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

Abstract

Mineralogical and geochemical data for ca. 1720 Ma Si-Fe-Mn seafl oor-hydrothermal sedimentary rocks (exhalites) near the Jones Hill Zn-Cu-Pb-Ag-Au volcanogenic massive sulfi de (VMS) deposit, northern NewMexico, provide valuable insights into the redox state of late Paleoproterozoic deep seawater. Distal exhalites ~1200 m south of the deposit form beds 0.5-2 m thick composed of interlayered iron formation and metachert. The iron formation consists mostly of quartz and magnetite, and includes 0.3-3-cm-thick laminae of fi ne-grained garnet-quartz rock, which in places contains as much as 9.4 wt% MnO that resides chiefl y in spessartine-rich garnet (coticule). Shale-normalized rare earth element data for an unaltered, low-Al quartz-magnetite iron formation show no Ce anomaly, which rules out fully oxic deep waters during exhalative mineralization. The garnet-quartz rocks and coticules mostly have small positive Ce anomalies, which are larger for calculated detrital-free compositions, thus precluding deposition in anoxic waters. Signifi cant amounts of ferric iron are inferred for protoliths of the iron formation, based on the presence of abundant magnetite laminae, and of magnetite inclusions in cores of the spessartine garnets. Protoliths of the garnet-quartz rocks and coticules probably consisted largely of clays and Fe-Mn oxyhydroxides. Together these mineralogical and geochemical data suggest that the Jones Hill exhalites were deposited from deep seawater having low concentrations of dissolved O 2 corresponding to suboxic conditions, and not the sulfi dic conditions proposed for late Paleoproterozoic deep seawater by other workers. Exhalites associated with Cu-rich VMS deposits, when effects of alteration and detrital components are considered, can be important proxies for evaluating the evolving redox state of ancient deep oceans. on 23 July 2009 geosphere.gsapubs.org Downloaded from Paleoproterozoic seafl oor-hydrothermal Si-Fe-Mn exhalites Geosphere,

Metamorphosed Proterozoic Zn-Pb-Ag Mineralization in the Foster River Area, Northern Saskatchewan, Canada

“…All three samples exhibit positive CePAAS anomalies (Fig. 9c), which are likely the result of oxidative surface weathering (e.g., Slack et al, 2009). Although quartz garnetite in or adjacent to Broken Hill-type mineralization in the Curnamona Province, Australia, shows little to no Ce anomalies, they have approximately equal proportions of detritus and hydrothermal sediment (Fig.…”

“…Normalized rare earth element plots of Precambrian iron formations have been used extensively as a proxy for redox conditions in the hydrosphere of the early Earth and for understanding how the ocean-atmosphere-lithosphere system operated during this time (e.g., Bau, 1993;Bau and Dulski, 1996;Slack et al, 2009). Normalized REE patterns of such iron formations generally show positive Eu anomalies and negative Ce anomalies, especially where associated with massive sulfides (Spry et al, 2000).…”

A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia

“…Our interpretation is also consistent with Mo concentration and isotope data for these ORM (Kendall et al, 2009(Kendall et al, , 2015b, which constrain the extent of ocean euxinia rather than general ocean anoxia (euxinic plus ferruginous conditions). The model does not preclude the possibility that a significant portion of the low-productivity regions of the deep oceans may have been covered by weakly oxygenated waters (Slack et al, 2007(Slack et al, , 2009, nor does it preclude oxygenated surface waters in highly productive regions (Sperling et al, 2014;Reinhard et al, 2016).…”

The Hadean-Archaean Environment

“…Nielsen et al (2006b) detected thallium isotope variations with the expected coupled chemical variations in recent Hawaiian basalts. The deep ocean became suboxic at $1.8 Ga (Slack et al 2007(Slack et al , 2009). …”