Geochemistry and Petrography of a Lower Carboniferous, Lacustrine, Hot Spring Deposit: East Kirkton, Bathgate, Scotland

[摘要] The East Kirkton, Brigantian "limestone'' was thought to be of hotspring origin by Hibbert, (1836) due to the occurrence of numerous, small, spherulitic growths, this was supported by Cadell, (1925) following his observations of active hot-springs in Yellowstone National Park and in New Zealand. The aim of the continuing research is to constrain this geological setting by examination of textural evidence and detailed geochemical analyses. The spherules occur in most horizons of this finely laminated sequence, which includes: bituminous shales; carbonate rich and silica rich sediments; and occasional limestone bands and lenses. The regular laminae are typical of a lake deposit, a low energy, environment of restricted circulation, with freshwater organisms preserved in many horizons. This study has focussed on the silicic sediments, unusual in a fresh water lake. At East Kirkton many chert textures suggest an origin as a gel-like precipitate: synsedimentary slumping without brittle fracturing indicates a silica precipitate capable of ductile movement. Cracks have formed perpendicular to bedding, without continuing from the chert into surrounding layers and these may be the shrinkage cracks of a cooled silica gel. Both silica and carbonate form primary precipitates, though both also form several replacive phases. Cathodoluminescence (CL), UV fluorescence and SEM were used to identify the presence of both carbonate and silica spherules,and also to differentiate a variety of carbonate phases, principally calcite and dolomite. Isotopic analysis of S180 and 8D are presented showing that the origins of the siliceous fluids can be constrained for the East Kirkton environment. The analyses (delta18O = 21.3 to 26.9‰, n=18; deltaD = -52.2 to -90.7‰, n=13) fall close to the "agate line" of Fallick et al (1986) and are consistent with equilibration of the cherts at 40 to 90 °C with L. Carboniferous meteoric water of delta18O = -4‰ and deltaD = -2‰. Fluids in equilibrium with this silica "gel" may have been trapped only as the "gel" cooled and hardened, upon mixing with the lake waters, to result in fine, laminae of chert.

[发布机构] University:University of Glasgow

[关键词] Geology, Geochemistry