World-Class PGE-Cu-Ni Talnakh Deposit : New Data on the Structure and Unique Mineralization of the South-Western Branch

The Talnakh deposit is one of the largest PGE-Cu-Ni deposits in the world. It is located inside the North-Western part of the Siberian Trap province and consists of three branches. The problem of massive ore origins has been discussed for several decades. The structure of the South-Western branch and the mineralogy of related its Cu-rich ore are discussed in this article. The Southern 2 orebody has a deep inclination (45◦) inside the intrusive body, close to its wall, in contrary to the horizontal orebodies of the North-Eastern branch. Mineral composition of the Southern 2 orebody differs from the composition the other orebodies of the Talnakh intrusion as well. It consists ofchalcopyrite, pentlandite with subordinate cubanite, and pyrrhotite. Its specific feature is a large amount of bornite and chalcocite. These minerals occur in disseminated and massive ores. The ores of the other branches of the Talnakh intrusion have chalcopyrite-pyrrhotite compositions whilethe Southern 2 massive ore is enriched in Cu (19.03–25.8 wt %; Cu/Ni = 3.8–8.6) and PGE: ΣPGE changes from 39.1 to 279 ppm, Pd/Pt = 1.3–32. Twelve minerals of the systems Pd-Sn-Cu, Pd-Pb-Bi, Pd-Ni-As, Au-Ag-Pd-Cu, and intermetallics of the Pt-Fe-Cu-Ni types were discovered in ores, but are also widespread in other orebodies. Isoferroplatinum, sperrilite and cooperitewere not found. The unusual structure and composition of the Southern 2 orebody suggest its origin from a separate magma impulse. The correlation between disseminated and massive ores of the Southern 2 orebody in term of chemical and mineralogical composition is evidence of the formation of massive oresin situ, without displacement along the bottom of the massif.


Introduction
The North Siberian Platform is a large metallogenic province including numerous PGE-Cu-Ni deposits.Three of them are huge sulfide deposits located in the Norilsk area: The Oktyabr'skoe, Talnakh, and Norilsk 1.They have been described in many publications .
These deposits have some specific features compared with the other Cu-Ni and PGE deposits.First of all, they are related to mafic-ultramafic intrusions located in the north-western part of the Siberian Trap province (251 ± 0.3) [36,37] (Figure 1: Schema of the geological structure of the Norilsk area), while the other deposits relate to the large Proterozoic plutons.As a result, massive sulfide orebodies (average thickness is 20-30 m, maximum is 56 m) in the Norilsk area are associatedwith thin gabbro-dolerites intrusions (150 m in thickness).This geological positioning of orebodies poses a question about the mechanism of their formation.
To solve the problem of the mechanism of massive ore formation it is necessary to study the mineral compositions of massive and disseminated ores together.In most cases a comparison of these two ore types does not give a result because they have ordinary compositions of pentlandite-chalcopyrite-pyrrhotite.Only one orebody is described in the literature in detail [18,[43][44][45][46][47][48][49] had unusual composition.It is the Octyabr'skoe deposit (2 km × 4 km × 0.05 km) hosted by the Kharaelakh intrusion which has been completely explored recently.The mineral zoning from the center to the contact has been described as talnakhite-cubanite-chalcopyrite-pyrrhotite ore types.The Octyabr'skoe deposit is characterized by mineralogical diversity, large quantities and size of many rare minerals, especially platinum group minerals discovered here for the first time: Talnakhite, putoranite, sobolevskite etc. [50][51][52][53][54][55][56][57][58][59][60][61][62].Nevertheless, the outer zone of this orebody consists of pyrrhotite, and disseminated ore in gabbro-dolerites has the same composition.This very common composition does not allow us to determine their origin.
However, there are a few other orebodies that have unusual chemical and mineralogical composition and that have never been studied and described before.These are orebodies of the Southern part of the Talnakh deposit.We present mineralogical and geochemical data on the Southern 2 orebody of the Talnakh intrusion which is rich in Cu and PGE.Comparison of compositions of disseminated and massive ore demonstrates their similarity, suggesting their formation by a filter-pressing process in situ, without essential movement of sulfide melt along the bottom of the gabbro-dolerite massif.In short, they were crystallized from separate impulse of the silicate melt which formed the Southern branch of the Taknakh intrusion.The Norilsk-Kharaelakh Fault played an essential role in the localization of the intrusive and ore bodies that is proved by the structure of this block of the Talnakh deposit.

General Characteristic of the Area
The Norilsk area is located North of the Siberian platform (Figure 1)consists of huge sequences (7-10 km) of carbonate-terrigenouse rocks (Rhiphae-Devonian) with interlayers of gypsum, anhydrite and salt [63].They are covered by coal-bearing sandstones and siltstones, combined in the Tunguska Group rocks (C 2 -P 3 ).Thicknesses of basalts and tuffs of the Siberian Trap formation covering terrigenous rocks reaches up 3.5 km.Its age was estimated as 251 Ma [36,37].Volcanic rocks were subdivided into 11 formations [64], the three lower onesare subalkaline with elevated TiO 2 content (up to 3.5 wt %) and high Gd/Yb ratio while upper formations have a typical tholeiitic composition with TiO 2 around 1 wt %.Numerous basic-ultrabasic intrusions coeval to lavas are located mostly in the Devonian and Tunguska Group rocks.Their morphology vary from sill-like to irregular bodies.

Deep Structure of the Area
The Norilsk region is located within the Norilsk megablock, which is restricted by the Yeniseysky, Malokhetsky and Keta-Irbinsky faults (respectively in the North, West and East).The last one separates the Norilsk megablock from the megablock corresponding to the Tunguska syneclise (Figure 1).The increased permeability of the earth's crust here is determined by the development of a long-lived rift system associated with the Yenisey-Khatangsky Ttrough (the Norilsk-Kharaelyakhsky Trough).The Eastern part of the area refers to the Tunguska syneclise.The latter is separated from the Norilsk-Kharaelyakhsky Trough by the Khantaysko-Rybninsky Swell.The formation of the Norilsk-Kharaelakhsky Trough began in the Late Paleozoic time, the maximum development occurred in the Permo-Triassic volcanic activity, and the final structural appearance was acquired during the post-volcanic period, when anticlines were formed.The permeability of the earth's crust in the extreme North-West of the platform is associated with the development of intracontinental rift structures of the West Siberian plate and the Yeniseysko-Khatangsky Troughs.All known Cu-Ni deposits of the Siberian Platform are located here.The surface of the crystalline basement within the central part of the Kharaelakhsky Trough lies at a depth of 7-8 km and rises to the Southern and South-Eastern directions to 5 km.

The Position of the Intrusions
Differentiated intrusions are combined into the Norilsk Intrusive Complex, three of them contain enormous sulfide contents forming the Oktyabr'skoe, Talnakh and Norilsk 1 deposits located inside two ore junctions, the Norilsk and Talnakh (Figure 1).The last one comprises the Oktyabr'skoe and Talnakh deposits.It is believed [9,10,18], that the Norilsk-Kharaelakh Fault (NKF) determined the position of these ore-bearing massifs located at different levels of the rock sequence.
The localization of the differentiated intrusions of the Talnakh ore junction shows a "branching bundle" [65] relative to the Norilsk-Kharaelakhsky Fault (NKF) (Figure 2).Intrusive bodies occur in the deep regions (around 500 m from the surface) and rise to the modern surface.It is important to emphasize that they do not reach the lava flows.The Talnakh intrusion is elongated in the meridian direction at around 20 km [7,9,65,66].
The intrusion's width is 1 km and the average thickness is 150 m.The roof is flat while the bottom is undulated, with many dips.It lies at a slight angle with respect to the host rocks of the Devonian and the Tunguska rock group.The Talnakh intrusion consists of two branches, South-Western and North-Eastern, separated by Norilsk-Kharaelakh and feathering faults.Some authors [38,65] also distinguish the North-Westernbranch (Figure 2, we follow this classification), sometimes referred to as the Kharaelakh intrusion.The connection of these branches is complex, because this area is broken by a series of discontinuous faults and fractures, in which displacements of up to 0.3 km are observed.The North-Eastern branch is located to East of the main NKF, with a shallow dip (5-6 degrees) to the North-East.Its length is more than 10 km, width is 1200 m, and the maximum thickness is 250 m.The South-Western branch is elongated along the NKF for 6 km and dips to the North-Eastat an angle of 4 • -6 • .The bottom has a keel-shaped chute.The North-Western branch is isometric and is located to the West of the NHF (Figure 2).
The internal structure of the Talnakh intrusion is typical of other ore-bearing intrusions of the Norilsk Intrusive Complex.It consists of gabbro-dolerites (from bottom to top): contactic, taxitic, picritic, olivine, olivine-bearing, olivine-free (we follow the nomenclature of rocks adopted to the Norilsk area and used by geologists for several decades) [4,9,13,38].The upper zone of intrusions Minerals 2018, 8, 124 5 of 33 comprises gabbro-diorites, leucogabbro, upper picritic and taxitic gabbro-golerites and magmatic breccias.Some horizons can be absent in this zone.
There are four types of sulfide ores related to the Talnakh intrusion: (i) disseminated pentlandite-chalcopyrite-pyrrhotite ore in the taxitic and picritic gabbro-dolerites; (ii) massive pentlandite-chalcopyrite-pyrrhotite ore in the lower contact zone of massif and surrounding rocks; (iii) disseminated chalcopyrite ore in surrounding rocks; and (iv) low-sulfide ore in upper zone of massifs (in the upper taxitic, picritic gabbro-dolerites and leucogabbro).As a rule the massive ores are concentrated close to the bottom of the intrusion, in several centimeters to several meters away from the contact in the Devonian carbonate-terrigenous sedimentary country rocks or in the Tunguska Group.They occupy a horizontal position and are related to troughs in the bottom of the intrusion [5,6].However, these geological features are typical only for North-Eastern part of the deposit.
All these types are enriched in PGE compared with other Cu-Ni deposits in the world.Many publications are devoted to the North-Eastern part of the Talnakh deposits [7,9,38,[66][67][68][69] and the Kharaelakh intrusion [6] but there is little data on the South-Western part deposit and absolutely none about related ores.This is the first description of Cu-rich ore in the South-Western branch.Furthermore, the study of the structure and composition of intrusions and ores allows one to answer a number of questions related to the mechanisms of formation of deposits.
chalcopyrite-pyrrhotite ore in the lower contact zone of massif and surrounding rocks; (iii) disseminated chalcopyrite ore in surrounding rocks; and (iv) low-sulfide ore in upper zone of massifs (in the upper taxitic, picritic gabbro-dolerites and leucogabbro).As a rule the massive ores are concentrated close to the bottom of the intrusion, in several centimeters to several meters away from the contact in the Devonian carbonate-terrigenous sedimentary country rocks or in the Tunguska Group.They occupy a horizontal position and are related to troughs in the bottom of the intrusion [5,6].However, these geological features are typical only for North-Eastern part of the deposit.
All these types are enriched in PGE compared with other Cu-Ni deposits in the world.Many publications are devoted to the North-Eastern part of the Talnakh deposits [7,9,38,[66][67][68][69] and the Kharaelakh intrusion [6] but there is little data on the South-Western part deposit and absolutely none about related ores.This is the first description of Cu-rich ore in the South-Western branch.Furthermore, the study of the structure and composition of intrusions and ores allows one to answer a number of questions related to the mechanisms of formation of deposits.[65,66]).Projection on the surface; (b) (after Geology of Ore Deposits [69]) and isopach map of the Talnakh intrusion with studied boreholes.

Materials and Methods
We have studied the Southern 2 orebody of the Talnakh deposit located in the South-Western branch (enriched in Cu and PGE) in detail and compared their intrusive rocks and ores with North-Eastern (boreholes OUG-2 and SF-10) and South-Western branches (borehole TG-21).[65,66]).Projection on the surface; (b) (after Geology of Ore Deposits [69]) and isopach map of the Talnakh intrusion with studied boreholes.

Materials and Methods
We have studied the Southern 2 orebody of the Talnakh deposit located in the South-Western branch (enriched in Cu and PGE) in detail and compared their intrusive rocks and ores with North-Eastern (boreholes OUG-2 and SF-10) and South-Western branches (borehole TG-21).
The South-Western portion of the Talnakh deposit is characterized based on data from the core material recovered by the borehole EM-6, which penetrated the Southern 2 orebody and the samples collected in the underground mine "Majak" (samples MA-12a-MA-17a).
Major elements in the rocks were analyzed by XRF method at the Institute Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry RAS (IGEM RAS), by analyst A.I. Yakushev.Determination of the concentration was done with Wavelength Dispersive X-ray Spectrometer, model Axios mAX production company PANalytical (Eindhoven, The Netherlands).The spectrometer is equipped with X-ray tube with a capacity of 4 kW Rh-anode.The maximum rated tube voltage 60 kV max anode current of 160 mA.Eleven elements were measured, and the relative standard deviation for the majority of these elements was between 1.2% and 4.5% (relative standard error of 5.6% for TiO 2 and 13% for Cr 2 O 3 ); detection limits are given in  Base and precious metals were analyzed at the Laboratory of the Geochemistry and Analytical Chemistry of Precious Metals at the Vernadsky Institute (analysts I.V. Kubrakova and O.A. Tyutyunnik).To analyze ore samples, they were decomposed by heating with a mixture of concentrated HF + HNO 3 and then dried to wet salts.The salts were treated with reverse aqua regia (HNO 3 :HCl = 3:1) to the complete oxidation of the elemental sulfur, and wet salts were then obtained.Upon complete dissolution of the salts, the solution was cooled and filtered into a 100 mL flask.The filter with precipitate was placed in a corundum crucible, ashed in a muffle furnace at 500 • C, fused with a double-excess quantity of Na 2 O 2 at 600 • C and then treated with HCl to convert the salts into chlorides.The solution obtained was mixed with filtrate.To analyze Au, Pt, Pd, and Rh, a preconcentration of noble metals on a complex-forming sorbent POLYORGS IV was performed; the sorbent as a suspension was analyzed by ETAAS on a Solaar MQZ (Thermo Electron Corp., Cambridge, UK) at 242.8, 265.9, 247.6, and 343.5 nm, respectively.Silver was analyzed in the initial solution by ETAAS at 328.1 nm with 1.25% ascorbic acid as a modifier.Fe, Cu, Ni, Co, and S were analyzed by ICP-AES on an Iris Intrepid II XDL (Thermo Electron Corp., Franklin, MA, USA).The accuracy of the determination was confirmed by replicated analysis of SRMs SARM-7B (Platinum ore) and KN-1 (Nickel concentrate).
Minerals were analyzed using a CAMECA SX 100 (Cameca, Gennevilliers, France) at the Vernadsky Institute of Geochemistry and Analytical Chemistry (analyst N.N.Kononkova) and JEOL JXA 8200 SuperProbe (3-1-2 Musashino, Akishima, Tokyo, Japan) at IGEM RAS (analyst E.V. Kovalchuk) at 40 nA beam current and 20 kV accelerating voltage; the counting times were 10 s when major components were analyzed and 20s for trace elements.Back-scattered electrons images BSE images were taken on an Jeol JSM-6480LV electron microscope (Jeol, Tokyo, Japan) at the Institute Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry RAS Laboratory of Analytical Techniques (beam current 10 nA, accelerating voltage 20 kV, live counting time 100 s; analyst L.O.Magazina) and by scanning electron microscope (SEM) a LEO 1413 VP equipped by an energy dispersive spectrometer (EDS) with spectrometer (EDS) "Oxford" at the V.S. Sobolev Institute of Geology and Mineralogy.

Morphology of the Branches of the Talnakh Intrusion
Based on the data of isohypses and isopaches maps [65,66] and the drilling results of Norilskgeologiya Ltd., we constructed several sections through the massif that help get additional information about the morphology of their branches (Figure 3).According to these data, the North-Eastern branch is a ribbon-like body.The intrusion plunges to the North-East at a steeper angle than the surrounding rocks which fall in the same direction.Thus, its Southern part (partially eroded) lies in the deposits of the Tunguska Group, while the Northern part is located in the Upper Devonian rocks (the Kalargonsky Formation).The intrusion's bottom is uneven, which results in a narrowing of the cross section of the intrusion [8].The massive ores have a subhorizontal position in the section.As a rule, they are localized in a few decimeters from the lower intrusive contact, while the transition from disseminated ores to massive ones at the base of the intrusion occurs gradually.Sulfides gradually penetrate into the host rocks and accumulate in the form of large (up to 40 m, borehole OUG-2 [23]) or small (first decimeters, borehole SF-10, TG-21) orebodies.
The morphology of the South-Western part of the Talnakh intrusion substantially differs from those described above.The intrusive body is localized in a graben-shaped block displaced 250 m to the South and formed by two branches of the NCF [8,65], the planes of which restricted the distributionof Minerals 2018, 8, 124 9 of 33 magma flow to the West and to East.As a result, the sides of the intrusion have a steep fall to the central part, caused by the steeply falling planes of tectonic disturbances (Figures 3 and 4).The Southern parts of the South-Western and North-Eastern branches are eroded.The Talnakh intrusion is submerged to the North-East, with a slight uplift in the center.The Southern part of it lies in the Tunguska Group of rocks, while the Northern part is located in the Devonian rocks.In the North, according to [65], the intrusion connects with the North-Eastern and North-Western parts by thin conductors at a depth of about 500 m from the surface.
Eastern branch is a ribbon-like body.The intrusion plunges to the North-East at a steeper angle than the surrounding rocks which fall in the same direction.Thus, its Southern part (partially eroded) lies in the deposits of the Tunguska Group, while the Northern part is located in the Upper Devonian rocks (the Kalargonsky Formation).The intrusion's bottom is uneven, which results in a narrowing of the cross section of the intrusion [8].The massive ores have a subhorizontal position in the section.As a rule, they are localized in a few decimeters from the lower intrusive contact, while the transition from disseminated ores to massive ones at the base of the intrusion occurs gradually.Sulfides gradually penetrate into the host rocks and accumulate in the form of large (up to 40 m, borehole OUG-2 [23]) or small (first decimeters, borehole SF-10, TG-21) orebodies.fall to the central part, caused by the steeply falling planes of tectonic disturbances (Figures 3 and 4).
The Southern parts of the South-Western and North-Eastern branches are eroded.The Talnakh intrusion is submerged to the North-East, with a slight uplift in the center.The Southern part of it lies in the Tunguska Group of rocks, while the Northern part is located in the Devonian rocks.In the North, according to [65], the intrusion connects with the North-Eastern and North-Western parts by thin conductors at a depth of about 500 m from the surface.

Internal Structure of the Intrusive Branches
The above alternation of rock varieties in the section of intrusions is preserved, but horizons thicknesses vary considerably, especially in the case of picritic gabbro-dolerites and gabbro-diorites.The section of the North-Eastern branch is represented (with a total thickness of 118 m, borehole SF-10, Figure 5a) by different gabbro-dolerites (contactic, taxitic, picritic, olivine, olivine-bearing, olivinefree) and gabbro-diorite (Table 1).In the upper part, thin horizons of picritic and taxitic gabbrodolerites are observed with elevated Cu and Ni contents (Table 1) as well as magmatic breccias with fragments of coals and cemented gabbro material.The most distinct variations in the composition of rocks are reflected in the distribution of MgO (Figure 5, Table 1), the content of which sharply increases in picritic gabbro-dolerites (both in the lower part of the section and in the upper section).However, massive ores are practically absent in the section because the borehole is drilled behind the contour of rich economically valuable ores.
An analogous structure of the section is observed in the previously described section in OUG-2 borehole [23], which penetrated 40 m into massive ores.The thickness of the horizon with disseminated ores reaches 27 m.The difference in the structure of the intrusive part of these sections (Figure 5a,b) is in the absence of upper picritic horizons and magmatic breccias in the borehole OUG-2.
The North-Western branch (borehole TG-21) is distinguished by the increased capacity of the taxitic gabbro-dolerites (Figure 5c).Thus, the total thickness of picritic and taxitic gabbro-dolerites, as well as partly ore-bearing olivine gabbros, forming a productive horizon of economicallyvaluable mineralization, reaches almost 50 m.The thickness of massive ore is around 1 m.
These branches have similar composition in rare element distributions reflected in spiderdiagrams (Figure 6, Table 2).

Internal Structure of the Intrusive Branches
The above alternation of rock varieties in the section of intrusions is preserved, but horizons thicknesses vary considerably, especially in the case of picritic gabbro-dolerites and gabbro-diorites.The section of the North-Eastern branch is represented (with a total thickness of 118 m, borehole SF-10, Figure 5a) by different gabbro-dolerites (contactic, taxitic, picritic, olivine, olivine-bearing, olivine-free) and gabbro-diorite (Table 1).In the upper part, thin horizons of picritic and taxitic gabbro-dolerites are observed with elevated Cu and Ni contents (Table 1) as well as magmatic breccias with fragments of coals and cemented gabbro material.The most distinct variations in the composition of rocks are reflected in the distribution of MgO (Figure 5, Table 1), the content of which sharply increases in picritic gabbro-dolerites (both in the lower part of the section and in the upper section).However, massive ores are practically absent in the section because the borehole is drilled behind the contour of rich economically valuable ores.
An analogous structure of the section is observed in the previously described section in OUG-2 borehole [23], which penetrated 40 m into massive ores.The thickness of the horizon with disseminated ores reaches 27 m.The difference in the structure of the intrusive part of these sections (Figure 5a,b) is in the absence of upper picritic horizons and magmatic breccias in the borehole OUG-2.
The North-Western branch (borehole TG-21) is distinguished by the increased capacity of the taxitic gabbro-dolerites (Figure 5c).Thus, the total thickness of picritic and taxitic gabbro-dolerites, as well as partly ore-bearing olivine gabbros, forming a productive horizon of economicallyvaluable mineralization, reaches almost 50 m.The thickness of massive ore is around 1 m.

Morphology
The Southern 2 orebody is confined to the Southern part of the massif, where its upper part is eroded and covered by Quaternary sediments (Figure 3).The area of intrusive rocks reaching the surface is 3 km 2 .It is difficult to judge the shape of the roof of an intrusive body, but the bottom strongly undulates.Complete sections of intrusive rocks occur only in the Northern part of this block where powerful magmatic breccias are widespread in the upper part.The horizons of contactic, taxitic and picritic gabbro-dolerites, as well as olivine gabbros are distinguished in this cross-section (from the bottom to top).Their thickness varies irregularly.In the Southern part of the intrusive body where the Southern 2 orebody is located, the following horizons were recognized (from top to botton): magmatic breccias, gabbro-diorites, gabbro-dolerites, olivine-bearing gabbro-dolerites, and taxitic gabbro-dolerites (Figure 7).
The Southern 2 orebody takes an unusual position for massive ore.It is not confined to the bottom of the intrusion, but occurs in its Western flank.It has a steep inclinations (up to 45 • ) near the roof changing to 10 • at the bottom (Figure 7).Its second specific feature is that it almost entirely lies in intrusive rocks (mostly in taxitic rocks).In general, it has a lenticular shape, with rare ramifications on individual branches.Its boundaries are clear, usually tectonic.Massive ores are surrounded by a halo of disseminated ores above and below it.The length of the Southern 2 orebody is 700 m, and the maximum thickness is 15 m.The Southern 2 orebody takes an unusual position for massive ore.It is not confined to the bottom of the intrusion, but occurs in its Western flank.It has a steep inclinations (up to 45°) near the roof changing to 10° at the bottom (Figure 7).Its second specific feature is that it almost entirely lies in intrusive rocks (mostly in taxitic rocks).In general, it has a lenticular shape, with rare ramifications on individual branches.Its boundaries are clear, usually tectonic.Massive ores are surrounded by a halo of disseminated ores above and below it.The length of the Southern 2 orebody is 700 m, and the maximum thickness is 15 m.

Chemical Composition of the Ores
The predominance of chalcopyrite and bornite in ores reflects the high-copper composition of the Southern ore body.Copper concentrations vary from 19 to 25 wt % (Table 3), but nickel concentrations are also high (3−6 wt %) due to the large amount of pentlandite.The ratio of copper to nickel is 3.2-8.6 in the ore (in disseminated ores is 4.8-5.1).The cobalt content is extremely low, a few hundredths of a percent.S/Se varies in massive ore in a small range from 0.31 to 0.44; it is 0.44 in disseminated ore.These ratios are different from the Cu/Ni and S/Se ratios in ore from the North-Western branch (TG-21) and in the semi-massive ore of the Norilsk 2 intrusion (Samples N2/2, N2/4; N2/4a, Table 3, No. [23][24][25] where the first is equal to 1 and the second riches 1.89.The contents of theplatinum group elements are extremely high in all samples of massive ores, ΣPt + Pd + Rh reaches

Chemical Composition of the Ores
The predominance of chalcopyrite and bornite in ores reflects the high-copper composition of the Southern ore body.Copper concentrations vary from 19 to 25 wt % (Table 3), but nickel concentrations are also high (3−6 wt %) due to the large amount of pentlandite.The ratio of copper to nickel is 3.2-8.6 in the ore (in disseminated ores is 4.8-5.1).The cobalt content is extremely low, a few hundredths of a percent.S/Se varies in massive ore in a small range from 0.31 to 0.44; it is 0.44 in disseminated ore.These ratios are different from the Cu/Ni and S/Se ratios in ore from the North-Western branch (TG-21) and in the semi-massive ore of the Norilsk 2 intrusion (Samples N2/2, N2/4; N2/4a, Table 3, No. [23][24][25] where the first is equal to 1 and the second riches 1.89.The contents of theplatinum group elements are extremely high in all samples of massive ores, ΣPt + Pd + Rh reaches 280 ppm, and their content in the disseminated sulfides reaches 350 ppm (calculations in 100% sulfides).The distributions of PGE in disseminated and massive ore penetrated by borehole EM-6 is shown in Figure 8. PGE contents are high across whole body of the massive ore.     3.   3.

Mineral Composition of the Ores
A distinctive feature of the ore assemblage of the Southern 2 orebody is the large predominance of chalcopyrite over other sulfides in comparison with the ores from the other branches, where pyrrhotite predominates (Figure 9).The pentlandite-cubanite-chalcopyrite assemblage is the main type of ores (Figure 10a-d) whereas the chalcopyrite-pyrrhotite type is rare.An unusual feature of these ores is the widespread bornite in massive and disseminated ores which occurs both over massive ores (endocontact) and under the orebody into exocontact (Figure 10e-h).
Chalcopyrite is represented only by tetragonal species (Tables 4 and 5), while the other minerals of the chalcopyrite group (talnakhite, moihkukite, putoranite) that are typical of the Oktyabr'skoe deposit were not found in the Southern 2 orebody.Cubanite is common in these ores similarly to the intermediate zone of the main orebody of the Kharaelakh intrusion, but an abundance of pentlandite in the Southern 2 distinguishes them from it.Pentlandite occurs as two morphological varieties (Figure 10b,d): (i) large euhedral crystals, due to ores sometimes acquiring the porphyry structure; and (ii) flame-like grains in chalcopyrite, cubanite, and pyrrhotite.The first morphological variety is more common.Crystals of pentlandite up to 1 cm in size are fractured and the cracks are filled with violarite replacing chalcopyrite.The composition of Southern 2 pentlandite differs from other deposits in the Norilsk region [19,24,43] by the lower concentrations of cobalt (0.38-0.73 wt % Co versus 1-2 wt %, respectively), as well as by the prevalence of Ni-rich pentlandite (Fe/Ni < 1).A characteristic feature of pentlandite from the Southern 2 orebody is frequent Cu in its composition (up to 3.4 wt %, Table 4).Inclusions of sphalerite, galena and veins of magnetite are found often in chalcopyrite.Magnetite occur sometimes as myrmecitic aggregates with chalcoprite.Minor copper amount there is in all ore minerals of the orebody: millerite (1-4.4 wt %), pyrite (0.18 wt %), pyrrhotite (0.11-0.12 wt %).Idaite, pyrite and MSS are rare minerals in the studied ores (Table 5).
Minerals 2018, 8, x FOR PEER REVIEW 15 of 32 pyrrhotite predominates (Figure 9).The pentlandite-cubanite-chalcopyrite assemblage is the main type of ores (Figure 10a-d) whereas the chalcopyrite-pyrrhotite type is rare.An unusual feature of these ores is the widespread bornite in massive and disseminated ores which occurs both over massive ores (endocontact) and under the orebody into exocontact (Figure 10e-h).Chalcopyrite is represented only by tetragonal species (Tables 4 and 5), while the other minerals of the chalcopyrite group (talnakhite, moihkukite, putoranite) that are typical of the Oktyabr'skoe deposit were not found in the Southern 2 orebody.Cubanite is common in these ores similarly to the intermediate zone of the main orebody of the Kharaelakh intrusion, but an abundance of pentlandite in the Southern 2 distinguishes them from it.Pentlandite occurs as two morphological varieties (Figure 10b,d): (i) large euhedral crystals, due to ores sometimes acquiring the porphyry structure; bornite and chalcopyrite-bornite mineralogical varieties in the endo-and exo-contacts of the intrusion: both in gabbro and in the sandstones underlying the intrusion (and in the hornfels developed on them).Sometimes only one bornite occurs in the orebody (Figure 10).Furthermore, small clusters of chalcopyrite (10-12 cm) surrounded by a border of bornite have been found in the upper part of the Southern 2 orebody.Chalcopyrite and bornite are in form of veins in disseminated ore, while the chalcocite is in the interstitium of rock-forming minerals as a rule (Figure 10g,h).A specific feature of the Southern 2 orebody is the presence of a large amount of chalcocite-bornite and chalcopyrite-bornite mineralogical varieties in the endo-and exo-contacts of the intrusion: both in gabbro and in the sandstones underlying the intrusion (and in the hornfels developed on them).Sometimes only one bornite occurs in the orebody (Figure 10).Furthermore, small clusters of chalcopyrite (10-12 cm) surrounded by a border of bornite have been found in the upper part of the Southern 2 orebody.Chalcopyrite and bornite are in form of veins in disseminated ore, while the chalcocite is in the interstitium of rock-forming minerals as a rule (Figure 10g,h).
Among the Pd minerals, stannopalladinite Pd5Sn2Cu predominates.It occurs in intergrowth with various PGMs: polarite, tetraferroplatinum, zvyagintsevite and unnamed phases.It differs from the mineral previously described in the literature by the excess of Cu in the composition of most of the grains, the amount of which above the one unit is added to Pd in the mineral formula: (Pd,Cu)5Sn2Cu.Rustenburgite and atokite are found in intergrowth together with a clear phase boundary between them (Figure 12a), although both these minerals form an extended series of solid solutions.Both these minerals contain Cu and Ni (Table 6).It differs from the mineral previously described in the literature by the excess of Cu in the composition of most of the grains, the amount of which above the one unit is added to Pd in the mineral formula: (Pd,Cu) 5 Sn 2 Cu.Rustenburgite and atokite are found in intergrowth together with a clear phase boundary between them (Figure 12a), although both these minerals form an extended series of solid solutions.Both these minerals contain Cu and Ni (Table 6).A significant proportion (24%) of the mineral associations is represented by Pd-Pb(Bi) compounds (polarite, zvyagintsevite and unnamed phases of this system) (Table 7).Pb-containing minerals are in intergrowths with the other minerals of PGE (Figure 12b-d).In this case, the decomposition structure of the solid solutions between zvyaginsevite and Pd 5 Pb 3 is often observed (Figure 12d).Between polarite Pb-rich composition are dominated: Pd 1.03 (Pb 0.82 Bi 0.15 ) 0.97 , although PdBi with a minor Pb as well as the Pd 3 (Bi,Pb) 2 alsooccur in this association (Table 7).The unnamed phase Pd 5 Pb 3 is widespread in this system.It contains a minor Ag (Table 7) unlike the other Pd-Pb minerals.In this case the compositions of this phase deviate from stoichiometry to the side of zvyagintsevite (Figure 13).Galena is found in intergrowths with Pb-bearing minerals.Pt-Fe-Cu-Ni alloys in the chalcopyrite ores of the Southern 2 orebody occur in association with stannopalladinite (Figure 12b,d).They are the latest phases in ores compared with other PGE minerals.These alloys have the same composition in the different grains (Table 8) and the points of their compositions lie in the tetraferroplatinum field on the diagram Pt-Fe-Ni (Figure 13), although they contain about 10 mol % of the tulameenite and about 15 mol % of ferronickelplatinum.Tetraferroplatinum contains Pd and Rh as minor elements (Table 8).Minerals of the Pd-Ni-As system are 7% of the total amount of PGM.Palladoarsenide, guanglinite, stillwaterite, majakite have also been found (Table 9).They occur as euhedral crystals (Figures 12c  and 14).The main feature of these minerals is the significant Ni concentrations in the Pd-As minerals, close to their nickel analogs in Pd-Ni-As system (Table 9).The unidentified phase (Pd,Cu) 3 (As,Sb,Sn), possibly is guanglinite (Pd 3 As) which is considerably enriched by other elements.
Tetra-auricupride Cu(Au,Pd) predominates among the minerals of the Au-Pd-Ag-Cu system while auricupride Cu 3 (Au,Pd), electrum and Ag-rich alloys are much less common (Table 10).They form intergrowths with other PGM taking the intersticial position between them.In this case, tetraauricupride is a precursor mineral in comparison withAu-Ag alloys (Figure 12c,e).A large (more than 1 mm) microaggregate of PGM has been studied in detail (Figure 15, Table 11).The euchedral grain of stannopalladinite (points 1 and 2 in Figure 15b) is bordered by the Pd 5 Pb 3 phase (points 3 and 4) which is included in stannopalladinite too (points 7) at upper part of the microaggregate.Tetraferroplatinum (points 9-11) is an outer rim after the Pd 5 Pb 3 phase.The top most part of this microaggregateis represented by the intergrowth of polarite (white, point 12) with undiagnosed phase due to its small size (as shown in Figure 15g).The outer rim of this intergrovth is composed of a gold-silver alloy in association with zvyagintsevite.The rounded grains of stanopalladinite include zvyagintsevite (point 38), and Pd 5 Pb 3 phase localized in interstices of grains in the middle part of the microaggregate (Figure 14d).Tetraferroplatinum is also included in the stannopalladinite grains (point 35) and occur as a rim around them (point 36).The lower part of the PGM cluster is represented by a porous aggregate of Au-Ag alloys containing Cu and Pd in which tetraferroplatinum and stannopalladinite are included.
Of particular interest are the crooked areas in the grain of chalcopyrite enriched with silver.They are located on the contact of chalcopyrite with PGM. Figure 15h shows the different brightness areas due to silver content ranging from 5.74 to 25.96 wt % (points [59][60][61].The concentration of Ag has a negative correlation with Fe content in this phase with R 2 = 0.9997.

Discussion
The origin of the Norilsk deposits assumes two stages of their formation: The appearance of the sulfide phase in the melt and its further evolution and crystallization.In this paper, we consider only the second aspect of genesis, namely, the structural position of the magmatic and sulfide bodies and possible mechanism of the formation of massive ores from an already existed sulfide melt.
The Norilsk-Kharaelakh Fault was the main structure which controlled the magmas intrusion, their localization and position of the orebodies.First of all, different magma portions ascended from the deep zones to the modern chambers using this large fracture.There were at least three pulses of the magma intrusions that formed three branches: North-Eastern, North-Western and South-Western.Their rear parts are located in the Norilsk-Kharaelakh Fault, while the heads are stretched in the Northern, Western and Eastern directions, respectively (Figure 2).The most complex structure is typical of the South-Western branch, due to the melt entered into the block, restricted by the branches of this major disturbance.Their steep fall to the center of the graben determined the steep inclination of the sides of the South-Western body.Contacts of the other two intrusive branches are more flat.During the cooling of the silicate melt contractile cracks were formed, they were located subparallel to the steeply sloping sides.The main part of the sulfide melt was concentrated inside these cracks.The cracks in the North-Western and North-Eastern branches occur a subhorizontal position, and the massive ores are subhorizontal as well.Thus, the position of the massive ores of the Talnakh deposit was largely determined by the prototectonics of the territory.The Norilsk-Kharaelakh Fault was a long-lived structure, because significant displacements along its branches and feathering cracks occurred after the crystallization of silicate and sulfide melts.As a result, the South-Western branch was shifted not only relative to the North-Eastern and North-Western branches, but it was also subdivided into the separate blocks (Figure 3, II-II cross-section).The Southern part of the Southern 2 orebody was descended at 40 m compared to the Northern one (Figure 7b, VI-VI cross-section).
The Southern 2 orebody composition supports our conclusion that the branches of the Talnakh intrusion were formed from different portions of the silicate melt, which carried a different sulfide melt.The silicate part of these branches were similar (Figure 6) while the sulfide melts were different: In the South-Western branch the sulfide melt was enriched with Cu and PGE in comparison with the melts of the North-Eastern and North-Western branches.The structure of the South-Western branch and the fundamental difference in the composition of ores indicate that the disturbances into the portions took place before intrusion of the magmas.We cannot estimate the time relationship of the intrusive branches, but it is likely that they were formed sequentially, from magmas flowing out of the one channel.
So far, two hypotheses of massive ore origin for the Norilsk-Talnakh ores have been suggested: they were formed either as a result of the intrusion of an independent portion of the sulfide melt [18,19], or as a result of filter pressing processes when the sulfide melt penetrated into the host rocks.
The last model was elaborated in detail by Likhachev [5,8], who demonstrated the pipe-like structure of the intrusive body based on the numerous cross-sections of the North-Western branch of the Talnakh intrusion.The bodies of massive ores are localized under the deflection of the massif bottom sole (Figure 16).The same position of massive ore is also visible in the distribution of ore bodies and isopaches of the Talnakh intrusion, shown in Figure 2. Indeed, the bodies of massive ores are confined to the areas of maximum intrusion thickness.Based on the study of the orientation of olivine crystals in rocks, Likhachev [6,8] proposed an explanation of this fact.At the narrowed and enlarged sections of the intrusion chamber (with a difference in the cross-sectional area), the nature of the flow varies from turbulent to laminar.With the settling of crystals from magma with laminar flow, a planar orientation of the grains relative to the bottom is formed, while with settling from turbulent flow, a non-oriented grain orientation is observed.The study of the crystals' distribution made it possible to conclude that the subsidence of the bulk of the crystals and sulfides settles due to a change in the flow velocity from the narrowed towards the extended sections.Thus, there is a correlation between the thicknesses of disseminated and massive ores in the bottom sagging.Sulfides, penetrated into the host rocks under the intrusion, formed massive ores.A similar picture arises when considering the Kharaelakh branch.Likhachev [8] concluded that the main deposit of the Oktyabr'skoe deposit was formed as a result of the introduction of sulfides with different portions of the melt into the same place.Conclusions on the formation of the Talnakh deposits from different portions of magma were also made by Stekhin [46].penetrated into the host rocks under the intrusion, formed massive ores.A similar picture arises when considering the Kharaelakh branch.Likhachev [8] concluded that the main deposit of the Oktyabr'skoe deposit was formed as a result of the introduction of sulfides with different portions of the melt into the same place.Conclusions on the formation of the Talnakh deposits from different portions of magma were also made by Stekhin [46].In the South-Western branch, the structure of the intrusion and associated mineralization is fundamentally different: the orebodies are confined to steep fractures in the endocontact of the intrusive body.Consequently, the sulfide melt did not penetrateinto the host rocks during the silicate crystallization, as described above, but was concentrated in compression fractures and faults.The mechanism of its concentration basically remained the same (filter-pressing), but the morphology of orebodies changed due to other tectonic conditions and the composition of the surrounding rocks.Different types of morphologies of ores, depending on the physical properties of rocks, were modeled by Saumur and Cruden [70].
The formation of the Southern 2 orebody from a high-copper sulfide melt remains an open question.Usually such a composition is considered as a residual sulfide melt formed during crystallization and moved some distance from the place of its crystallization [8].In this case, at first, the higher-temperature pyrrhotite melt crystallized and stayed in place, while the chalcopyrite melt moved.However, this scenario is possible only after the crystallization of the silicate melt.In this case, the disseminated ores have a chalcopyrite-pyrrhotite composition.However, the main conclusion of our study is that the melt did not migrate through fractures over long distances.It was formed in situ, confirmed by the composition of the disseminated ores accompanying the Southern 2 orebody: they are also enriched in copper and platinum metals and consist of the same Cu minerals (chalcocite, bornite) that are absent in other orebodies of the Norilsk-Talnakh region.This indicates that silicate melt carried out Cu-rich sulfide melt from the deep zone; it did not enter the place from outside.The mineral composition of the Southern 2 deposit also confirms that it crystallized under physical and chemical parameters that differed from those of the main deposit of the Oktyabr'skoe deposit (higher fS2).
Most of the PGE minerals were previously found in other disseminated, veinlet-disseminated and massive ores of the Norilsk-Talnakh region [51,53,55,58,59,[71][72][73][74].All minerals were enriched in copper due to its crystallization from Cu-rich melt.However, the ratio of mineral species and the In the South-Western branch, the structure of the intrusion and associated mineralization is fundamentally different: the orebodies are confined to steep fractures in the endocontact of the intrusive body.Consequently, the sulfide melt did not penetrateinto the host rocks during the silicate crystallization, as described above, but was concentrated in compression fractures and faults.The mechanism of its concentration basically remained the same (filter-pressing), but the morphology of orebodies changed due to other tectonic conditions and the composition of the surrounding rocks.Different types of morphologies of ores, depending on the physical properties of rocks, were modeled by Saumur and Cruden [70].
The formation of the Southern 2 orebody from a high-copper sulfide melt remains an open question.Usually such a composition is considered as a residual sulfide melt formed during crystallization and moved some distance from the place of its crystallization [8].In this case, at first, the higher-temperature pyrrhotite melt crystallized and stayed in place, while the chalcopyrite melt moved.However, this scenario is possible only after the crystallization of the silicate melt.In this case, the disseminated ores have a chalcopyrite-pyrrhotite composition.However, the main conclusion of our study is that the melt did not migrate through fractures over long distances.It was formed in situ, confirmed by the composition of the disseminated ores accompanying the Southern 2 orebody: they are also enriched in copper and platinum metals and consist of the same Cu minerals (chalcocite, bornite) that are absent in other orebodies of the Norilsk-Talnakh region.This indicates that silicate melt carried out Cu-rich sulfide melt from the deep zone; it did not enter the place from outside.
The mineral composition of the Southern 2 deposit also confirms that it crystallized under physical and chemical parameters that differed from those of the main deposit of the Oktyabr'skoe deposit (higher f S2 ).
Most of the PGE minerals were previously found in other disseminated, veinlet-disseminated and massive ores of the Norilsk-Talnakh region [51,53,55,58,59,[71][72][73][74].All minerals were enriched in copper due to its crystallization from Cu-rich melt.However, the ratio of mineral species and the geochemistry of most of them (minor elements) from sulfides of the Southern 2 orebody have a number of distinctive features.Minerals of Pt predominate over Pd minerals in disseminated ores of all ore occurrences of the Norilsk ore deposits, such as isoferroplatinum, sperrylite, cuperite, moncheite and rustenburgite (from 50% to 80% in different ore occurrences) [75].These platinum minerals also predominante as in massive ores of different composition and in the placer of the Medvezhy Creek [76], while the Pt minerals are in a subordinate amount (16%) (Figure 11) in the chalcopyrite ores of the Southern 2 orebody of the Talnakh intrusion.They are mainly represented by tetraferroplatinum and rustenburgite (rare).Typical platinum minerals of the Norilsk ores (isoferroplatinum, sperrylite and cooperite) were not found in the Southern 2 orebody.

Conclusions
1.The South-Eastern branch of the Talnakh intrusion is characterized by a specific structure of silicate and sulfide bodies.The tectonic fractures and faults determined their morphology: the intrusion sides and veins of massive ore fall at steep angle to the center in contrast to other massive ore which take a horizontal position.
2. Ore composition of the Southern 2 orebody differs from the composition of other massive ores including the Cu-rich main body of the Octyabr'skoe deposit.It is characterized by high Cu and PGE (ΣPt + Pd = 278 ppm) and specific mineralogical composition: chalcopyrite associates with chalcocite and bornite in massive and also as in disseminated ore.This is evidence of the formation of massive ore in situ, without displacement along the intrusion's contact.
3. PGE minerals contain Cu as impurity.Many minerals widespread in the Norilsk ore were not found in the Southern 2 orebody, they are isoferroplatinum, sperrylite and cooperite.
4. The South-West branch of the Talnakh intrusion and related ore crystallized from separate portion of the silicate melt combined with a Cu-rich sulfide melt.

Figure 3 .
Figure 3. Geological map of the Talnakh ore junction and cross-sections I-I and II-II (here and in Figure 4 based on data of Ltd.Norilskgeologia with author's corrections) with the longitude sections.The red lines on the map are line of cross section and their numbers.

Figure 3 .
Figure 3. Geological map of the Talnakh ore junction and cross-sections I-I and II-II (here and in Figure 4 based on data of Ltd.Norilskgeologia with author's corrections) with the longitude sections.The red lines on the map are line of cross section and their numbers.

Figure 4 .
Figure 4. Cross-sections III-III and IV-IV of the Talnakh branches (based on authors data and after Likhachev [5]).Captions in Figure 3.

Figure 4 .
Figure 4. Cross-sections III-III and IV-IV of the Talnakh branches (based on authors data and after Likhachev [5]).Captions in Figure 3.

Figure 7 .
Figure 7. Position of the Southern 2 orebody inside the South-Westbranch, the cross-sections V-V (a) and VI-VI (b) (after Ltd.Norilskgeologia data with author's corrections).

Figure 7 .
Figure 7. Position of the Southern 2 orebody inside the South-Westbranch, the cross-sections V-V (a) and VI-VI (b) (after Ltd.Norilskgeologia data with author's corrections).

Figure 8 .
Figure 8. Structure of South-Western branch in borehole EM-6 and PGE distribution in ore.Data in Table3.

Figure 8 .
Figure 8. Structure of South-Western branch in borehole EM-6 and PGE distribution in ore.Data in Table3.

Figure 11 .
Figure 11.Relationships of the PGE minerals in ores of the Southern 2 orebody.

Figure 11 .
Figure 11.Relationships of the PGE minerals in ores of the Southern 2 orebody.

Figure 16 .
Figure 16.Model of positions of disseminated and massive ore in the North-Western branch of the Talnakh intrusion (after Likhachev [8], with changes).

Figure 16 .
Figure 16.Model of positions of disseminated and massive ore in the North-Western branch of the Talnakh intrusion (after Likhachev [8], with changes).

Table
1, No. 24.Quality control of the results of the analysis performed on standard samples of rocks U.S. Geological survey (USGS) BCR-2, DTS-2, BHVO-1 (Table 1, No. 25-30).Analysis of rare elements in rocks was carried out by ICP-MS (Element-2, Thermo Scientific, Darmstadt, Germany) at Lomonosov Moscow State University by Ya.V. Bychkova.The detection limits of elements were from 0.1 ppm for elements of heavy and intermediate atomic weight to 1 ppm for light elements (Table 2, No. 10).The accuracy of the analyses was 1%-3%.The standards were certified samples BHVO-2 and COQ-(Table 2, No. 11-12).

Table 1 .
Chemical composition of the rocks from the Talnakh intrusion, wt %.

Table 2 .
Rare elements in rocks of the Talnakh intrusion, ppm.

Table 1 .
Chemical composition of the rocks from the Talnakh intrusion, wt %.

Table 3 .
Chemical composition of sulfide ore.

Table 3 .
Chemical composition of sulfide ore.

Table 4 .
Minerals of the Southern 2 orebody.

Table 5 .
Composition of ore minerals from the Southern 2 orebody, wt %.

Table 6 .
Mineral compositions in the system Pd-Pt-Cu-Sn from the Southern 2 orebody, wt %.

Table 7 .
Mineral compositions in system Pd-Pb-Bi in the Southern 2 orebody, wt %.

Table 11 .
Chemical composition of platinum group minerals, wt %.