Magnetite Skarn Research Articles

LA-ICP-MS U-Th-Pb Dating and Trace Element Geochemistry of Allanite: Implications on the Different Skarn Metallogenesis between the Giant Beiya Au and Machangqing Cu-Mo-(Au) Deposits in Yunnan, SW China

The giant Beiya Au skarn deposit and Machangqing porphyry Cu-Mo-(Au) deposit are located in the middle part of the Jinshajiang–Ailaoshan alkaline porphyry metallogenic belt. The Beiya deposit is the largest Au skarn deposit in China, whilst the Machangqing deposit comprises a well-developed porphyry-skarn-epithermal Cu-Mo-(Au) mineral system. In this paper, we present new allanite U-Th-Pb ages and trace element geochemical data from the two deposits and discuss their respective skarn metallogenesis. Based on the mineral assemblage, texture and Th/U ratio, the allanite from the Beiya and Machangqing deposits are likely hydrothermal rather than magmatic. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) allanite U-Th-Pb dating has yielded Th-Pb isochron ages of 33.4 ± 4.6 Ma (MSWD = 0.22) (Beiya) and 35.4 ± 9.8 Ma (MSWD = 0.26) (Machangqing), representing the retrograde alteration and magnetite skarn mineralization age of the two deposits. The Beiya and Machangqing alkali porphyry-related mineralization are synchronous and genetically linked to the magmatic hydrothermal activities of the Himalayan orogenic event. Major and trace element compositions reveal that the Beiya allanite has higher Fe3+/(Fe3+ + Fe2+) ratios, U content and Th content than the Machangqing allanite, which indicate a higher oxygen fugacity and F content for the ore-forming fluids at Beiya. Such differences in the ore-forming fluids may have contributed to the different metallogenic scales and metal types in the Beiya and Machangqing deposit.

Porphyry-Style Petropavlovskoe Gold Deposit, the Polar Urals: Geological Position, Mineralogy, and Formation Conditions

Geological and structural conditions of localization, hydrothermal metasomatic alteration, and mineralization of the Petropavlovskoe gold deposit (Novogodnenskoe ore field) situated in the northern part of the Lesser Ural volcanic–plutonic belt, which is a constituent of the Middle Paleozoic island-arc system of the Polar Urals, are discussed. The porphyritic diorite bodies pertaining to the late phase of the intrusive Sob Complex play an ore-controlling role. The large-volume orebodies are related to the upper parts of these intrusions. Two types of stringer–disseminated ores have been revealed: (1) predominant gold-sulfide and (2) superimposed low-sulfide–gold–quartz ore markedly enriched in Au. Taken together, they make up complicated flattened isometric orebodies transitory to linear stockworks. The gold potential of the deposit is controlled by pyrite–(chlorite)–albite metasomatic rock of the main productive stage, which mainly develops in a volcanic–sedimentary sequence especially close to the contacts with porphyritic diorite. The relationships between intrusive and subvolcanic bodies and dating of individual zircon crystals corroborate a multistage evolution of the ore field, which predetermines its complex hydrothermal history. Magmatic activity of mature island-arc plagiogranite of the Sob Complex and monzonite of the Kongor Complex initiated development of skarn and beresite alterations accompanied by crystallization of hydrothermal sulfides. In the Early Devonian, due to emplacement of the Sob Complex at a depth of approximately 2 km, skarn magnetite ore with subordinate sulfides was formed. At the onset of the Middle Devonian, the large-volume gold porphyry Au–Ag–Te–W ± Mo,Cu stockworks related to quartz diorite porphyry—the final phase of the Sob Complex— were formed. In the Late Devonian, a part of sulfide mineralization was redistributed with the formation of linear low-sulfide quartz vein zones. Isotopic geochemical study has shown that the ore is deposited from reduced, substantially magmatic fluid, which is characterized by close to mantle values δ34S = 0 ± 1‰, δ13C =–6 to–7‰, and δ18O = +5‰ as the temperature decreases from 420–300°C (gold–sulfide ore) to 250–130°C (gold–(sulfide)–quartz ore) and pressure decreases from 0.8 to 0.3 kbar. According to the data of microanalysis (EPMA and LA-ICP-MS), the main trace elements in pyrite of gold orebodies are represented by Co (up to 2.52 wt %), As (up to 0.70 wt %), and Ni (up to 0.38 wt %); Te, Se, Ag, Au, Bi, Sb, and Sn also occur. Pyrite of the early assemblages is characterized by high Co, Te, Au, and Bi contents, whereas the late pyrite is distinguished by elevated concentrations of As (up to 0.7 wt %), Ni (up to 0.38 wt %), Se (223 ppm), Ag (up to 111 ppm), and Sn (4.4 ppm). The minimal Au content in pyrite of the late quartz–carbonate assemblage is up to 1.7 ppm and geometric average is 0.3 ppm. The significant correlation between Au and As (furthermore, negative–0.6) in pyrite from ore of the Petropavlovskoe deposit is recorded only for the gold–sulfide assemblage, whereas it is not established for other assemblages. Pyrite with higher As concentration (up to 0.7 wt %) is distinguished only for the Au–Te mineral assemblage. Taking into account structural–morphological and mineralogical–geochemical features, the ore–magmatic system of the Petropavlovskoe deposit is referred to as gold porphyry style. Among the main criteria of such typification are the spatial association of orebodies with bodies of subvolcanic porphyry-like intrusive phases at the roof of large multiphase pluton; the stockwork-like morphology of gold orebodies; 3D character of ore–alteration zoning and distribution of ore components; geochemical association of gold with Ag, W, Mo, Cu, As, Te, and Bi; and predominant finely dispersed submicroscopic gold in ore.

Mineralogy and metasomatic evolution of the Mianeh iron skarn deposit, Norduz-Agarak border, NW Iran

The Mianeh iron skarn deposit lies in the Arasbaran region within the Qaradagh metallogenic district, NW Iran. This high-grade massive magnetite skarn originated by the interaction of Upper Cretaceous limestone with metasomatic ore-bearing fluids associated with hypabyssal Oligo-Miocene quartz diorite. Mineral chemistry of the primary clinopyroxenes demonstrates the sub-alkaline, volcanic arc setting of magmatism. Two general stages of skarnification are recognized: (1) silicate skarn (stage I) is composed essentially of grossular and low-Fe diopside formed before the main mineralization and (2) magnetite-garnet skarn (stage II) composed of strongly anisotropic coarse-grained garnets with a narrow compositional zoning radially formed by addictive infiltrating of silica and iron-rich metasomatic fluids which overprint and/or crosscut the early stage silicate skarn. Anhydrous prograde calc-silicate assemblages were replaced by a series of hydrous calc-silicates (epidote, tremolite-actinolite) and/or quartz, calcite, magnetite, hematite, and pyrite. Magnetite (±hematite) is the dominant hypogene ore mineral that initially precipitated coincident with the late prograde to the early retrograde metasomatic stages. Mineralogical studies suggest that silicate skarn formation commenced at temperatures about 560 °C, X(CO2)fluid ≤ 0.15, αSiO2∼−1.0, and fluid pressure 1.0 kbar. The magnetite-garnet skarn formed from H2O-rich fluids [X(CO2)fluid < 0.1] at a temperature of 525 to 450 °C and maximum log ƒO2 between −20.2 and −23. During the late stages of prograde skarn development, the stability field of andradite shifted to low ƒO2 and ƒS2 conditions resulting in main iron ore deposition (as magnetite). The andradite replacement temperature and presence of pyrite (instead of pyrrhotite) suggest that logƒS2 remained constant at about −6 to −7 during cooling of the system.

Age constraints on the hydrothermal history of the Prominent Hill iron oxide copper-gold deposit, South Australia

The Mesoproterozoic Prominent Hill iron-oxide copper–gold deposit lies on the fault-bound southern edge of the Mt Woods Domain, Gawler Craton, South Australia. Chalcocite–bornite–chalcopyrite ores occur in a hematitic breccia complex that has similarities to the Olympic Dam deposit, but were emplaced in a shallow water clastic–carbonate package overlying a thick andesite–dacite pile. The sequence has been overturned against the major, steep, east–west, Hangingwall Fault, beyond which lies the clastic to potentially evaporitic Blue Duck Metasediments. Immediately north of the deposit, these metasediments have been intruded by dacite porphyry and granitoid and metasomatised to form magnetite–calc–silicate skarn ± pyrite–chalcopyrite. The hematitic breccia complex is strongly sericitised and silicified, has a large sericite ± chlorite halo, and was intruded by dykes during and after sericitisation. This paper evaluates the age of sericite formation in the mineralised breccias and provides constraints on the timing of granitoid intrusion and skarn formation in the terrain adjoining the mineralisation. The breccia complex contains fragments of granitoid and porphyry that are found here to be part of the Gawler Range Volcanics/Hiltaba Suite magmatic event at 1600–1570 Ma. This indicates that some breccia formation post-dated granitoid intrusion. Monazite and apatite in Fe-P-REE-albite metasomatised granitoid, paragenetically linked with magnetite skarn formation north of the Hangingwall Fault, grew soon after granitoid intrusion, although the apatite experienced U–Pb–LREE loss during later fluid–mineral interaction; this accounts for its calculated age of 1544 ± 39 Ma. To the south of the fault, within the breccia, 40Ar–39Ar ages yield a minimum age of sericitisation (+Cu+Fe+REE) of dykes and volcanics of ∼1575 Ma, firmly placing Prominent Hill ore formation as part of the Gawler Range Volcanics/Hiltaba Suite magmatic event within the Olympic Cu–Au province of the Gawler Craton.

A magmatic source of hydrothermal sulfur for the Prominent Hill deposit and associated prospects in the Olympic iron oxide copper-gold (IOCG) province of South Australia

The Prominent Hill deposit is a world-class iron oxide copper–gold (IOCG) deposit in South Australia, characterized by a high Cu/S ratio of the dominant Cu-(Fe) sulfides hosted by hematite breccias. It contains a total resource of 278Mt of ore at 0.98% Cu and 0.75g/t Au. Prominent Hill is one of several IOCG deposits and numerous prospects in the Olympic IOCG province that are temporally associated with the 1603–1575Ma Gawler Range Volcanics, a large igneous province including co-magmatic granitoid intrusions of the Hiltaba Suite. Globally, IOCG deposits share many similar features in terms of their geological environment and mineral association. However, it is not yet clear whether sulfur and copper originate from the same source rocks and which hydrothermal redox processes created the characteristic iron oxide enrichment. Highly variable sulfur isotope compositions of sulfides and sulfates in IOCG deposits have previously been interpreted in terms of diverse sulfur sources that may include contributions from magmatic, sedimentary, seawater or evaporitic sulfur. In order to test these alternatives, we performed a detailed sulfur isotope study of Cu-(Fe) sulfides from Prominent Hill and IOCG prospects nearby. The Prominent Hill deposit shows a wide range in δ34SV-CDT between −33.5‰ and 29.9‰ for Cu-(Fe) sulfides, and a narrower range of 4.3‰ to 15.8‰ for barite. Iron sulfides (pyrite, pyrrhotite) show a narrow range in sulfur isotope composition, whereas Cu-bearing sulfides show a much wider range, and more negative δ34SV-CDT values on average. We propose a two-stage sulfide mineralization model for the IOCG system in the Prominent Hill area, in which all hydrothermal sulfur is ultimately derived from a magmatic source that had a composition of 4.4±2‰. The diversity in sulfur isotope composition can be produced by different fluid evolution pathways along reducing or oxidizing trajectories. A reduced sulfur evolution pathway is responsible for stage I mineralization, when intrusion-derived magmatic-hydrothermal fluids produced early pyrite and minor chalcopyrite at Prominent Hill, and iron±copper sulfides in regional magnetite skarns and in some pervasively altered volcanic rocks of the Gawler Range Volcanics. Shallow-venting magmatic-hydrothermal fluids and subaerial volcanic gases that became completely oxidized by reaction with atmospheric oxygen produced sulfate and sulfuric acid with a sulfur isotope composition equal to their magmatic source. This highly oxidized ore fluid probably consisted dominantly of water from the hydrosphere, but contained magmatic solute components, notably sulfate, acidity and Cu. Sulfate reduction produced hydrothermal Cu sulfides with a wide range in sulfur isotope compositions from very negative to moderately positive values. Partial reaction of the Cu-rich stage II fluid with earlier stage I sulfides resulted in mixing of sulfur derived from sulfate reduction and from sulfides deposited during stage I. Modeling of the sulfur isotope fractionation processes in response to reducing and oxidizing pathways demonstrates that the entire spectrum of sulfur isotope data from stage I and stage II mineralization can be explained with a single, ultimately magmatic sulfur source. Such a magmatic sulfur source is also adequate to explain the complete spectrum of sulfur isotope data of other IOCG prospects and deposits in the Olympic province, including Olympic Dam. The results of our study challenge the conventional model that suggests the requirement of multiple and compositionally diverse sulfur sources in hematite-breccia hosted IOCG style mineralization.

Geochemistry of rare earth elements in the Baba Ali magnetite skarn deposit, western Iran – a key to determine conditions of mineralisation

Abstract The Baba Ali skarn deposit, situated 39 km to the northwest of Hamadan (Iran), is the result of a syenitic pluton that intruded and metamorphosed the diorite host rock. Rare earth element (REE) values in the quartz syenite and diorite range between 35.4 and 560 ppm. Although the distribution pattern of REEs is more and less flat and smooth, light REEs (LREEs) in general show higher concentrations than heavy REEs (HREEs) in different lithounits. The skarn zone reveals the highest REE-enriched pattern, while the ore zone shows the maximum depletion pattern. A comparison of the concentration variations of LREEs (La–Nd), middle REEs (MREEs; Sm–Ho) and HREEs (Er–Lu) of the ore zone samples to the other zones elucidates two important points for the distribution of REEs: 1) the distribution patterns of LREEs and MREEs show a distinct depletion in the ore zone while representing a great enrichment in the skarn facies neighbouring the ore body border and decreasing towards the altered diorite host rock; 2) HREEs show the same pattern, but in the exoskarn do not reveal any distinct increase as observed for LREEs and MREEs. The ratio of La/Y in the Baba Ali skarn ranges from 0.37 to 2.89. The ore zone has the highest La/Y ratio. In this regard the skarn zones exhibit two distinctive portions: 1) one that has La/Y &gt;1 beingadjacent to the ore body and; 2) another one with La/Y &lt; 1 neighbouring altered diorite. Accordingly, the Baba Ali profile, from the quartz syenite to the middle part of the exoskarn, demonstrates chiefly alkaline conditions of formation, with a gradual change to acidic towards the altered diorite host rocks. Utilising three parameters, Ce/Ce*, Eu/Eu* and (Pr/Yb)n, in different minerals implies that the hydrothermal fluids responsible for epidote and garnet were mostly of magmatic origin and for magnetite, actinolite and phlogopite these were of magmatic origin with low REE concentration or meteoric water involved.

In-situ LA–ICP–MS trace elements analysis of magnetite: The Fenghuangshan Cu–Fe–Au deposit, Tongling, Eastern China

The Fenghuangshan deposit is a typical Cu–Fe–Au skarn deposit in the Tongling area, Anhui province, Eastern China. The deposit has a paragenetic sequence of a prograde skarn stage, followed by a retrograde skarn stage, and a final quartz–sulfide and carbonate stages. Magnetite in the Fenghuangshan deposit mainly formed in the retrograde and carbonate stages. According to the morphology of magnetite and mineral assemblage of ores, we divided magnetite-bearing ores into three groups. Group 1 (early retrograde skarn stage) is represented by a mineral assemblage of magnetite and chalcopyrite. Group 2 (late retrograde skarn stage) has a mineral assemblage of magnetite, chalcopyrite, and wollastonite with characteristic ring-like magnetite. Group 3 (carbonate stage) is characterized by large amounts of calcite veins crosscut or associated with magnetite and intensive hematization of magnetite crosscut by the veins.Laser ablation (LA)–ICP–MS was used to determine trace element concentrations of magnetite from the three mineralization stages. Positive correlations among Mg, Al, Ca, and Si in magnetite indicate that these lithophile elements have similar behavior during the skarn formation process. Calcium is an important discriminant element for magnetite in skarn deposits. Positive correlations are also evident for Pb, Sn and W in magnetite, which also indicates their similar behavior. In general, magnetite grains of different stages have similar normalized trace element patterns, indicating that they share a common source. However, some elements such as Co and Mn in magnetite decrease from early retrograde skarn stage, late retrograde skarn stage to carbonate stage, which may be attributed to the precipitation of coexisting minerals (sulfides and carbonates) or the decreasing temperature. Magnetite grains of the retrograde stage have higher Mg+Mn and Si+Al contents than those of the carbonate stage, indicating a decreasing degree of fluid–rock interaction during the skarn formation process. Trace element data of skarn magnetite indicate a more widely compositional variation than previously suggested. Magnetite from the Fenghuangshan Cu–Fe–Au deposit has similar composition to those from other Cu, Cu–Fe or Cu polymetallic skarn deposits, but different from those from Fe skarn deposits, such that magnetite composition is very powerful in establishing the origin of skarn deposits.

Major and Trace Elements of Magnetite from the Qimantag Metallogenic Belt: Insights into Evolution of Ore–forming Fluids

AbstractMagnetite, as a genetic indicator of ores, has been studied in various deposits in the world. In this paper, we present textural and compositional data of magnetite from the Qimantag metallogenic belt of the Kunlun Orogenic Belt in China, to provide a better understanding of the formation mechanism and genesis of the metallogenic belt and to shed light on analytical protocols for the in situ chemical analysis of magnetite. Magnetite samples from various occurrences, including the ore‐related granitoid pluton, mineralised endoskarn and vein–type iron ores hosted in marine carbonate intruded by the pluton, were examined using scanning electron microscopy and analysed for major and trace elements using electron microprobe and laser ablation–inductively coupled plasma–mass spectrometry. The field and microscope observation reveals that early–stage magnetite from the Hutouya and Kendekeke deposits occurs as massive or banded assemblages, whereas late–stage magnetite is disseminated or scattered in the ores. Early–stage magnetite contains high contents of Ti, V, Ga, Al and low in Mg and Mn. In contrast, late–stage magnetite is high in Mg, Mn and low in Ti, V, Ga, Al. Most magnetite grains from the Qimantag metallogenic belt deposits except the Kendekeke deposit plot in the “Skarn” field in the Ca+Al+Mn vs Ti+V diagram, far from typical magmatic Fe deposits such as the Damiao and Panzhihua deposits. According to the (MgO+MnO)–TiO2‐Al2O3 diagram, magnetite grains from the Kaerqueka and Galingge deposits and the No.7 ore body of the Hutouya deposit show typical characteristics of skarn magnetite, whereas magnetite grains from the Kendekeke deposit and the No.2 ore body of the Hutouya deposit show continuous elemental variation from magmatic type to skarn type. This compositional contrast indicates that chemical composition of magnetite is largely controlled by the compositions of magmatic fluids and host rocks of the ores that have reacted with the fluids. Moreover, a combination of petrography and magnetite geochemistry indicates that the formation of those ore deposits in the Qimantag metallogenic belt involved a magmatic–hydrothermal process.

인도네시아 파찌딴 광화대 함 금속 광체의 안정동위원소 특성

주요어 : 동-아연-연-금, 스카른, 열수 , 망상광체, 맥상광체, 안정동위원소, 파찌딴 광화대, 인도네시아 Extensive base-metal and/or gold bearing ore mineralizations occur in the Pacitan mineralized district of the south western portions in the East Java, Indonesia. Metallic ore bodies in the Pacitan mineralized district are classified into two major types: 1) skarn type replacement ore bodies, 2) fissure filling hydrothermal ore bodies. Skarn type replacement ore bodies are developed typically along bedding planes of limestone as wall rock around the quartz porphyry and are composed mineralogically of skarn minerals, magnetite, and base metal sulfides. Hydrothermal ore bodies differ mineralogically in relation to distance from the quartz porphyry as source igneous rock. Hydrothermal ore bodies in the district are porphyry style Cu-Zn-bearing stockworks as proximal ore mineralization and Pb-Zn(-Au)-bearing fissure filling hydrothermal veins as distal ore mineralization. Sulfur isotope compositions in the sulfides from skarn and hydrothermal ore bodies range from 6.7 to 8.2‰ and from 0.1 to 7.9‰, respectively. The calculated δ 34 S values of H2S in skarn-forming and hydrothermal fluids are 0.9 to 7.1‰ (5.6-7.1‰ for skarn-hosted sulfides and 0.9-6.8‰ for sulfides from hydrothermal deposits). The change from skarn to hydrothermal mineralization would have resulted in increased SO4/H2S ratios and corresponding decreases in δ 34 S values of H2S. The calculated δ 18 O water values are: skarn magnetite,

Multistage ore formation at the Ryllshyttan marble and skarn-hosted Zn–Pb–Ag–(Cu) + magnetite deposit, Bergslagen, Sweden

Numerous magnetite skarn deposits and marble- and skarn-hosted base metal sulphide deposits occur in polydeformed and metamorphosed, felsic-dominated metavolcanic inliers in the Palaeoproterozoic Bergslagen region of south-central Sweden, including the Ryllshyttan magnetite and Zn–Pb–Ag–(Cu) sulphide deposit, approximately 2.5km SW of the large Garpenberg Zn–Pb–Ag–(Cu–Au) deposit. The Ryllshyttan deposit, from which approximately 1Mt of Zn-rich massive sulphide ore and 0.2Mt of semi-massive magnetite were extracted, is located near a transition between magnesian skarn and dolomitic marble. The host unit consists of a 10–20m-thick former calcitic limestone of likely stromatolitic origin that is commonly pervasively altered to skarn, locally hosting magnetite skarn deposits. The ore-bearing unit is one of several mineralised marble units within a more than 1km-thick, felsic-dominated metavolcanic succession that includes a metamorphosed, large caldera-fill pyroclastic deposit, 800m stratigraphically above the Ryllshyttan host succession. The Garpenberg stratabound Zn–Pb–Ag–(Cu)–(Au) deposit is located higher in the stratigraphy, just below the caldera fill deposits. The metavolcanic succession is bounded to the NW by a large granitoid batholith and intruded by a microgranodiorite pluton less than a 100m from the Ryllshyttan deposit. Magnetite laminae in bedded skarns and metavolcanic rocks in the hanging wall of Ryllshyttan indicate an early (syngenetic) accumulation of Fe-rich exhalites. In contrast, the sulphide mineralisation consists of stratabound replacement-style ore associated with dolomitisation of the host and with discordant K–Mg–Fe±Si alteration of volcanic rocks and early porphyritic intrusions in the footwall and hanging wall. The microgranodiorite that intrudes the host succession crosscuts the K–Mg–Fe±Si alteration envelope and is overprinted by Na–Ca alteration (diopside and plagioclase-bearing mineral associations) that also overprints K–Mg–Fe±Si-altered rocks. The Na–Ca alteration is interpreted to be associated with the formation of calcic and magnesian iron skarn deposits semi-regionally at a similar stratigraphic position. Despite superimposed amphibolite facies regional metamorphism and substantial syn-D2–D3 remobilisation of sulphides concurrent with retrograde alteration of skarn assemblages, cross-cutting field relationships indicate that the Ryllshyttan magnetite and Zn–Pb–Ag–(Cu) sulphide deposit results from protracted VMS-style hydrothermal activity including early seafloor mineralisation (Fe-rich exhalites), closely followed by sub-seafloor carbonate-replacement-style mineralisation (base metal-bearing massive sulphides). Both mineralisation styles were overprinted by contact metasomatism associated with the formation of abundant magnetite skarn deposits during the emplacement of granitoid intrusions. As for other deposits in the Bergslagen region, the ore-forming system at Ryllshyttan thus has similarities to both metamorphosed VMS deposits and metasomatic Fe and Zn skarn deposits. Our results suggest that the sequence of volcanic, intrusive and hydrothermal events in this region is compatible with prograde heating of a long-lived hydrothermal system, wherein a shift from a convective seawater-dominated system to a contact metamorphic and/or metasomatic environment occurred during the early stage of the 1.9–1.8Ga Svecokarelian orogeny. This model partly resolves the controversy regarding genesis of the iron oxide and base metal sulphide deposits in Bergslagen, as we recognise that these deposits have a complex history of alteration, metamorphism, deformation and (re)mobilisation, and no unique established genetic model can account for all their features.

TEM study on particles transported by ascending gas flow in the Kaxiutata iron deposit, Inner Mongolia, North China

Metal migration processes, by means of gas-transported particles, from an iron magnetite skarn deposit beneath Quaternary transported overburden, to surface was studied at the Kaxiutata iron deposit. This study area is located in the desert of the Inner Mongolia region in northern China. Ascending gas-transported particles were collected using passive adsorption methods; a total of 50 samples were obtained over the known mineralization (30 within the mining area and 20 from the exploration area), and 15 samples were collected over background areas for comparison. The morphology, categorization, and chemical composition of particles in these samples were determined by transmission electron microscopy (TEM). In the mining and exploration area, the particles related to the subsurface mineralization (ore-bearing particles) are primarily Fe-, Cu-, Zn-, Pb-, and Bi-bearing, and particles show significant common characteristics. Sulphur is generally absent, or its content is notably low. However, based on the geological description of primary mineralization, Cu, Zn, and Pb occur mainly as sulphides, implying that the minerals were subjected to in situ or transport-associated oxidation. Most particles are actually aggregates of various smaller particles of diverse origin. Evidence of oxidation and agglomeration of particles may reflect the high porosity and gas permeability of the sandy overburden, which provides an oxidizing environment for gas-transported particles. The distribution of ore-bearing particles is correlated with the subsurface mineralization. Iron-rich particles (&gt; 30% Fe) and Cu-, Zn-, and Bi-bearing particles collected above the mining area were also collected in the exploration zone, and these types of particles are likely indicators of underlying iron mineralization. Samples collected from the background zone revealed no Cu-, Bi-, Mo-, and Pb-bearing particles. The migration mechanism of gas-transported particles in the desert area is summarized and discussed based on this and previous studies. Supplementary material: Some of the TEM images, high resolution TEM images and SAED patterns of the particles with no relation to mineralization are supplemented in Appendix 1 which is available at http://www.geolsoc.org.uk/SUP18810 .

Corrigendum to: New methods for interpretation of magnetic vector and gradient tensor data I: eigenvector analysis and the normalised source strength

Acquisition of magnetic gradient tensor data is likely to become routine in the near future. New methods for inverting gradient tensor surveys to obtain source parameters have been developed for several elementary, but useful, models. These include point dipole (sphere), vertical line of dipoles (narrow vertical pipe), line of dipoles (horizontal cylinder), thin dipping sheet, and contact models. A key simplification is the use of eigenvalues and associated eigenvectors of the tensor. The normalised source strength (NSS), calculated from the eigenvalues, is a particularly useful rotational invariant that peaks directly over 3D compact sources, 2D compact sources, thin sheets and contacts, and is independent of magnetisation direction. In combination the NSS and its vector gradient determine source locations uniquely. NSS analysis can be extended to other useful models, such as vertical pipes, by calculating eigenvalues of the vertical derivative of the gradient tensor. Inversion based on the vector gradient of the NSS over the Tallawang magnetite deposit obtained good agreement between the inferred geometry of the tabular magnetite skarn body and drill hole intersections. Besides the geological applications, the algorithms for the dipole model are readily applicable to the detection, location and characterisation (DLC) of magnetic objects, such as naval mines, unexploded ordnance, shipwrecks, archaeological artefacts, and buried drums.

Geochemistry of magnetite from porphyry Cu and skarn deposits in the southwestern United States

A combination of petrographic observations, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and statistical data exploration was used in this study to determine compositional variations in hydrothermal and igneous magnetite from five porphyry Cu–Mo and skarn deposits in the southwestern United States, and igneous magnetite from the unmineralized, granodioritic Inner Zone Batholith, Japan. The most important overall discriminators for the minor and trace element chemistry of magnetite from the investigated porphyry and skarn deposits are Mg, Al, Ti, V, Mn, Co, Zn, and Ga—of these the elements with the highest variance for (I) igneous magnetite are Mg, Al, Ti, V, Mn, Zn, for (II) hydrothermal porphyry magnetite are Mg, Ti, V, Mn, Co, Zn, and for (III) hydrothermal skarn magnetite are Mg, Ti, Mn, Zn, and Ga. Nickel could only be detected at levels above the limit of reporting (LOR) in two igneous magnetites. Equally, Cr could only be detected in one igneous occurrence. Copper, As, Mo, Ag, Au, and Pb have been reported in magnetite by other authors but could not be detected at levels greater than their respective LORs in our samples. Comparison with the chemical signature of igneous magnetite from the barren Inner Zone Batholith, Japan, suggests that V, Mn, Co, and Ga concentrations are relatively depleted in magnetite from the porphyry and skarn deposits. Higher formation conditions in combination with distinct differences between melt and hydrothermal fluid compositions are reflected in Al, Ti, V, and Ga concentrations that are, on average, higher in igneous magnetite than in hydrothermal magnetite (including porphyry and skarn magnetite). Low Ti and V concentrations in combination with high Mn concentrations are characteristic features of magnetite from skarn deposits. High Mg concentrations (<1,000 ppm) are characteristic for magnetite from magnesian skarn and likely reflect extensive fluid/rock interaction. In porphyry deposits, hydrothermal magnetite from different vein types can be distinguished by varying Ti, V, Mn, and Zn contents. Titanium and V concentrations are highly variable among hydrothermal and igneous magnetites, but Ti concentrations above 3,560 ppm could only be detected in igneous magnetite, and V concentrations are on average lower in hydrothermal magnetite. The highest Ti concentrations are present in igneous magnetite from gabbro and monzonite. The lowest Ti concentrations were recorded in igneous magnetite from granodiorite and granodiorite breccia and largely overlap with Ti concentrations found in hydrothermal porphyry magnetite. Magnesium and Mn concentrations vary between magnetite from different skarn deposits but are generally greater than in hydrothermal magnetite from the porphyry deposits. High Mg, and low Ti and V concentrations characterize hydrothermal magnetite from magnesian skarn deposits and follow a trend that indicates that magnetite from skarn (calcic and magnesian) commonly has low Ti and V concentrations.

Intermediate sulfates in barite-celestite isomorphic series: Composition and mode of occurrence

Intermediate (mixed) sulfates of the BaSO4-SrSO4 isomorphic series are relatively rare in nature. Nevertheless, large resources of such sulfates have been revealed in Russia at carbonatite-type deposits (Karasug in Tuva and Khalyuta in the western Transbaikal region) and at the Oktyabr’skoe skarn magnetite deposit in the Angara region. The compiled database of chemical compositions of mixed Ba-Sr sulfates (876 items) comprises data related to the above deposits and other worldwide occurrences. Ba-Sr sulfates are formed in a wide range of conditions from sedimentary basins to high-temperature magmatic systems, and their generalized compositional intervals completely overlap. At the same time, sulfates of various natural assemblages and generations occupy discrete intervals separated from one another in isomorphic series. The statistical boundaries of these intervals correspond to 15, 36, and 66 (±1) mol % SrSO4. Many Ba-Sr sulfates contain up to 7–11 mol % CaSO4.

Krasnotur’insk Skarn copper ore field, Northern Urals: The U-Pb age of ore-controlling diorites and their place in the regional metallogeny

The Krasnotur’insk skarn copper ore field known from the theoretical works of Academician K.S. Korzhinskii is located in the western part of the Tagil volcanic zone (in the area of the town of Krasnotur’insk). The ore field is composed of layered Devonian (Emsian) volcanosedimentary rocks intruded by small plutons of quartz diorites, diorites, and gabbrodiorites. Widespread pre-ore and intra-ore dikes of similar composition control the abundance of the andradite skarns formed after limestones and the magnetitesulfide and sulfide ore bodies formed after skarns. The LA-ICP-MS U-Pb concordant age of zircon from the quartz diorite of the Vasil’evsko-Moskalevskii pluton calculated by 16 analyses (16 crystals) is 407.7 ± 1.6 Ma (MSWD = 1.5). Taking into account the geological and petrogeochemical similarity of diorites of small plutons and intra-ore dikes, it is assumed that this age corresponds to the period of formation of the ore-magmatic system of the Krasnotur’insk skarn copper ore field. It was probably formed somewhat earlier than the Auerbakh montzonitic pluton and the accompanying skarn magnetite deposits in the south.

پترولوژی و سن سنجی زیرکن به روش U-Pb در توده های نفوذی مناطق A، C جنوبی و دردوی، معدن سنگ آهن سنگان خواف

معدن سنگ آهن سنگان خواف در 300 کیلومتری جنوب‌شرقی مشهد، در شرق کمربند آتشفشانی- نفوذی خواف- کاشمر- بردسکن قرار دارد. توده های گرانیتوئید در سه محدوده معدن سنگ آهن سنگان شامل نواحی A، C جنوبی و دردوی مورد مطالعه قرار گرفتند. سه توده نفوذی بیوتیت هورنبلند مونزونیت پورفیری، بیوتیت سینیت و سینوگرانیت در مناطق مورد مطالعه شناسایی شد. براساس روابط قطع‌شدگی صحرایی، عدم وجود گارنت اسکارن و مگنتیت در مرز این توده ها و آلتره‌شدن آنها توسط محلول کانه دار بعدی، این توده های گرانیتوئیدی از اسکارن مگنتیت دار قدیم‌ترند. سن سنجی زیرکن به روش U-Pb سن آنها را 42 میلیون سال قبل (ائوسن‌ میانی) تعیین می‌کند. مقدار پذیرفتاری مغناطیسی آنها بین SI5-10×310 تا SI5-10×900 است، بنابراین متعلق به گرانیتوئیدهای سری مگنتیت (اکسیدان) و از نوع I هستند. براساس بررسیهای ژئوشیمیایی، توده های نفوذی عمدتا از نوع متاآلومینوس، منیزیمی، آلکالی‌کلسیک تا آلکالیک و شوشونیتی تا اولتراپتاسیک می باشند. غنی‌شدگی نسبی عناصر LREE نسبت به HREE و عناصر LILE (Sr, Cs, Rb, K and Ba) نسبت به HFSE (Nb, Ta, Ti, Hf, Zr) مؤید تشکیل ماگما در زون فرورانش است. این ماگما از ذوب‌بخشی اندک (کمتر از 1) یک منشأ لرزولیتی گارنت- اسپینل دار (با مقدار کم گارنت) حاصل‌شده که با پوسته قاره ای آلودگی پیدا کرده است. وجود اندک گارنت به‌عنوان کانی باقی‌مانده در منشأ این توده ها با نسبت پایین (La/Yb)N (29/6 تا 73/34) توجیه‌پذیر است. مقدار بالای نسبت Rb/Sr و LILE/HFSE و مقدار بالای K2O، Th و Nb نقش اختلاط پوسته قاره ای را روشن می کند. بررسیهای پترولوژیکی و تعیین سن دقیق توده های نفوذی موجود در معدن سنگان به‌عنوان بخشی از کمربند آتشفشانی- نفوذی خواف- کاشمر- بردسکن، به شناخت هر‌چه بهتر جایگاه تکتونوماگمایی این کمربند و نیز چگونگی ارتباط توده های نفوذی با کانی سازی برای اکتشاف بیشتر در آینده کمک خواهد کرد.

Geology, mineralization, stable isotope geochemistry, and fluid inclusion characteristics of the Novogodnee–Monto oxidized Au–(Cu) skarn and porphyry deposit, Polar Ural, Russia

The Novogodnee–Monto oxidized Au–(Cu) skarn and porphyry deposit is situated in the large metallogenic belt of magnetite skarn and Cu–Au porphyry deposits formed along the Devonian–Carboniferous Urals orogen. The deposit area incorporates nearly contemporaneous Middle–Late Devonian to Late Devonian–Early Carboniferous calc-alkaline (gabbro to diorite) and potassic (monzogabbro, monzodiorite- to monzonite-porphyry, also lamprophyres) intrusive suites. The deposit is represented by magnetite skarn overprinted by amphibole–chlorite–epidote–quartz–albite and then sericite–quartz–carbonate assemblages bearing Au-sulfide mineralization. This mineralization includes early high-fineness (900–990 ‰) native Au associated mostly with cobaltite as well as with chalcopyrite and Co-pyrite, intermediate-stage native Au (fineness 830–860 ‰) associated mostly with galena, and late native Au (760–830 ‰) associated with Te minerals. Fluid inclusion and stable isotope data indicate an involvement of magmatic–hydrothermal high-salinity (>20 wt.% NaCl-equiv.) chloride fluids. The potassic igneous suite may have directly sourced fluids, metals, and/or sulfur. The abundance of Au mineralization is consistent with the oxidized character of the system, and its association with Co-sulfides suggests elevated sulfur fugacity.

New methods for interpretation of magnetic vector and gradient tensor data I: eigenvector analysis and the normalised source strength

Acquisition of magnetic gradient tensor data is likely to become routine in the near future. New methods for inverting gradient tensor surveys to obtain source parameters have been developed for several elementary, but useful, models. These include point dipole (sphere), vertical line of dipoles (narrow vertical pipe), line of dipoles (horizontal cylinder), thin dipping sheet, and contact models. A key simplification is the use of eigenvalues and associated eigenvectors of the tensor. The normalised source strength (NSS), calculated from the eigenvalues, is a particularly useful rotational invariant that peaks directly over 3D compact sources, 2D compact sources, thin sheets and contacts, and is independent of magnetisation direction. In combination the NSS and its vector gradient determine source locations uniquely. NSS analysis can be extended to other useful models, such as vertical pipes, by calculating eigenvalues of the vertical derivative of the gradient tensor. Inversion based on the vector gradient of the NSS over the Tallawang magnetite deposit obtained good agreement between the inferred geometry of the tabular magnetite skarn body and drill hole intersections. Besides the geological applications, the algorithms for the dipole model are readily applicable to the detection, location and characterisation (DLC) of magnetic objects, such as naval mines, unexploded ordnance, shipwrecks, archaeological artefacts, and buried drums.

Migration transformation of two-dimensional magnetic vector and tensor fields

SUMMARY We introduce a new method of rapid interpretation of magnetic vector and tensor field data, based on ideas of potential field migration which extends the general principles of seismic and electromagnetic migration to potential fields. 2-D potential field migration represents a direct integral transformation of the observed magnetic fields into a subsurface susceptibility distribution, which can be used for interpretation or as an ap riorimodel for subsequent regularized inversion. Potential field migration is very stable with respect to noise in the observed data because the transform is reduced to the downward continuation of a wellbehaved analytical function. We present case studies for imaging of SQUID-based magnetic tensor data acquired over a magnetite skarn at Tallawang, Australia. The results obtained from magnetic tensor field migration agree very well with both Euler deconvolution and the known geology.

3D Inversion of SQUID Magnetic Tensor Data

Developments in SQUID-based technology have enabled direct measurement of magnetic tensor data for geophysical exploration. For quantitative interpretation, we introduce 3D regularized inversion for magnetic tensor data. For mineral exploration-scale targets, our model studies show that magnetic tensor data have significantly improved resolution compared to magnetic vector data for the same model. We present a case study for the 3D regularized inversion of magnetic tensor data acquired over a magnetite skarn at Tallawang, Australia. The results obtained from our 3D regularized inversion agree very well with the known geology of the area.