Eva S. Schandl

Evolution of the Acipayam (Denizli, Turkey) Rodingites

Numerous diabase and gabbro dikes intrude the Lycian peridotite thrust sheet of southwestern Turkey. Some of the dikes are rodingitized and the host harzburgite is serpentinized. The present study demonstrates that the major episodes of serpentinization (of harzburgite) and rodingitization (of dikes) predated the obduction of the Lycian peridotite thrust sheet. Hydrogrossular veins, which represent the major event of calcium metasomatism in the dikes, are locally fractured, fragmented, and offset. High-field-strength-element concentrations and Ta/Yb-Th/Yb ratios in the mafic rocks suggest that the dikes were originally emplaced in an island-arc environment; thus, they must have been transported to their current location. Fluid-inclusion microthermometry obtained on primary fluid inclusions in hydrogrossulars indicates that the fluid involved in rodingitization and contemporaneous serpentinization was moderately saline (8 equiv. wt% NaCl) and within the range of 250°-450°C. The above salinity and temperature are consistent with microthermometric measurements obtained from rodingite minerals worldwide. It is apparent from our study that rodingite geochemistry and mineralogy provide valuable information on the tectonic and metamorphic evolution of a region. Trace-element and REE geochemistry can define the tectonic setting of dikes, detailed mineralogy of the dikes and host serpentinite provides information on the relative timing of serpentinization and contemporaneous rodingitization, and fluid-inclusion microthermometry provides information on the temperature and composition of the Ca-rich metamorphic fluids.

The origin of major talc deposits in the Eastern Desert of Egypt: relict fragments of a metamorphosed carbonate horizon?

Detailed geochemical and mineralogical investigation of four talc deposits in the Eastern Desert of Egypt (Atshan, Abu Gurdi, Darhib and Kashira) suggests that the deposits form a distinct lithological unit within the Shadli metavolcanic rocks. The talc crystallized from the replacement of siliceous carbonate beds locally intercalated with clastic sediments. Th/Yb vs. Ta/Yb ratios of the rocks suggest that the sediments and the host volcanic rocks formed in an active continental margin (ACM) environment. Thus, the talc deposits may represent relict fragments of an ancient, regionally extensive carbonate horizon within the arc-related metavolcanics. The talc-rich rocks, which contain relict carbonate, serpentinized olivine and tremolite, have low (<3 wt%) Al2O3, Cr, Ni (<20 ppm), Co and Sc (<15 ppm) concentrations, precluding mafic or felsic igneous protoliths. The deposits were locally affected by contact metamorphism, giving rise to pyroxene-hornfels and granulite facies assemblages, and by regional metamorphism which produced greenschist-amphibolite grade assemblages. Disseminated sulfides commonly occur in the talc-tremolite-rich rocks (having low Al2O3 concentrations), suggesting that the metals were probably present in the original carbonate beds, but were remobilized and reconcentrated during the various metamorphic events.

Geochemical and tectonic implications of igneous rocks from ODP leg 114, sub-antarctic South Atlantic

Igneous rocks recovered from three major tectonic features of the sub-antarctic South Atlantic Ocean during ODP Leg 114 have a range of compositions: Northeast Georgia Rise, basalt to andesite; Meteor Rise, basalt to trachyte; spreading center in the West South Atlantic Basin, MORB-like basalt. Major and trace element as well as Sr and Nd isotopic data clearly indicate that all rocks share the enriched elemental and isotopic signatures typical of both hotspot and mid-ocean ridge volcanism in the southern ocean basins. Data for the Northeast Georgia and Meteor Rises also suggest the influence of older, continental lithosphere.

Sb-enriched ultramafic lamprophyre in the Hemlo Au-Mo deposit of the Superior Province, Canada: Evidence for Post-Archean Sb mobility

An Sb-enriched ultramafic lamprophyre (UML) sill was identified within the Hemlo Au-Mo deposit (David Bell mine) of the Superior Province, Canada. The low SiO 2 (31 wt.%), high TiO 2 (4.2 wt.%), Nb (146 ppm), Ta (10 ppm), Cr (900 ppm), and Ni (720 ppm) contents, as well as the unusual mineralogy (olivine, perovskite, Ti-rich phlogopite, calcite) of the sill are consistent with the geochemistry and mineralogy of other ultramafic lamprophyres on the global scale. The fine-grained, dark greenish-black sill has unchilled margins and intrudes basaltic and felsic fragmentals. The lamprophyre contains 794 ppm Sb, which is concentrated in fine-grained ullmannite, a Ni(Co)-Sb sulfide, within pervasively altered olivine. The presence of ullmannite in altered olivine within the UML implies that Sb was mobilized after the emplacement of the sill. Because Sb is considered to be one of the pathfinder elements for Au at Hemlo, and because antimony sulfides (including ullmannite) and arsenides are commonly associated with Au at the Hemlo camp, the addition of Sb to the Proterozoic lamprophyre suggests that Sb mobilization (and by inference, Au) possibly occurred throughout the tectonic evolution of the deposit. This has implications for genetic models of Hemlo, as cross-cutting relationships of Archean plutons and multi-phase deformation of the deposit make it difficult to distinguish between the actual timing of Au, Mo and Sb introduction and their subsequent re-mobilization and re-concentration into structural traps during various stages of deformation. It also re-opens questions concerning the genetic link between gold metallogeny and tectonic evolution of the region.