Hydrothermal alteration and sulfide mineralization in gabbroids of the Markov Deep (Mid-Atlantic Ridge, 6° N)

Abstract

A new type of sulfide occurrence related to metasomatically altered brecciated gabbroids has been studied at the Sierra Leone site situated in the axial rift valley of the Mid-Atlantic Ridge (Markov Deep, 6° N). Two associations of plutonic, subvolcanic, and volcanic rocks were dredged: (1) mid-ocean ridge basalts (MORB) and their intrusive analogues and (2) rocks of the silicic Fe-Ti-oxide series with dominating gabbronorites and sporadic trondhjemites. Almost all igneous rocks at the Sierra Leone site are enriched in Pb, Cu, U, Ga, Ta, Nb, Cs, and Rb and depleted in Zr, Th, and Hf. The rocks of the Fe-Ti-oxide series are enriched in Zn, Sn, and Mo and depleted in Ni and Cr. The main ore-bearing zone is situated at the foot of the eastern wall of the deep, where it is hosted in cataclastic hornblende gabbro and gabbronorite of the Fe-Ti-oxide series. Ore mineralization in metasomatically altered rocks is composed of quartz-sulfide and prehnite-sulfide veinlets, disseminated sulfide, and veined copper sulfide ore. The ore consists of pyrite, chalcopyrite, sphalerite, pyrrhotite, bornite, chalcocite, and digenite. The δ34S value of sulfides varies from 3.0 to 15.3‰. At the foot of the eastern wall of the Markov Deep, directly downslope from the ore-bearing zone, loose sediments contain grains of native Cu, Pb, Zn, and Sn and intermetallic compounds (isoferroplatinum, tetraferroplatinum, and brass) apparently derived from the ore. Mineral assemblages of ore-bearing metasomatic rocks and fluid inclusions therein were studied. Ore metasomatism developed under a low oxygen potential within a temperature interval from 400 to 160°C, though initial hydrothermal alteration of rocks proceeded at temperatures of 800–450°C. The temperature of stringer-disseminated ore mineralization is estimated at 170–280°C. The hydrothermal fluids are considered to be of magmatic origin; as the hydrothermal system evolved, they became diluted with seawater that was contained in fractured oceanic crust. The ore matter could have been derived from magmatic fluids that were released from water-saturated melts of the Fe-Ti oxide series during their ascent and leached from host gabbroids in the process of metasomatic alteration.