Metalogênese do depósito aurífero Volta Grande, Domínio Bacajá (PA), Cráton Amazônico: aplicação de espectroscopia de infravermelho VNIR-SWIR.

Abstract

The world-class Volta Grande gold deposit contains measured reserves of ~6.0 Moz at 1.02 g/t, divided into north and south exploration blocks. It is inserted in the geological context of the Bacajá Domain and was affected by the Trans-Amazonian Cycle (2.26–1.95 Ga). Part of the mineralization is hosted in a group of gneisses and mylonitized granitoids in amphibolite facies of medium to high metamorphic grade of the Três Palmeiras Group (2.36 Ga). Recent research in the northern block has revealed the presence of late volcanics and plutonics, with isotropic texture and intermediate to felsic compositions, which host disseminated gold in different types and styles of hydrothermal alteration, as well as in quartz and carbonate (±sulfides) venules and veins. Thus, this Master's Thesis represents the continuity of research in the northern block of this repository with the application of the VNIR–SWIR (visible-near and short-wave infrared) infrared spectroscopy technique. This tool helps to explain in detail the configuration of the hydrothermal system, contributing to a better understanding of the genesis of the deposit. The mineralogy observed by spectroscopy in metamorphic rocks confirms the occurrence of potassic, propylitic, intermediate argillic, pervasive carbonate, and advanced argillic hydrothermal alterations types. The latter occurs associated with high levels of gold and alunite, a mineral indicative of epithermal systems with high-sulfidation. In turn, the isotropic volcanic and plutonic rocks present more developed, intense, and larger-volume hydrothermal alterations. They reveal greater diversification of hydrothermal minerals, where jarosite is the superior indicator of advanced clay alteration, which is also consistent with high-sulfidation epithermal mineralizations. In addition, the appearance of rhodochrosite, pyroxmangite, and galena, mainly related to volcanic rocks of andesitic and dacitic compositions, suggests an epithermal system of intermediate-sulfidation. The geological features present in the region and the hydrothermal alterations, especially the propylitic alteration in the rocks with allanite, clay minerals, montmorillonite, and zeolites, portray a typical epidote subzone of a low-temperature propylitic alteration that are genetically related to the medium-depth intrusions where they appear hydrated porphyry stocks. In this way, the Volta Grande gold deposit reveals characteristics compatible with rare and base metals porphyry and epithermal mineralizing systems, already identified in other regions of the Amazon Craton. The high-sulfidation conditions at the northwest portion of this repository and intermediate-sulfidation at the southeast region point to a transitional environment. The VNIR–SWIR spectroscopy method represents an important tool that identifies and characterizes hydrothermal minerals quickly and efficiently, as well as differentiating them from weathered ones. In general, it becomes a significant prospective guide when robustly analyzing minerals that are difficult to recognize by other methods such as conventional optical microscope or scanning electron microscope (SEM). The results presented here represent a remarkable contribution to the geological and metallogenetic knowledge of the Bacajá Domain, as well as the Amazonian Craton as a whole, pointing out the potential for identifying economically viable deposits of precious and base metals associated with volcanic and plutonic systems that occur in a vast area of this domain.