Massive Ore Research Articles

Sulfide Remobilization in the Metamorphosed Kayad Sedimentary Exhalative Zn-Pb Deposit, Western India: Evidence from Mode of Occurrence, Texture, Hydrothermal Alteration Features, and Trace Element Chemistry

Abstract The Kayad Zn-Pb deposit, situated within the Proterozoic Aravalli-Delhi fold belt in western India, is primarily characterized by sphalerite and galena along with pyrrhotite and chalcopyrite. The mineralization occurs as disseminated ores in quartzite, disseminated/laminated and massive ores in quartz-mica schist, and in pegmatite and quartz veins. The laminated ores conform to the regional schistosity and folding, whereas the massive Zn-Pb ores postdate the pervasive tectonic fabric, accumulating at the fold hinges. The massive ore is characterized by durchbewegung texture, discrete blebs of galena and chalcopyrite in a sphalerite matrix with low interfacial angles, and discrete intergrowths of sulfides and sulfosalts such as pyrargyrite, gudmundite, Ag-tetrahedrite, and breithauptite. Geochemical analyses of sulfides reveal microinclusions of sulfosalts comprising Ag, Sb, Cu, Tl, and As, which are regarded as low-melting chalcophile elements (LMCEs). Hydrothermal alteration is insignificant in the laminated and massive ores but prominent around Fe-Cu ± Zn-Pb and Zn-Pb ± Fe-Cu veins. The alteration assemblages in these veins evince a pervasive K + Na ± Fe alteration, later overprinted by a subsidiary Ca ± Na alteration. We interpret the laminated/disseminated ores to be of syndiagenetic sedimentary-exhalative (SedEx) origin formed within an euxinic basin. Conversely, the textural features, mineralogical composition, lack of associated hydrothermal alterations, and evident structural influence on the emplacement of the massive ores suggest they have been remobilized both via plastic flow and by sulfide partial melting. Temperature estimates of up to 650°C, derived from Ti-in-biotite geothermometry of the metamorphosed host rocks, indicate lower-middle amphibolite facies conditions during regional metamorphism. The initiation of melting at these temperatures was promoted by the desulfurization of pyrite to pyrrhotite in quartz-mica schist, aided by melting point depression due to the presence of LMCEs like Ag, Sb, and As.

Sulfide Copper‐Iron Isotopic Fractionation During Formation of the Kalatongke Magmatic Cu‐Ni Sulfide Deposit in the Central Asian Orogenic Belt

AbstractCopper and iron isotopic signatures in sulfide and silicate minerals are important genetic indicators in magmatic sulfide deposits. Kalatongke is a large‐scale magmatic Cu‐Ni sulfide deposit in the Central Asian Orogenic Belt, and one that experienced multiple stages of magmatism and contamination. It is an ideal deposit in which to study Cu‐Fe isotopic fractionation during multiple stages of magmatism and sulfide mineralization processes. The Kalatongke sulfide orebodies are hosted by three small mafic intrusions in which pyroxene and sulfides (pyrrhotite, pentlandite, and chalcopyrite) are the most common Fe‐rich minerals, and chalcopyrite is the dominant Cu‐rich mineral. Sulfide liquid and silicate melt ▵56FeSul‐Sil (0.03–0.19‰) and ▵65CuCcp‐Sil (−0.78–0.74‰) values are indicative of non‐equilibrium fractionation. Most of the Cu isotope compositions in the sulfide ores at Kalatongke can be modeled as subduction‐ metasomatized, oxidized mantle source‐derived silicate melt (initial δ57Fe = 0.15‰, δ65Cu = −0.07‰) that underwent lower crustal contamination, and then reacted with silicate melt, having an R factor of 100–1,000. Rapid silicate melt and sulfide liquid Fe isotope exchange and re‐equilibration between chalcopyrite and pyrrhotite in the massive ores is reflected in the similarity of their δ56Fe values. Sulfide in disseminated ores shows a range of Fe isotope ratios, influenced by the proportions of monosulfide solid solution (MSS) and intermediate solid solution (ISS) formed. Copper isotopes can be utilized to characterize crustal contamination and silicate melt‐sulfide liquid interaction, while the Fe isotope ratios of sulfide minerals record sulfide liquid segregation and evolution in magmatic sulfide deposits.

Morphological features and composition of noble metal minerals in disseminated and massive ores of the central part of the Oktyabrskoe deposit, Norilsk region

Morphological features and composition of noble metal minerals in disseminated and massive ores of the central part of the Oktyabrskoe deposit, Norilsk region

Mechanism of Fe-skarn formation in the Nanminghe iron deposit, China

Skarn iron deposits are important resources for iron-rich ores and their genesis has been a topic of debate. The Nanminghe iron deposit is a typical skarn iron deposit in Wu’an, Hebei Province, China which provides a good example to investigate the metallogenic mechanism. In this paper, we present a detailed study of the geology, petrography, and geochemical characteristics of magnetite in the Nanminghe iron deposit. Two types of magnetite can be identified in the massive ores: Mag1 is euhedral with radial cracks developed in the margin. It is a low-Si magnetite, with SiO2 content ranging from 0.07% to 0.92%, with an average value of 0.49%. The SiO2 content of Mag2 is from 0.90% to 3.29%, and an average value of 2.03%. The Mag2 is associated with quartz, goethite, and apatite. Geochemical characteristics of Mag1, together with the ore textures suggest its crystallization from an iron magma. There are significant differences in the geochemical characteristics between Mag1 and Mag2. Mag2 is rich in Nb, Ta, high field strength elements, large ion lithophile elements and has higher silica content. The Nb/Ta of Mag2 is higher than that of the original mantle. These characteristics as well as La/Ta vs Th/La, Th/U vs Sm/Nd and Nb/Ta vs Nb features show that Mag2 is formed in the environment of supercritical fluids. The δ56Fe of Mag1 is 0.157‰-0.172‰, with an average value of 0.166‰, and the δ56Fe of Mag2 is 0.119‰-0.160‰, with an average value of 0.139‰. The Fe isotope of Mag2 is lighter than Mag1. There is a negative correlation between δ56Fe and SiO2 of Mag1 and Mag2, suggesting that the lighter δ56Fe of Mag2 was caused by injection of Si-rich fluids. We propose a genetic model where we envisage that the iron-rich melt formed in the deep magma chamber by reaction of dioritic rocks and carbonates. Mag1 formed by cooling of the iron-rich magma, following which Si-rich supercritical fluid were injected into the deep chamber resulting in fluid overpressure. The iron-bearing melt-fluid migrated upward along the magmatic conduit, when Mag2 formed. Our study provides insights into the formation mechanism of skarn iron deposits.

The Ophiolite-Hosted Cu-Zn VMS Deposits of Tuscany (Italy)

Several Jurassic, ophiolite-hosted Cu-Zn VMS deposits occur in Tuscany. They are hosted by tectonic units of oceanic affinity (Ligurian Units), such as the well-known deposits of nearby Liguria. Industrial production was small and definitively ceased in the 1960s. Locally, massive ore (chalcopyrite-bornite-chalcocite) with an exceptionally high grade was found. The Montecatini Val di Cecina mine exploited the largest “bonanza” and, for few decades in the 19th century, became one of the most profitable copper mines in Europe. This study provides an updated review of these deposits. Tuscan Cu-Zn VMSs mostly occur in proximity of the contact between the serpentinite-gabbro basement and the overlying basalts. Chalcopyrite-pyrite stockworks occur in serpentinite-gabbro cut by dolerite dykes, while the largest massive sulphide bodies are hosted by polymictic-monomictic breccias at the base of pillow basalts. Early chalcopyrite ores were mechanically–chemically reworked and upgraded to bornite-rich nodular ore embedded in a chlorite, calcic amphibole, Fe-rich serpentine, quartz, andradite, ilvaite, and xonotlite assemblage. This bornite-rich ore contains substantial amount of sphalerite and pyrite and ubiquitous grains of clausthalite, hessite, tellurium, and gold. They represent a prime example of the sub-seafloor portion of a hybrid mafic-ultramafic oceanic hydrothermal system formed in an OCC along the slow spreading ridge of the Jurassic Piedmont-Ligurian Ocean. The peculiar mineralogical–textural character of the bornite-rich ore was driven by an interface coupled dissolution–precipitation process mediated by fluids.

Neoproterozoic-Early Cambrian Northern Chah-Farsakh Cu-Zn VMS-type deposit, Iran: Constraints from geology, geochemistry, fluid inclusions and geodynamic

Northern Chah-Farsakh is the first seafloor VMS described in the Torud-Chahshirin metallogenic belt of Iran. The deposit is hosted by Neoproterozoic-Early Cambrian mafic (basaltic andesitic)-siliciclastic rocks in volcano-sedimentary basin. The evolution of the ore-bearing basin is governed by the Proto-Tethys Oceanic crust subduction beneath the Central Iranian microcontinent. Three ore facies can be distinguished based on the ore textures studies: stringer zone ore facies, massive-replacement ore facies, bedded ore and distal facies. Pyrite, chalcopyrite, magnetite and sphalerite are the main minerals, together with minor amounts of pyrrhotite, bornite, tetrahedrite and tennantite. Textures include laminated, banded, replacement, massive, and vein-veinlet texture. There is a distinct metal zonation in the massive sulfide orebody; Gold, Ag, and Zn contents increase vertically from the stringer to the bedded ore facies; Cu and Co increase in the massive ore facies, and Pb is highest in the stringer zone. Ore mineralization in the Northern Chah-Farsakh deposit has emplaced in two stages. The fine-grained ore bands (stage 1) which are intricately interlayered with host rock beds, exhibit sedimentary structure such as laminae and bedding are consistent with a syn-sedimentary origin. Coarser-grained stage 2 ores have massive and vein-veinlet textures that are considered to form by replacement during sub-seafloor fluid flow. Based on SEM-EDS analysis, significant compositional variations were observed in the Py-1b grains. Py-1b is enriched in Pb compared to Py-1a. The elemental distribution map shows that the chalcopyrite (Cpy-2) minerals in Northern Chah-Farsakh deposit do not have variable composition and are homogeneous. Fluid inclusions in quartz minerals rom the footwall stringer zone have trapping temperatures between 203 and 500 °C with salinities (3.39–16 wt% NaCl equiv.) higher than modern seawater. Fluid inclusion studies suggest that the ore-forming fluids were derived from seawater. High level of Se in the massive ore facies of the Northern Chah-Farsakh supports the formation of mineralization from a high-temperature hydrothermal fluid. The Northern Chah-Farsakh Cu-Zn VMS deposit typify the mafic-siliciclastic type (Besshi-type), where the majority of the host stratigraphy is composed of a mafic flow-volcaniclastics and sedimentary rocks.

Genesis of the Large-Scale Kamado Magnesite Deposit on the Tibetan Plateau

Lacustrine strata-bound magnesite deposits associated with Alpine-type ultramafic rocks are hydrothermal in origin. The magnesite ores of the Kamado deposit are unconformably underlain by mid-Jurassic marine carbonate and ultramafic rocks of the Bangong-Nujiang ophiolite suite and are in fault contact with hanging wall rocks composed of siliceous sinter. Three types of cryptocrystalline magnesite ores can be identified in Kamado: (1) strata-bound massive magnesites, representing the main ore type in the upper part; (2) banded ores in the lower part; and (3) some vein and stockwork ore in the ultramafic wall rocks. Integrated scanning electron microscopy, C–O isotope analysis, and geochemical analyses were carried out on the Kamado deposit. The results indicate that: (1) the orebody is composed of magnesite, with accessory minerals of aragonite, opal, and chromite; (2) the siliceous sinter and relatively high B (32.0–68.1 ppm) and Li (14.7–23.4 ppm) contents of the magnesite ores reflect long-term spring activity in Kamado; (3) the light carbon (δ13CV-PDB: −4.7 ± 0.3‰ to −4.1 ± 0.6‰) and oxygen isotopic compositions (δ18OV-SMOW: +12.3 ± 0.3 to +16.3 ± 0.1‰) of the stockwork ores in the foot wall rocks indicated that the carbon in fractures in the ultramafic rocks is from a mixture of marine carbonate and oxidized organic-rich sedimentary rocks, reflecting a typical “Kraubath-type” magnesite deposit; and (4) the relatively heavy carbon isotopic (δ13CV-PDB: +8.7 ± 0.4‰ to +8.8 ± 0.3‰) composition of the banded magnesite ores in the lower segment may have formed from heavy CO2 generated by anaerobic fermentation in the lakebed. Additionally, the carbon isotopic (δ13CV-PDB: +7.3 ± 0.3‰ to +7.7 ± 0.7‰) composition of the massive magnesite ores in the upper segment indicates a decline in the participation of anaerobic fermentation. As this economically valuable deposit is of the strata-bound massive ore type, Kamado can be classified as a lacustrine hydrothermal-sedimentary magnesite deposit, formed by continuous spring activities under salt lakes on the Tibetan Plateau, with the Mg mainly being contributed by nearby ultramafic rocks and the carbon mainly being sourced from atmosphere-lake water exchange, with minor amounts from marine carbonate strata.

The first identification of cronstedtite in Cu–Ni–PGE ores of the Talnakh intrusion

We present new mineralogical data of cronstedtite from the Southern-2 orebody, located in the South-Western branch of the Talnakh intrusion (Noril’sk area) composed of massive sulfides in which the total amount of oxides and silicates does not exceed 1–3 vol%. The petrographic and mineralogical features of these ores indicated occurrence of fine-grained, fibrous needle like clusters < 50-µm-sized grains of cronstedtite (7.09 Å along its c-axis). This mineral confirmed by a number of analytical techniques (powder X-ray diffraction of balk samples, transmission electron microscopy, scanning electron microscopy, Raman and Infrared spectroscopy). Cronstedtite sporadically contains signals of Al, Ni, Ca and filling the cracks and cavities between sulfides of copper (chalcopyrite) and iron (pyrrhotite, pentlandite). In some cases, cronstedtite contains micron-sized PGM, and associates with magnetite. According the X-ray diffraction analysis of the bulk massive ores besides cronstendtite are established kaolinite, gypsum, calcite, quartz, and cristobalite. The findings of cronstedtite in Noril’sk area have never been mentioned publicly before. Its occurrence is the northernmost known locality in the world. Our results imply that the formation of cronstedtite in the Talnakh intrusion could be possible by the active participation low-temperatures fluids within the relatively near-surface (< 2 km of paleosurface) conditions of intrusion emplacement, in contrast to other deep-seated supergiant Cu–Ni–PGE deposits in the world. The conditions of formation in isolated cavities in fresh pyrrhotite-pentlandite-chalcopyrite massive ores of deep level of the Talnakh intrusion could be favorable for the formation of cronstendtite.

Microfabrics, In Situ Trace Element Compositions of Pyrite, and the Sulfur Isotope Chemistry of Sulfides from the Xitieshan Pb-Zn Deposit, Qinghai Province, Northwest China: Analysis and Implications

The Xitieshan deposit, located in the central segment of the northern margin of the Qaidam Basin, is among the largest massive Pb-Zn sulfide deposits in China. This deposit, along with its ore-bearing rock series known as the Tanjianshan Group, underwent greenschist facies metamorphism due to subsequent orogeny. We investigated the in situ sulfur isotopes of sulfides with different occurrences to define the origin of ore-forming fluids. The δ34S values of sulfides from stratiform ores, massive ores in schist, stockwork ores in marble, schist and discolored altered rocks that constitute a typical double-mineralization structure range from −5.3‰ to +5.6‰ and from −1.7‰ to +32‰, respectively, indicating distinct biological and thermochemical reductions in seawater sulfates. These are similar to the sulfur isotopic characteristics of VSHMS deposits. Pyrite, whose LA-ICP-MS trace element compositions can provide significant information about metallogenic evolution and deposit genesis, is ubiquitous throughout the whole mineralization process. In these stratiform, massive and stockwork ores, three pyrite types were identified: colloform pyrite (Py0), fine-grained anhedral spongy pyrite (Py1) and coarse-grained euhedral pyrite (Py2). The contents of most metallogenic elements, such as Cu, Pb, Zn, Ag, Mo, Mn and Sn, decrease from Py0 to Py2 with the enhancement of metamorphic recrystallization. This suggests that the expelled elements appear as inclusions in primitive pyrite, contributing to the precipitation of new sulfide phases, such as sphalerite and galena. Orogenic metamorphism played an important role in controlling further Pb-Zn enrichment of the Xitieshan deposit. Moreover, there is another mineralization type, primarily occurring as sulfide veins in the undeformed Formation C siltstones of the Tanjianshan Group, which also crosscut early-formed sulfides, showing close-to-zero S isotopic compositions. In this mineralization type, pyrite (Py3) displays high Se/Tl (&gt;10) and Co/Ni (&gt;2.2) ratios, both indicating a minor superimposed post-orogenic magmatic–hydrothermal event.

Controls on the metal tenors of sulfide ores in the Jinchuan Ni–Cu–PGE sulfide deposit, NW China: Implications for the formation of distinct textural types of sulfide ores in magma conduits

The giant Jinchuan Ni–Cu–platinum-group element (PGE) sulfide deposit is a magma conduit system comprising four main intrusive units termed segments III, I, II-W, and II-E. The deposit comprises disseminated, net-textured, massive sulfide, and Cu-rich sulfide ores, with variations in metal tenors occurring among these different ore types and among the segments. Controls on these metal tenor variations must be linked to metallogenic processes, but this remains poorly constrained. To asses this, we systematically compared metal tenors (Ni, Cu, and the PGE) of ores from two perspectives — i) different ore types in each segment and ii) a single ore type across all four segments. Two major styles of metal tenor variations are documented. Style 1: Segments III and I have higher metal tenors than segments II-W and II-E for any given type of ore. Based on sulfide segregation models, this variation is interpreted to be related to variable degrees of early removal of sulfide liquid from the magmas that formed the four segments. Style 2: Disseminated ores in segments III and I have higher metal tenors than the other ore types, whereas all of the ore types in segments II-W and II-E have similar metal tenors. This is interpreted to be the result of the disseminated ores in segments III and I having formed from sulfide liquids that experienced higher R factors than the sulfide liquids that formed the other ore types, whereas all of the ores in segments II-W and II-E formed from sulfide liquids that experienced similar R factors. This difference suggests that the different ore types in segments III and I formed via early percolation and accumulation of sulfide liquids (the net-textured and massive ores) followed by the capture of disseminated, high R factor sulfides (the disseminated ore), whereas the different ore types in segments II-W and II-E likely formed by the physical percolation of originally disseminated sulfide liquid through the inter-crystal pore space. This study demonstrates that both sulfide ore texture and their metal-tenor variations are critical to characterizing ore-forming processes in dynamic magma conduits.

Mineralogy and geochemistry of magnetite in the Zhdanov Ni-Cu-(PGE) deposit in the Pechenga ore field, Russia: Implications for the formation of magmatic sulfide ores

The mineralogy and chemical composition of magnetite and chromite series (simplified as magnetite) can provide valuable insights into the formation processes of magmatic Ni-Cu-(PGE) sulfide deposits. Previous studies mainly focus on magnetite in disseminated ores and massive ores, but very little is known about the magnetite in net-textured ores. In this study, magnetite in net-textured ores (type 1 magnetite) and massive ores (type 2 magnetite) from the Zhdanov Ni-Cu-(PGE) deposit in the Pechenga ore field, were recognized and analyzed using scanning electron microscope (SEM), electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Both the type 1 and type 2 magnetite grains show zoned textures with Cr concentration decreasing from the core to the rim. Type 1 magnetite has higher Al and Mg concentrations than type 2 magnetite. These observations indicate that type 1 and type 2 magnetite crystallized from and record the solidification process of silicate melt and sulfide liquid, respectively. Type 1 magnetite within sulfides of net-textured ores has identical chemical composition to that within silicates, which suggests that the chemical composition of magnetite is not significantly affected by sub-solidus re-equilibrium with sulfide minerals. It also suggests that the type 1 magnetite within sulfides was incorporated into sulfide liquid later during the downward percolation of sulfide liquid through the magnetite-bearing olivine crystal mush. On the other hand, type 2 magnetite has comparable concentrations of W, Sc, Ga, Cr, Mo, and Mn to the type 1 magnetite. Such compositional similarity can be explained by the crystallization of type 2 magnetite from the sulfide liquid pool that have been mingled with the downward percolated sulfide liquid. Accordingly, the downward percolation of sulfide liquid plays an important role in the formation of net-textured ores and massive ores in the Zhdanov deposit.

Re-Os Systematics in the Layered Rocks and Cu-Ni-PGE Sulfide Ores from the Dovyren Intrusive Complex in Southern Siberia, Russia: Implications for the Original Mantle Source and the Effects of Two-Stage Crustal Contamination

The Dovyren Intrusive Complex (Northern Baikal region, 728 ± 3 Ma) includes the dunite–troctolite–gabbronorite Yoko–Dovyren massif (YDM) associated with a sequence of underlying mafic-to-ultramafic sills, locally demonstrating interbedding relations with the most primitive rocks of the pluton. These sills and apophyses contain sulfide mineralization ranging from globular to net-textured and massive ores. Major types of the YDM cumulates and sulfide mineralization were examined for their PGE contents and Re-Os isotopic systematics. The ten analyzed samples included chilled and basal rocks, poorly mineralized troctolite, PGE-rich anorthosite, as well as three samples from a thick ore-bearing apophysis DV10 connected with the YDM. These samples yielded a Re-Os isochron with an age of 759 ± 36 Ma and an initial 187Os/188Os of 0.1309 ± 0.0026 (MSWD = 110), which is in consistent with the previously reported U–Pb zircon age. It is shown that being recalculated to γOs(t) at t = 728 Ma, these isotopic compositions demonstrate three clusters regarding the relationship between γOs(t) and 187Re/188Os: (i) the chilled gabbronorite (YDM) and subcontact olivine gabbronorite (DV10) yielded the most radiogenic values of γOs(t) 10.5 and 10.0 among basal ultramafics, (ii) plagiodunite, troctolite, and sulfide ores showed lower radiogenic compositions, with γOs(t) ranging from 7.3 to 8.7, (iii) olivine gabbronorite, plagioperidotite, and one sample of PGE-rich anorthosite yield very primitive γOs(t) in the range 4.5 to 5.6 (on average 5.2 ± 0.6). The lowest values of γOs(t) for the least fractionated rocks of the YDM suggest a primitive mantle source, formed from a partly contaminated Neoarchean protolith, which is considered to be anomalous in Upper Riphean due to very low εNd(t) of −16 for the most primitive Dovyren magma (Fo88-parent). The highest values of γOs(t) and relative enrichment in the 34S isotope in the chilled gabbronorite (YDM) and subcontact olivine gabbronorite (DV10) evidence that their primitive to evolved magmatic precursors could be affected by a metamorphic fluid enriched in radiogenic 187Os, originating in the exocontact halo due to the thermal decomposition of pyrite from the dehydrated country rocks. This is consistent with the second-stage contamination of the Dovyren magma by the hosting crustal rocks (probably of 10 wt% shists), generating more evolved Fo86-parent magma with higher εNd(t) of −14.

The ca. 349 Ma metamorphic enrichment of the Tianhu metamorphosed sedimentary Fe deposit in the eastern Tianshan: Evidence from apatite trace elements, oxygen isotopes, and U-Pb chronology

Apatite is a common accessory mineral in iron oxide-apatite (IOA), magmatic Fe-Ti oxide, and metamorphosed sedimentary Fe deposits. Apatite chronology and oxygen isotopes in the IOA deposits have been extensively studied, however, such information in the metamorphosed sedimentary Fe deposits remains poorly understood. Here, we report detailed trace elements, oxygen isotopes, and U-Pb chronology of apatite from the Tianhu metamorphosed sedimentary Fe deposit in the eastern Tianshan, NW China. To our knowledge, this is the first report on the in situ oxygen isotopes of apatite from the metamorphosed sedimentary Fe deposits. Cathodoluminescence imaging of the apatite grains from the Tianhu massive Fe ores show a homogeneous texture, suggesting that they were formed by a single event rather than multiple superimposed events. These apatite grains contain abundant fluid inclusions, and show relatively high δ18O values of 7.9 ‰-14.0 ‰ and low Th and U contents, which are consistent with their metamorphic hydrothermal origin. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) dating of the studied apatite grains yielded a U-Pb age of 348.5 ± 18.5 Ma, indicating that the metamorphic enrichment of the Tianhu metamorphosed sedimentary Fe deposit occurred considerably later than the Ordovician ore-hosting rocks. Combined with previous studies, we suggest that the metamorphic massive ores of the Tianhu metamorphosed sedimentary Fe deposit formed in an orogenic setting. Our study highlights that a combination of trace elements, oxygen isotopes, and U-Pb chronology of apatite can be used as powerful tools to constrain the genesis of metamorphosed sedimentary Fe deposits.

Textural relationships and mineral compositions of Fe-Ti oxide ores of the Panzhihua layered intrusion, SW China: Insights for magma emplacement and lateral extension of Fe-Ti oxide ore bodies

The Panzhihua gabbroic layered intrusion in the Emeishan large igneous province (SW China) hosts economically important Fe-Ti-(V) oxide ore deposits. The intrusion consists of six sections from northeast to southwest. To constrain the emplacement of parental magma across the sections and the magmatic processes associated with Fe-Ti oxide ore deposits, a combined study on field relations, rock textures, mineral compositions, and late-stage magmatic microstructure was carried out for the Fe-Ti oxide ores from the Zhujiabaobao, Lanjiahuoshan, Jianshan, and Nalaqing sections. A ∼7-meter-thick ultramafic portion comprising peridotite, olivine (Ol)-rich net-textured and massive ores is firstly recognized at the bottom of the Zhujiabaobao section in this investigation. The magnetite grains in the ultramafic portion are Cr-rich with 0.58 to 1.83 wt.% Cr2O3, and may have crystallized from an earlier injection of Fe-Ti-Cr-rich magma. On the other hand, in the predominate clinopyroxene (Cpx)/plagioclase (Pl)-rich net-textured to massive ores of the intrusion, the magnetite grains contain <0.2 wt.% Cr2O3, and may have crystallized from late injections of Fe-Ti-rich, Cr-depleted magma. We proposed that the parental magma may be emplaced into an originally “tilt-spoon” structure and initially unloaded in the Zhujiabaobao section, then infilled the Lanjiahuoshan to Jianshan sections and extended further to the Nalaqing section. The thick, massive Fe-Ti oxide ore layers in the lower part of the intrusion are attributed to the silicate immiscibility of interstitial liquid within the crystal mush, and the large amounts of Fe-Ti oxides likely crystallized from immiscible Fe-rich melt aggregated in the lower part of the magma chamber.

Geochronology, pyrite trace elements, and in-situ S isotopes of the giant Nagengkangqie’er silver deposit in the Eastern Kunlun Orogenic Belt, Northern Tibetan Plateau

The Nagengkangqie'er (Nageng) silver deposit, situated in the Eastern Kunlun Orogenic Belt (EKOB), Northwestern China. It contains 5,070 tons of contained silver at a silver grade of 325 g/t. Nonetheless, the genesis of the Nageng silver deposit is still under debate, due to the absence of definitive constraints the mineralization age and the metal sources. The U-Pb data analysis of a post-ore calcite vein established the minimum age of the mineralization at 207 ± 3 Ma, whereas the maximum age of 217 ± 4 Ma was determined from zircon U-Pb data obtained from the altered and mineralized rhyolitic porphyry host-rock. These temporal limitations suggest that the mineralization formed at ca. 217–207 Ma, and it is associated with the post-collisional setting of the Paleo-Tethys evolution in the EKOB. The distinct textural patterns, as revealed by petrographic observations, suggest three types of pyrites: py1 is composed of euhedral pyrite in the early barren quartz vein at stage 1, py2 is represented by subhedral-anhedral grains in massive ore veins at stage 2, and py3 forms euhedral crystals of varying sizes, which are interspersed with early-formed marcasite in the quartz + calcite + silver-bearing sulfosalt-dominated veins at stage 3. In-situ δ34S values of pyrites range between −2.33 ‰ and −0.37 ‰, indicating a dominantly magmatic origin. EPMA studies have revealed that silver in the Nageng deposit predominantly occurs as silver-bearing galena at stage 2, whereas silver is enriched in sulfosalts of stage 3. From py1 to py3, a decrease in the Co-content indicates a decrease in the ore-forming fluid temperatures. The presence of stage 3 sulfide, sliver-bearing sulfosalt, and calcite-quartz in a fracture-filled vein indicates low-pressure conditions during its formation, compared to stages 1 and 2. In summary, the Nageng deposit is a typical magmatic-hydrothermal vein-type Ag-Pb-Zn deposit. Fluid cooling and decrease in pressure, coupled with decreases in fO2 and fS2, are the parameters inferred to have led to a decrease of silver solubility in the hydrothermal fluids and, successively, promoted extensive Ag deposition.

Geology, geochemistry, fluid inclusion microthermometry and depositional environment of the Early Cretaceous Meymeh sub-seafloor replacement sideritic-ankeritic iron deposit, Malayer-Esfahan Metallogenic Belt, Iran

Meymeh sideritic-ankeritic iron deposit in the eastern part of Malayer-Esfahan Metallogenic Belt (MEMB) is formed in the sedimentary-volcanic rocks of the Early Cretaceous sequence. Ore mineralization in the study area based on stratigraphic location and type of host rocks divided in two ore horizons. Lower ore horizon located in the dolomitic-sandy limestone (Kc3) unit, which upper ore horizon is formed in thin-bedded limestone (Km) rocks. Petrographic studies indicates that mineralization comprises three ore facies: stockwork, bedded and massive ore facies. The most important primary minerals are siderite, ankerite, ferroan-dolomite, pyrite, pyrolusite, barite and minor chalcopyrite. The most frequent textures in the ore zones include laminae, replacement, vein-veinlet, massive and banded. Dolomitization and silicification are the main wall rock alteration styles; alteration intensity increases towards the ore zones. Based on relationships between ore minerals and rock forming minerals, ore mineralization (hypogene and supergene) in the Meymeh deposit formed during three main stages: fine-grained Fe-carbonate bands are intricately interlayered with dolomite beds. Sideritic-ankeritic bands exhibit classic sedimentary textures, such as laminations and bedding, indicative of a syn-sedimentary to early diagenesis origin. Coarser-grained stage two siderites and ankerites show breccia and vein-veinlet textures, and are considered to have formed by replacement during burial diagenetic sub-seafloor fluid flow. In stage three, siderite and ankerite were converted to secondary iron oxides such as oxide/hydroxide Fe minerals during meteoric water flow through the inverted normal and thrust faults and uplift. The primary two-phase fluid inclusions in the quartz-2 and ankerite-2 minerals that have been investigated from the ore mineralization section of the Meymeh deposit are homogenized at temperatures between 110.3 to 226.9 °C. Salinities of the primary fluid inclusions range from 3.39 wt.% to 14.77 % NaCl eq. This finding suggests that hydrothermal brine fluid mixing with seawater could be the primary mechanism that prompted ore formation. The similarity of REE patterns between siderite and ankerite in different ore facies and host rock carbonates indicates their derivation from the same ore fluids. The Meymeh deposit is considered a typical case of sedimentary hydrothermal diagenetic sideritic-ankeritic mineralization, in which minerals deposited when hydrothermal fluid was released from anoxic to suboxic water columns.

The Role of Te, As, Bi, and Sb in the Noble Metals (Pt, Pd, Au, Ag) and Microphases during Crystallization of a Cu-Fe-S Melt

Quasi-equilibrium directional crystallization was performed on a melt composition (at. %): 18.50 Cu, 32.50 Fe, 48.73 S, 0.03 Pt, Pd, Ag, Au, Te, As, Bi, Sb, and Sn, which closely resembles the Cu-rich massive ores found in the platinum-copper-nickel deposits of Norilsk. Base metal sulfides (BMS) such as pyrrhotite solid solution (Fe,Cu)S1±δ (Poss), non-stoichiometric cubanite Cu1.1Fe1.9S3 (Cbn*), and intermediate solid solution Cu1.0Fe1.2S2.0 (Iss) are progressively precipitated from the melt during the crystallization process. The content of noble metals and semimetals in the structure of BMS is below the detection limit of SEM-EDS analysis. Only tin exhibits significant solubility in Cbn* and Iss, meanwhile Pt, Pd, Au, Ag, As, Bi, Sb, and Te are present as discrete composite inclusions, comprising up to 11 individual phases, within their matrices. These microphases correspond to native Au, native Bi, hessite Ag2Te, sperrylite Pt(As,S)2, hedleyite Bi2Te, michenerite PdTeBi, froodite PdBi2, a solid solution of sudburite-sobolevskite-kotulskite Pd(Sb, Bi)xTe1−x, geversite PtSb2, and a multicomponent solid solution based on geversite Me(TABS)2, where Me = Σ(Pt, Pd, Fe, Cu) and TABS = Σ(Te, As, Bi, Sb, Sn). Most of the inclusions occur as thin layers between BMS grain boundaries or appear drop-shaped and subhedral to isometric grains within the sulfide matrix. Only a small fraction of the trace elements form mineral inclusions of sizes ≤ 0.5 μm in Poss, most likely including PtAs2 and (Pt,Pd)S. It is likely that the simultaneous presence of noble metals (Pt, Pd, Au, Ag) and semimetals (As, Te, Bi, Sb) in the sulfide melt leads to the appearance of liquid droplets in the parent sulfide melt after pyrrhotite crystallization. The solidification of droplets during the early stages of Cbn* crystallization may occur simultaneously with the cooling of later fractions of the sulfide melt, resulting in the formation of Iss. In addition, abundant gas voids containing micro-inclusions were observed in Cbn* and Iss. These inclusions showed similar chemical and mineral compositions to those in BMS matrices, i.e., the presence of gas bubbles did not affect the main features of noble metal fractionation and evolution. Therefore, it is reasonable to assume that ore particles suspended in the melt are either trapped by defects at the crystallization front or transported towards gas bubbles via the Marangoni effect.

Compositional variations of magnetite in different sulfide ore types in the Jinchuan Ni-Cu-PGE sulfide deposit, NW China: Insights into the mineralizing processes of conduit-type systems

The trace-element composition of magnetite in Ni–Cu–platinum-group element (PGE) sulfide deposits is influenced by magmatic evolution, fractional crystallization of sulfide liquid, and post-magmatic processes, and, therefore, may record ore-forming processes. The Jinchuan Ni–Cu–(PGE) sulfide deposit in NW China contains a variety of sulfide ore types. Despite the capabilities of magnetite to serve as a petrogenetic indicator, its textural–compositional variability in these different sulfide ore types has received little attention. Five types of magnetite with distinct textures and chemical composition are identified in this deposit: (1) Cr-magnetite that crystallized from silicate magma, (2) reactional magnetite produced by the diffusion of oxygen from sulfide liquid to silicate melt, magnetite that crystallized from (3) MSS and (4) ISS, and (5) magnetite that precipitated from hydrothermal fluids. The compositional variability of Cr-magnetite in net-textured and disseminated ores records fractionation of multiple pulses of silicate magmas in a conduit system. Particularly, the higher Ni/Cr and lower V content of Cr-magnetite in disseminated ores relative to those in net-textured ores suggests that the silicate magmas from which the disseminated ores formed were more evolved and had higher fO2 than the magmas from which the net-textured ores formed. Compositional differences between Cr-magnetite and MSS magnetite suggest that magnetite that crystallized from sulfide liquids has higher Ni/Cr ratio, and lower concentrations of lithophile and chalcophile elements. Thus, magmatic magnetite with such compositional features is indicative of sulfide saturation in Ni–Cu–(PGE) sulfide deposits. Compositional differences between MSS and ISS magnetite record fractional crystallization of sulfide liquid. Particularly, the lower V content of ISS magnetite relative to that of MSS magnetite suggests an increase in fO2 during fractionation of the sulfide liquid. Accordingly, this results in greater diffusion of oxygen between the sulfide liquid and surrounding silicate magma, and the preferential occurrence of reactional magnetite in Cu-rich ores. Hydrothermal magnetite records the interaction of magmatic sulfides with oxidized hydrothermal fluids. Based on the lowest chalcophile element contents of this type of magnetite, this interaction did not affect the metal budget of the sulfide ores. Lastly, the similar chalcophile element contents, and overall similar multi-element variation patterns of MSS magnetite in disseminated and massive ores demonstrates that the sulfide liquid that formed the different ore types may have had similar chalcophile element contents. Therefore, the massive ores likely formed by the draining and coalescance of originally disseminated sulfide liquid in a mush zone. Taken together, the compositional variability of texturally distinct magnetite at Jinchuan document a range of magmatic–hydrothermal processes that formed and modified this mineralized system. This contribution continues to highlight the potential for magnetite to serve as a petrogenetic indicator for magmatic Ni-Cu-PGE deposits globally.

The effect of platinum-group minerals on differentiation of platinum-group elements in magmatic sulfide deposits: Evidence from the Cu-Ni-PGE deposits in the Yangliuping area of the Emeishan large igneous province, SW China

Platinum-group minerals (PGM) are important host phases for platinum-group elements (PGE) in magmatic sulfide deposits. Recent studies have demonstrated that some PGM, such as sperrylite, can crystallize early from sulfide liquids. It is important to determine whether and how these PGM particles affect the enrichment and differentiation of PGE in Cu-Ni-PGE deposits. The magmatic sulfide deposits in the Yangliuping area (mainly consisting of the Yangliuping deposit and Zhengziyanwo deposit) are the largest Cu-Ni-PGE deposits in the Emeishan large igneous province, SW China. Primitive-mantle normalized PGE patterns and fractional crystallization modelling show that massive ores in the Yangliuping deposit are enriched in Pd and Pt. In contrast, massive ores, breccia ores and Cu-rich ores in the Zhengziyanwo deposit are depleted in Pt. Scanning electron microscopy (SEM), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), focused ion beam (FIB) and transmission electron microscopy (TEM) were used to constrain the occurrence of PGE in the massive ores from the two deposits. Mass balance calculation shows that <7% of Ir, Rh and Pt occur in base metal sulfides (BMS, mainly pyrrhotite, pentlandite and chalcopyrite) as solid solution in both the Yangliuping and Zhengziyanwo massive ores. 17% and 91% Pd are present in solid solution within BMS in the Yangliuping and Zhengziyanwo massive ores, respectively. In the Yangliuping massive ores, PtAs2 and Pd-PGM particles were commonly found within BMS crystals, and Ir-Rh-(Pt) sulfarsenides enclosed in cobaltite-gersdorffite solid solution (CGSS) are enriched in Pt. In contrast, in the Zhengziyanwo massive ores, no PtAs2 particles and fewer Pd-PGM particles were found, and Ir-Rh-(Pt)AsS particles enclosed in CGSS are poor in Pt. The distribution, mineralogy and microstructures of Ir-Rh-(Pt)AsS, PtAs2 and Pd-PGM particles in both the Yangliuping and Zhengziyanwo deposits suggest that they mainly crystallized from sulfide liquids or formed from Pd-Pt-semimetal-rich droplets. We suggest that the incorporation of Pd-PGM and PtAs2 particles into MSS cumulates led to the enrichment of Pd and Pt in the Yangliuping massive ores. In the Zhengziyanwo deposit, early fractional crystallization of PtAs2 reduced the Pt content in sulfide liquids and resulted in the formation of Pt-depleted sulfide ores and low-Pt Ir-Rh-(Pt)AsS particles, but no Pt-rich sulfide ores have been found to date. As a result, the role of PGM particles should be considered when using PGE to trace the formation process of Cu-Ni-PGE deposits. This study also provides additional evidence that the behavior of Pt in sulfide liquids is influenced by thermodynamic conditions and As, and suggests a potential for Pt-rich sulfide ores exploration in the Zhengziyanwo deposit.

Iron, boron, and sulfur isotope constraints on the ore-forming process of subseafloor replacement-style volcanogenic massive sulfide systems

The well-preserved Ashele subseafloor replacement-style volcanogenic massive sulfide (VMS) deposit in the Central Asian Orogenic Belt comprises two stages of Cu mineralization, i.e., early massive sulfides dominated by colloform and euhedral pyrite intergrown with chalcopyrite and sphalerite, which were replaced by late vein-dominated chlorite-chalcopyrite assemblages. In this study, a combined systematic Fe, B, and S isotope investigation was first applied to investigate the sulfide precipitation processes and the relative proportion of fluid sources in different alteration and mineralization stages of the Ashele deposit. Boron isotopes of Mg-rich tourmaline (δ11B from −5.57‰ to −2.73‰, average (avg.) −4.23‰) indicate significant seawater (∼19%) participated during the formation of massive sulfides. A two-component mixing model is used to estimate the contribution of seawater and igneous sulfur to the total sulfur budget, and the results show the increasing contribution of magmatic sulfur from the early (35%) to late (76%) stages. In addition, δ56Fe values of pyrite gradually increase from the massive ore (−0.46‰ to −0.02‰, avg. −0.24‰), quartz-pyrite (−0.09‰ to 0.07‰, avg. −0.01‰), chlorite-chalcopyrite-quartz-pyrite (0‰ to 0.21‰, avg. 0.08‰) to the quartz-sericite zone (−0.02‰ to 0.29‰, avg. 0.14‰), which is likely related to the different extent of isotopic exchange and formation temperature, and could be used in the exploration of VMS systems. The new two-stage ore-forming model shows that in the early stage, rapid mixing of hydrothermal fluid from underlying magma chamber with abundant cold seawater led to rapid deposition of pyrite and associated Cu mineralization under relatively oxidized condition, and long-term hydrothermal activities in relatively closed systems would promote the formation of upper massive ores, which resulted in an equilibrium system between pyrite, chalcopyrite, and associated fluid with wide ranges of δ56Fe in pyrite (−0.46‰ to −0.02‰) and chalcopyrite (−1.56‰ to −0.49‰). The late hydrothermal activities in relatively open system would contribute to stringer sulfides or stockworks underlying the massive ore in relatively reduced conditions with heavier Fe isotope compositions in pyrite (−0.09‰ to 0.29‰) and chalcopyrite (−0.60‰ to −0.04‰). Overall, our study demonstrates that the coupling of B, Fe, and S isotopes could be a useful tool to indicate long-term subseafloor infilling and replacement processes for subseafloor replacement-type VMS deposits, which are the prerequisite to form large-tonnage VMS deposits.