Sedimentary exhalative (SEDEX) deposits

Abstract

There are 132 SEDEX (including Irish and BHT subtypes) deposits worldwide with known grade and tonnage, and of these, 50 have geological resources equal to or greater than 20 Mt. In Canada, there are 35 deposits, seven with measured geological resources of more than 20 Mt, including the Sullivan deposit with 162 Mt. Twelve Canadian deposits are past producers, the largest of which is the Sullivan deposit, which produced 149 Mt of 5.33 percent Zn and 5.64 percent Pb. The morphology of SEDEX deposits is highly variable and includes mounds, lenses, and tabular or sheet-like bodies. Their internal architecture is controlled by the proximity of sea floor sulphides to fluid discharge vents. Vent-proximal deposits typically formed from buoyant hydrothermal fluids, whereas vent-distal deposits formed from fluids that are denser than seawater and pooled in bathymetric depressions that may be remote from sea floor vents. Most SEDEX deposits are hosted by organic-rich sedimentary rocks that were deposited in basins during periods in the Earth’s history when the oceans were stratiifed with a lower anoxic and H2S-rich water column. In the Paleozoic Selwyn Basin, for example, there is a close temporal relationship between upward-increasing δ34S secular trends in sedimentary pyrite, anoxic laminated carbonaceous shales and cherts, and three major SEDEX forming events in the Late Cambrian, Early Silurian, and Late Devonian. The typical basinal architecture of most SEDEX deposits is a continental rift basin with at least 2 to 5 km of syn-rift,coarse-grained, permeable clastics and related volcanic rocks and/or volcaniclastics overlain by post-rift relatively impermeable basinal shales or carbonates. Hydrothermal discharge to the sea floor was commonly focused at the intersection of extensional and transform faults. There is close temporal and, in many cases, spatial association of SEDEX deposits with basaltic volcanic rocks, dykes, and sills. The low rigidity, permeability, and thermal conductivity of host sediments served to focus and prolong hydrothermal discharge at a restricted number of vent sites, thereby generating deposits that are an order of magnitude larger on average than VMS deposits. SEDEX deposits most likely formed from oxidized and therefore H2S-poor fluids generated in geopressured hydrothermal reservoirs within syn-rift clastic (and evaporitic) sediments sealed by fine-grained marine sediments. The large variability in the temperature, salinity, metal content, and redox conditions of SEDEX fluids was controlled by a number of parameters including the local thermal regime, the redox state of the reservoir sediments, and the presence or absence of evaporites. Because most of the fluids that formed SEDEX deposits were probably depleted in reduced S, an essential requirement for this deposit type is a sufficient supply of reduced S at the site of deposition. In the case of well-bedded deposits that formed at the sea floor, the most likely S source is bacteriogenic H2S generated in an ambient anoxic water column.