Papers by Thomas Shepherd
A multidisciplinary project was carried out in an attempt to improve understanding of the evoluti... more A multidisciplinary project was carried out in an attempt to improve understanding of the evolution and inter-relationship of magmatism and metalliferous mineralization in the central and southern Andes. This has involved the co-ordinated application of geochronology, radiogenic and stable isotope geology, and fluid inclusion studies to both regional and specific magmatic and metallogenic occurrences throughout the area. A brief introduction to the principles and application of these techniques is given. Rb-Sr whole-rock geochronology (supplemented by K-Ar dating carried out in Chile) and Sr isotope methods have been used extensively, especially in Chile, where there was little previous data of this type, to elucidate the evolution of the Andean margin in space and time. Regional mineralization studies encompass sulphur and lead isotopes as well as geochronology. Finally, all techniques has been jointly applied in two case studies based on specific previous metal deposits
Mineralium Deposita, 1996
... Kusakabe (1975). Carbonate samples were prepared for stable iso-tope analysis following the m... more ... Kusakabe (1975). Carbonate samples were prepared for stable iso-tope analysis following the method of McCrea (1959). The quartz samples were processed according to the method of Clayton and Mayeda (1963). The galena ...
Chemical Geology, 2003
Stable isotope compositions of quartz (δ 18 O quartz) and fluid inclusion waters (δ 18 O FI and δ... more Stable isotope compositions of quartz (δ 18 O quartz) and fluid inclusion waters (δ 18 O FI and δD FI) were analysed from Profitis Ilias, a low-sulphidation epithermal gold mineralisation deposit on Milos island Greece, to establish if δ 18 O FI The data show systematic variations with sample elevation. Samples from the highest elevations (c. 650 m asl) have the lightest δ preserve a record of paleo-geothermal processes. Previous studies show that mineralisation at Profitis Ilias resulted from extreme boiling and vaporisation and a zone located at approximately 430 m asl represents the transition between a liquid-and vapour-dominated system [Mineral. Dep. 36 (2001) 43]. The deposit is also closely associated with an active geothermal system, whose waters have a well-characterised stable isotope geochemistry [Pagel and Leroy (1991) Source, transport and deposition of metals. Balkema, Rotterdam, 107-112]. The samples were collected over an elevation interval of 440 m (210 to 650 m asl) to give information on the liquid-and vapour-segments of the paleo-system. 18 O FI (-7.3 ‰) and δD FI (-68.0 ‰) whilst the deepest (c. 210 m asl) are isotopically heavier (δ 18 O FI-3.7 ‰; δD FI-19.0 ‰). Relative changes in δ 18 O FI closely parallel those in δD FI. δ 18 O quartz shows an opposite trend, from the lightest values (+13.9 ‰) at the lowest elevations to the heaviest (+15.1 ‰) at the highest. δ 18 O FI show correlations with other parameters. For example, variable fluid inclusion homogenisation temperatures in the vapour-dominated part of the system, correlate with a rapid shift in δD FI (-33.3 to-50.5 ‰) and δ 18 O FI (-4.1 to-6.2 ‰) and gold contents also increase in the same zone (up to 50 ppm). Comparable correlations in δ 18 O quartz or δ 18 O calculated (estimated geothermal fluid from fluid inclusion homogenisation data) are absent. δ 18 O calculated are always 5 to 10 ‰ heavier than δ 18 O FI. Comparison with the modern geothermal system shows that δD FI-δ 18 O FI are similar. Isotope data for the modern system and fluid inclusion waters fall on linear trends sub-paralleling the meteoric water line and project towards seawater values. Numerical modelling favours kinetically controlled fractionation to explain differences in δ 18 O calculated and δ 18 O fluid rather than diffusive posttrapping equilibration. The evidence suggests, that in low-temperature epithermal systems, δ 18 O FI Keywords: Fluid inclusions; oxygen and hydrogen isotopes; active Aegean arc; epithermal mineralisation may represent a better record of fluid process and the isotopic composition of the geothermal fluid than temperature-corrected quartz data.
Science of The Total Environment, 2009
Direct evidence of fluid mixing in the formation of stratabound Pb–Zn–Ba–F mineralisation in
A geochemical investigation of the West Cumberland meta somatic haematite deposits and related ve... more A geochemical investigation of the West Cumberland meta somatic haematite deposits and related vein deposits in the central Lake District was carried out to establish the origin of the mineralizing fluids. Ore samples we re analysed for a variety of major and trace elements and were found to contain a simple geochemical assemblage characterised by the abundance of arsenic and locally barium. The lack of regional variation in ore geochemistry and the equally uniform mineralogy suggests that the replacement and vein deposits are cogenetic and belong to the same metallogenic province. Minor differences which do exist can be explained, by wallrock - ore fluid interactions. In the Eskdale area the veins are associated with a particular phase of the host granite containing an abundance of free haematite. Relative trace element enrichments in the ores are matched by sympathetic depletions in the adjacent haematized granite. The implied geochemical relationship is verified by a spatial corr...
Economic Geology, 1981
Interest in fluid inclusions, the microscopic droplets of fluid preserved in naturally occuring m... more Interest in fluid inclusions, the microscopic droplets of fluid preserved in naturally occuring minerals, has grown rapidly during the past ten years. Once considered as mineralogical curios, they are now the focus of intense research, par-ticularly in the fields of economic ...
The Science of the total environment, Jan 25, 2016
One hundred soil samples were collected from urban spaces, in Lisbon, Portugal, in two surveys th... more One hundred soil samples were collected from urban spaces, in Lisbon, Portugal, in two surveys that were carried out in consecutive years, to assess the potential adverse human health effects following exposure to potentially toxic elements and organic compounds in the urban soils. The study hereby described follows on from the earlier work of the authors and aims at performing a source-pathway-fate analysis of lead (Pb) and polycyclic aromatic hydrocarbons (PAHs) in the urban soils in order to increase current knowledge on factors influencing exposure of the population. Various techniques were combined to achieve the proposed goal. Geogenic and anthropogenic sources were apportioned by means of Pb isotope mixing models. Isotope data was further coupled with geographic information system mapping to assess local mixed sources of Pb and PAHs. Unleaded vehicle exhaust and cement production show the largest relative contribution to the total soil-Pb, but their respective importance depe...
Geological Society London Petroleum Geology Conferenceseries, 2005
From a geological perspective, hydrogen has been neglected. It is not as common as biogenic or th... more From a geological perspective, hydrogen has been neglected. It is not as common as biogenic or thermogenic methane, which are ubiquitous in hydrocarbon basins, or carbon dioxide, which is common in geologically-active areas of the world. Nevertheless, small flows of hydrogen naturally reach the Earth's surface, occur in some metal mines and emerge beneath the oceans in a number of places worldwide. These occurrences of hydrogen are associated with abiogenic and biogenic methane, nitrogen and helium. Five geologic environments are theoretically promising for exploration based on field, palaeofluid and theoretical evidence: ophiolites (Alpine, Variscan and Caledonian in order of
Anal Chem, 1982
ACS; Journals; C&EN; CAS. ACS Publications. Journals A–Z Books|Authors & Reviewers|Librar... more ACS; Journals; C&EN; CAS. ACS Publications. Journals A–Z Books|Authors & Reviewers|Librarians|ACS Members|About Us|e-Alerts|Help. Quick Search. Advanced Search. Anal. Chem. All Journals/Website. Analytical ...
Proceedings of the Ussher Society, 1987
Mineralium Deposita, 1984
A fluid inclusion investigation of the Carrock Fell tungsten deposit, Northern England, confirms ... more A fluid inclusion investigation of the Carrock Fell tungsten deposit, Northern England, confirms that the quartz-wolframite-scheelite veins associated with the Caledonian Skiddaw Granite are almost exclusively related to an exocontact hydrothermal system developed at the margin of a local cupola. Fluid circulation, as defined by the spatial variation in temperature and H20/ CO2 ratios for inclusions in vein quartz, reveals a strong structural control. The zone of maximum flow, which extends 0-400 m out from the granite contact, is characterised by high H20/CO2 ratios and corresponds closely with the known distribution of high-grade oreshoots. Based on the fluid inclusion "gas" signature for the Carrock Fell deposit, a distinction can be made between potentially tungstaniferous quartz veins and those related to Cu-Pb-Zn deposits in the absence of diagnostic ore minerals. Also, a regional survey of quartz veins in the Lake District suggests that at several localities the fluids have a close affinity with those at Carrock Fell. This is interpreted as the high-level, distal expression of tungsten mineralisation at depth. Evidence for similar mineralisation elsewhere in the British Caledonides favours those granites in the paratectonic zones of Ireland and southern Scotland.
Mineralium Deposita, 1985
A regional fluid inclusion study of Cu-Au (+ Zn-Pb) mineralisation in the Harlech Dome area, Nort... more A regional fluid inclusion study of Cu-Au (+ Zn-Pb) mineralisation in the Harlech Dome area, North Wales, gives support to the concept of two distinct metallogenic episodes. The inclusion assemblages associated with the porphyry copper mineralisation at Coed-y-Brenin are consistent with a genetic model of early potassic-propylitic alteration overprinted by later phyllic alteration. High salinity fluids, normally characteristic of potassic alteration, are confined to the host rock quartz. The meteoric/hydrothermal system is closely linked to the emplacement of late-Cambrian diorites. Integrated fluid inclusion and mineralogical studies of the Gold-belt veins suggest that the mineralising fluids were probably dehydration waters released from weakly metamorphosed Cambrian and perhaps Precambrian sediments during hydraulic fracturing in a tensional zone at the close of the Caledonian orogeny. Localisation of economic concentrations of gold in veins at the level of the Clogau Formation is ascribed to a destabilisation of metal complexes caused by a change in fluid buffering from a pyrite-magnetite assemblage in the Lower Cambrian sediments to a pyrite-pyrrhotite-graphite assemblage in the Upper Cambrian sediments. Veining associated with the Coed-y-Brenin porphyry copper deposit and related breccia pipes can be distinguished from the copper-gold veins of the coextensive Dolgellau Gold-belt by the presence in the former of inclusions notably richer in CO2. Furthermore the Gold-belt fluids have a distinctive low CO2/CH4 + N2 + H2 ratio.
Mineralium Deposita, 1987
Mineralogical studies demonstrate that the Hercynian polymetallic antimony-rich deposit of Bourna... more Mineralogical studies demonstrate that the Hercynian polymetallic antimony-rich deposit of Bournac can be described by four stages of ore deposition and one of partial ore remobilization. Fluid inclusion data permit calculation of the composition and temperature of the fluids associated with each stage of hydrothermal mineralization and concomitant wall-rock alteration. Stages I and II (Fe-As and Zn) are represented by moderate-salinity H20-CO2-(NaC1) inclusions which correlate closely with early carbonate deposition. Stage III fluids which are responsible for the deposition of Pb-Sb ores are characterized by low-salinity H20-(NaC1) inclusions. During the final stage of mineralization (IV), corresponding to the main phase of stibnite deposition, abundant aqueous inclusions confirm the continued involvement of lowsalinity fluids and the intense development of potassic clays and secondary silica in the wall rocks. Homogenization temperatures suggest that the whole cycle of mineralization took place during a gradual decrease in fluid temperature of 380°-140°C. Stibnite deposition is restricted to the interval of 230°-140°C thus confirming an essentially epithermal environment. Stage V (partial remobilization) is distinguished by the presence of high-salinity CaC12-rich inclusions which are tentatively related to Triassic barite mineralization in the region and therefore postdate the Bournac antimony ores. Homogenization temperatures for this stage range 140°-60 °C.
Journal of Geophysical Research, 1990
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Papers by Thomas Shepherd