BUTTE, MONTANA: COPPER MINERALS

Several years ago, on a camping trip heading for the Canadian Rockies, I had the opportunity to visit Butte, Montana, home of the “Richest Hill on Earth.”  If I remember correctly the town was terribly polluted and “strange-looking” water poured into the creeks.  However, it was interesting to visit the Berkeley Pit, one of those giant (1.5 x .5 miles in size; nearly 1800 feet deep) open pit copper mines.  I had lived and worked in Utah and had seen the Bingham Pit several times so was not surprised at the size of the Berkeley. Copper mining started in Butte in 1955 (Anaconda Copper) and ceased in 1982.  Today the pit is filling with acidic water containing lots of nasty stuff like arsenic, cadmium, zinc and copper and in a few years will reach the natural water level (water table).  At that time, as I understand the situation, the nasty water will reverse flow and pollute the ground water and ultimately seep into the surrounding streams.  Federal SUPERFUND money is being used to try and slow/stop the upcoming environmental disaster.

Berkeley Pit from the air.  Public Domain Photo.

Mining at the “Richest Hill on Earth” did not always center on copper as placer gold was the draw after the precious metal discovery in the early 1860’s.  By the 1870’s silver made millionaires out of several miners and capitalists as a spur railroad from Utah reached the area and supplies became less expensive—and silver became more lucrative.  It is interesting to note that before the arrival of the Utah and Northern Railroad the metallic ore was shipped to Swansea, Wales for smelting—now that was a long haul by wagons, railroads, and ships!   By the 1880’s copper was king (with gold and silver as a byproduct).   By the 1890’s the Butte area was producing about 25% of the world’s copper (~50% of the U.S copper).  By the middle of the 20^th century open pit mining replace the underground mines and a much lower ore grade (less copper) became profitable.  However, in the 1970’s “cheaper” copper became available from localities like Chile and Mexico and the Butte mines “almost” became extinct. Today “Montana Resources operates an open pit copper and molybdenum mine.  The operation comprises the Continental mine, crushers, and a concentrator facility where tons of raw ore are processed into high-quality metal concentrates…Montana Resources employs approximately 350 people.”

Postcard ca. 1917.  Public Domain photo.

The Boulder Batholith (batholith: large and complex igneous intrusive structure; usually contains a number of smaller intrusive structures called plutons) in southwest Montana (around the Butte area) contains a very large pluton (Butte Granite but actual rock is a quartz monzonite) and “ approximately a dozen contiguous smaller plutons generally considered to be part of the Boulder Batholith, and numerous isolated small stocks and plugs that are especially abundant north and east of the Butte Granite.”  The emplacement of these plutons into rocks ranging from Precambrian to Mesozoic in age …”are probably byproducts of subduction-related processes, including back-arc magmatism that prevailed along the west edge of the North American plate during” the Late Cretaceous (77.6-73.7 Ma) (above from du Bray and others, 2009).

Meyer (1968) described the major ore deposits near Butte as the result of hydrothermal action along faults, fissures and fractures with most minable ore found in the hypogene although there is some supergene enrichment.  Any major oxidation zone, with associated minerals, seems absent. 

Rusk and others (2008) noted that the hydrothermal mineralization seemed to come from a single major event, and originated at a very great depth.  Any variation in the ore mineral assemblages was probably the result of the hydrothermal solutions reacting with the wall rock at different temperatures and pressure.  Several hydrothermal events seem absent. 

As I understand it, the primary hypogene minerals in ore deposits form from ascending hydrothermal solutions heated by the magmatic intrusions.  Supergene enrichment is caused by descending surface waters oxidizing primary ore deposits near the surface and redepositing metals below the ground water table. The oxidation zone minerals also react with surface waters but are deposited above the water table. 

The primary ore minerals (hypogene) are sulfides—a metal cation(s) bonds with the S^— (sulfur) anion.  In the enrichment zones of the supergene and oxidation, the sulfides are “oxidized” and the metal cation(s) now bond with: oxygen (oxides: O^--), oxygen and hydrogen (hydroxides: OH^-), sulfur and oxygen (sulfates: SO₄^-), and carbon and oxygen (carbonates: CO₃^--).

The major copper ore minerals (hypogene) at Butte are chalcocite (Cu₂S), bornite (Cu₅FeS₄), and enargite (Cu₃AsS₄).  Covellite (CuS), digenite (Cu₉S₅), djurleite (Cu₃₁S₁₆), and colusite (Cu₂₆V₂(As,Sn,Sb)S₃₂ are minor sources.  Other ores (silver) include supergene native silver, acanthite, stephanite, proustite, and pyrargyrite while molybdenite is “native.”    Minor zinc comes from supergene sphalerite. Other collectable supergene minerals include rhodochrosite, rhodonite, tennantite (Meyer, 1968). 

Silver (Ag) wire.  Width ~7 mm.

Sphalerite (ZnS) crystals. Maximum width ~2.6 cm.

A 2002 issue of the Mineralogical Record (Jenkins and Lorengo) was devoted to Butte minerals and listed 130 total minerals.  Previously Meyer and others (1968) had noted 46 supergene and 65 hypogene minerals. 

Massive bornite. Width ~3.4 cm.

Over the years I have acquired a small number of specimens from mines around Butte:  the copper minerals, colusite, chalcocite, enargite, covellite, digenite and bornite; and silver and sphalerite.

Bornite, a copper-iron sulfide (Cu₅FeS₄), is one of the hypogene ores in the Butte District, and is a major ore of copper in many porphyry copper deposits.  Interestingly, its buddy in many deposits, chalcopyrite (CuFeS₂), seems not a major copper ore at Butte.  The bornite specimen is my collection came from the Leonard Mine and is massive, somewhat granular, without good crystals (although there may be a few poorly defined blocky cubes).  The color is sort of a dark purple-black and there are a few iridescent surfaces; it would not be good peacock ore (see Blog    ).  The hardness (Mohs) of around ~3 allows it to be easily scratched by a steel knife blade.  It seems rather brittle and some fresh surfaces display a poorly-defined conchoidal fracture. Crude, sooty black chalcocite crystals.  Small covelite crystal shown below. Length (up-down) ~2 cm.

Crude, sooty black chalcocite crystals.  Small covellite crystal shown below. Length (up-down) ~2 cm. 

Chalcocite is a sulfide mineral (Cu₂S) and primary disseminated grains and/or massive grains are common in hypogene rocks at all levels at Butte, and is an important copper ore.  However, Butte chalcocite also was deposited as a secondary mineral in thick, rich copper beds and may have altered from bornite, and in some localities replaced pyrite.  Apparently, rich solutions deposited the secondary chalcocite in fissures and cracks and the mineral is the major copper ore (~60%) at many Butte mines.  The secondary chalcocite usually occurs as “sooty” black, crude crystals, and sometimes as actually replacing sulfides such as pyrite and enargite.

The secondary chalcocite in my collection is composed of the “sooty” black crystals that seem crudely formed.  It is soft at ~2.5-3.0 (Mohs) and has a gray-black streak.  The grains are associated with covellite. 

Photomicrograph, top of a covellite crystal composed of stacked plates.  With quartz (white) and chalcocite (dark).  width of crystal ~4 mm.  Material "under" the C is foam.

The occurrence of covellite (CuS) at Butte is similar to chalcocite in that disseminated grains/clusters/crystals are primary while secondary crystals are altered from bornite (most likely).  Covellite is not a major copper ore at Butte, nor elsewhere, but the Butte mines seem a major source of crystals produced for the specimen market.  Whereas Butte chalcocite seems unexciting (as a specimen mineral), covellite forms nice and “pretty” crystals.

Specimen with massive covellite with nice pyrite crystals and minor quartz.  Length ~3.7 cm.

Photomicrograph. Covellite and pyrite crystals.  Width FOV ~1 cm.

Photomicrograph covellite crystals with reflecting faces with pyrite crystals.  Width FOV ~1 cm.

The copper sulfide crystals are quite soft (~1.5-2.0 Mohs) and form thin platy hexagonal crystals.  The color is usually an indigo-blue or blue-black and commonly iridescent.  Crystal faces often reflect light and appear as deep red crystals or brassy yellow (the color of crystals on my specimen).  The only locality information is “Butte District.”
 

Small brittle fragment of massive digenite (blue-black) with a vein of brassy pyrite.  Width of specimen ~1.1 cm.  Collected at Leonard Mine, Butte District.

Digenite (Cu₉S₅)  is an interesting mineral first described from Germany in 1844 but rather uncommon in collections.  It is associated with hydrothermal action and can either be primary (hypogene) or secondary.  At Butte digenite is primary but in other localities it is often located near the intersection of the hypogene and supergene.  It may be replaced by chalcocite in the higher supergene.  As I understand it (and many times that is a stretch) digenite almost always contains some iron, and is usually not an important ore of copper.  Crystals are rare and most specimens are fairly small masses showing conchoidal fractures.  The specimen color is sort of a blue-black and have a metallic/submetallic luster.  Digenite is fairly soft at  2.5-3.0 (Mohs).  Small brittle fragment of massive digenite (blue-black) with a vein of brassy pyrite.  Width of specimen ~1.1 cm.  Collected at Leonard Mine, Butte District.

Striated crystals of enargite.  Width ~2 cm.

 

Mass of enargite crystals with pyrite and quartz. Width of specimen ~4.7 cm.
Enargite (Cu₃AsS₄) is a copper sulfide with arsenic added to the chemistry.  Copper and arsenic are commonly found together and the result is a fair number of copper arsenate minerals (see previous Blogs).  Tabular to prismatic, iron-black to gray-black, crystals are common and one key to identification are the striations present on the crystals. The crystals really do not show much of a distinct termination and essentially are flat faces.  The luster seems more metallic than minerals like chalcocite but the hardness is similar (~3.0 Mohs).  It has a black streak and twinning is common. Enargite is almost always a primary hypogene mineral and is an important copper ore at Butte.

Colusite with quartz.  Width specimen ~ 3.7 cm.

Photomicrograph colusite with quartz.  FOV width  ~1.0 cm.

Colusite (Cu₂₆V₂(As,Sn,Sb)₆S₃₂, is  a primary hydrothermal mineral at Butte and is intimately associated with chalcocite and enargite. This copper-vanadium sulfide is the only mineral that has the Butte District as its type locality---the Colusa claims.  Other than its relative abundance at Butte, colusite is a rather rare mineral.  My specimen, from the East Colusa Mine, is granular to massive with only a few poorly developed crystals—typical of the region.  It has a metallic luster with a bronze cast and a black streak.  Like most of the copper minerals, it is rather soft at 3.0-4.0 (Mohs).

This small article has been a rather simplified version of the copper minerals in the Butte District, Silver Bow County, Montana.  In actuality, the geology and mineralization at Butte is exceedingly complex with much zoning, and differential mining areas.  For an old soft rock stratigrapher like me, trying to understand the ore mineralization is quite difficult.  Hopefully, I was able to offer a brief synthesis that did not stray too far from the truth!

REFERENCES CITED

du Bray, E.A., K. Lund, R.I. Tilling, P.D. Denning, and E. DeWitt,  2009, Geochemical Database for the Boulder Batholith and its Satellitic Plutons, Southwest Montana:  U.S. Geological Survey Digital Data Series 454, 1 CD-ROM. MONTANA CONNECTION

Jenkins, R. E. and J.A. Lorengo, 2002, Butte, Montana Minerals, Mines and History: The Mineralogical Record, v. 33 no. 1.

Meyer, C., E.P. Shea, C.C. Goddard Jr., and Staff, 1968, Ore Deposits of Butte, Montana in Ore Deposits of the United States, 1933-1967: Graton-Sales Vol. 1, New York, AIME, v. 2.

Rusk, B. G., M.H. Reed, and J. H. Dilles, 2008, Fluid Inclusion Evidence for Magmatic-Hydrothermal Fluid Evolution in the Porphyry Copper-Molybdenum Deposit at Butte, Montana: Economic Geology, v. 103, n. 2.

MONTANA CONNECTION

My mother was born in 1913 and lasted for 94 years.  As a child, ~1919, her parents (along with five children) migrated from Kansas to Montana in order to “homestead’ on land.  They settled on a “ranch” near Winifred north of Lewistown.  By all accounts it was not a successful venture!  My grandfather was a poor Kansas “dirt farmer” and this Montana land was not suitable for crop farming.  According to my mother, her father had a case of “wanderlust.” Before Montana they tried living in east Texas---in a tent.  At least in Montana they were able to construct some sort of crude “log cabin.”  She remembered receiving a nickel and a banana for Christmas.  Having never seen the fruit before, several kids promptly chomped it down---skin and all. One child lost his nickel in the snow.  It was not a “merry” Christmas.  In the early-1920’s my grandmother fell off the horse and developed a compound fracture of the right leg.  With my grandfather away working in a wood mill (or cutting lumber), the young children removed the wooden “storm cellar” door, hauled it in a wagon to the pasture where my grandmother lay, loaded her on the door into the wagon and hauled her to town for a doctor/nurse.  That seemed to be the end of the homesteading adventure and back to Kansas they retreated. I come from a family of tough adventurers! 

In the early 1990’s the youngest sibling (too young for the Montana adventure) hauled my mother and her oldest sister up to the old homestead “looking for the lost nickel.”  The coin was never located; however, the old “log cabin” was still standing and being used for a storage shed on a modern ranch. 

Main Street, Winifred, Montana.  Public Domain photo.  

Milwaukee Railroad Depot, Winfred, Montana.  Winifred was founded in 1913 as the terminus of the Chicago, Milwaukee, St. Paul and Pacific railroad.  Photo from montanatom1950 at Flickr.