Monazite U-Th-Pb and sericite Rb-Sr dating of the Xiajiadian black shale-hosted gold deposit in the Qinling orogen, central China: Implications for regional gold metallogeny

Abstract

The Qinling orogen in central China contains several black shale-hosted gold deposits, but the age of their formation and a possible relation between gold mineralization and regional tectonism remain undetermined. Here we present results of in situ monazite U-Th-Pb dating and sericite Rb-Sr dating of the Xiajiadian black shale-hosted gold deposit to provide tight constraints on the time of gold deposition and information on the tectonic setting under which the deposit formed. The Xiajiadian gold deposit is mainly hosted in black shales of the Lower Cambrian Shuigoukou formation, with minor ores contained in sandy conglomerate of the Lower Devonian Xichahe formation. Gold ores in the black shales have been intensively oxidized and consist mainly of hematite, goethite, gypsum, quartz, calcite, native gold, and unrecognized iron oxides. However, gold ores in the sandy conglomerate are free of oxidation and thus provide an opportunity to date the formation of the Xiajiadian gold deposit. The ores consist of pyrite, marcasite, sericite, and quartz, which are associated with a variety of accessory minerals, including monazite, apatite, and titanite. Gold is mainly present as structure-bound ions or nanoparticles in pyrite and marcasite, but minor native gold grains also occur as inclusions in pyrite. Monazite is well developed in the ores, and has close textural association with Au-bearing pyrite and marcasite. Most monazite grains have zoned textures consisting of a core area (Mnz1) and an overgrowth zone (Mnz2). Mnz1 contains 4.8–13 wt.% ThO₂ and shows large variations in HREE and prominent negative Eu anomalies in the chondrite-normalized REE patterns. It yields common lead-corrected ²⁰⁸Pb/²³²Th dates ranging from 910±10 to 416±4 Ma (2σ). These dates are interpreted in terms of a detrital origin for monazite in the core area. Mnz2 has much lower ThO₂ contents of 0.8–1.7 wt.% and shows weak negative Eu anomalies in the chondrite-normalized REE patterns. It yields common lead-corrected ²⁰⁸Pb/²³²Th dates of 229–209 Ma (2σ) with a weighted mean of 217.6±3.0 Ma (2σ), which is considered as the time of Mnz2 crystallization. Sericite in gold ores and associated alteration zones has close relations to Au-bearing pyrite and marcasite, and yields a well-defined Rb-Sr isochron age of 222.2±3.3 Ma (2σ). The sericite Rb-Sr age is indistinguishable within analytical errors from the weighted mean Th-Pb age of Mnz2. The age data presented here indicate that the Xiajiadian gold deposit formed at ca. 220 Ma in the Late Triassic (Norian). We therefore suggest that gold mineralization at Xiajiadian occurred during the transitional stage from oceanic subduction to continental collision that led to the formation of the Qinling orogen. During this stage, metamorphic devolatization of the underlying Early Paleozoic to Proterozoic carbonaceous sequences likely supplied ore fluids from which the Xiajiadian gold deposit formed.