Ore Textures Research Articles

Early Cambrian Black Shale-Hosted Mo-Ni and V Mineralization on the Rifted Margin of the Yangtze Platform, China: Reconnaissance Chromium Isotope Data and a Refined Metallogenic Model

The transgressive basal unit of the Early Cambrian black-shale sequence along the rifted margin of the southeastern Yangtze platform hosts a wide spectrum of marine sedimentary rocks distributed over more than a length of 1,000 km. A few-centimeter-thick sulfide-rich black shale units have spectacular metal tenors (Mo and Ni in the percent range, PGE + Au around 1 ppm) and consist of submillimeter-scale laminated sulfide and carbonaceous material and centimeter-sized pebble-like rip-up clasts of Mo-S-C compounds, pyrite, and Ni-rich polymetallic sulfides in carbonaceous and phosphate-rich matrix. The δ 53/52 Cr authigenic values of Mo-Ni sulfide-rich black shale samples from the Zunyi mining district (Guizhou province), as well as from the Sancha district (Hunan province), 400 km northeast, have a mean of 0.96 ± 0.22‰ ( n = 8), while V-rich black shale from both districts has a mean of 1.34 ± 0.46‰ ( n = 5). These data indicate significantly positively fractionated values compared to igneous silicate Earth. The Cr isotope values of the studied shales compare with recent findings of positively fractionated δ 53/52 Cr values in Late Neoproterozoic-Phanerozoic marine carbonates and shale/mudstones and attest for the operation of an intensified oxidative surface Cr cycle from at least around ~0.75 Gyr ago. We propose that the major change in Cr cycling around the Precambrian-Cambrian boundary was caused by a significant rise of atmospheric oxygen levels. The Cr isotope data confirm earlier conclusions from Mo and Os isotopes, which indicate a seawater metal source with ultimate metal supply by oxidative weathering of continental crust. In particular, covariation trends of Re-Os and of platinum-group elements display a distinct seawater signature, and a degree of metal enrichment which is an extension of the normal black shale pattern. The Mo-Ni sulfide-rich black shale units are probably produced by winnowing of subaquatic hardground deposited under euxinic conditions. The same stratigraphic level also hosts anoxic/suboxic V-rich black shale (V in illite) and huge phosphorite deposits in oxic/suboxic settings, bedded barite, as well as 10s of meter-thick sapropelic alginite (combustible shale). We show that the extreme metal enrichment of the Mo-Ni sulfide-rich black shale can be understood as a combination of redox cycling and bottom-water/sediment-interface scavenging under euxinic conditions (Mo, noble metals), and of oxidation (remineralization) of organic matter settling from the photic zone, with sulfide fixation of Ni and other biophile elements under denitrifying and sulfate-reducing conditions. High biological activity (as in coastal upwelling settings with high nutrient supply), very low clastic accumulation rate (as in protected basins), and low organic matter deposition (high rate of remineralization) in stratified oxic-suboxic-euxinic basins are requirements for advanced metal enrichment in black shales. A special bathymetric condition of slope deposition with intrabasinal olistostrome-like mass movement and gravitational winnowing explains both the peculiar ore textures and the further metal enrichment to ore grade in the Early Cambrian rock units on the Yangtze platform. The Mo-Ni sulfide-rich sediments can be regarded as the euxinic variant of the marine hydrogenous ore deposit spectrum, where ferromanganese nodules/crusts represent the oxic variant of extreme fractionation from seawater.

Mineralogical Studies of Low-Grade Iron Ore from Jharkhand–Orissa Region, India

Due to increasing demand of steel in domestic market and depletion of high-grade iron ores day by day, industries as well as policy makers have started working on low-grade iron ores for iron and steel making industries. There are huge scopes for low-grade iron ores in iron and steel making. Aim of this study is to understand of morphology, texture, phase identification and properties of iron ore. Banded iron formation type low-grade iron ore is taken for the mineralogical studies. Characterizations of minerals are carried out by X-ray fluorescence spectroscopy, X-ray diffraction, scanning electron microscopy, energy dispersive analytical X-ray and optical microscopy techniques. Hematite is found as the dominant mineral; whereas quartz and kaolinite are as gangue minerals in low-grade iron ore. By this study, conventional liberation or beneficiation practice can be optimized for low-grade iron ore.

Iron ore textural information is the key for prediction of downstream process performance

Textural information or information about the presence of porosity, different material or mineral types and their structural arrangement in iron ore is crucial for understanding, predicting and optimising downstream processing performance. Ores with the same chemical and mineral composition may behave very differently during downstream processing due to differences in textural components.To produce a textural description of iron ore, it is preferable to use an automated system to avoid subjectivity and to collect additional information about mineral abundance, liberation and association. CSIRO created a unique dedicated optical image analysis software package for automated textural classification and characterisation of different minerals, sinters and coke. This software, called Mineral4/Recognition4, has been used extensively to collect data for this article.Four case studies of CSIRO research are presented to demonstrate the importance of textural information.•The first example shows that iron ore samples with different texture but similar mineralogy undergo different degrees of assimilation in compact sintering.•The second example shows that empirical modelling of sinter properties was improved considerably after introducing textural information.•The third example demonstrates the application of classification by ore texture to model and optimise hydrocyclone performance.•The last example is an experimental study of ultrasonic treatment of hematitic–goethitic iron ore fines. It demonstrates how the resulting breakdown or deagglomeration of different particles, and the mineral deportment, can be better understood when textural information is also considered.In all cases, the availability of textural information was critical, providing a better prediction of process performance or a deeper understanding of the unit process.

New isotopic evidence bearing on bonanza (Au-Ag) epithermal ore-forming processes

New Cu, S, and Pb isotope data provide evidence for a magmatic source of metal(loid)s and sulfur in epithermal Au-Ag deposits even though their ore-forming solutions are composed primarily of heated meteoric (ground) waters. The apparent isotopic discrepancy between ore metals and ore-forming solutions, and even between the ore and associated gangue minerals, indicates two different sources of epithermal ore-forming constituents: (1) a shallow geothermal system that not only provides the bulk of water for the ore-forming solutions but also major chemical constituents leached from host rocks (silica, aluminum, potassium, sodium, calcium) to make gangue minerals and (2) metals and metalloids (As, Te, Sb, etc.) and sulfur (±Se) derived from deeper magma bodies. Isotopic data are consistent with either vapor-phase transport of metal(loids) and sulfur and their subsequent absorption by shallow geothermal waters or formation of metallic (Au, Ag, Cu phases) nanoparticles at depth from magmatic fluids prior to encountering the geothermal system. The latter is most consistent with ore textures that indicate physical transport and aggregation of nanoparticles were significant ore-forming processes. The recognition that epithermal Au-Ag ores form in tectonic settings that produce magmas capable of releasing metal-rich fluids necessary to form these deposits can refine exploration strategies that previously often have focused on locating fossil geothermal systems.

Deriving the ideal ore texture for microwave treatment of metalliferous ores

High power density microwave treatments on metalliferous ores have historically been shown to reduce ore competency prior to beneficiation at economically feasible energy inputs. However, the relationship between mineralogical textural features and the extent of the microwave-induced fracturing had previously been limited to qualitative descriptions or simplistic two-phase numerical models, which could not account for the complex mineral assemblages in real ores. In this paper, mineralogy, grain size, dissemination, textural consistency and mineral associations were determined for 13 commercially exploited nickel, copper and lead–zinc ores using a Mineral Liberation Analyser (MLA). The ores were subjected to high power density microwave treatments at up to 25kW in a single mode cavity with microwave energy inputs of approximately 0.5–10kWh/t, and the subsequent reductions in ore competency were measured by the Point Load Test. The ores that demonstrated the greatest reductions in strength typically contained between approximately 2wt% and 20wt% of highly microwave-absorbing minerals, with a native grain size d50 greater than approximately 500μm, constrained by hard matrix minerals such as quartz and feldspar. Texturally consistent ores with a high proportion of amenable textures also demonstrated the highest average reductions in strength. These findings support the qualitative descriptions and numerical modelling results available in the literature and provide a baseline for selecting likely candidate ores for microwave treatments in the future.

ORE MINERALOGY AND MINERAL CHEMISTRY OF PYRITE, GALENA, AND SPHALERITE AT SORIPESA PROSPECT AREA, SUMBAWA ISLAND, INDONESIA

The Soripesa prospect area is located at Maria village, Wawo district, Bima region in the East Sumbawa Island, Indonesia. Lithology is dominantly composed of a lithic-crystal tuff of andesitic and dacitic composition and bedded limestone. The polymetallic epithermal quartz veins are hosted by andesitic volcaniclastic rocks. Within these veins, multiphases, colloform-crustiform, bedding to massive textures with pyrite, sphalerite, galena, chalcopyrite, chalcocite, azurite, and malachite are observed. Selected samples were analyzed by using ore microscopy and SEM-EDX. Ore minerals show replacement, ex-solution, colloform, and zonal textures. The paragenesis diagram was made from a careful study of polished sections and thin sections. Textures of ore minerals such as banded, exsolution, replacement, and zone, have been interpreted to correspond to the order of deposition. In pyrite, the average content of Co (0.45 wt.%) is higher than Ni content (0.14 wt.%) and it means that their origin may be hydrothermal origin. Average content ratio, Co:Ni is 2.81. Galena shows a low Ag content of 0.07 %in average. But they show a high Au content of 1.48 %in average. Sphalerite shows a low Fe content of 1.04 %in average and occasionally chalcopyrite inclusion/disease also occurred. Ga and Ge contents are also high in sphalerite. Co>Ni in pyrite, low content of Ag in Galena, low content of Fe and mole % FeS in sphalerite, high content of Ga and Ge, and log (Ga/Ge) in sphalerite, show that pyrite, galena, and sphalerite from Soripesa prospect area were formed under low temperature condition of hydrothermal fluid. Keywords: Ore textures, paragenesis, deposition, hydrothermal fluid, low temperature

Origin of the Luping Pb deposit in the Beiya area, Yunnan Province, SW China: Constraints from geology, isotope geochemistry and geochronology

The Luping Pb deposit, a newly discovered Pb–(Fe–Au–Ag–Zn) polymetallic deposit close to the Cenozoic Beiya porphyry-related Au–(Fe–Cu–Pb–Zn–Ag) deposit, is located in the central Sanjiang (three rivers: Jinshajiang, Lancangjiang and Nujiang) region (western Yangtze Block) in southwestern China. Ore bodies of the Luping deposit are mainly stratiform type, host in the Middle Triassic Beiya Formation limestone and Lower Triassic Qingtianbao Formation sandstone, and occur along NNW-trending bedding-plane faults within axis of Beiya–Qinghe syncline. Three ore types, namely the Pb–Au sulfide, Fe–Au, and Au ores, can be discerned regarding their ore metal association with reserves of 2.7million tonnes (Mt) (average grade: 3.06wt.% Pb, 0.66g/t Au, 41g/t Ag, and 0.82wt.% Zn), 3Mt (average grade: 30wt.% Fe, 0.96g/t Au and 31g/t Ag), and 0.67Mt (average grade: 2.16g/t Au and 13g/t Ag), respectively. Pb–Au sulfide ores contain mainly galena, siderite, native gold, sphalerite, and pyrite, with minor chalcopyrite, pyrrhotite and arsenopyrite, and gangue minerals such as quartz, calcite and dolomite. Sulfide ore textures include mainly brecciated, banded and vein, and minor massive and disseminated. LA-ICP-MS zircon U–Pb dating of alkali-rich quartz syenite porphyry from drill holes yielded a Late Eocene (Priabonian) age (36.2±0.4Ma), broadly coeval to the zircon U–Pb ages (ca. 38Ma–31Ma) of alkaline porphyries and molybdenite Re–Os age (36.9±0.3Ma) from the Beiya deposit near Luping. Calcite separates have δ13CPDB and δ18OSMOW values ranging from −5.8‰ to −5.1‰ (average: −5.6‰) and +13.2‰ to +14.7‰ (mean: +13.6‰), respectively, suggesting association with the mantle. The sulfides have a narrow range of δ34SCDT values (+0.8‰–+2.8‰, mean +1.8‰), which is similar to that of mantle-derived S (0±3‰). 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of the sulfides range from 18.62 to 18.69, 15.66 to 15.70 and 38.92 to 39.08, respectively, similar to those of the porphyry whole-rock (206Pb/204Pb 36 Ma=18.61–18.71, 207Pb/204Pb 36 Ma=15.64–15.77 and 208Pb/204Pb 36 Ma=38.88–39.32). Ore deposit geology and C–O–S–Pb isotopic data suggest that the sources of metals and ore-forming fluids were likely to be derived from the alkali-rich porphyries in the Beiya area. We propose that the Luping deposit is best classified as an epithermal Pb polymetallic deposit associated with the Beiya alkaline porphyries (ca. 36Ma–31Ma). Similar to other deposits related to the Cenozoic alkaline porphyries in the Jinshajiang–Red River (Ailaoshan) metallogenic belt, the Luping deposit may have been formed in the India–Eurasia post-collisional setting, when partial melting occurred in the Triassic stagnant slab that thickened the lower crust and the upper mantle, producing the alkaline magmatism and its associated mineralization.

Mineral transformations and textural evolution during roasting of bog iron ores

The processes occurring during roasting of bog iron ores were characterized using TG–DTG–DTA–QMS, XRD, FTIR and specific surface analysis. Removal of physically adsorbed water is followed by dehydroxylation of iron oxyhydroxides and oxidation of organic matter at 200–600 °C. The main product of the transformations is disordered nanocrystalline (proto)hematite or hematite/maghemite mixture, depending on organic matter content and heating conditions. The conversion of iron oxyhydroxides to hematite occurs at temperatures different than those reported for pure compounds. At higher temperatures, protohematite undergoes recrystallization to the stoichiometric hematite, and manganese oxides are partially reduced. At 1000 °C, the roasting products consist of hematite and cristobalite together with Mn–Fe spinels if the initial ore contained Mn oxides. The admixtures of various secondary silicates were encountered as well. Low- to moderate-temperature roasting slightly affects the specific surface area and lowers volume of micropores. The high-temperature transformations lead to decrease in the specific surface area and to the destruction of porous texture of the bog iron ores. Although the general course of the processes during roasting was similar in all the samples, some of their details as well as mineralogy and properties of the products are highly dependent on the composition of the initial material.

Photo- and digital-detectors for quantitative digital densitometry for distribution analysis of ores by means of gamma-activation

A method for photo processing of nuclear film detectors, which ensures higher reproducibility (about 2 %) of densitometry has been suggested. Application of the method allows for recovering fine mineralogical texture of a complex polymetallic ore of the Norilsk deposit with time series autoradiograms after computer processing. Some problems concerning interpretation of the results are discussed. It has been shown that application of digital imaging plates as a detector demonstrates high reproducibility suitable for precise quantitative autoradiography. The developed method of digital gamma-activation autoradiography via short-lived radionuclides may be successfully used for screening analysis of large area ore’s thin sections.

Copper Ores in Mawat Ophiolite Complex (Part of ZSZ) NE Iraq

Ore microscopy study for some samples from Mawat complex NE Iraq (part of Zagros Suture Zone) showed the presence of copper ore minerals was irregularly distributed, mainly throughout the basic rocks of Mawat Ophiolite Complex. The main sulfides minerals identified were: chalcopyrite, pyrite, bornite, chalcocite and covellite. Iron hydroxides/oxides including goethite and magnetite were also observed associated with sulfide ore. Chalcopyrite and pyrite were the main primary minerals from which other ores were formed. They were originated at the orthomagmatic stage, and also deposited from aqueous solution of final stage in the consolidation of a magma forming vein textures which was observed in many samples. Two types of pyrite generations were recognized. They were pre-tectonic pyrite and post-tectonic pyrite. The secondary recognized ore textures were rim, pseudo morph, zoning, relicts, atoll and vein replacement textures. Secondary textures resulting from exsolution were also presented. Three stages of mineralization could be concluded: orthomagmatic stage (Earliest), the hydrothermal activations stage (Hypogene) and the weathering stage (Supergene). Paragenaticaly, the primary copper sulfides including chalcopyrite and bornite were responsible for other copper ores formation. These minerals were observed disseminated throughout the host mafic rocks which were considered as endogenic deposits (magmatic and hypogene origin) and they were modified as small patches of chalcocite and covellite.

Skarn Textures of Yongping Copper Deposit, Yanshan, Jiangxi Province

Yongping copper deposit is located at the collisional belt between Yangtze Block and Cathaysia Block. It is a historic mine, and plays an important role in China’s copper production. Owing to the complicated geologic settings, the genesis of this deposit remains controversial for decades; VMS and generalized Skarn have been proposed. According to the VMS model (Gu et al., 2007), Yongping deposit formed in late Paleozoic basins, appearing as strata-bound massive sulfides, and underwent remobilization of ore and magmatic hydrothermal overprinting by Mesozoic magmatism. Nevertheless, Mao et al. (2011) proposed a tectono-genetic model for porphyry-skarn-stratabound Cu-Au-Mo-Fe which is really suitable for Yongping Deposit. However, all these studies focus on regional scale rather than local scale. Therefore, we try to concentrate on small scale, ore textures for instance, to search for evidence of the genesis of this deposit.

Geological features and S isotope composition of tin deposit in Dachang ore district in Guangxi

The geological investigation of the Dachang ore field was carried out in detail, and the geological characteristics of the deposits, consisting of the Tongkeng and Dafulou deposits, were observed and researched systematically. It suggests that the mineralization types of Changpo ore are composed of cracking vein, thin vein, bedded vein and thin vein-net vein disseminated types. The cracking vein ore is usually lens-shaped in the vertical direction. The thin vein ore is always characterized by a stable trend and tendency. The bedded ore always occurs along the strata in the way of filling and metasomatism in the fracture system. In terms of Bali-longtoushan ore, it is characterized by complicated mineral components and a variety of minerals. More generally, ore textures consist of the anhedral-subhedral shapes and thin particle, and secondary with the interstitial texture, solid solution separation texture, dissolution texture, corona texture, and crushing texture, yet ore structures include the massive, veinlet, disseminated, banded, miarolitic, biological residual and brecciated structure. In addition, the sulfur isotopes of the metal sulfide were analyzed. The results show that the d34S values of Tongkeng ore range from −0.30% to 1.38% with more dispersed characteristics, yet in terms of Dafulou ore, the d34S values are from −0.15% to 0.22% which are characterized by more focused. This indicates that the sulfur isotope composition has large difference between the different deposits. The sulfur isotope values of the Dafulou ore are concentrated relatively, yet are dispersed for the Tongkeng ore. Likewise, there are also divergences of sulfur isotopes for the different minerals. The sulfur isotope values of pyrrhotine are dispersed, yet are homogeneous for pyrite. In short, the divergence of the sulfur isotope is reflected in both the different deposits and minerals, all of these may account for the difference of sulfur sources.

Structure and texture of ores from the Pravourmiisky deposit in the Khabarovsk region of Russia as evidence for its formation conditions

The structure and texture of the ores and ore-bearing metasomatic rocks at the Pravourmiisky tin-tungsten deposit are discussed. The zonal distribution of various genetic and morphological types of structure and texture has been revealed in vertical sections of the ore zone. The most diversity with a prevalence of cataclastic texture is characteristic of the middle (in section) ore zone segment, where the main economic greisen-type tin mineralization is hosted. Upsection, this mineralization gives way to the quartz-tourmaline Cu ore with widespread exsolution microtextures indicating high-temperature ore deposition. The structural and textural diversity of the ore is caused by the long-term geological history of the deposit under conditions of low-amplitude strike-slip and thrust faulting when compression was repeatedly followed by periods of relative quiescence or weak extension.

The Mineral Characteristics and Occurrence of Gold in Nali Gold Deposit, Guangxi

Using the analysis techniques of polarizing optical microscope and electron probe, mineral composition, ore texture and structure and the occurrence of Au in the primary ore are studied. The research shows that the main ore minerals in the ore include realgar, pyrite and arsenopyrite etc. Sulfur-stibarsen is the main carrier of Au, and is the major associated mineral of realgar, surrounded mainly by realgar, and partly is the associated mineral of arsenopyrite. The existence of visible gold and microscopic gold in the ore of this mining area can be excluded, and the gold might exist mainly in the form of inclusion gold, which is the so-called “nanoAu”.

Significant Zn–Pb–Cu remobilization of a syngenetic stratabound deposit during regional metamorphism: A case study in the giant Dongshengmiao deposit, northern China

The giant Dongshengmiao Zn–Pb–Cu deposit is located in the Langshan district, northern China. The ores are hosted within a Proterozoic rift sequence, which underwent lower greenschist facies metamorphism and shear deformation during development of Early Cretaceous intraplate orogenic belt. Northwest-dipping thrust faults, which share similar orientations and dip angles with the orebodies, are well developed in the mining area. Syngenetic stratabound sulfides were formed during the Proterozoic rifting event, but syngenetic ore textures have seldom been preserved except for some pretectonic fine-grained pyrite. Petrological observation, 39Ar/40Ar geochronology, combined with previous isotopic and fluid inclusion studies indicates that significant Zn–Pb–Cu remobilization took place as a result of thrust faulting associated with metamorphic devolatilization of ore-hosting rocks at ca. 136Ma, coeval with the intraplate orogeny and regional crustal shortening. Sulfides were redistributed in shear structures or along grain boundaries of ore-hosting carbonates, and Fe-rich carbonates were ideal sites for Zn–Pb–Cu precipitation.

Metallogenesis of the Zhibo and Chagangnuoer volcanic iron oxide deposits in the Awulale Iron Metallogenic Belt, Western Tianshan orogen, China

The Zhibo and Chagangnuoer iron ore deposits occur at the same volcanic edifice and are situated along the eastern segment of the Awulale Iron Metallogenic Belt in the Western Tianshan orogen of China. The former deposit is exposed near the caldera and the latter lies on the northwestern flank of the volcanic structure. The ore mineralization, the ore texture, the ore bodies structures, the trachytic to trachy andesitic host rocks and the geological characteristics of the Zhibo and Chagangnuoer deposits indicate a magmatic source for the iron oxide mineralization. This is supported by the observation that the vast majority of the Zhibo and Chagangnuoer magnetites have δ18O values between >+0.9‰ and <+4.00‰ consistent with a precipitation from ‘normal’ subduction type intermediate to felsic magmas or high-temperature magmatic fluids. However some magnetites with low δ18O values (<+0.9‰) are thought to have formed in equilibrium with a low to moderate temperature hydrothermal fluid as is evidenced by pervasive and vein-type controlled (skarn-type) alteration of the ore and host volcanic rocks. Furthermore, the trace element composition of the equilibrium ortho-magmatic magnetites was reset during this hydrothermal alteration event. Major and trace element characteristics indicate that the volcanic rocks, the granitoids and the iron ore bodies were formed in a continental arc setting during the Early Carboniferous. The ore-hosting volcanic rocks probably originated from melting of a mantle wedge that was metasomatized during subduction. Laser ablation-ICPMS (LA-ICPMS) U–Pb zircon ages of 350±2Ma and 361±7 Ma were obtained for zircon from the trachy andesitic host rock of the iron ore exposed near the caldera of the volcano and the highly altered fine grained garnet-dominated host rock on the flank of the volcano, respectively. A U–Pb zircon age of 321±2Ma was gained for the rhyolite overlying the ore bodies at the Chagangnuoer deposit, which were intruded by a granitic pluton with a U–Pb zircon age of 319±2Ma. Furthermore, the minerals of the skarn-type alteration zone at the Chagangnuoer deposit yield an Sm-Nd isochron age of 313±7Ma (Grt-Act-Epi-whole rock). Considering previous geochronological studies, a 350–330Ma age range as observed in the host rocks of the Zhibo mine is believed to display a period of continuous volcanic activity and associated ortho-magmatic, high-temperature iron oxide mineralization during subduction. The iron oxide mineralization in both deposits is thought to have been derived from the same magma source, as the ore-hosting volcanic rocks are similar to ‘Kiruna-type’ iron oxide ores. Subsequently, the volcanic host rocks and the iron ore bodies were variably hydrothermally altered during subduction (ca. 319–313Ma). The alteration zones were probably facilitated by the infiltration of hydrothermal fluids along faults, which were created by the intrusion of a granitic pluton.

Lithology, mineralogy and geochemical characterizations of sediment-hosted Sr–F deposits in the eastern Neo-Tethyan region – With special reference to evaporation and halokinesis in Tunisia

The Neo-Tethyan basin is known for its sediment-hosted Sr deposits in Spain, Turkey, Cyprus, and the Gulf Region. Sediment-hosted Sr–F deposits with base metals formed in the rim sinks and on top of salt domes resulting from halokinesis of Triassic evaporites near the southern edge of the Mediterranean Sea in Tunisia. These evaporites delivered part of the elements, created a basin-and-swell topography and provided the local and regional unconformities to which many of the mineral deposits are related. Five mineralizing processes, each with characteristic sedimentary ore textures, are related to this subsurface salt movement: (1+2) Early- and late-stage replacement (“zebra rocks”), (3) hydraulic fracturing (“fitting breccia” sensuDill and Weber, 2010b), (4) remobilization (“spinifex structures”), and (5) open-space filling (“caves and vein-like deposits”). Basinal brines from Mesozoic aquifers delivered Pb, Zn, Cd, REE, Y, Hg, and Se, while Sr, Cs, Be, Li, Cu and Co have been derived from Cenozoic salinas of the Neo-Tethyan basin. Mixing of Mesozoic and Cenozoic brines between 28 and 19Ma provoked the emplacement of Sr–F mineralization at temperatures below 200°C under strong alkaline conditions. Epigenetic polyphase Sr–F deposits bearing base-metals which are closely related to salt domes (Tunisian-Type) may be traced into epigenetic monophase Sr deposits within bioherms (Cyprus-Type) devoid of Pb, Zn and F. Moving eastward, syndiagenetic monophase Sr deposits in biostromes (Gulf-Type) herald the beginning of Sr concentration in Miocene sabkhas of the Neo-Tethys. The current results are based upon field-related sediment petrography and on mineralogical studies, which were supplemented by chemical studies. The present studies bridge the gap between epigenetic carbonate-hosted MVT and syndiagenetic evaporite deposits, both of which developed during the same time span (Neogene) and were hosted by the same environment (near-shore marine marginal facies of the Neo-Tethys basin).

Mineragraphic Study of Manganese Ore Deposits of Kandri, Mansar, Beldongri and Satak Mines, Nagpur District (Maharashtra) Central India

In Nagpur, M.P.-Maharashtra Manganese Belt is exposed which is part of Sausar Belt. In Nagpur, four manganese mines viz Mansar, Kandri, Beldongri and Satak are selected for the ore microscopic study. These mining areas are situated in that part of Central India which are highly deformed and metamorphosed and are characterised by high grade amphibolite facies to low grade greenschist facies. The Precambrian manganese ore deposits of Nagpur are formed by the metasediments of Sausar belt which is rich in manganese sediments. The manganese ores of the study area are co-folded with distinct rock types i.e. “gondites” and predominantly composed of a mixture of braunite, bixbyite, hausmannite, hollandite, jacobsite, vredenburgite, pyrolusite, cryptomelane and psilomelane. The dominant rocks are tirodi migmatitic gneiss, argillaceous and calcareous rock types. This paper discusses the mineralogy of the manganese ores of Kandri, Mansar, Satak and Beldongri. After the careful assessment of mineralogy, texture and paragenesis of the manganese ore of the study area, it has been concluded that the manganese ores of the study area are formed by the multiple processes i.e. metamorphism and supergene enrichment. These supergene enrichments are evidence of colloidal influx. Occurrence of free braunite, free hausmannite and free jacobsite indicates the high grade metamorphism. Pyrolusite, rhodonite, and psilomelane-cryptomelane are formed by supergene enrichment. The presence of lamellar twinning in braunite indicates about shear pressure. In this area the different manganese mineral assemblages and their textures suggest about the occurrence of different metamorphic activity and depositional environment. The texture and paragenesis of manganese ore of the study area are discussed here.

Ore microscopic characterization of mineralized rocks at the Bi’r Tawilah gold prospect, Saudi Arabia

Ore microscopic investigation of core samples from the Bi’r Tawilah gold prospect enables to distinguish three main groups of ore paragenesis from oldest to youngest, namely magmatic, hydrothermal and supergene. The ore minerals are encountered in sheared and mineralized ophiolitic serpentinites and a variety of hydrothermally altered granitic rocks that include quartz–diorite, granodiorite, monzogranite and quartz–feldspar porphyry. Magmatic ore minerals are chromite, magnetite, ilmenite and pyrrhotite. Hydrothermal ore minerals are the most common (>90%) and they follow the paragenetic sequence: rutile, sphalerite, chalcopyrite, pyrite (with visible gold and electrum inclusions) and arsenopyrite. Supergene ore minerals are ferric Ca-arsenate and ferric oxyhydroxide (±goethite) which represent the weathering products of pyrite and arsenopyrite, respectively. The non-arsenian nature of pyrite and late formation of arsenopyrite all together suggest mobility of As in the hydrothermal fluids until late stages. There are no remarks of ore metamorphism as all events took place post-orogenically despite zonal arrangement of inclusions in pyrite that can result from paucity of crystallization. Ore textures indicate change from early relatively reducing hydrothermal fluids at low fS2 that crystallize pyrrhotite which is converted into pyrite when the fluid becomes relatively more oxidizing at high fS2 and pH <5. On the other hand, the supergene enrichment zone at the uppermost 40m is dominated by exclusively oxidizing fluids (pH >7) at low temperature up to ∼75°C. Supergene ferric Ca-arsenate is a cellular-structured hydrous phase with distinct As5+↔Fe3++Ca2+ ionic substitution in three distinct domains.

Structure of magnetite lodes at the Estyunino iron deposit in the central Urals

The structure of magnetite lodes is determined by iron and sulfur distribution, as well as texture and structure of ore. These features have been revealed by documentation of cores from ore intervals in exploration boreholes penetrating two main lodes 21 and 22 of the Estyunino iron deposit. The documentation of cores was accompanied by sampling for microscopic examination of texture and structure of ore and selection of Fe and S contents in ore. Then these data were summarized as sections of the lodes. It was established that the structure of magnetite lodes is characterized by conformable ore layers distinguished by texture, structure, and Fe and S contents. Banded and spotty ores containing less than 50% magnetite are predominant. Layers of homogeneous massive ore are infrequent. The textural pattern indicates a volcaniclastic nature of host rocks. The spotty texture is characteristic of hyaloclastites with vitreous shards. The banded texture with oriented distribution of fiamme is inherent to volcaniclastic rocks. In both cases, magnetite selectively replaces dark-colored vitreous fragments and is also dispersed in the salic matrix and lava fragments. No indications of crosscutting superposed relationships are observed. The available data can be satisfactorily explained by an impregnation-metasomatic mode of ore deposition.