Seafloor-hydrothermal Si-Fe-Mn exhalites in the Pecos greenstone belt, New Mexico, and the redox state of ca. 1720 Ma deep seawater

• DOI: 10.1130/ges00220.1
• Publication Date: 2009-06-09T17:49:29Z
• Authors: John F. Slack; Tor Grenne; Andrey Bekker
• Keywords: Banded iron formation; Protolith

Mineralogical and geochemical data for ca. 1720 Ma Si-Fe-Mn seafloor-hydrothermal sedimentary rocks (exhalites) near the Jones Hill Zn-Cu-Pb-Ag-Au volcanogenic massive sulfide (VMS) deposit, northern New Mexico, provide valuable insights into redox state of late Paleoproterozoic deep sea-water. Distal exhalites ~1200 m south deposit form beds 0.5–2 thick composed interlayered iron formation metachert. The consists mostly quartz magnetite, includes 0.3–3-cm-thick laminae fine-grained garnet-quartz rock, which in places contains as much 9.4 wt% MnO that resides chiefly spessartine-rich garnet (coticule). Shale-normalized rare earth element an unaltered, low-Al quartz-magnetite show no Ce anomaly, rules out fully oxic waters during exhalative mineralization. coticules have small positive anomalies, are larger calculated detrital-free compositions, thus precluding deposition anoxic waters. Significant amounts ferric inferred protoliths formation, based on presence abundant magnetite laminae, inclusions cores spessartine garnets. Protoliths probably consisted largely clays Fe-Mn oxyhydroxides. Together these mineralogical suggest were deposited from sea-water having low concentrations dissolved O2 corresponding to suboxic conditions, not sulfidic conditions proposed seawater by other workers. Exhalites associated with Cu-rich VMS deposits, when effects alteration detrital components considered, can be important proxies evaluating evolving ancient oceans.