The Mushiston Sn deposit in Tajik Tien Shan as the type locality for stannite-cassiterite-hydrostannate mineralization: New mineral chemistry data and genetic constraints
Journal of Geochemical Exploration
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Abstract
The Mushiston Sn deposit is located in Hercynian South Tien Shan fold and thrust belt on the territory of Tajikistan. Unique features of the deposit include sulphide-dominated composition of the primary ore with stannite being the main ore mineral, and a thick oxidation zone where stannite is replaced by various hydrostannate minerals. Therefore, the Mushiston deposit is a rare example of stannite-cassiterite-hydrostannate mineralization and the type locality where several new hydrostannate minerals such as mushistonite, natanite and vismirnovite were first described. The structure of the deposit is characterized by a series of subvertical, generally NE-trending mineralized veins crosscutting the limbs of the large submeridional anticline composed of Paleozoic sedimentary strata. The primary ore mainly consists of a quartz-stannite group minerals – cassiterite mineral assemblage with other subordinate sulfides and minor sulfosalts. A 200–400 m thick zone of supergene oxidation is developed in the upper part of the deposit. New mineral chemistry data reveal compositional variations of the stannite group minerals, sphalerite and sulfosalts. The chemical compositions of co-existing stannite group minerals and sphalerite indicate extensive mutual substitution between Fe and Zn. The use of stannite-sphalerite geothermometry constrains the temperature for the main stage of the primary ore formation in the range of 257–308 °C. The tetrahedrite geothermometry yields temperatures in the range of 170–200 °C, which is in agreement with geological evidence for the late crystallization of sulfosalts at lower temperatures. The geological and mineralogical features of the Mushiston Sn deposit correspond to the mesothermal hydrothermal vein system mainly developed under intermediate sulfidation conditions. The proposed genetic model for the Mushiston deposit suggests formation of brittle fractures in the thermal aureole above the concealed causative granitoid intrusion, by regional analogue of Permo-Carboniferous age, where magmatic fluids mixed with meteoric water to form the hydrothermal system responsible for deposition of cassiterite-sulfide ore.
Introduction
The Mushiston Sn deposit is situated in the Zeravshan ore district of Hercynian South Tien Shan fold and thrust belt in Tajikistan. Unique features of the Mushiston deposit include the composition of primary ore with stannite [Cu2(Zn,Fe)SnS4] occurring as the main ore mineral, and a thick supergene oxidation zone where stannite ore is replaced by various hydrostannate minerals. Therefore, the Mushiston deposit represents a rare example of stannite-cassiterite-hydrostannate mineralization and the type locality where several new hydrostannate minerals, such as mushistonite (Marshukova et al., 1984), natanite and vismirnovite (Marshukova et al., 1981) were first described. The oxidation zone of the Mushiston deposit is economically important with more than 75% of the proven tin reserves of the deposit contained in the secondary hydrostannate ore where mushistonite [(Cu,Zn,Fe)Sn(OH)6] is the principle ore mineral (Ivanov et al., 1993). Mushiston is known to have been exploited since the Bronze Age. In particular, C14 dates in the range 1645–1250 BCE were reported for the wooden logs from ancient underground workings (Boroffka et al., 2002; Garner, 2013: 127). There were possibly three phases of ore extraction in Mushiston, with the oldest being a surface or near surface extraction and the second leaving a complex system of extensive underground working. The third phase, characterized by the use of metal tools and dated by a piece of pottery to the Late Bronze Age could possibly represent the last period of mining in Mushiston (Garner, 2013:239) until it was re-discovered first as an arsenic deposit and then as a tin deposit in the 1930s (Pavlovskii et al., 1995).
The geology and ore composition of the Mushiston deposit were studied in detail by several authors (Marshukova et al., 1986; Shustikov, 1989; Mamadvafoev et al., 1992; Pavlovskii et al., 1995). However, the last published descriptions of the deposit date back to the 1990s and have to be revisited in view of the recent data and an enhanced understanding of principles of ore formation applying modern techniques. In this study, we present a generalized description of the deposit accompanied by new mineral chemistry data for primary and secondary mineralization. Recent results of detrital zircon study published by Biske et al. (2021) supported reconsideration of the depositional age of the host rocks of the Mushiston deposit as Neoproterozoic, which allowed for new interpretation of its geological structure. The obtained mineral chemistry data provided some constraints on the formation temperatures of the primary ore and allow assessment of the sulfur fugacity in the hydrothermal fluids. These results, in combination with previously published data, were used to propose a refined genetic model for the formation of the Mushiston deposit.
Section snippets
Regional geology and metallogeny
The Tien Shan orogen in the southern part of the Central Asian Orogenic Belt was formed by the late Paleozoic collision between the Precambrian Karakum and Tarim continents in the south and the early Paleozoic Kazakhstan continent in the north, and subsequently modified by a series of Meso-Cenozoic intracontinental deformation events (Zonenshain et al., 1990; Şengör et al., 1993; Windley et al., 2007; Jepson et al., 2018). The western Tien Shan is composed of three major tectonic units or ...
Materials and methods
In this study we used a representative collection of samples covering all major types of ore from the Mushiston deposit. Samples 1, 4, 9, 10 were collected from Adit 3 tailings during our last field campaign of 2019, and represent partially oxidized quartz vein ore type (Fig. 7). Samples 145–87, mu2–90, m-181, 22–017 were collected earlier and include specimens of unoxidized ore from deep horizons of the deposit. Description and preliminary identification of the ore minerals were conducted in ...
Cassiterite
In the studied samples of unoxidized ores, the cassiterite occurs as large corroded grains in association with stannite and sphalerite (Fig. 8g, h). In the oxidized ores, small well-shaped crystals of cassiterite were identified in the veinlets of secondary mushistonite and malachite (Fig. 8k, l). Representative analyses of cassiterite are given in Electronic Supplement Table 1. The concentrations of Sn vary from 74 to 80 wt%, which corresponds to 0.93–1.06 Sn p.f.u. The concentrations of Cu ...
Stannite-sphalerite and tetrahedrite geothermometry
Iron and Zn are partitioned in the co-existing stannite and sphalerite according to the reaction Cu2FeSnS4 (in stannite) + ZnS (in sphalerite) → Cu2ZnSnS4 (in stannite) + FeS (in sphalerite), which made it possible to use this mineral pair as a reliable geothermometer valid for all the mineral species from the stannite group (Tamas and Andrii, 2020 and references therein). Coefficient of Fe
Zn partitioning (Kd) for this reaction is calculated according to the formula ...
Conclusions
The Mushiston Sn deposit in the Tajik Tien Shan is an example of stannite-cassiterite-hydrostannate mineralization and the type locality for several new hydrostannate minerals such as mushistonite and natanite. The structure of the deposit is characterized by a series of subvertical generally NE-trending mineralized veins crosscutting the limbs of the large submeridional anticline composed of Paleozoic sedimentary strata. The primary ore mainly consists of quartz-stannite-cassiterite mineral...