Gold deposits in poly-deformed metasedimentary rocks: a case study of the C1-Santaluz gold deposit, Itapicuru Greenstone Belt, Northeast of Brazil

The C1-Santaluz gold deposit is located in the Itapicuru greenstone belt in Bahia State, Brazil. Metavolcanic and metasedimentary rocks dominate at the base and top, respectively. The ore minerals related to the auriferous mineralization are pyrite, arsenopyrite, sphalerite, chalcopyrite and stibnite. Arsenopyrite is related to, and associated with the presence of gold in quartz veins, a feature which is detected by scanning electron microscope analyses. The deposit is structurally complex, with tree deformational phases being recognized: D _n-1 , D _n , and D _n+1 . Phases D _n-1 and D _n have a direct relation to the mineralization, and mineralized quartz veins are parallel to the S _n foliation (S ₀ //S _n-1 //S _n ). The intersection between the S ₀ //S _n-1 bedding and the S _n foliation generates an intersection lineation parallel to the D _n fold axis, plunging to the NW, which has favored an increase in the volume of mineralized bodies.

Keywords:
Structural settings; auriferous mineralization; sulfide assemblage

Authorship

José Antonio Cirillo de Assis ^**Corresponding author

Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista “Júlio de Mesquita Filho” - São Paulo (SP), Brazil. E-mails: jcirillo.assis@gmail.com, georgell@rc.unesp.brUniversidade Estadual Paulista “Júlio de Mesquita Filho”BrazilSão Paulo, SP, BrazilInstituto de Geociências e Ciências Exatas, Universidade Estadual Paulista “Júlio de Mesquita Filho” - São Paulo (SP), Brazil. E-mails: jcirillo.assis@gmail.com, georgell@rc.unesp.br

http://orcid.org/0000-0001-6698-9083

George Luiz Luvizotto

http://orcid.org/0000-0002-6150-8292

*Corresponding author

SCIMAGO INSTITUTIONS RANKINGS

Figures

Figures (13)

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Figure 1.
(A) São Francisco Craton highlighted in the South American Plataform. (B) The basement terrains of the São Francisco Craton, as well as the Rio Itapicuru greenstone belt located in the Serrinha Block. (C) Geological map of the Rio Itapicuru greenstone belt. The green star represents the current study area, the C1-Santaluz deposit. Modified from Donatti Filho (2007Donatti Filho J.P. 2007. Reavaliação do Contexto Tectônico dos Basaltos do Greenstone Belt do Rio Itapicuru (Bahia), com base na Geoquímica de Elementos-Traço. MS Dissertation, Instituto de Geociências, Universidade Estadual de Campinas, Campinas, 62 p.).

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Figure 2.
Synthesis of the geochronological ages of the RIGB and its surrounding lithotypes.

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Figure 3.
Geological map of the C1-Santaluz deposit, Rio Itapicuru greenstone belt (universal transverse mercator; datum: SAD 1969_UTM zone 24S).

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Figure 4.
Schematic, proposed lithotectonic column for the C1-Santaluz gold deposit, based on Assis (2016Assis J.A.C. 2016. Geologia do Depósito de Ouro de C1-Santaluz no Greenstone Belt do Rio Itapicuru, Brasil. MS Dissertation, Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista “Júlio de Mesquita Filho”, Rio Claro, 123 p.).

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Figure 5.
Macroscopic samples (A, D, E, F, G, I) and photomicrographs (B, C, H, J) of the C1-Santaluz units. (A) Gray-colored, fine-grained metadacite with oxidized bands; (B) Representative example of metadacite. Notice igneous phases transformed into sericite (highlighted by dashed, red rectangle); (C) Matrix of the chlorite-sericite-quartz schist of the Metasedimentary Domain; (D) Chlorite-sericite-quartz schist with oxidized levels; (E) Drill core sample of carbonaceous phyllite showing folded quartz and pyrite veinlets; (F) Carbonaceous phyllite collected from a drill hole showing the main foliation (S_n) parallel to the banding (S₀), with both structures folded; (G) Carbonaceous breccia with quartz veinlets, clasts and oxidized features; (H) Carbonaceous breccia showing ductile-brittle features in the matrix; (I) Drill hole of green-gray-colored metadiorite; (J) Photomicrograph showing a metadiorite containing muscovite, plagioclase, quartz, and a carbonate-quartz vein. (B, C, H and J) Cross-polarized, transmitted light photomicrographs of thin sections.

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Figure 6.
Drill hole samples (A, B) and photomicrographs (C, D) of quartz veins and veinlets from mineralized zones of the C1-Santaluz deposit. (A) Concordant (in respect to the main foliation) and discordant quartz veins in hydrothermally altered or silicified carbonaceous phyllite; (B) Silicified carbonaceous breccia with a quartz vein containing arsenopyrite; (C) Sulfide paragenesis represented by pyrite and arsenopyrite in hydrothermally altered carbonaceous phyllite; (D) Free gold grains in quartz vein associated with arsenopyrite in carbonaceous breccia. (C and D) Cross-polarized, reflected light, photomicrographs of thin sections.

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Figure 7.
Intensity vs energy graphs (EDS) from the C1-Santaluz deposit obtained of: (A) Carbonaceous material present in carbonaceous breccia; (B) Ankerite in quartz veins present in carbonaceous phyllite; (C) Gold grain free in a quartz vein hosted in carbonaceous breccia; (D) Arsenopyrite in gold-bearing quartz vein of graph C; (E) Pyrite grain identified in carbonaceous breccia; (F) Chalcopyrite associated with pyrite in carbonaceous breccia; (G) Sphalerite, which represents a subordinate phase in carbonaceous phyllite; (H) Stibnite in carbonaceous phyllite, also a subordinate phase and described for the first time in the deposit.

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Figure 8.
Scanning electron microscope (SEM) images obtained from thin sections of the C1-Santaluz deposit. (A) Carbonaceous breccia with S₀ bedding parallel to an S_n-1 foliation, folded with planar-axial S_n foliation; (B) Free gold grains (analyses 5 and 6) associated with arsenopyrite (analyses 3 and 4) in carbonaceous breccia.

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Figure 9.
XRD diffractograms of 4 samples from the C1-Santaluz deposit. (A) Sample MP151-2 corresponds to a carbonaceous phyllite with white micas, chlorite and quartz; (B) Carbonaceous phyllite, which highlights graphite as well as microcline, chlorite and white micas; (C) Carbonaceous breccia composed of white micas, chlorite and quartz, and veins of quartz and albite; (D) Carbonaceous phyllite indicating the presence of muscovite, chlorite, quartz, and veins of quartz and albite.

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Figure 10.
(A) Schematic illustration showing structures associated with the deformation phases in the studied area; (B) Cross-polarized, transmitted light photomicrograph of carbonaceous breccia thin section illustrating the main deformation features shown in A; (C) Stereogram of the D_n-1 phase with the main S₀//S_n-1 direction (n = 83) observed mainly in chlorite-sericite-quartz schist, carbonaceous phyllite, and its contact with the metadiorite apophysis; (D) Part of the study area highlighting the pre-D_n folds that truncate the metadiorite/carbonaceous breccia contact (repeated layers of chlorite-sericite-quartz schist unit).

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Figure 11.
(A) Cross-polarized, transmitted light photomicrograph of thin section showing S_n foliation transposing S₀//S_n-1 in carbonaceous phyllite. (B) Stereogram of the S_n foliation (n = 127) of all lithotypes from the C1-Santaluz deposit. (C) Stereogram of intersection lineation of carbonaceous phyllite and chlorite-sericite-quartz schist. (D) Stereogram of D_n fold axes of the metasedimentary domain. (E) Inflection of the metadiorite/carbonaceous breccia contact marking of an overturned synform fold dipping to the northwest.

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Figure 12.
Cross-polarized, transmitted light photomicrographs of thin section showing: (A) An example of the S_n+1 foliation (crenulation cleavage) of carbonaceous phyllite; (B) S_n+1 crenulation cleavage highlighting to D_n+1 fold in a carbonaceous phyllite; (C) Stereogram of S_n+1 foliation in the metasedimentary domain.

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Figure 13.
Mineralogical sequence according to the main lithotypes and deformational phases at the C1-Santaluz deposit.

Figure 1. (A) São Francisco Craton highlighted in the South American Plataform. (B) The basement terrains of the São Francisco Craton, as well as the Rio Itapicuru greenstone belt located in the Serrinha Block. (C) Geological map of the Rio Itapicuru greenstone belt. The green star represents the current study area, the C1-Santaluz deposit. Modified from Donatti Filho (2007Donatti Filho J.P. 2007. Reavaliação do Contexto Tectônico dos Basaltos do Greenstone Belt do Rio Itapicuru (Bahia), com base na Geoquímica de Elementos-Traço. MS Dissertation, Instituto de Geociências, Universidade Estadual de Campinas, Campinas, 62 p.).

Figure 2. Synthesis of the geochronological ages of the RIGB and its surrounding lithotypes.

RIBG: Rio Itapicuru greenstone belt.

Figure 3. Geological map of the C1-Santaluz deposit, Rio Itapicuru greenstone belt (universal transverse mercator; datum: SAD 1969_UTM zone 24S).

Figure 4. Schematic, proposed lithotectonic column for the C1-Santaluz gold deposit, based on Assis (2016Assis J.A.C. 2016. Geologia do Depósito de Ouro de C1-Santaluz no Greenstone Belt do Rio Itapicuru, Brasil. MS Dissertation, Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista “Júlio de Mesquita Filho”, Rio Claro, 123 p.).

Figure 5. Macroscopic samples (A, D, E, F, G, I) and photomicrographs (B, C, H, J) of the C1-Santaluz units. (A) Gray-colored, fine-grained metadacite with oxidized bands; (B) Representative example of metadacite. Notice igneous phases transformed into sericite (highlighted by dashed, red rectangle); (C) Matrix of the chlorite-sericite-quartz schist of the Metasedimentary Domain; (D) Chlorite-sericite-quartz schist with oxidized levels; (E) Drill core sample of carbonaceous phyllite showing folded quartz and pyrite veinlets; (F) Carbonaceous phyllite collected from a drill hole showing the main foliation (S_n) parallel to the banding (S₀), with both structures folded; (G) Carbonaceous breccia with quartz veinlets, clasts and oxidized features; (H) Carbonaceous breccia showing ductile-brittle features in the matrix; (I) Drill hole of green-gray-colored metadiorite; (J) Photomicrograph showing a metadiorite containing muscovite, plagioclase, quartz, and a carbonate-quartz vein. (B, C, H and J) Cross-polarized, transmitted light photomicrographs of thin sections.

Op: opaque mineral; Ser: sericite; Qtz: quartz; Chl: chlorite; Cam: carbonaceous material; Pl: plagioclase; Ms: muscovite; Cb: carbonate mineral; Py: pyrite.

Figure 6. Drill hole samples (A, B) and photomicrographs (C, D) of quartz veins and veinlets from mineralized zones of the C1-Santaluz deposit. (A) Concordant (in respect to the main foliation) and discordant quartz veins in hydrothermally altered or silicified carbonaceous phyllite; (B) Silicified carbonaceous breccia with a quartz vein containing arsenopyrite; (C) Sulfide paragenesis represented by pyrite and arsenopyrite in hydrothermally altered carbonaceous phyllite; (D) Free gold grains in quartz vein associated with arsenopyrite in carbonaceous breccia. (C and D) Cross-polarized, reflected light, photomicrographs of thin sections.

Figure 7. Intensity vs energy graphs (EDS) from the C1-Santaluz deposit obtained of: (A) Carbonaceous material present in carbonaceous breccia; (B) Ankerite in quartz veins present in carbonaceous phyllite; (C) Gold grain free in a quartz vein hosted in carbonaceous breccia; (D) Arsenopyrite in gold-bearing quartz vein of graph C; (E) Pyrite grain identified in carbonaceous breccia; (F) Chalcopyrite associated with pyrite in carbonaceous breccia; (G) Sphalerite, which represents a subordinate phase in carbonaceous phyllite; (H) Stibnite in carbonaceous phyllite, also a subordinate phase and described for the first time in the deposit.

Figure 8. Scanning electron microscope (SEM) images obtained from thin sections of the C1-Santaluz deposit. (A) Carbonaceous breccia with S₀ bedding parallel to an S_n-1 foliation, folded with planar-axial S_n foliation; (B) Free gold grains (analyses 5 and 6) associated with arsenopyrite (analyses 3 and 4) in carbonaceous breccia.

Figure 9. XRD diffractograms of 4 samples from the C1-Santaluz deposit. (A) Sample MP151-2 corresponds to a carbonaceous phyllite with white micas, chlorite and quartz; (B) Carbonaceous phyllite, which highlights graphite as well as microcline, chlorite and white micas; (C) Carbonaceous breccia composed of white micas, chlorite and quartz, and veins of quartz and albite; (D) Carbonaceous phyllite indicating the presence of muscovite, chlorite, quartz, and veins of quartz and albite.

Figure 10. (A) Schematic illustration showing structures associated with the deformation phases in the studied area; (B) Cross-polarized, transmitted light photomicrograph of carbonaceous breccia thin section illustrating the main deformation features shown in A; (C) Stereogram of the D_n-1 phase with the main S₀//S_n-1 direction (n = 83) observed mainly in chlorite-sericite-quartz schist, carbonaceous phyllite, and its contact with the metadiorite apophysis; (D) Part of the study area highlighting the pre-D_n folds that truncate the metadiorite/carbonaceous breccia contact (repeated layers of chlorite-sericite-quartz schist unit).

Figure 11. (A) Cross-polarized, transmitted light photomicrograph of thin section showing S_n foliation transposing S₀//S_n-1 in carbonaceous phyllite. (B) Stereogram of the S_n foliation (n = 127) of all lithotypes from the C1-Santaluz deposit. (C) Stereogram of intersection lineation of carbonaceous phyllite and chlorite-sericite-quartz schist. (D) Stereogram of D_n fold axes of the metasedimentary domain. (E) Inflection of the metadiorite/carbonaceous breccia contact marking of an overturned synform fold dipping to the northwest.

Figure 12. Cross-polarized, transmitted light photomicrographs of thin section showing: (A) An example of the S_n+1 foliation (crenulation cleavage) of carbonaceous phyllite; (B) S_n+1 crenulation cleavage highlighting to D_n+1 fold in a carbonaceous phyllite; (C) Stereogram of S_n+1 foliation in the metasedimentary domain.

Figure 13. Mineralogical sequence according to the main lithotypes and deformational phases at the C1-Santaluz deposit.

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Gold deposits in poly-deformed metasedimentary rocks: a case study of the C1-Santaluz gold deposit, Itapicuru Greenstone Belt, Northeast of Brazil

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[1] *Corresponding author