TECTONIC CONSTRAINTS ON THE TIMING RELATIONSHIPS OF METAMORPHISM, FLUID PRODUCTION AND GOLD-BEARING QUARTZ VEIN EMPLACEMENT

Abstract

There is an ongoing discussion about the genetic significance of the postmetamorphic emplacement of many gold mineralizations. As a contribution to this discussion, this paper reviews the timing relationships between metamorphic devolatilization and metamorphism at different crustal levels for different tectonic processes. For example, denudation accompanying or following thickening causes metamorphism to occur later at depth than at shallow levels. Thus, many internally-heated terrains (e.g. Barrovian terrains) are of `deep-later' type; that is, peak metamorphism (and devolatilization) at deeper levels occurs at later times. Metamorphic processes of the `deep-later' type may produce and release metamorphic fluids during prograde metamorphism at depth and precipitate them in the form of `postmetamorphic' quartz veins at shallow levels.Magmatic heating from below, on the other hand, is a `deep-earlier' process causing inverse timing relationships. In many metamorphic terrains deep-later and deep-earlier processes may compete, for example if denudation at the surface accompanies heating from below. If denudation and heating from below are of similar duration, `deep-later' characteristics prevail and a strongly bi-modal age distribution of peak metamorphism in different crustal levels may arise: the upper crust will experience metamorphism early but largely contemporaneously and the lower crust will experience metamorphism much later. This process is particularly favourable for substantial simultaneous devolatilization of large sections of the lower crust to postdate metamorphism at shallow levels. Thus, such processes may result in late infiltration `events' in cooling rocks of low metamorphic grade. The common observation of postmetamorphic fluid infiltration and mineralization may therefore indicate, but does not require, that the mineralising fluids are related to the same metamorphic event.