Mineralogical Tracers of Gold and Rare-Metal Mineralization in Eastern Kazakhstan

: Replenishment of mineral resources, especially gold and rare metals, is critical for progress in the mining and metallurgical industry of Eastern Kazakhstan. To substantiate the scientiﬁc background for mineral exploration, we study microinclusions in minerals from gold and rare-metal ﬁelds, as well as trace-element patterns in ores and their hosts that may mark gold and rare-metal mineralization. The revealed compositions of gold-bearing sulﬁde ores and a number of typical minerals (magnetite, goethite, arsenopyrite, antimonite, gold and silver) and elements (Fe, Mn, Cu, Pb, Zn, As, and Sb) can serve as exploration guides. The analyzed samples contain rare micrometer lead (alamosite, kentrolite, melanotekite, cotunnite) and nickel (bunsenite, trevorite, gersdorfﬁte) phases and accessory cassiterite, wolframite, scheelite, and microlite. The ores bear native gold (with Ag and Pt impurities) amenable to concentration by gravity and ﬂotation methods. Multistage rare-metal pegmatite mineralization can be predicted from the presence of mineral assemblages including cleavelandite, muscovite, lepidolite, spodumene, pollucite, tantalite, microlite, etc. and such elements as Ta, Nb, Be, Li, Cs, and Sn. Pegmatite veins bear diverse Ta minerals (columbite, tantalite-columbite, manganotantalite, ixiolite, and microlite) that accumulated rare metals late during the evolution of the pegmatite magmatic system. The discovered mineralogical and geochemical criteria are useful for exploration purposes. process. The analyses reveal diverse Ta-bearing minerals with Au, Ag, Pt, Zr, Hf, W, U, and Ir impurities that remained mute for semi-quantitative spectral analyses. Mica minerals (muscovite, lepidolite) are rich in Li and, together with spodumene, can be a source of lithium. The results have expanded the knowledge on the composition of ores and rocks, as well as on gold and rare metal mineralization patterns, which can have theoretical and practical applications in mineral exploration.


Introduction
The territory of Eastern Kazakhstan is an exceptional natural metallogenic laboratory which stores iron, copper, lead, gold, metals, rare earths, and other mineral resources that maintain a large mining and metallurgical industry [1]. Most of known mineral deposits were discovered by the classical geological prospecting in the second half of the 20th century. However, the pool of easily detectable deposits has been exhausted while the resources of the previous discoveries will be spent in a few decades of mining. The vertical structure of ore zones and their host formations have been poorly constrained so far, and the conventional exploration approaches become ever less workable in the metallogenic provinces of Eastern Kazakhstan. The progressive depletion of old mining areas calls for advances in exploration, especially for base, rare, and precious metals.
Gold mineralization in the Great Altai is known from the Rudny Altai, Char, and West Kalba zones. It is especially abundant in the Char and West Kalba zones but occurs mostly as a minor component in sulfide and complex ores in the Rudny Altai zone. The gold-bearing Fe, Mn, Cu, Zn, Pb, Au, Ag, etc. ores in the latter are associated with Devonian basalt-andesite-rhyolite volcanism (D 1-3 ) and controlled by large faults [2,3]. Many large and giant Cu, Pb, and Zn deposits of Rudny Altai (Orlovka, Artemiev, Ridder-Sokolny, Maleevsk, etc.) contain gold at average grades of 0.8-1.0 g/t Au, which makes a considerable share in the ore reserves of the region [6].
The gold deposits and occurrences in the Char and West Kalba zones ( Figure 1) count more than 450, with the largest ones of Bakyrchik, Suzdal, and some others [2,3,5]. They show genetic links with gabbro-diorite-granodiorite-plagiogranite magmatism (C 2-3 ) and related fluid inputs channeled by a system of NW faults.
The Suzdal gold field is located at the boundary between the Char and West Kalba metallogenic zones. The host Arkalyk Fm. (C 1 v 2-3 ar) is composed of calcareous-carbonaceousclayey silt, fine sand, massive crinoidal limestone, and moderate amounts of porphyritic andesite and tuffite ( Figure 2). The intrusive rocks of the area are diorite porphyry (dikes) and granodiorites of the Kunush complex (C 3 ). The mineralization is mainly controlled by the 700-1300 m wide Suzdal fault zone of quasi-parallel NE faults dipping in the SE direction at 40-60 • . Four 10-25 m thick ore zones in the field have been explored to a depth of 500 m. Nest-like, vein, or disseminated gold-bearing sulfide mineralization occurs in heavily deformed sediments. The orebodies are mainly localized in the middle of ore zones ( Figure 3).
Some other occurrences and sites in the West Kalba and Char zones (Baibura, Zhaima, etc.) are potentially rich in gold-bearing sulfide ores as well ( Figure 1).
The mineralogical and geochemical exploration criteria were revealed in several reference rare-metal pegmatite deposits from the Kalba-Narym zone (Asubulak, Bakennoye, etc.).  Some other occurrences and sites in the West Kalba and Char zones (Baibura, Zhaima, etc.) are potentially rich in gold-bearing sulfide ores as well ( Figure 1).

Materials and Methods
The work began with field examination and sampling at typical gold and rare-metal

Materials and Methods
The work began with field examination and sampling at typical gold and rare-metal deposits in Eastern Kazakhstan. We collected mainly 0.5-1.0 kg samples of sedimentary, volcanic, and intrusive rocks and ores, including gold-bearing jasperoids, gold-quartzcarbonate-sulfide metasomatics, quartz veins, etc. (5-6 samples of each rock and ore type on average). The location of samples is shown on the Figure 3. Native gold particles of 0.05 to 0.375 mm sizes were extracted from six 12-15 kg samples of ironstones from weathered zones. The samples of hornfels and fresh or altered granites, as well as pegmatitic minerals of simple oligoclase-microcline to complex albite-spodumene assemblages represented the rare-metal pegmatite deposits.
The laboratory procedures included separation of monomineral fractions (pyrite, muscovite, lepidolite, spodumene, tourmaline, quartz, cassiterite, etc.) and various analyses. The compositions of rocks and minerals were determined, respectively, by mass spectrometry with inductively coupled plasma (ICP-MS) on an Agilent 7500cx (Agilent Technologies, Santa Clara, CA, USA) spectrometer and by electron probe microanalysis (EMPA) on a Cameca MS-46 (Cameca, Gennevilliers, France) analyzer that allowed detection and precise measurements of seventy three elements (Au, Ag, Pt, Cd, In, Ir, Y, Cd, TR, U, etc.). The analytical procedures were performed at VERITAS Laboratory of the D. Serikbaev East Kazakhstan Technical University (Ust'-Kamenogorsk) and at the Analytical Center of the V. Sobolev Institute of Geology and Mineralogy (Novosibirsk). Scanning electron microscopy and energy dispersive spectrometry (SEM-EDS) was applied to study micrometer inclusions of opaque and related minerals and to analyze impurity elements (Au, Ag, Pt, In et al.). The instruments were a Jeol-100C microscope with a Kevex-Ray detector and a Jeol ISM-6390 LV microscope with an Oxford INCA Energy system (JEOL, Tokyo, Japan). The major-element chemistry was analyzed by the X-ray fluorescence (XRF) method. The Au and Ag contents in ores were determined by atomic-absorption spectrometry (AAS).

Tracers of Gold Mineralization
The primary ores of the Suzdal gold deposit consist of pyrite, goethite, arsenopyrite, and gold as main minerals and accessory antimonite, pyrrhotite, chalcopyrite, galena, sphalerite, ilmenite, cinnabar, etc. The rock matrix minerals are quartz, siderite, calcite, and K-Al silicates. Gold is either disseminated in pyrite and arsenopyrite or appears as submicrometer to 100-150 µm particles in the primary sulfide ores. The gold mineralization is very unevenly distributed, with average grades reaching 9 g/t Au in economic ores. It is traced by Fe, As, Sb, Cu, Pb, and Zn sulfide minerals in ores and their limestone hosts.
The ores in the Baibura deposit are especially abundant in brecciated ironstones from the weathered zone, with numerous nests or disseminated particles of oxidized sulfide minerals and visible gold ( Figure 5).
The mineralization is traced by Fe, Mn, Al, As, Sb, Cu, Pb, Zn, Ag, Au, and other elements that were supplied into the system. Typical minerals in gold-bearing sulfide ores ( Figure 6) are goethite, magnetite, arsenopyrite, antimonite, galena, gold, silver, etc.
The ores in the Baibura deposit are especially abundant in brecciated ironstones from the weathered zone, with numerous nests or disseminated particles of oxidized sulfide minerals and visible gold ( Figure 5). The mineralization is traced by Fe, Mn, Al, As, Sb, Cu, Pb, Zn, Ag, Au, and other elements that were supplied into the system. Typical minerals in gold-bearing sulfide ores ( Figure 6) are goethite, magnetite, arsenopyrite, antimonite, galena, gold, silver, etc. We revealed a number of previously undetected phases (magnetite, brownite, PGE xenotime and rare-metal bearing cassiterite, scheelite, microlite), as well as rare phases of Pb (alamosite, kentrolite, melanotekite, cotunnite) and Ni (bunsenite, trevorite (?), gersdorffite, etc.), which shed more light on ore compositions.
Native gold was separated from jasperoids (with participation of S. Petrov), identified with reference to the morphological classifications of L. Nikolaeva and S. According to Cameca MS-46 microprobe data ( Figure 8) the gold particles have 150 to 400 μm sizes and flaky, platelet, or elongate shapes; some make nests or are intergrown with quartz, hydrogoethite, or rock. Gold and silver also exist in μm to tens of μm microinclusions detected under an ISM-6390 microscope (JEOL, Tokyo, Japan) at defect sites on the surface of jasperoids (Figure 9a  According to Cameca MS-46 microprobe data ( Figure 8) the gold particles have 150 to 400 µm sizes and flaky, platelet, or elongate shapes; some make nests or are intergrown with quartz, hydrogoethite, or rock.
Alamosite is a rare Pb phase known from weathered zones of lead deposits [25]. In Eastern Kazakhstan, we discovered this mineral at the Baibura deposit in the West Kalba zone, where gold and manganese fields are spatially proximal. Alamosite and other Pb minerals were found in manganese ores containing elevated concentrations of Pb, Cu, Zn and other metals. SEM analysis of pure alamosite varieties showed a relatively uniform Si:O:Pb proportion of 1:2.2:5.4, with a total of 100%. Micrometer alamosite grains in thin quartz-manganese veins have intricate shapes and rough surfaces. High concentrations of trace elements were determined in Zhezdy, Zhomart, and other hydrothermal Mn deposits of Central Kazakhstan [26]. Therefore, the presence of manganese ores coexisting with gold mineralization can be considered as a good exploration guide.  Alamosite is a rare Pb phase known from weathered zones of lead deposits [25]. In Eastern Kazakhstan, we discovered this mineral at the Baibura deposit in the West Kalba zone, where gold and manganese fields are spatially proximal. Alamosite and other Pb minerals were found in manganese ores containing elevated concentrations of Pb, Cu, Zn and other metals. SEM analysis of pure alamosite varieties showed a relatively uniform Si:O:Pb proportion of 1:2.2:5.4, with a total of 100%. Micrometer alamosite grains in thin quartz-manganese veins have intricate shapes and rough surfaces. High concentrations of trace elements were determined in Zhezdy, Zhomart, and other hydrothermal Mn deposits of Central Kazakhstan [26]. Therefore, the presence of manganese ores coexisting with gold mineralization can be considered as a good exploration guide.
Magnetite (up to 3-5 mm) in mineralization zones traceable by magnetic anomalies reaching 2250 nT also has diagnostic value. Thus, Suzdal-type apocarbonate gold ores have complex compositions, with phases of lithophile, chalcophile, siderophile and other elements. The presence of HREEs (Dy, Er, Yb), PGE (Ir, Pt), Fe, Ni, Co, Cr, Au, W, Ta, and U, as well as quite high contents of Sr, record a crust-mantle source of primary ores. The ores bear free gold of different particle shapes and sizes (from fine dust to fine fractions), which is amenable to gravity and flotation concentration. Typical opaque minerals include magnetite, hematite, goethite, pyrite, arsenopyrite, Mn phases, galena, chalcopyrite, and sphalerite coexisting with rare-metal and REE phases.
Such trace elements as Fe, Mn, As, Sb, Cu, Pb, Zn, Ag, Sn, W, and Ba are main tracers of gold mineralization. The revealed minerals and elements can be used in exploration as guides to new gold fields and occurrences.
The pegmatites evolved at variable pressure, temperature, pH, and alkalinity conditions while mineral assemblages underwent metasomatic alteration with formation of secondary microcline, albite, greisen, spodumene, etc. and changed from simple barren graphic and oligoclase-microcline pegmatite to Li-bearing microcline-albite, quartz-albite-muscovite (greisen), quartz-albite-spodumene varieties with progressively increasing concentrations of Ta, Nb, Be, Li, Cs, Sn, and TR. These trends are consistent with the general evolution of pegmatites and rare-metal (Ta, Be, Li, Cs, Sn, TR, etc.) mineralization [29][30][31][32]. Mineralization is especially high in pegmatite veins bearing Thus, Suzdal-type apocarbonate gold ores have complex compositions, with phases of lithophile, chalcophile, siderophile and other elements. The presence of HREEs (Dy, Er, Yb), PGE (Ir, Pt), Fe, Ni, Co, Cr, Au, W, Ta, and U, as well as quite high contents of Sr, record a crust-mantle source of primary ores. The ores bear free gold of different particle shapes and sizes (from fine dust to fine fractions), which is amenable to gravity and flotation concentration. Typical opaque minerals include magnetite, hematite, goethite, pyrite, arsenopyrite, Mn phases, galena, chalcopyrite, and sphalerite coexisting with rare-metal and REE phases.
Such trace elements as Fe, Mn, As, Sb, Cu, Pb, Zn, Ag, Sn, W, and Ba are main tracers of gold mineralization. The revealed minerals and elements can be used in exploration as guides to new gold fields and occurrences.

Tracers of Rare-Metal Mineralization
Pegmatite ore formation was modeled as pulse-like rhythmic inputs of metal-laden gas-liquid fluids (H 2 O, F, B, Cl, Ta, Sn, Be, etc.) from crustal magma sources through faulted crust, synchronously with granitic magmatism [10,23]. The Kalba-Narym raremetal pegmatites are spatially and genetically linked with medium-coarse porphyritic phase I Bi granites (P 1 ) of the Kalba complex [10].
Scanning electron microscopy has provided new data on mineralogy and chemistry of ores and distribution of impurities in Ta, Li, and other phases in pegmatites. It also revealed rare minerals, which remained undetectable by semi-quantitative laboratory methods, as well as typical minerals and trace elements that may be indicators of raremetal mineralization.
Tantalite (Fe, MnTa 2 O 6 ), a main opaque mineral, occurs as brownish to black shortprismatic or tabular crystals or as impregnation in albitized pegmatite ( Figure 13). Scanning electron microscopy has provided new data on mineralogy and chemistry of ores and distribution of impurities in Ta, Li, and other phases in pegmatites. It also revealed rare minerals, which remained undetectable by semi-quantitative laboratory methods, as well as typical minerals and trace elements that may be indicators of raremetal mineralization.
Lepidolite is a main Li mineral in the quartz-cleavelandite-lepidolite (greisen) assemblage and one of principal indicators of rare-metal mineralization. It can occur either as coarse and medium flaky particles coexisting with large aggregates of cleavelandite and quartz, or as fine to very fine flaky metasomatic quartz-lepidolite nests among other minerals. It contains anomalously high concentrations of rare alkalis (16,240 ppm Li, 10,300 ppm Rb, and 1350 ppm Cs), relatively high contents of many trace elements (7470 ppm Ta, 109.5 ppm Nb, 81.5 ppm Sn, and 70.4 ppm W), and some trace elements above the average upper continental crust values [24] (1101 ppm B, 15.9 ppm Ge, 9.7 ppm Tl, 3001 ppm Mn, and 1372 ppm P). Lepidolite sheets enclose micrometer grains of metasomatic quartz which, in its turn, contains randomly distributed pollucite inclusions.
Transparent muscovite (and its greenish or gilbertite varieties) from different assemblages are main hosts of rare metals and rare alkalis. The total of rare alkalis reaches 12,308 ppm, and the contents of metals determined by ICP-MS are 153.1 ppm Ta, 301.1 ppm Nb, 35.9 ppm Be, 509.5 ppm Sn, and 25.9 ppm W. The crystals bear fluid inclusions (tantalite-columbite, cassiterite, fluorite, ilmenite, tetrahedrite, zircon, pyrite, barite, halite) and metallic species of Fe, Pb, etc.; some samples contain Ga (165 ppm) and Ag (11.8 ppm). The trace element contents in mica minerals increase as the mineralization progresses.
Color tourmalines are diverse in color, crystal morphology, and chemistry. Black tourmaline (shorl) with low trace-element abundances corresponds to simple pegmatite. The color varieties include aggregates of dark green tourmalines of variable coloration, blue indigolite, pink rubellite, and black-head polychrome tourmaline. Paragenetic assemblages of color tourmaline with cleavelandite, lepidolite, and pollucite are indicators of rich Ta, Nb, Sn, Li, and Cs ores; polychrome tourmaline with high Cs contents (1354 ppm) traces Cs ores. Transparent muscovite (and its greenish or gilbertite varieties) from different assemblages are main hosts of rare metals and rare alkalis. The total of rare alkalis reaches 12,308 ppm, and the contents of metals determined by ICP-MS are 153.1 ppm Ta, 301.1 ppm Nb, 35.9 ppm Be, 509.5 ppm Sn, and 25.9 ppm W. The crystals bear fluid inclusions (tantalite-columbite, cassiterite, fluorite, ilmenite, tetrahedrite, zircon, pyrite, barite, halite) and metallic species of Fe, Pb, etc.; some samples contain Ga (165 ppm) and Ag (11.8 ppm). The trace element contents in mica minerals increase as the mineralization progresses.
Color tourmalines are diverse in color, crystal morphology, and chemistry. Black tourmaline (shorl) with low trace-element abundances corresponds to simple pegmatite. The color varieties include aggregates of dark green tourmalines of variable coloration, blue indigolite, pink rubellite, and black-head polychrome tourmaline. Paragenetic assemblages of color tourmaline with cleavelandite, lepidolite, and pollucite are indicators of rich Ta, Nb, Sn, Li, and Cs ores; polychrome tourmaline with high Cs contents (1354 ppm) traces Cs ores.
Miarolitic pegmatite formed late during mineralization and is of limited occurrence. It is a gemstone facies widespread in Siberia, Tajikistan, Mongolia, and USA. In Eastern Kazakhstan, miarole assemblages (topaz, morion, aguemarine, emerald) are known from the Delbegetei granitic intrusion, and crystal-bearing pegmatite occurs in the Akzhailau and Dungaly intrusions. The Yubileinoye miarolitic pegmatite mainly includes albite, microcline, quartz, muscovite, lepidolite, and apatite, and less often contains spodumene and fluorite. The typical assemblage in ores is tantalite-microlite-samarskite-fergusonite minerals, which is of minor economic value though.

Discussion
Mineral resources of base, noble, and rare metals in Eastern Kazakhstan are critical for sustainable development of the mining and metallurgical industry. Many complex ore, gold, and rare-metal deposits in the Great Altai province still retain high mineral potential [1,3]. Mineral exploration progress requires further research with advanced technologies and methods [4], including mineralogical and geochemical studies of minerals as tracers of ore formation conditions.
The new data we have obtained on compositions of weathered ironstones [17] show that they bear free gold of fine dust to fine size fractions, with Ag, Cu, W, and Pt impurities, which is amenable to gravity and flotation concentration. According to mass spectrometry, the ironstones contain heavy rare earths, PGE, Cr, Ni, Sr, and U. The typical main (goethite, limonite, magnetite, arsenopyrite, antimonite, and gold) and accessory (galena, chalcopyrite, sphalerite, brownite, cassiterite, etc.) phases and trace elements, such as Fe, Mn, As, Sb, Pb, Sn, Ag, and Au, can be used as exploration guides for deposits of this kind. The mineralogical exploration methods are especially useful in the case of hidden (buried) ores at initial prospecting stages [1].
The discussion on a large age range of primary gold mineralization at the Suzdal deposit [19] appears unreasonable as the gold-bearing structures were proven to be truncated by the Early Permian granites of the Kalba complex and volcanics of the Semeytau trough [34].
The deposits of Eastern Kazakhstan formed in an alkaline granite-pegmatite system. The system gained trace elements (Ta, Nb, Be, Li, and Cs) mainly late during the process, as it is common for many such deposits [35]. The richest pegmatite veins show multistage mineralization and store diverse assemblages. The compositions of Ta minerals evolved from columbite to tantalite-columbite, manganotantalite, ixiolite, and microlite, judging by the corresponding increase in Mn/Fe and Ta/Nb ratios. Tantalite is enriched in rare alkalis, Sn, W, and contains Au, Ag, Pt, Ir, and In. Microlite contains relatively high Zr, Hf, and U, in the same way as that from the Giraúl field in Angola where late-generation tantalite is rich in Ta, Mn, Zr, and other elements [35].
Rare-metal mineralization can be predicted proceeding from the presence of spodumene, muscovite, and lepidolite, which are likewise common to pegmatites and raremetal granites [9,36]. Li-bearing mica phases are important sources of lithium. In view of the growing demand for lithium in the world markets, it is pertinent to estimate pegmatite sites for the Sn-Be-Ta-Li potential.
Thus, the reported studies of gold and rare-metal mineralization provide new evidence for the composition of metal-laden fluids and the patterns of rare metals, REE, Au, and other elements that can be used as tracers of magmatism-related mineralization. The revealed mineral assemblages record the evolution of fluid regime and multistage ore formation, as one can infer by analogy with published evidence [18,29,32,37,38].

Conclusions
The studies of gold and rare-metal mineralization in Eastern Kazakhstan shed new light on the contents of rare metals, REE, Au, PGE, and other related elements in raremetal phases. The analyzed samples from the Kalba deposits contain disseminated and free gold with Ag, Cu, and Pt impurities, and newly discovered rare mineral phases (alamosite, kentrolite, bunsenite, microlite, ferrosilite, etc.). Gold-bearing sulfides are traced by such elements as Fe, Mn, As, Sb, Cu, Pb, Zn, Ag, Sn, W, and Ba. A number of unique minerals (cleavelandite, lepidolite, color tourmalines, spodumene, ambligonite, pollucite, and tantalite-columbite), which are commonly found in other such deposits worldwide (Bernic Lake, Zimbabwe, Koktogai, etc.), are guides to rare-metal mineralization. The concentrations of Sn-Ta and Li-Cs increased progressively at the late stage of the pegmatitic process. The analyses reveal diverse Ta-bearing minerals with Au, Ag, Pt, Zr, Hf, W, U, and Ir impurities that remained mute for semi-quantitative spectral analyses. Mica minerals (muscovite, lepidolite) are rich in Li and, together with spodumene, can be a source of lithium. The results have expanded the knowledge on the composition of ores and rocks, as well as on gold and rare metal mineralization patterns, which can have theoretical and practical applications in mineral exploration.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.