Fluid flow, alteration and polysulphide mineralisation associated with a low-angle reverse shear zone in the Lower Palaeozoic of the Anglo-Brabant fold belt, Belgium

In the Lower Palaeozoic rocks of the Brabant Massif (Belgium), a recently discovered polysulphide mineralisation is related to a low-angle reverse shear zone. This shear zone has been attributed to the main early Devonian deformation event. Data from boreholes and outcrops allow a detailed investigation of the alteration pattern and palaeofluid flow along this shear zone. Macroscopic observations of the mineralogy and quantitative changes in the phyllosilicate mineralogy indicate that this shear zone is characterised by an envelope of intense sericitisation and silicification. In addition, chloritisation is associated with this alteration. The alteration zone may reach a thickness of 250 m. Ore mineralisation occurred synkinematically and is spatially related to the shear zone. The mineralisation consists of pyrite, marcasite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, stibnite and smaller amounts of tetrahedrite and other sulphosalts. It is concentrated in quartz–sulphide veins or occurs diffusely in the host rock. The mineralising fluids have a low-salinity H₂O–CO₂–CH₄–NaCl–(KCl) composition and a minimum temperature of 250–320 °C. The δ¹⁸O values of quartz vary between +12.3‰ and +14.5‰ SMOW, and δD compositions of the fluid inclusions in the quartz crystals range from −65‰ to −35‰ V-SMOW. The δD and the calculated δ¹⁸O values of the mineralising fluids fall in the range typical for metamorphic fluids and partly overlap with that for primary magmatic fluids. The δ³⁴S values, between +4.7‰ and +10.6‰ CDT, fall outside the interval typical for I-type magmas. Important migration of likely metamorphic fluids, causing a widespread alteration and a polysulphide mineralisation along a low-angle shear zone, has, thus, been identified for the first time in the Caledonian Anglo-Brabant fold belt.     

Development of fluid conduits in the auriferous shear zones of the Hutti Gold Mine, India: evidence for spatially and temporally heterogeneous fluid flow

The gold mineralization of the Hutti Mine is hosted by nine parallel, N–S trending, steeply dipping, 2–10 m wide shear zones, that transect Archaean amphibolites. The shear zones were formed after peak metamorphism during retrograde ductile D₂ shearing in the lower amphibolite facies. They were reactivated in the lower to mid greenschist facies by brittle–ductile D₃ shearing and intense quartz veining. The development of a S₂–S₃ crenulation cleavage facilitates the discrimination between the two deformation events and contemporaneous alteration and gold mineralization. Ductile D₂ shearing is associated with a pervasively developed distal chlorite–sericite alteration assemblage in the outer parts of the shear zones and the proximal biotite–plagioclase alteration in the center of the shear zones. D₃ is characterized by development of the inner chlorite-K-feldspar alteration, which forms a centimeter-scale alteration halo surrounding the laminated quartz veins and replaces earlier biotite along S₃. The average size of the laminated vein systems is 30–50 m along strike as well as down-dip and 2–6 m in width.Mass balance calculations suggest strong metasomatic changes for the proximal biotite–plagioclase alteration yielding mass and volume increase of ca. 16% and 12%, respectively. The calculated mass and volume changes of the distal chlorite–sericite alteration (ca. 11%, ca. 8%) are lower. The decrease in δ¹⁸O values of the whole rock from around 7.5‰ for the host rocks to 6–7‰ for the distal chlorite–sericite and the proximal biotite–plagioclase alteration and around 5‰ for the inner chlorite-K-feldspar alteration suggests hydrothermal alteration during two-stage deformation and fluid flow.The ductile D₂ deformation in the lower amphibolite facies has provided grain scale porosities by microfracturing. The pervasive, steady-state fluid flow resulted in a disseminated style of gold–sulfide mineralization and a penetrative alteration of the host rocks. Alternating ductile and brittle D₃ deformation during lower to mid greenschist facies conditions followed the fault-valve process. Ductile creep in the shear zones resulted in a low permeability environment leading to fluid pressure build-up. Strongly episodic fluid advection and mass transfer was controlled by repeated seismic fracturing during the formation of laminated quartz(-gold) veins. The limitation of quartz veins to the extent of earlier shear zones indicate the importance of pre-existing anisotropies for fault-valve action and economic gold mineralization.     

Formation of low-temperature mylonites and phyllonites by alkali-metasomatic weakening of felsic volcanic rocks during progressive,subduction-related deformation

Dacitic to rhyolitic volcanic rocks of the Spruce Lake nappe experienced two phases of alkali-metasomatism as a result of fluids channelling along shear zones. The shear zones formed during a progressive, thrust-related deformation associated with underplating and incorporation of the volcanic rocks into the Brunswick subduction complex of northern New Brunswick. The fluids mainly represent chemically and isotopically modified seawater released by dewatering of the associated underthrusted shaly sedimentary rocks. Both phases of metasomatism weakened the felsic rocks, leading to strain localisation. Albitisation of felsic volcanic rocks as a result of Na-metasomatism during underthrusting facilitated formation of mylonites near peak high-pressure metamorphism (330–370°C, 600–800 MPa). The mylonites are preferentially preserved in the roof-thrust shear zone of the Spruce Lake nappe. Core-mantle structures, bulging and crystallographically preferred orientations indicate that albite behaved more ductilely than K-feldspar. The ductility of albite at these low temperatures is interpreted as a function of abundant intragranular fluids. Phengite-rich phyllonites formed after peak high–pressure metamorphism during uplift by out-of-sequence thrusting. These phyllonites are generally characterised by a slight gain in K and loss of Na and are best developed in the basal shear zones of the Spruce Lake nappe.     

The Daduhe gold field at the eastern margin of the Tibetan Plateau: He, Ar, S, O, and H isotopic data and their metallogenic implications

The Daduhe gold field comprises several shear-zone-controlled Tertiary lode gold deposits distributed at the eastern margin of the Tibetan Plateau. The deposits are hosted in a Precambrian granite–greenstone terrane within the Yangtze Craton. The gold mineralization occurs mainly as auriferous quartz veins with minor sulphide minerals. Fluid inclusions in pyrite have ³He/⁴He ratios of 0.16 to 0.86 R_a, whereas their ⁴⁰Ar/³⁶Ar ratios range from 298 to 3288, indicating a mixing of fluids of mantle and crust origins. The δ³⁴S values of pyrite are of 0.7–4.2‰ (n = 12), suggesting a mantle source or leaching from the mafic country rocks. δ¹⁸O values calculated from hydrothermal quartz are between − 1.5‰ and + 6.0‰ and δD values of the fluids in the fluid inclusions in quartz are − 39‰ and − 108‰. These ranges demonstrate a mixing of magmatic/metamorphic and meteoric fluids. The noble gas isotopic data, along with the stable isotopic data suggest that the ore-forming fluids have a dominantly crustal source with a significant mantle component.     

Role of fluid mixing and wallrock sulfidation in gold mineralization at the Semna mine area, central Eastern Desert of Egypt: Evidence from hydrothermal alteration, fluid inclusions and stable isotope data

The Semna gold deposit is one of several vein-type gold occurrences in the central Eastern Desert of Egypt, where gold-bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks. The Semna gold deposit is related to a series of sub-parallel quartz veins along steeply dipping WNW-trending shear zones, which cut through tectonized metagabbro and granodiorite rocks. The orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre-existing fault segments. Textural, structural and mineralogical evidence, including open space structures, quartz stockwork and alteration assemblages, constrain on vein development during an active fault system. The ore mineral assemblage includes pyrite, chalcopyrite, subordinate arsenopyrite, galena, sphalerite and gold. Hydrothermal chlorite, carbonate, pyrite, chalcopyrite and kaolinite are dominant in the altered metaggabro; whereas, quartz, sericite, pyrite, kaolinite and alunite characterize the granodiorite rocks in the alteration zones. Mixtures of alunite, vuggy silica and disseminated sulfides occupy the interstitial open spaces, common at fracture intersections. Partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins.Petrographic and microthermometric studies aided recognition of CO₂-rich, H₂O-rich and mixed H₂O–CO₂ fluid inclusions in the gold-bearing quartz veins. The H₂O–CO₂ inclusions are dominant over the other two types and are characterized by variable vapor: liquid ratios. These inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids. The generally light δ³⁴S of pyrite and chalcopyrite may suggest a magmatic source of sulfur. Spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing. Conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226–267 °C and 350–1100 bar, under conditions transitional between mesothermal and epithermal systems.The Semna gold deposit can be attributed to interplay of protracted volcanic activity (Dokhan Volcanics?), fluid mixing, wallrock sulfidation and a structural setting favoring gold deposition. Gold was transported as Au-bisulfide complexes under weak acid conditions concomitant with quartz–sericite–pyrite alteration, and precipitated through a decrease in gold solubility due to fluid cooling, mixing with meteoric waters and variations in pH and fO₂.     

Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit, Quadrilátero Ferrífero, Brazil

In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D₁, D₂ and D₃). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D₃. The orebodies constitute sulfide-bearing D₁-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H₂O-rich containing CO₂ + Na⁺ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H₂O is indicated. The CO₂ is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H₂O to CO₂-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.     

Geologic and tectonic setting of Deseado Massif epithermal deposits, Argentina, based on El Dorado-Monserrat

Middle–Late Jurassic bimodal volcanism, typical of a retroarc setting, developed during widespread extensional tectonism within the Deseado Massif, southern Argentina. This geologic environment led to the formation of numerous low-sulfidation epithermal deposits that are spatially and temporally related to the volcanic activity. The lack of significant high-sulfidation epithermal deposits may be because the tectonic and volcanic settings do not favor the formation of these types of deposits. El Dorado-Monserrat is a low-sulfidation epithermal prospect located near the southern boundary of the Deseado Massif. Mineralization is genetically linked to the Late Jurassic Chon Aike Formation and hosted by volcanic rocks of the middle Late Jurassic Bajo Pobre Formation. Two different mineralization areas have been identified. The Monserrat area is the most important, with veins hosted in a north-striking, left-lateral shear zone. The average thickness is 0.85 m, and the average metal content is 6.2 ppm gold and 153 ppm silver. The El Dorado area has discontinuous echelon veins within a right-lateral shear zone with low gold and silver grades. Hydrothermal alteration of the host rocks includes an inner zone of quartz-adularia and illite alteration and an outer zone of propylitic alteration. The main gangue mineral is quartz, which formed in successive pulses, plus adularia, pyrite, hematite, magnetite, and barite. Precious metals occur as zoned electrum. Ore mineral precipitation took place between 200 and 280 °C from low salinity fluids due to boiling.     

4: Transpressional tectonics, lower crust decoupling and intrusion of deep mafic sills: A model for the unusual metallogenesis of SW Iberia

SW Iberia is interpreted as an accretionary magmatic belt resulting from the collision between the South Portuguese Zone and the autochthonous Iberian terrane in Variscan times (350 to 330 Ma). In the South Portuguese Zone, pull-apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the Iberian Pyrite Belt. Massive sulphides precipitated in highly efficient geochemical traps where metal-rich but sulphur-depleted fluids of dominant basinal derivation mixed with sulphide-rich modified seawater. Massive sulphides formed either in porous/reactive volcanic rocks by sub-seafloor replacement, or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity. Crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks, while magmatic fluids probably had a minor influence on the observed geochemical signatures.The Ossa Morena Zone was a coeval calc-alkaline magmatic arc. It was the site for unusual mineralization, particularly magmatic Ni–(Cu) and hydrothermal Fe-oxide–Cu–Au ores (IOCG). Most magmatism and mineralization took place at local extensional zones along first-order strike-slip faults and thrusts. The source of magmas and IOCG and Ni–(Cu) deposits probably lay in a large mafic–ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust. Here, juvenile melts extensively interacted with low-grade metamorphic rocks, inducing widespread anatexis, magma contamination and further exsolution of hydrothermal fluids. Hypersaline fluids (δ¹⁸O_fluid > 5.4‰ to 12‰) were focused upward into thrusts and faults, leading to early magnetite mineralization associated with a high-temperature (> 500 °C) albite–actinolite–salite alteration and subsequent copper–gold-bearing vein mineralization at somewhat lower temperatures. Assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex. Further injection of both basic and sulphide-rich magmas into the upper crust led to the formation of Ni–(Cu)-rich breccia pipes.Younger (330 to 280 Ma?) peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis. These granitoids have W–(Sn)- and Pb–Zn-related mineralization that also shows geochemical evidence of major mantle–crust interaction. Late epithermal Hg–(Cu–Sb) and Pb–Zn–(Ag) mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex. In all cases, most of the sulphur seems to have been derived from leaching of the host sedimentary rocks (δ³⁴S = 7‰ to 20‰) with only limited mixing with sulphur of magmatic derivation.The metallogenic characteristics of the two terranes are quite different. In the Ossa Morena Zone, juvenile magmatism played a major role as the source of metals, and controlled the styles of mineralization. In the South Portuguese Zone, magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids, which are dominated by crustal signatures. Most of the magmatic and tectonic features related to the Variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion, which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs.     

Hydrothermal alteration and mineralisation of the Glen Eden Mo-W-Sn deposit: a leucogranite-related hydrothermal system,Southern New England Orogen,NSW, Australia

The Glen Eden Mo-Sn-W deposit in north-eastern New South Wales, Australia, is an example of a leucogranite-related, low-grade, large-tonnage hydrothermal system. It occurs in the southern part of the New England Orogen and is hosted within Permian felsic volcanic rocks, intruded at depth by dykes of porphyritic microleucogranite (Glen Eden Granite). The deposit is hosted within a pipe-like quartz-rich greisen breccia body about 500 m in diameter, surrounded by a greisen zone several hundred metres across, zoning out into altered volcanic rocks. The dominant ore minerals, largely hosted as open space fillings and disseminations in quartz and quartz-rich greisen, are molybdenite, wolframite and cassiterite; they are accompanied by minor to trace amounts of muscovite, fluorite, topaz, siderite, pyrrhotite, arsenopyrite, chalcopyrite, sphalerite, bismuth, bismuthinite, joseite A, cosalite, galenobismutite, beryl, anatase and late-stage dickite and kaolinite. Two types of breccia are recognised: (1) greisenised volcanic rock fragments (quartz + muscovite), cemented by hydrothermal quartz ± K-feldspar ± ore minerals, and (2) fragments of hydrothermal quartz ± cassiterite ± wolframite enclosed in quartz ± clay. In both types of breccia and in stockwork veins, there is evidence of early precipitation of Mo-Sn-W phases, followed by Bi minerals and base metal sulfides (± fluorite, siderite).Breccia formation and associated hydrothermal alteration (greisen, potassic, argillic, propylitic) are interpreted to be related to devolatilisation of the highly fractionated Glen Eden Granite of early Triassic age (240±1 Ma based on ⁴⁰Ar/³⁹Ar geochronology of greisen muscovite) as well as to fluid mixing with meteoric waters. The breccia pipe could have formed in part by rock dissolution and collapse, as well as by explosive degassing of boiling fluids. Fluid inclusion evidence is consistent with boiling, with breccia pipe formation and mineralisation having mainly occurred at 250–350 °C from fluids with salinity of 0.4–9 wt% NaCl equivalent in the dilute types and 30–47 wt% NaCl equivalent in the hypersaline types. Stable isotopic evidence (O, D, C, S) indicates a strong magmatic contribution to the hydrothermal fluids and metals in the breccia. The ¹⁸O values of quartz decrease outward from the breccia pipe (10.6–12.3 in the pipe to 3.4–8.7 in the peripheral quartz) indicating that there has been mixing with isotopically light (high latitude) meteoric fluids, mainly after formation of the breccia pipe.     

Volcanic lithogeochemistry and alteration at the Delbridge massive sulfide deposit, Noranda, Quebec

The Delbridge orebody occurs within a thick sequence (> 1 km) of porphyritic to aphyric massive rhyolite and rhyolite breccia of the Archean Blake River Group. The orebody produced ≈ 370,000 tonnes grading 0.61% Cu, 9.6% Zn, 110 g/t Ag and 2.1 g/t Au (1969–1971). The footwall consists of massive quartz porphyritic rhyolite mantled by proximal rhyolite breccias. An irregular chloritic alteration pipe with mineralization is subvertical to the ore lens. The orebody occurs at a thick cherty horizon within rhyolite breccia, and is overlain by a succession of mafic debris flows, porphyritic to aphyric massive rhyolite flows, and finally andesite. The main alteration assemblage in the rhyolite units is quartz-albite-sericite-chlorite-carbonate. Immobile element plots and rare-earth element data indicate that the footwall rhyolite flows and proximal breccias are tholeiitic to transitional (Zr/Y = 3.5–5.5; La_N/Yb_N = 1.7–2.6), whereas hangingwall rhyolite flows are mildly calc-alkaline (Zr/Y = 6.5–7.5; La_N/Yb_N = 2.8–3.8). These two rhyolite types also have separate alteration lines in Ti-Zr space and in various immobile element plots. The identification of chemically different rhyolites above and below the orebody provides markers that can be identified and traced even where strongly altered. An intrusive rhyolite mass in the footwall is chemically identical to the hangingwall aphyric rhyolite flows, and is interpreted as the feeder to these flows. Calculated mass changes in the footwall rhyolite commonly are large, and result from major silica change (±30%), significant loss of Na₂O + CaO, and important additions of K₂O and FeO + MgO. The margins of the pipe show net mass gain, whereas the interior of the pipe shows net mass loss. Hangingwall rhyolite shows mass changes that generally are much smaller than in the footwall. Felsic rocks in the silica-sericite alteration zone up to ≈ 200 m from the orebody have high δ¹⁸O values of 10–12‰, reflecting low-temperature alteration. The orebody occurs near the contact between a mainly tholeiitic rhyolite footwall and an overlying sequence of mildly calc-alkaline rhyolite then andesite.     

Geology and geochemistry of D–O–C isotope systematics of the Qolqoleh gold deposit, Northwestern Iran: Implications for ore genesis

The Qolqoleh gold deposit is located in northwestern part of the Sanandaj–Sirjan metamorphic belt, northwestern Iran. Igneous and sedimentary units exposed in the area have undergone greenschist metamorphism. The area was affected by a NE–SW trending shear zone and subsequent deformation. Two different types of mineralization are distinguished in the Qolqoleh gold deposit based on geological–structural conditions indicated by microtextural analysis: ductile and then brittle. Ore-forming processes are divided into three stages: Early (I), Middle (II) and Late (III), which include quartz–pyrite (I), sulfides and gold (II) and carbonate veinlets (III), respectively. The stage I fluids are characterized by δ¹⁸O = 15.5‰ at 440 ºC, and are thought to be deep-sourced metamorphic waters; the stage III fluids, with δ¹⁸O = 1.6‰, are shallow-sourced meteoric waters; whereas, the stage II fluids, with δ¹⁸O = 13.1‰, are a mixture of deep-sourced metamorphic and shallow-sourced meteoric fluids. Based on comparisons of the D–O–C isotopic systematics, the ore-forming fluids with characteristic high δ¹⁸O and δ¹³C and low δD originated from metamorphic devolatilization of Cretaceous volcano-sedimentary (felsic to mafic metavolcanic rocks–shale–carbonate–carbonaceous chert) sequences, locally rich in organic matter. During late Cretaceous continental collision of the Afro-Arabian continent and the Iranian microcontinent, a crustal slab consisting of felsic to mafic metavolcanic rocks, carbonate, shale and carbonaceous chert was underthrust northwards beneath the central Iranian microcontinent along the Zagros fault. During further contraction, deformation was localized in reverse oblique-slip structures with vergence toward south; shear zones generally follow contacts between more competent and less competent rock units. Metamorphic devolatilization of this underthrust slab is the source of the ore-forming fluids that generated the Au ore belt, which includes the Qolqoleh gold deposit.     

Geological features and origin of gold deposits occurring in the Baotou–Bayan Obo district, south-central Inner Mongolia, People's Republic of China

Located at western portion of northern margin of North China craton, the Baotou–Bayan Obo district is one of the most important Fe–REE–Nb and Au metallogenic provinces in China. Presently, about 52 gold deposits and prospects have been discovered, explored and mined, among which Shibaqinhao, Laoyanghao, Houshihua, Saiyinwusu, Wulashan and Donghuofang are the most important ones. All these gold occurrences can be subdivided into three groups (or types) according to its host rocks: (1) hosted by Archean high-grade metamorphic rocks; (2) hosted by Proterozoic sedimentary rocks; (3) hosted by or related to Hercynian alkaline intrusive rocks. The first group contains the Shibaqinhao, Laoyanghao and Houshihua gold deposits. Gold mineralization at these three deposits occurs within Archean amphibolite, gneiss and granulite as gold-bearing quartz veins and veinlet groups containing native gold, electrum, pyrite and chalcopyrite. The Saiyinwusu deposit belongs to the second group, and occurs within Proterozoic sandstone, quartzite and carbonaceous slate as quartz veins and replacement bodies along the fracture zones. Pyrite, marcasite, arsenopyrite, native gold and electrum are identified. The third group includes the Wulashan, Donghuofang and Luchang deposits. Gold mineralization at these three deposits occurs predominantly within the Hercynian alkaline syenite or melagabbro stocks and dyke swarms or along their contacts with Archean metamorphic wall rocks as K-feldspar–quartz veins, dissemination and veinlets. Pyrite, galena, chalcopyrite, native gold and calaverite are major metallic minerals.δ³⁴S value of sulfides (pyrite, galena and pyrrhotite) separates from groups 1 and 2 varies from −4.01‰ to −0.10‰ and −3.01‰ to 2.32‰, respectively. δ³⁴S values of Archean and Proterozoic metamorphic wall rocks for groups 1 and 2 deposits range from −20.2‰ to −17.0‰ and −15.8‰ to −16.2‰, respectively. The values are much lower than their hosted gold deposits. All these pyrite separates from Hercynian alkaline intrusions associated with the gold deposits show positive δ³⁴S values of 1.3‰ to 4.8‰, which is higher than those Precambrian metamorphic wall rocks and their hosted gold deposits. δ³⁴S values of the sulfides (pyrite and galena) from the Donghuofang and Wulashan deposits (group 3) increase systematically from veins (−14.8‰ to −2.4‰) to the Hercynian alkaline igneous wall rocks (2.8‰ to 4.8 ‰). All of these deposits in groups 1, 2 and 3 show relatively radiogenic lead isotopic compositions compared to mantle or lower crust curves. Most lead isotope data of sulfides from the gold ores plot between the Hercynian alkaline intrusions and Precambrian metamorphic wall rocks. Data are interpreted as indicative of a mixing of lead from mantle-derived alkaline magma with lead from Precambrian metamorphic wall rocks.Isotopic age data, geological and geochemical evidence suggest that the ore fluids for the groups 1 and 2 deposits were generated during the emplacement of the Hercynian alkaline syenite and mafic intrusions. The Hercynian alkaline magma may provide heat, volatiles and metals for these groups 1 and 2 deposits. Evolved metamorphic fluids produced by the devolatilization, which circulated the wall rocks, were also progressively involved in the alkaline magmatic hydrothermal system, and may have dominate the ore fluids during late stage of ore-forming processes. Most of these gold deposits hosted by Archean high-grade metamorphic rocks occur at or near the intersections of the NE- and E–W-trending fracture systems. The ore fluid of the group 3 deposits may have resulted from the mixing of Hercynian alkaline magmatic fluids and evolved meteoric waters. The deposits are believed to be products of Hercynian alkaline igneous processes along deep-seated fault zones within Archean terrain.     

Petrology of the Rainy Lake area,Minnesota, USA-implications for petrotectonic setting of the archean southern Wabigoon subprovince of the Canadian Shield

The Rainy Lake area in northern Minnesota and southwestern, Ontario is a Late Archean (2.7 Ga) granite-greenstone belt within the Wabigoon subprovince of the Canadian Shield. In Minnesota the rocks include mafic and felsic volcanic rocks, volcaniclastic, chemical sedimentary rocks, and graywacke that are intrucded by coeval gabbro, tonalite, and granodiorite. New data presented here focus on the geochemistry and petrology of the Minnesota part of the Rainy Lake area. Igneous rocks in the area are bimodal. The mafic rocks are made up of three distinct suites: (1) low-TiO₂ tholeiite and gabbro that have slightly evolved Mg-numbers (63–49) and relatively flat rare-earth element (REE) patterns that range from 20–8 x chondrites (Ce/Yb_N=0.8–1.5); (2) high-TiO₂ tholeiite with evolved Mg-numbers (46–29) and high total REE abundances that range from 70–40 x chondrites (Ce/Yb_N=1.8–3.3), and (3) calc-alkaline basaltic andesite and geochemically similar monzodiorite and lamprophyre with primitive Mg-numbers (79–63), enriched light rare-earth elements (LREE) and depleted heavy rare-earth elements (HREE). These three suites are not related by partial melting of a similar source or by fractional crystallization of a common parental magma; they resulted from melting of heterogeneous Archean mantle. The felsic rocks are made up of two distinct suites: (1)low-Al₂O₃ tholeiitic rhyolite, and (2) high-Al₂O₃ calc-alkaline dacite and rhyolite and consanguineous tonalite. The tholeiitic felsic rocks are high in Y, Zr, Nb, and total REE that are unfractionated and have pronounced negative Eu anomalies. The calcalkaline felsic rocks are depleted in Y, Zr, and Nb, and the REE that are highly fractionated with high LREE and depleted HREE, and display moderate negative Eu anomalies. Both suites of felsic rocks were generated by partial melting of crustal material. The most reasonable modern analog for the paleotectonic setting is an immature island arc. The bimodal volcanic rocks are intercalated with sedimentary rocks and have been intruded by pre- and syntectonic granitoid rocks. However, the geochemistry of the mafic rocks does not correlate fully with that of mafic rocks in modern are evvironments. The low-TiO₂ tholeiite is similar to both N-type mid-ocean-ridge basalt (MORB) and low-K tholeiite from immature marginal basins. The calc-alkaline basaltic andesite is like that of low-K calc-alkaline mafic volcanic rocks from oceanic volcanic arcs; however, the high-TiO₂ tholeiite is most similar to modern E-type MORB, which occurs in oceanic rifts. The conundrum may be explained by: (1) rifting of a pre-existing immature arc system to produce the bimodal volcanic rocks and high-TiO₂ tholeiite; (2) variable enrichment of a previously depleted Archean mantle, to produce both the low- and high-TiO₂ tholeiite and the calc-alkaline basaltic andesite, and/or (3) enrichment of the parental rocks of the high-TiO₂ tholeiite by crustal contamination.     

Gold and copper mineralization at the El Indio deposit, Chile

A Middle Tertiary volcanic belt in the High Andes of north-central Chile hosts numerous precious- and base-metal epithermal deposits over its 150 km north-south trend. The El Indio district, believed to be associated with a hydrothermal system in the late stages of development of a volcanic caldera, consists of a series of separate vein systems located in an area of 30 km² which has undergone intense argillic-sericitic-solfataric alteration. The majority of the known gold-copper-silver mineralization occurs within a structural block only 150 by 500 m in surface area, with a recognized vertical extent exceeding 300 m. This block is bounded by two high-angle northeast-trending faults oriented subparallel to the mineralized veins.Hypogene mineralization at El Indio is grouped into two main ore-forming stages: Copper and Gold. The Copper stage is composed chiefly of enargite and pyrite forming massive veins up to 20 m wide, and is accompanied by alteration of the wall rocks to alunite, kaolinite, sericite, pyrite and quartz. The Gold stage consists of vein-filling quartz, pyrite, native gold, tennantite and subordinate amounts of a wide variety of telluride minerals. Associated with this stage is pervasive alteration of the wall rocks to sericite, kaolinite, quartz and minor pyrophyllite. The transition from copper to gold mineralization is marked by the alteration of enargite to tennantite and by minor deposition of sphalerite, galena, huebnerite, chalcopyrite and gold. Mineral stability relations indicate that there was a general decrease in the activity of S₂ accompanied by variations in the activity of Te₂ during the Gold stage.Fluid-inclusion data show homogenization temperatures ranging from about 220 to 280°C, with salinities on the order of 3–4 eq. wt. % NaCl for the Copper stage. The Gold-stage inclusions indicate a similar range in homogenization temperatures, but significantly lower salinities (0.1–1.4 eq. wt. % NaCl). Fluid inclusions of transition minerals show a weak inverse relationship between homogenization temperatures (190–250°C) and salinities (3.4–1.4 eq. wt. % NaCl), which may represent mixing of hotter Gold-stage fluids with cooler late-Copper-stage fluids. No evidence of boiling was found in fluid inclusions, but CO₂ vapor-rich inclusions were identified in wall-rock quartz phenocrysts which pre-date copper and gold mineralization.Mineral stability calculations indicate that given a fairly restricted range of solution compositions, the Copper-, Transition- and Gold-stage minerals at El Indio could have been deposited from a single solution, with constant total dissolved sulfur which underwent reduction through time. Limited sulfur-isotope data indicates that pyrite from the Copper stage was not in isotopic equilibrium with Copper-stage alunite or Transition-stage sphalerite. The sulfur-isotope and fluid-inclusion data indicate that two fluids with comparable temperatures but different compositions flowed through the El Indio system. The earlier fluid deposited copper attended by sericite-alunite-kaolinite alteration, and later epithermal fluids deposited gold with quartz-sericite-kaolinite-pyrite alteration.     

A stable isotope and fluid inclusion study of the Qaleh-ZariCu–Au–Ag deposit, Khorasan Province, Iran

The Qaleh-Zari copper deposit, located in South Khorasan in the Central Lut region of Iran, is a polymetallic vein deposit with major amounts of Cu, Au, Ag and minor amounts of Pb, Zn and Bi. Mineralization occurs in a series of NW–SE trending fault planes and breccia zones in Paleogene andesitic to basaltic volcanic rocks. Argillization, sericitization and propylitization characterize alteration halos bordering mineral veins. The main ore minerals are chalcopyrite, pyrite, galena and sphalerite, with quartz, calcite and minor chlorite as the main gangue phases. Microthermometric measurements of fluid inclusions in cogenetic quartz indicate homogenization temperatures between 160 and 300 °C and salinities from 1 to 4 wt% NaCl equiv. Boiling occurred in the mineralising fluids at 160–1000 m below the paleo-water table at pressures of approximately 15−80 bar at various stages in the formation of the ore body. The wide range of pressures and temperatures reflects the multi-stage nature of the mineralization at Qaleh-Zari. The δ¹⁸O values in quartz (relative to SMOW) and δ³⁴S values in chalcopyrite and galena (relative to CDT) range from 6.5 to 7.5‰ and 0.0–1.5‰ (mean: 7.0‰), respectively. At 300 °C, calculated fluid δ¹⁸O values are close to 0‰. These data suggest a magmatic origin for sulfur and a surficial origin for the mineralizing fluid. Mineralization at Qaleh-Zari is interpreted as epithermal and low-sulfidation in style and was probably related to a deep-seated magmatic system. Ore deposition was the result of boiling, cooling and pressure reduction.     

Incipient blueschist metamorphism and metasomatism in the Tavşanli region,Northwest Turkey

Volcano-sedimentary rocks in an imbricate tectonic zone around a peridotite massif have been studied northeast of the town of Tavanli in Northwest Turkey. Basic volcanic rocks, which are the dominant rock type in this zone, show incipient blueschist metamorphism and associated metasomatism. While the igneous textures of the volcanic rocks are retained, augites are partially to completely replaced by sodic pyroxene, and plagioclase is albitised resulting in rocks with 6–8 wt.% Na₂O. The volcanic rocks are cross-cut by numerous veins of calcite, aragonite, quartz, pumpellyite, albite, lawsonite and sodic pyroxene. Pelagic limestones, which are interbedded with the basic volcanic rocks, consist of coarse aragonite grains showing partial replacement by calcite. The occurrence of aragonite, lawsonite and albite indicates conditions of metamorphism for the whole zone in the range of 5–8 kb and 150–200° C. Metasomatism, probably related to high pressure serpentinization, has occurred contemporaneously with the incipient high pressure metamorphism.     

小宛南山金矿地质特征及成因

小宛南山金矿是产于太古宙花岗岩-绿岩地体,受敦煌群D岩组上部层位和韧性剪切带控制的变质热液型金矿床.矿源岩为绿岩带的镁铁质火山岩,成矿流体主要为变质热液,主成矿期属中元古宙.基于金矿床区域地质背景、矿床地质特征、微量元素特征、硫铅同位素组成和包裹体特征,以金赋存层位、容矿岩石、韧性剪切构造、蚀变作用为基础,通过分析成矿地质条件,证明金矿床应属变质热液成因.     

Gold mineralization in the West Hoggar shear zone,Algeria

The Amesmessa gold prospect is located along a vertical N-S-trending crustal-scale ductile shear zone; stretching lineations are subhorizontal. This major shear zone is a Late Pan African dextral strike-slip fault of the Pharusian Belt of the Tuareg Shield (Algeria). The Amesmessa shear zone is asymmetric: strong thermal and deformational gradients are present along its western border where biotitic ultramylonites are in contact with a rigid Archean complex (In Ouzzal block), whereas there is a progressive gradation, through mylonite then protomylonite, to the Proterozoic gneiss of the Eastern block which displays co-axial Pan African structures. The Amesmessa shear zone is characterized by the presence of a felsic dike complex emplaced during shearing, and forming the most important parent material for ultramylonites. Basic magmas and carbonatites also intruded within the shear zone. The gold-rich quartz veins are located within the ultramylonitic western part of the shear zone. These N-S-trending laminated quartz veins formed during the late increments of shearing (plastic/brittle transition), by repeated syntectonic hydraulic fracturing along zones of rheological contrast parallel to foliation. The ore mineral association (pyrite, galena, native gold, sphalerite) crystallized in the deformed quartz matrix along late shear planes. Undeformed E-W trending banded quartz veins are present in the mylonitic eastern part of the shear zone; their gold content is low and no native gold has been observed. A strong hydrothermal alteration resulted in the development (along the walls of the N-S gold-bearing quartz veins) of a 5-m-wide carbonate-sericite-albite-pyrite secondary mineral association which implies an important CO₂ supply and moderate temperature conditions. There is no alteration halo around the E-W quartz veins. Ultramylonites, hydrothermally altered rocks and quartz veins display similar REE patterns characterized by strong LREE enrichments. Shear-related fluids could be likely parental fluids for the Amesmessa gold mineralization and the associated hydrothermal alteration. Hydrothermal fluids were drawn into dilation zones and filled opening fractures along the main planar discontinuity of the most deformed rocks. The supply of CO₂ may come from a deep-seated source as suggested by the presence of carbonatite dikes in the shear zones and the existence of CO₂-H₂O-rich fluid inclusions in quartz. The location of the gold-bearing quartz veins in the western part of the shear zone can be explained by the presence of strong thermal and rheological gradients.     

Boron metasomatism and behaviour of rare earth elements during formation of tourmaline rocks in the eastern Arunta Inlier,central Australia

Tourmaline rocks of previously unclear genesis and spatially associated with W- (Cu)-bearing calc-silicate rocks occur in Palaeoproterozoic supracrustal and felsic intrusive rocks in the Bonya Hills in the eastern Arunta Inlier, central Australia. Tourmalinisation of metapelitic host rocks postdates the peak of regional low-pressure metamorphism (M₁/D₁, ~500 °C, ~0.2 GPa), and occurred synkinematically between the two main deformation events D₁ and D₂, coeval with emplacement of Late Strangways (~1.73 Ga) tourmaline-bearing leucogranites and pegmatites. Tourmaline is classified as schorl to dravite in tourmaline–quartz rocks and surrounding tourmaline-rich alteration zones, and as Fe-rich schorl to foitite in the leucogranites. Boron metasomatism resulted in systematic depletion of K, Li, Rb, Cs, Mn and enrichment of B, and in some samples of Na and Ca, in the tourmaline rocks compared to unaltered metasedimentary host rocks. Whole-rock REE concentrations and patterns of unaltered schist, tourmalinised schist and tourmaline–quartz veins—the latter were the zones of influx of the boron-rich hydrothermal fluid—are comparable to those of post-Archaean shales. Thus, the whole-rock REE patterns of these rocks are mostly controlled by the metapelitic precursor. In contrast, REE concentrations of leucogranitic rocks are low (10 times chondritic), and their flat REE patterns with pronounced negative Eu anomalies are typical for fractionated granitic melts coexisting with a fluid phase. REE patterns for tourmalines separated from metapelite-hosted tourmaline–quartz veins and tourmaline-bearing granites are very different from one another but each tourmaline pattern mirrors the REE distribution of its immediate host rock. Tourmalines occurring in tourmaline–quartz veins within tourmalinised metasediments have LREE-enriched (La_N/Yb_N=6.3–55), shale-like patterns with higher REE (54–108 ppm). In contrast, those formed in evolved leucogranites exhibit flat REE patterns (La_N/Yb_N=1.0–5.6) with pronounced negative Eu anomalies and are lower in REE (5.6–30 ppm). We therefore conclude that REE concentrations and patterns of tourmaline from the different tourmaline rocks studied are controlled by the host rock and not by the hydrothermal fluid causing boron metasomatism. From the similarity of the REE pattern of separated tourmaline with the host rock, we further conclude that incorporation of REEs in tourmaline is not intrinsically controlled (i.e. by crystal chemical factors). Tourmaline does not preferentially fractionate specific REEs or groups of REEs during crystallisation from evolved boron- and fluid-rich granitic melts or during alteration of clastic metasediments by boron-rich magmatic-hydrothermal fluids.Editorial responsibility: J. Hoefs     

Regional-scale hydrothermal alteration in the Central Blake River Group,western Abitibi subprovince,Canada: implications for VMS prospectivity

The Late Archean Blake River Group is a thick succession of predominantly mafic volcanic rocks within the southern zone of the Abitibi greenstone belt. It contains a number of silicic volcanic centers of different size, including the large Noranda volcanic complex, which is host to 17 past-producing volcanogenic massive sulfide deposits. The Noranda complex consists of a 7- to 9-km-thick succession of bimodal mafic and felsic volcanic rocks erupted during five major cycles of volcanism. Massive sulfide formation coincided with a period of intense magmatic activity (cycle III) and the formation of the Noranda cauldron. Hydrothermal alteration in these rocks is interpreted to reflect large-scale hydrothermal fluid flow associated with rapid crustal extension and rifting of the volcanic complex. The alteration includes abundant albite, chlorite, epidote and quartz (silicification), which exhibit broad stratigraphic and structural control and correlate with previously mapped whole-rock oxygen isotope zonation. The Mine Sequence volcanic rocks are characterized by abundant iron-rich chlorite (Fe/Fe+Mg >0.5), hydrothermal amphibole (ferroactinolite) and coarse-grained epidote of clinozoisite composition (<10 wt% Fe ₂O ₃). Volcanic rocks of the pre-cauldron sequences, which contain only subeconomic stringer mineralization, are characterized by less abundant chlorite and mainly fine-grained epidote (>10 wt% Fe ₂O ₃) lacking the clinozoisite solid solution. Alteration in the Mine Sequence volcanic rocks persists along strike well beyond the limits of the main ore deposits (as far as several tens of kilometers) and can be readily distinguished from greenschist facies metamorphic assemblages at a regional scale. The lack of similar alteration in the pre-cauldron sequences is consistent with limited ¹⁸O-depletion and suggests that the early history of the volcanic complex did not support large-scale, high-temperature fluid flow in these rocks. Comparisons with a much smaller, barren volcanic complex in nearby Ben Nevis township reveal important differences in the alteration mineralogy between volcanoes of different size, with implications for area selection during regional-scale mineral exploration. The Ben Nevis Complex consists of a 3- to 4-km-thick succession of mafic, intermediate and felsic volcanic rocks centered on a small subvolcanic intrusion. Alteration of the volcanic rocks comprises mainly low-temperature assemblages of prehnite, pumpellyite, magnesium-rich chlorite (Fe/Fe+Mg <0.5), iron-rich epidote (>10 wt% Fe ₂O ₃) and calcite. Actinolite ± magnetite alteration occurs proximal to the intrusive core of the complex, but the limited extent of this alteration indicates only local high-temperature fluid circulation adjacent to the intrusion. A distal zone of carbonate alteration is located 4–6 km from the center of the volcano. Although iron-bearing carbonates are present locally within this zone, the absence of siderite argues against a high-temperature origin for this alteration. These observations do not offer positive encouragement for the existence of a fossil geothermal system of sufficient size or intensity to have produced a large massive sulfide deposit.