40 Ar/ 39 Ar geochronology of magmatism and hydrothermal activity of the Madjarovo base-precious metal ore district, eastern Rhodopes, Bulgaria

Abstract

AbstractThe Madjarovo volcanic complex and ore district comprise alteration styles from potassium silicate, advanced argillic and sericite alteration to adularia-sericite alteration/mineralization with a close and unambiguous spatial relationship to specific magmatic events. New 40Ar/39Ar laser fusion and incremental heating experiments on nine sanidine, biotite, adularia, K-feldspar, and alunite samples constrain the ages and time span of lavas and tephras comprising the complex and their relationship to the hydrothermal activity. These results demonstrate that high-K calc-alkaline to shoshonitic volcanic activity began c.32.7 Ma and terminated c.500 ka later with the extrusion of quartz latite lavas at 32.2 Ma. The final stage of volcanism was accompanied by intrusion of compositionally similar monzonite stocks and trachytic dykes (c.32.2–32.1 Ma) and associated barren advanced argillic and sericite alteration (lithocap) and adularia-sericite base/precious metal vein mineralization. A probable thermal event at c.12–13 Ma disturbed the ages of alunite and sericite-bearing alteration at low stratigraphic levels. However, field relations combined with a plateau age of 32.1 ± 0.2 Ma from adularia in low-sulphidation veins that cross-cut lithocap indicate that hydrothermal activity, including base- and precious-metal vein deposition, was coeval with the youngest magmatic activity.