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Silver is probably the closest isotopic proxy to track monoisotopic gold and has been shown to have great potential to yield new information on the origin and enrichment processes of gold. This study describes the development of a tailored analytical protocol for accurate analysis of Ag isotopes and provides the first Ag isotope data for the Paleoproterozoic Rajapalot Au-Co deposit, Finnish Lapland. Six native Au samples yield ε109Ag values (relative to NIST SRM 978a) from −6.8 to +2.1 and are within the range of Ag isotopic compositions reported for native Au samples. The mean of the analyzed Au samples is ε109Ag = −3.8 ± 1.7 (2SD) with most of the samples with negative ε109Ag values (−6.7 to −2.0); one sample has a positive ε109Ag value of +2.1 ± 0.5. Silver isotope fractionation in the Rajapalot Au deposit was likely associated with physicochemical processes related to deposition and/or re-mobilization of the ore rather than with source region inheritance. It is suggested that redox reactions involving Ag+ ↔ Ag0 phase change primarily account for the isotopic differences within the deposit. Our results also suggest that the Rajapalot Au-Co deposit was formed via multistage ore-forming processes and/or that the primary ore was re-mobilized, which caused isotope fractionation along fluid pathways. Silver isotope variation within a deposit may mark a fractional crystallization trend with the lightest isotopic composition representing the earliest precipitate. Hence, Ag isotopes show potential as an isotopic vectoring tool in search of Au-enriched domains. [ABSTRACT FROM AUTHOR]

This project follows on from an initial study of Celtic gold coins from the Middle Rhine/Moselle region, which was based on material found at the Martberg, a Late Iron Age/Roman sanctuary and settlement (River Moselle, western Germany; Bendall 2003 ). The earlier work was expanded to encompass over 100 examples of various other regional Celtic gold coinages from the collection of the Römisch-Germanisches Zentralmuseum, Mainz. The alloy (Au–Ag–Cu) and trace element compositions (in particular Ni, Sb and Pt) were determined by EPMA and LA–ICP–MS, respectively, and their Pb isotope signatures were measured by LA–MC–ICP–MS. Of the 28 trace elements measured, only Ni, Sb and Pt were found to show meaningful variations and so only they are presented here. In particular, differences in the Pt/Au ratios between various groups of earlier coinage (imitations of Philippus and Alexander/Nike staters, Sch. 23 and some early Boian coins) on the one hand, and the majority of the Boian and the southern rainbow cup coinages on the other, indicate a significant difference in the gold sources exploited for these regional coinages. The Pb isotope data confirm previous conclusions that the contribution of gold to the total lead in the Au–Ag–Cu coin alloys can be detected, especially for coins with over 70% gold, and show that possible gold sources include both eastern Mediterranean and Alpine sources. Combining the Pb isotope data with the Pt/Au ratios allows the potential gold sources to be further differentiated. [ABSTRACT FROM AUTHOR]

Abstract: Garnet from hydrothermally altered calcsilicate rocks of the Navachab gold deposit, Namibia, records complex zoning patterns that reflect the alteration history during mineralization. In order to study the origin of the zoning patterns, the major and trace element geochemistry was analyzed by EMP- and LA-ICP-MS techniques. Garnet essentially forms spessartine–grossular–almandine solid solutions, and records a two-stage growth history. Stage I garnet usually forms the cores of porphyroblasts, and records a bell-shaped zoning profile. This garnet generation is highly enriched in spessartine, and documents a progressive depletion of manganese in the fluid. Stage II garnet overgrows the earlier garnet, and marks the onset of increased fluid–rock interaction and hydrothermal gold mineralization. In the semi-massive sulfide breccias, stage I garnets were dissolved, and the overgrowing stage II garnet displays gradational compositional variations. In the surrounding mylonites, stage II garnet is characterized by oscillatory zoning profiles, documenting the episodic supply of hydrothermal fluid in these rocks. All garnet grains display similar REE pattern, suggesting a formation under similar P–T–X conditions. The grains are enriched in HREE, depleted in LREE, and have a weak to pronounced negative Eu-anomaly. The sum of the REE increases with increasing Ca, pointing to a progressive increase of these elements in the ore fluid. In conjunction with considerations on the mechanisms of hydrothermal fluid flow, the data demonstrate that the type of zoning pattern developed in hydrothermal garnet is mainly controlled by the permeability of the rock: bell-shaped growth zoning may develop in hydrothermal systems during pervasive hydrothermal alteration. Oscillatory zoning requires that the fluid flow is episodic, and possibly accompanied by periods of reduced growth rates between individual pulses of chanellized hydrothermal fluid flow. [Copyright &y& Elsevier]