VOLUME 52 (2)/August 2023

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 (An_10.1–33.2) and albite (An_0–7.6), which prevail over K-feldspar (Or_87.9–92.4), 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.

The seismic hazard assessment across the densely urbanized region in the Sofia Basin in Bulgaria can be improved through using fault data, in search of which we first aim to attain to a better knowledge on the past earthquake ground ruptures. Currently focused on the North-Sofia fault, we investigated whether, and how, the fluvial system responded to fault displacements at a site located near the village of Gorni Bogrov. At the studied site, a scarp delimits a distal alluvial fan on the upper surface from an alluvial plain on the lower surface. Although the scarp forms a complex structure that resembles a breached relay ramp, its fault origin is questionable, because the basin axial stream could have caused lateral erosion approaching the toe of the fan. The electrical resistivity profiles that we measured imply that recent deposits are associated with fault displacement. A fault located beneath the middle slope juxtaposes Neogene sand layers, covered by an alluvial fan in the footwall, with colluvial deposits laterally merged with deposits from the alluvial plain in the hangingwall. The scarp also contains a non-tectonic component related to lateral erosion affecting mainly the lower slope. Breaks on the scarp profile are interpreted to have originated through erosion repeatedly renewed due to multiple fault displacements in recent times. The interseismic intervals of the North-Sofia fault are inferred to be in order of thousands of years but not longer. The inferred interseismic intervals correspond approximately to those of the adjacent South-Ihtiman and distant Chirpan faults; however, they are noticeably shorter compared to the adjacent Zlatitsa and distant Krupnik faults. It seems likely that faults in a broad region on the Balkans release strain at remarkably different rates, independently from their proximity.

The adamas of Ancient peoples is proved to be the precious diamond. An ancient and uconventional diamond occurrence, the mines of Philippi in Macedonia (northern Greece), is geologically explained here. The most probable source is believed to be flat subduction-related (shoshonitic) lamprophyres that originated at a depth of >110 km within the mantle in the diamond stability field. The thick lithospheric mantle beneath Greece supports the occurrence of diamondiferous lamprophyric rocks in this area. However, no lamprophyre outcrops have been found, except for a few lamprophyre dikes 14 km SE of Philippi. Another explanation for the occurrence would implicate detrital diamonds that have been preserved in the local greenschist facies rocks. The XRD analysis of Philippi paragneiss showed the existence of the mantle mineral magnesiowüstite, which is a possible diamond indicator. The Philippi occurrence deserves detailed investigation involving alluvial sampling and ground magnetic surveys to locate the diamond source.