Archean Basement Research Articles

Geological and Geochemical Characteristics of the Archean Basement-Hosted Gold Deposit in Pinglidian, Jiaodong Peninsula, Eastern China: Constraints on Auriferous Quartz-Vein Exploration

The gold deposits that are hosted in the Archean metamorphic rock, have yet to be explored beyond Pinglidian gold deposit in the northwestern Jiaodong Peninsula, eastern China. This kind of gold deposit differs from those that are hosted in Mesozoic granitoids, showing good potential for the prospecting of auriferous quartz-vein gold deposits controlled by the structures in greenfield Archean metamorphic rock. Pinglidian gold deposit is located in the hanging wall of the Jiaojia fault and consists of eight separated orebodies that are enveloped by altered rock in Archean biotite plagiogneiss. These orebodies and wall-rock alterations are strongly controlled by local structures that formed during the Mesozoic rotation and kink folding of the foliated and fissile Archean basement host. The major wall-rock alterations comprise sericitization, silicification, pyritization, and carbonation, which is up to 18 m in width and progressively increases in intensity towards the auriferous quartz vein. The visible gold is present as discrete native gold and electrum grains, which have basically filled in all manner of fractures or are adjacent to galena. We recognize two types of gold bearing quartz veins that are associated with mineral paragenetic sequences during hydrothermal alteration in the Pinglidian gold deposit. The petrological features and geochemical compositions in the reaction fronts of the alteration zone suggest variations in the physicochemical conditions during ore formation. These minerals in the wall rock, such as plagioclase, biotite, zircon, titanite, and magnetite, have been broken down to hydrothermal albite, sericite, and quartz in a K–Na–Al–Si–O–H system, and sulfides in a Fe–S–O–H system. The major and trace elements were calculated by the mass-balance method, showing gains during early alteration and losses during late alteration. The contents of K2O, Na2O, CaO, and LOI varied within the K–Na–Al–Si–O–H system during alteration, while Fe2O3 and MgO were relatively stable. Rare-earth elements (REE) changed from gains to losses alongside the breakdown of accessory minerals, such as large ion lithophile elements (LILE). The Sr and Ba contents exhibited high mobility during sericite-quartz alteration. Most of the low-mobility high-field strength elements (HFSE) were moderately depleted, except for Pb, which was extremely high in anomalous samples. The behavior of trans-transition elements (TRTE) was related to complicated sulfides in the Fe–S–O–H system and was constrained by the parameters of the mineral assemblages and geochemical compositions, temperature, pressure, pH, and fO2. These factors during ore formation that were associated with the extents and intensity of sulfide alteration and gold precipitation can be utilized to evaluate the potential size and scale of an ore-forming hydrothermal system, and is an effective exploration tool for widespread auriferous quartz veins in Archean metamorphic basements.

Detrital zircon age spectra of the Yungou Formation and its constrain to the related block affinity, western Yunnan

滇西允沟岩组为一套低绿片岩相的泥质浅变质岩，岩性以云母石英片岩、云母石英千枚岩、云母片岩、细晶灰岩为主，局部夹硅质岩，一直以来被认为是前寒武纪变质基底的重要组成部分。其原岩以碎屑岩为主，含有部分灰岩、白云岩，是一套形成于特提斯洋被动大陆边缘的半深海-深海沉积物，因此，是研究特提斯洋构造演化的重要窗口。但是，对允沟岩组的形成时代至今仍存在较大争议，为此本文选取允沟岩组中的片岩进行了碎屑锆石U-Pb精确定年。分析结果表明，允沟岩组原岩沉积时代为新元古代晚期-寒武纪期间（551~491Ma），主要由新太古代、新元古代及少量古元古代碎屑物质组成。其中，大量太古宙晚期碎屑锆石表明其源区有太古宙基底的存在，而1749Ma这组碎屑锆石可能与哥伦比亚超大陆聚散有关，956Ma和848Ma这两组锆石记录的事件可能是对罗迪尼亚超大陆三阶段裂离事件的前两期的响应。此外，还含有少量泛非运动信息，但泛非运动对其源区的影响极其有限。结合前人研究成果和本文碎屑锆石年龄谱，显示允沟岩组形成于原特提斯洋的被动陆缘，与印度板块、南羌塘地块具有显著的亲缘性。;The Yungou Formation in western Yunnan Province, a suit of metapelites of lower greenschist facies, consists dominantly of mica quartz schist, mica quartz phyllite, mica schist and fine crystalline limestone, with some siliceous rocks. It has always been considered as an important part of the Precambrian metamorphic basement. The protoliths are mainly clastic rocks with some limestone and dolomite, belonging to a set of semi-deep sea sediments formed in the passive continental margin of the Tethys Ocean. So the Yungou Formation witnessed the origin, development and extinction of the Tethys Ocean, and played a great role in understanding the tectonic evolution of the Tethys Ocean. However, its forming age is still under controversial. In this paper, the zircon U-Pb dating of schists from the Yungou Formation, southwestern Yunnan Province, has been carried out. The analysis results show that the depositional age of the protolith of the Yungou Formation is in Late Neoproterozoic-Cambrian (551~491Ma), and mainly composed of Neoarchean, Neoproterozoic and some Paleoproterozoic detrital rocks. The discovery of massive Neoarchean detrital zircons indicates the existence of the Archean basement in the source area. The 1749Ma zircons may be related to the Columbia Supercontinent from assembly to breakup. The zircons of 956Ma and 848Ma may be a response to the former two stages of the three-stage-rifting evolution of the Rodinia Supercontinent. In addition, the dating result also revealed the existence of pan-African tectono-thermal event, but its impact on the source region of the Yungou Formation is extremely limited. Combined with previous study, the U-Pb age spectra of detrital zircons shows that the Yungou Formation developed on the passive continental margin of the Proto-Tethys Ocean and when it deposited this block has the good affinity of the Indian and South Qiangtang blocks.

Exploration of deep terrestrial subsurface microbiome in Late Cretaceous Deccan traps and underlying Archean basement, India

Scientific deep drilling at Koyna, western India provides a unique opportunity to explore microbial life within deep biosphere hosted by ~65 Myr old Deccan basalt and Archaean granitic basement. Characteristic low organic carbon content, mafic/felsic nature but distinct trend in sulfate and nitrate concentrations demarcates the basaltic and granitic zones as distinct ecological habitats. Quantitative PCR indicates a depth independent distribution of microorganisms predominated by bacteria. Abundance of dsrB and mcrA genes are relatively higher (at least one order of magnitude) in basalt compared to granite. Bacterial communities are dominated by Alpha-, Beta-, Gammaproteobacteria, Actinobacteria and Firmicutes, whereas Euryarchaeota is the major archaeal group. Strong correlation among the abundance of autotrophic and heterotrophic taxa is noted. Bacteria known for nitrite, sulfur and hydrogen oxidation represent the autotrophs. Fermentative, nitrate/sulfate reducing and methane metabolising microorganisms represent the heterotrophs. Lack of shared operational taxonomic units and distinct clustering of major taxa indicate possible community isolation. Shotgun metagenomics corroborate that chemolithoautotrophic assimilation of carbon coupled with fermentation and anaerobic respiration drive this deep biosphere. This first report on the geomicrobiology of the subsurface of Deccan traps provides an unprecedented opportunity to understand microbial composition and function in the terrestrial, igneous rock-hosted, deep biosphere.

On the so-called “leukogranulite formation” and “greenlevada suite” of Early Precambrian of the Ukrainian Shield

It has been shown using as an example the geological structure of the Precambrian basement of the Middle Bug area (the Ukrainian Shield) that the chronostratigraphic scheme of the Lower Archean needs to be modified, in particular, greenlevada suite of the Dniester-Bug series represented by supercrustal leucogranulite rock formation sho-uld be included. It has been found that leucogranulite formation of the Bug area are rocks (granitized biotite and garnet-biotite gneisses) which belong to formation-gene-tic type of quartz-feldspar metasomatites, particularly, to structural-formational association of metasomatites from regional fault zones. According to the data of geological, geochemical, tectonophysical, radioisotopic studies of the similar rocks in the exposures of the middle stream of the river South Bug, it has been proved that they were formed around 2.03 Ga ago as a result of diaftoresis of rocks of the Archean basement and their granulitic appearance as well as subvertical occurrence are related to dyna-mometamorphic including shear processes. Methodic problems of the studies of the age relations of the Catarchean rock complexes are under discussion. The well-known geological cross-section of Khoshchuvato-Zavalye site of the Middle Bug area has been considered and it has been shown that the stratigraphic method based on the principle of sequence of stratifying of supercrustal rocks is impossible to be effectively used in such complexes because of their strong tectonic and intrusive-metasomatic reprocessing, practical impossibility of their initial bedding to be found. The best way under these conditions is to be supported by more objective petrostructural, microstruc-tural, petrochemical, including radioisotopic and geophysical data.

Zircon U-Pb and Lu-Hf isotope constraints on Archean crustal evolution in Southeastern Guyana Shield

The southeastern Guyana Shield, northeast Amazonian Craton, in the north of Brazil, is part of a widespread orogenic belt developed during the Transamazonian orogenic cycle (2.26–1.95 Ga) that includes a large Archean continental landmass strongly reworked during the Transamazonian orogeny, named Amapá Block. It consists mainly of a high-grade metamorphic granulitic-migmatitic-gneiss complex, of Meso- to Neoarchean age and Rhyacian granitoids and supracrustal sequences. For the first time, coupled U-Pb and Lu-Hf isotope data were obtained on zircon by LA-ICP-MS from five tectono-stratigraphic units of the Archean basement and one Paleoproterozoic intrusive rock, in order to investigate the main episodes of crustal growth and reworking. Whole-rock Sm-Nd isotope data were compared to the zircon Lu-Hf data. Three main magmatic episodes were defined by U-Pb zircon dating, two in the Mesoarchean (∼3.19 Ga and 2.85 Ga) and one in the Neoarchean (∼2.69–2.65 Ga). Subchondritic ɛHf(t) values obtained for almost all investigated units indicate that crustal reworking processes were predominant during the formation of rocks that today make up the Amapá Block. Hf-TDMC model ages, ranging from 2.99 Ga to 3.97 Ga, indicate that at least two important periods of mantle extraction and continental crust formation occurred during the Archean in southeastern Guyana Shield, an older one in the Eoarchean (∼4.0 Ga) and a younger one in the Mesoarchean (∼3.0–3.1 Ga). The latter is recognized as an important period of crustal accretion worldwide. The recognition of an Eoarchean episode to the southeastern most part of the Guyana Shield is unprecedented and was not recorded by whole-rock Sm-Nd data, which were restricted to the Meso–Paleoarchean (2.83 Ga to 3.51 Ga). This finding reveals that continental crust generation in the Amazonian Craton began at least 500 Ma earlier than previously suggested by the Sm-Nd systematics.

Strain variations across the Proterozoic Penokean Orogen, USA and Canada

Strata in the Huron (2.5–2.0 Ga) and Animikie (2.2–1.85 Ga) basins were deposited on the southern margin of the Archean Superior province. These rocks were deformed during the Penokean orogeny (∼1850 Ma) followed by subsequent accretionary orogens to the south at 1750 Ma (Yavapai) and 1630 Ma (Mazatzal). Strain patterns are unique to each orogenic belt with no far-field effect: Archean Wawa terrane rocks in the Penokean foreland preserve deformation associated with Archean accretion with no younger Penokean, Yavapai or Mazatzal strain overprint. The Penokean orogeny deformed Huron-Animikie basin sediments into a north-vergent fold-and-thrust belt with no Yavapai or Mazatzal strain overprint. Yavapai orogen strains (SW-NE margin-parallel shortening) are unique when compared to the younger Mazatzal shortening (N20°W) shortening, with no strain overprint.Penokean deformation is characterized by shortening from the south including uplifted Archean gneisses and a northerly thin-skinned fold-and-thrust belt, with north-vergent nappes and a gently-dipping foreland. Our study of finite and calcite twinning strains (n = 60) along (∼1500 km) and across (∼200 km) the Penokean belt indicate that this orogeny was collisional as layer-parallel shortening axes are parallel across the belt, or parallel to the tectonic transport direction (∼N-S). Penokean nappe burial near the margin resulted in vertical shortening strain overprints, some of which are layer-normal. The Sudbury impact layer (1850 Ma) is found across the Animikie basin and provides a widespread deformation marker with many local, unique strain observations. We also report new geochronology (U-Pb zircon and apatite) for the gneiss-mafic dike rocks at Wissota (Chippewa Falls, WI) and Arbutus (Black River Falls, WI) dams, respectively, which bears on Penokean-Yavapai deformation in the Archean Marshfield terrane which was accreted during the Penokean orogen. Pseudotachylite formation was common in the Superior province Archean basement rocks, especially along terrane boundaries reactivated by contemporaneous Penokean, Trans-Hudson, Cape Smith and New Quebec deformation. In the hinterland (south), the younger Yavapai orogen (1750 Ma; n = 8) deformation is preserved as margin-parallel horizontal shortening (∼SW-NE) in Yavapai crust and up to 200 km to the north in the Penokean thrust belt as a strain and Barrovian metamorphic overprint. Mazatzal deformation (1630 Ma; n = 16) is preserved in quartzites on Yavapai and Penokean crust with layer-parallel and layer-normal shortening strains oriented N20°W.

A 2207 Ma radiating mafic dyke swarm from eastern Dharwar craton, Southern India: Drift history through Paleoproterozoic

We report new TE-TIMS precise Pb-Pb baddeleyite ages together with consistent paleomagnetic results on two Paleoproterozoic mafic dyke swarms, intruding Archean basement rocks in the northern and southern region of Cuddapah basin, Eastern Dharwar Craton. The distinct E-W to NW-SE striking dykes, which gives a weighted mean age of 2206.8 ± 2.1 Ma, confirms the presence of a radiating mafic dyke swarm with a fanning angle of 50°, convergence towards a focal point in the north-western part of Dharwar Craton. The North-South trending parallel dyke swarm yields an emplacement age of 2252.4 ± 1.2 Ma, making this the second oldest well dated dyke swarm of the Dharwar craton after the ∼2367 Ma event.Paleomagnetic results from the ∼2207 Ma radiating dykes have revealed a stable reverse polarity remanence directions with a mean direction of D = 26°, I = 65° (α95 = 6°) and corresponding paleopole situated at 57°N and 113°E (dp = 8°, dm = 10°). Reliable baddeleyite age determinations at paleomagnetic sampling sites, positive baked contact test and a secular variation correlation test proves that the reporting remanence directions are primary. We have also reviewed the published paleomagnetic results from similar N-S dykes, which are yielding a normal polarity with a paleopole position at 16°S and 299°E (dp = 13°, dm = 15°). We consider this result as a key paleopole for the ∼2252 Ma dyke swarm.We have attempted to reconstruct an updated APWP for Dharwar craton utilizing six well constrained key paleopoles from 2.37 Ga to 1.89 Ga periods. The path suggests that the Dharwar craton has relatively rapid drift rate (28.77 cm/yr) from 46° to 53° paleolatitude along with a clockwise rotation (30°) during the short time span of ∼9 Ma (2216 Ma–2207 Ma), when a magnetic polarity reversal occurred. However, we speculate that major tectonic activity or plate tectonic processes were responsible for rapid drift of Dharwar craton at that time. The identification of coeval mafic magmatic episodes (∼2.2 Ga) from several other Archean cratons over the globe firmly supports the popular hypothesis of supercraton breakup at ∼2.2 Ga time.

Lithotectonic elements of Archean basement on the Liaodong Peninsula and its vicinity, North China Craton, China

The Liaodong Peninsula, in the northeastern part of the Eastern Block in the North China Craton, China, consists of lithologic units from Archean to Cenozoic in age. The basement rocks consist of widespread amphibolite- to granulite-facies Archean supracrustal assemblages and granitoid gneisses, as well as Paleoproterozoic volcano-sedimentary successions that were intruded by granitic–mafic complexes, and then metamorphosed under greenschist- to amphibolite-facies conditions. The basement rocks are overlain by thick Mesoproterozoic–Cenozoic sedimentary sequences. A synthesis of the available petrological and geochronological data allowed us to establish a geological framework for the Precambrian basement on the Liaodong Peninsula and its vicinity. The basement can be subdivided into three tectonic units: the Neoarchean Liaonan Block, the Eo–Neoarchean Longgang Block, and the intervening Paleoproterozoic Jiao–Liao–Ji Belt. In this paper we delineate the characteristics of an Archean tectonothermal event, and in a companion paper we examine the Paleoproterozoic lithotectonic assemblages. Rock samples of the Hadean eon are rare worldwide, but Hadean zircons have been identified in rocks of the Liaodong Peninsula, and they provide one of the oldest known mineralogical records on Earth. The Archean gneisses in the Liaonan Block are dominated by quartz dioritic–granodioritic gneisses that were emplaced between 2.55 and 2.44 Ga, and these rocks later underwent a lower-amphibolite-facies metamorphism. On the other hand, the Archean basement in the Longgang Block is dominated by TTG (tonalitic–trondhjemitic–granodioritic) and granitic gneisses, charnockites, and small amounts of supracrustal sequences with much older protolith ages of up to 3.85 Ga, and these rocks have undergone amphibolite- to granulite-facies metamorphism. Post-tectonic magmatism (ca. 2.5 Ga) marked the end of the Archean tectonothermal event in the Eastern Block of the North China Craton.

Geochemistry and origin of the Rhyacian tholeiitic metabasalts and meta-andesites from the Vila Nova Greenstone Belt, Guyana Shield, Amapá, Brazil

The origin and significance of the metavolcanic rocks of the Vila Nova Greenstone Belt (VNGB) is an important issue in order to understand the evolution of the Guyana Shield in Paleoproterozoic times. This work presents new LA-MC-ICPMS U-Pb zircon data and geochemical analyses carried out on metavolcanic rocks of the VNGB. This information was supplemented by petrography, stratigraphic and structural data acquired through geologic mapping and boreholes descriptions. In the Vila Nova region, the VNGB units rest on the Archean basement composed of orthogneisses of the Tumucumaque Complex. The lower portion of the VNGB consists of metabasalts and andesitic metabasalts, covered by an upper metasedimentary domain with subordinate metavolcanic and exhalative chemical rocks. The metavolcanic rocks include metabasalts, meta-andesites, amphibolites and amphibole schists, deformed and elongated according to the regional NW-SE structure. U-Pb zircon data yielded an age of 2154 ± 6 Ma for a meta-andesite of the lower portion of VNGB. One orogenic metamorphism followed by three deformational events were recognized. The D1 and D2 events formed the schistosity (S1), preserved as intrafolial folds (F2) and the crenulation cleavage (S2) associated with the development of the oblique shear zones. Two distinct assemblages: 1) garnet, hornblende and biotite, and 2) hornblende, plagioclase and diopside define two distinct metamorphic peaks: M1 and M2 with temperatures of 450 and 650 °C, and lithostatic pressure between 4 and 6 kbar. The metabasites comprise geochemical compositions of Fe- and Mg-tholeiites with komatiitic affinity. The first group is enriched in LREE and HFSE. The second group shows MORB-like REE patterns in association with negative Nb, Ti and P anomalies. The observed features indicate that the metavolcanic rocks of the VNGB represent the evolution of back-arc basins and island arcs associated with the Trans-Amazonian Orogeny. These rocks characterize the Rhyacian evolution of the Maroni-Itacaiunas Province in the eastern portion of the Guyana Shield.

Fault Zone Evolution and Development of a Structural and Hydrological Barrier: The Quartz Breccia in the Kiggavik Area (Nunavut, Canada) and Its Control on Uranium Mineralization

In the Kiggavik area (Nunavut, Canada), major fault zones along, or close to, where uranium deposits are found are often associated with occurrence of thick quartz breccia (QB) bodies. These bodies formed in an early stage (~1750 Ma) of the long-lasting tectonic history of the Archean basement, and of the Proterozoic Thelon basin. The main characteristics of the QB are addressed in this study; through field work, macro and microscopic observations, cathodoluminescence microscopy, trace elements, and oxygen isotopic signatures of the quartz forming the QB. Faults formed earlier during syn- to post-orogenic rifting (1850–1750 Ma) were subsequently reactivated, and underwent cycles of cataclasis, pervasive silicification, hydraulic brecciation, and quartz recrystallization. This was synchronous with the circulation of meteoric fluids mixing with Si-rich magmatic-derived fluids at depth, and were coeval with the emplacement of the Kivalliq igneous suite at 1750 Ma. These processes led to the emplacement of up to 30 m thick QB, which behaved as a mechanically strong, transverse hydraulic barrier that localized later fracturing, and compartmentalized/channelized vertical flow of uranium-bearing fluids after the deposition of the Thelon Basin (post 1750 Ma). The development and locations of QB control the location of uranium mineralization in the Kiggavik area.

Relationships between magmatism and deformation in northern Yorke Peninsula and southeastern Proterozoic Australia

ABSTRACTThe ca 1600–1580 Ma time interval is recognised as a significant period of magmatism, deformation and mineralisation throughout eastern Proterozoic Australia. Within the northern Yorke Peninsula in South Australia, this period was associated with the emplacement of multiple phases of the Tickera Granite, an intensely foliated quartz alkali-feldspar syenite, a leucotonalite and an alkali-feldspar granite. These granites belong to the broader Hiltaba Suite that was emplaced at shallow crustal levels throughout the Gawler Craton. Geochemical and isotopic analysis suggests these granite phases were derived from a heterogeneous source region. The syenite and alkali-feldspar granite were derived from similar source regions, likely the underlying ca 1850 Ma Donington Suite and/or the ca 1750 Ma Wallaroo Group metasediments with some contamination from an Archean basement. The leucotonalite is sourced from a similar but more mafic/lower crustal source. Phases of the Tickera Granite were emplaced synchronously with deformation that resulted in development of a prominent northeast-trending structural grain throughout the Yorke Peninsula region. This fabric is associated with composite events resulting from folding, shearing and faulting within the region. The intense deformation and intrusion of granites within this period resulted in mineralisation throughout the region, as seen in Wheal Hughes and Poona mines. The Yorke Peninsula shares a common geological history with the Curnamona Province, which was deformed during the ca 1600–1585 Ma Olarian Orogeny, and resulted in development of early isoclinal and recumbent folds overprinted by an upright fold generation, a dominant northeast-trending structural grain, mineralisation, and spatially and temporally related intrusions. This suggests correlation of parts of the Gawler Craton with the Curnamona Province, and that the Olarian Orogeny also affected the southeastern Gawler Craton.

Nature and Age of Detrital Zircons from Rocks of the Shear Zone: The Problem of Occurrence of the Archean Basement in the Transangarian Yenisei Ridge

Petrogeochemical and geochronological study of boudined quartzite sandstones and tonalites, as well as host amphibolites, in the shear zone showed that potential sources of detrital zircons were Neoarchean–Paleoproterozoic rocks of the Angara–Kan block and products of their metamorphism. Interpretation of the available data does not confirm the inferred presence of the Early Precambrian basement of the Siberian Craton in the Transangara region.

Reply to the ‘Comment on Anatomy of impactites and shocked zircon grains from Dhala reveals Paleoproterozoic meteorite impact in the Archean basement rocks of Central India’ by Pati et al., 2018, Gondwana Research

Reply to the ‘Comment on Anatomy of impactites and shocked zircon grains from Dhala reveals Paleoproterozoic meteorite impact in the Archean basement rocks of Central India’ by Pati et al., 2018, Gondwana Research

Comment on “Anatomy of impactites and shocked zircon grains from Dhala reveals Paleoproterozoic meteorite impact in the Archean basement rocks of Central India” by Li et al., 2018, Gondwana Research, 54, 81–101

Comment on “Anatomy of impactites and shocked zircon grains from Dhala reveals Paleoproterozoic meteorite impact in the Archean basement rocks of Central India” by Li et al., 2018, Gondwana Research, 54, 81–101

Geochemistry and U–Pb–Hf zircon data for plutonic rocks of the Troia Massif, Borborema Province, NE Brazil: Evidence for reworking of Archean and juvenile Paleoproterozoic crust during Rhyacian accretionary and collisional tectonics

Rhyacian (c. 2.30–2.05 Ga) subduction-accretion and collisional processes are thought to represent an important event of crustal growth and reworking, often reported in the so-called Tranzamazonian/Eburnean granite-greenstone terranes of N/NE Brazil and West African craton. In this work, we present whole-rock geochemistry and U–Pb–Hf zircon ages for Rhyacian plutonic rocks that intrude Paleoproterozoic granite-greenstone terranes and Archean basement rocks of the Troia Massif, Borborema Province, NE Brazil. U–Pb SHRIMP zircon age of 2185 ± 4 Ma was obtained for a metatonalite sample (Mirador metatonalites). The metatonalites show geochemical affinity similar to those of adakites-like rocks, with all “classic” (slab-derived) adakitic signature (e.g. high Sr/Y, La/Yb and low HREE), but more akin to those evolved from partial melting of the metasomatized mantle wedge, followed by fractional crystallization. Zircon Hf crustal model ages of the metatonalite sample range between 2800 and 2660 Ma, evidencing that Archean crustal components contributed to its magma genesis, probably via subduction of continental-derived sediments to the mantle wedge. For potassic plutons of the Troia Massif (Bananeira suite) we obtained U–Pb SHRIMP zircon age of 2079 ± 4 Ma for a deformed quartz-monzonite with geochemical affinities similar to those derived from low-pressure partial melting of a K-rich mafic protolith. For less-deformed equigranular granites, we obtained a U–Pb LA-ICPMS zircon age of 2068 ± 5 Ma. They are high-K calc-alkaline and slightly peraluminous granites that probably derived from low-pressure partial melting of tonalitic crustal sources (metatonalites) and/or metagraywackes. Zircon Hf crustal model ages of a granite sample range from 2713 to 2535 Ma, evidencing Archean crustal contribution to magma genesis. However, two analyzed c. 2307 Ma inherited zircon grains show εHf(t) values of c. +4.87, indicating that crustal reworking of less-radiogenic Paleoproterozoic sources also participated in its genesis. Regional correlations of our results with the Paleoproterozoic record of the Troia Massif, Borborema Province and surrounding cratonic domains, suggest that the studied plutons are related to the c. 2.2–2.0 Ga Transamazonian/Eburnean orogenic cycle. The c. 2185 Ma metatonalites are associated to pre-collisional setting, while the c. 2079–2068 Ma quartz monzonites and granites evolved during collisional setting.

Provenance of lead (Pb) in soils in the Mt Isa urban area

This paper integrates more than 50 years of geological, geochemical and environmental research relevant to the Pb content of soils in Mt Isa to clarify the role of soil as a potential pathway for Pb exposure to local residents. Most of Mt Isa is underlain by thin residual soils that formed by weathering of the Proterozoic Mt Isa Group (MIG). The MIG contains carbonate and volcaniclastic sedimentary rocks largely derived from contemporaneous and earlier cycles of igneous activity and minor detritus from an Archean basement. The upper part of the MIG contains stratiform Pb ore deposits that are about 1650 Ma old. Deformation and erosion have exposed these rocks to weathering and soil forming processes several times since the Proterozoic, with the current landform developing over the last 20 Ma. Soil formation has generally depleted Pb from the subsoil and enriched the top soil. Pb has been widely distributed to the northwest and north of the city in wind-blown dust and water runoff from outcrops of oxidised ore for millions of years. Additional Pb emissions from smelting commenced at Mt Isa in 1931. Most of the urban area receives 50–100 mg/m2/yr of Pb from dust deposition and some of this is resuspended or washed away. Additional Pb in mine waste rock and contaminated river sand was brought into the residential area as fill or for sandpits. The Mt Isa community is exposed to Pb in air at levels regulated to minimise health effects of inhalation and ingestion. The observed Pb levels in air generally correlate with distance from the mine but not with the distribution of children with elevated blood Pb levels. Geological evidence of anomalous soil Pb derived from underlying layers of mineralised rock may partly explain the observed blood Pb distribution.

Orosirian magmatic episodes in the erepecuru-trombetas domain (southeastern Guyana shield): Implications for the crustal evolution of the Amazonian craton

The granitic rocks of the Erepecuru-Trombetas Domain, southeastern Guyana Shield, are part of two extensive volcano-plutonic associations that marked the central portion of the Amazonian Craton during the Orosirian. The older episode (2.0–1.97 Ga) encompasses the Igarapé Paboca volcanic Formation and Caxipacoré Suite and the younger episode (1.90–1.87 Ga) comprises the Água Branca and Mapuera plutonic suites and the pyroclastic/effusive rocks of the Iricoumé Group. LA-ICP-MS U-Pb zircon dating of granitoids provided ages of 1991 ± 5.9 and 1989 ± 7 Ma for the Caxipacoré Suite, 1887.5 ± 4.8 Ma for the Água Branca Suite and 1881 ± 8.1 Ma for the Mapuera Suite, which reinforce the occurrence of two Orosirian magmatic episodes in the southeastern Guyana Shield. The geochemical characteristics of the Caxipacoré granitoids suggest that they formed in an orogenic tectonic setting, related to a subduction environment, while the geochemical signatures of the Água Branca and Mapuera granitoids betray the evolution from a convergent context to an extensional intracontinental environment. Nd-TDM (2.19–2.22 Ga) and Sr-TUR (1.95–1.96 Ga) model ages and slightly positive εNd (+0.26 to +0.66) for the Caxipacoré granitoids point to an origin from melting of mantle-derived magmas with participation of Rhyacian sialic crust. The Água Branca and Mapuera granitoids display Nd-TDM (1.95–2.30 Ga) and Sr-TUR (1.84–2.02 Ga) model ages and positive to negative εNd (+2.92 to −1.96) that indicate parental magmas derived from melting of dominantly Rhyacian crustal sources with minor mantle contribution. In addition, the Nd signature and the lack of Archean inherited zircon in the plutonic rocks do not favor the existence of an Archean basement in this part of the southern Guyana Shield. This assumption together with similarities with the adjacent Uatumã-Anauá Orosirian Domain, westward, led to suggest that the Erepecuru-Trombetas Domain should not be longer interpreted as belonging to the Central Amazon Province, as defined in the current geotectonic models of the Amazonian Craton.

Timing of sediment-hosted Cu-Ag mineralization in the Trans-Hudson orogen at Janice Lake, Wollaston Domain, Saskatchewan, Canada

The Janice Lake Cu-Ag mineralization in the Wollaston Domain of northern Saskatchewan is hosted by a metasedimentary sequence in the upper part of the Wollaston Supergroup of the Trans-Hudson orogen. The Wollaston Supergroup was deposited between ~ 2070 and 1865 Ma in a foreland basin setting constructed over Archean basement of the Hearne craton. The Trans-Hudson orogen underwent final collision and peak metamorphism at ~ 1810 Ma, during consolidation of Laurentia and its amalgamation with the Columbia supercontinent. Titanite is a common constituent of the post-peak metamorphic assemblages of Trans-Hudson lithotectonic units and accompanied disseminated sediment-hosted Cu sulfide mineralization at Janice Lake. Titanite crystals, intergrown with chalcocite over a strike-length of ~ 2 km of Cu-bearing stratigraphy, were dated by the ID-TIMS and LA-ICP-MS U-Pb methods, returning an age range from 1780 to 1760 Ma and a weighted average age of 1775 ± 10 Ma. The titanite ages effectively date the associated chalcocite-dominated sediment-hosted Cu-Ag mineralization and its formation during initial post-orogenic uplift and cooling, ~ 30 myr after peak metamorphism. The age-range and tectonic setting of the Janice Lake mineralization confirms that sediment-hosted Cu mineralization was an integral part of the metallogenic endowment of Columbia and that its emplacement coincided with the continental-scale Trans-Hudson orogeny rather than with diagenesis and extensional basin development ~ 100 myr earlier.

Earth's oldest preserved K-bentonites in the ca . 2.1 Ga Francevillian Basin, Gabon

Bentonites are the alteration product of volcanic tephra typically preserved in low-energy, sedimentary environments below baseline. Although volcanic tuffs occur throughout the Earth’s history, bentonites older than ca. 1.5 Ga have not been described. We present the mineralogy, geochemistry, and age data for Kbentonite beds within the FB Formation in the unmetamorphosed Paleoproterozoic Francevillian Basin, Gabon. The clay mineralogy of the K-bentonites consists predominantly of illite with substantial amounts of kaolinite and trace amounts of long-ordered illite/smectite (R3) mixed layer. The kaolinite content and co-existing 1M and 2M1 illites are indicative of diagenetic smectite illitization over a prolonged period of time with minimal burial temperature. Their chemical characteristics suggest derivation from calc-alkaline intermediate to felsic magma, related to continental volcanic arc magmatism in a subduction setting. The zircon grains are relatively small, rounded to sub-rounded, and yield 207Pb/206Pb dates that have a narrow range with a weighted mean of 2971 13 Ma, consistent with the age of the underlying crystalline calcalkaline Archean basement granitoids. This age indicates incorporation of zircons from the Archean basement granitoids into the magma during magmatic activity. Considering that the FB Formation bentonites were derived from a volcanic arc developed along the margin of the West Gabonian block and are preserved in the lower part of the Francevillian Basin, we infer that this basin reflects high-rate, but short-lived sedimentation in a pro-foreland basin setting. Paleogeographically, these K-bentonites could thus serve as a potential correlation marker for the Paleoproterozoic Gabonian and adjacent cratons at ca. 2.1 Ga. Based on the current records, these are the world’s oldest bentonite beds so far reported.

Geochemical Features of Au-Te Epithermal Ores of the Samolazovskoye Deposit (Central Aldan Ore District, Yakutia)

The Samolazovskoye deposit (Central Aldan ore region, Russia) is confined to the porphyry syenite lopolith (J3-K1), localized between the granitic gneiss Archean basement and the series of the Vendian-Lower Cambrian carbonate cover rocks. Four hydrothermal-metasomatic parageneses have been identified within the deposit: skarn paragenesis, developed on the syenites and carbonate cover rocks contact; so called «gumbaite» paragenesis (kalifeldspar + fluorite + carbonate ± quartz), superimposed on the intrusive massif rocks; feldspatholitic paragenesis (quartz + feldspar), developed in the granitic gneisses of the crystalline basement; ore-bearing fluorite-roscoelite-carbonate-quartz paragenesis, superimposed on all of the above. The article compares ores evolved within gumbaitic syenites, basement feldspatholites and breccias, composed of all the above-mentioned rocks clasts. The geochemical study of given ores, resulted in two identified elements associations: gold-telluride (Au, Sb, As, V, Tl, Te, Hg, W) related to the fluorite-roscoelite-carbonate-quartz hydrothermal-metasomatic paragenesis and (uranium)-polymetallic (Bi, Cu, Pb, Zn, Mo, Se, Li, U), associated with the syenites gumbaitization (?). There is only gold-telluride association within the basement ore bodies, while the ore bodies localized in the syenites intrusion hold both associations, along with the Au and Ag contents being an order of magnitude higher. Breccia ores are characterized by the maximum concentrations of the ore elements. Gold-telluride association of the Samoazovsky deposit ores is specific to epithermal Au-Te mineralization associated with alkaline (A-type) magmatism.