Chemistry and U–Pb geochronology of cassiterite in the Xiasai deposit, central Yidun Terrane (SW China): Link between Sn and Ag–Pb–Zn mineralisation

Abstract

Tin and (Ag)–Pb–Zn mineralisation generally co-exist in SEDEX, porphyry, skarn, greisen, as well as magmatic-hydrothermal Sn and/or Pb–Zn polymetallic deposits. The genetic relationship between these two styles of mineralization, however, is not well defined. More than 20 skarn and hydrothermal-vein-type Ag–Pb–Zn–Sn deposits can be found in the central Yidun Terrane (YDT) of southwestern China. Amongst these is the large Xiasai deposit, which contains 1028 t Ag @ 337.8 g/t, 0.27 Mt Pb + Zn @ 8.33 %, and 20,000 t Sn @ 0.75 %. The deposit is zoned, comprising proximal cassiterite-quartz veins and distal Ag–Pb–Zn veins. We present micro-textural studies, LA–ICP–MS U–Pb dating results, as well as cassiterite major and trace element data that served to better constrain the link between the Sn and Ag–Pb–Zn mineralisation at Xiasai. Two types of cassiterite, i.e., Cst I and Cst II, can be distinguished using cathodoluminescence (CL) imagery. Cst I has a dull luminesce and oscillatory zonation, and it has been partially replaced by Cst II, which has a bright luminesce and a homogeneous texture. Whistle the SnO2 and TiO2 contents of Cst I and Cst II are similar, the latter has slightly higher FeO compositions (from bdl to 0.08 wt% and 0.01 to 0.11 wt%, respectively). LA–ICP–MS U–Pb dating of cassiterite yielded a Tera-Wasserburg U–Pb lower intercept age of 99.2 ± 0.8 Ma (n = 39, MSWD = 1.8), identical to the ages of granitic magmatism and Ag–Pb–Zn vein formations. LA–ICP–MS trace element analyses revealed that cassiterite is enriched in Ti and W, but depleted in Fe, Mn, Nb, and Ta. The Zr/Hf ratios of cassiterite range from 4.7 to 15.8 and Ti/Zr ratios range from 1.8 to 6.2. Overall, the measured Fe, W, Nb, and Ta concentrations, as well as the calculated Zr/Hf and Ti/Zr ratios, indicate that the cassiterite-quartz veins at Xiasai have a magmatic-hydrothermal origin. Our findings, when combined with previous micro-thermometrical fluid inclusion data (423–481 °C for Sn and 158–386 °C for Ag–Pb–Zn mineralisation), suggest that Sn and Ag–Pb–Zn mineralisation was cogenetic and linked to a common magmatic-hydrothermal system. The spatial relationship between Cretaceous A-type granitic magmatism and Ag–Pb–Zn–Sn mineralisation in the central YDT can be used as a guideline for mineral exploration of both skarn and hydrothermal veins.