Orogenic Au deposits with atypical metal association (Cu, Co, Ni): Insights from the Pohjanmaa Belt, western Finland
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The Laivakangas Au-Cu metallogenic area is characterised by orogenic Au deposits with both Au-only and with atypical (Au ± Cu, Co, Ni) metal associations. Here we study and compare four examples to better constrain the parameters controlling enrichment in base metals in addition to Au. We selected two typical Au-only deposits, the Laivakangas and the Huhta deposits and two orogenic Au deposits with atypical metal association, the Jouhineva Au-Cu-Co-Ag and the Kurula Au-Co deposits. All four deposits record multiple successive mineralisation events with local variations in their respective metal association. Two auriferous mineralisation events are identified, (1) a ubiquitous As-Au-(Co, Ni) event close to peak metamorphism (620–430 °C) where Au occurs either as invisible Au in arsenides or as inclusion in arsenopyrite; (2) a later Cu(-Au)-rich sulfide event on the retrograde path where Au locally occurs as free, native grains along with chalcopyrite. From S isotope studies of the sulfide and sulfarsenide minerals and relations between the deposits and surrounding rocks, we propose that the variation in metal association of the ore fluid is linked to the diversity of lithologies involved in metamorphic fluid production. Multi-event hydrothermal mineralisation and relatively reduced redox conditions appear critical to increase the Au endowment in a deposit and to introduce atypical metals. Results of this study provide a new comprehension of the variability of metal association in orogenic Au deposits of the Laivakangas Au-Cu metallogenic area and elsewhere.Keywords:
Base metal
Hypogene
Arsenopyrite
Nowadays, research on promoting the dissolution of chalcopyrite is important. As a natural symbiotic mineral of chalcopyrite, arsenopyrite will have an impact on the dissolution of chalcopyrite. This paper shows the influence of arsenopyrite on the dissolution of chalcopyrite in an acidic culture medium. The leaching results showed that adding arsenopyrite increased the leaching concentration of copper by 332 mg/L. The residues showed a decrease in sulfur through X-ray diffraction analysis (XRD) and an increase in dissolution degree through scanning electron microscope (SEM). Electrochemical experiments have shown that the rest potential of arsenopyrite is higher than that of chalcopyrite, so there is a galvanic interaction, and the impact on chalcopyrite is greater than that of arsenopyrite. The polarization curve also proves this. Under the interaction of galvanic couples, the reduction of S0 production and the enhancement of Cu2+ release can promote the dissolution of chalcopyrite. In addition, X-ray photoelectron spectrometer (XPS) analysis under the action of galvanic coupling indicates that more SO42− is generated on the surface of chalcopyrite, replacing Sn2−/S0, and SEM shows a stronger corrosion morphology. All results confirm that the electrochemical effect between arsenopyrite and chalcopyrite promotes the dissolution of chalcopyrite in the acidic culture medium.
Arsenopyrite
Galvanic cell
Hydrometallurgy
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The Numão gold deposit in the northern part of the Iberian Variscan belt is close to the regional first-order Vilariça-Manteigas fault system. The second and third-order structures controlled the gold mineralized bodies. These bodies consist of disseminated sulfides, arsenopyrite-quartz veins, and sulfides-scheelite-quartz veins. The scheelite occurrence predates the gold mineral assemblage and is lithologically controlled by host rocks with calc-silicate minerals. The ore mineral assemblage comprises scheelite, arsenopyrite, chalcopyrite, bismuth, and Au-Ag mineral phases, with minor pyrite, pyrrhotite, sphalerite, and galena. The hypogene gold minerals at Numão deposit are hosted by arsenopyrite, chalcopyrite, and silicate matrix in minor amounts. Chemically, gold fineness ranges from 663 to 841. The thermal evolution of the mineralization was estimated based on arsenopyrite (381° to 502 °C) and chlorite geothermometers (264° to 341 °C). The multi-element and principal component analysis information was used to better identify geochemical patterns and trends in hydrothermal alteration. The gold-metal association (Au-As-W-Bi) was found to be similar to other gold deposits in the European Variscan belt.
Arsenopyrite
Hypogene
Scheelite
Wolframite
Cassiterite
Gangue
Argillic alteration
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1961)
The authors have arrived at the following results upon studying the idaite-bearing ores from ten different localities in Japan. 1) Idaite have been found in several deposits in Japan. The specimens from Yakuwa Mine, Yamagata Prefecture, have been unequivocally identified by X-ray powder method. The number of sites yielding idaite in Japan is expected to increase with further study. 2) Idaite occurs in the secondary enrichment zone of copper deposit in the form of secondary decomposition product of chalcopyrite and bornite, except in two sites, where it is present as final crystallization product in copper deposits formed under low temperature and pressure. 3) Hypogene idaite is contained in small quantity in bornite, together with digenite and chalcopyrite. Supergene idaite forms lattices, lamellae and veinlets buried in chalcopyrite and bornite. In some cases, it forms films between chalcopyrite and supergene chalcocite, or replaces pyrite in company with supergene chalcocite, digenite and covellite. 4) The process of secondary enrichment and oxidation of chalcopyrite and bornite may be summarized as follows; chalcopyrite→supergene bornite hypogene bornite idaite chalcocite digenite→covellite cuprite tenorite native copper
Bornite
Chalcocite
Covellite
Supergene (geology)
Hypogene
Marcasite
Copper sulfide
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Inert
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Hypogene
Halite
Stockwork
Quartz monzonite
Porphyritic
Melt inclusions
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Brine
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The Sungun porphyry copper deposit (PCD) is located in EastAzarbaijan, in northwestern Iran. The felsic rocks occur as stocks and dykes ranging in composition fromquartzmonzodiorite through quartzmonzonite. The stocks are classified into porphyry stocks I and II. Porphyry stock II, hosting the copper ore, experienced an intense hydro-fracturing leading to the formation of stockwork-type veinlets and micro-veinlets of quartz, sulphides, carbonates and sulphates. Three distinct types of hydrothermal alteration and sulphide mineralization are recognized at Sungun (1) hypogene, (2) contactmetasomatic (skarn), and (3) supergene.Hypogene alteration is developed in four kinds: potassic, phyllic, propylitic and argillic. Three types of fluid inclusions are typically observed at Sungun: (1) vapour-rich, two-phase, (2) liquid-rich two-phase and (3)multi-phase.Halite is the principal solid phase inmultiphase inclusions. Primary multiphase inclusions (LVH type fluid inclusions) within the quartz crystals in quartz-sulphide and quartzmolybdenite veinlets (quartz associated with sulphideminerals) were selected formicro-thermometric analyses and considered to be suitable for pressure calculations and estimation of hydrothermal fluid density.Homogenization temperature, salinity, pressure and density were measured and calculated in forty-seven selected samples None of the variables could distinguish the potassic from phyllic alteration zones clearly. In the potassic alteration zone, the average of homogenization temperature is about 413oC, while in the phyllic alteration zone its average is about 375 oC. It was expected that the temperature in the potassic alteration zone would be higher than that in the phyllic zone, but the difference found was not very significant The fluid inclusion salinity within both alteration zones obviously relates to their homogenization temperature: the average salinity in the samples from the potassic zone is 46.3 (wt%NaCl equiv.), which is higher than that in the samples from the phyllic zone.. Based on the estimated depth of the potassic alteration domain, it is expected that the lithostatic pressure was higher than in the phyllic alteration zone.According to the fluid inclusion studies and pressure alculation, it is estimated that the average pressure for the potassic alteration zone was about 512 (bars) while the average pressure for phyllic zone was about 310 (bars). The average density of fluids in the samples from the potassic alteration zone is 1.124 (g/cm3), which is higher than that in the phyllic alteration zone (1.083 g/cm3).
Hypogene
Stockwork
Halite
Argillic alteration
Felsic
Porphyritic
Bornite
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Hypogene
Metasomatism
Halite
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This paper assesses chemical-mineralogical changes resulting from hydrothermal alteration associated with granite-hosted gold mineralization in southern Salamanca province, Spain. Within the mineralized veins, along planes of quartz growth, two types of fluid inclusions were observed. One type is rich in CH4 with minor CO2, the other is rich in H2O with CO2 (± CH4). These are interpreted as reflecting the immiscibility of an initial fluid rich in H2O-CH4 and some CO2. Inclusions with similar composition are seen at the silicification formed at the granite contact with host rocks. However, differences in P-T conditions and immiscibility of fluids are indicated by the microthermometric study and relation of the inclusions. These are consistent with the temperatures calculated from the arsenopyrite-pyrite geothermometer. Formation temperatures of 445 ± 15° C were deduced for the mineralization at the granite contact and temperatures not exceeding 386° C for the vein mineralization. Metasomatically altered granites are depleted in Na and K in comparison to fresh granites. A gain in CO2 has been measured in the altered rocks. No correlation was found between gold contents and any of the major or trace elements analyzed. δ34S arsenopyrite values suggest a variable source of sulfur. Calculated δDfluid values show significant variability (−66 to −37%0 SMOW), whereas δ18O fluid values show small variation (from +7.6 to +8.4%o SMOW). These values for the fluids are consistent with interaction between magmatic fluids and metamorphic rocks. Gold deposition in quartz veins could be explained by the loss of H2S during fluid immiscibility.
Arsenopyrite
δ34S
Wolframite
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