A associação geoquímica Au-As-B-W-Cu-(Sn) em solos, colúvios, crosta laterítica e gossans no alvo Águas Claras-Carajás

Abstract

The studied area called Alvo Águas Claras, is located in the Mineral Province of Carajás, southwest of the State of Pará, which represents one of the largest mineral provinces in the world. It is an important area for gold research, which has been prospected since 1991 by DOCEGEO, a subsidiary of Companhia Vale do Rio Doce – CVRD. This target is located in a region of dense rainforest cover, whose geological profiles are highly weathered, showing similarities with the gold deposits described in several other regions of West Africa and Australia. In the Amazon region, the older lateritic profiles are undergoing intense alteration since the end of their formation in the Lower Tertiary, with consequent distribution of these profiles and formation of soils or colluviums. The objective of this work is the detailed study of the behavior of gold, on the surface of this truncated lateritic terrain, and its relationship with trace elements such as B, W, Sn and Cu, in latosols, colluviums and outcropping laterite-gossanic crusts in the area. Thus, a detailed geochemical and mineralogical study of this material was developed, in order to assist in the identification of geochemical signatures indicative of the primary nature of mineralizations and associated rocks, in addition to the evaluation of the dispersion and mobility of these elements during the transformations of the lateritic and gossans crusts. in latosols and colluviums, to establish criteria for geochemical prospecting of these bodies in deeply weathered regions. The work methodology consisted of a field phase, developed on the mineralized bodies denominated by DOCEGEO as Corpo da Anomalia Au/As and Corpo do F-23, with the objective of geological reconnaissance of the area, description of the units and collection of samples of surface. The samples collected were mostly oxisols, colluviums, lateritic crusts, gossans, in addition to host rocks (sandstones and siltstones) and quartz veins. The analytical treatment consisted of several phases such as: drying, quartering and separation of 200g aliquots for pulverizing and 100g for separating heavy and insoluble minerals from pre-selected samples, through the highest levels of boron and gold. Petrographic analyzes were performed using an optical microscope with reflected and transmitted light. All samples were submitted to X-ray diffraction analysis, where both the total sample composition and the different domains of complex samples and isolated minerals were analyzed. A detailed mineralogical study was carried out on tourmalines, involving the calculation and refinement of the unit cell parameters. Quantitative analyzes of the minerals, as well as detailed photographs, were carried out by Scanning Electron Microscopy, with Dispersive Energy System, involving gold particles and other associated heavy minerals. In the geochemical analyses, the following major elements were analyzed: Fe2O3, TiO2, P2O5 through colorimetry; SiO2 and loss on ignition (P.F.) by gravimetry; Al2O3 by titrimetry; Na, Mn, K, Mg by atomic absorption, and some trace elements including Au. In the statistical treatment of data, correlation studies were mainly used, based on Parson's correlation coefficient (r), through binary correlation diagrams, correlation matrices and dendrograms of cluster analysis in R-mode. Isovalue maps were made for Au, B, W, and As, in order to better visualize the surface dispersion of these elements. The geology of the area is characterized by different outcropping and sub-outcropping supergenic materials, which can be juxtaposed in the form of the following composite profile, from top to bottom: Soil horizon (latosol), Colluvium horizon, Lateritic crust and Gossans, the latter being the main gold mineralized bodies prospected in the Águas Claras area. The mineralogy of the entire profile is basically represented by quartz, kaolinite and iron oxides-hydroxides, in different proportions. Accessory minerals such as tourmaline, wolframite, cassiterite and muscovites are frequently found, occurring in varying amounts, practically in all samples. The mineralogical composition found in the various materials analyzed is quite simple in terms of the variety of minerals, but the proportions of the mineralogical phases vary greatly, even within samples from the same horizon. The minerals that represent the greatest distribution among the samples are quartz and hematite. The first occurs abundantly in the oxisol, decreasing significantly in the lateritic crust samples with almost total disappearance in the gossans samples, while the behavior of the hematite is the opposite. The geochemical associations obtained through the multi-elementary analyses: Oxisols: 1) As – Cu – B – Au – W - (Mn) - (Pb); 2) Cr – V – Ga – Mo – Ni – Zn; Colluvium: 1) As – Y – Cu – Mn – Pb – Mo; 2) Au – W – B – F; Lateritic crust: 1) As – W – Au – B – F – Sn; 2) Cu – Ni – Pb – Zn – Sc. The most characteristic geochemical association in all horizons is that represented by Au-B-W-As, possibly with Sn-Cu-F, which must reflect the geochemical signature of the Au-sulfide primary mineralizations, in addition to the influence of the host sediments in addition to the granitic intrusions. Other associations or pairs of correlations involving Cr-V-Ga-Mo-Zn are related to the ease of these elements being incorporated into the structure of iron oxides and hydroxides, abundant in all studied horizons. The gold particles found in the various horizons, have quite varied morphology and purity, which may indicate the presence of several stages of mobilization of this metal in the supergenic environment. As for the trace elements, there are strong anomalies of W, Sn and B, with the minerals responsible for the high levels, respectively, wolframite, cassiterite and tourmaline, the latter belonging to the compositional field of dravita-schorlite, rich in iron , getting very close to the field of ferridravites. Within the above, it can be said that despite the mineralogical simplicity, the concentration of trace elements is quite heterogeneous, but with characteristic signatures that remain in different horizons and that allow delineating the mineralized supergenic bodies and inferring the composition of the primary bodies. The understanding of the dispersion and the characterization of these geochemical signatures prove to be of great importance in geochemical prospecting, in the exploration of other bodies of similar nature in the region.