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Hydrothermal alterations in the Vršnik ore body of the Bučim Cu porphyry deposit, Republic of North Macedonia

DEPOSIT OVERVIEW The Buchim Cu porphyry deposit is situated approximatelly 80 km SE from Skopje, Republic of North Macedonia (FIGURE 1) in the contact zone of the Serbo-Macedonian Massif and the Vardar zone. The deposit represents a part of the Lece- Chalkidiki metallogenic belt (Serafimovski et al., 2016). The genesis of the deposit is associated with several intrusions of latitic and latitic andesitic composition, which intruded mica-schists, gneisses, and amphibolites of the Precambrian Serbo- Macedonian Massif. The U-Pb ages obtained from zircons isolated from the intrusions range from 24.04±0.77 to 24.51±0.89 (Lehman et al., 2013). Vrshnik ore body represents one of four ore bodies at the Buchim deposit (FIGURE 2). The hypogene mineralization at the Buchim deposit mostly occurs along contacts of the intrusions and their host rocks but the mineralized intrusion of the Vrshnik ore body represents an exception. The mineralization is comprised of stockworks and disseminations with chalcopyrite, bornite, and cubanite, as the most common Cu ore minerals. The ore mineralization is associated with pyrite, magnetite, hematite, and minor amounts of native Au (Serafimovski et al., 2008). ALTERATION ZONES IN THE VRSHNIK ORE BODY Analyzed samples were collected from the Vrshnik open-pit (FIGURE 3). X-ray powder diffraction (XRPD) analyses, together with preliminary petrographic and ore microscopic analyses of the collected samples, revealed presence of four hydrothermal alteration zones: potassic, phyllic, propylitic and argillic alteration zones (FIGURE 3 & 4). Potassic alteration is characterized by a distinctive reddish color of the rock and abundant presence of K-feldspars. Microscopical and XRPD analyses revealed the presence of hydrothermal (“jagged”) biotite, hydrothermal K-feldspar (adularia), extensive albitization and sericitization of plagioclase. Chalcopyrite and bornite are the most common ore minerals within the zone of potassic alterations. In the outcrop, a transition from the potassic to phyllic alteration zone is marked by a color change from reddish to grey-greenish tint. The phyllic alteration zone is characterized by complete sericitization of plagioclases and predomination of pyrite over other sulphides. Chlorites, hematite, and magnetite veins are also present. Propylitic alteration envelopes a large portion of andesite in the Vrshnik ore body. It is characterized by an abundance of chlorite and pyrite, and the presence of carbonate veins, mostly composed of siderite and calcite. Some features of argillic alteration were observed, especially in the upper parts of the ore body, where clay minerals (kaolinite, illite) are present, and somewhat abundant. More research is needed to determine whether such parts belong to argillic or phyllic alteration. In some propylitised parts of the intrusion, covelline and cuprite were determined, indicating the presence of cementation zone.

North Macedonia, Buchim mine, powder X-ray powder diffraction (XRPD), Porphyry Cu deposit, hydrothermal alterations, sulphides

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