COEXISTING GALENA, PbS//S//S AND SULFOSALTS: EVIDENCE FOR MULTIPLE EPISODES OF MINERALIZATION IN THE ROUND MOUNTAIN AND MANHATTAN GOLD DISTRICTS, NEVADA.

Studies of galena and Pb-Bi-Ag-Cu(Hg) sulfosalts of varied compositions in the Round Mountain and Manhattan gold districts of Nevada corroborate the occurrence of several distinct mineralized systems characterized by different mineral compositions and assemblages. The different episodes of mineralization are related to different Cretaceous and Tertiary magmatic-hydrothermal events, although the assignment of specific mineral compositions and associations with dated mineralized systems is still incomplete. At the Fairview mine, galena associated with Pb-Bi-Ag sulfosalts and simple sulfides occurs as intergrowths of discretely different composition controlled by silver and bismuth content. A galena-type phase (Pbs//s//s) with solid solution toward matildite, AgBiS//2(composition Gal//6//0Mat//4//0) was found. If silver and, to a lasser extent, bismuth are absent, the galena formed will be almost pure PbS. Some minor antimony may substitute for bismuth. This study confirms a linear relationship between a cell edge and Ag-Bi(Sb) content for galena containing less than about 15 wt. percent Ag and Bi(Sb).