Publication Type:

Journal Article


Economic Geology and the Bulletin of the Society of Economic GeologistsEconomic Geology and the Bulletin of the Society of Economic Geologists, Economic Geology Publishing Company, Lancaster, PA, United States, Volume 106, Number 4, p.667-686 (2011)




Canada, chemical composition, clastic rocks, conglomerate, cores, eastern canada, gold ores, ICP mass spectra, in situ, ion probe data, isotope ratios, Isotopes, mass spectra, metal ores, Mineral exploration, Ontario, overgrowths, Paleoproterozoic, placers, Precambrian, proterozoic, pyrite, S-33/S-32, S-34/S-32, Sedimentary rocks, spectra, Stable isotopes, sulfides, sulfur, Trace elements, upper Precambrian, veins


Extensive pyrite-bearing, auriferous cobble and boulder conglomerates are present in the basal 30 m of the Mississagi Formation in Pardo and Clement Townships, Ontario, Canada. The sedimentology of the conglomerates, combined with regional geology, indicates limited fluvial transport in a gravel bed braided river with local hyperconcentrated flows, with material derived from a highly restricted catchment area. Postdepositional overprinting of the conglomerates is related to the Penokean orogeny at 1.85 to 1.5 Ga and alkali metasomatism at approximately 1.7 Ga. Several pyrite varieties, including detrital and postdepositional recrystallized and altered grains, are present in the conglomerates. Detailed in situ laser ablation inductively coupled plasma-mass spectrometry (LA ICPMS) analysis of pyrite revealed that the gold in the deposit is intimately associated to large detrital pyrite grains, as "invisible" Au, with lesser amounts of free gold. Postdepositional pyrite and pyrite overgrowths have very low Au content. Elements such as Pb, Bi, Sb, Te, Ag, and Hg, together with Au were mobilized during hydrothermal alteration and dissolution of detrital pyrite grains. Small-scale transport and reprecipitation formed part of the postdepositional pyrite and free gold in immediate contact with postdepositional grains. In contrast, Ni, Co, and As were not mobilized. External fluid infiltration is negligible and is indicated only by minor, compositionally distinct, late sulfide veinlets crosscutting the conglomerates. Sulfur isotope analysis of detrital pyrite yields generally positive delta (super 34) S values (0.97-9.26 per mil). The delta (super 34) S sulfur isotope composition of pyrite overgrowths and postdepositional grains overlaps the isotopic range of the detrital grains, suggesting a near-closed S system during postdepositional processes. However, the detrital pyrite tends to have slightly negative Delta (super 33) S while the postdepositional and overgrowth pyrite are either neutral or slightly positive. The S isotope composition of the detrital pyrite is compatible with an origin of S as dissolved sulfate in an ocean under a low oxygen atmosphere. The potential source of the Au-bearing detrital pyrite appears to have been an, as yet, undiscovered Archean deposit located within 1 to 8 km of the placer deposit.


GeoRef, Copyright 2018, American Geological Institute.<br/>2011-062887<br/>Clement Ontario<br/>Mississagi Formation<br/>Pardo Ontario