Sulﬁde Minerals as Potential Tracers of Isochemical Processes in Contact Metamorphism: Case Study of the Kochumdek Aureole, East Siberia

: Marly limestones from the Lower Silurian sedimentary units of the Tunguska basin (East Siberia, Russia) underwent metamorphism along the contact with the Early Triassic Kochumdek trap intrusion. At ≤ 2.5 m from the contact, the limestones were converted into ultrahigh-temperature marbles composed of pure calcite and sulﬁde-bearing calcsilicate layers. The sulﬁde assemblages in the gabbro and marbles were studied as potential tracers of spurrite-merwinite facies alteration. The gabbro-hosted sulﬁdes show Fe-Ni-Cu-Co speciation (pyrrhotite and lesser amounts of chalcopyrite, pentlandite, and cobaltite) and positive δ 34 S values (+2.7 to +13.1‰). Both matrix and inclusion sulﬁde assemblages of prograde melilite, spurrite, and merwinite marbles consist domi-nantly of pyrrhotite and minor amounts of troilite, sphalerite, wurtzite, alabandite, acanthite, and galena. In contrast to its magmatic counterpart, metamorphic pyrrhotite is depleted in Cu (3–2000 times), Ni (7–800 times), Se (20–40 times), Co (12 times), and is isotopically light (about –25‰ δ 34 S). Broad solid solution series of (Zn,Fe,Mn)S cub , (Zn,Mn,Fe)S hex , and (Mn,Fe)S cub indicate that the temperature of contact metamorphism exceeded 850–900 ◦ C. No metasomatism or S isotope resetting signatures were detected in the prograde mineral assemblages, but small-scale penetration of magma-derived K- and Cl-rich ﬂuids through more permeable calcsilicate layers was documented based on the distribution of crack-ﬁlling Fe-K sulﬁdes (rasvumite, djerﬁsherite, and bartonite).


Introduction
Petrological analysis of metamorphic rocks commonly aims to reconstruct peak metamorphic conditions and P-T paths from data on rock-forming minerals [1][2][3][4][5]. The scope of genetically informative minerals in petrology was improved with the application of the rare earth elements (REE), Ti, and Zr systematics due to the implementation of high-resolution and isotope geochemical analytical techniques [6,7]. The metamorphic conditions of the spurrite-merwinite facies are useful for examining the formation of high-temperature phases that accumulate trace elements [8][9][10]. Some Ti, U, Zr, Sn, and Sc host phases are The goal of the study is to demonstrate that detailed field examination coupled with systematic geochemical analyses of representative rocks and mineralogical studies of sulfides can shed light on the regime of prograde and retrograde igneous-related metamorphism.

General Information
Diverse sulfide mineralization was reported in the spurrite marbles of the Kochumdek contact aureole [31][32][33][34][35] located in the western termination of the Tunguska basin (the Podkamennaya Tunguska River catchment, East Siberia) [1,2,30,36]. The Tunguska basin is composed of thick sequences (3-7 km in thickness) of Middle Proterozoic to Paleozoic volcano-sedimentary rocks (PR 2 -P 2 ), which are interbedded with mafic sills and buried under flood basalts over a large part of the basin territory. Continental flood basalt magmatism produced the large igneous province of Siberian Traps at the Permian-Triassic boundary [37][38][39]. The flood basalts cover an area of~330,000 km 2 and are localized mainly within the Lower Paleozoic (O-S) strata of the Tunguska basin ( Figure 1) [37,40,41].

Site Description
The sampled contact aureole is located along the right bank of the Kochumdek River (62 • 27 54.59" N, 91 • 55 42.99" E) at an elevation of 107 m above sea level. This is a permafrost area with a hilly taiga landscape, deeply incised river valleys and flat watersheds that are partly covered by Quaternary glacial deposits (Q II-III ). Permafrost degradation in the Holocene post-glacial period produced an erosion cutout till at the top of a sill within 15 km in the lower reaches of the river. An outcrop in the steep river bluffs exposes a continuous sequence of Ordovician pelitic sediments and Llandovery marly limestone intruded by two concordant sills at elevations of 280-320 m and 360-410 m (Figures 1 and 2A).
The largest part of the marble-sill contact is inaccessible, being either submerged below the Kochumdek River level or buried under snow and aufeis. High-grade spurritemerwinite marbles [30] were sampled immediately in the river during a low stand from a horizontal contact between the sill and the overlying carbonate-marl marker bed of the Lower Kochumdek Subformation. Presumed remnants of a large aureole appear on the riverside and on a low right-side terrace above the contact as scattered blocks of tilleyitewollastonite marble and recrystallized marly limestone (Figure 2A-D), where most of the blocks have horizontal bedding and appear autochthonous ( Figure 2B,C). Their origin from an aureole, which was destroyed by postglacial erosion but survived in sags of the upper sill margin, can be inferred from the geological context ( Figure 4B).  The S-T1 sediments lying over the Kochumdek sill were about 700 m thick at the time of its emplacement in the Early Triassic, which corresponds to a pressure of 200 bar. Based on poly-mineral assemblages in the marbles, the intruding magma is inferred to have heated the sediments to 700 to 900 °C [1,2,30]: ≥ 900 °C in Zone 2; ≥ 750 °C in Zone 3; and ≥ 700 °C in Zone 4 ( Figure 4A); the temperature within Zone 5 recorded in the isobaric invariant assemblage of calcite + actinolite + K-feldspar + diopside + biotite was no higher than 450-500 °C [49]. The P-T conditions of the Kochumdek contact metamorphism have been reliably constrained, which makes it a natural reference system suitable for the study of the ensuing compositional changes.
In general, the aureole is free from evident imprints of closely related metasomatic changes or net transfer reactions. Deformation signatures are rare or absent. The rocks appear virtually homogeneous, free from textural effects of deformation and mechanical mixing (namely, local reorientation or recrystallization of mineral grains, strong preferred orientation, or crystal size patterns). Both sill and metamorphic or sedimentary rocks near the contact lack breccias, cataclastics, healed cracks, or veining networks. The~4 m thick ( Figure 4A) aureole results from prograde metamorphism of marly limestone by a heat flux rising from the sill, with an initial magma temperature of~1200 • C. The thermal event produced three high-grade mineral assemblages at progressively closer distances to the contact: (i) calcite + tilleyite + wollastonite + melilite (Zone 4: 1.5-2.6 m), (ii) calcite + spurrite + monticellite + melilite + perovskite (Zone 3: 1.0-1.5 m), and (iii) calcite + spurrite + merwinite + monticellite + melilite + perovskite (± rankinite, bredigite) (Zone 2: 0.3-0.5 m). The edge of the high-grade contact metamorphic aureole at~2.6 m is marked by an abrupt change from tilleyite-wollastonite marble to recrystallized marly limestone of a similar appearance (Zone 5). The top of the sill is delineated by a very thin (1-3 cm) and discontinuous skarn of Zone 1 [2,30].

Sampling
The S-T 1 sediments lying over the Kochumdek sill were about 700 m thick at the time of its emplacement in the Early Triassic, which corresponds to a pressure of 200 bar. Based on poly-mineral assemblages in the marbles, the intruding magma is inferred to have heated the sediments to 700 to 900 • C [1,2,30]: ≥ 900 • C in Zone 2; ≥ 750 • C in Zone 3; Minerals 2021, 11, 17 7 of 43 and ≥ 700 • C in Zone 4 ( Figure 4A); the temperature within Zone 5 recorded in the isobaric invariant assemblage of calcite + actinolite + K-feldspar + diopside + biotite was no higher than 450-500 • C [49]. The P-T conditions of the Kochumdek contact metamorphism have been reliably constrained, which makes it a natural reference system suitable for the study of the ensuing compositional changes.
In general, the aureole is free from evident imprints of closely related metasomatic changes or net transfer reactions. Deformation signatures are rare or absent. The rocks appear virtually homogeneous, free from textural effects of deformation and mechanical mixing (namely, local reorientation or recrystallization of mineral grains, strong preferred orientation, or crystal size patterns). Both sill and metamorphic or sedimentary rocks near the contact lack breccias, cataclastics, healed cracks, or veining networks.

Sampling
About 150 samples of gabbro, marble, and limestone were collected during the field trips of 1981 and 2017 to the Kochumdek River. The highest-grade spurrite-merwinite marbles were sampled in 1981 in the dried riverbed by Dr. V.Yu. Kolobov. The very top of the sill as well as lower-grade marble and marly limestone was sampled in 2017 ( Figure 2B-E). The sampling site was a 1-km-long and 70-100-m wide band from the intrusive contact into the sediments (Figure 2A). A total of sixty-six representative samples, including gabbro (12), spurrite (19) and tilleyite (10) marbles, and recrystallized marly limestones (25) were selected for a detailed mineralogical study.

Analytical Procedures
Analyses were mostly carried out at the Analytical Center for Multi-Elemental and Isotope Research (Sobolev Institute of Geology and Mineralogy (IGM, Novosibirsk, Russia)) and at the South Ural Research Center of Mineralogy and Geoecology (SU FRC MG, Miass, Russia). Major elements in bulk-rock samples were analyzed by the ICP-AES technique (inductively coupled plasma atomic emission spectroscopy) on a ThermoJarrell IRIS Advantage atomic emission spectrometer (ThermoJarrell Intertech Corporation, Atkinson, WI, USA) at IGM (Novosibirsk). The preconditioning procedure included fusion of powdered whole-rock samples with lithium borate [50]. Trace elements in bulk samples of gabbro, marble, and limestone were determined by mass spectrometry with inductively coupled plasma (ICP-MS) on an Agilent 7700x spectrometer (Agilent Technologies, Inc., Santa Clara, CA, USA) at the SU FRC MG (Miass, Russia). The method was slightly modified from those reported in [51,52] and is described in detail by Sokol et al. (2020) [53].
Scanning electron microscopy (SEM) was used to identify minerals and to characterize the phase distribution and morphology of sulfides, based on energy-dispersive spectra (EDS), backscattered electron (BSE) images, and X-ray element maps (EDS system). The measurements were performed at IGM (Novosibirsk) on a TESCAN MIRA 3LMU microscope (Tescan Orsay Holding, Brno, Czech Republic) equipped with an Oxford AZtec Energy Xmax-50 microanalyses system (Oxford Instruments Nanoanalysis, Abingdon, UK). For SEM examination, thin sections were sputter coated with~15 to 25-nm carbon films. EDS analyses of minerals were run in a high-vacuum mode at an accelerating voltage of 20 kV, a beam current of 1.5 nA, and a live spectra acquisition time of 20 s. The results were checked against reference standards of simple compounds and metals for most of the elements.
The trace-element composition of sulfides (pyrrhotite, sphalerite and rasvumite) was determined by laser ablation mass spectrometry with inductively coupled plasma (LA-ICP-MS) at the SU FRC MG (Miass, Russia). The LA-ICP-MS analyses were run on a NewWave Research UP-213 laser ablation system coupled with an Agilent 7700x (Agilent Technologies, Inc., Santa Clara, CA, USA) plasma mass spectrometer. The procedure is outlined in [54], with an Nd: YAG UV source, frequency quadruple (wavelength 213 nm) with fluence settings of 2.5-3.5 J/cm 2 , helium cell carrier gas and a gas flow rate of 0.6-0.7 L/min. Polished sections of marble, gabbro, and marly limestone were made using the standard equipment and following the procedure described in [55]. The texture of the samples was examined by SEM with a special focus on inhomogeneities in FeS, ZnS and KFe 2 S 3 crystals (e.g., zoning or mineral inclusions). The large (50-200 µm) grains selected for analyses were free from visible inclusions or signatures of alteration. The LA-ICP-MS method was of limited applicability for sphalerite and rasvumite, because they were commonly too small (only 20 µm across) and often contained sub-micrometer mineral inclusions. The element contents were calibrated against reference materials of USGS MASS-1 and USGS GSD-1g using 66 Zn as an internal standard for sphalerite, and 57 Fe for pyrrhotite and rasvumite, respectively [56,57]. All mass fractions for USGS MASS-1 and USGS GSD-1g were taken from the GeoReM base preferred values. The calibration standard was analyzed every 10-15 spots to account for the instrument drift. Data processing was carried out using the Iolite software package [58].
Raman spectroscopy was used for express diagnostics of ZnS and MnS polymorphic modifications and to discriminate between djerfisherite and chlorbartonite. Raman spectra were recorded on a Horiba Jobin Yvon LabRAM HR800 spectrometer (Horiba Jobin Yvon S.A.S., Lonjumeau, France). The spectra were excited with a 532-nm green line of a Thorlabs 50-mW power neodymium Nd:YAG laser working at double-harmonic frequency. The radiated laser power was attenuated with optical filters and the beam power onto the sample surface in range 1.5-0.05 mW. The scattered light was recorded by an Andor 1024-channel Peltier-cooled CCD detector (Oxford Instruments, Belfast, Northern Ireland) in a region of 0 to 1000 cm −1 at a resolution of 2 cm −1 . The acquisition time for the Raman spectra varied from 10 to 100 min. The excitation was with Olympus objectives at magnification of ×100 (WD = 0.2 mm) for green line. The Raman spectra were deconvolved into Voigt amplitude functions using the Model S506 Interactive PeakFit software (2002 Spectroscopy Software, Canberra Industries, Meriden, CT, USA) [59]. For method details see [33]. The collected spectra were compared with reference spectra from RRUFF databases (The RRUFF™ Project), as well as with the spectra of synthetic phases.

Marbles
The Kochumdek marbles are dense fresh rocks with a coarse banded structures consisting of light gray (mainly calcitic) and darker (silicate) bands inherited from the sedimentary protoliths ( Figure 3E-G). Merwinite marbles have the coarsest grains of 0.5-4 mm to 1-2 cm [2]. The marbles share major-element similarity with the precursor Lower Kochumdek Subformation marly limestone, except for depletions in Na2O and K2O and lower CO2 contents (Table 3; Figure 6A). The contents of other major oxides are controlled by silicate-to-carbonate ratios.  Figure 6B).

Marbles
The Kochumdek marbles are dense fresh rocks with a coarse banded structures consisting of light gray (mainly calcitic) and darker (silicate) bands inherited from the sedimentary protoliths ( Figure 3E-G). Merwinite marbles have the coarsest grains of 0.5-4 mm to 1-2 cm [2]. The marbles share major-element similarity with the precursor Lower Kochumdek Subformation marly limestone, except for depletions in Na 2 O and K 2 O and lower CO 2 contents (Table 3; Figure 6A). The contents of other major oxides are controlled by silicateto-carbonate ratios.  [34,61]. Melilite from Zone 4 has high soda-melilite (CaNaAlSi 2 O 7 , up to 16 mol%) and åkermanite (Ca 2 MgSi 2 O 7 , up to 68 mol%) contents, whereas that from Zone 2 approaches refractory gehlenite (Ca 2 Al 2 SiO 7 , up to 80 mol%). The accessories are widespread and randomly distributed, and include perovskite and Fe, K, Zn, Mn, Pb, and Ag sulfides, which mainly occur in the silicate layers ( Table 2).

Recrystallized Marly Limestones, Zone 5
The recrystallized marly limestone of Zone 5 (Table 2; Figures 7 and 10A-C) contains abundant sulfide mineralization (six primary and three secondary phases), which is mainly hosted by the metapelitic layers. Pyrrhotite is the most abundant sulfide occurring as 300-700 µm crystals or anhedral grains with a particular spongy structure that records its origin by integrated recrystallization ( Figure 10B,C). The mineral encloses thin (1-5 µm) wavy pyrrhotite-troilite exsolution lamellae, with the Fe/S ratio ranging from 0.9 to 1.0 (Table 4). Framboidal pyrite, which is a common phase in the sedimentary protolith, is very rarely preserved in Zone 5, as well as 3-10 µm inclusions of chalcopyrite, galena and arsenopyrite. Many pyrrhotite grains bear evidence of dissolution and are rimmed by secondary djerfisherite and rasvumite. The rims occasionally contain a phase with a composition resembling Cl-free djerfisherite, which we tentatively interpret as bartonite.
monly of Fe, Pb, and Ag compositions (pyrrhotite, galena, and acanthite), while Mn and Zn sulfides are limited to a few cases in pyrrhotite, and phases of Cu, Co, and Ni are absent.
Sulfide grains in the matrix are relatively abundant (up to 1%), diverse (Table 5), and localized in silicate layers or at their boundary with the carbonate layers. No sulfides were found in the matrix of calcite layers. Sulfide inclusions in calcite (mainly pyrrhotite and very rare unidentified varieties of ZnS) are extremely rare and restricted to the carbonatesilicate boundary.
Matrix sulfides are generally much larger than sulfide inclusions and reach up to 500 μm in size ( Figure 10D). These include Fe, Zn, K, and Mn compounds (Table 5), which are dominated by subhedral or euhedral pyrrhotite (100-500 μm) at the boundaries of spurrite, melilite and calcite grains ( Figure 10E). Pyrrhotite shows typical exsolution textures with fine (≤10 μm) lamellae and Fe/S ratio ranging from 0.9 to 1.0 (Table 4). Some pyrrhotite grains in samples from Zones 2 and 3 are overgrown by Mn-bearing magnetite containing 0.6 wt% TiO2 and up to 150 ppm V2O5. The inner and outer boundaries of ≤30 μm magnetite rims are roughly parallel and generally mimic the contours of the primary pyrrhotite grains ( Figure 10F), which indicates overgrowth without replacement reactions. Some matrix pyrrhotite grains host 10-30 μm inclusions of alabandite, sphalerite or wurtzite ( Figure 10F) and tiny blebs of galena and acanthite. The textural control of their distribution is independent of lamellae edges.

Marbles Sulfides of Merwinite-and Spurrite-Monticellite Zones 2 and 3
The marbles of Zone 2 contain large merwinite grains, along with other ultrahightemperature index minerals, including spurrite, gehlenite-rich melilite and sporadic rankinite, and bredigite. Sediments in this zone were heated to temperatures no lower than 900 • C. Some marbles from spurrite-monticellite Zone 3, where the temperature was at least 750 • C, likewise contain merwinite as relict phases or small inclusions.
Sphalerite occurs exclusively in the highest-grade marbles as 80-100 μm anhedral grains commonly intergrown with pyrrhotite (Table 6). Wurtzite, identified by Raman spectroscopy, forms sporadic 3-20 μm inclusions in matrix pyrrhotite or less often in coarse rasvumite-I grains ( Table 6; Figure 12G). Pyrrhotite frequently appears as altered pseudo-inclusions, partly or fully replaced by rasvumite, that fits the definition of a retrograde phase according to the criteria of Brown et al. (2014) [27]. The sulfide grains are connected with their melilite or perovskite hosts by tiny cracks resolvable in SEM images ( Figure 11). Many pseudo-inclusions contain unaltered blebs of galena, acanthite (<1-5 µm) or (Zn, Fe, Mn) S, like the inclusions of fresh pyrrhotite ( Figure 12I).  Sulfide inclusions in this zone are mainly <10 μm pyrrhotite grains usually trapped in melilite rims. Tiny blebs of alabandite, galena, and acanthite are very rare and are commonly enclosed in pyrrhotite (Table 5). Pyrrhotite is the only matrix sulfide and occurs as grains up to 500 μm in size with thin lamellae of troilite. In addition, pyrrhotite is occasionally intergrown with up to 100 μm round grains of alabandite (Table 6), but more often contains the latter as fine inclusions.

Retrograde sulfides
Retrograde sulfides in the Kochumdek marbles include K-Fe minerals, such as abundant rasvumite-II with minor bartonite and K-Fe(± Cu, Ni) sulfide-chloride (djerfisherite) ( Table 5; Figures 12 and 13). Djerfisherite and rasvumite-II (unlike prograde rasvumite-I) Thus, sulfide inclusions in the marbles of high-temperature Zones 2 and 3 are commonly of Fe, Pb, and Ag compositions (pyrrhotite, galena, and acanthite), while Mn and Zn sulfides are limited to a few cases in pyrrhotite, and phases of Cu, Co, and Ni are absent.
Sulfide grains in the matrix are relatively abundant (up to 1%), diverse (Table 5), and localized in silicate layers or at their boundary with the carbonate layers. No sulfides were found in the matrix of calcite layers. Sulfide inclusions in calcite (mainly pyrrhotite and very rare unidentified varieties of ZnS) are extremely rare and restricted to the carbonate-silicate boundary.
Matrix sulfides are generally much larger than sulfide inclusions and reach up to 500 µm in size ( Figure 10D). These include Fe, Zn, K, and Mn compounds (Table 5), which are dominated by subhedral or euhedral pyrrhotite (100-500 µm) at the boundaries of spurrite, melilite and calcite grains ( Figure 10E). Pyrrhotite shows typical exsolution textures with fine (≤10 µm) lamellae and Fe/S ratio ranging from 0.9 to 1.0 (Table 4). Some pyrrhotite grains in samples from Zones 2 and 3 are overgrown by Mn-bearing magnetite containing 0.6 wt% TiO 2 and up to 150 ppm V 2 O 5 . The inner and outer boundaries of ≤30 µm magnetite rims are roughly parallel and generally mimic the contours of the primary pyrrhotite grains ( Figure 10F), which indicates overgrowth without replacement reactions. Some matrix pyrrhotite grains host 10-30 µm inclusions of alabandite, sphalerite or wurtzite ( Figure 10F) and tiny blebs of galena and acanthite. The textural control of their distribution is independent of lamellae edges.
The (Mn,Fe)S and (Zn,Fe,Mn)S modifications were identified by Raman spectroscopy. Wide absorption bands at~228 and~342 cm -1 in the Raman spectra of the (Mn,Fe)S phase represent an α-MnS modification (Fm3m) [62] corresponding to alabandite. Wurtzite is identifiable from the 348, 419, and 628 cm -1 bands that fit the 350,~418 mon , and~630 cm -1 bands of standard ZnS hex [63]. The Raman spectra of Fe-bearing sphalerite show two strong bands at 301 and 330 cm -1 and match with the 300 and 331 cm -1 bands in the synthetic compound (Zn 0.84 Fe 0.16 S) cub [64].
Thus, the marbles from high-temperature Zones 2 and 3 share similarities between the inclusion and matrix assemblages. They mainly consist of Fe, Mn, Zn, Pb, and Ag sulfides (e.g., pyrrhotite, Zn-Mn-Fe sulfides, galena, and acanthite). Single-crystal rasvumite-I occurs only in the matrix, whereas rasvumite in inclusions are rather a retrograde secondary phase that replaces pyrrhotite. Cu, Co and Ni phases are absent and the presence of these elements was not detected in SEM-EDS point analyses of pyrrhotite or rasvumite-I in both inclusion and matrix assemblages from the highest-grade metamorphic zones.
Sulfide inclusions in this zone are mainly <10 µm pyrrhotite grains usually trapped in melilite rims. Tiny blebs of alabandite, galena, and acanthite are very rare and are commonly enclosed in pyrrhotite (Table 5). Pyrrhotite is the only matrix sulfide and occurs as grains up to 500 µm in size with thin lamellae of troilite. In addition, pyrrhotite is occasionally intergrown with up to 100 µm round grains of alabandite (Table 6), but more often contains the latter as fine inclusions.

Retrograde Sulfides
Retrograde sulfides in the Kochumdek marbles include K-Fe minerals, such as abundant rasvumite-II with minor bartonite and K-Fe(± Cu, Ni) sulfide-chloride (djerfisherite) ( Table 5; Figures 12 and 13). Djerfisherite and rasvumite-II (unlike prograde rasvumite-I) are common in all metamorphic zones but are absent in the sill top. Rasvumite-II commonly occurs in pyrrhotite reaction rims where the two phases are separated by a dissolution boundary which has an uneven profile. Pyrrhotite is partly replaced, and rasvumite-II crystallizes asymmetrically, being thickest on the crack-facing side of pyrrhotite grains. The rasvumite rims are the thinnest next to the contact melilite and monticellite (up to 15 µm) but reach 50-70 µm at the boundary with spurrite ( Figure 12B,C). Many rims of this kind are doublelayered, with a wider rasvumite-II zone adjacent to pyrrhotite and a thin and discontinuous outer zone of djerfisherite ( Figure 12B-E). Rasvumite-II microcrysts are limited to a few cases where rims have expanded into the open crack space ( Figure 12F). Fully replaced pyrrhotite grains were not observed in the matrix but often appear as pseudo-inclusions ( Figure 12I), while the tiny blebs of galena, (Zn, Fe, Mn) polymorphic modifications, alabandite, and acanthite remain intact ( Figure 13). Less often, rasvumite overgrows around pyrrhotite rather than replacing it, where it forms rims up to 85 µm thick. This kind of pyrrhotite-rasvumite boundary is usually even in shape and free from dissolution features ( Figure 12A).
Bartonite is a tetragonal analog of cubic djerfisherite with variable contents of Cl and S, where Cl may be absent in the ultimate (end-member) case [65][66][67]. The Cl-free analogs of djerfisherite in the Kochumdek aureole coexist with secondary rasvumite and djerfisherite phases in Zones 4 and 5. The identification of bartonite is quite tentative and stems from the absence of Cl in the SEM-EDS spectra. XRD and Raman analyses were hampered due to the small grain sizes and intricate intergrowths of bartonite with other K-Fe sulfides. Retrograde sulfide mineralization is especially abundant in the marbles of Zone 4, where pyrrhotite is most strongly affected by dissolution and partially-to-completely replaced by retrograde rasvumite-II, djerfisherite and bartonite ( Figure 13).
Rasvumite-II microcrysts are limited to a few cases where rims have expanded into the open crack space ( Figure 12F). Fully replaced pyrrhotite grains were not observed in the matrix but often appear as pseudo-inclusions ( Figure 12I), while the tiny blebs of galena, (Zn, Fe, Mn) polymorphic modifications, alabandite, and acanthite remain intact ( Figure  13). Less often, rasvumite overgrows around pyrrhotite rather than replacing it, where it forms rims up to 85 μm thick. This kind of pyrrhotite-rasvumite boundary is usually even in shape and free from dissolution features ( Figure 12A). Bartonite is a tetragonal analog of cubic djerfisherite with variable contents of Cl and S, where Cl may be absent in the ultimate (end-member) case [65,66,67]. The Cl-free analogs of djerfisherite in the Kochumdek aureole coexist with secondary rasvumite and djerfisherite phases in Zones 4 and 5. The identification of bartonite is quite tentative and stems from the absence of Cl in the SEM-EDS spectra. XRD and Raman analyses were hampered due to the small grain sizes and intricate intergrowths of bartonite with other K-Fe sulfides. Retrograde sulfide mineralization is especially abundant in the marbles of Zone 4, where pyrrhotite is most strongly affected by dissolution and partially-to-completely replaced by retrograde rasvumite-II, djerfisherite and bartonite ( Figure 13).  (Tables 4 and 7). LA-ICP-MS analysis of coarse pyrrhotite grains confirms the concentration of Co (3643-5260 ppm) and Ni (3027-9230 ppm) and reveals additional impurities of ≤192 ppm Se and <65 ppm Mn (Table 8; Figure 14). Other chalcophile elements are minor: 1.49 to 6.83 ppm Ag, V, As, Pb, Hg, Ge, Mo, Zn, and Cu, and 0.20 to 0.69 ppm Os, Ru, Tl, Pd, Bi, Te, Ga, and Sn. The trace-element compositions of cobaltite (CoAsS), sphalerite, and pentlandite remain unconstrained because of small grain sizes.  and Cu reaching, respectively, 0.81 wt%, 0.66 wt%, and 0.16 wt% (Tables 4 and 7). LA-ICP-MS analysis of coarse pyrrhotite grains confirms the concentration of Co (3643-5260 ppm) and Ni (3027-9230 ppm) and reveals additional impurities of ≤192 ppm Se and <65 ppm Mn (Table 8; Figure 14). Other chalcophile elements are minor: 1.49 to 6.83 ppm Ag, V, As, Pb, Hg, Ge, Mo, Zn, and Cu, and 0.20 to 0.69 ppm Os, Ru, Tl, Pd, Bi, Te, Ga, and Sn. The trace-element compositions of cobaltite (CoAsS), sphalerite, and pentlandite remain unconstrained because of small grain sizes.  (Table  8).

Marbles
Pyrrhotite (Fe 1-x S) from the highest-temperature merwinite assemblages of Zone 2 (T ≥ 900 • C) has a pure composition, with low contents of all trace elements, except for Co (≤0.15 wt%) and within 246 ppm Mn, 10 ppm Ni and Se, and 1 ppm Zn. Pyrrhotite from spurrite-monticellite assemblages in Zone 3 (T ≥ 750 • C) has comparable contents of Co, Se, and Ag reaching 515 ppm, 3.70 to 6.50 ppm, and 0.22 ppm, respectively, but contains more Ni and Mn (up to 913 ppm and 509 ppm, respectively). The contents of Zn (≤36 ppm) and Cu (≤32 ppm) are about ten times higher than the respective values for Zone 2 (Tables 4 and 8; Figure 14).
Alabandite grains are homogeneous and have a large range of MnS/FeS ratios (Mn 0.76-0.89 Fe 0.12-0.25 S) (Figure 15). The range of (Mn,Fe)S solid solutions narrows down with increasing temperature of metamorphism. The contents of Fe are especially variable (7.53 to 15.68 wt%) in alabandite from Zone 4, but is from 8.24 to 11.53 wt% in Zone 2 and 7.49 to 9.99 wt% in Zone 3 ( Table 6). The mineral is free from Mg, Cr, and V impurities, which are common in high-temperature alabandite from meteorites [68].
The rims of retrograde rasvumite-II replacing pyrrhotite were too narrow for traceelement analysis, but the impurities must be low as in prograde rasvumite-I based on the near 100% totals (Table 9). On the other hand, djerfisherite replacing rasvumite in the rims shows systematic enrichments in Ni (0.50-11.01 wt%) and Cu (0.25-5.22 wt%) ( Table 10). Persistence of oxygen revealed in X-ray elemental maps of rasvumite in rims and crackfilling rasvumite-II indicates both replacements by djerfisherite and partial oxidation of late-generation rasvumite ( Figure 16).  The rims of retrograde rasvumite-II replacing pyrrhotite were too narrow for traceelement analysis, but the impurities must be low as in prograde rasvumite-I based on the near 100% totals (Table 9). On the other hand, djerfisherite replacing rasvumite in the rims shows systematic enrichments in Ni (0.50-11.01 wt%) and Cu (0.25-5.22 wt%) ( Table 10). Persistence of oxygen revealed in X-ray elemental maps of rasvumite in rims and crackfilling rasvumite-II indicates both replacements by djerfisherite and partial oxidation of late-generation rasvumite ( Figure 16).   Djerfisherite has a highly variable composition, with relatively constant K (8.93-9.90 wt%), Fe in a range of 42.85 to 53.51 wt%, 1.08 to 1.57 wt% Cl, 32.75 to 34.89 wt% S, and Na content below the detection limits of both SEM-EDS and EPMA techniques ( Table 10). The ranges of Ni, Cu, and Co are quite large. Coarse particles of djerfisherite are often compositionally homogeneous, though Fe, Ni, Cu, and Co contents in individual grains may differ even within half of a thin section area ( Figure 17). The distribution of djerfisherite enriched or depleted in these metals shows some pattern: the contents of Ni and Co are the highest (reaching 11.01 wt% and 1.00 wt%, respectively) in djerfisherite near the sill margin in Zone 2 and the lowest (<2.00 wt% Ni and <0.01 wt% Co) 1.  (Table 11).

Stable Sulfur Isotopes in Pyrrhotite
The sulfur isotope composition was analyzed in hand-picked pyrrhotite fractions selected from the Kochumdek gabbro, marly limestone, recrystallized limestone (Zone 5), and marble (Zones 2 and 4) samples (Table 12; Figure 18). Pyrrhotite from both parent and recrystallized limestones has the lightest sulfur isotope composition with a narrow δ 34 S range of -28.68 to -23.3‰ corresponding to common S-bearing organic matter of marine anoxic sediments [88]. Pyrrhotite from marbles is likewise depleted in 34 S, with δ 34 S in the -25.36 to -25.18‰ range in Zone 2 and slightly less negative δ 34 S values in Zone 4 (-15.07‰). Pyrrhotite from gabbro has positive δ 34 S from +2.68 to +13.10‰, which can be a sign of different scale contamination of mantle-derived basic sulfides (~0‰) with evaporitic sulfate (δ 34 S about +20‰) derived from seawater [88,89]. Table 12. Sulfur isotope compositions of pyrrhotite from recrystallized marly limestones and marbles from the Kochumdek contact aureole, and gabbro from the Kochumdek trap, compared with stratotype marly limestone of the Lower Kochumdek Sub-formation.

Sample
Rock

Stable Sulfur Isotopes in Pyrrhotite
The sulfur isotope composition was analyzed in hand-picked pyrrhotite fractions selected from the Kochumdek gabbro, marly limestone, recrystallized limestone (Zone 5), and marble (Zones 2 and 4) samples (Table 12; Figure 18). Pyrrhotite from both parent and recrystallized limestones has the lightest sulfur isotope composition with a narrow δ 34 S range of -28.68 to -23.3‰ corresponding to common S-bearing organic matter of marine anoxic sediments [88]. Pyrrhotite from marbles is likewise depleted in 34 S, with δ 34 S in the -25.36 to -25.18‰ range in Zone 2 and slightly less negative δ 34 S values in Zone 4 (-15.07‰). Pyrrhotite from gabbro has positive δ 34 S from +2.68 to +13.10‰, which can be a sign of different scale contamination of mantle-derived basic sulfides (~0‰) with evaporitic sulfate (δ 34 S about +20‰) derived from seawater [88,89].  Table 12) and with data for S-rich organic matter after [88], mantle after [28], and Cambrian seawater sulfate after [90].  Table 12) and with data for S-rich organic matter after [88], mantle after [28], and Cambrian seawater sulfate after [90].

General Geochemical Features of Adjacent Igneous and Sedimentary Rocks
The limestones of the Tunguska basin underwent intense phase changes upon interaction with the high temperature (at least 1200 • C [30]) mafic magma intrusion during the Early Triassic. High temperature spurrite-merwinite metamorphism was restricted to a narrow zone within 2.5-3 m away from the contact. The top of the Kochumdek sill bears no evidence of auto-metasomatism or vein mineralization, while both the contact and interior parts of marbles lack breccias or vein stockworks, and skarns are limited to a 3 cm thick discontinuous zone. In general, the local geology of the aureole records a regime of thermal metamorphism close to an ideal model of isochemical transformations with only a minor role of metasomatic agents. In order to find out how much the natural process matched the model and why such exceptional conditions arose in the Kochumdek aureole, we compared the geochemical features of the rocks and studied the composition and distribution of sulfide mineralization.
The contrasting trace-element compositions of the Kochumdek limestone and igneous samples allow for the use of geochemical tracers to decipher the fluid flow patterns in the contact area by estimating the relative extent of the fluid flow, as well as the distribution of sulfide minerals as carriers of fluid components. The Cr, Ni, Co, Cu, V, Sc and Cl inputs and isotopically heavy sulfur would have left an imprint on the marbles if they were affected by metasomatic fluids released from the cooling igneous body. However, no such signatures of element transport would exist in the case of isochemical contact metamorphism. We focused the study on sulfides in the aureole rocks, largely due to their diversity where in addition to ubiquitous pyrrhotite, the sulfide assemblage includes numerous rare phases, each being a potential proxy of growth conditions. Another reason is that sulfides are sensitive to re-crystallization and re-equilibrate promptly in response to changes in the growth medium.
The interaction between the gabbro from the top of the Kochumdek sill and marly limestones exhibits a number of informative geochemical and mineralogical fingerprints. The Kochumdek gabbro samples correspond to intraplate basalts according to mineralogy, bulk-rock chemistry and trace-element composition [42,91]. They are marked by Fe-, Ti-, Vand Sc-enrichments, moderate-to-low Cr, Ni, Co, and Cu contents, and low concentrations of S, Mn, and Zn. Apatite-group minerals and late phases in the gabbro contain appreciable amounts of chlorine but lack sulfate sulfur, which is an indication of low oxygen fugacity in the crystallizing melt [21]. The same conditions, as well as the redox regime of cooling basaltic magma, controlled the accessory mineral assemblage. Magnetite, common to these rocks, is enriched in Ti and V and coexists with ilmenite. Sulfide mineralization is scarce (Fe, Cu, Ni, and Co sulfides), where pyrrhotite is predominant and enriched in Ni, Co, and Se, depleted in Mn, and contains trace amounts of Mo and Ag (Table 8; Figure 14). Pyrrhotite from the gabbro is defined by δ 34 S from +2.68‰ to +13.10‰, which differs significantly from the isotopically light sulfur in sulfides from the Lower Silurian limestones and marbles ( Figure 18; Table 12).
The variable and higher δ 34 S of igneous rocks in the Tunguska basin can be attributed to contamination of basaltic magma with Ordovician limestones that bear gypsum and/or anhydrite with high δ 34 S values (+21.0 to +32.2‰) [92] or Middle and Early Cambrian evaporates (δ 34 S from +9 to +2‰). The latter effect is especially prominent in the Norilsk area in the northern flank of the basin [28,68,93].
Intact and recrystallized marly limestones are depleted in Cr, Ni, Co, Cu, V, and Sc relative to the gabbro samples, but are notably more enriched in isotopically light sulfur and Mn, and occasionally show positive Zn spikes (Table 8; Figure 14). Their accessory authigenic mineralization is largely composed of pyrite (originally As-enriched, typical of marine anoxic sediments [94][95][96], As-bearing pyrrhotite, arsenopyrite, as well as unevenly distributed galena and sphalerite. The mineralogy of sulfides and in particular, exsolution textures, are of broad use as a proxy for reconstructing the cooling history of meteorites [87]. Multiphase and multistage sulfide assemblages have been documented extensively in skarn deposits [26]. However, little is known of the difference between inclusion and matrix sulfide assemblages in contact metamorphic rocks. In this respect, the zoned Kochumdek aureole may be an advantageous test site, as it appears to have undergone a relatively simple set of events and mainly quasi-isochemical metamorphism. There are several key features known about the aureole which render it a suitable reference: (i) the exact intrusion body, which triggered thermal alteration of sediments, (ii) the sedimentary precursor rocks, (iii) there was a single thermal event, and (iv) there are numerical constraints on the duration of heating in each metamorphic zone [1,2,29,30].

Sulfides in
The features and timing relationships of sulfides and their assemblages in the Kochumdek marbles are interpreted following the ideology of Brown et al. (2014) [27]. Sulfide inclusions in prograde rock-forming minerals are considered to be isolated and thus unmodified by subsequent retrograde fluid-mineral interactions. Monomineralic inclusions are considered to be relics of peak metamorphic conditions (except for possible polymorphic transformations). They reflect the scope of the respective solid solutions and the trace-element peculiarity of individual minerals which were stable under those conditions. Polyphase sulfide assemblages within a single inclusion are either intergrowths of syngenetic high-temperature phases or decomposition products of complex precursors which broke down upon cooling. The main isochemical changes in response to temperature drop, with no reaction between sulfide phases, include: (i) polymorphic transformations, (ii) exsolution (compositional change of solid solutions), and (iii) the breakdown of high-temperature stoichiometric phases to multiphase assemblages of the same overall bulk compositions but different stoichiometry for each newly formed phase [27]. As a consequence, the sulfide assemblages occurring as inclusions may be different from those originally incorporated under the peak temperature conditions [97]. However, the bulk chemistry of such multiphase inclusions also remains invariable and provides information about sulfides that existed at the time of their entrapment by crystallizing prograde rock-forming minerals [27].
The sulfide inclusions surrounded by tiny cracks or connected via radial cracks with the matrix were exposed to retrogressive fluids and can be classified as pseudoinclusions [27]. Their compositional difference from true (i.e., isolated) inclusions provides insights into some of the peculiarities of retrogressive fluids (e.g., sulfur activity, major-ion chemistry and oxygen fugacity patterns).
In long-lived metamorphic complexes in subduction zones, the common and large matrix sulfides are considered to be late-stage phases that formed after (possibly long after) the prograde sulfides were trapped as true inclusions [27], and the two groups of sulfide assemblages often show systematic differences. Matrix sulfides commonly form by transformation, replacement or alteration of their higher-grade precursors, but may also result from delayed re-incorporation of sulfur into the metamorphic rocks via cracks or any other lithological heterogeneities. The small-scale fluid pathways increase rock permeability and control the precipitation of new sulfide minerals. Major-and trace-element patterns of early and late sulfide phases, as well as their sulfur isotopic composition, have implications for the source(s) of fluids, metals, and sulfur, and thus can provide insights into the history of metamorphic rocks [26,27,98,99].
Crack-filling sulfides and minerals that rimmed and/or replaced earlier sulfides are attributed to the oldest retrogressive events. They may form long after both inclusion and matrix sulfides or follow immediately as a response to temperature and pressure drop.
The recrystallized limestones of the Kochumdek aureole retain all geochemical features of their sedimentary precursors and bear monotonic sulfide mineralization, with predominantly pyrrhotite, which has a negative δ 34 S values (-28.68‰ to -23.32‰) and contains Mn, Ni, and Co impurities, as well as trace amounts of Se (Figures 14 and 18; Tables 8 and 12). The morphology of pyrrhotite provides a strong indication that it formed by integrative recrystallization (Figure 10B,C). Chalcopyrite, galena and arsenopyrite occur sporadically, mainly as fine inclusions on the periphery of pyrrhotite grains, and thus most likely record the removal of impurities from the precursor pyrite and/or pyrrhotite during recrystallization. The sulfides in marbles from Zone 4 also include rare grains of acanthite and alabandite. Sulfides occur mostly in the matrix, except for a few pyrrhotite inclusions in calcite. Thus, the composition of sulfide inclusions in low-grade rocks is distinct from matrix minerals, where Fe-sulfides are predominant, while matrix pyrrhotite coexists with Mn, Ag, and Pb sulfides.
The merwinite-and spurrite-monticellite marbles of Zones 2 and 3 demonstrate geochemical inheritance from the protolith sediments, and their bulk trace-element compositions lack any notable signatures of material transport from the sill. The marbles show moderate enrichment in Mn and Zn and preserve low contents of Cu (≤60 ppm), Ni (≤30 ppm), and Co (≤15 ppm) (Table 3). However, sulfide assemblages in merwinite marbles differ from their counterparts from both lower-grade zones and precursor sediments in terms of greater compositional diversity (Fe, Mn, Zn, Pb, and Ag sulfides) and in similarity of inclusion and matrix sulfide assemblages.
The trace-element composition and isotopic signatures of pyrrhotite provide convincing evidence for the isochemical nature of high-temperature metamorphism in the Kochumdek aureole. Matrix pyrrhotite from merwinite assemblages were analyzed by LA-ICP-MS, and monofraction samples were selected from the same assemblages for sulfur isotope analysis. Hypothetically, sulfides from the matrix (unlike those of inclusions) would be better candidates for recrystallization by fluids released from the cooling magma. If this were the case, they would store geochemical fingerprints of mafic magma, such as Ni, Co, and Cu enrichment, as well as reintroduced isotopically heavy sulfur, especially in pyrrhotite from the merwinite zone. However, matrix pyrrhotite from merwinite marble is strongly depleted in Ni, Co, Cu, Se, Mo, and Ag and has δ 34 S values about -25‰ (Figures 14  and 18; Tables 8 and 12).
The obtained results show the compositional features of ultrahigh-temperature sulfides occurring under spurrite-merwinite facies conditions during thermal alteration of marly limestone, which so far has only been achieved for sphalerite from combustion metamorphic rocks [53]. Iron is the principal mineral-forming cation in the Kochumdek marble and, together with Mn, is among main components in the (Zn,Fe,Mn)S cub , (Zn,Mn,Fe)S hex , and (Mn,Fe)S cub solid-solution series, which coexists with the predominant pyrrhotite.
Alabandite (α-(Mn,Fe)S) is the only proper Mn phase in the Kochumdek marbles. Its origin is likely attributable to: (i) markedly higher bulk contents of Mn than Zn in the sedimentary protoliths, and to (ii) restricted storage capacity of pyrrhotite structure with respect to Mn 2+ [65]. The combined action of the two factors prevented total Mn dissemination in the (Fe,Mn)S or (Zn,Fe,Mn)S solid solutions. It is possible that α-(Mn,Fe)S formed as a primary high-temperature phase, judging by its presence as inclusions in the rock-forming minerals from spurrite marbles ( Figure 11). On the other hand, the systematic coexistence of fine and very fine (Mn,Fe)S particles on the boundary of larger pyrrhotite grains (both inclusions and matrix) indicates that (Mn,Fe)S may have also separated during high-temperature purification of pyrrhotite. The latter hypothesis is supported by direct comparison of impurities in pyrrhotite from different metamorphic zones (Figure 14), where the Mn contents are the highest in pyrrhotite from Zone 4 but low at the top of the sill. The Mn depletion rules out any notable transfer between the magma and limestone/marble material, and magma contamination by the latter.
The actual process of Mn fractionation may have been much more complicated. It is pertinent to note that Mn impurities occur in all Fe-bearing minerals from the Kochumdek marbles. The amount of Mn in rock-forming Ca-Mg silicates increases proportionally with increasing Mg:Ca ratio, and peaks at ≈3 wt% MnO in monticellite Ca(Mg 0.59-0.82 Fe 0.12-0.26 Mn 0.03-0.07 )(SiO 4 ) [30,61]. Manganese is present in magnetite rims around pyrrhotite (1.33-6.77 wt% MnO) but is absent from all Ca minerals (calcite, tilleyite, spurrite, rankinite). Its contents are below the detection limit even in wollastonite, which is known to readily incorporate Mg, Fe and Mn at high temperatures [100]. Tiny inclusions of Zn(Fe-Mn) and Mn(Fe) sulfides were commonly found trapped in melilite and might be interpreted as a consequence of Fe uptake during the growth of complex silicates. Less often, such inclusions are found in Fe-and Mn-free spurrite, but are absent in monticellite which is the main Fe and Mn host silicate phase in the Kochumdek marble. This provides evidence for efficient crystallochemical fractionation of Mn during ultrahigh-temperature low-pressure contact metamorphism, as well as for strong Fe-Mn coupling.
The solid solution series (Zn,Fe,Mn)S cub , (Zn,Mn,Fe)S hex , and (Mn,Fe)S cub has preserved unusual compositions consistent with equilibration at near extreme temperatures and thus may have grown at peak metamorphic conditions. Although the rapid kinetics of re-equilibration under cooling in sulfides [24] limits their use as geothermometers, they can yield some threshold estimates. At high temperatures, Fe and Mn can more easily incorporate into both sphalerite and wurtzite structural types [64,101]. Sphalerite from the ultrahigh-temperature merwinite marbles of the Kochumdek aureole contains about 20 wt% Fe on average and up to 5.9 wt% Mn, while Zn is below 0.6 atoms per formula unit (a.p.f.u.) (Zn 0.55-0.57 Fe 0.32-0.35 Mn 0.07-0.10 S). The minerals appear homogeneous and free from exsolution lamellae under SEM magnification (up to 0.5 µm), but intergrowths might exist on the submicron scale. In the natural environment, iron reaches the highest contents (25.3-32.1 wt% Fe or 42-50 mol% FeS) in sphalerite from meteorites which form at temperatures above 900 • C (1200 K) [87]. Experiments show that FeS in (Zn,Fe)S cub ranges from 20 to 55 mol% at 850 • C, but its content in (Zn,Fe)S cub equilibrated with pyrrhotite decreases to ≤13 mol% as the temperature drops to 742 • C at 1 bar [26].
The incorporation of large amounts of Mn stabilizes the wurtzite-type structure [101]. In the analyzed samples, wurtzite occurs only as inclusions in melilite from merwinite assemblages and contains comparable concentrations of Fe (12.0 to 19.1 wt%) and Mn (16.8 to 18.3 wt%), and anomalously low Zn (Zn 0.39-0.46 Fe 0.19-0.31 Mn 0.28-0.30 S). According to experimental evidence [102], synthesis of Mn-rich wurtzite is possible at temperatures no lower than 800 • C. The wurtzite inclusions are rare in the Kochumdek marbles and occur in coarse pyrrhotite grains. Therefore, it remains unclear whether they originated simultaneously with predominant pyrrhotite at the peak conditions or were separated while pyrrhotite was expelling Mn and Zn impurities.
Alabandite (MnS cub or α-MnS) belongs to the galena (or NaCl) structural type and the (Mn,Fe)S cub solid solution is more restricted than (Zn,Fe)S cub [65]. The MnS-FeS solid solutions in the Kochumdek marbles contain up to 15.7 wt% or 0.25 a.p.f.u. Fe (Mn 0.76-0.89 Fe 0.12-0.25 S) ( Table 6), or above the Fe-content in α-(Mn,Fe)S from meteorites [87]. This is the highest amount reported for α-(Mn,Fe)S (Mn 0.81 Fe 0.19 )S crystals synthesized by the gas-transport method in the Fe-Mn-Zn-S system at temperatures ranging from 900 to 800 • C [103]. According to [26], the FeS-MnS solid solution has the highest miscibility in the NaCl structure at 800 • C. Unfortunately, the alabandite compositions are unsuitable for subtler temperature reconstructions and in the FeS-MnS diagram [102] they fall into the broad stability field (1300-600 • C) of the alabandite + pyrrhotite assemblage ( Figure 19).
The presence of large rasvumite-I grains with perfect cleavage in the matrix of the highest-temperature merwinite marbles is intriguing, as rasvumite coexisting with pyrrhotite (in the absence of pyrite) provides a record of increasing activity of potassium and decreasing fugacity of sulfur in the mineral-forming medium [104][105][106]. In the context of our results, sulfur fugacity may decrease because most of the isotopically light sulfur derived from the authigenic pyrite and S-bearing organic matter of sedimentary protoliths became immobilized in the earliest ultrahigh-temperature metamorphic sulfides, mainly pyrrhotite (+(Zn,Fe,Mn)S and (Mn,Fe)S solid solutions). As for the greater potential of K at peak metamorphic conditions, it may result from crystallochemical fractionation because no ultrahigh-temperature Ca or Ca-Mg minerals contain K. The K-content is below the detection limit of EPMA (<0.02 wt% K 2 O), even in gehlenite-rich melilite which is the only potential K host among rock-forming minerals [30,61]. Currently, there is no information on the thermal stability of KFe 2 S 3 phase(s). Based on isostructural with rasvumite (KFe 2 S 3, space group Cmcm) fibrous thioferrates RbFe 2 S 3 and CsFe 2 S 3 obtained by pyro-synthesis at 950 • C, ambient pressure and flowing nitrogen [107], rasvumite having shorter and stronger K-S bonds than both Rb-S and Cs-S bonds, could be stable at higher T than 950 • C. 0.25S) ( Table 6), or above the Fe-content in α-(Mn,Fe)S from meteorites [87]. This is the highest amount reported for α-(Mn,Fe)S (Mn0.81Fe0.19)S crystals synthesized by the gastransport method in the Fe-Mn-Zn-S system at temperatures ranging from 900 to 800 °C [103]. According to [26], the FeS-MnS solid solution has the highest miscibility in the NaCl structure at 800 °C. Unfortunately, the alabandite compositions are unsuitable for subtler temperature reconstructions and in the FeS-MnS diagram [102] they fall into the broad stability field (1300-600 °C) of the alabandite + pyrrhotite assemblage ( Figure 19). Figure 19. The FeS-MnS equilibrium diagram [102]. Compositional field of alabandite from spurrite marbles (green field) plotted using author data. Alab = alabandite, L = liquid, Po = pyrrhotite.
The presence of large rasvumite-I grains with perfect cleavage in the matrix of the highest-temperature merwinite marbles is intriguing, as rasvumite coexisting with pyrrhotite (in the absence of pyrite) provides a record of increasing activity of potassium and decreasing fugacity of sulfur in the mineral-forming medium [104,105,106]. In the context of our results, sulfur fugacity may decrease because most of the isotopically light sulfur derived from the authigenic pyrite and S-bearing organic matter of sedimentary protoliths became immobilized in the earliest ultrahigh-temperature metamorphic sulfides, mainly pyrrhotite (+(Zn,Fe,Mn)S and (Mn,Fe)S solid solutions). As for the greater potential of K at peak metamorphic conditions, it may result from crystallochemical fractionation because no ultrahigh-temperature Ca or Ca-Mg minerals contain K. The K-content is below the detection limit of EPMA (<0.02 wt% K2O), even in gehlenite-rich melilite which is the only potential K host among rock-forming minerals [30,61]. Currently, there is no information on the thermal stability of KFe2S3 phase(s). Based on isostructural with rasvumite (KFe2S3, space group Cmcm) fibrous thioferrates RbFe2S3 and CsFe2S3 obtained by pyrosynthesis at 950 °C, ambient pressure and flowing nitrogen [107], rasvumite having shorter and stronger K-S bonds than both Rb-S and Cs-S bonds, could be stable at higher T than 950 °C.
Large crystals of rasvumite-I were analyzed by SEM and LA-ICP-MS only, while direct structural determination was not possible. Meanwhile, this phase is morphologically Figure 19. The FeS-MnS equilibrium diagram [102]. Compositional field of alabandite from spurrite marbles (green field) plotted using author data. Alab = alabandite, L = liquid, Po = pyrrhotite.
Large crystals of rasvumite-I were analyzed by SEM and LA-ICP-MS only, while direct structural determination was not possible. Meanwhile, this phase is morphologically different from needle-like rasvumite-II, and the angle between its cleavage directions, which are prominent in coarse grains, is 60 or 120 • rather than 90 • (perfect cleavage on {110} for orthorhombic rasvumite) ( Figure 12G,H). Therefore, the merwinite assemblages of the Kochumdek marbles include a previously unknown high-temperature modification of KFe 2 S 3 . In addition to orthorhombic rasvumite (KFe 2 S 3, space group Cmcm), at least two other polymorphic modifications have been synthesized so far: monoclinic KFe 2 S 3 (C2/m) [108] and low-temperature KFe 2 Se 3 and RbFe 2 S 3 phases that belong to another orthorhombic group, which break down above 450 • C [109].

Crack-Filling (Retrograde) Sulfides
The temperature constraints for the cooling of marbles can be used to reconstruct the thermal stability limits of crack-filling sulfides. Mineral assemblages in cracks differ drastically from those in both matrix and inclusions and consist of relatively abundant doublet or trio K-Fe sulfides, rasvumite + djerfisherite ± bartonite. Their ubiquity is unanticipated for K-poor calcareous rocks, even more so that such mineralization is absent from gabbro containing up to 1.4 wt% K 2 O (Tables 1-3). The first reconstruction of the thermal history and some chemical parameters of the retrogression processes in the Kochumdek aureole became possible with reference to recently published experimental results on the stability limits of various Fe and K sulfide assemblages and individual compounds [105,106,110]. Pyrite-bearing mineral assemblages (pyrrhotite + pyrite + rasvumite + sylvite) and pyritefree assemblages (pyrrhotite + rasvumite + sylvite + KFeS 2 ) are stable below and above 513 • C, respectively. The absence of pyrite in the sampled marbles indicates that rasvumite replaced pyrrhotite (forming rims and pseudo-inclusions) at temperatures above 513 • C.
The exposed rocks also lack both KCl and KFeS 2 phases because the former is highly soluble, while the latter is prone to hydrolysis.
The assemblage of pyrrhotite + rasvumite + djerfisherite fills cracks in marbles, as well as in recrystallized limestones. The same assemblage, plus a KCl phase (sylvite), was synthesized at 400 • C, but turned out to be unstable at 600 • C [110]. Djerfisherite was shown to melt at ambient pressure and temperatures slightly below 600 • C, where a new phase of unknown composition formed [110]. Systems of tiny cracks formed almost contemporaneously along grain boundaries in the minerals of marble and recrystallized marly limestone grains within a narrow (≤3.5 m) zone of heating around the cooling intrusion, apparently due to differences in thermal expansion coefficients of the phases in contact. The assemblages of silicates in pelitic limestone layers constrain the heating temperatures to within 450-500 • C ( Figure 4A), which appears to be the upper limit of djerfisherite formation in the aureole rocks.
Djerfisherite that replaces rasvumite-II in cracks is the only phase in the Kochumdek marbles which bears Ni, Cu, Co and Cl typical of gabbro. Its stability is controlled by the lower activity of sulfur (relative to rasvumite) and higher activity of chlorine in the fluid phase [67,72,74,80,104]. The distribution of djerfisherite in the Kochumdek marbles highlights the patterns of the magma-derived fluid flow and permeability in the cooling aureole. The localization of djerfisherite within the metapelitic layers reflects the permeability difference between calcite and Ca-Mg silicate layers. This effect was reference from sites of contact metamorphism in carbonate-silicate sequences where pure calcite (meta-carbonate) rocks act as aquitards while metapelitic layers can channel the fluid flow. Consequently, material transport is aligned with layering rather than being isotropic, and the silicate layers show distinct signatures of metasomatic alteration, mineral replacement and/or isotopic resetting [111]. In the case of Kochumdek, neither the calcite layers were infiltrated nor became the silicate layers affected by Si, Al, Mg and Fe metasomatism or S isotopic resetting of high-temperature sulfides. Therefore, there was no significant flux of material from the sill into the marly limestone. The aureole rocks were cracked only during the final stage of the event, and the deformation was restricted to micrometer-size weak zones along grain boundaries or, less often, produced tiny cracks in minerals (Figures 11 and 16). A few healed micrometer cracks contain sporadic ≤10 µm grains of Sc-rich garnet, besides djerfisherite (Table 2), which provides additional evidence for restricted upward fluid flow from the Sc-enriched sill into the cooling country rocks.
The filling of tiny cracks, fissures or voids in the marble minerals and at grain boundaries consists of retrograde mineral assemblages, which are of limited abundance in the Kochumdek rocks. These assemblages record minor fluid flow in the aureole and limited heterogeneous permeability of the marbles during cooling. The formation conditions of the fluid flow can be inferred from the presence of K-Fe sulfides, especially, djerfisherite. It is no longer an apparent rare phase due to high-resolution techniques that can detect it in numerous silica undersaturated lithologies, which include alkaline ultramafic, basic and syenitic complexes and related metasomatic rocks [82][83][84]; kimberlites, kimberlite-hosted mantle xenoliths, and carbonatites, where djerfisherite is frequently localized at sites of NaCl and KCl or in Cl-bearing nodules [33,72,[78][79][80][81]; mafic intrusions, with mainly Cu or Ni djerfisherite varieties [83,112]; skarns, where its presence is commonly attributed to infiltration of K-and Cl-bearing fluids from plutons [113][114][115]; and in few contactmetamorphic marble settings akin to Kochumdek [69]. The formation of djerfisherite in the Kochumdek marbles, which are depleted in K and Cl and have sedimentary protoliths devoid of evaporates [48], could be attributed to external fluids enriched in both K and Cl.
Studies of infiltration patterns and phase composition changes in fluids from cooling contact aureoles [49,116] demonstrate that brines should release inevitably from the fluids at temperatures below the second critical point. For the case of impure marbles with thin calcareous argillic bands (Bufa del Diente contact aureole, Mexico), these authors showed that CO 2 -rich fluid produced by decarbonation of limestone escaped as an immiscible low-density CO 2 -H 2 O fluid (with X CO2 ≥ 0.5). According to the suggested model [49,116], light internal fluid leaves its parent rock and rises into the overlying strata via grain-edge flow, while the residual lower-temperature evolved fluid (mainly external hypersaline (Na-K-Cl) brine of magmatic origin) becomes locked in metapelite layers which acts as a contact-metamorphic aquifer, unlike the largely impervious thick-bedded pure calcite marble acting as a specific aquitard.
The cooling marbles of the Kochumdek aureole were infiltrated by externally-derived saline fluids (with Ni, Co, Cl and Sc tracers) that separated from crystallizing magma. Modest amounts of such fluids can be inferred from low fluid saturation of poorly differentiated bodies of the Kuz'movsky complex, which is consistent with the absence of skarns within the aureole. The mechanically strong massive marbles in the contact zone acted as an aquitard, while highly restricted penetration of brines and their migration via silicate layers was only possible through a system of small cracks that formed during the cooling. The fluid distribution pattern and the limited scale of the event are evident from the distribution of djerfisherite in SEM elemental maps ( Figure 12B,E and Figure 16).

The
Kochumdek aureole of spurrite-merwinite marbles, though being almost free of skarn features, is a locality with an exceptional diversity of sulfide minerals (fifteen mineral species). The chemical difference of sulfides from marbles (Fe,K,Mn,Zn,Pb,Ag) and gabbro (Fe,Ni,Cu,Co), as well as contrasting δ 34 S values, indicate that the high-temperature (750-900 • C) contact metamorphism of marly limestone was nearly isochemical, without transport of isotopically heavy sulfur and metals (Cu,Ni,Co) across the gabbro-sediment interface. Such transport was impeded by limited amounts of fluid in the igneous body, as well as the massive structure of the host marly limestone which was originally free from weak zones as potential fluid conduits and thus acted as a specific aquitard during the brief period of prograde metamorphism.
2. The data on studied sulfides have implications for the conditions of spurritemerwinite metamorphism. Broad solid solutions of (Zn,Fe,Mn)S cub and α-(Mn,Fe)S, as well as the discovery of a (Mn,Fe)-rich wurtzite in the Kochumdek marbles, record unusually high peak temperatures of metamorphism at the intrusion contact to at least 850-900 • C. These temperature estimates, mainly from matrix sulfide data, are consistent with those based on silicate mineral assemblages [30]. Based on the similarity of sulfide assemblages in the matrix and in the inclusions, along with the compositional similarity of individual minerals, these texturally different sulfide assemblages in high-grade zones formed within a short time under comparable temperatures. According to numerical constraints [30], the hottest conditions (T ≥ 830 • C) near the intrusion margins (Zone 2 or merwinite marbles) were maintained for six to eight months. This brief thermal event has changed the micritic marly limestone into quite coarse marble. The similarity of the matrix and inclusion sulfide assemblages may be attributed to the brevity of geological processes. Restricted infiltration of magma-derived saline fluids (with specific Ni, Co, Cu and Sc geochemistry) took place during retrogression and led to the formation of crack-filling assemblages of K-Fe sulfides with sporadic Sc-garnet.
3. LA-ICP-MS examination of pyrrhotite, sphalerite, and rasvumite has provided the first evidence that spurrite-merwinite metamorphism of a carbonate protolith can induce crystallochemical fractionation and accumulation of chalcophile (Ni, Cu, Co, Zn, Mn, Cd, Tl, Hg, In, Se), as well as some incompatible large-ion (Rb, Cs, Ba) elements. Under the peak temperature (T > 900 • C), the majority of iron was incorporated into the pyrrhotite and sphalerite structural types, but only small amounts incorporated into the galena structural type, whereas Mn became preferably concentrated in the galena and wurtzite structural types. The selective accumulation of other elements was: Rb, Cs, Tl, Ba and Se in the KFe 2 S 3 phase (Rasv-I); Cd, In, and Hg in sphalerite; and Ni and Co in pyrrhotite.