New insights on fluids from the deep hydrogeosphere of the Rio das Velhas Greenstone Belt, SE Brazil

Abstract:

The potential for the Precambrian crust to be an environment that can sustain life in isolation has fueled interest in understanding the processes operating in the deep subsurface. Crustal fluids in crystalline shields can remain isolated over extended timescales and are associated with abiotic production of methane and hydrogen gas via water-rock reactions, and locally with deep microbial ecosystems. We present the first data for fluids and gases sampled in two underground gold mines, Córrego do Sítio and Cuiabá, located in the Quadrilátero Ferrífero (Iron Quadrangle) region, Southeastern Brazil.

The two mines exploit gold mineralizations hosted in rocks of the Nova Lima Group in the Rio das Velhas Greenstone Belt (RVGB; age ~2.8 Ga). While the Cuiabá mine lies in a metavolcanosedimentary sequence at the base of the RVGB, the Córrego do Sítio mine is hosted in the overlying metaturbiditic sequence. These rocks differ from previously studied Precambrian Shield environments of the Canadian Shield, Kaapvaal Craton, and Fennoscandian Shield because they have been substantially affected by later, Paleoproterozoic and Neoproterozoic deformational events, providing the opportunity for understanding how tectonic deformation may impact fluid production, evolution, and storage in the crust.

Fluids in the two mines include gas phases rich in methane and higher hydrocarbons, with He and N₂ both present in variable proportions. Unlike fracture fluids studied at other Precambrian Shield sites, the waters are not highly saline, and likely represent fracture fluids potentially impacted by both the local aquifer and by percolation of mine service waters. Further work is underway to constrain the origin and evolution of fluids in these Brazilian mines and will contribute to knowledge of the habitability and hydrogeology of these environments and the geological processes that support deep life.