Genetic features of tourmaline from the magnesian skarns of Kuhilal deposit, Southwestern Pamir

Abstract

Background. Although the Kuhilal deposit of Mg skarns has been thoroughly studied, tourmaline in their composition has never been considered [5, 6, 16]. Uvite, a tourmaline variety, was mentioned in the list of rare minerals only with its crystallochemical formula [17]. L.N. Shabynin, a great connoisseur of skarns, noted the rarity of tourmaline in Mg skarns of various deposits. Tourmaline of this type is characterized by a strong variation in ferruginosity and a significant content of Ca. At the same time, Ca:Na varies from 2:3 to 20:1 [15]. According to N.N. Pertsev, another major specialist in skarns, tourmaline is a rare mineral of Mg skarn deposits [11]. The boron-iron ore deposit of Tayozhnoe, Aldan shield, is an exception. This deposit belongs to skarns, where a widespread development of tourmaline was noted [12]. Tourmaline (dravite) of this deposit was found only in silicate metamorphic rocks containing marbles and Mg skarns.Aim. To determine the paragenesis, crystal morphology, chemical composition, and genetic characteristics of tourmaline from Kuhilal Mg skarns.Materials and methods. Large aggregates (more than 10 cm across) and crystals of apple-green tourmaline from the Mg skarns of the Kuhilal deposit were investigated. Samples of Mg skarns containing tourmaline were collected and documented by the authors during fieldwork research at the Kuhilal deposit in 2017. The samples were examined by mineralogical and petrographic methods, X-ray diffraction analysis using a DRONE-3M (analyst A.V. Fedorov, Sergo Ordzhonikidze Russian State University for Geological Prospecting), microprobe analysis using a Cameca SX 100 in 15 kV 30 nA shooting mode (analyst V.I. Taskaev, IGEM RAS), and X-ray fluorescence analysis using an AXIOS advanced spectrometer with an X-ray tube equipped with a 3 kW Rh anode and Philips PW-2400 with a sensitivity of 10-4% (analyst A.I. Yakushev, IGEM RAS). The majority of studies were conducted at the Department of Mineralogy and Gemology of the Sergo Ordzhonikidze Russian State University for Geological Prospecting.Results. Tourmaline from the Mg skarns of the Kuhilal deposit has been studied for the first time. This mineral is rare for skarn rocks. Two tourmaline mineral associations were identified: with spinel and forsterite, and with chlorite and serpentine. In terms of chemical composition, tourmaline isattributed to fluorine-containing uvite with a close to zero ferruginosity. Uvite is characterized by a lenticular shape with a rarely observed, underdeveloped prism. Its mineralogical properties, chemical composition, and formation conditions were analyzed. Tourmaline can be used to make inexpensive jewelry inserts.Conclusion. Tourmaline crystallization occurred under the conditions of granulite facies of regional metamorphism. For boron formation, an apo-sedimentary, evaporite source is assumed.