Abstract

Very well preserved fossil vent chimneys from the Silurian Yaman-Kasy volcanogenic-hosted massive sulfide deposit in the Southern Urals range in decreasing temperatures from chalcopyrite-pyrite black smoker to sphalerite-chalcopyrite-marcasite-pyrite gray smoker to sphalerite-quartz-barite white smoker assemblages. Laser-ablation ICPMS analyses show systematic trace element distribution patterns across chimneys. Coarse-grained layers of chalcopyrite in the central conduits are relatively high in Se and Sn but are low in other elements. Chalcopyrite at the margins of such layers is enriched in Bi, Co, Au, Ag, Pb, Mo, Te, and As, which reside in microinclusions of tellurides and/or sulfoarsenides. Sphalerite in the conduits and the outer chimney wall contains elevated Sb, As, Pb, Co, Mn, U, and V. Antimony, As, and Pb reside in microinclusions of a galena-fahlore assemblage, whereas the Co and Mn likely substitute for Zn2+ in the sphalerite structure. The highest concentrations of most trace elements are found in colloform pyrite within the outer wall of the chimneys and likely result from rapid precipitation under high- temperature-gradient conditions. The trace element concentrations in the outer-wall colloform pyrite decrease in the following order, from the outer wall inward: Tl > Ag > Ni > Mn > Co > As > Mo > Pb > Ba > V > Te > Sb > U > Au > Se > Sn > Bi, governed by the strong temperature gradient. In contrast, pyrite in the high- to mid-temperature central conduits exhibit concentration of Se, Sn, Bi, Te, and Au. The zone between the inner conduit and outer wall is characterized by recrystallization of colloform pyrite to euhedral pyrite, which becomes depleted in all trace elements except Co, As and Se.

The mineralogical and trace element variations between chimneys are likely due to increasing fO2 and decreasing temperature caused by mixing of hydrothermal fluids with cold oxygenated seawater. Average values of Se (a high-temperature element) decrease in the order from black to gray to white smoker chimneys. The medium-temperature association (Te, Bi, Co, Mo, and Au) is typically present in the gray smoker chimneys. The white smoker chimneys are depleted in most elements except for Ag, Tl, Te, Sb, and As, probably due to the dilution of the vent fluid by seawater which penetrates deeper parts of the hydrothermal system. U and V are concentrated in the outer wall of most chimneys due to their extraction from seawater associated with the more reduced fluids of black and gray smokers.

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