A sulphur isotope study of the Broken Hill deposit,New South Wales,Australia

Sulphur isotopic compositions of sulphides within garnet-rich rocks and high-grade ore from the Broken Hill deposit, New South Wales, Australia, have been determined and show a range of values of –3.3 to +6.7 per mil. Thermochemical considerations, including the spread of values of ³⁴S, suggest that the deposit was derived from a mixed source of sulphur in which seawater, reduced by inorganic processes, mixed with magmatic sulphur or that sulphate from contemporaneous seawater was reduced biogenically at low temperatures. Thermochemical considerations also suggest that pyrrhotite formed by desulphidation of pyrite so that the original Fe-S-O assemblage was pyrite ± magnetite.³⁴S measurements show a broad range which is considered to be due to isotopic reequilibration during retrograde metamorphism and analytical and sampling technique. These data should not be used to indicate original temperatures of deposition or metamorphic temperatures associated with the various metamorphic events.     

“Background” δ34S values of Kupferschiefer sulphides in Poland: pyrite-marcasite nodules

Regional background ³⁴S values of pyrite-(marcasite) nodules throughout the Zechstein basin in Poland have been measured to help estimate the proportion of externally derived sulphur in the Kupferschiefer Cu-Ag ores. The ³⁴S values of the 17 FeS₂ nodules measured range widely, from -25.2 to -51.9%., similar to the previously published -28 to -43%. range in disseminated pyrite in the Kupferschiefer. The wide variation cannot be attributed to pyrite versus marcasite mineralogy, amount of contained chalcopyrite or sphalerite, carbonate versus shale host rock, early versus late formation, percent of included calcite, or to size, shape, or texture. There is also no relation with proximity to the centres of copper mineralization in southwestern Poland where sulphides are typically isotopically heavier. The ³⁴S values do, however, vary directly with percent of host-rock fragments included in the nodules. Repeat samples that were washed with acid or hot water show the same wide variation, indicating that contamination by sulphate sulphur in the host rock is not a factor. Neither is organic sulphur because of its small volume. Instead, the sulphur composition may be fundamentally controlled by the formation mechanism of the nodule, whereby ³⁴S-rich sulphide is preferentially concentrated, possibly replacing anhydrite lenses. Alternatively, a network of host rock inclusions might act as a more accessible conduit for later, ³⁴S-rich fluids to infiltrate the nodule and add to earlier, ³⁴S-poor pyrite.In the ore deposits, higher ³⁴S values of ore nodules suggest less indigenous sulphur in limestone than shale lithologies. An isotopic temperature of 61 °C from a chalcopyrite-galena pair agrees with other estimates of <105°C. Higher values in ore nodules/veinlets than in adjacent disseminations, and the calculated ³⁴S_py value from a pyrite-bornite mixture support the idea that metal-bearing ³⁴S-rich fluids penetrated the Kupfer-schiefer through a network of fractures.Contribution to IGCP Project 254 Metalliferous Black Shales     

Sulfur isotope studies of the felbertal scheelite deposit,eastern alps

The Felbertal scheelite deposit is the largest known strata-bound tungsten concentration. It lies in an up to 400 m thick rock pile in the lowermost part of the volcanic rock sequence, probably of the Early Paleozoic Habach Formation. Both ore fields (eastern and western) have been affected by Variscan and Alpine metamorphism and tectonism, resulting in a remobilization of the ore mineralization. This ore deposit and the neighboring rocks show a strikingly low sulfur content. The eastern field with one major orebody has very little sulfide mineralization. The western field, with 8 orebodies (K1–K8) and two remobilized vein zones (S1 and S2), reveals somewhat more minor sulfide enrichments that are mainly within and around the K1 and K2 orebodies and in some parts of the interlayered schist sequence. Sulfur isotope compositions of 90 sulfide minerals (37 pyrrhotite, 20 chalcopyrite, 19 pyrite and 11 molybdenite and/or WS₂-MoS₂ solid solutions and 3 Pb-Bi sulfosalts, including 7 sulfides within scheelite grains) from 60 ore and host rock samples have been determined with a standard error of less than ±0.2 per mil. All data range from –3.6 to +4.3 ³⁴S. There are small differences in the sulfur isotope values from place to place and in time from the first and second to the third generation. In the western field, the K1 orebody differs from other orebodies (K2, K4, K7) due to isotopically heavier ³⁴S values. The three scheelite generations show differences in the ³⁴S values of the sulfide microphases within scheelite grains, from +1.0 to +4.3 per mil for the first and the second, and from –1.8 to –3.3 per mil for the third generation. Sulfide phases within molybdoscheelites may have crystallized under the same conditions as the other coeval sulfide minerals in the same orebody. They commonly formed later than scheelite. These changes may be explained using data from Ohmoto and Rye (1979): Small changes in temperature, pH, and/or may result in large changes in the ³⁴S values with the precipitation of isotopically heavier sulfides under more reducing conditions. Only four samples with sulfide mineral pairs show isotopic equilibrium. All others display some disequilibrium. We suggest that the sulfides in the ores and surrounding volcanogenic host rocks formed contemporaneously from the same hydrothermal ore fluids, and that the sulfur species in these fluids may have been dominantly H₂S.     

Sulphur isotope studies of the Mt Molloy,Dianne and O.K. stratiform sulphide deposits,Hodgkinson Province,North Queensland,Australia

Stratiform sulphide deposits which have been metamorphosed to lower greenschist facies occur within the Paleozoic strata of the Hodgkinson Province, northeast Queensland. Massive cupreous pyrite is ubiquitous and Mt Molloy and Dianne also have layered chalcopyrite-rich and sphalerite-rich lenses. Sulphide ³⁴S values for the mineralisation show a narrow spread, around 02030; at the Dianne and O.K. deposits, but a wider spread and an average several per mil higher at the Mt Molloy area. The minerals can not be used for geothermometry due to isotopic disequilibrium. However, metamorphic effects on the isotopic compositions appear not to have been significant. A decrease in temperature and contact of the ore fluid with sea water probably caused the precipitation of the ore minerals. A magmatic ore fluid with ³⁴S_S around 02030; predominated at the Dianne and OK deposits whereas the fluid at Mt Molloy mixed with sea water to acquire a higher ³⁴S_S value.     

Genesis of copper mineralisation,Myall Creek prospect,South Australia

The Myall Creek copper prospect is in unmetamorphosed carbonaceous dolosiltstone and sandstone at the base of the late Proterozoic (Adelaidean) Tapley Hill Formation. It contains disseminated, fine-grained chalcopyrite, zincian tennanite, bornite, chalcocite, pyrite, sphalerite and galena, and irregular to straight chalcopyrite-rich veinlets. Some ore minerals rim and/or partially replace pyrite or clastic grains. There is no evidence of hydrothermal activity. The ³⁴S_CDT values of pyrite and the other sulfides fall in the wide range –3.6 to +44.2. Dolomite in both mineralised and unmineralised samples has ¹³C_PDB values concentrated around –3, and ¹⁸O_SMOW values around +25. It is concluded that the mineralising fluids were near-neutral brines which leached metals from the basement and early Adelaidean rocks. They entered the Tapley Hill sediments at moderately low temperatures via permeable strata and faults. The metals were precipitated by biogenic H₂S, and also fixed by reaction with iron sulfides and, possibly, organic matter. Continuing ascent of brines into the mineralised strata caused breakdown of detrital feldspars and Fe-Ti oxides, and some solution-remobilisation of early-formed sulfides.     

Carbonatization and propylitic alteration of fragmental basaltic rocks, Quesnel River gold deposit, Central British Columbia

The Quesnel River gold deposit (1.2 million tonnes grading 5.22 g/t Au in three separate zones) occurs within Takla Group volcanic rocks of Upper Triassic age proximal to an alkalic stock. The deposit occurs in amphibole-augite phyric, fragmental, basaltic rocks. Alteration has produced an assemblage of epidote-chloritetremolite-calcite-quartz with lesser pyrite, chalcopyrite, pyrrhotite, sphalerite, marcasite, galena, arsenopyrite and gold.The West Zone comprises a tabular, conformable sulfide body underlain by bedded, variably altered fragmental basaltic rocks and overlain by siltstone and argillite. In the Main Zone, highest gold grades occur adjacent to a sharp discordant alteration front with barren, strongly carbonatized, pyritic basaltic lapilli-tuff. It is overlain by siltstone and argillite and bounded to the east and a depth by a west dipping reverse fault. To the west the auriferous, propylitically altered, rocks grade laterally into lower grade and barren basaltic rocks.Oxygen(¹⁸O = + 9 to + 15) and carbon (¹³O= -14 to –7) isotopic signatures of calcite from carbonate-altered and propylitically altered rocks are similar. However, sulfur isotopic values for pyrite are different, with gold-associated pyrite (³⁴S = –7 to –3) distinct from pyrite in carbonate altered rocks with (³⁴S = + 8 to + 13).The carbonization occurred before complete induration of the basaltic fragmental rocks, whereas propylitization and gold plus sulfide precipitation is clearly epigenetic.     

A sulphur isotopic study of the Bleikvassli Zn-Pb-Cu deposit,Nordland, northern Norway

The Bleikvassli Zn-Pb-Cu deposit occurs in the Uppermost Allochthon in the Caledonides of northern Norway. The orebody is enclosed in amphibolite-facies schists and gneisses, underlain by amphibolites, and it has been classified as a sediment-hosted massive sulphide (SEDEX) deposit. The stratiform ore is dominantly pyritic, with a basal layer of pyrrhotitic ore. Sulphide veins occur in the footwall. The orebody generally has a limited range of ³⁴S, from 0.3 to 4.5% (x = 2.4 ± 1.2, 1 , n = 26). The lowest ³⁴S values (0.3–2.3) were found in sulphide veins in the footwall and vent proximal stratiform ore. More distal pyritic Zn-Pb ore has heavier average ³⁴S values (up to 4.5). The ore sulphides were deposited from a hydrothermal solution with ³⁴S about 2 perhaps with the incorporation of a minor portion of sulphide from the ambient seawater. The hydrothermal solution probably acquired most of its sulphide from the underlying mixed lithology; notably basaltic rocks. Sulphide produced by thermochemical reduction of seawater in the deep conduit system may also have been incorporated. Bacteriogenic sulphide is not likely as a major source of ore sulphur in the massive ore. Sulphide incorporated in distal pyrite, which have ³⁴S from -12 to-10, could have formed either by oxidation of the hydrothermal sulphide, or by bacterial reduction of seawater sulphate in the depositional environment. Exchange of sulphur isotopes probably took place only on a localized scale during Caledonian metamorphism, the bulk sulphur isotopic composition of the ore being preserved in a hand specimen scale.     

Sulphur and sulphate-oxygen isotopes and the origin of the silvermines deposits,Ireland

New sulphur and sulphate-oxygen isotope measurements for the main discordant and stratiform lead-zinc-barite orebodies at Silvermines Co. Tipperary, allow reappraisal of previously offered differing interpretations (Graham, 1970; Greig et al., 1971) of the bearing of sulphur isotopes on the genesis of this important Irish deposit. The following aspects of the data are confirmed: barite ³⁴ S-values range from 17–21, similar to lower Carboniferous seawater sulphate: stratiform sulphide lens pyrites have ³⁴ S-values ranging from –13 to –36; vein sulphide ³⁴ S-values range from –8 to 4; sulphide ³⁴ S-values increase upwards and outwards respectively in the related discordant and stratiform G orebodies; galena-sphalerite isotope palaeotemperatures are not too consistent, ranging from 40 to 430°C (using the calibration of Czamanske and Rye (1974). New facts are as follows: barite ¹⁸O-values range from –13 to –17, stratiform barites ranging from 13 to 14.5; sulphides separated from a single stratiform ore lens hand specimen usually have ³⁴ S_sl > ³⁴ S_ga > ³⁴ S_py; the outward decrease in ³⁴ S-values in the stratiform G orebody is confined to the first few hundred feet only; pyrite ³⁴ S-values progressively increase downwards through one stratiform sulphide orebody; yet variations of 13 occur within a single colloform pyrite structure from another stratiform orebody. It is concluded that there were at least two sources of sulphur, seawater sulphate and deep-seated sulphur. The former was the dominant source of all sulphate and, via biogenic reduction, of the sulphur in the bulk of the stratiform sulphide. The latter was the source of the sulphur in the vein sulphides. There was minimal isotopic interaction between the cool seawater sulphate and the warm unwelling ore fluid sulphur species, even though the latter precipitated under near isotopic equilibrium conditions when the temperature dropped and/or the pH and Eh increased. The lack of isotopic equilibrium between pyrite and ore sulphides in the stratiform ore lenses may result from the latter having precipitated slightly later than the former because of solubility relationships. Overall the present isotopic evidence supports considerable geological evidence favoring a syngenetic origin for the stratiform Silvermines orebodies.     

Oxygen and hydrogen isotope evidence for meteoric water infiltration during mylonitization and uplift in the Ruby Mountains-East Humboldt Range core complex,Nevada

Stable isotope analyses of rocks and minerals associated with the detachment fault and underlying mylonite zone exposed at Secret Creek gorge and other localities in the Ruby-East Humboldt Range metamorphic core complex in northeastern Nevada provide convincing evidence for meteoric water infiltration during mylonitization. Whole-rock ¹⁸O values of the lower plate quartzite mylonites (95% modal quartz) have been lowered by up to 10 per mil compared with structurally lower, compositionally similar, unmylonitized material. Biotite from these rocks has D values ranging from -125 to -175, compared to values of -55 to-70 in biotite from unmylonitized rocks. Mylonitized leucogranites have large disequilibrium oxygen isotope fractionations ( ^{quartz-feldspar} up to 8 per mil) relative to magmatic values ( ^{quartz-feldspar}1 to 2 per mil)). Meteoric water is the only major oxygen and hydrogen reservoir with an isotopic composition capable of generating the observed values. Fluid inclusion water from unstrained quartz in silicified breccia has a D value of-119 which provides a plausible estimate of the D of the infiltrating fluid, and is similar to the isotopic composition of present-day and Tertiary local meteoric water. The quartzite mylonite biotites would have been in equilibrium with such a fluid at temperatures of 480–620° C, similar to independent estimates of the temperature of mylonitization. The relatively high temperatures required for isotopic exchange between quartz and water, the occurrence of fluid inclusion trails and deformed veins in quartzite mylonites, and the spatial association of the low-¹⁸O, low-D rocks with the shear zone all constrain isotopic exchange to the mylonitic (plastic) deformation event. These observations suggest thata significant amount of meteoric water infiltrated the shear zone during mylonitization to depths of at least 5 to 10 km below the surface. The depth of penetration of meteoric fluids into the lower plate mylonites was at least 70 meters below the detachment fault. In contrast, the upper-plate unmylonitized fault slices are dominated by brittle fracture and are often intensely veined (carbonates) or silicified (volcanic rocks and breccias). The fluids associated with the veining and silicification were also meteoric as evidenced by low ¹⁸O values of the veins, which are often 10 per mil lower than the adjacent carbonate matrix, and the exceptionally low ¹⁸O values (down to-4.4) of the breccias. Several previous studies have documented the infiltration of meteoric fluids into the brittley deformed upper plate rocks of core complexes, but this study provides convincing evidence that surface fluids have penetrated lower plate rocks undergoing plastic deformation. It is proposed that infiltration took place as the shear zone began the transition from plastic flow to brittle fracture while the lower plate rocks were being uplifted. During this period, plastic flow and brittle fracture were operating simultaneously, perhaps allowing upper plate meteoric fluids to be seismically pumped down into the lower plate mylonites.     

Influence of sedimentary environments on sulfur isotope ratios in clastic rocks: a review

Strata-bound sulfide deposits associated with clastic, marine sedimentary rocks, and not associated with volcanic rocks, display distributions of S³⁴ values gradational between two extreme types: 1. a flat distribution ranging from S³⁴ of seawater sulfate to values about 25 lower; and 2. a narrow distribution around value S³⁴ (sulfide)=S³⁴ (seawater sulfate) –50, and skewed to heavier values. S³⁴ (seawater sulfate) is estimated from contemporaneous evaporites. There is a systematic relation between the type of S³⁴ distribution and the type of depositional environment. Type 1 occurs in shallow marine or brackish-water environments; type 2 occurs characteristically in deep, euxinic basins. These distributions can be accounted for by a model involving bacterial reduction of seawater sulfate in systems which range from fully-closed batches of sulfate (type 1) to fully open systems in which fresh sulfate is introduced as reduction proceeds (type 2). The difference in the characteristic distributions requires that the magnitude of the sulfate-sulfide kinetic isotope effect on reduction be different in the two cases. This difference has already been suggested by the conflict between S³⁴ data for modern marine sediments and laboratory experiments. The difference in isotope effects can be accounted for by Rees' (1973) model of steady-state sulfate reduction: low nutrient supply and undisturbed, stationary bacterial populations in the open system settings tend to generate larger fractionations.

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Zusammenfassung Schichtgebundene Sulfid-Lagerstätten in Begleitung von klastischen, marinen Sedimentgesteinen ohne Beteiligung vulkanischer Gesteine zeigen kontinuierliche Verteilungen der S³⁴-Werte zwischen zwei Extremtypen: 1. Eine flache Verteilung im Bereich von S³⁴-Werten des Seewasser-Sulfats bis zu Werten, die etwa 25 niedriger liegen. 2. Eine eng begrenzte Verteilung um den S³⁴ (Sulfid)-Wert=S³⁴ (Seewasser-Sulfat) –50 und asymmetrischer Verteilungskurve mit stärkerer Besetzung bei den schwereren Werten. Das S³⁴ (Seewasser-Sulfat) wird von gleichaltrigen Evaporiten abgeleitet. Es besteht eine systematische Beziehung zwischen der Art der S³⁴-Verteilung und dem Milieu des Ablagerungsraumes. Typ 1 tritt im marinen Flachwasser oder in brackischer Umgebung auf. Typ 2 ist charakteristisch für tiefe euxinische Becken. Diese Verteilungen können erklärt werden mit Hilfe eines Modells mit bakterieller Reduktion von Meerwasser-Sulfat in Systemen, die von völlig abgeschlossenen Sulfat-Mengen (Typ 1) bis zu völlig offenen Systemen reichen, in die bei fortschreitender Reduktion frisches Sulfat zugeführt wird (Typ 2). Der Unterschied in den charakteristischen Verteilungen setzt voraus, daß die Stärke der kinetischen Sulfat-Sulfid-Isotopen-Wirkung auf die Reduktion in beiden Fällen verschieden ist. Dieser Unterschied wurde bereits wegen der Widersprüche zwischen den verschiedenen S³⁴-Werten heutiger mariner Sedimente und Laborexperimente vermutet. Der Unterschied in der Isotopen-Wirkung kann durch das Modell von Rees (1973) für kontinuierlich ablaufende Sulfat-Reduktion erklärt werden. Geringes Nahrungsangebot und ungestörte, gleichbleibende Bakterien-Populationen in offenen Systemen neigen zur Erzeugung stärkerer Fraktionierungen.

     

Hydrothermal alteration and oxygen and hydrogen isotope geochemistry of the D-68 zone Cu-Zn Massive Sulfide Deposit,Noranda District,Quebec, Canada

Summary Pervasive hydrothermal alteration zones in quartz-feldspar porphyry domes underly all massive sulfide lenses in the D-68 Zone Cu-Zn deposit, Noranda. Alteration pipes are mineralogically zoned and contain chloritic cores consisting of stringer sulfides, enveloped by sericitic haloes. Silicified rocks are found locally.Alteration took place at nearly constant volume. Na depletion, and K enrichment relative to the least altered rocks, are found in all alteration zones. Fe and Mg have been added to the chloritic zone and subtracted in the sericitic and silicic zones. Ca and Si are enriched mainly in the silicic zone. Al, Ti and Zr were the least mobile of the elements studied.Whole-rock ¹⁸O values vary from +5.6 to +6.2 per mil in chloritized rocks, +5.8 to + 7.3 per mil in sericitized rocks and + 7.2 to + 8.3 per mil in silicified rocks. D values for two chloritized samples are – 63 and – 70 per mil whereas in two sericitized samples they are close to –62 per mil. Quartz from the chlorite alteration zone is isotopically heavier (¹⁸O = 8.6 per mil) than that from the sericite alteration zone (¹⁸O = 6.4 per mil), suggesting equilibration with different hydrothermal fluid or different temperature of alteration. Assuming an alteration temperature of 300° + 50°C the fluid in equilibrium with quartz and chlorite had ¹⁸O and D values of about 1.5 ± 2.0 per mil and –23 ± 5 per mil, respectively. The fluid in equilibrium with quartz and sericite had ¹⁸O and D values of about –0.5 ± 2 per mil and –30 ± 5 per mil, respectively. On the basis of isotopic data, seawater was probably the major constituent of the hydrothermal fluids.

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Hydrothermale Umwandlung und Sauerstoff-Wasserstoff-Isotopengeochemie der Zone D-68 Cu-Zn Derberz Sulfidlagerstätte, Noranda District, Quebec, Canada

Zusammenfassung Hydrothermale Umwandlungszonen in porphyrischen Quarz-Feldspat Gesteinskörpern liegen unterhalb von Derberz Sulfidlinsen in der D-68 Zone Cu-Zn Lagerstätte, Noranda. Umgewandelte pipes sind mineralogisch zoniert; sie enthalten aus Sulfiden bestehende chloritische Kerne, die von sericitischen Höfen umhüllt werden. Lokal treten silicifizierte Gesteine auf.Die Umwandlung ging bei annähernd konstantem Volumen vor sich. Na-Verarmung und K-Anreicherung, bezogen auf die am wenigsten umgewandelten Gesteine, liegen in allen Umwandlungszonen vor. Fe und Mg wurden der Chloritzone zugeführt, in den Sericit- und Si-Zonen abgeführt. Ca und Si sind vor allem in der Si-Zone angereichert. Al, Ti und Zr waren von den untersuchten Elementen am wenigsten mobil.Gesamtgesteins-¹⁸O Werte variieren von +5,6 bis +6,2 in den chloritisierten Gesteinen, von +5,8 bis 7,3 in sericitisierten Gesteinen und von +7,2 bis +8,3 in den silicifizierten Gesteinen. Die D Werte für zwei chloritisierte Proben betragen –63 und –70, in zwei sericitisierten Proben liegen sie hingegen nahe bei –62. Quarz von der Chlorit-Umwandlungszone ist isotopisch schwerer (¹⁸O = 8,6) als von der Sericit-Umwandlungszone (¹⁸O = 6.4), was eine Gleichgewichtseinstellung mit verschiedenen hydrothermalen Lösungen oder eine verschiedene Umwandlungstemperatur nahelegt. Bei einer angenommenen Umwandlungstemperatur von 300 ± 50°C, hatte die im Gleichgewicht mit Quarz und Chlorit stehende Lösung ¹⁸O und D Werte von etwa 1,5 ± 2 bzw. –23 + 5. Die im Gleichgewicht mit Quarz und Sericit befindliche Lösung hatte ¹⁸O und D Werte von etwa –0,5 ± 2%o bzw. –30 ± 5. Aufgrund der Isotopendaten war wahrscheinlich Meerwasser der Hauptbestandteil der hydrothermalen Lösungen.

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With 7 Figures     

Stratiform and vein U,Mo and Cu mineralization in the Novoveská Huta area,CSFR

U-Mo and Cu mineralization occurs in horizons as well as in veins in the Permian formations near Novoveská Huta. Ore mineralization is represented by uraninite, U-Ti oxides, coffinite, molybdenite, chalcopyrite, tennantite and pyrite. The isotopic composition of S and C displays a larger variability in the stratiform ores (³⁴S from –32.7 to +2.7 and ¹³C from –27.1 to — 0.5) These data suggest mixing of meteoric solutions with fluids of volcanic origin and a complex history. There is a narrower range of ³⁴S from –18.8 to –4.6 and ¹³C from –6.3 to –2.5% in quartz-carbonate veins with Cu mineralization suggesting a deep source of ore-bearing solutions. The Permian volcanics were a significant source of ore elements.Their contents of U, Mo, Cu and Y are from two to eight times higher than in sedimentary rocks. Accumulations of ore elements in the horizons were formed by the reduction and adsorption processes 240 ± 30 Ma ago according to U-Pb isotopic dating. Due to Alpine tectonism, these low-grade ores (U<0.1 wt%) were remobilized and higher-grade U-Mo ores (U>0.1 wt%) were formed 130 ± 20 Ma ago at temperatures ranging from 110 to 120° C, according to fluid inclusions. Younger veins with Cu mineralization were formed 115 ± 10 Ma according to the model age of Pb at temperatures ranging from 95 to 190°C.     

A stable isotope study of retrograde alteration in SW Connemara,Ireland

Dalradian metamorphic rocks, Lower Ordovician meta-igneous rocks (MGS) and Caledonian granites of the Connemara complex in SW Connemara all show intense retrograde alteration. Alteration primarily involves sericitization and saussuritization of plagioclase, the alteration of biotite and hornblende to chlorite and the formation of secondary epidote. The alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs, and was the result of a phase of fluid influx into these rocks. In hand specimen K-feldspar becomes progressively reddened with increasing alteration. Mineralogical alteration in the MGS and Caledonian granites took place at temperatures 275±15°C and in the MGS P_fluid is estimated to be 1.5 kbar during alteration. The °D values of alteration phases are:-18 to-29 (fluid inclusions),-47 to-61 (chlorites) and-11 to-31 (epidotes). Chlorite ¹⁸O values are +0.2 to +4.3, while ¹⁸O values for quartz-K-feldspar pairs show both positively sloped (MGS) and highly unusual negatively sloped (Caledonian granites) arrays, diverging from the normal magmatic field on a - plot. The stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200–140°C. The retrograde fluid had D -20 to-30 in all lithologies, but the fluid ¹⁸O varied both spatially and temporally within the range-4 to +7. The fO₂ of the fluid that deposited the epidotes in the MGS varied with its ¹⁸O value, with the most ¹⁸O-depleted fluid being the most oxidizing. The D values, together with low (<0) ¹⁸O values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin. This meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration. The variation in retrograde fluid ¹⁸O values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid-rock interaction. Infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the Galway granite soon after intrusion at 400 Ma. However convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later.     

Oxygen isotope compositions of selected laramide-tertiary granitoid stocks in the Colorado Mineral Belt and their bearing on the origin of climax-type granite-molybdenum systems

Quartz phenocrysts from 31 granitoid stocks in the Colorado Mineral Belt yield ¹⁸O values less than 10.4, with most values between 9.3 and 10.4. An average magmatic value of about 8.5 is suggested. The stocks resemble A-type granites; these data support magma genesis by partial melting of previously depleted, fluorine-enriched, lower crustal granulites, followed by extreme differentiation and volatile evolution in the upper crust.Subsolidus interaction of isotopically light water with stocks has reduced most feldspar and whole rock ¹⁸O values. Unaltered samples from Climax-type molybdenumbearing granites, however, show no greater isotopic disturbance than samples from unmineralized stocks. Although meteoric water certainly played a role in post-mineralization alteration, particularly in feldspars, it is not required during high-temperature mineralization processes. We suggest that slightly low ¹⁸O values in some vein and replacement minerals associated with molybdenum mineralization may have resulted from equilibration with isotopically light magmatic water and/or heavy isotope depletion of the ore fluid by precipitation of earlier phases.Accumulation of sufficient quantities of isotopically light magmatic water to produce measured depletions of ¹⁸O requires extreme chemical stratification in a large magma reservoir. Upward migration of a highly fractionated, volatile-rich magma into a small apical Climax-type diapir, including large scale transport of silica, alkalis, molybdenum, and other vapor soluble elements, may occur with depression of the solidus temperature and reduction of magma viscosity by fluorine. Climax-type granites may provide examples of ¹⁸O depletion in magmatic systems without meteoric water influx.     

Possible non-equilibrium oxygen isotope effects in mantle nodules,an alternative to the Kyser-O'Neil-Carmichael18O/16O geothermometer

Kyser, O'Neil, and Carmichael (1981, 1982) measured the ¹⁸O values of coexisting minerals from peridotite nodules in alkali basalts and kimberlites, interpreting the nodules as equilibrium assemblages. Using Ca-Mg-Fe element-partition geothermometric data, they proposed an empirical¹⁸O/¹⁶O geothermometer: T(°C)=1,151–173–68 ², where is the per mil pyroxene-olivine fractionation. However, this geothermometer has an unusual crossover at 1,150 °C, and in contrast to what might be expected during closed-system equilibrium exchange, the most abundant mineral in the nodules (olivine) shows a much greater range in ¹⁸O (+4.4 to +7.5) than the much less abundant pyroxene (all 50 pyroxene analyses from spinel peridotites lie within the interval +5.3 to +6.5). On ¹⁸O-olivinevs. ¹⁸O-pyroxene diagrams, the mantle nodules exhibit data arrays that cut across the ¹⁸O=zero line. These arrays strongly resemble the non-equilibrium quartzfeldspar and feldspar-pyroxene ¹⁸O arrays that we now know are diagnostic of hydrothermally altered plutonic igneous rocks. Thus, we have re-interpreted the Kyser et al. data as non-equilibrium phenomena, casting doubt on their empirical geothermometer. The peridotite nodules appear to have been open systems that underwent metasomatic exchange with an external, oxygen-bearing fluid (CO₂, magma, H₂O, etc.); during this event, the relatively inert pyroxenes exchanged at a much slower rate than did the coexisting olivines and spinels, in agreement with available exchange-rate and diffusion measurements on these minerals. This accounts for the correlation between ¹⁸O pyroxene-olivine and the whole-rock ¹⁸O of the peridotites, which is a major difficulty with the equilibrium interpretation.Contribution No. 3978, Publications of the Division of Geological and Planetary Sciences, California Institute of Technology     

Contrasting fluid/rock interaction between the Notch Peak granitic intrusion and argillites and limestones in western Utah: evidence from stable isotopes and phase assemblages

The Jurassic Notch Peak granitic stock, western Utah, discordantly intrudes Cambrian interbedded pure limestones and calcareous argillites. Contact metamorphosed argillite and limestone samples, collected along traverses away from the intrusion, were analyzed for ¹⁸O, ¹³C, and D. The ¹³C and ¹⁸O values for the limestones remain constant at about 0.5 (PDB) and 20 (SMOW), respectively, with increasing metamorphic grade. The whole rock ¹⁸O values of the argillites systematically decrease from 19 to as low as 8.1, and the ¹³C values of the carbonate fraction from 0.5 to –11.8. The change in ¹³C values can be explained by Rayleigh decarbonation during calcsilicate reactions, where calculated is about 4.5 permil for the high-grade samples and less for medium and low-grade samples suggesting a range in temperatures at which most decarbonation occurred. However, the amount of CO₂ released was not anough to decrease the whole rock ¹⁸O to the values observed in the argillites. The low ¹⁸O values close to the intrusion suggest interaction with magmatic water that had a ¹⁸O value of 8.5. The extreme lowering of ¹³C by fractional devolatilization and the lowering of ¹⁸O in argillites close to the intrusion indicates oxgen-equivalent fluid/rock ratios in excess of 1.0 and X(CO₂)_F of the fluid less than 0.2. Mineral assemblages in conjunction with the isotopic data indicate a strong influence of water infiltration on the reaction relations in the argillites and separate fluid and thermal fronts moving thru the argillites. The different stable isotope relations in limestones and argillites attest to the importance of decarbonation in the enhancement of permeability. The flow of fluids was confined to the argillite beds (argillite aquifers) whereas the limestones prevented vertical fluid flow and convective cooling of the stock.     

Geology and geochemistry of the Changba SEDEX Pb-Zn deposit,Qinling orogenic belt,China

The Changba Pb-Zn SEDEX deposit occurs in the Middle Devonian sequence of the Anjiaca Formation of the Western Qinling Hercynian Orogen in the Gansu Province, China. The Changba-II orebody is hosted in biotite quartz schist and is the largest of 143 stratiform orebodies that are hosted either in biotite quartz schist or marble. The Changba-II comprises two types of mineralization: a bedded facies and an underlying breccia lens. The bedded section exhibits three sulfide sub-facies zoned from bottom to top: 1) banded sphalerite intercalated with quartz albitite; 2) interbedded massive pyrite and sphalerite ore; and 3) banded sphalerite ore intercalated with banded baritite. Major metallic minerals are sphalerite, pyrite, galena, with minor arsenopyrite, pyrrhotite, boulangerite, and rare chalcopyrite. The bedded sulfides are underlain by a lens of brecciated and albitized biotite-quartz schists cemented by sulfides and tourmaline.Massive and bedded sulfide ³⁴S values range from 8.1 to 29.3, whereas barite ³⁴S values range from 20.8 to 31.5. Disseminated pyrite in footwall schists has ³⁴S values ranging from 8.1 to 10.6, and increase to values ranging from 11.1 to 14.7 in the hangingwall. The lower ³⁴S values for massive and bedded sulfides are interpreted to be derived from progressive bacterial sulfate reduction (BSR) of Devonian seawater in a sulfate-restricted sub-basin. The higher ³⁴S values for massive and bedded sulfides could be a product of quantitative BSR but this is incompatible with barite being more abundant above the bedded sulfides. Instead, it is more likely that thermochemical sulfate reduction of seawater sulfate or of evaporite was the source of heavy hydrothermal sulfur. Heavy hydrothermal sulfur was injected into a sulfate-restricted sub-basin where it mixed with low ³⁴S BSR sulfide to form the massive and bedded sulfides. The REE patterns of sulfide layers and associated quartz albitite and baritite are similar to those of the host biotite quartz schists, suggesting that the hydrothermal fluids leached REE from the underlying rocks. Pb isotope ratios in galena form an array between the Upper Crust and the Mantle reservoir curves, which indicates that the lead is derived from upper crustal rocks comprising mafic igneous units. The Sr⁸⁷/Sr⁸⁶ ratio of 0.7101 for carbonate within the sulfide layers also suggests that Sr is derived from the mixing of Sr leached from upper crustal rocks with Middle Devonian seawater Sr. A Rb-Sr isochron age of 389.4 ± 6.4 Ma for sulfide layers and the interbedded hydrothermal sediments is consistent with the age of host Mid-Devonian strata. Ar³⁹/Ar⁴⁰ plateau age at 352.8 ± 3.5 Ma and Ar³⁹-Ar⁴⁰ isochron age of 346.6 ± 6.4 Ma for albite in the quartz albitite intercalated with sulfide layers indicate either albite formation after the sulfides or thermal resetting of the Rb-Sr system at about 350 Ma, the age of collision between the North China and Yangtze cratons.Editorial handling: E. Frimmel     

Sulfur isotope study of source and deposits of stibnite in the Turhal Area,Turkey

The Turhal antimony sulfide ore deposits are hosted by a Permian-Jurassic sequence which consists of black phyllites at the base followed by interbedded phyllites and calcareous quartzites with metabasite interlayers and then by brown-gray phyllites with marble blocks. Four different styles and three distinct episodes of mineralization were distinguished according to deposition features of the ores and kinkbands in the stibnite crystals. Stibnite from stratiform, disseminated and vein occurrences as well as pyrite from black phyllites showed the following sulfur isotope composition (³⁴S): +2.8 and +3.0 for stratiform stibnite (n = 2), +3.6 and +5.5 for disseminated stibnite (n = 2), +2.5 to +7.8 for vein stibnite (n = 11) and -6.1 to +0.1 for pyrite (n = 3). The ³⁴S compositions of stibnite are interpreted as suggesting an ultimately single source for sulfur in the various styles of mineralization, i.e. synsedimentary volcanic exhalations for the stratiform and disseminated together with ores and hydrothermal mobilisation of these as well as leaching of volcanic rocks to form the vein ores. Deep basinal fluids probably under normal geothermal gradient conditions caused the leaching of the primary sulfides as suggested by the oxygen isotope composition of vein quartz associated with the ores. By contrast sulfur in pyrite is essentially a derivation of seawater sulfate through bacterial and/or chemical reduction.     

Stable isotope (C,O, S) systematics of the mercury mineralization at Idrija,Slovenia: constraints on fluid source and alteration processes

The world-class Idrija mercury deposit (western Slovenia) is hosted by highly deformed Permocarboniferous to Middle Triassic sedimentary rocks within a complex tectonic structure at the transition between the External Dinarides and the Southern Alps. Concordant and discordant mineralization formed concomitant with Middle Triassic bimodal volcanism in an aborted rift. A multiple isotopic (C, O, S) investigation of host rocks and ore minerals was performed to put constraints on the source and composition of the fluid, and the hydrothermal alteration. The distributions of the ¹³C and ¹⁸O values of host and gangue carbonates are indicative of a fracture-controlled hydrothermal system, with locally high fluid-rock ratios. Quantitative modeling of the ¹³C and ¹⁸O covariation for host carbonates during temperature dependent fluid-rock interaction, and concomitant precipitation of void-filling dolomites points to a slightly acidic hydrothermal fluid (¹³C–4 and ¹⁸O+10), which most likely evolved during isotopic exchange with carbonates under low fluid/rock ratios. The ³⁴S values of hydrothermal and sedimentary sulfur minerals were used to re-evaluate the previously proposed magmatic and evaporitic sulfur sources for the mineralization, and to assess the importance of other possible sulfur sources such as the contemporaneous seawater sulfate, sedimentary pyrite, and organic sulfur compounds. The ³⁴S values of the sulfides show a large variation at deposit down to hand-specimen scale. They range for cinnabar and pyrite from –19.1 to +22.8, and from –22.4 to +59.6, respectively, suggesting mixing of sulfur from different sources. The peak of ³⁴S values of cinnabar and pyrite close to 0 is compatible with ore sulfur derived dominantly from a magmatic fluid and/or from hydrothermal leaching of basement rocks. The similar stratigraphic trends of the ³⁴S values of both cinnabar and pyrite suggest a minor contribution of sedimentary sulfur (pyrite and organic sulfur) to the ore formation. Some of the positive ³⁴S values are probably derived from thermochemical reduction of evaporitic and contemporaneous seawater sulfates.Editorial handling: P. Lattanzi     

An isotopic investigation of the environment of deposition of the McArthur mineralization

Additional measurements of ³⁴S/³²S, ¹³C/¹²C and ¹⁸O/¹⁶O ratios in metallic sulphides, carbonates and organic residues suggest a mode of genesis of the McArthur deposit generally consistent with geological and geochemical evidence. A very stable marine environment is indicated by the constant values for ¹³C and ¹⁸O observed throughout the entire deposit. However, ³⁴S contents of pyrites varied by 25 in a manner consistent with the water depths and sulphate availability postulated for the McArthur environment.