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Meikle, Rodeo and Griffin

Nevada, USA

Main commodities: Au
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The Meikle orebody is approximately 2.5 km to the north of the Betze-Post deposit, along the northward extension of the Post Fault on the northern boundary of Eureka County in north-eastern Nevada, USA, some 40 km to the NNW of the township of Carlin. The Goldbug, Barrel, Rodeo and Griffin underground orebodies are located between the Betze open pit and Meikle undeground mines. Both Meikle and Rodeo are exploited as part of the Goldstrike underground operation.

Meikle is a deeper, relatively low tonnage, high grade orebody, with reserves of:

      6.5 Mt @ 21 g/t Au (Proven+probable Reserve, Dec '92, Christensen, '93)
      7.6 Mt @ 23.5 g/t Au = 177 t Au (Proven+probable Reserve, Dec '94, Volk, et al., 1995)
      9.1 Mt @ 24.4 g/t Au = 222.11 t Au (Production+reserves+resources, 2002, Theodore, et al., 2003)
      7.54 Mt @ 11.4 g/t Au (Proved+probable reserves, 2007; Mining.com. 2008)
     ~10 Mt @ 21 g/t Au = 220 t Au (Meikle, pre-mining resource, Jory, 2002)
     ~3.8 Mt @ 12 g/t Au = 46 t Au (Rodeo, pre-mining resource, Jory, 2002)
     ~1.3 Mt @ 12 g/t Au = 15.5 t Au (Griffin, pre-mining resource, Jory, 2002)
     ~1 Mt @ 6 g/t Au = 6.2 t Au (Barrel, pre-mining resource, Jory, 2002)
     ~3.5 Mt @ 10.7 g/t Au = 37 t Au (Upper Goldbug, pre-mining resource, Jory, 2002)
     ~4 Mt @ 7.5 g/t Au = 31 t Au (Lower Goldbug, pre-mining resource, Jory, 2002)

      10.6 @ 7.6 g/t Au (Goldstrike underground proved+probable reserves, 2013; Barrick Gold, 2014)
      6.1 @ 9.2 g/t Au (Goldstrike underground measured+indicated resources, 2013; Barrick Gold, 2014)
      1.3 @ 9.5 g/t Au (Goldstrike underground inferred resources, 2013; Barrick Gold, 2014)

The orebody is compact and high grade when compared to that at the nearby Post-Betze deposit. In early 1995 it was being developed as a 2000 tpd underground mine (Volk, et al., 1995).

The regional geology, mineralisation and alteration of the Meikle deposit are as decribed in the Post-Betze and Carlin Trend records. In sumary, they comprise a series of silicified solution collapse breccias that occur within the silty limestones, micrites and fossiliferous limestone sequence of the Devonian Popovich Formation, and to a lesser extent the Siluro-Devonian Roberts Mountain Formation silty limestones.

Like the Post-Betze deposit, ore at Meikle is bounded to the east by the Post Fault which has been intruded by a series of monzonite dykes that are typically 30 to 60 m thick and are compositionally similar to the Goldstrike intrusive. The ore is underlain by the fossiliferous limestone of the Roberts Mountains Formation which forms a prominent palaeo-topographic high along the length of the Meikle orebody (Volk, et al., 1995).

The Meikle deposit is hosted by a complex series of breccias which are located within the Popovich Formation. Mineralisation has been traced from 240 to 580 m below the surface, and is open at depth. The mineralised breccias are generally heterolithic and contain clasts of all other sedimentary and intrusive lithologies, with the exception of the Quaternary Carlin Formation. They breccia bodies are typically pipe like in geometry and often occur along or within structural zones. The clasts indicate multiple stages of brecciation and silicification. While the majority of the ore is breccia hosted, significant ore grade mineralisation is also found within non-brecciated sediments and intrusives. In contrast to Post-Betze, no ore grade has been recorded in either the Vinini or Rodeo Creek sediments (Volk, et al., 1995).

The Meikle orebody has been divided into three ore zones. These are the:

Upper Main Zone - which is relatively flat lying and occurs immediately below the Rodeo Creek-Popovich contact. This contact often exhibits low angle brittle-ductile fabrics and is typically intruded by a series of latite and lamprophyre sills which are presumed to be of Jurassic age. This zone occurs along the apex of the palaeo-topographic high of the Roberts Mountains Formation mentioned above (Volk, et al., 1995).
Lower Main Zone - which dips at 65 to 75° to the north-east and is located in the footwall of the Post Fault system, along the eastern boundary of the palaeo-topographic high of the Roberts Mountains Formation mentioned above. This zone comprises the high grade core of the deposit and has an average thickness of around 60 m, strike length of 350 m and trend of 300 to 340°. It has been interpreted to be the main 'feeder' zone for mineralisation, and yields intersection of more than 30 g/t Au. Mineralisation in the upper portion of this orebody is confined between the footwall of the monzonite dyke system and the hangingwall of the palaeo-topographic high of the Roberts Mountains Formation (Volk, et al., 1995).
South Meikle Ore Zone - This zone occurs at the same stratigraphic position as the Upper Main Zone, but lies to the west of the palaeo-topographic high of the Roberts Mountains Formation. Ore occurs within breccias and mudstones underlying the lamprophyre sill complexes along the Rodeo Creek-Popovich contact, while in many cases the sills themselves host high grade mineralisation along their footwall contacts. The geometry of orebodies in this zone is typically synformal and lenticular, with marked reductions in grade and thickness towards their up dip edges. The average thickness of ore at South Meikle is 12 to 15 m. In places, clasts of green chert typical of the Vinini Formation are found within breccias underlying the South Meikle orebody. This section of the Vinini Formation is absent at this locality, suggesting the wholesale collapse of the whole section overlying the ore zone (Volk, et al., 1995).

The dominant control of mineralisation at Meikle appears to be the NNW striking structures of the Post-parallel fault system which are interpreted to have formed the main conduits for both intrusives and gold mineralisation. These zones apparently focused the intervals of breccia formation within the Popovich Formation and carbonate dissolution within the underlying Roberts Mountains Formation. Structural intersections tend to host high grade pods of mineralisation, with complex geometries. Low angle structures have apparently controlled the location of dykes and sills which appear to have formed upper barriers/caps to decarbonatisation/decalcification and gold mineralisation. The same low angle structures have also emplaced impervious caps of Vinini Formation and Rodeo Creek unit siliciclastics over reactive carbonate rocks of the Popovich Formation (Volk, et al., 1995).

The controls on mineralisation at both Meikle and Post-Betze appear to be: i). The presence of district scale NNW, NW and NE trending, long lived, high angle structures which have been subjected to multiple deformational events, including transpressional and transtensional phases ii). The occurrence of reactive carbonate proximal to these structures, particularly debris flow and sedimentary breccias of the middle and lower Popovich Formation, and secondary breccia zones; iii). The formation of 'map scale' antiforms providing caps to the mineralised zones, but with associated fracture systems on the hinges and limbs of these folds; and iv). The presence of thrust faults which have emplaced capping siliciclastics of the Rodeo Creek unit and Vinini Formation, resulting in impermeable barriers being created. Intrusives within these structures are interpreted as also having inhibited fluid flow. High grade mineralisation is apparently influenced by all of these factors, while in addition, the intersection of high angle faults has also controlled the localisation of high grade shoots with complex shapes; (Volk, et al., 1995).

The gold overprints an older base metals event, characterised by silica, sphalerite and pyrite. Solution collapse breccias associated with this early stage are overprinted by a second Jurassic collapse accompanying the intrusive activity that is prior to the introduction of gold. Both systems utilise the same fault system.

The most recent source geological information used to prepare this summary was dated: 2001.     Record last updated: 22/10/2014
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.


Meikle

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
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
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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