Geodiversity of hydrothermal processes along the Mid-Atlantic Ridge and ultramafic-hosted mineralization: A new type of oceanic Cu-Zn-Co-Au volcanogenic massive sulfide deposit

Abstract

Several hydrothermal deposits associated with ultramafic rocks have recently been found along slow spreading ridges with a low magmatic budget. Three preferential settings are identified: (1) rift valley walls near the amagmatic ends of ridge segments; (2) nontransform offsets; and (3) ultramafic domes at inside corners of ridge transform-fault intersections. The exposed mantle at these sites is often interpreted to be a detachment fault. Hydrothermal cells in ultramafic rocks may be driven by regional heat flow, cooling gabbroic intrusions, and exothermic heat produced during serpentinization. Along the Mid-Atlantic Ridge (MAR), hydrothermal deposits in ultramafic rocks include the following: (1) sulfide mounds related to high-temperature low-pH fluids (Logatchev, Rainbow, and Ashadze); (2) carbonate chimneys related to low-temperature, high-pH fluids (Lost City); (3) low-temperature diffuse venting and high-methane discharge associated with silica, minor sulfides, manganese oxides, and pervasive alteration (Saldanha); and (4) stockwork quartz veins with sulfides at the base of detachment faults (15°05′N). These settings are closely linked to preferential circulation of fluid along permeable detachment faults. Compared to mineralization in basaltic environments, sulfide deposits associated with ultramafic rocks are enriched in Cu, Zn, Co, Au, and Ni. Gold has a bimodal distribution in low-temperature Zn-rich and in hightemperature Cu-rich mineral assemblages. The Cu-Zn-Co-Au deposits along the MAR seem to be more abundant than in ophiolites on land. This may be because ultramafic-hosted volcanogenic massive sulfide deposits on slow spreading ridges are usually not accreted to continental margins during obduction and may constitute a specific marine type of mineralization.

Antimony Fixation in Solid Phases at the Hydrothermal Field of Kolumbo Submarine Arc-Volcano (Santorini): Deposition Model and Environmental Implications

“…Fouquet et al, 2010). The toxicity, the solubility, and adsorption properties of Sb to natural minerals depend on the oxidation state (Sb 3+ and Sb 5+ ) (Filella et al, 2002a), however, key aspects of the environmental chemistry and the mechanisms of concentration of Sb in natural systems, and its global biogeochemical cycle, remain poorly understood.…”

Composition and Formation of Gabbro-Peridotite Hosted Seafloor Massive Sulfide Deposits from the Ashadze-1 Hydrothermal Field, Mid-Atlantic Ridge

“…Gold is the typomorphic element of sulfides related to ultramafic rocks [2]. Our observations indicate at early high-temperature stages of hydrothermal system evolution gold does not reach significant values.…”

“…The most prominent feature is the high Cu + Zn content (~30 wt %). Such enrichment cannot be explained by gabbro-peridotites as source rocks because the concentrations of these elements are higher in basalts [2]. However, serpentinites of Rainbow field characterized by high values of Cu and Zn [2] could be considered as another source of metals for ultramafic sulfides.…”

“…Since 1985, more than 20 new hydrothermal fields with associated mineralization have been discovered on the Mid-Atlantic Ridge (MAR) [2,3]. Several of these systems have been discovered during Russian scientific and prospecting exploration activities in the Northern Equatorial MAR [4].…”

“…OCC are tectonically uplifted along detachment faults, which exhume these deep-seated rocks onto the seafloor. OCC-associated SMS deposits are widely distributed at slow-and ultra-slow spreading ridges and are characterized by high grades of copper and gold [2,4]. The Ashadze hydrothermal field is located at 12°58′ N, 15 miles to the north of the Marathon fracture zone (Figure 1).…”