GEOCHEMICAL EXPLORATION A CASE STUDY OF GOLD MINERALIZTION IN ILESHA NIGERIA AGY 410, APPLIED GEOLOGY SEMINAR PRESENTED BY ADEYEMO VICTOR ADEDAYO MATRIC NO: 092494 SUPERVISOR: DR. IPOOLA OKUNLOLA COMEG REGD.

CONTENTS Introduction Methods of geochemical survey Case study of gold mineralization in Ilesha Nigeria Regional geology Topography Climate Methodology Results Conclusion

INTRODUCTION Geochemical prospecting for minerals, as denned by common usage, includes any method of mineral exploration based on systematic measurement of one or more chemical properties of a naturally occurring material. The purpose of the measurements The discovery of a geochemical "anomaly" or An area where the chemical pattern indicates the presence of ore in the vicinity.

GEOCHEMICAL ANOMALY Anomalies may be formed either at depth by igneous processes metamorphic processes or At the earth's surface by agents of Weathering Erosion Surficial transportation.

WHAT TO MEASURE The chemical property measured is most commonly the trace content of some element or group of elements; in naturally occurring material like: Rock, Soil, Gossan, Glacial debris, Vegetation, Stream sediment, or water.

Geochemical Exploration Methods There are only two: Soil surveys/sampling Stream surveys/sampling Soil surveys give an indication of the chemistry of the underlying rocks (that have been weathered). If on a slope the rock will be further up slope from the soil. Nigerian Geoscientists during the Geochemical Mapping exercise MINISTRY OF MINES AND STEEL DEVELOPMENT Sustainable management of mineral resources world bank assisted project, December 2010.

Geochemical Exploration Method 2 STREAM SURVEYS/SAMPLING With stream sampling the chemistry of the water will be influenced by the rock it flows over. The concentration of an element will be highest just downstream from the ore. The concentration will then decrease as the water gets diluted further downstream. Nigerian Geoscientists sieving stream sediments with a personnel of the British Geological Survey MINISTRY OF MINES AND STEEL DEVELOPMENT Sustainable management of mineral resources world bank assisted project, December 2010.

CASE STUDY OF GOLD MINERALIZTION IN NIGERIA Ilesha, Nigeria ELUEZE AND OLADE (1985)

REGIONAL GEOLOGY The Ilesha case study described by Elueze and Olade (1985) covers an area approximately 250 km2 in southwest Nigeria. It is underlain by a north- south trending belt of Proterozoic rocks that forms

Fig.1 General geology and locality map of I1esha area (after Elueze and Olade, 1985).

As part of an extensive regional exploration program initiated by the Nigerian Mining Corporation, a reconnaissance stream ,sediment survey was undertaken in the Ilesha district, the main objective being to delineate areas with potential for gold mineralization.   The area of interest was delineated by geological mapping, the chief consideration being the well- known association of mafic rocks with gold mineralization.

The dominant rocks are metamorphosed mafic volcanics and volcaniclastics which include Amphibolites, Ta1c-tremolite schists, Chlorite and Mica schists   These rocks are intruded by granite gneisses. Economic interest focused principally on the alluvial gold deposits, which are believed to have been derived from auriferous quartz veins and stringers and disseminations within the amphibolites and associated mafic schists.

TOPOGRAPHY The topography of the Ilesha area is for the most part undulating, with an average elevation of 400 m above sea level, except in the southwest, where an inselberg landscape is quite well developed. The area is drained by a few main streams, but these possess numerous tributaries. The alluvial sediments in the upper parts of the drainage system are composed mainly of sandy silt, but downstream often comprise clays rich in organic matter.

CLIMATE The area has a humid tropical climate with an annual rainfall of 1500 mm. Although the area lies in a zone tropical rain forest, the vegetation now consists of secondary growth on account of intensive cultivation and deforestation. Chemical weathering is generally intense, penetrating to depths in excess of 20 m, particularly in areas that are underlain by greenstone rocks

METHODOLOGY More than 300 stream-sediment samples were collected from carefully selected sites which represents a sample density of about one sample per km2• From previous orientation surveys, The -100 mesh fraction regarded to be the most suitable, are selected for analysis. Statistical treatment of the analytical data and the construction of frequency distribution and probability plots assists in obtaining thresholds for the anomalous values.

MAPS SHOWING ELEMENTS DISTRIBUTION To facilitate assessment of the geochemical patterns maps are presented that show the distribution of elements of economic or exploration significance (Fig. 2 and 3). Elueze and Olade (1995) revealed that the only elements with distribution patterns that are related to known mineralization are As, Au and Ni.

Fig. 2 Distribution of As in stream-sediments in the Ilesha area (after Elueze and Olade, 1985)

RESULTS Arsenic values range from less than 10 to 97 ppm, those in excess of the regional threshold of 21 ppm being confined to areas around Itagunmodi and 3 km north, near Iyere (Fig. 2). In these areas the occurrence of sulphide-bearing auriferous quartz stringers and disseminations in amphibolite has been reported and the enhanced arsenic values may be related to these features.

Distribution of Au in stream-sediments in the Ilesha area (after Elueze and Olade, 1985)

RESULT CONT. Gold values in active stream sediments are generally low and erratic, only a few values exceeding 1 ppm (Fig. 3). The area around Itagunmodi, however, which contains the highest density of old gold workings, is characterized by Au values that generally exceed 0.1 ppm.

CONCLUSION Elueze and Olade (1985) concluded that the interpretation of stream-sediment reconnaissance geochemical data from the greenstone belt of the Ilesha area showed that the areal distribution of trace elements is subject to strong lithological and environmental controls.

  Outline of map showing favorable areas for follow-up exploration (after Elueze and Olade, 1985)

  (Eluze and Olade, 1985) Concluded that the rich gold ore Shoots are closely associated with abundant arsenopyrite veins. Eluze and Olade (1985) regarded the most promising area for follow-up exploration to be the area around Itagunmodi and extending northwards to Iyere (Fig. 3). This could be investigated further by a combination of detailed stream-sediment and soil survey.

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