June/July 2009

Economic Geology

Butte, Montana (Part 3)

By R. J. Cathro

“Secondary sulphide enrichment was first recognized at Ducktown, Tennessee, but it was at Butte that the theories of secondary enrichment first took definite form.”

~Emmons, 1933

Knowledge of the Butte deposit has advanced dramatically since Meyer et al. (1968) summarized almost a century of geological study, exploration and mining of the copper-rich polymetallic mineralization (the ‘classic’ Butte ore body). That date corresponds, more or less, with a transition from traditional studies of the youngest stage of mineralization to new types of research on the deeper porphyry zone, and the deposit as a whole. That includes modern tools, such as fluid inclusions, geochronology and geochemistry that were used to determine the genesis of the deposit.

The Butte ore body, one of the world’s great base-metal deposits, comprises a huge copper-dominated, polymetallic system of vein-faults that have been superimposed on a deeper copper-molybdenum porphyry system. It is hosted by the medium- to coarse-grained Butte quartz monzonite stock, which lies within the southern end of the late Cretaceous Boulder batholith, dated at about 76 Ma (but with some parts as old as 80 Ma). The batholith intruded Cretaceous andesite and rhyolite of the Elkhorn Mountains Volcanics, Proterozoic sedimentary rocks of the Belt Supergroup and Archaean crystalline basement.

The youngest of the two distinct stages of mineralization, called the Main Stage because it is closest to surface and was mined first, consists of large, through-going, Cordilleran-style vein-faults containing base metal lodes comprised mainly of copper, zinc, lead, silver and arsenic sulphides. The earlier copper-molybdenum event, called the Pre-Main-Stage, has a spatial overlap with the Main Stage, which suggests that the two are related, although the genetic relationship remains unresolved. Mine exposures and deep drilling have revealed that the porphyry mineralization occurs as two internally zoned domes, named the Anaconda to the west and the Pittsmont to the east, each about two kilometres in diameter. The top of the Anaconda dome, molybdenite mineralization was encountered near the 2800 level (about 900 metres below surface). In the eastern part of the deposit, the north-striking, moderately west-dipping, post-ore Continental fault offsets the Pittsmont dome and has dropped the west side downward by about 1.25 kilometres. The vertical range of the pre-faulted Butte deposit that has been sampled and studied was expanded to approximately 2.6 kilometres by means of a deep drill hole on the eastern side of the fault. That provided an exceptional opportunity to study the physical and chemical processes of this magmatic-hydrothermal system, and has suggested that Butte is one of the deepest porphyry copper-molybdenum deposits known. Fluid inclusion phase relationships indicate that it formed at a depth of five to nine kilometres.

The two domes are defined by zones of abundant magnetite-chalcopyrite-bearing veins and by molybdenum grade contours, and are separated by a bulb-shaped zone of intense pyritic veining and pervasive grey sericite alteration up to 1.2 kilometres wide. They are concentrically zoned with widely overlapping shells of alteration bordering centimetre-scale stockwork veins. The deep drill holes in the Pittsmont dome revealed an inner stockwork of barren quartz veins with minor molybdenite and thin sericite-biotite-potassium-feldspar alteration envelopes containing up to about two per cent copper as chalcopyrite. These envelopes contrast sharply, mineralogically and chemically, with the later sericitic and argillic alteration envelopes around Main Stage veins. The highest molybdenum grades occur in an area characterized by quartz-dominated veins containing a few per cent molybdenum with no alteration. They occur upward and outward from the barren quartz stockwork, but below the bulk of the chalcopyrite-magnetite mineralization. Early dark micaceous alteration passes upwards into a 300 metre-thick shell of abundant quartz-chalcopyrite-magnetite-pyrite veins with potassium-feldspar, chlorite, quartz, and green sericite alteration envelopes. In the outermost zone, veins contain pyrite, sphalerite, galena, rhodochrosite, chalcopyrite and epidote in potassium-feldspar-bearing propylitic alteration.

Geochronology has disclosed that mineralization began with the intrusion of the quartz porphyry dykes at about 66 Ma. The porphyry dykes are closely associated with the Pre-Main-Stage porphyry copper-molybdenum ores in the Pittsmont and Anaconda domes, dated at about 64 to 66 Ma based on the age of molybdenite. The economic phase ended with the Main Stage veins at about 62 to 64 Ma (Dilles et al., 2004).

Meyer et al. (1968) described how the major east-west-striking Anaconda system of veins, the earliest in the district, generally followed the direction of the quartz porphyry dykes. They were the major producers in the western third of the district, and also in the eastern third where the ‘horsetail’ zones break off from them. The Anaconda system strikes about N65ºE at the west end but curves to the southeast at the east end, while dips are steeply to the north in the upper levels but southerly below the 2800 level. The western portion consists of five major veins and 12 to 15 smaller veins, which are right-slip with minor normal movement, plus numerous splits and bifurcations. Individual oreshoots were up to hundreds of metres long and over 650 metres deep. The central portion consists of giant tensional veins similar to the western portion. The horsetail area consists of about six distinct subzones composed of both rich veins and low-grade mineralization disseminated in quartz monzonite. The upper part was later re-mined from surface in the Berkeley pit. Anaconda oreshoots averaged seven to 10 metres in thickness and reached up to 30 metres at changes of strike and junctions of veins. They hosted the original chalcocite “bonanza” veins and the great rhodocrosite ore bodies at the Emma mine. The spacing of the veins was irregular, ranging from less than 100 metres to more than a kilometre apart.

The modern interpretation of the relationship between the Anaconda system and the younger system of Blue (Northwest) veins is that the two sets occupy a conjugate set of strike-slip faults that formed at the same time and in the same stress field, that they were moving successively, and that they show mutually cross-cutting relationships when they intersect. The Anaconda veins are mainly right-slip whereas the Blue veins are mainly left-slip. The 1968 interpretation, on the other hand, was that the Anaconda veins were offset by the southwest-dipping veins of the Blue set, with mainly horizontal displacement of up to 100 metres.

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