The San Francisco manganese deposit, located in western central Mexico in a Tertiary, continental-volcanic province, is composed of Mn oxides and Fe oxides. The small geographic extent of the tuffs which enclose the oxides, and their sorting, rounding of constituents, and distinct stratification, point to deposition in a small lacustrine basin. The stratigraphic conformity of the oxides with the tuffs and their purity and zoning pattern suggest precipitation as a chemical sediment from volcanic solutions, at a time of decreased volcaniclastic contributions. As the mineralizing solutions entered the depositional basin, the two oxides precipitated separately, Fe oxides closer to the source and Mn oxides farther from it, in response to a gradual increase in pH and Eh of the solutions through mixing with alkaline, oxygenated waters in the depositional basin.Spectrographic analyses for minor and trace elements reveal that both the Mn oxides and the Fe oxides in the San Francisco deposit are strongly enriched in As, Ba, Cu, Mo, Pb, Sr, and V compared to crustal averages. Except for V, the concentrations of these elements are higher in the Mn oxides than in the Fe oxides by factors ranging from 3 to 140. Thus, the Mn oxides reflect the composition of the mineralizing solutions much more strongly than the Fe oxides. Mineralogy and solubility calculations suggest that the minor elements were removed from solution by adsorption rather than by precipitation in response to saturation. Since both oxides when freshly precipitated adsorb minor elements strongly, the distinct partition is interpreted as a result of diagenetic recrystallization processes. During recrystallization, Ba, Sr, and Pb adsorbed on precipitating hydrous Mn oxides form manganates of the cryptomelane-hollandite group, and Cu can substitute for Mn in the lattice of mixed valence Mn oxides. These elements would be desorbed from recrystallizing Fe oxides because of their inability to form minerals with trivalent Fe or to substitute for Fe in the lattice of its oxides. Depending on the geochemical characteristics of the depositional basin, they would form sulfates or carbonates in situ, be built into spatially associated Mn oxides, or reprecipitate in genetically related sulfidic sediments. The less distinctly partitioned minor elements As, Mo, and V can be accommodated in both Mn and Fe oxides, by formation of arsenates, molybdates, and vanadates or by substitution for either Mn or Fe in their oxides.High concentrations of the minor elements which characterize the San Francisco deposit have been reported from other Mn and Fe oxide deposits of volcanic association. The presence or unusual concentration of these elements in Mn and Fe oxides, or in genetically related sulfates, carbonates, or sulfides, is supporting evidence for volcanic contributions to their formation.
Located in the Eastern Cordillera of northwestern Argentina, the Aguilar mine and several smaller prospects are aligned north-south in a 30- by 5-km district. The Zn-Pb-Ag sulfide ores are hosted in intensely folded and faulted Lower Ordovician quartzites and hornfelses, near their contact with the Cretaceous Aguilar granite. The orebodies are stratigraphically conformable with the siliciclastic rocks and parallel to the contact metamorphic halo of the granite, in the pyroxene-hornblende and albite-epidote hornfels facies.The ores are associated with sediments formed in local depressions of a tectonically active, shallow-marine environment and consist of strata-bound and stratiform sulfide lenses and layers several hundreds of meters long and wide, and tens of meters thick. A distinctive ore stratigraphy is recognizable: disseminated and stockwork sulfides overlain by breccia-hosted sulfides and banded to massive sulfides. Lead-rich fissure veins and quartz veins cut these types of strata-bound sulfides; barite is abundant in one of the prospects. The ores are made up of fine- to medium-grained intergrowths of sulfides and sulfosalts in quartzite and calc-silicate gangue. The sulfides are intensely recrystallized and annealed in the Aguilar deposit, which is located in the pyroxene-hornblende metamorphic halo. In the Esperanza prospect, located in the lower grade albite-epidote halo, the sulfides are finer grained, less recrystallized, not annealed, and contain abundant framboidal pyrite and textures of soft-sediment deformation.The calc-silicate assemblage in the Aguilar deposit is characterized by subcalcic garnets with significant proportions of spessartine (Sp (sub 26-78) mole %), almandine (Al (sub 5-23) ), grossularite (Gr (sub 8-52) ), and andradite (Ad (sub 0-14) ). Pyroxenes are manganese rich and iron poor, with an endmember range of hedenbergite (Hd (sub 10-55) mole %), johannsenite (Jo (sub 15-35) ), and diopside (Di (sub 20-80) ). Late-stage skarn minerals include calcium-rich bustamite, subcalcic actinolite, chlorite, and vesuvianite with up to 20 wt percent rare earth elements.Sulfur isotope values range from 10.8 to 26.5 per mil for sulfides, and from 32.4 to 34.0 per mil for barite. The strongly positive delta 34 S values for sulfides and barite are consistent with fractionation of sulfide and sulfate from Lower Ordovician seawater. Lead isotope ratios in galenas from all Aguilar ores have means of 206 Pb/ 204 Pb = 18.04, 207 Pb/ 204 Pb = 15.64, and 208 Pb/ 204 Pb = 38.03. Potassium feldspar from the Cretaceous Aguilar granite is far more radiogenic, having ratios of 206 Pb/ 204 Pb = 19.28, 207 Pb/ 204 Pb = 15.67, and 208 Pb/ 204 Pb = 39.00. These isotope ratios suggest an early Paleozoic, crustal source for the lead in the ores, unrelated to the lead in the granite.The overall geologic setting and geometry of the Aguilar ores, the distinctive ore stratigraphy, the mineral composition and textures, the calc-silicate assemblage, and the stable and radiogenic isotope evidence suggest that the sulfides in the Aguilar district formed as exhalative accumulations in a Lower Ordovician, shallow-marine sedimentary basin. The present distribution of the ores and their metamorphic textures indicate overprinting by contact metamorphism during the emplacement of the Cretaceous Aguilar granite.
The San Francisco manganese deposit is a well-stratified mass of manganese oxides and iron oxides enclosed in a continental sequence of Tertiary volcanic rocks. The two oxides form irregular layers and lenses within the deposit and are clearly separated from each other: iron oxide layers predominate to the northwest, and manganese oxide layers and lenses to the southeast.The deposit's conformity with the surrounding tuffs and its textural and compositional stratification point to a syngenetic sedimentary origin. Ripple marks and cross-bedding in underlying sandstones and shales and an abundance of iron oxides in the adjacent tuffs indicate deposition in shallow water under oxidizing conditions. Empirical data from present-day environments and the mineralogic composition of the deposit give a probable range of Eh-pH for its formation and point to the mechanisms involved in the separation of iron and manganese in the depositional basin.The purity of the oxides, the large enrichment of manganese relative to iron as compared to crustal averages, and the trace element composition of the oxides suggest volcanic solutions as a source of the two metals.
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.
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