Porphyry Magma Research Articles

Fluid inclusion and isotope evidence for magmatic-hydrothermal fluid evolution in the Tuwu porphyry copper deposit, Xinjiang, NW China

The Tuwu deposit (557 Mt at 0.58% Cu) is the most typical and economically significant one of several porphyry copper deposits in the Dananhu-Tousuquan Arc Belt near the Turpan-Hami Basin, northwest China. The mineralization occurs as chalcopyrite and minor bornite in quartz-bearing veins and host porphyries (i.e. diorite porphyry and tonalite porphyry), and is closely associated with potassic (e.g. K-feldspar, magnetite, and hematite) and phyllic (e.g. sericite, pyrite, albite and chlorite) alterations. Fluid inclusions were studied in different generations of quartz, defined as QzIA+IB, QzII, QzIII, and QzIV. Four types of fluid inclusions were recognized, namely liquid-rich biphase (L0), vapor-rich biphase (V0) and triphase (V1, with one opaque mineral), as well as brine-rich (B0; with or without an opaque mineral) inclusions. Based on fluid inclusion microthermometry using FIA method, the fluids associated with QzIB (early ore stage) are characterized by relatively high temperatures (306–458 °C) and salinities (50.3–62.1 and 6.5–22.6 wt% NaCl + CaCl2 equiv. for B0 and L0 inclusions, respectively), under lithostatic pressure of 167 to 444 bars (depth of 0.6–1.6 km), and the fluids associated with QzII and QzIII (main ore stage) have progressively decreasing temperatures from 201° to 345 °C, under hydrostatic pressure of about 90 bars (depth of 0.9 km), which induced to large-scale mineralization at Tuwu. Mineralogy and physicochemical calculations suggest that the early stage occurred at alkaline and high-oxidized fluid environment (pH = 9.6 to 11.5; logfO2 = −26.4 to − 24.3), whereas the main ore stage took place at acidic and low-oxidized conditions (pH = 4.0 to 5.0; logfO2 = −30.8). The fluids associated with QzIV (late ore stage) show low homogenization temperatures (<204 °C) and salinities (0.6–4.2 wt% NaCl + CaCl2 equiv.), implying addition of meteoric water. The δ18O values of quartz and chlorite separates range from 7.7 to 12.1‰ (n = 21), with the calculated δ18Ofluid values of − 5.4 to 9.0‰, and the δD values of fluids range from −99 to −45‰. All these observations combined with previous geochronological and geochemical results reveal that the Tuwu deposit was formed by magmatic-hydrothermal fluids related to the Late Carboniferous felsic magmatism (i.e. tonalite porphyry magma) in a subduction setting. The Cu concentration in magma source is probably the key factor controlling the anomalous Cu mineralization at Tuwu, and the oxygen fugacity and temperature decreasing of the fluids resulted in abundant precipitation of Cu-riched sulfides.

Genesis of the Dachang Sn-polymetallic and Baoshan Cu ore deposits, and formation of a Cretaceous Sn-Cu ore belt from southwest China to western Myanmar

Large scale Cretaceous magmatism and SnCu mineralization occurred in an E-W-trending ore belt from southwest China to western Myanmar. We investigated the genesis of the world-class Dachang Sn-polymetallic and newly discovered Baoshan Cu deposit in Guangxi Province, southwest China, and the tectonic setting of this belt. The Dachang ore field contains a giant vein ore body (No. 100) and stratiform ore bodies (No. 91 and 92). The cassiterite U-Pb age (92.5 ± 4.4 Ma) of the No. 100 ore body is similar to the zircon U-Pb age of the Longxianggai biotite granite (90.8 ± 1.0 Ma), suggesting the vein ore body was related to Cretaceous magmatism. The fluid inclusions in the No. 92 ore body are similar to those in the No. 100 ore body, and contain CO2-H2O, liquid-rich and halite-bearing fluid inclusions, with homogenization temperatures of 300–410 °C. These data further suggest that mineralization was related to Cretaceous magmatism. The mineralization in the Baoshan Cu deposit occurs as veins in the Baoshan porphyry and adjacent country rocks. The Baoshan porphyry yielded a zircon U-Pb age of 90.6 ± 1.3 Ma, EuN/Eu* = 0.05, CeN/Ce* = 12.2 and εHf(t) = −11.39 to −7.37. These data indicate that the porphyry magma was reduced and crustally derived. Low Sr/Y ratios (<0.1) and large negative Eu anomalies (EuN/Eu* = 0.03–0.06), along with rare earth element tetrad effects further imply the porphyry magma was highly fractionated. The Baoshan porphyry was genetically associated with Cu mineralization, but is distinct from the typical Cu-mineralizing porphyry magmas.This Cretaceous Sn-Cu ore belt was likely related to subduction and roll-back of the Neo-Tethys Plate rather than northwestward subduction of the Paleo-Pacific Plate.

Origin and evolution of ore-forming fluid for the Gaosongshan gold deposit, Lesser Xing'an Range: Evidence from fluid inclusions, H-O-S-Pb isotopes

Epithermal gold deposits are typical precious metal deposits related to volcanic and subvolcanic magmatism. Due to the lack of direct geological and petrographic evidences, the origin of the ore-forming fluid is deduced from the spatial diagenesis-mineralization relationship, chronological data, physicochemical characteristics of mineral fluid inclusions, mineral or rock elements and isotopic geochemical characteristics. By objectively examining this scientific problem via a geological field survey and petrographic analysis of the Gaosongshan epithermal gold deposit, we recently discovered and verified the following points: (1) Pyrite-bearing spherical quartz aggregates (PSQA) occur in the rhyolitic porphyry; (2) the mineralization is structurally dominated by WNW- and ENE-trending systems and occurs mostly in hydrothermal breccias and pyrite-quartz veins, and the ore types are mainly hematite-crusted quartz, hydrothermal breccia, massive pyrite-quartz, etc.; (3) the alteration types consist of prevalent silicification, sericitization, propylitization and carbonation, with local adularization and illitization. The ore minerals are mainly pyrite, primary hematite, native gold, and electrum, with lesser amounts of chalcopyrite, magnetite, sphalerite, and galena, indicating a characteristic epithermal low-sulfidation deposit. The ore-forming fluid may have been primarily derived from magmatic fluid exsolved from a crystallizing rhyolitic porphyry magma. Further zircon U-Pb geochronology, fluid inclusion, physicochemical and isotopic geochemical analyses revealed that (1) rhyolitic porphyry magmatism occurred at 104.6 ± 1.0 Ma, whereas the crystallization of the PSQA occurred at 100.8 ± 2.1 Ma; (2) the hydrothermal fluid of the pre-ore stage was an exsolved CO2-bearing H2O-NaCl magmatic fluid that produced inclusions mainly composed of pure vapor (PV), vapor-rich (WV) and liquid-rich (WL) inclusions with a small number of melt- (M) and solid-bearing (S) inclusions; mineralization-stage quartz contains WL and rare PV, WV and pure liquid (PL) inclusions characterized by the H2O-NaCl system with low formation temperatures and low salinities; (3) the characteristics of hydrogen, oxygen, sulfur, and lead isotopes and those of rare earth elements (REEs) provide insight into the affinity between PSQA and orebodies resulting from juvenile crust or enriched mantle. Combined with previous research on the mineralogenetic epoch (99.32 ± 0.01 Ma), we further confirm that the mineralization of the deposit occurred in the late Early Cretaceous, which coincides with the extension of the continental margin induced by subduction of the Pacific Plate beneath the Eurasian Plate. The formation of the ore deposit was proceeded by a series of magmatic and hydrothermal events, including melting of enriched juvenile crust, upwelling, the eruption and emplacement of the rhyolitic magma, the exsolution and accumulation of magmatic hydrothermal fluid, decompression, the cooling and immiscibility/boiling of the fluid, and mixing of the magmatic fluid with meteoric water, in association with water-rock interaction.

Redox-controlled generation of the giant porphyry Cu–Au deposit at Pulang, southwest China

Some porphyry Cu–Au deposits with relatively reduced ore assemblages, characterized by high hydrothermal pyrrhotite contents and a lack of primary hematite and magnetite, are generally considered to be associated with reduced I-type granitoids. However, the role of magmatic oxygen fugacity (fO2) in controlling Cu–Au mineralization in such reduced porphyry deposits is poorly understood. The giant Late Triassic (ca 216 Ma) Pulang porphyry Cu–Au deposit of southwest China shows typical reduced ore assemblages. This study reported the systematical variation of upper crustal magmatic fO2 of Pulang deposit, based on detailed investigations of mineral crystallization sequences and compositional features of the mineralization-related porphyries (early P1 and late P2 porphyry). Results indicate that magma of the mineralization-related porphyries experienced complex fO2 fluctuations during its upper crustal evolution. The early primary magma had very high initial fO2, with ΔFMQ ≥ + 3.0 at depths of > 12 km [ΔFMQ is the deviation of logfO2 from the fayalite–magnetite–quartz (FMQ) buffer]. The fO2 of evolved parental magma subsequently decreased, with ΔFMQ ≤ + 1.9, due to injection of relatively reduced dioritic magmas (ΔFMQ = + 1.4 to + 2.3) from a deeper chamber (17–21 km depth) into the primary magma chamber at 10–12 km depth. Magma mixing had largely ceased at 6–10 km depth. The parental magma then ponded within the reduced Tumugou formation at a depth of ~ 3.7 km where magmatic fO2 decreased to a moderately oxidized state (ΔFMQ = ~ + 1.6), and finally to a moderately reduced state [reflected by log(Fe2O3/FeO) ratios of − 0.5 for P2 porphyry]. Results of this study of magmatic fO2 indicate that porphyry magmas associated with reduced Pulang ore assemblages were initially generated as highly oxidized magma which was subsequently reduced through magma mixing and contamination by reduced sedimentary rocks of the Tumugou Formation. The sharp fO2 decrease at very shallow depth prevented the early loss of Cu and Au because the magma remained oxidized until it was emplaced at ~ 3.7 km depth. Moderately reduced magmas may thus have a genetic association with porphyry Cu–Au mineralization.

Insight into zircon REE oxy-barometers: A lattice strain model perspective

Zircon rare earth element (REE) oxy-barometers have been widely used to calculate magma oxygen fugacity. However, sometimes confusing results are obtained, such as extremely reduced oxygen fugacity in Hadean magmas (e.g. ΔFMQ−8), extremely oxidized oxygen fugacity in post-Hadean magmas (e.g. ΔFMQ+10) and contradictory oxygen fugacities recorded in ore-related porphyry magmas (ΔNNO+5 vs ΔNNO+1). To address these issues, this work reviews the underlying assumption of zircon REE oxy-barometers, the lattice strain model. By introducing a geochemical filter to identify clean, inclusion-free zircon and a new parameter δK to constrain the Dzircon/wholerock deviation from the lattice strain model, we systematically re-evaluate the common zircon REE oxy-barometers, address the issues noted in Hadean, post-Hadean and ore-related magmas, and provide an alternate approach to using zircon REE in studies of magmatic processes.

Mineralogy of Bi-sulfosalts and tellurides from the Yaoan gold deposit, southwest China: Metallogenic implications

Bismuth-sulfosalts and -tellurides are commonly present in many hydrothermal gold deposits and can provide important genetic and physicochemical constraints on the metallogenesis. The abundant auriferous specular hematite veins in the Yaoan gold deposit of the Jinshajiang-Red River alkaline porphyry metallogenic belt (southwest China) contain an assemblage of Bi-sulfosalts and tellurides together with hematite, pyrite, and chalcopyrite. Bismuth-sulfosalts are characterized by replacement textures resulting from changes of fluid composition (e.g., Pb, Bi, Cu, and Ag) and mainly include bismuthinite derivatives (e.g., bismuthinite, paarite, krupkaite, lindströmite, and friedrichite-aikinite), paděraite, cuprobismutite homologues (probably submicroscopic intergrowths of hodrushite and cuprobismutite), and emplectite. Tellurides are tiny, comprising tsumoite, melonite, hessite, and calaverite, which coexist with bismuthinite derivatives. These Bi-sulfosalts and tellurides have direct contact with gold, indicating that the Yaoan gold metallogenesis was likely linked to the syenite porphyry magmatism that was originally sourced from a regional lithospheric mantle probably metasomatized by subduction of Te-rich sediments. The telluride assemblage also reveals that the gold was precipitated under oxidizing conditions that has a high tellurium fugacity at the beginning (fTe2 = ∼10−7.9 and fS2 = ∼10−8.5 at ∼300 °C).

Phenocryst Zonation in Porphyry-Related Rocks of the Baguio District, Philippines: Evidence for Magmatic and Metallogenic Processes

Mantle-derived mafic magmas may be the source of ore-forming metals in large Cu porphyry systems, but evidence of primary petrogenetic and metallogenetic processes can be masked by hydrothermal alteration. The Baguio district of Northern Luzon, Philippines is a world-class mineral province containing approximately 40 Moz of gold and 5 Mt of copper, distributed across many porphyry Cu and epithermal Au deposit systems which are spatially related to Pliocene pre-ore Liw-Liw Creek (LCC) basaltic dikes (3·59–4·73 Ma) and a syn-ore plagioclase–hornblende diorite porphyry (2·81–2·98 Ma). A range of phenocryst phases (plagioclase, amphibole and clinopyroxene) are well preserved in both the basaltic dike suite and diorite porphyry suite, and provide an excellent natural laboratory to investigate magmatic processes and the relationship between metallogenesis and mafic magma. Widespread normally zoned clinopyroxene and amphibole grains in the basaltic dike suite show sharp decreases in Mg#, but increasing Mn, Sr, Cu, Y, Zr and REE from core to rim. Mixing of evolved felsic magma into the mafic magma is the most likely cause of these trends as fractional crystallization would typically result in gradual variations. In the diorite porphyry suite, reversely zoned amphibole is characterised by sharp increases in Mg#, Cu and Ni, but decreasing Mn, while oscillatory zoned plagioclase with distinct regions of patchy zonation shows repeated variations of An, Mg, Fe and Sr from core to rim. These signatures indicate the addition of basaltic magma to the diorite porphyry magma. The modeled Mg# of the melts (estimated assuming mineral-melt equilibrium) is also consistent with magma mixing. For example, the high Mg# of 66–72 and low Mg# of 38–44 estimated from the LLC clinopyroxene cores and rims, respectively, support the presence of basaltic magma, mixed with evolved felsic magma. All normally zoned clinopyroxene and amphibole in the basaltic dike suite and reversely zoned amphibole in the diorite porphyry suite show consistent increases in Cu from core to rim, suggesting relative enrichment of Cu in both types of injected magma. The pre-ore basaltic and ore-forming dioritic hybrid melts are estimated to have contained 45–96 ppm and 319–351 ppm Cu, respectively, based on Cu partition coefficients and zone area percentage. The Cu content of the basaltic hybrid melt is typical of arc magmas (Cu 50–100 ppm), whereas the Cu in the dioritic melt was 3–6 times higher than typical arcs magma. Both the clinopyroxene-melt thermobarometer and Al-in-hornblende geobarometer indicate similar crystallization pressures for both suites (8·6∼8·8, 4·4∼4·7, 2·1∼2·8 kbar), suggesting that both pre-ore basaltic and syn-ore dioritic suites formed in different magma chambers at similar depths. The addition of Cu enriched mafic magma may contribute ore-forming elements to the mineralising magmas and significantly increase the mineralization potential of coeval felsic rocks. Studies of phenocrysts have the potential to elucidate the role of magmatic process in the formation of porphyry systems and allow for the recognization of the key characteristics of fertile magmatic systems.

Origin of dioritic magma and its contribution to porphyry Cu–Au mineralization at Pulang in the Yidun arc, eastern Tibet

The giant Pulang porphyry Cu–Au deposit in the Yidun arc, eastern Tibet, formed due to westward subduction of the Garze–Litang oceanic plate in the Late Triassic. The deposit is hosted in an intrusive complex comprising primarily coarse-grained quartz diorite and cored quartz monzonite. Here, we investigate a suite of simultaneous (216.6 ± 1.9 Ma) diorite porphyries within the complex. The diorite porphyries are geochemically similar to mafic magmatic enclaves (MME) hosted in coarse-grained quartz diorite, and both are characterized by low SiO2 (59.4–63.0 wt%) and high total alkali (Na2O + K2O = 7.0–9.2 wt%), K2O (3.5–6.4 wt%), MgO (3.2–5.5 wt%), and compatible trace element (e.g., Cr = 72–149 ppm) concentrations. They are enriched in large-ion lithophile and light rare earth elements (LILE and LREE, respectively), but depleted in high field-strength and heavy rare earth elements (HFSE and HREE, respectively), and yield variably high (La/Yb)N ratios (17–126, average 65) with weak to negligible Eu anomalies. Furthermore, they yield low (87Sr/86Sr)i ratios (0.7054–0.7067), weakly negative εNd(t) (−2.8 to −0.8) values, and variable zircon εHf(t) (−5.4 to +0.8) and δ18O (6.0‰–6.7‰) values. These geochemical features indicate that the diorite porphyry and MME formed through crustal assimilation of a magma produced during low-degree partial melting of metasomatized phlogopite-rich subcontinental lithospheric mantle. In contrast, the coarse-grained quartz diorite and quartz monzonite have relatively high concentrations of SiO2 (61.1–65.3 wt%), K2O (4.1–5.4 wt%), and total alkali (Na2O + K2O = 7.1–8.1 wt%), and low concentrations of MgO (generally <3.0 wt%) and compatible trace elements (e.g., Cr = 38–61 ppm). They yield high Sr/Y ratios (50–63) that indicate an adakitic affinity, and are enriched in LILE, depleted in HFSE, and yield lower (La/Yb)N values (13–20, average 17) than the diorite porphyry and MME. They yield low (87Sr/86Sr)i ratios (0.7046–0.7066), negative εNd(t) (−3.3 to −1.7) values, and zircon εHf(t) and δ18O values of −2.9 to −0.1 and 5.7‰–6.5‰, respectively, suggesting that they represent high-K calc-alkaline to shoshonitic adakitic magmas that were sourced from subduction-modified juvenile lower crust. Observations of the newly identified diorite porphyry and previously reported MME suggest that input of such dioritic magma into the upper crustal porphyry magma chamber would have contributed not only the necessary metals (e.g., Cu and Au), sulfur, but also H2O to the system, thus aiding in the generation of the giant Pulang porphyry Cu–Au deposit.

Cu-rich porphyry magmas produced by fractional crystallization of oxidized fertile basaltic magmas (Sangnan, East Junggar, PR China)

The concentration of Cu is crucial to the formation of porphyry Cu deposits. However, key petrologic evidence for the origin of Cu-rich magmas is scarce. Here, we describe newly discovered Cu-rich mafic porphyritic enclaves (CMPEs) with accumulated phenocrysts, porphyritic textures and disseminated sulfide minerals in a Carboniferous tonalite pluton in the Sangnan area, East Junggar. These CMPEs have diagnostic petrologic, geochronologic and geochemical signatures that indicate they are intruded globules of Cu-rich mafic magma. These signatures include microgranular elongated laths of zoned plagioclase, needle-shaped zircon, poikilitic K-feldspar, skeletal intergrowths of chalcopyrite and epidote, sharp contacts and different geochemical compositions than the host tonalite. They also display slight younger zircon U-Pb age but similar hornblende formation depths with their host tonalites. The host tonalites are calc-alkaline and adakitic, whereas the CMPEs are high-K calc-alkaline and dioritic and have the geochemical signature of normal arc rocks.The CMPEs contain groundmass magnetite in the carapace and Cu-Fe sulfide minerals in the core. Most Cu-Fe sulfide minerals are interstitial. The interstitial pyrite and chalcopyrite display late magmatic-hydrothermal textures, including intergrowths of chalcopyrite with magmatic oligoclase, K-feldspar and quartz, poikilitic chalcopyrite, skeletal and myrmekite-like intergrowths of chalcopyrite and epidote and sulfide-bearing miarolitic cavities. The interstitial sulfide minerals formed closely following the crystallization of the groundmass K-feldspar and quartz, and contemporary with albite and epidote. The CMPE magmas may be related to the formation of porphyry Cu deposits. The CMPE magmas are likely to have been produced by fractional crystallization of oxidized fertile basaltic magmas in a subcrustal chamber below the host tonalite chamber. The oxidized basaltic magmas were derived from metasomatised mantle wedges. The adakitic tonalite magmas are probably formed by the partial melting of mafic lower crust, that was triggered by the intrusion of the hot basaltic magmas.

Zircon LA-ICP-MS Dating and Geochemical Characteristics of I-type Granitoids from the Yanhu Area, West Segment of the Bangongco-Nujiang Suture (Western Tibet): Petrogenesis and Implications for the Southward Subduction of the Tethyan Ocean

ABSTRACT The Yanhu granitoids are located in the west segment of the Bangongco-Nujiang suture in the western Tibetan Plateau. The main rock types of the granitoids are diorite porphyry, quartz diorite, granodiorite, granite and granite porphyry. Here, their zircon LA-ICP-MS U-Pb ages and petrogeochemical data are reported. Three groups of magmatic events can be distinguished from the Yanhu area: group 1 includes samples AK01 and ZK01 of diorite porphyry, and sample D3658 of quartz diorite that yield mean zircon U-Pb ages of 121.0 ± 2.7 Ma, 116.6 ± 2.0 Ma and 116.0 ± 3.9 Ma, respectively; group 2 includes sample D0050 of diorite porphyry, samples D1393 and D3660 of granodiorite and sample D3065 of granite porphyry that yield mean zircon U-Pb ages of 104.9 ± 2.0 Ma, 105.4 ± 3.8 Ma, 104.2 ± 1.9 Ma and 104.2 ± 1.9 Ma, respectively; group 3 includes sample D3093 of granite that yields mean zircon U-Pb ages of 93.6 ± 1.5 Ma. The zircon LA-ICP-MS U-Pb ages suggest that the Yanhu granitoids were emplaced at 121.0 – 93.6 Ma, representing Cretaceous magmatism in the west segment of the Bangongco-Nujiang suture. The granitoids are composed of SiO2 (56.57 to 76.98 wt.%), Al2O3 (12.20 to 17.90 wt.%), Na2O (3.61 to 4.98 wt.%), K2O (2.06 to 4.71 wt.%) and CaO (0.27 to 5.74 wt.%). The Yanhu granitoids exhibit enrichment in LREE (light REE) and LILE (large ion lithophile elements) such as Rb, Th, U, Pb and K and depletion of HREE (heavy REE), P, Ti, Nb, Ta and Zr. Their A/CNK ratios of 0.85-1.06 are &amp;lt;1.1, implying that they are high-K, metaluminous-weakly peraluminous I-type granites. The Yanhu granitoids were generated mainly by partial melts of the meta-igneous lower crust and some arc-related materials. The Yanhu granitoids probably formed in VAG and syn-COLG tectonic settings related to the southward subduction of the Tethyan Ocean. Diorite porphyry and quartz diorite magmatism from 121.0 Ma to 116.0 Ma may be associated with the southward Bangongco–Nujiang Tethys oceanic crust subduction. Diorite porphyry, granodiorite, and granite porphyry magmatism from 105.4 Ma to 104.2 Ma may be associated with the rising asthenosphere induced by the slab breakoff. Granite magmatism from 93.6 Ma may be related to the crustal thickening induced by the final amalgamation of the Lhasa Terrane and the Qiangtang Terrane.

The effect of titanite crystallisation on Eu and Ce anomalies in zircon and its implications for the assessment of porphyry Cu deposit fertility

The redox sensitivity of Ce and Eu anomalies in zircon has been clearly demonstrated by experimental studies, and these may represent an important tool in the exploration for porphyry Cu deposits which are thought to be derived from oxidised magmas. These deposits are significant because they are the source of much of the world's copper and almost all of the molybdenum and rhenium, key elements in many modern technologies. However, Ce and Eu anomalies in zircon are also affected by the co-crystallisation of REE bearing phases, such as titanite. Here, we report the trace element chemistry of zircons from titanite-bearing intrusions associated with mineralisation at the world class Oyu Tolgoi porphyry Cu–Au deposit (Mongolia). Based on these data, we suggest that neither zircon Eu/Eu⁎, nor Ce4+/Ce3+ are robust proxies for melt redox conditions, because they are both too strongly dependent on melt REE concentrations, which are usually poorly constrained and controlled by the crystallisation of titanite and other REE-bearing phases. In spite of this, Eu/Eu⁎ can broadly distinguish between fertile and barren systems, so may still be an indicator of porphyry magma fertility, and a useful tool for exploration.

Oxygen fugacity and porphyry mineralization: A zircon perspective of Dexing porphyry Cu deposit, China

Oxygen fugacity (fO2) is a key factor that controls the formation of porphyry Cu deposits. Porphyry Cu deposits are typically oxidized, but when and how porphyry magmas gain their high oxygen fugacity signatures, and how oxygen fugacity controls porphyry mineralization, remains obscure. To trace the origin of the high oxygen fugacity in porphyry Cu deposits, we determined trace element compositions and U-Pb ages of magmatic and inherited zircon from Dexing porphyry Cu deposit, calculated Ce4+/Ce3+ of zircons and estimated the oxygen fugacity of their parental magmas. The Ce4+/Ce3+ ratios of Middle Jurassic (∼170Ma) magmatic zircons are high (550 on average), whereas the Ce4+/Ce3+ ratios of inherited zircons (200–880Ma) are much lower (263 on average). The relationship suggests that the Dexing porphyry magma was highly oxidized when the Jurassic magmatic zircons crystallized (estimated fO2: ΔFMQ+0.7 (±1.3) – ΔFMQ+1.9 (±1.3)), which cannot be attributed to partial melting of the Neoproterozoic arc crust (estimated fO2: ΔFMQ−2.4 (±1.1) – ΔFMQ+0.7 (±1.2)) as proposed previously. Instead, the high fO2 (ΔFMQ+1.5) of these magmas is a primary feature of the latest magmatism. Sulfate rather than sulfide is the dominant species at fO2>ΔFMQ+1.5. In general, the behavior of Cu and other chalcophile elements is controlled by sulfide, while the sulfur speciation is controlled by oxygen fugacity. Previous modeling results show that partial melting of mantle peridotite under high oxygen fugacity (even at >ΔFMQ+1.5) cannot form Cu-rich magmas, which plausibly explains the lack of porphyry Cu deposits in normal arc rocks. This is because mantle peridotite has low Cu and S contents. Our modeling shows that partial melting of subducted oceanic crust, under oxygen fugacities higher than ΔFMQ+1.5, is favorable for producing primary magmas with Cu contents sufficiently high for porphyry mineralization, which plausibly explains the close relationship between porphyry Cu deposits and oxidized magmas with adakitic affinities.

Magmatic Evolution of Granodiorite Intrusions at the El Salvador Porphyry Copper Deposit, Chile, Based on Trace Element Composition and U/Pb Age of Zircons

Uranium-lead ages and trace element compositions of zircon from a series of shallow porphyry intrusions document the temporal, chemical, and thermal magmatic evolution of magmatic-hydrothermal porphyry Cu (Mo-Au) ores in the El Salvador district, Chile. Zircons ( n = 240) from 15 Eocene age diorite, granodiorite, and granite porphyry intrusions were analyzed by SHRIMP-RG ion microprobe. The weighted means of 207 Pb-corrected 206 Pb/ 238 U zircon ages span 3 m.y. from about 44 to 41 Ma, with peak magmatic flux at 44 to 43 Ma. The granodiorite porphyries at the Turquoise Gulch copper deposit record waning stages of magmatism at 42.5 to 42.0 Ma and were followed by postmineral latite dikes at about 41.6 Ma. Porphyry copper ores formed contemporaneously with porphyry intrusion centers that progressed temporally from north to south, from the small deposits at Cerro Pelado (~44.2 Ma), Old Camp (~43.6 Ma), and at M Gulch-Copper Hill (~43.5–43.1 Ma) to the main ore deposit at Turquoise Gulch (~42 Ma). The Eocene porphyry intrusions contain a few Mesozoic ( n = 9) inherited zircons and numerous ( n ≥19) antecrystic zircons about 1 to 2 m.y. older than the host intrusion that provide evidence of extensive Eocene magmatic recycling. The Ti-in-zircon geothermometer provides estimates of 890° to 620°C for zircon crystallization and records both core to rim cooling and locally high-temperature rim overgrowths. Most zircon in ore-related K, L, and R porphyries yields near-solidus temperatures of 750° to 650°C and crystallized from compositionally diverse granodiorite porphyries that are a product of crystal fractionation of hornblende, apatite, and titanite with lesser crustal contamination and mixing with high-temperature deep-sourced mafic magma. During a 3-m.y. period, porphyry intrusions tapped an evolving granodioritic magma chamber that was periodically heated, locally remelted, and mixed with mafic magma during recharge events but cooled between recharge events to evolve ore fluids. Europium anomalies (chondrite-normalized Eu N /Eu N * ) in zircons become more pronounced with increased Hf content and cooling but display two distinct evolutionary paths: Eu N /Eu N * of early quartz porphyry evolves from 0.8 to 0.3, whereas the late synmineralization porphyries evolve from 0.8 to 0.65. The Eu N /Eu N * ratio of zircon reflects the Eu 3 +/Eu 2 + ratio of the melt, and therefore the granodiorite porphyries at Turquoise Gulch were the most strongly oxidized of the El Salvador magmas. The strongly oxidized trend porphyry magmas at Turquoise Gulch are apparently directly linked to magmatic degassing at ~700°C to produce large amounts of ore-forming copper, sulfur, and chlorine-enriched magmatic-hydrothermal aqueous fluids.

Tectonic environments of South American porphyry copper magmatism through time revealed by spatiotemporal data mining

Porphyry ore deposits are known to be associated with arc magmatism on the overriding plate at subduction zones. While general mechanisms for driving magmatism are well established, specific subduction-related parameters linking episodes of ore deposit formation to specific tectonic environments have only been qualitatively inferred and have not been formally tested. We develop a four-dimensional approach to reconstruct age-dated ore deposits, with the aim of isolating the tectono-magmatic parameters leading to the formation of copper deposits during subduction. We use a plate tectonic model with continuously closing plate boundaries, combined with reconstructions of the spatio-temporal distribution of the ocean floor, including subducted portions of the Nazca/Farallon plates. The models compute convergence rates and directions, as well as the age of the downgoing plate through time. To identify and quantify tectonic parameters that are robust predictors of Andean porphyry copper magmatism and ore deposit formation we test two alternative supervised machine learning methods; the ‘random forest’ (RF) ensemble and ‘support vector machines’ (SVM). We find that a combination of rapid convergence rates (~100 km/Myr), subduction obliquity of ~15°, a subducting plate age between ~25–70 Myr old, and a location far from the subducting trench boundary (>2000 km), represent favorable conditions for porphyry magmatism and related ore deposits to occur. These parameters are linked to the availability of oceanic sediments, the changing small-scale convection around the subduction zone, and the availability of the partial melt in the mantle wedge. When coupled, these parameters could influence the genesis and exhumation of porphyry copper deposits.

From a long-lived upper-crustal magma chamber to rapid porphyry copper emplacement: Reading the geochemistry of zircon crystals at Bajo de la Alumbrera (NW Argentina)

The formation of world class porphyry copper deposits reflect magmatic processes that take place in a deeper and much larger underlying magmatic system, which provides the source of porphyry magmas, as well as metal and sulphur-charged mineralising fluids. Reading the geochemical record of this large magmatic source region, as well as constraining the time-scales for creating a much smaller porphyry copper deposit, are critical in order to fully understand and quantify the processes that lead to metal concentration within these valuable mineral deposits. This study focuses on the Bajo de la Alumbrera porphyry copper deposit in Northwest Argentina. The deposit is centred on a dacitic porphyry intrusive stock that was mineralised by several pulses of porphyry magma emplacement and hydrothermal fluid injections. To constrain the duration of ore formation, we dated zircons from four porphyry intrusions, including pre-, syn- and post-mineralisation porphyries based on intersection relations between successive intrusion and vein generations, using high precision CA-ID-TIMS. Based on the youngest assemblages of zircon grains, which overlap within analytical error, all four intrusions were emplaced within 29 ka, which places an upper limit on the total duration of hydrothermal mineralisation. Re/Os dating of hydrothermal molybdenite fully overlaps with this high-precision age bracket. However, all four porphyries contain zircon antecrysts which record protracted zircon crystallisation during the ∼200 ka preceding the emplacement of the porphyries. Zircon trace element variations, Ti-in-zircon temperatures, and Hf isotopic compositions indicate that the four porphyry magmas record a common geochemical and thermal history, and that the four intrusions were derived from the same upper-crustal magma chamber. Trace element zoning within single zircon crystals confirms a fractional crystallisation trend dominated by titanite and apatite crystallisation. However, zircon cathodoluminescence imaging reveals the presence of intermediate low luminescent (dark) growth zones in many crystals from all intrusions, characterised by anomalously high Th, U and REE concentrations and transient excursions in trace element ratios. A return to the same fractionation trend after this excursion excludes external compositional forcing such as magma mixing. Instead we interpret the “dark-zones” to record zircon crystallisation during a transient event of rapid growth that resulted from mafic magma injection into the base of the magma chamber, releasing a CO2-rich vapour phase into the dacitic crystal mush. We propose that this vapour phase then migrated upwards to the apical part of the magma chamber from where it was expelled, together with successive batches of magma, to form the porphyry copper deposit within a short time-span of less than a few 10,000 years. The short duration of host rock emplacement, hydrothermal alteration and mineralisation presented in this study provides critical constraints on fluid storage in magma chambers and the genesis of large porphyry copper deposits.

Genesis of the Huangshaping W–Mo–Cu–Pb–Zn polymetallic deposit in Southeastern Hunan Province, China: Constraints from fluid inclusions, trace elements, and isotopes

The Huangshaping polymetallic deposit is located in southeastern Hunan Province, China. It is a world-class W–Mo–Pb–Zn–Cu skarn deposit in the Nanling Range Metallogenic Belt, with estimated reserves of 74.31Mt of W–Mo ore at 0.28% WO3 and 0.07% Mo, 22.43Mt of Pb–Zn ore at 3.6% Pb and 8.00% Zn, and 20.35Mt of Cu ore at 1.12% Cu. The ore district is predominantly underlained by carbonate formations of the Lower Carboniferous period, with stocks of quartz porphyry, granite porphyry, and granophyre. Skarns occurred in contact zones between stocks and their carbonate wall rocks, which are spatially associated with the above-mentioned three types of ores (i.e., W–Mo, Pb–Zn, and Cu ores).Three types of fluid inclusions have been identified in the ores of the Huangshaping deposit: aqueous liquid–vapor inclusions (Type I), daughter-mineral-bearing aqueous inclusions (Type II), and H2O–CO2 inclusions (Type III). Systematic microthermometrical, laser Raman spectroscopic, and salinity analyses indicate that high-temperature and high-salinity immiscible magmatic fluid is responsible for the W–Mo mineralization, whereas low-temperature and low-salinity magmatic-meteoric mixed fluid is responsible for the subsequent Pb–Zn mineralization. Another magmatic fluid derived from deep-rooted magma is responsible for Cu mineralization.Chondrite-normalized rare earth element patterns and trace element features of calcites from W–Mo, Pb–Zn, and Cu ores are different from one another. Calcite from Cu ores is rich in heavy rare earth elements (187.4–190.5ppm), Na (0.17%–0.19%), Bi (1.96–64.60ppm), Y (113–135ppm), and As (9.1–29.7ppm), whereas calcite from W–Mo and Pb–Zn ores is rich in Mn (>10.000ppm) and Sr (178–248ppm) with higher Sr/Y ratios (53.94–72.94). δ18O values also differ between W–Mo/Pb–Zn ores (δ18O=8.10‰–8.41‰) and Cu ores (δ18O=4.34‰–4.96‰), indicating that two sources of fluids were, respectively, involved in the W–Mo, Pb–Zn, and Cu mineralization.Sulfur isotopes from sulfides also reveal that the large variation (4‰–19‰) within the Huangshaping deposit is likely due to a magmatic sulfur source with a contribution of reduced sulfate sulfur host in the Carboniferous limestone/dolomite and more magmatic sulfur involved in the Cu mineralization than that in W–Mo and Pb–Zn mineralization. The lead isotopic data for sulfide (galena: 206Pb/204Pb=18.48–19.19, 207/204Pb=15.45–15.91, 208/204Pb=38.95–39.78; sphalerite: 206Pb/204Pb=18.54–19.03, 207/204Pb=15.60–16.28, 208/204Pb=38.62–40.27; molybdenite: 206Pb/204Pb=18.45–19.21, 207/204Pb=15.53–15.95, 208/204Pb=38.77–39.58 chalcopyrite: 206Pb/204Pb=18.67–19.38, 207/204Pb=15.76–19.90, and 208/204Pb=39.13–39.56) and oxide (scheelite: 206Pb/204Pb=18.57–19.46, 207/204Pb=15.71–15.77, 208/204Pb=38.95–39.13) are different from those of the wall rock limestone (206Pb/204Pb=18.34–18.60, 207/204Pb=15.49–15.69, 208/204Pb=38.57–38.88) and porphyries (206Pb/204Pb=17.88–18.66, 207/204Pb=15.59–15.69, 208/204Pb=38.22–38.83), suggesting Pb206-, U238-, and Th 232-rich material are involved in the mineralization. The Sm–Nd isotopes of scheelite (εNd(t)=−6.1 to −2.9), garnet (εNd(t)=−6.8 to −6.1), and calcite (εNd(t)=−6.3) from W–Mo ores as well as calcite (εNd(t)=−5.4 to −5.3) and scheelite (εNd(t)=−2.9) from the Cu ores demonstrate suggest more mantle-derived materials involved in the Cu mineralization.In the present study we conclude that two sources of ore-forming fluids were involved in production of the Huangshaping W–Mo–Pb–Zn–Cu deposit. One is associated with the granite porphyry magmas responsible for the W–Mo and then Pb–Zn mineralization during which its fluid evolved from magmatic immiscible to a magmatic–meteoritic mixing, and the other is derived from deep-rooted magma, which is related to Cu-related mineralization.

Geochronology, redox-state and origin of the ore-hosting porphyry in the Tongkuangyu Cu deposit, North China Craton: Implications for metallogenesis and tectonic evolution

Tongkuangyu Cu deposit, located at the southern edge of the North China Craton, is hosted by a suite of sedimentary-magmatic rocks. Genesis of this giant ore deposit has been debated for over half a century. New data from geochronological, geochemical and isotopic analyses on the ore-hosting monzogranitic porphyries are presented. Seventy four zircon U-Pb dates by laser ablation ICPMS suggest that the porphyries were emplaced at ca. 2180Ma. Geochemically, they show transitional features between typical Archean TTG and sanukitoid, e.g. displaying sodium enrichment with elevated K contents, and exhibiting strong REE fractionation with elevated HREE, Mg, Cr contents. Ti-in-zircon thermometer reveals a crystallization temperature of 675±16°C. Cerium anomalies in zircons indicated that the porphyry magmas have low oxygen fugacity of 0.5 log unit below the fayalite-magnetite-quartz buffer (ΔFMQ −0.5). Zircons from the porphyries have ɛHf(t) values of −4.9 to 2.5 with two-stage depleted mantle model ages of ∼2.8Ga, which point to a magma source resembles the newly-discovered 2.7Ga TTGs and diorite in this region. Zircons from the porphyries are slightly more enriched in 18O than that of mantle zircons, with δ18O values of 5.2–5.9‰ (averaging 5.6±0.1‰). Geochemical modeling suggests that the porphyry magmas were likely sourced from melting of the 2.7Ga diorite leaving behind hornblende and biotite as residues, with minor contribution (<20%) from juvenile, mantle-derived materials. The reducing nature of the porphyry magmas makes them incapable of sequestering sufficient Cu and S, thus rendering them unsuitable for porphyry-style copper mineralization. By tentative inference, we suggest that magmatic sulfide in the porphyries may have acted as a reducing agent to precipitate copper from the ore-forming fluids. Combining the above evidence and previous interpretations, we propose a tectonic evolution model envisaging an oceanic subduction at ∼2.7Ga, followed by a continental collision at ∼2.5Ga, and ended up with an extension at ∼2.2Ga.

Zircon record of the plutonic-volcanic connection and protracted rhyolite melt evolution

The potential petrogenetic link between a crystal-poor rhyolite (the Rhyolite Canyon Tuff) and its associated subvolcanic intrusion and crystal-rich post-caldera lavas from Turkey Creek, Arizona (USA), is examined using zircon chemical abrasion–thermal ionization mass spectrometry U-Pb geochronology and inductively coupled plasma mass spectrometry trace element analyses. U-Pb ages indicate that zircon growth within the rhyolite and the dacite-monzonite porphyry magmas was coeval over ∼300 k.y. prior to the large eruptive event. Trends in zircon trace elements (Hf, Y/Dy, Sm/Yb, Eu/Eu*) through time in the dacitic-monzonitic units and rhyolite reflect melt evolution dominated by crystal fractionation. Importantly, the Y/Dy ratio in zircons in both units remains mostly similar for the first ∼150 k.y. of the system’s evolution, but the dominant population in the rhyolitic unit diverges from that of the dacite-monzonite porphyry ∼150 k.y. before eruption. We interpret this divergence in trace element composition to record the assembly time of the melt-rich cap within its intermediate mush zone in the upper crustal reservoir. These results are consistent with (1) a connection between plutonic and volcanic realms in the upper crust, (2) a protracted time scale for constructing an intermediate mush large enough to hold 500 km 3 of rhyolite, and (3) the prolonged extraction of that melt prior to eruption.

Identifying tectonic niche environments of South American porphyry magmatism through geological time: a spatio-temporal data mining approach

Porphyry ore deposits are well known to be associated with arc magmatism related to subduction on the overriding plate. Furthermore, the regional mechanisms for magmatism and the resulting formations of porphyry deposits are well established. Specific parameters leading to these events have been inferred, but not formally tested. We aim to identify the specific set of tectono-magmatic parameters that result in a subducting slab producing particular types of magmatism on the overriding plate, and their link to the formation of ore deposits. We use a four-dimensional approach to reconstruct age-dated magmatism back through space and time to isolate the tectono-magmagic parameters leading to the formation of a metalliferous deposit during subduction. By utilising machine learning techniques we identify and quantify geodynamic parameters that are robust predictors of back-arc magmatism and porphyry formation. The 'random-forest' ensemble and 'support vector machines' learning classification methods are employed to prioritise parameters that are considered influential in the development of magmatism and the subsequent metallogenesis of porphyry ore deposits. We find that a combination of convergence rates and directions, seafloor age, subduction obliquity, and the distance to a trench edge help predict whether magmatism and related ore deposits occur.

Subterranean origin of accreted lapilli in cone-sheets of the houshihushan sub-volcanic ring complex, Shanhaiguan, China

In addition to syenite ring dykes and multiple alkaline granite stocks, the sub-volcanic Houshihushan alkaline ring complex near Shanhaiguan, Qinhuangdao City, contains cone-sheets of two types: a majority filled with granite porphyry and a minority filled with quartz syenite porphyry. Many cone sheets show evidence of multiple magma intrusion events. Some granite porphyry sheets’ multiple chilled margins display magmatic roll structures indicating that turbulent magma flowed up the fractures. In one upward-closing cone-sheet K-feldspar phenocrysts floated up through fluid granite porphyry magma and became concentrated at the top providing direct evidence of shallow-level crystal fractionation, confirmed by published rock analyses. Accreted lapilli with K-feldspar crystal cores occur only in the inner parts of a minority of cone-sheets and field relationships indicate that they must have formed beneath the ground surface. Similar lapilli occur in erupted ignimbrites preserved in the collapsed caldera. Voids between lapilli in cone-sheets indicate the presence of volumes of gas below the surface that could have flowed upwards as fast-moving hot gas streams. We propose a mechanism of formation that began with subterranean magmatic rolls with K-feldspar crystal cores that formed on dyke walls, and became detached. Then they were caught up in rising gas streams and erupted at the surface. Thus accreted lapilli formed underground, were erupted along with blobs of fluid glass in escaping gases, and transported over the surface in nuees ardentes, to settle and cool as ignimbrite flows.