New identification and significance of Early Cretaceous mafic rocks in the interior South China Block

Caledonian Tin Mineralization in the Jiuwandashan Area, Northern Guangxi, South China

The Jiangnan orogenic belt is located between the Yangtze and Cathaysia blocks in South China and is one of the largest W–Sn–Nb–Ta ore belts worldwide. Mineralization occurred from the Proterozoic to Mesozoic, but Caledonian Sn mineralization has rarely been reported. The Jialong cassiterite–sulfide deposit is located in the western Jiangnan orogenic belt. It is hosted by the Sibao Group and in contact with the northeastern part of the Yuanbaoshan granite. The deposit was overprinted by the Sirong ductile shear zone. Here, we present cassiterite U–Pb and mylonitic granite muscovite 40Ar/39Ar ages for this deposit. The cassiterite and muscovite yielded concordant U–Pb and 40Ar/39Ar ages of 422–420 Ma, indicating that Sn mineralization occurred during the early Paleozoic and was spatially and temporally related to the ductile shear zone. The cassiterite is depleted in Nb (0.51–5.46 ppm), Ta (0.01–1.09 ppm), Ti (32.84–423.15 ppm), Sc (0.02–1.45 ppm), Hf (0–1.11 ppm), and other high-field-strength elements. Elements, such as Pb (0.01–8.11 ppm) and Sb (9.92–56.45 ppm), are relatively enriched in the cassiterite, which indicate the Jialong deposit was not directly related to magmatism. Shearing along the Sirong ductile shear zone occurred at 419.6 ± 3.8 Ma, concurrent with the formation of the Jialong Sn–Cu deposit. Moreover, cassiterite in the deposit exhibits obvious shear and brittle deformation, and dissolution and regrowth, suggesting that Sn mineralization was closely related to ductile shearing. The Sirong ductile shear zone and secondary shear structures had a key role in controlling the Sn orebody. The heat generated during tectonic deformation in the ductile shear zone may have produced the ore-forming hydrothermal fluids, and NW–SE-trending fractures in the strata provided the space for mineralization. Metamorphic hydrothermal fluids generated by Caledonian shear deformation extracted Sn from Sn-rich strata, which then migrated along interlayer fractures produced by shearing. A decrease in pressure and water–rock reactions led to the mineralization of Sn and other elements. This deposit is the first example of Caledonian and shear zone-related Sn mineralization identified in the Jiuwandashan area of northern Guangxi.