Ore Textures Research Articles

Fe–Cu deposits in the Kangdian region, SW China: a Proterozoic IOCG (iron-oxide–copper–gold) metallogenic province

Numerous Fe–Cu deposits are hosted in the late Paleoproterozoic Dongchuan and Dahongshan Groups in the Kangdian region, SW China. The Dongchuan Group is composed of siltstone, slate, and dolostone with minor volcanic rocks, whereas the Dahongshan Group has undergone lower amphibolite facies metamorphism and consists of quartz mica-schist, albitite, quartzite, marble and amphibolite with local migmatite. Deposits in the Dongchuan Group are commonly localized in the cores of anticlines, in fault bends and intersections, and at lithological contacts. Orebodies are closely associated with breccias, which are locally derived from the host rocks. Fe-oxides (magnetite and/or hematite) and Cu-sulfides (chalcopyrite, bornite) form disseminated, vein-like and massive ores, and typically fill open spaces in the host rocks. The deposits have extensive albite alteration and local K-feldspar alteration overprinted by quartz, carbonate, sericite and chlorite. Deposits in the Dahongshan Group have orebodies sub-parellel to stratification and show crude stratal partitioning of metals. Fe-oxide ores occur as massive and/or banded replacements within the breccia pipes, whereas Cu-sulfide ores occur predominantly as disseminations and veinlets within mica schists and massive magnetite ores. Ore textures suggest that Cu-sulfides formed somewhat later than Fe-oxides, but are possibly within the same mineralization event. Both ore minerals predated regional Neoproterozoic metamorphism. Both orebodies and host rocks have undergone extensive alteration of albite, scapolite, amphibole, biotite, sericite and chlorite. Silica and carbonate alterations are also widespread. Ore-hosting strata have a LA-ICP-MS zircon U–Pb age of 1681 ± 13 Ma, and a dolerite dyke cutting the Fe-oxide orebodies has an age of 1659 ± 16 Ma. Thus, the mineralization age of the Dahongshan deposit is constrained at between the two. All ores from the two groups have high Fe and low Ti, with variable Cu contents. Locally they are rich in Mo, Co, V, and REE, but all are poor in Pb and Zn. Sulfides from the Fe–Cu deposits have δ34S values mostly in the range of +2 to +6 per mil, suggesting a mix of several sources due to large-scale leaching of the strata with the involvement of evaporites. Isotopic dating and field relationships suggest that these deposits formed in the late Paleoproterozoic. Ore textures, mineralogy and alteration characteristics are typical of IOCG-type deposits and thus define a major IOCG metallogenic province with significant implications for future exploration.

Sphalerite Associated with Pyrrhotite-Chalcopyrite Ore Occurring in the Kotana Fe-Skarn Deposit (Giresun, NE Turkey): Exsolution or Replacement

The Kotana prospect is located about 30 km south of Giresun (NE Turkey). The ore mineralization is a Fe-skarn occurring within the low-grade pre-Lower Jurassic Pınarlar metamorphics, consisting of marble-phyllite intruded by the Upper Cretaceous-Eocene Aksu biotite monzogranite. The principal primary ore minerals include pyrrhotite and magnetite along with minor pyrite (I) and chalcopyrite, accompanied by trace sphalerite. Sphalerite is closely associated with chalcopyrite and to a lesser extent with hexagonal pyrrhotite. Secondary ore minerals include pyrite (II), marcasite, martite, hematite, goethite, lepidocrocite, and intermediate Fe-oxides-hydroxides. Gangue minerals are mainly calcite and quartz. Oxidation of the primary sulphides resulted in formation of diverse secondary ore textures containing bird's eye, martitic, spheroidal, colloform, rim, and veinlets. Distinct crystal shapes of sphalerite are of particular interest to this investigation due mainly to their proposed formation mechanisms. Three alternative mechanisms for their formation were considered: (i) quasi-exsolved bodies developed by hexagonal pyrrhotite replacement of chalcopyrite, (ii) interstitial formation between coalescing pyrrhotite crystals during crystal growth, and (iii) as genuinely exsolved bodies, and as such conflict with previous experimental results. Although the general absence of solute mineral outside the solvent mineral suggests solid solution at high temperatures (favouring the third mechanism), textures, modal, microprobe and sulfur-isotope data suggest that these are more likely to be pseudoexsolved bodies formed as a result of replacement of chalcopyrite by hexagonal pyrrhotite. The \delta^3^4S values of pyrrhotite and chalcopyrite are between 5.23 and 6.73 per mil (n= 12) and 2.29 and 3.26 per mil (n= 8), respectively, indicating continuous enrichment in heavy sulphur isotopes from prograde stage to retrograde stage within the typical range for skarn-type mineralization. Fluid inclusion analyses of calcite and quartz gangues indicate that the minimum homogenization temperature (Th) averaged 400±20 ºC with salinities < 15 wt% NaCl equivalent.

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

AbstractOre 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.

Development of Microstructure and Texture of Hematite Ores Deformed to Large Strain in Torsion: Can Texture Identify the Prevailing Strength and Creep Mechanisms During Deformation?

AbstractSamples of fine‐grained (approximately 9 µm) and coarse‐grained (approximately 45 µm) hematite ores with almost random crystallographic preferred orientation (CPO) were deformed in high pressure, high temperature (400 MPa, 850–1 000 °C) torsion experiments up to shear strains of 4.7. Samples with large initial grain size, preferably deformed by dislocation creep attended by dynamic recrystallization, showed grain size reduction and a weak CPO (J ∼ 1.4 at 950 °C). In contrast, fine‐grained ores, deformed mainly by grain boundary sliding accompanied by dislocation activity with slip on the dominant basal glide system, showed grain growth and a strong CPO (J ∼ 2.9). At high strain both ores attained similar strength and grain size, but different CPOs. The experiments demonstrate that the texture intensity of highly deformed rocks strongly depends on initial microstructure and may not reflect unambiguously the prevailing deformation mechanism and strength as often assumed in field studies.

Bonded-particle modelling of microwave-induced damage in ore particles

Microwave heating of mineral ores offers a mechanism to induce fractures around grain boundaries due to the different rates of microwave power dissipation and the differences in thermal expansion coefficient among various minerals in the ore particles. As a consequence, this has the potential to reduce the energy required in subsequent grinding and to enhance liberation of valuable minerals. In this paper, numerical simulation using a bonded-particle model was undertaken to provide a better understanding of the mechanism of microwave-induced micro-fracture and to predict the effect of microwave power delivery and ore texture on microwave treatment of ores. Computational simulations of microwave heating and thermal damage have been carried out on two-phase conceptual ores. It was shown that the extent of damage and the crack pattern in an ore sample for the same microwave energy input depend strongly on the applied power density and the microwave absorbent phase grain size. It is possible both to reduce the energy input and to localize the microwave-induced damage around the grain boundaries by operating at high power density. It was also shown that high power pulsed equipment would be more efficient than continuous wave equipment for treating fine-grained ores.

Mineralogical and geochemical investigations of the Dajia Salento-type bauxite deposits, western Guangxi, China

The Dajia Salento-type bauxite deposit in western Guangxi is hosted within the Quaternary ferrallitic soil profile, and it formed via breaking up, weathering and oxidizing of Permian bauxite orebodies occurring as a semi-continuous layer in the upper Permian. Mineralogical analyses reveal that diaspore, hematite and kaolinite are the major minerals in bauxite ores with small amounts of anatase, chamosite, gibbsite, goethite, illite, zircon, quartz and pyrite. The ore texture and mineral assemblage reveal that the depositional/diagenetic environment of the Dajia bauxite was much close to phreatic environment. Both the ore texture and the morphology of zircon grains also indicate that most of the bauxitic soils were transported a short distance. Diaspore is suggested to be non-metamorphic in origin and mainly formed in a reducing condition of diagenetic environment, while kaolinite is the product of the in situ epigenetic replacement of alumina in diaspore by dissolved silica. Geochemical analyses indicate that Al 2O 3, Fe 2O 3, SiO 2 and TiO 2 are the main components of the bauxite ores and trace elements such as Zr, Hf, Nb, Ta, Th and U were enriched during the bauxitization process. Simultaneously, Zr vs. Hf and Nb vs. Ta show a high correlation. Geochemical indices such as Zr/Hf, Nb/Ta and Eu/Eu* (among others) denote that the magmatic rocks related to the Emeishan plume in western Guangxi and the carbonates in the underlying Maokou Formation provided the main sources of material for the bauxite ores.

Mineralogy, Microstructure, and Chemical Composition of Goethites in Some Iron Ore Deposits of Orissa, India

The Joda–Barbil region of the Jamda–Koira valley is one of the most important sources of iron ores in India. The mineralogy, texture, and chemical composition of iron ore of the region have been analyzed to understand the presence of different types of goethite and their compositional variation with respect to Fe, Al2O3, SiO2, and P that are essential for upgrading the ores for iron and steel making. The iron ore samples have been investigated by optical microscopy, EPMA, XRD, and TG-DTA. The optical microscopy studies have indicated three dominant textural types of goethite, such as (a) colloform-banded vitreous, (b) massive vitreous, and (c) earthy or ochreous goethite. The vitreous goethite is hard and crystalline, whereas ochreous goethite is clayey and consists of ultrafine crystallites often intimately admixed with kaolinite and gibbsite. EPMA studies indicated that (a) ochreous and colloform-banded goethite contain more alumina compared to massive vitreous goethite and (b) colloform-banded goethite has higher phosphorus contents (P2O5: 0.90–2.25%).

Organic and inorganic geochemistry of Ljubija siderite deposits, NW Bosnia and Herzegovina

The Ljubija siderite deposits, hosted by a Carboniferous sedimentary complex within the Inner Dinarides, occur as stratabound replacement-type ore bodies in limestone blocks and as siderite-sulfides veins in shale. Three principal types of ore textures have been recognized including massive dark siderite and ankerite, siderite with zebra texture, and siderite veins. The ore and host rocks have been investigated by a combination of inorganic (major, trace, and rare earth element concen- trations), organic (characterization of hydrocarbons includ- ing biomarkers), and stable isotope geochemical methods (isotope ratios of carbonates, sulfides, sulfates, kerogen, and individual hydrocarbons). New results indicate a marine origin of the host carbonates and a hydrothermal- metasomatic origin of the Fe mineralization. The differ- ences in ore textures (e.g., massive siderite, zebra siderite) are attributed to physicochemical variations (e.g., changes in acidity, temperature, and/or salinity) of the mineralizing fluids and to the succession and intensity of replacement of host limestone. Vein siderite was formed by precipitation from hydrothermal fluids in the late stage of mineralization. The equilibrium fractionation of stable isotopes reveals higher formation temperatures for zebra siderites (around 245°C) then for siderite vein (around 185°C). Sulfur isotope ratios suggest Permian seawater or Permian evaporites as the main sulfur source. Fluid inclusion composition confirms a contribution of the Permian seawater to the mineralizing fluids and accord with a Permian mineralization age. Organic geochemistry data reflect mixing of hydrocarbons at the ore site and support the hydrothermal-metasomatic origin of the Ljubija iron deposits.

Effects of Magmatic Processes, Serpentinization, and Talc-Carbonate Alteration on Sulfide Mineralogy and Ore Textures in the Black Swan Disseminated Nickel Sulfide Deposit, Yilgarn Craton

The Black Swan disseminated orebody occurs predominantly within the partially carbonated serpentinite core of a zoned serpentinite–talc-carbonate ultramafic cumulate body of komatiitic parentage. The Black Swan disseminated sulfides depart significantly from the characteristic pattern observed in otherwise similar deposits in Western Australia and Canada, which display magnetite-bearing pentlandite-rich sulfide aggregates (“blebs”) in serpentinites, and S-rich millerite-bearing assemblages in talc carbonates. The Black Swan disseminated ores in contrast are dominated by millerite-pyrite–(polydymite) assemblages in both serpentinites and talc carbonates, with no evident relationship to the progression of carbonation. Sulfides show three types of intergrowth with gangue minerals, which can occur separately or all together: intergrowth with magnetite, at least some of which appear to be primary magmatic in origin; fine intergrowth with interlocking lath-textured antigorite; and intergrowth with carbonate minerals. Carbonate commonly replaces the core of sulfide blebs while retaining the primary magmatic outline. Carbonate replacement of sulfide roughly parallels the overall progression of carbonation of the whole rock, but no systematic relationship of the other intergrowth types to carbonate content is observed in the serpentinites. Serpentine mineralogy, determined by microbeam Raman spectroscopy, comprises lizardite, which occurs in pseudomorphic mesh textures after olivine, and antigorite, formed by variable degrees of overprinting of original lizardite mesh textures, ranging to a completely overprinting interlocking texture with no relic igneous texture. This progression is interpreted as the result of mild thermal metamorphism of original lizardite serpentinite. This progression is completely independent of carbonate content and is also apparently independent of extent of sulfide-antigorite intergrowth, which is interpreted as being early, and related to volume expansion during initial sea-floor serpentinization. Talc is almost completely absent from serpentinites, while carbonate content varies widely. Whole-rock Ni and S data are very strongly correlated in millerite-poor, pentlandite-rich samples and poorly correlated in samples where millerite is the dominant Ni-bearing sulfide. On this basis we conclude that the pentlandite assemblages are slightly modified survivors of the original magmatic event, while the millerite-pyrite assemblages formed during the earliest stages of serpentinization. Overprinting of the early serpentinization event by talc carbonate had essentially no effect on sulfide mineralogy, other than inducing intergrowth with carbonate. A number of distinctive features, such as magnetite-rich bleb cores, the association of sulfides with segregation vesicles and chromite “shells,” the subspherical morphology of the blebs, and preferential replacement of bleb cores, are considered to be inherited from the magmatic phase and are attributed to an original high but variable oxygen and halogen content of the sulfide melt component. Exsolution of this component during solidification of the blebs in some cases produced sulfide-centered bubbles now preserved as segregation vesicles, while in other cases oxygen was retained within the bleb forming magnetite-sulfide intergrowths. Chromite shells formed as a result of exsolved oxygen interacting with komatiite magma. Volatile-rich blebs solidified in some cases with hollow or porous cores, which were subsequently exploited by carbonate replacement.

Geology and Ore Deposits of the Shakanai Mina(3).On the Mode of Occurrence and Texture of Yellow Ore in the No.4 and No.8 Ore Deposits.

Geology and Ore Deposits of the Shakanai Mina(3).On the Mode of Occurrence and Texture of Yellow Ore in the No.4 and No.8 Ore Deposits.

Some ore textures involving sphalerite from the Furutobe mine, Akita Prefecture, Japan.

Some ore textures involving sphalerite from the Furutobe mine, Akita Prefecture, Japan.

Continuous carbonatitic melt–fluid evolution of a REE mineralization system: Evidence from inclusions in the Maoniuping REE Deposit, Western Sichuan, China

The Maoniuping REE deposit is a world-class deposit with 1.2 Mt of REO grading on average 2.89 wt.% REO. It is the largest in the 270-km long Mianning–Dechang REE belt and is associated with Himalayan carbonatite–alkalic complexes in the eastern Indo-Asian collisional zone, Western Sichuan Province, China. The deposit is hosted by nordmarkite stocks and carbonatite sills that display a radiometric age of 40 to 24 Ma and which intruded a Yanshanian granite pluton. The 40.3 to 27.8 Ma REE mineralization occurs as vein systems hosted in nordmarkite and carbonatite with minor altered granite and rhyolite. Four ore types are recognized based on ore texture and mineral assemblage: (1) disseminated; (2) pegmatitic; (3) brecciated; and 4) stockwork (stringer) types. Five mineralizing stages are confirmed according to vein crosscutting relationships, mineral assemblage and microthermometric results, these are: 1) carbonatite stage, 2) pegmatite stage, 3) barite–bastnaesite stage, 4) later calcite stage and 5) epigenetic oxidation stage. Varied inclusion assemblages are found in fluorite, quartz, bastnaesite, barite and calcite from stages 1 through to stage 4. The dominant inclusion types include: melt, melt–fluid, CO 2-rich fluid and aqueous-rich fluid inclusions. Fluid, melt–fluid and melt inclusion studies indicate that the ore-forming fluid resulted from the unmixing of carbonatite melt and carbonatitic fluid. Initial ore-forming fluids were high-temperature (600 to 850 °C), high-pressure (> 350 MPa) and high-density supercritical orthomagmatic fluids, characterized by SO 4-rich and multi-component composition (e.g. K, Na, Ca, Ba, Sr and REE). The dominant anion is not Cl, but SO 4. The evolution of the ore-forming fluid is from a melt–fluid at high temperature, through a CO 2-rich fluid at high to medium temperature to aqueous-rich fluid at low temperature. REE precipitation occurred from a high to medium temperature CO 2-rich fluid. The main mechanism for REE precipitation was phase separation of CO 2 and aqueous fluids resulting in a decrease of temperature and pressure.

The Talgan massive sulfide deposit, the southern Urals, Russia, as an example of subseafloor ore deposition from magmatic fluid

The sequence of orebody formation at the Talgan massive sulfide deposit; morphology of sulfide orebodies; mineralogy, texture, and structure of ore; chemical composition of minerals; and fluid inclusions and relationships between stable isotopes (S, C, O) in sulfides from ores and carbonate rocks are discussed. The deposit is localized in the Uzel’ga ore field of the northern Magnitogorsk Megazone. The sulfide ore is hosted in the upper felsic sequence of the Middle Devonian Karamalytash Formation, composed of basalt, basaltic andesite, and rhyodacite. Orebodies are irregular lenses lying conformably with host rocks. Pyrite, chalcopyrite, sphalerite, and fahlore are the major ore minerals; galena, bornite, and hematite are of subordinate abundance. Sulfide mineralization bears attributes of deposition under subseafloor conditions. The carbonate and rhyolite interlayers at the roofs of orebodies and the supraore limestone sequence served as screens. Zoning typical of massive sulfide deposits was not established. The study of fluid inclusions has shown that the temperature of the hydrothermal solution varied from 375 to 110°C. δ34S‰ ranges from −2.4 to +3.2‰ in pyrite, from −1.2 to +2.8‰ in chalcopyrite, and from −3.5 to +3.0‰ in sphalerite (CDT). These parameters correspond to an isotopic composition of magmatic sulfur without a notable percentage of sulfate sulfur. δ13C and δ18O of carbonates vary from −18.1 to +5.9‰ (PDB) and from +13.7 to +27.8‰ (SMOW), respectively. The carbon and oxygen isotopic compositions of carbonates from ores and host rocks markedly deviate from the field of marine carbonates; a deep source of carbon is suggested. The results obtained show that the main mass of polysulfide ore at the Talgan deposit was formed beneath the floor of a paleoocean. The ore-forming system was short-lived and its functioning did not give rise to the formation of zonal orebodies. Magmatic fluid played the leading role in mineral formation.

Modelling and optimization of hydrocyclone for iron ore fines beneficiation — using optical image analysis and iron ore texture classification

A new modelling technique for simulating hydrocyclone performance has been developed, in which particles in every size fraction of the feed ore are classified based on ore texture type, taking into account that the same ore texture types in every size fraction of the feed ore have similar mineral contents and densities. Mineral tracking by optical image analysis and newly-developed texture classification software was used in this technique to classify the feed ore particles by texture type and to determine the average particle density of each class in every size fraction. Particle density calculations took into account the reduction of porosity with reduction of particle size and the effect of different imaging magnifications for different size fractions. The data obtained about each class in every size fraction was used to create a virtual feed which was input to the hydrocyclone model to simulate the ore processing performance. For model validation, pilot-scale hydrocyclone beneficiation experiments were performed on an iron ore blend, using different hydrocyclone pressures and percent solids in the feed pulp. Model parameters were determined from one set of experimental results and the calibrated model was then used to predict the outcomes of the two subsequent experiments. Comparisons of the model and experimental results are presented and discussed. This new approach enables prediction of the recovery of each mineral and texture type in the products, calculation of the total product iron grade and recovery, and optimisation of the hydrocyclone performance for a given ore.

The Rhyolite-Hosted Volcanogenic Massive Sulfide District of Cuale, Guerrero Terrane, West-Central Mexico: Silver-Rich, Base Metal Mineralization Emplaced in a Shallow Marine Continental Margin Setting

The Cuale mining district, situated in the Cordillera Madre del Sur, ~30 km southeast of Puerto Vallarta, Mexico, comprises a number of small, polymetallic Pb, Zn, Cu, volcanogenic massive sulfide deposits (VMS), and vein and stockwork-hosted deposits with low-sulfidation epithermal characteristics. Massive sulfide mineralization contains high Ag ± Au grades when compared to similar deposits in western Mexico. The massive sulfide bodies are hosted by an interbedded rhyolitic volcanic and volcaniclastic succession, herein termed the Cuale volcanic sequence. Regionally, the Cuale volcanic sequence is part of the Zihuatanejo subterrane belonging to the Guerrero terrane, which is interpreted to be a complex Mesozoic accreted arc terrane hosting the majority of VMS deposits in Mexico. The rocks of the Cuale volcanic sequence are not metamorphosed and are only weakly deformed but exhibit intense chlorite, sericite, and quartz hydrothermal alteration. Three stratigraphic units are identified within the Cuale volcanic sequence: (1) a more than 400-m-thick footwall unit of quartz and plagioclase phyric rhyolite flows, related hyaloclastites, and subordinate heterolithic volcaniclastic breccias; (2) an ore horizon characterized by sedimentary rocks ranging from volcaniclastic conglomerates to black shales, as well as aphyric to quartz and plagioclase phyric rhyolite; and (3) late intrusions composed of quartz and plagioclase phyric rhyolite and subordinate andesitic dikes intruding all units. The sedimentary rocks of the ore horizon are intercalated with rhyolite tuff horizons, some of which exhibit welding or accretionary lapilli that are interpreted as evidence for shallow submarine or subaerial deposition. The rhyolites of the Cuale volcanic sequence are calc-alkaline with only limited compositional variations. However, the trace element signatures reveal a weak trend from the footwall rhyolites, which are characterized by low Zr/Y (1‐1.9) and low chondrite normalized La/Yb ratios (9 of 11 samples between 1.24 and 3.3), to slightly higher values in the ore horizon and late intrusive phase (Zr/Y = 1.6‐7.6 and chondrite normalized La/Yb ratios between 2.25 and 15). This compositional evolution can be explained by slightly decreasing degrees of partial melting related to a transition from amphibole-free assemblages for the footwall to amphibole-bearing assemblages in the residuum for the ore horizon and intrusive rhyolites. This interpretation is supported by the observed volcanic stratigraphy and indicates increasing pressure at the site of melt generation as the volcanic edifice was built. Zircons from five samples of the Cuale volcanic sequence have been dated by the U-Pb method, using isotope dilution thermal ionization mass spectrometry (ID-TIMS). Although some samples show evidence of a minor inherited Pb component, an age range of 157.2 ± 0.5 to 154.0 ± 0.9 Ma has been established. Cuale represents the oldest dated VMS camp in western Mexico as other VMS districts of western Mexico are between 151.3 and 138 Ma. Two styles of mineralization are recognized: volcanogenic massive sulfide mineralization (VMS) and late lowsulfidation epithermal vein and stockwork mineralization characterized by high Au (1.89 ppm) and Zn (2.35%) but low Cu (0.2%) contents. The VMS mineralization is contained in more than 15 small orebodies, which exhibit a range of mineralization styles. Orebodies interpreted as proximal are characterized by stockwork vein networks with high Au and Cu contents, underlying massive pyrite bodies, which grade laterally and vertically into sphalerite- and galena-rich massive sulfide ore. Stockwork mineralization is locally hosted by aphyric rhyolite. More distal orebodies, rich in Pb, Zn, and Ag, are hosted by black shale and locally exhibit laminated or brecciated sedimentary textures in the galena-sphalerite-pyrite ore. The sedimentary textures observed in some massive sulfide orebodies suggest that the sulfide has been transported and deposited in anoxic basins adjacent to the rhyolite domes. Some of the more significant orebodies show evidence of both distal sedimentary ore textures and proximal features indicated by intense quartz-sericite-pyrite alteration in the immediate footwall.

THE GENESIS OF SULFIDE ASSEMBLAGES IN THE FORMER WILHELMINE MINE, SPESSART, BAVARIA, GERMANY

The vein-type copper mineralization in the abandoned Wilhelmine copper mine, Sommerkahl, Spessart, northwestern Bavaria, Germany, is hosted by metamorphic rocks of the Spessart Crystalline Complex. These are overlain by Permo-Triassic sedimentary rocks, including the stratabound base-metal mineralization of the Kupferschiefer. Ore textures in the sulfide veins demonstrate three stages of mineralization: (i) An early stage (I) is characterized by colloform textures, documented by spherical, cockade-like or garland-shaped, monomineralic or polymineralic aggregates of tennantite I, enargite I, pyrite I, chalcopyrite I, bornite I and digenite I. (ii) During a subsequent stage of recrystallization (II), the colloform textures were overgrown by, or enclosed in, anhedral grains of tennantite II, enargite II, bornite II, digenite II, pyrite II and chalcopyrite II. Minimum temperatures of about 175°C for this stage can be estimated from the bulk composition of fine-grained to submicroscopic digenite–bornite intergrowths, exsolved from an initial 1 a solid solution with up to 55 mole % of bornite. Conversely, the assemblage pyrite + chalcopyrite + tennantite provides an uppermost temperature limit of ~440°C [at an a (S 2 ) = 10 −5 ]. (iii) A late stage of alteration (III) led to the replacement of the primary sulfides by yarrowite, spionkopite and rare covellite, together with goethite, under decreasing temperatures and rising activity of sulfur. The close spatial association of the Sommerkahl vein-type Cu deposit with the overlying Kupferschiefer suggests that this metal-rich bituminous shale played an important role in the formation of the ore veins. Sulfur isotope analyses yielded negative δ 34 S of −12.8 to −23.9‰, which indicate that the sulfur was derived from the overlying Kupferschiefer, presumably by hydrothermal leaching. Such a low-temperature mobilization could also be discussed for all or part of the metals forming the Wilhelmine ore, which later on recrystallized in response to hydrothermal activity (modified hydrothermal model). However, we prefer a strictly hydrothermal model, according to which all or most of the metals were derived from deep-seated sources, transported upward by hydrothermal fluids, and precipitated by thermochemical reduction of sulfate due to interaction with the sulfur-bearing organic matter and the pyrite of the Kupferschiefer. Formation of the sulfide ore veins in the former Wilhelmine mine is related to post-Variscan hydrothermal activity that affected the Spessart area in Middle Jurassic to late Early Cretaceous time.

Utilization of optical image analysis and automatic texture classification for iron ore particle characterisation

Optical image analysis is a very convenient tool for obtaining comprehensive information about fine iron ore size fractions. Data can be obtained on mineral abundances, porosity, particle shape and ore textures with a high level of accuracy. A range of techniques has been used to characterise iron ore samples on a particle-by-particle basis. Automatic textural classification of iron ore particles was used to establish classes containing particles with very similar mineral composition and texture. Image analysis coupled with probe analysis and mineral density measurements provided information about the chemical composition and density of each particle class. The combination of these results enabled a “virtual feed” to be created, which can be a key input into a beneficiation unit model for predicting its performance. Identification and classification of the textural type of each particle was performed according to the CSIRO-Hamersley Iron Ore Group Classification Scheme. If more detailed classification is needed, further classification can be performed based on dimensional, chemical or mineral criteria, such as the presence of certain minerals in particles or total iron content. Some deficiencies of the current image analysis procedures and their further improvement and automation are also discussed.

A shear zone related greenstone belt hosted gold mineralization in the Archean of West Greenland. A petrographic and combined Pb–Pb and Rb–Sr geochronological study

Widespread gold showings have been found in an Archean greenstone belt on Storø in Godthåbsfjord, West Greenland. The Storø Greenstone Belt (SGB) comprises metasedimentary and metavolcanic rocks with minor magnetite-rich bands. Gold mineralization is hosted by amphibolites, mica schists and garnetite rocks. Gold occurs in various mineral associations, but its presence in löllingite–arsenopyrite–pyrrhotite parageneses is prevalent. To a lesser extent, native gold occurs within seriticized plagioclase, in sheeted quartz veins, and as inclusions in garnet. Based on petrographic observations and comparison with similar ore mineral textures from other deposits we propose the following model for the deposition of the gold-bearing löllingite–arsenopyrite–pyrrhotite assemblage on Storø: In the first stage, arsenopyrite likely formed with gold in solid solution. In the second stage, gold-bearing arsenopyrite was replaced by löllingite and pyrrhotite. This took place during regional prograde metamorphism. Gold accumulated within löllingite as discrete grains. During subsequent retrograde metamorphic conditions, löllingite and pyrrhotite reacted and formed arsenopyrite, and gold was subsequently exsolved and preferably deposited as discrete grains or films along arsenopyrite–löllingite grain boundaries. During a later event a garnet overgrowth partly affected the gold-bearing löllingite–arsenopyrite–pyrrhotite assemblage. The isotopic composition of Pb released from these garnets by Pb stepwise leaching experiments (PbSL) and the bulk Pb isotopic compositions of arsenopyrite and pyrrhotite indicate a common source of Pb in the fluid from which first the sulfides and subsequently garnet were formed. Bulk Pb analyses and PbSL experiments of arsenopyrite and PbSL experiments of garnet yield ages of 2863 ± 24 Ma (2 σ; MSWD = 0.2) for arsenopyrite and 2748 ± 62 Ma (2 σ; MSWD = 220) for garnet. The 2863 ± 24 Ma age is thus a minimum age for the mineralization. Rb–Sr isotopic analyses of cogenetic biotite from within strongly mineralized zones reveal Middle Proterozoic (ca. 1.65 Ga) mineral ages, well in accordance with a regional reheating event known to have affected the area, and are thus interpreted as cooling ages thereof. This regional reheating event has apparently not affected the Pb–Pb isotopic system of arsenopyrite, implying that the blocking temperature of the U–Pb system in arsenopyrite is likely to be above ∼ 300 °C. The Late Archean ages for arsenopyrite and garnet are consistent with the regional amphibolite facies metamorphism related to major terrane assemblies in the study area occurring at that time. The ore textures preserved today are interpreted as those formed under this regional metamorphic event.

Copper, gold and silver enrichment in ore mylonites within massive sulphide orebodies at Hongtoushan VHMS deposit, N.E. China

The Hongtoushan Archaean Cu–Zn Volcanic-hosted Massive Sulphide (VHMS) deposit occurs in the Hunbei granite–greenstone terrane, Liaoning Province of NE China. The ores were metamorphosed (3.0–2.8 Ga) to upper amphibolite facies at temperatures between 600 and 650 °C, and underwent three phases of deformation. The deformation has transposed the stratiform orebodies into a Y-shaped configuration. Ore minerals in the deposit were originally composed of pyrite, pyrrhotite, chalcopyrite and sphalerite. Prograde metamorphism obliterated almost all the sedimentary textures in the ores, and the present ore textures were formed substantially during peak and post-peak metamorphism. Intensive remobilisations have occurred in the Hongtoushan deposit at various scales. There are more than 30 oreshoots inside the main massive sulphide orebodies. The oreshoots range from 2 to 30 m in length, 0.1 to 1.5 m in width, and 2 to 30 m in depth extension. Most run parallel or sub-parallel to the strike of the massive sulphide ore layers with only a few being developed in metamorphic rocks in the immediate vicinity of the sulphide orebodies. Almost all the sulphide and gangue minerals in the oreshoots have been affected by intense shearing. Pyrite is characterised by cataclastic flow; pyrrhotite, chalcopyrite and sphalerite exhibit strong plasticity. Hydrothermal alteration minerals in the oreshoots are dominated by deformation-free hornblende, actinolite, epidote, clinozoisite, biotite, muscovite, albite, carbonate and quartz. Cu, Au and Ag in the ore mylonites average 11 wt.%, 1.74 and 235 g/t, respectively. Compared to the massive sulphide ores, these oreshoots show remarkable enrichment of these elements with enrichment coefficients of 5.3, 5.0 and 4.6, respectively. We draw the conclusion that these oreshoots are actually ore mylonites overprinted by late-stage fluids. The presence of annealing textures in pyrrhotite and the absence in pyrite suggest that formation and succeeding annealing of the ore mylonite took place over the temperature range from 550 to 450 °C. Such a temperature range, combined with the geology and mineralogy, indicates that these ore mylonites and the derivative oreshoots were formed during upper-greenschist-facies retrograde metamorphism. Similarity of the ore mylonites to the massive ores in metal varieties and lead isotope ages suggest that Cu, Au and Ag of the overprinting fluids on the ore mylonites were derived mainly from the nearby massive sulphide ores. However, lead isotope compositions of the ore mylonites also suggest that an external source, such as the metamorphic host rocks, has also contributed to their metal budget.

Application of a cellular automata based simulation model of size reduction in mineral processing

The prediction of particle size and minerals distribution in products from size reduction processes is one of the fundamental problems in mineral processing. In this paper, a 2D model based on cellular automata is proposed to simulate particles generation by discriminatory size reduction. This model generates particles on images of ore textures, through the growing of nucleation seeds. The size reduction process is simulated as a growing process of seeds over representative images of the ore texture. Each particle grows according to rules related with the probability of agglomeration between different seeds. The final pattern represents the sections of particles corresponding to the mill product, for a specified milling time. Preliminary results of areal size distributions and mineral grade distribution shows values quite similar to experimental ones. The incorporation of an optimisation algorithm, will allow calibration of the model to improve these results.