The first deep geothermal exploration borehole (995 m) to be drilled in the UK for over 20 years was completed at Eastgate (Weardale, Co. Durham) in December 2004. It penetrated 4 m of sandy till (Quaternary), 267.5 m of Lower Carboniferous strata (including the Whin Sill), and 723.5 m of the Weardale Granite (Devonian), with vein mineralization occurring to 913 m. Unlike previous geothermal investigations of UK radiothermal granites that focused on the hot dry rock concept, the Eastgate Borehole was designed to intercept deep fracture-hosted brines associated with the major, geologically ancient, hydrothermal vein systems. Abundant brine (≤46 °C) was encountered within natural fracture networks of very high permeability (transmissivity c . 2000 darcy m) within granite. Evidence for the thermal history of the Carboniferous rocks from phytoclast reflectance measurements shows very high values (≥3.3%) indicating maximum temperatures of 130 °C prior to intrusion of the Whin Sill. Geochemical analysis of cuttings samples from the Eastgate Borehole suggests radiothermal heat production rates for unaltered Weardale Granite averaging 4.1 μW m −3 , with a mean geothermal gradient of 38 °C km −1 . The Eastgate Borehole has significant exploitation potential for direct heat uses; it demonstrates the potential for seeking hydrothermal vein systems within radiothermal granites as targets for geothermal resources.

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The Boteti palaeo‐estuary in northern Botswana is located where the endoreic Boteti river, an overflow from the regional Okavango river system, enters the Makgadikgadi pans. The present work considers diagenetic silica and calcium carbonate dominated transformations. The aims are to help identify precursor conditions for the origin of microcrystalline silcrete–calcrete intergrade deposits while developing insight into pene‐contemporaneous silica and calcite matrix formation. General precursor conditions require the presence of cyclical endoreic freshwater inflow into a saline pan. The pan should be deep enough to sustain a permanent watertable under climatic conditions sufficient to cause carbonate fractionation within the groundwater. Freshwater inflow into a saline pan drives the geochemistry of the system (from freshwater to saline, from neutral to high pH). The geochemistry is controlled by the periodicity of inflow relative to salinity levels of phreatic groundwater in the receptor saline pan. The source of most silica and localized CaCO3 is derived from the dissolution and precipitation of micro‐fossils, while more general CaCO3 enrichment stems from saline pan based carbonate fractionation. Diagenetic change leads to colloidal then more consolidated bSiO2/CaO aggregate formation (amorphous silica) followed by transformations into opaline silica over time. Irregular zones of siliceous sediment forming in otherwise calcareous deposits may relate to the irregular occurrence of biogenic silica in the source sediments, inferring a source for local silica mobilization in intergrade deposits. The distribution of calcareous micro‐fossils may have a similar converse effect. Diagenetic evidence from an intergrade deposit with a low SiO2/CaO ratio suggests that transformation occurred more into the pan, while an intergrade deposit with a high SiO2/CaO ratio more likely formed closer to a land margin and was frequently inundated by freshwater. Pene‐contemporaneous silcrete–calcrete intergrade formation under the above conditions may take place where dissolved silica crystallizes out in the vicinity of calcite crystals due to local decreases in pH. The continuing consolidation of bSiO2/CaO aggregates may be facilitated by the presence of increasing amounts of calcite. It appears that CaCO3 may act as a catalyst leading to pene‐contemporaneous bSiO2/CaO aggregate formation. However the processes involved require further work. Copyright © 2013 John Wiley & Sons, Ltd.

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Abstract. The Upper Ordovician sedimentary succession of southeast Ireland is riddled with almost synsedimentary igneous intrusions. A study was required to determine any effects of these intrusions on Kübler Index (KI) so that further sampling for a regional study could be undertaken and the results would be unbiased. Sampling was carried out on the low-grade metapelites (Southeast Ireland) around the rhyolitic and doleritic intrusions, which were intruded into the still soft sediment. Clay mineralogy was determined using X-ray diffractometry and KI values were measured; SEM and whole rock elemental analyses were applied to complement the results of the XRD work. KIs immediately around rhyolitic intrusions were observed in all cases to be lower than the regional KI, however this was not seen around doleritic intrusions. SEM work shows illite grain sizes following the KI trend and K2O were noted to be highest where KI was lowest. Rare earth elements (REEs) show some mobilization which also follows the KI trend. This is considered to be a residual effect of hydrothermal fluids / volatiles hosted by the rhyolites but not encountered in the dolerites. Chlorite crystallinity is not affected as it most likely crystallized after the hydrothermal event. The KI is often used as a proxy for illite crystallinity in studies of low grade metamorphism but is influenced by many parameters and care should be taken when applying this technique to studies of regional metamorphism.