Metasomatism and ore formation at contacts of dolerite with saliferous rocks in the sedimentary cover of the southern Siberian platform

Abstract

The data on the mineral composition and crystallization conditions of magnesian skarn and magnetite ore at contacts of dolerite with rock salt and dolomite in ore-bearing volcanic-tectonic structures of the Angara-Ilim type have been integrated and systematized. Optical microscopy, scanning and transmission electron microscopy, electron microprobe analysis, electron paramagnetic resonance, Raman and IR spectroscopy, and methods of mineralogical thermometry were used for studying minerals and inclusions contained therein. The most diverse products of metasomatic reactions are found in the vicinity of a shallow-seated magma chamber that was formed in Lower Cambrian carbonate and saliferous rocks under a screen of terrigenous sequences. Conformable lodes of spinel-forsterite skarn and calciphyre impregnated with magnesian magnetite replaced dolomite near the central magma conduit and apical portions of igneous bodies. At the postmagmatic stage, the following mineral assemblages were formed at contacts of dolerite with dolomite: (1) spinel + fassaite + forsterite + magnetite ( T = 820-740°C), (2) phlogopite + titanite + pargasite + magnetite ( T = 600-500°C), and (3) clinochlore + serpentine + pyrrhotite ( T = 450°C and lower). Rock salt is transformed at the contact into halitite as an analogue of calciphyre. The specific features of sedimentary, contact-metasomatic, and hydrothermal generations of halite have been established. The primary sedimentary halite contains solid inclusions of sylvite, carnallite, anhydrite, polyhalite, quartz, astrakhanite, and antarcticite; nitrogen, methane, and complex hydrocarbons have been detected in gas inclusions; and the liquid inclusions are largely aqueous, with local hydrocarbon films. The contact-metasomatic halite is distinguished by a fine-grained structure and the occurrence of anhydrous salt phases ( CaCl 2 · KCl, CaCl 2 , n MgCl 2 · m CaCl 2 ) and high-density gases ( CO 2 , H 2 S, N 2 , CH 4 , etc.) as inclusions. The low-temperature hydrothermal halite, which occurs in skarnified and unaltered silicate rocks and in ore, is characterized by a low salinity of aqueous inclusions and the absence of solid inclusions. The composition and aggregative state of inclusions in halite and forsterite indicate that salt melt-solution as a product of melting and dissolution of salt was the main agent of high-temperature metasomatism. Its total salinity was not lower than 60%. The composition and microstructure of magnetite systematically change in different mineral assemblages. Magnetite is formed as a result of extraction of iron together with silicon and phosphorus from dolerite. The first generation of magnetite is represented by mixed crystals, products of exsolution in the Fe-Mg-Al-Ti-Mn-O system. The Ti content is higher at the contact of dolerite with rock salt, whereas, at the contact with dolomite, magnetite is enriched in Mg. The second generation of magnetite does not contain structural admixtures. The distribution of boron minerals and complex crystal hydrates shows that ...

Borate Minerals and Origin of the RNA World

“…Examples give the impression that extensive precipitation began only in the lower Cambrian, as in the case of the Angara suite (Fig. 1) in the Korshunovskoye and Nepskoye deposits on the Siberian Platform, Irkutsk Oblast', Russia (Mazurov et al 2007;Semeykina and Kozlova 1985). The only borate reported from a Precambrian evaporite is chambersite (Mn 3 B 7 O 13 Cl) associated with algal dolostone in the 1500 Ma Gaoyuzhuang Formation, China (Fan et al 1999;Shi et al 2008).…”

Optical absorption, infrared, Raman, and EPR spectral studies on natural iowaite mineral

“…7). As Raman spectroscopy of iowaite has already been reported [1], the analysis is not discussed here.…”

“…Iowaite belongs to R/3 m space group. The hexagonal unit cell parameters are a = 0.3116 nm, c = 2.4113 nm [1][2][3][4]. Its structure consists of three brucite-like layers and three disordered inter layers.…”

“…Iowaite from the Korshunovskoe, Siberia contains MgO = 39.80, Fe 2 O 3 = 24.71, and Cl = 5.41 wt%. Further ESR studies indicate the presence of Fe(III) in the crystal structure and Mn(II) as an impurity [1]. Also Raman and IR spectra were reported on this compound [2].…”

The basalt pipes of the Tunguska Basin (Siberia, Russia): High temperature processes and volatile degassing into the end-Permian atmosphere

“…Early halite investigations with Electron Paramagnetic Resonance (EPR)(Istomin et al, 2000;Mazurov et al, 2007) allow determining the specific characteristic of metamorphic halite. Three type of halite were distinguished on the basis of fluid inclusions accompanied with EPR data: sedimentary halite, high-temperature metamorphic halite, and low-temperature metamorphic (recrystallized) halite.…”