Revisiting the origin and significance of the Paleoproterozoic complex in the Himalaya: new insights from apatite geochemistry

The textures of outcrop and near-surface exposures of the massive magnetite orebodies (>90 vol % magnetite) at the Plio-Pleistocene El Laco iron oxide-apatite (IOA) deposit in northern Chile are similar to basaltic lava flows and have compositions that overlap high- and low-temperature hydrothermal magnetite. Existing models—liquid immiscibility and complete metasomatic replacement of andesitic lava flows—attempt to explain the genesis of the orebodies by entirely igneous or entirely hydrothermal processes. Importantly, those models were developed by studying only near-surface and outcrop samples. Here, we present the results of a comprehensive study of samples from outcrop and drill core that require a new model for the evolution of the El Laco ore deposit. Backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to investigate the textural and compositional variability of magnetite and apatite from surface and drill core samples in order to obtain a holistic understanding of textures and compositions laterally and vertically through the orebodies. Magnetite was analyzed from 39 surface samples from five orebodies (Cristales Grandes, Rodados Negros, San Vicente Alto, Laco Norte, and Laco Sur) and 47 drill core samples from three orebodies (Laco Norte, Laco Sur, and Extensión Laco Sur). The geochemistry of apatite from eight surface samples from three orebodies (Cristales Grandes, Rodados Negros, and Laco Sur) was investigated. Minor and trace element compositions of magnetite in these samples are similar to magnetite from igneous rocks and magmatic-hydrothermal systems. Magnetite grains from deeper zones of the orebodies contain >1 wt % titanium, as well as ilmenite oxyexsolution lamellae and interstitial ilmenite. The ilmenite oxyexsolution lamellae, interstitial ilmenite, and igneous-like trace element concentrations in titanomagnetite from the deeper parts of the orebodies are consistent with original crystallization of titanomagnetite from silicate melt or high-temperature magmatic-hydrothermal fluid. The systematic decrease of trace element concentrations in magnetite from intermediate to shallow depths is consistent with progressive growth of magnetite from a cooling magmatic-hydrothermal fluid. Apatite grains from surface outcrops are F rich (typically >3 wt %) and have compositions that overlap igneous and magmatic-hydrothermal apatite. Magnetite and fluorapatite grains contain mineral inclusions (e.g., monazite and thorite) that evince syn- or postmineralization metasomatic alteration. Magnetite grains commonly meet at triple junctions, which preserve evidence for reequilibration of the ore minerals with hydrothermal fluid during or after mineralization. The data presented here are consistent with genesis of the El Laco orebodies via shallow emplacement and eruption of magnetite-bearing magmatic-hydrothermal fluid suspensions that were mobilized by decompression-induced collapse of the volcanic edifice. The ore-forming magnetite-fluid suspension would have rheological properties similar to basaltic lava flows, which explains the textures and presence of cavities and gas escape tubes in surface outcrops.

Petrogenesis of the Nanling Mountains granites from South China: Constraints from systematic apatite geochemistry and whole-rock geochemical and Sr–Nd isotope compositions

A critical review on petrogenetic, metallogenic and geodynamic implications of granitic rocks exposed in north and east China: New insights from apatite geochemistry

To constrain the late Triassic tectonic evolution of the Songpan-Ganzi orogenic belt, we present new whole-rock and in situ apatite geochemistry for plutonic rocks in its eastern margin. The Taiyanghe pluton can be classified into two rock types: dioritic and granitic rocks. The former exhibits low SiO2 and MgO contents but high Al2O3, Th, LREE contents, and Th/Yb and Th/Nb ratios, as well as low Ba/La and Ba/Th ratios and enriched Sr-Nd isotopic compositions, which, together with apatite geochemistry and Nd isotopes, indicate that they were derived from low degrees of partial melting of lithospheric mantle metasomatized by sediment-derived melts. The latter is characterized by high Sr and low Y and Yb, with elevated Sr/Y and (La/Yb)N ratios, implying an adakitic affinity. Notably, their similar Sr-Nd isotopic compositions indicate an origin from partial melts of a newly underplated lower crust. The Maoergai granitic rocks, characterized by high Sr and low Y and Yb contents with high Sr/Y and (La/Yb)N ratios, are indicative of adakitic rocks. In combination with the enriched whole-rock Sr-Nd isotopes and the apatite Nd isotopic data, we suggest that they were generated by the partial melting of the ancient thickened mafic lower crust. The Markam and Yanggonghai felsic granitoid rocks are peraluminous and similar to typical S-type granitoids, indicating an origin from remelting of the Triassic metasedimentary rocks. Based on the temporal-spatial relationship of the late Triassic plutonic rocks in the orogenic belt, we suggest that these rocks were formed in association with the roll-back and subsequent break-off of a subducted slab of the Paleo-Tethys Ocean. During the subduction, the formation of the Maoergai adakitic rocks was triggered by slab roll-back, whereas the magmatic “flare up” (ca. 216–200 Ma) was likely caused by slab break-off. This indicates that the final closure of the Paleo-Tethys Ocean happened in the end of the Triassic or Early Jurassic.

We conducted in situ geochemical (major-, trace-element and Nd isotope compositions) analyses on apatite, together with whole-rock geochemistry from gabbro, granodiorite and granite in the Cretaceous Pingtan intrusive complex (SE China), aiming to investigate the roles of magmatic evolution and post-crystallization hydrothermal activity during its formation. Regardless of limited range in initial Nd isotopes ranges in both bulk rock [ɛNd(t) = − 2.0 to − 0.4] and associated apatite [ɛNd(t) = − 3.8 to − 0.4] from the Pingtan igneous complex, the apatite shows wide compositional and textural variations from gabbro to granite. Apatite from the gabbro (Group 1) displays a zoning structure characterized by increasing F and Sr but decreasing Cl and LREE from the core to rim. The increase of Sr from the core to rim is attributed to plagioclase accumulation, and the decreases of LREE and Cl from the core to rim is caused by post-crystallization hydrothermal activity. The high Cl content in the primitive Group 1 apatite further suggests derivation of the mafic magma from a mantle wedge metasomatized by Cl-rich sediment. In contrast, apatite from the granite (Group 2) has the lowest Cl and Sr but the highest F and Yb contents, which can be further divided into two subgroups of Group 2A and 2B based on texture and composition. Group 2A apatite shows homogenous composition with trace elements similar to apatite from I-type granite. The positive correlation between Sr and Eu/Eu* indicates that crystallization of Group 2A apatite is co-precipitated with a feldspar-dominated fractionation. However, Group 2B apatite contains mineral inclusion of monazite and has the highest U content and F/Cl ratio, resembling apatite from S-type or highly fractionated I-type granite. These features are consistent with the influence of post-crystallization hydrothermal activity. Apatite from the granodiorite (Group 3) has an intermediate composition between Group 1 and 2A apatite and shows a homogenous texture with trace element features similar to that from I-type granite. Group 3 apatite defined a negative correlation of Sr with La/Yb, which is attributed to fractional crystallization of hornblende and plagioclase. The geochemistry of apatite indicates that the gabbro and granitic rocks of the Pingtan intrusive complex were, respectively, derived from the mantle and crustal sources with similar ɛNd(t) values. Our study, therefore, demonstrates that apatite geochemistry has a potential to monitor the magma source, magmatic evolution and post-crystallization fluid activity of an igneous complex.

Neoproterozoic subducted materials in the generation of Mesozoic Luzong volcanic rocks: Evidence from apatite geochemistry and Hf–Nd isotopic decoupling

Processes controlling magma fertility at Buenavista del Cobre porphyry copper deposit (Cananea, México): A new petrogenetic model based on zircon U-Pb dating and apatite geochemistry

Diverse apatite geochemical compositions in early Paleozoic granitoids of the North Qinling orogen, China: Insights into their petrogenesis and magma sources

Multiple-pulse magmatic intrusion and fluid metasomatism in Mesozoic Qianlishan rare metal granite, South China: Records from apatite geochemistry

The Han–Xing (Handan–Xingtai) region is famous for its endowment of skarn iron deposits in China. These deposits are mainly spatially and genetically associated with diorite rocks, but these rocks show different Fe ore potential. Major and trace elements composition of apatite from the Kuangshan and Fushan diorite complexes were investigated to explore the potential of apatite as a proxy of petrogenesis and Fe fertility of these rocks. All the investigated apatite grains are identified as fluorapatite, which is typical for magmatic apatite. The Sr, Y, Mn, and Heavy Rare Earth Elements (HREE) contents of apatite in the Kuangshan diorite complex are positively correlated with the increase of melt SiO2 content compared to that in the Fushan diorite complex. Apatite geochemistry indicates that magmas of the Fushan complex mainly experienced the fractional crystallization of hornblende in the deep crustal reservoirs, whereas the Kuangshan complex has experienced the fractional crystallization of hornblende in the deep and the shallow plagioclase fractional crystallization. The F, Cl and S content of the Kuangshan complex estimated by apatite volatile (F = 2632 ppm, Cl = 4100 ppm, SO3 = 140 ppm) is significantly higher than that of the Fushan complex (F = 2488 ppm, Cl = 3400 ppm, SO3 = 90 ppm). The Eu, Ce anomalies, Mn, and SO3 contents of apatite show that both of the two complexes have higher oxygen fugacity (Δ FMQ), but the oxygen fugacity of the Kuangshan complex calculated by Mn and SO3 content (Δ FMQ + 2.41) is higher than that of the Fushan complex (Δ FMQ + 1.77), which may also be one of the reasons for the great difference in ore-forming scale between the two complexes. Our results suggest that the high volatile contents and oxidation states of magma estimated by apatite, as well as the lower Sr/Y in apatite reflect favorable conditions for skarn iron mineralization. Therefore, our study shows that magmatic apatite geochemistry may be a useful tool to distinguish the Fe fertility of plutonic rocks related to skarn deposits.

The thermal and structural history of the San Emigdio Mountain area, southern San Joaquin basin, California, was studied using inverse modeling and geohistory analysis of new fission-track (FT) data from 32 apatite samples from five wells and two outcrop traverses. Apatite composition was determined by microprobe analysis on eight of the samples in order to select appropriate kinetic parameters for modeling. Modeling of seven Eocene to Oligocene samples and a single basement sampled along the Pleito Creek traverse shows that the base of the sequence began to cool from {approximately} 115 {plus minus} 10C during the late Mohnian ({approximately} 13-7 Ma). Ten Eocene to Oligocene samples and a single basement sample along the San Emigdio Creek traverse cooled below {approximately} 85 {plus minus} 20C during latest Mohnian to early Pliocene ({approximately} 8-4 Ma). Assuming paleogeothermal gradients of {approximately} 25 to 30C/km, these data indicate that the San Emigdio Mountains experienced {approximately} 2 km of denudation from the late Mohnian to Pliocene at rates less than 0.5 mm/yr, and {approximately} 1.5 km of Pliocene to Recent denudation at rates less than 1 mm/yr. The study also shows that the area was affected by Miocene heating, probably associated with Tecuya Formation volcanismmore » (22.5 Ma). In addition, ten new zircon FT dates indicate a Sierra Nevada-type provenance from Eocene to Miocene. The apatite microprobe analyses indicate decreasing numbers of chlorine-rich grains from Eocene to Miocene, suggesting a change in the geochemistry of apatite in the source region over time.« less

Fingerprinting crustal anatexis with apatite trace element, halogen, and Sr isotope data

Evolution of alkaline magmas and enrichment of rare earth elements: Insights from the geochemistry of apatite in the Saima alkaline igneous complex, Liaodong Peninsula, China

In addition to its world-renowned W-Sn deposits, the Nanling region of South China also hosts several Cu-Pb-Zn polymetallic deposits that are genetically linked to the Mesozoic granodioritic plutons. In this study, we conducted in situ trace elemental and isotopic analyses on zircon and apatite along with whole-rock geochemical analysis to constrain the Cu-Pb-Zn ore formation. The granodiorites from the Baoshan, Tongshanling, and Shuikoushan deposits yielded zircon U-Pb ages of ca. 160 Ma, broadly coeval to the Cu-Pb-Zn mineralization. The average oxygen fugacity (fO2) for the granodiorites is ΔFMQ (logfO2 value relative to the fayalite-magnetite-quartz oxygen buffer) +0.74 (Baoshan), ΔFMQ +0.07 (Tongshanling), and ΔFMQ +1.67 (Shuikoushan), with the corresponding apatite Cl content of 0.36−0.46 wt%, 0.21−0.36 wt%, and 0.29−0.98 wt%, respectively. These fO2 and Cl values are higher than typical granite-related W-Sn mineralization (ΔFMQ −1.17, Cl = 0.01−0.48 wt%), and may have facilitated the Cu-Pb-Zn mineralization. Zircon Ce/Sm and Eu/Eu* ratios for the granodiorites, respectively, are 2.16−9.30 and 0.36−0.60 (Baoshan), 2.6−7.96 and 0.16−0.57 (Tongshanling), and 5.92−12.13 and 0.44−0.69 (Shuikoushan). This suggests that the Baoshan and Shuikoushan granodiorites may have undergone stronger amphibole fractionation than the Tongshanling granodiorite. This, in turn, implies that the granodiorites from Baoshan and Shuikoushan had higher water content than those from Tongshanling. The inferred “wetter” granodioritic magma at Baoshan and Shuikoushan may account for their larger deposit size than that of Tongshanling. Zircon grains of the granodiorites from Baoshan, Tongshanling, and Shuikoushan have εHf(t) values of −15.5 to −8.2, −16.9 to −8.2, and −10.6 to −7.4, while the apatite grains have εNd(t) values of −9.2 to −6.7, −8.8 to −6.0, and −8.7 to −4.2, respectively. The Nd-Hf isotopes are decoupled in the Baoshan and Tongshanling granodiorites, deviating from the terrestrial array. Thus, the Baoshan, Tongshanling, and Shuikoushan granodiorites were primarily derived from partial melting of the lower continental crust, while some melts from the subduction-metasomatized mantle were likely involved in the Baoshan and Tongshanling granodiorite formation. This may have led to the higher Cu/(Pb + Zn) ratio in the Baoshan and Tongshanling deposits. The formation of granodiorite-related Cu-Pb-Zn deposits in the Nanling region was likely associated with the Paleo-Pacific plate subduction and slab rollback in an extensional setting.

Abstract: Four types of apatite have been identified in the Ningwu region. The first type of apatite is widely distributed in the middle dark colored zones (i.e. iron ores) of individual deposits. The assemblage includes magnetite, apatite and actinolite (or diopside). The second type occurs within magnetite‐apatite veins in the iron ores. The third type is seen in magnetite‐apatite veins and (or) nodules in host rocks (i.e. gabbro‐diorite porphyry or gabbro‐diorite or pyroxene diorite). The fourth type occurs within apatite‐pyrite‐quartz veins filling fractures in the Xiangshan Group. Rare earth elements (REE) geochemistry of apatite of the four occurrences in porphyry iron deposits is presented. The REE distribution patterns of apatite are generally similar to those of apatites in the Kiruna‐type iron ores, nelsonites. They are enriched in light REE, with pronounced negative Eu anomalies. The similarity of REE distribution patterns in apatites from various deposits in different locations in the world indicates a common process of formation for various ore types, e.g. immiscibility. Early magmatic apatites contain 3031.48–12080 times 10−6 REE. Later hydrothermal apatite contains 1958 times 10−6 REE, indicating that the later hydrothermal ore‐forming solution contains lower REE. Although gabbro‐diorite porphyry and apatite show similar REE patterns, gabbro‐diorite porphyries have no europium anomalies or feeble positive or feeble negative europium anomalies, caused both by reduction environment of mantle source region and by fractionation and crystallization (immiscibility) under a high oxygen fugacity condition. Negative Eu anomalies of apatites were formed possibly due to acquisition of Eu2+ by earlier diopsite during ore magma cooling.The apatites in the Aoshan and Taishan iron deposits yield a narrow variation range of 87Sr/86Sr values from 0.7071 to 0.7073, similar to those of the volcanic and subvolcanic rocks, indicating that apatites were formed by liquid immiscibility and differentiation of intermediate and basic magmas.