Fluids and hydrothermal alteration assemblages in a Devonian gold-bearing hot-spring system, Rhynie, Scotland

Hydrothermal alteration at Rhyme, Aberdeenshire, Scotland, is concentrated along a fault zone, which juxtaposes surface deposits and the mineralised feeder zone to the Rhyme hotspring system. Mineralisation consists of breccias and veins filled with quartz, chert, calcite, K-feldspar and pyrite. Associated pervasive alteration comprises a high-temperature K-feldspar-quartz-illite facies (formed at 250-350degreesC), a medium-temperature mixed layered illite/smectite-quartz-K-feldspar-chlorite-calcite facies (formed at 150-200degreesC) and a low-temperature mixed layered illite/smectite-chlorite-calcite facies (formed at 100 to + 150degreesC). The fluids responsible for mineralisation were mainly moderate- to high-temperature (T-h=91-360degreesC), low-salinity (<0.2 to 2.9 wt.% NaCl eq.) H2O-NaCl-heated meteoric fluids comparable to modern and ancient hot-spring systems. The migration of these fluids was mainly restricted to a major fault zone bounding the Devonian basin. Fluids responsible for mineralisation, alteration and cementation elsewhere in the basin were low-temperature (T-h 57 to 161 degreesC), low- to high-salinity (<0.2 to 18 wt.% NaCl eq.) H2O-NaCl fluids, which resemble basinal brines.