12 It is widely believed that atmospheric oxygen saturation rose from <10 -5 Present 13Atmospheric Level (PAL) in the Archean to >10 -2 PAL at the Great Oxidation Event 14

Zircon (ZrSiO 4 ) is the most commonly used mineral in U-Pb geochronology. Although it has proven to be a robust chronometer, it can suffer from Pb-loss or elevated common Pb, both of which impede precision and accuracy of age determinations. Chemical abrasion of zircon involves thermal annealing followed by relatively low temperature partial dissolution in HF acid. It was specifically developed to minimize or eliminate the effects of Pb-loss prior to analysis using Thermal Ionization Mass Spectrometry (TIMS). Here we test the application of chemical abrasion to Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) by analyzing zircons from both untreated and chemically abraded samples. Rates of ablation for high alpha-dose non-treated zircons are up to 25% faster than chemically abraded equivalents. Ablation of 91500 zircon reference material demonstrates a ca. 3% greater down-hole fractionation of 206 Pb/ 238 U for non-treated zircons. These disparities necessitate using chemical abrasion for both primary reference material and unknowns to avoid applying an incorrect laser induced fractionation correction. All treated samples display a marked increase in the degree of concordance and/or lowering of common Pb, thereby illustrating the effectiveness of chemical abrasion to LA-ICP-MS U-Pb zircon geochronology. OPEN ACCESSMinerals 2014, 4 504

The Tasiast gold deposits are hosted within Mesoarchean rocks of the Aouéouat greenstone belt, Mauritania. The Tasiast Mine consists of two deposits hosted within distinctly different rock types, both situated within the hanging wall of the west-vergent Tasiast thrust. The Piment deposits are hosted within metasedimentary rocks including metaturbidites and banded iron formation where the main mineral association consists of magnetite-quartz-pyrrhotite ± actinolite ± garnet ± biotite. Gold is associated with silica flooding and sulphide replacement of magnetite in the turbidites and in the banded iron formation units. The West Branch deposit is hosted within meta-igneous rocks, mainly diorites and quartz diorites that lie stratigraphically below host rocks of the Piment deposits. Most of the gold mineralisation at West Branch is hosted by quartz-carbonate veins within the sheared and hydrothermally altered meta-diorites that constitute the Greenschist Zone. At Tasiast, gold mineralisation has been defined over a strike length >10 km and to vertical depths of 740 m. All of the significant mineralised bodies defined to date dip moderately to steeply (45º to 70º) to the east and have a south-southeasterly plunge. Gold deposits on the Tasiast trend are associated with second order shear zones that are splays cutting the hanging wall block of the Tasiast thrust. An age of 2839 ± 36 Ma obtained from the hydrothermal overgrowth on zircons from a quartz vein is interpreted to represent the age of mineralisation.

The Kolhan Group in the Singhbhum Craton of eastern India is a passive margin siliciclastic shelf to deep‐water carbonate ramp platform succession. It occupies a unique position in respect of the Central Indian Tectonic Zone (CITZ), widely considered as the suture joining the northern and southern Indian cratonic blocks at the time of amalgamation of the Mesoproterozoic supercontinent Columbia. The Kolhan succession is closely comparable to other Late Proterozoic ‘Purana’ successions of India including the siliciclastic‐carbonate Suasar Group from the central part of the CITZ. We report here the first U–Pb detrital zircon ages from the Kolhan Group. The youngest age population obtained from basal siliciclastics suggests it is not older than ca. 1,158 Ma. The absence of any concordant detrital zircon <1,000 Ma which is well represented in the adjoining Singhbhum Shear Zone and CITZ extensions, suggests that the sedimentation took place prior to ca. 1,000 Ma. The Kolhan Group, therefore, records an extensional event between two major episodes of supercontinent assembly (i.e., Columbia and Rodinia). A similar motif of extension and passive margin sedimentation across Mesoproterozoic Peninsular India together with the widespread Mesoproterozoic lamproite and kimberlite emplacement event suggests an episode of extension and major basin development across the Indian Shield in between the two supercontinent amalgamation cycles of Columbia and Rodinia.

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