Ore genesis of the Narenwula quartz-vein type W polymetallic deposit in the southern Great Xing’an Range W belt, NE China: Constraints from wolframite geochronology and individual fluid inclusion analysis

Abstract

The Narenwula deposit, situated in the southern Great Xing’an Range W belt, is a large-scale quartz-vein type W polymetallic deposit in NE China. The mineralization is spatially associated with Late Jurassic monzogranite and granite porphyry. In this study, we present detailed description of the ore geology, wolframite in-situ U-Pb dating, and fluid inclusion analyses including microthermometry, laser Raman spectra, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) microanalyses to precisely constrain the timing of ore formation, origin and evolution of ore-forming fluids as well as metal precipitation mechanisms. In-situ U-Pb dating of hydrothermal wolframite yields a W mineralization age of 136.4 ± 2.0 Ma (1σ, MSWD = 1.3), which is ∼14 Ma younger than the zircon U-Pb ages of W-bearing granitoids, indicating the W-bearing granitoids are not genetically related to the W mineralization at Narenwula. The hydrothermal process can be divided into four stages: stage I is characterized by abundant wolframite (I), which is the dominant stage of W mineralization; stage II is featured with the development of wolframite (II) together with minor amounts of pyrite and chalcopyrite; stage III is the predominant stage of sulfide mineralization; and stage Ⅳ is characterized by lack of sulfide or wolframite and occurrence of quartz, carbonate, and fluorite. Cathodoluminescence analysis of quartz further revealed four generations of quartz, corresponding to the four hydrothermal stages. Four types of fluid inclusions were distinguished for quartz, including CO2-rich (C1-type), CO2-bearing (C2-type), liquid-rich (l-type), and brine (B-type) fluid inclusions. Fluid inclusion microthermometry shows a decrease in homogenization temperatures and salinities from the early to late stages. The ranges of the homogenization temperature for stages I to Ⅳ are 440–280, 320–240, 260–180, and 200–160 ℃, respectively, and the ranges of salinities for stages II to Ⅳ are 2.9–17.8, 1.4–10.2, and 0.4–6.7 wt% NaCl equiv., respectively. The salinities of fluid inclusions from stage I have a bimodal distribution in the ranges of 9.6–17.4 and 31.1–35.6 wt% NaCl equiv. Individual fluid inclusion LA-ICP-MS analyses suggest a magmatic source for the fluids of all stages, evidenced by the high Rb and Mn concentrations, high Rb/Na, K/Na, Cs/Na, Li/Na, and Zn/Na ratios, which are distinctly different from those of basinal brines. The Rb/Sr and K/Rb ratios of fluid inclusions are similar to those of Late Mesozoic highly fractionated W-mineralized granitoids in NE China, indicating that the metallogenic parent rocks at Narenwula are highly evolved. In combination with evidence of consistent Cs/Rb and Cs/(Na + K) ratios of fluid inclusions from all stages, we suggest that the mineralizing fluids originated from an underlying, geochemically uniform, and highly fractionated granitic magma. Fluid immiscibility and fluid-rock interaction (especially greisenization) are the main mechanisms for wolframite precipitation, whereas natural cooling of fluids may be subordinate. Fluid inclusion data provide evidence that the addition of meteoric water to the magmatic-hydrothermal fluid initiated at the sulfide mineralization stage, and the fluid dilution and cooling due to fluid mixing played an important role in the process of sulfide precipitation. This study highlights that W and base metals (e.g., Cu, Pb, Zn) in ore-forming fluids display different migration and precipitation progresses, the knowledge of which is important for elucidating multi-stage ore-forming processes of quartz-vein type W polymetallic deposits in NE China.