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 ...
