Abstract

The Homestake ore bodies are spindle-shaped or pipelike replacements of the pre-Cambrian* Homestake formation where second-stage folds, or cross folds, intersect earlier isoclinal folds. Localization of ore in the cross folds is believed to be due dominantly to the permeability produced by dilatation of the previously well oriented schists.

Field relations and microscopic study enable subdivision of Homestake mineralization into four perhaps interrelated stages. The principal minerals in the first stage are quartz, chlorite, and arsenopyrite; in the second stage, quartz, ankerite, and pyrrhotite. In the third stage only pyrrhotite was deposited. In the fourth stage, following the intrusion of the Tertiary rhyolite dikes, were deposited mostly pyrite and calcite, and minor amounts of a great variety of other minerals. The four stages are separated by structural breaks, at first measured by very minor fracturing but between the third and fourth stages by widespread fracturing, faulting, and dike intrusion. Gold can be interpreted to have been deposited during the fourth stage but superimposed on the products of mineralization of the three earlier stages.

Several unanswered questions remain. The age of the gold mineralization cannot be established. It may be predominantly pre-Cambrian, wholly Tertiary, or partly of each age. It is not known why practically all the gold is confined to the Homestake formation although some of the adjacent formations differ very little from the Homestake in physical and even in chemical character. Finally, an answer to the question of why the mineralization occurred in a sequence of stages is needed not only for this deposit but for most ore deposits. The question is raised whether the widely accepted depth-zone classification of ore deposits should not be replaced by a classification based on composition of the ore-forming fluids.

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