Mineralization formed during local extensional regime before or during basin inversion
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High-angle of the controlling fault; High-angle of mineralized veins; High-angle extensional mineralized veins with no shear; Normal faults are efficient conduits for hydrothermal flux (Sibson 2000Sibson R.H. 2000. Fluid involvement in normal faulting. Journal of Geodynamics, 29(3-5):469-499. https://doi.org/10.1016/S0264-3707(99)00042-3 https://doi.org/10.1016/S0264-3707(99)00...
) and in many cases form barriers for hydrocarbon accumulations (Hardman and Booth 1991Hardman R.F.P., Booth J.E. 1991. The significance of normal faults in the exploration and production of hydrocarbons. In: Roberts A.M., Yielding G., Freeman B. (eds.). The Geometry of Normal Faults, 56. London: Geological Society, p. 1-13.); Disposition of mineralized breccias into two segments divided by an allochthonous phyllite. Emplacement of Serra do Garrote Formation rocks between carbonate rocks of Morro do Calcário occurred after main mineralization process, once the latter are deformed by the former. |
Which major regional extensional event was responsible for mineralization prior to complete inversion? Layers in both fault blocks show minimal thickness alteration when considering their association with normal faults related to Vazante Group deposition. Therefore, it appears that the extension occurred after the deposition of the Vazante Group. However, a significant event capable of causing such extension before the completion of basin inversion is the synorogenic formation of the foreland basin, which hosts the Bambuí Group sediments (Brito Neves et al. 1996Brito Neves B.B., Winge M., Carneiro M.A. 1996. Orogêneses precedendo e Tafrogêneses sucedendo Rodínia na América do Sul. Boletim IG-USP.Série Científica, 27:1-40. https://doi.org/10.11606/issn.2316-8986.v27i0p1-40 https://doi.org/10.11606/issn.2316-8986....
, Almeida et al. 2000Almeida F.F.M., Brito Neves B.B., Carneiro C.D.R. 2000. The origin and evolution of the South American Platform. Earth-Science Reviews, 50(1-2):77-111. https://doi.org/10.1016/S0012-8252(99)00072-0 https://doi.org/10.1016/S0012-8252(99)00...
, Alkmim 2004Alkmim F.F. 2004. O que faz de um craton um cráton? O Cráton do São Francisco e as revelações almeidianas ao delimitá-lo. In: Mantesso-Neto V., Bartorelli A., Carneiro C.D.R., Brito-Neves B.B. (eds.). Geologia do Continente Sul Americano: Evolução da obra de Fernando Flávio Marques de Almeida. São Paulo: Beca, p. 17-35., Reis and Alkmim 2015Reis H.L.S., Alkmim F.F. 2015. Anatomy of a basin-controlled foreland fold-thrust belt curve: the Três Marias salient, São Francisco basin, Brazil. Marine and Petroleum Geology, 66(Part 4):711-731. https://doi.org/10.1016/j.marpetgeo.2015.07.013 https://doi.org/10.1016/j.marpetgeo.2015...
). Reis et al. (2017)Reis H.L.S., Suss J.F., Fonseca R.C., Alkmim F.F. 2017. Ediacaran forebulge grabens of the southern São Francisco basin, SE Brazil: Craton interior dynamics during West Gondwana assembly. Precambrian Research, 302:150-170. https://doi.org/10.1016/j.precamres.2017.09.023 https://doi.org/10.1016/j.precamres.2017...
have described extensive normal faulting as a result of a forebulge uplift in the east. Nevertheless, the effects of this process on the Vazante Group rocks remain unclear. Additionally, similar to the Vazante Group, the Bambuí sediments were also affected by thin-skinned tectonics, and large thrust faults are identifiable along the foreland sedimentary sequence (Reis and Suss 2016Reis H.L.S., Suss J.F. 2016. Mixed carbonate-siliciclastic sedimentation in forebulge grabens: an example from the Ediacaran Bambuí Group, São Francisco basin, Brazil. Sedimentary Geology, 339:83-103. https://doi.org/10.1016/j.sedgeo.2016.04.004 https://doi.org/10.1016/j.sedgeo.2016.04...
, Reis et al. 2017Reis H.L.S., Suss J.F., Fonseca R.C., Alkmim F.F. 2017. Ediacaran forebulge grabens of the southern São Francisco basin, SE Brazil: Craton interior dynamics during West Gondwana assembly. Precambrian Research, 302:150-170. https://doi.org/10.1016/j.precamres.2017.09.023 https://doi.org/10.1016/j.precamres.2017...
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Mineralization generated during a local and regional compressive state regime during basin inversion
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Similar to the previous hypothesis, the fault promoted an increase in rock permeability within the damage zone and facilitated the connection of overpressured fluids from sealed reservoirs. These fluids then migrated to a precipitation site where favorable geochemical conditions were present. The inversion of inherited normal faults is often invoked to explain the formation of certain mineral deposits controlled by high-angle reverse faults, as described in Sibson’s model of fault-valve (Sibson et al. 1988Sibson R.H., Robert F., Poulsen K.H. 1988. High-angle reverse faults, fluid-pressure cycling, and mesothermal gold-quartz deposits. Geology, 16(6):551-555. https://doi.org/10.1130/0091-7613(1988)016%3C0551:HARFFP%3E2.3.CO;2 https://doi.org/10.1130/0091-7613(1988)0...
). This is because, under compressional conditions (horizontal sigma 1), the development of high-angle thrust faults is not favorable. For these reactivations to occur, fluid pressure must exceed the lithostatic load. Consequently, significant quantities of overpressured fluids accumulate below until they are suddenly discharged to higher structural levels once this pressure threshold is reached. This cycle may be repeated multiple times, as hydrothermal mineralizing fluids can seal the cracks, leading to pressure buildup once again. |
The high-angle fault itself is an issue, similar to the high-angle extensional veins. The mineralizing fault-valve process is well-known in orogenic gold deposits, where controlling high-angle faults often extend deep into the seismogenic crustal limit (Sibson et al. 1988Sibson R.H., Robert F., Poulsen K.H. 1988. High-angle reverse faults, fluid-pressure cycling, and mesothermal gold-quartz deposits. Geology, 16(6):551-555. https://doi.org/10.1130/0091-7613(1988)016%3C0551:HARFFP%3E2.3.CO;2 https://doi.org/10.1130/0091-7613(1988)0...
). However, there is currently no evidence to suggest that the Bonsucesso fault or any thrust fault in the Vazante Belt has such deep extensions. |
Controlling structures and mineralization were formed during local and regional compression regime
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The mineralized zones are controlled by a reverse fault. The high angle of the controlling fault may be attributed to a rotation caused by the absorption of regional shortening. |
Low-angle thrust faults are commonly associated with high dilation zones. Mineralized veins at the Bonsucesso deposit do not exhibit shear. Even when veins and fault planes are back-rotated to a position close to a common low-angle thrust fault, the mean veining attitude does not align with the expected Riedel-shear fractures. |