Petrogenetic links between rare metal-bearing pegmatites and TTG gneisses in the West African Craton: The Mangodara district of SW Burkina Faso

Abstract

We describe the geological context of rare metal-bearing pegmatites from the Mangodara district (South-West Burkina Faso, Paleoproterozoic West African Craton) and discuss their petrogenesis and links with the host rocks. The Mangodara district exposes a gneiss-granitoid complex structured in a regional-scale dome, mantled by granodioritic gneiss enclosing rafts of amphibolite, micaschist and paragneiss, and cored by tonalitic to trondhjemitic gneisses. These gneisses enclose granitoid plutons, and four populations of rare metal-bearing pegmatites: titanite-allanite-, apatite-zircon-, garnetcolumbite (Li, Nb) and garnet-REE (Ti, Y, HREE)-bearing varieties.2 Rafts of migmatitic amphibolite, micaschist and paragneiss are chemically equivalent to nearby Birimian greenstone belts. An origin of the gneiss-granitoid complex by partial melting is suggested by diffuse contacts of rafts with the gneisses as well as the presence of garnet-bearing or hornblendebearing leucosome in paragneiss and in amphibolites, respectively. Textural continuity between pegmatites and granitic veins concordant to the foliation of the gneisses point to syntectonic segregation of the pegmatite-forming magmas.The compositions of gneisses and plutonic rocks spread between a Na-rich pole and a K-rich pole.Granodioritic gneiss, hornblende-biotite granodiorite and potassic biotite granitoids define a K-rich series attributed to relatively low-pressure high-degree partial melting of dominantly paragneiss, which accounts for low to absent LILE and HFSE fractionation compared to the paragneiss. Tonalitictrondhjemitic gneiss, a trondhjemite and a peraluminous two-mica tonalite belong to a Na-rich series characterized by low K content and strong depletion in REE and HFSE, which might reflect plagioclase clustering after partial melting at a relatively high pressure of amphibolite, and fractionation of HFSE-REE-bearing minerals.U-Pb dating of zircon from a titanite-allanite-bearing pegmatite at 2094.3 ± 8.8 Ma and of apatite from tonalitic-trondhjemitic gneiss, apatite-zircon-bearing pegmatite and granodioritic gneiss at 2094 ± 21 Ma, 2055 ± 20 Ma, and 2041 ± 33 Ma respectively, confirm that partial melting, melt segregation and crystallization-cooling occurred during the Eburnean orogeny.Based on these data, we propose that (i) titanite-allanite-and apatite-zircon-bearing pegmatites result from syntectonic segregation of residual melt during crystallization of the tonalitic-trondhjemitic gneiss, (ii) garnet-columbite-bearing pegmatites originate from syntectonic melt segregation from the migmatitic paragneiss, and (iii) garnet-REE-bearing pegmatites derive from segregation of a residual melt within the granodioritic gneiss.

Insights from mineral trace chemistry on the origin of NYF and mixed LCT + NYF pegmatites and their mineralization at Mangodara, SW Burkina Faso

“…These peaks are coeval with major deformation phases D1 and D2 that are expressed by folds and penetrative foliation in greenstones, synmigmatitic foliation in migmatites and magmatic layering in granitoids (Baratoux et al 2011;Bonzi et al 2021;). These features have been interpreted as recording a combination of gravitational instabilities with diapirs cored by granitoid gneiss complexes deformed during lateral flow of the orogenic crust in a context of transpression (Allibone et al 2002;Baratoux et al 2011;Bonzi et al 2021;Chardon et al 2020;Feybesse and Milési 1994;Ganne et al 2012;Lompo 2009Lompo , 2010Milési 1989;Milési et al 1992;Vidal et al 2009).…”

“…Greenstone belts are affected by greenschist grade metamorphism during the Eburnean orogeny, rising to amphibolite grade in granitoid gneiss complexes. The greenschist-amphibolite facies transition has been interpreted to represent metamorphic aureoles around intrusions (Debat et al 2003;Doumbia et al 1998;Feybesse et al 2006;Ganne et al 2014;Gueye et al 2008;Pons et al 1992), or regional metamorphic gradient (Bonzi et al 2021;Vidal et al 2009;Vidal and Alric 1994). Crystallization ages of magmatic zircon from volcanic and plutonic rocks of greenstone belts and granitoid gneiss complexes, in South-Western Burkina Faso and Northern Ivory Coast, range between 2200 Ma and 2000 Ma (Baratoux et al 2011;Gasquet et al 2003;Hirdes et al 1996;Parra-Avila et al 2017.…”

“…Also, the strong partitioning dependence of rare-metals on residual and peritectic minerals might further contribute to rare-metals depletion in the anatectic melt (Bea 1996;Bea et al 1994;Bea and Montero 1999;Brown et al 2016). Despite these limitations, the anatectic model of pegmatite genesis has recently been invoked in several studies (Bonzi et al 2021;Müller et al 2015Müller et al , 2017Shaw et al 2016;Turlin et al 2019).…”

“…In the Paleoproterozoic West African Craton, previously described rare-metal-bearing pegmatites of Saraya in Senegal, Winneba in Ghana, Issia in Ivory Coast and Goulamina in Mali consist of LCT pegmatites mineralized in spodumene and CGM (Allou 2005;Allou et al 2005;Dampare et al 2005;Ndiaye et al 1997;Nude et al 2011;Wilde et al 2021). In a previous paper (Bonzi et al 2021), we have presented the geological context of the raremetal-bearing pegmatites of the Mangodara district (southwestern Burkina Faso). Based on structural, petrological, geochemical and geochronological data on the pegmatites and their hosts, we have proposed an origin by partial melting of paragneiss/amphibolite and melt segregation in tonalitic-trondhjemitic-granodioritic gneisses.…”

Mineralogical, fluid inclusion and radiometric studies on Wadi El-Dob pegmatites, northern Eastern Desert, Egypt.

“…Wadi with a peralkaline signature (Estrade et al, 2001;Schmitt et al, 2022). NYF, garnet-REE pegmatite containing ilmenite and Nb-Ta minerals should originate as the product of melt segregation within the granite during its crystallization (Bonzi et al, 2021;. Generally, the immiscibility supercritical CO 2 fluid and liquid rich in H 2 O coexist in many environments of the shallow crust (Kaszuba et al, 2006).…”