The three main Vantage disseminated gold deposits and the Yankee Basin Deposits 10 km to the SSE are located in the Alligator Ridge district, some 115 km to the SSE of the town of Carlin, in Nevada, USA. They lie on the projected extension of the Carlin Trend.
Previous mining in the district dates back to the 1870's, although no prior workings were obvious in the vicinity of the deposits. The orebodies are adjacent to a 900 m segment of a major normal fault. They are localised at the contact between the Devonian Devils Gate Limestone (equivalent to the Popovich Formation on the Carlin Trend and the Wenban Limestone of the Cortez-Battle Mountain Trend) and the overlying Devono-Carboniferous Pilot Shale (Ilchik, 1990).
The Devils Gate Limestone is underlain by the Devonian Nevada Formation, the oldest unit exposed in the district, which is a medium grey, saccharoidal, dolostone. The Devils Gate Limestone is composed of a lower dolostone and an upper micritic limestone. The Pilot Shale is a thinly laminated, calcareous, carbonaceous siltstone that is approximately 140 m thick, separated into two members by a 6 m thick chert bed. The lower member is the chief host to ore and is composed of 30 to 45% dolomite, 35 to 50% illite, 14% detrital quartz, 5% pyrite, 1.5% organic matter and 0 to 3% kaolinite. The Pilot Shale is overlain successively by the Joana Limestone and the Chainman Shale, both of lower Carboniferous age (Ilchik, 1990).
The orebodies were hemispherical to bell shaped in cross section and were circular to slightly elongate in plan. They ranged from 120 to 270 m in diameter and were up to 75 m thick. The highest grades of around 25 g/t Au, were in the central portion of each orebody, generally just above the contact between the Devils Gate Limestone and the Pilot Shale (Ilchik, 1990).
Gold mineralisation is preferentially developed in the Pilot Shale, accompanied by Ag, Sb and As. The main form of alteration associated with gold is decalcification and quartz replacement of the host rock carbonate. Quartz replacement in the Pilot Shale closely follows the contact between ore and waste, whereas in the Devils Gate Limestone, a stratabound jasperoid extends well beyond the ore zone. In the Pilot Shale the outer boundary of the ore corresponds closely with the outer boundary of silicification and an outward increase in the carbonate content of the hosts. Beyond the area of silicification, diagenetic dolomite in the Pilot Shale was replaced by hydrothermal calcite and calcite veins accounting for up to 10% of the rock (Ilchik, 1990).
Subsequent to gold mineralisation, intense oxidation, believed to be hydrothermal, destroyed organic matter, sulphides and detrital illite in much of the Pilot Shale and alunite±barite was deposited. This phase was barren of gold and had a similar distribution to that of the silicification. K/Ar dating of the alunite yields an age of 11.5 Ma. Surface oxidation and erosion produced a later overprinting jarosite and goethite zone (Ilchik, 1990).
Jasperoid, which was more extensive than ore, commonly contains >100 ppb Au, while limestone beneath the alteration contact was barren. The highest and most consistent gold grades were within carbonaceous siltstone, although visible gold was not evident. Visible gold in the oxidised and indurated ore was present as overgrowths on vein quartz, accounting for 15% of the gold and was recoverable by gravity techniques. The jasperoid and carbonaceous siltstones are taken to represent the primary ore (Ilchik, 1990).
Samples collected from a jasperoid body 200 m from the deposit in 1976 led to its discovery. The maximum value obtained was 0.5 ppm Au. Rock chip samples from a small outcropping portion of the first of the orebodies, the remainder of which was not exposed, gave a value of 9 ppm Au. A soil sampling grid on 60 x 150 m centres gave anomalous values of up to 160 ppm As, 70 ppm Sb and 983 ppb Hg, with detectable Au at four sample sites (Ilchik, 1990).
Total production to 1990 from the Vantage deposits was quoted as - 5.8 Mt @ 4 g/t Au for 23 tonnes of Au
The Yankee Basin deposits differ from most other Carlin type ores in that they contain abundant oil. There are 16 separate gold bodies in this group. These are generally smaller and lower grade than those at Vantage, with average grades of around 1.5 g/t Au, each with just 0.05 to 1.5 tonnes of contained gold. However their shallow depth, complete oxidation and proximity made them commercially viable. These deposits, as at Vantage, occur in the lower part of the Pilot Shale and in the upper part of the overlying Devils Gate Limestone. The contact between the two units is marked by vuggy, stratabound jasperoid up to 30 m thick which extends far beyond the individual deposits.
The Yankee Basin occurrences are localised in a domal antiform that may have acted as a trap for both the petroleum and hydrothemal fluids. These rocks have also been extensively disrupted by faulting which has served to localise hydrothermal fluid flow and alteration. Two types of body are recognised. The first is tabular, following faults and fracture zones and encompassing scattered dissolution breccias. The second are columnar in shape and are dominated by dissolution breccias.
The ore takes the form of complexly cross cutting veins of quartz and carbonate with lesser barite and associated pyrite with stibnite and realgar, abundant petroleum and gold.
(Source: PorterGeoconsultancy, www.portergeo.com.au, 1999)
