Hydrothermal dolomitization and porosity development: An example from Precambrian dolomitic rocks of Agua Clara Formation, Ribeira Belt, southern Brazil

摘要

Dolomitic rocks from the Rio Bonito Quarry, located in the Agua Clara Formation, record multistage dolomitization processes linked with porosity development in the southern Brazilian Shield. Dolomitic marble and breccia samples from this unit contain three textural types of dolomite that either are fabric destructive or void-filling phases. To study the role of dissolution in the porosity development in these dolomite types - from sedimentary to metamorphic domains - we combine petrographic methods, cathodoluminescence, Raman spectroscopy, micro-computed tomography, stable isotopes of carbon and oxygen, and fluid inclusion analysis. The protoliths of the dolomitic rocks of Rio Bonito Quarry are assumed as Mesoproterozoic (ca. 1.5 Ga) marls presenting shallow shelf to deep water lithofacies. The first dolomitization stage, forming a nonmimic replacement of the primary carbonate (Dol1) is fabric destructive and related to a burial setting (delta O-18: -12.42 to - 9.68 parts per thousand; delta C-13: -2.29-0.40 parts per thousand). The development of bedding-parallel stylolites of low amplitude led to increasing of porosity (up to 0.2%) and promoted the fluid circulation. During the Neoproterozoic Brasiliano tectonic events (ca. 650-550 Ma), the metamorphism of marls into dolomitic marble is intimately connected to the development of second replacement dolomite (Dol2). Penecontemporaneous brecciation is assumed as triggering factor to the fluid change, where Dol2 is akin to dolomites from MVT-deposits (delta O-18: -19.12 to -12.03 parts per thousand; delta C-13: -0.07 to -2.17 parts per thousand), and the total porosity is enhanced to 2%. The ubiquitous tectonic stress boosted the hydraulic fracturing, providing conditions to saddle dolomite (Dol3) precipitation, occurring as a replacement and void-filling phase. Lower isotopic values of Dol3 (delta O-18: -14.93 to -10.77 parts per thousand; delta C-13: -3.36 to -1.89 parts per thousand) suggest disequilibrium between the host-rock and the dolomitization fluid, while the porosity is augmented to 9%. The subsequent dedolomitization, generating void-filling calcite, is associated to meteoric phreatic input (delta O-18: -8.31 to -5.56 parts per thousand; delta C-13: -5.59 to -2.56 parts per thousand), similarly to the development of calcretes; therefore, diminishing the porosity towards 4%. Microthermometric data shows different signatures in the dolomitization fluid, which started at 100-150 degrees C and moderate-salinity (3-4 wt% eq.NaCl) in a burial setting, reaching up to 270 degrees C and low-salinity (0-2 wt% eq.NaCl) as a response to a shifting tectonic environment or fluid mixing. Additionally, the temperature shift between the host rock (120-130 degrees C) and the different dolomitization stages (from 140-150 degrees to 210-270 degrees C), above 10 degrees C offset, point to a hydrothermal setting.