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БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2017“BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2017”

Skarn mineralization from the North Petrovitsa Pb-Zn deposit, South BulgariaСкарнова минерализация от Pb-Zn находище Северна Петровица, Южна БългарияGeorgi Milenkov1, Rossitsa D. Vassileva2, Aaron Hantsche1, Kalin Kouzmanov1, Valentin Grozdev2

Георги Миленков1, Росица Д. Василева2, Аарон Хенчи1, Калин Кузманов1, Валентин Гроздев2

1 University of Geneva, Department of Earth Sciences, Rue des Maraîchers, 13, Geneva, Switzerland; E-mail: Georgi.Milenkov@etu.unige.ch2 Geological Institute, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria; E-mail: rosivas@geology.bas.bg

Keywords: distal skarn, clinopyroxene, rhodonite, Madan district, Central Rhodopes.

Introduction

A significant portion of the economic resources in the Tertiary Pb-Zn vein and replacement deposits in the Central Rhodopes, in Southern Bulgaria are concen-trated in metasomatic ore bodies developed on distal johannsenite-hedenbergite skarns, replacing marble lenses in a crystalline gneiss-amphibolite metamor-phic complex (Ivanov et al., 2000). In the skarn bod-ies, clinopyroxene is overprinted by retrograde pyrox-enoids, amphiboles, chlorite, carbonates, and quartz commonly associated with economic sulfide mineral-ization, dominated by galena-sphalerite-pyrite. Metal grade (as high as 8% total Zn+Pb) is variable between metasomatic and vein ore bodies (Milev et al., 1996).

Detailed field-oriented and geochemical studies of the prograde and retrograde skarn assemblages from the Madan deposits can help establishing the genetic relationships between skarn- and ore-forming pro-cesses in these complex ore bodies. In this contribu-tion we report new data on textural relationships and mineral chemistry of the distal skarn mineralization in new outcrops at the 865m mine level of the North Petrovitsa deposit (Fig. 1A).

Results

Field observations and textures. The clinopyroxene skarn bodies are formed entirely in a graphite-bear-ing marble layer of the gneiss-amphibolite meta-morphic sequence, however they are also associated with Al-bearing skarn assemblages in the adjacent aluminous rocks (gneiss, amphibolite, pegmatite). For example, the marble is in direct contact with a concordant pegmatite body that has been affected by such an Al-bearing skarn assemblage consisting of epidote, amphibole, and minor sericite, immediately

adjacent to the skarn bodies (Fig. 1A). In these clino-pyroxene skarns, decimetric prismatic aggregates of johannsenite-hedenbergite form three-dimensional radial structures with concentric successive growth bands. They consist of platy prismatic crystals of jo-hannsenite and minor hedenbergite, often intimately intergrown with rhodonite. Their elongation is paral-lel to the direction of the skarn front propagation. At the skarn-marble contact individual pyroxene crystals terminate as fibers or whiskers (Fig. 1B–C) represent-ing highly nonequilibrium crystals with large surface areas which result from rapid unidirectional growth from highly supersaturated solutions under a diffusion regime (Bonev, 1993).

In the skarn-hosted ore bodies, various textures of replacement, inheritance and overprinting are ob-served as a result of post-skarn infiltration and miner-alization. Sulphide mineralization is observed as nests, impregnations and rhythmic banded ores in the skarns. Concentric shell-like textures of replacement minerals suggest at least partial dissolution of the host carbon-ate rock, leaving empty space on top of the aggregates, often filled by later quartz and/or carbonates deposited at lower temperatures (Fig. 1B).

Mineral composition. Johannsenite (Mn-pyroxene) is typical for Zn-bearing distal skarns, where skarn silicates and later minerals are commonly enriched in Mn (Meinert et al., 2005). The primary skarn calc-sili-cate assembalge is dominated by high-Mn clinopyrox-enes (johannsenite and minor Mn-hedenbergite), and some rhodonite (Mn-pyroxenoide).The main variation in the pyroxene composition is in the major cations Mn-Fe-Mg. The Mn/Fe ratio varies in the range 2.3–7.5. The MnO content reaches 23.4 wt%, while FeO is 3.13–8.59 wt% and the MgO is generally low (up to 5.0 wt%). The average Al2O3 is 0.96 wt%, with a maximum of 2.11 wt%.

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Two morphological types of rhodonite are ob-served in the skarns, which show different physical and chemical characteristics: i) single crystals formed by topotaxic replacement after johansennite (Fig. 1C) inheriting the crystallographic orientation to the pre-cursor mineral (3.8–3.8 wt% CaO; ~5.0 wt% FeO; 0.85–1.09 wt% MgO), and ii) massive accumulation of rhodonite at the skarnification front (Fig. 1D), with-out inheritance of the crystallographic orientation to the johannsenite precursor (CaO 7.8–9.9 wt%; FeO <4.4 wt%; MgO up to 0.85 wt%).

Discussion and conclusionsIn the Madan deposits, pegmatite bodies are com-monly observed as layered or crosscutting intrusions relative to the metamorphic country rock. Skarns are often developed along the pegmatite-marble contact, however, considering the localized skarnification of both aluminosilicate and carbonate rocks, it appears that skarn-formation is not connected with pegmatite emplacement. Instead, the skarn-forming fluids used these lithological contacts as channels for infiltration-driven reactions. The same fluid pathways were used

by the lower-temperature ore-precipitating fluids which formed the economic Pb-Zn mineralization in the Madan deposits.

Acknowledgements: The results are part of the SNF 200021_165752 project “Distal skarns as an ore-forming environment”. Dr. Atanassova-Vladimirova is greatly acknowledged for the SEM-EDS analyses. The authors are grateful to Zlatko Kasabov, geologist of the Petrovitsa mine, for the assistance in the under-ground work.

ReferencesBonev, I. 1993. Non-equilibrium highly anisometric crystals

and whiskers of galena. – Mineral. Mag., 57, 231–240.Ivanov, Z., D. Dimov, S. Sarov. 2000. Structure of the Central

Rhodopes. – In: Ivanov, Z. (Ed.). Sructure, Alpine Evolution and Mineralizations of the Central Rhodopes Area (South Bulgaria). ABCD-GEODE Borovetz, Guide to Excursion B, 6–20.

Meinert, L., G. Dipple, S. Nicolescu. 2005. World skarn depos-its. – Econom. Geol., 100, 299–336.

Milev, V., V. Stanev, V. Ivanov. 1996. Mining Production in Bulgaria 1878–1995, Statistical Reference Book. Zemia–93 Press, 196 p. (in Bulgarian).

Fig. 1. Skarn mineralization in North Petrovitsa: A, photograph of the studied outcrop, showing relationships of the mineralized skarns with the host marbles, gneisses and concordant pegmatite dyke; B–D: BSE images of the prograde and retrograde skarn minerals; B, retrograde pyroxene alteration with formation of rhodonite-carbonate-quartz association; arrows indicate pyroxene whiskers; C, topotaxic pyroxene-rhodonite trans-formation; D, radial aggregates of altered pyroxene at the skarnification front, marked by secondary rhodonite and Mn-carbonates. Abbreviations: Amp, amphibole; C, calcite; Px, pyroxene; Q, quartz; Rdn, rhodonite; Rhz, rhodochrosite.