15e Colloque annuel du RÉDiST
Key attributes of stratiform Zn-Pb-Ag deposits in sedimented continental rifts: application to mineral exploration
Wayne D. Goodfellow, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8. Couriel: wgoodfel@nrcan.gc.ca
Résumé de conférence
Stratiform sulfide deposits hosted in sedimentary basins constitute the major global resource of zinc, lead and silver. Included within this group of deposits are SEDEX, VSHMS (includes BHT), and Irish-type deposits. Of the 21 supergiant deposits (>100 million tonnes), 11 are SEDEX, 7 are VSHMS, 2 are VHMS and 1 is a Besshi-type deposit. Compared to VHMS deposits, SEDEX deposits are fewer in number but an order of magnitude larger, on average.
Both SEDEX and VSHMS deposits formed in anoxic marine sedimentary basins (sag-phase) overlying a thick syn-rift sequence of permeable coarse-grained clastic sediments. SEDEX deposits occur in intracratonic rifts and reactivated passive margins and are concentrated in the Mesoproterozoic and Devonian, whereas VSHMS deposits are hosted by bimodal volcanic-sedimentary sequences in back-arc continental rifts. They occur in local third-order, fault-controlled basins within larger extensional basins. In nearly all cases, the deposits are located close to major syn-sedimentary faults which have tapped hydrothermal fluids from 2 to 10 km subsurface depth in the basin. A close temporal and, in many cases, spatial association of some deposits with volcanic rocks, dykes, and sills indicates that the hydrothermal systems were driven by high-level magma bodies.
The deposits are both distal and proximal, exhalative or replacive, and variable in morphology and architecture, mineralogy and chemical composition, hydrothermal alteration and the nature of distal hydrothermal sediments. The internal architecture of vent-proximal deposits is characterized by a zone-refined vent complex above a sulfide stringer zone cutting hydrothermally altered sediment. The hydrothermal alteration and distal sediments are laterally zoned about vents and are effective vectors for mineral exploration.
Ore-forming fluids are variable in temperature, salinity and redox conditions, and were likely generated in geopressured hydrothermal reservoirs within syn-rift clastic (and evaporitic) sediments capped by an impervious sequence of fine-grained basinal sediments. Most of the reduced sulphur was derived from an ambient anoxic water column, whereas metals were supplied by convective hydrothermal and, in the case of VSHMS deposits, convective and magmatic fluids.
The large size of sediment-hosted stratiform deposits, compared to those hosted by volcanic rocks, is due to a number of factors that include a) basinal and hydrothermal architecture, b) hydrothermal systems that are long-lived because of the thermal insulating of the hydrothermal reservoir by overlying sediments, c) focussed fluid discharge due to the low permeability of soft, fine-grained sediment, and d) anoxic bottom waters that facilitated the total capture of metals at the seafloor.
