Geology of Nimba 

County, Liberia 

GEOLOGICAL SURVEY BULLETIN 1540 

Prepared in cooperation with the Liberian 

Geological Survey under the auspices of the 

Government of Liberia and the Agency for 

International Development) U. S. Department 

of State 

Geology of Nimba 

County, Liberia 

By ERIC R. FORCE 

GEOLOGICAL SURVEY BULLETIN 1540 

Prepared in cooperation with the Liberian 

Geological Survey under the auspices of the 

Government of Liberia and the Agency for 

International Development) U.S. Department 

of State 

Precambrian rocks grouped in two initially 

separate tectonic terranes were juxtaposed, 

redeformed, intruded, metamorphosed, and 

then overthrust by rocks of a third, younger, 

Precambrian terrane 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1983 
UNITED STATES DEPARTMENT OF THE INTERIOR 

jAMES G. WATT, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

Force, Eric R. 

Geology of Nimba County, Liberia. 

"Prepared in cooperation with the Liberian Geological Survey under the auspices of the 

Government of Liberia and the Agency for International Development, U.S. Department 

of State. " 

Bibliography: p. 

Supt. of Docs. no: I 19.3:1540 

1. Geology-Liberia-Nimba County. I. Liberian Geological Survey. II. United States. 

Agency for International Development. III. Title. IV. Series. 

QE75.B9 no. 1540 557.3s [556.66'2) 82-600081 

[QE339.L5] AACR2 

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Afexandria, VA 22.304 
CONTENTS 

Page 

Abstract ---------------------------------------------------------- 1 

Introduction ------------------------------------------------------- 1 

Purpose ------------------------------------------------------- 1 

Character of Nimba County --------------------------------------- 1 

Status of geologic knowledge -------------------------------------- 4 

Methods used in this project--------------------------------------- 5 

Precambrian rocks -------------------------------------------------- 6 

Gneissic rocks -------------------------------------------------- 6 

Metasupracrustw rocks ------------------------------------------ 8 

Metamorphosed mafic igneous rocks -------------------------------- 12 

Igneous rocks -------------------------------------------------- 13 

Phanerozoic rocks and deposits ---------------------------------------- 13 

StrUcture --------------------------------------------------------- 14 

Folds --------------------------------------------------------- 14 Faul~--------------------------------------------------------- 16 

Metamorphism --------------------.... -------------------------------- 17 

Granulite facies ------------------------------------------------- 17 

Amphibolite facies ---------------------------------------------- 19 

Dynamic metamorphism ------------------------------------------ 20 

Precambrian stratigraphy and geologic history --------------------------- 20 

Older units and events of the Gbedin-Kahnple area--------------------- 20 

Older units and events of the Mt. Nimba area------------------------- 22 

Younger units and events of the entire Sanokole-Mt. Nimba area---------- 22 

Eburnean(?) and younger even~ of eastern Nimba County --------------- 23 

Miner& resources --------------------------------------------------- 23 

Iron ---------------------------------------------------------- 23 

Kyanite ------------------------------------------------------- 24 

Diamonds and graphite ------------------------------------------- 24 

Kaolinite ------------------------------------------------------ 25 

Tin ----------------------------------------------------------- 25 

References cited ---------------------------------------------------- 25 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map of Nimba County --------------------------------- 2 

2. Photograph of Nimba Range from the southwest ---------------- 3 

3. Geologic sketch map of Nimba County ------------------------- 8 

4. Annotated photographic panorama to the south from Mt. Bele ----- 10 

5. Map showing former and present drainage pattern in northern 

Nimba County ------------------------------------------ 14 

III 
IV GEOLOGY OF NIMBA COUNTY, LIBERIA 

6. Map showing distribution of folds of different ages in the Mt. NimbaSanokole area of Nirnba County----------------------------- 15 

7. Metamorphic-facies sketch map of Nimba County---------------- 18 

8. Chart showing Precambrian stratigraphy and geologic history of 

Nirnba County ------------------------------------------ 21 

TABLES 

Page 

TABLE 1. Radiometric ages of rocks from Nimba County ------------------ 7 

2. Chemical analyses of some Nirnba County iron-formation rocks----- 12 

3. Modal compositions of amphibolite and mesocratic pyroxeneplagioclase gneiss in lateral continuity----------------------- 13 
GEOLOGY OF NIMBA COUNTY, LIBERIA 

By ERIC R. FORCE 

ABSTRACT 

Precambrian rocks of Nimba County can be divided into three tectonic terranes: 

(1) a Nimba block of Nimba Supergroup supracrustal rocks containing isoclinal folds; 

(2) a Gbedin-Kahnple block, in which a folded granitic gneiss basement is unconformably overlain by iron-formation and leucocratic gneiss; and (3) a terrane of diorite, 

supracrustal rocks, and mafic rocks. The first two terranes were juxtaposed in an unknown manner and together were redeformed, metamorphosed (locally to granulite 

facies), and intruded by mafic rocks. The third terrane was later thrust over the first 

two. The histories of folding in the terranes were independent before juxtaposition 

but have been the same thereafter; four fold sets are recognized. 

INTRODUCTION 

PURPOSE 

From 1963 to 1975, a joint project of the U.S. Geological Survey 

(USGS) and the Liberian Geological Survey, funded by the Agency for 

International Development, U.S. Department of State (PASA 

no.AFR(IC) 11-00), produced a geologic map of the entire country of 

Liberia as 10 quadrangle sheets at 1:250,000 scale. From 1970 to 1972, 

the author was the principal investigator in Sanokole, Zwedru, and 

Gbanka quadrangles, which together make up most of Nimba County 

(fig. 1). This report attempts to synthesize the map information for the 

county. 

CHARACTER OF NIMBA COUNTY 

Nimba County is in northeastern Liberia and borders Guinea on the 

north and Ivory Coast on the east (fig. 1 ). 

Geomorphology.-Altitudes range from about 150 m, where the 

Cestos River flows out of the county, to more than 1,300 m at M t. 

Nimba, the highest point in Liberia Local relief is tens of meters between hills and streams on rolling terrain that characterizes most of 

1 
2 

7°00'N 

6°30'N 

6°00'N 

10 

GEOLOGY OF NIMBA COUNTY, LIBERIA 

LIBERIA~AFRICA 

NIMBA 

COUNTY } 

IVORY 

---) GUINEA 

GUINEA Lugbeh Yekepa 

• i.Lamc~~\.. ,...-

MT. YULITOHN X '.., " 

GBAHM RIDGE X "2.. 

Gbapa • 9.J~<v ~T. ~ COAST MT. TOKADEx -:...,S'~0 NIMBA ~ MT. YIDDINYE X ~~~ 

MT. DETO.HN X MT. BELE IVORY x Sanokole 

. ?-\lle1 • MT. KITOMA 

~Gt'' • Gbedin 

Gahnpa 

ZORZOR Cocopa• SANOKOLE 

QUADRANGLE ~ QUADRANGLE 

R! (j 

~(:, 

• Saale 

~si f(-o'?' 

G'o ~'---

GBANKA 

>I 

ZWEDRU 

QUADRANGLE QUADRANGLE COAST 

I-I 0<v • Tapeta 

81 '?"~ ~ ~_j\. 

U1 v'f ,_/-e,'f. -'9>-<i G6>/ ,.l.. ,.l.. ,? 

~I 1.1 oe '11. ~ ~ I 

G~ ~~ ;;5 "-", 

(J)I 0o. 8 -L ...... 

~I ~v: ~ 

z• ~~ ~ 

~I R 

~'<!(;' 

~-'t (j 

..... 

'"4 BUCHANAN JUAZOHN 

QUADRANGLE QUADRANGLE 

f 

20 30 40 50 KILOMETERS 

FIGURE !.-Quadrangle boundaries and geographic localities of Nimba County. 

the county. Long continuous ridges composed of metamorphosed 

supracrustal (sedimentary and volcanic) rocks stand as much as 800 m 

above the rolling terrain that is underlain by gneiss (fig. 2); these 
INTRODUCTION 3 

resistant ridges are a result of etching by chemical weath~ring of the 

landscape. Exposures of supracrustal rock on the ridges are much 

more common than are exposures of gneiss on the deeply weathered 

rolling terrain. 

The rivers have quiet stretches, about a kilometer long, separated by 

rapids or falls, where the rivers commonly divide into numerous 

channels separated by rock exposures in streambeds. In the streambeds, exposures of gneiss are generally more complete than those of 

some supracrustal rocks; because the gneiss is more resistant to mechanical weathering (in streambeds) and the supracrustal rocks are more 

resistant to chemical weathering (on the rest of the land surface), one 

can follow a river through a mountain range of supracrustal rocks, 

where there are no rock exposures in the banks, and come out on gentle 

country that has excellent streambed exposures of gneiss. 

Climate and vegetation.-Nimba County extends from about lat 6° 

to 8°N. The tropical climate has alternating wet and dry seasons. 

Toward the north, annual rainfall is as little as 200 em. Temperatures 

are moderate all year in the highest parts of the county. Rain forest 

dominates except for a few savannahs at higher altitudes, most of 

which have formed on canga (iron-formation-pebble conglomerate). 

Population and language.-The county seat of government is at 

Sanokole. Of the 100 or more towns having populations of more than 

200, many are accessible only by trail. Much of the population is dispersed in small tribal villages that show few modem influences. 

FIGURE 2.-Nimba Range viewed from the southwest, Mt. Bele (a synclinal keel(?)) on 

the right. The Nimba range is composed of metasedimentary and metavolcanic rocks 

of the Nimba Supergroup. Rolling terrain in the foreground is underlain by gneiss. 

Photograph by T. P. Thayer, 1949. 
4 GEOLOGY OF NIMBA COUNTY, LIBERIA 

Though English is spoken by many inhabitants, especially in the 

towns, the tribal languages Mano, Kpelle, Gio, and Gbi are most 

common. 

Land use.-The largest industry of Nimba County, indeed of Liberia, 

is iron mining. The Liberian American-Swedish Mining Co. (LAMCO) 

operates mines at M t. Nimba, Gbahm Ridge, and M t. Tokade, all in 

the northern part of Nimba County (fig. 1). 

Rubber plantations exist at several places in the county; the largest 

by far is Cocopa Plantation (fig. 1 ). Logging is also a large industry; 

hardwoods are logged selectively from magnificent stands of timber 

that remain in the Gio Forest and Gbi Range areas (fig. 1). 

The most common occupation of Nimba County residents, however, 

is upland rice farming. Small plots of mature timber are cleared and 

burned, farmed for rice and other vegetables for a couple of years, and 

then allowed to revert to forest. At Gbedin, rice irrigation is being attempted under a government program. 

Transportation system.-Most towns and villages are linked only by 

foot trails. The larger towns of Gahnpa, Sanokole, Y ekepa, Tapeta, and 

Kahnple are linked by all-weather laterite-surface roads. Spur roads 

lead to other towns and logging operations. Many towns have airstrips 

suitable for light planes and courageous pilots. An iron-ore railroad 

leads from the LAMCO operations at Yekepa to the coast. 

STATUS OF GEOLOGIC KNOWLEDGE 

Despite efforts by the author and his colleagues and predecessors, 

the geology of Nimba County remains poorly known. Belts of supracrustal rocks probably remain to be discovered, as do important 

mineral deposits. Certainly, the origin of the gneiss that forms·most of 

the county is poorly understood and needs further study. 

Previous work.-Geologic work in Nimba County began in the 

1940's with investigations by T. P. Thayer and Arthur Sherman, but a 

great impetus to further work was provided by the discovery of the 

Nimba iron ores by Clarke in 1955. Subsequent studies by LAMCO 

geologists were the first systematic geologic programs in the county. 

These studies resulted in reconnaissance geologic maps of an area that 

included most of Nimba County (Offerberg and Tremaine, 1961) and 

detailed geologic maps of the immediate area of the Nimba mines 

(Berge, 1968, 1971-1972). Berge (1966, 1974) also published on the 

petrology and origin of the iron ores. 

Previous products of this project.-This report is based on information gathered during a cooperative program by the Liberian Geological 

Survey and the USGS from 1963 to 1972. The United Nations 
INTRODUCTION 5 

Development Program (UNDP) was also involved in mineral exploration during part of this period. Geologic maps of Nimba County were 

published as the following 1:250,000-scale quadrangles: Sanokole 

(Force and Berge, 1977), Zwedru (Force and Beikman, 1977), Zorzor 

(Seitz, 1977), Gbanka (Force and Dunbar, 1977), Buchanan (Tysdal, 

1977a, 1978a), and Juazohn (Tysdal, 1977b, 1978b). Force and others 

(1971-1972) have described an aspect of the geology near Cocopa. 

METHODS USED IN THIS PROJECT 

Fieldwork was by the traverse method, as the veg~tation and lack of 

outcrop made it difficult to follow contacts. Most of the traverses were 

along trails and consisted of observation of float boulders as well as of 

outcrops. Road and railroad traverses included observations of saprolite as well as of bedrock and float. Some of the most valuable traverses 

were along the St. John River, Y a Creek, Cestos River, and Gwen 

Creek. Travel here was by rubber boat during low water. Rock 

exposures along rivers were far better than elsewhere. 

Data from magnetic and aeroradiometric surveys flown by Lockwood, Kessler, and Bartlett (Behrendt and Wotorson, 1974a-f; Wotorson and Behrendt, 197 4a, b) were available before most of the fieldwork was done and served to guide the work. These maps were also 

helpful in locating contacts and outlining rock units. Both types of 

maps were useful in distinguishing gneiss from supracrustal sequences 

and amphibolite, which are typically more magnetic and less radioactive. Magnetic data locally provided information on the dip of contacts, and magnetic textures were used wherever possible to differentiate gneiss of slightly different character. 

Base maps for fieldwork were prepared from uncontrolled air photomosaics by project personnel. The component aerial photographs are 

black and white infrared photographs of good quality at 1:40,000 scale. 

The geologic maps were published at 1:250,000 scale on shaded-relief 

maps derived from these bases (U.S. Geological Survey and Liberian 

Geological Survey, 1972a, b; 1973a, b). 

Systematic photogeologic interpretation, mostly by the geologic 

map compilers, was an integral part of extending map patterns from 

information gathered on widely spaced traverses. Contacts, marker 

beds, fractures, structural attitudes, and dikes interpreted from photographs were plotted on 1:40,000-scale base maps. Many "fractures" 

interpreted from photographs proved of dubious importance. In many 

places, both geophysical information and photogeology indicated the 

existence of a feature; many traces on the ground surface could be 

located more precisely from the photographs than from geophysical 
6 GEOLOGY OF NIMBA COUNTY, LIBERIA 

maps. This was especially true for contacts of supracrustal sequences 

and for the location of dikes. We were concerned that photogeology in 

the "jungle" would be unrewarding, and indeed it took some familiarization, but in general, photogeology was a key to mapping in Liberia 

In completely forested areas, the lack of confusing cultural boundaries 

and the excellence of ''etching'' produced by weathering compensated 

for the opacity of the trees. 

Lithologic information and structural attitudes from previous surveys were incorporated into maps made in this project. The numerous 

data in Offerberg and Tremaine (1961) were particularly useful, as their 

rock identifications are consistent and their locational accuracy good; 

their traverses were therefore not duplicated. Their correlations and 

interpretations were not accepted, however, because their work was 

done before the existence of adequate air photographs or geophysical 

information. Data from Contri (1966) and Lemarchand (1966) along the 

Ivory Coast border were used in a similar manner. 

PRECAMBRIAN ROCKS 

Liberia is in Precambrian crystalline terrane; radiometric ages on 

rocks from Nimba County range from 2,230 to 2,950 million years 

(m.y.) (table 1). Four clans of Precambrian rocks are apparent. The 

most abundant is quartzofeldspathic gneiss. Another important clan is 

the clearly metasedimentary-metavolcanic sequences of schist, quartzite, and iron-formation, which form the mountain ranges. Mafic rocks 

that locally formed by metamorphism of sills are a third component. 

The last clan consists of relatively unmetamorphosed dioritic rocks 

(fig. 3). 

GNEISSIC ROCKS 

Leucocratic gneiss.-Leucocratic gneiss (gnl) is used here as a general term for quartzofeldspathic gneiss that occupies huge tracts of the 

county. The unit occupies an area of low relief, and outcrops are sparse 

except along rivers. The gneiss is fairly homogeneous except in strain 

features and porphyroblast growth. Offerberg and Tremaine (1961) 

attempted to map these features. The typical mineral assemblage is 

quartz, two feldspars, and biotite. Foliation defined by biotite orientation is typically strong, and textures range from porphyroblastic 

augen gneiss to equigranular medium-grained gneiss. 

The leucocratic gneiss is believed to be partly metaigneous, for the 

following reasons: It is fairly homogeneous, the only readily apparent 
PRECAMBRIAN ROCKS 

TABLE I.-Radiometric ages of rocks from Nimba County 

[From Hurley and others, 1971; Hedge and others, 1975) 

Age (m.y.) Rock unit sampled Material analyzed 

2,950 ------- feldspathized Nimba whole rock 

Supergroup 

2,650 ------- feldspathized Nimba whole rock 

. Supergroup 

2,2001 ------- mica phyllite of whole rock 

Nimba Supergroup 

2,230 ------- mesocratic pyroxene- hornblende 

plagioclase gneiss (retrograde) 

1

Questioned by Hedge and others (1975). 

7 

Method 

Rb-Sr 

Rb-Sr 

Rb-Sr 

K-Ar 

inhomogenities being those of deformation fabric and those of amphibolite bodies that are probably metamorphosed dikes. Mineralogy is 

mostly simple and consistent with calc-alkalic original compositions. 

This evidence is admittedly weak, and more work is badly needed. 

Berge (1971-1972) has shown that in some parts of the county, leucocratic gneiss can be traced into metasedimentary sequences and 

thus is itself metasedimentary, at least in part. Other such places will 

be discussed individually later in the report. However, much of the leucocratic gneiss probably is older, partly metaplutonic rock on which 

supracrustal rocks were deposited. If this is true, then the most important contacts in the area, the unconformities at the bases of metasupracrustal sequences, must commonly be within leucocratic gneiss 

sequences where they are almost impossible to find. 

Granitic gneiss.-The term "granitic gneiss" (gng) is used here for a 

little foliated rock having coarse-grained granitoid texture and local 

anatectic features that occupies the Gbedin-Kahnple area in the northem part of the county. The mineral assemblage is also essentially 

quartz, two feldspars, and biotite; potash feldspar is more abundant 

than plagioclase. Amphibolite bodies are rare within this unit. Exposure of granitic gneiss is better than that of leucocratic gneiss, as the 

average relief is greater, and the unit locally forms exfoliation domes 

(fig. 4). Migmatite is present, apparently because of partial melting; 

the neosome of this migmatite locally intrudes adjacent leucocratic 

gneiss. For reasons discussed later, granitic gneiss appears to be the 

oldest rock in the county. It is almost certainly a metamorphosed igneous rock. 
8 GEOLOGY OF NIMBA COUNTY, LIBERIA 

0 1 0 20 30 40 50 KILOMETERS 

FIGURE 3.-Geologic sketch map of Nimba County. Units are described in text. 

METASUPRACRUSTAL ROCKS 

"Metasupracrustal" here refers to sedimentary and volcanic rocks 

formed at the Earth's surface but later metamorphosed. 
jd 

0 

~ 

D 

D 

sa 

[SJ 

__ i __ 

-~-

md 

I gnm I 

0 

PRECAMBRIAN ROCKS 

EXPLANATION 

u 

(!) 

Diabase dike rJ) 

<( 

Leucocratic gneiss 0:: 

:J 

'"":) 

Granitic gneiss 

Composite supracrustal 

assemblage 

Schist + z 

Amphibole schist ::s 

0:: 

Quartzite <( 

~ 

lXI ~ 

u 

LLJ 

Iron-formation 0:: . 0.. 

Silicate facies of 

iron-formation 

Metamorphosed diabase 

Mesocratic pyroxeneplagioclase gneiss 

Diorite 

Contact-queried where 

uncertain 

Fault-queried where 

uncertain 

Thrust fault-sawteeth on 

upthrust side 

Anticline, showing trace of 

axial plane and direction 

of plunge 

Overturned anticline 

9 

Assemblages of metasupracrustal rocks.-The most common assemblage of metasupracrustal rocks is a combination of mica or graphite 

schist, garnet, mica, magnetite-hematite, or kyanite quartzite, actinolite or hornblende-garnet-quartz amphibolite, and oxide or silicate 

facies of iron-formation. The assemblage is metasedimentary and 

metavolcanic and is believed to have been deposited on gneissic basement. A composite unit (z) has been used on the geologic maps and in 

figure 3 for the whole assemblage, where it cannot be further divided. 

At Mt. Nimba, the Nimba Supergroup of Berge (1968, 1974) contains amphibole schist, quartz-mica schist, quartz-mica-graphite 

schist, and oxide-facies iron-formation in a well-defined stratigraphic 

sequence. In the northern part of Gbi Range, the assemblage is pure 

quartzite, magnetite quartzite, garnet-kyanite quartzite, mica schist, 

and oxide-facies iron-formation; the stratigraphic order is unknown. 

Most metasupracrustal sequences outside the Nimba area probably do 

not correlate with the Nimba Supergroup, so the term is not extended. 

Schist.-The Nimba Supergroup contains quartz-mica schist and 

quartz-mica graphite schist (s). In addition, quartz-mica-garnet schist 

and minor amphibolitic schist are present along Gwen Creek in the 

southern part of the county. Natural exposures are few. 

Quartzite.-Quartzite (q) is common and characteristic in the Gio 

Forest, where it holds up long narrow ridges, and in the Gbi Range. 

The unit includes relatively pure quartzite, magnetite- or hematitebearing quartzite, mica quartzite, garnet quartzite, and kyanite-garnet 
10 GEOLOGY OF NIMBA COUNTY, LIBERIA 

FIGURE 4.-Annotated panorama to the south from Mt. Bele. In the middle distance are 

exfoliation domes (E) of granitic gneiss. The black line traces the north contact of the 

silicate facies of iron-formation, which forms hills where metamorphism is of amphibolite facies (isa) but plains at granulite facies (isg). Mt. Kitoma (K) is composed of 

quartzite. Quartzite with iron oxide and garnet is gradational into 

oxide and silicate facies of iron-formation, respectively. 

Amphibolite.-The amphibolite of the metasupracrustal assemblage (sa) is distinct from amphibolites that are metamorphosed dik~s 

and sills in gneissic terrane. The metasupracrustal amphibolite is 

either actinolite-cummingtonite rock, locally schistose, or hornblendeplagioclase-garnet-quartz rock, whereas the hypabyssal intrusive 

rocks are hornblende-plagioclase amphibolite or pyroxene-plagioclase 

gneiss. Metasupracrustal amphibolite is present in the Nimba Supergroup and on M t. Yiddinye near Sanokole, as well as in other composite metasupracrustal units. 

Iron-formation.-Nimba County is well endowed with iron-formation 

(i). The Nimba Supergroup contains the oxide-facies Nimba Itabirite 

(Berge, 1966, 1968, 1971-1972, 1974), which is mined at Mt. Nimba, 

Gbahm Ridge, and Mt. Tokade. Different depositional conditions produced both a simple oxide and a carbonate- and silicate-bearing oxide 

facies. Metamorphic grade ranges from greenschist facies to pyroxenegranulite facies, as indicated by the assemblages actinolite-chlorite 

and orthopyroxene-garnet. 

Oxide-facies magnetic iron-formation at Mt. Kitoma is possibly also 

of economic importance. 

A silicate-facies iron-formation (is) extends for more than 60 km, 

from near Gahnpa past Kahnple (figs. 3 and 4), as four discrete segments. Variable metamorphic conditions have produced a banded iron 
PRECAMBRIAN ROCKS 11 

oxide-facies iron-formation. Mt. Detohn (D) is composed of Nimba Supergroup rocks. 

The stream that carried diamonds from Gbapa flowed through the valley between the 

location of the camera and Mt. Detohn. Photograph by T. P. Thayer, 1949. 

orthopyroxene-garnet interval and a foliated grunerite interval (Force 

and others, 1971-1972). 

A belt of metasupracrustal rocks along the St. John River extends 

into Nimba County and contains magnetic itabirite as a central spine 

of several ridges. The Zua Range contains a similar sequence that 

extends to near Tapeta Another belt of oxide-facies iron-formation 

forms much of the Gbi Range and extends into Gio Forest, where 

orthopyroxene-bearing silicate facies of iron-formation is also present. 

Table 2 shows the few available chemical analyses of Nimba County 

iron-formations. 

Gneisses of supracrustal origin.-In a few localities, leucocratic 

gneiss clearly is interbedded or gradational with rocks known to be of 

supracrustal origin and thus must be either supracrustal in origin or 

the result of later injection. The important contacts (probably unconformities) between presumably older plutonic gneiss and metasupracrustal sequences must then be within leucocratic gneiss units at these 

localities. Three such localities are known in Nimba County. They are 

(1) the Nimba area where Berge (1974) found that the Nimba Supergroup of metasedimentary and metavolcanic rocks passes laterally 

into gneiss as metamorphic grade increases, only itabirite and graphite 

schist being left to mark their stratigraphic positions in the metasedimentary sequence; (2) the Gahnpa-Kahnple belt of silicate-facies ironformation, where a narrow belt of iron-formation seems to have been 

deposited above an unconformity on granitic gneiss, and leucocratic 

gneiss to the south must then overlie it; and (3) exposures of schist and 

quartzite on Gwen Creek, which pass gradationally into gneiss. 
12 GEOLOGY OF NIMBA COUNTY, LIBERIA 

TABLE 2.-Chemical analyses of some Nimba County iron-formation rocks 

[Values are in weight percent of oxides present, no entry indicates value not determined or less than 0.1. From 

Berge, 1971-1972, 1974; Force and others, 1971-1972] 

Sample 

Oxides No. 4 

Si02 ------------------ 40.4 39.7 40.2 40.4 

AJ203------------------ .4 .1 .3 .9 

Fe~0 3 ----------------- 35.5 44.0 55.6 41.4 

FeO------------------- 18.1 12.9 1.1 

MgO ------------------ 1.8 .6 .2 

CaO ------------------ 1.8 .3 .7 

N~O ------------------ .2 

K20 ------------------- .2 .1 

P205 ------------------ .2 .1 .2 

1

Average silicate-bearing itabirite, Mt. Tokade-Mt. Yulitohn. 2

Average magnetic itabirite, Nimba 3

Average hematite itabirite, Nimba 4

Average silicate-bearing magnetic itabirite, Nimba 5Average silicate-bearing itabirite, Gbahm Ridge. 6Average "blue" (hematite) ore. 7

Average "brown" (goethite) ore. 8

0rthopyroxene-garnet silicate facies, Cocopa area 9

Grunerite silicate facies, Kahnple area 

13.3 

.1 

1.2 

.2 

.2 

6 

39.8 1.5 

.3 .5 

33.1 97.0 

17.4 

1.8 

2.3 

.2 

METAMORPHOSED MAFIC IGNEOUS ROCKS 

8 9 

1.5 49.2 48.6 

.8 3.7 6.4 

93.7 6.5 3.2 

35.0 34.0 

3.3 4.1 

.6 .7 

.1 .1 

.1 .1 

Plagioclase-hornblende amphibolite and plagioclase-pyroxene gneiss 

of Nimba County are thought to represent metamorphosed mafic dikes 

and sills. 

In the Kahnple area, some bodies are clearly dikes in granitic gneiss, 

as their tabular and crosscutting nature is apparent in aerial photographs, and their texture and mineralogy are locally little altered from 

a diabase precursor. These rocks are massive, have hornblende coronas 

around earlier pyroxene, and locally grade into foliated amphibolite. 

On Cocopa Plantation, mesocratic pyroxene-plagioclase gneiss (gnm) 

probably originated as sills intruded into a fold hinge along an unconformity between granitic gneiss and iron-formation. Intrusion was 

probably synchronous with granulite-facies metamorphism, as 

domains that have original diabasic texture remain. East of Cocopa, 

the granulite-facies mesocratic gneiss can be traced into melanocratic 

amphibolite. Presumably, these rocks have the same composition and 

origin. The paradox of different color indices is explained by the metamorphic reaction hornblende - pyroxene + plagioclase, in which a 

dark mineral of the amphibolite reacts in part to produce a light 

mineral in the granulite. 
PHANEROZOIC ROCKS AND DEPOSITS 13 

TABLE 3.-Modal compositions of amphibolite and mesocratic pyroxene-plagioclase 

gneiss in lateral continuity 

Mineral analyzed 

Hornblende ----------

Hypersthene ---------

Diopside -------------

Plagioclase -----------

Other ---------------

1

These were andesine. 

~hese were andesine-labradorite. 

[From Berge, 1971-1972, table 1) 

Average of 8 amphibolites near Gbapa Average of 12 gneisses near Lugbeh 

(percent) (percent) 

64 

0 

0 

271 

9 

38 

10 

10 

402 

2 

Similar pyroxene-plagioclase rocks occupy the hinge of the same fold 

at the northern end of the county and in the Gbapa area Berge 

(1971-1972) found the same color-index differences between amphibolite and equivalent mesocratic pyroxene-plagioclase gneiss in that 

area (table 3). 

Mesocratic pyroxene-plagioclase gneiss also occupies a large area 

in the Gio Forest; its origin there is not known. Locally, it is blastomylonitic and forms the basal part of thrust sheets. 

IGNEOUS ROCKS 

The only nonmetamorphosed Precambrian igneous rock mapped in 

the county is a large body of diorite and quartz diorite (di), most of 

which is in tectonic contact with other units but which presumably 

was intrusive into leucocratic gneiss (cf. Tysdal, 1978a, b). The body is 

massive to slightly foliated, porphyritic, and contains primary tabular 

red plagioclase, hornblende, and deuteric biotite and epidote. Toward 

the western margin, a gently east-dipping shear foliation becomes progressively stronger until the rock becomes a gametiferous blastomylonite. The mylonitic rocks appear to structurally overlie ironformation and quartzite near Tapeta 

PHANEROZOIC ROCKS AND DEPOSITS 

Diabase forms northwest-trending dikes, mostly in the southeastern 

part of the county. These dikes are Jurassic in age on the basis of evidence elsewhere in Liberia (Dalrymple and others, 1975). Exposures in 

Gwen Creek show that the dikes are unaltered and locally are more 

than 10 m thick. 
14 GEOLOGY OF NIMBA COUNTY, LIBERIA 

Canga (conglomerate containing iron-formation clasts and ferruginous cement) and laterite form mappable bodies underlying relict land 

surfaces on the flanks of Nimba Range. Fluvial deposits are widespread though difficult to map; they include deposits in modem 

valleys and deposits formed in former valleys now stranded on drainage divides. The distribution of diamond-bearing gravels derived from 

the Gbapa area reveals a former drainage pattern significantly different from the present one (fig. 5). 

STRUCTURE 

FOLDS 

With the exception of folds mapped by Berge (1968), folds in Nimba 

County are not well known. However, there is evidence of relative fold 

ages in the Sanokole-Mt. Nimba area 

Four sets of folds have been recognized: Two older sets, probably 

formed in separate terranes, have been juxtaposed and deformed together by two younger fold sets, all mapped in the northern half of the 

county. The first set of older folds (F1 in fig. 6) is confined to granitic 

~:::;:::_:; Distribution of gravels worked 

for diamonds 

JC30'N ---J...- Present drainage (simplified) 

..,. <1111 Present drainage divide 

0 10 20 30 40 50 KILOMETERS 

I I I I I I 

GUINEA 

... . /"\_,.-""" 

IVORY 

COAST 

FIGURE 5.-Former and present stream drainage in northern Nimba County. The former 

drainage is interpreted from the distribution of pits in gravels worked for diamonds 

derived from the Gbapa area 
r3D'N 

STRUCTURE 

EXPLANATION 

-$-- Anticlinal fold axis 

----¥----- Synclinal fold axis 

--t:f-- Axis of overturned anticline 

Marker unit 

0 10 20 30 40 50 KILOMETERS 

I I I I I I 

GUINEA 

15 

FIGURE G.-Distribution of folds of different ages in the Mt. Nimba-Sanokole area F1 

and F2 are oldest, but they apparently formed separately, and their relative ages are 

unknown. 

gneiss of the Gbedin-Kahnple area. This set consists of a single large 

eastward-closing refolded antiform that extends about 30 km southeast, then northeast, from Gbedin. Hinges can be traced in aerial 

photographs. The foliation of granitic gneiss has been folded. In map 

view, the fold appears isoclinal, though on the Gahnpa-Sanokole road, 

foliation dips gently away from the axial plane to the northeast and 

steeply on the southwest. This fold is truncated by a younger unconformity or fault, which separates it from rocks on Mt. Kitoma. The fold 

also formed prior to emplacement of diabase dikes, now metamorphosed, which are undeformed by the fold but restricted to granitic 

gneiss. 

The second set of older folds (F 2 in fig. 6) is restricted to the Nimba 

Supergroup and is most obvious in the Mt. Nimba-Gbahm Ridge area, 

where the folds were mapped by Berge (1968). The folds are nearly isoclinal. In the northern part of the range, they plunge southwest, but 

farther south in the range, plunge direction is reversed. If itabirite at 

Mt. Bele, Mt. Tokade, and Mt. Yulitohn is correlative with Nimba 

Itabirite and collectively younger than surrounding gneiss, it must 

represent the keels of tight synclines similar to folds in the Mt. Nimba 

area. The discontinuity of synclines results from interference by later 

folds. 
16 GEOLOGY OF NIMBA COUNTY, LIBERIA 

The third fold set (F 3 in fig. 6) refolds both the older sets. It consists 

of a single large open fold that closes at the southwest; its axial plane 

can be traced for more than 50 km southwestward from Nimba Range 

as it folds the older folds of Nimba Supergroup (F 2), the older fold in 

granitic gneiss (F 1), and iron-formation that overlies granitic gneiss. 

Mesocratic pyroxene-plagioclase gneiss was probably intruded as sills 

along the hinge of this fold in the Cocopa area and the Gbapa area. 

The fourth set of folds (F 4 in fig. 6) consists of northwest-trending 

open upright folds that interfere with older folds in the Nimba Supergroup to produce the opposed plunges observed in Nimba Range and 

the dome-and-basin pattern of iron-formation to the west. These folds 

probably also produce the odd pattern of magnetic anomalies over the 

Nimba Range, in which large positive and negative anomalies alternate on the crest of the range. Similar folds were observed in the bed of 

the St. John River at the west end of the county, where foliation in leucocratic gneiss is folded into a slightly overturned open syncline. The 

trend of F4 folds parallels the characteristic trend of Pan-African 

(about 600 m.y.) deformation along the coast, and this fold set may be 

a distal effect of that deformation. 

Other conspicuous folds in the county are in iron-formation at the 

south end of Zua Range, in quartzite of the Gio Forest, and in ironformation of Gbi Range. The last fold appears in aerial photographs to 

have been refolded. Correlation of these folds with the tentative fold 

chronology for the Mt. Nimba-Sanokole area has not been attempted. 

FAULTS 

The major faults in the county, from oldest to youngest in presumed 

age, are: a folded suturelike fault, a thrust fault, northeast-trending 

shear zones, and a possibly conjugate set of fractures. A folded suturelike fault in the northern part of the county (fig. 3) separates the terranes of F 1 and F 2 folding; this fault seems to be required if those 

terranes were unrelated during that early folding but were both folded 

by F 3• The position of the fault is uncertain except along the east flank 

of Nimba Range where it was mapped by Berge (1971-1972); its westerly extent is shown merely in plausible locations. The fault must be 

folded by F 3 and F 4 folds. 

The most conspicuous faults in the county are imbricate thrusts in 

Gio Forest. They dip gently east and are marked by thick mylonitic 

zones, mostly blastomylonite, produced from rocks of the upper plate. 

The upper (eastern) thrusts have diorite on the upper plate, whereas 

lower (western) thrusts have mesocratic pyroxene-plagioclase gneiss 
METAMORPHISM 17 

on the upper plate. The eastern thrusts cut the boundaries of ironformation on the lower plate in Gbi Range. The northern end of the 

western segment is marked by discordant foliations rather than by 

mylonitic rocks. On the main road east from Tapeta, kaolinitic gouge 

crops out at the base of blastomylonitic diorite several hundred meters 

thick. The gouge probably represents continuation of faulting at a 

shallow crustal level. 

Nearly vertical northeast-trending shear zones are conspicuous 

throughout much of Liberia Nimba County has such zones along the 

Cestos River at its southeastern boundary (Cestos shear zone of 

Tysdal, 1977 a) and along the Mani River at its northwestern boundary. A conspicuous shear zone along the St. John River (Force and 

Dunbar, 1977) fades just at the western boundary of the county. The 

zone along the Cestos River contains flinty ultramylonite. The zone 

along the St. John River is intruded by pegmatite and locally contains 

cassiterite, both of which may extend into Nimba County. 

Immediately east of Cocopa Plantation, steep fractures divide the 

terrane into rhomb-shaped blocks. The fractures cut out the band of 

iron-formation at two places, providing good evidence of offset (probably vertical offset). Magnetic maps indicate that iron-formation is 

continuous at depth. 

METAMORPHISM 

The metamorphic assemblages are predominantly the result of a 

single event that postdates deposition of supracrustal sequences and 

the formation of two fold sets (fig. 6). This simplifies the study of metamorphism. Metamorphism is probably synchronous with F 3 folding 

and intrusion of sills as discussed later in the report. 

A later dynamothermal metamorphism accompanied thrusting of 

diorite over rocks already metamorphosed. 

GRANULITE FACIES 

Metamorphic orthopyroxene is preserved mostly in mafic rocks and 

iron-formation in Nimba County. Biotite persists in the more extensive 

granitic gneiss and leucocratic gneiss. Much extrapolation between 

minor lithologies was therefore necessary to prepare figure 7. Locally, 

leucocratic gneiss contains pyroxene, as along the LAMCO railroad 

south of Gahnpa 
18 

JC30'N 

rOO'N 

GEOLOGY OF NIMBA COUNTY, LIBERIA 

EXPLANATION 

?···----- Isograd, dashed where 

approximate, dotted 

where inferred, queried 

where uncertain 

-;;-J GUINEA 

;:;,.:::; /"-.. /\ ..... -"' ~ / ' 

(J:;;"< / 

0/ 

,/EPIDOTE 

//AMPHIBOLITE 

I 

I 

/ 

\ 

AMPHIBOLITE 

GRANULIT't-.... ,, 

?, . 

....... ? 

.....-. ............ ? 

AMPHIBOLITE 

:>-I 

!-I 

81 

Ul 

~~ DYNAMOTHERMAl 

r.n 1 AMPHIBOLITE 

~I ~I 

~I I 

l ... , 

,, 

0 10 20 30 40 50 KILOMETERS 

FIGURE 7.-Distribution of metamorphic facies in Nimba County. 

Iron-formation of both oxide and silicate facies locally contains 

pyroxenes, including orthopyroxene. Force and others (1971-1972) 

reported that the dominant orthopyroxene in silicate facies of iron-
METAMORPHISM 19 

formation in the Cocopa area is iron-rich, having the composition 

En14_

16 (eulite). This pyroxene-bearing iron-formation traces eastward 

into iron-cummingtonite or grunerite-dominated assemblages that 

have the same chemical composition (table 2). Almandine-spessartine 

garnet is present in both facies. Berge (1971-1972, 1974) documented 

coarse pyroxene in oxide facies of iron-formation at Mt. Tokade, Mt. 

Yulitohn, and nearby locations. The silicate facies of iron-formation 

contains orthopyroxene in the Gio Forest area, most notably near 

Saale. 

In the Cocopa area, orthopyroxene-bearing mesocratic gneiss is present only where adjacent iron-formation contains pyroxene (Force and 

others, 1971-1972). The pyroxene is clearly metamorphic at all but a 

few localities where original igneous textures are preserved. Berge 

(1971-1972) outlined an area of pyroxene-plagioclase mesocratic gneiss 

extending north and west from Gbapa; similar gneiss is widespread in 

the Gio Forest. The distribution of mesocratic pyroxene-plagioclase 

gneiss and of pyroxene-bearing iron-formation is thus the same. Force 

and others (1971-1972) suggested that the metamorphic significance 

of the iron orthopyroxene, eulite, is the same as that of ordinary 

hypersthene. Granulite-facies rocks of the Cocopa and Mt. Nimba 

areas are believed to be linked but are separated by granitic gneiss that 

does not contain pyroxene. The Gio Forest area is distinct and is 

separated from the Mt. Nimba and Cocopa areas by thrust faults. 

To a considerable extent, the metamorphic facies of rocks having 

amphibolite composition can be traced on magnetic maps. Pyroxeneplagioclase mesocratic gneiss contains ilmenite rather than magnetite, 

whereas the amphibolite-facies equivalent, hornblende amphibolite, 

contains more magnetite. On magnetic maps, a granulite-facies terrane 

(except iron-formation) is an area of low magnetic relief, whereas equivalent amphibolite-facies rocks contain discrete anomalies over large 

bodies of amphibolite or high microrelief over a terrane of gneiss and 

small amphibolite bodies. 

AMPHIBOLITE FACIES 

Most rocks of the county show upper amphibolite-facies assemblages. Pelitic rocks are uncommon but, where present, as in Gbi 

Range, contain kyanite. Leucocratic gneiss is biotite bearing; mafic 

gneiss is hornblende bearing. Amphibolite-facies assemblages are also 

present in the granulite-facies area as a late overprint; pyroxenes in 

pyroxene-plagioclase mesocratic gneiss have hornblende rims. Hedge 

and others (1975) reported a 2,230-m.y. age for retrograde hornblende 

from such a rock (table 1). 
20 GEOLOGY OF NIMBA COUNTY, LIBERIA 

Berge (1971-1972) described a part of Nimba Range where phyllite 

contains lower amphibolite-facies assemblages. Actinolite schist in 

metasupracrustal sequences, as at Mt. Yiddinye and near the St. John 

River, may also represent lower amphibolite-facies conditions. If so, 

the coincidences of metamorphic breaks with contacts are evidence of 

unconformable or tectonic contacts between those metasupracrustal 

sequences and adjacent gneiss. 

DYNAMIC METAMORPHISM 

Blastomylonite of the Gio Forest area records deep-seated shearing, 

as diopside and garnet are present only in sheared diorite of the eastern 

thrusts, and orthopyroxene and garnet are present in sheared 

mesocratic gneiss of the western thrusts. This orthopyroxene is not 

relict, as grain size is larger than that observed in unsheared pyroxeneplagioclase mesocratic gneiss. 

PRECAMBRIAN STRATIGRAPHY AND GEOLOGIC HISTORY 

A stratigraphic succession of Precambrian rocks (fig. 8) of the 

Sankokole-Mt. Nimba area is implied by tentative chronologies of folding, faulting, and metamorphism already discussed. The stratigraphy 

of other parts of the county is less certain. Some supracrustal sequences, for example, in Zua Range, are probably complex synclines on 

gneissic basement, but this structure could not be demonstrated. 

The two separate histories for the Gbedin-Kahnple area and the Mt. 

Nimba area together represent the oldest geologic events known in the 

county. 

OLDER UNITS AND EVENTS OF THE GBEDIN-KAHNPLE AREA 

Intrusion into an unknown host of the granitic rock (that subsequently became granitic gneiss) is the oldest event decipherable in the 

Gbedin-Kahnple area The tight (F 1) antiform was formed prior to 

intrusion of diabase dikes (now metadiabase found only in granitic 

gneiss). The unit was then unroofed. 

An unconformity on granitic gneiss is probably present around the 

entire unfaulted margin of the body but is most evident in the Gbedin 

area There, leucocratic gneisses and iron-formation discordantly 

overlie foliated granitic gneiss and are unaffected by the (F 1) fold. 

Stratigraphic contacts between leucocratic gneiss and iron-formation 
Gbedin-Kahnple area Mt. Nimba area 

Folding, faulting, t-olarng, Taultrng, 

and metamorphism Rocks (present form) Rocks (present form) and metamorphism 

I 

I 

Open folding (F4 ) I 

Thrusting in Gio I 

Forest I 

I Diorite and quartz diorite. I 

I 

Third fold (F3 ) and I Third fold (F3 Mesocratic pyroxene-plagioclase ) and 

intense metamorphism gneiss and amphibolite intense metamorphism 

I 

I 

I 

I Juxtaposition of I Juxtaposition of 

two areas I two areas 

> Iron formation and 

some leucocratic gneiss 

Unconformity 

Metadiabase 

Early folding (F1 ) /Mt. Alpha Phyllite Early folding (F2 ) 

Nimba ltabirite c. 

:J «< 0 - 1000 m Metamorphism to Gbahm Ridge Phyllite .C'-

gneiss E~ ·- 4l -500m Seka Valley Amphibole Zc. 

Granitic gneiss :J Schist Ill 

FIGURE B.-Precambrian stratigraphy and geologic history of Nimba County. Thickness scale applies to Nimba Supergroup only. 

1-C 

~ C":l 

> 

a:: 

ttl 

~ 

~ :!§ 

~ 

> 1-3 

0 

~ 

> 1-C 

::I: 

~ 

fZ 

t:l 

0 

tz:j 

0 

s 

0 

-

C":l 

::z:: 

~ ~ 

~ 

~ 
22 GEOLOGY OF NIMBA COUNTY, LIBERIA 

in the upper unit are parallel to the surface of the underlying unconformity. In the Kahnple area, the apparent lack of metadiabase dikes 

in iron silicate and leucocratic gneiss suggests that there, also, an unconformity separates older granitic gneiss from younger ironformation and leucocratic gneiss. 

OLDER UNITS AND EVENTS OF THE MT. NIMBA AREA 

The earliest recognized event in the Nimba area was deposition of 

the Nimba Supergroup (Berge, 1968, 1974), including associated rocks, 

which are now leucocratic gneisses containing marker graphite schist 

or iron-formation horizons traced into Nimba Supergroup (Berge, 

1971-1972). Figure 8 shows Berge's stratigraphy of th.e Nimba Supergroup. These rocks were folded into tight to isoclinal folds (F 2) described by Berge (1968). 

YOUNGER UNITS AND EVENTS OF THE ENTIRE SANOKOLE-MT. NIMBA AREA 

An open fold closed to the southwest (F 3) is the oldest structure certainly shared by the Gbedin-Kahnple and Mt. Nimba areas. Presumably, rock sequences of the two areas, and older features already 

formed, were juxtaposed along a suturelike fault before the formation 

of this fold. 

Metamorphism of the entire area is roughly synchronous with the 

formation of this open (F 3) fold, on the basis of the following evidence: 

Mesocratic pyroxene-plagioclase gneiss intruded into the hinge in both 

the Cocopa and Gbapa areas was almost completely metamorphosed 

after intrusion, along with other units. Local islands in this gneiss have 

relict igneous textures, suggesting intrusion into a metamorphic environment (that is, granulite facies) where its own mineralogy was 

stable. Thus, the F 3 fold, intrusion of sills into its hinge, and metamorphism all appear to be synchronous. Probably at the same time, the 

granitic gneiss became locally migmatitic. On the LAMCO railroad 

east of Gahnpa, the neosome of this migmatite intrudes and 

retrogrades pyroxene-plagioclase mesocratic gneiss along the contact. 

The events discussed above are probably Liberian in age (about 

2,700 m.y.) or older. Whole-rock rubidium•strontium metamorphic(?) 

ages of Nimba Supergroup (dates by Hurley and others, 1971, accepted by Hedge and others, 1975) are 2,650 and 2,950 m.y. The granitic gneiss unit probably is older yet. 
MINERAL RESOURCES 23 

EBURNEAN(?) AND YOUNGER EVENTS OF EASTERN NIMBA COUNTY 

Hurley and others (1971) showed that radiometric ages of about 

2,150 m.y. (Eburnean) are common in eastern Liberia The only reliable 

radiometric age on rocks of Nimba County that can be assigned to the 

Eburnean is one for retrograde hornblende from a mesocratic 

pyroxene-plagioclase gneiss near Cocopa (2,230 m.y., Hedge and 

others, 1975). In the eastern part of the county, intrusion of dioritic 

rocks and later westward thrusting of these and other rocks seem 

likely to have been Eburnean events. 

The diorite and quartz diorite are not metamorphosed except along 

thrust contacts, so they must postdate adjacent leucocratic gneiss. 

Unfortunately, intrusive contacts with leucocratic gneiss were not 

observed. 

Thrusting in the Gio Forest area was clearly later than diorite on the 

upper plate of the eastern thrust or mesocratic pyroxene-plagioclase 

gneiss on the upper plate of the western thrust; both rocks are mylonitic along the thrusts. Thrusting is thus the next event in the 

chronology. Metamorphic conditions associated with thrusting locally 

reached granulite facies. 

The last important event of the area is open upright folding (F 4) that 

interferes with older folds to give doubly plunging synclines in the Mt. 

Nimba area These folds may be Pan-African in age (about 600 m.y.). 

MINERAL RESOURCES 

Nimba County is an important producer of iron ore, and alluvial and 

bedrock mining for diamonds is conducted on a small scale. The county 

has kyanite, graphite, and kaolinite resources of unknown importance 

and cassiterite may also be present. 

IRON 

Iron ore is mined by LAMCO at Mt. Nimba, Gbahm Ridge, and Mt. 

Tokade; production capacity is about 12 million tons per year. The 

high-grade ore (63-68 percent iron (Fe)) is derived from itabirite (oxidefacies iron-formation) by some kind of metamorphic hydrothermal 

enrichment (hematitic "blue ore") and by. deep weathering (goethitic 

"brown ore," Berge, 1966). The ore was discovered in 1955, and reserves were estimated at 250 million tons (Berge, 1968); the 1978 estimate of the reserves of the western part of the area is 535 million tons. 
24 GEOLOGY OF NIMBA COUNTY, LIBERIA 

Much greater quantities of magnetite and hematite itabirite envelop 

the present ores and are themselves low-grade resources (see table 2 for 

composition). 

Other iron-formation resources at Mt. Kitoma, Mt. Bele, and at the 

northernmost end of the county have been evaluated by LAMCO. Sequences containing iron-formation in Zua Range, the northern end of 

Gio Forest, and other places were examined by Offerberg and Tremaine (1961). The Saale part of Gio Forest contains some oxide-faci~s 

as well as silicate-facies iron-formation. 

Oxide-facies iron-formation occupies much of the northern part of 

the Gbi Range, and judging from the similarity in aeromagnetic data 

and topographic expression of the two parts of the range, ironformation must be present in the southern part also. High-grade ore 

was not observed, but the traverses were few. Some of the itabirite consists of hematite rather than magnetite; all· is coarse grained, In the 

northern part of the range, iron-formation is nearly horizontal over a 

considerable area 

KYANITE 

In Gbi Range, abundant kyanite as much as 1 em long was observed 

in kyanite-garnet schist (and soil) adjacent to iron-formation in several 

localities. 

DIAMONDS AND GRAPHITE 

Minor digging for alluvial diamonds is being conducted in the 

valleys of Y aa River and Y a, Boo, and Nzra Creeks (fig. 5). This 

digging formerly must have been more intensive, as these valleys are 

heavily scarred with pits. About 1965, digging for diamonds in graphite schist began near Gbapa 

The alluvial diamond diggings are presently in the drainages of four 

separate drainage basins, but they line up in such a way that they 

must have once been part of a single drainage that has been broken up 

by stream piracy. Diamond pits in gravel have been found in drainage 

divides separating parts of the former single drainage. The reconstructed drainage system had its head at Gbapa (see fig. 5). 

Diamonds are being recovered from graphite schist at Gbapa 

UNDP personnel recovered diamonds from unweathered unsheared 

graphite schist, though mining is concentrated along sheared and 

weathered zones, probably because the material is easier to disaggregate. The alluvial diamonds are derived from this graphite schist. 
REFERENCES CITED 25 

The graphite schist is at most a few tens of meters thick and is concordant with adjacent muscovite gneiss. It is fine grained and contains 

perhaps 20 percent fine graphite. Rutile and tourmaline are the most 

abundant accessory minerals. 

The origin of diamonds in this rock is a question that has implications beyond this deposit. I believe that the diamonds are neither of 

detrital nor regional metamorphic origin. The mineralogy and texture 

of the graphite schist rule out a detrital origin, as rutile, tourmaline, 

diamonds, and fine quartz do not make a plausible complete detrital 

assemblage. It is more likely that the diamonds are genetically related 

to the graphite, yet regional metamorphism of amphibolite facies is 

much too low for diamond formation. Local impact metamorphism 

may be responsible, but this hypothesis has not been investigated. 

KAOLINITE 

Kaolinitic fault gouge is exposed where the eastern thrust at the 

base of a diorite sheet crosses the main Tapeta-Zwedru road. This 

material is possibly thick and pure enough to warrant exploitation. 

TIN 

UNDP personnel have found cassiterite in greisenized pegmatite 

and in other rocks along the St. John shear zone 40 km west of the 

boundary of Nimba County. Alluvial cassiterite has also been found in 

this area The shear zone, the pegmatite, and tin anomalies (Offerberg 

and Tremaine, 1961) extend roughly to the county boundary; thus, the 

presence of cassiterite in Nimba County is likely. 

REFERENCES CITED 

Behrendt, J. C., and Wotorson, C. S., 1974a, Aeromagnetic map of the Gbanka quadrangle, Liberia: U.S. Geological Survey Miscellaneous Investigations Series Map 

I-776-B, scale 1:250,000. 

__ 1974b, Aeromagnetic map of the Sanokole quadrangle, Liberia: U.S. Geological 

Survey Miscellaneous Investigations Series Map I-774-B, scale 1:250,000. 

__ 1974c, Aeromagnetic map of the Zorzor quadrangle, Liberia: U.S. Geological 

Survey Miscellaneous Investigations Series Map I-773-B, scale 1:250,000. 

__ 197 4d, Total-count gamma radiation map of the Gbanka quadrangle, Liberia: 

U.S. Geological Survey Miscellaneous Investigations Series Map I-776-C, scale 

1:250,000. 
26 GEOLOGY OF NIMBA COUNTY, LIBERIA 

__ 197 4e, Total-count gamma radiation map of the Sanokole quadrangle, Liberia: 

U.S. Geological Survey Miscellaneous Investigations Series Map I-774-C, scale 

1:250,000. 

__ 1974f, Total-count gamma radiation map of the Zorzor quadrangle, Liberia: 

U.S. Geological Survey Miscellaneous Investigations Series Map I-773-C, scale 

1:250,000. 

Berge, J. W., 1966, Genetical aspects of the Nimba iron ores: Geological, Mining, and 

Metallurgical Society of Liberia Bulletin, v. 1, no. 1, p. 36-43. 

__ 1968, A proposed structural and stratigraphic interpretation of the NimbaGbahm Ridge area, Liberia: Geological, Mining, and Metallurgical Society of Liberia Bulletin, v. 3, p. 18-44. 

__ 1971-1972, Geology of the Nimba Concession area, Nimba County, Liberia (pt. 1): 

Geological, Mining, and Metallurgical Society of Liberia Bulletin, v. 5, p. 29-92. 

__ 1974, Geology, geochemistry, and origin of the Nimba Itabirite and associated 

rocks, Nimba County, Liberia: Economic Geology, v. 69, no. 1, p. 80-92. 

Contri, J. P., 1966, Rapport de Fin de Leve' Couperes Toulepleu 4c-4d Guiglo 3c-3d: 

Societe' pour le De'veloppement Minier de la Cote D'Ivoire Rapport 175 (1). 

Dalrymple, G. B., Gromme, C. S., and White, R. W., 1975, Potassium-argon age and 

paleomagnetism of diabase dikes in Liberia; initiation of central Atlantic rifting: 

Geological Society of America Bulletin, v. 86, no. 3, p. 399-411. 

Force, E. R., and Beikman, H. M., 1977, Geologic map of the Zwedru quadrangle, Liberia: U.S. Geological Survey Miscellaneous Investigations Series Map I-777-D, 

scale 1:250,000. 

Force, E. R., and Berge, J. W., 1977, Geologic map of the Sanokole Quadrangle, Liberia: 

U.S. Geological Survey Miscellaneous Investigations Series Map I-774-D, scale 

1:250,000. 

Force, E. R., and Dunbar, J.D. N., 1977, Geologic map of the Gbanka quadrangle, Liberia: U.S. Geological Survey Miscellaneous Investigations Series Map I-776-D, 

scale 1:250,000. 

Force, E. R., Srivastava, S. P., and Phillips, E., 1971-1972, Eulite rock and associated 

granulite facies gneisses from Nimba County, Liberia: Geological, Mining, and 

Metallurgical Society of Liberia Bulletin, v. 5, p. 93-111. 

Hedge, C. E., Marvin, R. F., and Naeser, C. W., 1975, Age provinces in the basement 

rocks of Liberia: U.S. Geological Survey Journal of Research, v. 3, no. 4, p. 425-429. 

Hurley, P. M., Leo, G. W., White, R. W., and Fairbairn, H. W., 1971, The Liberian age 

province (about 2,700 m.y.) and adjacent provinces in Liberia and Sierra Leone: 

Geological Society of America Bulletin, v. 82, no. 12, p. 3483-3490. 

Lemarchand, R., 1966, Rapport de Fin de Leve' Couperes Toulepleu 1d-2d-3b-4a-4b: 

Societe' pour le De'veloppement Minier de la Cote D'I voire Rapport 181. 

Offerberg, J. 0., and Tremaine, J. W., 1961, Report on LAMCO Joint Venture's geological investigations in Liberia between Nimba and Lower Buchanan along the 

Railroad Concession Area: Stockholm, W. Reklam/Caslon Press Boktr. AB, 7 4 p. 

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