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| Author(s): |
Rodrigo Prudente de Melo
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | Rio Claro. 2016-06-23. |
| Institution: | Universidade Estadual Paulista (Unesp). Instituto de Geociências e Ciências Exatas. Rio Claro |
| Defense date: | 2016-04-28 |
| Advisor: | Marcos Aurelio Farias de Oliveira |
| Abstract | |
The Aguapeí belt comprises a narrow and long (~500Km) NNW-SSE zone of deformed and metamorphosed rocks structurally confined between the eastern margin of the Paraguá Craton and the reactivated basement of Rondonian San-Ignacio. Neoproterozoic (~0.92 Ga) reactivation of crustal scale structures has been pointed out as responsible for the tectonic and metamorphic evolution of the belt. It has caused the deformation and metamorphism of previously deposited sedimentary rocks in an aulacogen basin and its Mesoproterozoic basement as well. The metamorphic evolution of Aguapeí belt, and its relationship with gold mineralization occurring on its southernmost portion was investigated in this research through petrography coupled with fluid inclusions studies, geochemistry and geothermobarometry. The studied area is located to the south of Pontes e Lacerda city (MT), more or less on central portion of Alto Guaporé Gold Province (AGGP), which in turn comprises a series of gold deposits hosted along the belt. The structural architecture of the south portion of the studied area (Pau-a-Pique region) is given by a series of strike-slip shear zones, whereas trusting and folding are dominant on central and northern (Ernesto area). The structural pattern suggest that the deformation was controlled by strike-slip shearing due to oblique shortening. Geothermobarometry based on Si content in muscovite, Ti in quartz and oxygen isotopes fractionation between the pair quartz-magnetite occurring in quartz veins and in muscovite schist present in shear zones suggest that the conditions reached the greenschist-amphibolite facies transition with temperature above 490ºC and minimum pressure of 3.2Kbars. These P-T condition are in agreement with the petrographic features found in mafic diorite and andesite preserved in cores of low deformation along anastomosed shear zones in Pau-a-Pique area and are also more or less consistent with the presence GBM quartz recrystallization mechanism occurring within shear zones. Whole rock geochemistry suggest that fluid were present during strike-slip shearing in Pau-a-Pique area. Percolating fluids during shearing has caused major chemical transformation of the mylonite during the prograde conditions. In the extremely modified rocks, metagranitoids were transformed to micas schist due elemental remobilization. The P-T conditions found in shear zones contrast strongly with those recorded in most of low deformed metasedimentary rocks of Aguapeí Group, mainly present on central and northern portion of the studied area. The presence of biotite-absent pelite and bulging recrystallization mechanism of quartz suggest low temperature conditions (below 400ºC). Meanwhile b-parameter in white micas of metasedimentary rocks records low content of phengite generally found in low to intermediate pressure regions. These conditions are compatible with greenschist facies metamorphism and suggest that the metamorphic conditions was not uniform through the area. Percolation of reduced aqueous-CO2 fluids were responsible for gold mineralization as recorded in Pau a Pique and Ernesto (intermediate Level) fluid inclusions. The close relationship between chlorite, sulfide and carbonate occurring along fractures in early formed quartz albite veins in Pau-a-Pique was used to estimate the temperature of sulfide precipitation using chlorite thermometry. The results suggest that chlorite, carbonate and sulfide found in fractures of early formed quartz, has been crystallized at 325ºC at the retrograde path in Pau-a-Pique deposit. The textural aspects of pyrite suggest that previously formed Fe-Ti oxides occurring in the wall rock of mineralized veins in the Ernesto deposit (Intermediate level) and in mica schist in shear zones of the Pau a Pique deposit, were replaced by pyrite due to reaction of Fe-Ti oxides with reducing H2S rich fluids. Such process was the main mechanism responsible for gold precipitation. The characteristics of fluid inclusions and the hydrothermal alteration of the studied deposits are similar to those found in most of orogenic gold deposit around the world. δD and δ18O stable isotope geochemistry are in agreement with metamorphic source for the fluid-forming of the main guange phases present in Pau-a-Pique, Ernesto (intermediate level) and other six gold occurrences in the central portion of AGGP. Meanwhile δD signature of fluid inclusions are compatible with those observed in major orogenic gold provinces worldwide. The results allows to conclude that the studied gold deposits were formed latter on metamorphic evolution of Aguapeí belt, during retrograde conditions, and from mixed aqueos-CO2 fluids that has transported gold by complexation with H2S. (AU) | |
| FAPESP's process: | 12/17432-8 - Metamorphic characterization of the Aguapeí Mobile Belt and it's relationship to the metallogenesis of gold deposits in Pontes e Lacerda region, Mato Grosso state. |
| Grantee: | Rodrigo Prudente de Melo |
| Support Opportunities: | Scholarships in Brazil - Doctorate |