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Geofísica Internacional ISSN: 0016-7169 [email protected] Universidad Nacional Autónoma de México México Bakkali, Saad A resistivity survey of phosphate deposits containing hardpan pockets in Oulad Abdoun, Morocco Geofísica Internacional, vol. 45, núm. 1, january, 2006, pp. 73-82 Universidad Nacional Autónoma de México Distrito Federal, México Available in: http://www.redalyc.org/articulo.oa?id=56845105 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative Geofísica Internacional (2006), Vol. 45, Num. 1, pp. 73-82 A resistivity survey of phosphate deposits containing hardpan pockets in Oulad Abdoun, Morocco Saad Bakkali Earth Sciences Department, Faculty of Sciences and Techniques, Abdelmalek Essaadi University, Tangier, Morocco Received: March 29, 2005; accepted: August 11, 2005 RESUMEN En la gran cuenca de Oulad Abdoun en Marruecos se inició la explotación de un nuevo depósito de fosfatos para reemplazar al de Grand Daoui que se encuentra agotado. Existen inclusiones estériles de caliche que dificultan la extracción de las rocas fosfatadas y que son difíciles de detectar. La resistividad de los caliches excede un valor de 200 Ω-m contra 80 a 150 Ω-m para la roca fosfatada. Se llevó a cabo un proyecto de prospección eléctrica con un equipo Schlumberger sobre una extensión de 50 hectáreas. Se obtuvieron modelos del perfil geológico mediante análisis de Fourier, y se logró localizar las inclusiones de caliche y cubicar las reservas de fosfatos de manera más confiable. PALABRAS CLAVE: Prospección geofísica, resistividad, fosfatos, análisis de Fourier, filtrado, Marruecos. ABSTRACT In the great Oulad Abdoun Basin exploitation of a new phosphate deposit was begun after the Grand Daoui horizon was exhausted. Inclusions of sterile hardpan—so-called “disturbances” —are hard to detect and interfere with phosphate extraction. Their resistivity is above 200 Ω-m as against 80 to 150 Ω-m for the phosphate-rich mineral. A Schlumberger resistivity survey over an area of 50 hectares was carried out. Models of the geology were successfully obtained from Fourier analysis. A new field procedure was tested to deal with the presence of disturbances. Estimates of phosphate reserves were improved and better con- strained. KEY WORDS: Geophysical surveys, resistivity, phosphate, Fourier analysis, filtering, Morocco. INTRODUCTION phate rock interface produce an important resistivity con- trast. Other factors such as changes in lithofacies and clay Morocco is a major producer of phosphate, with an content and consistency appear to account for some addi- annual output of 19 million tons and reserves in excess of 35 tional resistivity difference. It was found that normal phos- billion cubic meters. This represents more than 75% of world phate-bearing rock has a resistivity of 80 to 150 Ω-m while reserves. Resistivity surveys have been successfully used in the hardpan typically features resistivity values of between the Oulad Abdoun phosphate basin in Khouribga Province 200 and 1000 Ω-m. (Figure 1), which is about 120 km south of Casablanca. The present survey was carried out in the Sidi Chennane deposit A pilot resistivity survey was performed over an area which is a part of Oulad Abdoun basin, extending over some of 50 hectares. The purpose of this experiment was to try 800 000 hectares. and map and constrain the anomalous regions correspond- ing to hardpan. A resistivity map was expected to allow the The Sidi Chennane deposit is sedimentary and contains electrical resistivity signals to be imaged in 3D. We used a several distinct phosphate bearing layers. These layers are Schlumberger array with a span of 120 m designed to reach found in contact with alternating layers of calcareous and a depth of 40 m. The so-called disturbances appear at ran- argillaceous hardpan. In this field, extraction was begun af- dom so that the apparent resistivity map may be used by the ter Grand Daoui deposit was exhausted. However, the new operating personnel as a kind of radar to plan the sequence deposit contains many inclusions or lenses of extremely tough of field operations. We use traditional Fourier analysis to hardpan locally known as dérangements or disturbances, determine the amplitudes of the effects of the random distur- found throughout the phosphate bearing sequence. The hard- bances, or any other resistivity anomalies that may be present. pan pockets are normally detected only at the time of drill- ing. They interfere with field operations and introduce a se- OVERVIEW OF THE AREA OF STUDY vere bias in the estimates of phosphate reserves. The Sidi Chennane phosphate deposit is within the Direct exploration methods such as well logging or Oulad Abdoun basin about 33 km south east of Khouribga surface geology are not particularly effective. However, the and 24 km SSW of Oued Zem (Figure 2). Its boundaries are: chemical changes which are detectable at the hardpan/phos- West, meridian 372500 (Lambert), South, meridian 228000 73 S. Bakkali Fig. 1. Location of the study area. (Lambert), East, highway RP22, and North, the outcrops of sedimentary cover is fairly thick (Michard, 1975). The up- the basement of the phosphate-rock sequence. The climate permost formations of the Maestrichtian and Eocene contain of the phosphate plateau is essentially arid. Rainfall is from the phosphate-bearing strata which are 30 to 50 m thick. The November to May and is usually below 400 mm. Vegetation earlier deposits, i.e. the lower 5 to 28 m, are clayey phos- is of sparse dwarf palm trees. Rural population subsists on phates of Maestrichtian age. The upper 20 to 30 m are less cattle ranching and seasonal agriculture in small villages, or homogeneous. They are layered phosphate marls and sand- douars. Ground water is increasingly scarce. Scattered wells stones with some limestones of Eocene age. depend on an aquifer in the Turonian limestones at depths of 100 m or more, which is sealed by the Senonian marls. This Below the phosphate-bearing strata one finds up to 70 aquifier is also the sole water supply for the various mining m of Senonian marls and limestone marls; 20 to 60 m of operations. Turonian limestones; a Cenomanian formation of alternat- ing gypsum marls and limestone marls; and finally 10 to 60 GEOLOGY m of red marls and mudstones of pre-Cenomanian age. The phosphate mineral was deposited over a long time The disturbances may be differentiated by size of the window from Maestrichtian (late Cretaceous, about 80 ma), pocket or inclusion, type of material, hardness, clay content, to Lutetian (early Eocene, 40 ma). However, deposition was or type of contact with the phosphate rock. Two main types irregular. Some layers are missing. Oulad Abdoun Basin oc- of disturbances are found. The first type is found throughout cupies most of the phosphate plateau which is bounded to- the mineral deposit: it appears to be a random mixture of ward the north by red outcrops of pre-Cenomanian sediments limestones, marls, clays, cherts and low-grade phosphate with forming anÄextension of the south edge of the Central Mas- large amounts of cherty limestone. The second type is highly sif. The Western boundary is the Rhamna Range, the Beni disturbed and lacks any dominant facies. It appears as an Amir plain is to the South and the Upper Atlas of Beni Mellal accumulation of low-grade phosphate limestone blocks with extends to the East. The geology of the study area is well large nodules of chert, marl, some fragments of chert and understood (see Figure 3 for stratigraphy). phosphate rock. The latter type forms inclusions of 10 to more than 150 m and is the most abundant during mining The geologic section rests unconformably on Paleozoic operations (Figure 4). These pockets are found both in the schists and quartzites. The basement is well located and the underlying formation and in the upper members of the phos- 74 Resistivity survey in Moroccan phosphate deposits Fig. 2. Main phosphate basins in Morocco. Fig. 3. General outline of the stratigraphy. 75 S. Bakkali Fig. 4. Adverse effects of « disturbances » on mining operations Fig. 5. A resistivity traverse over three « disturbances ». phate sequence, and as a result there are strong resistivity by geophysicists and engineering geologists and the proce- contrasts between the disturbances and the normal phosphate- dures are well established. The study area was selected for bearing rock. These contrasts were confirmed in the test runs its representativity and the resistivity profiles were designed (Figure 5). Geophysical prospection could thus be based on to contain both disturbed and enriched areas (Kchikach and prior evidence from field data. Hiyane, 1991). The sections were also calibrated by using vertical electrical soundings. FIELD PROCEDURES High values of apparent resistivity were encountered Resistivity is an excellent parameter and marker for dis- due to the presence of near vertical faulting between areas of tinguishing between different types and degree of alteration contrasting resistivity, and fault zones which may contain of rocks. Resistivity surveys have long been successfully used more or less highly conducting fault gouge. The gouge may 76 Resistivity survey in Moroccan phosphate deposits contain gravel pockets or alluvial material in a clay matrix The lateral inhomogeneities of the ground can be in- (Kchikach et al., 2002). Such anomalous sections are also vestigated by means of the apparent resistivity obtained from classified as disturbances. Apparent resistivity values in these the survey. As the surface extension of the layers is displayed profiles locally exceeded 200 Ω-m. we may infer the presence or absence of any disturbances as well as any facies variations. Our resistivity measurements In order to locate and define the anomalous areas or were performed by means of a Syscal2 resistivity meter by disturbances, an electric current of intensity I was passed BRGM Instruments using a rectangular array of 20 m x 5 m.

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