Chemical and boron-isotope variations in tourmalines from an S-type granite and its source rocks: the Erongo granite and tourmalinites in the Damara Belt,Namibia

Tourmaline is widespread in metapelites and pegmatites from the Neoproterozoic Damara Belt, which form the basement and potential source rocks of the Cretaceous Erongo granite. This study traces the B-isotope variations in tourmalines from the basement, from the Erongo granite and from its hydrothermal stage. Tourmalines from the basement are alkali-deficient schorl-dravites, with B-isotope ratios typical for continental crust (δ¹¹B average −8.4‰ ± 1.4, n = 11; one sample at −13‰, n = 2). Virtually all tourmaline in the Erongo granite occurs in distinctive tourmaline-quartz orbicules. This “main-stage” tourmaline is alkali-deficient schorl (20–30% X-site vacancy, Fe/(Fe + Mg) 0.8–1), with uniform B-isotope compositions (δ¹¹B −8.7‰ ± 1.5, n = 49) that are indistinguishable from the basement average, suggesting that boron was derived from anatexis of the local basement rocks with no significant shift in isotopic composition. Secondary, hydrothermal tourmaline in the granite has a bimodal B-isotope distribution with one peak at about −9‰, like the main-stage tourmaline, and a second at −2‰. We propose that the tourmaline-rich orbicules formed late in the crystallization history from an immiscible Na–B–Fe-rich hydrous melt. The massive precipitation of orbicular tourmaline nearly exhausted the melt in boron and the shift of δ¹¹B to −2‰ in secondary tourmaline can be explained by Rayleigh fractionation after about 90% B-depletion in the residual fluid. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Tourmaline geochemistry and δ<Superscript>11</Superscript>B variations as a guide to fluid–rock interaction in the Habachtal emerald deposit,Tauern Window,Austria

Tourmalines from the Habachtal emerald deposit in the Eastern Alps formed together with emerald in a ductile shear zone during blackwall metasomatism between pelitic country rocks and a serpentinite body. Electron microprobe and secondary ion mass spectrometric (SIMS) analyses provide a record of chemical and B-isotope variations in tourmalines which represent an idealized profile from metapelites into the blackwall sequence of biotite and chlorite schists. Tourmaline is intermediate schorl-dravite in the country rock and become increasingly dravitic in the blackwall zones, while F and Cr contents increase and Al drops. Metasomatic tourmaline from blackwall zones is typically zoned optically and chemically, with rim compositions rich in Mg, Ti, Ca and F compared with the cores. The total range in δ¹¹B values is −13.8 to −5.1‰ and the within-sample variations are typically 3–5‰. Both of these ranges are beyond the reach of closed-system fractionation at the estimated 500–550°C conditions of formation, and at least two boron components with contrasting isotopic composition are indicated. A key observation from tourmaline core analyses is a systematic shift in δ¹¹B from the country rock (−14 to −10‰) to the inner blackwall zones (−9 to −5‰). We suggest that two separate fluids were channeled and partially mixed in the Habachtal shear zone during blackwall alteration and tourmaline-emerald mineralization. A regional metamorphic fluid carried isotopically light boron as observed in the metapelite country rocks. The other fluid is derived from the serpentinite association and has isotopically heavier boron typical for MORB or altered oceanic crust. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Detrital,metamorphic and metasomatic tourmaline in high-pressure metasediments from Syros (Greece): intra-grain boron isotope patterns determined by secondary-ion mass spectrometry

The boron isotopic composition of zoned tourmaline in two metasediments from the island of Syros, determined by secondary-ion mass spectrometry (SIMS), reflects the sedimentary and metamorphic record of the rocks. Tourmaline from a silicate-bearing marble contains small (≤20 μm) detrital cores with highly variable δ ¹¹B values (−10.7 to +3.6‰), pointing to a heterogeneous protolith derived from multiple sources. The sedimentary B isotopic record survived the entire metamorphic cycle with peak temperatures of ∼500°C. Prograde to peak metamorphic rims are homogeneous and similar among all analysed grains (δ ¹¹B ≈ +0.9‰). The varying δ ¹¹B values of detrital cores in the siliceous marble demonstrate that in situ B isotope analysis of tourmaline by SIMS is a potentially powerful tool for provenance studies not only in sediments but also in metasediments. A meta-tuffitic blueschist bears abundant tourmaline with dravitic cores of detrital or authigenic origin (δ ¹¹B ≈ −3.3‰), and prograde to peak metamorphic overgrowth zones (−1.6‰). Fe-rich rims, formed during influx of B-bearing fluids under retrograde conditions, show strongly increasing δ ¹¹B values (up to +7.7‰) towards the margins of the grains. The δ ¹¹B values of metamorphic tourmaline from Syros, formed in mixed terrigenous–marine sediments, reflect the B signal blended from these two different sources, and was probably not altered by dehydration during subduction.     

Fluctuations in a magmatic sulphur isotope signature from the Pinnacles Mine,New South Wales,Australia

Sulphur isotopic compositions of 29 sulphide samples from the Broken Hill-type Pinnacles Deposit, NSW, are found to cluster at 0%. (mean −0.8‰). The restricted range of the (δ³⁴S) values between −3.5 and + 3.7‰ with a mean of −0.8‰, is interpreted as reflecting partial oxidation of a dominantly magmatic sulphur source. δ³⁴S data for galena samples fall into two groups: (1) isotopically heavier galenas (range −0.7 to 0.0‰; mean −0.4‰) which come mainly from the footwall Zn lode and (2) isotopically lighter galenas (range −3.5 to −0.8‰; mean −2.2‰) which are from the main Pb lode. Sphalerite, pyrrhotite and chalcopyrite have slightly heavier isotopic compositions (range −1.6 to +3.7‰ mean +0.3‰) but exhibit the same stratigraphic differentiation. These data are interpreted as representing fluctuating conditions at the site of ore deposition, in which upwelling hydrothermal fluids were subject to increasing fO₂ and decreasing temperature with time.     

Boron-isotope fractionation between tourmaline and fluid: an experimental re-investigation

The fractionation of boron isotopes between synthetic dravitic tourmaline and fluid was determined by hydrothermal experiments between 400 and 700°C at 200 MPa and at 500°C, 500 MPa. Tourmaline was crystallized from an oxide mix in presence of water that contained boron in excess. In one series of experiments, [B]_fluid/[B]_tour was 9 after the run; in another series it was 0.1. All experiments produced tourmaline as the sole boron-bearing solid, along with traces of quartz and talc. Powder XRD and Rietveld refinements revealed no significant amounts of tetrahedrally coordinated boron in tourmaline. ¹¹B always preferentially fractionated into the fluid. For experiments where [B]_fluid/[B]_tour was 9, a consistent temperature-dependent boron isotope fractionation curve resulted, approximated by Δ¹¹B_{(tour–fluid)} = −4.20 · [1,000/T (K)] + 3.52; R ² = 0.77, and valid from 400 to 700°C. No pressure dependence was observed. The fractionation (−2.7 ± 0.5‰ at 400°C; and −0.8 ± 0.5‰ at 700°C) is much lower than that previously presented by Palmer et al. (1992). Experiments where [B]_fluid/[B]_tour was 0.1 showed a significant larger apparent fractionation of up to −4.7‰. In one of these runs, the isotopic composition of handpicked tourmaline crystals of different size varied by 1.3‰. This is interpreted as resulting from fractional crystallization of boron isotopes during tourmaline growth due to the small boron reservoir of the fluid relative to tourmaline, thus indicating larger fractionation than observed at equilibrium. The effect is eliminated or minimized in experiments with very high boron excess in the fluid. We therefore suggest that values given by the above relation represent the true equilibrium fractionations.     

Imprint of meteoric water on the stable isotope compositions of igneous and secondary minerals,Kap Edvard Holm Complex,East Greenland

 Hydrogen and oxygen isotope analyses have been made of hydrous minerals in gabbros and basaltic xenoliths from the Eocene Kap Edvard Holm intrusive complex of East Greenland. The analyzed samples are of three types: (1) primary igneous hornblendes and phlogopites that crystallized from partial melts of hydrothermally altered basaltic xenoliths, (2) primary igneous hornblendes that formed during late–magmatic recrystallization of layered gabbroic cumulates, and (3) secondary actinolite, epidote and chlorite that formed during subsolidus alteration of both xenoliths and gabbros. Secondary actinolite has a δ¹⁸O value of −5.8‰ and a δD value of −158‰. These low values reflect subsolidus alteration by low–δ¹⁸O, low–δD hydrothermal fluids of meteoric origin. The δD value is lower than the −146 to −112‰ values previously reported for amphiboles from other early Tertiary meteoric–hydrothermal systems in East Greenland and Scotland, indicating that the meteoric waters at Kap Edvard Holm were isotopically lighter than typical early Tertiary meteoric waters in the North Atlantic region. This probably reflects local climatic variations caused by formation of a major topographic dome at about the time of plutonism and hydrothermal activity. The calculated isotopic composition of the meteoric water is δD=−110 ± 10‰, δ¹⁸O ≈−15‰. Igneous hornblendes and phlogopites from pegmatitic pods in hornfelsed basaltic xenoliths have δ¹⁸O values between −6.0 and −3.8‰ and δD values between −155 and −140‰. These are both much lower than typical values of fresh basalts. The oxygen isotope fractionations between pegmatitic hornblendes and surrounding hornfelsic minerals are close to equilibrium fractionations for magmatic temperatures, indicating that the pegmatites crystallized from low–δ¹⁸O partial melts of xenoliths that had been hydrothermally altered and depleted in ¹⁸O prior to stoping. The pegmatitic minerals may have crystallized with low primary δD values inherited from the altered country rocks, but these values were probably overprinted extensively by subsolidus isotopic exchange with low–δD meteoric–hydrothermal fluids. This exchange was facilitated by rapid self–diffusion of hydrogen through the crystal structures. Primary igneous hornblendes from the plutonic rocks have δ¹⁸O values between +2.0 and +3.2‰ and δD values between −166 and −146‰. The ¹⁸O fractionations between hornblendes and coexisting augites are close to equilibrium fractionations for magmatic temperatures, indicating that the hornblendes crystallized directly from the magma and subsequently underwent little or no oxygen exchange. The hornblendes may have crystallized with low primary δD values, due to contamination of the magma with altered xenolithic material, but the final δD values were probably controlled largely by subsolidus isotopic exchange. This inference is based partly on the observation that coexisting plagioclase has been extensively depleted in ¹⁸O via a mineral–fluid exchange reaction that is much slower than the hydrogen exchange reaction in hornblende. It is concluded that all hydrous minerals in the study area, whether igneous or secondary, have δD values that reflect extensive subsolidus isotopic equilibration with meteoric–hydrothermal fluids. Received: 22 March 1994 / Accepted: 26 January 1995     

Tourmaline in Proterozoic Massive Sulfide Deposits from Rajasthan, India

We have analyzed the chemical composition and boron isotope composition of tourmaline from tourmalinites, granite and a quartz-tourmaline vein from the Deri ore zone and from a pegmatitic band in the Rampura-Agucha ore body. These two Proterozoic massive sulfide deposits occur in the Aravalli-Delhi orogenic belt, Rajasthan, northwest India. Tourmaline from stratiform tourmalinites closely associated with the massive sulfides in the Deri deposit have preserved their original chemical compositions despite regional and thermal metamorphism in the area. These tourmalines have low Fe/(Fe + Mg) ratios (0.19–0.30; mean 0.26) that suggest formation close to the sediment-sea water interface. The δ¹¹B values (−15.5 and −16.4‰) are compatible with boron derived from leaching of argillaceous sediments and/or felsic volcanics underlying the original massive sulfide deposit during its formation. Boron isotope compositions measured in tourmaline from a post-ore granite and quartz-tourmaline vein in the Deri deposit indicate that boron in these tourmalines was derived from the tourmalinites produced during ore formation. The boron isotope systematics of a coarse brown tourmaline crystal from a pegmatitic band on the hanging wall contact of the Rampura-Agucha deposit indicate that 45 ± 25% of the boron within the original tourmaline was lost during upper amphibolite facies regional metamorphism. Received: 3 April 1996 / Accepted: 11 April 1996     

Gold mineralization in the Mazowe area, Harare-Bindura-Shamva greenstone belt, Zimbabwe: II. Genetic relationships deduced from mineralogical, fluid inclusion and stable isotope studies, and the Sm-Nd isotopic composition of scheelites

In the Mazowe area some 40 km NW of Harare in Zimbabwe, gold mineralization is hosted in a variety of lithologies of the Archean Harare-Bindura-Shamva greenstone belt, in structures related to the late Archean regional D2/3 event. Conspicuous mineralzogical differences exist between the mines; the mainly granodiorite-hosted workings at Mazowe mine are on pyrite-rich reefs, mines of the Bernheim group have metabasalt host rocks and are characterized by arsenopyrite-rich ores, and Stori's Golden Shaft and Alice mine, both in metabasalts, work sulfide-poor quartz veins. In contrast to the mineralogical diversity, near-identical fluid inventories were found at the different mines. Both H₂O-CO₂-CH₄ fluids of low salinity, and highly saline fluids are present and are regarded to indicate fluid mixing during the formation of the deposits. Notably, these fluid compositions in the Mazowe gold field markedly contrast to ore fluids “typical” of Archean mesothermal gold deposits on other cratons. Stable isotope compositions of quartz from the various deposits (δ¹⁸O=10.8 to 13.2‰ SMOW), calcite (δ¹⁸O=9.5 to 11.9‰ SMOW and δ¹³C=−3.2 to −8.0‰ PDB), inclusion water (δD=−28 to −40‰ SMOW) and sulfides (δ³⁴S=1.3 to 3.2‰ CDT) are uniform within the range typical for Archean lode gold deposits worldwide. The fluid and stable isotope compositions support the statement that the mineralization in the Mazowe gold field formed from relatively reduced fluids with a “metamorphic” signature during a single event of gold mineralization. Microthermometric data further indicate that the deposits formed in the PT range of 1.65–2.3 kbar and 250–380 °C. Ages obtained by using the Sm/Nd and Rb/Sr isotope systems on scheelites are 2604 ± 84 Ma for the mineralization at Stori's Golden Shaft mine, and 2.40 ± 0.20 Ga for Mazowe mine. The Archean age at Stori's is regarded as close to the true age of gold mineralization in the area, whereas the Proterozoic age at Mazowe mine probably reflects later resetting. Received: 30 September 1998 / Accepted: 17 August 1999     

Geology,fluid inclusion and sulphur isotope characteristics of the El Cobre VHMS deposit,Southern Cuba

The El Cobre deposit is located in eastern Cuba within the volcanosedimentary sequence of the Sierra Maestra Paleogene arc. The deposit is hosted by tholeiitic basalts, andesites and tuffs and comprises thick stratiform barite and anhydrite bodies, three stratabound disseminated up to massive sulphide bodies produced by silicification and sulphidation of limestones or sulphates, an anhydrite stockwork and a siliceous stockwork, grading downwards to quartz veins. Sulphides are mainly pyrite, chalcopyrite and sphalerite; gold occurs in the stratabound ores. Fluid inclusions measured in sphalerite, quartz, anhydrite and calcite show salinities between 2.3 and 5.7 wt% NaCl eq. and homogenisation temperatures between 177 and 300°C. Sulphides from the stratabound mineralisation display δ ³⁴S values of 0‰ to +6.0‰, whilst those from the feeder zone lie between −1.4‰ and +7.3‰. Sulphides show an intra-grain sulphur isotope zonation of about 2‰; usually, δ ³⁴S values increase towards the rims. Sulphate sulphur has δ ³⁴S in the range of +17‰ to +21‰, except two samples with values of +5.9‰ and +7.7‰. Sulphur isotope data indicate that the thermochemical reduction of sulphate from a hydrothermal fluid of seawater origin was the main source of sulphide sulphur and that most of the sulphates precipitated by heating of seawater. The structure of the deposit, mineralogy, fluid inclusion and isotope data suggest that the deposit formed from seawater-derived fluids with probably minor supply of magmatic fluids.     

The boron isotope geochemistry of tourmaline-rich alteration in the IOCG systems of northern Chile: implications for a magmatic-hydrothermal origin

Hydrothermal tourmaline is common in the iron oxide-copper-gold (IOCG) deposits of the Coastal Cordillera of Chile where it occurs as large crystals in the groundmass of magmatic-hydrothermal breccias, such as in the Silvita or Tropezón ore bodies, or as small grains in replacive bodies or breccia cement in the ore-bearing andesite, as seen at the Candelaria or Carola deposits. Tourmaline shows strong chemical zoning and has a composition of schorl–dravite with significant povondraite and uvite components. The observed boron isotope composition is fairly variable, between −10.4‰ and +6.0‰ with no major differences among the different deposits, suggesting a common genetic mechanism. The δ¹¹B values are significantly lower than those of seawater or marine evaporites and very similar to those of younger porphyry copper deposits and volcanic rocks in the region, indicating that the boron has a common, likely magmatic, origin. The predominant boron source was ultimately dewatering of the subducting slab with a significant contribution derived from the overlying continental basement. The range of δ¹¹B values is between those of the porphyry copper deposits and the porphyry tin deposits of the Andes, suggesting that the IOCG mineralization might be genetically related to fluids having more crustal contamination than the porphyry copper deposits; such an interpretation is at odds with current models that propose that the Andean IOCG deposits are related to juvenile melts or to the circulation of basinal brines. Furthermore, the obtained δ¹¹B data are markedly different from those of the tourmaline in the Carajás IOCG district (Brazil), suggesting that IOCGs do not form by a unique mechanism involving only one type of fluids.     

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

Stable and radiogenic isotope composition of stratiform Cu–Co–Zn mineralization and associated sedimentary rocks within the Boléo district of the Miocene Santa Rosalía basin, Baja California Sur, constrains the evolution of seawater and hydrothermal fluids and the mechanisms responsible for sulfide and oxide deposition. Stable isotope geochemistry of limestone and evaporite units indicates a strong paleogeographic influence on the chemistry of the water column. Near-shore limestone at the base of the Boléo Formation is characterized by modified marine carbon (δ ¹³C_PDB=−6.0 to +4.4‰) and oxygen (δ ¹⁸O_SMOW=+19.5 to +26.2‰) isotope composition due to the influx of ¹³C- and ¹⁸O-depleted fluvial water. Sulfate sulfur isotope composition (δ ³⁴S_CDT=+17.21 to +22.3‰ and δ ¹⁸O_SMOW=+10.7 to +13.1‰) for basal evaporite and claystone facies are similar to Miocene seawater. Strontium isotopes are less radiogenic than expected for Miocene seawater due to interaction with volcanic rocks. Low S/C ratios, high Mn contents and sedimentological evidence indicate the basin water column was oxidizing. The oxygenated basin restricted sulfide precipitation to within the sedimentary pile by replacement of early diagenetic framboidal pyrite and pore-space filling by Cu–Co–Zn sulfides to produce disseminated sulfides. Quartz–Mn oxide oxygen isotope geothermometry constrains mineralization temperature between 18 and 118°C. Sulfur isotopes indicate the following sources of sulfide: (1) bacterial sulfate reduction within the sedimentary pile produced negative δ ³⁴S values (<−20‰) in framboidal pyrite; and (2) bacterial sulfate reduction at high temperature (80–118°C) within the sedimentary pile during the infiltration of the metal-bearing brines produced Cu–Co–Zn sulfides with negative, but close to 0‰, δ ³⁴S values. Isotope modeling of fluid-rock reaction and fluid mixing indicates: (1) sedimentary and marine carbonates (δ ¹³C=−11.6 to −3.2‰ and δ ¹⁸O=+19.0 to +21.8‰) precipitated from basin seawater/pore water that variably mixed with isotopically depleted meteoric waters; and (2) hydrothermal calcite (δ ¹³C=−7.9 to +4.3‰ and δ ¹⁸O=+22.1 to +25.8‰) formed by dissolution and replacement of authigenic marine calcite by downward-infiltrating metalliferous brine and brine-sediment exchange, that prior to reaction with calcite, had mixed with isotopically depleted pore water. The downward infiltration of metalliferous brine is inferred from lateral and stratigraphic metal distributions and from the concentration of Cu sulfides along the upper contact of pyrite-bearing laminae. The co-existence and textural relationships among framboidal pyrite, base metal sulfides, carbonate and Mn–Fe oxides (including magnetite) within mineralized units are consistent with carbonate replacement and high-temperature bacterial reduction within the sedimentary pile occurring simultaneously below a seawater column under predominantly oxygenated conditions.     

Near-surface expression of a young mesothermal gold mineralizing system, Sealy Range, Southern Alps, New Zealand

Carbonate-limonite veins formed in steeply dipping fractures in the upper few hundred metres of basement greywacke in the actively rising Southern Alps of New Zealand. The veins are found commonly in extensional fractures near to, but not in, major faults associated with mountain uplift, and/or sinistral faults which bound mountain ranges. Some of the veins contain sulphides and minor gold deposited as part of incrementally formed fracture fillings. Oxygen isotope ratios of calcite range widely between +6 and +24‰, and calcite δ¹³C_PDB=−5.5 to −11.5‰. The veins formed from isotopically exchanged crustal fluid with a probable meteoric water component. The shallow vein network is the near-surface expression of a tectonically induced hydrothermal system which has deposited gold-bearing veins with a mesothermal style over several vertical kilometres. This vein network has formed in a dilatational zone of the oblique collisional orogen where near-vertical fractures tap deep-sourced fluids. Similar processes acting at the southern end of the Southern Alps in the Miocene resulted in locally rich mesothermal quartz-gold veins. Received: 21 May 1997 / Accepted: 30 June 1998     

Boron recycling in the continental crust of the central Andes from the Palaeozoic to Mesozoic, NW Argentina

Whole-rock chemical composition and ¹¹B/¹⁰B isotope ratios in tourmaline was investigated to study the geochemical recycling of boron during the evolution of the Andean basement from the Palaeozoic to Mesozoic. In the basement (Cambrian to Ordovician high-grade paragneisses, migmatites and orthogneisses, the Eocambrian Puncoviscana Formation, and Paleozoic-Mesozoic granitoid igneous rocks) whole-rock B contents are generally below 100 ppm, but B contents of ˜1 wt% are found in cogenetic aplite and pegmatite dikes and in tourmaline–quartz rocks. In the metasedimentary rocks, no systematic variation in B content because of metamorphic grade and no correlation of B with other incompatible elements are apparent. Tourmalines from the high-grade metamorphic basement yield δ¹¹B values ranging from −11.2 to −6.8‰ and isotope fractionation during migmatisation was small. Metamorphic tourmalines from the Puncoviscana Formation have δ¹¹B values between −6.3 and −5.8‰. The calculated (corrected for fractionation) δ¹¹B values of −6 to −2‰ for the sedimentary protolith of the metamorphic basement indicate a continental B source with subordinate marine input. Tourmalines from Palaeozoic and Mesozoic granitoids display an identical range of δ¹¹B values from −12 to −5.3‰ and indicate a similarly homogeneous B source throughout time. Tourmalines from pegmatites and tourmaline–quartz rocks record the average δ¹¹B values of the parental granitic magma. We assume that B in the Palaeozoic and Mesozoic granitoids is derived from the local metamorphic basement supporting the hypothesis that recycling of the lower Palaeozoic crust is the dominant process in granitic magma formation from Palaeozoic to Mesozoic. Received: 15 December 1999 / Accepted: 11 July 2000     

Mineralogy, geochemistry and geological significance of tourmaline-rich rocks from the Paleozoic Cinco Villas massif (western Pyrenees, Spain)

Tourmaline-rich rocks associated with clastic metasedimentary rocks of Carboniferous age occur in the Cinco Villas massif, western Pyrenees. Three types of tourmaline-rich rocks were distinguished: (1) Fine-grained stratiform tourmaline-rich rocks, which are associated with carbonaceous metapelites (TR1); (2) stratabound tourmaline-rich rocks, associated with metapelites in the contact aureole of the Aya granitoid pluton (TR2); (3) stratabound to massive tourmaline-rich rocks, associated with psammopelites in contact with granites and pegmatites (TR3). Tourmalines belong to the schorl–dravite solid solution series and have a wide compositional range, from nearly end-member dravite for TR1 tourmalines to schorl for TR3 tourmalines; TR2 tourmalines have intermediate compositions. The Fe/(Fe+Mg) typically varies between 0.02 and ≈0.55, increasing from TR1 to TR3. The TR1 tourmalines commonly display a discontinuous chemical zoning with Fe-rich green cores (8–8.5% FeO) and Mg-rich colorless rims (10–11% MgO). In contrast, crystals that exhibit fine growth lamellae appear to lack significant chemical zoning. Oxygen and hydrogen isotope compositions also reveal major differences between TR1 and TR3 tourmalines, the former displaying heavier δ¹⁸O values (17.7–19‰) and δD values (−35 to −42‰) than TR3 tourmalines 11 to 13‰ and −47 to −76‰, respectively. The TR2 tourmalines show intermediate values of 11.3 to 14.6‰ for δ¹⁸O and −40 to −55‰ for δD. Linear and continuous chemical variations obtained for major and trace elements of the whole rocks reflect mixing between clay-rich and quartz-rich end-members, indicative that some tourmaline-rich rocks contain a significant detrital component. Chondrite normalized REE (rare earth element) patterns of tourmaline-rich rocks are similar to those of surrounding unaltered clastic metasediments, except for some TR1 rocks which are characterized by low contents of ΣREE. Mass-balance calculations show that tourmaline-forming processes plus metamorphism led to mass and volume changes at mesoscopic scales (≈10% for the TR1 tourmalinites). Silicon, Fe, Mn, and REE elements were partially lost from sedimentary rocks, whereas Mg and particularly B were added to pelitic sediments. Available data, nevertheless, do not allow an assessment of the boron source. Formation of the TR1 tourmaline-rich rocks probably was the net result of several processes, including direct precipitation from B-rich hydrothermal fluids or colloids, early diagenetic reactions of carbonaceous pelitic sediments with these fluids, and subsequent recrystallization during regional metamorphism. The TR2 tourmaline-rich rocks mainly developed by metamorphic recrystallization of TR1. Tourmaline-rich rocks and veins adjacent to pegmatites and granitic rocks (TR3) are the result of boron metasomatism; the primary boron having been recycled from stratiform tourmalinites during regional metamorphism and magmatism. Received: 18 November 1996 / Accepted: 25 April 1997     

Carbon and oxygen isotope constraints on fluid sources and fluid–wallrock interaction in regional alteration and iron-oxide–copper–gold mineralisation, eastern Mt Isa Block, Australia

The source of metasomatic fluids in iron-oxide–copper–gold districts is contentious with models for magmatic and other fluid sources having been proposed. For this study, δ ¹⁸O and δ ¹³C ratios were measured from carbonate mineral separates in the Proterozoic eastern Mt Isa Block of Northwest Queensland, Australia. Isotopic analyses are supported by petrography, mineral chemistry and cathodoluminescence imagery. Marine meta-carbonate rocks (ca. 20.5‰ δ ¹⁸O and 0.5‰ δ ¹³C calcite) and graphitic meta-sedimentary rocks (ca. 14‰ δ ¹⁸O and −18‰ δ ¹³C calcite) are the main supracrustal reservoirs of carbon and oxygen in the district. The isotopic ratios for calcite from the cores of Na–(Ca) alteration systems strongly cluster around 11‰ δ ¹⁸O and −7‰ δ ¹³C, with shifts towards higher δ ¹⁸O values and higher and lower δ ¹³C values, reflecting interaction with different hostrocks. Na–(Ca)-rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks, and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks. However, igneous rocks in the eastern Mt Isa Block contain negligible carbon and are incapable of buffering the δ ¹³C signatures of CO₂-rich metasomatic fluids associated with Na–(Ca) alteration. In contrast, plutons in the eastern Mt Isa Block have been documented as having exsolved saline CO₂-rich fluids and represent the most probable fluid source for Na–(Ca) alteration. Intrusion-proximal, skarn-like Cu–Au orebodies that lack significant K and Fe enrichment (e.g. Mt Elliott) display isotopic ratios that cluster around values of 11‰ δ ¹⁸O and −7‰ δ ¹³C (calcite), indicating an isotopically similar fluid source as for Na–(Ca) alteration and that significant fluid–wallrock interaction was not required in the genesis of these deposits. In contrast, K- and Fe-rich, intrusion-distal deposits (e.g. Ernest Henry) record significant shifts in δ ¹⁸O and δ ¹³C towards values characteristic of the broader hostrocks to the deposits, reflecting fluid–wallrock equilibration before mineralisation. Low temperature, low salinity, low δ ¹⁸O (<10‰ calcite) and CO₂-poor fluids are documented in retrograde metasomatic assemblages, but these fluids are paragenetically late and have not contributed significantly to the mass budgets of Cu–Au mineralisation.     

High-temperature hydrothermal alteration of the Boehls Butte anorthosite: origin of a bimodal plagioclase assemblage

The Boehls Butte anorthosite consists predominantly of an unusual bimodal assemblage of andesine and bytownite–anorthite. Oxygen isotope compositions of the anorthosite were profoundly altered by high temperature, retrograde interaction with meteoric-hydrothermal fluids that varied in composition from isotopically evolved to nearly pristine meteoric water. Oxygen isotope ratios of bulk plagioclase separates are in the range +7.0 to −6.2‰ V-SMOW, however, secondary ion mass spectrometry indicates spot-sized isotope values as low as −16‰. Typical inter- and intra-plagioclase grain variability is 3–6‰, and extreme heterogeneity of up to 20‰ is noted in a few samples. High-temperature hydrothermal alteration of intermediate plagioclase is proposed to explain the origin of bytownite–anorthite in the anorthosite and creation of its unusual bimodal plagioclase assemblage. The anorthite-forming reaction created retrograde reaction-enhanced permeability which, together with rapid decompression, extension, and unroofing of the anorthosite complex, helped to accommodated influx of significant volumes of meteoric-hydrothermal fluids into the anorthosite.     

Fluid origin and structural enhancement during mineralization of the Jinshan orogenic gold deposit, South China

The Jinshan orogenic gold deposit is a world-class deposit hosted by a ductile shear zone caused by a transpressional terrane collision during Neoproterozoic time. Ore bodies at the deposit include laminated quartz veins and disseminated pyrite-bearing mylonite. Most quartz veins in the shear zone, with and without gold mineralization, were boudinaged during progressive shear deformation with three generations of boudinage structures produced at different stages of progressive deformation. Observations of ore-controlling structures at various scales indicate syn-deformational mineralization. Fluid inclusions from pyrite intergrown with auriferous quartz have ³He/⁴He ratios of 0.15–0.24 Ra and ⁴⁰Ar/³⁶Ar ratios 575–3,060. δ¹⁸O_fluid values calculated from quartz are 5.5–8.4‰, and δD values of fluid inclusions contained in quartz range between −61‰ and −75‰. The δ¹³C values of ankerite range from −5.0‰ to −4.2‰, and ankerite δ¹⁸O values from 4.4‰ to 8.0‰. The noble gas and stable isotope data suggest a predominant crustal source of ore fluids with less than 5% mantle component. Data also show that in situ fluids were generated locally by pervasive pressure solution, and that widespread dissolution seams acted as pathways of fluid flow, migration, and precipitation. The in situ fluids and fluids derived from deeper levels of the crust were focused by deformation and deformation structures at various scales through solution-dissolution creep, crack-seal slip, and cyclic fault-valve mechanisms during progressively localized deformation and gold mineralization.     

Contrasting stable isotope behavior between calcite and dolomite marbles,Lone Mountain,Nevada

 Late Proterozoic to Cambrian carbonate rocks from Lone Mountain, west central Nevada, record multiple post-depositional events including: (1) diagenesis, (2) Mesozoic regional metamorphism, (3) Late Cretaceous contact metamorphism, related to the emplacement of the Lone Mountain granitic pluton and (4) Tertiary hydrothermal alteration associated with extension, uplift and intrusion of silicic porphyry and lamprophyre dikes. Essentially pure calcite and dolomite marbles have stable isotopic compositions that can be divided into two groups, one with positive δ¹³C values from+3.1 to +1.4 ‰ (PDB) and high δ¹⁸O values from +21.5 to +15.8 ‰ (SMOW), and the other with negative δ¹³C values from –3.3 to –3.6‰ and low δ¹⁸O values from +16.9 to +11.1‰. Marbles also contain minor amounts of quartz, muscovite and phlogopite. Brown and blue luminescent, clear, smooth textured quartz grains from orange luminescent calcite marbles have high δ¹⁸O values from +23.9 to +18.1‰, while brown luminescent, opaque, rough textured quartz grains from red luminescent dolomite marbles typically have low δ¹⁸O values from +2.0 to +9.3‰. The δ¹⁸O values of muscovite and phlogopite from marbles are typical of micas in metamorphic rocks, with values between +10.4 and +14.4‰, whereas mica δD values are very depleted, varying from −102 to −156‰. No significant lowering of the δ¹⁸O values of Lone Mountain carbonates is inferred to have occurred during metamorphism as a result of devolatilization reactions because of the essentially pure nature of the marbles. Bright luminescence along the edges of fractures, quartz cements and quartz overgrowths in dolomite marbles, low δD values of micas, negative δ¹³C values and low δ¹⁸O values of calcite and dolomite, and depleted δ¹⁸O values of quartz from dolomite marbles all indicate that meteoric fluids interacted with Lone Mountain marbles during the Tertiary. Partial oxygen isotopic exchange between calcite and low ¹⁸O meteoric fluids lowered the δ¹⁸O values of calcite, resulting in uniform quartz-calcite fractionations that define an apparent pseudoisotherm. These quartz-calcite fractionations significantly underestimate both the temperature of metamorphism and the temperature of post-metamorphic alteration. Partial oxygen isotopic exchange between quartz and meteoric fluids also resulted in ¹⁸O depletion of quartz from dolomite marbles. This partial exchange was facilitated by an increase in the surface area of the quartz as a result of its dissolution by meteoric fluids. The negative δ¹³C values in carbonates result from the oxidation of organic material by meteoric fluids following metamorphism. Stable isotopic data from Lone Mountain marbles are consistent with the extensive circulation of meteoric hydrothermal fluids throughout western Nevada in Tertiary time. Received: 1 February 1994/Accepted: 12 September 1995     

Mineralogical and stable isotope studies of gold–arsenic mineralisation in the Sams Creek peralkaline porphyritic granite, South Island, New Zealand

At Sams Creek, a gold-bearing, peralkaline granite porphyry dyke, which has a 7 km strike length and is up to 60 m in thickness, intrudes camptonite lamprophyre dykes and lower greenschist facies metapelites and quartzites of the Late Ordovician Wangapeka formation. The lamprophyre dykes occur as thin (< 3 m) slivers along the contacts of the granite dyke. δ¹⁸O_magma values (+5 to +8‰, VSMOW) of the A-type granite suggest derivation from a primitive source, with an insignificant mature crustal contribution. Hydrothermal gold–sulphide mineralisation is confined to the granite and adjacent lamprophyre; metapelite country rocks have only weak hydrothermal alteration. Three stages of hydrothermal alteration have been identified in the granite: Stage I alteration (high fO₂) consisting of magnetite–siderite±biotite; Stage II consisting of thin quartz–pyrite veinlets; and Stage III (low fO₂) consisting of sulphides, quartz and siderite veins, and pervasive silicification. The lamprophyre is altered to an ankerite–chlorite–sericite assemblage. Stage III sulphide veins are composed of arsenopyrite + pyrite ± galena ± sphalerite ± gold ± chalcopyrite ± pyrrhotite ± rutile ± graphite. Three phases of deformation have affected the area, and the mineralised veins and the granite and lamprophyre dykes have been deformed by two phases of folding, the youngest of which is Early Cretaceous. Locally preserved early-formed fluid inclusions are either carbonic, showing two- or three-phases at room temperature (liquid CO₂-CH₄ + liquid H₂O ± CO₂ vapour) or two-phase liquid-rich aqueous inclusions, some of which contain clathrates. Salinities of the aqueous inclusions are in the range of 1.4 to 7.6 wt% NaCl equiv. Final homogenisation temperatures (Th) of the carbonic inclusions indicate minimum trapping temperatures of 320 to 355°C, which are not too different from vein formation temperatures of 340–380°C estimated from quartz–albite stable isotope thermometry. δ¹⁸O values of Stage II and III vein quartz range from +12 and +17‰ and have a bimodal distribution (+14.5 and +16‰) with Stage II vein quartz accounting for the lower values. Siderite in Stage III veins have δ¹⁸O (+12 to +16‰) and δ¹³C values (−5‰, relative to VPDB), unlike those from Wangapeka Formation metasediments (δ¹³C_{bulk carbon} values of −24 to −19‰) and underlying Arthur Marble marine carbonates (δ¹⁸O = +25‰ and δ¹³C = 0‰). Calculated δ¹⁸O_water (+8 to +11‰, at 340°C) and (−5‰) values from vein quartz and siderite are consistent with a magmatic hydrothermal source, but a metamorphic hydrothermal origin cannot be excluded. δ³⁴S values of sulphides range from +5 to +10‰ (relative to CDT) and also have a bimodal distribution (modes at +6 and +9‰, correlated with Stage II and Stage III mineralisation, respectively). The δ³⁴S values of pyrite from the Arthur Marble marine carbonates (range from +3 to +13‰) and Wangapeka Formation (range from −4 to +9.5‰) indicate that they are potential sources of sulphur for sulphides in the Sams Creek veins. Another possible source of the sulphur is the lithospheric mantle which has positive values up to +14‰. Ages of the granite, lamprophyre, alteration/mineralisation, and deformation in the region are not well constrained, which makes it difficult to identify sources of mineralisation with respect to timing. Our mineralogical and stable isotope data does not exclude a metamorphic source, but we consider that the source of the mineralisation can best be explained by a magmatic hydrothermal source. Assuming that the hydrothermal fluids were sourced from crystallisation of the Sams Creek granite or an underlying magma chamber, then the Sams Creek gold deposit appears to be a hybrid between those described as reduced granite Au–Bi deposits and alkaline intrusive-hosted Au–Mo–Cu deposits.     

Stable isotopes and amphibole chemistry on hydrothermally altered granitoids in the North Chilean Precordillera: a limited role for meteoric water?

Whole rock and mineral stable isotope and microprobe analyses are presented from granitoids of the North Chilean Precordillera. The Cretaceous to Tertiary plutonic rocks contain important ore deposits and frequently display compositional and textural evidence of hydrothermal alteration even in barren rocks. Deuteric alteration includes replacement of biotite and amphibole by chlorite and epidote, sericitization and saussuritization of feldspars, and uralitization of clinopyroxene and/or amphibole. While whole rock compositions are not significantly affected, compositional variations in amphiboles suggest two types of hydrothermal alteration. Hornblende with actinolitic patches and rims and tight compositional trends from hornblende to Mg-rich actinolite indicate increasing oxygen fugacity from magmatic to hydrothermal conditions. Uralitic amphiboles exhibiting irregular Mg-Fe distribution and variable Al content are interpreted as reflecting subsolidus hydration reactions at low temperatures. The δD values of hydrous silicates vary from −63 to −105‰. Most δ¹⁸O values of whole rocks are in the range of 5.7 to 7.7‰ and are considered normal for igneous rocks in the Andes. These δ¹⁸O values also coincide well with the oxygen isotope composition of geochemically similar recent volcanics from the Central Andean Volcanic Zone (δ¹⁸O = 7.0–7.4‰). Only one sample in this study (δ¹⁸O = 3.0‰) appears to be depleted by isotope exchange with light meteoric water at high temperatures. The formation of secondary minerals in all other intrusions is mainly the product of deuteric alteration. This also holds true for the sample from El Abra, the only pluton associated with mineralization. This indicates the dominant role of a magmatic rather than a meteoric fluid in the alteration of the Cretaceous and Tertiary granitoids in northern Chile. Received: 8 July 1998 / Accepted: 15 April 1999