Textural indicators of mineralisation potential in porphyry magmatic systems – A framework from the archetypal Yerington district, Nevada

Abstract

Porphyry-type deposits are spatially and temporally associated with the relatively shallow and texturally complex parts of magmatic systems. Whilst certain textures offer snapshots into the physical processes which result in fluid exsolution and hydrothermal mineralisation, their documentation and interpretation remains disjointed. To address this, we describe a suite of magmatic and magmatic-hydrothermal textures from the classic Yerington Cu(-Mo-Au) porphyry district, Nevada, where Cenozoic extension and tilting has exposed a unique, ∼8 km palaeodepth, cross-section through the magmatic system. Within the granite cupolas that underlie the Ann Mason and Yerington porphyry deposits, these textures include pegmatitic pods and massive silica bodies. Emplaced through the cupolas, and genetically associated with ore formation, are aplite dykes that host mineralised unidirectional solidification textures (USTs), pegmatitic segregations, miarolitic cavities and early A-type quartz veins. Based on field relations, including associations with hypogene mineralisation, petrography and Ti-in-quartz crystallisation temperatures, we highlight how these textures may record the timing and location of the magmatic-hydrothermal transition and ore-formation. By doing so we provide a textural framework for exploration geologists to assess the likely 3D spatial and temporal architecture of porphyry mineralisation at the district-prospect scale before employing more invasive and expensive techniques.