Evolution Of Hydrothermal Fluids Research Articles

Tectonic control on hydrothermal circulation and fluid evolution in the Pietratonda–Poggio Peloso (southern Tuscany, Italy) carbonate-hosted Sb-mineralization

This paper deals with the development of an exhumed Sb-ore deposit associated with Neogene–Quaternary magmatism of southern Tuscany. This epithermal mineralization represents a fossil hydrothermal system that was probably active during the volcanic activity that produced rhyolitic lava flows (~ 2.3 Ma), presently exposed close to the study area (Roccastrada volcanic complex). Volcanism was coeval with strike-/oblique-slip and normal faults that controlled the hydrothermal circulation. Faults were active until the Pleistocene. Our study, based on fluid inclusion and structural data, demonstrates that fluids were channeled in damage zones of the Pliocene–Quaternary strike-/oblique-slip and normal faults and permeated within damage zones of Middle–Late Miocene extensional detachments, consisting of fractured carbonate rock masses sandwiched between successions with very low permeability. Fluids produced a diffuse hydrothermal alteration both in the cataclasite and in carbonate damage zones where jasperoid strata bounds developed. Fluid inclusion investigation allowed tracing the evolution of the hydrothermal fluids that is interpreted in terms of a progressive cooling of the hydrothermal system. Such evolution could be related to large infiltration of shallow waters from the early mineralizing stage (fluids at about 200–225 °C, salinity around 3 wt.% NaCl equiv.) down to the later post-ore stages, characterized by more diluted fluids (salinity of about 2.1 wt.% NaCl equiv) having lower temperatures (T h around 155 °C). The latest hydrothermal activity in the area is indicated by fluid inclusions hosted in late calcite veins that are characterized by T h values of about 125 °C. This fossil hydrothermal system may provide a close analog to the active geothermal systems occurring in southern Tuscany, thus the results are useful for better understanding the relationships between brittle structures and fluids migration.

Mineral waters characterization in the Azores archipelago (Portugal)

A study of the hydrogeochemistry of more than 100 mineral waters in the Azores archipelago (Portugal) was conducted on São Miguel, Terceira, São Jorge, Faial, Pico Graciosa and Flores islands. About 75% of the discharges are located at São Miguel, whereas the rest are more equally distributed over the other islands. Springs are dominant (74%), but drilled wells and boiling pools comprise about 10% of the studied discharges. Only 2% of discharges represent hand-dug wells. Results show a large range of chemical characteristics and diverse water types: discharge temperature range between 15 °C and 99.5 °C (median = 35 °C), conductivity between 139 and 43,100 μS/cm (median = 906 μS/cm), and pH between 2.2 and 7.82 (median = 5.78). The dominant water types are Na-HCO 3 and Na-Cl, representing two main trends (“hydrothermal” and “marine”), despite a few samples of the Na-SO 4 type. Relative minor and trace element contents reinforce conclusions drawn from major-ion compositions that major hydrogeochemical processes are: evolution of deep hydrothermal fluids, adsorption of magmatic volatiles, steam-heating of perched-aquifers, and mixing of deep fluids with marine or near-surface hydrothermal fluids.

Oxygen Isotope Compositions of Iron Oxides from High-Grade BIF-Hosted Iron Ore Deposits of the Central Hamersley Province, Western Australia: Constraints on the Evolution of Hydrothermal Fluids

The Hamersley province of northwest Western Australia is one of the world’s premier iron ore regions with high-grade martite-microplaty hematite iron ore deposits mostly hosted within banded iron formation (BIF) sequences of the Brockman Iron Formations of the Hamersley Group. These high-grade iron ores contain between 60 and 68 wt percent Fe, and formed by the multistage interaction of hydrothermal fluids with the host BIF formation. The oxygen isotope compositions of magnetite and hematite from BIF, hydrothermal alteration assemblages, and high-grade iron ore were analyzed from the Mount Tom Price, Paraburdoo, and Channar iron ore deposits. The δ18O values of magnetite and hematite from hydrothermal alteration assemblages and high-grade iron ore range from −9.0 to −2.9 per mil, a depletion of 5 to 15 per mil relative to the host BIF. The δ18O values are spatially controlled by faults within the deposits, a response to higher fluid flux and larger influence on the isotopic compositions by the hydrothermal fluids. The oxygen isotope composition of hydrothermal fluids (δ18Ofluid) indicates that the decrease in the 18O content of iron oxides was due to the interaction of both basinal brines and meteoric fluids with the original BIF. Late-stage talc-bearing ore at the Mount Tom Price deposit formed in the presence of a pulse of 18O-enriched basinal brine, indicating that hydrothermal fluids may have repeatedly interacted with the BIFs during the Paleoproterozoic.

Oxygen Isotope Compositions of Iron Oxides from High-Grade BIF-Hosted Iron Ore Deposits of the Central Hamersley Province, Western Australia: Constraints on the Evolution of Hydrothermal Fluids

The Hamersley province of northwest Western Australia is one of the world’s premier iron ore regions with high-grade martite-microplaty hematite iron ore deposits mostly hosted within banded iron formation (BIF) sequences of the Brockman Iron Formations of the Hamersley Group. These high-grade iron ores contain between 60 and 68 wt percent Fe, and formed by the multistage interaction of hydrothermal fluids with the host BIF formation. The oxygen isotope compositions of magnetite and hematite from BIF, hydrothermal alteration assemblages, and high-grade iron ore were analyzed from the Mount Tom Price, Paraburdoo, and Channar iron ore deposits. The δ18O values of magnetite and hematite from hydrothermal alteration assemblages and high-grade iron ore range from −9.0 to −2.9 per mil, a depletion of 5 to 15 per mil relative to the host BIF. The δ18O values are spatially controlled by faults within the deposits, a response to higher fluid flux and larger influence on the isotopic compositions by the hydrothermal fluids. The oxygen isotope composition of hydrothermal fluids (δ18Ofluid) indicates that the decrease in the 18O content of iron oxides was due to the interaction of both basinal brines and meteoric fluids with the original BIF. Late-stage talc-bearing ore at the Mount Tom Price deposit formed in the presence of a pulse of 18O-enriched basinal brine, indicating that hydrothermal fluids may have repeatedly interacted with the BIFs during the Paleoproterozoic.

Multistage boron metasomatism in the Alamo Complex (Central Iberian Zone, Spain): Evidence from field relations, petrography, and 40Ar/39Ar tourmaline dating

The Alamo Complex consists of structural-metamorphic domes surrounded by low-grade metasedimentary rocks of Upper Proterozoic to Lower Cambrian age that form part of the Schist Graywacke Complex (Central Iberian Zone, Spain). Tourmaline is ubiquitous throughout the domes, in which it occurs in tourmalinites, psammo-pelitic schists, quartzites, gneisses, migmatites, leucogranites, aplo-pegmatites, and quartz veins. Overall, tourmaline compositions can be described within the four component system schorl-dravite-foitite-magnesiofoitite, with K 2 O < 0.23%, low Ca contents, Mg/(Mg+Fe) = 0.25-0.71 and X/(X+Na) = 0.18-0.43, where X = vacancies in the X site. Field relations and petrographic observations, combined with tourmaline 40 Ar/ 39 Ar data, provide evidence of intense boron metasomatism affecting this region. Tourmaline 40 Ar/ 39 Ar step-heating spectra for Cambrian-Ordovician orthogneisses are complex, yielding a pseudo-plateau age of ~370 Ma that is interpreted to reflect Variscan rejuvenation of older tourmaline. Tourmaline 40 Ar/ 39 Ar data for mylonitized and folded tourmalinites yield disturbed spectra with pseudo-plateau ages of ~355-342 Ma that are unsuitable for precise age determination. These ages, however, are consistent with published ages (340-350 Ma) for the second Variscan deformation (D 2 ). Tourmaline from fine-layered tourmalinite and metasedimentary rocks yield well-defined plateau ages of 317 and 315 Ma, respectively, recording an additional metasomatic event concomitant with anatexis and evolution of B-bearing granites, pegmatites, and hydrothermal fluids. The different tourmaline-forming stages reflect significant boron cycling within the continental crust of the Central Iberian Zone, driven by deformation, metamorphism, and magmatism during the Variscan orogeny. Boron-rich aqueous fluids related to Cambro-Ordovician magmatism are considered to be the primary source of boron.

Evolution of Magmatic Vapor to Gold-Rich Epithermal Liquid: The Porphyry to Epithermal Transition at Nevados de Famatina, Northwest Argentina

Fluid inclusions in the subeconomic porphyry Cu-Mo-Au system at Nevados de Famatina and closely associated high-sulfidation epithermal Cu-Au-(Ag-As-Sb-Te) veins at La Mejicana, northwest Argentina, were studied to reconstruct the evolution of hydrothermal fluids from their deep magmatic source to the shallow epithermal environment. Field geology, vein petrography, fluid inclusion microthermometry, and single-inclusion microanalysis by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were combined to determine the evolution of pressure, temperature, and ore metal concentrations in the fluids. Cathodoluminescence imaging complements transmitted light petrography to constrain the successive stages of quartz formation and the entrapment sequence of the fluid inclusion populations. Aqueous liquid inclusions of ~5 wt percent NaCl equiv salinity trapped in quartz-sericite-pyrite (QSP) veins between 360° and 325°C contain unusually high concentrations of Cu (400–5,000 ppm), As (~200 ppm), Sb and Te (both up to 100 ppm), and Au (several ppm) along with other ore-forming elements. These veins are paragenetically transitional between subeconomic porphyry copper mineralization exposed in the valley floor, and high-sulfidation epithermal veins with high Au grades, which are preserved along a high ridge adjacent to the porphyry stock. Low-density vapor and hypersaline liquid (i.e., brine) inclusions were trapped in early-formed quartz of the porphyry stockwork veins, between 450° and >600°C. The brine contains lower Cu and Au concentrations than coexisting vapor inclusions and texturally even earlier inclusions of intermediate density that are trapped in phenocrysts and some stockwork veins; the latter may be equivalent to a parental fluid. The low- and intermediate-density fluids compositionally overlap in salinity with the aqueous liquids recorded by the later transitional quartz-sericite-pyrite veins. Fluid inclusions and geologic time relations indicate that the transitional quartz-sericite-pyrite veins were the channelways for metal-rich aqueous liquids that generated the high-sulfidation epithermal Cu-Au deposit. These mineralizing liquids are interpreted to have evolved by continuous density increase (“contraction”) from a low-salinity, S-rich magmatic fluid of low to intermediate density. This vaporlike fluid was produced by exsolution from a magma that existed at greater depth during the late stages of cooling of the magmatic-hydrothermal complex, and probably underwent minor brine separation prior to reaching the present level of exposure. Epithermal mineral precipitation occurred upon dilution of the low-salinity magmatic fluid with meteoric water, which entered the hydrothermal system as it was cooled and successively eroded during continued magmatic fluid ascent.

Fluid Evolution and Metallogenic Dynamics during Tectonic Regime Transition: Example from the Jiapigou Gold Belt in Northeast China

AbstractThe Jiapigou gold belt, one of the most important gold‐producing districts in China, is located in the northern margin of the North China Craton (NCC). The tectonic evolution of the gold belt is closely related to the Siberian Plate (SP) in the north, Yangtze Craton (YC) in the south and Pacific Plate in the east. In order to investigate the nature of the tectono‐fluid‐metallogenic system, the authors investigated the relationships among the tectonic regimes, fluid evolution and metallogenesis. This paper examined the corresponding spatial–temporal relationship between the ore‐controlling tectonic regime and hydrothermal fluid evolution in the Jiapigou gold belt. There are two types of gold mineralization: disseminated ores that are distributed within the NW‐trending main ductile shear zone and gold‐bearing quartz veins and minor disseminated ores that are distant to the ductile shear zone. The fluid inclusions in quartz contain a large amount of CO2. Metamorphic fluids of middle to high temperatures and pressures and meteoric waters of low temperatures and pressures mixed together during mineralization. A proposed ore‐forming model is as follows: in the pre‐ore phase, the collision of SP and NCC resulted in the NS‐trending compression of the ore belt. This formed the NE‐trending and NW‐trending shear faults and EW‐trending folds. During the ore‐forming phase, the collision of YC and NCC resulted in dextral shearing of the NW‐trending Jiapigou fault and the NE‐trending Green faults. High‐pressure fluids caused by the compression flowed into the dilatant zone. This may have caused both phase separation of CO2‐bearing fluids and the mixing of meteoric waters, metamorphic waters and magmatic source fluids and finally resulted in the disequilibrium of the ore fluids and precipitation of ore minerals.

A geochemical study of the Sweet Home Mine, Colorado Mineral Belt, USA: hydrothermal fluid evolution above a hypothesized granite cupola

Deposition of quartz–molybdenite–pyrite–topaz–muscovite–fluorite and subsequent hübnerite and sulfide–fluorite–rhodochrosite mineralization at the Sweet Home Mine occurred coeval with the final stage of magmatic activity and ore formation at the nearby world-class Climax molybdenum deposit about 26 to 25 m.y. ago. The mineralization occurred at depths of about 3,000 m and is related to at least two major fluid systems: (1) one dominated by magmatic fluids, and (2) another dominated by meteoric water. The sulfur isotopic composition of pyrite, strontium isotopes and REY distribution in fluorite suggest that the early-stage quartz–molybdenite–pyrite–topaz–muscovite–fluorite mineral assemblage was deposited from magmatic fluids under a fluctuating pressure regime at temperatures of about 400°C as indicated by CO2-bearing, moderately saline (7.5–12.5 wt.% NaCl equiv.) fluid inclusions. LA-ICPMS analyses of fluid inclusions in quartz demonstrate that fluids from the Sweet Home Mine are enriched in incompatible elements but have considerably lower metal contents than those reported from porphyry–Cu–Au–Mo or Climax-type deposits. The ore-forming fluid exsolved from a highly differentiated magma possibly related to the deep-seated Alma Batholith or distal porphyry stock(s). Sulfide mineralization, marking the periphery of Climax-type porphyry systems, with fluorite and rhodochrosite as gangue minerals was deposited under a hydrostatic pressure regime from low-salinity ± CO2-bearing fluids with low metal content at temperatures below 400°C. The sulfide mineralization is characterized by mostly negative δ34S values for sphalerite, galena, chalcopyrite, and tetrahedrite, highly variable δ18O values for rhodochrosite, and low REE contents in fluorite. The Pb isotopic composition of galena as well as the highly variable 87Sr/86Sr ratios of fluorite, rhodochrosite, and apatite indicates that at least part of the Pb and Sr originated from a much more radiogenic source than Climax-type granites. It is suggested that the sulfide mineralization at the Sweet Home Mine formed from magmatic fluids that mixed with variable amounts of externally derived fluids. The migration of the latter fluids, that were major components during late-stage mineralization at the Sweet Home Mine, was probably driven by a buried magmatic intrusion.

Role of basement in epithermal deposits: The Kushikino and Hishikari gold deposits, southwestern Japan

The magma–ore deposit relationship of most low-sulfidation epithermal ore deposits is still unclear, partly because many stable isotopic studies of such deposits have indicated the predominance of meteoric waters within hydrothermal fluids. However, it is certainly true that hydrothermal systems are ultimately driven by magmatic intrusions, and epithermal gold deposits might therefore be produced by magmatic activity even in deposits having has no obvious links to a magma. We re-examine the genesis of two typical low-sulfidation epithermal gold deposits, the Kushikino and Hishikari deposits, using structural simulations and isotope data. Many epithermal gold deposits including the Kushikino and Hishikari deposits have been discovered in Kyushu, southwestern Japan. The Kushikino deposit comprises fissure-filling veins within Neogene andesitic volcanics that overlie unconformably Cretaceous sedimentary basement. The veins consist of gold- and silver-bearing quartz and calcite with minor amounts of adularia, sericite and sulfides. Although carbon and oxygen isotopic data for the veins indicate a meteoric origin of the ore fluid, finite element simulations suggest that the vein system might have formed in direct response to magma intrusion. In particular, geophysical data suggest that intruding magma has uplifted the basement rocks, thereby producing fractures and veins and a positive Bouguer anomaly, and providing the heat necessary to drive an ore-forming hydrothermal system. The second component of this study has been to investigate the nature and evolution of the Kushikino and Hishikari epithermal systems. Isotope data document the geochemical evolution of the hydrothermal fluids. We conclude that the existence of sedimentary basement rocks at depth might have affected the strontium and carbon isotopic ratios of the Kushikino and Hishikari ore fluids. The 87Sr/ 86Sr ratios and δ 13C– δ 18O trend reveal that major ore veins in the Hishikari deposit can be distinguished from shallow barren veins. It was suggested isotopically that fluids responsible for the barren veins in nearby shallow and barren circulation systems were only controlled by the shallow host rocks. Such multi-isotope systematics provide a powerful tool with which to determine the center of hydrothermal activity and thereby document the evolution of hydrothermal fluids.

GEOCHEMISTRY AND ORIGIN OF CALCIC TUNGSTEN-BEARING SKARNS, LOS SANTOS, CENTRAL IBERIAN ZONE, SPAIN

The mesozonal, calcic and reduced Los Santos skarn, in the Central Iberian Zone, is a low-tonnage, high-grade tungsten deposit that occurs as discontinuous stratabound lenses enriched in scheelite along the contact between Variscan granodiorite– monzogranite and early Cambrian calcic and dolomitic marble and calc-silicate and pelitic hornfelses. The δ18O values of the garnet and clinopyroxene in the prograde skarn are indicative of δ18Ofluid compositions near 9.5–10.9‰. The compositions of clinoamphibole and biotite of the retrograde skarn suggest equivalent δ18Ofluid values (9.1–12.0‰). These heavy δ18O values indicate that the mineralizing fluids equilibrated with deep crustal rocks, with no significant input of surficial waters, even during the formation of the retrograde skarn. The Sr and Nd radiogenic isotope data for the scheelite (87Sr/86Sr in the range 0.7117–0.7119; eNd300Ma between −9.4 and −8.5), garnet–pyroxene skarn (87Sr/86Sr in the range 0.7119–0.7128; eNd300Ma between −8.5 and −8.7) and plagioclase-rich skarn (87Sr/86Sr in the range 0.7124–0.7129; eNd300Ma between −9.7 and −7.0) trace the complex evolution of hydrothermal fluids circulating through deep perigranitic systems. These signatures, as well as the REE contents, suggest equilibration of an external fluid with the host metasedimentary sequence. The regional geology and the Nd isotopes indicate that the ultimate source of the hydrothermal fluids and the tungsten and fluorine is not the adjacent barren granodiorite–tonalite nor the host metamorphic rocks, but rather an unexposed granitic pluton geochemically equivalent to the (biotite ± tourmaline)-bearing, fine-grained leucogranite dikes that crop out nearby. These are characterized by high 87Sr values (87Sr/86Sr > 0.7112–0.7149) and intermediate eNd signatures (eNd300Ma in the range −5.6 to −4.6) and are similar to those hosting perigranitic W–(Sn) mineralization in other areas of the Variscan Belt. Thus, the geochemical data show that the magmatic fluids extensively interacted with the host aluminosilicate rocks but were not able to precipitate the scheelite and fluorite until the reaction with carbonate rocks. Scheelite from nearby regionally stratabound orebodies have isotopic signatures (87Sr/86Sr in the range 0.7108–0.7110; eNd300Ma = −8.6) similar to those of the Los Santos skarn, strongly suggesting that they are also of Variscan perigranitic origin and not exhalative or synmetamorphic, as had been previously proposed.

Fluid Evolution in the Formation of the Fenghuangshan Cu-Fe-Au Deposit, Tongling, Anhui, China

The Fenghuangshan Cu-Fe-Au deposit is located in Tongling, Anhui province in central-east China, within the Middle-Lower Yangtze River polymetallic belt. The mineralization is related to Late Yanshanian magmatic intrusions emplaced in Lower Triassic limestones and is divided into two episodes. The first and more important, episode A, is related to granodiorite-granodiorite porphyry and characterized by massive, disseminated, veinlet- and breccia-style mineralization in skarn formed at the contact zone, whereas the second, episode B, is related to a quartz monzodiorite porphyry stock which crosscuts the previous skarn and is characterized by disseminated and veinlet-style mineralization within the porphyry. Based on mineral assemblages and their crosscutting relationships, the hydrothermal evolution of episode A is further divided into five substages (A1–A5), and that of episode B into four (B1–B4), among which A4, A5, and B3 are the main copper mineralization stages. This paper focuses on microthermometric studies of fluid inclusions in the various stages, with an aim to characterize the thermal and compositional evolution of hydrothermal fluids and to discuss their implications for mineralization in terms of fluid sources and metal deposition mechanisms. Fluid inclusions were studied in garnet, quartz, and calcite from stages A1, A4, A5, B1, B2, B3, and B4. Four types and 13 subtypes of fluid inclusions were distinguished based on fluid composition and phase assemblages. Type I fluid inclusions contain one or more salt daughter minerals (halite and sylvite) and homogenize into the liquid phase, whereas type II inclusions homogenize into the vapor phase, both commonly containing an opaque daughter mineral. Type III fluid inclusions are aqueous inclusions that do not contain daughter minerals and homogenize into the liquid phase, and type IV inclusions are CO2 bearing, homogenize to the CO2 or the aqueous phase, and commonly contain an opaque daughter mineral. Primary fluid inclusions in stage A1 are of type I and II, interpreted to represent two immiscible phases resulting from boiling, and those in stages A4 and A5 are of type III. Stages B1 and B2 are characterized by type I primary inclusions, and stages B3 and B4 by type III. Type IV inclusions occur as secondary inclusions in B1 quartz and are interpreted to be related locally to B3 stage mineralization. Microthermometric data indicate a trend of decreasing temperatures and salinities from early to late stages for both mineralization episodes, reflecting a change from a magmatic fluid-dominated system to one invaded by fluids from the country rocks and meteoric water. The fact that the main stages of mineralization (A4, A5, B3) coincided with the development of fluid inclusions without daughter minerals of salt suggests that sulfide precipitation did not take place until the system was diluted by cooler fluids with lower salinities. However, the observation that the fluid inclusions hosted in minerals before sulfide mineralization (i.e., stages A1, B1, and B2) commonly contain opaque daughter minerals inferred to be sulfides suggests that the ore-forming components were likely derived from the higher temperature magmatic fluids, and the fluids derived from the country rocks mainly contributed to the precipitation of sulfides through mixing and cooling.

Geochemical evolution of hydrothermal fluids related to polymetallic mineralization in the Gyeongnam mineralized district, Korea

Geochemical evolution of hydrothermal fluids related to polymetallic mineralization in the Gyeongnam mineralized district, Korea

Platinum-Group Element, Stable Isotope, and Fluid Inclusion Investigation of the Ultramafic Rock-Hosted Gunes-Sogucak Ni-Cu-Sulfide Mineralization in the Gunes Ophiolite, East-Central Turkey

Gunes-Sogucak Ni-Cu-sulfide mineralization in the Divrigi district of Sivas Province is genetically related to Late Cretaceous ultramafic rocks. Disseminated mineralization is scattered through the ultramafic rocks, whereas vein-type mineralization is present in fracture zones in them. Both disseminated and vein-type Ni-Cu-sulfide mineralization show enrichment of platinum-group elements (PGE), Au, and Ag relative to primitive mantle. Average metal concentrations of the Cu-Ni-sulfide mineralization are: Ni: 106, Cu: 263 ppm; Ru: 293, Pt: 26, Pd: 126, Au: 87, Ag: 316 ppb. Enrichments in PGE, Au, and Ag may have resulted from the chemical evolution of hydrothermal fluids. δ34SΣS values range from 7.9 to 14.6‰, with most values at the higher end. Judging by the range of sulfur isotope values, a component of sedimentary sulfate evidently was present in the hydrothermal fluids. δD values of waters trapped in quartz crystals range from -69 to -90‰, and δ18O values of quartz range from 11 to 15‰. Based on δ18O and δD data, combined with the geologic setting of the deposits, we propose that the ore fluids were dominated by magmatic waters. Sulfur isotopes indicate that ultramafic rocks assimilated a portion of their sulfur from sedimentary sulfates during ascent through the crust. Fluid inclusion data indicate that hydrothermal mineralization with PGE enrichment formed at a minimum temperature of around 280-300°C during percolation of high-salinity fluids through the ultramafic rocks.

Fluid chemistry and evolution of hydrothermal fluids in an Archaean transcrustal fault zone network: the case of the Cadillac Tectonic Zone, Abitibi greenstone belt, Canada

Detailed fluid geochemistry studies on hydrothermal quartz veins from the Rouyn-Noranda and Val-d'Or areas along the transcrustal Cadillac Tectonic Zone (CTZ) indicate that unmineralized (with respect to gold) sections of the CTZ contained a distinct CO2-dominated, H2S-poor hydrothermal fluid. In contrast, both gold mineralized sections of the CTZ (e.g., at Orenada #2) and associated higher order shear zones have a H2O–CO2 ± CH4–NaCl hydrothermal fluid. Their CO2/H2S ratios indicate H2S-rich compositions. The Br/Cl compositions in fluid inclusions trapped in these veins indicate that hydrothermal fluids have been equilibrated with the crust. Oxygen isotope ratios from hydrothermal quartz veins in the CTZ are consistently 2‰ more enriched than those of associated higher order shear zones, which are interpreted to be a function of greater fluid/rock ratios in the CTZ and lower fluid/rock ratios, and more efficient equilibration of the hydrothermal fluid with the wall rock, in higher order shear zones. An implication from this study is that the lower metal endowment of the transcrustal CTZ, when compared with the higher metal endowment in higher order shear zones (ratio of about 1 : 1000), may be the result of the lack of significant amounts of H2O–H2S rich fluids in most of the CTZ. In contrast, gold mineralization in the higher order shear zones appear to be controlled by the high H2S activity of the aqueous fluids, because gold was likely transported in a bisulfide complex and was deposited during sulfidation reactions in the wall rock and phase separation in the quartz veins.

Primary aqueous fluids in rhyolitic magmas: Melt inclusion evidence for pre- and post-trapping exsolution

This study examines melt inclusions containing bubbles of aqueous fluid (L, V, and L + V), occurring in rhyolites from the Okataina Volcanic Centre, New Zealand, and the Río Blanco Cu–Mo deposit, Chile. We demonstrate that these aqueous fluids coexisted with silicate melts (magmas) and represent either post-trapping exsolution (in the case of Okataina), or co-trapping of phases already coexisting in the magma (in the case of Río Blanco). Microthermometry proves that some of the bubbles are a single-phase aqueous liquid, and all are shown by PIXE analysis to be metal rich saline solutions. As such, these aqueous fluids provide the closest approximation to direct testing of the proposition that cooling magmas exsolve metal-rich aqueous fluid. In the case of pre-trapping exsolution at Río Blanco we show that some inclusions record and preserve magmatic emulsions (melt + aqueous fluid) that are the first stage in the evolution of hydrothermal fluids. We demonstrate that heating experiments on volatile-rich melt inclusions can produce in-situ exsolution of hypersaline metal-rich aqueous fluid bubbles, potentially permitting magma-chamber processes to be experimentally modelled.

Isotopic constraints on fluid evolution and precipitation mechanisms for the Boléo Cu–Co–Zn district, Mexico

Stable and radiogenic isotope composition of stratiform Cu–Co–Zn mineralization and associated sedimentary rocks within the Boleo district of the Miocene Santa Rosalia basin, Baja California Sur, constrains the evolution of seawater and hydrothermal fluids and the mechanisms responsible for sulfide and oxide deposition. Stable isotope geochemistry of limestone and evaporite units indicates a strong paleogeographic influence on the chemistry of the water column. Near-shore limestone at the base of the Boleo Formation is characterized by modified marine carbon (δ13CPDB=−6.0 to +4.4‰) and oxygen (δ18OSMOW=+19.5 to +26.2‰) isotope composition due to the influx of 13C- and 18O-depleted fluvial water. Sulfate sulfur isotope composition (δ34SCDT=+17.21 to +22.3‰ and δ18OSMOW=+10.7 to +13.1‰) for basal evaporite and claystone facies are similar to Miocene seawater. Strontium isotopes are less radiogenic than expected for Miocene seawater due to interaction with volcanic rocks. Low S/C ratios, high Mn contents and sedimentological evidence indicate the basin water column was oxidizing. The oxygenated basin restricted sulfide precipitation to within the sedimentary pile by replacement of early diagenetic framboidal pyrite and pore-space filling by Cu–Co–Zn sulfides to produce disseminated sulfides. Quartz–Mn oxide oxygen isotope geothermometry constrains mineralization temperature between 18 and 118°C. Sulfur isotopes indicate the following sources of sulfide: (1) bacterial sulfate reduction within the sedimentary pile produced negative δ34S values (<−20‰) in framboidal pyrite; and (2) bacterial sulfate reduction at high temperature (80–118°C) within the sedimentary pile during the infiltration of the metal-bearing brines produced Cu–Co–Zn sulfides with negative, but close to 0‰, δ34S values. Isotope modeling of fluid-rock reaction and fluid mixing indicates: (1) sedimentary and marine carbonates (δ13C=−11.6 to −3.2‰ and δ18O=+19.0 to +21.8‰) precipitated from basin seawater/pore water that variably mixed with isotopically depleted meteoric waters; and (2) hydrothermal calcite (δ13C=−7.9 to +4.3‰ and δ18O=+22.1 to +25.8‰) formed by dissolution and replacement of authigenic marine calcite by downward-infiltrating metalliferous brine and brine-sediment exchange, that prior to reaction with calcite, had mixed with isotopically depleted pore water. The downward infiltration of metalliferous brine is inferred from lateral and stratigraphic metal distributions and from the concentration of Cu sulfides along the upper contact of pyrite-bearing laminae. The co-existence and textural relationships among framboidal pyrite, base metal sulfides, carbonate and Mn–Fe oxides (including magnetite) within mineralized units are consistent with carbonate replacement and high-temperature bacterial reduction within the sedimentary pile occurring simultaneously below a seawater column under predominantly oxygenated conditions.

Petrographic Characteristics and Alteration Geochemistry of Granite-hosted Tungsten Mineralization at Degana, NW India

Abstract. Vein type tungsten mineralization at Degana is genetically and spatially associated with the Degana Granite. The deposit is characterized by pervasive wall rock alteration around the mineralized quartz veins. Laterally three different alteration zones, greisen, silicification and potassic zones, are marked based on the field features, mineral assemblages and geo-chemical characteristics. In the present paper, systematic mineralogical and chemical variation in these alteration zones is reported. Thick mono-mineralic (zinnwaldite) selvages around the veins characterize the deposit. Plagioclase and alkali feldspar are low in the greisen zones while K-feldspar shows more increase than plagioclase in the potassic zone. Quartz is uniformly high in all the alteration zones, but it shows an anomalous value in the silicification zone. Al2O3 concentration shows initial depletion in greisen zone with gradual increase away from the contact. MgO and FeO are higher in greisen zone than silicification and potassic zones. The potassic zone is characterized by the depletion of Na2O and higher value of K2O. The common presence of topaz and fluorite as both primary and secondary minerals and fluorine-bearing micas suggest fluorine partitioning in substantial amount between granitic melt and coexisting aqueous fluid phase and higher HF activity during the evolution of hydrothermal fluid. The mutual relationship of the fluorine minerals (topaz and fluorite) in the different alteration zones suggests an increase in the Ca2+ activity and decrease of H+ activity during the fluid evolution from greisenization towards alkali-metasomatised granite and the fluid is assumed to change from low to high activity ratio of Ca2+/H+.

Reply to the comments by Marignac and Cathelineau on the paper by Sánchez-España et al.: Source and evolution of ore-forming hydrothermal fluids in the northern Iberian Pyrite Belt massive sulphide deposits (SW Spain): evidence from fluid inclusions and stable isotopes (Mineralium Deposita 38: 519–537)

Reply to the comments by Marignac and Cathelineau on the paper by Sánchez-España et al.: Source and evolution of ore-forming hydrothermal fluids in the northern Iberian Pyrite Belt massive sulphide deposits (SW Spain): evidence from fluid inclusions and stable isotopes (Mineralium Deposita 38: 519–537)

Comment on the paper by Sanchez-Espa�a et al.: Source and evolution of ore-forming hydrothermal fluids in the northern Iberian Pyrite Belt massive sulphide deposits (SW Spain): evidence from fluid inclusions and stable isotopes (Mineralium Deposita 38:519?537)

Comment on the paper by Sanchez-Espa�a et al.: Source and evolution of ore-forming hydrothermal fluids in the northern Iberian Pyrite Belt massive sulphide deposits (SW Spain): evidence from fluid inclusions and stable isotopes (Mineralium Deposita 38:519?537)

伊豆半島西部の酸性変質作用および明ばん石の地球化学

Acid alteration in the western Izu Peninsula, Shizuoka Prefecture, is distributed widely within Miocene and Pliocene formations, and overlain by Quaternary volcanoes. For the purpose of understanding its origins, we examined the acid alteration and the property of alunite from several locations throughout the district. The acid alteration zone consists of acid-leached andesite, vuggy silica and massively silicified rocks, surrounded by advanced and intermediate argillic alterations. They are underlain by district-wide propylitic alteration. The advanced argillic alteration is characterized by an assemblage of pyrophyllite, diaspore and alunite. The alunite has cores rich in Sr and PO4. Oscillatory zoning of K, Na and Ca in alunite, natroalunite and minamiite is found. δ34S values are +18 to +26 per-mil. The oscillatory zoning is ascribed to the variation in the composition of hydrothermal fluid precipitating the alunite crystals, whereas the increased trend in SO4/PO4 from core to margin reflects the chemical evolution of hydrothermal fluid. The mineral assemblage and isotopic data suggest that the acid alteration in Ugusu district was formed from higher temperature fluid, which contained reactive magmatic components, while the acid alteration in Shotsubotoge-Nishina district was formed from lower temperature fluid.