Ultramafic Zone Research Articles

Layered intrusions and volcanic sequences in Central Brazil: Geological and geochronological constraints for Mesoproterozoic (1.25 Ga) and Neoproterozoic (0.79 Ga) igneous associations

A 350-km long belt of layered complexes and associated volcano-sedimentary sequences forms a continental-scale feature exposed along the internal portion of the Neoproterozoic Brasília Belt in central Brazil. This study provides new geochronological results and a critical review of the available data of these igneous associations of central Brazil. Precise age dating combined with geological and petrological studies indicate that this belt consists of two distinct igneous rock associations. The 1.25 and 0.79 Ga igneous episodes are constrained by reliable U–Pb zircon ages (SHRIMP and ID-TIMS) obtained in different regions and lithotypes of this belt. Both igneous associations were affected by high-grade metamorphism and tectonism at ca. 0.76 Ga, which partially disrupted the original igneous stratigraphy of the layered complexes and volcano-sedimentary sequences. The present configuration of this belt results from later events, probably caused by final ocean closure and continental collision between the São Francisco and Amazonian continents at ca. 0.63 Ga. The 1.25 Ga magmatic event represents an igneous rock association comprising extensive bimodal volcanic rocks (Palmeirópolis, Juscelândia and Indaianópolis sequences) and associated large mafic layered intrusions (Serra dos Borges and Serra da Malacacheta complexes). Geochemical data of mafic volcanic rocks indicate compositions similar to MORB, suggesting a tectonic setting where continental rifting led to the opening of an ocean basin. Large layered intrusions coeval to the volcano-sedimentary sequence consist mainly of olivine plus plagioclase cumulates, with abundant troctolite and leucotroctolite, associated with olivine gabbro, gabbro, anorthosite and pyroxenite. These layered intrusions are characterized by moderately primitive olivine compositions (Fo 63–76), positive ɛ Nd(1250Ma) values, low contents of incompatible trace elements and LREE-depleted mantle-normalized-REE patterns. These features suggest an origin from the same depleted mantle source that produced the extensive MORB-type volcanism. Intrusion of layered complexes possibly occurred since the early stages of rifting, being emplaced both at the base of and within the volcanic pile. Different magmatic ages and fractionation trends of layered bodies suggest that they represent a cluster of intrusions that originated during the time span of the continental to oceanic ridge rifting event. The 0.79 Ga igneous association consists of three large layered intrusions (Niquelândia, Barro Alto, and Canabrava complexes). Despite intense tectonism, several striking geological similarities between the three separate bodies have been used to suggest that they originally constituted a single continuous magmatic structure. These layered intrusion are characterized by thick ultramafic zones, highly primitive compositions of olivine (up to Fo 93) and extensive Fe-enrichment trends of pyroxenes. Layered mafic–ultramafic rocks have abundant evidence of crustal contamination, including highly negative ɛ Nd(800Ma) values, as well as older crustal xenoliths and zircon xenocrysts. Cyclic units consisting of successive layers of dunite, harzburgite, orthopyroxenite, websterite and gabbronorite occur throughout the stratigraphy of these layered intrusions. The Niquelandia Complex ranks among the thickest layered intrusions in the world, and together with the Barro Alto and Canabrava layered intrusions represent an enormous volume of mantle-derived magma emplaced into the crust during a continental rifting event. The 1.25 and 0.79 Ga rifting events are not exposed in the Brasília Belt or in the São Francisco Craton. This supports previous interpretations suggesting that these igneous rock associations are part of an exotic block in the Brasília Belt. Equivalent Mesoproterozoic (ca. 1.25 Ga) and Neoproterozoic (ca. 0.79 Ga) extensional events have been reported in other continents (e.g., North America). These igneous associations described within an allochtonous block in the Brasília Belt provide additional constraints, therefore, for continental reconstruction models before the break-up of Rodinia.

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.

A conduit-related genesis of the Lengshuiqing intrusive assemblage (Sichuan, SW China)

Lengshuiqing is part of the late Proterozoic igneous province from the western margin of the Yangtze craton. The Lengshuiqing area comprises five ultramafic–mafic intrusions, emplaced in the late Proterozoic Yanbian Group. The intrusions from Lengshuiqing contain cumulate ultramafic zones (peridotite + olivine pyroxenite), with cumulus olivine and Cr-spinel, and intercumulus pyroxenes, hornblende, phlogopite and plagioclase. Ni–Cu ore (pyrrhotite + pentlandite + chalcopyrite) is hosted in the ultramafic zones. Olivine-free diorite–quartz diorite ± gabbro and granite zones commonly occur above the ultramafic rocks. The genesis of the intrusions (conduit-related accumulation or differentiation from stagnant magma) was investigated. The amount of sulphides in the intrusions from Lengshuiqing is one order of magnitude bigger than the sulphides that can be dissolved by a volume of mafic magma similar with the volume of the intrusions. Most intrusions from Lengshuiqing have bulk composition (peridotite ± diorite ± granite) more magnesian (MgO = 21–22%; Mg# > 78) than the deduced composition of their parental magma (MgO = 9–11%; Mg# = 64–67). This indicates the accumulation of sulphide and mafic silicates from a volume of magma much bigger than the volume of the intrusions, which can be explained by the fractionation from magma ascending through the intrusions to shallower depths. A continuous supply and vent of magma is consistent with the lack of chilled margins, the melting of the wall rocks and the generation of high-temperature mineral assemblages (K-feldspar, diopside, and sillimanite) in the Yanbian Group. The intrusions from Lengshuiqing are seen as microchambers on conduits draining olivine-, Cr-spinel-, and sulphide-bearing mafic magma from a larger staging chamber.

Deep gas well encounters ultramafic kimberlite-like material in the Sauk Sequence of southeastern Ohio, USA

The first indication of a potential deep-subsurface kimberlite-like material in the Appalachian Basin, without surface expression, has been discovered in oil and gas well drilling samples from a deep well drilled near Zanesville, Ohio, USA. The well was drilled into the Cambrian Sauk Sequence within a localized fault-bounded graben, where a zone of ultramafic rocks was encountered. The zone of ultramafic rocks is within Sauk carbonates at 1720 m below sea level and is not highly anomalous on geophysical wire-line well logs. Petrographic analyses of grain-mounted samples show a 20 m zone of ultramafic rock material near the top of the Conasauga Group. Well cuttings from the ultramafic zone include relatively fresh phenocrysts of phlogopite with calcite, apatite, and titaniferous magnetite, in a secondary matrix of amphibole, chlorite, Fe-oxides, and possibly serpentine. The rocks have undergone deuteric alteration and probably later hydrothermal alteration. A 20-m-thick, localized, natural gas–bearing sandstone unconformably overlies (Knox unconformity) the Sauk Sequence above the ultramafic zone. Structural mapping indicates that the localized sandstone is confined to a small seismic-defined graben originating in the faulted Precambrian Grenville basement complex. Mapping of newly acquired gravity and magnetic data does not indicate the presence of an anomaly coincident with the graben, suggesting that the ultramafic material must be a very small, localized intrusive body (pipe or diatreme). Alternative interpretations include a thin intrusive sill, extrusive tuff deposits, Grenville-aged tectonic fault slice, or eroded igneous deposits. Subsurface mapping of units above the Sauk Sequence (Ordovician Black River Group–Trenton Limestone) shows no significant variation in structural contours above the feature. Although the igneous material has not been isotopically dated, the stratigraphic position suggests that it was intruded prior to Middle Ordovician time.

Platinum-group mineral assemblages in the Platreef at the Sandsloot Mine, northern Bushveld Complex, South Africa

AbstractPlatinum group mineral (PGM) assemblages in the Platreef at Sandsloot, northern Bushveld Complex, in a variety of lithologies reveal a complex multi-stage mineralization history. During crystallization of the Platreef pyroxenites, platinum group elements (PGE) and base-metal sulphides (BMS) were distributed thoughout the interstitial liquid forming a telluride-dominant assemblage devoid of PGE sulphides. Redistribution of PGE into the metamorphic footwall by hydrothermal fluids has formed arsenide-, alloy- and antimonide-dominant assemblages, indicating a significant volatile influence during crystallization. Serpentinization of the footwall has produced an antimonide-dominant PGM assemblage. Parts of the igneous reef were subjected to alteration by a late-stage, Fe-rich fluid, producing ultramafic zones where the telluride-dominant assemblage has been recrystallized to an alloy-dominant one, particularly rich in Pt-Fe and Pd-Pb alloys. A thin, small-volume zone of PGE-BMS mineralization along the base of the hangingwall contains a primary PGM assemblage that is locally altered to one dominated by Pt/Pd germanides. This is thought to have formed when the new pulse of Main Zone magma entered the chamber, and scavenged PGE from the underlying Platreef pyroxenites. That each major rock type at Sandsloot contains a distinctive PGM assemblage reflects the importance of syn- and post-emplacement fluid and magmatic processes on the development of Platreef mineralization.

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.

Archean Layered Mafic-Ultramafic Intrusions in the West Pilbara Craton, Western Australia: A Synthesis of Some of the Oldest Orthomagmatic Mineralizing Systems in the World

Considerable exploration interest has been generated by the platinum-group element (PGE) and Ni-Cu potential of the Archean layered mafic-ultramafic intrusions in the West Pilbara Craton, Western Australia. The Munni Munni intrusion contains the largest resource of PGE associated with a layered intrusion in Australia, and the Radio Hill and Mount Sholl intrusions host significant resources of Ni-Cu-Co sulfides. Titaniferous magnetite layers, remobilized sulfides, and structurally controlled hydrothermal polymetallic deposits have also been a focus for exploration in recent years. The ca. 2.9 Ga Munni Munni, Andover, Radio Hill, Mount Sholl, and Sherlock layered intrusions are a cogenetic suite of high-level (<5 kbars) bodies that represent some of the oldest mineralizing systems of their type in the world. Although they display similar field relationships and mineralogical, geochemical, and isotopic features, their contrasting chalcophile metal distribution patterns show that the timing and mechanism of the S saturation event were critical for the development of PGE-enriched sulfide-bearing layers and basal segregations of base metal sulfides. The intrusions form thick (>5.5 km) dikelike bodies or relatively thin (0.5–2 km) sheets and sills emplaced at different levels along major lithological discontinuities in the upper crust. Rhythmically layered ultramafic components are generally thinner than, and occur along the northern sides of, more massive overlying mafic components. The ultramafic zones consist of dunite, lherzolite, wehrlite, olivine websterite, clinopyroxenite, and websterite. Inverted pigeonite gabbronorite, magnetite gabbro, olivine gabbro, anorthositic gabbro, and anorthosite comprise the mafic sequences. Olivine and clinopyroxene were generally the first minerals to crystallize, except in the Andover intrusion, where orthopyroxene preceded clinopyroxene and possibly reflects greater contamination of the parent magma by felsic crustal material. The crystallization of chromite was inhibited in the ultramafic zones by the partitioning of Cr into early crystallizing clinopyroxene, thus downgrading the potential for PGE-chromite associations. The PGE mineralization in the Munni Munni intrusion occurs in the upper levels of a porphyritic plagioclase websterite orthocumulate layer directly below the ultramafic-gabbroic zone contact. Mineral compositional trends and Nd isotope data indicate that a Pd-Pt-Au-enriched S-undersaturated magnesian basaltic magma was frequently injected into a small magma chamber during formation of the ultramafic zone. A major influx of more fractionated, S-saturated tholeiitic gabbroic magma related to the resident magnesian magma, rapidly inflated the chamber, and induced turbulent magma mixing that resulted in the formation of the PGE-bearing porphyritic websterite layer. In contrast, the parent magmas that formed the Mount Sholl, Radio Hill, Andover, and Maitland intrusions were saturated in S before they were emplaced into the magma chambers. In these intrusions gravitational and structural controls were important for the concentration of PGE-poor (5–400 ppb Pt + Pd + Au) massive Ni-Cu-Co sulfides in depressions and structural embayments along the basal contacts beneath the thickest sequences of mafic-ultramafic cumulates. The parent magmas to the West Pilbara intrusions were siliceous high magnesian basalts of Al-depleted komatiitic affinity (Barberton-type) with 9 to 12 percent MgO, 15 to 25 ppm Sc, 12 to 18 ppm Y, low Al2O3/TiO2 (ca. 11 or half chondrite ratios), and light rare earth enrichment (chondrite-normalized La/Sm = 2.7, La/Lu = 9.0). They were generated with garnet in the residual asthenospheric mantle with probable involvement of a pre-3.0 Ga subduction-modified lithospheric mantle. Isotopic and geochemical modeling suggests that the magmas were contaminated by ca. 3.0 to 3.3 Ga Archean tonalitic to granodioritic crust before being emplaced into high-level magma chambers.

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.

Petrology of the Early Proterozoic Burakovsky layered intrusion, southern Karelia, Russia: mineral and whole-rock major-element chemistry

The Early Proterozoic (2449 ± 1.1 Ma) Burakovsky layered intrusion of southern Karelia, Russia, is the largest (700 km2) mafic pluton in the Fennoscandian Shield. The layered series contains two main parts: (i) an ultramafic series (85% dunite), and (ii) a mafic series (mostly gabbros). Mineral and whole-rock chemical variations in the ultramafic zone indicate little variation in chemistry over the entire sequence (e.g., in olivine, variations are from Fo89 to Fo87 over the upper 1600 m). In contrast, the Mg# in pyroxenes from the gabbronorite zone varies from 85 to 58 over an interval of 500 m. On the basis of the current data, there are no apparent reversals in chemistry in the ultramafic zone, whereas there is evidence for several in the gabbronorite zone. Furthermore, there is a distinctive reversal to more primitive compositions in mineral and whole-rock chemistry at the top of the ultramafic zone. These differences indicate that either (i) the gabbronorite zone crystallized from the residue left from formation of the ultramafic zone and the subsequent "flushing" of the more evolved portions to form the Early Proterozoic Karelian volcanic fields, or (ii) there were two separate magmas. Plagioclase crystals from the gabbronorite zone are chemically zoned, providing evidence of the local processes affecting crystallization of this sequence. Plagioclase crystals from near the base commonly display relatively Na-rich cores that are believed to result from either supersaturation at the onset of nucleation or the prior crystallization of some other, Ca-rich phase (e.g., clinopyroxene). Many plagioclase grains also contain reversely zoned rims, indicative of either (i) exchange between trapped liquid and the main magma during times of replenishment; or (ii) an increase in the CIPW-normative plagioclase–liquid partition coefficient for Ca, due to an increase in the augite component of the melt. The lack of evidence for abundant magma influxes in the ultramafic zone contrasts with the chemical reversals observed in the gabbronorite zone, suggesting that replenishment was a prevailing process only in the upper half of the intrusion.

The economic significance of the Bell River Complex, Abitibi subprovince, Quebec

The exposed part of the Bell River Complex on the northern edge of the Abitibi subprovince represents the upper portion of a layered complex, with a massive gabbroic main zone being overlain by a magnetite-rich gabbroic–anorthositic layered zone and a granophyre. The cumulate rocks display cryptic variation in mineral and whole-rock compositions towards a more evolved signature with stratigraphic height. There is a decrease in An content of plagioclase (from An73 to An51), and in Mg# of augite (from Mg# 75 to Mg# 56), an increase in La/Yb ratio of whole rocks (from 0.5 to 2.9), and a decrease in Cr (from 770 to 5 ppm) and Ni (from 315 to 45 ppm) contents of whole rocks. Concentrations of platinum-group elements (PGE) are mostly below detection limit. Compositional comparison with the South African Bushveld Complex shows that the lowest exposed levels of the complex are equivalent to the critical–main zone boundary, that is, in the immediate hanging wall of the PGE reefs. Based on recent geophysical studies showing that the Bell River Complex is approximately 5 km thick, we consider the presence of a hidden ultramafic zone a distinct possibility and propose a site for an exploration drilling programme. A comparison of compositional data from the Bell River Complex and associated basalts and rhyolites of the Watson Lake and Wabassee groups suggests that the three suites are cogenetic. Based on the fact that the numerous important volcanogenic massive sulphide (VMS) deposits in the region are located around the intrusion, and the similar age of the intrusion and the volcanic rocks, we support earlier workers in suggesting that the intrusion and the VMS deposits are genetically linked, that is, that the Bell River Complex provided the heat source that drove the hydrothermal fluids responsible for forming the VMS deposits.

Archean Enriched Mantle Beneath the Baltic Shield: Rare-Earth-Element Evidence from the Burakovsky Layered Intrusion, Southern Karelia, Russia

In order to better understand the origin and character of late-Archean mantle beneath the Baltic Shield, we have analyzed mafic-ultramafic rocks from one of the best-preserved, least-metamorphosed regions of Karelia, Russia. Trace-element data for samples from the ultramafic and gabbronorite zones of the large (700 km2) Burakovsky layered intrusion (BLI) are presented. Samples from the ultramafic zone are LREE enriched, indicating that they formed from a LREE-enriched parental magma. Indeed, a calculated parental magma for the ultramafic zone has a (Ce/Yb)n ratio of 2.6, a (Nd/Sm)n ratio of 1.1, and a (Dy/Yb)n ratio of 1.6. The LREE enrichment in the parental magma suggests either that the source region was LREE enriched or that the melt was contaminated by crust en route to the BLI magma chamber. Samples from the gabbronorite zone also are LREE enriched and indicate two distinct parental magmas. Group-I magmas, from the lower part of the gabbronorite zone, have (Ce/Yb)n ratios of 6.9 to 13.9, whereas Gro...

Variation in the composition of apatite in the Munni Munni Complex and associated intrusions of the West Pilbara Block, Western Australia

Apatite is a ubiquitous intercumulus mineral in the Archean Munni Munni, Radio Hill, and Mount Sholl layered igneous complexes of the west Pilbara Block in Western Australia. Apatite compositions from the three intrusions are relatively rich in the fluorapatite endmember and become more so in cumulates derived from compositionally evolved magmas. The most extensive stratigraphic coverage is from the Munni Munni complex, where the mole fraction of fluorapatite increases from approximately 60–70% in cumulates of the ultramafic zone to over 90% near the top of the gabbroic zone. The chlorapatite component decreases from a high of 20–30 mol% in the ultramafic zone to less than 10% in the uppermost part of the gabbroic zone. The REE content of apatite parallels stratigraphic changes in whole rock incompatible element (Zr, Rb) concentrations and is consistent with equilibration with the bulk assemblage. The stratigraphic decrease in the Cl/F and OH/F ratio of apatite can be modeled by assuming that fluid saturation of intercumulus liquids is a function of pressure and occurs prior to apatite saturation. Imposed on the stratigraphic trend are the effects of the combined processes of continuing fluid loss and fractional crystallization of apatite (and locally biotite) during solidification of the intercumulus liquid. These processes produced relative compositional variations in endmember components on the order of 50 mol% or more in individual thin sections. Because apatite crystallizes late in the solidification of intercumulus liquids, it is estimated that the Cl/F ratio of the original trapped parent liquid prior to degassing could have been more than 2 orders of magnitude higher than is now recorded by intercumulus apatite compositions. A comparison of literature apatite compositions from other intrusions shows that, with the exception of the unusually Cl-rich assemblages of the Stillwater and Bushveld complexes, processes affecting halogen geochemistry of layered intrusions do not appear to have changed significantly over the past 3 Ga.

Constraints on the formation of cyclic units in ultramafic zones of large basaltic chambers

Petrological models for the formation of cyclic units in ultramafic zones at the base of large mafic layered intrusions are still few and simple. In this study, we develop simple physical constraints, such as the volume balance and density relationships between the various liquids involved. We consider the formation of an entire ultramafic zone, made of N cyclic units due to N successive reinjections. We tackle the following problems. Are all the injections of the same chemical composition? Are the N injections responsible for the formation of the whole magma chamber? Two end-member models are examined. In the first one, the chamber grows with each injection and does not lose any magma (variable volume model, VV). In the second one, the chamber initially forms with a large volume of magma and remains at a constant volume with subsequent reinjections of small volumes balanced by eruptions of equal volume (fixed volume model, FV). Several scenarios for the formation of a cyclic unit are envisioned. Petrological data such as thicknesses of the cumulate layers and their compositions, together with the fractionation density of the cumulate phases, allow the calculation of the density evolution, as a function of the initial density only. Thus, the model yields constraints on the density of the initial liquid, and hence on its chemical composition. The volumes of the injected magmas are calculated as well as the evolution of the densities of the different involved liquids. We apply the calculation to the specific case of the Ultramafic Series of the Stillwater complex. The multiple reinjection models require that the initial liquids densities vary between 2.71 and 2.72 g/cm3, corresponding to MgO-rich liquids for which there is no field evidence, either in dikes or in chilled margins. They also require that the source produces liquids of different chemical composition. Thus, the evolution of the igneous system requires another, larger reservoir where differentiation takes place prior to injection into the shallower one. Contrary to the multiple reinjection models, a closed system model where crystallization proceeds in an isolated bottom layer does not require any specific value of density for the liquids, provided, of course, that it is of basaltic composition. On the negative side, such a model is not supported by any physical processes which would account for the formation of cyclic units.

Die Verteilung von Platingruppen-Elementen im Dumont Sill, Quebec und ihre Bedeutung f�r die Bildung von Ni-Sulfid MIneralisationen

The Dumont Sill is an Archaean komatiitic intrusion, whichDuke (1986) subdivided into a mafic (MZ) and ultramafic zone (UMZ). The UMZ comprises an upper (UPZ) and a lower peridotite subzone (LPZ) consisting of olivine + chromite cumulates and a dunite subzone (DZ) between them, consisting of olivine ± sulfide cumulates.

Formation of platiniferous sulfide horizons by crystal fractionation and magma mixing in the Munni Munni layered intrusion, West Pilbara Block, Western Australia

The Munni Munni Complex (T Nd CHUR model age 2.85 Ga), located in the west Pilbara block of Western Australia, is one of the best preserved layered intrusions in Australia. Exposed over an area of 4 X 9 km, it is composed of a lower 1,850-m-thick ultramafic zone and an overlying gabbroic zone which has a minimum thickness of 3,630 m. The ultramafic zone contains rhythmically layered dunite, lherzolite, olivine websterite, clinopyroxenite, and websterite, with orthopyroxenite, norite, chromitite, and platiniferous websterite prominent near the top of the zone. The gabbroic zone consists of gabbronorite, anorthositic gabbro, and minor anorthosite which display a pronounced tholeiitic fractionation trend. The order of appearance of cumulus mineral assemblages in the complex is olivine, olivine + clinopyroxene, clinopyroxene + olivine, clinopyroxene, clinopyroxene + orthopyroxene, orthopyroxene + chromite, and plagioclase + clinopyroxene + ?orthopyroxene. This sequence is at variance with major platinum-group element-bearing intrusions in which crystallization of orthopyroxene generally precedes that of clinopyroxene.Trace-element data, obtained on samples collected across the entire intrusion to investigate the effects of crystal fractionation and S evolution on the distribution of the platinum-group elements, show that in the sulfide-undersaturated ultramafic zone, Pt, Pd, Au, Cu, S, Se, Cs, Rb, St, and Zr behaved incompatibly and were concentrated in the melt during fractionation. The S content of the melt began to increase above the 700-m stratigraphic level of the ultramafic zone, but Pt, Pd, and Au contents increased above background levels of approximately 3 ppb to 3 ppm Pt + Pd only with the attainment of sulfide saturation at approximately the 1,830-m stratigraphic level. The concentration trends of Zr, St, Cs, Rb, and Cu paralleled that of S, but Ir and Ni largely partitioned with early crystallizing olivine and decreased in concentration with increasing fractionation. In contrast to the ultramafic zone, Pt, Pd, It, and Au have depletion trends in the sulfide-saturated gabbroic zone. Hence, the evolution of S largely governed the behavior of the platinum-group elements during the fractionation of the Munni Munni magma(s).The platinum-group element mineralization occurs immediately below the ultramafic-gabbroic contact. It resulted from the combined magmatic processes of crystal fractionation (as evidenced by increasing Cu/(Cu + Ni) ratios and incompatible element trends with stratigraphic height), and magma mixing. Two models are presented. In model 1, a hot, buoyant sulfide-saturated tholeiitic magma (containing 1,700-2,600 ppm S) rose through the density stratified platinum-group element-enriched, sulfide-undersaturated resident ultramafic magma (containing 530 ppm S) until reaching its own density level near the top of the chamber, where it spread out laterally for a distance of at least 12 km. Due to crystallization of plagioclase and subsequent Fe-enrichment [of the melt], the density of the gabbroic melt increased until it overturned and mixed with the platinum-group element-enriched fractionated parts of the ultramafic magma. Model 2 is similar to model 1, except that it involves the fractionation and internal mixing of one magma. In both models, magma mixing triggered sulfide saturation in the hybrid magma and established a high R factor (the silicate/sulfide mass ratio). The chalcophile platinum-group elements, due to their high sulfide/silicate partition coefficients, were scavenged by immiscible sulfide droplets which precipitated slowly in a porphyritic plagioclase websterite in a zone up to 20 m below the ultramafic-gabbroic zone contact. The platinum-group minerals comprising arsenides, sulfarsenides, tellurides, native metals, and mercury-bearing phases form small (<10 mu m) grains now enclosed within silicate minerals or along silicate grain boundaries (78% of occurrences), or are associated with chalcopyrite-pyrrhotite-pentlandite blebs (22%).The Munni Munni mineralized websterite layer is defined by discontinuities in (Pt + Pd)/ Se, (Pt + Pd)/S, (Pt + Pd)/Cs, (Pt + Pd)/Zr, and Cu/Zr ratios and Cu, Cs, Zr, Sr, S, and Se concentrations. Hence, these parameters together with information on the intercumulus component to the cumulates are useful for indicating potentially mineralized units in layered intrusions. Sulfide saturation also occurred along the basal contact of the ultramafic zone, but platinum-group element concentrations (< or =92 ppb Pt + Pd) are low since sulfide precipitation was probably induced by rapid cooling and the sulfides equilibrated with a small volume of silicate melt, implying a low R value. The different settings of sulfide mineralization in the Munni Munni Complex illustrate that timing and physical characteristics of the sulfide-saturation event are critical for the development of platinum-group element-enriched sulfide units in layered intrusions.

Zambales ophiolite, Philippines I. Geology and petrology of the critical zone of the Acoje massif

The Acoje massif constitutes the northernmost massif of the Zambales Ophiolite Complex, an assemblage of relatively intact fragments of a Mesozoic oceanic lithosphere. The critical zone of the massif consists of interlayered dunite, harzburgite, lherzolite, wehrlite, pyroxenite and feldspathic units which grade upsection into gabrronorite and gabbro cumulates. This sequence occurs between highly deformed residual harzburgite and a 4 to 5.5 k thick cumulate and isotropic gabbro sequence. The critical zone can be subdivided into a 540 m thick ultramafic zone and a 390 m thick mafic (feldspathic) zone. The boundary between these zones is marked by the first appearance of cumulus plagioclase. The paragenetic sequence that best describes the cumulus mineralogy is chromite, chromite + olivine, chromite + olivine + orthopyroxene, chromite + olivine + clinopyroxene (± orthopyroxene), olivine + clinopyroxene + plagioclase (± orthopyroxene, ± amphibole). The lithologie and chemical (including ƒO 2) variation across the critical zone reflects multiple magma influx, assimilation of tectonized harzburgite, magma mixing, fractional crystallization, and intercumulus melt/crystal interactions in a sub-oceanic magma chamber. Therefore, this critical zone represents the basal cumulate sequence of the Acoje massif, and not the uppermost portion of the residual mantle. There is a general trend of decreasing of deformation from the bottom to the top of the critical zone. This is attributed to the rheological inhomogeneity of the partially crystallized basal cumulate sequence during deformation. Geothermometric estimates based on the Mg-Fe 2+ exchange between olivine and spinel indicate equilibration to subsolidus temperatures.

Zambales ophiolite, Philippines

The Acoje massif is part of a mafic-ultramafic complex, the Zambales ophiolite, and is a fragment of Mesozoic oceanic crust. This paper documents the occurrence and phase relations of sulfides and associated phases in the critical zone of the Acoje massif. The Acoje critical zone (ACZ) forms the basal cumulate sequence of the massif and consists of a variably serpentinized lower ultramafic zone and a relatively less altered upper mafic zone. Two distinct sulfide associations have been identified: (1) a troilite (±pyrrhotite)-dominated group hosted by the mafic zone and (2) a pentlandite-dominated group hosted by the ultramafic zone. Troilite-dominated assemblages represent the original mineralogy of magmatically precipitated sulfides in the entire cumulate sequence. The pentlandite-dominated group appears to have evolved from the primary magmatic sulfides during low-temperature re-equilibration. The paragenetic evolution from the magmatic assemblage to the low-temperature assemblage appears to have proceeded as follows: (1) S-rich hexagonal pyrrhotite+pentlandite+chalcopyrite (or cubanite)+magnetite, (2) S-poor hexagonal pyrrhotite+pentlandite+intermediate solid solution (iss) phase (and/or cubanite)+magnetite, (3) troilite (or mackinawite)+pentlandite+iss+magnetite, (4) troilite (or mackinawite)+pentlandite+iss+native Cu+magnetite, (5) pentlandite+native Cu+magnetite, and (6) pentlandite+native Cu+Fe-Ni alloy+magnetite. This evolutionary trend, in conjunction with the observed textural, chemical, and sulfur-isotopic relations, indicates that the native metal and alloy phases in the ACZ were produced by low-temperature reduction of the primary magmatic sulfides. Correlations between sulfide assemblages and coexisting silicate-hydrosilicate-oxide assemblages further indicate that this alteration occurred during retrograde serpentinization of the Acoje massif. Two end-member models that could explain the inferred low-temperature mineralogic evolution of the ACZ sulfides are described: (1) an isothermal reduction model and (2) a non-isothermal equilibration model. Both isothermal and non-isothermal effects apparently were involved in the development of variably reduced sulfide-oxide-metal assemblages from the initial magmatic sulfides.

Platinum-group minerals in chromite seams of the Stillwater Complex, Montana

Platinum-group minerals are present as inclusions in both disseminated and massive chromite but are modally more abundant in massive chromite, in the ultramafic zone of the Stillwater Complex. They are small, generally less than 20 mu in diameter and characterized by euhedral and subhedral habits. Their distribution is random (not crystallographically controlled) within the chromite host. The platinum-group mineral inclusions may occur with silicate and base metal sulfide inclusions in an unfractured chromite host. All identified inclusions in unfractured chromite are laurite, (Ru, Os, Ir)S 2 . The Ru/(Ru + Os + Ir)(= Ruranges from 0.92 to 0.83 with Ru Os substitution more extensive than Ru Ir. Both Pd and Rh abundances in laurite are less than 2 wt percent. No Pt was detected. At present, the chemical data do not indicate a correlation between stratigraphic height and the Ru ratio.Several Pt-, Pd-, and Rh-bearing interstitial minerals and a single unusual inclusion phase have been identified in the A chromite seam at West Fork and the B seam at Mountain View. The interstitial platinum-group minerals are sperrylite and isoferroplatinum, whereas the unusual inclusion is a polyphase intergrowth of Pt-Pd-Ni arsenides with chalcopyrite and pent-landite.Whole-rock platinum-group element analyses of the chromite seams range from a few ppb to nearly 16 ppm with Pd > Pt > Rh > Ru > Ir (Os was not analyzed). This is in opposition to the data for the inclusions which show high Ru, Ir, and Os, low Rh and Pd, and no detectable Pt.Thus, in the Stillwater magma chamber, Ru-, Ir-, and Os-bearing minerals appear to have precipitated early and have been included in chromite. In contrast, Pd, Pt, and Rh apparently remained in solution in the magma and are in phases that precipitated late and are interstitial to chromite.

The role of crustal contamination in magma evolution through geological time

The temperatures of the mantle-derived magmas that have ascended through an intruded continental crust have decreased through geological time. A possible consequence of this change is that crustal contamination could have been a more significant factor in magma genesis in the Precambrian than in the modern Earth. Calculations suggest that a peridotitic komatiite magma (28% MgO) has the potential to assimilate more than three times the amount of crust that can be assimilated by basalt. The greater potential for assimilation by komatiite is a consequence of the large crystallization interval ( ∼ 400°C) and the high heat of fusion of olivine. In favourable circumstances thermal constraints allow basaltic and andesitic magmas containing more than 50% crust to be derived by an assimilation and fractional crystallization (AFC) process from a komatiite parent. AFC in komatiites can generate derivative magmas that would not be expected in the modern Earth. Substantial contamination of komatiite with average Archean upper crust or tonalite-trondjhemite gneiss can generate silica-rich Mg-rich basaltic komatiites and Mg-rich andesites. Contamination may explain why many basaltic komatiite lavas cannot be generated by olivine fractionation of the komatiites with which they are closely associated. Highly contaminated magmas crystallize in the order ol-opx-plag-cpx and are appropriate parental magmas for the ultramafic zones of large Precambrian layered intrusions such as the Bushveld and Stillwater Complexes. Whereas modern basalts are unlikely to be substantially contaminated by mafic or ultramafic rocks, komatiites are also capable of assimilating large amounts of gabbro, mafic granulite and some ultramafic rocks such as pyroxenite. If the Archean lower crust were made of such rocks AFC processes with a komatiite parental magma can generate iron-rich and alumina-rich basaltic magmas. Such magma compositions would crystallize in the order ol-plag-cpx/opx at low pressure and bear some resemblance to the magmas required to form the anorthositic parts of intrusions such as Bushveld and Stillwater. A similar process could also have had a role in generating the parental magmas to the massive Proterozoic anorthosite massifs, by assimilation of lower crust into picritic parent magmas.