Top, Mahoney Canyon, and Sage Flat Deposits, Bald Mountain Mining District, White Pine County, Nevada, USAi

Regional Level Types
Top	Deposit
Mahoney Canyon	- not defined -
and Sage Flat Deposits	- not defined -
Bald Mountain Mining District	Mining District
White Pine County	County
Nevada	State
USA	Country

Structure: The Top pit contains an array of high-angle faults that appear to radiate circularly from the Bald Mountain Stock, and low-angle, bedding-parallel faults. Both N-trending faults and less prominent W-trending fractures are present. The Bald Mountain district is located in an area of thinned crust along the eastern side of the Late Proterozoic rift that split the North American craton. It is also in the west-central portion of the Late Devonian-Early Mississippian Antler foreland basin, and near the eastern edge of deformation related to the late Paleozoic Humboldt orogeny To the west of the Bald Mountain district, geologic interpretations are dominated by recognition of Paleozoic deformation, whereas to the east, interpretations emphasize Mesozoic contraction and plutonism and Tertiary extension.

Alteration: Alteration at the Top Deposit includes decalcification, pervasive silicification, and argillization.The quartz porphyry dike is altered variably from incipient argillic alteration of feldspars to total textural destruction forming a soft white clay-rich rock. Locally, calc-silicate skarn and recrystallized marble have developed along the contact between th intrusive rocks and the calcareous sediments.

Tectonics: The Bald Mountain district is located in an area of thinned crust along the eastern side of the Late Proterozoic rift that split the North American craton. It is also in the west-central portion of the Late Devonian-Early Mississippian Antler foreland basin, and near the eastern edge of deformation related to the late Paleozoic Humboldt orogeny.

Commodity: Ore Materials: free gold Gangue Materials: pyrite, clay, limonite, jasperoid

Deposit: The Top deposit is the largest known mineral resource in the district. Gold mineralization at Top is hosted in the main quartz monzonite stock, quartz feldspar porphyry, and in limestone, argillite, and dolomites of the Ordovician Antelope Valley Formation along the margin of the Bald Mountain stock. Locally, calc-silicate skarn and recrystallized marble have developed along the contact between th intrusive rocks and the calcareous sediments. NNE-striking shear zones and breccia bodies cut across the south contact between the dike and the prophyry. Higher-grade gold mineralization is controlled by northeast and northwest trending normal faults and is associated with limonitic breccia, gouge, and locally with skarn. The orebody forms a horseshoe to pipe-like shape, which rakes to the northeast along the contact between the quartz feldspar porphyry and the dolomite and along the plunge of the intersection of the major NW and NNE structural zones. Rare visible gold is seen in the eastern portion of the Top deposit.

Deposit type: Distal disseminated Ag-Au

Development: The Bald Mountain mining district was established on August 20, 1869, when G.H. Foreman staked the Nevada claim group on silver chloride-bearing outcrops near the present site of the Top Deposit. During the 1870s and 1880s minor Cu, Ag, Sb, and Au were produced from area claims. Minor placering was followed by a period of inactivity, after which mining activity recommenced in the 1890s. The Copper Basin mine, adjacent to the east side of the Top deposit, was reactivated with minor Cu and Au production. The modern era of exploration in the district began with porphyry copper exploration by Bear Creek (Kennecott) Mining Company between 1956 and 1965. Ed Hager, with Cordero and in partnership with Steve Damele, explored the area during the late 1960s and 1970s and was instrumental in getting Placer Amex, predecessor company of Placer Dome, involved in Bald Mountain. The Top deposit was discovered in 1981. In May of 1983, Placer Amex began their first heap leach production of Au from the 5-pit deposit. In 1984, Placer announced mineable reserves of 2.8 MT grading 0.090 opt Au contained in six separate deposits and full-scale production at Bald Mountain Mine was achieved in late 1985 when mining commenced from the Top deposit, the largest economic gold occurrence discovered in the district mining district to date. Placer developed the Top deposit in 6 stages and is currently (2004) developing the Stage 7 layback of the Top Pit area, the total amount of material remaining to be mined in Stage 6 is estimated to be 2.2 million tonnes at 2.15 g/t Au and 7 million tonnes of waste. The ultimate pit will be approximately 340 metres deep. Bench heights are 8 metres. Internal high walls vary from 38 to 45 degrees depending on geotechnical conditions. Mining of ore and waste averages 45,000 tonnes per day, seven days a week. The Top pit is being mined by conventional open pit mining methods. The mobile equipment fleet, which was upgraded in 1996 consists of nine Cat 785 Haul Trucks and three LeTourneau L-1100 loaders. Drilling equipment consists of two DML and one DM45 Ingersoll Rand drill rigs. In 2002 Placer Dome reported plans to extend mining into Sage Flats, a deposit almost adjacent to the Top Pit, for which a proposed small open pit was already permitted. In 2006, Barrick Gold Corp. acquired the Top, Mahoney Canyon, and Sage Flat deposits through its acquisition of Placer Dome U. S., Inc.

Geology: The Top and RBM deposits both have high Cu, Zn, and Ag contents associated with gold mineralization as well as anomalous values in Bi, Hg, As, Sb, and tellurides. Granitic intrusive with quartz phenocrysts and oxidized pyrite was noted in cut face of drill roads.

Ore(s): At the Top deposit, higher-grade gold mineralization is controlled by northeast and northwest trending normal faults.

Other Databases

Link to USGS MRDS:	10310443