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Goldstrike - Betze / Post - Geology

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The Betze/Post orebody of the Goldstrike mine is located on the boundary of Elko and Eureka Counties in north-eastern Nevada, some 40 km to the NNW of the township of Carlin, and 60 km NW of Elko. It is the largest known sediment hosted disseminated gold mine on the Carlin Trend.

SUMMARY

Pre-mining, the deposit contained around 1000 t of gold. At December 1996 proven + probable reserves totalled 122 Mt @ 6.6 g/t Au. Mineralisation is predominantly hosted by the Devonian Popovich Limestone, Devonian Rodeo Creek clastics the Jurassic Goldstrike Intrusive Complex and the Siluro-Devonian Roberts Mountain Formation silty limestones, in decreasing order. The host sequence was subjected to four periods of pre-gold deformation and three periods of post mineralisation stress. The early deformation periods comprised two regional phases and a third accompanying the 158 Ma Goldstrike Intrusive Complex. The fourth, which was post intrusive and extensional, straddled mineralisation, and may have involved sinistral movement on the Post Fault. The first post ore stress was compactional around ore caused by the contraction of the hydrothermal system. The remainder were due to basin and range movement. In the Post-Betze ore zone primary high angle faults strike NNW and dip both east and west, while secondary faults strike east to ENE and dip north. Gold mineralisation occurs where primary and secondary faults intersect allowing fluid flow over long distances. Secondary access was developed at fault intersections with anticlines of decalcified units and zones of solution collapse. Mineralisation occurs as vertically stacked ore zones in limestone dissolution intervals and collapse breccias, separated by impermeable layers. Five stages of pyrite have been identified. The early stages were barren. Early hydrothermal pyrite of the third stage is commonly coated with micron sized gold bearing hydrothermal pyrite of the fourth generation, while the fifth stage is barren. During hydrothermal activity, de-calcification and solution collapse reduced the rock column by around 30 m in the ore deposit. The mineralisation is dated at 40 Ma and is believed to have taken place at a depth of 3 km. The ore is spatially distributed above a small porphyry intrusion, speculated to be an apophyses from a main magma chamber at 5 to 6 km.

DETAILED DESCRIPTION

The Post/Goldstrike mine, also known as Post/Betze is located on the boundary of Elko and Eureka Counties in north-eastern Nevada, some 40 km to the NNW of the township of Carlin. The ore deposit lies on the northern tip of the Lynn Window, approximately 7 km to the north-west of the Carlin mine and less than 2.5 km to the north of the Blue Star/Genesis orebodies.

According to Arehart, et al., (1993), the Post/Goldstrike ore zone occupies a plan area of approximately 3x1 km, elongated in a north-westerly direction, parallel to the Carlin Trend. The Post/Goldstrike orebodies fall partly within property owned by the Newmont Gold Company and partly within claims held by Barrick Goldstrike Mines Incorporated. Published reserve figures include:

Post  (Newmont)
   Proven + probable reserve 1992 - 30 Mt @ 5.55 g/t Au = 165 t Au (Christensen, 1993)
   Total geological reserve, 1989 - 97 Mt @ 1.90 g/t Au = 189 t Au (McFarlane, 1991)
   Total geological reserve, 1989 - 177 Mt @ 2.2 g/t Au = 387 t Au (Coope, 1991)
   Upper Post, pre-mining resource - ~78 Mt @ 1.2 g/t Au = 93.5 t Au (Jory, 2002)
   Lower Post, pre-mining resource ~19.5 Mt @ 4 g/t Au = 77.5 t Au (Jory, 2002)
   Deep Post, pre-mining resource ~6 Mt @ 21 g/t Au = 125 t Au (Jory, 2002)
Goldstrike / Betze (Barrick)
   Pre-mining resource ~208 Mt @ 6 g/t Au = 1250 t Au (Jory, 2002)
   Proven + probable reserve, Dec 1994 - 98.3 Mt @ 6.96 g/t Au = 685 t Au (Volk, etal., 1995)
   Proven + probable reserve, 1994 - 107 Mt @ 6.2 g/t Au (AME, 1995)
   Proven + probable reserve, 1992 - 102 Mt @ 6.15 g/t Au (Christensen, 1993)
   Reserve, 1990 - 160 Mt @ 3.6 g/t Au (Jones & Huspeni, 1990)
   Geological resource, oxide Ore, 1989 - 73 Mt @ 1 g/t Au (Coope, 1991)
   Geological resource, sulphide Ore, 1989 - 85 Mt @ 5.8 g/t Au (Coope, 1991).
   Open pit - Proven + probable reserve, December, 2005 - 116.8 Mt @ 3.92 g/Au = 458 t Au (Barrick Gold, 2006).
   Open pit - Measured + indicated + inferred resource, December, 2005 - 21.5 Mt @ 1.53 g/Au = 33 t Au (Barrick Gold, 2006).
   Underground - Proven + probable reserve, December, 2005 - 7.46 Mt @ 11.60 g/Au = 86.5 t Au (Barrick Gold, 2006).
   Underground - Measured + indicated + inferred resource, December, 2005 - 3.30 Mt @ 11.82 g/Au = 39.00 t Au (Barrick Gold, 2006).
   Open pit proved + probable reserves, 2013 - 85.6 @ 2.9 g/t Au (Barrick Gold, 2014)
   Open pit measured + indicated resources, 2013 - 5.9 Mt @ 2.1 g/t Au (Barrick Gold, 2014)
   Open pit inferred resources, 2013 - 1.1 Mt @ 2.2 g/t Au (Barrick Gold, 2014)
   Goldstrike underground proved + probable reserves, 2013 - 10.6 @ 7.6 g/t Au (Barrick Gold, 2014)
   Goldstrike underground measured + indicated resources, 2013 - 6.1 @ 9.2 g/t Au (Barrick Gold, 2014)
   Goldstrike underground inferred resources, 2013 - 1.3 @ 9.5 g/t Au (Barrick Gold, 2014)
Satellite deposits
   Screamer, pre-mining resource ~28 Mt @ 5.5 g/t Au = 155 t Au (Jory, 2002)
   Bazza, pre-mining resource - ~1.6 Mt @ 1.5 g/t Au = 2.5 t Au (Jory, 2002)
   Long Lac, pre-mining resource - ~1.3 Mt @ 1.5 g/t Au = 2 t Au (Jory, 2002)
   Pancana, pre-mining resource - ~0.4 Mt @ 1.5 g/t Au = 0.6 t Au (2002)

The Goldstrike underground reserves and resources are in the Meikle and Rodeo mines to the north of Betze.

In 1993, the combined Newmont + Barrick Post/Goldstrike deposit was the largest known sediment hosted gold resource in Nevada in terms of contained gold, with more than 790 t of Au in proved and probable reserves (Christensen, 1993). In 1994 the sections of the Post-Betze and Meikle deposits within Barricks property contained a total proven+probable reserve of around 870 t Au (Volk, et al., 1995).

Near surface gold was known above the deposit as early as 1963, with several small occurrences having been mined by Western States Mining Corporation between 1975 and 1986. These included the Bazza, West Bazza, Long Lac and Lost Pancana pits. In 1982 the Post mineralisation was discovered by Western States Mining in a program of 100 to 170 m drill holes. By 1986 a reserve of 11 Mt @ 1.7 g/t Au had been delineated. The initial discovery of deep high grade mineralisation took place in late 1986 with an intersection of 120 m @ 6 g/t Au at a depth of more than 300 m in drill hole no. 118. In January 1987 American Barrick Resources, under the name Barrick Goldstrike Mines Incorporated, acquired Western States Mining's titles to the Post deposit (Arehart, etal., 1993; Thoreson, 1993). Further deep drilling by Barrick delineated a large sulphide gold orebody at depths of between 400 and 600 m which was named the Lower Post. Based on geophysical, geochemical and geological data, a deep exploratory drill hole was commenced in early 1987, some 1 km to the north-west of the Lower Post deposit. This hole intersected 91 m @ 12.7 g/t Au in two zones at depths of between 250 and 450 m and was labelled the Betze deposit. Subsequent drilling between the two occurrences, Lower Post and Betze, has shown them to be part of a single larger ore deposit (Arehart, etal., 1993).

Geology

The Post/Goldstrike ore deposit is hosted by Devonian carbonate and clastic sediments on the northern margin of the Jurassic (158 Ma) Goldstrike Stock. The Devonian carbonates overlie Silurian to Devonian carbonates at depth and are in turn overlain across a Devono-Carboniferous thrust by Ordovician silici-clastics. At the top of the Devonian carbonate sequence there is a silici-clastic unit. The Jurassic 'stock' cuts all of these sediments. The succession within the immediate mine area is as follows, from the structurally lowest:

Silurian to Devonian, Roberts Mountains Formation, which is not exposed at Post/Goldstrike, although its upper sections are encountered in deep drilling. Below the Betze orebody it is represented by a light grey, thin bedded to laminated, platy, silty limestone to dolomitic siltstone, with interbeds of bioclastic limestone. The upper contact with the Popovich Formation is gradational (Thoreson, 1993). At Meikle however there is a second facies represented, composed of a typically fossiliferous dolomite or limestone, often displaying oolitic or pelloidal textures. The fossiliferous facies shows evidence of well developed cavernous porosity, often partially or completely infilled with sediment or collapse breccia. Both facies host minor gold, generally associated with structural zones (Volk, etal., 1995).
Devonian, Popovich Formation, 350 m thick - comprising a:
* Basal Unit, a bioclastic limestone/debris flow member, which commonly contains clasts of the underlying Roberts Mountains Formation (Volk, etal., 1995).
* Lower unit, 100 to 120 m thick - of dark grey micritic limestone with 25% interbedded bioclastic limestone and debris flow, and 25% interbedded siltstone. Dissolution of carbonate has resulted in collapse brecciation within this unit. The lower Popovich Formation hosts the highest grades of the Lower Post (>3.5 g/t Au) and the high grade (>7 g/t Au) of the Deep Post deposit (Thoreson, 1993). The unit is well laminated (Volk, etal., 1995).
* Middle unit, 100 to 120 m thick - of dark grey, massive to medium bedded siltstone and silty limestone. Distinct lenticular bedding textures are observed. Pyrite is present as framboids and as lenses on bedding planes. It contains 5 to 30 cm thick bioclastic units which increase in frequency down sequence. This units hosts the lower grade refractory leach grade ore (0.2 to 1.7 g/t Au) of the Lower Post deposit (Thoreson, 1993). Volk, etal., (1995) describe this unit as a thin bedded micritic limestone with abundant soft sediment deformational features.
* Upper unit, 100 m thick - comprising a black to grey, thin to medium bedded (2 to 60 cm) siltstone and silty limestone. It also contains 2 to 15 cm thick bioclastic limestone beds. Clay rich carbonaceous zones are prominent. Pyrite is present as framboids and as lenses on bedding surfaces, as in the Middle unit. Where oxidised it is yellow brown to lavender in colour. The upper siltstones contain minor interbedded siliceous mudstones similar to the overlying Rodeo Creek Unit (Thoreson, 1993). According to Volk, etal., (1995) this unit is a calcareous to micritic mudstone. Where not decarbonatised, they describe it is as being a thick bedded to massive, dark grey to black micrite with 3 to 10% carbonate veinlets.
  Near the Goldstrike intrusives the Popovich Formation has been locally metamorphosed to skarn(?), calc-silicate hornfels and marble. This contact aureole is 60 to 90 m wide. The contact between the Popovich Formation and the overlying Rodeo Creek Unit appears to be gradational (Thoreson, 1993; Andrew, 1993). Over 85% of the reserves at Post-Betze and Meikle are hosted by the Popovich Formation (Volk, etal., 1995).
Devonian, Rodeo Creek Unit, 110 to 240 m thick - a black, well bedded and banded siliceous mudstone to argillite with interlaminated grey calcareous siltstone. Within the upper sections it contains distinct horizons of thin bedded siltstone and fine to medium grained sandstones. This unit is the hosts to the Upper Post oxide orebody (Thoreson, 1993). Over 60 t of Au having been taken from lithologies of the Rodeo Creek Unit at Post-Betze, mainly from oxidised calcareous siltstone and highly fractured argillite (Volk, etal., 1995).
Devono-Carboniferous, Roberts Mountains Thrust, over which the Western Assemblage Vinini Formation of Ordovician age was thrust to the east to overlie the Devonian Popovich Formation carbonates and upper Rodeo Creek Unit silicic-clastics of the Eastern and Transition Assemblage.
Ordovician, Vinini Formation, >750 m thick - consisting of black and grey, highly deformed, massive to poorly bedded siltstone, sandstone, chert and mudstone. It may exhibit a distinct boudin like fabric of light and dark composition parallel to bedding. It breaks into irregular, wavy fragments which often have a waxy lustre adjacent to faulted contacts with the Popovich Formation and Rodeo Creek Unit. Abundant, low angle, intra-formational structures are mapped. At the base of the formation a 0 to 15 m thick limestone is present above the Roberts Mountains Thrust surface in the north-west and eastern sections of the pit. This unit is conformable with the overlying Vinini mudstone. In the north-west Post pit it comprises a black to light grey, silty limestone, while in the east of the pit it is a grey-green, massive, calcareous, silt unit (Thoreson, 1993). The Vinini Formation hosts minor low grade gold mineralisation peripheral to the Post-Betze orebodies (Volk, etal., 1995).

Jurassic to Cretaceous, Goldstrike Stock, - In the Post/Goldstrike mine area this 'stock' has been differentiated into a number stages, comprising:
* The earlier 165 Ma Goldstrike Intrusive diorite to granodiorite which is only weakly mineralised on its north-eastern margin and forms a barrier to the Deep Post mineralisation. The major minerals are plagioclase, orthoclase, biotite, quartz and hornblende with trace amounts of pyroxene. It is little altered, except where cut by faults and fractures and on its margins where it is adjacent to ore. There is a 60 to 90 m wide metamorphic aureole surrounding the stock. The main stock has a sill like form, as shown on Figure 123. The eroded upper section of this sill like mass may originally have formed a cap to the orebody (Thoreson, 1993; Arehart, etal., 1993; Andrew, 1993);
* A later 110 Ma Quartz-Monzonite Porphyry intrusive occurs throughout the deposit as dykes and sills. In places dykes and sills up to a few metres thick of this pre-ore intrusive comprise up to 40% of the drill core. It has an adamellite to granite composition and is characterised by phenocrysts of altered plagioclase, and altered biotite and quartz, in an altered fine grained groundmass. Most of these dykes have been intensely altered to quartz and sericite although they retain their original texture. Several dykes are present as breccias, some with fluidised textures. Where present in sediments they commonly form a local barrier to mineralisation, with elevated gold grades immediately below them (Thoreson, 1993; Arehart, etal., 1993);
* A later 39 Ma intrusive, the Biotite-Feldspar Porphyry which occurs within the Post Fault zone and the Deep Post shear zone. It is characterised by porphyritic biotite and/or feldspar in an aphanitic matrix. This latter intrusive is post mineral and cuts the Deep Post orebody (Thoreson, 1993; Arehart, etal., 1993).
  The 'stock' is in fact a sill-dyke complex. The intrusive bodies are often strongly mineralised along both their footwall and hangingwall contacts. In the Betze-Post pit thick sills of the Goldstrike intrusive cut a 200 m thick interval spanning the Rodeo Creek-Popovich Formation contact. The intrusive forms the southern boundary to the Betze mineralisation. Several of the shallow oxide orebodies mined prior to the development of the Betze-Post mine were preferentially located along its margins. In the Meikle area (described below), an age equivalent monzonite dyke forms the hangingwall to mineralisation, and is often associated with lamprophyre dykes and sills (Volk, etal., 1995).
Tertiary, Carlin Formation, generally composed of 0 to 140 m of light grey, water lain tuffs that weather to a light brown colour. The tuffs are slightly indurated at their base. These may commonly be underlain by 0 to 60 m of gravels to tuffaceous conglomerates which are un-consolidated, with sub-rounded sand, gravel and cobbles derived from the Vinini Formation and/or the Goldstrike Intrusive (Thoreson, 1993).

Structure

Three dominant high angle fault sets are observed at Goldstrike. These trend NNW, NE and NW (Volk, etal., 1995). The most prominent structure in the mine area is the NNW Post Fault which strikes at 335° and dips at 70° to 85°NE. Movement on the Post Fault has a complex history of repeated activity, cross-cutting both the mineralised Palaeozoic sediments and the post-mineral Tertiary Carlin Formation. Drilling indicates a dip-slip offset which may exceed 750 m (Thoreson, 1993). Extensive fracturing, with localised zones of shearing and brecciation are associated with this fault direction, accompanied by mineralisation and the most intense alteration (Jones & Huspeni, 1990). At surface these structures are often reflected by silicified or jasperoid ribs, while at depth they control the distribution of Jurassic dykes (Volk, etal., 1995). A secondary fault system trending north-east and dipping at 70 to 75°NW offsets the NNW set (Jones & Huspeni, 1990).

The Post Fault limits gold mineralisation to the east (Thoreson, 1993). The deposit, like that at Blue Star/Genesis, is located on the Tuscarora Spur a NNW trending ridge which has been separated by a 'Basin and Range' basin from the main Tuscarora Mountains to the east.

The structural fabric of the Post/Goldstrike mine is dominated by a north-west trending antiform which has folded the sediments in the footwall, or south-western side, of the Post Fault that truncates the eastern limb of the fold. This fold, which is assumed to represent part of the regional Tuscarora Antiform, is asymmetric, with the east limb dipping more steeply than the west. The siliceous mudstones of the Rodeo Creek Unit are intensely folded on the limbs of the fold. This structure is typical of the tight, asymmetric folding in the mine area, which is intimately related to thrust faulting. Late movement along faults both parallel to the Post Fault and in a north-east antithetic direction, offsets gold mineralisation from 3 to 35 m on each structure (Thoreson, 1993; Jones & Huspeni, 1991; Jones & Huspeni, 1990).

The 'Goldstrike Stock' occupies the southern half of the pit. The upper part comprises stacked sills parallel to low angle structures which exhibit post Cretaceous movement (Andrew, 1993).

A more subtle NW, 290° to 310°, structural trend is interpreted along the brecciated interface between intrusive rocks and sediments. This is also the orientation of the Betze deposit. Structures trending NE at between 30° and 50° contain local mineralisation and typically offset north-west trending faults. Brecciation occurs as sedimentary debris flow breccias within the Popovich Formation, irregular hydrothermal breccias and as tectonic breccias along major structural zones (Jones & Huspeni, 1991).

The oldest major structure is the Devono-Carboniferous Roberts Mountains Thrust which forms the contact between the Ordovician Vinini Formation and the underlying Devonian Rodeo Creek Unit (Thoreson, 1993). Within the Betze-Post pit the Roberts Mountains Thrust is a 0.3 to 1.5 m thick shallow dipping fault which separates the Vinini Formation and Rodeo Creek Member. At Meikle it has become an imbricate zone, 15 to 30 m thick, of penetrative phacoidal cleavage. Thrust faults are common in the Rodeo Creek Member and Vinini Formation. The most common are elongated in a NW direction, apparently representing NE-SW shortening, perpendicular to the dominant fold axes (Volk, etal., 1995).

Consult the record Goldstrike - Betze / Post - Mineralisation for a detailed description of Mineralisation and Alteration.

For more detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2002.     Record last updated: 23/10/2014
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.


Betze-Post

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Almeida C M, Olivo G R, Chouinard A, Weakly C and Poirier G,   2010 - Mineral Paragenesis, Alteration, and Geochemistry of the Two Types of Gold Ore and the Host Rocks from the Carlin-Type Deposits in the Southern Part of the Goldstrike Property, Northern Nevada: Implications for Sources of Ore-Forming Elements, Ore Ge: in    Econ. Geol.   v105 pp 971-1004
Arehart G B, Foland K A, Naeser C W and Kesler S E,  1993 - 40 Ar/ 39 Ar, K/Ar, and fission track geochronology of sediment-hosted disseminated gold deposits at Post-Betze, Carlin Trend, northeastern Nevada : in    Econ. Geol.   v. 88 pp. 622-646
Emsbo P, Hofstra A H  2003 - Origin and significance of postore dissolution collapse breccias cemented with calcite and barite at the Meikle gold deposit, northern Carlin Trend, Nevada: in    Econ. Geol.   v98 pp 1243-1252
Emsbo P, Hofstra A H, Lauha E A, Griffin G L, Hutchinson R W  2003 - Origin of high-grade gold ore, source of ore fluid components, and genesis of the Meikle and neighboring Carlin-type deposits, Northern Carlin Trend, Nevada: in    Econ. Geol.   v98 pp 1069-1105
Groff J A, Campbell A R, Norman D I  2002 - An evaluation of fluid inclusion microthermometric data for Orpiment-Realgar-Calcite-Barite-Gold mineralization at the Betze and Carlin mines, Nevada: in    Econ. Geol.   v97 pp 1341-1346
Kesler S E, Fortuna J, Zaojun Ye, Alt J C, Core D P, Zohar P, Borhauer J, Chryssoulis S L  2003 - Evaluation of the role of sulfidation in deposition of gold, Screamer section of the Betze-Post Carlin-type deposit, Nevada: in    Econ. Geol.   v98 pp 1137-1157
Leach T M  2004 - Distribution of alteration and mineralisation in the northern Carlin Trend gold deposits, Nevada: in   Hi Tech and World Competitive Mineral Success Stories Around the Pacific Rim,  Proc. Pacrim 2004 Conference, Adelaide, 19-22 September, 2004, AusIMM, Melbourne,     pp 153-159
Lubben J D, Cline J S and Barker S L L,  2012 - Ore Fluid Properties and Sources from Quartz-Associated Gold at the Betze-Post Carlin-Type Gold Deposit, Nevada, United States : in    Econ. Geol.   v.107 pp. 1351-1385
Palenik C S, Utsunomiya S, Reich M, Kesler S E, Lumin Wang and Ewing R C  2004 - Invisible gold revealed: Direct imaging of gold nanoparticles in a Carlin-type deposit: in    American Mineralogist   v89 pp 1359-1366
Ye Z, Kesler S E, Essene E J, Zohar P B, Borhauer J L  2003 - Relation of Carlin-type Gold mineralization to lithology, structure and alteration: Screamer zone, Betze-Post deposit, Nevada: in    Mineralium Deposita   v38 pp 22-38


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