Perspectives on genetic models for lode gold deposits

Abstract

Structurally hosted lode gold vein systems in metamorphic terranes constitute a single, coherent group of epigenetic precious metal deposits. Their origin, however, has been contentious for over a century, and there remain polarized views on both the timing and mode of their formation. The status of thinking on lode gold deposits contrasts with that on several other deposit classes. For volcanic-hosted massive base-metal sulphide (VMS) deposits, copper-, and molybdenum -porphyry deposits, and epithermal precious metal deposits, amongst other classes, the results of research over the last two decades has led to a broad concensus on genetic origins. Results from diverse lines of investigation have been integrated into conceptual models that have proven effective in formulating exploration strategies for these classes of deposit. In this Preface I trace the historical development of ideas on the origin of lode gold deposits as a context for the papers in this Thematic Issue, and as a framework for evaluating the current crustal continuum model, as described in several papers in this Thematic Issue.Examples of this class of epigenetic precious metal vein deposit have variously been called Archaean gold, mesothermal gold, gold only or lode gold. The term Archaean gold deposit arose because of the relative abundance of lode gold deposits in late Archaean greenstone belts. Moreover, within such belts the mineralization may show a spatial association with komatiites or Algoman banded iron formations (BIF) -lithological types for the most part restricted to the Archaean Era. However, it is now recognized that lode gold deposits in metamorphic belts of Proterozoic to Cenozoic age share numerous characteristics with Archaean counterparts, and all likely formed by a single, common process (Kerrich and Wyman 1990). Gold only (Hodgson and MacGeehan 1982) is a misnomer, inasmuch as the deposits invariably contain significant silver, with Au/Ag ratios averaging 5, and sporadically have enrichments of W and Te. The term mesothermal is appropriate, given that the vast majority of deposits occur in greenschist facies metamorphic terranes:here, mesothermal has the connotation of mid-crustal environment, rather than referring to a process. Recently it has been recognized that lode gold deposits in subgreenschist, amphibolite and granulite facies rocks, share numerous characteristics, such as structural hosting, metal inventory, and element association of Au, Ag, W, As, Sb, Se, Te, Bi, and ore fluid source, in common with greenschist hosted mesothermal counterparts. Accordingly the term lode gold is preferable for this single class of structurally hosted Au, Ag vein deposits, which now may be viewed as forming over a crustal depth range, or crustal continuum, extending from granulite to subgreenschist facies environments hence the term crustal continuum model for these lode gold deposits Groves 1993 this issue).This class of structurally hosted lode gold deposits differ from "Carlin" type deposits, or epithermal and bonaza gold depo...

Understanding Mineral Deposits

“…The common occurrence of gold mineralization postdating peak metamorphism probably reflects a lag in thermal relaxation during collisional processes, with peak metamorphism at depths of 60 km postdating peak metamorphism at shallow ('" 1 0 km) depths by as much as 40 million years (Kerrich 1993). 6).…”

Effects of frictional–viscous oscillations and fluid flow events on the structural evolution and Re–Os pyrite–chalcopyrite systematics of Cu-rich carbonate veins in northern Norway

“…Mesothermal vein deposits are epigenetic, structurally controlled vein systems that are enriched in base and/or noble metals and are hosted in metamorphic terranes (Kerrich, 1993). Most of them are dominated by quartz and carbonate and are emplaced under greenschist facies metamorphic conditions during deformation along brittle-ductile shear zones and faults (Groves, 1993;Sibson et al, 1988).…”

Archean turbidite hosted orogenic gold mineralization in the Gadag greenstone belt, Western Dharwar Craton, Peninsular India

“…Kerrich, 1993;Ugarkar et al, 2000;Zhou et al, 2002;Mishra and Pal, 2008;Yakubchuk et al, 2002;Tomilenko et al, 2010;Sarma et al, 2011, and references there in). Thus, the distribution of gold in metamorphic terrains over geologic time has become fairly well established, as summarized by Kerrich et al (2000), Goldfarb et al (2005) and Groves et al (2003).…”