OLOGICAL SURVEY PROFESSIONAL PAPER 1212-A

TRATIGRAPHY AND DIAGENETIC 

[ISTORY OF THE LOWER PART 

IF THE TRIASSIC CHITISTONE 

IMESTONE, ALASKA

Stratigraphy and Diagenetic

History of the Lower Part of the 

Triassic Chitistone Limestone, 

Alaska

By AUGUSTUS K. ARMSTRONG and E. M. MACKEVETT, JR.

GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, 

WRANGELL MOUNTAINS, ALASKA

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1212-A

An examination of the sabkha fades and other 

sedimentary features of the host rock 

for theKennecott copper deposits

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1982
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Armstrong, Augustus K.

Stratigraphy and diagenetic history of the lower part of 

the Triassic Chitistone Limestone.

(Geologic relations of Kennecott-type copper deposits, 

Wrangell Mountains, Alaska) (Geological Survey Professional 

Paper; 1212-A)

Includes bibliographical references.

1. Limestone Alaska Wrangell Mountains. 2. Geology, 

Stratigraphic Triassic. I. MacKevett, E. M. (Edward Malcolm) , 

1918- . II. Title. III. Series. IV. Series: Geological 

Survey Professional Paper; 1212-A.

QE471.15.L5A75 552'.5 81-6608

AACR2

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402
CONTENTS

Page

Abstract _______________________________________________ Al

Introduction __________________________________________ 1

Purpose and scope ___________________________________ 1

Study area __________________________________________ 1

Work methods ______________________________________ 1

Acknowledgments ____________________________________ 1

Geologic setting ________________________________________ 2

Descriptions of measured sections __________________________ 3

Donoho Peak section __________________________________ 3

Erie Mine section ____________________________________ 8

Bonanza mine outcrop section __________________________ 8

Green Butte section __________________________________ 10

Nizina River section __________________________________ 10

Page

Discussion of specific sedimentary features _________________ _A12

Micritic matrix and mud limestone ______________________ 12

Peloids_____________________________________________ 12

Bioclastdebris ______________________________________ 12

Pellet, pisooid, and laminated caliche ____________________ 14

Sedimentary breccias _________________________________ 16

Zebra beds and vug fillings ____________________________ 17

Intraformational stratigraphic correlation and environments of

deposition ___________________________________________ 21

Diagenetic history of the Chitistone Limestone of Bonanza Ridge 23

Conclusions ____________________________________________ 24

Location of measured sections _____________________________ 24

References cited ________________________________________ 24

ILLUSTRATIONS

FIGURE 1. Index map of the McCarthy region, Alaska, showing location of study area _______________________________________ A2

2. Photograph showing cliffs on west side of Nizina Valley showing folded Triassic rocks ______________________________ 3

3. Stratigraphic correlation diagram of lower part of the Chitistone Limestone ______________________________________ 4

4-7. Photomicrographs and scanning electron micrographs of:

4. Specimens from Donoho Peak section __________________________________________________________________ 7

5. Specimens from Donoho and Nizina River sections___________________________________________ _____________ 9

6. Specimens from Donoho Peak section __________________________________________________________________ 11

7. Specimens from Donoho Peak section __________________________________________________________________ 13

8. Photograph of Bonanza Ridge showing Bonanza mine bunkhouse, power plant, and Glory Hole _______________________ 14

9. Photomicrographs and photograph of specimens from Nizina River section _______________________________________ 15

10. Diagram showing sequence of alterations of peloid-bioclastic grainstone _________________________________________ 16

11. Photograph of wallrock from Mother Lode mine _____________________________________________________________ 18

12. Photographs and photomicrographs of specimens in Mother Lode mine __________________________________________ 19

13. Diagram showing probable sequence of events in development of vug and cavity filling of zebra bed __________________ 20

14. Diagram showing depositional model for the Upper Triassic carbonate facies of the Wrangell Mountains _______________ 21

15. Idealized section showing carbonate depositional cycles for lower 130 m of the Chitistone Limestone __________________ 22

16. Idealized section showing interpretation of environment of deposition in Donoho Peak section _______________________ 23

GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS, ALASKA

STRATIGRAPHY AND DIAGENETIC HISTORY OF THE LOWER PART OF THE

TRIASSIC CHITISTONE LIMESTONE

By AUGUSTUS K. ARMSTRONG and E. M. MACKEVETT, Jr.

ABSTRACT

This paper focuses on detailed petrographic and stratigraphic 

studies of the lower part of the Chitistone Limestone (Triassic), the 

carbonate rocks that host massive copper sulfide lodes at the Kennecott mines. Some implications of the studied rocks relevant to ore 

formation are mentioned.

The Chitistone Limestone and superimposed Upper Triassic and 

Jurassic sedimentary rocks formed in a marine basin on and surrounded by the Nikolai Greenstone, a ihick, extensive, largely 

subaerial succession of tholeiitic basalt with an intrinsically high 

copper content. The lowermost 130 meters of the Chitistone contains 

three incomplete upward-shoaling lime-mud cyclic sequences, each 

consisting of shallow subtidal limestone that grades upward into intertidal stromatolitic fine-grained dolomite. The youngest cycle contains well-developed sabkha features and dolomitic pisolitic and 

laminate caliches and underlies shallow-marine limestone. These 

sedimentary structures represent a regional sabkha facies that developed between 90 and 130 meters above the Nikolai Greenstone. 

This facies originally contained abundant gypsum and anhydrite, but 

exposure to vadose weathering leached most of the gypsum and 

anhydrite and developed a vuggy zone interbedded with porous 

dolomitic caliche. Subsequent marine deposition capped the porous 

zone with an impermeable seal. The resulting rock, and possibly 

others, probably served as permeable conduits for the ore-forming 

solutions.

INTRODUCTION

PURPOSE AND SCOPE

This paper provides detailed stratigraphic and petrographic information on the carbonate rocks that constitute the lower part of the Chitistone Limestone 

(Triassic). Investigations leading to this report are part 

of broader studies of Kennecott-type copper sulfide 

deposits, which are localized in the lower parts of the 

Chitistone. These studies are still in progress and as yet 

inconclusive. Consequently, this report concentrates on 

detailed outcrop and underground descriptions and interpretations of the rocks and minimizes their relevance to the ore deposits. The stratigraphy and microfacies analysis of three lime-mud upward-shoaling 

cycles are described and illustrated, and the focus of 

the study is on the last cycle, which contains sabkha 

sedimentary stuctures and a dolomitic caliche bed at 

its top. Undoubtedly, carbonate rocks in lower parts of 

the Chitistone were important in localizing the massive 

copper-sulfide rich lodes that typify Kennecott-type 

deposits. In particular, permeable parts of the sabkhas 

probably were conduits for the ore-forming solutions, 

and the sabkhas may have been a source of sulfur for 

the ore.

The present report elaborates on an earlier report 

(Armstrong and others, 1969) and refines the previous 

work by concentrating on detailed studies of lower

parts of the Chitistone and by discussing additional 

localities (fig. l).

STUDY AREA

The study area is along the southern flank of the 

Wrangell Mountains where the Chitistone Limestone 

hosts the major Kennecott-type copper deposits and is 

well exposed (fig. 2). This paper is largely based on detailed studies of rocks from five stratigraphic sections 

of lower parts of the Chitistone that were measured in 

1974. The location of the five sections are shown on figure 1 and their stratigraphic relations on figure 3. The 

present report is supplemented by data from previously studied stratigraphic sections (Armstrong and 

others, 1969; Armstrong and MacKevett, 1976; fig. l) 

and, to some extent, by broad-scope geologic studies 

of the McCarthy quadrangle that are summarized in 

MacKevett (1976).

WORK METHODS

Field studies conducted in 1974 and 1977 focused on 

detailed stratigraphic studies of the lower 140 m of the 

Chitistone Limestone with particular attention given to 

sedimentary structures and sampling. Oriented thin 

sections of 350 specimens representative of the outcrop 

sections and underground exposures were studied 

under the microscope. Friedman's (1959) methods 

were used for staining thin sections, alizarin red for 

calcite and potassium ferricyanide for ferrous iron in 

dolomite. Polished rock specimens were etched for 60 

seconds in 1 molar HC1 for study with a scanning electron microscope (SEM).

The carbonate rock classification is Dunham's 

(1962).

ACKNOWLEDGMENTS

The underground studies in the Bonanza, Jumbo, and 

Mother Lode mines were conducted in 1974 in conjunction with M. L. Silberman of the U.S. Geological Survey, P. R. Holdsworth, former manager, Alaskan Exploration Inexco Mining Company, and Prof. Clay Smith 

and George Linn, New Mexico Institute of Mining and 

Technology. Dr. R. M. Lloyd, of Shell Development 

Company, helped Armstrong in examining the critical 

Donoho Peak section. Outcrop and underground 

studies at the Kennecott mines in 1977 were done with 

the generous support of W. D. McCullough and C. B. 

Douthitt of Coastal Mining Company. We appreciate 

the generous help, critical review, and suggestions 

given by Philip W. Choquette, Robert N. Ginsburg,

Al
A2 GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS. ALASKA

Eugene A. Shinn, Robert K. Park, Charles A. Repenning, J. W. Baxter, and George de Vries Klein. This 

paper was presented at the 1977 Society of Economic 

Paleontologists and Mineralogists' symposium, Predictive Sedimentary Models for Earth Resources: Fossil 

Fuels and Metals.

GEOLOGIC SETTING

The distribution and structural relations of the 

Chitistone Limestone are discussed in several reports 

relevant to the McCarthy quadrangle, notable MacKevett's (1965, 1970, 1974, 1976). The Chitistone occupies a northwest-trending belt along the southern 

flank of the Wrangell Mountains, almost entirely in the 

McCarthy 1° by 3° quadrangle. Outcrops are known at 

a few Alaskan localities north of the Wrangell Mountains, and in the Yukon Territory (MacKevett, 1976). 

The Chitistone disconformably overlies a thick platform of Nikolai Greenstone (upper Middle and (or) 

lower Upper Triassic) consisting of tholeiitic basalt that 

for the most part was extruded under subaerial conditions. Recent paleomagnetic investigations indicate 

that the Nikolai Greenstone formed in largely subaerial

EXPLANATION

Bedrock and surficial deposits 

_i74A-4T

Outcrop of Chitistone Limestone 

studied by authors in 1974 and 1977 

Number refers to stratigraphic section 

in figure 3

V

Location of section and outcrop 

studied by Armstrong and others 

(1969)

ARCTIC OCEAN

61°30'

environments at low paleolatitudes in the proto-Pacific 

region (Hillhouse, 1977). Inasmuch as the Nikolai and 

Chitistone are components of the same tectonic terrane, it is implicit that the Chitistone formed in a similar paleoenvironment. The lowermost approximately 

130 m of the Chitistone contains carbonate rocks 

formed in supratidal and intertidal depositional environments. This sequence is superseded by limestone of 

shallow marine origin, which constitutes most of the 

upper part of the Chitistone.

Age-diagnostic fossils are rare in the Chitistone 

Limestone. The Chitistone's age, Karnian Stage of the 

Late Triassic, is best documented by ammonites of the 

genus Tropites, which are widely but sparsely distributed in the stratigraphically medial and upper parts 

of the formation. Silberling ( m Armstrong and others, 

1969) stated, "A fauna comprising Tropites cf. T. wellen Smith, Arcestes, and Halobm cf. H. superba Mojsisovics, was collected about 152 m above the base of 

the Chitistone at Green Butte (USGS Mesozoic loc. 

M1707) and is indicative of a late Karnian age."

The Chitistone grades upward into more open marine 

carbonates of the Nizina Limestone, of Late Triassic

143°00' 142°30'

^* ^

Approximate area underlain 

by the Chitistone and Nizina 

| Limestones 

60°.

146° 142SS1S 

&>. PACIFIC OCEAN

0 200 KILOMETERS

FIGURE 1. Part of the McCarthy 1° by 3° quadrangle, Alaska, showing locations of stratigraphic sections and mines discussed in this 

report, and index map of Alaska showing approximate area underlain by Chitistone Limestone. Modified from Armstrong and others 

(1969).
STRATIGRAPHY AND DIAGENETIC HISTORY, LOWER PART OF THE TRIASSIC CHITISTONE LIMESTONE A3

age, which in turn grade upward into the McCarthy 

Formation, of Late Triassic and Early Jurassic age. 

This gradation reflects a general progressive deepening 

of the marine depositional environments. Deformation 

and local metamorphism of the Chitistone are mainly 

consequences of a regional orogeny during parts of the 

Late Jurassic and Early Cretaceous.

DESCRIPTIONS OF MEASURED STRATIGRAPHIC 

SECTIONS1

The outcrops of Chitistone Limestone at Donoho Peak 

(74A-4T), Green Butte (74A-3T), and Nizina River 

(74A-7T) are in stratigraphic order and unaffected by 

faulting or metamorphism throughout their lowermost 

140 m (fig. 3). The Erie mine section (74A-6T) was

In this report all stratigraphic positions are given in meters above the Nikolai 

Greenstone-Chitistone Limestone contact: 85 m means 8r> m above the base.

measured to a horizon some 105 m above its base where 

dolomitization and tectonic fracturing have obscured 

stratigraphic relations. Section 74A-2T was measured 

on Bonanza Ridge east of Bonanza Mine.

DONOHO PEAK SECTION (74A-4T)

A smooth flat surface marks the top of the Nikolai 

Greenstone, but evidence of extensive subaerial weathering of the Nikolai is lacking. The marine transgression that marks the base of the Chitistone probably 

removed any existing regolith on the Nikolai 

Greenstone.

The basal Chitistone consists of gray-yellow calcareous shale and nodular argillaceous lime mudstone, 

0.3-2 m thick, in sharp contact with the underlying 

Nikolai Greenstone. The Chitistone Limestone from 2 to 

10 m is massive, gray lime mudstone that contains 

fragments of mollusk shells preserved as micritic enChitistone Limestone * ' -.. . 

FIGURE 2. Cliffs on west side of Nizina Valley (sec. 36, T. 4 S., R. 15 E.); relief is approximately 1 km. Folded Triassic rocks are unconformably overlain by Albian and younger Cretaceous rocks: Location of Nizina River section (74A-7T) is marked. Photograph by David L. 

Jones.
A4 GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS, ALASKA

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FIGURE 3. Stratigraphic correlation of lower part of Chitistone Limestone, showing lithology and sedimentary structures. See figure 1 for

locations of sections.
STRATIGRAPHY AND DIAGENETIC HISTORY, LOWER PART OF THE TRIASSIC CHITISTONE LIMESTONE A5

Composition, in percent

0 10 50 90 100 Dolomite

100 90 50 10 0 Limestone

Greenstone Shale

Symbols for fossil particles

<TO/m> Algal mats

^ Calcareous algae

<e Foraminifers

6 Molluscan in general

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* Ostracodes

* Echinoderms

Sedimentary structures

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pseudomorphs after anhydrite

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after anhydrite with a cumulus 

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Mother Lod mineunderground section

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FIGURE 3. Continued
A6 GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS, ALASKA

velopes with sparry calcite fillings. The lime mudstone 

beds are typically composed of calcite crystals between 

2 to 6 //m in size and argillaceous material; above 10 m 

they consist of calcite crystals in the 10 to 20 /j.m size 

range (fig. 4A). The environment of deposition for the 

lime mudstone is interpreted to be shallow, restricted 

marine.

At the Donoho Peak section, a thin-bedded platy argillaceous, dark-gray pelletoid lime mudstone to 

packstone occurs between 24 to 26 m. It is overlain by 

a 1-m-thick pellet to mud-lump lime mudstone and 

packstone. A 1-m-thick bed of slightly argillaceous, 

dark-gray dolomitic lime mudstone at 30-31 m contains laminae that are interpreted as weakly developed 

algal mat structures (fig. 4B). Park (1976, fig. la) and 

Kingsman and Park (1976) stated that lamination is an 

intrinsic characteristic of stromatolites, and Park's figure la shows sedimentary features similar to those 

from 30-31 and 47-48 m levels. This laminated 

mudstone (fig. 4.B) is in part a reflection of alternating 

bands of different dolomite content. The limestones 

from the 31- to 47-m interval are similar to the underlying beds except for a well-developed layer (38 to 43 

m) of dark-gray argillaceous, platy pelletoid dolomitic 

lime mudstone and packstone. A pelletoidal packstone 

from 37 m (fig. 4C) is typical of the nonargillaceous 

carbonate rocks of this interval. Similar pelletoidal 

packstones are found up to 55 m in the Nizina River 

section. The pellets commonly contain subhedral dolomite rhombs and local micritic envelopes, and they are 

coated with fibrous calcite. Spaces between the pellets 

are filled by microspar or sparry calcite cement.

At the Donoho Peak section a stratigraphic level between 47 to 48 m is laminated dark-gray argillaceous 

dolomitic lime mudstone (fig. 4D) lithologically similar 

to the bed at 31 m. The higher bed contains thinner, 

better developed laminae. Alizarin red staining of thin 

sections reveals alternating bands of dolomite-rich and 

dolomite-poor carbonate minerals. The section from 49 

to 93 m at Donoho Peak and 59 to 85 m at the Nizina 

River section contains some dolomite rhombs (fig. 4E, 

F] but is primarily pelletoid-molluscan-echinoderm 

lime mudstone and wackestone. The scanning electron

micrograph of a specimen from Donoho Peak at 69 m 

(fig.4£") shows that the pellets are composed of 2- to 

6-/zm calcite crystals and micrite between pellets of 

calcite crystals 8 to 20 /j.m across and some subhedral 

dolomite in the 10- to 15-/zm size range. The limestone 

from 72 m at Donoho Peak is typical pelecypodgastropod-echinoderm-pelletoid dolomitic wackestone 

to packstone. Above 86 m bioclasts of mollusks and 

other invertebrate's are rare or absent. SEM pictures 

(fig. 4F) and photomicrographs (fig. 5A) of peloid 

packstone at 88 m and 115 m show that the pellets 

again are composed of calcite crystals 5 /zm long but 

contain more clay between the crystals than the adjacent micrite matrix, which has calcite crystals 5 to 20 

/zm in size. (A similar sequence in the Nizina River section from 59 to 85 m (fig. 5-B, C, D] is composed of 

lime mudstone and peloid packstone with 10 to 35 percent dolomite that forms euhedral to subhedral rhombs 

10-60 /zm in size.) At 94 to 98 m ( and at 86 to 122 m 

in the Nizina River sections) the rocks are brownishgray dolomite with calcite void fillings. Some beds contain well-preserved pelletoid texture and relict fragments of echinoderms. Dolomite (fig. 5E, F) containing vugs lined with large dolomite rhombs and filled 

with calcite is found at 95 m.

The texture from 97 to 105 m at Donoho Peak is complex (fig. 6), but the rocks are predominantly dolomitic. The stratigraphic interval between 97 to 99.5 m is 

composed of brownish-gray dolomite with abundant 2- 

to 15-cm vugs filled with dolomite and calcite. At about 

97-100 m is a 25- to 40-cm-thick bed of dolomitic 

limestone (fig. 6A, B, E; fig. 7) consisting of dolomitic 

mudstone, peloids, and laminated crust. The vugs or 

cavities in these beds are lined with large (0.5-1 mm) 

rhombs of limpid dolomite with superposed sparry calcite. A 5- to 10-cm-thick bed of yellowish-orange, pyritic, argillaceous dolomite is interbedded within this 

unit.

At Donoho Peak at about 101 to 104 m is calcitic 

dolomite which has the relict texture of a molluscanostracode wackestone-packstone. The fossil fragments 

were molds that are now filled by dolomite in 50- to 

100-/zm rhombs.

FIGURE 4. Specimens from the Donoho Peak section. A, B, D, photomicrographs, plane-polarized light; C, E, F, scanning electron micrographs. Stratigraphic positions given in meters above the Nikolai Greenstone-Chitistone Limestone contact. A, Lime mudstone. Aggrading 

neomorphism has produced calcite rhombs 10 to 20 jum in size. Faint outlines of peloids with smaller calcite rhombs are darker. 15 m. B, 

Laminated, dolomitic peloid lime mudstone. Laminations are interpreted to be intertidal algal mats. 31 m. C, Peloid dolomitic packstone 

to grainstone. Dolomite rhombs are larger than calcite rhombs. 37 m. D, Laminated dolomitic peloid lime mudstone. Laminations are 

interpreted to be intertidal algal mats. 48 m. E, Dolomitic lime mudstone; in thin section the rock shows peloids. Peloid (lower right) is 

composed of smaller calcite rhombs than calcite crystals in matrix. 69 m. F, Peloid packstone. Peloids are lighter colored rounded areas 

to left. Peloid contains greater amounts of clay minerals (white material) between calcite rhombs, and matrix between peloids is formed 

by larger 5- to 12-jum calcite rhombs. 88 m.
STRATIGRAPHY AND DIAGENETIC HISTORY, LOWER PART OF THE TRIASSIC CHITISTONE LIMESTONE A7

30 ^ 20 n-m
A8 GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS, ALASKA

The section from 104 to 122 m shows evidence of a 

marine transgression followed by an open platform environment of deposition, by a decrease in dolomite and 

an increase of limestone with abundant remains of mollusks, echinoderms, and brachiopods (fig. 5^4).

ERIE MINE SECTION (74A-6T)

The Erie Mine section, 74A-6T, was measured west of 

the Erie mine. The section was measured to a horizon 

some 105 m above the Nikolai Greenstone where 

dolomitization and tectonic fracturing has obscured the 

stratigraphic relations. The basal bed of the Chitistone 

is 0.7 m of calcareous, brownish-red shale and is overlain by 34 m of gray lime mudstone and echinodermmolluscan-peloid wackestone. The calcite rhombs in the 

micrite are 10 to 30 jum in size and are the result of aggrading neomorphism. Laminations, which are suggestive of algal mats, are found at 35 and 51m above the 

Nikolai Greenstone. Dolomitic peloid lime mudstone 

are found from 35 to 46 m. Dolomitic lime mudstone is 

present from 46 to 63 m. Light-gray dolomite in beds 5 

to 15 cm thick is present between 63 to 70 m, These 

beds contain vugs with sparry calcite filling and also 

have poorly developed algal laminations. These dolomites appear to have replaced a peloid lime mudstone 

with dolomite rhombs that are about 4 //m in size, 

Dolomitic lime mudstone and echinoderm wackestone 

are present from 77 to 83.5 m. Massive, chert-free 

dolomite is found from 83.5 to 105 m above the base.

BONANZA MINE OUTCROP SECTION(74A-2T)

The Chitistone Limestone section 74A-2T on Bonanza 

Ridge was measured about 600 m south of the Bonanza 

mine (fig. 8). The Chitistone near the main Kennecott 

mines from west of the Jumbo mine to McCarthy Creek 

has been extensively dolomitized, tectonically brecciated, and faulted. Outcrops on Bonanza Ridge are difficult to correlate with nearby outcrop sections at 

Donoho Peak to the west (74A-4T), or at Green Butte 

mine to the southeast (74A-3T) (fig. 3). However, 

limestone in the stratigraphic interval 4 to 18 m above 

the Nikolai provides reasonably valid stratigraphic correlations. These basal beds are 1 to 2 m of calcareous 

shale and thin-bedded argillaceous lime mudstone and 

10 to 12 m of bioturbated, argillaceous, gray, thinbedded lime mudstone. The characteristic structure of 

these beds is crinkly bedding surfaces formed by anastomosing tracks, trails, and burrows. These beds were 

deposited on a restricted platform. The lime mudstone 

is composed of 5- to 15-//m crystals of calcite. The 

Chitistone Limestone at about 20 m above the Nikolai 

has been brecciated and altered to medium- and 

coarse-grained dolomite.

The dolomite from 21 m above the Nikolai 

Greenstone has the relict fabric of a crinoid-peloid 

packstone. The crinoid bioclasts are now large dolomite 

crystals, and the outlines of the peloids are marked by 

dark, probably organic, inclusions within clusters of 

dolomite rhombs. The matrix is composed of 75- to 

125-juni subhedral dolomite rhombs. Some intercrystalline pores or vugs are present as well as 20- to 300- 

jum-wide fractures. Both are filled by sparry calcite.

A fractured dolomite at 44 and 48 m above the 

Nikolai Greenstone contact is composed of 80- to 

150 -fj.m subhedral dolomite rhombs. The fracture filling shows that some voids were partly lined with 1- to 

2- mm dolomite rhombs, and the remaining voids are 

filled with sparry calcite. Dedolomitization has affected 

both the large dolomite rhombs and the smaller dolomite rhombs of the matrix.

A dolomite at 81 m above the base has relict 

crossbedding and was derived from a peloidechinoderm wackstone-packstone. Thin section studies

FIGURE 5. Specimens from the Donoho Peak and Nizina River sections. A, B, E, F, photomicrographs, plane-polarized light; C, D, scanning 

electron micrographs. Stratigraphic position given in meters above the Nikolai Greenstone-Chitistone Limestone contact. A, Dolomitic 

molluscan eehinoderm peloid packstone, Donoho Peak section. Most molluscan fragments are now micritic envelopes, peloids contain 

dolomite rhombs; calcite has undergone aggrading neomorphism. Peloids have radial fibrous calcite cement on their surface, space between peloids is sparry caleite. Arrow points to dolomite rhombs that formed within the micrite of some peloids. 115 m. Specimens B~F 

are from the Nizina River section. B, Brachiopod( ?) shell fragment with possible geopetal filling in a matrix of micrite and pseudospar 

calcite, 41 m. C, Limestone composed of 5- to 25-^um caleite crystals, no apparent dolomite or clay minerals. This specimen in thin section 

(fig. 5£) is a peloid molluscan packstone that shows evidence of aggrading neomorphism. 41 m. D, Dolomitic limestone; dolomite is in 

clusters formed by 10- to 3Q-//m subhedral rhombs, ealeite matrix is 2 to 15 Mm in size. In thin section rock is a peloid packstone with 

selective dolomitization of some peloids. 74 m. E and Ft Dolomite and calcite, 95 m. Specimens have same fabric as underground zebra 

beds associated with Mother Lode and Bonanza mine ore bodies, dm, fine-grained dolomite matrix; Id, shows large and composite dolomite zoning. Crystals labeled Id consist of limpid dolomite only in their outermost margins. Some crystals are not entirely limpid dolomite, sc, final void-filling sparry calcite. Thin fracture filled with calcite can be seen on left of 5F. Fracture shows sequence of events; 

sparry caleite is youngest; it cuts across dolomite matrix, large dolomite rhombs, and vug-filling sparry calcite.
STRATIGRAPHY AND DIAGENETIC HISTORY, LOWER PART OF THE TRIASSIC CHITISTONE LIMESTONE A9

0.5 mm .
A10 GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS, ALASKA

show the rock is composed of dolomite crystal replacement of echinoderm bioclasts and a matrix of 100- 

to 200-/am subhedral dolomite rhombs. The rock at 86 

m is also characterized by vugs 1 to 6 mm in size. These 

are lined with 0.5- to 1-mm limpid dolomite rhombs, 

and filled with sparry calcite. The large rhombs show 

evidence of dedolomitization.

GREEN BUTTE SECTION (74A-JT)

The Green Butte section was measured adjacent to 

and north of the Green Butte mine. The Triassic carbonate rock section above the Nikolai Greenstone at 

Green Butte is some 1,067 m thick, of which 701 m is 

the Chitistone Limestone and 366 m is the Nizina 

Limestone (Armstrong and others, 1969). The section 

measured in 1974 is the lower 110 m of the Chitistone 

Limestone.

The top of the Nikolai Greenstone is a smooth surface 

not strongly altered by weathering. The basal Chitistone beds are 0.7 m of calcareous dark-gray to black 

shales, followed by 1.3 m of pale-yellow-weathering, 

thin-bedded, argillaceous lime mudstone. The section 

from 2 to 23.5 m is predominantly lime mudstone in 

which the calcite crystals have been subjected to aggrading neomorphism. The calcite crystals are now 

10-20 /am in size. The section from 24.5 to 87 m above 

the Nikolai Greenstone is predominantly dolomite that 

is pale yellowish orange to yell'owish gray in weathered 

outcrop, medium bedded, and chert free. Thin sections 

show that it is composed of subhedral rhombs, 100 to 

300 /am in size. The intercrystalline pores are filled by 

sparry calcite. Lime mudstone to peloid-algaemolluscan packstone is present between 87 and 90 m. 

Between 90 and 103.5 m is yellowish-brown-gray 

dolomite. At 102 to 103 m are dolomitic pisooids and 

laminated crust. At 103.5 m are massive, gray, algaemolluscan-echinoderm wackstone and packstone. These 

latter beds are interpreted as representing a regional 

marine transgression.

NIZINA RIVER SECTION (74A-7T)

The Chitistone Limestone is exposed on the west side 

of the Nizina River in a series of spectacular cliffs (fig.

2). The upper contact of the Nikolai Greenstone, beneath the Triassic sediments, is smooth and shows little evidence of pre-sedimentation weathering. The 

basal Chitistone beds consist of 0.2-0.3 m of gray shale 

that contains nodules of limestone. They are overlain 

by gray, argillaceous lime mudstones 1.5 to 2 m thick 

that contain abundant worm burrows and trails. Above 

these beds to 40 m above the base are gray, massive, 

chert-free, lime mudstones and peloid packstones with 

no obvious sedimentary structures. The lime 

mudstones, to a stratigraphic level of 55 m, are formed 

by calcite crystals that have been affected by aggrading 

neomorphism. The calcite rhombs are now 10 to 30 /am 

in size. These lime mudstones and peloid packstones 

are characterized by massive beds, a general lack of 

well-preserved fossil remains, and abundant peloids. 

The limestones from 35 to 59 m are massive, light 

gray, and chert free. They are composed of peloidrounded mud-lump molluscan-echinoderm packstone. 

The calcite crystals in these rocks have also been subjected to aggrading neomorphism. The fossil bioclasts 

are generally preserved as micritic envelopes. A thin 

section of a sample at 40 m shows it to be a peloidmolluscan packstone. This specimen when examined by 

the scanning electron microscope and EDAX analyzer is 

composed of calcite crystals in the 5- to 30-/mi size 

range. Examination of the areas between the calcite 

rhombs showed 1- to 3-/am-wide to 1- to 10-/mi-long 

crystals of anhydrite. The origin of this anhydrite in 

these peloid sediments is not known but possibly is related to late diagenetic fluids in the limestone which 

were rich with S04 ions. The fluids must have been low 

in Mg ions for the SEM studies reveal no dolomite in 

either limestone sample at 40 or 42 m. The dolomitic 

limestones from 59 to 85 m above the Nikolai contact 

are composed of lime^ mudstone and peloid packstone 

with 10 to 35 percent dolomite. The dolomite is composed of euhedral rhombs 10-60 /mi in size.

Sedimentary structures and vug fillings are found in 

the Nizina River section at 86, 90, and 94-96 m (fig. 

5E, F}. Algal mats at 94 m are associated with laminated crust and pisooids. The sedimentary structures 

at 95 m have vugs lined with large (1-2 mm) subhedral 

to euhedral limpid dolomite rhombs and partly filled

FIGURE 6. Photomicrographs, plane-polarized light of specimens from Donoho Peak section. Stratigraphic position given in meters above

the Nikolai Greenstone-Chitistone Limestone contact. A, Dolomitic-lime mudstone; dark areas are dolomitic lime mudstone composed of 

very small crystals of dolomite and calcite. Id, large limpid dolomite rhombs that line void walls; sc, sparry calcite that fills cavities. 99.7 

m. B, Peloid lime mudstone. Peloids are composed of a mixture of very small dolomite and calcite rhombs. Arrows point to subhedral and 

anhedral limpid dolomite rhombs that line voids. Remainder of void is filled by sparry calcite. Thin section is from caliche beds at 100.3 

m. C, Laminated crust, showing in lower part of view vesicles formed by overlapping upward lenses and parallel laminations. Above 

parallel lamination is breccia formed of broken crusts. 100 m. D, Enlarged view of laminae of crust (fig. 6C) with convex vesicles filled 

with small subhedral rhombs of clear dolomite and calcite. 100 m. E, Enlarged view of broken crust (fig. 6C) showing calcite filling. 100
STRATIGRAPHY AND DIAGENETIC HISTORY, LOWER PART OF THE TRIASSIC CHITISTONE LIMESTONE All
A12 GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS, ALASKA

with sparry calcite. The beds exposed at the 95-m level 

have the characteristic gray dolomitic matrix and white 

dolomite and calcite vug fillings that are lithologically 

similar to the zebra beds associated with the ore bodies 

in the Mother Lode, Bonanza, and Jumbo mines. Massive dolomite that is chert free and contains no obvious 

sedimentary structures occurs between 97 and 122 m. 

Some beds may be composed of dolomite rhombs as 

large as 1 mm. The brown dolomite from 122 to 135 m 

contains black nodular chert. Dolomite and calcite void 

fillings are found at 131 m above the base. An unusual 

sedimentary breccia occurs at 130 m (fig. 9A, D). 

Voids between the dolomite clasts of the breccia apparently were initially filled by radiaxial fibrous dolomite 

and subsequently by radiaxial calcite.

DISCUSSION OF 

SPECIFIC SEDIMENTARY FEATURES

MICRITIC MATRIX AND LIME MUDSTONE

Bathurst (1971, p. 511) stated that the upper size 

limits of crystals of lithified micrites is 3-4 yum. Scanning electron micrographs of micrite crystals from the 

Chitistone Limestone from the Donoho Peak section 

show that the size of the micrite calcite crystals at 4 m 

above the base range from 5 to 15 yum; at 37 m they are 

between 3 and 7 yu m; and at 88 m they are from 4 to 15 

yum. The Nizina River section shows similar large crystals in the lime mudstone. There the lime mudstone at 

10 m has micrite crystals from 5 to 20 yum in size and 

at 42 m from 5 to 30 yum in size. Dolomitic limestones 

at 74 m show micrite crystals composed of calcite from 

5 to 10 yum in size.

Studies of the Chitistone lime mudstone and micritic 

matrix shows the typical micrite crystals are generally 

in excess of 2 yum in size and have been affected by aggrading neomorphism. In a strict sense many of the 

Chitistone lime mudstone would be classified as microspars.

PELOIDS

McKee and Gutschick (1969) applied the term peloid 

to cryptocrystalline or microcrystalline material irrespective of size or origin. This would include structureless lime-mud (micrite) particles from 0.020 to 4 mm 

in size. Pellets or peloids can be formed as fecal pellets 

from any number of organisms, or by the process of

micritization. Bathurst (1971, p. 389) stated that micritization of skeletal debris forms a considerable part of 

the modern lime sand in Bimini Lagoon, Bahamas. 

Algae bore into the grains forming small tubes, which 

are subsequently filled with micrite. If this process proceeds the entire skeletoid grain will be replaced by 

micrite and will form a peloid indistinguishable from a 

fecal pellet.

The lower part of the Chitistone Limestone contains 

abundant peloids. They may have well-defined shapes 

or blurred outlines. Peloids with well-defined outlines 

when examined under the petrographic microscope 

show a diagenetic sequence of events. These events are 

the deposition of the peloids, a first-stage CaCOs cement, filling of voids between peloids by sparry calcite 

cement, and development within many of the peloids of 

euhedral rhombs of dolomite (fig. 10).

SEM examination of the peloids reveals in detail their 

crystal structure, inclusions of clay, and outlines. A 

specimen at 69 m from the Donoho Peak section shows 

(fig. 4E) the peloid to be composed of calcite crystals 

in the 3-4yum size range, whereas the interpeloid cement is in the 7- to 20-yum size range, and peloid 

packstone from 88 m (fig. 4F) shows the peloids to be 

100-300 yum in diameter, formed by 2- to 3-yum calcite 

rhombs, with abundant clay minerals between the calcite rhombs. The matrix between the peloids is very low 

in clay minerals and is formed by 5- to 20-yum calcite 

rhombs.

BIOCLAST DEBRIS

Bioclastic debris is relatively rare within the lower 

part of the Chitistone Limestone and consists primarily 

of ostracodes and fragments of pelecypods, gastropods, and echinoderms. Most of the pelecypods and 

gastropods were aragonite and are now preserved as 

micritic envelopes. These fragments do not show the 

details of the original shells, but only the original outlines are preserved as a dark thin outer zone that encloses an infilling of sparry calcite cement. Bathurst 

(1971, p. 333) considered these dark outer rims to be 

the results of the infilling of algal borings by carbonate 

mud, resulting in a micritic envelope. This process preserves the original shell outline after the solution of the 

aragonite and before the complete infilling by sparry 

calcite.

FIGURE 7. Photomicrographs, plane polarized light of specimens from Donoho Peak section. Stratigraphic position given in meters above 

Nikolai Greenstone-Chitistone Limestone contact. A-D, Dolomitic-pisooid caliche. A, View of textures illustrating packing and size range 

of pisooids; large clear areas are filled with rhombs of clear dolomite and sparry calcite. Note reverse grading of pisooid at bottom of 

figure. B, Enlarged view of a pisooid showing banding and light and dark areas. C, Pisooids and light-colored limpid dolomite (id) void 

filling and final sparry calcite (sc) void filling. D, Small pellets or pisooids beneath larger pisooid. Surface of pisooid is covered with 

small dolomite rhombs; centers of voids are filled with calcite. 97.2 m.
STRATIGRAPHY AND DIAGENETIC HISTORY, LOWER PART OF THE TRIASSIC CHITISTONE LIMESTONE A13
A14 GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS, ALASKA

PELLET, PISOOID, AND LAMINATED CALICHE

Two lithologically distinctive and stratigraphically 

useful caliche marker beds are in the Donoho Peak section at 97 to 97.8 m (fig. 1A-D) and at 99.7 to 101 m. 

Similar caliche beds are at 102 m in the Green Butte 

section, and at 135 m in the Nizina River section. The 

beds are from 0.3 to 1.3 m thick, weather a light gray 

to light-yellowish gray, and are easily recognized in the 

field by their color, the bands of laminated crust, and 

large peloids and pisolites up to 10 mm in diameter.

Thin sections and SEM studies show that the caliche

(fig. 6C--E; fig. 7A-D] is composed of dolomiticmicritic micropellets and large laminated pisolites in a 

micritic matrix with numerous voids filled by clear 

dolomite and sparry calcite (fig. QC-E; fig 1A-D). Nontectonic fractures are abundant. The interparticulate 

voids and nontectonic fractures are filled first by limpid dolomite and then sparry calcite. The laminated 

crusts (fig. 6B; fig. 5-F) are formed by wavy bands 10 

to 100 jum thick of dark micrite separated by thin 10- 

to 500 -jum bands of light to clear calcite and dolomite. 

The Chitistone Limestone laminated crust resembles

FIGURES. View of Bonanza Ridge looking west toward ruins of Bonanza mine bunkhouse and power plant ( center). The Glory Hole is on ridge 

above power plant. Chitistone Limestone on Bonanza Ridge is dolomitized and makes the irregular outcrop. Outcrop is vertically jointed. 

Location of section east of Bonanza mine (74A-2T) is marked. Line of traverse above measured section shown by dotted line. Photograph 

by David L. Jones.

FIGURE 9. Photomicrographs and photograph of sedimentary breccia from Nizina River section, from the Chitistone Limestone 130 m above 

its base. A. Thin section in plane light, dm, dolomite matrix; df, thin band of fibrous dolomite; Q, quartz; cf, radiaxial fibrous calcite; sc, 

sparry calcite. B, Polished slab of the breccia; dm, dark dolomite matrix. Birdseye-like structures and nontectonic brecciation are clearly 

visible, df, light-colored fibrous dolomite; cf, overlying darker radiaxial fibrous calcite; sc, white sparry calcite that fills center of voids. 

C, Thin section, crossed nicols. dm, fine-grained dolomite matrix; df, fibrous dolomite; cf, radial fibrous calcite; sc, sparry calcite. D, 

Thin section, crossed nicols, same symbols as above.
STRATIGRAPHY AND DIAGENETIC HISTORY, LOWER PART OF THE TRIASSIC CHITISTONE LIMESTONE A15
A16 GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS, ALASKA

closely the Quaternary laminated crust described by 

Read (1974, p. 260, figs. 8a, b) from Shark Bay, Australia. Some dark bands are separated by fibrous calcite up to 20 /im in length (fig. 6C); these may represent filled desiccation cracks.

The Chitistone caliches contain pisolites (fig. 7A-C) 

and large micritic peloids (fig. 5£, C, F). The pisolites 

and peloids may be compound; those over 1 mm in size 

are usually irregular in shape. Reverse grading of pisolites is common (fig. 7A, D). Fossil bioclasts are rare, 

and are generally small broken echinoderm fragments.

Scanning electron micrographs of the pisolite caliche 

at 135 m in the Nizina River section clearly show the 

pisolites to be composed of a mixture of 5- to 10-/im 

calcite crystals and 0.5- to 10-/im rhombs of dolomite. 

Large (30 to 150/im) crystals of calcite and dolomite 

occupy spaces between the pisolites and are readily 

distinguished from carbonate crystals that form the 

pisolites. The bands of the pisolites are characterized 

by differing proportions of dolomite and calcite crystals.

The caliche zone at 100 m in the Donoho Peak section 

is characterized by well-developed laminated crusts 

(fig.6C,D).

The caliche zone at 102-103 m in the Green Butte 

section contains well-developed peloids and laminated 

crusts. The pisolites are dolomitic and similar to those 

at the 135-m level in the Nizina River section.

The peloids, pisolites, and laminated caliche beds are 

all associated with carbonate sediments that indicate 

deposition in subtidal to supratidal environments. 

Esteban's (1976) classification would place these as 

vadose marine or vadose hypersaline caliche. Sedimentary structures in these beds are algal mats, dolomite 

or calcite pseudomorphs after gypsum-anhydrite, and 

mud chips.

Examination under the SEM of the pellets and pisolites shows that they are dolomitic, generally with

dolomite rhombs in the 0.5- to 10- /im size range. 

Scholle and Kinsman (1974) and Purser and Loreau 

(1973) have shown that caliche crust on Pleistocene 

limestone in Abu Dhabi, Trucial Coast, Persian Gulf, 

and Bonaire, Netherland Antilles, fo'rm in supratidal 

vadose areas that are characterized not by fresh water 

but by hypersaline brine. The resultant textures of 

these crusts are largely vadose, but the mineralogy 

aragonite and' dolomite and chemistry are of marine 

character. This environment could account for the 

abundance of microdolomite in the pisolites in the 

Chitistone Limestone. The Chitistone pisolites may 

have formed in part from dunes of skeletal grainstone, 

subjected to marine vadose weathering in hot semiarid 

conditions under the influence of fluctuating saline 

brines and rain water (schizohaline environment of 

Folk and Land, 1975). This environment would be the 

surface of a stable supratidal sabkha.

SEDIMENTARY BRECCIAS

The Nizina River section contains a sedimentary 

breccia at the 130-m level (fig. 9A-D). The sedimentary breccia occurs above a pisolitic caliche at 129 m 

and below beds with pseudomorphs of gypsum at 130- 

131 m, and pisolitic caliche at 135 m. The sedimentary 

breccias are associated with rocks of intertidal to supratidal to saline vadose-weathering origin. These breccia fragments are unlike any of the other breccias in 

the Chitistone Limestone. They may be up to 10 cm 

long and 3 cm thick and contain birdseye structure. Interclast cavities are elongated subparallel to bedding. 

The matrix of the clasts is composed of 100- to 150-/im 

subhedral dolomite rhombs commonly separated by 

black, partly oxidized pyrite cubes, 4 to 12 /im in size. 

Surfaces of the fragments are covered with white elongate crystals wich extend 2 to 5 mm into interclast 

cavities. The coarse-grained dolomite is in turn overlain by gray radiaxial calcite which extends 2 to 6 mm

FIGURE 10. Probable sequence of alteration of peloid-bioclastic grainstone as exemplified by a thin section from the Chitistone Limestone. A, 

Original sediments consisting of bioclasts of mollusks (ab) with a micritic envelope, echinoderm calcite bioclasts (cb), and peloids (P). 

B, First-stage CaCOs cement. C, Dissolution of aragonite clasts. Micritic envelope and first-stage CaCOs provide strength to molluscan 

bioclast molds. D, Complete cementation by second-phase CaCOa cement, syntaxial overgrowth on crinoid calcite bioclasts. E, Many 

peloids within the Chitistone Limestone contain dolomite (D) rhombs and microspar (C) from the micrite. Peloid-molluscan-echinoderm 

packstone would show similar sequence of events: micritic matrix may go to microspar, resulting in a rock very similar in appearance to 

the above grainstone.
STRATIGRAPHY AND DIAGENETIC HISTORY, LOWER PART OF THE TRIASSIC CHITISTONE LIMESTONE A17

into the cavaties. The remaining voids are partly filled 

by sparry calcite. Between the elongate dolomite 

rhombs and the radiaxial calcite is a diffuse band consisting of disjunct 100- to 300-jum quartz grains with 

calcite inclusions and some scattered 20- to 30-yum pyrite crystals.

The sequence of events for the formation of these 

breccias appears to be: (l) Carbonate muds were 

deposited in a subtidal-intertidal to possible supratidal 

environment. (2) The carbonate muds were partially 

cemented. (3) The partly lithified sediments were fractured. (4) The matrix of the sediments may have originally been dolomite or was progressively altered from 

calcium carbonate to dolomite during the above stages. 

Coarse dolomite rhombs were deposited on cavity walls 

of the fractured and porous sediments. (5) Radiaxial 

fibrous calcite was deposited over the dolomite that 

may represent recrystallized fibrous aragonite cement 

(Kendall and Tucker, 1973). (6) Voids were filled by 

coarse sparry calcite, probably in a fresh-water 

diagenetic environment. Eugene Shinn (written commun., 1976) interpreted the cements to be submarine 

(subtidal) in origin.

Evamy's (1973) study of the intertidal-supratidal 

sediments of the Trucial Coast of the Persian Gulf 

showed sedimentary structures similar to those in the 

Chitistone breccias.

The sequence of carbonate minerals that fill the voids 

and cavities of the breccia at 135 m in the Nizina River 

section provides clues to the history and genesis of the 

breccia. Kendall and Tucker (1973) suggested that 

radiaxial fibrous calcite, similar to that found in our 

breccias, is a replacement after acicular aragonite or 

fibrous magnesium calcite. Folk (1974) indicated that 

acicular carbonates are the results of slow sideward 

crystal growth in marine or tidal environments rich in 

magnesium. If these two assumptions are correct, then 

the radiaxial calcite was probably formed originally in 

marine waters.

ZEBRA BEDS AND VUG FILLINGS

In the Bonanza and Mother Lode mines, some massive copper sulfide ore bodies occur at stratigraphic 

levels. Many ore bodies are associated with what the 

miners term zebra beds, a unit that has specific carbonate rock fabrics, sedimentary structures, and environments of deposition. A photograph (fig. ll) and 

samples of these beds at the 503-m (1,650-foot) level, 

adjacent to a large massive ore body in the Mother 

Lode mine, show that zebra beds comprise a matrix of 

subhedral 50- to 20Q-/um light-brown-gray dolomite 

with abundant flat-bottomed, cumulus, enterolithic 

vugs lined with light-gray to white zoned coarse dolomite. Centers of the vugs are partly filled with sparry

calcite (fig. 12). Abundant 5- to 100-jum-wide fractures 

cut the dolomite matrix and both the dolomite and calcite vug fillings. These fractures are filled with sparry 

calcite and copper minerals. The intercrystalline pores 

between dolomite rhombs of the matrix contain sparry 

calcite. The outcrop specimen from the Nizina River 

section from 95 m above the Nikolai Greenstone (fig. 

5-E1 ; fig. 12C) shows the same mineralogy and sequence 

of mineralization except that the fine fractures in the 

Nizina River section are filled with calcite and are devoid of copper minerals.

Our underground studies at the Kennecott mines 

show that some ore bodies bottom in the zebra beds 

and that they probably are wider and best developed in 

the zebra beds, but their upper parts transect stratigraphically higher beds.

Fine-grained uniform dolomite that contains burrow 

and small-scale cut-and-fill structures underlies the 

zebra beds. The overlying beds are 1- to 3-m-thick beds 

of dolomitic-pisolitic-laminated caliche overlain in turn 

by coarse-grained dolomite with dolomite 

pseudomorphs after crinoid bioclasts.

The zebra beds of the Mother Lode (figs. 11 and 12) 

and Bonanza mines and the Nizina River section have 

sedimentary structures similar to Stromatactis, 

Stromatactis was first used taxonomically by Dupont 

(1881) for distinctive crystalline carbonate features 

that he considered stromatoporoids in Devonian limestones of Belgium. Lecompte (1937, 1954) thought 

these features were precipitated calcite resulting from 

the decomposition of algae. Bathurst (1959) concluded 

that Stromatactis refers only to an enigmatic flatbottom crystalline feature. Stromatactis sedimentary 

structures are associated with lime mud banks in England, Ireland, and Nevada. The lime mud banks may 

have hundreds of meters of relief above the adjacent 

deeper water sediments (Lees, 1961, p. 103, fig. 2; 

Lees, 1964; Schwarzacher, 1961, p. 1483, figs. 2-4; 

Ross and others, 1975). Most of the Stromatactis decribed are on inclined prograding beds. Lee (1961, 

fig.5) described a cross section of a Waulsortian bank 

with growth layers of lime mudstone, inclined beds of 

Stromatactis and bioclastic debris parallel with the inclination. The zebra beds of the Chitistone Limestone 

do not show these relations.

Zebra beds in the Chitistone Limestone are parallel 

with the underlying and overlying beds. This relation is 

well displayed in the inclined shafts of the Mother Lode 

and Bonanza mines. The inclined shafts parallel the 

beds just below the zebra beds. At various levels from 

the inclined shaft there are drifts into or across the 

zebra beds. This parallelism also is well displayed in 

the outcrops west of the Bonanza mine on Bonanza 

Ridge and on the Nizina River (fig. 2). There is no indi-
A18 GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS, ALASKA

cation in these or other outcrops examined of mounds 

or banks developed within the lower 150 m of the 

Chitistone Limestone.

Kendall and Skipwith's (1969,p. 859) photographs of 

the trenches they dug in Holocene sabkha deposits of 

Abu Dhabi show nodular and contorted anhydrite that 

is parallel to the surface and the underlying bedding

and has nearly identical shapes as the stratiform vugs 

within the zebra beds shown on the tunnel wall in figures 11 and 12 from the Mother Lode mine, and from 

the Nizina River outcrop section at 95 m above the 

Nikolai Greenstone.

Evidence for a sabkha deposition environment for the 

zebra beds includes: (l) their position in the top suite

FIGURE 11. Wallrock, adjacent to ore bodies, from 503-m (1,650-foot) level of Mother Lode mine. Beds dip about 30°. Dark area is dolomite 

matrix rock (dm), light areas are cavities lined with limpid dolomite (id) and final void filling by sparry calcite. Beds beneath the unit are 

uniform gray dolomite with burrows and small-scale cut-and-fill structures. Overlying these beds is coarse-grained dolomite with dolomite pseudomorphs of echinoderms that resemble Stromatactis of the literature. White areas are dolomite and calcite cavity fillings of 

what we interpret as pseudomorphs after interbedded anhydrite and gypsum.

FIGURE 12. Specimens from zebra beds in the Mother Lode mine at 503-m (1,650-foot) level adjacent to ore bodies. Stratigraphic position of 

specimen given in centimeters above mine floor. A, Polished surface of zebra rock. Dark areas are dolomite matrix, light areas are limpid 

dolomite and calcite pseudomorphs after anhydrite. 200 cm. B, Photomicrograph of thin section; dm, fine-grained dolomite matrix. 

Anhydrite pseudomorphs are lined first by large limpid dolomite rhombs, then by sparry calcite (sc). Tectonic fracture, now filled by 

sparry calcite (scf), crosscuts all textures. These fractures also contain copper mineralization. 250 cm. C, Polished surface. Dark areas 

are dolomite matrix, light areas are limpid dolomite and calcite pseudomorphs after anhydrite. 200 cm. D, Photomicrograph of thin 

section; fine-grained dolomite matrix. Anhydrite pseudomorphs are lined first by large limpid dolomite rhombs(ld), then by sparry calcite (SC). Tectonic fracturing crosscuts all textures and the fractures are filled by sparry calcite (SCF) and copper minerals (Cu). 200 

cm. E, Polished surface. Dark areas are dolomite matrix, light areas are limpid dolomite and calcite pseudomorphs after anhydrite and 

gypsum. 170 cm.
STRATIGRAPHY AND DIAGENETIC HISTORY, LOWER PART OF THE TRIASSIC CHITISTONE LIMESTONE A19

«** ^
A20 GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS, ALASKA

of beds of a sequence which goes from molluscanpeloid wackestone (subtidal) to beds dominantly composed of dolomite and devoid of fossil fragments; (2) 

the sedimentary structures of the zebra beds, and the 

shapes of the vugs which, with their flat bottoms and 

lenticular, enterolithic form suggest that they are 

pseudomorphs after anhydrite and gypsum; and (3) 

the finer grained subhedral dolomite matrix between 

the dolomite and calcite-filled vugs.

An alternative idea (P. W. Choquette, written commun., 1979) for the origin of the zebra beds is collapse 

brecciation due to solution of evaporites beneath the 

zebra beds.

The sequence of events for the formation of the zebra 

beds, as indicated from the study of the polished slabs, 

SEM and energy-dispersive X-ray analyzer examination, and thin sections, is probably the following (fig. 

13).

1. A coastal sabkha carbonate developed in the 

Chitistone Limestone at a stratigraphic level of some 

90-120 m above the Nikolai Greenstone.

2. Sabkha diagenesis of these carbonate sediments 

probably included dolomitization and concurrent 

emplacement of diagenetic gypsum crystals and 

anhydrite nodules within the sabkha sediments 

(Kinsman, 1966, 1969).

3. The gypsum crystals and anhydrite nodules dissolved from the zebra beds with associated dedolomitization of some of the dolomite matrix. Field and petrographic studies do not clearly define the time in which 

this event occurred. The outcrop sections at Donoho 

Peak, Green Butte, and Nizina River and the underground exposures in the Mother Lode mine have sequences that are interpreted as sabkha deposits containing beds with dolomitic caliche. The dolomitic 

caliche may represent formation of hypersaline brine in 

the vadose weathering zone (Scholle and Kinsman, 

1974). During periods of vadose weathering, meteoric 

ground water may have penetrated and dissolved the 

gypsum crystals or anhydrite nodules within the underlying sabkha sediments and formed the vugs.

4. Coarse-grained white zoned dolomite was deposited on the walls of the vug. Petrographic evidence 

does not define the time of this event. The Nizina River 

outcrop beds and the samples from the zebra beds in 

the mines contain voids that are filled by limpid dolomite rhombs and sparry calcite. The void fillings and 

mineralogy of outcrop samples and mine samples are 

identical. The outcrop example suggests an early 

diagenetic origin for the large clear dolomite crystals 

(fig. 5E, F}. The extensive voids created by the removal of the gypsum- anhydrite (figs. 1 1 and 12) indicates that a thick section of sediments above this zone 

would crush the dolomite matrix framework and brecciate the zebra zones. Thus it can be argued that the 

partial infilling of the vugs by coarse dolomite occurred 

at shallow depths and possibly in the sabkha environment. Folk and Land (1975, p. 63) gave theoretical arguments that dolomite probably forms most readily by 

a reduction in salinity, particularly in a schizohaline 

environment (alternating between hypersaline and 

near-fresh conditions) as in a floodable sabkha or a 

phreatic mixing zone. Flushing marine saline waters 

with fresh water lowers salinity but maintains a high 

Mg/Ca ratio; crystallization is slower and the interfering effect of foreign ions is reduced. Folk and Land

Sabka er

Q

Micrite 

Lime mudstone

1. Supratidal 

sabkha

0

/""\^

^Anhydrite j> / /

- Hypersaline water - 

table

2. Diagenetic 

dolomite and 

anhydrite

ivironment >-

^£P;O

<^DissolutionS 

^s-yoidx^

- Hypersaline water - 

table

Sabkha ~\ J~ 

surface 'V^1 CfeQ^^iWSTO3^ 

Pisolite and crust' 

laminate^XX.

 * 

Deposition of a 

thick sequence 

of Triassic and 

Jurassic sediments above 

porous sabkha 

bed

3. Dissolution of 4. Schizohaline 

anhydrite; for- environment. Limmation of vugs pid dolomite in 

vugs; dolomitic pisolites sabkha sediments with high 

porosity and permeability

Deposition of 

sparry calcite 

 and Cu2S > 

Pisolite bed ff

5. Late 

^

Jurassic 

and Early Cretaceous deformation, 

fractures in rocks, 

thick carbonate section above porous 

sabkha bed; voids 

in vugs and fractures 

filled with sparry calcite, final deposition 

of Cu2S

FIGURE 13. Probable sequence of events in devopment of vugs and cavity filling of zebra beds. Concepts are from samples from Mother Lode

mine and Nizina River section.
STRATIGRAPHY AND DIAGENETIC HISTORY, LOWER PART OF THE TRIASSIC CHITISTONE LIMESTONE A21

stated (p. 67) that dolomite formed from dilute solutions is characteristically limpid.

5. The event that formed the fractures that are 10 //m 

to 0.5 //m wide clearly followed limpid dolomite formation. The fracturing may have occurred during or after 

the deposition of the coarse-grained sparry calcite that 

fills the remaining pore space in the vugs. Sparry calcite fills the fractures within the matrix and the intercrystalline pores of the dolomite matrix. Deposition of 

this calcite was the last major event. If one accepts Folk 

and Land's (1975, p. 60) concept that blocky calcite 

can be formed only in low Mg/Ca water that is derived 

from meteoric water or where Mg has been removed 

from marine water by formation of dolomite or absorption clays, it is possible that the calcite is the product 

of phreatic fresh water. The purity of the calcite and its 

large crystals support this hypothesis. Fracturing and 

calcite void filling may have extended through several 

episodes as indicated by some calcite which fills voids 

and is then cut by fractures and filled by another cycle 

of calcite vein filling.

In the mines many of the fine fractures associated 

with the zebra beds contain not only sparry calcite but 

copper minerals; therefore copper mineralization occurred late in the long diagenetic history of these carbonate rocks.

INTRAFORMATIONAX STRATIGRAPHIC

CORRELATION AND 

ENVIRONMENTS OF DEPOSITION

The correlations shown in figure 3 for the lower part 

of the Chitistone Limestone are based on sedimentary 

features and microfacies. The interpretation of carbonate microfacies, sedimentary structure, and depositional models (fig. 14) follow the stratigraphic concepts 

published by Wilson (1970, 1975).

The lower 100 to 120 m of the Chitistone Limestone 

exposed from Donoho Peak to the Nizina River outcrops fits Wilson's (1975, p. 297) concept of upwardshoaling carbonate sediment cycles that are composed 

predominantly of lime mudstone (micritic) sediments 

whose faunas and sedimentary structures show a proDiagrammatic 

cross section 

for Mesozoic 

carbonate fades

Fades and 

facies numbers

Lithology

Color

M6an S63 I6V6I __-^&r'£S5-7Ss^^i£SBFBS5!S5:

Oxygenation

K, . . ^--rrrTtV'Sfalfe 0-800 --rf ,<"33c^s«==Normal wave bas^^t^j-r^^^SSg^y^^^^gg^^^X-i-^-L- Nor ' "ave

.._ Jevel -^^^rSr^^F^^^^^^ 

.VrS^J^^^^Ag^g i-". i i ' i ' i ' i i ^x=-+ Salinity increases * 

Basin-euxinic 

fades

1

Shale, radiolarian cherts and 

siltstones

Dark brown 

to black

Very even millimeter lami - 

nations, rythmic bedding, 

ripple cross 

laminations, 

burrows, and 

worm trails 

on bedding 

surfaces

Tidal shelf 

fades

2

Interbedded thin 

to medium-bedded lime mudstone; dolomite 

and chert

Gray to brown

Thoroughly burrowed. Thin to 

medium wavy 

to nodular 

beds; bedding 

surfaces show 

diastems

Toe of slope 

carbonates

3

Interbedded limestone and darkgray shale

Gray to dark 

brown

Laminations may 

be minor, massive 

beds common, 

lenses of graded 

sediments; lithoclasts and exotic 

blocks

Foreslope 

4

Variable depending on water energy upslope. Sedimentary breccia 

and lime sand or 

lime mudstone

Gray to light 

gray

Slumps in soft 

sediments, foreset bedding, exo-1 

tic blocks

Organic reef 

boundstone 

mass

5

Crust on accumulation of 

organic debris 

arid lime mud. 

Massive limestone dolomite

Light gray

Massive to thick 

beds, some cross 

bedding,to massive organic 

structures or 

open framework 

with roofed 

cavities

Sands on 

edge of 

platform

6

Crinoid-bryozoan 

to ooid packstone and grainstone

Light gray

Medium to largescale crossbedding; festoons 

common

Open 

platform

7

Variable.Carbonate bryozoancrinoid packstone; chert 

common

Light gray

Abundant 

burrows

Restricted 

platform

8

Finely and 

coarsely crystalline dolomite; 

dolomitic crinoid 

bryozoan,wackestone and packstone; dolomitic 

pelletoid lime 

mudstone

Light gray 

to gray

Birdseye structures,algal mats 

on or near intertidal zone, 

stromatolites in 

very shallow 

water, graded 

cross bedding

Intertidal to 

supratidal 

fades

9

Irregularly laminated dolomitic 

lime mudstone 

and dolomite; 

anhydrite and 

calcite pseudomorphs of anhydrite common; 

algal mats

Gray-yellow, 

brownish red

Anhydrite-gypsum rhombs, 

bedded anhydrite, cracks, 

chips, algal 

mats

Vadose weathering of sabkha 

surface; carbonate dune sands; 

caliche-calcrete 

10

Coated particles, 

peloid, pisolites, 

compound peloids, alternating 

microlaminae, 

limpid dolomite 

rhombs

Gray-light gray, 

yellowish brown

Solitary corals 

Colonial corals

Bryozoans 

Echinoderms

EXPLANATION 

O Ooid

-O Superficial ooid

© Pfeooid 

OJO Pelletoids

Algal mats 

AAA Anhydrite gypsum

FIGURE 14. Depositional model for Upper Triassic carbonate fades of the Wrangell Mountains. Concepts for model are from Wilson (1970,

1975).
A22 GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS, ALASKA

gressive change through restricted shallow marine carbonates with megafossils to dolomite and caliche 

(facies numbers 8-10,fig. 14). In these cycles a sharp 

upper contact occurs with open marine beds at the base 

of the next cycle.

Work on modern sabkhas has shown that the saline 

supratidal flats are aggradational features which 

gradually extend over subtidal, intertidal, and lagoonal 

sediments (Evans and others, 1964; Purser and Evans, 

1973). Wood and Wolfe (1969) describe in detail nine 

similar sabkha cycles from the Upper Jurassic and 

Lower Cretaceous Arab/Darb Formation of Saudi 

Arabia. Similar cycles were described by Wilson (1967) 

from the Duperow Formation (Devonian) of the Williston basin, western North Dakota and adjacent areas. 

The Duperow contains 12 such cycles. Wilson estimated 

the time of each cycle at about 500,000 to a million 

years. Evans and others (1969) described sabkha cycles deposited on the Trucial Coast on the Persian Gulf; 

each cycle lasts on the order of 1,000 years.

The well-exposed Donoho Peak section, 74A-4T, is 

the best outcrop for interpreting environments of deposition for the lower 135 m of the Chitistone Limestone (figs. 3, 15).

The Chitistone Limestone as shown on figure 3 contains two well-developed physical correlation lines. One 

is the contact with the Nikolai Greenstone. The second, 

which is exposed in three of the measured outcrop sections, is the dolomitic-peloid-pisolitic laminated caliche 

beds that range from 30 cm to greater than 1 m in 

thickness. The caliche beds are developed at the Nizina 

River section at 135 m, the Green Butte section at 

101-102 m and at the Donoho Peak 74A-4T at 97 to 98 

and 99.8 to 100 m (fig. 16). Our underground studies 

in 1977 found a 60-cm-thick bed of dolomitic-pisolitic 

caliche above the zebra beds in the 503-m (1650-ft) 

level of the Mother Lode mine.

The lower half of the Chitistone Limestone can be 

correlated between the Donoho Peak section and the 

Erie mine section by two weakly developed incomplete 

upward-shoaling lime-mud (micrite) cycles that are 

developed at the 30- to 35-m and 48- to 52-m levels 

(facies 8 and 9, fig. 14). These lower incomplete carbonate deposition cycles were not recognized in the 

Bonanza mine outcrop section (probably due to faulting) or at the Green Butte section.

The incomplete carbonate deposition cycles that 

occur at 48 to 52 m at the Donoho Peak and Erie mine 

sections are probably a time-stratigraphic equivalent to 

the laminate-pisooid caliche bed found between 86 to 

96 m in the Nizina River section.

The underground Mother Lode mine section shown 

on figure 3 is about 5 m thick. The sabkha facies contains algal mats, zebra beds, laminated beds, dolomite 

pseudomorphs after gypsum and anhydrite, peneconEnvironments 

of deposition

FIGURE 15. Idealized and interpreted carbonate depositional cycles 

for lower 130 m of Chitistone Limestone. Model is based on 

the Donoho Peak outcrop section. Lithologic symbols shown 

in figure 3.
STRATIGRAPHY AND DIAGENETIC HISTORY, LOWER PART OF THE TRIASSIC CHITISTONE LIMESTONE A23

temporaneous sedimentary dolomite breccia, and possible solution collapse structures. The sabkha facies in 

the outcrops and underground is overlain by dolomite 

caliche 0.3 to 1.3 m thick. The caliche is formed by 

bands of laminated crust, peloids, and pisolites. Similar rocks are found in the Jumbo and Bonanza mines 

near bases of the ore bodies. This rock type, referred to 

as the zebra beds, is associated with intertidal to supratidal sedimentary structures and from drill hole 

data is known to be about 103 m above the Nikolai 

Greenstone (P. R. Holdsworth, oral commun., 1974). 

The zebra beds are composed of a matrix of subhedral 

50- to 200 -/j.m light-brown-gray dolomite with abundant flat-bottomed, cumulus enterorithic vugs lined 

with light-gray to white zoned coarse dolomite. Centers 

of the vugs are partly filled with sparry calcite. The ore 

bodies at the Mother Lode mine are stratigraphically 

higher than the other known Kennecott ore bodies. 

They are associated with sedimentary features similar 

to those found at 96 to 104 m at the Donoho Peak section, 101 to 104 m at the Green Butte section, and 95 m 

and also 129 to 135 m at the Nizina River section. The 

dolomite and sedimentary structures in the Nizina 

River section are correlated, as shown in figure 3, with 

the other sections, because they are the final supratidal 

cycle in this outcrop, contain the best sabkha features 

and the best developed caliche zone, and are overlain 

by pelecypod-echinoderm packstone that represents 

the second major regional marine transgression above 

the Nikolai Greenstone.

DIAGENETIC HISTORY OF THE 

CHITISTONE LIMESTONE OF BONANZA RIDGE

The Chitistone Limestone on Bonanza Ridge from 

west of the Jumbo mine east to McCarthy Creek and in 

section 74A-2T is extensively dolomitized except for 

the lower 20-30 m of the section (figs. 3, 8). Outcrop 

and petrographic studies of the carbonate rocks from 

Donoho Peak east to the Nizina River suggest the following sequence of events.

1. Deposition of the Chitistone Limestone on the 

Nikolai Greenstone. Development of a few thin beds of 

early diagenetic dolomite is associated with the algal 

mats. The original stratigraphic sequence probably was 

simlar to the relatively unaltered sections (fig. 10) at 

Donoho Peak, 74A-4T, and Nizina River, 74A-7T.

2. A late diagenetic stage, possibly low-temperature 

hydrothermal dolomitization of the entire Chitistone

Limestone on Bonanza Ridge, except for the lower 

20-30 m. These lower beds may have remained undolomitized owing to the higher clay content of these

Nikolai Metersabove Greenstone-Chrtistone Limestone contact

110 ->

-

105 -

-

-

100 -

95-

90 - 

Peak Donoho74A-4T section Outcrop at Regal mine

0 I * I

* 0

,<?!,!

<> I *

<H « I

* v

/ i/ / /

/ I/ I/ I/ ^ ,/

./ I/ / ,/

U U

,$ I*

<*>!/ I

/ 0 Udo

do |/ |x

U * I/O

Q/ Si'

,/Gs ,/

/ /3

Q/ O,/

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o/o o /to

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/ I/ / /

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/ ,/ / ,/ / /

i/ S

/ /

I I

I I

I I

I ,

I i

0 / X

i 1 6

0 , V

10 I

Dunham's(1962) car- bonate classi- rocksize fication Crystal 

in micrometers

P

D

100 /zm

M

30-40 percent 

50-1 00 /zm 

dolomite 

rhombs

D 

D

D

D

D

50-1 00 /zm 

dolomite 

rhombs

M

40 percent 

dolomite 

50-1^5 M m 

dolomite 

rhombs

Environments of deposition

Subtidal marine; echinodermcalcareous algae-moll uscan 

peloid packstone

Intertidal; calcitic pale-yellowgray dolomite

Intertidal; dolomitic, pelletoid 

to peloid lime mudstone

Intertidal to supratidal; dolomite 

and calcite pseudomorphs after 

gypsum-anhydrite

Supratidal; fossil fragments and 

dolomite and calcite pseudomorphs after gypsum-anhydrite

'/Supratidal to vadose weathering 

on sabkha surface; pisooids 

and pisoliths, laminated crusts

Supratidal; dolomite and calcite 

pseudomorphs after gypsumanhydrite

Supratidal to vadose weathering 

on sabkha surface; pisooidpisoliths, laminated crusts, 

dolomite

Supratidal; dolomite and 

\ calcite pseudomorphs 

\ _ after anhydrite

Intertidal; pale-yellow-gray 

dolomite, uniform bedding 

0.5 to 0.8 meters thick, 

unfossiliferous

Subtidal; dolomitic peloid, unfossiliferous lime mudstone

FIGURE 13. Detailed section and interpretation of environment of deposition of complex carbonate rocks from 90 to 110 m in Donoho Peak 

section. Facies from 90 to 95 m is restricted platform to intertidal facies number 8 and 9 ( see fig. 14 ). Above 108 m the facies number is 8 

to 7, restricted to open platform. Note pisolite caliches at 97 to 97.8 m and 99.7 to 100 m interbedded with supratidal carbonates, facies 

number 9-10. Lithologic symbols shown in figure 3.
A24 GEOLOGIC RELATIONS OF KENNECOTT-TYPE COPPER DEPOSITS, WRANGELL MOUNTAINS, ALASKA

beds relative to the overlying carbonate rocks and their 

possible lower porosity and permeability to the 

magnesium-bearing fluids.

3. Tectonic fracturing, possibly in Jurassic and Early 

Cretaceous time, that resulted in the formation of 

numerous 10-^m- to 0.5-mm-wide fractures and, in 

some beds, the formation of angular dolomite breccias.

4. The circulation through the rocks of magnesiumrich fluids, which deposited or lined the larger cavities 

with 0.4- to 4-mm-zoned dolomite rhombs.

5. The introduction of calcium-carbonate-rich fluids 

that deposited sparry calcite in the fractures and remaining voids of the breccias and intercrystal spaces in 

the dolomite. This phase may be responsible for much 

of the dedolomitization (calcitification) of some of the 

dolomite.

The Bonanza Ridge outcrops are so altered to dolomite (formed as 0.5- to 2-mm-size rhombs) and so tectonically brecciated that many of the regional marker 

beds of the Chitistone Limestone are obliterated. These 

include the first and second laminated beds, which are 

well developed at Donoho Peak section 74A-4T and Erie 

mine section 74A-6T. Zebra beds crop out in the upper 

part of the Glory Hole diggings above the old power 

plant at the Bonanza mine. The Bonanza mine outcrop 

ore body was below the zebra beds. The zebra beds are 

a stratigraphic correlation for Bonanza Ridge outcrop.

The extensive, coarse-grained dolomite associated 

with the massive copper sulfide ore bodies is believed 

to reflect a late, probably low-temperature hydrothermal episode. The reasons for this interpretation 

are:

1. Dolomatization cuts across the stratigraphic section, irrespective of the original fabric or environment 

of deposition.

2.The dolomite is fairly coarse; rhombs are generally 

100 ;um or larger.

3. Stratigraphic sections at Donoho Peak and at the 

Nizina River contain dolomite zones restricted to layers 

with subtidal to supratidal sedimentary structures. This 

dolomite probably resulted from early diagenetic processes.

Dedolomitization or calcitification occurs in the 

rhombs of the fine-grained dolomite matrix and the 

limpid dolomite of the vugs adjacent to the sparry calcite and sparry calcite vein fillings of the zebra beds 

(fig. 5F). Von Morlot (1848), Evamy (1963, 1967), and 

Shearman, Khouri, and Tara (1961) showed that magnesium sulfate solution can be involved in the formation of dolomite, according to the equation 2CaCOa 

4-Mg24- +S0|~ ^CaMg(C03 )2+CaS04 , and observed 

that this is a reversible reaction and suggested that 

gypsiferous solutions could bring about dedolomitization. This dedolomitization can also be caused by

meteoric water with high Ca2+ /Mg2+ ratios. Some thin 

sections show extensive calcitification of dolomite 

rhombs adjacent to calcite-filled fractures.

CONCLUSIONS

Three lime-mud upward-shoaling carbonate cycles 

are present in the lower 135 meters of the Chitistone 

Limestone. The last cycle is characterized by a dolomitic caliche zorfe at its top. All of the known copper occurrences in the Chitistone are found within this 135-m 

interval. The overlying Chitistone Limestone above 135 

m was deposited on an open shallow marine platform 

(Armstrong and others, 1969).

Not all of the factors which formed the large copper 

sulfide ore bodies are known, and studies are in progress. There appears to be some relation between the 

sabkha facies, in particular the zebra beds, and caliche 

and the formation of the massive copper sulfide ore 

bodies of the Bonanza Ridge. We believe that the regional sabkha facies that developed between 90 and 

135 meters above the Nikolai Greenstone originally 

contained abundant gypsum and anhydrite, but exposure to vadose weathering leached most of the gypsum 

and anhydrite and developed a vuggy zone interbedded 

with porous dolomitic caliche. Subsequent marine deposition capped porous zones with an impermeable 

seal. This porosity served as permeable conduits for the 

ore-forming solutions.

LOCATION OF MEASURED SECTIONS 

SHOWN IN FIGURE 3

Donoho Peak, 74A-4T; outcrop at Regal mine; base of

section is 50 m east of Regal mine, SE X/4NW J /4

sec. 1,T. 4S.,R. 13 E. 

Erie mine, 74A-6T; outcrop 2 km west of Erie mine,

SW 1 /4sec. 4,T.4S.,R. 14 E. 

Jumbo mine, center sec. 15, T. 4 S., R. 14 E. 

Bonanza mine outcrop, 74A-2T; 2 km east of Bonanza

mine, SE */4 sec. 15, T. 4 S., R. 14 E. 

Green Butte, 74A-3T; 25 m north of Green Butte mine,

NE !/4SW !/4 sec. 30, T. 4 S., R. 15 E. 

Nizina River, 74A-7T; SW J /4 NE */4 sec. 36, T. 4 S., R.

15 E.

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