Ore Mineral Associations Research Articles

Geological Constraints on the Genesis of Jagpura Au-Cu Deposit NW India: Implications from Magnetite-Apatite Mineral Chemistry, Fluid Inclusion and Sulfur Isotope Study

The Jagpura Au-Cu deposit is situated within the Aravalli craton in the northwestern part of India. In the present work, petrography, mineral chemistry, fluid inclusion and sulfur isotopic compositions were used to study the Jagpura Au-Cu deposit. The ore mineral association of the deposit is arsenopyrite, loellingite, chalcopyrite, pyrrhotite and pyrite, along with native gold, magnetite and apatite. The gold fineness ranges from 914–937‰ (avg. 927‰). The presence of Au-Bi-Te phases, pyrite (>1 Co/Ni ratio), magnetite (≥1 Ni/Cr ratio, <1 Co/Ni ratio) and apatite (>1 F/Cl ratio) suggest the hydrothermal origin Au-Cu mineralization. A fluid inclusion study indicates the different episodes of fluid immiscibility with the homogenization temperatures varying between 120–258 °C and salinities range within the 8.86–28.15 wt% NaCl eq. The sulfur isotopic composition of sulfides varies from 8.98 to 14.58‰ (avg. 11.16‰). It is inferred that the variation in the sulfur isotopic compositions of sulfides is due to the cooling and dilution of the metalliferous fluid of mixed origin, derived from the basement meta-sedimentary rocks and the high saline basinal fluid. The iron oxide-copper-gold-apatite associations, structural control of mineralization, pervasive hydrothermal alteration, fluid salinity and sulfur isotope compositions indicate that the Jagpura Au-Cu deposit is similar to the iron oxide-copper-gold (IOCG)-iron oxide-apatite (IOA)types of deposits. Based on the ore geochemistry and the trace elements systematic of magnetite, the deposit is further classified as an IOCG-IOA type: IOCG-Co (reduced) subtype.

Geology, geochemistry and mineralogy of indium resources at Pitkäranta Mining District, Ladoga Karelia, Russia

This study is aimed at substantiating the prospects of the Pitkäranta Mining District for indium, assessing its resources and elucidating the mineralogical and geochemical features of the distribution of indium in ores. The skarn deposits genetically related to the Salmi anorthosite-rapakivi granite batholith of Early Riphean age were studied. The deposits were divided into two groups, depending on the degree of the greisen alteration of skarns: 1) Sn-Zn-Cu-Fe deposits (the Old Ore Field, Kitelä, Hopunlampi, Heposelkä and Kulismajoki) with little or no greisenization and 2) Be-Sn-Zn deposits with fluorite and magnetite (the New Ore Field, Lupikko, Uksa and Ristiniemi) with well-defined greisenization. Deposits of group 1 are located in the external zone of the Salmi Batholith with a steeply plunging roof (Kitelä), and deposits of group 2 commonly lie above LiF granite domes unexposed by erosion, and are spatially associated with the dykes of these granites and stockscheiders. The distribution of In and associated elements in skarn ores and major ore minerals (sphalerite, chalcopyrite, magnetite and cassiterite) were studied by ICP MS, LA ICP MS and electron probe microanalysis. A total of over 200 samples were analyzed. The results obtained show that In is closely associated geochemically with Zn and Cd, less closely with Cu, Bi and Te is not associated practically with Mo, W, As, Sn and Ag. The content of In in ores from various mines varies from <1 ppm to 0.33 %, and the average In content is 66 ppm. These variations are mainly due to different sphalerite contents in ores and the degree of manifestation of propylitic and greisen alteration of ore-bearing skarns. The main indium-bearing mineral is sphalerite (average In content is 1927 ppm). Other high-In minerals (stannoidite and mawsonite) are uncommon (<0.01 %) and, therefore, do not markedly affect the In balance in ore. The same applies with rare exception to roquesite, the only indium's mineral known here. In-richest (up to 1.5 %) sphalerites with 3–6 % Fe, up to 300–400 ppm Mn and 0.1–0.2 % Co were identified in ore mineral associations of propylitized skarns, which contain no or small amounts of chalcopyrite. Sphalerite-chalcopyrite association is characteristic of roquesite. It occurs mainly in low-tin aposkarn greisens with abundant arsenopyrite-löllingite-sphalerite mineralization. The formation of roquesite seems to have been supported by indium, which was released upon the decomposition of its solid solutions in chalcopyrite and sphalerite and upon the hydrothermal-temperature effect of the late intrusion phases of the Salmi Batholith, LiF granites, on ores. The indium distribution pattern in sphalerite, which is also rich Cd, Ag and Au, shows that roquesite-free propylitized skarns with Sn-Zn-Fe mineralization have the greatest potential for indium and these metals in the zones where the roof of the Salmi Batholith plunges steeply (40–50°) and LiF granites do not occur (Kitelä, Kulismajoki and Hopunlampi deposits). Predicted indium resources in the sphalerite ores of the Pitkäranta Mining District are estimated at 1585–2536 t (as shown by various calculations), cadmium resources at 18424 t, silver resources at 226 t and gold resources at 2 t.

MOLYBDENUM MINERALIZATION OF BALKA ZOLOTA Au-Mo DEPOSIT (MIDDLE DNIPRO, UKRAINIAN SHIELD)

The Balka Zolota gold-molybdenum deposit is located in the central part of the Solone ore field in the southern part of the Sura greenstone structure of the Middle Dnipro megablock of the Ukrainian Shield. Molybdenum mineralization is confined to the eastern flank of the deposit and is termed as the Solone occurrence. Main questions are address in this study of thе occurrence: i) the discovery and study of the deposit, ii) the composition of wall rock complex, iii) the structural position and localization of molybdenum mineralization, iv) the morphology of ore-bearing zones and ore bodies, (v) the composition of the ores, (vi) the ore mineral associations and the sequence of their formation, (vii) morphological features of molybdenite and (viii) a general analysis on the genesis of molybdenum mineralization. Mining of molybdenum ore of Balka Zolota deposit is possible in the case of output, first of all, gold ores. It can be realized more realistically after the start of mining operations within the Serhiivka gold-molybdenum deposit. Further geological exploration focusing on the Balka Zolota deposit is needed to assess the resources and reserves potential of the gold and molybdenum mineralization, especially on the eastern flank of the deposit.

An overview of some dating methods of mineralization

One of the most important achievements of applying radioactive isotope in the geological study is to determine the age of mineralization. Based on the synthesis, comparison, and evaluation of the popular isotopic systems in dating mineralization, the Rb - Sr, Re - Os, and Ar - Ar dating methods are discussed in this study. The results show that the Rb - Sr dating method is likely to be successfully applied to sulfide mineralization. If the mineralization contains little or no sulfide minerals, then the method is applied to other minerals in an ore mineral association. The Re - Os dating method has shown great success when it is applied to hydrothermal deposits, especially for the molybdenite - bearing vein deposits. The limitation of the method is that when the concentration of the Os element in molybdenite is low, it is difficult to establish the Re/Os ratio in a single mineral. The Ar - Ar isotope system for dating mineralization often uses mica or feldspar minerals, which are minerals with blocking temperatures ranging from 150÷350 0C, and later metamorphic periods have higher than 350 0C often overprinted these minerals. Generally, tectonic, magmatic, and metamorphic events, which occur after mineralization, can cause difficulty in determining the age of mineralization and collecting analysis samples.

Insights on Mineralogy and Chemistry of Fairview Gold Mine, Barberton Greenstone Belt, South Africa

DOI: 10.17014/ijog.8.1.73-99Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron microprobe analysis (EMPA) techniques were used to determine the mineralogy and chemistry of dominant sulphide and associated gold deposits at the Fairview Gold Mine of the Barberton Greenstone Belt (BGB). The rocks and ore mineral associations at the Fairview Mine indicate mesothermal (orogenic) conditions for the gold deposit. The whole rock chemistry of the Fairview Mine reflects calc-alkaline affinities, and felsic provenance is revealed by high SiO2, K2O, Al2O3, and TiO2, and low Fe2O3, MgO, Ni, and Cr contents. The mineral assemblages of the Fairview Mine rocks and ores indicate three phases of metamorphisms; regional prograde, thermal, and retrograde metamorphism, which caused zonation and heterogeneity of the ore minerals. The principal ore minerals in this deposit are pyrite, arsenopyrite, pyrrhotite, chalcopyrite, and rare sphalerite, as well as minor gold. The obtained results revealed that sulphides of the Fairview Mine are divided into two types (generations) varying in their morphology, association, and elemental composition. The old type 1 is porous, anhedral, heterogeneous, contains inclusions and rich in As, Ni, Co, and Au compared to type 2. Trace-element distribution and occurrence mode of gold at Fairview mine reflect two phases of gold mineralization. The first stage is associated with sulphides (mainly pyrite and arsenopyrite), whilst the second phase is free-lode hosted by silicates (mainly quartz).

Gold-tourmaline mineralization in carbonaceous shales of the Kumak deposit (South Ural)

The article discusses the geological structure of the Kumak gold ore deposit, confined to the Early Carboniferous Anikhov graben of the East Ural uplift. It is composed of sericite-quartz-carbon and quartz-carbon-tourmaline schists of the Bredy Formation (C1bd). Black shale deposits are carbonaceous type and belong to terrigenous-carbonaceous and silicon-carbonaceous formations. Among the main minerals in shales are noted: quartz, sericite, carbonates, sulfides, tourmaline and carbonaceous matter. The field is characterized by a wide variety of gold ore mineralization. Rich ore zones are noted at the intersection of the East Anikhov faults oriented in the north–south direction and the branching tensile fractures in the north-north-east and north-west directions. In the ore mineral association prevailing here, finely dispersed gold is associated with small crystals of pyrite, arsenopyrite and concentrates nearby abundant tourmaline areas. The fine intergrowth of tourmaline and gold indicates the synchronism of their formation and allows us to distinguish a quartz-tourmaline gold ore formation within the Kumak deposit, comparable with a number of objects in East Transbaikal and Tuva. The most probable source of tourmaline mineralization in sericite-quartz-carbon schists could be metamorphically transformed boron-containing marine sediments saturated with clay particles and Corg. The microprobe study of gold grains taken from carbonaceous shales and weathering crusts made it possible to attribute them to the high-grade (919–1000) type, which is the leading one in the gold ore mineralization of the considered deposit. It is established that in the zone of hypergenesis, gold grains are not homogeneous. Here, secondary redeposition of gold takes place in the form of small spongy high-grade aggregates, as well as the formation of a rim on some grains with clear signs of refinement and purification from impurity elements. The geological works were carried out under State Contract No. 0246-2019-0078. The analyses were supported by the regional Grant for Science and R&amp;D, Agreement No. 23 dated August 18, 2019. The studies into gold composition were executed in the framework of state-financed scientific topic No. AAAA-A19-119072390050-9. The authors express thanks to S. A. Yagudin for the analysis implementation and to E. O. Kalistratov for the help in description of polished sections. Further thanks are extended to R. S. Kisil and V. S. Panteleev for the help in the field works.

Brockite in wallrock metasomatites of the Safyanovskoe copper-sulphide deposit (Middle Urals)

The relevance of the work is due to the need to study ore copper-sulphide deposits in the Urals. Purpose of the work: description of accessory brockite in metasomatites of the Safyanovskoe copper-sulphide deposit. Research methodology: the chemical composition of minerals was determined using the Jeol JSM-6390LV scanning electron microscope with an INCA Energy 450 X-Max 80 energy dispersive attachment from Oxford Instruments (Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences, Ekaterinburg). Results and conclusions. For the first time for the Safyanovskoe copper-sulphide deposit (Middle Urals), an aqueous rare earth phosphate of calcium and thorium, brockite, has been determined; it belongs to the group of rhabdophane (Ca,Th,REE)[PO]4 ∙ _H2 O. The mineral is rare for the Urals and was described earlier in granite pegmatites of the Middle and South Urals, as well as in dikes of metaplagiogranites of the Bazhenov ophiolite complex. Brockite was found in the rocky metasomatites of the Safyanovskoe copper-sulphide deposit after crystalline lithoclastic tuff (tuffaceous sandstone) of acid composition. The main mass of the rock consists of quartz, kaolinite (sericite), carbonates (dolomite, Fe-magnesite) with rare inclusions of pyrite. Brockite is found in the dolomite-quartz matrix of the sample in intergrowths with REE-goyazite – strontium aluminophosphate. It is assumed for the Safyanovskoe copper-sulphide deposit that an alumina association with an ore mineral association and rare earth minerals, in particular, REE-alumophosphates and phosphates, will form closely at the same time as the temperature drops and the redox conditions of the mineral formation environment change.

УСЛОВИЯ ФОРМИРОВАНИЯ И ВОЗРАСТ РЕДКОМЕТАЛЬНОГО ОРУДЕНЕНИЯ КУКУЛЬБЕЙСКОГО РУДНОГО РАЙОНА (ВОСТОЧНОЕ ЗАБАЙКАЛЬЕ)

Актуальность. Определение современными методами условий формирования редкометального оруденения на примере типовых месторождений Кукульбейского рудного района, в совокупности с изотопно-геохронологическими исследованиями являются ключевыми аспектами в решении одной из важнейших фундаментальных проблем рудной геологии и геохимии – выяснение условий, источников, механизмов и последовательности формирования рудных концентраций металлов в различных геологических обстановках. Цель: получение новых данных о физико-химических условиях формирования редкометального оруденения, а также установление возраста оруденения. Объекты: Букукинское, Белухинское и Антоновогорское месторождения. Методы. Флюидные включения были изучены в прозрачно-полированных пластинках методами крио-термометрии и Рамановской спектроскопии. Крио-термометрические исследования выполнялись в микротермокамере THMSG-600 фирмы Linkam. Образцы последовательно охлаждались до температуры –190 °С и нагревались до полной гомогенизации вещества включений. В процессе наблюдения замерялись температуры эвтектики, плавления льда и гомогенизации. Температуры плавления льда для двухфазных и температуры плавления кристалликов соли для трехфазных флюидных включений позволили установить концентрацию солей в пересчете на NaCl-эквивалент. Состав газовой и минеральной фаз флюидных включений изучался методом Рамановской спектроскопии – спектрометр Ramanor U-1000 и детектор Horiba DU420E-OE-323 фирмы Jobin Yvon, лазер Millennia Pro фирмы Spectra-Physics; Confocal Raman Microscope alpha 300R фирмы WITec. 40Ar/39Ar метод изотопно-геохронологического датирования по методике ступенчатого прогрева применялся для установления абсолютного возраста оруденения. Результаты. Рудные минеральные ассоциации рассматриваемых месторождений сформировались при участии хлоридных высокотемпературных флюидов, насыщенных углекислотой. Повсеместно рудоносные флюиды содержат восстановленные газы, из которых превалирует метан, а сероводород, азот и водород находятся в подчиненном количестве. В рамках кристаллизации отдельных минеральных ассоциаций прослеживается эволюция рудоносного флюида: снижение температуры и концентрации растворов. Катионный состав растворов также изменился от существенно натрового до существенно калиевого. Для объектов исследования получены умеренные оценки глубин: 6–8 км для Букукинского, 4,5–6 км для Белухинского и 3–3,5 км для Антоновогорского месторождения. Возраст редкометального оруденения составляет на Антоновогорском месторождении – 146,7±1,7 млн лет (мусковит из кварц-мусковитовых грейзенов с вольфрамитом); на Букукинском месторождении – 135±2,8 млн лет (серицит из кварц-вольфрамитовых жил); на Белухинском месторождении – 155,6±1,8млн лет (серицит из кварц-вольфрамитовых жил). Выводы. Редкометальная минерализация Кукульбейского рудного района, на примере типовых месторождений, формировалась из высокотемпературных гидротермальных насыщенных восстановленными газами (метан, сероводород, азот и водород) углекислотно-водно-хлоридных растворов магматического происхождения. По глубине становления рассматриваемые рудоносные системы можно отнести к гипабиссальным от 8 до 3 км. Проведенные изотопно-геохронологические исследования показали, что в пределах Кукульбейского рудного района формирования вольфрамового оруденения происходило в позднеюрское-раннемеловое время, основная часть которого связана со становлением магматических пород кукульбейского комплекса, не исключено что позднеюрское оруденение связано со становлением магматических пород шахтаминского комплекса.

Mineral types of gold deposits and regularities of their distribution in south-eastern part of the East Sayan

Based on the mineral composition, gold deposits of the south-eastern part of the East Sayan were typed. The most informative classification criterion for typing is the composition of the gold producing ore mineral associations which are specific to each of the selected types. Whereas using other criteria different structural, composition or genetic characteristics of deposits will necessarily overlap. Eight mineral types of deposits, characterizing the main gold-bearing ore mineral associations were identified: gold-polysulfide, gold-quartz, gold-telluride, gold-tetradymite, gold-stibnite, gold-bismuth-sulfosalt, gold-pyrrhotite and gold-fahlore types. Regional metallogenic divisions such as structural metallogenic zones, differ somewhat in the characteristics of mineralization. Thus, within the Bokson-Garganskaya metallogenic zone, gold-quartz, gold-polysulfide and gold-pyrrhotite types dominate, although gold-telluride and gold-bismuth-sulfosalt types are common in the western part of this zone. In the Ilchir zone, gold-fahlore type deposits are developed. Khamsarinskaya zone is characterized by gold-tetradimite and gold-antimonite deposits existence. It has been established that the mineral types of deposits formation depend on the enclosing rock complexes composition. Thus, gold-quartz, gold-polysulfide and gold-pyrrhotite types are formed in association with the ophiolites and rocks of the Archean crystalline basement. At the deposits associated with granitoid massifs, minerals of metalloids, such as bismuth-bearing sulfosalts, stibnite, tetradimite and tellurides, take a leading role in ores. In the carbonate strata, a gold-fahlore type is formed. The proposed classification allows typing all known gold deposits in the south-eastern part of the East Sayan and can be applied to adjacent regions.

First insights on the molybdenum-copper Bled M'Dena complex (Eglab massif, Algeria)

Molybdenum-Copper showings in the Eglab massif (eastern part of the Reguibat rise of Algeria), are found in quartz-monzodiorite and granodiorite of the Bled M'Dena complex, a Paleoproterozoic circular structure of ∼5 km in diameter, comprising volcanic and intrusive suites. The latter consist of quartz-diorite, quartz-monzodiorite and granodiorite with a metaluminous normative composition. They display an “adakitic character” with moderate light rare-earth element (LREE) enrichment, minor Eu anomalies, high Sr/Y ratio and low Yb concentration, suggestive of a hydrous, arc magma of volcanic-arc affinity. The mineralization occurs mostly in quartz + molybdenite + chalcopyrite stockwork veins marked by widespread propylitic alteration along the selvages. Molybdenite and chalcopyrite are commonly associated with calcite, which precipitated at relatively late stages of the hydrothermal alteration. Fluid inclusions related to the mineralization stage, range from aqueous to aqueous-carbonic to solid bearing. The latter inclusions have the highest homogenization temperature (up to ∼400 °C), are salt saturated, and commonly contain molybdenite and/or chalcopyrite crystals. The petrology and geochemistry of the host rocks, the style of the hydrothermal alteration, the ore mineral associations, and the characteristics of the fluid inclusions, are all coherent in indicating that the Bled M'Dena represents a Paleoproterozoic porphyry style Mo mineralization, which is far unreported in the African continent.

The effect of Alpine metamorphism on an oceanic Cu-Fe sulfide ore: the Herin deposit, Western Alps, Italy

Herin mine (Champdepraz, Aosta, Italy), located in Aosta Valley, approximately between 1600 and 1800 m a.s.l., was exploited for at least 250 years for its Cu-Fe sulfide ore. The deposit host rocks belong to the metaophiolitic Zermatt-Saas unit, the eclogitic lower portion of the Piedmont Western Alpine Nappe. The ore mineral association mainly comprises pyrite and chalcopyrite, along with other sulfides such as pyrrhotite, sphalerite, cubanite and oxides (magnetite, rutile, ilmenite). The deposit consists in lenticular massive bodies and thin layers hosted in various greenschist-facies metamorphosed lithotypes. New data on geometric features, mineralogy, mineral chemistry, petrography, minerography were collected and compared with the existing models for massive-sulfide mineral deposits. We suggest a hydrothermal-volcanogenic primary origin of the mineralization with primary characters largely obliterated by subsequent metamorphic history. On the basis of previous works and our results, we identified two parameters as driving criteria for a comprehension of the multistage process that led to the present configuration of the ore: (a) textural characters of pyrite and (b) distribution of selected trace elements (Co, Ni, As) in sulfides. Spot analyses and atomic maps obtained by electron microprobe provided an integration of these two sets of data. Trace elements, in fact, show a zoned distribution, in particular in pyrite, that can be related to specific textural styles. We selected Co as a useful trace element, due to its high concentration and wide range in pyrite (270-22200 ppm). Besides we determined a critical concentration value for cobalt of 3160 ppm, as discriminating between two generations of pyrite. This led to outline a series of dissolution and crystallization events, that describes the metamorphic history of the sulfide ore.

Characteristics and genesis of Gangdese porphyry copper deposits in the southern Tibetan Plateau: Preliminary geochemical and geochronological results

Porphyry-type Cu (Mo, Au) deposits have been discovered along the Gangdese magmatic arc in the southern Tibetan Plateau. Extensive field investigations and systematic studies of geochemistry, S–Pb isotopic tracing, together with Re–Os and 40Ar/ 39Ar isotopic dating indicate that the mineralisation of the copper belt is genetically related to emplacement of late orogenic granitic porphyries during the post-collisional crustal relaxation period of the Late Himalayan epoch. These porphyries are petrochemically K-enriched and belong to shoshonitic to high-K calc-alkaline series. They display enrichment of large ion lithophile elements (LILE) Rb, K, U, Th, Sr, Pb and depletion of high field strength elements (HFSE) Nb, Ta, Ti and the heavy rare earth elements (HREE) and Y without Eu anomalies. These characteristics demonstrate that subduction played a dominant role in their petrogenesis and residual garnet was left in the magma sources. Pb isotope data show a linear correlation in the plumbotectonic framework diagram ranging from orogenic Pb in the eastern segment of the copper belt to mantle Pb in the western segment. These constitute a mixing line of the Indian Oceanic MORB with Indian Oceanic sediments and suggest that the porphyry magmas were dominantly derived from partial melting of subducted oceanic crusts mixed with a minor quantity of sediments and mantle wedge components. The Gangdese porphyry copper polymetallic belt has alteration characteristics and zonation typical of porphyry-type copper deposits which include potassic alteration (K-feldspathisation and biotitisation), silicification, sericitisation, and propylitisation. Mineralisation mainly occurs in strongly altered granitic cataclasite at the exo-contact with veinlet-disseminated textures. The porphyries themselves are weakly mineralised with disseminated pyrite and chalcopyrite. The copper deposits contain simple ore mineral associations consisting of chalcopyrite, pyrite, bornite, molybdenite, sphalerite and oxidised minerals of malachite, covellite and molybdite. During supergene oxidation, primary ores underwent secondary enrichment to form economic orebodies with Cu grade ranging from 1% to 5%. Ore sulphides of the copper belt display S and Pb isotopic compositions identical to the ore-bearing porphyries. Their δ 34S values vary between − 3.8‰ and + 2.4‰ and are typical of mantle sulphur. The 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb ratios vary in the ranges: 18.106 to 18.752, 15.501 to 15.638, and 37.394 to 39.058, respectively, and yielded radiogenetic lead-enriched signatures. Twelve molybdenite samples from the copper belt yielded isochron ages of 14.76 ± 0.22 Ma and 13.99 ± 0.16 Ma for the Nanmu and Chongjiang deposits and model ages of 13.5 to 13.6 Ma for the Lakang'e deposit. Meanwhile, 40Ar/ 39Ar isotopic dating of two biotite phenocrysts from the Chongjiang and Lakang'e deposits give plateau ages of 13.5 ± 1.0 Ma and 13.42 ±0.10 Ma, respectively. During the geodynamic evolution of the Gangdese collision-orogenic belt, intrusion of the ore-bearing porphyries took place just before the rapid uplift and E–W extension of the southern Plateau. And the ore-forming process may have occurred simultaneously with the uplift and extension (14 ± 0.1 Ma).

Genesis of the Shear Zone-related Gold Vein Mineralization of the Lega Dembi Gold Deposit, Adola Gold Field, Southern Ethiopia

The Lega Dembi Primary Gold Deposit in southern Ethiopia is related to the shear zone-hosted vein in the Neoproterozoic metamorphosed volcano-sedimentary succession of greenschist- to amphibolite-facies metamorphism. The rocks consists of a sequence of biotite-feldspar-quartz schists, carbonaceous mica-schists, amphibolites and basic to ultrabasic rocks. This unit is separated from a foot wall biotite gneiss by a major shear zone. The ore bodies are hosted in the volcano-sedimentary sequence and consist of swarms of quartz veins, lenses, and stockworks that propagated along mesoscale ductile to brittle-ductile shear zones. The mineralization is defined by a complex paragenesis of gold in association with Cu-Pb-Zn-Fe sulphides, tellurides and sulphosalts. The presence of Ni-bearing minerals in amphibolites of the host sequence, together with the ore mineral association, suggests an origin related to mafic volcanism.

Estudo microtermométrico dos fluidos hidrotermais relacionados com a mineralização aurífera de Montes Áureos, Nw do Maranhão

The Montes Aureos gold mineralization occurred within the Tentugal Shear Zone which is an approximately 100 km long, 15 km wide NW-SE- trending structure that marks the south-southwest boundary of the Sao Luis craton and corresponds to the most deformed rocks of the Gurupi mobile belt. The ore bodies are hosted by the Paleoproterozoic metavolcanic and metasedimentary rocks of the Gurupi Group, which have been metamorphosed under greenschist to low amphibolite facies conditions. A superimposed hydrothermal event generated a system of vein and veinlets both concordant and discordant to the rock foliation. Gold occurs in up to 2 cm thick, late-tectonic quartz + carbonate veins or veinlets associated with arsenopyrite, pyrite and minor chalcopyrite. The spatial distribution of the mineralized zones defines lenticular to tabular bodies with gold contents less than 2 ppm. Textural features and time relationships between hydrothermal and metamorphic assemblages indicate that the mineralization followed the metamorphic peak and that gold occurs in at least two different forms: 1) isolated grains deposited simultaneously with sulfides; and 2) in microfractures in arsenopyrite and pyrite. Gold had been most likely transported by the sulphur complex [Au(HS) 2 - ] in an aqueous-carbonic, low salinity fluid (2 - 8 wt % NaCl equiv.) at temperatures £ 450 o C. Deposition took place at a temperature range from 260 to 350 o C. For the prevailing value of 300 o C, confirmed by the chlorite geothermometer, pressure estimates fall between 1.3 and 2.8 Kbar, corresponding to depths of 4-8.5 km. Fluids related to the systems CO 2 ±CH 4 , H 2 O-CO 2 - NaCl±CH 4 ±MgCl and/or FeCl 2 and H 2 O-NaCl±MgCl and/or FeCl 2 circulated through the Montes Aureos rocks. The aqueouscarbonic fluids are considered most likely to be products of devolatization reactions of carbon-bearing sedimentary rocks at temperature probably above 500 o C. Initially homogeneous, these fluids have undergone immiscibility and were then trapped in fluid inclusions with different H 2 O/CO 2 ratios. As the metamorphic thermal regime decreased and carbonate precipitated, the aqueous-carbonic fluids became progressively impoverished in CO 2 and less saline. Mixture with cooler and low salinity surface waters may have occurred at the final stages of the Montes Aureos hydrothermal system. The geotectonic setting, the structural control of the mineralization by shear zone, the hydrothermal alteration features, the time relations between the metamorphic peak and hydrothermal alteration, the gold ore mineral association and the physico-chemical characteristics of the mineralizing fluids allow the Montes Aureos gold deposit to be classified in the orogenic category, as many others that occur in deformed and metamorphosed mid-crustal blocks.

ESTUDO MICROTERMOMÉTRICO DOS FLUIDOS HIDROTERMAIS RELACIONADOS COM A MINERALIZAÇÃO AURÍFERA DE MONTES ÁUREOS, NW DO MARANHÃO

The Montes Áureos gold mineralization occurred within the Tentugal Shear Zone which is an approximately 100 km long, 15 km wide NW-SE- trending structure that marks the south-southwest boundary of the São Luís craton and corresponds to the most deformed rocks of the Gurupi mobile belt. The ore bodies are hosted by the Paleoproterozoic metavolcanic and metasedimentary rocks of the Gurupi Group, which have been metamorphosed under greenschist to low amphibolite facies conditions. A superimposed hydrothermal event generated a system of vein and veinlets both concordant and discordant to the rock foliation. Gold occurs in up to 2 cm thick, late-tectonic quartz + carbonate veins or veinlets associated with arsenopyrite, pyrite and minor chalcopyrite. The spatial distribution of the mineralized zones defines lenticular to tabular bodies with gold contents less than 2 ppm. Textural features and time relationships between hydrothermal and metamorphic assemblages indicate that the mineralization followed the metamorphic peak and that gold occurs in at least two different forms: 1) isolated grains deposited simultaneously with sulfides; and 2) in microfractures in arsenopyrite and pyrite. Gold had been most likely transported by the sulphur complex [Au(HS) 2 - ] in an aqueous-carbonic, low salinity fluid (2 - 8 wt % NaCl equiv.) at temperatures ≤450°C. Deposition took place at a temperature range from 260 to 350°C. For the prevailing value of 300°C, confirmed by the chlorite geothermometer, pressure estimates fall between 1.3 and 2.8 Kbar, corresponding to depths of 4-8.5 km. Fluids related to the systems CO 2 ±CH 4 , H 2 O-CO 2 - NaCl±CH 4 ±MgCl and/or FeCl 2 and H 2 O-NaCl±MgCl and/or FeCl 2 circulated through the Montes Áureos rocks. The aqueouscarbonic fluids are considered most likely to be products of devolatization reactions of carbon-bearing sedimentary rocks at temperature probably above 500°C. Initially homogeneous, these fluids have undergone immiscibility and were then trapped in fluid inclusions with different H 2 O/CO 2 ratios. As the metamorphic thermal regime decreased and carbonate precipitated, the aqueous-carbonic fluids became progressively impoverished in CO 2 and less saline. Mixture with cooler and low salinity surface waters may have occurred at the final stages of the Montes Áureos hydrothermal system. The geotectonic setting, the structural control of the mineralization by shear zone, the hydrothermal alteration features, the time relations between the metamorphic peak and hydrothermal alteration, the gold ore mineral association and the physico-chemical characteristics of the mineralizing fluids allow the Montes Áureos gold deposit to be classified in the orogenic category, as many others that occur in deformed and metamorphosed mid-crustal blocks.

Geochemistry and Ore Genesis of Probably Gold-Bearing Rocks, Okote and Moyale Areas, Southern Ethiopia

The Moyale gold occurrence is located in southern Ethiopia and has been for placer gold mining by locals. Some first geological, geochemical and drilling studies have been done recently by the Ethiopian Institute of Geological Surveys, yielding sufficient geological and drilling core data for a general overview of the area. The Moyale region lies within the Mozambique Belt that record multiple deforrnational, magmatie ,and metamorphic events (Condie 1982). This belt is considered to be part of the Pan-African province with an age between 500700 Ma and consisting of a region comprising chiefly of rejuvenated older sialic crust and a region of island-arc and oceanic rocks. Some workers used the term Pan-African to include ages of 1100 to 500 Ma (Berhe 1990). Berhe (1990) considered the Moyale region as a part of Arabo-Nubian shield and, based upon the rock assemblages, structural trends, and trace element data, he indicated the mafic-ultramafic rocks of Moyale area to be of ophiolitic nature. The Moyale area is part of the pan-African Mozambique Belt forming Eastern margin of the Tanzanian Craton. The rocks of the Moyale region are comprised of hornblende (_+ biotite) gneiss, ultramafics, amphibolite and granodiorite in the west, feldspar-biotite gneiss, potassic feldspar-biotite gneiss, potassic feldspar-quartz-biotite schist, calcsilicate gneiss, granite gneiss, chlorite-amphibole schist and basic rocks and schists in the eastern, and tonalite, amphibole, metagabbro and ultramafics in the central part of the region. Regional heavy concentrate exploration for gold indicated the occurrence of gold; in addition, extensive placer mining was carried out mostly in weathered quartz vein and host rock in the mineralized zone. Regional exploration indicates gold and base metal type mineralization to occur dominantly in quartz vein and subordinately in their altered wall rocks. The gold content of quartz vein ranges from 0.1 to 215 ppm. The host rock contains up to 1.3 ppm in Chamuk, 5.7 ppm in the Haramsam area, and up to 1.0 ppm in the Hasemte area. The ore minerals present in quartz veins include gold, galena, sphalerite, pyrite, chalcopyrite and pyrrhotite in major part of the area. Similar ore mineral associations have been observed in the host rock in the Chamuk and Haramsam areas. The mineralization in the above areas is controlled by submeridional and East- West trending brittle-ductile shear that has affected the host rocks, granodiorite in Chamuk, amphibolite and quartzfeldspar-mica schist in Haramsam and metagabbro in Hasemte areas. Moreover, the mineralization is affected by intensive alteration We have used geological, petrological, and geochemical techniques to study the rocks of the Moyale and Okote areas in southern Ethiopia to get detailed data on their composition and formation and to establish the relation of gold mineralization with shear zone, host rock and alteration type. Aside from detailed petrographical studies using the optical and electron microscopes, our studies include major element analyses by XRF and trace element analyse by neutron activation analysis, XRF, and ICP-MS, as well as Rb-Sr and Sm-Nd isotope analyses to provide constraints on the provenance of the host rocks.

Nature, origin and evolution of the granitoid-hosted early Proterozoic copper-molybdenum mineralization at Malanjkhand, Central India - a reply

At Malanjkhand, Central India, lode-type copper (-molybdenum) mineralization occurs within calcalkaline tonalite-granodiorite plutonic rocks of early Proterozoic age. The bulk of the mineralization occurs in sheeted quartz-sulfide veins, and K-silicate alteration assemblages, defined by alkali feldspar (K-feldspar ≫ albite) + dusty hematite in feldspar ± biotite ± muscovite, are prominent within the ore zone and the adjacent host rock. Weak propylitic alteration, defined by albite + biotite + epidote/zoisite, surrounds the K-silicate alteration zone. The mineralized zone is approximately 2 km in strike length, has a maximum thickness of 200 m and dips 65°–75°, along which low-grade mineralization has been traced up to a depth of about 1 km. The ore reserve has been conservatively estimated to be 92 million tonnes with an average Cu-content of 1.30%. Supergene oxidation, accompanied by limited copper enrichment, is observed down to a depth of 100m or more from the surface. Primary ores consist essentially of chalcopyrite and pyrite with minor magnetite and molybdenite. δ34S (‰) values in pyrite and chalcopyrite (−0.38 to +2.90) fall within the range characteristic of granitoid-hosted copper deposits. δ18O (‰) values for vein quartz (+ 6.99 to +8.80) suggest exclusive involvement of juvenile water. Annealed fabrics are common in the ore. The sequence of events that led to the present state of hypogene mineralization is suggested to be as follows: fracturing of the host rock, emplacement of barren vein quartz, pronounced wall-rock alteration accompanied by disseminated mineralization and the ultimate stage of intense silicification accompanied by copper mineralization. Fragments of vein quartz and altered wall rocks and striae in the ore suggest post-mineralization deformation. The recrystallization fabric, particularly in chalcopyrite and sphalerite, is a product of dynamic recrystallization associated with the post-mineralization shearing. The petrology of the host rocks, hydrothermal alteration assemblages, ore mineral associations, fluid inclusions and the sulfur and oxygen isotopes of ores are comparable to those in Phanerozoic (and reported Precambrian) porphyry-copper systems, and the Malanjkhand deposit has important implications for both metallogenic models for, and mineral exploration in, Precambrian terrains.

Gold mineralization in the West Hoggar shear zone, Algeria

The Amesmessa gold prospect is located along a vertical N-S-trending crustal-scale ductile shear zone; stretching lineations are subhorizontal. This major shear zone is a Late Pan African dextral strike-slip fault of the Pharusian Belt of the Tuareg Shield (Algeria). The Amesmessa shear zone is asymmetric: strong thermal and deformational gradients are present along its western border where biotitic ultramylonites are in contact with a rigid Archean complex (In Ouzzal block), whereas there is a progressive gradation, through mylonite then protomylonite, to the Proterozoic gneiss of the Eastern block which displays co-axial Pan African structures. The Amesmessa shear zone is characterized by the presence of a felsic dike complex emplaced during shearing, and forming the most important parent material for ultramylonites. Basic magmas and carbonatites also intruded within the shear zone. The gold-rich quartz veins are located within the ultramylonitic western part of the shear zone. These N-S-trending laminated quartz veins formed during the late increments of shearing (plastic/brittle transition), by repeated syntectonic hydraulic fracturing along zones of rheological contrast parallel to foliation. The ore mineral association (pyrite, galena, native gold, sphalerite) crystallized in the deformed quartz matrix along late shear planes. Undeformed E-W trending banded quartz veins are present in the mylonitic eastern part of the shear zone; their gold content is low and no native gold has been observed. A strong hydrothermal alteration resulted in the development (along the walls of the N-S gold-bearing quartz veins) of a 5-m-wide carbonate-sericite-albite-pyrite secondary mineral association which implies an important CO2 supply and moderate temperature conditions. There is no alteration halo around the E-W quartz veins. Ultramylonites, hydrothermally altered rocks and quartz veins display similar REE patterns characterized by strong LREE enrichments. Shear-related fluids could be likely parental fluids for the Amesmessa gold mineralization and the associated hydrothermal alteration. Hydrothermal fluids were drawn into dilation zones and filled opening fractures along the main planar discontinuity of the most deformed rocks. The supply of CO2 may come from a deep-seated source as suggested by the presence of carbonatite dikes in the shear zones and the existence of CO2-H2O-rich fluid inclusions in quartz. The location of the gold-bearing quartz veins in the western part of the shear zone can be explained by the presence of strong thermal and rheological gradients.

Quelques précisions sur les caractères minéralogiques et chimiques des pyroxénolites à grenat du massif des Beni Bousera (Maroc) et sur leur signification pétrogénétique

The primary liquid of partial fusion, in the Beni Bousera ultra-basic Massif (Morocco), gives, by fractionnal cristallization, various assemblages that can be distinguished on the basis of ore mineral associations and silicate paragenesis. Residual liquids, of variable compositions, would have cristallized as garnet clinopyroxenites, with granulite facies primary assemblages (Ca Tschermak rich cpx + garnet + ilmenite) or eclogite facies primary assemblages (omphacitic cpx + garnet + rutile).