Geochemical and mineralogical characteristics of Fe-Ni- and bauxitic-laterite deposits of Greece

Contiguous vertical sample profiles from Ni-laterite deposits with in situ features (Kastoria, Profitis Ilias, and Tsouka) and allochthonous Ni- and bauxitic-laterite deposits in contact with basement limestone (Nissi) and bauxitic-laterites lying on peridorite (Parhari) were analyzed for major, trace (including Tn and U), rare earth and platinum-group elements (REE and PGE, respectively). In addition, minerals with emphasis to chromite grains found as residual components in these laterites, inherited from the ophiolitic parent rocks, were analyzed by microprobe. Low Al2O3, TiO2, REE, Th and U contents are common features of the Kastoria, Bitincka and Tsouka deposits. Tn contrast, elevated REE contents are present in the karst-type bauxitic- and Ni-laterite ores of Nissi and bauxitic-laterites of Parhari. The bauxitic-laterite deposit of Nissi attains REE contents of thousands parts per million in samples from the contact between the lowest part of the bauxitic-laterite and the footwall limestone. Highest contents of Th and U are found in the bauxitic-laterites, with Th ranging from 4 to 28 ppm and U from 4 to 66 ppm. In general, increasing Al contents are accompanied by elevated Ti, REE, Tn and U contents at the Parhari and Nissi laterite deposits. Goethite, is the dominant mineral in all Ni-laterite profiles studied, while boehmite co-existing with goethite is common in the bauxitic-laterites at Nissi and Parhari. Goethite exhibits variable Al contents, while the Al/Fe ratio increases towards the top of the profiles. The PGE concentrations are generally low, ranging from less than 100 ppb to a few hundred parts per billion. The lowest values - lower than in the bedrock - were recorded in the saprolite zone. A certain enrichment in Pt (up to 48 ppb), Pd (7 ppb), and Au (16 ppb) is recorded in the reddish altered peridotite overlying the saprolite zone at Profitis Ilias. The Fe-Ni ore overlying the reddish altered peridotite has the highest Os values (14 ppb), Ir (32 ppb), Ru (66 ppb), Rh (20 ppb), Pt (86 ppb), and Pd (186 ppb). Gold contents are below 36 ppb. An increasing Pt/Pd ratio from 3.0 in Fe-Ni-laterites to 6.0 in bauxitic-laterites is apparent. Both whole rock compositions and mineral chemistry of laterites indicate that major controlling factors of the composition of the bauxitic-laterites are the conditions during transportation/deposition of the weathered material and during diagenesis/metadiagenesis stage rather than parent mafic ophiolitic rocks. The comparison between the primary composition of chromite in the saprolite zone and the overlying Fe-Ni ore may provide evidence for the discrimination between Fe-Ni ore linked to in situ weathering and ore derived by transportation to some extent of elastic and chemical material. (C) 2000 Elsevier Science B.V. All rights reserved.