RELATIONSHIP BETWEEN ORGANIC-MATTER AND COPPER MINERALIZATION IN THE PROTEROZOIC NONESUCH FORMATION, NORTHERN MICHIGAN

Comparison of organic and inorganic geochemical data of stratigraphica lly equivalent, mineralized and unmineralized samples from strata host ing the White Pine stratiform copper deposit in northern Michigan prov ides insight into the ore forming process. Whole rock and organic extr act analyses were performed on mine and core samples from five gray be ds (''stripey'' shale, ''UZV'' siltstone, ''thinly'' shale, ''DGM'' si ltstone, ''domino'' shale) of the Proterozoic Nonesuch Formation and f rom the ''lower'' sandstone of the underlying Copper Harbor Conglomera te. Bitumen is degraded in both mineralized and unmineralized samples of the ''lower'' Nonesuch Formation. Presumably, this degradation is n ot a direct result of the mineralizing process, but represents either regional-scale water-washing or biodegradation. In contrast, the prist ane/n-C-17 and phytane/n-C-18 ratios are lower in mineralized samples than in unmineralized samples suggesting thermal maturation of the ''l ower'' Nonesuch Formation during mineralization. Copper mineralization most likely proceeded via three mechanisms: (1) deposition of native copper; (2) conversion of precursor pyrite to chalcocite; and (3) depo sition of chalcocite with copper and sulfur derived entirely from mine ralizing fluids. A positive correlation between organic carbon and cop per contents in mineralized strata implies that organic matter served as a reducing agent during copper mineralization. However, redox react ions must have conserved total organic carbon since the total organic carbon contents of mineralized and unmineralized samples are similar.