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Golden Sunlight |
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Montana, USA |
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Au
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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 Golden Sunlight gold mine is located in Jefferson County, south-western Montana, USA, some 50 km to the ESE of the Butte Cu-Mo porphyry/vein copper deposit.
SUMMARY
The gold-silver-telluride orebody is centred on the silica-cemented Mineral Hill breccia pipe which cuts arkosic, poorly sorted sandstone and minor calcareous interbeds, and a laminated siltstone-shale and carbonaceous black shale sequence, all belonging to the Middle Proterozoic Belt Supergroup. This breccia pipe is spatially related to a high level, late Cretaceous multi-phase, alkaline to sub-alkaline porphyry system of latite porphyries, quartz-monzodiorite and lamprophyres. The breccia pipe appears to be closely contemporaneous with the 84 ±18 Ma latite porphyry phase.
Three main styles of mineralisation are found in the Golden Sunlight mine area. These are stratabound sulphides within the shales of the Belt Supergroup; the Mineral Hill porphyry molybdenum system; and the auriferous Mineral Hill breccia pipe and related gold veins. The porphyry molybdenum mineralisation pre-dates the breccia pipe. It is embraced by a district wide alteration system covering 6 sq. km centred on the two largest latite bodies. Within the central portion of the alteration system disseminated and stockwork Mo mineralisation is developed.
The alteration comprises an inner pervasive zone of quartz, sericite, K feldspar, base metals, tellurides, molybdenite and native gold, and a distal zone of vein controlled alteration similar to the core, but lacking molybdenite. A base metal and rhodochrosite zone occurs farthest from the centre. The Mineral Hill Breccia Pipe, which is around 200 m in diameter, is located centrally within the alteration system, Clasts of the porphyry mineralisation are found in the breccia pipe. About 70% of the gold reserves are found in the breccia pipe. Mineralisation is present as disseminations and as structurally controlled veins and breccias, with at least four pulses of mineralisation. The veins, largely of quartz and auriferous pyrite, are generally <1 cm, but may be up to >10 m thick. Pyrite is common, constituting up to 20% of some rocks, in veins and breccia matrix. Free gold was deposited early, occurring interstitially to pyrite, whereas gold and silver tellurides were emplaced in the third of the four main pulses.
Prior to mining the deposit contained approximately 85 t Au. The reserve remaining in 1996 were 65 Mt @ 1.85 g/t Au, with production having been in progress since 1983 (Placer Dome 1997). Mining is scheduled to be completed during 2007 with no remaining reserves (Barrick Gold, 2006).
DETAILED DESCRIPTION
Gold was first discovered in the Golden Sunlight area in 1890. During the early parts of the twentieth century small amounts of gold were mined, principally from gold-quartz veins peripheral to the main deposit. Production to 1950 had totalled approximately 1.75 t Au. No alluvial gold was evident. In 1958 Placer Amex leased the property and in 1961 began geological mapping, soil sampling and geophysical investigations. A metallurgical study into heap leaching of the mineralisation identified to that date was carried out between 1968 and 1973, but was un-successful. Never-the-less, exploration drilling gained momentum between 1975 and 1979 as the price of gold rose, with 20 000 m of HQ drilling being completed for ore reserve calculations. Most of the holes were angled as the original target was the high grade quartz veins. A detailed feasibility study was commenced in 1980, and a decision to mine announced in August 1981. The open pit mine was in operation by December 1982 and the first refined gold produced in February 1983 (Porter & Ripley, 1985).
Published production and reserve figures are as follows:
0.15 Mt @ 11.3 g/t Au = 1.7 t Au (Production, 1890-1950).
3 Mt @ 2.2 g/t Au = 6.7 t Au (Production, 1983-85).
22.8 Mt @ 1.7 g/t Au = 39 t Au (Reserve <170 m, 1986).
25 Mt @ 1.7 g/t Au = 42 t Au (Reserve > 170 m, 1986).
42.5 Mt @ 1.85 g/t Au = 78.5 t Au (Reserve, 1988, Amer. Mines H'book, 1989).
43.3 Mt @ 1.9 g/t Au (Proven + Probable Reserve, 1994, AME, 1995).
65 Mt @ 1.85 g/t Au (Proven + Probable Reserve, 1996, Placer Dome).
Geology
The Golden Sunlight orebody is centred on a silica-cemented breccia pipe which cuts rocks of the Middle Proterozoic Belt Supergroup. Just to the south of the mine area crystalline, Archaean basement outcrops, separated from the Belt Supergroup rocks by a major east-west fault.
The ore deposit lies on the eastern flank of a north-south trending horst within the ESE trending Helena Embayment on the eastern margin of the Belt Basin. The Belt Supergroup sediments are overlain by Mesozoic volcanics to the north, while plugs and sills of Eocene 'latite porphyry' intrude the sequence. Regionally, gold mineralisation is distributed around the margins of the composite 78 to 68 Ma Boulder Batholith which intrudes the Belt Supergroup and partly intrudes, but is also locally overlain by, Cretaceous volcanics. The Boulder Batholith is largely composed of quartz monzonite and lies between the Butte and Golden Sunlight deposits. The Butte porphyry copper mineralisation is hosted by the Boulder Batholith and has been dated at 63 to 57 Ma (Brimhall, 1979). The mineralisation at Golden Sunlight has been tentatively dated at between 30 and 20 Ma.
The Golden Sunlight ore deposit is developed high in the upper plate of and eastward transported Cretaceous to Tertiary Sevier Orogeny thrust. It lies on the southern margin of the major ESE trending Lewis and Clark Lineament which also embraces the Butte porphyry copper and Coeur d'Alene Pb/Zn/Ag deposits to the WNW.
The Belt Supergroup sediments are developed extensively within the north-western United States and in western Canada, and are believed to have been deposited in the interval between 1450 and 870 Ma, but probably between 1450 and 1000 Ma (Oldow, et al., 1989), or 1600 to 1300 Ma (Burchfiel, et al., 1992). This sequence is 15 to 20 km thick and appears to have been deposited in an elongate, generally north-south trending intra-cratonic basin (Oldow, et al., 1989) with both a distinct NE and SW margin.
The stratigraphic succession within the district is as follows, from the base (Porter & Ripley, 1985):
Archaean Crystalline Basement.
Unconformity and Faulted Contact
Middle Proterozoic, Belt Supergroup, composed locally of:
- La Hood Formation - olive green to brownish weathering litharenite or arkose, which is poorly sorted, with relatively fresh feldspar clasts, and is generally medium to thick bedded.
- Newland Formation - limestone and calcareous shales. This is the main unit involved in the Golden Sunlight deposit. The shales are primarily buff to dark brown in colour, with some black units, while the limestones are generally black.
- Greyson Shale - a sequence of buff to light brown, blocky siliceous shales, with some zones of distinct fissility.
Unconformity
Tertiary, Bozeman Formation - which has been sub-divided into a:
- Basal, Probably Oligocene, Unit - of red breccia,
- Middle Unit - composed of red shales and calcareous siltstones,
- Upper Unit, Possibly Miocene to Pliocene - made up of arkosic rocks.
Igneous lithologies in the mine area include (Porter & Ripley, 1985):
Latite Porphyry - a light coloured porphyry composed of approximately 50% of a feldspar-biotite groundmass, and 50% plagioclase and orthoclase phenocrysts which are present in almost equal proportions. Quartz is only found in subordinate amounts within the groundmass. This is the most abundant intrusive found within the vicinity of the mine and occurs predominantly as sills. It is commonly cross-cut by the breccias and is itself brecciated. Clasts of latite porphyry up to several metres across can be seen in the breccia.
Lamprophyre - comprising a dark biotite-augite-plagioclase groundmass with phenocrysts of biotite and augite. These rocks are spatially associated with the latite porphyry, but cut across it, are never brecciated and lack sulphide mineralisation, although they are commonly highly altered.
Basalt Porphyry - occurs in the southern third of the property where a few bodies of intrusive basalt porphyry and extrusive olivine basalt flows are mapped. They are though to be younger than the latite. No mineralisation has been noted within them and none are altered.
Structure
Major faulting in the vicinity of the Golden Sunlight deposit consists of two faults marking the boundary of the upthrown, north-south horst block, the St Paul Gulch Fault to the west and the Golden Sunlight Fault to the east. These fault zones each vary in width up to 250 m. Both have been silicified and pyritised to varying degrees, while the Golden Sunlight Fault in particular is pervasively altered. The Latite Porphyry however is cut and displaced by the Golden Sunlight Fault. Recent movement on both faults is indicated by displacement of the Bozeman Formation (Porter & Ripley, 1985).
A number of minor discontinuous faults trend east west and transect the horst, cutting both the St Paul Gulch and Golden Sunlight faults. These are both high angle normal and reverse faults with dextral strike slip components. It has been suggested that they are related to the introduction of the latite intrusives. Fracturing and veining are common in the area, with two predominant sets. One set parallels the north-south Golden Sunlight Fault while the other trends approximately north-east (Porter & Ripley, 1985).
The breccia pipe, which hosts the orebody, is located within the upthrown horst block between the Golden Sunlight and St Paul Gulch Faults. Until the excavation of the open pit the only surface exposure of the orebody was a small knob known as Mineral Hill. The average diameter of the breccia pipe is around 200 m near the surface, widening with depth. The top of the breccia pipe is gradational with fractured lithologies of the Greyson and Newland Formation, the southern boundary is gradational with the latite porphyry, while the northern margin is faulted (Porter & Ripley, 1985).
Mineralisation and Alteration
Both mineralisation and alteration have influenced most rock types in the Golden Sunlight area and have occurred over a protracted time period, including both pre- and post-brecciation events. Pre-brecciation alteration appears to be the most pervasive and affects all rock types within a halo of around 800 m about the breccia pipe, except to the east of the Golden Sunlight Fault. This alteration is characterised by bleaching and silicification with disseminated pyrite and decarbonatisation/decalcification (Porter & Ripley, 1985).
The sediments of all three Belt Supergroup formations become more silicified towards the breccia pipe, while in and near the breccia pipe itself they are so altered as to be almost un-recognisable. The host rocks, particularly the calcareous lithologies, become increasingly fractured as the breccia is approached. The La Hood Formation is the most affected with highly variable colouration while bedding may be obscured and obliterated where altered. Petrographically it contains abundant silica and iron oxides. The Newland Formation completely loses its calcareous nature and becomes a blue grey colour in drill core, and dark brown to black in outcrop. The Greyson Formation is bleached in the mineralised zone. Both the Greyson and Newland Formations owe their change in appearance to silicification (Porter & Ripley, 1985).
The igneous rocks have also been altered in the area. The latite porphyry becomes hydrothermally altered near the pipe, with phenocryst being obscured or obliterated. Pyrite is a common accessory, being abundant in fractures and veins within the latite. The lamprophyres, although post brecciation, are also highly altered to clay minerals, although they exhibit no sulphide mineralisation (Porter & Ripley, 1985).
Post-brecciation alteration principally comprises sericite as a late stage in the breccia pipe and to a minor extent in the country rock sediments. Kaolinite is a late stage alteration product of the lamprophyre and latite. Carbonates, fluorapatite and barite are also considered part of this late alteration (Porter & Ripley, 1985).
Mineralised veins in the Golden Sunlight area vary from around 2 mm up to 2 m in width, although the majority are <1 cm. The mineralogy is similar to that of the breccia with predominantly quartz and pyrite, with minor chalcopyrite, galena and barite in places. Gold occurred in the larger veins and in this form was the object of the earlier mining activity (Porter & Ripley, 1985). When these veins intersect the breccia pipe there is a 'blossoming' of grade.
The breccia pipe is the principal focus of economic interest. The breccia clasts are pervasively silicified and pyritised in a similar fashion to the pre-breccia alteration seen in the surrounding country rock. In addition, in many breccia samples, the edges of the clasts have a rind of darker colouration which is composed almost entirely of quartz. These rinds are generally 2 to 6 mm in thickness. The cores of the clasts, while still silicified, contain phyllosilicates and remnant bedding textures (Porter & Ripley, 1985).
The areas interstitial to the clasts contain the economic mineralisation. The paragenesis of the interstitial minerals has four phases. An initial brecciation event led to the fracturing and crushing of both igneous and sedimentary rocks, with fragments ranging from metre sized clasts to rock powder of micron dimensions. Most however are between 5 and 50 mm. Fragments have jagged, sharp, edges and are almost never rounded (Porter & Ripley, 1985).
The first and by far most abundant mineralisation is composed of quartz and pyrite which are found on the edges of the clasts. The pyrite occurs as euhedral to subhedral grains from 0.5 to 2 mm across, while quartz is present as euhedral grains growing into the interstices, normal to the fragment surfaces. Hematite occurs as small laths included in both quartz and pyrite, and exhibits evidence of being early in this first phase of mineralisation and alteration. Various minute 1 to 50 µm inclusions of other minerals are found mainly with in the pyrite. These include chalcopyrite, bornite and other copper sulphides and a second group which includes gold, calaverite and tellurobismuthinite. The calaverite is present as 1 to 10 µm inclusions in quartz and pyrite, while the native gold and Bi telluride occur as somewhat larger 10 to 50 µm inclusions. This represents the main gold mineralising phase, with most gold being in the native form, although a small amount is present as calaverite1 (Porter & Ripley, 1985).
The second phase paragenetic suite is separated from the first by another distinct brecciation event, with the first phase pyrite being fractured and crushed. All of the minerals of the second event occur as coatings on brecciated pyrite fragments, as infillings on cracks and fractures and as inclusions along cleavage or fracture planes in pyrite. Chalcopyrite is the most common of these second phase minerals, although galena and sphalerite are also present. Minor gold extends into this period, while marcasite and rutile occur together as minor components (Porter & Ripley, 1985).
The third phase is only evident in a few samples and is identified by the presence of tennantite which replaces chalcopyrite. The tennantite locally has minor concentrations of Ag, Hg, Sb and Zn and sometimes has associated Au and Ag which are present entirely as tellurides (Porter & Ripley, 1985).
Late in the deposits history quartz was again introduced, both as interstitial fillings and as veins. The late quartz is cloudy and inclusion filled, with minor pyrite. There are also minor amounts of late barite, sericite and fluorapatite, with associated dolomite, magnesite and kaolinite (Porter & Ripley, 1985).
Mineralisation within the breccia pipes also extends outwards into the surrounding latite porphyries and the sediments of the Newlands Formation. While the mineralisation is relatively simple, with gold, silver, quartz and pyrite, there is anomalous copper and locally molybdenum. Lead and zinc are sparse and there is no appreciably anomalous As, Sb or Hg. Gold:silver ratios are approximately 1:1. However some 1.5 km to the west of the mine Pb and Zn are associated with Au mineralisation. Pyrite comprises 10 to 15% of the host breccia, extending outwards for 100 m beyond the limits of gold mineralisation. In the top of the system the gold mineralisation is more disseminated, but becomes more structurally controlled with depth.
The orebody is pipe shaped, pitching to the west, following the breccia, with a circular plan which has a radius of approximately 130 m. The gold is generally free, with up to 10% being tied to the pyrite.
The main deposit associated with the breccis pipe is known as the Mineral Hill ore body. Other structures adjacent to the breccia pipe, such as the Sunlight Vein, are also mineralised.
Three satellite deposits, which have not been exploited nor included in the current resource estimate are the:
East deposit, which is situated east of the Mineral Hill pitwith areal dimensions of 600x120 m and an average thickness of 75 m. This deposit is oxidised and amenable to conventional processing.
North deposit, located north of the Mineral Hill pit, is approximately 180x35 in plan and remains open at depth. The upper portion is oxidised and amenable to heap leaching.
Rattlesnake deposits, is located southeast of the Mineral Hill pit and is 300 m long by 40 m wide and 40 m thick. It is similar in composition to the East deposit.
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2001.
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.
Golden Sunlight
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Porter E W, Ripley E 1985 - Petrologic and stable isotope study of the Gold-bearing breccia pipe at the Golden Sunlight deposit, Montana: in Econ. Geol. v80 pp 1689-1706
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Spry P G, Paredes M M, Foster F, Truckle J S, Chadwick T H 1996 - Evidence for a genetic link between Gold-Silver Telluride and Porphyry Molybdenum mineralization at the Golden Sunlight deposit, Whitehall, Montana: fluid inclusion and stable isotope studies: in Econ. Geol. v91 pp 507-526
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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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