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The ore forming fluids in the Allchar polymetallic ore field, FYR Macedonia.

1. Introduction The Allchar polymetallic ore field is located at the southern margin of the Vardar zone, 110 km SE from Skopje, FYR Macedonia. Mineralization of Allchar is subdivided into three zones: (1) High temperature sulfide mineral paragenesis represented mainly by Sb and As mineralization ; (2) Sulfide mineralization barren of Sb and enriched in As and Tl and (3) Low temperature mineral paragenesis represented by barite, native sulfur, calcite, chalcedony and opal (IVANOV, 1986). The Allchar deposit is related to a Pliocene volcano-intrusive complex which occurs along a major regional fault zone between the rigid Pellagonian block on the west and the labile Vardar zone on the east. The basement is composed of Triassic sediments, the Jurassic ophiolites and the Cretaceous sediments (JANKOVIĆ & JELENOVIĆ, 1994). The rocks of volcano-intrusive complex range from andesite-quartz latite to rhyolite and trachyte (BOEV, 1993). Mineralization is spatially related to zonal hydrothermal alterations. The main types of alterations are silicification and argilization. The silification in the southern part of the ore field (zone I) represented by small quartz grains is related to antimony and gold mineralization. Argillitization is mainly developed in the volcanic rocks. In the northern part of the ore field (zone III) silification is represented by occurrence of chalcedony and opal (BOEV, 1993). 2. Samples and methods A fluid inclusion (FI) study was carried out to estimate the P-T conditions during mineralization and to characterize the mineralizing fluid. Microthermometric measurements were performed on primary FIs within doubly polished, ~0.5 mm thick, wafers of: (1) Quartz associated with high temperature sulfide mineralization (zone I) ; (2) Realgar and orpiment associated with sulfide mineralization barren of Sb and enriched in As and Tl (zone II) and (3) Opal associated with low temperature mineralization (zone III). Ultraviolet (UV) fluorescence microscopy was used to estimate the presence of hydrocarbons bearing inclusions. Analyses of evaporate mounds by scanning electron microscope equipped with EDX (SEM/EDX) were performed in order to constrain the chemical composition of mineralizing fluid. Analyses were focused on realgar and orpiment used for microthermometry. Evaporate mounds were prepared according to the procedure described by KONTAK (2004). 3. Results Microscopic examination of quartz, realgar and orpiment revealed the following types of FIs: Type I. Two phases (L+V) aqueous FIs frequently show progressive formation of negative crystal forms. Degree of fill (F) is between 0.85 to 0.95 ; Type II. Monophase, secondary, liquid FIs show mostly irregular forms. In quartz samples the FIs of Type III. are also present (monophase, anisotropic solid inclusions, with prismatic habitus and of primary origin). The following types of inclusion within opal were distinguish: Type I. Three phases (L1+L2+V) inclusions with irregular shapes and uniform degree of fill (0.95) ; Type II. Multiphase hydrocarbons inclusions usually occur as large, randomly distributed groups, restricted to healed fractures and sharp trails ; Type III. Monophase organic solid inclusions coexist with Type II inclusions. Microthermometric measurements were performed on FIs of Type 1 within quartz samples. Eutectic temperature (Te) between -44.0 and -58.0°C indicates presence of bivalent cations. Melting temperature of hydrate (Tmhyd1) varies between -42.0 and -24.2°C. Ice melting temperatures (Tmice) in the range from -2.4 to -18.1°C points to salinity between 4.0 to 21.3 wt.% NaCl equ. Homogenization into liquid phase is recorded in interval between 131.0 to 201.0°C. Microthermometry measurements within realgar confirmed presence of two different fluid inclusion types. Phase transitions were observed only within inclusions of Type I. Te is recorded in interval between – 50.3 and – 54.2°C. Melting runs in the temperature range between -35 and 0°C determined existence of two hydrates. Melting of the first hydrate (Tmhyd1) was recorded in temperature interval between -22.0 and -24.5°C. Temperature interval of the final melting of the second hydrate (Tmhyd2) is observed between -11.0 and -15.4°C. Tmice between -1.5 and -4.1°C corresponds to salinity of 2.56 to 6.85 wt.% NaCl equ. Homogenization (Th) follows disappearance of vapor at 120-141.1°C. Microthermometry measurements performed on FIs within orpiment samples gave data in the same range as those within realgar. Th was not recorded due to massive decrepitation of FIs at lower temperatures. Heating/freezing runs were performed on FIs of Type I and Type 3 within opal. Type I: Freezing was observed by the collapse of vapor bubble. During melting runs first phase change within FIs occur in temperature range from 2.4 to 7.0°C, by the abrupt appearance of vapor bubble. Total homogenization to liquid state was recorded at temperature range from 102.0 to 125.0°C. Complex hydrocarbons bearing inclusions of Type 2 did not freeze even at minimum stage temperature of -180°C. Th was not recorded due to massive decrepitation of FIs in the interval between 170 to 180°C. Analyses of realgar samples indicate Na and K chlorides composition of evaporate mounds. The presence of Al is also observed. A comparison of composition of evaporates mounds and microthermometric measurements points to composition of hydrates present within frozen inclusions. Hydrate with final melting temperature between -22.0 and -24.5°C (Tmhyd1) is hydrohalite (NaCl×2H2O). Hydrate with temperature between -11.0 and -15.4°C (Tmhyd2) is KCl×nH2O. Analyses of orpiment samples point that evaporate mounds consist only of Na and K chlorides. 4. Discussion and conclusion The Allchar deposit shows characteristics of Carlin-type mineralization: (1) strong structural control of mineralization by faults and folds ; (2) calcareous sedimentary host rocks of diverse facies, + igneous rocks ; (3) decarbonation, argillization, silification and sulfidation alterations ; (4) submicron gold in assocciation with pyrite, arsenian pyrite and arsenopyrite and (5) geochemical signature of Au, As, Hg, Sb and Tl. Gold was probably carried by bisulfide complexes and deposited in both intensely silicified parts and marginal argillic alteration zone, due to a combination of decreasing pressure and temperature, culminating in boiling and increasing pH. Thallium minerals (sulfides and oxides) can not be formed in neutral and acid conditions (Naumov, 2000). The hydrothermal fluids become alkaline as a result of degassing and removing CO2 in epithermal conditions. In such conditions the alkaline hydrotherms can deposit the thallium minerals in association with adular, zeolite, illite and other minerals - indicators of the high pH values. The role of organic matter in Carlin-type of mineralization is still unknown. BOEV, B (1993): Geology of Allshar polymetallic deposit, Macedonia. – Geologica Mecedonica, 7, 35-39. IVANOV, T. (1986): Allshar the richest ore deposit of Tl in the world. – In: NOLTE, E. (ed.): Proceedings of the Workshop on the feasibility of the Solar Neutrino Detection with 205Pb by geochemical and mass spectroscopical measurements. – Report GSI-86-9: Technische Universität München, pp. 1-6. JANKOVIĆ, S. & JELENOVIĆ, R. (1994): Thallium mineralization in the Allchar Sb-As-Tl-Au deposit. -N. Jb. Miner. Abh. 167: 283-297. KONTAK, D.J. (2004): Analysis of evaporate mounds as a complement to fluid-inclusion thermometric data: case studies from granitic environments in Nova Scotia and Peru. – Can.Mineral., 42, 1315-1329. NAUMOV, E. (2000): Behaviour of Thallium in Ore-Forming Processes on Gold-Mercury and Antimony-Mercury Deposits. -In: EMPG VIII Journal of Conference Abstracts, vol. 5, no. 1, Bergamo, Italy

gold; arsenic; thallium; Carlin-type; alteration

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