METASOMATISM IN THE MURUNTAU ORE FIELD (WESTERN UZBEKISTAN)

Abstract

The formation of the giant Muruntau gold-quartz deposit and ore field (western Uzbekistan) is defined as a prolonged (70 Ma) multiphase geological process related to shear tension and ductile deformations of rocks. The history of the ore field development is very complicated, and there is still much to be learned about it. We studied the conditions, zonation, and deformations of the metasomatism-related ore formation, as well as the behavior of carbonaceous matter (CM) in this process. Lower Paleozoic sheared carbonaceous host rocks are regionally metamorphosed in the muscovite-chlorite subfacies. This process, which evolved over a long time (C3-P1), started with the increase in temperature (the progressive megastage) and continued during its decrease when the bulk of gold was deposited. The beginning of the process was associated with ductile deformations that showed up in folding and shearing of rocks. At that time, pre-ore magnesium-potassium metasomatism took place in the temperature interval of 400-600°C. During the regressive megastage, calcium metasomatism occurred and skarn assemblages (actinolite, clinozoisite, diopside, and garnet) were deposited at T = 400-520°C. Later, ductile deformations were replaced by brittle deformations, and quartz-ore veinlets and lenses formed together with locally developed vein-related orthoclase-chlorite, albite-carbonate, sericite-tourmaline, and quartz-adular metasomatites. The metasomatic zonation showed up at early stages and was oriented along the direction of fluid filtration. The outer (front) zone is represented by a dome of biotitized rocks. Banded biotite-feldspar-quartz metasomatites occur within them. Their back (inner) zones are composed of the biotite-orthoclase-quartz assemblage. Younger skarn assemblages make up series of thin bands within biotite-feldspar-quartz metasomatites. Ore veinlets are enclosed within this metasomatic halo. CM took an active part in the hydrothermal process: it was output from metasomatic zones and redeposited along their outer boundary. CM interacted with the hydrothermal solution and maintained the reducing medium at the level of a QFM buffer. Temperatures established by the carbon isotope thermometer (CM + carbonate) are similar to values of mineral geothermometers (250-350°C during ore deposition). The distinctive features of metasomatism in the Muruntau ore field, hosted by carbonaceous sequences and shear zones, are the absence of vein-related zonation and a poor vertical zonation (a low T gradient), suppressed acid metasomatism, and the lack of an obvious relationship between ore mineralization and a particular magmatism.