Paleoproterozoic gold deposits at Alta Floresta Mineral Province, Brazil: two overprinted mineralizing events?

Abstract

Large gold provinces commonly show complicated mineralization histories, and the Paleoproterozoic Alta Floresta, one of Brazil's most exciting Au-Cu mineral provinces, is a good example. The current models defined four deposit types, all connected to a single (1.88-1.75 Ga) magmatic-hydrothermal event. However, long Province history, diverse geodynamic environment, and older ages of Type-1mineralisation weaken the single metallogenic event and enable the hypothesis of overprinted mineral events. By scale-integrated analyses, we revise the tectonic-geological context, structural-hydrothermal alterations, and chlorite-white mica geothermobarometer and propose the type-1 as an older, granitoid-hosted orogenic mineralisation, with subsequent overprinting by the magmatic-hydrothermal event. The older orogenic gold event developed orogenic gold deposits on WNW-trending shear zones in the Peixoto de Azevedo domain granitic-gneiss rocks. Phengite, biotite, chlorite-carbonate phyllonites (3.3-6.1 kbar, 300º-420ºC) host fault-fill quartz veins (pyrite-chalcopyrite-magnetite-pyrrhotite-gold-Bi-Ag tellurides). Mg-rich chlorite-phengite is the main alteration footprint for this mineralisation type. A younger magmatic-hydrothermal event in the Juruena magmatic-arc rocks produced Fe-rich chlorite-white mica alteration zones (0.6-4.6 kbar, 120º-380ºC) and disseminated and stockwork-breccia ore (pyrite-chalcopyrite-gold-molybdenite- Ti minerals-allanite) in porphyry-epithermal deposits. Where the younger mineralisation overprints the older, phyllic alteration destroyed the phengite orogenic gold phyllonite Sn+1 foliation. The ages of two pyrite populations (1979 and 1841 Ma) in the older fault-fill veins and molybdenite in late fractures (1805-1782 Ma) or disseminated in the ca. 1.79 Ga syenogranite porphyry suggest more than two episodes of mineralisation. These two events differ in their alteration styles, P-T conditions, and structural, mineralogical, and textural ore styles. The multi-scale approach enlightens the relationships between the various mineralisation events, allowing a new explorational potential within the province.Supplementary material at https://doi.org/10.6084/m9.figshare.c.6056324

An introduction to Recent Advances in Understanding Gold Deposits: from Orogeny to Alluvium: the importance of multi-method approaches and developing a characterization

“…1). However, the dynamic nature of the Earth's crust, particularly within mobile belts, typically results in various deposit types spatially overlapping or overprinting each other (Liu et al 2021;Mesquita et al 2022). Such overlaps can make understanding an individual deposit or a metallogenic district very challenging: characterizing the process(es) that formed a primary (hard-rock) deposit, or the detailed geometries in a changing river system for an alluvial deposit, typically requires a range of techniques for even a relatively basic characterization.…”

“…That is: (1) incomplete genetic models and classifications for most gold deposits; (2) nonunequivocal fluid and metal sources; (3) incomplete understanding of fluid migration, focusing, trapping and precipitation mechanisms at all scales, including the causes for the nugget effect in gold-bearing veins; and (4) the precise role and importance of both magmatism and CO 2 -rich fluids. In addition, the signatures in many gold deposits are complex and likely result from overlapping deposit types, formed at different time during the geological history, and unravelling these requires a very detailed understanding of the deposit and the regional geology (Liu et al 2021;Mesquita et al 2022). Finally, the realization that gold solubility is too low to account for the very high grade gold deposits via the traditional models invoking gold transportation as sulfur or, in certain cases, chloride complexes in aqueous crustal hydrothermal fluids (e.g.…”

“…The first five papers use multiple and regionalscale datasets to understand the larger-scale context of gold mineralization, including two papers discussing problems with the currently widely accepted genetic models on orogenic gold deposits (Liu et al 2021;Mortensen et al 2022;Zhao et al 2021;Babedi et al 2022;Mesquita et al 2022). These five papers are examples of approaches that are commonly conducted by academic researchers to give useful background information feeding into the relatively early stages of exploration, i.e.…”

“…Another example of multiple, overlapping mineralization episodes and the usefulness of large datasets comes from Mesquita et al (2022). They investigate multiple gold deposits and occurrences within the poorly exposed and only partially mapped Paleoproterozoic Alta Floresta district in the Amazonas Craton, Brazil, using a combination of both published and new structural data, chlorite and white mica geothermobarometry, and alteration assemblages.…”

GEOLOGIA E PETROGRAFIA DO GARIMPO DE Au ± Cu DA RAIMUNDA, NA PROVÍNCIA MINERAL DE ALTA FLORESTA (MT)

“…As ocorrências e depósitos do grupo (2), por sua vez, ocorrem em veios de quartzo + sulfetos ± carbonato encaixados em falhas e zonas de cisalhamento, tanto no embasamento granítico quanto em algumas unidades graníticas cálcioalcalinas de 1,96-1,98 Ga (Souza et al, 2005;Mesquita et al, 2015). Nesse contexto, as ocorrências do grupo (1) seriam equivalentes a sistemas do tipo ouro pórfiro (Assis, 2011;Moura et al, 2006;Silva & Abram, 2008;Assis et al, 2014;Assis et al, 2017), enquanto aqueles pertencentes aos grupos (3) e (4), similares a sistemas epitermais do tipo lowa intermediate sulfidation (Assis, 2011;Trevisan, 2015). Entretanto, relativo descompasso tem ocorrido quanto ao modelo genético vinculado aos depósitos e ocorrências do grupo (2), visto exibirem características descritivas bastante complexas e distintas dos demais grupos.…”