Hydrothermal Fluid-rock Interaction Research Articles

Mechanism of Opal-CT deposit from the Lam Narai Volcanics in Lop Buri, Thailand by using Petrography

Common opals were discovered in the rhyolite host rock, where various structural features such as cavities, fractures, and veins are observed. These voids served as pathways for the infiltration of silica-rich fluids originating from meteoric water, establishing a hydrothermal connection. The aqueous solution derived from meteoric water actively dissolves silica from the rhyolite host rock, thereby increasing the silica saturation within the fluid. Subsequently, the silica-saturated hydrothermal fluid fills the cavities and veins within the host rock. The margin of the host rock underwent alteration as it interacted with infiltrating hydrothermal fluid in a phenomenon known as hydrothermal fluid-rock interaction. At the periphery of the cavities, quartz crystals begin to form, effectively reducing the concentration of dissolved silica in the solution. As a result, the subsequent precipitation of amorphous opals occurs, predominantly within the inner part or central region of the vein or cavity. The formation of opals associated with Lam Narai volcanic activity occurred at a relatively high temperature of about 105-170 °C through a hydrothermal process.

U–Pb Geochronology and Stable Isotope Geochemistry of Terrestrial Carbonates, Lower Cretaceous Cedar Mountain Formation, Utah: Implications for Synchronicity of Terrestrial and Marine Carbon Isotope Excursions

The terrestrial Lower Cretaceous Cedar Mountain Formation, Utah, is a critical archive of paleoclimate, tectonics, and vertebrate ecology and evolution. Early Cretaceous carbon cycle perturbations associated with ocean anoxia have been interpreted from this succession, as expressed in stable carbon isotopes. However, refining the timing of the observed stable isotope excursions remains a key challenge in understanding how marine anoxia affects the Earth system, and is ultimately recorded in the terrestrial realm. The geochronology and geochemistry of a terrestrial carbonate near the base of this succession, which potentially records the Ap7 global carbon isotope excursion, is studied here. Petrographic and geochemical analyses are used to test plausible mechanisms for U incorporation into the calcite lattice in this sample. Using these methods, the hypothesis that the incorporation of U was at or close to the timing of carbonate precipitation is evaluated. U–Pb geochronology of calcite indicates a plausible Early Cretaceous age. However, comparison of the new U–Pb ages of calcite with detrital zircon maximum depositional ages immediately beneath the studied sample indicates a disparity in the apparent sedimentation rates if both types of geochronologic information are interpreted as reflecting the timing of sediment deposition. The totality of data supports an early, and high-temperature, diagenetic timing of U incorporation, with potential for minor leaching of U in subsequent fluid–rock interaction. The most likely mechanism for U transport and immobilization in these samples is hydrothermal fluid–rock interaction. Therefore, the radiometric ages, and corresponding stable isotope composition of U-bearing carbonate domains in this sample, indicate early subsurface fluid–rock interactions and not a record of atmosphere–soil geochemical reactions.

Magmatic hydrothermal fluid genesis of zeolite in the Paleogene Kongdian Formation of Zaoyuan oilfield, Bohai Bay Basin, China

Electronic probe, fluid inclusion homogenization temperature, Raman spectroscopy and laser ablation inductively coupled plasma mass spectrometry were utilized to identify the hydrothermal fluid-rock interactions in the second member of the Paleogene Kongdian Formation of Zaoyuan oilfield in Bohai Bay Basin (Kong 2 Member for short) of Well Z56 to find out the relationship between zeolite and hydrothermal fluid. The experimental results show that: (1) Pyrobitumen coexists with hydrothermal fluid characteristic minerals such as chlorite, barite, chalcopyrite, pyrite, natrolite and analcime in mudstone fractures. (2) The temperatures calculated from laser Raman spectrum of pyrobitumen, from the chlorite geothermometer and from measured homogenization temperature of natrolite inclusions are 324–354 °C, 124–166 °C and 89–196 °C, respectively; although vary widely, all the temperatures are obviously higher than the normal geothermal temperature. (3) The positive Eu anomaly of chlorite and barite, and the similar distribution pattern in rare earth elements between natrolite and basalt indicate they are from magmatic hydrothermal fluid. Moreover, drilling data shows that the Kong 2 Member in Well Z56 has several sets of basalt interlayers, suggesting there was geologic base of magmatic hydrothermal fluid activity. The magmatic hydrothermal fluid-rock interaction may be one of the reasons for the abnormal enrichment of zeolite in Kong 2 Member of the Cangdong Sag.

Imprint of chemical weathering and hydrothermalism on the Ge/Si ratio and Si isotope composition of rivers in a volcanic tropical island, Basse-Terre, Guadeloupe (French West Indies)

A significant portion of the disproportionally high chemical weathering flux in volcanic island arcs may originate from hydrothermal fluid-rock interaction, thereby compromising the accurate estimate of atmospheric CO2 consumption rates. The objective of this study is to evaluate how the riverine Ge/Si ratio and Si isotopes, two well-established tracers of weathering, respond to hydrothermal inputs. The work took place in Basse-Terre, Guadeloupe, a tropical volcanic island with a dense river network, high chemical weathering fluxes and various hydrothermal surface manifestations. We characterized the Ge/Si ratio and δ30Si of 15 thermal springs, nine non-impacted (NI) rivers and 13 hydrothermally-impacted (HI) rivers. The soil solution from a highly weathered soil profile (Ferralsol) and a clayey-rock corresponding to the material exposed in an extinct hydrothermal system were also measured. A new purification method was successfully developed in order to allow the reliable measurement of Si isotopes in SO42−- and Cl−-rich thermal spring and HI river waters by mass spectrometry. Basse-Terre's thermal springs have variable Ge/Si ratios (0.05–21.03 μmol.mol−1) and δ30Si (0.71–1.50‰), but with no apparent relationship to the water compositional type. The Ge/Si ratio (0.15–2.57 μmol.mol−1) and Si isotope composition (0.26–1.21‰) values of the NI rivers reveal differences in the watersheds' weathering degree. Dissolution of Ge- and 28Si-rich secondary minerals explains the high Ge/Si and isotopically light composition of the northern NI rivers draining strongly weathered terranes. The Ge/Si ratio and δ30Si values measured for the NI and HI rivers overlap, implying that they cannot be used to diagnose hydrothermal contributions to river basins unambiguously. However, when combined with the Cl− and SO42− concentrations, the analysis of Ge and Si in the HI rivers suggests that water seeping through an extinct hydrothermal system produces SO4-rich drainages with distinctively lower Ge/Si ratios than those inferred for watersheds receiving thermal spring discharges associated with an active hydrothermal system. Overall, our results provide new constraints for applying and interpreting Ge/Si and Si isotope measurements to study weathering in volcanic environments.

Microstructures of epidote-prehnite bearing damaged granitoids (northern Victoria Land, Antarctica): clues for the interaction between faulting and hydrothermal fluids

Exhumed faults in granitoids along the Lanterman Fault-Rennick Graben Fault system (northern Victoria Land, Antarctica) show superposed ductile to brittle deformation and pervasive hydrothermal fluid-rock interaction. These processes triggered multiple brittle slip events producing crosscutting epidote and prehnite-rich fault veins, ultracataclasites and pseudotachylytes of crushing origin. Combined microstructural and minerochemical investigations on fault damage zones show three types of alteration: (i) albitization of K-feldspar and Ca-plagioclase; (ii) crystallization of prehnite and calcite in veins; (iii) alteration of magmatic phases by secondary hydrous minerals (e.g. chlorite, white mica, epidote and prehnite). The fault experienced various episodes of strain weakening and hardening, due to alteration of minerals and precipitation of epidote and prehnite within ultracataclastic intervals, at decreasing temperature conditions (200 < T°C < 450) and varying CO2 fugacity of the fluids. Cyclic crystallization of epidote/prehnite within the fault cores caused cementation and locking of faults, concentration of deformation at weaker horizons and a progressive broadening of the fault zone. Our results indicate that multiple co-seismic slip and syntectonic fluid flow very likely occurred prior to the Cenozoic brittle reactivation of inherited anisotropies in the northern Victoria Land crust along the Lanterman Fault-Rennick Graben Fault system and underlines its high potential for polyphasicity.

The effect of hydrothermal fluids on Ordovician carbonate rocks, southern Ordos Basin, China

Hydrothermal processes have an important influence on the spatial distribution and formation of carbonate reservoirs, and play a critical role in certain deep reservoirs where hydrothermal fluid-rock interaction may result in permeability creation due to carbonate dissolution or permeability destruction due to the deposition of cementing minerals. This study analyzed certain hydrothermal indicator minerals (quartz, celestite, multi-stage calcareous fillings) in carbonate rocks from the Ordovician Majiagou Formation, southern Ordos Basin, China. The petrological and geochemical characteristics of these indicator minerals imply that the Majiagou Formation was affected by two metasomatic events related to the Hercynian–Indosinian and Yanshanian orogenies and linked to magmatic-hydrothermal fluids derived from basaltic melts. Most of the δCe and δEu values obtained from matrix dolomite samples were lower than one, but U/Th and V/C ratios showed positive anomalies. The δ13C and δ18O values of the matrix dolomite were concentrated between −12.8‰ and −6.0‰, whereas δ13C showed a bipolar distribution at the same depth. Elemental geochemistry results showed that the Ordovician carbonate rocks were subjected to freshwater leaching followed by evaporation and widespread hydrothermal dissolution successively. As a result of the multi-episodic hydrothermal activity that affected the southern Ordos Basin and the path of migration channel, a parental magmatic-hydrothermal fluid (fluid 1) that had been generated during the underplating of the southern North China Craton by Mesozoic alkaline basalt melts, separated into two types of fluids with different chemical properties: an acidic, hydrocarbon-bearing fluid (fluid 2a) mixed with Cambrian oil and gas and a weakly to moderately alkaline brine (fluid 2b), not in contact with hydrocarbons. Both fluids 2a and 2b then interacted with the Ordovician carbonate rocks with fluid 2a having undergone hydrothermal sulfate reduction with SO42- in the layers in which gypsum was developed, and convection of fluid 2a having resulted in brine formation. The reaction products of H2S and CO2 together with organic acid co-dissolved carbonate minerals in the Ordovician strata played a major role in the formation of a high porosity reservoir. Conversely, owing to the progressive reduction of temperature and solubility during fluid ascent, fluid 2b precipitated quartz-calcite fillings in the residual pore spaces, resulting in the localized destruction of the original reservoir permeability. In summary, this study analyzed the petrological and geochemical characteristics of 60 carbonate rock drill core samples, established the likely formation mechanism of fluids 2a and 2b, and determined the effects of these hydrothermal fluids on the Ordovician Majiagou Formation carbonate reservoir.

Microstructures reveal multistage melt present strain localisation in mid-ocean gabbros

In this work, we examine the tectono-metamorphic evolution of gabbroic rocks of the Atlantis Bank oceanic core complex, South West Indian Ridge, focusing on multistage strain localisation associated with exhumation. We study a sample from the core complex footwall below the depth of seawater hydrothermal fluid-rock interaction. We identify a succession of increasing strain localisation where early solid-state deformation of an olivine gabbro is succeeded by two episodes of melt present deformation resulting in increasing localisation of both strain and melt flux into a narrowing zone.The early melt-absent solid-state deformation occurs dominantly in the dislocation creep regime, and localises strain at the tens of metre scale. This evolved into melt present deformation characterised by (i) grain size reduction by replacement reactions, and (ii) dislocation creep and incipient grain boundary sliding in coarse- and fine-grained parts, respectively. This is based on (1) microstructures indicative of the former presence of melt, (2) a shift to lower calcium plagioclase, and (3) reaction textures involving partial replacement of olivine and diopside by fine grained enstatite and hornblende exhibiting only minor internal deformation features.A narrow (2–3 mm) high strain zone which cuts the earlier shear zone foliation at ~30°, exhibits strong and near complete grain size reduction by melt-rock reaction and tightly spaced foliation within a millimetre-wide high strain zone. Here, strain localisation is contemporaneous with a second melt influx shown by (1) complete olivine replacement by fine grained hornblende and enstatite exhibiting no crystallographic preferred orientation, (2) microstructures indicative of the former presence of melt, and (3) changes in the mineral chemistry of diopside, plagioclase, enstatite and hornblende. We suggest deformation was dominated by melt assisted grain boundary sliding allowing higher volumes of melt flux, a high degree of weakening and deformation focused in the narrow shear zone than during the porous melt flow in the wider, early shear zone.This combined microstructural and microchemical study highlights that the recognition of changes in deformation regime, their time relationships and potential melt related deformation are critical for understanding, and therefore modelling, progressive strain localisation, melt flux and the rheological evolution of oceanic detachment faults.

A fractal measure of mass transfer in fluid–rock interaction

Exploring the mechanism of mass transfer is critical for characterizing geochemical behaviour of elements in fluid–rock interaction and is useful for understanding the sequence of hydrothermal alteration. The focus of this study is on identifying the spatial relationship between the concentration variation of elements and the distance of samples to the source of the hydrothermal fluid. A mobility index (MI) was applied to represent the mass changes. A value of MI >1 indicates gain of elements, <1 loss of elements, =1 no mass changes. Two data sets are used to quantify the mass transfer in fluid–rock interaction at a decimeter–scale. A fractal relation between MI and distance (x) from alteration to the source of hydrothermal fluids is observed. For the enriched and depleted elements, the relation can be expressed as MI = cx−d and MI = cxd, respectively, where c is a constant, and d is the fractal dimension. Such relations indicate a power-law decrease/increase of MI with the decreasing degree of alteration. The fractal dimension provides an objective measure of the mass gain or loss. The larger the fractal dimension, the larger the mass change. The fractal dimension sequence quantitatively reveals the activity of ore–forming elements and the degree of mass changes in the process of hydrothermal fluid–rock interaction, which can provide critical information on hydrothermal alteration.

Variations in Large Synvolcanic Alteration Zones at Snow Lake, Manitoba, Canada, with Proximity to Associated Volcanogenic Massive Sulfide Deposits

Juvenile 1.89 Ga oceanic arc volcanic rocks of the Flin Flon volcanic belt at Snow Lake are characterized by extensive zones with anomalous 1.82 Ga metamorphic mineral assemblages, including porphyroblasts of garnet, staurolite, amphibole, biotite, gahnite, and/or kyanite. They were produced from altered rocks created during premetamorphic, 1.89 Ga, synvolcanic, hydrothermal fluid-rock interaction. Three separate episodes of hydrothermal alteration are recognized that span evolution of the host volcanic rocks from a primitive (nascent arc?) to mature arc geotectonic setting. The geologic, geochemical, mineralogical, and isotopic attributes of the zones indicate that they include volcanogenic massive sulfide (VMS)-related alteration, and they were produced at high and low temperatures and formed in seafloor/near seafloor and subseafloor (intrastratal) environments. Large-scale alteration zones at Snow Lake are up to 20 km in strike length and 0.8 km wide. Their large exploration “footprint,” which is 36 times greater in areal extent compared to associated “pipe-like” alteration zones, means that such zones provide a useful target to “vector-in” exploration to VMS depositional settings within volcanic belts. The VMS-related large-scale alteration zones at Snow Lake display diagnostic variations in intensity and style of alteration along strike toward VMS deposits, are stratigraphically underlain by altered portions of synvolcanic intrusions, are crossed by discordant zones of more intensely altered rocks, and can be demonstrated to have formed by interaction with high-temperature(>350°C) hydrothermal fluids. Cu-Zn–rich VMS deposits at Snow Lake formed in flow-dominated sequences, are hosted by large rhyolite flow complexes, and comprise lensoid orebodies. In contrast, Zn-Cu–rich VMS deposits, although also rhyolite associated, formed in volcaniclastic-dominated sequences, are spatially related to a district aquifer, and comprise stratiform, laterally continuous orebodies. This suggests that exploration for Cu-Zn VMS deposits could selectively target flow-dominated sequences and focus on rhyolite flow complexes that display significant alteration. Similarly, exploration for Zn-Cu VMS deposits could focus on volcaniclastic-dominated sequences in which a potential aquifer has been identified.

Cu-Ni-PGE Mineralization in the South Filson Creek Area, South Kawishiwi Intrusion, Duluth Complex: Mineralization Styles and Magmatic and Hydrothermal Processes

The South Filson Creek mineralization occurs above the basal units of the South Kawishiwi intrusion and represents an uncommon geologic setting of Cu-Ni-platinum-group element (PGE) mineralization within the Duluth Complex. Unlike other mineralization of economic interest in the basal heterogeneous troctolite units along the lower contact, it is located at an approximate stratigraphic height of 1,000 m above the contact with the footwall granitoids in the usually unmineralized homogeneous troctolite unit. Magmatic sulfide mineralization forms disseminated fine-grained patches and pockets in an area of approximately ½ km2 of homogeneous troctolitic rocks in the northeastern part of the South Kawishiwi intrusion. Pyrrhotite, chalcopyrite, pentlandite, and cubanite are the main sulfide minerals with subordinate amounts of other copper-bearing phases (bornite, covellite, talnakhite). Cu/Pd ratios for unaltered mineralization indicate that sulfides were formed from multiple injections of magma that segregated sulfides with different R factors or having different initial compositions. Variation in the halogen contents of apatite from pegmatoids within troctolitic rocks revealed that a fluid has segregated from the crystallizing melt into which fluid Cl and rare earth elements partitioned. Amphibole, chlorite, sericite, prehnite, pumpellyite, and carbonate are associated with alteration along brittle structures and isolated patches in the area. The hydrothermal fluid-rock interaction has affected the composition of primary magmatic sulfide mineralization and is also responsible for mineralization in the anorthositic hanging wall of the South Kawishiwi intrusion. Hydrothermal alteration resulted in a slight Pd depletion in magmatic sulfide mineralization, which might indicate remobilization of precious metals to an unknown location. The hanging-wall mineralization is characterized by relative Cu enrichment in comparison to the magmatic sulfide mineralization with no PGE enrichment. Partial remobilization of platinum group elements (PGE) has also occurred as a result of a separate alteration event, on the evidence that platinum group minerals (PGM) are also associated with serpentinization of olivine and Ca enrichment of plagioclase along grain boundaries with sulfides. This alteration was probably effective on a very local scale.

Formation mechanism of the lanthanide tetrad effect for a topaz- and amazonite-bearing leucogranite pluton in eastern Xinjiang, NW China

The Baishitouquan (BST) pluton is a topaz- and amazonite-bearing leucogranite intrusion located in the Middle Tianshan orogen of Xinjiang, northwestern China. This pluton exhibits five lithological zones gradational from the bottom upwards: leucogranite (zone-a), amazonite-bearing granite (zone-b), amazonite granite (zone-c), topaz-bearing amazonite granite (zone-d) and topaz albite granite (zone-e). Contents of REE and other trace elements were analysed on major and accessory minerals, including quartz, plagioclase, K-feldspar, white micas, topaz, fluorite, garnet, zircon and monazite, separated from above five zones of the BST pluton. Chondrite-normalized REE patterns of minerals from zone-a to zone-e display clear convex tetrad effect, and the TE 1,3 (quantification factor of tetrad effect) values of these minerals increases from zone-a to zone-e, except that a quartz sample from zone-b and zircon from zone-c exhibits W-type tetrad effect. Therefore, we conclude that REE tetrad features of the minerals, such as quartz, plagioclase, zircon, garnet and monazite, are inherited directly from the melt and have not significantly affected by both fractional crystallization and surface weathering. With progressing of magmatic differentiation, total REE contents decrease, Eu anomaly becomes more negative, whereas Nb/Ta, Zr/Hf and Y/Ho ratios decrease gradually from zone-a to zone-e with increasing TE 1,3 values. Therefore, we suggest that F-rich magmatic fluid–melt interaction above its solidus may leave the residual silicate melt with the features of very low contents of all REE, subchondritic Y/Ho, Zr/Hf and Nb/Ta values, and with an M-type tetrad pattern, including minerals crystallizing from it. Such a pattern is manifested by both whole-rock and separate minerals. Hydrothermal fluid–rock interaction played only a minor role to the convex tetrad effect of the minerals and their host rocks.

Evolution of a hydrothermal fluid-rock interaction system as recorded by Sr isotopes: A case study from the Schwarzwald, SW Germany

Metasomatic and Sr-isotopic changes, associated with formation of zoned alteration halos along hydrothermal veins, are documented for a gneiss from the Artenberg quarry near Steinach (Kinzigtal, Schwarzwald, SW Germany). Veins are postorogenic, SW-NE-oriented, and cut straight through metaquartzdioritic Variscan gneiss, where flow of low-temperature fluids (∼100–200°C) caused adularia-quartz-sericite-type alteration. Fluid-rock interaction occurred nearly 50 Ma after Variscan metamorphism, as constrained by a Rb–Sr multimineral isochron for unaltered gneiss of 327.1 ± 3.1 Ma, and by two independent ages of 279.2 ± 3.1 Ma and 274 ± 13 Ma, based on Rb–Sr systematics of late-stage quartz from the veins. In a profile from unaltered gneiss towards a vein, alteration-induced mineralogical changes correlate with metasomatic net addition of K, Rb, and Cl to the alteration zone, combined with net loss of Na, Ca, and Sr. Strontium isotopes give a more detailed insight into the fluid-rock interaction process. 87Sr/86Sr ratios in a profile across the alteration zone are incompatible with simple Sr leaching but reflect partial replacement of the rocks’ Sr by fluid-derived Sr, the isotopic composition of which varied with time. Early fluids, with high 87Sr/86Sr ratios compared to unaltered gneiss, evolved into fluids with somewhat lower ratios, and finally reached a second maximum in 87Sr/86Sr ratios. This Sr-isotopic fluid evolution is equally revealed by the mineral sequence of the vein mineralization. It appears that the compositional evolution of the fluids correlates with the sequence of mineral breakdown reactions in the gneissic host rock, and that the Sr-isotopic evolution of the fluids can be fully explained as the result of internal, progressive reaction of fluid with the local rocks. Results also show that the spatial distributions of Sr isotopes in metasomatic alteration zones may reflect the complex evolution of fluid-rock interaction systems, and ultimately constrain the factors controlling both fluid compositions and alteration patterns.

Origin of convex tetrads in rare earth element patterns of hydrothermally altered siliceous igneous rocks from the Zinnwald Sn–W deposit, Germany

The chondrite-normalized rare earth element (REE) patterns of whole rock samples from evolved granitic systems hosting rare metal deposits sometimes show a split into four consecutive curved segments, referred to as tetrads. In the present contribution, a rigorous statistical method is proposed that can be used to test whether geological significance should be attributed to tetrads that are only of limited size. The method involves a detailed evaluation of element and sample specific random and systematic errors that are constrained on the basis of independent repeated preparations and analyses of sample and reference materials. Application of the proposed method to samples from the granite-hosted Zinnwald Sn–W deposit, Germany, revealed that at least two tetrads in normalized whole rock REE patterns have to be analytically significant to rule out that fractional crystallization led to the unusual behavior of the REEs. Based on the analysis of altered albite granite and greisen samples from the endocontact of the Zinnwald granite massif, it is demonstrated that the lanthanide tetrad effect is responsible for the formation of the convex tetrads. Geological and petrological evidence suggests that the tetrads in the samples developed prior to greisenization and related cassiterite precipitation. In contrast to the endocontact samples, the rhyolitic wall rocks are typified by normalized REE patterns having tetrads that are variable in size and frequently close to the limit of analytical significance. The sizes of the tetrads apparently correlate with the intensity of albitization, but show no relation to subsequent alteration processes including greisenization and low-temperature argillization. This observation proves that curved segments in normalized whole rock REE patterns can be introduced during hydrothermal fluid–rock interaction.

Contact metamorphism in the Los Santos W skarn (NW Spain)

The intrusion of the Lower Permian Los Santos-Valdelacasa granitoids in the Los Santos area caused contact metamorphism of Later Vendian-Lower Cambrian metasediments. High grade mineral assemblages are confined to a 7 km wide contact aureole. Contact metamorphism was accompanied by intense metasomatism and development of skarns, and it generated the following mineral assemblages: diopside, forsterite, phlogopite (±clintonite) and humites and spinel-bearing assemblages or diopside, grossular, vesuvianite ± wollastonite in the marbles, depending on the bulk rock composition. Cordierite, K-feldspar, andalusite and, locally, sillimanite appear in the metapelitic rocks. Mineral assemblages of marbles and hornfelses indicate pressure conditions ranging from 0.2 to 0.25 GPa and maximum temperatures between 630 and 640 °C. 13C and 18O depletions in calcite marbles are consistent with hydrothermal fluid–rock interaction during metamorphism. Calcites are depleted in both 18O (δ18O = 12.74‰) and 13C (δ13C = −5.47‰) relative to dolomite of unmetamorphosed dolostone (δ18O = 20.79‰ and δ13C = −1.52‰). The δ13C variation can be interpreted in terms of Rayleigh distillation during continuous CO2 fluid removal from the carbonates. The δ18O values reflect hydrothermal exchange with an externally derived fluid. Microthermometric analyses of fluid inclusions from vesuvianite indicate that the fluid was water dominated with minor contents of CO2 (±CH4 ± N2) suggesting a metamorphic origin.

Origin of black cherts from the Marble Bar Drill Core #1 (Pilbara, W-Australia)

Black carbonaceous matter bearing cherts are abundant in Archean hydrothermal environments and may host traces of early life. In order to elucidate their origin, we studied black cherts from the Marble Bar drill core #1 (Archean Biosphere Drilling Project depths: 75.1 m, 94.2 m, 96.7 m, 100.4 m). Two types of black chert can be distinguished: (1) laminated black chert having preserved its sedimentary texture (75.10 m) and (2) hydro-brecciated black chert (94.2 m, 96.7 m, 100.4 m). The laminated chert is composed of alternating bright and dark layers, separated by wavy mat-like surfaces. Each layer is characterized by a pyroclastic texture with preserved protomineral shapes (100–500 lm). In dark layers, protominerals are more abundant showing grain contacts and are less-matrix supported than in bright layers. Protomineral shapes reveal the presence of former amphiboles and plagioclase. Rapid silicification preserved the texture and led to the precipitation of micro-quartz, disseminated lm Ni–Co–As bearing pyrite, Tiand Fe-oxides, sphalerite, galena, Fe–Mn– Mg carbonates, monazite and xenotimes. A second and third fluid influx occurred parallel and perpendicular to the lamination. Due to these fluids, K-feldspars, mica and illite precipitated around secondary pyrite and at the interface between dark lamina and a quartz vein. Laminated cherts have lower Se/S-ratio (10 ) compared to brecciated ones (10 ), indicating an increased hydrothermal influx. C1-normalized REE patterns of laminated and brecciated cherts show a strong positive Eu anomaly. The REE patterns and La(N)/Yb(N) ratios (0.75–1.6) of the laminated cherts resemble those of silicified basaltic precursor rocks. (La(N)/ Yb(N) = 1.51); and clearly differ from those of black cherts with black shale precursors (La(N)/Yb(N): 6.7–9.06, Orberger et al., 2006). Brecciated black cherts (La(N)/Yb(N) = 3.25–6.3) are enriched in LREE compared to the laminated ones. The studied cherts have a pyroclastic precursor of basaltic composition, silicified during hydrothermal fluid-rock interaction under reducing conditions. Si-CO2 bearing fluids remobilised metals (LREE, Fe, S, Pb, Zn.) from the laminated facies and mineralized the brecciated black chert.

Two Origins of Illite at the Dexing Porphyry Cu Deposit, East China: Implications for Ore-Forming Fluid Constraint on Illite Crystallinity

AbstractIllite is a distinctive clay mineral formed by K alteration within hydrothermal alteration zones in porphyry Cu deposits. Based on differences in spatial distribution, Kübler index, number of swelling layers, and polytype, two kinds of illite are recognized within the Dexing porphyry Cu deposit, East China. One is a hydrothermal mineral within hydrothermally-altered granodiorite porphyry and altered tuffaceous phyllite near the contact zone with the granodiorite porphyry cupola. The hydrothermal illite is formed by illitization of plagioclase and/or micas during hydrothermal fluid-rock interaction. The considerable variation of their higher Kübler indices (0.17–1.41°Δ2θ) with swelling layer is affected by fluid/rock ratio or fluid flux. The other type of illite is a product of low-grade metamorphism within tuffaceous phyllite away from the porphyry cupola (&gt;2 km), and has a lower Kübler index (0.06–0.13°Δ2θ), a 2M1 polytype, and no swelling layers. We suggest that, within the mineralized alteration zone, the lower the Kübler index, the stronger the degree of alteration, and the higher the copper grade. This is caused by a higher fluid/rock ratio in the middle-upper portions of the contact zone.

Spatial and Temporal Distribution of Gold Deposits in theUrals

During the past 10 years, the Paleozoic Urals mountain belt has been the subject of internationally based, deep seismic, and ancillary geologic studies aimed at improving our knowledge of the lithospheric evolution of the orogenic belt, in general, and its prominent crustal root, in particular. In view of its wide-ranging mineral endowment, especially for gold, this work also provides important keys to help clarify relationships between collisional tectonic processes and gold mineralization. Following an outline of current ideas on the tectonic evolution of the Urals, we present an overview of the distribution of the majority of known gold deposits. This information, together with the available isotopic and geologic age data of associated alteration assemblages, is compared to available geochronological data for various magmatic and deformation episodes. Spatial and temporal relationships suggest that hydrothermal mineralizing processes and the geodynamic evolution of the orogen are linked. In an attempt to analyze the deep structural framework of the orogen, down to the mantle, we have also examined a number of subcontinental- and regional-scale continuous geophysical datasets. The Urals have been subdivided traditionally into a series of north-south–striking tectonic zones but more recently into three principal tectono-magmatic sectors, which comprise a Suture sector along the Main Uralian fault zone and two sectors with tectonically imbricated island-arc, transitional (viewed as active continental margin), and continental zones. Gold deposits are found in all tectono-magmatic sectors of the orogen. Minor gold occurs in Siluro-Devonian volcanic-hosted massive sulfide deposits, magnetite(-copper) skarns, and porphyry copper deposits. In a number of cases, younger structures (shear zones, faults, and their intersections) have controlled the deposition of higher gold grades. In the Urals, this has led to the concept of progressive concentration of gold during later tectonic overprinting, and the understanding that regional deformation and hydrothermal fluid-rock interaction in the Late Carboniferous to Early Triassic upgraded gold contents of earlier deposits. During the Late Carboniferous to Early Triassic, gold-bearing quartz vein lodes, which effectively encompass the majority of the larger gold deposits in the Urals, were also formed in structural-chemical traps. Some deposits, such as Mindyak that is hosted by tectonic melanges along the Main Uralian fault zone, are clearly typical late-orogenic lode gold deposits. Others occur within and along the margins of early to middle Carboniferous older granite and, although local orthomagmatic relationships have been described, most recent observations now favor a strong structural control on the lodes and no direct genetic association with granite intrusion. In the two largest gold deposits, Kochkar and Berezovskoe, gold mineralization was controlled by structural and combined structural-chemical traps in dilational jogs in both shear and contact zones. Locally, gold mineralization has been shown to occur during a change from orthogonal to transpressional compression. The inferred ages of the lode gold deposits suggest they are mostly coeval with the generally undeformed Permian-Early Triassic younger granites. However, a spatial relationship with gold mineralization has not been observed. The younger granites were formed in an extensional regime following Ural-wide magma generation, caused by a thermal flux attributed to underplating of the crust by mantle-derived mafic magmas. The widespread distribution of this late thermal event, during changes in stress regime, and the involvement of magma-generating mantle processes suggest regional delamination in the lithosphere and concomitant upwelling of the asthenosphere. These processes may have largely occurred prior to the formation of the prominent crustal root of the Urals, which formed during subsequent transpressional convergence. Another, Triassic, Ural-wide phase of hydrothermal alteration has been recognized, but associated gold deposits have been documented in only two cases. We tentatively attribute this phase to destabilization of the lithosphere of the wider Altaids, reflected by the formation of the West Siberian and Central Asian basins, east of the Urals, during the Mesozoic. The integrated analysis of magnetic, Bouguer gravity, heat flow, and crustal thickness data suggests that a major indenter of the East European craton is responsible for a singular, transcrustal, sidewall ramp in the subsurface of the middle Urals. This ramp may extend into the mantle and formed the principal conduit zone for mineralizing fluids. The world-class Kochkar and Berezovskoe gold deposits are adjacent to this proposed conduit.

Ore-forming fluid constraints on illite crystallinity (IC) at Dexing porphyry copper deposit, Jiangxi Province

Illite, a distinctive kind of clay minerals of potassium alteration within the hydrothermal alteration zone, frequently occurs at the Tongchang porphyry copper deposit ore field. The illite crystallinity (IC) value and expandability are mainly affected by water/rock ratio or fluid flux. It was formed by illitization of plagioclase and micas during hydrothermal fluid-rock interaction within the porphyry body and near the contact zone with wall rocks. Moreover, the negative correlation between illite index (IC) and copper grade indicates that within the alteration zone, the smaller the illite crystallinity value, the higher the alteration degree, and the higher the copper grade due to higher water/rock ratio. At lower levels of the porphyry body, however, the illite crystallinity (IC) values are mainly controlled by temperature and time duration.

Formation and Transformation of Clay Minerals in the Hydrothermal Deposits of Middle Valley, Juan de Fuca Ridge, ODP Leg 169

Mineralogical, chemical, and isotopic characteristics of clays from Ocean Drilling Program (ODP) sites 1035, 856, 1036, and 858, drilled in Middle Valley at the northern end of the Juan de Fuca Ridge during Legs 139 and 169, were studied using X-ray powder diffraction, differential thermal analysis, infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray fluorescence, and inductively coupled plasma-mass spectrometry. Oxygen isotope measurements on authigenic clay minerals provide a record of mineral formation temperatures. Holes 1036A and 858B are located in the present-day hydrothermally active Area of Active Venting and reflect several stages of hydrothermal alteration. Smectite and some mixed-layer chlorite-smectite clays are the dominant phyllosilicates in the upper part of holes 1036A and 858B. Nearly pure chlorite-smectite mixed-layer clays and corrensite mainly occur more than 20 meters below the seafloor. Formation temperatures of these nearly pure chlorite-smectite mixed-layer clays and corrensite were calculated as 250° to 268°C. In the deepest unit, chlorite is the dominant phyllosilicate coexisting with authigenic quartz. The high-temperature alteration stages resulted in gains (50–85%) of Mg and Zn, an almost complete loss of K, Ca, Cu, Rb, Sr, and Ba (> 90%), and depletion (20–40%) in Si, Fe, Na, and Zr. Alteration effects in hemipelagic and turbiditic sediments from hole 856H, drilled near the summit of the Bent Hill massive sulfide deposit, are similiar to those of sediments in hole 1035H, which was drilled 350 m south of hole 856H near the southernmost peak of the Ocean Drilling Program mound. At both holes, the clay fraction of the sedimentary sequence between 140 and 470 m below seafloor is composed of varying amounts of authigenic quartz and chlorite indicating high-temperature alteration. The chlorite formation temperatures calculated from oxygen isotope data lie at 290° to 320°C in hole 1035H and 250° to 264°C in hole 856H. Mineralogical data from hole 1035D, drilled 75 m east of hole 856H, and hole 1035A, drilled 77 m west of hole 856H, indicate lower temperatures of hydrothermal alteration in the upper 50 m of their sedimentary sequences. These sequences are partly altered to chlorite-smectite mixed-layer clays coexisting with several detrital minerals and with a dolomite formation temperature of 74°C. Oxygen isotope evidence indicates authigenic chlorite in the deeper sedimentary section of hole 1035A that formed at relatively low temperatures (180°C), whereas chlorites from hole 1035D formed at higher temperatures (277°–292°C). Chemical changes in the strongly altered clay fractions of hole 856H and 1035D include gains (60–75%) of Fe, Mg, Zn, and Cu, a nearly complete loss (> 95%) of K, Na, Ca, Rb, Sr, and Ba, and a strong depletion (30–70%) in Si, Ti, and Zr. All of the chondrite-normalized rare earth element patterns of chlorite are characterized by negative Eu anomalies and varying levels of light rare earth enrichment. The chloritization observed at the Area of Active Venting is similar to the Mg-rich chlorite and to chloritic minerals observed at sediment-covered spreading centers in Escanaba trough on Gorda Ridge and in Guaymas basin on the East Pacific Rise. This observation suggests intense Mg metasomatism in a mixing zone where hydrothermal fluids interact with seawater and sediment at temperatures above 200°C. The similarity of the Fe-rich chlorites from the Bent Hill massive sulfide deposit to Fe-rich chlorites from several ancient massive sulfide deposits indicates that they probably reflect hydrothermal fluid-rock interaction at high temperatures below the zone of seawater mixing.

Fluid inclusion and stable isotope evidence for interaction between granites and magmatic hydrothermal fluids during formation of disseminated and pipe-style mineralization at the Zaaiplaats tin mine

Tin-tungsten-rare earth element mineralization at Zaaiplaats is hosted by the Lease and Bobbejaankop Granites--miarolitic, alkali feldspar granites which form part of the Lebowa Granite Suite of the Bushveld Complex. Pervasive hydrothermal alteration of the granites resulted in replacement of original biotite by chlorite, sericite, carbonate and Ti oxides, development of coarse vein and patch perthite and antiperthite in alkali feldspar, and precipitation of hydrothermal minerals in miarolitic cavities. Cassiterite occurs as a cavity infill and replacement mineral in tabular, subhorizontal zones within the Lease and Bobbejaankop Granites and in shallowly plunging and branching pipe systems in the granites. Disseminated mineralization exhibits no relationship to fracture systems, and hydrothermal fluids appear to have evolved essentially in situ during crystallization to produce miarolitic cavities. Minor subvertical veins containing fluorite, calcite, quartz, and sulfides crosscut the granites and pipes.Three major types of fluid inclusions have been identified in granite quartz and hydrothermal minerals from pipes and cavities. High-salinity inclusions characterized by the presence of solid phases such as halite, sylvite, and Fe chloride occur as primary inclusions in (1) granite quartz, (2) cavity-filling quartz, fluorite, and scheelite, and (3) cassiterite from the Iota pipe. These inclusions homogenize by dissolution of solid phases in the temperature range 160 degrees to 600 degrees C. Liquid-vapor inclusions occur in (1) late quartz from cavities, (2) quartz, fluorite, and calcite from cavities in the marginal pegmatite, and (3) late quartz and calcite infill from the Iota pipe. These inclusions homogenize to the liquid phase between 88 degrees and 240 degrees C, while ice melting temperatures range from -11 degrees to -30 degrees C. H 2 O-NaCl-CO 2 inclusions occur as secondary inclusions in granite quartz and early cavity-filling quartz, and primary inclusions in quartz from the quartz-tourmaline zone of the Bobbejaankop pipe. The compositional range and homogenization behavior of these inclusions is consistent with immiscibility around 350 degrees C and 1,000 bars.The oxygen isotope composition of granite whole rocks ranges from 9.2 to 9.9 per mil. Two samples of low-grade ore (8.4 and 9.0ppm) are depleted relative to the granites, possibly reflecting higher abundances of alteration and infill chlorite, sericite, and cassiterite. The delta 18 O values of quartz from granites and the marginal pegmatite range from 7.3 to 9.4 per mil, whereas those of quartz from cavities, low-grade orebodies, pipes, and veins range from 7.8 to 13.5 per mil. Most granite and cavity samples have quartz-feldspar fractionations less than 0 per mil. Vein and pipe samples tend to have higher delta 18 O feldspar and much higher delta 18 O quartz compared to granites, with quartz-feldspar fractionations of -0.6 to 1.2 per mil.The disequilibrium quartz-feldspar fractionations observed in granites and cavities reflect pervasive hydrothermal fluid-rock interaction at subsolidus temperatures. Modeling isotopic exchange between granites and a magmatic fluid phase in isotopic equilibrium with average Bobbejaankop Granite quartz at 600 degrees C indicates that whole-rock compositions result from final exchange at temperatures less than 400 degrees C, with fluid compositions in the range 4.2 to 5.2 per mil at 300 degrees C and 0.8 to 1.8 per mil at 200 degrees C. These results are consistent with fluid compositions calculated from delta 18 O quartz and pressure-corrected fluid inclusion homogenization temperatures, and indicate that oxygen isotope variation at Zaaiplaats reflects exchange between the granite and the magmatic fluid phase at temperatures down to 200 degrees C.