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Goldstrike - Betze / Post - Mineralisation |
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Nevada, USA |
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Au
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Super Porphyry Cu and Au
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The Betze/Post orebody and satellites of the Goldstrike mine are located on the boundary of Elko and Eureka Counties in north-eastern Nevada, some 40 km to the NNW of the township of Carlin. 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. The orebody is part owned by Newmont Gold Company and Barrick Goldstrike Mines Inc.
DETAILED DESCRIPTION
Consult the record Goldstrike - Betze / Post - Geology for a detailed description of Geology, Structure and reserves/resources.
Alteration
Mineralisation and alteration appear to have been controlled primarily by faults, crests of folds and permeability of the host rocks. As in the other gold deposits of the Carlin trend, the principal alteration styles are decalcification, silicification and argillisation with associated pyritisation and subsequent oxidation (Arehart, et al., 1993).
• Decalcification (or de-carbonatisation) - The earliest alteration style is decalcification, which supposedly removed up to 30% of the rock mass in places, commonly without an accompanying decrease in volume, thus greatly increasing the porosity and permeability of the altered rock. In areas of faulting some compression of the section occurred due to collapse of the decalcified rocks (Arehart, et al., 1993). Decalcification of the Popovich Formation limestones occurred along stratigraphically favourable horizons and along structurally prepared zones. The bioclastic limestones and debris flow members were favourable to decalcification (Thoreson, 1993). There is a distinct decrease in the extent of decalcification, up-section, away from faults. Carbonate units having lesser amounts of detrital material had sufficiently low initial permeability to resist significant porosity increase by decalcification. In contrast, the permeability and porosity of silty, debris flow and bioclastic units, with initial higher permeabilities, were enhanced. This enhanced permeability appears to have played a major role in controlling the subsequent location of mineralisation (Arehart, et al., 1993).
• Silicification - post dated decalcification and was strongly influenced by both stratigraphy and structure. Silicification occurs as quartz veins, stockwork veinlets and pervasive replacement (Jones & Huspeni, 1990). The products and degree of silicification ranges from weak veinlet and incipient silicification which are both fracture controlled, to complete replacement of bedded sedimentary rocks. Sedimentary structures are preserved in the latter replacement style which results in the formation of bedded jasperoids. Jasperoid breccias which are common at depth, represent subsequently silicified sedimentary slump, debris flow or karst breccias, as well as fault and explosion breccias. The possible explosion breccias include structures that are interpreted to have been diatreme material and pebble dykes (Arehart, et al., 1993).
Silicification occurs within fault zones and as stockwork veinlets within the siliceous Rodeo Creek Unit, while early pervasive silica flooded the permeable decalcified sediments. The early silicification was sulphide and gold poor. A second silica event penetrated the decalcified and brecciated sediments strongly silicifying the host sediments. The later event deposited fine grained euhedral pyrite and marcasite overgrowths onto the surface of pre-existing pyrite within the sediments. This pre-existing pyrite was probably of diagenetic origin. Gold is associated with late arsenian pyrite and authigenic pyrite that rims the larger pyrite grains, forms along fractures within the earlier pyrite, and is developed as fine grained disseminated sulphides (Thoreson, 1993).
Jasperoid was present above Post in the original outcrop (Andrew, 1993).
According to Volk, et al., (1995), at least five stages of silicification are evident. The earliest formed is coarse grained, white to grey bull-quartz, primarily in the Vinini and Rodeo Creek Member, and is pre-mineralisation. It is a syn-deformational product infilling thrust planes and concentrated in fold hinges. The second event appears to accompany decalcification, and possibly early stage mineralisation, but is often barren. The third silicification is associated with the main stage gold mineralisation, and is characteristically fine grained to microcrystalline and typically occurs with fine pyrite. Cross-cutting relationships observed in thin section suggest episodic or pulsating deposition. A fourth stage post mineralisation chalcedonic silica is common at higher structural levels, typically coating or infilling vugs and fractures in structural zones. The youngest silicification is hyaline in texture and is restricted to structural positions in the Vinini Formation. At Meikle, silicification is locally pervasive, with wholesale replacement of original sedimentary textures peripheral to high grade gold (Volk, et al., 1995).
• Argillisation - commonly occurs along faults and fractures which cross-cut silicified zones (Jones & Huspeni, 1991). Phyllo-silicate zoning in ore-hosting sediments at Post/Goldstrike comprises a core of kaolinite, where sericite-illite has been destroyed, grading outwards into the hydrothermal kaolinite dominant, kaolinite-sericite assemblage. The 'kaolinite only' zone is restricted to the most intensely altered rocks. In this core, the kaolinite is dominantly dickite. Isotope studies indicate that this kaolinite is hydrothermal rather than supergene in origin. Outside of the core of kaolinite, the sedimentary rocks contain detrital and/or diagenetic kaolinite and sericite, with many containing both types. Hydrothermal sericite is generally only present in small amounts. Regional studies have however indicated that the entire Carlin Trend was subjected to prolonged elevated temperatures during the early Mesozoic, probably resulting in the recrystallisation and upgrading of pre-ore diagenetic illite (Arehart, et al., 1993).
• Oxidation - is predominant in the Rodeo Creek Unit, with the base of oxidation occurring approximately 120 to 180 m below ground level, roughly corresponding to the top of the carbonaceous mudstones within the upper Popovich Formation. The oxide resource of the Upper Post deposit lies within this section (Jones & Huspeni, 1990).
• Pyritisation - There are multiple episodes of pyrite development, only one of which is associated with gold mineralisation. The earliest formed pyrite is brassy in colour and coarse, possibly diagenetic in occurrence. Pyrite is also associated with thermal metamorphism, occurring with quartz and sericite in the intrusive. The next stage is very fine grained pyrite/marcasite, imparting a dark grey colouration to the rock, with a slight bronzy sheen. A further stage of pyrite is arsenic rich and intimately associated with gold mineralisation. This gold-pyrite assemblage was deposited as thin rinds on earlier formed sulphide grains. In addition there is a late stage , brassy pyrite that fills vugs and fractures. Pyritisation occurs within both silicified and argillised zones, with the pyrite content ranging from 5 to 20% in ore (Volk, et al., 1995).
In addition to these forms of hydrothermal alteration, late diagenetic reactions and contact metamorphism related to the Goldstrike intrusive are also observed. Late diagenetic pyrobitumen veinlets are common throughout the upper Roberts Mountains Formation, particularly within the fossiliferous limestone facies. Enrichment of carbon is also observed along high angle structures and within the hinge zones of map scale anticlines. Cross-cutting relations indicate that the hydrocarbons were rendered immobile prior to gold mineralisation (Volk, et al., 1995).
Contact metamorphism related to the Goldstrike intrusive takes the form of quartz hornfels within clastic units of the Rodeo Creek Member and Vinini Formation, and pyroxene hornfels and marble with minor biotite hornfels within the Popovich Formation. Metasomatism associated with the same intrusive has produced diopside-grossular endo-skarn and diopside-grossular-vesuvianite exo-skarn. These skarns also contain retrograde tremolite-epidote-calcite assemblages and a later K feldspar-diopside overprint. The aureole varies from 10 m or so up to 600 m from the intrusive contact (Volk, et al., 1995).
Mineralisation
Gold mineralisation at Post/Goldstrike is classified into three zones, as follows:
Upper Post - The Upper Post orebody, now mined out, was predominantly hosted by oxidised and siliceous sediments of the Rodeo Creek Unit. Gold mineralisation was stratigraphically controlled with higher grade material occurring in argillised and bleached siltstone and sandstone lenses (Thoreson, 1993). High grade 3 to 9 g/t Au lenses appear to have been stratigraphically controlled, occurring along silicified and argillised sandstone lenses (Jones & Huspeni, 1991; Jones & Huspeni, 1990). Structurally controlled mineralisation was present as stockwork quartz veins and as silicified fault breccias within siltstone and mudstone (Thoreson, 1993; Jones & Huspeni, 1991).
Where the orebody was present within the upper unit of the Popovich Formation, the prime hosts of high grade mineralisation were the debris flow breccias within the siltstones and limestones in the centre of the pit. Carbonaceous members of the Popovich Formation were mainly waste, occupying the core of the antiform which had exposed the structural window. The Upper Post ore was wrapped over this core of carbonaceous waste (Andrew, 1993).
In the lower sections of the orebody where sulphides were present, high grade zones with realgar and orpiment assayed up to 20 to 25 g/t Au. In the sulphide zone, the gold was associated with fine 10 to 35 µm pyrite grains, with cores containing 50 to 70 ppm Au, encased in arsenical rinds carrying 400 to 700 ppm Au. The bulk of the production to 1993 was from the oxidised sections of this orebody (Andrew, 1993).
Lower Post/Betze - Newmonts Lower Post and Barrick's Betze ore blocks are connected and parts of the same orebody, the largest known to date on the joint venture property. Ore occurs beneath the Upper Post oxide zone, primarily within the variably carbonaceous and pyritic, mudstone-siltstone and silty limestones of the lower unit of the Popovich Formation with limited mineralisation in the underlying laminated siltstones of the Upper Roberts Mountains Formation. The Lower Post mineralisation is refractory and developed parallel to, and predominantly in the footwall of the NNW trending Post Fault Zone. The larger Betze orebody, also refractory, is located within the hinge zone of the WNW trending Betze Anticline.
Gold mineralisation is largely stratigraphically controlled and overall, forms a bedding parallel body. The highest grades of 5 to 12 g/t Au are associated with decalcified bioclastic (fossil hash) limestones, debris flow limestones and along pyrite filled shear zones. These higher grade zones are generally found near the base of the Popovich Formation, associated with pervasive silicification, pyritisation and hydrothermal brecciation. The average sulphide content of the Lower Post deposit is 4%, with locally abundant stibnite (Thoreson, 1993). The Lower Post and Betze orebodies within the Popovich Formation are 90 m thick, although some zones are up to 180 m thick. They occur at a depth of 240 to 300 m below the surface with a total length of 1200 m and width of 180 m.
Deep Post - This is a high grade, structurally controlled, refractory orebody, with a 7 g/t Au cut-off. Like the Lower Post/Betze orebody, it is hosted by the lower unit of the Popovich Formation. Mineralisation is associated with a 310° striking shear zone that dips steeply to the south-west. This shear zone forms the north-east margin of the Goldstrike Intrusive at Deep Post, then diverges from the intrusion, to the north-west towards Barrick's Betze deposit. Numerous blocks of igneous and metamorphic rocks have been caught up within the shear. Its hangingwall consists of un-mineralised Goldstrike intrusive and related contact metamorphosed sediments (Thoreson, 1993). The Deep Post orebody is underlain by a post mineral biotite-feldspar porphyry intrusive (Andrew, 1993).
High gold grades (>15 g/t Au) are associated with shearing and elevated concentrations of fine grained pyrite. The Deep Post orebody is characterised by its high pyrite content, usually >4%, its high total clays, usually >14% and by its low quartz of around 66% SiO2. Restricted mineralisation occurs along clay-pyrite-silica altered shears, veins, stockworks and fractures within the competent igneous and metamorphic rocks. Higher grade gold mineralisation is commonly found 'ponded' beneath 'impermeable' lenses of igneous and metamorphic rocks within zones of silicified and pyritic sediments. Arsenic occurs locally in the sediments below the hangingwall, while realgar and orpiment veins are associated with the high gold values (Jones & Huspeni, 1991; Thoreson, 1993).
High grade drill intersections included 122 m @ +30 g/t Au and 148 m @ 57 g/t Au. The arsenic levels within the orebody average 0.7% As (Andrew, 1993).
Smaller deposits such as Post Oxide, Long Lac, West Bazza , etc., are found in the Rodeo Creek/Vinini Formation, preferably in calcareous siltstones to fine grained sandstones interbedded with siliceous rocks. The smallest deposits, (Number 9, Lost Pancana, etc.,) are within the Goldstrike stock and are localised along structural zones which are usually low angle faults. The smaller deposits of Lost Pancana, Long Lac, Bazza, West Bazza and the larger Screamer body form a line of occurrences over a length of 2.75 km, in a NW-SE direction, parallel, and 500 m south west of to the Post-Betze mineralised zone (Jones & Huspeni, 1990). Volk, et al., (1995) however, indicate that the Betze and Screamer orebodies are connected and that an additional orebody, Winston, is located below and overlapping the old Long Lac pit.
In the larger deposits, a combination of favourable beds and structural control generally result in tabular deposits of irregular shape, or elongated in a north-west direction (Jones & Huspeni, 1990).
Controls on mineralisation at Post-Betze include high angle faults, anticlines, and fault related breccias. Deposit scale faults are the primary depositional sites for mineralisation, represented by the Post and Betze anticlines. The Post Anticline is doubly plunging and asymmetric, with numerous parasitic, eastward verging folds exposed on both limbs. The western limb is shallow, while the eastern is steep. A well developed fracture zone occurs in the fold hinge, corresponding to the location of the Upper Post orebody. A set of fractures parallel to the Post Fault are developed on both limbs. These appear to represent feeder structures to both the Lower and Upper Post deposits (Volk, et al., 1995).
The Betze Anticline has a WNW trend and is not compatible with the inferred compressional deformation of the Post Anticline and related folds. Its axis is however, sub-parallel to the northern margin of the Goldstrike intrusive, suggesting an association. A zone of ductile deformation, the Dillon Deformation Zone, which is believed to represent a NNE directed reverse fault, occurs on the northern limb of the Betze Anticline. The high grade core of the Betze orebody is preferentially located in the hinge zone of the Betze Anticline, with high grade pods documented in the Dillon Deformation Zone. The high grade south-western margin of the Betze deposit underlies sill complexes of the Goldstrike intrusive. The most favourable host lithologies in the Popovich Formation occur in its lower half. The micritic silty limestones in the upper half are not good hosts, except where they occur along the hinge line of the Betze Anticline and are intensely fractured at the contact with the intrusive. The zones of deformation and brecciation are interpreted to have provided a channel for ground water and hydrothermal fluids leading to the decalcification/decarbonatisation of carbonatic lithologies and the development of secondary porosity within large volumes (Volk, et al., 1995).
At Betze and Post, major mineralising faults have two dominant orientations. One is a continuous NNW to NW striking system, with steep east and west dips. Examples are the Post Fault System and the Buzzard Fault. The second set is less continuous and trends ENE to NE with moderate north dips, eg. the Winston and Adamite Faults. These faults are mineralised, although their intersection with NNW sets define high grade shoots. The NNW set faults are usually mineralised. One such structure, the Post Fault, is regarded as being the feeder zone to both the Lower and Upper Post orebodies. The NE faults connect the other set and appear to have acted as aquitards, as their hangingwalls are seldom mineralised (Volk, et al., 1995).
As described above (in the 'Alteration' segment), the gold mineralisation is associated with arsenic-bearing pyrite. This gold bearing arsenian-pyrite is bimodal, with a coarse 200 µm and a 10 to 20 µm fine population. In the larger size fraction gold occurs within thin 10 to 25 µm, high grade (200 to 700 ppm Au), arsenic rich euhedral rims enclosing arsenic poor pre-existing pyrite grains. The fine grained pyrite however appears to be both gold bearing and arsenic rich throughout. There is 5 to 20% pyrite in ore. Visible gold is observed at various locations, and although volumetrically minor, is most prevalent in carbonaceous silty limestone and mudstone. Non-sulphide gangue includes quartz, kaolinite, calcite, sericite and barite. Minor stibnite, sphalerite, realgar-orpiment are also observed (Volk, et al., 1995).
Pyrite occurs in both silicified and argillised rocks, with an average of 5% in ore grade sulphide mineralisation. Pyrite is more extensive than the mineralisation, occurring well outside of the orebody. Carbon occurs in the mineralised zone and along the peripheries as a remobilised front. Supergene and possibly hypogene oxidation has resulted in the destruction of pyrite, and the production of oxides and sulphates, such as goethite, hematite, jarosite, alunite and gypsum (Jones & Huspeni, 1990).
As in the other Carlin Trend deposits there is a geochemical association of As, Sb, Hg and Tl with Au, as well as low levels of Ag. Arsenic values are highest in the sheared sediments below the barren hangingwall metamorphics of the Deep Post orebody. Realgar and orpiment are observed in the Deep Post and upper Betze orebodies (Thoreson, 1993). Jasperoid outcrops were mapped at the surface showing three phases of silicification. Soil geochemistry returned high background values of 30 to 100 ppb Au, with peaks of 100 to 200 ppb. Induced polarisation geophysics was employed to identify the structural fabric which were observed as low chargeability linears (Andrew, 1993).
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1998.
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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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
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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
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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
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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
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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
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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
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Kesler S E, Riciputi L C and Zaojun Ye 2005 - Evidence for a magmatic origin for Carlin-type gold deposits: isotopic composition of sulfur in the Betze-Post-Screamer Deposit, Nevada, USA: in Mineralium Deposita v40 pp 127-136
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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
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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
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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
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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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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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