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

Abstract

This paper presents mineralogical and geochemical data on seafloor massive sulfides (SMS) from the Ashadze-1 hydrothermal field at the Mid-Atlantic Ridge (MAR). The Ashadze-1 deposit is associated with the uplifted lower crust and upper mantle (oceanic core complex, OCC) of the MAR segment characterized by asymmetric mode of accretion. The OCC is represented by deep-seated gabbro-peridotite rocks exhumed on the rift valley slope along the detachment fault, during seafloor spreading. Hydrothermal processes in OCC environments result in different deposit composition and morphology compared to basalt-hosted systems. Abundant chimneys and enrichment in particular metals, including copper, zinc, gold, cobalt and tin are typical for this type of SMS deposit. The Ashadze-1 deposit is considered an example of a hydrothermal system in the initial stage of evolution marked by the young age of the sulfides (<7.2 kyr). The mineralogy of Ashadze-1 reflects primary ore-forming processes unaffected by post formation alteration. We propose a model for the primary ore-forming hydrothermal process in an ultramafic-hosted environment on the modern seafloor.

Characterisation of Mineralised Material from the Loki’s Castle Hydrothermal Vent on the Mohn’s Ridge

“…Au content of high temperature chimney material is consistent with that of massive sulphides from the interior of the mounds and the underlying stockwork zones [7] which suggests the Au grades measured in this study could be continuous throughout the site. Similarly, Zn grades are good, Ag comparable, and Cu low when compared to other SMS deposits [7,12,16,52]; for example, the TAG has a central volume grading under 0.01% Zn, but over 2% Cu [36]. Land-based mafic-hosted VMS deposits have typical Cu, Zn, Au, and Ag grades of 2%, 1.9%, 2.5 ppm and 25 ppm, respectively [6], which are comparable with the values found in this study.…”

“…This has been done hydrothermal vent on the Arctic Mid-Ocean Ridge (AMOR). This has been done variably for other SMS localities such as the Mid Atlantic Ridge (MAR) [12][13][14], the Juan de Fuca ridge [15,16], and others [17][18][19], but not for the AMOR.…”

“…One site that shows potential for SMS-style mineralisation is the Loki's Castle (73°30′ N, 8° E), located on the ultraslow spreading (6)(7)(8)(9)(10)(11)(12)(13)(14)(15) mm/year [20]) AMOR, and discovered in 2008 [8]. The Loki's Castle is a site of active hydrothermal venting occurring just south of the junction between the Mohns and Knipovich Ridges, at 2400 m depth (Figure 1).…”

Geological and Chemical Factors that Impacted the Biological Utilization of Cobalt in the Archean Eon

“…It should be noted that anomalously high metal concentrations in hydrothermal fluids can form ore deposits for other metals including Zn and Pb (Wilkinson et al, ). Various hydrothermal sulfide deposits have shown high Co concentrations, such as in the 13°N hydrothermal field on the East Pacific Rise (Fouquet et al, ), West Magnitogorsk paleoisland arc of the South Urals (Artemyev & Zaykov, ), the southern portion of the Main Urals Fault (Melekestseva et al, ), and the MAR Ashadze‐1 Field (Firstova et al, ), which could leave open the possibility of high concentrations of Co in hydrothermal fluids. These potentially high Co concentrations in hydrothermal fluids, owing to alteration of ultramafic rocks by seawater at high temperature and pressure, and limited Co‐mineral formation before 2500 Ma (Figure ), could have resulted in enhanced organic complexation in the Archean ocean.…”

“…Rise (Fouquet et al, 1996), West Magnitogorsk paleoisland arc of the South Urals (Artemyev & Zaykov, 2010), the southern portion of the Main Urals Fault (Melekestseva et al, 2013), and the MAR Ashadze-1 Field (Firstova et al, 2016), which could leave open the possibility of high concentrations of Co in hydrothermal fluids. These potentially high Co concentrations in hydrothermal fluids, owing to alteration of ultramafic rocks by seawater at high temperature and pressure, and limited Co-mineral formation before 2500 Ma (Figure 1), could have resulted in enhanced organic complexation in the Archean ocean.…”