Mineralogy and Hydrothermal Alteration of Rocks Associated With Gold Mineralization in Akobo Greenstone Belt, Southwestern Ethiopia

Structurally hosted, mesothermal, lode-gold deposits in metamorphic terranes constitute a single, coherent group of epigenetic precious met al deposits. Many mesothermal gold deposits in the Quadrilatero Ferrifero (QF) region of Minas Gerais, Brazil are encompassed in the sub-amphibolite to greenschist facies group of Archean, greenstone belt deposits. In general, the hydrothermal alteration of these deposits characterizes a core of Fe-carbonates, normally with micas and albite, grading out into calcite and chlorite. Quartz veining and sulfidation are variably developed, with gold residing mainly within sulfides. Gold mineralizing fluids were CO2-H2O-rich, of relatively low salinity, with a near neutral to slightly alkaline pH and mainly reducing. In the QF, changes in the molar fraction Xco2, a(K+/H+) and a(Na+/H+) controlled the alteration assemblages of metamafic and meta-ultramafic rocks. Chlorite dominates the incipiente, and carbonate the advanced alteration assemblages, suggesting that a H2O-rich evolved from an original, higer CO2:H2O-ratio fluid, typical of the more advanced stages of alteration. Changes in ais and Xco2 controlled Al-poor banded iron formation (BIF) carbonate- and sulfide-alterations. The capacity of BIF mineral assemblage to buffer the incoming fluid fO2 determined 1) the evolving fluid ais (and Xco2), and thereby 2) the sulfide paragenesis in physical space, in chemical space and over geological time. Variations in ais and Pfiuid can also be deduced from the gold-associated alteration in BIF. Shear zones are occupied mainly by pyrrhotite, whereas pyrite + arsenopyrite occur as bedding-replacement zones in BIF. The incipient (buffered) and more advanced (unbuffered) stages of alteration reflect the gradual compositional evolution of a relatively uniform, external fluid through interaction with wallrocks, thus reflecting varying fluid versus rock ratios. The hydrothermal alteration and gold precipitation were roughly isothermal, in keeping with temperature and pressure calculations for deposits in other parts of the world. Suggested slight variations inf02, pH and ais in the QF deposits probably played an important role in gold deposition.

The Kalahari Goldridge Mine is located within the Archaean Kraaipan Greenstone Belt, about 60 km southwest of Mafikeng in the North West Province, South Africa. The ore body thickness varies from 15 to 45 m along a strike length of about 1.5 km within approximately N–S striking banded iron formation (BIF). The stratabound ore body is hosted primarily by BIF, which consists of alternating chert and magnetite–chlorite–stilpnomelane–sulphide–carbonate bands of millimetre- to centimetre scale. A footwall of sericite–carbonate–chlorite schist underlain by mafic amphibolite occurs to the west and carbonaceous metapelites in the hanging wall to the east. Overlying the hanging wall, carbonaceous metapelites, units of coarse-grained metagreywackes fining upwards, become increasingly conglomeratic up the stratigraphy. Small-scale isoclinal folds, brecciation, extension fractures and boudinage of cherty BIF units reflect brittle-ductile deformation. Fold axial planes have foliation, with subvertical plunges parallel to prominent rodding and mineral lineation in the footwall rocks. Gold mineralisation is associated with two generations of quartz–carbonate veins, dipping approximately 20° to 40° W. The first generation consists of ladder-vein sets (group IIA) preferentially developed in centimetre-scale Fe-rich mesobands, whereas the second generation consists of large quartz–carbonate veins (group IIB), which locally crosscut the entire ore body and extend into the footwall and hanging wall. The ore body is controlled by mesoscale isoclinal folds approximately 67° E, orthogonal to the plane of mineralised, gently dipping veins, defining the principal stretching direction and development of fluid-focussing conduits. The intersections of the mineralised veins and foliation planes of the host rock plunges approximately 08° to the north. Pervasive hydrothermal alteration is characterised by chloritisation, carbonatisation, sulphidation and K-metasomatism. Gold is closely associated with sulphides, mainly pyrite and pyrrhotite, and to a lesser extent, with bismuth tellurides and carbonate minerals. Mass balance transfer calculations indicate that hydrothermal alteration of BIF involved enrichment of Au, Ag, Bi, Te, S and CO2 (LOI), MgO, Ba, K and Rb, but significant depletion of SiO2 and, to a lesser extent, Fe2O3. Extensive replacement of magnetite and chlorite in BIF and other pelitic sedimentary rocks by sulphide and carbonate minerals, both on mesoscopic and microscopic scales, is evidence of interaction of CO2- and H2S-bearing fluids with the Fe-rich host rocks. The fineness of gold grains ranges from 823 to 921, similar to that of other epigenetic Archaean BIF-hosted gold deposits, worldwide.

Paramanahalli gold prospect is hosted by deformed metamorphosed volcano-sedimentary rocks of the Chitradurga Group. Mineralization is confined to altered host rocks and quartz veins, and structurally controlled by NNW-SSE trending ductile to brittle-ductile shear zones. Surface indication of mineralization is found in the form of sulfide-rich quartz-ankerite veins with pyrite disseminated in host rocks. Detailed petrographic studies indicate a mineral assemblage of chlorite-1 ± biotite ± albite ± quartz in metabasalt formed by low-grade metamorphism. Microstructures such as brecciation, microfractures, micro folds, and strain fringes indicate that the metamorphosed volcano-metasedimentary rocks (metabasalt, phyllite, greywacke) experienced brittle-ductile deformation. Carbonatization and sulfidation are common hydrothermal alterations in the areas of oxide facies of the Banded Iron Formation (BIF), where gold is enriched. Altered metabasalt and BIF contain a significant amount of sulfides with native gold. The above studies indicate that mineralization at Paramanahalli was associated with deformation and alteration processes. Hence, an attempt has been made to understand the genetic aspects of gold mineralization at the Paramanahalli gold prospect based on petrography and mineralogy of altered host rocks.

Geochemistry of banded iron formations and their host rocks from the Central Eastern Desert of Egypt: A working genetic model and tectonic implications

Hydrothermal alteration and mineralization in the Faina greenstone belt: evidence from the Cascavel and Sertão orogenic gold deposits

The Paleoproterozoic (~2.38–2.21 Ga) Yuanjiacun banded iron formation (BIF), located in Shanxi Province, is a Superior-type BIF in the North China Craton (NCC). This BIF is within a metasedimentary rock succession of the Yuanjiacun Formation in the lower Luliang Group. The clastic metasediments associated with the BIF are mainly represented by well-bedded meta-pelites (chlorite schists and sericite-chlorite phyllites) and meta-arenites (sericite schists), which have undergone greenschist-facies metamorphism. The Yuanjiacun Formation had been originally deposited in a passive margin setting, most probably on a stable continental shelf. Iron oxide (magnetite and hematite), carbonate, and silicate facies are all present within the iron-rich layers. Integration of petrographic and isotopic evidence indicates that the most likely precursor materials were comprised predominantly of probably hydrous, Fe-silicate gels of stilpnomelane-type composition, amorphous silica gels, and ferrihydrite. The \({\text{P}}_{{{\text{O}}_{2} }}{-} {\text{P}}_{{{\text{CO}}_{2} }}\) and pH-Eh fields of the mineral assemblages (and/or their precursors) in the Yuanjiacun BIF indicate anoxic and near-neutral to slightly alkaline conditions for the original depositional environment except for the hematite precursor field (that of Fe(OH)3), which is very small and exists only at relatively high \({\text{P}}_{{{\text{O}}_{2} }}\) values. The eastward transition from carbonate- into oxide-facies iron formations is accompanied by a change in mineralogical composition from siderite in the west through magnetite-ankerite and magnetite-stilpnomelane assemblages in the transition zone to magnetite and then hematite in the east. These distinct lateral facies are also observed vertically within the BIF, i.e., the iron mineral assemblage changes up section from siderite through magnetite into hematite-rich iron formation. The oxide-facies BIF formed near shore, whereas carbonate (siderite)- and silicate-facies assemblages formed in deeper waters. Based on detailed analyses of these variations on a basinal scale, the BIF precipitated during a transgressive event within an environment that ranged from deep waters below storm-wave base to relatively shallow waters. The BIF samples display distinctively seawater-like REE + Y profiles that are characterized by positive La and Y anomalies and HREEs enrichment relative to LREEs in Post-Archean Australian shale-normalized diagrams. Consistently positive Eu anomalies are also observed, which are typical of reduced, high-temperature hydrothermal fluids. In addition, slightly negative to positive Ce anomalies, and a large range in ratios of light to heavy REEs and Y/Ho, are present in the oxide-facies BIF. These characteristics, in combination with consistently positive δ56Fe values, suggest that deposition of the BIF took place along the chemocline where upwelling of deep, anoxic, iron- and silica-rich hydrothermal fluids mixed with shallower and slightly oxygenated seawater. The ankerite displays highly depleted δ13C values and the carbonate-rich BIF has a high content of organic carbon, suggesting dissimilatory Fe(III) reduction of a ferric oxyhydroxide precursor during burial of biomass deposited from the water column; that same biomass was likely tied to the original oxidation of dissolved Fe(II). The fact that the more ferric BIF facies formed in shallower waters suggests that river-sourced nutrients would have been minimal, thus limiting primary productivity in the shallow waters and minimizing the organic carbon source necessary for reducing the hematite via dissimilatory Fe(III) reduction. By contrast, in deeper waters more proximal to the hydrothermal vents, nutrients were abundant, and high biomass productivity was coupled to increased carbon burial, leading to the deposition of iron-rich carbonates. The deposition of the Yuanjiacun BIF during the onset of the Great Oxidation Event (GOE; ca. 2.4–2.2 Ga) confirms that deep marine waters during this time period were still episodically ferruginous, but that shallow waters were sufficiently oxygenated that Fe(II) oxidation no longer needed to be tied directly to proximal cyanobacterial activity.

The Mupane gold deposit, which is one of the numerous gold occurrences in the Tati Greenstone Belt in the northeastern part of Botswana, consists of four orebodies, namely Tau, Tawana, Kwena, and Tholo deposits. The present research, which focuses on the genesis of the Tau deposit, was based on ore petrography, mineral chemistry of sulfides, and sulfur isotope data. Mineralogical characteristics of the host rocks indicate that banded iron formation at the Tau deposit includes iron oxides (magnetite), carbonates (siderite and ankerite), silicates (chlorite and amphibole), and sulfides (arsenopyrite and pyrrhotite). The deposit features arsenopyrite‐rich zones associated with biotite‐chlorite veins, which are indicative of the precipitation of arsenopyrite concomitant with potassic alteration. The replacement of magnetite by pyrrhotite in some samples suggests that sulfidation was likely the dominant gold precipitation mechanism because it is considered to have destabilized gold‐thiocomplexes in the ore‐forming fluids. Based on textural relationships and chemical composition, arsenopyrite is interpreted to reflect two generations. Arsenopyrite 1 is possibly early in origin, sieve textured with abundant inclusions of pyrrhotite. Arsenopyrite 1 was then overgrown by late arsenopyrite 2 with no porous textures and rare inclusions of pyrrhotite. Gold mineralization was initiated by focused fluid flow and sulfidation of the oxide facies banded iron formation, leading to an epigenetic gold mineralization. The mineralogical assemblages, textures, and mineral chemistry data at the Tau gold deposit revealed two‐stage gold mineralizations commencing with the deposition of invisible gold in arsenopyrite 1 followed by the later formation of native gold during hydrothermal alteration and post‐depositional recrystallization of arsenopyrite 1. Laser ablation inductively coupled plasma mass spectrometric analysis of arsenopyrite from the Tau deposit revealed that the hydrothermal event responsible for the formation of late native gold also affected the distribution of other trace elements within the grains as evidenced by varying trace elements contents in arsenopyrite 1 and arsenopyrite 2. The range of δ34S of gold‐bearing assemblages from the Tau deposit is restricted from +1.6 to +3.9‰, which is typical of Archean orogenic gold deposits and indicates that overall reduced hydrothermal conditions prevailed during the gold mineralization process at the Tau deposit. The results from this study suggest that gold mineralization involved multi‐processes such as sulfidation, metamorphism, deformation, hydrothermal alteration, and gold remobilization.

The giant Karagba-Chauffeur-Durba (KCD) gold deposit and its peripheral gold satellite deposits are located within the Kibali gold district in the Neoarchaean Moto Greenstone Belt in the northeast of the Congo Craton, Democratic Republic of Congo. The gold mineralisation within the KCD area occurs within altered chemical sedimentary rocks such as banded iron formation (BIF) and chert, as well as in meta-carbonaceous argillite, meta-sandstones and poorly-sorted pebble meta-conglomerates. To elucidate and characterise the controls on gold mineralisation in the siliciclastic rocks of this sedimentary sequence dating from approximately 2630 to 2625 Ma, the present study critically considers an extensive data set comprising borehole logs, petrographic observations, gold assay data, and mineral chemistry. Detailed high magnification petrographic analyses of polished sections enabled evaluation of 3824 gold grains from meta-conglomerate and siliciclastic sedimentary host rock specimens. These were compared to an additional 985 gold grains from deformed quartz-carbonate (ACSA) altered rocks and BIF host rock types, representing end-members for a purely hydrothermal style of gold mineralisation. These analyses revealed that most of the gold grains in the meta-conglomerates occur within the gangue mineral assemblage in the form of free gold generally distal to any sulphide minerals, with a second population associated with the sulphides, mainly pyrite, followed by arsenopyrite and lesser pyrrhotite. The gold grains in the meta-conglomerates show larger average sizes (433 µm2) than in the grains in the ACSA (311 µm2) and BIF (180 µm2) host rock types. A large dataset of assayed gold results of 9,632,470 data from 3953 drill holes highlights that the ACSA-altered rocks and the chemical sedimentary rocks (chert and BIF) are the main hosts for gold, showing average grades of 3.9, 2.5 and 2.4 g/t when a 0.5 g/t cut-off is applied, respectively. Conversely, meta-conglomerates have a relatively low Au content (1.84 g/t), yet this dataset has an extensive set of outlying values, with 86 data points reflecting grades over 100 g/t. Scanning Electron Microscope (SEM-EDS/WDS) performed on 397 gold grains from meta-conglomerates and deformed quartz-carbonate (ACSA) altered rocks shows mainly Au-Ag alloys, dominated by native gold with averages of 85.46 wt% of Au and 9.76 wt% of Ag in the meta-conglomerates and 86.56 wt% of Au and 12.84 wt% of Ag within the ACSA. Minor amounts of Au-Ag alloys electrum and Au-Hg-Ag alloys amalgams are present predominantly at Gorumbwa and KCD deposits. Our analysis of textural features, mineral chemistry, morphology, assay results, and mineral associations suggests that there are significant differences between the purely hydrothermal mineralisation and the mineralisation hosted by the meta-conglomeratic units. We suggest that the pre-enrichment of these meta-conglomerates by placer processes has played a role in enhancing the fertility for gold mineralisation in the KCD area. This has far-reaching implications for the existing ore-genesis models and exploration paradigms employed in the KCD area. This work presents new opportunities for exploration by highlighting the significance of understanding the stratigraphy and identifying sub-basins that may have undergone early placer enrichments. To achieve this, the early sedimentary environment should be thoroughly investigated.

The Blue Dot gold deposit, located in the Archean Amalia greenstone belt of South Africa, is hosted in an oxide (± carbonate) facies banded iron formation (BIF). It consists of three stratabound orebodies; Goudplaats, Abelskop, and Bothmasrust. The orebodies are flanked by quartz‐chlorite‐ferroan dolomite‐albite schist in the hanging wall and mafic (volcanic) schists in the footwall. Alteration minerals associated with the main hydrothermal stage in the BIF are dominated by quartz, ankerite‐dolomite series, siderite, chlorite, muscovite, sericite, hematite, pyrite, and minor amounts of chalcopyrite and arsenopyrite. This study investigates the characteristics of gold mineralization in the Amalia BIF based on ore textures, mineral‐chemical data and sulfur isotope analysis. Gold mineralization of the Blue Dot deposit is associated with quartz‐carbonate veins that crosscut the BIF layering. In contrast to previous works, petrographic evidence suggests that the gold mineralization is not solely attributed to replacement reactions between ore fluid and the magnetite or hematite in the host BIF because coarse hydrothermal pyrite grains do not show mutual replacement textures of the oxide minerals. Rather, the parallel‐bedded and generally chert‐hosted pyrites are in sharp contact with re‐crystallized euhedral to subhedral magnetite ± hematite grains, and the nature of their coexistence suggests that pyrite (and gold) precipitation was contemporaneous with magnetite–hematite re‐crystallization. The Fe/(Fe+Mg) ratio of the dolomite–ankerite series and chlorite decreased from veins through mineralized BIF and non‐mineralized BIF, in contrast to most Archean BIF‐hosted gold deposits. This is interpreted to be due to the effect of a high sulfur activity and increase in fO2 in a H2S‐dominant fluid during progressive fluid‐rock interaction. High sulfur activity of the hydrothermal fluid fixed pyrite in the BIF by consuming Fe2+ released into the chert layers and leaving the co‐precipitating carbonates and chlorites with less available ferrous iron content. Alternatively, the occurrence of hematite in the alteration assemblage of the host BIF caused a structural limitation in the assignment of Fe3+ in chlorite which favored the incorporation of magnesium (rather than ferric iron) in chlorite under increasing fO2 conditions, and is consistent with deposits hosted in hematite‐bearing rocks. The combined effects of reduction in sulfur contents due to sulfide precipitation and increasing fO2 during progressive fluid‐rock interactions are likely to be the principal factors to have caused gold deposition. Arsenopyrite–pyrite geothermometry indicated a temperature range of 300–350°C for the associated gold mineralization. The estimated δ34SΣS (= +1.8 to +2.5‰) and low base metal contents of the sulfide ore mineralogy are consistent with sulfides that have been sourced from magma or derived by the dissolution of magmatic sulfides from volcanic rocks during fluid migration.

The resurgence of Banded Iron Formations (BIFs) during the Neoproterozoic, following a billion-year hiatus, reflects significant geodynamic and climatic transition. Newly discovered Neoproterozoic BIFs in the central Anti-Atlas region of Morocco provide key insights into these processes. The studied BIFs units are exposed within the Bou Azzer-El Graara inlier (Central Anti-Atlas), an oceanic paleo-suture zone between the Paleoproterozoic West African Craton and remnants of a Neoproterozoic magmatic arc. This inlier comprises 750 to 680 Ma magmatic arcs and ophiolitic remnants, both intruded by ~650 Ma dioritic plutons and overlain by Ediacaran metasedimentary sequences. The studied BIFs are hosted in meta-volcano-sedimentary units, intercalated between magmatic arc and ophiolitic complexes, and locally intruded by igneous bodies. Neither the BIFs nor their host volcano-sedimentary schists are associated with glacio-derived sediments.Petrological, geochemical, and geochronological analyses were conducted to reconstruct the paleo-depositional environment and identify the mechanisms of BIF formation. In situ U-Pb dating on hematite yielded a crystallization age of 641 ± 41 Ma. Hematite dating could be interpreted as an early diagenetic age probably close to BIF deposition.The whole-rock major and trace element composition of the Bou Azzer BIFs exhibits a high correlation among terrigenous proxies (e.g., Al, Zr, Hf) and silica content, with trends strongly aligning with the felsic host rocks. This suggests that the BIFs’ whole-rock geochemical signature, specifically the siliceous layers, is predominantly controlled by detrital inputs. Multi-element geochemistry, (e.g. mean La/YbSN ratio of 0.36, low TiO₂ content of 0.24 wt%, Y/Ho ratio of 26, Nb-Ta depletion) combined with Nd-Sr isotopic data from the host rocks (εNdᵗ +4.0 to +4.5), indicates a juvenile arc source, consistent with presence of igneous minerals, such as feldspar, epidote, and amphibole, in both the host rocks and BIF samples.Hematite in BIFs show two habitus: large euhedral grains surrounded by platy hematite. Petrographic evidence suggests that euhedral hematite precipitated at a more precocious stage, while platy hematite is distinctly aligned with the foliation of the host sediments. In situ LA-ICP-MS analyses of hematite from the two habitus reveal distinct geochemical signatures from each other and from the whole-rock compositions. Overall, hematite exhibits significantly lower ΣREE and superchondritic Y/Ho ratios up to 42, with a median value of ~28. Large euhedral hematite displays a pronounced negative Ce anomaly, indicative of precipitation from oxygenated seawater and distant from hydrothermal sources, as shown by low positive Eu anomaly (~1.06). The chemical composition of platy hematite shows no Eu or Ce anomalies, suggesting anoxic conditions during diagenetic crystallization. The Bou Azzer BIFs are Cryogenian and were deposited in an arc-bounded basin, with no evidence of glaciogenic influence. This paleo-depositional context emphasizes the role of limited arc-related basins during the Neoproterozoic, which facilitated the development of unique suboxic conditions.

Genesis of the Longwanggou iron deposit in the Yudongzi Complex, South China: Implications for the redox state of seawater at the onset of the Great oxidation Event

Depositional model for banded iron formation host to gold in the Archean Rio das Velhas greenstone belt, Brazil, based on geochemistry and LA-ICP-MS magnetite analyses

The Tucano gold deposit, located in the southeastern portion of the Guiana Shield, is structurally controlled and hosted in amphibolite facies metasedimentary rocks, mainly marble and banded iron formation (BIF), of the Paleoproterozoic Serra do Navio greenstone belt. The distinct high-pressure-temperature (P-T) hydrothermal alteration and relative timing of gold mineralization with respect to regional metamorphism and emplacement of leucogranite dikes make this deposit an ideal field laboratory to test the hypothesis that orogenic gold systems extend to hypozonal crustal levels. Combined detailed mapping, diamond drill core logging, petrography, mineral chemistry, and thermodynamic studies revealed that gold mineralization at Tucano is coeval with high-P-T replacement of peak metamorphic minerals at amphibolite facies conditions. Local preservation of open space-filling textures further supports the true high-temperature nature of hydrothermal alteration and gold mineralization. An early alteration stage (550°–600°C) is characterized by quartz-clinopyroxene-garnet and expressed as attenuated, boudinaged, or disrupted veins (pre- to early-kinematic). The gold-bearing main alteration stage (480°–590°C) has a dominant amphibole-phlogopite-magnetite-pyrrhotite ± calcite assemblage, with widespread replacement of metamorphic and early hydrothermal minerals. Sulfide precipitation increases toward the proximal alteration zone and is dominated by pyrrhotite with trace chalcopyrite. The assemblage pyrrhotite-loellingite-arsenopyrite is restricted to the proximal alteration zone in the marble host rock and indicates reduced fluid conditions. Gold-related hydrothermal textures, comprising both aligned alteration minerals within the shear zone fabric and poorly oriented euhedral to subhedral platy and prismatic alteration minerals, constrain the mineralization as a syn- to late-kinematic protracted event. Pressure estimates at 4.9 ± 1.2 kbar using the garnet geobarometer place this hydrothermal event at a hypozonal crustal level. Leucogranite dikes and stocks within the deposit are undeformed and are hence interpreted as postdating the gold mineralization event. Visible gold is in equilibrium with sulfide-arsenide assemblages and silicate-oxide minerals. Mass balance calculations indicate hydrothermal Na addition without concomitant K gain. Tucano is the first documented high-P-T orogenic gold deposit in the Guiana Shield, thus opening significant new exploration search space for orogenic-gold-style mineralization at deeper-crustal-level amphibolite facies terrains.

Interpretations and implications of LA ICP-MS analysis of chert for the origin of geochemical signatures in banded iron formations (BIFs) from the Meadowbank gold deposit, Western Churchill Province, Nunavut

Metal source and fluid–rock interaction in the Archean BIF-hosted Lamego gold mineralization: Microthermometric and LA-ICP-MS analyses of fluid inclusions in quartz veins, Rio das Velhas greenstone belt, Brazil