Mineralization of LS epithermal deposit in intracontinental: Evidence from the mineralogy, metallogenic age, and S-Pb isotopes of the Paodaoling Au deposit

Through the detailed field geological investigation, we found that the sulfide veins formed later than the granodiorite porphyry. The LA-ICP-MS U-Pb dating of dolomite results show that the Baodaoling gold deposit was formed at 137.3 ± 2.3 Ma, and the δ 34 S values shows that the ore-forming fluid mainly came from magmatic fluid and increased groundwater. Pb isotopes show that the Paodaoling Au deposit has similar provenance to the porphyry and skarn Cu(Au) deposits in the MLYB. According to the study of mineral association and occurrence of gold in the Paodaoling Au deposit, it is considered that Paodaoling Au deposit belongs to LS epithermal deposit. The results of this study provide a theoretical basis for finding epithermal deposits unrelated to volcanic activity in the MLYB. • The Paodaoling gold deposit, located in the MLYB, is classified as a LS epithermal deposit with a metallogenic age 137 Ma. • The lower part of the Paodaoling Au deposit holds potential for the presence of a porphyry Cu(Au) deposit. • The LS epithermal deposit can be formed in the non-volcanic environment under the background of intracontinental orogeny. The Paodaoling Au deposit is the largest Au deposit in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB), with Au reserves of 36.5 tonnes and an average grade of 1.93 g/t. Based on field geological surveys and mineralogical studies, LA-ICP-MS mapping, in-situ S-Pb isotope and LA-ICP-MS U-Pb dating of dolomite were carried out to identify the genetic type, metallogenic age and source of ore-forming materials of the deposit. In the Paodaoling Au deposit, hydrothermal fluids mainly filled in high angle tension joint with dip angles of 75° to 85°, as well as associated structure, and in shear joints with dip angles of 50°, suggesting that the fluids filled in the host granodioritic porphyry when it had been completely consolidated. The LA-ICP-MS U-Pb dating of dolomite associated with gold mineralization indicates a metallogenic age of 137.3 ± 2.3 Ma (n = 58; MSWD = 3.6; 2σ), which is obviouly later than the hosted granodioritic porphyry (147–141 Ma). Lead isotope data shows that 20 6 Pb/ 204 Pb = 18.190～18.450, 207 Pb/ 204 Pb = 15.594～15.646, 208 Pb/ 204 Pb = 38.467～38.625 , which fall into the mixed region between the mantle and the upper crust. The S isotope shows that ore-forming fluids of the Paodaoling Au deposit was derived from magma, and the sulfur from strata was added in the process of gold mineralization. Au exists mainly in the form of Au +1 solid solution in low sulfidation state as arsenopyrite and arsenian pyrite. The Paodaoling Au deposit was formed in an intracontinental orogeny setting without contemporaneous volcanic rocks, and its ores are characterized by enrichment in As and depletion in Te, Se, and Ag, with an Ag:Au < 1. The mineral association and ore-forming element characteristics of the Paodaoling Au deposit is similar to typical LS epithermal deposits related to subalkalic igneous rocks. Thus this study confirms that the first time the existence of LS epithermal deposits associated with porphyry and skarn Cu (Au) in the MLYB, it is of great significance to the metallogenic theory of porphyry and skarn-epithermal deposit in intracontinental orogeny and to guide regional geological prospecting.