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БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2014“BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2014”

Albite in the propylitic zone of alteration from the Chelopech high-sulphidation epithermal Cu-Au deposit, Bulgaria: new dataАлбит от пропилитовата зона на изменения във високосулфидизираното епитермално Cu-Au находище Челопеч, България: нови данниSylvina GeorgievaСилвина Георгиева

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia; E-mail: sylvina@geology.bas.bg

Key words: albite, adularia, plagioclase, propylitization, Chelopech high-sulphidation deposit.

Introduction and geological setting

The Chelopech high-sulphidation epithermal Cu-Au deposit is located in the Elatsite-Chelopech ore field in the northern area of the Panagyurishte ore region. The genesis of the deposit is related to the Late Cretaceous magmatism, controlled by the central parts of the Chelopech volcano (Popov et al., 1983). It is hosted by an Upper Cretaceous volcanic and volcano-sedi-mentary sequences consisting of sedimentary rocks, dome-like bodies, lava flows, breccias with volcanic elements and various tuffs. The igneous rocks are de-fined dominantly as andesites, latites, dacites to tra-chydacites (Stoykov et al., 2002).

In the host rocks of the Chelopech deposit are dis-tinguished 3 hydrothermal alteration zones, laterally developed outwards from the ore bodies (Georgieva et al., 2002). The innermost zone represents advanced argillic alteration (AAA) with “vuggy” and massive silica areas. The mineral assemblage is composed of quartz, dickite, kaolinite, pyrite, alunite, APS (alu-minium phosphate-sulphate) minerals mainly svan-bergite, woodhouseite, svanbergite-woodhouseite solid solution, florencite, crandallite and anatase/ru-tile. Diaspore, pyrophyllite and zunyite are formed at deeper levels. The AAA zone is spatially followed by sericitic zone, characterized with quartz, sericite, illite, kaolinite, pyrite, rutile ± APS minerals. The external, propylitic zone is composed of quartz, calcite, chlorite, albite, sericite, pyrite and epidote. These three altera-tion zones are formed in a single hydrothermal stage as a result of interaction of the fluid with the host rocks and change of the thermo-chemical parameters in the system respectively.

This work is focused on a study of albite. It is ini-tiated due to certain controversy discussed in previ-ous research, concerning the presence of the mineral in the propylitic alteration. According to Chipchakova (1966) the propylitic alteration in the deposit is ex-

pressed mainly as chloritization, carbonatization and adularization. Adularia is observed as veinlets or lim-pid pseudorhombic crystals in magmatic plagioclase. The existence of the mineral is supposed by micro-scopic observations and due to increased K2O content in the studied rocks. Later Radonova (1969) describes the epidote-chlorite-albite propylitic alteration, where albite is developed as clean, transparent veinlets, or in separate zones of the magmatic plagioclase. The au-thor accepts that albite is mistaken for adularia owing to its purity and transparency and emphasizes the in-creased content of K2О in the volcanic rocks as typical for the Srednogorie zone.

Results and discussionThe studied samples of the propylitic altered rocks were collected from the present-day surface area and from the underground galleries of the deposit. The chemical composition of the albite, product of the alteration and the magmatic plagioclase was determined using a Jeol “Superprobe 733” with EDS – HNU “System 5000” in Geological Institute, BAS, Bulgaria. The analyses were made with acceleration voltage 14 kV, beam cur-rent 1 nA and beam diameter 1 µm.

The main alteration minerals in propylitic zone are quartz, calcite, chlorite, albite, sericite, pyrite and rarely epidote (Georgieva, 2014). The rocks that have undergone this alteration are with preserved texture and often partially preserved magmatic minerals as quartz, plagioclase and apatite. Hydrothermal quartz and sericite in this zone are developed in fine-grained aggregates and thin veins in phenocrysts or in the matrix. Calcite is common as single crystals, nests or veins across the whole samples. Chlorite occurs main-ly as nests replacing magmatic amphibole and biotite. Epidote is established rarely in single fine grains, as-sociating with chlorite and sericite. Pyrite impregna-tion is typical for propylitic alteration. Albite is ob-

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served mainly as metasomatic replacement on primary magmatic plagioclase as thin veins, nests or in sepa-rate zones together with quartz and sericite. The min-eral often forms limpid veinlets and aggregates up to 100 µm in thickness, that as a rule preserve the orien-tation of the host plagioclase (Fig. 1).

The chemical composition of albite is almost con-stant and pure. Na2O content is 11.34–11.76 wt.% (0.96–1.00 apfu), Al2O3 varies within 18.12–18.69 wt.% (0.95–0.97 apfu), whereas SiO2 ranges from 68.16–70.02 wt.% (3.01–3.04 apfu). Certain amount of Fe2O3 – up to 0.22 wt.%, CaO – 0.17 wt.%, BaO – 0.16 wt.% and K2O – 0.12 wt.% were also detected.

The chemical composition of the magmatic plagio-clase, onto which is developed hydrothermal metaso-matic albite, corresponds to andesine (An40.08–43.9). The content of Na2O is 5.89–6.54 wt.% (0.52–0.57 apfu), CaO varies within 8.47–8.87 wt.% (0.41–0.43 apfu), Al2O3 is 25.12–25.78 wt.% (1.35–1.36 apfu), while SiO2 ranges from 57.42–58.33 wt.% (2.62–2.63 apfu). Small amount of K2O – up to 0.74 wt.%, BaO – 0.37 wt.% and Fe2O3 – 0.52 wt.% are established.

Conducted studies prove that the thin limpid vein-lets and aggregates, observed in the magmatic pla-

gioclase from the propylitic zone of alteration in the Chelopech deposit are composed of albite. The pres-ence of this mineral along with carbonate, chlorite, sericite and epidote is typical for this zone, related to advanced argillic style of alteration. According to Rusinov (1972) the formation of albite or adularia in propylites depends of Na2O and K2O relative activ-ity. The existence of adularia in the altered rocks in the area of the deposit is not rejected, but still need more studies. It occurrence should be related with dif-ferent hydrothermal events e.g. adularia-sericitic type (Hayba et al., 1985) or at boiling on the liquid-vapor boundary, where quartz-adularia alteration occurs (Rychagov, 2003).

ReferencesChipchakova, S. 1966. Adularization of propylitized rocks of

the Chelopech gold-copper-pyrite deposit, district of Pirdop. – Rev. Bulg. Geol. Soc., 27, 3, 329–332 (in Bulgarian with English abstract).

Georgieva, S. 2014. Mineralogy and Zonal Development of the Hydrothermal Alteration in Chelopech Deposit and Relation to the Ore Mineralization. PhD thesis. Geol. Inst., BAS, 167 p.

Georgieva, S., N. Velinova, R. Petrunov, R. Moritz, I. Chambefort. 2002. Aluminium phosphate-sulphate miner-als in the Chelopech Cu-Au deposit: Spatial development, chemistry and genetic significance. – Geochem., Mineral. and petrol., 39, 39–51.

Hayba, D., P. Bethke, P. Heald, N. Foley. 1985. Geologic, min-eralogic and geochemical characteristics of volcanic-hosted epithermal precious-metal deposits. – In: Berger, B. R., P. M. Bethke (Eds.). Geology and Geochemistry of Epithermal Systems. Soc. Econ. Geol., Rev. Econ. Geol., 2, 129–168.

Popov, P., V. Vladimirov, S. Bakardjiev. 1983. Structural mod-el of the polyformation Chelopech ore field. – Geol. Ore Dep., 25, 5, 3–11 (in Russian).

Radonova, T. G. 1969. Secondary quartzites and propylites in the Chelopech copper-pyrite deposit (Bulgaria). – Rev. Bulg. Geol. Soc., 30, 3, 251–267 (in Russian with English abstract).

Rusinov, V. L. 1972. Geological and physico-chemical Regularities of propylitization. Moscow, Nauka, 202 p. (in Russian).

Rychagov, S. 2003. Evolution of Hydrothermal-magmatic Systems of Island Arcs. Moskow, RAS, 50 p. (in Russian).

Stoykov, S., Y. Yanev, R. Moritz, I. Katona. 2002. Geological structure and petrology of the late cretaceous Chelopech volcano, Srednogorie magmatic zone. – Geochem., Mineral. and petrol., 39, 27–38.

Fig. 1. Microphotograph of relations between albite veinlets and mag-matic plagioclaseAb, albite; pl, plagioclase; Ser, sericite; Ap, apatite. Transmitted light, crossed nicols.