The structural controls of gold mineralisation within the Bardoc Tectonic Zone,Eastern Goldfields Province,Western Australia: implications for gold endowment in shear systems

The Bardoc Tectonic Zone (BTZ) of the late Archaean Eastern Goldfields Province, Yilgarn Craton, Western Australia, is physically linked along strike to the Boulder-Lefroy Shear Zone (BLSZ), one of the richest orogenic gold shear systems in the world. However, gold production in the BTZ has only been one order of magnitude smaller than that of the BLSZ (∼100 t Au vs >1,500 t Au). The reasons for this difference can be found in the relative timing, distribution and style(s) of deformation that controlled gold deposition in the two shear systems. Deformation within the BTZ was relatively simple and is associated with tight to iso-clinal folding and reverse to transpressive shear zones over a <12-km-wide area of high straining, where lithological contacts have been rotated towards the plane of maximum shortening. These structures control gold mineralisation and also correspond to the second major shortening phase of the province (D₂). In contrast, shearing within the BLSZ is concentrated to narrow shear zones (<2 km wide) cutting through rocks at a range of orientations that underwent more complex dip- and strike-slip deformation, possibly developed throughout the different deformation phases recorded in the region (D₁–D₄). Independent of other physico-chemical factors, these differences provided for effective fluid localisation to host units with greater competency contrasts during a prolonged mineralisation process in the BLSZ as compared to the more simple structural history of the BTZ.     

Multistage gold mineralization at the Lapa mine, Abitibi Subprovince: insights into auriferous hydrothermal and metasomatic processes in the Cadillac–Larder Lake Fault Zone

The Lapa gold deposit contains reserves of 2.4 Mt at 6.5 g/t Au and is one of the few deposits located directly within the Cadillac–Larder Lake Fault Zone (CLLFZ), a first-order crustal-scale fault that separates the Archean Abitibi Subprovince from the Pontiac Subprovince to the south. Gold mineralization is predominantly hosted in highly strained and altered, upper greenschist–lower amphibolite facies mafic to ultramafic rocks of the Piché Group. Auriferous ore zones consist of finely disseminated auriferous arsenopyrite–pyrrhotite?±?pyrite and native gold disseminated in biotite- and carbonate-altered wall rocks. Native gold, which is also present in quartz ± dolomite–calcite veinlets, is locally associated with Sb-bearing minerals, especially at depth ≤1 km from surface where the deposit is characterized by a Au–Sb–As association. At vertical depth greater than 1 km, gold is associated with arsenopyrite and pyrrhotite (Au–As association). The mineralogy and paragenesis of the Lapa deposit metamorphosed ore and alteration assemblages record the superposition of three metamorphic episodes (M1, M2, and M3) and three gold mineralizing events. Spatial association between biotitized wall rocks and auriferous arsenopyrite indicates that arsenopyrite precipitation is concomitant with potassic alteration. The predominant Au–As association recognized across the deposit is related to gold in solid solution in arsenopyrite as part of a pre-M2 low-grade auriferous hydrothermal event. However, the occurrence of hornblende?+?oligoclase porphyroblasts overprinting the biotite alteration, and the presence of porous clusters and porphyroblasts of arsenopyrite with native gold and pyrrhotite indicate an auriferous metasomatic event associated with peak M2 prograde metamorphism. Late retrograde metamorphism (M3) overprints the hornblende–oligoclase M2 assemblage within the host rocks proximal to ore by an actinolite–albite assemblage by precipitation of free gold and Sb–sulfosalts at lower P–T. The complex relationships between ore, structural features, and metamorphic assemblages at Lapa are related to the tectonometamorphic evolution of the Cadillac–Larder Lake Fault Zone at different times and crustal levels, and varying heat and fluid flow regimes. The Lapa deposit demonstrates that early, low-grade gold mineralization within the Cadillac–Larder Lake Fault Zone has benefited from late gold enrichment(s) during prograde and retrograde metamorphism, suggesting that multi-stage processes may be important to form gold-rich orogenic deposits in first order crustal-scale structures.     

Mossman orogenic gold province in north Queensland,Australia: regional metallogenic controls and undiscovered gold endowment

The Hodgkinson and Broken River provinces of the Mossman Orogen in north Queensland host numerous orogenic gold deposits and still remain under-explored. This paper discusses regional metallogenic controls and results of a probabilistic quantitative assessment of undiscovered gold potential in the region. Significant orogenic gold deposits in the region occur only within relatively small well-endowed metallogenic zones, likely to be controlled by the eastern margin of the Paleoproterozoic continental crust underlying the western Mossman Orogen. Three distinct styles of primary orogenic gold deposits are present in the area: gold–quartz veins, refractory gold associated with quartz–pyrite–arsenopyrite veins and stockworks and stibnite–quartz±gold veins. Refractory gold deposits are estimated to have the highest potential for significant undiscovered resources in the region. The Hodgkinson Province is estimated to host between one and ten significant undiscovered refractory gold ore fields, with a 50 % probability of at least 20 t of total contained gold and a 90 % probability of at least 1 t. The Broken River Province is estimated to host up to five significant undiscovered refractory gold ore fields, with a 50 % probability of at least 12 t of contained gold.     

The Syama and Tabakoroni goldfields,Mali

Gold deposits in the Syama and Tabakoroni goldfields in southern Mali occur along a north-northeast trending mineralised litho-structural corridor that trends for approximately 40 km. The deposits are interpreted to have formed during a craton-wide metallogenic event during the Eburnean orogeny. In the Syama goldfield, gold mineralisation in 9 deposits is hosted in the hanging-wall of the Syama-Bananso Shear Zone in basalt, greywacke, argillite, lamprophyre, and black shale. Gold is currently mined primarily from the oxidised-weathered zone of the ore bodies. In the Syama deposit, mineralisation hosted in altered basalt is associated with an intense ankerite–quartz–pyrite stockwork vein systems, whereas disseminated style mineralisation is also present in greywackes. In contrast, the Tellem deposit is hosted in quartz–porphyry rocks.In the Tabakoroni goldfield, gold mineralisation is hosted in quartz veins in tertiary splay shears of the Syama-Bananso Shear Zone. The Tabakoroni orebody is associated with quartz, carbonate and graphite (stylolite) veins, with pyrite and lesser amounts of arsenopyrite. There are four main styles of gold mineralisation including silica-sulphide lodes in carbonaceous fault zones, stylolitic quartz reefs in fault zones, quartz–Fe–carbonate–sulphide lodes in mafic volcanics, and quartz–sulphide stockwork veins in silicified sediments and porphyry dykes. The several deposit styles in the goldfield thus present a number of potential exploration targets spatially associated with the regional Syama-Bananso Shear Zone and generally classified as orogenic shear-hosted gold deposits.     

Deep seismic reflection profiling in the Archaean northeastern Yilgarn Craton, Western Australia: implications for crustal architecture and mineral potential

Deep seismic reflection data across the Archaean Eastern Goldfields Province, northeastern Yilgarn Craton, Western Australia, have provided information on its crustal architecture and on several of its highly mineralised belts. The seismic reflection data allow interpretation of several prominent crustal scale features, including an eastward thickening of the crust, subdivision of the crust into three broad layers, the presence of a prominent east dip to the majority of the reflections and the interpretation of three east-dipping crustal-penetrating shear zones. These east-dipping shear zones are major structures that subdivide the region into four terranes. Major orogenic gold deposits in the Eastern Goldfields Province are spatially associated with these major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world-class gold deposits plus many smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed metallogenically as the Laverton structure. The seismic reflection data have also imaged a series of low-angle shear zones within and beneath the granite–greenstone terranes. Where the low-angle shear zones intersect the major crustal-penetrating structures, a wedge shaped geometry is formed. This geometry forms a suitable fluid focusing wedge in which upward to subhorizontal moving fluids are focused and then distributed into the nearby complexly deformed greenstones.     

Invisible gold in arsenian pyrite and arsenopyrite from a multistage Archaean gold deposit: Sunrise Dam,Eastern Goldfields Province,Western Australia

The Sunrise Dam gold mine (11.1 Moz Au) is the largest deposit in the Archaean Laverton Greenstone Belt (Eastern Goldfields Province, Yilgarn Craton, Western Australia). The deposit is characterized by multiple events of fluid flow leading to repeated alteration and mineralization next to a major crustal-scale structure. The Au content of arsenian pyrite and arsenopyrite from four mineralizing stages (D₁, D₃, D_4a, and D_4b) and from different structural and lithostratigraphic environments was measured using in situ laser ablation inductively coupled plasma mass spectrometry. Pyrite contains up to 3,067 ppm Au (n = 224), whereas arsenopyrite contains up to 5,767 ppm (n = 19). Gold in arsenopyrite (D_4a stage) was coprecipitated and remained as “invisible gold” (nanoparticles and/or lattice-bound) during subsequent deformation events. In contrast, gold in pyrite is present not only as “invisible gold” but also as micrometer-size inclusions of native gold, electrum, and Au(Ag)–tellurides. Pristine D₁ and D₃ arsenian pyrite contains relatively low Au concentrations (≤26 ppm). The highest Au concentrations occur in D_4a arsenian-rich pyrite that has recrystallized from D₃ pyrite. Textures show that this recrystallization proceeded via a coupled dissolution–reprecipitation process, and this process may have contributed to upgrading Au grades during D_4a. In contrast, Au in D_4b pyrite shows grain-scale redistribution of “invisible” gold resulting in the formation of micrometer-scale inclusions of Au minerals. The speciation of Au at Sunrise Dam and the exceptional size of the deposit at province scale result from multiple fluid flow and multiple Au-precipitating mechanisms within a single plumbing system.     

Silver-rich telluride mineralization at Mount Charlotte and Au–Ag zonation in the giant Golden Mile deposit,Kalgoorlie, Western Australia

The gold deposits at Kalgoorlie in the 2.7-Ga Eastern Goldfields Province of the Yilgarn Craton, Western Australia, occur adjacent to the D2 Golden Mile Fault over a strike of 8 km within a district-scale zone marked by porphyry dykes and chloritic alteration. The late Golden Pike Fault separates the older (D2) shear zone system of the Golden Mile (1,500 t Au) in the southeast from the younger (D4) quartz vein stockworks at Mt Charlotte (126 t Au) in the northwest. Both deposits occur in the Golden Mile Dolerite sill and display inner sericite–ankerite alteration and early-stage gold–pyrite mineralization replacing the wall rocks. Late-stage tellurides account for 20 % of the total gold in the first, but for <1 % in the second deposit. In the Golden Mile, the main telluride assemblage is coloradoite?+?native gold (898–972 fine)?+?calaverite?+?petzite?±?krennerite. Telluride-rich ore (>30 g/t Au) is characterized by Au/Ag?=?2.54 and As/Sb?=?2.6–30, the latter ratio caused by arsenical pyrite. Golden Mile-type D2 lodes occur northwest of the Golden Pike Fault, but the Hidden Secret orebody, the only telluride bonanza mined (10,815 t at 44 g/t Au), was unusually rich in silver (Au/Ag?=?0.12–0.35) due to abundant hessite. We describe another array of silver-rich D2 shear zones which are part of the Golden Mile Fault exposed on the Mt Charlotte mine 22 level. They are filled with crack-seal and pinch-and-swell quartz–carbonate veins and are surrounded by early-stage pyrite?+?pyrrhotite disseminated in a sericite–ankerite zone more than 6 m wide. Gold grade (0.5–0.8 g/t) varies little across the zone, but Au/Ag (0.37–2.40) and As/Sb (1.54–13.9) increase away from the veins. Late-stage telluride mineralization (23 g/t Au) sampled in one vein has a much lower Au/Ag (0.13) and As/Sb (0.48) and comprises scheelite, pyrite, native gold (830–854 fine), hessite, and minor pyrrhotite, altaite, bournonite, and boulangerite. Assuming 250–300 °C, gold–hessite compositions indicate a fluid log f _Te2 of ?11.5 to ?10, values well below the stability of calaverite. The absence of calaverite and the dominance of hessite in the D2 lodes of the Mt Charlotte area point to a kilometer-scale mineral and Au/Ag zonation along the Golden Mile master fault, which is attributed to a lateral decrease in peak tellurium fugacity of the late-stage hydrothermal fluid. The As/Sb ratio may be similarly zoned to lower values at the periphery. The D4 gold–quartz veins constituting the Mt Charlotte orebodies represent a younger hydrothermal system, which did not contribute to metal zonation in the older one.     

Constraints on the composition of ore fluids and implications for mineralising events at the Cleo gold deposit,Eastern Goldfields Province,Western Australia

The Cleo gold deposit, 55 km south of Laverton in the Eastern Goldfields Province of Western Australia, is characterised by banded iron‐formation (BIF)‐hosted ore zones in the gently dipping Sunrise Shear Zone and high‐grade vein‐hosted ore in the Western Lodes.There is evidence that gold mineralisation in the Western Lodes (which occurred at ca 2655 Ma) post‐dates the majority of displacement along the Sunrise Shear Zone, but it remains uncertain if the ore in both structures formed simultaneously or separately. Overall, the Pb, Nd, Sr, C, O and S isotopic compositions of ore‐related minerals from both the Western Lodes and ore zones in the Sunrise Shear Zone are similar. Early low‐salinity aqueous‐carbonic fluids and late high‐salinity fluids with similar characteristics are trapped in inclusions in quartz veins from both the Sunrise Shear Zone and the Western Lodes. The early CO₂, CO₂–H₂O, and H₂O‐dominant inclusions are interpreted as being related to ore formation, and to have formed from a single low‐salinity aqueous‐carbonic fluid as a result of intermittent fluid immiscibility. Homogenisation temperatures indicate that these inclusions were trapped at approximately 280°C and at approximately 4 km depth, in the deeper epizonal range. Differences between the ore zones are detected in the trace‐element composition of gold samples, with gold from the Sunrise Shear Zone enriched in Ni, Pb, Sn, Te and Zn, and depleted in As, Bi, Cd, Cu and Sb, relative to gold from the Western Lodes. Although there are differences in gold composition between the Sunrise Shear Zone and Western Lodes, and hence the metal content of ore fluids may have varied slightly between the different ore zones, no other systematic fluid or solute differences are detected between the ore zones. Given the fact that the ore fluids in each zone have very similar bulk properties, the considerable differences in gold grade, sulfide mineral abundance, and ore textures between the two ore zones most likely result from different gold‐deposition mechanisms. The association of ore zones in the Sunrise Shear Zone with pyrite‐replaced BIF suggests that wall‐rock sulfidation was the most significant mechanism of gold precipitation, through the destabilisation of gold‐bisulfide complexes. The Western Lodes, however, do not exhibit any host‐rock preference and multistage veins commonly contain coarse‐grained gold. Fluid‐inclusion characteristics and breccia textures in veins in the Western Lodes suggest that rapid pressure changes, brought about by intermittent release of overpressured fluids and concomitant phase separation, are likely to have caused the destabilisation of gold‐thiocomplexes, leading to formation of higher‐grade gold ore zones.     

Granite-hosted molybdenite mineralization from Archean Bundelkhand craton-molybdenite characterization,host rock mineralogy,petrology, and fluid inclusion characteristics of Mo-bearing quartz

The dominantly high-K, moderate to high SiO₂ containing, variably fractionated, volcanic-arc granitoids (± sheared) from parts of Bundelkhand craton, northcentral India are observed to contain molybdenite (Mo) in widely separated 23 locations in the form of specks, pockets, clots and stringers along with quartz ± pyrite ± arsenopyrite ± chalcopyrite ± bornite ± covellite ± galena ± sphalerite and in invisible form as well. The molybdenite mineralization is predominantly associated with Bundelkhand Tectonic Zone, Raksa Shear Zone, and localized shear zones. The incidence of molybdenite is also observed within sheared quartz and tonalite–trondhjemite–granodiorite (TTG) gneisses. The fluid inclusion data show the presence of bi-phase (H₂O–CO₂), hypersaline and moderate temperature (100°–300°C) primary stretched fluid inclusions suggesting a possible hydrothermal origin for the Mo-bearing quartz occurring within variably deformed different granitoids variants of Archean Bundelkhand craton.     

Mineralogy and microchemistry of the Egat gold deposit, South Eastern Desert of Egypt

Gold-bearing quartz lodes from the Egat gold mine, South Eastern Desert of Egypt, are associated with pervasively silicified, highly sheared ophiolitic metagabbro and island-arc metavolcanic rocks. The mineralized quartz veins and related alteration haloes are controlled by NNW-trending shear/fault zones. Microscopic and electron probe microanalyses (EPMA) data of the ore and gangue minerals reveal that fine-grained auriferous sulfarsenides represent early high-temperature (355–382 °C) phases, with formation conditions as (fS₂?=??10, and fO₂ around ?31). A late, low-temperature (302–333 °C) assemblage includes coarse pyrite, arsenopyrite, and free-milling gold grains (88–91 wt.% Au), with formation conditions as (fS₂?=??8 and fO₂ around ?30). Gold was impounded within early sulfarsenides, while free-milling gold blebs occur along microfractures in quartz veins and as inclusions in late sulfides. Infiltration of hydrothermal fluids under brittle–ductile shear conditions led to mobilization of refractory Au from early sulfarsenide phases and reprecipitated free gold, simultaneous with silicification of the host rocks. The positive correlation between Au and As favors and verifies the use of As as the best pathfinder for gold targets, along the NNW-trending shear zones.     

The geology and mineralisation at the Golden Pride gold deposit,Nzega Greenstone Belt,Tanzania

The Golden Pride gold deposit (∼3 Moz) is located in the central part of the Nzega Greenstone Belt at the southern margin of the Lake Victoria Goldfields in Tanzania. It represents an inferred Late Archaean, orogenic gold deposit and is hosted in intensely deformed meta-sedimentary rocks in the hanging wall of the approximately E–W striking Golden Pride Shear Zone. The hanging-wall sequence also includes felsic (quartz porphyritic) to mafic (lamprophyric) intrusions, as well as banded iron formations. Hydrothermal alteration phases associated with mineralisation are dominated by sericite and chlorite. Two main ore types can be distinguished, chlorite and silica ore, both occupying dilational sites and structural intersections in the hanging wall of the main shear zone. Sulphide minerals in both ore types include pyrrhotite, arsenopyrite, pyrite and accessory sphalerite, galena, sulphosalts and Ni–Co–Bi sulphides. Gold and tellurides are late in the paragenetic sequence and associated with a secondary phase of pyrrhotite deposition. Sulphur isotope compositions range from −6 to 7 per mil and are interpreted to reflect contributions from two distinct sources to the mineralising fluids in the Golden Pride gold deposit. A redox change, potentially induced by the intrusion of mafic melts, together with structural elements in the hanging wall of the Golden Pride Shear Zone, are interpreted to be the main controls on gold mineralisation in this deposit.     

Re-Os geochronology of gold mineralization in the Fawakhir area,Eastern Desert,Egypt

We report the first Re-Os data on gold-associated arsenopyrite from mesothermal gold-quartz veins in the ancient Egyptian Fawakhir–El Sid gold mining district in the central Eastern Desert. This mining district has an ~5000-year-old history and is displayed in the Turin Papyrus Map (about 1150 BC), which is widely acclaimed as the world’s oldest geographic map, as well as the oldest geologic and mine map. The Fawakhir–El Sid district is part of a regional NNW-trending shear corridor (15 km wide) that hosts several other historic gold mines associated with left-lateral wrench structures and related granite intrusions. Vein-style gold mineralization is hosted within and at the margin of an I-type and magnetite-series monzogranite, the Fawakhir granite intrusion, and a Pan-African (~740 Ma) ophiolite sequence. The ore mineralogy of the mineralized quartz veins includes pyrite-arsenopyrite-pyrrhotite-sphalerite-galena-chalcopyrite-electrum plus a number of tellurides of Ag, Au, and Bi. The ¹⁸⁷Re/¹⁸⁸Os versus ¹⁸⁷Os/¹⁸⁸Os regression on 5 points of arsenopyrite gives an age of 601 ± 17 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os of 0.24 ± 0.07 (2 σ; MSWD = 17). This age coincides within error with the U-Pb age on zircon from the Fawakhir monzogranite (598 ± 3 Ma). The age coincidence and the hydrothermal Te and Bi metal signature suggest a foremost role of granite-related fluids in the quartz-vein system.     

金矿床中毒砂标型特征及金的赋存状态——以湖南金矿床为例

湖南含砷金矿资源储量大,分布广,类型多.矿床中的砷矿物主要为毒砂,几乎所有金矿床中的毒砂都含Au（一般120×10^-6~250×10^-6）,且比共生的黄铁矿含Au量高2~5倍,甚至1个数量级以上.毒砂中金的分布率高达64.3%~94.05%.毒砂的生成期有早、晚2期.化学成分为富S亏As型,并以富含微量元素Sb（Se）、Ni、Co而贫Mn及晶胞参数a₀值增大等为标型特征.大多数含Au毒砂均含有相当数量的“不可见金”,即使利用电子探针也难以发现.初步认为毒砂中的“不可见金”多呈纳米级微细粒状存在.     

Pyrite textures and compositions from the Zhuangzi Au deposit,southeastern North China Craton: implication for ore-forming processes

The Zhuangzi Au deposit in the world-class Jiaodong gold province hosts visible natural gold, and pyrite as the main ore mineral, making it an excellent subject for deciphering the complex hydrothermal processes and mechanisms of gold precipitation. Three types of zoned pyrite crystals were distinguished based on textural and geochemical results from EPMA, SIMS sulfur isotopic analyses and NanoSIMS mapping. Py0 has irregular shapes and abundant silicate inclusions and was contemporaneous with the earliest pyrite–sericite–quartz alteration. It has low concentrations of As (0–0.3 wt.%), Au and Cu. Py1 precipitated with stage I mineralization shows oscillatory zoning with the bright bands having high As (0.4–3.9 wt.%), Au and Cu contents, whereas the dark bands have low contents of As (0–0.4 wt.%), Au and Cu. The oscillatory zoning represents pressure fluctuations and repeated local fluid phase separation around the pyrite crystal. The concentration of invisible gold in Py1 is directly proportional to the arsenic concentration. Py1 is partially replaced by Py2 which occurs with arsenopyrite, chalcopyrite and native gold in stage II. The replacement was likely the result of pseudomorphic dissolution–reprecipitation triggered by a new pulse of Au-rich hydrothermal fluids. The δ³⁴S values for the three types of pyrite are broadly similar ranging from +?7.1 to +?8.8‰, suggesting a common sulfur source. Fluid inclusion microthermometry suggests that extensive phase separation was responsible for the gold deposition during stage II mineralization. Uranium–Pb dating of monazite constrains the age of mineralization to ca. 119 Ma coincident with a short compressional event around 120 Ma linked to an abrupt change in the drift direction of the subducting Pacific plate.     

Geology,geochemistry and sulphur isotopes of the Hat Han gold–antimony deposit,NE Vietnam

The Song Hien Rift basin is considered as one of the important regions for gold deposits in North East Vietnam. Host rocks of a number gold deposits in the Song Hien Rift basin are mainly in Lower Triassic sedimentary formations. However, there is the Hat Han gold deposit hosted in fined-grained mafic magmatic rocks with similar characteristics as gold deposit hosted in the Triassic sediments. Sulphur isotopic compositions of sulphide are similar to those in carbonaceous shale, suggesting that the sulphur was ‘borrowed’ from sedimentary rocks in filling the rift basin. Gold-bearing sulphides (pyrite and arsenopyrite) are the main form of Au presence in the ore. Gold in pyrite is present as Au^+ 1, and a minor amount of as nanoparticles of native Au (Au⁰); whereas in arsenopyrite, gold is chemically bound as the octahedral complex AuAs₂. Analysis of geology, as well as geochemical and isotopic studies show that the genesis of the Hat Han gold deposit is not related to the Cao Bang mafic magmatism; instead the latter only serves as (ore) host rock. The geochemical results presented above suggest that the gabbro host rock only supplies iron needed for sulphide formation. With regard to ore genesis, the Hat Han gold deposit in the Song Hien rift basin was generated in the similar way as sediment-hosted gold deposit. There are many similar typomorphic features between the Hat Han deposit and Carlin-like deposits in the Nanpanjang sedimentary basin in China.     

Syngenetic gold in western Victoria: occurrence,age and dimensions

In western Victoria, a widespread stratiform style of gold enrichment in Palaeozoic black mudstone and chert—clearly different from the classic mesothermal quartz vein deposits of the Victorian goldfields—has been confirmed by whole-rock geochemistry and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). This enrichment pre-dates compaction, deformation and low-grade metamorphism of the sedimentary host-rocks, and therefore possibly developed diagenetically during slow deposition of the thin carbonaceous black mudstone beds and the thinner layers of chert. These paired strata have been documented at many locations in three regional outcrop areas of chevron-folded Cambrian and Lower Ordovician metasediments in the Stawell and Bendigo Zones, where they are interbedded with quartz-rich turbidites. The layers were named ‘indicators’ by the early miners, who found locally rich nuggety gold deposits at intersections between these layers and mesothermal quartz veins. Gold is present in euhedral pyrite crystals in both black shale and chert. LA-ICPMS analysis of individual pyrite crystals in the indicator beds shows that pyrite is enriched in Au, As, Sb, Se, Te and Bi. The Au content of pyrite varies from 0.03 to 2.69 ppm with a mean of 0.58 ppm and shows a positive correlation with As, which varies from 1000 to 6000 ppm. Many pyrite crystals show enrichment of gold in their cores and depletion in their rims, confirming the likely syngenetic or syndiagenetic accumulation of gold during pyrite formation in the sediments. Prior to regional metamorphism, folding and faulting, the many indicator strata in the outcrop areas were parts of an extensive marine sequence of Late Cambrian and Early Ordovician age. The former primary source of this mineralisation is considered to have been one or more contemporaneously exhalative submarine hydrothermal systems. Thus, the older Palaeozoic sediments of the western Lachlan Fold Belt were significantly enriched in syngenetic gold in the Early Palaeozoic, at least 40 million years before emplacement of the quartz – gold vein deposits of the goldfields.     

Kilometre-scale structural setting of ore shoots in the Frasers gold deposit,Macraes mine,New Zealand

The Hyde-Macraes Shear Zone in southern New Zealand contains the circa 10 million ounce Macraes gold deposit, one of the larger Phanerozoic orogenic gold deposits discovered to date globally. Approximately 50% of this 10 million ounce resource is hosted by 5 major ore shoots up to 400 m wide and 1500 m long in the Frasers area at the southern end of the mine. Higher grade (>1.5 g/t Au) ore shoots are located along and immediately below the Hangingwall Shear, the principal strand of the Hyde-Macraes Shear Zone at the Frasers deposit. They typically trend parallel to the intersection of the shear and foliation in the underlying schist, commonly where the foliation dips more steeply that the overlying Hangingwall Shear. Especially thick zones of higher grade mineralised rock are located between the Hangingwall Shear and underlying second order splay shears whose position correlates with minor right-hand bends in the strike of the overlying Hangingwall Shear. Lower grade (<1.2 g/t Au), but economically significant, ore shoots are located within mineralised schists below the Hangingwall Shear. Outer margins of these lower grade ore shoots are generally parallel to the strike of the foliation in the host schist. They are most extensive where open disharmonic folding has resulted in the strike of the foliation diverging from that of the overlying Hangingwall Shear. No correlation exists between the position of any ore shoots and gently dipping jogs in the Hangingwall Shear, despite mineralisation occurring during reverse movement on the Hyde-Macraes Shear Zone. Instead the angular relationship between various strands of the Hyde-Macraes Shear Zone at Frasers and foliation in underlying schists is the most consistent structural feature likely to predict the location, extent, and orientation of ore shoots within the Frasers segment of the Hyde-Macraes Shear Zone.     

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

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

The nature of gold-bearing fluids in Atud gold deposit,Central Eastern Desert,Egypt

Electron microprobe analyses of gold and associated ore minerals as well as stable isotope analyses of sulphide and carbonate minerals were performed in order to determine the metal and fluid sources and temperature of the mineralizing systems to better understand the genesis of the Atud gold deposit hosted in the metagabbro–diorite complex of Gabal Atud (Central Eastern Desert, Egypt). The gold can be classified as electrum (63.6–74.3 wt.% Au and 24.6–26.6 wt.% Ag) and is associated with arsenopyrite and As-bearing pyrite in the main mineralization (gold-sulphides) phase within the main mineralized quartz veins and altered host rocks. Based on the arsenopyrite geothermometer, As-contents (29.3–32.7 atom%) in arsenopyrite point to deposition in the Log ?_S2 and T ranges of ?10.5 to ?5.5 and 305–450°C, respectively, during the main mineralizing phase. Based on the δ³⁴S isotopic compositions of the sulphides, they are originated from magmatic fluids in which the sulphur is either sourced directly from magma or remobilized from the magmatic rocks (gabbroic rocks). On the other hand, calcite formed from fluids having mainly magmatic mixed with variable metamorphic signatures based on its δ¹³C and δ¹⁸O values. This work concluded that the gold-bearing ores at Atud deposit have magmatic sources leaching from the country intrusive rocks during water/rock interactions then remobilized during a metamorphic event. Therefore, the Atud gold deposit is classified as an intrusion-related gold deposit, in which the gabbro–diorite host intrusion acted as the source of metals which were mobilized and deposited as a result of the effects of NW–SE shearing.     

Gold distribution and lithogeochemical discrimination of residual regolith and transported overburden,Minotaur gold deposit,Lake Lefroy,Western Australia

Abstract

The Yilgarn Craton of Western Australia hosts a number of Cenozoic paleochannels, which have been incised into the underlying Paleozoic and Precambrian rocks, filled with sediments and subsequently weathered. The paleochannels are of particular interest in mineral exploration as they may not only host placer-type deposits but also overlie significant supergene and primary mineralisation. Paleochannels also pose particular challenges during exploration as they mask underlying geochemical anomalies, including gold and pathfinder elements. This study investigates a method of distinguishing transported overburden from residual regolith utilising a combination of field and laboratory-based techniques. At the Minotaur deposit, the residual regolith and transported overburden are mineralogically similar, although the presence of biotite, chlorite and muscovite is more characteristic of the residual regolith. Geochemically, Zr, Ti, Co and Sc ratios form distinct groupings allowing more confident discrimination of transported overburden from the underlying residual regolith units. The presence of a distinct gold depletion zone at the redox front was also identified to be a feature of the Minotaur deposit, with Au enrichment occurring above (within transported overburden) and below (within saprolite) the redox front, similar to other gold deposits on Lake Lefroy. The lithological, stratigraphic and Au-enrichment characteristics at Minotaur have also been compared with regional studies and suggest the long-lived impact of paleovalleys on the depositional systems and Au distribution of Lake Lefroy. This work provides a basis for future studies of the region in particular the variation of depositional sequences within regional paleochannel networks.