Informe final: Elaboración de tendencias, patrones y conclusiones

The petrological variations from massive amphibolite through the biotite-actinolite schist to the biotite-chlorite-grunerite-calcite alteration zone in the Akjoujt Metabasalt unit can be correlated to the geochemical changes. These include changes in the major and trace element geochemistry in the rock as well as in the mineral chemistry.

7.2.2.1 Major element geochemistry (Alteration Box Plot)

The most prominent geochemical characteristics of the biotite-chlorite-grunerite-calcite alteration zone include substantial enrichment in K, Fe, and Mg, local enrichment in Cu, As, Ni, Co, REE and Au, and depletion in Na and Sr relative to least altered rocks (amphibolite,

86 biotite-actinolite schist). Mineralogically, this geochemical variation reflects the development of iron-rich biotite, chlorite, grunerite and locally sulfides in excess of plagioclase (albite) and amphibole. The geochemical trend in respect to the major element geochemistry from least altered amphibolite through the biotite-actinolite schist to the biotite-chlorite- grunerite-calcite alteration zone proximal to the ore bodies can be shown in a multi-element ratio plot (Alteration Box Plot; after Large et al. 2001). The Alteration Box Plot is a graphical representation that uses two alteration indices: The Ishikawa alteration index (AI) against the chlorite-carbonate-pyrite index (CCPI):

AI = (K2O+MgO)/(K2O+MgO+Na2O+CaO) x100 CCPI = (FeO+MgO)/(K2O+MgO+Na2O+FeO) x100.

The alteration index (AI) reflects gain of K₂O, and Na₂O and/or CaO loss; the chlorite- carbonate-pyrite index (CCPI) measures gain of FeO and MgO and total alkali depletion. In hydrothermally altered volcanic rocks (i.e. in VHMS deposits), the AI value increases as a result of plagioclase breakdown and biotite (or sericite) development. The CCPI measures total alkali depletion associated with Fe-Mg chlorite, magnetite and sulfide enrichment (Large et al. 2001). In Figure 7.2 is presented the alteration trend in the Akjoujt Metabasalt unit toward the metacarbonate.

Figure

Figure 7.2 Alteration Box Plot after Large et al. (2001) showing the geochemical trend in the Akjoujt Metabasalt unit, from least altered amphibolite through biotite-actinolite schist to the biotite-chlorite-grunerite-calcite alteration zone. The arrow marks the progressive alteration trend toward the ore bodies.

87 The least altered amphibolites plot within a field defined by the AI = 20 to 35, and the CCPI = 54 to 92, in the upper-central part of the diagram. In the D₂ shear zone, the biotite- actinolite schist and the biotite-chlorite-grunerite-calcite samples show a progressive increase in the AI and CCPI, forming a trend toward the chlorite/grunerite corner of the box plot. The biotite-actinolite schist samples trend toward the biotite and actinolite composition clustering between the AI = 32 to 43, and the CCPI = 74 and 85. The highly altered samples of the biotite-chlorite-grunerite-calcite alteration zone plot along the upper boundary of the diagram reflecting higher AI (41-95) and CCPI (69-99) values corresponding to the dominance of biotite, chlorite, and grunerite, and locally of magnetite and sulfides.

7.2.2.2 Trace and rare earth element geochemistry

Although the biotite-chlorite-grunerite-calcite alteration zone in the Akjoujt Metabasalt unit records a high degree of major element variation relative to least altered rocks, the contents of alteration-resistant trace elements (Nb, Zr, Y, Ti, Ta, Nd) show coherent concentration patterns except mobile elements such as the LILE elements K, Sr, Rb, and Ba. Similarly to the least altered rocks of the Akjoujt Metabasalt unit, the most distinctive feature in selected MORB-normalized multi-element diagrams is the high Th and Ce enrichment up to 60 and 22 times MORB, respectively (Figure 7.3a). In addition, Rb shows also a high enrichment up to 21 being in good correlation to the K2O and Cu enrichment indicating the evolved hydrothermal signature. In contrast, Sr is remarkably depleted up to 50 times relative to MORB, showing high mobility; it is positively correlated with Na depletion and is associated with plagioclase breakdown.

Figure 7.3 (a) MORB normalized spider diagram (values after Pearce 1982 and 1983), and

(b) chondrite (C1) normalized REE plot (values after Sun and McDonough 1989) for the

88 The chondrite (C1) normalized REE patterns show similar trends with that of the least altered amphibolite (Figure 7.3b). These are characterized by LREE enrichment (La/Yb_CN = 4.7-27.6) and limited Eu anomalies (Eu/Eu*=0.62-1.24). The consistent patterns of the REE indicate that in general the samples still may preserve the bulk of their original rare earth- element composition; although some samples with high metal contents (Cu, Co, Ni, and Au), are conspicuous for high REE enrichment (i.e. sample 8834; LaCN contents up to 514; Figure 7.3b).

7.2.2.3 Mineral chemistry

Chlorite and biotite grains from individual samples from the biotite-chlorite-grunerite-calcite alteration zone are compositionally homogenous on thin section scale (Table 12.17 and 12.20). However, the relative Fe/Mg proportion in the composition of the minerals is highly correlated with proximity of the sample to the ore zones. Samples with abundant grunerite- sulfide veinlets proximal to the ore bodies in the metacarbonate, have biotite and chlorite that show the highest proportion of Fe relative to Mg (Figure 7.4). In Figure 7.4 are illustrated the trends in variation of the Fe and Mg cations (p.f.u. per formula unit) in biotite and chlorite, respectively.

Figure 7.4 Compositional ranges of biotite (n=52) and chlorite (n=68) in samples from the biotite-chlorite-grunerite-calcite alteration zone. A linear relationship exists among the contents of Mg and Fe cations in both biotite and chlorite. Approaching mineralized zones the relative proportions of Fe/Mg for both biotite and chlorite change in the same way reaching the minimum Mg p.f.u. near the ore bodies.

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7.3 Summary

In respect to least altered metacarbonate, the breccia zones show enrichment in a broad suite of elements reflected by the high proportion of the arsenide-sulfide-gold phases in the breccia matrix. the high Fe₂O₃content (up to 65 wt.%) and the silica enrichment in the rock corresponds mineralogically to the development of Fe-Mg clinoamphiboles, magnetite and siderite in the breccias matrix. Pure Fe-Mg clinoamphibole zones from the matrix of the breccia are characterized by very high Au (up to 247 ppb), Cu, Co, Ni, As, Bi and REE concentrations representing the influx of these elements in the metacarbonate during the hydrothermal alteration. In massive ore zones the total amount of Cu, Au, Co, Ni, As, and S can exceed 20 wt.% of the rock composition.

The biotite-chlorite-grunerite-calcite alteration zone in the Akjoujt Metabasalt unit reflects a progressive increase of hydrothermal alteration toward the ore bodies in the metacarbonate. Several geochemical features show systematic variations with increasing proximity to the ore bodies. The most prominent bulk-rock geochemical characteristic is the progressive enrichment of Fe, K, Cu, As, Ni, Co, REE, Th and Au, and depletion of Na and Sr relative to least altered rocks (amphibolite, biotite-actinolite schist). Mineralogically, this geochemical variation reflects the development of iron-rich biotite, chlorite, grunerite and locally sulfides in excess of plagioclase (albite) and amphibole. The relative Fe/Mg proportion in the composition of the phyllosilicate minerals from the biotite-chlorite- grunerite-calcite alteration zone is highly correlated with proximity of the sample to the ore zones. Approaching mineralized zones the relative proportions of Fe/Mg for both biotite and chlorite change in the same way reaching the maximum near sulfide zones.

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8 STABLE ISOTOPE GEOCHEMISTRY

The source of hydrothermal fluids in iron oxide-copper-gold deposits is contentious with magmatic and other fluid sources having been proposed (i.e. evaporitic, meteoric etc.). For this study 18_{O, }13_{C and D and }34_{S ratios were measured from silicate, carbonate, iron} oxide and sulfide mineral separates that belong to the host rock and the ore-bearing, hydrothermal assemblage of the Guelb Moghrein deposit. The isotope analyses are supported by petrography and mineral chemistry of the paragenetic assemblages. The results were used to constrain the a) source of sulfur and metals, b) source of water of the hydrothermal fluid(s), c) temperature of mineral deposition, and d) origin of the host metacarbonate and graphite.

All isotope analyses were performed on pure, handpicked samples except sulfur analyses, which were performed on micro-drilled thick sections. Oxygen and hydrogen isotope ratios are reported relative to V-SMOW (Vienna Standard Mean Ocean Water), carbon isotope ratios relative to PDB (Peedee Belemnite), and sulfur isotope ratios relative to VCD (Vienna Canyon Diablo). The difference in the absolute isotopic ratios (notation) is expressed in parts per mil (‰).