Reid, Richard F.¹, Lauha, Eric A.¹, Kofoed, Reed K.¹, Vance, Randall B.¹, Blackmon, Jeffrey E.², Gopon, Phillip N.,³, (1) Newmont Mining Corporation, 1655 Mountain City Highway, Elko, NV 89801, (2) Blackmon Geosciences, LLC, 8790 W. Quarto Circle, Littleton, CO, 80128, (3) Department of Geoscience, University of Wisconsin-Madison, 1215 W Dayton St., Madison WI 53706
The Sandman gold deposits are located approximately 15 miles west of Winnemucca, Nevada, in the Krum Hills. Work is on-going to advance development of the deposits through a Joint Venture between Newmont Mining Corporation and Fronteer Development Group. The area is currently undergoing development and exploration drilling, feasibility studies, permitting activities and metallurgical testing.
Amazingly, prior to the 1980s, the area was unrecognized for any mineral potential. Following the discovery of the Sleeper deposit by Amax 20 miles to the northeast, Kennecott conducted a helicopter-supported exploration program in 1986. The color anomaly at North Hill was recognized and sampled returning gold values of up to 0.1 oz/st gold. Kennecott staked 35 square miles of BLM ground and drilled around 214 holes. Further exploration commenced with the consolidation of the intervening railroad checkerboard ground and a joint venture between Santa Fe Minerals and Kennecott. In 1996, Western States Minerals (now Fronteer) acquired the BLM sections and leased the JV ground gaining control of the project. Newmont and Fronteer entered into a joint venture on the project in June, 2008.
The Sandman area is underlain by basement rocks consisting of the Triassic Raspberry Formation, dominantly phyllite and quartzite. Overlying the Triassic basement in a mid-Tertiary graben is a complex mixture of volcaniclastic sediments, rhyolitic air fall and ash flow tuffs, conglomerates, and andesite/basalt flows. The volcanic field is early Miocene based on two age dates of 22.4 Ma and 22.6 from outcrops of basalt northeast and west of the Southeast Pediment deposit (Peters, 2003). The base of this basalt is locally hydrothermally altered. A source or vent for the volcanic material has not currently been identified.
The various areas of mineralization are associated with the intersection of north-south, and northeast structural trends and there are also northwest and east-west secondary structures. Most faults appear to be extensional with a minor strike-slip component. Growth faults, probably related to the development of the north-south graben, have an important influence on the volcanic lithologies and also control mineralization.
Alteration and mineralization is low sulfidation and characterized by silica-adularia flooding with subtle, discrete quartz-adularia veinlets. Highly fractured, structural breccias are common in the deposits and suggest that faulting was pre and post mineralization and a major control on mineralization. Mineralization may be stratigraphically controlled in some areas of the deposits. Large zones of strong argillic alteration surround the mineralization. Unger (2008) dated adularia west of Silica Ridge at mid-Miocene (16.2 Ma).
Four deposit areas have been identified to date: Southeast Pediment, Silica Ridge, North Hill and Abel Knoll. Together they contain approximately 250,000 ounces of gold. Gold-silver ratios decrease as gold grade increases (6.1:1 for all drill samples, 3.6:1 for gold >0.003 opt, 1.3:1 for gold >0.060 opt, 0.87:1 for gold >0.20 opt and 0.71:1 for gold >1.0 opt). Visible gold is generally present in the higher grade intervals.