Organic geochemistry of Mesoproterozoic Nonesuch Formation at White Pine, Michigan, USA

Abstract

The quality and quantity of the preserved organic matter (OM) in the Mesoproterozoic Nonesuch Formation at White Pine are evaluated in this project. Specifically, I have considered whether the rocks had source rock potential and whether there is a relationship between the OM and copper mineralization. The copper mineralization and hydrocarbons migration pathways are hypothesized to be related. There are three possibilities for the relationship. The copper ore fluid may have migrated with the hydrocarbons. The copper may also have precipitated where the hydrocarbons had accumulated, or the copper precipitated where there were accumulations of OM. Three cores (42C, 37F, and 30G) from in or near the White Pine mine were described and analyzed. The sampled core intervals are mostly from the Lower Nonesuch Formation with some from higher intervals. The overall lithology is gray laminated siltstone, with some sandy siltstone and lesser shale and sandstone. The core samples have various colors, with brown to dark brown samples hosting organic matter. The most abundant minerals are quartz, feldspar (plagioclase, orthoclase), mica, and some rock fragments; calcite and chlorite are mostly found as cement. Various analyses were done to investigate the organic matter. For estimating the maturity, kerogen type, and potential source rock quality, samples were subjected by the Rock Eval pyrolysis. Also other techniques were used for evaluating maturation, including ultraviolet microscopy and vitrinite reflectance microscopy; it was determined that the majority of samples have little to no vitrinite-like material. The organic matter as analyzed by the UV microscope is observed to be of three types, kerogen, bitumen, and oil inclusions. The oil inclusions are mostly found in the sandy siltstone samples. The organic matter is mostly not fluorescent possibly because it is overmature or immature; it contains less than about 10% pyrolyzable hydrocarbons. The organic matter may have been produced from remains of organisms like algae and fungus deposited within a lacustrine and/or transitional marine environment. The generative potential of the organic matter is in the poor to fair range. The range of TOC (total organic carbon) content is between 0.01 and 0.86 wt %. The highest value is detected above the mineralization zone in the Upper Nonesuch Formation. The kerogens of the Nonesuch Formation are types III and IV, types that usually are gas prone, or have no hydrocarbon potential. The samples may have been oxidized by copper bearing fluids which altered the organic matter and reduced its potential to produce hydrocarbons. Overall, the organic matter of the lower Nonesuch Formation at White Pine has no potential to produce hydrocarbons.