Iron–Titanium Oxide–Apatite–Sulfide–Sulfate Microinclusions in Gabbro and Adakite from the Russian Far East Indicate Possible Magmatic Links to Iron Oxide–Apatite and Iron Oxide–Copper–Gold Deposits

Abstract

Mesozoic gabbro from the Stanovoy convergent margin and adakitic dacite lava from the Pliocene–Quaternary Bakening volcano in Kamchatka contain iron–titanium oxide–apatite–sulfide–sulfate (ITOASS) microinclusions along with abundant isolated iron–titanium minerals, sulfides and halides of base and precious metals. Iron–titanium minerals include magnetite, ilmenite and rutile; sulfides include chalcopyrite, pyrite and pyrrhotite; sulfates are represented by barite; and halides are predominantly composed of copper and silver chlorides. Apatite in both gabbro and adakitic dacite frequently contains elevated chlorine concentrations (up to 1.7 wt.%). Mineral thermobarometry suggests that the ITOASS microinclusions and associated Fe-Ti minerals and sulfides crystallized from subduction-related metal-rich melts in mid-crustal magmatic conduits at depths of 10 to 20 km below the surface under almost neutral redox conditions (from the unit below to the unit above the QFM buffer). The ITOASS microinclusions in gabbro and adakite from the Russian Far East provide possible magmatic links to iron oxide–apatite (IOA) and iron oxide–copper–gold (IOCG) deposits and offer valuable insights into the early magmatic (pre-metasomatic) evolution of the IOA and ICOG mineralized systems in paleo-subduction- and collision-related geodynamic environments.

1. Introduction

Iron oxide–apatite (IOA; “Kiruna-type”) and iron oxide–copper –gold (IOCG; “Chilean-type”) deposits contain major resources of a wide range of critical (iron, copper, phosphorous, rare earths, uranium, etc.) and precious (gold, silver) metals [1,2,3,4,5,6,7,8,9,10] and are commonly found in arc-related, orogenic and post-orogenic tectonic settings [1,2,3,7,11,12,13,14,15,16,17,18,19,20]. The formation of most current models involves multi-stage magmatic–hydrothermal processes and hydrous halogen-rich fluids, which scavenge metals from primary mantle-sourced, metal-rich silicate melts [9,21,22,23,24,25,26,27,28,29,30,31]. Evaporitic basin-derived sources were also invoked for ore-forming fluids in some IOCG-IOA systems; for example, the giant Olympic Dam Fe-REE-Cu-Au-U district in Australia, Fe-Cu-Au-mineralized systems in Central Chile and magnetite–apatite deposits along the Middle and Lower Yangtze River in China [6,32,33,34,35,36,37,38]. Several IOA and IOCG deposits in orogenic and post-orogenic environments contain magnetite-rich lavas, suggesting the involvement of Fe-rich melts in their formation [10,39,40,41,42,43,44,45,46,47,48,49]. Although these melts occasionally carry a subduction-related geochemical signature, the presence of metal-rich iron oxide–apatite (with carbonate, sulfide and sulfate) melts at convergent margins is poorly documented and their possible sources and modes of origin are still rather inadequately constrained [50,51,52].

Most studies tend to agree that IOA-IOCG and related mineral systems reflect a complex interplay of crustal magmatic and hydrothermal processes [1,2,3,5,7,9,10,19,21]. The occurrence of magnetite lavas with andesitic cement along with cogenetic pyroclastics and even magnetite volcanic bombs [39,40,41,43,46,47,48] and the frequent association of IOA-IOCG deposits with plutonic and volcanic rocks of intermediate to felsic compositions (granodiorites and diorites, andesites and dacites) [4,5,10,11,15,20,21] indicate presence of some magmatic component, at least during the early stages of the evolution of IOA, IOCG and related mineral systems. Additional geochemical characteristics such as the Fe isotope signature [29,45] and trace element composition [31] of early-generation magnetite, distribution of F-rich and Cl-rich domains in apatite [28] and Os isotope geochemistry [40] also support petrologic models suggesting that IOA-IOCG deposits can be rooted in evolving lithospheric magmatic systems. On the other hand, evidence for participation of hydrothermal processes appears to be rather overwhelming and, on many occasions, overshadowing presence of magmatic precursors for these extremely complex magmatic-hydrothermal ore environments.

We report a new set of data generated during detailed Scanning Electron Microscopy–Energy-Dispersive Analysis (SEM-EDA) study on two samples representing (1) the plutonic root system of the Stanovoy Suture Zone (SSZ, Mesozoic Stanovoy convergent margin) and (2) Late Cenozoic adakitic dacite lava from the Bakening behind-the-front volcanic complex (BVC) in Kamchatka. The primary goal of this study includes documentation of iron–titanium oxide–apatite–sulfide–sulfate microinclusion assemblages in arc-related plutonic and volcanic rocks with inferences on their igneous origins and possible links to IOCG and IOA deposits. We also hope to shed some new light on the formation of these complicated ore systems and the potential role of magmatic processes in their evolution and genesis.

2. Geologic Background

The Stanovoy Suture Zone is located on the northeastern part of the Central Asian Orogenic Belt and is related to the northward subduction of the Mongol–Okhotsk ocean floor beneath the southern edge of the Siberian Craton [53,54,55] (Figure 1). The Central Asian Orogenic Belt is composed of various metamorphic, ophiolitic (both MORB and SSZ-types) and arc-related terranes [56,57,58], some of which were either accreted or subducted along the SSZ in Early Mesozoic [59]. The northward subduction of the Mongol–Okhotsk ocean floor beneath the SSZ is marked, in particular, by mineralized ultramafic-mafic plutonic complexes, which form a linear belt roughly parallel to the southern edge of the Siberian continent [55,60,61,62]. One of the best developed and certainly most studied (including drilling) intrusions in this magmatic belt is the Ildeus mafic–ultramafic complex, which carries multi-stage polymetallic Ni-Co-Cu-Pt-Pd-Au-Ag mineralization [55,60,61,62].

The Ildeus mafic–ultramafic intrusion consists of an ultramafic core composed of plagioclase-bearing dunite, harzburgite, lherzolite, wehrlite and websterite rimmed by norite, pyroxene–amphibole gabbro and gabbro-anorthosite. Both mafic and ultramafic rocks are intersected by numerous dikes of clinopyroxenite, websterite, granodiorite and Ti-lamprophyre [55,60,61,62]. Some core-related ultramafic cumulates and pyroxenitic dikes were locally subjected to several episodes of metasomatism, resulting in hydrated (talc + serpentine + chlorite ± carbonate) and alkali-rich (quartz + albite + potassic feldspar + biotite + sericite ± muscovite) assemblages. Ultramafic rocks display exotic disseminated sulfide-native metal-alloy mineralization [60,61,62] summarized in Table 1. Ildeus mafic-ultramafic rocks follow a calc-alkaline differentiation trend in AFM diagrams and display prominent high-field-strength element depletions coupled with large-ion lithophile and light rare earth element enrichments characteristic of subduction zone magmas [61,62]. Granodiorites display very low Y concentrations (<10 ppm) and high Sr/Y ratios (50–400) typical of adakites [63,64,65]. Lamprophyres exhibit high total alkalies, Ti and Nb content, and are classified as arc-related high-Nb basalts [65,66].

The volcanic province of Kamchatka in the NW corner of the Pacific Ring of Fire records protracted subduction of the Pacific plate beneath Eurasia and consists of three sub-parallel volcanic chains populated by more than 300 active and dormant volcanoes. Most active volcanoes are located in the Eastern Volcanic Front (EVF). The intra-arc rift of the Central Kamchatka Depression, which includes the most productive Eurasian volcano Kluchevskoi, separates the EVF from the Sredinny Range volcanic zone with four active volcanoes [67,68]. The EVF, in turn, is characterized by a complex crustal architecture and includes several cross-arc volcanic chains, such as the Kozelsky–Avachinsky–Koryaksky–Aag–Arik–Kupol–Bakening chain, located just north of the city of Petropavlovsk-Kamchatsky. Bakening is a predominantly andesitic-to-dacitic stratocone built upon older orthopyroxene–clinopyroxene–plagioclase–phyric basaltic andesites and andesites located approximately 110 km to the northwest of the Koryaksky volcano [68,69]. It is composed principally of amphibole–plagioclase–phyric dacites with adakite-like geochemical characteristics (Sr/Y = 30–60) surrounded by primitive olivine–pyroxene basaltic cinder cones and a single mantle xenolith-bearing high-Nb basalt lava flow [68]. Although most volcanic rocks of the Bakening center are consistent with derivation from a variably depleted mantle wedge fluxed by slab fluids [69], the prominent adakitic signature in younger cone dacites and the presence of high-Nb basalt indicates the involvement, however limited, of a slab melt component [68]. A fresh sample of an amphibole–plagioclase–phyric dacite from the main stratocone was selected for our detailed petrologic and SEM-EDA study.

Table 1. Metal and mineral assemblages in the Ildeus mafic–ultramafic intrusion (Stanovoy Suture Zone, Russian Far East).

Metal/Mineral Assemblages/Stages of Evolution	Native Metals	Alloys	Sulfides/Sulfosalts/ Halides/Sulfates/Tellurides	Associated Minerals
Early-stage magmatic	W, Pt, Zn, Bi, Pb, Au	Fe-W, Ti-Co-W, Fe-Pt, Cu-Pt, Ni-Rh-Pt, Pd-Pt, Ni-Cu, Cu-Zn, Cu-Ag, Sn-Zn-Cu, Zn-Cu-Ag, Cu-Ag, Cu-Ag-Au, Cu-Ag-Au-Zn-Ni	Pn, Co-Pn, Po, Mlr, Ccp, Bn, Cu-Ag-S, Fe-Ni-Co-Zn-S, Ag2S, Brt, Pb-Sn-Cl, AgCl, AgI	Ol, Mg-Opx (1), Cpx, Mg-Fe-Cr-Al Spl, Mag, Ilm, Ttn, Rt, Cl-Ap,
Late-stage magmatic	Au	Cu-Ag, Pb-Sb, Ni-Ag-Zn-Cu-Au, Zn-Cu-Au, Cu-Sn	Pn, Po, Ni-Po, Bn, Mlr, Co-Ni-Sp, Co-Ni-Zn-S, Ag2S, Cu-Ag-Pb-S, Ni-Gn, Sp, Brt	Fe-Opx (2), Amp (3), Bt, Pl, Mag, Ilm, Ttn, Ap, Bdy, Zrn, Aln, Qz, Cer, Aln, Dol
Metasomatic/Hydrothermal	Ag, Zn, Ni, Au	Cu-Ag-Au, Ag-Au, Cu-Ag, Cu-Zn	Pn, Ccp, Cct, Dg, Hzl, Py, Brt, Cst, Cu-Ag-S, Ag2S, Gn, Cu-Gn, Sb-Pb-Cl, Ag-Cl-S, Cu-Ag-Cl, AgCl, Bi-Cl, Cu-Sb-Ag-Se-S, Cu-Pb-Fe-As-S, Pb-As-S, Cu-Pb-As-S, Ag2S, Fe-Cu-Zn-Pb-S, Ni-Zn-Fe-Cu-S, Cu-Ag-Pb-Se-Te,	Tlc, Chl, Srp, Tr, Cb, Ep, Ab, Or, Ba-Or, Qz, Mag, Rt, Ttn, Aln, Mnz, Xtm

Table includes data from [60,61,62]. ⁽¹⁾ Mg-rich orthopyroxene (enstatite/bronzite). ⁽²⁾ Fe-rich orthopyroxene (hypersthene). ⁽³⁾ Al-rich (5–12 wt.% Al₂O₃) pargasitic hornblende [69]. Mineral abbreviations: Ol—olivine, Opx—orthopyroxene, Cpx—clinopyroxene, Pl—plagioclase, Amp—amphibole, Tr—tremolite, Bt—biotite, Ab—albite, Or—orthoclase, Ba-Or—Ba-rich orthoclase, Ep—epidote, Spl—spinel, Mag—magnetite, Ilm—ilmenite, Rt—rutile, Ttn—titanite, Ap—apatite, Cl-Ap—Cl-rich apatite, Bdy—baddeleyite, Zrn—zircon, Aln—allanite, Mnz—monazite, Xtm—xenotime, Cb—carbonates, Cer—cerussite, Dol—dolomite, Pn—pentlandite, Co-Pn—Co-rich pentlandite, Po—pyrrhotite, Ni-Po—Ni-bearing pyrrhotite, Py—pyrite, Ccp—chalcopyrite, Cct—chalcocite, Bn—bornite, Mlr—millerite, Gn—galena, Cu-Gn—Cu-bearing galena, Ni-Gn—Ni-bearing galena, Sp—sphalerite, Co-Ni-Sp—Co- and Ni-bearing sphalerite, Hzl—heazlewoodite, Dg—digenite, Cst—cassiterite.

Table 1. Metal and mineral assemblages in the Ildeus mafic–ultramafic intrusion (Stanovoy Suture Zone, Russian Far East).

Metal/Mineral Assemblages/Stages of Evolution	Native Metals	Alloys	Sulfides/Sulfosalts/ Halides/Sulfates/Tellurides	Associated Minerals
Early-stage magmatic	W, Pt, Zn, Bi, Pb, Au	Fe-W, Ti-Co-W, Fe-Pt, Cu-Pt, Ni-Rh-Pt, Pd-Pt, Ni-Cu, Cu-Zn, Cu-Ag, Sn-Zn-Cu, Zn-Cu-Ag, Cu-Ag, Cu-Ag-Au, Cu-Ag-Au-Zn-Ni	Pn, Co-Pn, Po, Mlr, Ccp, Bn, Cu-Ag-S, Fe-Ni-Co-Zn-S, Ag2S, Brt, Pb-Sn-Cl, AgCl, AgI	Ol, Mg-Opx (1), Cpx, Mg-Fe-Cr-Al Spl, Mag, Ilm, Ttn, Rt, Cl-Ap,
Late-stage magmatic	Au	Cu-Ag, Pb-Sb, Ni-Ag-Zn-Cu-Au, Zn-Cu-Au, Cu-Sn	Pn, Po, Ni-Po, Bn, Mlr, Co-Ni-Sp, Co-Ni-Zn-S, Ag2S, Cu-Ag-Pb-S, Ni-Gn, Sp, Brt	Fe-Opx (2), Amp (3), Bt, Pl, Mag, Ilm, Ttn, Ap, Bdy, Zrn, Aln, Qz, Cer, Aln, Dol
Metasomatic/Hydrothermal	Ag, Zn, Ni, Au	Cu-Ag-Au, Ag-Au, Cu-Ag, Cu-Zn	Pn, Ccp, Cct, Dg, Hzl, Py, Brt, Cst, Cu-Ag-S, Ag2S, Gn, Cu-Gn, Sb-Pb-Cl, Ag-Cl-S, Cu-Ag-Cl, AgCl, Bi-Cl, Cu-Sb-Ag-Se-S, Cu-Pb-Fe-As-S, Pb-As-S, Cu-Pb-As-S, Ag2S, Fe-Cu-Zn-Pb-S, Ni-Zn-Fe-Cu-S, Cu-Ag-Pb-Se-Te,	Tlc, Chl, Srp, Tr, Cb, Ep, Ab, Or, Ba-Or, Qz, Mag, Rt, Ttn, Aln, Mnz, Xtm

Table includes data from [60,61,62]. ⁽¹⁾ Mg-rich orthopyroxene (enstatite/bronzite). ⁽²⁾ Fe-rich orthopyroxene (hypersthene). ⁽³⁾ Al-rich (5–12 wt.% Al₂O₃) pargasitic hornblende [69]. Mineral abbreviations: Ol—olivine, Opx—orthopyroxene, Cpx—clinopyroxene, Pl—plagioclase, Amp—amphibole, Tr—tremolite, Bt—biotite, Ab—albite, Or—orthoclase, Ba-Or—Ba-rich orthoclase, Ep—epidote, Spl—spinel, Mag—magnetite, Ilm—ilmenite, Rt—rutile, Ttn—titanite, Ap—apatite, Cl-Ap—Cl-rich apatite, Bdy—baddeleyite, Zrn—zircon, Aln—allanite, Mnz—monazite, Xtm—xenotime, Cb—carbonates, Cer—cerussite, Dol—dolomite, Pn—pentlandite, Co-Pn—Co-rich pentlandite, Po—pyrrhotite, Ni-Po—Ni-bearing pyrrhotite, Py—pyrite, Ccp—chalcopyrite, Cct—chalcocite, Bn—bornite, Mlr—millerite, Gn—galena, Cu-Gn—Cu-bearing galena, Ni-Gn—Ni-bearing galena, Sp—sphalerite, Co-Ni-Sp—Co- and Ni-bearing sphalerite, Hzl—heazlewoodite, Dg—digenite, Cst—cassiterite.

3. Analytical Methods

Petrographic studies of gabbro from the Ildeus mafic–ultramafic complex and adakitic dacite from the Bakening volcano in Kamchatka were carried out using an Imager A2m petrographic microscope (Carl Zeiss, Jena, Germany).

A comprehensive study of metal and mineral microinclusions in rock-forming minerals in gabbro and adakite was completed using a VEGA 3 LMH TESCAN (TESCAN, Brno, Czech Republic) scanning electron microscope (SEM) with the Oxford X-Max 80 Gb energy-dispersive spectrometer (EDS) (Oxford Instruments, Abingdon, United Kingdom) with the following operating conditions: accelerating voltage of 20 kV, beam current of 530 nA and beam diameter of 0.2 µm. Reference samples including 37 natural and synthetic oxides, minerals and pure native metals (Oxford/108699 no. 6067) were used as standards. Co-standard Oxford Instruments/143100 no. 9864-15 was used for daily calibration of the SEM instrument. The accuracy of the EDS analyses was estimated to be ± 0.1 wt.%. Special sample preparation protocols reported in detail in [70] and designed to prevent contamination were utilized to expose metallic phases in situ and determine their relationships with host silicate and oxide phases as well as associated rock-forming and accessory minerals. Petrographic and SEM studies were completed at the Khabarovsk Innovative Analytical Center (KhIAC) of the Institute of Tectonics and Geophysics, Khabarovsk, Russian Federation.

Microprobe analyses of phenocrysts in adakitic dacite from the Bakening volcano in Kamchatka were carried out using the JEOL-8600 Superprobe at the University of Alabama (Tuscaloosa, AL, USA). Operating conditions were 15 kV accelerating voltage, 20 nA sample current, 40 s maximum counting time and beam diameters of 20 microns for plagioclase and 10 microns for all other mineral phases. A set of natural and synthetic standards was used and the data were processed using the corrections procedure of [71] modified by [72]. Additional details for microprobe procedures used in this study are summarized in [73].

4. Results

Both marginal gabbro from the Triassic Ildeus plutonic root complex in the Stanovoy Suture Zone and adakitic dacite from the Pliocene–Pleistocene Bakening volcano in Kamchatka carry iron–titanium oxide–apatite–sulfide–sulfate (ITOASS) microinclusions in association with a wide range of precious metal minerals and alloys. These mineral assemblages occur in igneous rocks with different textural, mineralogical and geochemical characteristics, which were formed and emplaced within the broad context of a Mesozoic (Ildeus) and Late Cenozoic (Bakening) subduction zone environment.

4.1. Petrology of Gabbro and Adakite

Marginal gabbro in the Ildeus mafic–ultramafic plutonic complex is characterized by a hypidiomorphic–granular texture (Figure 2a) composed of orthopyroxene, clinopyroxene and plagioclase with minor amphibole. Petrographic observations from this marginal gabbro as well as other mafic–ultramafic rocks throughout the Ildeus intrusion [55,62] suggest that Al-rich (SEM-EDA determinations) amphibole is always clearly interstitial (intercumulus, late-stage magmatic; cf. Figure 2c–e in [60]) and never a pyroxene replacement (metasomatic) phase. In fact, all rock-forming minerals in the Ildeus marginal gabbro are quite fresh and do not carry any signs of hydrothermal alteration [62]. Some parts of marginal gabbro display a poikilitic texture with amphibole forming euhedral to subhedral inclusions in calcic plagioclase (Figure 2d in [55]), attesting to the magmatic nature of both silicate minerals. Accessory minerals include abundant apatite, magnetite, ilmenite and sulfide with subordinate zircon, rutile and barite. Sulfides are represented by pentlandite, pyrrhotite, chalcopyrite, Ni-bearing pyrite and pyrite. In some cases, Ni-bearing pyrite appears to form pseudomorphs replacing corroded grains of primary pentlandite. Locally, marginal gabbro from the Ildeus mafic–ultramafic complex is cut by thin (several millimeters to centimeters across) veins of felsic plagioclase-rich material. Geochemical features, especially high Sr/Y (>50) and La/Yb (>30), identify these veins and veinlets as adakite [62]. Adakite veins contain abundant elongated euhedral apatite crystals and minor zircon (Figure 2b).

Adakitic dacite from the Bakening volcano contains euhedral phenocrysts and microphenocrysts of unzoned amphibole, zoned plagioclase and subordinate interstitial quartz and magmatic biotite (Figure 2c,d). Some amphibole grains appear to be corroded and partially abraded (Figure 2c) and most of them are characterized by well-developed opacitic (reaction) rims (Figure 2d), suggesting at least some degree of chemical disequilibria with the surrounding groundmass due to the degassing and slight temperature variations in the ambient melt [74,75,76,77]. The groundmass in Bakening adakite displays a classic trachytic texture and is composed of elongated euhedral plagioclase laths, equant Ti-magnetite, quartz, rare ilmenite and potassic feldspar crystals along with varying amounts of silica-rich glass (Figure 2c,d).

Amphibole phenocryst compositions are characterized by relatively high TiO₂ concentrations (1.11–2.66 wt.%;

Table 2. Representative phenocryst compositions (wt.%) in the Bakening volcano adakite.

	1	2	3	4	5	6	7	8	9	10	11	12
Mineral	Amp	Amp	Amp	Amp	Amp	Amp	Pl	Pl	Mag	Mag	Ilm	Ilm
SiO2	45.52	44.29	44.04	46.61	42.98	43.24	49.74	54.41	NA	NA	NA	NA
TiO2	2.01	2.03	2.08	1.11	2.66	2.30	0.00	0.01	5.63	12.71	44.15	40.34
Al2O3	10.87	11.42	11.29	10.46	12.95	12.38	31.39	28.30	3.30	3.01	0.14	0.13
Cr2O3	0.08	0.04	0.03	0.01	0.20	0.01	NA	NA	1.46	0.02	0.07	0.01
V2O5	NA	NA	NA	NA	NA	NA	NA	NA	0.49	1.11	2.67	2.11
FeO	10.47	11.40	11.70	13.34	10.86	11.35	0.17	0.29	79.55	77.06	46.81	50.05
MnO	0.15	0.15	0.21	0.37	0.16	0.19	0.00	0.01	0.09	0.14	0.34	0.71
MgO	14.39	14.09	14.23	12.33	13.68	14.03	0.01	0.05	3.49	2.90	3.47	1.80
CaO	11.78	11.40	11.11	11.81	10.98	12.09	14.83	10.97	NA	NA	NA	NA
Na2O	2.01	2.26	2.10	1.87	2.57	2.32	2.86	4.84	NA	NA	NA	NA
K2O	0.28	0.25	0.29	0.22	0.24	0.27	0.05	0.13	NA	NA	NA	NA
Total	97.56	97.33	97.07	98.14	97.27	100.34	99.05	99.01	94.02	96.95	97.65	95.15
Mg#	71.3	69.8	69.5	62.2	69.2	68.8	-	-	-	-	-	-
An	-	-	-	-	-	-	73.9	55.2	-	-	-	-
P (kb) *	4.8	5.1	5.0	4.8	5.8	5.5	-	-	-	-	-	-

Note. Mg# = Mg/(Mg + Fe). * —Ti-magnetite analysis in Mg-number: Mg/(Mg + Fe) (at.%). NA—not analyzed. Amp—amphibole, Cpx—clinopyroxene, Pl—plagioclase. * P (kbar)—pressure calculated using the empirical Al-in-hornblende geobarometer (Equation (5)) from [77].

), high Al₂O₃ content (10.46–12.95 wt.%; Table 2), Mg-numbers of 62.2–71.3 and variable Cr₂O₃ content (0.01–0.20 wt.%; Table 2). Plagioclase compositions vary from An₄₅ to An₉₄ and all plagioclase phenocrysts contain some iron and, in many cases, very minor but detectable amounts of magnesium and manganese (Table 2). Magnetite contains variable but generally high TiO₂ and V₂O₅ (up to 15 wt.% and 2 wt.%, respectively; Table 2) and is classified as V-bearing titano-magnetite characteristic of subduction-related magmas [78,79]. Ilmenite microphenocrysts and discrete equant grains in the groundmass can be sub-divided into two principal compositional types: (1) relatively MnO-rich and MgO-poor (analysis 12 in Table 2) and (2) relatively MgO-rich (up to 3–4 wt.% MgO) and MnO-poor (analysis 11 in Table 2). While the first type of ilmenite is quite common in evolved island-arc volcanic rocks such as dacites and rhyolites [78,79,80], the MgO-rich variety is rare and is possibly of a deeper megacrystic (high-pressure phase) or even xenocrystic origin. The elevated geikeilite content of dacitic ilmenites may also reflect the relatively low oxygen fugacity of magma differentiation as suggested by some experimental data [81].

Compositions of phenocrystic amphibole in Bakening adakitic dacites range from pargasite through pargasitic and edenitic hornblende to edenite (Figure 3a) chemically similar to amphibole phenocrysts from the Shiveluch volcano adakites in Central Kamchatka [82,83]. Plagioclase compositions vary from andesine to almost pure anorthite (Figure 3b). Anorthite megacrysts, frequently in association with Mg-olivine, are found in high-alumina basalts from the Japan [84], Izu–Mariana [85] and Kurile [86] arcs, but are extremely rare in more evolved dacitic-to-rhyolitic magmas [80]. Crystallization pressure for amphibole phenocrysts in Bakening adakite calculated using a refined Al-in-amphibole geobarometer (Equation (5) in [77]) ranges from 4.8 to 5.8 kbar, averaging at around 5 kbar (Table 1). Estimates of oxygen fugacity for the Bakening volcano lavas using Fe-Ti-Mn-Mg oxybarometers developed in [87,88] yielded values of 1–2 units above the quartz–fayalite–magnetite (QFM) buffer. This is similar to the redox conditions (0.5–2.5 units above QFM) typical of modern Kamchatka magmas in particular [89,90,91] and worldwide arc melts in general [92,93].

4.2. ITOASS Microinclusions in the Ildeus Arc Root Complex (Stanovoy Suture Zone)

Marginal gabbro from the hole ILN-009 in the Ildeus arc root complex contains several quasi-spherical (droplet-like ?) and partially resorbed (multiple textural embayments and locally uneven, undulating contacts with magmatic gabbroic matrix) magmatic segregations typically ranging in size from 0.25 to 5 mm, which are composed of various iron–titanium oxide, apatite, sulfide and sulfate minerals (ITOASS microinclusions; Figure 4). Some well-defined, quasi-spherical segregations have sharp contacts with host late-stage magmatic amphibole (Figure 4a) and contain microinclusions of chalcopyrite along with smaller grains of magnetite, apatite, pyrite and amphibole. Based on the SEM-EDS results, amphiboles inside and outside the ITOASS segregations have slightly different chemical compositions. Primarily, amphibole in the segregations contains elevated Al content (~5–6 wt.%). In comparison, all secondary amphiboles (actinolite, tremolite) with clear replacement textures in the Ildeus complex are characterized by low Al (<2 wt.%) content [55,60,61,62]. An isolated microinclusion of ilmenite was also observed immediately adjacent to the larger inclusion clusters (Figure 4a), although it is unclear from the textural data if this minute ilmenite could, at some point, have been considered an integral part of the larger ITOASS segregation in Figure 4a.

Other spherical-type microinclusions have slightly more diffuse boundaries with the host gabbroic rock (Figure 4b) and are surrounded by various minerals, including amphibole, plagioclase, apatite, quartz and pyrite. These segregations typically include magnetite, rutile, apatite, pyrite and barite, but may also contain minute crystals of orthopyroxene and amphibole (Figure 4b). Similar minerals, especially magnetite, ilmenite, rutile, apatite and pyrite, are disseminated in the pyroxene–amphibole–plagioclase–quartz gabbroic matrix and may possibly represent disintegrated ITOASS-type microinclusion clusters.

Other types of ITOASS microinclusions in marginal gabbro from ILN-009 are composed of the following mineral assemblages: ilmenite–rutile (Figure 5a), rutile–chalcopyrite–pyrrhotite–pyrite–barite (Figure 5b) and magnetite–chalcopyrite–barite (Figure 5c). These ITOASS assemblages are also found in the Ildeus marginal gabbro together with microinclusions of nickeliferous pyrite (Figure 5d), Cu–Ag–chloride (Figure 5e) and cupriferous silver (Figure 5f).

4.3. ITOASS Microinclusions in the Bakening Volcano (Kamchatka)

Adakitic dacite lava from the Bakening volcano in central Kamchatka contains several inclusions of ilmenite (Figure 6a) and magnetite (Figure 6b) hosted in siliceous glassy groundmass. Both larger ilmenite and magnetite oikocrysts contain euhedral to subhedral oval-shaped apatite microinclusions, which range in size from several microns to 10–15 microns across (Figure 6a,b). Some apatite microinclusions in ilmenite contain up to 0.7 wt.% of chlorine as determined by the SEM-EDA (Figure 6a). Both ilmenite–apatite and magnetite–apatite inclusions in the Bakening adakitic dacite are surrounded by numerous micron-sized magnetite crystallites included in silicic glass (Figure 6a,b).

Ilmenite–magnetite–apatite intergrowths are closely spatially and texturally associated with grains of magnetite and various sulfides (Figure 7). Euhedral to anhedral magnetite crystals ranging in size from 100 to 500 microns are included in amphibole (Figure 7a), primary magmatic biotite (Figure 7b) and K-Na feldspar (Figure 7c). In one particular case, euhedral magnetite contains a minute inclusion of silver chloride and is hosted by amphibole and glassy aphyric groundmass (Figure 7d). In another case, a euhedral magnetite crystal 50 microns across is included in Fe-K-rich glass (Figure 7e), which is also enriched in manganese (0.48 wt.% Mn; SEM-EDA). Another textural type of magnetite in the Bakening adakitic dacite is shown in Figure 7f, where numerous small (about one or two microns in size) euhedral angular-to-roundish magnetite grains are tightly packed into an ovoid-shaped crystalline form along the contact between amphibole and quartz crystals. This magnetite texture very closely resembles framboidal magnetite aggregations in some sedimentary rocks, where the formation of framboids is believed to be a reflection of changes in the overall redox conditions of mineralization [94].

Isolated sulfide grains in adakitic dacite lava from the Bakening volcano are less common in comparison with Fe-Ti oxides and are represented by pyrite and acanthite (Figure 8). Pyrite forms equant subhedral to anhedral grains 1 to 5 µm across which are included in quartz–plagioclase groundmass (Figure 8a) or silica-rich residual glass (Figure 8b). Silver sulfide (acanthite Ag₂S) occurs as an anhedral, possibly partially resorbed inclusion in quartz (Figure 8c). In addition, a single anhedral grain of barite approximately 3 microns across is included in quartz microphenocryst (Figure 8d) in the adakitic dacite lava from the Bakening volcano. Besides pyrite, acanthite and barite, amphibole and plagioclase phenocrysts and groundmass phases in the adakitic lava also contain spongy-like grains and aggregates of non-stoichiometric silver chloride (Figure 8e,f). We have previously shown that the non-stoichiometric ratio of Ag to Cl in the halide microinclusions in magmatic rocks is due to their exposure to light during sample preparation and to electron beams during SEM-EDA studies [61].

5. Discussion

Triassic marginal gabbro from the subduction-related mafic–ultramafic plumbing system in the Mesozoic Stanovoy convergent margin and Pliocene–Pleistocene adakites from the Bakening volcano in Kamchatka contain multiple microinclusions of iron–titanium oxide–apatite–sulfide–sulfate (ITOASS) composition. The iron–titanium oxides in the ITOASS are magnetite, ilmenite and rutile, the sulfides are chalcopyrite, pyrite and pyrrhotite, and the sulfate is barite.

ITOASS-type microinclusions in marginal gabbro from the Ildeus mafic–ultramafic arc root complex are characterized by the following mineral assemblages: (1) magnetite–apatite–chalcopyrite–pyrite–pyrrhotite–amphibole–barite; (2) magnetite–apatite–rutile–orthopyroxene–pyrite–barite; (3) rutile–chalcopyrite–pyrrhotite–pyrite–barite; (4) magnetite–chalcopyrite–barite; and (5) ilmenite–rutile (Figure 4 and Figure 5). Although no halogens were detected by SEM-EDA in apatite from the marginal gabbro, apatite in two-pyroxene gabbro, norite, pyroxenite and websterite from the Ildeus complex contained elevated chlorine and fluorine concentrations [55]. The marginal gabbro sample analyzed in this study also contained a single inclusion of Cu–Ag–chloride in amphibole (Figure 5e). Based on the textural and compositional features of ITOASS-bearing magmatic rocks summarized in [55,60,61,62] along with the occurrence of ITOASS microinclusions in unaltered magmatic rocks, the quasi-spherical shape and sharp contacts with the surrounding magmatic matrix, we infer that the assemblage of magnetite + apatite + ilmenite + chalcopyrite + pyrrhotite + barite I (inclusions inside the ITOASS segregations), along with pentlandite that occurs in some samples containing ITOASS-type associations, is of primary magmatic origin and crystallized from primitive metal-rich subduction-related melt under slightly reduced to slightly oxidized (−1 to +1 ΔQFM mineral buffer) crustal conditions [55,60,61,62]. It is worth noting here that barite is stable under a wide range of thermodynamic conditions and can be hosted by sedimentary, metamorphic and igneous rocks [95]. Based on the same textural and compositional criteria, mineral association of amphibole + quartz + pyrite + barite II (rims on ITOASS microinclusions and isolated anhedral grains in mafic and ultramafic intrusive rocks) are considered to represent the late magmatic stage in the evolution of the Ildeus magmatic plumbing system [61,62]. It is important to emphasize here that amphibole inside the ITOASS-type clusters appears to contain more Al in comparison with amphibole that hosts the iron–titanium oxide–apatite–sulfide–sulfate microinclusions. This can be interpreted as the reflection of protracted magmatic differentiation history of primary melt that leads to the formation of ITOASS mineral assemblages in the Ildeus arc root mafic–ultramafic complex, as well as similar magmatic hydrothermal systems [62,96,97]. Petrologic conditions of this differentiation inferred from the available mineral compositions and associations, along with the ubiquitous presence of chalcophile metal chlorides and the Cl-rich nature of apatite in the Ildeus arc root complex, suggest that the formation of ITOASS-type microinclusions most probably took place in mid-crustal magmatic plumbing conduits in the presence of a sulfur- and chlorine-rich fluid phase [55,60,61,62]. It was proposed previously that “IOA deposits typically evolve from subduction-related water-rich and chlorine-rich intermediate magmas under a wide temperature range, almost spanning the whole igneous-hydrothermal spectrum (~1000 to 300 °C)” [19]. Several other studies of well-exposed IOA and IOCG systems also emphasize the importance of chlorine-rich fluids for scavenging and transporting ore metals during the differentiation of primary metal-rich melt and subsequent construction of the upper crustal mineralized magmatic–hydrothermal systems [22,23,24,25,26,30,98,99,100]. Although the sources of these fluids vary from magmatic mantle- and crustal-derived to basinal evaporitic and meteoric [6,19,22,24], the involvement of sulfur- and chlorine-bearing fluids in the magmatic crystallization of the ITOASS-type microinclusions during development of the Ildeus arc root plutonic complex is well documented and supported by the ubiquitous presence of Cl-rich apatite and copper–silver–lead–antimony–bismuth chlorides in mafic and ultramafic rocks from the Ildeus intrusion [55,60,61,62].

Iron–titanium oxide–apatite–sulfide–sulfate (ITOASS-type) microinclusions in the adakitic lava from the Bakening volcano are represented by ilmenite–apatite and magnetite–apatite intergrowths (Figure 6) along with isolated individual magnetite and rare ilmenite crystals (Figure 7) in association with pyrite, acanthite and abundant non-stoichiometric silver chloride (Figure 8). Most iron–titanium oxides in the Bakening adakite contain variable V₂O₅ contents (up to 2.8 wt.%) [68,78], potentially indicating substantial variations in redox conditions during the crystallization of the parental subduction-related magmas [101,102]. Apatite microinclusions in ilmenite imbedded in silicic glass from the Bakening adakite (Figure 6a) contain up to 0.7 wt.% of chlorine, suggesting the involvement of Cl-rich fluids in magma genesis and evolution beneath the Bakening volcanic center in the Kamchatka volcanic arc. This is consistent with the ubiquitous presence of silver and silver–copper chloride microinclusions in the Bakening adakitic dacite (Figure 8e,f). Chlorine-rich fluids are integral in promoting the differentiation and evolution of volcanic arc magmas [103,104,105,106,107] and facilitating the transport of ore metals such as copper and gold in crust–mantle systems above active subduction zones [108,109,110]. High concentrations of chlorine and sulfur have been measured directly in melt inclusions from calc-alkaline lavas [111], as well as in thermal and mineral waters discharged from the volcanic edifices between active volcanoes in Kamchatka [112], suggesting the presence of chlorine-rich fluid in both the mantle wedge and island-arc crust beneath the Kamchatkan volcanic province [65,68,70,113,114,115].

Ilmenite–apatite and magnetite–apatite microinclusions appear to be in textural equilibrium with residual silicic glass in the adakitic dacite from the Bakening volcano (Figure 6), suggesting crystallization from evolved arc magma within a mid-crustal magma chamber beneath the southern segment of Kamchatka arc. Previous geophysical studies suggested the presence of such magmatic conduits beneath several volcanoes in the vicinity of the Bakening volcanic center, specifically Avachinsky and Koryaksky, located within 100 km to the southeast from the BVC [116,117]. Pressure estimates for the depth of fractionation of parental adakite magma beneath the BVC based on an Al-in-hornblende geobarometer (Table 2) are within 12–17 km, which is consistent with geophysical data from modern Kamchatka volcanoes [117,118,119,120] and petrologic constraints from arc-related plutonic complexes exposed at the surface [121,122,123]. Apatite is a common accessory mineral in most hydrous calc-alkaline magmas [124,125,126] and, together with magnetite, ilmenite and rutile, is stable under a range of pressures, temperatures and oxygen fugacity values typical of the moderately thick (30–40 km) island-arc crust [127,128,129]. Experimental and geochemical data suggest that sulfur partitioning between apatite and intermediate to felsic melts in typical crustal volcanic conduits is controlled by decreasing the temperature of crystallization and increasing oxygen fugacity and not so much by the original sulfur enrichment of the parental mantle-derived melt and presence in the fractionating melt of igneous sulfate minerals such as anhydrite [130]. Since no sulfur was detected in either apatite grains from the ITOASS microinclusions or individual apatite crystals in gabbro and adakite from the Russian Far East, we conclude that the decrease in oxygen fugacity during their formation in the crust was negligible and almost all sulfur was partitioned into early magmatic sulfides (pentlandite, pyrrhotite and chalcopyrite in the case of the Ildeus gabbro [55,61,62] and pyrite and acanthite in the case of the Bakening adakite, Figure 8a-c) and late magmatic barite in the case of both Ildeus gabbro (Figure 5b,c) and Bakening adakite (Figure 8d). This interpretation is consistent with the results of Fe-Ti oxide oxybarometry described in Section 4.1 of the Results. Volatile element budgets of apatites in the ITOASS inclusions in both gabbro and adakite from the Russian Far East were controlled by high-temperature hydrous chlorine-rich fluids that assisted differentiation in mid-crustal magmatic conduits and promoted the partitioning of iron, copper, gold, silver and associated critical metals into the later-stage exsolved mineralizing fluids associated with IOA and IOCG systems [7,8,9,10,19,25,29,50,51,52]. Several iron oxide–apatite deposits are known within the immediate vicinity of the Ildeus intrusion [131] and multiple IOCG mineralized systems were reported from the Aldan shield region north of the SSZ [132] (Figure 1). The presence of ilmenite–Cl–apatite and magnetite–apatite microinclusions in differentiated siliceous glass in the Bakening lava provides direct evidence for the crystallization of iron oxide–apatite assemblage from evolving adakite melt in a S-Cl-rich-fluid-saturated magmatic conduit beneath an active arc volcano in Kamchatka. If these ITOASS segregations represent actual melts, then we argue that these could give rise to IOA-IOCG systems.

In several cases, magnetite–apatite intrusions and associated IOA deposits are located in the vicinity of each other (e.g., the Ildeus intrusion is surrounded by multiple IOA metal showings; Figure 1) and even interlayered [11,12,133,134,135], suggesting close spatial, temporal and, perhaps, even genetic link between subduction-related magmas and Kiruna-type mineralization [5,9,20,49,135,136,137,138]. Several IOA deposits and showings are known in the vicinity of the Ildeus intrusion in the SSZ and IOCG-type mineralization occurring NW of the Bakening volcano in the Sredinny Range of Kamchatka (Figure 1). Similar geologic, petrologic and metallogenic relationships have been documented in magmatic terranes hosting IOCG deposits in the Andes of South America and elsewhere [1,3,10,13,15,20,139]. Arc-related magmatic roots of IOCG deposits are further emphasized by the frequent occurrence of magnetite lava (similar to the “classic” El Laco locality in northern Chile) in association with magnetite, hematite, chalcopyrite, bornite and gold mineralization [20,39,40,41,43,47,48,49,50], as well as the igneous geochemistry of early generations of magnetite and pyrite in such IOA and IOCG deposits as Carmen, Fresia, El Romeral, El Laco, Los Colorados, Cerro Negro Norte and many others [4,8,18,19,20,140,141,142,143,144]. We propose that the iron–titanium oxide–apatite–sulfide–sulfate (ITOASS-type) microinclusions in gabbro and adakite from the Russian Far East provide some potential links to the early magmatic stages of formation of IOA and IOCG deposits and offer new insights into the magmatic–hydrothermal evolution of subduction-related mineralized systems in orogenic terranes.

6. Conclusions

• Mesozoic gabbro from the Stanovoy active margin and Quaternary adakitic dacite lava from the Bakening volcano in Kamchatka contain iron–titanium oxide–apatite–sulfide–sulfate (ITOASS) microinclusions. Iron–titanium oxides are composed of magnetite, ilmenite and rutile; sulfides are composed of chalcopyrite, pyrite and pyrrhotite; and sulfates are represented by barite.
• Textural and compositional data suggest that ITOASS assemblages crystalized from metal-rich, mantle-derived (Ildeus) or slab-derived (Bakening) fractionating magma at mid-crustal levels (15–20 km below the surface) under slightly reduced to slightly oxidized conditions (from one unit below to one unit above the QFM mineral buffer).
• Magmatic crystallization and metal mobilization within the ITOASS microinclusions in Stanovoy gabbro and Kamchatka adakite were assisted by S-Cl-rich fluids, as indicated by the presence of Cu–Ag–chlorides in plutonic amphibole and volcanic phenocrysts in adakite lava and the elevated chlorine content of apatite in both gabbro and adakite.
• Although ITOASS microinclusions in the Stanovoy intrusion were possibly affected by hydrothermal processes during later collision and post-collision tectonic events, primary igneous ITOASS assemblages in the Russian Far East most probably represent the early magmatic roots of some mineralized IOA and IOCG systems.