Manganese mineralization in Archean greenstone belt,Joda–Noamundi sector,Noamundi basin,East Indian Shield

A Mesoarchean greenstone belt (3.5–3.0 Ga) in the western part of the East Indian Shield comprising the Iron Ore Group of the Noamundi basin contains economic resources of both iron and manganese ores in the NNE plunging regional synclinorium. Manganese mineralization in the central and eastern parts of this synclinorium, particularly in Joda–Noamundi sector, has taken place in multiple cycles starting from syngenetic sedimentary and exhalative type through mobilization and remobilization in different stages of tectonism, deformation and hydrothermal activities to latest lateritic or supergene type. A relatively high temperature metamorphic jacobsite–hausmannite–bixbyite–braunite assemblage, low temperature hydrothermal pyrolusite–psilomelane–hollandite assemblage and supergene pyrolusite–manganomelane–groutite–polianite assemblage are present and were formed by recycling of manganese in different stages of mineralization. A detailed structural study of the manganese ore bodies as well as their ore petrographic and mineralogical characteristics with mineral chemistry has revealed systematic mineralization and their relation to deformational phases. Such recycling of manganese and its structural control of mineralization in different phases is unique of its kind in comparison with other Archean manganese deposits in the world.     

Geology and geochemistry of fault-hosted hydrothermal and sedimentary manganese deposits in the Oakover Basin,east Pilbara,Western Australia

Manganese mineralisation in the Oakover Basin is associated with Mesoproterozoic extension, basin formation and deposition of the Manganese Group. The underlying basement architecture of the Oakover Basin (a local half-graben geometry), inherited from the Neoarchean rifting event, plays an important role on the distribution, style and timing of manganese deposits. Fault-hosted manganese deposits are dominant along the ‘active’ faulted eastern margin, whereas flat-lying sedimentary deposits are dominant along the western ‘passive’ margin reflecting differences in ore-forming processes. The large number of significant manganese deposits in the Oakover Basin, previously thought to reflect a spatial association with Carawine Dolomite, more likely reflects the restricted nature of the Mesoproterozoic basin and development of a large reservoir of Mn²⁺ and Fe²⁺ in an anoxic zone of a stratified basin. Low O₂ conditions in the basin were caused by a paleotopographic high forming a barrier to open ocean circulation. The western margin sedimentary deposits formed later than the fault-hosted hydrothermal deposits along the eastern margin, once a significant reservoir of Mn²⁺ and Fe²⁺ had developed, and when there was sufficient subsidence to allow migration of the redox front onto the shallow shelf, with Mn precipitation on and within the seafloor sediments. The sedimentary manganese deposits are not uniformly distributed along the western edge of the basin; instead they are concentrated into discrete areas (e.g. Mt Cooke–Utah–Mt Rove, Bee Hill, Skull Springs and the Ripon Hills districts), suggesting a degree of structural control on their distribution. Fault-hosted manganese is observed beneath and adjacent to many of the sedimentary deposits. Marked geochemical differences are observed between the Woodie Woodie hydrothermal deposits and the sedimentary deposits. Woodie Woodie deposits display higher Ba, U, Mo, As, Sn, Bi, Pb, S and Cu than the sedimentary deposits, reflecting the composition of the hydrothermal fluids. The Al₂O₃ values of the Ripon Hills and Mt Cooke deposits are much higher than the Woodie Woodie deposits, reflecting the composition of the dominant host rock, as Al₂O₃ is typically <5 wt% in the Carawine Dolomite, but is >10 wt% in basal shale units of the Manganese Group. Highly variable Mn:Fe ratios (?5:1) in the hydrothermal manganese at Woodie Woodie reflects rapid deposition of Mn in and around fault zones. In contrast, slower accumulation of Mn oxides on and within the seafloor to form the large sedimentary deposits results in Mn:Fe ratios closer to 1:1 and elevated Co + Ni and REE values.     

Proterozoic deformation in the east Pilbara Craton and tectonic setting of fault-hosted manganese at the Woodie Woodie mine

Neoarchean and Mesoproterozoic sequences in the Oakover Basin provide a record of deformation and sedimentation along the eastern edge of the Archean Pilbara Craton. The early extensional history of the Oakover Basin is overprinted by subsequent compressional events. Five distinct deformation events are recognised in the Woodie Woodie region; the Archean D₁ event, comprising west-northwest–east-southeast extension associated with formation of the Neoarchean Hamersley Basin; the Mesoproterozoic D_2a event, with northwest–southeast extension and basin formation associated with manganese mineralisation; the D_2b event, with renewed extension associated with intrusion of Davis Dolerite during the ca 1090–1050 Ma Warakurna event; the D₃ event, comprising northeast–southwest-directed compression attributed to the ca 900 Ma Edmundian Orogeny; the Neoproterozoic D₄ event, with east-northeast–west-southwest extension producing large D₄ grabens associated with the opening of the Officer Basin; and, the Neoproterozoic D₅ event comprising north–south-directed compression attributed to the ca 550 Ma Paterson Orogeny. Abundant manganese deposits are hosted by the Neoarchean and Mesoproterozoic sequences in the Oakover Basin, including the large high-grade manganese deposits at Woodie Woodie. The orebodies are predominantly hydrothermal in origin, with a late supergene overprint, and deposition of primary manganese mineralisation was synchronous with northwest–southeast Mesoproterozoic D_2a extension and basin formation. The manganese is associated with normal faults, and many of these represent growth faults related to basin formation. Stratabound manganese is found above or adjacent to fault-hosted manganese. An initial structural framework established during Archean rifting was reactivated in the D_2a event and provided a major structural control on manganese distribution. High-grade manganese deposits at Woodie Woodie mine appear to be located in a zone of oblique dextral extension on major north-northwest- to north-trending faults that mark the eastern ‘active’ or faulted margin of an early rift basin. These large north-northwest-trending normal faults are linked to a major northwest-trending transform fault zone (Jewel-Southwest Fault Zone) that separates the Oakover Basin into a northern and southern basin. The transform fault represents a major deep fluid conduit for hydrothermal fluids and most likely accounts for the concentration of significant manganese occurrences immediately to the north and south of this structure.     

Timing of supergene enrichment of low-grade sedimentary manganese ores in the Kalahari Manganese Field,South Africa

Low-grade carbonate-rich manganese ore of sedimentary origin in the giant Kalahari Manganese Field, South Africa, is upgraded to high-grade todorokite–manganomelane manganese ore by supergene alteration below the unconformity at the base of the Cenozoic Kalahari Formation. Incremental laser-heating ⁴⁰Ar/³⁹Ar dating of samples from the supergene altered manganese ore suggest that chemical weathering processes below the Kalahari unconformity peaked at around 27.8 Ma, 10.1 Ma and 5.2 Ma ago. Older ages are dominant in the upper part of the weathering profile, while younger ages are characteristic of the deeper part of the profile. Younger ages partially overprint older ages in the upper part of the weathering profile and demonstrate the downward progression of the weathering front by as little as 10 cm per million years. The oldest age obtained in the weathering profile, namely 42 Ma, is considered a minimum estimate for the onset of the post African I cycle of weathering and erosion that followed the break up of Gondwanaland and formation of the Cretaceous to early Cenozoic African land surface. The youngest ages, recorded at around 5 Ma, in turn, correspond well to the Pliocene transition from humid to arid climatic conditions in Southern Africa.     

Manganese oxide mineralogy,petrography and genesis,Pilbara Manganese Group,Western Australia

Supergene manganese oxides, occurring in shales, breccias and dolomites of Proterozoic Age, in the Western Australian Pilbara Manganese Group, have Mn/Fe ranging from 1.9 to 254 and Mn⁴⁺ to Mn (Total) of 0.49–0.94. The manganese mineralogy is dominated by tetravalent manganese oxides, especially by cryptomelane, with lesser amounts of pyrolusite, nsutite, manjiroite, romanechite and other manganese oxide minerals. The manganese minerals are commonly associated with iron oxides, chiefly goethite, indicating incomplete separation of Mn from Fe during Tertiary Age arid climate weathering of older, manganiferous formations. These manganese oxides also contain variable amounts of braunite and very minor hausmannite and bixbyite. The braunite occurs in three generations: sedimentary-diagenetic, recrystallised sedimentary-diagenetic, and supergene. The mode of origin of the hausmannite and bixbyite is uncertain but it is possible that they resulted from diagenesis and/or low-grade regional metamorphism. The supergene manganese deposits appear to have been derived from manganiferous Lower Proterozoic banded iron formations and dolomites of the Hamersley Basin and overlying Middle Proterozoic Bangemali Basin braunite-containing sediments.     

Reworked manganese ore bodies in Bonai-Keonjhar belt,Singhbhum Craton,India: Petrology and genetic study

The genetic evolution of three types of reworked manganese ore bodies namely: Detrital, Concretionary (Mangcrete) and Wad in the Precambrian Iron Ore Group occurring in Bonai-Keonjahr belt, Singhbhum Craton, India are reported. All the reworked Mn-ore bodies are developed in a restricted area and have a limited resource. Mangcrete and wad are commonly exposed at the surface and extend to a maximum depth of 10 m while detrital ores are observed below 10–20 m from the surface.Detrital ore bodies occur as large boulders and are buried under a thick zone of laterite. Mangcrete is concretionary in nature; oolitic, spherulitic and nodular in shape. Broken fragmented of ooloids and pisoloids, often observed in mangcrete, are indications of reworking. Wad exposures are noticed above low to medium-grade bedded manganese ore bodies. Among three reworked ore types, the detrital is of low to medium-grade having Mn:Fe ratio > 5, while wad and mangcrete are of sub-grade (Mn:Fe ~ 1) and off-grade type (Mn:Fe < 1) respectively.Detrital ore bodies are of allochthonous nature and developed through several stages such as fragmentation of pre-existing ore, leaching and cementation followed by transportation and deep burial. Mangcrete represent chemogenic precipitates at several stages of contemporary Mn-Fe-Al rich fluid under supergene environment. Wad is of bio-chemogenic origin and developed in a swampy region under marine environment due to slow chemical precipitation of Mn-Fe enriched fluid, in several stages nucleating quartz/hematite/cryptomelane detritals.     

New geochemical and mineralogical constraints on the genesis of the Vani hydrothermal manganese deposit at NW Milos island,Greece: Comparison with the Aspro Gialoudi deposit and implications for the formation of the Milos manganese mineralization

The Mn-Ba-Pb deposit at Aspro Gialoudi in NW Milos is shown to be a fossil inhalative-exhalative hydrothermal deposit that represents the deepest part of the Vani succession at the western extremity of the main Vani manganese deposit. The geology of the Vani-Aspro Gialoudi area is characterized by Upper Pliocene-Lower Pleistocene dacitic and rhyodacitic lava domes, which are overlain by the Vani volcaniclastic unit considered to be part of the 2.66–1.44 Ma magmatic event at Milos Island. The presence of in-situ and intrusive hyaloclastite breccias surrounding the coherent lava domes at Aspro Gialoudi and Vani areas indicates submarine emplacement for the domes. The dacitic-rhyodacitic domes are variously altered (mainly propylitic and/or argillic alteration, silicified and in some cases locally exhibiting adularia alteration). Both Aspro Gialoudi and main Vani deposit are located proximal to fault systems: the main Vani manganese deposit is adjacent to the NW-trending Kondaros-Katsimouti-Vani Dome fault, whereas the Aspro Gialoudi deposit is adjacent to the relatively minor NE-trending fault on the west coast of Milos. At Aspro Gialoudi, mineralization took place in a subseafloor and/or seafloor environment and is characterized by a stratabound Mn-barite-rich deposit mainly within a package of propylitized intrusive hyaloclastites and within the overlying sandstones. Banded epithermal veins trending NE-SW and composed of chalcedonic silica/quartz + barite + Mn-oxide ± sulfides crosscut the dacitic lavas, the hyaloclastites and the overlying volcaniclastic sequence at Aspro Gialoudi and are considered to represent the feeder zones of the manganese-barite mineralization. Within the veins, early sulfide (galena-sphalerite) barite and quartz deposition is followed by manganese oxides and aragonite, thus resembling the epithermal-style Pb-Zn-Ag-Mn mineralization across the NW-trending Katsimoutis-Kondaros-Vani fault. Mineralization in Aspro Gialoudi and Vani deposits seems to be controlled by alternating cycles of deposition of sulfides and hydrothermal manganese oxides within the faults. Manganese deposition in both deposits formed in a similar manner, namely by transport of hydrothermal fluids through the adjacent fault systems into a reservoir of volcanoclastic sandstone and hyaloclastites to produce a deposit initially consisting of principally of pyrolusite and occasionally ramsdellite, which were subsequently replaced by cryptomelane, hollandite, coronadite and hydrohaeterolite. Precipitation of hydrothermal manganese oxides took place very quick and under microbial Mn(II) oxidation. Compositional data show that metallic elements most enriched in the Aspro Gialoudi and Vani manganese deposits relative to the average continental crust, lie in the sequences Pb > Cd > Mn > As > Sb > Zn > W > Tl > Ba > Cu > Mo > Co > Bi and As > Sb > Pb > Mn > Tl > Cd > Zn > W > Cu > Ba > Mo > Co, respectively. Mineralogical and geochemical (e.g. REE) data from both Aspro Gialoudi and main Vani deposit are taken to indicate mainly a seawater source for the hydrothermal fluids. These two deposits are genetically and spatially related to base- and precious metal intermediate-sulfidation epithermal mineralization. They formed successively by similar processes and are considered to be integral parts of the same hydrothermal system.     

Geology,mineralogy and fluid inclusion data of the Kizilcaören fluorite–barite–REE deposit,Eskisehir, Turkey

The Kizilcaören fluorite–barite–Rare Earth Element (REE) deposit occurs as epithermal veins and breccia fillings in altered Triassic metasandstones and Oligocene–Miocene pyroclastics adjacent to alkaline porphyritic trachyte and phonolite. This deposit is the only commercial source of REE and thorium in Turkey. Most of the fluorite–barite–REE mineralisation at Kizilcaören has been formed by hydrothermal solutions, which are thought to be genetically associated with alkaline volcanism. The occurrence of the ore minerals in vuggy cavities and veins of massive and vuggy silica indicate that the ore stage postdates hydrothermal alteration. The deposit contains evidence of at least three periods of hypogene mineralisation separated by two periods of faulting. The mineral assemblage includes fluorite, barite, quartz, calcite, bastnäsite, phlogopite, pyrolusite and hematite as well as minor amounts of plagioclase feldspar, pyrite, psilomelane, braunite, monazite, fluocerite, brockite, goethite, and rutile. Fluid inclusion microthermometry indicates that the barite formed from low salinity (0.4–9.2 equiv. wt% NaCl) fluids at low temperatures, between 105 and 230 °C, but fluorite formed from slightly higher salinity (<12.4 equiv. wt% NaCl) fluids at low and moderate temperatures, between 135–354 °C. The depositional temperature of bastnäsite is between 143–286 °C. The local coexistence of liquid- and vapour-rich inclusions suggests boiling conditions. Many relatively low-salinity (<10.0 equiv. wt% NaCl), low and moderate temperature (200–300 °C) inclusions might be the result of episodic mixing of deep-saline brines with low-salinity meteoric fluids. The narrow range of δ³⁴S (pyrite and barite) values (2.89–6.92‰ CDT)suggests that the sulphur source of the hydrothermal fluids are the same and compatible with a volcanogenic sulphate field derived from a magmatic sulphur source.     

广东梅县锰矿物质成份的研究

随着我国钢铁工业和化学工业的迅猛发展,对锰矿资源的需求,日益增加.梅县锰矿公司对该县的锰矿地质和锰矿生产做了大量工作.在前人工作的基础上,我们在锰矿资源的调研中,曾对广东省梅县的宝山岗、白沙坪、桃尧大华、宝坑、仙水塘、磔角坑、车陂等地的锰矿体、进行过采样工作.经室内鉴定后、梅县的锰矿石有优质的放电锰矿石和冶金用锰矿石、矿床规模属于中小型.梅县锰矿资源的生产,继续已有20多年的历史,在矿床的质和量方面尚需做更深入的研究,以便为矿山开采和锰矿生产提供更充分的依据.本文是对锰矿物质成分初步研究的部分结果.     

Geology and chronology of the Zhaofayong carbonate-hosted Pb–Zn ore cluster: Implication for regional Pb–Zn metallogenesis in the Sanjiang belt,Tibet

Mississippi Valley type (MVT) Pb–Zn deposits can occur in orogenic thrust belts. However, the relationship between MVT ore-forming processes and thrusting is unclear. The 1500-km-long Sanjiang Metallogenic Belt in Tibetan Plateau is an important thrust-controlled MVT ore province with 860 Mt at 0.76–2.3% Pb, 0.3–6.1% Zn. The Zhaofayong MVT ore cluster in the Changdu area is a typical sample. The orebodies in this ore cluster are hosted in limestone, controlled by secondary faults to regional thrusts and forming along these faults. Two Pb–Zn mineralization stages in this cluster are recognized. Stage I is characterized by coarse and euhedral galena + sphalerite + calcite + fluorite + barite and Stage II by fine grained sphalerite + galena + pyrite + calcite. Sm–Nd isotopic dating of calcite forming in Stage I yields isochron ages of 41.1–38.1 Ma, suggesting the mineralization formed during extension following the first regional compression in the Changdu area. The connection between Stage I mineralization and the regional thrusting in the Changdu area can extend to the whole Sanjiang belt. Two stages of regional Pb–Zn mineralization are recognized between 65 Ma and 30 Ma and between 30 Ma and 16 Ma in the belt. The two Pb–Zn mineralization stages are consistent with those regional episodic thrusting activities and both of them immediately occurred after the episodic thrusting. An interpretation of the regional Pb–Zn mineralization is that regional compression forced the movement of hydrothermal fluids along regional thrust-nappe detachment faults and subsequent post-thrust extension caused the migration of hydrothermal fluids to the ore forming locations. The two mineralization stages in the Sanjiang Belt indicate complex processes related to India–Eurasia collision and the gradually younger mineralization ages from southeast to northwest indicate the collision follows the same direction.     

Petrography,mineralogy and geochemistry of the Late Eocene oolitic ironstones of the Jebel Ank,Southern Tunisian Atlas

The oolitic ironstones ore deposit of Jebel Ank (central Tunisia), is a simply folded stratiform ore body of about 2.5–8 m thickness located in the upper part of the epicontinental Souar Formation (Late Eocene) and is covered by the continental Segui Formation (Mio-Pliocene). The deposit contains about 20 Mt of ore with an average grade of 50% Fe. Generally, oolitic iron deposition occurs in shallow water lagoonal environments. The Jebel Ank deposit lies between two regional disconformities (Late Eocene and Miocene), and is evidence of a transitional stage at the end of regional regression before renewed transgression. The footwall of the oolitic iron ore-bearing bed consists of a fine-grained sandstone bed (10–20 cm-thick) pinching out laterally westward into green clays. The hanging wall is composed of thin-bedded limestone and clay alternations (2–3.5 m-thick).Iron occurs in the form of cryptocrystalline goethite with limited Al-Fe substitution. The goethite contains around 48% Fe, 5% Al and up to 1.5% P. Jarosite, alunite and manganese minerals (cryptomelane, psilomelane and manjiorite) are supergene secondary minerals, probably related to descending surface fluids. These manganese minerals occur as accessory minerals with the goethite and are most abundant at the lowermost part of the succession showing varied morphologies (local cement, space filling and free centimeter sized nodules). Fe-oolites in the deposit are similar to those documented in many other oolitic ironstone deposits. The dominant Fe-oolite type (>90%) has a concentrically laminated cortex with no nucleus. The nuclei of the oolites that do have a nucleus are most commonly detrital quartz grains.Major elements in high grade samples (Fe₂O₃ > 65%) vary within a limited range and show higher concentrations of SiO₂ (average 7.85%) and Al₂O₃ (average 5.1%), with minor TiO₂, MnO, MgO, Na₂O, K₂O, and SO₃ (less than 1%). PAAS-normalized trace elements of bulk samples and Fe-oolite generally show similar behavior, both are enriched in V, Co, Ni, Mo, As, Zn, and Y and are depleted in Cu, Rb, Zr, Nb, Ba, and Hf. Anomalous V, Cr, Ni, Zn, and REE-Y are correlated with goethite. PAAS-normalized REE-Y patterns of both bulk samples and Fe-oolite show slight HREE enrichment, positive Ce with negative Y anomalies.The mineralogy (goethite and cryptomelane) along with the geochemistry (Si vs. Al; As + Cu + Mo + Pb + V + Zn vs. Ni + Co binary plots; Zn–Ni–Co triangular diagram, REE-Y content and patterns and Ce/Ce¹ vs. Nd and Ce/Ce¹ vs. Y_N/Ho_N binary plots) of the studied oolitic ironstone are congruent with a hydrogenetic type. While two possible sources of iron for Jebel Ank ironstone can be proposed: (i) submarine weathering of glauconite-rich sandstone and (ii) detrital iron from adjacent continental hinterland, the later is the more plausible source of iron, based on paleogeographic setting, the occurrence of fine sandstone underlying the iron level, occurrence of Mn-ores in the lower part of the Fe-ores succession, high phosphorous, zinc, ∑REE-Y concentrations and Y/Ho ratios, and low La/Ce ratios.     

Jurassic microbial communities in hydrothermal manganese crust of the Rifian Calcareous Chain,Northern Morocco

The stratigraphic record of the Middle and Upper Jurassic in the Western Tethys is characterized by successive eustatic and tectonic events recorded as stratigraphic unconformities, which are revealed by hardgrounds, palaeokarsts, palaeosoils, and by the deposition of Fe–Mn crusts. The study of a Mn crust from the Middle-Upper Jurassic discontinuity in the Jbel Moussa Group (Rifian Cordillera), from stratigraphic, geomicrobiologic, mineralogical, and geochemical standpoints allows us to establish its hydrothermal origin. The manganese crust is composed by Ca-birnessite, cryptomelane and coronadite. Major- and trace-elements analyses of the whole crust show high contents in MnO (> 70 wt.%), a negative Ce anomaly and a positive Eu anomaly. Analysis of the microstructures under scanning electron microscopy reveals crystalline and microbial laminations, probably owing to fungal mycelium. Mineralogical and geochemical composition, together with microbial structures, suggest that this Mn crust formed as a result of venting hydrothermal fluids through synsedimentary faults. Chemosynthetic microbes were probably involved in the precipitation of Mn.     

Structural,textural, geochemical and isotopic signatures of synglaciogenic Neoproterozoic banded iron formations (BIFs) at Bafq mining district (BMD), Central Iran: The possible Ediacaran missing link of BIFs in Tethyan metallogeny

The Neoproterozoic magnetite–apatite–hematite–pyrolusite–jaspilite deposits in the Bafq mining district (BMD) contain more than 1.7 Gt ores with an average grade of 50 wt.% Fe and 0.01 to 7.78 wt.% P and were probably formed between 635 and 547 Ma in a riftogenic felsic submarine exhalative sequence of the Esfordi Formation. The ore zones occur in proximal zone of magnetite-rich albitized rhyolite (keratophyres), interdistal zone of rhyolitic tuff–tuffaceous sediments and distal zone of pyrolusite–jaspilite. These sequences are covered by the diamictites and cap carbonates. The BIFs are linked to the altered rhyolites–rhyodacites, jaspilites and diamictites and contain magnetite, hematite and apatite. The presence of Spriggina, Dickinsonia, Medusites and Persimedusites chahgazensis (Sennewald and Krüger, 1979; Hahn and, Pflug; McCall, 2006) in the Kushk shale member of the Esfordi Formation conforms to the Australian fauna of the Ediacaran period (635–540). This relative age is supported by some reliable Pb isotopic data (635–547 Ma) on galena, monazite and apatite (Huckriede et al., 1962; Torab, 2008; Stosch et al., 2011). The most frequent structures–textures in the ore zones include felsic autobrecciation, massive, colloidal, banded, detrital and glaciogenic. The BIFs are highlighted by high values of LREE fractionation and no significant Eu and Ce anomalies. The ores show high values of Fe₂O₃ (14–60%), and SiO₂ (4–34%), and low contents of Al (3.32%), Cr (21.48 ppm), Co (42.82 ppm), Ni (125 ppm), V (868 ppm), and Ti (0.13%) similar to those of the Ediacaran–Rapitan BIFs. The cap carbonates show depletion in δ¹³C, with a range from − 0.43 to − 6.6 per mil and then return to near excursion of about + 2.97‰ in the Lower Cambrian carbonates. These are followed by δ¹⁸O values, which range from − 6.64 to − 11.86‰. The presence of jaspilites, diamictites, C and O isotopic signatures, REE patterns, and immobile element relationships highlights a glaciogenic BIF. Importantly, the glaciogenic structures–textures and jaspilites do not support the misconception of the previously proposed magmatic–carbonatitic and metasomatic–hydrothermal IOCG–Kiruna ore deposits.     

Recognition of Yanshanian magmatic-hydrothermal gold and polymetallic gold mineralization in the Laowan gold metallogenic belt,Tongbai Mountains: New evidence from structural controls,geochronology and geochemistry

The Laowan metallogenic belt in China is an important metallogenic belt within the Tongbai orogenic belt, and contains the medium-sized Laowan and Shangshanghe gold deposits, the small Huangzhuyuan lead–zinc–silver–gold deposit and some gold and Cu–Pb occurrences. These deposits are hosted in Mesoproterozoic plagioclase amphibolite (or schist) and mica-quartz schist. The gold ores are mainly quartz veins and veinlets and disseminated altered ores. Subordinate ore types include massive sulfides and breccias. The Laowan gold deposit is characterized by three right-stepping en-echelon fracture-controlled alteration zones that dip gently to the south and includes disseminated, sheeted and stockwork ores. These lodes were formed by the interaction of ore-forming fluid with foliated-to laminated cataclasite within the transpressional faults. The Shangshanghe gold deposit is characterized by parallel ore lodes that dip steeply to the north, and includes quartz veins and breccias in addition to ores in altered wallrocks. These lodes were formed by focusing of fluids into transtensional faults. These ore controlling faults displaced early barren quartz veins 10 m horizontally with a dextral sense of motion. The ore-hosting structures at the Laowan and Shangshanghe deposits correspond to the P and R-type shears of a brittle dextral strike-slip fault system, respectively, which make angles of about 15° and − 15° to the Laowan and Songpa boundary faults. The ore-controlling fault system post-dated formation of a ductile shear zone, and peak regional metamorphism. This precludes a genetic relationship between hydrothermal mineralization and regional metamorphism and ductile shear deformation. These gold deposits are not typical orogenic gold deposits. The metallogenic belt displays district-scale-zoning of Mo → Cu–Pb–Zn–Ag → Au relative to Songpa granite porphyry dike zone, suggesting the mineralization may be closely related to the granite porphyry. Measured δ³⁴S of sulfides and δ¹⁸O and δD of fluid inclusion waters in auriferous quartz also are consistent with a magmatic source for sulfur and ore fluids. The similarity of Pb isotope ratios between the ores and Yanshanian granitoids suggests a similar source. As the age (139 ± 3 Ma) of granite porphyry obtained by zircon U–Pb isotope overlaps the mineralization age (138 ± 1 Ma: Zhang et al., 2008a), the gold and polymetallic metallogenesis of the Laowan gold belt has close spatial, temporal and possibly genetic relationships with Yanshanian high level magmatism.     

Young ores in old rocks: Proterozoic iron mineralisation in Mesoarchean banded iron formation,northern Pilbara Craton,Australia

The origin of bedded iron-ore deposits developed in greenstone belt-hosted (Algoma-type) banded iron formations of the Archean Pilbara Craton has largely been overlooked during the last three decades. Two of the key problems in studying these deposits are a lack of information about the structural and stratigraphic setting of the ore bodies and an absence of geochronological data from the ores. In this paper, we present geological maps for nearly a dozen former mines in the Shay Gap and Goldsworthy belts on the northeastern margin of the craton, and the first U-Pb geochronology for xenotime intergrown with hematite ore. Iron-ore mineralisation in the studied deposits is controlled by a combination of steeply dipping NE- and SE-trending faults and associated dolerite dykes. Simultaneous dextral oblique-slip movement on SE-trending faults and sinistral normal oblique-slip movement on NE-trending faults during initial ore formation are probably related to E–W extension. Uranium–lead dating of xenotime from the ores using the sensitive high-resolution ion microprobe (SHRIMP) suggests that iron mineralisation was the cumulative result of several Proterozoic hydrothermal events: the first at c. 2250 Ma, followed by others at c. 2180 Ma, c. 1670 Ma and c. 1000 Ma. The cause of the first growth event is not clear but the other age peaks coincide with well-documented episodes of orogenic activity at 2210–2145 Ma, 1680–1620 Ma and 1030–950 Ma along the southern margin of the Pilbara Craton and the Capricorn Orogen farther south. These results suggest that high-grade hematite deposits are a product of protracted episodic reactivation of a structural architecture that developed during the Mesoarchean. The development of hematite mineralisation along major structures in Mesoarchean BIFs after 2250 Ma implies that fluid infiltration and oxidative alteration commenced within 100 myr of the start of the Great Oxidation Event at c. 2350 Ma.     

Mineral chemistry and geochemical behavior of hydrothermal alterations associated with mafic intrusive-related Au deposits at the Atud area,Central Eastern Desert,Egypt

Vein-type gold deposits in the Atud area are related to the metagabbro–diorite complex that occurred in Gabal Atud in the Central Eastern Desert of Egypt. This gold mineralization is located within quartz veins and intense hydrothermal alteration haloes along the NW–SE brittle–ductile shear zone, as well as along the contacts between them. By using the mass balance calculations, this work is to determine the mass/volume gains and losses of the chemical components during the hydrothermal alteration processes in the studied deposits. In addition, we report new data on the mineral chemistry of the alteration minerals to define the condition of the gold deposition and the mineralizing fluid based on the convenient geothermometers. Two generations of quartz veins include the mineralized grayish-to-white old vein (trending NW–SE), and the younger, non-mineralized milky white vein (trending NE–SW). The ore minerals associated with gold are essentially arsenopyrite and pyrite, with chalcopyrite, sphalerite, enargite, and goethite forming during three phases of mineralization; first, second (main ore), and third (supergene) phases. Three main hydrothermal alteration zones of mineral assemblages were identified (zones 1–3), placed around mineralized and non-mineralized quartz veins in the underground levels. The concentrations of Au, Ag, and Cu are different from zone to zone having 25–790 ppb, 0.7–69.6 ppm, and 6–93.8 ppm; 48.6–176.1 ppb, 0.9–12.3 ppm, and 39.6–118.2 ppm; and 53.9–155.4 ppb, 0.7–3.4 ppm, and 0.2–79 ppm for zones 1, 2, and 3, respectively.The mass balance calculations and isocon diagrams (calculated using the GEOISO-Windows program) revealed the gold to be highly associated with the main mineralized zone as well as sericitization/kaolinitization and muscovitization in zone 1 more than in zones 2 and 3. The sericite had a higher muscovite component in all analyzed flakes (average X_Ms = 0.89), with 0.10%–0.55% phengite content in wall rocks and 0.13%–0.29% phengite content in mineralized quartz veins. Wall rocks had higher calcite (CaCO₃) contents and lower MgCO₃ and FeCO₃ contents than the quartz veins. The chlorite flakes in the altered wall rocks were composed of pycnochlorite and ripidolite, with estimated formation temperatures of 289–295 °C and 301–312 °C, respectively. Albite has higher albite content (95.08%–99.20%) which occurs with chlorite in zone 3.     

Modification of a Palaeoproterozoic porphyry-like system: Integration of structural,geochemical, petrographic,and fluid inclusion data from the Aitik Cu–Au–Ag deposit,northern Sweden

The Aitik Cu–Au–Ag deposit in the Gällivare area in northern Sweden is Sweden's largest sulphide mine with an annual production of 35 Mt of ore, and the biggest open pit operation in northern Europe. It is proposed in the present study that the Aitik deposit represents a Palaeoproterozoic, strongly metamorphosed porphyry copper deposit that was affected ca. 100 Ma later by a regional IOCG-type hydrothermal event. Consequently, the Aitik deposit might represent a mixed ore system where an early copper mineralisation of porphyry type has been overprinted by later regional IOCG mineralisation.Several attempts have previously been made to genetically classify the Aitik Cu–Au–Ag deposit as a distinct ore type. New geochemical, petrographic, structural, and fluid inclusion results combined with published data have provided the opportunity to present new ideas on the genesis and evolution of the Aitik Cu–Au–Ag deposit. The emplacement of a ca. 1.9 Ga quartz monzodiorite that host the ore at Aitik was related to subduction processes and volcanic arc formation, and synchronous with quartz vein stockwork formation and porphyry copper mineralisation. Highly saline aqueous (38 wt.% NaCl) fluid inclusions in the stockwork veins suggest entrapment at 300 °C and a pressure of nearly 3 kbar, a high pressure for a typical porphyry copper ore, but consistent with conditions at associated deep root zones of intrusion-related magmatic–hydrothermal systems. The highly saline fluid formed disseminated and vein-type ore of mainly chalcopyrite and pyrite within comagmatic volcaniclastic rocks, and caused potassic alteration (biotite, microcline) of the host rocks. The early porphyry copper mineralising event was followed, and largely overprinted, by CO₂ and aqueous medium- to high-salinity (16–57 wt.% salts) fluids related to a ca. 1.8 Ga tectonic and metamorphic event (peak conditions 500–600 °C and 4–5 kbar). Extensive deformation of rocks and redistribution of metals occurred. Magnetite enrichment locally found within late veins, and late amphibole–scapolite and K feldspar alterations within the deposit, are some of the features at Aitik implying that aqueous fluids responsible for IOCG-mineralisation (200–500 °C and ~ 1 kbar) and extensive Na–Ca alteration in the region during the 1.8 Ga tectonic event also affected the Aitik rocks, possibly leading to addition of copper ± gold.     

The age of supergene manganese deposits in Katanga and its implications for the Neogene evolution of the African Great Lakes Region

Supergene manganese deposits commonly contain K-rich Mn oxides with tunnel structure, such as cryptomelane, which are suitable for radiometric dating using the ³⁹Ar–⁴⁰Ar method. In Africa, Mn deposits have been dated by this method for localities in western and southern parts of the continent, whereas only some preliminary data are available for Central Africa. Here we present new ³⁹Ar–⁴⁰Ar ages for Mn oxide samples of the Kisenge deposit, in southwestern Katanga, Democratic Republic of the Congo. The samples represent supergene Mn oxide deposits that formed at the expense of primary Paleoproterozoic rhodochrosite-dominated carbonate ores. Main phases of Mn oxide formation are dated at c. 10.5 Ma, 3.6 Ma and 2.6 Ma for a core that crosses a mineralized interval. The latter shows a decrease in age with increasing depth, recording downward penetration of a weathering front. Surface samples of the Kisenge deposits also record a ≥ c.19.2 Ma phase, as well as c. 15.7 Ma, 14.2 Ma and 13.6 Ma phases. The obtained ages correspond to distinct periods of paleosurface development and stability during the Mio-Pliocene in Katanga. Because Katanga is a key area bordered to the North by the Congo Basin and to the East by the East African Rift System, these ages also provide constraints for the geodynamic evolution of the entire region. For the Mio-Pliocene, the Kisenge deposits record ages that are not systematically found elsewhere in Africa, although the 10.5–11 Ma event corresponds to a roughly simultaneous event in the Kalahari Manganese Field, South Africa. The rest of the Katanga paleosurface record differs somewhat from records for other parts of Africa, for which older, Eocene ages have been obtained. This difference is most probably related to the specific regional geodynamic context: uplift of the East African Plateau, with associated erosion, and the opening of the East African Rift System at c. 25 Ma are events whose effects, in the study area, interfere with those of processes responsible for the development of continent-wide paleosurfaces.     

Ore-structure relationships at Sishen Mine,Northern Cape,Republic of South Africa,based on fully-constrained implicit 3D modelling

A fully-constrained, implicit, 3D geological model of Sishen Mine reveals the original, pre-mining geometry of ore bodies, host rocks to mineralization and major structures. There are several overlapping controls, at a variety of scales, on the position, depth and geometry of laminated and conglomeratic ore. Most of these controls are structural or may be reconciled with the kinematic history of this part of the Maremane Dome. A series of near-horizontal sections, through the entire 3D model, demonstrates the manner in which these controls overlap and interact. First-order or large-scale controls comprise broad domes, which show preservation of laminated ore around their rims, outside of which conglomeratic ore occurs. Second-order controls comprise grabens and half-grabens, which are often bounded by strike-persistent normal faults, which show fault drag on their western flanks due to inversion, along with preservation of BIF-related supergene ore and conglomeratic ore. A type example is the thick, deep, linear ore to the west of the Sloep Fault. Third-order controls on the preservation of mineralization comprise downthrown blocks to the north of reactivated E-W, SE/ESE- or NE/ENE-trending conjugate faults. Upthrow to the south could be attributed to the 1.15–1.0 Ga NNW-directed Lomanian (Namaqua-Natal) Orogeny. Palaeosinkholes comprise fourth-order controls, which are superimposed on higher-order controls. Palaeosinkholes, which form the bulk of current mining, comprise deep, conical depressions with anomalous thicknesses of chert, chert breccia and haematite. Due to their limited size, the steepness of all units and the often chaotic nature of detached and slumped blocks in their centres, these volumes reflect longstanding models on palaeosinkhole development and very local ore control.     

Microplaty hematite ore in the Yilgarn Province of Western Australia: The geology and genesis of the Wiluna West iron ore deposits

The Wiluna West small (~ 130 Mt) high-grade bedded hematite ore deposits, consisting of anhedral hematite mesobands interbedded with porous layers of acicular hematite, show similar textural and mineralogical properties to the premium high-grade low-phosphorous direct-shipping ore from Pilbara sites such as Mt Tom Price, Mt Whaleback, etc., in the Hamersley Province and Goldsworthy, Shay Gap and Yarrie on the northern margin of the Pilbara craton. Both margins of the Pilbara Craton and the northern margin of the Yilgarn craton were subjected to sub-aerial erosion in the Paleoproterozoic era followed by marine transgressions but unlike the Hamersley Basin, the JFGB was covered by comparatively thin epeirogenic sediments and not subjected to Proterozoic deformation or burial metamorphism. The Joyner's Find greenstone belt (JFGB) in the Yilgarn region of Western Australia was exhumed by middle to late Cenozoic erosion of a cover of unmetamorphosed and relatively undeformed Paleoproterozoic epeirogenic sedimentary rocks that preserved the JFGB unaltered for nearly 2 Ga; thus providing a unique snapshot of the early Proterozoic environment.Acicular hematite, pseudomorphous after acicular iron silicate, is only found in iron ore and BIF that was exposed to subaerial deep-weathering in early Paleoproterozoic times (pre 2.2 Ga) and in the overlying unconformable Paleoproterozoic conglomerate derived from these rocks and is absent from unweathered rocks (Lascelles, 2002). High-grade ore and BIF weathered during later subaerial erosion cycles contain anhedral hematite and acicular pseudomorphous goethite. The acicular hematite was formed from goethite pseudomorphs of silicate minerals by dehydration in the vadose zone under extreme aridity during early Paleoproterozoic subaerial weathering.The principal high-grade hematite deposits at Wiluna West are interpreted as bedded ore bodies that formed from BIF by loss of chert bands during diagenesis and have been locally enriched to massive hematite by the introduction of hydrothermal specular hematite. No trace of chert bands are present in the deep saprolitic hematite and hematite–goethite ore in direct contrast to shallow supergene ore in which the trace of chert bands is clearly defined by goethite replacement, voids and detrital fill. Abundant hydrothermal microplaty hematite at Wiluna West is readily distinguished by its crystallinity.The genesis of the premium ore from the Pilbara Region has been much discussed in the literature and the discovery at Wiluna West provides a unique opportunity to compare the features that are common to both districts and to test genetic models.