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

Shallow, texturally complex granites can be associated, spatially INTRODUCTION and temporally, with hydrothermal alteration and mineralization. Granite-related ore deposits include porphyry and skarn The formation of a granite-related ore deposit represents the confluence type ores, and other hydrothermal deposits that are of many chemical and physical geological factors, including: magma spatially and temporally associated with granitic (sensu composition (e.g. concentrations of ore metals, chlorine, and water); lato) rocks. In this paper, I emphasize characteristics magma oxidation state; the relative timing of crystallization, magma found in porphyry style (Cu‐Mo‐Au) deposits, and some ascent, magmatic volatile phase exsolution; and the depth of em- related systems; however, my treatment here is not meant placement. Early magmatic crystallization of ore-metal-rich minerals to cover the full range of granitic rocks and mineralization coupled with late exsolution of the magmatic volatile phase is that make up the granite-related ore systems. The forpromoted by deeper emplacement levels and lower magmatic water mation of granite-related ore deposits by orthomagmatic‐ concentrations, and leads to low eYciencies of removal of compatible hydrothermal processes is dependent upon many physical elements from melts into the magmatic volatile phase(s). Water and chemical magmatic factors, including (but not limited concentrations in the melt and pressures of crystallization can be to) the composition and evolution of the associated estimated from phase equilibria and textural relationships of horn- magmas, the rate of magma ascent, the depth and blende in some felsic systems. Physical factors aVect the dynamics geometry of magma emplacement, and the local temof fluid (magma, melt and volatile phase) movement, which in turn perature and stress fields at the site of magma emaVect the size of zones of alteration and mineralization; granite placement. Detailed studies of the igneous rocks textures can record the partial history of some dynamic processes. associated with ore deposits can therefore yield critical Miarolitic cavities are good evidence for magmatic volatile phase information regarding the source of metals and ligands exsolution; both miarolitic cavities and pegmatitic texture exhibit in the ore-generative hydrothermal solutions that are evidence for external nucleation of crystals. Nucleation-controlled operative in the formation of granite-related ores. In this phenomena, expressed as variations in granite textures, can result review, I will present a series of narratives, consistent, in from a multiplicity of causes including undercooling of the melt; my view, with prevailing field and experimental data on textural elements such as pegmatite, aplite, skeletal crystals, and the orthomagmatic‐hydrothermal theory for the genesis graphic textures can form as a result of undercooling. Interconnected of some granite-related deposits. volumes of the magmatic volatile phase allow for advection of Some high-level granites (i.e. those that are emplaced hydrothermal ore fluids through the magma. In shallow granitic at depths of ~8 km or less) represent the crystallized systems, miarolitic cavities can be interconnected, consistent with remains of magmas that were associated with subvolcanic, this hypothesis. ore-generative, hydrothermal systems. Therefore, studies of shallow granites, aimed at understanding magmatic evolution and volatile phase exsolution at shallow levels, are necessary if we are to understand ore genesis and the deep inputs to present-day geothermal systems as well as evaluate hazards associated with the volcanic