V.V.Kozlov, S.K.Smirnova, N.G.Syromjatnikov, V.L.Levin
Institute of Geology and Geophysics, Tashkent, Uzbekistan
There is a small group of volcanic-hosted low-sulfidation gold deposits with an anomalous regime of fluid evolution, e.g. high temperature jumps after the main stage of epithermal ore deposition. As a result, calc-silicate, skarn-like assemblages are developed after carbonate in quartz-carbonate-adularia gold bearing veins. Most of the deposits are distributed in Mesozoic volcanic terrains in the Russian Far East (Sidorov et al., 1989) as well as in the Chatkal-Kurama late-Paleozoic volcanic terrain (Uzbekistan and Tadjikistan, Central Asia). Among these deposits are the Chadak, Kyzylcheku, Shkol'noe, and Bichanzor gold deposits, and several small occurrences not of commercial importance. As a rule, not all veins contain high temperature calc-silicates (wollastonite, garnet, epidote), but only those located in the most heated parts of deposits, e.g. near an intrusion cupola (Chadak) and/or intramineralization granitoid dikes (Shkol'noe). Few fluid inclusion and related stable isotope studies have examined the details of mineral-forming processes in the deposits.
The Chadak gold-silver deposit is situated in the north-eastern part of Kurama ridge, eastern Uzbekistan. It consists of four major NNW- trending vein systems (the Pirmyrab, Guzaksai, Akbulak-Karakutan and Aktash systems) cross-cutting the margins of an early Permian volcanic-tectonic depression structure. The vein field (>150 veins) occupies an area of more then 6 x 7 km. The veins extend over 800-1000m vertically but only the upper 150-200m of the vein systems contain significant gold values, and no more 30 veins contain any gold mineralization at all. The widths of the main veins range from 1.5-2 to 15-20m. The geologic units exposed in the Chadak vein field comprise late Paleozoic volcanic and volcanoclastic sedimentary rocks, which are intruded by volcanic plugs, porphyritic intrusions and dikes. Volcanic rocks are subdivided (after V.P.Korzhaev et al., unpublished data) into three major units: 1) late Carboniferous trachyandesite and dacite tuffs and flows, rhyodacite flows and volcanoclastic sediments (Minbulak and Nadak formations); 2) early Permian alkaline andesite-basalts, andesites, and volcanoclastic conglomerate and sandstones (Shurabsai and Ravash formations, total thickness is over 1km); 3) late Permian rhyolitic breccia flows, tuffs and extrusive bodies (Kyzylnura formation, thickness >250m). The intrusive rocks are mainly exposed in the north-western part of the vein field, as a large multiphase pluton. They are dominated by late Carboniferous - early Permian porphyry granodiorite and monzonite, intruded by alaskite stocks and quartz porphyry dikes. Intra-mineralization porphyry granite, syenite and basaltoid dikes and post-mineralization alkaline-basaltic dikes, cross-cutting the vein systems, also occur. Most of the veins are hosted by volcanic rocks of the Nadak formation and by the near-contact parts of granodiorite-monzonite pluton (Pirmyrab vein system). They are located below (-70 to -100m) the unconformity border between the Nadak and Shurabsai volcanic formations.
Mineralization and Fluid Characteristics
The mineralogy of the vein systems is very complex. 156 ore and gangue minerals, including 12 new unnamed minerals, have been identified. However, volumetrically, quartz is the most abundant mineral in the veins. Other common vein minerals are calcite, adularia, wollastonite, andradite, actinolite, hematite, and ankerite. The most important Au- and Ag-bearing phases are electrum of highly variable fineness, acanthite and Ag-Cu sulfides (jalpaite, mckinstryite etc.).
The Chadak vein systems developed during four major mineralization events, which represent different fluid pulses of different temperature and composition:
I. The oldest mineralization, comprises Au-Ag ore bearing quartz-calcite-adularia veins surrounded by a quartz-sericitic alteration halo. It is composed of two stages of ore deposition: an early quartz+calcite+pyrite+electrum stage and a late quartz+calcite+ adularia+electrum+acanthite stage. Bonanza ores, consisting of colloform rhythmically banded barren and electrum-rich fine- to coarse-crystalline quartz and adularia, occur in the upper parts of the Pirmyrab vein system.
Conditions of ore deposition were estimated by fluid inclusion (FI) microthemometry, mineral equilibria thermometry (the electrum-acanthite-sphalerite-pyrite geothermometer, N.Shikazono, 1985), bulk gas and leachate analyses, and stable isotope analyses of minerals.
Inclusions in gold-bearing quartz are generally small, and only nine samples (86 primary inclusions) were suitable for heating-stage tests. All these samples have been collected far from the most permeable parts of ore bodies, which are replaced by skarn-like mineralization during the post-ore period. The microthermometry gave the interval 272-364oC (av. 339oC) for primary FI in early stage quartz. Boiling conditions (coexisting vapor- rich and liquid-rich inclusions homogenizing at the same temperature) have been recognized in two samples. Pressures have been estimated in the interval 75-115 bar. For the late stage quartz, the homogenization temperature interval is 151-339oC (av. 263oC). Both types of homogenization, to liquid and to vapor phase, have been recognized. Liquid CO2-bearing inclusions (secondary?) also occur in the late stage quartz, but we can't specify their ages of formation. It is possible, that they have been trapped during post-ore mineralization events.
The mineral equilibria temperatures range from 263 to 349C (av. 306oC (24 tests)) for the first stage assemblages and from 192 to 292oC (av. 221oC (n=16)) for the second.
Atomic-absorption spectrometry, gas chromatography and ion-selective electrodes were used for bulk FI analyses in 11 quartz samples. The results were recalculated using water content in the samples. It was determined, that fluids were the chloride-bicarbonate Ca>Na >K>Mg solutions of low salinity (<5% NaCl eq.). They contain CO2 (1.7-5.0mol.%), CH4 (up to 0.2mol.%) and N2 (0.02-0.04mol.%); H2, C2H4 and O2 have not been detected.
Forty-three samples of quartz, adularia, calcite and wollastonite were analyzed for stable isotopes of oxygen and carbon. The oxygen isotope data combined with thermometry data show, that ore-forming fluids had inputs of magmatic and meteoric components (ca. 18Owater -5.0 to -2.3‰). Bonanza ores contain oxygen enriched in 18O (ca. 18Owater=+4.2‰).
II. Skarn-like calc-silicate, Ag-Bi-Pb-Zn-Cu-bearing mineralization replaced most of the older gold-bearing quartz-calcite-adularia veins in the Pirmyrab and Aktash vein systems and also occurs in the northern part of the Guzaksai and Akbulak-Karakutan vein systems. Skarn occurrences in the vein systems are concentrated in a NE-trending belt along the SW contact of the intrusive cupola in both intrusive and volcanic rocks. the skarn is composed of two major stage of formation: (1) an early wollastonite+garnet prograde stage and (2) a late quartz+actinolite+andradite+epidote+Fe-oxides+sulfides retrograde stage. Formation of the first stage minerals, notably wollastonite, is extensive at the quartz-calcite contacts. Calcite bands, blades, and lenses are usually replaced by wollastonite. Extensive recrystallization and partial redeposition of the older Au-Ag ores took place during both, prograde and retrograde stages.
Primary FI in the prograde stage minerals have not been found. Small secondary polyphase (liquid(L)+vapor(v)+1-2 anisotropic daughter minerals) usually occur in early quartz near thin veinlets of wollastonite. Mineral equilibria calculations suggest that mineral-forming fluids were the low CO2, oxidized alkaline solutions at the temperature range from 500 to 600oC. Based on the 18O value of wollastonite (+2.4‰), we suppose the mixed (magmatic-meteoric) origin of the fluid.
Six types of primary FI occur in quartz and epidote of the retrograde stage:
(1) 5-phase FI: L+gas+NaCl+2 anisotropic crystals (up to 40vol.%), Th=372-470oC, Td=410-531oC;
(2) 3-phase FI: L+gas+1 small anisotropic crystal, Th=370-411o;
(3) 3-phase CO2-bearing FI: L+CO2(l)+CO2(g), 10-15mol.%CO2, Th=283-323oC;
(4) CO2-rich low density FI: CO2(l)+CO2(g) with a small liquid rim, CO2 density 0.27-0.34g/cm3; Th=277-312oC;
(5) NaCl-bearing FI: L+v+NaCl, 29-38wt.% NaCl, Th=201-245oC;
(6) two phase FI: L+gas, Th=204-339oC.
The pressure, estimated from CO2-bearing inclusions, ranges from 250 to 450 bar.
III. Quartz-hematite-ankerite Cu-Bi-bearing mineralization comprise numerous quartz- hematite veins in the same vein systems. Only two-phase (L+v) primary inclusions occur in quartz crystals. The homogenization temperatures cluster between 150 and 272oC. The bulk gas and leachate analyses revealed the presence of CO2 (7.2-9.0 mol.%) and N2 (0.02 mol.%) in Ca>Mg>Na>K solutions. The oxygen isotope data for quartz (2) and carbonate (3) samples, combined with Th show the prevalence of meteoric water (ca. 18Ow.=-8.8‰) in the fluid.
IV. Quartz-barite-fluorite Pb-Zn-bearing mineralization comprise the small veins and veinlets cross-cutting all previously deposited mineralizations as well as latest alkaline basaltoid dikes. FI homogenization temperature ranges from 90 to 210oC.
Thus, the mineralogical and FI evidences suggest the invasion of high-temperature magmatic fluids into the buried meso-epithermal vein system and their progressive mixing with meteoric water and cooling.