Crustal Hydrothermal Fluids and Mesothermal Mineral Deposits

Abstract

In this chapter we examine a broad category of hydrothermal mineral deposits whose principal characteristic is their strong structural control, and whose genesis is attributed to the action of deep-circulating crustal fluids. The mineralisation is hosted in a variety of lithologies metamorphosed to lower-upper greenschist facies, and less commonly to lower amphibolite facies. In all cases, however, the mineralising events post-date peak metamorphism. The deposits are found within high strain zones in brittle (lower greenschist facies) to brittle-ductile structures (mid-upper greenschist facies). Nesbitt (1988) and Nesbitt and Muehlenbachs (1989) refer to these deposits, in which Au is the chief economic metal, as “mesothermal lode gold”. In the literature these are also referred to as metamorphogenic or metamorphic vein deposits (Groves and Phillips 1987; Groves et al. 1987; Nesbitt and Muehlenbachs 1989), or solution-remobilisation ores (Boyle 1979, Guilbert and Park 1986). Here we adopt Nesbitt and Muehlenbachs’ terminology and use for all of these deposits the general term mesothermal ore deposits. Within this broad category are included the mineralised quartz vein systems in Archean granite-greenstone terranes (Archean lode Au deposits), the quartz vein lode Ag deposits of Idaho (USA), the lode Au deposits of Pilgrim’s Rest in South Africa and turbidite-hosted Au deposits. The latter are generally of Phanerozoic age, and examples are found in the Canadian Cordillera, Nova Scotia and Alaska in North America, Victoria in Australia, and Otago-Marlborough in New Zealand.