Bisulfide Complexes Research Articles

Fluid sources for the La Guitarra epithermal deposit (Temascaltepec district, Mexico): Volatile and helium isotope analyses in fluid inclusions

The La Guitarra deposit (Temascaltepec district, South-Central Mexico), belongs to the low/intermediate sulfidation epithermal type, has a polymetallic character although it is currently being mined for Ag and Au. The mineralization shows a polyphasic character and formed through several stages and sub-stages (named I, IIA, IIB, IIC, IID, and III). The previous structural, mineralogical, fluid inclusion and stable isotope studies were used to constrain the selection of samples for volatile and helium isotope analyses portrayed in this study. The N 2/Ar overall range obtained from analytical runs on fluid inclusion volatiles, by means of Quadrupole Mass Spectrometry (QMS), is 0 to 2526, and it ranges 0 to 2526 for stage I, 0 to 1264 for stage IIA, 0 to 1369 for stage IIB, 11 to 2401 for stage IIC, 19 to 324 for stage IID, and 0 to 2526 for stage III. These values, combined with the CO 2/CH 4 ratios, and N 2–He–Ar and N 2–CH 4–Ar relationships, suggest the occurrence of fluids from magmatic, crustal, and shallow meteoric sources in the forming epithermal vein deposit. The helium isotope analyses, obtained by means of Noble Gas Mass Spectrometry, display R/Ra average values between 0.5 and 2, pointing to the occurrence of mantle-derived helium that was relatively diluted or “contaminated” by crustal helium. These volatile analyses, when correlated with the stable isotope data from previous works and He isotope data, show the same distribution of data concerning sources for mineralizing fluids, especially those corresponding to magmatic and crustal sources. Thus, the overall geochemical data from mineralizing fluids are revealed as intrinsically consistent when compared to each other. The three main sources for mineralizing fluids (magmatic, and both deep and shallow meteoric fluids) are accountable at any scale, from stages of mineralization down to specific mineral associations. The volatile and helium isotope data obtained in this paper suggest that the precious metal-bearing mineral associations formed after hydrothermal pulses of predominantly oxidized magmatic fluids, and thus it is likely that precious metals were carried by fluids with such origin. Minerals from base-metal sulfide associations record both crustal and magmatic sources for mineralizing fluids, thus suggesting that base metals could be derived from deep leaching of crustal rocks. At the La Guitarra epithermal deposit there is no evidence for an evolution of mineralizing fluids towards any dominant source. Rather than that, volatile analyses in fluid inclusions suggest that this deposit formed as a pulsing hydrothermal system where each pulse or set of pulses accounts for different compositions of mineralizing fluids. The positive correlation between the relative content of magmatic fluids (high N 2/Ar ratios) and H 2S suggests that the necessary sulfur to carry mostly gold as bisulfide complexes came essentially from magmatic sources. Chlorine necessary to carry silver and base metals was found to be abundant in inclusion fluids and although there is no evidence about its source, it is plausible that it may come from magmatic sources as well.

Physico-chemical conditions of ore-forming fluids associated with genesis of the Kalahari Goldridge deposit, Kraaipan Greenstone Belt, South Africa

The Kalahari Goldridge deposit is an Archaean lode gold deposit in the NorthWest Province of South Africa. Sub-greenschist facies banded iron-formation (BIF) dipping approximately 65°E, host the gold mineralisation. The D-zone orebody of the deposit is stratabound and varies from about 15 to 45 m in width along a strike length of approximately 1.5 km. Mineralisation is associated with two subhorizontal quartz–carbonate groups of veins (IIA and IIB), which dip approximately 20° to 40°W. Group IIA are ladder veins preferentially developed in centimetre-scale Fe-rich mesobands. Group IIB consists of large quartz–carbonate veins, which crosscuts the entire orebody and extends into the footwall and hanging-wall in places. Fluids responsible for gold deposition have low salinity H 2O–CO 2 ± CH 4 compositions, with elevated CH 4 contents attributed to localised hydrolysis reactions with interbedded carbonaceous sediment and ore fluid. The fluid has a significant CO 2 content (X CO 2 ≈ 0.06 to 0.19), and salinities below 7.0 wt.% NaCl equivalent (average of 3.5 and 3.0 wt.% NaCl equiv. for the first and second episodes of the mineralisations, respectively). Calculated values of fO 2, range from 10 − 33.8 to 10 − 30.5 bars, i.e., 4 to 5 log units below the QFM buffer boundary, indicate strongly reducing conditions of the ore fluid at deposition. Calculated total sulphur contents in the ore fluid range from 0.014 to 0.051 M and are consistent with the range (10 − 3.5 to 10 − 1 M) reported for sub-amphibolite facies ore fluids. The close association between sulphides and gold, together with the nature of the fluid indicate that Au was carried in solution as a Au(HS) 2 − complex. Extensive epigenetic replacement of magnetite and chlorite in BIF and other meta-pelitic sediments in the deposit by sulphides and carbonates indicates that interaction of a CO 2- and H 2S-bearing fluid with the Fe-rich host rocks facilitated Au precipitation by destabilisation of the reduced gold bisulphide complexes. Local fO 2 gradients may also account for gold precipitation within carbonaceous metasedimentary rocks. Evidence from light stable isotopes and fluid inclusions suggests that the mineralised veins crystallised from a homogeneous fluid under similar physicochemical conditions during the two episodes of mineralisation. Deposition occurred at temperatures ranging from 350 to 400 °C, and at fluid pressures ranging from 0.6 to 2.0 kbar. Calculated isotopic values of the ore fluid at these P– T conditions are in the following ranges: δ 18O = 6.7‰ to 10.5‰, δ 13C = − 6.0‰ to − 8.0‰ and δ 34S = + 1.7‰ to + 4.0‰. These data do not offer conclusive evidence for the source of mineralising fluid in the D-zone at the Kalahari Goldridge deposit, as they overlap the range suggested for fluid derived from prograde metamorphism and magmatic hydrothermal activity.

The aqueous geochemistry of gallium, germanium, indium and scandium

Relatively little information is available in the literature regarding the speciation and solubility of Ga, Ge, In and Sc in aqueous solutions, especially at elevated temperatures and pressures. In this paper we critically review stability constants for relevant aqueous complexes of these metals and solubility products for relevant solid phases. Most of the available data refer to standard conditions of temperature and pressure (25 °C and 1 bar), although for Ga and Ge, experimentally derived data are available for some geologically relevant species and phases up to 200–300 °C. The stable oxidation states of the four metals in aqueous solution are Ga(III), Ge(IV), In(III) and Sc(III). The ions of each of these metals are relatively hard in the Pearson [Pearson, R.G. 1963. Hard and soft acids and bases. Journal of the American Chemical Society 85, 3533–3539] sense, forming the strongest complexes with hard ligands such as hydroxide, fluoride, sulfate and phosphate, and weaker complexes with soft ligands such as chloride and bisulfide. The main exception to this rule is In(III), which forms reasonably stable chloride and bisulfide complexes. The hydrated Ga 3+, In 3+ and Sc 3+ ions are all octahedrally coordinated by water molecules, but there is some evidence, e.g., for GaCl 4 − and InCl 4 −, that a conversion to tetrahedral coordination may occur upon replacement of water by a sufficient number of other ligands. In most hydrothermal solutions, we predict that hydroxide complexes will be the most important forms of transport of Ga and Sc, although fluoride complexes will be important in environments where fluoride activities are relatively high (e.g., during greisen formation). In analogy with Si, the most important species for Ge is germanic acid, H 4GeO 4 0, but again fluoride complexes may be important at high fluoride activities. Chloride complexes are not expected to play a significant role in the transport of Ga, Ge or Sc at temperatures below approximately 300 °C. The behavior of In is expected to be the most variable, with, depending on conditions, hydroxide, chloride, fluoride or bisulfide complexes all contributing to its transport. Sulfate and phosphate complexes of Ga, In and Sc may play limited roles in the hydrothermal mass transfer of these elements, but only under special conditions; normally these complexes will be less important than hydroxide or fluoride complexes. Solubility calculations for 25 °C indicate that In-sulfide and Sc-phosphate are less soluble (i.e., more stable) than the corresponding oxyhydroxides, even when In-bisulfide and Sc–phosphate complexes are taken into account. However, the phases GaPO 4(s) and InPO 4(s) are generally more soluble than the corresponding oxyhydroxides. Solubilities of α-GaOOH(s) and GeO 2(tetragonal) have been estimated up to 300 °C and both increase with increasing temperature. The solubility of pure α-GaOOH(s) at 25 °C (< 10 − 6 m) is quite low from pH 3 to pH 8, consistent with the general immobility of Ga in the weathering environment and its concentration in bauxites.

Gold Enrichment and the Bi-Au Association in Pyrrhotite-Rich Massive Sulfide Deposits, Escanaba Trough, Southern Gorda Ridge

High gold contents (to 10.1 ppm, avg 1.4 ppm, n = 34) occur in pyrrhotite-rich massive sulfide samples from the sediment-covered floor of the Escanaba trough, the slow-spreading, southernmost segment of Gorda Ridge. These concentrations reflect the presence of primary gold, formed during high-temperature hydrothermal activity in mounds and chimneys, and secondary gold deposited during sea-floor weathering of massive sulfide. Primary gold occurs as fine-grained ( 2 μm) secondary gold grains have a porous, flaky morphology and occur in samples in which pyrrhotite is oxidized and replaced by Fe oxyhydroxides, Fe sulfate, and sulfur. Mounds and chimneys dominated by pyrrhotite and containing lesser amounts of isocubanite, chalcopyrite, and Fe-rich sphalerite were formed by high-temperature (estimated range 325°–275°C), reduced, low-sulfur vent fluids. The mineral and fluid compositions during this main stage of hydrothermal venting reflect subsurface interaction between circulating hydrothermal fluids and turbiditic sediment containing as much as 1.1 percent organic carbon. As the deposition of pyrrhotite, Cu-Fe sulfides, and sphalerite waned, a volumetrically minor suite of sulfarsenide, arsenide, Bi, and Au minerals was deposited from highly reduced, late main-stage fluids diffusing through mounds and chimneys. The low solubility of Au as a bisulfide complex and the absence of fluid mixing during this stage of hydrothermal activity apparently inhibited the precipitation of gold directly from solution. Instead, gold precipitation is thought to be linked to elevated concentrations of Bi in the late main-stage fluids. The textural relationships of Au and Bi minerals in pyrrhotite-rich samples, low melting point of native bismuth (271.4°C), and recent experimental results on Au and Bi in hydrothermal fluids contribute to the hypothesis that gold was effectively scavenged from the Escanaba trough vent fluids by coexisting droplets of liquid bismuth. Additional phase relationships of alloys in the Au-Bi system indicate that deposition of native bismuth and maldonite occurred at temperatures as low as 241°C. Bismuth droplets trapped in void space between main-stage mineral grains scavenged gold from ambient hydrothermal fluid to a greater extent than bismuth enclosed by late-forming pyrrhotite. The limited solid solution of Au in Bi can explain the apparent exsolution texture in which gold blebs are hosted by native bismuth. The electrum, native bismuth (with gold inclusions), and galena represent the last traces of gold mineralization from late main-stage fluids. During sea-floor weathering and the oxidation of pyrrhotite in the mounds and chimneys, secondary gold formed as aggregates of colloidal particles along pH gradients between acidic pore waters and ambient seawater. Gold was mobilized from earlier formed primary gold minerals and transported as aqueous chloride complexes. The reduction of Au(III) by residual Fe2+ in partly altered pyrrhotite and adsorption of colloids by Fe oxyhydroxides may have influenced the location of secondary gold grains within the alteration front. Solubility differences between gold and silver chloride complexes at low temperature account for the low Ag content of secondary gold grains. The high concentrations of Bi, and thus the association of Au and Bi minerals in pyrrhotite-rich massive sulfide, can be ascribed to the extensive interaction of hydrothermal fluids with sediment in the Escanaba trough. In contrast, the absence of the Au-Bi association in massive sulfides at other ridges, including other sediment-covered sites at Middle Valley and the Guaymas basin, reflects the predominantly basaltic source rocks.

The physical and chemical evolution of low-salinity magmatic fluids at the porphyry to epithermal transition: a thermodynamic study

Fluid-phase relationships and thermodynamic reaction modelling based on published mineral solubility data are used to re-assess the Cu–Au-mineralising fluid processes related to calc-alkaline magmatism. Fluid inclusion microanalyses of porphyry ore samples have shown that vapour-like fluids of low to intermediate salinity and density (~2–10 wt% NaCl eq.; ~0.1–0.3 g cm−3) can carry percentage-level concentrations of copper and several ppm gold at high temperature and pressure. In epithermal deposits, aqueous fluids of similar low to intermediate salinity but liquid-like density are ubiquitous and commonly show a magmatic isotope signature. This paper explores the physical evolution of low-salinity to medium-salinity magmatic fluids of variable density, en route from their magmatic source through the porphyry regime to the near-surface epithermal environment, and investigates the chemical conditions required for effective transport of gold and other components from the magmatic to the epithermal domain. Multicomponent reaction modelling guided by observations of alteration zonation and vein overprinting relationships predicts that epithermal gold deposits are formed most efficiently by a specific succession of processes during the evolution of a gradually cooling magmatic–hydrothermal system. (1) The low-salinity to medium-salinity fluid, after separating from the magma and possibly condensing out some hypersaline liquid in the high-temperature porphyry environment, must physically separate from the denser and more viscous liquid, and then cool within the single-phase fluid stability field. By cooling under adequate confining pressure, such a vapour will evolve above the critical curve and contract, without any heterogeneous phase change, to an aqueous liquid of the same salinity. (2) High concentrations of gold, transported as stable Au bisulphide complexes supporting >1 ppm Au even at 200°C, can be maintained throughout cooling, provided that the fluid initially carries an excess of H2S over Cu+Fe on a molal scale. This condition is favoured by an initially high sulphide content in a particularly low-salinity magmatic fluid, or by preferential partitioning of sulphur into a low-salinity vapour and partial removal of Fe into a hypersaline liquid at high temperature. (3) Acid neutralisation further optimises gold transport by maximising the concentration of the HS− ligand. This may occur by feldspar destructive alteration along pyrite±chalcopyrite±sulphate veins, in the transition zone between the porphyry and epithermal environments. An alternative acid/base control is the dissolution of calcite in sediments, which may enable long-distance gold transport to Carlin-type deposits, because of the positive feedback between acid neutralisation and permeability generation. The three physical and chemical transport requirements for high-grade epithermal gold mineralisation are suggested to be the common link of epithermal gold deposits to underlying magmatic–hydrothermal systems, including porphyry-Cu–Au deposits. Both mineralisation types are the result of gradual retraction of isotherms around cooling hydrous plutons in similar tectonic and hydrologic environments. As magmatic fluid is generated at increasing depths below the surface the importance of vapour contraction increases, leading to the typical overprinting of potassic, phyllic and advanced argillic alteration and their related ore styles.

HYDROTHERMAL As Bi MINERALIZATION IN THE NAKDONG DEPOSITS, SOUTH KOREA: INSIGHT FROM FLUID INCLUSIONS AND STABLE ISOTOPES

At the Nakdong As–Bi deposits, South Korea, Cambro-Ordovician sedimentary sequences are cut by numerous dykes of quartz monzodiorite and porphyritic granite. In the deposit, stage I involves arsenic mineralization, chiefly associated with arsenopyrite, pyrite, sphalerite and chalcopyrite, whereas bismuth mineralization characterizes stage II, with the coprecipitation of pyrrhotite, chalcopyrite, galena, bismuth, bismuthinite, cosalite, matildite, schirmerite, Au–Ag alloy, and argentite. The mineralization was initiated with the introduction of heterogeneous fluids of high salinity, presumably owing to the prominence of Ca, Mg, Na and K. At stage II, fluid immiscibility, which led to bismuth mineralization, produced (halite ± sylvite)-bearing highsalinity fluids of 27.6 to 49.3 wt.% NaCl equivalents, as well as low-salinity vapor-rich fluids. The homogenization temperatures of mineralizing fluids decreased only slightly from stage I, 283–416°C, to stage II, 222–395°C. A decrease in 18 O as well as 13 C in going from calcite in fresh limestone ( 18 O in the range +17.5 to +22.4‰, 13 C in the range +2.3 to +4.4‰) to silicified limestone ( 18 O in the range +13.3 to +18.3‰, 13 C in the range –2.5 to +1.3‰) were promoted not only by Rayleigh volatilization, but also by fluid–rock interaction, with the influx of magmatic fluids into carbonate rocks during the mineralization process. The fluid–rock interaction contributed to the CO2 and CH4 components in type-III fluids and also to the Ca-enrichment in type-I fluid inclusions. The prevailing species of sulfur in the mineralizing fluids is estimated to have been H2S. The 34 SH2S values obtained from the sulfide minerals increased from stage I, +3.2 to +4.4‰, to stage II, +4.1 to +4.8‰, values typical of magmatic sulfur, and the temperatures of homogenization of fluid inclusions increased as well. The decrease of sulfur fugacities, from 10 –9.1 –10 –6.4 atm at stage I to 10 –15.7 –10 –9.2 atm at stage II, through sulfide precipitation and H2S loss, induced the destabilization of bisulfide complexes and characterized the change of mineral stabilities from arsenopyrite – pyrite – sphalerite to bismuthinite – native bismuth assemblages.

Fluid inclusions and quantitative model for gold precipitation in the Tiouit deposit (Anti-Atlas, Morocco)

In the Tiouit gold deposit (Anti-Atlas, Morocco), a paragenetic study has revealed the existence mainly of two generations of gold, the earliest one was cogenetic with ferro-arseniferous stage I; the second generation (most important) is associated with the minerals of a cupro-zinciferous sulphide stage II. A microthermometric study of fluid inclusions demonstrates that the main part of the gold was introduced by an aqueous solution of relatively low salinity (1.7 m Nacl) and deposited at about 400 °C and 500 bars in microfractures crosscutting early sulphides. A calculated quantitative model for the fluid-mineral interaction demonstrates that a significant amount of gold (3 mg deposited from 1 kg of solution, i.e. 43% of total gold), transported as bisulphide complexes ( HAu ( HS ) 2 0 , AuHS 0 and Au ( HS ) 2 - ), can be deposited as a result of a decrease in both H 2S activity and oxygen fugacity produced by the alteration of pyrrhotite into pyrite.

Hydrothermal Alteration, Ore Fluid Characteristics, and Gold Depositional Processes along a Trondhjemite-Komatiite Contact at Tarmoola, Western Australia

Tarmoola is a structurally controlled Archean orogenic gold deposit hosted in greenschist facies metamorphosed komatiite and trondhjemite in the Leonora district of the Eastern Goldfields province, Yilgarn craton. High-grade (>1 g/t Au) orebodies are located in komatiite wall rock adjacent to the eastern and northeastern margins of the asymmetrical, north-south–striking, Tarmoola trondhjemite intrusion. Gold-bearing veins postdate trondhjemite emplacement (ca. 2700 Ma), quartz diorite dikes (ca. 2667 Ma), and regional greenschist facies metamorphism. Textures and crosscutting relationships in gold-bearing veins indicate two stages of hydrothermal fluid infiltration associated with a single gold-related hydrothermal event: a volumetrically dominant, but gold-poor, stage I fluid and a gold-rich stage II fluid. Gold-bearing veins contain stage I milky quartz and pyrite that are overprinted by stage II quartz-ankerite-muscovite-chalcopyrite-sphalerite-galena-gold-tellurides ± albite ± chlorite ± fuchsite ± epidote ± scheelite. Stage I hydrothermal alteration assemblages are different in trondhjemite and komatiite due to contrasting reactions between a common ore fluid and disparate wall-rock chemistry. Stage II fluid-wall rock interaction was minor compared to stage I and is indicated by the overprinting of stage I mineral assemblages by stage II microveins. Wall-rock alteration proximal to veins in trondhjemite is characterized by replacement of igneous plagioclase, amphibole, biotite, and metamorphic chlorite by hydrothermal quartz, muscovite, ankerite, calcite, pyrite, chalcopyrite, sphalerite, galena, tellurides, and gold, whereas in proximal alteration in komatiite, metamorphic chlorite and talc are replaced by ankerite, quartz, muscovite, albite, chlorite, fuchsite, pyrite, chalcopyrite, sphalerite, galena, tellurides, and gold. The stage II fluid was enriched in H2O, CO2, Si, Ca, K, Na, S, Au, Ag, Cu, Pb, W, Bi, As, Mo, Zn, and Te. Based on fluid inclusion studies and stage II mineral equilibria, gold deposited from a homogeneous, neutral to slightly alkaline (pH 5.1–5.5), reduced, low-salinity (<5.5 wt % NaCl equiv) fluid that had a bulk composition of 78 mole percent H2O and 21 mole percent CO2, and trace amounts of CH4, C2H6, H2, Ar, H2S, and He. Gold deposition occurred at 300° ± 50°C and 0.5 to 3.0 kbars. Assuming lithostatic fluid pressures, gold precipitated at a 2- to 10-km depth. Stage II gray quartz δ 18Ofluid values range from 5.9 to 7.5 per mil, whereas δ Dfluid values calculated from the dehydration of muscovite grains and measured directly from bulk fluid inclusion analyses of stage II gray quartz have ranges of –9 to –35 and –27 to –28 per mil, respectively. Hydrothermal ore fluids were transported from greater crustal depths to the site of gold deposition during the district-scale D3 event by shallowly W dipping, reverse brittle-ductile shear zones in supracrustal rock and along the steeply east dipping trondhjemite contact. Associated subhorizontal east-west shortening caused the reactivation of the eastern trondhjemite margin and subparallel foliation, which facilitated the transport of hydrothermal fluids and the generation of gold-bearing veins and hydrothermal alteration zones in komatiite. East-west–striking fractures in trondhjemite aided the lateral migration of ore fluids away from trondhjemite margins and the formation of east-west–striking gold-bearing veins and broad alteration zones. Gold was most likely transported in the stage II fluid as bisulfide complexes. The sulfidation of trondhjemite and komatiite wall rock by the stage II fluid caused the destabilization of Au bisulfide complexes and gold deposition. Potassium, Ca, and CO2 metasomatism of komatiite wall rock may have enhanced gold deposition via the acidification of the stage II fluid. The physicochemical characteristics of the Tarmoola ore fluid and relative timing of gold mineralization are consistent with the Yilgarn-wide, 2650 to 2630 Ma gold event. The host-rock chemistry, vein, and proximal wall-rock alteration mineral assemblages, and fluid temperature and pressures during gold deposition are also similar to that of earlier (ca. 2755 Ma) gold deposits in the Leonora district.

The Onaman Prospect, Ontario: An Unusual Occurrence of Cu-bearing Au Mineralization in a Shear Zone

The Onaman prospect is an unusual example of mesothermal gold mineralization in which native gold, electrum, and minor chalcopyrite are disseminated in metavolcaniclastic rocks in shear zones or concentrated in felsite dikes that intrude the metavolcaniclastic rocks. Mineralization was contemporaneous with carbonatization and involved early replacement of iron-bearing minerals in the wall rock by ankerite and pyrite and later replacement of pyrite by arsenopyrite, pyrrhotite, chalcopyrite, electrum, and native gold. Oxygen isotope ratios of ankerite suggest a metamorphic origin for the fluid, and carbon isotope ratios are similar to those of large mesothermal gold deposits in the Superior province for which a mantle origin has been proposed. Sulfur isotope ratios correlate positively with gold grades and may reflect decreasing f O2 during mineralization. A model is proposed in which deformation of the volcanic pile led to the formation of a shear zone and created dilational zones allowing dike intrusion. Metamorphic fluids rose along the shear zones and the dikes. Pyritization of the host rock was the major control on gold and chalcopyrite deposition, reducing a H 2 S and destabilizing the bisulfide complexes of Au and Cu. The Onaman occurrences provide an uncommon example of a mesothermal system in which pH was relatively low, and the correspondingly low solubility of copper allowed the fluid to saturate with and deposit chalcopyrite. In most other mesothermal gold systems, copper is too soluble to saturate the fluid with a copper mineral and is therefore flushed through the system.

Precious metals in high-temperature geothermal systems in New Zealand

High-temperature geothermal systems are the modern analogue of epithermal precious metal ore deposits. In these systems, gold and silver are transported primarily as bisulfide complexes, with the precious metals being deposited in response to boiling and mixing of the deep geothermal fluid. In order for the geothermal system to produce an economic precious metal deposit within the typical lifetime of a geothermal system, the deposition process must be efficient, and the gold and silver concentrations in the geothermal fluid must be sufficient. Thermodynamic data have been measured for gold and silver bisulfide complexes, but the question remains as to whether the deep geothermal fluid is saturated with respect to these complexes. Measurement of gold and silver concentrations in geothermal fluid sampled at the surface shows very low concentrations, as these metals precipitate down the well. Indirect measurements of the deep reservoir gold and silver concentrations have been calculated by estimating precipitate concentrations on scale deposits recovered from high-pressure apparatus at the surface. These estimates produce precious metal concentrations that are below saturation. We therefore designed and built a downhole sampling device specifically to measure precious and other trace metal concentrations in the deep reservoir fluids. The device is manufactured from titanium to be chemically inert, and therefore capable of scavenging the trace metals through acid rinsing. Analyses for precious and related metals on deep waters obtained from Kawerau and Ngawha geothermal systems, New Zealand, have been measured. Solutions were analysed by ICP-MS. Laboratory testing using blank solutions shows minimal sources of contamination from materials used in the construction and sampling procedure (i.e. titanium, MilliQ water and aqua regia). At Kawerau, samples were obtained from deep wells at ⩾1000 m depth at temperatures of 260 to 295 °C, while at Ngawha samples were taken from > 800 m depth at temperatures of 225–>235 °C. Gold, silver and thallium are at ppb levels, while arsenic, antimony and copper range from hundreds to thousands of ppb. Comparing these results with calculated solubilities of gold, silver and acanthite suggests deep waters are undersaturated in gold but close to saturation in silver.

P-T-X conditions of hydrothermal fluids and precipitation mechanism of stibnite-gold mineralization at the Wiluna lode-gold deposits, Western Australia: conventional and infrared microthermometric constraints

The Wiluna lode-gold deposits are located in the Archean Wiluna greenstone belt, in the northern sector of the Norseman-Wiluna belt in the Yilgarn Craton of Western Australia. They are hosted in subgreenschist facies meta-basalts, and controlled by the Wiluna strike-slip fault system and associated shear veins and breccias. The 13 individual lode-gold deposits have produced around 115 t Au from 1901 to 1946 and 1986 to today. Historically, they also produced 38.3 t As and 3.5 t Sb. Gold formed in two stages: stage 1 gold-pyrite-arsenopyrite is finely disseminated in the wallrock and breccia fragments, whereas stage 2 gold-stibnite is located in massive shear veins and breccia matrix, as fracture-fill and in banded-colloform textured veins. Stibnite-gold orebodies only occur in some of the deposits (e.g., Moonlight and northern part of the West Lode) and also display a restricted vertical extent, being preserved only in the uppermost 200 m of stibnite-bearing lodes. Petrographic, conventional, and infrared microthermometric and laser-Raman analysis on stibnite-bearing quartz veins and breccias reveal that the antimony- and gold-rich hydrothermal fluid was of mixed H2O-NaCl-CO2±CH4 type. Microthermometric measurements reveal maximum homogenization temperatures of 340 °C (average 290±25 °C), and a wide range of salinities between 0.2 and 23 eq. wt% NaCl. Aqueous-carbonic fluid inclusions contain variable XCO2+CH4 (0.03 to 0.82), with the carbonic phase containing a maximum XCH4 of 0.21. Combined petrographic and microthermometric evidence suggests that the fluid inclusion properties reflect fluid immiscibility of a low-salinity, medium XCO2+CH4, homogeneous parent fluid at about 290 °C and pressures between 700 and 1,700 bar. Fluid immiscibility was triggered by cyclic pressure release during fault-zone movement. The decompression (adiabatic cooling) of the hydrothermal fluids shifted the ore fluid to lower temperatures, significantly reduced the degree of stibnite undersaturation, and caused stibnite to precipitate. The deposition of stibnite reduced the ore-fluid H2S concentration, thereby destabilized gold bisulfide complexes in solution, and caused gold precipitation locally. This mechanism explains the intimate spatial association of stibnite and gold in quartz veins and breccias in the stibnite-gold orebodies at Wiluna.

Distribution of mercury, methyl mercury and organic sulphur species in soil, soil solution and stream of a boreal forest catchment

Concentrations of methyl mercury, CH3Hg (II), total mercury, Hgtot = CH3Hg (II) + Hg (II), and organic sulphur species were determined in soils, soil solutions and streams of a small (50 ha) boreal forest catchment in northern Sweden. The CH3Hg (II)/Hgtot ratio decreased from 1.2–17.2% in the peaty stream bank soils to 0.4–0.8% in mineral and peat soils 20 m away from the streams, indicating that conditions for net methylation of Hg (II) are most favourable in the riparian zone close to streams. Concentrations of CH3Hg (II) bound in soil and in soil solution were significantly, positively correlated to the concentration of Hgtot in soil solution. This, and the fact that the CH3Hg (II)/Hgtot ratio was higher in soil solution than in soil may indicate that Hg (II) in soil solution is more available for methylation processes than soil bound Hg (II). Reduced organic S functional groups (Org-SRED) in soil, soil extract and in samples of organic substances from streams were quantified using S K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Org-SRED, likely representing RSH, RSSH, RSR and RSSR functionalities, made up 50 to 78% of total S in all samples examined. Inorganic sulphide [e.g. FeS2 (s)] was only detected in one soil sample out of 10, and in none of the stream samples. Model calculations showed that under oxic conditions nearly 100% of Hg (II) and CH3Hg (II) were complexed by thiol groups (RSH) in the soil, soil solution and in the stream water. Concentrations of free CH3Hg+ and Hg2+ ions in soil solution and stream were on the order of 10−18 and 10−32M, respectively, at pH 5. For CH3Hg (II), inorganic bi-sulphide complexes may contribute to an overall solubility at concentrations of inorganic sulphides higher than 10−9M, whereas considerably higher concentrations of inorganic sulphides (lower redox-potential) are required to increase the solubility of Hg (II).

Genesis of the Auriferous C Quartz-Tourmaline Vein of the Siscoe Mine, Val d'Or District, Abitibi Subprovince, Canada: Structural, Mineralogical and Fluid Inclusion Constraints

The C quartz-tourmaline vein of the Siscoe deposit is a classic example of a high-grade Archean gold lode and averages 45 g/t Au, locally attaining 221 g/t Au. This vein cuts all dikes, the main regional foliation (S2) and several auriferous quartz-carbonate veins and is interpreted to be a shear vein, which formed during the development of a late to post-D2 reverse fault. The vein is composed of alternating tourmaline- and quartz-rich layers with minor amounts of rutile, scheelite, apatite, chlorite, muscovite, carbonate (mainly calcite), pyrite, chalcopyrite, tetradymite, and native gold. Vein filling occurred during the development of a reverse fault through a process involving repeated episodes of opening, mineral deposition, and plastic and brittle deformation. The vein geometry was controlled by local rheologic heterogeneities or by a west-northwest–east-southeast compressive event. The wall-rock alteration is characterized by the occurrence of tourmaline, pyrite, and calcite. The timing of gold deposition and the gold fineness in the C quartz-tourmaline vein differ from those of the wall rock. In the vein, native gold (4.7–7.3 wt % Ag) was the latest phase to deposit and commonly occurs in fractures that cut across the quartz and tourmaline layers; whereas in the wall rock, gold (7.9–13.2 wt % Ag) started to precipitate just before or synchronously with pyrite. Consequently, the textural and mineral chemical evidence suggests that the fluids in the vein and wall rock were not in equilibrium with each other during gold deposition and that the mechanisms of precious metal precipitation were different in the two environments. In the wall rock, gold started to precipitate synchronously with sulfides due to reaction of hydrothermal fluids with magmatic ilmenite. This reaction caused the destabilization of gold bisulfide complexes and prompted gold deposition; however, sulfides are very rare in the C quartz-tourmaline vein, and gold deposited after the main episode of vein filling, suggesting that its precipitation was not controlled by pyrite deposition. The occurrence of aqueous-carbonic (AC) fluid inclusions in healed fractures that radiate from gold grains suggests that these fluids were associated with the ore-forming process. Two types of aqueous-carbonic fluid inclusions were identified: H2O-NaCl-CO2-bearing (salinities between 4 and 7 wt % NaCl equiv) and H2O-NaCl-CO2-CH4-bearing (salinities varying between 7 and 9 wt % NaCl equiv). Both types homogenize at T > 190°C. Based on structural, textural, and fluid inclusion evidence, we propose that gold deposition in the C quartz-tourmaline vein took place after the main episode of vein filling as a result of continued fracturing and mixing of these two fluids.

Alteration geochemistry and fluid inclusion characteristics of the greenstone-hosted gold deposit of Hutti, Eastern Dharwar Craton, India

Gold mineralization of the Hutti mine, southern India, is situated in closely spaced laminated quartz veins and associated alteration haloes along steeply dipping shear zones within a sequence of rather uniform amphibolites. Intense shearing has resulted in large-scale mylonitization of the wall rocks. Anastomosing shear zones, with intervening lensoid bodies of unsheared amphibolites, are characteristic features of the deposit. The general pattern of symmetrical alteration comprises a distal zone of chlorite-rich rock, with a proximal biotite-rich zone adjacent to laminated quartz veins. Arsenopyrite thermometry yielded a temperature range of 350–477 °C for the biotite alteration zone, which preceded the formation of the laminated quartz veins. Mass balance calculations on the alteration zones indicate a gradual mass and volume loss during alteration. The alteration is accompanied by intense potash metasomatism and addition of sulfur, which resulted in the formation of arsenopyrite, pyrrhotite, and pyrite. Results of fluid inclusion studies suggest that low salinity (3.9–13.5 wt% NaCl equivalent) H2O–CO2 rich fluids were responsible for gold-rich laminated quartz vein formation in the Hutti deposit. These fluids constituted a later counterpart of the protracted fluid activity that first formed the biotite alteration zone. The estimated P–T values range from 1.0 to 1.7 kbar at 280–320 °C. These data, along with the alteration assemblages and the characteristic gold–sulfide association, both in the altered wall rock and laminated quartz veins, suggest that gold, transported as reduced bisulfide complexes, was deposited in response to sulfidation reactions in the wall rocks. Comparison of P–T conditions of formation of gold–quartz veins at Hutti with two other large gold deposits in the eastern Dharwar Craton, namely Kolar (1.8 kbar/280 °C) and western Ramagiri (1.45–1.7 kbar/240–270 °C), indicates broadly similar lode-gold forming conditions in the Dharwar Craton.

Orogenic disseminated gold in phanerozoic fold belts—examples from Victoria, Australia and elsewhere

Orogenic gold mineralisation in the Lachlan Orogen of southeastern Australia and in other Phanerozoic fold belts has generally been assumed to be confined entirely to quartz veins. However, an improved understanding of the tectonic, structural and geochronological framework in which these occurrences form, and recent advances towards the extent of wallrock alteration around these veins, cast doubt on the validity of this deeply entrenched ‘lode-gold paradigm’. Ore-grade orogenic disseminated gold mineralisation has been recognised in a number of fold belts that are known to host significant orogenic lode gold deposits (Lachlan Orogen, Australia; Buller Terrane and Otago Schist, New Zealand; Meguma Terrane, Canada; Tien Shan Mountains in Uzbekistan and Kyrgyzstan; Yana-Kolyma Province, Baikal, Verhoyansk and Allakh-Yun fold belts in north–central Mongolia and Far East Russia). Disseminated mineralisation in these collisional belts is invariably hosted by pervasively altered greenschist to subgreenschist facies rocks. These occurrences also share a number of characteristics with some Carlin-type gold deposits in the Great Basin of western North America (e.g., Getchell) regarding structural relationships, fluid composition, mineral paragenesis and a common association of carbonaceous matter with mineralisation. Features of alteration, physico-chemical characteristics of the ore-bearing fluids and strong structural control in all of the studied occurrences point to a close genetic association with lode gold mineralisation. The two styles are considered to represent end-members of a crustal continuum of orogenic gold emplacement in Phanerozoic fold belts, with disseminated mineralisation more likely to develop at shallower levels and within more permeable units during the waning stages of uplift and exhumation. These conditions facilitate ground preparation via brittle fracturing and the development of intricate stockwork systems. Efficient gold segregation from bisulphide complexes is largely controlled by fluid mixing of an evolved metamorphic fluid with either more oxidised ascending fluids or meteoric fluids, and accelerated by the presence of carbonaceous matter at the site of deposition.

Fluid Chemistry and Depositional Mechanism of the Epigenetic, Discordant Ores of the Proterozoic, Carbonate-Hosted, Zawarmala Pb-Zn Deposit, Udaipur District, India

The Proterozoic, carbonate-hosted, Zawarmala Pb-Zn deposit has stratiform, banded pyrite-sphalerite ore, as well as discordant, sphalerite-galena veins and rich massive galena ore shoots. The mechanisms of deposition of discordant vein and massive ores have been studied using speciation and solubility calculations based on ore fluid characteristics and physical parameters inferred from petrography, fluid inclusion studies, and leachate analysis. Massive galena ore, which is the economic ore type, cuts across the earlier, F2-related, vein-type ore and is distinct from the vein-type ore in terms of mineralogy, texture, and fluid inclusion characteristics. Fluid inclusions in quartz associated with the ores show that fluids forming vein-type ore are low-salinity (4.3–14.7 wt % NaCl equiv) H2O-NaCl fluids. The trapping temperature of vein-type ore fluid was estimated to be in the range of 395° to 290°C at a pressure of about 1,450 bars. Fluids associated with massive galena ore are H2O-CO2-NaCl fluids of lower salinity (about 3–4 wt % NaCl equiv). The wide variation in XCO2 of fluid inclusions that occur in close proximity to each other within the same sample of massive ore suggests heterogeneous entrapment of the low-salinity H2O-CO2-NaCl fluids. From the intersection of isochores of local, homogeneously entrapped CO2 and aqueous biphase fluids of unmixed CO2-H2O-NaCl fluid, trapping temperature fluid was estimated to be between 250° and 150°C at pressures of 2,000 to 750 bars. Despite the low salinity of the ore fluids, speciation studies show that chloride complexes are dominant over most of the temperature range of interest in both ore fluids. The only bisulfide complex of importance is \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Pb(HS)_{2}^{0}\) \end{document}, which is dominant below 175°C in fluids associated with massive galena ore. Data from fluid inclusions and solubility calculations reveal that cooling is the important mechanism of vein-type ore deposition. In contrast, physicochemical data indicate that an increase in reduced sulfur, by mixing a regional, high metal content, low sulfur fluid with a local, high sulfur fluid at the site of deposition, was the mechanism for massive galena ore deposition. The difference in oxidation state (and therefore also in metal content) of the two fluids is inferred to be a significant factor that caused differences in the modes of deposition of the two ore types and the relatively high value of XCO2 (0.2033) in fluids associated with massive galena ore.

Fluid regimes related to the formation of lode-gold deposits in the Rio Itapicuru greenstone belt, Bahia: a fluid inclusion review

The shear zone-hosted gold deposits of the Rio Itapicuru Greenstone Belt, northeastern Brazil, are located in its southern and northern sectors, the former including Fazenda Brasileiro Mine and Fazenda Canto deposit and the latter Fazenda Maria Preta, Mari and Ambrósio deposits. Fluid inclusion studies by microthermometry and laser Raman microspectroscopy in mineralized quartz veins of these deposits reveal two main populations of inclusions: primary, CO 2 - (± CH 4 ± N 2 ) inclusions (type I), and low salinity (&lt; 6 wt% NaCleq.) H 2 O - CO 2 - (± CH 4 ± N 2 ) inclusions (type II), representing fluid regimes, which were active during the gold mineralizing events. Both types of ore fluids are interpreted as part of a deep hydrothermal system, in which the fluid components may have been derived by devolatilization reactions during regional metamorphism and/or from mantle-magmatic sources (e.g. CO 2 , H 2 S). The nature of the CO 2 -rich fluid regime, atypical in mesothermal lode-gold deposits, still remains an open question. Gold deposition from bisulfide complexes took place at different crustal levels, at temperatures varying from 280 up to 500°C, and pressures from 1 up to 4.4 kb, probably closely linked with and dependent upon redox changes that accompanied the interaction of the fluids with mafic and carbonaceous host rocks.

Fluid regimes related to the formation of Lode-Gold Deposits the Rio Itapicuru Greenstone Belt, Bahia: a fluid inclusion review

The shear zone-hosted gold deposits of the Rio Itapicuru Greenstone Belt, northeastern Brazil, are located in its southern and northern sectors, the former including Fazenda Brasileiro Mine and Fazenda Canto deposit and the latter Fazenda Maria Preta, Mari and Ambrosio deposits. Fluid inclusion studies by microthermometry and laser Raman microspectroscopy in mineralized quartz veins of these deposits reveal two main populations of inclusions: primary, CO 2 - (± CH ± N 2 ) inclusions (type I), and low salinity (< 6 wt% NaCleq.) H 2 0 - CO 2 - (± CH 4 ± N ) inclusions (type II), representing fluid regimes, which were active during the gold mineralizing events. Both types of ore fluids are interpreted as part of a deep hydrothermal system, in which the fluid components may have been derived by devolatilization reactions during regional metamorphism and/or from mantle-magmatic sources (e.g. CO 2 H 2 S). The nature of the CO 2 -rich fluid regime, atypical in mesothermal lode-gold deposits, still remains an open question. Gold deposition from bisulfide complexes took place at different crustal levels, at temperatures varying from 280 up to 500°C, and pressures from 1 up to 4.4 kb, probably closely linked with and dependent upon redox changes that accompanied the interaction of the fluids with mafic and carbonaceous host rocks.

Cadmium Complexation with Bisulfide

We have used in situ dialysis to measure the solubility of CdS(s) in sulfidic solutions. The solubility product for the reaction CdS(s) + H+ ⇄ Cd2+ + HS- at 25 °C and 1 atm was found to be 10-14.82±0.03 for a crystalline product and to be 10-14.15±0.06 and 10-14.40±0.06 for two precipitates, respectively. We show that the solubility of these three solids at various pH values (4.2−8.6) and sulfide concentrations (10-4.3−10-1.3 M) can be reproduced adequately by the following four bisulfide complexes: CdHS+, Cd(HS)2, Cd(HS)3-, and Cd(HS)42-; the log Kn values for the general equation Cd2+ + nHS- ⇄ Cd(HS)n2-n are 7.38 ± 0.68, 14.43 ± 0.01, 16.26 ± 0.58, and 18.43 ± 0.05, respectively. The species CdOHS-, which has been reported previously, does not explain our experimental results. Calculations using estimated concentrations of organic ligands (humic substances and organic thiols) known to be present in natural waters indicate that sulfide complexes largely dominate Cd speciation in natural waters at ∑S(−II...

Fluid inclusion microthermometry and the P-T evolution of gold-bearing hydrothermal fluids in the Niuxinshan gold deposit, eastern Hebei province, NE China

Niuxinshan is a typical example of the numerous mesothermal gold deposits formed during Mesozoic tectono-magmatic reactivation of the Archean North China Craton in eastern Hebei province. Gold occurs in quartz-sulfide lodes in Archean amphibolites and also in greisen zones in the Mesozoic Niuxinshan granite stock. Four mineralization stages can be recognized from early to late: (1) quartz-K-feldspar, (2) quartz-pyrite, (3) quartz-polysulfide, and (4) quartz-carbonate. Gold mineralization mainly occurs in stages 2 and 3. Fluid inclusions in quartz and fluorite from greisen zones in the Niuxinshan granite, and inclusions in vein quartz and sphalerite from stages 1 to 3 in the amphibolites, have been studied by microthermometry. Three compositional types of inclusions are recognized: type 1 (Tp1) are H2O-CO2-bearing inclusions and include primary (Tp1-P) and secondary (Tp1-S) inclusions. These are found in quartz and fluorite from the greisen zones as well as in vein quartz and sphalerite from stages 1 to 3. The Tp1-P inclusions are considered to represent the gold-bearing hydrothermal fluids. Type 2 (Tp2-S) are secondary H2O-CO2 + solid phase inclusions in fluorite from the greisen zones. Type 3 (Tp3-S) are secondary aqueous inclusions with a solid phase which coexist with the Tp2-S in fluorite from the greisen zones. The Tp1-P inclusions show variable VCO2 (commonly 0.3 to 0.6) and XCO2 values (mainly 0.1 to 0.4). The salinities of inclusions cluster around 3 to 11 wt.% NaCl equivalent and their homogenization temperatures to the liquid phase (Th(L)) fall dominantly in the range of 260 to 360 °C. The compositional variations of inclusions in stage 1 probably result from exsolution of magmatic fluids at various stages; immiscibility or boiling of the fluids can be ruled out. The compositional variations of inclusions in the greisen zones and in vein stages 2 and 3 are attributed to cooling, mixing (dilution), and necking-down of the fluids. The Tp1-S and Tp2-S inclusions show salinities of 3 to 6 wt.% NaCl equivalent and XCO2 values of 0.04 to 0.17. Th(L) clusters at 240 to 260 °C. The Tp3-S inclusions have salinities of 3 to 6 wt.% NaCl equivalent and Th(L) of 170 to 240 °C. Isochoric reconstructions, combined with oxygen and sulfur isotope geothermometry of mineral pairs, give trapping P-T conditions for the gold-bearing fluids. The greisen zones formed at 310 to 460 °C and 1.3 to 3.7 kbar; stage 1 veins at 300 to 430 °C and 1.2 to 3.7 kbar; stage 2 veins at 290 to 380 °C and 1 to 3 kbar; stage 3 veins at 250 to 350 °C and 1 to 3 kbar. H2O-CO2 fluids with low to moderate salinities and moderate to high densities (0.66 to 1.01 g/cm3) dominated at early mineralization stages, and evolved towards H2O-richer and CO2- and less saline fluids through time. The retrograde P-T evolution probably resulted from regional uplift and cooling of gold-bearing hydrothermal fluids. The gold bisulfide complex was dominant in the fluids during mineralization and gold deposition was mainly induced by decreases of temperature and pressure, as well as destabilization of the bisulfide complex during sulfidization of wall rocks.