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Shale-Hosted Deposits of PB, ZN, and BA: Syngenetic Deposition from Exhaled Brines in Deep Marine Basins

By
J. B. Maynard
J. B. Maynard
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Published:
January 01, 1991

Abstract

Sediment-hosted deposits of lead and zinc can be divided into two end-member types: epigenetic in carbonate hosts and syngenetic in clastic hosts. The epigenetic (Mississippi Valley) type forms at temperatures of about 100-150°C from highly saline brines believed to be derived from the dewatering of sedimentary basins. Associated minerals are barite and fluorite. The clastic-hosted deposits form during deposition of the enclosing strata in relatively deep marine basins. Deposition seems to occur below wave base under conditions of low oxygen concentration in the bottom water. In some cases, free H<sub>2</sub>S was present in the water column in which case the basin is referred to as euxinic. Barite mineralization commonly accompanies Pb and Zn in these deposits, but fluorite is always absent. The Irish Pb- Zn-Ba-Ag deposits occupy an intermediate position, containing both epigenetic and syngenetic portions.

Barite may also be found alone, without base metal enrichment, in host rocks of similar lithology to those of the Pb,Zn-bearing deposits. The Pb,Zn-barren barite depos.its, however, differ in tectonic setting from the Pb,Zn-bearing deposits. A consideration of four examples from the Lower Paleozoic illustrates these two types of clastic-hosted mineralization. Well-studied examples from strata of other ages include the Proterozoic deposits of Australia (Gustafson and Williams, 1981; Lambert, 1983), which are very similar to those described here except for a general absence of barite, and the Carboniferous of Ireland (Samson and Russell, 1987), where barite is present in some deposits. The Irish deposits are distinctive in having extensive epigenetic mineralization

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Reviews in Economic Geology

Sedimentary and Diagenetic Mineral Deposits: A Basin Analysis Approach to Exploration

Eric R. Force
Eric R. Force
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J. James Eidel
J. James Eidel
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J. Barry Maynard
J. Barry Maynard
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Society of Economic Geologists
Volume
5
ISBN electronic:
9781629490120
Publication date:
January 01, 1991

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