Effects of hydrothermal alteration on Pb in the active PACMANUS hydrothermal field, ODP Leg 193, Manus Basin, Papua New Guinea: A LA-ICP-MS study

The conventional model of leaching volcanic rocks as a source of metals in a seafloor hydrothermal systems has been tested by examining the behavior of Pb and other trace elements during hydrothermal alteration. ODP Leg 193 drill sites 1188 (Snowcap) and 1189 (Roman Ruins) on Pual Ridge in the eastern Manus Basin offshore eastern Papua New Guinea provide a unique three-dimensional window into an active back-arc hydrothermal system. We investigate by means of a LA-ICP-MS microbeam technique the capacity of Pb to be leached from a host volcanic rock exposed to various types and intensities of alteration. Our results are in general agreement with previous studies that utilized bulk analytical techniques but provide a more detailed explanation of the processes.Fresh representative dacitic lavas from the Pual Ridge have an average whole rock Pb content of 5.2 ppm, an average interstitial glass Pb content of 5.6 ppm and an average plagioclase Pb content of 1.0 ppm. Altered matrix samples have highly variable Pb values ranging from 0 to 52.4 ppm. High Pb values in altered samples are associated with a low temperature chlorite and clay mineral assemblage, in some cases overprinted by a high temperature (up to 350 °C) silica-rich “bleaching” alteration. Only the most highly altered matrix samples have REE patterns that differ from the fresh Pual Ridge dacite. This may represent either different lava histories or alteration characteristics that have affected normally immobile REEs. Altered samples with the highest Pb values have similar REE patterns to those of the local unaltered lavas. They are compositionally similar to typical Pual Ridge dacites indicating a genetic relationship between the main regional volcanic suite and the subseafloor hydrothermally altered, Pb-enriched material.Relative loss/gain for Pb between the analyzed altered samples and a calculated precursor show a maximum relative gain of 901%. Samples with relative Pb gain from both drill sites are associated with lower temperature alteration mineral assemblages characterized by pervasive chloritization. The related lower temperature (220-250 °C) neutral to slightly acidic fluids have been ascribed by others to return circulation of hydrothermal fluids that did not interact with seawater. Because altered samples have a higher Pb content than the fresh precursor, leaching of fresh volcanic rocks cannot be the source of Pb in the hydrothermal systems.     

Variations in composition and abundance of white mica in the hydrothermal alteration system at Hellyer,Tasmania, as revealed by infrared reflectance spectroscopy

Short-wave infrared (SWIR) spectral reflectance of hydrothermally altered volcanic rocks in the footwall of the Hellyer massive sulfide deposit was measured with a portable PIMA-II infrared spectrometer. The Al–OH band was used to derive information on the octahedral Al content and the abundance of white mica (sericite) in the hydrothermal alteration and mineralization system. The range of the Al–OH band wavelength from 2192 nm to 2222 nm corresponds to the number of octahedral Al (Al^vi) in white mica approximately from 3.9 to 3.0 (based on 4 octahedral cations per formula). This Al^vi range represents a significant compositional variation, covering most of the compositional region between muscovite (Al^vi = 4.0) and phengite (Al^vi = 3.0). Furthermore, the spectral reflectance data show that the compositional variation of white mica is spatially related to hydrothermal alteration zoning, such that phengitic white mica tends to occur in 1) main upflow fluid channel, 2) intensely altered volcanic rocks, and 3) Pb–Zn mineralization, whereas muscovitic white mica was formed preferentially distal to massive sulfide mineralization on the margin of the footwall alteration system. The results suggest that the Al–OH band wavelength, and therefore the octahedral Al content, of white mica can be used as vectors to mineralization to map the hydrothermal system at Hellyer.     

Characterization of hydrothermally generated oil from the Uzon caldera,Kamchatka

This paper reports the analyses of unusual oils that accumulate in the Uzon Caldera, situated in the central volcanic region of Kamchatka, Russia. Gas chromatography–mass spectrometry (GC–MS) was used to determine the primary constituents, and the ¹³C and ¹⁴C compositions provided information about the potential source and age of the oils. The ¹⁴C ages determined are 1030 ± 40 a BP (measured) or 940 ± 40 a BP (conventional). The δ¹³C value is −30.6‰ versus the PDB standard, a value consistent with a biological origin. The nearly contemporary age of the C content indicates a geologically recent origin from biogenic detritus and not by synthesis from mantle C. The biogenic origin is supported by the presence of sterane and hopane biomarkers and the δ¹³C value of the bulk oil. The overall compositions of the oils indicate that they are derived from rapid hydrothermal alteration of algal/bacterial mat detritus buried by volcanic ashfall deposits of the Uzon Caldera. The oils represent the youngest hydrothermal petroleum reported to date.     

REE and HFS element behaviour in the alteration facies of the Erenler Dağı Volcanics (Konya,Turkey) and kaolinite occurrence

Mainly high-K, calc-alkaline, Late Miocene to Pliocene volcanic rocks cropped out of the Konya area in Central Anatolia, Turkey. The volcanic rocks are predominantly andesitic to dacitic in composition and rarely basalt, basaltic andesite, basaltic trachyandesite and pyroclastics. Kaolinite, illite, Ca-montmorillonite, alunite, jarosite, minamiite and silica polymorphs were formed by widespread and intense hydrothermal alteration in or around the volcanic products. To investigate the effects of hydrothermal alteration on the chemistry of volcanic rocks, the whole rock chemical composition (major and trace elements, including rare-earth elements (REE) was analysed. The results of the study demonstrate notable differences in the REE behaviour in the different sample groups. REE trends of fresh parent rocks to weakly-, moderately-altered, kaolinitic and alunitic rocks are characterised by strong LREE enrichment ((La/Lu)_cn = 14.57, 11,8 to 15.20, 4.54 to 13.30, 12.5 to 24.2 and 34.6 to 47.26, respectively). Most of the samples have pronounced negative and/or weakly-negative Eu anomalies ranging from 0.75 to 0.98 while three samples have weakly-positive Eu anomalies. LRE element contents are higher than those of HREE in the samples. The LRE elements were strongly enriched in the kaolinitic and alunitic alteration; in weakly- and moderately-altered rocks. LREE are nearly immobile whereas HRE elements show different behaviour in different rock groups. The HFS and TRT elements are slightly mobilised in weakly-altered rocks, but enriched in other alteration types. Elements commonly assumed to be immobile (e.g. Y, Zr, Nb, Hf, TiO₂, Al₂O₃, REE) show variation in mass calculation. LIL elements showed enrichment over LREE and MREE, and similar behaviour, in contrast with HFSE. A clear increment of trans-transition elements (TRTE) was found mainly in alunitic and partly in kaolinitic samples.     

Proterozoic volcanism in the Jack Hills Belt,Western Australia: Some implications and consequences for the World's oldest zircon population

Rare felsic volcanic rocks of dacitic to rhyolitic composition occur in the central part of the Jack Hills metasedimentary belt in the Narryer Terrane of Western Australia, interleaved with clastic sedimentary rocks and amphibolite. Representative samples of the four identified felsic volcanic units reveal a similar complex pattern of zircon age distribution, with all samples containing zircon populations at ∼3.3–3.4, ∼3.0–3.1, ∼2.6 and ∼1.8–1.9 Ga. The ∼3.3–3.4 Ga zircons show well-developed oscillatory zoning in cathodoluminescence (CL) images and are interpreted as inherited igneous zircon derived from granitic precursors, similar to the ∼3.3 Ga trondhjemitic granitoids currently exposed along the northern and southern margins of the belt. The ∼3.0–3.1 Ga zircons also reveal well-developed oscillatory zoning in CL and are most likely derived from granitoid and/or volcanic rocks of this age, as recorded in the Murchison domain to the south and possibly also present in the Narryer Terrane. The ∼2.6 Ga population matches the age of nearby late Archean granitoids intruding the Jack Hills belt and their oscillatory zoning and U–Th chemistry is consistent with their origin from such a source. The youngest discrete group of zircon grains, with ages ranging from ∼1970 to ∼1775 Ma, show strong oscillatory zoning and average Th/U ratios of 0.76, features consistent with an igneous origin. These younger zircons are therefore interpreted as defining the age of crystallisation of the volcanic rocks. These results establish that the Jack Hills metasedimentary belt contains significant post-Archean components. Taken together with similar results obtained from zircon occurring as detrital grains in clastic sedimentary rocks at Jack Hills, these results overturn the generally-accepted view that the belt is entirely Archean in age and that sedimentation was completed around 3.0 Ga ago. Instead, there is a distinct possibility that much of the material currently exposed in the Jack Hills belt formed in the Proterozoic. A further implication of this study is that the metamorphism affecting these rocks also occurred in the Proterozoic and consequently the rocks should not be considered as forming an Archean greenstone or metasedimentary belt. The paucity of zircons >4 Ga in the known Proterozoic sedimentary rocks and their total absence in the felsic volcanic rocks suggests that such ancient source rocks were no longer present in the area.     

Porphyry to epithermal transition in the Agua Rica polymetallic deposit, Catamarca, Argentina: An integrated petrologic analysis of ore and alteration parageneses

Agua Rica (27°26′S–66°16′O) is a world class Cu–Au–Mo deposit located in Catamarca, Argentina. In the E–W 6969400 section examined, the Seca Norte and the Trampeadero porphyries that have intruded the metasedimentary rock are cut by interfingered igneous and hydrothermal heterolithic and monolithic breccias, and sandy dikes. Relic biotite and K-feldspar of the early potassic alteration (370° to > 550 °C) with Cu (Mo–Au) mineralization are locally preserved and encapsulated in a widespread, white mica + quartz + rutile or anatase halo (phyllic alteration) with pyrite + covellite that suggests fluids with temperatures ≤ 360 °C and high f(S₂). The Trampeadero porphyry and the surrounding metasedimentary rock with phyllic alteration have molybdenite in stringers and B-type quartz veinlets and the highest Mo grades (> 1000 ppm).Multistage advanced argillic alteration overprinted the earlier stages. Early andalusite ± pyrite ± quartz is preserved in the roots of the argillic halo rimmed by an alumina–silica material and white micas. This alteration assemblage is considered to have been formed at temperatures ≥ 375 °C from condensed magmatic vapor. At higher levels, pyrophyllite replaces muscovite and illite in clasts of hydrothermal breccias in the center and east sector of the study section, suggesting temperatures of 280 to 360 °C. Clasts of vuggy silica in the uppermost levels of the central breccia, indicates that at lower temperatures (< 250 °C), fluids reached very low pH (pH < 2). In this early stage of the advanced argillic alteration, hydrothermal fluids seem to have not precipitated sulfides or sulfosalts.Hydrothermal brecciation was concurrent with fluid exsolution (↑? V), which precipitated intermediate-temperature advanced argillic alunite (svanbergite + woodhouseite) ± diaspore ± zunyite as breccia cement along with abundant covellite + pyrite + enargite ± native sulfur ± kuramite at intermediate depths and in lateral transitional zones to unbrecciated rocks. This mineral assemblage indicates temperatures near 300 °C, oxidized and silica-undersaturated hydrothermal fluids with high sulfur fugacity to prevent gold precipitation. Multiple generations of pyrite, emplectite, colusite, Pb- and Bi-bearing sulfosalts, and native sulfur with Au and Ag, accompanied by alunite introduction in the upper level breccias, probably occurred at lower temperatures, but still high sulfur and oxygen activity. An independent Zn and Pb (as galena) mineralization stage locally coincides with Au–Ag and sulfosalts, and advanced at depth, controlled by fractures and overprinting much of the previous mineralization. A later paragenesis of veinlets of alunite + woodhouseite + svanvergite + pyrite ± enargite that cut the phyllic halo suggests temperatures ~ 250 °C and without woodhouseite + svanvergite, temperatures ~ 200 °C. Kaolinite occurs in the phyllic halo as a late mineral in clots and in veinlets thus, in this zone, the fluid had cooled enough for its formation.     

Hydrothermal alteration in andesitic volcanoes: Trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)

The mineralogy and the trace element compositions of hydrothermally-altered volcanic materials collected from ash fall deposits and in four debris-avalanche deposits (DADs) at La Soufrière volcano in Guadeloupe have been determined. Phreatic explosions of the 1976 eruption and flank collapse events have sampled various parts of the active and ancient hydrothermal systems of the volcano. Hydrothermal mineral assemblages (smectite + silica polymorphs ± pyrite/jarosite ± gypsum) are typical of rock alteration by low-temperature acid-sulphate fluids. High-temperature mineral assemblages are rare, indicating that phreatic explosions and flank collapse events have sampled mainly the upper parts of the volcanic edifice.Andesitic eruptive products affected by shallow hydrothermal alteration are complex assemblages of volcanic materials (glass, phenocrysts and xenocrysts with complex magmatic histories) of different ages and compositions. The use of incompatible element ratios and REE compositions normalised to an unaltered reference material overcomes the interpretation difficulties related to mass balance effects of alteration processes and the petrologic heterogeneity of the initial material.REE and other incompatible elements (Th, U, Hf, Zr) are mainly concentrated in the glassy matrix of unaltered andesitic rocks. Secondary S-bearing mineral phases (e.g., gypsum, jarosite) that have precipitated from acid-sulphate fluids do not contain substantial incompatible elements (REE, U, Th, Hf, Zr). Compositional variations of incompatible elements in hydrothermally-altered andesitic materials reflect mainly volcanic glass–smectite transformation, which is characterised by (i) strong depletion of alkalis and alkaline earths (Ba, Sr) and first transition series elements (Zn, Cu, Cr, Co, Ni), (ii) immobility of highly incompatible elements (Th, Zr, Hf, LREE) and (iii) strong depletion of MREE and HREE. The sigmoid shape of normalised REE pattern is characteristic of glass–smectite transformation by low-temperature acid-sulphate fluids. This transformation also produces significant variations in U/Th values, which offer the opportunity to date the cessation of hydrothermal alteration and to reconstruct the evolution in space and time of hydrothermal activity in a volcanic edifice.     

福建松溪大林坑钼多金属矿床地质特征及成因初探

松溪大林坑钼多金属矿床位于浦城-宁德北西向钼多金属成矿带的中段,矿体产于大林坑破火山口的北东侧,辉钼矿主要分布在近火山口的南园组火山岩的裂隙和放射状断裂中,钼矿体呈等轴圆状,而铅锌矿体则呈脉状赋存在远离火山口的放射状断裂中,围岩蚀变以中低温面状蚀变为主,具水平分带和垂直分带,成矿作用与隐伏中酸性（偏碱性）斑岩关系密切,属于火山热液-斑岩型矿床成因类型.     

Mineralogy,geochemistry, and structural controls of a disseminated gold-bearing alteration halo around the schist-hosted Bullendale orogenic gold deposit,New Zealand

Orogenic gold-bearing quartz veins in the middle Tertiary Bullendale Fault Zone, New Zealand were mined historically for coarse gold in a narrow zone (ca. 5 m thick). However, recent drilling has revealed a broad hydrothermal alteration zone extending into the host schist, in which disseminated sulphide and gold mineralisation has occurred. The evidence of alteration is first seen over 150 m across strike from the fault zone, and the best-developed alteration halo is about 50 m wide. The extent and intensity of alteration is strongly controlled by local structures that developed during regional Tertiary kink folding of the pervasively foliated and fissile metasedimentary schist host. The earliest structures are foliation-parallel microshears (micron to millimeter scale) formed during flexural-slip folding. Later, but related, structures are predominantly normal faults and associated shear zones that have formed extensional sites during the regional folding event. All these structures facilitated hydrothermal fluid penetration and rock alteration, with localised vein formation and brecciation. Where fluid has followed structures, metamorphic chlorite, phengite, and titanite have been altered to hydrothermal ankerite, rutile, and muscovite or kaolinite. Ankerite with Fe/(Fe + Mg) < 0.4 formed in host rocks with Fe/(Fe + Mg) of 0.6, and iron released by ankerite alteration possibly formed pyrite and arsenopyrite that host disseminated gold. Fault zones were extensively silicified and veined with quartz, albite, sulphides, and gold. Host rocks have wide compositional variations because of centimeter-scale metamorphic segregation. However, the alteration halo is characterised by elevated CO₂ and S, as measured by loss-on-ignition (doubled to ca. 6 wt.%), elevated As (100–10,000 ppm), and weakly elevated Sb (up to 14 ppm). Strontium is elevated and Ba depleted in many altered rocks, so Sr/Ba ratio increases from < 1 (host rocks) to > 3 in the most altered and silicified rocks. Many altered and mineralised rocks have low Sr/Ba (< 0.5) as well. The subtle geochemical signature is not useful as a vector to ore because of the strong microstructural control on alteration. Likewise, there is no evidence for spatial mineralogical zonation across the alteration halo, although the most intense alteration is centred on the main fault zone, and intensity of alteration is controlled by microstructures at all scales. As documented in previous studies, hydrothermal alteration haloes enlarge the exploration target for some orogenic gold deposits, and may include disseminated gold, as in this Bullendale example.     

Initial behavior of granite in response to injection of CO2-saturated fluid

To understand the initial reactions of granite in a CO₂-saturated hydrothermal system, experiments were conducted using a batch-type autoclave over a temperature range of 100–350 °C at up to 250 bar and numerical computations of phase equilibria based on the experimental results were carried out. The experiments showed that the dissolution of granite and the deposition of secondary minerals were encouraged by the addition of CO₂. Solution chemistry and examination of the granite’s surface texture suggested that its initial dissolution is characterized by the release of Na and Ca (from the dissolution of plagioclase) and that initial precipitation occurs by deposition of some secondary minerals on to plagioclase and/or biotite in the CO₂-saturated system. However, the effect of CO₂ was small at 350 °C owing to the low activity of H₂CO₃. According to EDX analysis and numerical phase equilibrium calculations, the secondary minerals formed might be kaolinite, muscovite, smectite and calcite. That is, the granite as a whole might have the potential to take-up dissolved CO₂. The results suggest that the alteration of granite under CO₂-saturated hydrothermal conditions has the potential to capture CO₂ when it is injected at moderate temperatures (150–250 °C) into granite-hosted rock masses.     

Remobilisation of deep Na-Cl waters by a regional flow system in the Alps: Case study of Saint-Gervais-les-Bains (France)

The hydrothermal system of Saint-Gervais-les-Bains, France is located in a south western low-elevation point of the Aiguilles Rouges crystalline Massif. The crystalline rocks are not directly outcropping in the studied area but certainly exist beyond 300 m depth. Uprising waters are pumped from two different aquifers below the Quaternary deposits of the Bon Nant Valley. In the Lower Trias-Permian aquifer crossed by De Mey boreholes (27–36 °C), the ascending Na-SO₄ and high-Cl thermal water from the basement (4.8 g/L) is mostly mixed by a Ca-SO₄ and low-Cl cold water circulating in the autochthonous cover of the Aiguilles Rouges Basement. The origin of the saline thermal water probably results from infiltration and circulation in the basement until it reaches deep thrust faults with leaching of residual brines or fluid inclusions at depth (Cl/Br molar ratio lower than 655). The dissolution of Triassic halite (Cl/Br > 1000) is not possible at Saint-Gervais-les-Bains because the Triassic cold waters have a low-Cl concentration (< 20 mg/L). Water–rock interactions occur during the upflow via north–south strike-slip faults in the basement and later on in the autochthonous cover. For the De Mey Est borehole, gypsum dissolution is occurring with cationic exchanges involving Na, as well as low-temperature Mg dissolution from dolomite in the Triassic formations. The aquifer of imbricated structures (Upper-Middle Trias) crossed by the Lépinay well (39 °C) contains thermal waters, which are strongly mixed with a low-Cl water, where gypsum dissolution also occurs. The infiltration area for the thermal end-member is in the range 1700–2100 m, close to the Lavey-les-Bains hydrothermal system corresponding to the Aiguilles Rouges Massif. For the Ca-SO₄ and low-Cl end-member, the infiltration area is lower (1100–1300 m) showing circulation from the Mont Joly Massif. The geothermometry method indicates a reservoir temperature of probably up to 65 °C but not exceeding 100 °C.     

Metallogeny and environmental impact of Hg in Zn deposits in China

Zinc smelting is currently regarded as one of the most important atmospheric Hg emission sources in the world. In order to assess the potential environmental impacts of Hg from Zn smelting in China, the distribution of total Hg concentration (HgT) in Zn concentrates (ZCs) from 100 Zn deposits in China was investigated. It was found that HgT varies depending on the ore types and their geneses. Zinc concentrates from sedimentary-exhalative deposits (SEDEX, geometric mean = 48.2 μg/g) have the highest HgT. The possible explanation is that the sources of mineralizing solutions for SEDEX deposits are deep formational brines in contact with sedimentary rocks, and there are much higher background Hg contents in sedimentary rocks. Zinc concentrates from volcanic hosted massive sulfide deposits (VMS, geometric mean = 11.5 μg/g) and Mississippi Valley-Type (MVT, geometric mean = 10.1 μg/g) deposits have intermediate HgT. VMS may receive most of their Hg from fluid–rock interaction and/or by direct input of gaseous Hg from a mantle source. However, the source of metals within MVTs may be the low-temperature hydrothermal solution formed by diagenetic recrystallization of the carbonates. Intrusion related deposits (IRs) have the lowest HgT (Geomean = 2.4 μg/g), and the dispersion of Hg in the IRs seems to be influenced by the temperature of ore formation and/or the nature of wall–rock alteration. Finally, it was estimated that the annual Hg emission to the atmosphere from Zn smelting in China was about 107.7 tons in 2006.     

Influence of pH and temperature on the early stage of mica alteration

Mineral dissolution and precipitation reactions actively participate in controlling fluid chemistry during water–rock interaction. In this study, the changes in the biotite and muscovite basal surface nano-morphology were evaluated during interaction with fluids of different pH (pH = 1.1, 3.3 and 5.7) at different temperatures (T = 25°, 120°, and 200 °C). Results show that at the nanometre scale resolution of the atomic force microscope (AFM), dissolution generates etch pits with a stair-shaped pattern over the (0 0 1) surface. The flux of dissolved elements decreases when pH increases. However, at pH 5.7, a change was found in the flux after 42 h of reaction when abundant gibbsite and kaolinite coat the dissolving mineral surface. This phenomenon was widely observed at edges of the etch pits by AFM. It was also found that an increase in temperature produces an enhancement in the elemental flux in both micas. Dissolution regime changes after less than one day of interaction at high temperature because of abundant coating formation over the etch pits and edges. The results demonstrate the key role of nanometre size neogenic phases in the control of elemental flux from mica surfaces to solution. The formation of nanometre size coatings, blocking the sites active for dissolution, appears to control the alteration of phyllosilicates even at the early stage of the interaction.     

Uranium and arsenic dynamics in volcano-sedimentary basins – An exemplary study in North-Central Mexico

Uranium and As in deep groundwater of the volcano-sedimentary Villa de Reyes Graben around the city of San Luis Potosí in semi-arid North-Central Mexico (mean U: 7.6 μg L⁻¹, max. 138 μg L⁻¹; mean As: 11.4 μg L⁻¹, max. 25.8 μg L⁻¹) partly exhibit concentrations in excess of the WHO guideline values and thus endanger the quality of the most important drinking water source. To unravel the mechanisms for their enrichment in groundwater, the potential trace element sources, volcanic rocks and basin fill sediments, were characterized. A total of 131 solid and liquid samples were analyzed for major and trace element composition. The As/U hydrogeochemical signatures, their behavior during rock alteration and evidence from other major and trace element distributions, especially rare earth elements, strongly argue for dissolution of acid volcanic glass to be the dominating process of U and As release into groundwater. This natural baseline quality representing water–acid volcanic rock interaction is modified by additional trace element (preferentially As) mobilization from the sedimentary basin fill, representing a secondary source, in the course of decarbonatization of playa lake sediments and desorption from Fe-(hydr)oxide coated clastic material. The common behavior of both elements during magmatic differentiation and growing drift apart in sedimentary environments are important findings of this work. Comparison with recent findings in a similar environment suggests a common primary trace element source identification but significant differences in the evolution of As and U distribution. Geological and climatic similarity to numerous volcano-sedimentary basins makes the findings useful for water management purposes and transferable to other semi-arid regions facing challenges of geogenically impacted drinking water quality.     

浙江永嘉下龙银金矿蚀变分带及矿床成因

浙江永嘉下龙矿区属火山岩型金属矿床。分析下龙矿区银金地质特征和蚀变分带,发现矿区蚀变垂直分带有一定的规律性,自上而下分为绿泥石蚀变带、石英-绢云母-黄铁矿蚀变带、黄铁矿-绿泥石-碳酸盐蚀变带3个带,其中石英-绢云母-黄铁矿蚀变带蚀变在空间分布上与金银矿体分布一致,成因上与成矿密切相关,是该区良好的直接找矿标志。成矿作用与金银丰度较高的西山头组控矿层位、构造、潜火山岩侵入密切相关,矿床成因为中低温火山热液蚀变型矿床。研究其地质特征和成矿规律,可为该地区寻找同类型矿床提供借鉴。     

Hydrothermal alteration of granite by meteoric fluid: an example from the Carnmenellis Granite,United Kingdom

The interaction of granitic rock with meteoric fluid is instrumental in determining the chemistry of pore fluids and alteration mineralogy in downflow portions of convective groundwater circulation cells associated with many hydrothermal systems in the continental crust. Hydrothermal experiments and a detailed mineralogical study have been carried out to investigate the hydrothermal alteration of the Carnmenellis Granite, Cornwall, UK. Samples of drill chippings from a borehole 2 km deep in the Carnmenellis Granite have been reacted with a dilute Na-HCO₃-Cl fluid in hydrothermal solution equipment at temperatures of 80°, 150° and 250° C and a pressure of 50 MPa, with a water/rock mass ratio of 10, for experiment durations up to 200 days. Fluid samples were analysed for seventeen different chemical components, and solids were examined prior to, and after reaction using SEM, electron microprobe and conventional light optic techniques. Experimental fluids were mildly alkaline (pH 7–8.5) and of low salinity (TDS <800 mgl^–1). Mineral-fluid reaction was dominated by the dissolution of plagioclase and the growth of smectite, calcite (at all temperatures), laumontite (at 150° C), wairakite and anhydrite (at 250° C). Final fluids were saturated with respect to quartz and fluorite. Certain trace elements (Li, B, Sr) were either incorporated into solids precipitated during the experiments or sorbed onto mineral surfaces and cannot be considered as conservative (partitioned into the fluid phase) elements. Concentrations of all analysed chemical components showed net increases during the experiments except for Ca (at 250° C) and Mg (at all temperatures). A comparison of the alteration mineralogy observed in the experiments with that present as natural fracture infills in drillcore from the Carnmenellis Granite reveals that the solid products from the experiments correspond closely to mineral assemblages identified as occurring during the later stages of hydrothermal circulation associated with the emplacement of the granite.     

Naturally acid waters from Copahue volcano,Argentina

Volcanic acid sulfate–chloride brines form through absorption of volcanic vapors in shallow reservoirs of meteoric water. Reaction with surrounding volcanic rocks leads to partial neutralization of the fluids and precipitation of secondary minerals. Chemical data of such acid waters from Copahue volcano, Argentina, covering 8 years of observations, show evidence for changes in composition related to water rock interaction at depth prior to emergence of the fluids at the surface. The chemical composition changed dramatically during the 2000 eruption of Copahue, with enhanced concentrations and fluxes of Mg, Na, Fe and Al, followed in 2001 by rapidly declining concentrations and element fluxes. The subsequent 5 years saw more variable element ratios and strong depletions in K and Al. Most incompatible elements are released from the rock matrix stochiometrically, whereas some elements are enriched through vapor input from the magma (As, Pb, Zn). Most fluids have LREE enrichments relative to the rock matrix, but during periods of new magma intrusion the LREE enrichment decreases as does the magnitude of the negative Eu anomaly in the fluids. These observations are interpreted assuming early dissolution of plagioclase, olivine and volcanic glass that occurs during intrusion of new magma into the hydrothermal system. The high field strength elements are virtually immobile even in these hot acid fluids, with Nb and Ta more so than Hf and Zr. The mobility of U and Th in these fluids is comparable, at variance with Th behavior in neutral fluids. The local rivers and lakes of Copahue are fertilized by volcanic dissolved P, and most surface waters with pH < 3 have high levels of As. The acid fluids from Copahue may be surficial analogs for deep subduction fluids that evolve below zones of arc magma generation as well as for early Mars environments that are thought to have had large acid lakes.     

Search for the proverbial mantle osmium sources to the oceans: Hydrothermal alteration of mid-ocean ridge basalt

We present Os and Sr isotopes and Mg, Os, and Sr concentrations for ridge-crest high-temperature and diffuse hydrothermal fluids, plume fluids and ridge-flank warm spring fluids from the Juan de Fuca Ridge. The data are used to evaluate the extent to which (1) the high- and low-temperature hydrothermal alteration of mid-ocean ridge basalts (MORBs) provides Os to the deep oceans, and (2) hydrothermal contributions of non-radiogenic Os and Sr to the oceans are coupled. The Os and Sr isotopic ratios of the high-temperature fluids (265-353 °C) are dominated by basalts (¹⁸⁷Os/¹⁸⁸Os = 0.2; ⁸⁷Sr/⁸⁶Sr = 0.704) but the concentrations of these elements are buffered approximately at their seawater values. The ¹⁸⁷Os/¹⁸⁸Os of the hydrothermal plume fluids collected ∼1 m above the orifice of Hulk vent is close to the seawater value (=1.05). The low-temperature diffuse fluids (10-40 °C) associated with ridge-crest high-temperature hydrothermal systems on average have [Os] = 31 fmol kg⁻¹, ¹⁸⁷Os/¹⁸⁸Os = 0.9 and [Sr] = 86 μmol kg⁻¹, ⁸⁷Sr/⁸⁶Sr = 0.709. They appear to result from mixing of a high-temperature fluid and a seawater component. The ridge-flank warm spring fluids (10-62 °C) on average yield [Os] = 22 fmol kg⁻¹, ¹⁸⁷Os/¹⁸⁸Os = 0.8 and [Sr] = 115 μmol kg⁻¹, ⁸⁷Sr/⁸⁶Sr = 0.708. The data are consistent with isotopic exchange of Os and Sr between basalt and circulating seawater during low-temperature hydrothermal alteration. The average Sr concentration in these fluids appears to be similar to seawater and consistent with previous studies. In comparison, the average Os concentration is less than seawater by more than a factor of two. If these data are representative they indicate that low-temperature alteration of MORB does not provide adequate non-radiogenic Os and that another source of mantle Os to the oceans must be investigated. At present, the magnitude of non-radiogenic Sr contribution via low-temperature seawater alteration is not well constrained. If non-radiogenic Sr to the oceans is predominantly from the alteration of MORB, our data suggest that there must be a different source of non-radiogenic Os and that the Os and Sr isotope systems in the oceans are decoupled.     

邯郸娄里金矿床原生晕垂向分带特征研究

娄里金矿位于邯郸洪山杂岩体北侧的火山凝灰岩中。该类型金矿在太行山南段为首次发现,其成矿规律和深部成矿潜力的研究对圈定成矿远景区及增加矿山资源量有重要的指导意义。据此系统采集了矿区ZK11-2和ZK16-1钻孔岩心样品,在分析区域成矿地质规律和矿床地质特征的基础上,采用格里戈良分带指数法和广义衬值法两种方法计算了元素垂向分带序列,结果显示不同方法虽在元素排列顺序上存在一定差异,但在反映热夜活动期次及元素空间叠加模式上基本相同。两个钻孔中均出现了明显的"前尾晕共存"及"反向分带"现象。在此基础上结合微量元素垂向变化规律,分析了区内金矿体形成的热夜活动过程。研究表明两个钻孔的垂向分带都由两个完整的小循环组成,说明区内金矿体的形成至少经历了两次热夜活动叠加。     

河北沽源460铀钼矿床成因探讨

460铀钼矿是一大型铀钼共生矿床,矿床定位于多组断裂构造与火山机构的复合部位,铀钼矿体赋存于火山通道相次火山岩体和隐爆角砾岩中,以浸染状、网脉状矿化形式为主,围岩为酸性火山岩,矿物组合与蚀变有分带特点,并有高、低温矿物共存现象。火山岩、次火山岩中有较高的铀钍丰度。硫同位素表明成矿物质来源于基底岩石和火山岩围岩;氢氧同位素表明成矿热液主要为大气降水经深部加热形成。矿床类型为与次火山岩有关的热液矿床。