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Abstract: Whilst petrology, geochemistry and metal content of small mafic/ultramafic Ni–Cu bearing complexes have been extensively studied, their structural controls and emplacement mechanisms are still poorly documented. This study addresses the last two points with the Huangshan Ni–Cu ore belt (Chinese Eastern Tianshan) as a case study. The Huangshan intrusions are Early Permian; a period when the Tianshan orogenic belt recorded major right-lateral wrench tectonics, characterized by crustal-scale shear zones. Detailed mapping, petro-structural analysis and strain rate calculation within and around the intrusions allow us to establish that the Huangshan Ni–Cu-bearing mafic/ultramafic complexes are not layered intrusions. Instead, they emplaced by injection of several mafic/ultramafic magma batches within kilometre-scale tension gashes generated by Permian dextral shearing, and should be considered as synkinematic sheeted intrusions. Finite strain analysis across the Huangshan-Kangguer shear zone provides rather high shear strain rates (∼4.5). Considering the location and alignment of the Ni–Cu-bearing mafic/ultramafic bodies along regional shear zones throughout Eastern Tianshan, it appears that wrench tectonics most likely controlled and focussed the intrusion of parent magmas. As a consequence, rifting related to post-orogenic extension is not required to account for Permian magmatic features of the Tianshan Belt. Finally, the Huangshan Ni–Cu bearing mafic/ultramafic intrusions are neither parts of an ophiolitic suture, nor of a dunite-cored Alaskan-type ore deposit, as postulated in some previous studies. In the light of these new results, we believe that structural controls and emplacement mechanisms of many Ni–Cu sulphides deposits hosted by small intrusions (particularly funnel-shaped ones) should be (re-)evaluated from a structural and geophysical point of view. [Copyright &y& Elsevier]

[Display omitted] • Two stages of fluid exsolution are recognized in the Weijia W skarn deposit. • F-rich magmatic fluids are enriched in REEs, Y, Nb, Ta with flat REE patterns. • F consumption is critical for the compositional evolution of F-rich mineralizing fluids. • Combined garnet and scheelite trace elements are powerful in tracking W skarn mineralization. Fluid evolution is an essential subject in the studies of hydrothermal ore-forming processes. Garnet and scheelite prevail in W-bearing skarn deposits and record plentiful information on W skarn mineralization. In-situ LA-ICP-MS trace element analyses have been carried out on the garnet and scheelite from the F-rich Weijia W deposit in South China to constrain the processes of fluid evolution and W skarn mineralization. The Weijia W deposit mainly consists of calcic and magnesian skarns and was formed through six stages of alteration and mineralization. Two stages of fluid exsolution are recognized in the magmatic-hydrothermal system. The earlier exsolved fluids are Cl-rich with rightward-sloping REE patterns and result in pre-ore skarnization. The later exsolved fluids are F-rich with relatively flat REE patterns and are responsible for W mineralization. The pre-ore garnets are LREE-enriched and HREE-depleted whereas the syn-ore garnets are LREE-depleted and HREE-enriched with lower δ Eu values than the former, denoting a transition from wall rock-controlled to magmatic fluid-dominated fluid-rock interaction. Combined with the skarn occurrences, mineral assemblages, and garnet textures, it is revealed that the mechanism of skarn formation shifts from more diffusive metasomatism under lower water/rock ratios to more advective metasomatism at higher water/rock ratios. The advective metasomatism with a high flux of ore-forming fluids is hydrodynamically favorable for the consequent W mineralization. The earliest-formed scheelite is akin to the host granite porphyry in REE patterns and can reflect the features of initial mineralizing fluids. It has high ΣREE (1255–5059 ppm), Y (616–1416 ppm), Nb (585–2629 ppm), Ta (14–248 ppm), Mn (up to 1977 ppm) and low Mo (mostly < 1000 ppm) contents, indicating that the primary F-rich magmatic fluids are reduced and enriched in REEs, Y, Nb, and Ta. The contents of ΣREE, Y, Nb, and Ta in scheelite are decreasing with the evolution of the F-rich mineralizing fluids. The latest-formed scheelites have distinctly lower ΣREE (2.9–140 ppm), Y (0.01–4.1 ppm), Nb (1.7–77 ppm), and Ta (0.04–0.19 ppm) contents and higher (La/Yb) N ratios and δ Eu values. They show 1:1 positive correlation of Eu N vs. Eu N * and have high Mo (up to 129466 ppm) and low Mn (as low as 3.4 ppm) contents, implying involvement of meteoric waters in the evolved ore-forming fluids and an oxidizing condition. Fluid-rock interaction, mineral precipitation, and fluid mixing largely control the process of fluid evolution. Fluorine consumption due to the formation of fluorite and the other F-rich minerals is vital for the compositional evolution of F-rich mineralizing fluids. The decrease of F concentration in fluids will depress the solubilities of REEs, Y, Nb, and Ta and enhance the LREE/HREE fractionation. Greisenization and skarnization induced the neutralization of the acidic W-bearing fluids and the liberation of Ca from the granite porphyry and carbonate strata and then triggered the scheelite precipitation. The carbonate strata contain abundant Ca and the granite porphyry is poor in Ca, and thus the economic W mineralization dominantly occurs in the skarns. [ABSTRACT FROM AUTHOR]