High Magmatic Water Content Research Articles

Geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of the Late Jurassic granitoids from the Shuangyuangou deposit, South Qinling, China: Implications for petrogenesis and porphyry Cu metallogeny

The Shuangyuangou copper deposit is located in the southern segment of the Qinling orogenic belt in central China. The ore‐related intrusions include quartz diorite porphyry, granite porphyry and lamprophyre. In this study, we present whole‐rock geochemistry, zircon U–Pb geochronology, trace elements and Lu–Hf isotope data on the Shuangyuangou intrusions which provide critical constraints on the emplacement age, magma origin, petrogenesis and implications for porphyry Cu mineralization in this newly discovered deposit. Zircon U–Pb dating constrains the emplacement age of quartz diorite porphyry, lamprophyre and granite porphyry at 150–147 Ma. The quartz diorite porphyry and granite porphyry display high‐K calc–alkaline I‐type granite affiliation. The rocks show enrichment of LREE and LILE (Rb, Ba), depletion in HREE and HFSE (Nb, Ta, Ti) and absence of Eu anomaly. Zircon εHf(t) values of quartz diorite porphyry, lamprophyre and granite porphyry range from −7.5 to −1.7 with TDM2 ages of 1.3–1.6 Ga. Our data suggest that the parental magma was derived from partial melting of juvenile lower crust and metasomatic lithospheric mantle, with more involvement of mantle material for the lamprophyre. The zircon Eu/Eu* values of 0.55–0.78 suggest high magmatic water content which suppresses the fractionation of plagioclase. In addition, the ΔFMQ values calculated from zircon trace elements range from −0.90 to 2.96 which is a favourable factor for porphyry Cu mineralization at Shuangyuangou. The Late Jurassic Shuangyuangou intrusion and associated Cu mineralization were formed within an intracontinental setting after the collisional assembly of the Qinling orogenic belt.

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 >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 and metal fertility of Yinzu pluton in the Daye district, eastern China: Insights from whole-rock and mineral (zircon, apatite and amphibolite) geochemistry

The Daye district in eastern China is featured by numerous granitoid intrusions of Early Cretaceous ages that intrude Paleozoic to Early Triassic marine carbonate and clastic rocks. Several dozen skarn Cu-Fe-Au deposits occur along the intrusive contact zones of the Echeng, Tieshan, Jinshandian, Lingxiang, Tongshankou and Tonglushan intrusions. However, no economic skarn mineralization has been recognized in other intrusions and whether or not these intrusions are metal fertile remain unresolved. In this paper, we use whole-rock and mineral (zircon, apatite and amphibole) geochemistry to provide significant new insights into the petrogenesis and metal fertility of the Yinzu granitoid pluton.The Yinzu pluton mainly consists of quartz diorite and diorite phases that have zircon U-Pb ages of 148.4 ± 1 Ma and 146.6 ± 1.6 Ma, respectively. Whole-rock Sr-Nd isotopic compositions ((87Sr/86Sr)i = 0.70635 to 0.70741; εNd(t) = −8.74 to −3.73) and zircon hafnium isotopes (εHf(t) = −9.1 to −4.8) overlap with values of the Cu-fertile Tongshankou and Tonglushan granitoids, implying a common enriched lithospheric mantle source. The whole-rock geochemical data suggest that both the diorite and quartz diorite have experienced variable degrees of fractional crystallization of olivine, amphibole, titanite and magnetite. The Ce4+/Ce3+ ratios (16–445) and oxygen fugacity (logfO2) calculated using trace element concentrations of zircon (ΔFMQ = +0.2 to +2.4) suggest that both intrusive phases crystalized from oxidized melts similar to Cu-fertile magmas in the world, with the quartz diorite being well comparable to the Tongshankou and Tonglushan granitoids. Based on available melt-apatite partitioning models, the estimated S and Cl contents for the magmas parental to the Yinzu pluton are akin to those of the Cu-fertile granitoids in the Daye district. Rocks of the Yinzu pluton display high apatite Sr/Y (0.38–3.01) and Eu/Eu* (0.35–0.71) ratios as well as high zircon Eu/Eu* (0.51–0.86) and 10000*(Eu/Eu*)/Y (2–27) ratios, indicating significant fractionation of amphibole with limited plagioclase fractionation and high magmatic water contents (>4 wt% H2O). This view is consistent with the calculated melt H2O content of 4.3–6.1 wt% based on the amphibole compositions.Collectively, results presented here indicate that magmas generating the Yinzu pluton are hydrous, oxidized and S-rich, characters similar to the Cu-fertile granitoids in Daye and elsewhere. Thus, the Yinzu pluton is considered to be Cu-(Au) fertile. This consideration is partially supported by occurrences of several vein-type Cu prospects within and disseminated Au ores in Silurian clastic sedimentary rocks immediately to the southeast of this pluton. Based on the regional metallogeny and results of this study, we suggest that the Yinzu pluton is a promising target of future Cu-(Au) exploration.

Geology and factors controlling the formation of the newly discovered Beimulang porphyry Cu deposit in the western Gangdese, southern Tibet

The porphyry Cu deposits in the Gangdese metallogenic belt were mainly formed in the eastern portion. The Beimulang is a newly discovered deposit formed in post-collisional setting in the western part of the belt and adjacent to the super-large Zhunuo porphyry Cu deposit. There are three epochs of magmatism at Beimulang, including pre-mineralization quartz porphyry (PQP; 49.7 ± 0.4 Ma), inter-mineralization monzogranite porphyry (IMGP; 14.8 ± 0.2 Ma), monzogranite (IMG; 14.1 ± 0.2 Ma), and late-mineralization granite porphyry (LGP; 11.7 ± 0.1 Ma). The various degrees of alteration and sulfide occurred at the IMG(P) and LGP indicate that there are at least two episodes of fluid exsolution events associated with different intrusions spanning ∼ 3 m.y. at Beimulang, which are unlike other Gangdese porphyry Cu deposits showing single fluid exsolution around only one porphyry at Miocene (e.g., Qulong, Zhunuo, Dabu). The PQP is characterized by high SiO2 (75.6–81.4 wt%) and K2O (2.1–4.3 wt%) concentrations, high Nb/Ta (8.4–11.3) ratios, and low Mg# (22–43), Ni (1.5–2.7 ppm) contents. They have high (87Sr/86Sr)i (0.705098–0.707767) ratios and low εNd(t) (-4.9 to −1.9) values. These geochemical characteristics indicate that they were probably derived from a juvenile basaltic lower crust during the ongoing Indian-Asian collision. The IMGP and IMG show similar petrography and geochemical characteristics, such as high SiO2 (65.0–69.8 wt%), K2O (3.6–4.3 wt%), Sr (550–809 ppm) contents, high Sr/Y (79–89) ratios, and low Mg# (43–48), Th/Yb, and Nb/Yb values, combined with high (87Sr/86Sr)i (0.707254–0.708017) and low εNd(t) (-7.9 to −6.0), implying a similar source probably originated in the subduction-modified lower crust metasomatized by Neo-Tethyan oceanic slab-derived fluids. The LGP displays trends produced by fractionation of magma with a composition similar to the IMG(P), but wall rock assimilation plays a significant role during magmatic evolution. Zircons from the IMG(P) show higher Eu/Eu* (>0.3), 10000*(Eu/Eu*)/Y (>4), and (Ce/Nd)/Y (>0.04) ratios than those of the PQP, indicating the fertile magmas were more oxidized and hydrous. The high magmatic oxidation state for the IMG(P) is also supported by using the zircon (ΔFMQ = +4.1 to + 6.1), and amphibole (ΔFMQ =+2.6 to + 3.4) oxybarometers. Their high magmatic water contents are supported by evaluations using amphibole (∼3.5 wt%) and plagioclase (∼6.7 wt%) compositions, respectively. However, the LGP has higher magmatic oxygen fugacity (Eu/Eu* >0.3, ΔFMQ = +4.2 to + 7.3) caused probably by auto-oxidation of the magmas, but lower magmatic water contents (Sr/Y = 31 ± 15; V/Sc = 6 ± 4) caused presumably by previous fluid exsolution and/or fractionation of hydrous silicate minerals during magma ascent than those of the IMG(P). Therefore, the highly hydrous and oxidized magmas with juvenile material are favorable to form porphyry Cu deposits in Tibetan orogen. On the contrary, magmatic evolution can probably reduce the water content of magma chamber, which is not beneficial to porphyry Cu mineralization.

Uranium-Pb zircon ages, whole-rock and zircon mineral geochemistry as indicators for magmatic fertility and porphyry Cu-Mo-Au mineralization at the Bystrinsky and Shakhtama deposits, Eastern Transbaikalia, Russia

The Bystrinsky Cu-Au-Fe porphyry-skarn and Shakhtama Mo ± Cu porphyry deposits in Eastern Transbaikalia, Russia, are spatially associated with a series of fertile and non-fertile calc-alkaline to shoshonitic I-type granitoid suites, ranging in composition from porphyritic monzonites and granodiorites to granodiorite- and granite-porphyries. The obtained zircon U-Pb geochronological data reveal a narrow age interval of 1–3 Ma for the igneous suites within the Bystrinsky deposit (ca. 159–163 Ma), which is consistent with the short-magmatic activity at the Shakhtama deposit (ca. 164–161 Ma). Within both deposits, the fertile magmatic pulses comprise hybrid, high-silica and high- to moderate-magnesian, oxidized melts with relatively high magmatic water contents. These weakly fractionated melts represent the late-stage magmatic pulses of the Shakhtama Complex. By contrast, the barren magmatic pulses comprise a wide range of compositions, generally with lower magmatic water contents and oxidation states as well as significant plagioclase fractionation and crustal contamination. The Shakhtama Mo-related porphyries are more evolved when compared to the Bystrinsky Cu-Au intrusions reflecting significant crustal assimilation in addition to amphibole-plagioclase fractionation.Both whole-rock geochemistry and zircon mineral chemistry from the Bystrinsky and Shakhtama deposits may be used as fertility indicators in target generation and regional mineral exploration in Eastern Transbaikalia. More specifically, we propose critical thresholds of whole-rock Sr/Y and 10,000*(Eu/Eu*)/Y ratios of > 65 and > 850, respectively, and zircon mineral Eu/Eu*, Ce/Ce* and Yb/Dy ratios of > 0.4, >750 and > 4, respectively. Additionally, we propose an alternative method to calculating the Ce anomalies of zircons using MS Excel-based software which is provided in the Supplementary data of this paper.

Geochronology, geochemistry and Sr-Nd-Hf isotopes of the Heihuashan granite porphyry, NE China, and implications for Cu-Mo mineralization

Heihuashan is a newly discovered porphyry Cu-Mo prospect in Heilongjiang Province, northeastern China, with considerable Cu and Mo mineralization. Until now the ages of the Cu-Mo ores and the causative granite porphyry, the magma origin and characteristics, as well as the mineralization potential at depth are still unclear. In this contribution, whole rock geochemical and Sr-Nd isotopic compositions, zircon U-Pb ages and Hf isotopes, and molybdenite Re-Os dating, are presented to constrain the timing of the magmatism and mineralization, and the petrogenesis of the causative intrusion. The results also provide an opportunity for evaluating the potential of the deep Cu-Mo mineralization.Zircon U-Pb dating using LA-ICP-MS yields a crystallization age of 146.4 ± 1.8 Ma for the Heihuashan granite porphyry, which is consistent with the molybdenite Re-Os isochron age of 143.7 ± 3.1 Ma. These results indicate that the porphyry Cu-Mo mineralization at Heihuashan occurred at late Jurassic to early Cretaceous. The whole rockgeochemicaldatasuggest that the ore-related intrusion is mildly peraluminous and belongs to the high-Kcalc-alkalineseries, I-type granitoids. The granite porphyry samples are enriched in Rb, Ba, Th, U, and Sr, but depleted in Nb and Ta, showing typical arc magma affinities. They also have initial 87Sr/86Sr ratios of 0.70546–0.70550 and εNd(t) values between +0.4 and +0.7. The zircon εHf(t) values for the granitic intrusion range from +3.5 to +9.2, with two-stage Hf model ages of 977–614 Ma. The geochemical and Sr-Nd-Hf isotopic compositions suggest that the parental magma was dominantly derived from the slab-metasomatized mantle, which could have mixed with some lower crust components.The Fe2O3/FeO ratios of the Heihuashan granite porphyry are relatively high, ranging from 0.52 to 1.17, indicating a high magmatic oxidation state. This is also supported by the high zircon Ce/Ce*, Eu/Eu* and Ce/Nd ratios (33–2433, 0.22–0.70 and 2.4–44.0, respectively) and the high magmatic ΔFMQ values (+0.1 to +3.5 with an average of +2.1) calculated using the zircon trace elements. The intrusion has relatively high Sr/Y ratios (40–94) and displays negligible Eu anomalies (Eu/Eu* = 0.68–1.07); the zircons from the intrusion have high (Eu/Eu*)/Y (mostly > 0.0001) and (Ce/Nd)/Y ratios (mostly > 0.01), but low Dy/Yb ratios (generally < 0.4). These suggest that the fractionation of hornblende rather than plagioclase is dominant during the magma crystallization, which reveals a high magmatic water content. The above results indicate that the ore-related magma at Heihuashan is oxidized and hydrous, both of which are critical for porphyry Cu-Mo mineralization. The Heihuashan granite porphyry is geochemically similar to other Cu-related intrusions in NE China and also comparable to many porphyry Cu (-Mo) systems worldwide, implying a high Cu-Mo mineralization potential. Considering the surface and shallow levels at Heihuashan are characterized dominantly by sericite alteration, we suggest exploration and drilling focusing on the deeper levels beneath the currently mineralized domains, where major potassic alteration zones, likely associated with more extensive Cu-Mo mineralization, could be discovered.

Petrogenesis and oxygen fugacity of the Xintian complex associated with the Naozhi-Xintian porphyry–epithermal system in the Yanbian area, NE China

• The Xintian complex is associated with a telescoped porphyry-epithermal system. • Influxes of mafic melts into felsic melts result in the emplacement of this complex. • High oxidation state and magmatic water are conducive to the Cu–Au mineralization. The Naozhi polymetallic veins classified into the intermediate-sulfidation (IS) type and the 2 km northwest Xintian porphyry Cu–Mo ore occurrence in the Yanbian area, Northeast China, constitute a telescoped porphyry-epithermal system. IS veins are surrounded by Early Jurassic granodiorite stocks (189 ± 1 Ma; zircon U-Pb ) and Early Cretaceous andesite lava (128 – 130 Ma; zircon U-Pb ). They are partially covered by younger pyroclastic rocks dated at 108 ± 2 Ma (Zircon U-Pb ). The Xintian occurrence is connected in space and time with an intrusive complex including five sequential phases from gabbro diorite and granodiorite stocks through diorite porphyry dykes to granodiorite porphyry and granite porphyry stocks, with indistinguishable ages within errors (128 ± 1 Ma to 126 ± 1 Ma zircon 206 Pb/ 238 U by laser ablation inductively coupled plasma mass spectrometry). The broad compositional variations (eg., 52.24 – 69.23 wt% SiO 2 ) of the complex and the igneous mafic microgranular enclaves (MMEs) hosted by granodiorite stocks are likely responses to inputs of hot mafic melts into a shallow felsic magma chamber. Based on the trace element contents of zircons, the redox conditions during the crystallization of zircon minerals in equilibrium with the melts are evaluated by calculating the FMQ values. The early mafic phase (gabbro diorite; FMQ 4.60 ± 1.33) is more oxidized than later phases (granodiorite, diorite porphyry, granodiorite porphyry, and granite porphyry) with FMQ values of 1.70 ± 1.80 to 2.28 ± 2.36. The highly oxidizing conditions of associated magmas, as well as high magmatic water contents (>4%) because of ubiquitous biotite and amphibole in this complex, are conducive to escape of primary volatiles from shallow, fertile porphyry magmas, with consequent ore deposition in structurally optimal positions at Naozhi-Xintian.

Constraints on the Formation of the Giant Daheishan Porphyry Mo Deposit (NE China) from Whole-Rock and Accessory Mineral Geochemistry

AbstractThere are more than 80 porphyry (or skarn) Mo deposits in northeastern China with Jurassic or Cretaceous ages. These are thought to have formed mainly in a continental arc setting related to the subduction of the Paleo-Pacific oceanic plate in the Jurassic and subsequent slab rollback in the early Cretaceous. The Jurassic Daheishan porphyry Mo deposit is one of the largest Mo deposits in NE China, which contains 1·09 Mt Mo with an average Mo grade of 0·07 %. To better understand the factors that could have controlled Mo mineralization at Daheishan, and potentially in other similar porphyry Mo deposits in NE China, the geochemical and isotopic compositions of the ore-related granite porphyry and biotite granodiorite, and the magmatic accessory minerals apatite, titanite and zircon from the Daheishan intrusions, were investigated so as to evaluate the potential roles that magma oxidation states, water contents, sulfur and metal concentrations could have played in the formation of the deposit. Magmatic apatite and titanite from the causative intrusions show similar εNd(t) values from –1·1 to 1·4, corresponding to TDM2 ages ranging from 1040 to 840 Ma, which could be accounted for by a mixing model through the interaction of mantle-derived basaltic melts with the Precambrian lower crust. The Ce and Eu anomalies of the magmatic accessory minerals have been used as proxies for magma redox state, and the results suggest that the ore-forming magmas are highly oxidized, with an estimated ΔFMQ range of +1·8 to +4·1 (+2·7 on average). This is also consistent with the high whole-rock Fe2O3/FeO ratios (1·3–26·4). The Daheishan intrusions display negligible Eu anomalies (Eu/Eu* = 0·7–1·1) and have relatively high Sr/Y ratios (40–94) with adakitic signatures; they also have relatively high Sr/Y ratios in apatite and titanite. These suggest that the fractionation of amphibole rather than plagioclase is dominant during the crystallization of the ore-related magmas, which further indicates a high magmatic water content (e.g. &amp;gt;5 wt%). The magmatic sulfur concentrations were calculated using available partitioning models for apatite from granitoids, and the results (9–125 ppm) are indistinguishable from those for other mineralized, subeconomic and barren intrusions. Furthermore, Monte Carlo modelling has been conducted to simulate the magmatic processes associated with the formation of the Daheishan Mo deposit, and the result reveals that a magma volume of ∼280 km3 with ∼10 ppm Mo was required to form the Mo ores containing 1·09 Mt Mo in Daheishan. The present study suggests that a relatively large volume of parental magmas with high oxygen fugacities and high water contents is essential for the generation of a giant porphyry Mo deposit such as Daheishan, whereas a specific magma composition (e.g. with unusually high Mo and/or S concentrations) might be less critical.

RECOGNIZING PORPHYRY COPPER POTENTIAL FROM TILL ZIRCON COMPOSITION: A CASE STUDY FROM THE HIGHLAND VALLEY PORPHYRY DISTRICT, SOUTH-CENTRAL BRITISH COLUMBIA

Abstract The detrital zircons in tills overlying the Guichon Creek batholith, British Columbia, Canada, have trace element concentrations and ages similar to those of zircons from the bedrock samples from which they are interpreted to have been sourced. Rocks from the core of the batholith that host porphyry copper mineralization have distinct zircon compositions relative to the distal, barren margin. We analyzed 296 zircons separated from 12 subglacial till samples to obtain U-Pb ages and trace element compositions. Laser ablation U-Pb ages of the detrital zircons overlap within error with chemical abrasion-thermal ionization mass spectrometry U-Pb ages of the Late Triassic Guichon Creek batholith and confirm that the detrital zircons are likely derived from the batholith. The youngest intrusions of the batholith produced the Highland Valley Copper porphyry deposits and contain distinctive zircons with elevated Eu/EuN* &amp;gt;0.4 attributed to high magmatic water contents and oxidation states, indicating higher porphyry copper potential. Zircon from till samples adjacent to and 9 km down-ice from the mineralized centers have mean Eu/EuN* &amp;gt;0.4, which are indicative of potential porphyry copper mineralization. Detrital zircon grains from more distal up- and down-ice locations (10–15 km) have zircon Eu/EuN* mean values of 0.26 to 0.37, reflecting background values. We conclude that detrital zircon compositions in glacial sediments transported several kilometers can be used to establish the regional potential for porphyry copper mineralization.

Porphyry indicator zircons (PIZs): Application to exploration of porphyry copper deposits

Magmas with a high oxidation state (FMQ + 1 to + 2), a high-water content (≥4 wt% H2O) and a high degree of fractionation are necessary to form porphyry copper deposits (PCDs). Different proxies, such as Fe3+/Fe2+ ratio of whole-rock, minerals indicative of high oxygen fugacity (fO2) and water content as well as geochemical signatures of whole-rock (e.g. high Sr/Y and V/Sc ratios) are commonly used to trace these magmatic conditions. However, these proxies are of limited use for rocks hosting porphyry copper mineralization because the hydrothermal fluids associated with ore deposits obliterate the primary minerals and their initial geochemical signatures. Zircon is a common accessory mineral of igneous rocks that retains its original chemical composition from the time of crystallization, therefore provides information related to parental magmas. Several studies on PCDs worldwide have investigated the geochemical characteristics of zircons (e.g., Hf concentration, Ce4+/Ce3+, Eu/Eu*) as indicators of a high oxidation state, high magmatic water content and high degree of magma fractionation. Here, the petrographic and geochemical characteristics of zircons collected from Chuquicamata, Mirador, Esperanza and Escondida PCDs, located in the upper Eocene to lower Oligocene metallogenic belt of northern Chile, were determined in order to validate the use of zircon as a tracer for porphyry fertility. Our results show that zircon collected from all studied PCDs crystallized from monzogranitic to granodioritic magmas at temperatures below 750 °C and show euhedral morphologies mainly characterized by prismatic forms of type {110} and pyramidal forms of type {101}. Most zircons from PCDs have Hf concentrations > 8,750 (ppm) and Ce/Nd > 1, Eu/Eu* > 0.4, 10,000x(Eu/Eu*)/Y > 1, (Ce/Nd)/Y > 0.01, Dy/Yb < 0.3, Th/U 〈1−〉 0.1. From this group of geochemical values measured on zircon, we propose the Porphyry Indicator Zircon concept (PIZ), where each zircon from PCDs must comply with the previously mentioned geochemical framework. Our results show that studied PCDs have abundant zircons with PIZ characteristic (>54%); at Mirador and Esperanza porphyries the PIZ proportion exceeding 70%. On the other hand, the non-mineralized intrusive bodies have no zircons with PIZ characteristics. Therefore, these results combined with previously published studies from other porphyry belts worldwide, validate zircons as an indicator of metallogenic fertility and its use as a porphyry indicator mineral and a potential exploration tool in search for PCDs.

Multi-stage arc magma evolution recorded by apatite in volcanic rocks

AbstractProtracted magma storage in the deep crust is a key stage in the formation of evolved, hydrous arc magmas that can result in explosive volcanism and the formation of economically valuable magmatic-hydrothermal ore deposits. High magmatic water content in the deep crust results in extensive amphibole ± garnet fractionation and the suppression of plagioclase crystallization as recorded by elevated Sr/Y ratios and high Eu (high Eu/Eu*) in the melt. Here, we use a novel approach to track the petrogenesis of arc magmas using apatite trace element chemistry in volcanic formations from the Cenozoic arc of central Chile. These rocks formed in a magmatic cycle that culminated in high-Sr/Y magmatism and porphyry ore deposit formation in the Miocene. We use Sr/Y, Eu/Eu*, and Mg in apatite to track discrete stages of arc magma evolution. We apply fractional crystallization modeling to show that early-crystallizing apatite can inherit a high-Sr/Y and high-Eu/Eu* melt chemistry signature that is predetermined by amphibole-dominated fractional crystallization in the lower crust. Our modeling shows that crystallization of the in situ host-rock mineral assemblage in the shallow crust causes competition for trace elements in the melt that leads to apatite compositions diverging from bulk-magma chemistry. Understanding this decoupling behavior is important for the use of apatite as an indicator of metallogenic fertility in arcs and for interpretation of provenance in detrital studies.

Evaluation of crystallization and emplacement conditions of the McKenzie Gulch porphyry dykes using chemistry of rock-forming minerals: Implications for mineralization potential

Electron microprobe analyses of rock-forming minerals in the Middle Devonian (386.2 ± 3.1 and 386.4 ± 3.3 Ma) intermediate to felsic porphyry dykes of the McKenzie Gulch (MG) area, in northern New Brunswick, Canada are used to constrain important intensive parameters, such as oxygen fugacity (fO2), temperature, pressure, dissolved water (H2Omelt) content, and emplacement conditions. Biotite compositions plot just above the nickel-nickel oxide (NNO) buffer at fO2 values of ~−12.22, which together with the presence of magnetite indicate oxidizing conditions during crystallization. On a FeOtotal–MgO–Al2O3 discrimination diagram, biotite compositions plot in the calc-alkaline field, consistent with petrochemical observations, whereas in terms of log(XF/XOH) versus log(XMg/XFe), biotite plots in the moderately contaminated I-type field, suggesting possible interaction of these magmas with host rocks during their evolution.Pressure and temperature conditions of the magma estimated from aluminum-in-hornblende barometry and amphibole–plagioclase thermometry suggest that hornblende phenocrysts in these rocks crystallized at temperatures between ~700° and 810 °C and pressures between 4 and 6 kbar. Depth estimates based on these pressures suggest that these magmas resided at depths between ~13 and ≥19 km prior to final emplacement as dykes in the upper crust. The estimated water (H2Omelt) content of the magmas that fed the dykes indicates that they were highly hydrous (up to ~5.0 wt%), consistent with the presence of phases such as hornblende and biotite. High magmatic water content is a prerequisite for the formation of magmatic-hydrothermal ore deposits and can be used to assess the prospectivity of these slab failure magmatic suites for porphyry and skarn mineralization.

Control on the size of porphyry copper reserves in the North Balkhash–West Junggar Metallogenic Belt

The North Balkhash–West Junggar Metallogenic Belt is one of the most important areas within the Central Asian Metallogenic Domain. The belt comprises the North Balkhash and West Junggar regions, which have significantly different Cu reserves. We present mineral geochemistry, whole-rock major and trace element chemistry, zircon Hf and O isotope compositions, and zircon trace element chemistry, on the basis of which we conclude that porphyry copper deposits within the belt formed by different processes. We interpret ore-related magmas in the West Junggar region to have formed in an immature arc setting by the partial melting of lower crust (30–40 km), with some mantle input and sediments contamination. In contrast, ore-related magmas within the North Balkhash region are interpreted to have formed in a mature arc setting by the partial melting of lower crust (~40 km), with mantle input and possible upper-crustal contamination. The oxygen fugacities of ore-related intrusions within the North Balkhash region are typically higher than those within the West Junggar region. A high magmatic water content is considered to be important, but not sufficient, for the formation of large porphyry copper deposits. Thicker crust and high oxygen fugacity are the two main reasons for the higher Cu reserves in the North Balkhash region compared with the West Junggar region.

Extensive young silicic volcanism produces large deep submarine lava flows in the NE Lau Basin

New field observations reveal that extensive (up to ~ 402 km2) aphyric, glassy dacite lavas were erupted at multiple sites in the recent past in the NE Lau basin, located about 200 km southwest of Samoa. This discovery of volumetrically significant and widespread submarine dacite lava flows extends the domain for siliceous effusive volcanism into the deep seafloor. Although several lava flow fields were discovered on the flank of a large silicic seamount, Niuatahi, two of the largest lava fields and several smaller ones (“northern lava flow fields”) were found well north of the seamount. The most distal portion of the northernmost of these fields is 60 km north of the center of Niuatahi caldera. We estimate that lava flow lengths from probable eruptive vents to the distal ends of flows range from a few km to more than 10 km. Camera tows on the shallower, near-vent areas show complex lava morphology that includes anastomosing tube-like pillow flows and ropey surfaces, endogenous domes and/or ridges, some with “crease-like” extrusion ridges, and inflated lobes with extrusion structures. A 2 × 1.5 km, 30-m deep depression could be an eruption center for one of the lava flow fields. The Lau lava flow fields appear to have erupted at presumptive high effusion rates and possibly reduced viscosity induced by presumptive high magmatic water content and/or a high eruption temperature, consistent with both erupted composition (~ 66% SiO2) and glassy low crystallinity groundmass textures. The large areal extent (236 km2) and relatively small range of compositional variation (σ = 0.60 for wt% Si02%) within the northern lava flow fields imply the existence of large, eruptible batches of differentiated melt in the upper mantle or lower crust of the NE Lau basin. At this site, the volcanism could be controlled by deep crustal fractures caused by the long-term extension in this rear-arc region. Submarine dacite flows exhibiting similar morphology have been described in ancient sequences from the Archaean through the Miocene and in small batches on present-day seafloor spreading centers. This study shows that extensive siliceous lavas can erupt on the modern seafloor under the right conditions.

Zircon U–Pb and Hf–O isotopes trace the architecture of polymetallic deposits: A case study of the Jurassic ore-forming porphyries in the Qin–Hang metallogenic belt, China

The Qin–Hang intra-continental porphyry–skarn Cu polymetallic belt (QHMB) is among the economically important metallogenic belts in South China. The significant differences in the size and metal assemblage of the Jurassic magmatic–hydrothermal ore deposits in this belt remain as an enigma. Here we employ zircon U–Pb and Hf–O isotopes of the Tongshanling and Baoshan Cu–Pb–Zn deposits in the central part of the QHMB to investigate the contrasting metallogenic architecture. Our SIMS zircon U–Pb data indicate that the Tongshanling and Baoshan granodiorite formed at ~160Ma. These rocks show high Mg# values, and negative zircon εHf(t) and high δ18O values suggesting that the magmas of the granodiorite porphyries were mainly generated through the anatexis of older crustal components triggered by the input of mantle-derived magma. The minor content of amphibole phenocrysts, low Sr/Y ratios, negative Eu anomaly, and low zircon Ce4+/Ce3+ ratios indicate that the porphyries are relatively less oxidized with less water content compared with the ore-bearing porphyries in the Dexing and Yuanzhuding porphyry Cu deposits in the northern and southern part of the QHMB, suggesting that high magmatic water content and oxidation state are important prerequisites for the formation of large size porphyry–skarn copper deposits in the QHMB. The positive correlation between zircon εHf(t) values with the Cu reserves, as well as zircon δ18O values with the Cu/(Cu+Pb+Zn) ratios of the deposits indicate that the magmatic sources exerted a first-order control on the volume and metal assemblage of deposits in the QHMB. The Hf and Nd isotope contour maps indicate that the central part of the QHMB has high potential for Pb–Zn-dominated magmatic–hydrothermal deposits, whereas the northern and southern part of the QHMB are prospective for large Cu deposits. Our results have important implications in formulating regional exploration strategies for Jurassic porphyry–skarn Cu–Pb–Zn deposits in the Qin–Hang belt.

PARAGENESIS AND RE-OS MOLYBDENITE AGE OF THE CAMBRIAN AK-SUG PORPHYRY CU-AU-MO DEPOSIT, TYVA REPUBLIC, RUSSIAN FEDERATION

The Ak-Sug porphyry Cu-Au-Mo deposit is situated within the Altai Sayan region that forms part of the Tuva-Mongol arc of the Central Asian orogenic belt. The Ak-Sug intrusive complex is made up of a series of nested intrusions, including diorite, feldspar porphyry, quartz-feldspar porphyry, tonalite I, and tonalite II. Cu-Au-Mo mineralization occurs mainly within quartz-carbonate ± K-feldspar sulfide veins and later sericite-sulfide veins and alteration. In the northern zone, sulfide zoning consists of a central bornite core flanked by chalcopyrite and pyrite-chalcopyrite zones, with a pyrite zone on the hanging-wall side. Re-Os molybdenite dating of an early-stage quartz-carbonate ± K-feldspar sulfide vein and a late-stage molybdenite-only vein gives ages of 517.3 ± 3 and 517.4 ± 3 Ma, respectively. This indicates an Early Cambrian age for mineralization at Ak-Sug, making it one of the oldest porphyry copper deposits in the Central Asian orogenic belt. Published whole-rock geochemical data indicate that intrusive rocks at Ak-Sug and other Cambrian to Early Ordovician intrusive complexes in the Tuva-Mongol arc evolved by hornblende ± titanite fractionation due to high magmatic water contents, leading to adakite-like compositions with high Sr/Y and flat to listric-shaped rare earth element patterns that characterize many fertile arc magmas.

Origin of the Eocene porphyries and mafic microgranular enclaves from the Beiya porphyry Au polymetallic deposit, western Yunnan, China: Implications for magma mixing/mingling and mineralization

The origin of magmas with high H2O, S and metals that are linked to porphyry economic mineralization in continental collisional belts is controversial. In this paper, we studied the host porphyries and associated microgranular enclaves (MMEs) including gabbroic enclaves and dioritic enclaves from the large Beiya porphyry Au deposit in Jinshajiang-Ailaoshan mineralization belt, which provide first hand evidence of crust–mantle hybridization in the formation of fertile magma for porphyry deposit. Zircon LA-ICPMS U–Pb ages indicate coeval formation for the MMEs and the felsic porphyries in Eocene time (~36Ma). The MMEs show spheroidal shapes, acicular apatites, and oscillatory zoning with repeated resorption surfaces in plagioclases, which suggest that the MMEs are globules of a more mafic magma that was injected into and mingled with the host felsic magma. One gabbroic enclave sample exhibits high MgO (5.6wt.%), high TiO2 (1.7wt.%), enriched in large-ion lithophile element (LILE) and depleted in high field strength element (HFSE), with negative Nb, Ta and Ti anomalies. This indicates that the gabbroic enclave is similar to those of the coeval potassic mafic rocks in the Western Yunnan, which originated from the enriched mantle metasomatized by the slab-derived fluids. The dioritic enclave exhibit both potassic and adakitic-like features, and are characterized by high contents of MgO (4.4–4.9wt.%), K2O (6.4–7.4wt.%), and compatible trace elements (e.g. Ni: 115–142ppm; Cr: 214–291ppm), as well as by high Sr/Y ratios. The dioritic enclave samples yield εHf (t) values (−1.9 to +3.6) similar to those of the host porphyries. Petrographic, elemental, and isotopic evidence suggests that the Beiya dioritic enclave were formed by mixing between potassic mafic and adakitc-like melts, derived from metasomatized lithospheric mantle and lower crust, respectively. In contrast, the Beiya host porphyries are characterized by high SiO2 (66.9–71.9wt.%) and Al2O3 (13.8–15.9wt.%) contents, high Sr/Y ratios (35–79), low compatible element contents (e.g. Ni=0.9–7.2ppm; Cr =1.0–8.2ppm), low Mg# values (0.19–0.35), positive LILE anomalies, marked negative high HFSE anomalies, negative εNd(t) values (−6 to −2), and high (87Sr/86Sr)i values (0.7071–0.7081). These features indicate that the host porphyries were mainly formed by partial melting of subduction-modified lower crust with a certain assimilation of mafic melts represented by MMEs. MMEs hosted in the Beiya porphyries, formed from a hydrous, mafic magma although they formed in a non-arc setting. High magmatic water content is probably related to an enriched mantle metasomatized by fluids from a Proterozoic subducted oceanic slab. High Cu (341–626ppm) and Au contents of the MMEs suggest they crystallized from a metal-rich mafic melt. Complex assemblages of pyrite–chalcopyrite–apatite–rutile with hornblende are observed in the least altered MME. We suggest that the mafic melt probably supplied part of water, and metal to the Beiya porphyry Au system.

Relationships between arc maturity and Cu–Mo–Au porphyry and related epithermal mineralization at the Cenozoic Arasbaran magmatic belt

The actual study deals with the temporal and geochemical evolution of the arc magmatism and the Cu–Mo–Au mineralization in the Neo-Tethyan belt of the Arasbaran region in northwestern Iran. The magmatic activity in this section of the Neo-Tethyan belt periodically occurred from Upper Cretaceous to Quaternary. The Cretaceous magmatic rocks are calc-alkaline to tholeiitic in nature without any mineralization in the Arasbaran region. Cenozoic magmatism begins with alkaline (shoshonitic) volcanism at an early stage and is followed by alkaline to calc-alkaline plutonism. No significant mineralization is associated with the early stage alkaline magmatism and the most porphyry and epithermal mineralizations are related to the late stage adakite-like, calc-alkaline magmatism. The subsequent, post-collisional alkaline magmatism follows in the Plio-Quaternary. Magmatism and the related mineralization decrease in age from the north to the south, and the mineralization changes in time and space from a Cu–Mo to a Cu–Au association.Mineralization stages at the Arasbaran region consist of (a) Upper Oligocene to Lower Miocene (~20Ma) Cu–Mo porphyry and intrusion mineralization, and (b) Upper Miocene (~10Ma), Cu–Au porphyry and epithermal Cu–Au mineralization. Although the exact timing of the porphyry mineralization relative to the final collision is unclear, the presence of most porphyry and epithermal mineralizations in the Upper Oligocene to Upper Miocene, post-collisional, adakite-like intrusions, is related to crustal thickening and delamination of lower crust suggesting that they formed at a late stage of the Arasbaran magmatic arc evolution following a major volcanic stage in the Eocene. Mineralogical and geochemical characteristics of the mineralized porphyry bodies in Arasbaran region indicate that significant amounts of hornblende was fractionated prior to plagioclase crystallization as a consequence of high magmatic water contents. In contrast, the earlier, barren plagioclase-phyric magmas were relatively dry. Fertile plutonic suites in Arasbaran region may be recognized by their relatively late timing with respect to the voluminous Eocene volcanism. Widespread presence of hornblende phenocrysts, and major and trace element compositions with relatively high Sr/Y and La/Yb ratios, high Sr (>400ppm) and Nd (≥20ppm) indicate a long-lived mature arc setting.

High Sr/Y Magmas Reflect Arc Maturity, High Magmatic Water Content, and Porphyry Cu ± Mo ± Au Potential: Examples from the Tethyan Arcs of Central and Eastern Iran and Western Pakistan

The temporal and geochemical evolution of arc magmatism that culminated in porphyry Cu ± Mo ± Au deposit formation has been studied in three separate Neo-Tethyan arc systems in central and eastern Iran, and western Pakistan. Porphyry Cu-Au deposits in the Lut block of eastern Iran formed in the middle Eocene at the end of a period of extensive Paleocene-Eocene volcanism; porphyry Cu-Mo deposits in the Kerman belt of central Iran formed in the middle Miocene at the end of a period of voluminous Eocene-Oligocene volcanism; and porphyry Cu-Au deposits in the Chagai belt of western Pakistan formed in four pulses during the Eocene, early Miocene, middle-late Miocene, and late Miocene-Pliocene, after a prolonged period of arc magmatism that began in the Late Cretaceous (and is still active). In each region, the late porphyry-related magmas are more geochemically evolved and more hydrous (as evidenced by the presence of hornblende phenocrysts) than the preceding volcanic rocks. We suggest that this reflects maturation of the arc magmatic system over a period of tens of millions of years, leading to the generation of more evolved, volatile-rich magmas at later stages of the arc’s history. High magmatic water contents are a prerequisite for the shallow crustal emplacement of arc magmas and the subsequent generation of potentially ore-forming subvolcanic magmatic-hydrothermal systems. It is thus suggested that the fertility of arc magmas within a given arc terrane can be assessed by observing the relative timing of plutonic suites (later suites are more prospective), noting the common presence of hornblende or biotite phenocrysts (indicating high magmatic water contents), and through lithogeochemical fingerprinting of magmatic fractionation processes (relatively high Sr/Y and La/Yb ratios, and Eun/Eu* ratios ≈1, indicating abundant early hornblende fractionation and suppression of plagioclase crystallization in hydrous magmas).

HIGH Sr/Y ARC MAGMAS AND PORPHYRY Cu Mo Au DEPOSITS: JUST ADD WATER

Porphyry Cu ± Mo ± Au deposits are commonly associated with intermediate composition arc-related igneous rocks with high Sr/Y (and La/Yb) ratios, and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism (“adakites”). However, similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge-derived magmas by fractionation of amphibole and/or garnet, and/or by interaction with crustal materials during ascent through the upper plate lithosphere. While there is widespread evidence for such fractionation and contamination processes in porphyry magmas, there is little direct evidence for an origin by slab melting. The enhanced fertility of arc magmas relates primarily to their high water content, which is required for the formation of magmatic-hydrothermal systems upon shallow crustal emplacement. Magmatic water contents >4 wt percent H 2 O cause abundant amphibole (± garnet) fractionation but suppression of plagioclase crystallization at deep crustal levels, resulting in increasing Sr/Y and La/Yb ratios with differentiation, commonly into “adakitic” ranges, but not reflecting slab melting. Additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content, but these are secondary to the requirement for sufficient water. Thus, arc magmatic suites with high Sr/Y ratios and evidence for the presence of hydrous phenocryst phases (hornblende and/or biotite) are indeed prospective for porphyry Cu ± Mo ± Au deposits, but only because these parameters indicate high magmatic water content.