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The deposit is unique because of its extremely high Cu and Sn grades (45 000 000 t at 6% Cu plus 4 500 000 t at 12% Cu and 2.2% Sn). More than 300 000 t of Sn metal is contained in several types of mineralisation grading from 500 ppm to 60% Sn. The volcanic, volcaniclastic and sedimentary lithofacies that directly host the mineralisation consist of a rhyolitic dome-cryptodome-hyaloclastite complex. The studies indicate that ore formation resulted from a long-lived hydrothermal system, including two main mineralising events, an early stage of stringer and massive cassiterite deposition and a subsequent episode of massive sulphide ore generation. Two spatially independent stockworks fed these genetically related mineralising systems. The isotope signatures of the ore fluids are best explained by incorporation of magmatic and/or metamorphic fluids into a seawater-dominated hydrothermal system. Magmatic fluids, in particular, were a likely source for the Sn and much of the Cu., The deposit is unique because of its extremely high Cu and Sn grades (45 000 000 t at 6% Cu plus 4 500 000 t at 12% Cu and 2.2% Sn). More than 300 000 t of Sn metal is contained in several types of mineralisation grading from 500 ppm to 60% Sn. The volcanic, volcaniclastic and sedimentary lithofacies that directly host the mineralisation consist of a rhyolitic dome-cryptodome-hyaloclastite complex. The studies indicate that ore formation resulted from a long-lived hydrothermal system, including two main mineralising events, an early stage of stringer and massive cassiterite deposition and a subsequent episode of massive sulphide ore generation. Two spatially independent stockworks fed these genetically related mineralising systems. The isotope signatures of the ore fluids are best explained by incorporation of magmatic and/or metamorphic fluids into a seawater-dominated hydrothermal system. Magmatic fluids, in particular, were a likely source for the Sn and much of the Cu.

Contents:- Structures and chemical compositions of clay minerals, by G.W. Brindley, p.1-21, 38 refs.; X-ray identification (with ancillary techniques) of clay minerals, by G.W. Brindley, p.22-38, 56 refs.; X-ray diffraction identification of mixed-layer clay minerals, by J. Hower, p.39-59, 20 refs.; Shale diagenesis by J. Hower, p.60-80, 19 refs.; Formation damage and the crystal chemistry of clays, by W.R. Almon and D.K. Davies, p.81-103, 14 refs.; Classification and diagenesis of clay minerals in tight gas sandstones: case studies in which clay mineral properties are crucial to drilling fluid selection, formation, evaluation and completion techniques, by J.B. Thomas, p.104-118, 5 refs.; Depositional environment and diagenesis of Permian Rotliegende's sandstones in the Dutch sector of the southern North Sea, by W.R. Almon, p.119-147, 9 refs.; Clay diagenesis and effects on sandstone cementation (case histories from the Gulf Coast Tertiary) by J.R. Boles, p.148-168, 17 refs.; Application of thermodynamics to clay minerals and authigenic mineral equilibria, by I. Hutcheon, p.169-193, 35 refs., Contents:- Structures and chemical compositions of clay minerals, by G.W. Brindley, p.1-21, 38 refs.; X-ray identification (with ancillary techniques) of clay minerals, by G.W. Brindley, p.22-38, 56 refs.; X-ray diffraction identification of mixed-layer clay minerals, by J. Hower, p.39-59, 20 refs.; Shale diagenesis by J. Hower, p.60-80, 19 refs.; Formation damage and the crystal chemistry of clays, by W.R. Almon and D.K. Davies, p.81-103, 14 refs.; Classification and diagenesis of clay minerals in tight gas sandstones: case studies in which clay mineral properties are crucial to drilling fluid selection, formation, evaluation and completion techniques, by J.B. Thomas, p.104-118, 5 refs.; Depositional environment and diagenesis of Permian Rotliegende's sandstones in the Dutch sector of the southern North Sea, by W.R. Almon, p.119-147, 9 refs.; Clay diagenesis and effects on sandstone cementation (case histories from the Gulf Coast Tertiary) by J.R. Boles, p.148-168, 17 refs.; Application of thermodynamics to clay minerals and authigenic mineral equilibria, by I. Hutcheon, p.169-193, 35 refs.

In an effort to evaluate mineral-water isotopic exchange during cyclic steam stimulation (CSS), solutions and berthierine-dominated solids smaller than 2 micrometres from the Clearwater Formation oil sands of Alberta, Canada, were analysed for stable isotope compositions before and after reaction in autoclaves for 1 008 h at 250 degrees C. There was no significant change in solution delta18-O and delta2-H, which is consistent with the high water/mineral ratio used in the experiments. The solids showed a marked decrease in both delta18-O and delta2-H following the experiments. Pre-run solids have delta18O of +9.5 to +12.9 per mil and delta2-H of -114 to -113 per mil, whereas post-run solids have delta18-O of -4.7 to +2.1 per mil and delta2-H of -147 to -128 per mil. Neither oxygen- nor hydrogen-isotope equilibrium was established between the solids and the solutions. Calculation suggests that oxygen-isotope exchange (44-58%) was greater between the solids and the solutions than was the case for hydrogen isotopes (23-50%). It is proposed that this behaviour resulted from partial inheritance of the pre-run berthierine structure during formation of the post-run smectite, chlorite-smectite and chlorite. This process confounds the use of clay mineral stable isotope compositions as a temperature indicator of in situ steam/steam condensate interaction with oil-sands reservoirs. The results also suggest an additional mechanism by which new clay minerals can be formed during CSS-related, artificial diagenesis. (Authors.), In an effort to evaluate mineral-water isotopic exchange during cyclic steam stimulation (CSS), solutions and berthierine-dominated solids smaller than 2 micrometres from the Clearwater Formation oil sands of Alberta, Canada, were analysed for stable isotope compositions before and after reaction in autoclaves for 1 008 h at 250 degrees C. There was no significant change in solution delta18-O and delta2-H, which is consistent with the high water/mineral ratio used in the experiments. The solids showed a marked decrease in both delta18-O and delta2-H following the experiments. Pre-run solids have delta18O of +9.5 to +12.9 per mil and delta2-H of -114 to -113 per mil, whereas post-run solids have delta18-O of -4.7 to +2.1 per mil and delta2-H of -147 to -128 per mil. Neither oxygen- nor hydrogen-isotope equilibrium was established between the solids and the solutions. Calculation suggests that oxygen-isotope exchange (44-58%) was greater between the solids and the solutions than was the case for hydrogen isotopes (23-50%). It is proposed that this behaviour resulted from partial inheritance of the pre-run berthierine structure during formation of the post-run smectite, chlorite-smectite and chlorite. This process confounds the use of clay mineral stable isotope compositions as a temperature indicator of in situ steam/steam condensate interaction with oil-sands reservoirs. The results also suggest an additional mechanism by which new clay minerals can be formed during CSS-related, artificial diagenesis. (Authors.)

The deposits are very similar, both displaying typical amphibolite-facies paragenesis, but at Palmeiropolis oxygen isotopes do not distinguish hydrothermally altered from unaltered rocks. At Geco, the metamorphosed chlorite alteration zone has significantly lower delta 18-O values than the host rocks. These values were produced when the rocks interacted with hydrothermal solutions during deposit formation and were not disturbed by later high-grade metamorphism. At Palmeiropolis, the chlorite zone was isotopically rehomogenised. The process was facilitated by the smallness of the orebody (reserves are 4 000 000 t grading 1.23% Cu, 4.64% Zn and 0.72% Pb, compared with 50 000 000 t at Geco) and the presence of a large volume of unaltered rocks within the alteration zone. Whole-rock oxygen isotope analysis appears an effective method of identifying large areas of hydrothermal alteration in highly metamorphosed terranes, but is likely to miss small orebodies., The deposits are very similar, both displaying typical amphibolite-facies paragenesis, but at Palmeiropolis oxygen isotopes do not distinguish hydrothermally altered from unaltered rocks. At Geco, the metamorphosed chlorite alteration zone has significantly lower delta 18-O values than the host rocks. These values were produced when the rocks interacted with hydrothermal solutions during deposit formation and were not disturbed by later high-grade metamorphism. At Palmeiropolis, the chlorite zone was isotopically rehomogenised. The process was facilitated by the smallness of the orebody (reserves are 4 000 000 t grading 1.23% Cu, 4.64% Zn and 0.72% Pb, compared with 50 000 000 t at Geco) and the presence of a large volume of unaltered rocks within the alteration zone. Whole-rock oxygen isotope analysis appears an effective method of identifying large areas of hydrothermal alteration in highly metamorphosed terranes, but is likely to miss small orebodies.

The gold is hosted by the turbidites and greywackes of a Lower Proterozoic sediment-dominated accretionary complex and localised within the Ashanti structural belt, where a vertical section of about 4 km through the hydrothermal system is now preserved. Prestea ores occur in brittle-ductile rocks and Bogosu ores in the system's structurally higher cataclastic regime. Geochemical and isotopic evidence is consistent with the generation of a metamorphic fluid by devolatilisation of Birimian sediments and its upward migration along the permeable conduits provided by major structures during periods of uplift. Three important stages in the evolution of the hydrothermal system have been identified. Sedimentary country rocks and mafic dykes were altered under rock-predominant greenschist-facies conditions. Carbonate alteration of mafic dykes occurred during slow upward migration of fluids when the lateral diffusion of hydrogen permitted generation of methane, enriching residual CO2 and the resulting carbonate minerals in C-13. Rapid or explosive expulsion of deep-seated fluids and CO2-H2O phase separation caused precipitation of sulphides, arsenides and gold at relatively high crustal levels and low ambient pressure and temperature; fluid ascent was too rapid for isotopic and chemical re-equilibration between the ore fluid and adjacent country rocks., The gold is hosted by the turbidites and greywackes of a Lower Proterozoic sediment-dominated accretionary complex and localised within the Ashanti structural belt, where a vertical section of about 4 km through the hydrothermal system is now preserved. Prestea ores occur in brittle-ductile rocks and Bogosu ores in the system's structurally higher cataclastic regime. Geochemical and isotopic evidence is consistent with the generation of a metamorphic fluid by devolatilisation of Birimian sediments and its upward migration along the permeable conduits provided by major structures during periods of uplift. Three important stages in the evolution of the hydrothermal system have been identified. Sedimentary country rocks and mafic dykes were altered under rock-predominant greenschist-facies conditions. Carbonate alteration of mafic dykes occurred during slow upward migration of fluids when the lateral diffusion of hydrogen permitted generation of methane, enriching residual CO2 and the resulting carbonate minerals in C-13. Rapid or explosive expulsion of deep-seated fluids and CO2-H2O phase separation caused precipitation of sulphides, arsenides and gold at relatively high crustal levels and low ambient pressure and temperature; fluid ascent was too rapid for isotopic and chemical re-equilibration between the ore fluid and adjacent country rocks.

The Malala deposit is the only known porphyry Mo occurrence in Indonesia. It is typical of the F-poor class of Mo deposits and belongs to the plutonic sub-type. The mineralised system is associated with porphyritic intrusive rocks of predominantly granitic composition which occur as late differentiates in the roof zone of a composite pluton. Major oxide, trace element, isotope and mineralogical data indicate that the various intrusive phases of the Malala-Dondo suite had a common magma source. They are magnetite-series, LILE-enriched, Caledonian I-type granitoids, and belong to the high-K calc-alkaline series. Alteration and mineralisation are erratically, and in most places weakly, developed over an area of 4 km2, predominantly as a "shell" up to 50 m thick at the intrusive contact. Highest grades are found in the East Zone which has an estimated resource of 100 000 000 t at 0.14% MoS2. Petrographic, fluid inclusion and oxygen isotope data suggest that the hydrothermal system evolved from a hot (400-700 degrees C), hypersaline, magmatic fluid regime (stages I and II) to a cooler (150-400 degrees C) environment, characterised by dominantly meteoric, less saline fluids (stages III and IV). The early fluids produced potassic alteration, quartz veining and Mo mineralisation, whereas the later fluids deposited carbonate-ericite-chlorite assemblages accompanied by minor base metal mineralisation, followed by carbonate and kaolinite deposition., The Malala deposit is the only known porphyry Mo occurrence in Indonesia. It is typical of the F-poor class of Mo deposits and belongs to the plutonic sub-type. The mineralised system is associated with porphyritic intrusive rocks of predominantly granitic composition which occur as late differentiates in the roof zone of a composite pluton. Major oxide, trace element, isotope and mineralogical data indicate that the various intrusive phases of the Malala-Dondo suite had a common magma source. They are magnetite-series, LILE-enriched, Caledonian I-type granitoids, and belong to the high-K calc-alkaline series. Alteration and mineralisation are erratically, and in most places weakly, developed over an area of 4 km2, predominantly as a "shell" up to 50 m thick at the intrusive contact. Highest grades are found in the East Zone which has an estimated resource of 100 000 000 t at 0.14% MoS2. Petrographic, fluid inclusion and oxygen isotope data suggest that the hydrothermal system evolved from a hot (400-700 degrees C), hypersaline, magmatic fluid regime (stages I and II) to a cooler (150-400 degrees C) environment, characterised by dominantly meteoric, less saline fluids (stages III and IV). The early fluids produced potassic alteration, quartz veining and Mo mineralisation, whereas the later fluids deposited carbonate-ericite-chlorite assemblages accompanied by minor base metal mineralisation, followed by carbonate and kaolinite deposition.

The JC tin skarn deposit is a stanniferous skarn hosted within Mississippian sediments intruded by the mid-Cretaceous Seagull Batholith. The stable isotope compositions of silicate, carbonate and sulphide minerals have been used to interpret the evolution of fluid source during sequential stages of skarn formation., The JC tin skarn deposit is a stanniferous skarn hosted within Mississippian sediments intruded by the mid-Cretaceous Seagull Batholith. The stable isotope compositions of silicate, carbonate and sulphide minerals have been used to interpret the evolution of fluid source during sequential stages of skarn formation.

The Tom deposit is a stratiform, exhalative, massive sulphide deposit. It consists of a lens of massive ore that passes vertically upward and laterally into laminated ore. The footwall units host an area of ankerite and quartz veining, which is interpreted to represent the feeder zone for the mineralising fluids. The mineral assemblages and paragenesis suggest that early mineral deposition occurred in a sulphur-deficient environment. Local production of reduced sulphur eventually allowed the precipitation of sulphides. The mineral zonation exhibited by the massive and laminated ore is typical of hot, reduced hydrothermal fluids mixing with cooler, more oxygenated water., The Tom deposit is a stratiform, exhalative, massive sulphide deposit. It consists of a lens of massive ore that passes vertically upward and laterally into laminated ore. The footwall units host an area of ankerite and quartz veining, which is interpreted to represent the feeder zone for the mineralising fluids. The mineral assemblages and paragenesis suggest that early mineral deposition occurred in a sulphur-deficient environment. Local production of reduced sulphur eventually allowed the precipitation of sulphides. The mineral zonation exhibited by the massive and laminated ore is typical of hot, reduced hydrothermal fluids mixing with cooler, more oxygenated water.