Noritic Rocks Research Articles

Geology, sulphide geochemistry and geochronology of the Romsås Ni-Cu-Co mineralisation, Norway: Implications for ore formation and regional prospectivity

The Romsås intrusion, located in the southern portion of the Idefjorden lithotectonic unit, is part of a cluster of small to medium-sized mafic intrusive bodies which host NiCu sulphide mineralisation, referred to as the Indre Østfold metallogenic area. The Romsås is an almost 1 km-long intrusion with rounded shape, comprising undifferentiated quartz-norite with local orbicular texture. The assemblage, comprising cumulus orthopyroxene and plagioclase, is partly recrystallised and altered, thus orthopyroxene is partly replaced by amphibole, whereas plagioclase is locally replaced by epidote. Locally, garnet, amphibole, plagioclase and magnetite define a granoblastic texture as product of amphibolite-facies metamorphism. Several sulphide-rich lenses, normally up to 5 m wide, occur within the Romsås intrusion and display a sharp contact with the host noritic rocks. The textures vary from disseminated, net-textured and massive sulphides. The sulphide assemblage is homogeneous within the deposit and consists mainly of pyrrhotite, followed by pentlandite, and minor chalcopyrite, with variable alteration of pentlandite.This contribution provides the first systematic characterisation of the Romsås Ni-Cu-Co mineralisation and constraints for its formation and geodynamic setting. We have measured the concentrations of S, PGE, TABS+ (Te, As, Bi, Sb and Se) and other chalcophile elements in both whole rock and sulphide minerals from different ore textures, and UPb and Hf isotope systematics in zircon. The Ni and Cu concentrations in 100% sulphides are around 2% and 1%, respectively. The PGE concentrations in all sulphide-bearing samples are low, generally below 0.3 ppm, with Pt and Pd displaying a broad positive correlation and Pt/Pd ratios varying from 1 to 2. Different ore types are modelled to have formed from a mantle-derived parental magma with higher Cu/Pd ratios and lower Pd contents than mantle values, interpreted as the result of 5% prior crystallisation under sulphide-saturated conditions. Negative mantle-normalised Nb anomalies relative to Th, As and Sb suggest around 10% of assimilation of a component similar to lower crust by the parental magmas. Moreover, S/Se ratios greater than mantle values likely reflect the combined effect of Se depletion during early sulphide removal and external S addition upon crustal assimilation. The Romsås Ni-Cu-Co mineralisation likely formed as a result of sulphide segregation in response to external S addition during several magma pulses. The composition of the sulphide ores can be modelled by an R-factor of around 500. A significant budget of the chalcophile elements is not hosted by sulphide minerals and must be thus accounted for other discrete phases, such as platinum-group minerals. Nevertheless, trace elements distribution in sulphides supports only local remobilisation of chalcophile elements and records compositions comparable to sulphides from pristine NiCu deposits worldwide. Sulphide alteration may have led to exsolution of a parcel of chalcophile elements from sulphide lattice. A UPb zircon crystallisation age of 1551 ± 10 Ma, εHft values between ca. 3.5 and 6, and regional geological considerations suggest that the intrusion formed as part of an arc or back-arc rift setting in the Idefjorden lithotectonic unit. The Romsås sulphide mineralisation represents another example of magmatic sulphide deposit formed at an arc-related geodynamic setting, normally unconventional for the formation of this type of deposit. Therefore, our findings support that arc-related settings in Idefjorden lithotectonic unit, southern Norway, should also be regarded as potential for the formation of Ni-Cu-PGE deposits.

Complex Effects of Assimilation on Sulfide Saturation Revealed by Modeling with the Magma Chamber Simulator: A Case Study on the Duluth Complex, Minnesota, USA

Abstract Wall-rock assimilation can cause effective sulfide saturation in magmas and lead to the formation of base and precious metal sulfide deposits. Detailed assessments of how assimilation affects the sulfur content at sulfide saturation (SCSS) in magmas have been scarce because of the lack of suitable thermodynamic modeling tools. The Magma Chamber Simulator (MCS) is the first geochemical modeling software that accounts for thermodynamic wall-rock phase equilibrium in open magmatic systems experiencing recharge-assimilation-fractional crystallization. We used the MCS to model SCSS in a magmatic system corresponding to the parental melt of the Partridge River intrusion of the Duluth Complex, Minnesota. This intrusion hosts several Cu-Ni deposits in troctolitic and noritic rocks, which both show evidence of assimilation of the adjacent Virginia Formation black shale. Our simulations show that the dominantly troctolitic rocks can form via fractional crystallization if the parental melt is hydrous (≥ 1 wt % H2O), while gabbroic rocks dominate when the parental melt is H2O poor (≤ 0.14 wt % H2O). Formation of norite from the hydrous parental melt requires ~20–30% of selective assimilation of black shale partial melts or bulk assimilation of stoped blocks. In the fractional crystallization simulations, increasing the H2O content of the parental melt lowers SCSS. In the hydrous fractional crystallization scenarios, SCSS is lowered further by the depletion of FeO from the residual melt, owing to enhanced olivine stability. In the assimilation simulations, the residual melt in the magma subsystem becomes enriched in SiO2, Al2O3, K2O, and H2O with simultaneous depletion in FeO, MgO, CaO, and Na2O. These compositional changes promote sulfide saturation—an effect that is more pronounced in selective rather than in bulk assimilation scenarios. Trace element models, used as a proxy for the efficiency of sulfur assimilation, show that sulfur should behave as an incompatible element (DWR (S) ≤ 1) to wall rock in the selective assimilation simulations, i.e., enriched in early-assimilated wall-rock fluids and/or partial melts, in order to fulfill the natural sulfur isotope criteria of the Duluth Complex. Bulk assimilation may also be efficient enough to modify the sulfur isotope composition, but it requires a large amount of crystallization in the magma and is, hence, considered less likely to be the main process for sulfur assimilation. If wall-rock sulfur is effectively transported to the magma, in situ precipitation of sulfides without notable subsequent upgrading by dynamic processes could produce the sulfide grade of an average Cu-Ni deposit in the Partridge River intrusion.

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.

Impact melt breccia and surrounding regolith measured by Chang'e-4 rover

Chang'e-4 rover discovered a dark greenish and glistening impact melt breccia in a crater during its traverse on the floor of Von Kármán crater within the South Pole Aitken (SPA) basin on the lunar farside. The discovered breccia, being 52 × 16 cm, resembles the lunar impact melt breccia samples 15466 and 70019 that returned by the Apollo missions. It was formed by impact-generated welding, cementing and agglutinating of lunar regolith and breccia. Clods surrounds the breccia-hosting crater were crushed into regolith powders by the rover's wheels, indicating the regolith may be compacted slightly and becomes blocky and friable. Relative mineral fractions are estimated from the in situ measured spectra by a Hapke model-based unmixing algorithm. Unmixing reveals that plagioclase (PLG, 45 ± 6%) is dominant in the regolith, followed by almost equal fractions of pyroxene (PYX, 7 ± 1%) and olivine (OL, 6 ± 2%), indicating the regolith is likely related to noritic rocks. The regolith measured by Chang'e-4 rover was actually a highly mixture of multiple sources, with ejecta from Finsen crater being primary and possible contributions from Alder crater. Finsen and Alder craters are on the margin of the proposed impact melt pool produced by the SPA basin-forming event. Therefore, the ultimate source of the regolith might originate from a differentiated melt pool or from a suite of igneous rocks.

Apollo 12 breccia 12013: Impact-induced partial Pb loss in zircon and its implications for lunar geochronology

Apollo 12 breccia 12013 is composed of two portions, one grey in colour, the other black. The grey portion of the breccia consists mainly of felsite thought to have formed during a single crystallisation event, while the black part is characterized by presence of lithic fragments of noritic rocks and individual plagioclase crystals. In this study, U-Pb analyses of Ca-phosphate and zircon grains were conducted in both portions of the breccia. The zircon grains within the grey portion yielded a large range of ages (4154 ± 7 to 4308 ± 6 Ma, 2σ) and show decreasing U and Th concentrations within the younger grains. Moreover, some grains exhibit recrystallisation features and potentially formation of neoblasts. The latter process requires high temperatures above 1600–1700 °C leading to the decomposition of the primary zircon grain and subsequent formation of new zircon occurring as neoblasts. As a result of the high temperatures, the U-Pb system of the remaining original zircon grains was most likely open for Pb diffusion causing partial resetting and the observed range of 207Pb/206Pb ages. The event that led to the Pb loss in zircon could potentially be dated by the U-Pb system in Ca-phosphates, which have a weighted average 207Pb/206Pb age across both lithologies of 3924 ± 3 Ma (95% conf.). This age is identical within error to the combined average 207Pb/206Pb age of 3926 ± 2 Ma that was previously obtained from Ca-phosphates within Apollo 14 breccias, zircon grains in Apollo 12 impact melt breccias, and the lunar meteorite SaU 169. This age was interpreted to date the Imbrium impact. The zircon grains located within the black portion of the breccia yielded a similar range of ages (4123 ± 13 to 4328 ± 14 Ma, 2σ) to those in the grey portion. Given the brecciated nature of this part of the sample, the interpretation of these ages as representing igneous crystallisation or resetting by impact events remains ambiguous since there is no direct link to their source rocks via textural relationships or crystal chemistry. Similarly, the currently available zircon data set for all lunar samples may be distorted by partial Pb loss, resulting in meaningless and misleading age distribution patterns. Therefore, it is crucial to fully understand and recognize the processes and conditions that may lead to partial resetting of the U-Pb system in zircon in order to better constrain the magmatic and impact history of the Moon.

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.

HETEROGENEITY OF S ISOTOPE COMPOSITIONS RECORDED IN THE SUDBURY IGNEOUS COMPLEX, CANADA: SIGNIFICANCE TO FORMATION OF Ni-Cu SULFIDE ORES AND THE HOST ROCKS

Sulfur isotope ratios of sulfide ores and their host rocks, and the overlying main mass sequence of norites, quartz gabbros, and granophyres from the Sudbury Igneous Complex have been analyzed to establish the extent of isotopic heterogeneity of the Complex and the impact melt sheet from which it was produced. Noritic rocks located above the sublayer in the lower portion of the main mass of the Sudbury Igneous Complex are characterized by uniform sulfur isotope ratios with δ 34S values of 2.59 ± 0.48‰. Rocks from the granophyric portion of the main mass are characterized by elevated and variable δ 34S values from 4.4 to 14.1‰. The more 34S-enriched S in the granophyre was produced by secondary sulfide precipitation related to hydrothermal alteration systems developed in the upper altered granophyric part of the main mass as it cooled. Footwall and contact-style mineralization from the sublayer show variable δ 34S values from 0.2 to 4.3‰. South Range mineralization is characterized by low ratios from 0.2 to 2.5‰. North and Northeast Range mineralization exhibits locally elevated but variable δ 34S values; for example mineralization from Whistle mine varies from 3.1 to 4.3‰, whereas mineralization from Victor varies between 2.4 and 2.9‰. There is typically only a small difference in S isotope ratio of contact and proximal footwall mineralization. Country rocks in the area of the Sudbury Igneous Complex are also characterized by a wide range in δ 34S values and the sulfur isotope homogeneity of the noritic rocks can be explained by mixing of sulfur from several potential sources. However, the variability present between deposits at the lower contact of the Complex requires that country rocks of locally variable sulfur isotope composition interacted with magma at the base of the cooling melt sheet. There is no evidence that deposit size correlates with δ 34S value; variations between deposits may involve local S isotope exchange reactions between the country rock and the melt sheet as the melt digested the footwall. This study illustrates the application of S isotope data in constraining the scale of convective mixing and contamination in the largest known terrestrial melt sheet.

Revised mineral and Mg# maps of the Moon from integrating results from the Lunar Prospector neutron and gamma-ray spectrometers with Clementine spectroscopy

Mineralogical measurements from spectral remote sensing and remote geochemical measurements from gamma-ray and neutron spectrometers are complementary data sets that have been used together successfully to study the distributions of iron, titanium, and rare earth elements on the Moon. We compare neutron and gamma-ray data sets from Lunar Prospector and find them in good agreement with each other within the errors of previously developed equations that relate neutron flux with geochemistry, but find small adjustments to the nominal values are warranted. We used the neutron-validated LP GRS oxides to improve Clementine-based global mineral maps. The comparison was enabled by converting the minerals of Lucey (2004) to oxides using stoichiometry and assumptions about Mg#, calcium content of clinopyroxenes, and An#. We find that FeO and Al 2 O 3 derived from the maps of Lucey (2004) do not follow the expected negative correlation seen in lunar samples, but can be brought into agreement with samples and with LP GRS oxides by increasing plagioclase in proportion with orthopyroxene abundance, while simultaneously decreasing Mg#. We interpreted this to mean that plagioclase and orthopyroxene exist in rocks together (as in a noritic rock) with the spectrally difficult to detect plagioclase being masked by the strong spectral signature of the orthopyroxene. We generated a revised set of maps of the major lunar minerals and a map of Mg# for the mafic minerals that are consistent with Lunar Prospector neutron and gamma-ray spectrometer results and show greatly improved agreement with lunar soil samples over previous global mineral maps from Clementine.

Time scales of diffusive re-equilibration in bi-mineralic systems with and without a fluid or melt phase

Diffusion is a fundamental mass transfer process in magmatic and metamorphic systems. Simple recipes for evaluating time scales for diffusive re-equilibration of a trace element in bi-mineralic aggregates with and without the presence of intergranular fluid or melt are obtained through approximate solutions to diffusion equations. Diffusive re-equilibration times for the trace element depend on average grain sizes of the two minerals, diffusion coefficients and partition coefficients for the element in the two minerals, and volume fractions or volume fluxes of the minerals and fluid or melt in the system. There are two time scales for diffusive re-equilibration in fluid-bearing bi-mineralic systems. The fast time scale is proportional to fluid fraction and dominated by the mineral that has a faster diffusion rate. The slow time scale is bounded by the time scales of diffusion in the two minerals and determines the overall time scale of diffusive re-equilibration for the system. Presence of small amount of fluid or melt does not lead to a reduction in the time scale of diffusive re-equilibration for the bi-mineralic aggregate. Percolation of fluid or melt drives the system further away from equilibrium and increases the length and time scales for diffusive re-equilibration in the system. The time scales for subsolidus re-equilibration in bi-mineralic aggregates are determined by the minor’s rule: the mineral that contains a lesser amount of the trace element of interest in the system contributes more to the overall diffusive time.Applications to REE redistribution between clinopyroxene and orthopyroxene and between plagioclase and orthopyroxene highlight the importance of REE diffusion and partitioning in pyroxene and plagioclase. The time scales for subsolidus re-equilibration of REE in noritic rocks are dominated by light REE diffusion in orthopyroxene and by middle and heavy REE diffusion in plagioclase. The time scales for subsolidus re-equilibration of REE in two-pyroxene bearing rocks are controlled primarily by REE diffusion in orthopyroxene. The time scales for diffusive re-equilibration of REE in melt-bearing two-pyroxene aggregates are determined by diffusion in clinopyroxene for light REE and by diffusion in orthopyroxene for middle to heavy REE. Presence of melt flow further expands the role of clinopyroxene. This contrasting behavior in diffusive time scale may provide a simple explanation for the ubiquitous chemical disequilibrium in light REE between clinopyroxene and orthopyroxene observed in mantle peridotites from a wide range of tectonic settings.

Thermal history of Apollo 12 granite and KREEP-rich rock: Clues from Pb/Pb ages of zircon in lunar breccia 12013

With the mafic to ultramafic petrology of the Moon, the presence of granite appears anomalous. However, fragments of rocks with a definite granitic textures and mineralogy have been recovered from the maria, where their ages point to an ultimate origin in the highlands. Apollo 12 mare breccia 12013 is one of these unique lunar rocks, containing abundant granitic components. It consists of a black portion, composed mainly of fragments of plagioclase and noritic rock, with minor fragments of granitic rock, and a gray portion that is dominated by an overall granitic component. Zircon grains from this breccia were studied by combining cathodoluminescence (CL) imaging and micron-scale, ion-microprobe dating techniques. Zircon grains from the black portion have a large age variation (4.0–4.3Ga). A few of them exhibit complex age zoning within individual grains, corresponding to CL zoning. Zircon grains from the gray portion show no CL zoning features but also have a large age variation between different grains (4.2–4.3Ga).In this study, we suggest that the large age variations of zircon in 12013, among different grains and within individual grains were caused by post-crystallization impact events, and most of them are not original crystallization ages. The 4.3-Ga zircon age probably represents the minimum crystallization age of the granitic component in 12013, similar to lunar granites from the Apollo 14 and 17 landing sites. This old crystallization age also implies that various REE patterns of lunar granites might not be entirely related to their crystallization ages. Based on comparison with the 207Pb/206Pb ages for zircon from the Apollo 12, 14, and 17 landing sites, we suggest that there was at least one pre-3.9Ga major impact event on the Moon, at ∼4.2Ga.

Origin of PGE-Poor and Cu-Rich Magmatic Sulfides from the Kalatongke Deposit, Xinjiang, Northwest China

The Kalatongke Cu-Ni sulfide deposit in the Paleozoic Altay orogenic belt, NW China, is hosted in a Per mian mafic intrusion consisting of norite, troctolite, gabbro, and diorite. Disseminated Ni-Cu, massive Ni-Cu, and massive Cu-rich sulfide ores are mainly hosted in norite and gabbro. Some massive Ni-Cu ores also occur in the Carboniferous sedimentary rocks. The geologic and compositional relationships between various sulfide ores and the rocks of Kalatongke offer a new interpretation of the sequence of emplacement of the magmas, which underpins an understanding of the compositions of the ores and the formation of the Kalatongke deposit. Olivine grains from disseminated Ni-Cu ores have Fo values ranging from 71.6 to 78.0 mol % and Ni con tents from 1,000 to 2,200 ppm. Typically, Ni decreases from the cores to the rims from 2,000 to 1,000 ppm at constant Fo content, indicating the reaction of early-formed olivine with later-segregated sulfide melt. Cr spinels at Kalatongke are highly enriched in Fe 3+ and Fe 2+ , with relatively low Cr, Al, and Ti, reflecting reac tion with evolved trapped intercumulus melt. Norites are depleted in Nb, Ta, Zr, Hf, and Th and enriched in Sr and Ba, whereas disseminated Ni-Cu sul fide ores have considerable depletion of Rb and enrichment of Sr and Ba and lack depletion of Nb, Ta, Zr, and Hf, indicating their different origins. Disseminated Ni-Cu sulfide ores have bulk compositions with variable Cu and Ni contents which are much lower than those of massive Cu-rich and Ni-Cu ores, but disseminated and massive Ni-Cu ores have similar PGE contents with relatively low Pd/Ir ratios. Massive Cu-rich ores have much higher Pd and Pt with very high Pd/Ir ratios. The Kalatongke Cu-Ni sulfide deposit appears to have formed from two different pulses of PGE-poor and Cu-rich basaltic magmas that underwent different degrees of assimilation and fractional crystallization. The first magma pulse gained sulfide saturation because of minor crustal contamination and fractionated a small amount of sulfide (<0.03%); the evolved melt then intruded and assimilated crustal materials to attain sulfide saturation again. Sulfide liquid segregated from the magma to form the massive sulfide melts and residual magma formed the noritic rocks in the shallow magma chamber. The segregated massive sulfide melts then un derwent further fractionation to form massive Ni-Cu and massive Cu-rich ores. The second pulse of magma after removal of sulfides (<0.02%) experienced more crustal contamination and re attained S saturation. This new S-saturated and phenocryst-laden magma intruded the earlier formed massive sulfide ores and norites and formed the disseminated sulfide ores.

Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust

The Sept Iles layered intrusion (Quebec, Canada) is dominated by a basal Layered Series made up of troctolites and gabbros, and by anorthosites occurring (1) at the roof of the magma chamber (100–500 m-thick) and (2) as cm- to m-size blocks in gabbros of the Layered Series. Anorthosite rocks are made up of plagioclase, with minor clinopyroxene, olivine and Fe–Ti oxide minerals. Plagioclase displays a very restricted range of compositions for major elements (An 68–An 60), trace elements (Sr: 1023–1071 ppm; Ba: 132–172 ppm) and Sr isotopic ratios ( 87Sr/ 86Sr i : 0.70356–0.70379). This compositional range is identical to that observed in troctolites, the most primitive cumulates of the Layered Series, whereas plagioclase in layered gabbros is more evolved (An 60–An 38). The origin of Sept Iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts. The density of the FeO-rich tholeiitic basalt parent magma first increased from 2.70 to 2.75 g/cm 3 during early fractionation of troctolites and then decreased continuously to 2.16 g/cm 3 with fractionation of Fe–Ti oxide-bearing gabbros. Plagioclase (An 69–An 60) was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite. With further differentiation, plagioclase (<An 60) became negatively buoyant and anorthosite stopped forming. Blocks of anorthosite (autoliths) even fell downward to the basal cumulate pile. The presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and/or minor plagioclase nucleation within the main magma body. Dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid. The processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust, with (gabbro-)noritic rocks at the base and anorthositic rocks at the top. We propose that the lunar crust mainly crystallized bottom-up. This basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian-suite without KREEP contamination, while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust.

The Grader layered intrusion (Havre-Saint-Pierre Anorthosite, Quebec) and genesis of nelsonite and other Fe–Ti–P ores

The Grader layered intrusion is part of the Havre-Saint-Pierre anorthosite in the Grenville Province (Quebec, Canada). This intrusion has a basin-like morphology and contains significant resources of Fe–Ti–P in ilmenite and apatite. Outcropping lithologies are massive oxide alternating with anorthosite layers, banded ilmenite–apatite–plagioclase rocks and layered oxide apatite (gabbro-)norites. Drill cores provide evidence for stratigraphic variations of mineral and whole rock compositions controlled by fractional crystallization with the successive appearance of liquidus phases: plagioclase and ilmenite followed by apatite, then orthopyroxene together with magnetite, and finally clinopyroxene. This atypical sequence of crystallization resulted in the formation of plagioclase–ilmenite–apatite cumulates or “nelsonites” in plagioclase-free layers. Fine-grained ferrodiorites that cross-cut the cumulates are shown to be in equilibrium with the noritic rocks. The high TiO 2 and P 2O 5 contents of these assumed liquids explains the early saturation of ilmenite and apatite before Fe–Mg silicates, thus the nelsonites represent cumulates rather than crystallized Fe–Ti–P-rich immiscible melts. The location of the most evolved mineral and whole rock compositions several tens of meters below the top of the intrusion, forming a sandwich horizon, is consistent with crystallization both from the base and top of the intrusion. The concentrations of V and Cr in ilmenite display a single fractionation path for the different cumulus assemblages and define the cotectic proportion of ilmenite to 21 wt.%. This corresponds to bulk cotectic cumulates with ca. 8 wt.% TiO 2, which is significantly lower than what is commonly observed in the explored portion of the Grader intrusion. The proposed mechanism of ilmenite-enrichment is the lateral removal of plagioclase due to its relative buoyancy in the dense ferrodiorite melt. This plagioclase has probably accumulated in other portions of the intrusion or has not been distinguished from the host anorthosite.

Rock-strength and physical properties of Norites of the Merensky and Bastard Units, western Bushveld Complex

Platinum mining in the Bushveld Complex has expanded greatly over the last decade with many operations planned for, or already at depths greater than 1000 m. Mine infrastructures, rock fragmentation properties, rock failures and safety issues are important aspects of deeper mining operations and require an understanding of controls and properties which influence rock strength. This study, which includes rock classification of both fresh and altered material, chemical compositions, fabrics and structure, was carried out to investigate mainly the noritic rocks in the vicinity of the Merensky reef in the western Bushveld Complex. Uniaxial compressive strength (UCS) and tensile strength (TS) are shown to be broadly dependent on rock type, and strongly dependent on the degree of structural alteration as reflected by the Rock Quality Designation (RQD). In addition, there is an unexpectedly strong correlation between UCS values and both normative quartz and Zr contents of the whole rock. Both of these compositional parameters relate to the entrapment of a late-stage liquid which when solidified acted as cement and influenced the propagation of fractures under conditions of failure. Micrographic observations indicate that interstitial quartz, even in relatively small amounts, is effective in arresting fracture propagation. Relatively small variations in the quartz content could therefore significantly affect the rock strength of the norites. Other possible controls on rock strength include the relative development of foliation fabric, the distribution of crystal sizes, and the way that the rocks have annealed at high temperature.

The Sudbury Igneous Complex: A Differentiated Impact Melt Sheet

The Sudbury structure, Ontario, is the remnant of a 1.85 Ga old impact crater, which originally had a diameter of 200 to 250 km. The Sudbury Igneous Complex occurs within the Sudbury structure. The Sudbury Igneous Complex is a 2.5- to 3.0-km-thick, ~60- × 27-km elliptical igneous-rock body, which consists of four major lithologies (from top to bottom) traditionally termed granophyre, gabbro, norite, and sublayer (sulfide- and inclusion-bearing noritic rock). With the exception of the latter, all these lithologies are continuous across the structure. Modal analyses reveal that, following the IUGS system of nomenclature, quartz gabbro samples are in fact quartz monzogabbros, a few of the norite samples are quartz gabbros, and most norite samples are quartz monzogabbros. In view of these observations, and in order to clarify the nomenclature, an updated terminology is proposed (from top to bottom): upper unit, middle unit, lower unit, and contact sublayer. The bulk composition of the Sudbury Igneous Complex, from North Range data, is granodioritic. Continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the Complex behaved as a single melt system. All the Sudbury Igneous Complex lithologies have the same light to heavy rare earth element (REE) ratio and an overall pattern of increased light REE and depleted heavy REE. The occurrence of primary hydrous minerals (hornblende and biotite), deuteric alteration, and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in H2O. Moreover, the granophyric and other far-from-equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase. The Sudbury Igneous Complex differs from traditional layered mafic complexes in the following aspects. It has an overall intermediate composition, a hydrous nature, a crustal isotopic signature, normative corundum, and an unusually large volume of granophyre. The Sudbury Complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical, and therefore, mineralogical layering. Reported here for the first time, and similar to those found in impact melt rocks elsewhere, are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts. The well-known ore deposits of the Sudbury region are directly related to the genesis of the Sudbury Igneous Complex. Some ores precipitated from the Sudbury melt, whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex. It is concluded that the Sudbury Igneous Complex is the best exposed and only well-documented, to date, terrestrial impact melt sheet to have differentiated.

Chemical Evolution and Origin of Nickel Sulfide Mineralizationin the Sudbury Igneous Complex, Ontario, Canada

Chemical Evolution and Origin of Nickel Sulfide Mineralizationin the Sudbury Igneous Complex, Ontario, Canada

The Banpur-Balugaon and Bolangir Anorthosite Diapirs of the Eastern Ghats, India: Implications for the Massif Anorthosite Problem

Two anorthosite massifs in the Eastern Ghats share similar structural constitutions, internal differentiation histories, and overall thermal-tectonic patterns of evolution. The (1) circular to near-circular structural patterns both inside and close-to-border outside of the plutons; (2) the merging of these structures with the straight to gently flowing, essentially unidirectional structural trend of the granulite basement within short distances from the border of the plutons; (3) the increase of strain intensity near the border of the plutons; (4) the small but recognizable differences in the dip of the anorthosite flow layers; and (5) the foliation of the granulites, all are strong indications that these anorthosites were emplaced as syntectonic diapirs. In both massifs, anorthosite is by far the dominant lithology, noritic varieties being subordinate and generally formed as narrow dike-like bodies and pods, pockets, and irregular patches near the border. The striking features of the whole-rock and mineral chemistries of these massifs are increasing Fe, Ti, Mg, P, and REE, and decreasing Si and Al from the leucoanorthosites to the noritic rocks and wide Mg-Fe variation in the pyroxenes, in contrast to a relatively uniform plagioclase composition. These variations may be the result of initial differentiation in a layered complex serving as a precursor to the anorthosites, through modal sorting, rhythmic layering and cryptic changes, and a subsequent mixing of the modally sorted and cryptically enriched layers at different stages of anorthosite diapirism. Diapiraureole structural relationships, petrology, and thermobarometry suggest a moderately steep, counterclockwise cooling and exhumation path for both massifs to the P-T range: 5 to 6 kbar and 600 to 700°C. Such considerations, supplemented by a conductive cooling model for anorthosite emplacement and its subsequent evolution, lead to a branched path as the essential topology of the P-T-time history of the anorthosite-granulite associations of the Eastern Ghats. An important corollary of this inference is that a cycle of prograde and retrograde metamorphism of the aureole rocks—before and after the anorthosite invasion, respectively—is an essential consequence of the anorthosite emplacement.

The Origin of Anorthosites and Related Rocks from the Lofoten Islands, Northern Norway: II. Calculation of Parental Liquid Compositions for Anorthosites

Clinopyroxene, orthopyroxene and plagioclase in the Proterozoic INTRODUCTION Lofoten anorthosites are inferred to be in equilibrium with ferrodioritic Proterozoic massif-type anorthosites are huge intrusive melts, on the basis of literature partition coefficients for these phases bodies that contain hundreds to thousands of km of in equilibrium with basaltic melts. Ferrodiorites and high-Al gabbros nearly pure (>90%) plagioclase cumulates of intermediate occur as fine-grained dykes in spatial association with the ~1·8 composition (An50±10; Ashwal, 1993, and references Ga Lofoten anorthosites. They are linked to each other by an Fetherein). Most Proterozoic anorthosites are not associated enrichment fractionation trend typical of tholeiites, and they are with significant amounts of mafic or ultramafic cumulates. characterized by Al2O3 contents of 14–18·5 wt % and XFe [= They contain plagioclase that is relatively sodic (usually FeO/(FeO + MgO)] between 0·5 and 0·8. The association around An50) and this makes them distinct from other of high-Al gabbros, ferrodiorites and anorthosites is common in anorthositic rocks (Archean anorthosites, lunar anormany Proterozoic anorthosites. We infer that the high-Al gabbros thosites, anorthosites in layered intrusions), which contain to ferrodiorites represent the parental liquids from which these more calcic plagioclase. Proterozoic anorthosite comProterozoic anorthosites formed. The Lofoten rocks show fractionation plexes contain anorthosite sensu stricto (>90% plagioclase) trends that are similar to those of high-Al gabbros and ferrodiorites as well as variable amounts of leuco-gabbroic, -troctolitic from other Proterozoic anorthosite complexes. We interpret these trends and -noritic rocks which respectively may contain up to in terms of incorporation of intermediate composition plagioclase into 35% clinopyroxene, olivine or orthopyroxene. a typical tholeiitic magma with subsequent fractionation. The first The Proterozoic anorthosites of the Lofoten Islands in process produces positive Eu anomalies and the high Al2O3 contents northern Norway (Heier, 1960; Green & Jorde, 1971; of the melts, and explains the lack of negative Eu anomalies in Romey, 1971; Griffin et al., 1978; Wade, 1985) are most of the ferrodiorites from Lofoten and Laramie. associated with large amounts of temporally related intrusive mangerites, charnockites and gabbros (Markl et al., 1998). There are two different anorthosite intrusions present. One is a relatively calcic complex, which occurs on the island of Flakstadoy (Flakstadoy Basic Complex, FBC), and the other is a relatively sodic complex, which

Age and Petrogenesis of the Sondalo Gabbroic Complex (Central Alps, Northern Italy): Implications for the Origin Of Post-Variscan Magmatism

The Late Carboniferous-Early Permian gabbroic complexes of the Alpine belt are commonly ascribed to the extensional tectonics which postdate the Variscan orogeny (Dal Piaz, 1993). The best examples are provided by the Main Gabbro from the Ivrea-Verbano Zone (Quick et al., 1994) and by the Malenco Gabbro (Hermann et al., 1997). However, both Main and Malenco Gabbro have a widespread granulite-facies metamorphic overprint, which does not allow a complete petrological characterization of their igneous features. Very little is known about the Sondalo gabbroic complex of the Languard-Campo nappe (Austroalpine nappe system of Central Alps), although it is volumetrically important (it occupies an area of about 40 km 2) and almost devoid of metamorphic recrystallization. There are no recent studies on its petrogenesis and a determination of the intrusion age is lacking, despite geochronological determinations on adjacent granitoid masses suggesting that the Sondalo gabbroic complex was emplaced in Permian times (Del Moro and Notarpietro, 1987). The Sondalo gabbroic complex is a stock intrusive into high-grade metapelites of the Languard-Campo nappe (Austroalpine nappe system of the Central Alps). The pluton broadly consists of norites and minor troctolites at the core, and diorites and subordinate granodiorites at the rim. The pluton frequently contains xenoliths of granulite-facies restitic rocks and is crosscut by muscovite-bearing pegmatitic to aplitic dykes. This work aims to verify whether the Sondalo gabbroic complex is related to the Alpine gabbroic complexes which postdate the Variscan orogeny. For this purpose, we carried out a petrological study (petrography, major and trace element mineral chemistry, and Nd-Sr-O isotopic compositions) of the troctolite/norite association, thus unraveling the intrusion age, the affinity of the parental liquids and the role of crustal contamination. Combining the data presented with the literature information on the Late Carboniferous-Early Permian gabbroic complexes from the Alpine belt, we give some insights into the origin of the postVariscan magmatism. Troctolitic rocks commonly consist of euhedral to subhedral plagioclase and olivine (Fo = 73-78 mol %), and minor poikilitic clinopyroxene. The variation trend of anorthite content of plagioclase versus the forsterite content of olivine differs from that of arcrelated troctolitic rocks (Beard, 1986) and is similar to that of troctolitic rocks from slow spreadingridges. Noritic rocks are usually composed of euhedral to subhedral plagioclase and orthopyroxene, and minor poikilitic clinopyroxene. Moreover, noritic rocks may contain relatively high percentages of interstial ilmenite and biotite. Both troctolitic and noritic rocks have up to 15 % of titanian pargasite, which occurs as discontinuous rims around mafic minerals or as poikilitic grains. Pyroxenes and Tipargasite from troctolitic rocks have higher Mg, A1 and Cr than those from noritic rocks, whereas plagioclase does not show systematic compositional variations (An about 65 mol %). Plagioclase, pyroxene and Ti-pargasite were analysed for REE and selected trace elements by ion microprobe. Plagioclase shows a regular decrease from LREE to HREE and a marked positive Eu anomaly. Orthopyroxene contains low REE, with a steady increase from LREE to HREE. Clinopyroxene has depleted LREE, negative Eu anomaly and slightly depleted to almost fiat HREE. The REE pattern of titanian pargasite mimics that of the associated clinopyroxene, but at higher abundance levels. Chondrite normalization of incompatible trace elements in Ti-pargasite reveals that K, Ba and Sr (LILE) are slightly depleted relative to LREE, and Nb is commonly enriched relative to K and Ba. The trace element compositions of parental liquids,