Typomorphic Features of Placer Gold from the Billyakh Tectonic Melange Zone of the Anabar Shield and Its Potential Ore Sources (Northeastern Siberian Platform)

: Precambrian shields and outcropped Precambrian rock complexes in the Arctic may serve as the most important sources of various types of mineral raw materials, including gold. The gold potential of the Anabar shield in the territory of Siberia has, thus far, been poorly studied. A number of primary and placer gold occurrences have been discovered there, but criteria for the prediction of and search for gold mineralization remain unclear. The main purpose of this paper was to study the typomorphic features of placer gold in the central part of the Billyakh tectonic mélange zone in the Anabar shield and to compare them to mineralization from primary sources. To achieve this, we utilized common methods for mineralogical, petrographic, and mineragraphic analyses. Additionally, geochemical data were used. As a result of this investigation, important prospecting guides were identified, and essential criteria for the prediction of and search for gold deposits were elucidated. The characteristics of the studied placer gold were specific for gold derived from a proximal provenance. These characteristics included the poor roundness of the native gold grains, a cloddy– angular and dendritic form, an uneven surface, and a high content of coarse-fraction native gold (0.5–2 mm), which was as high as 24% of the volume of analyzed native gold. In addition, we conducted a study on the mineralogical features of the gold-sulfide mineralization that was disseminated throughout a small exposure area of paleo-Proterozoic para- and orthogneisses in the Anabar shield basement. A comparison of mineral inclusions in the coarse-fraction native gold and mineral assemblages in the ore deposits showed that one of the possible primary sources for placer gold might be small bodies of metasomatically altered orthogneisses associated with large granitoid plutons.


Introduction
The Anabar shield is a major element of the Arctic tectono-mineragenic belt. Despite the fact that most large and superlarge deposits of precious, radioactive, and rare-earth metals around the world are concentrated within shields, no economically important gold deposits have yet been discovered in the Anabar shield. Meanwhile, the method of studying the evolution of minerals in placers can help to identify their potential ore bodies [1,2]. Additionally, the study of the complex of placer gold typomorphic features is a highly effective way of predicting processes of gold mineralization on territories covered by Quaternary deposits, including the studied shield. The complex of placer gold typomorphic features (grain sizing, morphology, chemical composition, and mineral micro-inclusions) can be used as a means of exploration and evaluation throughout all stages of geological prospecting [3].

Materials and Methods
The grains of native gold were extracted from small-volume (1 m 3 ) samples taken from alluvial sediments in the upper reaches of the Borosku Unguokhtakh and Ulakhan Khaptasynnakh creeks. Sampling locations are shown in Figure 1. The alluvial material was subjected to panning in a "Siberian" wash pan, and small-volume samples (1 m 3 ) were screened through a trommel. A detailed analysis of the morphology and surface relief was conducted based on photographs of native gold particles that were obtained with a JEOL JSM-6480LV scanning microscope (JEOL, Ltd., Tokyo, Japan). Native gold fineness (Au, Ag, Hg, and Cu) was determined on a CAMECA Camebax-Micro microanalyzer (CAMECA, Gennevilliers, France) (accelerating voltage 20 kV, probe current 30 nA, and measurement time 7 s). Limits of element detection (in wt.%) were as follows: Au, 0.117; Ag, 0.061; Hg, 0.083; and Cu, 0.031. The fineness of one gold grain was identified by three points of the polished shear: central, lower, and upper marginal parts. More than 500 identifications of gold fineness were conducted. The gradation of the particle sizes and gold fineness was given in the classification of [4]. Eight samples of sulfidized igneous rocks were selected on the watershed of the Borosku Unguokhtakh and and Ulakhan Khaptasynnakh creeks. Atomic absorption, semiquantitative spectral, and wet chemistry analyses were conducted in the Diamond and Precious Metal Geology Institute by the specialists of Physical and Chemical Methods of Analysis Department. Analyses of mineral inclusions in the placer gold and the gold-sulfide disseminations were conducted on polished thin sections using a JSM-6480LV microscope equipped with Oxford Instruments wavelength and energy-dispersion spectrometers (accelerating voltage 20 kV, probe current 1.09 nA, and measurement time 7 s). The analytical lines were Au-Mα, Ag-α, and other elements-Kα.

Geological-Structural Setting of the Study Area
The study area covered the central part of the Billyakh tectonic mélange zone (TMZ) (Figure 1), which extends northwest for 250 km and is 30 km wide. This zone and other linear zones divide the Anabar shield into a series of granulitic block-terranes [18,19]. The Billyakh TMZ was formed by a system of closely spaced faults containing traces of blastomylonites and cataclasites, with variably diaphthorized granulites of the Upper Anabar and Khapchan Series between them ( Figure 1). In the central part of the Billyakh zone, the rocks of both series are punctuated with a large Billyakh pluton of porphyroblastic granodiorites and granites [8,18], as well as by small bodies and dikes of biotitehornblende gneisses with amphibolite facies (which include chemical analogues of quartz monzodiorites, granosyenites, and moderately alkaline granites with which potential Au mineralization is thought to be associated) [14,20]. The sampling site covered the upper reaches of the Borosku Unguokhtakh and Ulakhan Khaptasynnakh creeks and the interstream area ( Figure 1).

Typomorphism of Placer Gold
We studied the mineralogical-geochemical features of placer gold extracted from small-volume (1 m 3 ) samples taken from alluvial sediments in the upper reaches of the Borosku Unguokhtakh and Ulakhan Khaptasynnakh creeks ( Figure 1). See Section 2 for the research methodology.
It was discovered that the fineness of native gold does not depend on the grain size. Differences in fineness can probably be explained by unstable physical and chemical conditions of its deposition. Copper and mercury are found in native gold as an impurity, not as mineral inclusions.
It should be noted that a small gold nugget was found in this area. It was 11 mm in length, 7 mm in width, and 3 mm thick. The nugget had a platy form, a pitted surface, and was poorly rounded ( Figure 6). Its weight was about 0.48 g. The chemical composition of the native gold was uniform, and the fineness varied within a narrow range of 539‰ to 589‰. A high fineness shell associated with exogenic processes [4] was barely observed ( Figure 6). There were also inclusions of pyrrhotite, rutile, pseudorutile, quartz, kaolinite, and Fe-, Ti-, and Al-bearing phases. Therefore, the poor roundness and uneven surface of the mininugget and the relatively coarse gold particles (fractions of 0.5-2 mm) indicate that this placer gold was autochthonous and had a proximal bedrock source.

Mineralogical Features of the Disseminated Mineralization
In the divide between the Ulakhan Khaptasynnakh and Borosku Unguokhtakh creeks (in the distribution area of the Precambrian metamorphic rocks west of the Billyakh pluton), we identified a small zone of sulfidized igneous rocks traceable over a 300 m distance (Figure 1). The rocks had a massive texture and a gray or pinkish-grey color when they were freshly sheared. The color of their surfaces was brown, which was likely due to the decomposition of sulfides. Eight samples were collected at intervals of 30-50 m. An X-ray spectral analysis showed the presence of sulfide minerals, such as pyrrhotite, pyrite, and chalcopyrite ( Figure 7). A small native gold particle was found near quartz and chalcopyrite. Hessite also occurred as very fine streaks along the boundaries of the intergrown pyrite and chalcopyrite. Small and almost isometric crystals of monazite were also found. Minute galena grains were rarely observed. Atomic absorption analysis showed that the gold content in rocks was up to 2.8 ppm. From the information given above, it follows that the parageneses of the ore minerals in the sulfidized rocks were comparable to the mineral inclusions in the placer gold from the Borosku Unguokhtakh and Ulakhan Khaptasynnakh creeks. This suggests that these rocks may be the source of the placer gold discussed here.

Petrochemistry of Ore-Hosting Rocks
The chemical compositions of sulfidized rocks are given in Table 1, and figurative composition points are shown in Figure 8, together with datapoints for the Billyakh pluton and dike complex (these datapoints were obtained by previous investigators). Notes: 60/3, 60/4, 60/5, 60/7, 60/8, and 60/9: mineral composition that was diopside >> biotite and plagioclase << K-feldspar and a chemical composition corresponding to alkali feldspar syenites. 60/1, 60/6 (clinopyroxene + hornblende + biotite + plagioclase + K-feldspar (±quartz Q up to 1%)): syenites. Sulfides, apatite, and monazite were present. Chemical analyses were carried out at the Diamond and Precious Metal Geology Institute, Siberian Branch of the Russian Academy of Sciences.  Table 1; 4-granitoids samples. Analyses were conducted using samples from the collections of A.P. Smelov and B.B. Gerasimov, and data from [8,20] were also used. The samples from Table 1 and some samples from A.P. Smelov mentioned in the text are numbered.
The datapoints for the Billyakh pluton rocks revealed a pronounced trend in the lower part of the field of moderately alkaline rocks (quartz monzodiorites, granosyenites, and granites), with rare, smooth transitions into the field of normal-alkalinity rocks. Two samples strongly deviated from the trend toward a more alkaline field due to elevated and high K2O contents: granosyenite 320/3 with K2O = 6.2%, and alkali granite 301/4 with K2O = 10.5%. In many papers, the predominant rocks of the pluton are described as porphyry or porphyroblastic (K-feldspar) granodiorites. Massifs of these rocks have also been identified in other tectonic-mélange zones. In Figure 8, one can also see the datapoints for the granitoids occurring as numerous small bodies, exhibiting a trend that is different from that of the Billyakh granitoids. Pegmatites, aplites, and plagiogranites are also present.
In the eastern exocontact zone of the Billyakh pluton (in the interfluve of the Ulakhan and Ochchuguy Khaptasynnakh creeks), Lopatin [20] mapped a group of about 20 small diorite-syenite bodies that were among the rocks of the Khapchan Series. Another study [23] showed the presence of chemical analogues of monzogabbro (426/3), syenite (428/5), and quartz monzodiorites in these bodies, and their datapoints continued the trend of the pluton rocks. One of the bodies contained high-K granite (428/3: K2O = 8.3%, Na2O = 2.59%). The rocks of the small diorite-syenite bodies were united in a dike complex. One petrographic feature of the dike rocks was the absence of K-feldspar porphyroblasts, which is uncharacteristic of pluton rocks. In terms of the mineral content, the rocks of the dike complex were indistinguishable from the pluton rocks. According to current interpretations, the formation of the Billyakh tectonic mélange zone occurred due to a collision between the Daldyn and Khapchan terranes of the Anabar shield [24]. Presumably, the rocks of the Billyakh pluton (1983 ± 3 Ma) and the dike complex (1971 ± 4 Ma) are precollisional and late-or postcollisional formations, respectively [23]. Gold mineralization likely formed in the postcollisional stage in the transition to the platform stage of development [25,26]. A similar time sequence in terms of magmatism in porphyry-type gold deposits has been described in Baltic shield granitoids in Finland (aged 1.9-1.8 Ga) [27,28].
The figurative points for the rocks of the two studied samples (60/1 and 60/6) of the sulfidized dike-like body deviated from those obtained for the other samples. The rocks were analogous to syenites, and the values of the points followed a somewhat higher trend than that exhibited by the rocks of the Billyakh pluton and the dike complex ( Figure 8). Most of the samples were composed of high-K rocks (K2O > 8%), with similar amounts of oxides. They were analogous to alkali feldspar syenites (Table 1), which were absent from the pluton and the dikes. In terms of SiO2, Al2O3, and Na2O contents, they were comparable to quartz monzodiorites in the pluton and the dikes, but they had lower MgO, CaO, FeOtot, TiO2, and P2O5 contents. In the rocks of the studied dike-like body, such as in the eastern dikes, no K-feldspar porphyroblasts were observed. A characteristic feature of the high-K rocks was the dominance of diopside among the colored minerals, with hornblende and biotite being present as single grains. Therefore, a diopside-K-feldspar association was typical of these rocks.
According to unpublished material from previous studies, high-K rocks have also been found beyond the limits of the study area. An analysis of a syenite sample from the more northerly Chengelyakh migmatite pluton [29] showed values of K2O = 9.41% and Na2O = 1.68%, which correspond to alkali feldspar syenites. South of that, 10 samples from three localities had contents of K2O = 9%-15%, Na2O = 0.1%, and SiO2 = 44%-71%, and were united in a ''syenite complex'' [30]. Petrographically, they were highly feldspathic rocks, almost feldspathites.
High-K rocks also occurred among the alaskites, pegmatites, and gneisses. The importance of high-K rocks in the formation of gold mineralization (Figure 9a) has been established [31,32]. The Au/Ag ratio in the rocks had a direct correlation with sulfur and potassium and was inversely related to silicon and volatile values (Figure 9b-d). This particularity likely reflected the relationship between the amount of silver and the low-temperature replacement of minerals. Examples are shown in the following reactions: 3KAlSi3O8 orthoclase + H2O + CO2 = KH2Al3Si3O12 sericite + K2CO3 + 6SiO2 [33] and 2KAl3Si3O10(OH)2 muscovite + 2H + + 3H2O = 3Al2Si2O5(OH)4 kaolinite + 2K + [25] (Figure 9c). According to [34], mineral assemblages formed in the process of hydrothermal replacement reflect the compositional evolution of ore-forming fluid. The formation of gold mineralization commonly occurs after the peak of metamorphism and is related to multistage processes. In our case, the process of cooling the hydrothermal solution likely led, first, to the precipitation of K-feldspar, pyrite, and gold, and then quartz and silver. This process can be expressed by the following [34,35]: NaAlSi3O8 + CaAl2Si2O8 + SiO2 + 2KOH + H2O = 2KAlSi3O8 feldspar + CaO + 0.5Na2O2 + Al(OH) 3 (1) FeCO3 + Au(HS)2 − = FeS2pyrite + CO2 + OH -+ 0.5H2 + Au (2) 2Au(HS)2 − + H2O + 7.5O2 = 2Au + 4SO4 2− + 6H + NaAgCl2 + 0.5Na2SiO3 + 0.5H2 = Ag + 2NaCl + 0.5SiO2quartz + 0.5H2O The factors for Figure 9 can be suggested based on the position of the figurative points of the normative quartz and corundum. The factors are related to the degree of saturation of the system with silica: factor 1 is a high degree of saturation, while factor 2 is a low degree of saturation. Potassium metasomatites were possibly formed first. The silicon metasomatites and iron-alumina metasomatites were formed later. The stages of the ore formation process were not studied.

Discussion
The grain size analysis of placer gold from the studied area showed that the fraction of +0.5 mm is 24%, which is typical for placers of short-range drift [4]. The discovery of a small gold nugget also signifies the proximity of the ore body. It is important to emphasize that metal finds of this size are very rare for placers in eastern Siberia, which is typically dominated by very small (−0.25 mm) gold grains, representing more than 95% [36]. Morphological features of native gold, such as a low ratio of roundness and a primary tubercular and dimpled surface, indicate its short stay in hydrodynamic environments and short-range water drift [37]. It should be noted that the studied gold of all grades was of a low roundness. Gold fineness varied from low to extremely high. We have not identified the correlation between gold fineness and size. The wide variation of gold fineness is probably due to the unstable physical and chemical environment of the ore body formation. The absence of high-grade fringes in native gold excludes its prolonged stay in exogenous environments [4,36]. This is also confirmed by the detection of a small homogeneous gold nugget with very thin and short fringes of the initial stage in the peripheral part of the polished shear ( Figure 6). Admixture elements were displayed by Hg and Cu, which were identified in single grains of native gold. The small number of such grains does not allow us to conduct a thorough study.
On the basis of the above information, we can assume the presence of the nearby ore body of the studied placer gold.
According to the researchers in [3,37], mineral inclusions in gold can serve as indicators to predict placer gold ore bodies. Mineral micro-inclusions of pyrrhotite, pyrite, hessite, chalcopyrite, sphalerite, arsenopyrite, quartz, and potash feldspar were identified in the placer gold using X-ray spectroscopic microanalysis. A comparative analysis of mineral inclusions in placer gold and the ore mineral association of metasomatically altered high-potassium rocks with disseminated gold-sulfide mineralization, located in close proximity to placer manifestations, indicates that these formations may be one of the possible primary sources of the placer gold.
Due to gold mineralization having been found in the study area, the exocontact zone of the Billyakh pluton (and its associated dikes) has become the target of gold prospecting. Gold is located in the sulfidized rocks of the dike-like body, which includes chemical analogues of high-K alkali feldspar syenites that are not found in the pluton and the dikes. We attribute the origin of the highly potassic rocks to the manifestation of the intense metasomatic process of feldspathization. The K2O input was accompanied by the differentiated removal of MgO, FeOtot, CaO, TiO2, and P2O5, with SiO2, Al2O3, and Na2O showing inert behavior. It seems, however, that FeOtot was fixed-partly or completelyin the sulfides, which led to the formation of gold mineralization in high-K rocks, such as monzodiorites and quartz monzonites. Such metasomatically reworked rocks, which are analogous to syenites and alkali feldspar syenites, are the most promising in terms of the discovery of sulfides and native gold. The latter syenites, both when unaltered and slightly altered, are the most important types of rocks in the dike complex (quartz monzodiorites) and can also be found in the pluton. This suggests that finding gold-bearing rocks within the pluton is quite possible. It should be mentioned that the high-K ore-bearing rocks are characterized by a diopside-K-feldspar association, which may serve as a criterion for their discovery.
Presumably, potassic metasomatosis and sulfidization occurred in the last stages of formation of the Billyakh pluton and its associated dikes in the Early Proterozoic [14] period as a result of orebearing fluids reacting with the surrounding rocks.

Conclusions
The results from the field and laboratory studies and an analysis of the published literature and unpublished reports led us to the following conclusions: 1. Detailed studies on the mineralogy of gold from placers in the areas around the Borosku Unguokhtakh and Ulakhan Khaptasynnakh creeks revealed typomorphic characteristics of placer gold with a proximal provenance. These included the poor roundness of the native gold grains, a cloddy-angular and dendritic form, an uneven pitted surface with a thin discontinuous aluminosilicate shell, and an amount of coarse-fraction native gold in the studied small-volume (1 m 3 ) samples that was as high as 24%. Therefore, we predict that the primary source for the placer gold is located in the headwaters of the abovementioned streams; 2. An in-depth study of an unrounded, low-fineness gold nugget (0.48 g) found in the area of the Billyakh TMZ further supported the presence of a nearby bedrock source; 3. We studied mineral inclusions in the placer gold of the Billyakh TMZ, including pyrrhotite, pyrite, hessite, chalcopyrite, sphalerite, and arsenopyrite; 4. We also investigated sulfidized rocks found around the Precambrian metamorphic rock complexes, which were chemical analogues of syenites and alkali feldspar syenites. The diopside-K-feldspar association that was common in the high-K ore-bearing rocks could serve as a criterion in the search for gold in the Anabar shield area; 5. Our study of the ore mineral parageneses in the sulfidized alkali feldspar syenites-comparing them to the mineral inclusions in the placer gold-suggests that these rocks might be one of its primary sources.