Genesis of the Dingjiashan and Fengyan Zn-Pb polymetallic deposits in central Fujian, SE China: Evidence from magnetite geochemistry

Abstract

Central Fujian is an important polymetallic (Zn-Pb-Ag-Cu-Mo-Au) mineral province in southeastern (SE) China, yet its mineralization is still disputed between Proterozoic volcanogenic massive sulfide (VMS) or Mesozoic skarn-type. The Dingjiashan and Fengyan are two representative of them, with the ores mainly hosted in the Meso-Neoproterozoic Longbeixi Formation marble and schists. For the magnetite from Dingjiashan and Fengyan, their hydrothermal mineral assemblage and their low V, Al, Cr, Ni, Ga, and HFSEs contents (cf. igneous magnetite) are all typical of hydrothermal origin. LA-ICP-MS time-resolved depth profile and inter-element correlation suggest that: 1) Mn, Ti, V, Ni, and Cr are mainly present in solid solution; 2) Na, Mg, Al, K, Si, and Ca occur both as solid solution and the micro-silicate inclusions and/or halite-bearing fluid inclusions; and 3) Zn, Co, Cu, Ag, Pb, and Bi are intimately associated with sulfide micro-inclusions. For both Dingjiashan and Fengyan magnetite, the Ti, V, Al, Ga, and Mn contents are temperature-controlled, whilst Ti, V, Cr, and Sn contents are redox-controlled. Meanwhile, the Ca and Mg contents are controlled by fluid-rock interactions, and the Mn enrichment is inherited from the fluid composition. Coexisting mineral phases have likely exerted major geochemical control on the magnetite samples. We compiled a large published geochemical dataset on magnetite from well-studied VMS and skarn deposits worldwide. Compared with VMS-hosted magnetite, skarn-hosted magnetite has relatively high median contents for Ca, Al, Ge, Sn, Mn, and Mg, and low median contents for Si, Ga, Ti, V, Ni, and Cr. Overall, the Dingjiashan and Fengyan magnetite samples have similar trace element compositions (esp. Mn and V) to those of skarn-hosted (rather than VMS-hosted) magnetite. Magnetite geochemistry thus suggests that both the Dingjiashan and Fengyan are best classified as stratabound, distal Zn-Pb skarn deposits. Our findings highlight the effectiveness of magnetite trace element geochemistry in differentiating ore deposit types, which contributes to regional Zn-Pb ore exploration and research.