Gail E. Dunning and Joseph F. Cooper, Jr.
Bay Area Mineralogists

California Geology, Sept/Oct 1995, p. 119-128

Photo 1. View to the west showing the extensive Alta Mine dump. The reddish-brown area near the Alta shaft is composed of oxidized troilite and secondary copper minerals. PHOTOS BY GAIL DUNNING.


Troilite, stoichiometric iron sulfide (FeS), was named after Dominico Troili who observed the mineral in a meteorite that had fallen at Albareto, in Modena, Italy in 1766 (Haidinger, 1863). Since that time, Troilite has been recognized as a common constituent of meteorites. In terrestrial environments, it usually is associated with pyrrhotite and other sulfides and oxides, and has been identified from igneous intrusion and alkali ophiolitic sulfide deposits.

In 1922 Dr. Arthur S. Eakle of the University of California, Berkeley, described the first terrestrial occurrence of Troilite from samples submitted by Mr. Magnus Vonsen of Petaluma, California. Since the 1960's, the samplesı unconfirmed source has been the Alta mine, because of the abundant troilite found there (Photo 1). Recently, excellent samples of troilite matching Eakle's (1922) description have been collected from mines along the headwaters of Copper Creek, which begins just north of the Alta mine.


The Low Divide copper mining district is approximately 21 km northeast of Crescent City and 11 km east of the small town of Smith River, Del Norte County, California (Figure 1). The district can be reached by traveling east on Rowdy Creek road from Smith River to the divide between Hardscrabble and Copper creeks, where the road crossed the Alta mine property.

The mines in the Low Divide mining district consist of the Alta, Union, monument, and Occidental and are in Section 35, T18N, R1E, Humboldt Meridian, in Six Rivers National Forest (Figure 2). It should be remembered that, even though these mines have been inactive for more than half a century, they are still patented claims. The underground workings are mostly caved, inaccessible, and extremely hazardous. Detailed descriptions of the mines, including underground workings and ore production, are given by Lowell (1914) and Laizure (1925).

Topographical features of the area are typical of the northern California Coast Ranges and consist of steep, rugged mountains with deeply incised drainage systems. High areas have little relief and represent the remnants of a late Tertiary erosion surface known as the Klamath peneplane. Thin lateritic soils support a scrub forest. Landslides, some still active, have occurred on a large scale, particularly along streams that cut soft, easily eroded serpentinite.


Copper was discovered in the Low Divide district at the head of Copper creek in 1853. In 1860, copper prices rose in response to the material needs of the Civil War, and the mining camp of Altaville came into being (Laizure, 1925; Maxon, 1933; O'Brien, 1952).

In 1863, William H. Brewer, head of the Botanical Department of the Geological Survey of California, visited the camp of several hundred people and gives a colorful account of the life style during the heyday of mining activity in California (Brewer, 1865):

November 2 we footed it to Low Divide, or Altaville, about eighteen miles. The road was very crooked, running over high ridges and sometimes commanding grand views of the wide Pacific and of the surrounding rough landscape. The hills are covered with low brushes and here and there in the canyons heavy timber, as we approach the sea.

Low Divide is a little town on a sharp ridge - a "low divide," in truth, between higher hills. It is a regular mining town, of minersı cabins, a few stores, saloons, and a "hotel." At this last I stopped the entire week, and a filthier, dirtier, nastier, noisier place I have not struck in the state. The whole scene was truly Californian-everyone noisy. We found that the landlord had killed a pig that afternoon, and over sixty dollars had been lost or won in betting on its weight! You cannot differ with a Californian in the slightest matter without his backing his opinion with a bet...

Copper occurs here, scattered over quite an extent of country. A great number of claims are taken up and much work is being done on them. Only on of these mines, out of over thirty, had paid expenses, and this has produced as yet scarcely over five hundred tons of ore, sold at perhaps $50,000. But all hope to get rich. I trudged over the hills by day and sat in the dirty barroom or saloon during the evenings, and watched men lose their earnings at poker...

We had rainy weather there a part of the time, which increased my discomforts - standing at night in a crowded barroom, with seats for half a dozen, while twenty or thirty wet, dirty men from the mines steamed around the hot stove. To go to bed was no relief. We slept on the floor upstairs, some twenty or twenty-five of us - they kept running in and out all night. The noise from below prevented sleep until late, and the last of the card players would be getting to bed in the morning after the first risers were up.

But all things must come to an end, and so must this. Saturday, November 7, I footed it four miles down the road to a little Dutch tavern, where I got the luxury of a clean bed and a clean table, yet had to sit with wet feet and listen to a drunken mineral who was determined to enlighten me on the subject of geology.

Of all the district mines, the Alta, also known later as the Alta California, was the most prominent and produced the bulk of the ore shipped. Other mines in the district included the Occidental, Salt Lake-California (Union), and the Mammoth Group. Between 1860 and 1863, about 2,000 tons of ore were shipped from the district with a return of $41 to $102 a ton (Browne and Taylor, 1867). The first 42 tons of ore shipped from the Alta mine was composed chiefly of cuprite and came from a mass discovered near the surface that was nearly 1 m wide and 17 m long. It averaged 45 percent copper and was sold in San Francisco for $7,000, with the cost of extraction and delivery not exceeding $2,000 (Browne and Taylor, 1867).

Since copper smelters did not exist on the west coast of North America during that time, the Low Divide copper ore had to be shipped to Europe for smelting. The high grade ores were packed in bags and transported by wagon to the coastal settlement of Crescent City, where they were carried by coastal schooner to San Francisco. Upon arriving at the San Francisco docks, the ore was transferred to sailing ships for the long voyage around Cape Horn and across the Atlantic Ocean to smelters in Wales and Germany (Lowell, 1914; Laizure, 1925).

When the end of the Civil War caused the price of copper to drop, the mines shut down, although pockets of ore remained in most of them. Old records show most of the ore produced ran from 15 to 18 percent copper (Lowell, 1914).

The last recorded work in the district occurred in August 1951 when a partnership including John Noce, Joe Renarex, and Y. E. Yoder was engaged in cleaning out and retimbering the old Alta shaft that had burned out above the adit. By the end of August the shaft had been retimbered for 18 m (O'Brien 1952). There is no record of any further work having been done on the property. When we visited the property in 1963, the site was found abandoned, the head frame and hoist had been removed, and remaining buildings were in a state of disrepair. Today nothing remains of Altaville except some stone foundations; only the mine dump and some crumbling foundation can be found at the Alta mine. The underground workings are caved or flooded and inaccessible.


While in Crescent City, Mr. Magnus Vonsen obtained Troilite samples from a miner with no locality description other than "a copper claim northeast of Crescent City, in Del Norte County." In 1938, the late Charles W. Chesterman, a senior mining geologist with the California Division of Mines and Geology (DMG), examined Mr. Vonsenıs collection, became interested in Troilite, and did considerable research on the samplesı origin. In the early 1960's, Salem J. Rice of the DMG collected samples from various copper mines but found Troilite only at the Alta mine.

Our interest in Troilite began in 1963 when we learned of Eakle's work and the occurrence of troilite in Del Norte County. In July of that year we made a collecting trip to the Alta mine with the intent of examining the source of this unusual mineral. The mine dumps were found to contain large amounts of massive sulfide ore. Subsequent optical and chemical tests proved this material to contain Troilite, pyrrhotite, pentlandite, and other minor constituents. Contrary to Eakle's description, no magnetite was found in any of the sulfide samples examined.

In 1988, we re-examined the troilite samples from the Alta mine collected in 1963 and initiated a complete study that revealed major differences between the Alta mine material and that studied by Eakle. All the Alta mine troilite samples were very nearly identical, which heightened the mystery of the origin of Eakle's study samples.

Inquiries were made to the California Academy of Sciences, San Francisco (CASSF) and the University of California, Berkeley (UCB) to determine if samples of Vonsenıs troilite were available for study. Through the courtesy of Dr. Jean DeMouthe of the Academy, several excellent samples of troilite from the Vonsen collection were made available for study. Dr. DeMouthe stated that all detailed locality information that Vonsen had on the samples had been destroyed after his death. Ms. Peggy Gennero of UCB made available a suite of small troilite samples studies by Drs. Charles Meyer and Adolph Pabst. These samples had been provided by Mr. Vonsen and subsequently studied by Dr. Eakle. The only locality note for these samples was "Del Norte County."

The troilite samples from these two institutions were examined and found by optical microscopy, X-ray diffraction, and chemical analysis to be nearly identical and matched Eakle's 1922 description. To resolve this locality enigma, we visited the Low Divide district in 1990 to collect additional samples from the Alta mine (Photo 2). In addition, we examined other workings in the Hardscrabble Creek area but discovered no troilite samples. Next we visited the Union mine in the Copper Creek area, which revealed abundant troilite on the dumps. Further searching revealed a vein of in situ troilite containing veinlets of magnetite (Photo 3). All the Union mine workings produced abundant troilite containing chalcopyrite, the major copper ore mined. When we revisited the area in 1991, we examined the Copper Creek side of the Low Divide district in greater detail. We found that massive sulfide bodies rich in troilite were common in the area. This new material allowed us to compare troilite samples from the Alta mine, the Union mine and Vonsenıs collection.


Early geologic reconnaissance of this region were performed by Diller (1902), Hershey (1911), Maxson (1933), Wells et al. (1946), and Cater and Wells (1953). For the last two decades structural geologists, in addition to many other workers in closely related fields, have examined this region in increasing detail to better understand the complex tectonic events that have resulted in the present structural features, exposed and at depth.

This region of northwestern California and southwestern Oregon contains one of the largest intact ophiolites known, with an aerial extent of more than 800 square kilometers. This ophiolite, known as the Josephine Ophiolite, is a complex formation of igneous and metamorphic rocks that originated in a Late Jurassic back-arc sea basin adjacent to the then-existing coast. It is composed of a sequence of harzburgite tectonite, cumulative ultramafics, gabbro, high level gabbro and diorite, a sheeted dike complex, and upper layers of pillow lava and breccia (Harper, 1980, 1984; Harper et al. 1988).

The Josephine Ophiolite and the overlying Galice Formation are part of the Western Jurassic Belt of the Klamath Mountains geomorphic province. In general, the Klamath Mountains consist of arcuate, fault-bounded belts of diverse eugeosynclinal terranes. These belts display a general age progression with rocks ranging from early Paleozoic in the east to primarily Upper Jurassic in the west (Irwin, 1960, 1966, 1972, 1981).

Rock in the Low Divide district consists of Josephine peridotite and a harzburgite composed of olivine and enstatite. Along the joints and fractures, typically more than half the olivine and enstatite are replaced by serpentinite. The weathered harzburgite has a pyroxene-studded, tan to reddish-brown surface and weathers to a red lateritic soil.

The Copper Creek shear zone formed late in the sequence as indicated by cross-cutting relationships. This large north-striking shear zone appears to dip 50 to 75 degrees to the east and is bounded by the Alta Fault on the west and the Wimar Fault on the east. Locally, the Alta Fault dips steeply to the west and the Wimar Fault dips steeply to the east (Figure 2).

Detailed mapping of the Low Divide district by Dr. Gregory D. Harper (State University of New York, Albany, written communication, 1983) has shown that sulfide mineralization is restricted to the Copper Creek shear zone. This shear zone is bounded on the west by relatively unaltered peridotite and on the east by a fault-generated peridotite-serpentinite complex.


Sulfide-bearing samples for this study were collected from the Alta, Union, and Monument mines, the major mines along the Copper Creek shear zone. No sulfide-bearing rocks were noted in the several workings of the Occidental mine, although oxidized material was abundant. All field samples were carefully catalogued to maintain integrity among the several mines.

Using standard ore preparation methods, more that 100 polished sections were prepared for optical and SEM/EDS examination from the field samples. The data obtained from these polished samples constituted the sulfide vein mineralogy and textural relationships for this report. Sulfur isotopic analyses were performed by Dr. Robert A. Zierenberg, U.S. Geological Survey, Menlo Park, California; this data would be critical in determining the paragenesis of the sulfides identified in the Low Divide district.

Eakle's 1922 Troilite

Before Dr. Eakle had described the first terrestrial occurrence of troilite, the physical and chemical properties had not been determined sufficiently to differentiate it from pyrrhotite. Because of its abundance in Del Norte County it offered an opportunity to compare properties to the well-known occurrence in meteoric iron.

Eakle (1922) determined that troilite to be distinct from pyrrhotite in several respects, namely color, lack of magnetism, and solubility in sulfuric acid. The color of untarnished troilite (freshly broken) was observed to be light grayish-brown with no bronze-colored tinge but oxidation darkened exposed surfaces.

Dr. Eakle (1922) found the sulfide had inclusions of a bluish-black serpentinite containing much iron. The inclusions, however, did not affect the analysis because of their insolubility in sulfuric acid. A small amount of copper was detected but no nickel or chromium (chromium in the form of magnesiochromite is abundant in the district). "A magnetic phase in the sulfide was attributed to included magnetite, which was easily separated by a magnet. Further observations of troilite showed thin rims of magnetite grading into the troilite. The magnetite appears to be residual material from a conversion of a larger mass into troilite."

"The owner of the copper claim reported that the chalcopyrite bodies occur in a shear zone of serpentinite along an extensive fault zone. This shear zone consists of fractured and altered rock with slickensides, that has evidently been invaded by sulfide solutions that deposited the chalcopyrite. The owner stated that troilite was found in one of the tunnels, in rounded masses with slickensides." Eakle (1922) concluded that "these masses were probably magnetite in the original serpentinite, and solutions rich in hydrogen sulfide 'transposed' them into troilite. The large amount of iron in the rock in the form of magnetite appears to have conditioned the formation of the monosulfide instead of the more common pyrrhotite with its excess sulfur over iron."

The Alta Mine Troilite

Massive troilite samples recovered from the Alta mine dump are characteristically covered by a brown iron oxide crust and can be distinguished from the harzburgite by their high specific gravity and blocky fracture (Photo 4). Freshly broken troilite is silver-white to gray with a granular texture, unlike pyrrhotite or pyrite (Photo 5). Upon exposure to air the surface rapidly develops an iridescent tarnish that turns a dull bronze color in a few hours. All troilite specimens from the Alta mine are weakly magnetic because of admixed pyrrhotite.

In polished section, the troilite-rich sulfide matrix consists of a heterogeneous mixture of troilite, low-temperature pyrrhotite and pentlandite. The g rains of this mixture, as viewed under polarized light, show preferred orientation with respect to the flow direction during emplacement along the fracture system. Troilite generally makes up the larger fraction of the mixture with pyrrhotite second, followed by pentlandite. Troilite may be easily distinguished from low-temperature pyrrhotite and pentlandite optically in polished section. The troilite exhibits polarization colors ranging from deep gray to deep brown whereas pyrrhotite has polarization colors ranging from dark blue to gray to brown. Pentlandite shows no color change due to its isometric symmetry.

Inclusions of cobaltite, often showing a cubic form, are dispersed throughout the sulfide matrix. These crystals sections, some of which contain core of a gold-silver alloy, are usually fractured and surrounded by troilite.

Tochilinite and valleriite have formed as a direct replacement of troilite along individual grain boundaries and appear to be an alteration of the troilite during low-temperature hydrothermal serpentinization at depth.

No magnetite was observed in any of the polished sections from the Alta mine troilite, even though the surrounding harzburgite contains small grains of magnetite and nickel sulfides.

CASSF and UCB Troilite Samples

A macroscopic examination of the troilite samples from the Vonsen collection at CASSF and UCB indicates these closely agree with Eakle's 1922 description of troilite. Two excellent hand samples of troilite from CASSF, labeled V95 and V2227 were examined in detail for texture and mineralogy. The first, V95, is a mass of dark bronze-brown troilite, (11 x 6 x 2.5 cm) weighing about 475 grams (Photo 6). This sample shows distinct parting planes and extensive cracks. On one end is a thin slickenside composed of magnetite that extends into the troilite along the fractures.

The second sample, V2227, is a larger mass of troilite, measuring 10 x 7 x 6.5 cm, and weighing about 1000 gms. The color is quite similar to specimen V95 but somewhat brighter. The smaller end is a sawed surface that shows a number of magnetite veins penetrating the troilite 4 cm. One surface, presumably the one in contact with the serpentinite, is a slickenside composed of a bluish-black iron and chromium-rich chlorite.

Five of Vonsen's Del Norte County troilite samples labeled #1337, #1338, #1339, M3219 and #21327 were identified in the UCB collection. A macroscopic examination showed they were very similar to those obtained from CASSF.

Polished sections were prepared from thin slabs cut from CASSF V95 and V2227. also, small fragments from UCB samples 1337, 1338 and 1339 were mounted without cutting. Optical examination of these polished sections showed the general texture relationships between CASSF and UCB samples to be identical.

Union Mine and Monument Mine Troilite

The troilite samples collected from the in situ vein and the scattered dumps of the Union mine closely resemble those from the Alta mine but display certain textural and physical differences. When broken, the Union mine troilite displays a definite set of parting planes that the Alta material lacks. Although the Union mine material has the same silver-white to gray color on fresh surfaces, it tarnishes to a vivid blue within minutes of exposure to air. This color further darkens with time to a bronze color similar to the Alta mine troilite. Magnetite occurs as small octahedrons in the cavities in the ore and as thin veins cutting it. In polished section, troilite samples from the Union and Monument mines were found to be identical to those of CASSF and UCB.

Associated Minerals

The mineralogy of the Low Divide copper deposit is related to the deposition of massive sulfide bodies. Troilite composes 40 to 70 percent of the sulfide mass, and the other minerals occur as minute grains or segregations in the sulfide mass. Only rarely are the grains large enough to identify with the naked eye, cobaltite, magnetite, chalcopyrite, and the tochilinite-valleriite group being notable exceptions.

Minerals associated with troilite in the Low Divide district include bismuth, chalcopyrite, magnesiochromite, cobaltite, cobalt-pentlandite, gold-silver, magnetite, maucherite, pentlandite, pyrrhotite, sphalerite, tochilinite, valleriite, aragonite, calcite, chalcanthite, chalcocite, copper, cuprite, erythrite, goethite, melanterite, starkeyite, and tenorite.


The origin of the Low divide massive sulfide deposit is directly related to the formation of the Copper Creek shear zone, its intrusion by iron-rich sulfide solutions in a sub-oceanic environment, and subsequent serpentinization and uplift to its present position. The geochemical conditions required for the formation of natural troilite are not typical of normal sulfide deposition (Gronvold and Haraldsen, 1952; Konev et al. 1970; Nambu et al. 1976; Karup-Muller, 1978).

Possible sources for the massive sulfide bodies include (1) immiscible fluids derived from mafic magmas, which have been remobilized under geochemical conditions different from their origin, or (2) hydrothermal solutions derived from downwelling seawater that penetrated the upper mantle along fracture zones and reacted with sulfide-rich solutions. Isotopic sulfur analysis allows the differentiation of the sulfur source.

Troilite samples from the Alta and Union mines, and chalcopyrite from the Union mine represent material from the extremes of the mineralized shear zones. The results of the isotopic sulfur analyses on these samples show that the source of the sulfur was an immiscible melt rather than downwelled seawater (Zierenberg, USGS, Menlo Park, written communication, 1993, 1995).

The massive nature of the ore, troilite geochemistry, copper enriched stringers, and the control of mineralization by the shear zones indicate the sulfide bodies do not result from simple magmatic segregation. There has clearly been remobilization and concentration of the massive sulfide melt into fractures along the shear zones, as well as geochemical changes in the original monosulfide solid solution (MSS) that may have separated from the melt at high temperature. In order for troilite to form, a high iron-to-sulfur ratio has to be present. Low temperatures required for troilite formation by hydrothermal remobilization of the MSS is compatible with the sulfur isotopic data (R. Zierenberg, USGS, written communication, 1993, 1995).

An apparent paragenetic history for this unique troilite-bearing deposit can be summarized by the following statements:

1. The ultramafic rocks that hosted the initial sulfide-rich fluids formed deep below the ocean floor as a result of cooling of a mafic magma. As the mass cooled, magmatic segregation began, forming magnesiochromite and cobaltite (containing gold and silver), which were incorporated in the peridotite body.

2. Tectonic stresses on this peridotite body created faults and shear zones, channels for later fluid migration. 3. The immiscible fluids derived from magmatic segregation of the ultramafic magma formed an iron-nickel sulfide solid solution but the copper remained more mobile.

4. This solid solution was remobilized and redeposited as troilite, pyrrhotite, and pentlandite in fractures in the Copper Creek shear zone during the early stages of serpentization. Serpentinization, which produced an uplift of the entire area, subjected the rocks hosting the sulfides to minor alteration.

5. The geochemical conditions favoring the formation of troilite and low temperature pyrrhotite remained fairly constant as demonstrated by the uniform vein mineralogy.

6. Copper and zinc were enriched along the margins of the troilite veins as localized pods and stringers of chalcopyrite and sphalerite. Minor replacement of troilite by chalcopyrite took place in the Union mine ore body but was not widespread.

7. During the final stages of serpentinization magnetite penetrated along fractures in the troilite, and fibrous veins of tochilinite-valleriite formed.

8. Weathering of the copper-rich portions of the Alta mine sulfide body produced a supergene enrichment zone rich in cuprite, native copper and chalcocite.


Even though troilite occurs worldwide, it remains a very rare mineral. In at least three deposits noted in the literature, it has formed as exsolution lamellae within hexagonal pyrrhotite (Vorma, 1970; Nambu et al. 1985). In other occurrences it formed as the result of magmatic segregation in ultramafic rocks (Corlett, 1977).

Troilite from the Troodos Ophiolite near Pafkos, Cyprus, which is very similar to the Low Divide troilite, has been described by Panayiotou (1977). In this deposit, the primary sulfide-arsenide assemblage includes troilite, valleriite, maucherite, oregonite, chalcopyrite, pentlandite, gersdorffite, chromite, magnetite, graphite, molybdenite, and sphalerite.

Panayiotou (1977) suggests the formation of the Pafkos deposit is associated with the evolution of the Troodos Ophiolite during fractional crystallization of a basaltic magma thought to have been derived from the partial melting of a portion of the upper mantle. During this period immiscible fluids were segregated in the early ultramafic dunites and harzburgite. Subsequent pervasive serpentinization and tectonic movement of the dunites and harzburgite caused remobilization of the original sulfides with formation of troilite and minor changes in the mineralogy of the original sulfide assemblage.

The following conclusions have been drawn from recent geological field studies in the Low Divide and by a detailed microscopic examination of the massive sulfide ore from the district.

1. The troilite samples provided by M. Vonsen, upon which Eakle (1922) based his description, were most probably obtained from one of the tunnels of the Union mine, not the Alta mine as previously assumed.

2. The massive troilite-bearing sulfides originated below the sea floor as a result of magmatic segregation if immiscible fluids with subsequent remobilization during favorable geochemical conditions (high Fe/S ratios, low temperatures).

3. Isotopic sulfur data suggest the massive sulfide was most likely of magmatic origin deep in the earthıs crust and not from the reduction of seawater sulfate in downwelling oceanic waters.

4. The copper mineralization at the Alta mine was concentrated along the upper margins of the massive iron sulfide veins and pods and was subjected to supergene enrichment forming native copper, cuprite, and chalcocite.

5. The copper mineralization in the Union and Monument mines consists only of chalcopyrite with no evidence of post-depositional enrichment.

6. The paragenetic sequence derived from the ore microscopy studies suggests the following order of sulfide/oxide formation: 1) cobaltite, gold-silver, magnesiochromite, native bismuth; 2) troilite, pyrrhotite, pentlandite, cobalt-pentlandite, maucherite; 3) chalcopyrite, sphalerite; 4) chalcocite, native copper, cuprite; 5) magnetite, tochilinite, valleriite.


We take great pleasure in dedicating this paper to the late Charles W. Chesterman. Without his help and encouragement during the initial stages of this project much of his early research data would not have been known. Mr. Chesterman became interested in the troilite occurrence in 1938 and spent considerable research time in an attempt to isolate the locality of Vonsenıs samples. In 1960 he and the late Dr. Adolph Pabst initiated a study of troilite but, due to other commitments, put the study aside.

We hope our efforts in bringing this project to completion have fulfilled the goals of Mr. Chesterman and Dr. Pabst in understanding the paragenesis and location of the troilite samples from northern California. Charles was always a friend and mentor to professional and amateur mineral enthusiasts and his knowledge of California geology and mineralogy was invaluable during our early years in the mineralogy field.

We also wish t to express our sincere thanks to Dr. Jean DeMouthe of CASSF and to Ms. Petty Gennero of UCB for making available study specimens of troilite donated by Magnus Vonsen. Dr. Donald C. Harris, Geological Survey of Canada, provided important information and references on the identification of tochilinite and thoughts on he paragenesis of troilite and the supergene enrichment of copper sulfides.

We especially thank Dr. Gregory D. Harper, SUNY, who, during a chance meeting at the Alta mine and through subsequent correspondence, provided much information and references on the areaıs geology and paragenetic relationships. His keen interest in our work is much appreciated. Dr. Robert A. Zierenberg, USGS, Menlo Park, California, provided the isotopic sulfur analyses and many helpful thoughts on the role of the isotopic data in relation to the probable paragenesis of this unusual mineral deposit.

Critical reviews of this paper were made by Drs. Gregory Harper, Robert Zierenberg, M. Darby Dyar, Roger Kuhns, and David OıHanley. We sincerely thank each of them for their comments and constructive criticisms that improved this paper greatly. REFERENCES

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