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Abstract

Years ago it was assumed that nearly all metallic ore deposits were associated in some way with magmatic systems. Even deposits such as the Mississippi Valley lead/zinc deposits were hypothesized to be related to some granitic source at depth. Then, in the early 1970's, magmatic models began to fall out of favor as many economic geologists recognized that igneous processes were not required for the generation of many ore deposits. Today, the pendulum of opinion has returned to a middle position. We recognize that there are a number of metallic ore deposits whose existence does not necessarily require a direct magmatic input. There are, however, a number of classes of deposits whose close temporal and spatial relationships suggest a genetic connection with silicic plutonic or volcanic events.

Porphyry systems are disseminated deposits of copper, molybdenum, lead, and zinc with relatively low gold and silver. The mineral assemblages are mainly oxidized sulfide assemblages. In many cases there are several periods of mineralization involved in the formation of an economic deposits. In most cases, a protore stage in which the ore minerals are deposited in an intricate series of healed veins and veinlets is first developed. Where this stage is preserved, the fluids appear to have been hot and saline. Isotopic studies suggest that these early volatile phases were partly magmatic in origin. Thus, the protore stage of porphyry systems may be thought of as a deuteric magmatic process. Many porphyry systems are thought to be the roots of old volcanic

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