Abstract

Common to all models proposed for the origin of the concordant deposits at McArthur River is the formation of laminar and framboidal pyrite (Py 1 ) during early diagenesis by microbial sulfate-reducing reactions. In modern anoxic sediments the formation of diagenetic pyrite by these reactions produces a strong positive correlation between sulfide S and organic C, such that the frequency distributions of sulfide-S to organic-C ratios (S/C) in the sediments are unimodal and symmetrical. Ancient and unmineralized anoxic sedimentary rocks also have unimodal and symmetrical S/C frequency distributions indicating that the processes accompanying the lithification of anoxic sediments do not destroy the sulfide-S and organic-C fingerprint of diagenetic pyrite formation.The S/C frequency distributions of the concordant Ridge II and H.Y.C. (Here's Your Chance) deposits at McArthur River are unimodal but asymmetrical, suggesting a model in which the deposit of galena and sphalerite modified the sulfide-S and organic-C fingerprint of diagnetic pyrite formation by altering a symmetrical distribution to an asymmetrical one. The model is supported by the sulfide-S and organic-C relationships of the H.Y.C. deposit. Poorly mineralized samples in the deposit, containing mostly pyrite, have a near-symmetrical unimodal S/C frequency distribution like unmineralized anoxic sedimentary rocks. On the other hand, well-mineralized samples, containing galena and sphalerite as well as pyrite, have much higher S/C ratios than the poorly mineralized samples, and these ratios constitute the high S/C tail of the overall S/C frequency distribution of the H.Y.C. deposit. The model is not supported by the sulfide-S and organic-C relationships of the concordant Ridge II deposit which show that both the poorly and well-mineralized samples have similar unimodal and asymmetrical S/C frequency distributions. The different sulfide-S and organic-C relationships of the two deposits are attributed to the greater abundance of overgrowth pyrite (Py 2 ) in the Ridge II deposit.The different sulfide-S and organic-C relationships of the poorly and well-mineralized H.Y.C. samples are used to test the various mineralizing models previously proposed for the deposit. The only models which are consistent with the relationships are the epigenetic sulfate-reduction and syngenetic nonsulfide-exhalative models in which galena and sphalerite are deposited after Py 1 by reactions involving the oxidation of organic C in the host rocks and concurrent reduction (probably inorganic rather than microbial) of solution SO 4 (super -2) . The sulfide-S and organic-C relationships of the concordant Ridge II deposit are also consistent with these models. The relationships indicate that in the Ridge II deposit, the sulfate-reductions deposited galena, sphalerite, and some Py 2 in the well-mineralized samples and significant quantities of Py 2 in the poorly mineralized samples.

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