The Ag-Bi-Pb-Sb-S-Se-Te mineralogy of the Darwin lead-silver-zinc deposit, southern California

Abstract

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.

Bi-sulphotellurides associated with Pb – Bi – (Sb ± Ag, Cu, Fe) sulphosalts: an example from the Stan Terg deposit in Kosovo

“…Numerous Au-Ag-Bi-Te-S-bearing mineral systems have been described elsewhere, and they are commonly interpreted to have precipitated under either high-temperature hydrothermal (e.g., Czamanske & Hall 1975;Ren 1986), or under epithermal conditions (e.g., Oberthür & Weiser 2008). Various phases from the tetradymite group minerals (with unknown phases) together with sulphosalts were described from plenty of localities worldwide (Gu et al 2001;Cook & Ciobanu 2004;Pieczka et al 2009;Ciobanu at al.…”

Aleksite, PbBi2Te2S2, a new mineral1

“…Notes. 1 -"lead-bearing tetradymite -csiklovaite", Plimer (1974);2 vetskiy et al (1976); 3 -"bigh-Te phase", Czamanske and Hall (1975);4 -and Darling (1972). Formula calculated by us.…”

“…The third sulfotelluride, name "high-Te phase" (Czamanske and Hall, 1975) occurs in the Darwin deposit, southern California. In reflected light it has a rose color and is strongly anisotropic.…”

Bi-Sb energetics in sulfosalts and sulfides

“…Bartos, 1990;Boldyreva, 1970;Borodayev et al, 1986;Brooker and Jaireth, 1995;Bussell et al, 1990;Ruvalcaba-Ruiz and Thompson, 1988;Czamanske and Hall, 1975;Gibson et al, 1990;Gilmer et al, 1988;Goodell, 1974;Goodell and Petersen, 1974;Grrpper et al, 1991;Hayase, *Corresponding author 1955;Jeppson, 1987;Johnson and Meinert, 1994;Kelly and Tumeaure, 1970;Luetb, 1988;Lueth et al, 1990;Petersen et al, 1977;Petruk, 1968;Sillitoe et al, 1975;Springer, 1969;Stone, 1959;Turneaure, 1971). Among these minerals, bismuthinite-stibnites exhibit distinct compositional variations along flow paths of mineralizing fluids in ore deposits that are characterized by preponderances of molar Bi/(Bi+Sb) ratios near the end-members, with a conspicuous paucity of intermediate compositions (Fig.…”