Studies of the base metal sulfide deposits at McArthur River, Northern Territory, Australia; II, The sulfide-S and organic-C relationships of the concordant deposits and their significance

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. (Here9s 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.