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The experimental works of Russian and foreign researchers focused on the behavior of ore minerals, such as titanomagnetite and ilmenite in an oxidizing environment in different temperature ranges, have been analyzed. The optimum temperature conditions and time interval for oxidizing roasting of mineral aggregates have been substantiated. The Medvedevskoe ores are described in brief. Special attention is paid to the mineral assemblages of disseminated titanomagnetite–ilmenite ore subjected to secondary alterations, such as amphibolization, chloritization, saussuritization, albitization, and martitization. The evolution of the phase and structural heterogeneity of titanomagnetite microaggregates is observed as variations in the composition and structure of magnetite–ilmenite exsolution products under collective recrystallization complicated by martitization processes. Oxidizing roasting (T = 1000–1100°C) of titanomagnetite aggregates makes it possible to reduce the degree of their heterogeneity and form stable mineral phases, such as pseudobrookite, hematite, and rutile with subsequent enlargement thereof. The heterogeneous structure of titanomagnetite microaggregates affects the course of heterogeneous oxidation processes in microaggregates of different mineral assemblages. Due to solid-phase transformations, martitized titanomagnetite (with ilmenite lamellae) changes completely with the formation of pseudobrookite–hematite microaggregates. Ilmenite at the contact with hematite, as well as in individual grains, is transformed into complex fine-grained rutile–pseudobrookite aggregates. The newly formed minerals are mostly solid solutions. Pseudobrookite is characterized by a reduced Fe2O3 content (up to 62.71 wt %) with 33.51 wt % TiO2. Hematite as an transformation product of martitized magnetite contains up to 4.90 wt % TiO2. Rutile forms limited solid solutions with pseudobrookite, and Fe2O3 reaches 7.59 wt %. With a decrease in material size, the solid-phase transformation process becomes more active; therefore, the optimum size grades include –0.25 + 0.125 mm and –0.125 + 0.071 mm. The solid-phase transformation of primary ore minerals is almost completed during the roasting for 72 h. The homogenization of mineral aggregates and the enlargement of newly formed Fe- and Ti-bearing minerals makes them better detectable in technological products and improves the general Ti extraction in various products. The results obtained in the course of high-temperature oxidizing roasting of magnetite–ilmenite ores at the Medvedevskoe deposit can be of interest to geologists studying the formation processes of magmatic magnetite–ilmenite deposits, which vary widely in terms of both mineralization composition (from low-grade Ti ores to extremely hard-to-process high-grade Ti ores) and formation conditions (deposits of various depth facies) and depend considerably on the oxygen regime and composition of volatile mineralizing elements (H2O, Cl, F, etc.).

—The Akhunovo–Petropavlovsk area of the late Paleozoic granite magmatism is located in the northeast of the Magnitogorsk megazone (MMZ) in the South Urals. It is a series of successively intruded rocks (Petropavlovsk, Akhunovo, Karagai, and Uiskii Bor intrusions) differing not only in composition, the depth of formation, and ore content but also in the relationship with magmatic and fluid sources and in magma generation mechanisms. This area differs significantly in the number and composition of intrusive complexes from the igneous rocks and ore associations in the central and western parts of the MMZ. The granite magmatism pulses alternated with the collisional shearing/spreading and rifting stages. The Petropavlovsk mesoabyssal granite intrusion (347.0 ± 8.6 Ma) formed at the early stage of the area evolution. Its rocks are similar in composition to a suprasubductional series (melting products of a mantle source enriched not only in water fluid but also in Cl). Later (310–306 Ma), at the collision–compression stage, crustal intrusion of the Akhunovo–Karagai granodiorite–granite complex took place. The intruded rocks are similar to the Middle Urals continental-margin gabbro-tonalite–grano-diorite–granite plutons (320–290 Ma) bearing large gold–sulfide–quartz deposits (Berezovskoe etc.). At the final stage of the area evolution, during the transition from continental-margin regime to hard collision between the East European and Kazakhstan continents (late Carboniferous) and the intense shearing/spreading deformations, the Uiskii Bor granosyenite–granite intrusion (304.0 ± 4.8 Ma) rich in K and HFSE formed. Granite intrusions of this type have been revealed in the MMZ for the first time. Thus, the granitoid complexes of the Akhunovo–Petropavlovsk area formed under changes in geodynamic settings and are characterized by different compositions, depths of occurrence, and genesis. This permits us to consider the area a typical continental-margin center of the long-term mantle–crust interaction, where magma generation proceeded at different mantle and crust levels, with the participation of both suprasubductional and enriched plume-related rift sources.

—In the section of the sedimentary cover of the East European Platform, the new Sabantuy chromite ore occurrence was revealed. It is localized in Upper Kazanian sandstones of the erosional terrace above the floodplain of Malaya Berkutla Creek in the south-west of the Republic of Bashkortostan. The article presents the first results of geological, lithological, and mineralogical–geochemical studies of the new ore occurrence with signatures of an ancient coastal–marine placer. The Cr2O3 content in chromite sandstones (chromitolites) is 15.9–17.2 wt %. The bulk of the heavy fraction (67%) is composed of sub-angular and sub-rounded Cr-spinel grains 0.15–0.25 mm in size. Their mineralogical features indicate proximity to Cr-spinels from ultramafic rocks of ophiolite complexes.

1317 In this communication, we discuss new data on the radiological age and composition of parental rocks of the Voznesenskoe deposit and Karagaikul occurrence of the copper-porphyry type localized in the melange zone of the Main Uralian fault in the northwestern part of the Magnitogorsk megazone of the South Urals (Fig. 1). The Main Uralian fault zone represents here a system of steep eastwarddipping tectonic slices, which include serpentinites, serpentinite melange, pyroxenites, gabbrods, and blocks of volcanogenic, volcanosedimentary, and sedimentary (siliceous shales, limestones) variousage rocks deformed into synand antiform folds (7). The Voznesenskoe copper-porphyry deposit is located in a dikeshaped massif of diorite-quartz diorites steeply westwarddipping and extending in the NNE direction for a distance of 2.5 km. Its stringer-dis� seminated sulfide mineralization is developed in the northeastern endocontact of the massif being mostly confined to plagiogranite-porphyry dikes that intrude diorites parallel to their contact with host ser� pentinites (10). We have investigated the microelemental composi� tion of diorites and plagiogranite-porphyries with variable degrees of metasomatic alterations and have conducted U-Pb dating of zircons from quartz dior� ites. The Sm-Nd method was used for dating altered plagiogranite-porphyries. The microelemental composition of rocks from the productive intrusive body (Fig. 2) indicates all the ana� lyzed rock varieties belong to island arc calcalkaline magmatites. This is evident from negative Nb and Zr and positive Sr anomalies, low TiO 2 concentrations (0.03-0.16%), and plots illustrating the REE distribu� tion (Fig. 2b). By the distribution patterns and con� centrations of trace elements, REE included, plagiog� ranite-porphyries are similar to diorites from the Voznesenka massif. This implies their common origin and that they belong to different phases of formation of the massif.

Magmatism in the Western Magnitogorsk zone ofthe Southern Urals took place on the accretionarybasement (island arc basement partly thrust over thecontinental margin) and is represented by mafic (lesscommonly, intermediate, ultramafic, and acid) rocksthat constitute intrusions of variable morphology: sillshaped sheets, laccoliths, lopoliths, chonoliths,stocks, dikes [1]. Based on their geological positionand petrological properties, these intrusions are unitedinto several complexes, which were formed during several stages. Each magmatic stage was characterized bylocal tectonic settings that existed from the Tournaisian to the Bashkirian ages [2, 3]. In our opinion, theoldest, Carboniferous rocks are related to the late stageof the collision between the Magnitogorsk island arcand passive margin of the East European continent,which is termed in [4] as a “soft collision,” laterreplaced in the Middle Carboniferous [5] by a “hardcollision,” i.e., a collision between the Kazakhstanand East European blocks. Until recently, the age estimates of magmatism in the Western Magnitogorskzone were based on geological and petrological features: stratified complexes with migmatites, intersection of intrusive bodies and dikes, and a compositionalanalogy with Tournaisian–Visean basalts of the neighboring Magnitogorsk–Bogdanovsk graben [6]. TheKhudolaz complex of differentiated and layered bodies with sulfide Cu–Ni mineralization recentlybecame the sole exception: baddelyite and zircongrains extracted from two autonomous gabbroid bodies yielded similar isotopic dates of 324.78 ± 0.46 and328.90 ± 0.78 Ma, respectively [7]. The terminal stageof magmatism in the Western Magnitogorsk zone isrepresented by a dike belt (dike complex), whichcrosses all the complexes of the Khudolaz syncline andstrikes at NE 10°–13° (Fig. 1).The available data on the Khudolaz complex implyits formation in the environment of early stabilizationof the island arc basement [6], i.e., in the subplatformal regime. This stage likely characterizes the tectonicpause between the two abovementioned collisiontypes. For a long time, it remained unclear whether ornot the dike complex is synplutonic, i.e., representinga terminal stage of magmatism in the Nibearing Khudolaz complex as was assumed by E.K. Buchkovskii etal. in their stored reports of 1971 and 1974, or if it ispostmagmatic, i.e., reflecting an autonomous latemagmatic stage as proposed in [1]. At the same time,inasmuch as no data on the upper geological limit ofthe dike complex were available at that time, we conceded both the Late Carboniferous and Early Permianage for the latter. The recently obtained date of 321 ±15 Ma supports our previous conclusion that the dikecomplex represents an autonomous stage of magmatism and precedes the commencement of hard collision in the Western Magnitogorsk zone. This was thelast pulse of magmatism in this structure.The thickness of dikes in the complex under consideration is sustained along the strike averaging 0.5–1.2 m. In areas where dikes cross the sedimentarycomplexes, the length of their outcrops is maximum(>1 km). The dikes are frequently bifurcated to joinagain or extend parallel to each other over a significantdistance.The mineralogical composition and texture ofrocks in dikes allow the following types to be defined:hornblende dolerites, dolerite porphyrites, gabbrodiorites, lamprophyres (spessartite, malchite), andgabbro dolerites. The rocks are variably altered:pyroxenes are partly or entirely replaced by secondaryhornblende (uralite, actinolite, tremolite); plagioclaseis frequently sossuritized; basaltic hornblende is commonly replaced by uralites and ore mineral. Fissures inrocks are filled with serpentine, chlorite, and epidote.Some relatively fresh varieties exhibit both unalteredplagioclase and clinopyroxene.

We present new data on the geologic position, composition, and isotope characteristics of the Early Cretaceous granitoids of the Samarka terrane, Sikhote-Alin’, formed on a transform continental margin. Geological and geochronological data show that these granitoids were generated at two stages of magmatism: in the first half (Hauterivian–Barremian, 130–123 Ma) and second half (Albian–Cenomanian, 110–98 Ma) of the Early Cretaceous. Granitoids of the first stage form an autonomous (free of basic precursors) unimodal melanogranite–granite association and are characterized by normal alkalinity with domination of K over Na, low contents of Ca, and elevated contents of Al2O3. By composition, these are S-granites with a model Nd age of ∼1.3 Ga. Granitoids of the second stage are of more diverse petrogeochemical types. They show wider variations in K/Na and A/CNK, are richer in Ca and, sometimes, Sr, and are poorer in P than the granitoids of the first stage. Their compositions form a continuous trend from S- to I-granites, and their model Nd age is ≤1.2 Ga. Comparison of the petrochemical, trace-element, and isotope characteristics of the Early Cretaceous granitoids and upper-crustal rocks (sandstones and siltstones of the turbidite matrix of a Jurassic accretionary prism and basalts from the inclusions in it) of the Samarka terrane and the coeval garrboids has shown that the potassic S-granitoids formed at the early (Hauterivian–Barremian) stage of magmatism as a result of the anatexis of upper-crustal sedimentary rocks. At the late (Albian–Early Cenomanian) stage, the intrusion of mantle magmas led to a temperature increase in the lower crust, which favored more active anatexis, involvement of high-melting substrates (oceanic basalts) in the granite formation, and interaction of mantle and crustal magmas. This resulted in a great diversity of granitoids (from S- to I-type).

The study of the Nb isotopy of rocks of the conti� nental crustal is an effective tool for evaluating the nature of mechanisms of crustforming processes and ages of their manifestation (1). In the first approxima� tion, the Nd model isotope age characterizes the age of formation of the continental crust from the mantle source or, in other words, transformation from oceanic crust to continental. The Nd isotope systematics of sedimentary rocks of upper levels of the continental crust allows us to estimate the average model age and the possible paleogeographic source areas, as well as to determine the lower age limit of sedimentation if there are no paleontological data. Moreover, the use of two� stage Nd model ages of granitoides along with the geochemical and petrological data obtained allow us to obtain data about the deep levels of the continental crust (levels of generation of the granitic melt). In some cases, the combination of these approaches makes it possible to reconstruct the chemical compo� sition and the nature and mechanisms of formation of the continental crust of separate blocks (terranes) and large fold belts (2).

732 A band of layered gabbro intrusions belonging to the Kusa–Kopan Complex is confined to the zone of the Zyuratkul riftogenic fault in the eastern part of the Bashkir Meganticlinorium, where it consists of several massifs (Matkal, Kopan, Medvedevo, Kusa) with the Ryabinovka and Gubenskii granitoid massifs located in the hanging wall (Fig. 1). The gabbro massifs enclose synonymous ilmenite–titanomagnetite deposits of the Urals in the form of stratified series, vein, and lenticu� lar lodes of continuous and disseminated ores, which alternate with gabbroids. These deposits were discov� ered in the 1930s, and since that time their geological structure and composition of ores and host rocks have been studied by many researchers. The primary atten� tion was paid to the most debatable factors responsible for strong lateral variations in the composition of rocks and ores within the complex, represented by horn� blende gabbro and gneiss–granites in the Kusa and Gubenskii massifs and ilmenite–magnetite ores in the Kusa and Medvedevo deposits in the north and by gab� bronorites (Matkal and Kopan massifs), porphyroid granophyric granites (Ryabinovka Massif), and titano� magnetite ores in the south. Some researchers explain such differences in the mineral composition by the influence of superposed metamorphic processes on rocks and ores of the Kusa Massif [1, 2], which occurred substantially after the formation of intru� sions. Our petrological studies performed in recent years revealed that the formation depth of granitoids, gab� broids, and enclosed ores increases along this band in the northerly direction from 3–4 km in the south to 20–30 km in the north. Accordingly, the P–T param� eters of rock and ore formation vary in the same direc� tion: = 0.5–3.0 kbar and T = 900–1100°C in the south (Kopan and Matkal gabbro massifs and associ� ated deposits, Ryabinovka granite massif) and P = 6– 9 kbar and T = 600–900°C in the north (Kusa and Medvedevo gabbro and Gubenskii granite massifs

Gabbro-tonalite-granodiorite-granite (GTGG) plutons productive for gold are suprasubduction intrusive bodies formed at an Andean-type active continental margin 410–380, 365–355, and 320–290 Ma ago. The Devonian plutons are situated in the southeastern marginal continental zone, whereas the Carboniferous plutons occur in the northwestern zone. All GTGG plutons are mantle-crustal. Their formation started with hydrous basic magmatism and was accompanied by such magmatism up to the final stage. Mantle-derived amphibole gabbro and diorite experienced partial melting (anatexis) in the lower crust under a pressure of 6–10 kbar, giving birth to the tonalite-granodiorite members of the GTGG series. The latter, in turn, were involved in anatexis with the formation of adamellite and granite, immediately accompanied by hydrothermal gold mineralization. The multistep anatexis is the main petrogenetic process responsible for the gold resource potential of GTGG plutons. In the process of anatexis occurring under high fluid saturation, gold was repeatedly removed from rocks into fluid, facilitating its concentration in ore deposits.

This paper presents results of isotopic (Cameca IMS1270 NORDSIM and SHRIMP-II ion microprobes) and geochemical (LA-ICP MS) study of zircons in three dunite samples of the Uralian-Alaskan-type massifs of the Urals: Kosva, Sakharin, and Eastern Khabarny. The zircons in the dunites share common features. Each sample contains the following genetic and age groups of zircons: (1) xenogenic zircons of the Archean and Proterozoic age; (2) zircons of magmatic appearance, which in age and geochemistry are close to the zircons from associated gabbroids; (3) postmagmatic zircons that presumably crystallized from hydrothermal solutions. The xenogenic zircons of the Archean age in each of three samples comprise transparent fragments, which are depleted in U and other trace elements and presumably have mantle origin. Xenogenic zircons of the Proterozoic age (1500–2000 Ma) occur as oval grains with surface abrasion, the traces of their redeposition. The geochemical features of the xenogenic zircons unequivocally demonstrate their affiliation to the continental crust—the basement of the Uralian orogen.

The Seredovina gold-bearing massif is located in the Middle Urals, 2 km southeast of Nev’yansk in the well-known Nev’yansk gold district and is confined to the outer contact of the large Verkhisetsk tonalite‐granodiorite batholith (Fig. 1). The massif is a typical minor granitoid intrusion localized in the apical part of batholith-shaped bodies, which usually contains dike fields with gold-bearing quartz veins and wallrocks composed of listwaenite‐beresite metasomatites [1‐3]. The massif hosts the Nev’yansk Seredovina quartz‐sulfide gold deposit. The Seredovina Massif is composed of hornblende‐ biotite granodiorites crosscut by thin (up to 0.5 m thick) aplite dikes. The hornblende‐biotite granodiorites are light gray medium-grained rocks with a vague gneissic structure and porphyritic hypidiomorphic texture. The rock contains plagioclase, quartz, biotite, hornblende, and K-feldspar. Epidote, titanite, apatite, allanite, and zircon are accessory minerals. Plagioclase forms elongated prismatic and tabular grains with distinct rhythmic zoning. Potassium feldspar occurs as irregularly shaped interstitial grains of a cross-hatched microcline containing oikocrysts of tabular plagioclase and occasional amphibole. Quartz is observed as irregular grains. Amphibole is represented by common hornblende with brownish green pleochroism along Np and Nm to dark green along Ng . Biotite forms tabular and elongated plates with brownish green pleochroism. Biotite flakes often contain inclusions of apatite, zircon, and titanite, and often grow together with epidote. Zircons for isotope dating were extracted by the routine procedure: crushing of typical granodiorite sample (30 kg) of the massif to a fraction of ‐0.5 mm; extraction of the heavy fraction using the Wifley table and heavy liquid technique; and separation of zircon grains under binocular microscope. U‐Pb analyses were performed on a SHRIMP-II ion microprobe at the Center of Isotope Research, Karpinskii All-Russia Research Institute of Geology, using the standard technique [4, 5].

(1989). THE ROLE OF CHLORINE IN THE FORMATION OF MAGNETITE SKARNS. International Geology Review: Vol. 31, No. 1, pp. 63-71.