The origin and composition of carbonatite-derived carbonate-bearing fluorapatite deposits

Carbonate-bearing fluorapatite rocks occur at over 30 globally distributed carbonatite complexes and represent a substantial potential supply of phosphorus for the fertiliser industry. However, the process(es) involved in forming carbonate-bearing fluorapatite at some carbonatites remain equivocal, with both hydrothermal and weathering mechanisms inferred. In this contribution, we compare the paragenesis and trace element contents of carbonate-bearing fluorapatite rocks from the Kovdor, Sokli, Bukusu, Catalão I and Glenover carbonatites in order to further understand their origin, as well as to comment upon the concentration of elements that may be deleterious to fertiliser production. The paragenesis of apatite from each deposit is broadly equivalent, comprising residual magmatic grains overgrown by several different stages of carbonate-bearing fluorapatite. The first forms epitactic overgrowths on residual magmatic grains, followed by the formation of massive apatite which, in turn, is cross-cut by late euhedral and colloform apatite generations. Compositionally, the paragenetic sequence corresponds to a substantial decrease in the concentration of rare earth elements (REE), Sr, Na and Th, with an increase in U and Cd. The carbonate-bearing fluorapatite exhibits a negative Ce anomaly, attributed to oxic conditions in a surficial environment and, in combination with the textural and compositional commonality, supports a weathering origin for these rocks. Carbonate-bearing fluorapatite has Th contents which are several orders of magnitude lower than magmatic apatite grains, potentially making such apatite a more environmentally attractive feedstock for the fertiliser industry. Uranium and cadmium contents are higher in carbonate-bearing fluorapatite than magmatic carbonatite apatite, but are much lower than most marine phosphorites

Isotope zonation in basement crustal blocks of southeastern China: Evidence for multiple terrane amalgamation

Nd and Sr isotopic data for Mesozoic Yanshanian granites sampled across major NNE - trending faults in eastern Guangdong Province record conspicuous compositional variations in the crustal basement. Two principal crustal domains can be distinguished: Paleoproterozoic-dominated in the northwest and Mesoproterozoic-dominated in the southeast. Within the Mesoproterozoic-dominated domains, thin tectonic slivers of isotopically homogeneous Paleoproterozoic crust are interspersed with narrow belts displaying mantle-derived characteristics. Combined with other published data, the Nd-Sr isotopic signatures from eastern Guangdong show that the Precambrian basement in southeastern China is largely heterogeneous and made up of a series of narrow, crustal slices. The major domain boundaries coincide with large-scale NNE-trending strike-slip faults in the cover sequences. We postulate that these boundaries developed in the crust during the Mesoproterozoic and largely controlled the geological development of SE China throughout the Phanerozoic.link_to_subscribed_fulltex

Basement heterogeneity in the Cathaysia crustal block, southeast China

Isotope signatures and T DM model ages in Hong Kong and neighbouring Guangdong Province have indicated that the basement of the Cathaysia Block is probably an amalgamation of narrow crustal slices, ranging in age from latest Archaean to Mesoproterozoic. Inheritance ages from zircons contained within Mesozoic volcanic and plutonic rocks also show Proterozoic and Archaean components. Regional gravity survey studies display NNE- to NE-trending Bouguer anomalies that are indicative of sharp changes in rock densities at middle and lower crustal levels. The anomalies displayed on the gravity profile from Guangdong to Hong Kong have been modelled as narrow slices of Archaean and Proterozoic crust. A substantial E-W-trending Bouguer anomaly, which largely parallels the trend of the foliation in the Proterozoic schists of the region, is present to the east of Guangzhou. It is proposed that the basement of the Cathaysia Block consists of an amalgamation of NE- to NNE-trending Palaeo- to Mesoproterozoic and Archaean crustal terranes, which in places have retained the pre-amalgamation E-W-trending tectonic fabric. The discontinuities between the basement terranes, and the E-W structures have strongly influenced the geological evolution of the Phanerozoic sequences and igneous complexes in southeast China. These are most obviously manifest in the regional NE-trending fault and shear zones that displace the cover sequences. © The Geological Society of London 2004.link_to_subscribed_fulltex