Metasomatic halo of the saf'yanovskoe Zn-Cu massive sulfide deposit, central urals

By the example of the low-grade metamorphic (no higher than the zeolite fac ies) Early Devonian Saf'yanovskoe deposit in the eastern slope of the Centr al Urals, the complex and prolonged history of metasomatic halo formation i n an ancient massive sulfide system is demonstrated. The primary metasomati c zoning of the halo under massive sulfide ores is represented by a large b ody of chlorite-quartz metasomatic rocks (with high-grade chalcopyrite stri nger and dissemination mineralization) in the central part of the deposit, which change along dip and strike into chlorite-hydrosericite-quartz rocks. The metasomatic alteration ia medium temperature (up to 350 degreesC, illi te 2M(1)) in the central zone (feed channel) and changes gradually to low-t emperature away from it (250 degreesC and lower): the metasomatic rocks con tain hydrosericite t-1M and then I/S-hydrosericite c-1M (electron diffracti on data) in association with kaolinite. The fluid was characterized by an i ntermediate composition between those of magma-derived fluid and seawater ( delta O-18(H2O) = +3.2 parts per thousand and deltaD = -47 parts per thousa nd at T = 250 degreesC) and low carbon dioxide activity. Low-temperature hy drothermal activity continued after the burial of the massive sulfide body, which is recorded in the extensive development of sulfide-free kaolinite ( dickite)-ankerite-I/S-quartz association above the massive sulfide zone. Mi ca hydration and carbonatization of metasomatic rocks below the massive sul fide zone were related to this stage of mineral formation. The temperature of this process was 132-197 degreesC at a pressure of 0.30-0.37 kbar. The f luid was strongly enriched in carbon dioxide, and the isotope signature of carbonate (delta C-13 from 1.32 to -2.84 parts per thousand and delta O-18 from +18.8 to +22.2 parts per thousand) suggests that the fluid actively ex tracted carbon dioxide from the marble framework of the deposit. During met asomatic alterations, Al and Ti behave as immobile components and the geome tric volume was constant or increased on the expense of stringer and segreg ation silicification and sulfidization. In all cases, considerable addition of Si, Mg, and Fe and leaching of Na and Ca took place. The extensive deve lopment of chlorite-quartz metasomatic rocks in the Saf'yanovskoe deposit i s not typical of the Uralian-type massive sulfide occurrences and is simila r to the deposits of the Noranda region in Canada.