A Review of Shallow, Ore-related Granites: Textures, Volatiles, and Ore Metals

Abstract

Shallow, texturally complex granites can be associated, spatially and temporally, with hydrothermal alternation and mineralization. The formation of a granite-related ore deposit represents the confluence of many chemical and physical geological factors, including: magma composition (e.g. concentrations of ore metals, chlorine , and water); magma oxidation state; the relative timing of crystallization, magma ascent, magmatic volatile phase exsolution; and the depth of emplacement. Early magmatic crystallization of ore–metal-rich minerals coupled with the exsolution of the magmatic volatile phase is promoted by deeper emplacement levels and lower magmatic water concentrations, and leads to low efficiencies of removal of compatible elements from melts into the magmatic volatile phase(s). Water concentrations in the melt and pressures of crystallization can be estimated from phase equilibria and textural relationships of hornblende in some felsic systems. Physical factors affect the dynamics of fluid (magma, melt and volatile phase) movement, which in turn affect the size of zones of alternation and mineralization; granite textures can record the partial history of some dynamic processes. Miarolitic cavities are good evidence for magmatic volatile phase exsolution; both miarolitic cavities and pegmatitic texture exhibit evidence for external nucleation of crystals. Nucleation-controlled phenomena, expressed as variations in granite textures, can result from a multiplicity of causes including undercooling of the melt; textural elements such as pegmatite, aplite, skeletal crystals, and graphic textures can form as a result of undercooling. Interconnected volumes of the magmatic volatile phase allow for advection of hydrothermal ore fluids through the magma. In shallow granitic systems, miarolitic cavities can be interconnected, consistent with this hypothesis.