The metasedimentary-hosted Nyangoubé gold prospect, northwest Côte d'Ivoire: Geochemical and mineralogical characterization of associated hydrothermal alteration

Abstract

The Nyangoubé gold prospect, located in northwestern northwest of Côte d'Ivoire in Africa, is a feature of the Bagoé furrow of the Birimian of the West African Craton. This study is aimed at characterizing the geochemical and mineralogical signatures of hydrothermal alterations associated with the gold mineralization of Nyangoubé gold prospect to provide guidelines for mining exploration. Microscopic petrographic analysis and geochemical characteristics from elemental contents analyzed by X-ray fluorescence (XRF) and inductive coupled-plasma mass spectrometer (ICP-MS) were studied using alteration diagrams and by calculating mass balances and describing thin sections. The results indicate that the host rocks have been affected by silicification, carbonation, sericitization, chloritization, sulphidation and albitization. Hydrothermal alterations associated with the mineralization systems resulted in the destruction of plagioclase in the metasediments studied. The latter was replaced by sericite, chlorite, carbonates, quartz, and sulphides in varying proportions, depending on the intensity of each type of alteration linked to the formation of each mineral. The mass balance calculations show a gradual increase in the concentrations of Au, W, V, As and Pb as well as K₂O, CaO, Na₂O and Fe₂O₃ which could be vectoring parameters towards gold mineralization. The mineralogical assemblage as sericite-chlorite-pyrite, chlorite-pyrite±sericite, carbonate-sericite and chlorite-carbonate revealed by hydrothermal alteration trends in the host rocks could also help identify potential gold corridors in the area of study and its peripheries.

Keywords: Geochemistry; Gold; Hydrothermal alteration; Mineralization; Nyangoubé; Prospect.

Fig. 1

Simplified map of the geological units of the Ivory Coast [17]. 1: Ferké batholith, 2: two-mica granitoids associated with meridional dipping structures; 3: other two-mica granites; 4: sedimentary basins and volcano-sedimentary units; 5: Calc-alkaline granitoid localized in sedimentary basins; 6: Calc-alkaline granitoid localized in greenstone belts; 7: Undifferentiated granitoid, banded granites, gneisses, and migmatites (age above 2. 4 Ma); 8: Periods.

Fig. 2

Location of the study area.

Fig. 3

Geological map of the study area extracted from the Boundiali geological sheet at 1:200 000 [40]. 1. Andesitic breccia 2. Gabbro 3. Biotite granite 4. Granite with biotite and porphyritic facies 5. Biotite-hornblend granodiorite 6. Biotite migmatite 7. Mozonite mozogranite 8. Metaarenit 9. Metagabbro, amphibolite 10. Biotite metagranite 11. Biotite metagranodiorite 12. Quartzites 13. Undifferentiated shale 14. Faults 15. City 16. Boundiali permit.

Fig. 4

Classification and nomenclature of metasediments: A) diagram defining the potential sources of metasediments, B) diagram of [43] applied to metasediments, C) diagram illustrating the hydraulic fractionation of metasediments.

Fig. 5

Harker diagrams contrasting major oxides with SiO₂ of different geochemical groups of metasedimentary rocks.

Fig. 6

Ternary diagrams with poles showing hydrothermal alteration minerals in metasedimentary rocks hosting the Boundiali gold prospect; A-CNK-FM ternary diagram of [46] (A), and FeO + MgO/K₂O/Na₂O + CaO diagram (B) shows the hydrothermal alteration vectors approaching the chlorite and biotite poles.

Fig. 7

Binary saturation diagrams, A) sericite (3*K₂O/Al₂O₃), B) albite (Na₂O/Al₂O₃), C) sericite albite (3*K₂O + Na₂O/Al₂O₃), applied to the metasedimentry rocks hosting the Boundiali gold prospect.

Fig. 8

Box plots showing the variation of alteration indices AI (A) and CCPI (B) in the metasedimentary rocks hosting the Boundiali gold prospect.

Fig. 9

Binary plots showing AI alteration trends with (A) Na₂O and (B) K₂O in metasedimentary rocks hosting the Boundiali gold prospect.

Fig. 10

Box plot binary alteration diagram [50] applied to samples of metasedimentary rocks hosting the Nyangoubé gold prospect.

Fig. 11

Box plot showing mass changes during hydrothermal alteration for selected major elements in metasedimentary rocks hosting the Boundiali gold mineralization. Box plots are used to illustrate the mean (dot), median (line in the box) and scatter of the results. A box represents 50% of the values, two quantiles (the 25th and 75th percentiles). The lines attached to the boxes represent the 10th and 90th percentiles. The values that fall outside of the boxes are remoted.

Fig. 12

Box plot illustrating mass changes produced during hydrothermal alteration for selected trace elements in metasedimentary rocks hosting the Boundiali gold mineralization. Box plots are used to illustrate the mean (dot), median (line in the box) and scatter of the results. A box represents 50% of the values, two quantiles (the 25th and 75th percentiles). The lines attached to the boxes represent the 10th and 90th percentiles. The values that fall outside of the boxes are remoted.

Fig. 13

Photographs (A, D) and microphotographs (B, C) of silicification-associated alteration assemblages. A) quartz veinlet network on cores, B) quartz-carbonate veinlet network in thin slide, C) thin slide showing dissemination of quartz grains in metasedimentary rocks, D) quartz vein overprinting sericitization. V_QZ: quartz veinlet, Qz: quartz.

Fig. 14

Microphotographs (A, B) and photograph (C) of alteration assemblage associated with carbonation. A) destruction of feldspars in place of carbonate (calcite); B) thin slide showing a carbonate veinlet intersecting the carbonate (calcite) dominated rock matrix; C) mineralized quartz-carbonate vein. V_QZ: quartz veinlet, Qz: quartz, V_Qz + Cal: mineralized quartz-carbonate vein, VCal: calcite veinlet, Cal: calcite, Ser: sericite, Au: gold.

Fig. 15

Microphotographs of alteration assemblages associated with sericitization. A: Sericite locally replacing plagioclase; B: total replacement of plagioclase by sericite with the dissemination of pyrite.

Fig. 16

Microphotographs of alteration assemblages associated with chloritization. A, B: chlorite-dominated laminae along the direction of schistosity; C, D: disoriented chlorite in metasediments.

Fig. 17

Microphotographs of alteration assemblages associated with sulfidation. A) sulfide veinlet (pyrite, arsenopyrite and chalcopyrite) intersecting the rock matrix, B) gaining surficial porphyroblasts in a sericite-chlorite matrix; C) dissemination of disoriented sulfides due to low-intensity of deformation; D) sulfurs disseminated in metasediments and oriented in the direction of schistosity. Ser: sericite, Chl: chlorite, Py: pyrite, Apy: arsenopyrite, Ccp: chalcopyrite.