MINERALS FROM THE METALLIC ORE DEPOSITS OF THE AMERICAN SOUTHWEST

 
I recently had the opportunity to attend the annual symposium sponsored by the Friends of Mineralogy-Colorado Chapter, the Colorado School of Mines Geology Museum (CSMGM), and the Friends of the Colorado School of Mines Geology Museum.  The meeting, held on the campus of School of Mines, was entitled Minerals from the Metallic Ore Deposits of the American Southwest.  Rather than focus just on the ore minerals, the speakers presented quality information about the accessory minerals present at the mines.  All speakers had tremendous photos (including many photomicrographs) of both the ore minerals and the accessory minerals.  The meeting was well planned, parking (on the weekend was free) was available, lunch was close by, break snacks were plentiful and tasty, and the camaraderie among attendees was quite evident.  As a soft rock paleo person, I certainly enjoyed learning from mineralogy professionals and experienced rockhounds with much more mineral knowledge than is stuffed in my noggin. 

Mark Ivan Jacobson, one of Colorado’s pegmatite specialists, served as meeting moderator and introduced Ed Raines, a wonderful and informative storyteller who was the perfect speaker to lead off the sessions.  Ed is currently the Curator and Collections Manager for the CSMGM and has a wealth of information he is willing to share about Colorado mines and minerals.

To begin his presentation, Frontier Mining Methods, Ed gave a little history and terminology review for the non-miners like me.  I finally learned something about stopes, winzes and adits. The following diagram was nabbed from the Internet (imagersizetool.com) but does not approach the more detailed diagram featured in his talk; however, it gives some understanding to the complex terminology.

I was fascinated with the history of mining in the southwest but what stuck in my mine was how dangerous hard rock mining was, and probably still is! In order to drill holes for setting the charges (to “blow off” chunks of rock), single miners used a steel star drill (the jack) and whapped it with an 8-pound sledge hammer. They then rotated it a quarter turn and whopped it again.  This was a slow process, so someone decided teams of two men (females were never allowed in the mine) might be able to drill holes at a faster rate.  So, the shaker held the star drill while the hammer man hit it with a larger sledge hammer, maybe 16 pounds.  At times, there were two hammer men taking alternate turns whopping the drill—all of this activity took place with the light furnished, in the early days, by candles.  I presume, but am uncertain, that newly hired miners served as the shakers!  Got to trust the hammer man!!

Two miners with a star drill punching holes for the charges. Drawing courtesy of nevada-outback-gems.com

A miner operating a pneumatic drill punching a hole for charges.  Notice a single hose to the drill.  Photo courtesy of onlyinyourstate.com.

The next step in hard rock mining involved replacing the double jack miners with pneumatic drills powered by compressors supplying air through hoses.  The pneumatic drills allowed the bit to hammer, rather than turn, and this action produced a large about of dust that was inhaled by the miners and often produced a disease called silicosis. Nasty stuff that killed many miners.

A much large pneumatic drill.  Notice the screw brace in the bacground holding up ceiling.  Photo courtesy of USGS.

This pneumatic drill added a second hose to supply water. Notice the extra drill bits leaning on wall.  Photo courtesy of westernmining history.com.

Eventually someone thought of putting a hole through the center of the drill iron and pumping water to mix with the dust and produce a slurry. This action created quite a mess in the mine, but it did control the dust and silicosis.

After the charges and dust had settled down muckers came in and loaded the fragments of rock and pushed (later mules were used) the ore cars to a dump station where it was transferred to locations where the metals were extracted. 

Ore muckers with their cart.  Probably a posed phto before starting work as their clothes are clean.  Photo courtesy of miningartifacts.org.

I have not collected many minerals from New Mexico and therefore knew very little about the Cook’s Peak locality in Luna County, New Mexico, part of the Basin and Range.  Phil Simmons seemed quite excited about the gangue minerals he is collecting from the old lead-silver-zinc mines ($4.2 million), especially the fluorite.  A recent discovery of sidwellite, a rare molybdenum oxide [MoO₃-2H₂O], is also noteworthy.

Since I have a great interest in minerals from Utah, I especially appreciated Brent Thorne’s presentation on the secondary minerals found in the Tintic (copper, lead and gold) and Ophir (lead and zinc) mining districts.  Brent is a fantastic photographer (3,800 photos on MinDat.org) and specializes in photomicrographs of some pretty exotic and rare minerals (discovered and co-discovered 15 new mineral species).  His photomicrographs were of specific minerals scaled in microns (1000 microns are equal to 1 mm).  Unfortunately, my digital camera cannot come close to producing photomicrographs of this scale!

The geology of the Mining Districts is related to several large volcanoes that erupted in much of western Utah during the early Oligocene (Hintze and Kowallis, 2009) and covered Paleozoic rocks that were folded and faulted by an earlier mountain building event termed the Sevier Orogeny (Cretaceous).  During the later Oligocene, these volcanoes begin collapsing and large calderas formed.  The hydrothermal solutions associated with the volcanics followed the cracks and faults in the Paleozoic rocks and helped dissolve portions of the limestones.  As these solutions cooled the minerals begin to crystallize forming the ore bodies in the host rock.

One of the smaller mines in the Tintic District is the Carissa, a mine found on the slope of Mammoth Peak, home of the well-known Mammoth Mine.  It was connected by a tunnel to its more productive neighbor, the Northern Spy Mine. Carissa may not have been a large gold-silver producer; however, it was, in later years, a specimen producer of very nice crystals of the arsenates: adamite [(Zn,Cu)₂AsO₄OH], conichalcite  [(CaCuAsO4(OH)], mimetite [Pb₅(AsO₄)₃Cl], olivenite [CuAsO4(OH)], mixite [Cu₆Bi(AsO₄)₃(OH)₆-3H2O)], and the copper carbonates rosasite [(Cu,Zn)₂(CO₃)( OH)₂] and azurite [Cu₃(CO₃)₂(OH)₂].  All of these minerals are secondary and found in the oxidation zone where primary lead (argentiferous galena), zinc (hemimorphite?), bismuth (bismuth) and copper (copper, cuprite, enargite) were present.  The enargite [Cu₃AsS₄] could also have provided the arsenic for the arsenate (AsO₄) ion in the secondary minerals.

The copper zinc carbonate, rosasite (R) and the arsenate mixite (M) from the Tintic district..  FOV ~ 1 cm.

The most interesting specimen mineral collected from the Carissa, as least to me, is the rare copper bismuth arsenate named mixite.  Essentially a micromineral, mixite occurs as very tiny, slender, acicular needles that often congregate together in tuffs or radial sprays. Although the crystals are usually some shade of green to blue-green, occasionally they are white to light blue.  Individuals appear to have an adamantine luster although this is a difficult call. The tuffs are silky in nature. Crystals belong to the Hexagonal System and appear to be translucent to transparent.  Hardness is listed as 3.5-4 although that is tough for me to determine.

Tiny tuffs of mixite (M) along with hemimorphite (H) and goethite (G). FOV ~ 1 cm.

Microcubes of Blanchard  Blue. FOV ~1.2 cm.

 Erin Delventhal presented Dealers offering minerals “for sale” in many Colorado shows often display numerous chunks of fluorite. More than likely the larger hunks are labeled “Blanchard Blue” the signature mineral mined from the Blanchard Mine, Hansonburg District, Socorro County, New Mexico,information on the geology of the District (Pennsylvanian rocks on top of Proterozoic granite and gneiss), and the many secondary and gangue minerals of interest to collectors.  It seems that hard rock miners could not make a living producing lead from galena; however, as a specimen mine the place is a tremendous producer best known for the fluorite [CaF₂] and the world’s largest known linerite crystals [PbCu(SO₄)(OH)₂].

Penetrating twins of galena, Royal Flush Mine, Hansonburg District, New Mexico. FOV ~2 cm.

Cubes of fluorite, Royal Flush Mine Bingham, New Mexico. FOV ~11 cm.

 

 

 

Crystals of blue linerite and a spray of green brochantite. FOV ~1 cm (photomicrograph).

A "bundle" of brochantite fibers on linerite.  Width of photomicrograph ~3 mm.

Sprays of brochantite on quartz terminations (photomicrograph). Width FOV ~1 cm.

Robert Larson, a long-time geologist with much experience in southwestern Colorado, enlightened the audience with tales of exploration in the base metal mines of the San Juan Mountains.  Robert explained how many rockhounds concentrated their searches for metallic minerals and overlooked the many accessory minerals available in the mine dumps.  Some localities in the San Juans are famous for their pink specimens of a manganese carbonate, rhodochrosite [MnCO₃], and a manganese silicate, pyroxmangite [MnSio₃] (formally known as rhodonite).  We also learned  how the Sunnyside Mine, a rich gold mine, extended under Lake Emma—much to the apprehension of the knowledgeable miners.  On Sunday, June 4, 1978, when all miners were off work, Lake Emma broke through a glacial flour plug into the mine and the resulting “toilet flush”, completely emptied the Lake (but without loss of life).

The Creede area in the San Juan Mountains is famous for free silver, especially wire silver.  Width of specimen ~1 cm.

 

 

Translucent green fluorite, Sunnyside Mine Group. Width ~2.9 cm.

 

Rhodocrosite, Sunnyside Mine Group. Width large section ~9 mm.

 

Gold, quartz and sphalerite?, Silverton, Colorado. FOV ~1 cm.

Not many members attending the Symposium have visited the Republic of Kazakhstan, the largest country in central Asia!  However, Bob Embry, a mining engineer, spent several years of his career working there.  He also had time to explore the mines of the Magdalena District near Socorro, New Mexico, and presented information about mineral collecting, especially at the Kelly Mine.  Although the Kelly was originally mined to produce gold, silver, copper, zinc and lead, it is best known to modern rockhounds as a specimen mine.  Coloradans are very familiar with the Kelly as many groups have taken field trips to the area to collect the zinc carbonate, smithsonite [ZnCO₃]. 

Botryoidal smithsonite from the Kelly Mine.  FOV ~2.1 cm.

The Carlin Trend in east-central Nevada is one of the largest gold- producing areas in the world and the top producer in the United States.  According to presenter Jeffrey Blackmon, the Trend has produced over 100 million ounces of gold with another 30 million in reserve.  The Carlin Trend has carbonaceous limestone-hosted gold supplied by hydrothermal action.  Most of the gold is dissolved or disseminated in pyrite and/or arsenopyrite and is invisible to the naked eye.  Blackmon noted that over 100 species of microminerals have been identified from Carlin Trend rocks; however, minerals favored by rockhounds include stibnite (antimony sulfide), cinnabar (mercury sulfide), barite (barium sulfate), realgar (arsenic sulfide) and orpiment (arsenic sulfide created by decay of realgar).  Since Carlin Trend rocks are enriched in arsenic, antimony, mercury, barium and thallium, prospectors use these elements as indicators in their hunt for new gold deposits.  In fact, the Carlin gold deposits have supplied the name Carlin Type Deposits to several other similar gold producing areas, both in the United States and internationally. 

Carlin Type Deposits in northern Nevada. Map courtesy of USGS.

 

 

 

Carlin Type Deposits are enriched in arsenic.  This specimen of realgar (arsenic sulfide)was collected from the Getchell Mine located in the Getchell Trend (see map).  The longest crystal in the photomicrograph in ~1 cm in length.

 Karen Wenrich gave a quite technical presentation on the importance of Rare Earth Elements (REE) and uranium oxide in these rather strange/interesting solution collapse features found in northeastern Arizona.  These small “sink holes” (maybe 300 feet in diameter) can extend for hundreds of feet in a vertical direction.  She explained the collapse breccia inside the pipes perhaps contain 40% of the nation’s known uranium resources.  In addition, many pipes are rich in REE that could play a critically important role if supplies from China (producing over 90% of the world’s supply and controlling the market prices) became non-importable.  It seems as if these pipes really do not contain minerals of much interest to rockhounds.

Cross section of a "typical" breccia pipe in the Grand Canyon region.  Public Domain photo provided by the USGS from a paper authored by Warren Finch (2004).

After a long, but interesting day, the session adjourned and migrated to a nearby Inn for refreshments, the banquet, and a verbal auction of deaccessioned CSM minerals.

Sunday morning brought back storyteller Ed Raines with a presentation on the Gilman District north of Leadville, an area well known to many Coloradoans due to environmental concerns widely disseminated in the media.  Gilman was the largest mining area in Eagle County and was first claimed in the 1870s by prospectors looking for similar rock associations (Leadville Limestone cooked by an igneous intrusion) as found at nearby Leadville.  The mines originally produced silver and lead from an argentiferous (silver-bearing) galena.  By around 1900 the oxidized ore was generally “mined out” (or so they thought).  By 1905 production of zinc was going strong from iron-rich sphalerite called marmatite [(Zn,Fe)S] that produced spectacular shiny black crystals.  Today one can easily still acquire specimens of siderite (FeCO₃) marmatite and pyrite from Gilman.  These early mined ore sulfides were mostly flat bodies (mantos) replacing portions of the Leadville Limestone.  In the 1920s rich chimney (vertical) deposits of copper and silver were discovered and mined until production ceased in 1981.  The production of zinc had continued until 1974. Production over the years totals nearly 400,000 ounces of gold, 67.6 million ounces of silver, 212 million pounds of copper, 317 million pounds of lead, and 1.868 billion pounds of zinc, from 13.1 million tons of ore (Rodabaugh and others, 1968; Smith, 1977).

Argintiferous galena on siderite, Eagle Mine, Gilman District..  Width specimen ~4 cm.

Siderite and pyrite, Eagle Mine, Gilman District. Width ~ 4.6 cm.

Sphalerite v. marmatite with minor siderite. Gilman District.  Width ~5.1 cm.

The second presentation of the morning again moved south to New Mexico basin and range country when Michael Michayluk talked about his collecting at mines situated along the Torpedo-Bennett Fault Zone in the Organ Mountains.  These mountains are Tertiary volcanic and intrusive igneous rocks that were once part of an active caldera.  In fact, these rocks are all that remain of the volcano that blew its top.  The mineralization seems to occur, as hydrothermal replacements, along this fault zone where Precambrian granites are next to Paleozoic sedimentary rocks.  The ore production, silver, copper and zinc, was not spectacular in tonnage. Michayluk noted that the mines are most famous for “exceptional wulfenite specimens” although an entire suite of secondary and gangue minerals are present.  In fact, he showed several really nice photomicrographs of the secondary minerals collected from three mines along the fault zone.  In reality, I knew very little (perhaps nothing) about the area until listening to the presentation. I did go back to my collection to check on any wulfenites but only located hemimorphite.

Cluster of hemimorphite crystals each about 6-7 mm in length. Torpedo-Bennett Fault Zone.

 Pete Modreski came in and previewed a few minerals from Arizona (perhaps Arizona was slighted at the Symposium?) and then reviewed New Mexico districts and mines that “are of the most mineralogical interest.” He presented information about the important metallic ores from each district/mine along with the collectable secondary and nonmetallic gangue minerals.

I will throw in one of my photomicrographs from the Rowley Mine in Arizona: a single crystal of wulfenite (lead molybdate) along with acicular mimitite crystals (lead arsenate chloride). The length of the wulfenite crystal is ~2.5 mm.

Mark Jacobson capped the presentation portion of the symposium by regaling information about three gentlemen who, in the middle and late 1800s, played a critical part in expanding “our knowledge of minerals and mineral localities in Colorado”: 1) J. Alden Smith, Territorial and State Geologist; 2) Frederic Miller Endlich, helped develop mines in the San Juan Mountains; and 3) Jesse Summers Randall, newspaper writer and owner who advocated for development of mines around Georgetown. Jacobson noted that “most of the mineral localities that clubs collect at today were either exploited or documented by these three men.”  That fact seems quite an accomplishment.

The Symposium concluded with a choice of field excursions to either the Phoenix Gold Mine in Idaho Springs or the Hidee Gold Mine in Central City.  Previous commitments did not permit my attendance.

REFERENCES CITED

Hintze, L.F. and B.J. Kowallis, 2009, Geologic History of Utah: Brigham Young University Geology Studies, Special Publication 9.

Radabaugh, R. E., Merchant, J. S., and Brown, J. M.,1968, Geology and ore deposits of the Gilman (Red Cliff, Battle Mountain) district, Eagle County, Colorado, in Ridge, J. D., editor, Ore deposits of the United States, 1933-1967 (Graton-Sales volume): New York, American Institute of Mining Engineers.

Smith, D.A., 1977, Colorado Mining-A photographic history: Albuquerque, University of New Mexico Press, 176 p.