U-Pb Geochronology and Geochemistry of U-Rich Garnet from the Giant Beiya Gold-Polymetallic Deposit in SW China: Constraints on Skarn Mineralization Process

Abstract

The giant Beiya Au skarn deposit (over 300 tonnes Au metal reserve) is located in the middle part of the Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt. The deposit is the largest Au skarn deposit and third largest Au deposit in China. In this paper, we present garnet U-Pb ages and trace element geochemical data from two types of skarn-related U-rich garnet from Beiya, and discuss their implications on skarn metallogenesis. Based on optical characteristics and major element compositions, the older Grt I and the younger Grt II (both belong to the grossular-andradite solid solution) are distinguishable: Grt II (Adr 69.0 Grs 26.8-Adr 93.5 Grs 1.5) is slightly richer in Fe than Grt I (Adr 68.5 Grs 20.3-Adr 86.3 Grs 3.4), with the average FeO contents being 25.0% and 24.5%, respectively. LA-ICP-MS garnet U-Pb dating yielded lower intercept ages of 35.8 ± 0.8 Ma (MSWD = 0.9, n = 20) and 34.0 ± 2.1 Ma (MSWD = 1.2, n = 16), respectively. These ages represent the timing of the garnet crystallization, and are consistent with published alteration ages within error, which suggests that the technique presented provides a reliable alternative in dating skarn alteration. Grt I has higher U content and lower Th/U ratio than Grt II, indicating a lower oxygen fugacity for the earlier skarn alteration. Grt II shows weak (or no) Eu anomaly and lower LREE/HREE ratios than those of Grt I, which indicates that Grt I and Grt II may have formed under a mildly acidic and nearly neutral condition, respectively. From the early prograde skarn (Grt I) to late prograde skarn (Grt II) stage, the ore-forming fluids may have changed from a relatively reduced acidic to a relatively oxidized neutral pH condition.

Hunting the Datable Garnet using the LA‐ICP‐MS U‐Pb Method: Predicting Garnet U Concentration, based on Major and Minor Elements

“…With the development of LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry), in-situ garnet U-Pb dating provides a powerful solution for geochronological studies (Deng et al, 2017;Seman et al, 2017;Fu et al, 2018;Gevedon et al, 2018;Li et al, 2018Li et al, , 2019Wafforn et al, 2018;Yang et al, 2018;Zhang et al, 2018Zhang et al, , 2019Burisch et al, 2019;Zang et al, 2019;Duan et al, 2020;Millonig et al, 2020;Yan et al, 2020;Tang et al, 2021). For example, in metamorphic rocks, garnet typically has a prolonged growing history and records the P-T (pressure-temperature) conditions from prograde to retrograde stages as compositional zoning.…”

Emplacement ages of diamondiferous kimberlites in the Wafangdian District, North China Craton: New evidence from LA-ICP-MS U-Pb geochronology of andradite-rich garnet

“…It has been pointed out that incorporation of REE into garnets are in part controlled by crystal chemistry and mineral growth kinetics (i.e., substitution vs. interstitial solid solution, surface adsorption-occlusion processes, etc. ), but largely affected by external factors such as compositions/ physico-chemical properties of fluids/melts and host rocks, environmental conditions or metasomatism dynamics (Gaspar et al, 2008;Cheng et al, 2014;Fu et al, 2018;Xiao et al, 2018). Here, we attribute these similarities and distinctions to the different Ca sources, and variable oxygen fugacity of garnet-bearing assemblages.…”

“…The U-Pb isotopic results along with corresponding U-Th-Pb concentrations of LN30 and LN42 andradites are presented in Supplementary Table S3. Even though it has been proven that the matrix effects and elemental fractionation during LA-ICP-MS analysis of garnet samples can be effectively minimized just by optimizing the laser-ablation parameters or analytical modes (Deng et al, 2017;Fu et al, 2018;Wafforn et al, 2018;Li et al, 2019;Chen et al, 2021;Tang et al, 2021), it is still recommended to use suitable matrix-matched certified reference materials (rather than zircon) as external calibration standards for garnet U-Pb dating (Yang et al, 2018). In addition, garnet U-Pb isotope and trace element analyses affected by U-rich mineral inclusions were excluded during data processing.…”

“…Corroded Grt-III hydroandradite clasts (coexisting with pyrite) may have formed under mildly acidic, reducing conditions during late-stage hydrothermal alteration, which would enhance the stability of soluble Eu 2+ . Since Eu 2+ has a larger ionic radius and is thought to be easier to be adsorbed by crystal surface or much more likely to substitute into andradite than Eu 3+ , resulting in positive Eu anomalies (Cheng et al, 2014;Fu et al, 2018). Hence, the trace element patterns of subsequent Grt-III precipitates reflect the progressive chemical evolution of the residual/hydrothermal fluids.…”

Metallogenic Mechanism and Geodynamic Background of the Chang’an Chong Cu-Mo Deposit in Southern Ailaoshan Tectonic Belt: New Evidence from Garnet U-Pb Dating and In-Situ S Isotope

“…Notably, the detailed U mapping of garnet illustrates a uniform distribution, indicating that U occurs within the garnet crystal lattice. U-Pb ages of garnet therefore represent the crystallization ages of the rock, which are of critical importance to establish a temporal link with ore-related intrusive bodies [13][14][15][16][17][18][19][20][21][22][23][24][25]. Sulfide minerals with complex structures can record the chemical properties and the evolution process of ore-forming fluids, so the S isotope analysis can be used to analyze the geochemical environment and source of ore-forming materials [26,27], and track the changes in sulfur sources accurately.…”