Abstract

In the Coeur d'Alene district of northern Idaho, the conversion of diorite to monzonite and syenite through endomorphic processes, the development of zones of extensively bleached (sericitized) rock, and the metallization seem to be closely related phenomena and associated with the work of potash-rich solutions emitted from a deep magmatic source. In the beginning some of the magma was intruded locally to high levels and there consolidated as hornblende-pyroxene diorite, but the potash-rich solutions that followed the intrusion soaked upward through the still hot crystalline rock and introduced into it large quantities of potash feldspar (chiefly microcline), considerable amounts of albite, sphene, and magnetite, and lesser amounts of other minerals. As a result of these additions the originally consolidated diorite was changed to rock of monzonitic and syenitic composition. In places the bordering sedimentary rock (Belt series) was "granitized" by the intruding magma and the potash-rich solutions.Elsewhere the potash-bearing solutions were directed along zones of faulting in the sedimentary rocks where the cooler environment favored the formation of sericite rather than the higher-temperature feldspars. As a result, zones of extensively bleached (sericitized) rock came into existence.Structural adjustments along the zones of bleached rock and locally along the borders of the monzonitic rocks then provided openings for the movement of ore-bearing solutions with development of replacement veins in the bleached rocks and contact metamorphic deposits adjacent to intrusive rocks. The potash introduced into the solidified but still hot igneous rock and into the sedimentary rock along the zones of faulting presumably evolved as,a differentiation product of the deep source magma. Formation of potash-rich solutions at the magmatic source appears to be a prerequisite for mineralization in the Coeur d'Alene as in many other mining districts.

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