Structural control and hydrothermal evolution model of unusual, high-grade metasandstone-hosted iron deposits, mesoproterozoic eastern chapada diamantina, Brazil

Abstract

The Ibicoara high-grade, hypogene hematite deposits are located in the São Francisco Craton, in the Chapada Diamantina Fold Thrust belt. Iron mineralization host rocks are quartz and arkose sandstones from the Tombador Formation, which present planar stratification. Regionally, three deformational phases are identified. The first D1 phase developed kink with subordinate chevron folds, shear zones related to flexural slip with S/C structures, stretch lines (LX1), slickenlines and slickensides. The second phase D2 is represented by dextral and sinistral, reverse high-angle shear zones with NNW-SSE and NW-SE orientation trends, respectively, high-angle tension fractures with an ENE-WSW orientation. During the third deformation phase D3, dextral and sinistral shear zones with ENE-WSW and NE-SW orientations are developed. The richest hematite domains are massive, having Fe2O3total contents to 98%. These irregular-massive domains are distributed along the primary sedimentary foliation (plane-parallel stratification and tangential cross-bedding), of D2 shear zones, or tension fractures generated during D2. The irregular-massive hematite domains grade laterally into platy and stringer domains. In addition, there are vein, stockwork and breccia mineralization styles that crosscut irregular-massive domains or host rock. Chemical analyses of hematite in irregular-massive domains show that Fe2O3 correlates negatively with SiO2, Zr, Th, Sr, Nb, Hf and Ba. On the other hand, it correlates positively with Zn, V, U, Cu, Co, and with the sum of rare earth elements (REE). When normalized to the post-Archean Australian Shale (PAAS), the distribution of REE generally show negative Eu anomalies, and a weak enrichment of light rare earth elements (LREE) compared to the heavy rare earth elements (HREE). The paragenetic succession shows the existence of three groups of hematite. Groups 1 and 2 are mainly related to the irregular-massive domains controlled by rock stratification and the S0//S2 foliation in shear zones. The distinction between them is given by the finer grain size of the first generation of hematite and the presence of breccia domain in the second group. The third group is related to massive-vein domains. For each group, the hematite grain size increases successively, and there is a relative increase in the HREE in relation to the LREE. Furthermore, similarities between the chemical signatures of the first groups 1 and 2 hematite generation with the signatures of the host rock reveal an important influence of the chemical composition of the original rock. This is not the case for the late hematite of massive domains, as well as vein-style hematite, suggesting high fluid-rock conditions. The hematite hydrothermal alteration at Ibicoara has structural control, with dilational structures associated with D2 shear zones. The host lithotype was also a contributing factor due to its porosity and permeability, and the presence of sedimentary structures that favored flow of the hydrothermal fluid from shear zones. Replacement and direct precipitation took place and are responsible for the transfer of solutions and formation of hematite-rich areas. The hydrothermal fluid probably leached components from the Espinhaço Supergroup metasedimentary units and dikes that are regionally present, and carried these during the deformational phases that inverted the Paramirim Aulacogen. The Espinhaço Supergroup is characterized by a sequence of siliciclastic rocks, predominantly metasandstones, and felsic metavolcanic rocks, sterile in iron, which were deposited between the Statherian and Stenian periods. In the Chapada Diamantina area, the Espinhaço Supergroup is comprised from base to top by the Serra da Gameleira Formation and the Rio dos Remédios, Paraguaçu, and Chapada Diamantina groups. The Chapada Diamantina Group contains the Tombador Formation that hosts the high-grade iron deposits investigated in the present study. In light of the tectonic context, and existence of shear-related gold and baryte hydrothermal deposits nucleated during the Paramirim Aulacogen inversion, the Chapada Diamantina Thrust and Fold Belt is considered a potential regional exploration target.