METAL ACCUMULATIONS IN CARBONACEOUS ROCKS—DO CONVENTIONAL MODELS EXPLAIN OBSERVED ENRICHMENTS?

Enrichments of trace elements in carbonaceous sedimentary rock (‘black shales’) extending 1000s of km2 in basins generally distant from coeval volcanic influence are a product of episodic conditions capable of sequestering a large amount of metal. The commonly proposed source of these elements is seawater. Depositional conditions that are offered to explain the magnitude of observed enrichments (>1000 ppm Cr+Mo+Ni+V+U+Zn) typically include enhanced organic productivity, deposition in restricted suboxic to euxinic basins, and very slow sedimentation rates. Simple box models using published characteristics on the chemical composition and geologic setting of selected metalliferous ‘black shales’ were combined with the characteristics of modern oceans to simulate conditions of their formation.

Units considered in the modeling exercise include the Cretaceous Toolebuc Formation , Australia; the Mississippian Heath Formation, Montana; and units within the Paradox Formation, Colorado and Utah. Results of the modeling highlight a general inconsistency of published geologic and paleooceanographic conditions and the large mass of trace metals deposited. Typically, increased water depth, slower sedimentation, open circulation, or additional metal from other sources are necessary. An additional metal source seems likely for large metal accumulations such as the Toolebuc Formation which contains the equivalent of 400,000 times the annual recharge of dissolved vanadium to the ocean. Riverine input with elevated contents of vanadium has been proposed to supplement seawater but this source seems unlikely to account for the observed enrichments of Au. Redistribution of metals among ocean basins, and metals supplied by discharge of subsurface fluids were tested as alternatives.