Scheelite geochemistry in porphyry-skarn W-Mo systems: a case study from the Gaojiabang Deposit, East China

The study provides an opportunity to examine the geochemistry of scheelite from porphyry-skarn systems. For the first time, three distinct groups of scheelite are recognized in the same W-Mo porphyry-skarn deposit. Each shows different geochemical features in terms of (87Sr/86Sr)i, Mo, La, LREEs, MREEs, and HREEs. After emplacement of porphyry rocks, the initial W-Mo rich fluids derived from unconsolidated magma mixed with formation waters. During this process (M1), the initial fluids inherited the REE patterns with slight enrichment from magma and P-group scheelite was precipitated. This type of scheelite has a higher (87Sr/86Sr)i of 0.70893–0.71082, strong enrichment in Mo, and depletion in Ba, Ta, and U compared to the porphyry rocks. Fluids then migrated from the porphyry, infiltrating biotite or quartz hornfels and precipitating H-group scheelite. During this process (M2), fluids interacted with hornfels and mixed with formation waters, which have higher Sr-isotope compositions. At the contact zone between porphyry rocks and Cambrian limestone, skarn ores were formed. During this process (M3), mixing took place between magma-derived fluids and formation waters and interacted with limestones, allowing precipitation of S-group scheelite. This type of scheelite precipitated at an oxidized stage after precipitation of early skarn minerals which scavenged REE (especially HREE) from fluid.