Banca de DEFESA: Gabriel Angelo Barbosa Silva

Uma banca de DEFESA de MESTRADO foi cadastrada pelo programa.
STUDENT : Gabriel Angelo Barbosa Silva
DATE: 31/08/2023
TIME: 14:00
LOCAL: Sala de Videoconferência do IG e Plataforma Teams
TITLE:

Characterization and controls of gold mineralization associated with the Aurumina Granite Suite and Ticunzal formation, in the northeast of the State of Goiás, Brazil

KEY WORDS:

Aurumina Suite; Ticunzal Formation; gold; intrusion-related; peraluminous granite; graphite.

PAGES: 66
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUMMARY:

Various gold occurrences have been sporadically exploited since the XVIII century in the northeast portion of the Goias State, central Brazil, as small-scale and artisanal mines (“garimpos”). Study of the garimpos named Novo Horizonte, Morro dos Borges and Tucano is the main focus of this work. The region is located in the northern part of the Brasilia Fold Belt, in its exposed Paleoproterozoic basement rocks. The gold mineralizations are related to Paleoproterozoic peraluminous, reduced syn-tectonic granites of the Aurumina Suite, and metasedimentary country rocks of the Ticunzal Formation. The ore is hosted in shear zones, with directions varying from NE-SW to E-W, near the contact between biotite-muscovite monzogranite and graphite-bearing schist, in muscovite-quartz mylonite derived from granite shearing. Structural and textural elements indicate that mineralization is coeval with ductile shearing and is controlled by S-C fabric, with orebodies commonly having sigmoidal morphology with dextral rotation kinematic indicators and oblique mineral stretching lineation. Hydrothermal alteration resulted in intense silicification in narrow zones near the contact between the two lithologies, as well as the crystallization of gold and associated hydrothermal muscovite, chlorite, and sulfides. Ore sulfides include arsenopyrite, pyrite, chalcopyrite, galena, sphalerite, and pyrrhotite. Gold occurs as free grains along fractures in quartz veins, as elongated grains between muscovite lamellae, and as fine inclusions in sulfides. Granite samples in whole-rock chemical analysis have high SiO2 and Al2O3 contents, while Fe2O3 and MgO are greater in the schists. Au concentrations obtained from ore samples are up to 6,017 ppb in silicified mylonite from Morro dos Borges, and up to 16,273 ppb in a quartz vein from Novo Horizonte. Biotite-muscovite granite samples from Novo Horizonte have up to 24.2 ppb Pd and 14.4 ppb Pt. Mylonite and granite samples from the deposits show similar REE fractionation patterns, which are less fractionated than those of regional Aurumina granite samples. EPMA results of goldassociated arsenopyrite and chalcocite from the Novo Horizonte deposit indicates variable enrichments in Au, Ni, Co, Se, Bi, Sb, and Te, and temperatures estimated using arsenopyrite geothermometry yield crystallization temperatures of 390–535°C, interpreted as the crystallisation temperature interval for gold. In situ LA-ICP-MS spot analysis yelded ranges of δ 34S ratios for arsenopyrite of 5.62 to -1.76‰ and between +0.53 and +9.98‰ for pyrite, from a mineralized vein sample of the Novo Horizonte deposit, and together with values from bulk dissolved sulfide analysis performed on sphalerite (-4.7‰), pyrrhotite (-5.1‰) and pyrite (-6.4‰) from the Aurumina mine, suggest a metasedimentary component as the source, or one of the different sources of sulfur. Previous works define these deposits as orogenic gold deposits. However, despite having similarities with orogenic gold deposits, the mineralizations have some essential features that are also consistent with reduced intrusion-related gold deposits. Such features include its cordilleran hinterland tectonic environment, a regional association with tin and tungsten mineralizations, a strong association between the ore and the granitic pluton, with structures active during emplacement, cooling, and gold precipitation, high formation temperatures, and absence of mineralizations hosted in schist without a granite intrusion present. A viable model for the formation of the gold deposits involves a magmatic-hydrothermal process related to fluid exsolution from granitic bodies during shear zone-controlled emplacement, in a RIRG system forming intrusion-hosted type mineralizations.

COMMITTEE MEMBERS:
Interno - 418965 - CLAUDINEI GOUVEIA DE OLIVEIRA
Externo à Instituição - CLOVIS VAZ PARENTE - UFC
Presidente - 6404885 - NILSON FRANCISQUINI BOTELHO
Interno - 1057547 - VALMIR DA SILVA SOUZA