Platinum-Bearing Fe–Mn Oceanic Crust on Basalts: Mineralogy and Model of Formation

Fe–Mn oceanic crust on basalts of the guyot in the Mid-Pacific Seamount (Pacific Ocean, depth 2486 m, chemical composition (wt %): Mn 24.2, Fe 12.6, Ni 0.59, Сo 0.72, Cu 0.13; (ppm) Pt 0.35, Pd 0.0052), was studied using 3D-technology of mineralogical research. In addition to dominant vernadite and goethite, the following minerals are identified in the hydroseparation (HS) concentrates of the crust: 1) rock-forming and accessory minerals (clinopyroxene, plagioclase, potassium feldspar, biotite, ilmenite, titanomagnetite, Ti-chrome spinel, zircon, apatite); 2) sulfides that are identical to those from the basalt substrate (pyrite, chalcopyrite, bornite, chalcocite, tennantite, “nickel pentlandite” Ni4S3, sphalerite, galena, argentite/acanthite, molybdenite); 3) native metals (iron, nickel, copper, titanium, tungsten); 4) iron silicides (gupeiite Fe3Si, xifengite Fe5Si3, hapkeite Fe2Si); 5) platinum-group minerals (PGMs)—unnamed (Cu,Pt)4Si phase and rustenburgite (Pt,Pd)3(Sn,Sb). The ore mineral assemblages in basalts and Fe–Mn crusts are identical. Accessory minerals in basalts are hypothesized to have served as a source of native metals. “Microdroplets” of native iron Fe, (Fe,Ni), nickel Ni, (Ni,Cr), (Ni,Fe), and copper Cu (20–100 µm in size, sphericity up to 100%) are interpreted as crystallization products of metal melts in basalts, transported by deep-seated fluids into Fe–Mn crusts on these rocks. Zoned microglobules (20–70 µm) were identified, with cores composed of native iron or nickel and rims consisting of wüstite-magnetite and Fe–Mn hydroxides. Mineralogical data allow to propose a volcanogenic-fluid model for the formation of Fe–Mn crusts on submarine oceanic highlands.