Intermediate Intrusive Rocks Research Articles

Evaluation of the geochemical background of soil in a hyper-arid zone using a multivariate statistical methodology: The case of the city of Antofagasta in the Atacama Desert

This study investigates the geochemical background and factors influencing the variability of 19 environmentally relevant elements in the soils of Antofagasta, Chile, a region known for its extensive mining activities. Employing robust multivariate statistical techniques on a dataset of 94 soil samples, we identified four main factors explaining 70% of the total variance in elemental concentrations. These four factors reflect the influence of Jurassic volcanic rocks, intrusive rocks, marine sediments, and mafic to intermediate intrusive rocks. Cluster analysis revealed three distinct geochemical populations, each reflecting a unique combination of natural and anthropogenic influences. We established background concentrations for each element within these clusters using robust statistical methods. Geostatistical analysis, employing inverse distance weighted interpolation, produced factor distribution maps that, when integrated with geological data, provided insights into lithological and anthropogenic influences on soil geochemistry. Our findings highlight the complex interplay between natural geological processes, the region's unique arid climate, and anthropogenic activities in shaping the geochemical landscape of Antofagasta. This study contributes to the understanding of geochemical backgrounds in mining-intensive, arid regions and provides a methodological framework applicable to similar environments worldwide.

Late Carboniferous closure of the Junggar-Balkhash Ocean: Insights from the early Permian post-accretionary magmatism in the Barleik Mountains of West Junggar, NW China

Early Permian magmatic rocks in the West Junggar Terrane (WJT, NW China) contain crucial information about the final closure of the Junggar-Balkhash Ocean (JBO) in the southwestern Altaids, whereas their tectonic settings are inconclusive. In this study, we conduct an integrated study incorporating the geochronology, geochemistry and zircon Hf isotopes of a set of volcanic and intrusive rocks from the northwestern Barleik Mountains in the central WJT. Zircon UPb ages of eight volcanic and felsic–intermediate intrusive samples are ∼292–281Ma, corresponding to a regional magmatic flare-up in the central WJT. The volcanic samples include rhyolitic tuff, andesitic tuff, and rhyolitic andesite, characterized by high-K calc-alkaline to shoshonitic I-type granite features. They yield positive εHf(t) values with a narrow range of 9.40–13.52 and two-stage model ages from 421 Ma to 672 Ma. Above isotopic features, in combination with their relatively low Mg# values of 25–35, demonstrate that they were generated by partial melting of the juvenile lower crust. Felsic intrusive rocks include peraluminous I-type granitic porphyry and A2-type K-feldspar granite, and their low Mg# values (21–35) and trace element ratios suggest the partial melting from lower crustal rocks. By contrast, intermediate intrusive rocks include monzonite, diorite and a dioritic dyke, which represent fractionated partial melts sourced from the underplated depleted mantle-derived magma. Based on the occurrence of I-type and A2-type rocks, together with the coeval extensional strike-slip fault systems and a late Carboniferous angular unconformity, we infer that the early Permian magmatism in the central WJT resulted from an extensional post-accretionary setting. Taken together with similar geological evidence from the WJT and East Kazakhstan, the final closure of the JBO is suggested to occur in the late Carboniferous.

Petrogenesis of the late Cretaceous Budongla Mg-rich monzodiorite pluton in the central Lhasa subterrane, Tibet, China: Whole-rock geochemistry, zircon U-Pb dating, and zircon Lu-Hf isotopes

Mg-rich monzodiorite are found in the Budongla gold ore district, Zhongba County, Xizang (Tibet) Autonomous Region, P.R. China. Studying the petrogenesis of this intermediate pluton can provide effective information to explore the geological evolution of the Lhasa terrane. One monzodiorite sample yielded a weighted mean 206Pb/238U age of 92.7 ± 1.1 Ma (mean square weighted deviation=0.33) using LA-ICP-MS zircon U-Pb dating, which represents the late phase of Late Cretaceous magmatism. The rock-forming minerals in the Budongla Mg-rich monzodiorite mainly include K-feldspar, plagioclase, quartz, biotite augite, and amphibole, and its accessory minerals mainly include magnetite, titanite, zircon, and apatite. The rocks are rich in K2O+Na2O and K2O with medium contents of SiO2, CaO, and A12O3,, suggesting these rocks belong to the high-K calc-alkaline series. These rocks have high MgO, Fe2O3, and FeO, with high Mg# values and low DI, which implies they are Mg-rich intermediate intrusive rocks. The rocks are enriched in LREEs and LILEs and depleted in HREEs and HFSEs. They have negative Eu anomalies, no obvious negative Ce anomalies, and slightly negative εHf(t). We infer that the Budongla pluton is a high-K calc-alkaline metaluminous Mg-rich monzodiorite and intruded during the post-collisional period of the Lhasa and Qiangtang terranes.

Geodynamic Formation Conditions and Age of Granitoids from Small Intrusions in the West of the Yana–Kolyma Gold Belt (Northeast Asia)

Abstract We report results of geological, mineralogical-petrographic, geochemical, isotope-geochemical (Sm–Nd, Rb–Sr), and geochronological (U–Pb, 40Ar/39Ar) studies of acid and intermediate intrusive rocks (granodiorites, leucocratic granites, subalkaline granites, and subalkaline leucocratic granites, diorites, and quartz diorites) of the Bukeschen and Samyr small plutons in the western part of the Yana–Kolyma gold belt (northeast Asia). These rocks are combined with Late Jurassic (151–145 Ma) dikes of basic, intermediate, and acid compositions into a single complex of small intrusions. They intrude the Upper Triassic–Middle Jurassic terrigenous deposits of continental margin blocks in the eastern part of the Verkhoyansk–Kolyma folded area. Our new U–Pb data for zircon (SHRIMP-II) indicate that the Bukeschen and Samyr pluton granitoids formed in the Berriasian, at 144.5 and 143 Ma, respectively. The small-intrusion granitoids have geochemical and isotope (Sm–Nd and Rb–Sr) characteristics similar to those of Late Jurassic dikes of varying composition. Therefore, they can be united into a single complex of small intrusions generated from a mixed source with the participation of mantle (OIB- and E-MORB type), lower crust, and subduction components and with Paleoproterozoic–Mesoproterozoic Sm–Nd model age estimates for the magma sources. Late Jurassic–Early Cretaceous magmatic and postmagmatic events and cooling of the intrusions played an important role in the processes of gold localization in the western part of the Yana–Kolyma gold belt. This is reflected in two tectonothermal stages (accounting for closing temperatures of the U–Pb, 40Ar/39Ar, and Re–Os isotope systems for different minerals) estimated at 151–141 and 138–137 Ma. These results for the small-intrusion complex agree with the tectonic model of the evolution of an active continental margin (northeastern Siberia) in the Mesozoic era, whose final development stage in the Berriasian age saw the formation of mostly small granitoid plutons.

Biotite compositions and geochemistry of porphyry-related systems from the central Urumieh Dokhtar Magmatic Belt, western Yazd, Iran: Insights into mineralization potential

Commonly biotite occurs as a minor rock-forming mineral in a wide range of felsic to intermediate intrusive rocks. It can record the physicochemical conditions of the parent magma due to its complex crystal structure that allows multiple site substitutions of a wide range of elements. Geochemical studies of whole rocks and biotite (using EPMA and LA-ICP-MS methods) with petrographic analysis of biotite provide distinction between barren and fertile granitoids in western Yazd. Fertile suites are characterized by Si- and Mg-rich (3.47–4.35, ave. 3.75 a.p.f.u.) and total REE-poor biotite. In contrast, Fe (2.66–3.15, ave. 3.08 a.p.f.u.), Al, Ti, and REEs (in particular HREE+Y) increase in barren rocks. On the basis of biotite geobarometry, the fertile rocks (Darezereshk Porphyry Copper Deposit (PCD)) solidified at low pressure (ca. 0.6 kb, i.e., subvolcanic); in contrast, the barren rocks (Galvic and Shirkouh granodiorites) crystallized at higher pressures (1.1 to 3.6 kb). The temperature of the fertile granitoids is inferred to be about 750 °C, whereas T's of 625–675 °C for barren granitoids. The fertile suites have redox characteristic near the Ni-NiO buffer, whereas the barren suites are more reduced, near the quartz-fayalite-magnetite (QFM) buffer. Therefore, biotite major-element compositions, fertile magma bodies are emplaced at higher temperature and redox to lower pressures (subvolcanically) than the barren intrusive bodies. The whole-rock geochemical signatures show that both barren and fertile suites are represented by peraluminous to metaluminous granite to diorite that have calc-alkaline characteristics with an I-type volcanic arc (VAG) tectonic setting. However, fertile suites originated from the mantle-crust mixed magma exhibiting calc-alkaline affinity, whereas the barren suites are derived predominantly from recycling of continental crust.

Early Carboniferous Back‐Arc Rifting‐Related Magmatism in Southern Tibet: Implications for the History of the Lhasa Terrane Separation From Gondwana

AbstractThe Lhasa terrane is widely regarded as an integral unit that separated from Gondwana in the Carboniferous or late Triassic, but this terrane may more reasonably consist of two subterranes that separated from Gondwana at different times. Here, we describe newly identified early Carboniferous (355–344 Ma) basic and intermediate intrusive rocks in the Xiongcun area, southern Tibet. The basic rocks that yield negative whole‐rock εNd(t) values (−5.04 to −2.94) and zircon εHf(t) values (−9.2 to −1.7), combined with the chemical compositions, indicate that the magmas were derived by the partial melting of an enriched lithospheric mantle in a back‐arc extensional setting. The intermediate rocks yield εNd(t) and εHf(t) values of −5.78 to −5.36 and −9.2 to −4.5, respectively, which suggests that they were likely produced via crust‐mantle mixing. By combining our results with those of previous studies on the Lhasa terrane, we consider that a broad back‐arc rift system, including northern and southern branches, developed along the northern margin of Gondwana under the southward subduction of the Paleo‐Tethys oceanic lithosphere during the early Carboniferous. The northern branch evolved into the Sumdo Paleo‐Tethys Ocean, which led to the separation of the North Lhasa terrane from northern Gondwana during the late Carboniferous. The subsequent northward subduction of the Sumdo Paleo‐Tethys oceanic lithosphere beneath the North Lhasa terrane during the early to middle Permian may have triggered the southern branch to further evolve into the Neo‐Tethys Ocean and eventually may have resulted in the South Lhasa terrane separating from Gondwana.

Intermittent subduction of the Paleo-Tethys Ocean in the middle–late Permian: evidence from the mafic–intermediate intrusive rocks in the East Kunlun Orogenic Belt

In this study, we investigate the geochronology, geochemistry, zircon Lu–Hf isotopes and Sr–Nd isotopes of mafic–intermediate intrusive rocks in the Naxiguole area of the East Kunlun Orogenic Belt (EKOB), northwest China. Zircon U–Pb dating yields formation ages of 254.8 ± 1.2 Ma for gabbros and 251.9 ± 1.3 Ma for gabbrodiorites, suggesting that the mafic–intermediate intrusive rocks were formed in the late Permian. The gabbrodiorites show the geochemical characteristics of arc magmatic rocks, while gabbros were similar to those of backarc basin basalts. The gabbros yielded (87Sr/86Sr)i ratios of 0.7068–0.7097, low ε Nd(t) values of –4.40 to +0.16, and a limited range of ε Hf(t) values of –1.09 to +0.04. The gabbrodiorites display (87Sr/86Sr)i ratios of 0.7090–0.7098, ε Nd(t) values ranging between –3.99 and –3.55, and ε Hf(t) values of –0.81 to +0.34. The Sr–Nd and Lu–Hf isotopes from both types of samples resemble an enriched mantle but distinctly different from the crust. Based on their geochemical, and some trace elemental ratios (e.g. Ba/Th, La/Sm, Th/Yb and Sm/Yb), we suggest that the parental magmas of the gabbros and gabbrodiorites were formed by 1–5% partial melting of spinel lherzolite and garnet–spinel lherzolite, respectively. The gabbros originated from a mantle source modified by subducted slab fluids, while the gabbrodiorites were derived from a mantle source altered by subducted slab melts. By synthesising all the data, we propose that the gabbros were formed in a backarc extension setting and the gabbrodiorites in a continental arc setting. This suggests a geodynamic setting transition from backarc extension to subduction during the late Permian. In combination with previous research, we propose that the subduction mode in the Paleo-Tethys Ocean in the EKOB is intermittent. KEY POINTS Zircon U–Pb dating indicates an age of ca 254.8 ± 1.2 Ma for the gabbros and ca 251.9 ± 1.3 Ma for the gabbrodiorites (i.e. late Permian). The gabbros were derived from an enriched mantle source modified by slab-derived fluids whereas the gabbrodiorites had an enriched mantle source modified by subducted slab melts. The gabbros were in a backarc extension setting while the gabbrodiorites formed in a continental arc setting. This suggests that there was a transition in the geodynamic setting (from backarc extension to subduction).

Mafic–intermediate igneous rocks in the East Kunlun Orogenic Belt, northwestern China: Petrogenesis and implications for regional geodynamic evolution during the Triassic

Mafic–intermediate igneous rocks can provide important insights into the geodynamic evolution of orogenic belts. Here, we investigate the petrogenesis of mafic–intermediate intrusive igneous rocks in the East Kunlun Orogenic Belt (EKOB) to constrain regional geodynamic and magmatic evolution. These intrusive rocks are all located in the Permian-Triassic magmatic arc and parallel to the Paleo-Tethys ophiolite belt. Zircon UPb dating results indicate that these various igneous rocks crystallized during the Triassic. The 250–248 Ma appinites display low SiO2 contents, high MgO contents, and marked enrichments in large-ion lithophile elements and depletions in high-field-strength elements; combined with their enriched εHf(t) values, these appinites were considered to be derived from lithospheric mantle modified by subducted slab-derived fluids. The Harizha host granodiorites and MMEs have indistinguishable crystallization ages of 235 Ma but distinct SiO2 contents and δ18OSMOW values, which, together with enriched Sr–Nd–Hf–Pb isotopes, suggest that their primary magmas were derived from the Mesoproterozoic lower crust (MLC) and subduction-modified lithospheric mantle (SMLM), respectively. The 241 Ma Baidungou quartz diorite and the 218 Ma Huanglonggou diorite porphyry show characteristics of medium SiO2 contents, moderate Mg# and εHf(t) values relative to contemporaneous mafic and granitic rocks; therefore they probably represent the mixing products of MLC-derived felsic and SMLM-derived mafic magmas. Different from the Baidungou diorite and Harizha granodiorite, the Huanglonggou diorite porphyries have adakitic characteristics, which suggests crustal thickening during the Late Triassic. A synthesis of these new data with previous researches shows that the EKOB undergone three magmatic peaks corresponding to three extension events (slab-rollback, slab-breakoff, and delamination) during the Triassic evolution.

Carbonate-related metallic and non-metallic mineralization within and proximal to granites (Fichtelgebirge Pluton, Germany): “Mantle-crust marker mineralization”

Metacarbonate rocks (marble, calcsilicate rock, skarn) hosting strata- and structurebound Fe-, As-Sb, Bi-, Ni-Co, Cu-Pb-Zn, U-, W- ore minerals as well as talc, clay minerals, barite and fluorite are widespread in the country rocks south of the collisional calc-alkaline felsic to intermediate intrusive rocks of the Fichtelgebirge Pluton, Germany. At its western rim, thrustbound and vein-type mineral assemblages with Au-As-Sb and F-Ba minerals associated with carbonate gangue minerals developed. Only recently, a calcite-hosted Sb mineralization was encountered in a deep-seated lineamentary fault zone cutting through metaultrabasic rocks along the northern edge of the granites. The same structure zone forms the loci of calcite-bearing U episyenite situated within this pluton. A composite geological, mineralogical and chemical (major and trace elements, REE, C- and O isotopes) study has been conducted to distinguish the heat and element source in subcrustal or deep-seated crustal areas and in the largely exposed granite complex. Only Sn-W skarn deposits are genetically related to the highly fractionated granitic members of the Variscan pluton. Pegmatite skarn has a strong subcrustal component as to the heat source and the provenance of rare elements and a moderate crustal one as far as the silicates are concerned. Deep-seated fault zones were active over a rather long period of time and acted as conduits venting magmas and hydrothermal fluids from the waning stages of the Variscan deformation through the Neogene. Calcsilicate and carbonate mineral assemblages are an efficacious tool to constrain the physico-chemical regime in this mineral province, covering the temperature range from 745 °C down to 53 °C in a medium to low pressure regime at strongly varying redox conditions. The major and trace elements, the REE variation, the Ce and Eu anomalies as well as carbon and oxygen isotopes of the various mineral assemblages enable us to identify the fluid sources and depict the element concentration processes, e.g., mixing of fluids, connate fluid and meteoric fluid interaction. From the economic point of view, the mineralizing system is most prospective for rare element deposits, talc, kaolinite-group minerals, iron, and uranium. Accumulations of fluorite and barite are subeconomic in mineral assemblages inside as well as outside the granites, while base metals and precious metals are only of mineralogical interest.

Raising the Gangdese Mountains in southern Tibet

AbstractThe surface uplift of mountain belts is in large part controlled by the effects of crustal thickening and mantle dynamic processes (e.g., lithospheric delamination or slab breakoff). Understanding the history and driving mechanism of uplift of the southern Tibetan Plateau requires accurate knowledge on crustal thickening over time. Here we determine spatial and temporal variations in crustal thickness using whole‐rock La/Yb ratios of intermediate intrusive rocks from the Gangdese arc. Our results show that the crust was likely of normal thickness prior to approximately 70 Ma (~37 km) but began to thicken locally at approximately 70–60 Ma. The crust reached (58–50) ± 10 km at 55–45 Ma extending over 400 km along the strike of the arc. This thickening was likely due to magmatic underplating as a consequence of rollback and then breakoff of the subducting Neo‐Tethyan slab. The crust attained a thickness of 68 ± 12 km at approximately 20–10 Ma, as a consequence of underthrusting of India and associated thrust faulting. The Gangdese Mountains in southern Tibet broadly attained an elevation of >4000 m at approximately 55–45 Ma as a result of isostatic surface uplift driven by crustal thickening and slab breakoff and reached their present‐day elevation by 20–10 Ma. Our paleoelevation estimates are consistent not only with the C–O isotope‐based paleoaltimetry but also with the carbonate‐clumped isotope paleothermometer, exemplifying the promise of reconstructing paleoelevation in time and space for ancient orogens through a combination of magmatic composition and Airy isostatic compensation.

Triassic mafic and intermediate magmatism associated with continental collision between the North and South China Cratons in the Korean Peninsula

Triassic coeval mafic and intermediate magmatism occurred in the area suggested to be the southern margin of the North China Craton (NCC) in the Gyeonggi Massif (GM) of the Korean Peninsula. This study investigates aspects of the mafic and intermediate magmatism using SHRIMP zircon ages and whole-rock chemical and isotopic Sr–Nd data. The mafic and intermediate rocks intruded into a basement paragneiss in three areas (Yangpyeong, Odesan and Yangyang) within the GM at ca. 230Ma. The paragneiss was metamorphosed in both the Paleoproterozoic and Triassic. Gabbros (hornblende gabbro and pyroxene–mica gabbro) from the study areas exhibit strong light REE (LREE) enrichment relative to chondrite (LaN/YbN=11.1–30.6) and a high LILE/HFSE pattern, Ta–Nb–P–Ti troughs and positive Ba–K–Pb–Sr spikes on the N-MORB-normalized multi-element variation diagram. These features are typical characteristics of arc-related gabbros. The gabbros also show strongly enriched initial isotopic compositions (87Sr/86Sr(i)=0.7100–0.7137; εNd(t)=−13.1 to −19.7). The coeval intermediate intrusive rocks also exhibit whole-rock chemical and isotopic features (87Sr/86Sr(i)=0.7099–0.7143; εNd(t)=−10.8 to −18.6) similar to those of the gabbros. The mafic and intermediate intrusive rocks plot in the within-plate and/or post-collisional fields on tectonic discrimination diagrams. These data indicate that the mafic and intermediate magmatism in the study areas occurred during the Triassic post-collisional relaxation period via partial melting of sub-continental lithospheric mantle (SCLM) that was enriched in a subduction environment prior to (or during) the Permo-Triassic continental collision between the NCC and the South China Craton (SCC). The highly enriched mantle signatures revealed by the gabbros from the study areas are matched to the enriched features identified in Cretaceous mafic igneous rocks (ca. 130Ma) on the southern margin of the NCC. Thus, this study suggests that the lithospheric mantle beneath the Yangpyeong, Odesan and Yangyang areas are comparable to the SCLM of the NCC southern margin. The highly enriched nature of mafic and intermediate rocks from the study areas indicates that their source is the SCLM that was metasomatized by Permian to early Triassic subduction–continental collision processes.

Mineralogy, geochemistry, and genesis of the Garak Baghi kaolin deposit in the northwest of Saveh, Iran

The Garak Baghi kaolin deposit is located in Markazi province and in 75 km northwest of Saveh. It belongs to the Iranian central structural zone. Eocene-Miocene igneous rocks consist of dacite, andesite, tuff, and other pyroclastic rocks and acidic to intermediate intrusive rocks covered the area. Based on petrographic studies, the main minerals in the volcanic rocks are plagioclase, K-feldspar, and quartz. The tectonic processes have affected the area and formed the main faults and fractures that developed the hydrothermal alteration. Hydrothermal alteration has produced clay deposit. Clay deposits contain kaolinite, illite, and the residual materials of altered feldspar with impurity phase such as gypsum, calcite, anatase, and iron compositions. The quality of clay soils especially near the fault zones has been reduced by iron composition. Geochemical studies show that the rocks are medium-K calc-alkaline and meta-aluminous. Based on chemical index of alteration (CIA) and chemical index of weathering (CIW) standards, weathering in the Garak Baghi mine is low, and supergene processes have the main role in the formation of clay soils. The source rocks of clay minerals have been enriched with La, Ce, Th, U, and Rb and depleted of Ti, Nb, and Ba. Their geochemical features are strongly affected by parent magma and also the decomposition of feldspar during the alteration process. The chondrite-normalized rare earth element (REE) patterns display a steep decrease from light REE (LREE) to heavy REE (HREE) with Eu and Ce negative anomalies related to hydrothermal fluids. They show a positive anomaly in Tm because of oxidization conditions too. Geochemical studies demonstrate acidic volcanic rocks as a source of clay soils in the area.

Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria

Petrographic studies on the intrusive rocks from the Ishiagu area show that they are gabbro, dolerites, and diorite. The gabbro and diorite occur as stocks while the dolerites occur as sills in folded sedimentary sequence predominated by shale. Geochemical data confirmed the rocks as basic (gabbro and dolerites) and intermediate (diorite) rocks with silica contents ranging from 42.45 to 50.75 wt.% and 55.03 to 60.24 wt.%, respectively. They are predominantly alkaline (gabbro, dolerites, and diorites) with sparse tholeiites (few gabbro). The basic rocks are made up of calcic plagioclase (An49-54), clinopyroxene, olivine (altered), nepheline, and opaques, while the intermediate rocks consist of plagioclase (An41-49), hornblende, and clinopyroxene. Calcite occurs as a secondary mineral in both groups. Although many samples are moderately to heavily altered (loss on ignition: 2.12–10.46 wt.%), a consistent decrease in magnesium number (Mg#) (gabbro, 0.47; dolerites, 0.37; dolerites, 0.27) and immobile elements (Y, Zr, Hf, Ti, Nb, and Ta), which are more enriched in the intermediate than the basic rocks and similar patterns of spidergrams and rare earth elements (REEs), show that the rocks (gabbro, dolerites, and diorites) are genetically related. The rocks show stronger enrichment in incompatible elements and REEs [(La/Yb)n, 4.83–8.15 for the gabbro, 7.01–7.79 for the dolerites, and 8.85–9.63 for the diorites]. Primitive-mantle normalized trace-element patterns and trace-element ratios of both basic and intermediate rocks indicate similarity to ocean island basalts, which suggests a garnet-lherzolite mantle source. The ratios of La/Nb, Th/La, and Th/Nb and the high ratio of Nb/La (1.18–1.50 for gabbro, 1.37–1.95 for dolerites, and 1.21–1.32 for diorites) indicate a high 238U/204Pb (HIMU) mantle source region. There is no evidence of significant crustal contamination on the basic and intermediate rocks, which may be related to the rapid ascent of the magmas in an intracontinental rift setting similar to the East African rift.

Geochronology and fluid inclusion study of the Yinjiagou porphyry–skarn Mo–Cu–pyrite deposit in the East Qinling orogenic belt, China

The Yinjiagou Mo–Cu–pyrite deposit of Henan Province is located in the Huaxiong block on the southern margin of the North China craton. It differs from other Mo deposits in the East Qingling area because of its large pyrite resource and complex associated elements. The deposit’s mineralization process can be divided into skarn, sulfide, and supergene episodes with five stages, marking formation of magnetite in the skarn episode, quartz–molybdenite, quartz–calcite–pyrite–chalcopyrite–bornite–sphalerite, and calcite–galena–sphalerite in the sulfide episode, and chalcedony–limonite in the supergene episode. Re–Os and 40Ar–39Ar dating indicates that both the skarn-type and porphyry-type orebodies of the Yinjiagou deposit formed approximately 143Ma ago during the Early Cretaceous. Four types of fluid inclusions (FIs) have been distinguished in quartz phenocryst, various quartz veins, and calcite vein. Based on petrographic observations and microthermometric criteria the FIs include liquid-rich, gas-rich, H2O–CO2, and daughter mineral-bearing inclusions. The homogenization temperature of FIs in quartz phenocrysts of K-feldspar granite porphyry ranges from 341°C to >550°C, and the salinity is 0.4–44.0wt% NaCl eqv. The homogenization temperature of FIs in quartz–molybdenite veins is 382–416°C, and the salinity is 3.6–40.8wt% NaCl eqv. The homogenization temperature of FIs in quartz–calcite–pyrite–chalcopyrite–bornite–sphalerite ranges from 318°C to 436°C, and the salinity is 5.6–42.4wt% NaCl eqv. The homogenization temperature of FIs in quartz–molybdenite stockworks is in a range of 321–411°C, and the salinity is 6.3–16.4wt% NaCl eqv. The homogenization temperature of FIs in quartz–sericite–pyrite is in a range of 326–419°C, and the salinity is 4.7–49.4wt% NaCl eqv. The ore-forming fluids of the Yinjiagou deposit are mainly high-temperature, high-salinity fluids, generally with affinities to an H2O–NaCl–KCl±CO2 system. The δ18OH2O values of ore-forming hydrothermal fluids are 4.0–8.6‰, and the δDV-SMOW values are between −64‰ and −52‰, indicating that the ore-forming fluids were primarily magmatic. The δ34SV-CDT values of sulfides range between −0.2‰ and 6.3‰ with a mean of 1.6‰, sharing similar features with deeply sourced sulfur, implying that the sulfur mainly came from the lower crust composed of poorly differentiated igneous materials, but part of the heavy sulfur came from the Guandaokou Group dolostone. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of sulfides are in the range of 17.331–18.043, 15.444–15.575, and 37.783–38.236, respectively, which is generally consistent with the Pb isotopic signature of the Yinjiagou intrusion, suggesting that the Pb chiefly originated from the felsic–intermediate intrusive rocks in the mine area, with a small amount of lead from strata. The Yinjiagou deposit is a porphyry–skarn deposit formed during the Mesozoic transition of a tectonic regime that is EW-trending to NNE-trending, and the multiepisode boiling of ore-forming fluids was the primary mechanism for mineral deposition.

Quantified spatial relationships between gold mineralisation and key ore genesis controlling factors, and predictive mineralisation mapping, St Ives Goldfield, Western Australia

Determining the spatial relationships between various geological features and mineralisation is not only important for understanding the ore genesis of mineral deposits, but can also help to guide mineral exploration by providing predictive mineral maps. In this GIS-based study, we quantify the spatial relationships between gold mineralisation and plausible controlling factors in the central part of the St Ives area, Western Australia. We weigh plausible control factors including rock types, lithological boundaries and faults, using gold occurrences in drill-cores, and then apply the weights to the geological data to generate a predictive map for the entire St Ives area. The three major findings of this study are: (1) all major gold deposits are controlled by faults, and small-scale fault systems have a stronger correlation with gold mineralisation than large faults; (2) felsic to intermediate intrusive rocks show strong correlations with gold mineralisation, whereas metamorphic mafic rocks (greenstones) possibly acted as part of the broad regional environment for the mineral province rather than as a factor controlling the exact locations of the deposits; and (3) our predictive mapping indicates that the southeast part of the St Ives goldfield has a high potential for discovering new gold mineralisation.

Oligocene magmatism in the eastern margin of the east Himalayan syntaxis and its implication for the India–Asia post-collisional process

Post-collisional high Sr/Y-type magmatism in the southern Lhasa terrane extends ~1500km along the strike of the orogen. The origin and magma generation mechanism for these rocks remain controversial. We report a detailed study of such high Sr/Y-type magmatism in the east Himalayan syntaxis, SE Tibet, and explain its significance for understanding post-collisional geodynamical processes. The two-mica granites from the Beibeng, Damu, Bolonggong and Bomi plutons emplaced along the eastern margin of the east Himalayan syntaxis represent a high-K calc-alkaline suite (SiO2=67–75wt.%, Al2O3=13.3–15.8wt.%, K2O/Na2O=0.7–1.9). An outstanding geochemical feature is that they have high Sr/Y ratios (19–88, mostly >30), coupled with low Y and HREE, indicating that they were derived from partial melting of thickened lower crustal materials. Sr–Nd isotopic data show that the potential magma sources are the intermediate intrusive rocks related to the Jurassic–Eocene Gangdese magmatism and basement metasedimentary rocks. The contribution of the latter source increases from south to north. The two-mica granites from the Beibeng, Damu, Bolonggong and Bomi intrusions have magma crystallization ages of 29.9–27.5Ma, 27.6Ma, 27.1–26.5Ma and 23.7Ma, respectively. Our data show decreasing magma crystallization ages both from south to north along the eastern margin of the east Himalayan syntaxis and from east to west along the Gangdese belt. We suggest that such variations resulted from the diachronous breakoff of the subducted Indian continental slab. Beneath the east Himalayan syntaxis, the breakoff of the subducted Indian continental slab was initiated in the south and propagated towards north probably due to the thermal impact of a widening window opened by slab breakoff. Along the Gangdese belt, the breakoff of the subducted Indian continental slab commenced in east, and then propagated westwards.

Gravity evidence for a mafic intrusion beneath a mineralized zone in the Bondy gneiss complex, Grenville Province, Quebec — Exploration implications

A ground gravity survey over the Bondy gneiss complex and its mineralized iron oxide- and copper-rich hydrothermal system(s) in the Grenville Province of SW Quebec was undertaken to aid mineral exploration in mapping subsurface intrusions. Several kilometric-scale positive Bouguer anomalies were identified that coincide with outcropping mafic and intermediate intrusive rocks of the post peak-metamorphic, 1.17–1.16 Ga mafic to intermediate Chevreuil suite intrusions and a 1.09–1.07 Ga Rolleau ultramafic stock. An additional 4 × 3 kilometre positive gravity anomaly indicates a mafic body underlies part of the metamorphosed hydrothermal system in the area of magnetite , pyrite , pyrrhotite , and chalcopyrite mineralization . Advanced argilic alteration associated with sulphide enrichment here is however indicative of an epithermal system with a felsic intrusion fluid source. As a felsic intrusion cannot explain the positive Bouguer gravity anomaly both felsic and mafic bodies must be present beneath the mineralized zone. Our preferred interpretation based on integrating gravity data and 2D forward gravity modelling with the results of field and geochemical studies is that this anomaly corresponds to a ca. 500 m deep mafic 1.17–1.16 Ga Chevreuil suite pluton that may have provided the source for hydrothermal fluids associated with late ductile shear- and fault-related mineralization or remobilization of early mineralization associated with a felsic pluton into late structures. This interpretation is compatible with gabbro xenoliths in the 1.07 Ga Rivard lamprophyre dyke on the NW margin of the gravity anomaly that bear significant similarities with those of the Chevreuil intrusive suite. The presence of both early felsic and late mafic intrusions beneath a group of three mineral occurrences in the Bondy gneiss complex strengthens their prospectivity in comparison to other mineral occurrences in the area. That early, pre-metamorphic mineralization was upgraded late in the tectonothermal evolution during a subsequent igneous and deformation event agrees with interpretations of other IOCG-style deposits in the Lac Manitou area of the eastern Grenville Province. ► A ground gravity survey provides new data that aids mineral exploration. ► Most gravity anomalies are associated with mafic to intermediate intrusive rocks. ► A Gravity anomaly over mineralized outcrops may be generated by a deep gabbro. ► Mafic intrusion may be the driving source for mineralization or remobilisation.

Productos de erosión de las rocas intrusivas y plagio- granitos del complejo ofiolítico de Nicoya, Costa Rica

En este estudio se describe por primera vez la existencia de detrito plutónico en areniscas cretácicasdel noroeste de Costa Rica. Los fragmentos de rocas plutónicas son partículas accesorias, comúnmente presentes enlas areniscas piroxénicas que sobreyacen al Complejo Ofiolítico de Nicoya. Con base en el análisis modal de los granos del esqueleto, se analizan tres unidades de areniscas turbidíticas y neríticas de las Formaciones Rivas y El Viejo, respectivamente, cuya edad data del Campaniano Tardío-Maastrichtiano. Considerando los componentes prima-rios, estas areniscas muestran una composición muy similar a aquellas provenientes de arcos magmáticos. Se introducen dos parámetros modales para evaluar las contribuciones plutónicas. Éstos se definen como el índice de granos plutónicos con respecto al total de líticos (Lp+intercrec.QF/Lt) y el índice de piroxeno uralitizado con respectoal piroxeno total (uralPx/Px). Los valores modales del índice Lp+intercrec.QF/Lt indican, que los fragmentos plutó-nicos constituyen hasta un 9% del total de líticos. La relación relativamente proporcional entre ambos parámetrossugiere, que el piroxeno uralitizado también se deriva de la erosión de las rocas plutónicas. Las rocas-fuente de de-tríto plutónico corresponden con rocas intrusivas básicas hasta intermedias: gabros, doleritas y dioritas. Dos compo-nentes en particular - los intercrecimientos micrográficos de cuarzo y albita y el piroxeno uralitizado - son considerados como productos específicos de la erosión cretácica de las rocas intrusivas, principalmente plagiograníticas. Lasunidades de areniscas con detríto plutónico registran una erosión incipiente de los niveles inferiores de la secuenciaofiolítica de Costa Rica durante el Campaniano Tardío y Maastrichtiano. En este sentido, los fragmentos plutónicosy los granos de pedernales radiolaríticos asociados pueden ser usados como indicadores paleotectónicos del levan-tamiento vertical del orógeno de Costa Rica en el Cretácico Tardío. Este evento tectónico se correlaciona con el inicio de la Orogénesis Laramídica. Observaciones de campo evidencian que las rocas intrusivas del Complejo de Nicoya aún hoy día son una fuente miscelánea de sedimento en el noroeste de Costa Rica.

Geochronological and geochemical constraints on formation of the Tongling metal deposits, middle Yangtze metallogenic belt, east‐central China

The Tongling district is one of the most important non‐ferrous metal producers in China. The origin of Cu–Au deposits in the region is closely related to Late Mesozoic intermediate intrusions, which are mainly high‐K calc‐alkaline and shoshonitic series. Geochemical characteristics indicate that these granitic rocks are mixtures of more than two compositional end‐members, i.e. mantle‐derived melts and crust components incorporated through assimilation. Three important magmatic intrusions related to the Cu–Au deposits in the Tongling region – the Jiguanshi quartz monzodiorite, the Xishizishan quartz diorite and the Miaojia diorite porphyry – were selected for this study. Zircon U–Pb dating by LA ICP‐MS yielded two groups of ages (∼130–132 and 138–140 Ma) for these intrusions. Pyrite Re–Os age for the Xinqiao Cu–Fe–S deposit in the Tongling region is 126±11 Ma. Trace elements of zircon grains show that the earlier Cu–Au mineralization event was associated with adakitic rocks characterized by high positive Ce anomalies, produced at an elevated oxygen fugacity range. In contrast, later iron‐sulphur mineralization was closely related to low positive Ce anomalies, reflecting low oxygen fugacities. Considering that Pacific lithospheric subduction was the dominant factor that controlled major tectonic evolution in eastern China during the Early Cretaceous, the geochemical characteristics of these coeval ore‐forming intermediate intrusive rocks in the Tongling district were likely the result of Pacific plate underflow.

GEOCHRONOLOGY OF EPITHERMAL Au-Ag MINERALIZATION, MAGMATIC-HYDROTHERMAL ALTERATION, AND SUPERGENE WEATHERING IN THE EL PENON DISTRICT, NORTHERN CHILE

The El Penon epithermal gold-silver deposit, Region II, Chile, comprises multiple epithermal veins, which contain a geologic resource (measured + indicated + inferred) of 7 million oz (Moz) Au and 199 Moz Ag. The size, combined with bonanza Au grades, makes it the most important precious metal deposit in the Paleocene metallogenic belt of northern Chile. Quartz-adularia-carbonate veins are hosted in Paleocene and early Eocene mafic to felsic volcanic rocks that are hydrothermally altered up to 100s of meters from mineralized veins. Late Cretaceous and Eocene intermediate composition intrusive rocks also occur in the district and locally preserve alteration associated with magmatic-hydrothermal systems that are not related to precious metals mineralization of the El Penon deposit. Timing of hydrothermal activity in the district and age and duration of weathering are constrained by ages for vein adularia, hypogene alunite, and supergene alunite determined by Ar 39 /Ar 40 step-heating experiments. Adularias from the three largest veins of the El Penon deposit are dated at 52 to 53 Ma ( 40 Ar/ 39 Ar), indicating formation 1 to 3 m.y. later than host rhyolite domes. Magmatic-hydrothermal quartz-alunite alteration spatially associated with Late Cretaceous and Eocene intermediate composition intrusive rocks is dated at 73 to 65 and 50 to 49 Ma, respectively. Supergene alunite dates from the El Penon district indicate that weathering within a semiarid to arid climate occurred from 23 to 17 Ma, prior to the onset of hyperaridity.