Metamorphic dolomitic marble-hosted talc from the Mulvoj area in the Western Pamir Mountains, Tajikistan
, oraz | 04 sie 2023
Article Category: Original Paper
Data publikacji: 04 sie 2023
Zakres stron: 31 - 42
Otrzymano: 24 lut 2023
Przyjęty: 12 cze 2023
DOI: https://doi.org/10.2478/mipo-2023-0004
Słowa kluczowe
© 2023 Mohssen Moazzen et al., published by Sciendo
This work is licensed under the Creative Commons Attribution 4.0 International License.
Talc [Mg3Si4O10(OH)2] is a hydrous magnesium silicate with industrial, medical and cosmetic applications (Vitra 1998; Borowski et al. 2015). It can form by different mineralogical reactions in ultramafic, mafic and dolomitic metasedimentary rocks. On this basis, talc deposits can be classified into the serpentinite-hosted and carbonate-hosted deposits (Tosca et al. 2011; Ali-Bik et al. 2012). Talc forms from serpentine minerals in the hydrothermally altered ultramafic and mafic rocks. Serpentine changes to talc by influx of CO2-bearing hydrothermal fluids (Liu 1986; Muraishi 1988). Talc can form at the margins of a serpentinite body in contact with the country rocks (Gil et al. 2022). Dolomitic marble reacted with quartz and water-rich or Si-rich fluids, produces talc (Pieczka et al. 1998; Wilamowski, Wiewiora 2004; Saccocia et al. 2009; Woguia et al. 2021). Apart from the industrial importance, study of talc-bearing rocks can provide insights on the nature of high-pressure and low-temperature metamorphism and possible fluid recycling within the mantle by talc-bearing assemblages subduction (Zhang et al. 1995). Spandler et al. (2008) studied the talc-bearing blueschists and eclogites from New Caledonia and concluded that they subducted to depths of up to 70 km with potential to transport significant amounts of H2O to greater depths than serpentinite. Schreyer and Abraham (1975) studied high pressure assemblages of kyanite-gedrite and kyanite-talc from the Sar e Sang area of Afghanistan and talc formation in these rocks.
Abraham and Schreyer (1976) attributed the talc and phengite rocks in Piemontite Schist from Serbia to formation under very high-water pressures.
Talc is significant in geological studies also for its role in weakening the crust in regional-scale fault systems. Interconnected films of talc require small volume fractions to attain the frictional weakness in the fault zones (Moore, Rymer 2007). Collettini et al. (2008) reported talc plus serpentinites, tremolite and chlorite in the high strain regions of a fault core, which can weaken the fault zone and facilitate creeping. Talc can also be associated with mineralization. A good example is the Rędziny deposit in Sudetes, Poland which comprises a weathered polymetallic mineralization in dolomitic marbles, where talc is formed along with Ca-Fe arsenate, arsenopyrite and quartz (Pieczka et al. 1998) Talc occurs in SW Pamir Mountains of Tajikistan. Talc is formed in the ultra-magnesian rocks in this area along with kyanite, magnesio-hornblende, tourmaline and rare quartz. Grew et al. (1994, 1998) reported kornerupine-bearing rocks from three localities in SW Pamir Mountains in Kuhi-lal, Darai-Stazh and Mulvoj. Kornerupine occurs with all these minerals (Grew et al. 1998). They propose a prograde metamorphism at 650°C and 7 kbar for the formation of assemblages containing the most mineral phases (Grew et al. 1998). Considerable amount of talc mineralization in dolomitic layers in gneissic rocks in the Mulvoj area produced economically valuable deposits, similar to those in the Nangahar province of Afghanistan (Tahir et al. 2018). The main minerals in the Mulvoj talc mine are dolomite, quartz, calcite and tremolite. Field relations, petrography and geochemistry of these rocks are studied here to provide new data on protolith and CO2 mole fraction and temperature conditions of formation of talc.
Pamir Mountains are part of the Alpine-Himalayan orogen, located at convergence of Tien Shan, Karakoram, and Hindu Kush (Ruzhentsev, Shvolman 1981; Hubbard et al. 1999). Middle to Late Eocene metamorphism in the Pamir brought about by crustal thickening and pressure and temperature increase associated with the India–Eurasia collision (Fraser et al., 2001). Based on stratigraphical, lithological, and structural features, Pamir is divided into Northern Pamir, Central Pamir and Southern Pamir in Tajikistan (Burtman, Molnar 1993). The northern Pamir, which records a Pre to Late Carboniferous ocean basin, separates the Central Pamir from northern Eurasian lands by Palaeozoic sutures (Hildner 2003). The Kunlun arc and related arc magmatism (Jiang et al. 2008) indicate the subduction and following collision sutures (Fig. 1). The Central Pamir (Fig. 1) mainly consists of metamorphosed and deformed Precambrian and Palaeozoic rocks. It shares many similarities with Western Hindu Kush and represents more likely a continental fragment which collided with Eurasia in Permian following the closure of the Carboniferous oceanic basin (Rembe et al. 2021). Southern Pamir is separated from the Central Pamir by Rushan-Pshart Mesozoic suture (Fig. 1), which is characterized by Permian to Triassic marine sediments and ophiolitic rocks composed of pillow basalt, chert, andesite and serpentinized peridotite lenses (Zanchetta et al. 2018). The Southern Pamir is divided into South-eastern Pamir and the South-western Pamir. South-eastern Pamir consists of a Late Carboniferous to Early Permian sedimentary sequence made up by siltstone, clay. Sandstone, and limestone, covered by Triassic limestone, radiolarite and siltstone, and eventually by unconformable Jurassic limestone (Kukhtikov, Vinnichenko 2010). South Pamir experienced metamorphism at 750-800°C at a depth of ~55 km (Hacker et al. 2017). The South-western Pamir is similar to the Central Pamir in terms of rock types and deformation and metamorphism (Pashkov, Budanov 1990). The main rock types are metamorphosed rocks, intruded by several Mesozoic and Paleogene granitoids (Fig. 1). Drugova et al. (1976) and Kiselyov and Budanov (1986) propose an early granulite facies metamorphism for the South-western Pamir at 750°C temperature and 9.7 Kbar pressure, followed by a lower grade metamorphism at 600 to 650°C temperature and 3.5 to 5.5 kbar pressure. Grew et al. (1994) also suggest two stages of metamorphism in the South-western Pamir. The first phase was a moderate-pressure amphibolite facies at 650°C and ~7 kbar, followed by a decompression metamorphism at 5 kbar and slightly higher temperature of 650-700°C. Pamir peak metamorphism occurred during the Late Oligocene to Early Miocene (Searle et al. 2010). Peak metamorphism was followed by exhumation of a series of extensional gneiss domes in the Southern and Central Pamir terranes, that lasted until the Late Miocene to Early Pliocene (Stübner et al. 2013). Included among these gneiss domes are the Shahdara dome in the South-western Pamir terrane and the Yazgulem and Sarez domes in the Central Pamir terrane (Fig. 1). The largest area of Cenozoic mid to lower-crustal rocks in the Pamir are exposed in the Shahdara dome (Stearns et al. 2015), which is dominated by orthogneiss and granitoids (Schwab et al. 2004) and biotite and muscovite-rich schists.
Figure 1.
Geological Map of the Southwestern Pamirs (modified from Vlasov et al. 1991, Schwab et al. 2004 and Angiolini et al. 2013). The study area is indicated by a box.
The Mulvoj area mainly consists of foliated and folded high-grade gneiss and schist, with intercalations of dolomitic marble layers (Fig. 2), ranging from about 10 cm to few ten meters in thickness (Fig. 3A,B). The main minerals in gneiss are quartz, plagioclase, garnet, biotite, kyanite and sillimanite. It shows distinct gneissosity and occasional folding (Fig. 3C). The rocks are mainly fresh with minimum alteration effects and pale in colour in the field exposures. The schistosity in the pelitic schist is materialized by abundant oriented biotite flakes. Other minerals are quartz, occasionally sillimanite and rare plagioclase and opaque minerals. The lack of muscovite and chlorite in the schists supports their high-grade nature. Dolomitic marble is pale yellow in colour and appears as distinct layers. Marbles of the Mulvoj area contain relatively large amphibole (tremolite) crystals (Fig. 3D), which are aligned parallel in dolomitic marbles, indicating lineation in the rock due to deviatoric stress during their formation. Some patches of almost pure calcite can be found in the field. These calcite patches appear as well-crystallized calcites with rhombohedral structure (Fig. 3E). Talc appears as pure mineral in the field and it is along with calcite in considerable amounts (Fig. 3F) Mining to extract talc was active during the Soviet time in the Mulvoj area. The mine is at an elevation of ~800 m from the village level in a rough topography. Metal pillars and wires were used to carry talc ore from the mine site to the village. Figures 3G and H show some remaining mining structures in the area.
Figure 2.
Simplified geological map of the Mulvoj area in the Ishkashim district of Tajikistan (Based on Vlasov et al. 1991).
Figure 3.
Field photos from the metamorphic rocks in the Mulvoj area. (A) Dolomitic marbles within the gneiss and schist. (B) Large open fold in the metamorphic rocks. (C) Tight folds in gneiss. (D) Tremolite along with carbonate in the dolomitic marble. (E) Rhombohedral pure calcite. (F) Talc in the Mulvoj mine site. (G) and (H) Remaining of old mining metal zip-lines and pillars.
More than 30 samples of the talc and associated rocks were collected during two fieldworks in the Mulvoj area of the Ishkashim district in SW Pamir, during spring and autumn 2021. 15 thin sections were made from the selected samples for petrography studies. Whole rock major elements were analysed in 5 talc samples and one dolomitic marble, using X-ray fluorescence (XRF) method. The samples were crushed to less than 5mm in a steel jaw crusher and then were pulverized in a disc mill equipped with a tungsten carbide milling cup to <60μm. 0.5 g of powdered rock sample was mixed with Lithium-tetraborate (Li2B4O7) to make glass beads. The beads were used for XRF analyses. Then they were dissolved using microwave-assisted multiacid digestion in a mixture of pure nitric, hydrofluoric, and hydrochloric acids. The solution was used to analyse trace elements after required dilution using a Perkin Elmer SCIEX ELAN 6000 inductively coupled plasma mass spectrometer. International and internal standards were used for the calibrations. The results for major, minor and trace elements are shown in Tables 1 and 2, along with the detection limit for each method and element. One sample of talc deposit from the study area was analysed by X-ray diffraction (XRD, Phililps PW1730) diffractometer to find out the mineral composition. The powdered pellet of sample was pressed using a hydraulic press. CuKα 0.15418 nm radiation was generated using the Phillips PW1730 X-ray generator operated at 40kV and 30mA. Sample preparation and analyses were carried out in the Earth and Environmental Sciences laboratory of University of Central Asia in Khorog, Tajikistan and Zarazma Laboratories in Iran.
Major oxides of the studied samples by XRF (wt%).
| Sample No. | Detection | MVJ1 | MVJ2 | MVJ3 | MVJ4 | MVJ5 | MVJ6 |
|---|---|---|---|---|---|---|---|
| Rock type | Limit (wt %) | Talc | Talc | Talc | Talc | Talc | Marble |
| SiO2 | 0.05 | 60.76 | 61.07 | 60.41 | 59.73 | 60.12 | 3.21 |
| TiO2 | 0.05 | 0.08 | 0.08 | 0.07 | 0.07 | 0.08 | <0.05 |
| Al2O3 | 0.05 | 1.35 | 1.26 | 1.14 | 1.14 | 1.18 | 1.28 |
| BaO | 0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 |
| CaO | 0.05 | 0.19 | 0.12 | 0.14 | 0.19 | 0.18 | 29.07 |
| Fe2O3* | 0.05 | 0.64 | 0.71 | 0.75 | 0.61 | 0.59 | 0.62 |
| K2O | 0.05 | 0.31 | 0.32 | 0.30 | 0.29 | 0.31 | <0.05 |
| MgO | 0.05 | 32.29 | 33.21 | 31.98 | 32.55 | 32.64 | 20.76 |
| MnO | 0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | 0.17 |
| Na2O | 0.05 | 0.27 | 0.22 | 0.30 | 0.27 | 0.29 | 0.19 |
| P2O5 | 0.05 | <0.05 | <0.05 | 0.06 | <0.05 | <0.05 | 0.09 |
| SO3 | 0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 | <0.05 |
| Total | - | 95.89 | 96.99 | 95.15 | 94.85 | 95.39 | 55.39 |
all Fe as Fe2O3.
Trace and rare earth element (REE) composition of the analysed talc samples by ICP-MS; all in ppm.
| Sample | DL* | MVJ1 | MVJ2 | MVJ3 | Sample | DL | MVJ1 | MVJ2 | MVJ3 |
|---|---|---|---|---|---|---|---|---|---|
| Ag | 0.5 | <0.5 | <0.5 | <0.5 | Pb | 1 | 4 | 4 | 4 |
| As | 5 | 1.5 | 1.5 | 1.4 | Sb | 5 | 0.88 | 0.79 | 0.86 |
| Be | 1 | <1 | <1 | <1 | Sc | 0.5 | 0.8 | 0.8 | 0.7 |
| Cd | 0.1 | 0.35 | 0.34 | 0.35 | Sr | 2 | 6 | 6 | 6 |
| Ce | 1 | 1 | 1 | 1 | Th | 5 | <5 | <5 | <5 |
| Co | 1 | 1 | <1 | 1 | U | 5 | <5 | <5 | <5 |
| Cr | 1 | <1 | <1 | <1 | V | 1 | 12 | 12 | 11 |
| Cu | 1 | 3 | 2 | 2 | Y | 0.5 | <0.5 | <0.5 | <0.5 |
| La | 1 | 1 | 1 | 1 | Yb | 0.2 | 0.2 | 0.2 | 0.2 |
| Li | 1 | 16 | 17 | 16 | Zn | 1 | <1 | <1 | <1 |
| Mo | 0.5 | 0.55 | 0.53 | 0.55 | Zr | 5 | <5 | <5 | <5 |
| Ni | 1 | 11 | 9 | 9 |
DL = Detection limit, ppm
Mulvoj dolomitic marbles have a simple mineralogy and are made from dolomite (Dol) as the main mineral phase. Dolomite shows equigranular and mosaic texture (Fig. 4A), consistent with crystallization at temperature of ~500°C (e.g. Covey-Crump, Rutter 1989) during metamorphism. It is characterized by its cleavage and twinning planes under the microscope (Fig. 4A). Other mineral phases in the rocks are amphibole (Amph), rare quartz (Qtz), opaque minerals (Ore) and titanite (Ttn). Amphibole is the second abundant mineral in the studied marble samples. It ranges from 1mm to 5mm in size (Fig. 4B) and is colourless in the plain polarized light, indicating high Mg and low Fe content. This classifies amphibole in the Mulvoj marble as tremolite (Tr). Quartz appears as minor phase in the samples (Fig. 4A). Titanite (sphene) appears as wedge-shaped tiny crystals (~0.2mm across) to relatively large (1.5 mm across) grains in the rocks (Fig. 4C). It formed in textural equilibrium with other minerals in the rocks, since is well-crystalized and is in mutual contact with other minerals. Opaque minerals are more likely Fe-Ti oxides. Talc (Tlc) appears along with rare amount of calcite (Fig. 4D). Microscopic studies on eight samples of the Mulvoj area (from different outcrops and layers in one outcrop) shows that dolomite appears along with quartz in some samples with occasional occurrence of tremolite. Talc is accompanied by either calcite or calcite and quartz in the studied rocks. No equilibrium assemblages containing both calcite and dolomite was observed in the studied samples. Based on these observations, the main mineral assemblages in eight studies samples of the dolomitic marble can be classified as follows.
Figure 4.
Microscopic photos from different rock types in the Mulvoj area. (A) Dolomite with minor quartz. (B) dolomite, opaque minerals and idioblastic amphibole (tremolite) in a metamorphic dolostone. (C) Relatively large titanite in textural equilibrium with dolomite. (D) Talc sample with minor calcite.
One sample of talc from the study area was analysed by XRD method to find out the mineral composition. XRD graph with mineral phase peaks is shown in Figure 5A. As shown in the graph, talc is the main mineral in the studied sample.
Figure 5.
(A) XRD pattern for the studied talc sample. The vertical axis shows the intensity of peaks (proportional to the amount of the mineral phase in the studied sample) and the horizontal axis shows the 2Θ angle of the X-ray diffraction, which is indicative of the mineral phase. The sample is composed almost entirely from talc. (b) Classification of Mulvoj talc based on chlorite and carbonate content. Some talc occurrences are shown for comparison (Afghanistan from Tahir et al. 2018; Trimouns, France from Boutin et al. 2016; Montana, USA from Anderson et al., 1990; Austria, from Prochaska, 1989, Finland from GTK.
Chemical composition of five samples from Mulvoj talc along with one dolomitic marble are provided in Tables 1 and 2. As it can be seen in Table 1, the SiO2 content for the talc samples ranges from 59.73 to 61.07 wt%, while the SiO2 content for the marble sample is 3.21 wt% (Table 1). The MgO contents for talc samples range from 31.98 to 33.21 wt% and Fe2O3 content (as whole Fe in the samples) has very low amounts of 0.59 to 0.75 wt%. The contents of CaO, TiO2, Al2O3, Fe2O3, MnO, Na2O, K2O and P2O5 are low and range from <0.5 to 1.35 wt%. The CaO and MgO contents for the marble sample is 29.07 and 20.76 wt% respectively, characterizing it as dolomitic rock. Whole-rock trace element concentrations are given in Table 2. Most of the trace elements have similar concentration for all three analysed samples. The Ce and La contents in all samples are equal to 1 ppm, while the Y, Be, Cr, Zn and Zr contents are below the detection limit of the method used.
XRD analyses indicate almost pure talc for the samples from the Mulvoj mine (Fig. 5A). Commonly chlorite can be found along with talc in dolomite-hosted talc deposits (e.g., Moine et al. 1989; Schärer et al. 1999) and can form intergrowth with talc (Veblen 1983). No chlorite was found under the microscope or in the XRD studies in the Mulvoj talc samples. The lack of chlorite in our samples can be attributed to the low Al2O3 (1.28 wt%) content in the protolith marble. Figure 5B illustrates the mineralogical composition of the studied samples in the ternary diagram of talc-chlorite and carbonate/other minerals. Some talc deposits of the world are shown for comparison. Mulvoj talc is almost pure with low carbonate content and is similar to talc deposits from Afghanistan in this regard (Tahir et al. 2018). Hydration of ultramafic rocks produces talc along with serpentine (usually antigorite, Bucher, Grapes 2011). No serpentine was found in the studied samples, testifying for non-ultramafic protolith. Moreover, the absence of serpentine group minerals indicates that talc in the Mulvoj mine crystalized directly from reaction between dolomite and quartz in the presence of water-rich fluid (Tosca et al. 2011; Bjerga et al. 2015).
Geochemically, the composition of talc is diagnostic for its protolith type (e.g., ultramafic and Mg-carbonate; Prochaska 1989). Talc formed from minerals in ultramafic rocks hydration is enriched in Ni, Fe and Cr, compared to Mg-carbonate hosted talc (Prochaska 1989). Large ionic lithophile elements such as K and Li, indicative of crustal components, are usually higher in content in talc formed from Mg-carbonates (Yalçin, Bozkaya 2006). Mulvoj talc samples are rich in MgO and poor in Al2O3 and Fe2O3 (t) (Fig. 6). Major oxides such as TiO2, Fe2O3(t) and MnO and minor elements such as Cr are strikingly low in the studied samples (Table 1), compared to talc formed from serpentine minerals hydration. Na2O (0.22 to 0.30 wt%) and K2O (0.29-0.32) contents are instead relatively high, while Na2O and K2O contents of talc from hydration of peridotites are very low, mainly below the detection limits (Moine et al. 1989) of the conventional analytical methods used. Such geochemical features are in accordance with a sedimentary origin and show that the Mulvoj talc did not originate from peridotites. Mineral assemblage of talc-bearing rocks confirms this. Figure 7A shows concentration of some elements in the studied samples normalized to chondrite (normalization values are from McDonough, Sun 1995). Mulvoj samples show enrichment in Ti, K, Li and Pb, compatible with crustal material source. In particular Ti enrichment is materialized by the relative abundance of titanite in the studied rocks. One peridotite talc rock from Sivas, Turkey and one serpentinite-hosted talc sample from the Gilów deposit, WS Poland are shown for comparison. As visible in Fig. 7a, the concentration of studied elements in the Mulvoj samples are distinctly different from samples of Turkey and Poland. Figure 7B illustrates upper continental crust composition (Taylor, McLennan 1985) normalized diagram for the studied samples and samples from Turkey and Poland. Li, Mo, Ni, Pb and Ti show concentration in the samples similar to their concentration in the upper continental crust (close to 1), while La, Ce, Sr and Sc show relative depletion. Carbonate minerals can incorporate considerable amounts of Sr and incompatible elements (Andersson et al. 2014; Littlewood et al. 2017). Sr usually concentrates in carbonate or similar to La and Ce, can replace Ca in other Ca-bearing minerals (e.g., Vodyanitskii 2012). Low Sr, Ce and La contents can be attributed to the low concentration of Ca-bearing minerals in the analysed talc samples and/or the lack of intergrowth between Ca-bearing minerals (such as Ca-amphibole) and talc (e.g. Müller et al. 2003). Talc samples from Turkey and Poland, formed from peridotites, show different trends in Fig. 7b.
Figure 6.
Al2O3-Fe2O3(T)-MgO diagram classifies the Mulvoj talc as Mg-rich talc with very small Fe and Al content.
Figure 7.
(A) Selected elements concentration in the Mulvoj talc samples normalized to the chondrite values (McDonough, Sun 1995). (B) Elements concentration patterns normalized to the upper continental crust values (Taylor, McLennan 1985). Peridotite-related samples from Turkey (Yalçin, Bozkaya 2006) and Poland (Gil et al. 2022) are shown for comparison.
Pb, Cd and As are among the hazardous elements for human health as they pose several health problems. Recommended heavy elements content in talc, including Pb, in pharmacopeia is <10 ppm (TEP, 2005; TUSP, 2009). The Pb content is 4 ppm for all studied samples, the Cd content varies from 0.34 to 0.35 ppm and the As content is 1.4 to 1.5 ppm in Mulvoj talc samples. U and Th are radioactive elements and pose treat to human health. Uranium anomaly is reported around the Shinbo talc mine in Korea (Chung et al. 1998). Talc contamination with high-U bearing minerals can affect the suitability of talc in pharmaceutical and cosmetic applications. U and Th contents in the studied samples are below the detection limit of the used ICP-MS method. These chemical features make Mulvoj talc suitable in terms of possible application in medical and cosmetic materials production.
Sedimentary carbonate rocks are predominantly composed of dolomite, calcite and quartz. This mineral assemblage is characteristic of simple CaO-MgO-SiO2-CO2 metamorphic system. Hydrous minerals in the metamorphosed carbonate rocks are commonly talc and tremolite. The H2O necessary for the formation of these phases can be provided either from the pore fluids in the rocks or by infiltration into the rocks from the external sources (Ague 2003; Moazzen et al., 2009; Bucher, Grapes 2011). Pressure and temperature estimation of peak metamorphism in these rocks usually is problematic due to their simple mineral assemblages, the limited number of minerals in thermodynamic equilibrium (caused by the high degree of freedom in the metamorphic system), as well as the presence of a binary fluid (H2O-CO2). Hence, most conventional mineral geothermobarometric methods applicable for meta-basic and meta-pelitic rocks cannot be applied to the meta-carbonates (e.g. López Sánchez-Vizcaino et al. 1997). The only minor phase potentially suitable for temperature estimation is titanite as Zr content in titanite is used as a thermometer (Hayden et al. 2007). This thermometer is only applicable to system saturated in Zr (zircon) in the rocks. However, no zircon was found in the studied samples, hindering using this thermometer. Equilibrium reactions among mineral phases distinguished in the studied samples are therefore used to estimate the temperature and XCO2 during formation of talc, tremolite and calcite in the studied rocks.
According to the petrography and XRD studies, the main minerals in the Mulvoj samples are calcite (Cal), dolomite (Dol), quartz (Qtz), and tremolite (Tr). The phase relations among these minerals can be studied in the CMSH-CO2 system, where C is CaO, M is MgO, S is SiO2, and H is H2O. Considering H2O and CO2 as excess phases in the system, CMS components can be shown by a triangular phase diagram (Fig. 8). The arrangement of different tie lines (e.g. Dol-Qtz, Dol-Tlc, Tr-Cal...) defines the sequence of the following mineral reaction:
We propose that these reactions were responsible for formation of talc and tremolite in the Mulvoj rocks. All reactions are binary-fluid reactions (H2O-CO2) with CO2 release (e.g., Moazzen et al. 2009). More likely, H2O is provided from the adjacent metamorphic pelitic rocks (mica schists). Reaction curves in the T-XCO2 diagram (Fig. 9), constructed using thermodynamic data set of Berman (1988) and considering non-ideal mixing of H2O-CO2, used to estimate the T-XCO2 relations for the studied rocks (Tahir et al. 2018). The stability field of talc in terms of temperature and XCO2 (mole fraction of CO2 in the fluid) is studied by Gordon and Greenwood (1970), Skippen (1971, 1974) and Slaughter et al. (1975). Talc is stable at pressure greater than 2 kbar and temperature up to ~460°C and XCO2 up to 0.6 (Fig. 9). The diagram shows that there is a direct relation between the talc crystallization temperature and XCO2 value. Talc can form at much lower temperatures (350°C) when XCO2 values are low enough. Other factors including pressure and talc composition (especially Mg/Fe ratio in talc) will control its crystallization temperature.
Figure 8.
Phase relations for the studied rock samples in the CMSH-CO2 compatibility diagrams with H2O and CO2 as the excess phases. Tie lines define the mineral reactions. (A): Dol+Qtz assemblage, (B): Tlc+Cal±Qtz assemblage (C): Tr-forming reaction.
Calcite reacting with talc forms tremolite as a univariant reaction. It is possible to have talc and tremolite in samples as a univariant mineral assemblages, but they do not appear in the studied samples. This is more likely due to very limited exposure of the assemblage in the study area.
Talc formation by precipitation from hydrothermal fluids (Boutin et al. 2016) can be postulated for the Mulvaj area. Aqueous SiO2 and Mg2+ in fluid react with carbonate to form talc. This reaction releases H+ (Boutin et al. 2016). There is no evidence for Mg2+ metasomatism or presence of aqueous SiO2 in the Mulvoj area (e.g., silica veins accompanying talc mineralization). The released H+ can cause acidic alteration, which is not the case in the Mulvoj area.
Talc forms during metasomatism by fluid infiltration from the crystallizing pluton into the siliceous dolomitic rocks within the contact aureole (e.g., Chatir et al. 2022). No evidence of contact metamorphism observed in the studied rocks under the microscope. Furthermore, the Cretaceous intrusive rocks are at considerable distance from the Mulvoj area (Fig. 1), which rules out a contact metamorphic origin for the studied talc deposit.
Previous studies proposed a peak metamorphic event at 650°C to 750°C at ca. 7 to 9.7 kbar in the area (Kiselyov, Budanov 1986; Grew et al. 1994). No high-grade relict minerals (e.g. clinopyroxene, olivine, wollastonite) were observed in the studied metamorphic dolomitic marbles. This implies that the talc-bearing samples are not result of a retrograde metamorphism of high grade rocks. Talc and tremolite formation can be explained by a relatively low temperature trajectory of a prograde regional metamorphism at temperature <460°C, in which H2O was provided from dehydration metamorphic reactions of the interlayered pelitic (schist) rocks.
Figure 9.
Temperature-XCO2 diagram for talc formation in the siliceous system with binary H2O-CO2 fluid (Tahir et al. 2018). Stability field for talc + calcite and talc + calcite + tremolite are indicated.
Talc in the Mulvoj area of the Ishkashim district occurs in high-grade marble as interlayers in the South-western Pamir gneiss and schist. According to the petrography observations and XRD analyses results, talc is the main mineral in the deposit with subordinate amounts of quartz, calcite and amphibole. Geochemical studies indicate almost equal contents of CaO and MgO for marble, classifying it as dolomitic marble. Considering the minerals in dolomitic marble and talc samples, three main mineralogical reactions were responsible for talc and tremolite crystallization. The reaction at relatively lower temperature consumes dolomite, quartz and H2O to produce talc, calcite and carbon dioxide, the reaction at relatively higher temperature and almost similar XCO2 consumes talc, quartz and calcite to produce tremolite, H2O and carbon dioxide. The reaction at higher temperature produces tremolite and dolomite by talc and calcite breakdown. Co-existence of talc and calcite and also talc and tremolite in the studied samples put constraints on temperature-CO2 mole fraction (T-XCO2) of formation of the mineral assemblages of ~340°C up to ~460°C and XCO2 from zero (pure water) up to 0.6. This study shows that Mulvoj talc is a marble-hosted talc occurrence and is not related to ultramafic rocks, formed at the low-T trajectory of a prograde metamorphism. Most of the deposit is mined during the Soviet time and the remaining of the talc deposit is not considerable. Therefore, the mine cannot be considered economically valuable, however there is possibility for further exploration and extraction if the demand for talc increases. This will need considerable investment to do more prospecting, re-building the road and re-constructing the metal zip-line.