Phase equilibrium and stable isotope constraints on the formation of metasomatic garnet-vesuvianite veins (SW Adamello,N Italy)

 Metasomatic garnet-vesuvianite veins occur within the contact metamorphic marble sequence of the Lower Triassic Prezzo formation in a narrow, 1–5 m wide zone along an intrusive marble-granodiorite contact at the southwestern border of the Tertiary Adamello batholith. The metasomatic mineral assemblage is comprised of garnet, vesuvianite, clinopyroxene, wollastonite, and pyrrhotite, which were precipitated from the vein-forming fluid in a preexisting calcite matrix at conditions of about 2800 bars and 630° C. The veins are enriched in silicon, aluminum, iron, magnesium, titanium and depleted in calcium with respect to the unaltered contact metamorphic marble. Graphite, which is present in the unaltered Prezzo Marble is absent in the veins. Irregularly shaped mineralogically distinct zones with different degrees of silicification can be distinguished within the veins. The isotopic compositions of calcite (cc) in the unaltered marble are about δ¹⁸O (SMOW; Standard mean Ocean Water)=21.0‰ and δ¹³C(PDB; Peedee belemnite)=0.0‰. They are reset to significantly lower values within the veins, where δ¹⁸O_cc is 15.0 to 16.0‰ and δ¹³C_cc is −4.5 to −3.5‰. The isotopic front coincides with an abrupt change in the microscopic texture of matrix carbonate which occurs at the sharp boundary between graphite-bearing and graphite-free material. Within the veins the oxygen isotope fractionation between calcite and garnet (gar) varies systematically with distance from highly silicified zones. The variations in Δ¹⁸O_cc-gar are as large as 2‰, on a millimeter scale, indicating garnet-calcite isotopic disequilibrium. Vein formation was due to the infiltration of a water rich fluid of magmatic provenance into the carbonate country rock along fractures. Removal of graphite from the wall rock by dissolution through the metasomatic fluid induced recrystallization of matrix calcite. Permeability was enhanced during calcite recrystallization facilitating material transport into the wall rock and metasomatic alteration. Vein garnet was precipitated in isotopic equilibrium with the metasomatic fluid. The isotopic composition of preexisting calcite was initially out of equilibrium with the vein-forming fluid and it was shifted towards equilibrium by surface-reaction controlled calcite-fluid isotopic exchange during calcite recrystallization. Due to the short lifetime of the metasomatic system, calcite-fluid isotopic equilibrium was generally not attained. Within the veins, oxygen and carbon transport was fast relative to mineral-fluid exchange of their isotopes and the geometry of the isotopic pattern is largely controlled by the kinetics of mineral-fluid exchange. Received: 16 June 1994/Accepted: 20 May 1995     

A fluid inclusion and stable isotope study of 200 Ma of fluid evolution in the Galway Granite, Connemara, Ireland

Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies. The earliest fluid was a H₂O-CO₂-NaCl fluid of moderate salinity (4–10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V₁) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305–390 °C and 0.7–1.8 kbar. Homogeneous quartz δ¹⁸O across the batholith (9.5 ± 0.4‰n = 12) suggests V₁ precipitation at high temperatures (perhaps 600 °C) and pressures (1–3 kbar) from magmatic fluids. Microthermometric data for V₁ indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling. The second fluid was a H₂O-NaCl-KCl, low-moderate salinity (0–10 wt% NaCl eq.), moderate temperature (270–340 °C), high δD (−18 ± 2‰), low δ¹⁸O (0.5–2.0‰) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V₂) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5–2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H₂O-NaCl-CaCl₂-KCl fluid with variable salinity (8–28 wt% NaCl eq.), temperature (125–205 °C), δD (−17 to −45‰), δ¹⁸O (−3 to + 1.2‰), δ¹³C_CO2 (−19 to 0‰) and δ³⁴S_sulphate (13–23‰) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V₃). Correlations of salinity, temperature, δD and δ¹⁸O are interpreted as the result of mixing of two fluid end-members, one a high-δD (−17 to −8‰), moderate-δ¹⁸O (1.2–2.5‰), high-δ¹³C_CO2 (> −4‰), low-δ³⁴S_sulphate (13‰), high-temperature (205–230 °C), moderate-salinity (8–12 wt% NaCl eq.) fluid, the other a low-δD (−61 to −45‰), low-δ¹⁸O (−5.4 to −3‰), low-δ¹³C (<−10‰), high-δ³⁴S_sulphate (20–23‰) low-temperature (80–125 °C), high-salinity (21–28 wt% NaCl eq.) fluid. Geochronological evidence suggests V₃ veins are late Triassic; the high-δD end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-δD end-member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence. This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated. Received: 3 April 1996 / Accepted: 5 May 1997     

Epithermal mineralization and ore controls of the Shasta Au-Ag deposit, Toodoggone District, British Columbia, Canada

The Shasta gold-silver deposit, British Columbia, Canada, is an adularia-sericite-type epithermal deposit in which deposition of precious metals coincided with the transition of quartz- to calcite-dominant gangue. Mineralization is associated with stockwork-breccia zones in potassically altered dacitic lapilli tuffs and flows, and consists of pyrite, sphalerite, chalcopyrite, galena, acanthite, electrum and native silver. Pre- and post-ore veins consist solely of quartz and calcite, respectively. Fluid inclusion microthermometry indicates that ore minerals were deposited between 280 ° and 225 °C, from a relatively dilute hydrothermal fluid (˜1.5 wt.% NaCl equivalent). Abundant vapor-rich inclusions in ore-stage calcite are consistent with boiling. Oxygen and hydrogen isotopic data (δ¹⁸O_fluid = −1.5 to −4.1‰; δD_fluid = −148 to −171‰) suggest that the fluid had a meteoric origin, but was ¹⁸O-enriched by interaction with volcanic wallrocks. Initial (˜280 °C) fluid pH and log f O₂ conditions are estimated at 5.3 to 6.0, and −32.5 to −33 bar, respectively; during ore deposition, the fluid became more alkaline and oxidizing. Ore deposition at Shasta is attributed to localization of meteoric hydrothermal fluids by extensional faults; mineralization was controlled by boiling in response to hydraulic brecciation. Calcite and base metal sulfides precipitated due to the increase in pH that accompanied boiling, and the associated decrease in H₂S concentration led to precipitation of gold and silver. Received: 23 February 1995 / Accepted: 16 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     

The Darasun gold deposit,Eastern Transbaikal region: Chemical composition,REE patterns,and stable carbon and oxygen isotopes of carbonates from ore veins

The chemistry, REE patterns, and carbon and oxygen isotopic compositions of carbonates from ore veins of the Darasun deposit are discussed. In addition to the earlier described siderite, calcite, and carbonates of the dolomite-ankerite series, kutnahorite is identified. The total REE content in Fe-Mg carbonates of the dolomite-ankerite series (2.8–73 ppm) is much lower than in later calcite (18–390 ppm). δ¹³C of Fe-Mg carbonates and calcite varies from +1.1 to −6.7‰ and from −0.9 to −4.9‰, respectively. δ¹⁸O of Fe-Mg carbonates and calcite varies from +17.6 to 3.6‰ and from +15.7 to −0.5‰, respectively. The REE sum and carbon and oxygen isotopic compositions reveal zonal distribution relative to the central granodiorite porphyry stock. The correlation between the carbon and oxygen isotopic compositions and REE sum reflects variations in the physicochemical formation conditions and composition of ore-forming fluid. The isotopic composition of fluid is calculated, and possible sources of its components are considered. Earlier established evidence for a magmatic source of ore-forming fluid and participation of meteoric water in ore formation is confirmed. Geochemical evidence for interaction of ore-forming fluid with host rocks is furnished. The relationships between the REE sum, on the one hand, and carbon and oxygen isotopic compositions of hydrothermal ore-forming fluid, on the other, are established.     

S,Pb, C and O isotopic compositions and ore genesis of the strata-bound polymetallic sulfide deposits in central Inner Mongolia,China

The strata-bound Cu−Pb−Zn polymetallic sulfide deposits occur in metamorphic rocks of greenschist phase of the middle-upper Proterozoic Langshan Group in central Inner Mongolia. δ³⁴S values for sulfides range from −3.1‰ to +37.3‰, and an apparent difference is noticed between vein sulfides and those in bedded rocks. For example, δ³⁴S values for bedded pyrite range from +10.6‰ to +20.0‰, while those for vein pyrite vary from −3.1‰ to +14.1‰. δ³⁴S of bedded pyrrhotite is in the range +7.9‰–+23.5‰ in comparison with +6.5‰–+17.1‰ for vein pyrrhotite. The wide scatter of δ³⁴S and the enrichment of heavier sulfur indicate that sulfur may have been derived from H₂S as a result of bacterial reduction of sulfates in the sea water. Sulfur isotopic composition also differs from deposit to deposit in this area because of the difference in environment in which they were formed. The mobilization of bedded sulfides in response to regional metamorphism and magmatic intrusion led to the formation of vein sulfides. δ¹⁸O and δ¹³C of ore-bearing rocks and wall rocks are within the range typical of ordinary marine facies, with the exception of lower values for ore-bearing marble at Huogeqi probably due to diopsidization and tremalitization of carbonate rocks. Pb isotopic composition is relatively stable and characterized by lower radio-genetic lead. The age of basement rocks was calculated to be about 23.9 Ma and ore-forming age 7.8 Ma.²⁰⁷Pb/²⁰⁴Pb−²⁰⁶Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb−²⁰⁶Pb/²⁰⁴Pb plots indicate that Pb may probably be derived from the lower crust or upper mantle. It is believed that the deposits in this region are related to submarine volcanic exhalation superimposed by later regional metamorphism and magmatic intrusion.     

Isotope geochemistry of ore fluids for the Dongsheng sandstone-type uranium deposit, China

The Dongsheng sandstone-type uranium deposit is one of the large-sized sandstone-type uranium deposits discovered in the northern part of the Ordos Basin of China in recent years. Geochemical characteristics of the Dongsheng uranium deposit are significantly different from those of the typical interlayered oxidized sandstone-type uranium ore deposits in the region of Middle Asia. Fluid inclusion studies of the uranium deposit showed that the uranium ore-forming temperatures are within the range of 150–160℃. Their 3He/4He ratios are within the range of 0.02–1.00 R/Ra, about 5–40 times those of the crust. Their 40Ar/36Ar ratios vary from 584 to 1243, much higher than the values of atmospheric argon. The δ18OH2O and δD values of fluid inclusions from the uranium deposit are -3.0‰– -8.75‰ and -55.8‰– -71.3‰, respectively, reflecting the characteristics of mixed fluid of meteoric water and magmatic water. The δ18OH2O and δD values of kaolinite layer at the bottom of the uranium ore deposit are 6.1‰ and -77‰, respectively, showing the characteristics of magmatic water. The δ13CV-PDB and δ18OH2O values of calcite veins in uranium ores are -8.0‰ and 5.76‰, respectively, showing the characteristics of mantle source. Geochemical characteristics of fluid inclusions indicated that the ore-formation fluid for the Dongsheng uranium deposit was a mixed fluid of meteoric water and deep-source fluid from the crust. It was proposed that the Jurassic-Cretaceous U-rich metamorphic rocks and granites widespread in the northern uplift area of the Ordos Basin had been weathered and denudated and the ore-forming elements, mainly uranium, were transported by meteoric waters to the Dongsheng region, where uranium ores were formed. Tectonothermal events and magmatic activities in the Ordos Basin during the Mesozoic made fluids in the deep interior and oil/gas at shallow levels upwarp along the fault zone and activated fractures, filling into U-bearing clastic sandstones, thus providing necessary energy for the formation of uranium ores.     

Mineralogical, fluid inclusion, and stable isotope constraints on mechanisms of ore deposition at the Samgwang mine (Republic of Korea)—a mesothermal, vein-hosted gold–silver deposit

The Samgwang mine is located in the Cheongyang gold district (Cheonan Metallogenic Province) of the Republic of Korea. It consists of eight massive, gold-bearing quartz veins that filled NE- and NW-striking fractures along fault zones in Precambrian granitic gneiss of the Gyeonggi massif. Their mineralogy and paragenesis allow two separate vein-forming episodes to be recognized, temporally separated by a major faulting event. The ore minerals occur in quartz and calcite of stage I, associated with fracturing and healing of veins. Hydrothermal wall-rock alteration minerals of stage I include Fe-rich chlorite (Fe/(Fe+Mg) ratios 0.74-0.81), muscovite, illite, K-feldspar, and minor arsenopyrite, pyrite, and carbonates. Sulfide minerals deposited along with electrum during this stage include arsenopyrite, pyrite, pyrrhotite, sphalerite, marcasite, chalcopyrite, galena, argentite, pyrargyrite, and argentian tetrahedrite. Only calcite was deposited during stage II. Fluid inclusions in quartz contain three main types of C–O–H fluids: CO₂-rich, CO₂–H₂O, and aqueous inclusions. Quartz veins related to early sulfides in stage I were deposited from H₂O–NaCl–CO₂ fluids (1,500–5,000 bar, average 3,200) with T _htotal values of 200°C to 383°C and salinities less than about 7 wt.% NaCl equiv. Late sulfide deposition was related to H₂O–NaCl fluids (140–1,300 bar, average 700) with T _htotal values of 110°C to 385°C and salinities less than about 11 wt.% NaCl equiv. These fluids either evolved through immiscibility of H₂O–NaCl–CO₂ fluids as a result of a decrease in fluid pressure, or through mixing with deeply circulated meteoric waters as a result of uplift or unloading during mineralization, or both. Measured and calculated sulfur isotope compositions (δ³⁴S_H2S = 1.5 to 4.8‰) of hydrothermal fluids from the stage I quartz veins indicate that ore sulfur was derived mainly from a magmatic source. The calculated and measured oxygen and hydrogen isotope compositions (δ¹⁸O_H2O = −5.9‰ to 10.9‰, δD = −102‰ to −87‰) of the ore-forming fluids indicate that the fluids were derived from magmatic sources and evolved by mixing with local meteoric water by limited water–rock exchange and by partly degassing in uplift zones during mineralization. While most features of the Samgwang mine are consistent with classification as an orogenic gold deposit, isotopic and fluid chemistry indicate that the veins were genetically related to intrusions emplaced during the Jurassic to Cretaceous Daebo orogeny.     

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 isotope geochemistry and diagenetic mineralization associated with the Tono sandstone-type uranium deposit in Japan

The Tono sandstone-type uranium mine area, middle Honsyu, Japan is composed of Miocene lacustrine sedimentary rocks in the lower part (18–22 Ma) and marine facies in the upper part (15–16 Ma). Calcite and pyrite occur as dominant diagenetic alteration products in these Neogene sedimentary rocks. The characteristics of calcite and pyrite differ significantly between lacustrine and marine facies. Abundant pyrite, calcite, organic matter, and small amounts of marcasite or pyrrhotite occur in the lacustrine facies, whereas small amounts of calcite and framboidal pyrite, organic matter and no marcasite or pyrrhotite are found within the marine units. The δ¹³C values of calcite in the lacustrine deposits are low (−19 to −6‰ PDB) but those in marine formation are high (−11 to +3‰). This implies that the contribution of marine carbonate is larger in upper marine sedimentary rocks, and carbon in calcite in the lower lacustrine formation was derived both from oxidation of organic matter and from dissolved marine inorganic carbon. The δ³⁴S values of framboidal pyrite in the upper marine formation are low (−14 to −8‰ CDT), indicating a small extent of bacterial seawater sulfate reduction, whereas those of euhedral-subhedral pyrite in the lower lignite-bearing arkose sandstone are high (+10 to +43‰), implying a large extent of closed-system bacterial seawater sulfate reduction. The δ³⁴S and δ¹³C data which deviate from a negative correlation line toward higher δ¹³C values suggest methanogenic CO₂ production. During diagenesis of the lacustrine unit, large amounts of euhedral-subhedral pyrite were formed, facilitated by extensive bacterial reduction of seawater sulfate with concomitant oxidation of organic matter, and by hydrolysis reactions of organic matter, producing CH₄ and CO₂. Uranium minerals (coffinite and uraninite) were also formed at this stage by the reduction of U⁶⁺ to U⁴⁺. The conditions of diagenetic alteration within the lacustrine deposits and uranium mineralization is characterized by low Eh in which nearly equal concentrations of CH₄ and HCO₃ ⁻ existed and reduced sulfur species (H₂S, HS⁻) are predominant among aqueous sulfur species, whereas diagenetic alteration of the marine formations was characterized by a predominance of SO₄ ²⁻ among dissolved sulfur species. Modern groundwater in the lacustrine formation has a low Eh value (−335 mV). Estimated and measured low Eh values of modern and ancient interstitial waters in lacustrine environments indicate that a reducing environment in which U⁴⁺ is stable has been maintained since precipitation of uranium minerals. Received: 9 February 1996 / Accepted: 11 April 1997     

Geological and structural controls on gold mineralization in the Tanami District, Northern Territory

Gold mineralization in the Tanami district is hosted within moderately northwest dipping turbiditic sedimentary and basaltic volcanic rocks of the Paleoproterozoic Mt. Charles Formation. The gold occurs within a complex sinistral wrench-fault array and associated veins and alteration haloes. The main mineralized faults have a northerly trend and dip steeply east. Subsidiary structures trend at 030° and 070° and dip towards the southeast. Paleostress calculations based on fault striation populations and geometry (strike and dip) of faults indicate that at the time of the mineralizing event, σ ₁ was sub-horizontal and SE–NW directed with σ ₂ subvertical. Structural studies indicate that the mineralization occurred after the regional folding event and synchronous with the emplacement of felsic dykes into the mine sequence. Gold veins in the Tanami district are interpreted to be part of an outer thermal aureole gold system that formed during the emplacement of granitoids in the nearby ∼1,815 to ∼1,799 Ma Frankenia and/or Coomarie domes. Economic gold mineralization occurred late in the paragenetic history of the district. Gold is hosted by quartz-carbonate veins within shear zones, and also in the surrounding sericite- quartz- pyrite ± carbonate-altered wallrocks. Gold-mineralized veins precipitated at depths of 3 to 6 km from high temperature (∼300°C), low salinity (∼5 wt% NaCl equivalent) fluids with low CO₂ contents. Barren quartz, dolomite and calcite veins that occur in pre- and post-mineralization thrust faults formed from high salinity (∼20 wt% NaCl equivalent), low temperature (∼120–150°C) basinal brines. Pyrite in the gold mineralized veins and alteration halos has lower δ ³⁴S values (6.8 to 12.5‰) than local diagenetic pyrite (17.8 to 19.2‰) or pyrite in pre-mineralization thrust faults (31.7 to 37.1‰). The mineralizing fluids are inferred to have contained a well-homogenized mixture of magmatic and sedimentary-derived sulfur. Editorial handling: D. Huston     

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.     

Retrograde fluids in the Archean Shawmere anorthosite, Kapuskasing Structural Zone, Ontario, Canada

The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario, and is crosscut by numerous linear alteration veins containing calcite + quartz ± dolomite ± zoisite ± clinozoisite ± margarite ±paragonite ± chlorite. These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz ± calcite ± plagioclase show that the vein minerals were stable at T < 600 °C, XCO₂ < 0.4 at P ≈ 6 kbar. The stabilities of margarite and paragonite in equilibrium with quartz are also consistent with T < 600 °C and XCO₂ < 0.4 at 6 kbar. Additional assemblages consisting of calcite + clinochlore + quartz + talc + margarite indicate T < 500 °C with XCO₂ > 0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from H₂O-rich to CO₂-rich. The calcite in the retrograde veins has δ¹⁸O values that range from 8.4 to 11.2‰ (average = +9.7 ± 0.9‰) and δ¹³C values that range from −3.9 to −1.6‰ (average = −3.1 ± 0.6‰). These values indicate that the fluids from which calcite precipitated underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent Abitibi subprovince. Vein quartz contains CO₂-rich fluid inclusions (final melting T = −57.0 to −58.7 °C) that range in size from 5 to 17 μm. Measured homogenization temperatures (T h) range from −44.0 to 14.5 °C, however for most inclusions (46 of S1), T h = −44.0 to −21.1 °C (ρCO₂ ≈ 1.13 to 1.05 g/cm³). At 400 to 600 °C, these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of vein formation. It has been argued that some high density CO₂-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO₂-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO₂-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment. All fluid inclusion results are consistent with an initially semi-isobaric retrograde P–T path. Received: 2 April 1996 / Accepted: 15 November 1996     

Origin of ultramafic-hosted magnesite on Margarita Island,Venezuela

Ultramafic-hosted deposits of magnesite (MgCO₃) have been studied on Margarita Island, Venezuela, to elucidate the source of carbon and conditions of formation for this type of ore. Petrographic, mineralogic, and δ¹⁸O data indicate that magnesite precipitated on Margarita in near-surface environments at low P and T. δ¹³C ranges from −9 to −16‰ PDB within the magnesite and −8 to −10‰ PDB within some calcite and dolomite elsewhere on the island. The isotopically light dolomite fills karst and the calcite occurs as stock-work veins which resemble the magnesite deposits. These carbon isotopic ratios are consistent with a deep-seated source rather than an overlying source from a zone of surficial weathering. However, there is not much enrichment of precious metals and no enrichment of heavy rare-earth elements, as would be expected if the carbon had migrated upward as aqueous carbonate ions. The carbon probably has risen as a gaseous mixture of CO₂ and CH₄ which partially dissolved in near-surface water before leaching cations and precipitating as magnesite and other carbonates. The process probably is ongoing, given regional exhalation of carbonaceous gases.     

Sulphur, carbon and oxygen isotope studies of early Variscan mineralisation and Pb-Sb vein deposits in the Cornubian orefield: implications for the scale of fluid movements during Variscan deformation

The strongly deformed Middle Devonian-Lower Carboniferous metasedimentary-volcanic successions of the Trevone Basin (SW England) contain stratiform and Pb-Sb vein deposits that reveal a wide variation in δ³⁴S and δ¹³C, reflecting mineral deposition during diagenesis, regional metamorphism and basin inversion. Pre-Variscan metasedimentary sulphide (δ³⁴S=−33.7 to −26.7‰) and metabasite sulphide (δ³⁴S=+4.0 to +10.8‰) suggest two accessible source reservoirs for sulphur which were available for Sb-As-(Au) and Pb-Zn-(Ag) mineralisation (δ³⁴S=−3.3 to −15.0‰) during late Variscan semiductile-brittle shear. On the basis of pressure-corrected fluid inclusion temperatures, the calculated composition of fluid sulphur reveals an enrichment in δ³⁴S_H2S in the individual vein parageneses and depletion of the fluid sulphur reservoir during evolution of the vein systems. Carbonates in the same veins are partly contemporaneous with Pb-Sb mineralisation and late tensional deformation; their isotopic composition (δ¹³C=−3.2 and −13.4‰) appears strongly influenced by the host formation. Fluid inclusions in post-tensional quartz show a marked reduction in CO₂, suggesting that episodes of CO₂ degassing in response to punctuated reductions in pressure during uplift and brittle deformation was an important mechanism for vein carbonation. An origin for the Pb-Sb mineralisation involving local remobilisation of sulphur from the mixed metasedimentary-volcanic succession is probably inseparable from processes connected with Variscan metamorphism and deformation. Although the N Cornish Variscan deformation is part of a spatially large-scale event, the isotopic evidence suggests compartmentalisation of sulphur and carbon isotope features and short distances between sources and sinks. Received: 15 August 1998 / Accepted: 8 October 1999     

Hydrothermal uranium deposits containing molybdenum and fluorite in the Marysvale volcanic field, west-central Utah

Uranium deposits containing molybdenum and fluorite occur in the Central Mining Area, near Marysvale, Utah, and formed in an epithermal vein system that is part of a volcanic/hypabyssal complex. They represent a known, but uncommon, type of deposit; relative to other commonly described volcanic-related uranium deposits, they are young, well-exposed and well-documented. Hydrothermal uranium-bearing quartz and fluorite veins are exposed over a 300 m vertical range in the mines. Molybdenum, as jordisite (amorphous MoS₂), together with fluorite and pyrite, increase with depth, and uranium decreases with depth. The veins cut 23-Ma quartz monzonite, 20-Ma granite, and 19-Ma rhyolite ash-flow tuff. The veins formed at 19-18 Ma in a 1 km² area, above a cupola of a composite, recurrent, magma chamber at least 24 × 5 km across that fed a sequence of 21- to 14-Ma hypabyssal granitic stocks, rhyolite lava flows, ash-flow tuffs, and volcanic domes. Formation of the Central Mining Area began when the intrusion of a rhyolite stock, and related molybdenite-bearing, uranium-rich, glassy rhyolite dikes, lifted the fractured roof above the stock. A breccia pipe formed and relieved magmatic pressures, and as blocks of the fractured roof began to settle back in place, flat-lying, concave-downward, “pull-apart” fractures were formed. Uranium-bearing, quartz and fluorite veins were deposited by a shallow hydrothermal system in the disarticulated carapace. The veins, which filled open spaces along the high-angle fault zones and flat-lying fractures, were deposited within 115 m of the ground surface above the concealed rhyolite stock. Hydrothermal fluids with temperatures near 200 °C, ¹⁸O_H2O∼−1.5, D_H2O∼−130, log f O₂ about −47 to −50, and pH about 6 to 7, permeated the fractured rocks; these fluids were rich in fluorine, molybdenum, potassium, and hydrogen sulfide, and contained uranium as fluoride complexes. The hydrothermal fluids reacted with the wallrock resulting in precipitation of uranium minerals. At the deepest exposed levels, wallrocks were altered to sericite; and uraninite, coffinite, jordisite, fluorite, molybdenite, quartz, and pyrite were deposited in the veins. The fluids were progressively oxidized and cooled at higher levels in the system by boiling and degassing; iron-bearing minerals in wall rocks were oxidized to hematite, and quartz, fluorite, minor siderite, and uraninite were deposited in the veins. Near the ground surface, the fluids were acidified by condensation of volatiles and oxidation of hydrogen sulfide in near-surface, steam-heated, ground waters; wall rocks were altered to kaolinite, and quartz, fluorite, and uraninite were deposited in veins. Secondary uranium minerals, hematite, and gypsum formed during supergene alteration later in the Cenozoic when the upper part of the mineralized system was exposed by erosion. Received: 23 June 1997 / Accepted: 15 October 1997     

Water-rock interactions and chemical compositional variations during ductile deformation of the NW-striking shear zone in the Jiapigou gold belt,China

Two kinds of mylonite series rocks, felsic and mafic, have been recognized in the NW-striking shear zone of the Jiapigou gold belt. During ductile deformation, a large amount of fluid interacted intensively with the mylonite series rocks: plagioclases were sericitized and theAn values declined rapidly, finally all of them were transformed to albites; dark minerals were gradually replaced by chlorites (mostly ripidolite). Meanwhile, large-scale and extensive carbonation also took place, and the carbonatization minerals varied from calcite to dolomite and ankerite with the development of deformation. The δ¹³C values of the carbonates are −3.0‰ – −5.6‰ suggesting a deep source of carbon. The ductile deformation is nearly an iso-volume one (f _v≈1). With the enhancement of shear deformation, SiO₂ in the two mylonite series rocks was depleted, while volatile components suchs as CO₂ and H₂O, and some ore-forming elements such as Au and S were obviously enriched. But it is noted that the enrichment of Au in both the mylonite series rocks did not reach the paygrade of gold. The released SiO₂ from water-rock interactions occurred in the form of colloids and absorbed gold in the fluid. When brittle structures were formed locally in the ductile shear zone, the ore-forming fluids migrated to the structures along microfractures, and preciptated auriferous quartz because of reduction of pressure and temperature. Fluid inclusion study shows that the temperature and pressure of the ore-forming fluids are 245–292°C and 95.4–131.7 MPa respectively; the salinity is 12.88–16.33wt% NaCl; the fluid-phase is rich in Ca²⁺, K⁺, Na⁺, Mg²⁺, F⁻ and Cl⁻, while the gaseous phases are rich in CO₂ and CH₄. The δD and δ¹⁸O, values of the ore-forming fluid are −84.48‰ – −91.73‰ and −0.247‰ – +2.715‰ respectively, suggesting that the fluid is composed predominantly of meteoric water. This project is financially supported by the National Natural Science Foundation of China (No. 9488010).     

Young orogenic gold mineralisation in active collisional mountains, Taiwan

Gold-bearing vein systems in the high mountains of Taiwan are part of the youngest tectonic-hydrothermal system on Earth. Tectonic collision initiated in the Pliocene has stacked Eocene–Miocene marine sedimentary rocks to form steep mountains nearly 4 km high. Thinner portions of the sedimentary pile (∼5 km) are currently producing hydrocarbons in a fold and thrust belt, and orogenic gold occurs in quartz veins in thicker parts of the pile (∼10 km) in the Slate Belt that underlies the mountains. Metamorphic fluids (2–5 wt.% NaCl equivalent) are rising from the active greenschist facies metamorphic zone and transporting gold released during rock recrystallisation. Metamorphic fluid flow at the Pingfengshan historic gold mine was focussed in well-defined (4 km³) fracture zones with networks of quartz veins, whereas large surrounding volumes of rock are largely unveined. Gold and arsenopyrite occur in several superimposed vein generations, with ankeritic alteration of host rocks superimposed on chlorite–calcite alteration zones as fluids cooled and became out of equilibrium with the host rocks. Mineralising fluids had δ¹⁸O near +10‰, δ¹³C was between −1‰ and −6‰ and these fluids were in isotopic equilibrium with host rocks at ∼350°C. Ankeritic veins were emplaced in extensional sites in kink fold axial surfaces, formed as the rock mass was transported laterally from compressional to extensional regimes in the orogen. Rapid exhumation (>2 mm/year) of the Slate Belt is causing a widespread shallow conductive thermal anomaly without igneous intrusions. Meteoric water is penetrating into the conductive thermal anomaly to contribute to crustal fluid flow and generate shallow boiling fluids (∼250°C) with fluid temperature greater than rock temperature. The meteoric-hydrothermal system impinges on, but causes only minor dilution of, the gold mineralisation system at depth.     

Geology and geochemistry of telluride-bearing Au deposits in the Pingyi area, Western Shandong, China

Summary Telluride-bearing gold deposits of the Pingyi area, western Shandong, China, are located on the southeastern margin of the North China Craton. There are two main types of deposits: (i) mineralized cryptoexplosive breccia, e.g., Guilaizhuang; and (ii) stratified, finely-disseminated mineralization hosted in carbonate rocks, e.g., Lifanggou and Mofanggou deposits. In Guilaizhuang, the cryptoexplosive breccia is formed within rocks of the Tongshi complex and Ordovician dolomite. The mineralization is controlled by an E–W-trending listric fault. Stratified orebodies of the Lifanggou and Mofanggou deposits are placed along a NE-trending, secondary detachment zone. They are hosted within dolomitic limestone, micrite and dolomite of the Early-Middle Cambrian Changqing Group. The mineralization in the ore districts is considered to be related to the Early Jurassic Tongshi magmatic complex that formed in a continental arc setting on the margin of the North China Craton. The host rocks are porphyritic and consist predominantly of medium- to fine-grained diorite and pyroxene (hornblende)-bearing monzonite. SHRIMP U–Pb zircon dating of diorites give a ²⁰⁶Pb/²³⁸U weighted mean age of 175.7 ± 3.8 Ma. This is interpreted as representing the crystallization age of the Tongshi magmatic complex. Considering the contact relationships between the magmatic and host sedimentary rocks, as well as the genetic link with the deposits, we conclude that this age is relevant also for the formation of mineralization in the Pingyi area. We hence consider that the deposits formed in the Jurassic. The principal gold minerals are native gold, electrum and calaverite. Wall-rock alteration comprises pyritization, fluoritization, silicification, carbonatization and chloritization. Fluid inclusion studies indicate that all the analyzed inclusions are of two-phase vapor–liquid NaCl–H₂O type. Homogenization temperatures of the fluid inclusions vary from 103 °C to 250 °C, and the ice melting temperatures range from −2.5 °C to −13.5 °C, corresponding to a salinity range of 4.65 to 17.26 wt.% NaCl equiv. The δ³⁴S values of pyrite associated with gold mineralization exhibit a narrow range of −0.71 to + 2.99‰, implying that the sulfur was probably derived from the mantle and/or dioritic magma. The δ¹³C_PDB values of the fluid inclusions in calcite range from −7.3 to 0.0‰. The δ¹⁸O_SMOW values of vein quartz and calcite range from 11.5 to 21.5‰, corresponding to δ¹⁸O_fluid values of −1.1 to 10.9‰; δD values of the fluid inclusions vary between −70 and −48‰. The isotope data for all three deposits suggest mixing of ore-forming fluids derived from the mantle and/or magma with different types of fluids at shallow levels. Pressure release and boiling of the fluids, as well as fluid-rock interaction (Lifanggou and Mofanggou) and mixing of magmatically-derived fluids with meteoritic waters (Guilaizhuang) played an important role in the ore-forming processes.     

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.