010 KAPLAN AND RITTENBERG [CHAP. 23 



Germany, with shallow-water marine fossils, contains black shales rich in 

 chalcocite, bornite and chalcopyrite as Avell as sulfides of iron, zinc and lead, 

 together with small quantities of silver, nickel, cobalt, vanadium and molyb- 

 denum minerals. The Tri-State District in the Mississippi Valley is exception- 

 ally rich in zinc and lead sulfides, which occur in limestones, dolomites, cherts or 

 calcareous shales of the Paleozoic (Lindgren, 1928). 



Whether these non-ferrous sulfide ores are epigenetic or syngenetic is still a 

 lively (luestion. The main difficulty lies in finding the source of the metals. 

 The tendency has been to describe sulfide ores, even in sedimentary strata 

 separated by large distances from igneous intrusions, as being of a hydro- 

 thermal or metasomatic origin. Stanton (1955) found that in the lower Paleozoic 

 of the Bathurst region of New South Wales, Australia, sulfide ores occurred 

 adjacent to limestone and volcanic rocks. He interpreted this as indicating a 

 release of cations by volcanic exhalations then carried to the ocean in solution 

 and deposited in lagoons adjacent to actively growing reefs, probably containing 

 reducing sediments with a high sulfide content. 



The exact mechanism for pyrite formation in marine sediments is not yet 

 known, although it is commonl}'^ assumed to arise from hydrotroilite. This 

 concept is strengthened by the similarity in the isotoi^ic ratios of the hydro- 

 troilite and pyrite sulfur in the Milford Sound sediments (Table I). Pjri-ite is 

 found widely dispersed throughout reducing as well as oxidizing sediments. In 

 the surface oxidizing layers of the Santa Catalina Basin, pyrite constitutes the 

 most abundant sulfur compound, whereas in the Santa Monica Basin, which has 

 a high hydrotroilite content, pyrite is low. It thus appears that pyrite can form 

 in mildly oxidizing environments and at a comparatively rapid rate. 



Since sulfate reduction is undoubtedly involved as the first step in jowite 

 genesis, it might appear paradoxical that pyrite is abundantly formed in mildly 

 oxidizing sediments which should exclude sulfide production. The common 

 occiu"rence of pyrite within foraminiferal and diatom tests suggests that 

 reducing micro-environments adequate for sulfate reduction can occur in a 

 milieu that is predominately mildly oxidizing. 



Of great interest is the direct relationship between the pyrite content and 

 organic content of the sediment, which was also observed by Harmsen et al. 

 (1954) for marshes and soils in the low-lying coastal area of the Netherlands. 

 Because of the tendency for pyrite to concentrate in sediments Avith high 

 organic matter it is tempting to postulate a reaction occurring between the 

 hydrotroilite first formed and the protoplasmic matter, resulting in the 

 deposition of the disulfide. > 



With respect to the formation of other sulfides, Miller (1950) has shown that 

 sulfate-reducing bacteria can tolerate relatively high contents of base-metal 

 cationsand Baas Becking and Moore(1961) haverecentlydemonstrated the form- 

 ation of galena, sphalerite, covellite and digenite in bacterial cultures. Present 

 day neritic marine basins adjacent to areas of high volcanic activity or sub- 

 marine seepage such as Alaska, Japan or the South Pacific Island arcs, may, there- 

 fore, prove useful places to initiate studies on the formation of such minerals. 



