Geological Features and Genetic Models of Mineral Deposits
Richard Herrington
INTRODUCTION
Mineral deposits are natural concentrations of one or more mineral commodities that may be termed ore deposits if they reach levels where economic extraction is viable. They form as a result of many geological processes, but they form within specified geological settings. Similar genetic processes may operate to form deposits that have similar characteristics and, thus, may define a “class” of deposits. The geological processes that may concentrate minerals include fractional crystallization of magmas, release of volatiles from a crystallizing magma, magmatic interaction with host rocks, metamorphic reactions producing fluids, chemical changes in circulating heated groundwater, organic interactions, and weathering, among others.
Eckstrand et al. (1995) neatly summarize that “mineral deposit types” are mineral deposits that share a set of geological attributes and contain a particular commodity (or commodities) that collectively distinguish them from other types. Ideally, mineral deposit types should reflect how the mineral deposit was actually formed, and in this case, one might be able to use the term genetic model. However, in many cases there is considerable debate among geologists as to how a specific deposit was formed, and thus classifications based purely on a given genetic model will encounter problems. An alternative is to classify deposits based on empirical features (minerals, host-rock association, etc.), which will lead to the unique fingerprint of a particular deposit (i.e., a descriptive model). Even though no two mineral deposits are identical, empirical descriptions of deposits tend to show natural groupings into a small number of loosely definable categories or types. In turn, these categories tend to coincide with genetically derived models; so even by using purely physically descriptive classifications, there is often a close coincidence between these and models defined using genetic criteria. Prior to the 20th century, models for the formation of mineral deposits were subject to the often-polarized views of either plutonist (all deep igneous origins) or neptunist (all sedimentary origins) theories for the origin of rocks, and it was really only in the 20th century that modern views of mineral deposit formation emerged. From the late 1960s, plate tectonic theory has revolutionized geological thinking, and in the mid-1980s, Guilbert and Park (1986) devised a mineral deposit classification scheme based on plate tectonics. Figure 3.1-1 is a very broad attempt to show an interpretation of the environment of formation of key mineral hypogene deposit types with respect to plate tectonic settings in their broadest sense.
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