| dc.description.abstract | Porphyry-type deposits provide most of the world’s Cu and Mo, and a large proportion of Au and other metals, vital in the transition towards a green economy. They form from hydrothermal fluids exsolving in the upper parts of large, long-lived, trans-crustal magmatic systems. Whilst they are the most intensively studied of all ore-deposit types, our understanding of how they develop is fragmented as their 3D architecture is rarely well exposed. As such, it remains unclear why and which magmatic systems become capable of forming ore deposits, how mineralising fluids are transported from their source magmas in sufficient quantities, and over what time scales such magmatic-hydrothermal systems evolve. Understanding these aspects is vital for the development of exploration models to identify magmatic centres which may host porphyry-type deposits and where within them most mineralisation may lie.
To address this, the archetypal Yerington batholith, Nevada, USA, was studied as it is exceptionally well exposed in 3D and has provided constraints for most modern porphyry exploration models. From detailed field investigations and micro-textural, geochemical and high precision geochronological studies, the magmatic system underwent a major and abrupt change in chemical composition over a period of <200 kyrs that was coincident with the initiation of ore formation. This switch, which is documented across the plutonic to volcanic record, is attributed to a change in the source of magmas from the mid- to lower crust, to form the older McLeod quartz monzodiorite and Bear quartz monzonite plutons, to the lower crust (~30 km depth) to produce the Luhr Hill granite. The latter, which was emplaced within the older plutons at a depth of ~3-8 km, contains pegmatitic pods and massive silica bodies indicating high degrees of undercooling and volatile-undersaturation. Deeper and more evolved portions of the Luhr Hill granite magma were episodically intruded upwards as aplite dykes. These underwent progressive depressurisation and associated volatile exsolution to form a crystal mush of feldspar and quartz which contained a network of inter-crystal pathways for the upward migration of mineralising fluids from large volumes of the evolving, water-rich, deep regions of the underlying Luhr Hill granite pluton. As a result of these processes, the dykes contain unidirectional solidification textures (USTs), quartz segregations, mineralised miarolitic cavities, and finally A-type veins, with a continuum to B-type veins driven by cyclic hydrofracturing, just below and within the region of porphyry-style mineralisation.
These findings: i) Suggest that the increase in a magma’s ore-forming potential is not solely driven by a progressive multi-million year ramp-up in ore-forming constituents, but also through a rapid (<200 kyrs) switch in magma source and the nature of the plumbing system; ii) Narrow the temporal-geochemical footprint of magmas associated with porphyry mineralisation; iii) Provide evidence for the transport and focussing of magmatic-hydrothermal fluids through crystal mush dykes, from extensive regions of underlying magmas; and iv) Provide a spatial-temporal framework for exploration geologists to assess the 4D architecture of porphyry magmatic systems. These findings should be incorporated into future models of the nature and formation of porphyry- and similar-type deposits and will help provide an early filter in exploration before more invasive and expensive techniques are employed. | en_GB |
