REDISTRIBUTION OF CU, ZN, PB AND BA IN THE BOLLIBOKKA GROUP, EAST-SHASTA, CALIFORNIA, USA - IMPLICATIONS FOR METALLOGENY AND GEOCHEMICAL BALANCES

Basalt and andesite of the Bollibokka group underlie stratigraphic horizons with volcanogenic massive sulfide (VMS) deposits in the East Shasta district, but with no known deposits to the north. Least-altered rock, remote from known deposits, commonly retains clinopyroxene and titanomagnetite phenocrysts and contains abundant secondary pumpellyite and prehnite, in contrast to altered rock near known deposits. Least-altered rock contains on average 89 ppm Cu, 100 ppm Zn, 10 ppm Pb and 160-260 ppm Ba. Alteration within a few kilometers of the Bully Hill-Rising Star mines led to removal of approximately 59 ppm Cu, and possibly 7 ppm Zn, 2 ppm Pb and significant Ba from basalt. One km3 of altered basalt can account for 7 X the Cu and 1.7 X the Zn production of the mines. Basaltic volcaniclastic wacke in the area contains framboidal pyrite and kerogen and was enriched in Cu, Zn and Pb. Intense leaching within 1 km of the Afterthought mine led to removal of approximately 74 ppm Cu, approximately 34 ppm Zn and approximately 4 ppm Pb relative to unaltered basalt. Alteration of 1 km3 of basalt could supply 40 X the Cu, 8 X the Zn and 14 x the Pb produced by the Afterthought mine. Zones of maximum leaching of metals underlie known deposits, yet it cannot be proven that leached metals contributed to the deposits. The oceanic inventories of Cu and Zn can be established in thousands of years by hydrothermal and river-water fluxes. These fluxes cannot account for the rate of fixation of Zn in shale; a dominant detrital flux is required. A dominant detrital flux of Cu is permitted, but not required.