Sets Of Quartz Veins Research Articles

Structural Controls on Mineralization within the Huanggou Gold Deposit in the Southern Mesozoic Xuefengshan Orogen, South China

The Mesozoic Xuefengshan Orogen (XFSO) in South China hosts abundant gold ore deposits. The XFSO records a history of polyphase tectonic deformation and the structural controls on gold mineralization are poorly understood. The recently discovered quartz-vein type Huanggou gold deposit in the southern XFSO is characterized by multiple stages of deformation and represents an excellent natural laboratory for deciphering structural controls on gold mineralization in a complex orogenic belt. A systematic structural analysis indicates that the geometry of the Huanggou gold deposit and adjacent areas mainly resulted from four stages of deformation: (1) D1 top-to-the-NW sense of shearing; (2) D2 SE-directed back-folding and back-thrusting; (3) D3 NW–SE upright folding; and (4) D4 NW–SE normal faulting and fracturing. Two sets of quartz veins are exposed in the deposit: (1) SE-dipping Au-bearing quartz veins; and (2) NW-dipping non-mineralized quartz veins. The SE-dipping Au-bearing quartz veins were likely formed by the main deformation phase of the XFSO. During D1 top-to-the-NW simple shearing, these synkinematic Au-bearing quartz veins progressively evolved into sigmoidal shapes and rotated to a preferred SE-dipping orientation. Subsequently, D2 SE-directed back-folding and back-thrusting resulted in the formation of synkinematic NW-dipping non-mineralized quartz veins. D3 upright folding locally steepened the dips of both Au-bearing and non-mineralized quartz veins. Our results may provide new insights into the structural controls of gold mineralization in the XFSO.

Structural setting, wall rock alteration and gold mineralisation of the Mt. Percy gold deposit, Kalgoorlie, Western Australia

Field mapping, drill-core logging, petrography, laser ICP-MS analysis on pyrite, chlorite geothermometry and geochemical analysis of hydrothermally altered rocks reveal two distinct styles of gold mineralisation at Mt. Percy. The first style is D2 shear zone-hosted and correlates with Fimiston-style mineralisation documented in the Golden Mile. Laser ICP-MS analyses on pyrite indicate high As concentrations (average of 5823 ppm for pyrite in Hannans Lake Serpentinite and 635 ppm in feldspar-quartz porphyry). Chlorite geothermometry reveals average temperatures of 262 ± 50 °C and 357 ± 50 °C in distal and proximal alteration zones, respectively. Geochemical analyses reveal (1) positive gold-silver and gold-antimony relationships; (2) lack of a base metal-gold relationship; (3) systematic increase of carbonatisation and alkali saturation indices from distal to proximal alteration zones; and (4) addition of K, Ca, Na and V, with corresponding depletion of Mg, Fe, P and REE. The second style is related to D4 sheeted quartz vein sets and correlates with Mt. Charlotte-style mineralisation documented in the Mt. Charlotte deposit. Laser ICP-MS analyses on pyrites reveal low As concentrations (average of 51 ppm for pyrites in feldspar-quartz porphyry and 870 ppm in Devon Consols Basalt). Chlorite geothermometry reveals an average temperature of 325 ± 50 °C on vein selvage in the Devon Consols Basalt. High-grade gold mineralisation (> 50 g/t Au) and extensive (< 50 m wide) hydrothermal alteration zones coincide with D2 shear zones and D4 fault-controlled sheeted quartz vein sets, and particularly in areas where both of these structures intersect porphyry stocks.

Structural characteristic of the hydrothermal-ore system of the Bambukoy river basin, Western Transbaikalia

The hydrothermal ore system of the Bambukoy basin of the Western Transbaikalia is located within the South Muya Ridge is a set of quartz veins with different orientations, performing fissure paragenesis, formed during the development of thrust and shear deformations. The rocks enclosing are terrigenous deposits of the Amatkan Formation of the Vendian age, presented in the form of interbedded aleurolites and sandstones of varying grain size. The terrigenous deposits intensively deployed in inclined, recumbent, diving folds. In the structure of the zone, there are two types of quartz veins. The first type includes veins concordant stratification and cleavage, the second type are counter veins. Due to the kinematic reconstruction was revealed that the formation of sub - concordant veins is associated with the initial stages of the formation of the thrust system. In turn, the counter veins are derived from the subsequent stages of deformation, expressed in the development of left-side shifts that complicate the thrust system.

Structural controls of fluid flow and gold mineralization in the easternmost parts of the Karagwe-Ankole Belt of north-western Tanzania

Gold mineralization in the Biharamulo region of western Tanzania is confined to the sheared, low-angle basement-cover contact between Archaean basement gneisses of the Tanzania Craton and the structurally overlying, low-grade metamorphic metasediments of the Mesoproterozoic Karagwe-Ankole Belt. Regional-scale fluid flow along this detachment is indicated by the pervasive silicification and retrogression of wall rocks to pervasively foliated phyllonites and pyritization of particularly metasediments, commonly graphite-rich, in the hanging wall of the shear zone. Gold mining centres on specific structural sites along the detachment, but also in stratigraphically higher sections in the structurally overlying metasediments. Zones of gold mineralization along the detachment correlate with NE trending ramp structures (dip angles 20°–35°) that are most ideally orientated for slip and reactivation within the low-angle phyllonitic detachment. Repeatedly overprinted auriferous quartz-vein stockworks in quartzofeldspathic gneisses immediately below the detachment indicate brittle fracturing of the competent footwall lithotypes during slip along the weaker detachment. In cases of massive silicification, up to 50m thick quartz blows are formed along the contacts between detachment phyllonites and footwall gneisses. The multiple overprinting relationships of successive quartz-vein generations in these zones of massive silicification suggests that the quartz blows acted as competent blocks in the weak detachment, causing the repeated overprint of earlier silicification by later fracturing and quartz-veining events. Gold mineralization above the detachment and in stratigraphically higher metasediments is closely associated with fold structures that form part of the low-grade metamorphic fold-and-thrust belt. Veining is particularly abundant in competent lithotypes, such as quartzite and chemically reactive ferruginous mafic sills. Overprinting relationships between quartz vein sets illustrate fluid flow during fold amplification and, importantly, the final lock-up stage of folds, during which much of the mineralization was introduced. Oxygen isotope values for quartz veins indicate fluids were likely derived from clastic, mainly metapelitic sedimentary sequences of the Karagwe-Ankole Fold Belt. The data also implies that the partially reworked Archaean granitoid-greenstone basement of the Tanzania Craton has not contributed to the fluid evolution and possibly gold mineralization. The extent (>100km) of the basement-cover detachment and associated alteration is indicative for a regional-scale fluid system. Gold mineralization is, however, controlled by local structures and lithological contrasts that require the detailed mapping and sampling of the regional structure.

Porphyry Cu mineralization in the eastern desert of Egypt: inference from geochemistry, alteration zones, and ore mineralogy

Significant features of porphyry copper deposits were observed in the southern part of the Eastern Desert of Egypt. These include wide distribution of gossans cover and pyrite-rich zones, chalcopyrite disseminations, zoned hydrothermally altered mafic volcanics, hydrothermal breccia pipes, and quartz-vein sets in close association with granite porphyry intrusion. The investigated area is characterized by extensive magmatism forming a belt of late Neoproterozoic volcanic-intrusive complexes. The rock association consists of low-K calc-alkaline diorite/quartz-diorite intrusive rocks, granite porphyry intrusion, pitchstone porphyry, and basalt to basaltic andesite volcanics. Three zones of alteration have been identified in the study area arranged from the outer to inner as: (1) propylitic alteration in the mafic volcanics; (2) phyllic-argillic alteration with extensive silisification of mafic volcanics; and (3) silica-potassic alteration, overprinted by phyllic-argillic ones, toward the inner part of the pyrite-rich zones. Granite and pitchstone porphyries have similar geochemical characters of major, trace, and rare-earth element (REE) patterns indicate their formation from the same magma source. Mafic volcanics display enrichment in some lithophile elements (LILE; Cs, Ba, and K) and depletion in high field strength elements (HFSE; Nb, Ti) and relative depletion of light REE, which indicate a normal arc setting and substantial role of subduction in their petrogenesis. Mineralization occurs partly in the porphyritic granite intrusion itself and mainly in the altered mafic volcanics. The mineralized rocks display simple ore mineral assemblages consisting of pyrite and chalcopyrite, with minor amounts of pyrrhotite, covellite, and sphalerite. Pyrite is found as large crystals and aggregates containing numerous chalcopyrite inclusions of various sizes and shapes. Chalcopyrite also occurs as large masses intergrown with pyrite. The mineralized outcrops have high pyrite/chalcopyrite ratio; greater than 10, with goethitic-hematitic capping which possibly indicate to a pyrite shell in a shallow level porphyry copper system. The pyrite shell has anomalous values of Cu (up to 321 ppm) with low amounts of Zn, Pb, and Mo. These features suggests a porphyry Cu deposit in Egypt, which still needs more detailed studies that would involve exploration, core sampling, fluid inclusions, and stable isotope systematics.

The Pampe gold deposit (Ghana): Constraints on sulfide evolution during gold mineralization

The Ashanti belt, a world-renowned gold producing region in southwest Ghana, has received renewed attention in recent years. Most studies, however, focused on the major deposits situated along the Ashanti shear zone and in the adjacent Tarkwa Basin to the east, neglecting smaller yet important occurrences, such as the Pampe deposit which occurs few kilometers to the west of this shear zone, on the Akropong Trend. Nevertheless, investigating such simpler smaller-scale mineralizing systems is attractive, in that this can help shedding light on the processes that control gold deposition at the regional scale.At Pampe, gold endowment has been estimated at approximately half a million ounces with an average gold grade of 2.8g/t. The mineralization is related to two sets of quartz veins; a first set (V1), which has a NE trend and is sub-parallel to the main foliation (S1), and a second set (V2), which crosscuts this foliation. The V2 veins have a NNW–SSE trend with local conjugate geometries indicating that they formed during NNW–SSE shortening, which, regionally, is linked to major orogenic gold deposition. Gold mineralization is systematically associated with sulfides, which occur disseminated in the vein walls and in the surrounding host rocks. The ore sulfide paragenesis consists of 1) a first generation of pyrite, which is associated with V1 veins; 2) a second generation of sulfides, consisting of an intergrowth of arsenopyrite and pyrite that crystallized contemporaneously with the formation of the second vein set; 3) a late phase of pyrite growth which occurs as overgrowths on phase-2 sulfides and formed during the waning stages of V2 emplacement. Invisible (sub-microscopic) gold was detected in all sulfide generations by LA-ICP-MS. The analytical profiles for the Au signal are mimicked by those for Pb, Cu, As, Ag, Te, and Bi. Invisible gold is thus interpreted to have precipitated within sulfides in the form of nanoparticles (colloidal gold alloys). Conversely, visible native gold grains were recognized exclusively in association with arsenopyrite from late V2 veins, either as inclusions or, more commonly, at the boundary with other sulfides, as well as in micro-fractures that crosscut the sulfides. Gold precipitation was likely induced by sulfidation of the wall rock during fluid–rock interaction. The Pampe deposit exemplifies the mineralization processes that took place at larger scale in neighboring world-class deposits such as Obuasi, Bogoso and Prestea.

The Kalana vein-hosted gold deposit, southern Mali

The Kalana gold deposit occurs within metamorphosed Paleoproterozoic volcano-sedimentary series of the Birimian Baoulé-Mossi domain of the West African Craton, in the south of Mali. These formations are intruded successively by small dioritic bodies, as well as andesitic and tonalitic dykes, which exhibit volcanic arc-setting signatures. Mineralization is hosted by two sets of quartz veins that intersect the regional schistosity. The first set of veins is the most important in terms of grades and size, and consists of thick veins (up to meter size) that range in trend from N–S to E–W with shallow plunges. The second set consists of much thinner (centimeter size) sub-vertical quartz veins oriented NE–SW. The two sets of veins are interpreted to have formed during the evolution of late, gently dipping thrust faulting. Two episodes of gold precipitation are recognized: a first episode, during early stages of vein growth, formed micron-size native gold inclusions in arsenopyrite; a later episode, during vein shearing and fracturing of the quartz lodes, precipitated native gold in free form in quartz, in fractured arsenopyrite, and associated with chalcopyrite, galena, sphalerite, Bi sulfides and native bismuth. All evidence suggests that the Kalana deposit represents orogenic gold mineralization formed during a relatively long-lived hydrothermal system, at a late stage of the tectonic history of the greenstone belt.

Implications for Gold Mineralisation in the Gopapur Area, Keonjhar District, Orissa: A New Discovery

Gold deposits of Orissa can be broadly divided into three broad zones of Keonjhar, Mayurbhanj, and Sundargarh districts. In Keonjhar district gold occurrences have been reported from Saleikana-Dimirimunda-Kardangi sector in Telkoi block and Gopapur, Gajipur, Adal, Kushkala sector in Banspal block. Out of these localities Gopapur area seems to be the most promising one. The Precambrian metavolcanics of Gopapur in the Keonjhar district host auriferous quartz vein. In this area detailed exploration is carried out by GSI in 1:10000 scales with pitting, trenching, drilling, geological mapping and geochemical survey. The Quartz veins are found in the metavolcanics being emplaced along the prominent schistocity planes. Three sub parallel sets of quartz veins with high angle dip of are delineated. Gold mineralization is associated with the sulphides and oxides in quartz veins, basalts and laterites and ultramafic and is exposed on the surface as well as in underground trench faces. Gold occurs as refractory form and in free grains within quartz and intimate association with pyrite and chalcopyrite and oxides (haematite, goethite, magnetite and cuprite) in the form of refractory gold. The gold content of the mineralized quartz veins and lateritic wall rocks varies widely from 1 to 9 ppm. Fluid inclusion study shows the temperature ranges 250°C to 37°C and the gold deposit is of mesothermal type. The gold is formed by the remobilisation from sulphide bearing minerals to oxide bearing minerals. From the mineralogical and lithological assemblages the Gopapur prospect can be compared with Iron-formation-hosted vein and disseminated deposit that is Homestake gold deposit.

Fluid record of rock exhumation across the brittle–ductile transition during formation of a Metamorphic Core Complex (Naxos Island, Cyclades, Greece)

AbstractFluid inclusions trapped in quartz veins hosted by a leucogneiss from the southern part of the Naxos Metamorphic Core Complex (Attic‐Cycladic‐Massif, Greece) were studied to determine the evolution of the fluid record of metamorphic rocks during their exhumation across the ductile/brittle transition. Three sets of quartz veins (V‐M2, V‐BD &amp; V‐B) are distinguished. The V‐M2 and V‐BD are totally or, respectively, partially transposed into the foliation of the leucogneiss. They formed by hydrofracturing alternating with ductile deformation accommodated by crystal‐plastic deformation. The V‐B is discordant to the foliation and formed by fracturing during exhumation without subsequent ductile transposition. Fluids trapped during crystal–plastic deformation comprise two very distinct fluid types, namely a CO2‐rich fluid and a high‐salinity brine, that are interpreted to represent immiscible fluids generated from metamorphic reactions and the crystallization of magmas respectively. They were initially trapped at ∼625 °C and 400 MPa and then remobilized during subsequent ductile deformation resulting in various degrees of mixing of the two end‐members with later trapping conditions of ∼350 °C and 140 MPa. In contrast, brittle microcracks contain aqueous fluids trapped at 250 °C and 80 MPa. All veins display a similar δ13C pointing to carbon that was trapped at depth and then preserved in the fluid inclusions throughout the exhumation history. In contrast, the δD signature is marked by a drastic difference between (i) V‐M2 and V‐BD veins that are dominated by carbonic, aqueous‐carbonic and high‐salinity fluids of metamorphic and magmatic origin characterized by δD between −56‰ and −66‰, and (ii) V‐B veins that are dominated by aqueous fluids of meteoric origin characterized by δD between −40‰ and −46‰. The retrograde P–T pathway implies that the brittle/ductile transition separates two structurally, chemically and thermally distinct fluid reservoirs, namely (i) the ductile crust into which fluids originating from crystallizing magmas and fluids in equilibrium with metamorphic rocks circulate through a geothermal gradient of 30 °C km−1 at lithostatic pressure, and (ii) the brittle upper crust through which meteoric fluids percolate through a high geothermal gradient of 55 °C km−1 at hydrostatic pressure.

Geology and Fluid Characteristics of the Mina Velha and Mandiocal Orebodies and Implications for the Genesis of the Orogenic Chega Tudo Gold Deposit, Gurupi Belt, Brazil

The Chega Tudo gold deposit is located in the Gurupi belt of northern Brazil. Gold mineralization in the Mina Velha (upper level) and Mandiocal (intermediate and lower levels) orebodies is hosted in highly strained Paleoproterozoic (2148‐2160 Ma) felsic to intermediate metavolcanic rocks and schists that record greenschist facies metamorphism. The orebodies are broadly concordant with the regional northwest-oriented foliation, and mineralization comprises thick quartz vein sets and small quartz-carbonate-sulfide veinlets and enclosing hydrothermally altered host rocks. The hydrothermal mineral assemblage is postpeak metamorphism, syn- to late tectonic, and includes quartz, calcite, chlorite, white mica, pyrite, subordinate chalcopyrite, and traces of sphalerite and galena. Free-milling gold occurs in sulfide-poor veins in the upper and lower zones of the deposit, whereas refractory gold was deposited in fractures of pyrite and quartz in the more gold-enriched intermediate (±lower) level. Fluid inclusion and stable isotope (O, H, C, and S) data indicate that the two styles of gold deposition were produced by a combination of fluid immiscibility, fluid-rock reactions (sulfidation, carbonatization), and probably mixing, occurring in two discrete stages within a single mineralizing event associated with fluid flow within and around an active shear zone under fluctuating pressure conditions. The refractory gold was deposited at 340° to 370°C from a CO2-CH4-H2O‐NaCl fluid having salinity of 5.8 ± 2.7 wt percent NaCl equiv, variable CO2 contents (typically 12‐22 mol %), and up to 6 mol percent CH4. The δ18O and δD values of this fluid are 7.9 to 9.4 per mil and ‐29 to ‐37 per mil, respectively, indicating a metamorphic origin. Carbon isotopes show contrasting values in fluid inclusion CO2 (δ13C = ‐24.1‰) and calcite (δ13C = of ‐3.8‰). The strongly negative value of fluid inclusion CO2 is interpreted to be an organic signature acquired at the site of deposition by reaction of the deeply sourced ore-forming fluid with carbonaceous schists present in the deepest part of the deposit. The higher value for calcite probably records a crustal source. Oxygen fugacities calculated for the whole range of T-P-XCO2 conditions yielded log fO 2 between ‐28.7 and ‐30.5, indicating reduced conditions for the fluid, which is in agreement with the mineral paragenesis, fluid inclusion compositions, and sulfur isotope values. The second fluid, responsible for the deposition of the free-milling gold in veins and probably part of the gold of the lower zone of the deposit, had a lower temperature (330°‐340°C). This CO2-H2O‐NaCl fluid had a salinity of 1.6 to 2.5 wt percent NaCl equiv and contained 11 to 13 mol percent CO2. The δ18O and δD values of this fluid are 5.3 to 7.2 per mil and ‐12 to ‐30 per mil, respectively, also indicating a metamorphic source. The δ13C value of fluid inclusion CO2 is ‐6.9 per mil, likely representing a mantle-derived carbon source. Log fO 2 values between ‐30.5 and ‐31.5 also indicate slightly more reduced conditions for this fluid. Both fluids had near-neutral pH, between 5 and 6.2, and δ34S values of H2S between ‐0.2 and ‐1.0 per mil, interpreted to reflect derivation of sulfur from magmatic sulfides.

Numerical modelling of mechanical controls on coeval steep and shallow dipping auriferous quartz vein formation in a thrust zone, Macraes mine, New Zealand

The Hyde-Macraes Shear Zone (HMSZ) is a regionally continuous, low-angle, NE dipping (~15°) late-metamorphic thrust zone in the Mesozoic Otago Schist. The shear zone, which is host to large volumes of mineralised schist, consists of foliated fissile schist with some massive schist pods. Two sets of quartz veins are found within the HMSZ: thrust-related, shallowly dipping veins that were emplaced parallel or sub-parallel to the shears and swarms of steeply dipping extensional veins, which cut across the metamorphic foliation. The latter are restricted to the massive schist pods. Mutual cross-cutting relationships occur between steep extensional veins and shallow-dipping veins, suggesting that they formed contemporaneously. The co-existence of these two vein types locally implies local rotation of the principal stress axes to produce extensional veins within a regional thrust setting. The steep extensional veins are spatially related to lateral and oblique ramps within the HMSZ. Three-dimensional mechanical models show that these lateral or oblique ramps can produce favourable conditions for extensional vein formation when combined with a high fluid pressure and oblique convergence. Mechanical requirements include a reduced differential stress, a positive volumetric strain and an increase in the horizontal shear stress. Our models show that under certain conditions, it is possible for extension-related structures to form during shortening because of local changes in the stress state without the need for a regional scale switch in the imposed stress field. The convergence direction across the HMSZ during formation of the steep extensional veins was ~WNW.

Geometry and texture of quartz veins in Wadi Atalla area, Central Eastern Desert, Egypt

Several quartz vein sets with varying orientation, geometry and internal structure were recognized in the Atalla area. The veins were associated with the deformation phases affecting the area. En echelon and extensional veins are the main geometrical types. Syn-kinematic veins associated with the major northeast-over-southwest thrust faults were later boudinaged, folded and re-folded. En echelon veins, fibrous veins, and extensional veins are associated with the NNW–SSE faults. Other veins are associated with the NW–SE, N–S, NE–SW and E–W faults. Veins are concentrated at the intersection zones between faults. The internal structure of the veins comprises syntaxial, antitaxial, and composite types and reflects a change from a compressive stress regime to an extensional one. Chocolate-tablet structures and synchronous and co-genetic vein networks indicate later multi-directional extension of the area. Interaction between cracking and sealing of fractures is a common feature in the study area indicating that it was easy for the pore pressure to open pre-existing fractures instead of creating new ones. The reopening of pre-existing fractures rather than creating new ones is also indicated by the scattering of vein data around σ 3. There is an alteration and change in characteristics of the wall rock due to increase in fluid flow rate. Fault-valving probably is also a cause of the complex geometry of some veins.

Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relationship to District-Wide Zn, Pb, and Cu Mineralization

Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relationship to District-Wide Zn, Pb, and Cu Mineralization

Geometria fractal dos veios de quartzo da serra de Ouro Preto, flanco sudeste do anticlinal de mariana, Quadrilátero Ferrífero/MG

Fractal analysis statistics combined with textural and structural data of quartz vein sets have been used to address their evolution in the tectonic context of the Ouro Preto Range, which is the southern limb of the Mariana Anticline. Fractal statistics have become a convenient way to deal with the problem of the scale analysis of vein sets considering that vein dimensions (length, thickness and spacing) have analogous relationships and length and thickness vary according to a power law distribution. The vein sets have been mapped in three sections across the Ouro Preto Range, 2 km from each other. 613 veins have been mapped on the surface and in abandoned mine galleries. Measured data comprise thickness, length and spacing. 161 veins are hosted in quartzite, 60 in phyllite and 392 in the iron formation (BIF). Data have been plotted in cumulative log-plots of number of veins versus thickness, number of veins versus . length, thickness versus length and number of veins versus spacing. The older vein sets are parallel or subparallel to the regional cleavage. These veins underwent dynamic recrystalization and have been formed as a consequence of an extensional event. The younger vein set displays a crystalline fabric not truncated by the regional cleavage. The largest veins cross the quartzite layers and have larger axial ratios than the smaller ones that cross the phyllite and iron formation. Analysis of shape factors (thickness and length) using the fractal dimension D revealed that both vein sets display a flat shape (prolate in section) in the western portion of the studied area changing to a nodule shape (oblate in section) towards the east. In addition, vein density is greater forlarger values of D and D increases eastward for both sets of veins. These data may suggest that the source of the fluids is closer to the axis of the Mariana Anticline and/or that contractional strain increases towards the east as reported by regional geology data.

Structural controls on the Cowarra gold deposit near Bredbo, southeastern New South Wales

The Cowarra gold deposit is an epigenetic, structurally controlled, gold‐bearing sulfide vein system in a regional shear zone developed within isoclinally folded Ordovician turbidites. Gold occurs mainly in sulfide‐rich veins parallel to the axial cleavage or shears on fold limbs. There are seven sets of quartz veins, but they contain only minor gold. The mineralised shears lie within a zone of strong fold‐plunge variation, in the chlorite zone of a regional metamorphic complex, and to the west of the Bega Batholith. However, gold is not related to metamorphic outflushing, but more likely to sulfide‐rich fluids derived from depth during intrusion of I‐type granites of the Michelago Igneous Complex. The local control for sulfide veining was decompressional dilation of axial planar structures shortly after folding and shearing; this is unlike the structural control described for most deposits from this and similar terrains. Subsequent normal, strike‐slip and reverse fault movements in the shear zone develop...

The Coolgardie Goldfied, Western Australia: district-scale controls on an Archaean gold camp in an amphibolite facies terrane

Archaean lode gold deposits in the Coolgardie Goldfield, Western Australia, occur in mafic and ultramafic rocks which have been metamorphosed to the amphibolite facies. Mineralisation was broadly synchronous with peak metamorphism, the main phase of granitoid emplacement, and regional deformation. Several different structural styles are represented by the deposits of the Coolgardie Goldfield. Mineralisation occurs along sheared felsic porphyry-ultramafic rock contacts, in gabbro-hosted quartz-vein sets, in fault-bounded quartzvein sets, and in laminated quartz reefs sited in brittleductile shear zones. The structures hosting mineralisation formed in response to a progressive deformation event, related to granitoid emplacement in an east-west compressional far-field stress regime, but with local heterogeneous stress orientations. This occurred after an earlier period of thrust-stacking, with probable north-south directed tectonic transport. Two contrasting styles of goldrelated wallrock alteration are associated with the auriferous lodes of the Coolgardie Goldfield. A high-temperature assemblage (formation temperature >500°C), characterised by the proximal alteration assemblage garnet+ hornblende + plagioclase + pyrrhotite, contrasts with a medium-temperature assemblage (formation temperature <500°C), consisting of calcic amphibole + biotite + plagioclase + calcite + arsenopyrite + pyrrhotite. The distribution of the two styles of gold-related alteration is controlled by distance from voluminous syntectonic granitoids located to the west of the Coolgardie Goldfield, with the high-temperature style of alteration more proximal to the granitoid-greenstone contact than the medium-temperature style. The occurrence of gold deposits that formed under amphibolite facies conditions throughout the Coolgardie Goldfield supports a crustal continuum model for Archaean lode-gold deposits, which proposes that gold is deposited in metamorphic environments that range from the sub-greenschist to granulite facies. In addition, the data from Coolgardie suggest that syntectonic, synmineralisation granitic plutons may play a significant role in controlling the style of gold associated wallrock-alteration at deep crustal levels.

Contexte structural et minéralisations aurifères des gîtes Casa-Berardi, Abitibi, Québec

The Casa-Berardi gold deposits are located 180 km north of Rouyn-Noranda within Archean rocks of the Abitibi Subprovince. The Casa-Berardi deformation zone (CBDZ), which contains these deposits, is an east–west trending structure that is recognized along a minimum distance of 80 km. Ductile deformation has affected the lithologies in the central portion of the CBDZ. The CBDZ juxtaposes distinct sedimentary and volcanic lithostratigraphic packages which locally display opposite facing directions. Within the CBDZ the Casa-Berardi fault has been recognized; it is a brittle structure with a reverse motion. This fault represents a distinct element associated with the progression of deformation in the CBDZ and appears late in the geodynamical evolution of this deformation zone. Two other deformation zones have been identified in this region: (i) the east–west-trending Boivin-Paradis deformation zone, which has limited lateral extent and is located at the periphery of a granitoid intrusion, and (ii) the Laberge deformation zone, which has a northwest to east–west trend and crosscuts the regional structural grain. Most of the economic gold mineralization in the Casa-Berardi deposits is found in the second set of four recognized vein and fracture sets. The mineralized veins are found within zones of intense deformation which are preferentially developed close to contacts between sedimentary and volcanic units. The geometry of the different sets of quartz veins, their relative chronology, and their respective hydrothermal alterations can be explained by the progression of deformation within the CBDZ and bracket the gold mineralization as syn- to late-tectonic. The CBDZ is distinguished from the other two deformation zones by its greater lateral extent, the juxtaposition of distinct lithological domains, the complexity of its internal fabrics, and the relative length of the deformation event. These features indicate that in contrast with the other two deformation zones, the CBDZ was the site of a major mineralizing event. The recognition of contrasting characteristics between the different deformation zones has repercussions on the understanding of the different criteria that control gold deposition and, ultimately, on gold exploration.