Abstract

Economic quantities of scheelite occur in a sheared and feldspathized portion of the Illinois granodiorite stock, near Gabbs, Nevada. The granodiorite intrudes dolomite and shale of probable Late Triassic age.A westward projection of the stock, and adjacent sedimentary rocks, were subjected to strong compressive forces during or shortly after the emplacement of the granodiorite. The dolomite is thrust over the granodiorite, and the latter is intensely sheared for several hundred feet from the contact. The scheelite mineralization is one stage of a metasomatic replacement process; the metasomatizing solutions were guided by the shear and fracture zones. The mineralogical changes effected are concentrated where crushing and fracturing were most intense, and particularly where there was recurrent crushing. Four successive stages of metasomatic alteration can be recognized.(1) Feldspathization; the dominant process is replacement of quartz by orthodase and oligoclase.(2) Ore-type metasomatism; replacement of quartz and orthoclase by plagioclase, accompanied by minerals such as garnet, diopside and hastingsitic amphibole.(3) Scheelite mineralization; essentially confined to the ore-type rocks where they were subjected to crushing.(4) Quartz veining.In spite of the intense crushing no mylonitic rocks were observed, the feldspathized rocks are commonly coarse-grained, and low-temperature minerals such as clay minerals and chlorite are generally absent. These features indicate a high temperature for the mineralization, which probably took place while the intrusion was still hot.

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