Chemical Geology

Variations in molybdenite trace element chemistry represent a tool for discriminating discrete events in young magmatic-hydrothermal systems and constrain the role of granites in older greenstone belt-hosted gold systems. We show that besides Re and W (typical lattice-bound elements), molybdenite also concentrates chalcophile elements with chalcogenide affinity such as Bi, Pb, and Te (CEs). Elements from the latter group form nano-to micron-scale inclusions which also attract Au and Ag incorporation. High concentration of all elements is attributable to lattice-defects and coherent intergrowths between molybdenite and CE-minerals rather than polytypism. If both groups are used, trace element patterns are useful for interpreting superimposed ore-forming processes. We test the validity of this hypothesis by carrying out Laser-Ablation Inductively-Coupled Mass Spectroscopy spot analysis and element mapping on high-Re molybdenite from the Tertiary Au deposit at Hilltop (NV, USA) and the Archean Boddington Cu-Au deposit (Western Australia). Whereas W and CEs are affected by both deformation and interaction with subsequent fluids, Re is only affected by the latter. An epithermal overprint at Hilltop, recorded in a grain with a CE-rich halo surrounding a core with Re-W oscillatory zoning, upgrades Re content and is also traceable by measurable Au. At Boddington, granite-derived fluids dilute Re in precursor molybdenite, but both granite and orogenic deformation assist CE-mineral coarsening and Au release.

The Neoproterozoic Hekkelstrand Marble from the North-Central Norwegian Caledonides contains a calcite marble unit hosting a commercially exploited white dolomite marble deposit, which underwent polyphase amphibolite-facies, Scandian metamorphism. Major and trace element geochemistry, d 18 O, d 13 C and 87 Sr/ 86 Sr ratios have been used to define isotopic differences between interbedded calcite and dolomite marbles and to assess their post-depositional isotopic resetting. A high strontium concentration in the calcite marble (up to 1200 ppm) suggests an aragonitic lime mudstone precursor, and its leastaltered d 13 C (+4x V-PDB), d 18 O (27x V-SMOW) and 87 Sr/ 86 Sr (0.70615 to 0.70624) ratios are consistent with 700-600 Ma seawater. The dolomite marbles contain minor relics of the calcite (aragonite?) precursor, despite pervasive dolomitisation. The average Mn (44 ppm) and Fe (1041 ppm) contents of the dolomite marbles are comparable with those measured in the host calcite marbles and much lower than those reported from non-metamorphosed dolostones of a similar Neoproterozoic age. The dolomite marbles have an average Sr content of 115 ppm. An average Mg/Ca ratio of 0.56 is lower than that for stoichiometric dolomite (0.62) and is controlled mainly by remnant inclusions of precursor calcite. 87 Sr/ 86 Sr ratios range between 0.70725 and 0.71034. The least-altered d 13 C of +6x and d 18 O of~30x suggest a high degree of isotopic preservation of the dolomite. The dolomite marbles are isotopically heavier (both d 18 O and d 13 C) by~2x on average compared with the calcite marble, thus suggesting a pre-metamorphic, lower-temperature, isotopic equilibrium. The dolomite was apparently originally precipitated from 600 to 700 Ma seawater-dominated fluids. The 87 Sr/ 86 Sr ratio was modified by post-depositional processes. The high d 13 C and d 18 O values and the low Mn content of the bulk dolomite marbles are not consistent with common meteoric diagenesis or metamorphic alteration. The highest 87 Sr/ 86 Sr ratio of 0.70868 suggests that the earlier dolomite was apparently modified by ca. 560 Ma seawater or by meteoric water considerably buffered by ca. 560 Ma limestones. The most significant geochemical and isotopic modification of the dolomite took place during metamorphism. This has been observed across the dolomite marblesilicate rock contacts over a limited distance of b0.5 m where C and O isotope systems were reset by up to À4.5x and À9.7x, respectively, the 87 Sr/ 86 Sr ratio increased by up to 0.0012, whereas the Fe, Mn and Sr concentrations have increased by factors of~80,~15 and~1.5, respectively. The dolomite marbles, despite amphibolite-facies polyphase metamorphic alterations, have 0009-2541/$ -see front matter D .no (V.A. Melezhik).

The Tharsis massive sulphide deposit, one of the major VMS-type deposits in the Iberian pyrite belt (IPB) was severely deformed by the Variscan tectono-thermal events. The question of whether or not these events affected the metal distribution in the deposit has been addressed by simultaneously studying the mineral parageneses (Tharsis stockwork) and the fluid circulation (at local and regional scales). The results are: 0009-2541/02/$ -see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 -2 5 4 1 ( 0 2 ) 0 0 2 7 5 -9 (C. Marignac).

Detrital studies that utilize zircon U-Pb geochronology and fission-track (FT) thermochronometry are subject to a range of potential sources of bias that should be properly evaluated and minimized. Some of them are common to any single-grain mineral analysis (e.g., variable bedrock mineral fertility, hydraulic sorting during transport, selective grain loss during sample processing), whereas others are intrinsic to zircon, and are related to radiation damage and age discordance. In this article, we quantify the impact of intrinsic bias on detrital studies thanks to the analysis of modern detritus shed from the European Alps, and illustrate the general implications on geological interpretations. We show that detrital zircon U-Pb age distributions based on statistically robust datasets are highly reproducible and representative of the parent bedrock ages in the catchment. Arbitrary or selective removal of discordant grain ages can be minimized by using the Kolmogorov-Smirnov test to identify an appropriate cutoff level. Loss of metamict (α-damaged) zircon has a minor impact on data representativeness, and is mainly controlled by regional metamorphism rather than by mechanical abrasion during river transport. Zircon FT grain-age distributions were found to have poor reproducibility, although age spectra are consistent with bedrock data. However, unlike the U-Pb datasets, U-rich zircon grains (N 1000 ppm) are systematically missed, and undatable grains may exceed 50%. We identify two major sources of distribution bias specific to zircon FT datasets: (i) sediment sources dominated by U-rich zircon grains are markedly underrepresented in the detrital record, because such grains often have uncountable high densities of fission tracks ("U concentration bias"); (ii) sediment sources that shed zircon grains with high levels of α-damage are underrepresented, because these grains are lost during chemical etching for FT revelation ("etching bias"). In the case of multimethod dating on the same grains (e.g., FT and U-Pb double dating), bias affecting detrital zircon FT dating propagates to the entire dataset. These effects may not impact on exhumation-rate studies that utilize the youngest grain ages (i.e., lag-time approach). However, they represent a limiting factor for conventional provenance studies, and generally preclude application of zircon FT dating to sediment budget calculations.

Cryogenic cave calcite (CCC), formed by segregation of solutes during water freezing, was found in three Central European caves. This calcite type forms accumulations of loose calcite grains on cave floor. The calcite grains are of highly variable crystal morphology, and of sizes ranging from less than 1 mm to over 1 cm. The most typical feature is their accumulation as loose (uncemented) crystals. U-series dating indicates the formation of CCC in the studied caves during several climatic oscillations of the Weichselian (between 61 and 36 ka BP in the Chelsiowa Jama -Jaskinia Jaworznicka cave system in Poland, between 34 and 26 ka BP in the BUML Cave in the Czech Republic, and between 26 and 21 ka BP in the Stratenská Jaskyňa cave system, Slovakia). At the time of CCC formation, the studied caves were lying in a periglacial zone.

Cueva de Villa Luz, a hypogenic cave in Tabasco, Mexico, offers a remarkable opportunity to observe chemotrophic microbial interactions within a karst environment. The cave water and atmosphere are both rich in hydrogen sulphide. Measured H S levels in the cave atmosphere reach 210 ppm, and SO commonly exceeds 35 ppm. These gases, plus oxygen 2 2 ) Corresponding L.D. Hose . 0009-2541r00r$ -see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 0 9 -2 5 4 1 0 0 0 0 2 1 7 -5 ( ) L.D. Hose et al.r Chemical Geology 169 2000 399-423 400

Various diffusion raodels can be applied to Rb-Sr systems in cooling rocks. It is argued that a closed-system diffusioncontrolled model, inw3lving local equilibrium along grain boundaries, is most appropriate for describing the exchange of Sr isotopes between minerals in a rock sample during cooling. A finite-difference numerical method is used to solve the diffusion and mass-balance equations. Closure temperatures of mineral-pair Rb-Sr isochrons are predicted to depend on the factors involved in Dodson's infinite reservoir closure temperature formulation (cooling and diffusion rate, grain size and shape) of both the minerals. In addition, the closure temperature for mineral pairs is also dependent on the proportion of Sr in each mineral, which is dominantly a function of rock mode. This implies that Rb-Sr cooling ages from interbedded rocks having the same cooling history, but distinct modes, should differ: at slow cooling rates age differences could be > 100 Ma. Such effects, if unrecognised could result in erroneous cooling curves, but if recognised could be utilised in estimating true cooling rates. Our closed-system model :may also apply to other isotope decay schemes, such as the Sm-Nd system. A simple test is proposed which would allow the relevance of different diffusion models to be assessed.