Porphyry Dikes Research Articles

LA-ICP-MS Analysis of Crystallized Melt Inclusions in Olivine, Plagioclase, Apatite and Pyroxene: Quantification Strategies and Effects of Post-Entrapment Modifications

Abstract Melt inclusions represent a unique tool to reconstruct the composition and chemical evolution of silicate melts in magmatic systems. Laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS) is the most commonly used microanalytical technique to analyse crystallized melt inclusions without prior re-homogenization. Well-preserved melt inclusions can be quantified by subtracting the contribution of co-ablated host with a carefully selected internal standard. However, post-entrapment compositional re-equilibration commonly renders this task difficult, to the same degree as it would affect any quantification after prior re-homogenization. In this study, we first examine well-preserved, crystallized melt inclusions hosted in olivine, plagioclase, apatite, clinopyroxene and orthopyroxene from porphyry dikes and volcanic rocks to test various quantification strategies and evaluate the associated uncertainties, and then we use these strategies to quantify coarsely crystallized melt inclusions from gabbroic rocks at Marble Canyon (USA) and Laiyuan (China) that experienced severe post-entrapment modifications due to relatively slow cooling rates. The results demonstrate that even for well-preserved melt inclusions hosted in chemically complex minerals the uncertainty related to inclusion–host deconvolution can be rather high (up to 30‒50% for host-incompatible trace elements significantly above their limits of detection), though other uncertainties inherent to LA-ICP-MS analysis are relatively small (typically ≤5‒10%). The deconvolution-related uncertainty can be minimized to ca. 10% by (1) choosing whole rocks that are fresh and representative of magmatic liquids, (2) choosing the smallest possible spot size to ablate the melt inclusions, and (3) choosing a host endmember that is compositionally as similar as possible to the one ablated together with the inclusion. Results of coarsely crystallized melt inclusions from gabbroic rocks suggest that the range of elements affected by post-entrapment re-equilibration varies from intrusion to intrusion. Olivine-hosted melt inclusions from Marble Canyon appear to have diffusively lost Fe, Ti and Ca, whereas those from Laiyuan lost Fe, Na, Al, Ca, Ti and Y and gained V. However, the relative abundances of K, P, Rb, Sr, Zr, Nb, Mo, Cs, Ba, Ce, Ta, Pb, Th, U and ±Cu appear unchanged. Plagioclase-hosted melt inclusions from Marble Canyon are relatively well-preserved, whereas those from Laiyuan lost significant amounts of Fe, K, Mg, Mn, Rb and Co. Apatite-hosted melt inclusions seem well preserved with regard to most elements except for Cu. These results suggest that despite the post-entrapment modification of certain element concentrations and the associated difficulties in melt inclusion quantification, information on the approximate abundances of other elements that are invaluable for petrogenetic and metallogenic studies can still be retrieved from melt inclusions in gabbroic rocks using the LA-ICP-MS technique.

Hydrothermal fluid evolution at the Tiegelongnan porphyry-epithermal Cu(Au) deposit, Tibet, China: Constraints from H and O stable isotope and in-situ S isotope

The Tiegelongnan porphyry-epithermal Cu (Au) deposit is located in the Duolong porphyry district, north of the Bangong-Nujiang suture zone, Tibet, China. Mineralization is hosted by Jurassic sedimentary sandstone, and several phases of diorite and granodiorite porphyry dikes intruded between 123 and 116 Ma. The hydrothermal alteration is characterized by alunite-kaolinite-dickite overprinting quartz-muscovite-pyrite and biotite alteration zones. Porphyry chalcopyrite-pyrite ± molybdenite (Stage 1) mineralization is associated with biotite alteration. Porphyry chalcopyrite-bornite (Stage 2), and covellite (Stage 3) mineralization is associated with quartz-muscovite-pyrite alteration formed at ~121 Ma. Epithermal mineralization, consisting of pyrite-alunite (Stage 4), chalcopyrite-bornite-digenite (Stage 5), and tennantite-enargite (Stage 6), is hosted by two pulses of alunite-kaolinite breccia and veins at ~116 Ma and ~112 Ma. The fluid composition related to muscovite, with average δ18O of 8.9‰ and δD of −56‰, indicates a magmatic water origin. Fluid δ18O composition in equilibrium with quartz veins decreases from 6.7 to 2.3‰, which are likely the results of the water-rock isotopic exchange. Quartz fluid inclusions δD values between −50 to −84‰ are partly lower than that obtained from muscovite alteration fluids, which may result from H fractionation during fluid inclusions decrepitation. Epithermal stage fluid composition equilibrium with alunite yield δ18O from −1.2 to 2.7‰ and δD from −71 to −51‰, n = 11, which is comparable to the fluid composition equilibrium with Type Ⅰ kaolinite (hosting ores) with δ18O between −2.5 and 2.9‰, and δD between −72 and −51‰. It suggests that alunite and Type Ⅰ kaolinite formed with mixing between magmatic and high altitude Cretaceous meteoric water. Late Types Ⅱ and III kaolinite (filling alunite and quartz veins) fluid δ18O and δD values plot along a mixing line between magmatic and low altitude Cretaceous meteoric water, probably following the erosion and plateau subsidence. Porphyry mineralization sulfide stage 1 chalcopyrite and pyrite yield δ34S values between −5.8 and 0.9‰, with an average fluid δ34SH2S = −2.5‰ (n = 10), whereas stage 2 chalcopyrite returns δ34S values from −8.7 to −3‰ with an average δ34SH2S = −5.6‰ (n = 5). The lower fluid δ34SH2S values during sulfides stage 2 compared to that of stage 1, suggest that the chalcopyrite-bornite mineralization formed under higher oxidation conditions than that of the chalcopyrite-pyrite mineralization. Alunite yields δ34S values from 11 to 18.3‰ (n = 8), and the associated sulfide stage 4 pyrite have varying δ34S values from −32.2 to 5.4‰. Disequilibrium S isotope in alunite-pyrite pairs was likely because of rapid cooling and retrograde S isotope exchange during later sulfides emplacement. Epithermal mineralization sulfide Stage 4 S-equilibrated pyrite (−14.9 to −9.5‰), Stage 5 chalcopyrite (−11.6 to −8.2‰), and Stage 6 enargite (−5.4 to −2.6‰) display increasing δ34S values suggesting epithermal fluid compositions evolve towards more reducing conditions.

Triassic–Jurassic evolution of the eastern North China Craton: Insights from the Lushun-Dalian area, South Liaodong Peninsula, NE China

AbstractThe Lushun-Dalian area of the South Liaodong Peninsula, in NE China, located in the SE margin of the North China Craton (NCC) exposes a suite of Middle-Late Proterozoic low-grade metamorphic sedimentary rocks which can be divided into a lower competent layer, a middle incompetent layer, and an upper competent layer on the basis of lithology and deformation style. Two stages of deformation recorded both in the metasedimentary rocks and a magmatic complex intruded in them indicate that the Lushun-Dalian area is a key region to decipher the Triassic–Jurassic tectonic evolution of the eastern NCC. The earliest D1 deformation mylonitized the magmatic complex and thrusted it northeastward over the low-grade metasedimentary rocks, in which a series of NE-verging folds and NE-directed brittle thrust faults developed. The D2 deformation erased the D1 fabrics in the incompetent layer by a top-to-the-NW ductile shearing and refolded the D1 fabrics in the lower and upper competent units, producing a series of km-scale SW-plunging folds. New zircon secondary ion mass spectrometry and laser ablation–inductively coupled plasma–mass spectrometry U-Pb ages from the magmatic complex and the granite porphyry dikes intruded in it, combined with the unconformity between the low-grade metasedimentary rocks and the Early Cretaceous volcanic rocks, indicate that D1 and D2 occurred after 211 Ma and before the Early Cretaceous. The decrease of the deformation intensity of D1 and D2 from the Lushun-Dalian area toward the interior of the NCC in the NE and NW directions suggests that D1 was the structural response in the overriding plate to the NCC-South China Block convergence during the Late Triassic to Early Jurassic, and D2 was the structural response to the northwestward subduction of the Paleo–Pacific plate beneath the NCC in the Middle-Late Jurassic. The superimposition of D2 on D1 recorded a significant tectonic transformation from the nearly E-W–trending Tethysian domain to the NE-SW–trending Pacific domain.

Mineralogical and Isotopic Characteristics of Sodic-Calcic Alteration in the Highland Valley Copper District, British Columbia, Canada: Implications for Fluid Sources in Porphyry Cu Systems

AbstractThe Highland Valley Copper porphyry Cu (±Mo) district is hosted in the Late Triassic Guichon Creek batholith in the Canadian Cordillera. Fracture-controlled sodic-calcic alteration is important because it forms a large footprint (34 km2) outside of the porphyry Cu centers. This alteration consists of epidote ± actinolite ± tourmaline veins with halos of K-feldspar–destructive albite (1–20 XAn) ± fine-grained white mica ± epidote. The distribution of sodic-calcic alteration is strongly influenced by near-orthogonal NE- and SE-trending fracture sets and by proximity to granodiorite stocks and porphyry dikes. Multiple stages of sodic-calcic alteration occurred in the district, which both pre- and postdate Cu mineralization at the porphyry centers.The mineral assemblages and chemical composition of alteration minerals suggest that the fluid that caused sodic-calcic alteration in the Guichon Creek batholith was Cl bearing, at near-neutral pH, and oxidized, and had high activities of Na, Ca, and Mg relative to propylitic and fresh-rock assemblages. The metasomatic exchange of K for Na, localized removal of Fe and Cu, and a paucity of secondary quartz suggest that the fluid was thermally prograding in response to magmatic heating. Calculated δ18Ofluid and δDfluid values of mineral pairs in isotopic equilibrium from the sodic-calcic veins and alteration range from 4 to 8‰ and −20 to −9‰, respectively, which contrasts with the whole-rock values for least altered magmatic host rocks (δ18O = 6.4–9.4‰ and δD = −99 to −75‰). The whole-rock values are suggested to reflect residual magma values after D loss by magma degassing, while the range of hydrothermal minerals requires a mixed-fluid origin with a contribution of magmatic water and an external water source. The O-H isotope results favor seawater as the source but could also reflect the ingress of Late Triassic meteoric water. The 87Sr/86Srinital values of strongly Na-Ca–altered rocks range from 0.703416 to 0.703508, which is only slightly higher than the values of fresh and potassic-altered rocks. Modeling of those data suggests the Sr is derived predominantly from a magmatic source, but the system may contain up to 3% seawater Sr. Supporting evidence for a seawater-derived fluid entrained in the porphyry Cu systems comes from boron isotope data. The calculated tourmaline δ11Bfluid values from the sodic-calcic domains reach 18.3‰, which is consistent with a seawater-derived fluid source. Lower tourmaline δ11Bfluid values from the other alteration facies (4–10‰) suggest mixing between magmatic and seawater-derived fluids in and around the porphyry centers. These results imply that seawater-derived fluids can infiltrate batholiths and porphyry systems at deep levels (4–5 km) in the crust. Sodic ± calcic alteration may be more common in rocks peripheral to porphyry Cu systems hosted in island-arc terranes and submarine rocks than currently recognized.

Petrogenesis of Triassic granitoids in the Fengxian–Taibai ore cluster, Western Qinling Orogen, central China: Implications for tectonic evolution and polymetallic mineralization

The petrogenesis and tectonic setting of the Triassic granitoids, and their relationship with the coeval Au and Pb–Zn polymetallic mineralization in the Western Qinling Orogen (WQO) in central China remain debated. The Fengxian–Taibai Ore Cluster (FTOC) in the eastern part of the WQO hosts large-scale Triassic granitoids and numerous polymetallic deposits, providing an excellent opportunity to constrain the controversies mentioned above. In this work, we choose the Huahongshuping granodiorite pluton, Guji biotite monzogranite pluton, Taoerling biotite monzogranite dike, Dongtangzi granite porphyry dike and Dagou dioritic dikes in the FTOC for a combined study on the zircon U–Pb geochronology, whole-rock major, trace elements and Sr–Nd isotopes. LA–ICP–MS zircon U–Pb dating yields ages of ~225 Ma, ~224 Ma and 227–217 Ma for the Huahongshuping pluton, Guji pluton and granitoid dikes, respectively. All these granitoids have metaluminous and high-K calc-alkaline series affinities, and are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE) and depleted in high field-strength elements (HFSE) with a marked LREE/HREE fractionation ((La/Yb)N = 10.66–32.42), fractionation of LREE ((La/Sm)N = 2.72–6.60), and negative Eu anomalies (δEu = 0.45–0.99). They have initial Sr isotopic ratios of 0.7062–0.7082, and negative εNd(t) values of –9.42 to –8.09. The above features indicate that the FTOC granitoids were mainly derived from the partial melting of the Neoproterozoic meta-volcanic rocks induced by the decrease in pressure and the addition of water dehydrated from biotite, with direct contribution of mantle materials. Combined with previous data on sedimentation, deformation and metamorphism, and tectonics, we suggest that the WQO underwent the Early Mesozoic tectonic evolution from the subduction at 248–235 Ma, through syn-collision at 235–225 Ma, to post-collision at 225–195 Ma. Integrating with the ore geology, geochronology, mineralogy, and quartz and carbonate H–O, and in-situ S isotope data of sulfides from different mineralization stages at representative Au and Pb–Zn deposits in literatures, we propose a conceptive model that the post-collision process and granitic magmatic activity played an important role in the polymetallic mineralization within the FTOC.

The latest Aptian/earliest Albian age of the Kekura gold deposit, Western Chukotka, Russia: implications for mineralization associated with post-collisional magmatism

The Kekura gold deposit (76.2 t Au at 8.1 g/t) is situated in Western Chukotka, a region that hosts several Au, Ag, Cu, and Mo deposits and prospects. The Kekura deposit is related to the eponymous granite intrusion that is cut by porphyry dikes. The U-Pb zircon age of one of these dikes is 112 ± 1 Ma (2σ) that corresponds to the latest Aptian/earliest Albian. Both intrusion and dikes are hydrothermally altered and are cut by gold-quartz and molybdenite-quartz veins and stringers. Two molybdenite samples yield Re-Os model ages of 112.5 ± 0.6 and 112.3 ± 0.6 Ma (2σ). These Re-Os ages indicate the close temporal relationship between the molybdenite mineralization and the porphyry dikes. The age of the Kekura mineral system is similar to that of the post-collisional granitic plutons of the Anyui zone spatially scattered, between 140 and 210 km northwest of Kekura. We suggest that this temporal relationship may increase the likelihood of further discoveries of economic gold mineralization related to the currently underexplored Aptian post-collisional magmatic complexes of the Western Chukotka area.

Dikes of Quartz Porphyry and Their Role in the Formation of the Salmi Batholith (South Karelia)

New data are presented on the quartz porphyry dikes of the Salmi batholith belonging to anorthosite–rapakivi granite rock complexes. The relationships of various igneous rocks of the Salmi batholith and metasomatites were determined along with the isotopic U–Pb age by the LA–ICP–MS technique and the temperature of zircon formation by the Ti content.

Geochronology and fluid source constraints of the Songligou gold‐telluride deposit, western Henan Province, China: Analysis of genetic implications

AbstractThe Songligou gold‐telluride deposit, located in Songxian County, western Henan Province, China, is one of many gold‐telluride deposits in the Xiaoqinling‐Xiong'ershan district. Gold orebodies occur within the Taihua Supergroup and are controlled by the WNW F101 Fault, and the fault was cut across by a granite porphyry dike. Common minerals in gold orebodies include quartz, chlorite, epidote, K‐feldspar, calcite, fluorite, sericite, phlogopite, bastnasite, pyrite, galena, chalcopyrite, sphalerite, tellurides, gold, bismuthinite, magnetite, and hematite, and pyrite is the dominant sulfide. Four mineralization stages are recognized, including pyrite‐quartz stage (I), quartz‐pyrite stage (II), gold‐telluride stage (III), and quartz‐calcite stage (IV). This work reports the Rb–Sr age of gold‐telluride‐bearing pyrite and zircon U–Pb age of granite porphyry, as well as S isotope data of pyrite and galena. The pyrite Rb–Sr isochron age is 126.6 ± 2.3 Ma (MSWD = 1.8), and the average zircon U–Pb age of granite porphyry is 166.8 ± 4.1 Ma (MSWD = 4.9). (87Sr/86Sr) i values of pyrite and δ34S values of sulfides vary from 0.7104 to 0.7105 and −11.84 to 0.28‰, respectively. The obtained Rb–Sr isochron age represents the ore formation age of the Songligou gold‐telluride deposit, which is much younger than the zircon U–Pb age of the granite porphyry. Strontium and S isotopes, together with the presence of bastnaesite, suggest that the ore‐forming fluid was derived from felsic magmas with input of a mantle component and subsequently interacted with the Taihua Supergroup. Tellurium was derived from metasomatized mantle and was related to the subduction of the Shangdan oceanic crust and Izanagi plate beneath the North China Craton (NCC). This deposit is a part of the Early Cretaceous large‐scale gold mineralization in east NCC and formed in an extensional tectonic setting.

FORMATION OF THE KHEDOZEROBOLSHOZERSKAYA STRUCTURE OF THE BALTIC SHIELD IN THE LIGHT OF NEW GEOCHEMICAL AND GEOCHRONOLOGICAL DATA

Based on existing and newly obtained geological materials, complemeted by new petrogeochemical, geochronological and isotope-geochemical data, a new interpretation of the history of the geological development of the Khedozero-Bolshozerskaya structure in Central Karelia has been proposed. It is generally believed that the three strata identified within this structure consistently and successively overlap each other. This study established that sedimentation and volcanic activity were disparate both spatially and temporally, and occurred in different tectonic settings. The lower terrigenic and middle volcanogenic strata have tectonic contacts, while the upper volcanogenic stratum is a complex of thin transverse dikes. The accumulation of more ancient terrigenic sediments in the lower strata (of metagraywacke nature) occurred in the south of the structure, began no earlier than 2753 ± 6 Ma and was completed before the start of volcanic activity. The sedimentation conditions were similar to those of primitive oceanic island arcs. Volcanic activity (2712 ± 6 – 2703 ± 7 Ma), which is associated with the formation of the two upper strata, took place in the north of the structure, started with lava outflows of andesibasalts, andesites and dacites from the middle strata, and ended with the intrusion of upper-strata rhyolite porphyry dikes. The volcanism occurred in the settings similar to modern mature volcanic arcs. In the course of further geological development involving complex tectonic deformations and areal granitization, the southern terrigenic and the northern volcanogenic strata spatially converged.

Pervasive Miocene melting of thickened crust from the Lhasa terrane to Himalaya, southern Tibet and its constraint on generation of Himalayan leucogranite

The Oligocene to Miocene period is one of the most important stages during the formation and evolution of the Tibetan Plateau which generated the tectonic framework similar to the present. The rise of southern Tibet at this time was accompanied by widespread and intense tectonomagmatic activities. The Himalayan region was dominated by leucogranites considered as pure metasediments derived S-type granites, while the Lhasa terrane developed coeval thickened crust derived high Sr/Y calcalkaline rocks and metasomatized mantle derived ultrapotassic rocks. It is puzzling why the adjacent amalgamated regions show contrasting magmatic and dynamic process.This contribution presents a systematic study of newly found various dikes, including minette, aplite, granodiorite porphyry and granite porphyry, which are widespread from the southern Lhasa terrane, across the Yarlung Tsangpo suture zone (YTSZ) and into the Tethyan Himalaya. These dikes yield 16–11 Ma U-Th-Pb ages from zircon, titanite and monazite. The aplite and granodiorite porphyry dikes (high Sr/Y calcalkaline) and minette dikes (ultrapotassic) previal from the Lhasa terrane to the YTSZ. They are identical to the coeval high Sr/Y and/or ultrapotassic rocks from the Lhasa terrane, indicating a similar melting process of thickened lower crust and metasomatized lithospheric mantle. To the south of YTSZ, the middle Miocene granite porphyry in the Tethyan Himalaya exhibit high Sr/Y ratios (50–138), relatively low initial 87Sr/86Sr ratio (0.7064–0.7098), high εNd(t) value (−8.31 to −1.91) and positive εHf(t) value (+1.55 to +4.33), in contrast to the high 87Sr/86Sr and low εNd(t) Himalayan leucogranites. They were likely generated by partial melting of thickened lower crust dominated by amphibolite with significant contributions from juvenile magma. The two-mica granite of Bendui pluton from the Tethyan Himalaya is similar to the granite porphyry in element composition, while the highly evolved muscovite granite there resembles the Himalayan leucogranite. Together, the granite porphyry, Bendui two-mica granite and leucogranite from the Tethyan Himalaya show a transition trend in elemental and Sr-Nd isotopic compositions, implying that the high Sr/Y granite porphyry dikes may represent the more primitive magma of Himalayan leucogranites. In this case, the Himalayan leucogranites, at least in part, represent highly evolved I-type granites with extensive assimilation of ancient crustal materials instead of pure metasediment-derived partial melts. Therefore, we suggest that the thickened lower crust in southern Tibet underwent pervasive partial melting during Oligocene to Miocene as a result of the removal of subducting Indian plate.

Genesis of the Wulong gold deposit, Liaoning Province, NE China: Constrains from noble gases, radiogenic and stable isotope studies

The Wulong lode deposit contains over 80 tonnes of gold with an average grade of 5.35 g/t. It is one of the largest deposits in Dandong City, Liaoning Province in northeast China. Previous studies on the deposit focused on its geological characteristics, geochemistry, fluid inclusions, and the timing of gold mineralization. However, controversy remains regarding the origin of the ore-forming fluids and metals, and the genesis of the gold deposit. This paper presents zircon U–Pb and pyrite Rb–Sr ages and S, Pb, He, and Ar isotopic results along with quartz H and O isotopic data for all litho-units associated with the deposit. Laser ablation inductively coupled mass spectrometry measurements yielded zircon U–Pb dates for samples of pre-mineralization rocks like granite porphyry dike, the Sanguliu granodiorite, fine-grained diorite, and syn-mineralization diorite, as well as post-mineralization dolerite, and lamprophyre; their emplacement ages are 126 ± 1 Ma, 124 ± 1 Ma, 123 ± 1 Ma, 120 ± 1 Ma, 119 ± 2 Ma, and 115 ± 2 Ma, respectively. The pyrite Rb–Sr isochron age is 119 ± 1 Ma, indicating that both magmatism and mineralization occurred during the Early Cretaceous. The δ18OH2O values of ore-forming hydrothermal fluids from the quartz–polymetallic sulfide vein stage vary from 4.8‰ to 6.5‰, and the δDV-SMOW values are between −67.7‰ and −75.9‰, indicating that the ore-forming fluids were primarily magmatic. The noble gas isotope compositions of fluid inclusions hosted in pyrite suggest that the ore-forming fluids were dominantly derived from crustal sources with minor mantle input. Sulfur isotopic values of pyrite vary between 0.2‰ and 3.5‰, suggesting that S was derived from a homogeneous magmatic source or possibly from fluids derived from the crust. The Pb isotopic compositions of sulfides (207Pb/204Pb = 15.51–15.71, 206Pb/204Pb = 17.35–18.75, 208Pb/204Pb = 38.27–40.03) indicate that the Pb of the Wulong gold deposit is a mixture of crust and mantle components. Geochronological and geochemical data, together with the regional geological history, indicate that Early Cretaceous magmatism and mineralization of the Wulong gold deposit occurred during the rollback of the subducting Paleo-Pacific Plate, which resulted in lithospheric thinning and the destruction of the North China Craton (NCC), which indicates that the deposit is of magmatic–hydrothermal origin.

Alunite 40Ar/39Ar and Zircon U-Pb Constraints on the Magmatic-Hydrothermal History of the Zijinshan High-Sulfidation Epithermal Cu-Au Deposit and the Adjacent Luoboling Porphyry Cu-Mo Deposit, South China: Implications for Their Genetic Association

AbstractThe large Zijinshan high-sulfidation epithermal Cu-Au deposit, together with the adjacent Luoboling porphyry Cu-Mo deposit, constitutes a major porphyry-epithermal ore district in South China. Current debate centers on whether the Zijinshan and the adjacent Luoboling deposits are cogenetic or represent separate ore-forming events, which is a question of importance for exploration in the district. In this contribution, the magmatichydrothermal history of the relationship between Zijinshan and Luoboling is reconstructed based on new alunite 40Ar/39Ar ages from Zijinshan and zircon U-Pb ages of ore-related intrusions from both deposits. This study has been complemented by S isotope analysis on the dated alunite to assess their origin.Three types of coexisting alunite and sulfide assemblages exist at Zijinshan, namely, (1) alunite-quartz-covellite cemented breccias; (2) alunite-digenite veins and (3) banded alunite-pyrite veins. Their field occurrences and S isotope features suggest a magmatic-hydrothermal origin for alunite-quartz-covellite cemented breccias and alunite-digenite veins, whereas the origin of alunite-pyrite veins is likely to be related to magmatic steam. Given the intimate textural coexistence between sulfides and alunite, four undisturbed 40Ar/39Ar plateau ages obtained from alunite-quartz-covellite cemented breccia and alunite-digenite vein-type alunite define the timing of Zijinshan high-sulfidation mineralization from 102.86 ± 0.61 to 101.19 ± 0.60 Ma. These agree with bracketing zircon U-Pb ages of pre-ore dacite porphyry at 104.8 ± 0.9 Ma and 104.7 ± 0.5 Ma and a zircon U-Pb age of a post-ore granite porphyry dike at 99.5 ± 0.7 Ma. Combined with their field occurrences, the four alunite ages may imply episodic hydrothermal pulses and a possible time span of over 500 k.y. for the overall high-sulfidation mineralization. Two alunite samples from alunite-pyrite veins yield a slightly disturbed 40Ar/39Ar plateau age at 101.67 ± 0.61 Ma and an apparently undisturbed age at 99.91 ± 0.59 Ma, probably reflecting partial or complete thermal resetting related to the coeval granite porphyry dikes. At Luoboling, zircon U-Pb analysis yields an age of 133.6 ± 1.1 Ma for a dark, ore vein-bearing quartz-diorite porphyry sample and confirms the petrographic observation that they are xenoliths of early wall rocks for the porphyry mineralization. A granodiorite porphyry sample with abundant A-veins is interpreted as an intermineral porphyry phase, dated at 106.5 ± 1.4 Ma. This age is interpreted as the upper limit for porphyry Cu-Mo mineralization and agrees well with previously reported molybdenite Re-Os and biotite 40Ar/39Ar ages, which collectively suggest porphyry Cu-Mo mineralization and potassic alteration at Luoboling were formed within the interval between ca. 106 and 104 Ma.Taken together, the age of the Zijinshan high-sulfidation mineralization is substantially younger than the porphyry mineralization at Luoboling, and we conclude that there is no direct genetic link between the two deposits. However, the Zijinshan Cu-Au deposit is temporally associated with the deep porphyritic granodiorite, which gives zircon U-Pb ages from 102.1 ± 0.8 to 101.8 ± 0.5 Ma, overlapping with the alunite 40Ar/39Ar ages for the high-sulfidation mineralization. This finding has important implications for the ongoing exploration for the two mineralization types in the ore district and elsewhere.

Hydrothermal Alteration, Mineralization, and Structural Geology of the Zijinshan High-Sulfidation Au-Cu Deposit, Fujian Province, Southeast China

Zijinshan is a world-class Au-Cu district located in southwest Fujian Province, southeast China. It contains a diverse array of ore deposits hosted by the Zijinshan granite complex and surrounding volcano-sedimentary rocks. Associated deposits include high-sulfidation epithermal gold-copper, intermediate-sulfidation epithermal polymetallic silver-base metal, and porphyry molybdenum-copper deposits. The Zijinshan high-sulfidation Au-Cu deposit is located in the middle of the Zijinshan district, and ore zones are hosted in, below, and adjacent to the Zijinshan lithocap. The host rocks are part of a Jurassic granite complex emplaced between 165 and 157 Ma. 40Ar/39Ar age spectra of hydrothermal alunite indicate that hydrothermal activity at Zijinshan occurred at 102.86 ± 0.61 to 101.19 ± 0.60 Ma and was associated with dacite porphyry dikes that intruded at 104.8 ± 0.9 Ma. Hydrothermal activity produced pervasive silicic and advanced argillic alteration assemblages that are zoned from a central, massive quartz alteration domain, outward to massive quartz-dickite, disseminated quartz-alunite ± dickite, and disseminated quartz-muscovite-dickite zones. Copper sulfides occur in hydrothermally cemented breccias, veins, and minor disseminations in granite; copper minerals include covellite, digenite, and minor enargite. Oxide Au ores are associated with strongly weathered silicic alteration in the upper parts of the deposit. A set of subvertical NE-striking faults were active prior to mineralization at Zijinshan. During mineralization, a NW-striking fault system controlled the emplacement of veins, breccias, and dacite dikes. Kinematic indicators show that most of the NW-striking faults were active as normal oblique faults during mineralization. Postmineralization ENE-striking dextral strike-slip faults are associated with minor conjugate NNW-striking faults. The premineralization structures at Zijinshan likely formed during NW-directed compression. Synmineralization structures formed under subvertical contraction and subhorizontal NNE-plunging extension. Postmineralization structures developed in a strike-slip regime defined by a NW-oriented axis of compression and NE-trending axis of extension. Changes in the styles of pre- and synmineralization structures are related to fluctuations in the regional stress regime. The transition from compression to extension was fundamental to mineralization. Postmineralization faults are related to transient local stress regime variations at the deposit scale.

Chemical and boron isotope compositions of tourmaline in the Hadamiao porphyry gold deposit, Inner Mongolia, China

The Hadamiao porphyry gold deposit in Inner Mongolia, China, is located in the northern margin of the North China Craton (NCC). Tourmaline is a common mineral in hydrothermal veins and granitic rocks contemporaneous with the mineralized granite porphyry in this deposit. The tourmaline can be divided into six types, including magmatic tourmaline (Tur-M1) in granite porphyry dikes, magmatic-hydrothermal transitional tourmaline (Tur-M2) in nodular granitic rocks, and hydrothermal tourmalines in early tourmaline veins (Tur-V1), gold-bearing quartz-tourmaline-chalcopyrite veins (Tur-V2), breccia cements (Tur-V3) and late carbonate-quartz veins (Tur-V4). The gold mineralization is closely related to the occurrence of Tur-V2.Almost all the tourmalines from the Hadamiao deposit belong to the alkali group and show a schorlitic–dravitic composition, except for some Tur-M1 belonging to uvite. The concentrations of most elements in different tourmaline types overlap. Tur-M1 has higher Ca, W, Th, U, Pb, Mn, and Zr contents than other types, and the hydrothermal tourmalines (V1–V4) show slightly higher Ba, Cu and Y contents than Tur-M1 and Tur-M2. The concentration of Au in Tur-M1 and Tur-M2 can reach up to 80 ppb and is higher than that in Tur-V1; the Au content in Tur-V2 is an order of magnitude higher than that in other stages of tourmaline. Systematic contrasts in major and trace element compositions between magmatic and hydrothermal tourmalines are attributed to the assimilation of the country rock by the magmatic-hydrothermal system.The six types of tourmaline exhibit variable δ11B values ranging from −8.7‰ to +21.2‰. Tur-M1 shows the highest δ11B values of +15.1‰ to +21.2‰, and Tur-V2 yields the lowest values ranging from−8.7‰ to −5.4‰. The boron probably came from two sources with different isotopic compositions, one with high positive δ11B values and the other with negative δ11B values. Magmatic hydrothermal fluids could have provided isotopically light boron for hydrothermal tourmalines, whereas the heavy boron in magmatic tourmaline was probably derived from marine carbonates that are widespread in the study area. The magma related to the mineralization and the one containing magmatic tourmaline may have experienced different degrees of interaction with wall rocks, resulting in the diversity of δ11B values in Hadamiao tourmaline.

LA-ICP-MS zircon U-Pb age and Hf isotope data from the granitic rocks in the Iwakuni area, Southwest Japan: re-evaluation of emplacement order and the source magma

U-Pb-Hf isotopes of zircons from the granitic intrusive rocks in the Iwakuni area of Southwest Japan are determined by LA-ICP-MS analysis. The Gamano Granodiorite belonging to the Older Ryoke Granites of the Ryoke Belt yielded U-Pb mean age of 92.3 ± 2.6 Ma (MSWD = 1.8). The Namera Granite belonging to the Younger Ryoke Granites of the Ryoke Belt yielded U-Pb mean age of 106.3 ± 1.6 Ma (MSWD = 2.3). The Shimokuhara Granite belonging to the Hiroshima Granites of the San-yo Belt yielded U-Pb mean age of 103.8 ± 1.5 Ma (MSWD = 0.32). A granite porphyry sample taken from about 15 m thick dike discordantly intruded into pelitic schist of the Ryoke metamorphic rocks yielded U-Pb mean age of 92.5 ± 1.6 Ma (MSWD = 0.63). Hf isotope data, obtained from the same dated zircon grains, show initial 176Hf/177Hf isotopic ratios referred to as “eHf(t)” generally ranging between–4.8 and +1.1 except an anomalously low value (–15.0) from an inherited core. The eHf(t) values, calculated with average crustal 176Lu/177Hf ratio of 0.015, correspond to Hf isotope crustal model ages between 1474 to 1087 Ma (Mesoproterozoic). The Gamano Graodiorite has long been regarded to be a syn-tectonic intrusion, but the zircon ages and eHf(t) values are similar to those of the post-tectonic granite porphyry dike. The results of this study together with available U-Pb zircon ages of granitic rocks in the Iwakuni area reported elsewhere reveal that the order of emplacement indicated by the U-Pb zircon ages are much discrepant with the traditional classification, the Older Ryoke, the Younger Ryoke. It is better to refrain from the use of “Older” and “Younger” for the collective names of the granitic intrusive bodies.

40Ar/39Ar and U-Pb constraints on the age of the Zaozigou gold deposit, Xiahe-Hezuo district, West Qinling orogen, China: Relation to early Triassic reduced intrusions emplaced during slab rollback

The West Qinling orogen in central China contains dozens of world-class sediment-hosted gold deposits, but the ages of these deposits and their relation to regional tectonism, magmatism, and metamorphism are poorly understood. Zaozigou, in the western zone of the Xiahe-Hezuo district, is one of the larger deposits in the orogen with 142 t Au at an average grade of 2.69 g/t. It is hosted in a middle Triassic turbidite sequence and swarm of crosscutting ilmenite-series intermediate to felsic dikes. Ore bodies are localized along subvertical, NE- or NS-striking faults. Gold occurs in fine-grained arsenian pyrite and arsenopyrite disseminated in hydrothermally altered sedimentary and igneous rocks and in subordinate sulfide-quartz-ankerite veinlets. Such mineralization is locally crosscut and overprinted by NW-striking, gently-dipping, auriferous stibnite-quartz veins.Zircon grains from gold mineralized granodiorite and quartz diorite porphyry dikes have U-Pb ages of 248.9 ± 1.4 to 244.8 ± 1.4 Ma, whereas a post-ore diorite porphyry dike has a U-Pb age of 237.5 ± 1.4 Ma. Two samples of sericite associated with disseminated arsenian pyrite and arsenopyrite in sedimentary and igneous host rocks yield plateau ages of 245.6 ± 1.0 Ma and 242.1 ± 1.0 Ma, that are bracketed by the zircon U-Pb ages on pre- and post-ore dikes. Field relationships and geochronological results, thus, constrain magmatism and gold mineralization at the early Triassic.Previous work has shown that the Au-Cu skarns and intrusion-related gold deposits exposed at a deeper level in the eastern zone of the Xiahe-Hezuo district are genetically related to early-middle Triassic ilmenite series intrusions generated in an extensional back-arc setting during rollback of the subducted paleo-Tethyan slab. Our U-Pb and 40Ar/39Ar data show that the large Zaozigou gold deposit in the western zone of the Xiahe-Hezuo district is broadly coeval with the Au-Cu skarn and intrusion-related gold deposits in the eastern zone. This temporal consistency provides additional support for a genetic link between gold mineralization and magmatism in the Xiahe-Hezuo district. As such, this district may represent the only reduced intrusion related gold system in the West Qinling Orogen.

Two-event lode-ore deposition at Butte, USA: 40Ar/39Ar and U-Pb documentation of Ag-Au-polymetallic lodes overprinted by younger stockwork Cu-Mo ores and penecontemporaneous Cu lodes

The ore-genesis model for world-class deposits of the Butte mining district, Montana, USA, is deep pre-Main Stage porphyry Cu-Mo and overlying Main Stage Ag-Zn-Cu zoned-lode deposits, both of which formed from hydrothermal fluids driven by minor volumes of rhyolitic magma. The lode-specific model is that hydrothermal processes diminished in intensity outward from district center along lode veins, synchronously forming metal zones. The accepted models are controverted by new geologic and multi-method geochronologic studies.The new data reveal the following sequence of events: (1) Thermal study of country rock indicates that the 76.9-Ma Butte Granite cooled to 350–400 °C by 4 m.y. after emplacement. (2) Five quartz porphyry rhyolite dikes were emplaced at 67–65 Ma and another at 60 Ma (SHRIMP U-Pb) into the cooled Butte Granite without resetting 40Ar/39Ar ages in country rock. (3) Fifty-eight white mica and K-feldspar samples from alteration envelopes adjacent to Ag-Au-polymetallic lodes in outer parts of the district, Zn-rich lodes in intermediate parts, and Cu-rich lodes in the district center yield 40Ar/39Ar ages of 73–70 Ma for Ag-rich lodes, 65–64 Ma for Cu-rich lodes, and complex age spectra of 69–65 Ma for Zn-rich lodes.The data show that Ag-Au-polymetallic lodes occupied cross-district fractures by about 73 Ma, forming the greater Butte mining district. At 67–65 Ma, minor quartz porphyry dikes were emplaced into central and eastern parts of the rejuvenated fracture system but without evidence of related cupola or volcanic rocks or of thermal disturbance in the country rock. At 64.5 Ma, overlapping hydrothermal cells formed two stockwork Cu-Mo domes in deep parts of the fracture system. At 65–64 Ma and closely related to late-stage stockwork Cu-Mo activity, a penecontemporaneous hydrothermal pulse formed a high-sulfidation hydrothermal plume that (1) utilized the large re-opened fractures to cannibalize and remobilize Cu from autologous, stockwork, and older Ag-Au-polymetallic lodes, (2) deposited the rich, high-sulfidation Cu lodes, and (3) mobilized metals from early Ag-Au-polymetallic veins in middle parts of the district, transported the metals outward and redeposited them, enriching early veins, especially in the intermediate Zn plus Cu areas.Metals zones in lodes of the Butte district are the result of an intensely focused, Cu-rich hydrothermal plume that variably reworked the center of significantly larger, 10 m.y. older, Ag-Au-polymetallic lodes.

Petrogenesis and Assembly of the Don Manuel Igneous Complex, Miocene–Pliocene Porphyry Copper Belt, Central Chile

The 4·0–3·6 Ma Don Manuel igneous complex (DMIC), central Chile, provides a window into igneous processes involved in magma genesis associated with porphyry-style copper mineralization. This study uses petrographic, petrological, geochemical and isotopic data to examine the evolution of magmas from the mid- to lower-crustal source region to shallow emplacement. The data provide evidence for progressive oxidation of magma during differentiation and ascent, fractionation of Cl from S through degassing, and the late-stage, near-solidus removal of Cl from the system. Magmas of basaltic andesite to rhyolite composition were produced by polybaric differentiation of hydrous parental mafic magmas. Variations in crustal differentiation depths led to variable suppression of plagioclase saturation that is recorded in distinctive strontium versus anorthite evolution patterns. Hydrous, derivative magmas generated over a wide range of pressures were episodically emplaced into the shallow crust at depths between 3·5 and 5 km. Intermediate porphyry dikes closely associated with copper mineralization contain diverse crystal cargoes indicating significant magma mixing. These crystal cargoes represent samples of crystal mush entrained from different depths, as well as crystals originating in different magmas and crystals grown in situ from hybridized magmas. Mafic enclaves containing plagioclase and amphibole compositions that match those of the basaltic andesites occur within biotite tonalite, testifying to magma mingling during ascent. Sulfur and chlorine contents of apatite within the different DMIC units record variable degassing and decoupling of volatile components with sulfur showing variations of three orders of magnitude compared with one order of magnitude for chlorine. The hypabyssal nature of the DMIC affords a detailed, integrated record of magmatic differentiation processes occurring within trans-crustal magmatic systems of the sort thought to characterize many crustal arc settings and play a fundamental role in driving porphyry-style copper mineralization.

Genesis of the Weiquan Ag–Polymetallic Deposit in East Tianshan, China: Evidence from Zircon U–Pb Geochronology and C–H–O–S–Pb Isotope Systematics

The Weiquan Ag–polymetallic deposit is located on the southern margin of the Central Asian Orogenic Belt and in the western segment of the Aqishan–Yamansu arc belt in East Tianshan, northwestern China. Its orebodies, controlled by faults, occur in the lower Carboniferous volcano‐sedimentary rocks of the Yamansu Formation as irregular veins and lenses. Four stages of mineralization have been recognized on the basis of mineral assemblages, ore fabrics, and crosscutting relationships among the ore veins. Stage I is the skarn stage (garnet + pyroxene), Stage II is the retrograde alteration stage (epidote + chlorite + magnetite ± hematite ± actinolite ± quartz), Stage III is the sulfide stage (Ag and Bi minerals + pyrite + chalcopyrite + galena + sphalerite + quartz ± calcite ± tetrahedrite), and Stage IV is the carbonate stage (quartz + calcite ± pyrite). Skarnization, silicification, carbonatization, epidotization, chloritization, sericitization, and actinolitization are the principal types of hydrothermal alteration. LA–ICP–MS U–Pb dating yielded ages of 326.5±4.5 and 298.5±1.5 Ma for zircons from the tuff and diorite porphyry, respectively. Given that the tuff is wall rock and that the orebodies are cut by a late diorite porphyry dike, the ages of the tuff and the diorite porphyry provide lower and upper time limits on the age of ore formation. The δ13C values of the calcite samples range from −2.5‰ to 2.3‰, the δ18OH2O and δDV‐SMOW values of the sulfide stage (Stage III) vary from 1.1‰ to 5.2‰ and −111.7‰ to −66.1‰, respectively, and the δ13C, δ18OH2O and δDV‐SMOW values of calcite in one Stage IV sample are 1.5‰, −0.3‰, and −115.6‰, respectively. Carbon, hydrogen, and oxygen isotopic compositions indicate that the ore‐forming fluids evolved gradually from magmatic to meteoric sources. The δ34SV‐CDT values of the sulfides have a large range from −6.9‰ to 1.4‰, with an average of −2.2‰, indicating a magmatic source, possibly with sedimentary contributions. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of the sulfides are 17.9848–18.2785, 15.5188–15.6536, and 37.8125–38.4650, respectively, and one whole‐rock sample at Weiquan yields 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.2060, 15.5674, and 38.0511, respectively. Lead isotopic systems suggest that the ore‐forming materials of the Weiquan deposit were derived from a mixed source involving mantle and crustal components. Based on geological features, zircon U–Pb dating, and C–H–O–S–Pb isotopic data, it can be concluded that the Weiquan polymetallic deposit is a skarn type that formed in a tectonic setting spanning a period from subduction to post‐collision. The ore materials were sourced from magmatic ore‐forming fluids that mixed with components derived from host rocks during their ascent, and a gradual mixing with meteoric water took place in the later stages.

Zircon U-Pb geochronology, Hf isotopes and geochemistry of intrusive rocks in the Simorgh prospecting area, Lut Block, eastern Iran: petrogenesis and geological implications

The Simorgh prospecting area is located in the central part of the Lut Block in eastern Iran. The Lut Block, the eastern part of the Central Iranian Microcontinent (CIM), has a complex tectonic evolution and is characterized by extensive magmatic activities with a range of geochemical signatures. The Simorgh intrusions have features typical of calc-alkaline to high-K calc-alkaline rocks, metaluminous to slightly peraluminous and formed in a volcanic arc setting. Mineralization in the area is believed to be related to these intrusions. To better understand the petrogenesis of these intrusions, we report zircon U-Pb dating and Hf isotopes and whole rock geochemistry of two granites and one diorite. Age dating using Zircon U-Pb method on pyroxene diorite porphyry stock and two granite porphyry dikes revealed 24.85 ± 0.51 Ma, 25.37 ± 0.56 Ma and 25.94 ± 0.76 Ma ages respectively (late Oligocene, Chattian). Field observations and U-Pb dating indicate that there are at least two stages of emplacement: pre-mineralization intrusions (diorite porphyry) and syn-mineralization intrusions (granite porphyry, granodiorite porphyry and pyroxene diorite porphyry). Mantle-normalized, trace-element spider diagrams display enrichment in large ion lithophile elements (LILE) and light rare earth elements ((La/Yb)N = 9.26–14.48), and depletion in high field strength elements (HFSE) and heavy rare earth elements, as well as negative Eu anomalies (Eu/Eu* = 0.49–0.91). Zircon Hf isotope data from the Simorgh intrusive rocks show largely positive eHf(t) (average = 3.6 ± 2.0). Hf model ages are ranging from 720 to 1320 Ma, indicating its derivation from a relatively juvenile source. The presence of zircon xenocrysts may prove its contamination by older continental crust (Siderian).