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Gold deposits related to reduced granitic intrusions

by John F H Thompson, Rainer J Newberry
Reviews in Economic Geology ()

Abstract

The role of intrusions in the formation of many types of gold deposits has been widely debated. Magmatic-hydrothermal processes are proposed by some authors for gold-rich porphyry systems, while many other authors claim that intrusions only serve as convective heat engines or structural hosts for mesothermal gold deposits. Based largely on well-documented examples from Alaska and the Yukon Territory, we recognize a class of deposits that display a spatial and temporal relation to reduced (low f (sub O2) granitoids. Most intrusion-hosted and intrusion-proximal deposits and prospects of this class display a consistent and striking Au-Bi-Te-As (W,Mo,Sb) metal association; evidence for a series of alteration and mineralization events spanning a significant range of temperatures (>500 degrees -1 kbar generally lack evidence for rapid magmatic water exsolution (e.g., porphyritic textures, random stockworks, magmatic breccias). Deposits horizontally and vertically distal from intrusions within a given belt or district typically lack the strong Au-Bi association of proximal deposits and contain higher As, Sb, and base metals and only yield evidence for relatively low-temperature and low-salinity fluids. These latter types possess some characteristics of orogenic (mesothermal), epithermal, or Carlin-type deposits; however, their spatial and temporal association with the higher-temperature deposits suggests a common origin. Although the clearest examples of these reduced intrusion-related deposits (e.g., Fort Knox, Alaska; Dublin Gulch, Yukon Territory) are only of moderate size (<150 tonnes Au), worldwide deposits of apparently similar character and origin host major amounts of gold. Significant global examples include Mokrsko in the Czech Republic, Kidston in Australia, and possibly the world-class Murantau deposit in Kazakstan. Belts hosting deposits of this type occur in a variety of continental arc, back-arc, and collisional settings, most of which contain intrusion-related Sn-W deposits. A key characteristic of this class of deposit is the occurrence of a wide variety of mineralization styles, depending on formation depth, distance from the parent intrusion, and structural control. It is likely that new variants of this type of deposit are yet to be found in many areas of the world, an assertion supported by the relatively recent discovery of the unusually high-grade Pogo deposit in interior Alaska.

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