Orogenic gold deposits: A proposed classification in the context of their crustal distribution and relationship to other gold deposit types

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Abstract

The so-called `mesothermal' gold deposits are associated with regionally metamorphosed terranes of all ages. Ores were formed during compressional to transpressional deformation processes at convergent plate margins in accretionary and collisional orogens. In both types of orogen, hydrated marine sedimentary and volcanic rocks have been added to continental margins during tens to some 100 million years of collision. Subduction-related thermal events, episodically raising geothermal gradients within the hydrated accretionary sequences, initiate and drive long-distance hydrothermal fluid migration. The resulting gold-bearing quartz veins are emplaced over a unique depth range for hydrothermal ore deposits, with gold deposition from 15–20 km to the near surface environment.

On the basis of this broad depth range of formation, the term `mesothermal' is not applicable to this deposit type as a whole. Instead, the unique temporal and spatial association of this deposit type with orogeny means that the vein systems are best termed orogenic gold deposits. Most ores are post-orogenic with respect to tectonism of their immediate host rocks, but are simultaneously syn-orogenic with respect to ongoing deep-crustal, subduction-related thermal processes and the prefix orogenic satisfies both these conditions. On the basis of their depth of formation, the orogenic deposits are best subdivided into epizonal (<6 km), mesozonal (6–12 km) and hypozonal (>12 km) classes.

Introduction

This thematic issue of Ore Geology Reviews includes a wide variety of papers on a single type of quartz–carbonate lode-gold deposit. The deposit type in this issue alone is referred to as synorogenic, turbidite-hosted, mesothermal and Archaean lode-gold. This reflects the proliferation of such terms throughout the economic geology literature during the last ten years and a subsequent increase in confusion for the readers. For example, is a synorogenic Mother-lode type gold deposit different from an Archaean gold-only type or from a mesothermal greenstone–gold type? Many researchers working on such deposits would recognize these as essentially a variety of subtypes of a single deposit type, i.e. epigenetic, structurally-hosted lode-gold vein systems in metamorphic terranes (Kerrich, 1993). However, the consistent usage of a single and widely-accepted classification term for this deposit type as a whole is clearly warranted. `Mesothermal' is such a term that has been widely adopted during the last ten years, but is a term that, as originally defined by Lindgren (1933)for deposits formed at about 1.2–3.6 km, is more applicable to sedimentary rock-hosted `Carlin-type' deposits and the gold porphyry/skarn environment (Poulsen, 1996).

A principal aim of this introductory paper is to present and justify a unifying classification for these lode-gold deposits. An attempt is made to place these so-called `mesothermal' deposits into a broader class that emphasizes their tectonic setting and time of formation relative to other gold deposit types. A second aim is to review briefly their more significant defining features in the light of current inconsistent terminology and the recognition that this deposit group may form over a wider range of crustal depths and temperatures than commonly recognized (Groves, 1993; Hagemann and Ridley, 1993; Gebre-Mariam et al., 1995). The term orogenic is introduced and justified as a term to replace `mesothermal' and other descriptors for this deposit type. It is also suggested that the terms epizonal, mesozonal and hypozonal be used to reflect crustal depth of gold deposition within the orogenic group of deposits.

Section snippets

Definition of so-called mesothermal gold deposits

The so-called `mesothermal' gold deposits (Table 1) are a distinctive type of gold deposit which is typified by many consistent features in space and time. These have been summarized in a variety of comprehensive ore-deposit model descriptions that include Bohlke (1982), Colvine et al. (1984), Berger (1986), Groves and Foster (1991), Nesbitt (1991), Hodgson (1993)and Robert (1996). Kerrich (1993)summarizes many of the steps that led to these evolving modern-day models. A unifying tectonic theme

Problem of nomeclature

Prior to 1980, the so called `mesothermal' group of Archaean through Tertiary deposits was not widely recognized as a single special type of gold ore. Most classifications scattered the deposits among the mesothermal and hypothermal regimes of Lindgren (1933). Others, such as Bateman (1950), divided these deposits into groups within a very broad `cavity filling' type of epigenetic ore deposit. Hence, many Archaean lodes were classified as fissure filling type deposits, Otago was a shear zone

Proposed classification

These gold deposits, throughout the world's collisional orogenic belts, can actually be viewed as both syn- and post-orogenic in origin. Whereas host rocks for ore may already be undergoing uplift and cooling (thus `post-orogenic'), the ore-forming fluids may be generated or set in motion by simultaneous thermal processes at depth (thus `syn-orogenic') as described by Stuwe et al. (1993). For example, Kent et al. (1996)show that the main episode of gold mineralization in the Yilgarn craton

Acknowledgements

The authors acknowledge the input of past and present staff and students at the Key Centre at UWA, particularly Mark Barley, Kevin Cassidy and John Ridley. The research was funded largely by mining companies and supported by Key Centre Corporate Members, DEETYA, AMIRA, MERIWA and UWA. The paper was inspired as a result of a course given by F. Robert in Perth in February, 1996, and conferences on mesothermal gold deposits in Ballarat and Perth in July, 1996. The encouragement of Ross Ramsay is

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