Felsic Porphyry Research Articles

Oligocene magmato-tectonic events influential in the development of Pb-Zn, Ag, and Au mineralization at the Plomosas deposit, southwestern Sierra Madre Occidental, Mexico

ABSTRACT The Plomosas deposit is the product of a sequence of overlapping events of Pb–Zn, Ag, and Au low sulfidation mineralization that occurred in Mexico’s southwestern Sierra Madre Occidental (SMO). Field observations, detailed petrography, and U–Pb age dating of the entire stratigraphic sequence were used to characterize the magmatic evolution of the district and to obtain new insights into the evolution of the SMO. The stratigraphy at Plomosas is distinct from other areas to the north as it lacks Jurassic marine units, Cretaceous continental volcanic successions as well as Cretaceous and Palaeocene batholiths. The local basement is composed of undeformed Late Jurassic volcaniclastic units and coeval felsic intrusions. Early to late Oligocene silicic ignimbrites, intermediate lava flows, and occasional basaltic flows unconformably overlie the Jurassic basement. Several Oligocene intermediate to felsic porphyry bodies and rhyolitic domes crosscut the stratigraphy. The upper volcanic sequence is separated into two packages by an angular unconformity. Mineralization produced an early Pb–Zn-mineralized hydrothermal fault breccia, followed by Cu–Ag veins and late Au veins. The Pb–Zn mineralized breccia is hosted by ~30 Ma Oligocene volcanic units that form the basal sequence. The Cu–Ag and Au-rich veins overlap and crosscut the early hydrothermal features and cut the late Oligocene volcanic cover that unconformably overlie the basal sequence. Our results highlight the importance of episodes of Oligocene magmatism to the genesis of mineralization in the southwestern SMO specifically through a genetic link between Pb–Zn veins and intermediate Oligocene intrusive bodies, and of Ag and Au veins with late Oligocene volcanism.

Geochronology, Geochemistry, and Implications of Aplite Dyke in the Giant Jiama Porphyry Copper System, Tibet

AbstractJiama is a giant, high‐grade porphyry copper system in the Gangdese metallogenic belt, Tibet. Multistage intermediate–felsic porphyries intruded in this deposit, some of which are strongly associated with copper‐polymetallic mineralization. These ore‐bearing porphyries include monzogranite, granodiorite, and quartz diorite porphyries. A new granite aplite dyke was found in the south of Jiama. Its age, genesis, and relationship with ore‐related magmatism are obscure. Here, its emplacement age and petrogenesis were determined using mineralogy, zircon U‐Pb dating, geochemistry, and Sr‐Nd‐Pb isotope studies. The zircon LA‐ICP‐MS U‐Pb age of the aplite dyke is 16.66 ± 0.21 Ma (n = 14, MSWD = 0.66), earlier than that of the ore‐bearing porphyries (∼15 Ma) in Jiama. Furthermore, the aplite exhibits high amounts of silicon (SiO2 = 73.39%–74.74%), potassium (K2O = 5.12%–6.61%), aluminum (Al2O3 = 14.25%–14.69%), and light/heavy rare earth elements (LREE/HREE = 12.12–16.19) as well as negative europium (δEu = 0.47–0.72) and weak negative cerium anomalies (δCe = 0.84–0.93). The aplite dyke is characteristic of metaluminous–peraluminous I‐type granite, which is rich in large‐ion lithophile elements (Rb, Ba, Th and U) and depleted in high‐field‐strength elements (Nb, P and Ti). The aplite dyke and ore‐bearing porphyries in the Jiama deposit are the results of a partial melting of the juvenile lower crust, according to whole‐rock geochemistry and Sr‐Nd‐Pb isotope data, but the dyke and ore‐bearing porphyries were emplaced from the same magma chamber at different times. Thus, the aplite dyke shows the composition of the early evolution stage of shallow magma in the Jiama deposit and is the product of rapid condensation and crystallization.

Petrogenesis and tectonic implications of Eocene-Oligocene potassic felsic porphyries in the Sanjiang Region, southeastern Tibetan Plateau

• Eocene-Oligocene (36–33 Ma) potassic felsic rocks occur in the Sanjiang Region. • Eocene potassic felsic rocks produced by partial melting of mafic rocks in a thickened lower crust . • Eocene magmatic activities caused by post-orogenic delamination of lithospheric mantle and asthenosphere upwelling in the Eocene. This study presents zircon U-Pb dating and Hf isotope, whole-rock geochemistry and Sr-Nd isotopes of the potassic felsic porphyries from western Yunnan in the Sanjiang Region. Zircon U-Pb dating reveals that quartz-monzonite porphyry, granite porphyry, and adakitic granite porphyry were emplaced during 36–33 Ma. Geochemical data show that quartz-monzonite and granite porphyries have enriched LREE ((La/Yb) N = 39–67)and depleted HFSEs (e.g. Nb, Ta, P and Ti), with weak Eu anomalies. Moreover, they display similar 87 Sr/ 86 Sr ratios of 0.7068–0.7070 and negative εNd(t) values of −6.7 to −5.4, with zircon εHf(t) values of −5.65 to 3.06. The adakitic granite porphyries have high Sr (1273–1377 ppm), low Y (9.2–10.7 ppm), and low Yb (0.9–1.0 ppm) contents and high Sr/Y (128–138) and (La/Yb) N (17.7–19.8) values, typical of adakite rocks. In addition, these adakitic granite porphyries possess negative whole-rock εNd(t) values −0.2 to −0.3 and positive zircon εHf(t) values 2.7–6.0, associated with low Mg#, MgO, Ni and Cr contents. These geochemical features suggest that three kinds of potassic felsic porphyries could be produced by partial melting of mafic rocks (e.g., amphibolite) in a thickened lower crust. Our data, combined with previous studies and regional geology, suggest that delamination of lithospheric mantle and asthenosphere upwelling beneath the Sanjiang Region, both contributed to Cenozoic magmatic activities.

Timescale of a magmatic-hydrothermal system revealed by 40Ar\u201339Ar geochronology: the Mio-Pliocene Campiglia Marittima system (Tuscany, Italy)

Petrology and timing of magmatic-hydrothermal systems and the linkage between plutonic and volcanic domains are central topics in geosciences, because of broad implications for natural hazards and exploitation of natural resources. We investigated by the 40Ar–39Ar method the timescale of a well-characterized natural example, the Mio-Pliocene Campiglia Marittima magmatic-hydrothermal system (Tuscany, Italy). 40Ar–39Ar data from pristine and homogeneous trioctahedral micas and sanidine from the plutonic-hydrothermal-subvolcanic-volcanic sequence (from the Botro ai Marmi Granite to the San Vincenzo Rhyolite) record crystallization ages and define a temporal sequence lasting 973 ± 43 ka, starting from 5.409 ± 0.043 Ma. K-feldspar from mafic and felsic porphyries, unlike micas, are affected by submillimetre, micropore laden, alteration domains consisting of secondary K-feldspar and albite, and yielded staircase-shaped age spectra, compatible with a ternary mixing. Results document that the San Vincenzo Rhyolite consists of two diachronous batches, the first emplaced at 5.0024 ± 0.0062 Ma, closely following emplacement of mafic porphyries, the second at 4.4359 ± 0.0045 Ma. Bulk of hydrothermal deposits, consisting of skarns and associated Zn–Pb(-Ag) mineralization predating Fe–Cu ore, formed within the first ~ 400-ka lifetime of the whole sequence and was closely followed by the first eruption which should have run out most of the ore-forming potential of the system.

Petrogenesis and metallogenic implications of the Miocene granite porphyry in the Jiama Cu-polymetallic deposit, Gangdese belt, South Tibet

Geochronology, geochemical and isotopic studies were carried out on granite porphyry from the Jiama Cu-polymetallic deposit, Gangdese porphyry copper deposit belt, to constrain the genesis and metallogenic potential. LA-ICP-MS zircon UPb dating indicates that the granite porphyry was emplaced at 14.9 ± 0.3 Ma to 16.0 ± 0.3 Ma, which is consistent with other felsic porphyries in the deposit. The granite porphyry shows high-K calc-alkaline and peraluminous signatures. In addition, the granite porphyry shows enrichment in large ion lithophile elements and depletion in high field strength elements. However, it has low Sr (76.13–401.90 ppm) contents, which is different from other felsic porphyries with high Sr contents in the deposit. The initial 87Sr/86Sr (0.7055 to 0.7070) and εNd(t) (−0.53 to -2.37) values of the granite porphyry are close to those of other felsic porphyries in this deposit. The granite porphyry and diorite porphyry have almost identical zircon δ18O (5.71–5.82‰) and εHf(t) (4.1–3.97) values. These results suggest that all the felsic porphyries in the Jiama deposit were derived from the same source. Geochemical characteristics show that the magma of the granite porphyry experienced fractional crystallization of hornblende and plagioclase, whereas other felsic porphyries experienced hornblende-dominated fractional crystallization. Zircon Ce4+/Ce3+ values indicate the high oxygen fugacity of granite porphyries, which is key for CuMo mineralization.

Albite ± Actinolite-Altered Porphyry Dykes in Archean Gold Deposits of the Boulder Lefroy-Golden Mile Fault System, Yilgarn Craton, Western Australia: Petrography, Chronology, and Comparison to Canadian Albitites

The Boulder Lefroy-Golden Mile fault system in the Archean Yigarn Craton is the most productive gold-mineralized structure in Australia (>2300 t Au). The New Celebration deposit (51 t Au) is part of a group of hematite- and anhydrite-bearing mesothermal deposits and Fe-Cu-Au skarns associated with monzodiorite-tonalite intrusions in the strike-slip fault system. Ore-grade biotite-carbonate and late sericite-carbonate-alkali feldspar replacement is bound to the contacts of a felsic (low Cr, Ni, V) quartz-plagioclase porphyry dyke dated at 2676 ± 7 Ma. The sodic-potassic alteration of the felsic boudinaged dyke contrasts with the albite-actinolite alteration in the adjacent mafic (high Cr, Ni, V) plagioclase porphyry dated at 2662 ± 4 Ma, although both share the same sulfide-oxide assemblage: pyrite ± chalcopyrite, magnetite ± hematite. The younger porphyry locally crosscuts foliation and is bordered by post-kinematic actinolite-pyrite selvages overprinting talc-chlorite-phlogopite-dolomite schist. It contains auriferous pyrite (70 ppb Au; 610 ppb Ag) where sampled for zircon U-Pb chronology at +224 m elevation. Above the sample site, the dyke was mined as gold ore (1–6 g/t Au) at +300–350 m. Temperature estimates based on actinolite-albite pairs (300–350 °C) agree with the fluid inclusion trapping temperature of main-stage auriferous veins (330 ± 20 °C). These relationships are interpreted to indicate syn-mineralization emplacement. Gold-related albite-altered porphyry dykes (albitites) also occur in the world-class Hollinger-McIntyre (986 t Au) and Kerr Addison-Chesterville deposits (336 t Au), Abitibi greenstone belt, Canada.

Petrogenesis and U[sbnd]Pb (titanite) age of Cu[sbnd]Ag skarn mineralization in the McKenzie Gulch area, northern New Brunswick, Canada

Copper–silver skarn occurrences in the McKenzie Gulch (MG) area are spatially associated with Middle Devonian (386.3 ± 2.3 Ma; UPb zircon) intermediate to felsic dyke swarms. Mineralization occurs as veins and stockwork veinlets, disseminations, patches, and locally as replacement of calc-silicate skarns in argillaceous limestone. Fluid inclusion data from the MG deposit suggest that hydrothermal systems associated with skarn formation evolved from an early magmatic-dominated stage to a late formation water and/or meteoric water-dominated stage. An early prograde endoskarn and exoskarn formed from high temperature (444–865 °C) and high salinity (36–40 wt% CaCl2 + NaCl) fluids, whereas retrograde skarn formed from low temperature (97–257 °C) and low salinity (1–15 wt% NaCl equiv.) fluids. The salinity gap between prograde and retrograde fluids (~ 4-fold dilution) may be due to throttling in the magmatic-hydrothermal system resulting in a pressure regime that alternated between dominantly lithostatic and hydrostatic. The δ34S values of sulfides from the skarn deposit range from +4.6 to +9.1‰. These values are lower than the corresponding δ34S values from the adjacent gold-bearing quartz-calcite-sulfide veins whose values range from +7.9 to +10.0‰. These high positive values suggest that magmatic hydrothermal fluids interacted with and incorporated significant amount of sulfides from sedimentary rocks, which typically have higher δ34S values in the region.Hydrothermal titanites from retrograde skarn ores were successfully dated by laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS) in an effort to constrain the timing of mineralization. These analyses yield weighted mean 206Pb/238U age of 387.2 ± 3.6 Ma, in agreement with 206Pb/238U age of 386.3 ± 2.3 Ma of zircon from associated porphyry dykes. The felsic porphyry dykes spatially associated with CuAg skarn at MG, coupled with fluid inclusion data, provide evidence for magmatic activity coincident with the mineralizing events. In addition, Pb isotope compositions of galena and other sulfides (chalcopyrite, pyrite and pyrrhotite) are compatible with a model linking the skarns with Early to Middle Devonian magmatic activity at depth that eventually interacted with and incorporated significant volumes of sedimentary host rocks. Consequently, the MG CuAg skarn mineralization occurred contemporaneously with the intrusion of porphyry dyke swarms.

In Situ Elemental and Sr Isotope Characteristics of Magmatic to Hydrothermal Minerals from the Black Mountain Porphyry Deposit, Baguio District, Philippines

AbstractAt the Black Mountain porphyry Cu-Au deposit in the Baguio district, Northern Luzon (Philippines), pre- and synmineralized rocks preserve magmatic and hydrothermal minerals (e.g., plagioclase, amphibole, titanite, and epidote) spanning the complete paragenesis of the deposit. Strontium isotope values in early crystallized plagioclase phenocrysts from all felsic porphyries can be divided into two types. The type-I plagioclase crystals show relatively homogeneous Sr isotope values (0.7035–0.7038, 1σ <0.0001), indicating crystallization from a relatively stable and long-lived felsic magma chamber. The type-II plagioclase grains have a wider range of Sr isotope compositions (0.7032–0.7039, 1σ <0.0001), indicating mafic and/or felsic magma recharge. In magmatic titanite, Nb/Ta values are higher than those in the whole rock, while Zr/Hf and Y/Ho values are lower. In hydrothermal titanite and epidote, the ratios are similar to those in the whole rock. These patterns reflect crystallization effects imposed during the magmatic stage but an absence of differentiation during the hydrothermal stage. The consistent gradual decrease in total rare earth element, Y, Zr, and U contents in both hydrothermal titanite and epidote from early to late stages indicates the effect of hydrothermal fluid evolution with decreasing temperature. The variation of 87Sr/86Sr values in magmatic amphibole, plagioclase, and hydrothermal epidote in felsic and mafic rocks indicates the addition of mafic magma-derived fluid into the felsic magma-derived fluid. One extra source of fluid (probably derived from wall-rock limestone) was required to generate the highly radiogenic 87Sr/86Sr values of some epidote (0.7038–0.7053). Thus, in situ elemental and Sr isotope variation in minerals from different paragenetic stages can be used to interpret formation process and source for both magmas and hydrothermal fluids.

Contribution of magma mixing to the formation of porphyry-skarn mineralization in a post-collisional setting: The Machangqing Cu-Mo-(Au) deposit, Sanjiang tectonic belt, SW China

The Machangqing Cu-Mo-(Au) deposit in the central of Jinshajiang–Red River fold belt is a Cenozoic porphyry-skarn deposit in the western Yangtze Craton, southwestern China. Igneous rocks at Machangqing mainly consist of mafic and felsic rocks, including mafic dykes and barren syenite porphyry, as well as mineralized monzonite and granite porphyries with mafic microgranular enclaves (MMEs) in the ore-bearing porphyries. In this study, we compiled new and published geochemical and isotopic data for these intrusions, their MMEs and ore sulfides from the Machangqing deposit. Both mineralized and barren felsic porphyries, MMEs and mafic dyke were formed during the late Eocene (33–37 Ma), and have shoshonitic affinities (K2O + Na2O > 6 wt%, K2O/Na2O > 1 and Sr/Y > 40) ratios. They are enriched in LILE and LREE, but depleted in HFSE. Both mineralized and barren felsic porphyries are characterized by high SiO2 (64.67–72.81 wt%) and Al2O3 (13.43–16.10 wt%), but low Cr (average 37.63 ppm) and Ni (average 24.64 ppm) contents and low Mg# values (11–66). They have low whole–rock εNd(t) (–6.5 to –3.3) and relatively high (87Sr/86Sr)i (0.7061–0.7076), which fall inside the range of coeval mafic rocks in western Yunnan. The geochronological, geochemical and isotopic evidence suggest that the felsic magmas were formed by partial melting of the juvenile lower crust that had been injected by shoshonitic mafic magma as documented by the MMEs. The mafic dykes have high Mg# (71–76), Cr (average 492 ppm), Ni (average 195 ppm) contents, and were likely derived from partial melting of upper mantle sources metasomatized by paleo-subduction processes. The MMEs at Machangqing were produced by the mingling of metasomatized mantle-derived mafic melts with the juvenile Neoproterozoic thickened crust-derived felsic melts. The MMEs have spheroidal shapes and contain acicular apatite, indicating that the MMEs represent globules of coeval mafic melt injected into and partly assimilated by the felsic magmas. The ore sulfides have similar trace elemental and Pb isotopic features (206Pb/204Pb = 18.562–18.659; 207Pb/204Pb = 15.628–15.664; 208Pb/204Pb = 38.894–38.986) to the coeval mafic dykes, and their S-isotopic compositions (-2.24 to + 3.5‰) resemble typical mantle-source sulfides, suggesting a metallogenetic link with the coeval mafic magmas/melts and MMEs. Magmas of both the ore-bearing and barren porphyries are relatively oxidized in the FMQ to MH range (zircon Ce4+/Ce3+ = 87–950, log(fO2) = –22 to −5), although the ore-bearing ones are more oxidized. Petrological modelling suggests that the mineralized porphyries are derived from magma mixing dominated by partial melts of metasomatically-enriched mafic mantle, while the barren porphyries are products of magma mixing dominated by a juvenile lower crust component. We propose that magma mixing between lower crustal material with injected mafic mantle-derived magma has provided sufficient metals, sulfur and water to form the Machangqing Cu-Mo-(Au) deposit. The hydrous mafic magmas may have also remobilized the residual sulfides remaining in the continental lithospheric mantle (CLM), thus adding to the metal budget of these fertile magmas.

Petrochemistry and U–Pb (zircon) age of porphyry dykes at the McKenzie Gulch porphyry–skarn Cu–Ag–Au deposit, north-central New Brunswick, Canada: implications for emplacement age, tectonic setting, and mineralization potential

North- to northeast-trending intermediate to felsic porphyry dykes in the McKenzie Gulch (MG) area intrude Upper Ordovician through Silurian calcareous sedimentary rocks of the Matapédia Group. These dykes are spatially associated with numerous copper–silver skarn occurrences. In this area, two distinct suites of dykes are recognized: plagioclase–hornblende porphyry (P–H) and quartz–plagioclase porphyry (Q–P). These suites yielded U–Pb (zircon) ages of 386.2 ± 3.1 Ma and 386.4 ± 3.3 Ma, respectively, indicating a coeval relationship and similar genesis despite slight geochemical and petrographical differences. Geochemical data indicate that the dykes are granodioritic to granitic to slightly tonalitic in composition with I-type and slab failure signatures. When compared with other granitoids in the region, the Murdochville suite in Gaspésie, Québec, is the only intrusion that exhibits slab failure magmatic signatures similar to the MG dykes. Magmas with slab failure signatures are compositionally similar to tonalite–trondhjemite–granodiorite (TTG) suites and adakites. They exhibit low Y and Yb concentrations and subsequent high Sr/Y and La/Yb ratios, which are interpreted to be the result of melting of subducting slab and mantle during the waning stages of collision and consequent slab failure. These processes were followed by assimilation and fractional crystallization of minerals such as hornblende and clinopyroxene (± titanite), which preferentially partition Y and heavy rare earth elements (HREE) in the absence of significant plagioclase. Magmas with similar geochemical characteristics worldwide have been proposed to be an important source for metals in most porphyry Cu and skarn deposits and consequently are a target during mineral exploration.

A minimum Thanetian (Paleocene) age for the African Surface in the Eritrean highlands, Northeast Africa

Supergene alunite from a supergene leached capping zone in the Daero Paulos area near Asmara, Eritrea yields Thanetian (late Paleocene) K–Ar ages of 58.6 ± 2.3 and 59.4 ± 2.3 Ma. The leached capping is exposed in an erosional window through a laterite profile, with the supergene alunite forming by oxidation of pyrite contained in quartz veinlet stockworks hosted by porphyry copper-related, felsic porphyry and adjacent volcano-sedimentary country rocks. Geologic relationships show that the leached capping and contained supergene alunite formed simultaneously with the lateritic weathering event that accompanied regional planation of equatorial and subequatorial Afro-Arabia to form the African Surface. Landscape modification of the central Ethiopian-Eritrean plateau by incision of transverse valleys and canyons commenced in the Oligocene, with a principal exhumation event occurring in the Miocene in response to swell-induced rift-flank uplift during Red Sea opening.

Geochemistry and geochronology of mafic rocks from the Spanish Central System: Constraints on the mantle evolution beneath central Spain

The Spanish Central System (SCS) contains several suites of Palaeozoic mafic igneous intrusions with contrasting geochemical affinity: Ordovician tholeiitic metabasites, Variscan calc-alkaline gabbros (Gb1) and microdiorites (Gb2), shoshonitic monzogabbros (Gb3) and alkaline diabases and lamprophyres (Gb4). Not all of these rocks are accurately dated, and several aspects of their genesis are still poorly understood. We present new whole-rock geochemical data (major and trace elements, and Sr–Nd isotopes), U–Pb and Lu–Hf isotopic ratios on magmatic zircons and 40Ar/39Ar amphibole geochronology results in order to establish a precise chronology for the successive events of magmatism in the SCS, and discuss the nature of their mantle sources. Accurate ages have been determined for the Variscan gabbros (305–294 ​Ma), the microdiorites (299 ​Ma) and the accompanying felsic porphyries (292 ​Ma), the shoshonitic monzogabbros (285 ​Ma), and the alkaline diabases (274 ​Ma) and monzosyenites (271–264 ​Ma). According to this information, the Variscan mafic magmatism would be mainly concentrated in the range of 305–294 ​Ma, with a final manifestation represented by the minor shoshonitic dykes. The alkaline magmatism proved to be slightly older than previously thought and yielded at least two distinct pulses: diabases and lamprophyres–monzosyenites. Zircon Hf isotopes evidence the involvement of depleted and slightly enriched mantle sources. The bulk of the εHf values are in the broad range of −8 to +11, indicative of melting both depleted and enriched mantle regions. The high within-sample Hf isotope variation (up to ~11 epsilon units) shown by samples from the Variscan series (gabbros, microdiorites and monzogabbros) could be explained mainly by hybridisation of magmas derived from heterogeneous lithospheric mantle sources. Pressure estimates indicate that the Variscan mafic magmas were extracted from the lithosphere. The Nd–Hf isotopic composition of these suites of rocks suggests the recycling of pelitic sediments during the Cadomian orogeny. Deeper (asthenospheric) mantle levels were involved in the generation of the alkaline suite, whose anomalous negative εHf values (moderately decoupled with respect to radiogenic Nd) could be associated with subducted oceanic components raised by mantle upwelling associated with lithosphere thinning and extension during the Permian.

Geological mapping of intrusive rocks: a case study in the Garzón region, the Eastern Cordillera of the Colombian Andes

Jurassic granites (sensu lato) and related rocks are the most abundant rocks cropping out in the Eastern Cordillera of Colombia in the northern Andes. We present an attempt to map and characterize in detail the petrographic diversity of the Jurassic granites and related subvolcanic rocks emplaced in the Garzón region, southernmost the Eastern Cordillera. We also provide new geological maps for the Algeciras and Altamira Plutonic Massifs based on descriptive concepts of the petrographic facies and facies associations and detailed petrographic and mineralogical characterizations of the observed granite types. Eight plutonic and three subvolcanic petrographic facies or facies associations were mapped in the Algeciras Plutonic Massif, and another five in the Altamira Plutonic Massif. Hornblende biotite granites (syeno- and monzogranites) are the main plutonic types constituting the first massif, while leucogranites prevail in the second massif; a zoned facies distribution was mapped in the Algeciras Plutonic Massif. Felsic porphyries (dacite and rhyolite) crop out as minor irregular intrusive bodies or dikes and mafic-intermediate syn-plutonic dikes (mostly microdiorites) cross cut the main plutonic rocks in both massifs. Geological and petrographic characteristics (e.g. micrographic textures) suggest relatively shallow emplacement levels for the plutonic rocks as well as slightly high uplift and erosion rates at the north of the studied area. Local penetrative solid-state planar structures and dike orientations conform with dextral strike-slip movements along the SSW-NNE trending Algeciras Fault System in a transpressional regime associated with a major compressive and oblique convergence in Jurassic ages.

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

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

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.

The Archaean to Proterozoic igneous rocks of the Pilbara region, Western Australia –internationally significant geology of a globally unique potential geopark

The Pilbara region of Western Australia, covering some 500 km × 500 km, provides a diversity of Archaean to Proterozoic igneous rocks in a relatively compact area that records a younging southward crustal history of igneous activity, sedimentation, early life, tectonics, and metamorphism from the Archaean (3.6–2.7 Ga) to Proterozoic (2.5–1.8 Ga). The igneous rocks are variable in age, types of rocks, and mode of occurrence and, throughout the Precambrian, record varying igneous rock activity that appear related to several age-related geological settings: to north, the Archaean Pilbara Craton consists of a granitoid-and-greenstone complex; in the central region, there are Proterozoic sequences of volcanic rock, volcaniclastic rock, ironstone, chert, dolomite, shale, and intrusive dolerite sills and cross-cutting dolerite dykes; to the south, there are Proterozoic shale, dolomite, and chert with isolated granitic batholiths. Igneous activity begins in the Archaean with mafic and ultramafic volcanism alternating with sedimentation, and then granitoid cratonisation. This was followed by Proterozoic volcanic crustal accretion with mafic volcanic and volcaniclastic rocks, and by dolerite and gabbro sill and dyke intrusions, ending with isolated granite batholithic intrusions.Igneous rocks in the Pilbara region are diverse: komatiite; mafic volcanic/volcaniclastic rocks; basalt; tuff/volcanic breccia/accretionary lapilli; dolerite, gabbro, leucogabbro, pegmatitic gabbro, granite, and adamellite; xenolithic dolerite/gabbro; andesite, dacite, rhyodacite, rhyolite; granitoids: adamellite, monzogranite, syenogranite, granodiorite, tonalite, granite; granophyre; felsic dykes; and felsic porphyry. They are expressed as granitoid batholiths, komatiite and basalt sheets/lenses, mafic volcanic/volcaniclastic rocks in sheets, sills of dolerite, gabbro, ultramafic rocks, and diorite, dykes of dolerite, gabbro, and felsic rocks, structurally-oriented dolerite dyke swarms, tuff/volcanic breccia/accretionary lapilli in sheets/lenses, sheets of dacite, rhyodacite, rhyolite, and andesite, gabbroic plugs, apophyses, and a variety of host-rock to xenolith relationships. Today, the Pilbara region is arid, hence outcrop is excellent and many of these geological features are well exposed. The diversity of Archaean to Proterozoic igneous rocks in a relatively compact and well-exposed area and qualifies it as a globally unique potential Precambrian igneous-rock geopark.

High water contents of magmas and extensive fluid exsolution during the formation of the Yulong porphyry Cu-Mo deposit, eastern Tibet

The Yulong porphyry Cu-Mo ore district in eastern Tibet features a series of Eocene felsic porphyry intrusions, only one of which is intensively mineralized. In this study, zircon U-Pb ages, and major and trace element compositions of whole rock samples and in-situ accessory minerals (zircon, apatite and titanite) from these various intrusions were analyzed to determine the factors controlling Cu mineralization. New zircon U-Pb dating, together with published zircon U-Pb ages, suggest that these porphyry intrusions were emplaced over a relatively short period of time (43.9 ± 0.6 Ma to 40.9 ± 0.3 Ma), with the mineralized Yulong intrusion emplaced at a late stage of the magmatic activity (41.1 ± 0.3 Ma to 40.9 ± 0.3 Ma). These intrusions have indistinguishable whole-rock major and trace element compositions, but the mineralized Yulong intrusion has higher apatite and titanite La/Yb ratios (average apatite La/Yb = 115.1 ± 28.5, n = 52; average titanite La/Yb = 22.4 ± 4.9, n = 38) than the earlier subeconomic intrusions (average apatite La/Yb = 50.4 ± 12.3, n = 72; average titanite La/Yb = 12.4 ± 3.5, n = 58), suggesting more fractionation of amphibole from the magmas that sourced it. Additionally, a negative relationship between La/Yb and CeN/CeN* ratios in apatite and titanite are interpreted to reflect increasing oxidation states through magma evolution.Indistinguishable zircon EuN/EuN* values and apatite core SO3 contents from the mineralized (zircon EuN/EuN* = 0.69 ± 0.06, n = 25; apatite SO3 = 0.65 ± 0.24 wt%, n = 21) and subeconomic intrusions (zircon EuN/EuN* = 0.62 ± 0.05, n = 64; apatite SO3 = 0.61 ± 0.27 wt%, n = 15) suggest that the entire magmatic suite was relatively oxidized, hydrous and sulfur-rich, and was therefore fertile for ore-formation. However, apatite crystals from the mineralized Yulong intrusion have significantly lower Cl contents (0.08 ± 0.03 wt%, n = 72) and higher F/Cl ratios (47.80 ± 21.97, n = 72) than those from the subeconomic intrusions (Cl = 0.29 ± 0.29 wt%; F/Cl = 12.87 ± 5.16; n = 78). These data may reflect more extensive fluid exsolution from the parental magma chamber at the time of emplacement of the mineralizing Yulong magmas. Sudden and voluminous release of volatiles coeval with emplacement of the Yulong intrusion was triggered by an as yet unidentified process, but possibly including magmatic recharge.

Multiple Mesozoic porphyry-skarn Cu (Mo–W) systems in Yidun Terrane, east Tethys: Constraints from zircon U–Pb and molybdenite Re–Os geochronology

Porphyry Cu±Mo±Au deposits typically formed in volcanoplutonic arcs above subduction zones. However, there is increasing evidence for the occurrence of porphyry deposits related to magmas generated after the underplating arc has ceased. Post-subduction lithospheric thickening, lithospheric extension, or mantle lithosphere delamination could trigger the remelting of subduction-modified arc lithosphere and lead to the formation of post-subduction porphyry deposits. The NNW-trending Yidun Terrane, located in the eastern Tethys, experienced subduction of Garze–Litang oceanic plate (a branch of the Paleotethys) in the Late Triassic and witnessed two mineralization events respectively associated with the ca. 215Ma arc-related intermediate–felsic porphyries and the 88–79Ma mildly-alkaline granitic porphyries. It is, therefore, an ideal place to investigate the genetic linkage between the subduction-related porphyry deposits and post-subduction porphyry deposits. Our new in situ zircon U–Pb dating of the two granitic intrusions (biotite granite, 213.4±0.9Ma; monzogranite porphyry, 86.0±0.4Ma) in the Xiuwacu district, the molybdenite Re–Os age (84.7±0.6Ma) of the mineralization, and previously published geochronological data, together show the spatially overlapping distribution of the multiple Mesozoic porphyry systems in the Late Triassic Yidun arc system. Furthermore, the arc-like elemental signatures and the mixed Sr–Nd–Hf isotopic signatures of the Late Cretaceous ore-related porphyries (i.e., originating from a mixed components between the ∼215Ma juvenile arc crust and the Mesoproterozoic mafic lower crust) indicate a genetic linkage between the Late Triassic and Late Cretaceous porphyry systems. This suggests that the remelting of underplated arc-related mafic rocks formed during the subduction of the Garze–Litang Ocean could be responsible for the mixing between the mantle-derived components and the Mesoproterozoic lower crustal materials, when post-subduction transtension occurred in the Late Cretaceous. The formation of the Late Cretaceous porphyry–skarn Cu–Mo–W deposits could most likely be related to the remelting of Late Triassic residual sulfide-bearing Cu-rich cumulates in the subduction-modified lower crust that triggered by the Late Cretaceous transtension.

Contrasting Tectonic Settings and Sulfur Contents of Magmas Associated with Cretaceous Porphyry Cu ± Mo ± Au and Intrusion-Related Iron Oxide Cu-Au Deposits in Northern Chile

Porphyry Cu ± Mo ± Au and iron oxide copper-gold (IOCG) deposits share many similarities (e.g., Fe, Cu, and Au contents), but also have important differences (e.g., the predominance of sulfide minerals in porphyry deposits and iron oxides in IOCG deposits). Genetic comparisons are complicated by the broad definition of IOCG deposits; here we restrict our study to IOCG deposits that are related to igneous intrusive systems. In the Mesozoic Coastal Cordillera of northern Chile, both porphyry and IOCG deposits occur in close spatial and temporal proximity, offering the chance to examine what controls their different modes of formation. From detailed examination of the timing, geochemistry, and tectonic setting of associated igneous rocks, based on new and published data, we find that rocks associated with mid-Cretaceous IOCG deposits (~125–110 Ma) are largely indistinguishable from those associated with slightly earlier (>125 Ma) and later (<110 Ma) porphyry Cu ± Mo ± Au deposits. Magmas related to IOCG deposits were formed during a brief period of back-arc transtension in the mid-Cretaceous and are, on average, somewhat more mafic (dioritic), locally alkaline, and isotopically primitive compared to granodioritic magmas associated with porphyry deposits formed during normal contractional arc tectonics in the later Cretaceous. However, these compositional ranges overlap, and the differences are not clear enough to be diagnostic. We measured the SO3 content of igneous apatite from selected samples of these rocks to test the hypothesis that the difference in sulfur content of the ore deposits was due to differences in sulfur content of the associated magmas. Early igneous apatite crystals occurring as inclusions in silicate phenocrysts from the Carmen de Andacollo porphyry Cu-Au deposit (Re-Os molybdenite ages of 103.9 ± 0.5 Ma, 103.6 ± 0.5 Ma) are significantly richer in S (0.25 ± 0.17 wt % SO3, n = 69) than similar apatite crystals from two IOCG deposits (Candelaria, Casualidad) and a sample of regional mid-Cretaceous igneous rock from near Productora (0.04 ± 0.02 wt % SO3, n = 76). Using published partition coefficients for S between apatite and oxidized silicate melt, we semi-quantitatively estimate corresponding magmatic sulfur contents of ~0.02 wt % S in the Carmen de Andacollo magmas versus ~0.001 to 0.005 wt % S in the IOCG-associated magmas. This is an order of magnitude difference, and the opposite of what would be expected if the difference were due to bulk magma composition (sulfur solubility is generally higher in mafic magmas, whereas here the S content is higher in the more felsic porphyries). We conclude that the porphyry-forming magmas indeed had higher S contents than the IOCG-related magmas and suggest that these differences reflect different petrogenetic processes. During normal subduction, magmas derived from the metasomatized mantle wedge are hydrous, moderately oxidized, and S rich, and have the potential to generate S-rich porphyry-type deposits. In contrast, in back-arc extensional settings, upwelling asthenospheric melts carry a weaker subduction signature, including lower S contents. Interaction of these S-poor magmas with previously subduction modified upper plate lithosphere is more likely to give rise to S-poor IOCG deposits.

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

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