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. Laserablation 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 Zn 2+ 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 fO 2 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. the present paper is to document trace element variations in the sulfide phases (pyrite, chalcopyrite, and sphalerite) from the three sulfide chimney types found in the deposit and, in particular, to focus on the possible causes of trace element zonation from rim to core in the chimneys. Regional Geologic SettingSeveral reviews have described the geologic setting and composition of the VHMS deposits in the Urals (Filatov and Shiray, 1988;Koroteev et al.
The basalt-hosted Semenov-2 hydrothermal field on the Mid-Atlantic Ridge is host to a rather unique Cu-Zn–\ud rich massive sulfide deposit, which is characterized by high Au (up to 188 ppm, average 61 ppm, median\ud 45 ppm) and Ag (up to 1,878 ppm, average 490 ppm, median 250 ppm) contents. The largest proportion of\ud visible gold is associated with abundant opal-A, which precipitated after a first generation of Cu, Fe, and Zn\ud sulfides and before a second generation of Fe and Cu sulfides. Only rare native gold grains were found in earlier\ud sulfides. Fluid inclusions in opal-A associated with native gold indicate precipitation at 300° ± 40°C from\ud fluids of salinity higher than that of seawater (3.5–6.8 wt % NaCl equiv). According to laser ablation-inductively\ud coupled plasma-mass spectrometry analyses, invisible gold is concentrated in secondary covellite (23–227 ppm)\ud rather than in the primary sulfides (<1 ppm). Silver minerals (native silver, stutzite, and naumannite) rarely\ud occur in the sulfides and in aragonite associated with opal-A; invisible silver was detected in all sulfides, but,\ud again, covellite contains more Ag (>1,000 ppm) than all other sulfides (<250 ppm). Covellite replacing Zn\ud sulfides (covellite-A) is enriched in all analyzed trace elements relative to covellite replacing Cu-Fe sulfides\ud (covellite-B). The enrichment of covellite-A in trace elements may be related to the dissolution of inclusions\ud of various minerals hosted in former sphalerite, which were the source for Au and Ag (native gold), Pb and Tl\ud (galena), Se (chalcopyrite, Se-bearing galena, naumannite), Te and Bi (Bi tellurides), As (tennantite, chalcopyrite),\ud and Sb (tennantite). The formation of covellite-A was favored by hydrothermal fluid/seawater mixing or\ud direct oxidation of sulfides by seawater, as suggested by the relatively high contents of typical “seawater” elements\ud (U and V). The degree of seawater involvement was apparently lower for covellite-B.\ud Although the Semenov-2 field is basalt hosted, several geochemical features of the massive sulfides studied\ud are similar to those of the Mid-Atlantic Ridge ultramafic-hosted Cu-Zn–rich massive sulfides, such as Fe:Cu:Zn\ud ratios close to 1:1:1, high Sn, Se, Au ,and Ag contents, and high Au/Ag ratios. However, the strong enrichment\ud in SiO2, the moderate Mn and Co contents, very low Ni contents, and the Co/Ni ratio >1 are more consistent\ud with a mafic signature. Thermodynamic modeling of hydrothermal fluids produced by reactions between various\ud proportions of seawater and basalt or peridotite at 350°C shows that mineral assemblages broadly similar to\ud those of the Semenov-2 deposit can precipitate from fluids produced in a mafic environment, but that Au and\ud Ag minerals are not predicted to precipitate from such fluids over a wide temperature range. These results suggest\ud that an additional contribution to the hydrothermal system is required in order to achieve saturation in precious\ud metals. A magmatic input is suggested ...
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