Metamorphism and Copper Mineralization of the Portage Lake Lava Series, Northern Michigan.

Abstract

Permeable flow tops and interlayered conglomerates of the Keweenawan Portage Lake Lava Series (PLLS) were subjected to upper zeolite to prehnite-pumpellyite facies metamorphism and copper mineralization during subsidence of a thick, rift-related volcaniclastic sequence of late Precambrian tholeiitic basalts. Compositional changes of prehnite and mixed-layer chlorite-vermiculite correlatable with progressive metamorphic grade and studies of phase-assemblages establish a detailed zonation of secondary minerals. Phase equilibrium calculations combined with fluid inclusion and oxygen isotope thermometry demonstrate that (1) metamorphism, concordant copper mineralization and subordinate sulfide-bearing veins were concurrent with syndepositional tilting of the volcaniclastic pile, (2) best estimates of pressure and temperature are 0.5 kbar, 180(DEGREES)C at the top and 1.5 kbar, 280(DEGREES)C at the bottom of the exposed section, where errors in pressure are (TURN)0.5 kbar, and in temperature (TURN)40(DEGREES)C, (3) fluid pressure was close to lithostatic (P(,f(, ))>(, )0.85 P(,t)), (4) the paleogeothermal gradient was <(, )55 (+OR-) 21(DEGREES)C/km, and (5) the metamorphic fluid was presumably a Ca-Na-Cl brine low in carbon dioxide. Light stable isotope data suggest that basalts of the PLLS were enriched in ('18)O prior to solidification, and that siliceous flows and intrusives were probably derived from melting of continental crust. Flows were subjected to a pre-metamorphic hydration by meteoric water, and subsequently their permeable selvages equilibrated with a metamorphic/hydrothermal fluid of (delta)('18)O(,H(,2)O) (TURNEQ) 6 (+OR-) 1.5 per mil and (delta)D (TURNEQ) 0 (+OR-) 10 per mil. This fluid was probably seawater, modified by chemical and ('18)O exchange with rocks in excess of (TURNEQ)300(DEGREES)C deep in the basin, under initially low W/R ratios. The (delta)('34)S of vein chalcocite, bornite, anhydrite and barite (0 - 28 per mil) and the ('87)Sr/('86)Sr ratios of barites suggest that the source of sulfur and strontium, respectively, was presumably late Precambrian seawater. The favored mineralization model involves infiltration of seawater into the volcanics during or followed by burial. This resulted in heating and acidification of the confined fluid, leaching and dissolution of copper ( and other ions) by hydrothermal circulation and deposition of copper in a native state in the distal portions of the basin during ascent-induced cooling (Freda time). According to the proposed model, the copper shale mineralization at White Pine could have been produced by the same burial metamorphic fluids that mineralized the PLLS.