Gold-bearing Shear Zones Research Articles

Boron, sulphur and copper isotope systematics in the orogenic gold deposits of the Archaean Hattu schist belt, eastern Finland

The Hattu schist belt is located in the western part of the Archaean Karelian domain of the Fennoscandian Shield. The orogenic gold deposits with Au–Bi–Te geochemical signatures are hosted by NE–SW, N–S and NW–SE oriented shear zones that deform 2.76–2.73Ga volcanic and sedimentary sequences, as well as 2.75–2.72Ga tonalite–granodiorite intrusions and diverse felsic porphyry dykes. Mo–W mineralization is also present in some tonalite intrusions, both separate from, and associated with Au mineralization. Somewhat younger, unmineralized leucogranite intrusions (2.70Ga) also intrude the belt. Lower amphibolite facies peak metamorphism at 3–5kbar pressures and at 500–600°C temperatures affected the belt at around 2.70Ga and post-date hydrothermal alteration and ore formation. In this study, we investigated the potential influence of magmatic-hydrothermal processes on the formation of orogenic gold deposits on the basis of multiple stable isotope (B, S, Cu) studies of tourmaline and sulphide minerals by application of in situ SIMS and LA ICP MS analytical techniques.Crystal chemistry of tourmaline from a Mo–W mineralization hosted by a tonalite intrusion in the Hattu schist belt is characterized by Fe3+–Al3+-substitution indicating relatively oxidizing conditions of hydrothermal processes. The range of δ11B data for this kind of tourmaline is from −17.2‰ to −12.2‰. The hydrothermal tourmaline from felsic porphyry dyke swith gold mineralization has similar crystal chemistry (e.g. dravite–povondraite compositional trend with Fe3+–Al3+ substitution) and δ11B values between −19.0‰ and −9.6‰. The uvite–foitite compositional trend and δ11B ‰ values between −24.1% and −13.6% characterize metasomatic–hydrothermal tourmaline from the metasediment-hosted gold deposits. Composition of hydrothermal vein-filling and disseminated tourmaline from the gold-bearing shear zones in metavolcanic rocks is transitional between the felsic intrusion and metasedimentary rock hosted hydrothermal tourmaline but the range of average boron isotope data is essentially identical with that of the metasediment-hosted tourmaline. Rock-forming (magmatic) tourmaline from leucogranite has δ11B values between −14.5‰ and −10.8‰ and the major element composition is similar to that of the metasediment-hosted tourmaline.The range of δ34SVCDT values measured in pyrite, chalcopyrite and pyrrhotite is from −9.1 to +8.5‰, which falls within the typical range of sulphur isotope data for Archaean orogenic gold deposits. In the Hattu schist belt, positive δ34SVCDT values characterize metasediment-hosted gold ores with sulphide parageneses dominated by pyrrhotite and arsenopyrite. The δ34SVCDT values are both positive and negative in ore mineral parageneses within felsic intrusive rocks in which variable amounts of pyrrhotite are associated with pyrite. Purely negative values were only recorded from the pyrite-dominated gold mineralization within metavolcanic units. Therefore the shift of δ34SVCDT values to the negative values reflects precipitation of sulphide minerals from relatively oxidizing fluids. The range of measured δ65CuNBS978 values from chalcopyrite is from −1.11 to 1.19‰. Positive values are common for mineralization in felsic intrusive rocks and negative values are more typical for deposits confined to metasedimentary rocks. Positive and negative δ65CuNBS978 values occur in the ores hosted by metavolcanic rocks. There is no correlation between sulphur and copper isotope data obtained in the same chalcopyrite grains.Evaluation of sulphur and boron isotope data together and comparisons with other Archaean orogenic gold provinces supports the hypothesis that the metasedimentary rocks were the major sources of sulphur and boron in the orogenic gold deposits in the Hattu schist belt. Variations in major element and boron isotope compositions in tourmaline, as well as in the δ34SVCDT values in sulphide minerals are attributed to localized involvement of magmatic fluids in the hydrothermal processes. The results of copper isotope studies indicate that local sources of copper in orogenic gold deposits may potentially be recognized if the original, distinct signatures of the sources have not been homogenized by widespread interaction of fluids with a large variety of rocks and provided that local chemical variations have been too small to trigger changes in the oxidation state of copper during hydrothermal processes.

Modelling of deformation around magmatic intrusions with application to gold-related structures in the Yilgarn Craton, Western Australia

This study simulates rock deformation around high temperature granite intrusions and explores how gold bearing shear zones near intrusions were developed in the Yilgarn, using a new continuum damage mechanics algorithm that considers the temperature and time dependent elastic-visco-plastic constitutive behaviour of crustal materials. The results demonstrate that strain rates have the most significant effects on structural patterns for both extensional and compressional cases. Smaller strain rates promote the formation of narrow high-strain shear zones and strong strain localisation along the flank or shoulder areas of the intrusion and cold granite dome. Wider diffuse shear zones are developed under higher strain rates due to strain hardening. The cooling of the intrusion to background temperatures occurred over a much shorter time interval when compared to the duration of deformation and shear zones development. Strong strain localisation near the intrusion and shear zone development in the crust occurred under both extensional and compressional conditions. There is always clear strain localisation around the shoulders of the intrusion and the flanks of the “cold” granitic dome in early deformation stages. In the models containing a pre-existing fault, strain localisation near the intrusion became asymmetric with much stronger localisation and the development of a damage zone at the shoulder adjacent to the reactivated fault. At higher deformation stages, the models produced a range of structural patterns including graben and half graben basin (extension), “pop-up” wedge structures (compression), tilted fault blocks and switch of shear movement from reverse to normal on shear zones. The model explains in part why a number of gold deposits (e.g. Wallaby and Paddington deposits) in the Yilgarn were formed near the flank of granite-cored domes and deep “tapping” faults, and shows that the new modelling approach is capable of realistically simulating high strain localisation and shear zone development.

Characterization of gold mineralization in Garin Hawal area, Kebbi State, NW Nigeria, using remote sensing

Garin Hawal area, Kebbi State, NW Nigeria is part of the Neoproterozoic to Early Phanerozoic terrane separating the west African and Congo Cratons. Three main gold-bearing shear zones were detected in the study area from the processed Landsat ETM+ images and extensive ground investigation. Field and petrographical studies indicate that the Neoproterozoic rocks are represented by a highly folded and faulted belt constituted of hornblende, muscovite and graphite schist. They are intruded by granondiorites and late to post granitic dykes. Extensive alteration zones were identified using high resolution QuickBird image along Garin Hawal shear zone. The alteration zones and associated quartz veins are generally concordant with the main NE–SW regional structural trend and are dipping to the NW. Geochemical studies indicate that the gold content reaches 8 g/t in the alteration zones, while it reaches up to 35 g/t in the quartz veins. Mineralogical studies indicate that the alterations are strongly potassium-enriched. Pyrophyllite, kaolinite, illite, gypsum and quartz also occur. The main ore minerals are gold, chalcopyrite, arsenopyrite, pyrite, galena and iron oxides. This study indicates that the alteration zones and the associated quartz veins in the muscovite schist are promising and need more detailed exploration for Au and Ag mineralization to evaluate their potential.

Comportamento de elementos químicos em rochas mineralizadas em ouro na Faixa Seridó, Província Borborema

The Seridó fold Belt, central part of Rio Grande do Norte Domain, Borborema Province, NE Brazil, contain by some gold deposits associated with shear zones. The São Francisco and Ponta da Serra gold deposits are associated to and hosted in shear zones that crosscut the Seridó Group and basement orthogneisses, respectively. These gold-bearing shear zones are channels through where a great volume of hydrothermal fluids circulated and caused leaching and/or chemical precipitation. The pressure and temperature conditions of the original rock and the mylonite are estimated to have been 3.5±0.7 kbar and 574o±30oC, respectively. Petrographic studies indicate that biotite and plagioclase, in both deposits, were the most important minerals in the chemical reactions. These reactions were responsible for the formation of muscovite, cordierite and sillimanite in the mylonitic mica-schists, and of muscovite and microcline in the mylonitic orthogneisses. The study of chemical element mobility showed that during the event of deformation/hydrothermal metasomatism, Al, Na and Zr, (here) stayed immobile. The mobilized elements were K and Rb that show increase of the concentration in all transformed rocks, while Ca, Na and Sr show decrease of the concentration. The elements Sm and Nd behaved as immobile in the São Francisco deposit, while in the Ponta da Serra deposit, both elements lost mass. Regarding the formation of muscovite, K was liberated by the biotite brakdown and also entered the system via hydrothermal fluids. The loss of the Ca and Na was due to the incorporation of these elements in the fluid phase

Comportamento de elementos químicos em rochas mineralizadas em ouro na Faixa Seridó, Província Borborema

The Serido fold Belt, central part of Rio Grande do Norte Domain, Borborema Province, NE Brazil, contain by some gold deposits associated with shear zones. The Sao Francisco and Ponta da Serra gold deposits are associated to and hosted in shear zones that crosscut the Serido Group and basement orthogneisses, respectively. These gold-bearing shear zones are channels through where a great volume of hydrothermal fluids circulated and caused leaching and/or chemical precipitation. The pressure and temperature conditions of the original rock and the mylonite are estimated to have been 3.5±0.7 kbar and 574o±30oC, respectively. Petrographic studies indicate that biotite and plagioclase, in both deposits, were the most important minerals in the chemical reactions. These reactions were responsible for the formation of muscovite, cordierite and sillimanite in the mylonitic mica-schists, and of muscovite and microcline in the mylonitic orthogneisses. The study of chemical element mobility showed that during the event of deformation/hydrothermal metasomatism, Al, Na and Zr, (here) stayed immobile. The mobilized elements were K and Rb that show increase of the concentration in all transformed rocks, while Ca, Na and Sr show decrease of the concentration. The elements Sm and Nd behaved as immobile in the Sao Francisco deposit, while in the Ponta da Serra deposit, both elements lost mass. Regarding the formation of muscovite, K was liberated by the biotite brakdown and also entered the system via hydrothermal fluids. The loss of the Ca and Na was due to the incorporation of these elements in the fluid phase

Autochthonous and allochthonous strata of the El Callao greenstone belt: Implications for the nature of the Paleoproterozoic Trans-Amazonian orogeny and the origin of gold-bearing shear zones in the El Callao mining district, Guayana shield, Venezuela

The tectonic style of the Paleoproterozoic Trans-Amazonian orogeny in the Guayana shield was considered by previous workers to represent non-actualistic tectonics in which rising masses of granitoid magma of the Supamo complex deformed 2.1–2.0Ga supracrustal belts of volcanic and sedimentary rocks to form greenstone belts. Subsequent collision with the dominantly granulite-grade Imataca complex deformed the greenstone belts. However, the base of the greenstone belts is everywhere marked by a major mylonitic shear zone that placed rocks of the greenstone belts atop a regional unit of quartzite that sits unconformably upon both the Imataca and Supamo complexes. The simplest interpretation of these relationships is that the greenstone belts are allochthonous with respect to the Imataca and Supamo complexes. As the quartzite oversteps the contact between the Imataca and Supamo complexes, juxtaposition of the two complexes must predate deposition of the quartzite and deformation of the Trans-Amazonian orogeny.The quartzite and the thrust fault are used as paleohorizontal markers to unfold the greenstone belt in the El Callao area where auriferous shear zones, which clearly predate the thrusting, show reverse sense of movement. When restored to their pre-folding orientation they are normal faults. This, and the tholeiitic to calc-alkaline volcanic packages, are compatible with an extensional forearc to arc setting for both the mineralization and the volcano-sedimentary sequences.East–northeasterly trending folds of both the allochthon and the autochthon, coupled with the northerly thickening of the quartzite, suggest that the allochthon was part of an unknown craton that formerly lay to the north. A second regional fold set, which trends northerly, refolds the earlier structures. It is possibly related to collision of the San Carlos terrane with the amalgamated Imataca–Supamo–Pastora terranes. The unconformably overlying 1.98Ga Cuchivero arc terrane provides a minimum age for the two collisional events.

Products of 2.65–2.58 Ga orogenesis in the Slave Province correlated with Slave – Northern Cordillera Lithospheric Evolution (SNORCLE) seismic reflection patterns

Reflection patterns along Lithoprobe Slave – Northern Cordillera Lithospheric Evolution (SNORCLE) seismic reflection line 1 in the southwestern Slave Province are interpreted as products of tectonic wedging during late Archean lithospheric convergence. The interpretation is aided by application of new seismic imaging techniques, by correlation of upper crustal reflection patterns to known geology, and by a comparison of lithospheric reflection patterns to similar convergent zones elsewhere. In the Yellowknife area, reflection patterns consist of (1) east-dipping reflections at 12–14 s that project into the upper mantle, (2) a wedge-shaped body in the lower crust with an east-dipping reflection fabric that is truncated on the west by (3) a series of west-dipping reflections that outline thrust-and-fold structures in the upper crust. The similarity of these reflection patterns to those of the Proterozoic Fort Simpson – Hottah collision zone ~300 km to the west provides support for the interpretation that reflection patterns beneath the Slave Province are also products of collisional tectonics. Rocks within the Slave Province preserve evidence of a ~2.65–2.58 Ga pan-Slave orogenic event, in which the >2.9 Ga Central Slave Basement collided with the ~2.7 Ga juvenile eastern Slave Province. Their suture is interpreted to be a west-dipping surface at 4–5 s (12–15 km) beneath Yellowknife and to project to the surface east of the profile. In the lower crust and upper mantle, east-dipping reflections are interpreted to delineate a coeval subduction zone and accretionary wedge. The upper crustal thrust-and-fold structures are likely linked to gold-bearing shear zones at Yellowknife. These results provide tantalizing evidence that processes similar to those of modern convergent zones were operational at 2.65–2.58 Ga.

A Thrust Ramp Model for Gold Mineralization at the Archean Trondhjemite-Hosted Tarmoola Deposit: The Importance of Heterogeneous Stress Distributions around Granitoid Contacts

The trondhjemite-hosted Tarmoola gold deposit is situated in the Leonora district in the Eastern Goldfields province and is the largest granitoid-hosted gold deposit in the Yilgarn craton of Western Australia (>100 t Au). The massive trondhjemite pluton intrudes a northwest-striking, shallowly northeast-dipping, supracrustal sequence that comprises greenschist facies metamorphosed basalt, komatiite, chloritic siltstones, and a volcani-clastic unit. Several east-northeast-striking, quartz diorite dikes cut the trondhjemite and supracrustal sequence. Major ore zones (>1 g/t Au) occur (1) within the trondhjemite as discontinuous west-northwest-striking zones, (2) along the steeply dipping, eastern trondhjemite contact in the supracrustal and trondhjemite rocks, and (3) subparallel to the shallowly dipping, western trondhjemite margin in supracrustal rocks. Ore zones in the trondhjemite are controlled by west-northwest-striking, steeply south-west- and northeast-dipping, conjugate, quartz-carbonate veins that contain minor pyrite, chalcopyrite, sphalerite, galena, gold, and rare scheelite. Gold-bearing veins in supracrustal rocks have a similar vein mineralogy and timing to that of the auriferous veins in trondhjemite. Deformation at Tarmoola occurred during progressive, regional east-west-directed shortening. The first deformation event (D 1 ) caused early barren, isoclinally folded, quartz-carbonate veins and a pervasive foliation in supracrustal rock to be aligned subparallel to the steeply dipping eastern margin of the pluton. During the gold mineralization event (D 2 ), the trondhjemite pluton acted as a competent body within ductile supracrustal country rocks. Subhorizontal, northwest-southeast shortening caused auriferous conjugate veins in trondhjemite and the reactivation of the eastern contact and a preexisting subparallel foliation in the supracrustal rock. The reverse movement of supracrustal rock along the sheared western trondhjemite contact caused the formation of west-dipping, gold-bearing shear zones in the overlying supracrustal sequence. The reactivation of the eastern trondhjemite contact caused the development of low mean stress zones adjacent to the contact. Hy-drothermal ore fluids were focused into these regions to form ore zones. The third deformation event (D 3 ) produced brittle-ductile shear zones, quartz-carbonate veins, and brittle faults that offset lithological contacts and gold-bearing veins.

The metallogenic relationship between Birimian and Tarkwaian gold deposits in Ghana

The traditional concept of the Early Proterozoic gold deposits in Ghana — i.e. gold-bearing shear zones overlain by Tarkwaian paleoplacers containing reworked gold derived from the shear-zones — needs to be reconsidered in the light of recent research in Ghana, the Ivory Coast and French Guiana. This research has revealed a consistent pattern of geostructural and metallogenic evolution in which both the Birimian and the Tarkwaian rocks were deformed by a major Eburnean compression (D2). It has shown that the NE-SW faults controlling the Gold Coast Range shear-zone mineralization (Ashanti-Prestea) were formed during the Eburnean D2 episode of thrusting that was followed by hydrothermal activity with the emplacement of auriferous arsenopyrite and then by the development of quartz veinlets and native gold; thus the shear-zone mineralization could only have appeared during the D2 late-orogenic stage. It has also shown evidence of post-depositional D2 deformation in the gold sites examined in the Tarkwa gold-bearing conglomerate, although the effects are limited and primary lithological controls have been preserved that reveal these deposits to be “modified paleo-placers”. Thus, the Tarkwaian gold could not be derived from the gold-bearing shear-zones.

Application of till geochemistry to gold exploration, Ilomantsi, Finland

The Archean bedrock of the Ilomantsi area consists of narrow greenstone belts between dome-like granitoids. The main tectonic phases are isoclinal folding, dragging of isoclinal folds around subvertical axes and regional faulting. Conspicuous anomalies of As, Au, Mo and W were revealed in the course of regional geochemical mapping of till, which led to more detailed studies in Ilomantsi. The data presented in this study are based mainly on the results of 2960 till samples collected by light percussion drill at local scale (16 samples/km 2), but also on the results of regional (1 sample/4 km 2) and detailed (100–400 samples/km 2) sampling and on rock samples collected from outcrops. The analytical methods most commonly used were emission spectrometry, atomic absorption spectrophotometry and spectrometry. The results indicate that the lowest part of the basal till is dominated by local debris and that the geochemistry of the till reflects well the underlying bedrock. The highest Au values exist near the contacts of tonalite and supracrustal rocks. Arsenic follows intimately the directions of faulting. The anomalous contents of W and Mo are restricted to the southern part of the 15-km 2 Kuittila tonalite. Potassium, Na and Ca form conspicious anomalies in till, cutting both the supracrustal rocks and the granitoids. Local studies revealed in the Kuittila tonalite a notable network of Mo- and W-bearing quartz veins restricted to tensional zones of main fault directions, which are cut by gold-bearing shear zones and quartz vein nets. These shear zones are of en echelon type and they are silicified, sericitized, tournalinized, Na-depleted and partly carbonitized. The hosting tonalite is intensely biotitized. These metasomatic features of the bedrock can be correlated to the wider anomalies of K, Na and Ca in till. The implication is that significant migration of mobile elements has taken place in the bedrock of the study area and that, in places, gold has participated in these processes.

Application of fluid-inclusion studies to exploration in gold-bearing shear zones: the Cros-Gallet deposit (France) and the Sanoukou prospect (Mali)

A fluid-inclusion study was undertaken on samples from the Cros-Gallet deposit (Haute-Vienne, Massif Central, France) and the Sanoukou prospect (Kenieba district, Mali), both located in large shear zones several kilometres long. Ore mineral paragenesis for both deposits is similar: early arsenopyrite, pyrite and native Au, and later Pb-Ag-Cu sulfosalts, electrum and galena. Quartz samples were used for fluid-inclusion studies. Nondestructive microthermometry and Raman microprobe analysis were used to characterize the ore-forming fluids. Thermobarometric evolution was similar in both deposits and occurred in two stages: 1. (1) Fluid inclusions from the H 2 O-NaCl-CO 2 (CH 4 , N 2 ) system correspond to a drop of pressure in a lithostatic regime and accompanied early Au, As ore deposition. 2. (2) Fluid inclusions from the H 2 O-NaCl system correspond to a drop in temperature and an opening in a hydrostatic regime. It is associated with late Au, As, Pb, Sb ore deposition. This model was used to estimate the Au potential of quartz samples analyzed by standard global destructive analysis (ICP, Gas Chromatography). At Sanoukou, this method allowed assessment, in a given sample, of the relative importance of the two established stages and demonstrated that Au-rich quartz (Au content higher than 1 ppm) was probably genetically related to the aqueous stage. Utilization of this result could be considered in prospecting.

Geochemical signatures of gold-bearing shear zones in the Variscan basement, France

Abstract The geochemical signature of Au mineralization is a function of its sulphide parageneses and associated wall-rock alteration. This signature can be used to classify a prospect as a function of the anomalies it causes (in rocks or, if conditions are favourable, in soil) in a given geologic environment. The geochemical signature is made of three types of elements: 1. (1) The major elements initially present in the country rocks but whose concentrations can be modified on introduction of the mineralization. A convention the authors have defined denotes the elements of interest in square brackets [ ], preceded by + or −, respectively, indicating enrichment or depletion. 2. (2) Tracer elements, introduced by the mineralizing solutions, characteristic of the mineralized structure on a semi-regional scale. By convention they are written after those in square brackets. 3. (3) Pathfinder elements specific to mineralized zones, reflecting remobilization and concentration phenomena, constitute tracers on a detailed scale. These are denoted in parentheses ( ) at the end of the signature. Most of the Au-bearing shear zones in France have the following signature: [+ SiO 2 , +FeO 2 , − TiO 2 ] As,Sb,(Pb) The semi-regional tracer elements, As and Sb, appear to reflect mostly regional geological characteristics. Arsenic is always present in Au-bearing shear zones in the Variscan basement of France. This is similar to structures that cut volcano-sedimentary rocks of the Cadillac basin in Quebec, Canada. By contrast, As is usually scarce in shear zones cutting the mafic and ultramafic rocks of the Abitibi greenstone belt. In this case As can be replaced by Cu. Antimony, as a semi-regional tracer, appears characteristic of shear zones in France whereas Pb, as a detail pathfinder, is found in all geologic environments along the shear zone and is closely related to Au anomalies. Such as close correlation with Au limits its usefulness.

A new look at the stratigraphy of the Yellowknife Supergroup at Yellowknife, N.W.T. — implications for the age of gold-bearing shear zones and Archean basin evolution

Based on recent detailed mapping, a revised stratigraphic column is proposed for the rocks of the Archean Yellowknife Supergroup in the Yellowknife greenstone belt. The mafic volcanic rocks of the Kam Formation, previously thought to represent the oldest supracrustal rocks of the belt, overlap remnants of an earlier volcanic–sedimentary sequence, here referred to as the Octopus Formation. As its enormous thickness makes it too unwieldy to be described as a single formation, the Kam Formation is raised to group status and subdivided into four formations. It is proposed that the Kam Group should replace the Beaulieu Group in the Yellowknife area. The Chan Formation, at the base of the Kam Group, consists of multiple gabbroic intrusions that were emplaced into a carapace of pillowed flows. The intrusions locally resemble sheeted mafic dyke complexes in Phanerozoic ophiolites, thought to represent evidence for sea-floor spreading. The Crestaurum Formation, which overlies the Chan Formation, is characterized by massive and pillowed flows interlayered with a number of laterally continuous cherts and felsic tuffs. The Townsite Formation consists of rhyodacite breccias interbedded with felsic tuffs and pillowed dacites. The Yellowknife Bay Formation, at the top of the Kam Group and comprising massive and pillowed flows with pillow breccias and numerous interflow sediments, contains all the important gold deposits mined at Yellowknife. The Banting Formation, directly overlying the Kam Group and consisting of mafic to felsic volcanics, is also given group status and subdivided into two formations. Conglomerates and sandstones of the Jackson Lake Formation, formerly thought to separate the Kam and Banting groups, are considered to represent the youngest rocks of the Yellowknife Supergroup near Yellowknife. Gold-bearing shear zones clearly postdate deposition of the Banting Group, making the rocks of this group a potential target for gold exploration. The presence of remnants of a possible spreading center at the base of the Kam Group suggests that plate-tectonic processes were active during the formation of Archean supracrustal basins in the Slave Province.