Mafic Zone Research Articles

Origin of calcite by magma mixing in mingled rocks of the Ghansura Rhyolite Dome, Bathani volcano-sedimentary sequence, eastern India

Abstract The Ghansura Rhyolite Dome (GRD) is an integral part of the Bathani volcano-sedimentary sequence (BVSs), which in turn is a vital component of the Chotanagpur Granite Gneiss Complex (CGGC), eastern India. The rhyolite dome represents a shallow-level felsic magma chamber that underwent intrusion by basaltic magma during its evolution. The rocks present in the felsic dome such as basalts, rhyolites, and intermediate hybrid rocks preserve evidence of magma mixing and mingling. The objective of this contribution is to take into account, for the first time, that magma mixing processes can play an important role in the generation of calcite in magmatic rocks. The present study focuses on the mingled rocks exposed within the rhyolite dome. Previous reports have documented the existence of two distinct zones (mafic and felsic) interfacing each other in the mingled rocks of the rhyolite dome. The mafic zones dominantly consist of mineral phases such as actinolite, hornblende, biotite, plagioclase, ilmenite, calcite, and titanite. The felsic zones comprise quartz, K-feldspar, plagioclase, and biotite. When mafic magma intruded into the felsic magma chamber, interaction between the mafic and felsic magmas caused diffusion of H, Al, and K ions from the felsic to the mafic endmember. Such diffusive activities resulted in the breakdown of augite, already crystallized in the mafic magma, to form newer minerals like actinolite, hornblende, biotite, calcite, and ilmenite. The presence of ilmenite in the mingled rocks indicates the prevalence of reducing conditions during magma interaction and evolution. Breakdown of hornblende to biotite in such a reduced environmental condition led to the formation of calcite in the mingled rocks. From the results presented in this work, we are proposing that magma mixing can be a viable mechanism to form calcite in igneous rocks by magmatic processes.

Zircon U-Pb isotopic and geochemical study of metanorites from the chromite-mineralised Bacuri Mafic-Ultramafic Complex: Insights of a Paleoarchean crust in the Amapá Block, Guyana Shield, Brazil

The age and tectonic history of the chromite-mineralised Bacuri Complex, a layered mafic–ultramafic intrusion emplaced into Archean terranes of the Guyana Shield in the Amazonian Craton, are here investigated. The stratigraphy of the Bacuri Complex consists of an Ultramafic Zone composed of interlayered metaperidotites and chromitites and two mafic zones composed of metamafic rocks (amphibolites). In this study we present whole-rock lithogeochemical data integrated with zircon U-Pb isotopic and trace-element data from amphibolites from the Mafic Zone. The investigated amphibolites consist of hornblende + plagioclase ± diopside ± biotite with whole-rock compositions like noritic rocks of Archean layered intrusions. These samples are representative of amphibolites with variable proportions of hornblende and plagioclase, interpreted as resulting from amphibolite facies metamorphism of interlayered norite and leuconorite. We suggest that progressive fractionation of a primitive parental magma led to the formation of plagioclase and orthopyroxene cumulates in the mafic zones. We found that zircons from three metanorite samples comprise euhedral crystals with oscillatory zoning (interpreted to preserve a magmatic crystallisation age), as well as variably recrystallised metamorphic zircons. Their U-Pb 207Pb/206Pb ages span over 682 million years, from 3628 Ma to 2942 Ma. Trace element data suggest that magmatic zircons precipitated from late-stage magmatic fluids at ∼ 3.34 Ga (3343 ± 3.5 Ma weighted average age, n = 48, MSWD = 1.3), which defines the Bacuri Complex as one the oldest and best-preserved layered intrusions known. We show that fluid-mediated coupled dissolution-precipitation recrystallisation during metamorphism altered the composition and ages of magmatic zircons. We could not accurately define the number and age of metamorphic events in the 2942–3379 Ma age interval of metamorphic or ‘disturbed’ zircon domains. Our results indicate that continental crust was already established in the Amazonian craton in the Paleoarchean and possibly in the Eoarchean and that it has been reworked since at least the Mesoarchean.

Some factors affecting granitic pluton topography: Pluton topography in the southern Lachlan Foldbelt

Where neotectonics are inactive, granite batholith topography is determined largely by the dissolution rates of the minerals of the component plutons. In southern New South Wales, Australia, plutons in the Kosciusko, Corryong, Berridale, and Murrumbidgee Batholiths allow an assessment of the extent to which faults, joints, grainsize and foliation may control a pluton's topography. Changes in mean elevation of zoned plutons closely follow the changes in mineralogy; more felsic zones invariably occur at higher elevations than more mafic zones. Faults clearly dictate major landscape features, such as linear valleys. Neither grainsize nor foliation show any appreciable effect on these landscapes. Joint spacing in these plutons has a positive correlation with the rock's grainsize, with an intrinsic value varying linearly as the grain size varies. Not all joints at the intrinsic spacing may be evident. In perennially wet valleys most joints become weathered and so visible, leading to joints mostly spaced at that intrinsic value. In quickly drying crests and ridges more incipient joints remain closed, leading to larger boulders and a wider average joint spacing. Thus the evident joint spacing in granites is determined by grainsize and landscape position.

The Origin and Evolution of Ore-Bearing Rocks in the Loypishnun Deposit (Monchetundra Massif, NE Fennoscandian Shield): Isotope Nd-Sr and REE Geochemical Data

The Monchetundra massif is located in the north-eastern Fennoscandian Shield and refers to Paleoproterozoic massifs of the East-Scandinavian Large Igneous Province. The general section of the massif comprises two parts, the lower norite-orthopyroxenite and the upper mafic zones. The lower zone is of great interest due to its associated industrial platinum group elements (PGE) mineralization. The structure and peculiar features of rocks in the lower zone were studied using a drill core from the borehole MT-70 in the south-eastern slope of the Monchetundra massif intersecting the ore zone 1 of the Loypishnun deposit (according to the CJSC Terskaya Mining Company data). A comparison of the barren and ore-bearing varieties of norites and pyroxenites in the Loypishnun deposit shows that the ore samples have the lowest negative εNd values, a relatively more differentiated distribution spectrum with the Light rare earth elements (LREE) dominating over the Heavy REE (HREE), Eu/Eu* ≥ 1, and a higher mean content of alkali and large-ion lithophile elements (Ba, Rb, and Cs). New geochemical data indicated an origin of magmas for rocks from a layered series in the Loypishnun deposit by a high degree of melting of a LREE-rich source with a low mean content of REE. Negative εNd values, low ISr values, and a marked negative Nb indicate that the crustal material affected the evolution of rocks in the lower zone of the massif more than in the upper zone. The formation of ore bodies in the Loypishnun deposit was governed by the crust-mantle interaction, magmatic differentiation, and association with the most differentiated varieties, and by further concentration of the ore at the late and post-magmatic stages in a highly permeable environment for fluids in the Monchetundra fault zone.

Parental magma, magmatic stratigraphy, and reef-type PGE enrichment of the 2.44-Ga mafic-ultramafic N\xe4r\xe4nk\xe4vaara layered intrusion, Northern Finland

About 20 mafic-ultramafic layered intrusions in the northern Fennoscandian shield were emplaced during a widespread magmatic event at 2.5–2.4 Ga. The intrusions host orthomagmatic Ni-Cu-PGE and Cr-V-Ti-Fe deposits. We update the magmatic stratigraphy of the 2.44-Ga Näränkävaara mafic-ultramafic body, northeastern Finland, on the basis of new drill core and outcrop observations. The Näränkävaara body consists of an extensive basal dunite (1700 m thick), and a layered series comprising a peridotitic–pyroxenitic ultramafic zone (600 m thick) and a gabbronoritic–dioritic mafic zone (700 m thick). Two reversals are found in the layered series. The composition of the layered series parental magma was approximated using a previously unidentified marginal series gabbronorite. The parental magma was siliceous high-Mg basalt with high MgO, Ni, and Cr, but also high SiO2 and Zr, which suggests primary magma contamination by felsic crust. Cu/Pd ratio below that of primitive mantle implies PGE-fertility. The structural position of the marginal series indicates that the thick basal dunite represents an older wallrock for the layered intrusion. A subeconomic reef-type PGE-enriched zone is found in the border zone between the ultramafic and mafic zones and has an average thickness of 25 m with 150–250 ppb of Pt + Pd + Au. Offset-type metal distribution and high sulfide tenor (50–300 ppm Pd) and R-factor (105) suggest reef formation by sulfide saturation induced by fractional crystallization. The reef-forming process was probably interrupted by influx of magma related to the first reversal. Metal ratios suggest that this replenishing magma was PGE-depleted before emplacement.

Geology and metamorphism of the neoproterozoic Mangabal Complex: An example of Ni[sbnd]Cu-PGE mineralized intrusion in the Goiás Magmatic Arc, central Brazil

Inserted in the context of the Goiás Magmatic Arc, the Mangabal Complex represents a partially metamorphosed mafic-ultramafic intrusion, which is part of a cluster of neoproterozoic layered bodies formed during the Brasiliano Orogeny in central Brazil. The Mangabal Complex is composed of two exposures, the northern and the southern members, which exhibit the similar igneous mineralogy, crystallization sequence and were affected by the same metamorphic conditions. The stratigraphy of the Mangabal Complex can be divided into three main zones: i. the Lower Mafic Zone (LMZ), composed of adcumulatic norite and, forming the basal portion of the southern member; it is composed of adcumulatic norite; ii. the Ultramafic Zone (UZ), characterized by dunite and harzburgite cumulates, and iii. the Upper Mafic Zone (UMZ), consisting predominantly of norite, with restricted interlayeres of feldspathic dunite-orthocumulate. Petrographic data reveal a crystallization sequence for the Mangabal Complex formed by Olivine + Cr-Spinel > Olivine + Orthopyroxene > Orthopyroxene + Plagioclase > Clinopyroxene. In the olivine cumulus, the forsterita (Fo) content (Fo 78–85) suggest a moderately primitive parental magma. The NiCu-PGE mineralization occurs as disseminated sulfides, located in the UMZ, UZ and LMZ cumulate rocks, or forming massive to net-textured bodies in the metamorphic rocks of the UMZ. The main sulphide assemblage is composed of pyrrhotite + pentlandite + chalcopyrite ± pyrite, with minor pyrite + millerite. Disseminated sulphides are present in igneous rocks as well as in the metamorphic rocks that still partially preserve their original primary texture. This fact, together with the occurrence of abundant olivine and chromium-bearing spinel cumulates, suggests a fertile magma for the Mangabal Complex. However, the highest S, Ni, Cu, and PGE contents correlate positively with and are related to amphibolite and calc-silicate rocks associated with shear zones, indicating that metamorphism and deformation processes were critical in promoting the remobilization of sulphides and their deposition along shear zones. The primary mineral assemblage was largely replaced due to high-pressure upper-amphibolite facies metamorphism. The paragenesis representing metamorphic peak conditions is composed of kyanite + orthoamphibole + hornblende, which reveals pressures of approximately 8.5 kbar and a temperature up to 750 °C. The metamorphic mineral assemblage, together with the proximity to the São Luis de Montes Belos shear zone, suggests that kyanite crystallization in the Mangabal Complex and similar layered intrusions worldwide might be linked to the development of major crustal-scale structures.

Thick chromitite of the Jacurici Complex (NE Craton São Francisco, Brazil): Cumulate chromite slurry in a conduit

The Jacurici Complex, located in the NE part of the São Francisco Craton, hosts the largest chromite deposit in Brazil. The mineralized intrusion is considered to be a single N-S elongated layered body, disrupted into many segments by subsequent deformation. The ore is hosted in a thick, massive layer. Two segments, Ipueira and Medrado, have been previously studied. We provide new geological information, and chromite composition results from the Monte Alegre Sul and Várzea do Macaco segments located farther north, and integrate these with previous results. The aim of this study is to determine and discuss the magma chamber process that could explain the formation of the thick chromitite layer. All segments exhibit similar stratigraphic successions with an ultramafic zone (250m thick) hosting a 5–8m thick main chromitite layer (MCL), and a mafic zone (40m thick). The chromite composition of the MCL, Mg-numbers (0.48–0.72) and Cr-numbers (0.59–0.68), is similar to chromites from layered intrusions and other thick chromitites. Previous work concluded that the parental magma of the mineralized intrusion was very primitive based on olivine composition (up to Fo93) and orthopyroxene composition (up to En94) from harzburgite samples, and that it originated from an old subcontinental lithospheric mantle. We estimate that the melt from which the massive chromitite layer crystallized was similar to a boninite, or low siliceous high-Mg basalt, with a higher Fe/Mg ratio. The petrologic evidence from the mafic-ultramafic rocks suggests that a high volume of magma flowed through the sill, which acted as a dynamic conduit. Crustal contamination has previously been considered as the trigger for the chromite crystallization, based on isotope studies, as the more radiogenic signatures correlate with an increase in the volumetric percentage of amphibole (up to 20%). The abundant inclusions of hydrous silicate phases in the chromites from the massive ore suggest that the magma was hydrated during chromite crystallization. Fluids may have played an important role in the chromite formation and/or accumulation. However, the trigger for chromite crystallization remains debatable. The anomalous thickness of the chromitite is a difficult feature to explain. We suggest a combined model where chromite crystallized along the margins of the magma conduit, producing a semi-consolidated chromite slurry that slumped through the conduit forming a thick chromitite layer in the magma chamber where layered ultramafic rocks were previously formed. Subsequently, the conduit was obstructed and the resident magma fractionated to produce a more evolved composition.

Chromitites from the Luanga Complex, Carajás, Brazil: Stratigraphic distribution and clues to processes leading to post-magmatic alteration

The Neoarchean (ca. 2.75Ga) Luanga Complex, located in the Carajás Mineral Province in Brazil, is a medium-size layered intrusion consisting, from base to top, of ultramafic cumulates (Ultramafic Zone), interlayered ultramafic and mafic cumulates (Transition Zone) and mafic cumulates (Mafic Zone). Chromitite layers in the Luanga Complex occur in the upper portion of interlayered harzburgite and orthopyroxenite of the Transition Zone and associated with the lowermost norites of the Mafic Zone. The stratigraphic interval that hosts chromitites (∼150meters thick) consists of several cyclic units interpreted as the result of successive influxes of primitive parental magma. The compositions of chromite in chromitites from the Transition Zone (Lower Group Chromitites) have distinctively higher Cr# (100Cr/(Cr+Al+Fe3+)) compared with chromite in chromitites from the Mafic Zone (Upper Group Chromitites). Chromitites hosted by noritic rocks are preceded by a thin layer of harzburgite located 15–20cm below each chromitite layer. Lower Cr# in chromitites hosted by noritic rocks are interpreted as the result of increased Al2O3 activity caused by new magma influxes. Electron microprobe analyses on line transverses through 35 chromite crystals indicate that they are rimmed and/or extensively zoned. The composition of chromite in chromitites changes abruptly in the outer rim, becoming enriched in Fe3+ and Fe2+ at the expense of Mg, Cr, Al, thus moving toward the magnetite apex on the spinel prism. This outer rim, characterized by higher reflectance, is probably related to the metamorphic replacement of the primary mineralogy of the Luanga Complex. Zoned chromite crystals indicate an extensive exchange between divalent (Mg, Fe2+) cations and minor to none exchange between trivalent cations (Cr3+, Al3+ and Fe3+). This Mg-Fe zoning is interpreted as the result of subsolidus exchange of Fe2+ and Mg between chromite and coexisting silicates during slow cooling of the intrusion. A remarkable feature of chromitites from Luanga Complex is the occurrence of abundant silicate inclusions within chromite crystals. These inclusions show an adjacent inner rim with higher Cr# and lower Mg# (100Mg/(Mg+Fe2+)) and Al# (100Al/(Cr+Al+Fe3+)). This compositional shift is possibly due to crystallization from a progressively more fractionated liquid trapped in the chromite crystal. Significant modification of primary cumulus composition of chromite, as indicated in our study for the Luanga Complex, is likely to be common in non-massive chromitites and the rule for disseminated chromites in mafic intrusions.

Magmatic structure and geochemistry of the Luanga Mafic–Ultramafic Complex: Further constraints for the PGE-mineralized magmatism in Carajás, Brazil

The Luanga Complex is part of the Serra Leste Magmatic Suite, a cluster of PGE-mineralized mafic–ultramafic intrusions located in the northeastern portion of the Carajás Mineral Province. The Luanga Complex is a medium-sized layered intrusion consisting of three main zones: i. the lower Ultramafic Zone comprising ultramafic adcumulates (peridotite), ii. the Transition Zone comprising interlayered ultramafic and mafic cumulates (harzburgite, orthopyroxenite and norite) and iii. the upper Mafic Zone comprising a monotonous sequence of mafic cumulates (norite) with minor orthopyroxenite layers. Several PGE-mineralized zones occur in the Transition Zone but the bulk of the PGE resources are hosted within a 10–50meter thick interval of disseminated sulfides at the contact of the Ultramafic and Transition Zones. The compositional range of cumulus olivine (Fo78.9–86.4) is comparable to those reported for layered intrusions originated from moderate primitive parental magmas. Mantle normalized alteration-resistant trace element patterns of noritic rocks are fractionated, as indicated by relative enrichment in LREE and Th, with negative Nb and Ta anomalies, suggesting assimilation of older continental crust. Ni contents in olivine in the Luanga Complex (up to 7500ppm) stand among the highest values reported in layered intrusions globally. The highest Ni contents in olivine in the Luanga Complex occur in distinctively PGE enriched (Pt+Pd>1ppm) intervals of the Transition Zone, in both sulfide-poor and sulfide bearing (1–3vol.%) rocks. The origin of the PGE- and Ni-rich parental magma of the Luanga Complex is discussed considering the upgrading of magmas through dissolution of previously formed Ni-rich sulfide melts. Our results suggest that high Ni contents in olivine and/or orthopyroxene provide an additional exploration tool for Ni–PGE deposits, particularly useful for target selection in large magmatic provinces.

The growth and contamination mechanism of the Cana Brava layered mafic-ultramafic complex: new field and geochemical evidences

The Cana Brava complex is the northernmost of three layered complexes outcropping in the Goias state (central Brasil). New field and geochemical evidences suggest that Cana Brava underwent hyper- to subsolidus deformation during its growth, acquiring a high-temperature foliation that is generally interpreted as the result of a granulite-facies metamorphic event. The increase along the stratigraphy of the incompatible elements abundances (LREE, Rb, Ba) and of the Sr isotopic composition, coupled with a decrease in eNd(790), indicate that the complex was contaminated by the embedded xenoliths from the Palmeiropolis Sequence. The geochemical data suggest that the contamination occurred along the entire magma column during the crystallization of the Upper Mafic Zone, with in situ variations determined by the abundance and composition of the xenoliths. These features of the Cana Brava complex point to an extremely similarity with the Lower Sequence of the most known Niquelândia intrusion (the central of the three complexes). This, together with the evidences that the two complexes have the same age (c.a. 790 Ma) and their thickness and units decrease northwards suggests that Cana Brava and Niquelândia are part of a single giant Brasilia body grown through several melt impulses.

The Ni-Cu-PGE mineralized Brejo Seco mafic-ultramafic layered intrusion, Riacho do Pontal Orogen: Onset of Tonian (ca. 900 Ma) continental rifting in Northeast Brazil

The Brejo Seco mafic-ultramafic Complex (BSC) occurs at the extreme northwest of the Riacho do Pontal Orogen Internal Zone, in the northern margin of the São Francisco Craton in Northeast Brazil. The stratigraphy of this medium size (3.5 km wide and 9 km long) layered intrusion consists of four main zones, from bottom to top: Lower Mafic Zone (LMZ; mainly troctolite), Ultramafic Zone (UZ; mainly dunite and minor troctolite); Transitional Mafic Zone (TMZ; mainly troctolite) and an Upper Mafic Zone (UMZ; gabbro and minor anorthosite, troctolite, and ilmenite magnetitite). Ni-Cu-PGE mineralization occurs at the contact of the UZ with the TMZ, consisting of an up to 50 m thick stratabound zone of disseminated magmatic sulfides. An Mg-tholeiitic affinity to the parental magma is indicated by the geochemical fractionation pattern, by the magmatic crystallization sequence and by the elevated Fo content in olivine. A SmNd isochron yielded an age of 903 ± 20 Ma, interpreted as the age of crystallization, with initial εNd = 0.8. Evidence of interaction of the BSC parental magma with sialic crust is given by the Rare Earth and trace element patterns, and by slightly negative and overall low values of εNd(900 Ma) in between −0.2 and +3.3. Contrary to early interpretations that it might constitute an ophiolite complex, based mainly on the geochemistry of the host rocks (Morro Branco metavolcanosedimentary complex), here we interpret the BSC as a typical layered mafic-ultramafic intrusion in continental crust, related to an extensional regime. The BSC is chrono-correlated to mafic dyke swarms, anorogenic granites and thick bimodal volcanics of similar age and tectonic setting in the São Francisco Craton and surrounding areas. Intrusion of the BSC was followed by continued lithospheric thinning, which led to the development of the Paulistana Complex continental rift volcanics around 888 Ma and ultimately to plate separation and the generation of new oceanic crust (Monte Orebe Complex) around 820–650 Ma ago. Thus, the BSC provides a benchmark for the onset of Tonian continental rifting in this area, and is an important marker for the processes of Rodinia breakup and dispersion recorded in South America.

Geology, petrology and geochronology of the Lago Grande layered complex: Evidence for a PGE-mineralized magmatic suite in the Carajás Mineral Province, Brazil

The Lago Grande and Luanga mafic-ultramafic complexes are part of a cluster of PGE-mineralized layered intrusions located in the Carajás Mineral Province (CMP) in the Amazonian Craton. The Lago Grande Complex is a NE-trending medium-size (12-km-long and average 1.7-km-wide) layered intrusion consisting mainly of mafic cumulate rocks (Mafic Zone) and minor ultramafic cumulates (Ultramafic Zone). Geological sections indicate that igneous layers are overturned, such that the Ultramafic Zone overly the Mafic Zone. The Ultramafic Zone, about 4 km long and 500 m wide, comprises an up to 250 m-thick sequence of interlayered harzburgite and orthopyroxenite at the base and orthopyroxenite at the top. The Mafic Zone consists of a monotonous sequence of gabbroic rocks with an estimated thickness of up to 1000 m in the central part. Primary igneous minerals of the Lago Grande Complex are partially replaced by metamorphic assemblages that indicate temperatures up to the amphibolite facies of metamorphism. This metamorphic alteration is heterogeneous and characterized by an extensive hydration that largely preserves primary textures and bulk chemical composition.The composition of the parental magma of the Lago Grande Complex has been inferred from the crystallization sequences of the intrusion and lithogeochemistry of cumulate rocks. The compositional range of cumulus Ol (Fo82.5–85.7) is consistent with a moderately primitive composition for the parental magma. Cumulus minerals in the layered rocks indicate that the sequence of crystallization in the Lago Grande Complex consists of Ol + Chr, Opx + Chr, Opx, Opx + Pl and Opx + Pl + Cpx. The early crystallization of Opx relative to Cpx suggests that the primary magma was silica saturated. Mantle-normalized alteration-resistant trace element profiles of gabbroic rocks are fractionated, as indicated by relative enrichment in LREE and Th, with pronounced negative Nb and Ta anomalies. Nd isotopic data obtained for both mafic and ultramafic lithotypes render Nd model ages between 2.94 and 3.56 Ga, with variably negative εNd (T = 2.72 Ga) values (−0.32 to −4.25). The crystallization sequence of the intrusion and the composition of cumulus minerals, together with lithogeochemical and Nd isotopic results, are consistent with an original mantle melt contaminated with older continental crust. The contamination of mafic magma with sialic crust is also consistent with intra-plate rifting models proposed in several studies of the CMP. Lithogeochemical and isotopic data from the Lago Grande Complex may also be interpreted as the result of melting an old lithospheric mantle, and alternative models should not be disregarded.PGE mineralizations occur in chromitites and associated with base metal sulfides in the Lago Grande Complex. Chromitite has the highest PGE content (up to 10 ppm) and is characterized by high Pt/Pd ratio (4.3). Mantle-normalized profile of chromitite is highly enriched in PPGE and similar to those from Middle Group (MG) and Upper Group (UG) chromitites from the Bushveld Complex. Platinum group minerals (PGM) occur mainly at the edge of chromite crystals in the Lago Grande chromitite, consisting of arsenides and sulfo-arsenides. Sulfide-bearing harzburgite samples of the Lago Grande complex have PGE content of up to 1 ppm and low Pt/Pd (0.2–0.3) ratios.The 2722 ± 53 Ma U–Pb zircon age determined in this study for the Lago Grande Complex overlaps with the crystallization age of the Luanga Complex. Previous interpretation that the Lago Grande and Luanga layered intrusions are part of a magmatic suite (i.e., Serra Leste Magmatic Suite) is now reinforced by similar fractionation sequences, comparable petrological evolution and overlapped U–Pb zircon ages. The occurrence of the same styles of PGE mineralization in the Lago Grande and Luanga complexes, together with remarkably similar chondrite-normalized PGE profiles and PGE minerals for chromitites of both complexes, support the concept that they belong to the same PGE-fertile magmatic suite.U–Pb isotope analyses of zircon crystals obtained on bright luminescent rims with typical recrystallization features reveal a cluster of concordant to slightly discordant dates pointing at 2553 ± 61 Ma. These are likely to correspond to a significant disturbance of the U–Pb system of the Lago Grande Complex during the ca. 2.55 Ga regional scale hydrothermal event associated with the origin of IOCG type deposits. Common Bi–Cl minerals associated with highly transformed PGE-mineralized zones also suggest hydrothermal alteration by the regional IOCG type system. This type of alteration identified in the Lago Grande mafic-ultramafic layered intrusions may provide the explanation for common Ni–Co anomalies identified in several Cu–Au deposits, as well as hydrothermal Au–Pd mineralizations in Carajás.

Prograde transformations of amphibolites into eclogites and eclogite-like rocks in the low-pressure field of the eclogite facies (by the example of the Belomorian Mobile Belt)

Plagioclase-bearing garnet-omphacite (Grt-Omp) eclogites and garnet-augite eclogite-like (Grt-Aug) schists from the amphibolite and gneiss beds of the Belomorian Mobile Belt have been studied. They are spread over a large area. In most of the studied objects, these rocks have preserved primary concordant relations with the host amphibolite and gneiss strata; they are not disturbed by late tectonic processes and are not genetically related to tectonic-melange zones. Their protoliths were amphibolite lenses in gneisses or large mafic zones composed of amphibolites. The Grt-Omp eclogites formed in the low-pressure field of the eclogite facies (P = 12.5-13.0 kbar, T = 600-630 °C), and the eclogite-like Grt-Aug rocks, at the boundary between the amphibolite and eclogite facies (P = 9.6-11.1 kbar, T = 630-700 °C), under the intense impact of metamorphic fluid on the amphibolites. The compositional evolution of the rock-forming minerals during the formation of Grt-Omp eclogites and eclogite-like Grt-Aug rocks followed the same scheme. The petrographic diversity of apoamphibolite rocks (Grt-Omp eclogites and Grt-Aug schists) might be due to the difference both in the bulk composition of the metabasic protolith and in the ratios of CaO and Na2O activities in the metamorphic fluid. The relatively low content of CaO leads to the formation of Grt-Omp paragenesis in eclogites. Higher CaO contents give rise to eclogite-like Grt-Aug rocks containing jadeite-poor clinopyroxene.

Generation of High-Ni Sulfide and Alloy Phases During Serpentinization of Dunite in the Dumont Sill, Quebec

The Dumont sill in the Abitibi region of Quebec is a zoned intrusive body which is differentiated into a lower ultramafic zone, composed of peridotite and dunite, overlain by a mafic zone, with clinopyroxenite, gabbro, and quartz gabbro subzones. Although the dunite contains several layers enriched in primary Ni sulfide minerals, which add considerable value to the deposit, 47% of the resource consists of awaruite- and heazlewoodite-bearing serpentinized dunites which formerly contained Ni only in silicate minerals. Serpentinization occurred isochemically with respect to major components, apart from the addition of H2O and minor loss of CaO. Serpentinization occurred under conditions of very low water-rock ratio, resulting in buffering of fluid to extremely low activities of silica and H2O and allowing the widespread replacement of primary pentlandite by awaruite and heazlewoodite due to accompanying reductions in f O2 and f S2. In layers containing primary accumulations of magmatic sulfide, serpentinization has resulted in the remobilization of nickel from olivine to enrich cumulus sulfides. In layers lacking primary sulfide accumulations, serpentinization was accompanied by the formation of awaruite by reduction of Ni originally hosted by olivine. Early stages of serpentinization were marked by the generation of large volumes of Fe-rich serpentine containing abundant ferric iron as well as lesser amounts of brucite rich in ferrous iron. Later stages of serpentinization caused the replacement of early iron-rich serpentine and brucite by more magnesian serpentine and brucite and the formation of abundant magnetite. Assemblages that are completely serpentinized contain lower nickel in silicates, higher nickel tenor sulfides, and more modal awaruite compared to those that are weakly and partially serpentinized, which are characterized by higher nickel in silicates and lower Ni tenor sulfides. Although awaruite first appears early in the serpentinization process, the highest modal abundances of economically extractable heazlewoodite, awaruite, and Ni-rich pentlandite are observed in rocks that have been completely transformed to the Mg serpentine facies. Regardless of the final opaque mineral host for Ni, the key to removal of Ni from silicate minerals and its sequestration in phases amenable to beneficiation is the early formation of awaruite; otherwise, Ni partitions readily into serpentine and remains inaccessible to mineral extraction. Exploration for similar deposits should be focused on dunite adcumulates formed from unfractionated komatiites, which have undergone complete serpentinization under conditions of diffusion-controlled low water-rock ratio, as evidenced by the absence of wholesale deformation or veining.

O corpo Várzea do Macaco e as mineralizações de cromo, níquel e cobre, Complexo Máfico-ultramáfico Jacurici, Cráton São Francisco, Bahia

The Jacurici Mafic-ultramafic Complex, located in the northeastern portion of the Sao Francisco craton, consists of several N-S oriented layered bodies that host the largest chromite deposit in Brazil. The Varzea do Macaco body is at the northern part and also host a Ni-Cu sulfide mineralization. This study describes this body and its mineralization and compare with the southern intrusions (Ipueira-Medrado) where a petrological evolution was previous established. The Varzea do Macaco is stratigraphically inverted and disrupted in five blocks, laterally dislocated by late faults. It is constituted by dunite, lherzolite, ol-webesterite, chromitite and gabbronorite with variable serpentinization. The sulfide ore (Po ± Pn ± Cpy) is concentrated close to the main thick chromitite layer and occurs as: primary magmatic with interstitial sulfides associated with olivine and pyroxene; and as a remobilized ore, with sulfides associated to metasomatic veinlets or lenses that crosscut the primary layering. The interval where magmatic ores occur is characterized by the presence of magmatic amphibole that possible favoured late metamorphism and metasomatism transformation, stronger in this interval. The remobilized sulfide is enriched in chalcopyrite showing an increase in the Cu/Ni ratio. Comparing to Ipueira-Medrado, the Varzea do Macaco is enriched in clinopyroxene, but it can be subdivided in the same Ultramafic and Mafic zones. Possibly, both bodies are part of a single intrusive system characterized by a primitive magma with high Mg and Ni contents. The chromite mineralization is considered to be triggered by crustal contamination. At Varzea do Macaco, sulfur saturation was reached.

Review of Brazilian chromite deposits associated with layered intrusions: geological and petrological constraints for the origin of stratiform chromitites

Chromitite layers in mafic-ultramafic layered intrusions represent a special case of cumulate rock where chromite is the sole liquidus (cumulus) mineral. Different mechanisms for the formation of chromitite layers have been postulated. In order to provide geological and petrological constraints for these models, chromitites and hosting mafic and ultramafic rocks from the Bacuri, Niquelândia and Ipueira-Medrado layered complexes in Brazil were investigated. The Bacuri Complex (2·2 Ga) is a large layered intrusion of the Amazon Craton. Chromitite layers of the Bacuri mafic-ultramafic complex are restricted to a 30–120 m-thick sequence of ultramafic cumulates overlying mafic cumulates. Most of the chromite is concentrated in a several metres-thick chromitite layer (the main chromitite) located at the base of the ultramafic cumulates. Cryptic variation of olivine with stratigraphic height (from Fo89 at the base to Fo76 at the top) is consistent with extensive fractionation of the ultramafic sequence. The stratigraphic position of the main chromitite supports a model for its origin associated with a major new influx of primitive parental magma. The cause of chromite saturation must therefore result from changes in magma composition resulting from mixing new influxes of primitive magma with a fractionated magma resident in the magma chamber. The Niquelândia Complex (0·8 Ga) is a large layered intrusion in central Brazil. The Niquelaândia Complex has a thick (up to 4 km) Ultramafic Zone consisting mainly of dunite and interlayered harzburgite, websterite and several centimetres-thick discontinuous chromitite layers. A stratigraphic interval hosting relatively thicker and more consistent layers of chromitite occurs in the intermediate portion of the Ultramafic Zone. Compositional variation of olivine composition through this interval indicates that the magma located below the interval where chromitites occur was more primitive than the magma located above them. This feature is consistent with a model in which chromitites were formed by new influx of primitive magma, and mixing with slightly more fractionated resident magma. It is however intriguing to note that chromitite seams in the Niquelaândia Complex are mainly hosted by dunite. Different from chromitites observed in other large layered intrusions (e.g. Bushveld, Great Dyke), chromitite seams in the Niquelaândia Complex do not appear at the base of cyclic units. In the Niquelaândia Complex, the crystallisation of clinopyroxene as a cumulus mineral occurs immediately after the crystallisation of orthopyroxene. Extensive Cr depletion through clinopyroxene crystallisation is suggested as an explanation for the absence of chromitites at the base of cyclic units in the Niquelaândia Complex. The Ipueira-Medrado intrusion (2·0 Ga) is a small sill (7 km-long, 0·5 km-wide) consisting mainly of a basal Ultramafic Zone (∼250 m-thick) and an upper Mafic Zone (∼40 m-thick). At the upper part of the Ultramafic Zone a thick (5–8 m) and continuous chromitite layer (MCL) occurs. Cryptic variations of olivine suggest that the lowered sequence located below the MCL crystallised in a dynamic magma chamber undergoing frequent replenishment with primitive magma. Chromite compositions are very similar and show no systematic fractionation trends over the 5–8 m of the MCL, indicating that a continuous supply of primitive parental magma occurred during the time span of chromite crystallisation. Cryptic variation data for olivine rule out a model for the origin of the MCL from mixing new influxes of primitive parental magma with a fractionated magma resident in the magma chamber. In fact, the most primitive compositions of olivine occur in dunite and harzburgite located immediately below the MCL. Lithogeochemical and isotope data suggest that chromite crystallisation in the MCL of the Ipueira-Medrado sill was triggered by changes in physical–chemical parameters associated with crustal contamination. We propose that an increase in oxygen fugacity resulting from assimilation of carbonate-rich host rocks was the triggering factor leading to crystallisation of chromite as a sole cumulus phase in the Ipueira-Medrado sill. The unusually thick chromitite layer of the Ipueira-Medrado sill is interpreted to result from a large volume of magma passing through a conduit hosted by carbonate-rich crustal rocks. The examples of chromite deposits studied indicate that chromitite layers are associated with changes in the melt composition at stratigraphic horizons of the magma chamber. However, the specific trigger responsible for appropriate phase changes leading to chromite crystallisation may be different for each deposit.

Re-Os and Sm-Nd Isotope and Trace Element Constraints on the Origin of the Chromite Deposit of the Ipueira-Medrado Sill, Bahia, Brazil

The chromite deposit of the Paleoproterozoic Ipueira–Medrado sill is hosted in a single, thick (5–8 m), massive layer, which sets severe constraints for the origin of chromitites. It is divided from bottom to top into: (1) a Marginal Zone (5–20 m); (2) an Ultramafic Zone (<250 m) consisting of dunite and harzburgite that host the chromitite layer, in which intercumulus amphibole is important and more abundant toward the top; (3) a Mafic Zone (<40 m). The parental magma was large ion lithophile element and light rare earth element enriched and high field strength element depleted. Sm–Nd isotopic compositions are consistent with a 2 Ga age, but suggest a variable initial Nd isotopic composition that correlates with the abundance of amphibole. The more negative εNd (mean –6·5) of the amphibole-rich intervals argues for crustal contamination, although the εNd (mean –4·4) of the amphibole-free samples suggests an old, enriched, subcontinental lithospheric mantle source. Chromite separates have initial γOs values that range from –4·6 to +3. The negative γOs values are typical of old, Re-depleted, lithospheric peridotitic mantle and give Re-depletion model ages of up to 2·75 Ga. An integrated assessment suggests that the very high-Mg parental magma probably originated from Archean, subcontinental, metasomatized, peridotitic lithospheric mantle and was subsequently contaminated with up to 30% of crust, which triggered the chromitite crystallization.

The Chromite Deposit of the Ipueira-Medrado Sill, Sao Francisco Craton, Bahia State, Brazil

The Ipueira-Medrado mafic-ultramafic sill hosts the largest chromite deposit in Brazil. The sill is a 7-km-long and 300-m-thick elongated body consisting mainly of interlayered dunite and harzburgite but also containing minor mafic rocks. The most distinctive feature of the Ipueira-Medrado sill is a continuous, 5- to 8-m-thick massive chromitite layer that is currently being mined. The anomalous thickness of this chromitite layer provides important constraints on current genetic models for the origin of massive chromitites. The sill is divided into three zones: Marginal, Ultramafic, and Mafic. The Marginal zone (5–20 m thick) consists of sheared gabbro and orthopyroxene-rich harzburgite. The Ultramafic zone (up to 250 m thick) is subdivided into the Lower Ultramafic unit, the Main Chromitite layer, and the Upper Ultramafic unit. The Lower Ultramafic unit consists of dunite with minor harzburgite and chain-textured chromitite, the Main Chromitite layer is composed of chain-textured and massive chromitites, and the Upper Ultramafic unit consists mainly of harzburgite but also minor chain-textured chromitite and dunite. Pyroxene abundance increases toward the top in this zone and grades to an orthopyroxenite at the top. The presence of magmatic intercumulus amphibole is significant (up to 20 vol %) in this unit. The Mafic zone (up to 40 m thick) consists of leuconorites to melanorites that are partially metamorphosed under amphibolite facies conditions. The variation of olivine and orthopyroxene compositions throughout the sill layers reveals two intervals of distinct magmatic evolution. The interval located below the Main Chromitite layer is characterized by gradual upward evolution of mineral compositions toward more Mg-rich compositions (Fo93 and En94). The interval located above the Main Chromitite layer is characterized by rapid evolution toward more Fe-rich compositions (Fo85 and En84) with increasing stratigraphic height. The interval below the Main Chromitite layer suggests crystallization in a dynamic magma chamber undergoing frequent replenishment with primitive magma. The interval above the Main Chromitite layer suggests crystallization in a mainly closed, fractionating magma chamber. Mass balance considerations suggest that an enormous amount of magma (>10,000 m thick) was associated with the formation of the Main Chromitite layer at the Ipueira-Medrado sill. Such a volume of magma is obviously not represented in the stratigraphy of the Ipueira-Medrado sill. Mineral chemistry data indicate that the composition of the resident magma was most primitive at the stratigraphic interval immediately below the Main Chromitite layer. We suggest that the sill acted as a conduit through which a large volume of magma flowed. Under these conditions, contamination of the parental magma with crustal material and fluids is a likely mechanism for explaining the origin of the chromitite. Abundant intercumulus amphibole in the Upper Ultramafic unit and high Fe3+ in chromites, intercumulus orthopyroxene in Main Chromitite layer chromitites, and Sm-Nd and Re-Os isotope data for silicate rocks and chromitites, respectively, indicate that the Ipueira-Medrado sill formed from magmas contaminated by crustal material. The host rocks of the Ipueira-Medrado sill consist mainly of high-grade gneiss and marbles that would have been appropriate source of fluids and SiO2 and could have caused the increase in f O2.

Petrophysical Properties of the Goonumbla Volcanic Complex, NSW: Implications for Magnetic and Gravity Signatures of Porphyry Cu-Au Mineralisation

Petrophysical data are important for constraining the geophysical signatures of the Endeavour Cu-Au deposits within the Goonumbla Volcanic Complex (GVC). Susceptibilities vary systematically with lithology and, particularly, with alteration. Remanence tends to be subordinate to induced magnetisation in the GVC. Densities are predictably related to composition, Alteration effects on densities are generally minor, although rocks with particularly strong development of secondary magnetite, hematite or sulphides have higher densities.Modelling of magnetic and gravity profiles over the Goonumbla volcanic belt and the GVC, constrained by geological information and petrophysical properties, suggests that the GVC is underlain by a large zoned intrusion, representing the parent magma chamber, which has a substantial low density, weakly to moderately magnetic, core of alkali feldspar granite to monzonite composition, enclosed by marginal mafic monzonite and monzodiorite phases. The mafic roof zone and marginal phases of the GVC have high susceptibilities (> 0.08 SI, > 2.5 vol % magnetite).A prominent ridge of low density material occurs along the eastern margin of the mother intrusion. A zone of lower susceptibility occurs above the felsic ridge, probably representing magnetite-destructive alteration due to fluids emanating from the inferred underlying felsic intrusion. This zone appears to be related to the Endeavour lineament, which is thought to control the emplacement of many of the mineralising intrusions in the GVC. Magnetic signatures of deposits tend to be obscured by the heterogeneous magnetic environment, but reflect variably developed halos of enhanced magnetite content, associated with early potassic alteration, surrounding a core of reduced magnetite content, which represents the combined effect of felsic mineralising intrusives, mineralising phase (K-feldspar dominated) alteration and phyllic overprinting. Different signatures can be expected for lava-dominated wall rock sequences (weak to moderate annular high with well-developed central low) and volcaniclastic-dominated sequences (unimodal weak to moderate high).

The Chromite Deposits of the Bacuri Mafic-Ultramafic LayeredComplex, Guyana Shield, Amapa State, Brazil

The chromite deposits of the Bacuri mafic-ultramafic layered complex consist of 8.8 Mt of ore grading 34 percent Cr 2 O 3 and represent the second largest reserves of chromite in Brazil. The complex is a major stratiform complex overprinted by ductile deformation and associated regional amphibolite-facies metamorphism of the Transamazonian cycle (about 2.0 Ga), intrusive into gneiss-migmatite terranes of the Guyana Shield. The Bacuri mafic-ultramafic complex consists of a lower mafic zone (>500 m thick), an ultramafic zone (30–120 m thick) and an upper mafic zone (>300 m thick). The chromitite layers are restricted to the ultramafic zone. This zone consists of interlayered serpentinite (olivine cumulate) and chromitite (chromite cumulate). Most of the chromite is concentrated in a thick chromitite layer, known as the main chromitite, located at the base of the ultramafic zone in direct contact with the underlying lower mafic zone. The thickness of the main chromitite is highly variable owing to deformation and ranges from 3 to 30 m (average of 12 m). Several thinner layers of massive chromitite are located above the main chromitite within the ultramafic zone. Chromitites are generally massive and contain more than 60 vol percent of cumulus chromite. The chromite is mainly euhedral and fine-grained, ranging from 0.1 to 3 mm in diameter. The matrix of massive chromitite consists mainly of metamorphic silicates (serpentine, chlorite, and tremolite), except for the B1 orebody where large igneous orthopyroxene oikocrysts are preserved. Chromite grains in massive chromitite have a homogeneous core and an alteration rim. Microprobe traverses through chromite grains of massive chromitite indicate that the alteration rim is enriched in Cr and Fe 2+ and depleted in Al and Mg. The composition of chromite from massive chromitite changes significantly with stratigraphic height. The consistent stratigraphic variation is well indicated by progressive upward decrease in the Mg/(Mg + Fe 2+ ) ratio as well as the progressive upward increase in the Cr/(Cr + Al) ratio, the Fe 3+ /(Fe 3+ + Al + Cr) ratio, and TiO 2 content. The significant TiO 2 and Fe 3+ enrichments in chromite of the uppermost massive chromitites are typical of stratiform chromite deposits such as the Bushveld Complex. Olivine shows an equivalent trend of compositional variation characterized by an upward decrease in Fo and Ni content. The cryptic variation of chromite and olivine indicates extensive fractionation within the ultramafic zone. Reversals in the Fo content of olivine and in both Mg/(Mg + Fe 2+ ) ratio and TiO 2 content of chromite suggest that successive replenishments of the magma chamber with more primitive magma occurred during the formation of the ultramafic zone.