Felsic trans-porphyry deposits and associated ore (Sn, Ta, Nb, pegmatites) viewed from physics, chemistry (cDFT) and geologic contexts

Abstract

Some base metals (Cu, Mo, W, Sn) are extracted from silicate melts at the magmatic stage in porphyry deposits. Metal grade increases from some ppb or ppm up to percent levels. The common case is the porphyry type (Cu, Mo) above subduction zones. We introduce the trans-porphyry type deposits (Sn) within continental plates, resulting from large-scale shearing between cratons. The global process for ore generation has a three-fold effect, from magma generation, emplacement, and formation of a immiscible phase (MIP). Metals first segregate from the crust during melting slightly above 800 °C. During magma ascent, the MIP remains above the critical point (731 °C), and chemically attracts metals, transporting them faster than the melt. Metals segregation from the melt occurs in a subcritical state. It relies on the chemical potential contrast (chemical partitioning), enhanced by their differential motion (viscosity, density). Chemical diffusion negatively competes with advection, yielding enrichment. With cooling, the melt splits into two phases, silicate-rich and aqueous-dominated, both are immiscible with the melt and the matrix. Metals diffusivity, melt and matrix viscosity, cap rock permeability, and the amount of fluids are investigated in a parametric formulation. It rules out a gentle chemical differentiation though diffusion. Metals segregation (Sn, Nb, Ta) into pegmatite and greisen are the ultimate products of ore formation. Models based on the bulk composition of rocks (granite, pegmatites, greisens) reveal inadequate since they do not incorporate fluids in enough quantity. The physical parameters are also bracketed by the chemical descriptors (chemical potential, hardness) that rule melt and matrix evolution. The combined investigation of the physical context and chemical attractivity provides new insights into such ore formation in a magmatic context. 275 w.