The geology of the Steenkampskraal monazite deposit, South Africa; implications for REE-Th-Cu mineralization in charnockite-granulite terranes

Abstract

The ca. 1100 Ma Namaqualand Metamorphic Complex is host to a number of vein-type monazite-apatite-chalcopyrite-magnetite deposits, which cluster in a approximately 30-km 2 area 160 km south of the Okiep copper district. The largest of these veins, at Steenkampskraal, has been mined intermittently since the late 1950s for thorium. Field relationships demonstrate that the monazite-apatite vein of Steenkampskraal is parallel to a sheared, cuspate anticline strikingly similar to the well-known steep structures of the Okiep copper district. Quartz diorite and leucotonalite are found at the core of the anticline, in direct contact with the monazite-apatite vein, and comprise the newly proposed Roodewal suite. This association is also represented by veins and/or dikes of anorthosite, norite, enderbite, and charnockite that have a regional distribution in the Steenkampskraal area. Textural relationships indicate that monazite and orthopyroxene coexist in the charnockite veins and that monazite may be a minor constituent of the quartz diorite. Electron microprobe analyses of monazite from Steen-kampskraal and other localities display high ThO 2 contents (8-8.8 wt %), whereas bulk-rock REE analyses demonstrate a strong depletion in Eu and high La/Yb ratios.Sm-Nd isotope data for the monazite ore and closely associated rocks indicate model ages of 1010 to 1278 Ma (chondritic uniform reservoir mantle, CHUR) or 1450 to 1770 Ma (depleted mantle resevoir, DM). The CHUR model age is preferred as it compares to a published U-Pb age of 1180 + or - 40 Ma and to our new whole-rock U-Pb age for the ore of 1150 + or - 15 Ma.Age relationships, coupled with the coexistence of orthopyroxene and monazite, provide evidence that the mineralized veins of the newly proposed Steenkampskraal monazite district formed under water-undersaturated conditions in granulite facies (T approximately 800 degrees -860 degrees C, P approximately 5-6 kbars), a finding inconsistent with the previously proposed hydrothermal model. The new findings may indicate that the monazite-apatite ore was deposited by H 2 O-deficient, mineralizing fluids of crustal (metamorphic) or mantle origin. However, the lithological, mineralogical, and structural features of the Steenkampskraal monazite deposit resemble those of the silica-poor, magnetite-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 differences for the two areas, the structural style and associated intrusive rocks of the Roodewal and Koperberg suites are very similar. Protracted fractionation of a rare metals-enriched magma, yielding anorthositic cumulates and P-rich immiscible liquids, is therefore proposed as a model for the Steenkampskraal deposit and for the nelsonitc occurrences in the Okiep copper district.The ores of the Steenkampskraal monazite district may consequently represent a separate class of polymetallic, rare earth element (REE) mineralization with possible equivalents in other high-grade terranes, such as in Madagascar, in Mozambique, and in the southeastern United States.