COLLECTING MINERALS IN THE AREA OF DARWIN, Inyo Co., CA

 

Occasional Publication of the Bay Area Minerologists, February, 2003

by Fen Cooper and Gail Dunning

Introduction

The Darwin area is a fascinating place to collect and can produce some rare and very nice specimens. The area has been collected over for over 50 years so the easy stuff is long gone. (I know, I helped to take some of it.) While the collection of specimens will take work, the area still has great potential and there is little doubt that many species occur that are unrecognized and that new species probably occur both in the metallic mines and in the silicate skarn. The Darwin District is one of the many areas that require a lot of looking and thinking, but will continue to produce good specimens and rare species.

The following are some suggestions for a short field trip. Darwin has a large number of mines and the main mines are quite difficult to get into. But there are literally hundreds of smaller mines and prospects that will produce specimens.

While the Darwin area is mostly known as a silver-lead district, the mineralized solutions also carried other elements that produce localized quirks. To maximize your time, I recommend the small group of mines in Custer Canyon as they are close together. At the very least, you can collect some nice aurichalcite and hemimorphite. Incidentally the dump of the Custer adit contains one very nice hemimophite specimen. I collected some nice aurichalcite, hemimorphite and limonite pseudomorphs and then picked up half of a large piece of red brown massive limonite on impulse. I started washing up my specimens after I got home and found that a considerable portion of the limonite on that specimen was loose and washed away readily, exposing large, colorless, well formed hemimorphite crystals. This specimen, while not as good as some from Mapimi, Mexico, is the best I ever saw from Darwin and although I looked for the other half, I never did find it.

It would be well to note that much of the geologic literature on the area can be considered as being old and skewed heavily toward ŗore˛. Dunning, G.E., Moelo, Yves, Roberts, A.C., & Cooper, J. F., Ag-Cu-Pb-Bi Sulfosalts New To Darwin, Inyo County, California, is the latest published information and shows how much the technology of mineral identification has come. Incidentally, while the Silver Spoon Mine is not mentioned in this field trip description, it does contain some very rare, though ugly, specimens and would be an interesting place to collect if time permitted.

The report by Hall and Mackeverett on the Darwin area gives an overview of the geology of the Darwin Quadrangle and if you can obtain a copy of Hall, W.E., and Mackeverett, E.M., Economic Geology of the Darwin Quadrangle, it is well worth pursuing.

The mines in Custer Canyon include the Custer, Custer adit and St. Charles Mines and the Lucky Lucy Prospect. The next canyon south contains an interesting group of lead-silver- tungsten mines, the Durham-Fernando Mine being the largest.

Custer Canyon is located about 1 mile east of Darwin in the first canyon south of Zinc Hill Road. It is known locally as Custer Canyon although the name does not show up on most of the maps of the area. While a road is shown going down the canyon, it appears to have been washed out by a flash flood. Only the lower portion is passable if accessed from the bottom. The mine is in section 19, T. 19 S., R. 40 E., M.D.M. at latitude 36o 16š 20˛ North and Longitude 117o 34š 20˛ West on page 40 of Delormešs Southern and Central California Atlas and Gazetteer. A mine is shown on the map but is not named. Since the Custer Mine and the nearby St. Charles Mine share the same dump they essentially look like a single mine though they are actually two separate mines and had completely different ore mineral suites.

The St. Charles Mine

The St. Charles Mine is located in a tactite body that was formed at the contact between a quartz diorite and the Lost Burro Limestone. It has been extensively fractured and several intersecting minor faults cut the ore body. The ore was unusual because much of it consisted of veins of massive scheelite that could reach as much as two inches thick. These veins were usually encased in a tactite with a higher than normal garnet content.

The tactite is on an extension of the Custer vein system, but unlike the led-sliver ore body at the Custer Mine, the St. Charles Mine is situated at the top of a tungsten ore body. The tactite there is composed principally of massive green to brown andratite garnet but also contains veins of yellow-green massive epidot with small pockets of epidot crystal.

Sulfides were not common in the tactite and pyrite was the only sulfide found in the upper portion of the mine. The pyrite crystals can reach up to 1/2 inch and cubes are the most common form present. The crystals are typically heavily striated on the faces and occur in calcite filled cavities in the tactite. These pyrite crystals are often encased in calcite and when the calcite is removed they make attractive specimens.

The St. Charles Mine is developed by three separate workings.

St. Charles No. 1 Mine is a 140 foot shaft with over 700 feet of drifts and crosscuts. The bulk of the tungsten ore produced by the mine was produced from this working.
The St. Charles No. 2 Mine was a adit driven into the back of the ridge that the St. Charles No. 1 shaft sits on. This working was strictly an exploration project and no ore was found.
The St. Charles No. 3 Mine was another adit driven into the back of the ridge on a small lead-silver exposure. While a small amount of lead-silver ore was found the venture produced no commercial amounts of ore and was soon abandoned.

Bismuth Sulfosalt Complex

The St. Charles No. 1 Mine is of chief interest for the unusual bismuth sulfosalt bearing ore that came from its deep levels. This ore body closely resembles the ore found in the deep Levels of the nearby Custer Mine. Several of the many species that it contained were not described for many years until the necessary technology was developed to work on them.
The bismuth sulfosalts consist of a complex group of exsolved copper-silver-lead bismuth sulfosalts that include junoite, heyrovskyite, gustavite, friedrichite, eskimoite and pavonite. These minerals were found in rare pieces of an unusual pyrite bearing rock that is composed of minor garnet, pale orange orthoclase grains, white calcite and purple fluorite.

Calcite

Calcite is common in the St Charles Mine and small crystals are common in fractures in the tactite. On the 80 foot level of the St. Charles No. 1 Mine, a very interesting calcite occurs. The tunnel exposes a large fracture zone that is covered by calcite crystals. The calcite has two forms. A steep acicular scalenohedral crystal is the most common and a flat plate-like form is the other. At times the acicular crystals have small flat crystals perched on their points. Specimens showing this type of growth are interesting and attractive. As an added curiosity, the calcite often possesses a thin colorless coating of hyalite opal that fluoresces a brilliant greenish yellow.

 

Wulfenite

Wulfenite was found in the St. Charles No. 3 Adit that was dug to explore a silver lead showing. It occurs on tactite that is coated with manganese oxides and presents as small orange to brown crystals scattered on fracture faces. The crystals occur as small orange plates on a manganese stained portion of the vein. Only a small amount of this type of ore was ever found; the bulk was composed of low grade cerussite.

The major production at the St. Charles Mine was during the 1950's, but by 1955 the ore bodies were exhausted and the mine was abandoned. When the price of tungsten went back up in the '60, the mine was briefly reopened but failed to produce any signifient amount of ore and was closed. The claim is presently inactive, though sporadic exploration work has taken place over the years.

The Custer Mine

The Custer Mine is the next mine of any importance found as you continue down the canyon. The Custer Mine is composed of two separate workings. A shaft over 400 feet deep represents the major working while a tunnel was driven into a mineralized lead-zinc contact zone that occurs on the south side of the canyon.

The Custer Mine was first worked during the 1870šs when Darwin boomed and produced considerable rich high-grade lead-silver during this period. The rich ores were mined out after a few years and the Custer Mine went into decline along with the rest of the Darwin District. The mine was abandoned until 1950 when several of the Darwin mines were consolidated and reopened along with the Custer Mine. It was retimbered and additional mining carried out in the deep levels of the mine. The inclined shaft which was the major access ultimately reached a depth of over 400 feet and levels were driven at 50, 200, 250, 300 and 400 feet. The upper levels above 250 feet dated from the original days of operation and little ore was found there in the later efforts.

The Custer Adit was driven into the south wall of Custer Canyon to explore a showing of copper and zinc minerals. The tunnel is about 125 feet long and was dug during the 1950šs in hopes of finding new ore reserves. Although oxidized zinc ores were found no mixable ore reserves were found and the operation was ended.

The Main Mine

The Custer Mine is located on the contact between a dark brown calc-hornfels and a body of blue-gray limestone. Localized faulting has occurred and a quartz monzonite outcrops nearby, although these features appear to have had little relationship to the ore bodies that were formed.
Large masses of galena were formed in the calc-hornfels and these were weathered to form large masses of argentiferous cerussite. The upper portion of the ore body consisted of an almost pure mass of cerussite that was over 4 feet wide and this constituted much of the rich ore mined. As the mine passed out of the oxidized zone, sulfide ore in the form of galena and pyrite became more prominent. In the deep levels of the Custer Mine, sphalerite became more prominent.
In one area of the 400 foot level a tactite containing scheelite was encountered. This was the last area explored at the mine.

The Adit

The Custer Adit explored a completely different mineralized section of the calc-hornfels. Most of the minerals of interest from the Custer Mine come from this adit, which explored a weathered tactite that contains copper and zinc minerals. Here the highly weathered rock contained seams and pockets of hemimorphite associated with aurichalcite. No sulfides were ever encountered in this mine and only oxidized minerals were ever encountered.

Aurichalcite

Aurichalcite is relatively common in the Custer Adit where it occurs in association with hemimorphite. Specimens of aurichalcite consist of pieces of limonitic gossan containing cavities lined with radiating crystalline blue aurichalcite associated with white prismatic hemimorphite crystals. The crystals are often feathery and the color contrast with the associated limonite is striking.

Calcite

Calcite is locally abundant in the Custer Adit where it occurs in solution cavities. It forms on tactite with one face showing small hemimorphite crystals with flat plates of calcite perched on them.


Goethite psudomorphs after Pyrite

Near the opening of the tunnel of the Custer Adit an exposure of goethite pseudomorphs after pyrite cubes has been found. There are few associated minerals and the area is composed of a mass of small crystal groups that occasionally have small, dark gray, platy calcite crystals attached to their bases. The pseudomorphous crystal groups can be as much as 4 inches across with individual crystals reaching as much as 2 inches across a face. The crystals are simple cubes and no other form has been noted at this locality.

Hemimorphite

Excellent hemimorphite specimens have been found in the Custer Adit in vugs in the weathered tactite. The hemimorphite generally forms cavity coatings of 2-3 mm crystals, though occasionally limonite masses have been found that contain crystals up to 1 cm long. The larger crystals tend to be colorless while the smaller crystals often have a tan color from limonite inclusions. Normally the hemimorphite occurs alone but occasional specimens have crystallized aurichalcite or calcite associated with them.

Malachite

Malachite occurs sparingly in the Custer Adit and only a few specimens are known. It occurs in the limonitic gossan as pockets of felted crystals.

Pyrite

Pyrite was relatively common in the deep levels of the Custer Mine. Specimens of garnet tactite with bright cubic crystals of pyrite perched on one face. The crystals can be up to 1 cm across and frequently show striated faces.

Scheelite

Scheelite was found in the deep levels of the Custer Mine. It occurs in weathered quartz tactite that as rough gray crystals of scheelite. The scheelite fluoresces blue white under short-wave ultraviolet light.

The Custer Mine and the Custer Adit were last worked in the early 1950šs and when the Darwin Mines closed the Custer Mines also ceased production. When the mine closed, all activity ceased and the mine was abandoned. Entry appears to be possible without restriction and the dumps are readily accessible.

The Lucky Lucy Prospect

The Lucky Lucy Prospect is located on the northern slope of the canyon about 100 feet northwest of the Custer Mine Shaft. It is a small pit that was worked in the 1950šs when the larger mines in the area were working. Once the small copper-zinc ore body was mined out, the prospect was abandoned; the claim is now abandoned.

The Lucky Lucy Prospect is situated in a portion of the tactite body that formed at the contact between a calc-hornfels and the Lost Burro Limestone. A quartz monzonite outcrops nearby, although this feature appears to have had little relationship to the ore body that was formed. The tactite body was fractured by local fault movement and the ore body was formed as a replacement of the tactite.

The Lucky Lucy Prospect is one of those curious anomalies that make mineral collecting so interesting. It is completely different from the nearby mines and the minerals it contained were distinctly different. The brochantite and serpierite were well crystallized, and the serpierite find represented the first California locality for this rare and unusual mineral.

The ore body was unusual in the fact that, in an area of lead-silver and tungsten mines, the ores were predominately copper and zinc. The ore body consisted mainly of a mass of hemimorphite and chrysocolla. Sulfides occurred only as relict pieces. Specimens of chalcopyrite and sphalerite have been found on the dump, but galena was completely absent. While the bulk of the oxidized ore was silicates, a band of sulfates occurred near the edge of the ore body. Excellent specimens of brochantite and serpierite were found in this area and on the dump.

A specimen of the unusual tungsten ore bearing an the bismuth sulfosalt complex mentioned above was found on the Lucky Lucy Prospect dump. It closely resembles the ore found in the deep Levels of the nearby Custer Mine and the nearby St Charles Mine. But as no ore of this type was noted underground at the Lucky Lucy Prospect, it may be a manuport from the Custer or St. Charles Mines.

Probably the best specimen material found at the Lucky Lucy Prospect was a group of sulfates from the periphery of the ore body that contained a number of unusual oxidized copper-zinc minerals.

Aurichalcite

Aurichalcite is sparingly present in the ore of the Lucky Lucy Prospect. It occurs in the weathered limonite stained tactite in fractures as feathery blue aurichalcite crystals that are often associated with gypsum.

Bismutite

Bismutite is found sparingly on the dump of the Lucky Lucy Prospect. It occurs in weathered tactite that has been fractured and partially replaced by hemimorphite. The tactite contains long fibrous masses of bismutite, which are associated with serpierite and pseudomorphous after some unknown bismuth mineral.

Brochantite

Very nice specimens of brochantite have been found on the dump of the Lucky Lucy Prospect. It is well crystallized and ranges in form from stubby dark green crystals to acicular groups of dark green to greenish blue crystals that occur in fracture seams in limonite stained, weathered tactite. The acicular crystals can be several millimeters long and radiating groups can be 5 mm in diameter. The larger crystals are less common, but pockets containing dark green brochantite have also been found. These crystals are 3-4 mm long and are 1mm thick and often show well formed terminations on the ends of the prisms. The color contrast between between the green brochantite and the brown to black tactite can produce very colorful specimens.

Rosasite

Except for minor patches and stains of malachite carbonates are not common in the ore at the Lucky Lucy Prospect. The only exception to this is the local occurrence of rosasite. Rosasite was locally abundant in the upper highly oxidized limonitic portion of the Lucky Lucy Prospect, where it occurred in limonite stained tactite fractures as light green botryoidal masses.

Serpierite

Along with brochantite, the Lucky Lucy Prospect also produces the rare copper-zinc sulfate serpierite. It is found in the sulfate-bearing portion of the prospect and occurs on fracture faces in garnet tactite. These faces can be covered with sky blue radiating crystals of serpierite that are often associated with green brochantite or hemimorphite. Serpierite also occurs in impure vuggy veins as sky blue acicular crystals in hemimorphite lined vugs. Specimens of this type are very nice and acicular crystals of serpierite cam reach 5 mm long. The hemimorphite in this environment is often has a thin coating of hydrozincite. This is the first reported locality for serpierite in California.

Gypsum

Gypsum was found sparingly in the assemblage of secondary minerals. It occurred in the weathered tactite surrounding the ore body as radiating rod-like crystals associated with malachite balls and occasional sprays of aurichalcite.

Jarosite

Jarosite was also found on the dump of the Lucky Lucy Prospect in the gossan in seams that were composed of small orange crystals on fracture faces.

Sulfides

Sulfides are not common in the Lucky Lucy Prospect ore body and only rarely will specimens be found on the dump. Small masses of black chalcocite occur that have partially weathered to chalcanthite. Isolated lumps of pure chalcopyrite that are almost always covered by a thin coating of dark blue chalcocite. Covellite was relatively common in the secondary sulfide assemblage, where it occurs in the garnet tactite as isolated blue gray masses. Thin veins and individual grains of sphalerite have also been found. Sphalerite and chalcopyrite were probably the most important primary ore minerals present but only careful search will provide specimens of these two minerals.

Goethite & other olidized minerals

Except for goethite oxidized minerals are almost nonexistent. Thin films of cuprite have been found in fractures in tactite but these are extremely rare. At one time pyrite was extremely common on the periphery of the Lucky Lucy Prospect, but all that remains of this material is a bed of goethite pseudomorphs.

Hemimorphite

Hemimorphite is abundant at the Lucky Lucy Prospect, where it was an important part of the ore body. It occurred as massive white bands as much as 6 inches thick that occasionally contained pockets of crystals. The nearby weathered tactite also contained cavities lined with small hemimorphite crystals.

Chrysocolla

Chrysocolla is also abundant on the dump and in the rubble that partially fills the pit. It forms massive seams that are often associated with hemimorphite. Specimens of hemimorphite commonly contain pockets of crystals that are coated or entirely replaced by chrysocolla.

All in all the Lucky Lucy Prospect was a fascinating place to collect. I just wish that they had left more of the leaner ore around.

The Durham-Fernando Mine

The Durham-Fernanod Mine is located one canyon to the south about 1/2 miles southeast of Custer Canyon on the east flank of the Darwin Hills. The access road is shown on the map, but the workings are not. The access road runs southeast from a point on the Zinc Hill Road along the south face of the Darwin Hills and ends at the mines at a fork shaped intersection.

The Durham-Fernando Mine is a consolidation of two older mines that mined two different types of ore. The Fernando Mine is one of the older silver mines in the district and was first worked in the 1870s when the district was first being developed. It produced rich silver ore for several years but when the district declined in the 1880s production at the Fernando Mine ceased also. During its lifetime, the main shaft reached about 200 feet and there were several levels that were extensively stoped.

The Durham Mine was first worked in 1943 when its potential as a tungsten producer was recognized at a time when tungsten prices were high. When the war ended and tungsten demand dropped, the mine closed.

In 1952, The Fernando and Durham mines consolidated by the Brownstone Mining Company and the Durham Mine was reopened when it was found that the scheelite ore body of the Durham Mine extended into the deep levels of the Fernando Mine. The mining of this deep ore body continued until tungsten prices dropped again, and the mines were abandoned. In 1962, the mines were reopened and a small production of tungsten ore was made from the Durham Mine. After this production, the mines were again abandoned.

The Durham-Fernando Mine explores two differing mineralized zones. The Fernando Mine lead-silver ores originated in a mineralized fracture system in a dolomitic limestone that was created by local faulting. This zone produced a replacement type ore body that was mineralized mainly by argentiferous galena. The ore has been highly weathered and most of the silver ore mined occurred in the form of argentiferous cerussite that was associated with minor galena and anglesite in a gangue of jasper, calcite and limonite. Large bodies of this material were mined.

As the Fernando Mine went deeper a scheelite bearing tactite vein was encountered that cut the lead-silver ore body. This tactite also contained considerable bismuth, and bismuthinite was fairly common. The weathering reached the deep levels in the Fernando Mine, so much of the bismuthinite has been altered to green or yellow bismutite.

The Durham Mine explores a scheelite bearing tactite zone. The tactite is a fairly fine grained, green rock that has grains and crystals of scheelite embedded in it. The scheelite grains range in size from near microscopic to over 2 inches. Bismuthinite is also found in the tactite and much of it is weathered to bismutite.

The mineralogy of the two tactites is interesting as both contain bismuth, but no attempt was made to see if the tactites found in the two mines were from the same source. Minerals found on the Durham-Fernando Mine Dump include anglesite, bismuthinite, and bismutite. Cerussite is the ore mineral mined at the Fernando Mine and occurred as lenses of massive gray material. No crystals were ever noted.

Galena

Galena and pyrite formed the bulk of the original sulfides found in the Fernando Mine but little remains of them. Most have been converted to limonite or cerussite. Occasionally specimens of coarsely crystalline galena are found on the Fernando Mine dump. An assay on this material showed $60.00 a ton in silver.

Scheelite

Scheelite was found in both the Durham and Fernando Mines but was most common in the larger Durham Mine. It occurs as small gray octahedral crystals embedded in tactite with larger yellow 2 cm crystals occasionally found. It fluoresces blue white under shortwave ultraviolet light.

Collecting at Durham-Fernando

The Durham-Fernando Mine has not been worked for many years but is patented ground and as such is private property. The mine still contains tungsten reserves and periodically interest is revived in the property. Generally the area shows no activity and the dump area of the Fernando Mine is a good place to camp.

The Durham-Fernando Mine can produce some very interesting specimens. Scheelite crystals up to an inch embedded in tactite have been found up to an inch in size have been found and the bismuth minerals have never been studied. Specimens are not abundant but well worth the effort.

REFERENCES

Cooper, J.F., Unpublished field notes dated 10/25/64, page 261, 10/27/64, page 266, 12/24/66, page 400.

Dunning, G.E., Moelo, Yves, Roberts, A.c., & Cooper, J. F., Ag-Cu-Pb-Bi Sulfosalts New To Darwin, Inyo County, California. Occasional Web Publication of the Bay Area Mineralogists, 2000.

Hall, W.E., and Mackeverett, E.M., Economic Geology of the Darwin Quadrangle, Inyo County, California, California Division of Mines and Geology Special Report 51, 1958.

Norman, L.A., and Stewart, R.A., Mines and Mineral Resources of Inyo County, California, California Division of Mines and Geology, Vol. 47, No. 1, January 1951.