The Ambatolampy Group, central Madagascar, a Neoproterozoic rift-basin sequence?

2020 ◽  
Author(s):  
Wilfried Bauer ◽  
Imboarina T. Rasaona ◽  
Robert D. Tucker ◽  
Forrest Horton
Keyword(s):  
Gold Mineralization ◽  
Thin Layers ◽  
Detrital Zircons ◽  
Igneous Rocks ◽  
Heavy Minerals ◽  
Coarse Grained ◽  
Rift Basin ◽  
Zircon Age ◽  
Fine Grained ◽  
Magnetite Quartzite

<p>The crystalline basement of central Madagascar is composed of the Neoarchaean, high-grade metamorphic Antananarivo Domain, made up of granulite to upper-amphibolite orthogneisses and paragneisses, and intruded by Tonian igneous rocks of the Imorona-Itsindro suite (Archibald et al. 2016). Along its southern, western and northern margins several terranes were accreted between the Paleoproterozoic and the Neoproterozoic (Tucker et al. 2014) before Madagascar was affected by the collision of East- and West-Gondwana at the end of the Ediacaran.</p><p>Within the Antananarivo Domain, a more than 700 km long and up to 80 km wide belt of supracrustal amphibolite-facies rocks forms te Ambatolampy Group. It is characterized by abundant monotonous biotite schists and gneisses that are locally migmatised. The schists contain biotite, sillimanite, garnet and locally thick graphite-rich layers. Associated paragneisses are also biotite-rich and commonly carry sillimanite or hornblende. White quartzites ranging from thick-bedded ridge-forming units to fine, cm-scale interbeds are coarse-grained and contain often sillimanite. Dark quartzites rich in magnetite and heavy minerals occur as cm-thin layers throughout the whole group. Small bodies of pyroxenite, pyroxene-amphibolite, amphibolite ±garnet, and pyroxene gneiss are common, especially close to the base of the group.</p><p>The age of the Ambatolampy Group is highly controversial. A group of researchers from BGS and USGS reported a youngest detrital zircon age of 1054 Ma, whereas Archibald et al. (2016) assumed a Mesoproterozoic age, based on their youngest zircons of roughly 1.8 Ga. We present new near-concordant U-Pb detrital zircons ages as young as 800 Ma, indicating a sedimentary input from igneous rocks of the Imorona-Itsindro suite. Sedimentation must have ceased before 630 Ma which is constrained by the U-Pb zircon age of an intruding leucogabbro.</p><p>About half of Madagascar’s known 1050 gold occurrences are lying within the Ambatolampy Group. Fine-grained disseminated gold appears to be concentrated within relatively narrow stratigraphic intervals of the Ambatolampy Group, defined by the occurrence of boudinaged or fractured magnetite quartzite. In general, the gold grades in fresh rocks are below economic interest, the highest gold tenors were recorded in an up to 30 meter thick laterite zone above the basement. Another important commodity related to the Ambatolampy Group is graphite which had seen a mining boom in the 1910s and 1920s. The graphite is flaky with crystal diameters between 0.5 and 5 mm and contents of graphitic carbon between 6 and 15 %. Individual seams are up to 12 m wide and can be tracked for several kilometers.</p><p>We interpret the Ambatolampy Group as a mainly siliciclastic fill of a continental rift basin during a phase of crustal extension occurring contemporaneously with the intrusion of the Imorona-Itsindro Suite. The gold mineralization is most likely related to fluvial deposits from surrounding gold-bearing Archean basement.</p><p> </p><p><strong>References</strong></p><p>Archibald, D.B. et al. 2015. Tectonophysics 662, pp. 167-182.</p><p>Archibald, D.B. et al. 2016. Precambr. Res. 281, pp. 312–337.</p><p>Tucker, R.D. et al. 2014. J. African Earth Sci. 94, pp. 9-30.</p>

scholarly journals Quantification of pedogenic particles masked by geogenic magnetic fraction

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Marcin Szuszkiewicz ◽  
Hana Grison ◽  
Eduard Petrovský ◽  
Maria Magdalena Szuszkiewicz ◽  
Beata Gołuchowska ◽  
...  
Keyword(s):  
Environmental Magnetism ◽  
Igneous Rocks ◽  
Coarse Grained ◽  
Masking Effect ◽  
Magnetic Contribution ◽  
Magnetic Fraction ◽  
Magnetic Signal ◽  
Magnetic Parameters ◽  
Fine Grained ◽  
Natural Substances

AbstractPedogenic magnetic fraction in soils is attributed to fine-grained particles, i.e. superparamagnetic grains. In the case of a strongly magnetic geogenic fraction, pedogenic magnetic contribution is hard to detect. To the best of our knowledge, detailed research into the masking of pedogenic superparamagnetic grains and quantification of this effect has not yet been carried out. The principal aim of our research is to quantify the influence of coarse-grained ferrimagnetic fraction on the detection of the superparamagnetic grains. In order to describe the masking phenomenon, volume and frequency-dependent magnetic susceptibility were determined on a set of laboratory prepared samples composed of natural substances: a diamagnetic quartz matrix, detrital coarse-grained ferrimagnetic crystals from alkaline and ultra-alkaline igneous rocks, and superparamagnetic soil concretions formed in the Haplic Cambisol. Mineralogy, concentration, type and grain size of the tested material were described by parameters of environmental magnetism. The magnetic parameters distinguish both geogenic multidomain and pedogenic superparamagnetic grains. The magnetic signal of the superparamagnetic grains is gradually masked by the increasing proportion of multidomain grains of magnetite/maghemite. The experiment clearly describes the masking effect and brings new insight to studies dealing with strongly magnetic soils of natural and/or highly contaminated origin as a tool for estimation of superparamagnetic pedogenic contribution.

The relationship between sedimentary facies and faunal associations in the Llandovery siliciclastic Ross Brook Formation, Arisaig, Nova Scotia

1986 ◽  
Vol 23 (5) ◽  
pp. 705-726 ◽  
Author(s):  
J. M. Hurst ◽  
R. K. Pickerill
Keyword(s):  
Nova Scotia ◽  
Environmental Parameters ◽  
Sedimentary Facies ◽  
Great Distance ◽  
Thin Layers ◽  
Coarse Grained ◽  
Individual Species ◽  
Silty Mudstone ◽  
Evolutionary Pattern ◽  
Fine Grained

The Llandovery siliciclastic Ross Brook Formation of Arisaig, Nova Scotia, comprises three broadly defined sedimentary facies. These are a mottled silty mudstone facies (facies A), a laminated shale facies (facies B), and a laminated siltstone facies (facies C). Facies A consists of variably bioturbated silty mudstones, muddy siltstones, and fissile shales. It developed in relatively shallow water conditions; mudstones were presumably deposited from suspension, and siltstone laminae and thin layers were formed by increased wave and current activity. Mottling resulted from bioturbation by epifaunal and, particularly, infaunal benthic organisms. Within facies A, two subfacies are recognized: A(i) is mudstone dominated, and A(ii) is fine-grained siltstone dominated. Facies B consists of alternating laminae of undisturbed mudstone and fine-grained siltstone probably produced as a result of deposition from suspension during a temporary upward expansion of the oxygen-minimum layer. Facies C consists of 0.5–30 cm thick fine- to coarse-grained siltstones, which occur in lenses or layers of single, composite, or amalgamated units. Internally they are extremely variable, but all are interpreted as a result of deposition from storm-generated currents.The Ross Brook Formation formed on a shallow-marine, storm-influenced, subtidal inner–mid muddy shelf and is dominated by extensive but stratigraphically variable developments of facies A and facies C. Absolute water depth per se is difficult to assess, and although fluctuations occurred, much of the sequence is believed to have accumulated at or in the immediate vicinity of fair-weather wave base in water depths estimated to have been between 30 and 60 m. Silt supply was generally low, possibly reflecting great distance from source or the presence of a mud-dominated shoreline.Five brachiopod-dominated associations, which are stratigraphically the Eocoelia hemisphaerica, the Eocoelia intermedia – Eocoelia curtisi, the Visbyella nana, the "Camarotoechia" rossonia, and the Eocoelia sulcata associations, occur through the sequence. Associations change where die sum of the facies characteristics change, suggesting that the major physical controlling factor was substrate type and related environmental parameters. The development of discrete but intergrading associations is viewed as a consequence of the long-term persistence of a set of conservative animal–sediment relationships, superimposed on which is the evolutionary pattern of immigration and extinction of individual species.

Early-Paleozoic geodynamics of the Altai orogen: Constraints from geochemical and zircon U-Pb-Hf isotopic study of paragneissic rocks from the southern Chinese Altai

2020 ◽  
Author(s):  
Xing Cui ◽  
Min Sun ◽  
Guochun Zhao ◽  
Yunying Zhang ◽  
Jinlong Yao ◽  
...  
Keyword(s):  
Active Continental Margin ◽  
Detrital Zircons ◽  
Igneous Rocks ◽  
Cumulative Distribution ◽  
Geochemical Data ◽  
Accretionary Wedge ◽  
Early Paleozoic ◽  
Isotopic Study ◽  
Zircon Age ◽  
Southern Chinese

<p>The high-grade metamorphic complexes in the Chinese Altai were previously regarded as the Precambrian basement and thus important for unravelling tectonic evolution of the Altai orogen. This study reports detailed filed investigation, zircon U-Pb-Hf isotopic and whole-rock geochemical data for the paragneissic rocks from Northern Fuyun Complex (NFC), southern Chinese Altai. Detrital zircons from the paragneisses have a predominant early Paleozoic age population (ca. 535-435 Ma), with minor Neoproterozoic and sparse Mesoproterozoic to Archean ages. The geochemical analyses together with the euhedral shape of the detrital zircons suggest that their sedimentary protoliths mainly came from felsic-intermediate igneous rocks with low maturity. In combination with the cumulative distribution curves of zircon age spectra, the variable zircon εHf(t) values (-25 to +13), as well as the immature geochemical compositions, we infer that the protoliths were most likely deposited on an active continental margin in the early Paleozoic and sourced mainly from proximal igneous rocks, which are comparable to the Habahe Group. Similar detrital zircon age spectra of early Paleozoic sequences from the Chinese Altai, Mongolia Altai and Khovd Zone support the existence of a giant accretionary wedge developed along the western margin of the Ikh-Mongol Arc system, resulting from continuous northeast-dipping oceanic subduction. This research was financially supported by the National Key R&D Program of China (2017YFC0601205), Hong Kong RGC GRF (17302317 and 17303415) and NSFC Projects (41730213 and 41190075).</p>

scholarly journals Deformation of feldspar at greenschist facies conditions – the record of mylonitic pegmatites from the Pfunderer Mountains, Eastern Alps

Solid Earth ◽  
2019 ◽  
Vol 10 (1) ◽  
pp. 95-116 ◽  
Author(s):  
Felix Hentschel ◽  
Claudia A. Trepmann ◽  
Emilie Janots
Keyword(s):  
Grain Size ◽  
Granular Flow ◽  
Thin Layers ◽  
Size Reduction ◽  
Greenschist Facies ◽  
Coarse Grained ◽  
Dislocation Creep ◽  
Subsequent Growth ◽  
Dislocation Glide ◽  
Fine Grained

Abstract. Deformation microstructures of albitic plagioclase and K-feldspar were investigated in mylonitic pegmatites from the Austroalpine basement south of the western Tauern Window by polarized light microscopy, electron microscopy and electron backscatter diffraction to evaluate feldspar deformation mechanisms at greenschist facies conditions. The main mylonitic characteristics are alternating almost monophase quartz and albite layers, surrounding porphyroclasts of deformed feldspar and tourmaline. The dominant deformation microstructures of K-feldspar porphyroclasts are intragranular fractures at a high angle to the stretching lineation. The fractures are healed or sealed by polyphase aggregates of albite, K-feldspar, quartz and mica, which also occur along intragranular fractures of tourmaline and strain shadows around other porphyroclasts. These polyphase aggregates indicate dissolution–precipitation creep. K-feldspar porphyroclasts are partly replaced by albite characterized by a cuspate interface. This replacement is interpreted to take place by interface-coupled dissolution–precipitation driven by a solubility difference between K-feldspar and albite. Albite porphyroclasts are replaced at boundaries parallel to the foliation by fine-grained monophase albite aggregates of small strain-free new grains mixed with deformed fragments. Dislocation glide is indicated by bent and twinned albite porphyroclasts with internal misorientation. An indication of effective dislocation climb with dynamic recovery, for example, by the presence of subgrains, is systematically missing. We interpret the grain size reduction of albite to be the result of coupled dislocation glide and fracturing (low-temperature plasticity). Subsequent growth is by a combination of strain-induced grain boundary migration and formation of growth rims, resulting in an aspect ratio of albite with the long axis within the foliation. This strain-induced replacement by nucleation (associated dislocation glide and microfracturing) and subsequent growth is suggested to result in the observed monophase albite layers, probably together with granular flow. The associated quartz layers show characteristics of dislocation creep by the presence of subgrains, undulatory extinction and sutured grain boundaries. We identified two endmember matrix microstructures: (i) alternating layers of a few hundred micrometres' width, with isometric, fine-grained feldspar (on average 15 µm in diameter) and coarse-grained quartz (a few hundred micrometres in diameter), representing lower strain compared to (ii) alternating thin layers of some tens of micrometres' width composed of fine-grained quartz (<20 µm in diameter) and coarse elongated albite grains (long axis of a few tens of micrometres) defining the foliation, respectively. Our observations indicate that grain size reduction by strain-induced replacement of albite (associated dislocation glide and microfracturing) followed by growth and granular flow simultaneous with dislocation creep of quartz are playing the dominating role in formation of the mylonitic microstructure.

scholarly journals U-Pb (LA-ICP-MS) age of detrital zircons and the sources of terrigenous sediments of the Ipsit suite, Karagass series (Sayan segment of the Sayan-Baikal-Patom belt)

2018 ◽  
Vol 9 (4) ◽  
pp. 1313-1329 ◽  
Author(s):  
Z. L. Motova ◽  
T. V. Donskaya ◽  
D. P. Gladkochub ◽  
V. B. Khubanov
Keyword(s):  
Detrital Zircon ◽  
Source Area ◽  
Potassium Feldspar ◽  
Detrital Zircons ◽  
Igneous Rocks ◽  
Zircon Age ◽  
Genetic Types ◽  
Terrigenous Rocks ◽  
Detrital Material ◽  
Icp Ms

The petrographic, lithogeochemical and U-Pb (LA-ICP-MS) geochronological studies were carried out to investigate the terrigenous rocks sampled from the lower part of the Ipsit suite of the Karagass series (Sayan segment of the Sayan-Baikal-Patom belt). These rocks include sandstones, aleurite sandstones and aleurolites, and their mineral compositions are close to that of arkose. Most of the studied rock samples show petrographic features typical of the epigenetic changes at the stage of catagenesis: regeneration of quartz clastic grains, pelitization of potassium-feldspar clastic grains, occurrence of clay-hydromica aggregate, sericitization of plagioclase, chloritization of biotite, and silicification of dolomite pieces, and occurrence of authigenous tourmaline. The above was confirmed by the analysis of the concentrations of petrogenic elements in the studied rocks from the lower part of the Ipsit suite. The analysis results show that the concentrations of K2O are elevated, while the concentrations of Na2O are relatively very low, which may be due to the redistribution of these elements during epigenetic transformations. According to the classification by genetic types on the basis of the system of petrochemical modules, the rocks of the lower part of the Ipsa suite are of the petrogenic nature. The acidic igneous rocks are dominant in the source area, as evidenced by the presence of granitoid and quartzite fragments in the clastogenic component, as well as the set of accessory minerals typical of the igneous rocks of the acidic composition, and the distribution pattern of rare and trace elements. According to the U-Pb (LA-ICP-MS) dating of detrital zircons from the aleurite sandstone sampled from the lower part of the Ipsit suite, the zircons are exclusively of the Archean-Early Proterozoic ages. Such ages correlate with the age of the granitoids of the Sayan complex and the felsic volcanites from the Maltsev layer of the Elash series (Biryusa block). Furthermore, the detrital-zircon age spectra of the aleurite sandstone of the lower part of the Ipsit suite are identical to the detrital-zircon age spectra of the terrigenous rocks from the underlying strata of the Shangulezh and Tagul suites of the Karagass series. This study suggests that sedimentation of the Ipsit suite of the Karagass series took place due to the influx of detrital material from the southern part of the Siberian craton into the sedimentation basin, and the acidic igneous rocks of the Biryusa block were one of the main sources of detrital material.

scholarly journals Petrography, microthermometry, and isotopy of the gold veins from Vetas, Santander (Colombia)

2020 ◽  
Vol 24 (1) ◽  
pp. 5-18
Author(s):  
Sonia Rojas Barbosa ◽  
Juan Carlos Molano ◽  
Thomas Cramer
Keyword(s):  
Fluid Inclusions ◽  
Gold Mineralization ◽  
Coarse Grained ◽  
Fine Grained ◽  
Quartz Veins ◽  
Low Salinity ◽  
Second Stage ◽  
The Third ◽  
Jurassic Rocks ◽  
Low Sulfidation

The gold mineralization located in Vetas, Santander, consists of auriferous quartz veins hosted in Bucaramanga gneiss rocks, intrusive Jurassic rocks, and intrusive to porphyritic Miocene rocks. This study identified four mineralizing events: (1). Sericite, carbonate (ankerite and calcite?), massive and microcrystalline quartz, sphalerite, adularia, albite, galena, thin pyrite, pyrrhotite, chalcopyrite. The age for this stage is 10.78 ±0.23Ma (Ar/Ar on sericite). (2). Molybdenite, magnetite with exsolution of ilmenite, As-pyrite, sphalerite, fine-grained pyrite and little chalcopyrite quartz with huge, feathery, fine mosaic, flamboyant and microcrystalline textures and, tourmaline and sericite. (3). Gold and tennantite associated with sphalerite, fine- and coarse-grained pyrite, As-pyrite, chalcopyrite like inclusions, and quartz with flamboyant, mosaic, massive and “comb” textures, and tourmaline. Stage 2 and 3 happened from 7.58 ±0.15 Ma to 6,89±0,41Ma (Ar/Ar on sericite). (4). Thick, thin, and pyrite with arsenic, hematite and microcrystalline quartz (forming breccia texture), and sericite. The age for this stage is 5.24 ±0.10 (Ar/Ar on sericite). Post-mineral: quartz comb, alunite, halloysite, kaolinite, and ferrum hydroxides. The stable isotopes, ∂18O, ∂D, and ∂34S and fluid inclusions analysis infer that fluids were producing a mixture of meteoric and magmatic fluids with low salinity and minimum trapping temperatures between 200°C to 390°C. The mineralogy association, and fluid inclusions, in the first event show characteristic of low sulfidation epithermal. The second stage was hottest and with more magmatic signature over printed an intermediate sulfidation system; show a little more salinity on the fluids and more mineralogical diversity, the third and four events, could show an evolution of this fluid, where it was cooling and impoverishing on metals. Two initials stages are contemporaneous with two magmatic Miocene pulses on the area: the first one of granodiorite composition 10, 9± 0.2 Ma (U/Pb zircon), and the other one rhyodacite with 8.4 ±0.2 y 9.0 ± 0.2 Ma.

scholarly journals Sedimentological characterization of the quaternary deposits in the region between Marília and Presidente Prudente, São Paulo State, Brazil

2008 ◽  
Vol 61 (3) ◽  
pp. 343-356 ◽  
Author(s):  
Alethéa Ernandes Martins Sallun ◽  
Kenitiro Suguio
Keyword(s):  
Sedimentary Rocks ◽  
Sao Paulo ◽  
Heavy Minerals ◽  
Coarse Grained ◽  
São Paulo ◽  
Fine Grained ◽  
Paulo State ◽  
Grain Sizes ◽  
São Paulo State ◽  
Bauru Group

Sedimentologic characterization and provenance studies of quaternary colluvial deposits (9±1 to 980±100 ky B.P. - dated by luminescence) were based on textural and mineralogical analyses. These deposits occur extensively between Marília and Presidente Prudente (São Paulo State, Brazil), superimposed on the Bauru Group sedimentary rocks or the Serra Geral Formation basaltic rocks, both of Cretaceous age. They are distributed irregularly throughout the study area and are studied by sedimentologic and morphologic methods. These deposits are composed of very fine to coarse-grained unconsolidated sands, mostly constituted by quartzose monocrystalline grains reddened by iron oxide and hydroxide impregnation. Analyses of grain sizes indicated that total granulometric distributions are richer in the clay fraction than in the sedimentary rocks located. Non-micaceous transparent heavy minerals studied microscopically in fine-grained as well as in very fine-grained sands showed that zircon, tourmaline, staurolite and rutile are present in all the samples. Differences in grain sizes among the colluvial deposits and the Bauru Group sedimentary rocks are attributable to pedogenetic processes.

scholarly journals Geochemistry and geochronology of the Paleozoic sedimentary rocks in the Shar Khutul area, Central Mongolia

2018 ◽  
pp. 4-21
Author(s):  
Erdenechimeg D ◽  
Oyunchimeg T ◽  
Otgonbaatar D ◽  
Jitka Míková ◽  
Tomurchudur Ch ◽  
...  
Keyword(s):  
Shallow Water ◽  
Sedimentary Rocks ◽  
Thin Layers ◽  
Detrital Zircons ◽  
Source Rocks ◽  
Coarse Grained ◽  
Al2o3 Content ◽  
Oceanic Plate ◽  
Stratigraphic Unit ◽  
Middle Carboniferous

The study area is located in the central part of Tsetserleg terrane in the southwestern margin of the Khangai-Khentey orogenic system. The paper presents new data on geochemistry and geochronology of sedimentary rocks from the Shar Khutul area, where the Tsetserleg terrane consists of Silurian-Devonian oceanic plate stratigraphic unit and Carboniferous shallow water sediment. The Upper Silurian to Middle Devonian Erdenetsogt Formation (S3-D2er), which is an oceanic plate stratigraphic unit, is mainly composed of siliceous siltstone, volcanites, tuffs, quartzite, and cherts. The shallow water sediments are divided into Upper Devonian to Lower Carboniferous Tsetserleg Formation (D3-C1cc) and Lower–Middle Carboniferous Dzargalant Formation (C1-2dz). The Tsetserleg Formation (D3-C1cc) consists of only sedimentary rocks such as bluish-grey sandstones and siltstones, and Lower–Middle Carboniferous Dzargalant Formation (C1-2dz) is principally composed of medium- to coarse-grained, brown-greenish grey sandstones with thin-layers of dark siltstones and gravelites. The SiO2 content of the Shar Khutul area sandstones ranges from 63.85 to 67.95 wt.% and the average content of TiO2 is 0.72 wt.% and Al2O3 content is 14.38 wt.%. The Chemical Index of Alteration (CIA) value ranges from 48.71 to 56.94 and the range of Index of compositional variations (ICV) is from 0.98 to 1.24. Moreover, the samples studied show that most of the sandstones are generally immature and were derived from weakly weathered source rocks. The ratios of Eu/Eu* (0.83), La/Sc (3.81), La/Co (5.30), and Cr/Th (13.81) indicate that the derivation of the Shar Khutul area sandstones from felsic rock sources and confirm the signatures of a felsic igneous provenance and suggest an active continental margin tectonic setting of the source area. The clastic zircons from the medium grained sandstone (Erdenetsogt formation) yield ages between 2.5 Ga and 236 Ma and the detrital zircons exhibit four peak ages at 1.7-2.5 Ga (n = 13), 455-499 Ma (n = 6), 337-382 Ma (n = 13) and 236–250 Ma (n = 5).

SULFIDE RECRYSTALLIZATION AND GOLD REMOBILIZATION DURING THE 2.0 GA STAGE OF THE MINAS OROGENY: IMPLICATIONS FOR GOLD MINERALIZATION IN THE QUADRILÁTERO FERRÍFERO AREA, BRAZIL

2021 ◽  
Author(s):  
Mariana Brando Soares ◽  
David Selby ◽  
Laurence Robb ◽  
Atlas Vasconcelos Corrêa Neto
Keyword(s):  
Thermal Effects ◽  
Gold Mineralization ◽  
Gold Deposits ◽  
Coarse Grained ◽  
Gold Deposition ◽  
Mining District ◽  
Mean Square ◽  
Fine Grained ◽  
Tectonic Events ◽  
Quadrilátero Ferrífero

Abstract The historic Quadrilátero Ferrífero mining district in Brazil is a polydeformed Archean terrane recording several tectonic events. This history has hampered the accurate dating of its important gold deposits, given that most of the geochronological clocks have been affected by multiple thermal events. To determine an accurate time constraint for the mineralizing events in this region, we provide the first Re-Os dating for a gold deposit in the Quadrilátero Ferrífero, obtained from 13 mineral separates of disseminated sulfide phases (pyrrhotite, coarse-grained arsenopyrite, fine-grained arsenopyrite, and pyrite) at the recently discovered São Sebastião deposit (northwest Quadrilátero Ferrífero). Three distinct successive sulfidation stages are interpreted at the deposit. The final stage, texturally associated with gold, is marked by fine-grained arsenopyrite and pyrite (assemblage III) and is associated with high-temperature (~600°C) features. A Re-Os errorchron is obtained when considering the data of all samples collectively; however, a comprehensive Re-Os isochron age of 1987 ± 72 Ma (2σ; n = 4; IsoplotR model 3; initial 187Os/188Os = 2.1 ± 0.7) is obtained from assemblage III. A 187Re-187Osr isochron at 1988 ± 56 Ma (mean square of weighted deviates = 0.1, initial 187Osr = –0.1 ± 2.6 ppt; n = 4) is obtained if an initial 187Os/188Os composition of ca. 2.1, is used, this being consistent with the fact that the 187Os in the sulfides in assemblage III is largely radiogenic (187Osr). Earlier assemblages (I and II) individually show variable initial 187Os/188Os, indicative of disturbance to the Re-Os systematics at ~2.0 Ga. This age is considered the best estimate for the timing of gold mineralization at São Sebastião, being coincident with the waning stages of the Minas orogeny, the thermal effects of which are restricted to the southern Quadrilátero Ferrífero. Hence, we consider that the hot fluids from which assemblage III crystallized were responsible for resetting the Re-Os systematics in assemblages I and II and were central to Paleoproterozoic gold deposition in the region. This ~2.0 Ga age challenges previous notions of a late Archean, ~2.7 Ga mineralizing event in the Quadrilátero Ferrífero and makes it possible that gold remobilization occurred elsewhere in the region, particularly in areas representing deeper crustal levels.

scholarly journals Identification of Forearc Sediments in the Milin-Zedong Region and Their Constraints on Tectonomagmatic Evolution of the Gangdese Arc, Southern Tibet

Lithosphere ◽  
2020 ◽  
Vol 2020 (1) ◽  
pp. 1-20
Author(s):  
Shao-Hua Zhang ◽  
Wei-Qiang Ji ◽  
Hao Zhang ◽  
Guo-Hui Chen ◽  
Jian-Gang Wang ◽  
...  
Keyword(s):  
Early Cretaceous ◽  
Detrital Zircon ◽  
Age Distribution ◽  
Detrital Zircons ◽  
Coarse Grained ◽  
Southern Tibet ◽  
Fine Grained ◽  
Lhasa Terrane ◽  
Isotopic Analyses ◽  
History Of

Abstract The Xigaze forearc sediments revealed the part of the tectonomagmatic history of the Gangdese arc that the bedrocks did not record. However, the sediments’ development is restricted to the region around and west of Xigaze City. Whether the eastern segment of the arc had a corresponding forearc basin is yet to be resolved. In this study, a field-based stratigraphic study, detrital zircon U-Pb geochronology (15 samples), and Hf isotopic analyses (11 of the 15 samples) were carried out on four sections in the Milin-Zedong area, southeast Tibet. The analytical results revealed the existence of three distinct provenances. The lower sequence is characterized by fine-grained sandstone, interbedded mudstone, and some metamorphic rocks (e.g., gneiss and schist). The detrital zircon U-Pb age distribution of this sequence is analogous to those of the Carboniferous-Permian strata and metasediments of the Nyingtri group in the Lhasa terrane. The middle and upper sequences are predominantly composed of medium- to coarse-grained volcaniclastic/quartzose sandstones, which are generally interbedded with mudstone. The detrital zircon U-Pb ages and Hf isotope signatures indicate that the middle sequences are Jurassic to Early Cretaceous in age (~200–100 Ma) and show clear affinity with the Gangdese arc rocks, that is, positive εHft values. In contrast, the upper sequences are characterized by Mesozoic detrital zircons (150–100 Ma) and negative εHft values, indicative of derivation from the central Lhasa terrane. The overall compositions of the detrital zircon U-Pb ages and Hf isotopes of the middle to upper sequences resemble those of the Xigaze forearc sediments, implying that related forearc sediments may have been developed in the eastern part of the Gangdese arc. It is possible that the forearc equivalents were eroded or destroyed during the later orogenesis. Additionally, the detrital zircons from these forearc sediments indicate that this segment of the Gangdese arc experienced more active and continuous magmatism from the Early Jurassic to Early Cretaceous than its bedrock records indicate.

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