L. G. Filimonova and A. V. Chugaev
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Moscow
The data available on the geologic structure of the region, the mineralogy and geochemistry of the acid volcanics [Filimonova, 1995, 2002; Filimonova et al., 1990; Konstantinov et al., 1998; Sidorov, 1989], as well as the new data we have obtained recently for the products of volcanic, postmagmatic, and hydrothermal activity, suggest the great role of basic materials in their formation. Moreover, the territory of the Balygychan-Sugoi Trough can be ranked as a mantle plume province where the periods of endogenic activity were controlled by the reactivation of convection in the mantle.
The Balygychan-Sugoi Trough is located between the Kolyma and Omolon ancient massifs and extends for 120 km in a submeridional direction having a width of roughly 60 km (Figure 1). At the present time this region is an arch rising high above the sea level, complicated by ring structures of various sizes. The maximum absolute heights (1200 m) and relative elevations (about 400 m) are restricted to the Dukat volcanoplutonic system, namely, to the water divide of the rivers flowing to the Arctic Ocean and the Sea of Okhotsk. The level of the present-day erosion in the area of the trough is fairly high varying around 1500 m.
Kazanskii , Sidorov , and Konstantinov  believe that the trough is localized in the area of the junction of two global structural and metallogenic provinces (Figure 1). One of them is a fragment of the Kolyma-Kamchatka submeridional transform lineament, which crosses the Okhotsk-Chukotka volcanic belt, and the Kamchatka Peninsula which controlled the position of the Au-Ag mineralization. The depths of its faults were estimated from geophysical data [Shilo et al., 1979] to be as great as 100 km. Another structural element is a fragment of the Verkhoyansk-Chukotka silver-bearing belt surrounding the Kolyma-Omolon median massif. This arc-shaped zone surrounding the core of a huge dome, more than 2000 km across, controls the positions of silver-lead-zinc and cassiterite-sulfide ore deposits in the West Verkhoyansk region. In terms of this great domal structure, the Balygychan-Sugoi Trough is interpreted as one of the large radial crustal faults.
As follows from the data available for the geological structure of the Balygychan-Sugoi Trough and the adjacent areas, and also from the results of gravity measurements [Konstantinov et al., 1998; Shilo et al., 1979; Sidorov, 1989], the total thickness of the crust in the region is as great as 52 km. The depth to the top of the basalt layer is about 26 km. Its thickness (26 km) is greater significantly than the similar values for the areas of the adjacent anticlinal highs, where this value varies within 10-15 km. As follows from geophysical data, the axial region of the trough is marked by a zone of undulating meridional gravity anomalies, some of them controlling the distribution of ore fields.
The data available for mineral assemblages in crustal rock inclusions and for post-magmatic (PM) mineral associations were obtained from studying bedrock samples collected from natural outcrops, bulldozer cleanings, and boreholes. We examined and analyzed about 500 lump samples, from which we extracted xenocrysts, inclusions, and PM aggregates, restricted to networks of microcracks. In addition, rock samples weighing 10-15 kg were collected. The bulk of the study xenocrysts and inclusions of basic and ultrabasic rocks were extracted from man-made concentrates, prepared from the crushed rock material. The chemical compositions of individual minerals were studied using an MS-46 Cameca spectral analyzer by analyst I. P. Laputina. Our data on the chemical compositions of some minerals extracted from the concentrated specimens and found in thin sections showed good agreement [Filimonova, 2000]. The morphology and relationships between individual crystals in the mineral aggregates were studied visually, in thin sections, and using a scanning electron microscope by N. V. Trubkin. Electron microdiffraction patterns were obtained for microdispersed mineral phases using a JEM-100C scanning electron microscope equipped with an X-Ray energy-dispersion spectrometer capable of recording chemical elements from Na to U. The statistic frequency of various prismatic and pyramidal faces in zircon xenocrysts was estimated by A. D. Chervinskaya; some of the most characteristic grains were measured by N. N. Smolyaninova using a goniometer. The Rb-Sr isotope studies of the specimens were carried out in the Laboratory of Isotope Geochemistry and Geochronology, Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM), Russian Academy of Sciences, using conventional methods and techniques.
The evidence available for the geological structure of the study area, and of the Dukat volcanoplutonic system, in particular, as well as the K-Ar and Rb-Sr ages of the volcanic rocks and products of hydrothermal activity [Konstantinov et al., 1998], suggest the cyclic evolution of the postmagmatic and hydrothermal processes in the region.
The K-rhyolites contain ultramafic inclusions. Some of them are two-mineral aggregates with sizes < 0.25 mm. They consist of colorless translucent crystals of olivine and brown Ca-amphibole. The low Fe number of the olivine and the elevated contents of alkalis and Ti in the pargasite hornblende (Table 1) allows one to classify these inclusions as high-pressure hornblendite-cortlandite ultramafics [Zimin et al., 1989]. The other inclusions are represented by the microaggregates of aegirine xenocrysts of two generations. The earliest fine ( < 100 m m) grains contain Mg (Fe number = 0.57, Table 1). The late grains are almost devoid of this element.
During a pause between the early and late cycles of volcanic activity, multimetal disseminated mineralization (MDM) was formed in the sublatitudinal weak zones of the Buyunda-Gizhiga system of crustal faults. Its products are represented by grains, < 0.25 mm in size, located mainly in the leaching cavities of quartzose fluidal K-rhyolite.
The subsequent evolution of volcanic activity in the trough area was associated with the formation of the Okhotsk-Chukotka volcanic belt. A contrasting volcanogenic rock sequence composed of calc-alkalic andesite, basaltic andesite, dacite, and K-Na rhyolite accumulated during 20-25 Ma in the Late Cretaceous. Its diorite and gabbro-diorite are characterized by low 87 Sr/ 86 Sr ratios varying from 0.7044 to 0.7045 (Figure 2). The K-Na rhyolites of the Nayakhan Formation also showed the low values of this ratio (0.7047 3) with their Rb-Sr age being 90.5 2.5 Ma [Kolesnikov et al., 1998].
The xenocrysts of edenite hornblende from the Late Cretaceous K-Na rhyolites showed a high Fe content with the significant contents of alkalis (Table 1). These properties are very characteristic of amphiboles from anorogenic subalkalic rocks [Dobrokhotova, 1986]. Their 87Sr/86Sr ratio is 0.7057 with the Rb and Sr contents being 186 and 1425 ppm, respectively.
The zircon xenocrysts from the K-Na rhyolites resemble those from the Early Cretaceous K-rhyolites in some of their morphologic features (the presence of zircon prism faces and an obtuse pyramid). Characteristic of their habit is the ubiquitous presence of a poorly developed hyacinth-type facet 110 and acute bipyramid facets 331 and 113. This results in the expansion of the population of the morphotypes that formed at lower temperatures (700-750oC) and in the lower activity of alkalies. Characteristic of these zircons are the higher HfO2 contents compared to the zircons from the K-rhyolites (1.3-1.9 against 1.0-1.1 wt.%) and the lower Zr/Hf ratios (60-90), typical of zircons from gabbro.
After the final formation of the subaerial volcanic rock sequence the region experienced hydrothermal activity. A unique Au-Ag deposit (Dukat), as well as large sulfide-cassiterite (Sn-Ag) and cassiterite-silicate deposits, and also numerous zones of noncommercial mineralization, had been formed in the areas of old extensive geothermal fields.
A great role for the creation of a reducing environment during the early formation history of the Dukat ore-forming system was played by the compounds of carbon in its hydrothermal solutions. Carbon-bearing materials were characteristic of the preore dike-shaped zones of Ca-bearing metasomatites, carbonized rocks of explosive breccias, and some orebodies, especially those that had contacts with the host rhyolites. Konstantinov et al.  emphasized the trends toward the growing concentration of carbon dioxide in the mineral-forming solutions enriched in Au and Ag. The simultaneous migration of Ca compounds and ore metals has been proved by our findings of C-Pb-Tl-bearing todorokite [Filimonova, 2002].
Our data on the Pb isotope composition of galena from the Dukat ore deposit and from some tin and gold-silver deposits of the region suggest the contribution of some mantle source into the formation of some ore-producing hydrothermal solutions of the region. The 206Pb/204Pb ratios vary within 18.2-18.4 and are close to these ratios for basalts from a number of mantle-plume provinces reported by Grachev [.
The Late Cretaceous and Paleogene basalt dikes date the last (Late Cretaceous) period of igneous activity in the region. They are characteristic of the Dukat ore field and also of some large tin ore deposits, where they cut across all known igneous and hydrothermal rocks and are believed to be the root systems of the eroded basalts that had flowed over the ground surface. In terms of their chemical composition the dike basalts resemble continental tholeiite basalts, except for their higher contents of Na and K oxides and higher Fe numbers [Kalinin, 1985].
The data reported here on the geological structure of the Balygychan-Sugoi Trough and on the mineralogy and geochemistry of the products of magmatic, post-magmatic, and hydrothermal activities suggest a great contribution of basic and ultrabasic magmas into the formation of their chemical compositions. The direct products of these magmas are subvolcanic gabbro and quartz diorite bodies with low 87Sr/86Sr ratios (0.7044-0.7045), basalt dike swarms, and hornblende peridotite. These rocks are typical representatives of mantle plume materials. No acid volcanics have been found. However, the low values of the initial 87Sr/86Sr ratios (0.7031-0.7047) found for the Early and Late Cretaceous rhyolites of the region, as well as the inclusions of mafic and ultramafic rocks in them, allow us to rank them as the products of mantle plumes. We believe that the acid magma generated in a mafic magma source rose rapidly toward the surface, its contamination along this rise being extremely limited. The experimental data available for the conditions of Ca amphibole crystallization and Al, Fe, Mg, and Na redistribution among the coexisting minerals of deep inclusions [Johnson and Rutherford, 1989; Sobolev, 1974] suggest that edenite hornblende and spinel-clinopyroxene rock associations had been formed at pressures of 5-8 kbar and temperatures of 500-700oC. The values of these parameters do not contradict the data available for the crystallization temperature of zircon xenocrysts which might have been formed at 700-850oC. These values agree with the boundary conditions at the lower and upper boundaries of the crust in the region and suggest that the magma chambers where the acid magma had been generated and from where mafic and ultramafic inclusions had been derived had been a mantle diapir. The region of intersection between the submeridional rift, as deep as 100 km, and the peripheral zone of a very large dome was very favorable for the free rise of mantle diapirs into the lithosphere. The specific features of the outer appearance and chemical compositions of minerals in inclusions (intensive development of zircon prisms in zircon crystals, as well as the presence of pargasite and edenite hornblende, acmite, and acmite-bearing clinopyroxene) suggest that the basic magma had been highly alkalic. The maximum values of alkalinity had been characteristic of Early Cretaceous magma chambers, where acmite had crystallized. The alkalinity decline in the magmas of Early Cretaceous chambers seems to have been associated with crustal assimilation which might have been especially active during the orogenic tectonomagmatic history of the region when the processes concerned operated under compression conditions. This inference is supported by the high values of the initial Sr isotope ratios in the Late Cretaceous K-Na-rhyolites, where these values as high as 0.7047, compared to 0.7031 in the Early Cretaceous K-rhyolites.
The results obtained in this study for some mineralogic and geochemical features of the Late Cretaceous disseminated mineralization and the preore minerals of the Late Cretaceous Sn- and Ag-productive geothermal systems suggest that the products of this hydrothermal fluid activity had been associated with mantle plumes. The main source had been the material of differentiated mantle diapirs and the fluids separating from them. We believe that a great contribution to the formation of the Early Cretaceous metallic mineralization was made by the reduced fluids associated with the emplacement of basic and ultrabasic magmatic bodies into the lithosphere, typical of rift zones [Pierce et al., 1987]. The products of the post-magmatic activity were enriched in carbonaceous matter, graphite, HFSE ions (Ti, Nb, Zr, P, REE, Cr), and native elements.
The basic rocks of mantle origin also contributed to the formation of the chemical compositions of the preore minerals of some Late Cretaceous productive geothermal systems, including the Dukat ore field. This is indicated by the abundance of femic and Cr-bearing minerals and by the low Sr isotope ratios in them. However, some contribution was also made by mixing processes. Some portions of hydrothermal solutions, from which epidote-group minerals with the 87Sr/86Sr ratios of 0.7047-0.7053 crystallized might have been produced as a result of the mixing of components from the basic rocks and the deep lithosphere, having low Rb/Sr ratios. The later preore minerals of the western Dukat ore field originated from the mixing of two sources. One of them had low 87Sr/86Sr values ( < 0.7046), typical of mantle-plume formations, the other had higher values of this ratio ( > 0.7060) typical of the enclosing rhyolite.
The formation of a thick crust with a thick basalt layer in the area of the Balygychan-Sugoi Trough might have been associated also with the processes operating in a mantle plume region. The growth of the crust from below took place at the expense of the emplacement of basic and ultrabasic intrusions. The latter can be interpreted as the products of the rest magma that remained after the melting of acid magma from the ultrabasic melt, or, as suggested by Grachev [, as the intrusive analogs of plateau basalts, which usually represent magmatic activity associated with mantle plumes.
To conclude, the data available at the present time for the periodicity of volcanic and postvolcanic activities in the area of the Balygychan-Sugoi Trough and for the unique Dukat Au-Ag ore field located in its limits, as well as new data on the mineralogy and geochemistry of the resulting products, suggest that the impulses of endogenic activity might have been associated with the intrusion of mantle diapirs into the lithosphere. These high-temperature magma chambers and the fluids rising from them stimulated the development of volcanic, postvolcanic, and hydrothermal processes. These impulses of endogenic activity corresponded in their duration to two periods of the Meso-Cenozoic reactivation of mantle convection, first mentioned by Vogt , which operated roughly 70-80 and 110-120 million years ago.
The previously known peculiarities of the crustal structure of the rifting-related Balygychan-Sugoi Trough and our new data on the mineralogy and geochemistry of the products of the Meso-Cenozoic volcanic, postmagmatic, and hydrothermal activities suggest that this region can be classified as a mantle-plume province. The emplacement of thick Early and Late Cretaceous volcanic rocks and the associated products of dispersed and productive hydrothermal fluid activities were controlled significantly by deep basic magmas.
The periodic emplacement of the products of volcanic, post-volcanic, and hydrothermal activities is believed to have been associated with the reactivation of convection in the lower mantle and with the subsequent intrusion of mantle diapirs into the lithosphere, which controlled endogenic activity in various volcano-plutonic structures.
A thick volcanogenic and sedimentary rock sequence with its dispersed multimetal mineralization accumulated during a period of 30-35 million years in the Early Cretaceous time with the participation of subalkalic basalts and hydrocarbon fluids. The disseminated minerals are enriched in carbonaceous material, titanium, multivalent cation phosphate, and native metals, the main ore components of the region.
During the late Cretaceous a contrasting sequence of volcanic rocks and Au-Ag and Sn-Ag ores, including the unique Dukat Au-Ag ore deposit, accumulated during 10-15 million years under orogenic conditions. The compositional evolution of acid magmas and hydrothermal solutions involved the mixing of sources characterized by different 87Rb/86Sr ratios. Hydrocarbon fluids played a significant role in the formation of the Dukat ore deposit.
The latest period of endogenic activity in the Balygychan-Sugoi Trough (65-70 million years ago) was marked by the emplacement of basalt dikes which are interpreted as feeding channels for the now eroded lava plateaus.
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