BUTTE

A WORLD CLASS ORE DEPOSIT

Steve J. Czehura, Montana Resources, LLP, Butte, MT

Abstract mining of the early porphyry style mineralization has

sustained production over the past twenty years and mining
is expected to continue in the Continental pit with favorable
The Butte quartz monzonite is the host rock for early prices. Future mining will eventually develop a 500
porphyry copper style mineralization and subsequent million-ton geologic resource incorporated in a supergene
mesothermal-epithermal, enargite-gold style veining that enrichment blanket situated between the Berkeley pit and
transects the district. The east-west Anaconda system veins: the Continental pit. Grades in this enrichment blanket range
including the Emma, Anaconda-Original, Syndicate, in the neighborhood of 0.5% copper.
Badger-State, and Alice-Rainbow veins, each with strike
lengths of over 12,000 feet and vertical continuity in excess
of 4,500 feet, along with mining widths from 5 to 50 feet, Geologic Setting
are unique to Butte. The incredible dimensions and tenor of
these veins have yet to be surpassed and Butte is still touted
as being the “Richest Hill on Earth.” The Butte Mining District, spanning an area of some
The Anaconda Company was formed in 1895 to 25 square miles, is located in the southern portion of the
develop underground copper reserves in the district and Boulder batholith, a multiphased intrusive complex situated
eventually consolidated all operations under one company. in southwestern Montana. The dominant phase, the Butte
Production from underground mining continued through quartz monzonite (BQM), has been dated at 75.6 Ma and is
the mid-1970’s. Open pit mining in the Berkeley pit began a medium to coarse grained rock composed of 40%
in 1955 with the extraction of supergene ores, along with plagioclase, 20% quartz, 20% orthoclase, and 20%
the remnants of deeper horsetail zones inaccessible to the hornblende and biotite.
underground mines. Operations in the Berkeley pit were Although of least importance, aplite dikes and sills are
suspended in mid-1982 because of the rejection of high common throughout the district; but often, where they are
arsenic, copper concentrates at major smelting facilities intensely altered by hydrothermal fluids, they are mistaken
throughout the world. Bulk mining of porphyry style for quartz porphyry (QP). Aplite dikes and sills are
mineralization in the Continental pit began in January 1980 contemporaneous with the host rock and are cut by all other
to supplement copper production while stripping “C” features.
pushback in the Berkeley pit. The Anaconda Smelter closed Near surface, in the area of the Berkeley pit three sub-
in September 1980. A molybdenum circuit was added to parallel, E-W trending, steeply south dipping, sets of quartz
the Butte Concentrator in 1981 and mining continued in the porphyry (QP) dikes, 10 to 50-feet wide, transect the
Continental pit to produce copper and molybdenum district and cross the Continental Divide. With depth in the
concentrates. A blend of Berkeley and Continental ores Berkeley pit, multiple QP dikes have been mapped and in
were processed to dilute penalty metals in the final the area of the Continental pit at least three, through going
concentrates marketed to toll smelters. All mining dikes, are recognized along with a myriad of irregular
operations ceased mid-1983 when Anaconda closed the segments. In general, the dikes are all hydrothermally
Continental pit and offered the property for sale. altered and silica flooding extends outward in an envelope
Montana Resources reopened the Continental pit in in excess of 50 feet per side. Usually, the dikes terminate
June 1986 to produce copper and molybdenum upward with a breccia cap rafted in a biotite, k-silicate,
concentrates. Operations were suspended mid-2000, magnetite, and chalcopyrite matrix. Fine grained, biotite
because of exorbitantly high power prices, but resumed in breccias of similar composition, often, extend further
November 2003 with rising metal prices. Bulk open pit upward as the fissures narrow. All of the QP dikes appear

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to be of a similar composition and evidently, originally, rhyolite dike indicate vertical movement on the Continental
contained 30-40% phenocrysts of plagicoclase, biotite, and fault to be in excess of 3,500-feet.
quartz with megacrysts of k-feldspar. Remnant quartz eyes The Big Butte rhyolite plug, a conspicuous “cone
are distinctive features where the original textures have shaped butte” north and west of the city, is a younger, post-
been obliterated by hydrothermal alteration. Dating of mineralization, feeder that gave rise to extrusives on the
micas in the QP dikes indicates cooling ages between 64.5 west side of the district. Dating indicates emplacement at
and 63.1 Ma suggesting that they are closely related to both 51.5 Ma.
pre-Main Stage and Main Stage mineralization (Snee et al,
1999). In “C” pushback along the north highwall of the
Berkeley pit the northern most dike, the Modoc quartz Pre-Main Stage Mineralization
porphyry, is unique in that it incorporates an E-W,
elongated, body of igneous breccia that contains fragments
of earlier mineralized and altered QP. Pre-Main Stage, chalcopyrite (cp) and molybdenum
Immediately north and parallel to the Anaconda- (mb) mineralization is elongated along a S70E trend and
Original vein, underground workings have delineated a straddles the QP dikes at depth extending some four miles
post-mineralization rhyolite dike that, likewise, transects from the Steward shaft east to the Klepper fault.
the district trending E-W, dipping steeply to the south. On Molybdenum mineralization occurs below the 2000 Level
the east side of the district in the foothills at the base of the beneath the Berkeley pit and is exposed at surface in the
Continental Divide, it is exposed at surface in the hanging Continental pit east of the Continental fault. A deep
wall of the Continental fault. This dike dated around 58.8 diamond-drilling program completed by the Anaconda
Ma (Martin et al, 1999) cuts Main Stage veins as well as Minerals Company, between 1978 and 1981, provided
the quartz porphyry dikes and marks the end of multiple geologists with valuable data regarding the origin of this
thermal events that mineralized the district. Offsets in this unique mineralizing system.

Figure 1. Pre-Main Stage Geology after Reed (1981) and Dilles (2000, 2001, 2004).

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 Figure 2. Steward –Continental cross section, looking northeast, after Vian (1980), Reed (1981), and Dilles (2001).

pre-Main Stage, cp-mb, porphyry style, mineralization

characterized by swarms of interlacing, half-inch, veinlets
along with disseminations of chalcopyrite in mafic sites
along micro-fractures. Separating the two domes is a large
bulbous plug of hard (HGS), gray quartz (qz), sericite (ser),
and pyrite (py). Drill Hole No. 7 collared in the lower truck
parking lot in the Berkeley pit explored this mass to depths
below sea level. In fact, the peninsula left in the bottom of
the Berkeley pit between the lower parking lot and just
south of the Leonard Shaft was configured to avoid mining
this hard waste zone. The earliest veins in the porphyry
domes (Figure 3) are known as Early Dark Micaceous
(EDM) veins.
EDM veins are generally only about a half-inch wide
and are enclosed in an envelope characterized by green
biotite and k-silicate alteration, one-half to one-inch per
side. These irregular qz-cp-py veins, veinlets, and crackles
Figure 3. Pre-Main Stage zoning model for the are, usually, very abundant and carry chalcopyrite in their
Pittsmont dome (looking northeast), a composite of several alteration envelope. As these veins extend outward they
sections after Vian (1980) and Reed (1981). carry quartz, chalcopyrite, and magnetite. Typically, EDM
veins are characterized by an inner envelope of green
In total some 60,000 feet of drilling was completed sericite followed by an envelope of green biotite with
with ten holes probing to depths up to 7,000 feet below the disseminated chalcopyrite.
surface. From this data, along with information from Later sets of veins and veinlets consist of quartz,
underground and surface mapping, geologists have molybdenite, rare k-spar, and have no alteration selvage.
identified two centers, or domes (Figure 1 and Figure 2), of
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These are also abundant and are often rebroken by Main Expanding further outward, the Intermediate Zone is
Stage veins in the Steward-Anaconda dome and by quartz characterized by ore shoots of en/cc and/or bn/cc with
and pyrite veins and veinlets, contemporaneous with Main increasing sphalerite (sl) being precipitated outward, as the
Stage mineralization, in the eastern Pittsmont dome. mineralizing fluids were cooled and neutralized. Mineral
Exposures of these veinlet swarms in the up thrown limb of ratios, en/cc, decrease outward and upward. The presence
the Pittsmont dome can be seen in the eastern highwall of of sl with hypogene cc is indicative of the Intermediate
the Continental pit trending N50E, dipping steeply to the Zone. Although chalcopyrite (cp) often dominates the vein
south. mineralogy near the transition boundary, in the Peripheral
Zone copper minerals are largely absent. Here manganese
minerals, rhodochrosite and rhodonite, along with
Main Stage Mineralization sphalerite (sl) dominate the vein mineralogy. Galena and
pyrite are common.

Main Stage veins with persistent ore shoots are found

above the Steward-Anaconda dome and appear to be Alteration
emplaced in, roughly, conjugate sets of steeply dipping
structures striking N60-80E and N40-50W. The Anaconda
system veins trend N70E on the west side of the district and Wall rock alteration in the Butte district (Figure 4)
rotate slightly toward the south as they approach the center reflects thermal cooling, as well hydrogen ion activity and
of the district, in the vicinity of the Leonard shaft. Here, potassium ion metasomatism. Deep within the mine
along the Leonard-Belmont axis closely spaced SE striking, complex, haloing the early porphyry style mineralization,
en echelon, fractures give rise to the famous horsetail all hornblende laths, typical of the unaltered host rock, have
zones. Continuing eastward, these veins and horsetail zones been altered to a mossy, fine grained, biotite. This is the
die out against a massive bulbous, plug of qz-ser-py most outward expression of the early, massive porphyry
alteration. Why these structures are not through going is not copper system identified at depth in two, expansive domes
clearly understood, but Reed (1980) and Reed and Meyer (Brimhall and Roberts, 1973). Locally, vein alteration
(1999) postulate that the ductile behavior of this intensely envelopes, both EDM and sericitic-argillic Main Stage
altered rock mass prevented the development of through envelopes, overprint this pervasive biotitization of the host
going structures. rock.
The conjugate Main Stage veins striking N40-50W are
known as Blue veins. These cut and offset the Anaconda
structures, left laterally, up to several hundred feet. At
depth, these structures often merge with the Anaconda
system veins.
To describe the Main Stage ore geometry of the
deposit, geologists have delineated zoning patterns for the
sulfide minerals. Initially three zones were defined by Sales
(1913) and subsequently Meyer and others (1968) have
further detailed the zoning pattern. The Core of the
deposit, locally peripheral to the Leonard shaft at depth, is
characterized by highly sulfotaric minerals. Here massive
stringers of quartz (qz), pyrite (py), chalcocite (cc), enargite
(en) and/or bornite (bn) with covellite (cv) and digenite
(dg) dominate the vein mineralogy. Deeper in the system,
Main Stage veins carrying qz and tennantite (tn) transect
the Steward-Anaconda dome. Main Stage qz-cp veins are
also present deep in the system along with porphyry style Figure 4. Comparison of Main Stage and Pre-Main
mineralization. Stage alteration.
In general the Main stage veins are zoned both
vertically and horizontally. Concentrically upward and For the Main Stage features, alteration envelopes in the
outward, veins in the Central Zone are dominated by core of the district near the Leonard shaft are characterized
enargite (en), hypogene chalcocite (cc), and bornite (bn). by an advanced argillic assemblage within a zone of
Sphalerite (sl) and manganese minerals are absent. pervasive sericitic alteration. Here inner envelopes along

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Main Stage veins are dominantly quartz, sericite, and needed for steel production in the continental United States
kaolinite along with dickite, alunite, and pyrophyllite. during both World Wars. Likewise, there were production
Topaz and zunyite, although minor constituents, are quotes for copper and zinc to sustain the War efforts.
recognized as important indicator minerals. Typically, Main Metal production during the last twenty years accounts
Stage veins have distinct sericitic alteration envelopes that for about 6 percent of the total copper pounds produced,
grade outward into white argillic alteration (kaolinite, k- but less than two percent of the silver ounces. The
spar, bio-stable), followed by an envelope of green argillic accumulative molybdenum pounds result from ongoing
alteration (montmorillonite, k-spar, bio-stable). Where the mining in the Continental pit (Table 1).
veins are closely spaced the rock can be pervasively altered
as the various envelopes merge. Further out, the sericitic- Table 1. Total metal production compared to that produced
argillic envelopes on individual veins grade into in the last twenty years.
propylitized rock with a distinct greenish color resulting Production Butte Montana
from saussuritized plagioclase grains and altered 1880 - 2004 District Resources
hornblende laths. The presence of epidote distinguishes this Copper lbs 21,554,930,540 1,394,823,528
outer most, often pervasive, alteration. Zinc lbs 4,909,202,540 -
Manganese lbs 3,702,787,341 -
Lead lbs 854,797,405 -
Supergene Enrichment Silver oz 715,340,826 11,541,264
Gold oz 2,922,446 -
Molybdenum lbs 195,380,975 155,796,277
In the area of the Berkeley pit weathering of copper
mineralization produced a leach capping mottled with iron
oxides and secondary green copper minerals overlying a
thick zone of supergene enrichment conforming to Geologic Resource
topography. Enrichment of interlacing pyritic vein swarms
from the qz-ser-py plug, simultaneously, with deep
enrichment along the Main Stage veins, themselves, After over a century of mining, the Butte geologic
resulted in a blanket of rich sooty-chalcocite with molar resource still totals some 5.4 billion tons averaging 0.49
like roots that sustained open pit mining in the early years percent copper, 0.033 percent molybdenum, and 0.140 opt
of production. Roughly, half of Butte’s annual copper silver. The district resource incorporates estimates based on
production came from the secondary chalcocite blanket polygonal estimation, grade blocking, and ordinary kriging
during the first twenty years of mining in the Berkeley pit. (Table 2).
Similar enrichment on pyritic vein swarms has resulted
in a tabular orebody conforming to the paleotopography Table 2. Geologic reserves for the Butte District.
between the Berkeley pit and the Continental pit. This ore % % opt
Orebody Short Tons Cu Mo Ag Estimate
zone is exposed in the west highwall of the Continental pit
overlain by some 100 to 300 feet of leached capping Continental Area 538,844,000 0.26 0.033 0.01 Kriging
followed by 200 to 600 feet of alluvium (north to south,
respectively). The supergene enrichment blanket, Enrichment Blanket 504,992,000 0.48 0.000 0.10 Kriging
characterized by secondary chalcocite replacing pyrite, will
sustain production once copper-molybdenum ore is Pittsmont Dome 2,222,000,000 0.45 0.045 0.12 Polygon
exhausted in the Continental pit.
Anaconda-Steward 2,153,000,000 0.60 0.028 0.20 Grade
Even though a noticeable leached capping has
Dome/Underground Blocking
developed, ores in the Continental copper-molybdenum
orebody are not significantly enriched. Total 5,418,836,000 0.49 0.033 0.14

Included in the underground resource are 130 million tons

Historic Metal Production in a block caving configuration, averaging in excess of 1.00
percent copper, 0.345 opt silver, and 0.005 opt gold. The
Continental and Central Zone orebodies are amenable to
Total metal production reported for Butte places this open pit mining and incorporate the proven and probable
mining district among the top producers in the world. In reserves.
fact, the district produced 98 percent of the manganese

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 Ore Reserves Guidebook of the Twenty-fifth Annual Tobacco Root
Geological Society Field Conference, Butte, Montana
3-6 August, 2000 (TRGS, Missoula, Montana), p. 63-
As of January 1, 2005 ore reserves for the active mine 65.
complex totaled some 406 million tons averaging 0.34 10. Dillenbeck, E., 2001, Butte, Montana [geology]: in
percent copper and 0.027 percent molybdenum with a Unpublished guidebook Northwest United States -
0.074 opt silver credit. Copper and molybdenum are joint Southern British Columbia, Canada, Economic
products. Reserves are reported at a 0.23 percent copper Geology Fieldtrip, CSM Economic Geology Group,
equivalent cutoff. The numbers reported reflect remaining March 10-18, 2001, Section 11, p. 1-5.
proven and probable mining reserves based on the ore 11. Dilles, J.H., M.H. Reed, S. Roberts, L. Zhang, and R.
geometry detailed in a computer block model spanning over Houston, 1999, Early magmatic-hydrothermal features
six square miles. related to porphyry copper mineralization at Butte,
With sustained metal prices open pit mining is Montana: Geological Society of America Abstracts
expected to continue for the next 20 years. with Programs, v. 31, no. 7, p. 380.
12. Guilbert J.M., and L.G. Zeihen, 1964, The mineralogy
of the Butte district, Montana: Presented at the
References Northwest Mining Association Convention, Spokane,
Washington, December 4, 1964, 25 p.
13. Martin, M.W., J.H. Dilles, and J.M. Proffet, 1999, U-
1. Bodnar, R.J., 1999, Fluid inclusion evidence for a deep Pb geochronilogic constraints for the Butte porphyry
origin of porphyry copper-type mineralization at Butte, system: Geological Society of America Abstracts with
Mt: Geological Society of America Abstracts with Programs, v. 31, no. 7, p. 380.
Programs, v. 31, no. 7, p. 381. 14. Miller, R.N., Ed., 1978, Guidebook for the Butte field
2. Brimhall, G.H., Jr., 1977, Early fracture-controlled meeting of the Society of Economic Geologists: SEG,
disseminated mineralization at Butte, Montana: USGS, The Anaconda Company, August 18-21, 1973,
Economic Geology and the Bulletin of the Society of Second printing with additions, Feb. 15, 1978, 250 p.
Economic Geologists. v. 72, p. 37-59. 15. Meyer, C., 1962, Comments concerning mineralogical
3. Brimhall, G., H. Jr., 1980, Deep hypogene oxidation of zoning at Butte, Montana: Unpublished company
porphyry copper potassium-silicate protore at Butte, report, July, 1962, 21 p.
Montana: a theoretical evaluation of the copper 16. Meyer, C., 1965, An early potassic type of alteration at
remobilization hypothesis: Economic Geology and the Butte, Montana: American Mineralogist, v.50, p.1717-
Bulletin of the Society of Economic Geologists. v. 75, 1722.
no. 3, p. 384-409. 17. Meyer, C., E.P. Shea, C.C. Goddard Jr., and staff,
4. Burns, G.J., 1994, A review of the geology and historic 1968, Ore deposits of Butte, Montana in Ridge, J.D.
production of the Butte district: Presented at the 100th ed., Ore deposits of the United States, 1933 - 1967
Annual Northwest Mining Association Convention, (Graton-Sales Volume I): New York, AIME, v. 2, p.
Spokane, Washington, November 29 – December 2, 1363-1416.
1994, p. 12. 18. Proffet, J.M., and G. Burns, 1999, Tilting in the Butte
5. Burns, G.J., R.W. Vian, and K.C. Gee, 1979, Phase I district, Montana, and possible implications for
resource: Unpublished company report, Geological interpretation of metal zoning: Geological Society of
Dept., Underground Project, TAC, January 31, 1979, America Abstracts with Programs, v. 31, no. 7, p. 382.
118 p. 19. Reed, M.H., 1999, Zoning of metals and early potassic
6. Czehura, S., 2002, Butte geology: Invited paper and sericitic hydrothermal alteration in the Butte,
presented at the Butte Mineral Symposium hosted by Montana porphyry Cu-Mo deposit: Geological Society
The Mineral Museum and National Science of America Abstracts with Programs, v. 31, no. 7, p.
Foundation, Montana Tech of the University of 381.
Montana, Butte, Montana, April 5-6, 2002, 7 p. 20. Reed, M.H., and C. Meyer, 1999, The Butte Montana
7. Czehura, S., 1999, Continental pit reserve and resource Main Stage vein system: structural style, alteration, and
modeling, Montana Resources, Butte, Montana: mineral zoning, and their relationship to pre-Main
Geological Society of America Abstracts with Stage sercitic alteration: Geological Society of
Programs, v. 31, no. 7, p. 381. America Abstracts with Programs, v. 31, no. 7, p. 382.
9. Czehura, S., and G. Zeihen, 2000, The Continental 21. Rusk, B.G., and J.H. Dilles, 1999, Fluid inclusions in
orebody – Montana Resources, Butte, Montana: barren quartz and quartz-moly, Butte Montana:

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 Geological Society of America Abstracts with
Programs, v. 31, no. 7, p. 381.
22. Rusk, B.G., M. Reed, and J. Dilles, 2000, Porphyry
copper-molybdenum mineralization below Butte,
Montana [A field trip to the core shed]: Guidebook of
the Twenty-fifth Annual Tobacco Root Geological
Society Field Conference, Butte, Montana 3-6 August,
2000 (TRGS, Missoula, Montana), p. 60-62.
23. Sales, R. H. 1913, Ore deposits of Butte, Montana:
Transactions of AIME, p.1523-1626.
24. Snee, L., D. Miggins, J. Geissman, M. Reed, J. Dilles,
and L. Zhang, 1999, Thermal history of the Butte
porphyry system, Montana: Geological Society of
America Abstracts with Programs, v. 31, no. 7, p. 380.
25. Vian, R.W., 1980, Deep surface diamond drilling –
Results of exploration diamond drilling from the
surface, East Butte area: Internal company
memorandum, TAC, April 3, 1980, 62 p.
26. Zhang, L., J.H. Dilles, C.W. Field, and M.H. Reed,
1999, Oxygen and hydrogen isotope geochemistry of
pre-Main Stage porphyry Cu-Mo mineralization at
Butte, Montana: Geological Society of America
Abstracts with Programs, v. 31, no. 7, p. 381.

Butte – A World Class Ore Deposit by Steve J. Czehura

Session on: Classic Geology and Mining Deposits
An invited paper for presentation -

2006 SME Annual Meeting and Exhibit

& 7th ICARD
March 27-29, 2006
St. Louis, Missouri

Steve Czehura, Mgr Geology & Engineering

Montana Resources
600 Shields Avenue
Butte, Montana 59701
T (406) 496-3212
F (406) 723-9542
E sczehua@montanaresources.com