The origin of supergene nickeliferous chlorite in the Santa Fé Ni-Laterite Deposit, GO, Brazil

The Santa Fé Ni laterite deposit (Brazil) developed as a consequence of weathering of serpentinized dunites that produced a regolith in which the main Ni ore zone is a ferruginous saprolite with chlorite as the main ore-carrier along with Ni-bearing Fe-oxyhydroxides. The genesis of chlorite is related to hydrothermal alteration of spinel. This study found that part of the chlorite has a supergene origin. XRPD-Rietveld method and EMPA/SEM analyses coupled with geochemical and mass balance calculations have provided further information about the chemistry and mineralogy of the main ore minerals. An attempt is made to explain the mode of formation of secondary chlorite and associated ore minerals. The main Ni-bearing minerals in the green saprolite are serpentine and minor chlorite. In the ferruginous and ochreous saprolites chlorite is a major phase with Ni contents of the order of 3 and 7 wt% NiO, respectively. Fe-oxy-hydroxides (mean NiO = 1.8 wt%) are also abundant in the ferruginous and ochreous saprolites. According to the isocon method, Al and Ti are definitely enriched in the saprolite compared to dunite. The source of Al for chlorite formation probably derives from pyroxenites and peridotite. The lateral migration of Al in solution was fundamental for the supergene formation of chlorite.