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Meteoric incursion and oxygen fronts in the Dalradian metamorphic belt, southwest Scotland: a new hypothesis for regional gold mobility

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Abstract

Post-metamorphic quartz veins which occur over hundreds of square kilometres in the biotite zone of the Dalradian metamorphic belt consist of three principal types: anhedral quartz with pyrite, anhedral quartz with hematite, and prismatic quartz with hematite or rutile. The oxide minerals in anhedral veins have formed by oxidation of pre-existing sulphides, and gold was mobilized during this oxidation. Anhedral quartz veins formed from an aqueous fluid with up to 5 wt% dissolved salts and 16 wt% CO2 at about 300 °C. Texturally later prismatic quartz crystals formed from a compositionally similar fluid which was undergoing phase separation at the H2O-CO2 solvus at 160–200 °C and 500 to 1200 bars fluid pressure. Oxygen isotope ratios for quartz from the veins range from 12.0 to 15.3‰, with hematite-bearing veins generally isotopically heavier than pyrite-bearing veins. Calculated fluid oxygen isotope ratios range from + 8‰ for pyrite-bearing veins to -2‰ for late prismatic crystals. The mineralizing fluid contained a substantial component of meteoric water whose isotopic and chemical composition evolved with progressive water-rock interaction. Evolution of meteoric fluid composition involved migration of oxidation and oxygen isotope fronts in the down-flow direction as head-driven water passed through structurally controlled fractures in the schist pile. A gold solubility trough occurs for the observed fluid in the oxidation frontal zone. Gold remobilization and reprecipitation occurred progressively as the oxidation front migrated through the schist pile.

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Authors and Affiliations

  1. Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand

    D. Craw

  2. Department of Earth Sciences, Dartmouth College, 03755, Hanover, NH, USA

    C. P. Chamberlain

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  1. D. Craw
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  2. C. P. Chamberlain
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Craw, D., Chamberlain, C.P. Meteoric incursion and oxygen fronts in the Dalradian metamorphic belt, southwest Scotland: a new hypothesis for regional gold mobility. Mineral. Deposita 31, 365–373 (1996). https://doi.org/10.1007/BF00189184

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  • DOI: https://doi.org/10.1007/BF00189184

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