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

摘要

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 ~(40)Ar/ ~(39)Ar dating was performed on the skarn grunerite, yielding an isochron age of 412±15Ma. The ~(40)Ar/ ~(39)Ar 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 H _2O-CO _2, which has volatilized from carbonate and dehydrated from tuff, and the other is meteoric-derived surface water carrying minor meteoric CO _2 or organic matter-derived CO _2. 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.