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Gold Skarn Deposits—Geology and Exploration Criteria

By
Lawrence D. Meinert
Lawrence D. Meinert
Geology Department, Washington State University, Pullman, Washington 99164
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Published:
January 01, 1989

Abstract

About 106 kg (>30 Moz) of gold have been recovered from skarn deposits. These occurrences can be subdivided into five major skarn types: gold, iron, copper, porphyry copper, and lead-zinc. Each skarn type has distinctive characteristics and can be differentiated on the basis of geochemistry, mineralogy, pedogenesis, and tectonic setting. The most attractive exploration targets are the skarn deposits which can be mined for their gold content alone.

Most gold skarns contain only small amounts of copper, lead, and zinc and are associated with relatively mafic diorite and granodiorite plutons. They are also distinct in that they typically contain major amounts of arsenic, bismuth, and tellurium, elements which are not normally abundant in other skarn types. Gold grades are high, usually in the 5 to 15 ppm range. Gold skarn host rocks usually contain a significant clastic or volcaniclastic component. Pyroxenes from gold skarns tend to be more aluminum and iron rich (up to pure hedenbergite), amphiboles tend to be more aluminum rich, and garnets tend to be more aluminous grandites than are typical of many skarn types. Potassium feldspar, biotite, prehnite, idocrase, scapolite, apatite, and sphene are locally abundant. Arsenopyrite, pyrrhotite, and marcasite are the most common sulfide minerals with surprisingly large amounts of bismuth minerals (native bismuth, maldonite, wittichinite, hedleyite) and telluride minerals.

Most copper skarns are large but have relatively low gold grades; the highest grades are associated with zones of high sulfide content and intense retrograde alteration, especially low-temperature, silicified zones. Epidote, amphibole, and chlorite are the main retrograde alteration products of garnet and pyroxene and also occur with massive sulfide replacement of skarn. The highest average Ag and Au grades occur in skarn that is both sulfide rich and has abundant retrograde alteration. Calcic Fe skarns are also very large but appear to contain gold only in spotty high sulfide and high copper zones. These zones can be very high grade and might be attractive for small-scale mining, but the potential for larger targets, such as gold disseminated in the main magnetite skarn bodies, is limited. Pb-Zn skarns are small to moderate in size and often contain significant silver, but very little gold.

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Economic Geology Monograph Series

The Geology of Gold Deposits: The Perspective in 1988

Reid R. Keays
Reid R. Keays
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W. R. H. Ramsay
W. R. H. Ramsay
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David I. Groves
David I. Groves
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Society of Economic Geologists
ISBN electronic:
9781629490014
Publication date:
January 01, 1989

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