Geophysical Exploration of the Yanfolila-Sankarani Region, Mali By Richard (Dick) West1 and Mamadou Bathily2
BHP Mineral 5330 South 900 East, Suite 200 Salt Lake City, UT 2 BHP Mineral PO Box 2856 Bamako, Mali 1
Northwest Mining Association 1994
INTRODUCTION I would like to discuss the results of an airborne magnetic and radiometric survey in the YanfolilaSankarani region of southern Mali. This survey is part of BHP Minerals grass roots exploration program in Mali to cost-effectively identify economic Au and VMS deposits in the region. During the presentation I will discuss the following: ● Regional and local geology of the area as known before the airborne survey ● Airborne survey parameters ● Revised integrated geologic interpretation based on the airborne, geochemistry and geologic results ● Follow-up geophysical survey results as part of a VMS exploration program. Mamadou and I would like to thank BHP Minerals for allowing us to give this presentation today and to Demetrius Pohl (BHPM) for his geologic insights regarding West Africa and the Yanfolila-Sankarani area.
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SURVEY LOCATION
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SURVEY LOCATION (cont.)
Sankarani River
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REGIONAL GEOLOGY & AEROMAGNETICS
Yan-San Aeromag Survey
West African Regional Geology BHP West African Aeromagnetic Stitch - 1993 December 1994
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LOCAL GEOLOGY YANFOLILA-SANKARANI AREA Aeromagnetic Survey Boundary
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According to Milesi et al (1989), the concession is centered on a thin band of granites flanked by flysch basins on either side. Faults trend in N-S, NE-SW and NW-SE directions Kalana Au Mine lies south of the airborne survey Airborne survey area is approximately 70 km E-W by 100 km N-S
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EXPLORATION HISTORY
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AEROMAGNETIC RESULTS - FIRST VERTICAL DERIVATIVE OF TMI & TMI
Light areas represent relatively low magnetic susceptibility and dark areas are relatively high magnetic susceptibility)
First vertical derivative of TMI, HEQ stretch December 1994
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TMI, linear stretch 7
AEROMAGNETIC RESULTS - RTP OF TMI The reduced to pole (RTP) of the total magnetic intensity (dark areas are relatively low magnetic susceptibility and light areas are relatively high magnetic susceptibility) Note some vertical striping from high magnetic zones due to limitation of the algorithm in low magnetic latitudes
RTP of TMI, HEQ stretch December 1994
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RADIOMETRIC RESULTS ● ●
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Ternary Radiometric Image Potassium-Thorium-Uranium concentrations as represented by RedGreen-Blue (KUTh = RGB) White zones are areas of relatively high concentrations for all three elements
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INTERPRETED GEOLOGY ●
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The western third of the concession is dominated by metasediments & minor volcanics. South central zone is a major, sheared volcanic belt that is bounded by a sinistral shear zone. This belt was later intruded in the south by a late-to-post tectonic granite. Elongated volcanics and volcanosediments occur in the east & appear to wrap around a syn-tectonic granite which was subsequently intruded by a late granite. SE portion of the survey is composed of early granites as is the NE portion. Imbricate thrusting is invoked to account for the complexity in the magnetic images and the uniformity of the radiometric images in the north.
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GEOLOGIC COMPARISON – BRGM VS YANFOLILA-SANKARANI (1994) INTERPRETATION
Milesi (1989) December 1994
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GOLD CONCENTRATION DRAPED ON 1ST VD TMI IMAGE ●
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Gold concentration in soils is draped on the 1st vertical derivative of the TMI. Red denotes relatively high gold concentrations with blue denoting relatively low to no gold concentrations. Most Au anomalies are located in the southern half of the survey & associated with contacts or faults. Komana, Gouela and Kenieba areas have gold anomalies that lie along faults and contacts.
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VMS EXPLORATION – TERNARY CU-PB-ZN SOIL CONCENTRATIONS
Cu-Pb-Zn = RGB December 1994
Cu-Pb-Zn draped on 1VD TMI Northwest Mining Association 1994
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CONCLUSIONS ●
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The results of this combined airborne magnetic and radiometric survey provides a powerful geologic mapping tool for areas of deeply weathered terrain. In the case of the Yanfolila-Sankarani region, a new volcanic belt has been identified in an area previously thought to be dominated by metasediments. Using the interpreted geologic and structure map allows the exploration effort to be focused much faster than when relying on ground mapping alone and allows an improvement in the prioritization of soil geochemical anomalies. The Yanfolila-Sankarani Concession in the south has good gold potential which BHP Minerals is following up at present. The massive sulfide potential is unknown at this time but additional exploration is warranted. West Africa is very prospective terrain but discriminating between occurrences and large scale deposits is difficult and time consuming using traditional ground mapping and geochemical sampling methodology. We are convinced that airborne geophysics will assist in cost-effectively locating those mineral deposits that are of interest to BHP Minerals.
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PRESENTATION NOTES
These are the presentation speaker notes for the presentation at the NWMA in Spokane in December of 1994
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ABSTRACT An airborne magnetic and radiometric survey was conducted over the Yanfolila and Sankarani region of southern Mali to aid in a grass-roots exploration program for gold in this Birimian age (2100 mya) basin. The two airborne techniques were effectively used to map geology and structure and to provide a new exploration paradigm for the region. This approached was needed because deep weathering and limited outcrop hampered ground mapping and regional soil geochemical surveys were tedious and expensive. The airborne results indicate that the region is more complicated structurally than previously thought and that mafic volcanics do occur within the basin setting. The radiometrics were quite useful in delineating syntectonicto-late tectonic granitic intrusives within the sediments and a set of potassic intrusives along a major shear zone in the south. The shear zone was unknown prior to the survey and is a prime target for gold mineralization. These results place the geochemical results in geologic context and allow a more rigorous prioritization of the geochemical anomalies. Ground geophysical surveys (Max-Min, VLF-EM and magnetics) have been conducted in two areas and were useful for delineating detailed structures. No drilling has been done to date, but the geophysics has been very useful in focusing the exploration effort and providing new exploration targets in the area. December 1994
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SURVEY LOCATION (SLIDE 2)
The Yanfolila-Sankarani Concession lies in southwestern Mali along the Guinean-Malian border. The survey area is located approximately 100 km south-southwest of Bamako, the capital city of Mali. Mali is a francophone, West African nation which is rich in tradition, history and hospitality. The concession is 5,000 km2 in area in a semi-arid region that encompasses hilly, brushy terrain as well as low-lying marshy area. Access is poor during the rainy season and the Sankarani River divides the concession in half, making travel between the northern and southern halves very time consuming.
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GEOLOGY (SLIDE 4) The Birimian Province of West Africa extends from Eastern Senegal to SW Niger and from NE Burkina Faso to the Atlantic Coast of Ghana and Cote d'Ivoire and covers ~750,000 km2. The best synthesis of the mapped geology in the region was completed in 1989 by Milesi et al for the BRGM and is shown on the left. This province is part of the West African Craton and is bordered by the Pan African fold thrust belt to the east and the Man Shield on the west and Upper Proterozoic platform sediments to the North. Geologic mapping is very difficult since deep weathering covers the region and outcrops comprises less than 5% of the land surface. Thick rain forest in the south and tall grasses in the north also hamper geologic mapping. The Birimian is an assemblage of supracrustal rocks deposited between 2.2 and 2 billion years ago. During this time the Eburnean Orogeny was occurring and pre-, syn- and post tectonic granites were intruded into the volcanic and volcanosedimentary rocks that make up the Birimian. Referring to the regional geologic and magnetic maps, the Birimian Province can be divided into three basic domains based on the outcrop patterns: i) large equi-dimensional basins with minor volcanics such as the Comoe Basin of Cote d'Ivoire and the Siguiri Basin of Guinea (very subdued magnetic texture in magnetic image), ii) narrow, elongate volcanic & sedimentary belts trending NE-SW and N-S such as the Boundiali Belt of northern CI, Fetekro Belt of central CI and the Ashanti Belt of Ghana, (ropey texture in Bibiani Belt) and lastly, iii) an arcuate volcanic & sedimentary belts draped around & digested by granitic intrusions such as the Boromo Belt of Burkina Faso (arcing pattern in the magnetic image). December 1994
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GEOLOGY (cont. SLIDE 4) These three domains reflect the deformational history that they have undergone. The large basins are the least deformed and are distal to volcanic centers. The elongated volcanic belts are the result of major crustal shearing and not rifting which is clearly shown by their association with major structures. In Burkina Faso the volcanic belts have been wrapped around contemporaneous granitic intrusions. Economic gold mineralization is often associated with shear zones which form contacts between the volcanics and volcanosediments and also within the metasediments. The region has a number of major gold producers. In Ghana there is the Ashanti Mine with a total of 1,000 tons of contained gold (mined and in reserve), Tarkwa with 500 tons and Prestea with 400 tons of contained gold. In Mali there are the Kalana Mine with 22 tons and BHP's Syama, Mine with 100 tons of contained Au. The concession is centered on the thin band of granites flanked by flysch basins on either side. The next slide shows a more detailed view of this geologic map.
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LOCAL GEOLOGY YANFOLILA-SANKARANI AREA (SLIDE 5)
The red border is the airborne survey boundary which is slightly larger than the concession area. The survey area is approximately 70km wide by 100 km long. The Kalana gold mine lies 25 km south of the southern border of the concession. This map was the base map when exploration began since it was the best available at the time. As shown, a thin band of granites cuts through flysch sediments with some minor volcanics mapped in the eastern portion of the concession. NE, NW and N trending faults are shown within the area.
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EXPLORATION HISTORY (SLIDE 6) Mali was a major producer of gold in the 14th century and the caravan routes across the Sahara were forged to support trade with the rich kingdom. In 1992 BHP Minerals was granted a concession for mineral exploration in the Yanfolila-Sankarani area. Given the poor understanding of the geology of the area, regional mapping commenced as well as a regional soil geochemistry survey with 1000 m line spacing and 400 m sample spacing. A regional airborne survey was funded by the BHPM Geophysics Group to show the benefits of airborne surveys in West Africa and to help with the geologic mapping. GEODASS Pty Ltd of South Africa was the contractor for the combined aeromagnetic and radiometric survey. Survey specifications were: Flight Line Direction: Flight Line Spacing: Terrain Clearance: Radiometric sensor crystal size: Flight Path: Projection and datum:
E-W 400 m 90 m 2000 cu inches GPS navigation UTM Zone 29 CM=-9 Clarke 1880
This year (1994) more detailed soil geochemical surveys are being conducted in specific areas to better define the nature of the regional geochemical anomalies.
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EXPLORATION HISTORY (SLIDE 6) Mali was a major producer of gold in the 14th century and the caravan routes across the Sahara were forged to support trade with the rich kingdom. In 1992 BHP Minerals was granted a concession for mineral exploration in the Yanfolila-Sankarani area. Given the poor understanding of the geology of the area, regional geologic mapping commenced as well as a regional soil geochemistry survey with 1000 m line spacing and 400 m sample spacing. A regional airborne survey was funded by the BHP Minerals geophysics group to demonstrate the benefits of airborne surveys in West Africa and to help with the geologic mapping. GEODASS Pty Ltd of South Africa was the contractor for the combined aeromagnetic and radiometric survey. Survey specifications were: Flight Line Direction: Flight Line Spacing: Terrain Clearance: Radiometric sensor crystal size: Flight Path: Projection and datum:
E-W 400 m 90 m 2000 cu GPS navigation UTM Zone 29 CM=-9 Clarke 1880
This year more detailed soil geochemical surveys are being conducted in specific areas to better define the nature of the regional geochemical anomalies.
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AEROMAGNETIC RESULTS - FIRST VERTICAL DERIVATIVE OF TMI & TMI (SLIDE 7) On the left is a greyscale image of the 1st vertical derivative of the total magnetic intensity and on the right is the total magnetic intensity image. The 1VD image is equal area histogram normalized while the TMI is linear stretched. Since this survey was conducted within a few degrees of the magnetic equator, dark areas in both images indicate relatively higher magnetic susceptibilities and light areas indicate relatively lower magnetic susceptibilities (the opposite of what one expects at mid-to-high magnetic latitudes). Both images show that the area has been cut by E-W dolerite dykes which are post mineralization and which were unknown prior to this survey. The central area displays a complex pattern of magnetic anomalies which are flanked by more subdued textures. On the west, magnetic lineaments trend from N-S to NE-SW and reflect subtle magnetic variations within the metasediments of the Birimian. On the east side, the magnetic lineaments trend NE-SW and are more magnetic and more continuous than on the west side. The more magnetic units are interpreted as volcanics within a volcanosediment package. The areas with little or no character are interpreted as granites or metasediments. One elongated granitic intrusive occurs in the center and two other more equidimensional granites occur at the south end. The south central granite appears to have digested some of the volcanics units indicating that it is a late-to-post tectonic granite. The southern half of the disturbed zone is characterized by elongated anomalies with the ropey texture characteristic of shear zones. The north half has a more complex magnetic pattern with some linear zones at the north. December 1994
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RADIOMETRIC RESULTS (SLIDE 9) The radiometric results were surprising good. On the right is a ternary image of the Potassium-ThoriumUranium concentrations as represented by Red-Green-Blue (KUTh = RGB). The Niger and Sankarani River banks are red with potassium rich clays and the rivers and reservoir are black due to gamma ray attenuation in water. The granites in the west half of the image are anomalous in all three elements (represented by the color white) while the south central granite is enriched in Thorium and Uranium. This clearly shows compositional changes of the granite over time. The volcano-sedimentary package of rocks that arc around the syntectonic granite in the SE are thorium enriched as are the metasediments in the NW. The southwest ‘foot’ of the survey is potassium enriched and shows more lithologic variation than the magnetics do. Potassium anomalies are also associated with the south central shear and circular anomalies indicative of alkali intrusives. Potassium enrichment associated with sericitic alteration has been mapped at Syama by Ken Witherly using a spectrometer and substantiated by airborne radiometrics. The central white spine in the north indicating enrichment of all three elements and the interpretation is an enigma at this time.
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INTERPRETED GEOLOGY (SLIDE 10) Synthesizing the magnetic and radiometric interpretations in addition to the existing outcrop maps and soil geochemistry yielded the integrated interpretation on the right. This interpretation is not definitive, but rather the first pass at integrating various geoscientific data sets. Ground truthing of the interpretation and refining the interpretation are necessary to better match the actual geology in this deeply weathered terrain. Mamadou and I don't completely agree on this interpretation but continue to improve the interpretation! The western third of the concession is dominated by metasediments with only minor volcanics. The south central zone is a major, sheared volcanic belt that is separated from the western metasediments by a sinistral shear zone. This belt was later intruded in the south by a late-to-post tectonic granite that digested some of the volcanic belt. Elongated volcanics and volcanosediments occur in the east & appear to wrap around a syn-tectonic granite which was subsequently intruded by a late granite. There is a suggestion of large open folds. Shears occur along the contacts between the various lithologies. The SE portion of the survey is composed of early granites as is the NE portion. Imbricate thrusting is invoked to account for the complexity in the magnetic images and the uniformity of the radiometric images in the north. This thrusting has brought the metasediments eastward over the volcanics. This area is difficult to interpret given the conflicting interpretations from the magnetics & radiometrics & more work is needed here. The NE granites are mapped on the ground as extending south to the late granite & there is support for that interpretation by the radiometrics but not the magnetics. Granites are also mapped within the wedge of volcanics in the south. Additional exploration has been focused in the Gouela, Komana and Kenieba Prospects in western Yanfolila based on the geochemical surveys, old workings and to a lesser extent on the geophysics. December 1994
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GEOLOGIC COMPARISON – BRGM VS YANFOLILA-SANKARANI (1994) INTEPRETATION (SLIDE 11) The pre-existing BRGM map (on the left) and the newly interpreted geologic map based on the airborne geophysics (on the right) highlight the differences between the two images and how airborne geophysics and focused ground follow-up can improve the mapping. There is no doubt that the geophysics has been instrumental in improving upon the mapped geology and has provided the geologist with a new paradigm for exploration: a volcanic belt instead of a flysch basin.
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GOLD CONCENTRATION DRAPED ON 1ST VD TMI IMAGE (SLIDE 12) On the right is an image of the concentration of the gold in soils draped onto the 1st vertical derivative of the TMI. The color scheme for the Au concentration is warm (red) to cool (blue) colors respectively denoting relatively high to low concentrations. The majority of the anomalies are located in the southern half of the survey. Most of the anomalies are associated with interpreted contacts or faults; this is most easily seen in the foot of the survey in the southwestern part of the concession. Very few Au anomalies occur within the areas interpreted as being granites. The Komana, Gouela and Kenieba areas all have gold anomalies that lie along faults and contacts. Gold occurs throughout the Birimian and geochemical anomalies abound; the trick is to determine which anomalies are associated with significant resources. Geochemistry does not always do that, in part because so many of the areas were prospected and mined in the 14th century. This activity affected huge tracts of land and significantly disturbed the soil profiles. Airborne surveys help to identify the significant geologic and structural trends, especially in a regional sense, which can be used to filter out anomalies that have no geologic context.
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VMS EXPLORATION – TERNARY CU-PB-ZN SOIL CONCENTRATIONS (SLIDE 13) BHP is also exploring for base metals within the concession. In adjacent Burkina Faso, the Perkoa Deposit was discovered by the United Nations Development Program (UNDP) and the Bureau des Mines et de la Géologie du Burkina (BUMIGEB) and it contains @6 million tons @ 18% Zn. On the left is a ternary image of the concentration in soils of three base metal indicator elements: Cu-Pb-Zn shown respectively as Red-Green-Blue. On the right is a similar ternary image of Cu-Pb-Zn concentrations draped onto the 1VD of TMI image. Concentration levelling errors are indicated by the higher average concentrations in the north half of the concession relative to the south half. Ignoring this issue, one of the best anomalous trends indicating elevated concentrations in all three elements is the white area in the Gouela area (highlighted in dashed yellow ellipse). Based on the Gouela geochemistry and magnetic structure, a VLF-EM survey was conducted in the area and then was followed up by a Max-Min survey in order to detect conductive mineralization that can be associated with VMS deposits. The NW-SE trending geochemical anomaly is bordered by a significant VLF-EM & Max-Min conductive anomaly on its west side. This border is also denoted in both the mag & radiometrics as a contact. Unfortunately the conductor is extensive in length and no significant, discrete increases in the conductance were noted. Consequently the conductor is believed to be graphitic in nature & not a massive sulfide. As expected, the ternary image also delineates lithological units and fault boundaries such as: the late granite, the volcanosediments in the SE and the shear zone in the south. December 1994
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END
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