Mineralogical-petrographical features, geochemical characteristics, and S isotope variability of Pb-Zn deposits in the Sakarya fragment of the Biga Peninsula (NW Türkiye)

Abstract

The Biga Peninsula in northwestern Anatolia, a part of the Alpine-Himalayan orogenic belt, has a complex geology that was formed following the closure of the northern branch of the Neotethys. Intense volcanism and plutonism in the area from the Eocene to the Middle Miocene period caused several Pb-Zn-Cu±Ag±Au deposits to form. The geometry of the mineralizations is largely made up of polymetallic veins, manto-chimneys, and irregular replacement bodies. Ore-bearing and host rocks in the mineralization zones and the wall rocks outside the mineralization zones were compiled in this study. The most common minerals in the skarn zones are garnet, pyroxene, amphibole, epidote, chlorite, quartz, and calcite, while quartz, calcite, and sericite are formed in the hydrothermal alteration zones. Based on the geochemical analyses, the granitoidic rocks are granodiorite, whereas the volcanic rocks are dacite, andesite, trachyandesite, and basaltic andesite. According to the skarn-forming features, the Biga plutons have chemical characteristics that can form Pb-Zn, Cu, Fe, and Au skarns. The carbonate units in the area are made up of marble (fresh), marble (in the alteration zone), and ore calcite. The geochemical analyses of the carbonate units indicate that in composition the fresh marble is almost pure CaCO3 . However, the marble in the alteration zone has higher SiO2 , Fe2 O3 , MgO, and MnO contents than that of the fresh marble. These values are higher in ore calcite. The metasandstones mostly consist of arkose and may have contributed to the main mineralization as they have initial ore element enrichment. According to a hierarchical cluster analysis (HCA) result, three different element groups were detected, these being Sb, Rb, Zr, Ag, Y, Mo, Hg, and Nb (increased during alteration), Sr and Ba (decreasing in alteration), and Cu, Au, As, and Cd (mineralization-related). Rare earth element (REE) characteristics indicate that hydrothermal fluids, responsible for the formation of the Biga Peninsula Pb-Zn deposits, are characterized by very low REE concentrations, and consequently the origin of these fluids is predominantly meteoric. The ∑REE, (Pr/ Yb)cn, Ce/Ce* values (8.63-24.79 ppm, 2.84-8.23, 0.38-0.82, respectively) of the marble in the alteration zone and ore-bearing skarns (3.34-27.20 ppm, 2.32-5.87, 0.36-0.83) support the findings of the meteoric contribution. Based on the similarities of the general trends and the abundances of REE elements in ore-bearing skarns and wall rocks, it is thought that wall rocks may have contributed part of the lead. The δ34S isotope compositions of galena, sphalerite, pyrite, and chalcopyrite fall into a narrow range of around 0‰. This data indicates that the sulfur in the Pb-Zn±Cu sulfides in the Biga Peninsula is of magmatic origin (δ34Smin: -5.5, δ34Smax: 5.2, δ34Savg: -0.7; n: 40). According to the δ34S values, the mineralizations occurred under intermediate sulfidation conditions, in a reducing environment with H2 S-dominant fluids with a near neutral pH value.