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

Abstract

Hydrothermal alteration at Rhynie, Aberdeenshire, Scotland, is concentrated along a fault zone, which juxtaposes surface deposits and the mineralised feeder zone to the Rhynie 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-feldsparquartz-illite facies (formed at 250–350°C), a medium-temperature mixed layered illite/smectitequartz-K-feldspar-chlorite-calcite facies (formed at 150–200°C) and a low-temperature mixed layered illite/smectite-chlorite-calcite facies (formed at 100 to +150°C). The fluids responsible for mineralisation were mainly moderate- to high-temperature (Th =91–360°C), 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 (Th 57 to 161°C), low- to high-salinity (<0·2 to 18 wt.% NaCl eq.) H2O-NaCl fluids, which resemble basinal brines.

40Ar/39Ar dating of hydrothermal activity, biota and gold mineralization in the Rhynie hot-spring system, Aberdeenshire, Scotland

“…2B) suggesting a close temporal relationship with fluids responsible for chert deposition at the surface. The suite of alteration minerals in the RFZ is typical of that found in the feeders of hot-spring systems elsewhere (Baron et al, 2004), which supports a connection between these hydrothermal minerals and the hydrothermal activity responsible for the Rhynie cherts. Therefore, dating the hydrothermal K-feldspar from the veins and altered andesite allows us to date the hydrothermal activity responsible for the Rhynie cherts and the age of the fossilised flora and fauna.…”

“…Two alteration facies (Fig. 1D) have been recognised which are K-feldspar -quartz -illite and illite/smectitequartz -K-feldspar -chlorite -calcite (Baron et al, 2004). The former facies hosted by andesite contains some of the highest levels of gold (approximately 100 ppb), which is mainly located in arsenian pyrite.…”

A high-precision U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications

“…1) was available to channel hydrothermal fluids, while the un-or only partially lithified fluvio-lacustrine sediments of the Rhynie basin would have provided an ample reservoir of meteoric waters (Rice et al 1995(Rice et al , 2002. With regard to the heat source, stable isotope data, corroborated by fluid inclusion studies, indicate a primary magmatic contribution to the Rhynie hydrothermal fluids (Rice et al 1995;Baron et al 2004;Mark et al 2011). A shallow-crustal magma body was presumably present beneath Rhynie during the hydrothermal episode, acting as the thermal drive for the hot-spring system.…”

“…As it crystallized, the andesitic magma body released fluids, which migrated upwards and entered the hydrothermal system (Rice et al 1995;Baron et al 2004;Mark et al 2011). Fig.…”

A new Middle–Late Jurassic flora and hot spring chert deposit from the Deseado Massif, Santa Cruz province, Argentina

“…(2) Both deposits are closely associated with areas of cross-cutting chalcedony ± calcite veins, containing disseminated iron oxides after pyrite and elevated concentrations of As, Hg and W (Baron et al 2004 (Panteleyev, 1996b) and hot spring Au-Ag deposits (Panteleyev, 1996a). (4) Chert bed morphology matches the commonly lenticular to podiform and composite-lens structures of Rhynie/Windyfield and indicates deposition in hollows in the surface of active alluvial fan deposits.…”