Geology of the San Isidro iron ore deposit, Venezuela

Abstract

The iron ore deposits of Cuadrilatero Ferrifero de San Isidro represent the largest iron ore reserves in Venezuela. The district is a part of the iron metallogenic province of northern Guayana, one of the richest iron-bearing regions of the world. All presently known iron ore deposits of Venezuela are situated within this province: Cerro Bolivar, Altamira, Rondon, San Isidro, Maria Luisa, El Pao and others. Their total ore reserves amount to 2,000 million tons (disregarding the unenriched or slightly enriched iron-formation). The Imataca belt to which the iron ore deposits are confined consists of metamorphosed sedimentary and igneous rocks of Early Precambrian age, the oldest rocks presently known in South America. This belt extends some 450 km from the Orinoco delta southwesterly to the Cauro River. Iron ore is formed from banded iron-formation, a member of the Imataca complex, by removal of silica. The process of supergene enrichment is controlled to a certain degree by structural elements. There are five ore bodies in the San Isidro district, extremely varied in shape and size. Single bodies extend up to 3–4 km in length, approximately parallel to the regional structure pattern, and a few hundred meters in width. The morphology of the bottom of the ore bodies is rather irregular, particularly in transversal sections. Contacts between ore and the unaltered iron-formation beneath are gradational. Maximum vertical section through ore is 260 m; the average is 60 m approximately. The stratigraphic thickness of iron formation has been magnified by structural deformations. The primary stratigraphic thickness is estimated to be some 50–150 m. The iron ore is classified into two main types: a) hard, crustal ore, b) soft, friable ore. Hematite grains which remained after the leaching of silica, and goethite (as cement) are the two main constituents of crustal ore. Hematite and magnetite and a minor amount of quartz are almost the only constituents of friable ore. The crustal ore forms a 15–60 m thick mantle covering friable ore. The overall volume ratio between the friable and the crustal ore is about 2:1. However, it varies in different zones. The mean composition of iron ore on the basis of 10,800 chemical analyses is 64.41% Fe, 2.62% SiO2, 0.6% Al2O3. The ore contains a minor amount of Mn, P, Ti (no S, As, Ba). The ore reserves amount to 750 million tons; in addition, 180–300 million tons of possible ore reserves are estimated.