Mineralized Quartz Veins Research Articles

대화 Mo-W 열수 맥상 광상의 유체 진화 특성

대화광상은 경기육괴의 편마암류와 화강암류에 발달한 열극을 충진 발달한 함 Mo-W 열수 맥상 광상이다. 대화광상의 몰리브덴-텅스텐 광화작용과 관련된 주요 수반광물인 석영에서 관찰되는 유체포유물은 상온 (<TEX>$20^{\circ}C$</TEX>) 에서의 상(phase) 관계와 냉각 및 가열 실험을 통해 측정된 균일화 온도와 상변화를 기초로 하여 3가지 주요 유형 (Type I, 액상이 우세한 <TEX>$H_2O$</TEX>-NaCl 유형; Type II, 기상이 우세한 <TEX>$H_2O$</TEX>-NaCl 유형; Type III; <TEX>$CO_2-H_2O$</TEX>-NaCl 유형) 으로 분류된다. 또한, 함 <TEX>$CO_2$</TEX> Type III 유체포유물은 <TEX>$CO_2$</TEX> 균일화 및 최종 균일화 특성을 바탕으로 4가지 유형 (IIIa, IIIb, IIIc, IIId)으로 세분된다. 대화광상 Type I 유체포유물의 균일화 온도는 약 <TEX>$374^{\circ}C{\sim}161^{\circ}C$</TEX>로 넓은 범위를 보여주며, 염농도 역시 약 13.6~0.5 equiv. wt. % NaCl의 넓은 조성 범위를 보인다. Type III 유체포유물 냉각 실험 시 측정된 <TEX>$CO_2$</TEX> 상의 용융 온도는 <TEX>$-57.4{\sim}-56.6^{\circ}C$</TEX>이며, <TEX>$CO_2$</TEX> 균일화 온도는 <TEX>$29.0{\sim}30.8^{\circ}C$</TEX>이다. 또한 <TEX>$CO_2$</TEX> clathrate 용융 온도는 <TEX>$7.3{\sim}9.5^{\circ}C$</TEX>로 염농도는 5.2~1.0 equiv. wt. % NaCl이고, 최종 균일화 온도는 <TEX>$303^{\circ}C{\sim}251^{\circ}C$</TEX>로 비교적 좁은 범위로 확인되었다. <TEX>$CO_2-H_2O$</TEX>-NaCl계 (Type III) 유체포유물의 경우 온도가 감소함에 따라 염농도 역시 감소하는데, 이는 높은 염농도를 가진 <TEX>$H_2O$</TEX>-NaCl계 유체와 낮은 염농도를 가진 <TEX>$CO_2-H_2O$</TEX>-NaCl계 유체의 불혼화에 의해 열수의 진화가 이루어졌음을 의미한다. Type I 유체포유물은 온도 감소와 염농도 사이의 뚜렷한 변화가 인지되지 않았다. 따라서, 대화 열수계의 함 몰리브덴-중석 광화작용은 <TEX>$400^{\circ}C$</TEX>, 5.2 equiv. wt.% NaCl의 염농도를 가진 광화유체로부터 시작되어, 약 <TEX>$350^{\circ}C$</TEX> 부근에서 유체의 불혼화 용융에 의해 진행되었다. 이후 대화 열수계에 유입된 상대적으로 낮은 온도와 염농도를 갖는 유체 (천수 또는 상대적으로 높은 물/암석 비를 갖는 열수유체) 의 혼입 작용에 의해 후기 천금속 광화작용이 야기되었다. The Daehwa Mo-W deposit is located within the Gyeonggi massif. Quartz and calcite vein mineralization occurred in the Precambrian gneiss and Jurassic granites. Three main types (Type I: liquid-rich <TEX>$H_2O$</TEX> type, Type II: vapor-rich <TEX>$H_2O$</TEX> type, Type III: <TEX>$CO_2-H_2O$</TEX> type) of fluid inclusions were observed and are classified herein based on their phase relations at room temperature. Within ore shoots, type III fluid inclusions have been classified into four subtypes (type IIIa, IIIb, IIIc and IIId) based on their volume percent of aqueous and carbonaceous (<TEX>$CO_2$</TEX>) phase at room temperatures combined with their total homogenization behavior and homogenization behavior of <TEX>$CO_2$</TEX> phase. Homogenization temperatures of primary type I fluid inclusions in the quartz range from <TEX>$374^{\circ}C$</TEX> to <TEX>$161^{\circ}C$</TEX> with salinities between 13.6 and 0.5 equiv. wt.% NaCl. Homogenization temperatures of primary type III fluid inclusions in quartz of main generation, are in the range of <TEX>$303^{\circ}C$</TEX> to <TEX>$251^{\circ}C$</TEX>. Clathrate melting temperatures of the type III fluid inclusions were 7.3 to <TEX>$9.5^{\circ}C$</TEX>, corresponding to salinities of 5.2 to 1.0 equiv. wt. % NaCl. Melting and homogenization temperatures of <TEX>$CO_2$</TEX> phase of type III fluid inclusions were -57.4 to <TEX>$-56.6^{\circ}C$</TEX> and 29.0 to <TEX>$30.8^{\circ}C$</TEX>, respectively. Fluid inclusion data indicate a complex geochemical evolution of hydrothermal fluids. The Daehwa early hydrothermal system is characterized by <TEX>$H_2O-CO_2$</TEX>-NaCl fluid at about <TEX>$400^{\circ}C$</TEX>. The main mineralization occurred by <TEX>$CO_2$</TEX> immiscibility at temperatures of about 300 to <TEX>$250^{\circ}C$</TEX>. At the late base-metal mineralization aqueous fluid formed by mixing with cooler and less saline meteoric groundwater.

Sulfidic and non-sulfidic indium mineralization of the epithermal Au–Cu–Zn–Pb–Ag deposit San Roque (Provincia Rio Negro, SE Argentina) — with special reference to the “indium window” in zinc sulfide

At San Roque in Patagonia's Rio Negro Province, Argentina, an In–Au–Cu–Zn–Pb–Ag mineralization (<0.24wt.% In, <7ppm Au, <0.45wt.% Cu, <14.1wt.% Zn, <0.55wt.% Pb, <60ppm Ag) is bound to lava, and volcaniclastics of Triassic through Jurassic age. The polymetallic sulfidic and non-sulfidic indium mineralization is attributed to the low-sulfidation (LS) to intermediate sulfidation (IS) epithermal type of mineralization. Its vein-type and stockwork mineralization developed at 39.2bars under hydrostatic conditions, corresponding to a depth of 400m below the water level of the paleoaquifer. In the redox-controlled hypogene mineralization, the temperature increased from 130°C up to as much as 250°C at depth, while the pH regime changed from slightly acidic near surface to more alkaline conditions around pH 8 at a depth of approximately 150m. The monophase mineral associations composed of sphalerite, Ag–Bi-enriched and inclusion-free galena (<1.7wt.% Ag, <3.7wt.% Bi), chalcopyrite, pyrite, gold, silver, digenite, various In–Cu- and Pb–Zn–Ag “intermediate products”, wittichenite, roquesite, sakuraiite, dzhalindite, brochantite, antlerite, cerussite, and “manganomelane” in a quartz and muscovite-rich gangue have been subdivided into three different stages: (1) Stockwork mineralization of LS to IS epithermal type (hypogene), (2) quartz vein mineralization (hypogene), and (3) salar mineralization (supergene–hypogene).Salt–mud flats controlled the youngest mineralization with Mn, Li, Ca, Mg, V, Sr, Cu, Ag and In bound to oxides, hydroxides, sulfates and subordinate carbonates. The quartz vein mineralization is made up of oxides, hydroxides prevailing over sulfides and containing W, Fe, Au, As, Pb, In, and Cu. It formed at the passage from the vadose into the phreatic zones under oxidizing to slightly reducing conditions. The level marks the boiling level of the hydrothermal solutions involved in the mineralizing process. The hypogene stockwork mineralization is exclusively made up of sulfides containing Zn, Pb, Cu, In, Ag and Bi in the phreatic zones. It developed under reducing conditions. Indium is present at all levels within the volcanic rocks and has been derived from sphalerite rich in Cd (<1.6wt.% Cd), In (<7.3wt.% In) and Cu (<7.2wt.% Cu) while the Fe contents are moderate in sphalerite (<6.8wt.% Fe). Indium reached economic grade only through the segregation of a Cu–In–S phase in the “indium window” which is defined by a Cd content of sphalerite in the range 0.2–0.6wt.% Cd. This concentration of In is controlled by the crystal morphology and the lattice parameters of the minerals involved. It is described as a two-stage process with interdiffusion processes in an Fe-enriched system (stage I) and zoned replacement in an Fe-poor system enriched in indium (stage II). Cu-bearing sphalerite decomposed into sphalerite poor in trace elements and into Cu–In-bearing sphalerite. Further indium concentration took place, when roquesite and sakuraiite decomposed along with an increase in oxygen pressure under hypogene and supergene conditions into dzhalindite. The physical–chemical conditions of the mineralogy and chemical changes in the system In–Cu–Zn–Cd observed in nature have been approximated based upon the results obtained during laboratory studies in material sciences that were focused on solar energy.

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

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

Laser Ablation ICP‐MS U‐Pb Zircon Geochronology of Granitoids and Quartz Veins in the Shihu Gold Mine, Taihang Orogen, North China: Timing of Gold‐mineralization and Tectonic Implications

AbstractThe Shihu gold deposit, situated in the Taihang Mesozoic orogen of the North China Craton (NCC), is hosted by ductile‐brittle faults within Archean metamorphic core complex. The deposit is characterized by gold‐bearing quartz‐polymetallic sulfides veins. The Mapeng granitoids stock and intermediate‐basic dikes intruded the metamorphic basement rocks, and are spatially related to gold mineralization. Detailed laser ablation inductively coupled plasma mass spectrometry (LA ICP‐MS) U‐Pb zircon ages of the granitic rocks, dykes and mineralized quartz veins in the studied area reveal its magmatic and mineralized history. The mineralized quartz veins contain inherited zircons with ages of about 2.55 Ga and 1.84 Ga, probably coming from the basement. These two Precambrian events are coeval with those in other parts of the NCC. The Mapeng granitoid stock, the largest intrusion in the area, was emplaced at ca. 130 Ma, and is coeval with magmatic zircon populations from diorites and quartz diorite pophyrites in the same region. The ca. 130 Ma magmatism and gold mineralization were most likely related to an underplating event that took place in the Taihang orogen at Late Mesozoic. The timing of gold mineralization with respect to felsic magmatism in the area is similar to those observed in other major gold‐producing provinces in the NCC. This episode is simultaneous with those in the eastern margin of NCC, indicative of a widespread late Yanshanian metallogenic event that was a response to the Early Cretaceous lithosphere in the eastern NCC, in which the mesothermal gold deposits were formed from similar tectono‐magmatic environments.

Epithermal gold mineralization in Costa Rica, Cordillera de Tilarán &amp;mdash; exploration geochemistry and genesis of gold deposits

*Corresponding author Epithermal gold mineralization in quartz veins forms part of a large ore belt extending in the NW–SE direction parallel to the Cordillera de Tilaran, Costa Rica. It is confined to Miocene–Pliocene andesites and basalts of the Aguacate Group volcanic arc. Gold-bearing quartz veins are related to faults and fractures of steep inclinations, accompanied by pronounced hydrothermal alteration. The key tectonic zones strike NW–SE but the majority of the ore veins are controlled by local extensional structures of Riedel shear type in the NE–SW, N–S to NNW–SSE directions. The brecciation, mylonitization and healing of deformed structures suggest that three main pulses of mineralization took place during the hydrothermal process. The gold is present as electrum (30 and 42 wt. % Ag) tiny inclusions up to 25 µm in size enclosed in quartz, pyrite and arsenopyrite. The other ore minerals are chalcopyrite, galena, sphalerite and marcasite and less abundant to scarce acanthite, pyrargyrite, greenockite, covellite, bornite and cassiterite. The principal elements exhibiting significant positive correlations with Au are Ag, Sb, As, Pb and Hg. Fluid inclusions of the H 2 O type were found in quartz and sphalerite from several Au-bearing occurrences. Temperatures of homogenization of fluid inclusions from several quartz generations and sphalerite vary generally between 150 and 290 °C; the salinity of the aqueous solution was very low, not exceeding 5 wt. % NaCl equiv. The age of the mineralization is estimated in the period between the intrusion of the Guacimal Pluton and effusions of the discordant volcanic Monteverde Formation, which is barren (i.e. between c. 6.0 and 2.1 Ma). Geochemical study indicated altogether 14 promising gold-bearing areas in the Montes del Aguacate and Cordillera de Tilaran, of which four can be recommended for further exploration.

Oxide minerals in the granitic cupola of the Jálama Batholith, Salamanca, Spain. Part II: Sn, W and Ti minerals in intra-granitic quartz veins

*Corresponding author The mineralized quartz veins hosted by the distal granitic facies of the Jalama Batholith were generated by successive processes of the fractures opening and filling by hydrothermal fluids enriched in volatiles. Cassiterite I, wolframite I and rutile I crystallized during an early stage, followed – after a ductile deformation – by a main Fe–Zn sulphide deposition, composed initially by subordinate cassiterite II and wolframite II. Later during this main sulphide deposition, ixiolite and Nb–Ta-rich rutile II filled fractures. Finally, the late open spaces and cavities were sealed by quartz and carbonates. The crystallization of wolframite II indicates enrichment in Mn of the fluid, compared with wolframite I, suggesting a normal evolutionary trend from the early Sn–W deposition to the first stage of the main Fe–Zn sulphide precipitation in the quartz veins. The scarcity of Sn during the latest stages of the ore deposition prevented the crystallization of cassiterite, favoring instead the incorporation of this element into the ixiolite and rutile II structure. This hydrothermal evolution from the crystallization of wolframite II to ixiolite and rutile II responded to enrichment in Ti and Fe of the late fluid, thus favoring a partially reversed trend in the Nb–Ta–Ti oxides composition. The late enrichment in Ti and Fe of the hydrothermal fluid could be explained by contamination by external metamorphic and/or meteoric fluids en riched in these elements. The incorporation of Ti, Nb and Ta into the ixiolite and rutile structure was favored by high F concentrations in the hydrothermal fluids, inherited from the residual melts of the Jalama Batholith.

Geodynamic settings for Paleoproterozoic gold mineralization in the Svecofennian domain: A tectonic model for the Fäboliden orogenic gold deposit, northern Sweden

Northern Sweden is currently experiencing active exploration within a new gold ore province, the so called Gold Line, situated southwest of the well-known Skellefte VMS District. The largest known deposit in the Gold Line is the hypozonal Fäboliden orogenic gold deposit. Mineralization at Fäboliden is hosted by arsenopyrite-rich quartz veins, in a reverse, mainly dip-slip, high-angle shear zone, in amphibolite facies supracrustal host rocks. The timing of mineralization is estimated, from field relationships, at ca. 1.8Ga.The gold mineralization is hosted by two sets of mineralized quartz veins, one steep fault–fill vein set and one relatively flat-lying extensional vein set. Ore shoots occur at the intersections between the two vein sets, and both sets could have been generated from the same stress field, during the late stages of the Svecofennian orogen.The tectonic evolution during the 1.9–1.8Ga Svecofennian orogen is complex, as features typical of both internal and external orogens are indicated. The similarity in geodynamic setting between the contemporary Svecofennian and Trans-Hudson orogens indicates a potential for world-class orogenic gold provinces also in the Svecofennian domain.The Swedish deposits discussed in this paper are all structurally associated with roughly N–S striking shear zones that were active at around 1.8Ga, when gold-bearing fluids infiltrated structures related to conditions of E–W shortening.

Isotopic geochemistry of the Sawayaerdun orogenic-type gold deposit, Tianshan, northwest China: Implications for ore genesis and mineral exploration

The Sawayaerdun gold deposit is hosted by Carbonaceous metasediments and is considered to be the largest Muruntau-type gold deposit in the Chinese Tianshan metallogenic belt. Gold mineralization at Sawayaerdun occurs in quartz veins associated with three major hydrothermal events: an early, barren quartz vein stage, middle stage mineralized quartz veins with pyrite and late carbonate (-quartz) veins.The isotopic compositions of quartz and sulfides from the Sawayaerdun gold deposit show some variation but are generally comparable to those of other orogenic-type gold deposits. Fluids trapped in early-stage quartz have a δ18O range of 13.6‰ to 15.4‰, δD of −48‰ to −75‰, δ13C of 0.5‰ to 4.2‰ and δ30Si of −0.2‰ to 0‰. In contrast, isotopic compositions of fluids trapped in middle-stage quartz have δ18O values of 6.7‰ to 14.7‰, δD of −56‰ to −110‰, δ13C of 0.4‰ to 10.1‰ and δ30Si of −0.3‰ to 0‰. Diagenetic and hydrothermal pyrite have similar sulfur (−1.8‰ to 0.9‰) and Pb isotopic values that are associated with host rock compositions. The early-stage, 18O and 13C-rich fluids are probably derived from metamorphic decarbonation of the sedimentary host rock at depth, leading to the precipitation of early barren quartz veins. In the middle stage, a decrease in the regional pressure and temperature regime could have resulted in the incorporation of external fluids into the ore-forming system. These external fluids with isotopic signatures similar to that of the host rock and generally rich in 34S and radiogenic Pb mixed with original ore-forming fluids to generate extensive metal precipitation. Late-stage fluids trapped by calcite veins show isotopic compositions similar to meteoric water, indicating the cessation of hydrothermal fluid circulation at Sawayaerdun occurred at this time. The metallogenetic model illustrated by stable and Pb isotopes is also consistent with fluid inclusion studies in Sawayaerdun.The development of mineralization at Sawayaerdun is strongly linked to fluid mixing, as witnessed by the isotopic signatures of fluids from identified ore-bearing zones. The isotopic compositions of other anomalous zones at Sawayaerdun are similar to those of the mineralized zones, suggesting a high potential for further exploration.

Secondary tungsten minerals in quartz veins in the Ishidera area, Wazuka, Kyoto Prefecture, Japan: anthoinite, mpororoite, and Fe-free hydrokenoelsmoreite

Secondary minerals of tungsten that are the products of alteration of scheelite present in quartz veins in the Ishidera area, Wazuka, Kyoto Prefecture, have been examined using XRD, SEM-EDS, EPMA, and XRF. From the results, three tungsten minerals were identified: anthoinite, mpororoite, and hydrokenoelsmoreite. The two former minerals have not been reported to be found in Japan. This is, therefore, the first discovery of anthoinite and mpororoite in Japan. The two minerals form a white powdery mixture with pseudomorphing scheelite. Chemical analysis of the mixture shows that the Al/W ratio is approximate to 1 and that the Fe2O3 content is very low, suggesting that the ideal formulae of anthoinite and mpororoite are WAlO3(OH)3 and WAlO3(OH)3·2H2O, respectively, even though the original mpororoite had a high content of Fe2O3 substituting for Al2O3. In addition to these two minerals, another tungsten mineral was also found within the scheelite-pseudomorphs. It occurs as aggregates of regular octahedral crystals up to 50 μm in length. The XRD data are in good agreement with those for hydrokenoelsmoreite, but chemical analysis shows that the major components are WO3, Al2O3, and H2O with no Fe2O3. Up to this time, only Fe-containing hydrokenoelsmoreite, once termed ferritungstite according to the old nomenclature, has been widely reported to be found in Japan. This paper is the first to report the occurrence of such an Fe-free hydrokenoelsmoreite in Japan. It is likely that these three secondary minerals of tungsten at Wazuka were formed in an environment where the supply of H2O and Al2O3 and the leaching of calcium ions from scheelite took place simultaneously. The source of Al is ascribed to the decomposition of muscovite in the quartz veins.

Metal fractionation of cadmium, lead and arsenic of geogenic origin in topsoils from the Marrancos gold mineralisation, northern Portugal

The Marrancos gold mineralisation has a chemical assemblage of Fe-As-Se-Bi-Au-Ag-Te-(Cu-Pb-Zn-Sn-W). The -200 mesh of 144 topsoil samples was analysed by ICP-MS to determine total contents of 53 elements that include potentially harmful elements like Cd, As and Pb. The soil geochemistry shows that some trace elements occur in considerably high concentrations. On the basis of data for total metal concentrations, 10 topsoil samples were selected to carry out a metal fractionation study using a selective extraction method. A set of four leaches of increasing strength was used sequentially in the soil samples. Across the study area, there is some evidence of past mining and exploration activity, indicating that these soils may be locally disturbed. The shallow mineralised quartz veins were exploited for gold by the Romans. Several galleries were constructed during the 2nd World War, probably for the exploration of quartz-cassiterite-wolframite veins. However, the main mineralised body in depth was never explored. The results of metal fractionation show different partitions for the three elements. Total Cd concentrations in these soils are low, with a median value of 0.1 mg/kg. In average, 12% of total Cd is adsorbed by clays and/or co-precipitated with carbonates, and 19% is bounded to Fe-Mn oxyhydroxides. However, the low concentrations indicate that the metal does not represent an immediate risk to human health. For Pb, metal fractionation shows that, on average, 22% of Pb is adsorbed by amorphous Fe and Mn oxides, but the samples from the northern part of the area have the major fractions of Pb in soluble forms. The low probability of exposure in this part of the study area decreases the risk posed by this heavy metal. Total As concentrations in the Marrancos soils are extremely high. A large area has As concentrations above 1,000 mg/kg. For As, metal fractions in the sulphide phase vary between 84 and 98% in the studied samples. But one sample has 20% of total As in easily reducible forms, corresponding to a partial concentration of 1,800 mg/kg that has a high probability of being bioavailable. The most labile As forms occur at the southern part of the area, where the probability of exposure is higher and the risk of human health increases in the same order. From the three studied potentially harmful elements, As is certainly the element of concern.

Transpressional zones in ophiolitic mélange terranes: Potential exploration targets for gold in the South Eastern Desert, Egypt

Gold-bearing quartz ± carbonate veins at the El-Anbat mine area, in the South Eastern Desert of Egypt, are associated with pervasively silicified, highly sheared ophiolitic and island arc rocks. Structural analysis of the shear fabrics along the ore zones indicate that geometry of the mineralized quartz veins and alteration patterns are controlled by the regional, NNW-trending zone of transpression, known as the Wadi Kharit–Wadi Hodein shear system, which is related to the 655–540 Ma, Najd strike–slip fault system in the Eastern Desert of Egypt. Microscopic and electron microprobe studies reveal that gold is chemically bound in As-pyrite, arsenopyrite, and gersdorffite, while free-milling gold is rarely observed in the quartz veins and silicified host rocks. Hydrothermal alteration could have liberated additional traces of gold from early pyrrhotite disseminated in listvenite. Geochemical studies reveal that shear zones associated with lenticular masses of porphyritic granite in the middle of the transpression zone are heavily mineralized (ppm levels) compared with other parts of the alteration zone. Where no granite is observed, low-grade zones (ppb levels) of gold mineralization occur in the highly deformed serpentinite and metavolcaniclastic rocks. Mass balance calculations suggest that hydrothermal fluids infiltrated under brittle–ductile shear conditions led to addition of Au, As, Cu, Rb, and Sb, concurrent with silicification, carbonatization and sericitization of the host rocks. Dispersion of gold and associated pathfinder elements is reported in altered rocks beyond the lode zone, indicating the potential for extensive, low-grade zones outside the already known El-Anbat mine location.

Geological setting of the Paleoproterozoic Rosebel gold district, Guiana Shield, Suriname

Abstract The Rosebel gold district is hosted in a Paleoproterozoic greenstone belt of the Guiana Shield and has many characteristics that enable classification of the ores as an orogenic gold deposit. Host rocks have undergone several phases of deformation. However, gold deposition occurred late in the structural history of the belt, and is considered part of a late regional metallogenic event with respect to the geotectonic evolution of the Guiana Shield. Economic gold mineralization is hosted in felsic to mafic volcanic rocks and two sedimentary successions that are differentiated into turbiditic and arenitic depositional packages. The detailed lithostratigraphic characterization and the geochemistry enable the correlation of the local rock types with the Paramaka, the Armina, and the Rosebel formations respectively. The Rosebel district comprises eight discrete gold deposits distributed along two major structures. The northernmost structure is a sub-vertical WNW–ESE shear zone that preserves evidence of dextral strike–slip followed by normal faulting. The southern structure is an east–west reverse fault along which gold deposits are mainly hosted in the footwall. Gold mineralization is associated with quartz vein arrays developed along pre-existing structural heterogeneities, such as stratigraphic contacts and fold hinges. Four main sets of veins are recognized in the district: shear veins, north–south tension veins, stacks of north-dipping tension veins, and anticline-hosted tension veins. Mineralized quartz veins are typically associated with a wallrock alteration assemblage comprising sericite, chlorite, carbonate, tourmaline, pyrite, pyrrhotite, and plagioclase. The presence of a WNW–ESE dextral strike–slip structure, an east–west reverse fault, and north–south tension veins are consistent with the formation of a Riedel system during a simple shear event. All vein sets cut deformed sedimentary rocks that were deposited in a pull-apart basin, which together with their planar geometries, indicates a late-Transamazonian orogen timing for gold mineralization. Extensive dating of other orogenic gold deposits in the Guiana Shield has revealed ages between 2.023 and 1.955 Ga for gold mineralization, which is broadly coeval with the onset of late-Transamazonian calc-alkaline volcanism and plutonism (2.05–1.88 Ga) south of the Guiana Shield.

Mineralization and fluid evolution of the Jiyuan polymetallic Cu–Ag–Pb–Zn–Au deposit, eastern Tianshan, NW China

The newly discovered Jiyuan Cu–Ag–(Pb–Zn–Au) deposit is located in the southern section of the eastern Tianshan orogenic belt, Xinjiang, northwestern China. It is the first documented deposit in the large Aqikekuduke Ag–Cu–Au belt in the eastern Tianshan orogen. Detailed field observations, parageneses, and fluid inclusion studies suggest an epithermal ore genesis for the main Cu–Ag mineralization, accompanied by a complicated hydrothermal alteration history most likely associated with the multi-stage tectonic evolution of the eastern Tianshan. The Jiyuan Cu–Ag ore bodies are located along the EW-striking, south-dipping Aqikekuduke fault and are hosted by Precambrian marble and intercalated siliceous rocks. Early-stage skarn alteration occurred along the contact zone between the marble layers and Early Carboniferous diorite–granodiorite and monzogranite intrusions; the skarns are characterized by diopside–tremolite–andradite–pyrite–(magnetite) assemblages. Local REE-enriched synchysite–rutile–arsenopyrite–(clinochlorite–microcline–albite) assemblages are related to K–Na alteration associated with the monzogranite intrusions and formed under conditions of high temperature (310°C) and high salinity (19.9 wt.% NaCl). Subsequent hydrothermal alteration produced a series of quartz and calcite veins that precipitated from medium- to low-temperature saline fluids. These include early ‘smoky’ quartz veins (190°C; 3.0 wt.% NaCl) that are commonly barren, coarse-grained Cu–Ag mineralized quartz veins (210°C; 2.4 wt.% NaCl), and late-stage unmineralized calcite veins (140°C; 1.1 wt.% NaCl). Tremolite and Ca-rich scapolite veins formed at an interval between early and mineralized quartz veins, indicating a high-temperature, high-salinity (>500°C; 9.5 wt.% NaCl) Ca alteration stage. Fluid mixing may have played an important role during Cu–Ag mineralization and an external low-temperature Ca-rich fluid is inferred to have evolved in the ore-forming system. The Jiyuan auriferous quartz veins possess fluid characteristics distinct from those of the Cu–Ag mineralized quartz veins. CO2-rich fluid inclusions, fluid boiling, and mixing all demonstrate that these auriferous quartz veins acted as hosts for the orogenic-type gold mineralization, a common feature in the Tianshan orogenic belt.

Late Neoproterozoic overprinting of the cassiterite and columbite-tantalite bearing pegmatites of the Gatumba area, Rwanda (Central Africa)

The Mesoproterozoic Kibara belt in Central Africa has recently been redefined and subdivided into the Karagwe–Ankole belt (KAB) and the Kibara belt (KIB), separated by Palaeoproterozoic (Rusizian) terranes. The KIB and KAB are characterised by the presence of numerous rare metal mineralised (Nb–Ta–Sn) pegmatites and Sn–W mineralised quartz veins that are related to the youngest granite generation, i.e. the G4-granites in Rwanda, which formed at 986 ± 10 Ma. The pegmatites of the Gatumba area (western Rwanda) have historically been mined for their columbite-tantalite and cassiterite mineralisation, but contain also beryl, apatite, spodumene, amblygonite, and rare phosphates. Columbite-tantalite formed during the crystallisation of the pegmatites, followed by intense alkali metasomatism, i.e. widespread growth of albite and white mica. The major part of the cassiterite mineralisation is, however, concentrated in zones associated with intense phyllic alteration. U–Pb ages of columbite-tantalite samples vary between ∼975 Ma and ∼930 Ma. The oldest ages (975 + 8.2/−8.3 Ma and 966 + 8.7/−8.6 Ma) overlap with previous reported Rb–Sr ages of the emplacement of the pegmatites (∼965 Ma) and are interpreted to reflect the crystallisation of the Nb–Ta mineralisation. The youngest ages (951 ± 15 Ma to 936 ± 14 Ma) are apparently related to variable degrees of resetting by (metasomatic) post-crystallisation processes. The resetting could either be due to recrystallisation of early Nb–Ta minerals or due to the disturbance of the U–Pb isotopic signature of the Nb–Ta minerals. The 40Ar– 39Ar spectra of muscovite samples associated with different steps in the paragenesis of the pegmatites show a spread of apparent ages between ∼940 Ma and ∼560 Ma that reflect Late Neoproterozoic tectonothermal events. One plateau age of 592.2 ± 0.8 Ma is interpreted to reflect far-field effects of the East African orogeny on the Karagwe–Ankole Belt.

Significance of zircon trace element geochemistry, the Shihu gold deposit, western Hebei Province, North China

The Shihu gold deposit is characterized by gold-bearing quartz-polymetallic sulfides and quartz veins. Both Mapeng granitoids batholith and intermediate-basic dikes intruded the metamorphic basement rocks, and are spatially associated with gold mineralization. Trace element abundances in zircons from the Shihu gold deposit, determined by laser-ablation microprobe ICPMS analysis, are sensitive to source rock type and crystallization environment. Concentrations of 21 trace elements were determined for zircons from granitoid rocks, diorites, quartz diorite porphyrites and gold-bearing quartz veins revealed some elemental characteristics and chondrite-normalized trace element patterns from different samples. There were no distinctive differences in REE concentrations of zircons from plutonic rocks and quartz veins, indicating that they probably had the same origin. Relatively flat chondrite-normalized REE patterns with (Yb/Sm) N ratios less than 60 characterized zircons from quartz diorite porphyrites and quartz veins. The highest Nb/Ta ratios were found in zircons from quartz diorite porphyrites, whereas the lowest ratios were found in quartz vein zircons. The Nb/Ta ratios were broadly correlated with HREE+Y contents, and had weak positive correlations with the depth of the Eu negative anomalies. High values U up to 0.4% and Th up to 0.1%, as well as positive correlations with REE+Y characterized zircons from quartz vein. The lowest Th/U ratios of zircons present in quartz veins reflected the relatively high concentration of U in hydrothermal fluid, and high Pb concentrations only typified quartz vein grains relatively enriched in U and Th. Zircons from quartz diorite porphyrites showed the most pronounced Ce anomalies, whereas weak Ce anomalies were typical of zircons from quartz veins, in which Eu/Eu* of zircons had a broadly negative correlation with Ce/Ce*. Trace element geochemistry of zircons from mineralized quartz veins and plutonic rocks confirmed that the sources of ore-forming materials were from Precambrian host rocks. Our results significantly enhanced the usefulness of zircon in crustal studies of the North China craton and as an indicator mineral in mineral exploration of the Taihang region.

Structural Control on Banded Iron Formation (BIF) and Gold Mineralization at Abu Marawat Area, Central Eastern Desert, Egypt

Abu Marawat area is considered as a promising site for exploration of gold mineralization, where many ancient gold mines, hydrothermal alteration zones, and intervening quartz veins are present. This study is a field-based structural analysis that aims at revealing the relationship between mineral occurrences and local structural setting. The area is a part of a back-arc volcanosedimentary sequence associated with banded iron formation (BIF) that has undergone extensive ductile and brittle deformation history. This multiple deformation is manifested by four phases. D1 and D2 are the product of compressional stresses and are expressed by F1, F2 and F3 folds in low-grade regionally metamorphosed rocks. D1 was a progressive deformational phase started with F1 folds, which in a later stage were overprinted by F2 folds. It resulted from NW-SE-oriented pure shear and is associated with imbricate thrust stacks, which control the locations of listwanite bearing gold. NE-SW-oriented compressive stress during D2 is displayed by F3-slip folds at the early stage followed by N-S- to NW-trending dip-slip normal faults and related shear zones. Mineralized quartz veins (MQV) were developed post-D2 and pre-D3. Folding and refolding in addition to thrust movement play a significant role in shortening and thickening of the iron formation bands located at the summit of Gebel Abu Marawat. D3 and D4 are expressed by brittle deformation. D3 is displayed by conjugate shear planes represented by sinistral-NW-oriented and dextral-NE–oriented strike-slip faults that led to the dislocation and redistribution of gold mineraliztion associated with both MQV and listwanite. Barren quartz veins trending E-W were also developed along gash fractures formed during this phase of deformation.

Errata: 40Ar/39Ar ages of granitoids from the Truong Son fold belt and Kontum massif in Laos

Geochemical and geochronological studies were conducted on granitoids in central-southern Laos. S-type ilmenite-series granitoids associated with Sn mineralization are distributed in the Truong Son fold belt of central Laos. Plateau ages determined by 40Ar/39Ar incremental heating method for biotite and muscovite of the granitoids range from 253 Ma to 247 Ma and from 244 Ma to 199 Ma in the southwest and northeast of the fold belt, respectively. During this period, the fractionated granites in the early stage are related to mineralization of medium-large Sn deposits. The granitoids of the Truong Son fold belt formed in the Indosinian orogeny are contemporaneous with the Late Permian-Early Jurassic Sn-bearing granites in the Southeast Asian Sn Belt. In contrast, I-type magnetite-series granitoids are distributed in the Kontum massif of southeasternmost Laos. Plateau ages for biotite and previously reported mineral ages of the magnetite-series granitoids range widely from 414 Ma to 252 Ma. The Carboniferous-Permian granitoids represents adakitic characteristics and the Permian adakitic granodiorites are associated with Cu ± Au mineralized quartz veins.

40Ar/39Ar ages of granitoids from the Truong Son fold belt and Kontum massif in Laos

Geochemical and geochronological studies were conducted on granitoids in central-southern Laos. S-type ilmenite-series granitoids associated with Sn mineralization are distributed in the Truong Son fold belt of central Laos. Plateau ages determined by 40Ar/39Ar incremental heating method for biotite and muscovite of the granitoids range from 253 Ma to 247 Ma and from 244 Ma to 199 Ma in the southwest and northeast of the fold belt, respectively. During this period, the fractionated granites in the early stage are related to mineralization of medium-large Sn deposits. The granitoids of the Truong Son fold belt formed in the Indosinian orogeny are contemporaneous with the Late Permian-Early Jurassic Sn-bearing granites in the Southeast Asian Sn Belt. In contrast, I-type magnetite-series granitoids are distributed in the Kontum massif of southeasternmost Laos. Plateau ages for biotite and previously reported mineral ages of the magnetite-series granitoids range widely from 414 Ma to 252 Ma. The Carboniferous-Permian granitoids represents adakitic characteristics and the Permian adakitic granodiorites are associated with Cu ± Au mineralized quartz veins.

Listvenite–lode association at the Barramiya gold mine, Eastern Desert, Egypt

Several occurrences of gold-bearing quartz veins are situated along the east–northeast-trending Barramiya–Um Salatit ophiolitic belt in the central Eastern Desert of Egypt. In the Barramiya mine, gold mineralization within carbonaceous, listvenized serpentinite and adjacent to post-tectonic granite stocks points toward a significant role of listvenitization in the ore genesis. The mineralization is related to quartz and quartz–carbonate lodes in silicified/carbonatized wallrocks. Ore minerals, disseminated in the quartz veins and adjacent wallrocks are mainly arsenopyrite, pyrite and trace amounts of chalcopyrite, sphalerite, tetrahedrite, pyrrhotite, galena, gersdorffite and gold. Partial to complete replacement of arsenopyrite by pyrite and/or marcasite is common. Other secondary phases include covellite and goethite. Native gold and gold–silver alloy occur as tiny grains along micro-fractures in the quartz veins. However, the bulk mineralization can be attributed to auriferous arsenopyrite and arsenic-bearing pyrite (with hundreds of ppms of refractory Au), as evident by electron microprobe and LA-ICP-MS analyses. The mineralized quartz veins are characterized by abundant carbonic (CO 2 ± CH 4 ± H 2O) and aqueous-carbonic (H 2O–NaCl–CO 2 ± CH 4) inclusions along intragranular trails, whereas aqueous inclusions (H 2O–NaCl ± CO 2) are common in secondary sites. Based on the fluid inclusions data combined with thermometry of the auriferous arsenopyrite, the pressure–temperature conditions of the Barramiya gold mineralization range from 1.3 to 2.4 kbar at 325–370 °C, consistent with mesothermal conditions. Based on the measured δ 34S values of pyrite and arsenopyrite intimately associated with gold, the calculated δ 34S Σs values suggest that circulating magmatic, dilute aqueous-carbonic fluids leached gold and isotopically light sulfur from the ophiolitic sequence. As the ore fluids infiltrated into the sheared listvenite rocks, a sharp decrease in the fluid fO 2 via interaction with the carbonaceous wallrocks triggered gold deposition in structurally favorable sites.

Laser probe 40Ar/39Ar dating for quartz from auriferous quartz veins in the Shihu gold deposit, western Hebei Province, North China

The Shihu gold deposit is situated in the central shear zone of the Meso-Cenozoic Fuping metamorphic core complex in the middle-northern part of the Taihang Mountains. This kind of gold deposits named after ‘Shihu type’ widely occurs in this region. Gold-bearing quartz veins are their most important industrial orebodies. Detailed laser 40Ar/39Ar geochronology of the mineralized quartz veins in the study area reveals details of its tectono-magmatic history. The 40Ar/39Ar ages for the quartz are between 134.4 Ma and 155.9 Ma with a mean age of 141±4 Ma on the isochron line and 139±7 Ma on the inverse isochron line, respectively, which is interpreted to be the best estimate of the crystalline age of the quartz veins. The gold mineralization was most likely related to an underplating event that took place in the northern part of the Taihang Mountain at ca. 140 Ma. The timing of gold mineralization in the area is similar to those observed in other major gold-producing provinces in the NCC. This episode is simultaneous with those in the eastern NCC, indicative of a widespread late Yanshanian metallogenic event that was a response either to the subduction of the Izanagi-Pacific plate beneath eastern China or to the removal of the Early Cretaceous lithosphere in the eastern NCC. The ore-forming material sources may be the Early Precambrian metamorphic basement, instead of intermediate-acid rock bodies and dykes.