Origin of the Saibo-Lamasu Skarn Cu deposit, Western Tianshan, Xinjiang, NW China: Evidence from geology, mineral chemistry, fluid inclusions, stable isotopes, and organic geochemistry

Abstract

The Saibo-Lamasu Cu deposit is the largest skarn Cu deposit in Western Tianshan, Xinjiang, NW China. The Cu orebodies are predominantly hosted in the contact zone between the oxidized granodiorite porphyry (lg fO2 = FMQ + 1.4 to 4.4) and reduced carbonaceous limestones (effective source rocks, TOC = 0.19 to 0.32 %). Three hydrothermal stages have been recognized: (1) The pre-ore stage skarn minerals in the proximal locations are dominated by andradite (And84Gro15Pyr1) and diopside (Di76-81Hd17-23Jo1-2), while that in the distal locations are mainly wollastonite, grossular (And15Gro83Pyr2), and hedenbergite (Di31-35Hd64-68Jo1-2). The coexistence of halite-bearing multiphase and vapor-rich fluid inclusions have similar total homogenization temperatures (Th) indicating that fluid boiling happened during this stage. (2) The syn-ore stage is typified by abundant sulfides and quartz. The coexistence of fluid inclusions representing trapped vapor (vapor-rich fluid inclusions) and liquid phases (liquid-rich fluid inclusions) showing similar Th implies the occurrence of a later boiling. (3) The non-boiling fluid inclusions in post-ore stage calcite are dominated by the liquid-rich fluid inclusions that have variable Th and relatively consistent salinities. The hydrothermal fluids are mainly magmatic water, the positive δ34S values of sulfides could reflect magmatic sulfur influenced by the interaction with wall rocks. The CH4 generated by the thermal pyrolysis of organic matter in carbonaceous limestones was dissolved in oxidized magmatic fluids that contributed to the change of skarn mineral assemblage and fluid characteristics on the temporal and spatial scales at the Saibo-Lamasu skarn Cu deposit.