A Historical Perspective on the Nature and Genesis of Epithermal Gold-Silver Deposits
Published:January 01, 1991
Byron R. Berger, 1991. "A Historical Perspective on the Nature and Genesis of Epithermal Gold-Silver Deposits", Historical Perspectives of Genetic Concepts and Case Histories of Famous Discoveries, Richard W. Hutchinson, Richard I. Grauch
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Economic geologists have sought for most of this century to achieve a unified set of models for epithermal Au-Ag deposits to serve as springboards for exploration and new avenues of research. Prior to the 1860s, form and appearance served as the basis for ore deposit classification, with genetic concepts modifying these early theories in the latter part of the century. In 1913, Lindgren classified vein deposits into separate categories on the basis of pressure and temperature criteria gleaned from documented attributes of many different types of ore deposits and study of metamorphic rock assemblages. In 1922, he coined the...
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Historical Perspectives of Genetic Concepts and Case Histories of Famous Discoveries
Ideas about the genesis of sulfide nickel-copper deposits largely rely on the geology and type of major deposits or new discoveries of the times in which they were proposed. Nickel-copper sulfides in the Norwegian gabbronorite intrusions and in the Sudbury Complex were the early examples supporting the conclusion that mafic intrusions were the main host rocks for sulfide nickel-copper deposits. Experimental work by the Norwegian geochemist J. H. L. Vogt at the beginning of the twentieth century created a firm foundation for the hypothesis of magmatic segregation. However, some of the geologic observations of the Sudbury deposits were in contradiction to the magmatic segregation theory, and many authors considered hy-drothermal processes as the main accumulation mechanism of Sudbury-type Ni-Cu sulfides. These two main theories, magmatic and hydrothermal, still prevail in genetic considerations today.
The discovery of shock metamorphic structures around Sudbury led to the idea that the Sudbury Complex was an old meteorite impact site and that the mafic intrusion with Ni-Cu sulfides followed the structure of a meteorite crater. After lively discussion, this idea has been widely accepted, but the type of magmatism, composition of primary melts, assimilation of silicate magma, and emplacement as well as localization of sulfides are still under study.
Discoveries of ultramafic-hosted nickel sulfides in Manitoba, Canada, and, later on, the komatiite-associated massive nickel sulfides in Western Australia awakened the discussion of genetic models of nickel sulfides in ultramafic host rocks, which has continued until today. Some studies published in the 1980s indicated that the komatiitic ultramafic flows of the Kambalda area, Western Australia, formed deep thermal erosion channels in the underlying supracrustal sequence, and the lavas assimilated volcanic and sedimentary material. The massive Ni-Cu sulfides formed from the assimilated barren sedimentary sulfides and accumulated at the basal depressions of the komatiitic flows.
Experimental studies widened knowledge of the origin of the Ni-Cu sulfides. In the 1960s the sulfide mineral stabilities and composition of sulfide phases were the main targets of study. In the 1970s the sulfide-silicate melt system was examined through the calculation of distribution coefficients between sulfides and silicates. Also, the importance of the magma/sulfide mass ratio (R factor) was discovered. Sulfur isotope studies of the Norilsk deposit, Siberia, proved that sedimentary sulfur was extracted from underlying sediments and reacted with metals in mafic magma to form Ni-Cu sulfide deposits. Since then, the origin of sulfur in Ni-Cu sulfides has been a topic