Porphyry Copper Systems Research Articles

Primary fluid exsolution in porphyry copper systems: evidence from magmatic apatite and anhydrite inclusions in zircon

Primary fluid exsolution in porphyry copper systems: evidence from magmatic apatite and anhydrite inclusions in zircon

Metallogenic inception of the Arabian-Nubian Shield: Daero Paulos porphyry copper prospect, Eritrea

The Daero Paulos porphyry copper prospect, in the Asmara region of the central Eritrean highlands, is part of the Nakfa tectono-stratigraphic terrane, which, along with four other major and several minor terranes that constitute the Neoproterozoic Arabian-Nubian Shield, is characterized by juvenile, intra-oceanic island-arc crust. Alteration and mineralization at Daero Paulos occupy a north-northeast-trending corridor of multiphase, rhyolitic to dacitic intrusions emplaced into mafic and intermediate volcanic and subvolcanic rocks and derivative volcaniclastic strata. New U-Pb geochronological data for the volcanic rocks in the region confirm a >860-Ma age for the Neoproterozoic (Tonian) host sequence. The porphyry copper alteration evolved from early epidote-chlorite-albite through sericite-chlorite to late quartz-sericite. Local advanced argillic alteration is the erosional remnant of the lithocap that formerly dominated the shallow parts of the Daero Paulos system. A post-mineralization, Alaskan-type pyroxenite-gabbro body intruded the porphyry copper corridor, and is interpreted as the feeder for mafic dikes along the periphery of the porphyry copper system. Re-Os ages of 881.0 ± 5.5 and 864.2 ± 3.6 Ma for molybdenite from veinlets at Daero Paulos constrain an early Tonian timing for porphyry copper formation during early opening of the Mozambique Ocean after Rodinian supercontinent break-up. The ages also imply a rapid transition from initial rifting to the start of subduction and consequent establishment of an intra-oceanic island arc in what is now the Nakfa terrane. The molybdenite ages show that Daero Paulos is currently the oldest dated mineralization in the Arabian-Nubian Shield, and ~60–200 and, possibly, ~250 m.yr. older than porphyry copper mineralization elsewhere in the Shield. However, volcanogenic massive sulfide (VMS) mineralization could have been forming at broadly the same time, albeit not genetically associated with Daero Paulos. Preservation of such shallowly formed Neoproterozoic mineralization is mainly attributed to burial beneath thick post-amalgamation volcano-sedimentary basinal cover, collision-generated thrust sheets, and later Phanerozoic strata.

Management of Copper Heap Leach Projects: A Geologist’s Perspective

Editor’s note: The Geology and Mining series, edited by Dan Wood and Jeffrey Hedenquist, is designed to introduce early-career professionals and students to a variety of topics in mineral exploration, development, and mining, in order to provide insight into the many ways in which geoscientists contribute to the mineral industry. Abstract Copper production by heap leaching, coupled with solvent extraction and electrowinning (SX-EW), is a well-established technology, with an annual output of about 3.7 million tonnes (Mt) of copper metal. Ores presently amenable to copper heap leaching include copper oxides and secondary copper sulfides. Most copper deposits amenable to acid sulfate heap leaching result from supergene processes within porphyry copper systems, although copper heap leaching has been applied to sandstone and shale-hosted deposits, among others. Copper heap leaching is a rate-dependent process sensitive to copper mineralogy (copper oxides > secondary sulfides > hypogene sulfides), driven by the pH of the leach solution, the activity of ferric iron (Fe3+ (aq)) dissolved in the leach solution, and temperature. Acid consumption, a principal operating cost item, depends on the pH of the leach solution; the presence of reactive gangue minerals, notably carbonates, Ca plagioclase, pyroxene, Fe-rich amphibole, and olivine; and the cumulative surface area of material in the heap. There are three basic approaches to commercial copper heap leaching—run-of-mine, dedicated pad, and on-off pad leaching, with variables that include crushing, acid/ferric agglomeration, solution application rate, and leach solution pH. These approaches affect copper leach kinetics, overall copper recovery, acid consumption, and capital and operating costs. A successful copper heap leach evaluation program requires a systematic approach, beginning with geologic mapping, then drilling and hydraulic and metallurgical testing, and concluding with financial analysis, engineering, and permitting. As geologists are the unique party in the process, with a thorough understanding of the overall deposit geology, including ore and gangue mineralogy, the domains that comprise the deposit, and the geochemistry of leaching, they must remain fully involved in the project throughout the evaluation. At the outset, geologists must manage the drilling program and define the grade-mineral domains. Later, they must participate in the metallurgical and hydraulic testing programs, including the evaluation of test results; then, during financial modeling, they must collaborate with all of the other specialists.

Petrogenesis of the Middle Jurassic andesitic dikes in the giant Dexing porphyry copper ore field, South China: Implications for mineralization

The giant Dexing porphyry copper ore field, containing 9.7 Mt copper, is the largest porphyry copper system in South China. The petrogenesis of andesitic dikes in the Dexing ore field and their genetic link with porphyry mineralization remain unclear. This paper presents LA-ICP-MS zircon U-Pb dating, elemental and Sr-Nd-Hf isotopic data of the andesitic dikes in the Dexing porphyry copper ore field. The Dexing andesitic dikes were emplaced during the Middle Jurassic at ca. 166 Ma. The Dexing andesitic dikes have relative high MgO contents and Mg# ratios but low TiO2 contents. They also have obvious Nb, Ta and Ti depletion in the primitive mantle normalized spider diagrams. The geochemical affinity of arc magma and low Ti contents indicate that the magma source is probably lithosphere mantle modified by subduction process. A non-arc setting during the Middle Jurassic is inferred due to the absence of typical arc rocks and the presence of rifting-related magmatic rocks in the interior of South China. The non-arc setting implies that the Neoproterozoic Jiangnan Orogen was likely responsible for the arc geochemical features. The concentration of copper in the andesitic dikes is not high, supporting that remelting of the metal-rich Neoproterozoic juvenile lower continental crust instead of the mantle is the critical factor for the giant porphyry style mineralization. The mantle derived mafic magma may play a key role for providing significant heat for remelting of the metal-rich lower continental crust via underplating into the crust-mantle boundary.

Metallogenesis and major challenges of porphyry copper systems above subduction zones

Porphyry copper±molybdenum±gold deposits (PCDs) are the most representative magmatic-hydrothermal metallogenic system above subduction zones with important economic value. Previous studies revealed that large PCDs are generally formed from initial arc magmas (from subduction-induced partial melting of the mantle wedge), which eventually ascend to the shallow crust (3–5 km) for mineralization after a series of complex evolution processes. These processes include (1) the dehydration or partial melting of subducting slab, which induces partial melting of the metasomatized mantle wedge; (2) the ascent of mantle-derived magma to the bottom of the lower crust, which subsequently undergoes crustal processes such as assimilation plus fractional crystallization (AFC) or melting, assimilation, storage and homogenization (MASH); (3) the magma chamber formation at the bottom of the lower, middle and upper crust; (4) the final emplacement and volatilization of porphyry stocks; and (5) the accumulation of ore-forming fluids and metal precipitation. Despite the many decades of research, many issues involving the PCD metallogenic mechanism still remain to resolve, such as (1) the tectonic control on the geochemical characteristics of ore-forming magma; (2) the reason for the different lifespans of the long-term magmatic arc evolution and geologically “instantaneous” mineralization processes; (3) the source of ore-forming materials; (4) the relative contributions of metal pre-enrichment to mineralization by the magma source and by magmatic evolution; and (5) the decoupling behaviors of Cu and Au during the pre-enrichment. These issues point out the direction for future PCD metallogenic research, and the resolution to them will deepen our understanding of the metallogenesis at convergent plate boundaries.

Geochemical and Petrology Properties of Zafarghand Index and Kahang Deposit of Porphyry Copper-Molybdenum

The probability of the occurrence of porphyry copper ore associated with subduction zone of the Neo-Tethys ocean and volcanic arc of Iran (Urumieh-Dokhtar) justifies the necessity of exploration and prospection of this type of ore in Iran. Zafarghand index and Kahang deposit of porphyry copper-molybdenum located in Isfahan province, center of Iran using Satellite data interpretations were discovered in the years of 2010 and 2003, respectively. Geological studies have indicated the presence of argillic and propylitic alteration halos associated with porphyry copper systems. The 250 rock samples were systematically collected at a sampling distance of 100 m and in the center of the porphyry system by a distance o f 50 m in Zafarghand. Also, 377 samples of lithogeochemicals (185 rock samples and 192 soil samples) were extracted systematically from Kahang deposit. The study of geochemical data of rock and soil samples showed similarity of these two types of mineralization with other mineralization of porphyry copper-molybdenum elsewhere in the world. Finally, the comparison of geochemical anomalies copper with rock units and alteration zones showed that atmospheric waters had washed out the copper in some of these zones and probably the supergene zone was formed in depth as porphyry copper-molybdenum deposit.

Multiple generations of titanites and their geochemical characteristics record the magmatic-hydrothermal processes and timing of the Dongguashan porphyry-skarn Cu-Au system, Tongling district, Eastern China

Dongguashan is one of the largest porphyry-skarn Cu-Au deposits in eastern China and titanite is found in the ore system. Five types of both magmatic and hydrothermal titanites are distinguished, based on their textural and chemical characteristics. Types 1 and 2 titanites are magmatic in origin whereas types 3, 4, and 5 are hydrothermal. Magmatic titanites are wedge-shaped and display concentric and sector zoning, whereas hydrothermal titanites show a wide range of textures that vary with different alteration stages. REE, Sn, Mo, and Cu are enriched in magmatic titanites (types 1 and 2). In contrast, vanadium and HFSE are enriched in hydrothermal titanites, especially in type 4 titanite from the propylitic alteration zone. Magmatic titanite (type 1) coexists with magnetite and K-feldspar and crystallized in oxidized and H2O-rich magma at 730–760 °C. Type 2 titanite contains ilmenite inclusions reflecting changes in the fO2 of the magma chamber, possibly caused by input of mafic magma. The mobility and enrichment of HFSE and association with fluorapatite reflect the F-rich composition of the magmatic-hydrothermal fluid at Dongguashan. The relationship between titanite textures and chemistry indicates that titanite can serve as a recorder of magmatic-hydrothermal processes in porphyry copper systems. U-Pb dating of type 4 titanite from the propylitic alteration zone and type 5 titanite from skarn yielded ages of 139.0 ± 2.6 Ma and 137.0 ± 2.0 Ma, respectively, indicating that it formed synchronously with the associated quartz monzodiorite.

Titanium‐in‐biotite thermometry in porphyry copper systems: Challenges to application of the thermometer

AbstractEmpirical geothermometer dealing with Ti solubility in the Fe‐Mg biotites was originally proposed for biotites in graphitic, peraluminous metapelites containing ilmenite or rutile that equilibrated roughly at 4–6 kbar. Given that biotites are abundant in the porphyry copper systems, this geothermometer has frequently been used for the determination of magmatic–hydrothermal temperatures in the porphyry copper systems. Common associations of porphyry copper deposits (PCDs), that is, low Al content of biotite, biotite chloritization (causes the biotite to become more magnesian and to lose Ti), and biotite formation by amphibole replacement, as well as disequilibrium, local equilibrium, or re‐equilibration of biotites, especially through potassic alteration, may provide significant uncertainty in the temperatures estimated a by Ti‐in‐biotite geothermometer. In addition, besides the calibration range of thermometer for pressure (400–600 MPa), the temperatures of major sulfide precipitation in PCDs (>~400°C) does not fit with the temperature range of thermometer calibration (480–800°C). Worth noting, as confirmed by fluid inclusion data in the Sarkuh PCD, regardless of presence of mineralogical requirements, obtained temperatures of sulfide mineralization using Ti in biotite thermometer could be overestimated. This may be due to the difference between general conditions of sulfide mineralization and calibration range of Ti in the biotite thermometer for pressure and temperature, as well as the metaluminous nature of biotites in PCDs.

Geochemistry, tectonic setting and magmatic origin of the mineralized stock in SahebDivan porphyry copper system, NW Iran

Geochemistry, tectonic setting and magmatic origin of the mineralized stock in SahebDivan porphyry copper system, NW Iran

Copper Minerals at Vesuvius Volcano (Southern Italy): A Mineralogical Review

This work is part of a project focused on the Somma–Vesuvius volcano and aimed at identifying Cu minerals related to mineralizing processes associated with magmatic activity in an active magmatic-hydrothermal system. A mineralogical survey was carried out on a set of samples represented by sublimates and fumarolic products from the collection of the Mineralogical Museum of the University of Naples Federico II (Italy). These samples are mainly related to most recent eruptive episodes of Vesuvius activity, from 1631 onward. Copper-bearing minerals were characterized, as well as associated minerals, by X-ray diffraction (XRD) scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). An investigation on the structural complexity of Cu-mineral assemblages with different temperature formations was also carried out using the TOPOS software package. The main copper phases are sulfates, followed by vanadates, hydroxyhalides, oxides, carbonates, silicates and finally, phosphates. New mineral occurrences for Vesuvius, both Cu-bearing and Cu-free, are described. Nevertheless, the fumarolic/alteration minerals at Vesuvius cannot be considered of economic relevance as a copper reservoir, this type of mineralizations are significant for copper crystal chemistry and for the knowledge of the mineralogical variants. The obtained datasets can be of interest for the knowledge of volcanic byproducts of copper ore deposits (i.e., porphyry copper systems) and of (base) metal segregation processes.

The behavior of Fe and S isotopes in porphyry copper systems: Constraints from the Tongshankou Cu-Mo deposit, Eastern China

Iron isotopes have been widely applied to trace mineralization processes of magmatic-hydrothermal ore systems, yet the application of Fe isotopes in porphyry Cu deposits (PCDs) is still restricted by the lack of understanding of its behaviors during hydrothermal alteration and ore formation processes. In this study, Fe isotope systematics in whole-rocks and Fe-bearing minerals coupled with S isotope compositions of Fe-sulfides are investigated for the early Cretaceous Tongshankou porphyry Cu-Mo deposit from Eastern China. The results show that altered ore-hosting granodiorite porphyries display a small δ56Fe variation (0.04‰ to 0.17‰) with an average of 0.10 ± 0.08‰ (2SD, n = 10), indistinguishable from the values for global igneous rocks. The δ56Fe of magnetite (0.22‰) and biotite (-0.08‰ to 0.12‰) from the altered porphyries are also within the δ56Fe ranges of those from global igneous rocks. These results indicate that hydrothermal fluids have negligible effect on Fe isotope compositions of the porphyry host rocks, likely due to the insignificant contributions of fluid-derived iron relative to the total Fe budget of the host rocks.The Fe-sulfides in the porphyry system record geochemical and iron isotopic characteristics of the ore-forming fluids. Specifically, chalcopyrites show large Fe isotope variations with δ56Fe ranging from −0.60‰ to 0.61‰. The calculated Fe isotope compositions of ore-forming fluids based on the chalcopyrite δ56Fe are thus not uniform and fall in the range from −0.69 ± 0.18‰ to 0.52 ± 0.18‰ (2SD). On the other hand, pyrites exhibit a δ56Fe variation between −0.48‰ and 0.40‰ and show distinct Fe isotopic characteristics for disseminated and vein-type pyrite. The δ56Fe for disseminated pyrites are consistently higher than whole-rock values, ranging from 0.14‰ to 0.40‰, likely reflecting equilibrium Fe isotopic exchange between pyrite and dissolved Fe in fluid. By contrast, δ56Fe for pyrites in veins are highly variable (−0.48‰ to 0.34‰) and show an inverse correlation with the chalcopyrite δ56Fe. These observations, in concert with S isotope data, have been interpreted to reflect pyrite formation through the FeS pathway, whereas the wide range in δ56Fe for pyrite may record isotopic change from initial FeS-fluid equilibrium towards pyrite-fluid equilibrium due to different extent of reaction upon precipitation. Additionally, when compared to sulfides formed by non-magmatic, low-temperature processes, the sulfides in our study exhibit a relatively narrow range of δ56Fe and δ34S, implying a high-temperature magmatic origin for both elements. Overall, our study demonstrates that the coupling of Fe and S isotopes could be a useful tool for tracking the processes of sulfide mineralization in porphyry-related deposits.

Reduced fluids in porphyry copper-gold systems reflect the occurrence of the wall-rock thermogenic process: An example from the No.1 deposit in the Xiongcun district, Tibet, China

High fO2 conditions characterize the majority of global porphyry copper deposits and contain highly oxidized minerals, such as magnetite and anhydrite. In contrast, the No. 1 porphyry Cu–Au deposit in the Xiongcun district (Tibet, China) has abundant pyrrhotite, reduced fluids (CH4 ≫ CO2), and a relative lack of highly oxidized minerals, which are indicative of low fO2 conditions. Scanning electron microscopy, fluid inclusion, C–H–O–He–Ar isotopes, and whole-rock organic carbon contents and isotope analysis were used to constrain the evolution of ore-forming fluid, the origin of CH4 and metal deposition mechanisms for the No. 1 deposit. The He–Ar isotopic compositions (3He/4He = 0.11–0.96 Ra, 40Ar/36Ar = 418.7–2920.2) suggest that the ore-forming fluids predominantly derived from crust source with minor mantle input. The H–O isotopic analysis results (δ18OH2O = −1.8 to +5.2‰, δD = −106 to −89.9‰) indicate that the ore-forming fluids were derived from a magmatic source that mixed with some meteoric waters. The element compositions of zircons and fluid/melt inclusions from the mineralized Middle Jurassic quartz diorite porphyry reveal that the primary magma was characterized by high log fO2 (>NNO) conditions. The quartz diorite porphyry intruded into the carbon-bearing wall rocks produced abundant CH4 by thermal decomposition of organic matter (δ13CCH4 = −26.3 to −28.5‰), which changed the redox state of the porphyry copper system from oxidized to reduced condition. Ore elements were deposited via fluid boiling as a consequence of the rapid decrease in temperature and pressure.

Coarse muscovite veins and alteration in porphyry systems

Coarse muscovite veins and alteration occur in porphyry copper and porphyry molybdenum-copper systems within the Laramide arc in Arizona, as well as at the Yerington district in Nevada. This work describes coarse muscovite in veins and altered wall rock in porphyry systems in this region and documents mineral assemblages, mineral compositions, spatial and temporal relationships, and hydrogen isotopic compositions. Coarse hydrothermal muscovite is documented in the roots of porphyry Cu ± Mo systems, as well as in and above the ore bodies in porphyry Mo-Cu systems, and it is compared to coarse hydrothermal muscovite (greisen) in lode Sn-W-Mo systems.Basin and Range extension has exposed coarse hydrothermal muscovite in several Laramide and Jurassic porphyry Cu (±Mo) systems, at paleodepths of ~3 to 12 km: Miami-Inspiration, Sierrita-Esperanza, Copper Basin (Crown King), Granite Mountain (roots of the Ray porphyry system), Gunnison (Texas Canyon stock), Grayback (Kelvin-Riverside district), Sycamore Canyon, the New Cornelia mine (Ajo district), and two systems in the Yerington district. Muscovite is the dominant mica in these coarse muscovite veins and associated alteration, with common K-feldspar and albite (An00-06), common accessory hematite, rutile, pyrite, and apatite, and rare accessory chalcopyrite, fluorite, molybdenite, wolframite, and scheelite. Coarse hydrothermal muscovite yields δD compositions that suggest formation from fluids that are dominantly magmatic-hydrothermal in origin. Whole-rock compositions of coarse hydrothermal muscovite show common gains in K and loss of Ca ± Na. Coarse muscovite veins and alteration in porphyry copper systems postdate mineralized potassic veins and form too deeply to overlap with shallower acidic forms of alteration (sericitic, advanced argillic).Variation in mineral assemblage, mineral compositions, and mineralization of coarse hydrothermal muscovite correlate with the composition of Laramide stocks. Porphyry Mo-Cu systems contain coarse muscovite alteration assemblages with the highest mineral diversity and trace-element enrichment. Coarse muscovite veins and alteration in porphyry Mo-Cu systems related to stocks ranging from quartz monzonite to granite in composition form at shallower paleodepths and occur within and above the associated orebodies. In contrast, coarse muscovite veins and alteration associated with subalkaline porphyry copper systems occur at deeper levels, in some cases overlapping with the bottom of potassic alteration and the ore body but extending well into the roots of the system in the underlying granitoid cupola. In these latter systems, zones of coarse muscovite alteration typically are poorly mineralized and mineral assemblages are less varied. These characteristics suggest that coarse muscovite-forming fluids are predominately of magmatic-hydrothermal origin and exsolved from late-stage, fractionated magmas of the larger pluton that sourced porphyry stocks and dikes responsible for porphyry copper mineralization. In some instances, however, the exposed coarse muscovite alteration is associated with a petrologically unrelated, commonly more felsic, later intrusion, rather than being related to late exsolution of fluid from the same crystallizing stock or batholith.

Hydrothermal Aluminum-Phosphate-Sulfates in Ash from the 2014 Hydrothermal Eruption at Ontake Volcano, Central Honshu, Japan

Aluminum-phosphate-sulfates (APS) of the alunite supergroup occur in igneous rocks within zones of advanced argillic and silicic alteration in porphyry and epithermal ore environments. In this study we report on the presence of woodhouseite-rich APS in ash from the 27 September 2014 hydrothermal eruption of Ontake volcano. Scanning electron microscope coupled with energy dispersive X-ray spectrometer (SEM-EDS) and field emission (FE)-SEM-EDS observations show two types of occurrence of woodhouseite: (a) as cores within chemically zoned alunite-APS crystals (Zoned-alunite-woodhouseite-APS), and (b) as a coherent single-phase mineral in micro-veinlets intergrown with similar micro-veinlets of silica minerals (Micro-wormy-vein woodhouseite-APS). The genetic environment of APS minerals at Ontake volcano is that of a highly acidic hydrothermal system existing beneath the volcano summit, formed by condensation in magmatic steam and/or ground waters of sulfur-rich magmatic volatiles exsolved from the magma chamber beneath Mt. Ontake. Under these conditions, an advanced argillic alteration assemblage forms, which is composed of silica, pyrophyllite, alunite and kaolinite/dickite, plus APS, among other minerals. The discovery of woodhouseite in the volcanic ash of the Ontake 2014 hydrothermal eruption represents the first reported presence of APS within an active volcano. Other volcanoes in Japan and elsewhere with similar phreatic eruptions ejecting altered ash fragments will likely contain APS minerals derived from magmatic-hydrothermal systems within the subvolcanic environment. The presence of APS minerals within the advanced argillic zone below the summit vent of Ontake volcano, together with the prior documentation of phyllic and potassically altered ash fragments, provides evidence for the existence within an active volcano in Japan of an alteration column comparable to that of porphyry copper systems globally.

Petrogenesis and metallogenic implications of volcanic rocks from the Lawu basin, eastern Tibet: Insights into the intracontinental Eocene-Oligocene porphyry copper systems

Generally, porphyry Cu deposits are associated with the comagmatic porphyry (or subvolcanic)-volcanic systems of high magmatic H2O-fO2 conditions. The volcanic rocks, as the counterpart of the porphyries, thus can provide some significant insights into the fertility of the porphyries to some extent. For this reason, we have used the Lawu volcanic rocks and spatial-temporal closely-related porphyries in the newly discovered porphyry Cu prospects (e.g., Seli, Zongguo, Mamupu) in the southern segment of the Yulong intracontinental porphyry Cu belt to illustrate the relationship between the porphyries and volcanic rocks, petrogenesis of the volcanic rocks, and then to evaluate the ore potential of the porphyries based on the magmatic H2O-fO2 conditions.Both the Lawu volcanic rocks and Seli-Zonguo-Mamupu porphyries are shoshonitic and metaluminous, and have similar REE patterns, and Sr-Nd-Hf isotopic compositions, which suggest a comagmatic relation between the volcanic rocks and porphyries. An episodic magmatism model is proposed to explain the slightly younger age (~36–35 Ma) and less evolved nature of volcanic rocks than the porphyries. The Lawu volcanic rocks of mainly intermediate composition (SiO2 = 54.25–64.68 wt%) have high K2O (4.75–5.94 wt%) and high K2O/Na2O ratios (1.69–2.00), broadly similar to the coeval Yulong fertile granitic porphyries and the Nangqian mafic lavas. The (87Sr/86Sr)i and εNd(t) values, uniform zircon εHf(t) and δ18O values, and lack of inherited zircons of the Lawu volcanic rocks don’t support their formation by mixing between the mantle-derived Nangqian mafic lavas and crust-derived Yulong felsic porphyries or assimilation and fractional crystallization (AFC) of mafic magmas. They are characterized by high Ba/Th, Ba/La and listric-shaped normalized rare earth element profile with significantly negative Nb-Ta-Ti anomalies, and have high initial 87Sr/86Sr ratios (0.7071–0.7079) and low εNd(t) values (−5.71 to −3.05), and low zircon εHf (t) (−1.53 to 4.09) and clearly high δ18O values (6.67–8.42‰), suggesting that, they were probably formed by fractional crystallization (FC) of mantle-derived mafic magmas and originated from mantle domains modified by significant amount of H2O-rich marine sediments of the Paleo-Tethyan oceanic slab.Magmatic H2O contents calculated from deepest-crystallized amphiboles indicate that, the Lawu volcanic rocks and Zongguo porphyries have initial magmatic H2O contents as high as the Yulong fertile porphyries and typical porphyry Cu systems worldwide (commonly >4 wt% H2O). Magmatic fO2 (ΔFMQ) of the Lawu volcanic rocks (0.6–1.3, ave. 0.9 ± 0.1) and the Zongguo porphyries (0.9–1.7, ave. 1.4 ± 0.2) are clearly lower than the fertile porphyries in the giant Yulong deposit (ΔFMQ = 1.6–3.3, ave. 2.3 ± 0.5) and typical porphyry Cu deposits in the world (commonly ΔFMQ > 2). The slightly lower magmatic H2O contents and slightly higher magmatic fO2 of the Zongguo porphyries than the Lawu volcanic rocks were ascribed to variable degassing during magmatic evolution. These suggest that, in spite of the high magmatic H2O contents, the comagmatic porphyries (at least the Zongguo porphyries) of the Lawu volcanic rocks in the southern segment of the Yulong porphyry Cu belt are unlikely to produce large-scale porphyry Cu mineralization like the giant Yulong deposit, due to the low magmatic fO2 conditions.

Biotite compositional variations – A physicochemical approach to investigate crustal involvement and ore potential in Middle Jurassic plutonic rocks from the Malayer-Boroujerd Plutonic complex, W Iran

Biotite is the most frequent ferromagnesian phase in granitoids from the Malayer-Boroujerd Plutonic complex (MBPC), Zagros Orogen. The abundance of biotite decreases from granitoids towards dioritic intrusions, dioritic and gabbroic dykes to gabbroic intrusions coincident with increase in hornblende/biotite ratio. Primary magmatic biotite composition in the MBPC granitoids (Fe-rich type) changes from annite to siderophyllite, while Fe-depleted secondary biotites from the altered gabbros and gabbroic-dioritic dykes approximate to the phlogopite-eastonite series, and those from least-altered gabbros display average composition of these groups, however, extend towards high-Al end-members. Using Ti-in-biotite geothermometer, the crystallization temperature varies between 550 °C to 750 °C in MBPC granitoids (P 10–15 bars). Mode-III: biotite crystallized at higher oxidizing conditions (f(O2) > 10–10 bars) in gabbroic and dioritic dykes as well as gabbroic intrusions. The progressive decreases in f (O 2)–values and oxidizing conditions from I-type to S-type granites (above HM-to below QFM-buffer) reflect increasing incorporation of less-oxidized upper crustal materials during magmagenesis. In addition, the highest f (O2) in MBPC mafic intrusive rocks is consistent with a modest crustal contamination, if any. There is lack of evidence for porphyry type alteration zoning and subvolcanic porphyritic intrusions associated with porphyry copper system. The small patches of calc-silicate rocks from the NW-MBPC formed in contact with less oxidized and highly fractionated, crustal-derived granular granitoids and are less predisposed to develop Cu-Au mineraliza- tion. However, given the relatively higher f (O2)-values, the Middle Jurassic mafic intrusive rocks originated from the meta- somatised mantle-wedge beneath SaSiZ, appears to be the best candidate for further ore exploration programs in the area.

Magnetite Chemistry in the Porphyry Copper Systems of Kerman Cenozoic Magmatic Arc, Kerman, Iran

The present work attempts to characterize magnetite chemistry in some collisional (Chahfiruzeh and Keder; Miocene) and pre-collisional (Reagan and Daralou; Eocene–Oligocene) porphyry copper systems hosted in the Kerman Cenozoic Magmatic Arc (KCMA). EPMA results for magnetite samples show collisional deposits have higher average contents of Fe and Ca compared to those of pre-collisional systems. Nevertheless, magnetite samples of pre-collisional porphyry Cu systems contain elevated Si values. Highlightly, the frequency of V and Ca in magnetite have a direct relation with mineralization potential of studied deposits decreasing from Chahfiruzeh to Keder, Daralou and Reagan. V/Cr and V/Al in the analyzed magnetite samples show that Reagan porphyry system is quite different than other studied deposits. Moreover, average concentrations of Mo and Cu reach the highest in collisional and pre-collisional deposits, respectively. According to Fe/(V + Ti) diagram, most of the studied magnetite samples have re-equilibrated magmatic nature. Al + Mn versus V + Ti and Ni (Cr + Mn) versus Ti + V diagrams also confirmed that all magnetite samples from collisional and pre-collisional deposits are analogous to those reported previously for porphyry Cu systems. Regarding the condition of oxygen fugacity, based on Ni versus V diagram, the Reagan deposit (pre-collisional) has the highest oxygen fugacity, while the Daralou deposit (pre-collisional) and collisional deposits (Chahfiruzeh and Keder) exhibit moderate to high fugacity. Ti + V versus Al + Mn show that selected deposits were approximately formed in the same temperature conditions.

Hydrothermal fluid evolution in collisional Miocene porphyry copper deposits in Iran: Insights into factors controlling metal fertility

The Cenozoic Urumieh-Dokhtar Magmatic Belt (UDMB) in Iran is considered as one of the major Cu-bearing regions in the world with high potential for the occurrence of giant/large porphyry Cu ± Mo ± Au systems. In the UDMB, porphyry Cu mineralization is strictly associated with Miocene intrusions originating mostly from collisional-related partial melting of subduction-modified juvenile lower crust. In this study fluid inclusion coupled with laser Raman analysis is used to characterize and compare the fluid evolution of the large Meiduk deposit and smaller low grade porphyry copper systems (e.g., Keder, Sarkuh, and Iju) in the UDMB. It can be shown that enhancement of metal fertility of a porphyry system is controlled by several factors: (1) The salinity of early hydrothermal ore fluids; maximum ore-efficiency is seen in porphyry deposits showing supersaturation of NaCl in the pristine high temperature fluids (e.g., Meiduk), (2) Presence of CO2 and fast exhausting of CO2 vapor components during early stages of mineralization, (3) Temperature decrease especially during the main ore stage, and (4) High oxygen fugacity (near to the magnetite – hematite oxygen buffer) of the most primitive fluids in the early stages of hydrothermal system.

Geochemical characteristics of collisional and pre-collisional porphyry copper systems in Kerman Cenozoic Magmatic Arc, Iran: Using plagioclase, biotite and amphibole chemistry

Kerman Cenozoic Magmatic Arc (KCMA) located in the southeastern part of Urumieh-Dokhtar Magmatic Bet (UDMB) hosts a number of world class and moderate to small porphyry copper deposits. The present work compares the silicate chemistry (plagioclase, biotite and amphibole) of Miocene collisional porphyries (e.g., Chahfiruzeh and Keder) with Eocene-Oligocene pre-collisional porphyry systems (e.g., Reagan and Daralou) in the KCMA. P (<200Mpa) and H2Omelt, in the all studied porphyry systems is analogous to those reported previously for other porphyry copper systems. All analyzed biotites are high-Mg with subduction-related calc-alkaline and medium to high oxygen fugacity character. However, the Al2O3 and TiO2 contents of amphibole indicate that studied porphyries had a crust-mantle mixed source. Pre-collisional porphyry systems with lowest degree of Cu mineralization simultaneity show lowest An% and Al* compared with that of collisional porphyries. The amphiboles represent calcic – calc alkaline moderately oxidant magmatic character (NNO). The plagioclase chemistry indicates that magmatic system in the collisional deposit is relatively closed exhibiting the low rate of mixing compared with pre-collisional deposits. The biotite chemistry indicates temperature decreasing from Keder to Reagan, Daralou and Chahfiruzeh, respectively. The lack of regular temperature die line in the pre-collisional porphyry systems could be effective in low grade mineralization. High biotite halogen fugacity ratios (fH2O/fHF and fH2O/fHCl > 1) imply water-rich nature of studied porphyry systems. The relative enrichment F in the collisional porphyry deposits (e.g., Keder) could be linked with enrichment of Mg in these systems.

Experimental study of high to intermediate temperature alteration in porphyry copper systems and geological implications

Porphyry copper systems, which provide most of the world’s copper resource, are commonly associated with characteristic concentric zonation of alteration and mineralization. In-depth knowledge of the distribution and transport mechanism of elements in the alteration zones is essential for understanding the ore-forming processes. We employed flow-reaction apparatus to simulate the fluid-rock interactions during porphyry ore formation so as to investigate the mechanisms that govern the transport of elements and the development of zonation. The results indicate more heterogeneous distribution of elements in the experimental products at 450°C compared to those at lower temperatures, which implies a crucial role of temperature in controlling elements redistribution in hydrothermal systems. Heating advances potassic alteration and Ca leaching of wall rocks. To achieve the same degree of sodic alteration, it requires a higher concentration of Na+ in the fluid toward higher temperature. Temperature also facilitates the incorporation of Ti, Sr and Pb into silicate minerals through cation substitution. We infer from experimental results that from the center of intermediate to mafic volcanic wall rocks toward periphery, the contents of K and Ti should decrease and the contents of Ca, Zn and Mn should increase, whereas the trend for Si and Na could be non-monotonic. This study provides experimental and theoretical insights into a variety of vital geological observations, including anhydrite formation and the widespread development of potassic rather than sodic alteration in porphyry copper deposits.