The ore‐forming fluids characteristics of quartz‐vein type scheelite deposits in eastern Yanbian, NE China: Evidence from in situ LA‐ICP‐MS rare earth elements of Yangjingou and Sidaogou deposits

Abstract

The quartz‐vein type scheelite deposits distributed in the Hunchun SN‐trending gold‐copper‐tungsten belt in eastern Yanbian, Jilin Province, are a group of recent discovery for the past decade. To determine properties of the ore‐forming fluids and the mineralization mechanism, in situ laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) rare earth elements (REE) analysis of the ore mineral scheelite was carried out for the selected Yangjingou and Sidaogou scheelite deposits in this metallogenic belt. The results showed that the substitution of REE3+ for Ca2+ in scheelite from the Yangjingou scheelite deposit can be accounted for the substitution mechanism: 3Ca2+ = 2REE3+ + □Ca (where □ is a site vacancy). It shows a slightly right‐dipped flat REE pattern with distinct positive Eu anomalies. The Sidaogou scheelite deposit, completes the substitution via the 2Ca2+ = REE3+ + Na+ and Ca2+ + W6+ = REE3+ + Nb5+ mechanisms. It shows left‐dipped REE pattern with relative enrichment of MREEs (mainly Dy) with negative Eu anomalies in the scheelite core and no or insignificant positive Eu anomalies in the scheelite rim. By comparing with the REE of granites which are close related to mineralization in the area, the ore‐forming fluids of these two scheelite deposits are determined to be mainly derived from magmatism. The inapparent correlation between EuN and Eu*N of the Yangjingou scheelite deposit and the high EuN/Eu*N values indicate that its ore‐forming fluids are reducing fluids. Its strong positive Eu anomalies are not entirely inherited from the mineralized tonalite, but is due to the release of Eu from the water‐rock reaction. The good correlation between EuN and Eu*N and the low EuN/Eu*N values in the Sidaogou scheelite deposit indicate that the ore‐forming fluids are oxidizing fluids. This may be caused by the mixing of the original magmatic fluid with a large amount of meteoric water. This study suggests that the water‐rock reaction is an important mineralization mechanism for the quartz vein‐type scheelite deposits in eastern Yanbian. In addition, fluid mixing is also important for the Sidaogou scheelite deposit. All these mechanisms influence the REE compositional characteristics of scheelite. The initial ore‐bearing fluids metasomatized the metamorphic rocks of Wudaogou Group, resulting in water‐rock reaction, and enriched ore‐forming materials such as Ca and W. As the ore‐forming fluids migrated upward along the NW‐trending structures, the escape of CO2 and CH4 caused by the tectonic decompression disrupted the physiochemical balance of the ore‐forming fluid system, catalyzed the combination of Ca2+ and WO42−, resulting in the precipitation and enrichment of the scheelite.