Geology and origin of Ag–Pb–Zn deposits occurring in the Ulaan-Jiawula metallogenic province, northeast Asia

Abstract

Located at the conjunction area of China, Mongolia and Russia in NE Asia, the Ulaan-Jiawula (also referred as UJ) region, with an area of 400,000km2, is one of the most important Ag–Pb–Zn, U, Sn, W, Nb–Ta, and Au metallogenic provinces in Asia. At present, 2126 deposits and showings including 500 Ag–Pb–Zn deposits have been discovered, explored and mined since the late 1960s. These Ag–Pb–Zn occurrences can be subdivided into three types according to their geological setting, texture, alteration and mineral assemblages: (1) low sulfidation epithermal Ag–Pb–Zn deposits; (2) intermediate sulfidation epithermal Ag–Pb–Zn deposits; (3) mixed-type Ag–Pb–Zn deposit consisting of vein-like and tabular ore bodies. The Eren Tologoi and Tsagenbulagen deposits are representative of low-sulphidation type Ag–Pb–Zn mineralization in the UJ region, and are associated with intensive adularization and sericitization. Ore occurs as mineralized quartz veins, veinlet groups and altered-fracture zones within Mesozoic alkaline and high-K calc-alkaline volcanic rocks, Ore mineralogy includes native silver, electrum, pyrite, galena, sphalerite, arsenopyrite, pyrargyrite and chalcopyrite. The Tsav and Jiawula deposits are typical of intermediate sulfidation Ag–Pb–Zn mineralization. The δ34S value of sulfide (pyrite and galena) separates from groups 1 and 2 varies from 1.5‰ to 3.5‰ and 2.0‰ to 4.5‰, respectively. The δ34S values of the Mesozoic volcanic host rocks for groups 1 and 2 deposits also show the positive δ34S values of 1.5–4.8‰, while the δ34S value of pyrite separate from the pre-Jurassic schist range from −6‰ to −8‰ which are much lower than Mesozoic volcanic host rocks and their associated ore deposits. There is no difference between the δ34S value of sulfide (pyrite and galena) separates from vein-like ore bodies of the group 3 deposits and their wall rocks, having δ34S value of 1.0–5.0‰ and 1.2–4.5‰ which are similar to that of groups 1 and 2 deposits. For the Mesozoic monzogranite porphyry dykes and their associated tabular skarn ore bodies, the pyrite separates show δ34S values of 2–5‰ and 1.8–3‰. All of these deposits show relatively radiogenic lead isotopic compositions compared to mantle or lower crust curves. Most lead isotope data of sulfides from the Ag–Pb–Zn ores plot between the Mesozoic volcano–hypabyssal rocks and pre-Jurassic metamorphic rocks. Monzogranite dykes at Ulaan and Noyon Tologoi have εNd (T) values ranging from 1.5 to 4.5 that are similar to most of the Mesozoic granite with positive εNd (T) values in the Great Hinggan Mountains-Mongolia orogenic belt. Data are interpreted as indicative of a mixing of ore-forming materials from mantle-derived alkaline and high-K calc-alkaline magma with these from pre-Jurassic metamorphic wall rocks. Isotopic age data, geological and geochemical evidence suggest that the ore fluids for the Ag–Pb–Zn deposits were generated during eruption or emplacement of the Mesozoic alkaline and high-K calc-alkaline magma. The Mesozoic magmas may provide heat, volatiles and metals for the group 1 and 2 deposits. Evolved metamorphic fluids produced by devolatilization, circulated the wall rocks, were also progressively involved in the magmatic hydrothermal system, and may have dominated the ore fluids during late stage ore-forming processes. Most of the Ag–Pb–Zn bodies that occur along the contact of the pre-Jurassic marble and Cretaceous monzogranite porphyry dykes at Ulaan and Noyon Tologoi are closely associated with skarn. The ore fluid of these group 3 deposit may have resulted from the mixing of Mesozoic magmatic water and evolved metamorphic fluids. Ore deposition in this instance would be the product of the interaction of the Mesozoic intrusions and pre-Jurassic carbonate rocks.