Origins of ore-forming fluids in the Mežica carbonate-hosted Pb-Zn deposit inferred from sphalerite halogen systematics

Abstract

Halogen (Cl-Br-I) systematics in sphalerite from the Pb-Zn deposit of Mežica provide new insights into the origin and evolution of ore-forming fluids. The data confirm and refine previous interpretations based on stable isotope analyses and sphalerite trace-element compositions, indicating that the fluids were derived from evaporated seawater and subsequently modified by basinal brines. Two principal fluid types are identified: a Br-poor fluid consistent with evaporatively concentrated seawater, and a Br- and I-enriched fluid representing a basinal brine modified through interaction with organic matter. Particularly notable are Br-rich samples characterized by exceptionally high Br/Cl ratios coupled with pronounced iodine enrichment. These values significantly exceed the typical range observed in MVT-type deposits and constitute a key geochemical hallmark of the Mežica mineralizing system. Stratigraphic and regional geological constraints indicate that mineralization occurred from the Late Triassic to the Early Jurassic. Restricted lagoonal and shallow-marine environments facilitated repeated seawater evaporation and the formation of dense brines, which infiltrated deeply into the subsurface. Subsequent basin deepening following the Triassic-Jurassic transition likely curtailed further brine generation. Organic matter played a pivotal role in the system, contributing both reduced sulfur and a significant proportion of halogen elements. The evolution of the mineralizing system was governed by the interplay between fluid mixing and fluid-rock interaction, ultimately producing the distinctive halogen signature observed in Mežica sphalerite.