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

Abstract

<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>