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Origin and environment of mineralization atthe Siskon Mine, Siskiyou County, California

Item Type text; Thesis-Reproduction (electronic); maps

Authors Hackman, David Brent, 1942-

Publisher The University of Arizona.

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ORIGIN AIR) ENVIRONMENT OF MINERALIZATION AT THE SISKON MINE, SISKIYOU

COUNTY, CALIFORNIA

byDavid Brent Hackman

A Thesis Submitted to the Faculty of the DEPARTMENT OF MINING AND GEOLOGICAL ENGINEERINGIn Partial Fulfillment of the Requirements

For the Degree ofMASTER OF SCIENCE

WITH A MAJOR IN GEOLOGICAL ENGINEERING In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 7 1

STATEMENT BY AUTHOR

This thesis has been submitted in partial ful­fillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowl­edgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

SIGNED:

APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below:

w. c / lacyProfessor of

Mining and Geological Engineering

7 date

ACKNOWLEDGMENTS

The author is greatly indebted to Dr. Willard C. Lacy, Department Chairman and thesis advisor, for his suggestions and direction during the formulation of the thesis.

Acknowledgments are likewise due Dr. John S. Sumner and Dr. John F. Able for their critical review of the manu­script.

The field work was performed at the Siskon mine owned by the Siskon Corporation. The availability of the facilities and the permission to use the data are appre­ciated. The information and advice given by Mr. Dana D. Norman during the course of the field investigation are also appreciated.

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TABLE OF CONTENTS

LIST OF ILLUSTRATIONS . . .LIST OF T A B L E S .......... .ABSTRACT ................ .

I. INTRODUCTION ............II. REGIONAL GEOLOGY ........

III. LITHOLOGY ................IV. STRUCTURAL GEOLOGY . . . .V. MINERALIZATION ..........VI. GOSSAN AND ZONE OF OXIDATION

VII. WALL-ROCK ALTERATION . . .VIII. INDUCED POLARIZATION SURVEY

IX. ORIGIN OF THE MINERALIZATIONGeological Setting . .Sulfide Body ........Sequence of Events . .

X. CONCLUSION ..............LIST OF REFERENCES * . . .

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812162131354041

. 42444750

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LIST OF ILLUSTRATIONS

1. Location of the Siskon Mine in the KlamathMountains Province of California . . . . . . 2

2. Geologic Map of the Siskon M i n e ........ in pocket3. Geologic Map of the Florida P i t ........ in pocket

Figure Page

4. Geologic Map of the Florida Tunnel . • • • in pocket5. Geologic Section through Drill Holes

DH-SIS No. 1 and DH-TGS No. 4 . . . . in pocket6. Geologic Section through Drill Holes

DH-TGS No. 2 and DH-TGS No. 4 . . . . in pocket7. Faults and Joint Patterns of the Siskon

Mine (Lower Hemisphere, SchmidtEqual Area Nets)........................... 13

$. Mineralized Quartz Chlorite Schist, Medicine Creek, with Mineralization Oriented North-South............................... 18

9. Distribution of Copper in Drill Hole DH-SISNo. 1 .......................... 20

10. Siskon Mine Area Viewed from the Southeast • • • 2211. Strong Zone of Gossan Developed above the

Portal of the Georgia Tunnel.......... 2312. Florida Pit Viewed from the South with Zone

of Leached Sintery Quartz Located in the Northwestern Portion of the P i t .......... 24

13. Exotic Limonite from the Gossan Developed inthe Tennessee Pit • • • ................... 25

14. Leached Sintery Quartz from the Florida Pit . . 27

v

viLIST OF ILLUSTRATIONS— Continued

Page15. A Comparison of Gossan Material from the

Virginia Fault Zone with UnoxidizedSulfide Mineralization from MedicineCreek.............. 28

16. Method Used in Plotting Dipole-dipoleInduced-polarization and ResistivityResults................................... 36

17. Induced Polarization Profiles from theSiskon Mine Area ................... in pocket

18. Relation of Concordant Pyritic SulfideDeposits to Regional Geology ............ 49

LIST OF TABLES

1. Mineral Composition of Surface Rock Samplesfrom the Siskon M i n e ....................... 9

2. Mineral Composition of Rock Samples fromDrill Holes in the Ore Zone of theSiskon M i n e ............................... 17

Table Page

3. Typical Metal Conduction Factors, 0.1-3 Hz . . . 39

vii

ABSTRACT

This study was undertaken to determine the mineral potential of the Siskon mine. Of particular interest was the possibility that the deposit would contain a secondary chalcocite zone of sufficient tonnage and grade to warrant mining. The mine previously had been operated to extract gold from the gossan which had developed over the zone of mineralization.

The Siskon mine is located within the rocks of the Galice Formation of the Western Jurassic belt of the Klamath Mountains. The Galice Formation is composed of eugeosynclinal rocks which were intruded by ultramafic and granitic rocks. The ore zone is found within a quartz chlorite schist bounded by meta-andesite and phyllite.The dominant structural trend at the mine matches the regional north-south pattern.

The primary sulfide mineralization consists of pyrite with minor amounts of chalcopyrite. Quartz is a common gangue mineral associated with the primary sulfide mineralization. In the oxidized zone chalcocite and covellite were developed by supergene processes. Very small quantities of gold and silver are found with the primary mineralization.

viii

ixPropylitic wall-rock alteration in the mine area

is difficult to distinguish from the effects of regional metamorphism. However, a definite zone of hydrothermal alteration in the footwall was found to extend for more than 1,000 feet from the ore zone. Chlorite, the most diagnostic of the alteration minerals at the Siskon mine, changes from an iron-rich variety to a magnesium-rich variety as the ore zone is approached.

The Siskon mine, together with three other similar deposits, is found in a seventy-mile-long band located six to ten miles west of a thrust fault which separates the Western Paleozoic and Triassic belt from the Western Jurassic belt. This belt is believed to be part of a fossil Benioff zone or zone of underthrusting from the west. This tectonic activity took place during the Nevadan orogeny in conjunction with volcanism, regional metamorphism, and emplacement of plutonic rocks. The Siskon mine and similar deposits in the area appear to be associated with submarine volcanism which apparently took place prior to regional metamorphism and the emplacement of plutonic rocks.

CHAPTER I

INTRODUCTION

The processes by which a mineral deposit forms should play a fundamental role in determining how that deposit should be evaluated. This study was undertaken to evaluate the mineral potential of the Siskon mine. In pursuit of this objective, a genetic model was developed to trace the evolution of the mineral deposit. Of partic­ular interest was the possibility that the deposit would contain a secondary chalcocite zone of sufficient tonnage and grade to warrant mining.

The Siskon mine is located in Sisiyou County in the northwestern portion of California (Fig. 1). The nearest town is Happy Camp, California, located forty road miles or twenty-two air miles northeast of the mine. Elevations at the property range from 2,000 feet to 3,800 feet. The terrain is steep with heavily vegetated slopes. Annual precipitation averages about sixty inches, mostly occurring from November through March.

The Siskon mine was operated from 1953 until I960 as a gold mine. Gold was extracted from gossan material mined at the Florida, Tennessee, and Virginia pits. Total

1

2

SISKON MINE

KLAMATHMOUNTAINS(PROVINCE

Fig. 1. Location of the Siskon Mine in the Klamath Mountains Province of California.

3production from the mine was 450,000 tons of ore worth approximately $3.6 million.

Evaluation of the mine area as a copper prospect began during the summer of 1969 with an induced polarization survey. This was followed by detailed surface and under­ground mapping and a program of diamond-core drilling. The project continued through the fall of that year when it was determined that the prospect did not merit continued inves­tigation at that time.

CHAPTER II

REGIONAL GEOLOGY'

The Klamath Mountains consist of four concentric, arcuate lithic belts that are concave to the east. From east to west the belts are (l) the Eastern Palezoic belt,(2) the Central Metamorphic belt, (3) the Western Paleozoic and Triassic blet", and (4) the Western Jurassic belt. The lithic belts generally are separated by faults, linear ultra- mafic bodies, or granitic plutons. The dominate structural elements of the Klamath Mountains province appear to be a series of concentric moderate-to-high angle reverse faults over which the rocks of the province were thrust southwest- ward. The strata most commonly dip eastward, and small- scale isoclinal folds with eastward-dipping axial planes are reported in all belts (Irwin, 1966).

The principal rocks of the Klamath Mountains are eugeosynclinal and plutonic rocks that were involved in the Nevadan orogeny. These rocks will be distinguished from younger rocks by the use of the term "subjacent.” Ultra- mafic rocks are an abundant component of the subjacent terrain, and generally crop out as bodies whose linear trends accentuate the arcuate structure of the Klamath Mountains. Generally, these rocks are serpentinized, and at

4

many places, principally along their borders, they are highly sheared. Most of the larger ultramafic bodies are clearly part of the subjacent terrain, as they are cut by granitic rocks that are reasonably certain to have been intruded during the Nevadan orogeny. Most of the subjacent strata west of the eastern Klamath belt have been regionally meta­morphosed to grades ranging from greenschist to amphibolite facies.

Tectonism in the Klamath Mountains has included re­gional metamorphism, thrust faulting, and the emplacement of plutonic rocks. The most pervasive tectonic event occurred during the Middle and Late Jurassic and marked the change from a long-term eugeosynclinal environment to that of a continental shelf. The major thrusts are suggested to in­clude fossil Benioff zones (Hamilton, 1969).

The Siskon mine lies near the center of the western Jurassic belt within the rocks of the Galice Formation. The Galice Formation consists of a lower metavolcanic unit and an upper metasedimentary unit. The lower unit consists mostly of greenish meta-andesite flows and flow breccias and is thought to be at least 7,000 feet thick. The meta­sedimentary unit is chiefly slaty mudstone with interbedded graywacke (Irwin, 1966). Most of the clastic rocks appear to have been derived from volcanic sources. This assemblage formed synchronously with widespread plutonism in the Sierra Nevada and Klamath regions and may be the volcanic

5

6equivalent of plutons, although an island-arc setting could alternatively be inferred (Hamilton, 1969).

According to Hamilton (1969) stocks and small batholiths formed above a Benioff zone, the volcanic products of the magmatism were then spread across the terrain and be­came involved in subsequent deformation, intrusion, and metamorphism which accompanied the magmatism. All of the underthrusting, except that in the far west, and all of the granitic intrusion were completed by very late Jurassic or very early Cretaceous time. Hamilton^ theories are sup­ported by Lanphere, Irwin, and Hotz (1968) whose isotopic data conflicts with the hypothesis that all of the Nevadan orogeny occurred after the deposition of the youngest sub­jacent strata. The isotopic data and geologic relations indicate that the Nevadan orogeny was a distinct deforma- tional, metamorphic, and plutonic event during the Middle and Late Jurassic. This concept implies that units such as the Galice Formation are synorogenic sedimentary units de­posited during the Nevadan orogeny. Lanphere (personal communication, 1969) stated that the gabbro-ultramafic com­plexes of the Western Jurassic belt in Oregon crystallized and were intruded approximately 150 million years ago.Granitic plutons 10 to 15 miles east of the Siskon mine have been dated at 145 to 155 million years. Concordant hornblende- biotite dates were measured on samples from four plutons

7(Lanphere and others, 1968). From this information, it can be concluded that the Nevadan orogeny including volcanism, sedimentation, intrusion, structural deformation, and ra­tional metamorphism occurred nearly simultaneously in a geologically short period of time, about 150 million yearsago.

CHAPTER III

LITHOLOGY

On the Geologic Map of California, Weed Sheet, prepared by the California Division of Mines and Geology, the Siskon mine is located in Upper Jurassic marine rocks of the Lower Galice Formation and is bounded by ultramafic rocks on the east and west and by diorite on the north and south.

Locally, the Galice Formation is composed of meta­andesite, quartz chlorite schist, and phyllite (Figure 2, in pocket). The sulfide mineralization is found entirely within the quartz chlorite schist. The phyllite in the mine area occurs adjacent to and entirely on the west side of the ore zone. These rock units are bounded on the east and west by meta-andesite.

On the surface, especially in the Florida pit area (Figure 3, in pocket), there is a sharp contact between the phyllite and the quartz chlorite schist. However, in the Florida tunnel (Figure 4, in pocket), it was very difficult to determine the actual location of the contact. As can be seen from Table 1 (sample Nos. 12 and 15), the principal dif­ference between these two rock types is the relative content of quartz and chlorite. All of the sample locations

Table 1. Mineral Composition of Surface Rock Samples from the Siskon Mine

Rock Name Meta--andesiteMeta-diorite Phvllite

QuartzChloriteSchist Meta-andesite

Sample Number (West to East) 3 2 1 A 5 a 15 12 7 10 13 11Minerals (per cent of total content by thin section analysis)Plagioclase(oligoclase) 30 60 65 SO 45 33

•25 60 35 45

Quartz 15 10 2 1 1 75 30 10 1 10Epidote 5 10 2 tr. 25Zoisite 25 10 20 15Actinolite 30 30 25 2 15 35Biotite 10 tr. 20Chlorite 30 15 10 15 20 10 15 60 20 10 tr.Sericite 10 5 5 4 10 10 5Pyrite tr. 1 1 2 tr. tr.Hematite tr. tr. tr. tr. tr. tr. tr.Limonite tr. tr. tr.Apatite tr. tr. tr. tr. tr. 10Clay minerals 15 5

Note: tr trace vO

10discussed in the text are located on Figure 2. The dominate mineral in the phyllite is quartz, while the dominate mineral in the schist is chlorite. The only other mineral found in significant quantity in both rock types is sericite. Texturally, both rock types are holocrystalline, fine-grained, and foliated.

Two types of meta-andesite are distinguishable in the field. One is a very fine-grained vesicular meta­andesite which bounds the mineralized schist on the east, while the other is a non-vesicular meta-andesite. Miner- alogically, the two types of meta-andesite are very similar; however, the textural difference is sufficient to establish a mappable contact between the two units. The non-vesicular meta-andesite is generally light gray, megascopically un­altered and very hard. The vesicular meta-andesite is generally gray-green, somewhat more altered in appearance and easier to break. All of the andesite is probably spilitic as indicated by the uniform oligoclase composition of the plagioclase. The principal mineral content dif­ferences between the meta-andesite to the east and west of the ore zone are found among the metamorphic and alteration minerals. To the east, the rocks are richer in zoisite and actinolite, while those to the west are richer in epidote, chlorite, and sericite. Some of the meta-andesite to the west of the mine area (Fig. 2, sample Nos. 2 and 3) is

composed of a fine breccia with breccia fragments from 1 to 10 cm in diameter.

South of the South fault the rock appears to be either a slightly more coarse-grained meta-andesite or a fine-grained meta-diorite. This rock will be referred to as meta-diorite in order to distinguish it from the previously discussed meta-andesite. Dikes of meta-diorite were also found cutting the quartz chlorite schist in drill holes SIS No. 1, TGS No. 2, and TGS No. 4 (Figs. 5 and 6, in pocket).

North-south oriented pods and bands of serpentinite occur on the Siskon claims roughly parallel to the bands of strongest mineralization. On the east edge of the claim area, the serpentinite occurs with an essentially parallel band of phyllite. The serpentinite is never found in direct contact with the sulfide mineralization.

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CHAPTER IV

STRUCTURAL GEOLOGY

The dominate structural trend at the Siskon mine matches the regional north-south pattern. Most of the zones of strong sulfide mineralization are oriented roughly north-south and all of the serpentinite bodies have their long axis in this direction. One of the major faults at the Siskon mine, the Florida fault, strikes approximately north-south and appears to have an average dip to the east of about 40° to 50°.

However, two zones of strong fracturing, the South fault and the Virginia fault zone, cut across the general north-south structural grain. The South fault, which separates the mineralized quartz chlorite schist from the meta-diorite, strikes N50°V«T and dips 50° to the northeast.The Virginia fault zone, on the other hand, strikes roughly east-west with an unknown dip.

Figure 7 shows the relationship between faulting and joint patterns. The joint data were plotted on Schmidt equal-area nets which were placed on the map to indicate the general area from which the data were gathered. In the Florida pit, one set of joints strikes perpendicular to the Florida fault and dips nearly vertical. Another set of joints

12

13

Fig. 7<> Faults and Joint Patterns of the Siskon Mine (Lower Hemisphere, Schmidt Equal Area Nets).

strikes approximately parallel to the Florida fault and dips about S80°V/. This joint set seems to be perpendicular to the apparent dip direction of the fault plane. On the equal- area plot of joint data taken to the east and northeast of the Florida pit, the joints appear to describe a crude arc which roughly fits the pattern established in the Florida pit. The greater concentration of joints dipping north to northwest on this plot is probably the result of the data taken north of the Florida pit where the fault strikes slightly to the northeast. The plot of the joints to the south of the Virginia fault zone shows three distinct sets of joints; one of them parallel to the South fault with the other two parallel to the distinct segments of the Virginia fault zone.

Drill hole logs from SIS No. 1, TGS No. 2, and TGS No. 4 all show that the mineralized quartz chlorite schist has been cut by meta-diorite dikes. One of these dikes, about ten feet thick, was intersected by all three drill holes (Figs. 5 and 6). A descriptive geometry three- point solution indicates that this dike strikes N36°W and dips 5$°NE, very close to the orientation of the South fault.

Metamorphism has apparently obscured the original bedding plane direction of the volcanic flow rocks; however, a projection of the contacts of the bed of vesicular

14

15meta-andesite located east of the ore zone from a contour map on to a vertical plane, indicates that it dips roughly 40° or 50° to the east. The band of mineralization which passes through the Florida pit appears to be parallel to the bed of volcanic flow rock.

The foliation of the phyllite and the quartz chlorite schist (Fig. 2) strikes generally north-south and dips to the west. The undulation of the foliation surfaces probably ex­plains the wide variation in westerly dip.. The steepening of the dip of the foliation with depth in drill hole SIS No. 1 (Fig. 6) indicates that the bedding probably dips to the east and that a syncline may exist in that direction with a north-south trending synclinal axis (Billings, 1954)•

CHAPTER V

MINERALIZATION

Sulfide mineralization at the Siskon mine occurs almost entirely within the quartz chlorite schist. The primary sulfide mineralization consists of pyrite with minor amounts of chalcopyrite. The composition of the mineralized rock is indicated by the petrographic analysis of thin sections from drill holes SIS No. 1 and SIS No. 2 as shown in Table 2.

The pyrite occurs disseminated throughout the schist as small blebs and crystals. In the drill core it was also noticed that pyrite occurs in small stringers and occasionally as veinlets. Generally, the stringers and blebs of pyrite are aligned parallel to the north-south oriented foliation (Fig. S). Quartz is a common gangue mineral associated with the sulfide mineralization.

The chalcopyrite generally occurs as blebs associated with pyrite stringers. However, two strong veinlets of chalcopyrite, pyrite, and quartz were encountered at 640 feet and 760 feet in drill hole SIS No. 1. Each of these veinlets was about a foot thick and contained approximately 10^-15% chalcopyrite. In the vicinity of these veinlets,

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Table 2. Mineral Composition of Rock Samples from Drill Holes in the Ore Zone of the Siskon Mine.

Drill Hole SIS No. 1 SIS No. 2Depth (ft.) 324 762 769 53 1 162 196 282 387Minerals (per cent of total content by this section analysis,Quartz 55 60 60 95 '55 65 50 40Chlorite 30-35 35 40 trace 10 10 • 25-30 35-40Sericite 5 1-2 trace 1 15 15 20-25 10-20Pyrite 5~ 10 3-5 , trace 3 20 10 2 5Hematite trace traceLimonite trace traceChalcopyrite traceChalcocite trace

H

Fig. 8. Mineralized Quartz Chlorite Schist, Medicine Creek, with Mineralization Oriented North-South.

19however, the rock contained less than 1/2$ of chalcopyrite (Fig. 9).

Above 600 feet in drill hole SIS No. 1, chalcocite is the principal copper mineral. It generally occurs as a soft, sooty fracture filling and as a surface coating on the chalcopyrite and pyrite crystals. The chalcocite seems to preferentially replace chalcopyrite rather than pyrite crystals when both of these minerals are available. Below 700 feet, oxidation was negligible and only trace amounts of chalcocite were observed. Covellite was the only other sulfide mineral observed. It was found in trace amounts as a blue coating on the surface of pyrite crystals.

Gold and silver are found in very small quantities,<.02 oz./ton Au and <.40 oz./ton Ag, associated with the primary mineralization. Gold was mined by open pit methods from the gossan where it had been concentrated by residual enrichment. The majority of the gold was mined from the Florida pit with minor amounts of gold coming from the Tennessee and Virginia pits.

I

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GRADE OF COPPER DEPTH (F T )

— 100

— 200

— 3 0 0

— 4 0 0

— 500

— 600

— 700

— 8 0 0

Fig. 9. Distribution of Copper in Drill Hole DH-SIS No. 1.

CHAPTER VI

GOSSAN AND ZONE OF OXIDATION

The most prominent feature of the Siskon deposit is an extensive dark brovm-to-tan gossan visible from a dis­tance of several miles (Figs. 10, 11, and 12). The gossan essentially has been derived entirely from bodies of dis­seminated to semi-massive pyrite. Three forms of limonite after pyrite commonly found at the Siskon mine are, using terminology from Blanchard (1965), botryoidal and smeary crusts, flat crusts, and indigenous thick-walled cellular sponge. The dark brown massive gossan is best developed in three bands located south of the Virginia pit and in a band approximately 2,000 feet long extending south-southwest from the Florida pit (Fig. 2). The rest of the deposit is covered with a less prominent buff to ochreous gossan.

Sample No. 16 (Fig. 13) from the Tennessee pit is an example of the dark brown massive gossan. The iridescent smeary crusts and botryoidal limonite indicate that the limonote is exotic or transported goethite and was derived from semi-massive pyrite in gangue minerals with very little neutralizing power.

Sample No. 9 is an example of the indigenous thick- walled cellular sponge form and is composed of goethite with

21

22

Fig. 10. Siskon Mine Area Viewed from the Southeast

23

Fig. 11. Strong Zone of Gossan Developed above the Portal of the Georgia Tunnel.

Fig. 12. Florida Pit Viewed from the South with Zone of Leached Sintery Quartz Located in the Northwestern Portion of the Pit.

K>-F-

25

Fig. 13. Exotic Limonite from the Gossan De­veloped in the Tennessee Pit.

26minor jarosite. This was probably derived from pyrite as the only sulfide mineral with sericite supplying the potassium for the jarocite. Much of the limonite has re­mained in place probably as a result of a high ratio of chlorite to quartz in the gangue minerals. A substantial part of the acid derived from the pyrite was used to leach the iron and magnesium from the chlorite so that much of the limonite was deposited in place. This sample also shows examples of flat and smeary crusts of limonite in­dicating its exotic nature.

Sample No. 14 (Fig. 14) was taken from the western portion of the Florida pit and is composed of quartz, kaolinite, and minor goethite and jarosite. The original rock was very high in silica and probably contained from 5$ to 10% pyrite. Since there was little in the rock to neutralize the acidic solution almost all of the limonite has been removed and much of the sericite has been converted to kaolinite.

Sample No. 17 (Fig. 15) taken from the Virginia fault zone illustrates the residual texture of the primary material which appears to have been similar to sample No. 6 (Fig. 15). Sample No. 17 is composed mostly of silica and goethite with a trace of hematite. The pyrite content may have been as high as 20% but with the banded nature of the sulfide mineralization this is difficult to estimate. A

Fig. 14. Leached Sintery Quartz from the Florida Pit.

28

Fig. 15. A Comparison of Gossan Material from the Virginia Fault Zone with Unoxidized Sulfide Mineralization from Medicine Creek.

small amount of chalcopyrite can be observed in sample No. 6, taken from the bed of Medicine Creek; however, if sample No. 17 contained any chalcopyrite, its characteristic box- work pattern has been completely obscured by the large quantity of exotic limonite derived from the pyrite. A ratio of pyrite to chalcopyrite of 1:1 is generally high enough to completely obscure any evidence of chalcopyrite (Blanchard, 1968). At the Siskon mine visual estimates of the pyrite and assay results for copper indicate that the pyrite to chalcopyrite ratio was at least 10:1.

Direct evidence of copper mineralization in the zone of oxidation is very limited. A few small green stains and small seams of chalcocite were found in the Florida tunnel and one area of green stain was found on the north bank of Medicine Creek. The green stains are probably chrysocolla since no carbonate minerals were available to form malachite. In addition, the strongly acidic solutions derived as a result of weathering made available a great deal of silica to combine with the copper to form chrys­ocolla. Most of the copper would, however, have gone into solution as copper sulfate which is more soluble than iron sulfate and was, therefore, carried away by surface and ground water. The small amount of copper that has remained as chrysocolla has been largely masked by the large amount of exotic limonite deposited.

29

30Most of the oxidized material in drill hole SIS

No. 1 was found in the top 170 feet. Scattered streaks of oxidation were, however, found as deep as 700 feet below the surface. Figure 9 indicates that the average grade of copper decreases gradually from the zone of strong oxidation until veins of chalcopyrite are encountered below 500 feet. Traces of chalcocite were observed associated with the chalcopyrite as deep as 735 feet.

CHAPTER VII

WALL-ROCK ALTERATION

Wall-rock alteration at the Siskon mine is useful as an indicator of location with respect to the sulfide mineralization. The effects of hydrothermal alteration are somewhat difficult to distinguish from the effects of regional metamorphism, although a definite pattern of wall- rock alteration can be established. The alteration mineral assemblage at the Siskon mine falls into the general category of propylitic alteration (Meyer and Hemley, 196?). This assemblage includes quartz, epidote, zoisite, actin- olite, chlorite, sericite, and apatite. An unusual aspect of the alteration mineral assemblage at the Siskon mine is the complete absence of carbonate minerals.

The study of wall-rock alteration minerals was made almost entirely in the meta-andesite of the footwall and hanging wall of the ore zone. This uniformity of wall- rock composition makes the analysis of alteration relatively simple. The dominate mineral in the wall-rock is oligo- clase. It is possible, however, that the plagioclase was originally more calcic before regional metamorphism.

Table 1 shows the composition of the samples located on Figure 2 in a crudely west-to-east sequence. Sample

31

32Nos. 1, 2, 3, 4, 5, and 15 are located in the footwall; sample Nos. 7, 10, 11, and 13 are located in the hanging wall; sample No. 12 is located on the strike of the ore zone; and sample No. 8 is separated by the South fault from the zone of sulfide mineralization. Wall-rock altera- .tior••in...the hanging wall and footwall are markedly different.

Table 2 shows the composition of the samples from the ore zone. With the exception of the iron and copper minerals, the samples in Table 2 and sample No. 12 from Table 1 are composed entirely of quartz, chlorite, sericite, and clay minerals. The clay minerals are lumped with the sericite since they are so fine-grained that individual mineral types are extremely difficult to distinguish with a petrographic microscope.

The alteration minerals in the footwall are quartz, epidote, chlorite, and sericite. There are some clay minerals, but they may be largely the result of weathering. -All of a.h'e alteration minerals occur in veinlets and as pods and clusters scattered throughout the rock. The greater quantity of alteration minerals in sample Nos. 2 and 3 is due to brecciation which permits these minerals to develop in the interstices between breccia fragments which are primarily composed of well-formed plagioclase crystals.

Chlorite is found scattered throughout the rock as an alteration product of the mafic minerals and the plagioclase.

33Chlorite is also found associated with other alteration minerals in veinlets. The chlorite can be differentiated into iron-rich and magnesium-rich varieties (Miller, 196l). As the ore zone is approached, the chlorites change from poorly-formed to well-formed crystals, and the composition changes from the iron-rich to the magnesium-rich variety (Price, 1953)• Sample Nos. 2 and 3 contain only the iron- rich variety, while sample No. 1 contains both varieties. Sample No. 4 and the sample from the ore zone contain only the magnesium-rich variety. Quartz is found principally in veinlets and pods associated with the veinlets. Sericite and epidote are generally found as alteration products of plagioclase; however, as the ore zone is approached, they are also found in veinlets. The plagioclase crystals in sample Nos. 2 and 3 are somewhat altered but are clearly recognizable as plagioclase crystals. In sample Nos. 1 and 4» the plagioclase crystals are strongly altered with deeply eroded edges and scattered flecks of sericite and chlorite. Sample No. 5> on the other hand, shows few signs of being hydrothermally altered; the only indicator of the proximity of the ore zone is the presence of well developed clusters of magnesium-rich chlorite crystals.

The hanging wall alteration minerals are primarily actinolite, zoisite, and chlorite with small amounts of

34quartz, sericite, zoisite, and apatite. The actinolite, zoisite, and chlorite may be the result of low-grade re­gional metamorphism rather than hydrothermal alteration. However, sample Nos. 7 and 11 do show some effects of apparent hydrothermal alteration. Sample No. 7 contains some quartz veinlets and sample No. 11 shows some sericite veinlets through plagioclase crystals. Otherwise, the hanging wall samples appear to be metamorphased country rock.

The wall-rock alteration pattern indicates that the mineralizing fluid passed through the footwall rock into the ore zone along a path that was apparently perpendicular to the bedding. This process probably occurred before the flow rock was structurally deformed from its original horizontally lying position. The sulfide mineralization appears to have taken place in a near-surface submarine environment in which trapped sea water may have served as the medium of mineral transport. Mineralization may have continued as additional flow rock, extruded from nearby volcanic centers, covered the zone of mineralization.Minor leakage through the hanging wall rock may have then occurred as the process of mineralization continued.

CHAPTER VIII

INDUCED POLARIZATION SURVEY

An induced polarization survey was the only geo­physical work performed at the Siskon mine. The dual frequency method was employed with a dipole-dipole array (Fig. 16). Eight survey lines were located roughly perpen­dicular to the long axis of the zone of sulfide mineraliza­tion. The two frequencies used were 0.1 Hz and 1.25 Hz.

The induced polarization profiles (Fig. 17, in pocket) were interpreted using theoretical curves developed by Ludwig and Henson (196?)• It should be made clear that the two-dimensional profiles shown in Figure 17 do not represent an electrical cross section of the ground. The expected patterns from many geometries are known, and these patterns are compared with the observed field results to arrive at an interpretation. The particular model used for comparison with the field data was the vertical dike case. Interpretation of the data was complicated somewhat by the steep topography found in the mine area. The induced polari­zation anomalies are shown on Figure 2, where the horizontal extent of the anomalies is indicated by solid and broken lines. The anomalous zones correlate well with the outcropping areas of the sulfide mineralization. In the area covered by landslide

35

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37debris, the induced polarization survey was especially useful in detecting the presence of sulfide mineralization.

On lines 3 through 8, a sharp contrast with the rocks to the west of the ore zone is indicated by a marked increase in resistivity values. On these lines, no similar increase in resistivity values is found to the east of the ore zone; however, a rapid falling off of PFE values can be observed to the east of the ore zone. This contradictory information may be due to a greater amount of fracturing east of the ore zone resulting in a lower resistivity with­out accompanying induced polarization effects.

Topography can be readily related to resistivity measurements (Grant and West, 1965)• High resistivity measurements frequently accompany hills while low resis­tivity measurements are generally associated with valleys. These effects are the result of current concentration in valleys and current dispersion in hills. A number of ex­cellent examples of these effects can be found on the profiles from the Siskon mine. High resistivity between stations 4E and 8E on Line 1 is associated with a small hill. Low resistivity is associated with a valley between stations 4W and 12V/ on Line 2. These are small-scale features best seen with a dipole separation interval of one. A large-scale feature is indicated by the high resis­tivity measurements plotted between stations 4W and 8E on line 7 which probably indicates the presence of a large

hill located between these stations affecting the resistivity as plotted for dipole separation intervals of one, two, and three.

Utilizing Table 3 (Madden, 196?), metal conduction factors from the Siskon mine indicate that most of the mineralized zone would fall into the category of finely- disseminated sulfides with only two of the profiles showing values in excess of one hundred or in the category of dis­seminated sulfides. This is in close agreement with the amounts of sulfide minerals found in drill hole SIS No. 1 where the quantity ranges from Yfo to 2% sulfide minerals by volume. A few narrow veins with as much as 50/o sulfides were encountered; however, none of them exceeds six inches in width and none was found closely spaced.

When compared with resistivity values from Brant (1966), the country rock enclosing the ore zone appears to have a high resistivity probably indicating relatively un­fractured rock. This is in agreement with visual observa­tions in the field where the only strong fracturing was found in the mineralized quartz chlorite schist. However, even this rock had very little permeability below the oxidized zone. Information regarding the permeability of the rock mass is essential in the exploration of the mine area for a secondary chalcocite blanket type of deposit.

39

Table 3. Typical Metal Conduction Factors, 0.1-3 Hz.Rock Tvoe and Mineralization Metal Conduction FactorUnmineralized granites 1Unmineralized basic rocks 1-10Finely disseminated sulfides 10-100Disseminated sulfides 100-1,000Fracture-filling" sulfides 1,000-10,000Massive sulfides >10,000

Source: Madden (196?)

CHAPTER IX

ORIGIN OF THE MINERALIZATION

The mineralized body at the Siskon mine belongs to the class of concordant, pyritic sulfide deposits. A few well-known examples of this type are found in the Noranda district in Canada, the Mt. Lyell district in Tasmania, the Rio Tinto district in Spain, and the Rammelsberg district in Germany. Two examples of this type from the United States are the Jerome district in Arizona and the Shasta district in California. Some younger deposits of this type, rel­atively uncomplicated by later geologic events, are located in Cyprus and Japan. All of these deposits may be con­sidered to be examples from a linear series in which one end- member consists of deposits containing a gold-copper mineral­ization associated with eugeosynclinal volcanic rocks and the other end-member consists of silver-lead-zinc deposits found in eugeosynclinal sediments (Gilmour and Still, 1963).

There has been considerable controversy over the origin of the concordant pyritic sulfide type of deposit.Many geologists have proposed a structurally controlled hydrothermal replacement origin while others, especially in the last ten to fifteen years, have proposed an origin in­volving submarine volcanism.

40

41Geological Setting

The regional setting of the Siskon mine is geolog­ically similar to the setting of other deposits of the same type. Miller (I960) emphasized the occurrence of these deposits in eugeosynclinal belts throughout the wo rid.

Locally, the concordant pyritic sulfide deposits are usually located in a fine-grained, thin-bedded, uniformly layered siliceous and pyritic sedimentary rock which occupies a break in the volcanic succession— usually between acidic volcanic rocks below and more basic ones above (Hutchinson, 1965)• At the Siskon mine, the mineralization is located in a quartz chlorite schist found just above a layer of siliceous phyllite. These rocks are located in a break in the volcanic succession between meta-andesite flow rocks. Williams (1963) states that the base metal mineralization in northeastern Newfoundland occurs almost exclusively within altered green lavas or pyroclastic rocks of mafic-to- intermediate composition. This similarity is also of interest in view of the fact that the Newfoundland deposits, like the Siskon deposit, are entirely pyrite-chalcopyrite deposits.

Structurally, the area of the Siskon mine is de­formed to a lesser degree than the highly folded and faulted greenstone belts which enclose the Canadian deposits. How­ever, the area is structurally somewhat more complex than

42the districts containing the essentially imdeformed flat- lying Kuroko deposits of Japan and the massive sulfide deposits of Cyprus. The degree of deformation of all of these districts appears to be largely a function of the age of the rocks involved.

Sulfide BodyOne of the genetically most significant charac­

teristics of the concordant, pyritic sulfide deposits is the nature of the footwall and hanging wall contacts. Hutchinson (1965) describes the hanging wall contact as being sharp and the footwall contact' as being gradational into the under­lying stringer ore. Maruyama (196?) also describes a net­work of veins containing chalcopyrite, pyrite, and quartz in the footwall of the Kuroko deposits. Smirnov, Borodayev, and Starostin (1969) also discuss the footwall and hanging wall relations in the Kuroko deposits. They noted con­centrations of veinlet-disseminated ore along steeply dipping shatter zones in the footwall, and marine sedi­mentary rocks sharply bounding the hanging wall. At the Siskon mine there is a sharp contact on the hanging wall side with the vesicular meta-andesite. On the other hand, drill hole SIS No. 1 encountered veins of quartz, pyrite, and chalcopyrite as the footwall contact was approached. (Figs. 5, 6, and 9).

43Propylitic alteration is ubiquitous around con­

cordant pyritic sulfide deposits; however, regional low- rank metamorphism has produced mineral assemblages very similar to those produced by mineralizing fluids. Common alteration minerals are silica, sericite, chlorite, pyrite, epidote, and carbonate (Hutchinson, 196$). However, the spatial distribution of these minerals will frequently yield some clues to the origin of the deposit. In the Cyprus deposits, the hanging wall lavas show little alter­ation; but the lavas alongside and beneath most of the sulfide bodies are strongly altered to chlorite, clay minerals, silica, and pyrite (Kinkel, 1966). Boldy (1968), in writing about the Delbridge massive sulfide deposit, noted a footwall alteration zone, crudely pipe-like in form, which lies normal to the ore-bearing contact. He also pointed out that the ore mineralization is localized in a chert unit which could be a sinter product localized around an orifice above the chlorite alteration pipe. At the Siskon mine, the alteration minerals in the footwall rocks are distinctly different from those found in the hanging wall. Altered brecciated rocks found in the footwall in­dicate that a brecciated alteration zone may exist perpen­dicular to the ore zone. This could have been the conduit for the mineralizing fluid.

44Some apparent structural control exists at the Siskon

mine in the form of fault or shear zones located on the foot- wall contact of the mineralized quartz chlorite schist. Faulting and shearing in the Canadian deposits are pronounced and have led to the belief that these deposits are struc­turally controlled hydrothermal replacement deposits. A similar origin could be hypothesized for the Siskon deposit; however, the Cyprus massive sulfide deposits and the Japanese Kuroko deposits do not exhibit any structural control with the exception of some minor folding and slump faulting. Hutchinson (1965) explains the faulting and shearing as the result of differential stresses which would develop in more competent rocks near the mineral body during metamorphism. The schistose texture in mineralized host rock would also be expected to develop as a result of this metamorphic process.

Sequence of EventsThe sequence of events involved in the formation of

the mineral deposit at the Siskon mine appears to be very similar to the sequence proposed by Gilmour (1965) for the Noranda district, Hutchinson (1965) for the Cyprus deposits, and Horikoshi (1969) for the Kuroko deposits of Japan.

A cycle of volcanism began with the extrusion of submarine flows of andesitic composition. In the waning stage of this volcanic cycle, there was a period of fumarolic

activity and related steam explosions. Pyroclastic material,45

indicated by the brecciated texture found in some of the gossan material from the ore zone, was created as a result of the steam explosions. The steam explosions were probably also responsible for the creation of a breccia zone through which mineralizing fluids passed upward toward the surface leaving a siliceous sinter at the orifice. The passage of the fumarolic fluids altered some of the rocks through which they passed. Generally, iron and magnesium were added and silica was removed. There was probably an excess of sulfur available near the surface which extracted the weakly-heId iron from the chlorite. This process would explain the presence of more magnesium-rich chlorite in and around the ore zone. The phyllite beds may possibly be ex­plained as a mixture of volcanic ash and silica from the fumerole. Although much of the mineral deposit may be ex­plained by direct precipitation of mineralization derived from the fumarole, a large part of it may have resulted by replacement of the pyroclastic material expelled during the steam explosions.

Contemporaneous with the deposition of the ore zone mineralization was the onset of a new cycle of andesitic lava extruded from adjacent volcanic centers. This lava began to cover the zone of sulfide deposition while the

46deposit was still forming, thus causing alteration of the hanging wall lavas along paths of leakage from below.

After deposition of the ore zone mineralization, the area was folded as a result of continued regional under-thrusting from the west. Faulting and shearing then took place in a predominate north-south direction in the zone of mineralization. Ultramafic and dioritic intrusion took place during this period of deformation and meta­morphism.

Later, after the region had been uplifted, erosion removed the rock overlying the mineral deposit and oxidation of the ore zone occurred. Residual enrichment of the gold deposit took place during this period of oxidation and erosion, especially where the mineralization was the strongest and the topography was relatively flat. Here, again, the Siskon deposit resembles the Cyprus deposits where the gossans over most of the ore bodies were rich in gold (Hutchinson, 1965).

CHAPTER X

CONCLUSION

The mineral deposit at the Siskon mine probably formed in conjunction with eugeosynclinal volcanism during the Nevadan orogeny late in the Jurassic Period. It occurs at a break in the volcanic sequence associated with a pyllite unit and pyroclastic material. The dominate sul­fide minerals are pyrite and chalcopyrite with a ratio of more than 10:1 pyrite to chalcopyrite. Trace amounts of gold and silver arc associated with the primary sulfide mineralization.

Gold ore was developed in the gossan near the surface of the mineralized zone by the process of super­gene enrichment. The ore zone lacks sufficient porosity for the development of a blanket of supergene chalcocite of sufficient tonnage and grade to be mineable at the present time.

The Siskon mine, the Blue Ledge mine, the Gray Eagle mine, and the Copper Bluff deposit, occur within a seventy-mile-long band located six to ten miles west of the thrust fault separating the Western Paleozoic and Triassic belt from the Western Jurassic belt. The Blue Ledge deposit, the Gray Eagle deposit, and the Copper

47

43Bluff deposit appear to be similar to the Siskon deposit from their brief description by Albers (1966). These deposits probably have a similar origin related to the arcuate structure of the Klamath Mountains. Figure 13 shows the relationship of these deposits to major faults and ultramafic and granitic intrusive rocks. Hamilton (1969) considers the major thrust faults to be related to zones of subduction or under-thrusting from the west. The location of the concordant pyritic sulfide deposits relative to the major thrust faults located six to ten miles to the east of them indicates that the faulting and mineral deposi­tion were probably contemporaneous during the Nevadan orogeny. This relationship of the deposits to major thrust faulting could be of considerable value in developing re­gional exploration programs for this area and similar areas in other parts of the world.

49

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Fig. 18. Relation of Concordant Pyritic Sulfide De­posits to Regional Geology.

LIST OF REFERENCES

Albers, J. P., 1966, Economic Deposits of the Klamath Mountains, in Geology of Northern California: California Division of Mines and Geology,Bull. 190, p. 51-62.

Billings, M. P., 1954, Structural Geology: EnglewoodCliffs, New Jersey, Prentice-Hall, 514 p.

Blanchard, Roland, 1968, Interpretation of Leached Out­crops: Nevada Bureau of Mines, Bull. 66, 196 p.

Boldy, Julian, 1968, Geological Observations on the Del- bridge Massive Sulphide Deposits: Canadian Inst.Mining Metallurgy Trans., V. 71, p. 247-256.

Brant, A. R., and the Newmont Exploration Staff, 1966, Ex­amples of Induced-polarisation Field Results in the Time Domain, in Mining Geophysics, V. 1,Case Histories: Tulsa, The Society of Explora­tion Geophysicists, p. 288-305.

Gilmour, Paul, 1965, The Origin of the Massive Sulphide Mineralization in the Noranda District, North­western Quebec: Geol. Assoc. Canada Proc.,V. 16, p. 63-81.

Gilmour, Paul, and Still, A. R., 1968, The Geology of the Iron King Mine, in Ore Deposits of the United States, 1933-1967 (Gratton-Sales Volume):New York, Am. Inst. Mining Metall. Petroleum Engineers, p. 1238-1257•

Grant, F. S., and West, G. F., 1965, InterpretationTheory in Applied Geophysics: New York, McGraw-Hill, 583 p.

Hamilton, Warren, 1969, Mesozoic California and the Under­flow of Pacific Mantle: Geol. Soc. America Bull.,V. 80, p. 2409-2430.

Horikoshi, Ei, 1969, Volcanic Activity Related to the For­mation of the Kuroko— Type Deposits in the Kosaka District, Japan: Mineral Deposits, V. 4, P» 321-345•

50

51Hutchinson, R. VZ., 19&5, Genesis of Canadian Massive Sul­

phides Reconsidered by Comparison to Cyprus Deposits, in Symposium on Strata-bound Sulphides and their Formative Environment: Canadian MiningMetall. Bull., V. 58, P- 972-986.

Irwin, VZ. P., 1966, Geology of the Klamath Mountains Province, in Geology of Northern California: California Division of Mines and Geology,Bull. 190, p. 19-37.

Kinkel, A. R., Jr., 1966, Massive Pyritic Deposits Re­lated to Volcanism, and Possible Methods of Emplacement: Econ. Geology, V . 6l, p. 673-694•

Lanphere, Marvin, Geologist, U . S. Department of theInterior, Geological Survey, Menlo Park, Calif., Pers. Commun., letter to author, October 28, 1969.

Lanphere, M. A., Irwin, VZ. P ., and Hotz, P. E., 1968, Isotopic Age of the Nevadan Orogeny and Older Plutonic and Metamorphic Events in the Klamath Mountains, California: Geol. Soc. America Bull.,V. 79, p. 1027-1052.

Ludwig, C. S., and Henson, K. H ., 1967, Theoretical Induced Polarization and Resistivity Response for the Dual Frequency System Collinear Dipole-dipole Array: Tucson, Arizona, Heinrichs Geoexploration Company.

Madden, T. R., 1967, Induced Polarization and its Applica­tion to Mineral Exploration: Moscow, USSR, In­terregional Seminar of the UNO, 33 p.

Maruyama, Sadao, 1967, Kuroko Geology: World Mining,V. 3, p. 15-16.

Meyer, Charles, and Hemley, J. J., 1967, Wall Rock Alter­ation, in Geochemistry of Hydrothermal Ore Deposits: New York, Holt, Rinehart, and Winston,670 p.

Miller, L. J., I960, Massive Sulfide Deposits in Eugeo- synclinal Belts ^abSjJ: Econ. Geology, V. 55,p. 1327.

Miller, R. J., 1961, Wall-rock Alteration at the Cedar Bay Mine, Chibougamau District, Quebec: Econ.Geology, V. 56, p. 321-330.

52Price, Peter, 1953, Wall-rock Alteration in Northwestern

Quebec /abs//: Bull. Geol. Soc. Am., V. 64,p. 1464.

Smirnov, V. I., Borodayev, Y. S., and Starostin, V. I.,1969, Pyritic Ores and Deposits of Japan: Internat.Geology Rev., V. 11, p. 845-856.

Williams, H., 1963, Relationship between Base Metal Miner­alization and Volcanic Rocks in Northeastern Newfoundland: Canadian Mining Jour., V. 84,p. 39-42.

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HACKMAN,THESIS, GEOLOGICAL

1971ENGIN E ERING

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HACKMAN, 1971 , GEOLOGICALTHESIS ENGINEERING

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1971ENGINEERING

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HACKMAN,THESIS, GEOLOGICAL

1971ENGINEERING

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