Lithochemical Stream Sediments of the Dukat Gold–Silver Ore-Forming System (North–East of Russia)

minerals

Article Lithochemical Stream Sediments of the Dukat Gold–Silver Ore-Forming System (North–East of Russia)

Artem S. Makshakov * , Raisa G. Kravtsova and Vasiliy V. Tatarinov

A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Favorskiy str 1A, 664033 Irkutsk, Russia; [email protected] (R.G.K.); [email protected] (V.V.T.) * Correspondence: [email protected]

Received: 11 October 2019; Accepted: 10 December 2019; Published: 14 December 2019

Abstract: The composition, structure and formation features of the exogenous anomalous geochemical fields (AGFs) identified by lithochemical stream sediments (LSSs) are considered using the examples of the Dukat gold–silver ore-forming system and the deposit with the same name. The research was carried out in the Balygychan–Sugoy trough (Magadan region, north–east of Russia). Areal geochemical surveys on the 1:200,000 and 1:50,000 scales were conducted. Exogenous AGFs of basic element indicators of all known mineralization types were studied. It is shown that the surveys on the 1:200,000 scale are characterized by simplicity, relative depth and the possibility to obtain information operatively about the metallogeny of the area as a whole. At the same time, the anomalies were revealed as a result of surveys often making a relatively poor component composition and low contrast compared with ores. The violation of quantitative and sometimes qualitative relationships can be seen between elements, especially those related to gold–silver mineralization. In this situation, the most informative are surveys of LSSs on the 1:50,000 scale. The AGFs were revealed as a result of their performance to have a richer component composition and high contrast, conforming to different ore types. It is shown that, while prospecting for gold–silver mineralization with LSSs in cryolithogenesis zone conditions, the binding forms study of mineralization element indicators is effective. In watercourse heads, where alluvium is practically absent, mosses are proposed for sampling, as they hold the sandy silt material firmly. The obtained results are recommended for use at all stages of prospecting, not only in the north–east of Russia but also in other similar climatic regions.

Keywords: lithochemical stream sediments; anomalous geochemical ﬁelds; areal geochemical surveys; gold–silver mineralization; element indicators; element binding forms

1. Introduction The Dukat gold–silver ore-forming system (OFS) is located in the north–east of Russian territory in the central part of the Balygychan–Sugoy trough (Omsukchan ore district, Magadan region). There are also unique silver reserves of the Dukat gold–silver deposit here as well as a number of smaller gold–silver (Au–Ag), silver–polymetallic (Ag–Pb) and tin–silver (Sn–Ag) deposits. The Omsukchan ore district is one of the largest in the Magadan region by the number of Au and Ag deposits, occupying the third position in gold reserves and the fourth place for silver among the Russian regions [1,2]. Only one Dukat deposit contains about 17 thousand tons of silver [3]. However, despite the fact that the Magadan region’s reserves of gold and silver occupy leading positions, the mineral raw material demand for the extraction of these noble metals is growing every year, and the number of easily discovered deposits in this territory is constantly decreasing. The problem of revaluating old deposits and identifying new ones is becoming real.

Minerals 2019, 9, 789; doi:10.3390/min9120789 www.mdpi.com/journal/minerals Minerals 2019, 9, 789 2 of 40

The growth in the world price for gold and silver is causing increased interest not only in large and medium-sized but also in small deposits. The main prospecting objects for mining and processing enterprises today are either relatively small deposits on the periphery of the already-known developed objects or deposits that are difficult to access, located in hard-to-reach areas and as a rule not outcropping [4–8]. Prospecting for such objects following traditional methods bears significant financial and labour costs and is often impossible. A reliable basis for solving this problem is a complex of applied and fundamental research, including geochemical searching methods. It should be emphasized that the widespread use of geochemical methods in prospecting and exploring practice has led to the discovery of a significant number of deposits in recent decades. Accordingly, an important role in the prospecting for ore mineralization, in our case gold and silver deposits, is given to the study of exogenous anomalous geochemical fields (AGFs) identified during area surveys of lithochemical stream sediments (LSSs). The first results of small-scale area surveys of stream sediments in the former USSR were published in [9–11]. Great contributions to the study of actual watercourses’ loose sediments (river stream sediments) and the development of a geochemical prospecting method based on LSSs in the USSR were made by major scientists such as A.P. Solovov and V.V. Polikarpochkin [12–16]. Further research continued with the work of their disciples and followers, for example [17–30]. Books of foreign scientists translated into Russian are very popular [31,32]. Their early publications on the study of stream sediments for the purpose of ore deposit prospecting can be referred to, for example, in [33–42] and, more recently, in [43–55]. During ore mineralization prospecting, traditional geochemical surveys of LSSs are almost a basic method, which allows the evaluation of considerable territories. The experience of such surveys performed in the Omsukchan district area—one of the Magadan region’s largest Au–Ag ore areas—has shown that, in periglacial areas in mountainous relief conditions, there are certain difficulties involved in lithochemical sampling. In cryogenesis zone conditions, in which the main process forming LSSs is physical weathering, river and stream heads are mainly made of highly washed coarse clastic sediments. Alluvial sediments are either absent or formed with a sharp deficit of sandy silt material, which is the concentrator of most ore elements. As a consequence, the negative traits of exogenous AGFs identified through LSSs are relatively poor component composition, low contrast and violation of the quantitative and sometimes qualitative relations between the elements [25,28,29,56]. As a result, it is difficult to typify the anomalies and, as a consequence, the relation to a certain type of mineralization and zonality identification—the main indicator in assessing the level of the ore zone’s erosion profile. It becomes obvious that LSSs formed in the conditions of cryolithogenesis zones need to be studied more thoroughly to develop new methods for effective ore mineralization prospecting, primarily for Au–Ag. This is especially true for well-studied territories where a significant amount of prospecting and appraisal works have already been undertaken. The central part of the Balygychan–Sugoy trough is just such a territory in the north–east of Russia, where the research was performed within the framework of the Dukat Au–Ag OFS and associated areas.

2. Study Areas: Geology, Ore Mineralization, Geography and Climate

2.1. Geological Setting Structurally, the research area is located in the Okhotsk–Chukotka volcanic belt (OCVB) (Figure1), which is the East Asian system’s largest element of marginal continental volcanoplutonic belts. The beginning of the belt development corresponds to the lower and middle Albian boundary. The upper age limit refers to the upper Cretaceous–Santonian or possibly to the Campanian beginning. The duration of the formation was approximately 25 million years [57–59]. According to V.F. Belyi [57], the ﬂank and the inner and outer zones are distinguished in their composition. The most signiﬁcant in size and length is the belt’s outer zone. Regarding its metallogenic aspect, it is categorized as an Au–Ag zone and divided into four sectors—Okhotsk, Penzhina, Anadyr and Central Chukotka. Minerals 2019, 9, 789 3 of 40

Minerals 2019, 9, x FOR PEER REVIEW 3 of 40 One of the largest linear structures of this zone in the Okhotsk sector is the Balygychan–Sugoy trough. ItBalygychan–Sugoy is conﬁned to the largetrough. Omsukchan It is confined (Balygychan) to the large deep Omsukchan fault. The trough’s(Balygychan) construction deep fault. involves The atrough’s number construction of large volcano-tectonic involves a number structures of large (depressions), volcano-tectonic to which structures the regional (depressions), ore systems to which are conﬁned.the regional The ore largest systems one hereare confined. is the Dukat The Au–Ag largest OFS. one here The OCVB is the Dukat and the Au–Ag Balygychan–Sugoy OFS. The OCVB trough’s and detailedthe Balygychan–Sugoy structural, geological trough’s and detail metallogeniced structural, characteristics geological have and beenmetallogenic considered characteristics in several works, have forbeen example considered [3,57 in–70 several]. works, for example [3,57–70].

Figure 1.1. The OCVBOCVB tectonictectonic andand metallogenicmetallogenic zoningzoning schemescheme (1–4).(1–4). Constructed by the authors of the presentpresent paperpaper usingusing thethe datadata fromfrom [3[3,57,71],57,71].. 11 andand 2—inner2—inner zonezone andand subzones:subzones: 1—inherited1—inherited (gold–silver–copper–porphyry)(gold–silver–copper–porphyry) andand 2—newly 2—newly formed (gold–silver–copper–molybdenum–formed (gold–silver–copper–molybdenum– porphyry); 3—outerporphyry); zone 3—outer (gold–silver), zone (gol sectors:d–silver), O—Okhotsk, sectors: O—Okhots P—Penzhina,k, P—Penzhina, A—Anadyr andA—Anadyr CC—Central and Chukotka;CC—Central 4—ﬂank Chukotka; zones: 4—flank WO—West zones: Okhotsk WO—West (gold–porphyry), Okhotsk (gold–porphyry), EC—East Chukotka EC—East (gold–silver Chukotka and gold–porphyry);(gold–silver and 5—Siberian gold–porphyry); platform; 6—Preriphean5—Siberian medianplatform; arrays 6—Preriphean (Okh—Okhotsk, median Om—Omolon arrays and(Okh—Okhotsk, E—Eskimo); 7—UpperOm—Omolon Yana–Chukotka and E—Eskimo); folded 7—Uppe area (Mesozoic);r Yana–Chukotka 8—Koryak–Kamchatka folded area (Mesozoic); folded area (Cenozoic);8—Koryak–Kamchatka 9—sector boundaries folded area of (Cenozoic); the OCVB outer9—sec zones;tor boundaries and 10—the of the Dukat OCVB Au–Ag outer ore-forming zones; and system10—the (OFS). Dukat Au–Ag ore-forming system (OFS).

The Dukat Au–Ag OFS, located in the central part of the Balygychan–Sugoy trough, is a long-lived The Dukat Au–Ag OFS, located in the central part of the Balygychan–Sugoy trough, is a early Cretaceous volcano-tectonic depression [3]. Its folded base is composed of Upper Yana complex long-lived early Cretaceous volcano-tectonic depression [3]. Its folded base is composed of Upper Triassic terrigenous rocks. The depression is made up of lower Cretaceous vulcanites of Askold suite Yana complex Triassic terrigenous rocks. The depression is made up of lower Cretaceous vulcanites rhyolite formation, which flank the coal-bearing lower Cretaceous molasse of the Omsukchan suite. of Askold suite rhyolite formation, which flank the coal-bearing lower Cretaceous molasse of the The early Cretaceous rhyolite formation is represented mainly by ultra-acid effusive rocks with very Omsukchan suite. The early Cretaceous rhyolite formation is represented mainly by ultra-acid high potassium alkalinity (rhyolites and rhyolitic ignimbrites) lying with a sharp angular unconformity effusive rocks with very high potassium alkalinity (rhyolites and rhyolitic ignimbrites) lying with a on the rocks of the Upper Yana complex. The Omsukchan suite is represented by siltstone series, sharp angular unconformity on the rocks of the Upper Yana complex. The Omsukchan suite is coal-bearing argillites, sandstones, gravelites and conglomerates and includes layers and lenses of coals. represented by siltstone series, coal-bearing argillites, sandstones, gravelites and conglomerates and Late Cretaceous sediments (the overlaid part) are formed by effusive rocks of the Kakhovka lower–upper includes layers and lenses of coals. Late Cretaceous sediments (the overlaid part) are formed by Cretaceous and Shorokh upper Cretaceous suites. Kakhovka suite vulcanites are represented by andesites effusive rocks of the Kakhovka lower–upper Cretaceous and Shorokh upper Cretaceous suites. and dacites. They rest with stratigraphic unconformity on the rocks of the Omsukchan suite. Shorokh Kakhovka suite vulcanites are represented by andesites and dacites. They rest with stratigraphic suite vulcanites are represented by rhyolites and rhyolitic ignimbrites. Among the intrusive formations unconformity on the rocks of the Omsukchan suite. Shorokh suite vulcanites are represented by are early Cretaceous diorite porphyrites and subvolcanic rhyolites; late Cretaceous granites, granodiorites, rhyolites and rhyolitic ignimbrites. Among the intrusive formations are early Cretaceous diorite diorites, quartz diorites, diorite porphyrites and gabbro; and late Cretaceous subvolcanic rhyolites, porphyrites and subvolcanic rhyolites; late Cretaceous granites, granodiorites, diorites, quartz nevadites, felsites, andesites, diorite porphyritess and diorites (Figure2). diorites, diorite porphyrites and gabbro; and late Cretaceous subvolcanic rhyolites, nevadites, felsites, andesites, diorite porphyritess and diorites (Figure 2).

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Figure 2. Schematic geological map of the Balygychan–Sugoy trough’s central part. Constructed by Figure 2. Schematic geological map of the Balygychan–Sugoy trough’s central part. Constructed by the authors of the present paper using the data from [72,73] with additions by [3] and materials of the the authors of the present paper using the data from [72,73] with additions by [3] and materials of Dukat geological exploration enterprise. 1—Quaternary sediments; 2—upper Cretaceous rhyolites, the Dukat geological exploration enterprise. 1—Quaternary sediments; 2—upper Cretaceous rhyolites, ignimbrites and rhyolitic tuffs (Shorokh suite); 3—lower–upper Cretaceous andesites, ignimbritesandesite–basalts, and rhyolitic their tuffstuffs (Shorokhand tuffaceous suite); lavas, 3—lower–upper dacites, rhyolites, Cretaceous tuffaceous andesites, conglomerates andesite–basalts, and theirtuffaceous tuffs and tuffaceousaleurolites lavas,(Kakhovka dacites, suite); rhyolites, 4—lower tuffaceous Cretaceous conglomerates sandstones, and aleurolites, tuffaceous argillites, aleurolites (Kakhovkaconglomerates suite); 4—lowerand coal seams Cretaceous (Omsukchan sandstones, suite); aleurolites, 5—lower Cretaceous argillites, conglomerates rhyolites, tuffaceous and coal lavas seams (Omsukchanand tuffs of suite); rhyolites, 5—lower felsites, Cretaceous andesites, rhyolites,dacites, tuffaceous tuffaceous conglomerates, lavas and tuffssandstones, of rhyolites, aleurolites felsites, andesites,and argillites dacites, tuffaceous(Askold suite); conglomerates, 6—Jurassic sandstones, aleurolites,aleurolites argillites, andsandstones, argillites tuffaceous (Askold suite); sandstones, 6—Jurassic aleurolites,sandy argillites, argillites, tuffaceous sandstones, gravelites tuffaceous and sandstones,ash tuffs; 7—Triassic sandy argillites, argillites, tuffaceous aleurolites, gravelites sandstones, and ash tuffs;sandy 7—Triassic mudstones, argillites, tuffs, aleurolites, calcareous sandstones,sandstones sandyand coquinas; mudstones, 8–11—late tuffs, calcareousCretaceous sandstones intrusions: and coquinas;8—granites, 8–11—late 9—granodiorites, Cretaceous intrusions:10—diorites, 8—granites, quartz diorites 9—granodiorites, and diorite porphyrites 10—diorites, and 11—gabbro; quartz diorites and12—early diorite porphyrites Cretaceous and diorite 11—gabbro; porphyrites; 12—early 13 an Cretaceousd 14—subvolcanic diorite porphyrites;formations: 13—late 13 and 14—subvolcanic Cretaceous formations:rhyolites, 13—late nevadites, Cretaceous felsites, andesites, rhyolites, nevadites,diorite porphyrites felsites, andesites,and diorites diorite and 14—early porphyrites Cretaceous and diorites rhyolites; 15—faults: a—reliable, b—estimated; 16—the boundaries of the Dukat Au–Ag OFS; and 14—early Cretaceous rhyolites; 15—faults: a—reliable, b—estimated; 16—the boundaries of the Dukat 17–21—deposits (a) and prospects (b): 17—gold–silver (1—Dukat, 2—Krasin, 3—Barguzin and Au–Ag OFS; 17–21—deposits (a) and prospects (b): 17—gold–silver (1—Dukat, 2—Krasin, 3—Barguzin and 4—Zharok), 18—silver–polymetallic (5—Mechta, 6—Tidid, 7—Nachalny, 8—Struya, 9—Yasnaya 4—Zharok), 18—silver–polymetallic (5—Mechta, 6—Tidid, 7—Nachalny, 8—Struya, 9—Yasnaya Polyana Polyana and 10—Fakel), 19—tin–silver (11—Maly Ken, 12—Novo-Dzhagyn, 13—Razny, and 10—Fakel), 19—tin–silver (11—Maly Ken, 12—Novo-Dzhagyn, 13—Razny, 14—Finalny, 15—Severny, 14—Finalny, 15—Severny, 16—Shorokh and 17—Tovarishch), 20—tin (18—Upper Bastoy, 16—Shorokh19—Chuzhoye, and 17—Tovarishch), 20—Vengerka, 20—tin 21—K (18—Upperhataren, 22—Industrialnoye Bastoy,19—Chuzhoye, and 23—Nimfa), 20—Vengerka, 21—tin–rare 21—Khataren, 22—Industrialnoyemetal and rare metal; and 23—Nimfa), 22—area of 21—tin–raregeochemical metalsurvey and of lithochemical rare metal; 22—area stream sediments of geochemical of 1:50,000 survey of lithochemicalscale (a); Chaika stream site, sediments where the of 1:50,000detailed scalework (a);was Chaika carried site,out, whereincluding the bryolithochemical detailed work was research, carried out, includingand the bryolithochemical alluvium material research, composition and study the alluvium (b). material composition study (b).

2.2.2.2. Ore Ore Mineralization Mineralization TheThe zonal zonal mineralization mineralization distribution distribution is typical of of the the Dukat Dukat Au–Ag Au–Ag OFS. OFS. The The central central part part is is representedrepresented mainly mainly by by Au–Ag Au–Ag deposits deposits andand prospects,prospects, then then Ag–Pb Ag–Pb and, and, on onthe the periphery, periphery, Sn–Ag. Sn–Ag. OutsideOutside the OFS,the OFS, there there is widely is widely manifested manifested tin (Sn),tin (Sn), less tin–rareless tin–rare metal metal (Sn–W) (Sn–W) and rareand metalrare metal (Mo–W) mineralization(Mo–W) mineralization (see Figure (see2)[ 3Figure,56,72 –2)78 [3,56,72–78].]. TinTin mineralization mineralization is is not not typical typical of of the the DukatDukat Au–Ag OFS; OFS; the the Sn–W Sn–W and and Mo–W Mo–W mineralization mineralization overover its areaits area is poorly is poorly developed. developed. Spatially Spatially andand genetically,genetically, it it is is closely closely related related to tothe the Dukat Dukat granitoid granitoid massif,massif, namely namely the the formation formation of of skarns, skarns, greisensgreisens and stockwork zones zones that that have have been been uncovered uncovered by by boreholes at a depth of 900–1000 m in zones of the massif endo- and exocontact. The main vein- and boreholes at a depth of 900–1000 m in zones of the massif endo- and exocontact. The main vein- and rock-forming minerals are quartz, feldspar, and muscovite. Less common ones are biotite, fluorite, rock-forming minerals are quartz, feldspar, and muscovite. Less common ones are biotite, fluorite, apatite, carbonates and very rarely, chlorites, amphiboles, pyroxenes, beryl and tourmaline. As for ore Minerals 2019, 9, 789 5 of 40 minerals, scheelite, wolframite, molybdenite, pyrrhotite, cassiterite, arsenopyrite and chalcopyrite are observed. Pyrite, galena and magnetite can be found [56,74]. The Sn–W and Mo–W mineralization is diverse in its geochemical composition. Relatively high contents are typical of Mo, As, Sn and Bi; fewer high contents are typical of W and Zn and low contents of Ag, Cu, and Pb. The formation of hydrothermal mineralization originated in several stages. The earliest is unprofitable polymetallic sulphide (Pb–Zn) mineralization. It is related to diverse composition zones of disseminated sulphide mineralization (ZDSM), widely manifested throughout the area [56]. As for ore minerals, pyrite, pyrrhotite, galena, sphalerite, chalcopyrite, and arsenopyrite are present. Zn and Pb form higher concentrations and Cu, As, Bi, Sn and Ag low ones. Later hydrothermal mineralization within the Dukat Au–Ag OFS is represented by a number of Sn–Ag, Ag–Pb and Au–Ag deposits and prospects [3,56,72–80]. Sn–Ag mineralization is widely observable in the deposits of Maly Ken and Novo-Dzhagyn and in the prospects of Razny, Finalny, Severny, Shorokh and Tovarishch. This type of mineralization is also evident in the developing areas of Ag–Pb and Au–Ag mineralization but much less frequently. The main vein minerals are quartz, chlorite, feldspar, sericite, calcite, and ankerite. The main ore minerals are argentite, stannite, cassiterite, chalcopyrite, sphalerite and galena, and the rare ones are pyrite, arsenopyrite and fahlores. The elemental composition is diverse. High concentrations are typical of As, Sn, Ag, Sb, Zn and Bi and lower concentrations of Pb and Cu. As for the uprising of ore zones, the geochemical zonality ranged series is as follows: Pb, Zn, Cu As, Sn, Ag, Bi Sb. → → Ag–Pb mineralization is widely manifested in the deposits of Mechta and Tidid and in the prospects of Nachalny, Struya, Yasnaya Polyana and Fakel. In addition, this type of mineralization is identified in the Dukat deposit, mainly in its low ore horizons and flanks. The ore bodies have quartz–sericite–chlorite–hydromica composition. Deeper veins of sulphide–carbonate (calcite)–quartz composition appear. As for ore minerals, the most prevalent ones are galena, sphalerite, arsenopyrite, chalcopyrite and silver minerals (argentite, freibergite, pyrargyrite and stephanite). Less common are pyrite, pyrrhotite, boulangerite, marcasite and stannite. Ag, As, Pb and Sb form high concentrations, Hg, Zn and Sn fewer high concentrations and Cu and Bi low concentrations. The zonality is distinctly manifested in the distribution of ore elements. The series of vertical geochemical zonality (on the ore zones’ rising) is as follows: Sn, Bi Zn, Cu Bi, Pb Ag As Sb, Hg. The occurrence of Sn and → → → → → Bi at depth is related to Sn–Ag mineralization, which is usually spatially joined with Ag–Pb. In the final phases of the hydrothermal stage, Au–Ag and predominantly Ag ores are formed. “Primary” volcanogenic Au–Ag ores have survived only as relics. Most of the Au–Ag ores were rejuvenated under the influence of embedded granitoid intrusions and have a complex composition. Au–Ag and Ag ores are widely developed on the largest silver reserves of the Dukat Au–Ag deposit and to a much lesser extent are manifested in the prospects of Krasin, Barguzin and Zharok. The silver reserves in the Dukat deposit amount to 17 thousand tons, and the gold reserves consist of 40 tons, with average contents of 1 ppm Au and 500 ppm Ag. This deposit has a long and complex development history. The ore bodies here consist mainly of a series of converged veined and mineralized zones. Predominantly Ag ores are represented mainly by quartz–pyrolusite and quartz–rhodonite veins and Au–Ag ores by quartz–rhodonite–feldspar, quartz–feldspar and quartz–feldspar–sulphide ones. The vein minerals are quartz, feldspar (adularia), rhodonite, chlorite, calcite, rhodochrosite, hydromica and sericite. The main ore minerals are argentite (acanthite), native Ag, electrum, kustelite, proustite, pyrargyrite, galena, sphalerite and chalcopyrite. In the circum ore metasomatic rocks, quartz, sericite, hydromica, chlorite, carbonate and epidote are widely developed, and of the ore minerals, only pyrite is present [56,74,76,80–84]. Typical volcanogenic Au–Ag ores are characterized by the simplicity of the basic ore elemental composition. Ag, Sb and Au are specified by high concentrations, while Pb, As, Zn and Hg exhibit medium concentrations. The elements’ concentration level related to Ag and rejuvenated Au–Ag ores increases significantly. The ore element composition becomes complicated. Along with the elements listed above (Ag, Au, Sb, As, Hg, Pb and Zn), Cu and Bi appear. On the rising Minerals 2019, 9, 789 6 of 40 of the ore zones, the generalized series of geochemical zonality for Ag and Au–Ag ores is as follows: Bi Zn, Cu Pb Au Ag As Sb Hg. → → → → → → → 2.3. Geography and Climate The main mountain structure of the research area is the Omsukchan ridge, located within the Balygychan–Sugoy interfluve (right tributaries of the Kolyma river). The relief is close to the Alpine type. The absolute peak levels are 1000–1600 m, and the relative excess is 400–600 m. The mountain slopes are steep (25–30◦). The mountains’ peaks and slopes are covered with large-block eluvial–deluvial sediments with a capacity of up to 3–5 m. Their bottom is over covered with thick deluvial–colluvial sediments. TheMinerals surface 2019, 9, x isFOR swampy, PEER REVIEW and the exposure is poor [72,73]. 6 of 40

The climate2.3. Geography of the and region Climate is sharply continental. Winters are long and cold, and summers are short and hot. Atmospheric precipitates are distributed unevenly throughout the year. Most of The main mountain structure of the research area is the Omsukchan ridge, located within the them fall in theBalygychan–Sugoy summer in interfluve the form (right of rain.tributaries Solid of th snowe Kolyma cover river). is The established relief is close in to latethe Alpine September–early October andtype. finally The disappearsabsolute peak inlevels late are May 1000–1600 and m, early and the June. relative The excess region is 400–600 is characterized m. The mountain by widespread slopes are steep (25–30°). The mountains’ peaks and slopes are covered with large-block permafrost development. eluvial–deluvial sediments with a capacity of up to 3–5 m. Their bottom is over covered with thick The hydrodeluvial–colluvial network of sediments. the research The surface area is belongs swampy, and to thethe exposure Kolyma is poor and [72,73]. Viliga river basins. The main watercourses areThe the climate Omsukchan, of the region is Kakhovka,sharply continental. Dzhagyn, Winters are and long Chapchik and cold, and rivers.summers are The short valleys of the and hot. Atmospheric precipitates are distributed unevenly throughout the year. Most of them fall in smaller riversthe and summer streams in the flowingform of rain. into Solid them snow havecover is a established V-shaped in transverselate September–early profile October in the and upper reaches. The width offinally these disappears valleys in reaches late May 100and early m and June. that The ofregion the is riverbeds characterized 5 by m widespread at depths permafrost of 0.4–1.6 m. In the middle course,development. the valley profile is trapezoidal. The width of the valleys reaches 600–1000 m and that The hydro network of the research area belongs to the Kolyma and Viliga river basins. The main of the watercourseswatercourses 30 mare at the a depthOmsukchan, of 0.6 Kakhovka, m. Most Dzha ofgyn, the and side Chapchik streams rivers. (watercourses The valleys of of the the first-order) are temporary.smaller The rivers water and streams temperature flowing into in them summer have a V-shaped rarely transver exceedsse profile 4 ◦C, in and the upper in winter, reaches. the streams freeze at theThe bottom. width of The these flora valleys is reaches typical 100 ofm and the that mountain of the riverbeds forest–tundra 5 m at depths zone of 0.4–1.6 of them. In central the Kolyma. middle course, the valley profile is trapezoidal. The width of the valleys reaches 600–1000 m and that Daurian larchof foreststhe watercourses growin 30 them at valleys.a depth of The0.6 m. mountainMost of the side slopes streams are (watercourses covered with of thethickets first-order) of cedar elfin. At their peaks,are temporary. the vegetation The water typical temperature of areas in summer of alpine rarely exceeds tundra 4 dominates.°C, and in winter, Mosses the streams and lichens are widespread everywhere.freeze at the bottom. The flora is typical of the mountain forest–tundra zone of the central Kolyma. Daurian larch forests grow in the valleys. The mountain slopes are covered with thickets of cedar elfin. 3. Research MethodsAt their peaks, the vegetation typical of areas of alpine tundra dominates. Mosses and lichens are widespread everywhere.

3.1. Methods3. of Research Geochemical Methods Research

All the areal3.1. Methods geochemical of Geochemical surveys Research of LSSs were performed according to standard techniques [16,85]. Systematic lithochemicalAll the areal pointgeochemical sampling surveys wasof LSSs carried were performed out from according loose to alluvialstandard techniques sediments of actual watercourses[16,85]. (Figure Systematic3). lithochemical point sampling was carried out from loose alluvial sediments of actual watercourses (Figure 3).

Figure 3. A theoretical sampling scheme of 1:200,000 (a) and 1:50,000 (b) scale lithochemical stream Figure 3. A theoreticalsediments (LSS) sampling surveys shown scheme on a ofconventional 1:200,000 map (a )of and the 1:50,000drainage system. (b) scale 1—watercourses lithochemical stream sediments (LSS)(blue lines), surveys their shownorder (Roman on anumerals) conventional and their maprunoff ofdirection the drainage (arrows); 2—sampling system. points 1—watercourses (blue lines), their(a—double; order b—ordinary); (Roman and numerals) 3—the main and river. their Note: runo The actualﬀ direction alluvium (arrows);sampling places 2—sampling of the points LSS surveys on the 1:200,000 and 1:50,000 scales discussed in the present paper are shown in Figure (a—double; b—ordinary); and 3—the main river. Note: The actual alluvium sampling places of the LSS 4. surveys on the 1:200,000 and 1:50,000 scales discussed in the present paper are shown in Figure4. Minerals 2019, 9, 789 7 of 40

A survey on the 1:200,000 scale was performed by the Central Geochemical Expedition (Aleksandrov, Russia) under the leadership of G.I. Khorin and V.A. Lasman as part of the contract work with the production geological organization “Sevvostokgeologiya” (PGO “Sevvostokgeologiya”, Magadan, Russia). The survey on the 1:50,000 scale and scientific and methodological works were carried out by the A.P. Vinogradov Institute of Geochemistry SB RAS (IGC SB RAS, Irkutsk, Russia) under the leadership of R.G. Kravtsova in the course of the research of IGC SB RAS and contracts with PGO “Sevvostokgeologiya”. Since 2000, work on detailing, generalizing and comparing all the previously obtained data from the 1980s has been undertaken. Mineralogical and geochemical studies of the alluvium material composition and the mineral and non-mineral binding forms of the main element indicators of mineralization of different types, primarily Au–Ag, were continued. For the purpose of increasing the efficiency of ore prospecting through LSSs in cryolithogenesis zone conditions, bryolithochemical research was performed. The processing of all the previously and currently obtained data, their generalization and interpretation, the plotting of mono- and polyelement geochemical maps and detailed mineralogical and geochemical research were implemented by the authors. In the areal geochemical surveys of LSSs at the 1:200,000 scale, the routes started at 50–100 m above the sampled watercourse mouths of senior-orders and ended in the stream mouths of the second-order, and in rare cases, the first-order, with the selected endpoints of the double samples located 20–30 m from one another. The distance between the selection points of ordinary samples was 500 m, the average distance between the sampled riverbeds was 2 km and the sampling density was 1 sample/km2. The location of the alluvium samples in the LSS survey on the 1:200,000 scale is shown in Figure4a. In the areal geochemical survey of LSSs on the 1:50,000 scale, the routes started at 50–100 m above the sampled watercourse mouths of senior-orders and ended (whenever the material could be sampled) in the stream heads of the first-order. The sampling step was 200–250 m, the average distance between the sampled riverbeds was 0.5–0.7 km and the sampling density was 6–10 samples/km2. The location of the alluvium samples in the survey of LSSs on the 1:50,000 scale is shown in Figure4b. In all cases, samples were taken from the bed’s dry part surface of a temporary or permanent watercourse. Depending on the predominant material granulometric fraction, whether sand–gravel or sand (rarely sandy silt), alluvium fractions were sampled. The alluvium sample weight was 200–300 g. All the samples were thoroughly dried and sieved through a sieve with a 1 mm mesh. Abrasion was performed mechanically in steel cups on a vibrating grinder to the state of powder (~200 mesh). During the scientific methodical work, bryolithochemical research was implemented and the material composition of alluvial sediments from watercourses draining Au–Ag ore zones was explicitly studied. The bryolithochemical method is based on the sampling of aquatic and semiaquatic mosses growing on the banks and beds of the watercourse heads, together with the silt and sandy silt material that is firmly held by the moss cushion. All the selected bryolithochemical samples were first burnt to ashes in a muffle furnace and then (to purify them finally from the sandy fraction) were additionally sieved through a sieve with a 0.1 mm mesh and further analyzed for a wide range of elements. When performing the detailed investigation of the distribution and binding forms of mineralization element indicators in the watercourses’ loose sediments, bulk alluvium mineralogical–geochemical samples weighing up to 6 kg were taken, sieved into different fractions (+2 mm, 2 mm +1 mm, 1.0 +0.25 mm, and 0.25 mm) and mounted in briquette polished sections. − ÷ − ÷ − Further investigation of this material was carried out using the polarizing light microscope POLAR 3 (Micromed, Saint Petersburg, Russia) and the JXA-8200 SuperProbe electron probe microanalyzer (JEOL, Tokyo, Japan). Minerals 2019, 9, 789 8 of 40

Minerals 2019, 9, x FOR PEER REVIEW 8 of 40

Figure 4. The alluvium samples’ location (red points points)) in the survey of LSSs on the 1:200,000 (a) and 1:50,000 (b)) scales.scales.

3.2. Analysis Methods The analysisanalysis ofof thethe geochemical geochemical samples samples obtained obtained while while performing performing the the areal areal LSS LSS surveys surveys on theon 1:200,000the 1:200,000 and and 1:50,000 1:50,000 scales scales and scientiﬁc–methodicaland scientific–methodical works works was carriedwas carried out in out diﬀ inerent different years inyears the analyticalin the analytical laboratories laboratories of the Centralof the Central Geochemical Geochemical Expedition Expedition (Aleksandrov, (Aleksandrov, Russia) andRussia) IGC and SB RASIGC (Irkutsk,SB RAS (Irkutsk, Russia). Russia). All thethe samples samples underwent underwent spectral spectral semiquantitative semiquantitative analysis analysis (SSQA) (SSQA) for a wide for rangea wide of elementsrange of accordingelements according to the techniques to the techniques in [16,86,87 ].in The [16,86,8 method7]. The of sample method spilling of sample was used spilling on spectrographs: was used on thespectrographs: STE-1 (LOMO, the Saint STE-1 Petersburg, (LOMO, Russia)Saint andPetersburg, DFS-8 (Kazan Russia) Optical and andDFS-8 Mechanical (Kazan Plant,Optical Kazan, and Russia)Mechanical with Plant, an attachment Kazan, Russia) of USA-5 with (Kazan an attachment Optical andof USA-5 Mechanical (Kazan Plant, Optical Kazan, and Russia)Mechanical and Plant, Kazan, Russia) and DFS-458S (Kazan Optical and Mechanical Plant, Kazan, Russia) with a

Minerals 2019, 9, 789 9 of 40

DFS-458S (Kazan Optical and Mechanical Plant, Kazan, Russia) with a multichannel analyzer of atomic emission spectra MAES (VMK-Optoelektronika, Novosibirsk, Russia) with photodiode lines. Gold was determined through atomic absorption spectrometry with preliminary sample decomposition using an acid mixture and metal extraction using an organic sulphide solution [88,89]. The contents were measured using atomic absorption spectrometers M-303, M-403 and AAnalyst-800 (Perkin-Elmer, Waltham, MA, USA) with detection limits (ppm) of 0.004, 0.002 and 0.002, respectively. The method of atomic absorption spectrometry in its electro-thermal option with a graphite furnace atomizer HGA-72 (Perkin-Elmer, Waltham, MA, USA) was used for the determination of low Au contents (detailed investigations). Measurements were carried out using the spectrometer M-503 (Perkin-Elmer, Waltham, MA, USA). This method’s detection limit of Au is 0.0002 ppm. Mercury, while conducting areal LSS surveys on the 1:200,000 scale, was analyzed through the traditional method of SSQA. We used a sample-spilling method on the spectrograph STE-1 with USA-5 attachment, and this element’s detection limit was 1 ppm. Later, during areal LSS surveys on the 1:50,000 scale and detailed research, the atomic absorption method with acid decomposition was used with the techniques in [90,91]. The measurements were carried out on an atomic absorption spectrophotometer RAF-1M (Kazgeofizpribor, Kazan, Russia) and afterwards the mercury analyzer RA-915+ with the attachment RP-91S (Lumex, Saint Petersburg, Russia), the element detection limit in both cases being 0.005 ppm. Low Hg contents were analyzed using the atomic fluorescence method [92]. An atomic fluorescence spectrometer PSA 10.003 (P.S. Analytical, Orpington, UK) with a hydride generator was used for the measurements. The Hg detection limit was 0.0002 ppm. Substance composition determination was performed through local analysis methods using electron probe microanalysis (EPMA) on the JXA-8200 SuperProbe (JEOL Ltd., Tokyo, Japan) microanalyzer in accordance with the available techniques [93–96]. Grain surfaces were examined through a scanning electron microscope JXA-8200 SuperProbe (JEOL Ltd., Tokyo, Japan) with backscattered and secondary electrons to detect inclusions containing ore elements. With the help of an energy dispersive spectrometer EX-84055MU (JEOL Ltd., Tokyo, Japan), the inclusion compositions found were identified. The elements’ content change on the surface was studied on the distribution maps of the elements’ X-ray radiation obtained using wave spectrometers. The chemical composition of the inclusions found was determined by the survey monitoring and recalculating the recorded relative intensities in the concentration using the JXA-8200 SuperProbe microanalyzer software (Version 01.42, JEOL Ltd., Tokyo, Japan). In determining the composition of finely dispersed gold inclusions with a size smaller than the locality of the EPMA method, matrix effects were taken into account following the method of content trends [95,96].

3.3. Methods of Information Mathematical Processing To process a large amount of analytical data, mathematical methods were widely used. To obtain preliminary information on the distribution of chemical elements, as well as to obtain different mathematical data (statistics, pair correlation, etc.), the Golden Software Surfer (Version 11.2.848, Golden Software, Llc, Golden, CO, USA) and Microsoft Office Excel 2003 software (Version 11.8169.8172) were used. In the study of the exogenous AGF composition and structure identified by LSSs, the plotting of mono- and polyelement geochemical maps was executed using the multidimensional field method [97,98]. The main operations in the analysis of AGFs and geochemical map plotting were automated. To divide the analytical data sets into a system of homogeneous quantities, an automatic classification was developed, which helped to identify the most common combinations of ore elements in the samples at their close quantitative values. Monoelement geochemical fields were plotted and chemical element combinations (classes) were sought. The class separation (element association) is based on the percentage of the contrast ratio (CR) with regard to the dispersion measures and the dependence between the CRs of various elements (CR = Ci/Cb, where Ci is the element content and Cb the background). Then, the multidimensional field was plotted. The final result is presented in the form of mono- and polyelement AGF maps. The multidimensional field method has a number of Minerals 2019, 9, 789 10 of 40 advantages. It allows one to make a series of geochemical maps with a solution that is common to all elements; has a separate classification (selecting the group of elements that are typomorphic for this type of ores, this phase or this stage); shows only genetically homogeneous associations; contains mappings, in which we used the main (leading) elements; and so on. During the study of exogenous AGFs identified through LSSs, an important role was given to the Cb selection. The Cb that we accepted for the main element indicators of mineralization considered in this work are shown in Table1 (column 1) and, in general, are comparable with the average contents calculated for rocks of average and acidic composition for the earth’s crust (see Table1, columns 2–6). The same Cb values were accepted for endogenous geochemical fields. The aim is to have the possibility to compare exogenous and endogenous AGFs, which is of great importance in the plotting and interpretation of geochemical maps.

Table 1. Average element contents (ppm).

C In Average Rocks In Acidic Rocks In the Crust Element b 1 2 3 4 5 6 Au 0.005 - 0.0045 0.0043 0.00n 0.004 Ag 0.1 0.07 0.05 0.07 0.0n 0.07 Hg 0.005 - 0.08 0.083 0.n 0.08 Sb 0.2 0.2 0.26 0.5 0.n 0.2 As 2 2.4 1.5 1.7 n 1.8 Pb 10 15 20 16 20 12.5 Zn 50 72 60 83 40 70 Cu 10 35 20 47 100 55 Mo 1 0.9 1 1.1 n 1.5 W 2 1 1.5 1.3 50 1.5 Sn 2 - 3 2.5 n 2 Bi 0.1 0.01 0.01 0.009 0.0n 0.17

Note: 1—accepted in the work of Cb; 2–6—average content: 2–4—by [99], 5—by [100] and 6—by [101]. Dash—no data available, n = 1–9.

4. Results and Discussion From 1981 to the present time, within the framework of the IGC SB RAS scientific research in close cooperation with industrial organizations of the Magadan region (north–east of Russia), complex scientific research has been conducted. A significant amount of informative material from the areal geochemical surveys of LSSs on the 1:200,000 and 1:50,000 scales has been summarized. The exogenous AGF composition and structure identified by LSSs have been studied. They have been compared with the composition of primary ores and endogenous geochemical fields. The main factors that affect the LSS formation in cryolithogenesis zone conditions have been revealed. The sampling technique of the watercourse heads, where there are no alluvial deposits in the territories of subarctic landscapes, has been developed. Additional mineralogical and geochemical criteria for the prediction, prospecting, and evaluation of ore mineralization for such territories have been proposed.

4.1. Lithochemical Stream Sediments in the AGF Study of the Dukat Gold–Silver Ore-Forming System and Associated Areas—Regional Forecast The exogenous AGF study of the Dukat Au–Ag OFS identiﬁed by LSSs was based on regional survey data on the 1:200,000 scale, executed in the area within the two state sheets—the lower sheet part is P-56-XII and the upper sheet part is P-56-XVIII. This area covers the central part of the Balygychan–Sugoy trough, including the Dukat Au–Ag OFS (see Figures1 and2). The number of samples taken was more than 2500. See Figure4a (Section 3.1) for the samples’ location. Exogenous AGFs of Au, Ag, As, Pb, Zn, Cu, Mo, W, Sn, and Bi were identified throughout the studied area. The main elements forming these fields are Ag, As, Pb, Zn, W, Sn and Bi. Weakly manifested Minerals 2019, 9, 789 11 of 40

Minerals 2019, 9, x FOR PEER REVIEW 11 of 40 are Cu, Mo and especially Au. With regard to such elements as Hg and Sb, when surveying LSSs on the 1:200,000manifested scale, are no anomaliesCu, Mo and were especially revealed. Au. Hg With and Sbregard are elements to such forelements which theas Hg contents and Sb, are when difficult to detectsurveying through LSSs theon traditionalthe 1:200,000 SSQA scale, method. no anomal Thisies is duewere to revealed. this method’s Hg and insufficient Sb are elements limit for for their detection—1which the ppmcontents for Hgare anddifficult 20 ppm to detect for Sb. through Only a fewthe pointstraditional with SSQA content method. of 1 ppm This (CR is= due200) to Hg this and 20method’s ppm (CR insufficient= 100) Sb were limit observed. for their detection—1 Based on such ppm data, for it Hg is impossibleand 20 ppm to for talk Sb. about Only any a few regularities points in thewith distribution content of 1 of ppm these (CR elements = 200) Hg in and LSSs. 20 ppm (CR = 100) Sb were observed. Based on such data, it is impossible to talk about any regularities in the distribution of these elements in LSSs. Directly within the Dukat OFS, high-contrast exogenous AGFs form As (up to 2500/1250*), Directly within the Dukat OFS, high-contrast exogenous AGFs form As (up to 2500/1250*), Pb Pb (up to 3000/300), Ag (up to 30/300) and Bi (up to 25/250). Medium-contrast AGFs are characteristic (up to 3000/300), Ag (up to 30/300) and Bi (up to 25/250). Medium-contrast AGFs are characteristic of of Sn (up to 100/50) and Zn (up to 1500/30). Cu (up to 100/10), Mo (up to 10/10) and W (up to 20/10) are Sn (up to 100/50) and Zn (up to 1500/30). Cu (up to 100/10), Mo (up to 10/10) and W (up to 20/10) are manifested as low-contrast fields. When conducting surveys on the 1:200,000 scale, the anomalies of manifested as low-contrast fields. When conducting surveys on the 1:200,000 scale, the anomalies of suchsuch a maina main element element for for Au–Ag Au–Ag mineralization mineralization asas AuAu (up to 0.03/6) 0.03/6) are are manifested manifested very very poorly. poorly. *Here*Here and and later later in in the the text, text, in in the the numerator, numerator, the the average average element element contents contents (ppm)(ppm) areare givengiven andand in thein denominator the denominator the averagethe average values values of their of their contrast contrast ratios ratios (see (see Section Subsection 3.3). 3.3). GoldGold in in alluvial alluvial sediments sediments ofof LSSsLSSs isis practicallypractically not manifested manifested in in the the considered considered territory territory (Figure(Figure5). 5). Sporadic Sporadic AGFs AGFs of of low low contrast contrast (0.01–0.03 (0.01–0.03 ppm, ppm, CRCR = 2–6)2–6) are are observed. observed. Attention Attention should should bebe paid paid to to the the fact fact that that the the Au Au anomaly anomaly ofof suchsuch aa large Au–Ag deposit deposit as as the the Dukat Dukat deposit deposit is isnot not especiallyespecially di ffdifferenterent from from those those that that form form unprofitable unprofitable Au–Ag Au–Ag prospects prospects and and objects objects that that are are in in no no way relatedway related to this typeto this of type mineralization. of mineralization.

FigureFigure 5. 5.The The Dukat Dukat Au–Ag Au–Ag OFS OFS and and associatedassociated areas. Exogenous Exogenous Au Au an anomalousomalous geochemical geochemical fields ﬁelds (AGFs)(AGFs) identiﬁed identified through through LSSs LSSs (survey (survey onon thethe 1:200,0001:200,000 scale) scale).. For For the the legend legend to to Figures Figures 5–145–14 see see FigureFigure2 (Section 2 (Subsection 2.2). 2.2).

SilverSilveris is the the most most widely widely manifested manifested elementelement inin thethe entire studied studied area area (Figure (Figure 6).6). This This supports supports thethe unique unique metallogenic metallogenic specialization specialization of of the the considered considered territory territory as as well well as as all all of theof the OCVB OCVB outer outer zone forzone silver. for Au–Ag, silver. Au–Ag, Ag–Pb Ag–Pb and Sn–Ag and Sn–Ag ore objects ore objects located located within within the boundaries the boundaries of the of Dukat the Dukat Au–Ag OFSAu–Ag and beyond OFS and this beyond distinctly this fixdistinctly AGFs offix high AGFs (10–30 of high ppm, (10–30 CR =ppm,100–300) CR = and100–300) medium and (1–10medium ppm, CR(1–10= 10–100) ppm, CR Ag = contrast. 10–100) Ag The contrast. maximum The contentmaximum of thiscontent element of this in element the exogenous in the exogenous AGFs identified AGFs throughidentified LSSs through is 5–30 LSSs ppm is (CR 5–30= ppm50–300). (CR = These50–300). fields These are fields related are torelated the largest to the largest of the silverof the silver Au–Ag reservesAu–Ag in reserves the Dukat in deposit.the Dukat Anomalies deposit. Anomalie with contents with of content 1–5 ppm of (CR1–5 =ppm10–50) (CR were = 10–50) identified were in a circumferentialidentified in a circumferential direction of this direction deposit of andthis indeposit the areas and in of the Au–Ag areas (Zharok),of Au–Ag Ag–Pb(Zharok), (Fakel) Ag–Pb and Sn–Ag(Fakel) (Shorokh and Sn–Ag and Finalny)(Shorokh prospects and Finalny) as well prospects as the Ag–Pb as well deposits as the (MechtaAg–Pb deposits and Tidid). (Mechta Outside and the DukatTidid). OFS, Outside such fieldsthe Dukat fix a OFS, number such of fields Au–Ag fix anda number Sn–Ag of prospects.Au–Ag and They Sn–Ag are prospects. partly manifested They are in partly manifested in the development area of the Sn ore objects. Low-contrast (0.3–1 ppm, CR = 3–10) the development area of the Sn ore objects. Low-contrast (0.3–1 ppm, CR = 3–10) Ag AGFs have the Ag AGFs have the widest development. widest development.

Minerals 2019, 9, 789 12 of 40 Minerals 2019, 9, x FOR PEER REVIEW 12 of 40 Minerals 2019, 9, x FOR PEER REVIEW 12 of 40

FigureFigure 6. 6.The The Dukat Dukat Au–Ag Au–Ag OFS OFS and and associatedassociated areas. Exogenous Ag Ag AG AGFsFs identified identified through through LSSs LSSs Figure 6. The Dukat Au–Ag OFS and associated areas. Exogenous Ag AGFs identified through LSSs (survey(survey on on the the 1:200,000 1:200,000 scale). scale). (survey on the 1:200,000 scale). ArsenicArsenic, as, as well well as Ag,as Ag, is widely is widely manifested manifested over over the entire the entire area (Figure area (Figure7), although 7), although its detection its Arsenic, as well as Ag, is widely manifested over the entire area (Figure 7), although its limitdetection using limit the SSQA using methodthe SSQA is method no better is no than better 30 than ppm. 30 Directlyppm. Directly within within the Dukatthe Dukat Au–Ag Au–Ag OFS, detection limit using the SSQA method is no better than 30 ppm. Directly within the Dukat Au–Ag theOFS, maximum the maximum As concentrations As concentrations reach 200–2500 reach ppm,200–2500 and ppm, the contrast and the is 100–1250.contrast is These 100–1250. high-contrast These OFS, the maximum As concentrations reach 200–2500 ppm, and the contrast is 100–1250. These fieldshigh-contrast are confined fields to the are Mechta confined and Tididto the Ag–Pb Mechta deposits. and AGFsTidid withAg–Pb a concentration deposits. AGFs of 100–200 with ppma high-contrast fields are confined to the Mechta and Tidid Ag–Pb deposits. AGFs with a andconcentration CR = 50–100 of are 100–200 fixed by ppm the LSSsand CR ofthe = 50–100 Maly Ken are andfixed Novo-Dzhagyn by the LSSs of Sn–Ag the Maly deposits Ken and and the concentration of 100–200 ppm and CR = 50–100 are fixed by the LSSs of the Maly Ken and FinalnyNovo-Dzhagyn Sn–Ag prospect Sn–Ag asdeposits well as and in the the areas Finalny in whichSn–Ag the prospect ore objects as well were as in not the identified. areas in which AGFs the with Novo-Dzhagyn Sn–Ag deposits and the Finalny Sn–Ag prospect as well as in the areas in which the ore objects were not identified. AGFs with lower contents (30–100 ppm) and lower contrast (CR = lowerore objects contents were (30–100 not identified. ppm) and AGFs lower with contrast lower contents (CR = 15–50)(30–100 areppm) developed and lower more contrast significantly. (CR = 15–50) are developed more significantly. Specifically, for the Dukat Au–Ag deposit, the As AGFs are Specifically,15–50) are developed for the Dukat more Au–Agsignificantly. deposit, Specifical the Asly,AGFs for the are Dukat typical, Au–Ag with deposit, contents the As not AGFs exceeding are typical, with contents not exceeding 50–100 ppm and CR = 25–50. The same fields are characteristic 50–100typical, ppm with and contents CR = 25–50.not exceeding The same 50–100 fields ppm are an characteristicd CR = 25–50. ofThe a numbersame fields of Au–Ag, are characteristic Ag–Pb and of a number of Au–Ag, Ag–Pb and Sn–Ag prospects. The central part of the Dukat OFS, as well as Sn–Agof a number prospects. of Au–Ag, The central Ag–Pb part and ofSn–Ag the Dukat prospects. OFS, The as wellcentral as part some of Ag–Pbthe Dukat (Struya OFS, andas well Yasnaya as some Ag–Pb (Struya and Yasnaya Polyana) and Sn–Ag (Shorokh) prospects, is distinguished by Polyana)some Ag–Pb and Sn–Ag (Struya (Shorokh) and Yasnaya prospects, Polyana) is distinguishedand Sn–Ag (Shorokh) by fields prospects, with the lowestis distinguished contents ofby this fields with the lowest contents of this element (<30 ppm, CR < 15). elementfields with (<30 the ppm, lowest CR contents< 15). of this element (<30 ppm, CR < 15).

Figure 7. The Dukat Au–Ag OFS and associated areas. Exogenous As AGFs identified through LSSs FigureFigure 7. 7.The The Dukat Dukat Au–Ag Au–Ag OFSOFS andand associatedassociated areas. areas. Exogenous Exogenous As As AG AGFsFs identified identiﬁed through through LSSs LSSs (survey on the 1:200,000 scale). (survey(survey on on the the 1:200,000 1:200,000 scale). scale).

Minerals 2019, 9, 789 13 of 40

To the east of the Dukat OFS, the development area of the As AGFs increases significantly. Here, the As fields are related mainly to Sn, Sn–W and Mo–W and rarely to Sn–Ag objects. The contents of this element vary in the range of 30–500 ppm, and the contrast is 15–250. Moreover, AGFs with the maximum content of As (100–500 ppm) and contrast (CR = 50–250) are most often associated with development areas of prospects but with minimal content (30–100 ppm) and contrast (CR = 15–50) with deposits. In general, it can be seen that the As fields’ contrast in LSSs of deposits, prospects, and “ore-free” areas varies widely. The main factors affecting the As distribution, most probably, are the morphology of ore bodies and differences in their mineral composition. For example, exogenous AGFs of Au–Ag deposits and prospects, especially of such a giant as the Dukat Au–Ag deposit, are characterized by relatively low As contents. Ores belong to the vein type. Arsenic minerals are represented in them in a small quantity, mostly by sulphoarsenides (proustite and tennantite). These minerals are extremely unstable in hypergene environments. Entering watercourses, they are destroyed, part of the As transits into mobile water-soluble forms and is transported relatively short distances and the high-contrast fields are not formed. The highest-contrast As anomalies are related to Ag–Pb, Sn–Ag and Sn deposits and prospects, in which one of the main minerals of As is arsenopyrite. Here, the area’s arsenopyritization processes are widely manifested, related to the ore as well as the “ore-free” areas. In hypergene environments, the arsenopyrite is also unstable. It is fully oxidized and converted into mineral pitticite, soluble with difficulty and inactive in an aqueous medium, the development area of which in LSSs is comparable with the size of the areal arsenopyritization. Thus, high-contrast As anomalies are not related to Au–Ag deposits not only in the Dukat OFS but also beyond it. In addition, a significant part of such anomalies has no relation to profitable ore mineralization at all. Lead in the studied area is most manifested in the LSSs of the Dukat OFS and in the south-east near its borders (Figure8). High-contrast AGFs with concentrations of 1000–3000 ppm (CR = 100–300) are related to the Tidid Ag–Pb deposit and the Dukat Au–Ag deposit. These fields were also identified in the Dukat flanks and in the Au–Ag prospects. Fields of less contrast (250–1000 ppm, CR = 25–100) are confined to such prospects as Au–Ag Zharok, Ag–Pb Fakel, Sn–Ag Finalny, Shorokh, and Severny. Outside the Dukat OFS, such contrast AGFs are almost not manifested. AGFs with Pb 100–250 ppm (CR = 10–25) are fixed by other ore objects, including the Mechta Ag–Pb deposit and the Nachalny and Yasnaya Polyana Ag–Pb prospects, Krasin and Barguzin Au–Ag prospects, Razny Sn–Ag prospect and, much less so, Sn objects. The low-contrast AGFs (30–100 ppm, CR = 3–10) are developed much more widely than the others. Within the Dukat OFS, they are mainly related to Sn–Ag deposits and prospects, beyond the Dukat OFS, with Sn mineralization. The main part of low-contrast Pb anomalies is associated with unprofitable ZDSM, which is manifested throughout the area. Attention should be paid to the fact that the contrast of the fields related to some Au–Ag, Ag–Pb and even Sn–Ag prospects is higher than in the Mechta Ag–Pb deposit. This situation can be caused by a number of factors, such as the branching of the drainage system, the ore bodies’ morphology, their position in relation to the draining streams and the sod cover of the watercourse heads (a barrier to the material transfer). In addition, the low contents, to some extent, reflect the level of the ore zones’ erosion profile. For example, the upper ore intervals of Ag–Pb mineralization are not characteristic of high Pb content. In general, it can be seen that fields with Pb contents greater than 100 ppm (CR > 10) are confined to Au–Ag and Ag–Pb and rarely occur with Sn–Ag deposits and prospects. The relationship of contrast (CR = 10–100) and high-contrast (CR = 100–300) Pb AGFs with different ore objects is due to the fact that, in the series of vertical endogenous geochemical zonality, Ag–Pb mineralization takes an intermediate place between Au–Ag and Sn–Ag mineralization [56]. During the erosion of low ore horizons of Au–Ag ore objects and upper ore horizons of Sn–Ag ore objects by watercourses, the Ag–Pb ores will inevitably be affected. Zinc is widely manifested throughout the study area (Figure9). Medium-contrast fields with contents of 500–1500 ppm (CR = 10–30) are developed, mainly in the southern part of the Dukat OFS Minerals 2019, 9, 789 14 of 40

Minerals 2019, 9, x FOR PEER REVIEW 14 of 40 Minerals 2019, 9, x FOR PEER REVIEW 14 of 40 and along the periphery to the north–east and south-east of it. Within the Dukat OFS, such fields tend tendto be to attracted be attracted to Au–Ag to Au–Ag (Dukat, (Dukat, Barguzin Barguzin and Zharok)and Zharok) and Ag–Pband Ag–Pb (Tidid (Tidid and Fakel)and Fakel) deposits deposits and prospects.and prospects. Outside Outside the system, the system, exogenous exogenous Zn AGFs Zn AG areFs related are related to a number to a number of Au–Ag, of Au–Ag, Sn–Ag, Sn–Ag, and Sn prospects.and Sn prospects. Low-contrast Low-contrast fields with fields concentrations with concentrations of 150–500 of ppm150–500 and CRppm= and3–10 CR have = maximum3–10 have prevalence.maximum prevalence. They are identified They are bothidentified within both the with Dukatin the OFS Dukat and beyond OFS and it. beyond Such AGFs it. Such are relatedAGFs are to relateddifferent to ore different objects. ore Most objects. parts Most of the parts low-contrast of the low-contrast Zn anomalies, Zn anomalies, as well as Pb,as well have as no Pb, relation have no to relationdeposits to and deposits prospects and but prospects are associated but are withassociated unprofitable with unprofitable ZDSM. ZDSM.

Figure 8. The Dukat Au–Ag OFS and associated areas. Exogenous Pb AGFs identified through LSSs Figure 8. The Dukat Au–Ag OFS and associatedassociated areas. Exogenous Pb AGFs identiﬁedidentified through LSSs (survey on the 1:200,000 scale). (survey onon thethe 1:200,0001:200,000 scale).scale).

Figure 9. The Dukat Au–Ag OFS and associated areas. Exogenous Zn AGFs identiﬁed through LSSs Figure 9. The Dukat Au–Ag OFS and associated areas. Exogenous Zn AGFs identified through LSSs (surveyFigure 9. on The the Dukat 1:200,000 Au–Ag scale). OFS and associated areas. Exogenous Zn AGFs identified through LSSs (survey on the 1:200,000 scale). (survey on the 1:200,000 scale). Copper is manifested in the form of low-contrast AGFs throughout the territory (30–100 ppm, Copper is manifested in the form of low-contrast AGFs throughout the territory (30–100 ppm, CR =Copper3–10), and is manifested medium-contrast in the form AGFs of are low-contrast rarer (100–200 AGFs ppm, throughoutCR = 10–20 the) (Figureterritory 10 (30–100). Their ppm, main CR = 3–10), and medium-contrast AGFs are rarer (100–200 ppm, CR = 10–20) (Figure 10). Their main partCR = is3–10), concentrated and medium-contrast in the east of AGFs the Dukat are rarer OFS (100–200 and is conﬁnedppm, CR to= 10–20) the development (Figure 10). areasTheir ofmain Sn, part is concentrated in the east of the Dukat OFS and is confined to the development areas of Sn, Sn–Wpart is andconcentrated Mo–W mineralization. in the east of Withinthe Dukat the DukatOFS and OFS, is anomaliesconfined to with the Cudevelopment content of 50–100areas of ppm Sn, Sn–W and Mo–W mineralization. Within the Dukat OFS, anomalies with Cu content of 50–100 ppm (CR = 5–10) are attracted to its eastern edge. They are related to the Dukat Au–Ag deposit, to its flanks and to the Finalny Sn–Ag prospect. Lower-contrast fields (30–50 ppm, CR = 3–5) are

Minerals 2019, 9, 789 15 of 40

(CRMinerals= 5–10 2019) are, 9, x attracted FOR PEER toREVIEW its eastern edge. They are related to the Dukat Au–Ag deposit, to15 its of flanks 40 andMinerals to the 2019 Finalny, 9, x FOR Sn–Ag PEER REVIEW prospect. Lower-contrast fields (30–50 ppm, CR = 3–5) are developed15 of much40 developed much more widely. They are confined to the Au–Ag (Zharok prospect), Ag–Pb (Fakel more widely. They are confined to the Au–Ag (Zharok prospect), Ag–Pb (Fakel prospect), and Sn–Ag developedprospect), muchand Sn–Ag more (Malywidely. Ken They and are Novo-Dzhagyn confined to the deposits Au–Ag and (Zharok Razny prospect), and Severny Ag–Pb prospects) (Fakel (Maly Ken and Novo-Dzhagyn deposits and Razny and Severny prospects) objects. prospect),objects. and Sn–Ag (Maly Ken and Novo-Dzhagyn deposits and Razny and Severny prospects) objects.

FigureFigure 10. 10.The The Dukat Dukat Au–Ag Au–Ag OFS OFS andand associatedassociated areas. Exogenous Exogenous Cu Cu AG AGFsFs identified identiﬁed through through LSSs LSSs Figure 10. The Dukat Au–Ag OFS and associated areas. Exogenous Cu AGFs identified through LSSs (survey(survey on on the the 1:200,000 1:200,000 scale). scale). (survey on the 1:200,000 scale). MolybdenumMolybdenumin in the the consideredconsidered area area forms forms rare rare local local fields fields of medium of medium contrast contrast (10–30 ( 10–30ppm, CR ppm , CR== 10–30)10–30)Molybdenum andand wide wide in low-contrast low-contrastthe considered AGFs AGFs area (3–10 forms (3–10 ppm, rare ppm, localCR CR = fields =3–10)3–10) of (Figure medium (Figure 11). 11contrast ).Anomalies Anomalies (10–30 of ppm, ofmedium medium CR = 10–30) and wide low-contrast AGFs (3–10 ppm, CR = 3–10) (Figure 11). Anomalies of medium contrastcontrast are are manifested manifested only only outside outside thethe DukatDukat OFS and and attract attract to to Sn, Sn, Sn–W Sn–W and and Mo–W Mo–W objects. objects. contrast are manifested only outside the Dukat OFS and attract to Sn, Sn–W and Mo–W objects. Low-contrastLow-contrast AGFs AGFs are are identified identified both both withinwithin thethe Dukat OFS and and outside outside it itin in the the eastern eastern part part of ofthe the Low-contrastarea. Such fields AGFs attract are identified objects of both all known within types the Dukat of mineralization. OFS and outside it in the eastern part of the area. Such fields attract objects of all known types of mineralization. area. Such fields attract objects of all known types of mineralization.

Figure 11. The Dukat Au–Ag OFS and associated areas. Exogenous Mo AGFs identified through FigureFigureLSSs 11.(survey 11.The The Dukaton Dukat the Au–Ag1:200,000 Au–Ag OFS scale).OFS and and associated associated areas. areas. Exogenous Exogenous Mo Mo AGFs AGFs identiﬁed identified through through LSSs (surveyLSSs (survey on the on 1:200,000 the 1:200,000 scale). scale). Tungsten in LSSs is spread mainly outside the Dukat OFS (Figure 12). The most significant fieldsTungstenTungsten (10–200in inppm, LSSs LSSs CR isis = spreadspread 5–100) mainlymainlyof W anomalous outside outside the the concentrations Dukat Dukat OFS OFS (Figure in (Figure terms 12). 12of ).ThesizeThe mostand most contrastsignificant significant are fieldsfieldsattracted (10–200 (10–200 to the ppm, ppm, area CRCR located == 5–100) to the of of east W W anomalousof anomalous the Dukat concentrations concentrationsAu–Ag OFS. Theyin interms termsare reoflated ofsize size hereand and contrastto numerous contrast are are attractedattractedSn, Sn–W to to the andthe area areaMo–W located located deposits to to the the and east east prospects. of of the the Dukat Du katThe Au–Ag Au–Ag Dukat OFS.OFS.OFS TheyisThey characterized areare relatedrelated by herehere low-contrast to numerous W Sn, Sn,AGFs. Sn–W These and elementMo–W deposits concentrations and prospects. do not Theexceed Dukat 10 ppmOFS is(CR<5) characterized and were by identifiedlow-contrast in theW AGFs. These element concentrations do not exceed 10 ppm (CR<5) and were identified in the

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Sn–WMinerals and 2019 Mo–W, 9, x FOR deposits PEER REVIEW andprospects. The Dukat OFS is characterized by low-contrast16 W of AGFs. 40 These element concentrations do not exceed 10 ppm (CR<5) and were identified in the watercourse watercourse alluvium draining different ore objects—Au–Ag (Dukat and Zharok), Ag–Pb (Mechta) alluvium draining different ore objects—Au–Ag (Dukat and Zharok), Ag–Pb (Mechta) and Sn–Ag (Maly and Sn–Ag (Maly Ken, Novo-Dzhagyn and Razny). W anomalies’ contrast increases slightly (10–20 Ken, Novo-Dzhagyn and Razny). W anomalies’ contrast increases slightly (10–20 ppm, CR = 5–10) only ppm, CR = 5–10) only towards the north–eastern border of the Dukat OFS, where the Finalny Sn–Ag towards the north–eastern border of the Dukat OFS, where the Finalny Sn–Ag prospect is located. prospect is located.

FigureFigure 12. 12.The The Dukat Dukat Au–Ag Au–Ag OFS OFSand and associatedassociated areas. Exogenous Exogenous W W AG AGFsFs identified identiﬁed through through LSSs LSSs (survey(survey on on the the 1:200,000 1:200,000 scale). scale).

TinTinin in the the considered considered territory territory is is characterizedcharacterized by quite quite wide wide development development of of AGFs AGFs (Figure (Figure 13). 13 ). TheThe exogenous exogenous Sn Sn AGFs AGFs with with the the mostmost contrastcontrast (50–200 ppm, ppm, CR CR == 25–100),25–100), as aswell well as asthose those for forW, W, are observed outside the Dukat OFS. Such anomalies clearly fix Sn–Ag, Sn, Sn–W and Mo–W ore are observed outside the Dukat OFS. Such anomalies clearly fix Sn–Ag, Sn, Sn–W and Mo–W ore objects. Here, lower-contrast anomalies (<50 ppm, CR < 25) are the most developed. Within the objects. Here, lower-contrast anomalies (<50 ppm, CR < 25) are the most developed. Within the Dukat Dukat Au–Ag OFS, the Sn contents reach 50–100 ppm (CR = 25–50) only in single anomalies. Such Au–Ag OFS, the Sn contents reach 50–100 ppm (CR = 25–50) only in single anomalies. Such anomalous anomalous concentration fields are confined to the Severny Sn–Ag prospect and to the eastern flank concentration fields are confined to the Severny Sn–Ag prospect and to the eastern flank of the Dukat of the Dukat Au–Ag deposit. Fields of less contrast (20–50 ppm, CR = 10–25) are revealed by the Au–Agwatercourse deposit. LSSs Fields draining of less mainly contrast Sn–Ag (20–50 objects ppm, (Maly CR = Ken10–25) and are Novo-Dzhagyn revealed by thedeposits watercourse and LSSsRazny, draining Finalny mainly and Sn–AgShorokh objects prospects). (Maly To Ken a andlesse Novo-Dzhagynr extent, such fields deposits are manifested and Razny, in Finalny Au–Ag and Shorokh(Dukat) prospects). and Ag–Pb To (Fakel) a lesser deposits extent, suchand prospects fields are manifestedand on the inflanks Au–Ag of the (Dukat) Dukat, and where Ag–Pb Sn–Ag (Fakel) depositsmineralization and prospects was also and observed. on the flanks In general, of the fo Dukat,r exogenous where Sn Sn–Ag AGFs mineralization within the Dukat was Au–Ag also observed. OFS, In general,the low contrast for exogenous (6–20 ppm, Sn AGFs CR = within 3–10) is the characteristic. Dukat Au–Ag OFS, the low contrast (6–20 ppm, CR = 3–10) is characteristic.Bismuth forms exogenous AGFs (Figure 14) that are significant in size and contrast. Low-contrastBismuth forms Bi fields exogenous (CR < 10) AGFs are (Figurenot considered 14) that arehere significant due to the in poor size andBi detection contrast. limit Low-contrast of the BiSSQA fields method (CR < 10) (1 areppm). not The considered highest-contrast here due Bi toAGFs the poor(5–25 Bippm, detection CR = 50–250) limit of in the the SSQA area of method the (1Dukat ppm). Au–Ag The highest-contrast OFS are located Binear AGFs the north–eastern (5–25 ppm, CRborder.= 50–250) They are in confined the area mainly of the to Dukat the Sn–Ag Au–Ag OFSobjects—the are located Novo-Dzhagyn near the north–eastern deposit and border. the Razny They and are Finalny confined prospects. mainly In to addition, the Sn–Ag fields objects—the of such Novo-Dzhagyncontrast were depositidentified and in thesites Razny where and ore Finalnyobjects are prospects. not known. In addition, Sn–Ag objects fields of suchthe Dukat contrast OFS’s were identifiedwestern inpart sites (Maly where Ken ore deposit objects and are Tovarishch, not known. Severny Sn–Ag and objects Shorokh of theprospects) Dukat OFS’sare characterized western part (Malyby significantly Ken deposit lower-contrast and Tovarishch, Bi Severnyfields (<2.5 and ppm, Shorokh CR < prospects)25). The same are characterizedfields are related by significantlyto Au–Ag lower-contrastand Ag–Pb objects. Bi fields The (< only2.5 ppm, exception CR < is25). the The Mechta same Ag–Pb fields deposit, are related in which to Au–Ag the Bi and concentrations Ag–Pb objects. Theinonly LSSs exception reach 10 ppm is the (CR Mechta = 100). Ag–Pb Outside deposit, the Dukat in which OFS, thethe BiBi concentrationsfields of anomalous in LSSs concentrations reach 10 ppm are widely developed and confined to Sn, Sn–W, Mo–W and (in the south) Sn–Ag objects. The (CR = 100). Outside the Dukat OFS, the Bi fields of anomalous concentrations are widely developed maximum anomalous fields with the content of 5–25 ppm (CR = 50–250) are more prevalent here and confined to Sn, Sn–W, Mo–W and (in the south) Sn–Ag objects. The maximum anomalous fields than within the Dukat OFS. with the content of 5–25 ppm (CR = 50–250) are more prevalent here than within the Dukat OFS.

Minerals 2019, 9, 789 17 of 40 Minerals 2019, 9, x FOR PEER REVIEW 17 of 40 Minerals 2019, 9, x FOR PEER REVIEW 17 of 40

Figure 13. The Dukat Au–Ag OFS and associated areas. Exogenous Sn AGFs identified through LSSs FigureFigure 13. 13.The The Dukat Dukat Au–Ag Au–Ag OFS OFS andand associatedassociated areas. Exogenous Exogenous Sn Sn AGFs AGFs identified identiﬁed through through LSSs LSSs (survey on the 1:200,000 scale). (survey(survey on on the the 1:200,000 1:200,000 scale). scale).

Figure 14. The Dukat Au–Ag OFS and associated areas. Exogenous Bi AGFs identified through LSSs Figure 14. The Dukat Au–Ag OFS and associated areas. Exogenous Bi AGFs identified through LSSs Figure(survey 14. onThe the Dukat 1:200,000 Au–Ag scale). OFS and associated areas. Exogenous Bi AGFs identified through LSSs (survey(survey on on the the 1:200,000 1:200,000 scale). scale). Attention should be paid to the fact that the Bi AGFs of similar ore object mineralization types Attention should be paid to the fact that the Bi AGFs of similar ore object mineralization types areAttention manifested should in some be paid cases to and the factnot thatin others. the Bi AGFsFor example, of similar for ore the object Mechta mineralization and Tidid Ag–Pb types are are manifested in some cases and not in others. For example, for the Mechta and Tidid Ag–Pb manifesteddeposits, inthis some can casesbe explained and not in by others. the level For example,of their forerosion the Mechta profile and (see Tidid Figure Ag–Pb 14). deposits,High deposits, this can be explained by the level of their erosion profile (see Figure 14). High thisconcentrations can be explained of Bi in by the the AGFs level of of the their Mechta erosion deposit profile testify (see that Figure watercourses 14). High drain concentrations the lower ore of Bi concentrations of Bi in the AGFs of the Mechta deposit testify that watercourses drain the lower ore inintervals, the AGFs which of the are Mechta characterized deposit by testify high that Bi content watercourses in ores. drainWeakly the manifested lower ore Bi intervals, anomalies which or the are intervals, which are characterized by high Bi content in ores. Weakly manifested Bi anomalies or the characterizedlack of them by at highthe Tidid Bi content deposit in indicate ores. Weakly that the manifested watercourses Bi anomalies in this area or drain the lack the of upper them horizons, at the Tidid lack of them at the Tidid deposit indicate that the watercourses in this area drain the upper horizons, depositwhich indicate are characterized that the watercourses by low contents in this areaof Bi drain in the the ores. upper This horizons, corresponds which to are the characterized previously by which are characterized by low contents of Bi in the ores. This corresponds to the previously lowidentified contents endogenous of Bi in the geochemical ores. This corresponds zonality of these to the objects previously [56]. identified endogenous geochemical identified endogenous geochemical zonality of these objects [56]. zonalityWhen of these identifying objects [the56 ].typomorphic composition of exogenous AGFs (element associations) and When identifying the typomorphic composition of exogenous AGFs (element associations) and theWhen patterns identifying of their distribution the typomorphic in space composition (zonality), ofmultiple exogenous correlation AGFs (elementis effective. associations) Graphically, and the patterns of their distribution in space (zonality), multiple correlation is effective. Graphically, this correlation can be seen on a polyelement geochemical map plotted using the multidimensional thethis patterns correlation of their can distribution be seen on a in polyelement space (zonality), geochemical multiple map correlation plotted using is eff ective.the multidimensional Graphically, this field method (Figure 15, Table 2). correlationfield method can (Figure be seen 15, on Table a polyelement 2). geochemical map plotted using the multidimensional field method (Figure 15, Table2).

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Minerals 2019, 9, x FOR PEER REVIEW 18 of 40

FigureFigure 15. TheThe Dukat Dukat Au–Ag OFS and associatedassociated areas. Exogenous Exogenous po polyelementlyelement AGFs AGFs identified identiﬁed throughthrough LSSs LSSs (survey (survey on on the 1:200,0001:200,000 scale). 1–14—geochemical 1–14—geochemical associ associationsations of of ore ore elements elements (see (see TableTable 22):): 11 andand 2—predominantly2—predominantly Ag;Ag; 3–5—Ag–Pb;3–5—Ag–Pb; 6 and 7—Sn–Ag; 8 and 9—Mo–W and Sn–W; 10–12—Sn;10–12—Sn; and 13 andand 14—Pb–Zn14—Pb–Zn inin ZDSM. ZDSM. For For the the legend legend to to Figure Figures 15 see15 see Figure Figure2 (Section 2 (Subsection 2.2). 2.2). Table 2. The Dukat Au–Ag OFS and associated areas. Geochemical associations of ore elements (survey Tableof LSSs 2. on The the Dukat 1:200,000 Au–Ag scale). OFS and associated areas. Geochemical associations of ore elements (surveyNo. of LSSs on the 1:200,000 scale). Associations of Ore Elements No. AssociationsPredominantly of Ore Ag Elements 1 Ag(20/200) Pb(590/59) As(30Predominantly/15) Bi(1.4/14) Zn(650Ag /13) Cu(60/6) Sn(12/6) Au(0.01/2) 21 Ag(20/200) Ag(8.6 Pb(590/59)/86) Pb(260 As(30/15)/26) As(40 Bi(1.4/14)/20) Bi(1.1 Zn(650/13)/11) Zn(350 Cu(60/6)/7) Sn(10 Sn(12/6)/5) Cu(40 Au(0.01/2)/4) 2 Ag(8.6/86) Pb(260/26) As(40/20) Bi(1.1/11) Zn(350/7) Sn(10/5) Cu(40/4) Ag–Pb Ag–Pb 33 Pb(850/85) Pb(850/85) As(110/55) As(110/55) Ag(3.0/30) Ag(3.0/30) Zn(800/16) Zn(800/16) Bi(1.5/15) Bi(1.5/15) Sn(14/7) Sn(14/7) Cu(50/5) Cu(50/5) 44 As(150/75) As(150/75) Pb(450/45) Pb(450/45) Bi(3.5/35) Bi(3.5/35) Zn(600/12) Zn(600/12) Ag(1.0/10) Ag(1.0/10) Sn(18/9) Sn(18/9) Cu(40/4) Cu(40/4) 5 As(130/65) Bi(3.5/35) Pb(300/30) Zn(450/9) Ag(0.8/8) Sn(10/5) Cu(30/3) 5 As(130/65) Bi(3.5/35) Pb(300/30) Zn(450/9) Ag(0.8/8) Sn(10/5) Cu(30/3) Sn–AgSn–Ag 66 As(190/95) As(190/95) Bi(6.3/63) Bi(6.3/63) Sn(44/22) Sn(44/22) Pb(180/18) Pb(180/18) Ag(1.7/17) Ag(1.7/17) Zn(500/10) Zn(500/10) Cu(70/7) Cu(70/7) 77 As(110/55) As(110/55) Bi(3.3/33) Bi(3.3/33) Sn(20/10) Sn(20/10) Pb(100/10) Pb(100/10) Zn(400/8) Zn(400/8) Ag(0.6/6) Ag(0.6/6) Cu(40/4) Cu(40/4) Mo–W and Sn–W Mo–W and Sn–W 8 Bi(5.2/52) As(90/45) W(60/30) Mo(20/20) Sn(36/18) Pb(70/7) Zn(300/6) Cu(50/5) Ag(0.4/4) 89 Bi(5.2 Sn(100/50)/52) As(90 Bi(4.2/42)/45) W(60 As(80/40)/30) Mo(20 W(40/20)/20) Sn(36 Mo(8/8)/18) Pb(70 Pb(50/5)/7) Zn(300 Zn(200/4)/6) Cu(50 Cu(30/3)/5) Ag(0.4 /4) 9 Sn(100/50) Bi(4.2/42) As(80/40) W(40/20) Mo(8/8) Pb(50/5) Zn(200/4) Cu(30/3) Sn 10 Sn(130/65) Bi(5.9/59) As(90/45) W(16/8)Sn Zn(300/6) Cu(60/6) Mo(5/5) Pb(50/5) 1011 As(90/45) Sn(130/65) Bi(3.9/39) Bi(5.9/59) Sn(42/21) As(90/45) Zn(300/6) W(16/8) W(10/5) Zn(300/6) Mo(5/5) Cu(60/ 6)Cu(50/5) Mo(5/5) Pb(50/5) Pb(50/5) 1112 As(80/40) As(90/45) Bi(3.5/35) Bi(3.9/39) Sn(16/8) Sn(42/21) Zn(250/5) Zn(300/6) Pb(40/4) W(10/5) W(6/3) Mo(5/ 5)Mo(3/3) Cu(50 /Cu(30/3)5) Pb(50/ 5) 12 As(80/40) Bi(3.5/35) Sn(16Pb–Zn/8) Zn(250 in ZDSM/5) Pb(40 /4) W(6/3) Mo(3/3) Cu(30/3) 13 As(30/15) Bi(0.8/8) Sn(12/6)Pb–Zn Zn(250/5) in ZDSM Pb(40/4) Cu(30/3) Ag(0.3/3) 14 Pb(30/3) Zn(150/3) Cu(20/2) Ag(0.2/2) 13 As(30/15) Bi(0.8/8) Sn(12/6) Zn(250/5) Pb(40/4) Cu(30/3) Ag(0.3/3) Note:14 The Sb contents in all the element Pb(30/ 3)associ Zn(150ations/3) Cu(20 are below/2) Ag(0.2 the /2)detection limit of the SSQA method (<20 ppm). Note: The Sb contents in all the element associations are below the detection limit of the SSQA method (<20 ppm).

For exogenous AGFs of the Dukat Au–Ag OFSs and the ore objects related to it, the polymetallityFor exogenous and zonal AGFs construction of the Dukat are Au–Ag characteristic. OFSs and theIn the ore process objects relatedof the Dukat to it, the OFS’s polymetallity erosion, fromandzonal the central construction part to are the characteristic. periphery, Ag In thegeoc processhemical of associations the Dukat OFS’s are replaced erosion, fromby Ag–Pb the central then Sn–Ag.part to thePredominantly, periphery, Ag Ag geochemical element associations associations ha areve replacedlocal development by Ag–Pb thenand are Sn–Ag. manifested Predominantly, only at the Dukat Au–Ag deposit. The maximum contents here are typical of Ag and to a lesser extent of Pb and As. Such elements as Bi, Zn, Cu and Sn are weakly manifested. There is practically no Au, which

Minerals 2019, 9, 789 19 of 40

Ag element associations have local development and are manifested only at the Dukat Au–Ag deposit. The maximum contents here are typical of Ag and to a lesser extent of Pb and As. Such elements as Bi, Zn, Cu and Sn are weakly manifested. There is practically no Au, which does not allow the allocation of Au–Ag element associations that are characteristic of the deposit primary ores. The absence of Au and the strong appearance of Pb and Sn are not characteristic of typical epithermal Au–Ag mineralization. The unusually high Pb and Sn that occur can be explained by the fact that the sampled watercourses drain the middle-lower ore horizons of the deposit, where mainly Ag–Pb ores are manifested and Sn–Ag mineralization appears [56,102]. Ag–Pb and Sn–Ag associations are widely manifested throughout the studied area. Ag–Pb associations have an areal character of development. The highest concentrations of the main typomorphic elements (Ag, Pb and As) are confined to the flanks of the Dukat Au–Ag deposit and to the Ag–Pb objects on the periphery of the OFS with the same name. Such elements as Bi and Zn are manifested less, and even smaller amounts of Sn and Cu are evident. Sn–Ag associations are less developed, both on the periphery of the Dukat OFS and beyond it. Such associations are confined mainly to Sn–Ag objects. The highest contents are typical of As, Bi and Sn, relatively high for Pb and Ag and less high for Zn and Cu. Outside the Dukat Au–Ag OFS, at the most eroded site compared with the depression area with the same name, Mo–W, Sn–W and Sn associations are widely manifested. They are confined mainly to Mo–W, Sn–W and Sn ore objects. For Mo–W and Sn–W associations, the main ones are Bi, Sn, As, W, and Mo. Pb and Zn are manifested to a lesser extent and Cu and Ag even less. For Sn element associations, high contents are characteristic of Sn, Bi and As. W, Zn, Cu, Mo and Pb are weakly manifested. It should be noted here that Sn mineralization is quite often accompanied by rare metal mineralization [78]. In addition to the aforementioned associations related to ore mineralization, low-contrast element associations, mainly As, Bi, Ag, Zn, and Pb, related to ZDSM, are widespread throughout the area. Summarizing the results of the small-scale geochemical survey of LSSs on the 1:200,000 scale on the territory of the Dukat Au–Ag OFS and associated areas, it can be argued that such surveys are the main method for the regional forecasting of minerals in actively denudating mountain-folded subarctic regions in the north-east of Russia. They are the best way to solve the problem of projected evaluation of large-scale areas in a relatively short time and at a minimal cost. They are characterized by simplicity, relative depth and the possibility to obtain information operatively about the geochemical peculiarities and metallogeny of the region as a whole. Ore elements associations, their typomorphic composition and the regional exogenous geochemical zonality revealed by the multidimensional field method reflect primarily the geochemical peculiarities of the studied area and the ore mineralization distribution pattern here. As can be seen from the survey results of LSSs on the 1:200,000 scale, the metallogenic specialization in the studied area is clearly manifested only in Ag, Pb and Sn. The situation is ambiguous for Au AGFs, the main element indicator, which is of main interest for the Au–Ag mineralization type. Metallogenic specialization for this element is not manifested. Despite the positive aspects, surveys of LSSs on the 1:200,000 scale have significant drawbacks. The exogenous AGFs identified through them have a relatively poor component composition and low contrast. Not only quantitative but also qualitative relations are often violated. First of all, this situation is typical of anomalies related to Au–Ag deposits and prospects. Table3 shows the overall assessment of the quality and quantity relations in a conjugated system of endogenous AGFs–exogenous AGFs—ore + primary halo secondary halo stream sediments—for ore objects of the Dukat Au–Ag OFS. It can → → be seen clearly that the main typomorphic element composition is simplified and that alien elements, not peculiar to a particular type of mineralization, appear and the contrast reduces, especially that of Au–Ag AGFs. It is most probable that all these features are due to erosion by watercourses of deposits and prospects at various-level horizons, which in turn leads to mixing and concentrating of ore material of different types of mineralization in alluvial sediments. Minerals 2019, 9, 789 20 of 40

Table 3. General characteristics of qualitative and quantitative functional relations in a sequence of endogenous AGF exogenous AGF for ore objects of the Dukat Au–Ag OFS. → Minera- AGF Types Lization Endogenous Exogenous Exogenous Types (Ore + Primary Halo) (Secondary Halo) (Stream Sediments)

Ag, Ag Hg Ag Ag Au Sb As Pb (Zn, Cu, Bi) As Sb Au Pb Pb As (Bi, Zn, Sn, Cu, Au) Au–Ag 1000 910 400 300 100 74 300 150 100 20 10 200 59 15 Pb As Ag Zn Ag–Pb Ag Pb As Sb Hg Zn Ag Sb Pb As Hg Zn (Bi, Sn, Cu) 800 300 300 150 80 40 (Sn, Cu, Bi) 200 100 60 50 10 8 85 55 30 16 Sn–Ag As Sn Ag Sb Zn Bi Pb Cu As Ag Sn Zn Bi Pb Cu As Bi Sn Pb Ag Zn Cu 300 200 200 150 120 100 80 10 100 80 40 10 10 10 4 95 63 22 18 17 10 7 Note: The denominator below the element shows the CR average values in the maximum anomalous geochemical ﬁelds. The elements given in parentheses were not observed constantly. Bold indicates the main typomorphic elements for each type of mineralization. In the column for the stream sediments, Sb is absent (due to the insuﬃcient detection limit of the SSQA method).

All of the disadvantages listed above greatly complicate the study of the composition, structure and development scale of exogenous AGFs: their typification becomes difficult and, as a consequence, there is an unclear relationship with a specific type of mineralization as well as zonality identification—the main indicator used to assess the level of an area’s erosion profile. It is obvious that it is necessary to carry out a more detailed LSS study. As further research, performed in the area of the Dukat Au–Ag deposit and its flanks, has shown, the low contrast and poor component composition of the exogenous AGFs are related mainly to the watercourse sampling character. It is established that the most informative are the watercourse heads (streams of the first and second orders), which, in the surveying of LSSs on the 1:200,000 scale, are practically not sampled (see Section 3.1). In such situations, for the prospecting and evaluation of ore mineralization, primarily Au–Ag, in the conditions of cryolithogenesis zones, it is effective to perform surveys of LSSs on the 1:50,000 scale, in which all watercourses, including streams of the first-order, are sampled.

4.2. Lithochemical Stream Sediments in the AGF Study of the Dukat Gold–Silver Deposit—Prospecting and Evaluation The study of exogenous AGFs identified by LSSs for the purpose of the prospecting and evaluation of Au–Ag mineralization was carried out on the basis of geochemical survey data at the 1:50,000 scale. This area covers the southern part of the Dukat Au–Ag OFS, including the Au–Ag deposit of the same name, as well as the associated area (see Figure2, Section 2.2). The number of samples taken was more than 600. See Figure4b (Section 3.1) for the samples’ location. The exogenous AGFs of all the element indicators typical of ore mineralization in the Dukat Au–Ag OFS are distinctly manifested [56,102]. Gold is the most intensively manifested in LSSs directly related to the Dukat Au–Ag deposit (Figure 16). The contour line, combining the concentrations of 0.1–0.45 ppm (CR = 20–90), almost delineates the central part of the deposit (right tributaries of the streams Smely and Iskra) as having the richest content of this element. AGFs with a concentration of 0.02–0.1 ppm (CR = 4–20) are fixed on the flanks of the deposit (streams Smely, Levaya Brekchiya and Iskra). Outside the deposit, the content of this element usually does not exceed 0.02 ppm (CR 4). ≤ Silver at the considered site forms a very contrasting and quite large AGF development area (Figure 17). High-contrast Ag fields with contents of 10–100 ppm (CR = 100–1000) are clearly fixed within the unique Dukat Au–Ag deposit in the reserves of this metal as well as on its flanks. In addition, AGFs of such contrast are identified in the watershed of the Yagelny and Neponyatny streams. It should be noted that no ore objects have been identified on this watershed to date. The site may be promising in terms of silver mineralization detection. Exogenous fields of medium contrast with Ag content of 1–10 ppm (CR = 10–100) form smaller objects—the Nachalny and Fakel Ag–Pb prospects. Moving Minerals 2019, 9, x FOR PEER REVIEW 21 of 40

Minerals 2019, 9, 789 21 of 40 in the direction from the known ore objects, the contents in the LSSs gradually decrease and outside them usually do not exceed 1 ppm (CR < 10), which corresponds to low-contrast and near-background geochemical ﬁelds. Thus, higher levels of Ag concentrations in channel sediments of the area, as a rule, ﬁx ore mineralization and depend directly on the scale of its occurrence. Minerals 2019, 9, x FOR PEER REVIEW 21 of 40

Figure 16. The Dukat Au–Ag deposit and associated area. Exogenous Au AGFs identified through LSSs (survey on the 1:50,000 scale). For the legend to Figures 16–26 and 28 see Figure 2 (Subsection 2.2).

Silver at the considered site forms a very contrasting and quite large AGF development area (Figure 17). High-contrast Ag fields with contents of 10–100 ppm (CR = 100–1000) are clearly fixed within the unique Dukat Au–Ag deposit in the reserves of this metal as well as on its flanks. In addition, AGFs of such contrast are identified in the watershed of the Yagelny and Neponyatny streams. It should be noted that no ore objects have been identified on this watershed to date. The site may be promising in terms of silver mineralization detection. Exogenous fields of medium contrast with Ag content of 1–10 ppm (CR = 10–100) form smaller objects—the Nachalny and Fakel Ag–Pb prospects. Moving in the direction from the known ore objects, the contents in the LSSs gradually decrease and outside them usually do not exceed 1 ppm (CR < 10), which corresponds to low-contrastFigure 16. and The near-background Dukat Au–Ag deposit geochemical and associated fields. area.ar ea.Thus, Exogenous higher Au levels AGFs of identiﬁedidentifiedAg concentrations through in channelLSSs sediments (survey onon of thethe the 1:50,0001:50,000 area, scale).asscale). a rule, ForFor fix thethe ore legendlegend mineralization to to Figures Figures 16 16–26 –and26 and dependand 28 28 see see directly Figure Figure2 (Section on2 (Subsection the scale2.2). of its occurrence.2.2).

Silver at the considered site forms a very contrasting and quite large AGF development area (Figure 17). High-contrast Ag fields with contents of 10–100 ppm (CR = 100–1000) are clearly fixed within the unique Dukat Au–Ag deposit in the reserves of this metal as well as on its flanks. In addition, AGFs of such contrast are identified in the watershed of the Yagelny and Neponyatny streams. It should be noted that no ore objects have been identified on this watershed to date. The site may be promising in terms of silver mineralization detection. Exogenous fields of medium contrast with Ag content of 1–10 ppm (CR = 10–100) form smaller objects—the Nachalny and Fakel Ag–Pb prospects. Moving in the direction from the known ore objects, the contents in the LSSs gradually decrease and outside them usually do not exceed 1 ppm (CR < 10), which corresponds to low-contrast and near-background geochemical fields. Thus, higher levels of Ag concentrations in channel sediments of the area, as a rule, fix ore mineralization and depend directly on the scale of its occurrence.

Figure 17. The Dukat Au–Ag deposit and associated area.area. Exogenous Ag AGFs identiﬁedidentified through LSSs (survey on the 1:50,000 scale).

Mercury, likelike Ag,Ag, is is quite quite widely widely manifested manifested in thein the studied studied area area (Figure (Figure 18). High-contrast 18). High-contrast AGFs (0.5–1.5AGFs (0.5–1.5 ppm, CRppm,= 100–300)CR = 100–300) are related are related directly directly to the Dukatto the Dukat Au–Ag Au–Ag deposit. deposit. Fields Fields with lowerwith lowercontents contents (0.05–0.5 (0.05–0.5 ppm) and ppm) medium and medium contrast (CRcontrast= 10–100) (CR were= 10–100) identified were everywhere, identified everywhere, including in includingthe deposit, in itsthe flanks deposit, and its ore-free flanks areas. and ore-free In LSSs areas. of the In Nachalny LSSs of andthe FakelNachalny Ag–Pb and prospects Fakel Ag–Pb with prospectscontents of with 0.25–0.5 contents ppm, of the 0.25–0.5 field contrast ppm, the does fiel notd contrast exceed 50–100.does not Such exceed a wide 50–100. distribution Such a ofwide Hg distributionanomalies in of the Hg LSSs anomalies of the area in under the LSSs consideration of the area is probablyunder consideration due to the occurrence is probably of non-mineral due to the forms of this element. There is all the more reason for this as Hg’s own mineral forms at the Dukat deposit were detected neither in ores [56,81] nor in LSSs [29]. The extremely high mobility of Hg is well known. In the areas of an active tectonomagmatic process, it easily penetrates into the near-surface horizons along the fault zones and is sorbed by many minerals—ore and non-metallic ones. Hg is also fixedFigure in ore 17. minerals The Dukat as an Au–Ag impurity, deposit including and associated the Dukat area. depositExogenous [29 Ag,56 ].AGFs Based identified on all ofthrough the above, LSSs (survey on the 1:50,000 scale).

Mercury, like Ag, is quite widely manifested in the studied area (Figure 18). High-contrast AGFs (0.5–1.5 ppm, CR = 100–300) are related directly to the Dukat Au–Ag deposit. Fields with lower contents (0.05–0.5 ppm) and medium contrast (CR = 10–100) were identified everywhere, including in the deposit, its flanks and ore-free areas. In LSSs of the Nachalny and Fakel Ag–Pb prospects with contents of 0.25–0.5 ppm, the field contrast does not exceed 50–100. Such a wide distribution of Hg anomalies in the LSSs of the area under consideration is probably due to the

Minerals 2019, 9, x FOR PEER REVIEW 22 of 40 Minerals 2019, 9, x FOR PEER REVIEW 22 of 40 occurrence of non-mineral forms of this element. There is all the more reason for this as Hg’s own occurrence of non-mineral forms of this element. There is all the more reason for this as Hg’s own mineral forms at the Dukat deposit were detected neither in ores [56,81] nor in LSSs [29]. The mineral forms at the Dukat deposit were detected neither in ores [56,81] nor in LSSs [29]. The extremely high mobility of Hg is well known. In the areas of an active tectonomagmatic process, it extremelyMinerals 2019 high, 9, 789 mobility of Hg is well known. In the areas of an active tectonomagmatic process,22 of 40it easily penetrates into the near-surface horizons along the fault zones and is sorbed by many easily penetrates into the near-surface horizons along the fault zones and is sorbed by many minerals—ore and non-metallic ones. Hg is also fixed in ore minerals as an impurity, including the minerals—ore and non-metallic ones. Hg is also fixed in ore minerals as an impurity, including the Dukatit can bedeposit argued [29,56]. that Hg Based can beon transitedall of the farabove, away it bycan watercourses be argued that and Hg that can not be all transited anomalies far canaway be Dukat deposit [29,56]. Based on all of the above, it can be argued that Hg can be transited far away byassociated watercourses with oreand mineralization. that not all anomalies can be associated with ore mineralization. by watercourses and that not all anomalies can be associated with ore mineralization.

Figure 18. The Dukat Au–Ag deposit and associated area. Exogenous Hg AGFs identified through Figure 18. The Dukat Au–Ag deposit and associated area.area. Exogenous Hg AGFs identifiedidentified through LSSs (survey on the 1:50,000 scale). LSSs (survey on the 1:50,000 scale). AntimonyAntimony has an insufficient insufficient detection limit if de determinedtermined through through the the SSQA SSQA method, method, so so only only Antimony has an insufficient detection limit if determined through the SSQA method, so only high-contrasthigh-contrast AGFs (30–80 ppm, CR = 150–400),150–400), were were revealed revealed in in the studied area. These These anomalous anomalous high-contrast AGFs (30–80 ppm, CR = 150–400), were revealed in the studied area. These anomalous fieldsfields were identified identified with the streams draining the zones of Ag–Pb mine mineralizationralization (the Nachalny fields were identified with the streams draining the zones of Ag–Pb mineralization (the Nachalny Ag–Pb prospect and the watershed of the YagelnyYagelny andand NeponyatnyNeponyatny streams)streams) (Figure(Figure 1919).). Ag–Pb prospect and the watershed of the Yagelny and Neponyatny streams) (Figure 19).

Figure 19. The Dukat Au–Ag deposit and associated area. Exogenous Sb AGFs identified through Figure 19. The Dukat Au–Ag deposit and associated area. Exogenous Sb AGFs identified through LSSsFigure (survey 19. The on Dukat the 1:50,000 Au–Ag scale). deposit and associated area. Exogenous Sb AGFs identified through LSSs (surveyLSSs (survey on the on 1:50,000 the 1:50,000 scale). scale). Arsenic is manifested mainly as AGFs with concentrations up to 50 ppm and contrast up to 25 Arsenic is manifested mainly as AGFs with concentrations up to 50 ppm and contrast up to 25 (FigureArsenic 20). High-contrastis manifested AGFs mainly with as As AGFs concen withtrations concentrations of 200–1000 up ppm to 50 (CR ppm = and100–500) contrast are upfixed to (Figure 20). High-contrast AGFs with As concentrations of 200–1000 ppm (CR = 100–500) are fixed on25 the (Figure flanks 20 of). the High-contrast Dukat deposit—on AGFs with tributaries As concentrations of the Levaya of Brekchiya 200–1000 stream ppm (CR and= the100–500) watershed are on the flanks of the Dukat deposit—on tributaries of the Levaya Brekchiya stream and the watershed offixed the onIskra the and flanks Ersh of streams. the Dukat These deposit—on anomalies tributaries can also be of therelated Levaya to Ag–Pb Brekchiya mineralization. stream and The the of the Iskra and Ersh streams. These anomalies can also be related to Ag–Pb mineralization. The highest-contrastwatershed of the fields Iskra andare Ershidentified streams. in Thesethe watershed anomalies between can also the be relatedYagelny to Ag–Pband the mineralization. Neponyatny highest-contrast fields are identified in the watershed between the Yagelny and the Neponyatny stream,The highest-contrast where the silver-bearing fields are identified (possibly in theAg–Pb) watershed ore mineralization between the Yagelnyis assumed and to the be Neponyatny detectable. stream, where the silver-bearing (possibly Ag–Pb) ore mineralization is assumed to be detectable. Herestream, the where As thecontents silver-bearing in LSSs (possiblyreach very Ag–Pb) high ore values—1000–3000 mineralization is assumedppm (CR to be= 500–1500). detectable. Here thethe As As contents contents in LSSs in reachLSSs veryreach high very values—1000–3000 high values—1000–3000 ppm (CR = 500–1500).ppm (CR Lower-contrast= 500–1500).

As AGFs identiﬁed through LSSs (100–200 ppm, CR = 50–100) are related to Ag–Pb mineralization of the Nachalny and Fakel prospects. Arsenic is least manifested in LSSs of the Dukat Au–Ag deposit Minerals 2019, 9, x FOR PEER REVIEW 23 of 40 Minerals 2019, 9, x FOR PEER REVIEW 23 of 40 Lower-contrastMinerals 2019, 9, 789 As AGFs identified through LSSs (100–200 ppm, CR = 50–100) are related to Ag–Pb23 of 40 mineralizationLower-contrast of As the AGFs Nachalny identified and Fakelthrough prospects. LSSs (100–200 Arsenic ppm, is least CR manifested = 50–100) arein LSSs related of the to Ag–PbDukat Au–Agmineralization deposit of(20–50 the Nachalny ppm, CR and = 10–25). Fakel Asprospects. already Arsenicmentioned, is least the manifested arsenic minerals in LSSs in of the the deposit Dukat (20–50 ppm, CR = 10–25). As already mentioned, the arsenic minerals in the deposit are mainly areAu–Ag mainly deposit represented (20–50 ppm, by sulphoarsenides, CR = 10–25). As whichalready are mentioned, highly unstable the arsenic in hypergene minerals environmentsin the deposit represented by sulphoarsenides, which are highly unstable in hypergene environments (see Section 4.1). (seeare mainlySubsection represented 4.1). Occurring by sulphoarsenides, in ores in small which quantities are highly and unstablepassing intoin hypergene watercourses environments in mobile Occurring in ores in small quantities and passing into watercourses in mobile forms, As does not form forms,(see Subsection As does not 4.1). form Occurring high-contrast in ores fields. in small quantities and passing into watercourses in mobile high-contrast ﬁelds. forms, As does not form high-contrast fields.

Figure 20. The Dukat Au–Ag deposit and associated area. Exogenous As AGFs identified through LSSsFigure (survey 20.20. The on Dukat the 1:50,000 Au–AgAu–Ag scale). depositdeposit andand associatedassociated area.area. Exogenous As AGFs identifiedidentified through LSSs (survey on the 1:50,000 scale). Lead forms a very high contrast and very wide development area of AGFs (Figure 21). The fieldsLead of high formsforms contrast a a very very (1000–5000 high high contrast contrast ppm, and and veryCR very= wide 100–500) wide development developmentare related area to of areathe AGFs Dukat of (FigureAGFs Au–Ag (Figure21). deposit The 21). fields andThe of Fakelhighfields contrast Ag–Pbof high (1000–5000 prospect.contrast (1000–5000They ppm, are CR also= ppm,100–500) identified CR = are 100–500) relatedalong theare to the watershedrelated Dukat to Au–Ag thebetween Dukat deposit the Au–Ag Yagelny and Fakeldeposit and Ag–Pb andthe Neponyatnyprospect.Fakel Ag–Pb They stream,prospect. are also where identified They concentration are along also identified the watershed peaks alongof Ag between the and watershed As the are Yagelny observed between and the too. the Neponyatny The Yagelny fields and stream,of lessthe contrastwhereNeponyatny concentration (500–1000 stream, ppm, peakswhere CR of =concentration Ag50–100) and Asare are identified peaks observed of Agin too.LSSs and The ofAs the fields are Nachalny observed of less contrast Ag–Pb too. The prospect, (500–1000 fields ofalong ppm less, theCRcontrast =tributaries50–100) (500–1000 are of the identified ppm, Svetly, CR in Smely = LSSs 50–100) ofand the are Pravaya Nachalny identified Brek Ag–Pb chiyain LSSs prospect,streams. of the NachalnyDue along to the the tributariesAg–Pb high levels prospect, of of the Ag Svetly,along and PbSmelythe intributaries LSSs, and Pravaya this of thearea Brekchiya Svetly, may Smelybe streams. considered and DuePravaya promising to the Brek highchiya for levels streams.the of identification Ag Due and Pbto the in of LSSs, high indigenous thislevels area of Ag mayAg–Pb and be mineralization.consideredPb in LSSs, promising this area formay the be identification considered ofpromising indigenous for Ag–Pb the identification mineralization. of indigenous Ag–Pb mineralization.

Figure 21. The Dukat Au–Ag deposit and associated area. Exogenous Pb AGFs identified identified through Figure 21. The Dukat Au–Ag deposit and associated area. Exogenous Pb AGFs identified through LSSs (survey on the 1:50,000 scale). LSSs (survey on the 1:50,000 scale). Zinc in comparison with with Pb Pb is is manifested manifested less less in in the the LSSs LSSs of of the the considered considered area area (Figure (Figure 22).22). The most Zinc contrastingin comparison Zn AGFswith Pb with is manifested concentrations less of in 1000–10000the LSSs of ppm the considered (CR = 20–200)20–200) area have (Figure a local 22). distributionThe most contrasting and are fixed fixed Zn AGFs mainly with within concentrations the central of part 1000–10000 of thethe Au–AgAu–Ag ppm DukatDukat(CR = deposit.20–200)deposit. haveFields a with local contentsdistribution up toand 500–1000 are fixed ppm mainly (CR =within10–20) the have central the largest part of development the Au–Ag Dukat area. They deposit. are identifiedFields with in

Minerals 2019, 9, 789 24 of 40 Minerals 2019, 9, x FOR PEER REVIEW 24 of 40 Minerals 2019, 9, x FOR PEER REVIEW 24 of 40 contents up to 500–1000 ppm (CR = 10–20) have the largest development area. They are identified in the Ag–Pb Nachalny and Fakel prospects, the Dukat deposit ﬂanks and in the watershed between the thecontents Ag–Pb up Nachalny to 500–1000 and ppm Fakel (CR prospects, = 10–20) havethe Dukat the largest deposit development flanks and area.in the They watershed are identified between in Yagelny and the Neponyatny stream. thethe YagelnyAg–Pb Nachalny and the Neponyatny and Fakel prospects, stream. the Dukat deposit flanks and in the watershed between the Yagelny and the Neponyatny stream.

Figure 22. The Dukat Au–Ag deposit and associated area. Exogenous Zn AGFs identified through Figure 22. The Dukat Au–Ag deposit and associated area. Exogenous Zn AGFs identified through LSSsFigure (survey 22. The on Dukat the 1:50,000 Au–Ag scale). deposit and associated area. Exogenous Zn AGFs identified through LSSs (survey on the 1:50,000 scale). LSSs (survey on the 1:50,000 scale). CopperCopperforms forms AGFs AGFs in in their their contrast,contrast, configurationconfiguration and and confinement confinement to to watercourses, watercourses, similar similar to to Copper forms AGFs in their contrast, configuration and confinement to watercourses, similar to ZnZn AGFs AGFs (Figure (Figure 23 ).23). The The fields fields with with the the most most contrast contrast (500–1000 (500–1000 ppm, ppm, CR CR= =50–100) 50–100) are are related related to to the Zn AGFs (Figure 23). The fields with the most contrast (500–1000 ppm, CR = 50–100) are related to centralthe central part of part the of Dukat the Dukat Au–Ag Au–Ag deposit. deposit. Less contrast,Less contrast, with contentwith content of 200–500 of 200–500 ppm ppm (CR =(CR20–50), = 20–50),the central are establishedpart of the bothDukat in Au–Agthe central deposit. part ofLess the contrast,deposit andwith on content its flanks of 200–500as well asppm along (CR the = are established both in the central part of the deposit and on its flanks as well as along the watershed watershed20–50), are betweenestablished the both Yagelny in the andcentral the partNeponyatny of the deposit stream. and The on itsNachalny flanks as and well Fakel as along Ag–Pb the between the Yagelny and the Neponyatny stream. The Nachalny and Fakel Ag–Pb prospects are prospectswatershed are between characterized the Yagelny by Cu and AGFs the with Neponyatny contents notstream. exceeding The Nachalny 100–200 ppm and (CRFakel = 10–20).Ag–Pb characterized by Cu AGFs with contents not exceeding 100–200 ppm (CR = 10–20). Low-contrast Low-contrastprospects are AGFscharacterized (<100 ppm, by CRCu

Figure 23. The Dukat Au–Ag deposit and associated area. Exogenous Cu AGFs identified through FigureLSSsFigure 23.(survey 23.The The on Dukat Dukat the 1:50,000 Au–Ag Au–Ag scale). deposit deposit andand associatedassociated area.area. Exogenous Exogenous Cu Cu AGFs AGFs identified identiﬁed through through LSSsLSSs (survey (survey on on the the 1:50,000 1:50,000 scale). scale). Molybdenum and tungsten are the main typomorphic elements for Sn–W and Mo–W mineralization.MolybdenumMolybdenum Withinand andtungsten the tungsten Dukatare Au–Ag theare main the OFS, typomorphicmain the typomorphictin–rare elements metal elements forand Sn–W rare andformetal Mo–WSn–W mineralization mineralization.and Mo–W is Withinuncovered,mineralization. the Dukat basically, Au–AgWithin only the OFS, atDukat thea great tin–rare Au–Ag depth metalOFS, by boreholes andthe tin–rare rare metalin metalthe mineralization endo- and andrare exocontactmetal is uncovered, mineralization zones basically,of the is onlygranitoiduncovered, at a great massif depthbasically, with by boreholes onlythe same at a ingreatname the endo-depthand does andby boreholes exocontactnot outcrop. in zones theIn thisendo- of theregard, and granitoid exocontactW is massifnot manifested zones with theof the samein namealluvialgranitoid and sediments. does massif not with outcrop These the . elementsame In this name regard,concentrations and Wdoes is notnot in manifestedLSSsoutcrop. are near-backgroundIn inthis alluvial regard, sediments. W andis not do manifested not These exceed element in6 concentrationsppmalluvial at CRsediments. < 3. in Mo LSSs Theseforms are elementonly near-background low-contrast concentrations AGFs and in do(3–10 LSSs not ppm, are exceed near-background CR 6= ppm3–10) at(Figure CR and< 24).3. do MoIts not anomalies formsexceed only 6 low-contrastwithppm aat content CR AGFs< 3. ofMo 3–5 (3–10 forms ppm ppm, only (CR low-contrastCR= 3–5)= 3–10 are )develope (FigureAGFs (3–10 d24 almost). ppm, Its everywhere. anomalies CR = 3–10) with The(Figure content a content 24). Itsand anomalies of contrast 3–5 ppm with a content of 3–5 ppm (CR = 3–5) are developed almost everywhere. The content and contrast (CR = 3–5) are developed almost everywhere. The content and contrast increase slightly only in

Minerals 2019, 9, 789 25 of 40 Minerals 2019, 9, x FOR PEER REVIEW 25 of 40 Minerals 2019, 9, x FOR PEER REVIEW 25 of 40 relativelyincrease small slightly AGF only areas in (5–10relatively ppm, smallCR = AGF5–10 ).areas They (5–10 were ppm, identified CR = within5–10). They the Nachalny were identified and Fakel increase slightly only in relatively small AGF areas (5–10 ppm, CR = 5–10). They were identified Ag–Pbwithin prospects the Nachalny and onand the Fakel flanks Ag–Pb of the prospects Dukat Au–Ag and on deposit:the flanks in of the the north—in Dukat Au–Ag the watersheds deposit: in of within the Nachalny and Fakel Ag–Pb prospects and on the flanks of the Dukat Au–Ag deposit: in Iskrathe andnorth—in Ersh—and the watersheds in the east—in of Iskra the and watershed Ersh—and at the in the head east—in of the Yagelnythe watershed and Neponyatny at the head of streams. the the north—in the watersheds of Iskra and Ersh—and in the east—in the watershed at the head of the YagelnyTin, after and Ag,Neponyatny is the second streams. main element indicator of the Sn–Ag mineralization type, which is Yagelny and Neponyatny streams. manifestedTin, onlyafter inAg, the is deepthe second horizons main and element flanks indicator of the Dukat of the Au–Ag Sn–Ag deposit mineralization in the investigation type, which area.is Tin, after Ag, is the second main element indicator of the Sn–Ag mineralization type, which is Muchmanifested more rarely, only Snin the occurs deep in horizons Ag–Pb ores, and mainlyflanks atof depth,the Dukat where Au–Ag it has deposit relatively in the low investigation concentrations. manifested only in the deep horizons and flanks of the Dukat Au–Ag deposit in the investigation Inarea. this regard, Much Snmore in LSSsrarely, is alsoSn weaklyoccurs in manifested, Ag–Pb ores, mainly mainly in the at formdepth, of low-contrastwhere it has AGFsrelatively (6–20 low ppm , area. Much more rarely, Sn occurs in Ag–Pb ores, mainly at depth, where it has relatively low concentrations. In this regard, Sn in LSSs is also weakly manifested, mainly in the form of CRconcentrations.= 3–10) (Figure In 25 this). Theregard, anomalies Sn in ofLSSs low is contrast also weakly are revealed manifested, in watercourses mainly in the draining form bothof low-contrast AGFs (6–20 ppm, CR = 3–10) (Figure 25). The anomalies of low contrast are revealed in thelow-contrast Nachalny andAGFs Fakel (6–20 Ag–Pb ppm, CR prospects = 3–10) (Figure and the 25) Dukat. The anomalies Au–Ag deposit of low andcontrast its flanks are revealed to the in east. watercourses draining both the Nachalny and Fakel Ag–Pb prospects and the Dukat Au–Ag deposit Medium-contrastwatercourses draining Sn AGFs both with the concentrations Nachalny and ofFakel 20–100 Ag–Pb ppm prospects (CR = 10–50) and the have Dukat the localAu–Ag propagation deposit and its flanks to the east. Medium-contrast Sn AGFs with concentrations of 20–100 ppm (CR = 10–50) andand are its identified flanks to the in theeast. stream Medium-contrast Brekchiya toSn the AGFs east with of the concentrations field, which of is 20–100 related, ppm apparently, (CR = 10–50) to the have the local propagation and are identified in the stream Brekchiya to the east of the field, which is Sn–Aghave mineralizationthe local propagation outcropping and are onidentified its flanks. in the stream Brekchiya to the east of the field, which is related, apparently, to the Sn–Ag mineralization outcropping on its flanks. related,Bismuth apparently,, like Sn, to is the more Sn–Ag typical mineralization of Sn–Ag ores outcropping than of Au–Ag on its and flanks. Ag–Pb. Bi is poorly manifested Bismuth, like Sn, is more typical of Sn–Ag ores than of Au–Ag and Ag–Pb. Bi is poorly in alluvialBismuth sediments, like inSn, the is entire more investigation typical of Sn–Ag area. Itores forms than mainly of Au–Ag low-contrast and Ag–Pb. AGF with Bi is contents poorly of manifested in alluvial sediments in the entire investigation area. It forms mainly low-contrast AGF 0.3–1manifested ppm (CR in= alluvial3–10) (Figure sediments 26). in Concentrations the entire investigation increasing area. to 1–2 It forms ppm (CRmainly= 10–20) low-contrast were observed AGF with contents of 0.3–1 ppm (CR = 3–10) (Figure 26). Concentrations increasing to 1–2 ppm (CR = locally.with contents They were of identified0.3–1 ppm only(CR in= 3–10) LSSs (Figure of streams 26). drainingConcentrations the Nachalny increasing Ag–Pb to 1–2 prospect ppm (CR and = in 10–20) were observed locally. They were identified only in LSSs of streams draining the Nachalny 10–20) were observed locally. They were identified only in LSSs of streams draining the Nachalny LSSsAg–Pb of the prospect Brekchiya and stream in LSSs to theof the east Brekchiya of the Dukat stream deposit, to the which east isof related,the Dukat apparently, deposit, aswhich well asis to Ag–Pb prospect and in LSSs of the Brekchiya stream to the east of the Dukat deposit, which is Sn,related, to the Sn–Agapparently, mineralization as well as to outcropping Sn, to the Sn–Ag on its mineralization flanks. outcropping on its flanks. related, apparently, as well as to Sn, to the Sn–Ag mineralization outcropping on its flanks.

Figure 24. The Dukat Au–Ag deposit and associated area. Exogenous Mo AGFs identified through FigureFigure 24. 24.The The Dukat Dukat Au–Ag Au–Ag deposit deposit andand associatedassociated area. Exogenous Exogenous Mo Mo AGFs AGFs identified identiﬁed through through LSSs (survey on the 1:50,000 scale). LSSsLSSs (survey (survey on on the the 1:50,000 1:50,000 scale). scale).

Figure 25. The Dukat Au–Ag deposit and associated area. Exogenous Sn AGFs identified through Figure 25. The Dukat Au–Ag deposit and associated area. Exogenous Sn AGFs identified through FigureLSSs 25.(surveyThe Dukaton the Au–Ag1:50,000 depositscale). and associated area. Exogenous Sn AGFs identiﬁed through LSSs (surveyLSSs (survey on the 1:50,000on the 1:50,000 scale). scale).

Minerals 2019, 9, 789 26 of 40

Minerals 2019, 9, x FOR PEER REVIEW 26 of 40

FigureFigure 26. 26.The The Dukat Dukat Au–Ag Au–Ag deposit deposit and and associated associated area. area Exogenous. Exogenous Bi Bi AGFs AGFs identiﬁed identified through through LSSs (surveyLSSs (survey on the 1:50,000on the 1:50,000 scale). scale).

AsAs can can be be seen seen from from the the results results ofof thethe surveysurvey of LSSs on on the the 1:50,000 1:50,000 scale, scale, performed performed onon the the territoryterritory of of the the Dukat Dukat Au–Ag Au–Ag OFS’s OFS’s southernsouthern part,part, including the the deposit deposit with with the the same same name, name, AGFs AGFs of Ag, Sb, As, Pb, Zn, Cu and, most importantly, Au and Hg have the maximum contrast and the of Ag, Sb, As, Pb, Zn, Cu and, most importantly, Au and Hg have the maximum contrast and the largest area of development. All of the above elements are typomorphic of the Au–Ag and Ag–Pb largest area of development. All of the above elements are typomorphic of the Au–Ag and Ag–Pb mineralization types that are widely manifested here. The maximum anomalous concentrations are mineralization types that are widely manifested here. The maximum anomalous concentrations are fixed in the alluvial sediments of the first- and second-order watercourses directly draining the ore fixed in the alluvial sediments of the first- and second-order watercourses directly draining the ore zones. AGFs of Sn, Bi, Mo and W, characteristic of tin ore and rare-metal mineralization, which are zones.practically AGFs absent of Sn, in Bi, this Mo area, and are W, not characteristic manifested. of tin ore and rare-metal mineralization, which are practicallySampling absent the in thiswatercourse area, are heads not manifested. in conditions of cryolithogenesis zones when performing a geochemicalSampling survey the watercourse of LSSs on headsthe 1:50,000 in conditions scale is more of cryolithogenesis effective than taking zones only when double performing samples a geochemicalat the stream survey mouths of LSSsof the on second-order, the 1:50,000 rarer scale than is more the first-order, effective than as recommended taking only double for surveys samples on at thethe stream 1:200,000 mouths scale of (see the Subsection second-order, 3.1). rarer Au and than Hg the are first-order, illustrative as examples. recommended In their for concentration surveys on the 1:200,000distribution scale in (see the Section alluvial 3.1 sediments). Au and of Hg the are streams illustrative draining examples. the Dukat In their deposit concentration Au–Ag zones, distribution clear inpatterns the alluvial were sediments identified. of When the streams passing draining from the the first-order Dukat deposit watercourse Au–Ag to zones,the watercourse clear patterns of each were identified.subsequent When order, passing the content from the of first-order these elements watercourse in the samples to the watercourse is sharply reduced of each subsequent (Table 4). The order, thehighest content concentrations of these elements of Au in (maximum—0.45 the samples is sharply ppm; reducedaverage—0.078 (Table4 ppm)). The and highest Hg (maximum—1.5 concentrations of Auppm; (maximum—0.45 average—0.28 ppm; ppm) average—0.078 were found in ppm) the alluvium and Hg(maximum—1.5 of the first-order ppm; watercourses average—0.28 directly ppm) wereeroding found the in Au–Ag the alluvium ore zones. of theWhen first-order these streams watercourses enter larger directly watercourses eroding of the the Au–Ag second-order, ore zones. Whennoticeable these streams flow dilution enter largerand content watercourses decrease of occur. the second-order, The maximum noticeable (0.23 ppm) flow and dilution average and (0.041 content decreaseppm) Au occur. contents The maximumreduce twice. (0.23 For ppm) Hg, andthese average values are (0.041 even ppm) higher: Au contentsthe maximum reduce contents twice. For(0.33 Hg, theseppm) values reduce are by even almost higher: five the times maximum and the contents average (0.33 (0.10 ppm) ppm) reduce by almost by almost three five times. times When and the averageconsidering (0.10 ppm)the streams by almost of the three third-order, times. When there consideringis already a more the streams significant of the fivefold third-order, reduction there in is both the maximum (0.090 ppm) and the average (0.017 ppm) Au concentrations with regard to the already a more significant fivefold reduction in both the maximum (0.090 ppm) and the average first-order watercourses. The maximum Hg contents (0.12 ppm) decrease by more than twelve times (0.017 ppm) Au concentrations with regard to the first-order watercourses. The maximum Hg contents and the average (0.034 ppm) by eight times with regard to the first-order watercourses. (0.12 ppm) decrease by more than twelve times and the average (0.034 ppm) by eight times with regard to the first-orderTable 4. Distribution watercourses. of Au and Hg contents (ppm) in alluvial sediments depending on the watercourse order draining the Au–Ag zones of the Dukat deposit (survey of LSSs on the 1:50,000 Table 4. Distribution of Au and Hg contents (ppm) in alluvial sediments depending on the watercourse scale). order draining the Au–Ag zones of the Dukat deposit (survey of LSSs on the 1:50,000 scale). Watercourse Orders I II III WatercourseElements Orders Au IHg Au IIHg Au IIIHg Elements Au Hg Au Hg Au Hg Minimum content <0.006 0.017 <0.006 0.022 <0.006 0.005 MinimumMaximum content content <0.0060.45 0.0171.5 <0.230.006 0.33 0.022 0.090<0.006 0.120.005 MaximumAverage content content 0.45 0.078 1.50.28 0.041 0.23 0.10 0.33 0.017 0.090 0.034 0.12 Average content 0.078 0.28 0.041 0.10 0.017 0.034 Number of samples 34 23 44 21 42 26 Number of samples 34 23 44 21 42 26

Minerals 2019, 9, 789 27 of 40

Minerals 2019, 9, x FOR PEER REVIEW 27 of 40 Histograms, as seen in Figure 27, provide a more visual representation of the Au and Hg content Histograms, as seen in Figure 27, provide a more visual representation of the Au and Hg distribution depending on the order of the watercourse. It can clearly be seen in them that the maximum contentMinerals distribution 2019, 9, x FOR PEERdepending REVIEW on the order of the watercourse. It can clearly be seen in them27 of 40 that anomalousthe maximum contents anomalous of Au > contents0.05 ppm of Au (CR >> 0.0510) ppm and Hg(CR> > 0.110) ppmand Hg (CR > 0.1> 20) ppm are (CR most > 20) often are fixedmost in samplesoften fixed takenHistograms, in from samples watercourses as seen taken in Figurefrom of the watercourses27, first-order provide a (frequency ofmo there visualfirst-order of arepresentation given (frequency content of interval ofthe a Augiven forand Au—47%;content Hg forinterval Hg—61%).content for distribution Au—47%; Such contents dependingfor Hg—61%). exist moreon the Such rarely order contents inof thethe alluviumexistwatercourse. more of rarely second-order It can in clearly the alluvium watercoursesbe seen of in second-order them (frequency that forwatercourses Au—30%;the maximum for (frequency Hg—48%) anomalous andcontentsfor extremelyAu—30%; of Au > rarely 0.05for ppmHg—48%) in third-order (CR > 10) and and watercourses extremHg > 0.1ely ppm rarely (frequency (CR > in20) third-orderare for most Au—7%; forwatercourses Hg—8%).often fixed It in(frequency should samples be takenfor noted Au—7%; from that watercourses concentrations for Hg—8%). of Itthe of should Aufirst-order> 0.1be noted ppm (frequency (CRthat >concentrations 20)of a andgiven Hg content> of0.2 Au ppm > (CR0.1>interval ppm40) in (CR for third-order Au—47%;> 20) and for watercourse Hg Hg—61%). > 0.2 ppm sedimentsSuch (CR contents > 40) were existin third-order not more identified rarely watercourse in the at all.alluvium The sediments samplingof second-order were effi ciencynot of first-orderidentifiedwatercourses at watercourses all. (frequency The sampling in for the Au—30%; conditions efficiency for of of cryolithogenesisHg—48%) first-order and watercourses extrem zonesely for rarely Auin andthe in Hgconditionsthird-order anomalies of is revealing,cryolithogenesiswatercourses and, as (frequency a zones consequence, for forAu Au—7%;and a more Hg anomaliesfor reliable Hg—8%). determination is re Itvealing, should beand, of noted AGFs as a thatconsequence, belonging concentrations to a a more particular of Aureliable > type of mineralizationdetermination0.1 ppm (CR of> is 20) obvious.AGFs and Hgbelonging Other > 0.2 researchers,ppm to (CRa particular > 40) when in third-order studyingtype of themineralizationwatercourse Au stream sediments sedimentsis obvious. were of not RussianOther identified at all. The sampling efficiency of first-order watercourses in the conditions of auriferousresearchers, deposits when also studying formed the under Au stream the conditions sediments of cryolithogenesisof Russian auriferous zones deposits in the territories also formed of the cryolithogenesis zones for Au and Hg anomalies is revealing, and, as a consequence, a more reliable under the conditions of cryolithogenesis zones in the territories of the North Priamurye [20,103] and Northdetermination Priamurye [20of, 103AGFs] and belonging Pre Kolyma to a regions particular [104 ],type have of reachedmineralization the same is conclusion.obvious. Other Pre Kolyma regions [104], have reached the same conclusion. Theresearchers, typomorphic when studying composition the Au of stream exogenous sedime AGFsnts of (element Russian associations)auriferous deposits and their also distributionformed The typomorphic composition of exogenous AGFs (element associations) and their distribution patternsunder in the space conditions in the Dukatof cryolithogenesis Au–Ag deposit zones and in the the territories associated of the area North are Priamurye shown in the[20,103] polyelement and patterns in space in the Dukat Au–Ag deposit and the associated area are shown in the polyelement geochemicalPre Kolyma map regions (Figure [104], 28, have Table reached5). the same conclusion. geochemicalThe typomorphic map (Figure composition 28, Table 5). of exogenous AGFs (element associations) and their distribution patterns in space in the Dukat Au–Ag deposit and the associated area are shown in the polyelement geochemical map (Figure 28, Table 5).

Figure 27. Frequency histograms of Au (a) and Hg (b) contents in alluvial sediments depending on Figure 27. Frequency histograms of Au (a) and Hg (b) contents in alluvial sediments depending on the the order of watercourses draining the Dukat Au–Ag deposit (survey of LSSs on the 1:50,000 scale). orderFigure of watercourses 27. Frequency draining histograms the Dukatof Au ( Au–Aga) and Hg deposit (b) contents (survey in alluvial of LSSs sediments on the 1:50,000 depending scale). on the order of watercourses draining the Dukat Au–Ag deposit (survey of LSSs on the 1:50,000 scale).

Figure 28. The Dukat Au–Ag deposit and associated area. Exogenous polyelement AGFs identified Figure 28. The Dukat Au–Ag deposit and associated area. Exogenous polyelement AGFs identified Figurethrough 28. TheLSSs Dukat(survey Au–Ag on the deposit 1:50,000 and scale). associated 1–12—g area.eochemical Exogenous associations polyelement of ore AGFselements identified (see through LSSs (survey on the 1:50,000 scale). 1–12—geochemical associations of ore elements (see throughTable LSSs5): 1–4—predominantly (survey on the 1:50,000 Ag and scale). Au–Ag; 1–12—geochemical 5–9—Ag–Pb; 10 associations and 11—Sn–Ag; of ore elements and 12—Pb–Zn (see Table in5 ): Table 5): 1–4—predominantly Ag and Au–Ag; 5–9—Ag–Pb; 10 and 11—Sn–Ag; and 12—Pb–Zn in 1–4—predominantly Ag and Au–Ag; 5–9—Ag–Pb; 10 and 11—Sn–Ag; and 12—Pb–Zn in the ZDSM. thethe ZDSM. ZDSM.

Minerals 2019, 9, 789 28 of 40

Table 5. The Dukat Au–Ag deposit and associated area. Geochemical associations of ore elements (survey of LSSs on the 1:50,000 scale).

Nos. Ore Element Associations Predominantly Ag and Au–Ag 1 Ag(27/270) Pb(1800/180) Hg(0.3/60) Au(0.2/40) Zn(2000/40) Cu(350/35) As(20/10) Bi(0.4/4) 2 Ag(14/140) Pb(1000/100) Hg(0.26/52) Au(0.1/20) As(40/20) Cu(190/19) Zn(900/18) Bi(0.4/4) 3 Ag(11/110) Pb(800/80) As(60/30) Hg(0.12/24) Cu(210/21) Au(0.055/11) Zn(500/10) Bi(0.4/4) 4 Ag(6/60) Pb(500/50) As(80/40) Cu(180/18) Hg(0.05/10) Au(0.03/6) Zn(250/5) Bi(0.5/5) Ag–Pb 5 As(1350/675) Pb(2800/280) Sb(40/200) Ag(19.5/195) Cu(170/17) Zn(600/12) Hg(0.06/12) Bi(0.3/3) Sn(6/3) 6 Pb(1500/150) As(160/80) Ag(6/60) Cu(200/20) Zn(550/11) Hg(0.05/10) Bi(0.4/4) Sn(6/3) 7 Pb(600/60) Ag(5/50) As(80/40) Cu(180/18) Zn(400/8) Hg(0.04/8) Bi(0.6/6) Sn(8/4) 8 Sb(45/225) Pb(480/48) Ag(4.6/46) As(70/35) Zn(800/16) Cu(150/15) Bi(1/10) Sn(16/8) Hg(0.02/4) 9 Ag(2/20) As(30/15) Pb(100/10) Cu(50/5) Zn(200/4) Bi(0.3/3) Sn–Ag 10 Ag(5.2/52) Sn(70/35) As(60/30) Cu(200/20) Pb(190/19) Zn(800/16) Bi(1.5/15) 11 Ag(2.2/22) As(34/17) Sn(30/15) Cu(90/9) Pb(80/8) Zn(300/6) Bi(0.6/6) Pb–Zn in ZDSM 12 As(10/5) Ag(0.3/3) Bi(0.3/3) Pb(30/3) Zn(100/2) Cu(20/2) Note: For Sb, only contents > 30 ppm (CR > 150) are shown in element associations, above the detection limit of the SSQA method.

Predominantly Ag and Au–Ag element associations are distinctly fixed in the LSSs of the Dukat deposit. Spatially, they are combined with Ag–Pb and have a complex composition, which is related to the draining by watercourses not only in Au–Ag but also manifested here in Ag–Pb mineralization zones. Au–Ag associations with the maximum contents of the main typomorphic elements fix the most gold-rich central part of the deposit. High contents are typical of Ag and Pb and less high contents of Hg, Au, Zn and Cu. Such elements as As and Bi are almost not manifested. The main element contents are gradually reduced to the periphery with a simultaneous increase in the AGF development area. In their qualitative and quantitative composition, they are close to the Ag–Pb associations: the main typomorphic elements for the Ag–Pb mineralization type emerge in the first place. The concentrations of Ag, Pb, Cu, Hg, Au and Zn decrease. The concentrations of As, on the contrary, increase, and those of Bi remain almost unchanged. It is necessary to remember here that, for such an element indicator as Sb, due to the use of the SSQA method, it is not possible to detect concentrations of less than 30 ppm. Very few points were observed with contents of 30 ppm. They are confined to the contrast Au–Ag AGFs. Even such sporadic contents are very high for Sb (CR = 150), which confirms its existence in Au–Ag associations in one line with the main typomorphic elements. From the centre to the periphery, Au–Ag element associations are replaced by Ag–Pb associations, which are widely manifested throughout the area. They are identified on the flanks of the Dukat deposit and in the area of Fakel and Nachalny Ag–Pb prospects. The associations with the highest element indicator concentrations of the Ag–Pb ore mineralization were identified in the watershed of the Yagelny and Neponyatny streams. This may be due to the unidentified latent Ag–Pb mineralization here. The highest concentrations are observed for As, Pb, Sb and Ag and elevated concentrations for Cu, Zn and Hg. Such elements as Bi and Sn are almost not manifested. The AGFs of the Fakel prospect are characterized by associations with lower contents of the above-mentioned elements. In the Nachalny Ag–Pb prospect, the Sb, Pb, Ag, As, Zn and Cu form the maximum concentrations in the associations. To lower degrees, Bi, Sn, and Hg are manifested. The Ag–Pb element associations in the area’s eastern part are changed by Sn–Ag. Due to the weakly manifested Sn–Ag mineralization type in the investigation area, these associations have local development, a small size and relatively low element contents. The maximum anomalous contents here are typical of basic typomorphic elements, such as Ag, Sn, As, Cu, Pb, Zn and Bi. Minerals 2019, 9, 789 29 of 40

All the element associations related to ore mineralization are manifested against the background of the ZDSM. As can be seen from the polyelement geochemical maps, all the associations of ore elements detected within the considered area (predominantly Ag and Au–Ag, Ag–Pb and Sn–Ag) were identified more reliably when surveying LSSs on the 1:50,000 scale. It is important that, along with the predominant Ag, Au–Ag AGFs were not detected when surveying LSSs on the 1:200,000 scale. Distinctly, from the centre to the periphery, a sequence of lateral geochemical zonality was observed, expressed in the alternation of different types of polyelement AGFs (element associations): Ag and Au–Ag Ag–Pb Sn–Ag. This sequence is a reflection of the → → previously identified primary vertical geochemical zonality in the Au–Ag Dukat deposit [56,80,102,105]. It may be argued that the element composition of the exogenous AGFs, identified through LSSs when conducting the survey on the 1:50,000 scale, unlike the surveys on the 1:200,000 scale, is fully consistent with the composition and structure of drained and eroded ore zones. Despite the number of the above advantages, there is an important circumstance that makes it difficult to perform surveys of LSSs on the 1:50,000 scale for ore mineralization prospecting. It is impossible to take alluvial samples from the watercourse heads, where, in cryolithogenesis conditions, the loose sediments are poorly formed and often non-existent. This may be due to the sod cover of watercourses’ heads, their temporary nature, the effects of freezing temperatures on the mechanical and, especially, chemical weathering processes as well as the river material transfer. As a result, watercourses of the first-order are often made of coarse-grained sediments with rare manifestations of large, poorly rounded pebbles. The alluvial sediments required for sampling are fragmentary, absent or formed with a severe shortage of the sandy silt material that concentrates most of the ore elements. All this is especially true in cryolithogenesis zones, where the main process that forms LSSs is considered to be physical weathering. It is obvious that, in such situations, it becomes necessary to carry out detailed research to develop additional methods that are able to eliminate the above disadvantages. We have carried out this work. We have established that, when sampling the first-order watercourses, it is effective to carry out bryolithochemical research and study the binding forms (BFs) of element indicators in LSSs and in ores with their subsequent comparison.

4.3. Bryolithochemical Research When Prospecting for Gold–Silver Mineralization Based on the Stream Sediments G.P. Lapaev began to use aquatic mosses in the ore deposits for the first time when prospecting along watercourses in the USSR [106]. The essence of his method was to study the distribution of trace elements in surface fresh waters by selecting the aquatic mosses growing in them, carefully washing from them the sandy silt fraction. Later, aquatic mosses were also used by V.A. Zagoskin [107]. In contrast with G.P. Lapaev’s method, the element concentration determination is performed not with the ash of purified aquatic mosses but with the moisture obtained from pressing them. All the proposed methods have been named bryogeochemical ones and are, in fact, a kind of hydrogeochemical method. Foreign researchers have also used mosses to search for minerals [108–110]. Their proposed methods are similar to those used by G.P. Lapaev. The complexity of all the above bryogeochemical methods [106–110] stems from the necessity of searching for aquatic mosses. Their existence requires an aquatic environment, which is practically absent in temporary watercourses of the first-order. In addition, the ash microelement composition of aquatic mosses reflects not so much the biogeochemical stream’s sediment composition but the river water’s mineral composition. The chemical element content in the mosses supplying the water is significantly lower than that of the material of river sediments. Among other things, the burning of pure moss significantly reduces the ash content percentage, which, in turn, requires the initial sample selection to be sufficiently large by weight. One more serious drawback of the previously proposed methods is the operational complexity of separating the alluvium from the moss cushion. Our version of bryolithochemical method [111] is free from these disadvantages. Special bryolithochemical research was carried out for the development of this procedure. It was conducted at the Chaika site of the Dukat Au–Ag deposit along the first-order watercourses draining the Minerals 2019, 9, x FOR PEER REVIEW 30 of 40

MineralsSpecial2019, 9 ,bryolithochemical 789 research was carried out for the development of this procedure.30 of 40It was conducted at the Chaika site of the Dukat Au–Ag deposit along the first-order watercourses draining the Au–Ag ore zones (see Figure 2, Subsection 2.2, and Figure 29). The choice of just this site Au–Ag ore zones (see Figure2, Section 2.2, and Figure 29). The choice of just this site can be explained can be explained by the fact that there was an opportunity here to sample both the mosses together by the fact that there was an opportunity here to sample both the mosses together with the silt and with the silt and sandy silt material that is firmly held by the moss cushion (Figure 30), and the sandy silt material that is firmly held by the moss cushion (Figure 30), and the traditional lithochemical traditional lithochemical material. This is necessary for further comparing the element contents in material. This is necessary for further comparing the element contents in bryolithochemical stream bryolithochemical stream sediments (BSSs) and LSSs. The whole sampled material was divided into sediments (BSSs) and LSSs. The whole sampled material was divided into two samples—lithochemical two samples—lithochemical and bryolithochemical samples. In the first sample, silt–sand material and bryolithochemical samples. In the first sample, silt–sand material was separated from the moss was separated from the moss cushion. The second sample was a moss with mostly silt material that cushion. The second sample was a moss with mostly silt material that was difficult to separate. was difficult to separate. To compare the element contents in all the samples, at sites where alluvium To compare the element contents in all the samples, at sites where alluvium was formed fragmentarily, was formed fragmentarily, a lithochemical sample consisting mainly of sandy material was taken in a lithochemical sample consisting mainly of sandy material was taken in the traditional way. the traditional way.

Figure 29.29. The Dukat Au–AgAu–Ag depositdeposit atat thethe ChaikaChaika site.site. The schemescheme ofof bryolithochemicalbryolithochemical and lithochemical sampling sampling of of first-order ﬁrst-order watercourses. watercourses. 1—ore 1—ore bodies; bodies; 2—sampling 2—sampling points. points.

Figure 30. Sandy silt material in moss cushions growing on the banks and beds of watercourse heads. Figure 30. Sandy silt material in moss cushions growing on the banks and beds of watercourse heads.

The main attention was paid to Au and Ag, whichwhich are the main element indicators for the Au–Ag mineralization type. Their concentration levels in bryolithochemical samples significantlysignificantly exceed those that were identifiedidentified in the lithochemicallithochemical material, taken both from the moss cushion and inin thethe traditional traditional way. way. The The Au Au concentrations concentratio inns thein bryolithochemicalthe bryolithochemical samples samples reach reach 0.802 0.802 ppm, whichppm, which is much is much higher higher than thethan content the content of this of element this element in the in lithochemical the lithochemical material material selected selected from thefrom moss the moss cushion cushion (0.205 (0.205 ppm ppm and and less). less). Even Even lower lowe contentsr contents of of this this element element werewere identifiedidentified in lithochemical samplessamples takentaken usingusing the the traditional traditional method method (0.085 (0.085 ppm ppm and and less). less). The The Ag Ag distribution distribution is characterizedis characterized by by the the same same peculiar peculiar properties. properties. The The maximum maximum concentrationsconcentrations ofof AgAg werewere identifiedidentified

Minerals 2019,, 9,, 789x FOR PEER REVIEW 3131 of of 40

in bryolithochemical samples and reach 100 ppm. This element’s contents in all the lithochemical in bryolithochemical samples and reach 100 ppm. This element’s contents in all the lithochemical samples do not exceed 20 ppm. samples do not exceed 20 ppm. The initial data obtained regarding the material composition of bryolithochemical samples by The initial data obtained regarding the material composition of bryolithochemical samples by EPMA are of considerable interest. A large number of non-metallic mineral fragments (quartz and EPMA are of considerable interest. A large number of non-metallic mineral fragments (quartz and feldspar) and Fe and Mn hydroxides were detected in such samples. Ore minerals are represented feldspar) and Fe and Mn hydroxides were detected in such samples. Ore minerals are represented by by finely dispersed electrum particles (<10 μm), silver mineral relics and strongly destroyed small ﬁnely dispersed electrum particles (<10 µm), silver mineral relics and strongly destroyed small grains grains of pyrite, sphalerite, and galena. According to the preliminary data, Au (up to 0.61 wt %) of pyrite, sphalerite, and galena. According to the preliminary data, Au (up to 0.61 wt %) constantly constantly presents in small particles of arsenic pyrite in the form of an impurity. The particles, presents in small particles of arsenic pyrite in the form of an impurity. The particles, consisting of Au consisting of Au and Ag, do not often exceed 1–2 μm. They tend to connect in aggregates, the size of and Ag, do not often exceed 1–2 µm. They tend to connect in aggregates, the size of which reaches which reaches 50 μm (Figure 31). Inside these aggregates, the particles are distributed very 50 µm (Figure 31). Inside these aggregates, the particles are distributed very unevenly. They are unevenly. They are represented, as in ores, by electrum and the severely destroyed silver represented, as in ores, by electrum and the severely destroyed silver sulphosalts. sulphosalts.

Figure 31. Particles of electrum and silver sulphosalts in the bryolithochemical sample. Images are Figure 31. Particles of electrum and silver sulphosalts in the bryolithochemical sample. Images are given: (a)—in backscattered electrons; (b,c)—in X-rays. given: (a)—in backscattered electrons; (b,c)—in X-rays.

Based on the research carried out, we can conclude that the concentration levels of ore elements, primarily AuAu and and Ag, Ag, in bryolithochemical in bryolithochemical samples samp significantlyles significantly exceed those exceed identified those in lithochemicalidentified in samples.lithochemical Moss samples. cushions areMoss an ecushionsffective natural are an trap effective extracting natural finely trap dispersed extracting lithoparticles finely dispersed from the waterlithoparticles suspension from of the present water watercourses, suspension of even present temporary watercourses, ones. Along even temporary with the main ones. vein Along minerals with (quartzthe main and vein feldspar), minerals these (quartz particles and arefeldspar), represented these byparticles electrum, are silver represented mineral by relics electrum, and strongly silver destroyedmineral relics grains and of strongly simple sulphides, destroyed which grains largely of simp reflectle sulphides, the peculiar which properties largely ofreflect the primary the peculiar ore’s materialproperties composition. of the primary ore’s material composition.

4.4. The Binding FormsForms ofof ElementElement Indicators Indicators of of Gold–Silver Gold–Silver Mineralization Mineralization in in Lithochemical Lithochemical Stream Stream Sediments Sediments A detailed study of the distribution peculiarities and BFs of mineralization element indicators in LSSs,A detailed like bryolithochemical study of the distribution research, peculiaritie was carrieds and out BFs at the of mineralizati Chaika siteon of theelement Dukat indicators Au–Ag depositin LSSs, (seelike Figurebryolithochemical2, Section 2.2 research,, and Figure was 29carried, Section out 4.3at )[the23 Chaika,24,29]. site Ores, of the relative Dukat to Au–Ag typical epithermaldeposit (see Au–Ag Figure mineralization, 2, SubSection are2.2, preservedand Figure best 29, here Subsection [56]. The 4.3) study [23,24,29]. of the alluvium Ores, relative material to compositiontypical epithermal of the Au–Ag watercourses mineralization, draining are these preserved ores was best initiated here [56]. by means The study of a polarizingof the alluvium light microscope.material composition Quartz, feldspar,of the watercourses Fe oxides anddraining hydroxides these ores are was the initiated most prevalent. by means As of for a polarizing alluvium secondarylight microscope. minerals, Quartz, anglesite, feldspar, cerussite, Fe chalcocite, oxides and covellite, hydroxides manganese are the hydroxides most prevalent. and oxides As occur for morealluvium rarely. secondary Preserved minerals, close toanglesite, the primary cerussite, minerals, chalcocite, they are covellite, seldom manganese present. Poorly hydroxides destroyed and minerals,oxides occur oxidizing more rarely. to varying Preserved degrees close magnetite to the andprimary rarely minerals, galena, werethey wellare seldom diagnosed. present. Individual Poorly galenadestroyed grains minerals, can be oxidizing found either to varying in the degrees rim of cerussite–anglesitemagnetite and rarely or galena, in a relic were form. well diagnosed. Fragments ofIndividual other sulphides galena grains are even can more be found rare. Theseeither arein the pyrite, rim chalcopyriteof cerussite–anglesite and sphalerite. or in Smalla relic grainsform. (0.01–0.005Fragments mm),of other with sulphides a high reﬂective are even index, more which rare we. These could are not pyrite, identify chalcopyrite with an optical andmicroscope, sphalerite. areSmall presumably grains (0.01–0.005 attributable mm), to with silver-containing a high reflecti formations.ve index, which In the we course could of not the identify work, it with became an obviousoptical microscope, that the study are ofpresumably ore element attributable BFs in loose to silver-containing alluvial sediments formations. of watercourses In the course is ineﬀ ectiveof the usingwork, traditionalit became mineralogicalobvious that methods.the study The of successfulore element solution BFs ofin suchloose types alluvial of tasks sediments is directly of relatedwatercourses to the determinationis ineffective using of the traditional substance compositionmineralogical in methods. micron inclusions The successful and only solution through of localsuch types of tasks is directly related to the determination of the substance composition in micron

Minerals 2019, 9, x FOR PEER REVIEW 32 of 40 Minerals 2019, 9, 789 32 of 40 inclusions and only through local methods, such as EPMA [93–96]. The binding forms of Au, Ag, Hg, Sb, As, Pb and Zn, the basic element indicators for the Au–Ag mineralization type, were methods, such as EPMA [93–96]. The binding forms of Au, Ag, Hg, Sb, As, Pb and Zn, the basic investigated. element indicators for the Au–Ag mineralization type, were investigated. Gold in alluvial sediments, as well as in ores, is represented by electrum micro- and Gold in alluvial sediments, as well as in ores, is represented by electrum micro- and fine-dispersed fine-dispersed particles, which are heterogeneous in composition and reduced in fineness particles, which are heterogeneous in composition and reduced in fineness (330–480%). Electrum (330–480‰). Electrum submicron inclusions of the same fineness were identified in pyrite. Pyrite is a submicron inclusions of the same fineness were identified in pyrite. Pyrite is a main concentrator of main concentrator of Au in ores and a carrier in halo zones, including loose sediments of LSSs. In Au in ores and a carrier in halo zones, including loose sediments of LSSs. In this sulphide, 90% or this sulphide, 90% or more is shared with the “invisible” Au. A high content of admixed Au (up to more is shared with the “invisible” Au. A high content of admixed Au (up to 0.41 wt %) was detected 0.41 wt %) was detected in arsenic pyrite, which is still lower than in pyrite of bryolithochemical in arsenic pyrite, which is still lower than in pyrite of bryolithochemical samples (see Section 4.3). samples (see Subsection 4.3). In addition to pyrite, the mineral concentrators of “invisible” Au In addition to pyrite, the mineral concentrators of “invisible” Au include Fe hydroxides (up to 0.91 wt %) include Fe hydroxides (up to 0.91 wt %) and silver minerals—destroyed acanthite grains (up to 0.73 and silver minerals—destroyed acanthite grains (up to 0.73 wt %) and sternbergite (up to 0.70 wt %). wt %) and sternbergite (up to 0.70 wt %). Gold, bound with Fe hydroxides and sulphide minerals, Gold, bound with Fe hydroxides and sulphide minerals, after their destruction and oxidation, is easily after their destruction and oxidation, is easily leached by conventional natural acids, passes into a leached by conventional natural acids, passes into a mobile form and has a significant impact on the LSS mobile form and has a significant impact on the LSS formation of this element, even in the conditions formation of this element, even in the conditions of cryolithogenesis zones. Contrary to the traditional of cryolithogenesis zones. Contrary to the traditional view that the main role in the LSS formation in view that the main role in the LSS formation in permafrost regions is played by physical weathering, permafrost regions is played by physical weathering, it can be argued that no less important role it can be argued that no less important role belongs to the processes of chemical weathering (dissolution, belongs to the processes of chemical weathering (dissolution, oxidation, sorption, and oxidation, sorption, and chemisorption). This refers not only to Au but also to other element indicators chemisorption). This refers not only to Au but also to other element indicators of Au–Ag of Au–Ag mineralization, which are discussed below. mineralization, which are discussed below. Silver in LSSs, as well as in ores, is the most prevalent. Its mineral BFs are represented mainly by Silver in LSSs, as well as in ores, is the most prevalent. Its mineral BFs are represented mainly acanthite, the grains of which are corroded and oxidized to varying degrees. A significant part of them by acanthite, the grains of which are corroded and oxidized to varying degrees. A significant part of has a hypergene appearance. As impurities in acanthite, Pb, Cu and Se are observed (Figure 32, Table6). them has a hypergene appearance. As impurities in acanthite, Pb, Cu and Se are observed (Figure 32, Native silver, kustelite and sternbergite are much rarer. Native silver, as well as Au, is represented by Table 6). Native silver, kustelite and sternbergite are much rarer. Native silver, as well as Au, is finely dispersed particles (<10 µm). Hypergene segregation of native Ag and sternbergite was often represented by finely dispersed particles (<10 μm). Hypergene segregation of native Ag and observed during the decomposition of silver-containing base-metal minerals. Relic segregation of sternbergite was often observed during the decomposition of silver-containing base-metal minerals. acanthite–sternbergite was identified in the Fe hydroxides. Among the Ag sulphosalts, pyrargyrite Relic segregation of acanthite–sternbergite was identified in the Fe hydroxides. Among the Ag and proustite were observed—minerals that are characteristic only of the supra-ore and upper-ore sulphosalts, pyrargyrite and proustite were observed—minerals that are characteristic only of the horizons of the Dukat deposit and that, with a sufficient degree of probability, can speak in favor of the supra-ore and upper-ore horizons of the Dukat deposit and that, with a sufficient degree of positive prediction of deep mineralization. probability, can speak in favor of the positive prediction of deep mineralization.

FigureFigure 32.32. CorrodedCorroded andand oxidizedoxidized acanthiteacanthite grainsgrains inin looseloose sedimentssediments ofof watercourseswatercourses drainingdraining thethe Au–AgAu–Ag zoneszones ofof thethe DukatDukat deposit.deposit. The imagesimages areare presented:presented: (a,d)—in backscattered electrons; ((bb,,cc,,ee,,ff)—in)—in X-rays.X-rays. In In Figure Figure 32 ( 32a,d ):(a 1–20—the,d): 1–20—the identiﬁcation identification points poin of thets elementof the contentselement indicatedcontents inindicated Table6. in Table 6.

Minerals 2019, 9, 789 33 of 40

Table 6. Element composition (wt %) of corroded and oxidized acanthite grains in loose sediments of watercourses draining the Dukat deposit’s Au–Ag zones.

No. Ag Fe Pb Cu Se S Total Acanthite 1 84.87 0.53 <0.30 <0.30 <0.20 14.05 99.45 2 85.95 0.42 < < 0.29 14.05 100.71 3 88.39 0.35 < < <0.20 12.70 101.44 4 83.29 0.31 < < 0.49 12.45 96.54 5 84.79 0.42 < < 1.64 13.46 100.31 Weakly corroded part of acanthite grain 6 78.85 6.59 < < 0.51 12.64 98.59 7 72.79 10.25 < < <0.20 12.54 95.58 8 72.27 11.13 < < < 13.12 96.52 9 70.47 11.74 < < < 13.99 96.20 10 68.86 12.11 < < < 13.62 94.59 Acanthite 11 81.08 3.59 < 0.55 < 13.49 98.71 12 79.25 5.38 < 0.37 < 13.87 98.87 Acanthite + Fe hydroxides 13 42.25 26.47 0.39 1.14 < 9.79 80.04 14 53.79 20.68 0.52 1.40 < 9.43 85.82 15 46.74 19.41 0.92 1.45 < 9.39 77.91 16 46.97 25.09 <0.30 0.99 < 8.06 81.11 Fe hydroxides + acanthite 17 4.97 54.44 2.23 0.38 < 0.82 62.84 18 5.58 54.73 1.82 0.36 < 1.25 63.74 19 3.92 55.50 2.24 0.35 < 0.66 62.67 20 7.81 53.43 2.02 0.31 < 1.48 65.05 Note: The elements Au, Hg, Sb, As, Zn, Ni, Co, W, Mo, Sn, Bi, Ge, Te and Mn were not detected. For the point location of the composition determination, see Figure 32a,d.

Mercury is one of the important element indicators accompanying Au–Ag mineralization. It is typical of supra-ore and upper-ore horizons. This element’s main BFs in ores are adsorbed and isomorphous ones. The major mineral concentrators are sphalerite, galena, Ag sulphosalts, argentite, native Ag, kustelite and electrum. The Hg sulphide form, cinnabar, observed in other Au–Ag deposits in the north–east of Russia, was identiﬁed neither in ores nor in the halos of the Dukat deposit’s Au–Ag zones [56,102]. In the alluvial sediments of watercourses draining Au–Ag ores, along with relics of primary minerals containing Hg up to 1 wt %, secondary minerals are widely prevalent. These are Fe oxides and hydroxides containing up to 0.5 wt % Hg and secondary formations of Ag, Pb and Zn containing up to 0.2 wt % Hg. Mercury’s admixture is often associated with corroded grains of acanthite and sternbergite and more rarely with kustelite and native silver. Antimony and arsenic, as well as Hg, are typical of the supra-ore and upper-ore horizons of Au–Ag ore zones [56,102]. Despite the fact that, in the area of the Dukat deposit, these elements form a very contrasting LSS, their own mineral forms in alluvium are extremely rare, mainly appearing as pyrargyrite. Sb and As were identified as impurities in pyrite, chalcopyrite and Ag sulphides. These elements’ adsorbed forms are widely prevalent in Fe hydroxides. In addition, grains corresponding to pitticite composition were detected; this in turn is a product of arsenopyrite destruction, forming in hypergene environments in poorly soluble forms. Arsenopyrite, which is not typical of volcanogenic Au–Ag ores, has a fairly wide distribution in the Dukat deposit, mainly in the hosting rocks in the development zones of areal sulphide mineralization. Its appearance in the stream sediments indicates the influence of the hosting rocks on their formation and explains the wide manifestation of As AGFs throughout the entire Minerals 2019, 9, 789 34 of 40 deposit area. It can also explain the fact that a significant part of the As anomalies detected in the territory of the Dukat OFS are related not only to Au–Ag but in general to profitable ore mineralization. Lead in loose alluvial sediments of LSSs is rarely manifested as individual grains, mainly either in the rim of cerussite–anglesite or in relic form. The association of anglesite with acanthite is observed (see Table6). Fe hydroxides are often found, forming joint rims around galena and sometimes creating thin concentric zonal and looped structures. Galena needles are met, developing in largely corroded magnetite. In some cases, these segregations are developed quite intensively and almost completely replace the entire grain of magnetite. Zinc, against the background of relatively low contents in LSSs, is represented most often by an admixture, mainly in Fe hydroxides and in pyrite. It is extremely rare for its own mineral forms to be observed and to varying degrees, oxidized and corroded single crystals of sphalerite or intermetallic compounds of Zn with Cu. Similar compounds have been found in primary ores [112]. Most often sphalerite is in the rim of Ag-containing minerals. Comparing the BFs of Au, Ag and their accompanying elements (Hg, Sb, As, Pb and Zn) in LSS loose alluvial sediments of the Dukat deposit’s Au–Ag zones, we can see that their material composition is closely related to the Au–Ag ores’ composition (see Section 2.2). In watercourses’ alluvial sediments and in the ores, the ﬁnely dispersed native Au (electrum), “invisible” Au in sulphides, native Ag, kustelite, silver sulphides (acanthite and sternbergite) and silver sulphosalts (pyrargyrite), pyrite, galena, and sphalerite are the main ones.

5. Conclusions Surveys of LSSs on the 1:200,000 scale in the conditions of cryolithogenesis zones in the mountainous regions of the north-east of Russia are the most effective in poorly studied territories, where information about the geochemical features and metallogeny of the region as a whole is absent. A regional survey of LSSs allows basic information about the metallogeny of the entire region to be obtained in a relatively short time. Metallogenic specialization in the studied Au–Ag area is distinctly manifested only in respect of Ag, Pb and Sn. Small-scale surveys of LSSs allow to establish the regional geochemical zonality: less eroded areas in the west (the Dukat Au–Ag OFS) are characterized by Ag, Ag–Pb and Sn–Ag AGFs and more eroded areas in the east (associated with Dukat Au–Ag OFS areas) by Sn, Sn–W and Mo–W AGFs. However, there are significant drawbacks. Exogenous AGFs detected from such surveys have lower contrast. A decrease in the fields’ natural contrast was identified, resulting in the simplification of their main component composition in the direction of the primary halo secondary halo → → stream sediments. In some cases, violation of the qualitative and quantitative functional relationships happens between the elements. All this complicates the study of the composition, construction and AGF development scope. In other words, the exogenous AGF typification and zonality identification become difficult—the main indicator used to assess the level of ore zones’ erosive profile, the assessment of the mineralization type and, as a consequence, the estimation of their profitable value become less efficient. The survey of LSSs on the 1:50,000 scale carried out within the Dukat Au–Ag deposit and associated area showed that, in the interpretation of such anomalies at the stage of prospecting and evaluation, the sampling of first-order watercourses is effective. Alluvial sediments of watercourse heads are characterized by the highest element concentrations, primarily Au. As a result, the number of anomalies revealed increases significantly, and increased concentrations of elements in channel sediments, in most cases, are directly dependent on the presence and type of drained ore mineralization. Besides, the AGFs revealed during the survey of LSSs on the 1:50,000 scale are fully consistent with the composition and construction of eroded and drained ore objects, and the identified exogenous geochemical zonality is a reflection of the endogenous one. However, there is an important circumstance, which makes such survey difficult to carry out. The alluvial sediments of the first-order watercourses required for sampling are fragmentary, absent or Minerals 2019, 9, 789 35 of 40

formed with a sharp deficit of sandy silt material concentrating most of the ore elements. Discontinuity and extremely uneven distribution of mineralization element indicators while transporting material are typical of LSSs. This is especially true for cryolithogenesis zones, where physical weathering is considered to be the main process forming LSSs. In such situations, a positive result can be obtained using the bryolithochemical method that we developed. It not only allows researchers to carry out sampling in watercourse heads, where alluvial sediments are poorly or not at all formed, but also makes it possible to obtain more reliable information about the anomalies’ presence or absence. In the example of the behavior of Au and Ag in bryolithochemical stream sediments, which are the main element indicators of the Au–Ag mineralization type, it was found that their concentration in bryolithochemical samples is considerably higher than in lithochemical ones. It was shown that the element indicator BFs in BSSs and LSSs can be used as an effective additional criterion for the interpretation and evaluation of the identified anomalies. It was established that these BFs, generally agree with these elements’ BFs in ores, and sufficiently reflect the peculiarities of their mineral and geochemical composition. It was shown that moss cushions are an effective natural trap for finely dispersed fractions of alluvium. They extract not only small but also finely dispersed particles of minerals, up to submicron ones, from the water suspension of actual watercourses, even temporary ones. The implemented investigations have shown that the revealed regularities can be applied successfully at all stages of the geochemical study of ore-bearing areas, from expectation assessment in poorly studied territories to detailed work on already-known ore objects. The proposed methods for exogenous AGFs’ identification and the criteria developed for their evaluation are quite simple and effective. They may also be used at all stages of prediction and prospecting for ore mineralization, primarily Au–Ag, not only in the north–east of Russia but also in other regions where the LSS formation occurs in cryolithogenesis zones.

Author Contributions: Conceptualization, R.G.K.; methodology, A.S.M. and R.G.K.; formal analysis, A.S.M. and V.V.T.; investigation, R.G.K., A.S.M. and V.V.T.; writing–original draft preparation, A.S.M.; writing–review and editing, R.G.K.; visualization, A.S.M. and R.G.K. Funding: This study was performed within the framework of the State Assignment Project No. IX.130.3.1. (0350-2019-0010), with support from the Russian Federation President Foundation (Grant No. MK-2645.2019.5) and the Russian Foundation for Basic Research (Grant No. 18-33-00369). We used the scientific equipment of the “Isotope-Geochemical Research” Center for Common Use of IGC SB RAS, Irkutsk, Russia. Acknowledgments: The authors are grateful to the staff of IGC SB RAS (Irkutsk) for their assistance in analytical studies: I.E. Vasileva, E.V. Shabanova, S.S. Vorobyeva, N.E. Smolyanskaya, N.L. Chumakova (spectral analysis); S.Ts. Rinchinova, L.D. Andrulaitis, P.T. Dolgikh, V.N. Vlasova, T.S. Krasnoshchekova, O.S. Ryazantseva, E.V. Matveeva (atomic absorption analysis); and L.A. Pavlova (EPMA). The authors would like to express special thanks to the leadership of the PGO “Sevvostokgeologiya” (Magadan) for providing geological documentation and survey material on the 1:200,000 scale. The authors are also sincerely grateful to all the geological services of the Dukat geological exploration enterprise (Magadan) for their comprehensive assistance in performing fieldwork and their technical support. Conflicts of Interest: The authors declare no conflict of interest.

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