Geochemical Aspects of Stratiform and Red-Bed Copper Deposits in the Catskill Formation (Pennsylvania, USA) and Redstone Area (Canada). Sequence of Mineralization in Sediment-Hosted Copper Deposits (Part 3)

Abstract

Noneconomic red-bed copper occurrences in the Devonian Catskill Formation are localized by concentrations of reducing plant fragments, commonly in the basal parts of fining-upward fluvial (or tidal?) cycles. The occurrences are within and near thick zones of red mudstone to fine sandstone interpreted to have been deposited in relatively inactive parts of an extensive alluvial plain in an arid environment. Regionally, the occurrences are most abundant adjacent to areas of major sediment input, and in more detail are controlled in part by sandstone beds which were more mature and probably more permeable than the rest of the formation. The copper minerals were emplaced after earliest diagenesis, but before induration and formation of quartz overgrowths. Thin zones of transgressive marine sandstone are present near most occurrences. Copper has been redistributed within the Catskill, being greatly depleted from nearly all red (oxidized) sediments, but showing highly variable values in green and gray (reduced) sediments. Mobilization of copper from the red beds into chloride- bearing pore fluids of marine or evaporative origin is proposed, followed by flow of these fluids through the more permeable sandstones until organic reductants were encountered which precipitated the copper by bacterial sulfate reduction. The flow was driven by compaction, sea-level changes, and gravity flow down the alluvial plain. Red beds near the Coates Lake stratiform copper deposit in the Redstone area show similar depletion of copper.