Minor Shear Zones Research Articles

Lineaments Characterization and their Tectonic Significance in Mineralization using Landsat TM Data and Field Studies along Mambilla Plateau, Northeast, Nigeria

Lineament characterization and their tectonic significance in mineralization using Landsat TM data and field studies Along the Mambilla Plateau and environs, northeast Nigeria .The area forms part of the mineralized shear zone of the Northeastern, part of the Basement Complex. The Basement Complex rocks underlie more than two-thirds of the plateau and date back to the Precambrian to early Paleozoic eras. While the remaining part of the plateau and its surroundings are made up of volcanic rocks of the upper Cenozoic to Tertiary and Quaternary ages. Remote sensing imagery data and field measurements were integrated for mapping the structural elements and hydrothermal alteration zones related to mineralization. Tectonically the lineaments trends NE-SW, NNW-SSE, NW-SE, and NS, are the main trends controlling mineralization in the area based on data from remote sensing and field measurements. On the other hand, applying band ratios and the constrained energy minimization (CEM) technique to Landsat-8 data pointed out alteration minerals associated with the hydrothermal fluids. Silica, sericite, and chlorite are the common alteration minerals detected, while goethite and epidote are less common in the alteration zones of the area under study. A combination of CET structural complexity, circular features, and alteration zones produced a potential mineralization map, pointing out numerous prospective zones for mineralization that are proximate to areas where artisanal mining activities are carried out. The predictive map was validated by projecting the known prospect zones in the study area, reflecting an agreement between the known occurrences and the higher classes of the predictive zone. The techniques in the current research can be adopted as a direct indicator for structural mapping and detecting alteration zones related to mineralization elsewhere.

Thermo-tectonic evolution of the Neoarchaean Southern Marginal Zone of the Limpopo granulite Complex (South Africa)

Abstract Combined geophysical, structural geological, metamorphic, geochronological, and stable isotope information is employed to elucidate the Neoarchaean thermo-tectonic evolution of the Southern Marginal Zone (SMZ) within the Limpopo Complex (South Africa) during the Limpopo orogeny (2.72 to 2.62 Ga). The complex evolutionary history of the SMZ was controlled by an allochthonous SMZ granulite nappe that was extruded from a rising granulite diapir through a process of mid-crustal heterogeneous channel flow. This granulite nappe with its embedded structures (steeply plunging reclined folds and steep shear zones) was formed during emplacement of the diapir to mid-crustal level (6 kbar, 20 km depth) from where it was thrust south-westwards along the Hout River shear zone (HRSZ) sole thrust against the Kaapvaal Craton (KVC) at 2.72 to 2.69 Ga. Evidence for the thermo-tectonic interaction of the granulite nappe with the KVC includes (1) thrust complexes (referred to as hot-iron zones) that are developed at the frontal ramp sections of the HRSZ juxtaposed against the granite-greenstone belts of the KVC, and (2) strike-slip shear deformation associated with the lateral ramp section of the HRSZ, which developed against the KVC devoid of greenstone belts. The emplacement of the post-tectonic Matok granitic pluton at ~2.68 Ga into the SMZ signified the end of the thermo-tectonic event that established the regional fold- and shear deformational framework of the granulite facies SMZ. Post-Matok secondary shear zones reflect evidence for HRSZ-linked tectonism that continued intermittently to 2.65 to 2.62 Ga. Low H2O-activity fluids (H2O activity of 0.1 to 0.3) released from devolatilisation of underthrust greenstone material passively infiltrated and interacted with the overlying cooling granulites. This established a retrograde anthophyllite-in isograd at ~6 kbar and ~620°C that subdivides the SMZ into a northern granulite domain and a southern retrograde hydrated granulite domain. Simultaneously, gold-bearing fluids focused into these minor shear zones established shear zone-hosted orogenic gold mineralisation at 2.65 to 2.62 Ga. Emplacement of the post-tectonic Palmietfontein granite at ~2.46 Ga and associated sub-volcanic granitic dykes into both the retrograde hydrated granulite domain and the granulite domain signifies the end of all thermo-tectonic activity in the SMZ. A Palaeoproterozoic thermal overprint at ~2.1 Ga is recorded by Rb-Sr biotite and phlogopite ages derived from various rocks from the SMZ and adjacent KVC. This thermal event is not associated with deformation and did not result in the formation of new mineral assemblages. Integrated data presented and discussed in this paper contradict the interpretation of age and petrological data utilised to support alternative models for the evolution of the SMZ, including a proposed ~2.1 Ga Palaeoproterozoic polymetamorphic amphibolite-grade thermo-tectonic event.

An integrated study of Landsat ETM and Cartosat DEM data in identification of banded iron formations (BIFs) associated with sulphide mineralization

This paper deals with the identification of BIFs and associated sulphide mineralisation. An integrated approach, including the use of Landsat ETM-plus and Cartosat DEM data, GIS analysis, and geological data, is adopted for this purpose in the Nagavi area of Gadag Schist Belt (GSB), India. This integrated approach has enabled in identifying BIFs and structures. Band-7 of the ETM-plus sensor of Landsat-7 is used to identify BIFs and Band-5 for lineaments and shear zones. As a result of this study, the presence of gold mineralisation in sheared zones is noticed. BIFs are the economically prominent litho-units in the GSB hosting high-grade iron ore deposits along with sulphide mineralised shear zones. The strata bound ore is hosted primarily by BIF, consisting of chlorite, alternating chert and magnetite, sulphides and carbonate bands of a millimetre to centimetre scale.

Ore mineralogy and trace element (re)distribution at the metamorphosed Lappberget Zn-Pb-Ag-(Cu-Au) deposit, Garpenberg, Sweden

Qualitative and quantitative mineralogical and textural investigations were undertaken for the different sulfide minerals in the 1.89 Ga Lappberget deposit, with a focus on sphalerite and pyrite. Three sphalerite types were identified and associated with: (1) main massive sulfide mineralization, (2) Fe-Mn skarn ore, and (3) Ag-rich fissure veins. Sphalerite-1 contained a relatively higher amount of trace elements (Fe, Co, Ge, Cu, Sn, and Au) but is lower in Mn/Fe, Ga, and Hg compared to sphalerite-2. Evidence of tectono-metamorphic modification of sulfides is observed throughout the deposit, such as the formation of growth twins and deformation twins in sulfides, the development of high-angled triple-point junctions in recrystallized pyrite and sphalerite, and the formation of ‘ball ore’ or ‘durchbewegung’ textures within mineralized shear zones. The recrystallization of pyrite and galena may have led to the re-distribution of fluid-mobile elements (As, Cu, Zn, Pb, Ag, and Au), some of which were deposited in discordant Ag-rich fissure veins peripheral to the main massive sulfide lenses. Trace element signatures of sulfides in Lappberget are compatible with the other metamorphosed and deformed, subseafloor volcanogenic carbonate replacement (SVALS)-type deposits in the Bergslagen mining district.

Studies on uranium extraction from leach liquor of the mineralized shear zone at El-Missikat area using tri-n-octylamine

Fast extraction of uranium from Egyptian low-grade uranium mineralization sulfate leach liquor was studied with kerosene solutions of tri-n-octylamine (TOA). The parameters affecting uranium extraction were studied and the optimum conditions were determined. The equilibrium isotherm demonstrated the association of four moles of TOA per mole of uranium. Thermodynamic studies at different temperatures indicated that the extraction process was exothermic, favorable and spontaneous in nature. The organic phase loaded with uranyl sulfate under the optimum extraction conditions were examined by infrared spectrometry. The McCabe–Thiele plot for circuit of uranium with TOA predicted three extraction stages and two stripping stages.

Evidence for Multi-stage Melt Transport in the Lower Ocean Crust: the Atlantis Bank Gabbroic Massif (IODP Hole U1473A, SW Indian Ridge)

AbstractThe architecture of lower oceanic crust at slow- and ultraslow-spreading ridge is diverse, yet the mechanisms that produce this diversity are not well understood. Particularly, the 660-km2 gabbroic massif at Atlantis Bank (Southwest Indian Ridge) exhibits significant compositional zonation, representing a high magma supply end member for accretion of the lower ocean crust at slow and ultraslow-spreading ridges. We present the petrographic and geochemical data of olivine gabbros from the 809-metre IODP Hole U1473A at Atlantis Bank gabbroic massif. Structurally, the upper portion of U1473A consists of a ∼600-metre shear zone; below this, the hole is relatively undeformed, with several minor shear zones. Olivine gabbros away from the shear zones have mineral trace element compositions indicative of high-temperature reaction with an oxide-undersaturated melt. By contrast, olivine gabbros within shear zones display petrographic and chemical features indicative of reaction with a relatively low-temperature, oxide-saturated melt. These features indicate an early stage of primitive to moderately evolved melt migration, followed by deformation-driven transport of highly evolved Fe–Ti-rich melts to high levels in this gabbroic massif. The close relationship between shear zones and the reaction with oxide-saturated melts suggests that syn-magmatic shear zones provide a conduit for late-stage, Fe–Ti-rich melt transport through Atlantis Bank lower crust. This process is critical to generate the compositional zonation observed. Thus, the degree of syn-magmatic deformation, which is fundamentally related to magma supply, plays a dominant role in developing the diversity of lower ocean crust observed at slow- and ultraslow-spreading ridges.

A trace metal, stable isotope (H, O, S), and geochronological (U-Pb titanite) characterization of hybridized gold orebodies in the Missanabie-Renabie district, Wawa subprovince (Canada)

Mineralized shear zones in the Archean Missanabie-Renabie gold district (~ 1.1 Moz Au; Wawa, Ontario, Canada) locally define composite orebodies that record three hydrothermal events: (1) a pre-orogenic Au1 event (pre-D1 and pre-prograde-metamorphic); (2) a syn-orogenic, post-peak-metamorphic Retrograde event (syn-D3); and (3) a late syn-to post-orogenic Au2 event (late syn- to post-D4). Genetic considerations indicate the orebodies are hybrids with early intrusion-related (Au1) and later orogenic (Retrograde + Au2) events. Pearson product-moment correlation coefficients (log10) of whole-rock and LA-ICP-MS pyrite trace metal datasets distinguish Au1 from Au2 mineralization by Au-Ag, Au-Bi, and Au-Te correlations > 0.7 (p < 0.05) in the former, irrespective of sample medium and analytical method. An Au-Mo correlation in whole rock data (0.58–0.76; p < 0.05) further distinguishes Au1 from Au2 and supports an independently inferred intrusion-related origin for Au1. Sulfur isotope data is similar for both Au1 and Au2 pyrite with average δ34S values of − 5.5‰ ± 0.2‰ (1σ) and − 3.5‰ ± 0.3‰ (1σ) and average Δ33S values of 0.4‰ ± 0.1‰ (1σ) and − 0.3‰ ± 0.2‰ (1σ), respectively. SIMS δ18Oquartz values for the Au1, Retrograde, and Au2 events largely overlap and, like δ18Ocarbonate values of previous studies, tend to be lower than values typical of Archean gold deposits. The results of this study suggest that correlation coefficients in trace metal datasets are useful in discriminating and characterizing different gold events. Caution is emphasized with the use of S- and O-isotope datasets for these purposes. The presence of low δ18O values in vein quartz and carbonate is best explained by an 18O-depleted fluid formed during the Retrograde hydrothermal event. The latter is inferred at 2580 ± 21 Ma based on U-Pb geochronlogy of hydrothermal titanite, and relates to deformation and metamorphism in the nearby, amphibolite- to granulite-grade Kapuskasing metamorphic belt. Geochronological and geochemical evidence suggest that the 18O-depleted fluid may have formed via the devolatilization of biotite-bearing granitoids during deep-crustal metamorphism.

Source of gold in Neoarchean orogenic-type deposits in the North Atlantic Craton, Greenland: Insights for a proto-source of gold in sub-seafloor hydrothermal arsenopyrite in the Mesoarchean

Given that gold (Au) mostly remained in the incipient Earth mantle until ca. 3.9–3.8 Ga, a “proto-source” of gold may have been present in the dominantly mafic crust precursor born through first-stage melting of the early Earth mantle. In south-westernmost Greenland, a fragment of the North Atlantic Craton is characterised by greenstone belts comprising mafic volcanic and magmatic rocks, and harzburgite cumulates that were emplaced at ca. <3.19–3.01 Ga (e.g., Tartoq greenstone belt). Here, combining detailed sulphide petrography with rhenium-osmium-sulphur (Re-Os-S) isotope geochemistry of individual mineral separates of arsenopyrite from gold-sulphide mineralised shear zones, we pinpoint the precipitation of ca. 3.18–3.13 Ga (Re-Os model ages) hydrothermal arsenopyrite associated and coeval with arc-related magmatism of the Tartoq Group. We consider sub-seafloor hydrothermal alteration of the oceanic crust and magmatic activity to have supplied arsenic (As), Re, and Au, to result in the precipitation of the ca. 3.18–3.13 Ga arsenopyrite with primary invisible gold. Additionally, in major shear zones in a rigid juvenile continental crust, retrograde greenschist-facies metamorphism overprinted the ca. >3.0 Ga prograde amphibolite-facies metamorphic assemblages and caused local dissolution of arsenopyrite. During this retrograde tectono-metamorphic stage, in gold-rich shear zones, the Re-Os geochronometer in arsenopyrite was reset to a Neoarchean age while invisible gold was liberated and deposited as free gold with 2.66 Ga pyrite (Re-Os isochron ages). The initial Os isotope ratios of Neoarchean arsenopyrite (187Os/188Osi = 0.13 ± 0.02) and gold-bearing pyrite (0.12 ± 0.02) overlap with the estimated 187Os/188Os ratio of the Mesoarchean mantle (0.11 ± 0.01) and preclude contribution of radiogenic crustal Os from evolved lithologies in the accretionary arc complex, but instead, favour a local contribution in Os from basaltic rocks and serpentinised harzburgite protoliths by metamorphic fluids. Thus, the ca. 2.66 Ga lode gold mineralisation identified in the North Atlantic Craton may illustrate a gold endowment in shear zones in Earth’s stabilizing continental crust at the time of the 2.75–2.55 Ga Global Gold Event, through metamorphic upgrading of bulk gold which had originally been extracted from the Mesoarchean mantle and concentrated in hydrothermal arsenopyrite deposits in oceanic crust beneath the overall reduced Mesoarchean ocean.

Pan-African structural evolution of Paleoproterozoic basement gneiss and Cu-Co mineralized shear zones in the Domes Region of the Lufilian Belt, Mwombezhi Dome, Zambia

In the Domes Region of the Lufillian Belt, world-class stratiform Cu (±Co, ±Ni, ±U, ±Pb, ±Zn) deposits are closely associated with tectonized pre-existing basement unconformities, viz. the pre-Katanga-basement décollement (KD). In the KD footwall on the Mwombezhi Dome, the Lumwana Mine exploits mineralization within <60 m thick, laterally-extensive shear zones beneath kilometer-scale thrust sheets of Paleoproterozoic gneiss and schist. New field data and structural analysis highlight preferential strain localization, kinematics and sequential reactivation of multiple shear zone generations (D1-3) under high- to medium-grade, metamorphic conditions and clockwise P-T-t path during the Lufillian Orogeny (ca. 550-500). Regional, progressive, top-to-the-NNE fore-thrusting is assigned to D1, while D2 represents localized, opposite, top-to-the-SSW kinematics attributed to under-thrusting by dome-scale fore-thrusted basement slivers. Subsequent D3 shear zones transposed D1-2 structures and represent overall top-to-the-NNE extensional detachment and NNE-SSW-elongation along a maximum principal extension axis (L3) which defines kilometer-scale shear- or asymmetrical-boudins of competent gneiss in the KD footwall. A pre-existing NNE-SSW prolate dome asymmetry (L1-2) dictates a downlimb-directed sense of shear on the short axes of D3 asymmetrical boudins (WNW-ESE). D3 shear zones are preferentially localized along relict pre-Katanga anisotropy and D1-2 mylonitic horizons, while Cu–Co bearing ore shoots are associated with passive fold hinges in these asymmetrical boudin necks.

Control of Shear-Zone-Induced Pressure Fluctuations on Gold Endowment: The Giant El Callao District, Guiana Shield, Venezuela

The El Callao district, with a total endowment of more than 2000 t Au, is considered to be the most prolific gold resource in Venezuela. Mineralization is hosted by a vein system that is genetically associated with the El Callao transpressional shear zone. This vein system consists of a network of interconnected quartz–albite–ankerite veins enveloping a large number of metabasaltic fragments that host gold-bearing pyrites. Based on detailed mineralogical, microstructural, and fluid inclusion studies, a pressure-temperature pathway was established for the evolution of the mineralizing fluid during shear-zone development and exhumation. This path is characterized by repeated episodes of fluid pressure fluctuation from lithostatic (higher than 1.6 kbar) to near-hydrostatic values (&lt;0.4 kbar), recorded throughout the transition from the quasi-plastic to frictional deformation cortical domains. Each successive pressure drop induced boiling of the hydrothermal fluid, with the resulting fluid phase separation controlling: (i) pyrite and invisible gold crystallization, which occurred during ductile and ductile-brittle transition strain conditions, and (ii) primary gold remobilization with consequent native-refined gold precipitation, occurring mainly under brittle conditions. The metallogenic framework that was proposed for the El Callao shear zone can be used as a vector to explore and characterize other mineralized shear zones in the Guiana Shield and analogous orogenic systems worldwide.

Syn-tectonic sulphide remobilization and trace element redistribution at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, Bergslagen, Sweden

Mineralization types at the Palaeoproterozoic Falun base metal sulphide deposit are predominantly pyritic Zn-Pb-Cu-rich massive sulphide mineralization, disseminated to semi-massive Cu-Au mineralization, auriferous quartz veins, and mineralized shear zones of talc-chlorite-dominated schist. The massive and disseminated to semi-massive sulphide mineralization types were subject to polyphase ductile deformation (D1 and D2) and metamorphism under low-P, lower-amphibolite facies conditions, which led to the development of ore textures and paragenetic relationships indicating both mechanical and chemical remobilization of sulphides. In the massive sulphide mineralization, rare inclusion-rich pyrite occurs as relic cores inside inclusion-poor metamorphosed pyrite. Imaging and spot analysis using multielement laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) reveal that inclusion-poor pyrite was depleted in trace elements, which were originally present as non-stoichiometric lattice substitutions or in mineral inclusions. The inclusion-rich pyrite was shielded from depletion and, at least partly, retained its initially higher trace element concentrations, including Au.Gold is also associated with chalcopyrite in the disseminated to semi-massive Cu-Au mineralization and in the system of auriferous quartz veins hosted therein, the latter being also affected by the D2 ductile strain. It is inferred that emplacement of the vein system took place after the peak of metamorphism, which occurred between D1 and D2, but prior to and possibly even shortly after completion of the D2 deformational event. Similarities in trace element signatures in chalcopyrite are compatible with the interpretation that the quartz veins formed by local chemical remobilization of components from the Cu-Au mineralization. Transport of liberated Au from pyrite during grain growth in the massive sulphide mineralization may have upgraded the Au endowment in the quartz veins, leading to the additional formation of native gold in the veins. A strong correspondence between elements liberated from pyrite (e.g. Pb, Bi, Se and Au) and those forming discrete and characteristic mineral phases in the quartz veins (Pb-Bi sulphosalts, native gold) supports this hypothesis.Trace element signatures for the main sulphide minerals pyrite, chalcopyrite, sphalerite and galena are similar to previously published data from other metamorphosed massive sulphide deposits. The association of the Falun mineralization with elevated Bi is reflected by its occurrence in sulphide minerals (e.g. galena) and in abundant mineral inclusions of Pb-Bi sulphosalts (e.g. weibullite), especially in the disseminated to semi-massive Cu-Au mineralization. Elevated Sn concentrations in the lattice and/or as cassiterite inclusions in chalcopyrite, sphalerite and galena are compatible with a hot, acidic and reducing fluid during formation of the syn-volcanic, base metal sulphide mineralization and associated host-rock alteration.

НОВЫЙ ЗОЛОТОРУДНЫЙ РАЙОН В ТАНЗАНИИ

Tanzania is one of the leading gold mining countries in the world and the discovery of new gold resources on its territory is an actual task. Known gold deposits are concentrated mainly in the northwest of the country, in the metallogenic zone of Lake Victoria, where they are associated with the Archean greenstone belts, and to a lesser extent – in the southwest, in the ore regions of Lupa and Mpanda, confined to the Ubendian Paleoproterozoic mobile belt. With regard to the eastern regions of Tanzania, where the Proterozoic structures of the Uzagaran mobile belt are developed, until recently in this region any significant manifestations of gold mineralization were not known. As a result of our research in the northern part of the Morogoro province of the Republic of Tanzania, a new previously unknown gold deposit Mananila was discovered. It is represented by a large volume, up to 400–450 m long, up to 60–80 m thick, mineralized shear zone over intensely leached and schistosed migmatites, gneisses, amphibolites, penetrated by echelon systems of quartz veins and veinlet, steeply dipping bodies of quartz breccia up to 1.0–1.5 m thick. Gold contents range from 0.61 to 8.11 g/t, the average zone content is 2.5–3.0 g/t. Parallel to the main zone, similar structures are developed on the site, although they are of lower thickness. The forecast resources of the deposit are estimated at 20 tons of gold. 2.8 km to the east from the Mananila field, the recently discovered Mazizi gold deposit is located, and a number of small occurrences of gold are also known in the region. All these objects are located within a large shear zone of the northeastern strike, up to 4–5 km width, over 20 km in length. This serves as the basis for the identification of a new gold ore region in the northern part of the Morogoro province of the United Republic of Tanzania, within the Proterozoic mobile belt of Usagaran, the possible gold content of which has never been previously discussed in geological literature.

Orebody geometry, fluid and metal sources of the Omitiomire Cu deposit in the Ekuja Dome of the Damara Belt in Namibia

The Omitiomire Cu deposit (resource of 137 Mt at 0.54% Cu) in the Ekuja Dome of the Damara Belt in Namibia is hosted by an anastomosing, low-angle Pan-African (ca. 520 Ma) shear zone system developed around an older (ca. 1100–1060 Ma), late Mesoproterozoic intrusive breccia between a suite of mafic rocks (originally lava flows) and later tonalitic gneisses. High-grade ore shoots preferentially formed along contacts between tectonically interleaved biotite-epidote-quartz-chalcocite schists and felsic gneisses, and are directly related to an increase in the number and cumulative thickness of thin, contact-parallel mineralized shear zones. Alteration and mineralization are associated with elevated concentrations of K2O, Cr, Rb, S, and Cu and a loss of Na2O, CaO, and MgO. Oxygen isotope fractionation for quartz-biotite, quartz-feldspar, and quartz-amphibole mineral pairs support equilibrium temperatures of between 500 and 650 °C during the fluid/rock interaction. Mineral separates from amphibole-biotite gneisses and mineralized schists have similar ranges in δ18O values of about 1.2 to 2 ‰ relative to VSMOW. Coexisting minerals are arranged in an order of increasing δ18O values from biotite, to epidote, amphibole, and quartz, suggesting that the Omitiomire Shear Zone was a rock-dominated system. Similarly, H-isotope results for mineral separates from biotite-epidote schists and amphibole gneisses do not show any reversals for D/H fractionations, with δD values of between −48 and −82 ‰, typical of metamorphic-magmatic rocks. The homogeneous and low δ34S values (−6.1 to −4.7 ‰ CDT) are compatible with a local redistribution of sulfur from magmatic rocks and interaction with sulfur derived from metamorphic fluids of metasedimentary origin. The relatively low fluid/rock ratios and elevated Cu values (>1500 ppm) from unaltered amphibolite point to a local redistribution of an earlier (late Mesoproterozoic) Keweenaw-type Cu mineralization into later Pan-African shear zones during the exhumation of the Ekuja Dome. The timing, polyphase evolution, and tectonic setting of the Omitiomire deposit show remarkable similarities with the large Cu deposits of the Domes Region in the adjoining Lufilian Arc of northern Zambia. This suggests the presence of a much larger, regionally significant Cu province extending from central Namibia, through northern Botswana, and into Zambia.

Gold on the Kaapvaal Craton, outside the Witwatersrand Basin, South Africa

Gold occurs in a variety of ancient geological environments in South Africa, the most well-known being the unique Witwatersrand Basin in the central part of the Kaapvaal Craton. Outside of the Witwatersrand Basin significant quantities of gold have been recovered from Archaean greenstone belts of the 3500 to 2700 Ma Kaapvaal Craton as well as from various occurrences in the 2600 to 2200 Ma Transvaal Supergroup. Very little gold has been found in younger formations. Archaean greenstone belts in South Africa are characterised by substantial sequences of ultramafic (komatiitic) volcanic rocks generally overlain by mafic volcanic successions which, in turn, are overlain in some cases by greywacke-shale sequences and younger arenaceous lithologies. Highly sheared and carbonate altered ultramafic zones, together with the immediately surrounding country rock, host some of the most important gold deposits on the Kaapvaal Craton. In the northern part of the Barberton Greenstone Belt, shearing tends to be most intense along the contact of the ultramafic schists of the Zwartkoppie Formation and the overlying greywacke-shales, the contact being marked by the presence of a chert “bar” or banded iron formation (BIF) horizon. Gold mineralisation occurs below, above and/or within the more competent and fractured chert/BIF marker horizon. The locus of gold mineralisation is typically controlled by the rheological characteristics of the wallrocks in the proximity of the shear systems, e.g. the ductile greywacke-shale strata are tightly folded with sub-parallel mineralised shear zones, while competent arenaceous rocks have yielded in a brittle manner resulting in high-angle mineralised fractures. Extensive carbonate alteration in ultramafic rocks is often present along, and in the vicinity of, shear zones where potassium alteration results in a vivid green coloration due to the formation of fuchsite (Cr-mica) and where zones of silicification and quartz veining are closely associated with gold, arsenic and antimony mineralisation. Where mafic rocks act as more competent and fractured hosts to mineralisation, carbonate alteration results in the breakdown of ferromagnesian minerals and the liberation of considerable quantities of silica, manifested as silicification and as mineralised quartz veins. The breakdown of mafic minerals results in “bleaching” of the rocks as carbonate minerals replace ferromagnesian minerals, but may also result in the development of green, brown or black schists depending on the composition of the host rocks. One or more of the above alteration rock types and mineralisation styles is present in virtually all of the greenstone-hosted gold deposits of the Kaapvaal Craton. Wherever available, geochemical data is consistent with alteration fluids of relatively uniform composition but the style of the resultant mineralisation is ultimately dependent on the nature of the host rock. Greenstone gold deposits are irregularly distributed, both in terms of their size and distribution. Only about 1% of greenstone deposits can be regarded as successful mines, while the vast majority remain as showings. Geographically, these gold deposits are preferentially located along litho-structural features where both fluid flow and wallrock conditions were suitable for mineralisation. These criteria are restricted to major structural nodes, significant lineaments, or lithological units, with the result that the stronger mineralising systems tend to be concentrated in finite gold “camps” which host the larger deposits and numerous styles of mineralisation, e.g., the Sheba-Fairview Complex on the refolded arcuate Eureka and Ulundi synclines. Gold hosted in BIFs is a widespread association, but rarely economic except where the nature of the mineralisation allows for economic extraction, e.g., in the Kraaipan Greenstone Belt, where the volume of BIF is large enough to contain the mineralised fracture system. Gold in the Paleoproterozoic Transvaal Supergroup occurs in the lowermost Black Reef Formation quartzites as detrital accumulations in conglomerate channels, but more abundantly as epithermal veins in the dolomite of the overlying Malmani Subgroup and shale of the Pretoria Group as vertical cross-cutting veins and sub-horizontal reefs emplaced along parting planes. The quartz-carbonate-sulphide veins differ from Archaean ores in that gold rarely impregnates the wallrocks; they contain significant carbon which impedes gold recovery, and they contain silver, copper, arsenic, bismuth, lead and zinc in addition to gold.

Structural Controls on an Orogenic Gold System: The World-Class Siguiri Gold District, Siguiri Basin, Guinea, West Africa

Orebodies in the Siguiri district, a world-class Paleoproterozoic orogenic gold camp located in the Birimian of northeastern Guinea, are typically represented by cryptic subvertical damage zones that host a high density of mineralized veins. Although no large regional fault system was recognized, observations from five representative deposits of the Siguiri district (Sanu Tinti, Bidini, Kami, Kosise, and Sintroko PB1) show that these orebodies are locally controlled by incipient structures and spread across three distinct structural and lithostratigraphic domains. Two shale-dominated peripheral domains adjoin a central domain whose lithostratigraphy is dominated by medium- to coarse-grained graywacke and this domain hosts the bulk of the gold endowment of the district. The three domains exhibit similar structural elements that can be described within a four-stage deformation scheme. The first deformation event (D 1S ) is poorly constrained and interpreted to have been an N-S compressional event. It included development of minor folds with W- to WNW-gently plunging fold axes without a clear axial planar cleavage. The main and second deformation event (D 2S ) is interpreted to have been associated with E-W to ENE-WSW- directed compression. The D 2S event was responsible for forming the dominant N-trending structural grain of the district and creating interference patterns between F 1S and F 2S folds. The third event (D 3S ) developed progressively from D 2S compression into an early-D 3S E-W- to ENEWSW-directed transpression and a late-D 3S NNW-SSE-directed-transtension responsible for most of the gold mineralization in the Siguiri district. The fourth and last event (D 4S ) was an NW-SE-oriented compressional event responsible for the localized overprinting of veining by a steep to shallowly dipping NNE-SSW ductile cleavage. Late-D 3S gold-bearing mineral occurrences formed along subvertical N-S reverse faults, NE-trending dextral shear zones, WNW-trending sinistral faults, and E-trending normal relay faults developed or reactivated early-D 3S . Mineralization is expressed as mineralized shear zones or subvertical damage zones, characterized by a 10- to 15-m-wide zone of dense quartz-carbonate-sulfide veining, or disseminated gold-bearing sulfides. The mineralized veins consistently strike ENE-WSW, are steeply dipping, and commonly have a conjugate geometry at the mesoscale. Finite strain analysis of deformation, including analysis of folds, faults, and conjugate mineralized vein sets, is consistent with a stress switch from a compressional (D 2S ) to transpressional deformation (termed early-D 3S ). Results of paleostress analysis on conjugate mineralized vein sets that formed late during D 3S indicate that the stress field ranged from extensional to strike-slip, sometimes within the same vein locality. The late-D 3S deformation is interpreted to have been a transtensional event. The first change in the orientation of the principal stress axes is related to a switch from a far field-dominated to a body force-dominated stress field reducing the deviatoric component on the stress tensor. The second change in the orientation of the principal stress axes, from early-D 3S transpression to late-D 3S transtension, suggests that σ 1 and σ 2 were similar in magnitude, which facilitated localized stress switches. In the Siguiri district, the early-D 3S and late-D 3S stress switches, which occurred at both a local and regional scale, enhanced the fracture permeability and were critical for the establishment of active fluid pathways leading to the formation of a world-class gold system.

Shear-zone hosted copper mineralisation of the Omitiomire deposit — Structural controls of fluid flow and mineralisation during subduction accretion in the Pan-African Damara Belt of Namibia

The Omitiomire copper deposit is a relatively recent discovery in the Pan-African Damara Belt of central Namibia. The deposit is situated in Mesoproterozoic gneisses and amphibolites of the Ekuja Dome overlain by amphibolite-grade metaturbidites of the Southern Zone accretionary prism that formed during northward subduction of the Kalahari Craton below the Congo Craton between ca. 580–520Ma. Copper mineralisation is confined to an anastomosing system of shallowly-dipping, retrograde mylonitic shear zones within the Ekuja Dome. The shear zones are centred around a lithologically heterogeneous amphibolite-gneiss sequence. Mylonitisation and copper mineralisation are closely associated with the retrogression of particularly amphibolites and the partial or complete replacement of amphibolites by biotite–epidote and biotite–chlorite–epidote schists that host the chalcocite-dominated mineralisation.Deformation and mineralisation in the heterogeneous shear-zone system can be shown to describe a progression. Initial strain localization is confined to lithological (amphibolite-gneiss) contacts and associated quartz veining and fluid flow are preferentially developed around the margins of competent amphibolite units. Fluid infiltration and the retrogression of amphibolites to biotite–epidote schists leads to strain localization into the marginal schists that envelop amphibolites. Further veining and fluid flow are localised into the central parts of amphibolite units leading to the pervasive retrogression to biotite–epidote schists that dominate the central parts of the shear-zone system. Earlier quartz-vein generations appear as isoclinally folded and dismembered ribbons or boudins in mineralised schists. The clearly syntectonic introduction of the copper mineralisation is underlined by the intergrowth of chalcocite with the retrograde assemblages and chalcocite forming part of the mylonitic shear-zone fabric.3D modelling of drillhole data combined with limited surface exposure delineates a shallow east dipping, gently undulating ore body parallel to the regional gneissosity of the Ekuja Dome. The ore body comprises several mineralised lenses varying in thickness from 10m to >100m. Prominent ore shoots are gently doubly plunging to the N and S and parallel to the regionally developed L>S fabric in the gneisses. Kinematic indicators in the mineralised shear zone system point to a top-to-the S sense of shear, parallel to the regional L fabric and parallel to the southverging transport recorded in the structurally overlying prism metasediments.The regional setting of the Omitiomire deposit, kinematics, and retrograde, but high-temperature overprint of original mineral assemblages in the mineralised shear zones indicate deformation and fluid flow during the expulsion of the basement gneisses during N-ward direction subduction of the Kalahari Craton below the Congo Craton. Lithological, geochronological, structural and P–T data suggest numerous similarities and, indeed, correlations between the Omitiomire-style copper mineralisation of the Damara Belt with the large copper deposits hosted by basement gneisses in the Domes Region of the Lufilian Arc in Zambia.

Relationships between basin architecture, basin closure, and occurrence of sulphide-bearing schists: an example from Tampere Schist Belt, Finland

We present field observations from the Palaeoproterozoic volcano-sedimentary Tampere palaeobasin, where the primary structures have been exceptionally well preserved. We use the observations to construct a new tectonic model for the southeastern margin of the Tampere basin during its inversion and subsequent closure. The observed volcano-sedimentary and structural features suggest a change in the local structural style from thick-skinned inversion to thin-skinned thrusting, to accommodate the crustal shortening during basin closure. Furthermore, it is suggested that there is a genetic relationship between the interpreted palaeothrust and the sulphide-bearing schist horizons in the study area. On a more general note, the results imply that currently subvertical mineralized shear zones may have originally been gently dipping, further suggesting that the mineralized fluids may not necessarily have been sourced from great depths (i.e. from deep within the basement).

Tracking Late-Pan-African fluid composition evolution in the ductile crust of Madagascar: Insight from phase relation modelling of retrogressed gneisses (province of Fianarantsoa)

The basement of Madagascar has been submitted to a strong reworking during a late Pan-African (D2) shearing event. The influence of the relevant fluid transfer on phase relationships is investigated on a calc-silicate gneiss belonging to the Ikalamavony domain, deformed and retrogressed, from low-temperature amphibolite to upper-greenschist metamorphic conditions. The rock is characterized by different generations of pyroxene growing with epidote as coronitic structures around relics of garnet and magnetite. Phase diagram sections are commonly used to study phase relationships as a function of pressure and temperature in closed-systems. Here, the thermodynamic system has been considered as only open for the fluid components (i.e. H2O, CO2 and O2) in order to discuss the interaction between fluid flow and mineral phase evolution occurring during D2 deformation. Prediction in terms of phase composition and modal proportion of P–T–MH2O–CO2–O2 phase diagrams (using Perple-X calculations) are in good agreement with natural observations. They suggest that a large input of aqueous-carbonic fluid throughout the “paragneiss” during the exhumation of the rocks led to the destabilisation of the early Fe3+-rich garnet (andradite) and magnetite. Thermodynamic modelling illustrates how the replacement of such mineral assemblage by Fe2+-rich pyroxene (diopside), sphene, epidote and plagioclase (anorthite-rich) then amphibole (actinolite) and K-feldspar has preferentially been controlled by the amount of H2O and CO2 brought through minor shear zones during the D2 reworking, rather than by change of the effective bulk rock composition.

Mineral chemistry and genesis of Zr, Th, U, Nb, Pb, P, Ce and F enriched peralkaline granites of El-Sibai shear zone, Central Eastern Desert, Egypt

El-Sibai mineralized shear zone trending NNE-SSW is located at the northern segment of Gabal El-Sibai (500 m in length and 0.5 to 1.5 m in width). Rocks along the shear zone show different types of alterations such as hematization, kaolinitization, fluoritization, and silicification. These alterations are good traps for rare metals of thorite, ferrocolumbite, pyroclore, plumbopyroclore, fluorite, cerite-(Ce), zircon, Th-rich zircon, zirconolite (mixture of zircon & columbite), fluorapatite, titanite, and monazite minerals. The detailed mineralogical study of the El-Sibai shear zone revealed its enrichment in Th, Zr, Nb, Pb, U, F, P, LREE (Ce), especially concerning the hematization processes. The close correlation of ferruginated (hematitized) samples with high radioactivity is related to the high ability of iron oxides for adsorption of radioactive elements from their Solutions. The rare-metal minerals found in altered peralkaline granites (shear zone) are associated with hematitization, albititization, chloritization, fluoritization, and pyritization. Electron probe microanalysis (EPMA) provides an indication of a ränge of solid Solution between thorite and zircon, in which intermediate phases, such as Th-rich zircon were formed. These phases have higher sum of all cations per formula (2.0 to 2.09 atoms per formula unit, for 4 oxygen atoms) than that of ideal thorite and zircon. This is attributed to the presence of substantial amount of interstitial cations such as U, Y, Ca, and AI in these phases. Altered zircon enriched in Th and U (Th-rich zircon) preferentially involves coupled Substitution Ca2+ + (Th,U)4+ ↔ 2Zr4++ 2Si4+, implying that significant amount of U and Th may enter the Zr and Si position in zircon. Thorite and Th-rich zircon are related to hydrothermal fluid. Also the genesis of the studied zircon is related to metasomatic hydrothermal zircon (MHZ). The abundantly detected zircon, Th-rich zircon, Th-bearing minerals and fluorite of demonstrably hydrothermal origin can be attributed to the role of fluorine-rich fluids. Although Zr and Th are generally considered as highly immobile elements, yet the occurrence of zircon indicates that their significant concentrations can be transported under specified F-rich fluids. The hydrothermal origin could be accepted for the thorite, huttonite monazite, zircon, Th-rich zircon, ferrocolumbite, pyroclore, plumbopyrochlore, zirconolite, fluorite, cerite-(Ce), fluorapatite within the El-Sibai altered peralkaline granites (shear zone).

On the occurrence of gold mineralization in the Pala Neoproterozoic formations, South-Western Chad

The Pala region, in southwestern Chad, belongs to the northern part of the Central African Pan-African Fold Belt. It is made up of greenschist-facies schists and is characterized by bimodal, mainly mafic, magmatism. This schist unit named Goueigoudoum Series is intruded by pre- to post-tectonic plutonic rocks dated between 737 and 570Ma and dykes of quartz. Gold is mined artisanally from alluvial deposits and primary chalcopyrite–pyrite-bearing quartz veins, brecciated and silicified zones and shear zones. The majority of the mineralized shear zones and some quartz veins generally trend N–S to NNE–SSW or NW–SE and are interpreted as extensional shear fractures related to regional NE–SW-trending sinistral strike–slip shear zones. The geological context of the Pala region clearly indicates hydrothermal fluids formed along active continental margins during collisional orogenesis, and subsequent associated fluid migration typically occurred during strike–slip events. Although the origin of fluids may be varied (magmatic, metamorphic or meteoric fluids, Proterozoic seawater, or sedimentary basin formation waters), the distribution of the mineralizations along the granitoid intrusions suggests that magmatism played a major role in the dynamics of the mineralizing fluids.