Gold- and Silver-Rich Massive Sulfides from the Semenov-2 Hydrothermal Field, 13°31.13′N, Mid-Atlantic Ridge: A Case of Magmatic Contribution?

Abstract

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 ...

Trace Metal Distribution in Sulfide Minerals from Ultramafic-Hosted Hydrothermal Systems: Examples from the Kairei Vent Field, Central Indian Ridge

“…Both elements are likely incorporated into the chalcopyrite lattice at high temperatures, with high solubilities of Se and Te in reduced hydrothermal fluids of~350 • C [10,[49][50][51]. Similar enrichments of Te in chalcopyrite were described by Logatchev and Semyenov-2, with Te concentrations reaching 39.6 and 42.0 ppm, respectively (Table 3; [13,14]). However, the Te content of chalcopyrite in most other SMS deposits is typically low.…”

“…Chalcopyrite is commonly enriched in Co, Se, and Bi [7] and is the dominant sulfide phase in ultramafic-hosted SMS deposits [3]. Recently, several papers reported LA-ICP-MS-based trace element data of chalcopyrite from basaltic-, ultramafic-, and felsic-hosted SMS deposits worldwide (e.g., [13][14][15]). Wohlgemuth-Ueberwasser et al (2015) [13] documented that Au, Sb, As, Se, and Te enrichments in chalcopyrite are mainly controlled by submicroscopic inclusions (e.g., As-Au-Sb-Pb sulfosalts and/or tetrahedrite-tennantite), which are often elevated as a function of fluid temperature.…”

“…Bornite, digenite, and covellite are common Cu-(Fe)-sulfide minerals in SMS deposits which often precipitate under more oxidizing conditions caused by the ingress of seawater during the waning stages of hydrothermal activity (e.g., [3,4,51,55]). Due to the interaction of cooling hydrothermal fluids with cold, oxygenated seawater, U, V, and sometimes Ag may reach high concentrations in low temperature bornite and digenite formed during the latest hydrothermal phase or during secondary oxidation of primary sulfides (e.g., [14,15,47]). In contrast, bornite precipitated at higher temperatures has relatively high concentrations of Sn and In [15].…”

“…Previous studies suggest that the recognition of distinct trace element signatures of individual sulfide grains from seafloor hydrothermal systems can assist with tracking sequences of crystallization and stages of mineralization, sources of metals, and evolution of ore-forming fluids over time in SMS deposits (e.g., [10][11][12]). Recent work by Wohlgemuth-Ueberwasser et al (2015) [13], Keith et al (2016) [8], [14], and Wang et al (2017) [15] investigated trace element distributions using laser ablation ICP-MS analysis in sulfide minerals from SMS deposits in different geodynamic settings including ultramafic-hosted and basalt-hosted fields as well as a number of fields that are associated with submarine volcanic arcs and back-arc basins. Those studies demonstrated that all of the parameters mentioned above influence geochemistry and mineralogy and may account for the variable metal enrichment of SMS.…”

Silver-Rich Chalcopyrite from the Active Cerro Pabellón Geothermal System, Northern Chile

“…In modern hydrothermal settings, studies presenting trace element data of chalcopyrite are predominantly restricted to submarine hydrothermal environments, i.e., seafloor massive sulfides and active hydrothermal chimneys. For example, previous studies have reported LA-ICP-MS data showing that chalcopyrite from black smoker vents can host variable amounts of Zn, As, Sb, Ag, Pb, Bi, Se, Te, Co, and Ni, among other trace elements [12,[20][21][22][23][24][25][26]. Furthermore, [27] presented micro-analytical data of chalcopyrite retrieved from drill cuttings at the seawater-dominated Reykjanes geothermal system in Iceland.…”