Exploration and geology, 1962 to 2002, at the Goldstrike Property, Carlin Trend, Nevada

Выпуск 9
Автор(ы):Keith Bettles
Издание:Economic geology, 2002 г., 24 стр.
Exploration and geology, 1962 to 2002, at the Goldstrike Property, Carlin Trend, Nevada

The Goldstrike property, located in the Carlin Trend in Nevada, contains a diverse group of Carlin deposits, including some of the largest and highest grade examples known. The largest deposit, Betze-Post, has a gold endowment of approximately 1,250 metric tons (t) Au, and the Meikle deposit, which contains 220 t Au, has a grade of 24.7 g/t Au. Goldstrike is part of the larger Blue Star-Goldstrike subdistrict, which has an areal extent of 58.5 by 2 km and a total gold endowment of 1,970 t. The first discovery of gold at Goldstrike was in 1962. Subsequent exploration culminated in the discovery in 1986 of large high-grade orebodies beneath smaller, lower grade orebodies. Exploration over a 40-yr period has relied on the evolution in understanding of geology and ore controls, supported by the application of geochemical and geophysical exploration techniques.

The Goldstrike property is located close to the rifted margin of the North American craton, along an inferred deep crustal structure. Stratigraphy at Goldstrike consists of lower Paleozoic sedimentary rocks, including an autochthonous, miogeoclinal carbonate sequence and an allochthonous eugeoclinal silici-clastic sequence, separated by the early Mississippian Roberts Mountains thrust. Multiple periods of deformation are evident, dominated by contraction in the upper Paleozoic to Mesozoic, followed by extension beginning in the Eocene. This has resulted in a complex structural architecture that is a major control on the location, geometry, and size of the orebodies. Intrusive rocks at Goldstrike include a Late Jurassic calc-alkaline suite of diorite, rhyodacite, and lamprophyre and late Eocene calc-alkaline dacite dikes. These dikes, dated at approximately 39 Ma, are coeval with the Carlin gold mineralization and the onset of regional extension.

Gold in unoxidized ore is mainly found within arsenian pyrite and is associated with Hg, Sb, and Tl. The ore fluids were low salinity (<10 wt % NaCl equiv), had homogenization temperatures of 200° to 225°C, and are of meteoric origin. Alteration varies considerably between deposits and includes decarbonatization, argillization, and silicification. Dissolution of carbonate has produced collapse breccias, which often host high-grade ore. Supergene alteration has produced oxide ores, at depths up to 200 m. Mineralization at Goldstrike occurs in a variety of settings, reflecting an interplay of both structural and lithological controls. Structural controls include folds, low- and high-angle faults, particularly where faults intersect, and zones of fracturing and brecciation. Fracturing is enhanced in areas of rheological contrast, such as the contact of the Jurassic dioritic Goldstrike intrusion, which is the first-order control of the large Betze-Post deposit. The north-northwest–striking Post fault system is associated with the highest grade orebodies, such as the Meikle deposit and the Deep Post subdeposit. The majority of economic gold mineralization is hosted by the autochthonous rocks, mainly the limy to dolomitic mudstones of the Devonian Popovich Formation and brecciated limestones and dolomites of the Silurian-Devonian Bootstrap limestone. Lesser amounts are hosted in other autochthonous units and intrusive rocks. Characteristics of the host rocks that are believed to enhance their favorability to gold deposition are the presence of reactive carbonate, porosity, permeability, and the presence of iron, which can be sulfidized to form auriferous pyrite.

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