Mineralization Styles in the Orogenic (Quartz Vein) Gold Deposits of the Eastern Kazakhstan Gold Belt: Implications for Regional Prospecting

Abstract

The Eastern Kazakhstan Gold Belt is a major black-shale-hosted gold province in Central Asia where the main types of deposits comprise mineralized zones with auriferous sulfides (micro- and nano-inclusions of gold and refractory gold) and quartz veins with visible gold. The quartz vein deposits are economically less important but may potentially represent the upper parts of bigger ore systems concealed at depth. In this work, the mineralogy of the quartz vein deposits and related wall rock alteration zones was studied using microscopy and SEM-EDS analysis, and the geochemical dispersion of the ore elements in primary alteration haloes was documented utilizing spatial distribution maps and statistical treatment methods. The studied auriferous quartz veins are classified as epizonal black-shale-hosted orogenic gold deposits. The veins generally have linear shapes with an average width of ca. 1 m and length up to 150 m and contain high-grade native gold with minor amounts of sulfides. In supergene oxidation zones, the native gold is closely associated with Fe-hydroxide minerals cementing brecciated zones within the veins. The auriferous quartz veins are usually enclosed by the wall rock alteration envelopes, where two types of alteration are distinguished. Proximal phyllic alteration (sericite-albite-pyrite ± chlorite, Fe-Mg-Ca carbonates, arsenopyrite, and pyrrhotite) develops as localized alteration envelopes, and pervasive carbonation accompanied by chlorite ± sericite and albite is the dominant process in the distal alteration zones. The rocks within the alteration zones are enriched in Au and chalcophile elements, and three groups of chemical elements showing significant positive mutual correlation have been identified: (1) an early geochemical assemblage includes V, P, and Co (±Ni), which are the chemical elements characteristic for black shale formations, (2) association of Au, As, and other chalcophile elements is distinctly overprinting, and manifests the main stage of sulfide-hosted Au mineralization, and (3) association of Bi and Hg (±Sb and U) includes the chemical elements that are mobile at low temperatures, and can be explained by activity of the late-stage hydrothermal or supergene fluids. The chalcophile elements show negative slopes from proximal to distal alteration zones and form overlapping positive anomalies on spatial distribution mono-elemental maps. Thus, the geochemical methods can provide useful tools to delineate the ore elemental associations and to outline reproducible anomalies for subsequent regional gold prospecting.

1. Introduction

The Eastern Kazakhstan Gold Belt is one of the biggest black-shale-hosted gold provinces in Central Asia, containing several hundred placer and lode gold deposits with total endowment ca. 500 t Au [1,2,3,4]. Similar to other major black-shale-hosted gold provinces of the region, including the world-class Muruntau and Kumtor ore districts in the Tien Shan Gold Belt, the lode gold deposits of Eastern Kazakhstan occur as structurally controlled mineralization zones in metamorphosed carbonaceous flyschoid sediments [1,3,5,6,7,8]. Two main types of deposits distinguished in the Eastern Kazakhstan Gold Belt comprise the mineralized zones with auriferous sulfides and quartz vein-type deposits with visible native gold [7,8]. The largest Au deposits of the belt, including the Bakyrchik-Bolshevik mining district (Kyzyl Gold Project), Akzhal, Vasilievskoe, and Suzdalskoe, are classified as mineralized zones and are relatively well studied [4,9,10,11], while numerous small and sub-economic quartz vein deposits and occurrences, potentially representing upper parts of the hidden ore systems at depth, remain insufficiently investigated [4]. In order to fill in this gap, several newly discovered and re-examined quartz vein-type deposits have been studied in the course of the ongoing exploration projects. The mineralogical composition of the ore zones and related alteration envelopes was elucidated using conventional microscopy and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDS analysis), and the geochemical dispersion of the ore elements in primary alteration haloes was documented utilizing spatial distribution maps and statistical treatment methods. The obtained results help to better understand the ore-controlling factors playing an important role in formation of gold deposits and can be used as valuable guidelines for regional prospecting.

2. Geological Setting of Eastern Kazakhstan

Geological formations exposed in Eastern Kazakhstan are part of the Central Asian Orogenic Belt, one of the largest accretionary orogens comprising Precambrian and Paleozoic microcontinents and island arcs that amalgamated during the final closure of the Paleo-Asian Ocean in the late Paleozoic [12,13,14,15,16,17]. Tectonic units of Eastern Kazakhstan include several NW–SE elongated terranes formed during the evolution of the Ob-Zaisan branch of the Paleo-Asian Ocean. The Ob-Zaisan oceanic basin separated the early Paleozoic (Caledonian) micro-continent of Paleo-Kazakhstan from Peri-Siberian continental blocks of Altai-Sayan and was closed in the course of the late Paleozoic (Hercynian) collisional events. From west to east, the tectonic units of Eastern Kazakhstan include the Chingiz-Tarbagatai block, Zharma-Saur volcanic arc, West Kalba zone, Kalba-Narym belt, Irtysh shear zone, and Rudny-Altai terrane (Figure 1).

The westernmost Chingiz-Tarbagatai block is composed of deformed early Paleozoic volcanic arc formations comprising the basement of the Caledonian Paleo-Kazakhstan continent [19,20,21]. The early Paleozoic basement of the Chingiz-Tarbagatai is partially overlain by Devonian to early Carboniferous volcanics formed when subduction under the Paleo-Kazakhstan continent resumed in the middle–late Paleozoic [22].

The Zharma-Saur volcanic arc is a 10–15 km-wide discontinuous belt of early Carboniferous (Moscovian) volcanics stretching along the eastern margin of the Chingiz-Tarbagatai block. To the southeast, this arc can be traced in the Tarbagatai Range and further east in the Sawur Mountains of China [20,23,24,25,26].

The West Kalba, also known in the literature as the Chara zone, represents the largest (ca. 50–70 km wide) regional tectonic unit that hosts numerous Au deposits and occurrences, described as the Eastern Kazakhstan Gold Belt [8,10,27]. The West Kalba zone is interpreted as a superimposed Carboniferous sedimentary basin filled with flysch-like and turbiditic terrigenous sediments, derived from a juvenile intra-oceanic arc, which change upsection to coarse-grained clastics and limnic molasses [28]. It is suggested that the opening of this basin in the early Carboniferous (Visean) postdated the early stages of Hercynian collisional events in the area, while subsequent collisional compression and shearing resulted in strong deformation of the basin fill during the late Carboniferous–early Permian [20,27]. The deformations were accompanied by the greenschist facies metamorphism and small-scale granitoid magmatism, and can be generally characterized as top-to-the-west thrusting at the stage of collision and strong sinistral strike-slip faulting and shearing in the NW–SE direction at the post-collisional stage that occurred due to clockwise rotation of Siberia relative to the Paleo-Kazakhstan continent [27,29,30]. The thrusting produced numerous tectonic sheets of different geodynamic origin. One of them is the Char ophiolite mélange located in the central part of the West Kalba zone [31]. Late Carboniferous volcanics and associated rocks are developed in the southwestern part of the West Kalba zone. Intrusive rocks are relatively rare and comprise Carboniferous diorites and granodiorites with ages in the range 325–305 Ma and early Permian granites that form dikes and stocks in the Carboniferous sediments [27,32].

The main volume of the early Permian post-collisional granitoids was emplaced in the Kalba-Narym zone located to the east. In this area, batholiths of S-type granites, which also marginally intrude in the West Kalba zone, occupy up to 70% of the territory on the present-day erosion surface and host significant tin and rare metal mineralization [6,24,32,33]. The basement of the Kalba-Narym zone is composed of the late Devonian–early Carboniferous clastic sediments derived from the continental margins of Peri-Siberian blocks located to the east.

Further east, the Irtysh shear zone is a 5–10 km-wide band of deformations and high-grade metamorphic rocks developed along a system of continental-scale sinistral strike-slip faults separating the Kalba-Narym zone from the Rudny-Altai terrane, a Hercynian Peri-Siberian active continental margin hosting a number of world-class massive sulfide deposits [20].

3. Types of Deposits and Metallogenic Features of the Eastern Kazakhstan Gold Belt

The Eastern Kazakhstan Gold Belt, which is also known in the literature as the West Kalba or Chara Gold Belt, stretches for more than 300 km along the NW striking West Kalba zone and contains several hundred placer and lode gold deposits with total endowment estimated at ca. 500 t Au [1,5,6,8,34].

The lode gold deposits are hosted by the Carboniferous flyschoids (black shales) developed in the central and northeastern parts of the West Kalba zone and by Devonian to Carboniferous volcanics in the southern part of the zone (Figure 2). Based on field relationships and isotopic age determinations, it is generally accepted that major ore-forming events took place during the late Carboniferous–early Permian (310–280 Ma) and were related to the main phase of deformations and greenschist facies metamorphism in the West Kalba zone [7,35,36]. The deposits are classified as typical orogenic gold deposits [34,37]; however, based on geological and mineralogical features, they can be subdivided into two major types: (1) structurally controlled mineralized zones with auriferous sulfides (pyrite and arsenopyrite) and (2) quartz veins with visible native gold [7,8]. There is also one auriferous skarn and several gold occurrences in jasperoids, demonstrating the Carlin-type deposit signatures [38].

Figure 2. Part of the Mineral Deposits Map of Central Asia [39] showing Au deposits of the Eastern Kazakhstan Gold Belt and study areas shown as blue squares out of scale. Red frame indicates the territory shown in Figure 3.

Figure 2. Part of the Mineral Deposits Map of Central Asia [39] showing Au deposits of the Eastern Kazakhstan Gold Belt and study areas shown as blue squares out of scale. Red frame indicates the territory shown in Figure 3.

Most of the major lode gold deposits, such as the largest Bakyrchik (or Baqyrchik) deposit with estimated reserves of 240 t Au, and several smaller deposits, including Bolshevik (~30 t Au), Akzhal (~14 t Au), Vasilievskoe (~18 t Au), and Suzdalskoe (~24 t Au), can be defined as tectonically controlled mineralized zones, where strongly deformed host rocks undergo potassic, propylitic, and carbonate-phyllic alteration [4,7,9,10,11,40]. The ore zones in these deposits comprise disseminated or veinlet sulfide mineralization with main ore minerals represented by auriferous arsenopyrite and pyrite ± native gold with minor amounts of galena, chalcopyrite, pyrrhotite, sphalerite, and tennantite-tetrahedrite [7,9].

In contrast to mineralized zones, the ores of the quartz vein-type deposits are dominated by high-karat native gold with only minor amounts of sulfide minerals [4,10]. These deposits usually represent a series of individual quartz veins or stockwork vein systems often surrounded by alteration envelopes in the country rocks. The quartz veins can be generally classified as crack-seal-type; however, metasomatic replacement veins are also present in the areas of upper greenschist facies metamorphism [41]. The veins are generally characterized by linear or slightly bended shapes with average width 0.5–1.5 m and length up to 100–150 m. Deposits of this type are relatively small, with estimated reserves rarely exceeding 2–5 t Au; however, they often contain ultra-high-grade native gold ore-shoots, which represent attractive exploration targets for junior companies. Examples from the ongoing exploration projects conducted in the quartz vein deposits are presented herein and discussed in detail below.

Both types of deposits are structurally controlled. The regional-scale ore-controlling lineaments include the Kyzyl thrust-shear zone hosting the Bakyrchik and Bolshevik deposits and the Akzhal-Boko fault system, in which the Akzhal and Vasilievskoe deposits are located [4,6,7,10]. Character of deformations, ore-controlling faults, and spatial relationships between the two types of Au deposits are illustrated by the schematic geological map of the Bakyrchik ore field shown in Figure 3. In this map, late sublatitudinal top-to-the-south thrusts that formed in a thrust-shear deformation regime distorting the early NW trending structural pattern and controlling gold mineralization can be clearly seen. On a local scale, the emplacement of individual quartz veins and mineralized zones was controlled by transpressional structures, such as thrust planes, fold hinges, flexural folds, etc., where zones of boudinage and mylonitization and/or tectonic breccia were formed [7,42,43]. Extensional structures, as well as granitoid dikes and black shale intervals in the sedimentary section, are considered as important ore-controlling factors in the Eastern Kazakhstan Gold Belt [4,6,10].

Figure 3. Schematic geological map of the Bakyrchik ore field. Modified after [44].

Figure 3. Schematic geological map of the Bakyrchik ore field. Modified after [44].

4. Analytical Methods

The research methods used in this study aimed to characterize the mineralogical and chemical composition of the rocks from the studied gold deposits and to elucidate geochemical processes operating in the ore-forming systems. Description and preliminary identification of the ore minerals were conducted in the Analytical Centre of Saint-Petersburg University, Russia, utilizing optical microscopy. Further identification of minerals in the ores and altered rocks and determination of their chemical composition was conducted utilizing a Hitachi S-3400N scanning electron microscope equipped with an Oxford Instruments Energy-Dispersive Spectrometer X-Max20 (SEM-EDS) (Oxford Instruments, Abingdon, UK). The spectra were processed automatically using the AzTec Energy software package (version 3.1.) using the TrueQ technique. The electron beam accelerating voltage of 20 kV and a current of 1.6 nA were used. X-ray acquisition time was 30 s in spot-mode. The energy-dispersive spectra were processed using the AzTec Energy software package. Quantification of elemental compositions was conducted using natural and synthetic standards, including pyrite, InAs, PbTe, and pure metals Zn, Cu, Bi, W, Sn, Cd, Sb, and Mn. X-ray lines measured were Ag Lα, Sb Lα, As Lα, Pb Lα, Zn Lα, Sn Lα, Fe Kα, Cu Kα, and S Kα. The analytical errors for most elements were below 0.5–0.7 wt.% (1σ). The lower detection limits for the analyzed elements were estimated at ~0.1 wt.% [45].

The composition of native gold particles was analyzed using a scanning electron microscope Tescan VEGA-3 system equipped with an Oxford EDS detector in the South Ural Federal Scientific Center of Mineralogy and Environmental Geology in Miass, RF.

Lithogeochemical and section whole-rock samples were analyzed in the LLC “Stewart Geochemical and Assay” in Moscow, RF. A total of 23 chemical elements were determined. The analytical methods included multi-channel arc atomic emission spectrometry (Arc AES) for Ag, As, B, Bi, Cd, Co, Cr, Cu, Ge, Hg, Li, Mn, Mo, Ni, P, Pb, Sb, Sn, U, V, W, and Zn, and fire assay with atomic absorption spectroscopy (FA-AAS) for Au.

The visible gold-bearing quartz vein samples were investigated utilizing a Neoscan N80 X-ray microtomograph (Neoscan, Mechelen, Belgium) in the Analytical Centre of Saint-Petersburg University, RF. The data were processed with the Neoscan 80 software, and subsequent visualization of the data was performed using the Dataviewer (version 1.5.2.4) and CTVox software (version 1.5.2) applications (Bruker Corporation, Karlsruhe, Germany).

5. Description of the Studied Gold Deposits/Occurrences and Sampling

The studied mineralization sites are located 40–60 km southeast of the Bakyrchik deposit in the area hosting the majority of the quartz vein-type deposits of the Eastern Kazakhstan Gold Belt (Figure 2). This area is composed of flyschoid sediments of the lower Carboniferous (Serpukhovian) Dalankarninskaya Formation that changes upsection to coarse molasse sediments of the middle Carboniferous (Bashkirian) Taubinskaya Formation [4,46,47]. The sediments are strongly deformed. The northwest-trending folding dominates and is usually characterized by compressed, close to isoclinal folds inclined to the northeast at an angle of 60–70° (Figure 4A). North-dipping longitudinal faults and top-to-the-south thrusts are a characteristic feature. Along with the dominant northwestern linear folding, superimposed transverse folding of northeastern orientation is locally developed [46]. Both formations contain carbonaceous mudstones and siltstones that are turned into shales in the areas of green schist facies metamorphism. Carbonaceous sediments reach up to 150 m in thickness and contain from 0.44 to 1.37 wt.% organic matter, which is typical for black shales elsewhere [46].

Intensive prolonged deformation of the sediments under greenschist facies conditions enabled mobilization of silica and formation of numerous quartz veins emplaced in extensional structures all over the area. Individual quartz veins can be up to 5 m thick and 100–300 m long and are well preserved on the surface in the steppe regions of eastern Kazakhstan as large fragments of white quartz (Figure 4B). Auriferous quartz veins are virtually indistinguishable from the barren veins, which makes exploration in this area a challenging task because, statistically, only one out of several hundred veins may contain economic gold mineralization [4,46,47].

Another characteristic feature of this area comprises dikes and stocks of the Kunush gabbro-plagiogranite suite (Figure 4C). Dike swarms of the Kunush plagiogranite porphyry were reported from many ore fields and dated at ca. 310–300 Ma [27]. The dikes are deformed and hydrothermally altered together with surrounding sediments. Based on geophysical data, the presence of bigger concealed intrusions was supposed under several mineralized quartz vein systems of the region [4,7,47].

Currently operating quartz vein-type deposits in the study area include the Sentash, Dzhumba, and Kuludzhun gold mines. Three gold occurrences in the vicinity of the Sentash deposit where exploration projects resumed in the last years were studied in this work, and their geological features are briefly described below.

5.1. Sentash Deposit

The Sentash mine, where the exploration trenches and open pits were investigated in the course of this study, was used as a reference site to characterize the alteration and mineralization styles of a typical quartz vein-type deposit. The deposit includes 60 auriferous quartz veins on the territory of ca. 100 square km. Most veins were mined in the past and are now abandoned, however, several new exploration projects were initiated at the deposit in the last years.

The quartz veins crosscut deformed flyschoid sediments of the Dalankarninskaya Formation, which undergo phyllic alteration and contain altered dikes of granodiorite porphyry [46,47]. The veins predominantly have simple linear forms. The largest Udalaya vein that produced 2.2 t Au is a short tabular body emplaced in a tectonic detachment zone formed in the core of an anticlinal fold. The Udalaya vein is 1–5 m thick with a length of about 70 m and was traced to a depth of 300 m [46]. The other ore bodies with average reserves of about 0.1–0.3 t Au are represented by 0.1–1.0 m-thick quartz veins crosscutting the layering of the country rocks and filling the fractures of northwestern strike. These types of ore bodies are exemplified by a schematic geological map of exploration site No. 49 shown in Figure 5.

The auriferous quartz veins are the major host of gold in the Sentash deposit. However, altered country rocks enclosing the veins may also contain economic concentrations of Au represented by native gold particles hosted in quartz veinlets and surrounding altered sediments. Typical mineralized quartz veins and veinlets in altered sandy shale are illustrated in Figure 6A. More than 95% of gold in the ore is native high-karat gold. Minor sulfides are represented mainly by pyrite, and small amounts of pyrrhotite, chalcopyrite, and arsenopyrite. The average gold grade is usually in the range of 2–5 ppm Au, however, it may vary significantly within individual veins, reaching several kg per ton in the hyper-rich ore-shoots. Photographs of native gold from hyper-rich ore-shoots from the Sentash deposit, presented in Figure 6C–F, show that free gold in supergene oxidation zones usually associates with Fe-hydroxide minerals, filling the space between quartz fragments in brecciated veins.

Six representative rock samples collected at the Sentash deposit were used in this study to characterize the mineralogical and chemical composition of mineralized rocks. Three samples were investigated utilizing the SEM-EDS method (Supplementary Table S1). All samples were analyzed for 23 elements, and the results were included in the dataset for statistical treatment (Supplementary Table S1).

5.2. Chetyrekhletka Occurrence

The Chetyrekhletka occurrence located 16 km northeast of the Sentash deposit (Figure 2) contains one linear ore zone where the gold mineralization associates with auriferous quartz veins developed in contacts of strongly altered granitoid dike emplaced parallel to schistosity in surrounding flyschoid sediments (Figure 6A and Figure 7). Steeply dipping ore-body with Au grade 2–5 ppm is 1.5–3 m thick and can be traced for approximately 300 m [48]. Twenty representative rock samples collected along the profile across the ore zone were used in this study to characterize the mineralogical composition of the ore and alteration of the country rocks. Eight samples were investigated utilizing the SEM-EDS method (Supplementary Table S1). Au concentrations analyzed in 75 section samples collected from trench No. 1 (Figure 7) are given in Supplementary Table S1.

5.3. Valeria Prospect

The Valeria prospect is a 2 km × 2 km area located 7 km south of the Chetyrekhletka occurrence (Figure 2), where the positive geochemical As anomaly was detected in the course of 1:200,000 geological mapping in the 1960s [48]. Intercalating carbonaceous and sandy shales exposed in this area predominantly strike NW and enclose several 100–300 m-thick alteration zones, where the shales contain characteristic brownish porphyroblasts of carbonate minerals and abundant veinlets of quartz. Valeria prospect was re-examined within the frame of the ongoing exploration project, and lithogeochemical sampling conducted along ca. 2 km-long profiles across strike of alterations zones revealed elevated concentrations of gold in metasomatically altered shales. Eighty-eight lithogeochemical samples were analyzed for twenty-three elements, and the results were used to construct mono-element geochemical maps for main ore-forming metals and included in the dataset for statistical treatment (Supplementary Table S1). Two representative rock samples from Valeria prospect were investigated utilizing optical microscopy and SEM-EDS techniques in order to characterize the mineral composition of auriferous alteration zones (Supplementary Table S1).

5.4. Georgievka Occurrence

This mineralization site in fresh road-cut near Georgievka village (Figure 2) was recently discovered in the course of the ongoing exploration project. It represents a linear NW striking 300 m-thick alteration zone in sandy shales that contains 1–2 m-thick quartz veins with sulfide minerals and visible gold identified in polished thin sections. Three samples from the Georgievka occurrence were investigated utilizing optical microscopy and SEM-EDS techniques (Supplementary Table S1), and eleven samples analyzed for twenty-three elements were included in the dataset for statistical treatment (Supplementary Table S1). In addition, 2 quartz vein samples containing visible gold were studied using the computer tomography method.

6. Results

6.1. Petrography and Mineralogy of Alteration Zones

The mineralized alteration zones in the study area occur in the poorly sorted immature Carboniferous flyschoids that vary in composition from carbonaceous mudstones to coarse-grained graywacke sandstones [28]. The sediments are metamorphosed under lower greenschist facies conditions and characterized by a low degree of recrystallization and well-preserved sedimentary structures (Figure 8A) locally obliterated by schistosity, cleavage, and mylonitic foliations as a result of regional folding. Two types of alteration are distinguished: (1) Pervasive carbonate ± chlorite alteration is expressed in the field by the appearance of characteristic brownish porphyroblasts of iron-rich carbonate commonly replaced by Fe-hydroxide minerals (Figure 8B). This type of alteration is widely developed in the area and can be defined as distal to mineralization. (2) Proximal sericite-albite-pyrite alteration is expressed in the rocks as the light coloration zones often containing abundant porphyroblasts of pyrite, which can develop pervasively (Figure 8C) or aligned to the fractures serving as conduits for fluids (Figure 8D). Because these alteration styles are typical for all studied localities, they are described below collectively.

Brownish composite porphyroblasts predominantly composed of carbonate minerals is a characteristic feature of the distal alteration zones that may be partially related to regional metamorphism [49]. The porphyroblasts, comprising up to 30–40 vol.% of the rock, vary in size from 2 to 10 mm and usually preserve the shapes of original carbonate crystals. However, most of the porphyroblasts are characterized by complex internal structures, and several compositional varieties were distinguished as a result of microscopic investigations. Porphyroblasts of iron-rich carbonate (supposedly ankerite) completely replaced by Fe-hydroxide minerals are a typical feature of the wall rock alteration at the Sentash deposit and Chetyrekhletka occurrence (Figure 9A,B). Zoned porphyroblasts with fine-grained quartz-chlorite aggregate in the cores and ferrodolomite-iron hydroxide rims, shown in Figure 9C,D, were described in distal parts of alteration zones at the Chetyrekhletka occurrence. Calcite porphyroblasts are characteristic for iron-poor rock types (Figure 9E), and chlorite-dominated porphyroblasts with sericite rims were observed in black shales at several localities (Figure 9F).

Carbonation is also manifested in the groundmass of the rocks, where it is accompanied by the development of chlorite ± sericite and albite occurring as irregularly distributed clusters and/or branching microveinlets (Figure 9E). Monazite, rutile, and zircon occur as accessory and trace minerals (Figure 9E). SEM-EDS analysis has shown that the compositions of carbonate minerals in the groundmass correspond to those in the porphyroblasts and vary from ferrodolomite to calcite, with only one analysis plotting in the ankerite field on the classification diagram (Figure 10 and Supplementary Table S1). The composition of carbonate minerals is probably controlled by the iron content in the host rocks and by the degree of preservation of ankerite, which is easily decomposed and replaced by Fe-hydroxide minerals and dolomite in the supergene oxidation zones [49]. Distal alteration also includes silification expressed by the quartz-carbonate veinlets/veins typical for central parts of alteration zones. Gold mineralization is interpreted as part of this alteration assemblage, with highest concentrations (up to 0.2 ppm Au) found in veinlet-rich samples.

Proximal alteration zones usually occur as envelopes enclosing the auriferous quartz veins and comprise typical phyllic alteration assemblage, including sericite, albite, pyrite ± chlorite, Fe-Mg-Ca carbonates, arsenopyrite, and pyrrhotite. Pervasive phyllic alteration develops irrespectively of the host rock, affecting various types of sediments and granitoid dikes to the same extent (Figure 11A,B). Sericite predominantly replaces the matrix of sedimentary rocks and occurs as flakes and microveinlets comprising up to 50 vol.% of quartz-albite ± pyrite metasomatites (Figure 11C,D). Chlorite and calcite are characteristic secondary minerals in altered granitoid dikes, where the ferromagnesian phases are replaced by chlorite, and the primary feldspars are replaced by a fine-grained aggregate of quartz, albite, carbonate, and sericite with patches of Fe-hydroxide minerals (Figure 11E). Accessory minerals include apatite, ilmenite, rutile, zircon, and monazite (Figure 11C–F). Sulfide mineralization is mainly represented by pyrite, which is usually replaced by Fe-hydroxides as a result of supergene oxidation (Figure 11F). Concentrations of Au in proximal alteration zones may vary significantly, with native gold being the main host of this metal in the supergene oxidation zones.

6.2. Petrography and Mineralogy of Auriferous Quartz Veins

The auriferous quartz veins have sharp contacts with country rocks typical of crack-and-seal-type veins [46,47]. The main constituent of the veins is milky white quartz containing irregularly distributed patches of Fe-hydroxide minerals. Brecciation and recrystallization are common and point to multiple episodes of deformation and fluid circulation. Breccia zones usually contain coarse fragments of recrystallized quartz cemented together by a matrix composed of Fe-hydroxide minerals (up to 50 vol.%). Microscopic studies show that quartz displays multiple growth stages and a variety of textures, including sheared quartz, comb quartz, and a mosaic texture of variably recrystallized quartz (Figure 12A). All quartz veins contain variable amounts of carbonate minerals occurring as late-stage fracture-filling aggregates and some veins occasionally contain plagioclase and K-feldspar grains (Figure 12A,B). Accessory and trace minerals identified in the quartz veins include apatite, monazite, zircon, and ilmenite. Most quartz vein samples studied in this work represent supergene oxidation zones, and unoxidized sulfides were found only in the quartz veins from the Georgievka occurrence. In this locality, clusters of unoxidized pyrite in milky white quartz are enclosed by the oxidized zones, where the sulfides are replaced by Fe-hydroxide minerals and reddish vuggy quartz, and the transition between the oxidized zones and the unoxidized sulfides is abrupt (Figure 12C). SEM-EDS analysis of samples from the Georgievka occurrence has shown that sulfide minerals are predominantly represented by pyrite with small inclusions of chalcopyrite, arsenopyrite, and galena (Figure 12D). Botryoidal aggregate of Fe-hydroxide minerals with colloform banding, developed in the oxidized parts of the quartz veins, contains relics of pyrrhotite and microinclusions of iodargyrite (or iodyrite, AgI) and Bi phases (Figure 12E and Supplementary Table S1). Native gold in the quartz veins usually occurs as variably sized particles in association with Fe-hydroxide minerals (Figure 6C,F and Figure 12F) and as free grains and stringers filling the fractures between the quartz grains (Figure 6D,E). Microprobe analyses of gold particles recovered from the Chetyrekhletka occurrence have shown that most of the analyzed grains were represented by high- and very-high-grade gold with fineness generally in the range 970–995‰ and Au/Ag ratios from 1:50 to 1:170 (Supplementary Table S1).

In order to further corroborate the distribution of gold particles within Fe-hydroxide aggregate, a sample of visible gold-bearing quartz vein from the Georgievka occurrence was scanned utilizing X-ray computed micro-tomography. This method generates a shadowgraph of a sample, where the differences in greyscale represent degrees of X-ray absorption that is a function of the density and atomic number of the sample materials [50,51]. Figure 13 shows two images of the gold-bearing quartz vein sample where the quartz appears dark grey, the phases with the brightest grey values are Fe-hydroxide minerals, and native gold particles appear bright yellow. In the image, shown in Figure 13A, the sample is rendered opaque to illustrate the general distribution of Fe-hydroxide minerals in quartz, and in the image shown in Figure 13B, quartz and Fe-hydroxide minerals are rendered semitransparent, allowing the distribution and size of the gold particles in the central part of the sample to be seen. Thus, the computed micro-tomography data matches well the observations made in the hand specimens (Figure 6C,F) and under the microscope (Figure 12F), showing that aggregates of Fe-hydroxide minerals are the main host of native gold in the supergene oxidation zones.

6.3. Geochemistry of Mineralized Alteration Zones

Geochemical methods are widely used to analyze, visualize, and interpret data for further mineral exploration [52,53,54,55]. Extensive lithogeochemical sampling conducted in the course of this work allowed for the recognition of general metallogenic tenors in the mineralized areas, and corresponding datasets for the studied localities are compiled in Supplementary Table S1.

The wall rock alteration envelopes surrounding the auriferous quartz veins are enriched in Au and chalcophile elements, and the concentrations of the ore metals show positive slopes from distal to proximal parts of the alteration zones (Figure 7). Thus, the alteration zones in flyshoid sediments, which may or may not contain the auriferous quartz veins exposed on the surface, can be considered as the primary dispersion haloes, and their geochemical characteristics can provide important information relevant to ore genesis. Therefore, in addition to mineralogical characterization of the alteration zones, analysis of spatial distribution of the ore elements in the alteration zones and statistical analysis of geochemical data were applied to determine the ore-forming elemental association.

6.3.1. Spatial Distribution of Ore Elements in Alteration Zones

Spatial distribution of the ore elements was studied at the Valeria prospect, where several alteration zones, described in Section 5.3, were sampled in the course of lithogeochemical exploration. The mono-elemental maps illustrating the spatial distributions of Au and other relevant ore elements were created by interpolating the lithogeochemical data. The interpolated geochemical maps for Au, As, and Pb, presented in Figure 14A–C, were constructed by classifying the measured concentration values into six groups using the ArcGIS software (version 10.8.2, Environmental Systems Research Institute, Redlands, CA, USA), with each concentration group equally covering approximately 15% of the total variation range.

As seen in Figure 14A, the whole-rock gold anomalies reaching up to 0.15 ppm Au spatially overlap with the alteration zones in sandy shales that were identified and mapped during the field work. The gold anomalies mainly coincide with broad anomalies of As (up to 200 ppm; Figure 14B) and Pb (up to 25 ppm; Figure 14C). In addition, elevated concentrations of Cu (up to 300 ppm) and Zn (up to 200 ppm) were found in the same areas (Supplementary Table S1).

6.3.2. Statistical Treatment of Geochemical Data

Statistical treatment is widely used for interpretation of geochemical data and employs various statistical techniques that help to better understand the relationships between different geochemical variables and identify the ore-forming processes, e.g., [56,57]. Herein, the statistical treatment was applied to lithogeochemical datasets and included pairwise correlation coefficients and factor analysis. Before entering the statistical treatment, the data were processed following standard procedures including the log-ratio transformation and censoring the data below the detection limit and outliers [57,58], and statistical analysis was carried out utilizing the software platform IBM SPSS Statistics v30 [59,60].

Pairwise Correlation Coefficients

Correlation coefficients help to assess the degree of linear correlation between the concentrations of different chemical elements. Pairwise correlation coefficients, based on the Pearson’s correlation coefficient formula, were calculated for the data from each of the studied mineralized areas and collectively. Positive correlation of Au with As and strong mutual correlations in the distribution of the chalcophile elements (Zn, Cd, Cu, Pb, Sb, Ag, Ge, and Hg) were established in all datasets and are exemplified here by the correlation matrix calculated for lithogeochemical data from the Valeria prospect (

Table 1. Pairwise correlation coefficients between concentrations of chemical elements in the lithogeochemical dataset from Valeria prospect. Correlation coefficients greater than the critical value, indicating a statistically significant correlation, are marked in bold print.

	Au	Ag	As	B	Bi	Cd	Co	Cr	Cu	Ge	Hg	Li	Mn	Mo	Ni	P	Pb	Sb	U	V	W	Zn
Au	1
Ag	0.07	1
As	0.44	0.09	1
B	0.08	0.60	0.34	1
Bi	0.24	−0.21	0.14	−0.28	1
Cd	0.09	0.56	0.17	0.58	−0.34	1
Co	0.13	0.16	0.09	0.03	−0.36	0.13	1
Cr	0.20	0.07	0.30	0.17	0.18	−0.04	0.08	1
Cu	0.24	0.43	0.33	0.50	−0.39	0.41	0.59	0.21	1
Ge	0.13	0.18	0.22	0.46	0.21	0.31	−0.24	0.27	0.20	1
Hg	0.26	−0.19	0.10	−0.32	0.96	−0.28	−0.40	0.17	−0.42	0.23	1
Li	-	0.18	−0.20	0.39	−0.67	0.56	0.22	−0.38	0.31	−0.01	−0.65	1
Mn	0.13	−0.24	0.26	−0.09	0.13	−0.05	0.30	0.23	0.20	0.06	0.06	−0.21	1
Mo	0.01	−0.17	−0.21	−0.25	0.29	−0.36	−0.14	−0.07	−0.19	0.08	0.31	−0.38	-	1
Ni	0.28	0.28	0.40	0.16	0.35	0.13	0.28	0.71	0.42	0.30	0.32	−0.30	0.36	−0.01	1
P	-	0.29	0.23	0.56	−0.43	0.44	0.12	0.22	0.54	0.40	−0.48	0.36	0.10	−0.14	0.16	1
Pb	0.55	0.25	0.29	0.44	−0.45	0.40	0.37	0.22	0.45	0.10	−0.47	0.34	0.22	−0.25	0.14	0.50	1
Sb	0.49	0.07	0.18	0.09	0.68	0.12	−0.27	−0.03	−0.16	0.48	0.70	−0.20	0.01	0.06	0.18	−0.18	−0.01	1
U	0.25	−0.15	0.19	−0.26	0.98	−0.27	−0.36	0.21	−0.34	0.26	0.97	−0.63	0.09	0.24	0.39	−0.41	−0.44	0.73	1
V	0.01	0.50	0.13	0.79	−0.51	0.61	0.28	0.29	0.68	0.36	−0.55	0.50	0.17	−0.21	0.27	0.69	0.56	−0.17	−0.50	1
W	-	0.62	0.09	0.32	−0.20	0.56	0.04	0.04	0.19	0.06	−0.21	0.32	−0.17	−0.15	0.13	0.25	0.26	−0.01	−0.14	0.37	1
Zn	0.04	0.52	0.17	0.72	−0.41	0.52	0.30	0.15	0.71	0.49	−0.44	0.42	0.04	−0.17	0.21	0.80	0.48	−0.09	−0.40	0.75	0.27	1

). In this dataset, where a correlation coefficient greater than the critical value indicated a significant correlation (n = 39, α = 0.05, critical value = 0.41), gold showed a statistically significant positive correlation with As (r = +0.44), Pb (r = +0.55), and Sb (r = +0.49; Figure 15). Mutual correlation between the chalcophile elements is exemplified by Zn, which had positive correlation with Cd, Cu, Pb, Ag, and Ge. Apart from gold, Sb showed a strong positive correlation with Bi, Hg, and U, comprising association of the “low-temperature” chemical elements. Ni had a predictably strong positive correlation with Co, Cu, and V, and the latter also showed a positive correlation with other chalcophile elements and P. The results of the pairwise correlation analysis point to the existence of distinct geochemical associations in the analyzed database that are further corroborated by the factor analysis.

Factor Analysis

Principal component analysis and factor analysis are routinely applied in processing of geochemical data to reduce the dimensionality of large datasets, by transforming a large set of variables into a smaller one that still contains most of the information in the large set [57]. The factor analysis, in particular, aims to produce a small number of factors, which can explain the observed variance in the larger number of variables [57,61]. It was shown that the factor analysis was capable of uncovering relationships between the variables that were not recognizable in the pairwise correlation and cluster analyses [62,63]. It was shown that the factor analysis was capable of uncovering hidden relationships between chemical elements, potentially indicating specific geochemical processes and/or mineralization events that were not recognizable in the pairwise correlation and cluster analyses, even with limited datasets [57,62,63].

In this study, the factor analysis was applied to a dataset containing 115 whole-rock geochemical samples analyzed for 20 chemical elements (variables). The results are summarized in

Table 2. Results of factor analysis from the n = 115 whole-rock geochemical data, including the 20 variables in relation to the first 3 factors accounting for 55% of the total data variance. Elements with largest positive loadings onto each factor are marked in bold print.

	Factor 1	Factor 2	Factor 3
Au	0.83	0.09	0.04
Ag	0.89	−0.01	−0.01
As	0.74	0.04	0.17
B	−0.09	−0.41	0.51
Bi	0.16	0.82	0.01
Co	0.31	0.02	0.59
Cr	0.06	0.11	0.41
Cu	0.31	−0.09	0.73
Hg	−0.20	0.91	−0.08
Li	−0.28	−0.59	0.23
Mn	0.49	0.36	0.31
Mo	0.56	0.04	0.13
Ni	0.11	0.43	0.80
P	−0.01	−0.22	0.57
Pb	0.49	−0.33	0.46
Sb	0.18	0.58	0.12
U	−0.03	0.94	0.07
V	0.04	−0.50	0.63
W	0.04	−0.23	0.31
Zn	0.76	−0.14	0.28
Explained variance	3.77	4.07	3.27
Cumulative variance (%)	18.84	20.34	16.35

, and a full report is given in Supplementary Table S1. After extraction and Varimax rotation, three distinct linear components (or factors) accounting for 55% of the total variance with the eigenvalue >1 were delineated within the dataset. It is commonly suggested that the factors with eigenvalues above 1 could be considered significant and that the proportion of the total variance explained by the extracted factors should be greater than 50% [57,64]. The results of the data processing in this study show that 55% common variance shared by 20 variables can be accounted for by 3 factors with eigenvalues greater than one. Although the first three factors account for only 55% of the total variance, it is considered as a good outcome because the identified factors are geologically meaningful and are in line with the main principle of factor analysis that lithologically controlled and regionally distributed variables are generally extracted first and then the more locally controlled variables are identified [56,57].

The first factor accounting for 19% of the total variance is dominated by large positive loadings for main ore elements, including Au (0.83), Ag (0.89), As (0.74), and Zn (0.76). The second factor has explained 20% of the total variance. Variables that strongly contribute to the second factor with positive loadings are U (0.94), Hg (0.91), Bi (0.82), and Sb (0.58), representing the chemical elements that are mobile at low temperatures. Factor three, explaining 16% of the variance, is dominated by large positive loadings for Ni (0.80), Cu (0.73), V 0.63), Co (0.59), and P (0.57), which represent the chemical elements typical for the black shale formations.

The results of the factor analysis are in line with pairwise correlation coefficients and provide further details on the dispersion of chemical elements in primary haloes and internal structure of the dataset under study.

7. Discussion

7.1. Quartz Vein Deposits as Part of the Regional Ore-Forming System

Two major deposit types in the Eastern Kazakhstan Gold Belt comprise structurally controlled mineralized zones with auriferous sulfides and quartz veins with visible native gold [2,6,7,9,10,11,35,48]. Fluid inclusion data from the Bakyrchik mining district show that the mineralized zones with auriferous sulfides formed from aqueous-carbonic fluids at temperatures in the range 315–240 °C and pressures of ca. 1.3–1 kbar [7]. Comparable conditions with slightly lower temperatures were reported for the hydrothermal breccias and quartz veins with visible gold that were interpreted as the late manifestations of Au mineralization at the Bakyrchik ore field [7]. The stable isotope studies point to a variable (sedimentary to magma-derived) isotopic composition of sulfur and to a predominantly sedimentary to metamorphic source of carbon [7]. Similar results were reported for the Kostobe deposit in the Eastern Kazakhstan Gold Belt [65]. In general, the pressures reported for deposits of the Eastern Kazakhstan Gold Belt are slightly lower than pressure estimates for the Muruntau (~1.6 kbar) and Kumtor (~1.2–1.3 kbar) Au deposits in the Tien Shan orogenic belt and are comparable with the pressures reported for the Olympiada Au deposit in the Yenisei Ridge [7]. Thus, available pressure estimates for Au deposits of Eastern Kazakhstan correspond to lithostatic depths of ~4–5 km, which is typical for relatively epizonal black-shale-hosted orogenic Au deposits elsewhere [7,37].

The quartz vein-type deposits in the study area do not associate with economically significant sulfide-rich mineralized zones; however, in the other areas of Eastern Kazakhstan, the two types of deposits are closely associated, allowing for the recognition of vertical and lateral zonation within the regional ore systems. In particular, the vertical zonation was studied in several major mineralized areas, including the Espe, Kulundzhun, and Kostobe deposits [4,46,65], and the Bakyrchik ore field [7,46,66]. The general features of the vertical zonation delineated in these deposits include the appearance of auriferous sulfides (Espe and Kulundzhun) and granitoid stocks (Kostobe and Bakyrchik) at the deeper levels of quartz vein-type deposits, and the vertically zoned alteration pattern with carbonate-sericite-phlogopite-quartz zone occurring at deeper levels that changes upsection to typical phyllic alteration and to argillic alteration in the supergene oxidation zones [4,7,10,48,67]. In addition, an increase in the arsenopyrite/pyrite ratio with depth was reported for several ore systems that led some authors to propose an intrusion-related model for their genesis [7,48]. Although the detailed study of the genesis of Au deposits was not the aim of this work, the abovementioned features suggest that the quartz vein Au deposits, investigated herein, may represent the upper parts of bigger concealed ore systems with significant potential for future gold exploration.

7.2. Alteration Patterns, Geochemistry, and Implications for Regional Prospecting

Mineralogical characteristics of the alteration zones in combination with new lithogeochemical data, obtained in this study, enable a better understanding of principal geochemical processes operating in the ore-forming systems. Hydrothermal alteration assemblages associated with the studied quartz vein Au deposits can be subdivided into two types: distal and proximal to mineralized quartz veins. Pervasive carbonation locally accompanied by the development of chlorite ± sericite and albite is the dominant process in the distal alteration zones. The proximal phyllic alteration (sericite-albite-pyrite ± chlorite, Fe-Mg-Ca carbonates, arsenopyrite, and pyrrhotite) develops as localized several-hundred-meter-wide alteration envelopes in the wall rocks and is often superimposed onto the mineral assemblages of distal alteration. These alteration styles are similar to those described in the other Au deposits of the Eastern Kazakhstan Gold Belt [4,7,9,35,40,48] and in the black-shale-hosted orogenic gold deposits elsewhere [7,49,68,69,70,71].

Proximal alteration zones are characterized by elevated concentrations of Au (up to 0.2 ppm), As (up to a several hundred ppm), and other chalcophile elements, such as Zn, Pb, and Cu (Supplementary Table S1). The distribution of Au and chalcophile elements generally shows negative slopes from proximal to distal alteration zones, which is well seen in the profile lines across the ore zones (Figure 7 and Supplementary Table S1). The ore-forming elemental association can be further recognized utilizing statistical treatment of lithogeochemical data and mono-elemental geochemical maps. Pairwise correlation coefficients and factor analysis of the lithogeochemical dataset revealed three groups of chemical elements showing significant positive mutual correlations: (1) Au and As (±Ag, Sb, Cu, Pb, and Zn), (2) V and P (±Co, Ni, Mo, and Cu), and (3) Bi, Hg, U, and Sb (Table 1 and Table 2). The association of V and P, which are characteristic chemical elements of black shale formations, is interpreted as an early geochemical assemblage, and a positive correlation of V and P with Co (±Ni) can be explained by the presence of early authigenic pyrite enriched in these elements. The association of Au, As, and chalcophile elements is distinctly overprinting, and is explained by the development of gold-bearing sulfides (pyrite and arsenopyrite), manifesting the main stage of the gold mineralization in the area. Finally, association of Bi and Hg (±Sb and U) includes the chemical elements that are mobile at low temperatures and can be related to the late-stage hydrothermal or supergene fluids. Similar patterns of distribution of the ore elements have also been reported for the large Bakyrchik gold deposit by Soloviev et al., 2020 [7], and for black shale-hosted orogenic gold deposits of the Yenisei Ridge by Mansurov and Tarasov, 2017 [49].

Spatial distribution maps represent another method to visualize the ore elemental association of Au, As, and chalcophile elements in the alteration zones. Positive anomalies of Au, As, and Pb on mono-elemental maps constructed for the Valeria prospect broadly coincide with each other, and with the alteration zones in the sandy shales identified in the field (Figure 14). Distribution of concentrations shows that the alteration process has led to an increase in the content of Au and chalcophile elements by several orders relative to background values. Because the alteration zones at the Valeria prospect do not associate with any known auriferous quartz veins outcropped on the surface, they can represent the primary dispersion haloes above the concealed ore bodies, which make them potentially interesting for future gold exploration. Thus, the geochemical methods can provide useful tools to delineate the ore elemental associations and to outline reproducible regional anomalies for subsequent follow-up.

7.3. Bonanza Quartz Veins and Formation of Ultra-High-Grade Ore-Shoots

The auriferous quartz veins are usually enclosed by the wall rock alteration envelopes where the degree of alteration and concentrations of gold increase toward the central parts of the zones. This is illustrated by the sampling profile from the Chetyrekhletka occurrence showing gradual increase of gold concentrations toward the central part of the alteration envelope where a series of high-grade auriferous quartz veins is located (Figure 7). Similar geochemical patterns were observed in the other studied mineralized areas (Supplementary Table S1). Although the gradual increase of gold concentrations in the alteration zones is evident, the content of gold in the altered wall rocks usually do not exceed 0.1–0.2 ppm that, compared with high and ultra-high concentrations of gold in bonanza-type quartz veins, may indicate that the process responsible for the accumulation of gold in the ultra-high-grade ore-shoots was geochemically different. This conclusion is corroborated by the spatial distribution maps of As and Au for the Valeria prospect where the localized Au anomalies only partially overlap with broad anomalies of As (Figure 14). Such distribution patterns of Au and As anomalies, also reported for the Bakyrchik mining district [48] and for other black-shale-hosted gold provinces, e.g., [70], may indicate that the quartz vein-type gold mineralization was localized and superimposed onto larger pre-existing alteration zones containing disseminated sulfides.

It is generally accepted that the ultra-high-grade gold concentrations in bonanza veins, exceeding the Au solubility expected for fluids in orogenic gold deposits, can be explained by the physical transport of gold as solid colloidal nanoparticles and their aggregates in suspension [72,73,74,75]. This mechanism matches well the thermodynamic conditions typical for the black-shale-hosted gold provinces where hydrothermal activity, associated with deformations and greenschist facies metamorphism, resulted in the mobilization of large volumes of silica [71,73]. Rapid precipitation of amorphous silica was accompanied by precipitation and accumulation of gold particles and occurred when fluids experienced rapid changes in chemical composition and/or physical state (liquid–gas phase transitions) as a result of decompression, cooling, or flash vaporization in the fault valve systems [73,76,77]. In this model, the observed association of native gold with Fe-hydroxide minerals can be explained by oxidation of ankerite and sulfides by the late-stage hydrothermal or supergene fluids, and precipitation of Fe-hydroxide minerals in the brecciated zones within the quartz veins. This process resulted in recrystallization and further concentration of native gold and formed the low-temperature mineral assemblages, including Ag (AgI) and Bi phases typical for supergene oxidation zones [43,78].

8. Conclusions

The quartz vein deposits of the Eastern Kazakhstan Gold Belt are classified as epizonal orogenic Au deposits. The auriferous quartz veins, hosted in Carboniferous flyschoids (black shales), have linear or slightly bended shapes with average width 0.5–1.5 m and length up to 100–150 m, and contain high-karat native gold with only minor amounts of sulfide minerals. In supergene oxidation zones, the native gold is closely associated with Fe-hydroxide minerals cementing brecciated zones within the veins.

The auriferous quartz veins are usually enclosed by the wall rock alteration envelopes where two types of alteration are distinguished. Proximal phyllic alteration (sericite-albite-pyrite ± chlorite, Fe-Mg-Ca carbonates, arsenopyrite, and pyrrhotite) develops as localized alteration envelopes, and pervasive carbonation accompanied by chlorite ± sericite and albite is the dominant process in the distal alteration zones.

The rocks within the alteration zones are enriched in Au and chalcophile elements, and three groups of chemical elements showing significant positive mutual correlation have been identified: (1) an early geochemical assemblage includes V, P, and Co (±Ni)—chemical elements characteristic for black shale formations, (2) association of Au, As, and chalcophile elements is distinctly overprinting, and manifests the main stage of the sulfide-hosted Au mineralization, and (3) association of Bi and Hg (±Sb and U) includes the chemical elements mobile at low temperatures. The chalcophile elements form reproducible positive anomalies on spatial distribution maps and can be used as reliable pathfinders for regional gold prospecting.