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86 results on '"Barley, M. E."'

10. Petrography and geochemistry of the Dales Gorge banded iron formation: Paragenetic sequence, source and implications for palaeo-ocean chemistry

Author

Pecoits, E., Gingras, M. K., Barley, M. E., Kappler, A., Posth, N. R., Konhauser, K. O., Pecoits, E., Gingras, M. K., Barley, M. E., Kappler, A., Posth, N. R., and Konhauser, K. O.

Abstract

Banded iron formations (BIFs) have long been considered marine chemical precipitates or, as more recently proposed, the result of episodic density flows. In this study, we examined the mineralogy, petrography and chemistry of the Dales Gorge BIF to evaluate the validity of these models. Microbands reflect a compositionally variable primary precipitation/sedimentation pattern with diagenetic modifications. "Iron-rich" bands are characterized by massive anhedral aggregates and xenomorphic hematite, commonly showing overgrowths of subhedral magnetite with minor apatite and late diagenetic ankerite-Fe dolomite. "Iron-poor" bands consist of fine-grained quartz, siderite and Fe-talc in variable amounts. Amorphous silica, ferrihydrite (precursor of hematite), greenalite and possibly some siderite constitute the primary precipitates, while ankerite-Fe dolomite, Fe-talc and magnetite and the bulk of siderite are secondary mineral phases. Ankerite and Fe-dolomite most likely represent by-products of siderite dissolution and the subsequent reaction of dissolved bicarbonate with Ca, Mg and Fe. Ferroan-talc is thought to be formed from the following reactions: (i) greenalite + chert, and (ii) siderite + chert. Magnetite formed from the conversion of hematite, likely through bacterial Fe(III) reduction. Geochemical mineral analyses show that all the phases have very low concentrations of trace elements with the exception of Ba, As, Cr, Zn and Sr. This partitioning was presumably controlled by both sorptive reactions occurring during primary precipitation in the water column and secondary remobilization during diagenesis. Whole-rock analyses indicate two decoupled sources for BIF and S macrobands. While BIF macrobands have a major hydrothermal influence, data from the S macrobands supports a dominantly mafic provenance. Nonetheless, when all the lithologies (i.e., source rocks, S and BIF macrobands) are evaluated together, continuous geochemical trends can be observed. This sugges

Published
2009

41. Archaean carbon reservoirs and their relevance to the fluid source for gold deposits

Author

Groves, D. I., Golding, S. D., Rock, N. M. S., Barley, M. E., and McNaughton, N. J.

Abstract

It is commonly assumed1,2that seawater-derived carbon in altered volcanic rocks is the only major pre-metamorphic carbon-in-carbonate reservoir in Archaean greenstone belts. Thus carbonates from Archaean gold deposits with stable isotope ratios too negative (median δ13C ≈ −3‰) to have been derived from such carbon (δ13C usually assumed to be near 0‰) have been interpreted as being derived from a local felsic magmatic source1. Barley and Groves3, however, recognize two carbonate-alteration styles that predate regional metamorphism and gold mineralization in the Norseman–Wiluna greenstone belt of Western Australia: evidence is summarized below. These are seafloor alteration and fault-controlled regional alteration, which, as shown below, have completely different carbon isotope compositions. The latter (median δ13C = −4.8‰) implies a major juvenile carbon (CO2) flux from the mantle during greenstone-belt evolution, and is thus of fundamental importance in itself. However, here we show why the mere existence of two carbon-in-carbonate reservoirs and variation in carbon isotope ratios of carbonates between individual gold deposits in one area, negate some of the fundamental assumptions used to support magmatic-fluid models1.

Published
1988
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