Granite Emplacement Research Articles

Application of Gravity Survey for Tin Exploration at Bongsu Granite, Kulim, Kedah, Malaysia

Alluvial tin mining in Karangan area was closely related to the mineralization of Bongsu Granite and its contact zone with sedimentary rocks of Semanggol Formation possibly through greisenization process. A gravity survey focusing on high tin anomaly area in Gunung Bongsu Forest Reserve was conducted to define the subsurface geology and structural patterns related to tin mineralization. The gravity data were corrected for drift, free air, Bouguer, latitude and terrain corrections to produce final gravity data of the study area. The data were processed and analyzed using Oasis Montaj to produce Bouguer, regional, residual, Total Horizontal Derivative (THD) regional and THD residual gravity maps for qualitative interpretations. Comparison of the produced gravity anomaly maps indicate occurrence of partial ring fractured-style model of granite emplacement initialized by laccolith development, doming of sedimentary bedrock, magmatic diking through ring fractures and finally sinking and melting of surrounded sedimentary rock before magma cooling and crystallizing to become granites bedrocks. Greisenization occurs within the inner ring granite as the bedrock developed at later phase.

Mesozoic crustal growth in Mainland Southeast Asia: Zircon U-Pb and Hf isotopic evidence from the Late Cretaceous Luyingtang granitic pluton in the northernmost SE Asian granite Province, SW China

The Changning-Menglian suture in the Sanjiang region of SW China, the Chiang Rai line in Thailand, and the Bentong-Raub suture in Peninsular Malaysia are marked by remnants of the Paleo-Tethys oceanic lithosphere in Mainland Southeast Asia. We present U-Pb ages and Hf isotopic compositions of zircon from Luyingtang granites east of the Changning-Menglian suture, with the aim of identifying their sources and petrogenesis, which are significant for understanding the crustal evolution of the Mainland Southeast Asia. The Luyingtang granitic pluton is located in the northernmost SE Asian granite Province, SW China. New zircon U-Pb ages indicate that the granites were emplaced in the Late Cretaceous, in accordance with the regional scale inland magmatic event in Mainland Southeast Asia. Petrographically, the samples range from granodiorite, biotite granite, two-mica granite to pegmatite. The biotite granite, two-mica granite and pegmatite have low contents of TiO2, Al2O3, FeO*, MgO, CaO and P2O5, but high values of SiO2 and K2O, compared to the granodiorite. The granodiorite, biotite granite and two-mica granite invariably show enrichment in light rare earth elements (LREE). The biotite granite and two-mica granite show negative Eu anomalies, but the granodiorite displays weak Eu anomaly. The pegmatites show a clear tetrad effect and contain pronounced negative Eu anomalies. All the rocks demonstrate characteristic negative Nb, Ta, Sr and Ti anomalies, and a positive Pb anomaly, when plotted on spidergrams. Most of the granites are characterized by positive εHf (t) values (0.3–8) and fairly young TDM2 ages (612–1094 Ma), suggesting they contain a significant proportion of juvenile component. Hf isotopic modelling shows that the Luyingtang granites are likely to have been produced by partial melting of mixed sources, which were probably of basalts in proportions of 50–70% and Precambrian continental crust. The emplacement of granites with such a large proportion of mantle-derived component indicates crustal growth in Mainland Southeast Asia during Mesozoic. In contrast, one of the two-mica granite samples displays negative εHf (t) values (−7.6 to −3) and older TDM2 ages (1305–1599 Ma), which suggest a more crustal affinity. The arc-continent collision during the Cretaceous may have induced foundering of an over-thickened lithospheric mantle keel beneath the Paleo-Tethys orogenic belt, and then resulted to localized upwelling of asthenosphere mantle and basaltic underplating. The parental magmas of the hinterland I-type granites formed by melting of a mixed source, containing both Precambrian crust and mantle-derived basalt, followed by extensive magmatic differentiation.

4D history of the Nimbus VHMS ore deposit in the Yilgarn Craton, Western Australia

Due to its unique mineralogy in the Archaean Yilgarn Craton, the origin of the high-grade Nimbus Ag-Zn-(Au) deposit has been contentious for a number of years. Recent interpretations of the deposit as a shallow water and low temperature volcanic-hosted massive sulphide (VHMS) deposit with epithermal characteristics (i.e. a hybrid bimodal-felsic deposit), were based on detailed studies of its volcanology, mineralogy, hydrothermal alteration assemblages, geochemistry, multiple S isotopes and trace element content of sulphide. However, this model has been questioned in favour of a late, epithermal fault-hosted system. Effective greenfields exploration for similar deposits elsewhere requires a robust deposit model, and well understood timing of ore formation. We present a comprehensive multi-disciplinary study to further constrain the processes involved in the evolution of Nimbus and generate a 4D evolutionary model of the system.Host rock dacite formation is well constrained by SHRIMP U-Pb zircon geochronology to c. 2703 Ma. Re-Os ages for the first sulphide phase, gives an imprecise maximum age to the ore formation of 2682 ± 110 Ma. High precision Pb-isotope variations in ore-stage and late galenas track changes in the sulphides over time, complemented by new geochronology. The Pb-isotope data indicates that the polymetallic ore precipitation happened at the same time as the volcanism, with the second generation indicating minor, local remobilisation of Pb in late quartz-carbonate veins at c. 2630 Ma. 40Ar/39Ar dating of sericite-altered plagioclase from foliated dacite, and U-Pb SHRIMP dating of syn- to post-deformation monazites have yielded ages of ~2630 Ma. The late event at c. 2630 Ma is most likely related to widespread late low-Ca granite emplacement across the Eastern Goldfields.This study provides arguments that suggest that Nimbus is a syn-volcanic deposit. The ore timing is consistent with the model of replacement-type VHMS deposit, with the age of the main mineralisation similar to the age of the dacitic host-rocks at 2703 Ma. The late event modifies the pre-existing ore and overprints the previous ore-related alteration ages from the altered plagioclase. High precision, double spiked, Pb isotope constraints from late quartz-carbonate veins provides an opportunity to vector to hidden deposits.

A review of molybdenite, and fluorite mineralisation in Caledonian granite basement, western Ireland, incorporating new field and fluid inclusion studies, and Re-Os and U-Pb geochronology

The recent discovery of late-magmatic quartz vein hosted molybdenite, and exceptional gem quality vein fluorite, in the Caledonian Galway Granite Complex (GGC), has prompted a review of these contrasting styles of mineralisation in the late-Caledonian granite basement, Connemara, western Ireland. Existing published U-Pb and Re-Os chronometry and fluid inclusion microthermometry are combined with new: a) geological field observations, b) U-Pb zircon and Re-Os molybdenite geochronometry and c) fluid inclusion microthermometry to generate a new pressure-temperature-time model (P-T-t) of mineralisation for the GGC. Re-Os chronometry molybdenite indicates that granite related molybdenite mineralisation extended from 423 to 380Ma overlapping with the GGC emplacement history determined by U-Pb zircon chronometry. The P-T-t model reflects initial granite emplacement and Mo-mineralisation at ∼423Ma followed by lower P and T granite emplacement and related quartz vein hosted Mo-mineralisation at ∼410Ma (Carna pluton), ∼400Ma (Kilkieran pluton) and at ∼380Ma (Costelloe Murvey granite). The gem quality fluorite veins in the GGC represent late-Triassic hydrothermal mineralisation. These veins form part of a regional N Atlantic-European Triassic-Jurassic hydrothermal mineralisation province. Vein emplacement was triggered by the rifting of the N Atlantic facilitating crustal thinning and subsidence of continental crust and initiating hydrothermal activity at the margins of Mesozoic basins.

U–Pb ages of Miocene near-trench granitic rocks of the Southwest Japan arc: implications for magmatism related to hot subduction

AbstractWe present a new dataset of zircon U–Pb ages that document igneous activity in the SW Japan arc during middle Miocene time and discuss its relationship with the opening of the Japan Sea, Philippine Sea plate migration, and subduction of the young hot lithosphere of the Shikoku Basin. Precursory magmatism, characterized by dike and stock intrusions, startedc. 15.6 Ma in both Kyushu and the Kii Peninsula. Most plutonism occurred between 15.5 and 13.5 Ma in an area 600 km long and 150 km wide. No along-arc trend was recognized in the U–Pb ages of igneous activity near the trench. Our data indicate that all near-trench middle Miocene igneous activity occurred immediately after the opening of the Japan Sea ceased, i.e. after 16 Ma, implying that melt extraction and the emplacement of granites in the near-trench region had some influence on the back-arc opening. Our data also imply that the trench–trench–trench-type triple junction between the Japan arc and the Izu–Bonin–Mariana arc must have reached the east side of the Kii Peninsula by 15.6 Ma. The wide distribution of contemporaneous magmatic activity along the arc requires a trench-parallel heat source, such as the subduction of a trench-parallel ridge or a young and highly segmented ridge–fracture zone system in addition to the hot wedge mantle condition related to the opening of Japan Sea.

Whole-rock and zircon geochemistry of the Xiaoliugou granites, North Qilian Orogen (NW China): Implications for tectonic setting, magma evolution and W–Mo mineralization

Ordovician W–Mo polymetallic mineralization-related granites are widely developed in the western section of the North Qilian Orogen, NW China. In order to shed light on the tectonic setting, magma evolution and genesis of mineralization in the orogeny, highly evolved granitic rocks (monzogranites) from the Qibao and Qiqing deposits in the Xiaoliugou ore field were studied. Typical of subduction-related A-type granites, the Xiaoliugou granitic rocks are enriched in SiO2, Rb, Th, U, Ta, Nd, Zr and Hf but relatively depleted in Ba, Sr, P and Eu. The variable initial whole rock 87Sr/86Sr ratios (0.62937–0.70543) and wide range of zircon εHf(t) values (−18 to 6) suggesting mantle–crust interaction and mixing of depleted mantle and deep crust in the source area. Whole-rock elemental and Rb–Sr isotopic geochemistry suggest slow cooling, significant fractional crystallization, weak wall-rock assimilation/contamination and intense fluid differentiation/metasomatism during the genesis of the Xiaoliugou granitic rocks. Zircon U–Pb dating yielded three age clusters: Precambrian (>540 Ma) (n = 32), Ordovician (~450 Ma) (n = 40), and Cretaceous (~135 Ma) (n = 18). Zircon internal textures indicate a range of sources including residual/detrital zircons from the magma source region and local sediments, magmatic and fluid-altered zircons, and late stage hydrothermal zircons. Zircon trace element and Lu–Hf isotopic compositions suggest fluid metasomatism during granite emplacement in the Ordovician and overprinting by a more evolved, oxidized fluid in the Cretaceous. Involvement of a late, high-temperature, highly fractionated, F-rich fluid could have enhanced W–Mo mineralization. Consequently, a two-stage genetic model is proposed for the Xiaoliugou granitic-minerogenic system involving primary mineralization in the Ordovician (~450 Ma), followed by a Cretaceous magmatic hydrothermal overprint (~135 Ma).

Transpressional deformation during Ediacaran accretion of the Paranaguá Terrane, southernmost Ribeira Belt, Western Gondwana

The Paranaguá Terrane is mainly constituted by an Ediacaran arc-related granitic complex, spread in a NE-SW trending elongated stripe, located in the Southern Ribeira Belt, South-Southeastern Brazil. The basement of these granites occurs as a disrupted folded belt composed of metasedimentary and gneissic-migmatitic rocks. The integration of structural and geophysical (gamma spectrometry and magnetometry) data from the Paranaguá Terrane and the adjacent tectonic units, suggests that the deformation in the study area is partially controlled by the geometry of the Luis Alves Cratonic Block, as well as the northern part of Dom Feliciano Belt. The irregular shape of this cratonic unit associated with N-NW tectonic transport direction caused strain partitioning in the area and the development of two structural domains: southern and northern Paranaguá. The southern part is characterized by sinistral transpressional shear zones, while the northern sector is dominated by thrust shear zones that represent a large frontal collision with oblique components. The hierarchy of the structural and microstructural features in the Paranaguá Terrane associated with geochronologic data suggest two deformational phases (D1 and D2) during a transpressive collisional system. The D1 progressive deformational stage is correlated to the main period of granitic crystallization during the Ediacaran and was divided in two moments: i) early D1 (640–610) associated with the regional metamorphism and thrust and folding tectonics with N-NW tectonic transport and; ii) late D1 (610-580 Ma) linked to the strain partitioning and development of the southern (simple-shear dominated) and northern (pure-shear dominated) structural domains with sinistral kinematics, granite emplacement and high-temperature mylonites. The D2 deformation stage (540–500 Ma) is mainly represented by the low-temperature mylonites and deformation under retrograde conditions.

Permian felsic magmatism in the Neoproterozoic Nagar Parkar Igneous Complex of the Malani Igneous Suite: Evidence from zircon U–Pb age

AbstractWe report Permian (ca. 272 Ma ±5.4 Ma) felsic dykes that intrude into the Neoproterozoic (ca. 750 Ma) magmatic suite of the Nagar Parkar Igneous Complex (NPIC), the western extension of the Malani Igneous Suite (MIS). The NPIC consists of Neoproterozoic basement amphibolites and granites (riebeckite–aegirine gray granites and the biotite–hornblende pink granites), all of which are intruded by several generations of mafic and felsic dykes. Granitic magmatism occurred in the Late Neoproterozoic (ca. 750 Ma) due to the subduction‐, followed by the rift‐related tectonic regime during the breakup of the Rodinia supercontinent. U–Th–Pb zircon and monazite CHIME age data of 700–800 Ma from the earlier generation porphyritic felsic dykes suggest the dyke intrusion was coeval or soon after the emplacement of the host granites. Our findings of Permian age orthophyric felsic dykes provide new insights for the prevalence of active tectonics in the MIS during late Paleozoic. Textural features and geochemistry also make the orthophyric dykes distinct from the early‐formed porphyritic dykes and the host granites. Our newly obtained age data combined with geochemistry, suggest the existence of magmatism along the western margin of India (peri‐Gondwana margin) during Permian. Like elsewhere in the region, the Permian magmatism in the NPIC could be associated with the rifting of the Cimmerian micro‐continents from the Gondwana.

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

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

Editorial

Editorial

Skarn formation and tin enrichment during regional metamorphism: The Hämmerlein polymetallic skarn deposit

The Hämmerlein Sn-rich polymetallic skarn deposit (Erzgebirge, Germany), which is hosted by Variscan schists and gneisses, includes garnet, amphibole, pyroxene, and magnetite skarns that formed during regional metamorphism at ∼340 Ma. The skarn deposit was overprinted during the intrusion of the ca. 320 Ma Eibenstock granite. Tin was introduced into the system during regional metamorphism at 340 Ma (Sn mostly hosted in silicate minerals) and after the intrusion of the Eibenstock granite at c. 320 Ma (Sn hosted in cassiterite). The emplacement of late stage melts of the Eibenstock granite also resulted in the formation of cassiterite-quartz veins in the schists hosting the Hämmerlein skarns and at some distance in the formation of several cassiterite greisen.Major, trace, and rare earth element data of the skarn units and their hosts rocks demonstrate that skarns formed from impure carbonates with variable proportions of silicate minerals. Depending on the original modal compositions of the skarns and their host rocks, the mineralizing fluid selectively added and removed elements, which resulted for the skarns and the schist ore in a general enrichment in Sn, In, Cu, and W and a noticeable depletion in Al and Sr. The Sr, Nd, and Pb isotopic composition of the skarns, schists, and gneisses resembles those of the metamorphic rocks of the Erzgebirge rather than those of the granites, possibly indicating that the geochemical signature of granite-derived fluids had been severely altered during fluid-rock interaction before these fluids reached the skarns, schists, and gneisses. Similarly, the Li and B isotopic composition of the skarn samples reflects the likely range of possible source rocks for metamorphic fluids, rather than the composition of the granite-derived fluid. In contrast, the Li and B signature of greisen samples shows a variable mixture of material derived from the granite and the metasedimentary rocks, respectively.Our data show that a first Sn enrichment took place when the Paleozoic sedimentary rocks were affected by regional metamorphism at ca. 340 Ma. Fluids derived from these siliciclastic sedimentary protoliths mobilized and transported Sn and Fe. These fluids reacted with the impure carbonates, resulting in decarbonation and the formation of calc-silicate minerals and biotite in the various skarn types. The changing fluid composition as well as the incorporation of Fe in skarn silicates and formation of Fe-oxides resulted in the incorporation of Sn into silicate minerals. During or after the emplacement of the Eibenstock granite, the original skarn was to a variable extent retrogressed and Sn was released from Sn-bearing calc-silicates and biotite. During this event additional Sn was added to the skarn and the hosting schists. Released Sn and newly added Sn precipitated as cassiterite.

SHRIMP U-Pb zircon dating of Neoproterozoic granites from the easternmost São Roque Domain, Ribeira Belt

Four granitic plutons from the easternmost portion of the São Roque Domain (SRD) were dated by U-Pb zircon SHRIMP; the results indicate that the peak of granitic magmatism was concentrated in a short interval (606–589 Ma), the same as in the central portion of the SRD. The two porphyritic biotite granites (Morro Azul and Imbiruçu) yield the oldest ages (respectively, 601.0 ± 4.9 Ma and 606.3 ± 4.8 Ma) and have inherited zircons with similar ages (~2.1–2.0 Ga), but are not comagmatic. The Morro Azul granite has geochemical signature similar to other high-K calc-alkaline (HKCA) granites from the SRD, while the Imbiruçu granite has much higher Sr/Zr and other chemical features that demand different sources. Other two dated samples are very different from each other and from the HKCA granites. The age of the Morro do Pão tonalite (589.1 ± 5.5 Ma) indicates that, in spite of its more deformed character, it is younger; a similar value, although with large uncertainty, is shown by the Serra dos Índios peraluminous leucogranite (593 ± 12 Ma). Our analytic data combined with field relationships suggest that soon after the main episode of HKCA magmatism, the SRD witnessed the emplacement of granites of varied composition forming small bodies that recorded deformation associated with transcurrent tectonics.

Geochemical characteristics and origin of the Neoproterozoic high-K calc-alkaline granitoids in the northern part of Mandara hills, northeastern Nigeria

The high-K calc-alkaline granitoids in the northern part of the Mandara Hills are part of the well-exposed post-collisional plutons in northeastern Nigeria. The calc-alkaline rock association consists of quartz monzodiorite, hornblende biotite granite, biotite granites and aplite which intruded the older basement consisting mainly of low-lying migmatitic gneisses and amphibolites during the Neoproterozoic Pan-African Orogeny. Petrological and geochemical studies have revealed the presence of hornblende, iron oxide, and metaluminous to slightly peraluminous characteristics in the granitoids which is typical of I-type granite. The granitoids are also depleted in some high field strength elements (e.g. Nb and Ta) as well as Ti. Plots of Mg# versus SiO2 indicate that the granite was derived from partial melting of crustal sources. Lithospheric delamination at the waning stage of the Pan-African Orogeny possibly triggered upwelling of hot mafic magma from the mantle which underplated the lower crust. This, in turn, caused partial melting and magma generation at the lower to middle-crustal level. However, the peculiar geochemical characteristics of the quartz monzodiorite especially the enrichment in compatible elements such as MgO, Cr, and Ni, as well as LILE element (e.g. K, Ce, Cs, Ba, and Sr), signify that the rock formed from an enriched upper mantle source. The emplacement of high-K granites in the Madara Hill, therefore, marked an important episode of crustal reworking during the Neoproterozoic. However, further isotopic work is needed to confirm this model.

Petrology, geochronology, and geophysical characterization of Mesoproterozoic rocks in central Illinois, USA

The Precambrian basement rocks of the Eastern Granite-Rhyolite Province (EGRP) in central Illinois (midcontinent region of North America) exhibit a complex history of early volcanism, granite emplacement, and intrusion of mafic rocks. A comprehensive suite of dedicated petrographic analyses, geophysical logs, and drill core from four basement-penetrating wells, two-dimensional and three-dimensional seismic reflection data, and U-Pb age data from the Illinois Basin–Decatur Project (IBDP) and Illinois Carbon Capture Storage (ICCS) Project site provide new constraints for interpreting the Precambrian basement of the Illinois Basin. These new data reveal the basement to be compositionally and structurally complex, having typical EGRP felsic volcanic rocks intruded by the first reported gabbro in the Precambrian basement in Illinois. Zircons (n ​= ​29) from rhyolite give a U-Pb weighted mean average age of 1467 ​± ​9 ​Ma. Zircons (n ​= ​3) from a gabbro dike that intrudes the rhyolite yield a concordia age of 1073 ​± ​12 ​Ma, which corresponds to Grenville-age extension and represents the first Grenville-age rock in Illinois and in the EGRP. A high-resolution three-dimensional seismic reflection volume, coincident with the four wells, provides a context for interpreting the petrological data and implies a high degree of heterogeneity for basement rocks at the IBDP–ICCS site, as also shown by the drill cores. The occurrence of Grenville-age gabbro is related to a prominent bowl-like structure observed on local two-dimensional seismic reflection profiles and the three-dimensional volume that is interpreted as a deep-seated mafic sill complex. Furthermore, heterogeneities such as the brecciated EGRP rhyolite and later gabbro intrusion observed in the basement lithology at the IBDP–ICCS may reflect previously unknown distal elements of the 1.1 ​Ga Midcontinent Rift in the EGRP and more likely Grenville-age extension.

Martin Harold Phillips Bott. 12 July 1926—20 October 2018

Martin Bott was a geophysicist who made fundamental contributions to our understanding of gravity and magnetic anomalies and their geological interpretation. His research on the deep structure of the Earth's crust was both pioneering and innovative, and he showed how field geophysical measurements could be used to address geological problems such as the mechanics of granite emplacement, sedimentary basin formation and mountain building. When he began his research, the use of gravity and magnetic anomalies to understand deep crustal structure was in its infancy and largely confined to research laboratories in the oil and gas industry. Four decades later his lifetime efforts have seen the emergence of potential field methods as one of the principal means of constraining the structure, stress state and long-term strength of the Earth's crust and upper mantle in continents and oceans. Martin was an inspiring undergraduate teacher and outstanding supervisor, as reflected by his many research students who went on to prominent leadership positions in academia, government and industry. He leaves a legacy of more than 150 scientific papers in peer-reviewed journals and a lucidly written and beautifully illustrated textbook. As well as his many scientific achievements, Martin was an accomplished mountaineer, a dedicated churchgoer and an avid gardener. He saw no conflict between his science and his enduring Christian faith.

Biotite chemistry and the role of halogens in Archaean greenstone hosted gold deposits: A case study from Geita Gold Mine, Tanzania

Geita Hill is one of the largest Tanzanian gold deposits, located southwest of Lake Victoria and ∼2 km along strike from the giant Nyankanga deposit. The deposit is hosted by metasedimentary rocks – dominated by ironstones and intruded by numerous diorite dykes. Textural, structural and geochronological observations indicate that the gold mineralisation overprints all the ductile fabrics and the late orogenic D6 reverse faulting event, suggesting that gold was introduced during later D7 and D8 events. The metamorphic background is characterized by biotite + K-feldspar + actinolite ± pyrhotite, which is overprinted by the gold related alteration consisting of sulfidation fronts, veins and micro-fractures dominated by quartz + biotite + K-feldpar + pyrite. The almost identical mineralogy of the mineralised assemblage and the metamorphic background makes it difficult to identify the source, the effect and the spatial extent of the mineralising fluid. As part of this study detailed analyses of biotite were conducted in order to identify the nature of the mineralising fluid and its spatial effect across the Geita Hill deposit. Results show that the mineralised assemblage is slightly more oxidized (pyrite + magnetite) compared to the metamorphic background (pyrrhotite + magnetite). The intense sulfidation within the ore zone resulted in the formation of Mg-rich biotite, which grades into more Fe-rich biotite away from the ore zone. This change in biotite chemistry causes a shift in the SWIR Fe-OH 2250 nm absorption feature to lower wavelengths, which is also correlated with an increase in gold grade within the mineralised zone. The abundance of F in biotite significantly increases within the ore zone indicating the presence of F-rich hydrothermal fluids during mineralisation. Late timing of mineralisation, synchronous with the emplacement of voluminous high-K granites, and the presence of K- and F-bearing fluids indicate a genetic connection between the mineralising fluids and the late granites. Biotite grains from different deposits types (porphyry, IOCG, lode-Au) appear to have distinct composition suggesting that the source of the fluids involved in the formation of each deposit type has a characteristic halogen chemistry.

Li-Na-metasomatism related to I-type granite magmatism: A case study of the highly fractionated La Pedriza pluton (Iberian Variscan belt)

This study describes rare albitites associated with highly fractionated I-type granitic melts, showing a mineral paragenesis that resembles those of high-temperature metasomatized rocks (fenites) associated with alkaline and carbonatitic intrusions. The studied albitites occur as lenticular bodies on the meter scale, hosted in peraluminous granites, and are scattered in a narrow sector next to the peraluminous and highly fractionated I-type La Pedriza leucogranite (LPL) pluton in the Spanish Central System. Albitites show sharp contacts and preserve textural features of the host granites (i.e., isovolumetric metasomatism). These rocks consist of a variably albitized magmatic feldspar framework (70–85 modal %) and interstitial domains where singular Na-Li-Fe3+-rich amphiboles and pyroxenes crystallized with albite or quartz. Titanite, andradite-grossular garnet, Mg-Li-rich mica, magnetite, fluorite, and apatite are metasomatic accessory minerals. Mass balance indicates enrichments of Na2O, FeO, MgO, MnO, Li, F, Be, and some HFSE (high-field-strength elements, such as Zn, Sc, and Sn) and LILE (large-ion lithophile elements) depletion. U-Pb data from metasomatic titanite yield 307 ± 10 Ma, which is coeval with the LPL and within the range of the regional granite emplacement (about 309–298 Ma). The LPL also shows strong compositional affinities (F, Li, Na, Sn, and Zn enrichment) with albitites. Similar Sr-Nd-δD isotopic signatures between albitites and host granites suggest that magmatic fluids might be dominant during metasomatism. However, the heterogeneity of δ18O, which shifts toward lower values in the more distant outcrops, suggests contribution of meteoric in origin low-salinity waters during prograde metasomatism. P-T-fO2 conditions are estimated at 0.2–0.3 GPa, 645–700 °C, and relatively oxidized conditions. We propose that Na-HFSE-flux-rich residual melts of the LPL expelled two immiscible hot liquids: an alkaline low-density liquid and a high-density hydrosilicate liquid (HSL). The low-density fluid dissolved magmatic quartz and provided albitization of the magmatic feldspar framework by a diffusive mechanism. Quartz dissolution created microchannels for infiltration of the relatively low viscosity HSL, forming tubular channels. This fluid was enriched in network modifiers (Li, Na, F, Be) and cations (Ca, Al) released after albitization, and it precipitated in these microchannels to form the new metasomatic paragenesis (quartz, albite, uncommon Li-rich mafic silicates, and accessory minerals).

Defining early stages of IOCG systems: evidence from iron oxides in the outer shell of the Olympic Dam deposit, South Australia

The IOCG deposit at Olympic Dam (South Australia) is hosted within the Roxby Downs Granite, which displays a weakly mineralised contact to the orebody (hereafter ‘outer shell’). In a mineralogical-geochemical characterisation of Fe-oxides from the outer shell, we show silician magnetite (Si-magnetite) and HFSE-bearing hematite define the early stages of alkali-calcic alteration. This association forms in the presence of hydrothermal K-feldspar and calc-silicates via overprinting of magmatic magnetite and ilmenite breakdown. Geochemical modelling, at ≥ 400 °C, shows such reactions occur at pH-fO2 conditions coinciding with shifts from K-feldspar to sericite, and ilmenite to rutile stability. The subsequent Si-magnetite+siderite association forms down-T in the absence of K-feldspar. Transition from granular to bladed morphologies in Si-magnetite is part of a series of Fe-oxide interconversions, followed by formation of zoned, U-W-Sn-Mo-bearing hematite. Enrichment in REE, Y and U in Si-magnetite and the prevalence of U-W-Sn-Mo-bearing hematite support a granite-derived fluid. Combined, petrographic and geochemical evidence show a transition among Fe-oxides from the outer shell to the orebody attributable to the evolution of the same fluid. Unusual massive magnetite intervals and Fe-oxide nodules in granite are considered due to either the presence of inherited lithologies, metasomatic products, or the result of magnetite-rich, crystal mush forming in the melt. We propose a model, corroborated by recently published data including high-precision U-Pb dating of magmatic zircon and hydrothermal hematite, in which an ‘outer shell’ is initiated at the 6–8 km depth of granite emplacement during volatile release from fluids ponding at intrusion margins. Granite cupola collapse at shallower levels (2–3 km?) follows via uplift along faults, facilitating intense brecciation and ore formation.

Confirmation of the Major Grenville Event in the Qinling Complex of the Qinling Orogenic Belt, Central China

The Qinling Complex from the Qinling orogenic belt was generally considered to be part of the Caledonian orogeny, however information of the Grenville event of the Qinling Complex has been poorly recognized. Two granite samples of greenschist-facies and two paragneiss samples of amphibolite- faices are identified from the Qinling Complex. The granites occur along the regional gneissosity of the Qinling Complex, thus it is suggested that their magmatic zircon ages (~970 Ma) mark the lower boundary of the main metamorphic age (amphibolite-granulite facies). In addition, some early-formed metamorphic zircons (~1 000 Ma) are distinguished in the granites, which may reflect the information about the source area of the granites. Therefore the major metamorphism of amphibolite-granulite facies in the Qinling Complex is constrained at Early Neoproterozoic (~1 000 Ma), not Early Paleozoic as conventionally considered. In the Early Paleozoic, the Qinling Complex was characterized by multiple extension- shear activities, overprint of greenschist-facies metamorphism and emplacement of extensive granites. These granites and related thermal events could reset the U-Pb isotopic system of the early-formed zircons, leading to the apparent zircon ages younger than their protolith age. As a result, the Qinling Complex is a modified Early Neoproterozoic orognenic belt or an independent block, which became a continental margin arc during the Early Paleozoic, being accompanied by metamorphism, deformation, and emplacement of continental arc granites. The Erlangping, Kuanping, and Taowan groups to the north of the Qinling Complex show more intensive deformation of the Caledonian, and their oblique subduction towards the Qinling Complex caused the formation of eclogites. Afterwards, the Qinling Complex was amalgamated with the Erlangping, Kuanping, and Taowan groups, which all experienced the same Caledonian orogeny, and possible later orogenies.

Characterizing the permeability of drillhole core samples of Toki granite, Central Japan to identify factors influencing rock-matrix permeability

Rock matrix permeability is mainly controlled by microcracks and is important for long-term solute transport, but may be highly heterogeneous. This study aims to identify the factors influencing the permeability of granite matrix by analyzing: physical properties, crack distribution, mineral composition, and the concentration of main elements in 48 core samples taken from three deep drillholes in the Mizunami area, central Japan. All drillholes intersect the Tsukiyoshi fault at depth. Permeability of core samples, measured by a gas permeameter, largely increases in the fault and fracture zones, which may be caused by the development of cracks as the total length of cracks extracted from thin-sections is correlated with measured permeability. Although a significant correlation is identified between permeability and P-wave velocity, a representative elastic property of rock, this correlation is enhanced by classifying the samples into two groups by the Mn/Fe concentration ratio. This ratio is identified as a key factor to separate lithofacies differentiated by the source magma. Thus, lithofacies is another control factor for permeability due to the difference in mineral composition. Moreover, permeability shows significant negative and positive correlations with Si and Ca concentrations, respectively. These concentrations are probably affected by dissolution of silicate minerals and calcite generation in the hydrothermal alteration process, which can develop both pore space and crack network and consequently, increases rock permeability in addition to crack generation and development. Therefore, a combination of hydrothermal alteration before the emplacement of granite and strong faulting are the predominant processes for controlling permeability.