Tectonothermal events in the Olympic IOCG Province constrained by apatite and REE-phosphate geochronology

The Olympic iron oxide–copper–gold province in South Australia contains numerous deposits and prospects, including the Olympic Dam Cu–U–Au–Ag deposit and the Acropolis prospect. The Acropolis prospect comprises massive, coarse-grained magnetite–apatite veins partly replaced by a hematite-stable assemblage. The apatite grains in the veins contain zones with abundant inclusions of other minerals (including monazite and xenotime) and low trace-element concentrations relative to the inclusion-free zones. The inclusion-rich apatite zones are interpreted to be formed from the recrystallisation of the inclusion-free apatite and remobilisation of U, Th and rare earth element (REE) from apatite into monazite and xenotime. Apatite, monazite and xenotime are all established U–Th–Pb geochronometers and offer the potential to constrain the alteration history of the Acropolis prospect. The LA-ICPMS U–Pb age of inclusion-free apatite is within error of the age of the host volcanic units (ca 1.59 Ga). Inclusion-rich apatite yields both near-concordant analyses that are within error of the inclusion-free apatite as well as highly disturbed (discordant) analyses. The most concordant analyses of monazite (Th–Pb) inclusions and xenotime (U–Pb) inclusions and rim grains indicate an alteration event occurred at ca 1.37 Ga and possibly also at ca 500 Ma. The disparity in age of the inclusion-rich apatite and the REE-phosphate inclusions (and rim grains) is suggested to be owing to the apatite being initially recrystallised at ca 1.59 Ga and modified again by a later event that also formed (or coarsened) most of the inclusions. Partial resetting of the majority of the monazite inclusions as well as the presence of significant amounts of common Pb has complicated the interpretation of the monazite results. In contrast, xenotime is a more robust geochronometer in this setting. The ages of the two post-1.59 Ga events that appear to have affected the Acropolis prospect do not correspond to any events known to have occurred in the Gawler Craton. The earlier (ca 1.37 Ga) age instead corresponds best with metamorphic–magmatic–hydrothermal activity in Laurentia, consistent with the proximity of Laurentia and the Gawler Craton inferred from palaeogeographic reconstructions. The later (ca 500 Ma) event corresponds to the Delamerian Orogeny and has been shown by prior studies to have also affected the Olympic Dam deposit.     

Age constraints on Tarkwaian palaeoplacer and lode-gold formation in the Tarkwa-Damang district, SW Ghana

Two major epigenetic gold-forming events are recorded in the world-class gold province of southwest Ghana. A pre-Tarkwaian event was the source of the world-class Tarkwa palaeoplacers whereas post-Birimian and Tarkwaian deformation, which was related to the Eburnean orogeny, gave rise to the world-class (e.g. Prestea) to giant (e.g. Obuasi) orogenic gold deposits which have made the region famous for more than 2,500 years. A maximum age of 2133±4 Ma for Tarkwaian sedimentation is provided by 71 of 111 concordant SHRIMP II U–Pb dates from detrital zircons in Tarkwaian clastic rocks from Damang and Bippo Bin, northeast of Tarkwa. The overall data distribution broadly overlaps the relatively poorly constrained ages of Birimian volcanism and associated Dixcove-type granitoid emplacement, indicating syntectonic development of the Tarkwaian sedimentary basin. These zircon ages argue against derivation of the palaeoplacer gold from an orogenic gold source related to the compressional phase of an orogeny significantly older than the Eburnean orogeny. Instead, they suggest that the gold source was either orogenic gold lodes related to an earlier compressional phase of a diachronous Eburnean orogeny or ca. 2200–2100 Ma intrusion-related gold lode. The CO₂-rich fluid inclusions in associated vein-quartz pebbles are permissive of either source. At the Damang deposit, an epigenetic, orogenic lode-gold system clearly overprinted, and sulphidised low-grade palaeoplacer hematite–magnetite gold occurrences in the Banket Series conglomerate within the Tarkwaian sedimentary sequence. Gold mineralisation is demonstrably post-peak metamorphism, as gold-related alteration assemblages overprint metamorphic assemblages in host rocks. In alteration zones surrounding the dominant, subhorizontal auriferous quartz veins, there are rare occurrences of hydrothermal xenotime which give a SHRIMP U–Pb age of 2063±9 Ma for gold mineralisation. The similar structural timing of epigenetic gold mineralisation in Tarkwaian host rocks at Damang to that in mainly Birimian host rocks elsewhere in southwest Ghana, particularly at Obuasi, suggests that 2063±9 Ma is the best available age estimate for widespread orogenic gold mineralisation in the region. Argon–argon ages of 2029±4 and 2034±4 Ma for hydrothermal biotite from auriferous quartz veins appear to represent uplift and cooling of the region below about 300 °C, as estimates of the temperature of gold mineralisation are higher, at around 400 °C. If peak metamorphism, with temperatures of about 550 °C, is assumed to have occurred at about 2100 Ma, the biotite ages, in combination with the xenotime age, suggest a broadly constant uplift rate for the region of about 1 km per 10 million years from about 2100 to 2025 Ma.     

松辽盆地徐家围子营城组流纹岩地球化学特征及构造指示意义

松辽盆地徐家围子营城组发育一套以流纹岩为主的中酸性火山岩。岩石薄片观察和主量、微量元素研究发现,后期热液蚀变、区域埋深及低温水合作用对流纹岩的Si、K、Rb等元素含量产生一定影响; 流纹岩明显富集Rb、Th、U、Pb等强不相容元素,Ba、Sr、Ti、Eu、P 负异常,暗示其经历了斜长石、磷灰石和钛铁矿分离结晶作用。流纹岩的(⁸⁷Sr/⁸⁶Sr)_i(0.705265 ~ 0.711895)值变化范围较大,大多数ε_Nd(t)为正值(1.83 ~ 3.38),Pb同位素比值相对集中,(²⁰⁶Pb/²⁰⁴Pb)_i=18.09～18.32,(²⁰⁷Pb/²⁰⁴Pb)_i=15.50 ~ 15.54,(²⁰⁸Pb/²⁰⁴Pb)_i=37.80 ~ 38.10。研究表明,岩石的源区为大比例年轻成分与少量古老地壳的混合部分熔融,其演化过程中经历了不同程度的地壳混染。松辽盆地早白垩世火山岩为板内伸展环境喷发的产物,可能与太平洋板块俯冲导致的中国东部岩石圈减薄、软流圈上涌密切相关。     

Low‐temperature thermochronology of francolite: Insights into timing of Dead Sea Transform motion

Cambrian siliciclastic sequences along the Dead Sea Transform (DST) margin in southern Israel and southern Jordan host both detrital fluorapatite [D‐apatite] and U‐rich authigenic carbonate‐fluorapatite (francolite) [A‐apatite]. D‐apatite and underlying Neoproterozoic basement apatite yield fission‐track (FT) data reflecting Palaeozoic–Mesozoic sedimentary cycles and epeirogenic events, and dispersed (U–Th–Sm)/He (AHe) ages. A‐apatite, which may partially or completely replace D‐apatite, yields an early Miocene FT age suggesting formation by fracturing, hydrothermal fluid ascent and intra‐strata recrystallisation, linked to early DST motion. The DST, separating the African and Arabian plates, records ~105 km of sinistral strike‐slip displacement, but became more transtensional post‐5 Ma. Helium diffusion measurements on A‐apatite are consistent with thermally activated volume diffusion, indicating Tc ~52 to 56 ± 10°C (cooling rate 10°C/Ma). A‐apatite AHe data record Pliocene cooling (~35 to 40°C) during the transtensional phase of movement. This suggests that timing of important milestones in DST motion can be discerned using A‐apatite low‐temperature thermochronology data alone.     

Apatite U–Pb dating and geochemistry of the Kyrgyz South Tian Shan (Central Asia): Establishing an apatite fingerprint for provenance studies

This paper presents an apatite U–Pb and geochemistry archive for exposed plutons and metamorphic rocks of the Kyrgyz South Tian Shan (STS) within the Central Asian Orogenic Belt. Apatite U–Pb dates and trace-element geochemistry are provided for 17 samples from late Carboniferous–early Permian I-type granites in the Terktinsky complex and A-type granites in the Kokshaal Range; early Devonian granites in the Kembel complex; Cryogenian granitoids and tuffs from the Middle Tian Shan and gneisses from the Atbashi metamorphic complex. These samples form a comprehensive selection of igneous and metamorphic rocks within the cores of Mesozoic basement highs that supplied detritus to adjacent basins such as the Tarim, Ferghana and Yarkand-Ferghana Basins. Generally, the granitoid samples preserve primary igneous apatite U–Pb ages that are within uncertainty of previously published zircon U–Pb dates. The apatites from the Atbashi metamorphic complex record anomalous Ordovician dates with large uncertainties that are interpreted as mixing ages between Cryogenian protolith formation and Carboniferous metamorphism. Principal component analysis discriminates apatite samples from the different bedrock terranes in the Kyrgyz STS based on their geochemical fingerprint and categorizes the samples with respect to an extensive apatite geochemical archive. The combined apatite-zircon archive provides a novel framework for provenance studies on the Meso–Cenozoic sedimentary history of the Central Asian Orogenic Belt.     

Mineralogical and fluid characteristics of the fluorite-rich Monakoff and E1 Cu–Au deposits,Cloncurry region,Queensland, Australia: Implications for regional F–Ba-rich IOCG mineralisation

The Monakoff iron oxide–Cu–Au (IOCG) deposit, located to the north east of Cloncurry within the Eastern Succession of the Mount Isa Inlier, Queensland, Australia, is characterised by high concentrations of F and Ba, with a host of other enriched elements including Co, Ag, Mn, REE, U, Pb, Zn and Sr. This gives the deposit a characteristic gangue assemblage dominated by fluorite, barite and calcite. The nearby E1 deposit, located 25 km to the NNE of Monakoff, and the large Ernest Henry deposit, 3 km to the west of E1, also contain abundant fluorite, barite and calcite in late stage assemblages. The three deposits, therefore, constitute a distinct group of IOCG deposits within the district, based on their F-rich geochemical and mineralogical affinities.The Monakoff ore zone is hosted in dilational openings along a shear zone developed within metasediments and metavolcanic rocks at the boundary between competent hangingwall rocks of the Toole Creek Volcanics and footwall rocks of the Mount Norna Quartzites. Four stages of alteration and mineralisation are recognised: Stage 1 garnet–biotite alteration; Stage 2 biotite–magnetite alteration; Stage 3 main F–Ba-ore mineralisation; and a Stage 4 pyrite–alloclasite Au–Co–As overprint. The E1 deposit has a more complex history, but Stage 5 has veins of fluorite–barite–carbonate that are comparable to Monakoff's main stage. The Stage 3 assemblage at Monakoff comprises a sheared groundmass of fluorite, barite, manganoan calcite, magnetite, chalcopyrite, pyrite, galena and sphalerite, with coarser grained pods of the same mineralogy interpreted to be dilational structures infilled during syn-ore deformation. Accessory minerals include U–Pb-oxides, REE–F-carbonates and Ag–Pb–Bi-sulfosalts, with no discrete Au minerals. The sulfosalts are interpreted to have formed from an immiscible Bi-melt within the mineralising fluid at temperatures higher than the melting point of Bi. The Stage 4 overprint at Monakoff contains pyrite and alloclasite. Laser ablation analyses of the sulphide minerals at Monakoff reveal that Stage 3 sulphides contain only trace amounts of Au (0.04 ppm in pyrite), although galena and chalcopyrite contain significant concentrations of Ag. Stage 4 pyrite and alloclasite, however, contain ~ 1 ppm Au in solid solution and mass balance calculations indicate the majority of bulk rock Au to be present in these minerals, although the majority of bulk Ag is present in Stage 3 sulphides. The Stage 5 veins at E1 have an identical gangue and accessory mineralogy to Stage 3 at Monakoff and differ in the sulphide mineralogy only in the lack of galena and sphalerite.Four fluid inclusion populations are identified within the fluorite at Monakoff: Group 1 is CO₂ rich; Group 2 is complex solid–liquid–vapour inclusions, with two groups based on homogenisation temperature (> 450 °C and 300–375 °C). Laser ablation-ICP-MS analyses indicate that these inclusions contain Cu, Pb, Zn, Fe, Mn, Mg, Ag, REE, U and Ba, but significantly no S, Se or Au; Group 3 are solid–liquid–vapour inclusions with a T_h of 200–275 °C, and contain Ba, Na, Mg, K and Br; and Group 4 are low salinity liquid–vapour inclusions. Group 1, 2 and 4 inclusions are also present in fluorite at E1. The REE geochemistry of fluorite from Monakoff and E1 is comparable and is characterised by a distinct positive Eu anomalies in all analyses, interpreted to indicate oxidising conditions at the time of high temperature ore deposition. The presence of abundant fluorite and barite is indicative of fluid mixing due to the insolubility of barite and fluorite and thus Ba and S, and Ca and F must have been introduced via different fluids. We propose that the oxidised fluid represented by the Group 2 inclusions and containing F, Ba, REE, U and base metals, mixed with a reduced, S-bearing fluid in a zone of dilation in the host shear zone that acted as a conduit for fluid flow during D₃ deformation. The source of the metal and F-rich fluid is likely to be the nearby granitic intrusions of the Williams–Naraku batholith, probably the Malakoff granite. This granite is also likely to be the source of the CO₂ represented by Group 1 fluid inclusions, and the REE, U, base metals and possibly Au, although the high Pb and Zn content of Monakoff and not E1 may suggest a local input of those elements at Monakoff. Stage 4 mineralisation overprints the F–Ba stage and is characterised by a Co–As–Au signature. At present it is unclear if this is a late stage, more reduced, evolution of the main ore fluid, or a separate mineralising event entirely.The presence of this F–Ba-metal-rich fluid has produced a distinctive style of IOCG mineralisation in the area to the north of Cloncurry. The probable link to the Malakoff granite implies that similar deposits may be present within several kilometres of the granite in suitable structural traps. Monakoff illustrates that although structurally controlled, the presence of Na–Ca alteration and ‘red rock’ K-alteration and brecciation are not key exploration criteria for these deposits. In addition, the presence of the overprinting As–Co–Au assemblage may indicate that this is a separate mineralising episode that may be present at other localities in the district. This study has also shown that fluorite can provide a powerful tool for determining ore forming conditions in F-rich IOCG systems.     

Anomalously silver-rich vein-hosted mineralisation in disseminated-style gold deposits,Jiaodong gold district,China

Structural measurements and geochemical analyses, including bulk and in situ pyrite geochemistry, sulfur isotopes, and whole-rock geochemistry, are presented for the No. 3 orebody of the Jiaojia gold deposit (JJ3), located in the Jiaodong district of northeast China. The JJ3 orebody is distinct from the main orebody of the Jiaojia deposit (JJ1) because it is characterised by steeply dipping sub-metric quartz-pyrite veins with up to 300 ppm of gold, whereas the JJ1 orebody represents an archetypal example of the disseminated and veinlet style mineralisation characteristic of regional faults in the Jiaodong district. Measurements on JJ3 veins and the host Jiaojia-Xincheng regional fault are consistent with development of mineralised, steeply dipping extension fractures during normal faulting, which produced the fault-hosted disseminated-style JJ1 orebody. Trace element geochemistry of pyrite in these veins shows that JJ3 pyrite is geochemically distinct from those of the main Jiaojia and Xincheng orebodies, being relatively enriched in Ag and Pb, as well as Ba, Bi, Te and Au, and relatively depleted in Cu and As. Enrichment in Ag and Pb is possibly related to infiltration of a saline hydrothermal fluid, as both are effectively transported as chloride complexes; however, depletion of Cu, which is also mobile as chloride complexes, requires a low temperature saline fluid where Cu is no longer soluble. The textural setting of the ore minerals suggests that these cooler fluids likely infiltrated during the waning stages of the hydrothermal system. The relative abundance of barite in the JJ3 orebody, which formed from late-stage oxidised magmatic–hydrothermal fluids, also supports the interpretation that the JJ3 orebody represents a late mineralisation event. The pervasive alteration surrounding the JJ3 orebody is K-feldspathic with a minor sericitic overprint, indicating an earlier higher temperature pervasive fluid flow event that was followed by low-temperature mineralising fluids. This interpretation implies that fracture dilation post-dated the earliest alteration, and that mineralisation and pervasive alteration in the JJ3 orebody are geochemically disconnected. Thus structural analysis is expected to be the most effective targeting method in future exploration for similar ore bodies.     

Mineralogy and Geochemistry of Pyrite and Arsenopyrite from the Zaozigou Gold Deposit in West Qinling Orogenic Belt,Central China: Implications for Ore Genesis

The Zaozigou gold deposit lies in the West Qinling orogenic belt, Gansu Province, China. It is one of the largest gold deposits, and the orebodies are hosted in fine‐grained slates intercalated with limestone of the Middle‐Triassic Gulangdi Formation and varied dykes. The gold orebodies are strictly controlled by the NE‐, NW‐, and SN‐trending tensional and shearing faults with high dipping angle. The mineralogy and geochemistry of pyrite and arsenopyrite are measured by electron microprobe. Pyrite has up to 0.12 wt.% Au, and arsenopyrite contains up to 0.17 wt.% Au. The antithetic correlation between S and As indicates the substitution of As for S in pyrite, and arsenic occurs in anionic As^1? state in the pyrite structure under the reduced conditions. Pyrite has relatively high Co (~364–2248 ppm) but relatively low Ni (~109–497 ppm) contents, with Co/Ni ratios ranging from ~1.63 to 10.50, indicating that the deposit originated from a volcanogenic fluid and remobilized by hydrothermal fluid. Au in arsenopyrite occurs as cationic Au in solid solution, whereas Au in pyrite is in solid solution and metal nanoparticles (Au⁰). The texture characteristics and trace element geochemistry among cores, transition zones, and rims of pyrites demonstrate that there are at least four pulses of fluid participating in the generation of pyrite in the deposit. The calculated formation temperatures of the Zaozigou deposit vary from 148°C to 304°C, with an average temperature of 213°C based on Au contents in pyrite. The Pb isotopic compositions of pyrite samples suggest that the metallogenic materials of the Zaozigou deposit were derived from the mantle and upper crust. All the characteristics above lead us to draw the conclusion that the Zaozigou gold deposit is classified as an epithermal deposit.     

Thermochronology of Mesozoic Sandstone from the Beipiao Basin and Its Implication to Meso-Cenozoic Tectonic Evolution of the Eastern Yan-Liao Orogenic Belt

1 IntroductionThe Yan-Liao orogenic belt lies in the northern segmentof the North China Block (NCB) (Fig. 1). During Mesozoicto Cenozoic time, it experienced intense tectono-magmaticactivation, accompanied by the formation ofintracontinental basins and widespread magmatism and is avery important area to study continental dynamics andMeso-Cenozoic tectonic evolution in eastern China. Mostof previous work in this area has focused on the formationof basement, structural style and volcano-se…     

Proto-Andean evolution of the Eastern Cordillera of Peru

The Eastern Cordillera of Peru represents one of the longest (> 1200 km) Paleozoic metamorphic and magmatic belts exposed along the western Andean margin of South America. In this study, we examine the tectonothermal evolution of a key segment of the metasedimentary basement of the Eastern Cordillera of Peru (the Huaytapallana Complex) and demonstrate that it has experienced a hitherto undocumented high-grade orogenic event at 260 Ma (latest Middle Permian) based on U–Pb and Th–Pb monazite age data from paragneisses and U–Pb dating of zircon rims from leucosomes. These ages are interpreted as recording crystallization and are consistent with 255 Ma rutile growth in lower-grade units. U–Pb apatite data (c. 260–230 Ma) in all units are consistent with slow cooling from this 260 Ma metamorphic peak. U–Pb zircon geochronology of pre-tectonic plutons yield ages ranging from c. 302 Ma to c. 260 Ma. These geochronological data are augmented by new U–Pb apatite age data from other segments along the Eastern Cordillera of Peru. A regional synthesis of existing geochronological constraints from the Eastern Cordillera of Peru demonstrates that the margin has experienced a polycyclic orogenic history. Deformation and magmatism occurred at c. 480 Ma and c. 435 Ma during the Famatinian orogenic cycle, was followed by a Late Silurian to Early Carboniferous (c. 420–350 Ma) magmatic and metamorphic gap, and terminated with Gondwanide magmatism and metamorphism at c. 315 Ma and c. 260 Ma. These Famatinian and Gondwanide orogenic phases can be correlated into the Proto-Andean margin of Argentina and Chile and are thus of regional extent. The evolution of the Proto-Andean margin is thus best explained by changes in tectonic plate reorganization in a long-lived Paleozoic accretionary orogen which was undergoing phases of advance and retreat, resulting in magmatic pulses and orogenic phases which can be correlated along the length of the plate boundary.     

Young porphyries,old zircons: new constraints on the timing of deformation and gold mineralisation in the Eastern Goldfields from SHRIMP U–Pb zircon dating at the Kanowna Belle Gold Mine,Western Australia

The Kanowna Belle Gold Mine is a Late Archaean orogenic lode-gold deposit hosted by felsic volcaniclastic and intrusive rocks (porphyries) of the Kalgoorlie Terrane, Western Australia. Rare gold occurs in fragments of veins and alteration that form clasts within the Black Flag Group volcaniclastic rocks at the Kanowna Belle mine, indicating that epithermal gold mineralisation accompanied Black Flag Group volcanism. The SHRIMP U–Pb zircon age of the volcaniclastic unit is 2668±9 Ma, and xenocrystic zircons with ∼2.68, 2.70 and 2.71 Ga age groupings are common. The Black Flag Group rocks are faulted by a D1 thrust, and ∼2670 Ma is thus an older limit for regional D1 deformation. Although SHRIMP U–Pb zircon ages of felsic porphyries commonly give the best constraints on the timing of deformation and structurally controlled gold mineralisation, the data are complex and dates from single samples can be ambiguous. Four Porphyry samples from the Kanowna Belle Gold Mine were analysed. Backscattered electron and cathodoluminescence imaging show that most magmatic zircon in the porphyries is either high-U and metamict, or restricted to rims on older xenocrysts that are too narrow to be dated by SHRIMP. Some porphyries appear to have been saturated with zircon at source and contain only xenocrystic zircons. Zircons that are interpreted to be magmatic in a sample of the mineralised Kanowna Belle Porphyry gives a mean age of 2655±6 Ma. The Kanowna Belle Porphyry is cross cut by regional D2 fabrics and ∼2655 Ma is thus the maximum age for regional D2 deformation. This is a maximum age for epigenetic lode-gold mineralisation. The age of resetting of high-U zircon grains (2.63 Ga) and the age of ore-related Pb–Pb galenas (2.63 Ga) serves as an approximate date for lode-gold mineralisation. If the complex zircon history of the felsic porphyries at Kanowna Belle is typical of this suite throughout the Eastern Goldfields Province, it is clear that existing single zircon dates from this Province require reevaluation, backed up by careful backscattered and cathodoluminescence imaging and textural studies.     

Orogenic gold mineralisation hosted by Archaean basement rocks at Sortekap, Kangerlussuaq area, East Greenland

A gold-bearing quartz vein system has been identified in Archaean basement rocks at Sortekap in the Kangerlussuaq region of east Greenland, 35 km north–northeast of the Skaergaard Intrusion. This constitutes the first recorded occurrence of Au mineralisation in the metamorphic basement rocks of east Greenland. The mineralisation can be classified as orogenic style, quartz vein-hosted Au mineralisation. Two vein types have been identified based on their alteration styles and the presence of Au mineralisation. Mineralised type 1 veins occur within sheared supracrustal units and are hosted by garnet-bearing amphibolites, with associated felsic and ultramafic intrusions. Gold is present as native Au and Au-rich electrum together with arsenopyrite and minor pyrite and chalcopyrite in thin alteration selvages in the immediate wall rocks. The alteration assemblage of actinolite-clinozoisite-muscovite-titanite-scheelite-arsenopyrite-pyrite is considered to be a greenschist facies assemblage. The timing of mineralisation is therefore interpreted as being later and separate event to the peak amphibolite facies metamorphism of the host rocks. Type 2 quartz veins are barren of mineralisation, lack significant alteration of the wall rocks and are considered to be later stage. Fluid inclusion microthermometry of the quartz reveals three separate fluids, including a high temperature (T _h ?=?300–350 °C), H₂O–CO₂–CH₄ fluid present only in type 1 veins that in interpreted to be responsible for the main stage of Au deposition and sulphidic wall rock alteration. It is likely that the carbonic fluids were actually trapped at temperatures closer to 400 °C. Two other fluids were identified within both vein types, which comprise low temperature (100–200 °C) brines, with salinities of 13–25 wt%?eq. NaCl and at least one generation of low salinity aqueous fluids. The sources and timings of the secondary fluids are currently equivocal but they may be related to the emplacement of Paleogene mafic intrusions. The identification of this occurrence of orogenic-style Au mineralisation has implications for exploration in the underexplored area of east Greenland between 62 and 69°?N, where other, similar supracrustal units are known to be present.     

Petrogenesis and timing of emplacement of porphyritic monzonite,dolerite, and basalt associated with the Kuoerzhenkuola Au deposit,Western Junggar,NW China: implications for early Carboniferous tectonic setting and Cu–Au mineralization prospectivity

ABSTRACT

The Kuoerzhenkuola epithermal Au deposit is located in the northern part of the West Junggar region of NW China and is underlain by a recently discovered porphyritic monzonite intrusion that contains Cu–Au mineralization. Zircon LA-ICP-MS U–Pb dating of this intrusion yielded an age of 350 ± 4.7 Ma. The porphyritic monzonite is calc-alkaline and is characterized by high concentrations of Sr (583–892 ppm), significant depletions in the heavy rare earth elements (HREE; e.g. Yb = 0.96–2.57 ppm) and Y (10.4–23.3 ppm), and primitive mantle-normalized multi-element variation diagram patterns with positive Sr and Ba and negative Nb and Ti anomalies, all of which indicate that this intrusion is compositionally similar to adakites elsewhere. The composition of the porphyritic monzonite is indicative of the derivation from magmas generated by the melting of young subducted slab material. The area also contains Nb-enriched basalts that are enriched in sodium (Na₂O/K₂O = 1.20–3.90) and have higher Nb, Zr, TiO₂, and P₂O₅ concentrations and Nb/La and Nb/U ratios than typical arc basalts. The juxtaposition of adakitic rocks, Nb-enriched basalts, and dolerites in this region suggests that the oceanic crust of the expansive oceans within the West Junggar underwent early Carboniferous subduction. Magnetite is widespread throughout the Kuoerzhenkuola Au deposit, as evidenced by the volcanic breccias cemented by late hydrothermal magnetite and pyrite. In addition, the zoned potassic, quartz-sericite alteration, and propylitic and kaolin alteration in the deeper parts of the porphyritic monzonite are similar to those found in porphyry Cu–Au deposits. These findings, coupled with the mineralogy and geochemistry of the alteration associated with the Kuoerzhenkuola Au deposit, suggest that the mineralization in this area is not purely epithermal, with the geology and geochemistry of the porphyritic monzonite in this area suggesting that a porphyry Cu–Au deposit is probably located beneath the Kuoerzhenkuola Au deposit.     

New data for geology and geochemistry of the Rodion gold–quartz deposit,northeastern Russia

This paper is focused on the new data for geology, mineralogy, and geochemistry of stockworks consisting of steep and gentle quartz veins and veinlets forming a complex multilevel structure at the Rodion deposit. These stockworks range from 25 to 150 m in thickness. Average gold grade is 1.8 g/t. Ore minerals pyrite, arsenopyrite, chalcopyrite, sphalerite, galena, and native gold are predominantly concentrated on the vein and veinlet walls. Thermal metamorphism caused by the intrusion of the Ulakhan granodiorite pluton is the important singularity of the deposit. The deposit ore is enriched in chalcophile microelements Au, Ag, As, Sb, Cu, Pb, Zn, and Bi as compared to the average composition of the upper crust and hosting Permian sequences. The enrichment factors range from a few to hundreds of times. Bi, W, Pb, Ag, and Na₂O are positively correlated between each other and with Au. The highest correlation coefficient 0.59 is between Au and Bi. Au is negatively correlated with Ba, Li, Co, Ni, Mn, Ti, and Be. The stockwork ores were formed involving homogeneous low-saline (9.4–4.3 wt % NaCl equiv) substantially aqueous bicarbonate-chloride fluid at 275–330°C and 300–1840 bar fluid pressure. Fluid has a high concentration of CO₂ (up to 349 g/kg of water) and is reductive (СО₂/СН₄ = 17–37.3). Na and Ca are the major cations in the fluid, whereas K and Mg are minor. In addition, many microelements were detected in the fluid: As, Li, Rb, Cs, Mo, Ag, Sb, Cu, Zn, Cd, Pb, U, Ga, Ge, Ti, Mn, Fe, Co, Ni, V, Cr, Y, Zr, Sn, Ba, W, Au, Hg, and REE. The results obtained are consistent with the metamorphic–magmatic formation model of orogenic gold–quartz deposits within the Yana–Kolyma belt.     

Linking uplift and mineralisation at the Mount Novit Zn-Pb-Ag Deposit,Northern Australia: Evidence from geology,U–Pb geochronology and sphalerite geochemistry

The subeconomic Mount Novit Zn-Pb-Ag deposit is located approximately 20 km south of Mount Isa, Queensland. In contrast to the nearby Mount Isa, Hilton and George Fisher Zn-Pb-Ag deposits, mineralisation at Mount Novit is situated to the west of the regional-scale Mount Isa Fault and is hosted in the Moondarra Siltstone as opposed to the Urquhart Shale. Lower-grade (<4 wt.% Zn + Pb) Zn-Pb-Ag mineralisation primarily replaces pre-existing carbonate alteration and veining and consists of pyrrhotite, pyrite and sphalerite with lesser galena. Higher-grade (>10 wt.% Zn + Pb) mineralisation occurs as a matrix supported breccia dominated by sphalerite and pyrrhotite with galena, pyrite, and magnetite. In-situ U–Pb geochronology was completed on apatite and two textural varieties of monazite. Fine-grained (<50 µm) subhedral to anhedral monazite is located within highly foliated biotite alteration directly adjacent Zn-Pb-Ag mineralisation and yields a mean weighted ²⁰⁷Pb/²⁰⁶Pb age of 1527 ± 18 Ma (MSWD = 1.06). This age is consistent with the formation of highly foliated biotite alteration during D₃ deformation of the Isan Orogeny. Apatite from the same fabric yields a lower intercept age of 1443 ± 29 Ma (MSWD = 1.30). Consistent with previous studies, this age is interpreted to represent the age of a major thrusting event along the Mount Isa Fault that resulted in the cooling of the Mount Novit area below ～375 °C. Coarse-grained monazite is coeval with Zn-Pb-Ag mineralisation and yields a mean weighted ²⁰⁷Pb/²⁰⁶Pb age of 1457 ± 11 Ma (MSWD = 0.28). Sphalerite from Mount Novit has low concentrations (<1 ppm) of Ge and Ga and a relatively high concentration of In (5 to >10 ppm), possibly reflecting the leaching of the metals from an underlying basement unit. The GGIMFis geothermometer (Frenzel et al., 2016) produced a mean formation temperature of 345 ± 52 °C. The timing and temperature of Zn-Pb-Ag mineralisation is consistent with the age and cooling temperature of apatite presented in this study. Based on these correlations, we suggest that Zn-Pb-Ag mineralisation at Mount Novit was emplaced during an episode of major thrusting along the Mount Isa Fault, with the precipitation of Zn-Pb-Ag mineralisation driven by the cooling of the Mount Novit area below ～375 °C. A key implication of this study is a new model for synorogenic Zn-Pb-Ag mineralisation to the south of Mount Isa, which contrasts with the widely accepted regional-scale syngenetic metallogenic model.     

Cooling and exhumation of the oldest Sanqiliu uranium ore system in Motianling district,South China Block

The Sanqiliu uranium deposit belongs to a uranium ore system in Motianling district. It is the oldest uranium deposit in South China. Primary uranium mineralization occurred almost simultaneously with the emplacement of the host granites and subsequent dykes, and it has a relatively high grade of uranium (0.421%). We clarify the age of mineralization and investigate the cooling history through new pitchblende U–Pb and apatite fission‐track thermochronology. The pitchblende U–Pb results indicate that uranium mineralization occurred at ~801–759 Ma. Fractionation of uranium and lead at ~374–295 Ma is interpreted as remobilization and resetting of the original uranium. The Motianling area has apatite fission‐track ages of 57 to 18 Ma. By combining our results with previous work, we conclude that the deposit cooled slowly and was exposed at the surface during the Cenozoic. The timing and depth of exhumation helped to preserve and avoid erosion of the uranium deposit, and highlight the potential for regional uranium exploration.     

Alteration paragenesis and mineral chemistry of the Tjårrojåkka apatite–iron and Cu (-Au) occurrences, Kiruna area, northern Sweden

The northern Norrbotten area in northern Sweden, is an important mining district and hosts several deposits of Fe-oxide Cu-Au-type. One of the best examples of spatially, and possibly genetically, related apatite–iron and copper–gold deposits in the region is at Tjårrojåkka, 50 km WSW of Kiruna. The deposits are hosted by strongly sheared and metamorphosed intermediate volcanic rocks and dolerites and show a structural control. The Tjårrojåkka iron deposit is a typical apatite–iron ore of Kiruna-type and the Tjårrojåkka copper occurrence shows the same characteristics as most other epigenetic deposits in Norrbotten. The host rock has been affected by strong albite and K-feldspar alteration related to mineralisation, resulting in an enrichment of Na, K, and Ba. Fe and V were depleted in the altered zones and added in mineralised samples. REE were enriched in the system, with the greatest addition related to mineralisation. Y was also mobile associated with albite alteration and copper mineralisation. The Tjårrojåkka iron and copper deposits show comparable hydrothermal alteration minerals and paragenesis, which might be a product of common host rock and similarities in ore fluid composition, or overprinting by successive alteration stages. Mineralogy and mineral chemistry of the alteration minerals (apatite, scapolite, feldspars, amphiboles, and biotite) indicate a higher salinity and Ba/K ratio in the fluid related to the alterations in the apatite–iron occurrence than in the copper deposit, where the minerals are enriched in F and S. The presence of hematite, barite, and in SO₄ in scapolite suggests more oxidising-rich conditions during the emplacement of the Tjårrojåkka-Cu deposit. From existing data it might be suggested that one evolving system created the two occurrences, with the copper mineralisation representing a slightly later product.     

Implications of pyrite geochemistry for gold mineralisation and remobilisation in the Jiaodong gold district,northeast China

The Jiaodong gold district of eastern China, the largest gold producing district in China, is located on the eastern margin of the North China Craton. It consists of three mineralisation belts: the western Zhao-Ye belt, the middle Qixia belt, and the eastern Muping–Rushan (Muru) belt. Over 85% of mineralisation is hosted in the Zhao-Ye belt, which is bordered by the mantle-tapping Tan Lu fault zone. Pyrite crystals from three deposits in the Zhao-Ye belt and three deposits in the Muru belt were studied using a combination of optical petrography, bulk pyrite geochemistry, and in-situ laser ablation ICP-MS. Results show that although mineralisation is broadly similar between the two belts, there are significant differences in ore and gangue mineral textures, pyrite geochemistry, and style of gold mineralisation.Texturally, pyrite grains from the Zhao-Ye belt are generally cubic and do not exhibit zoning. In contrast, Muru pyrite grains are more often pyritohedral, commonly exhibit well-defined concentric zoning, and display textures in ore and gangue minerals indicative of open space growth. Bulk pyrite geochemistry suggests a distinct enrichment in Pb, Bi, Au, Ag and Te in the Zhao-Ye belt, whereas the Muru belt pyrite is significantly enriched in As, Cu and Co. In situ pyrite geochemistry indicates that Au and As are variably correlated in the Zhao-Ye belt, typically only exhibiting correlation at low Au concentrations. Most gold occurs as visible electrum along pyrite fractures and grain boundaries, with a minor generation of invisible gold formed through As-facilitated uptake into pyrite. In the Muru belt, Au and As have a strong correlation and there is limited occurrence of gold particles, indicating that most gold in the Muru belt is invisible gold contained in the crystal structure of As-rich pyrite.The differences in style of gold mineralisation between the belts indicates an inherent difference in timing of gold introduction: in the Zhao-Ye belt, the visible electrum accounting for most of the gold endowment is formed post-pyrite, whereas the invisible gold in the Zhao-Ye and Muru belts is formed syn-pyrite. The heterogeneity in gold distribution in the Jiaodong district is attributed to melting of metallogenically fertile Archean crust at the base of the well-endowed Zhao-Ye belt, and the lack of a similarly fertile source region beneath the Muru belt.     

Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust

The U–Pb isotopic system in zircon is the tool of choice to interrogate high-temperature geological processes, yet this system has potential to investigate lower temperature fluid–rock interaction as well. In some cases, removal of radiogenic Pb is incomplete, potentially allowing regression of discordant U–Pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid-driven isotopic disturbance. However, in rocks preserving more complex histories, simple regression is not effective at resolving multiple Pb loss events. Here, we use a ‘concordant–discordant comparison’ (CDC) test to establish the times of U–Pb disturbance in the Acasta Gneiss Complex (AGC), Canada. AGC c. 4.03 to c. 3.40 Ga orthogneisses experienced a long and complex post-crystallization history, for which U–Pb zircon data reflects not only the heterogeneous nature of the rock, but also the varying degrees and duration of crustal reworking that inevitably involved open system processes. The CDC test calculates the similarity between the concordant age structure and a modelled age structure, the latter inferred from discordant analyses, over a wide range of potential disturbance times. Our analysis reveals concordant zircon components implying new growth and/or recrystallization at 3992 ± 5, 3501 ± 6, 3442 ± 5 and 3126 ± 6 Ma. In addition, we establish episodes of radiogenic-Pb loss driven by fluid–rock interaction at 3150 ± 50 Ma, and probably at 2875 ± 50 Ma and c. 2590 Ma. These Pb-loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism, granite emplacement, and unroofing. Pb-loss within the AGC shows an antithetic relationship in different samples that are in close geographic proximity. We suggest that zircon alteration and associated new growth effectively rendered those grains that underwent Pb-loss at a particular time less susceptible to alteration during the next episode of fluid interaction.     

新疆东准噶尔金水泉金矿床地质特征、成矿时代及其地质意义

新疆奇台县金水泉金矿床是东准噶尔卡拉麦里成矿带上一个典型的造山型金矿床,产于清水-苏吉泉大断裂和卡拉麦里深断裂之间的次级断裂带中,其成矿时代尚无确切的年代学资料。通过阴极发光(CL)、背散射图像(BSE)和矿物包体确定含金石英脉中的锆石种类,进行LA-ICP-MS锆石U-Pb同位素定年。捕获锆石主要呈柱状、长柱状,振荡环带清晰,粒径较热液锆石大,包括岩浆锆石和变质锆石;~(206)Pb/~(238)U年龄为365~418Ma、265Ma。热液锆石呈短柱状、棱角状、不规则状、双锥状,常含继承锆石内核,内部包裹体为磷灰石或含金磷灰石,发育不明显的波缓状振荡环带或无振荡环带;~(206)Pb/~(238)U年龄加权平均值为314.9±3.2Ma,指示金矿化作用发生于晚石炭世早期碰撞造山向造山后伸展转变的构造环境。成矿时代晚于赋矿围岩姜巴斯套组的年龄(约343Ma),与总结区域构造演化规律所限定的时间范围(320~310Ma)吻合。综合研究认为,金水泉金矿床成矿作用可划分为350~320Ma造山阶段含金流体生成、315Ma挤压-伸展转换阶段金迁移就位、二叠纪陆内变形阶段金矿体破碎变形3个阶段。