Metallogenesis in the Harlech Dome, North Wales; a fluid inclusion interpretation

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Authors:Shepherd, T. J.; Allen, P. M.
Author Affiliations:Primary:
Br. Geol. Surv., Isot. Geol. Unit,, London, United Kingdom
Volume Title:Mineralium Deposita
Source:Mineralium Deposita, 20(3), p.159-168. Publisher: Springer-Verlag, Berlin, Federal Republic of Germany. ISSN: 0026-4598
Publication Date:1985
Note:In English. 33 refs.; illus. incl. 4 tables, sketch maps
Summary:A regional fluid inclusion study of Cu-Au (+ Zn-Pb) mineralization in the Harlech Dome area, North Wales, gives support to the concept of two distinct metallogenic episodes. The inclusion assemblages associated with the porphyry copper mineralization at Coed-y-Brenin are consistent with a genetic model of early potassic-propylitic alteration overprinted by later phyllic alteration. High-salinity fluids, normally characteristic of potassic alteration, are confined to the host-rock quartz. The meteoric/hydrothermal system is closely linked to the emplacement of late Cambrian diorites. Integrated fluid inclusion and mineralogical studies of the Gold-belt veins suggest that the mineralizing fluids were probably dehydration waters released from weakly metamorphosed Cambrian and perhaps Precambrian sediments during hydraulic fracturing in a tensional zone at the close of the Caledonian orogeny. Localization of economic concentrations of gold in veins at the level of the Clogau formation is ascribed to a destabilization of metal complexes caused by a change in fluid buffering from a pyrite-magnetite assemblage in the Lower Cambrian sediments to a pyrite-pyrrhotite-graphite assemblage in the Upper Cambrian sediments. Veining associated with the Coed-y-Brenin porphyry copper deposit and related breccia pipes can be distinguished from the copper-gold veins of the coextensive Dolgellau Gold-belt by the presence in the former of inclusions notably richer in CO2. Furthermore the Gold-belt fluids have a distinctive low CO2/(CH4 + N2 + H2) ratio. [L.G.L.]
Subjects:Breccia; Carbon dioxide; Clastic rocks; Copper ores; Economic geology; Fluid inclusions; Gold ores; Inclusions; Lead-zinc deposits; Metal ores; Metallogeny; Mineralization; Paleosalinity; Porphyry copper; Sedimentary rocks; Veins; Europe; Great Britain; Harlech Dome; United Kingdom; Wales; Western Europe; Clogau Formation; Coed-y-Brenin; North Wales
Abstract Numbers:86M/0348
Record ID:1987063343
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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