Analyses by electron microprobe have disclosed unexpected complexity in the mineralogy of the Ag-Bi-bearing galena ores at Darwin, yet reveal that these elements are carried predominantly in solid solution in PbS. Darwin "galena" can be divided into three groups on the basis of Ag-Bi content, mineral assemblage, and location in the mine. Ninety percent of the galena, aptly named, is free from inclusions, and contains less than 0.22 weight percent Ag; a fine-textured PbSss typical of shallow workings contains 1.7 to 3.3 weight percent Ag and 3.9 to 7.3 weight percent Bi in solid solution and, moreover, contains several types of small exsolved laths (to •'0.25 mm long) that are enriched in Ag, Bi, and Sb; a third distinct PbSs•, containing 1.5 to 4.6 weight percent Ag and 3.8 to 10.8 weight percent Bi, occurs only as a breakdown product in a localized late sulfosalt stage of the mineralization.At least two, and perhaps three, distinct types of sulfosalt that relate compositionally to the minerals cosalite (Pb2Bi2Ss) , gustavite (AgPbBiaS6) , and "schirmerite" (AgPb2BiaS,?) have exsolved from the fine-textured PbS•, as have lesser amounts of matildite and tetradymite. A chemically complex sulfosalt similar in structure to
heyrovsk•ite (6Pbx_xBi•x/aS.Bi•Sa) is the predominant natural associate of the third PbS•s, and wisps of a minor, high-Te sulfosalt are typical in this association. Compositions of the heyrovsk•ite-like phase are variable, apparently as the result of breakdown below 250øC of a primary sulfosalt of approximate composition: Pb-41; Bi-31;Ag-7; S-10; Se-9; and Te-1 weight percent.One-to-one coupling of Ag and Bi is approached in the two groups of PbS• (the average atomic ratio of Bi:Ag is 1.29) but is not characteristic of the sulfosalts. Consideration of sulfosalt bonding characteristics provides an apparent explanation for this lack of one-to-one coupling between Ag and Bi, as well as for the variable ratios of Pb to Bi measured for the several exsolved sulfosalt phases. Extensive, near-ideal substitution of Se for S is typical of the various PbS•,-sulfosalt assemblages. Significant substitution of Sb for Bi is noted only in the exsolved sulfosalts found in the finetextured PbSs,.Because exsolved mineral phases persist in samples annealed at 350øC, ore deposition at Darwin is presumed to have occurred at higher temperatures.
