Contribuição ao estudo da gênese de minério de ferro na mina N4E, Carajás

Abstract

The nine plateaus that form the Serra Norte of Carajás make up one of the largest accumulations of iron ore in the world, being the jaspilite of the Carajás Formation protore generator all this deposit. Numerous studies have dealt the petrography of jaspilite, highlighting all its primary sedimentary and diagenetic features. However, few studies use petrography to discuss the origin of iron ore. Among the areas of thought that discuss the ore forming processes include the formation of supergene alteration and hypogenic alteration, despite they come from distinct origins they assume that the dissolution of silica was an agent that acted during the formation of the deposit. Thus, samples collected in the N4E Mine, specifically in protore/ore interface were studied with special attention to the dissolution cavities that were present. Based on the degree progression of the alteration of the rock, the following categories were individualized : Jaspilite not Changed, Jaspilite Little bit Changed, Jaspilite Changed and Ore (Compact and Friable). The dissolution features show themselves preferably rounded lenticular according to the rock layering and restricted to the siliceous bands and silica levels in the opaque band. The size of these features increases significantly with increasing degree of alteration of the rock. It was found that the chert is characterized as the first mineral to be dissolved, followed by granular quartz and jasper, respectively. The minerals neoformation that has been identified by quartz and hematite recrystallization do not certify the entry of new elements in the system, indicating only rebalancing level of the mineral . The majority part of the hematite in the friable ore is essentially the same as Jaspilite not Changed, with few signs of subsequent crystallization. Therefore, it is observed that the passage solution during formation of the ore basically do not change the hematite to that depth level. Thus, added to the lack of minerals accessory of high temperature indicative of hypogenic origin, the characteristics make supergene origin more suitable for the generation of iron ore in Mina N4E. The friable ore would then be the result of intermittent periods of greater intensity supergene attack from since the maxim of 70 Ma until today. The alteration individualized typical beds silica leaching and allowed the partial mobilization of iron oxides under the action of descending meteoric solutions and oscillation of the underground water table.