Analytical studies on the hand specimens from the Blagojev Kamen W-Au ore field: An approach to its ore genesis

Abstract

The Blagojev Kamen, a.k.a. Grabova Reka, deposit located in Eastern Serbia played a key role in the underground exploitation of the significant metals like W-Au and occasionally Sn and Ag in the 1950-1960s of Yugoslavia, which was later to be ceased. The mine is currently flooded. So as to rekindle the expectancy for revealing the potential W-Au in the area, a comprehensive study has been conducted in the scope of iTARG3T (Innovative Targeting & Processing of W–Sn–Ta–Li Ores) project supported by EIT (European Institute of Technology). The study area belongs in the orogenic belt of East Serbian Carpatho-Balkanides that spans through Romania, Serbia and Bulgaria, and is affiliated with the Neresnica-Beljanica metallogenic zone of Eastern Serbia. The metamorphic facies of green schist and amphibolite of Precambrian period and of Phanerozoic eon and mafic volcano-sedimentary sequence are intruded by Variscan granitoids that ultimately introduced a variety of metallic minerals into the system. These metamorphic facies mark a progressive increase in pressure and temperature southwards along the sampling locations. The host rock assemblages consist principally of chlorite schist, chloritic schist of an igneous origin and some meta-volcanoclastics. The prominent hydrothermal activities led to wolfram and gold mineralizations within quartz veins and stockworks along the fractures of greenschist. The placer gold obtained that were obtained along the River Pek values up to 30g/ton (10g/ton on average), and WO3 content varies between 0.2 and 2.0 wt. %. In the vicinity of the study area are also Pb-Zn ore bodies which are believed to have contaminated the W-Au bearing quartz veins judging by the peak anomalies observed in the geochemical analyses. A total of twenty hand specimens have been procured from the outcrops that are situated in the vicinity of Blagojev Kamen in order that their mineralogical, petrographical and geochemical characteristics could be revealed via analyses. To start off, rock samples that might have had the possibility of bearing important metallic minerals have been determined and analyzed. The sample T10-D has showed a notable concentration for the manganese dioxide mineral termed pyrolusite in the XRD profile. According to the XRD analyses, four major alteration types have been characterized: (1) Propylitic (quartz, Ca-rich albite, Ca-rich anorthite, pargasite, ferropargasite, dravite (Mg-rich tourmaline), tremolite (amphibole group), phlogopite (end-member of biotite), clinochlore (chlorite), montmorillonite and epidote; (2) argillic (quartz, muscovite mica and clay minerals); (3) sericitic (phyllic) (quartz and muscovite mica); (4) potassic (quartz, K-feldspar, biotite and anorthite). In addition, pyritization and sericitization have been distinguished in the select samples. Pyrite, chalcopyrite, goethite, sphalerite, ilmenite (?) and pyrolusite minerals have been encountered in the polished sections of the green schist and amphibolite under the ore microscopy. Pyrite, being the most abundant, usually has euhedral to subhedral cubic crystal structures, while showing off a epigenetic cataclastic texture in one of the sections. The pyrolusite anomaly seen in the XRD diffractogram has been verified by microscopic observations, as well. Sphalerite shows as a replacement product of pyrite and on occassion, of chalcopyrite. The chemical compositions of the metamorphic rock samples have been found out. The results yielded very low concentrations of W (9.28 ppm on average) and Mn (1052 ppm on average) meanwhile Au has been detected in none of the samples. A small amount of Pb concentration could be attributed to the contamination from the neighbouring Pb-Zn ore bodies. Stable isotope analyses for S and O isotope ratios have been carried out. 34S/32S (pyrite) and 18O/16O (quartz) values for the Blagojev Kamen deposit correspond to an average of 2.15‰ and 11.60‰, respectively. δ18Owater has been calculated to be 3.24‰ and δDwater to be 35.92‰. Based on these values and assumptions, the sulfur-rich fluids have an igneous origin while the water that contributed to the precipitation of the metallic ore minerals originated from a metamorphic genesis. For the purpose that the salinity and temperature conditions of the ore-bearing fluids could be presented, fluid inclusion studies have been conducted on the quartz minerals of the hand specimens. The measurement results indicated a low to intermediate salinity window (2.57 – 13.29 equiv. wt.% NaCl) and an intermediate homogenization temperature range (186.1 oC – 390.3 oC) for primary inclusions. The Th-Salinty plot of both primary and secondary inclusions illustrated that a likely retrograde boiling event might have occurred during the mineralization stage of the metallic ore minerals. However, some inclusions have been interpreted to contain a noticeable CO2-Clathrate gas combination as the Tm values for these inclusions fall in the +0 oC - +10 oC array. Utilizing the thermal profiles correlated with vapor pressures and densities, the following factors for the Blagojev Kamen W-Au deposit have been deduced: (1) Fluid pressure≈ 61 bar (6.1 MPa); (2) Fluid depth≈ 695m; and (3) Fluid density≈ 0.82 g/cm3. The crucial mechanism behind the W-Au mineralization for the Blagojev Kamen deposit is believed to be interconnected with regional metamorphism that caused the ore-forming solutions to be transported through fractures and micro-cracks in the host rock.