Leucocratic magmatic melts with the maximum fluorine concentrations: Experiment and relations in nature

Experimental data indicate that high F concentrations in leucocratic aluminosilicate melts (of granite and nepheline syenite composition) bring about the crystallization of F-rich minerals (topaz, villiaumite, and cryolite) on the liquidus. The crystallization of the minerals is controlled by the silicity, agpaitic coefficient, and proportions of alkalis in the system SiO₂-Al₂O₃-Na₂O-K₂O-F-H₂O. Our earlier experimental data on this system are compared with petrographic and petrochemical data on granites and nepheline syenites containing accessory topaz, cryolite, and villiaumite. The composition of topaz- and cryolite-bearing rocks is proved to correspond to the experimentally established equilibrium fields of F-rich aluminosilicate melt with these minerals. It is proved that the high-F minerals can crystallize from melt. The partial substitution of K and Na for Li modifies phase relations in the system, first of all, significantly expands the equilibrium field of aluminosilicate melt and alkaline aluminofluoride melts. The two melts are proved to be immiscible within broad compositional ranges in the SiO₂-Al₂O₃-Na₂O-Li₂O-F-H₂O system at 800–650°C and 1 kbar. Experimental data indicate that fluoride brine can coexist with aluminosilicate melts in nature. This finds support data on melt inclusions in granites and alkaline rocks whose contents of major components, water and fluorine are close to those in the experimental glasses. Our data lend support to the hypothesis that large cryolite bodies at the Ivigtut, Pitinga, Ulog-Tanzek, and other deposits were formed by fluoride salt melts that separated from F-rich aluminosilicate magmas late in the course of their differentiation. It is experimentally established that fluoride salt melts are able to concentrate valuable trace elements, such as Li, W, Nb, Hf, Sc, U, Th, and REE, which suggests that such melts can play an important role in the origin of rare-metal deposits genetically related to rocks that crystallize from magmas rich in F.     

Wall-Rock alteration of biotite hornfels at the Tyrnyauz deposit,Russia

A new zonation type of W–Mo-bearing altered biotite hornfels at the Tyrnyauz deposit is reported. The review of zonation indicates a subsequent transition into the mobile state of CaO, MgO, FeO, and Al₂O₃ and retention of volume owing to dissolution or deposition of quartz as an excess mobile mineral. The main features of zonation are similar to those in acid leaching columns; the input of strong CaO base into the outer zone is unusual.     

Processes of Replacement by Melt at Interaction between Refractory Materials and Industrially Produced Melts

In a number of industries (ferrous and nonferrous metallurgy, glass-making and silicate-producing technologies), interaction between refractory materials with melts results in sequences of reaction zonation (reaction columns) that show all principal features of diffusion-controlled metasomatic zoning. However, in contrast to the latter, reaction melt is generated together with crystalline phases in the rear zones of the columns. This melt is neither mechanically displaced melt that affects the refractory materials, nor produced by melting. The process generating this melt is most adequately defined as replacement by melt. The principal characteristics of the zoning are discussed below with reference to the corrosion of chromite–periclase refractory materials with melted slag in nickel-producing metallurgy. Similarities between the relations observed under different conditions and in different systems and the evolutionary dynamics of the process, specifics of melt generation and changes in its composition in the zones are demonstrated below with the use of data on other technologies and their experimental modeling. The mechanism of melt replacement is applicable to describing natural reaction processes of magma interaction with host rocks (magmatic replacement), with the following unobvious implications. (1) It is reasonable to expect that the minerals of the rocks should host melt inclusions. (2) It is reasonable to expect that certain minerals should be found in two distinct populations: (i) those in equilibrium with melt in the reaction column and (ii) those crystallizing from the cooling melt. (3) Two or more zones of the column can consist of the same minerals, but their proportions should be different. (4) Plastic deformations in the rear zones of the column (magmatic replacement) should be associated with brittle ones in the pristine host rocks and frontal (metasomatic) zones. (5) In contrast to the rocks of metasomatic columns, the material of magmatic-replacement zones can flow through fractures cutting across the host metasomatic rocks and thereby intersect the outer metasomatic zones.     

Mineral Associations and Mo–W Ore Types of the Slepaya Zalezh’ Orebody at the Tyrnyauz Deposit

Geology of Ore Deposits - Slepaya Zalezh’, the largest orebody of the Tyrnyauz Mo–W deposit, is considered a first-priority object should mining operations at the deposit be resumed....