δ18O Values Of Quartz Research Articles

Infiltration of meteoric fluids in an extensional detachment shear zone (Kettle dome, WA, USA): How quartz dynamic recrystallization relates to fluid-rock interaction

We document the interplay between meteoric fluid flow and deformation processes in quartzite-dominated lithologies within a ductile shear zone in the footwall of a Cordilleran extensional fault (Kettle detachment system, Washington, USA). Across 150 m of shear zone section, hydrogen isotope ratios (δD) from synkinematic muscovite fish are constant (δD ∼ −130‰) and consistent with a meteoric fluid source. Quartz-muscovite oxygen isotope thermometry indicates equilibrium fractionation temperatures of ∼365 ± 30 °C in the lower part of the section, where grain-scale quartz deformation was dominated by grain boundary migration recrystallization. In the upper part of the section, muscovite shows increasing intragrain compositional zoning, and quartz microstructures reflect bulging recrystallization, solution-precipitation, and microcracking that developed during progressive cooling and exhumation. The preserved microstructural characteristics and hydrogen isotope fingerprints of meteoric fluids developed over a short time interval as indicated by consistent mica 40Ar/39Ar ages ranging between 51 and 50 Ma over the entire section. Pervasive fluid flow became increasingly channelized during detachment activity, leading to microstructural heterogeneity and large shifts in quartz δ18O values on a meter scale. Ductile deformation ended when brittle motion on the detachment fault rapidly exhumed the mylonitic footwall.

Formation age and genesis of the Gongchangling Neoarchean banded iron deposit in eastern Liaoning Province: Constraints from geochemistry and SHRIMP zircon U–Pb dating

The Gongchang iron deposit, a typical Algoma-type banded iron formation (BIF) deposit located in the Eastern Block of the North China Craton (NCC), is hosted in Neoarchean metamorphic rocks, including amphibolite, biotite leptynite, garnet biotite schist, plagioclase-hornblende schist, garnet chlorite schist, muscovite quartz schist and actinolite magnetite quartzite. Geochemical data suggest that protoliths of the amphibolites and biotite leptynite should be basalt and dacite, respectively. SHRIMP U–Pb dating of zircons from the plagioclase-hornblende schist shows that the Gongchangling iron deposit was formed at 2554±12Ma. Major elements and REE data of the ores indicate that the Gongchangling BIF precipitated from seawater, and the ultimate iron and silica origins may have been related to nearby volcanic activity. The δ18O values of quartz from the BIFs range between 9.1 and 12.9‰, suggesting that exotic high δ18O fluids were involved during metamorphism. The pyrite have δ34S values of range between −3.5‰ and 14.7‰, suggesting that mineralization were originally from metamorphism. Comparison of the mineralization age of the Gongchangling BIF with those of other local BIFs in North China and other cratons throughout the world indicates that the Gongchangling BIF formed coevally with the global BIF mineralization at ∼2.5Ga. Taken together, these data suggest that the Gongchangling iron deposit was formed in a volcanic submarine environment in which hydrothermal fluids associated with volcanic activity supplied both iron and silica for the banded iron formations.

Fault mechanics at the base of the continental seismogenic zone: Insights from geochemical and mechanical analyses of a crustal-scale transpressional fault from the Argentera crystalline massif, French–Italian Alps

Faulting mechanics is examined by combining geochemical and mechanical analyses from a key exposure of a major fault zone crossing the Argentera crystalline massif (French–Italian Alps). The analyses are carried out on co-genetic and syn-tectonic quartz and chlorite extension veins and shear veins associated with late stage deformation in the fault zone. Paleothermometry based on chlorite compositions gives formation temperatures of 300 °C ± 20 °C, that is near the base of the seismogenic zone. δ18O values of quartz and δD and δ18O values of chlorite indicate that veins crystallized from a metamorphic fluid. In parallel, a mechanical analysis based on the Mohr-Coulomb theory shows that the pore fluid factor λv required to simultaneously reactivate the studied fault and to open the extension veins was close to a lithostatic value (λv ∼ 1).Comparisons with the 2003–2004 Ubaye seismic swarm, having occurred in the continuity of the studied fault zone, suggest that the base of the seismogenic zone may act as a limit separating an upper permeable reservoir saturated with meteoric waters under hydrostatic to supra-hydrostatic pressures from a lower low-permeability reservoir containing metamorphic waters under lithostatic pressure. This study suggests that overpressured fluids can be released upwards in the brittle crust by shear-enhanced permeability and can trigger earthquakes.

Oxygen isotope signatures of quartz from major Asian dust sources: Implications for changes in the provenance of Chinese loess

We present a systematic investigation of the oxygen isotopic composition of quartz in both fine and coarse fractions (<16 and 16–63μm) from major dust source regions in East Asia, including the Mongolian Gobi, the northern Chinese deserts, the Taklimakan desert, and the Qaidam Basin. The results demonstrate that the quartz oxygen isotope ratios of the Taklimakan desert and the Mongolian Gobi are more heterogeneous compared with the other areas. The quartz δ18O values of both the fine and coarse fractions from the various sources are overlapped to varying degrees, thus making it difficult to differentiate them. Nevertheless, the quartz δ18O values of both fractions exhibit an increasing trend from the Mongolian Gobi, to the northern Chinese deserts, and then to the Taklimakan desert. This implies that the geological settings of the source areas are different, which in turn results in differing contributions of high-temperature igneous rocks. The combination of quartz δ18O results with other quartz-based provenance tracers can clearly differentiate the three major source areas, i.e., the Taklimakan desert, the Mongolian Gobi, and the northern Chinese deserts. In addition, comparison of our results with previous δ18O measurements of fine-grained quartz from the Luochuan loess sequence suggests the likely glacial–interglacial fluctuations in dust provenance. Finally, we suggest that the combination of quartz δ18O signatures and other dust provenance tracers can potentially improve the recognition of long-term fluctuations in the provenance of Chinese loess-red clay deposits.

Oxygen isotope ratio of quartz veins from the auriferous Ramagiri–Penakacherla schist belt and surrounding granitoids in the Eastern Dharwar craton: A case for a possible link between gold mineralization and granite magmatism

In the Eastern Dharwar craton, among the many shear zone-hosted lode gold deposits, those at Ramagiri and Penakacherla are located near the western margin of the craton. Mineralized quartz (±sulfide±carbonate) veins are hosted by the schistose (metavolcanic and carbonaceous metasedimentary) rocks, in close spatial association with granitoids having quartz and quartzofeldspathic veins representing hydrothermal activities associated with them. Mineralized quartz veins from the ore zones (in Ramagiri and Penakacherla regions) and quartz (or pegmatitic) veins from the surrounding granitic terrane were chosen for δ18O analysis. Samples from the schistose and granitic domains show δ18Oquartz values in the range of 10.4–14.9 and 9.3–10.9‰ respectively. The ore-zone fluids from the Ramagiri and Penakacherla regions give δ18O values of 7.9±1.5 and 5.1±0.8‰, calculated at pressure-corrected temperatures obtained from fluid inclusion microthermometry. The late-magmatic fluid is relatively 18O-poor with δ18O values estimated at 4.5±0.7‰ and the value is closer to what is obtained for the ore zones. Based on the δ18O values reported and a possible magmatic contribution to ore fluid deciphered from fluid inclusion characteristics, a genetic relationship between granitic magmatism and gold mineralization is surmised. The observed increase in the 18O/16O ratio from the magmatic fluid to ore fluid in the shear zone is attributed to interaction of the magmatic fluid with host metasediments, that agrees well with the variation in the CO2/CH4 ratio of carbonic component in such fluids.

Characteristics of Triassic epithermal Au mineralization at the Q prospect, Chatree mining area, Central Thailand

AbstractThe Chatree deposit is located in the Loei‐Phetchabun‐Nakhon Nayok volcanic belt that extends from Laos in the north through central and eastern Thailand into Cambodia. Gold‐bearing quartz veins at the Q prospect of the Chatree deposit are hosted within polymictic andesitic breccia and volcanic sedimentary breccia. The orebodies of the Chatree deposit consist of veins, veinlets and stockwork. Gold‐bearing quartz veins are composed mainly of quartz, calcite and illite with small amounts of adularia, chlorite and sulfide minerals. The gold‐bearing quartz veins were divided into five stages based on the cross‐cutting relationship and mineral assemblage. Intense gold mineralization occurred in Stages I and IV. The mineral assemblage of Stages I and IV is characterized by quartz–calcite–illite–laumontite–adularia–chlorite–sulfide minerals and electrum. Quartz textures of Stages I and IV are also characterized by microcrystalline and flamboyant textures, respectively. Coexistence of laumontite, illite and chlorite in the gold‐bearing quartz vein of Stage IV suggests that the gold‐bearing quartz veins were formed at approximately 200°C. The flamboyant and brecciated textures of the gold‐bearing quartz vein of Stage IV suggest that gold precipitated with silica minerals from a hydrothermal solution that was supersaturated by boiling. The δ18O values of quartz in Stages I to V range from +10.4 to +11.6‰ except for the δ18O value of quartz in Stage IV (+15.0‰). The increase in δ18O values of quartz at Stage IV is explained by boiling. PH2O is estimated to be 16 bars at 200°C. The fCO2 value is estimated to be 1 bar based on the presence of calcite in the mineral assemblage of Stage IV. The total pressure of the hydrothermal solution is approximately 20 bars at 200°C, suggesting that the gold‐bearing quartz veins of the Q prospect formed about 200 m below the paleosurface.

Oxygen Isotope Diversity in the Anorogenic Koegel Fontein Complex of South Africa: a Case for Basement Control and Selective Melting for the Production of Low- 18O Magmas

Koegel Fontein, about 350 km north of Cape Town, is the only known early Cretaceous anorogenic igneous complex along the volcanic rifted margin of South Africa. The oldest rocks of the complex are minor granite and syenite intrusions at 144 Ma, which were followed by tholeiitic and alkaline basalt dykes, then by microsyenite and quartz porphyry dykes. The youngest and largest igneous unit is the 135 Ma Rietpoort Granite, with an exposed diameter of about 20 km. The country rocks are Mesoproterozoic gneisses of the Namaqua–Natal Province, which in many places were deformed and retrogressed by Pan-African tectonism. Whole-rock δ18O values from the Rietpoort Granite and smaller plutonic units (syenite, granite) are in the range 6–9‰ (outliers 4‰ and 17‰). Quartz δ18O values from all units are in a narrow range and indicate magma δ18O values between 6 and 8‰. In contrast to the syenites and granites, most mafic and silicic dyke units have δ18O

Evolution of a Carboniferous carbonate-hosted sphalerite breccia deposit, Isle of Man

A newly discovered, extensive sphalerite-bearing breccia (~7.5 wt.% Zn) is hosted in dolomitised Carboniferous limestones overlying Ordovician–Silurian metasedimentary rocks on the Isle of Man. Although base metal sulphide deposits have been mined historically on the island, they are nearly all quartz vein deposits in the metamorphic basement. This study investigates the origin of the unusual sphalerite breccia and its relationship to basement-hosted deposits, through a combination of petrographic, cathodoluminescence, fluid inclusion, stable isotope and hydrogeologic modelling techniques. Breccia mineralisation comprises four stages, marked by episodes of structural deformation and abrupt changes in fluid temperature and chemistry. In stage I, high-temperature (T h > 300°C), high-salinity (20–45 wt.% equiv. NaCl) fluid of likely basement origin deposited a discontinuous quartz vein. This vein was subsequently dismembered during a major brecciation event. Stages II–IV are dominated by open-space filling sphalerite, quartz and dolomite, respectively. Fluid inclusions in these minerals record temperatures of ~105–180°C and salinities of ~15–20 wt.% equiv. NaCl. The δ34S values of sphalerite (6.5–6.9‰ Vienna-Canyon Diablo troilite) are nearly identical to those of ore sulphides from mines in the Lower Palaeozoic metamorphic rocks. The δ18O values for quartz and dolomite indicate two main fluid sources in the breccia’s hydrothermal system, local Carboniferous-hosted brines (~0.5–6.0‰ Vienna standard mean ocean water) and basement-involved fluids (~5.5–11.5‰). Ore sulphide deposition in the breccia is compatible with the introduction and cooling of a hot, basement-derived fluid that interacted with local sedimentary brines.

δ18O and chemical composition of Libyan Desert Glass, country rocks, and sands: New considerations on target material

– Oxygen isotope and chemical measurements were carried out on 25 samples of Libyan Desert Glass (LDG), 21 samples of sandstone, and 3 of sand from the same area. The δ18O of LDG samples range from 9.0‰ to 11.9‰ (Vienna Standard Mean Ocean Water [VSMOW]); some correlations between isotope data and typological features of the LDG samples are pointed out. The initial δ18O of a bulk parent material may be slightly increased by fusion due to the loss of isotopically light pore water with no isotope exchange with oxygen containing minerals. Accordingly, the δ18O of the bulk parent material of LDG may have been about 9.0 ± 1‰ (VSMOW). The measured bulk sandstone and sand samples have δ18O values ranging from 12.6‰ to 19.5‰ and are consequently ruled out as parent materials, matching the results of previous studies. However, separated quartz fractions have δ18O values compatible with the LDG values suggesting that the modern surface sand inherited quartz from the target material. This hypothesis fits previous findings of lechatelierite and baddeleyite in these materials. As the age of the parent material reported in previous studies is Pan-African, we measured the δ18O values of bulk rock and quartz from intrusives of Pan-African age and the results obtained were compatible with the LDG values. The main element abundances (Fe, Mg, Ca, K, Na) in our LDG samples conform to previous estimates; Fe, Mg, and K tend to be higher in heterogeneous samples with dark layers. The hypothesis of a low-altitude airburst involving silica-rich surface materials deriving from weathered intrusives of Pan-African age, partially melted and blown over a huge surface by supersonic winds matches the results obtained.

Granitic Pegmatite of the Umanotani-Shiroyama Quartz-Feldspar Mine, Shimane Prefecture, Southwest Japan

The pegmatite deposits at the Umanotani-Shiroyama mine, the largest producer of K-feldspar and quartz in Japan and characterized by a simple mineralogy, dominated by quartz and K-feldspar with minor muscovite and plagioclase, are hosted in the ilmenite-series Masago Granite.The ore zones are classified into the following three zones based on the distance from the host granite inwards: (1) marginal; (2) transitional; and (3) central ore zones. Crystal grain size increases in this order with the resultant occurrence of gigantically grown crystals of quartz in the central ore zone. The evidences presented here reveal that the pegmatite deposits were formed during the latest stage of crystallization and differentiation of the granitic magma being responsible for the Masago Granite. They include:(1) close temporal and spatial association of the granite and the pegmatite deposits (about 95 to 90 Ma), (2) common occurrence of macroscopic “graphic intergrowth" displayed by quartz and K-feldspar in orebodies, (3) common occurrence of melt inclusions trapped in “ore quartz" and “ore K-feldspar", (4) common occurrence of perthites observed in “ore K-feldspar" and (5) inheritance of oxygen isotopic signature, especially of quartz, from the surrounding biotite granite. The δ18O values of quartz, whether in the related igneous rocks or in ores, are almost the same, around +12 ‰, while those of K-feldspar in the igneous rocks is around +11‰ and those in ores are significantly depleted in 18O, about +7 to +8 ‰. It might be due to the difference in the exchange reaction rate of oxygen between quartz and K-feldspar. Abundant two-phase (liquid+gas) fluid inclusions are trapped in “ore quartz" and “ore K-feldspar", as well as in the related igneous rocks. Their presence strongly indicates that hydrothermal fluids, possibly admixture of magmatic fluids released from the Masago Granite and circulating meteoric water, entered to the magmatic-hydrothermal system and circulated through the orebodies mainly during the later phase of the pegmatite formation. Measured homogenization temperatures of two-phase inclusions trapped in “ore quartz" are in the following ranges: 250 to 400 ℃ in the marginal ore zone; 230 to 370 ℃ in the transitional ore zone; 240 to 340 ℃ in the central ore zone. Also suggested is that late circulation of hydrothermal fluids lasted for not so long time as to affect the δ18O values of quartz within the magmatic-hydrothermal system, resulting in escape of the fluids from the system through ubiquitously observed quartz±muscovite veins and fissures.

Foreword to the special volume on "Mineralogical, geochemical and isotopic links between granitic pegmatites and their parental granites"

This special issue of Journal of Geosciences contains several papers drawing attention to the genetic, spatial and temporal relationships of granitic pegmatites and related granites – one of the paramount problems of granitic pegmatite genesis. Wise and Brown address the granite–pegmatite link at the famous Sebago system, southern Maine, USA. The authors show mineralogical, petrological and geochemical data that suggest that pegmatitic leucogranites are the likely parent to most LCT granitic pegmatites in this region, although another fertile granite–pegmatite system may be responsible for some anomalous pegmatite dikes showing NYF affinity. Swanson and Veal describe relationships between pegmatites and host granitoids in the high-pressure (0.7–1.1 GPa) Spruce Pine district, North Carolina, USA. They focus on the textural transition from granitoids to pegmatites and argue that the transition was controlled mainly by different nucleation rates as related to water content of the melts. Colombo et al. focus on mineralogy and crystal chemistry of micas from the A-type El Portezuelo granite and related miarolitic pegmatites at Catamarca, northwestern Argentina. The micas (biotite, muscovite, zinnwaldite) exhibit compositional evolution characteristic of NYF-type granite– miarolitic pegmatite systems. Beal and Lentz present detailed mineralogical and isotopic study of aquamarine beryl in A-type pegmatite–aplite dykes and associated quartz veins and greisen at the Zealand Station Be–Mo deposit, New Brunswick, Canada. High δ18O values of quartz and beryl indicate a considerable crustal component to the magmas and low δD values of the channel water in beryl reflect crystallization from a volatile-undersaturated magmatic source before pegmatite dyke emplacement. Uher et al. describe primary and secondary compositional trends in beryl in LCT pegmatites of the beryl–columbite subtype in western Carpathians, Slovakia. In the primary trend, Cs, Cs/Na, Mg and Mg/Fe decrease in most beryl crystals from less evolved to more fractionated pegmatites. In the secondary trend, there is decreasing Cs and increasing Mg/Fe that may represent a late replacement. These contributions demonstrate the complexity of granite–pegmatite links through mineralogical, geochemical and isotopic data and field relationships. The papers enhance our understanding of the processes that lead to the links.

Channelized fluids in subducted continental crust: constraints from δD–δ18O of quartz and fluid inclusions in quartz veins from the Chinese Continental Scientific Drilling Project

Fluid inclusions hosted by quartz veins in high-pressure to ultrahigh-pressure (HP-UHP) metamorphic rocks from the Chinese Continental Scientific Drilling (CCSD) Project main drillhole have low, varied hydrogen isotopic compositions (δD = −97‰ to −69‰). Quartz δ18O values range from −2.5‰ to 9.6‰; fluid inclusions hosted in quartz have correspondingly low δ18O values of −11.66‰ to 0.93‰ (T h = 171.2∼318.8°C). The low δD and δ18O isotopic data indicate that protoliths of some CCSD HP-UHP metamorphic rocks reacted with meteoric water at high latitude near the surface before being subducted to great depth. In addition, the δ18O of the quartz veins and fluid inclusions vary greatly with the drillhole depth. Lower δ18O values occur at depths of ∼900–1000 m and ∼2700 m, whereas higher values characterize rocks at depths of about 1770 m and 4000 m, correlating roughly with those of wall-rock minerals. Given that the peak metamorphic temperature of the Dabie-Sulu UHP metamorphic rocks was about 800°C or higher, much higher than the closure temperature of oxygen isotopes in quartz under wet conditions, such synchronous variations can be explained by re-equilibration. In contrast, δD values of fluid inclusions show a different relationship with depth. This is probably because oxygen is a major element of both fluids and silicates and is much more abundant in the quartz veins and silicate minerals than is hydrogen. The oxygen isotope composition of fluid inclusions is evidently more susceptible to late-stage re-equilibration with silicate minerals than is the hydrogen isotope composition. Therefore, different δD and δ18O patterns imply that dramatic fluid migration occurred, whereas the co-variation of oxygen isotopes in fluid inclusions, quartz veins, and wall-rock minerals can be better interpreted by re-equilibration during exhumation. Quartz veins in the Dabie-Sulu UHP metamorphic terrane are the product of high-Si fluids. Given that channelized fluid migration is much faster than pervasive flow, and that the veins formed through precipitation of quartz from high-Si fluids, the abundant veins indicate significant fluid mobilization and migration within this subducted continental slab. Many mineral reactions can produce high-Si fluids. For UHP metamorphic rocks, major dehydration during subduction occurred when pressure–temperature conditions exceeded the stability of lawsonite. In contrast, for low-temperature eclogites and other HP metamorphic rocks with peak metamorphic P–T conditions within the stability field of lawsonite, dehydration and associated high-Si fluid release may have occurred as hydrous minerals were destabilized at lower pressure during exhumation. Because subduction is a continuous process whereas only a minor fraction of the subducted slabs returns to the surface, dehydration during underflow is more prevalent than exhumation even in subducted continental crust, which is considerably drier than altered oceanic crust.

NATURE AND ORIGIN OF AN LCT-SUITE PEGMATITE WITH LATE-STAGE SODIUM ENRICHMENT, BRAZIL LAKE, YARMOUTH COUNTY, NOVA SCOTIA. II. IMPLICATIONS OF STABLE ISOTOPES ( 18O, D) FOR MAGMA SOURCE, INTERNAL CRYSTALLIZATION AND NATURE OF SODIUM METASOMATISM

The 395 Ma Brazil Lake granitic pegmatite, a rare occurrence of an LCT-type (Li–Cs–Ta) pegmatite in the Meguma Terrane of Nova Scotia, Canada, is hosted by metasedimentary and metavolcanic rocks of the Silurian White Rock Group. Previous mineralogical studies showed the pegmatite formed in two distinct stages, an early spodumene – K-feldspar – quartz ± muscovite stage that was followed by a pervasive metasomatic stage in which albite and secondary muscovite grew at the expense of earlier K-feldspar. The economically important oxide phases (Ta–Nb) are intimately associated with secondary albite. Determination of the stable isotopic (δ18O, D) values for silicate phases (quartz, K-feldspar, spodumene, albite, muscovite, garnet, tourmaline) provide insight into the origin of the pegmatite-forming melt and its subsequent internal evolution. A primary δ18O signature for the melt of +8‰ is calculated using the δ18Ovalues of quartz and Δquartz – whole-rock values from studies on other granitic suites. Interpreted in the context of the geological setting, the δ18O data preclude a dominantly metasedimentary source and suggest, instead, that the pegmatite-forming melt originated via partial melting of a mixed igneous ( i.e., volcanic)- sedimentary source. The calculated δ18OH2O–δDH2O values, at 500°C, correspond to the field for magmatic waters, but some isotopic data, including δD for fluid-inclusion extracts, suggest late-stage incursion of metamorphic water that had equilibrated with the surrounding metasedimentary rocks. Mineral–mineral fractionation ( e.g., ΔQtz–Ms) does not reflect equilibrium within the pegmatite, which may have resulted from several processes, including non-equilibrium crystallization, variable degrees of isotopic exchange during protracted cooling, or interaction with an external reservoir(s). The data for albite (δ18O = +9.1‰, n = 6) are consistent with a magmatic origin, albeit late in the evolution of the pegmatite, on the basis of textural arguments. A model is proposed for the origin of the extensive albite-enriched zones, which involves introduction of a sodic melt derived at depth after protracted fractionation of the same parental melt from which the Li-rich parts of the pegmatite originated.

The world-class Natalka gold deposit, northeast Russia: REE patterns, fluid inclusions, stable oxygen isotopes, and formation conditions of ore

REE patterns of hydrothermally altered rocks, fluid inclusions, and stable oxygen isotopes of quartz were studied at the Natalka gold deposit. Metasomatic rocks formed under decompression reveal gradual depletion in LREE and HREE relative to siltstone of the protolith. The HREE patterns of metasomatic rocks formed under decompression are uniform; an insignificant removal of LREE can be noted. The progressive extraction of REE with increasing alteration of rocks could have been due to the effect of magmatogenic or meteoric fluid. Because a Ce anomaly is absent, the participation of oxidized meteoric water was limited. The inverse correlation between the total REE content and the Eu anomaly value in altered rocks indicates a substantial role of magmatogenic fluid. The REE patterns of altered rocks formed under compression show that the role of metamorphic fluid was not great. All metasomatic rocks are enriched in LREE, so that the enrichment of fluid in LREE as well may be suggested. Three fluid compositions were captured as fluid inclusions: (1) H2O-CO2-NaCl-MgCl2 with a salinity of 1.0–4.9 wt % NaCl equiv, (2) CO2-CH4, and (3) H2O-NaCl-MgCl2 with a salinity of 7.0–5.6 wt % NaCl equiv. Compositions (1) and (2) coexisted in the mineral-forming system at 250–350°C and 1.1–2.4 kbar as products of phase separation under conditions of decreasing P and T. The interaction of this fluid with host rocks resulted in the formation of extensive halos of beresitized rocks with sulfide disseminations. The precipitation of arsenopyrite and pyrite led to the substantial depletion of mineral-forming fluid in H2S and destabilization of the Au(HS)2− complex. The fluid with the third composition arose due to the boiling of the H2O-CO2-CH4-NaCl-MgCl2 liquid and was responsible for metasomatic alteration of host rocks. The late mineral assemblages were deposited from this fluid at the initial stage of ore formation. The high methane concentrations in the ore-forming fluid were likely caused by interaction of hydrothermal ore-bearing solutions with carbonaceous host rocks. The δ18O values of quartz from quartz-scheelite-pyrite-arsenopyrite and sulfide-sulfosalt mineral assemblages vary from +11.6 to +14.1‰ and +11.2 to +13.5‰, respectively. The parental fluids of the early and late mineral assemblages probably were derived from a magmatic source and were characterized by \( \delta ^{18} O_{H_2 O} \) = +6.3 to +8.8‰ at 350°C and +3.6 to +5.9‰ at 280°C, respectively. The narrow interval of oxygen isotopic compositions shows that this source was homogeneous. The data obtained allow us to suggest that the deposit formation was related to magmatic activity, including the direct supply of ore components from a magma chamber and mobilization of these components in the processes of dehydration and decarbonation during contact and regional metamorphism.

18O/16O Evidence for Contrasting Hydrothermal Regimes Involving Magmatic and Meteoric-Hydrothermal Waters at the Valhalla Metamorphic Core Complex, British Columbia

Modeling of published oxygen isotope analyses of 115 whole-rock, quartz, and feldspar samples from the Valhalla metamorphic core complex provides evidence for three distinct stages of hydrothermal activity affecting the detachment faults and upper plate. (1) Lithostatically pressured aqueous fluids having δ18O ~10 per mil were expelled from the lower plate and moved upward into the upper plate through an oblique-sinistral transfer zone between the southern terminus of the Columbia River detachment fault and the northern terminus of the Slocan Lake detachment fault. These fluids ascended through fractures and exchanged with Slocan Group shales and graywackes to produce the marked zonation of vein siderite δ18O (11.4–18.4‰) observed in previous studies of the Sandon group of Ag-Pb-Zn lode deposits. (2) Later, as brittle deformation along the Slocan Lake detachment fault became the dominant tectonic process, these magmatic and/or metamorphic fluids intermittently mixed with even larger quantities of downward circulating, hydrostatically pressured meteoric-hydrothermal fluids. In the upper plate Nelson granodiorite adjacent to the Slocan Lake detachment fault, δ18O values of quartz (8.9–11.9‰) and feldspar (−5.0 to +9.6‰) define a steep array in δ-δ space (slope = 25), suggesting that synextensional meteoric-hydrothermal activity along the Slocan Lake detachment fault (initial δ18Owater ~−15‰ and T ~300°C) was relatively short-lived (~106 yr), but very intense, with W/Ropen-system = ≤1.05 and fluid flux = 0.1 to 3 m/yr. The δ18O values of quartz (10.8–12.4‰) and feldspar (6.4–10.1‰) from lower plate greenschist mylonites indicate that small quantities of the low 18O meteoric-hydrothermal fluids penetrated into the lower plate. The systematic decrease in feldspar δ18O toward the Slocan Lake detachment fault in both the upper and lower plates shows that these low 18O waters were channeled along the detachment fault. (3) Meteoric-hydrothermal activity along the Valkyr shear zone postdates ductile deformation and was associated with the emplacement of the Coryell plutons in the upper plate. The δ18O values of quartz (7.6–11.0‰) and feldspar (0.0–8.6‰) from the Valkyr shear zone define a much shallower array in δ-δ space (slope = 2.3) than those at the Slocan Lake detachment fault, indicating a higher temperature and/or longer lived episode of hydrothermal activity. The geographic distribution of pronounced disequilibrium δ18O effects in Coryell quartz (5.8–8.2‰) and feldspar (−4.4 to +7.4‰) indicates that meteoric-hydrothermal activity was most intense along inferred coeval extensional fracture zones that lie beneath Arrow Lake. This modeling suggests that transfer zones that accommodate deformation between detachment faults may be a very attractive exploration setting for lode-type mineral deposits.

Geology and geochemistry of jasperoids from the Gold Bar district, Nevada

Gold Bar is one of several Carlin-type gold mining districts located in the Battle Mountain–Eureka trend, Nevada. It is composed of one main deposit, Gold Bar; five satellite deposits; and four resources that contain 1.6 Moz (50 t) of gold. All of the deposits and resources occur at the intersection of north-northwest- and northeast-trending high-angle faults in slope facies limestones of the Devonian Nevada Group exposed in windows through Ordovician basin facies siliciclastic rocks of the Roberts Mountains allochthon. Igneous intrusions and magnetic anomalies are notably absent. The Gold Bar district contains a variety of discordant and stratabound jasperoid bodies, especially along the Wall Fault zone, that were mapped and studied in some detail to identify the attributes of those most closely associated with gold ore and to constrain genetic models. Four types of jasperoids, J0, J1, J2, and J3, were distinguished on the basis of their geologic and structural settings and appearance. Field relations suggest that J0 formed during an early event. Petrographic observations, geochemistry, and δ18O values of quartz suggest it was overprinted by the hydrothermal event that produced ore-related J1, J2, and J3 jasperoids and associated gold deposits. The greater amount of siliciclastic detritus present in J0 jasperoids caused them to have higher δ18O values than J1,2,3 jasperoids hosted in underlying limestones. Ore-related jasperoids are composed of main-ore-stage replacements and late-ore-stage open-space filling quartz with variable geochemistry and an enormous range of δ18O values (24.5 and −3.7‰). Jasperoids hosted in limestones with the most anomalous Au, Ag, Hg, ±(As, Sb, Tl) concentrations and the highest δ18O values are associated with the largest deposits. The 28‰ range of jasperoid δ18O values is best explained by mixing between an 18O-enriched fluid and an 18O-depleted fluid. The positive correlation between the sizes of gold deposits and the δ18O composition of jasperoids indicates that gold was introduced by the 18O-enriched fluid. The lowest calculated δ18O value for water in equilibrium with late-ore-stage quartz at 200°C (−15‰) and the measured δD value of fluid inclusion water extracted from late-ore-stage orpiment and realgar (−116‰) indicate that the 18O-depleted fluid was composed of relatively unexchanged meteoric water. The source of the 18O-enriched ore fluid is not constrained. The δ34S values of late-ore-stage realgar, orpiment, and stibnite (5.7–15.5‰) and barite (31.5–40.9‰) suggest that H2S and sulfate were derived from sedimentary sources. Likewise, the δ13C and δ18O values of late-stage calcite (−4.8 to 1.5‰ and 11.5 to 17.4‰, respectively) suggest that CO2 was derived from marine limestones. Based on these data and the apparent absence of any Eocene intrusions in the district, Gold Bar may be the product of a nonmagmatic hydrothermal system.

Silicification of sulphate evaporites and their carbonate replacements in Eocene marine sediments, Tunisia: two diagenetic trends

AbstractSamples of chert nodules, diagenetic carbonates and evaporites (gypsum/anhydrite) collected from the gypsiferous limestones of the Kef Eddour Member (Ypressian‐Priabonian) near Metlaoui and Sehib (Tunisia) show selective silicification with great variety in the silicified by‐products. Based on δ13C values, which support an organic origin for the carbon, carbonates replaced evaporites microbially through bacterial sulphate reduction. Observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites (anhydrite/gypsum). In the absence of early diagenetic carbonate phases, silica with δ18O values from +25 to +28·6‰ [standard mean ocean water (SMOW)] replaced the outer parts of anhydrite nodules at pH &lt; 9. In contrast, pore‐fluid pH values &gt; 9 in the innermost parts of the anhydrite nodules prevented silica precipitation. The record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus. The microbial diagenetic replacement of evaporites (bacterial sulphate reduction) by carbonates (calcite, aragonite and dolomite) favoured silica replacement of carbonates rather than evaporites. Silica, with δ18O signature of +21 to +26‰ (SMOW), replaced carbonates on a volume‐for‐volume basis, yielding a more siliceous groundmass, and accounting for 90–95% of the nodules. The relatively higher δ18O values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed (meteoric/marine) to marine water. The lower δ18O values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage. The silica supply for chert formation could be derived from the reworked bio‐siliceous deposits (diatomites) to the west of the basin [vestiges of an opal‐CT precursor undetectable by X‐ray diffraction (XRD) were revealed by δ29Si magic‐angle‐spinning nuclear magnetic resonance investigations], diagenesis of the extraformational and overlying clay‐rich beds (the host limestones are clay‐poor as shown by XRD measurements), and minor volcanogenic and hydrothermal contributions during early diagenetic stages.

Hydrothermal Alteration and Mineralization in the Neves-Corvo Volcanic-Hosted Massive Sulfide Deposit, Portugal. II. Oxygen, Hydrogen, and Carbon Isotopes

The oxygen, hydrogen, and carbon isotope compositions of hydrothermally altered rocks and minerals from massive and stockwork ores of the Neves-Corvo volcanic-hosted massive sulfide deposit have been used to infer its thermal history and the origins of the hydrothermal fluids. Quartz and cassiterite separated from the stringer and massive cassiterite ores of the Corvo orebody (“tin corridor”) have δ18O values of 12.5 to 13.6 and −1.4 to −0.4 per mil, respectively. Quartz-cassiterite oxygen isotope fractionation indicates temperatures of 174° to 207°C (avg 191°C) for precipitation of this mineral pair. The calculated oxygen isotope composition of the fluid (δ18OH2O = −0.4 to +0.7‰; avg 0.1‰) suggests that cassiterite precipitation resulted from extensive mixing of a tin-bearing fluid with seawater at sea-floor hydrothermal vents. The whole-rock oxygen isotope composition of the hydrothermally altered, felsic volcanic rocks hosting the sulfide stockwork of the Corvo orebody varies from the central stockwork (chloritic alteration zone I, δ18O = 8.7–9.6‰), to the peripheral zones (sericitic and paragonitic alteration zones IIa, δ18O = 11.6–13.1‰, and IIb, δ18O = 11.0–14.1‰). The δ18O values of quartz also increase from alteration zone I (avg 13.9‰) to alteration zone IIa (avg 14.4‰) to alteration zone IIb (avg 15.0‰). Quartz from the hanging-wall jasper unit yielded higher δ18O values of 15.5 to 17.9 per mil. Chlorite from alteration zone I has δ18O values of 6.1 to 8.4 per mil (avg 7.4‰). Sericite has δ18O and δD values that vary slightly from δ18O = 9.3 to 10.1 per mil (avg 9.9‰) and δD = −59 to −47 per mil (avg −53‰) in alteration zone I, to δ18O = 9.8 to 11.4 per mil (avg 10.5‰) and δD = −50 per mil in alteration zone IIa, to δ18O = 9.4 to 11.7 per mil (avg 10.6‰) and δD = −61 to −36 per mil (avg −45‰) in alteration zone IIb. Siderite associated with waning hydrothermal activity has low δ13C values (−11.1 to −5.8‰), suggesting the incorporation of oxidized, organic carbon from the footwall sequence and/or the addition of magmatic carbon. In alteration zone I, quartz-chlorite fractionation indicates temperatures of 249° to 402°C (avg 335°C) and δ18OH2O values of 6.1 to 10.2 per mil (avg 8.3‰). Quartz-sericite pairs gave isotopic equilibrium temperatures of 250° to 289°C (avg 269°C) and δ18OH2O values of 5.8 to 7.0 per mil (avg 6.4‰) in alteration zone IIa, and 170° to 318°C (avg 223°C) and δ18OH2O = 1.4 to 8.3 per mil (avg 4.5‰) in alteration zone IIb. The oxygen isotope data show that in the core of the feeder system (alteration zone I), the temperature and the δ18OH2O values were very high. The δD values calculated for the ore fluids (δD = −37 to −11‰) are also significantly lower than seawater. Outward from the central stockwork (alteration zones IIa and IIb) the ore fluid was depleted in 18O and the temperatures were lower. The oxygen, hydrogen, and carbon isotope signatures of the Neves-Corvo ore fluids are best explained by incorporation of magmatic and/or metamorphic fluids into a seawater-dominated hydrothermal system. Magmatic fluids, in particular, were a likely source for the tin and much of the copper in the Neves-Corvo ores.

Stable isotope geochemistry of the Archean Val-d‘Or (Canada) orogenic gold vein field

A systematic study of the auriferous quartz veins of the Val-d’Or vein field, Abitibi, Quebec, Canada, demonstrates that the C, O, S isotope composition of silicate, carbonate, borate, oxide, tungstate and sulphide minerals have a range in composition comparable to that previously determined for the whole Superior Province. The oxygen isotope composition of quartz from early quartz–carbonate auriferous veins ranges from 9.4 ‰ to 14.4 ‰ whereas later quartz-tourmaline-carbonate veins have δ18Oquartz values ranging from 9.2 ‰ to 13.8 ‰. Quartz-carbonate veins have carbonate (δ18O: 6.9–12.5 ‰; δ13C: −6.2– −1.9 ‰) and pyrite (δ34S: 1.2 and 1.9 ‰) isotope compositions comparable to those of quartz-tourmaline-carbonate veins (δ18O: 7.9–11.7 ‰; δ13C: −8.0 – −2.4 ‰; δ34S: 0.6–6.0 ‰). δ18Oquartz values in quartz-tourmaline-carbonate veins have a variance comparable to analytical uncertainty at the scale of one locality, irrespective of the type of structure, the texture of the quartz or its position along strike, across strike or down-dip a vein. In contrast, the oxygen isotope composition of quartz in quartz-tourmaline-carbonate veins displays a regional distribution with higher δ18O values in the south-central part of the vein field near the Cadillac Tectonic Zone, and which δ18O values decrease regularly towards the north. Another zone of high δ18O values in the northeast corner of the region and along the trace of the Senneville Fault is separated by a valley of lower δ18O values from the higher values near the Cadillac Tectonic Zone. Oxygen isotope isopleths cut across lithological contacts and tectonic structures. This regional pattern in quartz-tourmaline-carbonate veins is interpreted to be a product of reaction with country rocks and mixing between (1) a deep-seated hydrothermal fluid of metamorphic origin with minimum δ18O=8.5 ‰, δ13C=0.6 ‰ and δ34S=−0.4 ‰, and (2) a supracrustal fluid, most likely Archean seawater with a long history of water-rock exchange and with maximum δ18O=3.9 ‰, δ13 C=−5.6 ‰ and δ34S=5.0 ‰.

Geological and Geochemical Characteristics of the Zhulazhaga Gold Deposit in Inner Mongolia, China

Abstract Located in Alxa Zuoqi (Left Banner) of Inner Mongolia, China, the Zhulazhaga gold deposit is the first large‐scale gold deposit that was found in the middle‐upper Proterozoic strata along the north margin of the North China craton in recent years. It was discovered by the No. 1 Geophysical and Geochemical Exploration Party of Inner Mongolia as a result of prospecting a geochemical anomaly. By now, over 50 tonnes of gold has been defined, with an average Au grade of 4 g/t. The ore bodies occur in the first lithological unit of the Mesoproterozoic Zhulazhagamaodao Formation (MZF), which is composed mainly of epimetamorphic sandstone and siltstone and partly of volcanic rocks. With high concentration of gold, the first lithological unit of the MZF became the source bed for the late‐stage ore formation. Controlled by the interstratal fracture zones, the ore bodies mostly appear along the bedding with occurrence similar to that of the strata. The primitive ore types are predominantly the altered rock type with minor ore belonging to the quartz veins type. There are also some oxidized ore near the surface. The metallic minerals are composed mainly of pyrite, pyrrhotite and arsenopyrite with minor chalcopyrite, galena and limonite. Most gold minerals appear as native gold and electrum. Hydrothermal alterations associated with the ore formation are actinolitization, silicatization, sulfidation and carbonation.A total of 100 two‐phase H2O‐rich and 7 three‐phase daughter crystal‐bearing inclusions were measured in seven gold‐bearing quartz samples from the Zhulazhaga gold deposit. The homogenization temperatures of the two‐phase H2O‐rich inclusions range from 155 to 401°C, with an average temperature of 284°C and bimodal distributions from 240 to 260°C and 300 to 320°C respectively. The salinities of the two‐phase H2O‐rich inclusions vary from 9.22wt% to 24.30wt% NaCl equiv, with a mode between 23wt% and 24wt% NaCl equiv. Comparatively, the homogenization temperatures of the three‐phase daughter crystal‐bearing inclusions vary from 210 to 435°C and the salinities from 29.13wt% to 32.62wt% NaCl equiv. It indicates that the ore‐forming fluid is meso‐hypothermal and characterized by high salinity, which is apparently different from the metamorphic origin with low salinity. It suggests a magmatic origin of the gold‐bearing fluid.The δ18O values of quartz from auriferous veins range from 11.9 to 16.3 per mil, and the calculated δ18O values in equilibrium with quartz vary from 1.06 to 9.60 per mil, which fall between the values of meteoric water and magmatic water. It reflects that the ore‐forming fluid may be the product of mixing of meteoric water and magmatic water. Based on geological and geochemical studies of the Zhulazhaga gold deposit, it is supposed that the volcanism in the Mesoproterozoic might make gold pre‐concentrate in the strata. The extensive and intensive Hercynian tectono‐magmatic activity not only brought along a large number of ore‐forming materials, but also made the gold from the strata rework. It can be concluded that the ore bodies were mainly formed in late hydrothermal reworking stage. Compared with typical gold deposits associated with epimetamorphic clastic rocks, the Zhulazhaga deposit has similar features in occurrence of ore bodies, ore‐controlling structure, wall‐rock alterations and mineral assemblages. Therefore, the Zhulazhaga gold deposit belongs to the epimetamorphic clastic rock type.