Greenstone Terrane Research Articles

Geologic interpretation of aeromagnetic and chemical data from the Oaks Belt, Wabigoon subprovince, Minnesota: implications for Au-rich VMS deposit exploration

The Oaks Belt (OB) is a Neoarchean volcanic complex located in northwestern Minnesota, USA. It is part of the Wabigoon granite–greenstone terrane that hosts the world-class Rainy River gold deposit in nearby Ontario, Canada. Rocks in the OB form a north-dipping homocline in the fault-bounded pressure shadow of a sigma-shaped volcano-plutonic wedge that spans east–west for 220 km across the Minnesota, USA – Ontario, Canada border. Exploration drilling in the area delineated pyrrhotite–pyrite massive sulfide deposits, iron formation, chert, and semi-massive sphalerite mineralized zones. High-resolution aeromagnetic data indicate a large (∼60 km2) composite subvolcanic intrusion underlies these iron-rich strata in the OB. The position of this inferred intrusion elucidates the low base metal content of known massive sulfide deposits, as they were too far away (6–10 km) from a heat source to have been favorable sites for base metal deposition. The relative abundance of Au and Zn in the OB, alongside correlation coefficients between metals in massive sulfide deposits, iron formation, and chert, indicates the rocks were affected by a low-temperature hydrothermal system under relatively shallow water conditions (<1000 m). Negative correlation between Na2O and CaO in basalt, and their mutual moderate positive correlation with immobile corundum (Al2O3), implies alteration in the upper part of the volcanic pile did not result in substantial element mobility in most samples. Geochemical data from mafic and felsic volcanic rocks plot mainly in the calc-alkaline field. Thus, the OB is most prospective for hosting Au-rich VMS deposits and future exploration should focus on paleo-thermal corridors and favorable stratigraphic horizons near the newly inferred composite subvolcanic intrusion.

Igneous Rock Associations 19. Greenstone Belts and Granite−Greenstone Terranes: Constraints on the Nature of the Archean World

Igneous Rock Associations 19. Greenstone Belts and Granite−Greenstone Terranes: Constraints on the Nature of the Archean World

Petrogenesis of basalt–high-Mg andesite–adakite in the Neoarchean Veligallu greenstone terrane: Geochemical evidence for a rifted back-arc crust in the eastern Dharwar craton, India

The ∼2.7Ga Veligallu greenstone terrane in the eastern Dharwar craton is one of the least studied Neoarchean greenstone belts in India. To our knowledge, there is no published dataset that provides a detailed account of the petrogenetic processes, which culminated into the evolution of this greenstone belt. This paper presents a comprehensive account of field, petrography and geochemistry of the basalt–high-Mg andesite–adakitic volcanic rock association in the Veligallu greenstone belt.The basalts are tholeiitic in nature and are characterized by primitive geochemical compositions with ∼49wt% SiO2, ∼9wt% MgO, Mg# 54, Cr∼197ppm, Ni∼122ppm, Y∼27ppm, Yb∼2.7ppm; Zr/Y<3 and Ce/Yb<5. They do not plot on a mixing line between Mesoarchean TTG and N-MORB, therefore, inconsistent with any interaction with the upper continental crust. Accordingly, they are interpreted to have erupted in an oceanic setting. On the basis of Nb/Th ratio the Veligallu basalts are divided onto two groups. The basalts with Nb/Th>8 display systematically flat to light-REE depleted chondrite normalized REE patterns (LaN/YbN=0.76–1.0), in contrast to the samples with Nb/Th<8, which display slight enrichments in light-REE (LaN/YbN=1.0–1.6). The depleted basalts are characterized by primitive mantle normalized Nb concentrations lower than La but equal to Th, in contrast to the enriched basalts, which are characterized by slight depletions of Nb relative to both La and Th. Collectively, as a group, they display Nb/Yb ratio higher than the N-MORB. The enrichment in Nb is attributed to moderate to low degree (∼10%–4%) partial melting of a depleted MORB source. The trace, REE and their interelement ratios in the Veligallu basalts exhibit transitional characteristics between the average N-MORB and the typical Phanerozoic island arc basalts. Overall, the geochemical characteristics of the Veligallu basalts are identical to basalts erupted in the Phanerozoic back-arc basins.The Veligallu high-Mg andesites are orthopyroxene-bearing calc-alkaline andesites and are characterized by moderate SiO2 (∼56wt%), high MgO (∼8wt%), Mg# (61–67), Cr (174–330ppm) and Ni (173–242ppm) contents; low Y (∼13ppm) and Yb (∼1.2ppm) concentrations, relative to typical island arc andesites. They exhibit light-REE enriched (LaN/SmN∼1.7) and heavy-REE depleted (GdN/YbN∼1.9) fractionated REE patterns. On a primitive mantle normalized trace element variation diagram they exhibit negative Nb and Ti, and contrasting positive Zr anomalies relative to the neighboring REE. The La–Yb systematics suggest that these high-Mg andesites were generated by hydrous partial melting of a slab-melt metasomatized mantle wedge in the sub-arc mantle. The felsic volcanic rocks are characterized by high SiO2∼73wt%, Na2O∼4.5wt%, and significantly low concentrations of MgO (∼0.72wt%), Cr∼19ppm, Ni∼23ppm, Y∼5.9, and Yb∼0.44ppm; strongly fractionated chondrite normalized REE patterns (LaN/YbN∼30) with occasionally minor positive Eu anomaly, and primitive mantle normalized negative Nb and Ti, and contrasting positive Zr anomaly. The geochemical attributes of the Veligallu felsic volcanic rocks are analogous to the Cenozoic high silica-type adakites that are produced by partial melting in the sub-arc mantle, and presumably without any significant interaction of the slab-melts with the overlying mantle wedge.The juxtaposed occurrence of basalts with the banded iron formations, and their close spatial association with the high-Mg andesites, in the Veligallu greenstone belt, indicates that the petrogenesis of these volcanic rocks is consistent with their eruption in an intraoceanic setting. The close proximal association of the back-arc type basalts, and the high-Mg andesites and adakites within the Veligallu greenstone belt potentially suggests that this greenstone belt represents a rifted back-arc crust. This event appears to have significantly contributed to the peak of crustal growth activity in the eastern Dharwar craton at 2.7Ga.

Archaean granulite-facies paragneisses from central Swaziland: inferences on Palaeoarchaean crustal reworking and a complex metamorphic history

We present a petrographic, petrological, geochemical, and geochronological study (U–Pb/Lu–Hf) on granulite-facies paragneisses of the Mahamba Gneiss Complex in central Swaziland, eastern Kaapvaal Craton. Our data suggest that prograde metamorphism occurred at c . 3.07 Ga. Dating of detrital zircons of a metapelite in combination with geochronological and ambiguous structural relationships with granitoid gneisses suggests two possible scenarios: (1) the time of deposition of the sedimentary protoliths is prior to c . 3.58 Ga; (2) c . 3.58 Ga granitoid crust was the basement for the sedimentary protoliths. Furthermore, enrichment in Ni and Cr in the Mahamba metasediments may originate from erosion of a greenstone terrane similar in composition to the dominantly mafic and ultramafic lithologies of the Barberton Greenstone Belt or Dwalile supracrustal belt. Evidence for an older crustal basement is provided by the oldest detrital zircons, which yield ages up to c . 3.7 Ga. Because the metamorphic conditions of metapelitic rocks have no similarity to P–T paths of modern subduction–collision tectonic settings we conclude that the metamorphic history of the Mahamba Gneiss Complex cannot be explained by this model. We propose that deformation and metamorphism occurred through intracratonic contraction and a thermal event possibly related to Mpuluzi batholith emplacement. Supplementary material: Sample localities, mineral chemistry, analytical methods, and mineral equilibria modelling method are available at www.geolsoc.org.uk/SUP18786 .

Role of fluids in the exhumation of the Southern Marginal Zone of the Limpopo Complex, South Africa

The Limpopo Complex (LC) of southern Africa displays structural, metamorphic, and geochronological evidence for a complex exhumation-controlled evolutionary history that followed after burial and attainment of peak metamorphic conditions (P>9kbar, T>900°C) in the root zone (Central Zone, CZ) of the orogen before ∼2.72Ga. The Southern Marginal Zone (SMZ) of the LC, in addition, provides evidence for thrust-controlled emplacement of a hot granulite facies nappe onto the adjacent low-grade granite greenstone terrane of the northern Kaapvaal Craton (NKVC) in the interval ∼2.69–2.62Ga. A tectono-metamorphic scenario in which initial near vertical channeling (exhumation) at ∼2.69Ga was transferred into horizontal channeling (thrusting) at ∼2.67–2.64Ga is interpreted to have commenced with in situ partial melting of crustal rocks in the root zone of the orogen (CZ). This resulted in formation of melt-weakened crust (migmatized gneisses) that with continuous shortening led to vertical transfer of lower crustal material, accompanied by fluid-fluxed decompression melting that resulted in production of large volumes of leucocratic granitic material. This material detached to form buoyant diapirs that accompanied the emplacement at ∼2.65Ga of mega sheath folds at the middle crustal level throughout the entire CZ of the LC. This early stage of near-vertical exhumation is reflected (i) by the fact that linear elements of sheath folds and shear zones plunge consistently at steep angles to the SW throughout the entire CZ, and (ii) by decompression-cooling (DC) segments of P–T paths constructed for metapelitic rocks. DC paths in the far northern part of the SMZ that are associated with the steep north-dipping portion of the Hout River Shear Zone (HRSZ) traversed from P∼8.5kbar, T∼850°C to P∼6.5kbar, T=720°C. Early near-vertical exhumation transferred into horizontal channel flow that controlled emplacement at the middle crustal level of the granulite facies nappe (SMZ). Horizontal channeling (thrusting) is recorded in metapelitic granulite by near-isobaric cooling (P∼6kbar, T∼700–550°C) segments of earlier DC P–T paths that are associated with the shallow north-dipping portion of the HRSZ. Devolitilization of underthrusted greenschist due to thrusting hot granulite over cool greenschist released large volumes of high temperature aqueous fluids from the underthrusted rocks that constantly infiltrated the overriding granulite during southwards migration of the nappe. Immiscible CO2-rich and brine fluids thus derived persistently infiltrated into and reacted with cooling granulite and triggered the main pulse of anatexis at P>7.5kbar, T>750°C. Similar fluids established a regional retrograde isograd and associated zone of 4500km2 retrograde hydrated granulite at P∼6kbar, T=600–630°C located in the hanging wall of the shallow north-dipping HRSZ that bounds the SMZ in the south. Shear zone-hosted metasomatic alteration including formation of lode-gold deposits occurred at T=600–630°C, P∼6kbar in the zone of rehydrated granulite, and at higher P–T conditions in the granulite zone. The granulite facies SMZ of the LC thus preserves clear evidence for a direct link between persistent fluid flux and fluid-rock interaction as an integral component of a conveyor-like tectonic mechanism driven by vertical tectonics that operated from the core region of the orogen (CZ). This mechanism explains the important link between the thrust-controlled emplacement of the SMZ onto the NKVC and deep crustal fluid flow.

Arc‐nascent back‐arc signature in metabasalts from the Neoarchaean Jonnagiri greenstone terrane, Eastern Dharwar Craton, India

The Neoarchaean Jonnagiri greenstone terrane (JGT) is located at the centre of the arcuate Hutti–Jonnagiri–Kadiri–Kolar composite greenstone belt in the eastern Dharwar Craton. High MgO (MgO = ~14 wt.%; Nb = 0.2 ppm), low Nb (LNB) (MgO = 7.8–12 wt.%; Nb = 0.1–5.1 ppm) and high Nb basalts (HNB) (MgO = 5.6–10.1 wt.%; Nb = 9.0–10.6 ppm) metamorphosed to lower amphibolite facies are identified based on their geochemical compositions. These metabasalts exhibit depleted HFSE (Nb–Ta, Zr–Hf), pronounced LREE and LILE enrichments suggesting contribution from subduction‐related components during their genesis. Th and U enrichment over Nb–Ta indicates influx of fluids dehydrated from subducted oceanic lithosphere. The high MgO basalts with higher Mg# (51) than that of the associated LNB and HNB (Mg# = 34–47) represent early fractionated melts of subduction‐modified mantle peridotite. The LNB were produced by partial melting of mantle wedge metasomatized by slab‐dehydrated fluids, whereas the HNB represents melts of subducted oceanic crust and hybridized mantle wedge. Lower Dy/Yb and variable La/Yb ratios suggest their generation at shallower depth within spinel peridotite stability field. The low Ce–Yb trend of these metabasalts reflects intraoceanic type subduction which straddles the fields of arc and back‐arc basin basalts, resembling the Mariana‐type arc basalts. The Jonnagiri metabasalts were derived in a paired arc‐back‐arc setting marked by nascent back‐arc rift system that developed in the proximity of an intraoceanic arc. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Neoarchaean felsic volcanic rocks from the Shimoga greenstone belt, Dharwar Craton, India: Geochemical fingerprints of crustal growth at an active continental margin

The felsic volcanic rocks of Neoarchaean Shimoga greenstone terrane of western Dharwar Craton in India are dominantly represented by rhyolites occurring at stratigraphically upper horizons. The Shimoga rhyolites are associated with conglomerates, quartzites, argillites, limestones, cherts, basalts and intermediate volcanic rocks clearly suggesting an accretionary package. The rhyolites of Daginkatte and Shikaripura areas are potassic, with porphyritic alkali feldspar and quartz as essential minerals and chlorite, biotite and opaques as accessory phases. Geochemically, the rocks show enrichment in LILE and depletion in HFSE relative to primitive mantle values, with negative Nb-Ta, Zr-Hf anomalies and positive Th anomalies. These features of the Shimoga rhyolites compare well with the geochemical characteristics of magmas generated in subduction-related tectonic settings. Their alkaline compositions, intermediate to low HFSE abundances, moderate to high Zr/Y values (1.5–8.3), with La/Ybn (2–28), pronounced negative Eu anomalies, and variable LREE/HREE fractionation trends resemble the FI and FII rhyolites of Wabigoon and Uchi belts of Superior Province, Canada. The Shimoga rhyolites are interpreted to be products of melting of thick basaltic crust metamorphosed to amphibolite/eclogite grade, with garnet- and amphibole-bearing mantle residue. The rhyolites show prominent negative Eu and Ti anomalies, moderate to strong LREE fractionation, flat to mildly fractionated HREE patterns and are geochemically analogous to Type 1 and Type 3 rhyolites of Superior Province, Canada suggesting their derivation from intracrustal melting and fractional crystallization of basaltic liquids with prominent contribution from mantle wedge and slab components. Our data suggest the contribution of Neoarchaean active continental margin processes for the growth and evolution of continental crust in the western Dharwar Craton.

Structure, alteration, and geochemistry of the Charlotte quartz vein stockwork, Mt Charlotte gold mine, Kalgoorlie, Australia: time constraints, down-plunge zonation, and fluid source

The Kalgoorlie district in the Archean Yilgarn Craton, Western Australia, comprises two world-class gold deposits: Mt Charlotte (144 t Au produced to 2013) in the northwest and the Golden Mile (1,670 t Au) in the southeast. Both occur in a folded greenschist-facies gabbro sill adjacent to the Golden Mile Fault (D2) in propylitic alteration associated with porphyry dikes. At Mt Charlotte, a shear array of fault-fill veins within the Golden Mile Fault indicates sinistral strike-slip during Golden Mile-type pyrite–telluride mineralization. The pipe-shaped Charlotte quartz vein stockwork, mined in bulk more than 1 km down plunge, is separated in time by barren D3 thrusts from Golden Mile mineralization and alteration, and occurs between two dextral strike-slip faults (D4). Movement on these faults generated an organized network of extension and shear fractures opened during the subsequent infiltration of high-pressure H2S-rich fluid at 2,655 ± 13 Ma (U–Pb xenotime). Gold was deposited during wall rock sulphidation in overlapping vein selvages zoned from deep albite–pyrrhotite (3 g/t Au) to upper muscovite–pyrite assemblages (5 g/t Au bulk grade). Chlorite and fluid inclusion thermometry indicate that this kilometre-scale zonation is due to fluid cooling from 410–440 °C at the base to 350–360 °C at the top of the orebody, while the greenstone terrane remained at 250 °C ambient temperature and at 300 MPa lithostatic pressure. The opened fractures filled with barren quartz and scheelite during the retrograde stage (300 °C) of the hydrothermal event. During fracture sealing, fluid flux was periodically restricted at the lower D3 thrust. Cycles of high and low up-flow, represented by juvenile H2O–CO2 and evolved H2O–CO2–CH4 fluid, respectively, are recorded by the REE and Sr isotope compositions of scheelite oscillatory zones. The temperature gradient measured in the vein stockwork points to a hot (>600 °C) fluid source 2–4 km below the mine workings, and several kilometres above the base of the greenstone belt. Mass balance calculations involving bulk ore indicate enrichment of both felsic (K, Rb, Cs, Li, Ba, W) and mafic elements (Ca, Sr, Mg, Ni, V, Cr, Te), a source signature compatible with the local high-Mg porphyry suite but not with the meta-gabbro host rock. The initial 87Sr/86Sr ratios of the vein scheelites (0.7014–0.7016) are higher than the mantle ratio of the meta-gabbro (0.7009–0.7011) and overlap those of high-Mg monzodiorite intrusions (0.7016–0.7018) emplaced along the Golden Mile Fault at 2,662 ± 6 Ma to 2,658 ± 3 Ma.

Gold estimation in low grade deposits example from Penakacherla greenstone terrane

Gold estimation in low grade deposits example from Penakacherla greenstone terrane

Paleoproterozoic high δ13Ccarb marbles from the Ruwenzori Mountains, Uganda: Implications for the age of the Buganda Group

The Paleoproterozoic Buganda Group forms part of the arcuate Ruwenzori Fold Belt, in southern Uganda, separating the Archean granite–greenstone terranes of the Tanzanian craton from the Archean high-grade gneisses of the North Uganda terrane. It consists of a metasedimentary succession of mica schists, phyllites, quartzites, and rare marbles and calc-silicates, overlain by a thick succession of amphibolites and mafic metavolcanic rocks, including tholeiitic pillow lavas, that have a MORB-like composition. The volcano-sedimentary succession was once part of the ocean floor, and was later obducted onto the Tanzanian craton during the Ruwenzori orogeny at around 1.9Ga. The Ruwenzori Fold Belt and its rock protoliths are poorly dated, and the age of the volcano-sedimentary succession is only constrained between c. 2.13 and 1.99Ga. We report here on the stable isotope (C and O) geochemistry of marbles from the Ruwenzori Mountains in western Uganda, which has a bearing on the age of the Buganda Group.A suite of samples of impure dolomitic and calcitic marbles from the High Peaks Schist and the Kilembe Schist subgroups, Buganda Group, was analyzed. The marbles, which are metamorphosed to upper amphibolite grade, show a wide spread of δ13Ccarb values, ranging from +11.3 to −9.3‰ VPDB, and a range of δ18O values from −9.9 to −24.0‰ VPDB. There is a generally positive correlation between δ13C and δ18O values in the marbles as well as between δ13C and Mn/Sr ratios. Six of the 12 measured samples from the High Peaks Schist Subgroup have anomalously high δ13Ccarb values ranging between +11.3 and +10.0‰ VPDB.The Ruwenzori carbonate rocks are interpreted to have been deposited during the c. 2.22–2.06Ga Lomagundi positive carbon isotope excursion in seawater composition, and these rocks were subsequently affected by regional metamorphism during the c. 1.9Ga Ruwenzori orogeny, contributing to the wide range of δ13C and δ18O values found in this study. The Ruwenzori marbles, being interbedded in the lower part of the Stanley Volcanics Formation, also indicate an age for the volcanics between c. 2.22 and 2.06Ga, during the peak of the Lomagundi carbon isotope excursion. These age constraints are broadly consistent with a recently obtained age of c. 2.13Ga for a granulite in the basement for the Buganda Group.

Petrogenesis of mafic alkaline dikes from the ~ 2.18 Ga Mahbubnagar Large Igneous Province, Eastern Dharwar Craton, India: Geochemical evidence for uncontaminated intracontinental mantle derived magmatism

In this paper we present detailed field observations, petrology, and geochemical data, and discuss the petrogenetic aspects of the unusual high Mg-mafic dikes of alkaline to subalkaline nature from the widely known Paleo-proterozoic Mahbubnagar Large Igneous Province. These dikes traverse the granitoids of the peninsular Gneissic Complex and the 2.7Ga metabasalts in the adjacent Gadwal greenstone terrane. The study area is geologically embedded between the eastern margin of the Gadwal greenstone terrane and the northwest margin of the Proterozoic Cuddapah basin, in the eastern Dharwar craton of peninsular India. The dikes are fine to medium grained, massive, mesocratic to melanocratic in appearance, and range in dimension from a few hundred metres to over ten kilometres in strike length extending in the northwest to the southeast direction. These dikes are neither deformed nor metamorphosed, and they exhibit well-preserved igneous textures. They are predominantly composed of clinopyroxene (augite) and plagioclase (oligoclase–andesine) with subordinate biotite, amphibole, chlorite, and accessory titanite and ilmenite. They exhibit uniform geochemical compositions with ocean island basalt-like characteristics. Their compositions preclude any significant alteration, element mobility, or contamination by the upper continental crust during their ascent and emplacement. Therefore, their major and incompatible trace element characteristics reflect a primary source feature. Numerical calculations suggest a melt segregation temperature of ~1390°C at 2.2GPa corresponding to an estimated depth of ~71km. Geochemical modelling indicates low-percentage partial melting (~2%–6%) of a peridotitic mantle source in the garnet±spinel stability field. The regional tectonic framework and field observations corroborated by detailed geochemical studies favour an intracontinental setting for the emplacement of these high-Mg mafic dikes of alkaline affinity. Further, we propose that the Gadwal alkaline dikes evolved from a common parental magma as a consequence of the partial melting of a metasomatised subcontinental lithospheric mantle by adiabatic upwelling of an asthenospheric plume source coupled with lithospheric thinning and extension.

U–Pb dating constraints on the felsic and intermediate volcanic sequence of the nickel-sulphide bearing Cosmos succession, Agnew-Wiluna greenstone belt, Yilgarn Craton, Western Australia

The Cosmos succession, within the Agnew-Wiluna greenstone belt of the Yilgarn Craton, consists of ultramafic, intermediate and felsic volcanic lithologies, and contains several komatiite-hosted nickel sulphide ore bodies. The volcanic succession that forms the stratigraphic footwall to the mineralised komatiite lavas consists of an intercalated sequence of fragmental and coherent extrusive lithologies, ranging from basaltic-andesites through to rhyolites. It also contains cross cutting felsic intrusions. The stratigraphic hangingwall consists largely of reworked volcanic-derived sedimentary rocks that include polymictic conglomerates containing granitic clasts. SIMS U–Pb dating was undertaken on zircons from nine felsic lithologies spanning the established stratigraphy. The footwall succession gives emplacement ages ranging from 2736±4 to 2724±3Ma. The overlying Cosmos Ultramafic sequence consists of two units; the basal ultramafic UMu1 package and the upper ultramafic UMu2 package, which are separated by a discontinuous felsic volcanic horizon that sits approximately 30–60m above the basal contact of the Cosmos Ultramafic sequence. This discontinuous felsic horizon is dated at 2685±8Ma and represents a break in accumulation of komatiite lavas and may be associated with a major hiatus. The succession also features two felsic intrusions emplaced at 2653±7Ma and 2670±6Ma, where the latter cross-cuts the upper contact of the UMu2 package. These results bracket the age of UMu1 package between 2724±3Ma and 2685±8Ma and that of UMu2 package between 2685±8Ma and 2670±6Ma. This stratigraphic division of the Cosmos ultramafic is further strengthened by the contrasting styles of mineralisation above and below the discontinuous felsic horizon and associated unconformity; UMu1 hosts several massive nickel sulphide deposits situated at the contact with the footwall volcanic succession, while UMu2 is typified by high-tenor disseminated nickel sulphide deposits situated just above the discontinuous felsic horizon. Our data indicates that the Cosmos volcanic succession was accumulated via punctuated volcanism spanning ∼50Ma that was followed by intrusive activity lasting for at least a further 32Ma. The results also imply that construction of this relatively small, mineralised greenstone succession lasted longer than anticipated on stratigraphic considerations alone, an outcome that may have implications for the timing of emplacement of other bi-modal volcanic successions in similar greenstone terranes.

Geochemistry of two stratigraphically-related ultramafic (komatiite) layers from the Neoarchean Sigegudda greenstone terrane, Western Dharwar Craton, India: Evidence for compositional diversity in Archean mantle plumes

Two compositionally different ultramafic units are present in the Neoarchean Sigegudda greenstone terrane, Western Dharwar Craton, India. These ultramafic units occur in the same volcano-sedimentary sequence and are separated by a fault-bounded volcanic arc association. Because of deformation and amphibolite facies metamorphism, the primary igneous textures have been extensively modified in both ultramafic units. Given their spatial and temporal association with sedimentary and volcanic rocks and high MgO contents (12–29wt.%), these ultramafic rocks are interpreted as metamorphosed komatiites and komatiitic basalts. Field relationships and rock types indicate that the lower section of the Sigegudda volcano-sedimentary sequence was deposited in a peri-cratonic, continental rift setting. The lower ultramafic unit is characterized by 12.4–29.6wt.% MgO, Mg#=76–91, and 526–1150ppm Ni. The upper ultramafic unit is overall more magnesian and compositionally restricted: MgO=21.6–25.9wt.%, Mg#=86–89, and Ni=610–1000ppm. The lower unit features a combination of relatively elevated TiO2 (0.40–0.90wt.%), Al2O3/TiO2≤the chondritic ratio of 21, and (Gd/Yb)N ratios >1; these are Ti-enriched komatiites reported for the first time from greenstone belts in the Dharwar Craton. The upper unit is compositionally comparable to Al-undepleted komatiites and characterized by Al2O3/TiO2≥21, LREE-depletion coupled with near-flat chondrite-normalized HREE patterns. Geochemical data indicate that the Sigegudda komatiites and komatiitic basalts were variably contaminated by either continental crust or sub-continental lithospheric mantle, or both. The geochemical differences between the two units are explained by variable depths and degrees of partial melting. The Ti-enriched lower unit appears to have been formed through lower degrees of partial melting at depths ≥90km, whereas the Al-undepleted upper unit was generated by higher degrees of partial melting at depths ≤90km. Field relationships and geochemical characteristics of the Sigegudda komatiites can be explained by a geodynamic model in which the stratigraphically lower Ti-enriched komatiites were erupted from a mantle plume onto a rifting continental margin, whereas the stratigraphically upper Al-undepleted komatiites originated from a younger mantle plume and erupted onto a volcanic arc sequence that accreted to the rifted continental margin.

Zircon U[sbnd]Pb geochronology and Hf isotopes of major lithologies from the Yishui Terrane: Implications for the crustal evolution of the Eastern Block, North China Craton

As an important component of the Western Shandong Complex in the Eastern Block of the North China Craton, the Yishui Terrane consists of Neoarchean high-grade supracrustal rocks, granitoid gneisses and charnockites. LA-ICP-MS zircon UPb dating and Hf isotopic analyses on these lithologies have been carried out and the results provide new insights into the Neoarchean crustal evolution of the Eastern Block of the North China Craton. New zircon dating results reveal that the magmatic precursors of the supracrustal metapelites and granitoid gneisses were generated at 2.54–2.53Ga and 2.57–2.55Ga, respectively, and the charnockites were emplaced contemporaneously around 2.56–2.53Ga. Single metamorphic zircon grains and overgrowth rims from these rocks document consistent metamorphic ages at ~2.50Ga, suggesting that the Yishui Terrane experienced a high-grade metamorphic event at the end of the Neoarchean. Detrital zircons from pelitic gneisses yield 207Pb/206Pb ages of 2.89–2.65Ga, with a major age peak at ~2.53Ga and a subordinate age peak at ~2.70Ga, implying that the sedimentary protoliths of the pelitic gneisses may have been sourced from ~2.53Ga rocks in the Yishui Terrane and ~2.70Ga rocks in the adjacent Luxi Granite-Greenstone Terrane. Hf isotopic compositions show that the Neoarchean magmatic zircons have positive εHf(t) values ranging from +1.4 to +7.8 and depleted mantle model ages of 2.92–2.60Ga with a prominent peak at 2.8–2.7Ga, suggesting that the Neoarchean crust was derived mainly from juvenile sources and partly from the recycling of old continental crust in the Yishui area. Combined with previous data from the adjacent Luxi Granite–Greenstone Terrane and other complexes in the Eastern Block, it is concluded that the Neoarchean was an important period of crust accretion in the Eastern Block of the North China Craton.

Late Neoarchean potassic high Ba–Sr granites in the Taishan granite–greenstone terrane: Petrogenesis and implications for continental crustal evolution

A series of Late Neoarchean high Ba–Sr granites with a potassium-rich signature has been identified from the Taishan granite–greenstone terrane (TSGT), within the Eastern Block of the North China Craton (NCC). LA-ICP-MS zircon dating shows that the Wanghailou, Hujiazhuang and Xiajiazhuang granitic intrusions were emplaced at ~2.54Ga, ~2.52Ga and ~2.47Ga, respectively. Geochemically, they are alkali-rich, with high K2O contents and high K2O/Na2O ratios, revealing their high-potassium calc-alkaline nature. They are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to typical A-, I- and S-type granites, and show negative anomalies in HFSE (Nb, Ta, Ti) and weak Eu anomalies. Their geochemical and isotopic characteristics, such as extremely low Y and Yb concentrations, high Sr/Y and (La/Yb)cn ratios, strongly fractionated REE patterns and elevated εNd(t) and εHf(t), imply that the parental magmas were derived directly from partial melting of the sub-arc enriched mantle at depths >15kbar with residual garnet in the source. This mantle was previously depleted lithospheric mantle and was subsequently metasomatized by subduction-related fluids and/or melts. Fractional crystallization and crustal assimilation of these magmas during ascent resulted in their variable geochemical and isotopic characteristics. Taking into account the space–time patterns of late Archean magmatism in the area, an active subduction regime is favored to account for the generation of these potassic high Ba–Sr granites in the TSGT.The two-stage Nd and Hf model ages of these granites suggest that continental crust older than 2.80Ga is preserved beneath the TSGT. In combination with previous data from the region, it is noted that, apart from rapid crustal growth at ~2.7Ga, other additions of mantle-derived magma, such as minor 2.6–2.5Ga sanukitoids, siliceous high-Mg basalts (SHMBs) and high Ba–Sr granites, also made a significant contribution to continental crustal evolution in the TSGT.

Neoarchean siliceous high-Mg basalt (SHMB) from the Taishan granite–greenstone terrane, Eastern North China Craton: Petrogenesis and tectonic implications

Siliceous high-Mg basalt (SHMB) is a rare rock type that occurs mainly at or near the Archean–Proterozoic boundary. It shares some geochemical similarities with Phanerozoic boninites, but there is a clear distinction. Whether the petrogenesis of SHMB resembles that of Phanerozoic boninites or is related to the komatiitic magmatism is controversial. Neoarchean SHMBs are identified for the first time from the Taishan granite–greenstone terrane within the Eastern Block of the North China Craton (NCC). Zircon U–Pb dating indicates that they were emplaced at ∼2.54Ga, contemporaneous with the generation of sanukitoids and adakitic rocks, but later than the eruption of the Late Archean (2.71Ga) komatiites and komatiitic basalts in the area. The high MgO (>8%), high SiO2 (>52%), and Al2O3/TiO2 ratio (12.4–45.0), together with low TiO2 (<0.5%) and HFSE contents and strong enrichment in LREE and LILE in the Taishan SHMBs are comparable with typical Phanerozoic boninites, except for distinct HREE depletion, lack of U-shaped REE patterns and conspicuous positive Zr anomalies. In conjunction with their more depleted Nd isotopic characteristics (ɛNd(t=2.54Ga)=+4.42 to +1.05) relative to Late Archean komatiites in the region, it suggests that these SHMBs were derived from partial melting of refractory depleted mantle which experienced earlier basalt extraction and was subsequently enriched in LILE and LREE by subduction-related metasomatization, rather than the products of assimilation–fractional crystallization (AFC) of komatiitic magma. A slab-derived adakitic melt was likely the metasomatizing agent, along with minor aqueous fluids released from the subducting oceanic slab. In combination with regional studies, the generation of these magmas was probably related to slab rollback, which is ascribed to the arrival of an oceanic plateau and/or residual thickened lithospheric keel at the subduction zone at that time. This mechanism might have played a crucial role in the formation of Archean granite–greenstone belts and was an important factor in continental crustal growth, particularly during the Late Archean.

Mineral Potential Mapping for Orogenic Gold Deposits in the Rio Maria Granite Greenstone Terrane, Southeastern Para State, Brazil

The Rio Maria granite-greenstone terrain is characterized by extensive surficial cover and a lack of outcrop. Therefore, airborne geophysical measurements play a major role in mineral exploration in this region. A high-resolution airborne survey was used to build a prospectivity model for gold targeting employing a fuzzy logic technique. Within the Rio Maria granite-greenstone terrain, a total of 57 new potential orogenic gold targets were identified. The ability of this processing technique to identify favorable targets with potential for economic gold mineralization was verified by comparing the new predicted targets with known gold occurrences (e.g., Mamao Mine and Lagoa Seca deposits). Geographic Information System (GIS)-based automated processing methods employing fuzzy logic techniques were used to derive spatial models for generating orogenic gold exploration targets. Two metallogenic approaches were used. The first approach considers orogenic gold deposits hosted at the contact between mafic and felsic rocks. The second approach considers shear zone-hosted veins associated with mafic rocks and iron formations in the greenstone terrain. Detailed models were constructed for different blocks of the study area. A subset of these targets, i.e., Marcinho, Resende, and Votuporanga, were subsequently assessed using field evaluations that consisted of geological mapping and geochemical sampling. A follow-up drilling program is currently in progress and will be used to assess the main target areas where grid soil and rock sampling have indicated anomalous trends. The remaining predicted targets warrant further investigation. Data integration using GIS modeling and interpretation resulted in the following main conclusions with respect to the orogenic gold exploration potential of the Rio Maria Province: (1) as shown using GIS-based prospectivity analysis, there is considerable potential for orogenic gold deposits along the Andorinhas greenstone belt, and several prospective areas are associated with mafic and iron formation units; (2) several deposits within the Rio Maria and Identidade greenstone belt are associated with felsic units, as demonstrated by the fuzzy logic models; (3) many of the previously known targets that have been re-identified should be reevaluated to identify those the most promising targets for the discovery of a gold deposit; (4) the final prospectivity model shows that many of the most important gold deposits known are located within areas of high favorability, and several other new potential gold-bearing targets were selected; and (5) the proposed method identifies 57 geologically consistent targets and led to the discovery of the Marcinho deposit.

Geochemistry of a komatiitic, boninitic, and tholeiitic basalt association in the Mesoarchean Koolyanobbing greenstone belt, Southern Cross Domain, Yilgarn craton: Implications for mantle sources and geodynamic setting of banded iron formation

Komatiite–basalt magmatism and Algoma-type banded iron formation (BIF) of Archean greenstone terranes have common time series, but there are few constraints on the geodynamic setting in which BIF were precipitated. The ∼3.0Ga Koolyanobbing greenstone belt (KGB), Southern Cross Domain, Yilgarn craton, exhibits an greenschist to lower amphibolite facies metamorphosed lithostratigraphic series, including four predominantly mafic to ultramafic volcanic sequences (S1–S4) with prominent BIF horizons (and local basal pyrite-dominated massive sulfide lenses) in between. Petrography, XRD, high-precision major, trace high field strength, and REE element chemistry of representative ultramafic and mafic rocks from S1, S2, and S3 are consistent with a komatiitic, boninitic, and tholeiitic basalt association. Komatiites are Al-depleted and Al-undepleted (Al2O3/TiO2 12–28 and (Gd/Yb)N, ≥1) endorsing a zoned plume and/or melting over a range of depths. The boninite-series rocks (Al2O3/TiO2 28–45 and (Gd/Yb)N, <1) preserve characteristic U-shaped REE patterns and positive Zr–Hf/MREE anomalies. Komatiites include uncontaminated members and counterparts contaminated by metasomatized continental mantle lithosphere, suggesting eruption proximal to a rifted arc setting, whereas boninite-series were erupted at an intraoceanic arc, collectively consistent with plume eruption into a convergent margin. Al-depleted and Al-undepleted komatiites plot close together to the mantle array on the Th/Yb versus Nb/Yb discrimination plot of Pearce (2008), implying a depleted, mildly heterogeneous, source for the mantle plume from which komatiites erupted. Boninites lie parallel to and above the mantle array, in the field of intraoceanic arcs, in keeping with their inferred oceanic convergent margin setting. Ratios of Nb/Yb extend to lower and higher values than komatiites implying a more heterogeneous upper mantle source of the subarc wedge.The KGB exhibits the first reported boninite-suite rocks in the Southern Cross Domain, Yilgarn craton. Conclusively, the komatiitic, boninitic, and tholeiitic basalt association suggests that BIF and basal massive pyrite lenses of the Koolyanobbing greenstone belt were precipitated from low-temperature submarine discharge of a seawater-dominated hydrothermal system driven by magmatic systems of a mantle plume erupting in a forearc convergent margin suprasubduction ophiolite.

Geochemistry and tectonic setting of basalts from the Eastern Goldfields Superterrane

A database of more than 650 whole-rock analyses on basaltic rocks from the Eastern Goldfields Superterrane has been compiled from the literature and from public-domain datasets. The data fall into three distinct geochemical categories: a High-Th Siliceous Basalt group, a Low-Th Basalt group and an Intermediate-Th Basalt group. The Low-Th Basalt group shows SiO2 values between 50 and 53 wt%; Al2O3 around 15 wt%; elevated Cr and Ni; MgO mostly between 5.5 and 8 wt%; and flat REE–HFSE patterns with slight depletion in Th and minor positive and negative Nb anomalies. The High-Th Siliceous Basalt group has high SiO2, commonly >54 wt%; MgO between 6 and 9 wt% with higher values reflecting presence of accumulated olivine or pyroxene; low Fe and Ti compared with Low-Th Basalt group; depleted Ni for given Mg#; enriched LREE and Th, combined with strongly negative Nb anomalies and mantle-like Zr/Nb, Nb/Y, Al/Ti and HREE ratios. The Intermediate-Th Basalt group is intermediate between these two end-members in almost all respects. All three groups are represented across the entire Eastern Goldfields Superterrane. The most widespread group, the Low-Th Basalt, is remarkably homogeneous across terranes and domains. It is interpreted as the Archean analogue of plume-head related Large Igneous Province basalts, showing a close match to flood basalts associated with late-stage continental rifting. The High-Th Siliceous Basalt group displays a distinctive geochemical signature, which is evidently unique to Archean greenstone terranes. Derivation by contamination during fractionation of komatiites, probably deep in the crust, and followed by mid-crustal homogenisation in magma chambers, is the most likely hypothesis. Basalts with characteristic island arc signatures such as Nb depletion and low Ni and Cr contents, have not been recorded in the Eastern Goldfields Superterrane. This poses a significant challenge to uniformitarian models, which attempt to explain the evolution of the entire east Yilgarn craton in terms of modern arc accretion tectonics. Distribution of mafic and ultramafic mafic magmatism across the superterrane at ca 2700 Ma can be explained by emplacement of a major driving plume under the ‘lid’ of the Youanmi Craton. The keel of thickened, buoyant lithosphere under the archon diverted the plume head towards the craton margin, where it induced continental rifting. Voluminous eruption of plume-tail komatiite was concentrated and focused through this zone of rifting along the eastern margin of the Youanmi craton in the Kalgoorlie Terrane, while plume-head basalts and less voluminous komatiites were erupted over a much wider area.

Arc picrite–potassic adakitic–shoshonitic volcanic association of the Neoarchean Sigegudda greenstone terrane, western Dharwar craton: Transition from arc wedge to lithosphere melting

A rare association of basalt, andesite, dacite and ultrapotassic volcanic flows is present, together with basalts, komatiites, and banded iron formation (BIF), in the Neoarchean Sigegudda greenstone terrane of the western Dharwar craton. Andesites and dacites are porphyritic, preserving biotite, K-feldspar, and relict fine-grained leucite. Ultrapotassic flows preserve phenocrysts of relict leucite, relict pyroxene, and K-feldspar. Compositionally, basaltic samples are tholeiitic, andesites and dacites are calc-alkaline, and the ultrapotassic samples shoshonites to leucitites. High-Mg basaltic flows have Mg# 0.67–0.52, Ni 1126–260ppm, fractionated REE [(La/Sm)N=1.02–1.75, (Gd/Yb)N=1.41–1.74], and negative anomalies of Nb, Ta, and Ti on primitive mantle normalized diagrams; they are interpreted as arc picrites generated by slab dehydration and high-temperature, low degree, melts of subarc mantle wedge. Andesites qualify as high-Mg andesites. With dacites, these flows have some of the characteristics of adakitic rocks in being aluminous (Al2O3=14–24, 15–18wt.%), with fractionated REE [(La/Sm)N=1.90–2.56, 1.99–2.85, (Gd/Yb)N=1.60–2.64, 1.31–2.50 respectively], with flat to positive anomalies of Zr–Hf relative to MREE, but are potassic relative to “normal” arc andesites–dacites, and possess low abundances of Cr, Co, Ni relative to true adakites. Given the relict mineralogy, with LREE-enrichment the potassic character appears primary. These flows are interpreted as melts of lower arc crust, akin to potassic adakites documented from orogenic belts. Shoshonites and leucitites (SiO2=53–66, 48–53, K2O=3–8, 8–9wt.% respectively) are aluminous at 15–33wt.% Al2O3, with extreme enrichment of LREE at La 200–1000 times chondrite, fractionated REE [La/Sm)N=2.01–3.06, 1.32–3.94, (Gd/Yb)N=2.01–3.65, 1.33–4.50, respectively], and pronounced negative troughs at Nb–Ta–Ti: these compositions are consistent with decompressional melting of arc mantle lithosphere, previously metasomatized by subduction-derived fluids, during arc rifting. Accordingly, this volcanic association records a transition from subarc wedge melting to melts of arc crust and mantle lithosphere, likely in response to arc rifting.