A DEVONIAN AURIFEROUS HOT-SPRING, SYSTEM, RHYNIE, SCOTLAND

The Early Devonian Rhynie hot spring system is the oldest known and is of the low surphidation type. It extends for at least 1.5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments, plant-bearing sinters and andesitic lavas. The age of sedimentation and hydrothermal activity has been determined by palynological (Pragian) and radiometric (396 +/- 12 Ma) techniques. The outlier is a half graben with a complex stepped western margin. The Devonian rocks show intense hydrothermal alteration along the fault zone. The main alteration minerals are quartz, K-feldspar, calcite, hematite and illitic and chloritic clays. Multiple chert veining and brecciation are widely developed, and geyserite and vent material are also present. Pyrite occurs in veins and all alteration facies. Sinters and altered rocks contain high concentrations of Au, As, Sb, Hg, W and Mo. Gold occurs in arsenian pyrite and as sub-micron particles in oxidized rocks. The fluid(s) responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water. However, incursions of high temperature (300-440 degrees C) magmatic fluids occurred with delta D -65 parts per thousand and delta(18)O around +8.5 parts per thousand. delta(34)S (pyrite) and initial Sr-87/Sr-86 ratios (vein calcite) lie mainly within the ranges +3.4 parts per thousand to +8.50 parts per thousand and 0.71138 to 0.71402 respectively. These data indicate that late Proterozoic Dalradian metasediments are a likely source for S and Sr but other sources are possible. delta(13)C values for caliche and vein calcite imply derivation of carbon from non-organic sources. The Rhynie cherts were deposited from a low salinity fluid of probable meteoric origin (delta(18)O(chert) +13.1 parts per thousand to +16.5 parts per thousand) which had interacted with the basement rocks and sediments (high Xe/Ar, Br/Cl and I/Cl ratios). Plant-bearing chert yielded an 40Ar/39Ar ratio (292.1 +/- 0.6) significantly less than that of modern air and may be the first valid determination of a sample of ancient atmosphere. The Early Devonian Rhynie hot spring system is the oldest known and is of the low surphidation type. It extends for at least 1.5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments, plant-bearing sinters and andesitic lavas. The age of sedimentation and hydrothermal activity has been determined by palynological (Pragian) and radiometric (396 +/- 12 Ma) techniques. The outlier is a half graben with a complex stepped western margin. The Devonian rocks show intense hydrothermal alteration along the fault zone. The main alteration minerals are quartz, K-feldspar, calcite, hematite and illitic and chloritic clays. Multiple chert veining and brecciation are widely developed, and geyserite and vent material are also present. Pyrite occurs in veins and all alteration facies. Sinters and altered rocks contain high concentrations of Au, As, Sb, Hg, W and Mo. Gold occurs in arsenian pyrite and as sub-micron particles in oxidized rocks. The fluid(s) responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water. However, incursions of high temperature (300-440 degrees C) magmatic fluids occurred with delta D -65 parts per thousand and delta(18)O around +8.5 parts per thousand. delta(34)S (pyrite) and initial Sr-87/Sr-86 ratios (vein calcite) lie mainly within the ranges +3.4 parts per thousand to +8.50 parts per thousand and 0.71138 to 0.71402 respectively. These data indicate that late Proterozoic Dalradian metasediments are a likely source for S and Sr but other sources are possible. delta(13)C values for caliche and vein calcite imply derivation of carbon from non-organic sources. The Rhynie cherts were deposited from a low salinity fluid of probable meteoric origin (delta(18)O(chert) +13.1 parts per thousand to +16.5 parts per thousand) which had interacted with the basement rocks and sediments (high Xe/Ar, Br/Cl and I/Cl ratios). Plant-bearing chert yielded an 40Ar/39Ar ratio (292.1 +/- 0.6) significantly less than that of modern air and may be the first valid determination of a sample of ancient atmosphere. The Early Devonian Rhynie hot spring system is the oldest known and is of the low surphidation type. It extends for at least 1.5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments, plant-bearing sinters and andesitic lavas. The age of sedimentation and hydrothermal activity has been determined by palynological (Pragian) and radiometric (396 +/- 12 Ma) techniques. The outlier is a half graben with a complex stepped western margin. The Devonian rocks show intense hydrothermal alteration along the fault zone. The main alteration minerals are quartz, K-feldspar, calcite, hematite and illitic and chloritic clays. Multiple chert veining and brecciation are widely developed, and geyserite and vent material are also present. Pyrite occurs in veins and all alteration facies. Sinters and altered rocks contain high concentrations of Au, As, Sb, Hg, W and Mo. Gold occurs in arsenian pyrite and as sub-micron particles in oxidized rocks. The fluid(s) responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water. However, incursions of high temperature (300-440 degrees C) magmatic fluids occurred with delta D -65 parts per thousand and delta(18)O around +8.5 parts per thousand. delta(34)S (pyrite) and initial Sr-87/Sr-86 ratios (vein calcite) lie mainly within the ranges +3.4 parts per thousand to +8.50 parts per thousand and 0.71138 to 0.71402 respectively. These data indicate that late Proterozoic Dalradian metasediments are a likely source for S and Sr but other sources are possible. delta(13)C values for caliche and vein calcite imply derivation of carbon from non-organic sources. The Rhynie cherts were deposited from a low salinity fluid of probable meteoric origin (delta(18)O(chert) +13.1 parts per thousand to +16.5 parts per thousand) which had interacted with the basement rocks and sediments (high Xe/Ar, Br/Cl and I/Cl ratios). Plant-bearing chert yielded an 40Ar/39Ar ratio (292.1 +/- 0.6) significantly less than that of modern air and may be the first valid determination of a sample of ancient atmosphere. The Early Devonian Rhynie hot spring system is the oldest known and is of the low surphidation type. It extends for at least 1.5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments, plant-bearing sinters and andesitic lavas. The age of sedimentation and hydrothermal activity has been determined by palynological (Pragian) and radiometric (396 +/- 12 Ma) techniques. The outlier is a half graben with a complex stepped western margin. The Devonian rocks show intense hydrothermal alteration along the fault zone. The main alteration minerals are quartz, K-feldspar, calcite, hematite and illitic and chloritic clays. Multiple chert veining and brecciation are widely developed, and geyserite and vent material are also present. Pyrite occurs in veins and all alteration facies. Sinters and altered rocks contain high concentrations of Au, As, Sb, Hg, W and Mo. Gold occurs in arsenian pyrite and as sub-micron particles in oxidized rocks. The fluid(s) responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water. However, incursions of high temperature (300-440 degrees C) magmatic fluids occurred with delta D -65 parts per thousand and delta(18)O around +8.5 parts per thousand. delta(34)S (pyrite) and initial Sr-87/Sr-86 ratios (vein calcite) lie mainly within the ranges +3.4 parts per thousand to +8.50 parts per thousand and 0.71138 to 0.71402 respectively. These data indicate that late Proterozoic Dalradian metasediments are a likely source for S and Sr but other sources are possible. delta(13)C values for caliche and vein calcite imply derivation of carbon from non-organic sources. The Rhynie cherts were deposited from a low salinity fluid of probable meteoric origin (delta(18)O(chert) +13.1 parts per thousand to +16.5 parts per thousand) which had interacted with the basement rocks and sediments (high Xe/Ar, Br/Cl and I/Cl ratios). Plant-bearing chert yielded an 40Ar/39Ar ratio (292.1 +/- 0.6) significantly less than that of modern air and may be the first valid determination of a sample of ancient atmosphere.