Abstract

Five lava flows in the Keweenawan series of the Michigan copper district have been studied and chemical analyses made, with special attention to: (1) chemical and mineralogic variations within the upper, pegmatitic portions of the flows; (2) processes of differentiation; (3) comparison of the types of rocks found within a single flow with the types of flows that make up the lava series; (4) the relation of differentiation in the flows to the origin of the copper deposits.

The primary factors in the differentiation of the flows were probably: (1) cooling at a moderate rate, and predominantly from the top; (2) crystal sorting by differential settling; (3) migration and local concentration of volatiles; (4) periodic convective overturn of the magma. Convection currents probably were not active in the thinner flows and may not have been in the thicker. Alternate hypotheses have been presented, one assuming that convective overturn was important, the other that it was not.

All types of flows in the series can be approximated by differentiation products of the Greenstone flow, indicating that all the flows could have been differentiated from a magma similar to, but perhaps slightly more mafic than, that of the Greenstone flow.

In considering the origin of the copper deposits, the writer has briefly reviewed the literature and presented new evidence. The following modes or origin of the deposits appear still open to consideration: (1) The deposits are epigenetic, and formed by deposition from hot solutions ascending from an underlying intrusive. (2) The deposits are the result of a combination of syngenetic and epigenetic processes. Primary concentrations of copper in fumaroles, thermal springs, or playa lakes were reworked by later, ascending igneous emanations. (3) The amygdaloidal deposits are primarily syngenetic and have been only moderately modified by the action of later ascending solutions.

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