International Geologiical Congress - Oslo 2008

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MRD-13 Ore deposits associated with black shales: from their origin to their environmental impacts

 

Metalliferous organic-rich black shales: Are exhalative hydrothermal processes fundamental to their genesis?

 

Poul Emsbo, US Geologic Survey (United States)
 

 

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 abundant metal (and carbon) in MBS reflects oceanographic conditions that enhance organic productivity/preservation and optimize extraction of metals from seawater. Yet, to date, no modern analogs of MBS have been found and despite intense study the genetic processes responsible for these differences remain unexplained.

MBS are commonly found in sedimentary basins that also host a range of different types of syngenetic metal deposits in correlative strata. The relationship between MBS and hydrothermal synsedimentary exhalative (SEDEX) Zn-Pb-Ag, Ba, Au deposits has previously been ascribed to euxinic ocean conditions enhancing deposition of metal sulfide minerals. Correlative strata commonly contain sedimentary PO4, Mn, and Fe deposits that have traditionally been explained by upwelling of cold nutrient-rich ocean waters onto continental shelves. Increased nutrients in shallow marine environments are thought to cause, as along modern upwelling coastlines, a surge in bioproductivity and consequent eutrophication, anoxia/dysoxia, and sequestration of organic carbon. In this model, metal and phosphate in the shale are sourced from seawater and accumulated through various biogenic and chemical mechanisms.

However, 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 be an inadequate source for the amounts of metal present in some MBS. Alternatively, it can be shown that the fluxes of metal delivered to the ocean during SEDEX events can rival or surpass the modern riverine fluxes into the ocean. Another overlooked aspect of SEDEX systems is that the flux of nutrients (e.g. NH4, reduced C, trace metals, Ba, Si) can in some cases exceed the modern riverine fluxes. Such an immense nutrient flux into a single sedimentary basin would undoubtedly cause large increases in primary bioproductivity and could potentially trigger eutrophication and basin-wide anoxia/euxinia.
Given that the flux of metals and biolimiting nutrients delivered to the ocean by a single SEDEX hydrothermal discharge can rival the entire modern riverine flux, systems of this type are a plausible source for the metals and nutrients required to form MBS. An appealing aspect of the SEDEX trigger model is that it explains the temporal and spatial coincidence of anoxia/euxina, MBS, and various Zn-Pb-Ag, Ba, Au, PO4, Mn, Fe deposits at numerous times in Earth history.

 

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