Origin of the Yangwantuan Gold Deposit in the Jiangnan Orogen (South China): Constraints from Sericite Rb-Sr Isotopes and Quartz Trace Elements

The Jiangnan Orogen (South China) hosts abundant gold deposits, but the absence of accurate constraints on the ore-forming age and process has resulted in significant controversy regarding their origins. The Yangwantuan gold deposit, located in the central part of the Jiangnan Orogen, is characterized by multi-stage quartz veins linked to mineralization and alteration. The mineralization can be divided into three stages, namely the barren quartz–sericite stage (I); the quartz–sericite–native gold–polymetallic sulfide stage (II, including the quartz–sericite–dolomite–native gold–polymetallic sulfide (IIA) and quartz–chlorite–sericite–native gold–arsenopyrite (IIB) substages); and the quartz–dolomite–calcite–arsenopyrite (III) stage. On the basis of the mineralization and alteration sequence and quartz’s internal texture, 11 generations of quartz are determined, including gray QzIa and dark QzIb in Stage I; oscillatory-zoning QzIIa, homogeneous QzIIb, and veined QzIIc in Stage IIA; homogeneous QzIId, QzIIe trapping sulfide inclusions, and veined QzIIf in Stage IIB; and gray QzIIIa, dark QzIIIb, and veined QzIIIc in Stage III. The decrease in Al content corresponds to an increase in pH from QzIa to QzIb, favoring the transportation of gold in the fluid. The sharp drop in temperature and the increment of pH, revealed by Al and Ti content variations from QzIIa to QzIIb, indicates a strong water–rock interaction, consistent with the occurrence of arsenopyrite in the wall rock. Therefore, the gold precipitation in Stage IIA may be triggered by the consumption of H2S through water–rock interaction, whereas during Stage IIB and III, the precipitation of gold is attributed to the consumption of H2S as a result of the formation of abundant sulfide, which is supported by the coexistence of sulfide and QzIIf and QzIIIc. The Stage IIA sericite Rb-Sr isochron age of 397 ± 11 Ma (MSWD = 0.8, n = 32) suggests that the mineralization age is closely related to the Devonian Orogeny. The absence of contemporaneous magmatic rock and quartz Al and Ti concentrations both indicate that the Yangwantuan deposit may be classified as an orogenic gold deposit.