OROGENIC GOLD DEPOSITS - A PROPOSED CLASSIFICATION IN THE CONTEXT OF THEIR CRUSTAL DISTRIBUTION AND RELATIONSHIP TO OTHER GOLD DEPOSIT TYPES

The so-called 'mesothermal' gold deposits are associated with regional ly metamorphosed terranes of all ages. Ores were formed during compres sional to transpressional deformation processes at convergent plate ma rgins in accretionary and collisional orogens. In both types of orogen , hydrated marine sedimentary and volcanic rocks have been added to co ntinental margins during tens to some 100 million years of collision. Subduction-related thermal events, episodically raising geothermal gra dients 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 e nvironment. On the basis of this broad depth range of formation, the t erm 'mesothermal' is not applicable to this deposit type as a whole. I nstead, the unique temporal and spatial association of this deposit ty pe with orogeny means that the vein systems are best termed orogenic g old 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) c lasses. (C) 1998 Elsevier Science B.V. All rights reserved.