Triassic gold-polymetallic mineralization in the middle section of the Central Asian orogenic belt: In-situ garnet U-Pb dating and composition analyzing on the garnets from Laodonggou deposit

• The Triassic is a critical period for metal mineralization in the middle section of the CAOB. • The Laodonggou gold–polymetallic deposit is attributed to a magmatic origin. • Gold enrichment and precipitation occurred during the retrograde skarn stage. The Beishan Orogenic Belt, situated in the middle section of the Central Asian Orogenic Belt (CAOB), is a key region for endogenous metal mineralization. Within this belt, the Laodonggou gold–polymetallic deposit is a representative example of such deposits. Skarn-type ore is the most important ore type, and the skarn stage represents the initial phase of enrichment and mineralization of metallic minerals, including gold and copper. Garnet, the dominant alteration mineral during the skarn stage, provides crucial insights into the physicochemical characteristics, fluid environment, and timing of the early mineralization at Laodonggou. Two generations of garnets were identified in the deposit. Garnets formed during the prograde skarn stage primarily belong to the andradite-grossular transitional series, while those formed during the retrograde skarn stage are predominantly andradite. Findings revealed that the early zoned garnets from the prograde skarn stage developed under relatively neutral, weakly oxidizing to weakly reducing conditions, with a low water–rock ratio and relatively closed fluid conditions. During the retrograde skarn stage, the fluid environment became weakly acidic and weakly oxidizing, with an open fluid system, higher water–rock ratios, and increased oxygen fugacity. Moreover, fluid metasomatism during this stage was characterized by infiltrative metasomatism, which facilitated gold enrichment and mineralization. Garnet in-situ U–Pb ages of 243.5 ± 9.1 Ma and 245.6 ± 5.3 Ma were obtained for the Laodonggou deposit for the first time, which serves as a representative example of the Triassic gold–polymetallic mineralization event in the middle section of the CAOB. During the Triassic, the middle CAOB experienced three major subduction phases, namely, 258–247 Ma, 234–230 Ma, and 222–211 Ma. Similarly, regional intermediate-felsic magmatic intrusions occurred in three phases, namely, 248–233 Ma, 225–217 Ma, and 207–209 Ma. The prolonged and intermittent subduction provided abundant deep-seated metallogenic materials, making the Triassic one of the most critical periods for metal mineralization in the region, which can be divided into two relatively concentrated phases. The period of 250–220 Ma saw the peak for gold and polymetallic mineralization, while 213–203 Ma experienced weaker gold–polymetallic mineralization, with rare metal mineralization dominating. Isotope studies of S, Pb, and C–O indicate that the metallogenic materials of the deposit originated from a mixed source, including deep-seated magmas and country rocks, with magmatic fluids playing a dominant role in the mineralization process. Based on the characteristics of rare earth elements, we conclude that the skarn, gold ore bodies, and diorite porphyrite of the deposit are products of the same magmatic-hydrothermal mineralization system. Therefore, the Laodonggou gold–polymetallic deposit is interpreted as having a magmatic origin.