Alligator Ridge, Alligator Ridge Mining District, White Pine County, Nevada, USAi

Regional Level Types
Alligator Ridge	Mine
Alligator Ridge Mining District	Mining District
White Pine County	County
Nevada	State
USA	Country

Structure: Basin and Range-type high angle normal faults. Low amplitude anticlines and synclines that strike N-S and plunge 20S, with limbs that dip nearly 20 degrees. N- to NE-striking normal fault ; possible N-NE striking anticline.

Alteration: Alteration consists predominantly of quartz replacement of host rock carbonate. Quartz replacement in Pilot Shale closely follows the contact between ore and waste, whereas in Devils Gate Limestone, a stratiform jasperoid replacement body extends well beyond the ore zones. Beyond the area of silicification, diagenetic dolomite in Pilot Shale was replaced by hydrothermal calcite, and calcite veins comprised as much as 10% of the host rocks. Following gold deposition, intense oxidation destroyed organic matter, sulfides, and detrital illite in Pilot Shale and alunite and/or barite veins were deposited; this event is thought to be hydrothermal. Lastly, oxidation related to weathering deposited jarosite and goethite. Shifts in the isotopic compositions of carbonate are associated with gold mineralization.

Commodity: Ore Materials: native gold Gangue Materials: drusy quartz, barite, calcite, gypsum, alunite, kaolinite in oxidized ore; stibnite, pyrite, orpiment, realgar, calcite in carbonaceous ore. Also jarosite, specular hematite, stibiconite, goethite

Deposit: The ore zones lie directly above an unmineralized jasperoid unit which has replaced the lowest portion of the Pilot Shale. The mineralization is related to a NNE-striking normal fault exposed in the north and east walls of the Vantage I open pit. This fault flattens at depth to a low-angle or bedding plane shear. The three Vantage deposits form a mineralized zone that covers an area 3000 feet long by 1000 feet wide. Dimensions of the Vantage one orebody are approximately 250 by 250 by 250 feet with an average grade of 0.1 opt gold. The Vantage II orebody covers an area approximately 800 feet long by 500 feet wide and 300 feet thick averaging 0.11 opt gold. The Vantage III deposit covers an area 200 feet long by 200 feet wide and 150 feet thick with an average grade 0.08 opt gold. The Vantage Ine orebody was exposed at surface. Mineralization in the remaining deposits becomes progressively deeper from north to south. UPON CHECKING PLACER DOME 10K, 1994 AND AN. RP. 1994, IT APPEARS THAT PLACER DOME U.S., INC. HAS INCLUDED THIS PROPERTY WITH ITS BALD MOUNTAIN OPERATION, (0320330503).

Deposit type: Sediment-hosted Au

Development: The Alligator Ridge mine was discovered in an area of no previous exploration or mining history where claims were first staked by a prospector in 1976, working on a grubstake for AMSELCO Minerals. The area was subsequently mapped, sampled and drilled. Reserves were announced in 1981 when mining began. A new carbon in-leach facility was finished in 1987 to improve recovery of gold from ore. The mine was first operated by AMSELCO Minerals, later BP Minerals Inc., and later by a 50/50 joint venture with NERCO Minerals Co. (1988). Reserves at the Alligator Ridge mine were exhausted in December, 1989, but the mine continued to produce intermittently. The mine last produced in 1996, but production was included with that of the Bald Mountain mines of Placer Dome U.S. Inc., and not reported separately. In 2006, Barrick acquired the Alligator Ridge Mine properties through its acquisition of Placer Dome U.S. Inc..

Geology: The disseminated gold ore occurs in silicified siltstones of the pilot shale with the high grade ore occurring in discontinuous, carbonaceous lenses. Just east of the vantage pit the vantage normal fault strikes n10e and dips 70 degrees east and has downthrown the underlying devils gate limestone on east against the upthrown pilot and joanna sequence, which lie to the west of the fault. Gold deposition is accompanied by ag, sb, as, +/-hg. Upper age limit of gold mineralization set by movement along the vantage fault, which offsets 39-24 ma volcanics. Lower age limit set by k-ar dating of post mineral alunite, which yields an age of 11.5 ma. There are four separate but adjacent areas of mineralization containing submicron sized to visable particles of gold in iron-stained siltstone. Each deposit occurs at greater depth As evidenced by an increase in sulfides and carbonaceous material. All deposits contain some oxide material. The origin of the mineralization is believed to be young geologically, maybe less than five million years, and related to volcanic events still evidenced by nearby hot springs and geysers. At present, there does not appear to be any relationship between the four mineralized areas. There are four classes of ore: primary, partly deleterious, deleterious, and carbonaceous. Deleterious is defined As 1-2% sulfides, occurring As realgar, orpiment, and stibnite. Mineralization for m1 and m2 is the same. Stibiconite is also present. Ore is related to hot spring activity that caused silicification and alteration of the pilot shale, and deposition of the micron sized gold particles. The ore bodies occur As lenses that conform to the attitude of the shale, and align with a set of north-northeast striking fractures. Vantage 1 is roughly circular, 200x200x40 meters. Vantage 2 is 200x100x40 meters. Vantage 3&4 are estimated to be 200x100x50 and 200x200x50 meters, respectively. All 4 deposits are irregular in shape and occur near the base of the shale. Other types of wall rock alteration are jasperoid silicification, oxidation, and decarbonization. General size of ore body is medium. Deepest ore body is 120 meters.

Ore(s): Stratigraphic controls: The greatest portion of precious metal mineralization occurs in the lower 300 feet of the Pilot Shale; a small portion of the mineralization is in the upper 25 feet of the underlying Devils Gate Limestone. This conformable sedimentary contact was the focus for lateral movement of hydrothermal fluids. The upper 100 feet of the pilot shale is a less permeable claystone and may have capped hydrothermal fluids. Structural controls: the Vantage fault served as a conduit for rising hydrothermal fluids. There is also evidence of a north-northeast striking asymmetrical anticline running the length of the deposits. The axis of the anticline would lie along the strike of the three vantage deposits and plunges gently to the southwest. Along the crest of the anticline numerous extensional fractures and extensive brecciation may have provided conduits for the ore-bearing solutions.

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Link to USGS MRDS:	10310371

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