An overview of the geology and major ore deposits of Central Africa: Explanatory note for the 1:4,000,000 map “Geology and major ore deposits of Central Africa”

This paper is prepared within the frameworks of IGCP Project 470 and the associated BRGM scientific project “Africa 1999–2004” to accompany the 1:4,000,000 scale map “Geology and major ore deposits of Central Africa, presented at the 20th Colloquium of African Geology in Orleans in June 2004. It incorporates geological and metallogenic data from eight countries in Central Africa (Angola, Cameroon, Chad, Central African Republic, Congo Brazzaville, Democratic Republic of Congo (DRC), Equatorial Guinea and Zambia). The map is a harmonised and geo-referenced preliminary map, based on a GIS at 1:2,000,000 scale, and focusses on the spatial and temporal distribution of selected major deposits.     

Recent population trends of sooty and light-mantled albatrosses breeding on Marion Island

Sub-Antarctic Marion Island is one of the few islands where both species of Phoebetria albatrosses breed sympatrically. The last published assessment of their population trends, which reported counts up to 2008, concluded that the numbers of breeding pairs of sooty albatross P. fusca (Endangered) were decreasing, whereas numbers of light-mantled albatross P. palpebrata (Near Threatened) were increasing. Extending the counts to 2014 reversed these trends, with numbers of sooty albatrosses increasing from 2006 to 2014, and numbers of light-mantled albatrosses decreasing from 2007 to 2014. Confidence in island-wide counts is low due to the cryptic nature of the albatrosses on their largely inaccessible cliff-side nest sites, as well as counts for sooty albatrosses taking place late in the incubation period when 10?20% of nests have already failed. Given the greater conservation concern for the sooty albatross, we recommend that dedicated annual counts be conducted during the early incubation period, and be repeated shortly after the chicks hatch (late December), mid-way through the nestling period (late February) and prior to fledging (late April), to give a better idea of breeding success. Count zones also should be revised to facilitate more accurate counts, ensuring more reliable estimates of sooty albatross population trends at Marion Island.     

Major and trace element geochemistry of plutonic rocks from Francistown, NE Botswana: evidence for a Neoarchaean continental active margin in the Zimbabwe craton

The Neoarchaean Tati granite–greenstone terrane occurs within the southwestern part of the Zimbabwe craton in NE Botswana. It comprises 10 intrusive bodies forming part of three distinct plutonic suites: (1) an earlier TTG suite dominated by tonalites, trondhjemites, Na-granites distributed into high-Al (Group 1) and low-Al (Group 2) TTG sub-suite rocks; (2) a Sanukitoid suite including gabbros and Mg-diorites; and (3) a younger high-K granite suite displaying I-type, calc-alkaline affinities.

The Group 1 TTG sub-suite rocks are marked by high Sr/Y values and strongly fractionated chondrite-normalized rare earth element (REE) patterns, with no Eu anomaly. The Group 2 TTG sub-suite displays higher LREE contents, negative Eu anomaly and small to no fractionation of HREE. The primordial mantle-normalized patterns of the Francistown TTGs are marked by negative Nb–Ti anomalies. The geochemical characteristics of the TTG rocks are consistent with features of silicate melts from partial melting of flat subducting slabs for the Group 1 sub-suite and partial melting of arc mafic magmas underplated in the lower crust for the Group 2 sub-suite. The gabbros and high-Mg diorites of the Sanukitoid suite are marked by Mg#>0.5, high Al₂O₃ (>>16%), low TiO₂ (<0.6%) and variable enrichment of HFSE and LILE. Their chondrite-normalized REE patterns are flat in gabbros and mildly to substantially fractionated in high-Mg diorites, with minor negative or positive Eu anomalies. The primordial mantle-normalized diagrams display negative Nb–Ti (and Zr in gabbros) anomalies. Variable but high Sr/Y, Sr/Ce, La/Nb, Th/Ta and Cs/La and low Ce/Pb ratios mark the Sanukitoid suite rocks. These geochemical features are consistent with melting of a sub-arc heterogeneously metasomatised mantle wedge source predominantly enriched by earlier TTG melts and fluids from dehydration of a subducting slab. Melting of the mantle wedge is consistent with a steeper subduction system. The late to post-kinematic high-K granite suite includes I-type calc-alkaline rocks generated through crustal partial melting of earlier TTG material. The Neoarchaean tectonic evolution of the Zimbabwe craton is shown to mark a broad continental magmatic arc (and related accretionary thrusts and sedimentary basins) linked to a subduction zone, which operated within the Limpopo–Shashe belt at 2.8–2.65 Ga. The detachment of the subducting slab led to the uprise of a hotter mantle section as the source of heat inducing crustal partial melting of juvenile TTG material to produce the high-K granite suite.