The McIntosh Layered Troctolite-Olivine Gabbro Intrusion, East Kimberley, Western Australia

The well-preserved ?lower Proterozoic McIntosh intrusion consistsof 96 macro-layers with a total stratigraphic thickness of about6 km. The lowermost rocks in this possible cone-shaped intrusionare hidden, and the roof and the upper layers were removed byerosion. The layered sequence is dominated by 40 bimodal cyclicunits of troctolite and olivine gabbro. Minor gabbronorite layersoccur throughout the sequence, and are more abundant and morefractionated higher in the sequence. Six imperfect megacycicunits are developed in the upper 2700 m, each unit consistingof several troctolite-olivine gabbro cyclic units followed bya Fe-Ti oxide-bearing gabbronorite. The overall cumulus crystallizationorder in each megacyclic unit was plagioclase first, closelyfollowed by olivine, then augite, orthopyroxene, and magnetitesuccessively. Cryptic composition data for troctolites and olivine gabbrosshow a slight overall decrease of 10 mol per cent An and Fofrom the base to the top of the layered sequence (approximateranges An_80–70 and Fo_78–68). Several major fluctuationsoccur however, and are generally associated with the oxide gabbronorites,which are significantly more fractionated than the adjacentlayers (plagioclase An_53–60, orthopyroxene Mg_52–69Each fluctuation comprises a marked progressive discontinuity(rapid normal fractionation) followed by a gradual to rapidregressive discontinuity (or reversal) in the overlying troctolitesand olivine gabbros. Apparently, such marked progressive discontinuitieshave not been described in layered intrusions. A chilled margin and the overall composition of the intrusionsuggest an olivine tholeiite parent magma, inferred to havecrystallized at P 6 kb, relatively low _PH2O and high f_O2 (>NNO buffer). The troctolite-olivine gabbro cyclic units areinferred to have formed by fractional crystallization of periodicadditions of new magma. However, the oxide gabbronorites seemtoo fractionated relative to the underlying layers to have formedby conventional crystal fractionation mechanisms, and they couldhave resulted from a ‘liquid fractionation’ processin which fractionated residual magma, instead of rising, periodicallybecame denser and ponded on the temporary floor (a density crossover).Gradual, reversed cryptic trends in the cyclic units above theoxide gabbronorite layers may reflect mixing of this fractionatedmagma with successive magma additions.     

Small Scale Primary Cumulus Igneous Layering in the Kalka Layered Intrusion, Giles Complex, Central Australia

Small scale primary cumulus layers occur in various parts ofthe Kalka Intrusion in central Australia, and generalized modelshave been developed from them to explain the formation of primarysmall scale layering features in stratiform gabbroic intrusions.Primary small scale layering appears to be produced by repeatedbursts of discontinuous nucleation, followed by differentialgravitational settling of pyroxene or olivine relative to plagioclase.Dependent on the distance of the nucleation zone above the pileof settled cumulus crystals, the layering may be mineral gradedor isomodal as a result of either incomplete or complete separationof cumulus pyroxene or olivine and plagioclase on settling.Reverse or continuous mineral grading may result from settlingoverlap of plagioclase by pyroxene or olivine formed in laternucleation cycles. Conditions of continuous nucleation willgenerally result in massive unlayered sequences, irrespectiveof any differential settling which may occur between phases.     

The geochemistry of pyroxenes from the lower layered series of the Jimberlana intrusion,Western Australia

Most of the Al³⁺ entering the pyroxenes does so by substituting for tetrahedral Si⁴⁺. This creates a charge imbalance that requires the simultaneous entry of Cr³⁺, Ti⁴⁺, Fe³⁺ or Al³⁺ into octahedral sites. Cr³⁺, because of its high crystal field stabilisation energy (CFSE), is the most important of these elements to enter the early-formed pyrosenes but it is replaced by Ti⁴⁺ later in fractionation when the Cr³⁺ content of the melt becomes depleted. The dependence of Cr³⁺ and Ti⁴⁺ on charge balance controls their partition between coexisting pyroxenes and olivines. Ca-rich pyroxene which contains more Al³⁺ than Ca-poor pyroxene also has more Ti⁴⁺ and Cr³⁺ whereas olivine, which contains negligible Al³⁺, has low Cr³⁺ and Ti⁴⁺. The Al³⁺ content of pyroxenes is influenced by changes in P, T, \(a_{{\text{SiO}}_{\text{2}} }\) and \(a_{{\text{Al}}_{\text{2}} {\text{O}}_{\text{3}} }\) of the magma and by the nature of the ion providing charge balance in the octahedral site. Of these \(a_{{\text{SiO}}_{\text{2}} }\) is dominant and variations in the Al³⁺ content of the Jimberlana pyroxenes correspond closely with the expected changes in the \(a_{{\text{SiO}}_{\text{2}} }\) of the melt. The substitution of divalent ions, such as Mn²⁺ and Ni²⁺, in the pyroxene lattice is by replacement of Fe²⁺ or Mg²⁺ in the octahedral M ₃ and M ₂ sites and is therefore independent of charge balance. If there are no size restrictions, the principal factor to be considered is the CFSE the ion receives in octahedral co-ordination. Ni²⁺, which receives a high CFSE, partitions strongly between the early-formed pyroxenes and olivines and therefore becomes depleted in the magma with fractionation. Conversely Mn²⁺, which receives zero CFSE, concentrates in the magma with fractionation and becomes a more important substitute in the later-formed pyroxenes. Its geochemical behaviour is controlled by its size. The narrow miscibility gap of the Jimberlana pyroxenes and the high En content of the Ca-poor pyroxenes at the bronzite pigeonite changeover suggest that these pyroxenes crystallised at a higher temperature than pyroxenes of comparable composition from other intrusions.     

The Skaergaard Layered Series, Part VII: Sr and Nd Isotopes

The initial isotopic ratios of strontium and neodymium in theSkaergaard Layered Series vary both vertically and laterally,on every scale from the intrusion as a whole down to coexistingminerals in a single rock. The magma that filled the Skaergaardchamber was contaminated to various degrees with the metamorphicrocks through which it rose and was never completely homogenizedafter being intruded. The contamination was most pronouncedin contact zones and aureoles around rare xenoliths. The greaterconcentrations of lithophile trace elements in the Upper BorderSeries was previously attributed to assimilation of buoyantfragments of gneiss that collected under the roof, but mostof the rocks of the Upper Border Series are isotopically indistinguishablefrom those of the Layered Series. It is doubtful, therefore,that this part of the intrusion assimilated much more of themetamorphic basement than did the rest of the magma. Similarly,the marked increase in the concentrations of excluded elementsin the upper part of the Layered Series is not matched by achange in the isotopic character of the rocks and cannot beattributed to a later influx of new magma. Analyses of mineralsseparated from rocks with exceptionally mafic or felsic modalcompositions revealed marked inhomogeneities in the isotopiccompositions of their constituent minerals. For example, coexistingplagioclase and pyroxene from closely associated anorthositesand pyroxenites have very different initial isotopic ratiosof both strontium and neodymium. The same is true of mafic andfelsic layers in modally graded gabbros. These differences areunrelated to the low-temperature alteration shown by oxygenisotopes. They must have been introduced when the original gabbrowas largely crystallized and underwent local metasomatic replacementby nearly mono-mineralic mafic and felsic assemblages. KEY WORDS: Nd isotopes; Skaergaard; Sr isotopes     

The Skaergaard Layered Series: I. Structure and Average Compositions

Re-examination of the Skaergaard Layered Series in the lightof more extensive field work and sampling shows that the lithologiczones vary laterally as well as vertically, in both their bulkchemical composition and their mineralogical assemblages. Themargins of the zones differ from both the central part of theLayered Series and Marginal Border Series in being richer inFeO^*, TiO₂, K₂, P₂O₂, and most excluded elements. Mafic mineralstend to be more abundant and more iron-rich, plagioclase ismore albitic and more strongly zoned, and apatite and biotiteare more abundant near the margins. When the average compositions of successive zones are compared,the abundances of most excluded components are seen to declineupward as far as Middle Zone then reverse their trends and increasethrough Upper Zone. P₂O₅ and K₂O are negatively correlated inUpper Zones B and C, owing, perhaps, to separation of immisciblefelsic liquids from the iron-rich magma. No evidence has beenfound for introduction of a new batch of less differentiatedmagma. Layered rocks have an average composition that is more maficthan that of homogenous rocks at the same level. Blocks thatfell from the roof have the opposite relation; they are greatlyenriched in felsic components compared to the original compositionsof the Upper Border Series from which they came. Although some of the compositional variations may be consistentwith differing degrees of fractionation of trapped liquids,no consistent relation has been found between the degree offractionation and rates of crystal accumulation or cooling atthe walls. Contamination with the metamorphic wall rocks, eitherby assimilation or by hydrothermal fluids, seems to have hadonly local effects and cannot account for the large-scale variations.At least some of the compositional differences must have resultedfrom late-stage processes that redistributed certain componentsafter the intrusion reached advanced stages of solidification.     

Geochemical exploration for base and precious metal sulphides associated with the Jimberlana Dyke, Western Australia

A geochemical exploration case history is described, dealing with the search for Ni and Cu sulphides, platinoids and chromite associated with the Jimberlana Dyke in Western Australia. The Jimberlana Dyke is a strongly differentiated mafic-ultramafic intrusive exhibiting similarities with the Great Dyke of Rhodesia.Soil sampling was used as the main search technique, the minus 80-mesh fraction of the near-surface soils being analysed for Ni, Cu, Co and Cr by atomic absorption spectrophotometry. Evaluation methods relied initially on visual inspection of element profile plots. Follow-up soil sampling and auger drilling were carried out in anomalous areas to define targets for deeper drill-testing. Subsequent statistical evaluation of the geochemical data has largely confirmed the initial interpretation but has in addition helped to elucidate the excellent correlation existing between the near-surface soil sampling and bedrock geochemistry.The programme was successful in locating a number of occurrences of sulphide mineralization, many of which contained copper and nickel sulphides. The most significant anomaly was at Bronzite Ridge, being related to a small pipe-like body of sulphides with grades up to 2% Ni and 2% Cu. All sulphide intersections tested recorded low values for the platinoid metals. No significant chromite segregations were located and high Cr values in soils (> 1%) can be attributed to a combination of lateritic and eluvial concentration processes.Although no economic mineralization was located the programme demonstrated the extreme sensitivity of Cu as an indicator of sulphides even through moderately thick soil cover.     

Isotopic Studies of Processes in Mafic Magma Chambers: I. The Kiglapait Intrusion, Labrador

The initial Nd and Sr isotopic ratios of cumulate rocks andminerals of the Kiglapait intrusion are pertinent as indicatorsof the processes that affected the Kiglapair magma while itwas resident in the crust. A Sm–Nd mineral isochron indicatesthat the crystallization age of the intrusion is 1305?22 millionyears. The initial _Nd values range from—1?6 in the LowerZone to 6 in the Upper Zone and correlate with the anorthitecontent of plagioclase. The initial ₈₇Sr/⁸⁶Sr ratios are 0.70407to 0.70433 in the Lower Zone, and increase monotonically up-section(decreasing plagioclase anorthite content) to 0.7068 in theUpper Zone. These variations are attributed to assimilationof roof rock concurrently with crystallization. It is evidentthat replenishment of the chamber with uncontaminated magmawas important during the accumulation of the lower zone rocks,but did not occurduring crystallization of the Upper Zone. Amathematical model relating isotope ratio shifts to the relativerates of crystallization, assimilation, and replenishment ispresented. It is estimated that the rate of assimilation wasbetween 0.01 and 0.04 of the crystallization rate, and thatduring the accumulation of the Lower Zone, the rate of replenishmentwas about half of the crystallization rate. A formulation for‘assimilation efficiency’is presented that relatedthe actual amount of assimilation (determined from the isotopedata) to the thermodynamic maximum allowable amount. The assimilationefficiency of the Kiglapait intrusion is not tightly constrained,but appears to be of the order of 0.01 near the end of crystallizationand perhaps as high as 0.07 near the beginning. Further quantitativeestimates of assimilation efficiency in different intrusionsemplaced under different conditions may aid in understandingassimilation mechanisms and in assessing the role of assimilationin magmatic evolution in general. Many layered intrusions studiedto date have initial Nd isotope ratios close to the chondriticvalue (_Nd= 0), suggestive of mantle magma sources that are chemicallyundepleted, and thus different from the sources of mid-oceanridge basalt and island-arec basalt.     

Sedimentary Structures and Magma Current Velocities in the Kalka Layered Intrusion, Central Australia

Examples of reworked sedimentary structures (asymmetric rippleforms, planar cross-stratification, and a scour structure) andmodified sedimentary structures (load casts) are described fromthe layered, gabbroic Kalka Intrusion in central Australia.The reworked structures are thought to be produced by the actionof vigorous, unidirectional, localized currents of convectionalorigin. Current velocities have been estimated at between 0.001and 5 cm sec^–1 for the lower flow regime structures, andgreater than 0.1 cm sec^–1 for the scour structure. Thesecompare well with theoretical convection velocities in magmas.Pyroxene/plagioclase mobility within basaltic magmas is greaterthan sand mobility in aqueous systems. The advent of the current activity which produced the scouris associated with a change from discontinuous to continuousnucleation (Goode, 1976), probably due to the increased turbulencein the crystallization zone. Erosional depths within unconsolidaredcrystal mushes that later became anorthositic adcumulates indicatethat melt diffusion can occur through at least 1 metre of cumulusaggregate.     

Studies of Igneous and Tectonic Textures and Layering in the Rocks of the Gosse Pile Intrusion, Central Australia

The Gosse Pile mafic-ultramafic intrusion is a laycred igneousbody, the upper part of which was involved in localized ductiledeformation (low angle faulting) soon after crystallization.A complete gradation between rocks showing typically igneoustextures and layering and those showing typically tectonic,or metamorphic, textures and layering can be observed. Textures and preferred orientations in the undeformed part ofthe intrusion are very similar to those found in other layeredigneous bodies. Orthopyroxenes show very strong preferred orientationsin some parts of the body. The metamorphic textures and layering of the deformed rocksare identical to ‘flow-layering’ which is used asan essential criterion for distinguishing ‘alpine-type’from ‘stratiform’ bodies. The Gosse Pile Body thusdemonstrates that at least some of the ‘alpine-type’bodies may be produced byin situ deformation of ‘stratiform’bodies.     

Relationship between the Layered Series and the overlying evolved rocks in the Bjerkreim-Sokndal Intrusion, southern Norway

The Bjerkreim-Sokndal layered intrusion (BKSK) consists of a > 7000-m-thick Layered Series comprising anorthosites, leuconorites, troctolites, norites, gabbronorites and jotunites (hypersthene monzodiorites), overlain by an unknown thickness of massive, evolved rocks: mangerites (hypersthene monzonites; MG), quartz mangerites (QMG) and charnockites (CH). The Layered Series is subdivided into six megacyclic units that represent the crystallisation products of successive major influxes of magma. We have studied a ca. 2000-m-thick section that straddles the sequence from the uppermost part of the Layered Series to the QMG in the northern part of the intrusion. Mineral compositions in 37 samples change continuously in the lower part of the sequence up to the middle of the MG-unit (plagioclase An37-18; olivine Fo40-7; Ca-poor pyroxene Mg#57-15; Ca-rich pyroxene Mg#65-21). Above this compositions are essentially constant in the upper part of the MG-unit and in the QMG (An21-13; Fo6-4; Mg#opx17-13; Mg#cpx25-20). The amount of interstitial quartz and the amount of normative orthoclase, however, both increase systematically upwards through the QMG-unit, implying that these rocks are cumulates. There is no evidence of a compositional break in the MG-QMG sequence that could reflect influx of relatively primitive magma.

Two types of QMG/CH are known in the uppermost part of BKSK. Olivine-bearing types are comagmatic with the underlying Layered Series; the studied stratigraphic sequence belongs to this suite. Two-pyroxene QMG and amphibole CH define a separate compositional lineage related to jotunites. An intrusive unit of dominantly two-pyroxene QMG is discordant to the olivine-bearing jotunite-MG-QMG sequence near Rapstad, confirming the presence of two compositionally distinct suites of QMG and related lithologies in the upper part of BKSK.

A xenolith-rich unit near the olivine-bearing MG-QMG boundary represents a major collapse of the roof of the magma chamber during the final stages of crystallisation.     

Crystallization History of the Upper Layered Series, Kap Edvard Holm, East Greenland

Field studies of the Upper Layered Series of the Kap EdvardHolm Complex indicate that consolidation occurred by accumulationof primocrysts near the floor of the magma chamber; nucleationoccurred continuously near the roof of the chamber and intermittentlynear the base. Over an exposed thickness of 4000 m the plagioclaseshows no detectable cryptic variation, and a hypothesis of combinedliquid and crystal fractionation processes is invoked to explainthis feature. The crystallization history of mafic intrusivesis considered on the basis of available liquidus and soliduscurves for anhydrous and water–saturated magmas.     

鲁西下寒武统层状金矿成矿地质特征及找矿方向

下寒武统层状金矿是鲁西地区继归来庄隐爆角砾岩型金矿以后,近年来所发现的又一新类型金矿.该类金矿赋存于寒武纪朱砂洞组灰质白云岩和白云质灰岩中.矿体产出层位稳定,矿石类型单一,矿化连续,具有广阔找矿前景和较大的资源潜力.矿床形成与中偏碱性次火山杂岩及其次火山热液作用密切相关,表现为“二长斑岩—断裂裂隙—碳酸盐岩层”三位一体的控矿机制.该类金矿最早在磨坊沟被发现和评价,因此,笔者将其称为“磨坊沟式”金矿.     

鲁西下寒武统层状金矿成矿地质特征及找矿方向

下寒武统层状金矿是鲁西地区继归来庄隐爆角砾岩型金矿以后,近年来所发现的又一新类型金矿.该类金矿赋存于寒武纪朱砂洞组灰质白云岩和白云质灰岩中.矿体产出层位稳定,矿石类型单一,矿化连续,具有广阔找矿前景和较大的资源潜力.矿床形成与中偏碱性次火山杂岩及其次火山热液作用密切相关,表现为“二长斑岩—断裂裂隙—碳酸盐岩层”三位一体的控矿机制.该类金矿最早在磨坊沟被发现和评价,因此,笔者将其称为“磨坊沟式”金矿.     

The Major Element Variation of the Layered Series of the Skaergaard Intrusion and a Re-estimation of the Average Composition of the Hidden Layered Series and of the Successive Residual Magmas

Using variation diagrams for the major elements in the layeredrocks, estimates are made of the average amounts of the variouselements in the total rock separating at successive stages.From the analyses of the chilled marginal gabbro, taken to representthe composition of the initial magma, and with the further likelyassumption that the Skaergaard intrusion is a closed system,at any rate for most of the elements, various hypotheses onthe relative volumes of the different parts of the intrusionare tested to find the one best fitting the known distributionof the elements in the observable rocks. Estimates are thenmade of (1) the overall composition of the hidden part of theintrusion by subtracting the amounts of an element in the observedrocks from the total in the initial magma, and (2) the compositionof the successive residual magmas formed as a result of thecrystal fractionation.     

The Major Element Variation of the Layered Series of the Skaergaard Intrusion and a Re-estimation of the Average Composition of the Hidden Layered Series and of the Successive Residual Magmas

Using variation diagrams for the major elements in the layeredrocks, estimates are made of the average amounts of the variouselements in the total rock separating at successive stages.From the analyses of the chilled marginal gabbro, taken to representthe composition of the initial magma, and with the further likelyassumption that the Skaergaard intrusion is a closed system,at any rate for most of the elements, various hypotheses onthe relative volumes of the different parts of the intrusionare tested to find the one best fitting the known distributionof the elements in the observable rocks. Estimates are thenmade of (1) the overall composition of the hidden part of theintrusion by subtracting the amounts of an element in the observedrocks from the total in the initial magma, and (2) the compositionof the successive residual magmas formed as a result of thecrystal fractionation.     

鲁西下寒武统层状金矿成矿地质特征及找矿方向

下寒武统层状金矿是鲁西地区继归来庄隐爆角砾岩型金矿以后 ,近年来所发现的又一新类型金矿。该类金矿赋存于寒武纪朱砂洞组灰质白云岩和白云质灰岩中。矿体产出层位稳定 ,矿石类型单一 ,矿化连续 ,具有广阔找矿前景和较大的资源潜力。矿床形成与中偏碱性次火山杂岩及其次火山热液作用密切相关 ,表现为“二长斑岩—断裂裂隙—碳酸盐岩层”三位一体的控矿机制。该类金矿最早在磨坊沟被发现和评价 ,因此 ,笔者将其称为“磨坊沟式”金矿。     

A Petrological and Geochemical Account of Subsurface Noritic Intrusion in the Western Part of Bundelkhand Massif,Shivpuri District,M.P.

Subsurface exploration for uranium in the northwestern part of Bundelkhand massif, near Khor area, Shivpuri dist., M.P., resulted in intercepting a substantial thickness of mafic rock within Bundelkhand granitoid. Intercepts of this mafic rock at various levels in the boreholes, indicate that the rock mainly occurs as dykelike intrusion and fracture-fills within Bundelkhand granite. It is essentially composed of hypersthene and plagioclase, with or without olivine, leading to the characterisation as hypersthene microdolerite, noritic dolerite and norite (±olivine), depending on the grainsize and variation from intergranular to ophitic texture. Chemically, the rock is characterised by av. 49.09% SiO₂, 2.46%TiO₂, 2.33 Fe₂O₃, 9.45% FeO, 5.75% MgO, 8.37% CaO and 0.96% K₂O. The normative composition ranges from 3.53% quartz, 46.86% plagioclase, 12.58% diopside, 19.24% hypersthene. The olivine normative samples show av. 5.65% olivine. Geochemical plots indicate an intra-plate affinity along with oceanic signature, while presence of mineralogical and normative olivine, together with the REE pattern, point towards a lower crustal or mantle source. The mineralogical and normative presence of either quartz or olivine in these mafic rocks implies that it has an intermediate character between the tholeiitic dolerite dykes and the komatiite-type ultramafics reported from Bundelkhand craton. The complex geochemical signature of the rocks also reveals that both intra-continental as well as a mixture of oceanic- to upper mantle signatures are evident. The present study is a first time report of the occurrence of this hitherto unknown noritic body at depth within the Bundelkhand granite, which has no visible surface expression. The findings may strengthen the existing concept of a continuum between Rajasthan craton in the west and Bundelkhand craton in the east, as a single protocontinent.     

The Geology of the Great Dyke, Zimbabwe: Crystallization, Layering, and Cumulate Formation in the P1 Pyroxenite of Cyclic Unit 1 of the Darwendale Subchamber

The P1 layer of the Great Dyke is an 200 m thick pyroxenitesuccession in Cyclic Unit 1 and, as the topmost lithology ofthe Ultramafic Sequence, represents the transition from ultramaficto mafic rocks. Of critical importance to this part of the stratigraphyis the strong lateral environmental change from axis to marginas a result of the flared structure of the Great Dyke. Duringthe formation of the P1 layer the axial zone was underlain bya great thickness of hot ultramafic cumulates whereas the samelayer in the marginal zone progressively offiaps the lower ultramaficlayers and is in close proximity to the underlying wall/floorrocks. Heat loss through the floor was therefore much greaterin the marginal zone than in the axis. Major lateral variations are observed, with all lithologicalunits and layers thinning towards the margins of the subchambertogether with a progressive change in the form of the cumulates.Discordant relationships towards the margin between layer types(modal, cryptic, and form) are a feature of the P1 unit whichhas also been recognized in other parts of the Great Dyke (Prendergast,1991). Pyroxene compositions show significant variations withinan overall fractionation trend and decoupling occurs betweenmajor and minor element components of bronzite, suggesting strongcompositional heterogeneity of the magma. This type of crypticlayering has not previously been described and is informallycalled ‘cryptorhythmic’ layering. Pyroxene compositional variation is related to reaction andmodification by trapped intercumulus liquid, and few mineralspreserve liquidus compositions. A similar situation must existfor most layered intrusions. The strong dependence of pyroxenecompositions on incompatible element content in the whole-rockshows that the original liquidus compositions were modifiedby postcumu-lus overgrowth and reaction with the trapped intercumulusliquid. Well-constrained data arrays indicate that most cumulatesin the P1 layer behaved as a closed system with little or nomigration of intercumulus liquid. Liquidus compositions cantherefore be deduced and the residual porosity and degree ofpostcumulus formation were modelled using a computer program.Residual porosity is shown to be between 1 and 13% (by mass).Rocks in the marginal facies have a relatively large proportionof discrete postcumulus phases but instead of representing crystallizationof trapped liquid these are shown to be mainly heteradcumulusphases, i. e., interstitial minerals that have grown largelyby adcumulus processes in equilibrium with the main body ofmagma. The heteradcumulus component can be as high as 27%. Thesephases occur as oikocrysts which give rise to a well-developednodular pyroxenite (the ‘potato’ reef). The formationof the nodules caused local redistribution of primary sulphideliquid. The liquid layers which gave rise to cumulates in the marginalfacies are shown to be enriched in iron and incompatible elementscompared with the axial zone, indicating that the P1 pyroxenitelayer formed by crystallization of a magma which was eithercompositionally stratified or exhibited a strong lateral compositionalgradient.     

A primary platinum occurrence in the Freetown Layered Intrusion, Sierra Leone

The primary source of the alluvial platinum-group minerals (PGM) of the Freetown Layered Intrusion, Sierra Leone, has been unknown since their discovery 70 years ago. This is a report on work in progress to establish a primary occurrence related to the igneous layering. New whole-rock major and minor element analyses and precious metal assays show that at key horizons there is a significant change in the silicate mineral compositions and in Cu, Cr and Ni, as well as in Pt and Pd, comparable to those encountered in other PGM-bearing layered intrusions. Anomalous (0.1 to 0.4 g/t) Pt levels occur in pyroxene troctolites, which maintain these levels along strike. Received: 30 July 1999 / Accepted: 2 December 1999     

A Strontium and Neodymium Isotopic Investigation of the Fongen--Hyllingen Layered Intrusion, Norway

The Fongen—Hyllingen Intrusion, situated 60 km SE of Trondheim,Norway, is a synorogenic layered mafic intrusion of Caledonianage . The intrusion is divided into four evolutionary stages based on cryptic variations: StageI—a basal reversal; Stage II—unchanged mineral chemistryor slight normal evolution; Stage III—a gradual regression;Stage IV— a strong normal fractionation trend Magma replenishmentdominated during most of the crystallization, i.e. during StagesI, II and III Replenishing magma was more dense than resident,evolved magma, and continuing influx eventually caused a compositionallystratified magma column to form. Cryptic lateral variation isan important feature in the southern part of the complex andformed by in situ crystallization from a stratified magma alongan inclined floor, where modal layering formed parallel to thecrystallization front. Initial Sr- and Nd-isotopic ratios inthe cumulates vary as a result of assimilation of country rockand subsequent mixing between uncontaminated, replenishing magmaand contaminated, resident magma. The parental magma had a moderatelydepleted isotope composition, relative to Bulk Earth, with _Nd=584and Sr_i=070308, whereas the main contaminant was a partialmelt of metapelitic country rock with _Nd=-874 and Sr_i=07195(Sr_i is the initial ⁸⁷Sr/⁸⁶Sr). Sr_i in the analysed cumulatewhole-rock samples ranges from 070308 to 070535 and initial_Nd ranges from. 158 to 584. There is a strong correlationbetween mineralogical composition and isotopic trends in mostof the cumulates: the most primitive samples are the least contaminated,as reflected by relatively high ed and low Sr,, and more evolvedsamples have progressively lower eNi and higher Sry A gradualregression of several hundred metres thickness characterizesStage III; stratigraphically upwards mineral compositions becomemore primitive and isotope compositions more depleted (higher_Nd and lower Sr_i), implying a process of. progressive mixing-inof replenishing, primitive and uncontaminated magma. Magma influxin Stage III took place by fountaining, whereas magma additionwas more tranquil in the earlier stages. The fountaining influxentrained resident, relatively evolved and contaminated magma,resulting in a hybrid magma which ponded at the floor. Duringprolonged magma addition with concomitant crystallization, thelowermost magma layer was replaced by progressively more primitivehybrid magma, creating a gradual regression in the crystallizingcumulate sequence. A detailed two-dimensional study revealslateral variations in mineral compositions both at the baseand top of Stage III, whereas lateral variations in Sr- andNd-isotopic compositions are present at the top, but not atthe base. This implies that the lowest crystallizing part ofthe magma column was essentially isotopically homogeneous, butcompositionally stratified, before influx in Stage III. Isotopicgradients in the magma were strong close to the roof, wheremost of the assimilation occurred, and decreased downwards,merging into isotopically homogeneous magma. This stratifiedsystem was destroyed by turbulent mixing between replenishingand resident magma during fountaining influx in Stage III, anda new stratification was established with both an isotopic anda compositional gradient. After the final influx, crystallizationcontinued in an essentially closed system, in which the remainingmagma column eventually became homogenized, as magma layersmixed when their densities converged owing to release of buoyant,residual liquid during fractional crystallization. Corresponding author