Abstract

Sedimentary rocks near 3 base metal deposits were analyzed to determine possible changes in Zn, Cu, and Fe content caused by alteration or contact metamorphism. Of the sediments in the Hanover, New Mexico district of "contact metasomatic" Zn-Cu deposits, only the Devonian (Percha) fissile shale contains sufficient Zn (70 p. p. m. Zn, 48 p. p. m. Cu, and 4% Fe) to contribute significantly to the metal content of the deposits. However, this shale has the same metal content (within + or -20 p. p. m. Zn and Cu and within + or - 1% Fe) when unaltered, as in the recrystallized and silicified contact zone (excluding the skarn). Extraction of 20 p. p. m. Zn from the altered area of the shale is quantitatively insufficient to account for the 600,000 metric tons Zn in the skarn and ore deposits of the contact zone. In the northern Mississippi valley district, neither the Maquoketa shale (35 p.p.m. Zn, 65 p.p.m. Cu, and 3% Fe), nor the oil rock (20 p. p. m. Zn, 60 p. p. m. Cu, and 7% Fe) has a sufficient metal content in comparison with the background level given by average igneous rock (100 p. p. m. Zn, 70 p. p. m. Cu, and 5% Fe) to be the source for the metals in the deposits. Near the margins of the San Francisco del Oro district, Chihuahua, Mexico, the gradient of Zn content from the veins into the silicified wall rocks passes through a minimum below that of unaltered shale (123 p. p. m. Zn, 75 p. p. m. Cu, and 5% Fe). However, the quantity of Zn shown by this gradient to have been extracted from the shale is only a negligible fraction of the metal content of the entire deposit. It is quantitatively impossible for the metal content of these 3 districts to have been derived from the surrounding Paleozoic or younger sediments by lateral secretion.

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