Geochemical and isotopic study of metasomatic apatite: Implications for gold mineralization in Xindigou, northern China

Abstract

The 0.7 Moz Xindigou gold deposit lies on the north margin of North China Craton. The orebodies are controlled by NWW-trending shear zones and hosted by an Archean metavolcanosedimentary sequence. These host schist, amphibolite and marble rocks were intruded by late Paleozoic granites and mafic dikes. Two independent magmatic-hydrothermal assemblages were identified based on petrographic evidence: a pre-ore magmatic-hydrothermal magnetite – apatite ± pyrite ± zircon assemblage and an ore-related potassium feldspar – muscovite – sulfide ± minor monazite assemblage.Apatite that has undergone later alteration shows significant intracrystalline spatial variability with respect to the concentrations of rare earth elements (REEs), Si, and Sr. Scanning electron microscope backscattered images and statistical interrogation of the compositional data acquired by LA-ICP-MS indicate that apatite grains in the pre-ore magnetite – apatite assemblage are homogeneous and enriched in REEs and Si (Ap1). Where sulfide-related alteration of mineralization stage was overlapped, these apatite grains display a distinctive texture of homogeneous cores and turbid rims. This dissolution-reprecipitation process preferentially transported elements from the original apatite. REEs and Si gradually decrease from the core (Ap1) to the altered rim (Ap2 & Ap3) whereas Sr increases. Localized leaching of REEs (especially LREEs) from apatite was followed by precipitation of secondary monazite grains in the metasomatic apatite rims and the muscovite – sulfide – quartz – phosphate sequence. In situ Sr isotope analyses of the apatite show the 87Sr/86Sr ratios varying from the lowest value, 0.707874 in unaltered Ap1, to the highest, 0.714160 in altered Ap3. Zircon grains from migmatite display a peak metamorphic age at 2512 ± 9 Ma, whereas zircons from the early magnetite-apatite association formed near 1776 ± 12 Ma, and late hydrothermal monazite yields a Th-Pb age of 301.0 ± 1.7 Ma.These geochronology results show that the mineralization occurred during the late Paleozoic orogenesis, long after Neoarchean regional metamorphism and late Paleoproterozoic rift-related magmatism. The typical hydrothermal features of orogenic gold deposit could be inferred from the fluorapatite. The elemental patterns of fluorapatite grains indicate that a carbon and Sr enriched, acid, slightly oxidized and low salinity hydrothermal system was responsible for apatite metasomatism and gold mineralization. Strontium isotopes further reveal that the fluid related to gold deposition was derived from mafic lower crust, with extensive radiogenic Sr accumulation during fluid-rock interaction. This study demonstrates the potential application of apatite in exploration for orogenic gold deposits and its use in tracking hydrothermal REE re-mobilization during metasomatism.