Geology, mineralization, geochemistry, and skarnization processes in the Qaradash Pb–Zn occurrence (NW Zanjan)


1 Graduated MSc student, Department of Geology, University of Zanjan

2 Department of Geology, University of Zanjan

3 Department of Geology, Faculty of Sciences, University of Zanjan


The geological units exposed in the Qaradash area are Paleozoic metamorphic rocks. Microscopic studies reveal that the skarnoid aureole in Qaradash is composed of garnet hornfels, garnet-pyroxene hornfels, pyroxene hornfels, epidote-garnet-pyroxene hornfels, epidote hornfels and marble subzones. Pb–Zn mineralization at Qaradash occurred as quartz-sulfide vein-veinlets within skarnoid aureole.The ore zone can be traced 200 m along strike and up to 15 m in width. The hydrothermal alteration includes of decarbonatization, actinolization-epidotization, carbonatization, and silicification. The ore minerals at Qaradash are pyrite, chalcopyrite, galena, sphalerite, and hematite. The gangue minerals include garnet, clinopyroxene, calcite, quartz, epidote, chlorite, and actinolite. Ore minerals display disseminated, vein-veinlet, brecciated, replacement, and relict textures. Similar Chondrite–normalized rare elements and REE patterns (McDonough and Sun, 1995) of ore and skarnoid aureole sub-zones samples indicate that they are genetically related. Based on mineralogical and textural studies, skarnification processes in the Qaradash occurrence can be divided into 2 stages including: (1) prograde metasomatic stage and (2) retrograde metasomatic stage. Pb–Zn mineralization occurred during retrograde metasomatic stage. Based on mineralogical and textural evidence, prograde metasomatic stage was formed simultaneously in 430–550 °C and ƒO2 was equal to 10-23 to 10-26 (e.g., Einaudi, 1982). According to mineralogical complex of the early retrograde stage, it seems that metasomatic fluids had ƒS2 ≈ 10-6.5 (e.g., Einaudi, 1982). Based on field evidences as well as ore geology, skarnoid aureole sub-zones, marble host rock, structure and texture and paragenetic sequences, we conclude that the Qaradash occurrence is a calcic Pb–Zn skarn mineralization.


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