Metalliferous jurassic rocks of the Fan-Y agnob syneclise (Central Tajikistan)

Background. The Fan-Yagnob syneclise is an imposed structure within the Hercynian Zeravshan-Gissar zone. It formed on a Paleozoic folded basement as series of sedimentary rocks from T3 to N1, including Jurassic, Cretaceous, and Paleogene terrigenous complexes with a total thickness of ~ 2.7 km. The syneclise hosts the Fan-Yagnob coal-mining field. In the vicinity, there are two deposits within the Paleozoic frame: the Takfon tungsten deposit with high concentrations of Sn, As, Bi, Cu, Ag, Au in the north and the Dzhizhikrut antimony deposit with Au, Ag, Hg, Tl, Se, Pb, Bi, Sn in the south. Previous geological prospecting within the Zeravshan-G issar zone of Central Tajikistan indicated that the Fan-Y agnob syneclise had limited potential for ore mineralization. This conclusion was based on the absence of magmatism and hydrothermal processes in its contours. In 2024, we carried out a study of the Jurassic terrigenous stratum, including its mineralogy and geochemistry, which calls into question the presumed absence of ore mineralization within the syneclise. Exploration works were conducted along the watershed between the Yagnob and Fandarya rivers, at Mount Kuhi-Malik, above zones of gas emissions associated with known coal fires. At this site, the Jurassic sandstone sequence, with a thickness of approximately 800 m, was investigated. Samples were collected from all lithological varieties of the terrigenous rocks.

Aim. To assess the metalliferous prospects of the Jurassic terrigenous rocks of the Fan-Yagnob syneclise.

Research object. The Jurassic sedimentary clastic rocks of the Fan-Yagnob syneclise.

Materials and methods. From the most complete section of the Jurassic system located on the watershed of the Yagnob and Fandarya rivers, 26 specimens, each weighing 1.5 kg, were collected from all rock varieties. These specimens were used to prepare polished thin sections, polished grain mounts, and geochemical samples. Analysis was carried out using a portable X-ray fluorescence (XRF) spectrometer Vanta-M  (Olympus, USA); X-ray tube anode: Rh, tube power 4 W, voltage 50 kV, SSD detector. The composition of heavy mineral grains and their semi-quantitative analysis were conducted using a JSM-840 scanning electron microscope (JEOL) equipped with a Si(Li) semiconductor detector and an INCA-350 analytical system (Oxford Instruments) at an accelerating voltage of 20 kV and a beam current of 2 nA. The elemental composition of the rocks was determined by emission spectroscopy and mass spectrometry using inductively coupled plasma (ICP) spectrometers. For analysis, the samples were divided into two types: easily and hardly decomposable. The former were dissolved using a Hot Block system, while the latter were brought into solution using the same acid mixture in microwave autoclaves (MARS-5). Element concentrations were measured using an ICP-OES VARIAN-720 spectrometer and an HP 4500 series mass spectrometer. Multi-element standards from MERCK and SPEC were used as calibration standards. All analytical work was carried out at the A.E. Fersman Mineralogical Museum of the Russian Academy of Sciences (analysts L.A. Pautov and M. Saymudasiri) and at a laboratory of the Russian State Geological Prospecting University (MGRI-RSGPU) named after Sergo Ordzhonikidze (analyst M. Saymudasiri).

Results. In the terrigenous rocks of the Jurassic system, along an 800 m profile, anomalously high concentrations of V, Zn, Ce, As, Co, Eu, Se, Sb, Sm, Cd, and Ag were identified, exceeding Clarke values by three or more times. Along the geochemical profile, ore minerals containing the above- mentioned elements were identified, including stibnite, galena, argentite, native gold, and silver, as well as monazite-(Ce), xenotime-(Y), and florencite-(Ce). In addition, unidentified phases enriched in Se, Ta, Ni, Mo, Bi, Tl, Pb, Ge, F, Sn, and W were found.

Conclusion. The identification of Ag-, Pb-, Sb-, Sn-, W-, and Au-bearing ore minerals within Jurassic terrigenous rocks along an 800 m profile, occurring over extended intervals (tens of meters) within the geochemical anomaly characterized by V, Co, Zn, Ge, As, Sc, Ag, Cd, Sb, Ce, Sm, and Eu, suggests a possible manifestation of ore mineralization. This finding highlights the need for further investigation of the entire Mesozoic sedimentary succession of the syneclise, where ancient placer-type accumulations of certain metals may be present. Most of the detected ore minerals are extremely fine-grained, typically less than 40 microns, which likely explains why they were not identified in previous explorations. These results raise the question of the metallogenic potential of the Fan-Yagnob syneclise and possibly other Mesozoic syneclises in Central Tajikistan, warranting more detailed studies across the Jurassic formations.

1. Vadilo P.S. An underground fire in Central Tajikistan. Nature. 1958. No. 8. C. 88–91 (In Russ.).

2. Geology of the USSR. Tajik SSR. Moscow: Publishing House of Scientific and Technical Sciences. Publishing house of literature on geology and protection of mineral resources. 1959. Vol. XXIV. Part II. 732 p. (In Russ.).

3. Litvinenko A.K., Saymudasiri M. Prospects of ore bearing in the Fan-Yagnob syneclise of Zeravshan-Gissar, Central Tajikistan. Proceedings of higher educational establishments. Geology and exploration. 2025. No. 4. P. 134–142 (In Russ.).

4. Mirakov M.A., Fayziev A.R., Pautov L.A. Native selenium in the products of the underground fire of the FanYagnob coal deposit (Central Tajikistan). Reports of the Academy of Sciences of the Republic of Tajikistan. 2017. Vol. 60. No. 9. P. 456–460 (In Russ.).

5. Mirakov M.A., Pautov L.A., Makhmadsharif S., Karpenko V. Yu., Shodibekov M.A. The first find of mercury minerals — tiemannite and cinnabar in the sublimates of a natural underground fire in the Kuhi-Malik stow at the Fan-Yagnob coal deposit (Tajikistan). New data on minerals. 2020. Vol. 54. Issue 4. P. 96–106 (In Russ.).

6. Okhunov R.V., Yerov Z.E., Negmatov I.I. Atlas handbook of the Von-Yagnob coal deposit. Dushanbe. 2017. 170 p. (In Russ.).

7. Pautov L.A., Mirakov M.A., Karpenko V. Yu., Makhmadsharif S. Ge-containing cassiterite from the ignitions of a natural underground fire in the Kuhi-Malik tract at the Ravat site of the Fan-Yagnob coal deposit (Tajikistan). New data on minerals. 2022. Vol. 56(1). P. 12–23 (In Russ.).

8. Pautov L.A., Mirakov M.A., Makhmadsharif S., Karpenko V. Yu., Fayziev A.R. The discovery of native tellurium in the ignition of a natural underground fire in the Kuhi-Malik tract at the Fan-Yagnob coal deposit (Tajikistan). New data on minerals. 2019. Vol. 53. P. 95–99 (In Russ.).

9. Dissection of stratified and intrusive formations of Tajikistan. Dushanbe: Donish, 1976. 207 p. (In Russ.).

10. Saymudasiri M., Litvinenko A.K. Modern mineral formation at the Fan-Yagnob coal deposit of the Southern Tien Shan (Tajikistan). Young — Geosciences. 2024. Vol. 3. P. 268–271 (In Russ.).

11. Saymudasiri M., Litvinenko A.K. Taliy (Tl) and Pb-Bi containing minerals from the ignitions of a natural underground fire at the Fan-Yagnob coal deposit (Tajikistan). Proceedings of the XVII International Scientific and Practical Conference «New Ideas in Geosciences”. Moscow: Publishing House of the Sergo Ordzhonikidze Russian State University for Geological Prospecting, 2025. P. 168–171 (In Russ.).

12. Tadjibekov M., Radjabov I. Morphological types and the most important patterns of the Neogene intragastric depressions of the Gissar-Alai. Problems of Geology of the Republic of Tajikistan. Dushanbe. 1999. Issue 1–2. P. 73–85 (In Russ.).

13. Tadjibekov M. The Gissaro-Alai intracranial depressions in the latest stage of geological development. Dushanbe: Donish. 2005. 256 p. (In Russ.).