Double-vein (ore-bearing vs. ore-free) structures in the Xitian ore field, South China: Implications for fluid evolution and mineral exploration

Abstract Two types of spatially and temporally associated hydrothermal veins, ore-bearing and ore-free, occur as double vein structures in the Xitian ore field, South China. Here, we investigated the genetic relationship, material provenance, fluid properties, and prospecting significance of the co-developed veins in the granite-related, fault-controlled vein-type Xiangdong W, Goudanlan W, Chaling Pb–Zn and Xinggao fluorite deposits of the Xitian ore field, focusing on two common minerals: quartz and zircon. Petrographic, CL, microthermometric and H–O isotopic analyses of Stage I and Stage II quartz in both the ore-bearing and ore-free veins of these deposits document two episodes of hydrothermal activity. The first episode involved a high-temperature (up to 370 ℃), high-salinity (up to 28% NaCl equiv.), high-δ18O (–2.1‰ to +7.2‰), and high-δ2H (–82‰ to –54‰, mean = –68‰, n = 13) fluid that corresponds to the main magmatic-hydrothermal stage of the deposits, and the second episode involved a low-temperature (down to 90 ℃), low-salinity (down to 1% NaCl equiv.), low-δ18O (–8.8‰ to –5.3‰), and low-δ2H (–83‰ to –60‰, mean = –73‰, n = 6) fluid that represents an accessary mineralization stage. Variations in fluid inclusion types and compositions indicate that the processes of fluid mixing, boiling, cooling, and degassing played important roles in mineralization, and that magmatic fluid became less important than meteoric water along during the formation of ore-free veins. LA-ICPMS U–Pb dating reveals totally different age populations for these veins: zircons from the ore-bearing quartz veins yield older and more variable ages (pre-Devonian: > 400 Ma, n = 68) that are similar to that of the local basement/sedimentary rocks, whereas zircons from the ore-free quartz veins yield younger and less variable ages (Triassic: 250–200 Ma, n = 31; and Jurassic: 170–140 Ma, n = 13) that are identical to those of the host granite zircons. In addition, the ore-free veins show lower temperatures, salinities, and total REEs (as recorded by quartz) but greater alteration (as reflected by CL and trace element features of hydrothermally-altered zircons) and wider vein size than the ore-bearing veins. We propose a “mineral–water separation” model to explain the formation of the double-vein structures. In this model, ore-bearing veins and ore-free veins formed in succession during faulting events: ore-bearing veins formed first in the smaller fractures and were the dominant sites of mineralization, whereas the ore-free veins served as dissipating passageways for residual water and silica in larger fractures. We suggest that the mineral-water separation model has good application prospects in mineral exploration, and that zircons can be applied as an indicative mineral to differentiate the mineralization potential of different types of quartz veins.