Synkinematic mafic/ultramafic sheeted intrusions: Emplacement mechanism and strain restoration of the Permian Huangshan Ni-Cu ore belt (Eastern Tianshan, NW China)

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 (similar to 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. (C) 2012 Elsevier Ltd. All rights reserved.