Geology, mineralogy and geochemistry of the Cerro Matoso nickeliferous laterite, Córdoba, Colombia

Abstract

The Cerro Matoso nickeliferous laterite, located in northern Colombia near the town of Montelibano, was developed in pre-Late Cretaceous ultramafic rocks consisting principally of slightly serpentinized harzburgite. The peridotite body is flanked by ferruginous sandy sediments with interbedded coal beds of the Cienaga de Oro Formation (early Oligocene-early Miocene) and Recent alluvial gravels and sands. The harzburgite consists predominantly of olivine with lesser amounts of orthopyroxene and secondary serpentine. Intense serpentinization is primarily confined to areas of faulting and brecciation particularly along the western and eastern boundaries of the peridotite body. Development of the nickel laterite profile was greater in weakly serpentinized peridotite both in vertical extent and in the degree of enrichment of nickel-rich secondary products than in strongly serpentinized peridotite. The Cerro Matoso peridotite was affected by two major tectonic events. Compressional stresses associated with the late phases of the Pre-Andean orogeny in middle Eocene-late Eocene time brought the peridotite to the surface and also generated a major NE trending fault system in the peridotite body arid local serpentinization particularly along fault zones. Lateritization of the harzburgite probably began in late Eocene-early Oligocene time and chemical weathering and erosion favoured by a tropical humid and rainy climate with probable alternating wet-relatively dry seasons, continued throughout the Oligocene period. A major NW trending fault system developed in the peridotite body during the late Miocene-Pliocene Andean orogeny and the southwestern part of the weathered peridotite body was uplifted relative to the northeastern part. This uplift apparently was sufficient for intense erosion to remove most of the laterite profile from the uplifted block while the northeastern block was being only slightly modified. The laterite profile consists of an upper Canga zone underlain respectively by a Limonite zone and Upper and Lower Saprolite zones. The economic silicate-type nickel mineralization at Cerro Matoso is confined to the saprolite zones. Nickel content in parent peridotite ranges from 0.28 to 0.36%, and ore grade cutoff for materials comprising the laterite profile is 1.5%. Ore occurs as massive and fracture filling types, the massive ore constituting the most important part of the deposit economically. Smectite and serpentine are the dominant Ni-bearing minerals in the massive ore and are particularly abundant in the Upper Saprolite zone. Pimelite, nimite and Ni-bearing sepiolite fill fractures in both the Upper and Lower Saprolite zones. Nickel averages 3% in the massive ore and ranges to as much as 7% locally. Nickel grades in fracture fillings range to as much as 30%. Variations in pH, and to a lesser extent Eh, of the weathering solutions influenced the selective differential element accumulation and mineral formation throughout the laterite profile. Oxidizing and slightly acidic conditions apparently were present in the Limonite and Canga zones and led to precipitation of Fe as goethite. Slight increase in pH in the Limonite zone as compared to the Canga zone favored Mn precipitation at the lower part of the Limonite zone as Mn oxide. Ni was highly mobile in the Canga and Limonite zones and alkaline conditions present in the saprolite zones promoted its deposition in garnierite. Co was less mobile than Ni and the zone of maximum Co concentration formed near the base of the Limonite zone. Moderate mobility of silica occurred in upper profile levels but mobility decreased rapidly in the Upper Saprolite zone apparently due in part to the presence of higher amounts of dissolved Mg in the weathering solutions. Cr mobility was very low in this weathering environment. Alumina behaved as a highly immobile component and concentrated primarily in the near surface part of the Canga zone.