Montana Tunnels

Montana, USA

Main commodities: Au Ag
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The Montana Tunnels gold silver deposit in south-western Montana, USA, is some 30 km SSE of Helena, 50 km NNW of the Golden Sunlight gold mine, and 55 km NE of the Butte Cu-Mo porphyry/vein copper deposit.

Mining in the surrounding Colorado District was initially for silver and lead produced from at least 20 steeply dipping vein deposits hosted by shear zones. Gold was only a by-product. The largest and economically most important was the Alta Mine, 2.5 km to the east of Montana Tunnels. The vein worked at this mine was up to 45 m wide, 1900 m long and was stoped to a depth of 420 m. Although the veins of the district surround the Montana Tunnels diatreme, they predate it, being overlain unconformably by the younger of the hosts cut by the diatreme. The main period of mining in the Colorado District was from 1864 to 1900. During this time there was no significant production from Montana Tunnels which was named after two close exploratory adits driven in about 1900. Some gold was produced from placer operations in two creeks near and draining the diatreme, probably derived from the upper parts of the orebody (Sillitoe, et al., 1985).

Between 1975 and 1983 six major mining companies drilled the prospect, one searching for porphyry copper, the others for bulk gold mineralisation. All terminated their option agreements with the owners. In 1983 Centennial Minerals Ltd, under a joint venture agreement with the owner, US Minerals Exploration Co., completed a detailed drilling program and announced reserves (Sillitoe, et al., 1985). Production commenced in 1987. In 1992, 4.15 Mt of ore was mined at a head grade of 0.62 g/t Au, 15 g/t Ag, 0.61% Zn, 0.22% Pb. The open pit mine has a stripping ratio of 1.7:1, waste:ore. Gold recovery was 87% from a conventional milling operation to produce 2.3 t Au, 41.2 t Ag, 6532 t Pb and 17 630 t Zn (AME, 1995).

The bulk mineable gold-silver orebody is located in the central portion of a diatreme which was emplaced along the faulted contact between the andesitic rocks of the late Cretaceous Elk Horn Mountains Volcanics, and a sequence of quartz-latite ignimbrites that belong to the middle Eocene Lowland Creek Volcanics. The Elk Horn Mountains Volcanics in the Montana Tunnels area occur as a 9x8.5 km roof pendant in the north-central part of the late Cretaceous (78 to 68 Ma) Boulder Batholith. Both are unconformably overlain by the 50 to 48 Ma Lowland Creek Volcanics. A NNE striking swarm of quartz-latite porphyry dykes was emplaced during the waning stages of the Lowland Creek Volcanism. The two largest dykes have been dated at 50 and 45 Ma respectively, and were intruded into the diatreme prior to the cessation of brecciation and mineralisation (Sillitoe, et al., 1985).

The diatreme covers a surface area of 1 sq. km and is known to extend to a depth of at least 300 m. It is believed to have near vertical walls. The main rock type of the diatreme is a matrix breccia which is characterised by a sand sized (<2 mm), easily disaggregated, tuffaceous matrix of quartz-latite (rhyodacite) composition which contains fragments of adjacent volcanic wall rocks and intrusive rocks derived from the underlying Boulder Batholith. The matrix comprises 70 to 90% of the breccia and is predominantly sand sized, but has decreasing amounts of silt and clay sized material. The sandy matrix comprises up to 50% subrounded to subangular crystals and crystal fragments of quartz, biotite, plagioclase and K feldspar, the last three of which are largely destroyed by alteration. The clasts are normally <3 cm across, but may be up to 20 cm, and are usually subangular to well rounded and appear to have been polished. The Elkhorn Mountain Volcanics constitute the principal clast lithology, with quartz-monzonite, alaskite, aplite, quartz-latite porphyry and Lowland Creek Volcanics also recognised (Sillitoe, et al., 1985).

Blocks of volcanic wall rock are found lying at various angles against the walls of the diatreme and within it. Those blocks, principally of Elkhorn Mountains Volcanics, underwent hydraulic brecciation to form mono-lithologic breccias with little matrix, after being incorporated into the diatreme. Such brecciated blocks comprise little moved, but slightly expanded arrays of angular clasts. The largest such block is 270x120x30 m in size. Blocks that are interpreted to represent pyroclastic base surge deposits form a sub-aerial tuff ring to the diatreme, while carbonised wood is also found within the breccia of the diatreme. The diatreme is sub-divided into two unequal segments by a WNW trending oblique slip fault (Sillitoe, et al., 1985).

The orebody occupies about 10% of the diatremešs surface area, and is located near the centre of its southern segment, as divided by the WNW fault that cuts it in two. Although the gold values in the orebody appear to be distributed erratically, individual ore shoots are indicated to plunge to the north-west (Sillitoe, et al., 1985).

Some 70 to 90% of the sulphide minerals in the diatreme are disseminated in the breccia matrix with lesser, widely spaced, multi-directional veinlets, and as 3 to 10% of the diatreme matrix between brecciated blocks of Elkhorn Mountains Volcanics. Pyrite dominates all of the sulphide mineralisation styles, accompanied in decreasing order of abundance by dark brown sphalerite, galena, minor chalcopyrite, very minor tetrahedrite and rare electrum, in a gangue of manganocalcite, siderite and minor quartz. Disseminated mineralisation is homogeneously distributed throughout the matrix of the breccia as both monominerallic and polyminerallic grains <2 mm in size. Disseminated sulphides are most abundant adjacent to, and up to several cm's outwards from, veinlets, while thin halos of sulphides rim a few of the larger breccia clasts. Some of the sulphides also formed from the replacement of matrix components, particularly feldspars (Sillitoe, et al., 1985).

Veinlets that cut the breccia range from a few to 40 mm thick and appear to be multi-directional. They are generally widely spaced, more than 30 cm apart (Sillitoe, et al., 1985).

Fragments and clasts of, or containing sulphide and gangue minerals are common within the diatreme, implying repeated episodes of brecciation and mineralisation. Abundant manganocalcite and siderite within the ore zone were accompanied by pervasive sericite alteration, but only weak kaolinisation and silicification. Sericitisation is the most abundant alteration style, accounting for 20 to 30% of the altered diatreme breccia, replacing all of the biotite and much of the feldspar in the matrix. The other alteration products, in decreasing order of importance are, carbonates, quartz and kaolinite. Sericitisation grades outwards, beyond the ore zone, into a chlorite-montmorillonite-carbonate alteration assemblage which is also found within the weakly mineralised interiors of 'late mineral dykes' that cut the diatreme. Subsided blocks of Lowland Creek ignimbrites within the diatreme are similarly altered to chlorite-clay-carbonate with sericite (Sillitoe, et al., 1985).

Gold occurs in electrum with a fineness of about 550, and as inclusions of less than 200 µm in pyrite and sphalerite. Silver mostly occurs as a solid solution with galena, which was precipitated with the electrum. The Ag:Au ratio in the orebody averages 10:1. The deposit is characterised by geochemically anomalous concentrations of Zn, Pb and Mn, and compared to most volcanic associated orebodies, is low in As, Sb and Hg (Sillitoe, et al., 1985).

Published production and reserve figures include:

    7.4 Mt @ 1.47 g/t Au = 11 t Au (Proven +Probable Reserve, 1986, USBM).
    120 Mt @ 0.75 g/t Au = 90 t Au (Inferred. Resource, 1986 USBM).
    61 Mt @ 0.96 g/t Au, 12 g/t Ag, 0.67% Zn, 0.28% Pb (Reserve, 1983, Sillitoe, 1985).
    67.6 Mt @ 0.96 g/t Au, 12 g/t Ag, 0.67% Zn, 0.28% Pb (proven+probable geological reserves in 1986 - 0.5 g/t Au equiv cutoff).
    54.6 Mt @ 1.61 g/t Au
equiv (proven+probable geological reserves in 1986 - 0.6 g/t Au equiv cutoff).
    32.2 Mt @ 2.13 g/t Au
equiv (proven+probable geological reserves in 1986 - 0.9 g/t Au equiv cutoff).
    45.6 Mt @ 0.86 g/t Au (Reserve, 1987, Min J, Gold Serv, v. 1, p. 8, 1989).
    35.7 Mt @ 0.59 g/t Au, including 27.5 Mt @ 11.7 g/t Ag, 0.2% Pb, 0.6% Zn (Reserve, 1993, AME, 1994).

For detail see the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1985.    
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.

  References & Additional Information
   Selected References:
Sillitoe R H, Grauberger G L, Elliott J E  1985 - A diatreme-hosted Gold deposit at Montana tunnels, Montana: in    Econ. Geol.   v80 1707-1721

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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