Mineral chemistry and in-situ U-Pb geochronology of altered pegmatite from the vicinity of the Strashimir Pb-Zn vein deposit

Abstract

The current study presents new mineralogical, geochemical and geochronological data for a pegmatite body hosted in gneisses and marbles from the vicinity of the Strashimir Pb-Zn vein deposit, Madan ore district, South Bulgaria. The mineral composition of the studied pegmatite is represented by oligoclase–andesine (An10.1–33.2) and albite (An0–7.6), which prevail over K-feldspar (Or87.9–92.4), and quartz. The established accessory minerals are allanite-(Ce), titanite, apatite and zircon. The pegmatite/marble contact is affected by later hydrothermal silicate-carbonate alteration without detected ore mineralization, despite the spatial proximity with the Strashimir Pb-Zn vein deposit. Epidote-group minerals in pegmatite are defined as members of the clinozoisite–epidote series. As a major constituent of the hydrothermal alteration zone, they are manifested in two well-distinguished generations along with chlorite, quartz and carbonates. The calculated temperature of chlorite mineralization yields T° of crystallization in the range of 223–266 °C. As a result of the hydrothermal fluid circulation, the accessory allanite-(Ce) is transformed to REE-rich epidote-clinozoisite, marked by depletion of REE and Fe and enrichment of Si, Al, and Ca. Due to the limited mobility of REE in fluids, after leaching these elements are incorporated in nearby crystallized epidotes. According to the occurrence, mineral association and chemical properties, three titanite populations are distinguished in the pegmatite. Two of them were in-situ dated by the LA-ICP-MS U-Pb method and reveal overlapping ages of 39.9±2.1 Ma and 39.5±2.2 Ma (39.3±1.2 Ma combining all titanite analyses). The ages are interpreted as titanite growth in a pegmatite body related to granitic melts in the Late Alpine high-metamorphic units (Madan Unit) of the Central Rhodopes. Hydrothermal fluids either did not affect the U-Pb isotope system of the titanites or were derived from the same fluid-rich melt.

Crystal chemical investigations of epidote group minerals from two Bulgarian localities: Effects of Mn content and Mn/Fe ratio on the structural peculiarities

“…According to a previous study (Georgieva et al, 2023), the major element chemical composition of the Pechinsko Pass epidotes reveals members of the clinozoisite-epidote solid solution (Armbruster et al, 2006), predominantly Fe-rich clinozoisite towards epidote in the second generation. Despite the chemical inhomogeneity, the crystal chemical formula of the pink generation, as determined by representative EDS data, ranges as follows: Ca 1.97-2.01 Mn 0.0-0.003 (Al 2.47-2.60 Fe 0.35-0.48 Mn 0.01-0.03 ) 2.97-3.00 [Si 2 O 7 ][SiO 4 ]O(OH) (Si 3.02-3.03 apfu); whereas, for the green generation, it is Ca 1.98-2.04 Mn 0.005-0.01 (Al 2.01-2.50 Fe 0.48-0.91 Mn 0.0-0.02 ) 2.96-3.00 [Si 2 O 7 ][SiO 4 ]O(OH) (Si 2.98-3.04 apfu).…”

“…The second locality represents an altered pegmatite outcropping around the Pechinsko Pass near the Strashimir Pb-Zn vein deposit with well-developed late epidote alteration zone. These epidotes, previously classified as members of the clinozoisite-epidote series, are manifested in two well-distinguished texturally, chemically and by color generations (pink and green) in association with chlorite, quartz and carbonates (Georgieva et al, 2023).…”

“…The pegmatite body crops out in the area of the Pechinsko Pass, close to the upper part of the Strashimir Pb-Zn vein deposit, Madan district. The pegmatite is emplaced at ~40 Ma (U-Pb titanite age, Georgieva et al, 2021Georgieva et al, , 2023 along the contact between marbles and gneisses with amphibolite lenses. Together with numerous similar pegmatite bodies, it intrudes the high-grade metamorphic rock succession with Jurassic protolithic age referred to as the Madan lithotectonic unit (Sarov et al, 2006b(Sarov et al, , 2010Jahn-Awe et al, 2012) of the Rhodope Metamorphic Complex.…”

“…In the pegmatite-altering hydrothermal mineralization, epidote associates with chlorite, quartz and minor carbonates. The morphology, mineral association and chemical properties of two well-distinguished epidote generations (pink and green) have been studied in details by Georgieva et al (2021Georgieva et al ( , 2023.…”