Addressing ore formation and exploration

Abstract

Common base and noble metals represent an important economic factor in the actual industrial development. For instance the world resources for copper are actually estimated for about the next 30 years only. The situation requires rethinking the way major ore deposits form, leading to new guides for exploration. The present paper briefly examines the processes leading to ore formation, in relation with granitic or granodioritic intrusions. It identifies the importance of metal enrichment during the magmatic stage. Within the magma chamber that forms porphyry intrusions, metals may incorporate to first formed crystals, becoming inert; concentrate into the residual melt of a mush; or segregate by diffusion into the exsolved magmatic volatile phase (MVP) into which they are transported and precipitated. A competition results between elements partitioning and diffusivity. Hence, a specific Péclet number for each metal (Cu, Au, Ag, Mo, W, Sn, and REE) controls the ratio between the diffusive and the advective flux. Metals diffusivity in the melt shows differential behavior relative to a threshold of about 10−13 m2/s. Metals with slower diffusivity values (e.g. As) will not concentrate. Conversely, fast diffusive metals (Au, Ag, Cu) may rapidly incorporate the MVP, provided an adequate component (halogens or S) is attractive for metals. The chemistry of the MVP escaping the magma induces different alteration patterns. Their relative content in F, Cl or S, attested by the composition of biotites and apatites, links with the preferential content of metals in ore deposits, representing a valuable tool for exploration. Finally the model is replaced in a set of coupled mechanical-chemical instabilities, within a three phase material.