Major and trace element signature of epidote-group minerals in altered pegmatites from the Petrovitsa Pb–Zn deposit of the Madan ore region, Central Rhodopes, Bulgaria: Evidence of allanite/epidote transformation

Abstract

: The geochemical behaviour of major, rare, and trace elements in members of epidote-group minerals formed at different stages of magmatic and hydrothermal activity in pegmatites from the Madan ore district was studied. Accessory allanite-(Ce) and two generations of hydrothermal clinozoisite–epidote occur in hydrothermally-altered pegmatite bodies at the 820 mine level in the Petrovitsa Pb–Zn deposit. Abundant large concordant and crosscutting pegmatites (age 49.63 ± 0.94 Ma) with a thickness of more than 2 m are embedded in a high-grade metamorphic complex composed of various gneisses, amphibolites, and marbles in the area of the Petrovitsa deposit. The pegmatites consist mainly of feldspars and quartz, with plagioclases (albite–oligoclase, oligoclase–andesine, anorthite) predominating over K-feldspars. The main accessory minerals are allanite-(Ce), zircon, apatite, and an abundance of titanite. The overprinted hydrothermal mineral association is characterised by the formation of clinozoisite–epidote, adularia, chlorite, Ca-garnet, titanite, leucoxene, carbonates, hematite, and quartz. Based on petrographic observations, mineral relationships, and chemical properties, two generations of epidotes ( sensu lato ) were recognised: early epidote (Ep1) and late epidote (Ep2). Chemically, allanite-(Ce) contains high amounts of La and Th. The mineral suffered alteration due to several multiphase hydrothermal events and is partly or entirely transformed into REE-rich clinozoisite, causing depletion in REE and Th as well as enrichment of Si, Al, and Ca. The epidote 1 generation is defined chemically as clinozoisite to clinozoisite– epidote, whereas the late epidote 2 generation is clinozoisite–epidote, reaching epidote members. The REE contents in the studied epidotes from both generations are equally low with minor exceptions. The influx of later fluids percolated through and probably extracted REE from allanite and thereafter concentrated them in late-generation epidotes. This is also evidenced by some high REE contents in epidotes from both generations, formed in close proximity to allanite or titanite and/or overgrowing them, which is a sign of restricted REE transport.