Granitoids of the Ergelyakh Intrusion-Related Gold–Bismuth Deposit (Kular-Nera Slate Belt, Northeast Russia): Petrology, Physicochemical Parameters of Formation, and Ore Potential

This paper describes features of petrographic and chemical compositions and isotopic dating of the Ergelyakh and Sokh plutons, located within the Kular-Nera slate belt, Verkhoyansk-Kolyma folded region (VKFR), Northeast Russia. Intrusion of the massifs took place approximately 145–150 million years ago. Different isotopic systems on the whole rock samples and mineral separates record at least two stages of later tectono-magmatic activity 130–120 and 110–100 million years ago. Granitoid magmas for the Ergelyakh and Sokh plutons were formed at high temperatures (950–1060 °C) within the amphibolitic lower crust of an island arc setting. The ages of crustal protoliths for granitoids of the Ergelyakh intrusion-related gold–bismuth deposit, calculated on Rb–Sr and Sm–Nd two-stage models, are 1109–1383 and 1199–1322 million years, respectively. Formation of the Ergelyakh and Sokh plutons took place within a significant temperature interval (<450 to 901 °C) and, with regard to the superposition of later events, lasted for a long time. During the cooling process of granitoid melts, at the time of biotite crystallization in both massifs, a significant increase of oxygen fugacity was registered. The ore potential of granitoids of both massifs seems to be similar, but due to some differences in the physicochemical parameters of their formation (redox conditions), it was partially realized only within the Ergelyakh massif with the generation of several minor intrusion-related gold–bismuth deposits. Granitoid melts of the Ergelyakh massif were formed in relatively heterogeneous and oxidizing conditions (∆Ni–NiO = +3.26 to –3.60). Granitoid melts for the Sokh massif (∆Ni–NiO = –2.88 to –9.27) were formed in reducing conditions.

Various magmatic bodies are found in the fault zone, including dolerite, andesite, diorite porphyry, and basalt dikes, and small granitoid plutons, of which the largest are the Ergelyakh, Sokh, Kurdat, Samyr, and Saryllakh massifs (from SE to NW) forming ore clusters [40][41][42][43][44][45]. A series of intrusive granitoid bodies of the EIRGD extend northeasterly across the fold structures following the strike of the Ergelyakh transverse fault (Figures 1 and 2) [6]. The massifs are located in the eastern limb of the Malo-Taryn syncline. The largest is the Ergelyakh massif (50 km 2 ). Other plutons are no larger than 2 km 2 , including the Sokh stock, about 1.7 km 2 in area. Some massifs form a single granodiorite-granite body at depth. The granitoids intrude into the Norian and Carnian sedimentary formations, which are metamorphosed to biotite and biotite-cordierite hornfels in exocontact zones of the plutons. The Carnian rocks include sandstones, siltstones, and clay shales. The Norian deposits are divided into the lower (sandstones and shales) and upper (shales and siltstones) substages. The alteration of the rocks is represented by greisenization.
The Ergelyakh massif has a zonal structure. Granodiorites (42%) make up the periphery of the massif, adamellites (20%) its apical part, and leucocratic granites (38%) the core. Dikes and veins of aplitic granites and aplites are widely developed, and pegmatite bodies occur locally. Relationships between various rock types of the massif are not well understood. Both sharp contacts and gradual transitions are observed.
The Sokh massif is composed of granodiorite porphyries intruded by leucocratic granites with an aplitic margin [6]. The aplite veins in the granodiorites of the massif transect quartz veinlets and are in turn cut by them. Between the Ergelyakh and Sokh massifs there are widespread dikes and veins of leucocratic and aplitic granites and aplites.

Petrography of Igneous Rock of the Ergelyakh and Sokh Massifs
Granitoids of the Ergelyakh massif are represented by light gray equigranular rocks with a grain size up to 0.5 cm. They have a hypidiomorphic granular texture with elements of porphyritic texture ( Figure 3A). The mineral contents of the granodiorites are plagioclase (42-60%), orthoclase (8-19%), quartz (17-35%), and biotite (7.5-16%). Plagioclase is present as phenocrysts of prismatic form with signs of sericitization both in the center and at the periphery of crystals ( Figure 3B). It is zonal, with 33-45% An in the core and 15-31% An in the rim. K-feldspar occurs as porphyric crystals, rarely of microperthitic texture (albite ingrowths in orthoclase) ( Figure 3). Very rarely, the grain cores are partly replaced by the secondary alteration products (pelitization), which gives them a light-brownish color. Quartz has an irregular form and occurs interstitially between the grains of salic minerals. Dark-colored minerals are dominated by biotite, which occurs in two generations ( Figure 3C). The first generation is represented by idiomorphic frequently chloritized porphyric grains, and the second generation occurs as small irregular grains.
Granites make up the central part of the massif and also form numerous dike-like bodies in adamellites of the apical zone of the massif and in sedimentary rocks beyond its limits. Their contacts with granodiorites are both gradual and sharp. The quantitative mineralogical compositions of the granites are plagioclase (30-36%), K-feldspar (30-40%), quartz (25-34%), and biotite (3-6%). They have a hypidiomorphic granular texture ( Figure 3G). Plagioclase is present as prisms of irregular form, often as polysynthetic and simple twins or zonal individuals, and is partly sericitized ( Figure 3H). The An content is 20-30% in the core and 4-13% in the rim. K-feldspar occurs as irregular and rectangular crystals, is weakly pelitized, and has a perthitic texture ( Figure 3I). Quartz forms grains of various forms and sizes (up to 2 mm) with mosaic and wavy extinction. Biotite has isomorphous form, a characteristic pleochroism, and straight, sometimes wavy extinction.
Aplitic granites and aplites of the veined facies are widespread in both the granitoids and the enclosing sedimentary rocks. They are made of plagioclase (22-28%), K-feldspar (29-41%), quartz (36-42%), and biotite (0.1-0.7%). They have a granitic, aplitic texture ( Figure 3J). Plagioclase is unzonal (5-12% An) and partly sericitized ( Figure 3K). K-feldspar is perthitized and contains inclusions of plagioclase and granular quartz ( Figure 3G). Quartz in the form of irregular and rounded grains is included in plagioclase and K-feldspar, and also forms small veinlets ( Figure 3L). Rare inclusions of reddish-brown biotite are found in all of the rock-forming minerals.
The Sokh massif is composed of granodiorite porphyries, adamellites, granites, and aplites. Granodiorite porphyries are represented by phenocrysts of light-gray plagioclase (up to 1 cm in size) set in the fine-grained groundmass ( Figure 4A). The mineral contents of the rocks are plagioclase (41-44%), K-feldspar (17-23%), quartz (25-26%), and biotite (9-13%). Plagioclase is present in two generations. The early generation is represented by porphyric prismatic crystals. Simple and polysynthetic twins ( Figure 4B) and zonal individuals are observed. Most of the crystals show evidence of insignificant sericitization both in the core and at the periphery. Plagioclase of the late generation is observed in the groundmass. Cores of the crystals contain 50-55% An and the rims contain 23-45% An. K-feldspar forms xenomorphic phenocrysts in the rock matrix. The amount of albite component in it is 15.4%. Quartz forms grains of various forms and sizes (up to 2 mm). Biotite is reddish-brown in color, occurs in peripheral zones of plagioclase, and is often corroded by quartz. It is normally partly chloritized on cleavage or in the form of spots ( Figure 4C).  The mineral contents of adamellites are plagioclase (31-50%), K-Na-feldspar (16-30%), quartz (up to 15%), and biotite (up to 5%). The texture of the rocks is hypidiomorphic granular with porphyric elements ( Figure 4D). Two generations of plagioclase are present. The first generation includes intensely sericitized porphyric prismatic crystals represented by polysynthetic and zonal individuals ( Figure 4E). The An content is 20-30% in the core and 4-13% in the rim. Plagioclase of the second generation occurs in interstices between porphyric grains of K-feldspar and plagioclase. Kfeldspar is also of two generations. The early generation includes irregular porphyric grains, and the The mineral contents of adamellites are plagioclase (31-50%), K-Na-feldspar (16-30%), quartz (up to 15%), and biotite (up to 5%). The texture of the rocks is hypidiomorphic granular with porphyric elements ( Figure 4D). Two generations of plagioclase are present. The first generation includes intensely sericitized porphyric prismatic crystals represented by polysynthetic and zonal individuals ( Figure 4E). The An content is 20-30% in the core and 4-13% in the rim. Plagioclase of the second generation occurs in interstices between porphyric grains of K-feldspar and plagioclase. K-feldspar is also of two generations. The early generation includes irregular porphyric grains, and the later one is represented by small grains set in the groundmass. Biotite is tabular and irregular in form. Along with quartz, it fills interstices between salic minerals of the first generation or includes K-feldspar and plagioclase of the later generation ( Figure 4F). Minerals 2019, 9, x FOR PEER REVIEW 7 of 32 later one is represented by small grains set in the groundmass. Biotite is tabular and irregular in form. Along with quartz, it fills interstices between salic minerals of the first generation or includes Kfeldspar and plagioclase of the later generation ( Figure 4F). The quantitative mineralogical composition of the grains is plagioclase (30-45%), K-feldspar (up to 30%), quartz (up to 20%), and biotite (up to 5%). The texture is hypidiomorphic granular and granitic ( Figure 4G). Plagioclase of the first generation occurs as prisms of irregular form, and is mainly represented by fractured zonal individuals that are partly sericitized ( Figure 4H). The secondgeneration plagioclase is found in the groundmass in the form of simple and polysynthetic twins. An content amounts to 20-30% in the core and 4-13% in the rim. Irregular and rectangular crystals of K- The quantitative mineralogical composition of the grains is plagioclase (30-45%), K-feldspar (up to 30%), quartz (up to 20%), and biotite (up to 5%). The texture is hypidiomorphic granular and granitic ( Figure 4G). Plagioclase of the first generation occurs as prisms of irregular form, and is mainly represented by fractured zonal individuals that are partly sericitized ( Figure 4H). The second-generation Minerals 2019, 9, 297 8 of 35 plagioclase is found in the groundmass in the form of simple and polysynthetic twins. An content amounts to 20-30% in the core and 4-13% in the rim. Irregular and rectangular crystals of K-feldspar are weakly pelitized. Quartz forms grains of various forms and sizes with mosaic and/or wavy extinction. Biotite occurs as isomorphous grains of two generations. The first generation includes large individuals present in interstices between porphyric grains of K-feldspar and plagioclase, and the second is represented by late-magmatic biotite in the groundmass ( Figure 4I).

Geology of the Ergelyakh Gold-Bismuth Deposit
The EIRGD occurs above the cupola of the Ergelyakh granitoid massif [6]. The granitoids and adjacent biotite hornfels are cut by en echelon lens-like, steeply dipping quartz veins up to 1 m thick and 250 m long. The main ore bodies contain a successive series of mineral associations: muscovitetourmaline-quartz metasomatic, wolframite-tourmaline-quartz, pyrrhotite-loellingite-danaitearsenopyrite, and bismuth-sulfotelluride [6]. Finely disseminated (0.006-0.1 mm) native gold is present in sulfoarsenides in the amount of 50-150 g/t, and free small-size gold with widely ranging fineness (750-960% ) is associated with bismuth minerals. As-bearing minerals are Co-Ni-loellingite, gersdorffite, Ni-danaite, and arsenopyrite. The late gold-bearing association of bismuth minerals includes tetradymite, A-joseite, B-joseite, and tellurobismuthite. Within the deposit and in near intrusive zones, polymetallic gold-silver mineralization has been reported [6].
Early quartz of the Ergelyakh deposit crystallized at 265-305 • C and 0.2 kbar from dilute Na and Mg chloride solutions with salinity of 4.5 wt. % to 8.6 wt. % NaCl equivalent [46]. Quartz from productive Au-bearing associations are characterized by inclusions of concentrated Na-Ca chloride solutions with salinity of 32.9-32.7 wt. % NaCl equivalent, which homogenize to liquid at temperatures of 360-255 • C, and by inclusions of Na-Mg chloride solution (3.7-6.9 wt. % NaCl equivalent) with homogenization temperatures from 360 to 190 • C and a pressure of 0.06 kbar [44]. The depth of mineralization formation is 1-2 km [1].

Chemical Composition of Rock-Forming Minerals
The most informative rock-forming minerals of the studied massifs are K-feldspars, plagioclase, and biotite. In our study we paid special attention to them. The data obtained show that K-feldspar in all granitoid types is characterized by wide variations in orthoclase (54.5-93.2%) and plagioclase (11.7-40.8%) components (Tables 1 and 2). Plagioclase is high in orthoclase component (2.4-8.4%) irrespective of its basicity and the type of granitoids. The content of anorthite mineral in plagioclase ranges from 42.0% to 7.6%, i.e., from andesine to albite, indicating that formation of plagioclase from the EIRGD granitoids took a long time and occurred under nonequilibrium conditions. Chemically, it contains, on average, 25-33% anorthite and 4.6-6.3% orthoclase components.
Garnets from the Sokh massif do not exhibit a distinct trend in composition. The amount of almandine in garnets from granitoids (Table 4) varies within a narrow range (69.7-78.2%). Garnets from adamellites are somewhat higher in almandine (70.8-78.2%) and those from granodiorites and aplitic granites have lower almandine content: 71.7-72.9% and 69.7-71.8%, respectively. A more pronounced trend is observed for spessartine in garnets, which is high in granodiorite porphyries and aplitic granites (24.6-25.6%) and low in adamellites (1.7-5.5%). Garnets from adamellites are also marked by elevated pyrope mineral content (13.5-22.0%), which is much higher than in granodiorites and aplitic granites (0.4%). This promotes differences in Mg# value, which is higher in garnets from adamellites (0.02-0.23) than from granodiorites and aplitic granites (0.03-0.14). Garnets from adamellites also exhibit a higher degree of iron oxidation (Fe 3+ /ΣFe = 1.2-2.6) as compared to granodiorites and aplitic granites, which is zero. These differences in the composition of garnets are due to different conditions of their formation.
A higher content of spessartine mineral and a lower amount of pyrope component may indicate relatively low pressures during the rock emplacement [48].
Granitoids of the early phases of both massifs differ in Li and Rb contents (Table 5). In granodiorites and adamellites of the Ergelyakh massif, the Li content is 52.8 ± 7.2 and 45.01 ± 5.3 ppm, and in analogous rocks of the Sokh massif it is 85 ± 2.9 and 73.5 ± 2.1 ppm, respectively. In granites and aplites of both massifs, the Li content is nearly identical: 46.9 ± 22.5 and 18.5 ± 11.0 ppm in the Ergelyakh massif and 45.3 ± 8.0 and 18.6 ± 5.6 ppm in the Sokh massif, respectively ( Figure 5D). In general, there is a tendency for decreased Li content from early to late phases of the two massifs. No regularity is observed in Rb distribution in granitoids of different phases of the massifs. Its content is somewhat higher in the Sokh massif rocks. Granodiorites and adamellites of the Ergelyakh massif contain 108.3 ± 18.3 and 101.4 ± 37.1 ppm Rb, respectively, and analogous rocks of the Sokh pluton contain 132 ± 20 and 147.6 ± 18.7 ppm Rb. However, average Rb content in the granites of the Ergelyakh massif (158.2 ± 21) is higher than in those of the Sokh massif (125.3 ± 7.5). In aplites from these massifs, Rb content is rather close (117.9 ± 21 and 125.6 ± 7.5 ppm, respectively). The K/Rb value for granitoids of the Ergelyakh massif increases from granodiorites (227 ± 27) through adamellites (277 ± 84) and granites (244 ± 17) to aplites (362 ± 89). A similar tendency for K/Rb variation is characteristic of the Sokh massif granitoids: granodiorites (254 ± 36) and adamellites (232 ± 2) have low K/Rb ratios, while granites (277 ± 11) and aplites (372 ± 51) exhibit higher values. On the K/Rb-Rb diagram (Figure 6), granitoids of the studied massifs do not follow a general trend typical of granitoids differentiated from a single magma chamber, but rather form a series of individual fractionation trends for the rocks of different phases in the massif emplacement. The K/Rb ratio increases from granitoids of the early to late phases, which corresponds to anatectic granites [49].  Granitoids of the Ergelyakh massif have a generally close Sr content (granodiorites, 259 ± 53 ppm; adamellites, 238 ± 136 ppm; granites, 244 ± 17 ppm), with a sharply decreased aplite content (74 ± 51 ppm). In the Sokh massif, Sr content in granodiorites is 235 ± 21 ppm, close to that in analogous rocks of the Ergelyakh pluton, but decreases from adamellites (180 ± 21 ppm) through granites (117 ± 50 ppm) to aplites (58 ± 33 ppm). The Rb/Sr ratio varies in different phases of the massifs depending on the distribution of these elements in the rocks. A tendency is observed for an increase in this value from granodiorites to aplites: 0.41 ± 0.1 and 0.57 ± 0.14 in granodiorites, 0.55 ± 0.31 and 0.84 ± 0.24 in adamellites, 2.43 ± 1.38 and 1.28 ± 0.77 in granites, and 2.51 ± 1.85 and 2.87 ± 1.55 in aplites of the Ergelyakh and Sokh massifs, respectively.
The distribution of the discussed elements in granitoids is controlled by the mineral content of the rocks and the amounts of these elements in the minerals. Particularly, K-feldspar and biotite are the main carriers of Rb in the rocks. K-feldspars and biotites from granodiorites of the Ergelyakh massif contain 199 ± 58 and 544 ± 68 ppm Rb, respectively. In adamellites, these minerals are richer in Rb (258 ± 93 and 568 ± 65 ppm). The highest Rb values are found in K-feldspar and biotite from granites (514 ± 334 and 665 ± 191 ppm). Sr in the EIRGD granitoids is mainly concentrated in feldspars. K-feldspars from the Ergelyakh massif granodiorites contain 257 ± 57 ppm Sr; from adamellites, 150 ppm; and from granites, 134 ± 93 ppm. Plagioclase from granodiorites is high in Sr (417 ± 104 ppm) Granitoids of the Ergelyakh massif have a generally close Sr content (granodiorites, 259 ± 53 ppm; adamellites, 238 ± 136 ppm; granites, 244 ± 17 ppm), with a sharply decreased aplite content (74 ± 51 ppm). In the Sokh massif, Sr content in granodiorites is 235 ± 21 ppm, close to that in analogous rocks of the Ergelyakh pluton, but decreases from adamellites (180 ± 21 ppm) through granites (117 ± 50 ppm) to aplites (58 ± 33 ppm). The Rb/Sr ratio varies in different phases of the massifs depending on the distribution of these elements in the rocks. A tendency is observed for an increase in this value from granodiorites to aplites: 0.41 ± 0.1 and 0.57 ± 0.14 in granodiorites, 0.55 ± 0.31 and 0.84 ± 0.24 in adamellites, 2.43 ± 1.38 and 1.28 ± 0.77 in granites, and 2.51 ± 1.85 and 2.87 ± 1.55 in aplites of the Ergelyakh and Sokh massifs, respectively.
The distribution of the discussed elements in granitoids is controlled by the mineral content of the rocks and the amounts of these elements in the minerals. Particularly, K-feldspar and biotite are the main carriers of Rb in the rocks. K-feldspars and biotites from granodiorites of the Ergelyakh massif contain 199 ± 58 and 544 ± 68 ppm Rb, respectively. In adamellites, these minerals are richer in Rb (258 ± 93 and 568 ± 65 ppm). The highest Rb values are found in K-feldspar and biotite from granites (514 ± 334 and 665 ± 191 ppm). Sr in the EIRGD granitoids is mainly concentrated in feldspars. K-feldspars from the Ergelyakh massif granodiorites contain 257 ± 57 ppm Sr; from adamellites, 150 ppm; and from granites, 134 ± 93 ppm. Plagioclase from granodiorites is high in Sr (417 ± 104 ppm) and that from adamellites contains 177 ± 209 ppm Sr. K/Rb ratios in K-feldspars and plagioclases of the massif show a similar trend. K-feldspars from granodiorites have the highest K/Rb ratio (538 ± 102).

Isotope Systematics and Ages of the Massifs
The geologic age of magmatism at the EIRGD is not well constrained. The lower age limit is defined by the age of the overlying sedimentary rocks (T3-J1). The oldest manifestations of magmatic activity in the region are mafic dikes (Rb-Sr, 145-162 Ma) of the orogenic Malo-Tarynskoe gold deposit located north of the EIRGD [41]. The upper age limit of magmatism at the EIRGD is not established. The first K-Ar and Rb-Sr isotope dates obtained from granitoids of the Ergelyakh and Sokh massifs were discussed previously [6]. Based on these data, we compiled a summary of isotope ages of the rocks in these massifs determined by different isotope methods (Table 6).
Isotope ages of the EIRGD granitoids, irrespective of the determination method, vary within a wide range . This is probably due to the effects of various geological processes, including hybridism and assimilation of the enclosing rocks (sample 204, 196 ± 5 Ma) and the younging of isotope dates as a result of superposed later processes. This is discussed in detail in [6], where a conclusion is made that formation of the EIRGD granitoids occurred in the time interval 160-145 Ma with the subsequent cooling of the thermal field until 136 Ma. These results are in agreement with Ar-Ar ages of the Ergelyakh pluton (biotite, 142.9 ± 0.4 Ma [50]) and Rb-Sr ages of the Samyr and Kurdat plutons (bulk sample, 140-145 Ma [10]) and other intrusives of the Tas-Kystabyt plutonic belt [50,51].
In the interval 125-120 Ma, a new episode of tectono-magmatic activity occurred at the EIRGD as inferred from Rb-Sr dates of biotites from the Ergelyakh massif granitoids and K-Ar ages of K-feldspars from adamellites of the Sokh pluton. One of the late geological events probably took place at 110-100 Ma. These processes, likely of hydrothermal-tectonic nature, caused destruction of the isotopic systems of minerals and rocks. The possibility of a late superposed process is confirmed by young Sr biotite dates (101-106 Ma) and K-Ar plagioclase ages obtained from some EIRGD granitoid samples. The process may be synchronous with the manifestation of hydrothermal metamorphism. For example, sericite-quartz from a quartz vein with wolframite and bismuthine has a K-Ar age of 114 Ma. These events are associated with subduction-accretion processes in the rear part of the Uda-Murgal and Okhotsk-Chukotka magmatic belts of the East Asian active continental margin.
The initial Sr isotope composition (I 0 ) for the EIRGD granitoids varies within a wide range (0.7065-0.7093). Sr isotope heterogeneity may be due to primary and secondary reasons. Primary isotope heterogeneity (I 0 = 0.707-0.708) may be related to the heterogeneous composition of the substratum, whose melting occurred without homogenization of magma upon ascent to the emplacement level and caused local isotope microheterogeneity. Secondary isotope heterogeneity in granitoids (I 0 > 0.708) was likely caused by late superposed processes conditioned by a long-term tectono-magmatic evolution of the whole ore-magmatic system of EIRGD. In general, there is a reverse correlation between the Rb-Sr dates of the rocks and minerals and the I 0 value, which is indicative of the evolution of the Sr isotope composition of granitoids under conditions of a thermostatic ore-magmatic system. I 0 values obtained for the EIRGD granitoids (0.707-0.709) suggest their lower crustal protoliths, which agrees well with the Nd isotope composition (εNd(T) = -4.4 and -2.90) of the Ergelyakh massif magmatites [52].

Petrogenesis
Petrogenetically, the EIRGD granitoids are transitional between the S-and I-types (Figure 7).

Petrogenesis
Petrogenetically, the EIRGD granitoids are transitional between the S-and I-types (Figure 7). To determine the primary sources of the EIRGD granitoids, we recalculated the data on the chemical composition of rocks according to [54] and plotted them on a discrimination diagram (Figure 8), where the fields of magma-generating substrata are outlined from the results of experimental studies [54]. Data points for granodiorites and adamellites of both massifs plot into the field of amphibolites just as those for the Sokh massif granites, whereas the Ergelyakh pluton granites fall mainly into the field of amphibolites and partly greywackes, with some altered varieties falling into the field of metapelites. The age of protoliths calculated from the Rb-Sr model dates (Table 7) with the use of the method described in [55] varies within a narrow range (1035-1383 Ma), coinciding with their Sm-Nd model ages (1199-1322 Ma) (Table 8) [52]. Their formation was likely related to Mesoproterozoic (Riphean) geodynamic events. Figure 9 shows the plot of 87 Rb/ 86 Sr vs 87 Sr/ 86 Sr. For all igneous rocks of the Ergelyakh and Sokh massifs, a positive relationship is observed between the 87 Rb/ 86 Sr and 87 Sr/ 86 Sr parameters. To determine the primary sources of the EIRGD granitoids, we recalculated the data on the chemical composition of rocks according to [54] and plotted them on a discrimination diagram (Figure 8), where the fields of magma-generating substrata are outlined from the results of experimental studies [54]. Data points for granodiorites and adamellites of both massifs plot into the field of amphibolites just as those for the Sokh massif granites, whereas the Ergelyakh pluton granites fall mainly into the field of amphibolites and partly greywackes, with some altered varieties falling into the field of metapelites. The age of protoliths calculated from the Rb-Sr model dates (Table 7) with the use of the method described in [55] varies within a narrow range (1035-1383 Ma), coinciding with their Sm-Nd model ages (1199-1322 Ma) (Table 8) [52]. Their formation was likely related to Mesoproterozoic (Riphean) geodynamic events. Figure 9 shows the plot of 87 Rb/ 86 Sr vs 87 Sr/ 86 Sr. For all igneous rocks of the Ergelyakh and Sokh massifs, a positive relationship is observed between the 87 Rb/ 86 Sr and 87 Sr/ 86 Sr parameters. Notes: f Rb/Sr and ε Sr are deviations of the 87 Rb/ 86 Sr ratio and Sr isotope ratio relative to primitive mantle reservoir in the protolith of granitoids; T DM -2st is the two-stage model age of the protolith formation. Notes: f Sm-Nd and ε Nd are deviations of the 147 Sm/ 144 Nd ratio and Nd isotope ratio relative to the primitive mantle reservoir in the protolith of granitoids; T DM and T DM -2st are the one-and two-stage model ages of the protolith.  Notes: fRb/Sr and εSr are deviations of the 87 Rb/ 86 Sr ratio and Sr isotope ratio relative to primitive mantle reservoir in the protolith of granitoids; TDM-2st is the two-stage model age of the protolith formation.  Notes: fSm-Nd and εNd are deviations of the 147 Sm/ 144 Nd ratio and Nd isotope ratio relative to the primitive mantle reservoir in the protolith of granitoids; TDM and TDM-2st are the one-and two-stage model ages of the protolith.
Some data available on the rare elements from the Ergelyakh massif granitoids plotted on the discrimination geodynamic diagrams suggest that they were formed in an island arc or a continental arc setting (Figures 10 and 11). The new data obtained indicate that the granitoids of the Ergelyakh and Sokh massifs were formed in relation to subduction in the Uyandina-Yasachnaya volcanic arc. Some data available on the rare elements from the Ergelyakh massif granitoids plotted on the discrimination geodynamic diagrams suggest that they were formed in an island arc or a continental arc setting (Figures 10 and 11). The new data obtained indicate that the granitoids of the Ergelyakh and Sokh massifs were formed in relation to subduction in the Uyandina-Yasachnaya volcanic arc.

Physicochemical Conditions of Granitoid Formation
The conditions of formation of the EIRGD granitoids were determined by different methods based on the chemical composition of the rocks and their minerals (biotite, feldspars). Temperatures of the melt generation based on the empirical petrochemical geothermometer [24,25] are similar for granitoids of the Ergelyakh and Sokh plutons: granodiorites, 1010-1065 °C; adamellites, 977-1033 °C; and granites, 958-985 °C (Tables 9 and 10).
Aplitic melts of the Ergelyakh pluton were formed at a higher temperature (952-980 °C) than aplitic granites of the Sokh massif (970-718 °C). Water content in the parent melt for the Ergelyakh massif granitoids, determined with a model from [25], was 2.0-5.0% (3.2 ± 0.9% for granodiorites and 3.1 ± 0.1% for adamellites). The melt for the Sokh massif granitoids contained 3.4-3.7% water. Formation of the melt for granodiorites of the Ergelyakh massif occurred at a pressure of 8.5 ± 3.6 kbar, for adamellites at 10.0 ± 1.6 kbar, and for granitoids of the Sokh massif at 11.2 kbar. Assuming that the density of the overlying rocks is 2.7g/cm 3 , these pressures approximately correspond to melt formation depths of 22.9 ± 8.7 km, 27.0 ± 4.2 km, and 30.2 km, respectively. Magma ascending to the surface and its cooling-initiated crystallization of Ti-bearing phases. The highest crystallization temperatures were obtained from the Ti geothermometer [26] for granodiorites and adamellites (827-901 °C) and the lowest ones for granites and aplites (602-793 °C) of both massifs. The temperatures of rocks saturated with apatite (TAp = 630-892 °C) indicate that its crystallization began practically synchronously with Ti minerals but lasted somewhat longer. High temperatures of apatite saturation in some rocks (>900 °C) ( Table 9) may be due to the presence of restitic apatites. Temperatures of

Physicochemical Conditions of Granitoid Formation
The conditions of formation of the EIRGD granitoids were determined by different methods based on the chemical composition of the rocks and their minerals (biotite, feldspars). Temperatures of the melt generation based on the empirical petrochemical geothermometer [24,25] are similar for granitoids of the Ergelyakh and Sokh plutons: granodiorites, 1010-1065 °C; adamellites, 977-1033 °C; and granites, 958-985 °C (Tables 9 and 10).
Aplitic melts of the Ergelyakh pluton were formed at a higher temperature (952-980 °C) than aplitic granites of the Sokh massif (970-718 °C). Water content in the parent melt for the Ergelyakh massif granitoids, determined with a model from [25], was 2.0-5.0% (3.2 ± 0.9% for granodiorites and 3.1 ± 0.1% for adamellites). The melt for the Sokh massif granitoids contained 3.4-3.7% water. Formation of the melt for granodiorites of the Ergelyakh massif occurred at a pressure of 8.5 ± 3.6 kbar, for adamellites at 10.0 ± 1.6 kbar, and for granitoids of the Sokh massif at 11.2 kbar. Assuming that the density of the overlying rocks is 2.7g/cm 3 , these pressures approximately correspond to melt formation depths of 22.9 ± 8.7 km, 27.0 ± 4.2 km, and 30.2 km, respectively. Magma ascending to the surface and its cooling-initiated crystallization of Ti-bearing phases. The highest crystallization temperatures were obtained from the Ti geothermometer [26] for granodiorites and adamellites (827-901 °C) and the lowest ones for granites and aplites (602-793 °C) of both massifs. The temperatures of rocks saturated with apatite (TAp = 630-892 °C) indicate that its crystallization began practically synchronously with Ti minerals but lasted somewhat longer. High temperatures of apatite saturation in some rocks (>900 °C) ( Table 9) may be due to the presence of restitic apatites. Temperatures of Figure 11. Discrimination Rb/Zr-Nb diagram for Ergelyakh massif granitoids [57].

Physicochemical Conditions of Granitoid Formation
The conditions of formation of the EIRGD granitoids were determined by different methods based on the chemical composition of the rocks and their minerals (biotite, feldspars). Temperatures of the melt generation based on the empirical petrochemical geothermometer [24,25] are similar for granitoids of the Ergelyakh and Sokh plutons: granodiorites, 1010-1065 • C; adamellites, 977-1033 • C; and granites, 958-985 • C (Tables 9 and 10).
Aplitic melts of the Ergelyakh pluton were formed at a higher temperature (952-980 • C) than aplitic granites of the Sokh massif (970-718 • C). Water content in the parent melt for the Ergelyakh massif granitoids, determined with a model from [25], was 2.0-5.0% (3.2 ± 0.9% for granodiorites and 3.1 ± 0.1% for adamellites). The melt for the Sokh massif granitoids contained 3.4-3.7% water. Formation of the melt for granodiorites of the Ergelyakh massif occurred at a pressure of 8.5 ± 3.6 kbar, for adamellites at 10.0 ± 1.6 kbar, and for granitoids of the Sokh massif at 11.2 kbar. Assuming that the density of the overlying rocks is 2.7g/cm 3 , these pressures approximately correspond to melt formation depths of 22.9 ± 8.7 km, 27.0 ± 4.2 km, and 30.2 km, respectively. Magma ascending to the surface and its cooling-initiated crystallization of Ti-bearing phases. The highest crystallization temperatures were obtained from the Ti geothermometer [26] for granodiorites and adamellites (827-901 • C) and the lowest ones for granites and aplites (602-793 • C) of both massifs. The temperatures of rocks saturated with apatite (T Ap = 630-892 • C) indicate that its crystallization began practically synchronously with Ti minerals but lasted somewhat longer. High temperatures of apatite saturation in some rocks (>900 • C) ( Table 9) may be due to the presence of restitic apatites. Temperatures of zircon (T Zr = 680-786 • C) and monazite (TREE = 660-851 • C) saturation indicate that these minerals were formed under conditions of decreasing magmatic melt temperature.
Crystallization temperatures of feldspars determined on the basis of the albite-orthoclase-anorthite thermometer [28] and two-feldspar thermometer [29] range widely (939-327 • C) (Table 12). This is indicative of the long history of their formation under nonequilibrium conditions during the magma evolution. Lower temperatures obtained from K-feldspar geothermometers reflect the effect of superposed processes. Plagioclase is among the first rock-forming minerals to crystallize at high temperatures. It continues crystallizing with decreasing temperature simultaneously with later minerals (biotite, quartz). The basicity of plagioclase reduces, and at the lowest magma temperatures formation of albite begins, likely in relation to post-solidus conditions of the pluton emplacement. It is probable that crystallization of a portion of biotite and more basic plagioclase already began at the ascent of magma to the pluton emplacement level. Final crystallization of more acidic plagioclase, quartz, and K-feldspar from the melt occurred in the magma chamber with formation of plutonic bodies. Subsequent post-solidus and post-magmatic processes caused re-equilibration of the mineral composition and formation of secondary minerals (chlorite, sericite, albite).
in reducing conditions, more strongly reducing in the Sokh massif. The degree of reduction increased irregularly from the early to late intrusion phases, which was likely related to the initial heterogeneity of their primary protoliths. Cooling of the melts down to the crystallization temperature of micas led to a higher degree of oxidation of a magmatic system and to an increase in ∆Ni-NiO values as determined from the biotite composition (Table 13). Biotites from granodiorites of the Ergelyakh massif were formed at ∆Ni-NiO equal to -0.01 ± 0.8 (-1.1 to +0.4), those from adamellites at +0.5, and from granites at +1.8 ± 2.7 (-0.7 to +6.3). Biotites from granodiorites of the Sokh massif crystallized at ∆Ni-NiO equal to +0.7 ± 0.2 (+0.5 to +0.9), from adamellites at -0.6, and from granites at +0.3 ± 0.9 (-0.4 to + 1.0). In general, there is a tendency toward varying oxygen fugacity during the evolution of biotites from granitoids of the massifs (r(To-∆Ni) = -0.8), which is indicative of the increasing oxidation degree of the rocks with decreasing emplacement temperature. The lowest-temperature biotites (441 • C) are characterized by the maximum ∆Ni-NiO value (+6.3), which suggests that they were formed under conditions of a magnetite-hematite buffer. They are likely to be the products of re-equilibration during late geological processes.
Thus, the P-T data obtained for the granitoids of the studied massifs characterize wide variations of their temperature regime (901 to <450 • C, with consideration of late chloritization and albitization) and pressures of 2.3 ± 1.1 kbar (Ergelyakh massif) and 2.2 ± 0.4 kbar (Sokh massif) during the long emplacement period.

Mineragenic Potential
According to [30], the formation of ore occurrences associated with granitoids is controlled by the pressure conditions in the course of their emplacement. Particularly, pressure less than 1 kbar is favorable for the formation of Pb-Zn and Mo mineralization, while pressure of 1-2 kbar is responsible for Cu-Fe and Sn and 2-3 kbar for W occurrences. At pressures exceeding 3 kbar, no mineralization can form. From the Ti-biotite geobarometer data, the EIRGD granitoids were formed within a wide pressure range from 1.67 to 3.4 kbar (Table 12), and thus can be considered promising for certain types of mineralization. Mason [60] believes, however, that granitoids that initially formed at low fO 2 values can hardly be prospective of any mineralization, even with subsequently increasing oxygen fugacity. The data we obtained on oxygen fugacity in EIRGD rocks (Table 13) show that at the onset of the Ergelyakh granitoid crystallization, their parent magma was inhomogeneous in terms of oxygen fugacity (∆Ni-NiO from +3.3 to -3.6), becoming more oxidized in the period of biotite formation. Granitoids of the Sokh massif initially formed under reducing conditions (∆Ni-NiO = -8.7 to -2.9 for granitoids, -8.8 to -3.9 for adamellites, -9.3 to -8.2 for granites). On the FeO-Fe 2 O 3-log fO 2 diagram (Figure 12), data points for the EIRGD granitoids fall into fields of different mineralization types (Cu-Mo, Mo, W, Sn, and, partly, moderately reduced lithophile element Au association). In general, there is a tendency toward varying oxygen fugacity during the evolution of biotites from granitoids of the massifs (r(To-ΔNi) = -0.8), which is indicative of the increasing oxidation degree of the rocks with decreasing emplacement temperature. The lowest-temperature biotites (441 °C) are characterized by the maximum ΔNi-NiO value (+6.3), which suggests that they were formed under conditions of a magnetite-hematite buffer. They are likely to be the products of re-equilibration during late geological processes.
Thus, the P-T data obtained for the granitoids of the studied massifs characterize wide variations of their temperature regime (901 to <450 °C, with consideration of late chloritization and albitization) and pressures of 2.3 ± 1.1 kbar (Ergelyakh massif) and 2.2 ± 0.4 kbar (Sokh massif) during the long emplacement period.

Mineragenic Potential
According to [30], the formation of ore occurrences associated with granitoids is controlled by the pressure conditions in the course of their emplacement. Particularly, pressure less than 1 kbar is favorable for the formation of Pb-Zn and Mo mineralization, while pressure of 1-2 kbar is responsible for Cu-Fe and Sn and 2-3 kbar for W occurrences. At pressures exceeding 3 kbar, no mineralization can form. From the Ti-biotite geobarometer data, the EIRGD granitoids were formed within a wide pressure range from 1.67 to 3.4 kbar (Table 12), and thus can be considered promising for certain types of mineralization. Mason [60] believes, however, that granitoids that initially formed at low fO2 values can hardly be prospective of any mineralization, even with subsequently increasing oxygen fugacity. The data we obtained on oxygen fugacity in EIRGD rocks (Table 13) show that at the onset of the Ergelyakh granitoid crystallization, their parent magma was inhomogeneous in terms of oxygen fugacity (ΔNi-NiO from +3.3 to -3.6), becoming more oxidized in the period of biotite formation. Granitoids of the Sokh massif initially formed under reducing conditions (ΔNi-NiO = -8.7 to -2.9 for granitoids, -8.8 to -3.9 for adamellites, -9.3 to -8.2 for granites). On the FeO-Fe2O3-log fO2 diagram (Figure 12), data points for the EIRGD granitoids fall into fields of different mineralization types (Cu-Mo, Mo, W, Sn, and, partly, moderately reduced lithophile element Au association). Data points for biotites on the IV(F/Cl)-IV(F) diagram are plotted into the field of rocks promising for Sn-W-Be and Cu-porphyry mineralization ( Figure 13). While schematic, the discrimination diagrams presented here well illustrate the mineragenic specialties of the EIRGD. Figure 12. Schematic diagram of relationship between fractionation degree and oxidation state of magma and dominant metal paragenesis with reference to granitoids of Ergelyakh and Sokh massifs [11].
Data points for biotites on the IV(F/Cl)-IV(F) diagram are plotted into the field of rocks promising for Sn-W-Be and Cu-porphyry mineralization ( Figure 13). While schematic, the discrimination diagrams presented here well illustrate the mineragenic specialties of the EIRGD.

Conclusions
In summary, it can be concluded that localization of the EIRGD granitoids in the Adycha-Taryn fault zone, at the boundary of the Verkhoyansk fold-and-thrust and Kular-Nera slate belts, suggests a long, multistage history of tectono-magmatic activity there. Emplacement of the granitoid massifs in the region occurred no later than 145 Ma. Various isotope systems of the rocks and minerals record at least two more stages of tectono-magmatic activity at 130-120 and 110-100 Ma. These events were likely responsible for modification of initial features of magmatic rocks and minerals and reequilibration of their isotope systems.
Formation of granitoid magmas occurred at high temperatures (1060-950 °C) within the lower amphibolite crust, in an island-arc setting. The ages of protoliths for the EIRGD granitoids calculated from two-stage Rb-Sr and Sm-Nd models are 1109-1383 and 1199-1322 Ma, respectively. Emplacement of the Ergelyakh and Sokh massifs took place within a wide range of temperatures (900-450 °C) over a long period, taking into account late superposed processes. Parent melts for the Ergelyakh granitoids were formed in heterogeneous, more oxidizing conditions (ΔNi-NiO = +3.3 to -3.6) in contrast to granitoid melts of the Sokh massif (ΔNi-NiO = -2.9 to 9.3) that originated under reducing conditions. As the granitoid melts cooled down, a slight increase in oxygen fugacity occurred by the time of biotite crystallization in both massifs. The mineragenic potential of granitoids in both massifs seems to be similar, but owing to differences in physicochemical parameters of their formation (redox conditions), it was only partly developed in the Ergelyakh massif with the formation of small intrusion-related gold-bismuth deposits.

Conclusions
In summary, it can be concluded that localization of the EIRGD granitoids in the Adycha-Taryn fault zone, at the boundary of the Verkhoyansk fold-and-thrust and Kular-Nera slate belts, suggests a long, multistage history of tectono-magmatic activity there. Emplacement of the granitoid massifs in the region occurred no later than 145 Ma. Various isotope systems of the rocks and minerals record at least two more stages of tectono-magmatic activity at 130-120 and 110-100 Ma. These events were likely responsible for modification of initial features of magmatic rocks and minerals and re-equilibration of their isotope systems.
Formation of granitoid magmas occurred at high temperatures (1060-950 • C) within the lower amphibolite crust, in an island-arc setting. The ages of protoliths for the EIRGD granitoids calculated from two-stage Rb-Sr and Sm-Nd models are 1109-1383 and 1199-1322 Ma, respectively. Emplacement of the Ergelyakh and Sokh massifs took place within a wide range of temperatures (900-450 • C) over a long period, taking into account late superposed processes. Parent melts for the Ergelyakh granitoids were formed in heterogeneous, more oxidizing conditions (∆Ni-NiO = +3.3 to -3.6) in contrast to granitoid melts of the Sokh massif (∆Ni-NiO = -2.9 to 9.3) that originated under reducing conditions. As the granitoid melts cooled down, a slight increase in oxygen fugacity occurred by the time of biotite crystallization in both massifs. The mineragenic potential of granitoids in both massifs seems to be similar, but owing to differences in physicochemical parameters of their formation (redox conditions), it was only partly developed in the Ergelyakh massif with the formation of small intrusion-related gold-bismuth deposits.