Hydrothermal Mineral Assemblages Research Articles

A TEXTURAL AND GEOCHEMICAL GUIDE TO THE IDENTIFICATION OF HYDROTHERMAL MONAZITE: CRITERIA FOR SELECTION OF SAMPLES FOR DATING EPIGENETIC HYDROTHERMAL ORE DEPOSITS

Hydrothermal monazite is one of the most reliable geochronometers for U-Pb dating of epigenetic or hydrothermal ore deposits because its blocking temperature, at ca.700°C, is higher than that of most metamorphic conditions. Although igneous monazite is routinely used for age dating igneous rocks, only a few ore deposits have been dated by hydrothermal monazite. This is due to the similar morphology of hydrothermal and igneous monazite, which prevents the reliable distinction of hydrothermal monazite, in particular. We demonstrate that hydrothermal monazite can be distinguished from igneous monazite by its unique geochemical signature, its local abundance, and to common association with hydrothermal mineral assemblages. Our data and the data of others reported in the literature suggest that the low ThO 2 content of hydrothermal monazite (0–1 wt %) is distinct from that of igneous monazite (3 to >5 wt %) and may be used to determine their genesis.

Henderson Porphyry Molybdenum System, Colorado: II.Decoupling of Introduction and Deposition of Metals during GeochemicalEvolution of Hydrothermal Fluids

Henderson Porphyry Molybdenum System, Colorado: II.Decoupling of Introduction and Deposition of Metals during GeochemicalEvolution of Hydrothermal Fluids

Henderson Porphyry Molybdenum System, Colorado: I.Sequence and Abundance of Hydrothermal Mineral Assemblages, Flow Paths ofEvolving Fluids, and Evolutionary Style

At Henderson, Colorado, 12 rhyolitic stocks of Oligocene age crystallized in three intrusive centers whose tops are 1 km below the surface of Red Mountain: Henderson (oldest), Seriate, and Vasquez (deepest and youngest). A variety of fluorine-rich hydrothermal mineral assemblages are grouped according to their temperatures of formation, either high-, moderately high, moderate-, or low-temperature (the latter three groups are termed lower temperature assemblages). The groupings were based on relative age relationships in areas free of cyclical events, and absolute temperature estimates based on fluid inclusion study later were made for many mineral assemblages, as detailed in a companion paper. Numerous crosscutting contacts of individual stocks combined with limits of associatd high-temperature assemblages can be interpreted as spatially extensive time lines, thereby enabling temporal correlation of spatially separated events. High-temperature silicic (quartz-fluorite) and intense potassic (quartz-K-feldspar-molybdenite) assemblages developed in numerous cycles, each corresponding to emplacement of a stock and deposition of molybdenite. Molybdenite occurs without pyrite mostly in the high-temperature assemblages. In contrast, lower temperature assemblages broadly envelop intrusive centers, rather than single stocks. Some of the moderately high temperature, less intense potassic assemblages (including mottled K-feldspar-quartz and magnetite-K-feldspar) also formed during multiple cycles. Moderate-temperature sericitic assemblages (including topaz with pyrite or magnetite and sericite with pyrite or magnetite) only locally exhibit evidence for multiple cycles, and low-temperature intermediate argillic assemblages (pyrite-clay and several assemblages containing base metal sulfides, F-bearing, Mn-rich garnet, and rhodochrosite) were deposited in only a single event. These lower temperature assemblages terminate 700 m below the present surface. A zone of relatively fresh rocks extends upward and outward from this termination, separating the lower temperature assemblages from the first appearance of propylitic alteration in a more peripheral position, beyond the area of study. At no time did an advanced argillic assemblage form. Lower temperature assemblages are subdivided into two suites on the basis of position: above intrusive centers, formed from fluids that flowed upward out the top of mineralizing stocks from several intrusive centers; and on the flanks of the Seriate center, formed from fluids released from the apex of the Seriate stock and injected laterally and downward along the flanks of the stock. Na- and Na-K-feldspars (± K-feldspar ± topaz ± micas) are members of some assemblages of the second suite. The upper and lower limits of the hydrothermal system, for much of its history, are preserved within the exposures studied. The upper limit of the hydrothermal system rose to 250 m above the apices of the stocks for the first time during development of low-temperature assemblages, which extended higher above Henderson than Seriate. Prior to that time, the region where thermally and chemically evolving, saline hydrothermal fluid was reacting with wall rocks was confined to a vertical interval of approximately 0.5 km. The base of the system rose as moderately high and moderate-temperature assemblages formed but fell rapidly in elevation upon development of low-temperature assemblages. There is no evidence for system-scale convective circulation of fluids during development of the assemblages studied. Sericitic alteration did not begin to form anywhere in the system until after intense potassic alteration had terminated in the vicinity of stocks in both the Henderson and Seriate centers; proximal to stocks, the multiplicity of intrusive events repeatedly reversed the overall trend toward lower temperatures. Sericitic alteration was followed by a late cycle of intense potassic alteration and molybdenite deposition associated with emplacement and crystallization of the Vasquez stock prior to formation of low-temperature assemblages. The hydrothermal system that formed the Henderson orebody can be viewed as a series of thermally retrograding cycles, all except the last of which was truncated by a reversal in the general trend of declining temperatures. Hence, the evolutionary style of Henderson is termed variably cyclical, in contrast to the nearly perfectly cyclical style embodied in the original Climax model for porphyry molybdenum deposits and the nearly perfectly unidirectional style envisaged for many porphyry copper deposits. An evaluation of evolutionary style can lead to geologic tests for geochronologic interpretations and proposed genetic origins of deposits.

Chemical reaction path modeling of hydrothermal mineralization in the Tongonan geothermal field, Leyte (Philippines)

The study of hydrothermal processes in an active geothermal system provides an excellent opportunity to test the equilibrium between the hydrothermal mineral assemblage and present-day fluid. Simulations of selected hydrothermal processes in the Tongonan geothermal system were performed to ascertain the effectiveness of different depositional mechanisms for hydrothermal minerals. Hydrothermal processes, such as boiling, fluid mixing and water–rock interaction were simulated using the reaction path programme CHILLER. Adiabatic boiling was done from 300 to 100 °C. The steam-heated end-member used for the mixing was generated by titrating the vapor phase at 100 °C after boiling of the primary water and mixing with a sample of the groundwater. Water-rock interaction in the Tongonan geothermal field was simulated by titrating andesite into 1 kg of primary water at 300 °C. Adiabatic boiling resulted in the precipitation of a sequence containing paragenetically early traces of gold with quartz, followed by quartz, acanthite, chalcocite and late quartz and base metals. Mixing of the steam-heated water with the boiled primary water resulted in the formation of the ore minerals acanthite, bornite, chalcocite, covellite, galena, pyrite, and sphalerite. Gangue minerals include anhydrite, Mg-chlorite, alunite, kaolinite, muscovite, and quartz. The mineral assemblages predicted using the water–rock interaction calculations provide a close approximation to the hydrothermal alteration commonly observed for measured temperatures of 300 °C. Minerals predicted to precipitate include tremolite, actinolite, diopside, and epidote, together with anhydrite. These minerals are observed to occur in hot regions of the geothermal system. The results of the chemical modeling suggest that the combination of boiling, mixing and water–rock interaction could precipitate the hydrothermal minerals in a Tongonan-like geothermal system. However, the absence of some minerals predicted by the simulation (e.g. acanthite and calcite) suggests that physico-chemical conditions other than those assumed in the simulations are prevailing in the system.

Mobilization and enrichment of high-field strength elements during late- and post-magmatic processes in the Shuiquangou syenitic complex, Northern China

The Shuiquangou syenitic complex consists of melasyenite and leucosyenite. In the northeastern part, there is a small area where the leucosyenite was affected by alteration. The altered leucosyenite contains much higher concentrations of high-field strength elements (HFSE), especially Nb, Zr, REE and Y compared to fresh leucosyenite. REE patterns of the altered leucosyenite show moderate negative Eu anomaly, in contrast to the slightly positive to non-anomalous Eu signature of the fresh syenitic rocks. Allanite from the hydrothermal mineral assemblage in the altered leucosyenite also contains much higher concentrations of REE and Ti than the euhedral magmatic allanites in unaltered leucosyenite. The enrichment of HFSE in altered leucosyenite was interpreted to be caused by the hydrothermal fluids based on mineral chemistry and whole-rock data. Within the fresh leucosyenite, allanite occurs in two habits, euhedral ones indicating a primary magmatic origin and anhedral ones suggesting a late-magmatic origin. The latter has higher concentrations of REE and Ti than the former. The anhedral allanite was presumed to be formed coeval with the alteration of the euhedral allanite as a result of residual magmatic fluid derived directly from the syenitic magma. The alteration of magmatic euhedral allanite and the enrichment of REE and Ti in the anhedral late-magmatic allanite suggest that these elements were mobilized by the residual magmatic fluid. Both residual late-magmatic and hydrothermal fluids have an inferred composition characterized by high alkalic and F contents, suggesting that F may be the most ligand that complexed the high-valence cations.

A hydrothermal system associated with the Siljan impact structure, Sweden--implications for the search for fossil life on Mars.

The Siljan ring structure (368 +/- 1.1 Ma) is the largest known impact structure in Europe. It isa 65-km-wide, eroded, complex impact structure, displaying several structural units, including a central uplifted region surrounded by a ring-shaped depression. Associated with the impact crater are traces of a post-impact hydrothermal system indicated by precipitated and altered hydrothermal mineral assemblages. Precipitated hydrothermal minerals include quartz veins and breccia fillings associated with granitic rocks at the outer margin of the central uplift, and calcite, fluorite, galena, and sphalerite veins associated with Paleozoic carbonate rocks located outside the central uplift. Two-phase water/gas and oil/gas inclusions in calcite and fluorite display homogenization temperatures between 75 degrees C and 137 degrees C. With an estimated erosional unloading of approximately 1 km, the formation temperatures were probably not more than 10-15 degrees C higher. Fluid inclusion ice-melting temperatures indicate a very low salt content, reducing the probability that the mineralization was precipitated during the Caledonian Orogeny. Our findings suggest that large impacts induce low-temperature hydrothermal systems that may be habitats for thermophilic organisms. Large impact structures on Mars may therefore be suitable targets in the search for fossil thermophilic organisms.

Water–rock interaction in the active geothermal system of Pantelleria, Italy

The geothermal fluid sampled in a deep (990 m b.s.l.) well drilled into the peralkaline volcano of Pantelleria consists for the most part of seawater. The fluid is depleted in Ca and Mg and enriched in K, Rb, Cs and F due to interaction with rocks of trachytic composition. A forward geochemical model has been used to simulate the water–rock interaction processes occurring within the geothermal system in the southern part of the island. The simulation assumes different mixtures of seawater with groundwater and volcanic gas. The system model that emerges from the simulation is the following: the seawater flows through fractures and faults, reaching temperatures of over 300°C in the southern part of the island where an upflow zone of volcanic gas is present. Volcanic gases increase the amount of C and S in the system, lower the pH, and enhance the dissolution of trachyte. After reaction with trachyte the pH increases and the fluid reaches saturation conditions with respect to albite, quartz, saponite, K-feldspar and muscovite, in agreement with the natural hydrothermal mineral assemblages. The model confirms that the reservoir fluid is a mix between seawater and meteoric water, more saline fluids possibly existing in deeper levels. The fluid chemistry and the hydrothermal minerals are similar to those found in the Icelandic geothermal systems of Reykjanes and Svartsengi.

CONTRASTING FLUID TYPES AT THE NEVORIA GOLD DEPOSIT IN THESOUTHERN CROSS GREENSTONE BELT, WESTERN AUSTRALIA: IMPLICATIONS OFAURIFEROUS FLUIDS DEPOSITING ORES WITHIN AN ARCHEAN BANDED IRON-FORMATION

Two main hydrothermal mineral assemblages have been identified in the banded iron-formation-hosted gold mine at Nevoria. The earlier quartz ± garnet ± clinopyroxene ± calcite vein group formed pre- or synmetamorphic, and the later quartz ± pyrrhotite ± pyrite vein group appears to be postmetamorphic and related to gold mineralization. Fluid inclusion characteristics are obviously different in those two vein groups. Microthermometric analysis indicates that the fluids with metamorphic alteration are aqueous, CO2-rich or CO2-absent solutions; no or very small amounts of CH4 were involved in this fluid. Mineralizing fluids were a CH4-CO2-H2O solution. The initial auriferous fluids were CH4 dominant. Heterogeneous trapping, interaction of the hydrothermal fluid with graphite-bearing rocks, or fluid mixing may cause large variations of CH4/CO2 ratios or a XCH4 of CH4-CO2-H2O inclusions, particularly in mineralized quartz-pyrrhotite veins. Phase mixing or separating, resulting in an increase in pH and f O2, together with loss of reduced sulfur by mineral-fluid reactions and precipitation of sulfides, led to the breakdown of the gold-transporting complexes.

Petrography, mineralogy and geochemistry of the Zarshuran Carlin-like gold deposit, northwest Iran

Gold mineralisation at Zarshuran, northwestern Iran, is hosted by Precambrian carbonate and black shale formations which have been intruded by a weakly mineralised granitoid. Granitoid intrusion fractured the sedimentary rocks, thereby improving conditions for hydrothermal alteration and mineralisation. Silicification is the principal hydrothermal alteration along with decalcification and argillisation. Three hydrothermal sulphide mineral assemblages have been identified: an early assemblage of pyrrhotite, pyrite and chalcopyrite; then widespread base metal sulphides, lead-sulphosalts and zoned euhedral arsenical pyrite; and finally late network arsenical pyrite, massive and colloform arsenical pyrite, colloform sphalerite, coloradoite, and arsenic–antimony–mercury–thallium-bearing sulphides including orpiment, realgar, stibnite, getchellite, cinnabar, lorandite and a Tl-mineral, probably christite. Most of the gold at Zarshuran is detectable only by quantitative electron microprobe and bulk chemical analyses. Gold occurs mainly in arsenical pyrite and colloform sphalerite as solid solution or as nanometre-sized native gold. Metallic gold is found rarely in hydrothermal quartz and orpiment. Pure microcrystalline orpiment, carbon-rich shale, silicified shale with visible pyrite grains and arsenic minerals contain the highest concentrations of gold. In many ways Zarshuran appears to be similar to the classic Carlin-type sediment-hosted disseminated gold deposits. However, relatively high concentrations of tellurium at Zarshuran, evidenced by the occurrence of coloradoite (HgTe), imply a greater magmatic contribution in the mineralising hydrothermal solutions than is typical of Carlin-type gold deposits.

Geologic Setting, Geochemistry of Alteration, and U-Pb Age of Hydrothermal Zircon from the Silurian Stog'er Tight Gold Prospect, Newfoundland Appalachians, Canada

The early Paleozoic accretionary tectonic regime that was established along the Laurentian margin of the Newfoundland Appalachians provided a favorable setting for shear-hosted gold mineralization along a major terrane suture, the Baie Verte-Brompton Line. The Stog’er Tight prospect is one of several mesothermal-style gold occurrences hosted by shear zones within accreted ophiolites and oceanic arc terranes on the Baie Verte Peninsula. It is an epigenetic, stratabound deposit that is confined to shallow-level gabbro sills within the volcanic cover sequence of the allochthonous Point Rousse ophiolite complex. Gold mineralization at Stog’er Tight is associated with hydrothermal mineral assemblages represented by chlorite-calcite, sericite-ankerite, red albite-pyrite (±Au) and chlorite-magnetite alteration zones. Gold occurs with pyrite within the intensely altered gabbro, along the margins of syn- to late-shear, quartz-rich, replacement veins. The Stog’er Tight gabbro served as a rigid body conducive to shear deformation and fluid penetration, whereas, its high Fe-Ti oxide content induced fluid oxidation and gold-pyrite precipitation. Hydrothermal alteration involved progressive CO 2 , S, Na, and LILE metasomatism, along with significant enrichments in the REE, HFSE, and Th in the high-grade ore zone. Temperatures of vein formation and alteration are constrained by oxygen isotope thermometry to be between 250°C and 480°C. Isotopic compositions of vein quartz from the Stog’er Tight prospect are uniform (δ 18 O = +12.5 ± 1‰) and fall within the range cited for many shear-hosted, auriferous quartz veins. A U-Pb zircon igneous age of 483 +3/−2 Ma for the Stog’er Tight gabbro is consistent with its stratigraphic correlation with other Ordovician ophiolitic and volcanic arc/back-arc assemblages in Newfoundland. The U-Pb age of a rare variety of hydrothermal zircon recovered from the high-grade ore zone directly constrains the timing of gold mineralization to 420 ±5 Ma, in accord with a major orogenic episode of Silurian age that produced many of the magmatic and metamorphic rock suites in north-central Newfoundland. Our results are consistent with a post-peak metamorphic, late-magmatic model for gold mineralization that occurred during the waning stages of Silurian orogenesis, driven by emplacement of I-type granitoid intrusions into the crust.

Estimation and testing of standard molar thermodynamic properties of tourmaline end-members using data of natural samples

Different estimation methods have been combined with calorimetric data for a natural Mg-rich tourmaline and dumortierite, a structurally similar borosilicate, to estimate thermodynamic properties of five tourmaline end-members (dravite, schorl, alkali-free dravite, aluminofoitite, and ferrifoitite). The results were tested by predicting the solubility of Fe-rich tourmaline in natural, low-variance hydrothermal mineral assemblages, for which the B concentration was independently known from Laser Ablation-ICP-MS analysis of coexisting fluid inclusions. Agreement within the uncertainty of experimental and thermodynamic data supports the validity of the estimation methods that used the fictive properties of the IIIB2O3 polyhedron for calculating standard enthalpy and entropy, and reactions involving dumortierite to estimate the heat-capacity functions. This study indicates more generally that the development of new microanalytical techniques for measuring the composition of natural fluid inclusions in well-constrained mineral assemblages represents a promising approach to estimating or testing thermodynamic data for complex silicates.

Noble Gases and Carbon Isotopic Composition of Fluid Inclusions from the Larderello Geothermal Field (Italy), Preliminary Data

The intracontinental geothermal field of Larderello (Tuscany, Italy) is a vapour-dominated system, which has been exploited for electricity production throughout this Century. At Lardereil0, studies on hydrothermal mineral assemblages and related fluid inclusions evidenced the presence of several fluid types of different sources and a complex evolution of the geothermal system, from early stage circulation, related to granite emplacement, to present-day condition (Valori et al., 1992; Cathelineau et al., 1994). In this study we measured N2/Ar ratio and the isotopic compositions of noble gases and carbon of CO2 extracted by undervacuum crushing of fluid inclusions in order to: 1) define the origin of the palaeogeothermal fluids, 2) compare noble gas and CO2 isotopic compositions of the fluid trapped in the inclusions with present-day discharged fluid and evaluate if the source of these fluids has been changed with time. The stratigraphy of the deep geothermal wells (up to 4.5 km below the ground level, b.g.l.) schematically consists of (from top to bottom): 1) a cover of siliciclastic sediments, carbonatic rocks, evaporite and ophiolite rocks (Late Triassic to Pliocene), 2) a complex of tectonic slices which include the lower formation of the sedimentary rocks and the uppermost part of the metamorphic basement, 3) a metamorphic basement (Palaeozoic), consisting of phyllitic-quartzitic rocks, micaschists and gneisses. The deepest wells at Larderello have also encountered contact metamorphic rocks and granite dykes and intrusion with ages between 1.3 and 3.8 Ma. Geophysical data suggest that a large intrusion is present below the Larderello area; the geothermal activity can be related to the presence of this intrusion. The productive horizons were found in Triassic carbonate and anhydrite formations (shallow reservoir) of the upper units and in some permeable levels of the Palaeozoic metamorphic basement (deep reservoir).

The Pantelleria caldera geothermal system: Data from the hydrothermal minerals

This paper proposes, on the basis of petrographic and mineralogic data on cutting and cores from two deep wells (“Pantelleria 1” and “Pantelleria 2”), the first model of the active hydrothermal system of the island of Pantelleria. Phyllosilicates were studied in detail because they are considered key minerals in the identification of hydrothermal processes. The results of these studies emphasize differences between the intracaldera and pericaldera areas of the island. Within the 45 ka caldera there is a high-temperature (240–260 °C at 600–800 m depth) active hydrothermal system with five zones of characteristic alteration minerals with increasing depth. Rocks are unaltered to a depth of 200 m, contain smectite and mixed-layer chlorite-smectite (C/S) between 200 and 380 m, chlorite, illite, chalcedony and quartz from 380 to 500 m, albite, adularia and saponite from 500 to 680 m, mixed-layer biotite-vermiculite from 680 m to the depth drilled (1100). Outside the caldera, but near the rim, a low-temperature and low-permeability (< 140 °C) hydrothermal system is characterized by smectite, dolomite and ankerite at depths of 390 to 650 m, chlorite and calcite at 650–900 m, and mixed layers of chlorite-smectite, illite-smectite and iron carbonates (ankerite, siderite) from 900 m of the well at 1203 m. The superimposition of hydrothermal mineral assemblages is evidence for cooling in the hydrothermal system both inside and outside the caldera. We propose that a high-temperature hydrothermal system developed inside the caldera. In an early stage in the area surrounding the subvolcanic body, biotite isograd is reached and an alkali-metasomatism zone develops inside the body itself. This phase may also account for the development of a chlorite-albite-adularia zone extending to 400 m. A cooling phase (nearly 50 °C) followed, resulting in the substitution of biotite by mixed-layer biotite-vermiculite and by the crystallization of Fe-rich saponite instead of chlorite, within the currently active reservoir. A cooling phase has also been identified in the well outside the caldera.

Hydrothermal alteration in the Aluto-Langano geothermal field, Ethiopia

The hydrothermal mineral assemblages found in eight wells (with a depth range of 1320–2500 m) of the active geothermal field of Aluto-Langano (Ethiopia) indicate a complex evolution of water-rock interaction processes. The zone of upflow is characterized by high temperatures (up to 335°C) and the presence of a propylitic alteration (epidote, calcite, quartz and chlorite, as major phases) coexisting with calcite and clay minerals. The zone of lateral outflow is characterized by mixing of deep and shallow waters and the occurrence of a calcite-clay alteration that overprints a previous propylitic assemblage. Clay minerals have a mushroom-shaped zonal distribution consistent with the present thermal structure of the field. Microprobe analyses have been carried out on chlorite and illite in order to apply several geothermometers. Most of the chlorite is iron-rich chlorite. It is found that the temperatures calculated from the chlorite geothermometer (159–292°C) after Cathelineau and Nieva [ Contrib. Mineral. Petrol. 91, 235–244 (1985)] are in good agreement with in-hole measured temperatures (155–300°C). In the upflow zone, temperatures calculated from this geothermometer (217–292°C), together with fluid inclusion data of Valori et al. [ Eur. J. Mineral. 4, 907–919 (1992)], and computed saturation indices of alteration minerals, indicate thermal stability or slight heating. On the other hand, evidence of a significant cooling process (up to 171°C) in the outflow zone is provided by the comparison between fluid inclusion homogenization temperature (240–326°C) and in-hole temperature (155–250°C). The apparent salinities (0.8–2.3 wt% NaCl eq.) of the fluid inclusions are generally higher than the salinity of the present reservoir fluid (0.29–0.36 wt% NaCl eq.). Clay minerals (illite, smectite, Ill/S mixed layers, vermiculite and chloritic intergrades) generally occur at temperatures consistent with their stability fields.

Modelling of equilibrium and non equilibrium segregation to grain boundaries : R.G. Faulkner (Loughborough University of Technology, Loughborough, UK)

The volcanic geothermal system of Olkaria in Kenya has been extensively drilled for development purposes. The well data show that the reservoir is liquid-dominated. Often downhole temperatures in thermally stabilized wells follow the boiling point curve with depth to at least 2 km. In one part of the field (Olkaria East), a vapor-dominated cap existed above the liquid reservoir when the first deep wells were sunk in the area. Many of the producing wells have excess discharge enthalpy, i.e. the enthalpy is higher than that of steam-saturated water at the temperature of producing aquifers. In some cases, inflow from feed zones in the vapor cap may contribute to the excess enthalpy. Geothermometer results indicate, however, that excess enthalpy is dominantly produced by phase segregation in the depressurization zone around producing wells, the liquid being partially retained in the aquifer by its capillary adsorption onto mineral grain surfaces while the steam flows into the wells. The flashed water discharged from wells is of the Na–Cl or Na–HCO3 types containing 50–1200 ppm of Cl. The most abundant dissolved constituent (average on a molal basis) in the aquifer fluid is CO2, followed by Na, Cl, Si, F, K and H2S. Modelling of aquifer fluid compositions using the phase segregation model reveals that the concentrations of only one of the major reactive components (CO2) may be externally controlled, i.e. by its flux to the hydrothermal fluid. All the other reactive components (Al, Ca, F, Fe, K, Mg, Na, Si, S) are controlled by close approach to local equilibrium with hydrothermal minerals. In the Olkaria West and Domes sectors, the CO2 aquifer fluid concentration is controlled by its flux from the magma heat source, but in other parts of the Olkaria field, it closely approaches equilibrium with the epidote–prehnite–calcite–quartz hydrothermal mineral assemblage. The mineral assemblage pyrite–pyrrhotite–magnetite controls aquifer water H2S and H2 concentrations. Departure from equilibrium is larger for individual hydrothermal minerals than for mineral pairs and especially mineral assemblages. In the case of mineral pairs, the only aqueous species entering the reactions are either a pair of ions or gases, but for minerals assemblages it is a single gas only. If a gas is in equilibrium with a mineral assemblage, all its minerals should also be in equilibrium with the gas. There are several reasons for the observed larger departure from equilibrium between solution and individual minerals than between mineral pairs and assemblages:(1) the stoichiometry of the reactions, (2) in the case of iron, inadequate quality of thermodynamic data on its hydrolysis constants and(3) changes in fluid component concentrations between undisturbed aquifer and wellhead due to precipitation/dissolution reactions. This last mentioned process is most important for components present in low concentration in the fluid. Specifically for Ca-bearing minerals, the apparent under-saturation in the reservoir water is considered to be an artefact caused by removal of Ca from solution by calcite precipitation in the depressurization zone around wells where the water is strongly degassed by its extensive boiling. The initial vapor fraction in the reservoir fluid has been estimated from the modelled H2S and H2 concentrations in that fluid assuming their concentrations to be fixed by specific mineral-solution equilibria in the initial aquifer liquid water. The results indicate that very small amount of vapor is present in the initial aquifer fluid, or 0.03% by weight on average (a few % by volume). This number is thought to carry considerable uncertainty when all the errors and approximations made in the calculations are taken into account. Yet, some vapor must be present, generated by depressurization boiling of rising hot water.

Chemical reaction path modeling of ore deposition in mississippi valley-type Pb-Zn deposits of the Ozark region, U.S. Midcontinent

The Ozark region of the U.S. midcontinent is host to a number of Mississippi Valley-type districts, including the world-class Viburnum Trend, Old Lead Belt, and Tri-State districts and the smaller Southeast Missouri barite, Northern Arkansas, and Central Missouri districts. There is increasing evidence that the Ozark Mississippi Valley-type districts formed locally within a large, interconnected hydrothermal system that also produced broad fringing areas of trace mineralization, extensive subtle hydrothermal alteration, broad thermal anomalies, and regional deposition of hydrothermal dolomite cement. The fluid drive was provided by gravity flow accompanying uplift of foreland thrust belts during the Late Pennsylvanian to Early Permian Ouaehita orogeny.In this study, we use chemical speciation and reaction path calculations, based on quantitative chemical analyses of fluid inclusions, to constrain likely hydrothermal brine compositions and to determine which precipitation mechanisms are consistent with the hydrothermal mineral assemblages observed regionally and locally within each Mississippi Valley-type district in the Ozark region. Deposition of the regional hydrothermal dolomite cement with trace sulfides likely occurred in response to near-isothermal effervescence of CO 2 from basinal brines as they migrated to shallower crustal levels and lower confining pressures. In contrast, our calculations indicate that no one depositional process can reproduce the mineral assemblages and proportions of minerals observed in each Ozark ore district; rather, individual districts require specific depositional mechanisms that reflect the local host-rock composition, structural setting, and hydrology.Both the Northern Arkansas and Tri-State districts are localized by normal faults that likely allowed brines to rise from deeper Cambrian-Ordovician dolostone aquifers into shallower carbonate sequences dominated by limestones. In the Northern Arkansas district, jasperoid preferentially replaced limestones in the mixed dolostone-limestone sedimentary packages. Modeling results indicate that the ore and alteration assemblages in the Tri-State and Northern Arkansas districts resulted from the flow of initially dolomite-saturated brines into cooler limestones. Adjacent to fluid conduits where water/rock ratios were the highest, the limestone was replaced by dolomite. As the fluids moved outward into cooler limestone, jasperoid and sulfide replaced limestone. Isothermal boiling of the ore fluids may have produced open-space filling of hydrothermal dolomite with minor sulfides in breccia and fault zones. Local mixing of the regional brine with locally derived sulfur undoubtedly played a role in the development of sulfide-rich ore runs.Sulfide ores of the Central Missouri district are largely open-space filling of sphalerite plus minor galena in dolostone karst features localized along a broad anticline. Hydrothermal solution collapse during ore deposition was a minor process, indicating dolomite was slightly undersaturated during ore deposition. No silicification and only minor hydrothermal dolomite is present in the ore deposits. The reaction path that best explains the features of the Central Missouri sulfide deposits is the near-isothermal mixing of two dolomite-saturated fluids with different H 2 S and metal contents. Paleokarst features may have allowed the regional brine to rise stratigraphically and mix with locally derived, H 2 S-rich fluids. The Viburnum Trend and Old Lead Belt ores are galena rich with lesser amounts of sphalerite; they replace the most permeable dolostone facies in the Bonneterre Dolomite. Hydrothermal dissolution of host dolostone was concurrent with sulfide deposition, but dolomite deposition occurred episodically between periods of sulfide deposition. The important ore controls in these districts are a variety of sedimentary and geologic features that allowed cross-stratigraphic fluid flow and provided opportunities for fluid mixing. The reaction path which best reproduces the broad features of the Viburnum Trend and Old Lead Belt ores is one in which a dolomite-saturated, lead-rich, zinc- and H 2 S-poor brine mixes with a less saline, H 2 S-rich fluid. The brine became enriched in K, Mg, and Pb and depleted in H 2 S as it flowed through sandstone and redbed aquifers prior to entering the district. This mixing model is consistent with the abundant fluid inclusion and stable isotope evidence for fluid mixing in the districts. Small amounts of cooling associated with the mixing may have contributed to sulfide deposition.

Petrology and alteration geochemistry of the epithermal Balya Pb-Zn-Ag deposit, NW Turkey

The 1.5 km-large hydrothermal system of Balya is characterized by three alteration styles which from the outer halo towards the center are: (i) propylitic alteration with the hydrothermal mineral assemblage of calcite-daphnite-albite-epidote-quartz-pyrite; (ii) argillic/phyllic alteration with the hydrothermal mineral assemblage of sericite/muscovite-kaolinite-rutile-quartz ± pyrite; (iii) advanced argillic alteration with the hydrothermal mineral assemblage of alunite-jarosite-kaolinite-quartz-sericite ± pyrite. Hornblende andesite is the protolith of the hydrothermal alteration system. Enrichment in Si, Sb and Rb, and depletion in Na, Ca, Mg, Fe, Mn, P, Ba, Sr, and Zn distinguishes the argillic/phyllic and advanced alteration types from propylitic alteration and the unaltered hornblende andesite protolith. REE distribution patterns indicate an essentially immobile behaviour of REEs during the alteration cycle. K-Ar age data for unaltered and hydrothermally altered rocks define a synchronous age of 25.3 ± 1.2 Ma for both igneous and hydrothermal activity.

Roter Kamm impact crater, Namibia: Geochemistry of basement rocks and breccias

The Roter Kamm crater in the southern Namib Desert has previously been identified as an impact structure on the basis of crater morphology and the presence of impact melt breccias which contain shock metamorphosed quartz and lithic clasts. To better define the variety of target rocks and breccias, we studied the petrography and chemical composition of a new suite of twenty-eight basement and breccia samples from the Roter Kamm crater. Based on chemical data for target lithologies and breccias we suggest that the crater was formed in a two-layer target region: an upper layer of Gariep metasediments (schist, marble, ± quartzite and sandstone) overlying the crystalline basement of the Namaqualand Metamorphic Complex. The basement was also heavily intruded by coarse-grained quartz veins and quartz- and quartz-feldspar pegmatites. The clast population in the melt breccias indicates that impact-induced melting involved mainly metasedimentary target rocks, with rarely detected contributions from pegmatite and granite/granodiorite. Three varieties of melt breccias can be defined: (1) “schistose,” (2) quartzitic melt breccias, (3) “true” impact melt breccias. These melt breccia types are chemically heterogeneous, and even the impact melt breccias may have been produced in situ and not from a coherent melt body. The shapes of the schistose melt breccias, previously thought to be ejected impact breccias, are most likely caused by erosion, and these breccias are now interpreted to be locally derived. The crater basement as exposed at the rim was structurally severely affected and, at least locally, considerable thermal energy was generated during formation of large volumes of cataclastic, mylonitic, and pseudotachylitic breccias. Analyses of mylonite and pseudotachylites from the crater rim, as well as their respective host rocks, show that these breccias were mainly formed from local material. Analyses of pseudotachylite-like breccias indicate that these possible friction melts are generated by preferential melting of hydrous ferromagnesian minerals and feldspar, similar to their tectonically produced counterparts. Although no significant fluid effects resulting from formation of mylonites or pseudotachylites are indicated, several breccias (compared to their host rocks) do show evidence of severe chemical alteration (chloritisation and sericitisation). The presence of large vesicles filled with hydrothermal mineral assemblages in some schistose breccias and other petrographic and chemical data support the hypothesis of an impact-induced hydrothermal event in the crater area.

Water-rock interaction processes in the Aluto-Langano geothermal field (Ethiopia)

Aluto-Langano is a water-dominated geothermal field in the Main Ethiopian rift system. The hydrothermal mineral assemblage is characterized by the presence of illite-montmorillonite ( T < 150°C) and illite-chlorite (150°C < T < 310°C). In the hottest part of the wells (up to 335°C) the mineral assemblage is characterized by the presence of Ca-Al-silicates (epidote, garnet, actinolite). However these phases also occur as relics in wells where lower temperatures are measured. Calcite, quartz and chlorite are widespread minerals in all the wells, both at low and high temperatures. Fluid inclusion data from quartz and calcite of two wells (LA-3 and LA-6) reveal fluids with an apparent salinity of 1.5–2.0 for LA-3 and 0.85–1.36 wt.% NaCl eq. for LA-6. Homogenization temperatures ( T b) range from 240°C to 350°C. The calculated CO 2 concentration in the liquid phase ranges from 0.07 to 0.66 mol. Thermodynamic properties and phase relations of calc-silicates show that the presence or absence of these minerals is mainly controlled by the P CO2 in the system, even where temperature has played an important role. Fluid inclusion data from two deep wells (LA-3 and LA-6) indicate an increase in temperature and CO 2 content of the geothermal fluid after the trapping of the inclusions. The wells produce an alkali-chloride-bicarbonate water. Enthalpy-chloride and Na/1000-K/100-√Mg diagrams indicate that none of the discharge waters can be taken as representative of the deep fluid in full equilibrium with the reservoir rock, and that important mixing processes characterize even the hottest zone of the system. These mixing processes could be responsible for the continuous dynamic evolution of the hydrothermal system, as evidenced by the authigenic mineralogy.

Geologic and geochemical controls on gold-rich stringer mineralization in the Que River Deposit, Tasmania

The Que River mine in western Tasmania was developed on a small (3.3 million metric tons) but high-grade (>20 wt % Zn + Pb) polymetallic massive sulfide deposit hosted by Cambrian volcanics and volcaniclastic sediments of the Mount Read Volcanics. The massive ore is surrounded by an extensive zone of low-grade stringer vein and disseminated mineralization. This stringer zone formed during hydrothermal alteration of andesitic lavas and related volcaniclastics in the stratigraphic footwall. The hydrothermal alteration mineral assemblage (sericite-silica-pyrite + or - base metal sulfides + or - potash feldspar) survived subsequent lower greenschist-grade regional metamorphism.Part of the stringer zone has elevated Au (up to about 5 ppm), Sb (up to 1,000 ppm), and Ba (up to 2,000 ppm) values. Much of the gold-enriched stringer zone occurs in a coarse polymictic volcaniclastic containing distinctive millimeter-size patches of cream-white alteration. In thin section the cream-white patches are seen to comprise intergrowths of coarse white mica and base metal sulfide. The gold-enriched zone is also characterized by the presence of solid inclusion-rich potash feldspar not observed elsewhere in the stringer zone.The entire stringer zone is crudely zoned with a Cu-bearing core beneath the central part of the massive orebody (Large et al., 1988) surrounded by a Pb-Zn-bearing stringer mineralization which merges with a distal Pb-Zn-Ag-Au-As-Sb-Ba stringer mineralization in the gold-enriched zone. Sulfur isotope signatures show a progression from lightest values in mineral separates from massive ore (delta 34 S pyrite ca. 8ppm) to heaviest values in gold-enriched zone samples (delta 34 S pyrite up to 17ppm).The gold-enriched stringer zone is interpreted to be the more distal part of the footwall alteration zone developed during formation of the main Que River massive sulfide orebody. Best development of this distal gold-bearing stringer mineralization occurred in the more permeable coarse volcaniclastics. It is interpreted that the Au was transported as a bisulfide complex and precipitated in the outer portions of the stringer when fluids were oxidized and cooled during mixing with seawater