Showing papers in "Mineralium Deposita in 2002"
TL;DR: The Jiaodong gold province of China as discussed by the authors is one of the largest gold deposits in the world, containing >900 tonnes of gold and gold-bearing quartz veins.
Abstract: The Jiaodong gold province, along the southeastern margin of the North China craton, has combined production and resources of >900 tonnes of gold. More than 95% of the gold in this province is hosted in Mesozoic granitoids, which intruded Late Archean metamorphic basement rocks episodically at ca. 160–150 and 130–126 Ma. The deposits of the Jiaodong Peninsula form one of the largest provinces of granitoid-hosted lode-gold deposits in the world. The Jiaodong gold deposits occur as massive auriferous quartz veins hosted in subsidiary second- or third-order faults, and as disseminated- and stockwork-style mineralization hosted in alteration zones along major regional-scale faults. Mineralization took place during brittle reactivation (D3) of pre-existing D2 ductile shear zones. Wall rock alteration is dominated by K-feldspar and sericite, with also sulfide minerals and extensive silicification closest to the orebodies. The alteration was accompanied by the introduction of volatiles, Fe, Mg, K, Rb, Ag, As, Au, Bi, and Sb. Enrichment of base metals in the gold lodes is generally low, although locally significant. Gold deposition took place at temperatures of 250 to 350 °C for most gold deposits, and the ore fluids are CO2-bearing and have calculated salinities of 6–13 wt% NaCl equiv. Deposits from the Zhao–Ye, Xixia, and Muping–Rushan belts have similar, if not identical, structural controls, alteration patterns, ore mineral assemblages, and stable isotope signatures. Lead isotope data indicate that the ore-stage leads were buffered by their host rocks. The granitoid-hosted gold deposits in the Zhao–Ye belt are most radiogenic, the greenstone-hosted deposits in the Xixia belt is the least radiogenic and the granitoid-hosted deposits in the Muping–Rushan belt have intermediate compositions. Gold mineralization in the Zhao–Ye, Xixia, and Muping–Rushan belts in the Jiaodong gold province appears to be broadly contemporaneously at ca. 130–120 Ma, as constrained by SHRIMP U–Pb zircon dating on host rocks and a post-gold dike, as well as by Ar–Ar, Rb–Sr, and K–Ar dating of hydrothermal alteration minerals and ores. This Early Cretaceous age is about 100 million years younger than the peak of ultra-high pressure metamorphism in the Qinling–Dabie–Sulu orogen between the North China and Yangtze cratons, and about 500 km south of the gold province. However, it overlaps with the extension and exhumation stage of the collisional orogeny, and the subduction of the Pacific plate beneath the Eurasian continental crust. Whereas the majority of the Jiaodong gold deposits occur on the margin of the North China craton, some also occur in the suture zone between the two cratons. They are significantly landward relative to the subduction zone of the Pacific Plate compared with typical orogenic lode-gold deposits globally. Lode-gold deposits of ca. 130–120 Ma occurred on both sides of the Tan-Lu fault zone. Available data favor a temporal link of gold mineralization with Pacific Plate subduction.
231 citations
TL;DR: The gold-rich Xiaoqinling-Xiong'ershan region in eastern Shaanxi and western Henan provinces, central China, lies about 30-50 km inland of the southern margin of the North China craton as discussed by the authors.
Abstract: The gold-rich Xiaoqinling–Xiong'ershan region in eastern Shaanxi and western Henan provinces, central China, lies about 30–50 km inland of the southern margin of the North China craton. More than 100 gold deposits and occurrences are concentrated in the Xiaoqinling (west), Xiaoshan (middle), and Xiong'ershan (east) areas. Late Archean gneiss of the Taihua Group, and Middle Proterozoic metavolcanic rocks of the Xiong'er Group are the main host rocks for the deposits. Mesozoic granitoids (ca. 178–104 Ma) are present in most gold districts, but deposits are typically hosted in the Precambrian basement rocks hundreds of meters to as far as 10 km from the intrusions and related hornfels zones. Deposits in the Xiaoqinling and Xiaoshan areas are best classified as orogenic gold deposits, with ores occurring in a number of distinct belts both in quartz veins and disseminated in altered metamorphic rocks. Alteration assemblages are dominated by quartz, sericite, pyrite, and carbonate minerals. The ore-forming fluids were low salinity, CO2-rich, and characterized by isotopically heavy δ18O. Four deposits (Dongchuang, Wenyu, Yangzhaiyu, and Dahu) in the Xiaoqinling area each contain resources of about 1 Moz Au. Some of the gold deposits in the Xiong'ershan area represent more shallowly emplaced tellurium-enriched orogenic systems, which include resources of approximately 1–1.5 Moz Au at Shanggong and Beiling (or Tantou). Others are epithermal deposits (e.g., Qiyugou and Dianfang) that are hosted in volcanic breccia pipes. Isotopic dates for all gold deposits, although often contradictory, generally cluster between 172–99 Ma and are coeval with emplacement of the post-kinematic granitoids. The gold deposits formed during a period of relaxation of far-field compressional stresses, clearly subsequent to the extensive Paleozoic–early Mesozoic accretion of arc terranes and the Yangtze craton onto the southern margin of the North China craton. Hydrothermal and magmatic events occurred locally where extension-related Precambrian basement uplifting took place along the craton margin. Fluids for the orogenic gold deposits in the Xiaoqinling, Xiaoshan, and Xiong'ershan areas may have been released from evolving magmas or resulted from prograde metamorphic reactions within the uplift zones. Alternatively, for the epithermal gold deposits at shallower levels in the Xiong'ershan area, gold-transporting fluids were mainly exsolved from coeval magmas, although meteoric water was also involved in these hydrothermal systems.
208 citations
TL;DR: The major gold provinces in China formed during the main episodes of Phanerozoic tectonism as discussed by the authors, which involved interaction of China's three major Precambrian cratons, North China, Tarim, and Yangtze (or South China when combined with Cathaysia block), with the Angara (or Siberian), Kazakhstan-Kyrgyzstan, and Indian craton.
Abstract: Gold exploration in China has expanded rapidly during the last two decades since a modern approach to economic development has become a national priority. China currently produces 180 tonnes (t) of gold annually, which is still significantly less than South Africa, USA, and Australia. However, China is now recognized as possessing significant gold resources in a wide range of mineral deposit types. Present estimates of gold resources in China exceed 4,500 t, which comprise 60% in gold-only deposits, more than 25% in base metal-rich skarn, porphyry, and vein deposits, and more than 10% in placer accumulations. The major gold provinces in China formed during the main episodes of Phanerozoic tectonism. Such tectonism involved interaction of China's three major Precambrian cratons, North China, Tarim, and Yangtze (or South China when combined with Cathaysia block), with the Angara (or Siberian), Kazakhstan–Kyrgyzstan, and Indian cratons. Resulting collisions included deformation of accreted oceanic sequences between the cratonic blocks. The most important ore-forming orogenies were (1) the late Paleozoic Variscan (405–270 Ma), which led to amalgamation of the Angara, North China and Yangtze cratons, (2) the Indosinian (270–208 Ma), which led to the collision of North China and South China cratons, (3) the Yanshanian (208–90 Ma), which was largely influenced by the subduction of the Izanagi–Pacific plates beneath eastern China, and (4) the Himalayan (<90 Ma) indentation of the Indian continent into Eurasia. No important Precambrian gold systems are recognized in China, mainly because of reworking of exposed Precambrian rocks by these younger orogenies, but there are a few Caledonian (600–405 Ma) gold-bearing systems in northern Xinjiang. Most of China's orogenic, epithermal, and Carlin-like gold deposits are in the reworked margins of major cratonic blocks and in metasedimentary rock-dominated fold belts adjacent to these margins. Accordingly, the major gold provinces are present along the northern, southeastern and southern margins of the North China craton, along the southwestern and northwestern margins of the Yangtze craton, in the Tianshan and Altayshan orogenic belts in northern Xinjiang, and throughout the southeastern China fold belt. Gold-placer deposits derived from these primary deposits are concentrated in the northernmost part of northeastern China and along the northwestern margin of the Yangtze craton. The major provinces with significant gold in porphyry-related copper systems and base metal skarns are present in the Yangtze River area along the northeastern and southeastern margin of the Yangtze craton, in the fold belt in southwestern China, and scattered through northern China. Three-quarters of the Chinese gold-only deposits occur within the North China craton margins. Half are located in the uplifted Precambrian metamorphic rocks and most of the remainder are hosted in the Phanerozoic granitoids that intruded the reworked Precambrian terranes. The abundance of granite-hosted gold contrasts the North China craton with other Precambrian cratons, such as those in Western Australia, central Canada, and Zimbabwe, where gold is mainly hosted in the Archean greenstone belts. This difference may be explained by the multiple episodes of Phanerozoic tectonism along the North China craton margins resulting from the collision of the Angara, North China, and South China cratons, and from subduction of the Izanagi–Pacific oceanic plates underneath the eastern China continent.
191 citations
TL;DR: The Carlin-type gold deposits in China lie mostly near the margins of the Proterozoic Yangtze and Aba cratons Submicron sized gold in micron-sized arsenian pyrite is disseminated in the fractured Cambrian through Triassic carbonaceous shale and carbonate rocks, and is associated with anomalous Hg, Sb, As, U, and Tl Alteration typically includes silicification, argillization, and sulfidation Aqueous fluid inclusions contain CO2, have relatively low temperatures of homogenization (250
Abstract: The Carlin-type gold deposits in China lie mostly near the margins of the Proterozoic Yangtze and Aba cratons Submicron-sized gold in micron-sized arsenian pyrite is disseminated in fractured Cambrian through Triassic carbonaceous shale and carbonate rocks, and is associated with anomalous Hg, Sb, As, U, and Tl Alteration typically includes silicification, argillization, and sulfidation Aqueous fluid inclusions contain CO2, have relatively low temperatures of homogenization (250–150 °C), and salinities (8–2 wt% equiv NaCl) that typically decrease from early to later stages The indicated pressures of 105–330×105 Pa correspond to depths in excess of 10 to 30 km, assuming hydrostatic conditions The δD values of fluid inclusions (–87 to –47‰) and the calculated δ18O values for water in ore fluids (–21 to 162‰) reflect interactions between meteoric water and sedimentary rocks The δ13C values of calcite (–9 to 2‰) and δ34S values of sulfides (–24 to 17‰) suggest that C and S were derived from marine carbonate (and organic carbon) and diagenetic sulfides (and organic sulfur) in the vicinity of the deposits Geologic relationships and geochronologic evidence indicate the deposits formed during a late phase of the Yanshanian orogeny (140–75 Ma)
190 citations
TL;DR: In this paper, the authors used 40Ar-39Ar geochronology on 16 samples from gangue and alteration phases, combined with unpublished SHRIMP U-Pb dates on associated granitoids, suggest that gold mineralizing events occurred during Variscan, Indosinian, and Yanshanian orogenies at circa 350, 250, 200, 180, 150, and 129.
Abstract: The northern margin of the North China craton is well-endowed with lode gold deposits hosting a resource of approximately 900 tonnes (t) of gold. The ~1,500-km-long region is characterized by east-trending blocks of metamorphosed Archean and Proterozoic strata that were episodically uplifted during Variscan, Indosinian, and Yanshanian deformational and magmatic events. At least 12 gold deposits from the Daqinshan, Yan-Liao (includes the Zhangjiakou, Yanshan, and Chifeng gold districts), and Changbaishan gold provinces contain resources of 20–100 t Au each. Most deposits are hosted in uplifted blocks of Precambrian metamorphic rocks, although felsic Paleozoic and Mesozoic plutons are typically proximal and host ~30% of the deposits. The lodes are characterized by sulfide-poor quartz veins in brittle structures with low base metal values and high Au:Ag ratios. Although phyllic alteration is most common, intensive alkali feldspar metasomatism characterizes the Wulashan, Dongping, and Zhongshangou deposits, but is apparently coeval with Variscan alkalic magmatism only at Wulashan. Stepwise 40Ar–39Ar geochronology on 16 samples from gangue and alteration phases, combined with unpublished SHRIMP U–Pb dates on associated granitoids, suggest that gold mineralizing events occurred during Variscan, Indosinian, and Yanshanian orogenies at circa 350, 250, 200, 180, 150, and 129 Ma. However, widespread Permo-Triassic (~250 Ma) and Early Jurassic (~180 Ma) thermal events caused variable resetting of most of the white mica and K-feldspar argon spectra, as well as previously reported K–Ar determinations. Compiled and new stable isotope and fluid inclusion data show that most δ18O values for ore-stage veins range from 8 to 14‰, indicating a fluid in equilibrium with the Precambrian metamorphic basement rocks; δD values from fluid inclusions range widely from –64 to –154‰, which is indicative of a local meteoric component in some veins; and highly variable δ34S data (+7 to –17‰), even within individual deposits, indicate various local country-rock sources for sulfur. Fluid inclusions from all districts show variable homogenization temperatures between 240 and 400 °C, and are consistently characterized by low salinity, H2O–CO2 ± CH4, N2 solutions. Although the data are largely consistent with that from orogenic gold veins, intrusion-related veins and epithermal veins are also recognized. The multiple episodes of mineralization are coincident with episodic tectonic reactivations and associated magmatism along the northern margin of the North China craton. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00126-001-0239-2.
158 citations
TL;DR: In this paper, the late Paleozoic-early Mesozoic tectonic evolution of Xinjiang Autonomous Region, northwestern China provided a favorable geological setting for the formation of lode gold deposits along the sutures between a number of the major Eastern Asia cratonic blocks.
Abstract: The late Paleozoic–early Mesozoic tectonic evolution of Xinjiang Autonomous Region, northwestern China provided a favorable geological setting for the formation of lode gold deposits along the sutures between a number of the major Eastern Asia cratonic blocks. These sutures are now represented by the Altay Shan, Tian Shan, and Kunlun Shan ranges, with the former two separated by the Junggar basin and the latter two by the immense Tarim basin. In northernmost Xinjiang, final growth of the Altaid orogen, southward from the Angara craton, is now recorded in the remote mid- to late Paleozoic Altay Shan. Accreted Early to Middle Devonian oceanic rock sequences contain typically small, precious-metal bearing Fe–Cu–Zn VMS deposits (e.g. Ashele). Orogenic gold deposits are widespread along the major Irtysh (e.g. Duyolanasayi, Saidi, Taerde, Kabenbulake, Akexike, Shaerbulake) and Tuergen–Hongshanzui (e.g. Hongshanzui) fault systems, as well as in structurally displaced terrane slivers of the western Junggar (e.g. Hatu) and eastern Junggar areas. Geological and geochronological constraints indicate a generally Late Carboniferous to Early Permian episode of gold deposition, which was coeval with the final stages of Altaid magmatism and large-scale, right-lateral translation along older terrane-bounding faults. The Tian Shan, an exceptionally gold-rich mountain range to the west in the Central Asian republics, is only beginning to be recognized for its gold potential in Xinjiang. In this easternmost part to the range, northerly- and southerly-directed subduction/accretion of early to mid-Paleozoic and mid- to late Paleozoic oceanic terranes, respectively, to the Precambrian Yili block (central Tian Shan) was associated with 400 to 250 Ma arc magmatism and Carboniferous through Early Permian gold-forming hydrothermal events. The more significant resulting deposits in the terranes of the southern Tian Shan include the Sawayaerdun orogenic deposit along the Kyrgyzstan border and the epithermal and replacement deposits of the Kanggurtag belt to the east in the Chol Tagh range. Gold deposits of approximately the same age in the Yili block include the Axi hot springs/epithermal deposit near the Kazakhstan border and a series of small orogenic gold deposits south of Urumqi (e.g. Wangfeng). Gold-rich porphyry copper deposits (e.g. Tuwu) define important new exploration targets in the northern Tian Shan of Xinjiang. The northern foothills of the Kunlun Shan of southern Xinjiang host scattered, small placer gold deposits. Sources for the gold have not been identified, but are hypothesized to be orogenic gold veins beneath the icefields to the south. They are predicted to have formed in the Tianshuihai terrane during its early Mesozoic accretion to the amalgamated Tarim–Qaidam–Kunlun cratonic block.
139 citations
TL;DR: In the Songpan-Ganzi basin, Carlin-like gold deposits have only been identified during the last decade in the southern zone of the western Qinling belt.
Abstract: Gold deposits of the western Qinling belt occur within the western part of the Qinling–Dabie–Sulu orogen, which is located between the Precambrian North China and Yangtze cratons and east of the Songpan–Ganzi basin. The early Paleozoic to early Mesozoic orogen can be divided into northern, central, and southern zones, separated by the Shangdan and Lixian–Shanyang thrust fault systems. The northern zone consists of an early Paleozoic arc accreted to the North China craton by ca. 450 Ma. The central zone, which contains numerous orogenic gold deposits, is dominated by clastic rocks formed in a late Paleozoic basin between the converging cratonic blocks. The southern zone is characterized by the easternmost exposure of Triassic sedimentary rocks of the Songpan–Ganzi basin. These Early to Late Triassic turbidities, in part calcareous, of the immense Songpan–Ganzi basin also border the western Qinling belt to the west. Carlin-like gold deposits are abundant (1) along a westward extension of the southern zone defined by a window of early Paleozoic clastic rocks extending into the basin, and (2) within the easternmost margin of the basinal rocks to the south of the extension, and in adjacent cover rocks of the Yangtze craton. Triassic and Early Jurassic synkinematic granitoids are widespread across the western Qinling belt, as well as in the Songpan–Ganzi basin. Orogenic lode gold deposits along brittle–ductile shear zones occur within greenschist-facies, highly-deformed, Devonian and younger clastic rocks of the central zone. Mainly coarse-grained gold, along with pyrite, pyrrhotite, arsenopyrite, and minor base metal sulfides, occur in networks of quartz veinlets, brecciated wall rock, and are disseminated in altered wall rock. Isotopic dates suggest that the deposits formed during the Late Triassic to Middle Jurassic as the leading edge of the Yangtze craton was thrust beneath rocks of the western Qinling belt. Many gold-bearing placers are distributed along the river systems that flow south from the lode-bearing central zone. Carlin-like gold deposits have only been identified during the last decade in the southern zone of the western Qinling and in the northeastern corner of the Songpan–Ganzi basin. The deposits mainly contain micron-diameter gold in arsenical pyrite; are characterized by the common occurrence of cinnabar, stibnite, realgar, and orpiment; exhibit strong silicification, carbonatization, pyritization, and decalcification dissolution textures; and are structurally controlled. The lack of reactive host lithologies may have prevented development of large (>100 tonnes of gold), stratigraphically-controlled orebodies, which are typical of the Carlin deposits in the western USA. These deposits are hosted by Triassic turbidities and shallow-water carbonates, and an early Paleozoic inlier in the Songpan–Ganzi basin that extends in an east–west belt for about 300 km. Rather than true "Carlin" deposits, these Carlin-like deposits may be some type of shallow-crustal (i.e., epithermal) hybrid with features intermediate to Nevada-style Carlin deposits and the orogenic gold deposits to the immediate north. These Carlin-like deposits also overlap in age with the early Mesozoic orogenic gold deposits and, therefore, also formed during the final stages of collision between the cratons and intermediate basin closure.
138 citations
TL;DR: The 1,500km-long Banatitic Magmatic and Metallogenetic Belt (BMMB) of Romania, Serbia and Bulgaria is a complex calc-alkaline magmatic arc of Late Cretaceous age.
Abstract: The 1,500-km-long Banatitic Magmatic and Metallogenetic Belt (BMMB) of Romania, Serbia and Bulgaria is a complex calc-alkaline magmatic arc of Late Cretaceous age. It hosts a variety of magmatic-hydro- thermal Cu, Au, Mo, Zn, Pb and Fe deposits, including Europe's only world-class porphyry-copper deposits. Regional metallogeny can be linked to subduction of the Vardar Ocean during the Late Cretaceous, as part of the closure of the Neotethys Ocean that had separated Europe and Africa in the Mesozoic. Porphyry Cu-(Au)- (Mo) and intimately associated epithermal massive sul- phides dominate in the central segments of the belt in southernmost Banat (Romania), Serbia and north-west Bulgaria. These districts are the economically most im- portant today, including major active Cu-Au mines at Moldova Noua˘ in Romania, Majdanpek, Veliki Krivelj and Bor in Serbia, and Elatsite, Assarel and Chelopech in Bulgaria. More numerous (and mostly mined in the past) are Fe, Cu and Zn-Pb skarns, which occur mainly at the two ends of the belt, in Eastern Bulgaria and in Romania. This paper summarises some of the deposit characteristics within the geodynamic framework of terminal Vardar subduction. Heterogeneous terranes of the belt, including the Apuseni Mountains at the western end, are aligned parallel to the Vardar front following continental collision of the Dacia and Tisza blocks. All available geochronological data (numerous K-Ar and some U-Pb and Re-Os ages) are compiled, and are complemented by a new high-precision Re-Os date for the Dognecea skarn deposit, south-west Romania (76.6±0.3 Ma). These data indicate that magmatism extended over at least 25 million years, from about 90 to 65 Ma in each segment of the belt. Within Apuseni Mountains and Banat, where magma emplacement was related to syn-collisional extension in the orogenic belt of Carpathians, ore formation seems to be restricted in time and maybe constrained by a shared tectonic event.
127 citations
125 citations
TL;DR: Porphyry molybdenum-gold and high-sulphidation epithermal gold are commonly broadly similar to those hosted by their calc-alkaline counterparts.
Abstract: Porphyry copper-gold, skarn copper-gold, sediment-hosted (Carlin-style) gold, breccia pipe, low-sulphidation epithermal gold, pluton-related (mesothermal or orogenic) gold vein and volcanogenic massive sulphide deposits associated with alkaline rocks are commonly broadly similar to those hosted by their calc-alkaline counterparts. In contrast, porphyry molybdenum-gold deposits are confined to alkaline igneous centres. Alkaline suites are notably deficient in high-sulphidation epithermal gold deposits and even in the advanced argillic lithocaps which host them. This is surprising, given that the required oxidised sulphur species are seemingly more abundant than in calc-alkaline igneous centres. Highly efficient buffering of acidic fluid by metasomatised alkaline rocks may offer a viable explanation. All types of intrusion-related zinc deposits also appear to be poorly developed in alkaline provinces. The characteristics of several gold and copper deposits associated with alkaline rocks, including the giant Porgera, Cripple Creek, Ladolam, Olympic Dam and Phalaborwa examples, are judged to diverge appreciably from their most closely related deposit types, rendering them arguably unique. Most of these aberrant mineralisation styles may be due to variations in magmatic-fluid compositional and liberation characteristics consequent upon the extreme diversity of ore-related alkaline magmas. Most types of gold and copper deposits developed in calc-alkaline provinces also constitute exploration targets in and around alkaline igneous centres, although porphyry copper-gold and low-sulphidation epithermal gold deposits are considered to possess the greatest potential. Perhaps of even greater interest, however, is the possibility of encountering unconventional giant gold and copper deposits which lack closely analogous examples. If already-defined aberrant deposits are truly unique, then exploration designed specifically to detect additional examples is pointless. Furthermore, exploration for new unique deposits is difficult because their defining geological parameters are unknown. Alkaline rocks within or behind calc-alkaline arcs at convergent plate boundaries probably offer the greatest exploration potential, although anorogenic intracontinental extensional settings should not be ignored.
115 citations
TL;DR: The Bingham porphyry Cu-Au-Mo deposit, Utah, may only be world-class because of substantial contributions of sulfur and metals from mafic alkaline magma to an otherwise unremarkable calc-alkaline system as discussed by the authors.
Abstract: The Bingham porphyry Cu–Au–Mo deposit, Utah, may only be world-class because of substantial contributions of sulfur and metals from mafic alkaline magma to an otherwise unremarkable calc-alkaline system. Volcanic mafic alkaline rocks in the district are enriched in Cr, Ni, and Ba as well as Cu, Au, platinum group elements (PGE), and S. The bulk of the volcanic section that is co-magmatic with ore-related porphyries is dacitic to trachytic in composition, but has inherited the geochemical signature of high Cr, Ni, and Ba from magma mixing with the mafic alkaline rocks. The volcanic section that most closely correlates in time with ore-related porphyries is very heterogeneous containing clasts of scoriaceous latite, latitic, and minette, and flows of melanephelinite, shoshonite, and olivine latite in addition to volumetrically dominant dacite/trachyte. Bingham ore-related porphyries show ample evidence of prior mixing with mafic alkaline magmas. Intrusive porphyries that have not been previously well-studied have several chemical and mineralogical indications of magma mixing. These "mixed" lithologies include the hybrid quartz monzonite porphyry, biotite porphyry, and minette dikes. Even some of the more silicic latite and monzonite porphyries retain high Cr and Ba contents indicative of mixing and contain trace amounts of sapphire (<1 mm). The heterogeneous block and ash flow deposits also contain sapphire and are permissively correlated with the intrusions based on chemical, mineralogical, and isotopic data. Magma mixing calculations suggest about 10% of the monzonitic/latitic ore-related magma may have been derived from mafic alkaline magma similar to the melanephelinite. If the original S content of the mafic magma was about 2,000–4,000 ppm, comparable with similar magmas, then the mafic magma may have been responsible for contributing more than half of the S and a significant portion of the Cu, Au, and PGE in the Bingham deposit.
TL;DR: The world-class Imiter silver deposit, in the Anti-Atlas Mountains of Morocco, is a Neoproterozoic epithermal vein deposit genetically associated with a felsic volcanic event, and formed within a regional extensional tectonic regime.
Abstract: The world-class Imiter silver deposit, in the Anti-Atlas Mountains of Morocco, is a Neoproterozoic epithermal vein deposit genetically associated with a felsic volcanic event, and formed within a regional extensional tectonic regime. Rhyolitic volcanism related to ore formation has been dated at 550±3 Ma by ion-probe U/Pb on zircons. The economic silver mineralization is superimposed on an older, discrete base-metal assemblage associated with calc-alkaline granodioritic magmatism. The magmatism is dated at 572±5 Ma by ion-probe U/Pb dating on zircons, and by 40Ar/39Ar dating on hydrothermal muscovites. In the Anti-Atlas Mountains, the Precambrian–Cambrian transition appears as an important period for the formation of major, productive precious-metal deposits associated with volcanic events and extensional tectonics. The Imiter silver deposit constitutes a Precambrian analogue to modern epithermal deposits.
TL;DR: The Hongge-type intrusions in the Pan-Xi area along the western margin of the Yangtze block, southwestern China, are spatially associated with the Late Permian Emeishan flood basalts.
Abstract: Mafic and ultramafic intrusions in the Pan-Xi area along the western margin of the Yangtze block, southwestern China, are spatially associated with the Late Permian Emeishan flood basalts of the Emeishan Large Igneous Province. The Hongge layered intrusion is one of the plutonic bodies of this province and hosts a giant Fe–V–Ti deposit. This intrusion has three zones: a lower, olivine clinopyroxenite zone, a middle, clinopyroxenite zone, and an upper, gabbro zone. Each of these zones consists of one or two compositional cycles, which have distinct Mg# values, TiO2 concentrations and total REE contents. The middle clinopyroxenite zone and upper gabbro zone contain thick (14–84 m) magnetite-mineralized layers. Samples from the lower olivine clinopyroxenite zone and middle clinopyroxenite zone of the Hongge intrusion are enriched in platinum and palladium relative to iridium and ruthenium. Rocks of the lower olivine clinopyroxenite zone have Pd/Ir ratios (1.8–22.3) lower than those of the middle clinopyroxenite zone (6.2 to 83 in its lower part and 3.6 to 49 in its upper part). The Pd/Ir ratios increase progressively upwards in each cyclic unit. Chromite is the major phase controlling concentrations of iridium and ruthenium. The Cu/Pd ratios at the bottom of each cyclic unit are close to that of the mantle and increase upwards. This variation suggests that each cyclic unit represents a new batch of magma, and sulfide mineral segregation removes PGE (platinum-group elements), Ni and Cu. Each cyclic unit in the Hongge layered intrusion might have resulted from crystal fractionation and mixing between a primary and evolved magma. The high Cu/Pd, Ti/Pd, Ni/Pd and Cu/Ir ratios of the intrusion suggest that the sulfide mineral segregation may have played an important role in PGE differentiation. If this interpretation is correct, then there is a potential to find economic PGE-rich horizon in the Hongge-type intrusions in the region.
TL;DR: The Cadia porphyry gold-copper district is the largest hydrothermal, intrusion-related gold deposit in eastern Australia and is considered to be an excellent example of the association of alkaline, potassic magmas and gold -copper style mineralisation as mentioned in this paper.
Abstract: The Cadia porphyry gold–copper district is the largest hydrothermal, intrusion-related gold deposit in eastern Australia. Discovered in 1992 by Newcrest Mining Limited, pre-mine resources in the district were in excess of 585 t Au and 2.35 Mt Cu. The Cadia district lies within shoshonitic subaqueous volcanic rocks of the Late Ordovician Molong Volcanic Belt in the eastern Lachlan Fold Belt (LFB) of New South Wales. An island arc tectonic setting is envisaged for the formation of these volcanic rocks, but the nature of the arc basement and geometry of the subduction zone(s) is debated. Mineralisation occurs in four principal porphyry deposits (Cadia Hill, Cadia Ridgeway, Cadia East/Cadia Far East and Cadia Quarry) as sheeted and stockwork quartz–sulfide veins, and locally as broadly stratabound disseminated mineralisation (Cadia East) and skarn (Big Cadia and Little Cadia). All of the porphyry deposits show a close spatial association with shoshonitic monzodiorite to quartz monzonite dykes and stocks of the Cadia Intrusive Complex (CIC). Gold–copper mineralisation is hosted by these intrusions and also by the enclosing Forest Reefs Volcanics (FRV) wall rocks. Hydrothermal alteration associated with mineralisation is potassic, which is overprinted by selectively pervasive propylitic and silica–albite assemblages. Petrological studies and major and trace element analysis of rocks of the CIC and FRV, as well as of other intrusions in the Cadia district that appear unrelated to mineralisation, have been conducted. Unaltered samples from the CIC are characterised by high K2O contents (up to 6.5 wt%) and molecular K/Na ratios consistently >1, confirming the alkalic and shoshonitic nature of the complex. The CIC samples also fall just within the shoshonite field on Ce/Yb–Ta/Yb and Th/Yb–Ta/Yb plots, although immediately adjacent to the calc-alkaline field. The FRV are geochemically similar to the CIC, although lower in P2O5 and Ce (LREE). Dacitic and hornblende porphyry intrusions from within the district, but spatially unrelated to mineralisation, have a distinctive geochemical signature and are not shoshonitic in composition. The close spatial association between gold–copper mineralisation and the shoshonitic, monzonitic CIC argues strongly for a genetic link between the two. A similar association occurs at the economic Goonumbla porphyry gold–copper deposits in the eastern LFB, while sub-economic gold–copper mineralisation at Cargo and Copper Hill is associated with calc-alkaline quartz diorite and dacitic intrusions. The Cadia district is considered to be an excellent example of the association of alkaline, potassic magmas and gold–copper porphyry style mineralisation.
TL;DR: A geochronology study using U-Pb isotope dilution TIMS analyses of zircon has been conducted to determine the ages of volcanic-associated massive sulfide (VMS) deposits in the Iberian Pyrite Belt (IPB), the world's most prolific VMS province.
Abstract: A geochronology study using U–Pb isotope dilution TIMS analyses of zircon has been conducted to determine the ages of volcanic-associated massive sulfide (VMS) deposits in the Iberian Pyrite Belt (IPB), the world's most prolific VMS province. Ages have been determined for host rocks to four VMS systems that span the IPB: the giant Rio Tinto and Aljustrel districts in the central region, Lagoa Salgada to the west, and Las Cruces to the east. A sample of chloritized quartz porphyritic dacite/rhyolite in the footwall of the San Dionisio massive sulfide deposit of the Rio Tinto district is 349.76±0.90 Ma. This is taken as the best age estimate of the mineralization in the Rio Tinto district, probably the world's largest volcanogenic massive sulfide system. Two xenocrystic zircons from the same sample yielded 207Pb/206Pb ages of 414 and 416 Ma, which provide a minimum estimate for the age of the inherited component. A biotite tonalite from the Campofrio area, 3.5 km north of the center of the Rio Tinto district, is chemically similar to the felsic host rock protolith at Rio Tinto. The Campofrio sample has an age of 346.26±0.81 Ma, slightly younger and outside of the 2σ error for the Rio Tinto age; therefore, this phase of this intrusion was not a heat source for the hydrothermal system that formed the deposits of the Rio Tinto district. The Campofrio sample also has three zircon analyses with 207Pb/206Pb minimum ages of 534, 536, and 985 Ma, indicating inheritance from Ordovician and Neoproterozoic sources. In the Aljustrel VMS district, a U–Pb zircon age of 352.9±1.9 Ma characterizes the altered Green Tuff host rock of the Algares deposit, which is slightly older than the Rio Tinto age. Two zircons with 207Pb/206Pb ages of 531 and 571 Ma from this sample indicate inheritance from a Cambrian or older source. The age of mineralization at Lagoa Salgada is given by essentially identical ages of 356.21±0.73 and 356.4±0.8 Ma, for footwall and hanging wall samples, respectively. The hanging wall sample has two zircon analyses with 207Pb/206Pb ages of 464 and 466 Ma, indicating inheritance from an Ordovician or older source. The age for an altered dacite tuff sample from the hanging wall of the Las Cruces deposit is 353.97±0.69 Ma. One zircon analysis from the Las Cruces sample has a 207Pb/206Pb age of 1048 Ma, indicating inheritance from a Neoproterozoic source. These U–Pb ages refine the IPB geochronology provided by previous studies, and they suggest that either volcanism progressed toward the center of the IPB, or that volcanism was broadly static and the strata were progressively rifted to the margins during transtensional basin formation. The zircon inheritance provides direct evidence for Proterozoic to Ordovician sources, reflecting either basement rocks beneath the Phyllite–Quartzite Group during VMS formation in late Tournaisian times, or a Proterozoic to Ordovician detrital component in Phyllite–Quartzite Group source rocks. The presence of an older crustal component is consistent with VMS formation during rift development at a continental margin.
TL;DR: In this article, the authors suggest that a majority of the world's sediment-hosted exhalative (SEDEX) lead-zinc deposits are vent-distal.
Abstract: A majority of the world's sediment-hosted exhalative (SEDEX) lead–zinc deposits are vent-distal. They are not underlain by a discordant alteration zone or stockwork vent complex that would indicate the path by which ore fluids reached the seafloor. The absence of a vent complex, together with sulfide mineral replacement of host rock mineral assemblages has led several investigators to suggest that, in spite of the well-layered nature of these deposits, mineralization was formed by sub-seafloor lateral migration of ore fluids along permeable strata. Field observations, supported by simple laboratory experiments, however, suggest an alternative process for characterizing the genesis of vent-distal SEDEX deposits. Cool, saline brines (e.g., ~120 °C and >15 wt% NaCl equiv.) are denser than seawater and, upon discharging into the sea, would flow away from the discharge vent as bottom-hugging fluids, similar to the behavior of turbidity currents. Their high densities and velocities prevent them from mixing with overlying seawater, thereby precluding significant cooling and dilution of the ore fluid. Upon coming to rest in a seafloor depression, the addition of H2S and/or dilution of the ore fluids to lower salinities result in the eventual precipitation of a vent-distal SEDEX deposit. Furthermore, the dense ore-forming fluid can sink into permeable sediments beneath the brine pool by displacing less dense pore water. The ore fluids are thus capable of effectively overprinting and/or replacing pre-existing minerals in the consolidating sediment pile.
TL;DR: The exceptionally large gold resource at Ladolam (>1,300 metric tons of gold), Lihir Island, resulted from the transition of an early-stage, low-grade porphyry gold system to a low-sulfidation epithermal gold event as discussed by the authors.
Abstract: The exceptionally large gold resource at Ladolam (>1,300 metric tons of gold), Lihir Island, resulted from the transition of an early-stage, low-grade porphyry gold system to a low-sulfidation epithermal gold event. This transition was probably triggered by rapid decompression during the partial slope failure of Luise stratovolcano and accompanied by the ingress of seawater. The original porphyry stage is indicated by remnant hydrothermal breccia clasts of strongly biotite–magnetite altered monzodiorite with disseminated pyrite ± chalcopyrite and poorly developed pyrite ± quartz stockwork veins. The breccias are overprinted by biotite–magnetite alteration and their matrix is strongly mineralized with disseminated auriferous pyrite. The breccias are cut by late-stage epithermal quartz–chalcedony–illite–adularia–pyrite veins and associated illite–adularia alteration that locally contain bonanza gold grades of up to 120 g/t. Isotope data suggest a magmatic source of sulfur in the gold-bearing fluids at Ladolam. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00126-001-0230-y
TL;DR: The Maronia copper-molybdenum deposit is hosted by a porphyritic microgranite, located 30 km south-east of Komotini in Greece in the Rhodope metallogenic province as mentioned in this paper.
Abstract: The Maronia copper-molybdenum deposit is hosted by a porphyritic microgranite, located 30 km south-east of Komotini in Thrace (north-eastern Greece) in the Rhodope metallogenic province. The geology of the area is dominated by metamorphosed Mesozoic sedimentary and volcanic rocks, and Tertiary plutonic and subvolcanic intrusions. The metamorphic rocks belong to the Makri Unit of the Circum Rhodope Belt and consist mainly of marbles and calc-schists in the lower part and schists in the upper part. The marbles and schists of the Makri Unit are intruded by the Oligocene Maronia pluton of gabbroic-monzogabbroic-monzonitic composition and a younger porphyry microgranite, which is the host of the copper-molybdenum mineralisation. Three hydrothermal alteration zones have been recognised in the porphyritic microgranite: an argillic zone, a phyllic zone, and a propylitic zone which extends into the surrounding rocks. Additionally, three highly silicified zones crop out at the north-eastern, south-eastern and southern parts of the microgranite. Chalcopyrite-pyrite-molybdenite mineralisation, concentrated mainly in the silicified zones and associated with areas of phyllic and propylitic alteration, occurs as disseminations, veinlets and segregations. Surface samples of altered rock contain as much as 7,600 ppm Mo, 5,460 ppm Cu and 1 ppm Au. Geochemical data from a drill core revealed a 10-m-thick horizon containing as much as 12 ppm Au, 17 ppm Ag and 2.00% Cu. Argillic and phyllic alteration zones are characterised by relatively low REE abundances (average total: 88 and 95 ppm respectively) and negative Eu anomalies, compared to rocks in the propylitic alteration zone where REE contents are higher (average total: 177 ppm) and there is a positive Eu anomaly. The ore-related mineral assemblage consists of sulphides (pyrite, chalcopyrite, cubanite, pyrrhotite, pentlandite, molybdenite, sphalerite, galena and bismuthinite), sulphosalts (tetrahedrite, tennantite, zinkenite, chalcostibite, famatinite, bournonite, boulangerite, meneghinite) and oxides (magnetite). The molybdenites revealed unusually high and variable rhenium concentrations, from 0.12 to 4.21 wt%. Microthermometric investigation of four types of fluid inclusions in ore-related quartz revealed salinities from 5 to 55 wt% NaCl equiv, with homogenisation temperatures varying mainly from 280 to 460 °C. The application of the chlorite geothermometer yielded temperatures between 308 and 331 °C for lowest-temperature propylitic alteration. The estimated trapping pressures of the ore-forming fluids range from 150 to 510 bar. Boiling is considered to be the main process of ore formation. Sulphur isotopic compositions for the pyrite and molybdenite suggest an igneous derivation of sulphur. Ascending melts and hydrothermal fluids interacted with the sulphide-bearing metavolcanic rocks, incorporating sulphur and ore metals (including Cu, Mo, Pb, Zn and Au) from the lithosphere. Geological, mineralogical and geochemical data indicate that the Maronia deposit has the potential of containing economic quantities of exceptionally high-grade porphyry Cu-Mo ores, with economic gold grades, in an easily accessible, yet under-explored region.
TL;DR: Alkaline-related epithermal vein, breccia, disseminated, skarn, and porphyry gold deposits form a belt in the southern Rocky Mountains along the eastern edge of the North American Cordillera as discussed by the authors.
Abstract: Alkaline-related epithermal vein, breccia, disseminated, skarn, and porphyry gold deposits form a belt in the southern Rocky Mountains along the eastern edge of the North American Cordillera. Alkaline igneous rocks and associated hydrothermal deposits formed at two times. The first was during the Laramide orogeny (about 70–40 Ma), with deposits restricted spatially to the Colorado mineral belt (CMB). Other alkaline igneous rocks and associated gold deposits formed later, during the transition from a compressional to an extensional regime (about 35–27 Ma). These younger rocks and associated deposits are more widespread, following the Rocky Mountain front southward, from Cripple Creek in Colorado through New Mexico. All of these deposits are on the eastern margin of the Cordillera, with voluminous calc-alkaline rocks to the west. The largest deposits in the belt include Cripple Creek and those in the CMB. The most important factor in the formation of all of the gold deposits was the near-surface emplacement of relatively oxidized volatile-rich alkaline magmas. Strontium and lead isotope compositions suggest that the source of the magmas was subduction-modified subcontinental lithosphere. However, Cripple Creek alkaline rocks and older Laramide alkaline rocks in the CMB that were emplaced through hydrously altered LREE-enriched rocks of the Colorado (Yavapai) province have 208Pb/204Pb ratios that suggest these magmas assimilated and mixed with significant amounts of lower crust. The anomalously hot, thick, and light crust beneath Colorado may have been a catalyst for large-scale transfer of volatiles and crustal melting. Increased dissolved H2O (and CO2, F, Cl) of these magmas may have resulted in more productive gold deposits due to more efficient magmatic-hydrothermal systems. High volatile contents may also have promoted Te and V enrichment, explaining the presence of fluorite, roscoelite (vanadium-rich mica) and tellurides in the CMB deposits and Cripple Creek as opposed to deposits to the south. Deep-seated structures of regional extent that formed during the Proterozoic allowed the magmas to rise to shallow crustal levels. Proterozoic sites of intrusions at 1.65, 1.4, and 1.1 Ga were also important precursors to alkaline-related gold deposits. Many of the larger gold deposits are located at sites of Proterozoic intrusions, and are localized at the intersection of northeast-trending ductile shear zones formed during Mesoproterozoic deformation, and an important north-trending fault formed during 1.1 Ga rifting.
TL;DR: In the Hutti mine, southern India, gold mineralization is located in closely spaced laminated quartz veins and associated alteration haloes along steeply dipping shear zones within a sequence of rather uniform amphibolites as discussed by the authors.
Abstract: Gold mineralization of the Hutti mine, southern India, is situated in closely spaced laminated quartz veins and associated alteration haloes along steeply dipping shear zones within a sequence of rather uniform amphibolites. Intense shearing has resulted in large-scale mylonitization of the wall rocks. Anastomosing shear zones, with intervening lensoid bodies of unsheared amphibolites, are characteristic features of the deposit. The general pattern of symmetrical alteration comprises a distal zone of chlorite-rich rock, with a proximal biotite-rich zone adjacent to laminated quartz veins. Arsenopyrite thermometry yielded a temperature range of 350–477 °C for the biotite alteration zone, which preceded the formation of the laminated quartz veins. Mass balance calculations on the alteration zones indicate a gradual mass and volume loss during alteration. The alteration is accompanied by intense potash metasomatism and addition of sulfur, which resulted in the formation of arsenopyrite, pyrrhotite, and pyrite. Results of fluid inclusion studies suggest that low salinity (3.9–13.5 wt% NaCl equivalent) H2O–CO2 rich fluids were responsible for gold-rich laminated quartz vein formation in the Hutti deposit. These fluids constituted a later counterpart of the protracted fluid activity that first formed the biotite alteration zone. The estimated P–T values range from 1.0 to 1.7 kbar at 280–320 °C. These data, along with the alteration assemblages and the characteristic gold–sulfide association, both in the altered wall rock and laminated quartz veins, suggest that gold, transported as reduced bisulfide complexes, was deposited in response to sulfidation reactions in the wall rocks. Comparison of P–T conditions of formation of gold–quartz veins at Hutti with two other large gold deposits in the eastern Dharwar Craton, namely Kolar (1.8 kbar/280 °C) and western Ramagiri (1.45–1.7 kbar/240–270 °C), indicates broadly similar lode-gold forming conditions in the Dharwar Craton.
TL;DR: The porphyry-copper systems in the central part of the Srednogorie zone (Bulgaria) are represented by three major deposits (Elatsite, Medet and Assarel) and several smaller deposits and occurrences, all of them within the Panagyurishte ore district.
Abstract: The porphyry-copper systems in the central part of the Srednogorie zone (Bulgaria) are represented by three major deposits (Elatsite, Medet and Assarel) and several smaller deposits and occurrences, all of them within the Panagyurishte ore district. The hydrothermal systems are related to Late Cretaceous calc-alkaline igneous complexes. Ore mineralisation is developed predominantly in the apical parts of subvolcanic and intrusive bodies as well as within the volcanic and basement metamorphic rocks. Several of the porphyry systems are spatially associated with shallow-level intermediate and high-sulphidation volcanic-hosted epithermal deposits of economic importance, such as the major gold-copper mine at Chelopech located 10 km from the Elatsite porphyry-copper deposit. Mineralisation processes in the porphyry deposits start with intensive hydrothermal alteration of the wall rocks. K-silicate alteration is characteristic for pre-ore hydrothermal activity in all of them, and it is located mostly in their central parts. Propylitic alteration is prominent in the Medet and Assarel deposits. The Assarel deposit is located in the central part of a palaeovolcanic structure and shows a large spectrum of pre-ore alterations, including propylitic, sericitic, and advanced argillic assemblages. The initial stages of the hydrothermal systems are characterised by high temperatures (>550–500 °C) and highly saline (50–20 wt% NaCl equiv.) and vapour-rich fluids of likely magmatic origin. The composition of the fluids gradually changes from H2O–NaCl±FeCl2 to H2O–NaCl–KCl and H2O–NaCl-dominated as the fluids cool, react with wall rocks, and may become diluted with meteoric water. Fe-Ti-oxide mineral associations were formed early in all deposits, later followed, in the Elatsite deposit, by an assemblage of bornite, chalcopyrite, platinum group element (PGE) phases, Co-Ni thiospinels, Ag- and Bi-tellurides, and selenides. The main ore stage in all deposits is dominated by chalcopyrite ± pyrite, associated with propylitic alteration at Medet, K-silcate-sericite alteration at Elatsite, and chlorite-sericite alteration at Assarel. Specific geochemical features of the ore systems are marked by precipitation of numerous fine-grained mineral inclusions in chalcopyrite and pyrite (palladoarsenide, Pd-rammelsbergite, Cu-Ni sulpharsenides in Elatsite; Co-and Ni-pyrite, carrolite, vaesite, sulvanite and colusite in Medet; enargite, As-sulvanite, colusite, goldfieldite, wittichenite, calaverite and aikinite in Assarel). Native gold and electrum are observed as small grains, except in Elatsite where larger aggregates up to several millimetres are found. Molybdenite is present as small flakes within chalcopyrite, or forms felted aggregates within thin quartz veinlets in the Elatsite and Medet deposits, and very rarely at Assarel. Thin veinlets of quartz-galena-sphalerite are found at the upper parts of the deposits. Supergene alteration of the primary sulphide minerals is developed in the Assarel deposit where a thick blanket of secondary copper mineralisation (chalcocite, covellite) has formed. In the other two deposits, secondary mineral associations (Cu and Fe oxides and hydroxides) are limited to near the surface. Differences in the deposits studied suggest that Elatsite and Medet are formed probably at deeper levels of the porphyry systems, whereas Assarel is developed at rather shallower and lower-temperature conditions typical of the apical part of a subvolcanic body intruding its volcanic superstructure.
TL;DR: In this paper, a new type of gold mineralisation containing minor amounts of platinum and palladium has been found proximal to the secondary redox interface located below the Cu-Ag Kupferschiefer orebody of the Polkowice-Sieroszowice mine in the southwestern part of the Lubin Sieroszekice district, Poland.
Abstract: A new type of gold mineralisation containing minor amounts of platinum and palladium has been found proximal to the secondary redox interface located below the Cu–Ag Kupferschiefer orebody of the Polkowice-Sieroszowice mine in the south-western part of the Lubin–Sieroszowice district, Poland. This deposit can be classified as redbed-type gold. Our study shows that gold, platinum and palladium occur in secondary red-coloured sections of the basal Zechstein sedimentary rocks and in the uppermost Weissliegendes sandstone. Noble metal mineralisation occurs within an average interval of 0.22 m, which lies directly below the copper ores. The average grade of the horizon is 2.25 ppm Au, 0.138 ppm Pt and 0.082 ppm Pd with a metal content of several tens of tonnes of gold. A transition zone has been recognised between the gold-bearing horizon and the copper deposit. This transition zone is characterised by the presence of low grades of copper ( 0.5 ppm). Native gold accompanied by electrum, mercury-bearing gold, haematite, covellite, chalcocite, bornite and chalcopyrite has been identified in the gold-bearing horizon. In some sections, Pd-arsenides, tetra-auricupride, Co-arsenides, clausthalite, tennantite, digenite, yarrowite, spionkopite and galena have also been noted.
TL;DR: The Apacheta mine as mentioned in this paper is located in the Shila district, 600 km southeast of Lima in the Cordillera Occidental of Arequipa Province, southern Peru.
Abstract: The Au-Ag (±Pb-Zn) Apacheta deposit is located in the Shila district, 600 km southeast of Lima in the Cordillera Occidental of Arequipa Province, southern Peru. The vein mineralization is found in Early to Middle Miocene calc-alkaline lava flows and volcanic breccias. Both gangue and sulfide mineralization express a typical low-sulfidation system; assay data show element zoning with base metals enriched at depth and higher concentrations of precious metals in the upper part of the veins. Three main deposition stages are observed: (1) early pyrite and base-metal sulfides with minor electrum 1 and acanthite; (2) brecciation of this mineral assemblage and cross-cutting veinlets with subhedral quartz crystals, Mn-bearing calcite and rhombic adularia crystals; and finally (3) veinlets and geodal filling of an assemblage of tennantite/tetrahedrite + colorless sphalerite 2 + galena + chalcopyrite + electrum 2. Fluid inclusions in the mineralized veins display two distinct types: aqueous-carbonic liquid-rich Lw-c inclusions, and aqueous-carbonic vapor-rich Vw-c inclusions. Microthermometric data indicate that the ore minerals were deposited between 300 and 225 °C from relatively dilute hydrothermal fluids (0.6–3.4 wt% NaCl). The physical and chemical characteristics of the hydrothermal fluids show a vertical evolution, with in particular a drop in temperature and a loss of H2S. The presence of adularia and platy calcite and of co-existing liquid-rich and vapor-rich inclusions in the ore-stage indicates a boiling event. Strong H2S enrichment in the Vw-c inclusions observed at –200 m, the abundance of platy calcite, and the occurrence of hydrothermal breccia at this level may indicate a zone of intense boiling. The vertical element zoning observed in the Apacheta deposit thus seems to be directly related to the vertical evolution of hydrothermal-fluid characteristics. Precious-metal deposition mainly occurred above the 200-m level below the present-day surface, in response to a liquid/vapor phase separation due to an upward boiling front.
TL;DR: In the case of the Cadia Igneous Complex, the authors of as discussed by the authors showed that the enrichment was not progressive over time, and that the oldest intrusions in the Ordovician are not necessarily the least compositionally evolved.
Abstract: Mineralised igneous complexes of Ordovician age from New South Wales range in composition from quartz-rich medium-K dacites (e.g. Copper Hill), to quartz-poor, high-K to 'shoshonitic' monzodioritic to monzonitic complexes (Goonumbla and Cadia). Despite Ordovician igneous suites being considered mostly 'shoshonitic' by some authors, only the Cadia Igneous Complex is potassic in the sense of having molecular K/Na greater than unity. A feature of the intrusive complexes is that each is spatially associated with volcanic rocks of similar compositional character, and that these relationships may occur over long periods of time within local regions (e.g. Goonumbla Volcanic Complex), or show progressive compositional evolution over probably shorter time periods as in the case of the Cadia District. The igneous suites are variably enriched in K and LILE elements, with uniformly high K/Rb ratios, low mantle-compatible element abundances and marked depletions in Ti, Nb and Ta, which is consistent with a subduction-related tectonic setting. Patterns of enrichment and depletion between each suite are similar, and are consistent with the magmas having been derived from sources variably enriched by slab derived materials prior to fusion. Recent dating results from the Goonumbla Volcanic Complex suggest that this enrichment was not progressive over time, and that the oldest intrusions in the Ordovician are not necessarily the least compositionally evolved. These results confirm the overall 'calc-alkaline' nature of Ordovician magmatism and do not support models that argue against contemporaneous subduction on the grounds that Ordovician magmatism was almost exclusively shoshonitic in character. Economic size and significance of associated deposits increase with the degree of K-enrichment (but not total alkali contents) observed in the associated igneous complexes, and supports conceptual models that link mineralisation potential and fertility with processes related to the production of K-enriched protoliths.
TL;DR: The chemistry and morphology of gold grains, which were separated from disintegrated ores, were systematically studied by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA).
Abstract: Native gold is an important economic component of the complex ores of the Chelopech high-sulphidation volcanic-hosted epithermal Au–Cu deposit (Bulgaria) The ore consists of pyrite, chalcedonic silica, chalcopyrite, enargite, luzonite, tennantite, bornite, sphalerite, galena, and numerous other sulphide, arsenide and telluride minerals Gold is paragenetically associated with most of the arsenic-bearing and base metal sulphide minerals The chemistry and morphology of gold grains, which were separated from disintegrated ores, were systematically studied by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) The gold is characterised by high fineness (~950) and a mean composition of 941 wt% Au, 527 wt% Ag, 053 wt% Cu and 010 wt% Fe Gold grains show variable morphology: subhedral flakes, irregular grains, euhedral isometric crystals, elongated rods, wires and fine-fibrous crystals, {111} twins, fine dendrite-like formations, spongy gold, and polycrystalline grains The densest faces {111} are morphologically the most important Morphometric measurements show a pronounced flatness of the gold particles The mean value of the Corey factor (a measure of flatness from 0 to 1 = spherical) is 014 and the mean length to width value is 164 It is suggested that crystal growth of gold took place in small voids, fine cracks and intergranular space, the geometry of which have controlled the highly variable grain shapes No direct correlation between the size and composition of gold grains exists, but larger grains tend to be of higher fineness
TL;DR: In this paper, the Radka epithermal, replacement type, volcanic rock-hosted copper deposit, Bulgaria, has been studied using near-infrared (IR) microscopy.
Abstract: Pyrite samples from the Radka epithermal, replacement type, volcanic rock-hosted copper deposit, Bulgaria, have been studied using near-infrared (IR) microscopy. Two generations of pyrite based on their textures, composition and behaviour in IR light can be distinguished. Electron microprobe analyses, X-ray elemental mapping and Fourier transform infrared spectroscopy were used to study the relationship between crystal zoning, trace element contents and IR transmittance of pyrite. The observed crystal zoning is related to variable arsenic contents in massive fine-grained and colloform pyrite from the early pyrite–quartz assemblage, and cobalt contents in pyrite crystals from the late quartz–pyrite vein assemblage. There is a negative correlation between trace element content and IR transmittance of pyrite. The IR transparency of pyrite is thus a sensitive indicator of changes in trace element concentrations. Fluid inclusions have only been found in the second pyrite generation. Scanning electron microscopy observations on open fluid inclusion cavities permitted the crystallographic features of vacuoles to be determined. A characteristic feature of primary fluid inclusions in pyrite is a negative crystal habit, shaped mainly by {100}, {111} and {210}. This complicated polyhedral morphology is the reason for the observed opacity of some isometric primary inclusions. Secondary fluid inclusion morphology depends on the nature of the surface of the healed fracture. Recognition of the primary or secondary origin of fluid inclusions is enhanced by using crystallographically oriented sections. Microthermometric measurements of primary inclusions indicate that the second pyrite generation was deposited at maximum P–T conditions of 400 °C and 430 bar and from a fluid of low bulk salinity (3.5–4.6 wt%), possibly KCl-dominant. There are large ranges for homogenisation temperatures in secondary inclusions because of necking-down processes. Decrepitation features of some of pyrite-hosted inclusions and of all inclusions in associated quartz indicate reheating of the veins to 500–550 °C. The late cobalt-rich quartz–pyrite vein assemblage in the Radka deposit may be the shallow manifestation of deeper and genetically related porphyry copper mineralisation. This is a common observation of many intermediate- to high-sulfidation epithermal replacement-type ore bodies in this ore district and possibly the Cretaceous Banat–Srednogorie metallogenic belt in general.
TL;DR: In this paper, lead isotope analyses have been performed on the two major Miocene mining districts of Romania, Baia Mare and Apuseni Mountains, and the results suggest a magmatic signature for the Pb (and possibly other metals) in the hydrothermal fluids.
Abstract: Lead isotope analyses have been performed on the two major Miocene mining districts of Romania, Baia Mare and Apuseni Mountains. These two districts have different non-overlapping 206Pb/204Pb isotopic signatures ranging from 18.752 to 18.876 and 18.497 to 18.740. In the Baia Mare district, epithermal deposits are overall homogeneous in their lead isotopic compositions and have values similar to the average of the calc-alkaline volcanic rocks. These results suggest a magmatic signature for the Pb (and possibly other metals) in the hydrothermal fluids. However, magmas in this district show isotopic evidence of crustal assimilation. In the southern Apuseni Mountains, the lead isotope compositions of sulfide minerals in porphyry copper deposits are clustered, confirming that Pb, and probably other metals, were derived principally from associated porphyry stocks. On the other hand, lead isotope data on sulfides in epithermal ore deposits are much more scattered, indicating a notable contribution of Pb from local country rocks. In the Apuseni Mountains, 'fertile' volcanics are few and appear to come from a more primitive mantle-derived source. Most of the analysed volcanic rocks seem 'barren'. Differences in lead isotopic compositions between the Baia Mare district and the Apuseni Mountains are due to a different basement, and probably to variations in crustal assimilation superimposed on variations in the mantle source composition. In the Apuseni Mountains, Pb may be partly inherited from the previous Mesozoic magmatic–hydrothermal stage. From a geodynamic point of view, it seems that the nature and the source of volcanic rocks and their position related to the collision area of the Carpathian arc are not the only factors controlling the 'fertility' of a volcanic district.
TL;DR: In this article, a combination of fluid-inclusion microthermometry, Raman spectroscopy, bulk crush-leach analysis, laser ablation-optical emission spectrometry (LA-OES) of individual fluid inclusions and stable isotope data has been used to characterize the ore-forming solutions involved with gold/polymetallic mineralization in three of the largest deposits.
Abstract: The Baia Mare district (Romania) represents a complex segment of the Neogene/Quaternary, subduction-related, volcanic chain of the Carpathians. In this province, the calc-alkaline volcanism and associated mineralization are closely related to a major strike-slip fault and an underlying pluton. The ore deposits of the district have many features typical of low-sulfidation (or adularia-sericite) epithermal deposits. The combination of fluid-inclusion microthermometry, Raman spectroscopy, bulk crush-leach analysis, laser ablation-optical emission spectrometry (LA-OES) of individual fluid inclusions and stable isotope data has been used to characterize the ore-forming solutions involved with gold/polymetallic mineralization in three of the largest deposits. Ore fluids at Baia Sprie, Cavnic and Sasar deposits had temperatures of 320 to 150 °C and salinities in the range 0–21 wt% NaCl equiv. During phreatomagmatic activity, fracturing and periods of vein dilation, the pressure fluctuations may have caused boiling. The hydrothermal fluids were Na–K–Li–Ca chloride solutions with lesser amounts of SO4 and F. The Na/K ratios and the stable isotope compositions are consistent with equilibration of the fluids with country rocks at progressively lower temperatures. The halogen ratios (Br/Cl×103 in the range 0.13–1.21), together with sulfur isotopic systematics (δ34S=0.3–5.2 per mil), are consistent with a dominantly magmatic origin of the solutes, and probably of the ore fluids themselves.
TL;DR: The Skouries porphyry Cu-Au deposit as discussed by the authors, containing an indicated reserve of 206 Mt at 0.54% Cu and 0.80 g/t Au, is hosted by at least four hypabyssal monzonite-porphyry phases.
Abstract: The Skouries porphyry Cu–Au deposit, containing an indicated reserve of 206 Mt at 0.54% Cu and 0.80 g/t Au, is hosted by at least four hypabyssal monzonite–porphyry phases. In decreasing age, they are: (1) pink monzonite, (2) main monzonite, (3) intra-mineral monzonite, and (4) late-stage porphyry. High-grade ore is directly associated with the main and intra-mineral monzonite phases. All intrusive phases are cut by late-stage monzonite dykes that are barren. The monzonites have porphyritic textures with phenocrysts of plagioclase, alkali feldspar and amphibole as well as apatite and titanite microphenocrysts in a fine-grained feldspar-dominated groundmass. Mineralized samples are affected to varying degrees by potassic alteration, ranging from weak biotite–magnetite disseminations, through cross-cutting veinlets of hydrothermal orthoclase, to zones with pervasive orthoclase flooding. The high halogen contents of the Skouries intrusions are reflected in the high Cl and F concentrations of mica phases (up to 0.19 and 2.48 wt% respectively). The presence of magmatic magnetite in all intrusive phases implies high oxygen fugacities of the parental melts. All four monzonite phases have relatively evolved compositions, as reflected by their high SiO2, low MgO and low mg#, and variable but low contents of mantle-compatible elements such as V, Ni and Co. However, their mg# suggests increasing degrees of fractionation of the parental melts with decreasing age. Their high K2O (up to 5.8 wt%) and K2O/Na2O ratios (>1), as well as their high Ce/Yb and Th/Yb ratios (>34 and >21 respectively), which are believed to have been unaffected by alteration processes, are typical of alkaline rocks of the shoshonite association. Importantly, the Skouries intrusions are characterized by very high U and Th contents (up to 18.9 ppm and 62 ppm, respectively) that are consistent with accessory thorite and rare allanite in several samples. The high initial 87Sr/86Sr ratios (0.7082) for the Skouries intrusions suggest crustal contamination during emplacement. The use of geochemical discrimination diagrams assigns the rocks to a continental arc setting in accord with the interpretation of previous workers.