U-Pb, Re-Os, and Ar/Ar Geochronology of Rare Earth Element (REE)-Rich Breccia Pipes and Associated Host Rocks from the Mesoproterozoic Pea Ridge Fe-REE-Au Deposit, St. Francois Mountains, Missouri

Rare earth element (REE)-rich breccia pipes (600,000 t @ 12% REO) are preserved along the margins of the 136 Mt Pea Ridge magnetite-apatite deposit, within Mesoproterozoic (~1.47 Ga) volcanic-plutonic rocks of the St. Francois Mountains terrane in southeastern Missouri, USA. The breccia pipes cut the rhyolite-hosted magnetite deposit, and contain clasts of nearly all local bedrock and mineralized lithologies. Grains of monazite and xenotime were extracted from breccia pipe samples for SHRIMP U-Pb geochronology; both minerals were also dated in one polished thin section. Monazite forms two morphologies: (1) matrix granular grains composed of numerous small (<50 μm) crystallites intergrown with rare xenotime, thorite, apatite, and magnetite; and (2) coarse euhedral, glassy, bright yellow grains similar to typical igneous or metamorphic monazite. Trace element abundances (including REE patterns) were determined on selected grains of monazite (both morphologies) and xenotime. Zircon grains from two samples of host rhyolite and two late felsic dikes collected underground at Pea Ridge were also dated. Additional geochronology done on breccia pipe minerals includes Re-Os on fine-grained molybdenite and 40Ar/39Ar on muscovite, biotite, and Kfeldspar. Ages (± 2-sigma errors) obtained by SHRIMP U-Pb analysis are as follows: (1) zircon from the two host rhyolite samples have ages of 1473.6 ± 8.0 and 1472.7 ± 5.6 Ma; most zircon in late felsic dikes is interpreted as xenocrystic (age range ca. 1522-1455 Ma); a population of rare spongy zircon is likely of igneous origin and yields an age of 1441 ± 9 Ma; (2) pale yellow granular monazite—1464.9 ± 3.3 Ma (no dated xenotime); (3) reddish matrix granular monazite—1462.0 ± 3.5 Ma and associated xenotime—1453 ± 11 Ma; (4) coarse glassy yellow monazite—1464.8 ± 2.1, 1461.7 ± 3.7 Ma, with rims at 1447.2 ± 4.7 Ma; and (5) matrix monazite (in situ) —1464.1 ± 3.6 and 1454.6 ± 9.6 Ma, and matrix xenotime (in situ) —1468.0 ± 8.0 Ma. Two slightly older ages of cores are about 1478 Ma. The young age of rims on the coarse glassy monazite coincides with a Re-Os age of 1440.6 ± 9.2 Ma determined in this study for molybdenite intergrown with quartz and allanite, and with the age of monazite inclusions in apatite from the magnetite ore (Neymark et al., this volume). A 40Ar/39Ar age of 1473 ± 1 Ma was obtained for muscovite from a breccia pipe sample. Geochronology and trace element geochemical data suggest that the granular matrix monazite and xenotime (in polygonal texture), and cores of coarse glassy monazite precipitated from hydrothermal fluids during breccia pipes formation. The second episode of mineral growth at ca. 1443 Ma may be related to faulting and fluid flow that rebrecciated the pipes. The ca. 10 m.y. gap between the ages of host volcanic rocks and breccia pipe monazite and xenotime suggests that breccia pipe mineral formation cannot be related to the felsic magmatism represented by the rhyolitic volcanic rocks, and hence is linked to a different magmatic-hydrothermal system

A systematic review of how emotional self-awareness is defined and measured when comparing autistic and non-autistic groups

We would like to sincerely thank all the authors who shared their data with us. We would also like to thank Ira Lesser, Taylor Graeme, and Arvid Heiberg for kindly sharing their articles for the historical review. Review was conduced as part of CFH's PhD studies. We would like to thank the Northwood Trust, UK for their financial support for this research. Research data available upon request from first author.Peer reviewedPublisher PD

Constraints on the origins of fluids forming Irish Zn-Pb-Ba deposits: Evidence from the composition of fluid inclusions

The numerous Zn-Pb deposits in the Irish midlands, together with the quantity and grade of the ore, make this a world-class base metal province. Despite significant exploration and research, there is still disagreement on the origin of the mineralizing fluids. In this study, we have measured the composition of fluid inclusions using a crush-leach technique. These data constrain the possible sources of the fluids both during and after mineralization. Chloride and Br concentrations indicate that the main ore fluid at Tynagh and Silvermines was seawater that had evaporated until the salinity was between 12 and 18 wt percent. However, Na, K, and Li data show that water- rock interactions have resulted in the depletion of Na and the enrichment of K and Li relative to the concentrations expected for evaporated seawater. The fluids that produced postore dolomitization were also derived from seawater that had evaporated to higher salinities and had precipitated halite. The concentrations of Na, K, and Li are what would be expected for seawater at this degree of evaporation, and show that these fluids did not interact with the same lithologies as the ore fluids. The major change to their fluid composition was exchange of Mg for Ca during dolomitization. We suggest that the ore fluids could only have reached their high salinity by evaporation of seawater on the extensive shallow water shelf regions that existed over much of the Irish midlands

Aspects of the geochemistry of zinc: a journey to sphalerite

Our journey to sphalerite begins with a review of the occurrence and behavior of Zn in a wide spectrum of natural fluids. Measured Zn concentrations in crustal fluids vary over at least six orders of magnitude, from around 0.01 to 20,000 ppm (2 wt %). Whist water-rock interaction and boiling influence Zn concentration, the origin of the fluids and in particular their temperature, pH and ligand concentrations are major factors in their capacity to transport Zn. However, in contrast to previous reviews, our compilation shows no significant overall correlation between Zn and Cl concentrations. Fluid chemistry (mainly mCl-, mΣS, pH and fO2) and temperature, and the resultant speciation determine the solubility of Zn and control precipitation of sphalerite. Bisulphide complexes only dominate under relatively high ΣS, low Cl-, and high pH, and are favoured by low temperatures, in most natural systems though chloride complexes dominate. The limited oxidation state of Zn leads to a narrow range of Zn isotope variation in ore systems, with a mean of all published ore-related δ66Zn data around 0.2 ± 0.3 ‰. Nonetheless, kinetic fractionation of Zn isotopes leads to enrichment in 66Zn in residual fluids, following precipitation of sphalerite from hydrothermal fluids. This has resulted, in a wide variety of systems, in a potentially useful exploration vector to feeder zones. We describe the basic crystallography of sphalerite, and show that commonly occurring layered sphalerite, in particular the colloform texture, has the ability to record the extraordinary dynamism of the ore depositional environment through careful petrographic studies followed by in situ laser S isotope and EMPA trace element analyses. Our journey ends in the Galmoy deposit in the Irish orefield, where we show that three distinct ore sulphide-precipitating events and the full isotopic spectrum of the dual ore sulphide source of the Irish-type deposits are recorded across a 2 cm colloform specimen

Origin of mineralising fluids in Irish-type deposits: Constraints from halogens

No abstract available