Metallogeny of the Făgăraş Mountains (South Carpathians, Romania): an overview with focus on the gold-bearing rocks

This study evaluates the relationship between the metamorphic and mineralization processes in the Făgăraş Mountains (South Carpathians, Romania). Recent emphasized goldbearing areas are discussed in this work. We present the preliminary data on the gold contents in different rocks and minerals, in relation with their geologic and tectonic setting.
   The Făgăraş Crystalline, representing the crystalline basement of the Făgăraş Mountains, is the eastern sector of the Getic Crystalline, the only one affected by Alpine low grade metamorphism (M3). There is a lithologic contrast, with genetic connotation, between the lower (gneissic) and the upper (mostly micaceous) structural levels and the Pre-Alpine metamorphic history with two medium grade events of regional metamorphism, M1 (Cadomian) and M2 (Variscan). Within Făgăraş Crystalline, the upper structural level corresponds to the Făgăraş Series, comprised of two formations, Şerbota (micaceous and phaneroblastic) and Suru (quartz-micaceous and microblastic, plus interlayered amphibolites closely associated with carbonate rocks). The lower structural level corresponds to the Cumpăna Series, also comprised of two formations, Cumpăna (augen gneisses) and Topolog (mica gneisses with lenses of amphibolites closely associated with quartzo-feldspathic gneisses).
   On the basis of the published data and our observations, the mineralizations from Făgăraş Mountains could be classified as follows: A. premetamorphic regionally metamorphosed (A.I. Pb-Zn stratiform mineralization within carbonate-dominated rocks; A.II. Ni-Cu-Co mineralization associated with bodies of metabasic and metaultrabasic rocks); B. Alpine or Alpine remobilization in metamorphic domains (B.I. pollymetalic mineralization: Pb-Zn-Cu, Pb-Zn ± Au, Ag; B.II. mineralization of Fe-Ti oxides, graphite ± sulfides and Au); C. associated to post-metamorphic (post-M3) tectonic planes (C.I. sulphide ± Au mineralization associated to major faults from Făgăraş Mountains).
   Concerning the gold concentrations, we analysed representative samples from: 1) both levels of Făgăraş Crystalline; 2) hydrothermalized tectonic breccias from major tectonic nodes (e.g., Perişani, Nucşoara); 3) quartz segregations from Cozia-Lotru and Sebeşul de Jos-Nucşoara major faults; 4) basalt dyke from a NW-SE strike fracture.
   The analyses show gold presence (as “invisible gold”) within rocks and minerals belonging to the Făgăraş Series: up to 3.12 ppm Au in quartz segregations within the mylonitic rocks of the Suru Formation and up to 0.98 ppm Au in quartz segregations within the micaceous rocks of the Şerbota Formation. The concentration of the gold is higher (5.46 ppm) in graphitic mylonites occurring on the Suru Nappe overthrust plane. According to the above classification, the mineralization associated with these mylonites is of type B.II (related to a shear zone). Unmineralized amphibolitic and carbonatic rocks exibit very low contents of gold. The basalt dyke contains 0.005 ppm Au. Gold concentrations in rocks belonging to the Cumpăna Series are lower than of rocks from the Făgăraş Series. However, some of the orthoamphibolites (+ sulfides) lenses from Topolog Formation have significant gold values. The highest gold contents were found in hydrothermalized tectonic breccias generated at the intersection between the Palaeogene longitudinal faults and Miocene – Quaternary transversal faults. The mineralization associated with these breccias is of type C.I. (see above) and consists of pyrite ± galena, sphalerite, marcasite, etc. Thus, on the Curmatura Oticului Fault, the Cernat Valley sector, a sample of hydrothermalized breccia contains 4.47 ppm Au. The matrix of the breccia consists of chlorite, sericite, clay minerals and fine disseminated pyrite; the composition of the clasts is dominated by quartz. On the Cozia-Lotru Fault, in a hydrothermalized breccia from the Perişani tectonic node, a content of 29.97 ppm Au was determined in the sulphide mineralization (predominantly pyrite and marcasite) and 0.98 ppm Au in the adjacent chloritic zone. Further research is necessary to understand the relationship between the gold-bearing mineralizations and their settings.