Constraints of S–Pb–Sr Isotope Compositions and Rb–Sr Isotopic Age on the Origin of the Laoyingqing Noncarbonate-Hosted Pb–Zn Deposit in the Kunyang Group, SW China

Abstract

The Laoyingqing Pb–Zn deposit is located on the southwestern margin of the Yangtze block and on the east side of the Xiaojiang deep fault in the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic triangle area (SYGT). This deposit was first discovered in the silty and carbonaceous slate of the Middle Proterozoic Kunyang Group that is structurally controlled by thrust faults and anticlines. This study is aimed at investigating whether the Laoyingqing deposit has the same ore-forming age and type as other Pb–Zn deposits related to the Pb–Zn metallogenic system and prospecting prediction of the deep and peripheral areas of the deposits in the SYGT. Based on the sphalerite Rb–Sr age dating and S–Sr–Pb isotopic composition analysis of the Laoyingqing Pb–Zn deposit, the following results were obtained. First, the Rb–Sr isochron age of sphalerite is 209.8 ± 5.2 million years (Ma), consistent with the ages of most Pb–Zn deposits in the SYGT (approximately 200Ma), thereby potentially indicating that these Pb–Zn deposits may have been formed synchronously during the late Indosinian orogeny. Second, the Pb isotopic compositions of sulfides show a linear trend on the average crustal Pb evolution curve in 207Pb/204Pb vs. 206Pb/204Pb plot. In addition, Pb isotopic ratios were consistent with the age-corrected Pb isotopic ratios of basement rocks, consequently suggesting that the source of mixed crustal Pb is mainly derived from basement rocks. Combined with the initial 87Sr/86Sr ratios of sphalerite between the (87Sr/86Sr)200Ma value of the basement rocks and that of the Upper Sinian–Permian carbonates, it can be concluded that the ore-forming metals were mainly derived from basement rocks. Third, sulfur isotopic composition of sphalerite from the Laoyingqing deposits shows δ34SCDT values that range mainly from -2.62‰ to 1.42‰, which is evidently lower than the δ34SCDT values of sulfides (8–20‰) from other Pb–Zn deposits in the SYGT. This can be interpreted as a result of mixing with reduced S that was mainly derived from the thermochemically reduced S in the overlying strata and a small amount of reduced S produced by the pyrolysis of S-containing organic matter. We conclude that the Laoyingqing deposit and most of the Pb–Zn deposits in the SYGT are Mississippi Valley-type deposits, thereby providing new ideas for investigating the deep and peripheral areas of Pb–Zn deposits.

Geological and Geochemical Characteristics and Genesis of the Laoyingqing Zinc Deposit in Northeastern Yunnan

“…It cannot be completely ruled out that there is Emeishan basalt in the deep part of this deposit activated by ore-forming fluids, which provides a small amount of metallic elements for this deposit. This conclusion is consistent with the conclusion obtained by Gong et al [7] through Pb and Sr isotope research. The Y and Ho elements have the same valence state and ion radius, and their ion radii with eight co-ordination numbers are almost identical.…”

“…Based on the research conclusions regarding rare-earth elements and H-O isotopes, combined with previous research results, the mineralization process of the Laoyingqing zinc deposit can be described as a collision between the Indosinian landmass and Yangtze landmass in the late Indosinian period (about 200 Ma), which led to the closure of the ancient Tethys Ocean and the formation of the Wuxing anticline [7]. The basin fluids underwent large-scale migration under the action of gravity, and a small amount of gypsum-salt layer sulfate from marine carbonate rocks of the Dengying Formation of the Sinian system was extracted.…”

“…The Laoyingqing zinc deposit is about 19 km in horizontal distance from Huize County, Yunnan Province, with a geotectonic location on the southwestern edge of the Yangtze block. Previous research on this deposit mainly includes the following: (1) Yang Xiaoxia [5] and Bian Shantao et al [6] believed that the ore body is controlled by fault fracture zones, and the genesis of this deposit is a fracture-filling vein-type Pb-Zn deposit, whereas (2) Gong et al [7] studied the composition of Pb and Sr isotopes and believed that the ore-forming metals mainly came from surrounding rocks (Kunyang Group).…”

Metallogenic Model for Pb-Zn Deposits in Clastic Rocks of the Dahai Mining Area, Northeast Yunnan: Evidence from H-O-S-Sr-Pb Isotopes

“…The Rb-Sr isotopic compositions obtained for the Laoyingqing deposit are also listed in Table 3. The isochron age of sphalerite was determined to be 209.8 ± 5.2 Ma [17].…”

“…The complex metallogenic structural system controls the widespread distribution of Pb-Zn deposits (Figure 1b). The Pb-Zn deposits in the metallogenic district in northeast Yunnan are mainly hosted in Mesoproterozoic epimetamorphic slate [17] and Neoproterozoic-Permian carbonate rocks. Layered, lenticular, and vein-shaped orebodies are controlled by oblique thrust fault zones.…”