Origin of ore-forming fluids of Mississippi Valley-type (MVT) Pb-Zn deposits in Kangdian area, China

Analyses of fluid-inclusion leachates from ore deposits show that Na/Br ratios are within the range of 75–358 and Cl/Br 67–394, respectively, and this variation trend coincides with the seawater evaporation trajectory on the basis of the Na/Br and Cl/Br ratios. The average Cl/Br and Na/Br ratios of mineralizing fluids are 185 and 173 respectively, which are very close to the ratios (120 and 233 ) of the residual evaporated seawater past the point of halite precipitation. It is suggested that the original mineralizing brine was derived from highly evaporated seawater with a high salinity. However, the inclusion fluids have absolute Na values of 69.9-2606.2 mmol kg-1 and Cl values of 106.7-1995.5 mmol kg-1. Most of the values are much less than those of seawater: Na, 485 mmol kg-1 and Cl, 566 mmol kg-1, respectively; the salinity measured from fluid inclusions of the deposits ranges from 2.47 wt% to 15.78 wt% NaCl equiv. The mineralizing brine has been diluted. The δ18O and δD values of ore-forming fluids vary from −8. 21% to 9. 51% and from −40. 3% to −94.3%, respectively. The δD values of meteoric water in this region varied from −80% to −100% during the Jurassic. This evidenced that the ore-forming fluids are the mixture of seawater and meteoric water. Highly evaporated seawater was responsible for leaching and extracting Pb, Zn and Fe, and mixed with and diluted by descending meteoric water, which resulted in the formation of ores.