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

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.