Mineralization age and sources of ore‐forming material of the Nanmushu Zn‐Pb deposit in the Micangshan Tectonic Belt at the northern margin of the Yangtze Craton, China: Constraints from Rb‐Sr dating and Sr‐Pb isotopes

Abstract

The Nanmushu Zn‐Pb deposit, hosted by the Neoproterozoic Dengying Formation dolostone, is located in the eastern part of the Micangshan tectonic belt at the northern margin of the Yangtze Craton, China. This study involves a systematic field investigation, detailed mineralogical study, and Rb‐Sr and Pb isotopic analyses of the deposit. The results of Rb‐Sr isotopic dating of coexisting sphalerite and galena yield an isochron age of 486.7 ± 3.1 Ma, indicating the deposit was formed during the Late Cambrian to Early Ordovician. This mineralization age is interpreted to be related to the timing of destruction of the paleo‐oil reservoir in the Micangshan tectonic belt. All initial 87Sr/86Sr ratios of sphalerite and galena (0.70955–0.71212) fall into the range of the Mesoproterozoic Huodiya Group basement rocks (0.70877–0.71997) and Dengying Formation sandstone (0.70927–0.71282), which are significantly higher than those of Cambrian Guojiaba Formation limestone (0.70750–0.70980), Cambrian Guojiaba Formation carbonaceous slate (0.70766–0.71012), and Neoproterozoic Dengying Formation dolostone (0.70835–0.70876). Such Sr isotope signatures suggest that the ore strontium was mainly derived from a mixed source, and both of the Huodiya Group basement rocks and Dengying Formation sandstone were involved in ore formation. Both sphalerite and galena are characterized by an upper‐crustal source of lead (206Pb/204Pb = 17.849–18.022, 207Pb/204Pb = 15.604–15.809, and 208Pb/204Pb = 37.735–38.402), and their Pb isotopes are higher than, but partly overlap with, those of the Huodiya Group basement rocks, but differ from those of the Guojiaba and Dengying Formations. This suggests that the lead also originated from a mixed source, and the Huodiya Group basement rocks played a significant role. The Sr and Pb isotopic results suggest that the Huodiya Group basement rocks were one of the most important sources of metallogenic material. The geological and geochemical characteristics show that the Nanmushu Zn‐Pb deposit is similar to typical Mississippi Valley type, and the fluid mixing may be a reasonable metallogenic mechanism for Nanmushu Zn‐Pb deposit.

LA-ICP-MS Trace Element Geochemistry of Sphalerite and Metallogenic Constraints: A Case Study from Nanmushu Zn–Pb Deposit in the Mayuan District, Shaanxi Province, China

“…[26]. According to the geological features and mineral paragenesis relationships of the deposit, it is inferred that the Nanmushu deposit had experienced pre-ore sedimentary and the diagenetic stage, main-ore hydrothermal ore-forming stage, and post-ore supergene stage [22]. In the sedimentary and diagenetic stage, the fine-grained (10 to 50 µm) euhedral pyrite is disseminated within dolomite.…”

“…They are related to the paleo oil reservoir but not associated with igneous activity [21,25,26]. Previous studies show that the ore-forming ages are mainly 468-488 Ma [21][22][23], suggesting that Nanmu belongs to an epigenetic hydrothermal deposit. Furthermore, the ore-forming fluids are basinal brines trapped in a heterogenous fluid system with medium-low temperature and medium-high salinity [19].…”

“…The Mayuan Zn-Pb ore field contains a metal resource of over 10 Mt Zn (1.05-10.82 wt%) and Pb (0.55-7.54 wt%) which are associated with some beneficial elements, such as Ag (2-35 ppm), Ge (0.002-0.05 wt%), Cd (0.002-0.10 wt%), and Cu (0.03-0.35 wt%) [15,16]. Many studies have been conducted on the Mayuan Zn-Pb ore field, including the geological features of the ore deposit [15,17], the source of ore-forming materials [16,[18][19][20], the ore-forming age [21][22][23], the relationship between hydrocarbon organic matters and mineralization [21,[24][25][26], etc. Regarding the genesis of the deposit, the initial understanding was that it was an epigenetic medium-low temperature hydrothermal deposit [27] or a syn-sedimentary hydrothermal deposit [28], but later, most scholars agreed that it was a Mississippi Valley-type (MVT) deposit [16,[18][19][20][21][22][23].…”

“…Many studies have been conducted on the Mayuan Zn-Pb ore field, including the geological features of the ore deposit [15,17], the source of ore-forming materials [16,[18][19][20], the ore-forming age [21][22][23], the relationship between hydrocarbon organic matters and mineralization [21,[24][25][26], etc. Regarding the genesis of the deposit, the initial understanding was that it was an epigenetic medium-low temperature hydrothermal deposit [27] or a syn-sedimentary hydrothermal deposit [28], but later, most scholars agreed that it was a Mississippi Valley-type (MVT) deposit [16,[18][19][20][21][22][23]. However, studies on the associated beneficial elements are still scarce.…”