Dating iron skarn mineralization using hydrothermal allanite-(La) U–Th–Pb isotopes by laser ablation ICP-MS

Abstract

Trace elements and U–Th–Pb isotopes of hydrothermal allanite from the Beiminghe iron skarn deposit (eastern North China Craton) were simultaneously analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to demonstrate the suitability and robustness of the U–Th–Pb system in this phase as a geochronometer of ore-forming processes. Allanite grains from the Beiminghe deposit, typically exhibit optical and chemical zoning and are intergrown or texturally equilibrated with prograde and/or retrograde skarn assemblages mainly consisting of tremolite, diopside, magnetite, and garnet. In addition, they contain abundant aqueous two-phase or daughter mineral-bearing three-phase aqueous fluid inclusions, confirming their hydrothermal origin. Most grains have high La/Ce ratios (1.19–1.74) and thus are classified as allanite-(La). A notable feature of the hydrothermal allanite-(La) is the lack of common Pb, with values comparable with those of the zircon standards 91500 and GJ-1 used in this study. LA-ICPMS spot analyses reveal remarkable variation in Ca, REEs (rare earth elements), Fe, Al, and Mg within a single grain, consistent with the substitution of REE3++Mg+Fe2+ for Ca+Al+Fe3+ in the mineral structure. Traverses of zoned grains show that the rims have Th/U ratios ranging from 0.73 to 11.55, significantly higher than the core areas (0.07–1.81). Fifty-nine spot analyses (beam size of 32μm or 60μm) on 5 allanite-(La) grains yield reproducible 206Pb/238U ages of 134±7 to 139±6Ma, with a weighted mean 206Pb*/238U age (206Pbexcess-corrected 206Pb/238U age) of 136±1Ma (2σ). Of these, 34 analyses on allanite rims have a weighted mean 208Pb/232Th age of 139±2Ma (2σ) that is within error of the 206Pb*/238U age. The dating results showed that an increase of beam size from 32 to 60μm significantly reduces the analytical uncertainties (2σ). Petrographic and textural data have demonstrated that the allanite-(La) grains formed during skarn alteration and notably have close paragenetic relationships with retrograde skarn assemblages and iron oxide minerals; their U–Th–Pb ages therefore are interpreted as the timing of iron skarn mineralization at Beiminghe. Thirteen zircon grains from the ore-related diorite intrusion have a weighted mean 206Pb/238U age of 136±2Ma (2σ), consistent with and thus confirming the reliability of the allanite-(La) U–Th–Pb ages when interpreted as the formation age of the Beiminghe iron deposit. Our results suggest that hydrothermal allanite with low Th and U is a promising U–Th–Pb geochronometer and can provide reliable constraints on the timing of hydrothermal mineralization without requiring a matrix-matched external standard. The present dating results also indicate that the U–Pb, rather than Th–Pb, isotope system is preferable for U–Th–Pb dating of low Th/U allanite.