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Butte, Continental, Berkely, Anaconda Vein |
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Montana, USA |
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Cu Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Butte ore deposits lie entirely within the composite 78 to 68 Ma Boulder Batholith in Montana, USA. This batholith is elongated generally NNE-SSW, bounded to the north by the Lewis and Clark Line, and to the south by an east-west fault that marks the local northern margin of Archaean crystalline basement. The tectonic block between these two structural zones has been sporadically active since the Middle Proterozoic, largely characterised by recurrent subsidence. The Boulder batholith predominantly intrudes rocks of the Middle Proterozoic Belt Supergroup, and is overlain by, and partly intrudes, Cretaceous volcanics, which are overlain in turn by andesitic to rhyolitic Tertiary volcanics of the Challis and Tuscarora Arcs (Meyer, etal., 1968).
The earliest intrusions in the batholith, near both the northern and southern ends, were mafic, with massive granodiorite subsequently emplaced, also localised near the same extremities. These was followed by the Butte Quartz-Monzonite which is the dominant rock type of the batholith (Meyer, etal., 1968), dated at 71 Ma at Butte (Hutchinson & Albers, 1991). It comprises a medium to coarse grained quartz-orthoclase rock making up 40 to 45% of the rock, with 35 to 40% plagioclase, accompanied by hornblende and biotite. Isolated tabular dykes and masses of aplite and pegmatite are found throughout. Quartz-porphyry dykes intrude the quartz-monzonite and aplites. These are generally 3 to 15 m thick in the mine area and are characterised by embayed quartz phenocrysts in a strongly altered groundmass that may have been quartz and alkali feldspar, suggesting a rhyolitic or quartz-latitic composition. The quartz-porphyry intruded the quartz-monzonite along directions subsequently followed by the NE-SW to east-west Anaconda vein system, the earliest major veins in the district (Meyer, etal., 1968). Narrow breccia zones containing wall rock fragments in a matrix of alkali feldspar, biotite and quartz, with disseminated pyrite and chalcopyrite are often associated with the quartz porphyry dykes, and within the surrounding Butte Quartz-Monzonite. This represents the first Cu mineralisation in the district (Brimhall, 1979).
Mineralisation and Alteration
The NE-SW to east-west Anaconda veins are faulted by NW striking veins of the Blue vein system, which are most prominent in the central third of the district. The Anaconda veins generally dip at 60 to 80° to the south-east, with anastomosing/interconnecting steeper and shallower companions. The Anaconda veins were the major producers in the western and the eastern thirds of the district. To the west, 5 principal veins, 12 to 15 smaller and numerous splits and bifurcations were mined. These strike at 60 to 70° but further east curve to the east, with a central zone in which the Anaconda veins are generally absent, before reappearing further to the east where they strike to the south-east. This central zone, which is devoid of Anaconda veins, is approximately 1500 x 900 m across at the 900 m below surface level. In the eastern third of the district the Anaconda veins are present in an en echelon pattern. Numerous small, closely spaced, south-east striking fractures break away from the larger Anaconda veins in this area to form 'horsetail zones' of rich Cu mineralisation. These zones of SE trending 'horsetail veins' however, strike at about 70° (Meyer, etal., 1968).
The NW striking veins of the Blue vein system are best developed in a NE striking and NW dipping zone through the centre of the district, corresponding to the interval that is generally devoid of Anaconda veins, and also where the Anaconda veins change strike. To the north the Blue veins dip steeply to the south, but to the south shallow to 45° in the same direction (Meyer, etal., 1968). The Anaconda and Blue vein systems are known as the Main Stage of mineralisation. Individual ore shoots in both the Main stage vein systems are very extensive, commonly more than a kilometre in both vertical and horizontal extent. Individual Anaconda system veins average 6 to 9 m in thickness, but may be locally as much as 30 m wide in the central zone, while Blue system veins average 1.5 to 6 m in thickness (Meyer, etal., 1968).
Viewed broadly, the mineralisation in the big vein systems is arranged in crudely concentric zones of Zn and Mn around a central Cu-zone. Three crudely concentric zones are described, namely 1). a 'central core' occupying an area of altered quartz-monzonite in which ores are characteristically free of sphalerite and manganese minerals; 2). an 'intermediate zone' in which the ores are predominantly Cu, but are seldom free of sphalerite; and 3). a 'peripheral zone' in which Cu has not been found in commercial quantities, but sphalerite and manganese predominate. In the peripheral zone Mn, Zn and Ag are the most important commodities (Meyer, etal., 1968).
The pattern of distribution of Cu-Fe-S series minerals in the large Main Stage veins of the Cu-zone is not symmetrical with respect to the metal zoning. There is a fringe of chalcopyrite-bornite all around the Cu front, at the outer edge of the 'intermediate zone'. However, within the Cu-zone, pyrite-chalcocite-enargite-covellite-digenite assemblages are prevalent near its eastern edge, from a depth of 1200 m to the surface, whereas deep on the large Anaconda veins in the western part of the Cu-zone chalcopyrite is dominant. Here the chalcopyrite grades upwards into bornite-chalcocite ores, and eastwards into the higher sulphur assemblages of the east as detailed above. The veins of the Main stage have well developed alteration selvages. In the 'intermediate' and 'peripheral zones' Zn is present in all of the major Anaconda veins and many Blue veins as sphalerite with ex-solution blebs of chalcopyrite. Rhodochrosite is an important component of the veins in the 'peripheral zone', with a width of 30 m of the coarsely crystalline Mn mineral having been mined at one locality. Rhodonite is commonly present as the rhodochrosite becomes more 'impure'. Some 23 255 t of Ag, 1.9 mt of Mn and 245 000 t of Zn were produced from Butte between 1880 and 1972 (Meyer, etal., 1968).
The Main stage veins are fringed by alteration selvages which in the Cu-zone comprise an inner zone of silicification, sometimes with an advanced argillic fringe to the vein. The silicification is followed outwards by intense sericitisation, and at depth by an argillic halo which has an inner kaolinite and outer montmorillonite sub-zone. These selvages range from less than a metre in width adjacent to a 1 m thick vein, to a pervasive zone continuous between adjacent veins. In the 'intermediate' and 'peripheral zones' where Zn and Mn are present, the veins are fringed by sericitisation, the two argillic sub-zones described above, and an outer propylitic sub-zone, before passing into fresh quartz-monzonite (Meyer, etal., 1968)
Only slightly offset from the core of the three mineral zones described above, but structurally earlier, is a zone in which quartz-molybdenite veinlets are prominent, forming a dome shaped volume deep in the mine, below the majority of the Main Stage mineralisation. The top of this zone, known as the 'pre-Main Stage', or the Mo-zone, is some 900 m below the surface, widening with depth.
The pre-Main stage predates the Main Stage, and comprises abundant veinlets and gashes which are narrow (veinlets are generally less than 25 mm, averaging 5 to 10 mm wide; gashes may be up to 250 mm thick) and discontinuous (from 1 to 10 m or so in length). These veins cut the quartz porphyry dykes and the breccia zones, but are in turn cut by the Main Stage veins. The veinlets of the pre-Main Stage represent a number of different morphologies, phases of mineralisation and compositions, only some of which have well developed alteration selvages. Those without selvages cut those with selvages and include barren quartz and banded quartz-molybdenite veinlets, the latter of which contribute the bulk of the mineralisation in the Mo-zone. The veins with selvages are generally 3 to 25 mm thick, enveloped by a selvage up to 20 cm across which comprises biotite, K-feldspar and sericite with disseminated chalcopyrite, often assaying up to 2% Cu across the width of the selvage (Meyer, etal., 1968; Brimhall, 1979). Many of the earlier disseminated pyrite-chalcopyrite occurrences are within the Mo-zone. On the basis of the selvage composition the pre-Main Stage is described as the 'potassic core' of the orebody (Brimhall, 1979). Due to the depth and grade the pre-Main Stage mineralisation has not been exploited.
Age dating of the Main and pre-Main Stage alteration minerals yields dates of 57.5 Ma (or 59 to 57 Ma) and 62.8 Ma (or 63 to 62 Ma) respectively, compared to the 71 Ma age of the Butte Quartz Monzonite. The temperatures of formation of the Main and pre-Main Stage mineralisation selvages are apparently 200 to 350° and 650 ± 100° C respectively (Brimhall, 1979).
Supergene enrichment produced a chalcocite blanket that was mined in the Berkely pit, in the eastern portion of the district, where pervasive sericitisation is most widespread. In this vicinity primary mineralisation is characterised by more abundant pyrite, both in veins and in the wall rock as an alteration of biotite. Primary Cu sulphides are mainly enargite and chalcocite with subordinate bornite. Extensive kaolinite developments were produced by acid attack on the original sericitic altered rock. The depth of oxidation and thickness of the blanket were quite irregular, owing to the control of permeability by veins and faults. Generally oxidation was complete to 60 to 90 m. Chalcocite is the main supergene mineral, coating pyrite and other sulphides.
Published production and reserve figures at Butte include:
Butte/Berkely/Continental:
Underground & open pit production 1880 to 1972 - 452 Mt @ 2% Cu, 0.23 g/t Au, 51 g/t Ag (Tooker, 1991),
Reserve 1971 - 500 Mt @ 0.8% Cu (Gilmour, 1982),
Reserve Berkely open-cut, 1968 - 414 Mt @ 0.74% Cu (Skillings, 1978).
Reserve Continental orebodies, 1986 - 680 Mt @ 0.6% Cu (USBM),
Reserve Continental open-cut, 1992 - 360 Mt @ 0.3% Cu, 0.07% Mo (Am. Mines H'book, 1994),
Reserve , 1993 - 496 Mt @ 0.35% Cu (AME, 1994).
Butte vein system - (included in figures above)
Reserve 1968 - 12.5 Mt @ 4.27% Cu, 50 g/t Ag (Skillings, 1978),
Production 1880-1964 - 327 Mt @ 2.5% Cu, 60 g/t Ag (Bartos, 1989)
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1996.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Gammons C H, Snyder D M, Poulson S R and Petritz K, 2009 - Geochemistry and Stable Isotopes of the Flooded Underground Mine Workings of Butte, Montana: in Econ. Geol. v104 pp 1213-1234
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Houston R A and Dilles J H, 2013 - Structural Geologic Evolution of the Butte District, Montana: in Econ. Geol. v.108 pp. 1397-1424
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Mercer C N and Reed M H 2013 - Porphyry Cu-Mo Stockwork Formation by Dynamic, Transient Hydrothermal Pulses: Mineralogic Insights from the Deposit at Butte, Montana : in Econ. Geol. v.108 pp. 1347-1377
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Nahnybida T, Gleeson S A, Rusk B G and Wassenaar L I, 2009 - Cl/Br ratios and stable chlorine isotope analysis of magmatic–hydrothermal fluid inclusions from Butte, Montana and Bingham Canyon, Utah: in Mineralium Deposita v.44 pp. 837-848
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Reed M, Rusk B and Palandri J, 2013 - The Butte Magmatic-Hydrothermal System: One Fluid Yields All Alteration and Veins: in Econ. Geol. v.108 pp. 1379-1396
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Rusk B G, Reed M H and Dilles J H, 2008 - Fluid inclusion evidence for magmatic-hydrothermal fluid evolution in the porphyry copper-molybdenum deposit at Butte, Montana: in Econ. Geol. v103 pp 307-334
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Rusk B G, Reed M H, Dilles J H and Kent A J R, 2006 - Intensity of quartz cathodoluminescence and trace-element content in quartz from the porphyry copper deposit at Butte, Montana : in American Mineralogist v91 pp 1300-1312
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Rusk B, Reed M 2002 - Scanning electron microscope-cathodoluminescence analysis of quartz reveals complex growth histories in veins from the Butte Porphyry Copper deposit, Montana: in Geology v30 pp 727-730
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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