Adjacent Granite Research Articles

Petrogenesis of Mesozoic pegmatites in the Dahongliutan Li-mineralized belt (Western Kunlun, NW China)

The genesis of pegmatite and the associated Li-mineralization mechanism remain debated. The intimate temporal-spatial relationship between the pegmatites in the Dahongliutan Li-mineralized belt (incl. #509 Daoban and Akshay Li-poor pegmatites, and Akshay Li-rich pegmatite) and the Dahongliutan granite pluton allows us to discuss the origin of pegmatite and the Li-mineralization mechanism by conducting detailed geochemical and geochronological investigations on the two rock units in the Western Kunlun orogen (NW China). The Li-poor pegmatites in the Dahongliutan Li-mineralized belt exhibit similar or much lower Li, Rb, and Cs compared to the adjacent granite, and their εNd(t) values (-8.7–-10.0) are significantly lower than those of the granite (-7.5–-8.1) as well. However, the Akshay Li-rich pegmatite shows extremely high Li, Rb, Nb, Ta, and initial Sr isotopic ratios (0.7223–0.7227, obtained from apatite), in contrast to the Dahongliutan granite (ISr = 0.7108–0.7157). The whole-rock δ7Li values of the Dahongliutan granite, pegmatites, and the wall-rock Bayanharshan Group metapelite are + 0.76–+5.67 ‰, +0.8–+14.4 ‰, and −4.2–+2.8 ‰, respectively. Zircon and cassiterite U-Pb dating yielded 206Pb/238U ages of 216 ± 2 Ma and 195 ± 2 Ma for the #509 Daoban Li-poor pegmatite and Akshay Li-rich pegmatite, respectively. Modeling of trace elements and Li isotopes suggests that the pegmatites in the Dahongliutan Li-mineralized belt did not originate from fractional crystallization of granitic magma. Instead, they may have formed through direct anatexis under amphibolite-facies conditions. Abundant Li-rich claystone, occasionally accompanied by evaporite, may exist in the magma source of the Li-rich pegmatite, which is the key factor controlling pegmatitic Li mineralization.

Unravelling the genetic relationship between pegmatites and granites in the Jiajika Li-Be polymetallic district, Songpan-Ganze Orogenic Belt, Southwestern China: Insights from monazite U-Pb geochronology and trace element geochemistry

The Jiajika Li-Be Polymetallic District is located in the Songpan-Ganze Orogenic Belt, Southwestern China. The Li-Be mineralization is genetically, temporally, and spatially, related to pegmatites in the Jiajika District. Hundreds of pegmatite dikes were emplaced surrounding the adjacent Majingzi granite pluton. Drill core samples from the 3,200 m deep Jiajika Scientific Drilling project (JSD-1) were acquired to determine the vertical extent and the petrogenesis and metallogeny of the granites, and the pegmatites, respectively, and to obtain a better understanding of their genetic relationship. The vertical zonation of the pegmatites from shallow to deep is as follows: shallow albite pegmatites; albite-microcline pegmatites; microcline pegmatites; and deeper more albite pegmatite. Spodumene occurs mainly in the shallow albite pegmatites (55–105 m depth), and disseminated beryl, columbite-tantalite, and cassiterite, mainly occur in pegmatites at depths less than 700 m. Eight samples were selected for monazite U-Pb geochronology and trace element geochemistry analyses from pegmatites and granites at different depths. LA-ICPMS monazite U-Pb geochronology for the pegmatites indicates two episodes of magmatic activity; one at 211–205 Ma (2σ, 523–2,494 m depth) and another at 193–192 Ma (2σ, 3,170–3,211 m depth). The crystallization age of the older pegmatites is in good agreement with the monazite U-Pb geochronology of the adjacent Majingzi granite (208–205 Ma, 2σ, 1,265–2,568 m depth) within analytical uncertainty, indicating their close temporal relationship. The younger albite pegmatite (193–192 Ma) is associated with the columbite-tantalite Nb-Ta rare-metal mineralization. Compared to those in the deep pegmatites (excluding the younger albite pegmatite in 3,170–3,211 m depth), and the adjacent granites, monazites in the shallow pegmatites and the adjacent granites are relatively enriched in Li, Th, and U, indicating the continuous evolution of a felsic magmatic system; with the residual melt more enriched in various incompatible rare-metal elements. Hence, the monazite U-Pb geochronology and trace element geochemistry indicate that a highly evolved felsic magma plays an important role in the petrogenesis of the Jiajika Li-Be pegmatites.

Feldspar traces mineralization processes in the Qongjiagang giant lithium ore district, Himalaya, Tibet

Recently, a large spodumene pegmatite deposit was discovered in the Higher Himalayan orogenic zone near Qongjiagang. The granites exposed on about 1500 vertical meters of the outcrop (with elevation varying from 4077 m to 5533 m) show an evolution from two-mica granite to tourmaline muscovite granite and tourmaline albite granite. Various types of pegmatites are dispersed throughout the granite, creating an opportunity to examine the evolutionary relationship between these rock types and their potential as ore deposits rich in lithium. This study collected feldspar samples from various granites and pegmatites along a vertical cross-section, and analyzed their structures using various techniques including Raman spectroscopy, and transmission electron microscopy. Elemental and isotopic compositions were determined using electron microprobe and laser ablation inductively coupled plasma mass spectrometry, providing valuable mineralogical information on the formation of widespread lithium deposits in the Qongjiagang area. The observed variations in feldspar composition suggest that changes in granite lithology and ore-bearing potential of pegmatites with increasing altitude result from upward fractional crystallization of magma formed by partial melting of Himalayan crystalline basement. This process is likely driven by decreasing temperature, resulting in a significant decrease in concentrations of alkaline earth metals (e.g., Ca, Sr, Ba), and gradual enrichment of trace elements including Rb, Cs, and Li. As a result, pegmatites attain a melt environment similar to that of the adjacent granite at similar elevations. Structural features of feldspar suggest that the decrease in Li concentration in K-feldspar and the formation of lithium-rich zonation of pegmatites are a result of simultaneous growth of spodumene and blocky orthoclase during the magmatic stage, as well as deuteric fluid extraction by syntectonic processes during the transition to the stable stage of microcline feldspar.

Geochronological constraints for a two-stage history of the Sveconorwegian rare-element pegmatite province formation

Most pegmatites of southern Norway seem to be derived from anatectic melting of metamorphic rocks during the Sveconorwegian orogeny rather than to be highly evolved residual melts derived from granites. We test this hypothesis by providing new age data for thirteen pegmatites and one granite. Based on these new age data, we distinguish two age groups of Sveconorwegian pegmatites (>1,000 m3 in size); Group 1: 1100–1030 Ma and Group 2: 930–890 Ma. All pegmatites except those from the Østfold area crystallized significantly earlier or later than adjacent granites. The Tørdal granite, yielding an age of 946 ± 4 Ma, is about 40 Ma older than the adjacent pegmatites. Field evidence and the age difference between pegmatites and granites supports an anatectic origin for these pegmatites. Sources of these pegmatite melts are biotite- and biotite-amphibole gneisses and amphibolites. Group 1 pegmatites formed in transpressional regimes after peak metamorphism, whereas Group 2 pegmatites formed in an extensional regime and the required heat for partial melting was provided by mafic magma underplating.Differences in the rheological behavior of amphibolite and granitic gneiss during extensional tectonics are the major reason why Group 2 pegmatites occur preferentially in large amphibolite bodies. Under mid-crustal conditions, amphibolite reacts brittle to semi-brittle forming open structures in an extensional tectonic regime where partial melts drained into. Granitic gneisses react in a ductile manner and do not have the ability to drain partial melt.Pegmatite formation in the Grenville Province, i.e., the Laurentian part of the Grenville–Sveconorwegian orogenic belt, formed between ca. 1090 and 980 Ma peaking at 1010 to 980 Ma. Thus, the Grenville peak postdates the Sveconorwegian Group 1 peak by about 30 Ma. These pegmatites formed in similar orogenic settings, implying that similar tectono-metamorphic developments along the Grenville–Sveconorwegian orogenic belt were diachronous.We conclude that local anatexis is the major pegmatite-melt forming process in the Sveconorwegian as well as Grenville orogen. Local anatexis also may be important in other pegmatite provinces.

Mesoarchean gabbro-anorthosite complex from Singhbhum Craton, India

The Singhbhum Craton in Peninsular India preserves some of the oldest continental crust on the globe with Hadean and Paleoarchean zircon record. Here we investigate a gabbro-anorthosite complex from the eastern margin of the craton exposed at Besoi, Kuliana, Sarat and Jashipura areas. These rocks display moderate SiO2 (47.94–53.77 wt%), TiO2 (0.42–2.17 wt%), Al2O3 (8.97–14.96 wt%) MgO (6.14–10.30 wt%) and CaO (10.73–15.07 wt.) at low to moderate Cr (20–862 ppm), Ni (32–636 ppm) and V (81–666 ppm) and are classified as noritic gabbros and gabbros. Their rare earth and trace element patterns exhibit mild to moderate fractionation with coherent patterns (La/Yb = 1.71–14.59), negative to positive Eu anomalies (Eu/Eu* = 0.62–1.44) negative Nb, Ti and sparingly negative to positive ZrHf anomalies. The UPb analysis of zircon grains from the Jashipura gabbros yield upper intercept ages of 3047 to 3099 Ma and the 3047 Ma age is considered to mark the timing of emplacement. Our geochemical data suggest that these rocks were generated through fractional crystallization of magma in a magma chamber at a transitional depth of the spinel-garnet regime with subsequent crustal contamination at an active continental margin supporting a continental arc setting. The gabbro-anorthosite suites in this region are synchronous with the adjacent granites and we propose that the mafic magma provided the heat for crustal melting that generated the granitoids. This magmatic event coincides with the ca. 3.1 Ga widespread crustal growth event in the Singhbhum Craton.

Hydrotermálny bastnäsit-(Ce) zo štôlne Elisabeth pri Gemerskej Polome (Slovenská republika)

Bastnäsite-(Ce), ideally CeCO3F, was recently found at the dumps of the Elisabeth adit near Gemerská Poloma, Rožňava Co., Košice Region, eastern Slovakia. It forms orange-brown aggregates up to 2 × 1 cm with vitreous to greasy lustre, which occur in the hydrothermal quartz veins crosscutting the coarse-grained, porphyritic rare metal S-type granite. Bastnäsite-(Ce) is closely associated with white, pale-green to purple fluorite, siderite and minor pyrite. It is hexagonal, space group P-62c with refined unit-cell parameters: a 7.1354(1) Å, c 9.7954(2) Å and V 431.90(1) Å3. The empirical formula of bastnäsite-(Ce) from the Gemerská Poloma based on sum of all cations = 1 apfu is (Ce0.49 La0.22Nd0.15Pr0.05Sm0.03Th0.02Ca0.02Gd0.01Y0.01)Σ1.00(CO3)1.00F0.83(OH)0.17. The Raman and infrared spectra of bastnäsite-(Ce) as well as tentative assignment of observed bands are given in this paper. Bastnäsite-(Ce) and associated minerals were formed from the early-hydrothermal post-magmatic fluids related to the adjacent granite.

The Tashisayi nephrite deposit from South Altyn Tagh, Xinjiang, northwest China

The Tashisayi nephrite deposit is located in South Altyn Tagh, in Qiemo County, Xinjiang Province, northwest China. It is a recent discovery in the vast, well-known Kunlun-Altyn nephrite belt distributed along the south of the Tarim Basin, producing more than half of the nephrite from the whole belt in 2017. Field investigations revealed that it is a dolomitic marble-related (D-type) nephrite deposit, but little is known about its age of formation and relationships between the granites and marble. Here we report field investigations, petrography of the nephrite, as well as petrography, geochemistry, geochronology of the zoisite-quartz altered intrusive rock and adjacent granites. An A-type granite is identified with a SHRIMP U–Pb zircon age of 926 ± 7 Ma, suggesting it was emplaced in an extensional tectonic environment at that time. The altered intrusive rock has a cluster of U–Pb zircon age of 433 ± 10 Ma, with similar trace element features to the A-type granite, suggesting it was formed in an extensional regime at this later time. Nephrite formed because of the metasomatism of dolomite marble by hydrothermal fluids. It is inferred that Ca2+ was released from the dolomitic marble by metasomatism forming Ca-rich fluids, which caused alteration of both the intrusive rocks (6.00–8.22 wt.% CaO) and granite (1.76–3.68 wt.% CaO) near the nephrite ore bodies. It is also inferred that Fe2+ from the granite migrated towards the dolomite marble. The fluids gave rise to the formation of Ca-minerals, such as zoisite, in the nephrite and altered intrusive rock, and epidote in the granite. Based on the contact relationships, similarity in hydrothermal processes, and consumption of Ca2+, the Tashisayi nephrite is considered to have formed at the same time as the alteration of the intrusive rocks, i.e. ∼433 Ma. The geochronological similarity (∼926 Ma, 433 Ma) of South Altyn and North Qaidam may suggest that tectonically they belong to one single complex in the past, which was offset by the Altyn Tagh fault (ATF). The similar formation ages of the nephrites from Altyn Tagh (433 Ma) and the previously studied areas of West Kunlun (378–441 Ma) and East Kunlun (416 Ma) indicate that these nephrites formed during the closure of Proto-Tethys and in the accompanying post-collisional, extensional environment.

Internal framework and geochemistry of the Carboniferous Huaco granite pluton, Sierra de Velasco, NW Argentina

The A-type Huaco granite pluton of the Velasco range (Sierras Pampeanas of northwest Argentina) is formed by three coeval granitic facies and contains subordinate coeval-to-late facies, as well as enclaves, dikes and stocks that show different temporal relations, textures and compositions. The dominant facies (Regional Porphyritic Granite; RPG) is a porphyritic two-mica monzo- to syenogranite, with abundant microcline megacrysts up to 12 cm in size. It was emplaced in a dominant extensional setting and has a mainly crustal source but with participation of a mantle-derived component. The RPG transitions towards two coeval and co-genetic granite facies, at its margins (Border Granite; BG) and around Be-pegmatites (Adjacent Porphyritic Granite; APG). These two facies have a finer-grained texture and smaller and less abundant megacrysts. They are also monzo- to syenogranites, but a slight decrease in the biotite/muscovite ratio is observed from the BG to the RPG to the APG. Trace element modeling suggests that the RPG, BG and APG differentiated from the same magma source by fractional crystallization. Temporally older mafic (ME) and felsic (FE) enclaves are common in the pluton. The ME can be considered partially assimilated remnants of a mafic component in the genesis of the RPG, whereas the FE seem to be remnants of premature aplites. Other subordinate rocks intrude the RPG and are, hence, temporally younger: felsic dikes (FD), dioritic dikes (DD) and equiganular granites (EqG) are clearly posterior, whereas coeval-to-late Be-pegmatites (BeP) and orbicular granites (OG) formed during the final stages of crystallization of the pluton. The BeP, OG and FD indicate the presence of abundant water and volatiles. The EqG form small stocks that intrude the RPG and were possibly originated from purely crustal sources. The DD probably correspond to a younger unrelated episode of mafic magmatism.

LA-ICP-MS U–Pb zircon, columbite-tantalite and40Ar–39Ar muscovite age constraints for the rare-element pegmatite dykes in the Altai orogenic belt, NW China

AbstractThe Chinese Altai is renowned for its rich rare-element resources. Nine representative rare-element (REL) pegmatites were dated using LA-ICP-MS and40Ar–39Ar methods. The columbite grains yield a weighted mean206Pb/238U age of 239.6±3.8 Ma for the Dakalasu (Be-Nb-Ta) pegmatite and concordia U–Pb ages of 258.1±3.1 Ma and 262.3±2.5 Ma for the Xiaokalasu (Li-Nb-Ta) pegmatite. The zircons display a weighted mean206Pb/238U age of 198.5±2.5 Ma for the Husite (Be) pegmatite and concordia U–Pb ages of 194.3±1.6 Ma and 248.2±2.2 Ma for the Qunkuer (Be) and Taerlang (barren) pegmatites. The muscovite40Ar–39Ar dating gives plateau ages of 286.4±1.6 Ma, 297.0±2.6 Ma, 265.2±1.5 Ma, 178.8±1.0 Ma, 162.2±0.9 Ma, 237.7±1.3 Ma, 237.4±1.2 Ma and 231.9±1.2 Ma for the Talate (Li-Be-Nb-Ta), Baicheng (Nb-Ta), Kangmunagong (barren), Husite (Be), Qunkuer (Be-Nb-Ta), Xiaokalasu (Li-Nb-Ta), Weizigou (Be) and Taerlang (barren) pegmatites, respectively. These new ages coupled with previous geochronological work suggest that the REL pegmatites in the Chinese Altai formed during early Permain – Late Jurassic time. The REL pegmatites located in the Central Altaishan terrane are younger than those in the Qiongkuer–Abagong terrane, showing a correlation with the coeval and adjacent granites. The formation of the REL pegmatites and these granites indicates frequent and strong magmatic activity in the post-orogenic and anorogenic setting. The spatial and temporal distribution of pegmatites and granites reveals a magmatism path from the SE (of age early–middle Permian), to the NW (middle Permian – Middle Triassic) and finally to the central part (Middle Triassic – Jurassic) of the Chinese Altai.

Phosphate minerals from the hydrothermal quartz veins in specialized S-type granites, Gemerská Poloma (Western Carpathians, Slovakia)

An interesting association of phosphate minerals (fluorapatite, triplite, arrojadite-group minerals and viitaniemiite) was studied from intra-granitic hydrothermal quartz veins with minor amounts of albite, orthoclase, muscovite, fluorite, rhodochrosite, arsenopyrite, pyrite, bismuthinite and kobellite. The veins occur in highly evolved, Permian topaz-zinnwaldite leucogranite at the Elisabeth adit near Gemerska Poloma, Gemeric Unit, Western Carpathians (eastern Slovakia). Fluorapatite is enriched in Mn (similar to 4 wt. % MnO, 0.3 apfu Mn) and frequently replaced by triplite, representing the first known triplite occurrence in Western Carpathians. This mineral forms irregular aggregates (<= 7 cm across), Mn/(Mn + Fe) atomic ratio of which attains 0.68 to 0.78 and F/(F + OH) = 0.89-0.92. Fluorarrojadite-(BaNa) to its Mn-dominant analogue fluordickinsonite-(BaNa) (both minerals still not approved by IMA-CNMNC) occurs as aggregates up to 2 cm across, showing Sr-rich (similar to 1.7 wt. % SrO, similar to 0.36 apfu Sr) and Sr-poor (<= 0.6 wt. % SrO, <= 0.13 apfu Sr) compositions with W site F/(F + OH) = 0.74-0.80 and M site Mn/(Mn + Fe) = 0.39-0.52. Rare viitaniemiite is Mn-rich (10 to 11 wt. % MnO, 0.34-0.38 apfu Mn). The phosphate mineralization in quartz represents a high-temperature hydrothermal assemblage. The F-rich Mn, Fe, Ca-bearing phosphates, fluorite, and muscovite precipitated most likely in presence of alkali-and fluorine-bearing post-magmatic fluids which altered primary magmatic minerals (especially Li-rich micas and alkali feldspars) and liberated some elements (Fe, Mn, Al, Ba, Sr, Na, K) from the adjacent granite.

Predicting natural arsenic contamination of bedrock groundwater for a local region in Korea and its application

A logistic regression model for the probability of arsenic exceeding the drinking water guidelines (10 μg/L) in bedrock groundwater was developed for a selected county in Korea, where arsenic occurrence and release reactions have been investigated. Arsenic was enriched naturally by the oxidation of sulfide minerals in metasedimentary rocks and mineralized zones, and due to high mobility in alkaline pH conditions, concentrations were high in groundwater of the county. When considering these reactions of arsenic release and water quality characteristics, several geological and geochemical factors were selected as influencing variables in the model. In the final logistic regression model, geological units of limestone and metasedimentary rocks, the concentrations of nitrate and sulfate, and distances to closed mines and adjacent granite were retained as statistically significant variables. Predicted areas of high probability agreed well with known spatial contamination patterns in the county. The model was also applied to an adjacent county, where the groundwater has not previously been tested for the presence of arsenic, and a probability map for arsenic contamination was then produced. Through the analysis of arsenic concentrations at the wells of high probability, it was determined that the applied model accurately indicated the arsenic contamination of groundwater. The logistic regression approach of this study can be applied to predict arsenic contamination in areas of similar geological and geochemical conditions to the county used in this model.

Geochronology of the transition of eclogite to amphibolite facies metamorphism in the North Qinling orogen of central China

Quartz-rich veins, together with their host high-pressure (HP) to ultra-high-pressure (UHP) rocks, are ideal for deciphering the fluid–rock interaction history in subduction zones. Previous studies of samples from Dabie–Sulu have shown that most quartz-rich veins within UHP/HP eclogite were formed during the prograde or early exhumation stage of host rocks. In contrast, a combined study of petrology and U–Pb, Sm–Nd and Lu–Hf geochronologies for a quartz-rich vein and its host retrogressed eclogite from North Qinling, which is associated with a granite intrusion and obliterated with amphibolite-facies overprint, reveals that the amphibolite-facies overprint is likely due to the late regional magmatism and the quartz-rich vein might derive from the nearby granite. A pool of U–Pb analyses of zircon grains from the retrogressed eclogite reveals zircons grew over an extended period of time but with two caveats at ca. 490 Ma and ca. 473 Ma. Analyses of zircon grains from the quartz vein yield a weighted mean 207Pb/ 206Pb ages of 428 ± 5 Ma (1σ), which is indistinguishable from that of formation of the adjacent granite. Combined elemental zoning patterns in the garnet with the spherical geometry effect, the Lu–Hf age of 416 ± 5 Ma (2σ) of the retrogressed eclogite is likely a mix age of eclogite-facies and amphibolite-facies metamorphism while the garnet–amphibole Sm–Nd age of 400 ± 8 Ma (2σ) approximates amphibolite-facies retrogression metamorphism. The latter is consistent with the garnet–biotite Sm–Nd age of 393 ± 4 Ma (2σ) of the adjacent granite, suggesting contact metamorphism. The > 70 Myr gap between the eclogite-facies metamorphism and the amphibole-facies overprint and the consistency of the timing the amphibolite-facies overprint for the eclogite and the cooling of the magma intrusion in this area indicate that the eclogite-facies metamorphism and amphibolite-facies metamorphism might be distinct metamorphic events, and the latter might be trigged by the granite intrusion.

The Millungera Basin—new geoscience supporting exploration

An unknown sedimentary sequence was first recorded during a Geoscience Australia/ Geological Survey of Queensland/ pmd*CRC deep seismic reflection survey in the Mount Isa Inlier and adjacent undercover terrains, during 2006/07. The sequence occurs unconformably underneath the Carpentaria Basin succession in the Julia Creek area, east of Cloncurry in north Queensland, and is named the Millungera Basin. A section through the basin is recorded along seismic line 07GA–IG1, recorded between north of Cloncurry to east of Croydon. In this section three internal sequences are noted—with two strongly reflective units separated by a poorly reflective unit. As well as deep crustal seismic reflection profiles, magnetotelluric profiles were collected along the same traverse. These data show a moderately conductive Millungera Basin underlying the strongly conductive Carpentaria Basin. Zones of limited reflectors beneath the basin in the seismic sections have been interpreted as granites, raising the possibility of raised geothermal gradients. The Millungera Basin may comprise a potential geothermal target. The Millungera Basin sequence is interpreted to overlie granites. Adjacent Proterozoic granites of the Williams Batholith are known to be high heat producing granites, containing high levels of potassium thorium and uranium. The hydrocarbon potential of the basin is similarly uncertain. Strong reflectors in the seismic sections may be coal beds. Although the depth of the basin in the seismic section is insufficient to have reached the oil window, interpretation of gravity profiles by Geoscience Australia suggest the basin deepens to the south, possibly reaching 4,000 m. If fertile beds have reached the oil window, the structurally more complex eastern side of the basin may contain petroleum traps. The age of the rocks in the Millungera Basin is not known. Constraints from the seismic suggest between the early Mesoproterozoic and the Middle Jurassic. Investigations into the nature of the basin are continuing. A more detailed magnetotellurc survey is being undertaken to better define the shape of the basin. In order to reliably describe the basins components, a deep drilling program is required.

THE GEOLOGY AND STRUCTURE OF THE MASVINGO GREENSTONE BELT AND ADJACENT GRANITE PLUTONS FROM GEOPHYSICAL DATA, ZIMBABWE CRATON

Research Article| December 01, 2009 THE GEOLOGY AND STRUCTURE OF THE MASVINGO GREENSTONE BELT AND ADJACENT GRANITE PLUTONS FROM GEOPHYSICAL DATA, ZIMBABWE CRATON O. GWAVAVA; O. GWAVAVA Department of Physics, University of Zimbabwe, P. O. Box MP167, Mt Pleasant, Harare., Present Address: Department of Geology, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa. Corresponding author: e-mail: ogwavava@ufh.ac.za Search for other works by this author on: GSW Google Scholar R.T. RANGANAI R.T. RANGANAI Department of Physics, University of Zimbabwe, P. O. Box MP167, Mt Pleasant, Harare., Present Address: Physics Department, University of Botswana, Private Bag UB 0704, Gaborone, Botswana. e-mail: ranganai@mopipi.ub.bw Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2009) 112 (3-4): 277–290. https://doi.org/10.2113/gssajg.112.3-4.277 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation O. GWAVAVA, R.T. RANGANAI; THE GEOLOGY AND STRUCTURE OF THE MASVINGO GREENSTONE BELT AND ADJACENT GRANITE PLUTONS FROM GEOPHYSICAL DATA, ZIMBABWE CRATON. South African Journal of Geology 2009;; 112 (3-4): 277–290. doi: https://doi.org/10.2113/gssajg.112.3-4.277 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The structure and subsurface configuration of the Masvingo Greenstone Belt (MGB) in the Zimbabwe Craton (ZC), were investigated using aeromagnetic and gravity data. Gravity data from 158 new and 202 old gravity stations were combined to produce a Bouguer gravity map covering the greenstone belt and surrounding granites. The greenstone belt and ultramafic complexes are characterised by Bouguer gravity highs, whereas the granitic terrain are lows, with the lowest values over the post-volcanic plutons. Gravity modelling, constrained by surface geology and density measurements, was carried out on a profile crossing the greenstone belt. The modelling results indicate a depth extent of around 6 km for the dense greenstone belt with a thickness of about 8.5 km for the adjacent younger granite plutons, which are significantly less dense. Granite-greenstone contacts along the profile are near-vertical.Magnetic maps compiled from the Zimbabwe Geological Survey national aeromagnetic data set show the bulk of the greenstone belt to have low magnetic anomaly values, but with high magnetic anomaly values along its northern and western margins over ultramafic units and banded iron formations. The younger granites generally have higher magnetic anomaly values compared to both the greenstone belt and the older gneisses. In particular, a pronounced east-northeast-trending broad magnetic high is observed over the Charumbira pluton, and its edges mark the boundaries with the greenstone belt to the north, and the Limpopo belt to the south; with the latter characterised by low magnetic values. Several linear, small amplitude magnetic lows, which are mostly due to faults, trend northwest to southeast and north to south, with some revealing an apparent dextral movement. Derivative maps for both gravity and magnetic data bring out some of the linear features and the edges of the greenstone belt and granite plutons. Shallow and deep depth slices of the magnetic field allow separation of high and low frequency anomalies; such as the east-northeast-trending Limpopo dyke swarm of Karoo age.The information obtained from modelling, derivative maps and depth slicing suggest a trough-shaped configuration for the MGB, which is consistent with other known greenstone belts on the ZC and elsewhere. This information is consistent with an intracratonic, extensional rift environment for emplacement of the greenstone belt. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Distribution and mineralogy of platinum-group elements in altered chromitites of the Campo Formoso layered intrusion (Bahia State, Brazil): control by magmatic and hydrothermal processes

Polyphase, penetrative hydrothermal metasomatism in chromitites of the Campo Formoso layered intrusion produced spectacular chromite – ferrian chromite zoning and transformed the primary intercumulus silicates into a chlorite – serpentine – carbonate – talc assemblage. Alteration did not substantially modify the composition of chromite cores and the distribution of platinum-group elements (PGE) through the sequence of chromitite layers, which still are consistent with magmatic fractionation processes. Texture and composition of laurite and Os–Ir–Ru alloys included in chromite cores indicate that these PGM were not altered, and are probably magmaticin origin. In contrast, the PGM located in the intergranular chlorite matrix (laurite, Ir–Ru–Rh sulfarsenides and Pt–Pd compounds with Sb, Bi and Te) display evidence of hydrothermal reworking. These PGM are intimately intergrown with low-temperature Ni-sulfides. The paragenesis suggests that the Ni-sulfides-PGM assemblage formed at the expenses of unknown PGM precursors, which must have been originally present in the intercumulus silicate matrix. Mechanism of formation involves a sequence of dissolution-precipitation events controlled by variation of redox conditions during chromite alteration. The presence of a secondary ore mineral assemblage consisting of galena, bismuthinite, native antimony, and various Pb–Sb compounds suggests a possible contribution of fluids derived from the adjacent granite.

Relict Proterozoic basement in the Nanling Mountains (SE China) and its tectonothermal overprinting

The basement beneath the Caledonian belt in SE China has been overprinted by Indosinian and Yanshanian events and is poorly known. In situ U‐Pb and Hf‐isotope analysis of zircons from granites and surrounding rocks in the Nanling Mountains area of the fold belt reveal that the Lanhe felsic gneiss is an outcrop of Mesoproterozoic basement, probably with a partly Late Archean protolith. This is consistent with Nd model ages of granites in this region, and exposures of Cathaysia basement in the coastal area of SE China. In the Nanling Mountains, this basement was reactivated to generate granitoid rocks during the Caledonian, Indosinian, and Yanshanian tectonothermal events. The Caledonian overprint is documented in extensive exposures of the Zhuguang migmatites (425–470 Ma) and by previous U‐Pb zircon dating of adjacent granites. The Jiangnan granite may be of similar age, but strongly reworked. The Indosinian overprint resulted in the formation of granites such as the Luxi and Xiazhuang bodies (233–246 Ma) in the Guidong complex. The Yanshanian overprint produced the most widely distributed granites (circa 160 Ma) such as the Jiufeng and Fogang bodies, and parts of the Guidong complex. Hf‐isotope data show that all of the Phanerozoic magmatic rocks were derived from the Mesoproterozoic basement (and minor Archean components), without obvious contributions from juvenile sources. The presence of zircons with Archean age (2.5–2.7 Ga) in both the Lanhe gneiss and Devonian sediments indicates that the Badu and Mayuan groups may not be the oldest basement of the Cathaysia block. Some Archean basement (circa 2.5–2.7 Ga) may still be exposed but unrecognized in this region, and Cathaysia may include Archean microcontinental fragments.

Oxygen isotope evidence for crustal assimilation and magma mixing in the Granite Harbour Intrusives, Northern Victoria Land, Antarctica

The stable isotope composition (O,H) of whole-rock and mineral separates of Cambrian-Ordovician gabbros, diorites, granodiorites and granites forming the Mt. Abbott composite intrusions (Northern Victoria Land, Antarctica) was measured to constrain the origin and evolution of the magmas postdating the Ross Orogen. The δ 18O values of olivine gabbros plot in the field of slightly evolved mantle-derived melts ( δ 18O WR=6.8–7.4‰). The O-isotope character of the mantle source inferred from the δ 18O values of cumulous olivine in gabbros (5.7–6.8‰) is enriched in 18O compared to modern arc-related magmas. Geochemical data and concurrent high δ 18O values, and initial strontium ( 87Sr/ 86Sr=0.7060) and neodymium ( 143Nd/ 144Nd=0.5122) isotope ratios indicate that the olivine gabbros formed by crustal contamination of a primary calc-alkaline basaltic melt. The diorites have high δ 18O values, among the highest ever measured for dioritic rocks (8.7–10.3‰), and Sr-isotope ratios that partially overlap with the adjacent and mingled felsic lithologies (0.708–0.710). The diorites have pyroxene with high, nearly constant δ 18O values (8.2–8.6‰) that are independent from the silica content of the rocks; thus, they did not increase in response of the chemical evolution of the rocks. The diorites originated from the same primary calc-alkaline basalt experiencing different amounts of crustal contamination, and underwent different degrees of mixing with the adjacent granites, producing granodioritic facies and quartz/feldspar xenocrystic diorites. The δ 18O, 87Sr/ 86Sr and 143Nd/ 144Nd compositions of the granites and granodiorites overlap (10.8–12.1‰, 0.7096–0.7108, 0.5119–0.5120). They are distinct from the values of the mafic rocks and indicate that gabbros and granites were not cogenetic. The granites are a separate melt component likely derived from nonmodal partial melting of fertile meta-igneous protoliths.

Strain localization due to a positive feedback of deformation and myrmekite-forming reaction in granite and aplite mylonites along the Hatagawa Shear Zone of NE Japan

With increasing mylonitization of aplite veins and their adjacent granite in the Hatagawa Shear Zone of NE Japan, the modal content and grain size of feldspar porphyroclasts decrease, while the modal content of fine-grained (≤50 μm) polymineralic aggregate of plagioclase, K-feldspar, quartz and biotite increases up to 60–80% in the granite and aplite ultramylonites. Deformation-induced myrmekite lobes are ubiquitously developed around K-feldspar porphyroclasts. The occurrence and chemical composition of plagioclase and K-feldspar in fine-grained polymineralic aggregates indicate that the plagioclase is mostly derived from myrmekite, while the interstitial K-feldspar originates from isolated porphyroclast rims between growing myrmekite lobes and/or precipitations from solution. Modal and bulk chemical compositions of granite and aplite indicate that the mylonites are preferentially developed in aplite veins richer in K-feldspar than the surrounding granite. Deformation-induced K-feldspar replacement by myrmekite is then more favored during deformation to produce more abundant fine-grained polymineralic aggregates. Subsequent reaction softening leads to more enhanced deformation, which promotes further myrmekite-forming reaction. Such a positive feedback of deformation and myrmekite-forming reaction in aplite veins and their adjacent granite may be responsible for strain localization in the granite and aplite mylonites along the Hatagawa Shear Zone.

Potarite (Pd-Hg) in thermally metamorphosed dunite from the Inazumi-yama ultramafic complex, southwestern Japan: an implication for the behaviour of mercury in PGE mineralization in peridotite

Abstract Pd-rich PGM (platinum-group minerals) have been found in sulphide-bearing dunites and harzburgites from the Inazumi-yama ultramafic complex in southwestern Japan. In the dunite, potarite (PdHg0.85–0.88) is the most abundant PGM but other associated PGM include stibiopalladinite (Pd5Sb2) and rare sperrylite (PtAs2) and Pd-rich alloys. A Pd telluride has been found in the harzburgite. The PGM are enclosed usually by pentlandite-heazlewoodite composite grains and, rarely, by altered chrome-spinel. These minerals are characteristic of an ultramafic assemblage but they are accompanied by ubiquitous galena and minor sphalerite not usually associated with these ultramafic assemblages. The ultramafic part of this complex has been thermally metamorphosed (olivine-talc zone) within the contact aureole of an adjacent granite. The PGM, sulphides and altered spinel are all intergrown with antigorite and/or chlorite, indicating a metamorphic overprint on the primary igneous mineralogy. The Pd/Pt ratios of 9 suggest a process of hydrothermal concentration for the Pd and it is proposed that the Pd has been remobilized and reconcentrated by hydrothermal solutions derived from the granitic magma which reacted with the Pd concentrated in the primary magmatic sulphides. Mercury may have been leached from surrounding sediments by the hydrothermal solutions. It is possible that potarite and the other PGM formed at temperatures of up to 650°C and are likely to be found in other thermally metamorphosed ultramafic rocks showing a depleted character (spinel Cr/[Cr+Al]&gt;0.7), common in orogenic belts.

The Big Picture for Exploration in the Eastern Goldfields Province, Western Australia, defined by Regional Magnetic and Gravity Data

Geological understanding of the Eastern Goldfields has been limited in the past by poor outcrop owing to regolith and surficial cover. The economic importance of the greenstone belts relative to gneiss and granitoids, has led to geological maps and models concentrating on the geology and geological evolution of the greenstone belts. The regional aeromagnetic and gravity data are used to derive models for the whole area, and therefore also to subdivide the non-greenstone rock. lnterpretation of these regional geophysical data sets indicates the presence of features which are tens to hundreds of kilometres long. They include gneiss-granite domains, gneiss-granite domes, greenstone terranes and large bodies of granite. Recent more detailed surveys acquired under the National Geoscience Mapping Accord provide better resolution of the internal structure of these domains and enhance characteristic anomaly patterns used to discriminate between domains. Greenstone-belt deformation has been influenced by the shape of adjacent gneiss-granite domains, granites and lineaments. The geophysical models developed provide a valuable framework in which to assemble a wide variety of geological, geochemical, and geophysical information. It is through the generation of comprehensive regional models of the geology that effective exploration of areas of poor outcrop will be developed.