The hydrothermal fluid of Archæan lode-gold deposits at different metamorphic grades: compositional constraints from ore and wallrock alteration assemblages

A low-salinity, mixed aqueous-carbonic fluid is common to all Archæan lode-gold deposits throughout the range of mineralising conditions from sub-greenschist to lower-granulite facies temperatures. Alteration assemblages and fluid-inclusion data give constraints on the fluid composition. Fluid X_{CO 2} is 0.1–0.3 in typical greenschist-facies (mesothermal) deposits. At higher temperatures, the assemblages are consistent with formation from a fluid of similar composition, but slightly higher or lower X_{CO 2} cannot be ruled out, and fluid-inclusion data indicate that CH₄ may be an important component in ore fluids at these temperatures. Fluid pH is neutral or weakly alkaline at all conditions. A range of relative oxidation states of four orders of magnitude f_{O 2} is indicated at any temperature, with deposits more oxidising relative to QFM at lower temperature. Sulphur contents of the fluids vary from ≈ 10 to 10^?3.5m_∑S, with a trend towards lower sulphur contents at lower temperatures. The relative concentrations of major cations in solution are similar at all conditions with Na ? K ≥ Ca, although Ca may be less abundant at low temperatures. The broad similarities in ore-fluid composition at all temperatures give support to ‘crustal-continuum’ models, in which Archæan lode-gold mineralisation involved either a single fluid moving through the middle and upper crust, or derivation of ore fluids by similar processes at different crustal levels. Many of the compositional differences between high- and lowtemperature ore-fluids may be attributed to evolution of deep-sourced hydrothermal solutions as they rise along structurally-controlled conduits. The constancy of major ore-fluid component concentration (e.g. CO₂, Cl, ± K) suggests fluid-buffering and high fluid-rock ratios along fluid pathways. Fluid-buffered conditions can also explain the ore-fluid f_{O 2}-temperature relations; with equilibria between oxidised and reduced aqueous carbon or sulphur species controlling the oxidation state. In contrast, the concentrations of components present in lesser abundance in Archæan gold ore-fluids (e.g. S, Ca, H⁺) were probably controlled either by saturation of one or more mineral phases brought on by decreasing temperature, or were rock-buffered through fluid-rock reactions. Extrapolation to high temperatures of the K, Na and Ca contents of the gold-bearing fluids indicates that their composition is consistent with derivation from, or final equilibration with, rocks of intermediate-granitic composition, thus giving support to isotopic and geological arguments for ore-fluid source regions external to the greenstone belts. The fluid oxidation states are characteristic of a wide range of potential source rocks, including mantle-derived igneous rocks, calc-alkaline granitoids and magmas, and seaflooraltered metabasalts. Strongly oxidised magmatic sources or unusually oxidising source processes (e.g. CO₂-streaming during granulitisation of the lower crust) are therefore not required in the genesis of Arch?an lode-gold deposits.     

Application of iron and zinc isotopes to track the sources and mechanisms of metal loading in a mountain watershed

Here the hydrogeochemical constraints of a tracer dilution study are combined with Fe and Zn isotopic measurements to pinpoint metal loading sources and attenuation mechanisms in an alpine watershed impacted by acid mine drainage. In the tested mountain catchment, δ⁵⁶Fe and δ⁶⁶Zn isotopic signatures of filtered stream water samples varied by ∼3.5‰ and 0.4‰, respectively. The inherent differences in the aqueous geochemistry of Fe and Zn provided complimentary isotopic information. For example, variations in δ⁵⁶Fe were linked to redox and precipitation reactions occurring in the stream, while changes in δ⁶⁶Zn were indicative of conservative mixing of different Zn sources. Fen environments contributed distinctively light dissolved Fe (<−2.0‰) and isotopically heavy suspended Fe precipitates to the watershed, while Zn from the fen was isotopically heavy (>+0.4‰). Acidic drainage from mine wastes contributed heavier dissolved Fe (∼+0.5‰) and lighter Zn (∼+0.2‰) isotopes relative to the fen. Upwelling of Fe-rich groundwater near the mouth of the catchment was the major source of Fe (δ⁵⁶Fe ∼ 0‰) leaving the watershed in surface flow, while runoff from mining wastes was the major source of Zn. The results suggest that given a strong framework for interpretation, Fe and Zn isotopes are useful tools for identifying and tracking metal sources and attenuation mechanisms in mountain watersheds.     

Major element chemistry of glasses in Apollo 14 soil 14156

Glasses in a soil sample (14156) from the middle layer of the trench at the Fra Mauro landing site show a wide range of compositions clustered around certain preferred compositions. Ninety per cent of the glasses are of two major types—Fra Mauro basalt (63 per cent) with high K and 17 weight per cent Al₂O₃ and Highland basalt or anorthositic gabbro (27 per cent) with low K and 25–26 weight per cent Al₂O₃. The glass population is almost identical with that of the comprehensive soil 14259 (Apollo Soil Survey, 1971).     

Intercomparison of NO,NO2 and HNO3 measurements with photochemical theory

The simultaneous measurements of NO, NO₂ and HNO_A mixing‐ratio profiles carried out on the Stratoprobe balloon flight of 22 July 1974 have been simulated with a time‐dependent model using the measured temperature and ozone profiles. The calculated ratios of NO/NO₂, HNO₃/NO₂ using currently accepted photochemistry are consistent with the measured ratios within the experimental errors of the measurements. The measured NO₂/NO ratio is almost a factor of two smaller than predicted, although the discrepancy is still within the experimental errors. A remarkable proportionality in the NO₂ and O₃ profiles has been noted and is unexplained. A time‐dependent simulation has been employed to convert the measurements into diurnally‐averaged profiles suitable for intercomparison with two‐dimensional stratospheric models and a comparison with constituent profiles from Prinn et al. (1975) is carried out as an example. The NO_V mixing ratio, formed from the sum of the NO, NO₂ and HNO₂ measurements is similar to the NO_V mixing ratio from several one‐ and two‐dimensional models used to predict the effects of SST's on the ozone layer. The odd nitrogen mixing ratio is roughly constant from 20 to 35 km at 11 ppbv.     

Temporospatial dynamics and public health significance of bacterial flora identified on a major leatherback turtle (Dermochelys coriacea) nesting beach in the Southern Caribbean

Grande Riviere beach, on the island of Trinidad, supports the largest nesting population of leatherback turtles in the Caribbean region. Throughout the nesting season, nests are naturally disturbed by newly nesting females, resulting in egg breakage and loss of some nest viability. This environment is ideal for the growth and proliferation of microorganisms. The range of bacterial flora present in beach sand and egg shells was examined, with emphasis on bacteria that may pose a threat to public and animal health. The extent to which the bacterial load and genera on the beach changed throughout the season was also assessed. Twenty‐five genera were identified, with Pseudomonas spp. found to be the most predominant environmental bacteria. Four genera identified possess zoonotic potential, while five additional genera are known to be of public and animal health significance. Distinct shifts in the density and distribution of bacteria were observed along the beach from early to peak nesting season. Shifts were seen across heavily traversed zones, thus highlighting the potential exposure threats posed to beach visitors and animals alike. Further studies aimed at speciating this population of bacteria, as well as isolating potential fungal pathogens may mitigate this threat. Identification of bacterial agents that are specifically pathogenic to leatherback turtles, turtle eggs, hatchlings and those who may interact with these animals will serve to enhance and guide efforts to better conserve this species and protect the health of all who visit this ecologically significant site.     

Luna 20 soil: abundance and composition of phases in the 45–125 micron fraction

Glass compositions in the Luna 20 soil indicate a minor contribution of mare rocks and a major contribution of highly feldspathic highland material. Glasses with the composition of Highland basalt (anorthositic gabbro or norite) predominate in a range of highly aluminous glasses.The analyses of minerals in the soil show that the highland rocks have a unique assemblage of minerals that can readily be distinguished from the mineral assemblages of either mare or KREEP basalts. The soils are characterized by abundant anorthitic (An_92–99), low-Fe plagioclase. Highly magnesian orthopyroxenes, pigeonites and augites are the most prominent pyroxenes. Unlike mare basalt pyroxenes, clinopyroxenes with intermediate Ca values are not abundant, but extreme iron enrichment towards pyroxferroite does occur. Olivines are more abundant than at other sites and are Mg-rich, low in Ca and Cr. Spinels with compositions approaching MgAl₂O₄ predominate over pleonastes and chromites. Ilmenite and metal are present but not abundant.These data establish the unique nature of the minerals in the highland soils. The mineral compositions are consistent with derivation from a suite of highly feldspathic rocks in which Highland basalt compositions predominate. Some of the mineral data, particularly from the pyroxenes, are suggestive of surface or near-surface processes, rather than plutonic crystallization.     

Metamorphic evolution of the central Southern Cross Province,Yilgarn Craton,Western Australia

Two contrasting styles of metamorphism are preserved in the central Southern Cross Province. An early, low‐grade and low‐strain event prevailed in the central parts of the Marda greenstone belt and was broadly synchronous with the first major folding event (D₁) in the region. Mineral assemblages similar to those encountered in sea‐floor alteration are indicative of mostly prehnite‐pumpellyite facies conditions, but locally actinolite‐bearing assemblages suggest conditions up to mid‐greenschist facies. Geothermobarometry indicates that peak metamorphic conditions were of the order of 250–300°C at pressures below 180 MPa in the prehnite‐pumpellyite facies, but may have been as high as 400°C at 220 MPa in the greenschist facies. A later, higher grade, high‐strain metamorphic event was largely confined to the margins of the greenstone belts. Mineral assemblages and geothermobarometry suggest conditions from upper greenschist facies at P–T conditions of about 500°C and 220 MPa to upper amphibolite facies at 670°C and 400 MPa. Critical mineral reactions in metapelitic rocks suggest clockwise P–T paths. Metamorphism was diachronous across the metamorphic domains. Peak metamorphic conditions were reached relatively early in the low‐grade terrains, but outlasted most of the deformation in the higher grade terrains. Early metamorphism is interpreted to be a low‐strain, ocean‐floor‐style alteration event in a basin with high heat flow. In contrast, differential uplift of the granitoids and greenstones, with conductive heat input from the granitoids into the greenstones, is the preferred explanation for the distribution and timing of the high‐strain metamorphism in this region.