http://www.sciencedirect.com/science/article/pii/S1674987111001113

The Rheic Ocean was one of the most important oceans of the Paleozoic Era.It lay between Laurentia and Gondwana from the Early Ordovician and closed to produce the vast Ouachita-Alleghanian -Variscan orogen during the assembly of Pangea.Rifting began in the Cambrian as a continuation of Neoproterozoic orogenic activity and the ocean opened in the Early Ordovician with the separation of several Neoproterozoic arc terranes from the continental margin of northern Gondwana along the line of a former suture.The rapid rate of ocean opening suggests it was driven by slab pull in the outboard lapetus Ocean.The ocean reached its greatest width with the closure of lapetus and the accretion of the periGondwanan arc terranes to Laurentia in the Silurian.Ocean closure began in the Devonian and continued through the Mississippian as Gondwana sutured to Laurussia to form Pangea.The ocean consequently plays a dominant role in the Appalachian-Ouachita orogeny of North America,in the basement geology of southern Europe,and in the Paleozoic sedimentary,structural and tectonothermal record from Middle America to the Middle East.Its closure brought the Paleozoic Era to an end.     

‘Invisible gold’ in bismuth chalcogenides

Gold concentrations have been determined by LA-ICPMS in bismuth chalcogenides (tellurides and sulfosalts, minerals with modular structures; chalcogen X = Te, Se, and S) from 27 occurrences. Deposit types include epithermal, skarn, intrusion-related and orogenic gold. The samples comprised minerals of the tetradymite group, aleksite series, bismuth sulfosalts (cosalite, lillianite, hodrushite, bismuthinite, and aikinite), and accompanying altaite. Gold concentrations in phases of the tetradymite group range from <0.1 to 2527 ppm. Phases in which Bi > X tend to contain lower gold concentrations than Bi₂X₃ minerals (tellurobismuthite and tetradymite). Cosalite and lillianite contain Au concentrations ranging up to 574 and 3115 ppm, respectively. Bismuthinite derivatives have lower Au concentrations: <2 ppm in bismuthinite and up to 542 ppm in aikinite. In our samples, Au concentrations in altaite range from <0.2 to 1662 ppm.Smoother parts of the LA-ICPMS profiles suggest lattice-bound gold, whereas irregularities on the profiles are best explained by the presence of gold particles (?1 μm in diameter). Plotting Au vs. Ag for the entire dataset gives a wedge-shaped distribution, suggesting that Ag underpins Au uptake in both bismuth tellurides and sulfosalts. In the tellurides, correlation trends suggest statistical substitution of Ag(Au), together with Pb, into the octahedral site in the layers. In sulfosalts, Au follows coupled substitutions in which M¹⁺ (Ag, Cu) enters the structure. In tellurides, the presence of van der Waals gaps at chalcogen-chalcogen contacts provides for p-type semi-conductive properties critical for gold scavenging from fluids. Such weak bonds may also act as sites for nucleation of Au (nano)particles. In sulfosalts, contacts between different species that replace one another are also highly predictable to act as traps for (nano)particulate gold.Invisible gold in Bi-chalcogenides is useful to (i) identify trends of orefield zonation, (ii) discriminate between ‘melt’ and ‘fluid-driven’ scavenging, and (iii) interpret replacement and remobilisation processes. Bismuth chalcogenides have the potential to be significant Au carriers in sulfide-poor Au systems, e.g., intrusion-related gold, with impact on the overall Au budget if mean Au concentrations are high enough and the minerals are sufficiently abundant.     

Trace and minor elements in sphalerite from metamorphosed sulphide deposits

Sphalerite is a common sulphide and is the dominant ore mineral in Zn-Pb sulphide deposits. Precise determination of minor and trace element concentrations in sulphides, including sphalerite, by Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS) is a potentially valuable petrogenetic tool. In this study, LA-ICP-MS is used to analyse 19 sphalerite samples from metamorphosed, sphalerite-bearing volcanic-associated and sedimentary exhalative massive sulphide deposits in Norway and Australia. The distributions of Mn, Fe, Co, Cu, Ga, Se, Ag, Cd, In, Sn, Sb, Hg, Tl, Pb and Bi are addressed with emphasis on how concentrations of these elements vary with metamorphic grade of the deposit and the extent of sulphide recrystallization. Results show that the concentrations of a group of trace elements which are believed to be present in sphalerite as micro- to nano-scale inclusions (Pb, Bi, and to some degree Cu and Ag) diminish with increasing metamorphic grade. This is interpreted as due to release of these elements during sphalerite recrystallization and subsequent remobilization to form discrete minerals elsewhere. The concentrations of lattice-bound elements (Mn, Fe, Cd, In and Hg) show no correlation with metamorphic grade. Primary metal sources, physico-chemical conditions during initial deposition, and element partitioning between sphalerite and co-existing sulphides are dominant in defining the concentrations of these elements and they appear to be readily re-incorporated into recrystallized sphalerite, offering potential insights into ore genesis. Given that sphalerite accommodates a variety of trace elements that can be precisely determined by contemporary microanalytical techniques, the mineral has considerable potential as a geothermometer, providing that element partitioning between sphalerite and coexisting minerals (galena, chalcopyrite etc.) can be quantified in samples for which the crystallization temperature can be independently constrained.     

Do S-type granites commonly sample infracrustal sources? New results from an integrated O, U–Pb and Hf isotope study of zircon

In contrast to I-type granites, which commonly comprise infracrustal and supracrustal sources, S-type granites typically incorporate predominantly supracrustal sources. The initial aim of this study was to identify the sources of three Scottish Caledonian (~460 Ma) S-type granites (Kemnay, Cove and Nigg Bay) by conducting oxygen, U–Pb and Hf isotope analyses in zircon in order to characterise one potential end-member magma involved in the genesis of the voluminous late Caledonian (~430–400 Ma) I-type granites. Field, whole-rock geochemical and isotopic data are consistent with the generation of the S-type granites by melting their Dalradian Supergroup country rocks. While Hf isotope compositions of magmatic zircon, U–Pb data of inherited zircons, and high mean zircon δ¹⁸O values of 9.0 ± 2.7‰ (2SD) and 9.8 ± 2.0‰ for the Kemnay and Cove granites support this model, the Nigg Bay Granite contains zircons with much lower δ¹⁸O values (6.8 ± 2.1‰), similar to those found in Scottish I-type granites. This suggests that the Nigg Bay Granite contains low-δ¹⁸O material representing either altered supracrustal material, or more likely, an infracrustal source component with mantle-like δ¹⁸O. Mixing trends in plots of δ¹⁸O vs. εHf for S-type granite zircons indicate involvement of at least two sources in all three granites. This pilot study of Scottish Caledonian S-type granites demonstrates that, while field and whole-rock geochemical data are consistent with local melting of only supracrustal sources, the oxygen isotopic record stored in zircon reveals a much more complex petrogenetic evolution involving two or more magma sources.