structurally overlie the Carichapo Group 

(Menendez, 1968, 1972, 1974). Our mapping confirms that 

the contact between the two formations is faulted. 

Volcanogenic sedimentary rocks of the Early Proterozoic 

Caballape Formation overlie rocks of the Pastora Supergroup (Menendez, 1972). Rocks of the Pastora Supergroup 

and the Caballape Formation were regionally intruded by 

diabasic to gabbroic sills and (or) dikes.

INTRODUCTION

The Lo Increible mining district provides a unique 

opportunity to unravel the structural setting of Precambrian 

lode gold deposits of the Guayana Shield of Venezuela and 

to date the Early Proterozoic Yuruari Formation of the Pastora Supergroup (fig. 1). Although Gibbs and Olszewski 

(1982) dated rocks equivalent to the Pastora Supergroup in 

Guyana to the east, this study provides the first published 

U-Pb date on zircon from a sample of the Pastora Supergroup in Venezuela. The mapping described herein was a 

part of a joint U.S. Geological Survey-Corporation Venezolana de Guayana, Tecnica Minera, C.A. (CVG-TECMIN) 

geologic field mapping course taught by W.C. Day in June 

1989. R.M. Tosdal provided the U-Pb date on zircon from a 

sample of the Early Proterozoic Yuruari Formation. The 

remaining co-authors participated in the field course.

The Lo Increible mining district is in the eastern part of 

Estado Bolivar, approximately 150 km southeast of Puerto 

Ordaz and 10 km northeast of the El Callao mining district. 

The study area is covered by a thick weathered profile and 

dense tropical jungle vegetation, and thus our observations 

were limited to linear traverses cut through the jungle and 

sites along roads and mines and creek bottoms.

GEOLOGY 

REGIONAL STRATIGRAPHIC SETTING

Rocks of the Lo Increible mining district are Early 

Proterozoic volcanic, plutonic, and sedimentary rocks generally metamorphosed to low grade (middle to upper 

greenschist facies) and to a lesser extent medium grade 

(lower amphibolite facies). Regional mapping studies by

GEOLOGY AND GEOCHEMISTRY OF THE 

LO INCREIBLE MINING DISTRICT

The northern part of the study area is underlain by rocks 

of the Early Proterozoic Yuruari Formation (fig. 2). Rocks of 

the Yuruari Formation are poorly exposed, except in mine 

workings near the shear zone that separates the Yuruari and 

the El Callao Formations in the central part of the study area. 

The southern part of the study area is dominated by a large 

gabbro sill that forms a prominent ridge. The gabbroic sill is 

enfolded within the Caballape Formation.

EL CALLAO FORMATION

The oldest rocks in the study area are basalts of the 

Early Proterozoic El Callao Formation, which forms the 

basal unit in the Pastora Supergroup. This formation is characterized by tholeiitic basaltic pillow lava flows (unit Xec, 

fig. 2) and interlayered flow breccia (unit Xecb). The shape 

and orientation of the pillow structures, which are well 

exposed along the highway in the western part of the study 

area (fig. 2), indicate that the unit is upward facing and 

youngs stratigraphically to the southeast in the study area. 

The formation weathers light brown and in many places is 

recognized only by light-brown colluvial soil and float. 

Although regionally the unit is of low metamorphic grade, 

within and immediately adjacent to the shear zone (<250 m) 

in the central part of the study area the El Callao Formation 

has been recrystallized to hornblende-biotite schist of 

medium metamorphic grade.

Two sheets of basaltic breccia lie within the El Callao 

Formation (unit Xecb, fig. 2). These sheets are each about 

50 m thick and contain angular basaltic clasts as much as 15 

cm in diameter in a volcaniclastic basaltic matrix. The brec-
GEOLOGY OF LO INCREIBLE MINING DISTRICT E3

ESTADO 

\ DELTA AMACURO

Boundary of Venezuelan 

Guayana Shield

Figure 1. Location of the Lo Increible study area, Estado Bolivar, Venezuela.

cia sheets formed as interflow breccias within the basaltic 

flows of the El Callao Formation.

Rocks of the El Callao Formation are metaluminous, 

quartz-normative, low-potassium tholeiitic basalt. Plotted on 

a Jensen (1976) diagram, our samples of the El Callao Formation range from high-iron to high-magnesian tholeiite

(fig. 3A). The Mg-numbers are from about 47 to 63 (table 1), 

typical of tholeiitic basalt (Basaltic Volcanism Study 

Project, 1981, p. 132-192). The rare earth element patterns 

(fig. 44) are relatively flat and are about 10 times chondrite 

in abundance. All of these characteristics are similar to modern mid-ocean ridge, low-potassium island arc, and back-arc
E4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

3 KILOMETERS

-400-

Figure 2 (above and facing column). Geologic map of the Lo Increible mining district, north of El Callao, Estado Bolivar, Venezuela. 

Numbered solid circles are sample localities.

basalts (see Jakes and Gill, 1970, p. 19). The proportions of In order to characterize the environment of formation,

MnO, TiC>2, and ?2O5 are similar to those of modern island the trace element geochemistry for samples of the El Callao

arc tholeiite (fig. 3fi). Rocks of the El Callao Formation Formation is compared with that for island arc low-potascontain less TiC>2 than tholeiite formed in mid-ocean ridge sium basalt and normal (or N-type) mid-ocean ridge basalt

environments. (MORE), which may have formed in similar tectonic
GEOLOGY OF LO INCREIBLE MINING DISTRICT E5

40

 60

EXPLANATION

Alluvium 

Tectonic breccia 

Gabbro

Diabase

Pastora Supergroup Divided into:

Caballape Formation 

Yuruari Formation

El Callao Formation of the Carichapo Group Includes 

volcanic breccia (unit Xecb)

Contact Approximately located; dashed where inferred

Fault Dashed where inferred; arrows show direction of

movement. Bar and ball on downthrown side 

Thrust fault Sawteeth on upper plate

Shear zone

Syncline Dashed where inferred

Strike and dip of foliation

Bearing and plunge of lineation

Pillow lava showing younging direction

Mine or prospect

Paved road

Unimproved road

Town

Lake

environments (fig. 5A). The data are normalized to an average composition for island arc low-potassium tholeiite 

formed near Benioff zones (Holm, 1985, p. 311). A sample 

with a trace element composition identical to that of the average island arc low-potassium tholeiite will have a ratio of 

unity, a sample having lower elemental abundances will plot 

below the unity line, and, conversely, a sample having higher 

abundances will plot above.

Tholeiite of the El Callao Formation is relatively 

depleted in the large-ion lithophile elements K, Sr, and Ce 

but has abundances of the high field strength elements P, Zr, 

Ti, Y, and Yb comparable to those of island arc low-potassium tholeiite (fig. 5A). The rubidium concentration of El 

Callao tholeiite is similar to that of island arc tholeiite. By 

comparison, normal mid-ocean ridge basalt (Pearce, 1982, p. 

527) is also relatively depleted in large-ion lithophile elements but has higher high field strength element abundances 

than the El Callao Formation or island arc low-potassium 

tholeiite.

YURUARI FORMATION

The Yuruari Formation is a quartz-biotite-muscovite 

schist containing white quartz pods and veinlets that are parallel with the foliation. The unit is highly susceptible to 

weathering and forms low-lying topography. Outcrops are 

rare except near mines. The formation has a characteristic 

bright orange to red soil that contains residual pods of white 

quartz. The schist has been metamorphosed to medium grade 

(amphibolite facies), but ubiquitous alteration of primary 

biotite to chlorite indicates retrograde to low-grade (greenschist facies) metamorphism. Less weathered exposures 

within the shear zone show that the protolith was dacite and 

felsic volcaniclastic graywacke and locally interlayered 

basalt.

The intensely weathered nature of the Yuruari Formation limits geochemical sampling to determine the composition of the protolith. One sample of the Yuruari Formation, 

although weathered, is listed in table 1 (sample 11). The 

sample plots as calc-alkaline dacite on the Jensen diagram 

(fig. 3A) but has rhyolitic SiO2 content (75.6 weight percent). The relatively higher SiO2 may reflect the effects of 

weathering in which other constituents are leached. The rare 

earth element data also suggest similar leaching effects. 

Assuming that the composition of the protolith was generally 

similar to typical shale or graywacke (Menendez, 1972), 

approximately an order of magnitude loss has occurred in the 

abundance of the light rare earth elements (fig. 4fi).

CABALLAPE FORMATION

The Caballape Formation consists of calc-alkaline 

andesite, dacite, and rhyolite tuff and felsic volcaniclastic 

sedimentary rocks that have been metamorphosed to low 

grade (greenschist facies). The formation is poorly exposed 

in the study area; outcrops are in the creek bed adjacent to the 

high-angle normal fault north of the gabbroic sill and at isolated locations in the core of the east-plunging syncline in the 

central and southern parts of the study area (fig. 2).

One sample from the core of the syncline in the study 

area is low-potassium calc-alkaline basalt (sample 7, table 1, 

fig. 3A). In the Anacoco area, approximately 100 km southeast of this study area, the Caballape Formation ranges in 

composition from calc-alkaline basalt through dacite (Day 

and others, 1989). In both the Anacoco and Lo Increible 

areas, the major-, minor-, and trace-element content of the 

unit is similar to that of calc-alkaline low-potassium basalt 

and basaltic andesite typical of island arc sequences (figs. 

3fi, 4Q.
E6 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

GABBRO

A large gabbroic body (unit Xgb, fig. 2) was intruded 

as a sill and then infolded with the Caballape Formation to 

form an eastward-plunging syncline in the central and southern part of the study area (fig. 2). Fresh surfaces are dark 

green to black. The texture of the gabbro grades from 

fine-grained diabasic margins inward to a medium- to 

coarse-grained interior. The gabbro has undergone regional 

greenschist facies metamoiphism. It is relatively resistant to 

weathering and forms outcrops along the prominent ridge in 

the southern part of the study area.

The gabbro is younger than supracrustal rocks of the 

Pastora Supergroup. Several smaller diabasic sills (or dikes) 

are present within the El Callao and Yuruari Formations 

(unit Xid, fig. 2). The relative ages of the main gabbro and 

the diabase sills are not established; however, the units are 

considered coeval.

The gabbro is quartz normative, metaluminous, and 

tholeiitic in composition (samples 8-10, table 1). The 

Mg-numbers are relatively high (less evolved), ranging from 

approximately 65 to 69 (table 1). The gabbro is distinctly different from the El Callao Formation in that it has higher 

Mg-numbers and greater abundances of the light rare earth 

(figs. 4A, 4D) and large-ion lithophile (fig. 5B) elements. 

Although the major element contents of the gabbro are similar to komatiitic compositions (fig. 3A), the light rare earth 

element contents and higher MnO abundances (fig. 3fi) 

show that the gabbro has affinities with calc-alkaline basaltic 

magmas. The calc-alkaline affinity of the trace elements may 

imply that the gabbro evolved during the same calc-alkaline 

magmatic episode as the Caballape Formation, which it 

intrudes.

U-PB ZIRCON AGE OF THE 

EARLY PROTEROZOIC 

YURUARI FORMATION

Zircon extracted from a metadacite tuff within the 

Yuruari Formation (sample location 25, fig. 2) provides a 

reliable radiometric date for the formation and, therefore, for 

the upper part of the Pastora Supergroup. Although the 

Yuruari Formation is highly weathered, reliable age data can 

be extracted from zircon in units that have undergone 

extreme tropical weathering (Gibbs and Olszewski, 1982).

Five zircon fractions analyzed from the sample metadacite tuff of the Yuruari Formation have discordant 

206pb/238|j and 207pb/235Tj ages (table 2). All the fractions

define a chord on a concordia diagram (mean square 

weighted deviation=0.3) (Ludwig, 1983) with an upper 

intercept of 2,131±10 Ma and a lower intercept that is essentially present-day (fig. 6). This pattern of discordance in the

FeO+Fe2O3+TiO2

EXPLANATION

Gabbro

Caballape Formation 

Yuruari Formation 

El Callao Formation

-High- / 

magnesium Ultramafic 

basalt / komatiite

/

\Dacite\Andesite 

Rhyolite \ \

Andesite

Dacite

Rhyolite

A1203 

A

TiO2

MgO

EXPLANATION 

Gabbro

Caballape Formation 

El Callao Formation

MnOxlO 

B

P2O5 x10

Figure 3. Ternary plots showing chemical characteristics of samples from the Lo Increible mining district, Estado Bolivar, Venezuela. A, Jensen diagram. Modified from Jensen (1976). B, 

MnO/TiO2/P2O5 discriminant diagram for basalt and basaltic 

andesite. Fields: CAB, island arc calc-alkaline basalt; IAT, island 

arc tholeiite; MORB, mid-ocean ridge and marginal basin basalt; 

OIT, ocean island tholeiite; OIA, ocean island basalt (Mullen, 

1983).

ages indicates that lead was lost from zircon during a subsequent event (Silver and Deutsch, 1963). The upper concordia 

intercept age of 2,131±10 Ma is interpreted to indicate an 

Early Proterozoic crystallization age for the dacitic tuff. This 

interpretation is confirmed by the good agreement between 

the 207Pb/206Pb ages of the individual zircon fractions and 

the upper intercept age on the concordia diagram. The lower
Table 1. Major- and trace-element abundances for rocks of the Lo Increible mining district, Estado Bolivar, Venezuela.

[Locations of samples shown by sample numbers in figure 2. All field numbers are prefixed by WDV. Major-element oxides in weight percent; trace elements i

Rock type

Formation

Sample No.

Field No.

Basalt

El Callao

1

89-019

Basalt

El Callao

2

89-030

Basalt

El Callao

3

89-055

Basalt

El Callao

4

89-031

Basalt

El Callao

5

89-034

Basalt

El Callao

6

89-068

Andesite

Caballape

7

89-073

Gabbro

8

89-037

Gabbro

9

89-071

Major elements

Si02

A1203

Fe2O3t

MgO

CaO

Na2O

K2O

TiO2

P205

MnO

CO2

LOI900°C

Total

Mg-number1

47.60

15.60

13.90

5.58

13.00

0.85

0.04

0.89

0.06

0.17

<0.01

2.70

100.39

51.00

48.50

14.60

11.50

8.52

12.50

1.38

0.04

0.66

0.06

0.19

<0.01

2.45

100.40

65.76

49.00

14.40

11.40

8.16

11.90

2.03

0.04

0.67

0.06

0.19

0.01

2.16

100.01

64.98

49.90

14.10

10.50

7.93

12.70

2.31

0.03

0.63

0.06

0.18

0.09

1.78

100.12

66.19

49.40

13.50

14.90

7.23

8.63

2.09

0.04

1.01

0.09

0.23

0.13

2.86

99.98

55.71

50.00

14.20

10.60

7.60

12.70

2.26

0.06

0.64

0.06

0.17

0.02

1.77

100.06

65.02

55.60

14.80

10.00

4.99

7.08

2.98

0.38

0.78

0.23

0.13

<0.01

2.76

99.73

56.40

49.30

11.60

12.20

10.10

11.60

1.69

0.50

0.74

0.09

0.19

0.09

2.20

100.21

68.21

49.40

12.30

11.00

10.80

12.30

1.11

0.17

0.49

<0.05

0.19

0.01

2.50

100.26

71.79

Trace elements

Nb

Rb

Sr

Zr

Y

Ba

Cu

Ni

Zn

Cr

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

<10

7

443

46

19

11

61

178

87

297

3.6

8.0

1.2

6.7

2.20

0.93

2.6

0.73

4.5

0.87

2.6

0.42

2.9

<10

<2

165

38

10

9

120

141

76

439

2.0

5.1

0.8

4.3

1.40

0.60

1.8

0.31

2.5

0.52

1.5

0.26

1.4

<10

7

108

45

18

10

108

110

79

431

2.2

5.7

0.9

5.0

1.50

0.67

2.0

0.36

2.9

0.54

1.6

0.25

1.6

<10

<2

82

39

11

8

111

117

68

384

1.7

4.9

0.7

4.3

1.20

0.60

1.7

0.34

2.5

0.59

1.5

0.21

1.6

<10

6

105

60

28

11

117

91

107

45

2.7

7.3

1.1

6.5

2.10

0.91

2.4

0.51

3.8

0.80

2.4

0.38

2.2

<10

<2

65

42

17

18

82

118

78

400

1.7

4.6

0.7

4.2

1.40

0.54

1.5

0.30

2.4

0.52

1.4

0.22

1.4

<10

17

522

130

18

201

2

105

60

264

28.0

53.0

6.2

27.0

4.90

1.40

4.2

0.64

3.4

0.62

1.7

0.24

1.6

13

11

165

73

17

199

71

184

83

307

12.0

25.0

2.8

14.0

3.30

1.00

3.2

0.59

3.6

0.69

2.2

0.30

1.8

<10

5

139

44

12

86

142

156

68

100

4.8

10.0

1.3

5.8

1.70

0.56

1.7

0.32

1.8

0.41

1.1

0.14

0.9

^g-numbei^lOOxMg/CMg+Fe2*), where Fe2+=0.85x (Fe2O3t/79.85x0.8995)
E8

1,000

100

10

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

1,000

100

10

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

1,000 F

100 r

10

I I I I I I I I

1,000

100

10

Sample 8

Sample 10

I I I I I I I I I I

LaCePrNd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

B D

Figure 4. Chondrite-normalized rare earth element (REE) diagrams for rocks of the Lo Increible mining district, Estado Bolivar, 

Venezuela. A, El Callao Formation. B, Weathered sample of the Early Proterozoic Yuruari Formation (open squares). Typical 

post-Archean Australia shale (PAAS; heavy line) and North America Shale Composite (NASC; light line) (Taylor and McLennan, 1985) 

are shown for comparison. C, Meta-andesite sample from the Caballape Formation (open squares). Typical calc-alkaline island arc 

andesite (Gill, 1981) (heavy line) is shown for comparison. D, Samples from a gabbroic sill that intrudes the Caballape Formation.

intercept age is interpreted to indicate that the time of lead 

loss was recent. The lower intercept (70±50 Ma) (fig. 6) 

probably reflects lead loss due to surficial weathering during 

the Cretaceous to Paleogene, similar to the lead loss 

described by Gibbs and Olszewski (1982) for rocks

equivaenslent to the Pastora Supergroup in Guyana. The 

geomorphological development of the Venezuelan Guayana 

Shield and the effects of weathering on shield rocks in the 

Mesozoic and Tertiary are discussed by Bricefio and 

Schubert (1990) and by references contained therein.
GEOLOGY OF LO INCREIBLE MINING DISTRICT E9

10.00

1.00

0.10 -

0.01

10.00

2 1.00

0.10

0.01

> Gabbro ( 

Yuruari Formation (A)

B

Rb Sr Ca Zr Ti Yb

Figure 5. Spider diagrams for rocks of the Lo Increible mining district, Estado Bolivar, Venezuela. Normalized to island arc lowpotassium tholeiite (Holm, 1985, p. 311). A, El Callao (circles) and Caballape Formations (squares). Normal mid-ocean ridge basalt 

(N-type MORE) (Pearce, 1982, p. 527) is also shown (diamonds). B, Yuruari Formation (triangles) and gabbro unit (squares) that 

intrudes the Caballape Formation.

Neither prograde medium-grade metamorphism within 

the shear zone nor subsequent retrograde (greenschist facies) 

metamorphism during the Trans-Amazonian orogeny (about 

2.15-2.0 Ga) affected the U-Pb isotopic systematics in zircon from the sample of the Yuruari Formation. Lead loss

during any younger event is enhanced by radiation damage 

to the zircon lattice that accumulates through geologic time. 

The low uranium content (100-220 ppm 238U) of the zircon 

from the felsic tuff effectively precluded any extensive 

radiation damage to the zircon lattice due to the decay of
E10 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

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Figure 6. U-Pb concordia diagram showing U-Pb isotopic data for 

zircon from a metadacite tuff in the Yuruari Formation of the Pastora 

Supergroup, Lo Increible mining district, Estado Bolivar, Venezuela. Solid diamonds represent data points; analytical errors at the 

two-sigma (2o) confidence level on each analysis are smaller than 

the square and are not shown. Solid chord is regression line through 

all five zircon fractions.

uranium and thorium in the period between the crystallization age of the tuff and its subsequent metamorphism and 

tectonic burial during the Trans-Amazonian orogeny.

STRUCTURAL SETTING OF GOLD 

DEPOSITS

STRUCTURAL OBSERVATIONS

The shear zone that separates the Yuruari and El Callao 

Formations in the study area is a northeast-striking zone of 

intense ductile deformation. Although rocks of the Yuruari 

Formation are dominant within the shear zone, locally rocks 

of the El Callao Formation are incorporated and have been 

recrystallized to amphibolite schist. The orientation of the 

shear zone is presumably parallel to the generally southeast 

dip of the tectonic foliation within the zone. Such an orientation indicates that the El Callao Formation south of the shear 

zone structurally overlies the Yuruari Formation to the north. 

Therefore, assuming the stratigraphy outlined by Menendez 

(1972), the shear zone has a component of thrusting, placing 

the older El Callao Formation over the younger Yuruari Formation.

Rocks within the shear zone are highly foliated and are 

locally deformed into small-scale folds of S and Z symmetry; the latter is the dominant geometry. An S-C mylonitic 

fabric is well developed within the shear zone. Two foliations characterize the S-C mylonitic fabric. One foliation (C 

surface) forms the dominant foliation and is the plane of 

strongly localized ductile shearing (see Lister and Snoke, 

1984). The other foliation (S surface) is developed oblique to
Table 3. Gold and trace element abundances for samples associated with gold deposits of the Lo Increible mining district, Estado Bolivar, Venezue

[In parts per million. Location of samples shown by sample number in figure 2. All field numbers are prefixed by WDV]

Rock type Schist Quartz vein Quartz vein Quartz vein

Sample No. 12 13 14 15

Field No.

Au

As

Co

V

Rb

Sr

Zr

Y

Ba

Ce

La

Cu

Ni

Zn

Cr

Formation. 

In general, goldconcentrations inthese samples ares 

high as 18ppm.Samples thathave highergold valuesalso

89-016 89-013 89-014 89-061

<0.002 0.004 0.200 0.008

<10 <10 <10 50

2 12 16 45

23 6 34 170

10 <2 12 42

526 37 153 114

162 12 67 34

6 3 9 10

12 165 295 92

34 100 21 9

32 2 <2 5

10 3 9 47

48 <5 22 100

92 12 36 57

95 <20 <20 343

sericite, chlorite, pyrite, quartz, and carbonate minerals. 

Analyses ofgrabsamples fromquartz veins insurface 

deposits are listed intable 3.Because weathering withinthe 

shear zone wasintense, theeanalyses donot represent original protolith compositions. Sample12, froma 5-10-m-deep prospect pit, isan unmieralized sampleof schist. Itisa 

weathered fine-grained maficvolcanic tuffhat contins fine-grained feldspar phenocrysts andhas superimposed tectonic foliation. Theprotolith was probably fine-grained mafic tufwithin theYuruari Formation. Samples 13-16are 

composite samples fromseveral golddeposits inthedistrict. Samples 17-22are paired samples ofhostbiotite-sericite 

schist and corresponding quartz- vein material (sample 17with sample18, 1920and 2122).Protolith forthehost biotite-sericite schistof thepaired samples was proba- bly volcaniclastic sedimentary rockofthe Yuruari

Quartz vein

16

89-053

2.100

60

3

52

4

109

16

4

16

<2

<2

15

31

82

140

Two generations ofquartz chracterize thegold-bearing veins.An early dark-gray pyrite-bearing tourmaline-rich quartz is cutbylater, whitebull quartz veins. Alteration min-erals associated withthe golddeposits include tourmaline,

Schist host

17

89-009

0.030

30

17

101

44

360

112

3

670

46

35

31

36

38

67

biotite-amphibole schistof recrystallized ElCallao Forma- tion. Therefore, thehost samples listedin table 3coudhave been derived originally fromeither formation, butwere 

tectonically juxtaposed within theshear zone.

Quartz vein

18

89-008

7.000

50

13

59

18

151

94

9

314

47

9

41

29

55

50

pinch and swell withinplanes ofshear foliation thehost schistandre both suparallel withdiscordant to the tectonic foliation. Thehost schist isa tectonic melange of

biotite-sericite schistof theYuruari Formation and

Schist host

19

89-46A

<0.002

20

4

239

8

184

57

11

203

<2

8

45

94

28

392

Gold deposits intheLo Increible miningdistrict are 

low-sulfide shear-zone-hosted lodegold (rmesothermal) type. Native goldiswithin discontinuous quartz veins thatare 0.5-20 cm wide ands log 100m. The quartz veins

Quartz vein

20

89-46B

<0.002

<10

3

47

3

109

22

7

9

2

<2

10

74

48

114

GOLD DEPOSITS

Schist host

21

89-47A

0.150

70

18

111

68

153

215

31

704

65

26

115

45

112

75

Qua

89

6

<

2

2

5

3

<2

laaYuForm(assuthstratigou

lined byMenendez, 1972)indicate thahesear zone is a

right-oblique slip-thrust (or reverse) faultzone. Therefore, rocks ofthestudy area undrwent a dextral oblique compres- sion, or dextral transpression, alonga northwest-oriented 

tectonic axisofmaimum compression.
E12 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

have elevated arsenic contents. The quartz veins are relatively enriched in gold, but they are depleted in most of the 

other elements analyzed.

The overall nature, mode of occurrence, and origin of 

the gold deposits are similar to those described by Berger 

and Bliss (1986) for low-sulfide gold-quartz veins and by 

Col vine (1989) for shear-zone-hosted lode-gold deposits in 

the Canadian Shield. The gold deposits are restricted to the 

quartz veins in or adjacent to the shear zone separating the 

Yuruari and El Callao Formations. Formation of alteration 

minerals in the host rock during deformation within the shear 

zone is indicated by the parallel foliation within the secondary micaceous minerals and the unmineralized host schist. 

Therefore, gold deposition accompanied deformation and 

alteration within the shear zone.

Historical grade and tonnage information for the gold 

deposits of the Lo Increible mining district is limited. Data 

for mines within the district (Locher, 1974) show that the 

district has produced at least 1,300 kg of gold at a grade of 

about 12 grams per metric ton (53.3 metric tons at 10.6 

grams per metric tons for the Experiencia mine, 3.3 metric 

tons at 6.2 grams per metric ton for the Garrapata mine, 45.3 

metric tons at 14.7 grams per metric ton for the Increible 

mine, and 1.7 metric tons at 11.4 grams per metric ton for the 

Talisman mine). The limited production data are a result of 

the ad hoc mining within the district in dominantly hand dug 

surface trenches and primitive underground workings. Gold 

has been recovered primarily from mercury amalgamation of 

hand-cobbed quartz-vein material, and a quantitative assessment of the gold potential of the district would be speculative 

at best. The assay values reported in table 2 indicate, however, that high-grade veins are present in the district. High 

potential at a high degree of certainty is present for undiscovered shear-zone-hosted lode-gold deposits within and along 

the extension of the shear zone. Further exploration of the 

shear zone, especially to the east of the study area, should 

result in the discovery of additional gold deposits.

CONCLUSIONS

The paleotectonic environment of the Lo Increible mining district of Estado Bolivar, Venezuela, and, by extension, 

the entire Pastora Supergroup, can be deciphered by integrating geologic and geochemical data from all of the units in the 

area. Major-, minor-, and trace element data for the El Callao 

Formation show an affinity of these rocks with modern 

low-potassium island arc tholeiite. Metagraywacke of the 

Yuruari Formation may have formed along a convergent 

plate margin. Calc-alkaline basalt from the younger Early 

Proterozoic Caballape Formation is similar in composition 

to low-potassium calc-alkaline basalt and basaltic andesite 

typical of island arc sequences.

We suggest that the rocks in the Pastora Supergroup 

formed in an immature intraoceanic island arc setting that

underwent subsequent oblique dextral thrusting along a 

northwest-oriented axis of tectonic compression. Similar 

relationships are observed in the Early Proterozoic Birimian 

greenstone belts of West Africa (Sylvester and Attoh, 1992), 

which Sidder and Mendoza (1991, this volume) correlate 

with rocks of the Pastora Supergroup of Venezuela.

REFERENCES CITED

Basaltic Volcanism Study Project, 1981, Basaltic volcanism on the 

terrestrial planets: New York, Pergamon Press, 1286 p.

Berger, B.B., and Bliss, J.D., 1986, Descriptive model of 

low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., 

eds., Mineral deposit models: U.S. Geological Survey Bulletin 

1693, p. 239-243.

Briceno, H.O., and Schubert, Carlos, 1990, Geomorphology of the 

Gran Sabana, Guayana Shield, southeastern Venezuela: 

Geomorphology, v. 3, p. 125-141.

Colvine, A.C., 1989, An empirical model for the formation of 

Archean gold deposits Products of final cratonization of the 

Superior Province, Canada, in Keays, R.R., Ramsay, W.R.H., 

and Groves, D.I., eds. The geology of gold deposits The perspective in 1988: Economic Geology Monograph 6, p. 37-53.

Gibbs, A.K., and Olszewski, W.J., 1982, Zircon U-Pb age of Guyana greenstone belts: Precambrian Research, v. 17, p. 199-214.

Gill, J.B., 1981, Orogenic andesites and plate tectonics: Berlin, 

Springer-Verlag, 390 p.

Holm, P.E., 1985, The geochemical fingerprints of different tectonomagmatic environments using hygromagmatophile element abundances of tholeiitic basalts and basaltic andesites: 

Chemical Geology, v. 51, p. 303-323.

Jakes, P., and Gill, J. 1970, Rare earth elements and the island arc 

tholeiitic series: Earth and Planetary Science Letters, v. 9, p. 

17-28.

Jensen, L.S., 1976, A new cation plot for classifying subalkalic volcanic rocks: Ontario Ministry of Natural Resources Miscellaneous Paper 66, 22 p.

Krogh, T.E., 1973, A low contamination method for hydrothermal 

dissolution of zircons and extraction of U and Pb for isotopic 

age determinations: Geochimica et Cosmochimica Acta, v. 37, 

p. 485^94.

Lister, G.S., and Snoke, A.W., 1984, S-C mylonites: Journal of 

Structural Geology, v. 6, p. 617-638.

Locher, Eugenio, 1974, Oro en Venezuela: Conferencia Geologica 

Inter-Guayanas, Ciudad Guayana, Venezuela, 1972, Memoria, 

Publication Especial 6, p. 558-587.

Ludwig, K.R., 1983, Plotting and regression programs for isotope 

geochemists, for use with HP-86/87 microcomputers: U.S. 

Geological Survey Open-File Report 83-849,94 p.

Mattinson, J.M., 1987, U-Pb ages of zircons A basic examination 

of error propagation: Chemical Geology (Isotope Geoscience 

Section), v. 66, p. 151-162.

Menendez V. de V., Alfredo, 1968, Revision de la estratigrafia de 

la Provincia de Pastora segiin el estudio de la region de Guasipati, Guayana Venezolana: Boletm de Geologia, Caracas, v. 

10, no. 19, p. 309-338.
GEOLOGY OF LOINCREIBLE MINING DISTRICT E13

 1972, Geologia de la region Guasipati, Guayana Venezolana: Congreso Geologico Venezolano, 4th, Caracas, 1971, 

Memoria, Publicacion Especial 5, v. 4, p. 2001-2046. 

-1974, Guia de la excursion geologica Guasipati-El

Callao-Canaima: Conferencia Geologica Inter-Guayanas, 9th, 

Ciudad Guayana, Venezuela, 1972, Memoria, Publicacion 

Especial 6, p. 49-67.

Mullen, E.D., 1983, MnO/TiO2/P2O5 A minor element discriminant for basaltic rocks of oceanic environments and its implications for petrogenesis: Earth and Planetary Science Letters, 

v. 62, p. 53-62.

Pearce, J.A., 1982, Trace element characteristics of lavas from 

destructive plate boundaries, in Thorpe, R.S., ed., Andesites: 

New York, John Wiley and Sons, p. 525-548.

Sidder, G.B., and Mendoza, Vicente, 1991, Geology of the 

Venezuelan Guayana Shield and its relation to the entire

Guayana Shield: U.S. Geological Survey Open-File Report 

91-141, 59 p.

Silver, L.T., and Deutsch, S., 1963, Uranium-lead isotopic variations in zircons A case study: Journal of Geology, v. 71, 

p. 721-758.

Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial 

lead isotope evolution by a two-stage model: Earth and Planetary Science Letters, v. 26, p. 207-221.

Sylvester, P.J., and Attoh, Kodjopa, 1992, Lithostratigraphy and 

composition of 2.1 Ga greenstone belts of the West African 

craton and their bearing on crustal evolution and the 

Archean-Proterozoic boundary: Journal of Geology, v. 100, 

p. 377-393.

Taylor, S.R., and McLennan, S.M., 1985, The continental 

crust Its composition and evolution: Oxford, England, 

Blackwell Scientific Publications, 312 p.

Geology and Geophysics of the

Sierra Verdun-Cerro Piedra del Supamo Area,

Estado Bolivar, Venezuela

By Floyd Gray, Fernando J. Nunez, Jeffrey C. Wynn, Gary B. Sidder, 

Freddy Davila, and Angel Baez

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-F

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENTS

Abstract........................................................................................................................... Fl

Resumen......................................................................................................................... 1

Introduction.................................................................................................................... 1

Geology.......................................................................................................................... 2

Regional Geologic Setting...................................................................................... 2

Geology of the Study Area..................................................................................... 3

Geochemistry.................................................................................................................. 6

Analytical Methods................................................................................................. 6

Analytical Results................................................................................................... 9

Volcanic Rocks............................................................................................... 9

Mafic-Ultramafic Rocks................................................................................. 9

Aeromagnetic Data......................................................................................................... 11

Mineral Deposits............................................................................................................. 18

Conclusions.................................................................................................................... 19

References Cited............................................................................................................. 19

	FIGURES

1. Map showing location of Sierra Verdun-Cerro Piedra del Supamo study area.......................................................... F2

2. Map showing geology of study area and adjacent region........................................................................................... 4

3. Jensen diagram for volcanic rocks from study area.................................................................................................... 6

4. Diagrams showing geochemical characteristics of volcanic rocks from study area................................................... 7

5. Discriminant diagrams for volcanic rocks from study area......................................................................................... 7

6. Magnesia-variation diagrams for volcanic rocks from study area.............................................................................. 8

7. Jensen diagram for mafic-ultramafic rocks from study area....................................................................................... 10

8. AFM diagram showing geochemical characteristics of mafic-ultramafic rocks from study area............................... 10

9. Magnesia-variation diagrams for mafic-ultramafic rocks from study area................................................................. 11

10. Aeromagnetic interpretation map of study area showing suggested boundaries of buried magnetic sources............ 13

11. Profile of magnetic body that crosses and partially parallels Rio Yurudn.................................................................. 15

12. Profile of magnetic body across east-trending body near Rio Yurudn........................................................................ 16

13. Profile of magnetic body at southern end of line B-B'.. .............................................................................................. 17

14. Profile of magnetic sources along line A-A' in western part of study area................................................................. 18
Geology and Geophysics of the

Sierra Verdun-Cerro Piedra del Supamo Area,

Estado Bolivar, Venezuela

By Floyd Gray, 1 Fernando J. Nunez,2 Jeffrey C. Wynn,3 Gary B. Sidder,4 

Freddy Davila,2 and Angel Baez2

ABSTRACT

The Sierra Verdun-Cerro Piedra del Supamo area is in 

an Early Proterozoic greenstone-granite terrane in the 

Guayana Shield of southeastern Estado Bolivar, Venezuela. 

Rocks of the greenstone belts, mainly mafic flows and minor 

associated tuffaceous units, are strongly foliated and are 

metamorphosed to the greenschist facies. These rocks have 

been locally metamorphosed to the amphibolite facies, 

particularly near their contact with granite. A mafic-ultramafic complex was emplaced in older greenstone-belt rocks of 

basaltic andesite to basalt probably near the end of deformation of the greenstone belts. Historical and active placer and 

alluvial gold mining has stimulated recent interest in the 

area, which also has potential for deposits of 

shear-zone-hosted gold (low-sulfide gold-quartz veins), 

intrusion-related gold and precious-metals, chromite, placer 

and lode platinum-group elements, nickel-copper ore, and 

lateritic nickel.

RESUMEN

El area de Sierra Verdun-Piedra del Supamo esta 

ubicado en un ambiente granito-rocas verdes de edad Proterozoico Temprano, el cual es parte del Escudo de Guayana 

en el Estado Bolivar, Venezuela. Las rocas del cinturon de 

rocas verdes, principalmente flujos maficos y unidades tobaceas menores, estan fuertemente foliadas y metamorfizadas

^.S. Geological Survey, Corbett Building., 210 E. 7th Street, Tucson, 

Arizona 85705

2Corporacion Venezolana de Guayana, Tecnica Minera, C.A., C.C. 

Chilemex, Piso 1, Puerto Ordaz, Venezuela.

3U.S. Geological Survey, Unit 62101, APO AE 09811-2101, USA.

4U.S. Geological Survey, Denver Federal Center, MS905, Denver, 

Colorado 80225.

a la facie del esquisto verde, y a la facie de la anfibolita cerca 

del contacto con rocas graniticas. Un complejo 

mafico-ultramafico fue emplazado en rocas mas antiguas del 

cinturon de rocas verdes, las cuales tienen una composition 

andesitica a basaltica. Este plutonismo mafico-ultramafico 

probablemente ocurrio hacia el final de deformation de las 

rocas que conforman el cinturon de rocas verdes. Trabajos de 

mineria de oro, tanto antiguos como moderaos, han estimulado el interes en el area, la cual tambien tiene potencial para 

yacimientos de oro formados en zonas de cizalla (vetas de 

cuarzo con oro y bajo contenido de sulfuros); metales preciosos y oro relacionados con intrusiones graniticas; depositos 

de cromita; placeres y vetas con elementos del grupo del 

platino; mena de niquel-cobre, y lateritas niqueliferas.

INTRODUCTION

The Sierra Verdun-Cerro Piedra del Supamo study area 

covers an area of 544 km2 about 250 km south-southeast of 

Puerto Ordaz and about 60 km south of the town of Tumeremo in Estado Bolivar, Venezuela. The study area is centered near the confluence of the Rio Supamo and the Rio 

Yuruan and is between lat 6°36/ and 6°50' N. and long 

62°00' and 61°47' W. (fig. 1). It can be reached by helicopter; a poorly maintained logging road from El Callao or a 

5-hour boat trip from El Dorado yields some limited, difficult access. The terrain is rugged and hilly and is characterized by a north-northwest-trending chain of mountains that 

has local relief of as much as 300 m. Although excellent outcrops are present on ridge tops and in the rivers during the 

dry season, the area is generally heavily forested. Much of 

the fieldwork was conducted along traverses cut through the 

jungle at 2-km intervals perpendicular to axes that are 

roughly parallel with the strike of structures and the trend of 

geologic units.

Fl
F2

62°30

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

62°00' 61°30'

6°30' -

61°00'

6°00'

0 10 20 30 40 50 KILOMETERS

Figure 1. Location of the Sierra Verdun-Cerro Piedra del Supamo study area, Estado Bolivar, Venezuela.

The Sierra Verdun-Cerro Piedra del Supamo area was 

chosen for study for the following reasons:

1. Numerous gold placers and other surficial prospects, some of which are currently worked, are present in the 

area.

2. Radar images and reconnaissance mapping indicate that the area includes a diverse lithologic and stratigraphic section of greenstone belt.

3. A large mafic and ultramafic intrusive body within 

the greenstone belt, similar to rocks in other greenstone-belt 

areas in Venezuela, enables description of the nature of the 

contact relations for this particular suite of rocks and its 

mineral resource potential.

Geologic mapping was done in the area along and south 

of the Rio Yuruan during March through July of 1990. 

Because much of the area is covered by forest and deeply 

weathered soil, detailed analysis of aeromagnetic data was 

used to better define the geology and structure.

GEOLOGY 

REGIONAL GEOLOGIC SETTING

Early Proterozoic greenstone-granite terranes of the 

Pastora Supergroup and the Botanamo Group crop out in the 

central and eastern parts of the Venezuelan Guayana Shield 

(Sidder and Mendoza, this volume, fig. 2). They cover an 

area of about 360 km by 250 km, or about 90,000 km2. The 

Pastora Supergroup consists of the El Callao and Cicapra 

Formations of the Carichapo Group and the Yuruari 

Formation. Rocks of the Botanamo Group, which contains 

the Caballape and Los Caribes Formations, lie discordantly 

on the Pastora Supergroup. Cox and others (1993) restricted 

the use of formation names to their type locations and considered the sequence of volcanic rocks in the Sierra 

Verdun-Cerro Piedra del Supamo area approximately age 

equivalent to other mafic greenstone sequences such as the 

El Callao Formation.
GEOLOGY AND GEOPHYSICS, SIERRA VERDUN-CERRO PIEDRA DEL SUPAMO F3

Granitic plutons and domes of the Supamo Complex 

were intruded into the metasedimentary and meta-igneous 

rocks of the greenstone-granite terranes and separate the 

greenstone belts into branching synclinoria (Menendez, 

1968, 1972; Benaim, 1972). Felsic to intermediate volcanic rocks of the Early Proterozoic Cuchivero Group and 

clastic sedimentary rocks of the Early to Middle Proterozoic Roraima Group overlie and cover rocks of the greenstone-granite terranes to the west and south of the study 

area. The contact between the greenstone-granite terranes 

and the Archean Imataca Complex to the north is the 

Guri shear zone (Gibbs and Olszewski, 1982; Onstott and 

others, 1989).

Mafic to ultramafic intrusive rocks are present in the 

stratigraphic sequence of the greenstone belts south of El 

Callao (Tosiani and Sifontes, 1989; Nunez, unpublished 

data, 1990; Nunez, Gray, and Davila, unpublished data, 

1990; Wynn and others, this volume). The intrusive rocks 

commonly form layered complexes that include cumulate-textured rocks such as pyroxenite and peridotite associated with gabbroic cumulus and noncumulus rocks and 

lesser anorthosite and diorite. Country rocks of this intrusive sequence are basalt and basaltic andesite. The study 

area is part of an extensive north-striking ridge that originates in the El Callao area and continues southward to the 

Rio Chicanan, interrupted by structures associated with the 

Rio Yuruan.

GEOLOGY OF THE STUDY AREA

Within the study area, an Early Proterozoic 

supracrustal sequence of strongly deformed metavolcanic 

flows, tuffs, and hypabyssal mafic rocks forms the country rocks of an ultramafic-mafic complex (fig. 2). The 

supracrustal sequence is intruded and fragmented at its 

margins by granitic, monzonitic, and dioritic intrusive 

rocks of the Supamo Group. The western margin of the 

intrusive rocks is exposed in the Rio Yuruan near the 

mouth of the Rio Mochila. The granitic rocks display a 

strong foliation parallel with the contact with rocks of the 

greenstone belt that trends N. 43°-55° W. and dips 

steeply 60°-80° S. Basalt, minor gabbro, and interflow 

tuffs form xenolithic breccias and are extensively veined 

and sloped by a large granitic dome in a 0.5-km-wide 

zone.

In general, the southern part of the study area, including the area of the Rio Yuruan, is characterized by 

low-lying volcanic rocks and the prominent north-northwest-striking Sierra Verdun ridge and its flank of curvilinear ridges. Northeastward (upsection) from the basal basalt 

are basaltic andesite and minor tuff and tuff breccia. The 

Sierra Verdun ridge consists of gabbroic rock; minor clinopyroxenite and feldspathic clinopyroxenite are in the

eastern part of the ridge. The topography of the contact of 

the gabbro ridge at its northern limit suggests a fault contact with the volcanic rocks. The eastern margin of the 

greenstone belt is virtually identical to the western margin 

in its fragmental intrusive character. East of the study 

area, outcrops of dacite, rhyolitic tuff, flow-banded rhyolite, cherty chemical sedimentary rocks, and minor lenses 

of intercalated arkosic rock in an adjacent body of greenstone-belt rocks (separated from greenstone-belt rocks in 

the study area by granite) are present.

Although ultramafic rocks probably account for 1-2 

percent of the volume of igneous rocks in greenstone belts of 

the Guayana Shield in Guyana (Gibbs, 1987), the area near 

Cerro Piedra del Supamo contains a significantly greater volume of ultramafic-mafic intrusive rocks that are generally 

unfoliated and little affected by regional metamorphism as 

compared to volcanic country rocks. Sulfide-bearing, 

fine-grained hornfels and recrystallized basaltic volcanic 

rocks that contain coarse acicular amphibole are in contact 

zones adjacent to gabbroic intrusive rocks. Medium-grained 

cumulate-textured rocks, sheared and mylonitized talc 

schist, and serpentinite are the most common rocks in the 

contact areas where horafelsic rocks are found. Although 

neither spinifex textures nor features indicative of an extrusive origin for these rocks have been observed in the study 

area, komatiite was identified in the area on the basis of 

reported relic spinifex texture (Tosiani and Sifontes, 1989). 

The so-called spinifex texture is, however, rosettes of coarse 

acicular amphibole after pyroxene in contact metamorphic 

zones, and an origin other than komatiite is probable.

The ultramafic-mafic complex in the Sierra 

Verdun-Cerro Piedra del Supamo area is composed of 

crudely zoned, layered, cumulus dunite, wehrlite, olivine clinopyroxenite and minor feldspathic pyroxenite surrounded 

by locally foliated gabbroic rocks. The gabbroic rocks, 

which volumetrically are the most abundant rocks in the 

area, range from homogeneous textured gabbro to locally 

phase transitional pyroxene cumulate containing interstitial 

plagioclase and magnetite, plagioclase-pyroxene cumulate, 

and plagioclase cumulate containing oikocrysts of postcumulus pyroxene. The ultramafic rocks are cumulates composed of varying proportions of cumulus olivine, 

clinopyroxene, and orthopyroxene (?) and postcumulus 

plagioclase, orthopyroxene, brown hornblende, and magnetite. Layering and lamination strike variably north and dip 

between 20° and 70° to the west, although faults generally 

disrupt the orientation of the sequence. Textures are 

generally well preserved; however, locally, primary 

minerals are partly to totally altered to serpentine minerals, 

talc, amphibole, and magnetite. Large bodies of talc-serpentine schist are associated with fractures or shear zones within 

the ultramafic rocks. Pyrite, pyrrhotite, and chalcopyrite in 

amounts as much as 15-20 percent form ubiquitous 

accessory minerals in the ultramafic rocks.
F4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD
GEOLOGY AND GEOPHYSICS, SIERRA VERDUN-CERRO PIEDRA DEL SUPAMO F5

EXPLANATION

Diabase (Mesozoic and Early Proterozoic) Dark-gray to 

greenish-gray, fine- to coarse-grained tholeiitic intrusive rocks. 

Present as dikes, sills, and laccoliths. Includes rocks about 

1,850-1,650 Ma and rocks about 270-219 Ma (Sidder and 

Mendoza, this volume)

RORAIMA GROUP (EARLY AND 

MIDDLE PROTEROZOIC)

Canaima Formation of Yanez (1985) Quartzarenite and arkose, 

conglomeratic arenite, conglomerate siltstone, and shale. 

Abundant cross-strata in arenite and arkose units. Weathers to 

form cliffs on resistant arenite, arkose, and conglomerate units 

and slopes on relatively nonresistant siltstone, shale, and silty 

arenite units. About 1,1000-2,000 m thick. Equivalent to Uairen 

and Cuquenan Formation, and lowermost part of Uaimapue 

Formation, of Reid and Bisque (1975)

GRANITIC ROCKS (EARLY PROTEROZOIC)

Granitic rocks Mostly monzogranite and granite. Porphyritic 

medium to coarse grained, some inclusions of granodiorite

Supamo Complex Sodic granodiorite and trondhjemite, 

paragneiss, and migmatite; commonly in domeshaped intrusions. Sparse pegmatite. About 2,230- 

2,050 Ma (Sidder and Mendoza, this volume)

EUGEOSYNCLINAL METAMORPHIC ROCKS 

(EARLY PROTEROZOIC)

Mica schist and phyllite Quartz + muscovite ± chlorite ± 

chloritoid ± ankerite schist and phyllite and subordinate 

quartzite or metachert derived from sedimentary and felsic 

volcanic rocks. Ankeritic rocks weather to ferruginous schist 

and phyllite

Mafic to intermediate metalava and metatuff Chlorite ±

epidote ± actinolite schist and phyllite and greenstone, commonly 

containing relict pyroxene phenocrysts. Relict textures suggest 

amygdaloidal flows and lithic- and crystal-rich tuff

ROCKS OF THE GREENSTONE BELTS 

(EARLY PROTEROZOIC)

Metagabbro Saussuritized and locally amphibolitized. Locally has

cumulus texture 

Ultramafic rocks Mainly metaperidotite, metapyroxenite,

serpentinite, and talc schist 

Mica schist and phyllite Quartz+muscovite+chlorite+ ankerite

schist and phyllite and subordinate quartzite or metachert derived

from sedimentary and felsic volcanic rocks. Ankeritic rocks

weather to ferruginous schist and phyllite 

Felsic metatuff and flows Quartz+muscovite+chlorite schist

containing relict phenocrysts of partially resorbed quartz and

broken feldspar 

Mafic to intermediate metalava and metatuff Chlorite+ albite+

epidote+actinolite schist and schistose greenstone, locally

amygdaloidal. Subordinate albite-epidote amphibolite. Minor

chert 

Amphibolite Mainly highly deformed hornblende schist containing

plagioclase (An 17). Locally shows outline of original phenocrysts

replaced by hornblende

Contact

Fault Dashed where concealed

Figure 2 (above and facing page). Geology of the Sierra 

Verdun-Cerro Piedra del Supamo study area (outlined by box) and 

adjacent region, Estado Bolivar, Venezuela. Modified from Cox 

and others (1993).

Cumulus dunite, wehrlite, and olivine clinopyroxenite 

are volumetrically minor rock types in the ultramafic parts of 

the intrusive complex. Olivine cumulate generally consists 

of euhedral to subhedral olivine partly altered to serpentine 

and magnetite and postcumulus poikilitic clinopyroxene 

altered to talc, amphibole, and serpentine minerals. Sulfide 

minerals are present in interstitial, postcumulus areas. Olivine clinopyroxenite and wehrlite consist of euhedral to 

subhedral cumulus clinopyroxene, anhedral and locally network textured cumulus olivine, and postcumulus pyroxene, 

brown amphibole, and magnetite.

Clinopyroxene cumulate and feldspathic clinopyroxenite are the most abundant rock types in the ultramafic 

part of the intrusive complex. The transition between 

feldspathic clinopyroxenite and true gabbroic rocks 

(pyroxene cumulate, pyroxene-plagioclase cumulate, and 

homogenous textured gabbro) is difficult to define as a 

contact boundary. Initial mapping placed the contact 

between pyroxenite and gabbroic rocks generally at 

approximately 20 percent modal plagioclase (visual 

estimate); however, further systematic examination of hand 

specimens collected during traverses across the contact 

zone demonstrated that the rocks differ only slightly in 

modal abundance of interstitial postcumulus plagioclase 

and that the increase in abundance of plagioclase is 

irregular and gradual. Therefore, at least the more 

melanocratic varieties of gabbroic rock form part of a 

sequence consanguineous with pyroxenite. Clinopyroxene 

cumulate consists of medium- to coarse-grained, euhedral 

to subhedral clinopyroxene that is locally twinned, has 

iron-titanium oxide exsolution lamellae, and commonly 

contains interstitial plagioclase and less commonly 

magnetite and amphibole. Postcumulus plagioclase is 

altered to tremolite-actinolite, chlorite, and clay minerals. 

Fine-grained, disseminated sulfide minerals such as 

pyrrhotite, pyrite, and less abundant chalcopyrite occupy 

interstitial sites. Feldspathic clinopyroxenite consists of 

cumulus, euhedral to subhedral clinopyroxene, and varying 

amounts of interstitial plagioclase. Magnetite and amphibole are postcumulus phases.

The mafic part of the ultramafic-mafic complex is 

volumetrically more abundant. Mafic rocks are locally foliated and fine grained near the margins of the complex. They 

consist of clinopyroxene cumulate and plagioclase-clinopyroxene cumulate and lesser amounts of plagioclase cumulate, and they are locally laminated and modally layered. 

Plagioclase-clinopyroxene cumulate is composed of 

euhedral to subhedral, cumulus plagioclase and clinopyroxene; postcumulus clinopyroxene, magnetite, and brown 

hornblende fill interstitial spaces. Alteration is widespread, 

and secondary minerals include amphibole, chlorite, and 

epidote.
F6 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

FeO+Fe2O3+TiO2

A1203 MgO

Figure 3. Jensen (1976) diagram for volcanic rocks from the Sierra Verdiin-Cerro Piedra del Supamo study area showing their 

distribution relative to the cation percentages of A12O3, MgO, and (FeO+Fe2O3+TiO2).

GEOCHEMISTRY 

ANALYTICAL METHODS

Rocks collected for chemical analysis in the Sierra 

Verdun-Cerro Piedra del Supamo study area vary in quality 

from fresh outcrops in and along the banks of the Rio Yuruan 

and in the Cerro Piedra del Supamo to weathered float samples along ridges or sides of hills south of the Rio Yuruan. 

The weathering rind on float samples was removed before 

chemical analysis. Each sample collected for geochemical 

analysis was 5-10 kg in size and was crushed and ground to 

-100 mesh powder. The major element oxides A12O3, total 

Fe (reported as FeTO3), MgO, CaO, Na2O, K2O, TiO2, and 

MnO were analyzed by Marta Garcia in the laboratories of 

Tecnica Minera, C.A., by atomic absorption using a

Perkins-Elmer 5000 spectrograph, for SiO2 by the 

gravimetry, and for P2Os and TiO2 by photocolorimetry. 

Abundance of the volatile elements H2O, CO2, F, and Cl and 

the loss on ignition were not determined.

Figure 5 (facing page). Discriminant diagrams for volcanic 

rocks from the Sierra Verdun-Cerro Piedra del Supamo study area, 

Estado Bolivar, Venezuela, showing the chemical affinity of the 

rocks and their tectonic environment of deposition. A, FeO*/MgO 

versus SiO2. FeO* is total iron as FeO. B, SiO2 versus 

(FeO+Fe2O3)/FeO+Fe2O3+MgO. C, TiO2 versus Mg-number. D, 

MnO (x!0)-TiO2-P2O5 (xlO). Fields: CAB, island arc calc-alkaline basalt; IAT, island arc tholeiite; MORE, mid-ocean ridge and 

marginal basin basalt; OIT, ocean island tholeiite; OIA, ocean island basalt (Mullen, 1983).
SiOi, IN WEIGHT PERCENT FeOVMg

ro co

Ti02, IN WEIGHT PERCENT
TOTAL IRON AS Fe2O3, IN WEIGHT PERCENT A1 2 O 3 , IN WEIGHT PERCENT

S

orooioo

 a

-O »

m

3) 

O 

m 

Z _

i i , i , i , i ,

vc

,,,,,,,1,

i | 

1 1 1 1 

K2O, IN WEIGHT PERCENT Na2O, IN WEIGHT PERCENT 

I

V BF i
1

0 

0.4

0.3

5 0.2

0 

1.0

0.5

GEOLOGY AND GEOPHYSICS, SIERRA VERDUN-CERRO PIEDRA DEL SUPAMO F9

ANALYTICAL RESULTS 

VOLCANIC ROCKS

In the Sierra Verdun-Cerro Piedra del Supamo area, all 

of the Early Proterozoic supracrustal rocks have been regionally metamorphosed to greenschist facies. The volcanic 

rocks were classified on a ternary plot that utilizes the cation 

percentages of Al, (Fe2++Fe3+4Ti), and Mg (fig. 3) (Jensen, 

1976) and on a total alkalis-silica diagram (fig. 4A) (Le Bas 

and others, 1986, 1992; Le Bas and Streckeisen, 1991). 

Figures 3 and 4 show that proportions of basalt and basaltic 

andesite are subequal. The volcanic rocks are dominantly 

tholeiitic (figs. 4, 5), and some samples plot as calc-alkalic 

basalt. Two samples that plot as rhyolite on the total 

alkali-silica diagram are altered granite or rhyolite dike. 

Suggested bimodality of the volcanic rocks in the greenstone 

belts of the Guayana Shield (Gibbs, 1987) could not be 

confirmed in this study because of the paucity of silicic 

samples.

The concentrations of the major oxides in these rocks 

are dominantly controlled by fractionation and by the 

compositional changes of plagioclase, pyroxene, and other 

mafic and iron-titanium oxide minerals during magma 

evolution. Magnesia-variation diagrams (fig. 6) show 

characteristic variations of the major-element oxides versus 

MgO. Titania plotted versus Mg-number shows a clear 

evolutionary trend (fig. 5C). A ternary plot of the oxides of 

manganese, titanium, and phosphorous may indicate the 

tectonic environment of deposition of volcanic rocks 

(fig. 5D) (Mullen, 1983). The volcanic rocks of the Sierra 

Verdun-Cerro Piedra del Supamo area were probably 

deposited in an island arc setting.

MAFIC-ULTRAMAFIC ROCKS

Mafic intrusive rocks of the study area are clinopyroxene cumulate, plagioclase-clinopyroxene cumulate, and 

lesser amounts of plagioclase cumulate. Their whole-rock 

major-element-oxide chemical compositions reflect the 

dominant cumulus minerals. The composition of plagioclase-bearing rocks is similar to that of tholeiitic high-iron 

and high-magnesium basalt (fig. 7). On the Jensen cation 

plot (fig. 7), peridotitic rocks plot in the ultramafic komatiite 

field. Oxide abundances plotted on AFM (fig. 8) and 

MgO-variation (fig. 9) diagrams show compositional gaps 

that may correspond to the occurrence of plagioclase on the 

liquidus. Basalt and basaltic andesite in the Sierra Verdun-Cerro Piedra del Supamo area probably are not 

consanguineous with these intrusive ultramafic-mafic rocks.

4 6 8 10 

MgO, IN WEIGHT PERCENT

12 14

Figure 6 (above and facing page). Magnesia-variation diagrams 

for volcanic rocks from the Sierra Verdun-Cerro Piedra del 

Supamo study area, Estado Bolivar, Venezuela.
F10 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

FeO+Fe2O3+TiO2

A1203 MgO

FeO*

V V V V \/ V V V V

Figure 7 (above). Jensen (1976) diagram for mafic-ultramafic 

rocks from the Sierra Verdun-Cerro Piedra del Supamo study area, 

Estado Bolivar, Venezuela, showing their distribution relative to 

the cation percentages of A12O3, MgO, and (FeO+Fe2O3+TiO2).

Figure 8. AFM ((Na2O+K2O)-FeO*-MgO) diagram showing 

geochemical characteristics of mafic-ultramafic rocks from the 

Sierra Verdun-Cerro Piedra del Supamo study area, Estado 

Bolivar, Venezuela. FeO* is total iron as FeO.
GEOLOGY AND GEOPHYSICS, SIERRA VERDUN-CERRO PIEDRA DEL SUPAMO Fll

i i

i i i i i

i , ,

I I I I

, , I ,

10 15 20 25 30 

MgO, IN WEIGHT PERCENT

35 40

20 

18

LLJ 14 

U 

DC

o_ 12 I5 10

LU

^ o .> o

z 

0 6

4

2

0 

3

ILU 

oc 2

m

Q.

1- 

C3

LLJ

Z

o 1

M

2

0 

0.4

t; 0.3

LU

u

DC

LU

o_

I0 0-2

LU

Z

c5

0

c

- ' -

1 '. 1 " f J

 -

 

- -

- ' -

- -

~ i 1 i 1 i N i 1 i. 1 i 1 i 1 i ~

i | , | , | , | i | i | i | i

*_

+ '

m 

' ' , .

. i , i , ^ i i ." i i r . T ,

, | , | , | i | , i i i . i i

*

- " m

" 

- * t .

, i , i , N i i , i , r . T -I

) 5 10 15 20 25 30 35 4

MgO, IN WEIGHT PERCENT

mafic-ultramafic rocks from the Sierra Verdiin-Cerro Piedra de

Supamo study area, Estado Bolivar, Venezuela.

AEROMAGNETIC DATA

Aeromagnetic data for the Guri l:500,000-scale quadrangle (lat 6°-8° N., long 60°-63° W.), including the area of 

the present study, were acquired in 1959 and 1961 by Hunting Surveys (Canada) for the Venezuelan Ministry of Energy 

and Mines through a contract with Goodyear Aeroservice 

(Aeroservicio de Venezuela, C.A.). Flight lines were spaced

approximately 1 km apart and were flown at 152-m mean 

terrain clearance in a N. 22° W. direction that was chosen to 

cut across the strike of regional foliation. Internal evidence, 

such as poor correlation between surveys spaced two years 

apart, suggests that incorrect registration of fiducial marks 

may have led to errors in horizontal location of as much as 

2.5 km. A synoptic interpretive diagram (fig. 10) was manually derived using forward two-dimensional magnetic mod-
F12 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

2.0

1.5

5 1.0

0.5

0 

0.3

i r

i I i I i

=. 0.1

0 

0.4

0.3

a 0.2

i 1 r i T i r

10 15 20 25 30 

MgO, IN WEIGHT PERCENT

35 40

eling for control. The anomalies were classified using the 

steepest tangent method (Vacquier and others, 1951) into 

two rough depth ranges, from superficial (probably outcrop 

or rocks under only a thin veneer of saprolite) to 2.5 km 

depth. Data were reduced and specific areas modeled using 

Geosoft (1988), a magnetic data modeling software package.

Figure 10 shows the interpreted boundaries or buried 

contacts of the source bodies, probably granitic material, 

causing the anomalies derived from various l:50,000-scale 

aeromagnetic contour maps. Areas of almost nonmagnetic 

basement complex (mafic to felsic metalavas and metatuffs 

and mafic-ultramafic intrusive rocks of the greenstone belt) 

were not interpreted. Surprisingly, these rocks exhibited 

only a weak to nondetectable magnetic signature. Preliminary examination of the magnetic susceptibility of samples 

collected in the field area yielded inconsistent results; basaltic rocks showed variations in magnetic susceptibility of 

almost two orders of magnitude, and granitic and gneissic 

rocks had susceptibilities that were 3-10 times greater than 

that of the basalt.

The interpretation presented herein fits in the context of 

a larger interpretation of the Guri 1:500,000-scale quadrangle (Wynn and Sidder, 1991; Cox and others, 1993). The 

crosshatched areas shown in figure 10 represent bodies 

whose magnetic signatures indicate that their tops are within 

500 m of the surface. The enclosed areas without hatching 

represent the outlines of bodies 500 m or deeper. Smaller 

shallow bodies are commonly present within these deeper 

bodies; these are possible superficial expressions of deeper 

plutonic bodies such as apophyses of a deeper intrusion.

Figure 105 shows a detailed geophysical interpretative 

anomaly map of part of the study area (area 1). The detailed 

map shows several lensoidal, west- and northwest-striking 

bodies near the Rio Yuruan and a number of other bodies 

north of the river. Gold is found in placer, alluvial, and 

weathered bedrock workings associated with weathered 

quartz veins and quartz-vein float in the central part of the 

study area and across the Rio Yuruan (see mine symbols). 

These occurrences are superimposed on the eastern half of a 

shallow west-striking feature (anomaly A). Intrusion-related 

gold deposits such as Vuelvan Caras and Omai are present in 

other parts of the Guayana Shield. Other plutonlike expressions in the study area are similar to that of anomaly A and 

may be exploration targets.

Figure 11 is the profile of the magnetic body 

anomaly A. The source body may crop out locally, and it 

dips to the north. Figure 12 shows a geophysical model 

across the west-striking body (anomaly B, fig. IOB) to the 

south of anomaly A. Because the edges of the anomaly are 

compromised by interference with adjacent anomalies, the 

model can only approximately define the source body. The 

model suggests a depth to source of about 500 m.
GEOLOGY AND GEOPHYSICS, SIERRA VERDUN-CERRO PIEDRA DEL SUPAMO F13

6°45'

i i i i

10 KILOMETERS

0

EXPLANATION

Picas (traverses) Shown in figure lOB

Magnetic terrane boundary or extension of narrow dike

Outline of shallow magnetic source Top within 500 m of the surface

Outline of intermediate-depth magnetic source Plutonic source >500 m depth

Outline of reversely polarized magnetic body

 Station

y Mine or prospect

Figure 10 (above and following page). Aeromagnetic interpretation maps of the Sierra Verdun-Cerro Piedra del Supamo study area, 

Estado Bolivar, Venezuela, showing suggested boundaries of buried magnetic sources. A, Map of study area. B, Detail of area 1 shown in A.
F14 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

62°00' 61°52'30"

6°45'

6°39'

A large plutonic source without any shallow expressions 

is at the southern end of the line of profile B-B' (fig. 10A). 

The model of this source (fig. 13) suggests that the source is 

a plutonic body about 3 km wide (north-south) and buried at 

least 2 km deep. It is distinct from the plutonic bodies to the 

north and to the southeast.

Figure 14 shows an attempt to model as a single discrete 

body one of the complex, multilevel anomaly areas that has a 

deep source and superficial expressions along profile A-A' 

(fig. 10). Because the modeling program only used tabular 

bodies, the resulting model is approximate. Nevertheless, the 

model (fig. 14) suggests that the buried body probably has

surface outcrops. This body is almost 5 km in maximum 

diameter and trends N. 22° W.; it may cross the Rio Yuruan 

west of the Rio Supamo intersection.

In the northeastern part of the study area (fig. 1QA, area 

2), two separate strings of long, tabular bodies strike east to 

northeast. The bodies in the southern set of these strings 

probably are deep; those of the northern set crop out on the 

northern bank of the Rio Yuruan and may control its 

direction. The signatures of the anomalies indicate that both 

sets of tabular bodies comprise the same rock type. From 

previous experience in the Guayana Shield, it is possible to 

speculate that the source bodies are diabase dikes.
GEOLOGY AND GEOPHYSICS, SIERRA VERDUN-CERRO PIEDRA DEL SUPAMO F15

10,000

9,000

COui 

O 

< 8,000

EXPLANATION

 Observed 

 Fitted

 Base level

7,000

6,000

5,000

0

2,000

4-000

ui 6,000

8,000

4,000 8,000 12,000 

DISTANCE, IN METERS

16,000 20,000

MODEL PARAMETERS

Model type

Depth X

Half width F

Length left X

Length right X

Dip F

Thickness F

Susceptibility F

Remnance ratio X

Remnance incl X

Remnance decl X

Position F

Base level F

Base slope F

TABULAR 2

393m

2,275 m

1,200m

1,200 m

50°

17,203 m

0.0282 emu

0°

0°

0°

12,386 m

6,558.0604 nT

0.0170704 nT/m

GEOMAGNETIC FIELD 

Field strength 35,000 nT 

Inclination 35° 

Declination -11°

PLAN DIRECTIONS 

Strike prep -22° 

Line direction -22° 

Sensor height 150 m

(F-fitted, X-fixed)

Figure 11. Profile of magnetic body that crosses and is partially parallel with the Rio Yurudn. Line of profile is shown in figure 10.
F16 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

B1 B"

6,900

6,800

CO<

co 6,700

P 

O

z 

<

? 6,600

Q

01

O

O

6,500

6,400

6,300

0

EXPLANATION

 Observed 

 Fitted

500 

1,000

1,500

2,000

16,000 17,000 18,000 19,000

DISTANCE, IN METERS

20,000 21,000 22,000

MODEL PARAMETERS

Model type

Depth

Half width

Dip

Susceptibility

Remnance ratio

Remnance incl

Remnance decl

Position

Base level

Base slope

(F-fitted, X-fixed)

F

F

F

F

X

X

X

F

F

F

TABULAR

462m

493m

49°

0.0137 emu

0

0°

0°

21,836.07 m

6,0 13.1 46 nT

-0.0466693 nT/m

GEOMAGNETIC FIELD 

Field strength 35,000 nT 

Inclination 40° 

Declination -11°

PLAN DIRECTIONS 

Strike perp 0° 

Line direction 0° 

Sensor height 150m

Figure 12. Profile of magnetic body across east-trending body near the Rio Yuruan. Line of profile is shown in figure 10.
GEOLOGY AND GEOPHYSICS, SIERRA VERDUN-CERRO PIEDRA DEL SUPAMO

B

F17 

B1

6,800

6,700

6,600

Q 6,500

UJ

u_ 

O

t

O 6,400

2

6,300

0

1,000

2,000

CO 

QC 

LLJ

H 3,000

-_ 4,000

I

g 5,000

6,000

7,000

EXPLANATION 

 Observed

 Fitted 

 Base level

4,000 8,000 12,000 

DISTANCE, IN METERS

16,000 18,000

MODEL PARAMETERS

Model type

Depth

Half width

Length left

Length right

Dip

Thickness

Susceptibility

Remnance ratio

Remnance incl

Remnance decl

Position

Base level

Base slope

(F,-fitted, X, fixed)

F

F

X

X

F

F

F

X

X

X

F

F

F

TABULAR 2

2,117m

1,368m

1,800m

1,800 m

82°

72,342 m

0.0127 emu

0

0°

0°

15,222.54m

6,447.996 nT

0.0150515 nT/m

GEOMAGNETIC FIELD 

Field strength 35,000 nT 

Inclination 40° 

Declination -11°

PLAN DIRECTIONS 

Strike perp 0° 

Line direction 0° 

Sensor height 150m

Figure 13. Profile of magnetic body on the southern end of line B-B'. Line of profile is shown in figure 10.
F18 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

7,000

6,800

< 6,700

z

6,600

6,500

6,400

0

200

EXPLANATION

 Observed

Z 600

t

g 800

1,000

1,200

22,000 22,800 23,600 

DISTANCE, IN METERS

24,400

MODEL PARAMETERS

Model type

Depth

Half width

Dip

Susceptibility

Remnance ratio

Rem nance incl

Remnance decl

Position

Base level

Base slope

L

F

F

F

X

X

X

F

F

F

TABULAR

9.42m

465 m

55°

0.00400 emu

0

0°

0°

23,068.26 m

6,406.858 nT

0.1016821 nT

GEOMAGNETIC FIELD 

Field strength 35,000 nT 

Inclination 40° 

Declination -11°

PLAN DIRECTIONS 

Strike perp 0° 

Line direction 0° 

Sensor height 150m

(F-fitted, X-fixed, L-limit)

Figure 14. Magnetic profile of a complex, multilevel anomaly area along line A-A' (left to right). Line of profile is shown in figure 10.

MINERAL DEPOSITS

Mining in the Sierra Verdun-Cerro Piedra del Supamo 

area has concentrated on saprolitic and placer alluvial gold 

deposits in the weathered horizon. Newly opened and abandoned prospect pits scattered in the area immediately south 

of the Rio Yuruan in weathered saprolite overlying the 

greenstone-belt sequence may represent oxidized primary

lode deposits. The greenstone belt may contain other types of 

mineral deposits common in Proterozoic greenstone belts 

elsewhere in the world.

A strong penetrative fabric was observed in the 

supracrustal rocks of the study area, and, although major 

ductile shear zones were not observed, they cannot be ruled 

out because of the general lack of outcrop. Several prominent lineaments observed in side-looking airborne radar
GEOLOGY AND GEOPHYSICS, SIERRA VERDUN-CERRO PIEDRA DEL SUPAMO F19

images are interpreted as faults. Quartz-tourmaline veins and 

abundant quartz chips were observed as float immediately 

north of the Rio Yuruan. The combination of quartz veins, 

panned free gold, active prospects, and shallowly buried 

intrusive rocks suggests that the area surrounding and immediately adjacent to the Rio Yuruan, east of the Rio 

Supamo-Rio Yuruan confluence, is an excellent target for 

mineral exploration.

Mafic and ultramafic complexes associated with the 

greenstone belts may host minor deposits of chromite; they 

are sources of placer and lode platinum-group elements and 

are permissive for synorogenic-synvolcanic nickel-copper 

sulfide ore and nickel laterite. Several ultramafic samples in 

the Sierra Verdun-Cerro Piedra del Supamo area contain 

greater than accessory amounts of sulfide minerals, mainly 

pyrrhotite, as intercumulus grains. Altered shear zones that 

cut these bodies may be good targets for talc and serpentine.

CONCLUSIONS

Greenstone-belt rocks in the Sierra Verdun-Cerro 

Piedra del Supamo area of Estado Bolivar, Venezuela, correlate with other Early Proterozoic mafic rocks in the Guayana 

Shield and are age equivalent to the El Callao Formation. 

Mafic-ultramafic intrusive rocks in the area, by contrast, represent a younger regional magmatic event in the development of the Guayana Shield, an event slightly younger to 

contemporaneous with the granitic doming event. The intrusive rocks constitute a zoned, layered complex of cumulus 

dunite, wehrlite, olivine clinopyroxenite, and minor feldspathic pyroxenite enclosed in locally foliated gabbroic rocks. 

Low-lying volcanic rocks consist of intermixed basalt and 

basaltic andesite with minor tuff and tuff breccia. Granitic 

intrusive rocks are scattered throughout the greenstone belt.

A strong penetrative foliation in the greenstone-belt 

rocks of the study area is parallel with the regional contact 

with granitic domes. Although exposure in the jungle is 

poor, several prominent lineaments, schist-rich zones, and 

quartz-tourmaline vein float suggest buried shear zones.

Geochemical data indicate that mafic volcanic rocks in 

the study area are tholeiitic basalt and basaltic andesite. The 

mafic-ultramafic intrusive rocks are not, however, related to 

the volcanic rocks.

Geophysical data suggest that shallow buried intrusive 

rocks are present in the study area. At least one magnetic 

anomaly near the Rio Yuruan suggests an association 

between a magnetic polarizing body, quartz veins, and a gold 

prospect.

Economic concentrations of gold, platinum-group 

elements, nickel-copper sulfide, nickel laterite, serpentine, 

and talc may be present in the study area. The distribution 

and abundance of these commodities are speculative and 

merit more detailed study.

REFERENCES CITED

Benaim, Nesin, 1974, Geologia de la region El Dorado-Anacoco-Botanamo, Estado Bolivar: Conferencia Geologica 

Inter-Guayanas, 9th, Ciudad Guayana, Venezuela, 1972, 

Memoria, Publicacion Especial 6, p. 198-206.

Cordell, L.E., and McCafferty, A.E., 1989, Terracing operator for 

physical property mapping with potential field data: Geophysics, v. 54, p. 621-634.

Cox, D.P., Gray, Floyd, Acosta, Juan, Stewart, J.H., Arespon, 

Jesus, Brooks, W.E., Franco, Luis, Salazar, Edixon, and Lopez, 

Yolanda, 1993, Geologic map of the Guri and southern part of 

the Tucupita 2°x3° quadrangles, Bolivar State, Venezuela: 

U.S. Geological Survey Miscellaneous Field Studies Map 

MF-2242, scale 1:500,000.

Geosoft, Inc. 1988, MAGMOD3 magnetic data modeling software 

package: Suite 500, 204 Richmond Street West, Toronto, 

Canada.

Gibbs, A.K., 1987, Proterozoic volcanic rocks of the northern 

Guiana Shield, South America, in Pharoah, T.C., Beckinsale, 

R.D., and Rickard, D., eds., Geochemistry and mineralization 

of Proterozoic volcanic suites: Geological Society of London 

Special Publication 33, p. 275-288.

Gibbs, A.K., O'Day, P.A., and Renner, Rebecca, 1984, Recent 

work on the Barama-Mazaruni Supergroup of Guyana: Symposium Amazonico, 2nd, Manaus, Brazil, 1984, Anais, p. 

39-52.

Gibbs, A.K., and Olszewski, W.J., Jr., 1982, Zircon U-Pb ages of 

Guyana greenstone-gneiss terrane: Precambrian Research, 

v. 17, p. 199-214.

Gray, Floyd, 1992, Report on reconnaissance look at the Las Cristinas deposit Observations for a new deposit model: U.S. 

Geological Survey Administrative Report, 23 p.

Jensen, L.S., 1976, A new cation plot for classifying subalkalic 

volcanic rocks: Ontario Division of Mines Miscellaneous 

Paper 66, 22 p.

Le Bas, M.J., Le Maitre, R.W., Streckeisen, A.L., and Zanettin, 

Bruno, 1986, A chemical classification of volcanic rocks based 

on the total alkali-silica diagram: Journal of Petrology, v. 27, 

p 745-750.

Le Bas, M.J., Le Maitre, R.W., and Woolley, A.R., 1992, The construction of the total alkali-silica chemical classification of 

volcanic rocks: Mineralogy and Petrology, v. 46, p. 1-22.

Le Bas, M.J., and Streckeisen, A.L., 1991, The IUGS systematics 

of igneous rocks: Journal of the Geological Society, London, 

v 148, p. 825-833.

Menendez V. de V., Alfredo, 1968, Revision de la estratigrafia de 

la Provincia de Pastora segiin el estudio de la region de 

Guasipati, Guayana Venezolana: Boletin de Geologia, Caracas, v. 10, no. 19, p. 309-338.

 1972, Geologfa de la region Guasipati, Guayana Venezolana: Congreso Geologico Venezolano, 4th, Caracas, 1971, 

Memoria, Publicacion Especial 5, v. 4, p. 2001-2046.

Mullen, E.D., 1983, MnO/TiO2/P2O5 A minor element discriminant for basaltic rocks of oceanic environments and its 

implications for petrogenesis: Earth and Planetary Science 

Letters, v. 62, p. 53-62.
F20 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Onstott, T.C., Hall, C.M., and York, Derek, 1989, 40Ar/39Ar 

thermochronometry of the Imataca Complex, Venezuela: 

Precambrian Research, v. 42, p. 255-291.

Reid, A.R., and Bisque, R.E., 1975, Stratigraphy of the 

diamond-bearing Roraima Group, Estado Bolivar, Venezuela: 

Colorado School of Mines Quarterly Bulletin, v. 70, no. 1, 

p. 61-82.

Sidder, G.B., Acosta, E., Brooks, W.E., Contreras, G., Day, W.C., 

Earhart, R.L., Estanga, Y., Franco, L., Garcia, A., Guerra, A., 

Ludington, S., Marcano, L, Marsh, S.P., Martinez, F., Nunez, 

F., Page, N.J, Quintana, E., Rivero, L, Sanchez, H., and Wynn, 

J.C., 1988, Preliminary mineral resource evaluation of the 

Guayana Shield, Bolivar State, Venezuela: Geological Society 

of America, Abstracts with Programs, v. 20, no. 7, 

p. A277-A278.

Tosiani D., Tommaso, and Sifontes G., Ramon, 1989, Asociacion 

de rocas maficas-ultramaficas en la region del Cerro Piedra del

Supamo, sur de El Callao: Congreso Geologico Venezolana,

7th, Barquisimeto, Venezuela, 1989, Memoria, v. 1,

p. 163-174. 

Vacquier, Victor, Steenland, N.C., and Henderson, R.G., 1951,

Interpretation of aeromagnetic maps: Geological Society of

America Memoir 47, 151 p. 

Wynn, J.C., McCafferty, A.E., and Salazar, Edixon, 1989, Geologic

information derived from digital aeromagnetic data: Simposium Sudamericano de COGEODATA, 5th, Caracas, Resumen

de los Trabajos, p. 32-33. 

Wynn, J.C., and Sidder, G.B., 1991, The mineral resource potential

of the NB-20-4 quadrangle, Eastern Guayana Shield, Bolivar

State, Venezuela: U.S. Geological Survey Bulletin 1960, 16 p. 

Yanez, Galo, 1985, Geologia y geomofologfa del Grupo Roraima

en el sureste de Venezuel: Congreso Geologico Venezolan,

6th, Caracas, Ministerio de Energia y Minas, v. 2,

p. 1243-1306.
Gold Prospecting in the Cerro Arrendajo Study Area, 

Estado Bolivar, Venezuela

By William E. Brooks, Acenk Guerra, and Fernando J. Nunez

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-G

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENTS

Abstract........................................................................................................................... Gl

Resumen......................................................................................................................... 1

Introduction.................................................................................................................... 1

Geologic Setting............................................................................................................. 3

Gold Occurrences........................................................................................................... 3

Low-Sulfide Gold-Quartz Vein Model........................................................................... 5

Conclusions ................................................................................................................... 5

References Cited............................................................................................................. 8

FIGURES

1. Map showing location of Cerro Arrendajo study area................................................................................................ G2

2. Geologic map of Cerro Arrendajo study area.............................................................................................................. 4

TABLES

1. Analytical results for rocks collected in the Cerro Arrendajo study area.................................................................... G6

2. Analytical results for rocks collected in the Cerro Arrendajo study area.................................................................... 7
Gold Prospecting in the Cerro Arrendajo Study Area, 

Estado Bolivar, Venezuela

By William E. Brooks, 1 Acenk Guerra,2 and Fernando J. Nufiez2

ABSTRACT

The Cerro Arrendajo study area is in dense jungle in the 

eastern part of Estado Bolivar, Venezuela, approximately 75 

km southwest of the mining town of El Dorado. Early 

Proterozoic greenstone of the study area comprises two 

lithologic terranes: mafic flows in the southern terrane and 

gabbro in the northern terrane. Small areas of granite crop 

out in both terranes. Quartz veins, with and without tourmaline, are present in rocks of the southern terrane. The grade 

of metamorphism of rocks in the study area is low, and the 

rocks are not sheared.

Gold prospecting is indicated in both terranes by the 

presence of numerous prospect pits and tailings. Visible gold 

is present only in the southern terrane, mainly in 

stream-sediment samples from a drainage downslope from a 

tourmaline-bearing quartz vein. The gold content of the 

mafic flows and the granite is as high as 9 ppb and 16 ppb, 

respectively. A fine-grained silicic dike in the southern 

terrane contains 13 ppb gold. Tourmaline-bearing quartz 

veins, and possibly granite, are the more promising 

exploration targets in the area.

RESUMEN

El area de estudio en el Cerro Arrendajo esta localizada 

en selva densa en la parte oriental del Estado Bolivar, aproximadamente a 75 km al suroeste del pueblo minero de El 

Dorado. Las rocas verdes de edad Proterozoico Temprano 

del area comprenden dos terrenos litologicos: (1) flujos volcanicos maficos en la parte sur y (2) gabro en la parte norte. 

La rocas graniticas forman pequenos afloramientos en 

ambos terrenos. Vetas de cuarzo con o sin turmalina estan 

presentes en rocas del terreno sur. El grado metamorfico de 

todas las rocas es bajo y no estan cizalladas.

^.S. Geological Survey, Denver Federal Center, MS905, Denver, 

Colorado 80225.

2Corporaci6n Venezolana de Guayana, Tecnica Minera, C.A., C.C. 

Chilemex, Piso 1, Puerto Ordaz, Venezuela.

La exploration aurifera en toda el area esta indicada por 

la presencia de numerosos huecos de prospeccion y colas 

asociadas. El oro visible esta presente solamente en el 

terreno sur, principalmente en muestras de sedimentos en 

una quebrada cuesta abajo de una veta de cuarzo con turmalina. El contenido de oro de los flujos maficos y del granito 

alcanza 9 ppb y 16 ppb respectivamente. Un dique siliceo de 

grano fino en el terreno sur contiene 13 ppb de oro. Las vetas 

de cuarzo con turmalina y posiblemente el granito son areas 

con potencial mineral.

INTRODUCTION

The Cerro Arrendajo study area is in Estado Bolivar, 

Venezuela, in dense jungle 75 km southwest of El Dorado 

(fig. 1). Abandoned huts in the study area indicate seasonal 

occupation by indigenous people. Access to the 10-km2 

study area is by helicopter and trails. Rio Chicanan is the 

major named geographic feature in the study area.

The study area is underlain by Early Proterozoic greenstone of the Guayana Shield, which forms the northern part 

of the Amazon platform (Gibbs and Barron, 1983). Gabbro 

and granite comprise the northern part of the study area, and 

mafic flows, granite, and quartz veins comprise the southern 

part (fig. 2).

During previous reconnaissance of the Cerro Arrendajo 

study area, Guerra and others (1987) found numerous 

flooded prospect pits and tailings in both the northern and 

southern areas that indicated recent gold prospecting. A 

summary of work in the Cerro Arrendajo study area was 

included in an overview of three auriferous zones near El 

Dorado by Alberdi and Contreras (1988). During our work 

in 1988, visible gold was found in stream-sediment samples 

taken from drainages within the approximately 10-km2 area 

that was traversed (fig. 2). Because topographic maps have 

not been published, sample sites and contacts were compiled 

on a drainage base made from aerial photographs; due to 

dense jungle cover, locations of faults and contacts are 

inferred.

Gl
G2 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

69'

Boundary of Venezuelan 

Guayana Shield

Cerro Arrendajo

study area 

Kilometer 88

Figure 1. Location of Cerro Arrendajo study area, Estado Bolivar, Venezuela.
GOLD PROSPECTING, CERRO ARRENDAJO G3

GEOLOGIC SETTING

The Cerro Arrendajo study area is within an Early 

Proterozoic (2,250-2,100 Ma) greenstone terrane (Gibbs 

and Barron, 1983; Sidder and Mendoza, this volume) in the 

Venezuelan part of the Guayana Shield. It is approximately 

150 km south of the El Callao and Lo Increible mining 

districts. Gold ore at El Callao and Lo Increible is associated 

with metamorphosed Early Proterozoic basaltic to andesitic 

lavas of the El Callao Formation that host quartz veins containing tourmaline, native gold, pyrite, and calcite (Pasquali 

and Bisque, 1975; Vasquez, 1988; Day and others, this 

volume). The El Callao Formation has been regionally metamorphosed to greenschist facies, and the gold mines and 

prospects are spatially associated with quartz veins within 

shear zones (Day and others, this volume).

Greenstone of the study area comprises two lithologic 

terranes (fig. 2). The northern terrane, which is rarely 

exposed and is deeply weathered, is composed of greenish-gray, fine-grained gabbro. Quartz veinlets and mafic 

dikes a few centimeters thick cut the gabbro. The southern 

terrane is composed of weakly metamorphosed, foliated to 

nonfoliated, greenish-gray, mafic volcanic rocks. Lava flows 

are common, and subordinate flow breccias and pillow lavas 

are present in small areas in the headwaters of Quebrada 

Paranpin and in an unnamed stream approximately 1 km 

northeast of the heliport (fig. 2). Rocks of the southern terrane range from basalt to andesite in composition (47-56 

weight percent SiC>2).

The gabbro of the northern terrane and the mafic flows 

of the southern terrane contain small areas of biotite granodiorite to granite containing 69-77 weight percent SiC>2 

(table 1). These greenish-gray intrusive rocks are weakly 

metamorphosed and contain clots and phenocrysts of 

albitized feldspar. Several milky quartz veins and a tourmaline-bearing quartz vein in the southern terrane cut the mafic 

units.

In the southern terrane, an inclusion of mafic rock 

(approximately 1 m by 0.5 m) in the southernmost granite 

outcrop (unit Xg, fig. 2) is interpreted to be a clast of the 

adjacent mafic country rock. This relation indicates that 

intrusion of the granite occurred after emplacement of the 

mafic flows. This outcrop was under a meter of water in a 

pool, and within the limited exposure in the stream juxtaposition of the rocks due to shearing was not apparent.

A northeast-striking inferred fault was mapped in the 

north-central part of the study area in Quebrada Arrendajo.

Despite the intense weathering and thick jungle cover that 

obscure most geologic relations, inference of the fault is 

allowed based on the following field observations: outcrops 

of mafic flows are present in the streams east of the fault and 

are absent to the west; topography changes abruptly with 

hills to the east absent to the west; and mafic flows and 

quartz veins along the inferred fault are brecciated.

The contact between the gabbro (unit Xgb) and the 

mafic flows (unit Xm) is inferred to be along a drainage 

divide between Quebradas Paranpin and Arrendajo (fig. 2). 

The nature of the contact is unknown.

Numerous quartz veins, having apparently random 

orientation, cut the rocks of the southern terrane. The veins 

are commonly a few centimeters thick, but may be as much 

as a meter thick; they are milky white, unbrecciated, and 

unsheared. Neither calcite nor sulflde minerals were identified in the veins. One tourmaline-bearing quartz vein was 

mapped in the central part of the southern terrane. 

Stream-sediment samples taken downslope from the tourmaline-bearing quartz vein contained as many as 15 flakes of 

gold (sample CH402S, fig. 2).

Both rock and stream-sediment samples were taken in 

the study area. Rock samples were sent to U.S. Geological 

Survey laboratories in Denver for analytical work. 

Stream-sediment samples were panned in the field, the number of flakes of gold per sample was recorded, and the 

panned concentrates were retained by Corporation Venezolana de Guayana, Tecnica Minera, C.A. Stream-sediment 

samples were taken at every stream crossing; however, only 

six contained flakes of gold. Locations of analyzed rock 

samples and the stream-sediment samples containing gold 

are shown in figure 2.

GOLD OCCURRENCES

The northern and southern parts of the Cerro Arrendajo 

study area differ in geology and gold potential. Alluvial gold 

was reported by Guerra and others (1987) in six stream-sediment concentrates from the study area; however, the 

number of flakes of gold per sample was not recorded. During this study, gold was not found in panned concentrates 

from the northern terrane. Physical evidence of alluvial prospecting was found downstream from a granite outcrop. Gold 

content of the granite is 1 and 14 ppb (samples CH405R, 

CH404R, respectively; table 1).
G4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

62°00' 61°50'

..- -

: Vm . J^''/^i i j

r r T\ JT

CH405S, 7/4 N CH409 (A,B,C) R 

CH403S,7/4 / ' ^ ~vSv--CH410R 

AX V^-XqCH411R 

CH407R f :' X, \ * *T>°

\ CH400R 

\CH401R

Xm ., A

2000 METERS

J______I

Figure 2. Geology of the Cerro Arrendajo study area, Estado Bolivar, Venezuela. Latitude and longitude are approximate. Drainage base 

by A. Guerra, taken from aerial photographs; misi6n 0501109, fotos 1.105-1.108, escala 1:50,000, Proyecto Inventario, Recursos Naturales, 

Gerencia General, Desarrollo Regional, Corporation Venezolana de Guayana.
GOLD PROSPECTING, CERRO ARRENDAJO G5

CH402R

CH4035,

7/4

A

EXPLANATION

Granite (Early Proterozoic) Weakly metamorphosed, 

grayish-green to white. Clots and phenocrysts of 

albitized feldspar as much as 3 mm, quartz, biotite, 

chlorite, and sparse zircon

Gabbro (Early Proterozoic) Grayish-green, fine-grained, 

weakly metamorphosed equigranular gabbro containing 

albitized feldspar, amphibole, and opaque minerals. Cut by 

millimeter-sized quartz veinlets. Poorly exposed

Mafic flows (Early Proterozoic) Greenish-gray, fine-grained, 

basaltic to andesitic flows, breccia, and pillow lavas. 

Weakly metamorphosed, contains chlorite and epidote, 

foliated to nonfoliated. Sparse pyrite

Contact Approximately located 

Fault Approximately located 

Quartz vein

Tourmaline-bearing quartz vein 

Mafic dike 

Prospect 

Sample locality

Analyzed rock sample and field number Analytical 

results shown in table 1

Stream-sediment sample containing flakes of gold 

Sample number, flakes of gold per number of 

pans are also shown

Heliport (cleared for this study)

LOW-SULFIDE GOLD-QUARTZ VEIN 

MODEL

The presence of quartz veins in this greenstone terrane 

suggests that the low-sulfide gold-quartz vein deposit model 

(Berger, 1986) could serve as an exploration guide at Cerro 

Arrendajo, especially if the model is expanded to include the 

presence of tourmaline-bearing quartz veins. Low-sulfide 

gold-bearing quartz veins are present in greenstone belts as 

massive veins in regionally metamorphosed volcanic rocks 

and volcanic-derived sedimentary rocks. Quartz veins 

mapped from float on hill slopes and gold recovered from the 

quebradas (ravines) indicate that deposits of this model type 

are permissive in the study area; however, volcanic-derived 

sedimentary rocks were not found in the study area nor are 

sulfide minerals present in the quartz veins. The identification of tourmaline-bearing quartz veins, even though not 

strictly included in the model, is important in the origin of 

gold in the study area. Therefore, the presence of quartz 

veins, even without sulfide minerals, and gold in streams 

downslope from tourmaline-bearing quartz veins indicates 

low potential for low-sulfide gold-quartz veins in the study 

area.

CONCLUSIONS

Alluvial gold is present in six stream-sediment samples 

only in the southern terrane (fig. 2). Rakes of gold were 

found primarily in stream-sediment samples taken downstream from tourmaline-bearing veins in the central part of 

the study area (fig. 2).

Rakes of gold were not found in stream-sediment samples taken near the granitic rocks; however, chemical analyses indicate that the gold content of the granite is ~1 and 16 

ppb (samples CH405R and CH422R, respectively, table 1). 

A fine-grained silicic dike contains 13 ppb gold (sample 

CH408, table 2). Gold content of the mafic flows is low; 

however, the sample having the highest gold content (9 ppb, 

sample CH409BR; table 1) was taken near the silicic dike 

that contains 13 ppb gold.

Similarities between the Cerro Arrendajo study area 

and the El Callao mining district of Estado Bolivar, 

Venezuela, include possibly correlative mafic flows cut by 

quartz veins. In the Cerro Arrendajo study area, however, the 

degree of metamorphism is less and the rocks are not 

sheared, two important factors in the genesis of gold at El 

Callao.

Tourmaline-bearing quartz veins may be the primary 

source of gold in the Cerro Arrendajo study area based on the 

quantity of flakes of gold collected downstream from these 

types of veins. The gold content of the granite and granitic 

dikes (-1-16 ppb) indicates a possible secondary source of 

gold because gold was not found downstream from the granite outcrops. Reconnaissance mapping would be useful in 

finding more tourmaline-bearing quartz veins, granite outcrops, and, perhaps, shear zones. The low-sulfide 

gold-quartz vein model, if expanded to include the tourmaline-bearing quartz veins, is an important part of exploration 

strategy at Cerro Arrendajo.
Table 1. Analytical results for rocks collected in the Cerro Arrendajo study area, Estado Bolivar, Venezuela.

[Location of samples shown by field number in figure 2; rock units described in figure 2. Major element oxides (weight percent, uncorrected) were determined by 

Taggart, A. Bartel, and D. Siems; FeTOs indicates total iron reported as Fe2C>3. Trace elements (in parts per million) determined by an energy-dispersive analyzer C

and an Am95 source for Ba; analyst: E. Rivera; N, not detected at value listed; L, limit of determination. Spectrographic analyses are reported in parts per million u

Lower limit of detection is given in parentheses after element symbol; N, not detected at lower limit of determination shown; L, detected but below lower limit of d

absorption analyses are reported in parts per million; analyst J. McHugh]

Mafic flows (unit Xm)

Lab No.

Field No.

D-3 16942

CH400R

D-3 16943

CH402R

D-3 16944

CH403R

D-3 16947

CH406R

D-3 16948 D-3 16951 D-3 16952

CH407R CH409BR CH409CR

D-3 16955

CH413R

D-3 16945

CH404R

G

D-3 

CH

Major elements

SiO2

A1203

FeTO3

MgO

CaO

Na2O

K2O

TiO2

P205

MnO

LOI 900°C

Total

50.8

13.6

16.7

6.30

7.43

2.46

0.15

1.34

0.12

0.25

1.41

100.56

49.2

14.8

11.3

8.59

13.9

1.29

0.1

0.63

0.05

0.18

0.77

100.81

51.3

14.4

12.4

7.19

9.99

1.85

0.52

0.92

0.1

0.20

1.75

100.62

56.5

15.1

10.0

4.72

6.37

2.29

0.64

0.78

0.08

0.14

3.30

99.92

47.2 50.8

14.9 14.3

12.9 11.1

8.92 7.90

11.5 12.4

1.22 2.19

0.03 0.11

0.67 0.63

<0.05 0.05

0.22 0.19

2.54 0.73

100.10 100.40

46.6

16.3

11.7

8.10

12.8

1.17

0.09

0.55

<0.05

0.17

2.30

99.78

50.8

13.4

12.1

9.38

10.2

2.06

0.04

0.69

0.06

0.22

1.30

100.25

74.1

13.2

1.97

0.34

1.45

2.87

4.19

0.18

<0.05

<0.02

0.85

99.15

77

1

0

3

2

0

<0

0

0

99

Trace elements

Rb

Sr

Y

Zr

Nb

Ba

14

178

24

76

7

12L

9

113

18

31

6L

12L

30

148

22

89

7

202

17

161

19

204

10

300

8L 12

197 119

16 14

39 39

6L 6L

24 36

8L

114

18

37

6L

14

8L

85

18

38

6L

21

108

135

19

143

9

939

71

142

23

103

1,116

Spectrographic analyses

Ca (percent) (0.05)

Fe (percent)(0.05)

Mg (percent)(0.02)

Na (percent)(0.2)

P (percent)(0.2)

Ti (percent) (0.002)

Ag(0.5)

As (200)

Au (10)

B(10)

Ba (20)

Be(l)

Bi (10)

Cd (20)

Co (10)

3

7

3

1.5

N0.2

0.3

N0.5

N200

N10

N10

30

Nl

N10

N20

50

7

7

3

1

N0.2

0.3

N0.5

N200

N10

N10

N20

Nl

N10

N20

70

7

7

3

1.5

N0.2

0.5

N0.5

N200

N10

N10

300

Nl

N10

N20

50

3

5

1.5

2

N0.2

0.3

N0.5

N200

N10

N10

500

<1

N10

N20

30

10 10

10 7

3 3

1.5 1.5

N0.2 N0.2

0.3 0.3

N0.5 N0.5

N200 N200

N10 N10

N10 N10

20 30

Nl Nl

N10 N10

N20 N20

100 70

10

5

3

1.5

N0.2

0.15

N0.5

N200

N10

10

20

Nl

N10

N20

30

10

7

7

2

N0.2

0.3

N0.5

N200

N10

N10

30

Nl

N10

N20

70

0.7

2

0.5

2

N0.2

0.1

N0.5

N200

N10

10

1,500

2

N10

N20

10

N

N

N20

N1

1

1,00

N1

N2

1
GOLD PROSPECTING, CERRO ARRENDAJO G7

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>n o «n o

8

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«n o o >n

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>n o o o

8

>n o o o i O O

§

«n o o o V ;-; o o

8

r-H O 04

§

0000 

en C >n o

82288

1 1 S g fi

o o o o o

 04 04 O O

* S"

^z v|2

o o «n o o en 04 «- 1 O «n 

Z go o o o o

en 04 04 O O

 7; O4 O4

x z

o o o o o

«n 04 04 O en *"H z rj

z

o o >n o o

 n 04 >-H o en

 Z ^

Z

o o o o o

>n 04 04 o v>

» i "7 O4

z z

§

0000 04 04 O «n

« Z g

o o o o o

«n 04 04 o o

^H !Z 04 > i A Z

o o o o o 

>n 04 04 o o

- Z g -

S

o o o o 04 04 o en 04 *Z. 04 A z

o o o o o

O 04 04 O O

04 Z 7j

2 o?2 cl 

> ^ JH N Nl

'5

"a

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i

0

Cfl a

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1

8

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V

VO

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o

oo 

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8

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0

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v

^

Table 2. Analytical data for rocks collected in the

Cerro Arrendajo study area, Estado Bolivar, Venezuela.

[Location of samples shown by field number in figure 2. In parts per

million unless noted. Spectrographic analyses, analyst R. Hopkins;

lower limit of detection is shown in parentheses following element

symbol; N indicates not detected at lower limit of determination shown;

L indicates detected but below lower limit of determination shown.

Atomic absorption analyses, analyst J. McHugh]

Altered inclusion

Silicic dike Quartz vein in granite

Lab No. D-316949 D-316950 D-316953

Field No. CH408 CH409A CH410

Spectrographic analysis

Ca (percent) (0.05) 0.7 3 7

Fe (percent) (0.05) 3 35

Mg (percent) (0.02) 0.7 0.5 5

Na (percent) (0.2) 2 N0.2 2

P (percent) (0.2) N0.2 N0.2 N0.2

Ti (percent) (0.002) 0.15 0.1 0.2

Ag(0.5) N0.5 N0.5 N0.5

As (200) N200 N200 N200

Au(10) N10 N10 N10

B(10) <10 10 N10

Ba(20) 2000 20 70

Be(l) <1 Nl Nl

Bi(10) N10 N10 N10

Cd(20) N20 N20 N20

Co (10) <10 15 50

Cr(10) 50 150 500

Cu(5) 7 30 50

Ga(5) 30 15 20

Ge(10) N10 N10 N10

La (50) <50 N50 N50

Mn(10) 300 200 1000

Mo (5) N5 N5 N5

Nb(20) N20 N20 N20

Ni(5) 7 20 70

Pb(10) 20 <10 10

Sb(lOO) N100 N100 N100

Sc(5) <5 7 20

Sn(10) N10 N10 N10

Sr(lOO) 200 N100 N100

Th(lOO) N100 N100 N100

V(10) 70 100 150

W(20) N20 N20 N20

Y(10) <10 N10 20

Zn(200) N200 N200 N200

Zr(10) 150 20 20

Atomic absorption analysis

Au 0.013 0.001 0.003
G8 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

REFERENCES CITED

Alberdi, M, and Contreras, G., 1988, Tres prospectos auriferos en 

las cabeceras del Rio Chicanan: El Piton de Uroy, El Cerro 

Arrendajo y El Hacha: Primer Simposio Internacional del Oro 

en Venezuela, Caracas, p. 336-352.

Berger, B.B., 1986, Descriptive model of low-sulfide Au-quartz 

veins (model 36a), in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 

239.

Gibbs, A.K., and Barron, C.N., 1983, The Guiana Shield reviewed: 

Episodes, v. 1983, no. 2, p. 7-14.

Guerra, Acenk, Franco, Luis, and Alberdi, Margarita, 1987, Trabajo 

de campo geologico-geoquimico en el area del Cerro Arrendajo: Corporation Venezolana de Guayana, Tecnica Minera, 

C.A., Prospecciones Geologicas, Proyecto Canaima, Informe, 

5 PPasquali Z., Jean, and Bisque, R.E., 1975, Interpretation of soil 

geochemical expression of mineralization within El Callao 

gold mining district, Venezuela: Quarterly of the Colorado 

School of Mines, v. 70, no. 1, p. 83-107.

Vasquez, J.C., 1988, El Ministerio de Energia y Minas y la exploration aurifera: Primer Simposio Internacional del Oro en 

Venezuela, Caracas, p. 300-324.
Gold Resources of the Rio Chivao Study Area, 

Estado Bolivar, Venezuela

By William E. Brooks, Acenk Guerra, and Fernando J. Nunez

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-H

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENTS

Abstract........................................................................................................................... HI

Resumen......................................................................................................................... 1

Introduction..................................................................................................................... 1

Geologic Setting............................................................................................................. 1

Description of Deposit.................................................................................................... 4

Conclusions..................................................................................................................... 4

References Cited............................................................................................................. 4

FIGURES

1. Map showing location of Rio Chivao study area........................................................................................................ H2

2. Outcrop and sample location map of Rio Chivao study area...................................................................................... 3

TABLE

1. Gold content of samples collected in the Rio Chivao study area................................................................................ H4
Gold Resources of the Rio Chivao Study Area, 

Estado Bolivar, Venezuela

By William E. Brooks, 1 Acenk Guerra,2 and Fernando J. Nunez2

ABSTRACT

The Rio Chivao study area, about 20 km2, is in the eastern part of Estado Bolivar, Venezuela, in moderately dense 

jungle near the junction of Rios Chicanan and Chivao. It is 

approximately 50 km southwest of the mining center of El 

Dorado.

In the Rio Chivao study area, steeply dipping, highly 

altered quartz-mica schist that is cut by quartz veins is 

exposed in an irregularly shaped, 3-4-km2 open pit that consists of numerous individual excavations. These Early Proterozoic rocks contain as much as 34 ppm Au, and visible 

gold was present in panned concentrates collected from 

drainages near the open pit. Other rock types in the area 

include pegmatite, indicated by sparse float, and mafic 

volcanic rocks.

Gold is mined by spraying the walls of the open pit with 

a strong stream of water, and the resulting slurry is pumped 

to a shaker table for recovery of the gold. Miners also pan 

gold directly from the mud using wooden gold pans.

RESUMEN

El area de estudio en el rio Chivao es de 20 km2, esta 

localizada en la parte oriental del Estado Bolivar cerca de la 

confluencia de los rios Chicanan y Chivao y esta moderadamente cubierta por selva densa. El area esta aproximadamente a 50 km al sur del centre minero de El Dorado.

En el area de estudio estan expuestos esquistos cuarzo 

micaceos cortados por vetas de cuarzo en una excavation 

minera que consiste de numerosos huecos de prospeccion 

individuates en un area de 3 a 4 km2. Estas rocas de edad 

Proterozoico Temprano contienen hasta 34 ppm de oro. Oro 

visible esta presente en concentrados de batea colectados en

'U.S. Geological Survey, Denver Federal Center, MS905, Denver, 

Colorado 80225

2Corporacion Venezolana de Guayana, Tecnica Minera, C.A., C.C. 

Chilemex, Piso 1, Puerto Ordaz, Venezuela.

drenajes cerca de los huecos mineros. El area incluye otros 

tipos litologicos tales como rocas volcanicas maficas y pegmatita, las cual es estan indicada por fragmentos aislados.

El oro es minado aplicando un fuerte chorro de agua a 

las paredes de los huecos, el agua remueve el material aurifero el cual a su vez es bombeado a un cajon de recuperation 

(sluice box). El oro tambien es recuperado directamente del 

sedimento con bateas de madera.

INTRODUCTION

The Rio Chivao study area is in Estado Bolivar, 

Venezuela, approximately 50 km southwest of El Dorado 

(fig. 1). Access is by helicopter from El Dorado or Kilometer 

88 or by motorboat. The study area was described by 

Gutierrez (1987), and his reconnaissance of the 20-km2 

study area indicates an active open-pit gold mine and gold in 

nearby drainages (fig. 2). Existing mining trails and new 

trails cut for this study were followed in order to collect rock 

and stream-sediment samples from the open pit and adjacent 

areas.

GEOLOGIC SETTING

The Rio Chivao study area is in a part of the Guayana 

Shield that contains undifferentiated Early Proterozoic 

granitic and metamorphic rocks (Gibbs and Barren, 1983). 

Rock types described in the study area by Gutierrez (1987) 

include schist of the Yuruari Formation, granite and pegmatite of the Supamo Complex, and unnamed mafic volcanic 

rocks.

During our traverses, quartz-mica schist (mainly in the 

open pit), pegmatite (float sample only), and mafic volcanic 

rocks were observed (fig. 2). The quartz-mica schist (unit 

Xs) is steeply dipping, commonly weathered to clay, and 

usually red to yellow. This highly altered schist is gold bearing, has been flooded with quartz and minor sericite, and is 

cut by gold-bearing quartz veins. Millimeter-sized cubic 

vugs that may have hosted pyrite (?) are now filled with

HI
H2

69°

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

66° 63° 60°

VENEZUELA

Boundary of Venezuelan 

Guayana Shield

ESTADO 

\ DELTA AMACURO

Figure 1. Map showing location of Rio Chivao study area, Estado Bolivar, Venezuela.
GOLD RESOURCES OF THE RIO CHIVAO STUDY AREA H3

6°19'

6°17' -

2000 METERS

EXPLANATION

Schist (Proterozoic?) Pink-white to red, fine-grained, thinly 

laminated quartz-mica schist. Intense weathering and 

alteration have leached original pyrite

Pegmatite (Proterozoic?) No outcrop. One heavily stained float 

sample containing quartz and centimeter-sized feldspar was 

collected

Metabasalt (Proterozoic) Dark-gray to green aphanitic mafic 

volcanic rock containing minor pyrite

Contact Approximately located 

Vertical foliation 

Inclined foliation

H I I Quartz vein

 

/\ *« Open pit 

' .

X Prospect

CV007

3/3 

P23

A

Sample locality Gold content and sample type 

are given in table 1

Stream-sediment sample Flakes of gold and 

number of pans are shown

Stream-sediment sample of Guttierrez (1987) 

containing visible gold

Heliport (if river level is low)

Figure 2. Outcrop and sample location map of the Rio Chivao study area, Estado Bolivar, Venezuela. Drainage base from Catastro 

Minero Nacional, hoja 7834-IV-NE, escala 1:25,000.
H4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 1. Gold content of samples collected in the Rio Chivao 

study area, Estado Bolivar, Venezuela. 

[In parts per million. Locations shown by field number in figure 2. 

Atomic absorption analysis by B. Roushey]

Field No.

CV002 

CV006 

CV007 

CV009 

CV011

Lab No.

D3 16962 

D3 16963 

D3 16964 

D3 16965 

D3 16966

Gold

0.05 

0.50 

2.70 

0.80 

34.0

Sample description

Schist. 

Quartz vein. 

Schist. 

Schist. 

Quartz vein.

polycrystalline quartz. A piece of deeply weathered 

pegmatite float (unit Xp) (8-10 cm long) containing a 

centimeter-size euhedral feldspar was found about 2 km east 

of the open pit. In the sands of Rio Chivao, abundant muscovite (5 mm long), quartz, and feldspar grains indicate limited 

transport of detritus from a nearby coarse-grained silicic 

plutonic source, perhaps Supamo granite. During a river 

traverse, a small (about 25 m2) isolated outcrop of 

pyrite-bearing metabasalt (unit Xm) was found approximately 2-3 km east of the active mine (fig. 2).

The nature and location of lithologic contacts in the 

study area are unknown because of the heavy jungle cover, 

sparse outcrops, and distance between outcrops, which may 

be as much as 2 km. During this study, scintillometer readings taken during traverses were uniform and did not help to 

locate contacts or differences in radiation between rock units 

that might be used to infer a contact.

DESCRIPTION OF DEPOSIT

The center of mining activity is an irregularly shaped 

open pit, approximately 3-4 km2, that consists of numerous 

individual excavations (fig. 2). Discussions with the miners 

indicated that gold is present within the schist, along faults, 

and in the quartz veins. Gold values in the mine area are 

0.8-2.7 ppm for schist and 0.5-34 ppm for quartz veins 

(table 1). The dark-gray to yellow quartz veins are called 

"piedra brava" (fierce or resistant rock) by the miners.

Individual excavations, some as deep as 15 m, are dug, 

then the walls are sprayed with strong streams of water, and 

the sediments are pumped to a shaker table for recovery of

the ore. Gold is panned directly from the mud washed down 

from the walls of the mine and from sediments derived from 

small outcrops of schist in Rio Chicanan. A drop of mercury 

is usually added to the gold pan by the miners in order to 

amalgamate the gold. Gold-bearing sands and gravels in Rio 

Chicanan and its tributaries in the study area result from eluvial decomposition of the underlying gold-bearing schist.

During traverses in the Rio Chivao area, stream-sediment samples were washed and examined for visible gold. 

This method utilizes a conical wooden gold pan, called a 

batea, that is about 50 cm in diameter. Visible gold was 

found in panned concentrates taken at stream crossings north 

and south of the open pit (fig. 2). Gutierrez (1987) reported 

visible gold in several stream-sediment samples from drainages in the study area (fig. 2); however, the number offtakes 

of gold in each sample was not reported.

CONCLUSIONS

The regional geologic setting at Rio Chivao, Estado 

Bolivar, Venezuela, is little known due to poorly exposed 

and sparse outcrops, distance between those outcrops, and 

the difficulty in detecting and defining contacts. For the Rio 

Chivao area, the high gold content of the quartz veins and 

schist in the mine and the presence of gold in stream-sediment samples outside of the mine area fits the descriptive 

model of low-sulfide gold-quartz veins (Berger, 1986). 

There is high potential for gold in the area of the mine but, 

too little is known of the geology outside the mine area to 

make a sound regional appraisal.

REFERENCES CITED

Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz 

veins (model 36a), in Cox, D.P., and Singer, D.A., eds., 

Mineral deposit models: U.S. Geological Survey Bulletin 

1693, p. 239.

Gibbs, A.K., and Barren, C.N., 1983, The Guiana Shield reviewed: 

Episodes, v. 1983, no. 2, p. 7-14.

Gutierrez, Jose, 1987, Informe de avance sobre el trabajo de campo, 

efectuado en el Rio Chivao desde el 29/10 al 05/11/87: 

Corporation Venezolana de Guayana, Tecnica Minera, C.A., 

Informe, 5 p.
Estratigrafia del Grupo Roraima Interpretation Petrologica

en la Region Noreste del Parque Nacional Canaima,

Estado Bolivar, Venezuela

(Stratigraphy of the Roraima Group and Petrologic Interpretation

in the Northeast Region of Canaima National Park,

Estado Bolivar, Venezuela)

By Margarita Alberdi and Gloria Contreras

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-1

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENIDOS

Resumen......................................................................................................................... II

Abstract........................................................................................................................... 1

Introduction.................................................................................................................... 2

Resultados....................................................................................................................... 2

Description Litoldgica de las Formaciones.................................................................... 15

Formation Urico, Miembro I.................................................................................. 15

Formation Urico, Miembro II................................................................................ 15

Formation Urico, Miembro III............................................................................... 16

Capasde Abaren..................................................................................................... 16

Ambientes Sedimentarios............................................................................................... 16

Formation Urico, Miembro I.................................................................................. 16

Formation Urico, Miembro II................................................................................ 17

Formation Urico, Miembro III............................................................................... 17

Capas de Abaren..................................................................................................... 17

Problema de Correlation con Secuencias Conocidas..................................................... 17

Conclusiones................................................................................................................... 18

Recomendaciones para Trabajos en el Future................................................................ 18

Bibliografia..................................................................................................................... 18

	FIGURAS

1. Piano regional mostrando el area de estudio............................................................................................................... 13

2. Piano mostrando las posiciones de las ocho secciones estratigraficas........................................................................ 4

3. Columna estratigrafica de Rio Urico-Occidental........................................................................................................ 5

4. Columna estratigrafica de Rio Urico-Oriental............................................................................................................ 6

5. Columna estratigrafica de la Quebrada la Franela...................................................................................................... 7

6. Columna estratigrafica de la Quebrada Abaren........................................................................................................... 8

7. Columna estratigrafica del Rio Uroy-Occidental........................................................................................................ 9

8. Columna estratigrafica del Rio Uroy-Oriental............................................................................................................ 10

9. Columna estratigrafica en el Rio Chivao..................................................................................................................... 11

10. Columna estratigrafica en el Hacha-Serrania de Lema, Quebrada Hacha.................................................................. 12

11. Correlation de las ocho columnas estratigraficas ....................................................................................................... 13

12. Columna estratigrafica generalizada reuniendo las observaciones de las ocho columnas a una general.................... 14

13. Estructuras sedimentanas del miembro II en la seccion Rio Urico............................................................................. 15

14. Direcciones de paleocorrientes en la Quebrada Abaren que indica fuentes de aporte desde el noreste..................... 16
Estratigrafia del Grupo Roraima Interpretation Petrologica 

en la Region Noreste del Parque Nacional Canaima,

Estado Bolivar, Venezuela

(Stratigraphy of the Roraima Group and Petrologic Interpretation

in the Northeast Region of Canaima National Park,

Estado Bolivar, Venezuela)

By Margarita Alberdi 1 and Gloria Contreras2

RESUMEN

Ocho secciones sedimentarias estudiadas a lo largo de 

la Serrania de Lema indican que el Grupo Roraima en esta 

region contiene una secuencia turbiditica. Se distinguieron 

dos secuencias sedimentarias disconformes con notorias 

diferencias petrologicas. Se propone el nombre de 

Formacion Urico para la secuencia basal, la cual tiene su 

section tipo en las cabeceras del Rio Urico. El nombre de 

capas de Abaren se postula para la secuencia suprayacente, 

la cual tiene dos secciones de referencia en las Quebradas 

Franela y Abaren.

La Formacion Urico esta compuesta por tres miembros. 

El miembro I contiene grauwacas y brechas sedimentarias 

con abundantes clastos volcanicos. El miembro II suprayace 

en contacto disconforme y esta compuesto por lutitas y 

limolitas de color gris oscuro, con pirita y alto contenido de 

carbonate. El miembro III tambien esta en contacto disconforme y esta caracterizado por un componente volcaniclastico. La base contiene areniscas cuarzosas interestratificadas 

con lutitas tobaceas verdes. La parte media son lutitas 

tobaceas verdes con lamination piano paralela y pirita. Por 

encima de la parte media, afloran, en contacto abrupto y 

concordante, limolitas tobaceas rojas de gran persistencia 

lateral. El miembro III se subdividio en 3 partes. La Formacion Urico infrayace a una secuencia de areniscas cuarzosas y paraconglomerados polimicticos la" cual se denomina 

capas de Abaren. Las capas de Abaren presentan las

'Corporacion Venezolana de Guayana, Tecnica Minera, C.A., Puerto 

Ordaz, Venezuela.

2Voie de la Croix, 55000 Behonne, France; formerly Corporation 

Venezolana de Guayana, Tecnica Minera, C.A., Puerto Ordaz, Venezuela.

caracteristicas sedimentologicas y petrologicas tipicas del 

Grupo Roraima.

La Formacion Urico se interpreta como depositos de 

ambientes marines, posiblemente de talud y quizas de 

plataforma. Las capas de Abaren se interpretan como 

sedimentos fluviales de rios con canales entrelazados.

ABSTRACT

Eight stratigraphic sections of the Roraima Group were 

studied along the Lema Range in the northeastern part of the 

Gran Sabana of Venezuela. Two distinct sedimentary 

sequences that have markedly different lithology are present. 

The type section of the older, basal, turbiditic sequence, 

herein named the Urico Formation, is exposed in the headwaters of the Rio Urico. Reference sections for an overlying, 

younger section, herein referred to as the Abaren beds, are 

along the Quebrada Abaren and Quebrada La Franela (Piston 

de Uroy).

The Urico Formation consists of three informal members. The oldest, member I, consists of graywacke and sedimentary breccia that contain abundant volcanic clasts. The 

overlying member, member II, is in disconformable contact, 

consists of dark-gray shale and mudstone, and contains 

abundant pyrite and carbonate. Member III disconformably 

overlies member II and is characterized by a volcaniclastic 

component. The basal part of member III contains quartzarenite interstratified with green tuffaceous shale, and the 

middle part consists of green tuffaceous shale with planeparallel lamination. The upper part of member III rests on a 

sharp, but concordant, contact. It consists of laterally persistent red tuffaceous mudstone. Quartzitic and polymictic conglomerate and sandstone, herein called the Abaren beds,
12 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

overlie the Urico Formation. The Abaren beds form a 

sequence of rocks typical of the Roraima Group.

The Urico Formation is interpreted to represent a 

marine environment of deposition, possibly continental 

slope and platform. The Abare"n beds are fluvial sediments 

that were deposited in channels of anastomosing streams.

INTRODUCTION

Las unidades que forman el Grupo Roraima son extensas. En este trabajo, se estudid una pequena regi6n donde hay 

secciones estratigraficas bien desarrolladas con las cuales se 

construyo una seccion completa del Grupo Roraima. 

Ademas se evaluo el ambiente de deposition.

Las capas sedimentarias que afloran en el Monte 

Roraima y que ocupan un area de 450,000 km2 desde Tafelberg en Surinam hasta la Sierra de La Macarena en Colombia 

fueron denominadas por Dalton (1912) como "Serie 

Roraima." Aguerrevere et al (1939) elevaron la secuencia al 

rango de formation. Gansser (1954) y Yanez (1977) distinguieron tres miembros para la Formacidn Roraima. Por 

ultimo, Reid (1973) y Ghosh (1977) le confirieron a la formacion el rango de grupo. Los estudios geocronologicos de 

McDougall et al (1963), Snelling (1963), Priem et al (1973), 

y Basel y Teixeira (1975) determinaron una edad Proterozoicodel,800Ma.

Las rocas del Grupo Roraima son conocidas en Surinam 

a traves de los trabajos de Bisschops (1969) y en Brasil, principalmente, a traves de los trabajos de De Loczy (1973) y 

Gansser (1954). Reid (1973), Simon et al (1985), Yanez 

(1985), y Ghosh (1977) realizaron descripciones estratigraficas en Venezuela, especificamente en el area del Monte 

Roraima, Canaima, y Estado Amazonas. No obstante, a 

pesar del numero de trabajos dedicados a la geologia del 

Grupo Roraima, existe un concenso general en cuanto a la 

falta de information estratigrafica detallada en grandes areas 

inexploradas. Por lo cual, este trabajo contribuye al conocimiento del Grupo Roraima por describir macroscopicamente y microscopicamente rocas de ocho secciones 

sedimentarias a lo largo de la Serrania de Lema. Se realiza 

una correlation de las ocho secciones, y se presenta una 

columna estratigrafica generalizada. Se presenta informacion y observaciones que permiten determinar los posibles ambientes sedimentarios y se postula una nueva 

formacidn con el nombre de Formacion Urico, la cual 

infrayece sedimentos continentales correlacionables en 

terminos generales con el Grupo Roraima. Estos sedimentos 

continentales se denominan "capas de Abaren."

Esta investigation esta fundada en observaciones directas sobre los escarpes verticales de los tepuyes. La informacion estratigrafica se complement© con el estudio de 55

secciones finas y 20 analisis de difraccion de rayos X, lo que 

condujo, junto con los datos estratigraficos, a obtener una 

interpretaci6n sedimentologica.

El trabajo se efectuo como parte del Proyecto Prospecciones Geologicas que la Corporation Venezolana de Guayana-Tecnica Minera, C.A., desarrollo al norte del Parque 

Nacional Canaima. El area de trabajo esta ubicado entre lat 

6°00/-6°45/ N. y long 61°45/-62°15/ O. en el Distrito Sifontes del Estado Bolivar (fig. 1). Las secciones estratigraficas 

fueron estudiadas en las cabeceras del Rio Urico (afluente 

del Yuruan), en la Quebrada La Franela (afluente del Rio 

Chicanan), en las cabeceras de la Quebrada Hacha (afluente 

del Rio Tukupo), en las cabeceras del Rio Uroy, en las 

cabeceras del Rio Chivao, y a lo largo de la Quebrada 

Abaren (figs. 1 y 2).

RESULTADOS

A lo largo de la Serrania de Lema, se estudiaron ocho 

secciones sedimentarias, seis de ellas son correlacionables 

por la presencia de dos capas tobaceas que sirvieron como 

capas guias. Para la mayoria de las secciones se pudo 

establecer correlaciones secundarias utilizando para ello las 

grauwacas mas antiguas, los conglomerados polimicticos de 

matriz rosada, y las areniscas interestratificadas con limolitas rojas. Desafortunadamente, en la seccion sedimentaria en 

la Quebrada Hacha no se observaron las capas gums y lo que 

se conoce hasta los momentos de su litologia se presenta en 

este estudio hasta que trabajos mas detallados arrojen mas 

informacion.

Las figuras 3-10 son columnas estratigraficas y la figura 11 presenta una correlacidn entre las ocho secciones. La 

combinacidn de las secciones estratigraficas correlacionables arroja la columna litoestratigrafica generalizada (fig. 

12), en donde se muestra una secuencia sedimentaria en la 

base que se denomino Formacion Urico y una secuencia 

superior llamada las capas de Abaren. Las capas de Abaren 

son correlacionables en forma general con las capas basales 

del Grupo Roraima en el area de Santa Elena de Uairen.

Los afloramientos de la Formacion Urico se encuentran 

expuestos casi en su totalidad en la seccion occidental levantada en las cabeceras del Rio Urico, por lo que se postula a 

esta secuencia como la seccion tipo de la Formacion Urico. 

Sin embargo, tambien se observaron afloramientos parciales 

de ella en las secciones sedimentarias de los Rios Uroy, 

Chivao, y Quebrada La Franela. La capas de la Formacion 

Urico no presentan buzamientos aparentes, fallas o pliegues 

que indiquen tectonismo. Al igual que para el Grupo 

Roraima, estas capas no fueron deformadas por la Orogenesis Transamazonica propuesta por Mendoza (1977).
STRATIGRAPHY AND PETROLOGY OF RORAIMA GROUP

70° 65° 60° 55°

13

12°30'

10°

7°30'

2°30'

OCEANO 

ATLANTICO

Orinoco

ESTADO Ti^meremoO 

BOLIVAR

62°00'/ 61°00'

6°30'

6°00'

8 16 24 32KILOMETROS

Figura 1. Piano regional mostrando el area de estudio.
14 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

62° 15' 62°00' 61°55' 61°50' 61°45'

6°15' 

Figura 2. Piano mostrando las posiciones de las echo secciones estratigraficas.
STRATIGRAPHY AND PETROLOGY OF RORAIMA GROUP 15

COLUMNA ESTRATIGRAFICA RIO URICO - OCCIDENTAL, NORESTE DEL 

PARQUE NACIONAL CANAIMA, ESTADO BOLIVAR

UNIDADES

Abaren de Capas 

Formacion Uric

Miembro III

Miembro II

Miembro I

ESPESOR(m)

70.8 

51.8

44.3

31.3

24 

0

ESPESOR

INDIVIDUAL

(m)

17

2

1.5

3

0.6 

0.6

1

0.8

1

6

2

5

0.8

3.2

2.5

0.5

24

NO. DE 

MUESTRA

CH-220RCH-219 RCH-218RCH-217RCH-216RCH-215 RCH-214RCH-213R 

CH-212BR

CH-212A R 

CH-211 R

CH-210R 

CH-209 R

CH-207 R 

CH-208 R 

CH-207 R 

CH-206 R 

CH-205 R 

CH-204 R

CH-203 R 

CH-202A R 

CH-202B R 

CH-201CR 

CH-201 R

CH-200 R

LITOLOGIA GRAFICA

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LITOLOGIA DESCRIPTIVA

PAQUETE DE ARENISCAS ROSADAS, SIN DIFERENCIAR 

QUE METEORIZAN A ROJO CON TAMANO DE GRAND 

MEDIO Y ESTRATIFICACION CRUZADA PLANAR A GRAN 

ESCALA. EL PROMEDIO DE LAS CAPAS ES 2 m

ARENISCA CUARZOSA DE GRAND MEDIO

ARENISCA DE GRAND MEDIO A GRUESO

ARENISCAS ROSADAS DE GRAND FIND A MUY FIND

ARENISCA ROSADA 

CUARZO ARENITA DE GRAND MEDIO 

WACKA CUARZOSA

PAQUETE DE ARENISCAS CONGLOMERATICAS

CONGLOMERADO ROJO POLIMICTICO CON 

n PREDOMINANCE DE GRANOS DE CUARZO 

SUB - REDONDEADOS Y FRAGMENTOS DE 

EPICLASTICOS DE ROCAS VOLCANICAS

LIMOLITA TOBACEA ROJA CUARZO - MICACEAARCILLOSA

LUTITAS TOBACEAS COLOR VERDE CON CUBOS 

DE PIRITA BIEN DESARROLLADOS

LIMOLITA GRIS OSCURO CON PIRITA

ARENISCA CONGLOMERATICA COLOR GRIS

LIMOLITA GRIS OSCURO CON PIRITA EN CUBOS Y 

PIRITA FRAMBOIDAL

TILOIDE PIRITOSO, VERDOSO CON LAMINACION 

HACIA LA BASE, CON MATRIZ WACKA LITICA COLOR 

VERDE

^"WACKAS INTERESTRATIFICADAS CON BRECHAS

-xBRECHA POLIMICTICA PIRITOSA

ARENISCA VERDE MASIVA

GRANIITO DEL COMPLEJO DE SUPAMO

Figura 3. Columna estratigraflca de Rio Urico-Occidental en el Parque Nacional Canaima, Estado Bolivar.
16 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

COLUMNA ESTRATIGRAFICA RIO URICO - ORIENTAL, NORESTE DEL 

PARQUE NACIONAL CANAIMA, ESTADO BOLIVAR

UNIDADES

Abaren Capas de 

Formacion Uric

III

n

I

ESPESOR

(m)

42.5

22.5

15.5

10.5 

5.5

0

ESPESOR

INDIVIDUAL

(m)

20

1

6

1.5

1.5

2

1

2

2

4

1.5

NO. DE 

MUESTRA

CH-229 R

CH-228 R

CH-227 R 

CH-226 R

CH-225 R

CH-224 R 

CH-223 R

CH-222 R 

CH-221 R

LITOLOGIA GRAFICA

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LITOLOGIA DESCRIPTIVA

PAQUETE DE ARENISCAS ROSADAS QUE 

METEORIZAN A ROJO, CON TAMANO DE 

GRAND MEDIO A GRUESO

ARENISCA CONGLOMERATICA CON MATRIZ ARENOSA 

COLOR ROSADO

LIMOLITA TOBACEA CUARZO-MICACEA ARCILLOSA 

FERRUGINOSA

LUTITA TOBACEA VERDE CON PIRITA CUBICA Y 

ABUNDANTE CLORITA

QUARZO WACKA

LUTITAS CRISES CON ABUNDANTE PIRITA

CUARZO WACKA CON INTERCALACIONES DE 

ARENISCA GRAND MEDIO Y FIND

CAPA DE LIMOLITA GRIS VERDOSA CON PIRITA 

ESFEROIDAL Y CUBICA

CUARZO WACKA DE GRAND FIND, COLOR GRIS 

VERDOSO

CAPA DE LIMOLITA GRIS OSCURO FINAMENTE 

LAMINADA, QUE METEORIZA A OCRE. PASAN 

GRADUALMENTE A UNA ARENISCA CONGLOMERATICA 

CON GUIJARROS DE ORIGEN VOLCANICO Y PIRITA 

ESFEROIDAL

CONGLOMERADO (i, BRECHA ?) POLIMICTICA 

COLOR GRIS VERDOSO

GRANITO DE SUPAMO

Figura 4. Columna estratigrafica de Rio Urico-Oriental en el Parque Nacional Canaima, Estado Bolivar.
STRATIGRAPHY AND PETROLOGY OF RORAIMA GROUP 17

COLUMNA ESTRATIGRAFICA QUEBRADA LA FRANELA 

PISTON DE UROY, CANAIMA, ESTADO BOLIVAR

UNIDADES

Capas de Abaren

o

O

r5 

-o

Formac

embro III

^.

ESPESOR 

(m)

86.0 

46.0

38.9 

30.8

20.3 

0

ESPESOR 

INDIVIDUAL 

(m)

40

4

.1

3.0

0.1

2.5

2.0

0.1

1.5

1.8

3

1.5

6

0.5

1.8

12

2

2

2

NO. DE 

MUESTRA

CH - 51 R

CH-50

CH-48 

CH-47

CH-44 

CH - 35 R

CH - 37 R 

CH - 38 R

CH - 36 R

CH - 45 R 

CH-35

LITOLOGIA GRAFICA

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LITOLOGIA DESCRIPTIVA

ARENISCA DE GRAND MEDIO SIN DIFERENCIAR. 

ESPESORES INDIVIDUALES DE 2 m COLOR ROJO - 

ROSADO

CUARZO WACKA DE GRAND FIND COLOR ROJO - 

ROSADO

LIMOLITA ROJA

ARENISCA (SIN DIFER) DE GRAND MEDIO

LIMOLITA ROJA

ARENISCA (SIN DIFER) GRAND MEDIO 

ZO-IMOLITA ROJA

SUBLITOARENITA DE GRAND MEDIO

XLIMOLITA ROJA

SUBLITOARENITA DE GRAND GRUESO

ARENISCA CONGLOMERATICA BLANCA CON 5% 

GRANOS DE CUARZO

CONGLOMERADO ROSADO MATRIZ SOPORTA 

LOS GRANOS TAM MAX: 9 cm SEDIMENTOS 

ALTERNADO CON ARENA GRUESA

ARENISCA DE GRAND MEDIO ROSADA

PARACONGLOMERADO POLIMICTICO ROSADO 

CON FRAGMENTOS REDONDEADOS DE ROCAS 

VOLCANICAS Y ROCAS SEDIMENTARIAS

-^ARENISCA CUARZOSA DE GRAND MEDIO

PARACONGLOMERADO POLIMICTICO COLOR ROJO 

-^REDONDEADO Y FRAGMENTOS DE ROCAS VOLC.

PAQUETE DE CUARZO - WACKA DE GRAND MEDIO

ARENISCA GRAUWAQUICA COLOR ROJO

LIMOLITA TOBACEA ROJA

LUTITA TOBACEA VERDE CON CUBOS DE PIRITA 

DISEMINADOS, CON MATRIZ DE MICAS Y ARCILLAS

ROCA ULTRAMAFICA

Figura 5. Columna estratigraflca de la Quebrada la Franela en el Parque Nacional Canaima, Piston de Uroy, Estado Bolivar.
18 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

COLUMNA ESTRATIGRAFICA EN LA QUEBRADA ABAREN, 

CANAIMA, ESTADO BOLIVAR

UNIDADES

Capas Abaren de 

ESPESOR

(m)

30

20

10 

0

ESPESOR 

INDIVIDUAL 

(m)

5

2

2

1

1

3

3

3

1

7

2

NO. DE 

MUESTRA

CH-19B

CH-19A 

CH-18R

CH-16R 

CH-15R

CH-14R

CH-10R

CH-9R 

CH-8R

CH-7R 

CH - 7 - 1 R 

CH - 7 - 2 R 

CH - 7 - 3 R

CH-6R

O .' ' xO

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LITOLOGIA DESCRIPTIVA

ARENISCA CONGLOMERATICA

ARENISCA CUARZOSA DE GRAND MEDIO

CUBIERTO

ARENISCA CONGLOMERATICA

ARENISCA CUARZOSA DE GRAND MEDIO

ARENISCA CUARZOSA DE GRAND MUY GRUESO

ARENISCA CUARZOSA DE GRAND GRUESO

ARENISCA CUARZOSA DE GRAND GRUESO, 

CAPAS PLANAR

ARENISCA CUARZOSA DE GRAND GRUESO , SIN 

CAPAS CRUZADAS

CONGLOMERADO ROJO POLIMICTICO

ARENISCA CONGLOMERATICA COLOR ROSADO

BASAMENTO NO OBSERVADO

Figura 6. Columna estratigrafica de la Quebrada Abar6n en el Parque National Canaima, Estado Bolfvar.
STRATIGRAPHY AND PETROLOGY OF RORAIMA GROUP 19

COLUMNA ESTRATIGRAFICA EN EL RIO UROY OCCIDENTAL, 

CANAIMA, ESTADO BOLIVAR

UNIDADES

Capas de Abaren

Formacion Uric 3mbro III

ESPESOR

(m)

169.6

79.6 

47.1

0

ESPESOR 

INDIVIDUAL 

(m)

93

10

12

0.50

10

20

27

10cm

NO. DE 

MUESTRA

UR-57 

UR-56

UR-54

UR-52 

UR-51

 ' .:. .- . .' ';' ." .-}

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LITOLOGIA DESCRIPTIVA

PAQUETE DE ARENISCA SIN DIFERENCIAR, COLOR 

ROSADO

LIMOLITAS ROJAS

ARENISCA CUARZOSA DE COLOR ROSADO Y 

INTERESTRATIFICADA LIMOLITA ROJA

CONGLOMERADO CUARZOSO COLOR ROJO

LIMOLITAS TOBACEAS ROJAS CUARZO MICACEA 

ARCILLOSA FERRUGINOSA

INTERESTRATIFICACIONES DE LUTITA-LIMOLITA 

ARENOSA COLOR VERDE. ABUNDANTE PIRITA Y 

FRAGMENTOS VOLCANICOS

PAQUETE DE ARENISCAS CUARZOSAS COLOR 

BEIGE E INTERSTRATIFICACIONES DE CAPAS DE 

LUTITAS VERDES QUE SE HACEN MAS POTENTES 

HACIA EL TOPE

CUARZO WACKA DE COLOR GRIS, GRAND FIND

GRANITO DE GRAND MEDIO A GRUESO.

Figura 7. Columna estratigrafica del Rio Uroy-Occidental en el Parque Nacional Canaima, Estado Bolfvar.
110 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

COLUMNA ESTRATIGRAFICA EN EL RIO UROY ORIENTAL, 

CANAIMA, ESTADO BOLIVAR

UNIDADES

Capas de Abaren

Formacion Urico Miembro III

ESPESOR 

(m)

148

98 

79

49

24 

0

ESPESOR 

INDIVIDUAL 

(m)

50

7

4

8

30

25

9

15

NO. DE 

MUESTRA

UR-32 

UR-33 

UR-34

UR - 35 - 2

UR - 35 - 1 

UR - 36 - 2

UR-36 

UR-36

 - .' ... ' ' ' ' ' . ..' )

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: ' :-.. . .. : ' : : )

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.-.- -.-, ': :. : )

.: . :. :. ' ' : -.:-. 3

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LITOLOGIA DESCRIPTIVA

ARENISCAS CUARZOSAS

ARENISCAS CUARZOSAS CON 

INTERESTRATIFICACIONES DE LIMOLITA

ARENISCAS CONGLOMERATICAS

PAQUETE DE CONGLOMERADOS 

POLIMICTICOS COLOR ROJO

LIMOLITA TOBACEA CUARZO ARCILLOSA 

FERRUGINOSA

LUTITA TOBACEA COLOR VERDE ABUNDANTE 

PIRITA CUBICA Y CLORITA

PAQUETE DE ARENISCAS CUARZOSAS DE 

GRAND FIND

CUBIERTO

GRANITO DEL COMPLEJO DE SUPAMO

Figura 8. Columna estratigrafica del Rio Uroy-Oriental en el Parque Nacional Canaima, Estado Bolivar.
STRATIGRAPHY AND PETROLOGY OF RORAIMA GROUP 111

COLUMNA ESTRATIGRAFICA EN EL RIO CHIVAO, 

CANAIMA, ESTADO BOLIVAR

UNIDADES

Capas Abaren de 

Formacion Uric Miembro III

ESPESOR

(m)

137

87

54 

30

0

ESPESOR 

INDIVIDUAL 

(m)

50

15

10

8

15

2

1

6

10

20

NO. DE 

MUESTRA

UR-32

UR-33 

UR-35

UR-26

UR-34 

UR-37

UR-39 

UR-41

 : T-T- i

. - --.:.- :: :: : :: : '

' ' " ' ' ' i

.-'. -. ' ' . ' )

... . . ''':"; . ; j

.... .... ..... ....

 . . .'. . .- : v

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 .......... .. \

' ' ' ".. ' -.-.' ' /

. .-..: \

 '. .-. . .- . 'I

'&' -^' °& ^'os <y]

'c^-^^.^^^^J

:^-^-E:^^T

^^===-===^Z^\

:r. -, .: ;. - ^;t; >; )

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LITOLOGIA DESCRIPTIVA

PAQUETE DE ARENISCAS CUARZOSAS SIN 

DIFERENCIR DECOLOR BEIGE Y BLANCO

ARENISCAS CUARZOSAS DE GRAND FINO

PAQUETE DE CONGLOMERADOS POLIMICTICOS 

COLOR ROJO

LIMOLITATOBACEA ROJA

LUTITAS TOBACEAS. COLOR VERDE 

ABUNDANTE PI RITA Y CLORITA

CUBIERTO

^ARENISCA LITICA. GRAND GRUESO A MEDIO COLOR 

^VERDE - GRIS

GRAUWACA LITICA. GRAND MEDIO A FINO 

MASIVA. COLOR VERDE OSCURO, PIRITA

CUARZO WACKA, GRAND GRUESO, 

MASIVA COLOR VERDE

CUBIERTO

GRANITO DE SUPAMO SIN DIFERENCIAR

Figura 9. Columna estratigrafica en el Rio Chivao en el Parque Nacional Canaima, Estado Bolivar.
112 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

COLUMNA OESTRATIGRAFICA EN EL RIO HACHA - SERRANIA DE 

LEMA NACIMIENTO QDA. HACHA, CANAIMA, ESTADO BOLIVAR

UNIDADES

GRUPO RORAIMA

ESPESOR 

(m)

123.6

103.6 

23.6

14.1

4.0 

2.5

ESPESOR 

INDIVIDUAL 

(m)

20

80

4.5

2

3

2.4

7.7

1.5

0.5

2

NO. DE 

MUESTRA

CH - 86 R

CH-85

CH - 84 R 

CH - 83 R

CH - 82 R 

CH - 81 R

CH - 80 R 

CH - 79 R

CH - 78 R 

CH - 77 R

CH - 76 R

' '. ' >; '>' '"/ '

'.' ' ''. ''': '/' '.'

 ' ' '-.. ' . .- ! . ':

x

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^o>%S? " '&(£ $

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LITOLOGIA DESCRIPTIVA

ARENISCA CUARZO COLOR BLANCO

CUBIERTO POR DERRUBIOS DEL TEPUY

SUBLITOARENITAS DE GRAND MEDIO COLOR 

BLANCO-GRIS

ARENISCA LITICA DE GRAND GRUESO COLOR 

GRIS VERDOSO

ARENISCAS CONGLOMERATICAS CON CLASTOS 

REDONDEADOS DE CUARZO (5%) EN UNA MATRIZ 

DE ARENISCA DE GRAND MEDIO A GRUESO

PAQUETE DE CONGLOMERADOS 

INTERESTRATIFICADOS CON GRAUWACAS 

LITICAS BLANCO GRISACEAS

CONGLOMERADO BLANCO-GRIS 

OLIGOMICTICO, CON CLASTOS REDONDEADOS 

DE CUARZO QUE SOPORTAN LA MATRIZ

GRAUWACA LITICA COLOR GRIS

CONGLOMERADO DE COLOR BLANCO

META - TOBA LITICA CRISTALINA

Figura 10. Columna estratigrdfica en Quebrada Hacha-Serrania de Lema, Quebrada Hacha en Parque Nacional Canaima, 

Estado Bolivar.
STRATIGRAPHY AND PETROLOGY OF RORAIMA GROUP 113

ill

11iBisEJa

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ts

2

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13

13

I

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114 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

COLUMNA ESTRATIGRAFICA GENERALIZADA DE CANAIMA, ESTADO BOLIVAR

UNIDAD

de Abaren Capas 

Formacion Uric

III

II

I

ESP. 

TOTAL

(m)

190 

128

34 

20

0

ESPESOR 

DELA 

UNIDAD (m)

62

94

14

20

LITOLOGIA GRAFICA

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LITOLOGIA DESCRIPTIVA

PAQUETE DE ARENISCAS CUARZOSAS DE GRAND 

MEDIO

PAQUETE DE ARENISCAS CUARZOSAS, COLOR 

ROSADO INTERSTRATIFICADAS CON LIMOLITAS 

ROJAS

ARENISCA CONGLOMERATICA

PAQUETE DE CONGLOMERADOS POLIMICTICOS 

FRAGMENTOS DE CUARZO Y ROCAS VOLCANICAS, 

MATRIZ SOPORTA GRANOS

CUARZO WACKAS ROJAS DE GRAND MUY FIND

LIMOLITAS TOBACEAS ROJAS CUARZO 

MICACEA - ARCILLOSAS - FERRUGINOSAS

LUTITAS TOBACEAS VERDES FINAMENTE LAMINADAS 

CON CUBOS DE PIRITA BIEN DESARROLLADOS, 

ABUNDANTE CLORITA

ARENISCAS CUARZOSAS DE GRAND FIND QUE 

METEORIZAN A BEIGE - BLANCO. 

INTERESTRATIFICACIONES DE LUTITAS 

TOBACEAS VERDES

LUTITAS Y LIMOLITAS GRIS - OSCURO CON 

INTERESTRATIFICACIONES DE ARENISCAS GRIS 

VERDOSO MASIVAS Y ARENISCAS CONGLOMERATICAS 

CON CARBONATO ABUNDANTE, PIRITA

BRECHAS POLIMICTICAS PIRITOSAS 

INTERESTRATIFICADAS CON GRAUWACAS

GRAUWACA MASIVA, VERDE CON SULFUROS

BASAMENTO: GRANITOS + ROCAS ULTRAMAFICAS

Figura 12. Columna estratigraflca generalizada reuniendo las observaciones de las ocho columnas a una general.
STRATIGRAPHY AND PETROLOGY OF RORAIMA GROUP 115

DESCRIPCION LITOLOGICA DE LAS 

FORMACIONES

La Formation Urico yace sobre rocas graniticas del 

Complejo de Supamo y se dividio en ties miembros.

FORMACION URICO, MIEMBROI

El miembro I fue observado en las secciones de los Rios 

Urico y Chivao (figs. 3, 4, y 9). Rocas graniticas del 

Complejo de Supamo, las cuales afloran en la base de los 

tepuyes, infrayacen al mimebro I. El contacto no fue observado ya que esta cubierto por derrubios.

El miembro I tiene 20 m de espesor y esta formado por 

una secuencia que se inicia con cuarzo wackas de color 

verde, masivas de grano grueso a muy grueso. Contiene 

aproximadamente 40-60 por ciento de pirita diseminada, 

algunos granos tienen bordes corroidos. La matriz como epimatriz y ortomatriz (Dickinson, 1970) se compone de 

carbonatos, clorita, ilita-sericita, caolinita, pirofilita, feldespato, y cuarzo criptocristalino.

Las cuarzo-wackas pasan transicionalmente a 

grauwacas liticas, areniscas liticas en las cabeceras del Rio 

Chivao (fig. 9), y a brechas polimicticas y conglomerados 

brechoides interestratificados con grauwacas en las 

cabeceras del Rio Urico (fig. 3). Macroscopicamente, las 

brechas polimicticas contienen 45 por ciento de granos 

detriticos angulosos (tamano maximo, 11 cm; tamano 

minimo, 0.5 cm), 3-5 por ciento pirita, y 50-55 por ciento 

matriz grauwaquica. La matriz esta formada por epidota, 

carbonatos recristalizados, clorita, arcillas blancas, caolinita, 

feldespatos, y cuarzo criptocristalino y contiene como 

fraction gruesa fragmentos de roca volcanica, hornblenda 

basaltica, cuarzo mono-y policristalino, pirita, 

ilmenita-magnetita, y leucoxeno.

En la parte superior del miembro I se encuentra una 

capa de 2.5 m de espesor la cual esta caracterizada por clastos maficos redondeados (dimension maxima 15 cm) y 

fragmentos de cuarzo anguloso embebido en una matriz 

piritosa. Petrograficamente, fu6 clasificada como wacka 

litica. La matriz es laminada hacia la base y se hace masiva 

hacia el tope, petrograficamente se caracteriza por presentar 

cuarzo monocristalino, feldespatos, plagioclasa alterando 

completamente o parcialmente a carbonate, fragmentos de 

rocas volcanicas y sedimentarias, y minerales maficos sin 

diferenciar alterado completamente o parcialmente a clorita 

y calcita. Los clastos maficos de color gris oscuro estan 

dispuestos paralelos a la estratificacion, al igual que sus ejes 

may ores; la distancia aproximada entre clasto y clasto de 

40cm.

TOPE

0.5m

Arenisca, lamination cruzada planar 

Arenisca, lamination cruzada concava 

Arenisca, lamination cruzada planar

Arenisca, lamination paralela

Arenisca conglomeratica con gradation 

normal

Limolita gris-oscura

BASE

Figura 13. Estructuras sedimentarias del miembro II en la 

section Rio Urico en Parque National Canaima, Estado Bolivar.

FORMACION URICO, MIEMBRO II

En las secciones sedimentarias que se cartografiaron en 

las cabeceras del Rio Urico (figs. 3 y 4), el miembro I infrayace disconforme al miembro II el cual consiste de estratos de 

lutitas color gris oscuro, laminadas, con un alto porcentaje de 

carbonatos y abundante pirita (framboidal y ciibica). Las 

lutitas gradan a limolitas. En la section occidental del Rio 

Urico (fig. 3), las limolitas gradan hacia el tope a areniscas y 

areniscas conglomerdticas (tamano maximo, 5 mm) que cierran dos ciclos de engrosamiento, y en la section oriental del 

Rio Urico (fig. 4), las limolitas se interestratifican con 

cuarzo wackas.

En la section occidental del Rio Urico, las areniscas del 

primer ciclo regresivo muestran progresivamente de base a 

tope estructuras de gradation normal, lamination paralela 

planar, cruzada planar, y cruzada concava (fig. 13). Petrograficamente, la arenisca conglomeratica esta constituida por 45 

por ciento de matriz, 25 por ciento de arcillas, y 30 por ciento 

de parches de carbonato. El carbonato se encuentra principalmente como pseudoespato, producto de la recristalizacion 

de un lodo micritico original que paso progresivamente a 

microespato, y finalmente a pseudoespato. Dentro de los 

granos detriticos una matriz de cuarzo mono- y policristalino, fragmentos de rocas sedimentarias, fragmentos de 

rocas volcanicas, y trazas de leucoxeno, epidoto, biotjta, y 

pirita. En esta misma section sedimentaria, el segundo ciclo 

regresivo grada trancionalmente a una arenisca de grano fmo 

con lamination convoluta. Las direcciones de paleocorrientes en la estratificacion cruzada son suroeste indicando 

fuentes de aporte al noreste para esta parte de la Formation 

Urico.
116 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

FORMACION URICO, MIEMBRO III

El miembro III se caracteriza por presentar una lutita 

tobacea, color verde, fmamente laminada, con cubos de pirita bien desarrollados. Las lutitas afloran por distancias no 

menores de 50 km lineales sin cambios laterales de facies, y 

forma una capa gufa. La lutita tobacea verde tiene fragmentos de pomez alterados, cuarzo retrabajado, clorita 

autigenica, muscovita, epidoto, trazas de carbonate, y fragmentos volcanicos. En seccion fina, se observan a nivel 

micrometrico estructuras de deformacion ("slumping").

En la seccion sedimentaria en las cabeceras del Rio 

Uroy (fig. 7), las lutitas verdes estan interestratificadas en su 

base con areniscas de color beige-blanco de grano fino a 

medio y estratificaci6n cruzada. Las areniscas en el tope 

alcanzan un espesor continuo de 25 m en las secciones del 

Rios Uroy oriental (fig. 8) y Chivao (fig. 9). La lutita tobacea 

verde infrayace en contacto abrupto y concordante a una 

capa de limolitas tobaceas rojas, con laminacion planar, 

gradation inversa y laminacion paralela con clorita y 

feldespatos.

En la seccion sedimentaria de La Franela (fig. 5), la limolita tobacea roja, infrayace un paquete de cuarzo wackas 

con abundante muscovita y minerales opacos. El tamano de 

grano grada inversamente de muy fino a medio. Las areniscas presentan en la base rizaduras a pequena escala y en el 

tope laminacion cruzada concava a gran escala. Su position 

estratigrafica y la concordancia con las limolitas tobaceas 

rojas infrayacentes sugiere la ubicacion de este paquete 

dentro de la Formation Urico. El contacto entre la Formacion Urico y las capas de Abaren es una disconformidad.

Figura 14. Dirrecciones de paleocorrientes en la 

Quebrada Abaren que indica fuentes de aporte desde 

el noreste.

de arenisca cuarzosa color rosado, interestratificadas con 

limolita roja.

Todas las secciones estratigraficas correlacionables 

terminan con un paquete de areniscas sin diferenciar de 

grano grueso, color rosado a blanco, con estratificacidn 

cruzada planar y concava que llega a alcanzar espesores 

totales de 90 m en las cabeceras del Rio Uroy. Las 

direcciones de paleocorrientes indican, principalmente, 

fuentes de aporte desde el noreste similar a la Formaci6n 

Urico. El estudio de paleocorrientes a lo largo de la Quebrada Abaren se muestra en la figura 14.

CAPAS DE ABAREN

Las capas de Abaren estan por encima y en contacto 

discordante con la Formacion Urico. Las capas se inician con 

un paquete de conglomerados polimicticos cuya matriz 

soporta los granos color de meteorizacion rosado; presentan 

estratificacion gradada normal. Este conglomerado marca un 

intervalo de levantamiento y erosion. Los clastos son de 

diferentes litologias, principalmente granos de cuarzo y fragmentos de rocas volcanicas y sedimentarias. Los guijarros 

son redondeados a subredondeados, con dimensiones que 

alcanzan 13 cm. Estan embebidos en una matriz que varia 

entre arenisca cuarzosa, sublitarenita, y arenisca litica.

En la mayoria de las secciones levantadas, el conglomerado pasa transicionalmente a una arenisca conglomeratica 

con estratificacion cruzada concava a gran escala, sin 

embargo, en la seccion de Abaren, el conglomerado infrayace a un paquete de arenisca cuarzosa de grano grueso bien 

escogida, que presentan estratificacion cruzada planar 

gruesa y hacia el tope estratificacion cruzada concava. En 

Quebrada la Franela (fig. 5) y en las cabeceras del Rio Uroy 

(fig. 7), se observo arenisca conglomeratica sobre un paquete

AMBIENTES SEDIMENTARIOS

FORMACION URICO, MIEMBRO I

La capa superior del miembro I (fig. 3) se caracteriza 

por clastos disperses embebidos en una matriz grauwaquica, 

clastos de cuarzo anguloso, laminacion paralela hacia la 

base, y clastos alineados con la estratificacion y con los ejes 

mayores a la misma. El estudio de la petrografia textural y 

mineralogica de las grauwacas y su estratificacion masiva 

indica un ambiente turbiditico con action de corrientes marinas de turbidez como agentes de sedimentaci6n. La estratigrafia sigue la secuencia de Bouma (Reading, 1986). En las 

cabeceras del Rio Urico, la aparicidn de brechas polimicticas 

interestratificadas con grauwacas pone de manifiesto la incapacidad del medio sedimentario, para seleccionar los sedimentos transportados. Por otra parte, la pirita diseminada, 

apunta hacia un ambiente fisico-quimico reductor. La recristalizacion de carbonato autigenico en la matriz sustenta la 

hipotesis de un ambiente marino por ser muy pocos los ambientes continentales capaces de precipitar carbonato (Fliigel,
STRATIGRAPHY AND PETROLOGY OF RORAIMA GROUP 117

1982) Ademas, la asociacion litologica y estratigrafica 

observada en este trabajo no coincide con los escasos medios 

de sedimentacion carbonatica continental.

A la luz de los conocimientos de conientes de turbidez 

y torrentes de barro asociados, es probable suponer que estas 

rocas resulten de conientes subacuaticas de arena movil. La 

capacidad de transportar clastos de gran tamano por distancias considerables en estado semi-solido es consistente con 

el propuesto para las brechas y grauwacas del miembro I de 

la Formation Urico las cusles fueron depositadas en un 

ambiente marino, quizas bajo la action de corrientes de 

turbidez por encima de la capa de carbonate.

FORMACION URICO, MIEMBRO II

El miembro II fue depositado en un ambiente marino. 

El paso gradual de limolitas y lutitas en la base del miembro 

II hacia areniscas de grano medio-fino y conglomerates en 

el tope, junto con la secuencia de estructuras mostradas en la 

figura 14 y la aparicion de la laminacion convoluta, permiten 

postular pequenos ciclos regresivos, desde una sedimentacion en aguas tranquilas hasta una sedimentacion en aguas 

turbulentas. El ambiente marino se infiere a partir de la abundancia de carbonate en las lutitas y el alto porcentaje de 

pseudoespato en la arenisca conglomeratica de la section 

Rio Urico occidental (fig. 3).

FORMACION URICO, MIEMBRO III

Las rocas volcaniclasticas del miembro III contienen un 

componente piroclastico procedente de un evento volcanico 

desconocido. Las estructuras sedimentarias (laminacion 

paralela bien desarrollada y "flaser") a nivel macroscopico y 

estructuras de deformation a nivel microscopico, junto con 

la presencia de carbonate sugieren un ambiente de sedimentacion subacuatico, posiblemente marino tanto para la lutita 

como para la limolita tobacea. Para la lutita tobacea, las 

condiciones reductoras en el ambiente sedimentario se 

manifiestan con el desarrollo conspicuo de pirita cubica. El 

tope de este miembro esta erosionado presentando un 

delgado "paleosol."

CAPAS DE ABAREN

La base de las capas de Abaren comienza con un 

paquete de conglomerados polimicticos que descansan en 

contacto disconforme sobre el miembro III de la Formacion 

Urico. La abundancia de minerales estables en las areniscas, 

las superficies subredondeadas de los clastos, y el 

escogimiento de los granos junto con las siguientes observacanales entrelazados. Evidencia para esto es presenta a 

continuation:

1. Los estratos presentan colores de meteorizacion 

rojizos y rosados, producto de un cemento hematitico 

temprano (observation petrografica) que permite proponer 

un ambiente de sedimentacion continental oxidante (Tucker, 

1981; Reading, 1986).

2. Los conglomerados muestran un predominio de 

matriz sobre clastos, textura caracteristica de alta a media 

energia en rios de canales entrelazados (Cant, 1981). La 

marcada gradation normal es tambien propia de este ambiente (Nemec y Stell, 1984).

3. Se observan nueve ciclos "fining upward" o 

secuencias de disminucion de tamano de grano hacia arriba. 

Los ciclos comienzan con conglomerados en la base, los 

cuales corroboran la proposition del punto anterior en 

cuanto a la geomorfologia del ambiente fluvial (William y 

Rust, 1969). Los ciclos "fining upward" a mediana escala 

(interestratificaciones de conglomerados y areniscas), parecen corresponder a ligereas fluctuaciones en la velocidad de 

la corriente (Harms et al, 1975). Los ciclos "fining upward" 

a gran escala se interpretaron como el proceso hidraulico de 

abandono de canales a largo plazo (Nemec y Stell, 1984; 

Walker y Cant, 1984).

4. En la Quebrada Abaren, se observaron estratos de 

arenisca cuarzosa con estratificacion cruzada planar gruesa 

(30 cm) y con buzamientos de alto angulo, los cuales fueron 

interpretados como paleobarras de canales entrecruzados 

(Cant, 1981; Walker y Cant, 1984). Esta explication se hizo 

extensiva a las potentes secuencias de areniscas con estratificacion cruzada planar que conforman la parte superior de 

los tepuyes. En la mayoria de los casos, la estratificacion 

cruzada concava se interpreto como depositos de rizaduras 

linguoides que normalmente se desarrollan en la parte 

superior de los canales fluviales.

5. En la parte media de las capas de Abaren se cartografiaron capas centimetricas de limolitas rojas, que fueron 

interpretadas como paleo-llanuras de inundation segun los 

modelos de facies (Walker y Cant, 1984; Reading, 1986).

Para finalizar, cabe recordar que en secuencias antiguas, las corrientes entrelazadas pueden ser confundidas con 

facilidad con meandros. Sin embargo, en las facies de 

canales meandricos, los espesores de limolitas son potentes 

y son escasos los conglomerados en la base. Se concluye que 

las capas de Abaren se sedimentaron en un medio fluvial de 

canales entrelazados.

PROBLEMA DE CORRELACION CON 

SECUENCIAS CONOCIDAS

La correlation de las secuencias levantadas en este 

estudio, con secciones conocidas en otras partes del escudo,

ciones de campo, permiten definir un ambiente fluvial de es dificil. Las capas de Abaren estan compuestas
118 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

principalmente por areniscas cuarzosas con un porcentaje 

subordinado de wackas cuarzosas y sublitarenitas (figs. 

3-9). Su madurez mineralogica y textural moderadamente 

alias e interestratificiones de paraconglomerados polimicticos y lentes de limolitas indican una sedimentation en un 

ambiente oxidante. Las fuentes de aporte principalmente son 

desde el noreste (fig. 14). Las capas de Abaren presentan 

similitudes con las descripciones hechas por Reid (1973) y 

Simon et al (1985) en la unidad basal del Grupo Roraima en 

las cercanias de Santa Elenea de Uairen.

La Formation Urico presenta una secuencia litologica 

no descrita anteriormente dentro del Grupo Roraima. Su 

ambiente sedimentario, aparentemente marino-reductor, 

junto con su importante componente volcanico, la presencia 

de carbonato y la baja madurez textural y mineralogica de los 

depositos clasticos mas antiguos soportan su diferenciacion 

de las grandes secuencias continentales de ambiente proximo costeros que conforman al Grupo Roraima.

CONCLUSIONES

Este estudio presenta nueva information litologica, 

petrologica, y sedimentologica de ocho areas en el noreste 

del Parque Nacional Canaima. La estratigrafia de estas 

secuencias corresponden a dos grupos sedimentarios separadas por una disconformidad, y depositado en ambientes differentes. En la parte superior de la Serrania de Lema, los 

escarpes constituyen afloramientos de una secuencia sedimentaria compuesta por areniscas cuarzosas y conglomerados polimicticos con matriz de color rojo. Esta secuencia fue 

nombrada capas de Abaren, cuyas caracteristicas petrologicas, estructurales, y sedimentologicas son similares a las de 

la parte inferior del Grupo Roraima. Las capas descansan en 

contacto disconforme sobre otra secuencia, no tectonizada 

que se postula como la Formacion Urico, cuyas caracteristicas distintivas no permiten su correlation con el Grupo 

Roraima, tal y como ha sido defmido en trabajos anteriores.

La Formacion Urico tiene su section tipo en las 

cabeceras del Rio Urico y consiste de tres miembros. El mas 

antiguo tiene brechas polimicticas y grauwacas con un componente volcanico importante. El segundo miembro suprayace en contacto discordante. Esta compuesto por lutitas y 

limolitas de color gris oscuro, con pirita framboidal y cubica, 

lamination plano-paralela bien desarrollada, y una cantidad 

considerable de carbonato. El tercer miembro consiste de 

lutitas tobaceas verdes piritosas y limolitas tob£ceas rojas de 

gran persistencia lateral.

Las estructuras sedimentarias, las relaciones de campo, 

y las observaciones petrograficas permiten suponer que la 

Formacion Urico representa depositos marinos, posiblemente de talud o plataforma, mientras que las capas de 

Abaren representan paleocanales de rios entrelazados.

RECOMENDACIONES PARA 

TRABAJOS EN EL FUTURO

1. Cartografiar lateralmente la Formacion Urico y las 

capas de Abaren para relacionar su position con otras partes 

del Grupo Roraima.

2. Analizar quimicamente la secuencia sedimentaria 

en las cabeceras del Rio Urico, con especial enfasis sobre las 

lutitas gris-piritosas cuyo ambiente sedimentario pudiese 

haber favorecido la precipitation o deposition de oro, 

uranio, y otros elementos de interes.

3. Determinar el ambiente tectonico de sedimentacion para la Formacion Urico y la transition a las capas de 

Abaren.

4. Estudiar las fuentes de aporte y concentrados pesados en las capas de Abaren y en la Formacion Urico con el 

objeto de considerar las posibles correlaciones, fuentes para 

minerales, e interpretar el ambiente tectonico.

5. Realizar estudios geocronologicos para conocer la 

edad de la Formacion Urico.

6. Explorar el contacto entre la Formacion Urico y las 

capas de Abaren para posibles yacimientos economicos.

Agradecemos. Este trabajo fue publicado parcialmente en la memoria del septimo Congreso Geologico Venezolano en 1989. El trabajo siguiente fue colaborado por la 

Corporation Venezolana de Guayana-Tecnica Minera, C.A. 

(CVG-TECMIN), y el Servicio Geologico de los Estados 

Unidos. El honor de este trabajo va principalmente a los 

autores Alberdi y Contreras y se estima la colaboraci6n del 

Sr. John-Mark Staude por la revision. Nuestro ma's sincero 

agradecimiento al Dr. Vicente Mendoza, Dr. Santosh Ghosh, 

Dr. Jean Pasquali, Dra. Maria Lourdes de Gamero, Dr. Gary 

Sidder, Dr. Norman Page, Dr. Jeffrey Wynn, Ricardo 

Azuale, Dr. Jose Gutierrez, Acenk Guerra, Ricardo Cedeno, 

Alexis Cordova, Jorge Penott, Luis Carlos Genolet, a la seccion de cartografia de CVG-TECMIN y al dibujante Reinaldo Salazar, Bruce Galoob, y Patricia Covarrubias.

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Geology, Geochemistry, and Geophysics of 

Gold-Quartz Veins in the Piston de Uroy Area, 

Estado Bolivar, Venezuela

By Jeffrey C. Wynn, Norman J Page, Gloria Contreras, 

RJ. Quesada, Barry C. Moring, and Robert L. Oscarson

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-J

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENTS

Abstract........................................................................................................................... Jl

Resumen......................................................................................................................... 1

Introduction.................................................................................................................... 1

Geology.......................................................................................................................... 3

Geochemistry.................................................................................................................. 7

Geophysics...................................................................................................................... 14

Data Acquisition, Processing, and Analysis........................................................... 14

Magnetic Data................................................................................................. 14

Very Low Frequency Electromagnetic Data.................................................. 15

Induced Polarization Data.............................................................................. 16

Integration of the Geophysical Data....................................................................... 19

Conclusions.................................................................................................................... 20

References Cited............................................................................................................. 21

PLATE

[Plate is in pocket]

1. Schematic geologic map and cross sections, Piston de Uroy area, Estado Bolivar, Venezuela.

FIGURES

1, 2. Maps showing:

1. Location of Piston de Uroy area......................................................................................................................... J2

2. Quartz-vein system, geologic grid, and geophysical grid, Piston de Uroy study area....................................... 3

3. Scanning electron microprobe images and spectrum of sperrylite in composite grain of pyrite-chalcopyrite in

altered clinopyroxenite............................................................................................................................................... 7

4. Chondrite-normalized patterns for clinopyroxene, clinopyroxene-olivine, clinopyroxene-plagioclase,

olivine-chromite, and olivine-clinopyroxene-chromite cumulates............................................................................ 7

5. Total-field magnetic map ........................................................................................................................................... 15

6. Magnetic profile showing inferred causative body, pica 11....................................................................................... 15

7. Very low frequency electromagnetic profile along baseline of geologic grid........................................................... 16

8, 9. Maps showing contours of:

8. Gradient-array resistivity data from induced polarization survey acquired on geophysical grid....................... 18

9. Chargeability data from induced polarization survey......................................................................................... 18

10. Resistivity and chargeability profiles along pica 12 of geophysical grid.................................................................. 19

11. Map showing interpretation of magnetic, very low frequency electromagnetic, and induced polarization data

and results of geological mapping of quartz-vein system.......................................................................................... 20

TABLES

1. Trace-element and platinum-group-element analyses of unmineralized rocks from the Piston de Uroy area.......... J5

2. Gold and trace-element analyses of rock and soil samples from quartz veins in the Piston de Uroy area................ 8

3. Statistical parameters for unqualified values of analyses of quartz-vein material..................................................... 14
Geology, Geochemistry, and Geophysics of

Gold-Quartz Veins in the Piston de Uroy Area,

Estado Bolivar, Venezuela

By Jeffrey C. Wynn, 1 Norman J Page,2 Gloria Contreras,3 R.J. Quesada,4 

Barry C. Moring,5 and Robert L. Oscarson6

ABSTRACT

Placer mining has been active in the Piston de Uroy area 

of Estado Bolivar, Venezuela, on the middle Rio Chicanan 

since the 1940's. A recently discovered mineralized 

quartz-vein system in the area is hosted by a mafic-ultramafic complex emplaced in older greenstone-belt rocks of 

andesitic to basaltic composition. North of the mineralized 

area, sandstone and quartz-pebble conglomerate of the 

Roraima Group overlie rocks of the Early Proterozoic greenstone belts. The vein is 1-8 m or more wide and as long as 

3,000 m, and it has a significant gold content, as demonstrated both by direct sampling and by the extensive placer 

mining activity downstream. In addition, sperrylite (a platinum-bearing mineral) is present in the ultramafic (pyroxenite) rocks. A magnetic survey shows that the pyroxenite dips 

moderately to the north, and very low frequency electromagnetic data were used to map faulted offsets in the quartz-vein 

system. Gradient-array resistivity data outline the western 

extension of the quartz-vein system; the vein system probably extends 700-800 m farther to the west than mapped on 

the surface.

RESUMEN

La mineria de oro en placeres en la parte media del rio 

Chicanan ha estado activa desde la d6cada de 1940. 

Recientemente se ha descubierto un sistema mineralizado de

'U.S. Geological Survey, Unit 62101, APO AE 09811-2101, USA

2U.S. Geological Survey, Corbett Building., 210 E. 7th Street, Tucson, 

Arizona 85705.

3Voie de la Croix, 55000 Behonne, France; formerly Corporation 

Venezolana de Guayana, Tecnica Minera, C.A., Puerto Ordaz, Venezuela.

4Urbanizaci6n Quebrada de Cariaco, Calle Llamo Adentro, No. 276, 

Municipio Vargas, Caracas, Venezuela; formerly Corporation Venezolana 

de Guayana, Tecnica Minera, C.A., Puerto Ordaz, Venezuela.

5U.S. Geological Survey, 345 Middlefield Road, MS984, Menlo Park, 

California 94025.

6U.S. Geological Survey, 345 Middlefield Road, MS916, Menlo Park, 

California 94025.

vetas de cuarzo, las cuales estan encajadas en un complejo 

mafico-ultramafico que a su vez esta emplazado en rocas 

verdes (greenstones) mas antiguas de composition basaltica 

a andesitica. Al norte de la zona mineralizada, areniscas y 

conglomerados de cantos redondeados de cuarzo yacen por 

encima de los cinturones de rocas verdes de edad Proterozoico Temprano. La veta principal tiene 1 m de ancho y puede 

llegar a medir al menos 8 m de ancho, se extiende hasta 

3,000 m y tiene un contenido de oro significativo, lo cual esta 

indicado por muestreo directo y por la cantidad de actividad 

minera en placeres aguas abajo. Ademas, el mineral 

sperrylita, el cual contiene platino, esta presente en las rocas 

ultramaficas (pyroxenita). La prospeccion magnetica muestra que la pyroxenita esta buzando moderadamente hacia el 

norte. La prospeccion electromagnetica de baja frequencia 

(VLF) fue usada para ubicar segmentos fallados en el 

sistema de vetas de cuarzo. "Gradient array resistivity data" 

revelan que el sistema de vetas de cuarzo se extiende entre 

700 y 800 m mas hacia el oeste que lo indicado en superficie.

INTRODUCTION

The Piston de Uroy area is southwest of El Dorado in 

Estado Bolivar, north of the Rio Chicanan between lat 6° 15' 

and 6°20' N. and long 61°51' and 61°56/ W. (fig. 1). Access 

to the area is either by air or river; roads to the area do not 

exist.

Placer deposits of gold have been exploited on the Rio 

Chicanan and its tributaries near Piston de Uroy since the 

1940's, thus indicating that the area might contain lode 

sources of gold. During regional reconnaissance, gold was 

identified in alluvial deposits in the La Franela stream channel and elsewhere in the Piston de Uroy area, and a complex 

suite of mafic and ultramafic rocks was identified that is 

overlain unconformably by sedimentary rocks of the Early to 

Middle Proterozoic Roraima Group (Alberdi, 1988). Subsequently, in March and November 1988 and February and 

May 1989, N.J Page, G. Contreras, J.C. Wynn, D. Freitas, J. 

Quesada, and L. Guilloux conducted the fieldwork that

Jl
J2 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

7°00'

6°00'

Puerto raz _ sT^DO DELTA 

Ciudad Bo!ivat^>7 v^AMACURO 

Logo de X Vx'^\/^= 

Guri

Boundary of Venezuelan 

Guayana Shield

Figure 1. Maps showing location of the Pist6n de Uroy study area, Estado Bolivar, Venezuela.
GOLD-QUARTZ VEINS, PISTON DE UROY AREA J3

6°20'

61°52'30"

6°16'

3 KILOMETERS

Figure 2. Map showing the quartz-vein system (railroad tracks), geologic grid (smaller, east-west grid parallel with the quartz-vein 

system), and geophysical grid (larger, regular grid with eje (baseline) trending N. 80° E. and picas trending north-northwest and 

numbered from east to west), Piston de Uroy area, Venezuela. The geophysical camp is shown by a triangle and the local heliport as a 

circle with four tick marks.

forms the basis for this report. Although some outcrops are 

present in the drainages and on ridges in the area, the area is 

heavily forested, and much of the study was conducted using 

cleared paths or profiles (called picas) at 100-m intervals 

perpendicular to axes that are subparallel with the strike of 

the vein structure and the geologic units. The location of the 

geophysical and geological grids established in the study 

area and the location of the quartz-vein system are shown in 

figure 2. Within the context of the regional geology, which 

is not well known, the structure, lithology, petrology, and

geochemistry of the gold-quartz veins and their host rocks in 

the Piston de Uroy area are described. Because of the extensive soil and jungle coverage of the area, resistivity, 

magnetic, and very low frequency electromagnetic geophysical techniques were used to better define the geology.

GEOLOGY

The Piston de Uroy area forms part of an extensive 

greenstone belt in which rocks of the Pastora Supergroup
J4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

and the younger Botanamo Group have been reported (Mendoza, 1977; Tosiani and Sifontes, 1989). To the north, 

between the Rio Chicanan and the town of El Dorado, 

Tosiani and Sifontes (1989) identified gabbro, pyroxenite, 

and olivine cumulates. More recent work (Floyd Gray, U.S. 

Geological Survey, oral commun., 1990) confirms the existence of mafic and ultramafic cumulates in this area. This 

mafic-ultramafic complex was emplaced into andesitic and 

more siliceous volcanic rocks. Areas east and northeast of 

Piston de Uroy are underlain by basaltic and andesitic volcanic rocks intruded by granitic rocks of the Supamo Complex. 

The greenstone-granite terrane is overlain by rocks of the 

Roraima Group (Alberdi and Contreras, this volume).

Folded flows, tuffs, and volcaniclastic rocks of 

andesitic to basaltic composition form the country rocks of a 

mafic-ultramafic complex that is host to the mineralized 

quartz-carbonate veins north of Rio Chicanan (plate 1). Both 

groups of rocks are cut by diabase dikes; in addition, the 

mafic-ultramafic complex is cut by basaltic to andesitic brecciated and porphyritic dikes that are locally mineralized with 

pyrite and chalcopyrite. These volcanic and plutonic rocks 

underlie the relatively low (50-100 m relief), rounded hills 

north of the river. The steep escarpment farther to the north 

is formed by unconformably overlying rocks of the Roraima 

Group including sandstone, quartzite, conglomerate, and 

interlayered mudstone. Extensive colluvial and alluvial 

deposits cover much of the pre-Roraima Group bedrock, and 

close to the escarpment large blocks of quartzite, some as big 

as houses, form talus piles. Major structural features in the 

area include faults and east-striking quartz veins; the veins 

are cut by north-northwest-striking faults. On the southeast 

side of the mafic-ultramafic complex, veins and faults are cut 

by a northeast-striking fault filled by a diabase dike.

The mafic-ultramafic complex is composed of layered 

cumulus dunite, wehrlite, and clinopyroxenite and locally 

foliated, laminated, generally homogeneous gabbro. Layering and lamination strike northwest and dip to the north; 

modal-, phase-, and size-graded layering are present in the 

ultramafic rocks. Locally, the gabbro forms phase layers of 

two-pyroxene plagioclase cumulate and plagioclase 

cumulate containing oikocrysts of pyroxene. The ultramafic 

rocks are composed of varying proportions of cumulus 

olivine, chromite, and clinopyroxene and postcumulus 

plagioclase, orthopyroxene, and brown hornblende. Textures are generally well preserved; however, the primary 

minerals are partly to totally altered to mixtures of serpentine 

minerals, talc, amphiboles, and magnetite. Accessory sulfide 

minerals include chalcopyrite, pyrrhotite, and pyrite; 

chalcopyrite is the most abundant sulfide mineral in clinopyroxene cumulates.

Olivine and olivine-chromite cumulates contain euhedral to subhedral olivine altered to serpentine minerals and 

magnetite, accessory amounts of pyrrhotite, chalcopyrite, 

and pyrite, and euhedral chromite that has secondary 

magnetite rims. Locally, oikocrysts of orthopyroxene are

altered to talc, amphibole, and serpentine minerals. Most of 

the sulfide minerals are in interstitial spaces or at the margins 

of cumulus crystals, but rare inclusions of pyrrhotite and 

chalcopyrite in chromite are present.

Clinopyroxene cumulates consist of euhedral to subhedral, cumulus clinopyroxene that is locally twinned, has 

spinel exsolution lamellae, and is replaced to some degree by 

postcumulus brown amphibole. Interstitial postcumulus plagioclase is altered to tremolite-actinolite, chlorite, epidote, 

clay, and possible zeolite minerals. Some rocks contain 

oikocrysts of orthopyroxene altered to serpentine minerals 

and tremolite-actinolite. Primary brown hornblende is 

present interstitially. Euhedral to subhedral magnetite having exsolution lamellae of ilmenite (?) is also a primary magmatic mineral; both the magnetite and ilmenite are altered to 

secondary iron oxide minerals. One sample contained both 

interstitial biotite altered to chlorite and quartz associated 

with sulfide clots.

Secondary minerals in the clinopyroxene cumulates 

include green-brown to colorless amphibole that locally 

forms radiating clusters of crystals, as well as talc, chlorite, 

magnetite, epidote, and serpentine minerals. Sulfide minerals such as pyrrhotite, chalcopyrite, and pyrite are present as 

either single- or multi-phase grains and form inclusions in 

primary magnetite and clinopyroxene; they also are present 

in interstitial spaces. During the development of secondary 

amphiboles and other alteration minerals, the sulfide minerals were remobilized to varying degrees and formed 

intergrowths with the amphiboles. In the more altered rocks, 

pyrrhotite and chalcopyrite are replaced marginally and 

along fractures by magnetite. Sparse clusters of pyrite 

crystals, as much as 1 mm in diameter, are present. In these 

clusters, chalcopyrite and pyrrhotite are present both as 

inclusions in individual pyrite crystals and as interstitial 

material between pyrite crystals.

Clinopyroxene-olivine, clinopyroxene-orthopyroxene, 

and clinopyroxene-olivine-chromite cumulates are also 

present in the mafic-ultramafic complex. Primary and 

secondary minerals have the same cumulus and alteration 

textures as in the single-phase cumulates. The clinopyroxene-olivine-chromite cumulate contains only traces of 

pyrite, and the clinopyroxene-olivine cumulate contains only 

traces of pyrrhotite and chalcopyrite. These two latter types 

of cumulates are notably deficient in sulfide minerals in 

comparison with other cumulates.

The mafic part of the complex consists of plagioclase-clinopyroxene cumulates and minor amounts of 

plagioclase-clinopyroxene-orthopyroxene and plagioclase 

cumulates; all three rock types are locally laminated and 

modally layered. The plagioclase-clinopyroxene cumulate is 

composed of euhedral to subhedral cumulus plagioclase and 

clinopyroxene; postcumulus brown hornblende fills interstitial spaces and replaces clinopyroxene. Primary magnetite is 

also present. Secondary alteration minerals include variously 

colored amphiboles, chlorite, and epidote. Chalcopyrite,
Table 1. Trace-element and platinum-group-element analyses of unmineralized rocks from the Piston de Uroy area, Venezuela.

[Location of samples shown by number on plate 1. Semiquantitative six-step emission spectrographic analyses are in parts per million except for Ti, which is in percent;

analyst, R. Hopkins. Fire-assay inductively coupled mass spectrometry analyses are in parts per billion; analysts, P. Aruscavage and G. Riddle. Elements looked for but not detected at

levels indicated given (in parts per million): Ag (0.5), As (200), Au (10), Be (1), Bi (10), Cd (20), Ge (10), La (50), Mo (5), Nb (20), Sb (200), Th (100), W (20), and Zn (200).

N indicates not detected at level shown]

UGP-1 UGP-3 UGP-5 UGP-8 UGP-14 UGP-17 UGP-25 UGP-28 UGP-29 UGP-30 UGP-35 UGP-36 UGP-37 UGP-38 UGP-39 UGP-^41

Semiquantitative emission spectrography

TiBBa

Co

CrCu

GaMn

NiPb

SeSr

V

Y

Zr

0.3 0.2 0.3 0.15

15 10 10 N10

1,500 50 N20 N20

15 200 200 30

70 5,000 1,500 700

30 10 30 1530 10 10 5

700 1,000 1,000 700

30 1,500 300 10030 <10 <10 N10

10 10 50 15

500 N100 N100 N100

100 150 150 70

15 N10 15 <10

100 N10 20 10

0.5

15

20

300

5,000

70

7

1,000

1,500

N10

15

N100

200

<10

10

0.5

<10<20

100

3,000

50

7

1,000

700

N10

50

N100

150

15

20

0.7

20

1,500

70

1,000

15

20

1,000

200

15

30

500

150

20

100

0.5 0.3

10 10

20 <20

70 200

1,500 1,500

300 70

10 10

1,500 1,500

300 700

N10 N10

50 30

N100 N100

150 100

15 10

30 15

0.3 0.2

10 10

20 2070 70

1,000 50

300 300

10 20

0.15

10

20

7050

200

15

1,000 1,000 1,000

200 150

<10 <10

30 30

150 300100 150

15 10

10 10

150

<10

20

300

100

<10

10

0.3

<10

N10

100

1,000

30

10

1,000 1

200

<10

30

N100

150

15

30

1.0

<10

707030

70

20

,000

30

10

20

300300

20

100

0.5

10

<20

70

3,000

50

10

1,500

200

N10

30

N100

150

20

15

0.3

10

200

70

700

70

15

1,000

150

N10

20

300

150

10

15

Inductively coupled mass spectrometry

Pt

Pd

RhRu

Ir

<0.5 13 86 1.5

<0.8 16 39 8.0

<0.5 0.6 12 <0.5<0.5 20 19 <0.5

<0.5 3.5 10 <0.5

1.8

17

0.8

13

3.0

250

260

21

41

18

9.2

12

0.7

0.9

<0.5

1.5 <0.5

2.3 1.1

<0.5 <0.5<0.5 <0.5

<0.5 <0.5

1.0 0.9

0.9 1.3

<0.5 <0.5

<0.5 <0.5<0.5 <0.5

0.6

2.6

<0.5

<0.5<0.5

0.6

<0.8<0.5

<0.5

<0.5

170100

2522

14

170

190

15

30

13

0.5

1.3

<0.5<0.5<0.5

Sample descriptions

UGP-1UGP-3

UGP-5

UGP-8

UGP-14

UGP-1 7

UGP-25

UGP-28

Porphyritic andesite, altered

Olivine-chromite cumulate, serpentinizedCinopyroxene cumulate

Clinopyroxene cumulate, altered

Olivine-pyroxene-chromite cumulate, serpentinizedClinopyroxene-plagioclase(?) cumulate, altered

Volcanic rock, totally alteredClinopyroxene cumulate

UGP-29UGP-30UGP-35

UGP-36

UGP-37UGP-38

UGP-39UGP-44

Clinopyroxene cumulate, alteredClinopyroxene-orthopyroxene cumulate

Clinopyroxene-plagioclase cumulate

Clinopyroxene-plagioclase cumulate

Clinopyroxene cumulateClinopyroxene-olivine cumulate

Clinopyroxene cumulate, alteredPlagioclase-clinopyroxene cumulate, laminated

I

onis

> 

>

Contains 3 ppm Ag.
J6 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

pyrrhotite, and pyrite are present interstitially, and their 

intergrowth with secondary amphiboles indicates some 

degree of remobilization of elements in the sulfide minerals 

during alteration. Locally, pyrite is present as sparsely 

distributed single crystals as much as 2 mm in size. In more 

altered rocks, both chalcopyrite and pyrrhotite are replaced 

by magnetite, and covellite and chalcocite locally replace 

chalcopyrite.

The quartz-vein system was mapped in detail between 

the El Piston drainage and slightly west of the La Franela 

drainage (fig. 2, plate 1). The veins generally strike approximately east and dip steeply south or north. The veins and 

bands of altered country rock are 1-8 m wide. Outcrops are 

sparse in the area; the veins are marked only by blocks of 

quartz, rare outcrops, and, in places, extensive float. Where 

outcrop was sufficient, profiles perpendicular to the width of 

the vein were described, measured, and sampled by chipping 

across the profile over a measured thickness (plate 1). Elsewhere, chip samples from blocks were collected; sampling 

was biased toward the more oxide stained parts of the blocks. 

The contact between gabbro and ultramafic rocks was not 

observed but was mapped using geophysical information 

(Quesada and Wynn, 1990).

Quartz-vein material is present in four different geological settings (plate !):(!) near the southern contact between 

gabbro and the ultramafic rocks; for example, from east of 

the La Franela drainage to 0+430 W on the southern eje; (2) 

near the northern contact between gabbro and ultramafic 

rocks; for example, between 0+150 and 0+650E on the 

northern eje; (3) in gabbro; for example, between 1+100 and 

1+100E on southern eje; and (4) in ultramafic rocks; for 

example between 0+500 and 1+OOOE on the northern eje. As 

shown in the cross sections (plate 1), the characteristics of the 

veins change along their strike and across their width. For 

example, in outcrops along the La Franela drainage (see profile, plate 1), the vein system consists of several thin (as thick 

as 20 cm) quartz veins and quartz stockwork over a width of 

about 5 m in sheared and highly altered ultramafic rocks consisting of talc and serpentine, carbonate, and iron oxide minerals. East of La Franela, the vein is represented by 

2-5-m-wide blocks of white quartz or quartz and oxidized 

material. In general, veins in ultramafic rocks contain less 

quartz by volume than veins in gabbroic rocks.

The vein material includes white, massive, barren 

quartz; quartz-carbonate; gray quartz; manganese-ironoxide-brecciated quartz; and vuggy quartz crystals. Alteration minerals consist of talc, amphiboles (?), chlorite, and 

serpentine, carbonate, and iron oxide minerals in highly 

sheared rock. All of the surface exposures are oxidized and 

apparently leached. Sulfide minerals were found only in vein 

material from a dump of an exploration adit at the eastern end 

of the vein system.

The veins formed in shear zones, near lithologic contacts or in faults, that strike approximately east. This early 

generation of structures is cut by faults that trend
GOLD-QUARTZ VEINS, PISTON DE UROY AREA J7

Figure 3 (above and facing column). Scanning electron microprobe images and spectrum of sperrylite in a composite grain of 

pyrite-chalcopyrite in altered clinopyroxenite, sample UGP-39, Pist6n de Uroy area, Venezuela. A-C, Backscattered electron images. 

White area, sperrylite (s); light-gray areas, chalcopyrite (c) and pentlandite (pn); medium-gray area, pyrite (py); black area, silicate minerals. 

B is an enlargement of the area in A that is shown by the box. C, Images for iron, copper, sulfur, platinum, arsenic, and nickel. D, Spectrum 

from the sperrylite grain.

0.1

0.001

Ir Ru Rh Pt Pd

EXPLANATION

 UGP-17 A UGP-38 n UGP-14 V UGP-25 

A UGP-39 O UGP-5 UGP-3

Figure 4. Chondrite-normalized patterns for clinopyroxene 

(UGP-39, UGP-5), clinopyroxene-olivine (UGP-38), clinopyroxene-plagioclase (UGP-17), olivine-chromite (UGP-3), and olivine-clinopyroxene-chromite (UGP-14) cumulates and a sample of 

volcanic rock (UGP-25), Pist6n de Uroy area, Venezuela. Values 

used for normalization are from Naldrett (1981).

north-northwest and offset the veins. These faults are cut by 

a third generation of structures that probably are major 

regional shear zones, such as the faults causing the El Piston 

drainage, and are clearly visible in the very low frequency 

electromagnetic data (described later in this chapter).

GEOCHEMISTRY

A variety of materials was sampled during the course of 

the study. Panned concentrates were examined visually for 

gold (plate 1, VP-number series). In drainages that cross the 

vein system, samples collected below the veins contain gold. 

One hundred and nineteen samples of vein material, soil, and 

weathered adjacent rock were analyzed by semiquantitative 

six-step emission spectrography (Grimes and Marranzino, 

1968) and for platinum-group elements by fire-assay inductively coupled plasma mass spectrometry (Meier and others, 

1988). Vein materials and soils were analyzed for Se by 

hydride-generation atomic absorption, for Te and Ti by 

flameless atomic absorption, for Hg by cold-vapor atomic 

absorption, for F by specific-ion electrode, for Au by graphite-furnace atomic absorption, and for As, Bi, Cd, Sb, and Zn 

by inductively coupled plasma; methods are described by 

O'Leary and Meier (1986). Table 1 presents analytical 

results for unmineralized rocks and table 2 analytical results 

for vein material, soil, and weathered rock samples.
Table 2. Gold and trace-element analyses of rock and soil samples from quartz veins in the Piston de Uroy area, Venezuela.

[Location of samples shown by number on plate 1. In parts per million except for Au, which is in parts per billion. N indicates not detected at level shown; leaders (--

Analysts: R. Hopkins, P. Hageman, E. Welsch, F. Tippih, J. Shankey, D. Fey]

VI V2 V3 V4 V5 V6 V7 V8 V9 V10 V

Gold and trace elements by various methods

Au

As

Cd

Hg

Sb

Se

Te

Tl

Zn

7

100

-

<0.02

-

0.3

<0.05

<0.05

-

42

1.0

-

<0.02

-

<0.1

<0.05

<0.05

-

<2

0.4

-

<0.02

-

<0.1

<0.05

<0.05

--

<2

4.8

-

<0.02

-

0.1

<0.05

<0.05

-

4,640

5.5

-

<0.02

-

0.2

0.05

0.20

-

320 25 <2

170 1.8 18

..

<0.02 <0.02 <0.02

..

0.4 <0.1 <0.1

0.05 <0.05 <0.05

0.15 <0.05 <0.05

<2

0.2

-

0.02

-

<0.1

0.15

<0.05

<2

35

-

<0.02

-

0.2

<0.05

<0.05

-

<2

-

<0

-

<0

<0

<0

-

Semiquantitative emission spectrography

Ag

Ba

Co

Cr

Cu

Mn

Ni

Pb

V

N0.5

20

200

1,000

50

1500

2,000

10

100

V13

N0.5

30

N10

15

<5

30

7

N10

<10

V14

N0.5

20

N10

30

N5

10

<5

N10

15

V15

N0.5

<20

N10

30

10

15

<5

N10

30

V16

N0.5

700

500

20

300

>5,000

150

<10

50

V17

N0.5 N0.5 N0.5

30 20 <20

200 <10 30

1,000 70 700

300 30 50

1,000 500 200

150 7 100

15 N10 N10

150 30 150

UV-1-89 UV-2-89 UV-3-89

N0.5

20

N10

20

7

50

<5

20

20

UV^-89

N0.5

<20

20

100

20

500

50

N10

30

UV-5-89

N0

<20

N10

20

7

150

N10

15

UV

Gold and trace elements by various methods

Au

As

Cd

Hg

Sb

Se

Te

Tl

Zn

60

87

-

<0.02

-

0.8

<0.05

0.60

-

1100

430

<0.02

-

0.9

0.25

0.15

-

220

210

-

<0.02

~

0.3

<0.05

0.15

-

<2

4.6

-

<0.02

--

0.5

<0.05

<0.05

-

<2

8.2

-

<0.02

~

0.3

<0.05

<0.05

-

84 40 <2

1,310 170 <5

0.7 0.4 <0.1

0.20 0.02 N0.02

6.0 2 <2

5.8 1.6 <0.2

0.4 0.05 <0.05

0.15 0.10 <0.05

40 40 7

6

<5

<0.1

N0.02

4

<0.02

0.10

<0.05

53

6

12

<0.1

N0.02

<2

0.2

<0.05

<0.05

11

<2

1

<0

N

<2

<0

<

<

1

Semiquantitative emission spectrography

Ag

Ba

Co

Cr

Cu

Mn

Ni

Pb

V

N0.5

<20

20

300

50

300

100

<10

50

N0.5

<20

70

700

50

700

500

10

70

N0.5

70

150

1,500

100

1,000

500

10

100

N0.5

30

30

200

15

700

30

N10

70

N0.5

20

N10

70

15

100

7

<10

20

N0.5 N0.5 N0.5

70 50 100

50 50 50

1,000 700 50

70 70 30

1,500 1,000 1,500

300 200 70

15 20 N10

150 100 30

N0.5

300

300

50

700

>5,000

700

N10

100

N0.5

70

30

100

50

1,500

50

N10

70

N

10

3

15

5

1,50

10

N1

7
UV-9-89 UV-10-89 UV-1 1-89 UV-1 2-89 UV-1 3-89 UV-1 4-89 UV-1 5-89 UV-1 6-89 UV-17-89 UV-1 8-89 U

Gold and trace elements by various methods

Au

As

Cd

Hg

Sb

Se

Te

Tl

Zn

14

15

<0.1

N0.02

<2

<0.02

<0.05

<0.05

2

12

<5

<0.1

0.02

<2

<0.02

<0.05

<0.05

<2

22

7

<0.1

N0.20

<2

<0.02

<0.05

<0.05

4

38

<5

<0.1

0.02

<2

<0.02

<0.05

<0.05

2

1,400 400 22

9 33 21

1.2 1.3 0.4

N0.20 1.7 0.04

<2 <2 <2

<0.02 0.20 0.9

<0.05 0.15 0.05

0.30 0.40 0.25

55 35 5

22

57

2.0

0.04

<2

0.9

0.20

0.10

39

350

43

0.7

0.06

<2

1.2

0.10

0.20

18

1,600

19

0.8

0.02

<2

<0.2

0.05

0.05

37

Semiquantitative emission spectrography

Ag

Ba

Co

Cr

Cu

Mn

Ni

Pb

V

N0.5

20

<10

70

30

200

10

N10

30

UV-22-89

N0.5

50

70

20

30

1,500

30

N10

30

UV-23-89

7

70

100

50

70

1,500

30

N10

30

UV-24-89

N0.5

30

70

50

30

1,000

20

N10

30

UV-25-89

<0.5 0.5 N0.5

700 200 200

300 30 N10

70 500 200

300 200 100

>5,000 300 150

100 50 70

N10 15 15

50 150 1 50

UV-26-89 UV-27-89 UV-28-89

N0.5

70

70

700

300

200

70

15

150

UV-29-89

N0.5

200

30

200

150

200

70

15

150

UV-30-89

N0.5

20

30

50

150

200

50

10

70

UV-31-89U

Gold and trace elements by various methods

Au

As

Cd

Hg

Sb

Se

Te

Tl

Zn

950

20

0.9

0.06

<2

1.2

0.20

0.20

23

120

26

0.9

0.04

<2

0.8

0.10

0.25

19

300

20

1.2

0.02

<2

0.2

<0.05

0.20

36

350

32

0.9

0.04

<2

0.7

0.10

0.20

25

4 42 200

24 <5 366

<0.1 <0.1 0.9

0.04 N0.02 N0.02

<2 <2 <2

0.8 0.3 1.5

0.05 <0.05 0.20

0.05 0.05 0.05

4 <2 51

4200

46

2.6

0.04

<2

1.0

0.35

0.15

57

14

140

1.8

N0.02

<2

<0.2

0.05

<0.05

111

20

84

0.4

N0.02

<2

<0.2

<0.05

<0.05

107

Semiquantitative emission spectrography

Ag

Ba

Co

Cr

Cu

Mn

Ni

Pb

V

N0.5

150

30

500

300

300

70

15

200

N0.5

200

30

300

500

300

70

15

200

N0.5

200

70

150

300

200

100

10

150

N0.5

200

50

300

300

500

100

15

200

N0.5 N0.5 N0.5

70 30 30

<10 N10 30

150 100 500

50 10 50

300 150 700

50 7 150

<10 <10 15

100 70 100

N0.5

100

70

1,000

150

1,500

300

15

150

N0.5

150

100

2,000

70

1,500

500

15

150

N0.5

200

100

2,000

150

1,500

500

<10

150

2
Table 2. Gold and trace-element analyses of rock and soil samples from quartz veins in the Piston de Uroy area, Venezuela Continued.

UV-35-89 UV-36-89 UV-37-89 UV-38-89 TjV-39-89 UV-40-89 UV-41-89 UV-42-89 UV-^3-89 UV^5-89 UV

Gold and trace elements by various methods

Au

As

Cd

Hg

Sb

Se

Tl

Ti

Zn

10

68

1.4

N0.02

<2

<0.2

<0.05

0.10

92

4

48

1.9

N0.02

<2

<0.2

<0.05

0.05

116

8

47

<0.1

N0.02

<2

<0.2

<0.05

<0.05

25

10

109

1.2

N0.02

<2

<0.2

<0.05

<0.05

73

282

304 182 <5

0.7 0.5 <0.1

0.02 0.02 0.02

4 <2 <2

0.3 0.4 0.2

0.05 0.10 <0.05

0.15 0.05 0.05

58 40 2

300

10

<0.1

0.04

<2

0.3

0.10

0.10

2

16

6

<0.1

N0.02

<2

<0.2

<0.05

<0.05

<2

28

8

<0.1

0.02

<2

1.2

0.05

<0.05

31

<

1

<

N

<

<

<

<

Semiquantitative emission spectrography

Ag

Ba

Co

Cr

Cu

Mn

Ni

Pb

V

N0.5

300

100

2,000

200

1,500

700

<10

150

UV-49-89

N0.5

500

150

3,000

100

1,500

700

<10

150

UV-50-89

N0.5

100

70

2,000

200

700

500

<10

150

UV-51-891

N0.5

150

200

5,000

300

1,000

1,000

<10

200

UV-52-892

N0.5 N0.5 N0.5

200 20 30

100 30 <10

700 700 100

50 30 30

1,500 1,000 500

150 100 30

10 <10 <10

150 150 30

UV-53-893 UV-54-89 UV-55-89

0.7

30

30

500

300

1,000

150

N10

100

UV-56-89

N0.5

70

N10

70

20

300

7

N10

30

UV-57-89

N0.5

<20

70

1,500

70

1,500

200

N10

100

UV-58-89

N

2

<1

15

5

30

3

N1

7

UV-

Gold and trace elements by various methods

Au

As

Cd

Hg

Sb

Se

Tl

Ti

Zn

<2

26

<0.1

N0.02

<2

0.6

0.10

<0.05

<2

<2

12

<0.1

N0.02

<2

<0.2

<0.05

<0.05

2

2

62

<0.1

0.08

2

<0.2

0.30

<0.05

29

10

49

0.1

0.02

3

0.3

0.10

<0.05

18

68 22 30

64 270 295

0.2 0.2. 0.2

0.02 N0.02 N0.02

<2 3 <2

0.3 <0.2 0.3

0.10 0.10 0.05

<0.05 <0.05 <0.05

21 80 70

6

245

0.2

0.02

3

0.6

0.10

<0.05

90

14

38

0.1

N0.02

<2

<0.2

0.15

<0.05

50

<2

20

0.4

0.02

<2

0.7

<0.05

0.05

20

<

<

<

<

4

Semiquantitative emission spectrography

Ag

Ba

Co

Cr

Cu

Mn

Ni

Pb

V

N0.5

30

N10

150

30

200

15

<10

70

N0.5

30

100

150

20

700

10

N10

50

N0.5

<20

300

200

50

1,500

150

<10

70

N0.5

20

30

300

30

1,000

150

<10

100

N0.5 N0.5 N0.5

30 100 70

70 70 200

700 700 1,000

70 100 150

700 1,000 1,500

150 300 700

15 <10 <10

150 150 150

N.5

100

200

3,000

150

1,500

1,000

<10

200

N0.5

30

30

150

50

700

100

<10

100

N0.5

50

70

700

50

1500

100

<10

150

N

5

7

1,00

5

2,00

15

1

20

'Contains 20 ppm Mo. 

2Contains 5 ppm Mo. 

'Contains 10 ppm Mo.
UV-62-89 UV-63-89 UV-64-89 UV-65-89 UV-66-89 UV-67-89 UV-81-89 UV-82-89 UV-83-89 UV-84-89 U

Gold and trace elements by various methods

Au

As

Cd

Hg

Sb

Se

Tl

Ti

Zn

<2

<5

0.1

0.02

<2

<0.2

0.20

<0.05

5

<2

<5

0.3

N0.02

<2

<0.2

0.25

<0.05

14

<2

17

0.8

0.04

<2

0.6

0.30

<0.05

22

<2

5

<0.1

N0.02

<2

0.3

<0.05

<0.05

6

<2

151

0.4

N0.02

<2

0.3

<0.5

<0.05

27

<2 <2 <2

11 375 39

<0.1 0.4 <0.1

N0.02 N0.02 N0.02

<2 <2 <2

<0.2 0.9 <0.2

<0.05 0.25 <0.05

<0.05 <0.05 <0.05

<2 9 2

<2

29

<0.1

N0.02

<2

<0.2

<0.05

<0.05

3

4

427

0.3

N0.02

5

1.0

<0.05

<0.05

15

Semiquantitative emission spectrography

Ag

Ba

Co

Cr

Cu

Mn

Ni

Pb

V

N0.5

300

15

150

30

700

70

<10

150

UV-88-89

N0.5

50

20

200

30

700

100

<10

150

UV-89-89

N0.5

300

70

300

200

3,000

150

15

200

UV-91-89

N0.5

100

200

70

70

3,000

100

N10

30

UV-92-89

N0.5

200

100

2,000

100

1,500

500

10

150

UV-93-89

N0.5 N0.5 N0.5

30 20 20

N10 10 N10

20 500 200

5 70 30

700 300 150

5 100 30

N10 10 <10

20 150 70

UV-94-89 UV-95-89 UV-96-89

N0.5

30

N10

150

15

500

15

<10

50

UV-97-89

N0.5

100

70

700

50

1,500

300

<10

150

UV-98-89

N

N

U

Gold and trace elements by various methods

Au

As

Cd

Hg

Sb

Se

Tl

Ti

Zn

2

64

<0.1

N0.02

<2

0.3

<0.05

<0.05

3

650

695

0.9

N0.02

6

1.1

0.40

<0.05

32

2

138

<0.1

N0.02

<2

1.1

0.35

<0.05

4

2

235

<0.1

N0.02

2

0.5

0.65

<0.05

7

18

50

<0.1

N0.02

<2

<0.2

<0.05

<0.05

<2

4 2 <2

33 47 79

<0.1 <0.1 <0.1

N0.02 0.04 N0.02

<2 5 4

<0.2 0.3 0.3

<0.05 N0.05 N0.05

<0.05 N0.05 N0.05

<2 4 4

<2

110

<0.1

N0.02

3

0.3

N0.05

N0.05

4

10

48

<0.1

N0.02

<2

0.3

N0.05

N0.05

<2

Semiquantitative emission spectrography

Ag

Ba

Co

Cr

Cu

Mn

Ni

Pb

V

N0.5

20

<10

150

30

150

50

N10

70

N0.5

30

30

700

300

300

200

15

100

N0.5

30

<10

200

50

30

50

N10

70

N0.5

30

15

200

70

70

100

<10

70

N0.5

20

N10

100

30

50

15

N10

30

N0.5

30

N10

150

20

50

20

N10

50

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-
Table 2. Gold and trace-element analyses of rock and soil samples from quartz veins in the Pist6n de Uroy area, Venezuela Continued.

Sample descriptions

V1 Chip sample of quartz vein 

V2 Chip sample of quartz vein

V3 Chip sample of quartz vein

V4 Chip sample of quartz vein

V5 Chip sample of quartz vein

V6 Chip sample of quartz vein

V7 Chip sample of quartz vein

V8 Chip sample of quartz vein

V9 Chip sample of quartz vein

V10 Chip sample of quartz vein

V11A Chip sample of quartz vein

V11B Chip sample of quartz vein

V12 Chip sample of quartz vein

V13 Chip sample of quartz vein

V14 Chip sample of quartz vein

V15 Chip sample of quartz vein

V16 Chip sample of quartz vein

V17 Chip sample of quartz vein 

UV-1-89 Chip sample of white quartz vein,

approximately 5 m thick 

UV-2-89 Chip sample of oxide material and quartz from

blocks 

UV-3-89 Chip sample of white quartz, 0.5 m thick

UV-4-89 Chip sample of gray brecciated quartz, 2.5 m

thick

UV-5-89 Chip sample of foliated white quartz, 2 cm thick 

UV-6-89 Chip sample of altered, foliated quartz, 30 cm

thick 

UV-7-89 Chip sample of altered ultramafic rock, 2 m

thick

UV-8-89 Chip sample of foliated oxide quartz, 1 m thick 

UV-9-89 Chip sample of white quartz, 50 cm thick

UV-10-89 Chip sample of oxide-rich quartz, 1 m thick 

UV-11-89 Chip sample of altered, oxide rich quartz vein,

1 m thick 

UV-12-89 Chip sample of white quartz with limonite, 3 m

thick 

UV-13-89 Chip sample of foliated quartz blocks

UV-14-89 Yellow soil, channel sample across 25-cm

interval 

UV-15-89 Red-brown soil, channel sample across 50-cm

interval 

UV-16-89 Yellow soil, channel sample across 30-cm

interval 

UV-17-89 Red-brown soil, channel sample across 35-cm

interval 

UV-18-89 Oxidized quartz vein, channel sample across

40-cm interval 

UV-19-89 Red-brown soil, channel sample across 40-cm

interval 

UV-20-89 Yellow soil, channel sample across 20-cm

interval 

UV-21-89 Red-brown soil, channel sample across 40-cm

interval 

UV-22-89 Red-brown soil, channel sample across 20-cm

interval 

UV-23-89 Red-brown soil, channel sample across 135-cm

interval 

UV-24-89 Red-brown soil, channel sample across 30-cm

interval 

UV-25-89 Red-brown soil, channel sample across 150-cm

interval 

UV-26-89 Red-brown soil, channel sample across 150-cm

interval

UV-27-89 Chip sample from blocks of quartz vein 

UV-28-89 Chip sample from altered quartz-carbonate

boxwork, about 5 m thick 

UV-29-89 Chip sample from quartz-carbonate boxwork, 1

m thick 

UV-30-89 Chip sample from altered ultramafic rock, 50

cm thick 

UV-31-89 Quartz-carbonate vein, 2 cm thick

UV-32-89 Chip sample of altered rock, 50 cm thick 

UV-33-89 Quartz-carbonate vein, 2 cm thick

UV-34-89 Chip sample of altered rock, 120 cm thick

UV-35-89 Quartz-carbonate vein, 2 cm thick 

UV-36-89 Chip sample of altered rock, 47 cm thick

UV-37-89 Chip sample o

UV-38-89 Chip sample o

UV-39-89 Chip sample fr

vein

Grab sample o

minerals

Grab sample o

quartz crystals 

Grab sample o

UV-46-89 Grab sample o

rock 

UV-47-89 Grab sample o

UV^8-89 Grab sample o

UV-49-89 Grab sample o

UV-50-89 Grab sample o

UV-51-89 Chip sample o

thick 

UV-52-89 Chip sample o

thick 

UV-53-89 Chip sample o

m thick 

UV-54-89 Quartz stockw

UV-55-89 Quartz veinlet

UV-56-89 Chip sample o

UV-57-89 Chip sample o

UV-58-89 Chip sample o

m thick 

U V-59-89 Chip sample o

m thick 

UV-60-89 Chip sample o

m thick 

UV-61-89 Grab sample f

UV-62-89 Chip sample o

UV-63-89 Chip sample o

UV-64-89 Chip sample f

UV-65-89 Grab sample o

UV-66-89 Chip sample f
GOLD-QUARTZ VEINS, PISTON DE UROY AREA J13

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The trace-element content of the rocks is typical of that 

of ultramafic and mafic rocks. The copper content (table 1) 

correlates with the amount of chalcopyrite in polished 

sections and thus reflects well the amount of sulfide minerals in the rocks (table 2). The chromium content in the ultramafic rocks represents the amount of chromite. Some of the 

rock samples (UGP-5, UGP-17, UGP-38, UGP-39) 

contain anomalous amounts of platinum-group elements. 

Although a platinum-group mineral was not identified in the 

polished sections, sperrylite (PtAs2) was identified in 

scanning electron microscope images (see fig. 3). 

Chondrite-normalized diagrams for the platinum-group 

elements show that cumulates containing cumulus chromite 

have a different type of pattern than those for the other types 

of cumulates, which show positive slopes reflecting the 

concentration of platinum-group elements by a process 

associated with sulfide mineral formation (fig. 4). The 

saw-toothed chondrite-normalized pattern for rocks that 

contain cumulus chromite may be the result of iridium, 

ruthenium, and rhodium concentration at the time of 

chromite precipitation and platinum and palladium 

concentration at the time of sulfide formation.

Samples UGP-5, UGP-17, UGP-38, and UGP-39 

have the highest platinum-group element values and were 

examined for platinum-group minerals using the scanning 

electron microscope. Platinum-group minerals were not 

found in samples UGP-5 and UGP-17. Sample UGP-39 

contained a grain of platinum and arsenic enclosed in a composite grain of chalcopyrite (fig. 3). The extremely small 

size of the grain precluded quantitative analysis; however, 

the qualitative data suggest that the mineral is sperrylite 

(PtAs2>. Sample UGP-38 also contained sperrylite, as well 

as a (Pt, Te, Ni, As) mineral (possibly moncheite), a (Pt, Fe, 

Ir) alloy, and a (Pt, Ir, Rh, As, S) mineral (possibly hollingworthite).

Gold in vein material and soil samples ranges from less 

than 2 to 4,640 ppb; the arithmetic mean of unqualified 

values is 292.6 ppb, and the geometric mean is 33.1 ppb 

(table 2). Arithmetic and geometric means and standard 

deviation for the number of unqualified values for the 

analyses of vein material from table 2 are listed in table 3. 

Large differences between the arithmetic and geometric 

means and the relatively large standard deviations indicate 

the skewed nature of the normal distributions for most elements. Significant one-to-one correlations of lognormalized 

data exist between Mn, Co, Cr, Cu, and Ni; Cr, Mn, and Ni; 

Ni and Zn; As, Ni and Se; Se and Tl; Tl and Hg; Ba, Co, and 

Cu; and Au and Cd. The correlation with manganese reflects 

the development of manganese-iron oxide minerals and the 

absorption of other elements such as copper during 

weathering. The associations of arsenic, selenium, and 

tellurium probably reflect introduced materials related to 

mineralization.

In order to ascertain relations between metallic elements and elements other than gold, the data set in table 2
J14 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 3. Statistical parameters for unqualified values of

analyses of quartz-vein material given in table 2.

[In parts per million except for gold, which is in parts per billion]

Au

As

Cd

Hg

Sb

Se

Te

Tl

Zn

Ag

Ba

Co

Cr

Cu

Mn

Ni

Pb

V

Mean

292.6

103.8

0.83

0.08

4.1

0.65

0.19

0.18

37.1

3.8

188.0

109.9

598.0

134.7

1,683.4

205.0

13.6

104.0

Standard 

deviation

793.6

189.8

0.58

0.26

1.5

0.76

0.21

0.17

39.7

3.7

548.2

146.7

805.2

178.9

1,262.5

292.6

2.8

55.9

Geometric 

mean

33.1

35.5

0.63

0.03

3.8

0.48

0.14

0.13

19.2

2.0

75.4

66.2

260.5

70.9

547.6

89.5

13.3

85.3

Number of 

unqualified values

69

93

47

42

14

62

46

37

75

4

88

74

98

96

98

95

36

96

was split into sets of groups. In one split, samples containing 

less than 2 ppb Au were separated from samples having 

unqualified values. Mean values for arsenic, copper, and 

zinc of these two sample groups are different at a 99 percent 

confidence level, and mean values for cadmium, chromium, 

mercury, and nickel are different at a 95 percent confidence 

level. In another split, samples whose gold content is above 

the mode of gold contents and below the mode were tested; 

only the means for cadmium, zinc, and copper showed significant differences between the two groups of samples. The 

significant differences in means of elements of the sample 

groups split on gold content may indicate that these elements 

were introduced to the vein system during mineralization. 

Copper and zinc probably are the best pathfinder elements 

for rocks that contain gold.

GEOPHYSICS

Soil and jungle cover is extensive in the Piston de Uroy 

area, and several geophysical methods were used in order to 

obtain information on the buried rocks that host the 

quartz-vein system. Surveys were made using magnetic, 

induced polarization, and very low frequency electromagnetic methods. In addition, a group of students from Universidad de Simon Bolivar (Caracas) conducted a gravity 

survey at Piston de Uroy (V.R. Graterol, written commun., 

1989), but the data are not usable because local topography 

is steep, and topographic data were not collected for topographic corrections.

Following the initial geologic mapping, a grid was carefully surveyed and marked in anticipation of the geophysical 

surveys (plate 1). This grid is subsequently referred to as the 

geophysical grid to distinguish it from the original geologic 

(reconnaissance) grid (fig. 2). Traverse lines, called picas, 

were spaced 200 m apart along the 4-km-long baseline, 

called the eje. The eje is oriented N. 80° E., and the picas are 

oriented perpendicular to the eje. In part because the topography is more rugged near the scarp of the Roraima Group to 

the north, all of the northern part of the quartz-vein system 

discovered during the geological mapping (see fig. 2) was 

not adequately sampled using the geophysical methods.

DATA ACQUISITION, PROCESSING, AND

ANALYSIS

MAGNETIC DATA

Total-field magnetic data were acquired in January and 

May 1989 at 20-m intervals along both the geological and 

geophysical grids using Geometries model 816 and Scintrex 

model MP-2 magnetometers. A base station magnetometer 

was unavailable, but base stations were remeasured, on average, every one or two hours. The data were given UTM coordinates and reduced for drift (diurnal) correction using a 

computer program called SPDM3 (All Fernandez, Corporacidn Venezolana de Guayana, Tecnica Minera, C.A., written commun., 1989) and were gridded and plotted using a 

commercial software package called GDM developed by the 

Bureau de Recherches G6ologique et Minieres (fig. 5). The 

contoured data show a distinctive linear anomaly in the 

northern part of the geophysical grid caused by a strongly 

magnetic body trending roughly N. 75° E. This anomaly correlates closely with peridotite outcrops and may be caused, 

by the ultramafic body mapped by Page and Contreras (written commun. from Norman J Page, U.S. Geological Survey, 

and Gloria Contreras, Tecnica Minera, C.A., March 9,1988, 

to Vicente Mendoza and Cesar Gutierrez, T6cnica Minera, 

C.A., and February 16,1989, to Vicente Mendoza, Fernando 

Susach, and Nestor Angulo, T6cnica Minera, C.A.) (see also 

Contreras and Freites, 1989). The anomaly suggests that a 

larger continuous peridotite body is present in the subsurface.

A series of two-dimensional models was calculated 

along the picas using the MAGMOD3 software by Geosoft 

in an attempt to outline the surface trace of the peridotite 

body. This modeling is particularly important because the 

inclination of the Earth's magnetic field at Piston de Uroy of 

about 31° leads to strong distortion and to lateral shifts of the 

anomalies from their source rocks. Figure 6 shows the magnetic data with the modeled causative body from pica 11, 

approximately in the middle of the geophysical grid. When
620000E

GOLD-QUARTZ VEINS, PISTON DE UROY AREA

621000E 622000E

J15

623000E

697000N

696000N

CONTOUR INTERVAL 50 NANOTESLAS

1000 METERS

Figure 5. Total-field magnetic map for the Piston de Uroy area, Venezuela. Geophysical grid is shown in figure 2. Contour interval 50 

nanoteslas (nT); dots show station locations along picas (traverse lines).

SOUTH NORTH

44,000

WUJ

O 40,000

z 

z

o 36,000

_i

UJ

u_ 

O

fc 32.000

1

S 28,000

(A

2 o

I I 

EXPLANATION

 Observed 

o o o o o Fitted

 Base level

100

t 200

200 400 600 

DISTANCE, IN METERS

800 1,000

Figure 6. Magnetic model profile showing inferred causative 

body, pica 11 of geophysical grid shown in figure 2, Piston de Uroy 

area, Venezuela. The best susceptibility fit uses a long, narrow 

body that is buried 24 m below the surface, dips 10° N., and has a 

half-width of 72 m. The modeled susceptibility is 0.358 emu. The 

Earth's field strength is taken to be 32,700 nT, field inclination 31°, 

and declination -11°. See text for discussion.

the model results were combined, it was possible to infer, in 

map view, the location, size, and dip of the peridotite body.

VERY LOW FREQUENCY ELECTROMAGNETIC DATA

Recognition of offsets and truncations of the quartz 

veins early in the geological mapping phase (Norman J Page 

and Gloria Contreras, written commun., March 9, 1988, to 

Vicente Mendoza and Cesar Gutieirez) led to speculation 

about a late-stage series of offsetting faults, apparently striking north. In an attempt to correlate probable offsets in two 

parts of the quartz-vein system, very low frequency 

electromagnetic measurements were made. Because the 

transmitting station was oriented almost due north, only the 

positive crossovers (that is, where dip angles of the very low 

frequency electromagnetic field are positive to the left, negative to the right) indicate the presence of a conductor (for 

instance, a water- and gouge-filled fault or narrow shear 

zone) at the midpoint (see fig. 7). The very low frequency 

electromagnetic method allows rapid reconnaissance (data 

can be acquired at almost the speed at which one can walk), 

but it is relatively limited in depth penetration (about 60-100 

m in resistivities such as those encountered at Piston de
J16 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

EAST

-50

200 400 600 

DISTANCE, IN METERS

800 1,000 1,200

Figure 7. Very low frequency electromagnetic profile along the baseline (eje) of the geologic grid shown in figure 2, Piston de Uroy area, 

Venezuela. The vertical dashed bars indicate probable water-filled faults that offset the quartz-vein system.

Uroy). The depth of penetration depends on the resistivity of 

the uppermost materials being measured.

Very low frequency electromagnetic data were 

acquired in January 1989 using a Geonics EM-16 system 

and were plotted (fig. 7) using conventions discussed in 

Eraser (1969). An insufficient number of parallel profiles 

were acquired to justify a derivative filter approach (Eraser, 

1969). Two profiles were made, one along the geologic baseline or eje (fig. 2) and one parallel with it but 100 m to the 

north. Correlation of crossovers between these two parallel 

lines supports the possibility that a series of faults striking 

about N. 10° W. offsets the quartz-vein system in several 

places (shown by the vertical bars in fig. 7). In addition, large 

(as much as 30°) dip angles at 00 m and 1,000 m suggest that 

two major conductors (large, water-filled fault zones) are 

coincident with the La Franela and El Piston stream channels 

(fig. 2).

INDUCED POLARIZATION DATA

Since World War II, the induced polarization (IP) 

method has been used successfully to detect disseminated 

sulfide minerals in rocks (Sumner, 1976). During the 1970's, 

it was determined that the induced polarization method could 

be used as a mapping tool by outlining systematic variations 

in clay and metallic sulfide mineral contents (Zonge and 

Wynn, 1975). In some cases, the induced polarization 

method can detect the presence of pyrite in amounts of less 

than 0.1 percent and thus is an ideal tool to assist in separating otherwise indistinguishable lithologies. The induced 

polarization effect is caused by frequency-dependent variations in resistivity; consequently, resistivity data are 

obtained during induced polarization measurements as additional information. Variations in resistivity normally correlate closely with porosity, at least in the absence of
GOLD-QUARTZ VEINS, PISTON DE UROY AREA J17

heterogeneities such as clay layers or metallic sulfide minerals. The more water present (for example, in a fault zone), 

the lower the apparent resistivity. Induced polarization data 

were collected in the Piston de Uroy area to assist in mapping 

hidden fault zones and to delineate the buried rock units that 

may have controlled emplacement of the quartz veins.

Time-domain induced polarization data were collected 

over most of the geophysical grid in May 1989 using the 

gradient array configuration (Sumner, 1976; All Fernandez, 

written commun., 1989) and a Scintrex TSQ series transmitter and IPR-10A time-domain receiver. A transmitter dipole 

3,000 m long was set up along a pica, and smaller receiver 

dipoles were used both inline that is, along the same pica, 

but only inside the center 2,000-m zone of the transmitter and as many as two picas (400 m) away offline. Both 

40-m and 80-m receiver dipole lengths were used, and 

receiver dipoles were moved 20 m between measurements. 

The 40-m receiver dipole data are not shown here because 

they are incomplete due to noise; commonly, the field operators did not record the values because the receiver would 

not stabilize. The 80-m data were given UTM coordinates 

and reduced to apparent resistivity values using a computer 

program called POLARIN (All Fernandez, written commun., 1989). They were smoothed using a five-point 

low-pass filter and plotted using Lotus Symphony (for profiles) and SURFER4 (for plan maps).

Figure 8 represents a plan view of the resistivity data for 

80-m dipoles acquired along the picas of the geophysical 

grid in the Piston de Uroy area. The data show four anomalous (resistive) zones in a host-rock resistivity background of 

about 1,000 ohm-m.

In the western half of the map area (fig. 8), north of the 

eje, a high-resistivity (as much as 4,000 ohm-m) linear 

anomaly strikes about N. 80° E. and probably coincides with 

the eastern part of the western quartz-vein system. Mapping 

of the quartz-vein system in this area shows true widths of as 

much as 5 m. The resistive anomaly is truncated toward the 

east at about pica 11, in exactly the same manner as the 

mapped quartz vein, and probably extends westward at least 

as far as pica 2. This coincidence is remarkable for two reasons: one, it implies that the quartz-vein system has sufficient volume to be recognizable even with dilution of the 

resistivity by the surrounding rock, and, two, it signifies that 

the vein may extend at least 700-800 m farther to the west 

than is shown by surface mapping. This latter observation 

also implies that the vein is subparallel with, but eventually 

cuts, the peridotite body at an oblique angle, a relation that 

makes the quartz-vein system one of the youngest geological 

features in the area.

The unusual size or volume of the vein system contributes to its potentially economic importance. Note that with 

an 80-m receiver dipole, the induced polarization system is 

sampling and averaging over a roughly hemispherical volume of rock centered around the dipole. Dilution of the resistivity by the surrounding rocks and soils to the resistivity

contribution of a 2-5-m-wide quartz vein should therefore be 

substantial. Initially, it was expected that the induced polarization data would help in mapping rock types. A weak 

induced polarization anomaly over this part of the 

quartz-vein system is remarkable, which suggests that the 

total volume of the quartz-vein system is large.

Just south of the linear, so-called quartz-vein anomaly 

shown in figure 8 is a strong, discrete anomaly that is only 

expressed on picas 8 and 9. Resistivity values are greater 

than 9,000 ohm-m. The source body strikes N. 40° E. and is 

as wide as 200 m and at least as long as 350 m. The exact 

nature of the source is not clear, but it may be related to a 

gabbro inferred to be present from isolated float in the area 

of the anomaly. The source body may connect with another 

body to the east.

The third resistivity anomaly is an apparent eastern 

extension of the second. A profile across it is discussed following. In one place (pica 14), resistivity of more than 

20,000 ohm-m suggests a dense, extremely low porosity, 

possibly silica-flooded rock. Although it is possible that current leakage caused this high apparent resistivity, it is 

unlikely because the high resistivity is coherent across several picas.

A fourth resistivity anomaly of as much as 9,000 

ohm-m is parallel with the third anomaly but farther to the 

south. Comparison of the 40- and 80-m receiver dipole data 

for this southeastern anomaly suggests that the source body 

may dip south-southeast. This interpretation is not as certain 

as the dip (calculated from magnetic models) of the 

peridotite body farther to the north, which dips north.

Figure 9 represents a plan view of the chargeability data 

from the induced polarization survey. Though filtered by the 

contouring package, the substantial amplitude of the noise 

nevertheless is evident; the noise is even greater in the 40-m 

receiver dipole data. Two or three anomalies are clearly visible in figure 9. The westernmost anomaly is south of the eje, 

completely on pica 7, and probably is unrelated to the resistivity anomaly on picas 8 and 9. This anomaly has characteristics that suggest it is cultural in origin. Although a geologic 

origin cannot be completely excluded, the anomaly probably 

reflects buried debris left by miners working in the area.

The second most obvious chargeability anomaly shown 

in figure 9 is a linear feature about 100 m wide along profile, 

parallel with and about 150 m north of the eje, starting about 

pica 9 and extending at least 700 m through pica 12. Chargeabilities are as high as 60 msec on pica 10 and as high as 50 

msec on pica 12 (see below). These relatively high values are 

probably due to disseminated pyrite (or possibly ilmenite), 

probably at least 2-3 percent by volume. This anomaly is 

close to, but not coincident with, the resistivity anomaly that 

is correlated with the western quartz-vein system. The area 

of the anomaly is a primary drill target.

Figure 10 shows a resistivity and chargeability profile 

along pica 12; these data are for an 80-m receiver dipole 

length. The chargeability data are relatively noisy, primarily
J18

619000E

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

620000E 621000E 622000E 623000E

697000N 

696000N 

695000N

CONTOUR INTERVAL 2000 OHM-METERS

Figure 8. Map showing contours of gradient-array resistivity data (3,000-m transmitter dipole and 80-m receiver dipole) from the 

induced polarization survey acquired on the geophysical grid shown in figure 2, Piston de Uroy area, Venezuela. This array is analogous 

to AB=3,000 m and MN=80 m in Schlumberger array resistivity surveys for the limiting case of the receiver dipole inline and centered 

in the middle of the transmitter dipole. Contour interval 2,000 ohm-m. Dots indicate station locations along picas (traverse lines).

619000E 620000E 621000E 622 OOOE 623000 E

697000N

696000N

500 

I

1000 METERS

CONTOUR INTERVAL 10 MILLISECONDS

695000N

Figure 9. Map showing contours of chargeability data (polarization component) from the same induced polarization survey shown in 

figure 8. Contour interval 10 msec.

because the field operators did not average sufficient readings at each station. The 40-m receiver dipole chargeability 

data along this same profile, although providing better resolution, are in general too noisy to use except to roughly

estimate dip direction of the polarized bodies at depth. A 

five-point digital filter (fig. 10, solid line) was applied to 

the 80-m chargeability data in order to distinguish between 

rock types along the profile. The resistivity profile (fig. 10)
GOLD-QUARTZ VEINS, PISTON DE UROY AREA J19

NORTH

60,000

 50,000

 40,000

 30,000

 20,000

 10,000

-600 -400 -200 0

(EJE) 

LOCATION, IN METERS

400

Figure 10. Resistivity (squares) and chargeability (pluses) profiles along pica 12 of the geophysical grid shown in figure 2, Piston de 

Uroy area, Venezuela. Data were acquired using an 80-m receiver dipole spacing. Pluses represent original, noisy chargeability data, 

whereas the smooth curve represents a five-point (weighted) filtered version.

shows two distinct resistivity highs that probably represent 

silica-flooded (and therefore lower porosity) rocks. The 

southern high, centered about stations -300 and -400, can be 

traced from profile to profile, and probably represents a distinct lithostratigraphic unit. The apparent resistivity of the 

unit is greater than 10,000 ohm-m, a value that could reflect 

a true inherent resistivity as high as 30,000-50,000 ohm-m. 

The southern high is associated with a significant chargeability anomaly of as much as 35 msec. The apparent resistivity 

and chargeability anomalies together indicate a 

100-150-m-wide unit containing perhaps 2 percent pyrite 

and having much lower porosity (late-stage silica flooding or 

a relatively nontectonized intrusive rock) than the surrounding (volcanic?) rocks. The second resistivity high, around 

station -140, is similar to the first but is lower in amplitude, 

both in resistivity and in chargeability. It probably represents 

similar rock but a narrower width.

On the north side of the eje, centered around station 120 

(figs. 9, 10), is a chargeability anomaly; the resistivity is at

or less than the background average of 1,000 ohm-m. The 

anomaly is significant: three stations in sequence have 

chargeabilities of about 50 msec. The anomaly extends from 

pica 12 west to pica 9 and cannot be dismissed as mining 

debris. It is probably caused by a discrete geologic unit, only 

50-100 m wide, containing several percent by volume of 

either clay (for instance, montmorillonite) or possibly sulfide 

minerals or ilmenite (but probably pyrite). It is significant 

that this anomaly is subparallel with the western quartz-vein 

system but extends several hundred meters beyond it to the 

east. The anomaly may represent a completely hidden 

mineralized body.

INTEGRATION OF THE GEOPHYSICAL DATA

Figure 11 combines the known (mapped) expressions 

of the quartz-vein system and an interpretation of the three 

geophysical data sets. The western quartz-vein system may
J20 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

620000E 621000E 622000E 623000E

EXPLANATION

Polarizing body 

Peridotite 

Resistive unit 

Fault 

Drainage

697000N

| [ | -|_f_j- Quartz veins

696000N

1000 METERS 

I

Figure 11. Map showing interpretation of magnetic, very low frequency electromagnetic, and induced polarization data and results of 

geological mapping of the quartz-vein system, Piston de Uroy area, Venezuela. Geophysical grid is shown in figure 2.

extend 700-800 m farther to the west than mapped; however, 

it may or may not be offset 600 m by a left-lateral fault that 

is roughly parallel with, and between, picas 11 and 12. In 

fact, the western quartz-vein system may be a separate 

entity. Nevertheless, a N. 10° W.-striking conductor, 

probably a fault, is present at this point. Although the very 

low frequency electromagnetic data and field mapping show 

several short north-south offsets along the western vein 

system, the presence of a large-displacement transcurrent 

fault between picas 11 and 12 is belied by the relative continuity of two different (roughly east trending) bodies mapped 

by the induced polarization survey and by the relatively continuous peridotite body inferred from the magnetic data. It is 

likely, therefore, that the eastern vein system has always 

been separate from the western vein system.

The peridotite body on the north edge of the geophysical grid trends east-northeast, dips north for most of its 

length, and is more extensive in the subsurface. Magnetic 

susceptibility values for the peridotite (in outcrop) are about 

ten times those of the surrounding gabbro.

The highly polarized unit just north of the geophysical 

eje, between picas 9 and 12, is an enigma. It probably has a 

significant sulfide content and is subparallel with, and apparently related to, the eastern part of the western quartz-vein 

system.

The two parallel resistive units on and south of the geophysical eje in the central and eastern parts of the Piston de 

Uroy area probably represent rocks that contain small 

amounts of sulfide minerals and have low porosity. This low 

porosity could be caused by silica flooding or by metamorphism; the source body could be a relatively dense volcanic 

rock without subsequent fractures due to tectonism. 

Although comparison of the 40-m and 80-m dipole data 

suggests some south-with-depth displacement of these 

resistive bodies, the strong noise in these data makes that 

interpretation far from certain.

CONCLUSIONS

In the Piston de Uroy area of Estado Bolivar, Venezuela, a quartz-vein system is subparallel with the strike of, and 

is hosted by, an ultramafic body. The cumulate ultramafic 

rocks and gabbro intrude an older greenstone belt that 

contains volcanic rocks of andesitic to basaltic character. 

Unconformably overlying these rocks is the basal section of
GOLD-QUARTZ VEINS, PISTON DE UROY AREA J21

sandstone and quartz-pebble conglomerate of the Early 

Proterozoic Roraima Group.

The gold-bearing quartz-vein system in the Piston de 

Uroy area is roughly along the geologic baseline and north 

of the geophysical baseline. Although it appears that two 

segments of a single quartz-vein system are offset 600 m in 

a left-lateral direction, the continuity of adjacent subparallel 

units indicates that the eastern and western veins are probably distinct from each other. The quartz veins are unusually 

wide, 8 m or more at one location on the eastern vein, and, 

by virtue of their significant effect on the electrical 

resistivity data, they must constitute a large volume of rock. 

The overall length of both systems, including a probable 

700-800 m extension of the western vein as suggested by the 

geophysical data, is at least 3,000 m. Gold values are as high 

as 4,640 ppb in the western vein system and as high as 220 

ppb in the eastern vein system.

Both geologic mapping and geophysical data suggest 

that at least two sets of faults are present in the area. One set 

cuts the quartz-vein system and strikes approximately 

N. 10° W., probably postdating the quartz veins, and has offsets typically less than 100 m. The other set probably is part 

of a regional series of major shear zones that possibly follow 

older Precambrian structures. The shear zones are younger 

than the quartz veins because they are now highly conductive (water filled) and have not been recemented during 

metamorphism. The quartz-vein system has not been identified east of the El Piston stream drainage. If the shear-zone 

system has had any significant movement since emplacement of the quartz veins at Piston de Uroy, then movement 

has been greater than 1 km, which is the size of the vein-free 

ground examined east of the shear zone. This is unlikely.

Geophysical data suggest the presence of additional 

lithologic units not mapped on the surface. A hidden, 

strongly polarizing body, nonmagnetic and probably silica 

rich, apparently extends for several hundred meters to the 

east of the western quartz-vein system. Several additional 

highly resistive, but nonpolarizing features, probably thick 

nontectonized, dikelike bodies or silica-flooded units, are in 

the central-southeastern part of the Piston de Uroy area.

Without direct exploration such as trenching or drilling, 

the resource potential of the quartz-vein system of Piston de 

Uroy is difficult to assess. The characteristics of the veinsystem are similar to those of the low-sulfide gold-quartz 

vein model (Berger, 1986). A median grade for the 

low-sulfide gold-quartz vein model is 16 grams per metric 

ton, or 16 ppm (Bliss, 1986), a value greater than any determined in the surface samples. Further direct exploration,

such as more complete magnetic data and systematic 

dipole-dipole induced polarization surveys, is needed to 

more fully evaluate this prospect.

The probability of large concentrations of chrornite and 

nickel and base-metal sulfide minerals in the Piston de Uroy 

area is low based on the lack of high concentrations of 

chromite or sulfide minerals in any samples collected to date 

or observed in the field. Some rock samples contain anomalous amounts of platinum-group elements, but the 

distribution and content of these elements have not been 

determined.

REFERENCES CITED

Alberdi, Margarita, 1988, Informe geologico-geoquimico de la 

zona El Piston de Uroy-Abaren, in Corporacion Venezolana 

de Guayana, T6cnica Minera, C.A., Coordinaci6n General de 

Prospecciones, Annual Report, 1987, v. 2: Available from Corporacion Venezolana de Guayana, Tecnica Minera, C. A., Calle 

Mexico cruce con Calle Estados Unidos, C.C. Chilemex, Piso 

1, Puerto Ordaz, Venezuela.

Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz 

veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit 

models: U.S. Geological Survey Bulletin 1693, p. 239.

Bliss, J.D., 1986, Grade and tonnage of low-sulfide Au-quartz 

veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit 

models: U.S. Geological Survey Bulletin 1693, p. 239-243.

Contreras, Gloria, and Freites, Dennis, 1989, Informe 

geologico-geoquimico de las zonas de Canaima-Uroy-Chivao, 

Rio Chican£n, Piston de Uroy, y Rfo Cuyuni-Rio Uey, in 

Corporacion Venezolana de Guayana-T6cnica Minera, C.A., 

Coordinaci6n General de Prospecciones, Annual Report, 1987, 

v. 1, p. 282-455: Available from Corporaci6n Venezolana de 

Guayana, Tecnica Minera, C. A., Calle Mexico cruce con Calle 

Estados Unidos, C.C. Chilemex, Piso 1, Puerto Ordaz, 

Venezuela.

Fraser, D.C., 1969, Contouring of VLF-EM data: Geophysics, 

v. 34, p. 958.

Grimes, D.J., and Marranzino, A.P., 1968, Direct-current and spark 

emission spectrographic field methods for semiquantitative 

analysis of geologic materials: U.S. Geological Survey 

Circular 591,6 p.

Meier, A.L., Carlson, R.R., Lichte, F.E., Aruscavage, P.J., and 

Riddle, G.O., 1988, Determination of the platinum-group 

elements by fire assay-inductively coupled plasma-mass spectrometry [abs]: The Geochemical Society, V.M. Goldschmidt 

Conference, Baltimore, May 11-13,1988.
J22 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Mendoza, Vicente, 1977, Evolution tectonica del Escudo de Guayana: Congreso Latinoamericano de Geologia, 2nd, Caracas, 

1973, Memoria, Publicacidn Especial 7, v. 3, p. 2237-2270.

Naldrett, A.J., 1981, Platinum-group element deposits, in Cabri, 

L.J., ed., Platinum-group elements Mineralogy, geology, 

recovery: Canadian Institute of Mining and Metallurgy Special 

Volume 23, p. 197-232.

O'Leary, R.M., and Meier, A.L., 1986, Analytical methods used in 

geochemical exploration, 1984: U.S. Geological Survey Circular 948,48 p.

Quesada, R.J., and Wynn, J.C., 1990, M6todos geofisicos aplicados 

a sistemas de vetas de cuarzo aurifero en el Estado Bolivar,

Venezuela: Congreso Geofisico Venezolano, 5th, Caracas, 

1990,6 p.

Sumner, J.S., 1976, Principles of induced polarization for geophysical exploration: New York, Elsevier, 277 p.

Tosiani D., Tommaso, and Sifontes G., Ramdn, 1989, Asociacidn 

de rocas maTicas-ultramaficas en la region del Cerro Piedra del 

Supamo, al sur de El Callao, Estado Bolivar, Venezuela: 

Congreso Geoldgico Venezolano, 7th, Barquisimeto, 1989, 

Memoria, v. 1, p. 163-174.

Zonge, K.L., and Wynn, J.C., 1975, Recent advances and applications in complex resistivity measurements: Geophysics, v. 40, 

no. 5, p. 851-864.
Cenozoic Landscape Evolution of the 

Southern Part of the Gran Sabana, 

Southeastern Venezuela Implications for the 

Occurrence of Gold and Diamond Placers

By John C. Dohrenwend, Galo Yanez P., and Glenda Lowry

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-K

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENTS

Abstract........................................................................................................................... Kl

Resumen......................................................................................................................... 1

Introduction.................................................................................................................... 2

Lithologic and Structural Framework of the Southern Part of the Gran Sabana............ 4

General Stratigraphy of the Roraima Group........................................................... 4

Regional Deformation of the Roraima Group........................................................ 4

Geomorphology of the Southern Part of the Gran Sabana............................................. 5

Cenozoic Landscape Stripping............................................................................... 6

Late Cenozoic Degradational Processes................................................................. 7

Surficial Deposits................................................................................................... 8

Lateritic Soils of the Gran Sabana.................................................................................. 9

Distribution and Significance of Relict Lateritic Soils........................................... 11

Cenozoic Evolution of the Gran Sabana Landscape...................................................... 13

Occurrence of Placer Deposits ....................................................................................... 14

Paleoplacers within the Lower Part of the Roraima Group.................................... 16

Placers In Colluvial-Alluvial Deposits of Low-Order Drainages .......................... 16

Placers in Modern Channels of the Larger Streams............................................... 17

References Cited............................................................................................................. 17

FIGURES

1. Map showing location of Gran Sabana................................................................................................................... K3

2-6. Photographs showing:

2. South-facing cuesta scarp developed on quartzose sandstone of lower part of Uairen Formation

near axis of broad northwest-plunging anticline at Chiricayen..................................................................... 5

3. Detail of pseudokarst labyrinth on summit platform of Cerro Roraima........................................................ 6

4. Prominent benches in almost flat lying strata of Uaire"n Formation.............................................................. 7

5. Flat, bedrock-floored channel of Quebrada de Jaspe cut into jasper beds of upper part of

Uaimapue" Formation .................................................................................._^ 8

6. Valley of Pio Cuquenan downstream from its confluence with Rio Yuruani................................................ 9

7. Map showing distribution of laterite-capped paleosurface remnants within and adjacent to valley of

Rio Cuquenan in vicinity of Cerro Chiricayen........................................................................................................ 10

8-13. Photographs showing:

8. Surface of relict lateritic soil capping cuesta ridge along southwestern flank of valley of Rio Cuquenan ... 11

9. Typical lateritic soil roadcut exposure........................................................................................................... 12

10. Upper part of a relict lateritic soil profile....................................................................................................... 12

11. Detail of strongly developed nodular ferricrete crust.................................................................................... 13

12. Surface of relict ferricrete crust capping series of low fluvial terrace remnants ........................................... 14

13. Laterite-capped cuesta ridge developed on gently dipping strata of upper part of Uairen Formation.......... 15

14. Topographic profiles across valley of Rio Cuquenan showing distribution of relict lateritic soils

capping remnants of middle Tertiary paleolandscape............................................................................................. 16
Cenozoic Landscape Evolution of the

Southern Part of the Gran Sabana,

Southeastern Venezuela Implications for the

Occurrence of Gold and Diamond Placers

By John C. Dohrenwend, 1 Galo Yanez P.,2 and Glenda Lowry3

ABSTRACT

The Gran Sabana of southeastern Venezuela is a degradational landscape that is well adjusted to the lithologic and 

structural variations of the underlying Early to Middle Proterozoic Roraima Group (fig. 1). The Roraima Group consists 

of a sequence of flat-lying to gently folded, generally 

unmetamorphosed, terrigenous to shallow coastal marine 

strata that is 2,000-3,000+ m thick. Cuesta ridges and high 

mesalike tepuis (table mountains), having elevations as high 

as 2,800 m, are underlain by silica-cemented quartz sandstone. The larger valleys, at elevations of 800-900 m, are 

underlain by feldspathic sandstone, siltstone, claystone, and 

shale. Surficial deposits are generally limited to discontinuous colluvial-alluvial mantles, isolated colluvial aprons, thin 

(generally less than 2 m thick) fluvial terrace deposits, and 

somewhat thicker fluvial deposits within and adjacent to the 

channels of the largest rivers.

Within this degradational landscape, ferricrete-capped 

remnants of a middle (?) Tertiary geomorphic surface extend 

discontinuously from low strath terraces to high, structurally 

controlled ridges. The ferricrete remnants are particularly 

well developed on the finer grained and more feldspathic 

rocks that underlie the lower dip slopes, hills, and terraces 

within or adjacent to the larger valleys. These relict soils are 

characterized by a 1-3-m-thick, nodular to pisolitic ferricrete 

crust that contains abundant fragments of weathered bedrock 

and local concentrations of hematite and (or) gibbsite. This 

ferricrete crust is typically partly stripped and rubbled and is 

underlain by a deeply weathered, red- and yellow-mottled,

^.S.Geological Survey, 345 Middlefield Road, MS905, Menlo Park, 

California 94025.

2Corporacion Venezolana de Guayana, Tecnica Minera, C.A., Ciudad 

Bolivar, Venezuela.

3Corporaci6n Venezolana de Guayana, Tecnica Minera, C.A., Puerto 

Ordaz, Venezuela.

clay-enriched horizon as thick as 15 m. In most respects, the 

middle Tertiary landscape defined by these ferricrete-capped 

remnants was as well adjusted to underlying lithology and 

structure as is the modern landscape and had an overall relief 

comparable to present-day relief. Thus, during middle and 

late Cenozoic time, landscape change in the Gran Sabana 

was limited to an undefined amount of upland dissection 

accompanied by less than 10 m of general lowering of the 

largest valleys. This scenario does not accord with previous 

geomorphic models, which emphasize cyclic planation as 

the principal mode of landscape evolution in northern South 

America.

Gold and diamond resources in the southern part of the 

Gran Sabana are primarily limited to areas underlain by or 

downstream from extensive exposures of the Uairen Formation, the oldest formation within the Roraima Group. Within 

these areas, geologic-geomorphic environments that host 

gold and diamonds include active channels and Holocene 

floodplains of the larger rivers, colluvial-alluvial placer 

deposits of small low-order drainage basins, and paleoplacer 

deposits associated with conglomeratic lenses and beds 

within the lower part (lower 500-600 m) of the Uairen Formation. Additional gold and diamond resources may lie 

within or immediately beneath ferricrete crusts developed on 

surfaces underlain by the Uairen Formation. For the most 

part, these placer resources are limited in both grade and 

extent.

RESUMEN

La Gran Sabana al sur de Venezuela es un paisaje 

degradacional que esta bien ajustado a las variaciones litologicas y estructurales de Grupo Roraima el cual infrayace la 

region. El Grupo Roraima, de edad Proterozoico Temprano 

a Medio, consiste de una secuencia de 2,000 a 3,000 m de 

espesor compuesta por estratos continentales y costeros

Kl
K2 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

marines someros. Los estratos estan dispuestos horizontalmente o plegados suavemente. Areniscas cementadas con 

sflice forman las crestas de las cuestas e infrayacen las mesas 

(tepuis) los cuales alcanzan una elevation de 2,800 m. Los 

valles mas extensos, a una elevation de 800-900 m, estan 

formados sobre arenisca feldespatica, limolita, arcillita, abanicos coluviales aislados, depositos delgados (generalmente 

menos de 2 m de espesor) de terrazas fluviales y sobre 

depositos fluviales algo mas espesos dentro y adyacentes a 

los canales de los rios de orden mayor.

Dentro de este paisaje degradacional, hay depositos 

remanentes cubiertos por una laterita ferruginosa endurecida 

(ferricrete). Dichos remanentes son de una superficie 

geomorfica de edad Terciaria media (?). Estos se extienden 

discontinuamente desde las terrazas mas antiguas 

localizadas en valles bajos hasta crestas controladas 

estructuramente.

Los remanentes ferruginosos estan particularmente 

bien desarrollados sobre rocas feldespaticas de grano mas 

fmo las cuales infrayacen las partes bajas de las pendientes 

de buzamiento, colinas y terrazas dentro o adyacentes a los 

valles mas grandes. Estos suelos relictos estan caracterizados 

por una costra ferruginosa nodular a pisolftica la cual contiene abundantes fragmentos de roca infrayacente y concentraciones locales de hematita y/o gibbsita. Esta costra 

ferruginosa esta tipicamente desnuda y desmantelada, por 

debajo hay un horizonte enriquecido en arcilla, muy meteorizado, de color rojo moteado de amarillo, el cual puede alcanzar 15 m de espesor. En general, el paisaje durante el 

Terciario medio, defmido por los remanentes con tope lateritico, estaba tambien ajustado a la litologia infrayacente y 

a la estructura, generando una fisiografia comparable a la 

moderna. Asi pues, durante el Cenozoico medio y tardio, el 

cambio del paisaje en la Gran Sabana estaba limitado por una 

cantidad no definida de diseccion en las areas mas alias 

acompanada por un rebajamiento general de 10 m en los 

valles mas grandes. Este modelo no esta de acuerdo con 

modelos geomorficos previos, los cuales enfatizan la 

planacion ciclica como el modo principal de evolution del 

paisaje en la parte norte de America del Sur.

Los recursos de oro y diamantes en la parte sur de la 

Gran Sabana estan primordialmente limitados a areas que 

suprayacen la Formation Uairen o que estan aguas abajo de 

dicha formation la cual es la mas antigua del Grupo 

Roraima. Dentro de estas areas se distinguen varies ambientes geologico-geomorficos que contienen oro y diamantes. 

Estos incluyen canales activos y planicies de inundation de 

rios mayores de edad Holoceno, depositos aluviales-coluviales de tipo placer en pequenas cuencas de orden menor, y en 

depositos de paleoplaceres asociados con lentes y capas conglomeraticas localizados en la base (500-600 m) de la Formation Uairen. Los recursos adicionales de oro y diamantes 

podrian estar presentes en o inmediatamente por debajo de 

las costras ferruginosas desarrolladas sobre superficies que a 

su vez suprayacen a la Formacidn Uairen. Los recursos de

tipo placer, en su mayoria son limitados tanto en tenor como 

en extension.

INTRODUCTION

The Gran Sabana of southeastern Venezuela is a degradational landscape that is well adjusted to the lithologic and 

structural variations of the underlying Early and Middle 

Proterozoic Roraima Group. This 2,000-3,000+-m-thick 

sequence of terrigenous to shallow coastal marine strata is 

flat lying to gently folded and generally unmetamorphosed. 

Cuesta ridges and high mesalike tepuis (table mountains), 

which are formed on silica-cemented quartz sandstone, have 

elevations of as much as 2,800 m. In contrast, the floors of 

the larger valleys, which are underlain by interbedded 

feldspathic sandstone, siltstone, claystone, and shale, are at 

elevations between 800 and 900 m. Benched hillsides and 

piedmonts are widespread, and numerous lithologically controlled waterfalls and rapids interrupt the otherwise gentle 

longitudinal profiles of most streams. Surficial deposits are 

mostly limited to discontinuous colluvial-alluvial mantles on 

hill slopes and piedmonts, isolated colluvial aprons preserved beneath residual gravel lags, 1-2-m-thick fluvial 

deposits overlying strath terraces in the larger valleys, and 

5-10-m-thick channel deposits within the narrow late Quaternary floodplains of the largest rivers.

As many as six planar geomorphic surfaces, defined on 

the basis of regional correlations of similar elevations of 

summit levels and areas of low relief, have been recognized 

in the region of the Guayana Shield (James, 1959; McConnell, 1968; Zonneveld, 1985; Schubert and others, 1986; 

Briceno and Schubert, 1990). The formation of these surfaces is generally considered to be the result of alternating 

periods of tectonic uplift and stability, landscape dissection 

in response to the uplift, and parallel slope retreat during the 

succeeding interval of stability. The ages of these surfaces 

are generally thought to range from Mesozoic to Holocene; 

however, it has been pointed out that many of these surfaces 

coincide with lithologic units and that the steps which separate them commonly coincide with lithologic discontinuities 

(Kroonenberg and Melitz, 1983). Moreover, within the area 

of the southern part of the Gran Sabana, ferricrete-capped 

remnants of a middle (?) Tertiary geomorphic surface cut 

across several of these regional planation surface levels 

(extending almost continuously from dissected strath terraces lying less than 10 m above modern streams to high, 

structurally controlled ridges rising more than 200 m above 

the floors of the larger valleys). It is likely, therefore, that 

present-day relief within the Guayana Shield is mostly a 

function of lithologic resistance and that at least some of 

these so-called planation surfaces are primarily a manifestation of lithologic control in a relatively stable landscape that 

is developed on a thick sequence of very gently dipping to 

flat-lying sedimentary strata.
CENOZOIC LANDSCAPE EVOLUTION OF THE GRAN SABANA

66° 63°

K3

Boundary of Venezuelan 

Guayana Shield

Figure 1. Location of the Gran Sabana, Estado Bolfvar, Venezuela. Most of the geographic locations mentioned in the text are shown 

on plate 1 of Wynn, Sidder, and others (this volume).
K4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

LITHOLOGIC AND STRUCTURAL

FRAMEWORK OF THE SOUTHERN

PART OF THE GRAN SABANA

GENERAL STRATIGRAPHY OF THE 

RORAIMA GROUP

The Roraima Group is a laterally extensive sequence of 

Early and Middle Proterozoic, fluvial to shallow coastal 

marine strata that mantle a large part of the Guayana Shield 

and underlie most of the Gran Sabana (Reid, 1974; Yanez, 

1984; Sidder and Mendoza, this volume). These strata 

unconformably overlie an igneous and metamorphic basement complex of Early Proterozoic age, and they are 

intruded by about 1.65-Ga diabase dikes and sills. The 

Roraima Group is dominated by quartzose sandstone but 

also contains significant amounts of conglomerate, 

conglomeratic sandstone, pebbly quartz sandstone, arkosic 

to feldspathic sandstone, siltstone, mudstone, shale, and tuff. 

Cross stratification is common to abundant throughout the 

sequence. Within the Gran Sabana, deformation is limited to 

broad open folds; dips are generally less than 7°, although 

dips of as much as 15° are present locally.

The stratigraphy and structure of the Roraima Group 

have been described in some detail by Reid (1974), Yanez 

(1984), and Ghosh (1985) and are summarized by Sidder and 

Mendoza (this volume). As defined by Reid (1974), the 

Roraima Group in the southern part of the Gran Sabana is 

divided into four units, the Uaire"n, Cuquenan, Uaimapue, 

and Mataui Formations.

The Uaire"n Formation can be usefully divided into 

upper and lower parts that are recognizable both geomorphically and lithologically throughout the southern part of the 

Gran Sabana. The lower part (Canaima Formation, Unit I of 

Y£nez, 1984) is about 600 m thick and consists primarily of 

well-sorted, generally coarse to medium grained, moderately 

bedded to massive, cross-stratified quartzose sandstone and 

intercalated lenses and beds of polymictic and quartzose 

pebble conglomerate and minor thin beds and laminae of 

shaly siltstone. These rocks form the high cliffs and extensive dip slopes of Chiricayen, Cerro el Abismo, and other 

cuestas along the southern margin of the Gran Sabana. The 

upper part of the Uairen Formation (Canaima Formation, 

Unit II of Yanez, 1984) is 100-300 m thick and consists primarily of medium-grained sandstone with abundant trough 

cross-stratification and intercalated channel gravels. These 

rocks form benched scarp slopes and irregular ridges, particularly along the southern margin of the valley of the Rio 

Cuquenan. A ferricrete-capped lateritic soil, interpreted by 

Reid (1974) as a Precambrian paleosol and used by him to 

define the top of the Uairen Formation, is in fact part of a 

deep weathering profile of middle (?) to late Tertiary age that 

mantles the middle part of the Roraima Group (from the

upper part of the Uairen Formation to at least the middle of 

the Uaimapue Formation). This soil is highly significant to 

understanding the Cenozoic landscape evolution of the Gran 

Sabana and is described and discussed in some detail in a following section of this report.

The Uairen Formation is overlain by the Cuquenan Formation (Canaima Formation, Units III and IV of Yanez, 

1984). Exposures of the Cuquen&i Formation are limited, 

and it has not been comprehensively described. This unit 

underlies the valley of the Rio Cuquenan from the vicinity of 

Moroc Meru to the confluence of the Rio Cuquen&i with the 

Rio Aponguao. In this area, deeply weathered outcrops are 

present almost continuously along the Rio Cuquenan wherever the modern channel is actively eroding the late Pleistocene strath surface, which underlies most of the valley 

floor. These outcrops reveal well-bedded to massive, fine- to 

medium-grained sandstone interbedded with medium- to 

thin-bedded and laminated siltstone, claystone, and shale.

The Uaimapue' Formation, which overlies the 

Cuquenan Formation, can also be divided into two geomorphically distinct parts. The lower part (Canaima Formation, 

Unit V of Yanez, 1984) consists primarily of pervasively 

channelled and cross-stratified, fine- to coarse-grained sandstone and thin-bedded to laminated fine-grained sandstone, 

siltstone, and mudstone. The topography developed on these 

rocks is characterized by benched scarp slopes similar to the 

topography of the upper part of Uairen Formation. The upper 

part of the Uaimapue Formation (Guaiquinima Formation of 

Yanez, 1984) contains abundant beds and lenses of tuff and 

jasper that are interbedded with cross-stratified arkosic to 

quartzose sandstone. These resistant beds form conspicuous 

benches along hill slopes and numerous waterfalls and cascades along both major and tributary streams.

The Mataui Formation (Auyantepuy Formation of 

Yanez, 1984) consists primarily of pervasively cross stratified, fine- to medium-grained quartzarenite. This unit underlies the spectacular, vertical-walled high tepuis such as Cerro 

Roraima and Cuquenan Tepuy that dominate the landscape 

of the northern part of the Gran Sabana. It is not present 

within Gran Sabana Sur and therefore is not described in any 

detail in this report.

REGIONAL DEFORMATION OF THE 

RORAIMA GROUP

In the southern part of the Gran Sabana, the Roraima 

Group displays an apparent north-south deformational gradient ranging from relatively tight folds with conspicuous axial 

plane foliation in the south near the Brazilian border to very 

gently dipping to horizontal strata without foliation in the 

north in the vicinity of Chimata, Cuquenan, and Roraima 

tepuis. This deformational gradient is most conspicuous in 

the western part of the study area between the valley of the 

Rio Icabaru and the Rio Carom, but it is also present in the
CENOZOIC LANDSCAPE EVOLUTION OF THE GRAN SAB ANA

.* *^' ' * _» ^'-- "$'~ . *;*

Figure 2. South-facing cuesta scarp developed on quartzose sandstone of the lower part of the Uairen Formation near the axis of a broad 

northwest-plunging anticline at Chiricayen, Venezuela (lat 4°50' N., long 61°15' W.). Aerial view looking west.

eastern part of the study area, east of Santa Elena de Uairen. 

In the Icabaru-El Pauji area, the axial plane foliation is 

sufficiently well developed to impose a conspicuous 

geomorphic overprint on the ridge and valley topography. It 

is likely that this north-south deformational gradient has 

been a significant factor in the geomorphic evolution of the 

region, facilitating the deep dissection and general stripping 

of the Roraima Group south from the Pacaraima Mountains 

into Brazil and the northward encroachment into the Gran 

Sabana by the southward-flowing drainages of the northern 

Amazon basin.

GEOMORPHOLOGY OF THE

SOUTHERN PART OF THE

GRAN SABANA

The Gran Sabana is a degradational landscape that is 

mostly adjusted to the lithologic and structural variations of 

underlying bedrock (fig. 2). Indications of middle to late 

Cenozoic tectonism are not apparent within the region;

however, it is unlikely that a broad regional upwarping of the 

present landscape would generate any readily recognizable 

geomorphic expression of tectonic activity. Indications of 

general landscape maturity are abundant.

1. General topographic features are well adjusted to 

lithology and structure. Relatively resistant rock types 

underlie high tepuis and cuestas, whereas relatively nonresistant rock types underlie the larger valleys and adjacent 

low, rounded hills. In areas of folded, gently to moderately 

dipping rocks, ridges and valleys outline principal structures. 

Major streams are present as subsequent streams that are 

deflected around the noses of plunging folds. In the western 

part of the region between the Rio Icabaru and the Rio 

Carom, a classic ridge and valley topography has developed 

over the gently plunging, east-trending folds of the area.

2. Local geomorphic indicators of structural and 

lithologic control are common. Hillside and piedmont 

benching is widespread, particularly in areas underlain by 

either the Uairen or Uaimapue Formations. Lithologically 

controlled waterfalls and rapids are abundant along most 

lower order streams, and indeed such waterfalls are common
K6 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Figure 3. Detail of the pseudokarst labyrinth on the summit platform of Cerro Roraima, Venezuela. The pervasively cross-stratified 

quartzose of the Matauf Formation that forms the summit platform has been deeply etched by orographically intensified rainfall estimated 

at about 5 m per year. High-angle-oblique aerial view from a height of about 300 m.

even along some reaches of the principal drainages of the 

Rio Cuquenan and Rio Icabaru.

3. Paleosurface remnants are limited in abundance, 

extent, and state of preservation. In fact, the only indications 

of former geomorphic surfaces that remain in the present 

landscape are (1) possible paleosurface remnants defined by 

generally accordant ridge crests in broadly folded areas of 

ridge and valley topography; (2) laterite-capped hills, ridges, 

and low terraces that have survived only because of the 

potection afforded by their resistant ferricrete caps; and (3) 

low, generally accordant, and completely rounded bedrock 

hills within the broad valleys of the principal streams at the 

general level of the laterite-capped terrace remnants.

CENOZOIC LANDSCAPE STRIPPING

Ubiquitous geomorphic evidence indicates pervasive 

late Tertiary and Quaternary stripping of this landscape. 

These geomorphic features include:

1. Deep etching and almost continuous bedrock 

exposure on most tepuis and higher cuestas cover (cover, 

fig- 3)

2. Conspicuous, widespread bedrock outcrops and 

benching on most cuesta scarp slopes and extensive bedrock 

outcrops on many cuesta dip slopes (fig. 4)

3. Almost continuous bedrock exposure and 

numerous waterfalls and rapids along the channels of most 

tributary streams (fig. 5)

4. Almost continuous exposures of deeply weathered 

bedrock along the channels of the major rivers wherever 

these channels impinge upon pre-Holocene valley surfaces 

(fig- 6)

5. Extensive and widespread occurrences of rubbled 

ferricrete, lateritic lag gravels, and truncated deep 

weathering profiles developed directly on bedrock

6. A general lack of continuous surficial deposits 

(>l-2 m thick) in all areas of the landscape except local colluvial wedges on hill slopes armored by lateritic and (or) 

conglomeratic rubble and thin fluvial channel and overbank 

deposits within the modern floodplains of the largest rivers
CENOZOIC LANDSCAPE EVOLUTION OF THE GRAN SAB ANA K7

Figure 4. Prominent benches in almost flat lying strata of the Uairen Formation. Surficial deposits in this area are limited to discontinuous 

colluvial aprons along the base of the bench scarps. Aerial view north along the Brazilian frontier approximately 35 km northeast of Santa 

Elena de Uairen, Venezuela.

The principal reasons for this stripping are threefold:

1. The region is mostly underlain by weakly to moderately indurated, essentially unmetamorphosed, generally 

medium to very fine grained sedimentary rocks.

2. The humid-tropical conditions that likely dominated the middle to late Cenozoic climate of the region have 

deeply leached and weathered these rocks such that when 

eroded they rapidly disaggregate into their constituent mineral grains or decompose into finer grained weathering products.

3. The wet-dry seasonality of the present-day climate, 

and of probable late Cenozoic paleoclimates, facilitates seasonal flushing from the landscape of all available surficial 

materials.

LATE CENOZOIC 

DEGRADATIONAL PROCESSES

Middle to late Cenozoic degradation of the uplands of 

the Gran Sabana has been dominated by the following

processes: (1) intense leaching and solutional etching of 

upland areas (particularly the tepuis and higher cuestas 

underlain by the quartzitic sandstone and conglomerate of 

the Mataui and lower part of the Uairen Formations); (2) 

solutional etching and rubbling of ferricrete crusts; (3) 

discontinuous slumping and sliding along laterite-capped 

hill slopes underlain by the upper part of the Uairen 

Formation and both laterite- and sandstone-capped scarp 

slopes underlain by the lower part of the Uaimapue 

Formation; and (4) basal sapping of hill slopes and cuesta dip 

slopes, particularly those underlain by interbedded sedimentary rocks of varying permeability and erosional resistance. 

The basal sapping is manifest on two distinct geomorphic 

scales: (1) conspicuous, locally intensive hillside and 

piedmont slope piping has formed narrow, almost vertical 

sided gullies and small box canyons as much as 30-40 m in 

width and 20 m in depth; and (2) more general large-scale 

basal sapping has cut large, steep-sided, generally U-shaped 

valleys into many of the larger cuestas and tepuis. These 

valleys and cuestas are strikingly similar, in both planimetric 

and cross-sectional form, to those canyons and escarpments
K8 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Figure 5. Detail of the flat, bedrock-floored channel of Quebrada de Jaspe cut into jasper beds of the upper part of the Uaimapue 

Formation.

of the Colorado Plateau region where sapping erosion has 

been a principal degradational process (Laity and Malin, 

1985; Howard, 1988). Many of the large, U-shaped valleys 

in the Gran Sabana are now beheaded, indicating that a 

significant amount of slope retreat has likely occurred along 

at least some of the cuesta scarps of the region and that basal 

sapping has been a significant valley-forming process during 

most of Cenozoic time. The relatively minor (generally less 

than 5-10 m) general lowering of the larger valley floors has, 

most likely, been principally accomplished by high 

discharge stream flow during wet-season flooding.

SURFICIAL DEPOSITS

Surficial deposits of the Gran Sabana are mostly limited 

to a discontinuous veneer of colluvium and alluvium, generally less than 1-2 m thick, on most hillsides and piedmont 

slopes. Gray to dark-brown, weakly to moderately developed soils are on these deposits. In adjacent areas, 14C age 

determinations made on similar surficial deposits indicate a 

Holocene age (Schubert and others, 1986). Locally,

significant colluvial aprons have been preserved on hill 

slopes armored by pebble to cobble lags of weathered conglomerate or by cobble- to boulder-sized fragments derived 

from well-indurated sandstone or ferruginous lateritic crust; 

however, these occurrences are limited to a small proportion 

of all hill slopes. Alluvial fans are not present within the 

study area; instead, piedmont surfaces are erosional straths 

discontinuously veneered by thin alluvial and colluvial 

deposits. Significant fluvial and valley bottom deposits are 

limited to the valleys of the largest tributaries and principal 

rivers, and even in the broadest valleys these deposits are 

generally thin, except within the modern floodplains of the 

larger rivers where they may be 5-10 m thick. Aerial observations and analysis of large-scale aerial photographs show 

that most tributary channels are developed in bedrock and 

contain little fluvial sediment. Waterfalls and rapids also are 

abundant along many of these lesser streams. Within the valley of the Rio Cuquenan, between Moroc Meru and the Rfo 

Yuruanf, bedrock exposure is almost continuous along the 

modern channel and only 1-3 m of fluvial deposits lie 

beneath the surface of this 3-8-km-wide fluvial valley.
CENOZOIC LANDSCAPE EVOLUTION OF THE GRAN SABANA K9

Figure 6. Valley of the Rio Cuquenan downstream from its confluence with the Rio Yuruani, Venezuela. This flat-floored strath valley, 

7-8 km wide at this point, is the most extensive middle to late Cenozoic planation surface in the southern part of the Gran Sabana.

LATERITIC SOILS OF THE 

GRAN SABANA

Relict lateritic soils cap numerous cuesta dip slopes, 

ridges, piedmont slopes, and strath terrace remnants within 

and adjacent to the valleys of the Rio Cuquenan, Rio Aponguao, and Rio Yuruani (fig. 7). Laterite also is present as 

isolated remnants on cuesta dip slopes and other structurally 

controlled surfaces on high interfluve areas between the Rio 

Cuquenan and the Rio Yuruani and on the high stripped 

structural surface west of the upper valley of the Rio Yuruani. In most areas the well-indurated ferricrete crust that 

characterizes these relict soils has been at least partly 

stripped and rubbled (fig. 8). These relations suggest periods 

of landscape stability and instability in the Gran Sabana.

The relict lateritic soils of the Gran Sabana are generally similar to lateritic soils described from other tropical and 

subtropical areas of the world (Nahon, 1986). In most areas 

of the southern part of the Gran Sabana, these soils are 

characterized by the following general profile from bedrock

upward to the surface (layer designations from Nahon, 

1986):

Layer I. Weathered bedrock having preserved original 

structures

Layer II. Highly irregular transitional zone between 

layer I and a pervasively mottled, clay-rich layer (layer III). 

This transitional zone may have as much as several meters of 

relief

Layer III. Pervasively red and yellow mottled, 

clay-enriched zone wherein many preexisting rock structures have been destroyed. This layer locally is as thick 15 

m or more (fig. 9)

Layer IV. Transitional zone 0.5-2 m thick between the 

mottled layer (III) and the ferricrete crust (V)

Layer V. Ferricrete layer having nodular and (or) 

pisolitic structure and containing numerous fragments of the 

underlying weathered bedrock. The upper part of this layer, 

commonly vesicular and weathered to dark brown or black, 

superficially resembles basic volcanic flow rocks. This layer 

is typically at least 1 m thick and locally may be as thick as
K10 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAY ANA SHIELD

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CENOZOIC LANDSCAPE EVOLUTION OF THE GRAN SAB ANA Kll

Figure 8. Surface of a relict laterite soil capping a cuesta ridge along the southwestern flank of the valley of the Rio Cuquenan. These 

relict lateritic soils are typically degraded such that the upper part of the ferricrete crust is discontinuously exposed, etched, and partly rubbled to form a dark-red-brown to black, rough and irregular surface that is superficially reminiscent of the surface of a basaltic lava flow. 

View north toward Cerro Roraima (on skyline at center).

3 m. Locally, it contains conspicuous concentrations of 

hematite and gibbsite (figs. 10, 11)

Layer VI. Thin (generally <10 cm thick) surface layer of 

ferruginous pebbles and other lateritic rubble generated by 

degradation of the underlying ferricrete crust

DISTRIBUTION AND SIGNIFICANCE OF 

RELICT LATERITIC SOILS

The present distribution of these relict ferricrete soils is 

apparently controlled by both stratigraphic (lithologic) and 

geomorphic factors Lateritic soils are not present on the 

high cuestas and tepuis that are underlain by the deeply 

leached and pervasively etched quartzitic sandstone and conglomerate of the lower parts of the Uairen and Matam 

Formations; however, they are particularly well developed in 

areas underlain by the finer grained and more feldspathic 

rocks of the upper part of the Uairen Formation, the

Cuquenan Formation, and the lower part of the Uaimapue 

Formation. They are present, but are generally somewhat 

less well developed, on strata of the upper part of the 

Uaimapue Formation.

Within and adjacent to the valley of the Rio Cuquenan, 

a relict lateritic soil having a strongly indurated, 1-3-m-thick 

ferricrete crust mantles (1) dissected and rounded remnants 

of strath terraces of the Rio Cuquenan that are within 5-10 

m of the level of the modern floodplain (fig. 12), (2) terrace 

remnants of tributary streams along the valley flanks and 

within some of the larger tributary valleys, and (3) dip slopes 

and cuesta ridges developed on the upper part of the Uairen 

Formation and on the lower part of the Uaimapue Formation 

(figs. 8, 13). Locally, these landscape relics range almost 

continuously over 200-300 m of elevation, from the level of 

the strath terraces along the Rio Cuquenan to the crests of the 

cuesta ridges that form the valley sides (figs. 7, 14).

The paleolandscape defined by these surface remnants 

was similar to the modern landscape. All of the major
K12 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

'£';J^^ A-^' '^Qw&£jf&'' f^ » «^' ».SOi?xMi* ->*^**.i

Figure 10. Typical lateritic soil roadcut exposure showing upper 

part of layer III (clay-enriched, red and yellow mottled zone), layer 

IV (ferricrete-mottled zone transition), and layer V (nodular 

ferricrete). Road cut is approximately 4 m high.

Figure 11. Upper part of a relict lateritic soil profile showing 

layer IV (ferricrete-mottled zone transition), layer V (nodular to 

pisolitic ferricrete crust; here about 2.5 m thick), and layer VI (disaggregated ferruginous pebble gravel lag). This relict soil profile is 

on the summit surface of Yuoromota tepuy at an elevation of about 

1,000 m, approximately 180 m above the level of the modern 

floodplain of the Rio Cuquenan.

landscape components (such as stream valleys and cuesta 

ridges) occupied essentially the same relative and absolute 

positions as they do within the present landscape. Local 

relief between valley bottoms and ridge crests was about the 

same as now, and regional base level was within 10 m of 

modern base level. Upland dissection may have been somewhat less extensive than at present; however, geomorphic 

evidence to support this possibility is not preserved within 

the present landscape.

The age of this relict lateritic soil, and of the paleolandscape whose remnants it mantles, is not well constrained. 

Theoretical calculations suggest that a period of at least 1 

m.y. would be required (under climatic conditions suitable 

for lateritic soil formation) to form a lateritic soil similar to

that mantling much of the landscape of the southern part of 

the Gran Sabana (Fritz, 1975). Field studies also suggest that 

development of ferricrete profiles requires a period of more 

than 1 m.y. of favorable climatic conditions (Nahon, 1986). 

For example, Nahon and Lappartient (1977) determined that 

formation of a ferricrete soil profile derived from basic 

volcanic rocks of the Senegalese-Mauritanian Basin 

occurred within 6 m.y.; however, ferricrete profile development on feldspathic to arkosic sedimentary rocks may take 

somewhat longer. Whatever the case, planation of the broad 

valley of the Rio Cuquenan, development of a thick, strongly 

indurated ferricrete on that surface and on the surrounding 

hillsides and subsequent dissection of the valley sides and 

degradation of the ferricrete would probably require an
CENOZOIC LANDSCAPE EVOLUTION OF THE GRAN SABANA KB

Figure 11. Detail of a strongly developed nodular ferricrete crust 

containing abundant rinds and nodules of hematite and lesser 

amounts of segregated gibbsite. Hammer is shown for scale.

aggregate period of several million years. Thus, the landscape remnants capped by this lateritic soil are likely to be at 

least as old as early Pliocene or late Miocene.

CENOZOIC EVOLUTION OF THE 

GRAN SABANA LANDSCAPE

As many as six planar geomorphic surfaces have been 

interpreted in the Guayana Shield (James, 1959; Short and 

Steenken, 1962; McConnell, 1968; Zonneveld, 1982; Schubert and others, 1986; Briceno and Schubert, 1990). These 

geomorphic surfaces have been defined on the basis of 

regional correlations of similar elevations of summit levels 

and areas of low relief. The surfaces are generally interpreted 

as remnants of a series of regional planation surfaces that 

have been uplifted in the main shield area and downwarped 

along the coast and continental shelf (with a hinge line along

the north coastal plain of the continent). Formation of multiple erosion surfaces is generally considered to be the result 

of alternating periods of tectonic uplift and stability, landscape dissection in response to the uplift, and parallel slope 

retreat during the succeeding interval of stability. The surfaces are generally thought to range from Mesozoic to 

Holocene in age; however, it has been pointed out by 

Kroonenberg and Melitz (1983) that many of these surfaces 

coincide with lithologic units and that the steps which separate them commonly coincide with lithologic discontinuities. 

Therefore, it is likely that present relief within the Guayana 

Shield is mostly a function of lithologic resistance and that 

at least some of these geomorphic surfaces are primarily a 

manifestation of lithologic control in a relatively stable landscape that is developed on a thick sequence of very gently 

dipping to flat-lying sedimentary strata.

Within the southern part of the Gran Sabana, the 

existence of an extensive laterite-capped paleolandscape that 

has 200-300 m of local relief and cuts across several of the 

regional planation surface levels hypothesized by previous 

workers argues strongly for a middle to late Cenozoic history 

wherein (1) vertical dissection, controlled at least in part by 

variations in lithologic resistance to weathering and erosion, 

has been the dominant mode of landscape evolution and 

(2) lateral planation was limited to the formation of broad 

erosional floodplains along the major rivers. Indeed, landscape change in the Gran Sabana during middle and late 

Cenozoic time has been primarily, if not entirely, denudational. This denudational history has been marked by periods 

of general landscape dissection and stripping interrupted by 

intervals of relative landscape stability. The broad strath terraces that occupy the larger stream valleys most likely 

formed during the intervals of relative landscape stability. It 

is unlikely that any significant episodes of aggradation took 

place during most, if not all, of Cenozoic time.

In the mid-Tertiary, the landscape of the Gran Sabana 

was sufficiently stable to permit formation of broad strath 

floodplains, as wide as 7-8 km, along major drainages of the 

region and widespread development of deep weathering 

profiles capped by thick, well-developed ferricrete crusts. In 

most other respects, however, this mid-Tertiary landscape 

was much like the landscape of today. The position and general morphology of the major landscape elements (tepuis, 

ridges, and major valleys) were essentially as they are today, 

and the overall landscape was well adjusted to underlying 

lithology and structure. Overall relief between ridge crests 

and valley bottoms may have been somewhat greater, 

however, than present-day relief.

Following this mid-Tertiary period of relative landscape stability and lateritic soil development, a probable 

long-term climatic change destabilized the landscape. 

Upland areas were dissected perhaps as much as 100-150 m, 

and the lateritic soils were gradually stripped, etched, and 

rubbled; however, downwearing of the valley floors was
K14 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Figure 12. Surface of a relict ferricrete crust capping a series of low (5-10 m high) fluvial terrace remnants. This dissected broad valley 

surface can be traced almost continuously from this location, adjacent to the modern channel of the Rio Cuquenan, to the crests of the low 

ridges along the valley margin on the left skyline (approximately 7-8 km distant and 200 m higher in elevation).

generally less than 5-10 m. During the waning stages of this 

period of landscape destabilization and dissection, probably 

in late Tertiary or earliest Quaternary time, a shorter interval 

of relative landscape stability permitted formation of a 

2-3-km-wide strath floodplain that is inset 5-10 m below the 

general level of the middle (?) Tertiary broad valley surface. 

General upland degradation likely slowed during this time, 

but available stream power was sufficient to rapidly transport essentially all available detrital material out of the local 

landscape system. This interval of renewed lateral stream 

planation was followed by renewed incision of the valley 

floors and a probable acceleration of upland stripping that 

apparently began during the late Quaternary, probably in 

response to a late Pleistocene-early Holocene climate 

change when an increase in mean annual temperature of as 

much as 5°C-7°C and a substantial increase in mean annual 

precipitation likely occurred in northern South America 

(Hammen, 1972; Liu and Colinvaux, 1985; Schubert and 

others, 1986). The net result of this Cenozoic degradational 

history is an erosional landscape, well adjusted to underlying

lithology and structure, that is discontinuously mantled by 

generally less than 2 m of late Quaternary colluvial, alluvial, 

and fluvial deposits.

OCCURRENCE OF PLACER DEPOSITS

The presence of numerous paleosurface remnants 

developed on bedrock and mantled by thick ferricrete crusts 

and the almost complete absence of Tertiary or Quaternary 

deposits of any significant thickness indicate a lack of significant periods of sedimentary accumulation in the region of 

the southern part of the Gran Sabana during middle and late 

Cenozoic time. Surficial deposits in the Gran Sabana are limited to (1) generally less than 1-2 m of discontinuous colluvial-alluvial mantles on hill slopes and piedmonts, although 

somewhat thicker colluvial aprons are preserved locally 

beneath pebble to cobble lag gravels weathered from conglomerate lenses and beds within the Roraima Group or 

beneath cobble- to small boulder-size rubble derived from 

degraded ferricrete crusts; (2) thin fluvial deposits in the
CENOZOIC LANDSCAPE EVOLUTION OF THE GRAN SAB ANA K15

Figure 13. Laterite-capped cuesta ridge developed on gently dipping strata of the upper part of the Uairen Formation. A strongly developed ferrugenous crust, 2-4 m thick, caps the gently northeast sloping surface of this cuesta from the floor of the valley of the Rio Cuquenan 

(out of view to the right) to the crest of the ridge. Local relief in this view, from the tree-covered floodplain of the small stream in the foreground to the cuesta summit, is about 160 m.

larger valleys that are generally 1-2 m thick over Pleistocene 

(?) strath surfaces and 5-10 m thick within the narrow modern floodplains of the largest rivers; and (3) numerous, 

widely scattered landslide deposits, particularly in areas 

underlain by the upper part of the Uairen and the lower part 

of the Uaimapue Formations. Tertiary deposits have not 

been identified in the southern part of the Gran Sabana.

Consequently, gold and diamond resources in the 

southern part of the Gran Sabana are primarily restricted to 

three general geologic-geomorphic environments:

1. Placer deposits within the active channels and 

Holocene floodplains of the larger rivers downstream from 

extensive exposures of the Uairen Formation

2. Colluvial-alluvial placer deposits of small 

low-order drainage basins within or adjacent to extensive 

exposures of the Uairen Formation. These placers typically 

are associated with pebble to cobble gravels lying on or 

within about 1 m of the basal colluvial-alluvial contact with 

the underlying bedrock

3. Paleoplacer deposits associated with conglomeratic lenses and beds within the lower 500-600 m of the 

Uairen Formation

These placer deposits have been mined in several locations where the conglomerates presently lie within a few 

meters of dip-slope surfaces. Additional gold and diamond 

resources might be present within or immediately beneath 

lateritic ferricrete crusts developed on surfaces underlain by 

the Uairen Formation. Gold concentrations beneath the iron 

crusts of lateritic soils have been reported from Western 

Australia (Mann, 1984), from the Mato Grosso area of Brazil 

(Michel, 1987), and from Dondo Mobi, Gabon (Colin and 

Viellard, 1991).

These gold and diamond resources are generally widely 

scattered and probably limited in both grade and tonnage, 

and economic exploitation of these resources is generally 

limited to surface mining techniques. Therefore, the 

resources are, for the most part, probably best suited for 

exploration and exploitation by small independent operators.
K16 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

SOUTHWEST 

1200-1

Profile 1

1000-

NORTHEAST

SOUTHWEST 

800-1

600-

NORTHEAST

EAST

800

8 10 12 

DISTANCE, IN KILOMETERS

14 18 20

Figure 14. Topographic profiles across the valley of the Rio Cuquenan showing the distribution of relict lateritic soils (shaded) capping 

the remnants of the middle Tertiary paleolandscape. Profile locations are shown in figure 7. Vertical exaggeration times 10.

PALEOPLACERS WITHIN THE LOWER PART 

OF THE RORAIMA GROUP

Gold and diamond paleoplacers are present at several 

different stratigraphic levels within the lower part of the 

Roraima Group. In northern Brazil, gold and diamonds are 

present within small conglomerate lenses approximately 

160-210 m above the base of the local Roraima section near 

the summit of Serra Tepequem (approximately lat 3°47' N., 

long 61°42' W.) (Borges and D'Antona, 1988). In Venezuela, along the southern margin of the Gran Sabana, several 

mines are producing diamonds and (or) gold from conglomerate within the Uairen Formation. Four mines, apparently 

inactive at present, are between approximately 500 and 530 

m elevation on the gentle dip slopes of low cuestas that rim 

the inner valley of the Rio Icabaru in the vicinity of the town 

of Icabaru. Production from these mines was probably 

mainly from gravel lenses and beds that are within the same 

general stratigraphic horizon immediately beneath this 

dip-slope surface. Moreover, at least two gold and diamond 

prospects are in conglomerate and associated deeply leached 

sandstone near the axis and on the east-dipping flank of the

Chiricayen anticline (near lat 4°50' N., long 61°15' W., and 

lat 4°42' N., long 61°06' W., respectively). At both prospects, the productive conglomeratic layers are at least 

500-600 m above the base of the Uairen Formation.

PLACERS IN COLLUVIAL-ALLUVIAL 

DEPOSITS OF LOW-ORDER DRAINAGES

Six active gold and diamond placer mines are within 

headwater areas of tributary drainages of the Rio Icabaru and 

the Rio Cuquenan. Three of these mines are immediately 

south of the north-dipping cuesta ridge, underlain by lower 

Roraima Group rocks, that bounds the southern margin of 

the strike valley of the Rio Icabaru (in the vicinity of lat 

4°10/ N., long 61°12' W.). The other three mines are in a 

similar geologic and geomorphic setting at the base of the 

scarp slope that defines the nose of the west-plunging syncline that underlies Paraitepuy (between lat 3°32' and 3°39' 

N. and long 61°25' and 61°28' W.). These mines are all 

underlain by lower Roraima rocks, and they are all within a 

few kilometers of the headwaters of low-order streams drain-
CENOZOIC LANDSCAPE EVOLUTION OF THE GRAN SAB ANA K17

ing deeply dissected strata of the Uairen Formation. Production from these mines is derived from thin basal lenses of late 

Quaternary (Holocene?) pebble to cobble gravel that lie on 

or close to irregular bedrock surfaces. In addition, at least 

some production from two of these mines is derived from 

conglomerate lenses within the underlying, deeply weathered bedrock. Gold and diamond colluvial-alluvial prospects 

also occur near the axis of the north-northwest-plunging 

anticlinal nose of the Chiricayen anticline (within the drainage of Quebrada Nunque between lat 4°48' and 4°52' N. and 

long 61°15' and 61°20' W.). The gold and diamonds in these 

prospects are in basal colluvial-alluvial gravels of probable 

late Quaternary age that were likely derived from local 

conglomerates several hundred meters above the base of the 

Uairen Formation.

PLACERS IN MODERN CHANNELS OF THE 

LARGER STREAMS

Approximately 40 chupadoras (pontoon-mounted 

air-lift pumps) were present during 1989 on the Rio Icabarii 

between Los Caribes and the confluence of the Rio Icabaru 

and the Rio Carom, a distance along the river channel of 

about 140 km. These chupadoras are not uniformly 

distributed along the river channel; rather, they tend to be 

concentrated along limited reaches of the river. Most notably, about 60 percent were operating along a 30-km-long 

reach starting about 17 km downstream from Los Caribes 

and extending to the first major salto (waterfall), about 5.5 

km downstream from the Rio Hacha. About half of these 

were within 6 km of junctions with major tributaries. The 

reasons for this nonuniform distribution are not entirely 

clear. The distribution may indicate geologic and (or) fluvial 

geomorphic controls on the location of the richest channel 

placer or practical limitations of chupadora mining. It is 

probably significant, however, that almost all of these operations were within the region that is either underlain by or 

adjacent to uplands underlain by the lower part of the 

Roraima Group.

REFERENCES CITED

Borges, F.R., and D'Antona, R. de J.G., 1988, Geologia e mineralizfoes da Serra Tepequem: Congreso Brasileiro de Geologia, 

35th, Belem, Para, Brazil, Annals, v. 1, p. 155-163.

Briceno, H.O., and Schubert, Carlos, 1990, Geomorphology of the 

Gran Sabana, Guayana Shield, southeastern Venezuela: 

Geomorphology, v. 3, p. 125-141.

Colin, Fabrice, and Viellard, P., 1991, Behavior of gold in the lateritic equatorial environment Weathering and surface dispersion of residual gold particles, at Dondo Mobi, Gabon: Applied 

Geochemistry, v. 6, p. 279-290.

Fritz, Bertrand, 1975, Etude thermodynamique et simulation des 

reactions entre mineraux et solutions, application a la geochimie des alterations et des eaux continentales: Universite de 

Strasbourg, Sciences Geologiques, Memoire, 41, 152 p.

Ghosh, S.K., 1985, Geology of the Roraima Group and its implications: Simposium Amazonico, 1st, Caracas, 1981, Publicacion 

Especial 10, p. 31-50.

Hammen, Thomas van der, 1972, Changes in vegetation and 

climate in the Amazon Basin and surrounding areas during the 

Pleistocene: Geologic en Mijnbouw, v. 51, p. 641-643.

Howard, A.D., 1988, Introduction to cuesta landforms and sapping 

processes on the Colorado Plateau, in Howard, A.D., Kochel, 

R.C., and Holt, H.E., eds., Sapping features of the Colorado 

Plateau A comparative planetary geology field guide: 

National Aeronautics and Space Administration Special 

Publication SP-491, p. 6-56.

James, P.E., 1959, The geomorphology of eastern Brazil as interpreted by Lester C. King: Geographical Reviews, v. 49, 

p. 240-246.

Kroonenberg, S.B., and Melitz, P.J., 1983, Summit levels, bedrock 

control, and the etchplain concept in the basement of Surinam: 

Geologic en Mijnbouw, v. 62, p. 389-399.

Laity, J.E., and Malin, M.C., 1985, Sapping processes and the 

development of theater-headed networks in the Colorado 

Plateau: Geological Society of America Bulletin, v. 96, 

p. 203-217.

Liu, Kam-biu, and Colinvaux, P.A., 1985, Forest changes in the 

Amazon Basin during the last glacial maximum: Nature, v. 

318, p. 556-557.

Mann, A.W., 1984, Mobility of gold and silver in lateritic weathering profiles Some observations from Western Australia: 

Economic Geology, v. 79, p. 38-49.

McConnell, R.B., 1968, Planation surfaces in Guyana: Geographical Journal, v. 134, p. 506-520.

Michel, D., 1987, Concentration of gold in in-situ laterites from 

Mato Grosso: Mineralium Deposita, v. 22, p. 185-189.

Nahon, D.B., 1986, Evolution of iron crusts in tropical landscapes, 

in Colman, S.M., and Dethier, D.P., eds., Rates of chemical 

weathering in rocks and minerals: Orlando, Florida, Academic 

Press, 603 p.

Nahon, D.B., and Lappartient, J.R., 1977, Time factor and 

geochemistry in iron crust genesis: Catena, v. 4, p. 249-254.

Reid, A.R., 1974, Stratigraphy of the type area of the Roraima 

Group, Venezuela: Conferencia Geologica Inter-Guayanas, 

9th, Ciudad Guayana, Venezuela, 1972, Memoria, Publicacion 

Especial 6, p. 343-353.

Schubert, Carlos, Briceno, H.O., and Fritz, Peter, 1986, Paleoenvironmental aspects of the Caroni-Paragua River basin (southeastern Venezuela): Interciencia, v. 11, no. 6, p. 278-289.

Short, K.C., and Steenken, W.F., 1962, A reconnaissance of the 

Guayana Shield from Guasipati to the Rio Aro, Venezuela: 

Asociacion Venezolana de Geologia, Mineria y Petroleo, 

Boletm Informative, v. 5, no. 7, p. 189-221.

Yanez, Galo, 1984, Geology and geomorphology of the Roraima 

Group, southwestern Venezuela: West Lafayette, Indiana, 

Purdue University, Ph.D. dissertation, 145 p.

Zonneveld, J.I.S., 1982, Summit levels in Surinam: ITC Journal, 

1982, p. 237-242.

 1985, Geomorphological notes on the continental border in 

the Guy anas (N. South America): Zeitshrift fur 

Geomorphologie, N.F., supplement band 54, p. 71-83.

Gold and Diamond Resources of the 

Icabani Sur Study Area, 

Estado Bolivar, Venezuela

By William E. Brooks, Richard M. Tosdal, and 

Fernando J. Nufiez

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD 

U.S. GEOLOGICAL SURVEY BULLETIN 2124-L

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENTS

Abstract........................................................................................................................... LI

Resumen......................................................................................................................... 1

Introduction.................................................................................................................... 2

Geologic Setting............................................................................................................. 4

Weathering Effects on Trace Elements.......................................................................... 6

Gold and Diamond Prospecting...................................................................................... 7

Assessment of Mineral Potential.................................................................................... 8

Conclusions.................................................................................................................... 8

References Cited............................................................................................................. 8

FIGURES

1. Map showing location of Icabaru................................................................................................................................ L2

2. Geologic map of Icabaru Sur study area..................................................................................................................... 3

3. Concordia diagram for sample IC-001....................................................................................................................... 6

TABLES

1. Analytical data for rock samples of the Early Proterozoic Caicara Formation of the Cuchivero Group,

Icabaru Sur study area................................................................................................................................................. L5

2. U-Pb geochronologic data for felsic tuff (IC-001) from the Icabaru Sur study area.................................................. 7

3. Trace-element contents of unweathered and weathered sample pairs from the Early Proterozoic

Caicara Formation of the Cuchivero Group, Icabaru Sur study area.......................................................................... 8
Gold and Diamond Resources of the

Icabaru Sur Study Area,

Estado Bolivar, Venezuela

By William E. Brooks, 1 Richard M. Tosdal,2 and Fernando J. Nufiez3

ABSTRACT

The Icabaru Sur study area is in the southern part of 

Estado Bolivar, Venezuela, in a transition zone between 

savanna and jungle. The study area is 15 km southwest of 

Icabaru, the supply town for miners in the area. Access to the 

50-km2 study area is by helicopter or by unimproved road 

from Santa Elena de Uairen, approximately 80 km to the 

east.

Exposed in the study area are Early Proterozoic rhyolitic ash-flow tuffs and a porphyritic rhyolite of the 

Cuchivero Group and sandstone of the Early to Middle 

Proterozoic Roraima Group. These rocks have not been 

affected by regional metamorphism; the ash-flow tuffs have 

well-preserved compaction foliation, shards, and pumice. 

Three zircon fractions from one pyrite-bearing sample of the 

Cuchivero Group have consistent U-Pb ages of about 1.98 

Ga. Northeast- to east-striking faults separate the Roraima 

Group to the north from the volcanic province to the south.

One diamond mine is active in the study area, and 

numerous small prospects dot streams. Diamonds are panned 

from gravels derived from the Roraima Group. The presence 

of placer diamonds suggests a kimberlitic source; however, 

diamonds were not found in stream-sediment samples from 

the study area, and minerals indicative of a kimberlitic 

source have not been identified in the study area or in the 

outcrop area of the Roraima Group elsewhere in the shield. 

The primary source of the diamonds remains undetermined.

The presence of silicic volcanic rocks in the study area 

suggests a potential for epithermal gold deposits. Minor 

pyrite, a fluorite veinlet in one sample, and small quartz

^.S. Geological Survey, Denver Federal Center, MS905, Denver, 

Colorado 80225.

2U.S. Geological Survey, 345 Middlefield Road, MS901, Menlo Park, 

California 94025.

3Corporacion Venezolana de Guayana, Tecnica Minera, C.A., C.C. 

Chilemex, Piso 1, Puerto Ordaz, Venezuela.

veins cutting the volcanic rocks are the only indications of 

mineralized rock.

RESUMEN

El area de estudio Icabaru Sur tiene 50 km2 de area, esta 

localizada a 15 km al suroeste del poblado minero de Icabaru 

al sur del Estado Bolivar, Venezuela, y esta ubicado en una 

zona transicional entre vegetation sabana y selva. El acceso 

se realiza desde Santa Elena de Uairen, a 80 km al este de 

Icabaru, ya sea por helicoptero o por carreteras para vehiculos de doble traction.

En el area de estudio estan expuestas toba riolitica de 

flujo de ceniza y riolita porfiritica, ambas del Grupo 

Cuchivero, ademas aflora arenisca perteneciente al Grupo 

Roraima de edad Proterozoico Temprano a Medio. Estas 

rocas no han sido afectadas por metamorfismo regional; la 

toba riolitica de flujo de ceniza presenta foliation por 

compactaci6n, esquirlas y fragmentos de pomez. Las tres 

fracciones de circon de una muestra que contiene pyrita 

arrojaron edades consistentes por el metodo U-Pb de 

aproximadamente 1.98 Ga. El Grupo Roraima, al norte, esta 

separado de la provincia volcanica, al sur, por fallas que 

tienen un rumbo noreste a este.

Hay una mina de diamante activa en el area, y numerosos prospectos pequenos a lo largo de las quebradas. Los diamantes son extraidos con suruca (wire-mesh pan) a partir de 

grava derivada del Grupo Roraima. La presencia de diamante en depdsitos tipo placer sugiere una fuente kimberlitica, sin embargo, no se encontraion diamantes en 

sedimentos de quebradas, ademas, no se han encontrado 

minerales indicatives de una fuente kimberlitica en el area de 

estudio o en rocas del Grupo Roraima a traves del Escudo de 

Guayana. La fuente primaria de los diamantes permanece 

indeterminada.

La presencia de rocas volcanicas siliceas en Icabaru Sur 

sugiere un potencial de depositos auriferos epitermales. La 

unica indication de mineralization en el area es trazas de pirita, una venilla de fluorita en una muestra y vetas pequenas 

de cuarzo que invaden las rocas volcanicas.

Ll
L2 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

ESTADO 

\ DELTA AMACURO

Boundary of Venezuelan 

Guayana Shield

Figure 1. Location of Icabaru in southeastern Estado Bolivar, Venezuela. The Icabaru Sur study area is 15 km southwest of the town 

of Icabaru.

INTRODUCTION

The Icabaru Sur study area is in the southern part of 

Estado Bolivar, Venezuela (fig. 1). The study area is approximately 50 km2 in size and is 15 km southwest of Icabaru, a 

mining supply town for the region, in a transition zone

between savanna in the north and dense jungle to the south. 

Access to the study area is by hard surface to unimproved 

road from Santa Elena de Uairen, approximately 80 km to 

the east, or by helicopter.

In this part of the Guayana Shield, Early Proterozoic 

rhyolitic ash-flow tuffs, rhyolite of the Cuchivero Group,
GOLD AND DIAMOND RESOURCES, ICABARU SUR AREA L3

and sandstone of the Early to Middle Proterozoic Roraima 

Group (Gibbs and Barren, 1983; Sidder and Mendoza, this 

volume) are exposed. Rocks of the Roraima Group are commonly considered to be the source of placer gold and 

diamonds recovered from streams in the region. During a 

geologic reconnaissance of the area, Contreras and Page 

(1988) recommended additional field studies to investigate 

the potential of epithermal gold deposits in the region.

GEOLOGIC SETTING

The Icabaru Sur study area is within the central part of 

the Guayana Shield and contains volcanic rocks of the Early 

Proterozoic Uatuma Supergroup and sedimentary rocks of 

the Early to Middle Proterozoic Roraima Group (Gibbs and 

Barron, 1993) (fig. 2). Recent work by Sidder and Mendoza 

(this volume) indicates that the ash-flow tuff units and 

rhyolite are included in the Early Proterozoic Caicara Formation of the Cuchivero Group in Venezuela.

Silicic, relatively unmetamorphosed, volcanic rocks of 

the Caicara Formation of the Cuchivero Group crop out in the 

southern part of the study area. Three distinctive ash-flow 

tuffs and a porphyritic rhyolite are exposed. Compaction foliation visible in hand sample and outcrop indicates deposition 

as a subaenal ash-flow tuff. The silica content of these volcanic rocks ranges from 67.1 to 73.8 weight percent SiO2- 

Descriptions of the tuffs and the rhyolite are given in figure 

2, and analytical data are in table 1. A source for these tuffs 

has not been identified.

Three fractions of a zircon concentrate from one of the 

ash-flows (IC-001, fig. 2) have consistent U-Pb ages that 

imply the rock is about 1.98 Ga (fig. 3). The U-Pb geochronologic data are listed in table 2. Abundant sulfide minerals, 

which if not removed can contribute lead to the analysis, and 

the very fine grain size of the zircon were problems in the 

analysis; however, the age is relatively accurate based on the 

transparent character of the zircons, which indicates that not 

much damage has occurred to the zircon crystal lattice, on the 

fact that the least discordant data point is close to concordia, 

and on comparison with other published ages (Sidder and 

Mendoza, this volume).

Red to orange, well-sorted sandstone of the Uairen 

Formation of the Roraima Group crops out in the northern 

part of the study area. Grains are subrounded, 0.3-0.5 mm in 

diameter, and cemented with hematite. Bedding is massive, 

and sedimentary structures are rare. The Roraima Group is a 

thick continental sedimentary sequence composed of four 

formations (Reid and Bisque, 1975) and is present in Venezuela, Guyana, Suriname, and northern Brazil. Regional 

thickness of the Roraima Group is estimated to be 2,600 m or 

more (Dalton in Reid and Bisque, 1975). Rocks mapped as 

the Roraima Group in southeastern Venezuela are at least

1,670 Ma in age; some are possibly as old as about 1,900 Ma 

and others as young as about 1,500 Ma or younger (Priem 

and others, 1973; Sidder and Mendoza, this volume). A study 

of heavy minerals in streams that flow through and over 

rocks of the Uairen Formation in the region near Icabaru (Del 

Olio and others, 1989) indicates that the original source of 

the Roraima Group sediments was complex and included 

granitic, volcanic, alkaline, and metasedimentary rocks.

Sandstone of the Roraima Group is separated from the 

volcanic rocks to the south by northeast- to east-striking 

high-angle normal faults (Contreras and Page, 1988) that 

juxtapose rocks of the Roraima Group against the volcanic 

rocks of the older Caicara Formation. The amount of vertical 

displacement is unknown. Neither quartz veins, hydrothermal alteration, nor other physical evidence of mineralization 

is associated with these faults.

WEATHERING EFFECTS ON TRACE 

ELEMENTS

Exposures in the study area present an opportunity to 

study trace element variation between weathered and 

unweathered volcanic rocks in a tropical environment. The 

behavior of trace elements, especially zirconium, during 

weathering is not well known (Erlank and others, 1978). In 

chemical variation studies of metamorphic rocks in a tropical 

environment, Montero and others (1989) used zirconium 

content as an immobile reference element.

Five rock samples from the study area were collected 

and then sawed in order to separate the unweathered core 

from the weathered rind. These two splits were crushed, pulverized, and analyzed on an energy-dispersive analyzer that 

allows rapid analysis for Zr, Rb, Sr, Y, Nb, and Ba. Results 

of this study are presented in table 3. The zirconium content 

of the weathered samples is dramatically higher than that of 

the unweathered samples. Weathering did not affect the 

niobium content in most of the samples, but the strontium 

content is lower in weathered samples. Changes in content of 

rubidium, yttrium, and barium with weathering are erratic.

Presumably, most of the zirconium in these silicic rocks 

is contained in zircon, a mineral that commonly is resistant to 

chemical weathering (Deer and others, 1982). Therefore, it is 

likely that zirconium is released during the weathering of 

minerals other than zircon or from minerals of which 

zirconium is a minor component (Erlank and others, 1978). 

Dennen and Anderson (1962) compared the compositions of 

various fresh rocks with their respective weathering rinds. 

The zirconium content was little changed or slightly enriched 

(as much as twofold) in most samples and was depleted in 

only one sample.
L4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

61°49:

61°53;

4° 10'

Xcc

JIC-020 V018

Xcc U 

\

Xcc

Xcc

IC-028

S'\\

IC-021...-

'."^ 

' IC-024|

\

+

\

\ ' IC-025«

 '' ^'"

BRAZIL"

/' / \

(" / ^

/ /

/ I

 ^ ! 

\

\ \ ." """ )

2 KILOMETERS

Figure 2. Geology of the Icabaru Sur study area, Estado Bolivar, Venezuela. Drainage base from Icabaru Sur, 

1985, hoja 7828. Geologic mapping by Brooks and Nunez, 1988.
GOLD AND DIAMOND RESOURCES, ICABARU SUR AREA L5

IC-014

EXPLANATION

Roraima Group (Early to Middle Proterozoic) Red to orange, 

well-sorted (-0.3-0.5 mm diameter) quartz sandstone. 

Grains of subrounded quartz and polycrystalline quartz 

predominate; sparse subrounded zircon, muscovite, and 

opaque minerals are present; cement is hematite. Massive 

bedding. Neither metamorphic textures nor quartz 

overgrowths are present

Caicara Formation of the Cuchivero Group (Early Proterozoic)

Rhyolite (Early Proterozoic) Green to 

gray porphyritic rhyolite. Phenocrysts and clots of sanidine 

are from 0.5 to 1 cm in length. Devitrified groundmass 

contains opaque minerals and trace zircon

Ash-flow tuff Gray-green to black, crystal-rich plagioclase 

ash-flow tuff. Groundmass is devitrified and contains 

minor iron-titanium oxide minerals and zircon. 

Phenocrysts include plagioclase, sanidine, quartz, and 

altered biotite. Vitroclastic texture and compaction 

foliation are well preserved, d indicates a dikelike 

feature, 20 m long and 5 m wide, apparently a crystalrich ash-flow tuff, that has horizontal cordwoodlike 

structure and polygonal faces 30-40 cm on a side

Ash-flow tuff Gray-green, lithic ash-flow tuff. Lithic 

fragments are fine grained, angular, and from 0.5 to 

2 cm in diameter. May be white to black

Ash-flow tuff Dark-gray-green, crystal-poor, quartzbearing ash-flow tuff. Groundmass is devitrified and 

contains trace iron-titanium oxide minerals and 

zircon. Well-preserved vitroclastic texture. U-Pb age 

of 1.98 Ga (sample IC-001)

 Contact Approximately located

 Fault Approximately located; bar and ball on downthrown 

side

Strike and dip of sedimentary rock

Strike and dip of compaction foliation in tuff

Inclined

Vertical

Sample locality and number Analytical results are given in 

tables 1 and 3

Mine 

Heliport

GOLD AND DIAMOND PROSPECTING

Gold and diamonds are recovered from alluvium down 

slope from conglomeratic horizons in the Uairen Formation 

of the Roraima Group (Reid and Bisque, 1975). Dohrenwend 

and others (this volume) indicate that the gold and diamond 

resources in the Gran Sabana are in the following 

geologic-geomorphologic environments: (1) placer deposits 

downstream from extensive exposures of the Uairen Formation; (2) colluvial-alluvial placer deposits adjacent to 

exposures of the Uairen Formation; and (3) paleoplacer 

deposits associated with conglomeratic lenses in the lower 

500-600 m of the Uairen Formation.

The Roraima Group is commonly considered to be the 

immediate source of the modern placers and kimberlite in 

West Africa or possibly Brazil or Venezuela the initial 

primary source. Diamond placer deposits in Venezuela are 

the result of the following sequence of events, as proposed by 

Reid (1974): (1) intrusion of diamond-bearing kimberlite,

0.37 -

0.35 -

0.33 -

0.31

0.29

Intercepts at

1,978±43 and -30±620 Ma 

(MSWD=1.73)

5.0 5.4 5.8 

207pb/235u

Figure 3. Concordia diagram for felsic tuff from the Icabani Sur 

study area (sample IC-001), Estado Bolivar, Venezuela. Geochronologic data are given in table 2.

possibly in West Africa; (2) uplift and erosion of the 

kimberlite and transport of the diamonds to an area in South 

America now known as Suriname before the separation of 

Africa and South America by rifting and continental drift; (3) 

erosion of the diamond-bearing rocks and redeposition as 

Roraima Group sediments; and (4) uplift and erosion of 

Roraima Group sediments and concentration of the diamonds 

in alluvium derived from the Roraima Group. In an overview 

discussion of diamonds in Venezuela, Nixon (1988) attributed the source of the diamonds to be recently discovered 

kimberlitic intrusions in the Quebrada Grande area in the 

northern part of the shield in Venezuela; however, sedimentological features such as cross-stratification, ripple marks, 

and pebble orientation indicate that sediments in the lower 

part of the Roraima Group were derived from an easterly, not 

a northwesterly, source (Keats, 1974; Reid and Bisque, 1975; 

Ghosh, 1985). Meyer and McCallum (1993) concluded, on 

the basis of both physical characteristics of the diamonds and 

their distribution, that the source for diamonds both in and 

derived from the Roraima Group is neither West Africa nor 

the Quebrada Grande area and that a primary source remains 

to be discovered.

Rock and stream-sediment samples were collected in the 

study area; however, only rock samples were returned to U.S. 

Geological Survey laboratories in Denver. The stream-sediment (panned concentrate) samples were kept by Corporation Venezolana de Guayana, Tecnica Minera, C.A., in 

Tumeremo. Stream-sediment samples, which initially 

weighed 2-3 kg, were collected from 29 localities in the 

study area. These samples were panned in the field; none of 

the panned concentrates contained visible gold, and only 1 of 

the 29 samples contained a diamond. This milli- meter-size 

diamond was found at Agua Colorada, an active prospect in 

the northern part of the study area (fig. 3), when the prospector allowed us to sample his workings.
Table 1. Analytical results for rock samples of the Early Proterozoic Caicara Formation of the Cuchivero Group, Icabani Sur study area, Estado Bolfvar, Venezuela. 

[Location of samples shown in figure 2 by field number. Major-element oxides (weight percent, uncorrected) were determined by X-ray spectroscopy; analysts: J. Taggart, A. Bartel, and D. Siems; 

FeTO3 indicates total iron reported as Fe2C>3. Trace elements (in parts per million) determined by an energy-dispersive analyzer Cd109 source for Rb, Sr, Y, Zr, and Nb and Am95 source for Ba; analyst: E. Rivera; N, not detected at value listed; L, limit of determination. Spectrographic analyses are in parts per million unless noted; analyst R. Hopkins; lower limit of detection is given in parentheses after 

element symbol; N, not detected at lower limit of determination shown; L, detected but below lower limit of determination shown. Atomic absorption analyses are in parts per million; analyst J. McHugh; L, detected but below lower limit of determination shown]

Rhvolite (unit Xcr)

Lab No.

Field No.

D-3 16929

IC-004

D-3 16930

IC-005

D-3 16931

IC-006

D-3 16932

IC-007

Crvstal-rich ash-flow tuff (unit Xcc)

D-316934 D-316935

IC-014 IC-018

D-3 16937

IC-021

D-3 16938

IC-024

D-316939

IC-025

D-316940

IC-028

Lithic 

ash-flow

tuff (unit Xcl)

D-316933

IC-009

Crystal-poor 

ash-flow tuff

(unit Xca)

D-3 16928

IC-001

Major elements

SiO2

A1203FeT03MgO

CaO

Na2O

K2O

TiO2P205

MnO

LOI 900°CTotal

71.7

14.1

2.19

0.270.453.625.20

0.410.050.09

1.19

99.27

69.9

14.9

3.00

0.320.13

3.805.66

0.490.080.12

1.03

99.43

71.9

14.7

1.86

0.400.092.835.570.440.08

L0.02

1.49

99.36

70.2

13.5

3.34

0.300.783.65

4.95

0.480.11

0.12

1.62

99.05

73.8 73.0

13.3 13.3

1.63 1.70

0.23 0.370.52 0.82

3.48 3.82

5.14 5.010.34 0.33

<0.05 0.05

0.04 0.060.95 0.79

99.43 99.25

72.9

13.6

1.79

0.310.62

4.35

4.600.340.050.080.60

99.24

72.4

13.7

1.91

0.350.71

3.995.15

0.39

<0.05

0.060.39

99.05

70.4

14.4

2.260.46

1.10

4.14

5.070.460.080.070.55

98.99

72.3

13.8

2.010.370.77

3.94

5.04

0.380.060.070.49

99.23

67.1

15.1

4.68

0.940.353.46

4.39

0.600.150.342.20

99.31

74.9

12.8

1.71

0.210.68

1.28

6.540.26

<0.05

0.06

1.51

99.95

Trace elements

Rb

SrYZrNb

Ba

98

123

57

595

17

1,314

130

84

44

662

13

1,939

142106

42

354

17

1,525

102187

40

391

17

2,685

125 136

94 86

28 37

316 293

18 15

1,153 1,310

129

96

36

297

15

1,246

165

120

36

347

19

1,438

147179

36

416

16

1,900

158108

32

339

17

1,368

101

241

34

292

14

1,581

171

96

53

373

26

694

Spectrographic analyses

Ca (percent) (0.05)

Fe (percent) (0.05)

Mg (percent) (0.02)Na (percent) (0.2)P (percent) (0.2)Ti (percent) (0.002)

Ag(0.5)

As (200)Au (10)

B(10)Ba (20)

Be(l)Bi (10)

Cd (20)Co (10)

0.2

20.2

3

N0.2

0.3

N0.5

N200

N10

L10

1,500

3

N10

N20

N10

0.07

2

0.2

3

N0.2

0.3

N0.5

N200

N10N10

1,500

2

N10

N20

L10

0.07

20.3

2

N0.2

0.3

N0.5

N200

N10N10

1,500

3

N10

N20N10

0.2

20.22

N0.2

0.3

N0.5

N200

N10

N10

2,000

3

N10N20

10

0.2 0.32 20.2 0.5

2 3

N0.2 N0.2

0.2 0.2

N0.5 N0.5

N200 N200

N10 N10N10 N10

1,500 2,000

2 3

N10 N10

N20 N20

N10 N10

0.2

2

0.3

2

N0.2

0.3

N0.5

N200

N10

N10

1,500

3

N10N20

N10

0.220.3

2

N0.2

0.3

N0.5

N200

N10N10

1,500

3

N10N20

N10

0.3

20.3

2

N0.2

0.3

N0.5

N200

N10

N10

2,000

3

N10N20

N10

0.3

1.5

0.3

3

N0.2

0.2

N0.5

N200

N10N10

2,000

3

N10

N20N10

0.153

0.7

2

N0.2

0.3

N0.5

N200

N10

N10

1,500

3

N10N20

15

0.3

2

0.1

2

N0.2

0.2

N0.5

N200

N10

L10

1,000

3

N10

N20

N10

c/3

DC
Cr (10)

Cu(5)

Ga(5)

Ge(10)

La (50)

Mn (10)

Mo (5)

Nb(20)

Ni(5)

Pb (10)

Sb (100)

Sc(5)

Sn (10)

Sr (100)

Th(lOO)

V(10)

W(20)

Y(10)

Zn (200)

Zr (10)

L10

L5

50

N10

200

700

N5

20

N5

30

N100

7

N10

N100

N100

10

N20

50

N200

700

L10

5

30

N10

100

1000

50

L20

N5

50

N100

10

N10

N100

N100

10

N20

30

N200

1000

L10

L5

30

N10

70

300

N5

L20

N5

50

N100

7

N10

L100

N100

30

N20

30

N200

300

10

5

30

N10

70

1000

N5

20

N5

50

N100

7

N10

150

N100

50

N20

30

N200

500

L10

5

30

N10

100

700

N5

20

N5

30

N100

7

N10

100

N100

30

N20

30

N200

300

L10

L5

30

N10

100

700

N5

L20

L5

30

N100

7

N10

100

N100

15

N20

30

N200

200

N10

5

30

N10

100

700

N5

20

L5

30

N100

7

N10

L100

N100

20

N20

30

N200

300

L10

L5

30

N10

70

700

N5

L20

N5

50

N100

7

N10

L100

N100

20

N20

30

N200

300

L10

5

30

N10

70

500

N5

20

N5

50

N100

7

N10

150

N100

30

N20

20

N200

500

L1

L

5

N1

10

50

N

L2

N

3

N10

N1

10

N10

3

N2

3

N20

30

Atomic absorption analyses

Au L0.001 0.001 0.001 0.001 0.001 L0.001 0.001 0.001 0.001 

Table 2. U-Pb geochronologic data (zircon) for felsic tuff (IC-001) from the Icabaru Sur study area, Estado Bolfvar, Venezuela.

[Location of sample shown in figure 2; major- and trace-element data for sample given in table 1. Asterisk (*) following element symbol denotes radiogenic Pb. S

chemistry modified from Krogh (1973) and Mattinson (1987). Fraction: N, nonmagnetic; M, magnetic; amperes/side slope on a Franz Isodynamic separator; 140 

Observed ratios corrected for 0.125 per unit mass fractionation, based on replicate analyses of NBS 981. Uncertainties in the measured 208Pb/206Pb and 207Pb/206Pb 

uncertainties in the measured 206Pb/204Pb ratios are between 0.5 and 2 percent at the 2 sigma level(a). Isotopic data measured on a Finigan-Mat MAT 262 multipl

U.S. Geological Survey in Menlo Park. Atomic ratios calculated using the following constants: 238U/235U=137.88; 235U=0.98485xlO~9yr~1 ; 238U=0.155125xlO~

(1987)]

Fraction

N(1.8/l, <140) 

M(1.8/1.5,>230) 

M(1.8/1.5,<230)

Weight 

(mg)

1.7

2.7 

4.1

206pb*

(ppm)

43.7 

39.4 

75.6

238U 

(ppm)

142.8 

145.3 

266.0

206pb 

204pb

104.7 

316.5 

183.8

Observed ratios

207pb 

206pb

0.2502 

0.1637 

0.1937

208pb 

206pb

0.5774 

0.3499 

0.4276

2oepb*

235U

0.348723 

0.313032 

0.328726

Atomic ratios

207pb*

235u

5.91752 

5.25247 

5.48515

207^* 

206pb*

0.123072 

0.121696 

0.121019

206pb*

238u

1,928±2 

1,756±2 

1,832+2
L8 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 3. Trace-element contents of unweathered and weathered sample pairs from the Early Proterozoic Caicara Formation of the 

Cuchivero Group, Icabaru Sur study area, Estado Bolivar, Venezuela.

[In parts per million. Location of samples shown in figure 2 by number; map unit symbols are defined in figure 2. Determined by an energy-dispersive 

analyzer Cd109 source for Rb, Sr, Y, Zr, and Nb, Am95 source for Ba; analyst E. Rivera. U indicates unweathered sample; W indicates weathered 

sample. More complete analytical data for samples ICU-001, -004, and -014 are given in table 1]

Sample No.

Map unit

Sample type

Rb

Sr

Y

Zr

Nb

Ba

IC-001

Xcq

U

171

96

53

373

26

694

W

167

20

40

490

26

207

IC-004

Xcr

U

98

123

57

595

17

1,314

W

108

70

40

769

16

1,390

IC-014

Xcc

U

125

94

28

316

18

1,153

W

152

73

34

400

23

1,117

IC-015

Xcc

U

139

144

36

339

19

1,227

W

173

90

24

388

21

1,430

IC-019

Xcr

U

127

81

45

678

20

1,346

W

206

60

48

1,270

36

1,498

The presence of silicic volcanic rocks, ash-flow tuff, 

and rhyolite of the Caicara Formation in the Icabaru Sur 

study area suggests a possible epithermal source for the gold 

(Contreras and Page, 1988). Quartz veins (2-3 mm) cut the 

ash-flow tuff, but alteration minerals such as alunite, sericite, 

montmorillonite, adularia, silica, tourmaline, and carbonate 

are not present in the veinlets or host rocks. Gold was not 

present in stream-sediment samples taken downstream from 

the veinlets. One sample of ash-flow tuff (IC-021, fig. 3) 

had a millimeter-thick veinlet of fluorite.

Ten rock samples and one quartz-vein sample were submitted for spectrographic and trace element (gold) analysis 

(table 1). Results for gold are uniformly low (~1 ppb), and 

neither arsenic nor silver were detected. One sample of 

porphyritic rhyolite (IC-005) contains 50 ppm Mo. Anomalous concentrations of metals are not present in the sample 

that contains the fluorite veinlet.

ASSESSMENT OF MINERAL 

POTENTIAL

The geologic setting of the Icabaru Sur study area permits consideration of the following deposit models: epithermal veins and diamond-bearing kimberlite pipes. The only 

feature common to both the Icabaru Sur study area and a generalized (Sado, Creede, Comstock) epithermal quartz-gold 

model is the presence of silicic volcanic rocks. Hypabyssal 

intrusive rocks are not present in the area, unless further 

mapping proves that the porphyritic rhyolite is intrusive. 

Similarly, neither faults and fractures related to doming nor 

ring-fracture zones associated with mineralized calderas 

have been identified (Mosier, Berger, and Singer, 1986; 

Mosier, Sato, and others, 1986; Mosier, Singer, and Berger, 

1986). Therefore, the Icabaru Sur study area is assigned a 

low potential for an epithermal gold system; however, additional regional mapping, with attention to altered rock, intrusive rocks, quartz, and fluorite veins, would better define this 

assessment.

The presence of placer diamond production in the area 

of Icabaru is the strongest evidence for consideration of the 

descriptive model for diamond-bearing kimberlite pipes 

(Cox, 1986). Kimberlite is commonly highly fractured and 

deeply weathered, which typically produces topographic 

lows and which would be inconspicuous in such an environment (W.H. Raymond, U.S. Geological Survey, written 

commun., 1991). Therefore, the apparent absence of kimberlite pipes, perhaps unrecognized due to their small diameter 

and inconspicuous weathering style, might be due to the 

relatively small degree of coverage of the area by geologic 

mapping. Nonetheless, kimberlitic indicator minerals have 

not been identified from any of the alluvial diamond-bearing 

deposits associated with the Roraima Group (Briceno, 1984; 

Meyer and McCallum, 1993). Potential for diamond-bearing 

kimberlitic pipe deposits is, therefore, low.

CONCLUSIONS

In the Icabaru Sur study area of Venezuela, sandstones 

of the Early to Middle Proterozoic Roraima Group and 

ash-flow tuffs and rhyolite of the Early Proterozoic Caicara 

Formation of the Cuchivero Group are exposed. The absence 

of visible gold in stream-sediment samples and the minor 

amount of gold (~1 ppb) in analyses of rock samples eliminate the volcanic terrane as a possible source for the gold 

commonly found in placer deposits in the region. Therefore, 

conglomerates of the Uairen Formation of the Roraima 

Group remain the immediate source for gold and diamonds 

in the area. The geologic setting of the Icabaru region is 

generally favorable for mineral deposits, but the resource 

potential of the Icabaru Sur study area is apparently low.

REFERENCES CITED

Briceno M., H.O., 1984, Genesis de yacimientos minerales 

Venezolanos II Placeres diamantiferos de San Salvador de 

Paul: Acta Cientifica Venezolana, v. 36, p. 154-158.
GOLD AND DIAMOND RESOURCES, ICABARU SUR AREA L9

Contreras, Gloria, and Page, N.J, 1988, Informe sobre las actividades de campo realizadas en la zona de Paraytepuy-Icabaru-Los Caribes: Corporation Venezolana de

Guayana, Tecnica Minera, C.A., internal report, 4 p. 

Cox, D.P., 1986, Descriptive model of diamond pipes (model 12),

in Cox, D.P., and Singer, D.A., eds., Mineral deposit models:

U.S. Geological Survey Bulletin 1693, p. 54. 

Deer, W.A., Howie, R.A., and Zussman, J., 1982, Rock forming

minerals Orthosilicates (2nd ed.): London, Longman, v. 1 A,

919 p. 

Del Olio, D., Tapia, Jhonny, Sifontes, Ramon, 1989, Estudio

geologico-geoqufmico en la region Hoyada-Icabani, Gran

Sabana, Estado Bolivar: Congreso Geologico Venezolano, 7th,

v. 4, p. 1788-1812. 

Dennen, W.H., and Anderson, P.J., 1962, Chemical changes in

incipient rock weathering: Geological Society of America

Bulletin, v. 73, p. 375-384. 

Erlank, A.J., Smith, H.S., Marchant, J.W., Cardoso, M.P., and

Ahrens, L.H., 1978, Zirconium (section 40-G), in Wedepohl,

K.H., ed., Handbook of geochemistry: Berlin, Springer-Verlag,p. 40-A-1-40-O-16.

Ghosh, S.K., 1985, Geology of the Roraima Group and its implications: Simposium Amazonico, 1st, Caracas, 1981, Publication

Especial 10, p. 31-50. 

Gibbs, A.K., and Barron, C.N. 1983, The Guiana Shield reviewed:

Episodes, v. 1983, no. 2, p. 7-14. 

 1993, The geology of the Guiana Shield: New York, Oxford

University Press, Oxford Monographs on Geology and

Geophysics 22, 246 p. 

Keats, W., 1974, The Roraima Formation in Guyana: Congreso

Latinoamericano de Geologia, 2nd, Caracas, 1973, Memoria,

Publication Especial 7, v. 2, p. 901-940. 

Krogh, T.E., 1973, A low contamination method for hydrothermal

dissolution of zircons and extraction of U and Pb for isotopic

age determinations: Geochimica et Cosmochimica Acta, v. 37,

p. 485^94. 

Mattinson, J.M., 1987, U-Pb ages of zircons A basic examination

of error propagation: Chemical Geology (Isotope Geoscience

Section), v. 66, p. 151-162.

Meyer, H.O.A., and McCallum, M.E., 1993, Diamonds and their 

sources in the Venezuelan portion of the Guyana Shield: 

Economic Geology, v. 88, p. 989-998.

Montero, R.L., Yanez, Galo, and Lo Monaco, S., 1989, Estudio de 

las variaciones quimicas y mineralogicas en la secuencia roca 

fresca-halo costra de meteorizacion en rocas felsicas y maficas 

en la cuenca del Rio Paragua: Congreso Geologico Venezolano, 7th, v. 2, p. 892-911.

Mosier, D.L., Berger, B.R., and Singer, D.A., 1986, Descriptive 

model of Sado epithermal veins (model 25d), in Cox, D.P., and 

Singer, D.A., eds., Mineral deposit models: U.S. Geological 

Survey Bulletin 1693, p. 154.

Mosier, D.L., Sato, T., Page, N.J, Singer, D.A., and Berger, B.R., 

1986, Descriptive model of Creede epithermal veins (model 

25b), in Cox, D.P., and Singer, D.A., eds., Mineral deposit 

models: U.S. Geological Bulletin 1693, p. 145.

Mosier, D.L., Singer, D.A., and Berger, B.R., 1986, Descriptive 

model of Comstock epithermal veins (model 25c), in Cox, 

D.P., and Singer, D.A., eds., Mineral deposit models: U.S. 

Geological Survey Bulletin 1693, p. 150.

Nixon, P.H., 1988, Diamond source rocks from Venezuela: Industrial Diamond Quarterly, no. 51, p. 23-29.

Priem, H.N.A., Boelrijk, N.A.I.M., Hebeda, E.H., Verdurmen, 

E.A.Th., and Verschure, R.H., 1973, Age of the Precambrian 

Roraima Formation in northeastern South America Evidence 

from isotopic dating of Roraima pyroclastic volcanic rocks in 

Suriname: Geological Society of America Bulletin, v. 84, 

p 1677-1684.

Reid, A.R., 1974, Proposed origin for Guianian diamonds: Geology, v. 2, no. 2, p. 67-68.

Reid, A.R., and Bisque, R.E., 1975, Stratigraphy of the diamond-bearing Roraima Group, Estado Bolivar, Venezuela: 

Quarterly of the Colorado School of Mines, v. 79, no. 1, 

p 61-82.

Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial 

lead isotope evolution by a two-stage model: Earth and 

Planetary Science Letters, v. 26, p. 207-221.

Bauxita en Superficie de Planacion de la 

Guayana Venezolana 

(Bauxite on a Planation Surface in 

Venezuelan Guayana)

By Galo Yanez P.

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-M

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENIDOS

Resumen......................................................................................................................... Ml

Abstract........................................................................................................................... 1

Introduccion.................................................................................................................... 1

Metodologia.................................................................................................................... 3

La Superficie de Planacion de Los Pijiguaos................................................................. 6

Nuevas Areas Bauxiticas en la Superficie de Planacion de Los Pijiguaos..................... 7

Recomendaciones........................................................................................................... 8

Referencias..................................................................................................................... 8

FIGURAS

1-5. Mapas de:

1. Ubicacion del aiea de Los Pijiguaos.......................................................................................................................... M2

2. Areas de lateritas alummicas y bauxita al este de Puerto Ayacucho.......................................................................... 3

3. Areas de lateritas aluminicas y bauxita al sur y este de Los Pijiguaos...................................................................... 4

4. Area investigada en el helipuerto H-17..................................................................................................................... 5

5. Area investigada en el helipuerto H-20..................................................................................................................... 6

TABLAS

1. Resultados de analisis quimicos del area investigada en el helipuerto H-20............................................................ M6

2. Resultados de analisis quimicos del area investigada en el helipuerto H-17............................................................ 7
Bauxita en Superficie de Planacion de la

Guayana Venezolana

(Bauxite on a Planation Surface in

Venezuelan Guayana)

tfyGaloYanezP. 1

RESUMEN

La investigation geomorfologica es parte de los trabajos que la Corporacion Venezolana de Guayana, Tecnica 

Minera, C.A., realiza para el inventario de los recursos naturales de la region Guayana en Venezuela. Para el caso del 

norte del Estado Amazonas y en el drea del presente trabajo 

(entre lat 5°-7' N. y long 66°00'-67°30' O.) se ha determinado que la evolution del relieve ha estado mayormente controlada por un proceso de formation o destruction de 

superficies de planacion; debido en parte a la naturaleza predominantemente granitica del area y a la relativa estabilidad 

del escudo igneo-metamorfico.

Se ha estudiado, con mayor detalle, el area de Los 

Pijiguaos, la cual ha desarrollado y preservado lateritas 

aluminicas y bauxita. Con este modelo geomorfologico, se 

han delimitado nuevas areas de evolution geomorfo!6gica 

semejante, de las cuales las dos mas importantes fueron verificadas mediante la construction de helipuertos, encontrandose abundante costra bauxitica de origen secundario, las 

cuales podrian triplicar el potencial ya conocido de Los 

Pijiguaos. Tales areas se encuentran a 50 km hacia S. 40° E. 

de Los Pijiguaos y a 40 km hacia S. 75° E. de Puerto Ayacucho. Se abrieron calicatas cercanas a los helipuertos, de las 

que se tom6 muestras para analisis quimicos y cuyos resultados se anexan. Tales resultados indican la presencia de un 

potencial bauxitico muy importante que debe ser investigado 

con mayor detalle.

Corporacion Venezolana de Guayana, Tecnica Minera, C.A., Ciudad 

Bolivar, Venezuela.

ABSTRACT

Geomorphological research is part of the inventory of 

natural resources that the Corporacion Venezolana de 

Guayana, Tecnica Minera, C.A., is conducting in the Guayana Shield of Venezuela. In the northern part of Estado 

Amazonas, an area of relative tectonic stability between lat 

5° and 7° N. and long 66°00' and 67°30' W., the evolution of 

geomorphological relief has been controlled chiefly by 

constructional and destructional processes associated with 

the development of planation surfaces on a substratum made 

up mostly of granitic rocks.

The Los Pijiguaos area was studied in more detail 

because of the economic value of its aluminous lateritic cap. 

Application of the geomorphological model for this area to 

other areas in which a similar geomorphological evolution 

was identified allowed identification of new areas containing 

bauxitic caps of secondary origin. These new areas were 

later verified by helicopter-supported fieldwork. One area is 

about 50 km S. 40° E. of Los Pijiguaos, and the other is 40 

km S. 75° E. of Los Pijiguaos. Further investigation of 

mineralized rock in these areas may yield substantial 

reserves of aluminum, exceeding those of Los Pijiguaos.

INTRODUCTION

El presente trabajo se ha desarrollado en el marco de las 

investigaciones geomorfo!6gicas que se hacen para el inventario de los recursos naturales de la region Guayana (Estado 

Bolivar, Estado Amazonas, y Estado Delta Amacuro) a 

escala 1:250,000. El a"rea estudiada corresponde a las hojas 

de radar NB-19-8 y NB-19-12 a escala 1:250,000, situadas 

entre lat 5°00'-7°00' N. y long 66°00'-67°30' O. El 

mencionado inventario, desarrollado por la Corporacion

Ml
M2 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

7° 

6° -

Areas bauxiticas de origen 

semejante a Los Pijiguaos

Figure 1. Mapa de ubicacion del area de Los Pijiguaos.
BAUXITE ON A PLANATION SURFACE

67°30"

5°50' -

5°30' -

Borde escarpado de superficie

de planacion 

Area baja y plana, mayormente

aluvial 

Elevation (metres)

Figura 2. Areas de lateritas aluminicas y bauxita al este de Puerto Ayacucho (zona A).

Venezolana de Guayana, Tecnica Minera, C.A. 

(CVG-TECMIN o TECMIN), comenzo en 1985 en la 

frontera con Guyana y en la actualidad se esta investigando 

el Estado Amazonas.

Entre los trabajos previos relatives al area estan Rios 

(1969) y McCandless (1965) sobre la estratigrafia y 

petrologia; Mendoza et al (1977) hace un estudio petrologico 

de la parte norte del Estado Amazonas; Menendez et al 

(1985) y Menendez y Sarmentero (1985) describen el marco 

geologico de la bauxita de Los Pijiguaos.

En lo que respecta a la naturaleza, origen y evolucion de 

superficies de planacion, King (1967) en su estudio "Morphology of the Earth" describio los fundamentos en que se 

basa el concepto de superficies de planacion y paralelamente 

lanzo la hipotesis de la evolucion del relieve por pedimentaci6n, como un fenomeno generalizado e independiente del 

factor climatico. King establecio varias superficies de 

planacion en Africa, denominadas, de mas antigua a mas 

joven, Gondwana (Jurasico), Post-Gondowana (Cretaceo 

antiguo a medio), Africana (Oligoceno), entre las mas

importantes; las mismas tienen sus equivalentes en Brasil, 

denominadas Gondwana, Post-Gondwana y Sur-Americana, 

respectivamente.

Por otra parte, los factores que pueden influir en el origen y evolucion de las superficies de planacion son analizados por Melhorn y Dorland (1975). Los factores incluyen el 

factor tectonico, el factor eustatico y el factor climatico; asi 

como tambien el significado de los registros sedimentarios 

resultantes de un sostenido proceso de denudation continental. En el Escudo Guayanes, varios autores han reconocido la 

existencia de superficies de planacion, tales como McConnell (1969) en Guyana, Zonneveld (1969) en Surinam, Short 

y Steenken (1962) en Venezuela.

METODOLOGIA

En julio de 1988 se realiz6 una interpretaci6n geo!6gica 

y geomorfol6gica de las hojas de radar NB-19-8 y 

NB-19-12 a escala 1:250,000; luego, se hizo una interpret-
M4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

66°40' 66°30'

6°30

6°10'

SERRANIA LA CERBATANA

LEYENDA 

11111111 ' Borde escarpado de superficie de planacieon 0 Area baja mayormente aluvional

 Divisoria Elevacion (metres)

Figura 3. Areas de lateritas aluminicas y bauxita al sur y este de Los Pijiguaos (zona B).
BAUXITE ON A PLANATION SURFACE

67°07'11"

M5

5°32'34" -

Calicata

Helipuerto H.17 y calicata

Quebrada

Figura 4. Area investigada en el helipuerto H-17.

acion geologica y geomorfologica de fotografias aereas a 

escala 1:50,000 del proyecto 172 y fotografias aereas a 

escala 1:25,000 de la mision 050294 y la imagen de satelite 

CPDI003-055 a escala 1:250,000. El area total interpreta da 

fue de 36,000 km2.

La information obtenida fue disenada en transparencias 

sobre las hojas de radar. Paralelamente y mediante el empleo 

de mapas altimetricos del Catastro Minero Nacional, a escala 

h25,000 con lineas de nivel cada 20 m se establecieron las 

relaciones altimetricas de los distintos aspectos geologicos y 

geomorfologicos.

De esta manera, se determin6 que en las extensas areas 

de composition granitica que existen en el Estado Amazonas 

se ha desarrollado un modelo de evolucion del relieve caracterizado por la formaci6n y destruction de superficies de 

planaci6n, de las cuales, las mas conspicuas por su mejor 

preservation se encuentran a los niveles 600-750 m,

1,100-1,250 m y 1,500-1,600 m. Las ma's elevadas se situan 

escalonadamente hacia el sur. La influencia o el significado 

que estas superficies de planacion pudieran tener en la naturaleza de los recursos naturales esta en proceso de investigaci6n.

Sin embargo y tomando en consideration que la superficie de planacion 600-750 m es un yacimiento de lateritas 

aluminicas y bauxita gibbsitica, en Los Pijiguaos, se realiz6 

un estudio muy detallado, cualitativo y cuantitativo, en sus 

mas variados parametros. Luego, se procure identificar areas 

semejantes dentro de los 36,000 km2, consiguiendose como 

resultado las areas que se muestran en el mapa de la figura 1.

En marzo de 1989 y como parte del trabajo de campo, 

propio de las labores del inventario de los recursos naturales, 

TECMIN decidio la apertura de 20 helipuertos para obtener 

informaci6n de las areas que por su inaccesibilidad no tenian 

information previa. De los 20 helipuertos, dos estaban local-
M6 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

LEYENDA

Pica 

Quebrada

Campamento (helipuerto) 

H-20

Calicata

.3108101

( 5 ) Numero de la 

calicata

3308031 Numero de la 

muestra

3108051

3108011 

3102-01 

3102-02 

3102-03

400 METROS

Figure 5. Area investigada en el helipuerto H-20.

izados en areas interpretadas como lateritas aluminicas: 

H-17 con coordenadas lat 5°32.2' N., long 67°07.5' O. (fig. 

2), y H-20 con coordenadas lat 6°08.0' N., long 66°28.0' O. 

(fig. 3). Las coordenadas fueron tomadas instrumentalmente, 

del helicoptero, y no coinciden exactamente con las 

coordenadas aproximadas de las ima'genes de radar, que 

presentan los mapas de las figuras 2 y 3. En ambos 

helipuertos, bloques de coraza bauxitica en medio de la 

laterita se encontraron.

En julio de 1989 se procedio a la apertura de calicatas 

que permitirian obtener muestras para su andlisis quimico. El 

trabajo de campo en el drea del helipuerto H-20 fue realizado por el Geologo Andres E. Garcia Gerdes y en el helipuerto H-17 por el Geologo Frank Tovar. Se abrieron dos 

calicatas en H-17 (fig. 4) y cinco calicatas en H-20 (fig. 5) 

a diferentes profundidades, dependiendo de la dureza de la 

costra. En ambos casos, se tomaron muestras de bloques 

duros de bauxita y muestras de canal en laterita no 

consolidadas.

Los an£lisis quimicos para determinar el porcentaje de 

los principales oxidos se realizaron tanto en el laboratorio de 

BAUXIVEN en Los Pijiguaos, como en el Laboratorio de 

Ferrominera del Orinoco en Puerto Ordaz. Los resultados de 

tales andlises quimicos se exponen en las tablas 1 y 2.

LA SUPERFICIE DE PLANACION DE 

LOS PUIGUAOS

El aiea de Los Pijiguaos de 5,864 has. (fig. 2) el cual es 

en la actualidad un yacimiento de bauxita gibbsitica y lateritas aluminicas en explotacion, se ha originado de una 

superficie de planacion en el granito de Parguaza con textura 

rapakivi (Mendoza, 1974). Esta superficie esta considerada 

como parte de la denominada superficie Nuria (Short y 

Steenken, 1962) de Venezuela, llamada tambien superficie 

Imataca (Martin et al, 1975). A la superficie Nuria, se le 

atribuye una edad Cret£cica Superior a Terciario Inferior por

Tabla 1. Resultados de andlisis quimicos del area investigada en el helipuerto H-20.

No.de 

muestra

3108011

3108081

3108031

3108101

310201

310202

310203

310301

310302

310303

Tipo de 

muestra

Bloque

Bloque

Bloque

Bloque

Canal

Canal

Canal

Canal

Canal

Canal

Profundidad

a3m

a2m

aOm

al m

0-1 m

1-2 m

2-3 m

0-1 m

1-2 m

2-3 m

A1203 

(percentaje)

49.61

54.76

50.48

55.89

43.47

43.85

43.21

38.05

42.34

45.36

Si02 

(percentaje)

5.75

4.65

10.26

2.39

9.27

9.63

8.03

12.37

9.93

13.40

Fe2O3 

(percentaje)

15.16

14.87

8.29

11.58

16.13

16.42

16.55

17.18

16.90

10.85

Laboratorio1

F.M.O.

F.M.O.

F.M.O.

F.M.O.

F.M.O.

F.M.O.

F.M.O.

F.M.O.

F.M.O.

F.M.O.

Ubicaci6n

Calicata 2

Calicata 3

Calicata 4

Calicata 5

Calicata 2

Calicata 2

Calicata 2

Calicata 3

Calicata 3

Calicata 3

'Ferrominera del Orinoco, Puerto Ordaz.
BAUXITE ON A PLANATION SURFACE M7

Tabla 2. Resultados de analisis qufmicos del area investigada en el helipuerto H-17.

Tipo de 

muestra

Bloque

Bloque

Bloque

Canal

Canal

Canal

Canal

Bloque

Bloque

Profundidad

aO m

a2m

aOm

0-1 m

1-2 m

2-3 m

3-4 m

aO m

a2m

A1203 

(percentaje)

59.39

50.57

60.55

42.21

46.62

46.36

43.22

59.29

55.94

SiO2 

(percentaje)

3.37

14.90

3.37

13.80

10.07

8.60

10.20

2.29

5.11

Fe203 

(percentaje)

4.79

6.88

3.35

14.11

14.30

13.58

16.84

2.63

8.15

Laboratorio1

BAUXIVEN

BAUXIVEN

BAUXIVEN

F.M.O.

F.M.O.

F.M.O.

F.M.O.

F.M.O.

F.M.O.

Ubicacion

H-17

H-17

H-17

Calicata 1

Calicata 1

Calicata 1

Calicata 1

H-17

H-17

'Bauxita de Venezuela, Los Pijiguaos; Ferrominera del Orinoco, Puerto Ordaz.

su semejanza con la superficie Kopinang de Guyana (McConnell, 1969). Tomando en cuenta el criterio de la altitud, se 

engloba con este nombre a todos los restos erosivos situados 

entre 600-700 m sin importar lo disimil que sea su naturaleza o el proceso evolutive de su formation. En el presente 

trabajo se ha tratado de interpretar el modelo geomorfologico del area de Los Pijiguaos y delimitar otras areas de 

evolution y naturaleza semejante, por esta razon se le 

denomina superficie de planacion de Los Pijiguaos. Esta 

superficie contiene mena bauxitica de interes economico por 

meteorizacion del granito de Parguaza. La mena se formo 

mediante un proceso de pedimentaci6n con una elevation 

aproximada de 600 a 700 m y un desnivel abrupto de aproximadamente 500 m.

El 85 por ciento de las bauxitas que ocurren en el 

mundo se encuentran sobre este tipo de superficies de 

planaci6n que se desarrollan "in situ" sobre rocas magmaticas mediante procesos de meteorizaci6n de la roca original. 

El desarrollo de una superficie de planacion implica un prolongado periodo de denudation continental en condiciones 

sub-aereas y si las condiciones de clima lo permiten, pueden 

desarrollarse procesos de lateritizacion como paso previo al 

desarrollo de lateritas aluminicas y bauxita. El enriquecimiento en alumina se produce por emigration del hierro en 

la forma de hidr6xidos solubles y por lixiviacion de la silice. 

Se trata, pues, de un enriquecimiento "in situ" con mayores 

concentraciones de altimina en los niveles superiores, tal 

como acontece en el caso de Los Pijiguaos.

Evidentemente, las condiciones estructurales de la roca 

parental son una condici6n inicial indispensable para que se 

desarrolle la meteorizacion profunda. En el caso de Los 

Pijiguaos, se lleva a efecto mediante diaclasas verticales y 

horizontales.

El desarrollo de un espeso manto de meteorizacion va a 

depender de las condiciones del clima, siendo favorables las 

condiciones de clima tropical-humedo para una meteorizacion mas profunda y mas rapida de la roca original. En este 

aspecto, las condiciones topograficas juegan un papel decisive para que en la relation entre la meteorizacion y la

erosion prevalezca el ritmo de la meteorizacion dando como 

resultado el incremento de espesor de la zona meteorizada. 

El analisis de estas condiciones en el area de Los Pijiguaos 

exige una reconstruction paleo-geografica que apenas 

comienza a visualizarce.

El enriquecimiento en alumina y la bauxitizaci6n 

requieren de excelentes condiciones de drenaje interno y 

altos niveles de precipitation pluvial. Aqui juega un papel 

estelar el levantamiento que ha experimentado la superficie 

de planacion de Los Pijiguaos y el proceso que ha establecido un desnivel de 500 m entre la superficie de planacion y 

los valles pianos cercanos donde discurren los mayores rios. 

El drenaje interno se facilita a travel de las diaclasas verticales de la roca infrayacente. De hecho, las redes de drenaje 

sobre la superficie de planaci6n de Los Pijiguaos es un 

paleo-drenaje controlado por el sistema de diaclasas, el 

mismo que ha sido capturado en muchos sitios por el sistema 

de drenaje que acompana el proceso de pedimentacion inferior. Los procesos de pedimentaci6n son ampliamente discutidos por Twidale (1983).

En el caso de la superficie de planaci6n de Los 

Pijiguaos, el levantamiento ha producido una profunda 

incision de los cursos de agua a traves de la capa de meteorizaci6n, lo que ha originado interfluvios pianos y pendientes 

convexas y abruptas en los flancos de los cursos de agua. 

Cuando la incision llega a la roca fresca aparece una superficie lateral lisa en el granito, como paso previo a la 

forrnaci6n de domos graniticos.

NUEVAS AREAS BAUXITICAS EN LA

SUPERFICIE DE PLANACION DE LOS

PIJIGUAOS

Con las condiciones geomorfo!6gicas anteriores se nan 

delimitado nuevas areas semejantes al area de Los Pijiguaos. 

Dichas areas son remanentes erosionales de una superficie 

mayor la cual se ha denominada superficie de planaci6n de 

Los Pijiguaos, debiendo aclararse que unicamente se ban
M8 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYAN A SHIELD

delimitado las areas de mayor extension y continuidad lateral 

que pudieran conformar un yacimiento bauxitico de interns 

economico (figs. 2 y 3). Desde el punto de vista geologico, 

todas las areas que se ban indicado se encuentran sobre el 

granito de Parguaza de textura rapakivi, con exception del 

area sobre la serrama de La Cerbatana (fig. 2) que se ha 

desarrollado en el granito biotitico de Santa Rosalia 

(McCandless, 1965).

En el aspecto relativo a la altitud, todas tienen una altitud semejante, aun cuando una misma altitud de las formas 

del relieve no significa necesariamente contemporaneidad. 

Segun el concepto de "equilibrio dinamico" (Hack, 1960), 

superficies de planacion de igual altitud pueden tener edades 

distintas e, inversamente, superficies de planaci6n de diferente altitud pueden desarrollarse simultaneamente.

En las areas verificadas (H-17 y H-20), la forma de los 

interfluvios indica la presencia de una espesa zona de meteorizacion que ha sido preservada. El estado de desarrollo del 

relieve y, especialmente, su accesibilidad y condiciones 

topograficas semejantes a Los Pijiguaos, posibilitaria un 

sistema de explotacion como el que se ha implementado en 

Los Pijiguaos. En el caso de Los Pijiguaos, la mena bauxitica 

superficial tiene un espesor de 7 m y debido a la incision de 

las redes de drenaje el area de 5.804 has (fig. 2) se reduce a 

1.600 has, en el tope de los interfluvios, lo que totaliza un 

potencial certificado de 200x106 toneladas metricas 

de bauxita.

Las dos areas investigadas totalizan 23,505 has., en el 

caso de que estas areas presenten una mineralizaci6n semejante a Los Pijiguaos, significaiian SOOxlO6 toneladas 

metricas de bauxita.

RECOMENDACIONES

Tomando en consideration que el origen y evoluci6n de 

superficies de planacion es un problema esencialmente geomorfologico (Aleva, 1981), el cual tiene influencia importante en los recursos naturales en general y en el origen de 

recursos minerales lateriticos, en particular, y que es un 

problema complejo que debe ser enfocado inter-disciplinariamente, tanto desde el campo geologico como desde el 

campo geograTico, se recomienda formar un equipo de 

investigadores con el personal de los distintos entes publicos 

y privados que de una u otra forma estudian las superficies 

de planacion en Venezuela. Es preciso continuar las exploraciones de estas y otras superficies de planacion, asi como 

de los depositos sedimentarios asociados, a fin de definir el 

modelo de evolution geomorfologica de la region Guayana 

y su significado en el origen de los recursos naturales del 

area.

REFERENCIAS

Aleva, G.J.J., 1981, Bauxitic and other duricrusts on the Guiana 

Shield, South America: A.A. Balkema-Rotterdam,

Proceedings of the International Seminar on Laterization 

Processes, p. 261-269.

Hack, J.T., 1960, Interpretation of erosional topography in humid 

temperate regions: American Journal of Science, v. 258-A, 

p. 80-97.

King, L.C., 1967, Morphology of the Earth (2nd ed.): New York, 

Hafner Publishing Company, 726 p.

Martin Bellizzia, C, coordinator, and Ascanio, G., Candiales, L.J., 

Rios, J.H. and Luchsinger, S.E., guides, 1975, Excursidn 

geo!6gica No. 6, Puerto Ordaz-La Vergarena: Congreso 

Latinoamericano de Geologia, 2nd, Caracas, 1973, Memoria, 

Publicacion Especial 7, v. 1, p. 371-388.

McCandless, G.C., 1965, Reconocimiento geoldgico de la regi6n 

Nor-Occidental del Estado Bolivar: Boletin de Geologfa, v. 7, 

no. 13, p. 19-28.

McConnell, R.B., 1969, The succession of erosion bevels in 

Guyana: Interguiana Geological Conference, 8th, Georgetown 

1969, Geological Survey of Guyana, Records, v. 6, Paper 8, 

Proceedings, p. 1-16.

Melhorn, N.M., and Dorland, E.E., 1975, The case for episodic 

continental-scale erosion surfaces A tentative geodynamic 

model, in Theories of landform development: Binghamton, 

State University of New York, chapter 13, p. 243-276.

Mendoza, Vicente, 1974, Geologfa del area del Rio Suapure, parte 

noroccidental del Escudo de Guayana, Estado Bolivar, 

Venezuela; informe de progreso: Conferencia Geologica 

Inter-Guayanas, 9th, Venezuela, Memoria, Publicacion 

Especial 6, p. 306-338.

Mendoza, Vicente, Moreno, L.A., Barrios, Fernando, Rivas, 

Duggar, Martinez, Jesus, Lira, Pedro, Sardi, Gustavo, and 

Ghosh, S.K., 1977, Geologia de la parte norte del Territorio 

Federal Amazonas, Venezuela (informe en progreso): Congreso Geol6gico Venezolano, 5th, Caracas, 1977, Memoria, 

v. 1, p. 363^04.

Menendez V. de V., Alfredo, Rios, J.H., Weingarten, B., and Ticona, I., 1985, Caracteristicas geo!6gicas de la parte noreste del 

yacimiento de bauxita de "Los Pijiguaos," Estado Bolivar, 

Venezuela: Simposium Amazonico, 1st, Caracas, 1981, 

Publicacion Especial 10, p. 548-570.

Menendez V. de V., Alfredo, and Sarmentero, Alberto, 1985, 

Exploration de bauxita en la Guayana Venezolana con particular referencia a la Serrama de Los Pijiguaos: Simposium 

Amazonico, 1st, Caracas, 1981, Publicacidn Especial 10, 

p. 571-586.

Rios, J.H., 1972, Geologia de la regidn de Caicara, Estado Bolivar: 

Congreso Geologico Venezolano, 4th, Caracas, 1971, Memoria, Publicacion Especial 5, v. 3, p. 1759-1782.

Short, K.C., and Steenken, W.F., 1962, A reconnaissance of the 

Guayana Shield from Guasipati to the Rio Aro, Venezuela: 

Asociacidn Venezolana de Geologia, Mineria y Petroleo, 

Boletin Informative, v. 5, no. 7, p. 189-221.

Twidale, C.R., 1983, Pediments, peneplaines and ultiplaines: 

Revue de Geomorphologie Dinamique, v. 32, no. 1, p. 1-35.

Zonneveld, J.I.S., 1969, Preliminary remarks on summit levels and 

the evolution of the relief in Surinam (S. America): 

Proceedings, Guiana Geological Conference (Paramaribo, 

Suriname), 7th, 1966; Verhandelingen van het Koninklijk 

Nederlands Geologisch Mijnbouwkundig Genootschap, v. 27, 

p. 53-60.
Geochemistry of Six Gold-Bearing 

Quartz-Carbonate-Tourmaline Vein Systems, 

Estado Bolivar, Venezuela

By Sherman P. Marsh, Herbert A. Pierce, Henry Sanchez, and 

Norman J Page

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-N

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995

CONTENTS

Abstract........................................................................................................................... Nl

Resumen.......................................................................................................................... 1

Introduction..................................................................................................................... 2

Methods.......................................................................................................................... 2

Sample Collection................................................................................................... 2

Sample Preparation and Analysis........................................................................... 2

Data Analysis.......................................................................................................... 5

General Geologic Setting of Quartz-Carbonate-Tourmaline Veins ............................... 5

El Callao District............................................................................................................ 7

Mina Colombia....................................................................................................... 7

Lolncreible District................................................................................................ 7

Bochinche Area............................................................................................................... 9

Trench Tl................................................................................................................ 9

Trench 2A............................................................................................................... 11

Statistical Analysis.................................................................................................. 11

VLF-EM Survey..................................................................................................... 11

Bochinche Road Cut............................................................................................... 16

Marwani Area................................................................................................................. 16

Geology................................................................................................................... 21

Geochemical Sampling and Analysis..................................................................... 26

Mineralogy.............................................................................................................. 31

Geochemical and Genetic Model for Gold-Bearing Quartz-Carbonate-Tourmaline

Veins in Venezuela..................................................................................................... 33

Conclusion...................................................................................................................... 33

References Cited............................................................................................................. 34

Appendix Data Tables ................................................................................................ 37

FIGURES

1. Map showing location of El Callao, Bochinche, and Marwani areas.................................................................... N3

2. Geologic map of Lo Increible district showing locations of Mina-A, Mina Lapa, Mina Esperiencia, and

Mina San Pedro...................................................................................................................................................... 8

3. Photograph of Mina-A open-pit mine, showing large gold-bearing quartz-carbonate-tourmaline

vein boulder........................................................................................................................................................... 10

4. Photomicrograph of native gold from quartz-carbonate-tourmaline vein at Mina-A,.......................................... 11

5. Plot of gold values for rock samples collected from trench in open-pit mine at Mina San Pedro ....................... 12

6. Map showing eastern Distrito Roscio and northwest part of Zone en Reclamacion............................................. 13

7-9. Photographs of Bochinche areas showing:

7. Main road between Tumeremo and Bochinche............................................................................................ 14

8. Trench Tl.................................................................._ 14

9. Main quartz-carbonate-tourmaline vein in trench Tl................................................................................... 16

10. Plot of gold values for rock samples collected across quartz veins exposed in trench Tl, Bochinche area......... 17

11, 12. Photographs of Bochinche area showing:

11. Trench 2A...................................................................._ 18

12. Quartz-carbonate-tourmaline vein in floor of trench 2A.............................................................................. 20

13. Plot of gold values for sample collected across vein system at trench 2A, Bochinche......................................... 23
CONTENTS

14, 15. Profiles measured by VLF-EM receiver along trench 2A, Bochinche area, showing:

14. Resistivity.............................................^ 24

15. Phase angle.............................................._ 25

16. Photograph showing north side of road cut near Bochinchita.............................................................................. 25

17. Plot of gold values across quartz veins exposed in road cut near Bochinchita..................................................... 27

18, 19. Maps of zone 4, Marwani concession, showing:

18. Schematic geology....................................................................................................................................... 29

19. Rock sample locations ................................................................................................................................. 30

20. Photograph showing geology field camp near Esperanza along the main road

between Tumeremo and Bochinche...................................................................................................................... 30

21. Map showing panned-concentrate sample locations, zone 4, Marwani concession ............................................. 32

TABLES

1, 2. Limits of determination for semiquantitative emission spectrographic analysis of:

	1. Rock samples ............................................................................................................................................... N4

	2. Panned concentrates from stream sediments................................................................................................ 4

3. Analytical methods used and limits of determination for inductively coupled plasma emission

	spectroscopy and flame atomic absorption spectrophotometry methods.............................................................. 5

4. Univariate statistics for selvage and vein-rock samples, Mina Colombia............................................................ 6

5. Cohen geometric means for host-rock samples and vien-rock samples, Mina Colombia.................................... 6

6. Univariate statistics for vein, host, and silicified rock samples, Mina San Pedro................................................. 10

7. Comparison of Cohen geometric means of elemental values for vein and host-rock

	samples, Mina San Pedro.................................................................................................................................... 11

8. Univariate statistics for quartz-vein rock samples from trench Tl, Bochinche.................................................... 13

9. Correlation coefficients for quartz-vein samples from trench Tl, Bochinche...................................................... 15

10. Univariate statistics for host-rock samples from trench Tl, Bochinche............................................................... 15

11. Correlation coefficients for host-rock samples from trench Tl, Bochinche......................................................... 15

12. Univariate statistics for quartz-vein rock samples from trench 2A, Bochinche.................................................... 18

13. Univariate statistics for host-rock samples from trench 2A, Bochinche............................................................... 19

14. Univariate statistics for silicified host-rock samples from trench 2A, Bochinche................................................ 19

15. Correlation coefficients for quartz-vein rock samples from trench 2A, Bochinche.............................................. 20

16. Correlation coefficients for silicified host-rock samples from trench 2A, Bochinche.......................................... 21

17. Correlation coefficients for host-rock samples from trench 2A, Bochinche......................................................... 22

18. Element associations for the six-factor, R-mode factor analysis for rock samples, trench 2A, Bochinche.......... 26

19. Eigenvalues for the six-factor R-mode factor analysis for all rock samples, trench 2A, Bochinche.................... 26

20. Univariate statistics for host- and vein-rock samples from a road cut near Bochinche........................................ 28

21. Correlation coefficients for host- and vein-rock samples from a road cut near Bochinche.................................. 28

22. Univariate statistics for vein-rock samples from Marwani concession, zone 4.................................................... 31

23. Correlation coefficients for vein-rock samples from Marwani concession, zone 4.............................................. 31
Geochemistry of Six Gold-Bearing

Quartz-Carbonate-Tourmaline Vein Systems,

Estado Bolivar, Venezuela

By Sherman P. Marsh, 1 Herbert A. Pierce,2 Henry Sanchez,3 and Norman J Page2

ABSTRACT

Rock samples collected from six vein systems and 

panned-concentrate samples collected from streams draining 

one of the vein systems were analyzed as part of a study of 

gold-bearing quartz-carbonate-tourmaline veins in the northeastern Guayana Shield of Venezuela. Samples are from 

Precambrian metavolcanic host rocks, mostly greenstone, 

and associated (and probably slightly younger) gold-bearing 

quartz-carbonate-tourmaline veins. All samples were analyzed by atomic absorption, atomic emission spectroscopy, 

and ion-specific electrode techniques. Field observations 

and analysis of samples suggest a deposit type consistent 

with low-sulfide gold-quartz veins. Analyzed rock samples 

from the surface, the near surface (3-15 m), and depth (300 

m) show variations in gold concentrations from less than 

0.002 to 10 ppm.

R-mode factor analysis on sample data from a trench in 

the Bochinche area identified four elemental associations: 

(1) a host-rock metavolcanic suite; (2) a mafic suite; (3) a 

tourmaline suite; and (4) a gold mineralization suite. The 

gold probably is in carbonate parts of the quartz veins, and 

carbonate fluids probably were the transport media.

A very low frequency geophysical survey parallel with 

the trench in the Bochinche area indicates that this electromagnetic technique can identify buried quartz veins (as deep 

as 10 m for veins about 1 m thick) in the deeply weathered 

tropical soils of Venezuela.

'U.S. Geological Survey, Denver Federal Center, MS973, Denver, 

Colorado 80225.

2U.S. Geological Survey, Corbett Building, 210 E. 7th Street, Tucson, 

Arizona 85705.

3Corporacion Venezolana de Guayana, Tecnica Minera, C.A., C.C. 

Chilemex, Piso 1, Puerto Ordaz, Venezuela.

RESUMEN

Se analizaron un grupo de rocas colectadas en seis sistemas de vetas y concentrados pesados, los cuales provienen 

de quebradas que drenan uno de los sistemas de vetas. Estos 

analisis se llevaron a cabo como parte de un estudio sobre 

vetas de cuzirzo con carbonato-turmalina-oro en la parte noreste del Escudo de Guayana en Venezuela. Las muestras 

incluyen (1) rocas metavolcanicas, principalmente rocas verdes "greenstones," las cuales forman la roca caja de edad 

Precambrico, y (2) vetas de cuarzo con carbonato-turmalina-oro, las; cuales son probablemente un poco mas jovenes 

de las rocas verdes. Todas las muestras fueron analizadas por 

absorcion atomica, espectroscopia de emision atomica y tecnicas con electrodes de iones especificos. Las observaciones 

de campo y analisis de muestras sugieren un deposito caracterizado por vetas de cuarzo con oro y bajo contenido de sulfuros. Las muestras de superficie, de baja profundidad (3-15 

m), y profundas (300 m) muestran una varicion de <0.002 

ppm a 10 ppm de contenido de oro. La mayoria de las muestras contienen cantidades anomalas de oro, cine, selenio, 

sflice y hierro.

El amilisis factorial R-modal, realizado en datos de 

muestras de la trinchera 2A en el area de Bochinche, permitio ideni:ificar cuatro asociaciones de elementos que 

describen los siguientes rasgos geologicos: (1) rocas 

metavolcanicas (roca caja), (2) rocas maficas, (3) turmalina, 

y (4) mineralizacion de oro. El oro esta probablemente en la 

fraction carbonatica de las vetas; ademas, fluidos ricos en 

carbonate fueron probablemente el medio de transports del 

oro.

Un levantamiento geofisico de muy baja frecuencia 

(VLF), el cual fue realizado paralelo a la trinchera 2A, indica 

que este metodo electromagnetico puede ser usado para 

identificar vetas de cuarzo que no afloran. Este metodo es 

efectivo para detectar vetas de 1 m de espesor hasta una profundidad de 10 m en suelos tropicales de Venezuela.

Nl
N2 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

INTRODUCTION

As part of the cooperative project between the U.S. 

Geological Survey (USGS) and the Corporation Venezolana 

de Guayana, Tecnica Minera, C.A. (CVG-TECMIN), 

gold-bearing low-sulfide quartz-carbonate-tourmaline veins 

in greenstone belts of the Guayana Shield of Venezuela were 

investigated during site visits in 1988 to determine their 

geochemical signature. Three types of vein occurrences were 

examined: surface exposures, near-surface exposures (3-15 

m depth), and occurences at depth (300 m). Samples were 

collected and analyzed from six areas: Mina Colombia, Lo 

Increible, three areas near Bochinche, and zone 4 of the Marwani concession (fig. 1). Two of the studied areas are near 

the pueblo El Callao: the underground mine, Mina Colombia, 2.2 km south southeast of El Callao, and an open-pit 

operation, Mina San Pedro, in the Lo Increible district 15 km 

north of El Callao. Near Bochinche, two trenches, Tl and 

2A, and a road cut between Esperanza and Bochinchita were 

studied. The sixth study area, zone 4 of the Marwani 

concession, is in the jungle near the Guyana border.

The gold-bearing low-sulfide quartz-carbonate-tourmaline veins are in volcanic and volcaniclastic rocks and 

graywacke in the eastern greenstone belts of the Guayana 

Shield. General geologic characteristics of the Guayana 

Shield are described by Sidder and Mendoza (this volume); 

a more detailed geologic and geophysical description of the 

Bochinche area is given by Wynn, McCafferty, and Salazar 

(this volume), and the Lo Increible area is discussed by Day 

and others (this volume). The geology and geochemistry of 

zone 4 of the Marwani concession is discussed in this report, 

and the geology of Mina Colombia is described by 

Rodriguez(1986).

The Early Proterozoic greenstone belts of the Guayana 

Shield are composed of submarine sequences of tholeiitic to 

calc-alkalic basalt, rhyolite, turbiditic graywacke, volcaniclastic rocks, and chemical sedimentary rocks, in approximately this stratigraphic sequence from bottom to top. The 

belts also contain some ultramafic and mafic rocks, such as 

at Bochinche (Wynn, McCafferty, and Salazar, this volume). 

Metamorphism ranges from low-grade greenschist facies to 

high-grade amphibolite facies in the northern part of the 

shield near the Guri fault; shearing, folding, and faulting 

characterize postdepositional events in the greenstone belts. 

The belts formed between 2,250 and 2,100 Ma (Gibbs and 

Olszewski, 1982; Gruau and others, 1985); granitic rocks 

ranging in age from about 2,230 to 2,050 Ma have intruded 

the belts (Klar, 1979), and the granitic rocks and greenstone 

belts are crosscut by diabase dikes.

Acknowledgments. We thank Michael Alien for his 

help with the statistics and intricacies of STATPAC and 

USGS analysts R. Hopkins and D. Fey. Without the help of 

CVG-TECMIN field camp crew Manuel Ramirez, Miguel 

Loreto, Armand Ledezma, Wilmer Maita, Carlos Maurera, 

and Javier Mendez the work in Marwani zone 4 would not

have been possible. We express our appreciation to Jeffrey 

Wynn who added his insight to the Marwani zone 4 area 

from his interpretation of the aeromagnetics and for his contribution of the VLF-EM data for Bochinche trench 2A.

METHODS

SAMPLE COLLECTION

Samples were collected from the surface, from the near 

surface in trenches and a road cut, and from subsurface exposures in Mina Colombia. Most samples were taken from 

quartz-carbonate-tourmaline veins or their adjacent metavolcanic and metavolcaniclastic host rocks. Some surface rock 

samples were collected from float or colluvium where outcrops could not be found, and these may have been transported from their place of origin. Samples collected from 

many sites are single grab samples and may or may not be 

representative of their source materials on a larger scale. 

Some samples were collected because of the observed alteration and (or) mineralization (including visible gold) or 

because visible gold was identified in concentrate samples 

taken downstream from an outcrop. Some samples collected 

in trenches were composite chip samples, designed to be as 

representative as possible of the exposed rock unit, vein, or 

silicified zone. In general, host-rock samples were collected 

adjacent or in close proximity (usually within 10 m) to the 

quartz-carbonate-tourmaline veins. This form of sampling 

was done to determine if gold mineralization extended into 

the host rocks and to study alteration (usually silicification) 

in the host rocks resulting from introduction of the veins. 

One hundred and forty-five rock samples were collected and 

analyzed (appendix tables 1-6).

In addition to rock samples, 86 panned-concentrate 

samples were collected from streams draining areas containing known or possible quartz-carbonate-tourmaline veins in 

zone 4 of the Marwani concession (appendix table 7). All 

panned concentrates were taken from active first-order 

streams. Concentrate samples were purposefully biased by 

collection in parts of streambeds favorable for concentration 

of heavy minerals by natural stream processes.

SAMPLE PREPARATION AND ANALYSIS

Rock samples were crushed, divided into splits of 

approximately 500 g, and pulverized using a disk mill with 

ceramic plates to less than 100 mesh (0.15 mm). 

Panned-concentrate samples were reduced in the field to 

samples of 100-400 g and were further concentrated by 

floatation of the low-density minerals in bromoform (specific gravity 2.86). The remaining high-density fraction was 

separated into three fractions based on degree of magnetism.
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS

66° 63°

Boundary of Venezuelan 

Guayana Shield

Figure 1. Map showing location of El Callao, Bochinche, and Marwani areas, Estado Bolivar, Venezuela.

After removal of a highly magnetic magnetite fraction with 

a hand magnet, the sample was split into magnetic and 

nonmagnetic fractions using a modified Franz isodynamic 

separator set at 2 amperes; the mineralogy of the resultant 

nonmagnetic heavy-mineral fraction was studied under a 

stereographic binocular microscope to identify the contained

minerals. This nonmagnetic fraction was then ground to a 

powder for analysis.

All samples were analyzed for 35 elements using a 

semiquantitative direct-current arc emission spectrographic 

method (Grimes and Marranzino, 1968). Spectrographic 

results were obtained by visual comparison of spectra
N4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 1. Limits of determination for semiquantitative emission spectrographic analysis of rock samples, based on 10-mg sample. 

[Values from Adrian in Arbogast (1990)]

Element

Lower limit

of determination

Upper limit

of determination Element

Lower limit

of determination

Upper limit

of determination

Percent

Calcium (Ca)

Iron (Fe)

Magnesium (Mg)

0.05

0.05

0.02

20

20

10

Sodium (Na)

Phosphorus (P)

Titanium (Ti)

0.2

0.2

0.002

5

10

1

Parts per million

Silver (Ag)

Arsenic (As)

Gold (Au)

Boron (B)

Barium (Ba)

Beryllium (Be)

Bismuth (Bi)

Cadmium (Cd)

Cobalt (Co)

Chromium (Cr)

Copper (Cu)

Gallium (Ga)

Germanium (Ge)

Lanthanum (La)

Manganese (Mn)

0.5

200

10

10

20

1

10

20

10

10

5

5

10

50

10

5,000

10,000

500

2,000

5,000

1,000

1,000

500

2,000

5,000

20,000

100

100

1,000

5,000

Molybdenum (Mo)

Niobium (Nb)

Nickel (Ni)

Lead (Pb)

Antimony (Sb)

Scandium (Sc)

Tin (Sn)

Strontium (Sr)

Thorium (Th)

Vanadium (V)

Tungsten (W)

Yttrium (Y)

Zinc (Zn)

Zirconium (Zr)

5

20

5

10

100

5

10

100

100

10

20

10

200

10

2,000

2,000

5,000

20,000

10,000

100

1,000

5,000

2,000

10,000

10,000

2,000

10,000

1,000

Table 2. Limits of determination for semiquantitative enission spectrographic analysis of panned concentrates from stream sediments. 

[Values from Adrian in Arbogast (1990)]

Element

Lower limit

of determination

Upper limit

of determination Element

Lower limit

of determination

Upper limit

of determination

Percent

Calcium (Ca)

Iron (Fe)

Magnesium (Mg)

0.1

0.1

0.05

50

50

20

Sodium (Na)

Phosphorus (P)

Titanium (Ti)

0.5

0.5

0.005

10

20

2

Parts per million

Silver (Ag)

Arsenic (As)

Gold (Au)

Boron (B)

Barium (Ba)

Beryllium (Be)

Bismuth (Bi)

Cadmium (Cd)

Cobalt (Co)

Chromium (Cr)

Copper (Cu)

Lanthanum (La)

Manganese (Mn)

Molybdenum (Mo)

Niobium (Nb)

1

500

20

20

50

2

20

50

10

20

10

50

20

10

50

10,000

20,000

1,000

5,000

10,000

2,000

2,000

1,000

5,000

10,000

50,000

2,000

10,000

5,000

5,000

Nickel (Ni)

Lead (Pb)

Palladium (Pd)

Platinum (Pt)

Antimony (Sb)

Scandium (Sc)

Tin (Sn)

Strontium (Sr)

Vanadium (V)

Tungsten (W)

Yttrium (Y)

Zinc (Zn)

Zirconium (Zr)

Thorium (Th)

10

20

5

20

200

10

20

200

20

100

20

500

20

200

10,000

50,000

1000

1000

20,000

200

2,000

10,000

20,000

20,000

5,000

20,000

2,000

5,000

derived from the sample with spectra obtained from 

standards made from pure oxides and carbonates. Standard 

concentrations are geometrically spaced over any given 

order of magnitude of concentration such that values 

reported for each sample are reported in the geometric 

sequence 10, 15, 20, 30, 50, 70, 100, and so forth. The elements determined and their limits of determination are listed 

in table 1. The precision of the method is plus or minus one 

reporting interval at 83 percent confidence or two reporting 

intervals at 96 percent confidence (Motooka and Grimes, 

1976). Values determined for the major elements (Ca, F, Fe,

Mg, Na, P, and Ti) are given in weight percent; all other 

values are in parts per million. Analytical data from the spectrographic analyses are given in the appendix. Because the 

nonmagnetic heavy-mineral concentrate samples contain 

mineral species commonly high in iron, titanium, calcium, 

and zirconium, the sample weight used for analysis was 

reduced by half and the resulting values from the observed 

spectra were doubled. This modification of the method was 

made to eliminate the special interferences produced by high 

concentrations of these elements. Similarly, the lower limits 

of detection were also doubled (table 2).
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N5

Table 3. Analytical methods used and limits of determination for inductively coupled plasma emission spectroscopy (ICP) and 

flame atomic absorption spectrophotometry (AA) methods.

[In parts per million. Lower and upper limits of determination for the ICP method listed in this table are nominal and may be variable. The 

variability in limits of determination for an element may be due to variable sample aliquot weight, dilution of an analytical aliquot, or 

instrumental interference correction]

Element

Silver (Ag)

Arsenic (As)

Gold (Au)

Bismuth (Bi)

Cadmium (Cd)

Copper (Cu)

Molybdenum (Mo)

Lead (Pb)

Antimony (Sb)

Zinc (Zn)

Tellurium (Te)

Thallium (Tl)

Fluorine (F)

Gold (Au)

Analytical method

ICP

ICP

ICP

ICP

ICP

ICP

ICP

ICP

ICP

ICP

AA

AA

Ion-selective electrode

AA (graphite furnace)

Lower limit 

of determination

0.04

0.6

0.15

0.6

0.03

0.03

0.09

0.6

0.6

0.03

0.1

0.2

0.01

0.002

Upper limit 

of determination

51,500

3,000

2,400

1,500

500

1,200

1,500

12,000

800

500

20

~

-

--

Other methods of analyses used on samples from this 

study are summarized in table 3. Gold analyses were done 

using atomic absorption spectrophotometry after hydrobromic acid digestion of a 10-g aliquot and an organic solvent 

extraction (O'Leary and Meier, 1984). Arsenic, bismuth, 

cadmium, antimony, and zinc were determined using an 

inductively coupled plasma-atomic emission spectroscopy 

method described by Crock and others (1982, 1983) and 

Lichte and others (1988). Mercury was determined using a 

modification of the atomic absorption method described by 

Wilson and others (1988). Fluorine was determined using an 

ion-selective electrode method described by Hopkins 

(1977). Tellurium and thallium were determined by the 

flame atomic absorption method (Hubert and Chao, 1985). 

Results of these analyses are given in the appendix.

DATA ANALYSIS

The analytical results were entered into a computer 

database that contains both descriptive geological information and analytical data. Information may be retrieved and 

converted to a binary form and then processed using the U.S. 

Geological Survey statistical package STATPAC for 

computerized analysis or publication (VanTrump and 

Miesch, 1976).

STATPAC programs were used to analyze data sets 

from the six areas. Variables (elements) that displayed values below the threshold of detection for more than 50 percent 

of the samples were removed from the data set and not used 

in the statistical analysis. The number of valid elements 

ranges from 23 to 28 depending on the data set. Basic 

univariate statistics were run on lognormalized data. These 

values can be compared using Cohen-estimated geometric

means and deviations that account for qualified data where 

some values fall below the detection limit of the chemical 

analysis. Correlation matrices were constructed for most 

data sets to delineate various affinities (or disaffmities) 

between element pairs. In addition, one data set, Bochinche 

2A, had enough samples to allow R-mode factor analysis to 

be performed.

GENERAL GEOLOGIC SETTING OF

QUARTZ-CARBONATE-TOURMALINE

VEINS

Gold-bearing quartz-carbonate-tourmaline veins are 

present in greenstone-belt rocks and slightly younger 

volcanic rocks in the Guayana Shield in the east-central part 

of Estado Bolivar, Venezuela. The greenstone belts form 

parts of two Precambrian rock groups, the Pastora Supergroup (Korol, 1965; Menendez, 1968) and the structurally 

overlying volcanic rocks that crop out in the area of Rio 

Botanamo. Volcanic rocks of the Pastora Supergroup are 

regionally metamorphised to greenschist facies and in the 

northern part of the shield, within 30 km of the Guri fault, to 

amphibolite facies. Localized zones of amphibolite schist are 

developed in mafic volcanic rocks near contacts within the 

Supamo Complex. Large parts of the volcanic areas can be 

described as greenstone belts because of their elongate form, 

greenschist facies metamorphism, and flanking granitic 

batholiths. In the El Callao area, the Pastora Supergroup was 

subdivided by Menendez (1968) into the Carichapo Group 

and the overlying Yuruari Formation. The Carichapo Group 

comprises a lower pillow basalt containing local talc schist 

lenses and an upper andesitic metatuff. The Yuruari
N6 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 4. Univariate statistics for selvage and vein-rock samples, Mina Colombia, El Callao district, Venezuela.

[In parts per million unless otherwise noted. All analyses are by semiquantative spectrographic method except for As, Cd, Zn, and Au, which

were done by atomic absorption method. L, detected but below lower limit of determination; N, not detected at lower limit of determination]

Ranee

Element

Fe%

Mg%

Ca%

Ti%

Mn

Ag

B

Ba

Co

Cu

Ni

Pb

Sc

V

Y

Zr

Na

Ga

As

Cd

Zn

Te

Au

Minimum

0.07

0.07

0.01

0.007

30

1

1

20

30

10

7

10

7

10

20

20

0.2

15

5

0.3

20

0.25

0.003

Maximum

10

1.5

7

1

1,500

7

150

70

100

300

50

15

30

500

30

70

5

50

87

1.9

110

0.8

10.0

Geometric

Mean

3.2

0.7

2.8

0.3

595.4

2.0

34.5

28.9

47.3

41.4

18.0

11.4

22.9

132.7

26.2

47.7

1.9

30.1

18.9

0.9

72.9

0.47

0.56

Deviation

5.2

2.3

3.6

4.9

3.0

2.4

2.5

1.7

1.7

2.9

2.5

1.2

1.6

2.9

1.2

1.5

2.5

1.6

2.7

1.9

1.8

1.6

14.0

Number of samples

Valid

12

12

12

12

12

4

8

7

9

9

4

9

10

12

9

9

10

9

7

9

10

4

10

L

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

N

0

0

0

0

0

8

4

1

3

2

4

1

2

0

3

1

2

2

0

0

0

0

0

Cohen

Mean

3.2

0.7

2.8

0.3

595

0.2

17.1

20.1

27.8

19.3

2.3

10.6

16.7

133

19.1

28.4

1.2

16.6

6.8

0.4

35.6

0.02

0.16

Deviation

5.17

2.34

3.59

4.9

3.05

6.89

3.73

1.89

2.90

5.28

6.48

1.25

2.30

2.91

1.80

2.70

4.03

3.08

4.63

4.29

5.50

15.3

42.2

Table 5. Cohen geometric means for five host-rock samples and seven vein-rock samples, Mina Colombia,

El Callao district, Venezuela.

[In parts per million unless otherwise noted. Elements too heavily qualified on either the upper or lower detection limit to calculate a Cohen

geometric mean are not included]

Element

Fe%

Mg%

Ca%

Ti%

Mn

Ba

Ni

Co

Cu

V

Vein

1.7

0.5

1.9

0.4

408

22

1.2

16.5

9.6

96.6

Host

7.5

1.0

5.0

0.7

1,010

13

3.5

42

33.11

207

Element

Pb

Sc

Y

Zr

Na

Ga

As

Cd

Zn

Au

Vein

10.1

10.9

30.0

17.7

0.6

8.0

14.4

0.2

15.0

1.42

Host

10.8

27.7

23.5

43.0

2.7

40.8

4.5

1.3

99.2

0.007

Formation is an extensive unit of mica schist and phyllite and 

felsic metatuff. Similar types of rock are present in other 

greenstone areas, which are separated from El Callao by 

wide areas of granite, but in these belts topography is more 

subdued and jungle cover more dense, and stratigraphic 

relations between rock types are difficult to determine. It is 

likely that cycles of mafic and felsic volcanism of several 

different ages have contributed to the various greenstone 

belts in the northeastern part of the shield. In addition to 

volcanic rocks, extensive areas of gabbro and peridotite are

present in the greenstone belt extending south from El 

Callao.

Metavolcanic and metasedimentary rocks in the 

Bochinche and Marwani areas may represent a volcanic 

environment distinctly different from that of other greenstone belts in the Guayana Shield. The rocks are uniformly 

metamorphosed to greenschist facies and are strongly 

folded, judging from the steep dips of bedding and schistosity (Benaim, 1974) and the northeast-trending grain in the 

radar images (U.S. Geological Survey and Corporation
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N7

Venezolana de Guayana, Tecnica Minera, C.A., 1993, plate 

3). The area of exposure is broad rather than beltlike, and 

limited petrochemical data from the Anacoco area (Day and 

others, 1989) indicate that the volcanic rocks belong to a 

calc-alkaline suite rather than to a bimodal assemblage such 

as described by Gibbs (1980) for more typical greenstone 

belts in adjacent areas of Guyana.

The Pastora Supergroup and overlying volcanic rocks 

are separated by a large northwest-trending shear zone that 

is interpreted to be a low-angle fault. The greenstone belts 

contain differentiated volcanic, pyroclastic, and related 

metavolcanic rocks that are metamorphosed to greenschist 

facies. In general, the quartz-carbonate-tourmaline veins cut 

across foliation at an oblique angle.

Many of the low-sulfide gold-quartz veins in the Guayana Shield are related either to dislocation zones within the 

greenstone belts (such as in the El Callao and Lo Increible 

areas) or to inferred buried intermediate-composition 

plutons that intruded rocks of the greenstone belt.

EL CALLAO DISTRICT

The El Callao district contains a series of deposits from 

which the total production to date is about 180 metric tons of 

gold or roughly US$1.8 trillion (1991 dollars). One mine in 

the district, Mina Colombia, was sampled for this study.

MINA COLOMBIA

Mina Colombia, an active underground gold mine operated by Compania General de Mineria de Venezuela, C.A. 

(Minerven), is south-southeast of the town of El Callao. 

Mina Colombia produces ore from a zone of quartz-carbonate-tourmaline veins that strike N. 60°-70° E. and dip 

35°-45c S. Production is about 700 metric tons of ore per day 

with grades as high as 30 grams per metric ton and averaging 

about 9.9 grams per metric ton. The gold is present in pyrite 

(Ing. Fructuoso Rolando Salazar, oral commun.) and as free 

gold. The gold-bearing quartz-carbonate-tourmaline veins 

are in greenstone (metavolcanic and metavolcaniclastic 

rocks) that has been faulted and fractured. Two main 

veins the America and the Colombia currently are being 

mined on three levels. The veins are 2-5 m wide including 

adjacent fracture and breccia zones. The vein systems 

occupy fault structures and tension fractures in the greenstone; associated breccia zones and silicified areas are in 

both the veins and the host rocks. A zone of smaller 

secondary quartz veins normal to the primary veins extends 

approximately 10 m from the foot- and hanging-walls into 

the host rock. The secondary veins occupy fractures 

interpreted as result of tension and dilation of the primary 

America and Colombia veins. The secondary veins are 

deformed by folding and are also gold producing (appendix

table 1). Host rocks adjacent to the veins contain abundant 

pyrite (cubes as large as 0.5 cm) and tourmaline.

Twelve rock samples of vein and host material were 

collected and analyzed. Univariate statistics and Cohen-estimated geometric means and deviations are given in table 4. 

Vein rocks collected from Mina Colombia are enriched in 

yttrium, barium, arsenic, and gold relative to the host rocks 

(table 5). Lead is the same for veins and host rocks, whereas 

other elements are depleted from 15 to 70 percent.

The enrichment of gold and arsenic is typical of the 

low-sulfide gold-quartz vein model (Bliss, 1986); however, 

measured levels of arsenic are low, 5-87 ppm (appendix 

table I). The lack of an increase in iron probably indicates 

that most of the gold in this deposit is free gold and not 

chemically associated with the pyrite in the mine.

LO INCREIBLE DISTRICT

The Lo Increible district, 5-10 km northwest of El 

Callao (fig. 1), contains a series of small placers, open pits, 

and underground mines on and in a northeast-trending southeast-dipping shear zone (fig. 2). The shear zone contains 

many discontinuous quartz-carbonate-tourmaline veins, and 

it separates rocks of the Yurauri Formation and rocks of the 

El Callao Formation. The Yurauri Formation at Lo Increible 

is composed of amphibolite-grade, medium-grained 

quartz-biotite-muscovite schist whose protolith ranges from 

felsic volcaniclastic tuff and tuff breccia to mafic volcanic 

rock. The El Callao Formation consists of magnesium-rich 

tholeiitic metabasalt that was deposited as subaqueous flows 

and has been locally strongly deformed and metamorphosed 

to chlorite-amphibolite schist near the shear zone. These 

flows are locally crossed by some northwest-trending 

fractures (Day and others, this volume).

Gold in the Lo Increible district is in the shear zone as 

native gold in discontinuous quartz-carbonate-tourmaline 

veins that pinch and swell along strike. The shear zone is 

mined along strike for a distance of several kilometers.

Mining operations in the Lo Increible district use both 

underground (Mina Lapa) and open-pit methods (Mina-A 

and Mina San Pedro, figs. 2-4). Many of the open-pit mines 

in this area use bulldozers to break up surface exposures of 

the quartz veins, and commonly these mines occupy sites of 

former underground mines. Quartz boulders are as large as a 

meter or more in diameter, and gold is commonly visible on 

freshly broken surfaces. The deposit at Mina-A is extremely 

rich; at the time of sampling the mine had been in operation 

for about three months and had produced almost 45 kg of 

gold. Approximately two-thirds of the gold production at 

Mina-A is from outcrop, and one-third is from reworking 

tailings from previous mining. The mine currently produces 

about 350 metric tons of vein quartz per month at an average 

grade of 80 grams per metric ton (28 kg of gold per month).
N8 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Figure 2. Geologic map of the Lo Increfble district, Estado Bolivar, Venezuela, showing locations of Mina-A, Mina Lapa, Mina 

Esperiencia, and Mina San Pedro. Modified from Day and others (this volume).

About 12 kg of gold per month is recovered from the tailings 

of the old underground mine.

Mina San Pedro is an east-trending open pit along a 

shear zone. A north-south trench, roughly 15 m below the 

natural grade at its deepest, crosses the trend of the 

vein-shear zone system. Nine samples collected from the 

trench wall reflect the altered bedrock lithology, quartz-carbonate-tourmaline veining, and silicified breccia zones. The 

sampled trench is roughly perpendicular to the trend of the 

quartz-carbonate-tourmaline veins, which strike east-west 

and dip 80° S. The rocks in this area are brecciated, silicified, 

and altered to clays, sericite, and iron oxide minerals, and 

even the quartz-carbonate-tourmaline veins are brecciated

and silicified. The veins contain pockets and holes where a 

carbonate mineral has apparently weathered out.

Analytical data for Mina San Pedro are given in appendix table 2. Univariate statistics and Cohen geometric 

corrections for qualified data for elements are given in table 

6. A comparison of vein and host rock statistics using Cohen 

geometric means is given in table 7. At Mina San Pedro, only 

boron and gold are enriched in the veins relative to the host 

rock. Iron is unchanged between host and vein rocks, and the 

other elements are depleted in the veins relative to the host 

rock. Unlike Mina Colombia, arsenic, yttrium, and barium 

are not enriched in the veins. The lack of enrichment may 

reflect weathering in the near-surface environment.
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N9

EXPLANATION

Alluvium 

Tectonic breccia 

Gabbro

Diabase

Pastora Supergroup Divided into:

Caballape Formation 

Yuruari Formation

El Callao Formation of the Carichapo Group Includes 

volcanic breccia (unit Xecb)

Contact Approximately located; dashed where inferred

Fault Dashed where inferred; arrows show direction of

movement. Bar and ball on downthrown side 

Thrust fault Sawteeth on upper plate

Shear zone

Syncline Dashed where inferred

Strike and dip of foliation 

60 Bearing and plunge of lineation

Pillow lava showing younging direction

Mine or prospect 

= Paved road

= = = = = = Unimproved road

O Town 

Lake

Figure 5 shows a plot of gold values in the Mina San 

Pedro trench. The irregular distribution of gold values in relation to the spatial distribution of the quartz veins and the 

secondary brecciation of the quartz veins suggest at least two 

episodes of silicification. The large quartz vein at 4 m shows 

low gold values, whereas the two relatively small quartz 

veins (at 1 and 2 m horizontal scale) show elevated gold values. The irregularity of gold values from the trench may be 

caused by an insufficient number of samples, sample collection bias, or a nugget effect.

BOCHINCHE AREA

Three locations in the Bochinche area were sampled: 

trench Tl, trench 2A, and a quartz vein that crops out in a 

road cut along the mam road between Esperanza and 

Bochinchita (figs. 6, 7).

TRENCH Tl

Trench Tl trends north-south, is about 70 m long, and 

at its deepest point is about 6 m below the natural grade (fig.

8). It exposes a main quartz-carbonate-tourmaline vein 2.5 m 

wide and smaller zones of quartz veining on either side. The 

main vein zone strikes N. 65° E. and dips 75° N., and the 

smaller veins adjacent to the large vein generally strike N. 

70° E. and dip steeply 70°- 80° N. Manganese oxide minerals commonly are developed on the margins of the smaller 

veins. Many small silicified zones pinch and swell along 

small fractures. These silicified zones begin with manganese 

oxide staining along a fracture, broaden out into small zones 

of silicification, and then pinch out entirely.

The veins contain abundant tourmaline, and the quartz 

in the veins contains small inclusions of tourmaline and 

pyrite. Other sulfide minerals are probably present in the 

inclusions but were not recognized in the field. The altered 

schistose wallrock is silicified, sericititzed, and very weathered, in places almost to clay, but the schistose texture is 

mostly preserved. Manganese oxide and iron oxide staining 

is evident on surfaces and margins of the veins.

Twenty-four samples of host schist, silicified host, and 

quartz-carbonate-tourmaline veins were collected. Data for 

the samples were divided into a host-rock group and a 

quartz-vein group. Because only three samples of silicified 

host were collected, statistics were not done for this group. 

Geochemical data are given in appendix table 3. Basic 

statistics, Cohen corrections, and correlation coefficients 

were calculated for both groups. Data for the quartz-vein 

group from trench Tl are given in tables 8 and 9, and data for 

the host-rock group are given in tables 10 and 11.

Cohen-corrected geometric means for the near-surface 

quartz-vein rock group from trench Tl show that barium, 

boron, nickel, and, marginally, zirconium concentrations are 

elevated as compared to those elements at Mina Colombia. 

This is similar to the elemental relations for Mina San Pedro 

with the exception that in trench Tl arsenic concentrations 

are about the same in host and vein rocks.

Cohen-corrected geometric means for the schist 

host-rock group and the quartz-carbonate-tourmaline vein 

group show that gold and manganese are elevated in the 

veins. Base metals, copper, and zinc are enriched twofold in 

the host rocks. The other elements are neither enriched nor 

depleted.

Samples of the quartz-carbonate-tourmaline veins (fig.

9) show correlations between iron, nickel, vanadium, 

copper, and zirconium that probably represent a pyrite or 

other sulfide mineral suite. Correlations for manganese, 

titanium, chromium, and cobalt probably represent the 

signature of a lithic group comprising metavolcanic host 

rocks. Significant correlations between gold, zinc, and nickel 

most likely represent the mineralizing suite.

Gold values are variable across the sampled trench in 

terms of geology, but they are relatively high for some
N10 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Figure 3. Mina-A open-pit mine, Lo Increible district, Estado Bolivar, Venezuela; looking 

northeast. Large gold-bearing quartz-carbonate-tourmaline vein boulder is in middle foreground.

Table 6. Univariate statistics for vein, host, and silicified rock samples, Mina San Pedro, El Callao district, Venezuela. 

[In parts per million unless otherwise noted. All analyses are by semiquantative spectrographic method except for As, Cd, Zn, Te, Tl, and Au, which 

were done by atomic absorption method. L, detected but below lower limit of determination; N, not detected at lower limit of determination; G, 

greater than upper limit of determination]

Range

Element

Fe%

Mg%

Minimum

1 5

0.1

Ti% 0.1

Na%

Mn

B

Ba

Co

Cr

Cu

La

Ni

Pb

Sc

Sr

V

Y

Zr

Ga

F

As

Cd

Zn

Hg

Te

Tl

Au

0.7

30

500

150

10

10

7

50

7

10

5

200

50

10

30

15

0.01

21

0.2

6

0.02

0.05

0.1

0.012

Maximum

5.0

1.0

0.7

2

200

2,000

1,500

20

100

50

70

50

20

15

700

1,50

20

200

50

0.02

95

2.4

31

0.04

0.1

0.15

8.3

Geometric Number of samoles

Mean Deviation Valid

2.8 1.5 9

0.3 2.2 9

0.4 1.7 9

1.3 1.4 8

92 2.1 9

1,123 .8 7

553 2 9

12.6 .5 3

39.2 2.0 9

28.9 .8 9

59.2 1.2 4

21 1

15.6

7.8

389.1

97.9

16.2

104.5

23.9

0.02

49.4

.7 9

.4 8

.4 7

.6 8

.5 9

.3 6

.8 9

.5 8

.3 8

.7 9

0.5 2.2 9

15.6 1

0.04 1

.8 9

.3 6

0.05 1.3 7

0.12 1 .2 6

0.08 8.8 9

L

0

0

0

0

0

0

0

3

0

0

5

0

1

2

0

0

3

0

1

1

0

0

0

0

2

3

0

N

0

0

0

1

0

0

0

3

0

0

0

0

0

0

1

0

0

0

0

0

0

0

0

3

0

0

0

G

0

0

0

0

0

2

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Cohen

Mean Deviation

2.8

0.3

0.4

1.0

91.6

1,410

553

7.5

39.2

28.9

47.1

20.9

14.6

6.7

324

97.9

12.5

105

19.6

0.021

49.4

0.46

15.6

0.03

0.05

0.08

0.08

1.50

2.2

1.7

2.16

2.11

2.02

2.03

1.70

2.05

1.85

1.32

1.72

.42

.55

.98

.48

.56

.81

.97

0.39

1.73

2.17

1.82

1.65

1.32

1.93

8.81
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS Nil

Figure 4. Photomicrograph of native gold from quartz-carbonate-tourmaline vein at Mina-A, Lo Increfble district, Estado 

Bolivar, Venezuela. Gold grain is about 0.5 mm square.

massive and small quartz veins (fig. 10). The apparently 

inconsistent values are probably due to the nugget effect, to 

variations in the weathering of the carbonate host rocks, or 

to small sample number bias. It is clear that the gold is 

related to the veins and that the schist has low gold values.

TRENCH 2A

Trench 2 A trends north-south and is 120 m long and as 

deep as 15 m (figs. 11,12). Exposed material in the trench is 

red, thinly bedded, micaceous, metavolcanic schistose rock 

containing abundant black layers and lenses. Schistosity 

strikes N. 75°-80° E. and dips 72° N. Quartz-carbonate-tourmaline veins obliquely cut the host schist and generally 

strike N. 60°-70° W. and dip 65°-70° S. in the southern part 

of the trench. Northward along the trench these veins are 

more northeast trending, N. 45°-55° E., and also dip steeply

to the south. Zones near the quartz-tourmaline veins are silicified, and the host rock in places is bleached.

Fifty-four samples of host schist, silicified host, and 

quartz-carbonate-tourmaline veins were taken, and the analytical data are shown in appendix table 4. Basic statistics 

and Cohen-corrected geometric means and deviations are 

given in tables 12-14, and correlation coefficients are given 

in tables 15-17.

Comparison of Cohen-corrected geometric means for 

the quartz-carbonate-tourmaline veins and the host rocks 

shows an enrichment of cobalt, tellurium, and gold (fig. 13). 

All other elements are depleted or show little change.

STATISTICAL ANALYSIS

R-mode factor analysis on the entire data set for trench 

2A calculated the scores and loadings (table 18). An elemental suite of Ga, Na, Ti, Sc, Zr, Fe, Sr, Y, V, Tl, Be, Mn, Ni, 

and F (factor 1) probably reflects rock-forming minerals in 

the metavolcanic schistose host rocks. A second elemental 

suite consisting of cobalt, copper, (nickel), mercury, and 

selenium (factor 2) probably represents a mafic rock suite. A 

third element suite consisting of gold and zinc represents a 

mineralizing group (factor 3). The elements boron magnesium, and (fluorine) (factor 4) probably represent the mineral 

tourmaline. Factors 5 and 6 do not easily fit into any group 

but are probably part of the host rock group.

Table 19 lists eigenvalues for the six-factor varimax 

calculations. We chose the six-factor analysis because it 

resolved approximately 79 percent of the variance in the 

data. A seven-factor model, which created a single-element 

factor of selenium, only increased the amount of variance 

that could be resolved by 4 percent (to 83 percent).

VLF-EM SURVEY

A very low frequency electromagnetic (VLF-EM) survey was performed along trench 2A near Bochinche. The 

technique uses low-frequency (18-24 kHz) radio signals

Table 7. Cohen geometric means for vein and host-rock samples, Mina San Pedro, Lo Increfble district, Venezuela.

[In parts per million unless otherwise noted. Elements too heavily qualified on either the upper or lower detection limit to calculate a Cohen

geometric mean are not included]

Element

Fe%

B

Hg

Ti%

Mn

Ba

Ni

Cr

Cu

V

Vein

2.9

1,910

0.02

0.38

87.3

367

18.0

37.6

22.9

94

Host

2.7

841

0.03

0.45

97.4

926

25.1

41.4

38.7

103

Element

Pb

Sc

Y

Zr

Na%

Ga

As

Sr

Zn

Au

Vein

12.4

6.3

10.8

86.1

0.7

14.5

41.1

231

13.0

0.209

Host

17.3

7.1

14.9

133.0

1.6

27.8

62.1

479

19.5

0.048
N12 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

i i i il ill | 141 i i i i i i | i i i i i i i i i | i i i i i i i i i | i i 

EAST

i i I I i I I I I I I I i I I I I I I I I I I I

WEST

0.1

Q Q-] I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I | | I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

01 234 

DISTANCE, IN METERS

567

Brecciated schist 

Silicified schist

EXPLANATION

Iron oxide vein 

Sericite

Quartz vein

Figure 5. Plot of gold values for rock samples collected from a cross section in a trench in the open-pit mine at Mina San Pedro, Lo 

Increfble district, Estado Bolivar, Venezuela. Generalized rock types for the sampled cross section are also shown. Data on which the plot 

is based are given in appendix table 2.
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N13

Bochinchita

Bochinche GUAYANA

} 0 20 40 KILOMETERS

i i i I______|

Figure 6. Map showing eastern Distrito Roscio, Estado Bolivar, 

Venezuela, and the northwest part of the Zona en Reclamation, 

Guayana. Trenches Tl and 2A are just north of Bochinche, and the 

road cut is along the road between La Esperanza and Bochinchito.

Table 8. Univariate statistics for quartz-vein rock samples from trench Tl, Bochinche, Venezuela, including all samples of vein, 

host, and silicified rocks.

[In parts per million unless otherwise noted. All analyses are by semiquantative spectrographic method except for Cd, Zn, Te, Tl, and Au, which 

were done by atomic absorption method. L, detected but below lower limit of determination; N, not detected at lower limit of determination; G, 

greater than upper limit of determination]

Ranee

Element

Fe%

Mg%

Ti%

Mn

B

Ba

Be

Co

Cr

Cu

Ni

Pb

Sc

Sr

V

Y

Zr

Na%

Ga

Cd

Zn

Te

Tl

Au

Minimum

0.3

0.02

0.015

50

30

50

1

10

10

5

5

10

5

100

15

15

10

0.2

5

0.1

3

0.05

0.05

0.004

Maximum

10

0.5

0.7

2,000

500

1,500

1.5

300

500

100

150

20

15

200

150

30

200

0.7

50

0.5

100

0.65

0.45

0.6

Geometric

Mean

2.4

0.1

0.14

764

189

366

1.1

55

41

29

40

14

11

144

62

20

56

0.4

17

0.3

24

0.2

0.2

0.1

Deviation

3.4

2.2

4.4

2.9

2.7

2.8

1.2

2.5

3.7

2.6

2.8

1.4

1.7

1.4

2.2

1.3

2.5

1.8

2.1

2.2

2.8

2.6

2.2

5.5

Number of samples

Valid

12

12

12

11

11

12

5

10

12

12

12

6

7

3

12

5

10

5

6

7

11

7

8

12

L

0

0

0

0

0

0

1

0

0

0

0

6

1

2

0

3

2

2

2

5

1

5

4

0

N

0

0

0

0

0

0

6

2

0

0

0

0

4

7

0

4

0

5

4

0

0

0

0

0

G

0

0

0

1

1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Cohen

Mean

2.4

0.1

0.14

937

240

366

1.1

38

41

29

40

9.5

6.1

64

62

9

38

0.16

5.3

0.13

19

0.08

0.09

0.1

Deviation

3.39

2.2

4.4

3.40

3.46

2.76

1.20

3.45

3.67

2.61

2.77

1.60

2.37

1.91

2.24

2.23

3.46

2.52

4.03

3.55

3.63

5.34

3.35

5.5
N14 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Figure 7. Along main road between Tumeremo and Bochinche, Estado Bolivar, Venezuela. View 

looking east.

Figure 8. Trench Tl, Bochinche area, Estado Bolivar, Venezuela. View looking north.
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N15

Table 9. Correlation coefficients for quartz-vein rock samples, trench Tl, Bochinche area, Venezuela. 

[Number of sample pairs used to calculate the correlation is shown in parentheses]

Ti

Ba 0.8705(12)

Fe

Ti

Ni

Cr

Cu

Zr

Zn

Mn Ni

0.8653(11) 0.7822(12)

0.8016(12)

Co Cu

0.8626(10) 0.8150(12)

0.8911(12)

V

0.8700(12)

0.7181(12)

0.9475(12)

0.8156(12)

Zr

0.8394(1 1)

0.7837(1 1)

0.8887(10)

Zn

0.9073(11)

0.9141(11)

0.8794(11)

0.9421(10)

Au

0.7854(12)

0.7521(10)

Table 10. Univariate statistics for host-rock samples from trench Tl, Bochinche area, Venezuela.

[In parts per million unless otherwise noted. All analyses are by semiquantative spectrographic method except for Cd, Zn, Te, Tl, and Au, which 

were done by atomic absorption method. L, detected but below lower limit of determination; N, not detected at lower limit of determination]

Element

Range

Minimum Maximum

Geometric

Mean Deviation

Number of samples 

Valid L

Cohen

Mean Deviation

F

Ba

Fe%

Mg%

Ti%

Mn

B

Ni

Be

Co

Cr

Cu

V

Pb

Sc

Sr

Y

Zr

Na%

Ga

Cd

Zn

Te

Tl

Au

0.01

150

3

0.07

0.3

200

20

30

0.5

15

50

30

70

15

7

100

15

70

0.3

20

0.1

18

0.05

0.1

0.012

0.02

2,000

15

0.5

1

5,000

1,500

100

1.5

300

500

150

200

20

20

300

50

200

2

50

1.4

72

1.5

0.7

1

0.02

811

6.8

0.17

0.61

700

302

63

1.2

53

153

60

138

18

15

184

21

133

0.9

41

0.4

37

0.3

0.4

0.05

1.4

1.9

1.5

1.8

1.4

2.6

5.4

1.4

1.4

3.1

2.0

1.5

1.4

1.2

1.4

1.6

1.4

1.4

1.8

1.4

2.2

1.7

4.4

1.7

4.2

10

12

12

12

12

12

11

12

9

11

12

12

12

12

12

7

12

12

11

12

11

12

10

11

12

2

0

0

0

0

0

0

0

3

1

0

0

0

0

0

2

0

0

0

0

1

0

2

1

0

0

0

0

0

0

0

1

0

0

0

0

0

0

0

0

3

0

0

1

0

0

0

0

0

0

0.014

811

6.77

0.17

0.61

699

211

63.6

1.04

43.4

153

60.5

138

17.7

14.8

115

21.3

133

0.800

40.8

0.326

37.4

0.164

0.335

0.05

.51

.95

.47

.8

.4

2.65

7.44

1.39

1.48

3.58

1.95

1.51

1.35

1.16

1.41

2.02

1.38

1.37

2.16

1.38

2.45

1.68

5.49

2.33

4.22

Table 11. Correlation coefficients for host-rock samples from trench Tl, Bochinche area, Venezuela. 

[Note the lack of any significant correlation with gold]

Ba

Fe

Mg

Ti

B

Ni

Co

V

Mn Co V Sc Y Cd Te

0.7047(12)

0.8529(12) 08824(11)

0.7508(12)

0.9420(12)

0.7191(10)

0.8954(9)

0.7441(12)

Tl

0.8964(11)
N16 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Figure 9. Main large quartz-carbonate-tourmaline vein in trench 

Tl, Bochinche area, Estado Bolivar, Venezuela.

transmitted from several locations around the world. The 

instrument allows a calculation of resistivity and a direct 

reading of phase angle in the field. Figure 14 is a plot of distance versus resistivity and figure 15 a plot of distance versus 

phase angle. Both figures 14 and 15 also show the location 

of veins and the general rock types along the trench. The 

major quartz veins are shown in figure 14 as resistive highs. 

Background resistivity for the schist is about 500 ohm-m. 

The low-sulfide quartz-carbonate-tourmaline gold-bearing 

veins have a resistivity of about 2,000-3,000 ohm-m. It 

should be noted that only the larger veins or silicified zones 

can be identified using this technique. Many of the veins 

depicted in the lithology section below the plots are only 2-3 

mm wide, not sufficiently wide to be resolved by this type of 

geophysical survey. The plots of resistivity and phase are 

slightly shifted in relation to the geology because the resistivity data were collected outside of the trench and the veins 

cut diagonally across the trench where the geochemical samples were collected and the geology was mapped.

The resistivity contrast of approximately five to one and 

the relatively high background resistivity values for lateritic 

clayey soil should lend themselves well to an airborne electromagnetic survey. A survey designed to use multiple

frequencies such as 22,000, 7,200, and 900 Hz over areas 

suspected to contain large quartz-carbonate-tourmaline 

veins should easily map the extent and the trend of the veins 

and provide estimates of the depth to veins where the veins 

are covered by several meters of soil.

BOCHINCHE ROAD CUT

A large, vertical quartz-carbonate tourmaline vein that 

strikes N. 20° E. and crops out in a road cut between Bochinchita and Esperanza was also sampled. The road cut is about 

5 km southwest of Esperanza on the main road from Tumeremo to Bochinche (figs. 16,17). The vein is about 10 m wide 

and obliquely cuts host rocks of schist and fine-grained 

greenstone that have foliation striking N. 50° E. and dipping 

55° S. Twelve rock samples of host schist and quartz-carbonate-tourmaline veins were collected, and the analytical data 

are given in appendix table 5. Cohen-corrected geometric 

means and deviations are given in table 20, and correlations 

are given in table 21.

The quartz-carbonate-tourmaline veins in the road cut 

are relatively barren, and only boron is elevated. The surrounding schist and greenstone host rocks contain little B but 

are enriched in Cu, Cr, Fe, Mg, Ni, Ti, Zn, and Cd as compared to the veins (appendix table 5). Gold values for the 

veins are low, generally 0.002-0.030 ppm, generally lower 

than those of the host metavolcanic rocks. The lack of gold 

in this normally favorable environment of quartz-carbonate-tourmaline veins perhaps can be explained by weathering or by the localized distribution of gold in this 

environment. Where gold was observed elsewhere in this 

type of vein, the gold distribution was irregular.

MARWANI AREA

The Marwani concession is a large rectangular area that 

spans both sides of the Rio Marwani in the extreme eastern 

part of Estado Bolivar. It extends northwest-southeast and is 

in inaccessible jungle 110 km east-southeast of Tumeremo 

and 60 km south-southeast of Bochinche (fig. 1). It borders 

the area in dispute between Venezuela and Guyana on the 

northeast and the Rio Cuyuni on the south and southwest. 

Zone 4 of the concession is in the southwestern part of the 

concession and is the largest of the zones studied. The area 

is included on the 8036-1 NO and 8036-1 NE 

l:25,000-scale topographic maps. Access to the Marwani 

concession is by helicopter to a number of heliports cut into 

the jungle'. Further access to the area is by foot along handcut 

trails.

Zone 4 is an area of generally low relief and a northeast-trending chain of hills 180-200 m high. These hills form 

topographic linear features that are an expression of
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N17

1 

0.01 

DISTANCE, IN METERS 

\S1

Schist

EXPLANATION

Small quartz vein Massive quartz vein

Figure 10. Plot of gold values for rock samples collected across quartz veins exposed in trench Tl, Bochinche area, Estado Bolivar, 

Venezuela. Data on which the plot is based are given in appendix table 3. Host rock and quartz veins are shown in a schematic cross section.

northeast-trending resistant formations, in many places 

quartz-carbonate-tourmaline veins.

Geologists from TECMIN spent more than a year working in zone 4, and the results of this work are discussed in 

Sanchez (1986, 1987) and Acosta (1987). Our decision to 

focus on zone 4 was made on the basis of this previous work, 

which indicated abundant gold in streams draining the area 

and the presence of quartz-carbonate-tourmaline veins (figs. 

18, 19). Maps of the area used for our study show topography, drainage, trails (picas and ejes), location of concentrate 

samples containing gold, and location of past mining activity 

and were generated from the previously cited works. An 

excellent interpretation of the aeromagnetic survey in the

area of zone 4 was provided by Jeffrey Wynn (U.S. Geological Survey, oral commun., February 12, 1988). This interpretation delineates a north-northwest-trending pattern that 

corresponds to a topographic trend, indicates the possibility 

of several northwest-trending faults, and gave guidance to 

our fieldwork for locating the quartz-carbonate-tourmaline 

veins.

During the period February 14-25,1988, fieldwork was 

conducted in zone 4. Personnel and equipment were transported by truck to Esperanza and from there by helicopter to 

heliport 10 (figs. 18, 20). The camp was 0.5 km northeast of 

the heliport. Operations were carried out from this camp for 

the first eight days of the field work. On the eighth day the
N18 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 12. Univariate statistics for quartz-vein rock samples from trench 2A. Bochinche area, Venezuela.

[In parts per million unless otherwise noted. All analyses are by semiquantative spectrographic method except for Cd, Zn, Hg, Se, Te, Tl, and

Au, which were done by atomic absorption method. L, detected but below lower limit of determination; N, not detected at lower limit of

determination]

Ranee

Element

F

Ba

Fe%

Mg%

Ti%

Mn

B

Ni

Be

Co

Cr

Cu

V

Pb

Sc

Sr

Y

Zr

Na%

Ga

Cd

Zn

Hg

Se

Te

Tl

Au

Minimum

0.01

30

0.7

0.02

0.07

0.03

70

5

1

10

10

10

30

10

5

100

10

15

0.2

5

0.1

6

0.02

0.1

0.1

0.05

0.002

Maximum

0.02

1,500

7

0.7

0.5

1,000

2,000

500

1.5

2,000

70

700

150

1,000

15

200

30

150

0.5

15

4.6

300

170

0.5

7.2

0.25

1.0

Geometric

Mean

0.01

249

1.5

0.16

0.12

0.27

673

14

1.1

71

16.6

42

68

75

8.0

168

16

37

0.35

8.1

0.43

21

0.19

0.16

0.60

0.10

0.02

Deviation

1.46

3.71

1.69

2.72

1.89

3.06

3.00

3.35

1.22

4.61

1.80

2.83

1.68

4.67

1.57

.41

.58

.90

.65

.65

4.30

2.83

44

1.77

4.42

1.79

5.84

Number of samples

Valid

5

19

19

19

10

12

16

18

4

15

18

19

19

18

12

4

4

19

5

6

13

18

8

12

16

10

19

L

14

0

0

0

9

0

0

]

13

4

1

0

0

1

7

1

8

0

6

7

6

1

0

3

0

4

0

N

0

0

0

0

0

0

0

0

2

0

0

0

0

0

0

14

7

0

8

6

0

0

11

0

3

5

0

Cohen

Mean

0.007

249

1.52

0.16

0.05

67.5

899

12.9

0.78

37.2

15.9

42.0

67.7

64.7

5.76

50.5

4.87

36.9

0.109

3.27

0.181

17.8

0.004

0.136

0.345

0.05

0.02

Deviation

1.78

3.71

1.69

2.72

2.91

3,310

3.43

3.48

1.32

6.69

1.84

2.83

1.68

5.13

1.82

2.37

2.42

1.90

2.57

2.29

6.28

3.30

125.00

1.90

6.60

2.50

5.82

Figure 11. Trench 2A, Bochinche area, Estado Bolivar, Venezuela. View looking west.
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N19

Table 13. Univariate statistics for host-rock samples from trench 2A, Bochinche, Venezuela.

[In parts per million unless otherwise noted. All analyses are by semiquantative spectrographic method except for Cd, Zn, Hg, Se, Te, Tl, and

Au, which were done by atomic absorption method. L, detected but below lower limit of determination; N, not detected at lower limit of

determination]

Ranee

Element

F

Ba

Fe%

Mg%

Ti%

Mn

B

Ni

Be

Co

a

Cu

V

Pb

Sc

Sr

Y

Zr

Na%

Ga

Cd

Zn

Hg

Se

Te

Tl

Au

Minimum

0.01

300

2

0.05

0.15

150

20

10

1.0

10

10

30

70

20

7.0

100

15

50

0.2

5.0

0.1

3.0

0.02

0.1

0.005

0.1

0.002

Maximum

0.02

1,000

10

1.5

1.0

2,000

2,000

70

1.5

300

200

200

200

700

20

300

50

300

2.0

50

1.4

150

43

0.8

7.2

0.35

0.05

Geometric

Mean

0.01

682

5.2

0.35

0.52

415

421

33

1.3

23

105

67

126

51

17

262

24

137

0.9

39

0.25

46

0.97

0.23

0.17

0.18

0.005

Deviation

1.42

1.47

1.38

2.49

1.44

2.05

4.21

1.69

1.22

2.13

.86

.54

.34

2.16

.34

.32

.41

.42

1.67

1.62

2.22

2.13

60

1.93

4.89

1.46

2.48

Number of samples

Valid

3

26

26

26

26

24

17

26

22

22

26

26

26

26

26

25

25

26

26

26

16

25

4

23

8

26

24

L

3

0

0

0

0

0

0

0

4

3

0

0

0

0

0

0

1

0

0

0

10

1

0

2

1

0

2

N

0

0

0

0

0

0

0

0

0

1

0

0

0

0

0

1

0

0

0

0

0

0

2

0

17

0

0

Cohen

Mean

0.01

682

5.22

0.35

0.52

517

1,150

33.2

1.22

18.9

105

66.7

126

50.9

17.2

251

23.5

137

0.94

39.1

0.13

40.2

2.97

0.21

0.01

0.17

0.005

Deviation

1.46

1.47

1.38

2.49

1.44

2.80

6.71

1.69

1.27

2.37

1.86

1.54

1.34

2.16

1.34

1.41

1.48

1.42

1.67

1.62

3.05

2.69

60.3

2.04

10.9

4.89

2.64

Table 14. Univariate statistics for silicified host-rock samples from trench 2A, Bochinche area, Venezuela.

[In parts per million unless otherwise noted. All analyses are by semiquantative spectrographic method except for Cd, Zn, Hg, Se, Te, Tl and

Au, which were done by atomic absorption method. L, detected but below lower limit of determination; N, not detected at lower limit of

determination]

Range

Element

F

Ba

Fe%

Mg%

Ti%

Mn

B

Ni

Be

Co

Cr

Cu

V

Pb

Sc

Sr

Y

Zr

Na%

Ga

Cd

Zn

Hg

Se

Te

Tl

Au

Minimum

0.01

100

1.5

0.1

0.07

0.07

700

7

1

10

15

30

100

20

5

200

15

50

0.2

20

0.1

3

0.02

0.1

0.1

0.05

0.002

Maximum

0.03

1,000

7

2

0.7

5,000

2,000

150

1.5

2,000

150

700

200

700

20

500

70

300

1

50

6.8

430

130

0.5

0.75

0.25

2.6

Geometric

Mean

0.001

511

3.9

1.0

0.4

978

1,425

52

1.4

394

74

160

140

129

13

311

27

157

0.7

30

1.6

82

36

0.26

0.32

0.16

0.27

Deviation

0.008

358

1.7

0.72

0.23

1789

675

43

0.24

650

44

205

32

218

5.4

78

18

69

0.25

13

2.37

143

49

0.17

0.3

0.09

0.8

Number of samples

Valid

8

9

10

10

8

9

4

10

10

9

10

10

10

10

10

9

10

10

9

8

7

8

6

7

4

6

10

L

2

0

0

0

2

0

0

0

0

1

0

0

0

0

0

1

0

0

1

1

3

2

0

0

0

2

0

N

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

1

0

0

4

0

6

2

0

Cohen

Mean

0.01

517

3.59

0.671

0.169

61.9

2,560

38.5

1.33

79.5

58.5

98.1

137

56.9

11.7

268

23.5

141

0.590

18.1

0.242

12.8

0.08

0.03

0.06

0.009

Deviation

1.76

3.51

1.62

2.78

3.45

98.4

2.13

2.34

1.22

7.32

2.19

2.59

1.26

3.21

1.62

1.55

1.66

1.68

1.93

2.59

7.65

8.99

336.

7.30

3.05

9.03
N20 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 15. Correlation coefficients for quartz-vein rock samples from trench 2A, Bochinche area, Venezuela.

[Matrix shows significant correlations at or greater than the 95 percent confidence level. Number of sample pairs used to calculate the correlation is

shown in parentheses]

Fe

F 0.7186(19)

Ba

Fe

Mg

Ti

B

Ni

Co

Cr

Cu

V

Pb

Sc

Sr

Zr

Pb

F

Ba 0.5855(18)

Fe

Mg 0.7080(18)

Ti

B

Ni

Co

Cr

Cu 0.7909(18)

V 0.7065(18)

Pb

Sc

Sr

Zr

Ti B Ni Co

0.8954(10)

0.7134(16) 0.5621(18)

0.9743(15)

Sc Zr Cd Zn

0.6822(12)

0.6809(19)

0.7523(9) 0.7578(10) 0.8129(8) 0.8864(9)

0.7191(11) 0.8974(18) 0.7583(12) 0.7997(17)

0.6723(19)

0.5646(18)

0.6971(12)

0.7384(13) 0.6767(18)

Cr

0.6366(19)

0.8297(10)

0.9989(18)

0.9766(15)

Hg

0.7626(8)

0.9750(8)

0.9584(8)

0.7882(8)

0.9598(8)

0.9194(8)

0.9163(7)

0.7941(8)

Cu V

0.6588(19)

0.6309(19) 0.8757(19)

0.5309(16)

0.7338(17)

0.6843(15)

0.6286(18)

0.5280(19)

Se Au

0.8828(12) 0.5156(19)

Figure 12. Small (2-3 cm) 

quartz-carbonate-tourmaline vein 

in floor of trench 2A, Bochinche 

area, Estado Bolivar, Venezuela.
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N21

Table 16. Correlation coefficients for silicified host-rock samples from trench 2A, Bochinche area, Venezuela.

[Matrix shows significant correlations at or greater than the 95 percent confidence level. Number of sample pairs used to calculate the correlation

are shown in parentheses]

Mg Co Cr Pb Sc Sr Zr

Fe

Mg

Mn

Ba

Be

Co

Cr

Cu

Ni

Pb

Sr

V

Y

Zr

Na

Zn

Te

0.8768(10) 

-0.8308(9)

0.7668(8)

0.7390(10) 

0.7676(10)

0.9147(9)

0.7640(10) 

0.8419(10)

0.9109(10)

0.8063(9)

0.7744(9)

0.9030(10) 

0.8259(10)

Na Ga Cd Zn Hg Tl Au

Fe

Mg

Mn

Ba

Be

Co

Cr

Cu

Ni

Pb

Sr

V

Y

Zr

Na

Zn

Te

0.9147(6)

0.8559(6)

0.9756(6)

0.8758(8) 0.9213(10)

-.9803(6)

0.7011(9)

0.8990(6)

0.7446(9)

0.8433(8)

0.9844(8)

0.9230(7)

camp was moved to near heliport 11,3 km to the west, and 

the remainder of the fieldwork was done from this camp.

The central part of zone 4 was chosen for study on the 

basis of previous work consisting of geology, analysis of 

panned concentrates and soils, and examination of rocks 

from the area, all of which indicate the presence of 

quartz-carbonate-tourmaline veins. Rock exposure, although 

not abundant, was sufficient, especially in the small streams, 

to provide an indication of rock types present in the area. The 

exposures of the quartz-carbonate-tourmaline veins were 

excellent because they are more resistant to weathering than 

the surrounding schistose host rocks.

The purpose of our study was to establish the presence 

of the quartz-carbonate-tourmaline veins, sample the veins 

and surrounding host rocks for chemical analysis, and 

prepare a geologic map of the central area showing structures and the location and extent of the quartz-carbonate-tourmaline veins. This study also helped to determine 

exploration techniques and methodology for gold-bearing

quartz-carbonate-tourmaline veins in deeply weathered, 

highly vegetated tropical terranes.

GEOLOGY

Zone 4 is on the southwest side of a large northwest-trending structural zone that separates rocks of the 

Pastora Supergroup on the northeast from younger rocks of 

the Botanamo Group to the southwest (see Sidder and Mendoza, this volume). The rocks in zone 4 consist of metavolcanic and metasedimentary rocks intercalcalated with 

metalavas of intermediate composition. These rocks have 

been intruded by a large diabase-gabbro sill and many small 

diabase dikes (Acosta, 1987). In the central part of zone 4 is 

a layered schist-phyllite unit metamorphosed to upper greenschist facies. Quartz-carbonate-tourmaline veins cut the foliation of the host rocks at an oblique angle and are offset by 

at least one northwest-trending fault. Much of the geology is
N22 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 17. Correlation coefficients for host-rock samples from trench 2A, Bochinche area, Venezuela.

[Matrix shows significant correlations at or greater than the 95 percent confidence level. Number of sample pairs used to calculate the correlation are

shown in parentheses]

Ti Mn B Co

Ba 

Fe 0.6579(26) 0.4903(24) 

Mg 0.7237(17) 

Ti 

Mn 

B 

Ni 0.4483(22) 

Co 

Cr 

Cu 

Pb 

Sc 

Sr 

Zr 

Na 

Ga 

Cd 

Hg 

Se

Zr Na Ga Cd

Ba 

Fe 

Mg 0.4567(26) 0.5040(26) 

Ti 

Mn 0.4289(26) 0.4448(26) 0.6263(14) 

B 

Ni 

Co 

Cr 

Cu 

Pb 0.4346(26) -0.4195(26) -0.5355(26) 

Sc 

Sr 0.4495(25) 0.4480(25) -0.8950(15) 

Zr 0.4190(26) -0.7183(16) 

Na 0.4161(26) 

Ga -0.6185(16) 

Cd 

Hg 

Se

Cr Cu V Pb Sc Sr

-0.4620(26) 

0.53344(26) 0.4663(26) 0.5281(26)

0.5076(26) -0.4898(26) 0.5606(25) 

-).5959(17) 

0.4704(26) 

0.6628(22) 

0.5222(26) 0.6020(26)

-0.5847(25)

Zn Se Te TI Au

0.5128(26) 

-0.8078(8) 

0.5566(23) 0.6571(24) 

-0.8566(8)

0.5871(16) 

0.6716(20) 

0.4398(25)

0.4404(25) 

-0.9998(8) 

0.5178(24) 

0.6289(23) 

-0.9074(8) 0.4284(26) 

0.9886(6) -0.5826(16) 

0.8000(7) 

0.4730(22)

obscured by alluvial fill in the areas of low relief and by 

siliceous iron oxide material ("kanga") that forms over many 

of the higher hills. In this study we concentrated on the location and extent of the quartz-carbonate-tourmaline veins 

because these had the highest potential for gold mineralization.

There are numerous signs of past placer activity in the 

area, but by all indications the placer activity is more than 20 

years old. In some places holes as much as 15 m in diameter 

and 2 m in depth go to bedrock; that is, to the interface 

between the zone of weathering and laterized host rock. Host 

rock for the quartz-tourmaline veins is a deeply weathered 

quartz-mica schist-phyllite of volcanic or volcaniclastic protolith that contains zones of oxidized pyrite and pods and

stringers of massive white quartz, commonly with sericite 

on fracture surfaces. This quartz was barren where sampled 

and showed no visible sign of tourmaline nor any indication 

of gold. This quartz is interpreted to be part of the 

schist-phyllite, perhaps a metamorphic segregation, and not 

related to the younger, mineralized quartz-tourmaline veins. 

Detailed mapping of the quartz-tourmaline veins established a strike length of more than 2 km and widths of as much 

as 10 m (fig. 18). The veins appear to be dipping steeply to 

the south. Field observations indicate that these veins are 

zoned, in some places for as much as 30 m. This zonation 

consists of an outer zone of sugary textured quartzite and 

abundant small (<1 mm) magnetite crystals and common 

pyrite. Adjacent to the quartz-carbonate-tourmaline veins
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N23

0.1

0.01

i i i i i i i i i i i

0.001 I I I I I I I I I I I I I I I I I I I I I I I 

0 50 100

DISTANCE, IN METERS

EXPLANATION

Silicified host rock Schist Quartz vein

Figure 13. Plot of gold values for sample collected across the vein system at trench 2A, Bochinche area, Estado Bolivar, Venezuela. 

Generalized geologic section is shown below the plot.
N24

5,000

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

100 150

DISTANCE, IN METERS

EXPLANATION

Silicified host rock Schist

 

Quartz vein 

_

Figure 14. Resistivity measured by a VLF-EM receiver along trench 2A, Bochinche area, Estado Bolivar, Venezuela. Trench is approximately perpendicular to the strike of the gold-bearing quartz-carbonate-tourmaline veins.

this quartzite is microvesicular and dominated by boxwork 

quartz veins and some tourmaline. It is speculated that the 

microvesicular nature of the quartz veins is caused by dissolution of carbonate and (or) magnetite. These boxwork veins 

are as wide as 2-3 cm but more commonly are 3-5 mm wide. 

This quartzite may represent a silicified zone around the 

quartz-carbonate-tourmaline veins and may originally have

been the schist-phyllite host rock. Observations from trench 

Tl at Bochinche indicate that similar characteristics may be 

present on a much smaller scale next to quartz-carbonate-tourmaline veins. Examination of drill core from 

Bochinche that cuts the quartz-carbonate-tourmaline veins 

indicates that at depth the veins contain carbonate minerals, 

but no carbonate minerals were observed in the surface
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N25

100

DISTANCE, IN METERS

Silicified host rock

EXPLANATION

 ' --* """i*' 

llM^M^iy Schist

150

Quartz vein

Figure 15. Phase angle measured by a VLF-EM receiver along trench 2A, Bochinche area, Estado Bolivar, Venezuela. Trench is approximately perpendicular to the strike of the gold-bearing quartz-carbonate-tourmaline veins.

Figure 16. North side of road 

cut approximately 500 m northeast of Bochinchita, Estado 

Bolivar, Venezuela. Outcrop of 

schist and greenstone host rock is 

in right center of photograph. 

Small (3-4 cm) quartz-carbonate-tourmaline vein is near white 

sample bag.
N26 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 18. Element associations for a six-factor R-mode factor

analysis for rock samples from trench 2A, Bochinche area,

Venezuela.

[In decreasing order of factor loadings. Parentheses indicate loadings

that are not decreasing in order]

Ga

Na

Ti

Sc

Zr

Fe

Sr

Y

V

Tl

Be

Mn

Ni

F

Co

Cu

Hg

Se

Au

Zn

B

Mg

Pb

Te

Cd

Ba

(Cu)

Factor 1 Factor 2

0.97

0.93

0.92

0.92

0.89

0.88

0.88

0.81

0.75

0.69

0.65

0.60

0.54 (0.71)

0.53

0.82

0.79

0.67

054

Factor 3 Factor 4 Factor 5 Factor 6

(0.46)

(0.41)

0.89

0.82

0.87

0.77

0.79

0.68

0.83

0.67

0.41

exposures of the veins at zone 4. This lack of carbonate 

minerals may be the result of deep weathering; irregular cavities observed in the quartz veins in many places may represent locations of former carbonate minerals. The quartzite 

and the boxwork quartz veins from zone 4 were also sampled.

GEOCHEMICAL SAMPLING AND 

ANALYSIS

During this study 33 rock and 7 panned-concentrate 

samples from streams draining the area were examined. In 

addition, 80 panned-concentrate samples that had been collected in previous TECMIN studies in zone 4 were analyzed 

(appendix tables 6, 7). Sample locations for rock and 

panned-concentrate samples are shown in figures 19 and 21, 

respectively. Univariate statistics and Cohen-corrected geometric means and deviations for vein-rock samples are given 

in table 22, and correlations are given in table 23.

Although significant amounts of gold were not detected 

in the rock samples, gold was detected in each rock sample 

taken. Twenty-three of the rock samples were collected from 

quartz-carbonate-tourmaline veins; gold values ranged from 

less than 0.002 to 0.25 ppm. These results indicate that even 

at the surface of this deeply weathered terrane detectable 

gold remains in the quartz-carbonate-tourmaline veins.

Panned-concentrate samples were taken from small 

streams draining the Marwani zone 4 area (fig 21). Analytical results from the panned-concentrate samples are shown 

in appendix table 7.

Table 19. Eigenvalues for the six-factor R-mode factor analysis for all rock samples from trench 2A, Bochinche area,

Venezuela.

[Six-factor element associations are given in table 18]

Number 

of factors

1

2

3

4

5

6

7

8

9

10

Eigenvalue

11.5899

3.3407

2.2363

1.8977

1.2940

1.0465

0.9386

0.8201

0.7289

0.6204

Cumulative 

percent

0.4293

0.5530

0.6358

0.7061

0.7540

0.7928

0.8275

0.8579

0.8849

0.9079

Number 

of factors

11

12

13

14

15

16

17

18

19

20

Eigenvalue

0.4910

0.3797

0.3360

0.2428

0.2091

0.1586

0.1279

0.1054

0.0873

0.0789

Cumulative 

percent

0.9261

0.9401

0.9526

0.9616

0.9693

0.9752

0.9799

0.9838

0.9871

0.9900
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N27

i i i i i i i i i i i i i i i i i i i i I I i i i i i i i i i i i i i i

0.01

0.001

10 20 

DISTANCE, IN METERS

30 40

Quartz vein with tourmaline

EXPLANATION

Greenstone Schist

Figure 17. Plot of gold values across the quartz veins exposed in a road cut approximately 500 m northeast of Bochinchita, Estado Bolivar, 

Venezuela. Generalized geologic section below the plot shows locations of quartz-carbonate-tourmaline veins.
N28 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

Table 20. Univariate statistics for host- and vein-rock samples from a road cut near Bochinche, Venezuela. 

[In parts per million unless otherwise noted. All analyses are by semiquantative spectrographic method except for Cd, Zn, Hg, Se, Te, Tl, and Au, 

which were done by atomic absorption method. L, detected but below lower limit of determination; N, not detected at lower limit of determination; 

G, greater than upper limit of determination]

Ranee

Element

Ba

Fe% 

Mg% 

Ca 

Ti% 

Mn 

B

Ni

Be

Co

Cr

Cu

V

Pb

Sc

Y

Zn

Zr

Na%

Ga

Cd

Zn

Hg

Se

Te

Tl

Au

Minimum

30 

0.5 

0.2 

0.05 

0.02 

70 

20

5

1

15

10

7

100

10

5

20

200

10

0.2

5

0.3

6

0.02

0.1

0.05

0.05

0.002

Maximum

2,000 

7 

3 

10 

0.5 

1,500 

1,500

200

2

500

700

500

200

20

30

70

700

150

1.5

30

1.2

970

0.28

0.4

0.35

0.1

0.03

Geometric

Mean Deviation

118 3.1 

2.8 2.9 

0.8 2.5 

0.2 6.4 

0.1 2.9 

451 2.6 

164 4.5

42 4.0

1.4 1.4

49 3.0

107 4.1

49 4.0

142

14

16

47

402

25 :

0.5 :

14

0.6

.3

.3

.8

.6

.7

-.5

,.0

.9

.6

64 7.5

0.04 2.4

0.2 2.2

0.1 2.0

0.08 1.5

0.004 2.5

Number of samples

Valid

12 

12 

12 

7 

12 

11 

7

11

5

11

11

12

12

12

12

7

5

12

8

9

7

12

7

4

7

3

11

L

0 

0 

0 

5 

0 

0 

0

1

6

1

1

0

0

0

0

0

2

0

1

1

5

0

0

7

1

1

1

N

0 

0 

0 

0 

0 

0 

2

0

1

0

0

0

0

0

0

5

5

0

3

2

0

0

5

0

4

8

0

G

0 

0 

0 

0 

0 

1 

3

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Cohen

Mean

118 

2.83 

0.810 

0.159

574

33.3

0.873

40.8

82.6

48.6

142

14.1

16.6

16.9

167

24.7

0.309

9.63

0.190

64.5

0.022

0.061

0.073

0.029

0.004

Deviation

3.15 

2.89 

2.51

6.42

3.39

4.75

1.73

3.35

4.99

3.98

1.27

1.32

1.85

3.86

2.52

2.46

2.80

2.49

4.82

7.52

3.15

3.35

3.71

2.20

2.65

Table 21. Correlation coefficients for host- and vein-rock samples from a road cut near Bochinche, Venezuela.

[Matrix shows significant correlations at or greater than the 95 percent confidence level. Number of sample pairs used to calculate the correlation is

shown in parentheses]

Ti

Ba

Fe 0.9245(12)

Mg 0.5829(12)

Ti

Mn

Ni

Co

Cr

Cu

V

Zr

Ga

Cd

Mn Ni

0.6677(11) 0.6623(11)

0.6775(11) 0.7668(11)

0.6505(11)

0.7241(11) 0.7897(11)

Co Cr Cu

0.9658(11) 0.9406(11)

0.7118(11)

0.8780(11)

0.6089(1 1)

0.6614(10) 0.7805(11)

0.9559(11)

Sc

0.7940(12)

0.7225(12)

0.6678(12)

0.7465(1 1)

0.7734(12)

Zr Ga

0.6284(12) 0.7739(9)

0.8589(12) 0.8322(9)

0.7777(9)

0.9195(11)

0.7003(9)

0.6924(9)

Zn

0.8322(9)

0.9246(11)

0.7460(12)

0.7882(9)

0.8189(7)
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS

7°00'

EXPLANATION

Qal

Esq

Man

Mar

Mga

Can

Alluvium 

Schist

Meta-andesite 

Meta-arenite 

Metagabbro 

Canga

N29

60°35'

Contact 

Fault

Quartz vein 

Silicified zone 

Visible gold 

Heliport 

Mining activity

Figure 18. Schematic geologic map, zone 4, Marwani concession, Estado Bolivar, Venezuela. Gold bearing quartz-carbonate tourmaline 

veins, ejes (control lines cut through the jungle), picas (traverse lines cut through the jungle perpendicular to ejes at regular intervals), old 

mine locations, and helicopter landing sites are also shown. Modified from Jeffery Wynn (written commun., 1989).
N30

60°42' 

7°03'

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

60°41' 60°40' 60°39'

MW008R 

MW013R

MW014R, MW015R 

MW009F 

MW010R 

MW024R 

MW023R

MW005R 

MW001R

7°02' -

MW0303R 

MW033R A

MW032R

1 KILOMETER

J

7°or

Figure 19. Map showing rock sample locations, zone 4, Marwani concession, Estado Bolivar, Venezuela. Sample numbers are 

plotted near triangles showing location; elemental concentrations for the samples are given in appendix table 6.

Figure 20. Typical geology 

field camp near Esperanza along 

the main road between Tumeremo and Bochinche, Estado Bolivar, Venezuela.
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N31

Table 22. Univariate statistics for vein-rock samples from zone 4 of the Marwani concession, Venezuela.

[All values in parts per million unless otherwise noted. All analyses are by semiquantative spectrographic method except for Cd, Zn, Hg, Te, and 

Au, which were done by atomic absorption method. L, detected but below lower limit of determination; N, not detected at lower limit of 

determination; G, greater than upper limit of determination]

Ranee

Element

F

Ba

Fe%

Mg%

Ti%

Mn

B

Ni

Co

Cr

Cu

V

Pb

Sc

Y

Zr

Na%

Ga

Cd

Zn

Hg

Te

Au

Minimum

0.01

20

0.15

0.02

0.002

10

10

5

10

10

5

10

10

5

10

10

0.2

5

0.1

2

0.02

0.05

0.002

Maximum

0.04

200

20

1.5

0.3

5,000

2,000

50

100

150

50

150

20

20

100

150

1.5

70

3.1

270

0.08

4.2

0.25

Geometric

Mean

0.02

33

1.8

0.19

0.03

36

164

14

16

37

16

33

12

11

18

22

0.5

13

0.4

9.6

0.03

0.2

0.009

Deviation

1.8

1.7

3.7

3.4

4.5

3.7

6.1

2.1

2.3

2.7

2.1

2.4

1.3

1.7

2.4

2.4

2.0

2.8

2.7

5.3

2.2

3.7

4.8

Number of samples

Valid

6

27

34

26

29

27

20

18

7

16

17

34

16

13

6

24

11

13

12

17

3

16

21

L

28

3

0

8

5

7

3

15

9

15

8

0

17

3

1

8

2

14

22

17

31

18

13

N

0

4

0

0

0

0

3

1

18

3

9

0

1

18

27

2

21

7

0

0

0

0

0

G

0

0

0

0

0

0

8

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Cohen

Mean

0.003

27.4

1.77

0.087

0.016

22.4

-

5.48

3.42

8.89

5.27

33.1

9.25

3.75

2.53

14.1

0.109

2.91

0.048

1.66

0.031

0.040

0.003

Deviation

3.35

1.84

3.67

6.03

6.80

4.59

-

0.44

3.37

5.40

3.90

2.44

1.46

2.80

4.15

2.90

3.79

4.78

7.39

10.6

2.23

7.59

7.84

Table 23. Correlation coefficients for vein-rock samples from zone 4 of the Marwani concession, Venezuela.

[Matrix shows significant correlations at or greater than the 95 percent confidence level. Number of sample pairs used to calculate the

correlation is shown in parentheses.]

Ba 

Fe

Mg 

Ti

Ni 

Te

Ti

0.6635(22) 

0.5412(23)

Mn Ni

0.7894(21) 

0.6473(25) 0.8155(16)

Pb Sc Zr Cd Te

0.7400(12) 0.5926(24) 0.9204(12) 

0.6430(21) 

0.7347(13) 

0.8192(9)

Au

0.6963(12)

MINERALOGY

All of the nonmagnetic fraction of each panned-concentrate sample was examined under a microscope for mineralogy. A total of 87 samples were taken, and 59, or 68 

percent, contained visible gold in amounts from 1 to 43 

grains. In general, the grain size was less than 1 mm. All 

samples also contained rutile, in some cases as much as 50 

percent of the sample.

The rutile is in thin acicular crystals, many exhibiting 

the typical knee-shaped twinning. Other common minerals 

are zircon, apatite, tourmaline, and the zinc spinel ghanite.

All of the observed minerals, with the exception of the gold 

and tourmaline, probably reflect the metavolcanic host rocks. 

The central part of zone 4 is in greenstone terrane that is 

permissive for the occurrence of low-sulfur gold-quartz 

veins. Results from our study indicate that this area contains 

(1) quartz-carbonate-tourmaline veins related to northeast-trending dislocations, (2) offsetting northwest-trending 

faults, (3) zones of silicification around the quartz-tourmaline 

veins, and (4) gold, commonly abundant, in small streams 

that drain the areas of quartz-tourmaline veins. On the basis 

of these results, there is a high probability of the occurrence 

of lode gold deposits similar to those in the greenstone belts 

at Lo Increfble, El Callao, and Bochinche.
N32 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

60°46'

7°04'

60°38'

7°00'

6°56'

MCH008 n

MCH001,-, D MOC100 

r-,MOC105A

MOC004 n MOC097

H

MW009CQ

M' 

MOC090!-!

MOC109D

D MCH006 

MW002C 

D MCH005

MCH003

MOC021

MOC20

MOC087 

rO MOC008 MW011C MOC082

|-j

MCH014 MCH014

MCH022

MCH025

MCH026 nn MCH°27 

g MCH028

n MOC061

n MOC059

MCH016

MWO13C

MOC011

MCH024

1 MOC039

 , MOC005

MOC012

MCH035D

MCH030 

MCH032 D

D MCH037

MCH047D n MCH°48 D MCH038 

MCH072 D D MCH045 DMCH040

g MCH061

MCH055

MCH044 Q MCH036 

D MCH049 MCH042

D MCH073 

um 1 I l Ml^n(Jb4 H071 M°H050

MCH069 U

MCH055 D MCH062D 

n MCH051

_ MC.unxi

MCH052 0

o

D MCH065

MCH066n

Dn OMCH070

u MCH067 

DMCH063

2 KILOMETERS

MCH059

MCHO53

Figure 21. Map showing panned-concentrate sample locations, zone 4, Marwani concession, Estado Bolivar, Venezuela. Sample numbers are plotted near squares showing location; elemental concentrations for samples are given in appendix table 7.
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N33

GEOCHEMICAL AND GENETIC

MODEL FOR GOLD-BEARING

QUARTZ-CARBONATE-TOURMALINE

VEINS IN VENEZUELA

Work done by Colvine and others (1984) indicates that 

much of the world's gold has come from quartz veins in 

Archean greenstone belts similar to the greenstone belts in 

southeastern Venezuela. Gold in quartz veins in regionally 

metamorphosed metavolcanic rocks in the Precambrian 

greenstone belts in southeastern Venezuela is typical of the 

low-sulfide gold-quartz vein model (Berger, 1986; Bliss and 

Jones, 1988). Some Venezuelan gold-quartz veins contain 

tourmaline crystals, and the high boron content and associated tourmaline factor discussed in the R-mode factor analyses for Bochinche trench 2A support this observation. Many 

of veins contain measurable zinc and copper but generally in 

amounts only equal to or less than those in the analyzed host 

rocks. According to the model, amounts of lead and silver 

should be anomalous, but in the Venezuelan gold-bearing 

quartz-carbonate-tourmaline veins that we studied these elements are generally near or below the standard detection limits of the method used for analysis. Arsenic, considered the 

best pathfinder for this type of deposit, is present in samples 

collected underground at Mina Colombia at statistically significant levels and in the Mina San Pedro open pit at low levels, but it is present in only a few samples from the other 

locations studied. Many elements in the quartz-carbonate-tourmaline veins are depleted, and only gold and manganese and possibly boron are enriched. It may be that tropical 

weathering has leached away elements that could be used to 

target these deposits at the surface. This is likely because in 

this study the chemistry of the quartz-carbonate-tourmaline 

gold-bearing veins varies little between the surface, the 

near-surface, and depth (300 m or more) where the rocks are 

relatively fresh.

Bliss and Jones (1988) compiled reported mineralogical data from low-sulfide gold-quartz veins and ranked the 

frequency of occurrence reporting of sulfide minerals, which 

form less than 5 volume percent in this type of deposit. 

About 89 percent of the deposits contain pyrite; galena, arsenopyrite, and chalcopyrite are present in more than half of 

the deposits. Sphalerite, pyrrhotite, stibnite, tetrahedrite, and 

scheelite are present in 39, 22, 11, 9, and 7 percent of the 

deposits, respectively. These figures imply that for about 

half of the deposits the geochemical signature of the ore 

should contain iron, lead, arsenic, and copper and, less commonly, zinc, antimony, silver, and tungsten contained in sulfide minerals. Based on frequency of occurrence of the 

gangue minerals calcite (10 percent), ankerite (6 percent), 

mariposite (5 percent), sericite (4 percent), chlorite (4 percent), albite (3 percent), biotite, tourmaline, dolomite, rhodochrosite, and talc (a combined total of 2 percent), and 

ilmenite, barite, rutile, and siderite (a combined total of 1

percent), the geochemical elemental signatures could 

include elements such as manganese, boron, titanium, chromium, magnesium, and calcium. No one deposit includes a 

complete suite of either ore or gangue minerals, and thus the 

geochemical signature could be quite irregular.

In low-sulfide gold-quartz veins in Estado Bolivar, the 

base metals copper, lead, and zinc, iron, boron, and manganese are a consistent part of a geochemical signature. Thus, 

these veins meet much of the geochemical criteria for 

low-sulfide gold-quartz veins.

Based on grade and tonnage information compiled to 

1974 (Rodriguez, 1986), low-sulfide gold-quartz veins in the 

Guayana Shield fit the tonnage and grade model presented 

by Bliss (1986) for this deposit type. Reported tonnages 

range from 1,666 metric tons at Carmen Rosa to 1,588,428 

metric tons at Laguna, and average grades range from 32.7 

gram per metric ton at Botanamo to 10.8 grams per metric 

ton at Union San Serfin. Reported production within 0.5 km 

of Mina Colombia is 429,442 metric tons (to 1974) at an 

average grade of 11.3 grams per metric ton, and production 

at Lo Increible is 45,341 metric tons at 12.2 grams per metric 

ton. Thus, in the Bliss (1986) tonnage and grade model Mina 

Colombia is above the 80th percentile in tonnage and below 

the 40th percentile in grade, and Lo Increible is above the 

50th percentile in tonnage and below the 50th percentile in 

grade.

CONCLUSION

Reconnaissance geochemical sampling of the gold 

bearing quartz-carbonate-tourmaline veins in Estado Bolivar, Venezuela, in surface, near-surface, and subsurface 

environments indicates that the chemistry of these veins does 

not vary significantly. Field observation, chemical analysis, 

and statistical analysis indicate that the veins are low sulfide 

and gold bearing. Statistical analysis of the chemical data 

defines a host-rock suite of elements, a base-metal suite of 

elements, and gold. The strong lack of correlation between 

gold and other elements indicates that gold is present as elemental gold. Only in two cases is the gold associated with 

tellurium, indicating a possibility of gold tellurides; these 

occurrences are in the surface and near-surface environments where alteration is the most pronounced. Carbonate 

minerals were observed only in samples from depth, either 

from mines or from drill core. Most commonly, the carbonate minerals have been weathered from the veins leaving 

irregular cavities containing particulate gold. Copper, lead, 

and zinc are present in the veins at very low levels and probably represent remobilization of these elements from the host 

metavolcanic rocks. The solutions forming the veins apparently did not carry base metals and were probably not chloride brines. The veins are depleted in most elements except 

gold and, in some cases, manganese and boron. The solutions forming the veins were probably carbonate bearing
N34 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

(calcium, magnesium, strontium, and barium) and contained 

gold, boron, and manganese. The manganese content of the 

veins is relatively constant except in the surface environment 

where weathering has removed most of the manganese.

As part of this study, several exploration techniques 

were used, and four were particularly useful. Before going 

into the dense jungle of zone 4 of the Marwani concession, 

aeromagnetic maps and side-looking airborne radar (SLAR) 

imagery were examined. From the SLAR imagery linear 

topographic features were identified that might indicate 

resistant quartz-vein systems. Field checking of several of 

these features showed that this was a valid exploration 

approach, especially in the initial stages of exploration. 

Interpretation of aeromagnetic maps of the area prior to 

fieldwork identified faults and offsetting linear features. 

These faults were confirmed during field examination of 

zone 4, and their location made mapping of the discontinuous and offset quartz veins much easier. Once in the field, the 

gold pan was the most effective tool for determining the 

extent of the gold-bearing quartz-carbonate-tourmaline 

veins. In the tropical weathering environment carbonate in 

the veins weathers, freeing particulate gold, which then 

accumulates at the clay-sediment interface in the streams 

draining the area. Because the areas examined are of generally low relief (50-100 m), the distribution of the gold in sediments is proximal to the quartz-carbonate-tourmaline veins. 

The very low frequency electromagnetic (VLF-EM) survey 

done along trench 2A near Bochinche indicates that this 

technique may also be useful as an exploration tool. The 

quartz-carbonate-tourmaline veins were recorded as resistive highs as compared to background. An effective exploration procedure would be to select potential areas from 

interpretation of SLAR imagery and aeromagnetic maps, 

field check using VLF-EM and the gold pan, and then trench 

the most promising sites and collect rock samples for 

geochemistry. Very little of the greenstone terrane in the jungle-covered Guayana Shield has been explored, especially 

with modern techniques, and there is a high potential for the 

occurrence of more of these gold-bearing quartz-carbonate-tourmaline vein systems.

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Acosta, J., 1987, Informe geologico, Zona Marwani: Informe Interno, PPGZF, 24 p.

Arbogast, B.F., ed., 1990, Quality assurance manual for the Branch 

of Geochemistry, U.S. Geological Survey; U.S. Geological 

Survey Open-File Report 90-688, 184 p.

Benaim, Nesin, 1974, Geologia de la region El Dorado-Anacoco-Botanamo, Estado Bolivar: Novena Conferencia Geologica 

Inter-Guayanas, Ciudad Guayana, Venezuela, 1972, Memoria, 

Publication Especial 6, p. 198-206.

Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz 

veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit 

models: U.S. Geological Survey Bulletin 1693, p. 239.

Bliss, J.D., 1986, Grade and tonnage model of low-sulfide 

Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral 

deposit models: U.S. Geological Survey Bulletin 1693, 

p. 239-243.

Bliss, J.D., and Jones, G.M., 1988, Mineralogical and grade 

tonnage information on low-sulfide Au-quartz veins: U.S. 

Geological Survey Open-File Report 88-229, 99 p.

Colvine, A.C., Andrews, A.J., Cherry, M.E., Durocher, M.E., Fyon, 

A.J., Lavigne, Jr., M.J., Macdonald, A.J., Maramont, Souissan, 

K.H., Poulsen, K.H., Springer, J.S., and Troop, D.G., 1984, An 

integrated model for the origin of Archean lode gold deposits: 

Ontario Geological Survey Open-File Report 5524, 98 p.

Crock, J.G., and Lichte, F.E., 1982, The determination of rare-earth 

elements in geological materials by inductively coupled argon 

plasma-optical emission spectroscopy: Analytical Chemistry, 

v. 54, p. 1329-1333.

Crock, J.G., Lichte, F.E., and Briggs, P.E., 1983, Determination of 

elements in National Bureau of Standards geological reference 

materials SRM 278 obsidian and SRM 688 basalt by inductively coupled argon plasma-atomic emission spectroscopy: 

Geostandards Newsletter, v. 7, no. 2, p. 335-340.

Day, W.C., Martfnez, L.F., and Quintana, Enot, 1989, Bedrock 

geology and geochemistry of the Anacoco Sur II area, Bolivar 

State, Venezuela: U.S. Geological Survey Open-File Report 

89-0305, 14 p.

Gibbs, A.K., 1980, Geology of the Barama-Mazaruni Supergroup 

of Guayana: Cambridge, Mass., Harvard University, Ph.D. 

dissertation, 385 p.

Gibbs, A.K., and Olszewski, W.J., Jr., 1982, Zircon U-Pb ages of 

Guayana greenstone-gneiss terraine: Precambrian Research, v. 

17, p. 199-214.

Grimes, D.J., and Marranzino, A.P., 1968, Direct-current arc and 

alternating current spark emission spectrographic field 

methods for the semiquantitative analysis of geologic 

materials: U.S. Geological Survey Circular 591, 6 p.

Gruau, G., Martin, H., Leveque, B., Capdevila, R., and Marot, A., 

1985, Rb-Sr and Sm-Nd geochronology of Lower Proterozoic 

granite-greenstone terrains in French Guiana, South America: 

Precambrian Research, v. 30, p. 63-80.

Hopkins, D.M., 1977, An improved ion-selective electrode method 

for the rapid determination of fluorine in rocks and soils: U.S. 

Geological Survey Journal of Research, v. 5, p. 589-593.

Hubert, A.E., and Chao, T.T., 1985, Determination of gold, indium, 

tellurium, and thallium in the same sample digestion of 

geological materials by atomic-absorption spectroscopy and 

two-step solvent extraction: Talanta, v. 32, p.

Klar, Giampaolo, 1979, Geochronology of the El Manteco-Guri 

and Guasipati areas, Venezuelan Guiana Shield: Cleveland, 

Case Western Reserve University, Ph.D. dissertation, 163 p. 

[Dissertation Abstracts International, Sec. B, Sciences and 

Engineering, v. 39, p. 5284-B.]

Korol, Bohdan, 1965, Estratigrafia de la Serie Pastora, region Guasipati-El Dorado: Ministerio de Minas e Hidrocarburos, 

Boletin de Geologia, v. 7, no. 13, p. 3-18.

Lichte, F.E., Meier, A.L., Crock, J.G., 1987, Determination of the 

rare-earth elements in geological materials by inductively
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N35

coupled plasma mass spectrometry: Analytical Chemistry, 

April 15, 1987, p. 1150-1157.

Menendez V. de V., Alfredo, 1968, Revision de la estratigrafia de 

la Provincia de Pastora segun el estudio de la region de 

Guasipati, Guayana Venezolana: Ministerio de Minas e 

Hidrocarburos, Boletfn de Geologfa, v. 10, no. 19, p. 309-338.

Motooka, J.M., and Grimes, D.J., 1976, Analytical precision of 

one-sixth order semiquantitative spectrographic analysis: U.S. 

Geological Survey Circular 738, 25 p.

O'Leary, R.M., and Meier, A.L., 1984, Analytical methods used in 

gee chemical exploration: U.S. Geological Survey Circular 

948, 48 p.

Rodriguez M., S.E., 1986, Recursos minerales de Venezuela: 

Caracas, Ministerio de Energfa y Minas, Direction General 

Sectorial de Minas y Geologfa, Boletin de Geologfa, v. 15, 

no. 27, 228 p.

Sanchez, H., 1986, Informe de advance trabajo de campo de 

Marwani: Informe Interno PPGZF.

 1987, Planification trabajo de Campo Zona de Marwani:

Informe Interno PPGZF.

VanTrump, George, Jr., and Miesch, A.T., 1976: The U.S. 

Geological Survey RASS-STATPAC system for management 

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Geosciences, v. 3, p. 475-488.

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Tecnica Minera, C.A., 1993, Geology and mineral resource 

assessment of the Venezuelan Guayana Shield: U.S. 

Geological Survey Bulletin 2062,121 p.

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methods of separation for optical emission, atomic absorption 

spectroscopy and colorimetry: U.S^ Geological Survey 

Bulletin 1770, p. D1-D14.

APPENDIX 

DATA TABLES

The designation S on element headings indicates that analysis was done using semiquantitative emission spectrography; the designation /p on element 

headings for As, Bi, Cd, Sb, and Zn indicates that analysis was done using inductively coupled plasma-atomic emission spectrometry (ICP-AES) following 

a partial sample digestion with hydrochloric acid/hydrogen peroxide and solvent extraction of the metals as tertiary amine-halide complexes. Te and Tl were 

analyzed by graphite furnace atomic absorption spectroscopy (AA); Hg was analyzed by continuous-flow cold-vapor atomic absorption spectrophotometry 

(CVAA); Au was analyzed by graphite furnace atomic absorption spectrophotometry (AA) following sample digestion with hydrobromic acid-bromine and 

solvent extraction; and F~ was analyzed using ion-selective electrode potentiometry. N indicates that an element was looked for but not observed; L indicates 

that an element was observed but was below the lowest reporting value, and the lower limit of determination is given; G indicates that an element was 

observed but was above the upper reporting value, and the upper limit of determination is given; and B indicates that no analysis was obtained. Values 

determined for the major elements Ca, F, Fe, Mg, Na, P, and Ti are given in weight percent; all other values are in parts per million (micrograms/gram). 

Detailed descriptions of analytical methods are given in Arbogast (1990).
Appendix table 1. Rock samples collected from Mina Colombia, El Callao, Venezuela. 

MC001R

MC002R

MC003R

MC004R

MC005R

MC007R

MC008R

MC009R

MC010R

MC011R

MC012R

MC013R

MC001R

MC002R

MC003R

MC004R

MC005R

MC007R

MC008R

MC009R

MC010R

MC011R

MC012R

MC013R

MC001R

MC002R

MC003R

MC004R

MC005R

MC007R

MC008R

MC009R

MC010R

MC011R

MC012R

MC013R

Latitude

7°22'30"

7°22'30"

7°22'30"

7°22'30"

7°22'30"

7°22'30"

7°22'30"

7°22'30"

7°22'30"

7°22'30"

7°22'30"

7°22'30"

Co-S

ION

70

100

70

30

100

ION

ION

30

30

30

30

W-S

20N

20N

20L

20N

20L

70

20N

20N

20N

20N

20N

20N

Longitude Ca%-S

61°56'00" 1.5

61°56'00" 3

61°56'00" 5

61°56'00" 7

61°56'00" 7

61°56'00" 3

61°56'00" 0.1

61°56'00" 0.7

61°56'00" 7

61°56'00" 7

61°56'00" 7

61°56'00" 3

Cr-S Cu-S

15 5L

10L 100

ION 20

10L 20

10L 30

ION 300

ION 5N

10L 5N

ION 100

ION 50

10L 20

10L 10

Y-S Zn-S

ION 200N

20 200L

30 200N

20 200L

30 200N

30 200

ION 200N

ION 200N

20 200L

30 200N

30 200L

30 200N

F%

0.01L

0.01L

0.01

0.01L

0.01L

0.01L

0.01L

0.01L

0.01L

0.02

0.01L

0.01L

Ga-S

5L

50

20

50

15

20

5N

5N

30

30

50

30

Zr-S

10L

20

50

30

50

50

ION

10L

70

70

70

50

Fe%-S

0.7

10

7

10

7

10

0.07

0.3

5

7

7

7

Ge-S

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

As/p

12

5L

34

5L

21

87

5L

5L

7

33

5

5L

Mg%-S Na%-S

0.5 0.2

1 3

1 3

1 5

0.7 1

1.5 3

0.07 0.2N

0.3 0.2N

1.5 3

1 3

0.7 1.5

1 2

La-S Mn-S

SON 300

SON 1,000

SON 1,500

SON 1,500

SON 1,000

SON 700

SON 30

SON 200

SON 1,000

SON 1,000

SON 1,000

SON 700

Au Bi/p

1.4 2L

0.002L 2L

7.8 2L

0.034 2L

7.5 2L

10 2L

0.030 2L

0.048 2L

0.002L 2L

10 2L

0.250 2L

0.03 2L

P%-S

0.2N

0.2N

0.2L

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2L

0.2N

0.2N

Mo-S

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

Cd/p

0.1L

0.6

0.3

1.3

0.4

1

0.1L

0.1L

1.4

1.3

1.8

1.9

Ti%-S

0.07

0.7

0.7

0.7

0.7

1

0.007

0.03

0.5

0.5

0.5

1

Nb-S

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

Hg

0.02

0.02

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.04

0.02N

Ag-S

0.5N

0.5N

1.5

0.5N

1

1.5

0.5N

0.5N

0.5N

7

0.5N

0.5N

Ni-S

7

10

5L

5L

5L

50

5N

5N

30

5N

5N

5L

Sb/p

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

As-S

200N

200N

200N

200N

200N

200L

200N

200N

200N

200N

200N

200N

Pb-S

10L

10

15

15

10

15

ION

10L

10

10

10

10

Te

0.05L

0.05L

0.55

0.05L

0.25

0.45

0.05L

0.05L

0.05L

0.8

0.05L

0.05L

Au-S

ION

ION

15

ION

15

ION

ION

ION

ION

70

ION

ION

Sb-S

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

Tl

0.05L

0.05L

0.05L

0.05L

0.05L

0.05L

0.05L

0.05L

0.05L

0.1

0.1

0.05L

B-S

100

ION

30

ION

50

15

10

150

ION

20

30

ION

Sc-S

7

30

30

30

20

30

5N

5N

30

20

20

30

Zn/p

20

96

50

110

40

100

2L

2L

91

110

110

91

Ba-

20 

20L 

20L 

20L 

20 

30 

20L 

20 

20 

50 

70 

20N 

Sn-

ION 

ION 

ION 

ION 

ION 

ION 

ION 

ION 

ION 

ION 

ION 

ION 

Quar

Host 

Brec

Silic

Seco

Qua

Clea

Vein 

Han

Mai

Host 

Host 
Appendix table 2. Rock samples collected from Mina San Pedro, El Callao district, Venezuela.

LI006R

LI013R

LI014R

LI020R

LI027R

LI038R

LI041R

LI044R

LI060R

LI006R

LI013R

LI014R

LI020R

LI027R

LI038R

LI041R

LI044R

LI060R

LI006R

LI013R

LI014R

LI020R

LI027R

LI038R

LI041R

LI044R

LI060R

Latitude

7°25'00"

7°25'00"

7°25'00"

7°25'00"

7°25'00"

7°25'00"

7°25'00"

7°25'00"

7°25'00"

Co-S

ION

10L

10

ION

10

10L

ION

20

10L

W-S

20N

20N

20N

20N

20

20

20N

20N

20N

Longitude

61°48'00"

61°48'00"

6r48'00"

6r48'00"

6r48'00"

61°48'00"

6r48'00"

61°48'00"

61°48'00"

Cr-S

70

50

50

10

100

70

30

30

20

Y-S

20

10

15

10L

20

20

10L

15

10L

Ca%-S

0.05L

0.05L

0.05L

0.05N

0.05N

0.05L

0.05L

0.05L

0.05L

Cu-S

30

30

30

7

50

50

20

50

30

Zn-S

200N

200N

200N

200N

200N

200N

200N

200N

200N

F%

0.02

0.02

0.01

0.01L

0.02

0.02

0.02

0.02

0.02

Ga-S

50

20

30

5L

20

30

15

20

20

Zr-S

150

100

150

30

150

200

70

150

70

Fe%-S

3

3

5

1.5

5

3

2

3

2

Ge-S

ION

ION

ION

ION

ION

ION

ION

ION

ION

As/p

47

31

76

21

95

69

25

82

56

Mg%-S

0.3

1

0.7

0.2

0.3

0.3

0.7

0.1

0.15

La-S

70

SOL

50

SOL

SOL

50

SOL

70

SOL

Au

0.012

8.3

0.044

0.6

0.1

0.014

0.018

0.016

0.15

Na%-S

2

0.7

1.5

0.2N

1.5

2

1

1

1.5

Mn-S

30

50

150

150

150

200

30

150

100

Bi/p

2L

2L

2L

2L

2L

2L

2L

2L

2L

P%-S

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2L

0.2L

Mo-S

5N

5N

5N

5N

5N

5N

5N

5N

5N

Cd/p

0.3

0.3

0.6

0.2

0.7

0.6

0.2

2.4

0.4

Ti%-S

0.7

0.5

0.5

0.1

0.5

0.5

0.3

0.3

0.3

Nb-S

20N

20N

20L

20N

20N

20N

20N

20N

20N

Hg

0.02N

0.02N

0.02

0.04

0.04

0.04

0.02N

0.04

0.04

Ag-S

0.5N

0.5N

0.5N

0.5L

0.5N

0.5N

0.5N

0.5N

0.5N

Ni-S

20

30

20

7

30

20

15

50

20

Sb/p

2L

2L

2L

2L

2L

2L

2L

2L

2L

As-S

200N

200N

200N

200N

200N

200N

200N

200N

200N

Pb-S

15

10

20

10L

20

20

10

20

15

Te

0.05

0.05

0.05

0.05L

0.1

0.05

0.05

0.05L

0.05

Au-S

ION

10L

ION

10

ION

ION

ION

ION

ION

Sb-S

100N

100N

100N

100N

100N

100N

100N

100N

100N

Tl

0.1

0.05L

0.1

0.05L

0.15

0.15

0.05

0.15

0.1

B-S

1,000

2.000G

2,000

1,500

1,500

2,000

2,OOOG

500

500

Sc-S

7

7

7

5L

10

15

5

7

5L

Zn/p

12

11

25

6

28

26

8

31

15

B

1,5

1

70

30

70

1,0

30

70

70

A

B

W

Q

S

W

B

S

W
Appendix table 3. Rock samples collected from trench Tl, Bochinche area, Venezuela.

BCOOOR

BC014R

BC016R

BC027R

BC028R

BC048R

BC063R

BC072R

BC074R

BC080R

BC082R

BC090R

BC095R

BC100R

BC114R

BC106R

BC131R

BC133R

BC141R

BC156R

BC157R

BC164R

BC189R

BC135R

BCOOOR

BC014R

BC016R

BC027R

BC028R

BC048R

BC063R

BC072R

BC074R

BC080R

BC082R

BC090R

BC095R

BC100R

BC114R

BC106R

BC131R

BC133R

BC141R

BC156R

BC157R

BC164R

BC189R

BC135R

Latitude

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

7°33'00"

Co-S

30

70

15

10

50

15

300

100

20

30

100

30

100

70

ION

ION

70

200

300

15

70

30

10L

200

Longitude

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

60°48'00"

Cr-S

500

20

100

20

50

150

70

100

150

20

200

15

500

200

10

20

10

100

100

500

200

150

150

100

Ca%-S

0.05N

0.05L

0.05L

0.05L

0.05N

0.05N

0.05L

0.05L

0.05N

0.05L

0.05L

0.05L

0.05L

0.05L

0.05L

0.05L

0.05L

0.05N

0.05L

0.05N

0.05L

0.05N

0.05L

0.05L

Cu-S

50

30

30

20

50

70

70

50

150

30

50

20

70

50

5

5

30

70

50

100

50

100

50

70

F%

0.02

0.01L

0.01

0.01L

0.02

0.01L

0.01L

0.01L

0.02

0.01L

0.02

0.01L

0.01

0.01L

0.01L

0.01L

0.01L

0.01

0.02

0.01

0.01L

0.01L

0.02

0.02

Ga-S

50

5L

50

5L

50

50

15

20

30

5

50

5N

20

15

5N

5N

5N

50

30

50

20

50

30

50

Fe%-S

7

1.5

7

3

15

7

7

7

7

3

7

0.5

10

7

0.3

0.5

1.5

7

10

7

3

7

5

5

Ge-S

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

Mg%-S

0.3

0.05

0.1

0.07

0.15

0.07

0.1

0.15

0.15

0.15

0.5

0.1

0.07

0.5

0.1

0.07

0.02

0.2

0.2

0.1

0.2

0.15

0.1

0.3

La-S

SOL

SOL

SOL

SON

SON

SOL

70

50

SOL

SOL

100

SON

100

SOL

SON

SON

SON

50

50

SOL

SOL

SON

SOL

100

Na%-S

2

0.2L

1

0.2N

1.5

0.2N

0.2

0.5

0.7

0.2L

1.5

0.2N

0.2

0.5

0.2N

0.2N

0.2N

2

1

0.7

0.5

0.7

0.3

0.7

Mn-S

300

1,500

1,000

1,500

5,000

500

5,OOOG

2,000

300

700

1,500

700

1,500

1,500

50

700

1,000

200

1,500

300

1,000

300

300

1,500

P%-S

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2L

0.2N

0.2N

0.2N

0.2L

0.2N

0.2N

0.2N

0.2N

0.2N

0.2L

0.2L

0.2L

0.2N

0.2N

0.2L

Mo-S

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

Ti%-S

1

0.3

0.7

0.1

0.7

0.7

0.7

0.5

0.7

0.2

1

0.07

0.5

0.3

0.015

0.020

0.015

0.5

0.3

0.7

0.7

0.7

0.3

0.5

Nb-S

20L

20N

20L

20N

20L

20L

20L

20L

20L

20N

20L

20N

20L

20N

20N

20N

20N

20L

20N

20N

20N

20N

20N

20L

Ag-S

0.5N

0.5N

0.5N

0.5N

0.5N

0.5L

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

Ni-S

70

70

50

20

70

50

150

70

70

70

100

30

150

50

5

10

30

70

100

70

50

50

30

70

As-S

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

Pb-S

20

10L

15

10L

15

20

10

15

15

10

20

10L

20

10

10L

10L

10L

20

20

20

15

20

20

15

Au-S

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

Sb-S

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

B-S

500

200

20

500

30

ION

100

200

30

500

1,500

500

30

2,OOOG

500

300

70

300

1,500

300

1,000

70

700

1,500

Sc-S

20

5

15

5L

20

10

15

15

20

5

20

5N

15

15

5N

5N

5N

15

15

20

15

15

7

10

Ba

1,000

700

700

500

2,000

150

1,500

1,000

700

300

1,500

150

700

500

50

100

200

1,500

1,000

500

700

700

700

1

Sn

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO
BCOOOR

BC014R

BC016R

BC027R

BC028R

BC048R

BC063R

BC072R

BC074R

BC080R

BC082R

BC090R

BC095R

BC100R

BC114R

BC106R

BC131R

BC133R

BC141R

BC156R

BC157R

BC164R

BC189R

BC135R

W-S Y-S Zn-S Zr-S

20N 20 200N 200

20N 10L 200N 70

20N 20 200N 200

20N 10L 200N 30

20N 30 200N 150

20N 20 200N 150

20N 20 200N 150

20N 20 200N 100

20N 20 200N 150

20N 10L 200N 50

20N 50 200N 150

20N ION 200N 20

20N 30 200N 200

20N 15 200N 50

20N ION 200N 10L

20N ION 200N 10

20N ION 200N 10L

20N 20 200N 150

20N 20 200N 100

20N 20 200N 100

20N 15 200N 70

20N 20 200N 100

20N 15 200N 100

20N 20 200N 150

As/p

5L

5L

5L

5L

5

5L

5L

5L

5L

5L

8

5L

5L

5L

5L

5L

5L

5L

7

5L

5L

5L

5L

5L

Au

0.2

0.012

0.020

0.004

0.028

0.038

0.6

0.150

0.1

0.450

0.1

0.1

0.450

0.350

0.1

0.1

0.014

0.020

1

0.3

0.1

0.012

0.012

0.014

Bi/p

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

Cd/p

0.1L

0.1L

0.1

0.1

1.4

0.4

0.5

0.5

0.2

0.1

0.6

0.1L

0.5

0.5

0.1L

0.1L

0.1L

0.5

1.2

0.3

0.3

0.5

0.2

0.3

Hg

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.020

0.02L

0.02L

0.02L

0.02L

0.02L

0.020

0.020

0.02L

Sb/p

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

Te

0.25

0.05L

0.05L

0.05

0.05

0.05

0.65

0.3

0.05

0.2

1.1

0.05L

0.1

0.45

0.05L

0.65

0.05L

1

1.5

0.05

0.65

0.05L

0.05L

0.7

Tl Zn/p

0.4 34

0.1 17

0.3 23

0.05 14

0.55 72

0.05L 22

0.45 73

0.35 35

0.45 51

0.1 20

0.7 48

0.05L 10

0.2 100

0.1 42

0.05L 2L

0.05L 3

0.05L 18

0.65 47

0.7 72

0.4 65

0.35 18

0.4 64

0.1 18

0.4 35

Sch

Qua

Sch

Qua

Sch

Sch

Qua

Qua

Sch

Qua

Sili

Qua

Qua

Tou

Qua

Qua

Tou

Sch

Sili

Sch

Sch

Qua

Sch

Sch

Appendix table 4. Rock samples collected from trench T2, Bochinche area, Venezuela.

BC201R

BC202R

BC203R

BC204R

BC205R

BC206R

BC207R

BC208R

BC209R

BC210R

BC211R

BC212R

BC213R

BC214R

BC215R

BC216R

BC217R

BC218R

BC219R

BC220R

BC221R

Latitude Longitude Ca%-S F%

7°33'00" 60°48'00" 0.05L 0.01 L

7°33'00" 60°48'00" 0.05L 0.01L

7°33'00" 60°48'00" 0.05 0.01L

7°33'00" 60°48'00" 0.05 0.02

7°33'00" 60°48'00" 1.00 0.01

7°33'00" 60°48'00" 0.05L 0.0 1L

7°33'00" 60°48'00" 0.07 0.02

7°33'00" 60°48'00" 0.05L 0.01L

7°33'00" 60°48'00" 0.05L 0.01

7°33'00" 60°48'00" 0.05L 0.02

7°33'00" 60°48'00" 0.05L 0.01 L

7°33'00" 60°48'00" 0.10 0.03

7°33'00" 60°48'00" 0.05L 0.02

7°33'00" 60°48'00" 0.05L 0.02

7°33'00" 60°48'00" 0.07 0.01

7°33'00" 60°48'00" 0.05 0.01

7°33'00" 60°48'00" 0.05L 0.01 L

7°33'00" 60°48'00" 0.05L 0.01

7°33'00" 60°48'00" 0.05 0.03

7°33'00" 60°48'00" 0.05L 0.01L

7°33'00" 60°48'00" 0.07 0.01

Fe%-S

7

1.5

5

5

7

1.5

5

1.5

5

5

1

5

5

0.7

3

5

0.7

5

3

1

7

Mg%-S

0.7

0.2

1

0.7

0.7

0.2

0.7

0.05

0.15

0.15

0.02

1.5

0.3

0.15

0.2

1

0.07

0.2

0.7

0.1

1.5

Na%-S

1

0.2L

2

0.7

1.5

0.2L

1

0.2L

0.7

1

0.2N

0.7

1

0.2N

0.7

1.5

0.2N

1

1

0.2N

1.5

P%-S

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

Ti%-S

0.5

0.15

0.5

0.7

0.7

0.07

0.5

0.07

0.5

0.5

0.1

0.5

0.5

0.05L

0.5

0.5

0.05

0.5

0.5

0.03

0.7

Ag-S

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

As-S

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

Au-S B-S

ION 2,000

ION 1,500

ION 1,500

ION 2,OOOG

ION 2,OOOG

ION 2,000

ION 2,OOOG

ION 200

ION 150

ION 200

ION 70

ION 2,OOOG

ION 500

ION 1,000

ION 1,000

ION 2,OOOG

ION 300

ION 300

ION 2,OOOG

ION 500

ION 2,OOOG

Ba

700 

50 

1,000 

1,000 

1,000 

200 

700 

1,000 

1,000 

700 

1,000 

150 

700 

7,000 

700 

700 

70 

700 

1,000 

50 

300 
N42 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

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BC216R

BC217R

BC218R

BC219R

BC220R

BC221R

BC222R

BC223R

BC224R

BC225R

BC226R

BC227R

BC228R

BC228RA

BC229R

BC230R

BC231R

BC232R

BC233R

BC234R

BC235R

BC236R

BC237R

BC238R

BC239R

BC240R

BC241R

BC242R

BC243R

BC244R

BC245R

BC246R

BC247R

BC248R

BC249R

BC250R

BC267R

BC268R

BC275R

BC201R

BC202R

BC203R

BC204R

BC205R

BC206R

BC207R

BC208R

BC209R

BC210R

BC211R

BC212R

20

10L

10

10

10L

20

15

20

2,000

10

700

20

15

2,000

50

10

ION

70

10L

30

10L

15

30

10L

20

15

30

500

700

100

15

15

15

10

10L

10L

70

300

100

W-S

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

100

10

150

100

10

150

150

150

30

150

15

200

200

100

10

150

70

30

150

150

100

20

100

15

200

200

10

20

15

70

70

15

100

10L

50

30

15

50

70

Y-S

30

10L

50

30

30

ION

30

10L

20

50

10L

20

70

15

50

50

20

100

70

70

700

50

150

100

50

300

20

50

30

70

100

50

50

30

50

10

70

150

70

150

700

100

50

20

50

20

50

30

50

150

100

Zn-S

200L

200N

200N

200L

200L

200N

200N

200N

200N

200N

200N

200N

50

5N

50

50

5N

30

50

50

15

30

5L

50

30

305

50

307

50

30

50

5L

50

5N

50

505

5L

5N

20

50

5L

50

5L

50

20

5L7

15

Zr-S

150

50

150

150

150

15

300

30

150

150

50

200

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

As/p

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

SOL 1,000

SON 300

SOL 150

SOL 200

SON 200

SOL 150

SON 200

SON 300

SOL 5,OOOG

SON 500

SON 5,OOOG

SON 700

SON 300

SON 5,000

SON 700

SON 200

SON 200

70 1,000

SOL 150

SON 300

SON 300

SON 700

SON 300

SON 300

SON 700

SON 2,000

SOL 5,OOOG

SOL 5,OOOG

SOL 5,OOOG

70 3000

SON 300

SON 700

SON 300

SON 300

SON 300

SOL 150

SON 5,OOOG

SOL 5,OOOG

SOL 5,OOOG

Au Bi/p

0.012 2L

0.040 2L

0.004 2L

0.018 2L

0.012 2L

0.1 2L

0.038 2L

0.014 2L

0.002 2L

0.006 2L

0.008 2L

0.150 2L

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

Cd/p

0.1L

0.1L

0.1L

0.1L

0.3

0.2

0.1L

0.3

0.1L

0.1L

0.2

0.1L

20L

20N

20L

20L

20N

20N

20L

20L

20N

20L

20N

20L

20N

20L

20N

20L

20N

20N

20N

20L

20L

20N

20L

20N

20L

20L

20N

20N

20N

20L

20L

20N

20L

20N

20N

20L

20N

20N

20L

Hg

0.02

0.02

0.02N

0.02N

0.04

0.02

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

305

30

205

50

30

50

500

50

70

50

50

150

10

30

15

20

30

50

105

50

5L

50

30

15

30

30

50

205

207

1577

50

20

Sb/p

2L

2L

2L

2L2

2L

2L

2L

2L

2L

2L

2L

30

10

50

70

30

30

50

50

1,000

70

150

300

50

300

20

30

30

50

70

30

30

300

30

20

50

100

700

50

30

30

30

10

50

15

30

700

300

500

300

Te

0.05N

0.55

0.05N

0.05N

0.15

1.45

0.2

0.05N

0.05N

0.05N

0.05N 7

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

Tl

0.2

0.05L

0.15

0.2

0.15

0.05N

0.15

0.05N

0.25

0.25

0.05

0.05N

20

5L

20

15

5L

20

20

20

15

207

20

20

207

207

15

20

20

15

5L

15

5L

20

20

1075

15

15

5L

205

15

107

10

15

Zn/p

1507

82

53

69

10

19

39

33

49

12

29

S

Q

S

S

S

Q

S

Q

S

S

Q

S
Appendix table 4. Rock samples collected from trench T2, Bochinche area, Venezuela Continued.

BC213R

BC214R

BC215R

BC216R

BC217R

BC218R

BC219R

BC220R

BC221R

BC222R

BC223R

BC224R

BC225R

BC226R

BC227R

BC228R

BC228RA

BC229R

BC230R

BC231R

BC232R

BC233R

BC234R

BC235R

BC236R

BC237R

BC238R

BC239R

BC240R

BC241R

BC242R

BC243R

BC244R

BC245R

BC246R

BC247R

BC248R

BC249R

BC250R

BC267R

BC268R

BC275R

W-S

20N

20N

20N

20N

20N

20N

20L

20L

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

30

20N

20N

20N

20N

20N

70

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

Y-S

20

ION

70

20

ION

20

20

ION

20

20

30

15

20

10L

20

30

15

ION

30

15

15

20

20

50

10L

15

ION

20

20

10L

20

20

50

20

ION

20

10L

15

20

10L

10

30

Zn-S

200N

200N

200N

200N

200N

200L

200N

200N

200L

200N

200N

300

200N

200N

200N

200N

200L

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200L

200N

200N

200N

200

200N

200N

200N

200N

200N

500

200N

200L

200

Zr-S

100

30

150

150

20

150

150

15

200

150

150

70

150

30

150

70

200

20

150

100

50

100

150

200

30

300

30

100

100

50

70

50

150

200

30

150

30

150

150

30

150

150

As/p

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

22

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

18

6

5L

5L

5L

5L

5

5L

5L

5L

5L

260

5L

5L

15

Au

0.002L

0.008

0.008

0.002L

0.010

0.002

0.002

1

0.050

0.002

0.004

0.1

0.002

0.006

0.006

0.004

0.004

0.014

0.004

0.002

0.850

0.008

0.002

0.004

0.150

0.002

0.004

0.006

0.002

0.012

0.014

0.002

0.030

0.004

0.002

0.004

0.008

0.004

2.6

0.004

0.006

0.032

Bi/p

2L

2L

2L

2L

2L

2L

2L

5

2L

2L

2L

2L

2L

3

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

17

2L

2L

2L

2

6

2L

2L

2L

2L

2L

2L

2L

2L

15

2L

6

2L

Cd/p

0.1L

0.1

0.3

0.1

0.1L

0.2

0.1

0.1

0.2

0.1

0.1L

0.1

0.1L

0.1L

0.1

0.5

0.1L

0.1L

0.2

0.1

0.2

0.1L

0.1L

0.2

2.7

0.1

0.1L

0.7

0.8

1.4

1.3

6.8

1.2

0.2

0.1L

0.3

0.1

0.2

1.2

1.4

2.7

4.6

Hg

0.02N

0.02

0.02N

0.02N

0.02

0.02N

0.02N

0.04

0.02N

0.02N

0.02N

0.03

0.02N

0.04

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.02N

0.02

0.02

0.02N

0.02

0.02N

0.02N

0.02N

0.02N

0.02N

0.02

0.02N

0.02N

0.02N

Sb/p

2L

2L

2L

2L

2L

2L

2L

2L

3

2L

2L

7

2L

2L

2L

2L

3

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

3

11

2L

2L

2L

2L

2L

2L

2L

2L

3

2

Te

0.05N

0.15

0.05N

0.05N

0.1

0.05N

0.05N

1.8

0.15

0.05N

0.05

1.1

0.05N

0.05N

0.1

0.05N

0.05N

0.5

0.05N

0.05L

1.2

0.05N

0.05N

0.05N

7

0.1

0.2

0.05N

0.05N

7.2

0.05N

0.05N

0.1

0.05N

0.1

0.1

0.15

0.05

0.75

0.15

3.85

0.2

Tl

0.2

0.05N

0.05L

0.3

0.05L

0.25

0.25

0.05N

0.1

0.2

0.15

0.15

0.2

0.2

0.25

0.15

0.05N

0.05

0.25

0.1

0.1

0.35

0.05N

0.25

0.05N

0.05

0.05L

0.1

0.1

0.1

0.05

0.15

0.25

0.25

0.05

0.25

0.05L

0.2

0.2

0.15

0.1

0.25

Zn/p

43

9

61

40

6

120

56

34

26

44

50

170

35

71

74

57

37

10

55

24

28

45

14

26

60

14

8

93

100

43

36

62

130

29

18

58

21

50

430

19

55

300

Sam

Schi

Quar

Blac

Blea

Quar

Schi

Silic

Quar

Schi

Schi

Schi

Quar

Schi

Quar

Schi

Schi

Silic

Quar

Schi

Schi

Quar

Schi

Silic

Schi

Quar

Silic

Quar

Schi

Schi

Quar

Silic

Silic

Silic

Silic

Quar

Wea

Quar

Schi

Blac

Qua

Qua

Qua
Appendix table 5. Rock samples collected from the Bochinche road traverse area, Venezuela.

BI001

BI002

BI003

BI004

BI005

BI006

BI007

BI008

BI009

BI010

BIO 11

BIO 12

BI001

BI002

BI003

BI004

BI005

BI006

BI007

BI008

BI009

BIO 10

BI011

BIO 12

BI001

BI002

BI003

BI004

BI005

BI006

BI007

BI008

BI009

BI010

BIO 11

BI012

Latitude

7°34'30"

7°34'30"

7°34'30"

7°34'30"

7°34'30"

7°34'30"

7°34'30"

7034.30-

7034.30-

7°34'30"

7034.30-

7034.30-

Co-S

30

15

70

150

500

15

10L

70

15

30

70

50

w-s

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

Longitude

60°51'45"

60°51'45"

60°51'45"

60051'45"

60°51'45"

60°51'45"

60°51'45"

60°51'45"

60°51'45"

60°51'45"

60°51'45"

60°51'45"

Cr-S

200

30

700

150

200

10L

15

10

50

500

150

300

Y-S

50

ION

70

20

70

ION

ION

ION

ION

30

70

50

Ca%-S

0.1

0.07

0.05L

0.05L

0.07

0.05L

0.05

0.05L

0.07

0.05L

0.15

10

Cu-S

70

10

150

150

500

15

7

20

10

100

150

70

Zn-S

700

200N

500

200

500

200N

200N

200N

200N

200L

300

200L

F%

0.01L

0.01L

0.03

0.01L

0.01L

0.01L

0.01L

0.01L

0.01L

0.01L

0.01L

0.01L

Ga-S

20

7

30

20

30

5N

5L

5N

5

20

10

10

Zr-S

30

10

150

30

50

10

10

10

10

50

30

50

Fe%-S

7

2

7

7

7

0.5

0.7

0.7

1.5

7

3

7

Ge-S

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

As/p

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

Mg%-S Na%-S

2 1

1 0.5

2 0.2N

0.5 0.2

1.5 1.5

0.2 0.2N

0.7 0.2

0.3 0.2L

0.7 0.7

0.2 0.2N

1.5 0.5

3 0.7

La-S Mn-S

SON 700

SON 700

SOL 1,500

SON 1,500

SOL S.OOOG

SON 500

SON 150

SON 300

SON 70

SON 300

SOL 300

SON 1,000

Au Bi/p

0.0020 2L

0.0020 2L

0.0020 2L

0.0100 2L

0.0040 2L

0.0040 2L

0.0020L 2L

0.0020 2L

0.0020 2L

0.0100 2L

0.0280 2L

0.0100 2L

P%-S

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

0.2N

Mo-S

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

Cd/p

1

0.1L

1.2

0.4

0.8

0.1L

0.1L

0.1L

0.1L

0.6

0.6

0.3

Ti%-S

0.3

0.07

0.5

0.3

0.3

0.02

0.05

0.03

0.07

0.3

0.2

0.3

Nb-S

20N

20N

20L

20N

20N

20N

20N

20N

20N

20L

20N

20N

Hg

.02

.02N

.02N

.04

.28

.02N

.02

.02N

.02N

.04

.04

.04

Ag-S

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5L

0.5N

0.5N

0.5N

0.5N

Ni-S

150

20

150

70

200

5

5

5L

10

50

100

100

Sb/p

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

As-S

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

Pb-S

20

20

20

15

15

10

10

10

15

15

10

15

Te

0.05N

0.35

0.05L

0.35

0.35

0.05N

0.05N

0.05

0.15

0.15

0.15

0.05N

Au-S B-S

ION 20

ION 2,OOOG

ION 50

ION 300

ION 100

ION 700

ION 2,OOOG

ION 1,500

ION 2,OOOG

ION 100

ION ION

ION ION

Sb-S Sc-S

100N 30

100N 20

100N 20

100N 20

100N 30

100N 5

100N 7

100N 7

100N 20

100N 20

100N 20

100N 30

Tl Zn/p

0.05L 780

0.05N 16

0.10 790

0.05 190

0.10 970

0.05N 13

0.05N 6

0.05N 7

0.05N 7

0.05N 53

0.05N 400

0.05N 35

Ba

150 

100 

200 

150 

2,000 

100 

30 

30 

30 

100 

200 

150 

Sn

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

IO

Sa

Wea

Qua

Wea

Wea

Wea

Qua

Qua

Qua

Qua

Wea

Wea

Gre
N46 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

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MW015R

MW016R

MW017R

MW018R

MW019R

MW020R

MW021R

MW022R

MW023R

MW024R

MW025R

MW026R

MW027R

MW028R

MW029R

MW030R

MW031R

MW032R

MW033R

MW59650S

MW001R

MW002R

MW003R

MW004R

MW005R

MW006R

MW007R

MW008R

MW009R

MW010R

MW011R

MW012R

MW013R

MW014R

MW015R

MW016R

MW017R

MW018R

MW019R

MW020R

MW021R

MW022R

MW023R

MW024R

MW025R

MW026R

MW027R

MW028R

MW029R

MW030R

MW031R

10L

100

10L

ION

ION

10

ION

10L

15

10

ION

ION

10L

10L

20

ION

ION

10L

10L

10

W-S

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

50

30

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

70

150

ION

ION

10L

10

10L

100

10

10L

10L

50

30

10L

100

10L

10

100

20

30

Y-S

ION

20

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

20

ION

ION

ION

ION

ION

ION

10

10L

100

ION

ION

ION

10

ION

ION

5L

30

30

10

10

5L

5N

5N

5N7

5N

30

15

50

30

5N

5L

307

15

Zn-S

200N

200L

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200L

200N

200N

200N

200N

200N

200N

300

200

200N

200N

200N

200N

300

200N

200N

5N

5L5

5L

5L5

5L

5L

70

70

5L

5L5

5L

30

5N

5N75

15

Zr-S

15

150

15

15

10

100

10

10

10

10L

10

15

15

10L

ION

30

10

10L

10L

15

30

10

50

70

50

15

10L

101

10L

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

As/p

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L8

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

5L

SON 100

SON 5,000

SON

SON

SON

SON

SON

SON

SOL

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

Au

0.004

0.004

0.002L

0.002L

0.002L

0.002L

0.002

0.002L

0.002L

0.002L

0.048

0.23

0.002L

0.002

0.002

0.01

0.008

0.002

0.05

0.002L

0.01

0.034

0.002L

0.002

0.01

0.002

0.014

0.04

0.25

0.004

0.002L

20

10L

10

10

15

15

20

20

10L

30

10L

30

30

15

10L

30

10L

15

Bi/p

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

5N

5N

5N

5N

5N

5N

5N

5N

5N

10

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

Cd/p

0.1L

0.7

0.1L

0.1L

0.1L

0.1L

0.1L

0.1L

0.1L

0.1L

0.1L

0.3

0.1L

0.1L

0.1L

0.6

0.2

0.2

0.1

0.1L

0.1L

0.1L

0.1L

0.1L

0.1L

0.1L

0.3

0.2

3.1

0.1L

0.1L

20N

20N

2ON

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

Hg

0.02L

0.02

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.02L

0.08

0.02L

0.02L

5

5075

5L7

5N

30

30

30

5L

15

20

5L

50

5L5

10

15

10

Sb/p

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

2L

10 100N

10L 100N

10 100N

10 100N

10L 100N

10 100N

10L 100N

10L 100N

20 100N

10 100N

10L 100N

ION 100N

10L 100N

10L 100N

20 100N

10L 100N

10L 100N

10L 100N

10L 100N

20 100N

Te Tl

0.05L 0.05L

0.15 0.05L

0.05L 0.05L

0.05L 0.05L

0.05L 0.05L

0.05L 0.05L

0.05L 0.05L

0.1 0.05L

0.05L 0.05L

0.05L 0.05L

0.05L 0.05L

0.45 0.05L

0.05L 0.05L

0.05L 0.05L

0.05L 0.05L

0.05L 0.05L

0.45 0.05L

0.05 0.05L

2 0.05L

0.4 0.05L

0.05 0.05L

0.05L 0.05L

0.05L 0.05L

0.2 0.05L

0.05L 0.05L

0.05L 0.05L

0.05 0.05L

0.2 0.05L

4.2 0.05L

0.25 0.05L

0.05L 0.05L

5L

20

5N

5N

5N7

5N

10

20

20

5L5

5N

5N

15

5N

5N5

5N7

Zn/p

2L

92

2L

2L

2L

2L

2L

2L

2L

2L

2L59

2L

2L

120

1553

2L

2L

2L

2L5

2L4

2L7

270

2L

2L

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

Q

Ph

Q

Q

Q

Q

H

Q

Q

Q

Q

Q

Q

Q

Q

Q

Q

Q

Q

Q

Q

Q

T

Q

Q

Q

Q

Q

Q

S

Q
Appendix table 6. Rock samples collected from zone 4 of the Marwani concession, Venezuela Continued.

W-S

MW032R 20N 

MW033R 20N 

MW59650S 20N

Y-S

ION 

ION 

10

Zn-S

200N 

200N 

200

Zr-S

10L 

10L 

50

As/p

11 

5L 

5L

Au

0.002L 

0.002L 

0.002

Bi/p

2L 

2L 

2L

Cd/p

0.3 

0.4

2

Hg

0.02L 

0.02L 

0.02L

Sb/p

2L 

2L 

2L

Te

0.1 

0.05 

0.3

Tl

0.05L 

0.05L 

0.05

Zn/p

5 

2L 

160

Qua

Qua

Appendix table 7. Heavy-mineral separates from stream concentrate samples, zone 4, Marwani concession, Venezuela.

MOC005

MOC011

MOC012

MOC017

MOC020

MOC021

MOC038

MOC039

MOC059

MOC061

MOC082

MOC086

MOC087

MOC088

MOC090

MOC097

MOC100

MOC105A

MOC109

MCH001

MCH003

MCH004

MCH005

MCH006

MCH007

MCH008

MCH010

MCH011

MCH012

MCH013

MCH014

MCH015

MCH016

MCH018

MCH022

MCH024

MCH025

MCH026

MCH027

MCH028

Latitude

7°00'21"

7°00'49"

7°00'01"

7°01'25"

7°01'22"

7°01'21"

7°00'11"

7°00'11"

6°59'29"

6°59'49"

7°01'30"

7°01'43"

7°01'47"

7°01'52"

7001-57-

7°02'43"

7°03'10"

7°02'55"

7°02'39"

7°03'08"

7°02'36"

7°02'40"

7°02'12"

7°02'28"

7°03'00"

7°02'50"

7°02'20"

7°02'17"

7°02'16"

7°01'48"

7°01'12"

7°01'25"

7°0r09"

7°01'09"

7°00'55"

7°00'37"

7°00'40"

7°00'11"

7°00'13"

7°00'01"

Longitude

60°41'13"

60°40'24"

60°40'44"

60°41'11"

60°41'46"

60°41'59"

60°41'50"

60041'46"

60°42'41"

60°42'41"

60°39'41"

60°40'28"

60°40'41"

60°40'39"

60°40'57"

60°39'08"

60°39'29"

60°39'58"

60°39'58"

60°40'03"

60°39'36"

60°39'34"

60°40'06"

60°40'02"

60°40'44"

60°40'32"

60°40'28"

60°40'26"

60°40'17"

60°40'53"

60°40'08"

60°41'20"

60°41'32"

60°41'56"

60°42'05"

60°41'49"

60°42'12"

60°42'58"

60°42'51"

60°43'03"

Ca%-S

0.1L

0.1L

0.15

0.1L

0.1L

0.1N

0.1L

0.1L

0.1N

0.1N

0.1L

0.1L

0.1L

0.1N

0.1L

0.1N

0.1N

0.1N

0.1L

0.1L

0.1N

0.1L

0.1L

0.1L

0.1L

0.1N

0.1N

0.1L

0.1L

0.1L

0.1L

0.1L

0.1L

0.1L

0.1N

0.1L

0.1N

0.1L

0.1N

0.1L

Fe%-S

0.15

0.20

0.30

0.30

0.30

0.30

0.1L

0.1L

0.30

0.50

0.20

0.15

0.20

0.10

0.1L

0.1L

0.10

0.10

0.10

0.10

0.1L

0.1L

0.1L

0.1L

0.10

0.1L

0.15

0.15

0.1L

0.1L

0.1L

0.50

0.15

0.1L

0.30

0.70

0.70

0.20

0.70

0.70

Mg%-S

0.05

0.05L

0.05L

0.05L

0.05L

0.05L

0.05

0.05

0.05L

0.07

0.07

0.10

0.05

0.05

0.07

0.10

0.05

0.07

0.07

0.07

0.05

0.05

0.07

0.07

0.07

0.05

0.05

0.05

0.05L

0.05

0.05

0.05L

0.05L

0.05

0.05

0.05

0.05

0.15

0.05L

0.10

Na%-S

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5L

0.5N

0.5N

0.5N

P%-S

0.5L

0.5L

0.5L

0.5L

0.5L

0.5N

0.5L

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

0.5L

0.5N

0.5N

0.5N

0.5N

0.5N

0.5L

0.5N

0.5N

0.5N

0.5N

0.5L

0.5N

0.5L

0.5N

0.5L

0.5L

0.5L

0.5N

0.5N

0.5L

0.5N

0.5N

0.5N

0.5N

0.5N

0.5N

Ti%-S

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

2G

Ag-S

IN

IN

IN

IN

IN

IN

IN

IN

IN

10

IN

IN

IN

20

IN

IN

IN

IN

IN

IN

IN

IN

100

IN

IN

IN

IN

IN

10

IN

1L

IN

IN

IN

IN

IN

IN

IN

IN

IN

As-S

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

Au-S

20L

20N

100

20N

50

50

20N

20N

30

700

20N

20N

20N

1,000

20L

20N

20L

20N

20N

20N

20N

20N

l.OOOG

20N

20N

20N

20N

70

300

20N

50

30

20

20N

20L

20N

20N

20L

20N

20

B-S

100

50

50

30

70

30

200

50

20L

70

200

100

70

100

70

200

100

70

70

30

20L

20L

150

150

100

150

70

20L

30

100

100

20N

20

100

70

100

70

700

100

30

Ba-S

50

50

70

50

50

SOL

70

50

SOL

50

SOL

SOL

SOL

70

70

SOL

70

50

SOL

50

300

500

SOL

50

300

70

70

70

70

70

100

300

200

150

100

70

150

150

70

100

Be

2

2

2

2

2

2

2

2

2

2

2

2

2

5

2

2

2

2

2

2

2

2

2

2

2

3

2

2

2

2

2

2

5

2

2

2

3

5

2

5
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N49

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p-JZ _5 ZjZ J p-J ZZZZpJZZZZ cS<nenen<ncSCSCSCNenenpgen(^en^r^enencSc^en<nencsRencNenc^encsencscSr^entNCSeNtNCSCSCStS

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Appendix table 7. Heavy-mineral separates from stream concentrate samples, zone 4, Marwani concession, Venezuela Continued.

MOC005

MOC011

MOC012

MOC017

MOC020

MOC021

MOC038

MOC039

MOC059

MOC061

MOC082

MOC086

MOC087

MOC088

MOC090

MOC097

MOC100

MOC105A

MOC109

MCH001

MCH003

MCH004

MCH005

MCH006

MCH007

MCH008

MCH010

MCH011

MCH012

MCH013

MCH014

MCH015

MCH016

MCH018

MCH022

MCH024

MCH025

MCH026

MCH027

MCH028

MCH029

MCH030

MCH032

MCH033

MCH035

MCH036

MCH037

MCH038

MCH039

Cr-S

300

1,000

500

100

200

700

700

500

500

1,500

500

500

500

200

200

500

300

300

500

200

500

500

500

300

70

100

1,500

300

500

200

300

500

200

300

300

700

300

300

1,000

300

300

700

500

500

300

700

500

300

2,000

Cu-S

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

10L

ION

ION

ION

ION

ION

ION

ION

10L

ION

ION

ION

ION

ION

ION

10L

ION

15

20

15

ION

20

30

30

ION

15

ION

15

15

15

ION

15

10

ION

ION

10L

Ga-S

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

10L

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

Ge-S

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

La-S

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

100N

Mn-S

20L

20

20L

30

150

100

20

20

20L

20

100

20

20L

20L

30

20L

20L

20L

20

20

20L

20L

20L

30

30

30

20

30

30

20L

50

100

20L

30

20

50

50

20L

20

50

70

20L

20L

20

30

100

20L

20

20

Mo-S

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

Nb-S

70

50

50

100

SON

SON

70

70

70

50

70

70

100

100

70

70

70

70

50

100

70

70

70

50

70

70

SOL

100

70

100

70

SOL

200

70

200

70

150

150

100

300

70

100

150

200

100

100

100

SOL

100

Ni-S

ION

50

ION

ION

ION

20

ION

ION

ION

20

20

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

ION

20

ION

20

ION

10L

20

ION

ION

30

70

50

50

50

10

10

10

ION

15

10

15

ION

ION

30

Pb-S

20

20

20

30

20

20

30

20

50

20

20

20L

30

30

50

20L

50

50

20L

30

20L

20L

20

30

30

50

30

30

30

50

70

20L

20

20

20L

30

50

30

20L

30

30

30

30

30

30

30

20

20

20L

PdS

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

5N

Pt-S

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

20N

Sb-S

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

200N

Sc

70

70

70

50

100

70

70

70

70

70

50

70

30

70

30

70

70

50

50

30

50

50

20

50

30

70

70

50

70

30

70

100

70

50

100

70

100

70

70

100

150

100

150

100

100

150

100

100

100
GOLD-BEARING QUARTZ-CARBONATE-TOURMALINE VEIN SYSTEMS N51

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ZZ ZJZZZZZZZZZZZJZZZZZZZJZZZZZZZZZZZZZ

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Z Z Z ZZ JZZJZZZJZJZZZ ZZZZZJZZZ

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ffi 

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Appendix table 7. Heavy-mineral separates from stream concentrate samples, zone 4, Marwani concession, Venezuela Continued.

MOC005

MOC01 1

MOC012

MOC017

MOC020

MOC021

MOC038

MOC039

MOC059

MOC061

MOC082

MOC086

MOC087

MOC088

MOC090

MOC097

MOC100

MOC105A

MOC109

MCH001

MCH003

MCH004

MCH005

MCH006

MCH007

MCH008

MCH010

MCH011

MCH012

MCH013

MCH014

MCH015

MCH016

MCH018

MCH022

MCH024

MCH025

MCH026

MCH027

MCH028

MCH029

MCH030

MCH032

MCH033

V-S

200

150

200

150

200

200

300

200

300

300

300

200

300

150

150

150

200

200

200

200

300

300

200

200

70

100

200

200

200

200

200

200

150

200

200

200

200

200

300

150

200

300

300

200

W-S

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SON

SOL

SON

SOL

SON

150

SOL

SOL

100

SON

SOL

SOL

SOL

Y-S

150

500

150

200

1,500

300

70

70

100

150

150

100

50

500

150

100

200

150

50

150

70

100

30

150

200

500

300

200

300

150

300

700

300

500

300

150

200

500

300

700

200

100

150

500

Zn-S

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

SOON

500

700

SOON

SOON

SOON

500

500

SOON

500

1,000

1,000

SOON

500

SOON

SOON

SOON

SOON

SOON

500

SOON

SOON

SOON

SOON

SOON

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Mineral Deposits of the Venezuelan Guayana Shield

By Gary B. Sidder

GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

U.S. GEOLOGICAL SURVEY BULLETIN 2124-O

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
CONTENTS

Abstract........................................................................................................................... Ol

Resumen......................................................................................................................... 1

Introduction.................................................................................................................... 2

Geology of the Venezuelan Guayana Shield.................................................................. 2

Gold................................................................................................................................ 3

Diamonds........................................................................................................................ 6

Iron.................................................................................................................................. 7

Aluminum....................................................................................................................... 9

Manganese...................................................................................................................... 10

Tin................................................................................................................................... 11

Niobium, Tantalum, and Rare Earth Elements............................................................... 12

Uranium.......................................................................................................................... 12

Molybdenum................................................................................................................... 13

Titanium.......................................................................................................................... 13

Platinum.......................................................................................................................... 14

Tungsten......................................................................................................................... 14

Chromite......................................................................................................................... 14

Summary......................................................................................................................... 14

References Cited............................................................................................................. 15

PLATE

[In pocket] 

1. Mineral deposits of the Guayana Shield, Venezuela.
Mineral Deposits of the Venezuelan Guayana Shield

^GaryB.Sidder 1

ABSTRACT RESUMEN

The Venezuelan Guayana Shield is a relatively unexplored area, and its jungle cover inhibits easy discovery. 

Gold, iron, aluminum from bauxite, and diamonds are the 

only commodities produced at present. Gold is commonly in 

low-sulfide gold-quartz vein-type deposits hosted by 

metavolcanic and metasedimentary rocks of the Early Proterozoic greenstone belts. The El Callao district, containing 

the underground Colombia mine, is historically the biggest 

gold producer in Venezuela and has total production of about 

200 metric tons of gold. The Lo Increible and Kilometer 88 

districts are two other areas where mining activity is extensive. The Early Proterozoic Cuchivero Group has potential 

for gold deposits in epithermal veins, and the Early Proterozoic greenstone belts and possibly the banded iron formation 

of the Archean Imataca Complex have potential for gold in 

Homestake-type deposits. Superior-type deposits of 

enriched banded iron formation in the Imataca Complex, 

such as those at Cerro Bolivar, San Isidro, and Los Barrancos, are among the largest in the world. The Los Pijiguaos 

bauxite deposit developed through intense tropical weathering of the Middle Proterozoic Parguaza Granite. Other prospects of bauxite and lateritic bauxite formed on felsic and 

mafic volcanic and plutonic rocks elsewhere in the shield. 

Placer diamond deposits are predominantly derived from 

paleoplacer conglomerate beds in the lower 600 m of the 

Early to Middle Proterozoic Roraima Group. Kimberlite has 

been identified only in the Quebrada Grande area, the richest 

diamond-producing area in Venezuela. Also favorable for 

exploration are prospects of other metals such as platinum, 

chromium, and manganese in the greenstone belts, tin in the 

Parguaza Granite and in rhyolite of the Early Proterozoic 

Caicara Formation, and rare earth elements in carbonatite 

such as at Cerro Impacto or associated with potential Olympic Dam-type iron-copper-uranium-gold-rare earth element 

deposits in the Cuchivero Group and the Parguaza Granite.

'U.S. Geological Survey, Denver Federal Center, MS905, Denver, 

Colorado 80225.

El Escudo de Guayana en Venezuela es un area relativamente poco explorada, ademas la densa selva inhibe los 

descubrimientos faciles. Los unices minerales actualmente 

producidos son oro, hierro, aluminio a partir de bauxita y 

diamantes. El oro ocurre comunmente en vetas de cuarzo las 

cuales tambien llevan una baja proportion de sulfuros. 

Dichas vetas estan encajadas en rocas metavolcanicas y 

metasedimentarias de los cinturones de rocas verdes de edad 

Proterozoico Temprano. Historicamente, El Callao, el cual 

contiene la mina subterranea Colombia, es el distrito que 

mas oro produce en Venezuela, con una produccion anual 

(total) de 200 toneladas metricas. Otras dos areas donde la 

produccion de oro es extensa son El Kilometre 88 y Lo 

Increible. El Grupo Cuchivero tiene potencial para la existencia de depositos de oro en vetas epitermales. Los cinturones de rocas verdes de edad Proterozoico Temprano y 

posiblemente las formaciones bandeadas de hierro de edad 

Arqueana tienen potencial para depositos de oro del tipo 

Homestake. En el Complejo de Imataca, los depositos de 

Cerro Bolivar, San Isidro y Los Barrancos, los cuales son 

formaciones bandeadas de hierro enriquecidas del tipo Superior, estan entre los mas grandes del mundo. El deposito de 

bauxita de Los Pijiguaos se desarrollo durante la intensa 

meteorizacion bajo clima tropical del granito de Parguaza. 

Otros prospectos de bauxita y bauxita lateritica se formaron 

en otros lugares del escudo sobre rocas volcanicas felsicas y 

maficas y sobre plutones. Las acumulaciones de diamantes 

en depositos de tipo placer son predominantemente derivadas de paleoplaceres en capas conglomerates en los 600 m 

basales del Grupo Roraima el cual tiene una edad Proterozoico Temprano a Medio. Kimberlitas han sido identificadas 

solamente en el area de Quebrada Grande, la cual es el area 

mas rica en diamantes en Venezuela. Otros metales, tales 

como platino, cromo y manganese, son favorables para la 

prospeccion en los cinturones de rocas verdes. El granito de 

Parguaza y rocas rioliticas de la Formation Caicara de edad 

Proterozoico Temprano son favorables para depositos de 

estafio. Elementos del grupo tierras raras son de interes prospective en la carbonatita del Cerro Impacto o en el granito 

de Parguaza y posiblemente las rocas del Grupo Cuchivero

Ol
02 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

los cuales son favorables para acumulaciones de 

hierro-cobre-uranio-oro-elementos tierras raras del tipo 

Olympic Dam.

INTRODUCTION

Principal products of the Venezuelan mining sector currently are gold, iron, bauxite, and diamonds. Kaolin, sand 

and gravel, building stone, and other nonmetallic and industrial mineral commodities are also produced (Doan, 1994), 

but are not discussed herein. This paper describes known 

metallic mineral prospects and deposits in the Venezuelan 

Guayana Shield and suggests possible new, as yet undiscovered, types of deposits. The location and type of commodity 

for all known metallic mineral prospects, mines, and mining 

districts in the Venezuelan Guayana Shield are shown on 

plate 1. Information varies from a location and commodity 

symbol only (unnumbered on pi. 1) to a location, commodity 

symbol, and name (numbered prospects). Site-specific 

detailed geologic descriptions and, in some cases, reserve 

data on grade and tonnage are presented herein for some 

deposits. A list of prospects, mines, and mining districts, 

both those with production and those having potential for but 

without recorded production, is shown on plate 1.

This paper is a companion to Sidder and Mendoza (this 

volume), in which a comprehensive review of the geology of 

the Venezuelan Guayana Shield and its relation to the entire 

Guayana Shield of northern South America and the West 

African craton is presented. A brief outline of the geology of 

the Venezuelan Guayana Shield is provided herein. The 

reader is referred to Sidder and Mendoza and other papers in 

this volume for detailed descriptions of the geology of the 

Venezuelan Guayana Shield. A geographic map of the Venezuelan Guayana Shield is given as plate 1 of Wynn, Sidder, 

and others (this volume).

GEOLOGY OF THE VENEZUELAN 

GUAYANA SHIELD

The Guayana Shield of Venezuela consists predominantly of Archean and Early to Middle Proterozoic rocks that 

are divisible into five lithotectonic provinces: (1) Archean 

Imataca amphibolite- to granulite-facies gneiss terrane, (2) 

Early Proterozoic greenstone-belt rocks and associated granite-gneiss terrane of the Supamo Complex, (3) Early Proterozoic unmetamorphosed volcano-plutonic complex of the 

Cuchivero Group, (4) Early to Middle Proterozoic continental sedimentary rocks of the Roraima Group, and (5) Middle 

Proterozoic anorogenic rapakivi-type Parguaza Granite. 

Early Proterozoic and possibly Archean rocks in Estado 

Amazonas of western Venezuela are undivided, and their 

relation to other rocks of the Venezuelan Guayana Shield is

unclear. Early to Middle Proterozoic continental tholeiitic 

dikes, sills, and small irregular intrusive bodies and Mesozoic dikes emplaced during the opening of the Atlantic 

Ocean cut all of the lithotectonic provinces. Figure 2 in Sidder and Mendoza (this volume) is a simplified geologic 

province map, and table 1 in Sidder and Mendoza is a simplified stratigraphic explanation of the rock units and tectonic events of the Venezuelan Guayana Shield.

The Archean Imataca Complex (unit Ai, fig. 2, Sidder 

and Mendoza, this volume), the oldest unit of the Guayana 

Shield, consists predominantly of quartzofeldspathic gneiss 

and granulite and lesser amounts of intermediate to mafic 

gneiss and granulite and minor dolomitic marble and banded 

iron formation. Large isoclinal folds, which have been 

refolded by relatively open folds, are common in the Imataca 

Complex. Metamorphic grade ranges from granulite facies 

in the northeastern part of the belt to amphibolite facies in the 

southwestern part. During the pre-Trans-Amazonian tectonomagmatic event, between about 2,800 and 2,700 Ma, 

granitic rocks intruded the Imataca Complex, and injection 

gneiss and migmatite were developed.

The Early Proterozoic greenstone belts (unit Xg, fig. 2, 

Sidder and Mendoza, this volume), which formed between 

about 2,250 and 2,100 Ma, consist of a submarine sequence 

of tholeiitic mafic volcanic rocks, a sequence of tholeiitic to 

calc-alkalic basalt to rhyolite, and a succession of turbiditic 

graywacke, volcaniclastic rocks, and chemical sedimentary 

rocks that characterize the basal, middle, and upper parts, 

respectively. Layered mafic-ultramafic complexes also are 

present in the greenstone belts. The greenstone-belt rocks are 

metamorphosed to the greenschist facies and locally to the 

amphibolite facies near granitic domes of the Supamo Complex (unit Xs, fig. 2, Sidder and Mendoza, this volume), 

which intruded the greenstone-belt rocks between about 

2,230 and 2,050 Ma. Granite of the Supamo Complex 

divides the metasedimentary and meta-igneous greenstone-belt rocks into branching synclinoria between intrusive uplifts. The Trans-Amazonian orogeny was a period of 

continental collision and accretion between about 2,150 and 

1,960 Ma during which the Imataca Complex and the greenstone-granite terranes were deformed and metamorphosed.

Postcollisional, post-Trans-Amazonian magmatism 

between about 1,930 and 1,790 Ma produced volcanic, subvolcanic, and plutonic rocks of the Cuchivero Group (unit 

Xc, fig. 2, Sidder and Mendoza, this volume) and its equivalents throughout the Guayana Shield. Silicic rocks (rhyolite 

and granite to granodiorite) are dominant, and associated 

intermediate to mafic dikes and lava flows are less abundant. 

Undivided Proterozoic and possibly Archean rocks in Estado 

Amazonas of western Venezuela include granitic rocks, 

gneiss, and migmatite. Peak metamorphism and magmatism 

occurred between about 1,860 and 1,730 Ma.

Unmetamorphosed, posttectonic sedimentary rocks 

such as quartzarenite, conglomerate, arkose, siltstone, and 

shale of the Roraima Group (unit YXr, fig. 2, Sidder and
MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD 03

Mendoza, this volume) were deposited in fluvial, deltaic, 

shallow coastal marine, and lacustrine or epicontinental 

environments. The Roraima Group is possibly as old as 

about 1,900 Ma and as young as about 1,500 Ma.

Continental tholeiitic dikes, sills, and irregular intrusive 

bodies of the Avanavero Suite (unit Xa, fig. 2, Sidder and 

Mendoza, this volume) cut all older rocks of the Guayana 

Shield. These intrusive rocks are about 1,650 Ma in age, and 

possibly as old as about 1,850 Ma. Middle Proterozoic, about 

1,545 Ma, undeformed granite having rapakivi texture (unit 

Yp, fig. 2, Sidder and Mendoza, this volume) is characteristic of the Parguaza province.

Continental collision in the westernmost part of the 

Guayana Shield during the Nickerie orogeny reset potassium-argon and rubidium-strontium mineral ages of 

Archean and Early Proterozoic rocks in the central and eastern parts of the shield to about 1,200 Ma. Tholeiitic diabase 

dikes (unit Mzd, fig. 2, Sidder and Mendoza, this volume) 

intruded the Guayana Shield during the opening of the 

Atlantic Ocean from about 210 to 200 Ma. Erosion of the 

Precambrian terranes and uplift during the Mesozoic and 

Cenozoic Eras produced at least six planar geomorphic surfaces in the Guayana Shield at distinct elevations between 

about 2,900 and 50 m above sea level.

GOLD

Venezuela has a long history of gold production, and 

recent changes in investment and tax laws in Venezuela have 

led to a mineral exploration boom in the past five years; gold 

is the primary metal of interest. Total reported gold production between 1829 and 1971 was 187 metric tons (Locher, 

1974). Gold production between 1984 and 1991 totalled 

about 29.4 metric tons and was reportedly 7.6 metric tons in 

1992 (Ensrninger and others, 1990; Ensminger, 1992; U.S. 

Bureau of Mines, 1993; Doan, 1994). These numbers do not 

include production by unlicensed miners, and thus the actual 

level of production may be higher. In 1987, rudimentary 

sluices worked by mineros or garimpeiros (small-scale miners) produced an estimated 15 metric tons of gold (Engineering and Mining Journal, 1987; Hennius, 1988). Newspaper 

and unofficial reports imply that more than 10 metric tons of 

the mineros' production is smuggled out of the country each 

year. Indeed, as much as 10 metric tons of Brazil's annual 

reported gold production may have been obtained from mineros working in the Venezuelan Guayana Shield (Engineering and Mining Journal, 1987; Hennius, 1988). A total of 

more than 62 metric tons of gold is thought to have been 

removed by illegal immigrants from surface operations 

using primitive mining techniques (Doan, 1994).

Mendoza (1985) proposed that as much as 8,000 metric 

tons of gold had yet to be discovered in the Venezuelan 

Guayana Shield, and Newman (1989) reported that Venezuelan gold reserves are an estimated 358 metric tons. The

similarity between the geology of the Precambrian Guayana 

Shield and the Precambrian shields in Canada, Australia, and 

West and South Africa, supported by information from 

regional reconnaissance exploration in Venezuela (Gibbs 

and Barron, 1983,1993; Sidder and others, 1988; Sidder and 

Mendoza, this volume), indicates that potential for new discoveries of gold and other mineral deposits in Venezuela is 

high. Recent studies by the U.S. Geological Survey and the 

Corporacion Venezolana de Guayana, Tecnica Minera, C. A. 

(TECMIN) (a Venezuelan government-owned mineral 

exploration company) have estimated the number of undiscovered deposits for some deposit types such as low-sulfide 

gold-quartz veins, volcanogenic massive sulfide, and 

nickel-copper-platinum-group elements associated with 

mafic and ultramafic intrusive rocks (Wynn and Sidder, 

1991; U.S. Geological Survey and Corporacion Venezolana 

de Guayana, Tecnica Minera, C.A., 1993).

Early Proterozoic greenstone-belt rocks have produced 

the most gold in Venezuela. The richest gold lodes mined to 

date are in the El Callao mining district (loc. 37, pi. 1), where 

more than 260 gold-bearing quartz veins are present. The El 

Callao mine was the most productive gold mine in the world 

during the latter part of the 19th century (Newhouse and 

Zuloaga, 1929; Knox, 1942). Cumulative production in the 

El Callao district between 1829 and 1990 was about 200 

metric tons of gold (Bellizzia and others, 1981; Vicente 

Mendoza, Tecnica Minera, C.A., written commun., 1991), 

and peak production was in 1885 when 8,194 kg was produced (Locher, 1974). Mineria de Venezuela (MINERVEN) 

(a Venezuelan government-owned mining company) produced 1,820 kg of gold in 1990 from 188,800 metric tons of 

ore having an average grade of 9.64 grams of gold per metric 

ton from the underground Colombia mine in the El Callao 

district (Santaella, 1991). Reserves to a depth of 250 m at the 

Colombia mine are estimated to be about 14,300 kg from 

1.55 million metric tons of ore having an average grade of 

9.2 grams of gold per metric ton (Engineering and Mining 

Journal, 1988). Locher (1974) estimated that potential 

reserves in the El Callao district are about 84 metric tons of 

gold from 4.6 million metric tons of ore having an average 

grade of 18.33 grams of gold per metric ton. Tailings from 

old mines in the El Callao district have been dumped at 

Mocupia Gorge since 1928 (Engineering and Mining Journal, 1988). The tailings were initially estimated to contain 

4.6 million metric tons that averages more than 1.34 grams 

of gold per metric ton (Ensminger, 1992); however, recent 

estimates indicate that less than 600,000 metric tons having 

about 1.5 grams of gold per metric ton are present (Paul A. 

Rose, Monarch Resources, 1993, oral commun.). Slightly 

less than 1 metric ton of gold was produced from the tailings 

in 1990 and 1991 (Ensminger, 1992; U.S. Bureau of Mines, 

1993).

Veins in the El Callao district cut Early Proterozoic 

meta-igneous and metasedimentary rocks that are 

metamorphosed to the greenschist facies and locally to the
O4 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

amphibolite facies (Menendez, 1968). The protoliths consisted of a submarine sequence of tholeiitic mafic volcanic 

rocks, a sequence of tholeiitic to calc-alkalic basalt to rhyolite, and an interval of turbiditic graywacke, volcaniclastic 

rocks, and chemical sedimentary rocks in the basal, middle, 

and upper parts, respectively. Mafic metavolcanic rocks of 

the basal El Callao Formation of the Pastora Supergroup 

(Sidder and Mendoza, this volume, table 1) are the most 

common ore host, and metavolcaniclastic and metasedimentary rocks in other parts of the greenstone-belt sequence less 

commonly host ore (Menendez, 1972). Faults and shear 

zones localized the mineralization. Quartz veins are typically 

from 2 cm to more than 10 m in thickness, and the quartz is 

milky white to gray and locally banded. Native gold and 

minor to trace amounts of pyrite, tetrahedrite, chalcopyrite, 

bornite, scheelite, molybdenite, galena, pyrrhotite, and 

sphalerite are present in the quartz veins. Carbonate minerals 

(commonly ankerite, siderite, and calcite) are also present, 

and carbonate flooding, as well as propylitization, serialization, and silicification, has affected the wallrocks as much as 

several tens of meters away from the veins (Bundy, 1943; 

Banerjee and Moorhead, 1970; Menendez, 1974). Tourmaline and mariposite (chrome mica) are variably present in the 

veins and in the alteration assemblage (Macdonald, 1968).

The low-sulfide gold-quartz vein model (Berger, 

1986a; Bliss and Jones, 1988; Klein and Day, 1994), also 

known as shear-zone-hosted, mesothermal, metamorphic, or 

orogenic gold, best characterizes mineral deposits in the El 

Callao district. Characteristics of alteration and mineralization in deposits at El Callao are similar to those of Archean 

vein-type deposits in the Superior province of the Canadian 

Shield (Card and others, 1989; Klein and Day, 1994) and 

lode-gold deposits of the Juneau gold belt in southeastern 

Alaska (Goldfarb and others, 1988, 1991). It is possible that 

the pinchout of metasedimentary and metavolcanic rocks of 

the Cicapra Formation and the fault contact between the 

Yuruari and El Callao Formations in the El Callao area represents a "break" (a terrane boundary or tectonically disturbed zone) similar to the Larder Lake "break" in the 

Abitibi greenstone belt of the Superior province (Hinse and 

others, 1986; Kerrich and Wyman, 1990). Low-sulfide 

gold-quartz vein deposits in the El Callao and other districts 

in Venezuela are also similar to gold deposits in the Early 

Proterozoic Birimian greenstone belts of the West African 

craton, such as the Syama deposit in Mali and deposits in 

Ghana (Leube and others, 1990; Milesi and others, 1991, 

1992; Eisenloh;, 1992; Olson and others, 1992; Dzigbodi-Adjimah, 1993; Oss, 1993a; Mumin and others, 1994). 

The deposits in Ghana, such as Ashanti, Prestea, and Bibiani, 

are also located in extensive shear zones and near breaks or 

transition zones between predominantly volcanic and predominantly sedimentary rocks (Leube and others, 1990; 

Appiah and others, 1991; Oberthiir and others, 1991; Milesi 

and others, 1992; Dzigbodi-Adjimah, 1993; Watkins and 

others, 1993). The transition zones are characterized by

chemical sediments such as chert, iron-magnesium 

carbonate, and carbon-rich argillite and are the foci for the 

gold deposits (Leube and others, 1990; Milesi and others, 

1992). West Africa is also currently experiencing an exploration boom (Martineau, 1994). Gold, diamonds, and heavy 

minerals are the premier exploration and development 

targets, and nickel, copper, manganese, and graphite deposits are also being sought. Reported gold production in Ghana 

alone almost tripled between 1987 and 1993 from 10.2 to 

26.3 metric tons (Oss, 1993a), and new discoveries have 

been made in several other countries including Mali, Ivory 

Coast, and Guinea (Oss, 1993b; Martineau, 1994).

The Kilometer 88 (Iocs. 99, 100, and 104, pi. 1) and the 

Lo Increible (loc. 38) districts were two other areas of extensive exploration and mining activity in 1992. Mines in the Lo 

Increible district exploit gold-quartz-tourmaline veins, 

similar to those in the El Callao district, in a shear zone 

(Ferrand and others, 1984; Day and others, this volume). 

Most of the recent exploration boom in Venezuela is 

centered on the Kilometer 88 district (Danielson, 1993; 

Doan, 1994). Rocks in the Kilometer 88 district are intensely 

weathered, and the geology is not well understood. Hydraulic methods are commonly used to mine the ore. Pedro Lira 

(Ministerio de Energia y Minas, oral commun., 1988) and 

Floyd Gray and others (U.S. Geological Survey, written 

commun., 1994) described the weathered host rocks as schist 

similar to that in the El Callao area and granitic rocks. Veins 

in the Kilometer 88 area contain native gold and pyrite, trace 

to minor amounts of chalcopyrite and molybdenite, and 

gangue of quartz, dolomite, hematite, tourmaline, and sericite. Wallrock alteration consists of propylitization, carbonatization, and argillization. Faults and shear zones localized 

mineralization. Gold production in 1987 at the Cristina 5 

mine in the Kilometer 88 district was 1,700 kg from about 

142,000 metric tons containing an average of 12 grams of 

gold per metric ton, 0.36 percent copper, and 50 ppm lead 

(Pedro Lira, oral commun., 1988). High-grade ore contains 

as much as 23 grams of gold per metric ton, 7.3 percent copper, 11.0 percent molybdenum, and 7.0 percent lead. An estimated 40 metric tons of gold has been mined using primitive 

techniques in the last decade in the Kilometer 88 district 

(The Mining Record, 1992). Recent drilling at Las Cristinas 

indicates reserves of about 8.6 million ounces at a grade of 

about 0.04 ounces per ton (The Northern Miner, 1994). 

Characteristics of alteration and mineralization in the Kilometer 88 district indicate that the prospects and deposits may 

be porphyry copper±molybdenum±gold or low-sulfide 

gold-quartz veins.

Tecnica Minera, C.A., has explored several properties 

in eastern Venezuela. Bochinche (loc. 24, pi. 1), Marwani 

(Iocs. 26, 27), and Marwani-Los Caribes (loc. 31) are 

quartz-tourmaline vein prospects. The host rocks at 

Bochinche include a metamorphosed turbiditic sequence of 

graywacke, siltstone, and shale and mafic to intermediate 

tuff (Wynn and Sidder, 1991). Major faults and shear zones
MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD 05

cut these rocks, and gold-bearing quartz veins both parallel 

with and at an angle to foliation cut the metasedimentary and 

metavolcanic rocks. Bochinche resembles some turbidite-hosted gold deposits such as Loulo in Mali and possibly 

Dorlin in Guyana (Boyle, 1986; Milesi and others, 1989, 

1992; Dommanget and others, 1993; Fouillac and others, 

1993). Host rocks at Marwani consist of mafic to felsic volcanic and volcaniclastic rocks, siliceous chert, lean iron formation or ferruginous chert, and phyllite. Exhalative 

chemical sedimentary rocks associated with a quartz-tourmaline vent breccia at Marwani (loc. 26, pi. 1) (Sidder and 

others, 1988; Marsh and others, this volume) indicate that 

stratabound Algoma-type banded iron formation-hosted 

gold deposits may be present in the Marwani area. The prospect at Piston de Uroy (loc. 98) contains quartz±carbonate-sulfide veins (without tourmaline) that cut a 

mafic-ultramafic complex of cumulate rocks (Wynn and others, this volume). Gold has been mined from placers in 

streams that drain these areas.

Other lode gold prospects shown on plate 1 have little 

or no recorded production. La Camorra (loc. 91) reportedly 

contains as much as 11,000 kg of gold from 570,000 metric 

tons that grades from 9.1 to 24.4 grams of gold per metric ton 

and averages about 20 grams of gold per metric ton to 300 m 

depth (Engineeing and Mining Journal, 1988; Barnard, 

1990). The gold is hosted in quartz-tourmaline veins that cut 

Early Proterozoic greenschist-facies rocks. Gold in a prospect near Cerro La Pinto (loc. 58) is in shear zones that cut a 

sequence of mafic metabasalt and metagabbro (Sidder and 

Mendoza, this volume). These deposits also may be representative of the low-sulfide gold-quartz vein model (Berger, 

1986a; Bliss and Jones, 1988).

Vuelvan Caras (loc. 25, pi. 1) and Payapal (loc. 88) are 

the only known gold deposits in Venezuela hosted by granitic rocks, which are of probable Early Proterozoic age. The 

deposits are similar to deposits in Guyana, such as those at 

Omai, Peters' Mine, and Eagle Mountain, that are in stockwork veins within and at the periphery of granitic to dioritic 

plutons that formed from about 2,100 to 1,800 Ma, as dated 

by whole-rock rubidium-strontium isochrons (Macdonald, 

1968; Barren, 1969; Carter and Fernandes, 1969; Elliot, 

1986; Barnard, 1990; Bertoni and others, 1991; Gibbs and 

Barren, 1993). Several authors have suggested that gold 

deposits in greenstone-belt rocks in Guyana, Suriname, and 

French Guiana are spatially, if not genetically, related to granitic intrusions (Carter and Fernandes, 1969; Banerjee and 

Moorhead, 1970; Dahlberg, 1975; Blanc and others, 1980). 

In Venezuela, the association of granitic plutons and 

gold-bearing quartz veins is not well documented. Granitic 

rocks in the Early Proterozoic greenstone-granite terrane of 

the West African craton are estimated to contain only about 

3 percent of the gold reserves (Milesi and others, 1992).

Placer deposits of gold have been mined extensively in 

the Venezuelan Guayana Shield. Mining methods vary from 

panning and amalgamation with mercury to large dredges

with a series of shaker tables, jigs, grizzlies, sorters, and mercury plates. Dohrenwend and others (this volume) identified 

three types of placer deposits in the southern Gran Sabana 

area of southeasternmost Venezuela: (1) diamond placers 

within modern channels of major rivers, (2) gold and diamond placers in colluvial-alluvial deposits of low-order 

drainages; and (3) gold and diamond paleoplacers associated 

with conglomeratic lenses and beds at several different 

levels within the lower part of the Roraima Group (lower 

500-600 m of the Uairen Formation). Thus, the basal part of 

the Early to Middle Proterozoic Roraima Group is the source 

for modern placer gold and diamonds (Mendoza, 1985). 

Holocene paleoplacers (possibly about 8,000-10,000 years 

old) have also been mined in some areas, such as at Chiricayen (loc. 134, pi. 1), and probably are the immediate 

source of gold in some modern alluvial placers.

Pre-Roraima Early Proterozoic metaquartzite and metaconglomerate such as rocks of the Los Caribes and Cinaruco 

Formations are also possible exploration targets for gold, 

diamonds, and perhaps uranium. These rocks are correlative 

with metasedimentary rocks such as the gold-bearing Orapu 

Formation in French Guiana and metamorphosed conglomerate, arkose, sandstone, and minor shale of the Tarkwaian 

Series in the West African craton (Black, 1980; Ghosh, 

1985; Norman and Appiah, 1989; Vinchon, 1989; Leube and 

others, 1990; Ledru and others, 1991; Eisenlohr and Hirdes, 

1992; Milesi and others, 1992; Ledru and others, 1994). 

Deposits of the Tarkwa district in Ghana have produced 

about 165 metric tons of gold (Milesi and others, 1992). 

Rocks of the Birimian greenstone belts are the source of the 

Tarkwaian sediments, and paleoplacer deposits in 

quartz-pebble conglomerate of the Tarkwaian Series in the 

Ashanti belt in Ghana were derived from the earliest 

shear-zone-hosted gold-quartz vein deposits in Birimian 

rocks (Vinchon, 1989; Milesi and others, 1991,1992; Eisenlohr, 1992; Eisenlohr and Hirdes, 1992; Davis and others, 

1994).

A few gold placer deposits, such as La Planada (loc. 13, 

pi. 1), are within the area of outcrop of the Archean Imataca 

Complex or are near the Guri fault, which separates the 

Imataca Complex from the Early Proterozoic greenstone-belt rocks. The Imataca Complex contains deposits of 

enriched banded iron formation; however, the precious-metal content of these deposits is reportedly low 

(Engineering and Mining Journal, 1987). Hence, the source 

of the gold in placer deposits within the Imataca terrane is 

uncertain. Gold-bearing greenstone-belt rocks have been 

assumed to be the source of the gold because the placers are 

downstream from these rocks; however, it is possible that the 

Imataca Complex contains unknown, undiscovered gold 

resources.

Other types of gold-bearing deposits may also be 

present in the greenstone-belt rocks. Gold in a thin sequence 

of sheared rocks (finely laminated, rhythmically bedded 

exhalative metachert, graphitic metashale, and mafic
O6 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

metavolcanic rocks) at the Cerro La Pinto prospect is similar 

to Homestake-type gold deposits (Berger, 1986b; Sidder and 

others, 1991; Klein and Day, 1994). Homestake-type gold 

deposits may also be present in other areas within the greenstone belts such as Eldorado, Kaburi, and Honey Camp in 

Guyana (Gibbs and Barron, 1993).

Epithermal and bonanza-type precious-metal vein 

deposits are uncommon in Precambrian rocks (Hutchinson, 

1987). Propylitic and potassic hydro thermal alteration in felsic to intermediate pyroclastic volcanic rocks in the Early 

Proterozoic Cuchivero terrane, geochemically anomalous 

amounts of silver (<3 ppm), bismuth (< 1,000 ppm), and 

molybdenum (<20 ppm) in quartz-sulfide veins such as at 

Merevari (loc. 151, pi. 1), and gold in panned concentrates 

imply, however, that epithermal deposits may be hosted by 

these rocks (Sidder and Martinez, 1990). Deposition of sedimentary rocks of the Early to Middle Proterozoic Roraima 

Group soon after the volcanic rocks, as evidenced by locally 

conformable contacts (Sidder and Mendoza, this volume), 

may have preserved deposits of this type in the Guayana 

Shield.

A potentially important type of gold deposit in tropical 

areas is saprolitic or lateritic gold. These eluvial deposits 

generally have higher grades of ore in the weathered zone 

than in underlying primary ore (Macdonald, 1968; Lecomte 

and Colin, 1989; Barnard, 1990; Smith and others, 1991; 

Minko and others, 1992; Olson and others, 1992; Wilhelm 

and Essono Biyogo, 1992). Deposits such as Omai, Akaiwong, and Tassawini in Guyana have been mined in the past, 

and additional reserves may be present (Macdonald, 1968; 

The Northern Miner, 1989; J.D. Bliss, U.S. Geological Survey, written commun., 1989). At Omai, the weathered zone 

(as deep as 3 m) contains 1.5 million metric tons of ore that 

averages 3.19 grams of gold per metric ton; saprolitic material between 3 and 17m below the surface contains about 10 

million metric tons of ore that averages 1.44 grams of gold 

per metric ton; and the primary zone contains indicated and 

inferred reserves of 27 million metric tons of ore that averages 1.47 grams of gold per metric ton (Elliot, 1986; Ortslan, 

1987). Barron (1973) described several other prospects in 

Guyana in which gold-enriched lateritic soils overlie lower 

grade, gold-bearing quartz veins in greenstone-belt or granitic rocks. S.D. Olmore (oral commun., 1991) noted that at 

some prospects in greenstone-belt rocks of Venezuela 

gold-enriched soils overlie lower grade mineralized rocks. 

Mines in the Kilometer 88 area may be representative of 

enriched ore in the weathered zone overlying mineralized 

greenstone-belt or granitic rocks. The Ity deposit in the Ivory 

Coast of West Africa is an open-pit, heap-leach mine in 

auriferous laterite; reserves are 500,000 metric tons grading 

7 grams of gold per metric ton (Milesi and others, 1992; Oss, 

1993b). Recent investigations in West Africa (Danti and 

Brimhall, 1989; Davies and others, 1989; Minko and others, 

1992; Beauvais and Colin, 1993; Bowell, 1993, Bowell and 

others, 1993; Colin and others, 1993; Costa, 1993) on the

mineralogy and geochemistry of tropical rain forest soils and 

the mobility and distribution of gold in lateritic profiles are 

helpful guides to exploration for gold in the Guayana Shield. 

Secondarily enriched surface and near-surface gold deposits 

in the Guayana Shield are particularly attractive exploration 

targets because they can be mined by open-pit heap-leach 

methods.

The lithologic, petrologic, and tectonic characteristics 

of the greenstone belts are favorable for volcanogenic massive sulfide deposits; however, exploration throughout the 

Guayana Shield to the present has not identified any 

base-metal sulfide prospects in the greenstone belts of 

Venezuela or any massive sulfide deposits elsewhere in the 

shield (Gibbs and Barron, 1983). The Aremu airstrip 

prospect in Guyana is a possible massive sulfide deposit 

(Macdonald, 1968; Barron, 1973). There, a 1.6-m-thick horizon containing disseminated pyrite and chalcopyrite 

averages 0.67 percent copper for a strike length of about 

150 m (Barron, 1973). Mineralized rock pinches out in all 

directions. This horizon is conformable within a sequence of 

lava of intermediate composition, tuff, and minor carbonaceous sedimentary rocks (Barron, 1973). Groete Creek in 

Guyana is possibly a sediment-hosted massive sulfide 

deposit. Metamorphosed argillite, graywacke, and minor 

mafic volcanic rocks host a conformable zone of pyrite-chalcopyrite-gold-mineralized rock that has been traced along 

strike for more than 1,500 m (Macdonald, 1968; Barron, 

1973; Fred Barnard, oral commun., 1991). Proven and indicated reserves are about 30 million metric tons having an 

average grade of about 0.29 percent copper and 0.5-1.0 

grams of gold per metric ton (Barron, 1973; Gibbs and Barron, 1993). Vertically dipping, low-grade gold-bearing 

quartz veins and stringers containing disseminated 

chalcopyrite (Macdonald, 1968) may represent stockwork 

feeders below a massive sulfide deposit. Perkoa in Burkina 

Faso is the only known stratabound polymetallic massive 

sulfide deposit in Early Proterozoic rocks of West Africa. It 

contains about 4.5 million metric tons of ore at 17 percent 

zinc and about 60 grams of silver per metric ton (Milesi and 

others, 1989, 1992). A lead-lead date on galena from the 

deposit yielded an age of 2,120±41 Ma (Marcoux and others, 

1988). Amphibolite-facies metasedimentary rocks that have 

been hydrothermally altered and intensely deformed host the 

two main ore bodies (Ratomaharo and others, 1988; Milesi 

and others, 1989, 1992). The deposit formed in an intercontinental rift or a back-arc basin tectonic setting (Ratomaharo 

and others, 1988; Milesi and others, 1992).

DIAMONDS

Diamonds are recovered from placer mining operations 

throughout the Venezuelan Guayana Shield. Production in 

1991 totalled 213,557 carats, about 48 percent of which was 

gem quality and 52 percent industrial quality. Reported
MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD O7

production in 1992 was 478,000, about 63 of which percent 

was gem quality. These totals compare with peak production 

in 1974 of 1,248,979 carats, and total production between 

1913 and 1992 of 14,917,744 carats, about 27 percent of 

which was gem quality (Anez, 1985; Baptista and Parra, 

1985; Newman, 1989; Ensminger and others, 1990; Ensminger, 1992; Doan, 1994). The Quebrada Grande area (loc. 

66, pi. 1) and the San Salvador de Paul mine (loc. 81) are the 

largest producers of diamonds. The former accounted for 

about 3,654,830 carats between 1975 and 1980, or 76 percent of all diamond production, whereas the latter totalled 

about 823,920 carats, or 17 percent of production (Anez, 

1985). The majority of deposits are along the Rio Caroni (for 

example, Iocs. 9, 17, 18, 20, 41, 80, 82, 117, 118, 120, 121, 

124, and 146) and along the Rio Icabaru and on the south end 

of the Gran Sabana area (for example, Iocs. 125-130, 132, 

133, and 135-145). All of the diamonds produced thus far 

have been recovered from placer deposits. Some placer 

deposits, particularly in the Gran Sabana area, have produced gold as well as diamonds.

Conglomerate in the lower part of the Roraima Group 

has been proposed as the source of diamonds by many investigators (Pollard and others, 1957; Bricefio, 1984; Mendoza, 

1985). Reid (1974a) suggested that kimberlite, possibly in 

Brazil or even West Africa, was the source for the paleoplacer deposits in the Roraima Group. Studies at San 

Salvador de Paul (Bricefio, 1984) and elsewhere along the 

Rio Caroni and in the Gran Sabana area (Reid, 1974b; Reid 

and Bisque, 1975; J.C. Dohrenwend, U.S. Geological 

Survey, 1989, oral and written communs.) confirm that conglomerate of the Uairen Formation within the Early to Middle Proterozoic Roraima Group was the source of the alluvial 

diamonds. Bricefio (1984) stated that conglomerates in the 

Uairen Formation are themselves paleoplacers and were the 

source for diamond-bearing gravels deposited about 8,000 

years ago at San Salvador de Paul. These Holocene paleoplacers are the source of diamonds in some deposits in currently mined drainages (Bricefio, 1984).

Minerals indicative of kimberlite, such as chrome 

pyrope and magnesian ilmenite, were identified in the 

1970's in stream sediments of the Quebrada Grande area 

(loc. 66, pi. 1) (Baptista and Svisero, 1978; Meyer and 

McCallum, 1993). More than a dozen diamond-bearing kimberlitic dikes and sills containing chrome pyrope, titanium-rich phlogopite, chromite, and yimengite 

(K(Cr,Ti,Fe,Mg) 12019), a rare alteration product of chromite 

previously recognized only in kimberlite from China, have 

also been discovered recently (Nixon, 1988; Nixon and 

Condliffe, 1989; Nixon and others, 1989; Meyer and McCallum, 1993). Leached whole-rock kimberlite samples from 

the Quebrada Grande area yielded a rubidium-strontium isochron date of about 1,732±82 Ma (Nixon and others, 1992). 

Although neither kimberlite nor its indicator minerals has 

been identified elsewhere in Venezuela, it is possible that 

diamond-bearing kimberlite was intruded along north-northwest-striking fractures that extend throughout the western 

part of the Guayana Shield in Estado Bolivar, Estado Amazonas, and into Brazil. Sediments of the Roraima Group may 

have incorporated diamonds from the older Early Proterozoic kimberlite during transport and deposition. Alluvial diamonds mined in Ghana and elsewhere in West Africa are 

presumed to have a source in rocks of the Early Proterozoic 

Birimian greenstone belts (Kesse, 1985). Neither kimberlitic 

rocks nor kimberlitic indicator minerals have been identified; however, highly altered, metamorphosed, and 

deformed ultramafic rocks within the Birim diamond field in 

Ghana have major- and trace-element compositions, such as 

extreme light rare earth element enrichment and lanthanum 

to lutetium ratios greater than 30, similar to those published 

for kimberlite elsewhere (Nixon and others, 1992; McKitrick 

and others, 1993).

IRON

Deposits of enriched banded iron formation in the 

Guayana Shield provide Venezuela with abundant resources 

of iron. Production of iron ore and concentrate in Venezuela 

between 1950 and 1992 totalled approximately 650 million 

metric tons (Suarez and others, 1981; Rodriguez, 1987; 

Newman, 1989; Ensminger and others, 1990; Ensminger, 

1992; Doan, 1994). Four mines were in operation in 1992: 

Cerro Bolivar (loc. 52, pi. 1), San Isidro (loc. 51), Los 

Barrancos (loc. 53), and El Pao (loc. 16). Reserves at Cerro 

Bolivar, San Isidro, Los Barrancos, and the surrounding area 

are greater than 1,855 million metric tons and have a grade 

of about 63 percent iron. At a 55 percent cutoff grade, 

reserves total an additional 8,000-10,000 million metric tons 

(Rodriguez, 1986, 1987). Deposits containing more than 55 

percent iron, such as Cerro Bolivar, Los Barrancos, and San 

Isidro, are generally in hills between 400 and 700 m above 

sea level, an elevation that corresponds with the 

Imataca-Nuria planation surface that developed in the early 

Tertiary (Menendez and Sarmentero, 1985; Schubert and 

others, 1986; Bricefio and Schubert, 1990). In contrast, 

deposits at elevations less than 400 m are small and have 

grades of less than 55 percent iron (Rodriguez, 1986; 

Marcano and others, 1989).

The beds of banded iron formation are associated with 

the Archean Imataca Complex. The complex consists of 

amphibolite- to granulite-facies metasedimentary and 

metaigneous rocks comprising more than 80 percent 

quartzofeldspathic gneiss and granulite, 10-15 percent intermediate to mafic gneiss, granulite, and charnockite, 1 percent metamorphosed banded iron formation, and minor 

dolomitic marble and anorthosite (Dougan, 1976; Sidder and 

Mendoza, this volume). Protoliths of the Imataca Complex 

are possibly as old as 3,700-3,400 Ma (Montgomery and 

Hurley, 1978). The beds of banded iron formation vary in 

primary stratigraphic thickness from a few centimeters to
O8 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

10 m and uncommonly to 200 m. Folds and reverse faults 

have produced an apparent thickness of the banded iron formation of as much as 350 m (Ruckmick, 1963; Ferencic, 

1969). Some individual beds extend for 20 km along strike, 

such as at Cerro Bolivar. The entire stratigraphic sequence of 

the Imataca Complex has been isoclinally folded, and 

enriched banded iron formation ore generally occupies the 

limbs and centers of synclines (Ferencic, 1969). 

Oxide-facies banded iron formation is predominant; hematite and magnetite are the dominant iron minerals in the 

oxide facies (Ruckmick, 1963; Ferencic, 1969). The 

iron-rich beds are intimately interbedded with layers of silica, present as quartz, and iron-bearing silicate minerals such 

as greenalite, grunerite, cummingtonite, chlorite, and sericite. Enriched ore is composed predominantly of goethite and 

limonite and minor hydrohematite or turgite (Fe2O3'nH2O) 

(Ferencic, 1969; Gruss, 1973; Ascanio, 1985; Moreno and 

Bertani, 1985a; Bertani and Moreno, 1989; Marcano and 

others, 1989).

Banded iron formation in the Imataca Complex is commonly compared to deposits of Superior-type banded iron 

formation (Cannon, 1986a), such as those in the Lake Superior region of the United States, at Minas Gerais in Brazil, 

and in the Hamersley Range of Australia (Ruckmick, 1963; 

Kalliokoski, 1965; Ferencic, 1969; Gruss, 1973; Sidder, 

1991). Indeed, Superior-type banded iron formation and 

banded iron formation in the Imataca Complex have many 

similarities. For example, the primary stratigraphic thickness 

of banded iron formation at Cerro Bolivar and San Isidro is 

from less than 5 to 200 m, minor horizons of banded iron formation about 2 m thick are interlayered with metasedimentary rocks, and the banded iron formation is thinly laminated 

and banded (Ruckmick, 1963; Ferencic, 1969). Differences 

with Superior-type banded iron formation and similarities to 

Algoma-type banded iron formation (Cannon, 1986b) are 

also present. For example, strike lengths of banded iron 

formation in the Imataca Complex are generally 20 km or 

less (Ferencic, 1969; Gruss, 1973), and, most importantly, 

banded iron formation at Cerro Bolivar is interpreted to be 

interbedded with metavolcanic rocks, not metasedimentary 

rocks (Dougan, 1977). In addition, laminated calc-silicate 

quartzite is associated with laminated quartz-magnetite and 

orthopyroxene-quartz-magnetite iron formation near Cerro 

Bolivar (Dougan, 1977). The calcareous rocks may represent 

carbonate-facies banded iron formation or carbonate 

alteration in banded iron formation. Algoma-type 

banded-iron-formation-hosted gold deposits are commonly 

associated with carbonate-facies rocks (Phillips and others, 

1984); however, banded iron formation in the Imataca 

Complex is reportedly gold poor (Engineering and Mining 

Journal, 1987), and the metavolcanic rocks are subaerial, 

calc-alkaline, felsic continental arc rocks, not submarine 

mafic to felsic island arc or oceanic rocks (Dougan, 1977). 

Hence, banded iron formation in the Imataca Complex is 

predominantly Superior type, although some Algoma-type

banded iron formation may be present. Both types of banded 

iron formation have been identified in Archean rocks correlative to the Imataca Complex in the Reguibat and Leo 

Shields of the West African craton (Rocci and others, 1991).

Ascanio (1985) divided iron ore deposits in the Imataca 

Complex into three groups on the basis of grain size of the 

banded iron formation: (1) coarse grained (>1 mm), (2) 

medium grained (about 1 mm); and (3) fine grained (<1 

mm). Coarse-grained deposits include El Pao (loc. 16, pi. 1), 

Las Grullas (loc. 10), and Piacoa (loc. 7); Cerro Maria Luisa 

(loc. 19) is a medium-grained deposit; and Cerros Bolivar, 

San Isidro, Los Barrancos, El Trueno (loc. 59), Altamira 

(loc. 49), Redondo (loc. 47), Toribio (loc. 46), Arimagua 

(loc. 45), and others in the immediate area of Cerro Bolivar 

are fine-grained deposits. These three types of deposits are 

separated geographically; coarse-grained deposits are north 

of the El Pao fault toward the east, medium grained deposits 

are between the El Pao and Rio Carapo faults, and fine 

grained deposits toward the west are south of the Rio Carapo 

fault. It is likely that the grade of metamorphism affected the 

grain size of the banded iron formation protore, and subsequently that of the enriched ore, because both grade and 

grain size decrease toward the west (Gruss, 1973). Alternatively, differential erosional stripping that resulted from 

Cenozoic block uplift may have exposed ore at different levels in the complex (Olmore and others, 1993). For example, 

fresh fault scarps along the El Pao fault near El Pao indicate 

that recent vertical movement took place along an older, 

reactivated fault (Short and Steenken, 1962).

Deposits of Algoma-type banded iron formation 

(Cannon, 1986b) may be present in the Early Proterozoic 

greenstone belts. Reports of "ferruginous quartzite," "hematitic-manganiferous siliceous shale," and "jasper" in volcano-sedimentary sequences of turbiditic graywacke and 

felsic to intermediate tuffs (Menendez, 1968; Aguilar, 1972; 

Lira and others, 1985) indicate that this deposit type may be 

present. Early Proterozoic banded iron formation is present 

in greenstone-belt rocks in Suriname, Guyana, and Brazil 

(Gibbs and Barron, 1993). Gold is commonly associated 

with this type of banded iron formation (Cannon, 1986b), as 

previously noted. Early Proterozoic banded iron formation 

within greenstone belts (Algoma type) in West Africa is the 

source of some alluvial and eluvial gold (Milesi and others, 

1992). Manganese-rich rocks and other chemical sediments 

are important exploration guides for gold deposits in the 

Early Proterozoic rocks of the greenstone belts in Ghana 

(Leube and others, 1990). The manganese deposits are on the 

flanks of the volcanic belts and in transition zones from volcanic rocks to basinal sediments. These transition zones, or 

breaks, are characterized by chemical sediments such as 

chert, iron-magnesium carbonate, and carbon-rich argillite 

and are the foci for the gold deposits (Leube and others, 

1990; Milesi and others, 1992). As in the Guayana Shield, 

however, the West African craton is poor in banded iron 

formation (Milesi and others, 1992).
MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD 09

Iron-rich deposits of Early and Middle Proterozoic age 

(1.89-1.45 Ma) such as Kiruna, Sweden, Pea Ridge and others in the St. Francois terrane, Missouri, and Olympic Dam, 

Australia, are associated with subalkalic to alkalic, 

metaluminous to peraluminous granite-rhyolite terranes 

(Meyer, 1988; Sims and others, 1987, 1988). Hematite, 

apatite, pyrite, barite, fluorite, quartz, sericite, monazite, 

actinolite, and chlorite are commonly intermixed in varying 

amounts with the magnetite ore, and Au, Ag, Cu, U, Th, light 

rare earth elements, P, Ba, and F are typically enriched in 

anomalous to economic concentrations. These Olympic 

Dam-type iron-copper-uranium-gold-rare earth element 

magmatic-hydrothermal deposits are genetically related to 

magmas, especially those of intermediate to mafic composition, that formed the granite-rhyolite terrane (Sidder and 

Day, 1993; Sidder and others, 1993). The similarities in age, 

composition, and tectonic environment between the Early 

Proterozoic Cuchivero Group and Middle Proterozoic (1,545 

Ma) Parguaza Granite in Venezuela and the granite-rhyolite 

terranes of the St. Francois Mountains and Olympic Dam 

area suggest that Olympic Dam-type deposits are a possible 

exploration target in the Venezuelan Guayana Shield. A 

highly magnetic area and a superimposed uranium anomaly 

detected in aeromagnetic and aeroradiometric data over the 

Parguaza Granite in Estado Bolivar (lat 5° 10' N., long 64°20' 

W.) (Wynn, 1993) is a good site to investigate.

ALUMINUM

The development of bauxite ore and processing of alumina in Venezuela currently represents the second largest 

source of foreign currency revenue after petroleum (Newman, 1988; Ensminger, 1992; Doan, 1994). The production 

of alumina totalled 1,295,000 metric tons in 1991 and a 

reported 1,308,000 metric tons in 1992 (Doan, 1994); production of aluminum metal as unalloyed ingot totalled 

600,544 metric tons and a reported 561,354 metric tons in 

1991 and 1992, respectively, (Doan, 1994). Bauxite production has increased from 245,157 metric tons in 1987 to a 

reported 1,052,052 metric tons in 1992 (Ensminger, 1992; 

Doan, 1994). The principal deposits of bauxite are at Los 

Pijiguaos (loc. 64, pi. 1), Upata (loc. 15), Nuria (loc. 22), Los 

Guaicas (loc. 79), and in the Gran Sabana (loc. 105-107, 

131).

Deposits of bauxite and aluminum-rich laterite are 

products of intense weathering of granitic, gabbroic, and diabasic rocks in the Guayana and West African Shields. The 

deposit of bauxite at Los Pijiguaos developed on the Middle 

Proterozoic Parguaza rapakivi granite (Moreno and Bertani, 

1985b). The protolith granitic rocks contain from 65 to 73 

percent SiO2 and from 13.5 to 15 percent A^Os. Mining 

started at Los Pijiguaos in 1987, and Bauxita Venezolana, 

C.A., (BAUXIVEN) produced about 245,157 metric tons of 

ore. BAUXIVEN produced a reported 771,422 metric tons

of bauxite in 1990 (Ensminger, 1992). Upgrade of the mine 

facilities has increased capacity to 6 million metric tons per 

year, and projected capacity is 8 million metric tons per year 

(Doan, 1994). Initial measured and indicated reserves of 

bauxite were 201.8 million metric tons averaging 48.7 percent A^Os and 10.9 percent SiO2- These reserves included 

70.1 million metric tons averaging 51.8 percent A12O3 and 

6.4 percent SiO2 (Menendez and others, 1985). Newman 

(1989) reported that proven (200 million metric tons) and 

probable (500 million metric tons) reserves of 700 million 

metric tons are present in the Los Pijiguaos area. Three new 

bauxite deposits on the Parguaza Granite were discovered in 

1989 between the Los Pijiguaos mine and Puerto Ayacucho 

on the Rio Orinoco (Ensminger, 1992). The average thickness of ore at Los Pijiguaos is about 7.5 m, and overburden, 

where present, is less than 1 m thick. Gibbsite is the dominant ore mineral, and lesser amounts of kaolinite are present. 

The ratio of gibbsite to kaolinite decreases with depth.

The richest ore at Los Pijiguaos is at an erosional level 

known as the Imataca-Nuria erosion surface between 620 

and 690 m above sea level that formed during an intense 

weathering cycle in the early Tertiary (Short and Steenken, 

1962; Menendez and Sarmentero, 1985; Schubert and others, 1986). This weathering cycle corresponds to the cycle 

that formed the enriched iron ore at Cerro Bolivar, San 

Isidro, Cerro Altamira, and other deposits of enriched 

banded iron formation, as well as the bauxite deposits at 

Nuria, Upata, and Los Guaicas. It also correlates with a 

bauxite event elsewhere in South America, as well as in 

West Africa and India (Prasad, 1983). Structural and topographic features such as joints, fractures, and steepness of 

slope control the enrichment of A12O3 and depletion of SiO2 

in the granitic rocks (Moreno and Bertani, 1985b; Lo 

Monaco and Yanes, 1990). Those ore zones most enriched in 

alumina and depleted in silica correlate with zones of highest 

fracture density. A slope between 2° and 10° is most favorable for formation of ore because the thickness of ore and 

intensity of silica leaching are more affected by the slope 

than the degree of alumina enrichment (Moreno and Bertani, 

1985b).

The Upata district (loc. 15, pi. 1) contains 13 deposits 

of bauxite, 5 of which are possibly economic (Candiales, 

1961). These include El Chorro, La Mesa de la Carata, El 

Baul, Los Guamos, and Cerro Once. El Chorro has reserves 

of 1,259,250 metric tons, and the other four have combined 

reserves of 2,904,666 metric tons. The grade varies from 39 

to 67 percent A12O3, from 3 to 29 percent Fe2O3, and from 

less than 1 to 23 percent SiO2- The ore is apparently a weathering product of gabbro or amphibolite (Candiales, 1961). A 

large-tonnage, low-grade bauxite deposit that has a high silica content recently has been discovered north of El Palmar 

on the Palsapa Plateau (Vicente Mendoza, written commun., 

1992).

Nuria is a high plateau that forms an amphitheater and 

has a flat center and an elevated ringlike perimeter. The hills
O10 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

are between 600 and 700 m in altitude. Granitic gneiss comprises the central part of the structure, and diabase and gabbro form the peripheral hills. Intense chemical weathering 

enriched the mafic rocks in alumina and formed a low-grade 

ore deposit. The total of measured, indicated, and inferred 

reserves is 50 million metric tons containing 37.5 percent 

A12O3,28.9 percent Fe2O3, and 8.7 percent SiO2 (Candiales, 

1961; Bellizzia and others, 1981).

Deposits of aluminum-rich laterite and locally bauxite 

at Los Guaicas and in the Gran Sabana are weathering products of Proterozoic and possibly Mesozoic diabasic intrusive 

rocks (Bellizzia and others, 1981; Rodriguez, 1986). These 

deposits are relatively low in grade (about 35 percent A12O3, 

30-40 percent Fe2O3, and 3-9 percent SiO2) and generally 

small (much less than 100 million metric tons). The deposit 

at Los Guaicas contains 3.1 percent TiO2 (Bellizzia and others, 1981). The erosion surface in the Gran Sabana area, at 

altitudes between 900 and 1,200 m, is called the Kamarata or 

Wonken surface and is tentatively correlated with the 

Kanuku surface in Guyana, which developed during or prior 

to the Jurassic (Menendez and Sarmentero, 1985; Briceno 

and Schubert, 1990).

MANGANESE

Secondarily enriched deposits of manganese are associated with rocks of the Early Proterozoic greenstone belts and 

with banded iron formation of the Archean Imataca Complex. Deposits at San Cristobal (loc. 23, pi. 1) and La Esperanza (loc. 56) are in the area of outcrop of greenstone-belt 

rocks, and those at El Palmar (loc. 11), Guacuripia (loc. 12), 

El Manganese (loc. 14), Upata (loc. 15), and El Pao (loc. 16) 

are within the Imataca Complex (Bellizzia and others, 1981).

Manganese deposits at San Cristobal are secondarily 

enriched residual accumulations (Aguilar, 1972). Metamorphosed manganiferous chert and siliceous manganiferous 

rocks of sedimentary-volcanogenic origin that have been 

correlated with the Caballape Formation of the Early Proterozoic Botanamo Group form the protore. These rocks are 

intercalated with beds of ferruginous quartzite, dark-gray 

phyllite, greenish chloritic phyllite, bluish-gray kaolinitic 

argillite, and siltstone. The beds of siliceous manganiferous 

rocks are from 0.2 to 2.0 m in thickness, and beds of metachert are as thick as 15 cm. The protore consists of manganese silicate minerals, and enriched secondary ore contains 

psilomelane, pyrolusite, cryptomelane, wad (an impure, 

amorphous mixture of manganese oxide and oxides of other 

elements such as copper, cobalt, and silica), and spessartine. 

Samples of siliceous manganiferous protore and metachert 

contain 17.93 and 6.87 percent MnO, respectively, whereas 

residual secondary ore contains about 50 percent MnO. 

Aguilar (1972) determined that the poorly developed weathering profile and the thinness of the beds make these deposits 

subeconomic. Manganese prospects in the San Cristobal area

are similar to and possibly a continuation of ore-bearing 

strata at Matthews Ridge in Guyana (Holtrop, 1965; Carter 

and Fernandes, 1969; Gibbs and Barron, 1993). Enriched 

spessartite-bearing ore at Matthews Ridge is interlayered 

with metachert, quartzite, and sericite or carbonaceous 

schist, (Holtrop, 1965). Braunite ore and manganite are also 

present at Matthews Ridge. Isoclinal folding and later crossfolds are the principal controls of the secondary ore at Matthews Ridge (Gibbs and Barron, 1993). Serra do Navio in 

Brazil is the largest manganese deposit in the Guayana 

Shield and has measured reserves of 15.7 million metric tons 

at 39.3 percent MnC>2 and total reserves of 20.1 million metric tons (Nagell, 1962; Damascene, 1982; Schobbenhaus and 

others, 1984). Amphibolite-facies quartz-biotite-garaet 

schist of the Vila Nova Group hosts the protore manganiferous carbonate and gondite. Graphite and iron and copper-iron sulfide minerals such as pyrite, pyrrhotite, 

chalcopyrite, and bornite are associated with the rhodochrosite and spessartite-bearing protore horizons. Enrichment of the protore by weathering and oxidation has formed 

secondary oxide ore containing pyrolusite and cryptomelane 

(Nagell, 1962; Holtrop, 1965; Damascene, 1982; Gibbs and 

Barron, 1993). Manganiferous beds and manganese deposits 

such as Nsuta in Ghana, Tambao in Burkina Faso, and Ziemougoula in the Ivory Coast are present in the greenstone 

belts of the West African Shield (Milesi and others, 1992). 

Reserves at Nsuta are about 5 million metric tons of oxide 

ore at 48.9 percent manganese and about 28 million metric 

tons of carbonate ore at 15.3 percent manganese (Kesse, 

1985; Milesi and others, 1992). Manganiferous phyllite, 

schist, and carbonate are the host rocks at Nsuta, and nickeland cobalt-bearing sulfide minerals such as millerite, linnaeite, and pentlandite are associated with the manganese 

ore. The presence of tholeiitic volcanic rocks and siliceous 

beds in the Nsuta area suggests that the manganese ore is 

sedimentary-volcanogenic in origin (Sidder, 1991; Milesi 

and others, 1992).

Beds of manganiferous rocks in the Upata-El Palmar-Guacuripia area are interstratified with rocks of the 

Imataca Complex. These beds crop out within a stratigraphic 

sequence that is less than 500 m thick, and they have strike 

lengths of about 20 km (Drovenik and others, 1967). Individual manganiferous beds are generally less than 10 m thick. 

Manganese protore, which contains from 11 to about 30 percent MnO, is typical of gondite, a metamorphic rock containing spessartine and quartz that is derived from 

manganese-bearing rocks. The protore consists of spessartine garnet, quartz, and graphite and rhodonite-spessartine-rich rocks. These rocks are associated with quartzite, 

ferruginous quartzite, quartz-biotite schist, amphibole-bearing schist, and feldspathic gneiss that commonly contains 

cordierite and sillimanite. Dolomitic marble containing as 

much as 4 percent MnO also is present in the stratigraphic 

sequence and is commonly in direct contact with manganese-rich horizons as at Guacuripia (Drovenik and others,
MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD Oil

1967). Sulfide minerals such as pyrrhotite, pyrite, chalcopyrite, sphalerite, valeriite (2(Fe,Cu)2S2 '3(Mg,Al)(OH)2), and 

millerite (NiS) make up as much as 2 modal percent in 

gondite as small grains from 0.001 to 0.15 mm in diameter. 

Beds of manganese protore in the Upata-El Palmar-Guacuripia area are not intimately associated with banded iron 

formation; however, banded iron formation deposits at El 

Pao and Los Barrancos are enriched in manganese and contain as much as 34 percent manganese and 4-16 percent 

MnO2, respectively (Drovenik and others, 1967). Drovenik 

and others (1967) concluded that the sedimentary-nonvolcanogenic manganese deposit model best represents the protore manganese mineralization in the Imataca Complex; 

however, the lack of manganese carbonate ore minerals and 

the absence of metapelitic rocks suggest that some of the 

gneissic sequence may have an igneous protolith of felsic to 

intermediate volcanic rocks (Dougan, 1977), rather than a 

sedimentary protolith. Thus, the sedimentary-volcanogenic 

manganese model (Sidder, 1991) or the Cuban-type volcanogenic manganese model (Mosier and Page, 1988) may 

characterize some of the manganese deposits in the Imataca 

Complex.

Manganese orebodies in the Upata-El Palmar-Guacuripia area are secondarily enriched. Ore consists of manganese, iron oxide, and iron oxide and hydroxide minerals such 

as cryptomelane, psilomelane, pyrolusite, goethite, lithiophorite [(Al,Li)MnO2(OH)2], and nsutite (or gamma-MnO2) 

[(Mni_x+4Mnx+2O2_2X(OH)2x)] (Drovenik and others, 

1967). These minerals are associated with quartz and silicate 

minerals such as spessartine and mica and clay minerals. 

Minor amounts of hypogene minerals such as magnetite, 

mangano-magnetite, spessartine, rhodonite, braunite, hausmannite, and traces of rhodochrosite are also present. Four 

types of ore have been recognized: (1) earthy, (2) hard, (3) 

pisolitic; and 4) detrital. The first two types form beds that 

have relict protore textures, whereas the latter two are only 

present at the surface. Estimated total reserves in the area are 

434,000 metric tons of earthy ore that averages 25 percent 

manganese, 25 percent iron, and 10 percent SiO2; 21,700 

metric tons of high-grade (46-48 percent manganese), hard 

metallurgical ore; 441,000 metric tons of pisolitic ore that 

averages 20.9 percent manganese, 17.8 percent iron, 12.41 

percent SiO2, and 14.63 percent A^Os; and 104,910 metric 

tons of detrital clastic ore that averages 27.96 percent 

manganese, 14.97 percent iron, and 13.49 percent SiO2 

(Drovenik and others, 1967). These ores do not constitute a 

large resource. Some of these deposits have been mined in 

small quantities during the past 20 years.

The manganese prospects at La Esperanza are similar to 

those at San Cristobal. The protore in bedded manganiferous 

quartzite (apparently metachert of exhalative origin) has 

been secondarily enriched to form a residual deposit that 

contains reserves of about 40,000 metric tons of relatively 

low grade ore that averages about 20-30 percent MnO (Martin, 1976; Bellizzia and others, 1981).

TIN

Placer, eluvial, and lode prospects of cassiterite and tantalum-, niobium-, zirconium-, and titanium-bearing minerals 

have been identified in the extreme western part of Estado 

Bolivar and in Estado Amazonas. The best studied area is 

that surrounding Cano Aguamena (loc. 63, pi. 1), where a 

sequence of highly weathered complex pegmatite dikes, 

quartz veins, and aplite cuts the Middle Proterozoic (about 

1.55 Ga) Parguaza Granite, near or along large regional 

north-northwest-striking faults (Gaudette and others, 1978; 

Rodriguez and Perez, 1982; Perez and others, 1985). The 

granite is massive, unmetamorphosed, coarsely crystalline, 

porphyritic with rapakivi (wiborgite-type) texture, and 

apparently anorogenic in origin. The Parguaza Granite correlates in age and composition with granite in the Surucucus 

area of northernmost Brazil and may correlate with granite in 

the Agua Boa batholith in northern Brazil (Dall'Agnol and 

others, 1994). The Surucucus granite has rapakivi texture, 

formed about 1.6-1.5 Ga, and has potential reserves of 

20,000 metric tons of tin (Schobbenhaus and others, 1984; 

Jones and others, 1986; Dall'Agnol and others, 1994). 

Pitinga, one of the world's largest tin mines, is in the Agua 

Boa batholith and has measured placer reserves, as of 1984, 

of about 60,000 metric tons of tin and measured lode 

reserves of 143,000 metric tons of tin; total measured, indicated, and inferred placer and lode reserves are 98,000 and 

171,000 metric tons of tin, respectively (Schobbenhaus and 

others, 1984; Daoud and Antonietto, 1985, 1988; Damascene, 1988; Thorman and Drew, 1988). Pegmatites and 

quartz veins in the Carlo Aguamena area host the mineralized lode prospects identified thus far (Rodriguez and Perez, 

1982; Perez and others, 1985), unlike the lode ore at Pitinga, 

which is hosted by greisenized granite (Daoud and Antonietto, 1985, 1988; Horbe and others, 1985, 1991; Gibbs and 

Barren, 1993). Neither hydrothermal alteration nor greisenization of granite have been detected in the Cano Aguamena 

area. Minerals in the pegmatites and quartz veins include 

cassiterite, tantalum-rich rutile or struverite, tantalum-niobium-iron-manganese-bearing rutile, tantalite-columbite, 

and stanniferous tantalite or ixiolite (Aarden and Davidson, 

1977). In addition, hafnium-rich zircon, simpsonite 

(Al4(Ta,Nb)3(O,OH,F) 14), and a uranium-bearing lead tantalate mineral are present as inclusions in rutile. Tungsten-bearing minerals and sulfide minerals have not been 

observed in any samples. Although resources of tin or other 

metals have not been defined to date, analyses of shallow 

drill holes in hills near Cano Aguamena indicate that 11-13 

kg of heavy minerals per cubic meter are present and contain 

0.01-0.77 percent tin, 0.01-0.23 percent niobium, 1.8-29,0 

percent titanium, and 0.5-11.1 percent zirconium (Perez and 

others, 1985). Exploration continues throughout Estado 

Amazonas and western Estado Bolivar for secondary 

alluvial and eluvial deposits, as well as for primary pegmatite- or granite-hosted deposits (Rodriguez and Perez, 1982).
O12 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

At Alto Paragua (loc. 149, pi. 1), cassiterite is in peraluminous, high-silica rhyolite of the Early Proterozoic Caicara Formation of the Cuchivero Group and in panned concentrates. Cassiterite, barite, apatite, and copper-bearing 

pyrite are small (as much as 8 ^im by 4 (J,m) disseminated 

grains in rhyolitic tuff; several grains of cassiterite were 

present in heavy-mineral samples collected from creeks that 

drain into the Rio Paragua (Sidder 1990). Geochemically 

anomalous concentrations of tin in panned-concentrate, 

stream-sediment, and whole-rock samples in the upper Rio 

Paragua area (Sidder and others, 1991) are typical of rhyolite-hosted tin deposits (Duffield and others, 1990). Granitic 

rocks of the Uatuma magmatic event, such as the Mapuera 

and the Maloquinha suites in Brazil, host some tin prospects 

and deposits and are correlative with plutonic rocks of the 

Cuchivero Group in Venezuela (Damascene, 1988).

NIOBIUM, TANTALUM, AND RARE 

EARTH ELEMENTS

Some prospects of niobium, tantalum, and rare earth 

elements are associated with complex pegmatites in the Parguaza Granite (pi. 1, loc. 63, prospect near Cano Aguamena), 

as discussed in the section on tin, and with pegmatites in the 

Imataca Complex (pi. 1, prospects near Ciudad Bolivar). The 

richest resource of rare earth elements, niobium, thorium, 

and barium in Venezuela is Cerro Impacto (loc. 67).

The Cerro Impacto prospect is interpreted to be a 

deeply weathered carbonatite (Aarden, Iturralde de Arozena, 

Navarro, and others, 1978). The carbonatite is associated 

with an oval ring structure about 10 km in diameter. A thick 

lateritic cover at Cerro Impacto that extends to at least 200 m 

depth does not retain any traces of the original rock (Garcia 

and Aarden, 1977). The laterite is enriched in iron (as much 

as 61 weight percent), manganese (as much as 35 weight 

percent), aluminum (as much as 25 weight percent), barium 

(as much as 58 weight percent), thorium (as much as 0.5 

weight percent), niobium (as much as 1.5 weight percent), 

rare earth elements (as much as 7 weight percent cerium, 3 

weight percent lanthanum, and 0.8 weight percent 

neodymium), titanium (as much as 7 weight percent), zinc 

(as much as 0.8 weight percent), lead (as much as 0.5 weight 

percent), and other elements. Rocks that crop out in the 

area immediately surrounding the enriched laterite 

are alkalic granite, monzonite, granodiorite, quartz diorite, 

tonalite, and gabbro, some of which are fenitized. Fragments 

from the bottom part of some drill holes consist of fenitized 

rock, barite, and quartz pseudomorphs after carbonate 

crystals. Other minerals and mineral fragments in 

the laterite include goethite, pyrolusite, wad, gibbsite, 

kaolinite, gorceixite (BaAl3(PO4)2(OH)5H2O), goyazite 

(SrAl3(PO4)2(OH)5 H2O), florencite (CeAl3 (PO4)2(OH)6), 

bastnaesite [(Ce,La)CO3(F,OH)J, and monazite (Aarden,

Holm, and others, 1978; Aarden, Iturralde de Arozena, Moticska, and others, 1978). These characteristics suggest that 

an intensely weathered carbonatite is present at Cerro 

Impacto.

The Cerro Impacto complex is in proximity to large 

northwest-striking fractures. These fractures may be extensions of those along which kimberlite intruded in the Quebrada Grande area (loc. 66, pi. 1) northwest of Cerro 

Impacto, and they are parallel with large regional fractures 

that apparently controlled emplacement of pegmatitic dikes 

into the Parguaza Granite. These fractures extend throughout 

the western part of the Guayana Shield in Estado Bolivar, 

Estado Amazonas, and into Brazil.

The age of intrusion of Cerro Impacto is unknown. 

Mendoza and others (1977) suggested that the carbonatitic 

complex intruded plutonic rocks of the Cuchivero Group 

during the Mesozoic between 150 and 80 Ma; however, the 

date of about 1,732182 Ma reported by Nixon and others 

(1992) for kimberlite in the Quebrada Grande area suggests 

that the carbonatite at Cerro Impacto may be much older than 

Mesozoic. Cerro Impacto may correspond in age with Early 

Proterozoic kimberlite.

URANIUM

Reconnaissance geologic and airborne geophysical 

investigations for uranium in the Guayana Shield have 

detected numerous areas that have anomalously high radiometric signatures (Pasquali, 1981). The most notable anomalies in Venezuela are associated with the Churuata ring 

structure in Estado Amazonas, with gneiss of the Imataca 

Complex, and with conglomerate of the Roraima Group. 

Two unnumbered prospects in Estado Amazonas (pi. 1) were 

identified from airborne radiometric anomalies and are possibly related to the Parguaza Granite or to rocks of the 

Cuchivero Group (U.S. Geological Survey and Corporation 

Venezolana de Guayana, Tecnica Minera, C.A., 1993). Viable resources of uranium have not been identified to date 

elsewhere in the shield; the only identified occurrences in the 

West African Shield are associated with pegmatite (Kesse, 

1985).

The Churuata ring structure (loc. 152) is an Early (?) to 

Middle (?) Proterozoic alkalic complex that intrudes sedimentary rocks of the Roraima Group (Scares, 1985). Plutonic rocks of the ring structure, which include syenite, 

quartz syenite, nepheline syenite, granite, and alaskite, are 

deeply weathered. Whole-rock rubidium-strontium isochrons for granite and syenite in the complex yielded dates 

of 1,318±41 Ma and 1,274±34 Ma, respectively; potassium-argon analyses of amphibole and nepheline in syenite 

yielded dates of 1,288±40 Ma and 1,230±40 Ma, respectively. These dates may be reset ages because many rubidium-strontium and potassium-argon analyses in the western 

part of the shield yield dates of about 1,200±100 Ma for
MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD O13

Archean and Early Proterozoic rocks due to the Nickerie 

orogeny (Kroonenberg, 1982; Sidder and Mendoza, this volume). The concentration of uranium in whole-rock samples 

is from less than 1 to 165 ppm, and thorium is from 8 to 560 

ppm. Uranium-bearing minerals have not been observed, but 

uranium is probably associated with refractory minerals such 

as zircon or monazite. Other elements such as tin, lanthanum, yttrium, arsenic, lead, zinc, tungsten, zirconium, titanium, rubidium, and niobium are enriched in eluvial and 

alluvial material (Scares, 1985).

Anomalous radioactivity related to gneiss of the 

Imataca Complex and conglomerate of the Roraima Group 

has been detected during ground reconnaissance surveys 

(Pasquali, 1981). For example, samarskite was observed in 

pegmatitic phases of granitic migmatite within the Imataca 

Complex (Dougan, 1975). Analysis of samarskite in pegmatitic veinlets that cut rocks of the Imataca Complex indicated 

230 ppm equivalent uranium (Wyant and others, 1953). Chip 

samples of granitic gneiss contained 50 ppm equivalent uranium. Although these observations are of scientific interest, 

they have not led to the discovery of any uranium prospects. 

If, however, the Imataca Complex is slightly enriched in uranium, its rocks may have served as a source to as yet undiscovered deposits in younger sedimentary rocks within the 

Orinoco Basin or in small intracratonic basins on the Guayana Shield (Audemard, 1977).

Anomalous radioactivity has been measured in rocks of 

the Roraima Group at the southern end of the Gran Sabana 

between Santa Elena de Uairen and Icabaru (loc. 139, pi. 1) 

(Brooks and Nunez, 1991). The apparent similarity between 

basal quartz-pebble conglomerate of the Roraima Group and 

gold-uranium-bearing quartz-pebble conglomerates of the 

Witwatersrand, South Africa, Jacobina, Brazil, and Blind 

River, Canada, and the reported presence of authigenic 

pyrite (Gallagher, 1976; Bellizzia and others, 1981) led to 

the proposal that the basal conglomerate of the Roraima 

Group is a viable exploration target for gold and uranium 

deposits, as well as a possible source for younger alluvial 

deposits (Mendoza, 1985). Rocks of the Roraima Group are, 

however, significantly younger (about 1,900-1,500 Ma) 

than the Early Proterozoic and Archean (3,100-2,200 Ma) 

gold-uranium deposits and were deposited under a more 

oxygenated atmosphere. Uranium potential in the Roraima 

Group is low.

Although pre-Roraima metasedimentary rocks such as 

the Los Caribes and Cinaruco Formations are lithologically 

similar to those of the Roraima Group and may represent the 

same environments of deposition (Ghosh, 1985), the Los 

Caribes Formation is unexplored, and its mineral potential is 

unknown. Equivalent Early Proterozoic metasedimentary 

rocks of the Tarkwaian Series in the West African craton 

host gold-bearing paleoplacer deposits (Black, 1980; Norman and Appiah, 1989; Milesi and others, 1991; Eisenlohr 

and Hirdes, 1992). Uranium has not been identified in the 

Tarkwa-type gold deposits. The Tarkwaian-hosted paleoplacer deposits were concentrated and subsequently 

deformed during the Eburnean orogeny; however, the source 

of the Tarkwaian-hosted gold deposits is unknown (Milesi 

and others, 1991; Eisenlohr, 1992).

MOLYBDENUM

Molybdenite has rarely been reported in any descriptions of mineral prospects or deposits in the Venezuelan 

Guayana Shield. Molybdenum is uncommonly associated 

with tin in pegmatite within the Parguaza Granite and as a 

trace metal associated with vanadium, gallium, and tin at 

Cerro Impacto (Bellizzia and others, 1981). Molybdenite has 

recently been reported as an accessory ore mineral in the 

Kilometer 88 district, and molybdenum is present at ore 

grade together with gold, copper, and silver (The Mining 

Record, 1992). The molybdenum prospects shown on plate 

1 are taken from the metallogenic maps of Venezuela 

(Rodriguez and others, 1976; Bellizzia and others, 1980) and 

are identified as minor isolated veins that cut Middle 

Proterozoic granitic rocks (probably of the Early Proterozoic 

Cuchivero Group or the Middle Proterozoic Parguaza 

Granite).

Mendoza and others (1977) reported that molybdenite 

is relatively common in contact zones between felsic 

volcanic rocks and biotite granite of the Cuchivero Group in 

Estado Amazonas, but did not identify locations of particular 

prospects. In northern Brazil, molybdenite is disseminated in 

biotite granite and in small quartz veins at faulted contacts 

between granite and volcanic rocks of the Surumu Formation 

(equivalent to the Caicara Formation) (Montalvao and 

others, 1975; Schobbenhaus and others, 1984). Field and 

geochemical evidence (Montalvao and others, 1975; 

Mendoza and others, 1977; Sidder, unpublished data, 1988) 

indicates that the shallow porphyritic granitic bodies that cut 

volcanic rocks of the Caicara Formation in western Estado 

Bolivar and Estado Amazonas and their equivalents in Brazil 

are favorable for porphyry copper-molybdenum-type 

deposits.

TITANIUM

Ilmenite and rutile are abundant in heavy-mineral 

concentrates from numerous rivers in the Guayana Shield; 

however, commercial amounts of titanium have not been 

produced. Rio Nichare (loc. 68, pi. 1) is an alluvial prospect 

that contains abundant ilmenite. Creeks that drain into the 

Rio Marwani (loc. 28) contain high concentrations of rutile 

(Sherman Marsh, U.S. Geological Survey, oral commun., 

1988). Exploration is continuing in other rivers for mineable 

deposits of titanium (Charles Connolly, consultant, oral 

commun., 1989).
O14 GEOLOGY AND MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD

PLATINUM

Prospects of platinum-group elements are not well 

documented in Venezuela. Rodriguez (1987) reported that a 

newly discovered alluvial gold province in the area of the 

Rio Guapuchi in central Estado Amazonas (pi. 1) contains 

high concentrations of palladium. Other alluvial gold and 

diamond deposits in the eastern and northern parts of the 

Venezuelan Guayana Shield are said to contain high concentrations of platinum-group elements (Charles Connolly, oral 

commun., 1989). Tecnica Minera, C.A., and the U.S. Geological Survey recently discovered anomalous concentrations of platinum-group elements in the Piston de Uroy area 

(Wynn and others, this volume) (loc. 98, pi. 1) and in the La 

Esmeralda area (Sidder and others, 1991). The presence of 

cumulate mafic-ultramafic rocks and concentrations of as 

much as about 300 ppb platinum and 360 ppb palladium in 

the Piston de Uroy area (Wynn and others, this volume), in 

addition to placer prospects of platinum-group elements in 

French Guiana and Suriname (Choubert, 1974), indicate that 

platinum-group elements may be associated with deposits of 

nickel-copper sulfide minerals in these and similar rocks 

elsewhere in the Guayana Shield. Metabasalt and metagabbro in the Cerro La Pinto prospect (loc. 58) area near La 

Esmeralda may contain anomalous amounts of 

platinum-group elements (Sidder, 1990, 1994, unpublished 

data; Sidder, and others, 1991). Anomalous contents of platinum-group elements and rare platinoid minerals are present 

in several gold placers and associated with mafic and 

ultramafic rocks in Guyana and Suriname (Gibbs and 

Barron, 1993) and in the West African Shield (Kesse, 1985; 

Milesi and others, 1992).

TUNGSTEN

Tungsten is associated with deposits or prospects of 

other metals in the Guayana Shield. For example, scheelite 

has been identified in trace amounts within gold-quartz veins 

in the El Callao (loc. 37, pi. 1) and Botanamo (loc. 33) districts, and minor elevated concentrations of tungsten have 

been detected in tin-niobium-tantalum-bearing veins in the 

Parguaza Granite (Korol, 1961; Bellizzia and others, 1981).

CHROMITE

Trace amounts of chromium (as much as 0.5 weight 

percent C^Os) are present in mafic and ultramafic rocks of 

the Imataca Complex and in the greenstone belts. Minor 

amounts of chromite are present in alluvium within the area 

of outcrop of the Imataca Complex (Vicente Mendoza, oral 

commun., 1988). Cumulate mafic-ultramafic rocks at Piston 

de Uroy (loc. 98, pi. 1) may contain prospects for chromite

(Wynn and others, this volume). Small occurrences of alluvial and lode chromite are associated with ultramafic rocks 

in Suriname and Guyana (Gibbs and Barron, 1993) and in 

the West African Shield (Kesse, 1985; Milesi and others, 

1992).

SUMMARY

The Guayana Shield of Venezuela is a poorly explored, 

relatively unknown area. Gold, iron, diamonds, and bauxite 

are the only commodities currently mined. Gold is present in 

low-sulfide gold-quartz veins within Early Proterozoic 

greenstone-belt rocks cut by shear zones. Granite-hosted, 

lateritic, epithermal, and banded-iron-formation-associated 

gold prospects are also present. World-class deposits of 

enriched Superior-type, and possibly Algoma-type, banded 

iron formation are in the Archean Imataca Complex. The Los 

Pijiguaos bauxite deposit developed through intense tropical 

weathering of the Middle Proterozoic Parguaza Granite. 

Placer diamond deposits are predominantly derived from 

paleoplacer conglomerate beds in the lower 600 m of the 

Early to Middle Proterozoic Roraima Group. Kimberlite has 

been identified only in the Quebrada Grande area, where the 

richest placer diamond deposits in Venezuela are found. 

Also favorable for exploration are minor prospects of other 

metals such as platinum-group elements, chromium, and 

manganese in the greenstone belts, tin in the Parguaza Granite and in rhyolite of the Early Proterozoic Caicara Formation, and rare earth elements in carbonatite such as at Cerro 

Impacto or associated with Olympic Dam-type iron-copper-uranium-gold-rare earth element deposits in the Cuchivero Group and in the Parguaza Granite.

Descriptions of the deposits and prospects herein may 

provoke new thinking in mineral exploration of the Guayana 

Shield. For example, gold associated with banded iron formation might be present within the Imataca Complex in 

areas of carbonate±sulfide-facies rocks such as in the 

Upata-El Palmar-Guacuripia area and near Cerro Bolivar or 

in areas of ferruginous chert and other exhalative rocks associated with greenstone-belt rocks such as at Marwani, San 

Cristobal, La Esperanza, and Cerro La Pinto. Kuroko-type 

volcanogenic massive sulfide base- and precious-metal 

deposits may be hosted by the greenstone-belt rocks. Epithermal vein deposits of precious-metals may be present in 

felsic to intermediate volcanic rocks of the Cuchivero Group 

in the upper Rio Caura area, and Olympic Dam-type deposits 

may be associated with the Cuchivero Group or with the Parguaza Granite and its coeval volcanic rocks. Cumulate mafic 

and ultramafic rocks in the Piston de Uroy area may contain 

deposits of nickel-copper sulfide, platinum-group elements, 

and (or) chromium. Diamonds and rare metals may be associated with as yet undiscovered kimberlite and carbonatite 

that may intrude the Cuchivero Group in the western part of 

the shield.
MINERAL DEPOSITS OF THE VENEZUELAN GUAYANA SHIELD O15

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