Brittle-ductile Shear Zones Research Articles

Structural Modification of the Komatiite-Associated Harmony Nickel Sulfide Deposit, Leinster, Western Australia

The Harmony nickel sulfide deposit (3.2 Mt of ore) is situated along the eastern margin of the Agnew-Wiluna greenstone belt, Eastern Goldfields, Western Australia. The deposit occurs 2 km north of the larger Rocky’s Reward (15.9 Mt) and Perseverance (50.7 Mt) deposits. The mine stratigraphy trends north-northwest–south-southeast, dips moderately (30°–80°) to the west, is overturned, and metamorphosed to middle-amphibolite facies grades (about 550°C, 3 kbars). Stratigraphic footwall units to the west of the nickel-bearing komatiite include ca. 2720 to 2725 Ma (volcano-) sedimentary rocks and rhyodacite volcanic and volcaniclastic units. Hanging-wall units comprise mainly reworked sedimentary or volcaniclastic rocks interlayered with at least two barren spinifex-textured komatiite units exposed in the mine area. Exhalative pyrite-rich pelitic units and thin amphibolite units are locally present in the footwall and hanging wall. The nickeliferous komatiite is thickest in the center of the deposit and thins to the north, south, and at depth. Although massive sulfides and minor disseminated sulfides are mainly located along the sheared western footwall contact, minor massive sulfides occur throughout the komatiite and country rock in fold hinges, boudin neck areas, brittle-ductile shear zones, piercement cusps, piercement veins, and brittle faults. Footwall and hanging-wall units have a strong composite fabric defined by compositional layering and several subparallel axial planar foliations. East-west shortening resulted in the folding of the composite fabric to form abundant north-northwest–trending, tight, asymmetric, moderately (20°–40°) N- and S-plunging folds. The asymmetric folds thicken the komatiite and orebodies in the center of the pit and control deposit-scale orebody plunge directions. The attenuated north-northwest–trending komatiite fold limbs are boudinaged and concentrate pods of massive sulfides in boudin neck positions. Relaxation in horizontal shortening coincided with open recumbent folding of the steeply dipping limbs of earlier upright asymmetric folds. A resumption of east-west shortening in a brittle stress regime resulted in the remobilization of massive sulfides into east-west–trending brittle faults. The concentration of massive sulfides in brittle-ductile to brittle structures indicates that massive sulfide remobilization spanned a diverse range of deformation and metamorphic conditions. The present geometry and distribution of stratigraphy and orebodies at Harmony reflect a complex interplay of primary magmatic and secondary overprinting processes. Remobilized massive sulfide orebodies are an important exploration target in other deformed and metamorphosed terranes that host primary Ni-Cu-(PGE) deposits. Although footwall units are the preferential host to remobilized massive sulfide shoots, massive sulfides are commonly remobilized within shear zones and faults zones and hosted by hanging-wall units up to 50 m from their primary footwall position. In cases of folding, orebodies tend to be elongate parallel to the plunge direction of fold axes or mineral stretching lineation, and potential exploration targets occur along the trend of known folded ore shoots and more specifically along the plunge line of such shoots.

The tectono-metamorphic evolution of the central part of the Neoproterozoic Allaqi–Heiani suture, south Eastern Desert of Egypt

The Precambrian basement of the Um El Tuyor area represents the central part of the ca. 750–720 Ma Allaqi–Heiani suture in the extreme south of the Egyptian Eastern Desert. It is composed of allochtonous ophiolitic thrust slices and detached sheets, island arc metavolcanic–volcanoclastic and metasedimentary assemblages, and syn- to post-orogenic granitoid intrusions. These rocks are collectively referred to as a fold-and-thrust belt in view of their deformational styles, and as ophiolites and their lithologic association are concerned. Structural analysis reveals three major deformation events, namely: (a) D1: an early thrust-and-fold system, through which overturned to recumbent folds and major thrusts were developed and later overprinted by a sinistral transpression, (b) D2: a NE–SW-compressional regime that deformed the D1 structures by major folds (F 2) and penetrative axial planar NW–SE foliations (S 2) and associated major NW–SE sinistral faults and shear zone, and (c) D3: a late ENE–WSW compressional strain increment expressed by a series of NNW–SSE striking folds and crenulations (F3). Related right-lateral NNW–SSE strike slip faults and discrete brittle–ductile shear zones indicate a later transcurrent shear which is best developed in the metasedimentary successions. Mineral associations and porphyroblast-matrix fabrics in the pelitic metasediments indicate three phases of regional metamorphism (M1 → M3), syn-kinematic with D1, D2 and D3 respectively. Based on the petrographic and microprobe data, the peak metamorphic conditions were attained at 534°–561 °C/5.26–6.20 kbar, under which garnet, staurolite and sillimanite coexisted during D2. Although associated with the most significant metamorphic event, D2 corresponds roughly to a late collisional stage (650–620 Ma) during which the accreted Pan-African terrains attached to the East Saharan Craton. This is further confirmed by the D2-related sinistral strike slip faults that affiliate to the ca. 630–540 Ma Najd strike slip fault system in the Arabian–Nubian Shield. Criteria including an anticipated fold–thrust vergence generally to northwesterly directions, discrete mesoscopic duplexes, shear bands, and minor folds and thrust zones all indicate that the ophiolites were emplaced roughly from northwest to southeast. However, deformation of the ophiolite basal contacts and associated fabrics argue that the northwestward rooting of the ophiolitic massif in the study area cannot be considered as a criterion for the original direction of tectonic transport, which was likely from north to south if the entire Allaqi–Heiani belt is considered. Such deviation is best attributed to rotation through deformation by the N–S Hamisana shear zone. This principally agrees with the tectonic evolution of the Arabian–Nubian Shield in south Egypt proposed previously, which includes an early N–S arc–arc collision followed by a stage of E–W crustal shortening.

Geology and geochemical aspects of lode gold mineralization at Dimako–Mboscorro, SE Cameroon

The Dimako–Mboscorro lode gold deposit has two styles of mineralization: quartz veins (4.5–54 ppm Au) and altered wallrock (up to 2.6 ppm Au). The veins occur in steeply dipping brittle–ductile shear zones in a foliated biotite metagranite. The alteration envelop around the gold-bearing quartz veins shows lateral zoning comprising a silicified/ ferruginized and mylonitic zone (SFZ) at the centre, through a transitional sericite zone (SZ), to an outer, slightly sheared, but mineralogically less altered, though bleached, metagranite zone (BZ) at the shear zone boundaries. Hydrothermal alteration in these zones is characterized by silicification (SFZ, SZ), hematite formation (SFZ), alkali metasomatism with muscovite/sericite formation (SFZ, SZ, to a much lesser extent BZ), and pyritization (SFZ, SZ). The auriferous quartz veins are concordant to the host structure but discordant to the foliation in the metagranite. The main ore minerals in the veins and in the SFZ and SZ zones are gold, hydrothermal hematite, sphalerite, galena, pyrite, chalcopyrite, arsenopyrite, chalcocite, covellite and colloidal supergene hematite. Gold occurs as inclusions in sphalerite (15.7±0.4 wt% Ag; 83.6±0.2 wt% Au), as coarse grains associated with euhedral hematite pseudomorphs (18.5±1.0 wt% Ag; 82.0±1.1 wt% Au), as stringers within fractures in quartz (13.3±0.1 wt% Ag; 87.1±0.5 wt% Au), as well as fine to coarse grains related to limonite, goethite and colloidal hematite (8.1±1.8 wt% Ag; 92.1±2.2 wt% Au). A hydrothermal metal association of Ag, Au, As, Ba, Pb, V, W, and Zn is identified in the wallrock and quartz veins. These metals, together with K-enrichment, are potential pathfinder elements for wider lithogeochemical exploration in this region, especially when combined with the lateral wallrock zonation pattern. Preliminary fluid inclusion data from the auriferous quartz vein samples indicate that gold was probably deposited from low salinity (<1 to 8 wt% eq. NaCl, with mean at 5.52 wt% eq. NaCl) H 2 O–CO 2 fluids that have total homogenization temperatures of 263–335°C.

Interferência entre duas faixas móveis neoproterozóicas: o caso das faixas Brasília e Ribeira no Sudeste do Brasil

Neoproterozoic metasedimentary successions of the Andrelândia Sequence, gneisses of the Guaxupe Massif and Paleoproterozoic ortogneisses, with igneous zircon of 2.088_26 Ma, constitute the Precambrian units in the Tres Coracoes region, southern Minas Gerais State. Deformational structures and metamorphic associations indicate two collisional events related to the Neoproterozoic orogeny, that originated the Brasilia and Ribeira fold belts.Two continuous deformational phases (DB1 and DB2) and a relatively high pressure metamorphic event (MB) were related to the Brasilia fold belt. Superposed and related to the Ribeira fold belt, two deformational phases (DR1 and DR2) and a lower pressure metamorphic event (MR) are recognized. DB1 and DB2 produced nappes with movement to ESE, E and ENE. MB metamorphism started during DB1, as indicated by metamorphic isograds that are dislocated by DB1 and DB2 structures. K-feldspar-in/muscovite-out and sillimanite-in isograds related to the granulite facies are concordant with DB2 thrust faults, showing that the MB peak was sin-DB2. Folds with steep axial planes and axes plunging toWSWwere produced during DR1 that folded the nappes lateraly. The fact that metamorphic isograds related to theMRmetamorphic event cutMBisogrades and pre-existing structures shows the superimposition of this event on the Brasilia fold belt evolution. Finally, the last deformational phase DR2 produced dextral transcurrent ductile-brittle shear zones with SW-NE strike, that cut structures and metamorphic isograds of both fold belts.

Na-Fe-Ca Alteration and LREE (Th-Nb) Mineralization in Marble andGranitoids of Sierra de Sumampa, Santiago del Estero, Argentina

The Jasimampa area in the Sierra de Sumampa de Santiago del Estero, Argentina, contains light rare earth element (LREE: Th-Nb) mineralization associated with several stages of carbonates and widespread fenitization of marble, granite (496 ± 2.9 Ma), and alkaline and siliceous igneous comendite dikes (U/Pb zircon age of 390 Ma) derived from fractionation of oceanic island basalts. This is the third discovery of LREE mineralization in Argentina and marks a new alkaline intracontinental magmatic event of Devonian age. Albite (Ab 97-99 ) + aegirine (Ae 70-92 Ja 1-11 Q 6-19 )± hematite ± Mg-arfvedsonite ± fluorine-rich apatite and late Ca-Fe-Mn carbonates ± monazite ± pyrochlore overprint and crosscut granite and comendite outcrops in a 15 km 2 area; locally, the Na-rich assemblage is pervasive and results in total destruction of host rocks. Na-Fe metasomatism near granite and comendite produced aegirine (Ae 86-91 Ja 1-8 Q 6-11 )± Mg-arfvedsonite and minor albite (Ab 98-99 ) in adjacent marble, with multiple stages of late Ca-Mn-Mg-Fe carbonates associated with iron and manganese oxides, hematite, and traces of sulfides (sphalerite, pyrite, galena). Monazite-(Ce) is the main LREE-bearing mineral and occurs with minor ferrocolumbite, pyrochlore, strontiopyrochlore, and apatite associated with a second generation of fine-grained hematite, barite, and celestine. Structural controls on Na-Fe-Ca alteration and LREE mineralization in granitoid and marble protoliths include brittle-ductile shear zones, brittle fracture systems, and a series of monolithologic and heterolithologic breccias with hydrothermal aegirine, carbonate, and chalcedony matrices that occur at granite-marble-comendite contacts. Fragments of multiple breccia types within breccias indicate multiple episodes of brecciation, alteration, and mineralization. The fluid responsible for early fenitization of granite and comendite was aqueous-carbonic with CO 2 of relatively high density (between 0.85 and 0.95 g/cm 3 ), salinity near 9 wt percent NaCl equiv, and minimum temperatures in the range of 250° to 280°C. Pervasively altered comendite and granite have high total REE content (1,750 ppm and 2,126 ppm, respectively), mainly LREE. Early banded and mottled aegirine ± arfvedsonite replacements of marble have high absolute REE abundance (2,513–4,663 ppm) compared to the least altered marble (476 ppm). Subsequent hydrothermal stages with carbonates, hematite, pyrolusite, monazite, and subordinate celestine and barite caused alteration of marble that was accompanied by an increase in absolute REE abundance, up to 15.45 wt percent, and high Sr (10.5 wt %), Th (4,390 ppm), and Ba (1.8 wt %). Alteration and mineralization at Jasimampa are the result of late-stage carbonatitic magma fractionation and fluid exsolution. This is indicated by the strongly alkaline character of early alteration, the composition of the hydrothermal carbonates, apatite, and LREE-Sr- and Ba-rich minerals, the chondrite-normalized REE patterns showing strong LREE enrichment without an Eu anomaly, and the replacement of marble by assemblages with very high Ba, Th, LREE, and Sr, and strong K, Zr, and Ti depletions. The alteration style and mineralization at Jasimampa are similar to the giant Fe-LREE-Nb deposit Bayan Obo and other hydrothermal LREE deposits hosted in carbonatites of China.

Gold mineralisation throughout about 45�Ma of Archaean orogenesis: protracted flux of gold in the Golden Mile, Yilgarn craton, Western Australia

The Golden Mile deposit was discovered in 1893 and represents today the largest Archaean orogenic lode gold system in the world (50 M oz produced gold). The Golden Mile deposit comprises three major styles of gold mineralisation: Fimiston, Oroya and Charlotte styles. Fimiston-style lodes formed at 250 to 350 °C and 100 to 200 MPa and are controlled by brittle–ductile fault zones, their subsidiary fault zone and vein networks including breccias and open-cavity-infill textures and hydrothermally altered wall rock. Fimiston lodes were formed late D1, prior to D2 regional upright folding. Hydrothermal alteration haloes comprise a progression toward the lode of diminishing chlorite, an increase in sericite and in Fe content of carbonates. Lodes contain siderite, pyrite, native gold, 17 different telluride minerals (Au–Ag tellurides contain ~25% of total gold), tourmaline, haematite, sericite and V-rich muscovite. Oroya-style lodes formed at similar P–T conditions as the Fimiston lodes and are controlled by brittle–ductile shear zones, associated dilational jogs that are particularly well developed at the contact between Paringa Basalt and black shale interflow sedimentary rocks and altered wall rock. The orebodies are characterised by micro-breccias and zones of intense shear zone foliation, very high gold grades (up to 100,000 g/t Au) and the common association of tellurides and vanadian mica (green leader). Oroya lodes crosscut Fimiston lodes and are interpreted to have formed slightly later than Fimiston lodes as part of one evolving hydrothermal system spanning D1 and D2 deformation (ca. 2,675–2,660 Ma). Charlotte-style lodes, exemplified by the Mt Charlotte deposit, are controlled by a sheeted vein (stockwork) complex of north-dipping quartz veins and hydrothermally altered wall rock. The Mt Charlotte orebody formed at 120 to 440 °C and 150 to 250 MPa during movement along closely spaced D4 (2,625 Ma) and reactivated D2 faults with the quartz granophyre in the Golden Mile Dolerite exerting a strong lithological control on gold mineralisation. Veins consist of quartz–carbonate–minor scheelite, and wall-rock alteration comprises chlorite destruction and growth of ferroan carbonate–sericite–pyrite–native gold. Pyrite–pyrrhotite is zoned on the scale of vein haloes and of the entire mine, giving a vertical temperature gradient of 50–100 °C over 1,000 vertical metres. The structural–hydrothermal model proposed consists of four major stages: (1) D1 thrusting and formation of Fimiston-style lodes, (2) D2 reverse faulting and formation of Oroya-style lodes, (3) D3 faulting and dissecting of Fimiston- and Oroya-style lodes, and (4) D4 faulting and formation of Mt Charlotte-style sheeted quartz vein system. The giant accumulation of gold in the Golden Mile deposit was formed due to protracted gold mineralisation throughout episodes of an Archaean orogeny that spanned about 45 Ma. Fluid conduits formed early in the tectonic history and persisted throughout orogenesis with the plumbing system showing a rare high degree of focussing, efficiency and duration. In addition to the long-lasting fluid plumbing system, the wide variety of transient structural and geochemical traps, multiple fluid sources and precipitation mechanism contributed towards the richest golden mile in the world.

Late tectonic evolution of the Northern Apennines: the role of contractional tectonics in the exhumation of the tuscan units

Structural analysis performed in the continental tuscan units of the Northern Apennines (Italy) highlight the presence of transversal and parallel structures, with respect to the main trend of the belt (NW-SE), affecting the tectonic nappe pile. These structures, following the main synmetamorphic phases associated to isoclinal folds with axial plane foliations, are upright folds, brittle and brittle-ductile shear zones. Nearly upright folds are related to NW-SE and NE-SW directions of shortening. The two perpendicular directions of shortening, associated with a contraction setting, induced the vertical growth of the metamorphic domes enhancing the process of exhumation of the metamorphites. The presence of later NE-verging brittle-ductile shear zones crosscutting both systems of folds testifies that contraction long lasted during the orogenesis. Fluid inclusion analyses, performed in syntectonic veins coeval with NW-SE trending upright folds, point out that, after the first tectonic phase, pressure values decrease supporting that this tectonic unit was exhuming during the development of upright folds. Post-collisional extensional tectonics gave rise to collapse folds and low- and high-angle normal faults in the latest stages of deformation.

Hydrothermal Alteration, Ore Fluid Characteristics, and Gold Depositional Processes along a Trondhjemite-Komatiite Contact at Tarmoola, Western Australia

Tarmoola is a structurally controlled Archean orogenic gold deposit hosted in greenschist facies metamorphosed komatiite and trondhjemite in the Leonora district of the Eastern Goldfields province, Yilgarn craton. High-grade (>1 g/t Au) orebodies are located in komatiite wall rock adjacent to the eastern and northeastern margins of the asymmetrical, north-south–striking, Tarmoola trondhjemite intrusion. Gold-bearing veins postdate trondhjemite emplacement (ca. 2700 Ma), quartz diorite dikes (ca. 2667 Ma), and regional greenschist facies metamorphism. Textures and crosscutting relationships in gold-bearing veins indicate two stages of hydrothermal fluid infiltration associated with a single gold-related hydrothermal event: a volumetrically dominant, but gold-poor, stage I fluid and a gold-rich stage II fluid. Gold-bearing veins contain stage I milky quartz and pyrite that are overprinted by stage II quartz-ankerite-muscovite-chalcopyrite-sphalerite-galena-gold-tellurides ± albite ± chlorite ± fuchsite ± epidote ± scheelite. Stage I hydrothermal alteration assemblages are different in trondhjemite and komatiite due to contrasting reactions between a common ore fluid and disparate wall-rock chemistry. Stage II fluid-wall rock interaction was minor compared to stage I and is indicated by the overprinting of stage I mineral assemblages by stage II microveins. Wall-rock alteration proximal to veins in trondhjemite is characterized by replacement of igneous plagioclase, amphibole, biotite, and metamorphic chlorite by hydrothermal quartz, muscovite, ankerite, calcite, pyrite, chalcopyrite, sphalerite, galena, tellurides, and gold, whereas in proximal alteration in komatiite, metamorphic chlorite and talc are replaced by ankerite, quartz, muscovite, albite, chlorite, fuchsite, pyrite, chalcopyrite, sphalerite, galena, tellurides, and gold. The stage II fluid was enriched in H2O, CO2, Si, Ca, K, Na, S, Au, Ag, Cu, Pb, W, Bi, As, Mo, Zn, and Te. Based on fluid inclusion studies and stage II mineral equilibria, gold deposited from a homogeneous, neutral to slightly alkaline (pH 5.1–5.5), reduced, low-salinity (<5.5 wt % NaCl equiv) fluid that had a bulk composition of 78 mole percent H2O and 21 mole percent CO2, and trace amounts of CH4, C2H6, H2, Ar, H2S, and He. Gold deposition occurred at 300° ± 50°C and 0.5 to 3.0 kbars. Assuming lithostatic fluid pressures, gold precipitated at a 2- to 10-km depth. Stage II gray quartz δ 18Ofluid values range from 5.9 to 7.5 per mil, whereas δ Dfluid values calculated from the dehydration of muscovite grains and measured directly from bulk fluid inclusion analyses of stage II gray quartz have ranges of –9 to –35 and –27 to –28 per mil, respectively. Hydrothermal ore fluids were transported from greater crustal depths to the site of gold deposition during the district-scale D3 event by shallowly W dipping, reverse brittle-ductile shear zones in supracrustal rock and along the steeply east dipping trondhjemite contact. Associated subhorizontal east-west shortening caused the reactivation of the eastern trondhjemite margin and subparallel foliation, which facilitated the transport of hydrothermal fluids and the generation of gold-bearing veins and hydrothermal alteration zones in komatiite. East-west–striking fractures in trondhjemite aided the lateral migration of ore fluids away from trondhjemite margins and the formation of east-west–striking gold-bearing veins and broad alteration zones. Gold was most likely transported in the stage II fluid as bisulfide complexes. The sulfidation of trondhjemite and komatiite wall rock by the stage II fluid caused the destabilization of Au bisulfide complexes and gold deposition. Potassium, Ca, and CO2 metasomatism of komatiite wall rock may have enhanced gold deposition via the acidification of the stage II fluid. The physicochemical characteristics of the Tarmoola ore fluid and relative timing of gold mineralization are consistent with the Yilgarn-wide, 2650 to 2630 Ma gold event. The host-rock chemistry, vein, and proximal wall-rock alteration mineral assemblages, and fluid temperature and pressures during gold deposition are also similar to that of earlier (ca. 2755 Ma) gold deposits in the Leonora district.

Genesis of the Shear Zone-related Gold Vein Mineralization of the Lega Dembi Gold Deposit, Adola Gold Field, Southern Ethiopia

The Lega Dembi Primary Gold Deposit in southern Ethiopia is related to the shear zone-hosted vein in the Neoproterozoic metamorphosed volcano-sedimentary succession of greenschist- to amphibolite-facies metamorphism. The rocks consists of a sequence of biotite-feldspar-quartz schists, carbonaceous mica-schists, amphibolites and basic to ultrabasic rocks. This unit is separated from a foot wall biotite gneiss by a major shear zone. The ore bodies are hosted in the volcano-sedimentary sequence and consist of swarms of quartz veins, lenses, and stockworks that propagated along mesoscale ductile to brittle-ductile shear zones. The mineralization is defined by a complex paragenesis of gold in association with Cu-Pb-Zn-Fe sulphides, tellurides and sulphosalts. The presence of Ni-bearing minerals in amphibolites of the host sequence, together with the ore mineral association, suggests an origin related to mafic volcanism.

LATE OROGENIC STRUCTURAL EVOLUTION IN THE LIGURIAN ALPS: CASE HISTORY FROM THE VOLTRI GROUP

This work deals with ductile, brittle and brittle-ductile shear zones developed in ultramafic rocks of the Voltri Group (VG), Eastern Ligurian Alps (Italy), during the late Alpine orogeny. The late orogenic events (upper Eocenelower Miocene) are characterized by a complex superposition of syn-metamorphic (greenschist facies) and non-metamorphic structures linked to the exhumation of the subducted slabs during Alpine orogeny. In the VG these deformations are represented by low angle reverse shear zones and associated vein networks in mafic and ultramafic rocks. Many of these shear zones show multiple reactivation and thus several types of fault rocks, linked to different structural levels and stress regime, overprint in a single shear zone. In the ultramafites shear zones are associated with significant vein networks and wallrock metasomatic alteration and are characterized by a full range of fault rocks from serpentinite mylonite to fault gouge. Fault zones thickness and displacement, recrystallization of the host rocks and the brittle vs ductile style of the structures are strictly controlled by the fluid-induced reactions and are only partially linked to the structural level of development. Highest thickness of the fault zone occurs where fault rocks have been affected by huge quartz-carbonate vein networks and carbonate dominated metasomatic alteration; in this case cataclasite and fault breccia develop and brittle deformation overprint ductile ones. Superposition of ductile over brittle structures occurs along discrete low thickness shear zones, where sepiolite and phyllosilicate minerals replace serpentine minerals and pyroxene and/or in presence of very fine grained fault gouge. We investigated the complex interaction between brittle and ductilestructuresby constructing detailed structural maps, and logs of fault rock distribution and textures and studying the associated vein networks in order to understand the role of fluids in deformation. We discuss the implication for regional structural analysis and the late orogenic evolution.

Brittle-ductile shear zone evolution and fault initiation in limestones, Monte Cugnone (Lucania), southern Apennines, Italy

Abstract The processes of brittle-ductile shear zone evolution and fault initiation by the coalescence of en echelon arrays of tensile cracks are quantitatively analysed in terms of displacement and temperature conditions at which they took place in very low-grade, well bedded micritic limestones from the southern Apennines, Italy. Three different types of structures are distinguished: (i) conjugate arrays of en echelon, calcite-filled tension gashes, showing extensional shear offsets; (ii) en echelon vein arrays showing incipient development of discontinuous shear-parallel fractures cutting through the tension gashes; and (iii) faulted vein arrays, in which vein array breaching by a continuous, discrete normal fault has occurred. Fluid inclusion microthermometry from vein calcite sampled from the different sets of structures (i) to (iii) above indicates that environmental conditions remained roughly constant during the different stages of vein array evolution and fault development, with average homogenization temperatures from primary fluid inclusions being in the range 130–140°C. Our results show how displacement accumulation and shear strain essentially control vein array evolution by rotation of en echelon tension gashes, fracture linkage and, eventually, fault nucleation, at approximately constant temperature.

Electronically conducting brittle-ductile shear zones in the crystalline basement of Rittsteig (Bohemian Massif, Germany): Evidence from self potential and hole-to-surface electrical measurements

Crustal-scale shear zones may act as prominent electrical conductors given that sufficient amounts of graphite or fluids are present. There are several graphite-enriched shear zones within the 583-m-deep Rittsteig drilling (Bavaria, Germany), two of which are of major importance. One shear zone cuts through Moldanubian biotite-muscovite schists at 320 m depth. The other shear zone separates Tepla-Barrandian amphibolites from Moldanubian biotite-muscovite schists at 460 m depth. To detect these shear zones adjacent to the drilling, the self-potential method and the non-linear impedance spectroscopy have been applied. From the new data, obtained from hole-to-surface and surface measurements, we conclude that (1) graphite is pervasively distributed within the Rittsteig shear zones resulting in powerful electronic conductors, (2) the graphite-bearing shear zones are dipping moderately towards the S, and (3) the shear zone drilled at 320 m depth extends to the Earth’s surface where it has been detected ca. 400 m to the N of the drilling. As the graphite-bearing shear zones of both the Rittsteig drilling and the German Continental Deep Drilling (KTB) developed in the brittle-ductile regime of quartzofeldspathic rocks, we argue that the brittle-ductile boundary layer may act as a significant graphite attractor in the continental crust.

Caracterização geológica e estrutural através da interpretação do produto integrado TM-Landsat 5 e dados aerogamaespectrométricos, região de Alta Floresta - MT

Este artigo apresenta a caracterização geológica e tectono-estrutural de uma região da Província Aurífera de Alta Floresta, Mato Grosso, através da interpretação do produto integrado TM-Landsat 5 e de dados aerogamaespectrométricos (contagem total). O produto integrado TM-contagem total possui características importantes para a interpretação geológica: apresenta num único produto, feições estruturais fornecidas pela imagem TM, representadas pelos elementos de relevo e drenagem e os valores radiométricos (RGB) que contribuem de forma bastante satisfatória na discriminação das unidades litoestruturais. A geração do mapa geológico foi baseada na interpretação dos valores radiométricos de contagem total (levando-se em consideração quais fatores, tais como topografia e atividade antrópica, poderiam influenciar a reposta radiométrica), nas feições de relevo e drenagem (que auxiliam na caracterização tectono-estrutural) fornecidas pela imagem TM e também nos dados de campo. Como a área apresenta intensa atividade antrópica e relevo muito arrasado, o que prejudica a interpretação geológica das imagens de sensoriamento remoto, a geração do produto integrado TM-contagem total foi decisiva para a obtenção de dados necessários ao mapeamento. Como resultado final foi elaborado o mapa geológico da área, em escala 1:250.000, sendo delimitadas 10 unidades litoestruturais relacionadas diretamente às unidades radiométricas e às feições estruturais. Um importante trend de direção N60-70W e altos mergulhos relacionados às zonas de cisalhamento transcorrentes sinistrais dúcteis a dúcteis rúpteis foi caracterizado através da interpretação visual de produtos integrados e dados de campo.

Late Carboniferous dextral wrench-dominated transpression along the North African craton margin (Eastern High-Atlas, Morocco)

The Paleozoic terranes of the Moroccan Eastern High-Atlas crop out mainly in the Tamlelt inlier. Two structural domains are recognized: (i) the Northern Domain, primarily structured by N-S Eovariscan polyphased structures (D1 and D2) overprinted by a E–W D3-deformation allotted to a late Carboniferous phase; (ii) the Central-Southern Domain (CSD) only deformed by D3. This paper focuses on the CSD (100 × 40 km) which shows, on the large-scale, a typical S–C structure simultaneously developed during D3, in a principal ductile-brittle shear zone. Second-order shear zones subparallel to the principal one and limited by Shear bands (SB) are also recognized. The N70–90° trending F3-folds and late imbricate structures have a weak general southern vergence. Strain analysis generally shows a planar finite strain ellipsoid and a vertical foliation–horizontal lineation fabric. This fabric and different scales of structures show that D3-deformations were generated by dextral wrench-dominated transpressional tectonics. The CSD displays a large degree of displacement partitioning and efficient slip on the E–W SB. Consequently, we suggest that the D3-structures developed in the late Carboniferous, during an oblique convergence and massive dextral transpression event, as the Moroccan Mesetian domains slid along the North African craton boundary. The oblique convergence may have been related to the indentation model, and the location of the Tamlelt inlier along the northwest African craton. The suggested dextral transpression during the Tamlelt Hercynian evolution of SE Morocco is related to the geodynamic model of a West European and Laurentian promontory indenter.

Geology, geochemistry and genesis of the Hongshijing gold deposit in Ruoqiang, Xinjiang

The Hongshijing gold deposit, which occurs in the Middle and Late Carboniferous volcanic and pyroclastic rocks, is of the brittle-ductile shear zone type controlled by a rift belt. The Hongshijing gold deposit is one controlled by a brittle shear zone located in the Late Paleozoic rift zone. The altered-rock type and quartz type orebodies are contained in the gold-bearing formation, which consists of basalt and tuffaceous sandstone. The major mineralizing stage is at 267 - 261 Ma and reiteration mineralizing stage at 220 - 209 Ma. The ore minerals include pyrite, magnetite, copper, bornite, ferrohydrite, native gold, and the gangue minerals include quartz, sercite, calcite, Fe-dolomite, leucoxene, anorthose, biotite, baria, cajuelite, and agustite. The wall rock alteration associated with gold mineralization comprises silicification, carbonization, pyritization, sericitization and chloritization. The contents of gold are 2.4 × 10−9 in the gold-bearing formation, 5.7 × 10−9 in the tuffaceous sandstone and 1.4–1.5 × 10−9 in the basalt. Au is associated with Te, Se, Ni, Cu. Au = 74.331 × Te +0.0335 × Ni−0.0211 × Cu-2.650. Geochemical investigations revealed that the mineralizing materials came from the basalt and tuffaceous sandstone in the gold-bearing formation. Under the action of ductile brittle shear structure, the gold-bearing formation was metamorphosed and altered, and the ore-forming materials activated, migrated and mineralized. The mineralizing fluid shows three mineralizing stages, with the characteristics of middle to low temperature (381-115°C) and middle to low mass fractions of NaCl (2.18%-16.77%). The fluid is of the Ca2+ -Mg2+ -Na+ -Cl− type and H2O-NaCl system. According to the data from hydrogen and oxygen isotopic composition (δD = −114.6%= −68.8%, δ18OH 2O = −2.47%−5.91%), the conclusion can be drawn that the mineralizing fluid of the Hongshijing gold deposit was a kind of mixed hydrothermal solution composed mainly of meteoric water, magmatic water and formation water in basalt and tuffaceous-sandstone or metamorphic water. The results show the mineralizing fluid and mineralizing materials came from the basalt and suffaceous-sandstone in the gold-bearing formation in Late Variscan orogeny. Under the action of brittle shearing, the gold-bearing formation was metamorphosed and altered, and the ore-forming material activated, migrated and mineralized withT andP going down and changes in pH, Eh,f O 2 andf S 2 under the earth’s surface from 1.57 to 2.25 km.

Re?Os and Pb?Pb geochronology of the Archean Salobo iron oxide copper?gold deposit, Caraj�s mineral province, northern Brazil

Rhenium–osmium ages were determined for two molybdenite samples and a Pb–Pb age was derived from bornite–chalcopyrite–magnetite at the Salobo iron oxide copper–gold deposit to determine the timing of mineralization and its relation to the nearby Old Salobo Granite. Rhenium–osmium dating of molybdenite spatially associated with copper sulfide minerals yields ages with weighted means of 2576±8 and 2562±8 Ma. Removing the error multiplier introduced by the decay constant uncertainty, appropriate for comparing ages from the same isotopic system, these data convincingly argue for two temporally separated pulses of molybdenite deposition at 2576.1±1.4 Ma (n=2) and 2561.7±3.1 Ma (n=3). The 2576±8 Ma age coincides with a previously published U–Pb age of 2573±2 Ma for the Old Salobo Granite, suggesting that main stage ore formation may have been contemporaneous with granite magmatism. The slightly younger 2562 Ma age most likely represents new molybdenite precipitation associated with the development or reactivation of local shear zones. Lead–lead stepwise leaching of copper sulfide minerals yields a less precise isochron age of 2579±71 Ma, and supports an Archean age for the Salobo ores. This is the first documentation of an Archean iron oxide copper–gold deposit, and the Re–Os and Pb–Pb geochronology herein support 2580–2550 Ma estimates for basement reactivation and regional granite magmatism associated with the development of brittle–ductile shear zones.

Characteristics and genesis of Kanggur gold deposit in the eastern Tianshan mountains, NW China: evidence from geology, isotope distribution and chronology

The Kanggur gold deposit lies in East Tianshan mountains, eastern section of Central Asia orogenic belt. The gold mineralization occurs on the northern margin of the Aqishan–Yamansu island arc in the Tarim Plate. It is mainly hosted in Lower Carboniferous volcanic rocks, and controlled by syn-tectonic intrusions and the ductile shear zone. The host rock is a calc-alkaline volcanic rock series consisting of andesite, dacite, tuff and subvolcanic rocks. The gold ore bodies were distributed in transition zone from strong ductile deformation to weak deformation. Ore-controlling structure is EW-trending normal brittle–ductile shear zone. The gold ore bodies are composed of Au-bearing altered rocks and a few Au-bearing sulfide-quartz veins. The mineralization shows vertical zonation in which Au was enriched in the upper orebodies, Au–Pb–Zn were enriched in the middle and Au–Cu were enriched in the lower orebodies. According to mineral assemblages and alteration intensity, hydrothermal alteration could be divided into three zones: silicification, chloritization, pyritization and magnetization zone (major ore body); pyrite-phyllic rock and/or sericite alteration zone; and sericite–chlorite alteration zone. Five mineralization stages are identified on the basis of mineral assemblage and crosscutting relationship of veins. The δ 34S values of sulfide in the ores range from −0.9‰ to +3.3‰, similar to those of the volcanic and subvolcanic rocks (−3.21∼+1.3‰), indicating that the sulfur isotope composition is characterized by unfractionated mantle sulfur. The lead isotope compositions of ores are between those of the volcanic rocks and subvolcanic rocks. The initial 87Sr/ 86Sr ratios of the ores vary between 0.7077 and 0.7106, similar to those of the host rocks (0.7079–0.7125). According to the Sr, S and Pb isotopic data, it is indicated that the ore-forming materials were derived from the surrounding volcanic rocks and feature deep source. The δ 18O values of the ore fluids vary from +3.83‰ to −1.77‰ and the δ D values from −46‰ to −66‰. The δ 18O vs. δ D and δ 18O vs. 87Sr/ 86Sr diagrams indicate that ore fluids were derived from dynamic metamorphic water, magmatic water and meteoric water. The ore-forming ages of Kanggur gold deposit are determined by Rb–Sr and Sm–Nd isochron methods. The results show that the main ore-forming stages of Kanggur gold deposit are 290–282 Ma, secondary mineralization stages are 275–254 Ma. Studies on regional geology, ore geology, isotope geochemistry and ore-forming ages have proved that the Kanggur gold deposit was formed in the compression–extension transition stage during the latest Paleozoic continental collision, and is characterized by orogenic gold deposits.

Paleozoic orogenic gold deposits in the eastern Central Andes and its foreland, South America

In the eastern Central Andes and its foreland (6°–34°S), abundant quartz veins emplaced along brittle–ductile deformation zones in Ordovician to Carboniferous granites and gneisses and in saddle-reefs in lower Paleozoic turbidites represent a coherent group of middle to late Paleozoic structurally hosted gold deposits that are part of three major Au (±Sb±W) metallogenic belts. These belts, extending from northern Peru to central Argentina along the Eastern Andean Cordillera and further south in the Sierras Pampeanas, include historical districts and mines such as Pataz–Parcoy, Ananea, Santo Domingo, Yani–Aucapata, Amayapampa, Sierra de la Rinconada and Sierras de Córdoba. On the basis of the available isotopic ages, two broad mineralization epochs have been identified, with Devonian ages in the Sierras Pampeanas Au belt (26° to 33°30′S), and Carboniferous ages for the Pataz–Marañón Valley Au-belt in northern Peru (6°50′ to 8°50′S). The absolute timing of the southeastern Peruvian, Bolivian and northwestern Argentinian turbidite-hosted lodes, which form the Au–Sb belt of the southern Eastern Andean Cordillera (12° to 26°S), is poorly constrained. Field relationships suggest overlap of gold veining with Carboniferous deformation events. The northernmost belt, which includes the Pataz province, is over 160-km-long and consists of sulfide-rich quartz veins hosted by brittle–ductile shear zones that have affected Carboniferous granitic intrusions. Gold mineralization, at least in the Pataz province, occurred a few million years after the emplacement of the 329 Ma host pluton and an episode of molassic basin formation, during a period of rapid uplift of the host units. The two southern belts are associated with syn- to post-collisional settings, resulting from the accretion of terranes on the proto-Andean margin of South America. The Au–Sb belt of the southern Eastern Andean Cordillera presumably formed in the final stages of the collision of the Arequipa–Antofalla terrane and the Sierras Pampeanas Au belt is considered concurrent with the late transpressional tectonics associated with the accretion of the Chilenia terrane. The three Devono–Carboniferous Andean belts are the South American segments of the trans-global orogenic gold provinces that were formed from Late Ordovician to Middle Permian in accretionary or collisional belts that circumscribed the Gondwana craton and the paleo-Tethys continental masses. A paleogeographic map of the Gondwana supercontinent in its Middle Cambrian configuration appears as a powerful tool for predicting the location of the majority of the Paleozoic orogenic gold provinces in the world, as they develop within mobile belts along its border. The three South American belts are sited in the metallogenic continuation of the Paleozoic terranes that host the giant eastern Australian goldfields, such as Bendigo–Ballarat and Charters Towers, with which they share many features. When compared to deposits in the French Massif Central, direct counterparts of the Andean deposits such as Pataz and Ananea–Yani are respectively the Saint Yrieix district and the Salsigne deposit. Considering the ubiquity of the Au (±Sb±W) vein-type deposits in the Eastern Cordillera and Sierras Pampeanas, and the relatively little attention devoted to them, the Devonian and Carboniferous orogenic gold deposits in the eastern section of the Central Andes constitute an attractive target for mineral exploration.

Oldest Gold: Deformation and Hydrothermal Alteration in the Early Archean Shear Zone-Hosted Bamboo Creek Deposit, Pilbara, Western Australia

The Early Archean Bamboo Creek gold deposit contrasts with most other orogenic deposits because of its relatively early timing in the tectonic evolution of the Pilbara granitoid-greenstone terrane. The Bamboo Creek deposit is situated in a bedding-parallel, brittle-ductile shear zone (the Bamboo Creek shear zone) within a komatiite sequence. The laminated quartz-carbonate gold lodes occur in carbonate-altered boudins within the Bamboo Creek shear zone and are associated with early sinistral, northeast-up deformation in the shear zone, whereas dextral reactivation of the zone postdates gold deposition. Gold-related alteration zones reflect an increase in X CO 2 toward the mineralized zone. Variations in original host-rock composition give rise to asymmetric alteration zoning, with a fuchsite-carbonate zone in the more Mg- and Cr-rich cumulate-textured footwall and a chlorite-quartz zone in the more aluminous spinifex-textured hanging wall. The alteration envelope is enriched in Na 2 O, K 2 O, Rb, Pb, As, and Sb. Whereas pyrite and minor chalcopyrite occur in all alteration zones, tetrahedrite, galena, and sphalerite are strongly associated with gold in the lodes. The alteration and metal enrichment of the Bamboo Creek gold deposit are indistinguishable from those of other orogenic (mesothermal) lode gold deposits in Archean terranes. Carbonate δ 13 C (PDB) and δ 18 O (SMOW) isotope signatures are consistent throughout the alteration envelope at 0.2 ± 0.6 and 14.6 ± 0.6 per mil, respectively. The δ 13 C value, in particular, is higher than typical values for orogenic gold deposits, implying interaction of auriferous fluids with preexisting marine carbonates that formed during an early sea-floor alteration event. The temperature of deposition, estimated from chlorite thermometry and alteration assemblages, is about 250°C, which is within the lower part of the range for orogenic gold deposits. Lead-lead model ages for galena, together with the relationships between the Bamboo Creek shear zone and dated granites, indicate a relatively early age of gold deposition of ca. 3400 Ma. Correlation of structures associated with gold deposition and regional structural phases shows that gold deposition was most likely related to an extensional tectonic phase. The early timing and association with extension is unlike the tectonic setting of other Archean gold deposits, which tend to form during the final, compressional or strike-slip stages of orogenesis. The Bamboo Creek gold mineralization may have been related to an Early Archean lower crustal delamination event. This may explain the anomalous timing and the low gold endowment of the Pilbara relative to Late Archean greenstones.

Invisible gold within sulfides from the Archean Hutti–Maski schist belt, Southern India

The Hutti–Maski schist belt is an important gold producing Archean greenstone belt in India. Mesothermal gold mineralization occurs within smoky quartz–sulfide veins occupying brittle–ductile shear zones. Principal ore minerals observed are arsenopyrite, pyrite, pyrrhotite, chalcopyrite, sphalerite, loellingite and gold. Although most of the gold occurs in native form, a substantial amount of gold is also present in the sulfides, especially within arsenopyrite in invisible form. Two stages of gold deposition occur in this area: (1) lattice bound invisible gold within sulfides, restricted to the shear fractures, a part of the primary gold depositional history; (2) native gold within sulfides as inclusions and as separate grains which may belong to another phase of hydrothermal fluid influx. Remobilization, reconstitution and concentration of the early phase might have formed the later phase. The substitution of Fe by Au within the sulfidic sites, adsorption–reduction and fluid rock interaction/phase immiscibility may be the plausible mechanisms for the invisible gold, microscopic substrates and visible gold respectively.