Organic matter and metal enrichment in black shales of the Illinois Basin, USA

Abstract

The New Albany Shale of Indiana is one of several Devonian to Mississippian-age shales that are locally enriched in organic matter and a suite of metals. Previous studies of metal enrichment in black shales have been numerous. Mechanisms of metal concentration in the black shales have been shown to be variable, ranging from primary syngenetic metal enrichment, through diagenetic and hydrothermal (epigenetic) processes of concentration. End-member environments may be viewed as those related to syngenetic metal enrichment through sedimentary processes with no evidence of thermal anomalies or hydrothermal fluid infiltration, to those where the organic-rich shales have played a major role in the localization of mineralization that originated from hot, and generally oxidized, metal-bearing brines. Shales from the Devonian-Mississippian Appalachian and Illinois Basins in the United States (e.g. Levanthal, 1987, Ripley et al., 1990; Figure 1) are examples of the former, whereas the Mississippian black shales that host the giant Red Dog Pb/Zn deposit in Alaska (Moore et al., 1986; Warner et al., 1996) provide an example of the latter. Given this wide range of geological environments where metal enrichment may occur, the role of organic material in metal localization may range from that of primary concentrator to a flow inhibitor and local reducing agent for metal precipitation from hydrothermal fluids. In this communication we will review the nature of metal enrichment in the New Albany Shale of Indiana, where processes such as sedimentation rate, degree of anoxia in the basin, and organic matter type appear to be the primary controls of metal concentration. We will contrast two members of the New Albany Shale with different degrees of metal enrichment, but whose geological and geochemical signatures suggest deposition under euxinic or at least low oxygen conditions.