Hydrothermal Vein Systems Research Articles

A deep geothermal exploration well at Eastgate, Weardale, UK: a novel exploration concept for low-enthalpy resources

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.

Partitioning and Leaching Behavior of Actinides and Rare Earth Elements in a Zirconolite-bearing Hydrothermal Vein System

AbstractChemical extraction techniques and scanning electron microscopy were used to study the distribution and behavior of actinides and rare earth elements (REE) in hydrothermal veins at Adamello, (Italy). The six samples discussed in this paper were from the phlogopite zone, which is one of the major vein zones. The samples were similar in their bulk chemical composition, mineralogy, and leaching behavior of major elements (determined by extraction with 9M HCl). However, there were major differences in the extractability of REE and actinides. The most significant influence on the leaching characteristics appears to be the amounts of U, Th and REE incorporated in resistant host phases. Uranium and Th are very highly enriched in zirconolite grains. Actinides were more readily leached from samples with a higher content of U and Th, relative to the amount of zirconolite. The results show that REE and actinides present in chemically resistant minerals can be retained under aggressive leaching conditions.

Nitrogen 15–enriched Precambrian kerogen and hydrothermal systems

Nitrogen isotope data are reported for kerogen from Precambrian black shales at prehnite‐pumpellyite to greenschist facies and for micas from postmetamorphic hydrothermal quartz vein systems. Nitrogen in micas was acquired from N‐bearing aqueous metamorphic hydrothermal fluids generated, from breakdown of sedimentary kerogen and dehydration of K‐silicates, at deeper crustal levels. The 2.7 Ga kerogens and hydrothermal K‐micas yield δ15N values of 15–24‰, compared to existing data of 2–6‰ in Phanerozoic counterparts. Paleoproterozoic equivalents have intermediate δ15N of 7–12‰, implying a secular decrease in δ15N of shale kerogen. The 15N‐enriched nitrogen in Archean shales and hydrothermal vein systems cannot be caused either by N isotopic shifts accompanying metamorphism or Rayleigh fractionation because premetamorphic and postmetamorphic samples from the same terrane are both enriched and lack covariation of δ15N with N, C/N ratios, or metamorphic grade. The magnitude of shifts during progressive metamorphism of sedimentary rocks in previous studies is constrained at <1‰ to greenschist and about 3‰ to amphibolite facies. Furthermore, 15N‐enriched values cannot stem from long‐term preferential diffusional loss of 14N as samples were selected from terranes where 40Ar/39Ar ages are within a few million years of concordant U‐Pb ages; nitrogen is structurally bound in micas, whereas Ar is not. It is possible that the 15N‐enriched values stem from a different N cycle in the Archean, with large biologically mediated fractionations, yet the magnitude of the fractionations observed exceeds any presently known, and chemoautotrophic communities tend to depleted values. Earlier results on Archean cherts show a range from −6 to 30‰. Given the temporal association of chert‐banded iron formation with mantle plumes, the range is consistent with mixing between mantle N2 of −5‰ and the enriched marine reservoir identified in this study. We attribute the secular trend from Archean kerogen of 15–24‰ to Phanerozoic kerogen averaging 3–4‰ to a secondary atmosphere derived from CI‐chondrite‐like material and comets with δ15N of 30–42‰. Shifts of δ15N to its present values of 0‰ can be accounted for by a combination of sequestration of atmospheric N2 into sedimentary rocks and mantle degassing of −5‰ mantle N2.

Laser combustion analysis of δ 34S of sulfosalt minerals : determination of the fractionation systematics and some crystal-chemical considerations

Laser sulfur combustion in the presence of O 2 is the most commonly used method of in situ sulfur isotope analysis. Previous workers indicated that a small but reproducible fractionation of 34S/ 32S exists between the product SO 2 gas and the mineral. The magnitude of this fractionation varies with bond strength, as reflected in Gibbs free energy of formation at 298.15K. The correction factors are known for common sulfides and anhydrite but not, hitherto, for stibnite and the sulfosalt minerals, which are important constituents of many classes of ore deposits. We present the correction factors for the following chemically and crystallographically well-characterized minerals: stibnite (weighted mean = −1.2‰), bournonite (+0.6‰), tetrahedrite (+1.3‰), and boulangerite (+1.4‰). The Gibbs free energies of formation of these phases have been approximated by the sulfide summation method, and the correlation of ΔG 298 0 with the correction factor for the sulfosalts fits well with the trend previously established for simple sulfides. There is an excellent correlation between the fractionation factor and mineral composition, a parameter that does result in bond strength variations (e.g., mol fraction of PbS in sulfosalts), allowing estimation of the correction factors for simple intermediate compositions. Finally, bond strength also varies with variation in interatomic distances, and we have, therefore, investigated the behaviour of stibnite, a strongly anisotropic mineral. Our results indicate that there is a significant variation in fractionation factor, depending on crystal orientation. The fractionation factor along the prismatic b-axis, which displays the strongest chemical bonds (as monitored by the shortest bond lengths), is more negative (−1.7‰) than along the other crystallographic directions (−0.7 to −1.0‰), which is in full agreement with theoretical predictions. We demonstrate the application of the technique in unravelling source- and process-related sulfur isotope systematics in two hydrothermal vein systems in the classic mining area of the NE Rhenish Massif, both studies requiring resolution beyond the scale of conventional sulfur isotope analysis.

Remnants of an Early Archaean (>3.75 Ga) sea-floor, hydrothermal system in the Isua Greenstone Belt

Localised low-strain domains in the ∼3.75 Ga Isua Greenstone Belt, West Greenland, contain a wealth of well-preserved primary structures. These include pillow lavas with dark, fine-grained cooling rims, pillow breccias, debris flows and conglomerates. Here we describe quartz globules from a relatively undeformed pillow breccia, which contains unstrained quartz. Most quartz globules contain only few quartz grains, but some of the larger ones preserve a subidiomorphic rim reminiscent of former geodic lining. The globules are associated with a complex system of quartz veins. Metamorphic conditions recorded in pelites associated with the pillow breccia represent the early stage of a prograde event at 460–480 °C and ∼4 kb. Both globules and quartz veins pre-date the metamorphic fabric, and are interpreted as remnants of a sea-floor type hydrothermal system, operating at the time of lava eruption and pillow formation at ∼3.75 Ga. The globules, which are interpreted as former gas vesicles filled with quartz (and some carbonate), are embedded in an altered basaltic matrix now comprising biotite, muscovite and quartz±tourmaline and carbonate. Silica-filling in the vesicles was contemporaneous with the formation of an intricate hydrothermal vein system. During deformation, the strain was partitioned into the mica-rich rock matrix, when the vesicles behaved as competent objects, eventually cutting or deforming the thinner veins. Amphibolite-facies metamorphic overprint involved a limited flattening of some vesicles and the development of a very faint, sporadic schistosity, preserving most of the pre-metamorphic hydrothermal features. Major metamorphic structures correspond to quartz annealing, yielding curved grain boundaries and triple point junctions. We report rare primary fluid inclusions from quartz in the vesicles. The inclusions contain remnants of two independent fluid/mineral systems comprising pure methane and highly saline aqueous fluids (about 25 wt.% NaCl equivalent), and co-precipitating carbonates (calcite). Especially for the aqueous system, these fluids bear a strong resemblance to present-day sea-floor hydrothermal fluids. They are interpreted as relics of the sea-floor hydrothermal fluid, responsible for the alteration of the pillow fragments and co-precipitation of quartz and carbonate.

Geochemical and electron paramagnetic characteristics of quartz from a multi-stage vein environment, Cowarra gold deposit, New South Wales

The characterisation of quartz using trace element geochemistry and electron paramagnetic resonance measurements has been tested for different generations of quartz veining in the multi-stage hydrothermal vein system at Cowarra. Quartz closely associated with gold mineralisation can be distinguished from earlier and later quartz generations in the mineralised environment, and both types can be distinguished from regional, barren quartz veins. These separate stages of quartz veins are also recognised from their fluid inclusion characteristics. Geochemical differences in the quartz partly reflect the presence of minor mineral inclusions, particularly sulfides and arsenopyrite. The study highlights the importance of understanding the paragenetic sequence of quartz vein development in a mineralised environment and the need for careful, thorough sampling before attempting geochemical and EPR characterisation of quartz as an exploration technique.

Structural setting, textures, and timing of hydrothermal vein systems in the Val d'Or camp, Abitibi, Canada: implications for the evolution of transcrustal, second- and third-order fault zones and gold mineralization

In the Val d'Or camp, Archean Abitibi greenstone belt, Canada, numerous gold-mineralized second- and third-order fault zones are spatially associated with the transcrustal Cadillac Tectonic Zone (CTZ). This situation is used to test whether fluid systems in the CTZ have a similar structural timing to those in the gold-hosting structures, and hence the CTZ could represent the main fluid conduit in the camp. The transcrustal CTZ at Orenada No. 2 contains structurally complex vein systems, with mineralized quartz-tourmaline veins related to both D2 oblique-reverse faulting and F3 dextral asymmetric folding, both of which have been overprinted by unmineralized subhorizontal and subvertical quartz veins. Quartz ± tourmaline veins within second- and third-order shear zones at Paramaque and Rivière Héva also formed during D2 deformation and have been, at least at Rivière Héva, deformed by F3 asymmetric folding. In contrast, mineralized quartz vein systems at Cartier Malartic are controlled by F3 folding and overprinted by late-stage D3 faults which host late quartz-tourmaline veins. Quartz vein textures are consistent with these timing relations, because D2-controlled veins contain deformed quartz grains, whereas quartz in D3-controlled veins is unstrained. The D2 and D3 timing of mineralized quartz veins in the transcrustal CTZ and in second- and third-order structures is consistent with the notion that the CTZ represents the main fluid conduit and that mineralization occurred in linked second- and third-order structures. The different timing of quartz-tourmaline veins in different shear zones indicates that the veins were probably hydraulically linked to the CTZ during at least two different episodes. The location of Cartier Malartic structurally below the CTZ indicates that fluids travelled either downward from the main conduit or that the shear zone was part of the CTZ.

Structural setting, textures, and timing of hydrothermal vein systems in the Val d'Or camp, Abitibi, Canada: implications for the evolution of transcrustal, second- and third-order fault zones and gold mineralization

Structural setting, textures, and timing of hydrothermal vein systems in the Val d'Or camp, Abitibi, Canada: implications for the evolution of transcrustal, second- and third-order fault zones and gold mineralization

Modeling coupled heat and mass transport applied to the hydrothermal system of the Upper Harz Mountains (Germany)

Numerical modeling of coupled heat and mass transport is applied to simulate the temporal and spatial evolution of temperature and quartz precipitation in a hydrothermal vein system. In detail, the computer model simulates the time-dependent temperature evolution of a shear zone and its vicinity by convective and conductive heat transport. A reaction term allows calculation of precipitation and dissolution of vein minerals, here quartz, due to flow-controlled mass transport. Assuming different flow scenarios, examples for the probable evolution of temperature and mineralization of hydrothermal systems are presented. The concept for the model is given by the geological, geochemical, and isotopic studies of hydrothermal vein systems of the Upper Harz Mountains. The suggested scenarios seek to approximate conditions typical for veins showing repeated opening and sealing.

Large‐scale right‐slip displacement on the East San Francisco Bay Region fault system, California: Implications for location of late Miocene to Pliocene Pacific plate boundary

A belt of northwardly younging Neogene and Quaternary volcanic rocks and hydrothermal vein systems, together with a distinctive Cretaceous terrane of the Franciscan Complex (the Permanente terrane), exhibits about 160 to 170 km of cumulative dextral offset across faults of the East San Francisco Bay Region (ESFBR) fault system. The offset hydrothermal veins and volcanic rocks range in age from .01 Ma at the northwest end to about 17.6 Ma at the southeast end. In the fault block between the San Andreas and ESFBR fault systems, where volcanic rocks are scarce, hydrothermal vein system ages clearly indicate that the northward younging thermal overprint affected these rocks beginning about 18 Ma. The age progression of these volcanic rocks and hydrothermal vein systems is consistent with previously proposed models that relate northward propagation of the San Andreas transform to the opening of an asthenospheric window beneath the North American plate margin in the wake of subducting lithosphere. The similarity in the amount of offset of the Permanente terrane across the ESFBR fault system to that derived by restoring continuity in the northward younging age progression of volcanic rocks and hydrothermal veins suggests a model in which 80–110 km of offset are taken up 8 to 6 Ma on a fault aligned with the Bloomfield‐Tolay‐Franklin‐Concord‐Sunol‐Calaveras faults. An additional 50–70 km of cumulative slip are taken up ≤ 6 Ma by the Rogers Creek‐Hayward and Concord‐Franklin‐Sunol‐Calaveras faults. An alternative model in which the Permanente terrane is offset about 80 km by pre‐Miocene faults does not adequately restore the distribution of 8–12 Ma volcanic rocks and hydrothermal veins to a single northwardly younging age trend. If 80–110 km of slip was taken up by the ESFBR fault system between 8 and 6 Ma, dextral slip rates were 40–55 mm/yr. Such high rates might occur if the ESFBR fault system rather than the San Andreas fault acted as the transform margin at this time. Major transpression across the boundary between the Pacific and North American plates at about 3 to 5 Ma would have resulted in the transfer of significant slip back to the San Francisco Peninsula segment of the San Andreas fault. Since that time, the ESFBR fault system has continued to slip at rates of 11–14 mm/yr. If this interpretation is valid, the ESFBR fault system was the Pacific‐North American plate boundary between 8 and 6 Ma, and this boundary has migrated both eastward and westward with time, in response to changing plate margin geometry and plate motions.

Some new aspects of metallogeny of the Bohemian Massif (Central Europe)

The paper gives a summary and generalization based on the author's extensive studies of the metallogeny of the Bohemian Massif in Central Europe, presented in several papers and in an unpublished 1,500-page report (Bernard et al., 1990). Two principal units are distinguished in the metallogenic province of the Bohemian Massif: the Bohemian-Moravian metallogenic zone and the Thuringian-Saxonian-Lower Silesian metallogenic zone, separated from each other by the deep-seated Litomerice fault (Fig. 1). Within each of these two zones, several metallogenic subzones have been defined. The intensity of mineral development of the subzones (Nos. 1-9), from the Precambrian through the Quaternary, has been determined (Fig. 2). The paper focuses on and compares three hydrothermal vein systems associated with the Variscan orogeny: 1 - The Lower Permian vein-type Fe-Zn-Pb-(Ag-Cu) sulfide mineralization referred to as the k-pol association. These are large ore deposits preferentially occurring in anticline structures, connected with intrusion of middle-Variscan granite massifs. Here belongs, in particular, the Kutna Hora and the Havlickuv Brod districts in Bohemia, and the Freiberg district in Saxony (Germany). The predominant strike of large ore veins is NE-SW and NNE-SSW (Fig. 6). 2 - The Lower Permian vein-type Pb-Zn-(Ag-Cu) sulfide mineralization, referred to as the pol-association. The deposits are characteristically bound on principal deep-seated sutures and fault system striking NNE-SSW to ENE-WSW and on feather structures connected with them. Here belongs the Pribram ore district in central Bohemia, the deposits along the Blanice and Jihlava sutures, a. o. (Fig. 7). 3 - The Triassic (?) vein-type Pb-Zn mineralization, referred to as the p-pol association is essentially bound on one extensive zone in western Bohemia, generally striking NW-SE to NNW-SSE (Fig. 8). The strike of the differently old ore veins of the k-pol, pol, and p-pol associations shows a tendency to change from NE-SW to NW-SE, i.e. anti-clockwise, the same tendency as observed in the Freiberg District in Saxony (Fig. 9). From the regional distribution of the above three types of Variscan mineralizations of special interest is that the anticlines with the associated ore-deposits of the k-pol association do not extend to the east beyond the major NW-SE trending regional boundary called the Elbe or Dlouha mez lineament (Fig. 6). A discussion is given in the text.

Overview of some Colombian gold deposits and their development potential

Source rock gold deposits in Colombia include Proterozoic unconformity-related paleoplacers, Palaeozoic stratabound Au/Ag-Cu-Pb-V-U red bed-type ores, and Mesozoic/Tertiary hydrothermal vein systems of Cordilleran magmatic arcs. The younger metallogenic episode occurred some 8 Ma ago in the Western Cordillera. Epithermal deposits are generally small (> 10,000–1 Mt grading 5–10 g/t Au) and the precious-base metal (Cu, Zn, Fe) association was probably deposited within the temperature range 180–300 °C. Widespread palaeoplacers in Quaternary terraces with enrichment in old channels, as well as extensively dredged alluvial deposits in presently active sediments, are connected with ongoing Andean uplift and erosion. Palaeochannel and alluvial gold concentrations represent the most promising targets for future exploration. An additional output could also derive from the clastic-hosted, stratiform deposits of the shield and E Cordillera.

Conditions for fault-valve behaviour

Abstract Evidence for fluid pressures well in excess of hydrostatic during crustal deformation is provided by direct measurements of pressure levels in active tectonic regions and by hydrothermal vein systems in ancient orogenic belts. Faults that act as impermeable seals except immediately postfailure, when they become highly permeable channelways for fluid discharge, may behave as fluid-pressure-activated valves wherever they transect a suprahydrostatic fluid pressure gradient. Such fault-valve behaviour, causing abrupt fluctuations in fluid pressure linked to the earthquake cycle, is particularly likely for faults that remain active while unfavourably oriented for frictional reactivation in prevailing stress fields. The most extreme fault-valve action is likely to be associated with high-angle reverse faults. Valve activity may be especially prevalent near the base of the seismogenic zone when unstable frictional faulting gives way to aseismic shearing with increasing depth. At such structural levels, cyclic variations in the style of deformation may accompany fluid pressure cycling. Fault-valve activity plays an important role in the development of much fault-hosted mineralization, and may also assist the migration of hydrocarbon fluids in some structural settings.

Comparison of metasomatic reactions between a common CO 2 -rich vein fluid and diverse wall rocks; intensive variables, mass transfers, and Au mineralization at Alleghany, California

The gold deposits at Alleghany, California, are typical of many epigenetic gold-bearing hydrothermal vein systems in metamorphic terranes worldwide. Detailed analyses of alteration halos in serpentinite, mafic amphibolite, and granite wall rocks at Alleghany indicate that widely contrasting deposit types, ranging from fuchsite-carbonate schists to pyrite-albitites, resulted when different wall rocks interacted with the same externally derived CO 2 -rich hydrothermal vein fluid. Patterns of element redistribution within halos and among lithologic units suggest a complex process involving fluid flow along vein fractures and diffusion (+ or - infiltration) normal to the veins. Wall rocks locally controlled both the directions and magnitudes of chemical fluxes across vein walls.Vein fluids at constant temperature and pressure, with relatively constant ratios of mobile species (Na (super +) /K (super +) /H (super +) and CO 2 /H 2 S/H 2 ), caused contrasting (divergent) reactions in proximal alteration zones because the wall rocks controlled the ratios of relatively immobile species (Al/(Cr + Fe + Mg) and Fe/Mg), and the component activities, of hydrothermal mineral solid solutions. Thus, pyrite and albite were stable with respect to the relatively pure siderite and K mica that occur sparingly in proximal granite zones, but they were not stable with respect to the relatively impure siderite and K mica that characterize proximal serpentinite zones. Partly as a result of divergent reactions, altered rocks exhibit contrasting mobile element concentration gradients (e.g., Ca, Ba, Sr, Rb, Na/K, C/S) and imply local sources and sinks, and possibly short residence times, for some of these components along discordant fluid flow paths.Calculated reaction coefficients for H (super +) , H 2 , and H 2 S indicate that different wall rocks at different stages of alteration may have inhibited or promoted gold precipitation. The distribution of gold and its calculated solubility are consistent with its having been precipitated from Au(HS) 2 (super -) complexes in response to pyritization of granite (fluid desulfidation and reduction), and possibly in part, in response to dolomitization and sericitization of serpentinite (fluid acidification?). Strata-bound disseminated orebodies may be expected in a variety of other host rocks with higher than average Fe/Mg ratios, where pyrite was stable with respect to intermediate magnesite-siderite, as in the granite.It is proposed that the CO 2 -rich vein fluids approached partial equilibrium with alteration assemblages in the average, or dominant, rocks along their flow paths. Simpler, but radically different, proximal assemblages were then produced where the fluids encountered chemically extreme rock types. Fluid-rock interaction equilibria can account for much of the diversity in the mineralogy and chemistry of gold deposits in heterogeneous metamorphic terranes, while maintaining a simple genetic classification based on the distinctive chemistry of the hydrothermal fluids. Comparative studies of multicomponent phase equilibria and mass exchanges can provide both rationalizations and predictions of local wall-rock effects on geochemical anomalies and the locations of ore deposits in lithologically heterogeneous terranes. Simultaneous consideration of contrasting high-variance (phase-deficient) assemblages along discordant veins is suggested as a general method for quantifying fluid-intensive variables in metasomatic systems.

Hydrothermal subsilicic sodium gedrite from the Gåsborn area, West Bergslagen, central Sweden

AbstractSubsilicic sodium gedrite has been found in a hydrothermal vein together with sekaninaite (Fe-cordierite), andalusite, plagioclase, topaz, sillimanite, quartz, biotite, magnetite, ilmenite, hercynite, wolframite, pyrrhotite, chalcopyrite and pyrite. This vein is part of a hydrothermal vein system consisting predominantly of quartz-feldspar veins, some of which contain andatusite, sekaninaite, biotite, muscovite, fluorite and accessory oxides and sulphides, which was formed during the later stages of crystallization of a high-level anorogenic Svecofennian granite. Petrographic observations suggest the following crystallization sequence for the quartz-feldspar veins: plagioclase-quartz-andalusite-sekaninaite-microcline-biotite-albite-oxides and sulphides-muscovite-fluorite, and for the subsilicic sodium gedrite-bearing vein: andalusite-sekaninaite-subsilicic sodium gedrite-biotite-quartz-albite-sillimanite-topaz-oxides and sulphides. Electron microprobe analysis revealed that all subsilicic sodium gedrite is relatively homogeneous with only the following compositional variation: Na(A) 0.57–0.81, Aliv 2.31–2.57, XMg 0.15–0.21. The temperature (T) for the formation of the hydrothermal vein system is estimated at 550–600°C and the pressure (P) is estimated to be less than 3 kbar.

Contrasting styles of Sn-W mineralisation in peninsular Thailand and SW England

The Sn-W deposits of SW England and SE Asia are associated with crustally derived granitic rocks with late volatile-enriched (F, Li, B, P) differentiates. In peninsular Thailand, primary ores are principally pegmatitic, and hydrothermal vein systems are only locally important. In SW England, wolframite and cassiterite mainly occur in hydrothermal vein systems, and are associated with greisening and tourmalinisation; mineralised pegmatites are rare. These two styles of mineralisation are thought to arise because of differences in the character of late magmatic processes. In peninsular Thailand, late-stage tourmaline-bearing granitic rocks are enriched in B, but not Li and F, compared to earlier biotite granites. Similar late-stage granitic rocks occur also in SW England, but a later topaz granite, enriched in F, Li and P, also occurs. The Thai pegmatitic Sn-W deposits are thought to have formed by late magmatic crystallisation from an aqueous phase enriched in metals and derived by exsolution from a B and metal-rich magma, whereas the SW England mineralisation involved essentially post-magmatic hydrothermal processes. Complexing agents (especially F) and metals may have been derived from granitic or country rocks during hydrothermal circulation at the current level of emplacement.

Wolframite deposition in a hydrothermal vein system; the Grey River tungsten prospect, Newfoundland, Canada

Mineralization in the Grey River tungsten prospect, Newfoundland, is contained within a sheared Devonian K-feldspar megacrystic granite and metamorphic rocks which are typical of the gneissic terrain of the Gander zone. The mineralization is genetically related to a suite of post-tectonic, highly differentiated, alkali-rich leucogranite dikes.The tungsten-bearing hydrothermal veins range from tensional veinlets to lodes containing several injections of quartz. Major composite lodes have greisen alteration envelopes which are characterized by enrichment in volatiles, K 2 O, Al 2 O 3 , Rb, Li, F, heavy rare earth elements, and W. The heavy rare earth elements enrichment appears to be related to deposition of carbonate and fluorite in the greisen. The mineralization is divided into four stages: stage I, characterized by quartz + feldspar + molybdenite veins; stage II, composite lode formation, consisting of five vein types (in paragenetic order), quartz + bismuthinite (IIa), quartz + Fe-rich wolframite (IIb), greisen (IIc), quartz + sulfide (IId), and quartz + Fe-poor wolframite veins (IIe); stage III, characterized by silver-bearing quartz + galena + sphalerite veins; and stage IV, composed of fluorite + calcite + barite veins. A spatial mineral zonation from south to north matches the temporal sequence outlined above, with the exception of the stage IV veins which crosscut stage II veins.Fluid inclusion data indicate a complex evolutionary history for the hydrothermal fluid. The simultaneous trapping of CO 2 -rich and H 2 O-rich fluid inclusions, as well as solid inclusions of calcite, in quartz of the quartz+feldspar+ molybdenite and quartz + bismuthinite vein types, is evidence for the existence of a heterogeneous fluid during stage I and early stage II mineralization. Fluid phase equilibria indicate that the inclusions were trapped at temperatures of 330 degrees to 360 degrees C and fluid pressures of less than 500 bars.In composite lodes, quartz + sulfide and quartz + Fe-poor wolframite veins were deposited in open spaces created by normal faulting. These movements prompted rapid decrease in fluid pressure and temperature and caused retrograde boiling of the hydrothermal fluid (at 200 bars and 300 degrees to 330 degrees C) during deposition of quartz + sulfide veins. Deposition of wolframite in quartz + Fe-poor wolframite veins occurred at 270 degrees to 300 degrees C from an aqueous fluid of low salinity (<0.5 wt % NaCl) and low CO 2 content (<1.0 wt % CO 2 ) and after separation of a CO 2 vapor phase by retrograde boiling. Isotopic and fluid inclusion data suggest that deposition of Fe-rich wolframite may have occurred at a slightly higher temperature (330 degrees C) and from a fluid of higher CO 2 content than the bulk of the wolframite which has an Fe-poor composition.Progressive CO 2 loss from the hydrothermal fluid by immiscibility and retrograde boiling had a marked effect on the solution pH which shifted from 5 weakly acid) to 6 (weakly alkaline at 300 degrees to 350 degrees C) during tungsten deposition. Thermodynamic data on the solubility of wolframite suggest that a pH increase would provide an effective mechanism for tungsten deposition and explain the variation in wolframite composition, from Fe rich to Fe poor, with time.The association of hematite, Fe-rich wolframite, and muscovite in the greisen suggests high f (sub O 2 ) (10 (super -20) -10 (super -30) ) conditions and is supported by Eu behavior during alteration. Sulfides formed after tungsten deposition at temperatures less than 300 degrees C and under f (sub S 2 ) conditions of less than 10 (super -11) .CO 2 -rich fluids are commonly associated with tungsten deposits from a variety of environments and the evidence from the Grey River tungsten prospect shows that CO 2 plays an important role in the transport and deposition of tungsten in the hydrothermal environment.

Mountkeithite, a new pyroaurite-related mineral with an expanded interlayer containing exchangeable MgSO4

AbstractMountkeithite [(Mg,Ni)9(Fe3+,Cr,Al)3(OH)24]3+[(CO{in3,SO4)1.5(Mg,Ni)2(SO4)2(H2O)11]3− occurs with pyrite, magnetite, magnesite, hexahydrite, and morenosite in a low-temperature hydrothermal vein system in the Mount Keith disseminated nickel sulphide deposit, Western Australia. Electron-microprobe analyses gave the following average composition: MgO 31.7, NiO 6.1, CuO 0.1, Cr2O3 6.1, Fe2O3 8.3, Al2O3 2.6, SO3 14.7; microanalysis for C and H gave CO2 3.9, H2O 30.6; total 104.1. The X-ray powder diffraction pattern has the following strongest lines: 11.30(10), 5.63(8), 4.63(2), 3.765(6), 2.645(4), 2.545(4), 1.554(5), and 1.505(3); these can be indexed on a hexagonal cell with a = 10.698 Å, c = 22.545 Å.Mountkeithite occurs as friable aggregates and rosettes composed of soft, pearly to translucent, pale pink to white flakes with perfect basal cleavage. Optical properties are uniaxial negative with ɛ = 1.51 and ω = 1.52; weakly dichroic from colourless to palest pink. Specific gravity is 2.12 (calc. 1.95). Mountkeithite is soluble, with efferves-cence, in dilute HCl. Mountkeithite changes to a pyroaurite-like phase, with a 7.8.Å basal spacing when immersed in water, but can be changed back to an 11.3 Å phase by immersion in 1M MgSO4; mountkeithite also collapses to a 7.8 Å basal spacing under vacuum. Phases similar to mountkeithite have been synthesized by immersing grains of stichtite in 1M MgSO4, at 20, 50, 105, and 200°C.The structure of mountkeithite is interpreted as consisting of positively charged brucite-like hydroxyl layers, containing both divalent and trivalent cations, separated by about 7 Å-thick, negatively charged interlayers containing carbonate and sulphate anions, magnesium sulphate, and water. Carrboydite, motukoreaite, and hydrohonessite are minerals with similarly large basal spacings of 10–11 Å, and are also believed to have pyroaurite-related structures with expanded interlayers.

The Tungsten Queen Deposit, Hamme District, Vance County, North Carolina; a stable isotope study of a metamorphosed quartz-huebnerite vein

The Tungsten Queen mine, Vance County, North Carolina, is a large quartz-huebnerite vein deposit of late Precambrian or early Paleozoic age that has undergone middle Paleozoic regional metamorphism of amphibolite grade. The preservation of large banded huebnerite crystals and the low delta 18 O values in the vein minerals and wall rocks indicate a premetamorphic hydrothermal origin for the vein which involves meteoric waters. Quartz-sericite oxygen isotope distributions in the vein and wall rocks indicate metamorphic temperatures of 550 degrees + or - 50 degrees C. All other isotope distributions except possibly those for hydrogen appear to reflect primary hydrothermal distributions that were not significantly disturbed by the metamorphic event. The stable isotope systematics of the vein are remarkably similar to those of younger huebnerite/wolframite-quartz veins associated with shallow intrusions in erogenic belts. Sulfur isotope fractionations among coexisting sulfides in two mineral assemblages may indicate primary mineralization temperatures of about 260 degrees and 320 degrees C. The hydrothermal sulfur was derived from a deep-seated source. The carbon isotope data suggest that a sedimentary organic component was present in the fluids during carbonate deposition. Except for reequilibration between coexisting quartz and sericite, the 18 O systematics of the vein minerals appear to reflect primary hydrothermal distributions and indicate that the vein was largely a closed system during metamorphism. Hydrous minerals and waters in fluid inclusions in the vein minerals show nearly a 50 per mil range in delta D values which may also reflect primary variations in the hydrothermal vein system.

Archaean precious-metal hydrothermal systems, Dome Mine, Abitibi Greenstone Belt. II. REE and oxygen isotope relations

The Porcupine District, Abitibi Greenstone Belt, is one of the most extensive areas of Archaean auriferous mineralisation. At least two stages of lode-gold emplacement are recognised: the first stage involves gold-bearing carbonate–chert chemical sediments within the lower mafic volcanic sequence; the second stage is represented by auriferous hydrothermal quartz veins which postdate deformation of the greenstone assemblage and transect diverse host rocks.Rare-earth element (REE) concentrations in the stratiform carbonates are typical of the distinctive patterns recorded for Archaean chemical sediments. Chert in these rocks has a δ18O value averaging 17.1‰, implying exchange from heavier 18/16 ratios during diagenesis and metamorphism. Metabasic volcanic rocks and quartz–feldspar porphyry stocks with background gold abundances have mean whole-rock δ18O values of 9.1‰ and 10.7‰ respectively. This enrichment in 18O relative to primary igneous rocks is attributed to oxygen isotope exchange with seawater at low temperatures during fluid transport through the oceanic crust.Quartz in all of the five hydrothermal vein systems present has a δ18O of 14‰ to 15‰, and quartz-muscovite fractionations are 3.4‰ to 3.8‰. Ambient temperatures of mineralisation are estimated to have been 400 °C to 450 °C, from oxygen isotope thermometers, fluid inclusion filling temperatures, and metamorphic mineral assemblages. The calculated δ18O of the mineralising solutions is~10‰, implying fluids of metamorphic origin. REE patterns in hydrothermal quartz veins suggest that they have been derived from high-temperature solutions in equilibrium with source rocks having relatively flat (chondrite normalised) REE distributions, such as tholeiitic and komatiitic volcanics. Adjacent to hydrothermal veins, quartz in igneous rocks approaches isotopic equilibrium with vein quartz, at 15‰, and whole-rock δ18O values for metabasalts shift to ~11‰, implying extensive water-rock interaction. Strong depletions in heavy REE of metabasic schists adjacent to veins provides further evidence for pervasive hydrothermal alteration. The Eu enrichment of all lode gold deposits analysed at Dome Mine is consistent with the reduced state of the solutions involved in their deposition, as recorded by the predominance of Fe2+. The gold-bearing veins are believed to have formed by focussed flow of fluids outgassed at the greenschist–amphibolite transition. Source volumes for Au in the Porcupine District exceed 600 km3, the carrier fluid volume for mineralisation was 60–90 km3, the Au solute concentration in the low nanogram mL−1 range, and transport distances were of the order of 10 km. Such veins may be the precursors of precious-metal-bearing chemical sediments if fluids debouche into the hydrosphere.