Geology and origin of the hematite ores of the Nimba Range, Liberia

Abstract

The Main Orebody of the Nimba Range, northeast Liberia, contains 125 to 150 million tons of high-grade hematite ore in a single deposit approximately 250 to 300 m thick, 800 m long, and to a maximum explored depth of 670 m below the premining surface. The orebody consists almost entirely of anhedral hematite and martite with quartz as the only significant impurity. The ore is formed by alteration of gray itabirite (magnetite + quartz) of the Precambrian age Nimba Itabirite. The Main Orebody is enveloped (except along a southeast phyllite contact) by a 1,300-m-long lens of blue itabirite consisting of hematite and quartz which was formed by local oxidation of the gray itabirite.Medium-hard and hard ores are distinguished from soft ores on the basis of mineralogical, structural, and physical characteristics. Medium-hard ores consisting of compact but uncemented anhedral hematite and hard ores consisting of compact cemented anhedral hematite are of synmetamorphic origin. The distribution of these ores, which increase in proportion to soft ores with increasing depth, demonstrates that hard ores are neither physically bound to a weathered surface nor of supergene origin. Synmetamorphic alteration of gray itabirite oxidized and recrystallized euhedral and subhedral magnetite to anhedral hematite in blue itabirite and medium-hard and hard ores and leached silica from itabirite to form medium-hard and hard ores. A complementary effect was the alteration of phyllite adjacent to the Main Orebody, resulting in silicification at the expense of iron and magnesia, and sericitization. Despite concomitant recrystallization of phyllite, schistosity is retained. Foliation is also retained in hard and mediumhard ores.An extensive survey and analysis of joints and minor folds in the ore and waste rock in and adjacent to the Main Orebody pit indicates that these structures may have resulted from the removal of large quantities of silica from the Main Orebody under conditions of moderate metamorphic temperature and pressure. Structural control of localization of mineralization is implied.Leaching of silica from itabirite during greenschist facies regional metamorphism resulted in gentle infolding of the phyllite formations and the lateral collapse of the itabirite, resulting in a multiplicity of cross joints. A dual source of solutions (meteoric and connate) is proposed. The peculiar Archean crust condition, namely, a relatively thin crust with abrupt temperature gradients, is thought to contribute to introduction of meteoric solutions and to localize both meteoric and connate solutions.Soft ores occurring in the central part of the orebody consist of martite with some quartz, and exhibit slump folding. These ores are thought to have formed from blue itabirites through leaching of silica by postmetamorphic meteoric waters. However, the residual soft martite ores are in distinct contrast to the goethite-martite brown ores elsewhere along the Nimba Range, which resulted from supergene leaching of silica from itabirites. Whereas brown ores form close to the surface where Eh and pH variations are significant, it is suggested that the soft ores were formed by meteoric waters that descended through the adjacent porous medium-hard and hard ores and leached silica at levels below which Eh and pH variations are significant.