The origin of the super-large Lijiagou spodumene pegmatite deposit in Songpan-Garze Fold Belt, West Sichuan, China

<p>The Lijiagou spodumene deposit in the central Songpan-Garze Fold Belt (SGFB), West Sichuan, is a spodumene pegmatite-hosted deposit within the giant Songpan-Garze polymetallic belt. Systematic zircon, cassiterite and coltan U-Pb dating, Hf isotope and whole-rock geochemical analysis were undertaken. Two-mica granite and muscovite albite granite have S-type granite affinities and chemical compositions suggesting a post-orogenic setting, and the spodumene pegmatites belong to the LCT (Li-Cs-Ta) group of pegmatites. Zircon LA-MC-ICP-MS U-Pb dating of the two-mica granite, muscovite albite granite, barren albite pegmatite and albite spodumene pegmatite give crystallization ages of 219.2 ± 2.3 Ma (MSWD = 0.55), 217 ± 2.8 Ma (MSWD = 0.47), 202.8 ± 4.9 Ma (MSWD = 3.9), 200.1 ± 4.6 Ma (MSWD = 3.1), respectively. Zircons from the same units yield εHf(t) values of  −39.17 to 13.84, −22.73 to −2.83, −11.17 to 8.14, and −4.92 to −2.4, respectively, consistent with mixed crustal sources for the pegmatite and granite magmas. Cassiterite from spodumene pegmatite yields a concordia intercept age of 211.4 ± 3.3 Ma (MSWD = 2.9), while coltan yields a weighted mean age of 211.6 ± 0.5 Ma (MSWD = 0.61). The U-Pb zircon ages of the two-mica granite and muscovite albite granite are interpreted as ages of magmatic crystallization. Metamictization of zircon in both barren and spodumene pegmatites makes their U-Pb zircon ages liable to inaccuracy due to Pb loss. The coltan U-Pb age is regarded as an accurate measure of the magmatic crystallization age of the spodumene pegmatite. Given the differences in magmatic ages and εHf(t) ranges between spodumene pegmatite and both the two-mica granite and the muscovite albite granite, the spodumene pegmatite probably represents an anatectic melt and not a fractionation product of either of the granitic magmas. U-Pb coltan and U-Pb cassiterite dating are more likely to provide accurate crystallization ages of spodumene pegmatites than U-Pb zircon ages. Spodumene mineral exploration in such geological environments requires consideration both of the mineralogy and geochemistry of potential metasedimentary source rocks, and of the effects of granite intrusion in creating fertile mineral assemblages.</p><p>Two key conclusions for spodumene pegmatite mineral exploration follow: 1) prior intrusion of granite may be required to generate a metasedimentary mineral assemblage that may later yield albite-spodumene pegmatite magma, and 2) a focus on the mineralogy and Li concentration of potential metasedimentary source rocks is required to identify geological environments in which albite-spodumene pegmatite magmas may have been generated.</p>

Carbon, Nitrogen and Sulphur concentration and δ13C, δ15N values in Hypogymnia physodes within the montane area – preliminary data

The contribution of C, N and S, as well as the isotopic composition of C and N of atmospheric pollutants, are assumed to be reflected in the organic compounds inbuilt into the lichen thallus. The chemical and isotopic analyses were carried out on lichen Hypogymnia physodes samples gathered from Picea abies and Larix decidua, collected in 13 sampling points located in Karkonoski National Park and its closest vicinity in 2011. The results for %C, %N and %S varied from 43.44 to 46.79%, from 0.86 to 1.85% and from 0.07 to 0.27 %, respectively. The δ13C values ranged from −26.6 to −24.6‰, whereas δ15N values varied from −13.0 to −6.8‰. The ranges in isotope composition suggest different sources of C and N for Karpacz compared to the remaining sampling sites. For Karpacz, the δ13C values suggest (in case the fractionation product-substrate does not exist and Δ=0) that the dominant sources are coal combustion processes, whereas for remaining sampling points, the δ13C values are ambiguous and are masked by many mixed natural and anthropogenic processes. With the same assumption that Δ=0, the δ15N values suggest that transport is not a dominant source of nitrogen within Karpacz city. Moreover, in this study we tested the possible fractionation (Δ) for carbon and nitrogen, assuming that within the investigated area, the source of carbon is probably CO2 and/or DIC (HCO3−) dissolved in precipitation, while the source of nitrogen is NOx and/or NO3− ion. The calculated fractionation factors were: (i) for gaseous carbon compounds ΔCO2-Corg value from −13.4 to −11.4‰, whereas for the ions form ΔHCO3−-Corg value from −16.6 to −14.6‰, (ii) for nitrogen gaseous compounds ΔNOx-Norg value between apx. −17 and −5‰, whereas for the ions form ΔNO3−-Norg value between −9.9 and −3.7‰.

An unusual sapphire–zircon–magnetite xenolith from the Chanthaburi Gem Province, Thailand

A sapphire, zircon and magnetite-bearing xenolith from Khao Wua, near Chanthaburi in Thailand, conclusively demonstrates a common origin for the sapphire, zircon and magnetite found in alluvial deposits in the Chanthaburi gem fields. The original aluminium- and titanium-rich octahedral magnetite crystal in the xenolith exsolved into hercynite, magnetite and hematite during cooling. It includes minor anhedral jarosite–alunite, possibly originating as an iron-sulphide-rich immiscible liquid. Uranium-lead isotope dating of zircon in the xenolith gives an age of 1–2 (± <1) million years (Ma). This falls within fission track ages for alluvial zircons (2.57 ± 0.20 Ma) from the Chanthaburi—Trat gem fields and within the potassium-argon ages of 0.44 to 3.0 Ma for the alkali basaltic volcanism in the Chanthaburi Province. These data suggest a common origin for sapphire, zircon and magnetite, and link them with the processes involved in alkali basaltic magma generation. The high iron and zirconium, low magnesium, and the inferred sulphides suggest pegmatite-like crystallization in an incompatible-element enriched, silica-poor magma (partial melt or fractionation product) in the deep crust or upper mantle. Etch features on exposed surfaces of the xenolith indicate that it was transported out of its equilibrium environment by the rise of later magma.