Carbon and Oxygen Isotope Compositions of Calcite and Rhodochrosite from Geothermal Exploratory Drills TH‐4 and TH‐6 near the Toyoha Deposit,Hokkaido, Japan

We studied calcite and rhodochrosite from exploratory drill cores (TH‐4 and TH‐6) near the Toyoha deposit, southwestern Hokkaido, Japan, from the aspect of stable isotope geochemistry, together with measuring the homogenization temperatures of fluid inclusions. The alteration observed in the drill cores is classified into four zones: ore mineralized zone, mixed‐layer minerals zone, kaolin minerals zone, and propylitic zone. Calcite is widespread in all the zones except for the kaolin minerals zone. The occurrence of rhodochrosite is restricted in the ore mineralized zone associated with Fe, Mn‐rich chlorite and sulfides, the mineral assemblage of which is basically equivalent to that in the Toyoha veins. The measured δ¹⁸O_SMOW and δ¹³C_PDB values of calcite scatter in the relatively narrow ranges from ?2 to 5‰ and from ?9 to ?5‰, respectively; those of rhodochrosite from 3 to 9‰ and from ?9 to ?5‰, excluding some data with large deviations. The variation of the isotopic compositions with temperature and depth could be explained by a mixing process between a heated surface meteoric water (100°C δ¹⁸O =?12‰, δ¹³C =?10‰) and a deep high temperature water (300°C, δ¹⁸O =?5‰, δ¹³C =?4‰). Boiling was less effective in isotopic fractionation than that of mixing. The plots of δ¹⁸O and δ¹³C indicate that the carbonates precipitated from H₂CO₃‐dominated fluids under the conditions of pH = 6–7 and T = 200–300°C. The sequential precipitation from calcite to rhodochrosite in a vein brought about the disequilibrium isotopic fractionation between the two minerals. The hydrothermal fluids circulated during the precipitation of carbonates in TH‐4 and TH‐6 are similar in origin to the ore‐forming fluids pertaining to the formation of veins in the Toyoha deposit.     

A chlorite solid solution geothermometer the Los Azufres (Mexico) geothermal system

Chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites, in the active geothermal system of Los Azufres (Mexico). Electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition. Correlation coefficients among chemical constituents were calculated. It is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content, and there is almost no correlation with the octahedral aluminium and magnesium content. A procedure is proposed to select end-members and substitution vectors, and to give a general formula for these chlorites.Their formation temperatures are estimated with great accuracy, combining results of microthermometric data on fluid inclusions from gangue minerals of chlorites (quartz, calcite), direct measurements in wells (Kuster equipment), and chemical geothermometers. Correlations between chlorite compositions, range and nature of site occupancy, and temperature are good. Formation temperatures of chlorites range from 130° C to 300° C. As no other thermodynamic parameter varies significantly in the studied field (composition of the host rocks, nature of the geothermal fluids, pressure, ...), these variations of site occupancy (mainly Al(IV) and the octahedral occupancy (6-Al(VI)-(Mg+Fe(2+)) = VAC) are considered mainly as temperature dependent.Molar fractions of each end-member show very different variations with increasing temperature: X-kaolinite decreases, and X-chamosite increases, while X-talc-3 brucite does not show significant change. From these data, activity coefficients and standard state chemical potential of major components, and molar free energy formation of chlorite have been calculated for each temperature of crystallisation.     

Mineralogy and Mineral Chemistry of the Cretaceous Duolong Gold‐Rich Porphyry Copper Deposit in the Bangongco Arc,Northern Tibet

The Early Cretaceous Duolong gold‐rich porphyry copper deposit is a newly discovered deposit with proven 5.38 Mt Cu resources of 0.72% Cu and 41 t gold of 0.23 g t^?1 in northern Tibet. Granodiorite porphyry and quartz diorite porphyrite are the main ore‐bearing porphyries. A wide range of hydrothermal alteration associated with these porphyries is divided into potassic, argillic and propylitic zones from the ore‐bearing porphyry center outward and upward. In the hydrothermal alteration zones, secondary albite (91.5–99.7% Ab) occurs along the rim of plagioclase phenocryst and fissures. Secondary K‐feldspar (75.1–96.9% Or) replaces plagioclase phenocryst and matrix or occurs in veinlets. Biotite occurs mainly as matrix and veinlet in addition to phenocryst in the potassic zone. The biotite are Mg‐rich and formed under a highly oxidized condition at temperatures ranging from 400°C to 430°C. All the biotites are absent in F, and have high Cl content (0.19–0.26%), with log (X_Cl/X_OH) values of ?2.74 to ?2.88 and IV (Cl) values of ?3.48 to ?3.35, suggesting a significant role of chloride complexes (CuCl₂^‐ and AuCl₂^‐) in transporting and precipitating copper and gold. Chlorites are present in all alteration zones and correspond mainly to pycnochlorite. They have similar Fe/(Fe+Mg), Mn/(Mn+Mg) ratios, and a formation temperature range of 280–360°C. However, the formation temperature of chlorite in the quartz‐gypsum‐carbonate‐chlorite vein is between 190°C and 220°C, indicating that it may have resulted from a later stage of hydrothermal activity. Fe³⁺/Fe²⁺ ratios of chlorites have negative correlation with Al^IV, suggesting oxygen fugacity of fluids increases with decreasing temperature. Apatite mineral inclusions in the biotite phenocrysts show high SO₃ content (0.44–0.82%) and high Cl content (1–1.37%), indicating the host magma had a high oxidation state and was enriched in S and Cl. The highest Cl content of apatite in the propylitic zone may have resulted from pressure decrease, and the lowest Cl content of apatite in the argillic zone may have been caused by a low Cl content in the fluids. The low concentration of SO₃ content in the hydrothermal apatite compared to the magmatic one may have resulted from the decrease of oxygen fugacity and S content in the hydrothermal fluid, which are caused by the abundant precipitation of magnetite.     

Hydrothermal alteration and magnetic properties of rocks in the Carolina de Michilla stratabound copper district,northern Chile

In the Carolina de Michilla district, northern Chile, stratabound copper mineralization is hosted by Jurassic volcanic rocks along the trace of the Atacama fault system. In this study, we present the overall effects of hydrothermal alteration on the magnetic properties of rocks in this district. Two types of metasomatic alteration associations occur, one of regional extent and the other of local hydrothermal alteration associated with copper mineralization (e.g., Lince–Estefanía–Susana). Regional alteration is interpreted as a low-grade “propylitic association” characterized by an epidote–chlorite–smectite–titanite–albite–quartz–calcite association. The local hydrothermal alteration is characterized broadly by a quartz–albite–epidote–chlorite–calcite mineral assemblage. The most pervasive alteration mineral is albite, followed by epidote and, locally, actinolite. These minerals contrast sharply against host rock minerals such as chlorite, calcite, zeolite, prehnite, and pumpellyite, but alteration is constrained to mineralized bodies as narrow and low contrast alteration halos that go outwards from actinolite–albite to epidote–albite, to epidote–chlorite, and finally to chlorite. Hydrothermal alteration minerals, compared to regional alteration minerals, show iron-rich epidotes, a lower chlorite content of the chlorite–smectite series, and a nearly total albite replacement of plagioclase in the mineralized zones. Opaque minerals associated with regional alteration are magnetite and maghemite, and those associated to hydrothermal alteration are magnetite, hematite, and copper sulphides. We present paleomagnetic results from nine sites in the Michilla district and from drill cores from two mines. Local effects of hydrothermal alteration on the original magnetic mineralogy indicate similar characteristics and mineralogy, except for an increase of hematite that is spatially associated with the Cu–sulphide breccias with low magnetic susceptibilities. Results indicate that it is impossible to magnetically differentiate mineralized bodies from unmineralized lavas, except for pyrite-rich hydrothermal breccias. In conclusion, for stratabound copper deposits of the Michilla type, the overall effect of hydrothermal alteration on the paleomagnetic properties of rocks is of low contrast, not clearly discernable even at a small scale. From an exploration point of view, magnetic exploration surveys should not discern mineralized bodies of Cu–sulphide breccias except in detailed ground surveys due to the small size of contrasting bodies. Unoriented drill cores with primary ore mineralization record a characteristic remanent magnetization of reverse polarity. Taking into account the azimuth and dip of the drill cores, we were able to compare the magnetization of the mineralized bodies with the characteristic directions from sites drilled in situ from Late Jurassic–Early Cretaceous intrusives mostly. The characteristic direction recorded by the Pluton Viera is similar to the magnetization of the ore bodies of the Estefania mine. If copper mineralization mostly postdates the tilt of the volcanic flows, the low paleomagnetic inclinations suggest an age for the mineralization near 145 Ma, the time of the lowest paleolatitude for the South American plate during the Mesozoic.     

黑龙江金厂金矿床18号矿体围岩蚀变及短波红外光谱特征

金厂金矿18号矿体围岩蚀变发育顺序从早到晚为:钾化、硅化、绿泥石化、绢云母化、碳酸盐化、高蛉土化,从内往外依次发育青磐岩化带、绢英岩化带和钾化带.矿化出现在泥化和绢英岩化叠加处,以及泥化和青磐岩化叠加处.通过短波红外光谱测试技术,识别出本矿区有26种蚀交矿物,其中白云母含量与金矿体呈正相关,说明绢云母化与金矿化关系密切;青磐岩化带蚀变矿物组合为绿泥石+绿帘石+伊利石±埃洛石±蒙脱石±石英;钾化带蚀变矿物组合为钾长石+高岭石+埃洛石±蒙脱石±石英;绢英岩化带蚀变矿物组合为绢云母+埃洛石±蒙脱石±高岭石±石英.     

都龙锡锌矿床绿泥石特征及其成矿意义

都龙锡锌超大型矿床是中国第三大锡石硫化物矿床,其中的绿泥石化相当普遍,并与矿化关系密切。本文在岩矿鉴定基础上,利用电子探针对绿泥石进行了微区化学成分研究。研究结果表明,该矿床绿泥石为富铁种属的假鳞绿泥石、鲕绿泥石、蠕绿泥石(铁绿泥石)及铁镁绿泥石,指示形成于还原环境;绿泥石为泥质岩或铁镁质岩受热液交代蚀变的产物,绿泥石结构的离子置换主要体现为Fe对Mg的置换,反映其形成与含铁建造有关;绿泥石的形成温度为231～304℃,平均为269℃,属于中-低温范围。绿泥石的形成机制主要表现为溶蚀-结晶、溶蚀-迁移-沉淀结晶2种。绿泥石可能与锡成矿同期形成,其与矿石矿物的生成关系表明,燕山期岩浆活动对都龙矿床的叠加改造作用显著。     

Origin of Ore-forming Fluids Responsible for Gold Mineralization of the Pongkor Au-Ag Deposit, West Java, Indonesia: Evidence from Mineralogic, Fluid Inclusion Microthermometry and Stable Isotope Study of the Ciurug–Cikoret Veins

The Pongkor gold–silver mine is situated at the northeastern flank of the Bayah dome, which is a product of volcanism in the Sunda–Banda Arc. The hydrothermal alteration minerals in the Ciurug–Cikoret area are typical of those formed from acid to near‐neutral pH thermal waters. On the surface, illite/smectite mixed layer mineral (I/Sm), smectite and kaolinite, and spotting illite, I/Sm and K‐feldspar alteration occur at the top of the mineralized zone. Silicification, K‐feldspar and I/Sm zones are commonly formed in the wall rock, and gradually grade outwards into a propylitic zone. The mineralization of precious metal ore zone is constrained by fluid temperatures between 180 and 220°C, and with low salinity (<0.2 wt% NaCl equivalent) and boiling condition. The minimum depth of vein formation below the paleo‐water table is approximately 90–130 m for the hydrostatic column. Hydrogen and oxygen isotope data for quartz and calcite show relatively homogeneous fluid composition (?53 to ?68‰δD and ?5.7 to +0.3‰δ¹⁸O H₂O). There is no specific trend in the data with respect to the mineralization stages and elevation, which suggests that the ore‐forming fluids did not significantly change spatially during the vein formation. The stable isotope data indicate mixing between the hydrothermal fluids and meteoric water and interaction between the hydrothermal fluids and the host rock.     

Rosia Poieni copper deposit,Apuseni Mountains,Romania: advanced argillic overprint of a porphyry system

The Rosia Poieni deposit is the largest porphyry copper deposit in the Apuseni Mountains, Romania. Hydrothermal alteration and mineralization are related to the Middle Miocene emplacement of a subvolcanic body, the Fundoaia microdiorite. Zonation of the alteration associated with the porphyry copper deposit is recognized from the deep and central part of the porphyritic intrusion towards shallower and outer portions. Four alteration types have been distinguished: potassic, phyllic, advanced argillic, and propylitic. Potassic alteration affects mainly the Fundoaia subvolcanic body. The andesitic host rocks are altered only in the immediate contact zone with the Fundoaia intrusion. Mg-biotite and K-feldspar are the main alteration minerals of the potassic assemblage, accompanied by ubiquitous quartz; chlorite, and anhydrite are also present. Magnetite, pyrite, chalcopyrite and minor bornite, are associated with this alteration. Phyllic alteration has overprinted the margin of the potassic zone, and formed peripheral to it. It is characterized by the replacement of almost all early minerals by abundant quartz, phengite, illite, variable amounts of illite-smectite mixed-layer minerals, minor smectite, and kaolinite. Pyrite is abundant and represents the main sulfide in this alteration zone. Advanced argillic alteration affects the upper part of the volcanic structure. The mineral assemblage comprises alunite, kaolinite, dickite, pyrophyllite, diaspore, aluminium-phosphate-sulphate minerals (woodhouseite-svanbergite series), zunyite, minamyite, pyrite, and enargite (luzonite). Alunite forms well-developed crystals. Veins with enargite (luzonite) and pyrite in a gangue of quartz, pyrophyllite and diaspore, are present within and around the subvolcanic intrusion. This alteration type is partially controlled by fractures. A zonal distribution of alteration minerals is observed from the centre of fractures outwards with: (1) vuggy quartz; (2) quartz + alunite; (3) quartz + kaolinite ± alunite and, in the deeper part of the argillic zone, quartz + pyrophyllite + diaspore; (4) illite + illite-smectite mixed-layer minerals ± kaolinite ± alunite, and e) chlorite + albite + epidote. Propylitic alteration is present distal to all other alteration types and consists of chlorite, epidote, albite, and carbonates. Mineral parageneses, mineral stability fields, and alteration mineral geothermometers indicate that the different alteration assemblages are the result of changes in both fluid composition and temperature of the system. The alteration minerals reflect cooling of the hydrothermal system from >400 °C (biotite), to 300–200 °C (chlorite and illite in veinlets) and to lower temperatures of kaolinite, illite-smectite mixed layers, and smectite crystallization. Hydrothermal alteration started with an extensive potassic zone in the central part of the system that passed laterally to the propylitic zone. It was followed by phyllic overprint of the early-altered rocks. Nearly barren advanced argillic alteration subsequently superimposed the upper levels of the porphyry copper alteration zones. The close spatial association between porphyry mineralization and advanced argillic alteration suggests that they are genetically part of the same magmatic-hydrothermal system that includes a porphyry intrusion at depth and an epithermal environment of the advanced argillic type near the surface.Editorial handling: B. Lehmann     

Chloritization of the hydrothermally altered bedrock at the Igarapé Bahia gold deposit,Carajás,Brazil

The Igarapé Bahia gold deposit has developed from weathering of a near-vertical hydrothermal Cu (Au) mineralization zone. The unweathered bedrock composed of chlorite schists is mainly metamorphosed basalts, pyroclastic and clastic sedimentary rocks and iron formation. Contents and Fe/(Fe + Mg) ratios of chlorites increase from distal country rock towards the mineralization zone, which can be attributed to different water/rock ratios and locations in a hydrothermal system. In the hydrothermal system high salinity fluids convected through basin-floor rocks, stripping metals from the recharge zones with precipitation in discharge zones. The chlorite with lower Fe/(Fe + Mg) ratios indicates alteration by relatively unreacted Mg-rich fluids, occurring within recharge zones. By contrast, the chlorite with higher Fe/(Fe + Mg) ratios in the mineralization zone formed from solutions rich in Fe, Mn, Au, Cu, H₂S and SiO₂ within a discharge zone. The iron formation could also be formed within the discharge zone or on the basin floor from the Fe-rich fluids. The distal country rock with less chlorite content is a hydrothermal product at low water/rock ratios whereas the proximal country rock and the host rock with more chlorite content formed at high water/rock ratio conditions. The Al(IV) contents of chlorites indicate that the formation temperatures of these rocks range from 204 to 266 °C, with temperatures slightly increasing from distal country rock towards the mineralization zone.     

Mineral chemistry and geochemical behavior of hydrothermal alterations associated with mafic intrusive-related Au deposits at the Atud area,Central Eastern Desert,Egypt

Vein-type gold deposits in the Atud area are related to the metagabbro–diorite complex that occurred in Gabal Atud in the Central Eastern Desert of Egypt. This gold mineralization is located within quartz veins and intense hydrothermal alteration haloes along the NW–SE brittle–ductile shear zone, as well as along the contacts between them. By using the mass balance calculations, this work is to determine the mass/volume gains and losses of the chemical components during the hydrothermal alteration processes in the studied deposits. In addition, we report new data on the mineral chemistry of the alteration minerals to define the condition of the gold deposition and the mineralizing fluid based on the convenient geothermometers. Two generations of quartz veins include the mineralized grayish-to-white old vein (trending NW–SE), and the younger, non-mineralized milky white vein (trending NE–SW). The ore minerals associated with gold are essentially arsenopyrite and pyrite, with chalcopyrite, sphalerite, enargite, and goethite forming during three phases of mineralization; first, second (main ore), and third (supergene) phases. Three main hydrothermal alteration zones of mineral assemblages were identified (zones 1–3), placed around mineralized and non-mineralized quartz veins in the underground levels. The concentrations of Au, Ag, and Cu are different from zone to zone having 25–790 ppb, 0.7–69.6 ppm, and 6–93.8 ppm; 48.6–176.1 ppb, 0.9–12.3 ppm, and 39.6–118.2 ppm; and 53.9–155.4 ppb, 0.7–3.4 ppm, and 0.2–79 ppm for zones 1, 2, and 3, respectively.The mass balance calculations and isocon diagrams (calculated using the GEOISO-Windows program) revealed the gold to be highly associated with the main mineralized zone as well as sericitization/kaolinitization and muscovitization in zone 1 more than in zones 2 and 3. The sericite had a higher muscovite component in all analyzed flakes (average X_Ms = 0.89), with 0.10%–0.55% phengite content in wall rocks and 0.13%–0.29% phengite content in mineralized quartz veins. Wall rocks had higher calcite (CaCO₃) contents and lower MgCO₃ and FeCO₃ contents than the quartz veins. The chlorite flakes in the altered wall rocks were composed of pycnochlorite and ripidolite, with estimated formation temperatures of 289–295 °C and 301–312 °C, respectively. Albite has higher albite content (95.08%–99.20%) which occurs with chlorite in zone 3.     

Oxygen and Carbon Isotope Study of Calcite and Dolomite in the Disseminated Au-Ag Telluride Bulawan Deposit, Negros Island, Philippines

Abstract: The disseminated Au‐Ag telluride Bulawan deposit, Negros island, Philippines, is hosted by dacite porphyry breccia pipes which formed in a Middle Miocene dacite porphyry stock. Electrum and Au‐Ag tellurides occur mostly as grains intergrown with or filling voids between sphalerite, pyrite, chalcopyrite, galena and tennantite. Calcite, quartz and rare dolomite are the principal gangue minerals. Four types of alteration were recognized in the deposit, namely; propylitic, K‐feldspar‐sericitic, sericitic and carbonate alteration. Carbonate alteration is correlatable to the gold deposition stage and occurs mostly along fault zones. The δ¹⁸O and δ¹³C compositions of calcite and dolomite in propylite zone and ore‐stage dacite porphyry breccia were determined. The δ¹⁸O values of calcite in propylitized andesite range from +12.2 to +14.7%, and their δ¹³C values range from ‐6.1 to ‐1.0%. The δ¹⁸O values of calcite and dolomite in sericite‐ and carbonate‐altered, mineralized dacite porphyry breccia and dacite porphyry rocks range from +15.1 to +23.1%, and the δ¹³C values of calcite and dolomite range from ‐3.9 to +0.9%. The δ¹⁸O and δ¹³C values of the hydrothermal fluids were estimated from inferred temperatures of formation on the basis of fluid inclusion microthermometry. The δ¹⁸O values of hydrothermal fluid for the propylitic alteration were calculated to be +8.5 ‐ +9.5%, assuming 375°C. On the other hand, the δ¹⁸O values of ore solutions for base metal and Au mineralization were computed to be +13.6 ‐ +14.6%, assuming 270°C. The hydrothermal fluids that formed the Bulawan deposit are dilute and ¹⁸O‐enriched fluids which reacted with ¹⁸O‐ and ¹³C‐rich wallrocks such as limestone.     

Geochemistry of clay minerals for uranium exploration in the Grants mineral belt,New Mexico

Clay mineralogy studies of ore rocks versus barren rocks in the Grants mineral belt, New Mexico, show that some combination of chlorite (rosette form), illite, mixed-layer illite-montmorillonite, (±Mg-montmorillonite) are penecontemporaneous with uranium minerals in trend ore; these same clay minerals plus kaolinite are related to the roll-type ore near the main redox front of the Grants mineral belt. Clay minerals from barren rocks are characterized by a greater abundance of Na-montmorillonite, kaolinite, and face-to-edge form chlorite. Chlorites from ore zones contain much more vanadium than do chlorites from barren rocks. Trend orr probably formed from southeasterly flowing waters following paleochannels in the Late Jurassic. These deposits are found almost entirely in reduced rocks, and organic carbon may have been an important reductant to remove U-V-U-V-Se-Mo from solution as carbonate from ore zones contains some organic carbon based on stable isotope studies. Uplift, remobilization, and reprecipitation of some of the trend ore resulted in the formation of redistributed ore, some of which possesses a roll-type geometry. Mineralization for the roll-type ore was apparently controlled by sulfide-sulfate equilibria at or near the main redox front in the Grants mineral belt. Trend and roll-type ore possess different assemblages of clay minerals and different trace element abundances. Laramide-age faults cut both trend ore and some roll-type ores. Stack ore is found in Laramide-age fault zones. Limited oxygen isotopic data from clay minerals collected from two mines at Ambrosia Lake in reduced rocks indicate probable preservation of ancient, formational waters and show no evidence of infiltration by young meteoric waters. This information, plus the pre-Laramide-age faults, suggest, but do not unequivocally prove, that the main redox front has been relatively stable since its formation, probably some time in the Cretaceous. Younger encroachment of the redox front in post-Laramide time is proposed by others, however, and the problem is unresolved. Uranium for the deposits was most likely carried in solution as an oxyion, probably as (UO₂(CO₃)₂·2H₂O^2– or (UO₂(HPO₄)₂)^2–, although organic transport cannot be entirely ruled out. Oxyions of vanadium, selenium, molybdenum and possible arsenic and antimony, are interpreted from Eh-pH diagrams to have been segregated and transported with U⁶⁺ soluble species and precipitated when a chemically reducing environment was reached. The rare-earth elements are also enriched in ore zones, but is it not clear if they were transported with the U-V-Se-Mo-(As)-(Sb) suite or derived from a more local source.     

Mineralogy of New Caledonian metamorphic rocks

Variations in chemistry and related physical properties of sheet silicates in the Ouégoa district with metamorphic grade are investigated. Weakly metamorphosed rocks prior to the crystallization of lawsonite contain phengite (d ₀₀₆=3.317–3.323 Å), chlorite and occasionally paragonite while interstratified basaltic sills contain chlorite, minor phengite and stilpnomelane. Pyrophyllite crystallizes before lawsonite in some metamorphosed acid tuffs and is also stable in the lawsonite zone. Paragonite, phengite and chlorite appear to be stable through the sequence from weakly metamorphosed rocks into high-grade “eclogitic” schists and gneisses. Optical, chemical and some X-ray diffraction data is given for representative sheet silicates. Electron probe analyses of 55 phengites, 21 paragonites, 57 chlorites, 12 vermiculites, 2 stilpnomelanes, and 2 chloritoids are presented in graphical form. All K-micas analysed are consistently phengitic (3.29–3.55 Si^iv ions per formula unit) and show limited solid solution with paragonite (4 to 13% Pa). The K∶Na ratio of the phengite is strongly dependant on the assemblage in which it occurs; the amount of phengite component and its Fe∶Mg ratio depends on bulk-rock composition. Phengites from acid volcanics have the highest Fe∶Mg ratio, highest phengite component and β refractive indices. Phengites from basic volcanics and metasediments of the epidote zone have the lowest Fe∶Mg ratio. Phengites from lawsonite-zone metasediments have intermediate Fe∶Mg ratios. The phengites show a small decrease in phengite component with increasing metamorphic grade. d ₀₀₆ for phengites varied from 3.302 to 3.323 Å but at least in the lawsonite and epidote zones appears to reflect composition and had little systematic variation with metamorphic grade; phengites from very low-grade rocks showed the longest values of d ₀₀₆. Paragonite shows almost no phengite-type substitution and only limited solid solution (4–12%) with muscovite. All paragonites (6) and most phengites (20) which have been examined are 2M₁ polymorphs; one Fe²⁺-phengite appears to be a 1M polymorph. The chemistry of chlorites closely reflects parent-rock chemistry. Chlorites from metasediments have distinctly higher Fe/(Fe+Mg) ratios than chlorites from basic igneous rocks; chlorites from the lawsonite and lawsonite-epidote transitional zone metasediments have the highest Fe/(Fe+Mg) ratios. In metabasalts Fe/(Fe+Mg) ratios appear to reflect individual variations in bulk-rock chemistry and show no direct correlation with metamorphic grade. There is little difference in Al/(Si+Al) ratio between chlorites from sediments and basic igneous rocks although in both lithologies the chlorites from the epidote zone appear to be slightly more aluminous. Fe-rich chlorites of the lawsonite zone metasediments have been altered by a process involving leaching of Fe and Mg and introduction of alkalies to a brown pleochroic Fe-vermiculite. Chemical and physical data for this vermiculite are given. The decrease in Fe/(Fe+Mg) ratio in chlorites and phengites on passing from the lawsonite to the epidote zone can be correlated with the crystallization of Fe-rich epidote and almandine in the epidote zone. Elemental partitioning between coexisting minerals has shown Ti to be partitioned into phengite, while Fe and Mn are strongly partitioned into chlorite. When either stilpnomelane or chloritoid coexists with phengite or chlorite, Fe and Mn are slightly enriched in the stilpnomelane or chloritoid relative to the chlorite.     

赣东北地区矽卡岩典型矿物形成与演化的光谱证据:以朱溪钨多金属矿为例SCIEI北大核心CSCD

朱溪是近年来在江西塔前-赋春成矿带发现的一个世界级超大型钨铜矿床。本文采用短波红外+热红外光谱技术对矿区13个钻孔进行了光谱测量,结合岩石-矿物地球化学分析,探讨了朱溪矿床矽卡岩中典型蚀变矿物的形成与演化过程,厘定了矽卡岩形成不同阶段矿物组合的光谱特征,构建了朱溪矿床的短波红外+热红外光谱勘查模型。研究发现:(1)区内不同矿物组合形成了明显的蚀变分带,由内向外依次为绢云母(富Si)+长石(岩体顶层蚀变,多期流体叠加综合作用)→外矽卡岩:钙铝榴石+透辉石+(绢云母)→透辉石+蛇纹石+绿泥(帘)石+滑石→绢云母(富Al)+绿泥(帘)石(基底不整合面蚀变);(2)Al-OH波长的移动可指示成矿流体压力、温度及pH值的变化;(3)研究区透辉石的形成、演化与矿体之间关系密切,虽然富矿体赋存于矽卡岩形成早期的透辉石-石榴子石蚀变带,但大量矿体则赋存于矽卡岩退蚀变阶段的蛇纹石-绿泥石蚀变带;(4)矿体的形成与流体的混合作用关系极大,伊利石光谱吸收特征能够指示外部冷水(大气降水或地下水)的灌入轨迹。     

The Crater Mountain Deposit,Papua New Guinea: Porphyry‐related Au–Te System

Ore mineralization and wall rock alteration of Crater Mountain gold deposit, Papua New Guinea, were investigated using ore and host rock samples from drill holes for ore and alteration mineralogical study. The host rocks of the deposit are quartz‐feldspar porphyry, feldspar‐hornblende porphyry, andesitic volcanics and pyroclastics, and basaltic‐andesitic tuff. The main ore minerals are pyrite, sphalerite, galena, chalcopyrite and moderate amounts of tetrahedrite, tennantite, pyrrhotite, bornite and enargite. Small amounts of enargite, tetradymite, altaite, heyrovskyite, bismuthinite, bornite, idaite, cubanite, native gold, CuPbS₂, an unidentified Bi‐Te‐S mineral and argentopyrite occur as inclusions mainly in pyrite veins and grains. Native gold occurs significantly in the As‐rich pyrite veins in volcanic units, and coexists with Bi‐Te‐S mineral species and rarely with chalcopyrite and cubanite relics. Four mineralization stages were recognized based on the observations of ore textures. Stage I is characterized by quartz‐sericite‐calcite alteration with trace pyrite and chalcopyrite in the monomict diatreme breccias; Stage II is defined by the crystallization of pyrite and by weak quartz‐chlorite‐sericite‐calcite alteration; Stage III is a major ore formation episode where sulfides deposited as disseminated grains and veins that host native gold, and is divided into three sub‐stages; Stage IV is characterized by predominant carbonitization. Gold mineralization occurred in the sub‐stages 2 and 3 in Stage III. The fS₂ is considered to have decreased from ～10^?2 to 10^?14 atm with decreasing temperature of fluid.     

Sulfur isotopic zonation in the Cadia district,southeastern Australia: exploration significance and implications for the genesis of alkalic porphyry gold–copper deposits

The alkalic porphyry gold–copper deposits of the Cadia district occur in the eastern Lachlan Fold Belt of New South Wales, Australia. The district comprises four porphyry deposits (Ridgeway, Cadia Quarry, Cadia Hill, and Cadia East) and two iron–copper–gold skarn deposits (Big Cadia and Little Cadia). Almost 1,000 tonnes of contained gold and more than four million tonnes of copper have been discovered in these systems, making Cadia the world’s largest known alkalic porphyry district, in terms of contained gold. Porphyry gold–copper ore at Cadia is associated with quartz monzonite intrusive complexes, and is hosted by central stockwork and sheeted quartz–sulfide–(carbonate) vein systems. The Cadia porphyry deposits are characterized by cores of potassic and/or calc–potassic alteration assemblages, and peripheral halos of propylitic alteration, with late-stage phyllic alteration mostly restricted to fault zones. Hematite dusting is an important component of the propylitic alteration assemblage, and has produced a distinctive reddening of feldspar minerals in the volcanic wall rocks around the mineralized centers. Sulfide mineralization is strongly zoned at Ridgeway and Cadia East, with bornite-rich cores surrounded by chalcopyrite-rich halos and peripheral zones of pyrite mineralization. The Cadia Hill and Cadia Quarry deposits have chalcopyrite-rich cores and pyrite-rich halos, and Cadia Hill contains a high-level bornite-rich zone. Distinctive sulfur isotopic zonation patterns have been identified at Ridgeway, Cadia Hill, and Cadia East. The deposit cores are characterized by low δ³⁴S_sulfide values (−10 to −4‰), consistent with sulfide precipitation from an oxidized (sulfate-predominant) magmatic fluid at 450 to 400°C. Pyrite grains that occur in the propylitic alteration halos typically have δ³⁴S_sulfide values near 0‰. There is a gradual increase in δ³⁴S_sulfide values outwards from the deposit cores through the propylitic halos. Water–rock interaction during propylitic alteration caused magmatic sulfate reduction and concomitant oxidation of ferrous iron-bearing minerals, resulting in enrichment of ³⁴S in pyrite and also producing the distinctive reddened, hematite-rich alteration halos to the Cadia deposits. These results show that sulfur isotope analyses have potential applications in the exploration of alkalic porphyry-style deposits, with zones of depleted δ³⁴S_sulfide values most prospective for high-grade mineralization.     

Gold Mineralization in Banded Iron Formation in the Amalia Greenstone Belt,South Africa: A Mineralogical and Sulfur Isotope Study

The Blue Dot gold deposit, located in the Archean Amalia greenstone belt of South Africa, is hosted in an oxide (± carbonate) facies banded iron formation (BIF). It consists of three stratabound orebodies; Goudplaats, Abelskop, and Bothmasrust. The orebodies are flanked by quartz‐chlorite‐ferroan dolomite‐albite schist in the hanging wall and mafic (volcanic) schists in the footwall. Alteration minerals associated with the main hydrothermal stage in the BIF are dominated by quartz, ankerite‐dolomite series, siderite, chlorite, muscovite, sericite, hematite, pyrite, and minor amounts of chalcopyrite and arsenopyrite. This study investigates the characteristics of gold mineralization in the Amalia BIF based on ore textures, mineral‐chemical data and sulfur isotope analysis. Gold mineralization of the Blue Dot deposit is associated with quartz‐carbonate veins that crosscut the BIF layering. In contrast to previous works, petrographic evidence suggests that the gold mineralization is not solely attributed to replacement reactions between ore fluid and the magnetite or hematite in the host BIF because coarse hydrothermal pyrite grains do not show mutual replacement textures of the oxide minerals. Rather, the parallel‐bedded and generally chert‐hosted pyrites are in sharp contact with re‐crystallized euhedral to subhedral magnetite ± hematite grains, and the nature of their coexistence suggests that pyrite (and gold) precipitation was contemporaneous with magnetite–hematite re‐crystallization. The Fe/(Fe+Mg) ratio of the dolomite–ankerite series and chlorite decreased from veins through mineralized BIF and non‐mineralized BIF, in contrast to most Archean BIF‐hosted gold deposits. This is interpreted to be due to the effect of a high sulfur activity and increase in fO₂ in a H₂S‐dominant fluid during progressive fluid‐rock interaction. High sulfur activity of the hydrothermal fluid fixed pyrite in the BIF by consuming Fe²⁺ released into the chert layers and leaving the co‐precipitating carbonates and chlorites with less available ferrous iron content. Alternatively, the occurrence of hematite in the alteration assemblage of the host BIF caused a structural limitation in the assignment of Fe³⁺ in chlorite which favored the incorporation of magnesium (rather than ferric iron) in chlorite under increasing fO₂ conditions, and is consistent with deposits hosted in hematite‐bearing rocks. The combined effects of reduction in sulfur contents due to sulfide precipitation and increasing fO₂ during progressive fluid‐rock interactions are likely to be the principal factors to have caused gold deposition. Arsenopyrite–pyrite geothermometry indicated a temperature range of 300–350°C for the associated gold mineralization. The estimated δ³⁴S_ΣS (= +1.8 to +2.5‰) and low base metal contents of the sulfide ore mineralogy are consistent with sulfides that have been sourced from magma or derived by the dissolution of magmatic sulfides from volcanic rocks during fluid migration.     

Mineral types of hydrothermal alteration zones in the Dukat ore field and their relationships to leucogranite and epithermal gold-silver ore,northeastern Russia

The paper considers the localization of potassic and propylitic hydrothermal alteration zones in the domal volcanic-plutonic structure controlling the position of the Dukat ore field with the eponymous unique epithermal Au-Ag deposit. Comprehensive mineralogical and geochemical data on rocks and minerals in hydrothermal alteration zones and associated intrusions have shown that quartz-jarosite-sericite, quartz-pyrite-sericite, and quartz-adularia-chlorite alterations were formed with the participation of fluid flows related to a fingerlike projection of a high-K leucogranite porphyry intrusion with large phenocrysts. These hydrothermal alterations developed in the rifted graben under conditions of divergent plate boundaries, whereas quartz-clinozoisite-calcite, epidote-chlorite, and garnet-calcite-chlorite alterations were linked to K-Na leucogranite intrusive bodies and developed under conditions of convergent plate boundaries reactivated as a result of formation of the marginal Okhotsk-Chukotka volcanic belt. Phase separation and coagulation of specific portions of ascending fluids resulted in the formation and stabilization of small-sized particles of native silver and other ore components, which enabled involvement in flows of secondary geothermal solutions and ore-forming fluids. The Sr, Nd, and Pb isotopic compositions of rocks and minerals from the hydrothermal alteration zones, associated intrusions, and economic orebodies at the Dukat deposit indicate that their components have been derived from the juvenile continental crust, which was altered in pre-Cretaceous periods of endogenic activity. The components of gangue minerals of potassic and propylitic hydrothertmal alterations and associated intrusions have been taken from deep sources differing in 87Sr/86Sr and 143Nd/144Nd at similar U/Pb and Th/Pb ratios. Chalcophile lead in products of hydrothermal activity and melanocratic inclusions in leucogranite has been taken from regions with elevated U/Pb and Th/Pb ratios.     

Ore and Alteration Types of the Gold and Copper Mineralization in the Lascogon Project, Surigao del Norte, Mindanao Island, Philippines

Two ore and three alteration types were identified in the Lascogon Project of Philex Gold Philippines, in Surigao del Norte, Mindanao Island, Philippines. The jasperoid ore is the host to the Carlin‐like gold mineralization in the Lascogon and Danao prospects. The ore occurs in a decalcified and silicified horizon, with minor chlorite and goethite, stibnite, pyrite and quartz crystals ranging from cryptocrystalline to botryoidal. The stringer–stockwork type Cu‐Au mineralization in the Suyoc prospect is hosted in argillized andesitic rocks of the Mabuhay Formation. The primary ore minerals are chalcopyrite with minor amounts of sphalerite. The alteration types identified are propylitic alteration, argillic alteration and silicification. The propylitized basaltic and andesitic flows of the Bacuag Formation bound the jasperoid mineralization in the Lascogon prospect. Stratigraphically, the relationship between propylitized basalts and stringer–stockwork Cu‐Au is not clear but a lateral change can be inferred from jasperoid in the center and stringer–stockwork towards the east.     

基于红外光谱技术研究云南普朗斑岩铜矿的蚀变和矿化特征

近年来红外光谱技术作为一种绿色、快速、无损、精确探测矿物的技术手段而倍受关注,针对斑岩型矿床蚀变矿物高度叠加、蚀变分带界线不明显、细粒蚀变矿物多、黏土蚀变矿物多等特征,该技术在蚀变矿物识别和勘探信息解读等方面优势突出。本文应用红外光谱技术对云南普朗斑岩铜矿区钻孔ZK1801岩心进行矿物识别和蚀变分带划分的研究,识别出钾硅酸盐化带、绿帘石-绿泥石化带、绿泥石-伊利石化带、石英-伊利石化带和泥化带。研究表明:普朗铜矿整个钻孔的蚀变矿物主要有石英、钾长石、绢云母、绿泥石、绿帘石、高岭石、蒙脱石、伊利石等;根据矿化特征,发现铜矿体广泛赋存在钾硅酸盐化带和绿帘石-绿泥石化带中,与矿化关系密切的蚀变矿物"石英+钾长石+绢云母"和"绿帘石+绿泥石",可以作为普朗矿床勘查的标型蚀变矿物组合;研究区广泛发育的绢云母Al—OH波长随钻孔深度增加而逐渐从2210～2205nm减小到2202～2198nm, Al—OH波长2210～2205nm(长波绢云母)与矿化关系密切,可以作为普朗矿床勘查的指示信息。