OUTER SHELF AND SLOPE SETTINGS FOR SEDIMENT-HOSTED STRATIFORM ZN-PB DEPOSITS: EVIDENCE FROM ASSOCIATED PHOSPHORITES AND PHOSPHATE-RICH STRATA

The predominant model for stratiform Zn-Pb deposits hosted in black shales includes syngenetic to early diagenetic mineralization in anoxic or sulfidic basins. Although many such Zn-Pb deposits have sedimentary environments that support this model, the lithofacies of some deposits do not. A prime example is the Howards Pass (HP) district in western Canada, which in total (15 deposits) contains 423.5 Mt of indicated + inferred resources at 4.79 % Zn and 1.56 % Pb. Host strata are Silurian black carbonaceous and/or calcareous mudstone with minor limestone and chert. Beds of phosphorite (>18 wt % P₂O₅) ca. 10 cm to 0.8 m thick occur 0.5 to 3.0 m stratigraphically below the Zn-Pb deposits, approximately at the Ordovician-Silurian boundary, and display uniformly low Fe/P ratios (avg 0.11 ± 0.05). Thin phosphorites also occur 3 to 117 m above the deposits, as multiple 0.5- to 1.5-cm-thick laminae in black mudstone; these thin phosphorites also have low Fe/P ratios. Such ratios argue for a phosphorite origin by upwelling, and not by a Fe-Mn shuttle process that should, in theory, produce much higher Fe/P ratios of >1.0 based on modern analogs (e.g., Baltic Sea). On present-day continental margins, upwelling occurs mainly on outer shelf and slope settings, which is our favored environment for the HP deposits; this interpretation is supported by geochemical redox proxies such as Mo, Re/Mo, and Ce/Ce* that suggest mainly mild oxygenation of bottom waters and not persistent anoxic or sulfidic conditions. Close stratigraphic association of phosphorites or phosphate nodules with other sediment-hosted Zn-Pb deposits (Irecê Basin, Brazil; Prades Mountains, Spain; Gamsberg, South Africa) provides additional support for our model, which includes a hydrothermal (not upwelling) source of metals. Future exploration for stratiform Zn-Pb deposits in black shales therefore should include outer shelf and slope settings and not focus exclusively on anoxic or sulfidic basins.