On the timing relationship between fluid production and metamorphism in metamorphic piles: some implications for the origin of post-metamorphic gold mineralisation

Details On the timing relationship between fluid production and metamorphism in metamorphic piles: some implications for the origin of post-metamorphic gold mineralisation On the timing relationship between fluid production and metamorphism in metamorphic piles: some implications for the origin of post-metamorphic gold mineralisation

Feb 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.114..417s&link_type=abstract

Earth and Planetary Science Letters, Volume 114, Issue 4, p. 417-430.

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Scientific paper

The relative timing of metamorphism at different crustal levels is quantified for a range of metamorphic environments associated with convergent deformation. We characterise different metamorphic environments by making simple assumptions about the nature of heat input and the nature of heat withdrawal from the terrane. If internal heat production is the major heating agent during crustal thickening, then deep crustal levels reach their metamorphic temperature peak later than shallow levels. In terranes heated from below, such relationships apply only if erosion commenced early in the thermal history or if erosion rates are fast. Quantitative knowledge of these relations can be used together with simple assumptions about prograde fluid release to predict the likely timing of fluid passage through rocks higher in the column. In terranes with deep-later characteristics the evidence for fluid penetration may be preserved in the form of mineralised quartz veins or late stage alteration. Within our simple parameterisation of crustal heat sources we can use the grade of a metamorphic rock together with field evidence on the timing of metamorphism, igneous intrusion and deformation in order to constrain the time scale of the underlying thermal perturbation and erosion process for a given terrane. We demonstrate that time intervals up to several tens of million years may separate metamorphism and fluid emplacement in model evolutions that are scaled to describe greenschist facies metamorphic terranes that formed at low pressure. Such timing relationships are in fact recorded in many greenschist facies metamorphic terranes that host late stage mesothermal gold deposits. We suggest therefore that there is no conflict between the observation that the emplacement of many gold deposits occurred demonstrably after peak metamorphism of their host rocks and the interpretation that metamorphic fluids are responsible for the mineralisation.

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