Abstract

The strata-bound Cu–Fe–S mineralization of the Lower Proterozoic Dunphy Formation forms the largest known sediment-hosted copper deposit of the Labrador Trough. Occurring in three thin stratiform dolomitic intervals, the sulfides are mainly (1) disseminated in a primary porosity (birdseyes) cemented by dolospar and (2) vein bound in early tectonic spar-filled tension gashes. The first of these formed during diagenesis, the second during synkinematic metamorphism. Their parageneses and chemical and sulfur-isotopic compositions are similar.The history of cementation of the host dolomite of one of the intervals has been traced by cathodoluminoscopy of dolospar(–sulfide)-filled birdseyes. Four concentric rim cements, locally disrupted, are followed by two to three central cements. Electron-probe microanalysis shows increasing Fe and Mn in solid solution in these cements from rim to center, pointing to a decrease in Eh toward the center during cementation, corroborated by the presence of semi-centrally located sulfides.The degradation of Recent cyanobacterial mats is used, with few modifications, as a model for the diagenetic evolution of the dolomite. A syndepositional stage of microbial calcification was penecontemporaneous with biochemical dolomitization and followed by microbial diagenetic sulfate reduction of pore waters. The fenestral fabrics of the mats were the major channels of transport of cement-forming substances and the only channels that governed the lateral component of movement of the major pulses of metalliferous fluids.Copper and iron, presently bound to sulfur, were possibly provided by leaching of underlying redbeds and channelled by synsedimentary faults to the porous, degrading cyanobacterial mats.

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