The generation of a stratiform skarn and volcanic exhalative Pb–Zn deposit (Sawusi) in the southern Chinese Altay Mountains: The constraints from petrography, mineral assemblage and chemistry.

Abstract: Numerous Pb–Zn and Fe ore deposits occur in the volcanic–sedimentary basins along the southern Chinese Altay Mountains. The mineralization of these deposits is hosted by Early–Middle Devonian volcanics and sedimentary rocks and is directly associated with skarns. The genesis of these deposits is uncertain or hotly debated. This study considers the Sawusi Pb–Zn deposit from the southern Chinese Altay. The host rock of the Sawusi deposit is rhyolitic crystal tuff with intercalated bands of Fe–Mn calcic carbonates and a small amount of rhyolite within the upper section of the Early Devonian Kangbutiebao Formation. On the basis of a field investigation, the petrographic features of the skarn were described. Detailed electron microprobe analysis was performed on the skarn minerals, and the O, H and C isotopic compositions of quartz and its fluid inclusions were determined. Petrographic observation revealed a progressively intensifying alteration from the tuff and its intercalated bands to the stratiform skarn characterized by alternate bands of dark garnet–biotite skarn and light grunerite–biotite skarn. Mineral–chemical analysis shows that the stratiform skarn of the Sawusi deposit consists mainly of spessartine, almandine, Mn-rich grunerite, biotite and Fe–Mn calcic carbonate minerals. A mass balance consideration of the cations in the skarn minerals suggests that the stratiform skarn formed through dimetasomatism between the tuff and intercalated bands of sedimentary Fe–Mn calcic carbonates; however, the scale of the metasomatism is limited to a few centimeters. Laser ⁴⁰Ar/³⁹Ar dating was performed on the skarn grunerite, yielding an isochron age of 412±15Ma. The ⁴⁰Ar/³⁹Ar age is identical, within analytical error, to the zircon U–Pb age (401±2.7Ma) of the rhyolite from the Kangbutiebao Formation that hosts the skarn and Pb–Zn mineralization, and the age (401±3.1Ma) of the Biesisala porphyry, Sawusi deposit. The O, H and C isotopic compositions show that the ore-forming fluid includes two components. One is H₂O–CO₂, which has volatilized from carbonate and dehydrated from tuff, and the other is meteoric-derived surface water carrying minor meteoric CO₂ or organic matter-derived CO₂. Therefore, the stratiform skarn and Pb–Zn mineralization of the Sawusi deposit are quite different from the contact-metasomatic skarn and related deposits, and suggest a volcanic exhalation–sedimentation origin instead. This study has broad relevance to the Pb–Zn and Fe deposits along the southern Altay Mountains in the areas of ore genesis study and ore prospecting. [Copyright &y& Elsevier]