Simon Tapster

Duration and nature of the end-Cryogenian (Marinoan) glaciation

The end-Cryogenian glaciation (Marinoan) is portrayed commonly as the archetype of snowball Earth, yet its duration and character remain uncertain. Here we report U-Pb zircon ages for two ash beds from widely separated localities of the Marinoan-equivalent Ghaub Formation in Namibia: 639.29 ± 0.26 Ma and 635.21 ± 0.59 Ma. These findings verify, for the first time, the key prediction of the snowball Earth hypothesis for the Marinoan glaciation, i.e., longevity, with a duration of ≥4 m.y. They also show that the nonglacial interlude of Cryogenian time spanned 20 m.y. or less and that glacigenic erosion and sedimentation, and at least intermittent open-water conditions, occurred 4 m.y. prior to termination of the Marinoan glaciation.

From continent to intra-oceanic arc: Zircon xenocrysts record the crustal evolution of the Solomon island arc

The first U-Pb ages from a ca. 26–24 Ma pluton on Guadalcanal, in the intra-oceanic Solomon island arc (southwest Pacific Ocean), reveal Eocene- to Archean-aged zircon xenocrysts. Xenocryst populations at ca. 39–33 Ma and ca. 71–63 Ma correlate with previously obtained ages of supra-subduction magmatism within the arc. A ca. 96 Ma zircon population may be derived from Cretaceous ophiolite basement crust or region-wide continental rift-related magmatism. Xenocryst age populations alternate with periods of oceanic basin formation that fragmented the East Gondwana margin. Early Cretaceous to Archean zircon xenocryst ages imply continental origins and a cryptic source within the arc crust; they may have been introduced by Eocene interaction of a continental fragment with the arc, and concealed by ophiolite obduction. The data demonstrate that continentally derived zircons may be transported thousands of kilometers from their source and added to intra-oceanic arc magmas, a process likely facilitated by cyclical subduction zone advance and retreat. The findings highlight the continuum of arcs that occurs between continental and oceanic end members, and the caution with which zircons should be used to determine the provenance and setting of ancient arc terranes accreted to the continental crust.

Geology of the Don Manuel igneous complex, central Chile: Implications for igneous processes in porphyry copper systems

The Don Manuel igneous complex and associated porphyry copper mineralization in the Andean Cordillera of central Chile demonstrate similarities between intrusive complexes associated with porphyry copper deposits and arc volcanoes that generate porphyritic volcanics. The Don Manuel igneous complex intrusions progressed from quartz monzonite through rhyolite and biotite tonalite to intermediate porphyritic and basaltic andesite dikes, which intrude the older units. Mineralization is associated with the biotite tonalite and intermediate porphyries, which also contain the greatest abundance of mafic enclaves. Zoning patterns within plagioclase phenocrysts suggest that the later intermediate porphyries comprise a hybridized suite formed by magma mixing. New zircon U-Pb ages and whole-rock Ar-Ar ages indicate that the Don Manuel igneous complex was emplaced between ca. 4 and 3.6 Ma. The time scale for the episodic intrusion of the Don Manuel igneous complex units is similar to observed episodicity of eruption and degassing events in active arc volcanoes. Observations from the Don Manuel igneous complex are consistent with the close spatial and temporal association of mineralization with episodic intrusion and interaction between silicic and mafic magmas during emplacement. The observations are also consistent with the hypothesis that mafic magma provides a source of sulfur for porphyry copper deposit formation.

Steps to developing iron-oxide U-Pb geochronology for robust temporal insights into IOCG and BIF mineralisation

distinct zones of Cu and Pb–Zn mineralisation. The textural and structural relationships associated with the mineralised lodes suggests that they formed through oblique-slip faulting within a transtensional regime. The geochemical and economic significance of the Copper Coast mineralisation is not yet resolved, although the structural and petrographic evidence, revealed by this study, indicate that the system may represent remobilization from a proximal VMS deposit (Figures 1–3).

Petrogenesis and Assembly of the Don Manuel Igneous Complex, Miocene–Pliocene Porphyry Copper Belt, Central Chile

The 4·0–3·6 Ma Don Manuel igneous complex (DMIC), central Chile, provides a window into igneous processes involved in magma genesis associated with porphyry-style copper mineralization. This study uses petrographic, petrological, geochemical and isotopic data to examine the evolution of magmas from the mid- to lower-crustal source region to shallow emplacement. The data provide evidence for progressive oxidation of magma during differentiation and ascent, fractionation of Cl from S through degassing, and the late-stage, near-solidus removal of Cl from the system. Magmas of basaltic andesite to rhyolite composition were produced by polybaric differentiation of hydrous parental mafic magmas. Variations in crustal differentiation depths led to variable suppression of plagioclase saturation that is recorded in distinctive strontium versus anorthite evolution patterns. Hydrous, derivative magmas generated over a wide range of pressures were episodically emplaced into the shallow crust at depths between 3·5 and 5 km. Intermediate porphyry dikes closely associated with copper mineralization contain diverse crystal cargoes indicating significant magma mixing. These crystal cargoes represent samples of crystal mush entrained from different depths, as well as crystals originating in different magmas and crystals grown in situ from hybridized magmas. Mafic enclaves containing plagioclase and amphibole compositions that match those of the basaltic andesites occur within biotite tonalite, testifying to magma mingling during ascent. Sulfur and chlorine contents of apatite within the different DMIC units record variable degassing and decoupling of volatile components with sulfur showing variations of three orders of magnitude compared with one order of magnitude for chlorine. The hypabyssal nature of the DMIC affords a detailed, integrated record of magmatic differentiation processes occurring within trans-crustal magmatic systems of the sort thought to characterize many crustal arc settings and play a fundamental role in driving porphyry-style copper mineralization.

GZ7 and GZ8 - Two Zircon Reference Materials for SIMS U-Pb Geochronology

Here, we document a detailed characterisation of two zircon gemstones, GZ7 and GZ8. Both stones had the same mass at 19.2 carats (3.84 g) each; both came from placer deposits in the Ratnapura district, Sri Lanka. The U‐Pb data are in both cases concordant within the uncertainties of decay constants and yield weighted mean 206Pb/238U ages (95% confidence uncertainty) of 530.26 Ma ± 0.05 Ma (GZ7) and 543.92 Ma ± 0.06 Ma (GZ8). Neither GZ7 nor GZ8 have been subjected to any gem enhancement by heating. Structure‐related parameters correspond well with the calculated alpha doses of 1.48 × 1018 g−1 (GZ7) and 2.53 × 1018 g−1 (GZ8), respectively, and the (U‐Th)/He ages of 438 Ma ± 3 Ma (2s) for GZ7 and 426 Ma ± 9 Ma (2s) for GZ8 are typical of unheated zircon from Sri Lanka. The mean U mass fractions are 680 μg g−1 (GZ7) and 1305 μg g−1 (GZ8). The two zircon samples are proposed as reference materials for SIMS (secondary ion mass spectrometry) U‐Pb geochronology. In addition, GZ7 (Ti mass fractions 25.08 μg g−1 ± 0.18 μg g−1; 95% confidence uncertainty) may prove useful as reference material for Ti‐in‐zircon temperature estimates.

Textural, spatial and temporal variation within the igneous suites of the Spence porphyry copper deposit, northern Chile

trations of major elements, with finer zoning visible in CL as a result of variations in REE content. The compositions of the major concentric zones suggest paired substitution of cations (e.g. 2Ca=Na+REE) linked to halogen variations. There is difficulty in obtaining accurate halogen concentrations in apatite due to the effects of migration parallel to its c-axis during electron beam analysis; however, preliminary studies like this will be used to develop methods, standards and quality checks for later analyses. References

Age models for mineral exploration: effective strategy for end-user geochronology

fluid inclusion data. Interpreting deposit evolution also requires a conceptual understanding of the genesis and construction of the SW England batholith. We have extrapolated a new high-resolution zircon U–Pb and Lu–Hf regional model for granite genesis to illustrate how, over the lifetime of crustal melting, the thermal evolution of melt extraction zones, potential role of mantle-derived magmas and incremental assembly of upper crustal plutons might influence the nature of associated ore deposits. We shed light on the timing and origins of magmas associated with other W deposits (e.g. Cligga Head) and major boron-rich fluid exsolution events in the region (e.g. Wheal Remfry). However, the fundamental question of which magmatic event generatedW mineralisation at Hemerdon remains. This is currently being addressed by dating individual vein sets at Hemerdon using high-precision U–Pb geochronology of hydrothermal monazite to constrain the timing of ore formation relative to local granite emplacement.

Age and geochemistry of the Charlestown Group, Ireland: implications for the Grampian orogeny, its mineral potential and the Ordovician timescale

a LODE (London Centre for Ore Deposits and Exploration) Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK b iCRAG (Irish Centre for Research in Applied Geosciences) and School of Earth Sciences, University College Dublin, Belfield, Dublin 4, Ireland c Geological Survey Ireland, Beggars Bush, Haddington Road, Dublin 4, Ireland d Geological Survey of Northern Ireland, Dundonald House, Upper Newtownards Road, Belfast BT4 3SB, UK e Imperial College, Exhibition Road, London SW7 2AZ, UK f School of Ocean and Earth Science, National Oceanography Centre, European Way, Southampton University, Southampton SO14 3ZH, UK g NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK h Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK i Core Research Laboratories, The Natural History Museum, London SW7 5BD, UK

High-resolution temporal evidence for cyclic magmatic-hydrothermal evolution of the world class Qulong porphyry Cu–Mo system, Tibet

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