ISSN 10757015, Geology of Ore Deposits, 2016, Vol. 58, No. 2, pp. 134–148. © Pleiades Publishing, Ltd., 2016. Original Russian Text © B.B. Damdinov, S.M. Zhmodik, P.A. Roshchektaev, L.B. Damdinova, 2016, published in Geologiya Rudnykh Mestorozhdenii, 2016, Vol. 58, No. 2, pp. 154–170.
Composition and Genesis of the Konevinsky Gold Deposit, Eastern Sayan, Russia B. B. Damdinova, S. M. Zhmodikb, P. A. Roshchektaevc, and L. B. Damdinovaa a
Geological Institute, Siberian Branch, Russian Academy of Sciences, ul. Sakh’yanovoi 6a, UlanUde, 670047 Russia b Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, prosp. Akademika Koptyuga 3, Novosibirsk, 630090 Russia c Rifei OOO, ul. Babushkina 185, UlanUde, 670031 Russia Received March 24, 2014
Abstract—The Konevinsky gold deposit in southeast Eastern Sayan is distinguished from most known depos its in this region (ZunKholba, etc.) by the geological setting and composition of mineralization. To elucidate the cause of the peculiar mineralization, we have studied the composition, formation conditions, and origin of this deposit, which is related to the Ordovician granitoid pluton 445–441 Ma in age cut by intermediate and basic dikes spatially associated with metavolcanic rocks of the Devonian–Carboniferous Ilei Sequence. Four mineral assemblages are recognized: (1) quartz–pyrite–molybdenite, (2) quartz–gold–pyrite, (3) gold–polysulfide, and (4) telluride. Certain indications show that the ore was formed as a result of the super position of two distinct mineral assemblages differing in age. The first stage dated at ~440 Ma is related to intrusions generating Cu–Mo–Au porphyry mineralization and gold–polysulfide veins. The second stage is controlled by dikes pertaining to the Devonian–Carboniferous volcanic–plutonic association. The second stage is characterized by gain of Hg and Te and formation of gold–mercury–telluride paragenesis. DOI: 10.1134/S1075701516020033
INTRODUCTION A significant number of gold deposits and prospects are known in southeast Eastern Sayan. The largest gold– quartz–sulfide deposits make up the Urik–Kitoi gold ore zone (Mironov and Zhmodik, 1999). The basemetal ores at these deposits are characterized by quartz veins and mineralized zones composed of massive sulfide ore. Itrusionrelated deposits and occurrences are less abun dant. They are localized in the Tissa–Sarkhoi and Ospa gold ore clusters (Damdinov et al., 2007; Mironov et al., 2001). These deposits are distinguished by widespread telluride mineralization. One more object, revealed recently, is represented by the intrusionrelated Pogran ichny Au–Bi occurrence (Garmaev et al., 2013), which is characterized by the gold–bismuth mineral assemblage widespread in northeast Russia (Goryachev and Gamya nin, 2006). A study of the Au–Bi–Te mineralization inherent to the Konevinsky deposit has shown that the latter significantly differs from other gold deposits known in this region. To elucidate the cause of the peculiar min eralization, we have studied the composition, formation conditions, and origin of the deposit. We present the results in this paper. ANALYTICAL METHODS The used techniques include conventional meth ods applied to provide insights into ore composition: Corresponding author: B.B. Damdinov; Email:
[email protected]
petrographic and ore microscopy combined with examination on a Leo1430 SEM equipped with Inca Energy EDS (analyst S.V. Kanakin). The bulk chemi cal composition of rocks was determined by chemical analysis at the Geological Institute, SB, RAS; the con tents of trace elements and gold were measured by XRF, AAS, and the chemical–spectral method (analysts A.A. Tsyrenova, B.Zh. Zhalsaraev, E.M. Tat’yankina). The isotopic Ar/Ar age of micas was measured at the Analytical Center of Institute of Geology and Miner alogy, SB, RAS, using the technique described by Travin (2009). The sulfur isotopic composition was determined at the Far East Geological Institute, FEB, RAS (analyst T.A. Velivetskaya). To study inclusions of mineralforming media, we used a Linkam THMS600 microscopic stage with a temperature measurement range from –196 to +600°С at the Institute of Geology and Mineralogy, SB, RAS. Approximate estimation of salt contents in inclusions and solution density were calculated with the FLINCOR program (Brown, 1989). GEOLOGY OF DEPOSIT The Konevinsky gold deposit is located in south east Eastern Sayan within the Oka lithotectonic zone as a constituent of the Khuzhir ore field. The deposit is spatially related to the Sailag granite–granodiorite pluton pertaining to the Early Paleozoic Tannuola intrusive complex (Fig. 1). This pluton is a round
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intrusive body 12 × 7 km in plan view, which cuts through carbonate rocks of the Irkut Formation and comes into contact with volcanic rocks of the Ilei Sequence in the west. The medium to finegrained granodiorite contains numerous mafic inclusions and is affected by extensive beresitization, silicification, and feldspathization. The Early Paleozoic age of the Tannuola intrusive complex is confirmed by intrusive contacts with the Vendian–Lower Paleozoic sedimentary rocks of the Irkut Formation and by crosscutting relationships of Devonian dikes (Ognit complex of alkaline rocks) with granitoids. The isotopic age of Tannuola granitoids is 450–480 Ma (Fedotova and Khain, 2002). The Sailag pluton is also cut through by basic and intermediate subvolcanic dikes of the Ilei volcanic complex. The Ilei volcanic sequence is characterized by a twomember structure. The lower part of the section 480 m in thickness contains interlayers, extended beds, and lenses of volcanic–terrigenous rocks (tuf faceous conglomerate, gravelstone, tuffaceous sand stone, siltstone) incorporated into intermediate and felsic tuffs. The upper part no less than 250 m in thick ness consists of subalkaline rhyolite–rhyodacite lavas. The Ilei Sequence overlaps the Irkut Formation, as well as granitoids of the Urik and Tannuola complexes with angular and stratigraphic unconformity. In turn, the Ilei Sequence is crosscut by microdiorite dikes comagmatic to the lavas pertaining to this sequence. Thus, the Ilei Formation is no older than the Lower Paleozoic. The Ar/Ar age of muscovite from beresitized gran ite is 445 ± 4.5 Ma; the biotite age is 441.3 ± 4.6 Ma. The age interval 441–445 Ma corroborates the Paleo zoic age of granitoid magmatism (Late Ordovician). Biotite from microdiorite dike is dated at 324 Ma. Although this date does not strictly correspond to the Devonian age of volcanic rocks pertaining to the Ilei Sequence, it nevertheless indicates a younger age of volcanic–plutonic association. It should be noted that isotopic datings of volcanics from the Ilei Sequence are not available, and their age is based only on geolog ical position. The structure of deposit is controlled by two fault systems: (1) the nearly latitudinal Zhombolok Deep Fault and the NWtrending Sailag Normal Fault (Fig. 2). The fault systems are traced by subvolcanic intermedi ate and basic dikes of the Ilei volcanic–plutonic asso ciation. Almost all dikes are close in age. They are localized in the same fault systems and frequently crosscut one another. The relationships between dikes and ore zones also indicate their synchronism. In some localities, the dikes are crosscut by orecontrol ling zones, and in this case, they undergo metasomatic alteration. In other cases, dikes crosscut the mineral ized zones and remain unaltered. The dikes are char acterized by branching along the strike and updip approaching the surface. The thickness of dikes varies GEOLOGY OF ORE DEPOSITS
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from 10–20 cm to 3–5 m. The contacts with country rocks are distinct, even, or wavy. The dikes locally con tain xenoliths of country granodiorite and leucogranite. The veinlike orebodies at the Konevinsky deposit reveal the following morphology. The pivotal quartz vein or several veinlets occur in axial zone; the mar ginal zones are composed of Aubearing beresite enriched in sulfides up to 2–3%. The Au grade of ber esite gradually decreases at the distance of veins; how ever, beresite and beresitized granite here and there contain thin quartz–sulfide veins with elevated Au concentrations up to 1–2 gpt and higher. The total thickness of orebodies is 0.4–1.3 m, and they reach 472 m in extent. Quartz varies in color from light to darkgray. The quartz vein containing massive fahlore aggre gates is localized in the western ore field, where it is localized in a metabasic dike that transformed into listvenite among marmorized limestone of the Irkut Formation. The NEtrending dike 2.5 m thick (strike azimuth 65°–75° NE, dip angle 90°) contains an ore vein 0.1–0.3 m in thickness. The vein is located in the midst of the dike and is traced for 14 m; further the vein is covered with talus. The channel sampling yields the fol lowing results: 24.9 gpt Au, 528.6 gpt Ag, 22.16 % Cu, 3.3 % Zn, 0.055 % Cd, and 0.029% Bi. In contrast to the main ore of this deposit hosted in the granitoid pluton, the fahlore mineralization occurs in the listvenitized dike that cuts through carbonate rocks at the contact with the granitoid pluton. It should be noted that a great number of occurrences and deposits pertaining to the gold–fahlore mineral type occurs in the region under consideration. They are related to zones of silicified carbonate rocks (Dinamitny, Yuzhny, and other deposits) (Airiyants et al., 2007). In addition, small and poorly studied gold–silver–basemetal ore occurrences localized in terrigenous–carbonate rocks are frequent at the con tact with the Sailag granitoid pluton, which hosts the Konevinsky deposit. HOST ROCKS The host granitoids at the Konevinsky deposit are represented by granite and granodiorite. It should be noted that almost all intrusive rocks are subject to sec ondary alteration (sericitization, feldspathization, epidotization, chloritization) to one or another degree. Granite is composed of plagioclase, Kfeldspar, quartz, and biotite; zircon, apatite, and magnetite are accessory minerals. Feldspars are, as a rule, sericitized and chloritized. Granodiorite mainly consists of pla gioclase and small amounts of Kfeldspar, quartz, hornblende, secondary epidote and chlorite; acces sory apatite, zircon, titanite, and ore minerals. Grani toids are characterized by inequigranular up to por phyry structure.
DAMDINOV et al. R.
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Fig. 1. Schematic geological–structural map of the Oka–Sailag–Mungorga interfluve. (1) Quaternary boulders, sandstone, sand, clay, and cobbles; (2) Quaternary basalts; (3–5) Early–Middle Ordovician Tannuola complex: (3) second phase: granite, grano diorite, granodiorite porphyry, (4) first phase: granite, granodiorite, granite porphyry, (5) plagiogranite, granite, granodiorite, gra nodiorite porphyry; (6) Lower Riphean Sarkhoi Group: conglomerate, sandstone, felsic volcanic rocks; (7) Mesoproterozoic Irkut Formation: marble, limestone, metasandstone, carbonate–carbonaceous slate, amphibolite, and gneiss; (8) faults: (a) exposed and (b) traced beneath loose sediments; (9) Konevinsky ore zone; (10) contour of Koveninsky deposit shown in Fig. 2.
In bulk chemistry, granitoids of the Sailag pluton correspond to granodiorite, tonalite, and granite of K–Na series (Na2O/K2O = 0.64–2.24) and are charac terized by elevated alkalinity, containing up to 5.41 wt % Na2O and 5.12 wt % K2O (Table 1). In concentrations of trace elements, granite and granodiorite of the Konevinsky deposits are close to granitoids of calc alkaline series (Tauson, 1977), which, as known, are formed in a geodynamic setting of active continental margins. The distribution of trace elements, including REE, was considered by Zhmodik et al. (2006). The REE pattern has a weak negative slope typical of crustal rocks (Fig. 3a). The spidergram of trace ele ments closely corresponds to the upper crust composi
tion (Kerrich and Wyman, 1997) (Fig. 3b). This implies that granitoids were formed with predominant involvement of upper crustal matter. At the same time, the Nb, Ti, and Pb minima in the spidergram are char acteristic of the islandarc setting. By HFSE (Y, Nb, Zr) contents, the chemical composition of rocks fits those of islandarc granites (Pearce et al., 1984), evi dence that they formed at the active continental mar gin. Granitoids of the Sailag pluton are distinguished by an elevated Au content (0.05–0.25 ppm) and uni form fine sulfide (pyrite, chalcopyrite) dissemina tions. In addition, thin quartz–pyrite–molybdenite veinlets occur in slightly altered granitoids (Fig. 4a). The elevated Au concentrations and occurrence of GEOLOGY OF ORE DEPOSITS
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Fig. 2. Geological map of Konevinsky deposit. (1) Quaternary sediments, unspecified; (2) dolerite (β), porphyritic dolerite and diorite (δπ), gabbrodiorite and microdiorite (mδ) dikes; (3–5) Tannuola complex: (3) first phase: granodiorite and monzograno diorite; (4) second phase: diorite; (5) leucogranite; (6) faults: (a) mapped and (b) inferred; (7) geological boundaries: (a) mapped and (b) with gradual transition; (8) spot height; (9) orebodies: quartz veins and beresitization zones.
dispersed stringer–disseminated sulfide mineraliza tion in granitoids along with gold–basemetal occur rences near the massif contact are evidence for the high oreforming potential of granitoids. Dikes are strongly altered. In outer appearance, these are compact finegrained dark gray or greenish gray rocks, frequently foliated and deformed into folds; less frequently, the dikes consist of massive fine grained porphyritic rock. Under a microscope, these rocks are represented by mica–chlorite or quartz– albite–carbonate–mica aggregate. Microstructures are lepidoblastic or blastomylonitic. Sporadic apatite and zircon grains are accessory minerals. Judging by secondary alterations, the intermediate–basic rocks GEOLOGY OF ORE DEPOSITS
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served as a protolith. In chemistry, the dike rocks cor respond to metavolcanic rocks of the Ilei Sequence. ORE MINERALS The orebodies at this deposit are represented by quartz veins, as well as linear vein and veinlet zones with beresitization zones as selvages. Ore minerals in veins occur as clusters composed of sulfides, sulfosalts, and tellurides. In addition, stringer–disseminated sul fide mineralization occurs in the groundmass of gran itoids. The chemical compositions of minerals are given in Table 2.
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Table 1. Contents of major (wt %) and minor (ppm) elements in granitoids of Sailag pluton Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total Ba V Cr Ni Cu Zn Ga Rb Sr Y Zr Nb Mo Pb Th U
BC75
BC76
BC81
BC82
67.09 65.62 76.4 74.25 0.57 0.665 0.164 0.12 15.44 15.63 11.01 11.99 3.3 4.21 2.68 3.07 0.059 0.075 0.03 0.03 1.13 1.64 0.1 0.1 1.78 3.06 0.18 0.28 4.15 4.08 2.99 3.26 3.79 3.41 5.11 5.12 0.13 0.159 0.03 0.03 1.76 1.04 1.46 1.82 99.20 99.59 100.01 99.91 1080 860 350 430 41 44 26 15 7 8 15 7 10 14 15 4 10 11 33 6 31 58 14 12 16 16 15 14 104 86 163 149 431 396 68 91 17 21 3 4 181 225 45 46 20 21 16 7 0.69 0.92 2.2 1.45 21 21 63 47 8.2 9 8.6 13.1 3.1 2.4 4.1 1.8
BC83
BC101
70.39 65.66 0.423 0.6 14.97 15.53 2.21 3.53 0.043 0.058 0.88 2.11 1.82 2.52 3.87 4.59 3.6 3.52 0.088 0.148 1.15 1.40 99.45 99.666 900 1020 54 97 10 19 9 16 9 40 50 44 21 14 108 89 379 407 13 14 84 180 11 15 0.47 15 21 2 6 7 2.6 2.3
BC90 66.95 0.62 15.52 2.9 0.054 1.42 1.55 4.93 3.6 0.136 1.30 98.98 890 128 34 24 42 67 18 104 360 14 196 16 2.5 14 8.7 1.8
BC91
BC80
66.2 73.8 0.6 0.16 16.07 6.9 3.13 2.13 0.06 0.064 1.39 1.53 1.82 7.22 3.77 2.11 3.83 0.94 0.144 0.039 1.59 5.01 98.604 99.893 1010 150 103 17 27 17 24 11 31 3 69 9 16 7 117 23 331 565 14 9 145 81 22 4 5 3.15 10 47 8 3.5 2 3.1
BC77
BC98
66.48 65.64 0.56 0.57 15.17 14.64 3.18 3.3 0.057 0.05 1.93 1.71 3.92 2.35 3.55 5.41 3.06 3.35 0.139 0.14 0.94 2.29 98.986 99.45 700 90 94 92 120 45 25 18 37 26 56 40 18 15 67 76 517 155 12 11 153 119 16 17 3 4 14 12 7.5 8.5 2.8 0.9
Analyses were performed with XRF at Analytical Center, Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sci ences.
Molybdenite forms tabular grains and aggregates as veinlets reaching 3 cm across. The mineral occurs in the slightly altered granodiorite as thin films or flucans along fracture planes and in orebearing beresite and quartz veinlets as clusters or single grains (Fig. 4a). Ore pockets in quartz veins are devoid of molybdenite except for sporadic grains. Pyrite is the most abundant mineral at the deposit, which is represented by two generations. The early generation (pyrite1) occurs as disseminations and veined segregations of pyrite grains in association with chalcopyrite in beresites and beresitized granite, also as crystals in quartz–molybdenite veinlets. The sec ond generation (pyrite2) occurs as hypidiomorphic granular aggregates in selvages of veins and as corroded relics of euhedral crystals in aggregates of later chalcopy rite, fahlore, sphalerite, and galena (Figs. 5a, 5c). Sepa
rate quartz veinlets contain monomineralic aggregates of pyrite2 up to 3 cm across (Fig. 4b). No admixtures in pyrite have been identified above the detection limit (0.1 wt %). Chalcopyrite is observed in two generations. Chal copyrite1 occurs in association with pyrite2 as outer rims overgrowing the latter in beresite or beresitized granite; chalcopyrite2 is represented by allotriomor phicgranular aggregates in ore veins, emulsion in sphalerite, as well as inclusions, veinlets, and rims that corrode pyrite2 grains. Occasionally chalcopyrite rims sphalerite grains as inclusions within fahlore (Fig. 5b). Galena forms xenomorphic aggregates irregular in shape and with uneven margins in association with fahlore, sphalerite, and tetradymite. The grain size varies from fractions of a millimeter to 3.0 mm. The GEOLOGY OF ORE DEPOSITS
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Fig. 3. (a) REE pattern and (b) spidergram of granitoids from Sailag pluton, after Zhmodik et al. (2006); UCC, upper crust; LCC, lower crust; OIB, ocean island basalt.
mineral is often intergrown with native gold and occurs as galena–aikinite–tetradymite–fahlore and galena–sphalerite–chalcopyrite–fahlore aggregates (Figs. 5c, 5f). Sphalerite, which occurs as rounded isometric or less frequent oblong grains and aggregates, is inter grown with galena and often contains emulsion chal copyrite disseminations (Figs. 5b, 5c). Admixtures of Fe (up to 1.27 wt %), Cu (up to 1.27 wt %), and occa sionally, Cd (up to 0.60 wt %) are noted. Fahlore corresponds to tetrahedrite in chemical com position (Moёlo et al., 2008) and contains 2–4 wt % As. Ag (up to 1.97 wt %), Zn (up to 6.49 wt %), and Fe (up to 2.15 wt %) also occur in fahlore. The shape of grains is frequently rounded or irregular isometric; grain size is up to 2 mm; color is light gray in reflected light. As a GEOLOGY OF ORE DEPOSITS
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rule, fahlore occurs in association with sphalerite and galena aggregates, native gold, and tetradymite; it fre quently contains chalcopyrite inclusions (Figs. 5a–5d). Tetradymite is represented by elongated xenomorphic or, less frequently, small isometric grains up to 3 mm in size, which are frequently intergrown with galena, native gold, fahlore, and aikinite (PbCuBiS3). Bar shaped tetradymite crystals have been identified in galena aggregates (Fig. 5d). Tellurides (petzite, silvanite, hessite, altaite, colo radoite) make up the aggregates that fill the intergran ular space between earlier minerals and form rims that corrode these minerals (Fig. 5e). Native gold is diverse in morphology, but the most abundant are cloddy grains with uneven, often dentate rounded margins and uneven surfaces; in addition,
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(а)
(b) Fig. 4. Photographs of samples: (a) granodiorite with quartz–molybdenite veinlet, (b) beresite with quartz–pyrite veinlet and pyrite2 aggregate. Dotted line is veinlet boundary; dashed line is boundary of pyrite–molybdenite aggregate.
stringerlike and rounded grains are also frequent. Gold grains vary from 0.02 to 0.5 mm in size. Two genera tions of native gold are distinguished. Gold1 enters into the gold–quartz–pyrite assemblage and forms microinclusions, microstringers, intergrowths with pyrite, and less frequent grains in quartz of quartz– pyrire veinlets. Gold of this generation ranges in fine ness from 856 to 976‰ with a singlemode distribu tion and a maximum within 925–950‰. The elevated Hg and Cu concentrations as impurities in gold1 are not detected (Fig. 6a). Gold2 is a component of the gold–sulfide assem blage, where, in turn, two parageneses are distin guished: (i) gold–galena–sphalerite–fahlore and (ii) gold–galena–aikinite–fahlore–tetradymite (Figs. 5a, 5d, 5f). The fineness of gold2 is lower (719–943‰). The maximum of fineness falls in the 825–850‰ interval; smooth peaks are noted in the 775–800‰ and 875–900‰ intervals (Fig. 6b). In some cases, Hg (up to 6.51 wt %) and Cu (up to 0.91 wt %) are detected in gold2 as impurities. Gold2 is intergrown with galena, fahlore, and tetradymite or it occurs as stringers in quartz. The ore with the fahlore assemblage recorded in western part of the ore field is considered by Damdi nov et al. (2012). In particular, its composition has been described in detail; because of this, we restrict ourselves here to a brief description. The ore is com posed of fahlore aggregate represented by tetrahedrite, Agbearing tetrahedrite, tennantitetetrahedrite, and hakite (Cu7.355Ag2.733Hg1.812Fe0.206)12.06Sb4.042S12.87) with rare pyrite and chalcopyrite inclusions. This ore is characterized by abundant Au–Ag–Hg minerals that form fine dropshaped, rounded, or triangular, less fre quently, dendritic grains light yellow in color, +0.5, 38 sam ples in selection) between the above metals and with semimetals (Sb, Bi, Te), whereas Mo, which is related to the early assemblage, does not correlate with them.
The correlation between chemical elements of the basemetal assemblage is illustrated by the Cu–Sb binary (Fig. 7a). Positive Au–Cu correlation is noted for Aubearing beresite that hosts the earliest mineral assemblage (Fig. 7b). The Au content in the analyzed samples reaches 33.1 gpt, while the Ag content increases up to 550 gpt; GEOLOGY OF ORE DEPOSITS
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however, these noble metals do not show a significant correlation (Fig. 7c), because they differ in the mode of occurrence: native in sulfosalts (fahlore) and as late tellurides. The sulfur isotopic compositions are available only for one pyrite monofraction from a quartz pyrite vein let, molybdenite, and fahlore. The following δ34S val ues have been obtained: –1.2‰ for pyrite, +0.2‰ for molybdenite, and +2.8‰ for fahlore (tetrahedrite). In general, these values are close to one another and correspond to magmatic or mantlederived sulfur. The failed attempt to calculate the temperature from iso tope equilibrium in the pyrite–molybdenite couple is regarded as evidence for the formation of these metals at different stages of ore deposition. A single determi nation of oxygen isotope composition for orebearing quartz yielded δ18O = +13.3‰. RESULTS OF FLUID INCLUSIONS STUDY The studied fluid inclusions (FI) are contained in veined quartz that hosts minerals of gold–polysulfide and telluride assemblages. Most FI are extremely fine (1–3 μm), cluster into flattened trains (healed microf ractures), and are classified as secondary. The rare, relatively large (5–8 μm and up to 10–12 μm in excep tional cases) single FI do not belong to any observed trains of secondary inclusions and are classified as pri mary. The latter are frequently intersected by trains of small secondary inclusions or are opened. Thus, they are not representative and are excluded from consider ation. Only seven primarily FI have been studied. These are twophase (liquid Ⰷ vapor) inclusions of homogeneous capture mainly isometric and sporadi cally rhombshaped (Fig. 8). These inclusions com monly contain a vapor bubble without visible liquid СО2 and do not contain solid phases. Liquid СО2 is not recorded either visually or by freezing. The homogenization temperature (Thom) of such inclusions GEOLOGY OF ORE DEPOSITS
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in separate quartz grains varies from +106 to +206°C (Table 4). In cases when two primary inclusions are detected in a single quartz grain, their Thom differ by ~50°C. FI from the central part of quartz grain has a higher Thom (up to +206°С) than FI from the marginal zone (up to +161°С). Most likely this implies that the vein quartz crystallized with a drop in temperature. In the course of FI cooling, the first melting of the frozen solid phases took place at –21 to –22°C. This implies that the eutectic temperature Teut corresponds to NaCl ⋅ H2O solution. The temperature of ice melt ing in these FI varies from –10 to –12°С. The salt concentration in fluid calculated using the FLINCOR program varies from 13.94 to 15.96 wt % NaCl equiv. DISCUSSION Inasmuch as main orebodies are represented by quartz veins with beresite selvages, the Konevinsky gold deposit was earlier regarded as a quartzvein object pertaining to the gold–quartz type (Zololto …, 2000). However, such indications as elevated Au con tents, occurrence of stringer–disseminated sulfide chalcopyrite–molybdenite–pyrite mineralization hosted in the altered granitoids, positive correlation of Au with Cu in stringer–disseminated ore, and Au– Ag–basemetal occurrences at the contact with gra nitic pluton make it possible to classify the Konevinsky deposit as belonging to the porphyry Cu–Mo–Au type (Zhmodik et al., 2006), despite the elevated con centrations of Bi, Te, and Hg in the ore are not char acteristic of Cu–Mo–Au porphyry systems (Krivtsov et al., 1986; Sillitoe, 2010). The gold–bismuth association is typical of Au–Bi or intrusionrelated type (Lang and Baker, 2001; Gamyanin et al., 2003; Goryachev and Gamyanin, 2006). In contrast to Aubearing porphyry deposits, gold deposits of this type are characterized by predom inance of arsenopyrite and low contents of Fe, Pb, and
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Table 3. Contents of trace elements (ppm) in ore of Konevinsky deposit No.
Sample
1 2 3 4 5 6 7 8 9 10 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
KO5 KO9 KO17 KO18 KO23 KO27 KO31 KO3 KO6 KO21 KO30 KO33 KO39 KO40 KO41 KO42 KO43 KO44 KO46 KO1 KO20 KO11 KO2 KO4 KO13 KO25 KO32 KO10 KO12 KO45 KO16 KO29 KO24 KO35 KO34 KO28 KO14
Cu 32 41 180 34 4100 130 50 21 41 870 50 26 50 330 91 50 360 1960 670 420 1400 98 34 7400 180 5300 9000 1550 24 50 50 31 50 50 50 18 2900
Pb
Zn
Cd
Sn
Mo
66
53
2
1600
8 13 2500 28
47 26 4460
2
4 35 2 1850
43
Bi
46 26 1080 36 19
600 59 100 350 600 12
280 20 49 330 820 62
12 14 220 1400 1900 2480 62
36 20 590 550 1700 2070 54 16 36 3190 48 38 87 10 34 230
2
2 2 2 2 6 5 2 9 5 8
2
500 1300 9 17 73 17 15 153
4 4 15 10 1
570 1
8
8 2 7
Te
28 37 260 41 17
19 6 360 11 4 54 6 3 7 2300 2 4 1 8 5 6
Sb 2
4 7 450
As
10 50
28
220 170
240
395 5 11 15 2 4 6 29 25
350 21 84
9
30
52 330
80 21 6 69 54 730 740 17 24 11 12 80 1970 78
42 6 4 160 3 2 7 340 140 10 240 1830 370 217 800 146 9 1230 17 4470 6100 110 17 10 17 18 12 34 3000 2 840
56
7 4 25 8 234 62 3 8 204 58 330 8 17 3
30
Ag
Au
1.0 1.0 1.4 1.0 44.0 11.6 1.1 1.1 1.0 9.0 1.2 1.0 1.2 14.0 11.3 1.6 10.7 53.0 16.5 30.4 46.0 6.8 5.0 74.0 14.0 550 360 16.0 2.0 7.0 7.7 1.5 2.0 2.0 530 1.2 12.0
0.18 0.21 0.51 0.31 2.31 6.05 0.26 0.25 0.084 5.38 0.08 0.21 0.26 1.46 0.36 1.92 26.8 7.17 0.81 27.1 33.1 28.9 20.8 19.9 17.6 17.6 17.5 17.3 14.1 12.3 3.12 0.63 0.49 0.21 0.15 0.076
1–20, beresite with quartz veinlets; 21–38, quartz veins; blank cells, not detected. Analyses were performed with XRF (analyst B.Zh. Zhalsaraev); Au, with chemical–spectral method; Ag, with AAS (analysts A.A. Tsyrenova, E.M. Tat'yankina) at Geological Insti tute, Siberian Branch, Russian Academy of Sciences.
Zn sulfides. At the same time, bismuthinite and native Bi occur in small amounts at some porphyry deposits. Tellurium is contained in porphyry deposits; how ever, as a rule, Te mineralization is a product of epith ermal oreforming systems, which are associated with porphyry intrusions but recognized as an independent type of gold mineralization (Lindgren, 1933; Heden quist et al., 2000). The deposits of Au–Te geochemical type are more characteristic of volcanic belts at conti nental margins (Konstantinov, 1984; Stepanov and Moiseenko, 1993). In ore of the Konevinsky deposit,
the telluride assemblage is the latest rather than coeval with early minerals. The Hg minerals and Hgbearing gold are com monly formed at volcanogenic Au–Ag and telether mal Au–Sb–Hg deposits related to islandarc volca nic–plutonic associations, as well as at the deposits related to mafic–ultramafic rocks; in addition, mer curous gold is known from placer deposits (Borisenko et al., 2006; Damdinov et al., 2004; Zhmodik et al., 2008; Krylova et al., 1979; Mironov et al., 2004; Murzin et al., 1981; Murzin and Malyugin, 1987; Naz’mova and Spiridonov, 1979; Pokrovsky et al., GEOLOGY OF ORE DEPOSITS
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7000
(b)
30 10
Au
1000
Sb
145
100
1
10
1 10
100
0.1 0.07 20 9000
1000
100
1000
5000
Cu
Cu (c) 40 1 10
Au
2
1
0.1 0.07 1
10
100
600
Ag Fig. 7. Pair correlation: (a) Sb–Cu, (b) Au–Cu, and (c) Au–Ag in ore of Konevinsky deposit. (1) quartz veins, (2) beresite with quartz veinlets.
1977; Leistel et al., 1998; Nisten et al., 1986). Hg bearing deposits are confined to deep fault zones. The mantlederived fluids are sources of ore components at these deposits as evidence for the known fact of mercurial degassing of the mantle (Ozerova, 1986; Stepanov and Moiseenko, 1993). The mercuric min eralization at the Konevinsky deposit formed as a con stituent of the latest telluride assemblage. Thus, the supply of Hg and Te is related to lowtemperature ore formation. The occurrence of elevated Hg concentra tions in native gold is explained by the high affinity of mercury to gold. As soon as mercury appears in the system, native gold immediately reacts with it, form ing Au–Ag–Hg intermetallides and amalgamides. Thus, the Konevinsky deposit is distinguished by an unusual mineralogy with the occurrence of miner als typical of Cu–Mo–Au porphyry, Au–Bi, Au–Te, and Au–Hg gold deposits. The sulfur isotopic compositions of minerals from all these deposit types are close to one another and corre spond to mantlederived or magmatic sulfur. The oxygen GEOLOGY OF ORE DEPOSITS
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isotopic composition of quartz is also characteristic of igneous rocks. These isotope ratios are also inherent to porphyry copper deposits (Grabezhev, 2009). It is known that the early mineral parageneses of porphyry deposits form at temperatures of 400–600°C (Sillitoe, 2010). In particular, the quartz–molybdenite veinlets at the Bugdaya Cu–Mo–Au porphyry deposit formed at temperatures of 560–300°C (Kovalenker et al., 2011). A close temperature is suggested for the formation of the quartz–pyrite–molybdenite assem blage at the Konevinsky deposit. The formation of this assemblage is related to autometasomatic alteration during crystallization of the Sailag pluton expressed in extensive Kfeldspathization and beresitization of rocks and formation of stringer–disseminated miner alization. According to geochronological data, the granitoids crystallized 445–441 Ma ago. The development of quartz–gold–pyrite and gold–polysulfide mineral assemblages was caused by formation of quartz veins and veined zones related to nearlatitudinal and NWtrending fault systems; how
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DAMDINOV et al. (a)
(b)
(c)
vapor
vapor 8 μm
+29 (d)
+100
8 μm
(e)
vapor
5 μm
+30
8 μm
+199 (f)
vapor
+132
5 μm
5 μm
+166
Fig. 8. Photomicrographs of fluid inclusions (FI) in quartz at various temperatures: (a–c) FI irregular in shape at T = +29°С (a), +100°С (b), and +199°С (c); (d–f) rhombshaped FI at T = +30°С (d), +132°С (e), and +166°С (f).
ever, there is no direct evidence to which of these two systems, one or the other, mineral assemblages are related. Because the quartz–gold–pyrite assemblage is distinctly earlier, we suppose that its formation is related to the earlier nearly latitudinal Zhombolok Fault. The temperature formation conditions of the aforementioned mineral assemblages can be estimated only from indirect data, because the measured homogenization temperature of FI in quartz corre sponds only to the latest stage of ore formation. The formation temperature of the gold–pyrite assemblage obtained earlier for gold porphyry occurrences in the Tissa–Sarkhoi gold ore cluster is higher than 325°C (Damdinov et al., 2007). According to the experimen tal data, the homogeneous terahedrite solid solution is stable at a temperature of 359–127.5°C (Mozgova and Tsepin, 1983). This solid solution breaks down above and below these temperature limits. Because in our case decomposition products are not recorded, we can
apply the temperature conditions of fahlore stability to the gold–polysulfide assemblage. Dikes of the volcanic–plutonic association intruded approximately 324 Ma ago along the same fault systems that host vein zones. This is supported by the observed relationships between dikes and ore zones. Under the effect of relatively lowtemperature fluids related to volcanic–plutonic association, epith ermal paragenesis with Hg minerals and tellurides formed. As is known, telluride mineralization forms within a temperature interval of 110–280°C (Bortni kov et al., 1988). The study of fluid inclusions from orebearing quartz shows that they homogenized at a temperature of 206–160°C. These values correspond to the minimal temperature of oreforming hydrother mal solutions and fit the formation temperature of epi thermal ores. This implies that quartz transformed under the effect of late fluids. The hydrothermal solu tions that fill the studied FI contain 13.94–15.96 wt % NaCl equiv, are free of CO2, and are associated with
Table 4. Generalized results of thermometry and cryometry for seven fluid inclusions in quartz Temperature, °C
Composition of FI, wt %
Teut
Thom
Tice melt
CO2
NaCl equiv
Composition of salt system
–21…–22
+160… +206
–10…–12
b.d.l.
13.94–15.96
NaCl ⋅ H2O
b.d.l., below detection limit. GEOLOGY OF ORE DEPOSITS
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COMPOSITION AND GENESIS OF THE KONEVINSKY GOLD DEPOSIT
lowdensity gas phase; NaCl apparently dominates in the salt composition of solutions in FI. Based on the available data, the following genetic concept is proposed for the Konevinsky gold deposit. Taking into account that granitic rocks of the Sailag pluton overall are enriched in gold and accompanying ore components and are surrounded by Au and Ag bearing polymetallic occurrences, it cannot be ruled out that the granitic melt was enriched in oreforming components. This enrichment gave rise to crystalliza tion of ore minerals (molybdenite, chalcopyrite, pyrite) and the formation of Cu–Mo–Au porphyry ore–magmatic system expressed in stringer–dissemi nated mineralization hosted in granitoids. Afterward, veined quartz–gold–pyrite and gold–polysulfide assemblages formed in connection with two fault sys tems. The appearance of the late Hg and Tebearing assemblage was related to emplacement of dikes per taining to the volcanic–plutonic association, supply of Hg and Te, and formation of epithermal mineraliza tion. Mercury, owing to its high activity, immediately reacted with minerals, primarily with gold, as well as with tellurides and other compounds, e.g., fahlore. It is known that in some cases, porphyry and epith ermal deposits represent different hypsometric levels of combined complex porphyry–epithermal systems with gradual change of mineral assemblages from hypabyssal to nearsurface (Sillitoe, 1983; Kovalenker et al., 2006). In our case, however, we are dealing with different chronological stages of ore formation. The first stage corresponds to the age of ~440 Ma and is related to magmatism at active continental margin of the Tuva–Mongolia paleomicrocontinent, where intrusionrelated Cu–Mo–Au mineralization is formed. The second stage is related to emplacement of dikes pertaining to the Carboniferous volcanic–plu tonic association (~324 Ma). In the course of this pro cess, Hg and Te are supplied into the system and epi thermal gold–mercury–telluride paragenesis is formed. ACKNOWLEDGMENTS This study was supported by the Russian Founda tion for Basic Research (project nos. 130512056, 14 0500339, 150506950). REFERENCES Airiyants, E.V., Zhmodik, S.M., Mironov, A.G., et al., Gold mineralization in siliceous–carbonate rocks of south eastern East Sayan, Russ. Geol. Geophys., 2007, vol. 48, no. 5, pp. 389–399. Borisenko, A.S., Naumov, E.A., and Obolenskii, A.A., Types of gold–mercury deposits and their formaton condi tions, Russ. Geol. Geophys., 2006, vol. 47, no. 3, pp. 342– 354. Bortnikov, N.S., Cramer, Kh., Genkin, A.D., et al., Parageneses of gold and silver tellurides in the Florence gold GEOLOGY OF ORE DEPOSITS
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Translated by V. Popov
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