THE GEOLOGY OF THE STEENKAMPSKRAAL MONAZITE DEPOSIT, SOUTH-AFRICA - IMPLICATIONS FOR REE-TH-CU MINERALIZATION IN CHARNOCKITE-GRANULITE TERRANES

The ca. 1100 Ma Namaqualand Metamorphic Complex is host to a number of vein-type monazite-apatite-chalcopyrite-magnetite deposits, which clu ster in a approximately 30-km2 area 160 km south of the Okiep copper d istrict. The largest of these veins, at Steenkampskraal, has been mine d intermittently since the late 1950s for thorium. Field relationships demonstrate that the monazite-apatite vein of Steenkampskraal is para llel to a sheared, cuspate anticline strikingly similar to the well-kn own steep structures of the Okiep copper district. Quartz diorite and leucotonalite are found at the core of the anticline, in direct contac t with the monazite-apatite vein, and comprise the newly proposed Rood ewal suite. This association is also represented by veins and/or dikes of anorthosite, norite, enderbite, and charnockite that have a region al distribution in the Steenkampskraal area. Textural relationships in dicate that monazite and orthopyroxene coexist in the charnockite vein s and that monazite may be a minor constituent of the quartz diorite. Electron microprobe analyses of monazite from Steenkampskraal and othe r localities display high ThO2 contents (8-8.8 wt %), whereas bulk-roc k REE analyses demonstrate a strong depletion in Eu and high La/Yb rat ios. Sm-Nd isotope data for the monazite ore and closely associated ro cks indicate model ages of 1010 to 1278 Ma (chondritic uniform reservo ir mantle, CHUR) or 1450 to 1770 Ma (depleted mantle resevoir, DM). Th e CHUR model age is preferred as it compares to a published U-Pb age o f 1180 +/- 40 Ma and to our new whole-rock U-Pb age for the ore of 115 0 +/- 15 Ma. Age relationships, coupled with the coexistence of orthop yroxene and monazite, provide evidence that the mineralized veins of t he newly proposed Steenkampskraal monazite district formed under water -undersaturated conditions in granulite facies (T approximately 800-de grees-860-degrees-C, P approximately 5-6 kbars), a finding inconsisten t with the previously proposed hydrothermal model. The new findings ma y indicate that the monazite-apatite ore was deposited by H2O-deficien t, mineralizing fluids of crustal (metamorphic) or mantle origin. Howe ver, the lithological, mineralogical, and structural features of the S teenkampskraal monazite deposit resemble those of the silica-poor, mag netite-ilmenite-apatite-rich rocks (nelsonite) of the Koperberg Suite in the Okiep copper district. Although age of intrusion, Sm-Nd isotope systematics, and composition of the mineralization present clear diff erences for the two areas, the structural style and associated intrusi ve rocks of the Roodewal and Koperberg suites are very similar. Protra cted fractionation of a rare metals-enriched magma, yielding anorthosi tic cumulates and P-rich immiscible liquids, is therefore proposed as a model for the Steenkampskraal deposit and for the nelsonite occurren ces in the Okiep copper district. The ores of the Steenkampskraal mona zite district may consequently represent a separate class of polymetal lic, rare earth element (REE) mineralization with possible equivalents in other high-grade terranes, such as in Madagascar, in Mozambique, a nd in the southeastern United States.