Silver sulfosalts of the Santo Nino vein, Fresnillo District, Zacatecas, Mexico

The Santo Nino vein in the Fresnillo Ag-Pb-Zn district, Mexico, contains silver in acanthite and in the sulfosalts pyrargyrite, proustite, polybasite, selenian polybasite, tetrahedrite and stephanite, members of three silver-bearing sulfosalt solid-solution series. The sulfosalts, excepting proustite, are Sb-rich end members of their respective solidsolution series. The mineral assemblage evolved from (CuAg)-bearing to Ag-bearing: a) tetrahedrite, polybasite and pyrargyrite; b) polybasite, pyrargyrite and stephanite; c) pyrargyrite and acanthite; and d) acanthite. The average sulfosalt formulas are: pyrargyrite Ag3.02(Sbl.ooAso.03)S3; proustite Ag3.08(Sb0.03Aso.9S)S3; polybasite (Ag1S.40Cu1.1S) (Sb l.88AsO.19)SII; selenian polybasite (AgI4.S0CU1.1s) (Sbl.92AsO.d(Se1.8SS9.1S); tetrahedrite (CuS.92Ag4.21) (Fe1.43ZnO.73)(Sb3.91Aso.II)SI3; and stephanite (A~.81CuO.13)(Sb1.0SAso.~S4'Compositional tie-lines and relative semi-metal [Sb/(Sb + As)] values for coexisting pairs of pyrargyrite-polybasite, pyrargyrite-tetrahedrite, and polybasite-tetrahedrite, and for triplets of pyrargyritepolybasite-tetrahedrite show systematic trends (pyrargyrite > tetrahedrite > polybasite) and appear to be temperature-dependent [higher temperature, lower Sb/(Sb + As) values]. Sulfosalt (Sb-As) and (Ag-Cu) patterns indicate that element substitution, not fractional crystallization, determines the semi-metal and metal variations in the silver-bearing sulfosalts of the Santo Nino vein. Pyrargyrite shows only (Sb-As) substitution, tetrahedrite shows coupled (Sb-As) and (Ag-Cu) substitution, whereas polybasite shows noncoupled (Sb-As) and (Ag-Cu) substitution. The distribution of Sb/(Sb + As) values in polybasite and of Ag/(Ag + Cu) values in tetrahedrite indicates that the ore-bearing solutions flowed from a deeper, southwestern part of the district to shallower levels in the northeast.