Mineral Geochemistry Research Articles

Behavior of Platinum-Group Elements during Hydrous Metamorphism: Constraints from Awaruite (Ni3Fe) Mineralization

Abstract Natural Fe-Ni alloys are common in meteorites and, presumably, the Earth’s core, where they host significant platinum-group elements (PGE). However, little is known on PGE concentrations in hydrothermal or metamorphic Fe-Ni alloys (i.e., awaruite Ni3Fe) from terrestrial rocks. In this work, we examine the geochemistry of awaruite and related minerals from several placer deposits sourced from the suprasubduction ophiolitic (Kamchatsky Mys, Karaginsky Island, and Mamet) and Ural-Alaskan (Galmoenan) complexes of Kamchatka and the Koryak Highlands (Far East Russia) in order to assess the abundance of PGE in awaruite and constrain their mobility under metamorphic and hydrothermal conditions. Studied awaruite from ophiolitic and Ural-Alaskan type complexes formed via desulfurization of pentlandite during serpentinization. Three groups of platinum-group minerals (PGMs) are associated with awaruite from Kamchatsky Mys: (1) Pt-Fe alloys such as ferronickelplatinum (Pt2FeNi) or unnamed Ni2FePt alloys; (2) Os-Ir-Ru alloys of various composition; (3) Pd-Sb minerals which form together with serpentine during hydrothermal alteration. Despite the abundance of PGM inclusions, no significant PGE concentrations were measured in awaruite from the Kamchatsky Mys, Karaginsky Island, or Mamet ophiolites. In contrast, pentlandite relicts in awaruite from placers related to the Galmoenan Ural-Alaskan type complex contain exceptionally high, previously unreported, Os (up to 540 ppm). Awaruite that forms on behalf of this pentlandite does not show any significant Os enrichment. Rare Galmoenan awaruite analyses yield up to 3 ppm Pd. The new data are not in complete accordance with previous studies that reported relatively high (up to first 10 ppm) PGE content in awaruite. We attribute this to low PGE concentration in precursor sulfides and preferential partitioning of PGE into discrete secondary PGM within awaruite. Nevertheless, abundant inclusions of secondary PGM in awaruite provide evidence of PGE mobility during metamorphic and hydrothermal alteration of ultramafic rocks.

Petrogenesis and implications for tin mineralization of the Beidashan granitic pluton, southern Great Xing'an Range, NE China: Constraints from whole-rock and accessory mineral geochemistry

The Beidashan pluton, consisting of quartz monzonite in the northeast and biotite granite in the southwest, is a representative granitic intrusion in the southern Great Xing'an Range. To investigate its petrogenesis and relationship with tin mineralization, whole-rock and accessory minerals (zircon and apatite) geochemical analyses were carried out. The quartz monzonite mainly belongs to high-K calc-alkaline strongly peraluminous granite, and the biotite granite is high-K calc-alkaline weakly peraluminous granite. Zircon U-Pb dating indicates a younger age for the biotite granite (138 Ma) than the quartz monzonite (140 Ma). The zircon Hf and apatite Nd isotopic compositions of the quartz monzonite (εHf(t) = 5.2 to 9.1 and εNd(t) = −1.1 to 1.3) and those of the biotite granite (εHf(t) = 5.6 to 8.0 and εNd(t) = −0.9 to 1.0) are indistinguishable, implying that they were derived from the same sources. Modeling results using apatite Nd isotope data show that they were from a mixing source of the Xilinhot Group and mantle-derived basaltic melt. Whole-rock and apatite trace elements reveal that the granitic magma mainly experienced fractional crystallization of amphibole, biotite, plagioclase, K-feldspar, apatite, and monazite from the quartz monzonite to the biotite granite. Mineralogical, whole-rock geochemical, and apatite trace element data indicate that the Beidashan pluton has an I-type granite affinity. A crystal mush model was proposed accounting for the composition gap between these two granites (e.g., SiO2, Al2O3, and Ba), in which the biotite granite represents the highly fractionated interstitial melt extracted from the crystal mush system whereas the quartz monzonite is the residual crystal mush consisting of the cumulate crystals and trapped interstitial melt. The melt was extracted when the crystallinity of the crystal mush reached about 33–37%, and the residual crystal mush trapped about 38–46% interstitial melt. Both Ce anomalies in zircon and Eu anomalies in apatite imply reducing conditions. The calculated values of oxygen fugacity for the Beidashan pluton range from −19.3 to −15.7, and most of them are lower than the fayalite-magnetite-quartz (FMQ) oxygen buffer. Apatite in the Beidashan pluton has high F (2.35–3.61%) and low Cl (0–0.41%) concentrations, which are similar to those of Sn-W deposits worldwide. The Sn contents of apatite increase from the quartz monzonite (mean 0.30 ppm) to the biotite granite (mean 1.09 ppm), suggesting that magma evolution is important for the enrichment of Sn in the melt.

P-T-H2O conditions during multistage fractional crystallization, magma replenishment and mixing through the nephelinitic–phonolitic series and their role on the eruptive styles on the Trindade Island (South Atlantic Ocean, Brazil)

Magmas are affected by several differentiation processes as they migrate through the lithospheric plate, which are recorded by mineralogy, textures and geochemistry in igneous rocks. These processes and their related physico-chemical conditions can be reconstructed by petrography and mineral geochemistry, providing a detailed magmatic history from magma source to crystallization. We present a detailed petrography coupled with single-spot and multiple-profile mineral chemistry and whole-rock geochemistry that allows the reconstruction of P-T–H2O–depth conditions that affected the nephelinite-phonolite magmatic series and influenced eruptive styles. Results on nephelinites and phonolites suggest their cogeneticity and led to the recognition of multistage fractional crystallization, magma replenishment and mixing during their storage and eruptive events. Our data allow us to reconstruct nephelinite magma storage within the oceanic crust (20-5 km depth) with higher H2O content, and chambers lodged below the volcanic centers (<5 km depth) with lower H2O contents, which represent pre-eruptive magma storage subjected to progressive boiling. On the other hand, the main phonolite magma storage, crystallization and mixing with nephelinitic pulses occurred within the lithospheric mantle at depths of 57–37 km, indicating that the main magmatic differentiation of this series occurred at this depth. This process is accompanied by H2O increase, which may be related to magma ascension from the mantellic depths to the volcanic environment and eruption controlled by volatile contents. These interpretations are consistent with Strombolian to Hawaiian eruptive styles reported previously for nephelinite episodes and Vulcanian eruptive styles for phonolite ones.

Identification of recycled oceanic crust in the mantle source of sulfide-bearing lower crust of the Gangdese magmatic arc, Southern Tibet: Constraints from Sr–Nd–Li isotopes

Post-collisional porphyry copper deposit (PCD) is a new genetic type of PCDs discovered in the Himalayan–Tibetan orogenic belt. It is believed that the formation of ore-forming magmas results from the re-melting of a sulfide-bearing, thickened, juvenile mafic lower crust in a magmatic arc associated with the oceanic plate subduction. However, the material contribution of subduction processes to the formation of this particular arc magma remains unclear. In this paper, we attempted to present a study that includes petrology, mineral geochemistry, whole rock Sr–Nd–Li isotopes, of Late Cretaceous Lilong ultramafic igneous rocks, south Lhasa sub-terrane, Tibet. The target ultramafic igneous rocks are sulfide-bearing cumulates. They show relative depletion in whole rock Nd isotopic compositions, slightly elevated (87Sr/86Sr)i ratios (0.7043–0.7059), and variable δ7Li values (2.8–9.0‰). Simulation results showed that trace element of melts in equilibrium with cumulus hornblende grains exhibit typical arc-like trace element distribution patterns, indicating that these ultramafic igneous rocks crystallized from arc magma. Additionally, the modeled melts showed enrichment in melt-mobile elements such as Ba, Pb, Sr, Th, and light rare earth elements (LREE), and higher Th/Yb ratios, as compared with normal mid-ocean ridge basalt (MORB). These geochemical properties can be explained by the metasomatism of depleted MORB mantle peridotites by hydrous melts derived from subducted seafloor sediments. Whole rock Sr isotopes show correlations with Ba/Nb and Th/La ratios for the modeled melts, showing that fluids from the basaltic igneous crust, in addition to hydrous melts from the seafloor sediments, contribute to their mantle source. In addition, the heterogeneity of whole rock δ7Li values for ultramafic igneous rocks also indicates that their mantle sources have been metasomatized by terrigenous sediment- and altered oceanic crust-derived fluids. Depleted MORB mantle peridotites react with the mixed fluids at sub-arc depths to produce ultramafic metasomatites, and model calculations for trace element contents and Sr–Nd–Li isotopic compositions further validate that the geochemical compositions of the Lilong ultramafic rocks can be clarified by this process. The results show that the deeply subducted oceanic crust may have played an important role in the formation of sulfide-bearing juvenile mafic lower crust of this magmatic arc.

The propylitic alteration in the Ponce Enriquez Gold Mining district, Azuay province, Ecuador: genetic constraints from a mineral chemistry and fluid inclusions study

Wide areas of the Southern sector of Ecuador are characterized by Cretaceous mafic volcanic rocks, pervasively altered by a propylitic mineralogical assemblage with epidote, chlorite, and quartz with minor titanite, illite and prehnite. These propylitically altered rocks host significant gold mineralization in veins, deeply exploited in the last decades. Porphyry Au-Cu deposits also occur in the area. This work focuses on the study of this propylitic alteration to assess the genetic conditions and the relation with the distance from the porphyry system, through the chemical composition of the secondary minerals (particularly trace elements in epidote and chlorite) and fluid inclusion data. The aim is to improve knowledge of the trace element geochemistry of secondary minerals in propylitic alteration and their relationship to the causative porphyry systems. The results of this work indicate that alteration affecting the rocks of the Pallatanga Fm. can be related to the propylitic hydrothermal alteration halo of porphyry copper deposits. Fluid inclusion investigation in quartz veins associated with epidote and chlorite evidenced at least two pulses of hydrothermal fluids characterized by temperature around 345°C for the first one and around 305°C for the second one. The salinity of the fluids is always low (around 1.5 wt% NaClequiv.) suggesting a dominantly meteoric component. Evidence of boiling processes have not been observed. We hypothesize that the hydrothermally altered “green rocks” of the Ponce Enriquez Mining District could be envisaged as the propylitic halo of the Miocene hornblende-bearing Au-Cu porphyry deposits of Gaby-Papa Grande, and discuss the epidote and chlorite trace element composition in this scenario. The results help improving the general knowledge of the propylitically altered rocks in southern Ecuador and add new data for the use of trace element chemistry of epidote and chlorite in mineral exploration elsewhere.

Big data analytics for mining geochemistry of gold mineralization: The Gandy gold deposit, the Toroud-Chah Shirin (TCS) belt, north Iran

Due to the complexity of the gold element, the mineralogical and geochemical investigations are significant to trace the gold behavior in sedimentary, metamorphic, magmatic, and post-magmatic processes and to identify the physicochemical conditions (i.e., pH, Eh, temperature, pressure) of concentration in the ore bodies. The trace elements may be concentrated as the primary or secondary haloes in endogenic and exogenic deposits with multi-formational types. Identification and separation of multi-formational geochemical halos associated with ore bodies have been the most significant challenge of gold deposits. Mining Geochemistry and Big Data analytics have become essential to identify the multi-formational geochemical anomalies for gold exploration in the Commonwealth of Independent States (CIS) countries. During the last three decades, pattern recognition techniques such as Bayesian and Geochemical spectra have been used to determine mineralogical and geochemical types of multi-formational anomalies. Big Data analytics has been used for integrating multi-type geoscience data and pattern recognition techniques to assess geochemical anomalies. In this study, Big Data analytics were used to generate the mineralogical and geochemical type (MGT) quantitative models based on the Bayesian, geochemical spectrum and hybrid methods for investigating the distribution of trace element contents of pyrite, galena, and sphalerite in the Gandy Gold Deposit, the Toroud-Chah Shirin (TCS) belt, north Iran. The multi-formational geochemical anomalies associated with gold mineralization in the Gandy deposit indicated a multi-MGT anomaly superposition, which is a combination of two MGTs: Au + Polymetals and Au + sulfide. The results of this study demonstrated a new perspective on the distribution of geochemical anomalies and the multi-MGT geochemical modeling for gold exploration in the TCS belt. Consequently, the instigated approach is greatly conceivable and applicable to reveal contrasting near-ore multi-formational geochemical haloes for gold exploration in many other metallogenic provinces.

Natural Analogue Studies in Support of Post-Closure Safety Assessment of Deep Geological Disposal

Natural analogues are systems that have evolved over geological timescales with features similar to one or several components of a deep geological repository (DGR). Natural analogues complement short-duration laboratory studies since they are existing reflections of many long-term processes that might affect the performance of a repository. Mathematical models are often used for the post-closure safety assessment of a DGR. Confidence in the models’ predictions is enhanced when the models successfully simulate the past evolution of a natural analogue. This paper summarizes the Canadian Nuclear Safety Commission’s (CNSC’s) recent research on natural analogues to inform on (1) glacial erosion, (2) engineered barrier system, and (3) uranium reactive transport in the context of DGRs for radioactive wastes. Glaciation and its erosion are prominent factors impacting the performance of future DGRs at high latitudes in the northern hemisphere. The authors have reviewed the field data from the Greenland Analogue Project, developed a conceptual and mathematical model for the simulation of the thermal conditions within the Greenland ice sheet, as well as the thermal-hydraulic conditions at its base and the ice sheet velocity, and eventually estimated the erosion rate at the site. The Cigar Lake Analogue demonstrates the long-term radionuclide containment capability of the illite clay zone enveloping the ore body, serving as an analogy to the engineered clay barriers. The CNSC and University of Ottawa analyzed 129I in the Cigar Lake core samples, and modeled and correlated the diffusion-dominated transport of radionuclides over the geological evolution of the Cigar Lake deposit. The results provide information on the mobility of fission products and significant radionuclides in conditions analogous to the source, engineered barriers, and near-field host rock of a DGR. The reactive transport and geochemistry of the Kiggavik-Andrew Lake uranium deposit mineralization and remobilization was another natural uranium deposit analogue studied by the CNSC. A reactive transport model was established according to the conceptualized geochemical processes and run under specified boundary and initial conditions to validate the geochemical processes. The geometry, timing, geochemistry, and fluid composition were used as model constraints.

Characterisation and rare-metal potential of the Winneba-Mankoadze pegmatites, Southern Ghana: Evidence of two pegmatite fields

In southern Ghana, the region along the coast between Accra and Cape Coast hosts a large number of pegmatites mineralized in lithium, niobium-tantalum and tin. The pegmatites occur in many distinct groups, each extending over several kilometers. They intrude metasedimentary units of the Birimian Supergroup, and are associated with early to late orogenic granite intrusions which are metaluminous, sterile, and too old to be potential parental granites for the pegmatites. In this study, we characterized the Winneba-Mankoadze group of geographically coeval pegmatites, using field description, petrography, rare-metal mineralogy and accessory mineral geochemistry on micas, garnet and Nb–Ta–Sn minerals, in order to determine its rare-metal potential and to investigate its origin. The results indicate that the pegmatites are part of the albite-spodumene type of the Lithium–Cesium–Tantalum (LCT) family. The rare metal mineral assemblages are particularly complex and display relevant oxide species such as columbite- and wodginite-group minerals, tapiolite, microlite, cassiterite and rutile, which are evidences of an extremely evolved magmatic system. Based on mineral assemblages, whole-rock geochemistry, and mineral geochemistry on garnet, micas and the CGM, two pegmatite fields are distinguished in the Winneba-Mankoadze group, and an anatectic origin is proposed. For the first time in West Africa, we fully describe a highly fractionated LCT-family pegmatite field comparable to the most evolved pegmatite bodies in the world.

Two-Stage Evolution of the Altyn Tagh Fault System during the Tertiary: Constraints from Heavy Mineral Chemistry in Sediments of the Northwestern Qaidam Basin, Western China

The tectonic evolution of the Altyn Tagh Fault (ATF) remains controversial during the Tertiary. Qaidam Basin is the largest and highest plateau inland basin inside the Tibet Plateau. Sediments in the basin provide sedimentary records of the evolution history of its surrounding orogens, such as the ATF, located on the northwest margin of the Qaidam Basin. Comprehensive analyses of sandstone petrography, heavy mineral assemblages, and mineral geochemistry were adopted to effectively indicate the tectonic evolution history of ATF. The result indicates that the sediments in a wide range of the northwestern Qaidam Basin (e.g., the Xichagou section, the Yueyashan section) were derived from the Altyn Mountains. The increasing immaturity of sediments, increased denudation, and sedimentation processes from the early-middle Miocene to the Pliocene can be explained by the active tectonic setting of the ATF. During the early Miocene (ca. 22 Ma), there was an abrupt change in the heavy mineral composition of sediments in the northwestern Qaidam Basin. This change may be attributed to the large-scale slip motion along the ATF. Therefore, ~22 Ma is the key transforming period of the ATF system. On the foundation of the above, we suggest a two-stage evolution model of the ATF during the Tertiary: (1) From the late Eocene to the Oligocene, the tectonic setting of the ATF was relatively calm; (2) During the early Miocene period, the ATF underwent large-scale tectonic activation. It is likely to be a strike-slip tectonic activity, accompanied by an uplift of the Altyn Mountains. The active tectonic setting of the ATF was sustained after the Miocene.

Controls on metal fertility of dioritic intrusions in the Laiwu district, North China Craton: Insights from whole-rock geochemistry and mineral compositions

Several large high-grade iron skarn deposits in the Laiwu district of Shandong Province (North China Craton) are developed along the contact zone between the Early Cretaceous Kuangshan diorite pluton and Middle Ordovician dolomitic limestone. In sharp contrast, no iron skarn mineralization has been recognized in association with other contemporaneous dioritic intrusions (Jiaoyu, Jinniushan, Tietonggou) in this district. The reason for this contrast remains elusive. Here we present a comparative study of the ore-related Kuangshan pluton and its barren equivalents using whole-rock and mineral (zircon, apatite) geochemistry combined with existing Sr-Nd isotope data to provide new insights into the critical controls on iron skarn mineralization and shed light on future mineral exploration. Geochemical and Sr-Nd isotope data suggest that both the ore-related and barren intrusions were derived from a common enriched lithospheric mantle source following variable degrees of fractional crystallization and crustal assimilation. Modeling based on whole-rock Sr-Nd isotopes implies that the magmas responsible for the Kuangshan and Jinniushan diorite plutons assimilated significant quantities of the Ordovician evaporite-bearing carbonates. Both the fertile and barren diorites show high whole-rock Sr/Y ratios, high zircon Eu/Eu* and Eu/Eu*/Y ratios, and low zircon Dy/Yb ratios. These data, combined with high magmatic water contents (4.4−8.0 wt%) calculated from amphibole compositions, indicate that those four plutons crystallized from hydrous magmas. High whole-rock Fe2O3/FeO, and high ΔFMQ values (mostly &amp;gt;1; fayalite-magnetite-quartz oxygen buffers) estimated from zircon, apatite, and amphibole suggest that the magmas from which the diorite intrusions crystallized were all oxidized. Using available partitioning coefficients for Cl between apatite and magma, the parental melts of the Kuangshan and Jinniushan diorite are estimated to have had higher Cl contents (0.15−0.77 wt%) than the Jiaoyu and Tietonggou diorites (0.06−0.34 wt%). Two types of apatite from the intrusions studied are recognized based on their paragenetic relations: Type 1 apatite (Ap1) is an early-formed phase enclosed in euhedral amphibole, plagioclase, and biotite, whereas type 2 (Ap2) is a late-formed variety hosted in, or intergrown with, anhedral K-feldspar (± anhedral amphibole ± subhedral to anhedral quartz). In the fertile Kuangshan diorite, Cl and OH contents of apatite decrease, but Sr contents increase from Ap1 to Ap2. However, those parameters show reverse trends or remain constant from Ap1 to Ap2 in barren diorites. Such distinctive apatite geochemical signatures are best interpreted as reflecting more efficient fluid exsolution from the parental magma of the Kuangshan diorite than from the barren intrusions. This view is supported by the presence of abundant fluid inclusions in apatite and quartz exclusively from the mineralized intrusion, indicating its crystallization from volatile oversaturated melts. The results presented here indicate that elevated magma Cl contents and efficient separation of magmatic hydrothermal fluid from cooling magmas have been the critical factors controlling iron skarn mineralization in the Laiwu district. Mass balance constraints on Cl budget indicate that a minimum of 44−60 km3 of magma was required to form the Fe deposits associated with the Kuangshan pluton. Our preliminary results suggest that decreasing OH and Cl contents from early to late apatite in intermediate rocks can be used as an indicator for efficient fluid exsolution and consequently metal fertility of those rocks.

Petrogenesis of the Yunling Sn-rich magma in the Baoshan Block, SW China: Constraints from mineral and whole-rock geochemistry

Southwest Yunnan is one of the most important tin (Sn) metallogenic belts in China. The Sn mineralization is mainly linked with the Cretaceous and Cenozoic magmatism. Triassic granitoids are also widely developed in this region, where the Yunling granitoids are considered to represent an essential target for the localization of Sn mineralization. However, the source characteristics of these ore-related granitoids are still unclear. Here we present geochronology, petrology, and mineral geochemistry to reveal the petrogenesis of Sn-rich magma. Zircon U-Pb dating indicates that the Yunling samples were emplaced at 220–209 Ma, suggesting that the Late Triassic magmatism in the Yunling district along the S–N trending Changning-Menglian suture zone (CMSZ) was generated at the post-collisional stage. Samples from the Yunling pluton exhibit a variable SiO2 content of 54.8–68.5 wt% and are characterized by a high normative corundum content (>1 wt%) and aluminum saturation index (A/CNK = 0.84–1.17). Systemic zircon geochemistry suggests that these samples belong to S-type granite. Moreover, the granitoids have extremely enriched Nd-Hf isotopic signatures with εNd(t) values of −11.5 to −10.7 and εHf(t) values of −21.6 to −10.6. These findings, together with the elevated Ba/Pb ratios and high zircon Ti temperatures (≥800 °C), indicate that the Yunling S-type granitoids were likely to have been derived from the dehydrating partial melting of biotite in the metasedimentary rocks at high temperatures. In addition, the magmatic oxygen fugacity obtained from the magmatic accessory minerals suggests that the ore-related granitic magmas are highly reduced, with the estimated ΔFMQ (fayalite-magnetite-quartz buffer) generally ranging from −5.45 to −0.22 (-3.48 on average). Based on these results, we conclude that the decomposition of biotite in the source effectively extracted Sn, thus forming initial Sn-rich melts which crystallized and differentiated at a low oxygen fugacity and were eventually deposited in the shallow crust to form the Yunling Sn-bearing granitoids.

Sabkha and salina dolomite preserves the biogeochemical conditions of its depositional paleoenvironment

Precipitation of Mg-carbonate minerals, ranging from Mg-calcite to Ca-rich dolomite, is typically associated with microbial activity, and commonly takes place in saline environments along with other evaporitic minerals. The geochemistry of such evaporitic minerals can contribute to the interpretation of the environmental conditions and those (bio)geochemical processes occurring at the time of deposition. We use a multiproxy approach that combines stratigraphic, petrographic, mineralogical and geochemical data, such as Fe and Sr isotope values, to describe the environmental and biogeochemical conditions under which dolomite precipitated in a sabkha/salina environment using the Miocene Vilobí Gypsum Unit (Penedès Basin, NE Spain) as a case study.The lithofacies of Vilobí Gypsum Unit are interpreted to indicate sedimentary environments alternating between sabkha and coastal salina. The δ13C and δ18O values of Vilobí dolomites (from −6.3 to −0.4‰ and from 0.2 to 2.3‰, respectively) compare well with microbially influenced dolomite from modern sabkha and coastal environments. The dolomite 87Sr/86Sr ratios (0.70816 to 0.70874) show significant variations and indicate that the coastal sabkha/salina was strongly influenced by seawater. Microbial activity is evidenced by petrography (microbially induced sedimentary structures interpreted as cyanobacterial mats, and spheroidal dolomite with empty bacterial voids), and geochemical data (elemental concentrations enrichment in Fe2+ and Mn2+, up to 1388 and 605 ppm, respectively; supported by δ56Fe values of between −0.16 and 0.44‰). Collectively, these data suggest that the studied dolomite is a primary precipitate that records the geochemistry of the saline depositional environment influenced by microbial activity. The Vilobí Gypsum Unit is here dated to 16.05 Ma and 16.30 Ma (Burdigalian) by correlating the 87Sr/86Sr values of dolomite and shell of the oysters (shells between 0.70870 and 0.70872) with the global Sr curve. Moreover, dolomite records marine (porewater) values (C-O-Fe-Sr) that can be used to infer the depositional environment and geochemical conditions at the time of deposition. Results of non-traditional isotopes and trace metal concentrations support using dolomite formed in evaporitic environments as an archive of paleoenvironmental conditions.

Petrogenesis of Sn-related granitoids and implications for the formation of the world-class Gejiu Sn district, South China: Insights from whole-rock and accessory mineral geochemistry

Petrogenesis of Sn-related granitoids and implications for the formation of the world-class Gejiu Sn district, South China: Insights from whole-rock and accessory mineral geochemistry

Editorial: Accessory mineral geochemistry and its application in mineral exploration

Editorial: Accessory mineral geochemistry and its application in mineral exploration

Open-system differentiation of mafic magmas before the formation of layered Fe-Ti(V) deposits in Southeast China

Mafic magmatic differentiation is an important mechanism to drive Fe-Ti enrichments in melt and final formation of layered Fe-Ti-(V) oxide deposits, but how the differentiation operates and how the state of magmatic reservoirs controls differential metallogenic potential are unclear. The Xialan magmatic complex in northern Guangdong Province is the only complex hosting layered Fe-Ti (V) deposits in Southeast China and the formation of such oxide deposits is not addressed in detail. Here, we present an integrated study of geochronology, mineral geochemistry, whole-rock major and trace elements, as well as Sr-Nd-Pb isotopes, on the mafic rocks associated with the Xialan layered Fe-Ti (V) oxide deposits. The investigated mafic rocks are cogenetic and can be classified into three types according to field and textural features: diabase, Fe-Ti rich gabbro, and Fe-Ti barren gabbro. Based on petrographic observation and major and trace element geochemistry, the gabbros and diabases represent cumulates and residual melts, respectively. However, the Fe-Ti rich gabbros show different Nd isotopes from the other rocks, which indicates an open-system differentiation for the Xialan parental magma. A two-stage evolution process for the Xialan parental magma before the final crystallization fractionation to form the oxide deposits is further proposed based on MELTs modeling results and the characters of mineral assemblages. At the early stage, the magmas started differentiation with 0.5 wt% initial H2O content and moderate oxygen fugacity (△FMQ = ca. + 0.5 – +1.5) at 5 kbar and generated Fe-Ti enriched melts. At the later stage, the Fe-Ti rich melts gradually evolved to a much higher H2O content (>3 wt%), which stabilized the Fe-Ti oxides and hornblendes. Wall-rock assimilation during the late-stage magma evolution is suggested to be the key factor to elevate the H2O contents and oxygen fugacity of the magma which finally produced the layered Fe-Ti oxides. As such, this study demonstrates that open-system differentiation of mafic magmas can play a key role in the final generation of some hornblende-bearing layered Fe-Ti oxide deposits.

Trace elements and Li isotopes distinguish between liquid and solid phases contributing to mafic arc magmas

It is generally accepted that metasomatism of the mantle wedge peridotite by subducting slab-derived fluids plays a key role in dictating the composition of mafic arc magmas. However, this fluid metasomatism model has been challenged recently by the mélange diapirism model. In order to distinguish between liquid and solid phases that would exclusively contribute to mafic arc magmas, we have carried out an integrated study of whole-rock and mineral geochemistry for two types of contemporaneous mafic igneous rocks (gabbroic diorites and diorite dikes) from an accretionary orogen. The results indicate primary contributions from liquid phases composed of not only slab-derived aqueous solution but also seafloor sediment-derived hydrous melt. Although these mafic rocks all exhibit typical arc-like trace element distribution patterns, they show a series of differences in stable Li and radiogenic Sr–Nd–Hf isotope compositions as well as other geochemical variables. In general, the gabbroic diorites are characterized by enrichment in fluid-mobile elements (e.g., Rb, Ba, K, Pb, and Sr), high δ7Li and δ18O values, and depletion in Sr–Nd–Hf isotope compositions. By contrast, the diorite dikes exhibit enrichment in melt-mobile elements (e.g., LILE, LREE and Th), relatively low δ7Li values, and slight enrichment in Sr–Nd–Hf isotope compositions. There are also systematic differences and pertinent variations between Li isotope compositions and slab-derived fluid indicators such as whole-rock Rb/Nd, Sr/Nd, Nb/U, Th/La, Th/Nd and Th/Yb ratios for the two types of mafic igneous rocks. These indicate that the mantle sources of these mafic rocks would possess geochemical signatures that were acquired through chemical metasomatism by slab-derived fluids rather than physical mixing with mélange diapirs. Furthermore, the geochemical signatures allow for distinction between the oceanic slab-derived aqueous solution and the seafloor sediment-derived hydrous melt. This qualitative interpretation is quantitatively confirmed by model calculations of trace elements and Sr–Nd–Li isotopes in the two types of mafic igneous rocks. Therefore, the target mafic igneous rocks provide the geochemical evidence for the formation of their mantle sources through chemical metasomatism by incorporating the two types of metasomatic agent into the mantle wedge. This demonstrates a strong preference for the fluid metasomatism model over the mélange diapirism model for the origin of mafic arc magmas.

Superimposed mineralization and emplacement paleodepth of the Laozuoshan Cu-Au deposit in the Jiamusi block: Evidence from garnet U-Pb geochronology and mineral geochemistry

The Laozuoshan deposit is a large superimposed Cu-Au deposit composed of three ore zones (eastern, middle, and western). A large number of studies have revealed that the middle zone and eastern zone are skarn gold deposits formed by the contact metasomatism between a Late Paleozoic granodioritic magma and Mashan Group carbonate strata, while the western zone is a porphyry copper-gold deposit related to the hypabyssal emplacement of an Early Cretaceous dioritic-granodioritic magma. Therefore, is there mineralization related to this Early Cretaceous dioritic-granodioritic magmatism in the middle and eastern zones? To determine whether there was Early Cretaceous magmatism in the middle and eastern zones, diorite, diorite porphyrite and granodiorite porphyry, which are closely related to the skarn copper-gold mineralization in the eastern zone, were selected for study in this paper. Accurate garnet U-Pb isotope dating and chemical analysis of minerals reveal the following: (1) The formation age of the skarn copper-gold mineralization (104 Ma) is basically consistent with the emplacement time of the dioritic magma (103–106 Ma), which occurred in the Early Cretaceous. (2) The high temperature (722–802 °C), weak oxidation–reduction conditions (Δ FMQ = −4.82 ∼+0.24; log(ƒO2) = −21.2 ∼ −19.0), high water content (5.69–7.13 wt%) and shallow emplacement characteristics of the Early Cretaceous dioritic magma indicate that this magma had the potential to provide thermogenic minerals. (3) The geochemical characteristics of the skarn minerals reveal that the Early Cretaceous mineralization of the Laozuoshan deposit represents a calcareous reducing skarn copper-gold metallogenic system. Combining the mineralization and alteration characteristics of the three zones and previous work, this study further concludes that the Laozuoshan deposit is a superimposed metallogenic system involving a Late Permian skarn Au metallogenic system (eastern zone and middle zone) and an Early Cretaceous porphyry (western zone)-skarn (eastern zone and middle zone) Cu-Au system. The deep part of the mining area has the potential for searching for porphyry Cu-Au deposits.

Origin of Cretaceous alkaline annular structures in the peri-cratonic terranes of the Reguibat Rise in Morocco: New constraints from carbonatite–silicate pairs and subsolidus processes

We report an integrated field, petrographic, and geochemical study of three major Cretaceous carbonatite annular occurrences from the Oulad Dlim massif (Adrar Souttouf, NW margin of the West African Craton, south Morocco): Twihinate, Lamlaga, and Lahjayra. These complexes are commonly considered isolated carbonatite structures partially to completely overlain by silica breccias and iron oxide rocks. The carbonatites include two calcio‑carbonatites facies with distinct textural features and bulk-rock and mineral geochemistry: the “white” facies present characteristics consistent with a primary magmatic stage, while the “grey” facies likely resulted from hydrothermal alteration of the white carbonatites. We provide evidence that the annular structures include ultrapotassic alkaline syenites, probably derived from the same primary carbonated/silicated melt as the carbonatites, after partial melting of a K-rich lithospheric mantle source. The overlying silica breccias retain sandstone/quartzite-like stratifications, whereas their trace element signature is analogous to those of the carbonatites, suggesting that they were subjected to intense hydrothermal activity during carbonatite magmas emplacement. The iron-oxide rocks are chemically comparable to worldwide references of carbonatite-related laterite profiles, indicating a similar origin. All subsolidus (hydrothermal and weathering) alteration stages were accompanied by significant REE-Nb concentration resulting in high-grade deposits of monazite, synchysite, apatite and pyrochlore. Like other peri-Atlantic alkaline occurrences, the investigated Cretaceous alkaline‑carbonatite occurrences are likely related to the Atlantic Ocean opening and reactivation of pre-existing Pan-African tectonic structures.

Geochemical mapping by stream sediments of the NW portion of Quadrilátero Ferrífero, Brazil: Application of the exploratory data analysis (EDA) and a proposal for generation of new gold targets in Pitangui gold district

Quadrilátero Ferrífero (QF) is a worldwide known metallogenetic province and has in its NW portion the Pitangui greenstone belt, one of the new frontiers for gold exploration in that region, presenting important gold deposits, such as Turmalina and São Sebastião. Among several actions of the geological and mineral resources projects carried out by Geological Survey of Brazil (SGB/CPRM) in the QF region, a high-density geochemical survey (1 sample/4 km2) by stream sediments was accomplished in its NW portion to better understand the behavior and distribution of chemical elements along the main geological units, especially those related to gold mineralization (Au, As, Sb, Bi, etc.). The exploratory data analysis (EDA), regarding uni-, bi-, and multivariate analyses, point to an important chemical control of the stream sediments by the geological units and help to describe some lithological features and to understand the behavior and distribution of hydrothermal processes and gold mineralization. This procedure defined the main geochemical associations, including the association As, Sb, Cu and Bi linked to Ni, Cr and Co, interpreted as gold mineralization hosted in the metavolcanosedimentary lithotypes. The staged factor analysis (SFA) considering compositional data (CoDa), logistic function, and fractal model approaches provided the generation of the geochemical mineralization probability index (GMPI), which produce a GMPI map for gold mineralization. The calculations did not take into account the gold concentrations but only the pathfinder elements and the model validation by the prediction-area plot, confirm the high performance of this approach. The gold mineralization map, in addiction to satisfactorily predicting the location of most known gold occurrences, was able to indicate new potential targets for prospecting new gold deposits.

Sulfide Trace Element Signatures and S- and Pb-Isotope Geochemistry of Porphyry Copper and Epithermal Gold-Base Metal Mineralization in the Elatsite–Chelopech Ore Field (Bulgaria)

The Elatsite–Chelopech ore field in the northern part of the Panagyurishte district in Central Bulgaria comprises numerous spatially associated porphyry copper and epithermal gold deposits and prospects. In addition to the mineralization and alteration features, trace elements, lead and sulfur isotope signatures of sulfide minerals from porphyry copper, base metal and gold-base metal deposits/prospects have been studied. LA-ICP-MS analyses of pyrite, arsenopyrite and sulfosalt minerals validate them as major carriers for Au, Ag, Sb, Se and Co. Pyrite from the three types of mineralization has specific geochemical characteristics. Pyrite from the porphyry copper deposits/prospects has generally lower total trace element content compared to pyrite from the epithermal prospects, except for Se, Co and Ni. Pyrite from the base metal and gold-base metal veins is enriched in As, Au, Ag, Sb and Pb. In pyrite from the base metal deposits, Co and Ni have contents comparable to the pyrite from the porphyry copper deposits, while pyrite from the gold-base metal veins shows lower Co and Ni. Arsenopyrite from these deposits shows similar features. Similarly, sphalerite from the gold-base metal veins also has lower Co content compared to sphalerite from the base metal veins but higher In and Cu contents. In addition to the close spatial relationships between the Elatsite and Gorna Kamenitsa porphyry Cu deposits and Negarstitsa-West and Dolna Kamenitsa base metal prospects, as well as similarities in the mineralization and alteration styles, the lead isotopic (206Pb/204Pb = 18.61–18.68, 207Pb/204Pb = 15.64–15.65 for porphyry and 206Pb/204Pb = 18.55–18.67, 207Pb/204Pb = 15.64–15.68 for base metal) and sulfur isotopic (δ34S values of −3 to +1‰ for porphyry and δ34S values of −1.7 to +3.5‰ for base metal) signatures of sulfides support the idea of a genetic link between these two types of deposits. The porphyry and base-metal mineralization result from a common major ore-forming event during the Late Cretaceous, corresponding to deep/higher-temperature and shallower/distal/lower-temperature environments, respectively. In particular, more radiogenic lead (206Pb/204Pb = 18.41–18.47, 207Pb/204Pb = 15.67–15.76) and slightly different sulfur isotopic compositions (δ34S values of +3.5 to +10.6‰) of sulfides from the distal gold-base metal veins of Kordunsko Dere, Svishti Plaz and Shipkite might be a consequence of the interaction of the ore-forming fluids with an external older crustal and isotopically positive S source. Alternatively, a different fluid source/event for the formation of these gold-base metal veins may be suggested.