Banca de DEFESA: DANIEL AUGUSTO DE MIRANDA

Uma banca de DEFESA de MESTRADO foi cadastrada pelo programa.
DISCENTE : DANIEL AUGUSTO DE MIRANDA
DATA : 09/03/2020
HORA: 09:00
LOCAL: Instituto de Geociências - Auditório B
TÍTULO:

A MINERAL SYSTEM APPROACH ON THE STRUCTURALLY-CONTROLLED AU-BEARING QUARTZ VEINS OF SERRA DE JACOBINA, SÃO FRANCISCO CRATON

PALAVRAS-CHAVES:

Contendas-Mirante-Jacobina Lineament. Fluid inclusion. Chlorite geothermometry. Orogenic gold. Paleoproterozoic.

PÁGINAS: 92
GRANDE ÁREA: Ciências Exatas e da Terra
ÁREA: Geociências
RESUMO:

The Serra de Jacobina is located northeast of Bahia state, Brazil. It consists of a 250-km-long, N-S mountain chain at the eastern border of the Gavião-Lençóis paleoplate and corresponds to the Contendas-Mirante-Jacobina Lineament northern portion. There are several structurally controlled Au-bearing quartz veins hosted by metasedimentary rocks of the siliciclastic Jacobina Group and Vale do Coxo metaultramafic rocks exploited by artisanal miners. Four of these occurrences, named Maravilha, Jaqueira, Morro da Palmeirinha and Mina Velha were mapped. They are hosted by two fault systems of Paleoproterozoic age, named from east to west, Pindobaçu and Maravilha. The Pindobaçu fault system is a transcrustal structure and represents the contact between the Gavião-Lençóis and Mairi paleoplates. The Maravilha fault system is a shallower structure limited to the upper crust. Mineralized veins are hosted by second order structures related to these two fault systems. The hydrothermal alteration varies according to the host rock. Disseminated sericitic alteration is widespread within Jacobina Group quartzites and conglomerates, probably replacing clay minerals. At Vale do Coxo schists, the alteration is pervasive, with chlorite replacing biotite, and of the fracture-filling fracture type as quartz veins. Sulfidation is subordinated to the sericitic alteration, both at quartzites and schists from the Jacobina Group and Vale do Coxo, respectively. Gold occurs like isolated crystals within quartz veins or subordinated to sericitic and/or sulfidation assemblages at host rocks. Petrography and fluid inclusion microthermometry revealed three main types, with subtypes, of fluids (density and salinity, expressed as g/cm³ and wt. % NaCl eq. respectively): (i) Type WC, two-phase H₂O+CO₂+NaCl fluid occurs at Maravilha (bulk density 0.7-1.0 , salinity 0.6-11.1) and Jaqueira (bulk density 0.8-1.0, salinity 2.0-14.2); (ii) Type W, two-phase H₂O +NaCl or three-phase H₂O +NaCl+solid fluid, subtypes We (density 0.6-0.9, salinity 0.8-17.9) and Wh (density 1.1-1.2, salinity 30.9-35.7) at Jaqueira, subtype Wmp (density 0.6-0.9, salinity 0.8-17.9) at Morro da Palmeirinha, vapor rich subtype Wmv (density 0.6-0.9, salinity 13.4-21.1) at Mina Velha, and subtype Wl (density 0.8-1.1, salinity 0.2-20.9) which occurs at Jaqueira, Morro da Palmeirinha and Mina Velha; (iii) Type C, dark color, one- or two-phase CO₂-CH₄ and/or N₂ fluid (Tm CO₂ -57.0 to -58.2°C, Th CO₂ 4.4 to 23.2°C) which occurs at Maravilha and Morro da Palmeirinha. At Jaqueira, combined microthermometric results with chlorite geothermometry, reveals that the hydrothermal alteration and gold deposition occurs with P-T conditions of 1.6-2.0 kbar and 303-346°C. The main mechanisms of gold deposition were fluid immiscibility and fluid-rock interaction. The hydrothermal mineralization at Serra de Jacobina is interpreted as an orogenic mineral system and the following elements were characterized: (1) The production of orogenic fluids with variable composition; (2) the fault systems in a compression-driven fluid flow type are conduits and driver; (3) the inductors of gold precipitation were fluid immiscibility and fluid-rock interaction.

MEMBROS DA BANCA:
Presidente - 003.403.165-00 - AROLDO MISI - UFBA
Interno - 268656 - LUIZ CESAR CORREA GOMES
Externo à Instituição - CARLOS DINGUES MARQUES DE SÁ - UFS