Fig. 1. Continued
tion so that to discuss the possible environments of formation of the series. This volcanic and
plutonic assemblage may represent: 1. an association of basalt and layered intrusions,
2. all of the crustal members of an ophiolitic suite,
3. the plutonic roots of the lower levels and the coeval volcanic rocks of an island arc.
2. Geology
The characteristics of the Birimian formations have been defined from a study of the Loraboue´
prospect, in the northern part of the Poura district located 160 km south-west of Ouagadougou. The
Loraboue´ prospect terrain crops out in the me- dian part of the Boromo greenstone belt Fig. 1b
which trends north-south and is 500 km long and 40 km wide Feybesse et al., 1990; Lompo et al.,
1991. The Boromo greenstone belt is 2.15 – 2.10 Ga old Feybesse and Milesi, 1994 and belongs
to the Paleoproterozoic Eastern subprovince defined by Hirdes et al. 1996. This terrain was
chosen because of the presence of numerous bod- ies of ultramafic-mafic rock Syndicat BRGM-
BUVOGMI, 1983; Fig. 1c together with drill cores obtained by the SOREMIB Company
SOREMIB, 1990 which allow us to observe some unaltered samples and their relationships.
The Loraboue´ prospect forms an area over 4 km long and 2 km wide Fig. 1d. All rocks have been
subjected to greenschist-facies metamorphism and hydrothermal alteration, and are affected by a
N-S-trending steeply dipping regional foliation. The Boromo belt hosts several types of mineral-
ization Mile´si et al., 1992 including a massive Zn-Ag sulphide deposit at Perkoa Marcoux et
al., 1988; Napon, 1988; Ouedraogo, 1989. Dis- seminated gold is associated with albitite and
listvenite at Loraboue´ Be´ziat et al., 1998, and with albitite at Larafella Bamba et al., 1997. As
with most of the gold of West Africa, gold-bear- ing quartz veins are observed at Poura Oue-
draogo, 1989; Sanogo, 1993.
3. Petrography
The original sequence is composed of volcanic rocks interlayered with metasediments, within
which are developed assemblages of mafic and ultramafic rocks and some isolated gabbroic bod-
ies. The volcanic sequence is dominated by fine- grained basalt flows, with volumetrically minor
differentiates occurring as metafelsic and pyro- clastic rocks, dolerite dykes up to several metres
thick and thin rhyolitic dykes. The basalts are generally aphyric and yield typical greenschist-fa-
cies alteration assemblages. They can be greatly affected by carbonate alteration. Dolerites exhibit
the same mineral assemblage as basalt, but an intersertal to ophitic texture is commonly pre-
served. Small cross-cutting rhyolitic dykes are typ- ically composed of plagioclase phenocrysts and
corroded quartz Fig. 2A. The plutonic suite consists of ultramafic rocks
associated with gabbroic rocks, as well as gab- broic rocks cropping out as isolated bodies in
restricted areas. The ultramafic rocks exhibit an heteradcumulate
texture, with
sub-rounded olivine crystals up to 4 mm across poikilitically
enclosed by megacrysts 2 – 3 cm of pyroxene Fig. 2B and brown amphibole. Varying propor-
tions of the cumulus and post-cumulus minerals generally give rise to wehrlitic and, more rarely, to
dunitic cumulates. The post-cumulus minerals are essentially represented by clinopyroxene and
brown amphibole Fig. 2C, as well as accessory orthopyroxene Fig. 2D and biotite. The latter
occurs as a selvage around amphibole crystals Fig. 2E. Chromite is located both in olivine and
post-cumulus minerals Fig. 2B, C and D. The degree of alteration is highly variable, but olivine
is generally pseudomorphed into serpentine ac- companied by veinlets of magnetite Fig. 2D and
occasionally heazlewoodite Ni
3
S
2
. Orthopyrox- ene and brown amphibole are altered into tremo-
lite, which is in turn replaced by chlorite; chromite is locally transformed to ferritchromite and biotite
to chlorite Fig. 2E.
The gabbroic rocks associated with the ultra- mafic cumulates are usually layered. Composi-
tional layering is made up of modal variations, with layers of wehrlitic composition grading into
layers of gabbroic composition devoid of olivine and chromite crystals, in which plagioclase is
much more abundant Fig. 2F. In the gabbroic suite, amphibole is markedly more abundant than
clinopyroxene. Locally, gabbros show some dif- ferentiated facies with sub-pegmatitic texture cor-
responding
to late-stage
segregation. These
dioritic facies contain up to 70 modal plagio- clase up to 1 cm across including zircon, and
more frequently, apatite crystals. Clinopyroxene is partially recrystallized to clinozoisite, and ilmenite
replaced at the margins by titanite Fig. 2G.
The gabbros occurring as isolated bodies con- sist of two petrographic types:
1. layered gabbro, similar to the gabbro associ- ated with ultramafic rocks;
2. massive gabbro, sometimes associated with do- lerite, composed of clinopyroxene altered into
green amphibole, plagioclase and-in contrast
Fig. 2. Sample photographs and photomicrographs showing: A. Rhyolitic dykes cross-cutting tholeiitic basalts, typically with plagioclase Pl phenocrysts and corroded quartz Qz; B. Heteradcumulate texture of a wehrlite; serpentinized olivine S.Ol,
chromite Chr; C. Post-cumulus clinopyroxene Cpx and brown amphibole Am in wehrlite; D. Relic grains of fresh olivine Ol enclosed in orthopyroxene Opx; magnetite Mt; E. Post-cumulus amphibole and biotite Bi, partially pseudomorphosed into
chlorite in wehrlite; F. Layered gabbro; G. Differentiated gabbro; ilmenite Ilm; H. CaKa X-ray map of a clinopyroxene containing thin lamellae of chlorite.
with the layered gabbro-magnetite with thin lamellae of exsolved ilmenite.
4. Mineralogy
