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Beal |
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Montana, USA |
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Au
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Super Porphyry Cu and Au
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The Beal gold deposit is located some 26 km to the WSW of the Butte porphyry copper deposit in western Montana, USA.
Placer gold was first discovered in the district in 1864, followed by lode gold mineralisation during subsequent periods of alluvial exploitation. Between 1932 and 1976 a number of companies undertook underground testing and drilling programs of the hardrock gold lodes, but all withdrew. In 1980 Pegasus Gold acquired the property, undertook additional drilling in several programs, and following a feasibility study was in production in 1989. Production in 1993 was 1.8 t of Au from 1.54 mt of ore averaging 1.62 g/t Au and 70% recovery (Hastings & Harrold, 1988; AME Mining Quarterly, Dec, 1994).
Mineralisation at Beal is hosted by late Cretaceous continental clastic sedimentary rocks within the Sevier-Cordilleran Thrust Belt, between the 78 to 68 Ma Boulder Batholith to the east, and the slightly older late Cretaceous Idaho Batholith to the west. Sediments of the Middle Proterozoic Belt Supergroup and lower to middle Palaeozoic have been transported eastward, over separate west dipping thrusts, onto the host Cretaceous sediments (Hastings & Harrold, 1988).
Host rocks to the mineralisation are thick bedded to laminated, gently north dipping Kootenai/Colorado Formation sandstones, mudstones and conglomerates that have been metamorphosed to pyroxene-hornfels facies quartzite, hornfels and meta-conglomerate. The most abundant lithologies in the orebody area are quartzites which are white to green, fine to coarse grained and laminated to thick bedded. The darker coloured, coarse grained quartzite may contain up to 8% sulphide, while the lighter coloured and finer grained varieties have less sulphide and more chlorite (Hastings & Harrold, 1988).
The metamorphic mineral assemblage includes biotite, diopside, K-feldspar, chlorite, scapolite, quartz, actinolite, tremolite and hornblende. The contact metamorphism is interpreted to have been caused by granodiorite and diorite intrusives related to the Boulder Batholith. Subsequent recurrent high angle faulting has brecciated and strongly fractured the meta-sediments along a major east-west structure known as the Beal Shear (Hastings & Harrold, 1988).
The gold deposit has a diameter of at least 250 m and is localised within and adjacent to the Beal Shear. It is limited by a prominent north-west trending fault on the east, and by a meta-diorite intrusive to the west. Mineralisation in the deposit has been divided into two general types, sulphides and precious metals. Sulphide mineralisation occurs as disseminated grains and/or clots in favourable lithological units of the clastics. Sulphides are also found as narrow cross-cutting veins with quartz, chlorite or calcite within both the sediments and within the diorite. Pyrrhotite is the most abundant sulphide, followed by pyrite and chalcopyrite. Arsenopyrite and molybdenite are found in minor to trace amounts. Sulphide mineralisation, though apparently temporally and spatially related to precious metals, typically contains only trace amounts of Au and Ag (Hastings & Harrold, 1988).
Gold occurs in the free state as 1 to 5 µm particles disseminated in the coarser grained meta-sediments, as visible flakes on fractures and in quartz and quartz-adularia veins. Secondary gold is associated in minor amounts with lead-bismuth tellurides. The Au:Ag ratio is approximately 1:1. It appears that favourable lithology has been the primary control for both sulphides and precious metals at Beal. The best mineralised units have been meta-conglomerates, followed by quartzites, diopside-hornfels and finally K-feldspar hornfels. Sulphides and precious metals may also occur in veins which are typically less than 3 cm in thickness and are parallel the Beal Shear. These veins are generally either chlorite-quartz-sulphide; quartz and quartz-adularia; or quartz-carbonate (Hastings & Harrold, 1988).
Gangue minerals associated with gold mineralisation include sulphides, quartz, calcite, chlorite, sericite and various clays. The precious metal mineralisation has been dated at 71.8 Ma, and is interpreted to have followed the crystallisation of the sulphide minerals. The paragenetic sequence has been inferred to have been as follows, in sequential order: pyrrhotite, pyrite, molybdenite, arsenopyrite, chalcopyrite, telluride minerals and free gold. Hg and Sb are not anomalous (Hastings & Harrold, 1988).
Alteration within and near the Beal deposit has resulted in bleaching, chloritisation, local silicification and minor sericitisation of the meta-sediments. Bleaching is most evident at intrusive contacts and adjacent to most cross-cutting veins. Silicification has affected most rock types and is in part related to the mineralising event. It is most apparent in the breccia matrix east of the Gully Fault and adjacent to quartz veins throughout the deposit. Chlorite is most common in the quartzite matrix, along fractures and as envelopes adjacent to quartz, carbonate or sulphide veins. Sericite occurs in the quartzite matrix and within, or adjacent to, quartz or quartz-adularia veining. Limonite, jarosite and hematite coatings are common on fractured surfaces in the oxidised zone (Hastings & Harrold, 1988).
Published reserve figures include:
10.7 Mt @ 1.69 g/t Au = 18 t Au (Reserve, 1986, USBM)
8.1 Mt @ 1.46 g/t Au (Reserve, 1993, AME, 1994)
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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Selected References:
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Hastings J S, Harrold J L, 1988 - Geology of the Beal gold depsoit, German Gulch, Montana: in Schafer R W, Cooper J J, Vikre P G (Eds), 1988 Bulk Mineable Precious Metal Deposits of the Western United States Geol Soc of Nevada, Reno, pp 207-220
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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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