Arc magmatism and porphyry-type ore deposition are primarily controlled by chlorine from seawater

Abstract

A better understanding of magmatism and ore metal deposition at convergent plate margins is essential to the safety and benefit of society. Despite their spatial and, to a certain extent, temporal association, a direct connection between these two geologic processes is enigmatic. A detailed evaluation of available data reveals that seawater is key to the connection between these processes as it is the source of the ubiquitous, hot aqueous fluid from the subducting slab that is central to the generation of primary arc magmas and transfer or recycling of crustal materials at convergent margins. Particularly regarding the transfer of fluid-mobile elements or the so-called subduction component from the slab to arc lavas, however, the slab-derived hydrothermal fluid is perceived as weak and, thus, other transfer agents are proposed. Most petrogenetic studies also do not connect the source and transfer of metals in the spatially associated ore deposits to the same fluid. Chlorine from seawater can provide the crucial link between the two processes. The presence of Cl makes the slab-derived fluid a potent solvent not only to transfer but also to solubilize both the subduction component and ore trace metals from the slab, mantle wedge and lower-middle arc crust to arc lavas and ore deposits, respectively. Moreover, the dissolution and mobility of many elements in the Cl-rich fluid are primarily dependent upon their respective ionic potential (i.p.) values. In general, elements with cations that have low (soft cations) and high (hard cations) i.p. are mobile whereas those with in-between (intermediate cations) values are immobile in the Cl-rich fluid. Subducted sediment provides the bulk of subduction component and ore metals. These results led to the creation of a unified conceptual model claiming that the mobility, or immobility, of almost all elements in hydrothermal fluids at or near the Earth's surface is primarily dependent upon the amount of Cl in the fluids.