Isotopic and geochemical data consistent with

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Unformatted text preview: ochemistry • Shales are abundant on Earth’s surface and are typically very well homogenized - provide a key indicator of crustal evolution (e.g., Condie’s work, McLennan, etc.). • Provide outstanding indicators of anoxia and oxygenation of basins (i.e., basin ventilation) - key implications for large massive sulphides through time. • Often very useful in understanding ore genesis as many large SEDEX and VMS systems are associated with oceanic anoxic events. • More on their role as vectors to hydrothermal fluids in lecture on chemical sediments. Monday, 13 August, 12 Rock/Upper Crust 10 1 Post-Archean Shale Global Subducted Sediment .1 Monday, 13 August, 12 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Rock/Upper Crust 10 1 Wolverine Shales .1 Monday, 13 August, 12 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Howards Pass Broken Hill (Aus.) Windy Craggy Neves Corvo Aljustrel Rio Tinto Brunswick No. 12 H.Y.C. Sullivan La Zara Gamsberg Horne (Zone 5) Red Dog Kidd Creek Mt. Isa Century Saladipura Tharsis Mt. Lyll Big Syn Furnteheridos Broken Hill (Africa) Black Mtn. Azacollar Hilton Masa Valerde Los Frailes Sindesar Navan Ruttan Flin Flon Rampura-Agucha Crandon Sotiel Geco Faro Matsumine-Shakanai Cirque Dariba-Rajpura Lousal Mount Morgan Tonnage Distribution of Supergiant and Giant Deposits Sediment-hosted SEDEX/BHT VSHMS BESSHI Volcanic-hosted VHMS 0 100 200 300 400 500 Million Tonnes of Slide courtesy of Wayne Goodfellow Monday, 13 August, 12 Shale Geochemistry, Anoxia, and Massive Sulphide Deposits • Periods of anoxia (i.e., stratified oceans with lower H2Srich anoxic waters and upper oxygenated waters) are commonly associated with a proliferation of massive sulfide mineralization, in particular VMS and SEDEX. • Shales provide an excellent record of this anoxia and provide ability to delineate anoxic versus oxic basins. • Key for organic preservation (i.e., it isn’t oxidized back into carbonate). • Key for monitoring climate, atmosphere, and oceanic evolution. • Shales can form under both oxic and anoxic conditions but their trace element budgets will vary accordingly. Monday, 13 August, 12 From Goodfellow et al. (2003) Monday, 13 August, 12 BMC & FLD VHMS Model Peter & Goodfellow, 1996 Monday, 13 August, 12 Hanging wall shales, Wolverine deposit, YT Significance of Black Shales in VMS/SEDEX: • • • • • Impermeable cap rocks? Reducing agent? Anoxic bottom waters? Source of reduced H2S? Metal sponges? Subseafloor replacement of shales by Zn-rich massive sulphides, Wolverine deposit, YT Monday, 13 August, 12 Identifying Anoxia Using S-C • Goldhaber and Kaplan (1974) noted that bacterial reduction of seawater sulphate via carbohydrate reduction proceeded as follows: 7CH2O+4SO4+Fe2+ → 2FeS2+7HCO3-+3H++2H2O • This occurs in most ocean basins and is a kinetically controlled reaction whereby: Cpy → KxSpy, • • Cpy = # of carbon atoms metabolized • K - rate constant Spy = # of reduced sulphur precipitated as sulphide minerals. • Under normal oxidizing conditions this will yield a S/C for modern oxidized sediments of ~0.32. Steady state H2S oxidation back to form SO42-. • Anoxia...
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