METALLIFEROUS ORGANIC-RICH BLACK SHALES: WHERE DO THE METALS COME FROM?

The amount of metal contained in metalliferous black shales (MBS) can rival or surpass any metal deposit on Earth. Historically there has been little consensus as to the source, means of transport, and deposition of these metals. The paradigm established over the last twenty years, however, suggests that the anomalous metal and carbon contents of these shales reflect oceanographic conditions, which enhance organic productivity/preservation and optimize extraction of metals from seawater.

To date, no modern analogs of these ancient MBS have been found. Despite intense study, modern, euxinic and productive environments fail to form sediments with comparable metal enrichments and MBS. Moreover, mass balance constraints that consider the total mass and recharge of metal to the ocean, ocean circulation, and sedimentation rate suggest that typical seawater may not be an adequate source for the mass of metal present in some metalliferous black shales. Alternatively, the flux of metal delivered to the ocean during sedex hydrothermal events can be shown to rival their entire modern riverine fluxes into the ocean. Another overlooked aspect of sedex systems is the flux of nutrients (i.e. NH4, reduced C, trace metals, Ba, Si) that can also exceed the modern riverine flux. Significant nutrient fluxes into a sedimentary basin would undoubtedly trigger massive increases in primary bioproductivity and potentially basin-wide anoxic/euxinic conditions needed to form MBS. This model explains the temporal and spatial coincidence of anoxic/euxinic conditions, MBS, and sedex deposits through geologic time.