INTRODUCTION

I recently spent seven weeks in Niger following up on the recommendations of a BHP-Utah International delegation which spent 6 days there in January of 1988. My objective was to examine as many of the Birimian greenstone belts and sample as many mineralized areas as was practical. During the course of my trip, I met with many of the government officials in Niger who would participate in negotiations with BHP-Utah if we choose to initiate an exploration program. I was also able to map and sample 15 mineralized areas where local miners are or have recently been exploiting surface and shallow subsurface gold mineralization.

The Birimian greenstone belts of west Africa consist of numerous remnants of a major Lower Proterozoic volcano-sedimentary province spread throughout the countries of Mali, Niger, Burkina Faso, Ghana, and the Ivory Coast. In general, the western-most and southern-most remnants consist of rhythmically bedded greywacke and meta-shale with minor volcanic rocks. The eastern-most and northern-most rocks consist of mafic to intermediate volcanic rocks with minor volcaniclastic sediments. The remnant sequences in the Liptako region of Niger fall into the later group. Though the Birimian greenstone belts have been worked since pre-historic times, production has come primarily from the Ashanti gold fields of Ghana which were developed in the 1890's.

THE LIPTAKO REGION

LANDSCAPE, CLIMATE, AND TRANSPORTATION

The Liptako region of southwest Niger covers 26,000 km2, bounded by the Niger River on its northeast edge and the border with Burkina Faso elsewhere (map 1). The terrain is composed of 110º trending stabilized dunes and inter-dune deflation surfaces in a band 10-50 km wide along the south bank of the Niger River. Outcrops are rare. South of this band, the country becomes hilly with scattered plateaus whose caps are often composed of lateritic cuirasse. Hills in the areas underlain by high-grade metamorphic and plutonic rocks are apt to be bedrock. The low areas are covered by a moderate to thin residual soil, patches of cuirasse, and wind-blown alluvium. Outcrops are common.

The Liptako is part of the Sahel region where the wet season starts in May or June and ends in August or September. Despite the recent publicity about the two draughts in the last 15 years and growing alarm about desertification, the landscape of stabilized dunes, dissected laterite, and the Niger River’s anatomizing fluvial regime all suggest a long term change of climate from heavy regular seasonal rains to one of increasing aridity. During the dry season, trunk streams feeding the Niger are nearly dry and the soil is dry, making transport possible. After the first rain of more than 10 mm, runoff is so massive that the streams may be 2 m deep within hours and not be passable for 10 days. Clay-rich deflation surfaces and areas of residual soil develop patinas of mud, making them impassable. The result of the first rains is to make all transport by motor vehicle in the outlying parts of the Liptako impossible for 4 months of the year.

GEOLOGY

The first geologic maps of the Liptako where prepared as part of a mineral evaluation project carried out by the BRGM in the 1950’s and 60’s. Map 1 shows 3 large and several small areas of Birimian age schist separated by older high-grade metamorphic and plutonic rocks. The boundaries of these schist belts appear to be reasonably accurate.

Within the schist belts the BRGM mapped 3 main lithologies: mafic to intermediate volcanic rocks, mildly metamorphosed immature sedimentary rocks, and younger felsic intrusive rocks. The identification of major rock types on this map is not entirely consistent with my own observations.

I saw more of the stratigraphy of the eastern-most schist belt which occupies the Sirba River Valley than any of the other belts. Most of my generalizations are therefore biased by my observations there. Map 2 is a reconnaissance geologic map of the routes I took from one gold camp to another in the Sirba River Belt. The volcanic rocks in this belt are universally pyritic. Those with <2-3% pyrite and which are massive to weakly foliated consist of dark green quartz-chlorite schists (photo 1) and have been mapped by the BRGM as meta-basalt. Those which had a higher initial pyrite content and or strong foliations crop out as brown, strongly weathered, quartz-chlorite schist (photo 2) which the BRGM mapped as meta-sediments. The main volcanic rock types that I saw where massive to pillowed basalt (photo 3), lithic lapilli to block meta-tuff and hornblende porphyry of basaltic andesite (photos 4 and 5), fine-grained laminated to massive medium-grained lithic meta-tuff f of mafic and intermediate composition (photos 6 and 7), and minor porphoryritic andesite (photo 8). My general impression is that basaltic andesite is more prevalent .than basalt, flows and massive meta-tuff are more common than bedded meta-tuff, and sediments are a volumetrically insignificant portion of the whole. However, within the single unequivocal sequence of sediments that I did see, very fine-grained siliceous beds with manganese oxides and graphitic meta-shale are common (photos 9 and 10).

Metamorphic grade is generally low and the grain sizes of the original rocks have not been significantly enlarged. Lower greenschist facies prevails with a few notabable exceptions. First, serpentinized amphibolite facies mafic and ultramafic intrusive rocks occur near the margins of all the schist belts. I have interpreted these as dikes, sills, and magma chambers associated with the volcanism. The observation that these are most common near the contacts with basement rocks suggests that the schist belts are synclinoria in which the most deeply buried rocks are now exposed at the base of the limbs. Second, a highly pyritic contact metamorphic halo more than 10 meters wide exists at the contact between the volcanics and a composite tonalite-granodiorite pluton in the village of Toure and similar hornfels exists at Manda (see below).

Deformation is difficult to assess due to the absence of distinctive marker horizons and the primitive state of geologic mapping. Small-scale folding is ubiquitous and apparent wherever enough outcrop and contrast in lithology exists to follow individual beds. Fold axes are steep to vertical. I was not able to confirm the existence of large scale folding but suspect folding is present on all scales. No major fault systems could be identified but follow up work with air photos, landsat, and geophysics will aid in their recognition. Strong foliations are common in many of the pyroclastics and much weaker to non-existent in the flows (photos 11 and 12). The regional trend of foliations is sub-parallel to the elongation of the schist belts themselves (~60º ).Ductile shear zones are present and some of these, like Boubene and Al Mani Gountou are strongly mineralized (see below).

Foliations and stretching lineations become pronounced in these zones and may decrease in intensity over many tens of meters away from a maximum or decrease abruptly over a few meters. The most common trend is 45-70º.

Massive chloritic meta-basalt or meta-andesite exposure near Al Mani Gountou in the northern part of the Sirba River Schist Belt

PHOTO 1.

Massive chloritic meta-basalt or meta-andesite exposure near Al Mani Gountou in the northern part of the Sirba River Schist Belt.

Limonitic foliated kaolinized chloritic meta-andesite near Boubene in the northern part of the Sirba River Schist Belt. The BRGM has mapped this lithology as meta-sediments.

PHOTO 2.

Limonitic foliated kaolinized chloritic meta-andesite near Boubene in the northern part of the Sirba River Schist Belt. The BRGM has mapped this lithology as meta-sediments.

Massive chloritic pillow basalt near Tchalkam, in the central part of the Sirba River Schist Belt.

PHOTO 3.

Massive chloritic pillow basalt near Tchalkam, in the central part of the Sirba River Schist Belt.

Chloritic lapiIli and block meta-tuff near Al Mani Gountou in the northern part of the Sirba River Schist Belt.

PHOTO 4.

Chloritic lapiIli and block meta-tuff near Al Mani Gountou in the northern part of the Sirba River Schist Belt.

Massive chloritic hornblende meta-basaltic andesite porphyry from Boubene in the northern part of the Sirba River Schist Belt.

PHOTO 5.

Massive chloritic hornblende meta-basaltic andesite porphyry from Boubene in the northern part of the Sirba River Schist Belt.

Laminated fine grained meta-tuff near Deba in the southern put of the Sirba River Schist Belt.

PHOTO 6.

Laminated fine grained meta-tuff near Deba in the southern put of the Sirba River Schist Belt.

Massive bedded chloritic lapilli lithic meta-tuff near Deba in the southern part of the Sirba River Schist Belt.

PHOTO 7.

Massive bedded chloritic lapilli lithic meta-tuff near Deba in the southern part of the Sirba River Schist Belt.

Massive chloritic meta-andesite porphyry from the northern part of the Sirba River Schist Belt.

PHOTO 8.

Massive chloritic meta-andesite porphyry from the northern part of the Sirba River Schist Belt.

Bedded limonitic siliceous manganiferous met sediment from the southern part of the Sirba River Schist Belt.

PHOTO 9.

Bedded limonitic siliceous manganiferous met sediment from the southern part of the Sirba River Schist Belt.

Close-up of the interior of the beds in photo 11. Limonitic siliceous manganiferous meta-sediment from the southern part of the Sirba River Schist Belt.

PHOTO 10.

Close-up of the interior of the beds in photo 11. Limonitic siliceous manganiferous meta-sediment from the southern part of the Sirba River Schist Belt.

Strongly foliated bedded meta-tuffs from the northern part of the Sirba River schist Belt.

PHOTO 11.

Strongly foliated bedded meta-tuffs from the northern part of the Sirba River schist Belt.

Massive relatively undeformed meta-basalt flow. Note the preservation of details of the flow top surface.

PHOTO 12.

Massive relatively undeformed meta-basalt flow. Note the preservation of details of the flow top surface.

PEDOLOGY

Fossil ferrilitic soil covers most of the Liptako and has been dissected by the present drainage net. The A and B horizons have been removed by erosion, exposing a well developed scoriaceous to pisolitic cuirasse 2-3 m thick (photo 13). Erosion has removed these cuirasses over large areas leaving isolated plateaux separated by extensive plains overlain by thin alluvial soils, remnant mottled ferrilitic soil, saprolite, and bedrock outcrops. Unconsolidated pisolite horizons (carapace) are also present in some areas.

Much of the gold mined by local miners appears to be concentrated from a pyritic protore by pedogenic processes. Intense bleaching and kaolinization are common and in many places confined to the areas of high initial pyrite concentration marked by limonite filled and empty pyrite molds. Reports by local miners at the deposits with intensive kaolinization consistently identify rich surficial concentrations of medium to coarse-grained gold, a barren zone of 10 or more meters, and gradual increases in grade of finer-grained gold as the water table is approached. The pattern of bleaching, gold enrichment and depletion, and its correlation with initial pyrite content is consistent with intense acid leaching of the host rock and supergene enrichment of gold by oxidation of pyrite. It is possible that the upper enrichment zone is related to the first phase of ferrilitic weathering since it is commonly at the base of the cuirasse or between the top of the saprolite and base of the cuirasse. The barren zone and weaker gold enrichment near the water table suggests remobilization under the present weathering regime.

Massive hematitic scoriaceous cuirasse near Kossa in the western part of the Dorbel Schist Belt. This highly resistant material commonly caps low plateaux throughout the schist belts of the Liptako region.

PHOTO 13.

Massive hematitic scoriaceous cuirasse near Kossa in the western part of the Dorbel Schist Belt. This highly resistant material commonly caps low plateaux throughout the schist belts of the Liptako region.