Gold In Pyrite Research Articles

Influence of clays on fine particle filtration

Abstract Filtration of fine particle slurries is known to be inefficient in most plant operations and the problem is exacerbated by the presence of even small quantities of clay minerals. This paper discusses the influence of the commonly encountered clay types, bentonite and kaolin, in the filtration of a pyritic gold ore and how the adverse effects can be mitigated by the addition of Ca2 + ions and a hydrophobic reagent respectively, depending on the clay type. The presence of both kaolin and bentonite clays resulted in increasing specific cake resistance, filter medium resistance and final cake moisture content while effects of bentonite clay being more detrimental. The addition of hydrophobic reagent DAH (dodecylamine hydrochloride) improved the filtration rate of kaolin-containing ore. The moisture content of the filter cake was also reduced. By contrast, in the case of the bentonite-containing ore, the presence of DAH did not affect the filtration rate and the moisture content. Filtration of the bentonite-containing ore was significantly improved with the addition of Ca2 + ions.

Geology and Mineral Chemistry of Gold Mineralization in Mirge‐Naqshineh Occurrence (Saqez, NW Iran): Implications for Transportation and Precipitation of Gold

AbstractThe Mirge‐Naqshineh gold district is situated at northwest of Iran with a NW‐trending brittle‐ductile shear zone. It is hosted by Precambrian meta‐sedimentary and meta‐volcanic units traversed by mineralized quartz veins. In terms of cross‐cutting relationships and sulfide content three types of quartz veins are identified in the region. Among those, parallel to bedding quartz vein (type I) is the main host for gold mineralization. Gold is found in three different forms: 1) submicrometer‐size inclusions of gold in arsenian pyrite, 2) as electrum and 3) in the crystal lattice of sulfides (pyrite, galena and chalcopyrite). Six types of pyrite (Py1‐Py6) were identified in this ore reserve. Py3 coexists with arsenopyrite and contains the greatest As‐Au concentrations. There is a negative correlation between the As and S contents in Py2 and Py3, implying the substitution of sulfur by arsenic. Pyrites and mineralized quartz veins were formed via metamorphic‐hydrothermal fluid and reflect the gold‐transportation as Au(HS)−2 under reducing and acidic conditions. The gold precipitation mainly controlled by crystallization of arsenian pyrite during fluid/rock interactions and variation of fO2. The volcanic host rock has played an important role in gold concentration, as Py3 in this rock contains inclusion of gold particles, but gold is within the lattice of pyrite in phyllite or other units.

Alteration paragenesis and the timing of mineralised quartz veins at the world-class Geita Hill gold deposit, Geita Greenstone Belt, Tanzania

The world-class Geita Hill deposit is one of the largest gold deposits located within the Geita Greenstone Belt in NW Tanzania. The deposit is hosted within a complexly deformed sedimentary package dominated by ironstone and intruded by diorite dykes. The gold mineralisation is spatially associated with the Geita Hill Shear Zone which, is a NE-trending, moderately NW dipping deformation zone consisting of a network of discontinuous shear fractures that record early thrusting overprinted by later strike-slip and normal events. The regional metamorphic assemblage in the meta-sediments is characterised by biotite+chlorite+actinolite+K-feldspar+magnetite±pyrrhotite±pyrite indicating upper greenschist facies conditions. The gold-related alteration overprints the regional metamorphic assemblage, and is characterised by silicification and sulfidation fronts that generally extend out from the mineralised zone by no more than one meter. The alteration assemblage includes sub-vertical, mineralised quartz veins that trend approximately E-W. The mineralised quartz veins are accompanied by alteration halos of quartz+biotite+K-feldspar+pyrite which overprints the peak metamorphic assemblage. Gold is closely associated with secondary pyrite and occurs as free gold and gold tellurides (sylvanite, calaverite and nagyagite). It occurs mainly as inclusions in pyrite and as invisible gold in pyrite but also as gold inclusions in biotite and along quartz grain boundaries. Two distinct textural styles of auriferous pyrite can be distinguished: inclusion rich subhedral pyrite, hosting invisible gold, and inclusion free euhedral pyrite, hosting visible gold grains. It is common for the inclusions rich pyrite to have thick rims of inclusion free pyrite. The mineralising alteration is overprinted by barren, multiphase quartz-carbonate, and carbonate-chlorite veins. This alteration is characterised by the assemblage calcite+siderite+chlorite±quartz±pyrite±barite. Palaeostress analysis of mineralised shear fractures along the Geita Hill Shear Zone are indicative of sigma 1 being vertical and sigma 3 trending N-S, indicating N-S extension, which is consistent with the orientation of the mineralised quartz veins.

The Occurrence Mechanism of Gold in Pyrite from the Qiucun Gold Deposit in Fujian Province, China

The Occurrence Mechanism of Gold in Pyrite from the Qiucun Gold Deposit in Fujian Province, China

Magmatic-hydrothermal origin of the early Triassic Laodou lode gold deposit in the Xiahe-Hezuo district, West Qinling orogen, China: implications for gold metallogeny

The Xiahe-Hezuo district in the West Qinling orogen contains numerous Au-(As-Sb) and Cu-Au-(W) deposits. The district is divided into eastern and western zones by the Xiahe-Hezuo Fault. The western zone is exposed at a shallow level and contains sediment-hosted disseminated Au-(As-Sb) deposits, whereas the eastern zone is exposed at a deeper level and contains Cu-Au-(W) skarn and lode gold deposits within or close to granitic intrusions. The Laodou gold deposit in the eastern zone consists of auriferous quartz-sulfide-tourmaline and minor quartz-stibnite veins that are structurally controlled by fault zones transecting the Laodou quartz diorite porphyry stock and enveloped by potassic and phyllic alteration. Both the veins and alteration halos commonly contain quartz, sericite, tourmaline, pyrite, and arsenopyrite, with minor galena, sphalerite, chalcopyrite, tetrahedrite, and enargite. Gold occurs mainly as invisible gold in pyrite or arsenopyrite and locally as inclusions less than 50 μm in diameter. The zircon U-Pb age of 247.6 ± 1.3 Ma (2σ) on the host quartz diorite porphyry and the sericite 40Ar/39Ar plateau ages of 249.1 ± 1.6 and 249.0 ± 1.5 Ma (2σ) on two ore-related hydrothermal sericite samples are within analytical errors of one another. At the formation temperature (275 °C) inferred from microthermometric measurements of fluid inclusion, sericite and tourmaline yield calculated δDH2O values of −70 to −45‰ and δ 18OH2O of 5.8 to 9.7‰, while quartz yields calculated δ 18OH2O values of 5.1∼5.7‰. Hydrothermal tourmaline in quartz-sulfide-tourmaline veins has δ 11B of −11.2 to −0.9‰ (mean of −6.3‰) that are similar to the values of magmatic tourmaline (−8.9 to −5.5‰ with a mean of −6.8‰) in the host quartz diorite porphyry. The δ 34S values of sulfide minerals range from −5.9 to +5.8‰ with a mean of −0.6‰ that is typical of magmatic sulfur. Pyrite from hydrothermally altered quartz diorite porphyry and quartz-sulfide-tourmaline veins have relatively homogeneous lead isotopic compositions, compatible with granitic intrusions in the district. The geochronological and isotopic data combined support a magmatic origin for the Laodou gold deposit, most likely formed from fluids exsolved from the Laodou quartz diorite porphyry or associated intrusive phases at deeper levels beneath the stock. Orogenic and Carlin-like gold deposits in the West Qinling orogen have been commonly thought to have formed from metamorphic fluids. This study, however, highlights the role of magmatic-derived fluids in the formation of lode gold deposits. Synthesis of geochronological, geological, and geochemical data on magmatic rocks and ore deposits in and surrounding the Xiahe-Hezuo district indicates that gold mineralization predominantly occurred within a subduction-related magmatic arc prior to collision between the Yangtze and North China cratons that produced the West Qinling orogen.

Influence of clays on the slurry rheology and flotation of a pyritic gold ore

The presence of clay minerals in ores has been known to adversely affect the flotation performance. This work investigated the influence of kaolin and bentonite clays on the flotation of gold-bearing pyrite in the gold ore. The changes in rheological properties of the flotation pulp under normal operating conditions were also studied with the additions of kaolin and bentonite clays at various solid concentrations. The zeta potential measurements also were carried out to understand the mechanism of adsorption of potential determining ions. It was found that bentonite clay reduced the pyrite recovery, and modified the pulp rheology more than kaolin. The addition of Ca2+ ions to the pulp containing bentonite improved the pyrite flotation recovery by its ability to modify the rheological characteristics. It is consistent with the published literature that Ca2+ ions suppressed the swelling properties of bentonite, and altered the surface charge properties of bentonite clay particles. Additionally, the rheological characteristics such as the flow index and the consistency index affected the flotation performance as the former indicates the extent of movement of bubbles and particles within flotation pulp. This suggests the possibility of using the rheological measures to monitor the changes in the flotation performance in routine plant operations.

The interaction between copper species and pyrite surfaces in copper cyanide solutions

The adsorption of copper ions and the formation of a copper sulfide phase on pyrite surfaces are of vital importance to alter the surface property of pyrite and determine its fate either to be rejected in the flotation of polymetallic sulfide ores or to be recovered in the flotation of pyritic gold ores. Cyanide and copper may co-exist in the process water with complicated speciation. The objective of this study is to understand the interaction between copper cyanide species and pyrite and clarify the possible adsorption of copper on pyrite surfaces from cyanide-bearing solutions. Surface-enhanced Raman spectroscopy and electrochemical measurements were used to determine the reaction products formed on pyrite surfaces. It was found that Cu(I)-bearing species were incorporated into pyrite, forming a CuS-like sulfide from copper cyanide solutions at a more oxidizing potential, while a Cu2S-like sulfide formed at a more reducing potential. The amount of copper deposited on pyrite was significantly improved at a more reducing potential at which the pyrite surface tended to be FeS-like. In addition, these Cu(I)-sulfides on pyrite surfaces were dissolved by cyanide-bearing species at a high CN/Cu ratio, compromising the total amount of copper uptake.

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.

Composition and genesis of the Konevinsky gold deposit, Eastern Sayan, Russia

The Konevinsky gold deposit in southeast Eastern Sayan is distinguished from most known deposits in this region (Zun-Kholba, etc.) by the geological setting and composition of mineralization. To elucidate the cause of the peculiar mineralization, we have studied the composition, formation conditions, and origin of this deposit, which is related to the Ordovician granitoid pluton 445–441 Ma in age cut by intermediate and basic dikes spatially associated with metavolcanic rocks of the Devonian–Carboniferous Ilei Sequence. Four mineral assemblages are recognized: (1) quartz–pyrite–molybdenite, (2) quartz–gold–pyrite, (3) gold–polysulfide, and (4) telluride. Certain indications show that the ore was formed as a result of the superposition of two distinct mineral assemblages differing in age. The first stage dated at ~440 Ma is related to intrusions generating Cu–Mo–Au porphyry mineralization and gold–polysulfide veins. The second stage is controlled by dikes pertaining to the Devonian–Carboniferous volcanic–plutonic association. The second stage is characterized by gain of Hg and Te and formation of gold–mercury–telluride paragenesis.

Relationships Between Gold and Pyrite at the Xincheng Gold Deposit, Jiaodong Peninsula, China: Implications for Gold Source and Deposition in a Brittle Epizonal Environment

The Xincheng gold deposit, hosted by the Early Cretaceous 132 to 123 Ma Guojialing-type granitoids in northwest Jiaodong Peninsula, southeast North China craton, formed about 2 billion years later than regional metamorphism of the Archean Jiaodong basement rocks. The Xincheng deposit comprises mineralized zones with three types of hydrothermal pyrite associated with gold, tellurides, and a variety of sulfides: py1 as disseminated euhedral to subhedral grains in altered granitoids around quartz veins; py2 as subhedral grains with brittle cataclastic textures and fractures in quartz-pyrite veins; and py3 as subhedral, partially corroded crystals in sulfide-rich veins or veinlets. All three generations of pyrite are unzoned and have low trace element contents, including very low lattice-bound gold contents: (py1: 0.180 ppm; py2: 0.053 ppm; py3: 0.060 ppm). Given that there is 10 to 15% pyrite in the ore zone at Xincheng, its very low gold content indicates that it contributes <0.2% of gold to the 7.75 g/t gold in the orebody. Instead, over 99% of the gold is present as discrete electrum and/or gold (total range 0.02–59% silver) grains, which are largely sited in fractures at all scales in pyrite, other ore minerals, and quartz. Importantly, visible gold in py3 is also sited on solution-corroded pyrite grains. The pyrite textural and geochemical data indicate that it is impossible to derive the high gold-grade orebodies through local remobilization of originally lattice-bound gold in pyrite. Instead, the gold is interpreted to have been deposited through sulfidation reactions and phase separation of a H2O-CO2 ore fluid during progressive brittle cataclastic deformation associated with seismic activity and regional sinistral transtensional shear movement. This concomitant fluid infiltration and deformation caused episodic deposition and fracturing and corrosion of earlier formed pyrite and deposited visible gold in dilational cracks. The coupled development of the transtensional, rather than normal transpressional setting, and precipitation of gold within dilational veins and wall-rock alteration facilitated the deposition of visible gold and an exceptionally high gold tenor. All deposit characteristics indicate that the Xincheng gold deposit is a member of the epizonal orogenic deposit class.

Occurrence of gold in hydrothermal pyrite, western Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ) on the North Island of New Zealand is located above the subducting Pacific slab and hosts hydrothermal systems related to subduction, arc magmatism and crustal extension. In these systems, gold is transported primarily as sulphide complexes, with gold being deposited in response to boiling and mixing of the deep geothermal fluids. Conglomerate cobbles and hydrothermal fumaroles from the upper Retaruke River in the western TVZ are mineralised deposits that have been eroded from the nearby Miocene alluvial sedimentary plateau. Abundant gold-bearing pyrite was precipitated in the conglomerates and fumaroles by late hydrothermal process, primarily occurring as veinlets, disseminations and fine-grained aggregates which consists discrete euhedral microcrystals. Scanning electron microscopy combined with X-ray energy disperses spectroscopy shows that pyrites are commendably affected by late hydrothermal fluids, possibly by the carbonate fluid. Electron probe microanalysis revealed that the pyrite contains 20–120 ppm Au (averaging 60 ppm). Wavelength dispersive spectral elemental mapping suggests that gold is distributed uniformly in pyrite, indicating structurally bound gold (solid solution) in pyrite. Gold mineralisation recognised in the conglomerates and fumaroles demonstrates that the upper Retaruke River is a promising target for future gold exploration.

Mineralogy of the Boroo Gold Deposit in the North Khentei Gold Belt, Central Northern Mongolia

AbstractMineral assemblages and chemical compositions of ore minerals from the Boroo gold deposit in the North Khentei gold belt of Mongolia were studied to characterize the gold mineralization, and to clarify crystallization processes of the ore minerals. The gold deposit consists of low‐grade disseminated and stockwork ores in granite, metasedimentary rocks and diorite dikes. Moderate to high‐grade auriferous quartz vein ores are present in the above lithological units. The ore grades of the former range from about 1 to 3 g/t, and those of the latter from 5 to 10 g/t, or more than 10 g/t Au. The main sulfide minerals in the ores are pyrite and arsenopyrite, both of which are divisible into two different stages (pyrite‐I and pyrite‐II; arsenopyrite‐I and arsenopyrite‐II). Sphalerite, galena, chalcopyrite, and tetrahedrite are minor associated minerals, with trace amounts of bournonite, boulangerite, geerite, alloclasite, native gold, and electrum. The ore minerals in the both types of ores are variable in distribution, abundance and grain size. Four modes of gold occurrence are recognized: (i) “invisible” gold in pyrite and arsenopyrite in the disseminated and stockwork ores, and in auriferous quartz vein ores; (ii) microscopic native gold, 3 to 100 µm in diameter, that occurs as fine grains or as an interstitial phase in sulfides in the disseminated and stockwork ores, and in auriferous quartz vein ores; (iii) visible native gold, up to 1 cm in diameter, in the auriferous quartz vein ores; and (iv) electrum in the auriferous quartz vein ores. The gold mineralization of the disseminated and stockwork ores consists of four stages characterized by the mineral assemblages of: (i) pyrite‐I + arsenopyrite‐I; (ii) pyrite‐II + arsenopyrite‐II; (iii) sphalerite + galena + chalcopyrite + tetrahedrite + bournonite + boulangerite + alloclasite + native gold; and (iv) native gold. In the auriferous quartz vein ores, five mineralization stages are defined by the following mineral assemblages: (i) pyrite‐I; (ii) pyrite‐II + arsenopyrite; (iii) sphalerite + galena + chalcopyrite; (iv) Ag‐rich tetrahedrite‐tennantite + bournonite + geerite + native gold; and (v) electrum. The As–Au relations in pyrite‐II and arsenopyrite suggest that gold detected as invisible gold is mostly attributed to Au+1 in those minerals. By applying the arsenopyrite geothermometer to arsenopyrite‐II in the disseminated and stockwork ores, crystallization temperature and logfs2 are estimated to be 365 to 300 °C and –7.5 to –10.1, respectively.

Invisible and microscopic gold in pyrite: Methods and new data for massive sulfide ores of the Urals

Au speciation in sulfides (including “invisible” Au), which mostly controls the loss of Au during ore dressing, is discussed. Modern methods of analysis of Au speciation, with discussion of limitations by locality and sensitivity, are reviewed. The results of sulfide investigation by the methods of scanning and transmission electron microscopy, mass spectrometric analysis with laser ablation (LA–ICP–MS), the thermochemical method (study of ionic Au speciation), and automated “quantitative mineralogy,” are demonstrated for weakly metamorphosed VHMS deposits of the Urals (Galkinsk and Uchaly). Significant content of Au is scattered in sulfides, such as pyrite, chalcopyrite, and sphalerite, with quantitative predomination of pyrite. The portion of such “invisible” gold ranges from <10% (Galkinsk deposit) to 85% (Uchaly deposit). Major part of “invisible” gold occurs as micron- to nanoscale particles of Au minerals. The portion of gold structurally bound in pyrite lattice (from the bulk concentration of Au in pyrite) is estimated to be from few % (the Galkinsk deposit) to 20–25% (the Uchaly deposit). The presence of As and Sb in pyrite and sphalerite, as well as other trace elements (Te, Co, Mn, Cu, Hg, and Ag in both as well as Fe in sphalerite) stimulates the incorporation of Au in sulfide, but mostly in defect-associated, not isomorphic form. Micron particles of Ag sulfosalts (pyrargyrite, freibergite, stephanite, polybasite, pyrostilpnite, argentotetrahedrite, pearceite, proustite), Au–Ag alloys (from gold of high fineness to kustelite), Ag and Au–Ag tellurides (hessite, empressite, calaverite), and occasional Au–Ag sulfides (petrovskaite, uytenbogaardtite) were registered in the areas of Au enrichment of both deposits; selenotelluride (kurilite) particles were found on the Galkinsk deposit. Nanoscale (1–50 nm) native gold (spherical and disk-shaped particles, flakes) with a monocrystal diffraction pattern of some particles and a ring diffraction pattern of other particles was registered in the ores of these deposits by the methods of transmission electron microscopy. The low degree (or absence) of metamorphic recrystallization results in (1) predomination of thin intergrowths of sulfides, which is the main reason for the bad concentration of ores (especially for the Galkinsk deposit) and (2) the high portion of “invisible” gold in the massive sulfide ores, which explains the low yield of Au in copper and zinc concentrates, since it is lost in tailings with predominating pyrite.

Supergene gold mobility in orogenic gold deposits, Otago Schist, New Zealand

Gold has been chemically mobilised by groundwater from host sulphide minerals in orogenic gold deposits of Otago. Mobilisation occurred near the Cenozoic Otago Schist erosional surface beneath a sedimentary cover. Initial Au mobilisation, on a scale of micrometres, occurred when solid solution and microparticulate gold in pyrite and arsenopyrite grains were liberated by sulphide oxidation to iron oxyhydroxide pseudomorphs. Larger-scale mobilisation involved leaching of gold from up to 100-m-thick zones which were the target of historic mining, with up to 10× enrichment of Au in reprecipitation zones. Gold in the supergene zones is commonly crystalline with octahedral shapes and nuggety forms which fill cavities and coat prismatic quartz crystals. This gold retains some or all of the Ag (typically 2–8 wt%) from the primary source gold. Oxidised groundwaters that have interacted with sulphides become enriched in dissolved sulphate, but retain high pH (7–8.5). Under these conditions, metastable thiosulphate ions can dissolve and transport Au and Ag to be precipitated later by either oxidation or reduction.

A Preliminary Study on the Occurrence State and Precipitation Mechanism of Gold in Pyrite of Zhengguang Au Deposit

A Preliminary Study on the Occurrence State and Precipitation Mechanism of Gold in Pyrite of Zhengguang Au Deposit

Geological characteristics and ore-forming process of the gold deposits in the western Qinling region, China

The western Qinling, belonging to the western part of the Qinling–Dabie–Sulu orogen between the North China Block and South China Block, is one of the most important gold regions in China. Isotopic dates suggest that the Mesozoic granitoids in the western Qinling region emplaced during the Middle-Late Triassic, and the deposits formed during the Late Triassic. Almost all gold deposits in the western Qinling region are classified as orogenic, Carlin-type, and Carlin-like gold deposits, and they are the products of Qinling Orogenesis caused by the final collision between the North China Block and the South China Block. The early subduction of the Mian-Lue oceanic crust and the latter collision between South Qinling Terrane and the South China Block along the Mian-Lue suture generated lithosphere-scale thermal anomalies to drive orogen-scale hydrothermal systems. The collision-related magmatism also provided heat source for regional ore-forming fluids in the Carlin-like gold deposits.Orogenic gold deposits such as Huachanggou, Liziyuan, and Baguamiao lie between the Shang-Dan and Mian-Lue sutures and are confined to WNW-trending brittle-ductile shear zones in Devonian and Carboniferous greenschist-facies metasedimentary rocks that were highly-deformed and regionally-metamorphosed. These deposits are typical orogenic gold deposits and formed within a Late Triassic age. The deposits show a close relationship between Au and Ag. Ores contain mainly microscopic gold, and minor electrum and visible gold, along with pyrite. The ore-forming fluids were main metamorphic fluids. Intensive tectonic movements caused by orogenesis created fluid-migrating channels for precipitation locations. Although some orogenic gold deposits occur adjacent to granitoids, mineralization is not synchronous with magmatism; that is, the granitoids have no genetic relations to orogenic gold deposits. As ore-forming fluids converged into dilated fractures during the extension stage of orogenesis, changes of physico-chemical conditions resulted in fluid immiscibility that played a key role in gold and sulfide deposition.The geochemical and mineralogical characteristics of the Carlin-type deposits in the western Qinling region are similar to those in the Carlin trend, Nevada, USA. Gold deposits such as La’erma and Jinlongshan occur mostly in the southeastern margin of the western Qinling regionic region whereas some deposits occur in its eastern part. These deposits are hosted in slightly metamorphosed Cambrian to Triassic sedimentary rocks, showing structurally- and stratigraphically-controlled features. The deposits mainly contain submicroscopic and microscopic gold in arsenian pyrite and arsenopyrite, with characteristic ore-forming elements of Au–As–Sb–Ba. The ore-forming fluids are early-stocked formation water and later-recharged meteoric water. Meteoric water apparently evolved in ore-forming fluids by circulation, indicating the extensional setting, and led to the deposition of Au and other elements in cool reactive permeable rocks at shallow levels, forming the disseminated ores.Carlin-like gold deposits occur between the Shang-Dan suture and the Fengxian-Zhen’an fault. The host rocks are mainly sedimentary rocks that underwent reconstruction through reworking by structural metamorphism. These deposits are structurally controlled by brittle-ductile shear zone and occur adjacent to granitoid plutons. The most important characteristic that differ to the orogenic and Carlin-type gold deposits is the genetic relationship with the synchronous magmatism. Gold occurs mainly as microscopic gold. Pyrite and arsenian pyrite can be recognized as gold-bearing minerals. The ore-forming fluids are main magmatic water mixed with metamorphic and/or formation water. Similar to orogenic gold deposits, fluid immiscibility caused the deposition of gold Carlin-like gold deposits.

Gold in Pyrite within Altai – Sayan Folded Belt Gold Deposits

This article describes the result analysis of gold in pyrite for a number of some Altai – Sayan folded belt gold deposits. The analyzed pyrite samples showed gold grade from 0.04 to 20.0 g/t., which proves the fact that the gold concentrations in pyrites is irregular. Pyrites in productive mineral associations include significant metal enrichment, while pyrites of early poor gold-bearing associations and wallrock metasomatites are less prolific.

Controls on quartzite bearing base-metal sulfide and invisible gold mineralization at Gabal Abu Houdied area, South Eastern Desert, Egypt

The Neoproterozoic basement rocks cropping out in the Gabal Abu Houdied (GAH) area comprises mainly three lithotectonic assemblages; mafic–ultramafic ophiolitic rocks, island arc-related metasediment–metavolcanic rocks and syn- and post-tectonic intrusions. The base-metal sulfide and invisible gold mineralization are restricted to quartzites, which are considered as member of island arc-related metasediment rocks. The mineralized quartzite rocks are cropped out at three localities, at Wadi El Haussioum where cover a small area (0.25 km2), at Bir Abu Houdeid, cover small area (0.5 km2), and at Bir Shani (3.5 km long with different width ranging from 4 to 75 m). Based on textural characteristics and chemical features, documented by electron probe microanalysis (EPMA) and inductively coupled plasma mass spectrometry analyses (ICP-MS), two phases of sulfide minerals assemblages in the investigated quartzites of GAH area. An early mineralization phase includes pyrite, pyrrhotite, argentite (acanthite) and invisible gold, and the late phase, dominated by pyrite, galena, sphalerite, chalcopyrite, covelite, and ± invisible gold. Analyses of the structural data collected from the quartzite rocks and surroundings indicated that the base-metal sulfide and associated gold mineralization in the GAH area are mostly controlled by D3 transperssional deformation event. This event is main Hamisana shear zone (HSZ) deformation event and characterized by transperssional stress regime, with σ1 trending E–W and σ3 trending N–S governed conjugate NNW sinistral and NNE–SSW dextral strike slip faulting.

Lithologically controlled invisible gold, Yukon, Canada

The newly discovered Cretaceous Coffee orogenic gold deposit (>4 Moz resource) consists of an extensive oxidised zone developed on primary sulphidic rock. The primary mineralised rock is characterised by invisible gold in arsenian pyrite that has replaced biotite in selected host rocks. The deposit has a cryptic surface expression and is an example of an extremely subtle exploration target. Hydrothermal emplacement was controlled by extensional fractures, with breccias, but most mineralisation was focused on biotite-bearing granitic gneiss, metasedimentary gneisses, and younger biotite granite. Fine-grained (<0.1 mm) arsenian pyrite replaced biotite along mineral cleavage planes and followed biotite-rich metamorphic and post-metamorphic structural fabrics. Arsenian pyrite also formed overgrowths on earlier coarse-grained (up to 2 mm) barren hydrothermal pyrite. Arsenian pyrite is concentrically zoned on the 1–10-μm scale with respect to As, Sb, and Au contents and typically contains ∼5 wt% As, ∼500 mg/kg Sb, and ∼500 mg/kg Au, in solid solution. Biotite replacement was accompanied by sericitisation, silicification, and ankerite impregnation. Hydrothermal alteration involved dilution and localised depletion of K, Na, and Al in silicified host rocks, but most Ca, Mg, and Fe concentrations remained broadly constant. Magnesium-rich ultramafic host rocks were only weakly mineralised with auriferous arsenian pyrite and have fuchsite and magnesite alteration. Near-surface oxidation has liberated nanoparticulate and microparticulate supergene gold, which remains essentially invisible. Varying degrees of oxidation extend as deep as 250 m below the present subdued topographic surface, well beyond the present vadose zone, and this deep oxidation may have occurred during post-mineralisation uplift and erosion in the Cretaceous. Oxidation has leached some As from the surficial mineralised rocks, decreasing the geochemical signal, which is also obscured by the localised presence of high background As (up to 100 mg/kg) in metasedimentary quartzites in the region. Antimony provides more reliable soil anomalies than As, but most Sb anomalies are <100 mg/kg. The persistence of invisible gold into the extensive supergene zone, with little gold particle size enhancement, has ensured that no placer deposits have formed in nearby streams, further restricting the surface footprint and Au dispersal halo of this subtle exploration target.

MECHANISMS OF CRYSTAL GROWTH AND ZONING OF HYDROTHERMAL PYRITE FROM Sb-Au DEPOSITS IN NIZKE TATRY, SLOVAKIA

Pyrite is one of the most common ore minerals in hydrothermal ores and a hallmark of many types of hydrothermal deposits, e.g. , mesothermal quartz veins with pyrite, arsenopyrite, and visible or invisible gold. In addition to any other mineralogical investigation, it is possible to examine and interpret the crystal size distribution (CSD) of the mineral which is under investigation. The conditions of mineral-forming processes can be diverse in terms of temperature, pressure, and compositional variables, but the CSD shapes are usually convergent and by far the most common CSD type is the lognormal distribution. In this study, we determined the CSD for pyrite crystals from mesothermal quartz-pyrite-arsenopyrite-stibnite-gold ores from the Nizke Tatry Mountains (Western Carpathians, Slovakia). A large number of crystals were measured manually from ore sections or from stereographic images. All samples show a crystal size spectrum from ≈2 μm up to ≈550 μm. We discovered that all CSDs show a lognormal or pseudo-lognormal shape with low logarithmic size variances (β2). In order to understand the history of nucleation and growth of pyrite formed in hydrothermal milieus, we compared our measurements to simulations with different growth mechanisms using the GALOPER (Growth According to the Law of Proportionate Effect) program (after [Eberl et al. 1998][1]). From the shape of the distributions, the logarithmic mean (α) and β2, we inferred that the crystals grew in an open system with its associated growth mechanisms (surface-controlled growth and supply-controlled growth), since no evidence for ripening processes in the data or in the numerical simulations of our data could be found. Since ripening as a growth mechanism can be excluded, we have to assume that the growth of the pyrite crystals and so the formation of the whole ore body occurred on a relatively short geological time scale. This preliminary result opens more questions related to the emplacement, circulation, and supersaturation of hydrothermal fluids and to the formation of large ore bodies over short time scales. [1]: #ref-10