Crustal Scale Controls on Gold Ore Fluid Movement and Deposition as Revealed From Electrical Resistivity Structure: Examples from Compressional and Extensional Regimes

Abstract

Structurally-controlled, meso- and epithermal gold deposition in both compressional and extensional settings is a function of local and regional stresses, rheological contrasts, and thermochemical gradients. The influence of these factors can be illustrated through their effects on electrical geophysical structure, since this structure reflects fluid composition, porosity, interconnection and pathways. In the compressional, amagmatic New Zealand South Island, magnetotelluric (MT) data imply a concave-upward (``U'-shaped), middle to lower crustal conductive zone beneath the west-central portion of the island. The deep crustal conductor suggests a volume of fluids arising from prograde metamorphism and radiogenesis within a thickening crust of paleo deep-water clastic rocks. Change of the conductor to near-vertical orientation at middle-upper crustal depths is interpreted to occur as fluids cross the brittle-ductile transition during uplift, and approach the surface through induced hydrofractures. Near the brittle-ductile pressure breakthrough are deposited modern hydrothermal veining, gold mineralization, and graphite of deep crustal provenance, subsequently exposed by erosion. In Nevada, Carlin Trend deposits appear to overlie central intrusives of late Eocene age which occupy the transition from conductive paleo-abyssal pelitic sediments (potential gold source rocks) eastward to more resistive shelf carbonate/quartzite sequences along an ancient continental margin normal fault. The intrusive is flanked by conductive, apparent accommodation fault zones which may possess higher porosity as well as possible graphite flushed from sediments near the high-T system core and redeposited in the periphery. Exposed deposits are modeled to form at fluid pressure breakthroughs across permeability barriers such as organic shales or thrust planes, producing strong and favorable gradients in temperature, pressure and fluid oxidation.