Depositional-Tectonic Setting of Clastic-Hosted Lead-Zinc Sulfide Deposits: ABSTRACT

Abstract

Clastic-hosted, stratiform lead-zinc sulfide deposits (MacArthur River H.Y.C., Mt. Isa, Broken Hill, Australia; Sullivan, Howard's Pass, Anvil Range, Jason-Tom, Canada) precipitated from exhaled, hydrothermal fluids in similar depositional and tectonic settings. Host rocks suggest sedimentation in anoxic water below storm waves in areas devoid of active bottom currents. Depths were at least 150 to 200 m as inferred from the absence of storm-influenced deposits. Such numbers are suggested by calculations involving fetch distances of the ancient basins and by End_Page 643------------------------------ effective wave base in modern seas. Supporting evidence comes from the lack of bottom-dwelling metazoans in Phanerozoic examples indicating anaerobic conditions. In the examples, laminated to thickly bedded sulfides are interstratified with shales, siliciclastic or carbonate-sediment gravity-flow deposits, and some cherts. Where not obscured by deformation and metamorphism, sandstones show grading, flutes and grooves, load casts, and Bouma sequences. Submarine mudflow units may be common. Lacking are hummocky cross-stratification, wave ripples, mudcracks, abundant medium-scale cross-stratification, or other evidence in ore zones of shallow-marine processes or of subaerial exposure. Thicknesses of clastics point to basin depths much greater than 200 m. These deposits apparently formed in extensional tectonic regimes. Stratigraphic thickness variations, facies changes (especially the presence of local, fault-derived slumps), or geophysical evidence suggest the presence in many of active faults during ore genesis. These faults formed the basins or formed local traps, provided conduits for hydrothermal fluids, and positioned convective cells. End_of_Article - Last_Page 644------------