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Journal of the Geological Society

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An hydrothermal origin for copper-impregnated prehnite and analcime from Boylestone Quarry, Barrhead, Scotland

¹ Department of Applied Geology, University of Strathclyde, Glasgow G1 lXJ, UK
³ Scottish Universities Research and Reactor Centre, East Kilbride G75 0QU, UK

Field, petrographic, mineralogical, fluid inclusion and stable isotope studies of prehnite, analcime and calcite in basalt lava of Visean age (about 360 Ma) from Boylestone Quarry, Barrhead, Scotland are all consistent with an hydrothermal origin. In the hydrothermal system, water interacted with hot alkaline basalt soon after eruption. Feldspars were digested in an alkaline solution and alumina and silica were reprecipitated in fractures and veins as the hydrous alumino-silicates analcime and prehnite. The prehnite generally occurs below the analcime in more oxidized host basalt. Minor calcite crystallized late, usually on earlier analcime or prehnite. Fluid inclusion measurements give homogenization temperature (Th) of 83–184 °C (mean 145 °C) and salinities c. 1.5 equiv. wt% NaCl for analcime; and c. 46-234°C and c. 26 equiv. wt% NaCl for calcite. Fluid inclusions in analcime appear to contain some SO^2–₄ or CO^2–₃. Inclusions in calcite associated with prehnite have a lower Ca²⁺ :Na⁺ ratio than those in calcite associated with analcime. Using a fluid inclusion temperature of 145 °C for analcime of ¹⁸O = 15.6% (V-SMOW) implies ^l8O for the fluid was 1.4% and this leads to 184 °C for the temperature of precipitation of prehnite of ¹⁸O = 12.5%. The D values of –35% for prehnite are consistent with surface water (seawater or low latitude meteoric water) heated to 180 °C. Two groups of calcite are recognised on fluid inclusion and isotopic evidence: both groups have D values ranging from —61 to — 117% but one group is characterized by Th> 100 °C and ¹⁸O = 20 to 22% (¹⁸O_water = 10.3%) and is believed to have formed from surface waters which had exchanged ¹⁸O with igneous rocks at a high temperature whereas the second group with Th = 85 °C and ^l80i = 17.2% crystallized from normal surface waters of ¹⁸O_water = –0.4%. The carbonate ¹³C data (range –8.3 to –42.4% PDB) attest to the presence of organic-derived carbon dioxide in the hydrothermal fluid, as expected since organic material is observed in some fluid inclusions. The low ^I3C value, —42.4%, indicates oxidation of methane whereas the higher values point to decarboxyla-tion of organic matter

Copper was oxidized and remobilized from disseminated chalcopyrite in basalt and probably from disseminated copper minerals which had resulted from Carboniferous weathering of underlying basalts. Dissolved copper was reprecipitated on cooling and reduction, generally within and above greenish prehnite.

The fluid inclusion and isotope studies indicate that the water could well have been mainly meteoric with in addition, some warm Lower Carboniferous seawater and/or saline groundwaters; the water interacted with organic-rich sediments to provide CH₄ and CO₂ then to various degrees with hot igneous rock. The hydrocarbons may have been mobilized into groundwaters by baking of organic sediments in the Cementstone Group. The model is consistent with active volcanogenic geothermal activity during Lower Carboniferous times in the Midland Valley of Scotland.

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