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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 The geochemical variability of massive sulfides from mid-ocean ridges is explored with multivariate statistical analysis. The observed variability is mostly related to the temperature of deposition, the ridge spreading rate and zone refining. 2 The nature of the substrate (mafic vs. ultramafic rocks) seems to play a secondary role in controlling metal associations.

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Subject. The article presents the results of study of Mechnikovskoe gold deposit associated with listvenites and beresites of the Miass region of the Southern Urals.Materials and methods. Materials were sampled during the field work of 2010– 2012. The chemical composition of rocks is analyzed by methods of classical chemistry (rock-forming oxides) and ICP MS (trace elements). The mineral composition is determined on an electron microscope with EDS.Results. The deposit is composed of tectonic sheets of serpentinites, carbonatized serpentinites and listvenites (sheet I), metadiabases and plagioclase metabasalts of the Irendyk Formation and beresites and volcanosedimentary rocks and metabasalts of the Karamalytash Formation (sheet II). In the central part of the deposit, the volcanic rocks are intruded by a dike of finegrained island-arc granites. Chromites of serpentinites are characterized (on average) by high Cr# (89) and low Mg# (29) values and low contents of Al2O3 (6.94 wt %) and MgO (5.5 wt %). Gold-bearing rocks include listvenites, beresites and carbonaceous shales. The major ore mineral is pyrite; accessory minerals are Au and Ag minerals, chalcopyrite, fahlores, galena, sphalerite, pyrrhotite, cubanite, vaesite, melonite, secondary copper sulfides, barite, rutile, monazite and xenotime. Gold of the deposit contains low Ag contents (3.52 wt %) and minor amount of Cu and Hg (<1 wt % in most analyses).Conclusions. The listvenites and beresites of the deposit were formed after ultramafic and mafic rocks, respectively. The discovery of gold in various rocks indicates that gold mineralization was deposited after the formation of the geological structure of the deposit. The source of gold was most likely related to a magmatic fluid.