assessment of undiscovered nickel sulphide resources kalgoorlie terrane western australia part 1 deposit and endowment density models abstract the use of mineral deposit density regression models to estimate the number of undiscovered deposits is gaining acceptance in mineral resources assessments the deposit density regression models currently in use are based on well established power law relationships between deposit density deposits km 2 and the areal extent of the host rocks in well explored regions control areas worldwide although these generalized or global deposit density models can generate guideline estimates that are useful at the terrane scale locally derived terrane based deposit density regression models may potentially yield more relevant estimates at the terrane scale using 12 selected komatiite defined control areas in the kalgoorlie terrane western australia we found that the size km 2 of the control areas had power law relationships with i nickel sulphide deposit density and ii nickel endowment density nickel metal km 2 regression analyses showed that both power law relationships are statistically significant at the 5 level this suggests that nickel sulphide deposit and endowment density models could be used to estimate the number of undiscovered nickel sulphide deposits and amount of nickel metal endowment in less explored komatiites in the kalgoorlie terrane this study shows that global geological relationships can be viably downscaled onto local geological terranes thereby supporting the hypothesis that the processes of mineral deposit formation and preservation are scale independent and self similar

the chemical and mineralogical variations in the nickel mineralization associated with the kambalda dome western australia the volcanic peridotite associated nickel deposits at kambalda western australia consist of a number of orebodies or shoots located on the flanks of a broad domal structure within an archean greenstone belt in the yilgarn block chemical and mineralogical analyses of carted ore samples demonstrate that there is a systematic variation in sulfide bulk composition and mineralogy between shoots this is shown by positive correlations between the ni tenor of the shoots and their co cu pyrite and spinel contents these variations permit division of the shoots into three broad groups occupying distinct geographic positions in relation to the kambalda dome the variation in ni tenor between shoots is suggested to be caused by oxidation occurring prior to or during extrusion resulting in disequilibrium in f sub o 2 between the sulfide and silicate melts as equilibrium is reapproached oxidation of the sulfide proceeds resulting in a change in the partitioning behavior of fe and consequently the ni content of the sulfide melt increases tapping of the silicate sulfide melts at different degrees of departure from equilibrium could account for the observed compositional variation between the shoots emplacement of the sulfides is interpreted to have taken place as a single eruptive event the coincidence of spinel rich zones with high pyrite and cr contents at most major sulfide rich sulfide poor interfaces e g between massive and disseminated sulfides within the ore zone suggests that post extrusive but premetamorphic oxidation of the sulfides occurred at these positions this oxidation is due to disequilibrium in f sub o 2 between the silicate and sulfide during cooling and crystallization of the magma

tectono stratigraphy of late archaean greenstone terranes in the southern eastern goldfields western australia abstract late archaean greenstone terranes in the southern part of the eastern goldfields are regionally extensive faultbounded entities defined on the basis of small but distinct differences in stratigraphy and structure the greenstones comprise volcano sedimentary successions that were all deposited at the same time c 2720 2675 ma on sialic crust hence the terranes are interpreted to represent contemporaneous probably adjacent basins these basins were affected by the same history of granite plutonism and regional deformation c 2675 2620 ma early subhorizontal deformation is poorly understood but includes at least locally significant thrust stacking d1 with a south to north movement direction this was followed by regional east west extension affecting all supracrustal successions above a province wide detachment now lying at depths between 4 and 7 km this detachment may have evolved from the greenstone basement contact but only after extensive reworking clastic basins developed along or above major extensional faults but no volcanism is associated with this extension subsequent east west d2 shortening resulted in regional scale thrusting folding and associated late stage transcurrent fault movements the d2 event is contemporaneous with a major thermal event represented by voluminous granite emplacement c 2660 ma and associated peak regional metamorphism in the greenstone succession predominantly monzo granite derived from older sialic crust was emplaced throughout the period of basin formation and deformation early synvolcanic granite c 2719 2710 ma is a minor component late synvolcanic granite c 2685 2675 ma is widespread and syn d2 granite c 2660 ma is abundant there are a few small late tectonic intrusions c 2640 ma to 2600 ma the lateral lithostratigraphic persistence of the contemporaneous basins the syn d2 low pressure regional metamorphism and the province wide monzo granite magmatism suggest that the greenstone basins possibly developed in a tectonic setting similar to that of failed rifts above aborted spreading centres the basins closed in response to far field compressional stresses

internal stratigraphic architecture of the komatiitic dunite hosted mkd5 disseminated nickel sulfide deposit mount keith domain agnew wiluna greenstone belt western australia the mkd5 nickel deposit is hosted by the mount keith ultramafic complex mkuc a dunite body of komatiitic affinity located in the agnew wiluna greenstone belt western australia the internal architecture of the mkuc comprises seven distinct internal units that range from extreme adcumulate dunite to relatively fractionated pyroxenitic and gabbroic horizons the mkuc is divided into three packages of units the main adcumulate domain mad which is situated in the lower portion of the complex contains the bulk of disseminated nickel sulfide and is dominated by coarse adcumulate olivine textures overlying the mad is an upper fractionated zone which is dominated by mesocumulate to orthocumulate peridotite with domains containing oikocrystic pyroxenite and gabbroic lenses an aerially restricted unit comprising texturally and chemically distinct olivine cumulate rocks is known as the western mineralized zone wmz the fractionation trend between the mad and upz indicates a westerly facing for the mkuc conformable with the bounding stratigraphy in the mkuc truncation of some of the uppermost internal stratigraphic units by the hangingwall contact indicates that the unit has undergone structural modification since its emplacement and suggests that a proportion of the upper section has been removed by faulting furthermore vertical and lateral textural transitions within the internal stratigraphy suggest that the mkuc excluding the wmz was emplaced from an essentially continuous magma flow with the mad representing the period of highest magma flux and the major constructional period of emplacement conversely the wmz is interpreted to represent a later pulse of ultramafic magma emplaced stratigraphically above the main mkuc comparison of the mkuc with other komatiitic dunites from both within the agnew wiluna greenstone belt and worldwide indicates that despite apparent geometrical differences between lens and sheet komatiitic dunites a broad facies architecture can be defined we suggest that the differences in geometry are related to differing degrees of flow localization within dunitic units and are a function of both differences in the thermal characteristics and the degree of litho facies heterogeneity of the enclosing lithologies

a new method for spatial centrographic analysis of mineral deposit clusters abstract a centrographic method for analysing mineral deposit clusters is illustrated using the komatiite hosted kambalda nickel sulphide deposit cluster yilgarn craton western australia in this method the standard distance circle divides the cluster into a more endowed inner part and a less endowed peripheral part the standard deviational ellipse another centrographic object depicts the preferred northwest southeast trend of nickel orebodies at kambalda weighted centrography shows that nickel endowment is greater in the eastern than western part of the cluster the spatio geometric interaction of the circle and ellipse splits the cluster into several partitions the relative concentration of nickel orebodies or endowment within a partition in relation to their concentration within the entire cluster is termed capture efficiency komatiite areal trace exhibits higher nickel orebody capture efficiency than spatio geometric partitions however some spatio geometric partitions exhibit nickel endowment capture efficiencies comparable to that of komatiite furthermore nickel orebody and endowment capture efficiencies of komatiite are elevated only within the standard distance circle these results suggest that at kambalda i the standard distance circle is a prime window for understanding the komatiite hosted nickel system and ii spatio geometric partitions are plausible locales for spatial analysis of nickel orebodies and endowment the proposed centrographic method is potentially useful in mineral resource estimations and mineral exploration targeting

ruthenium chromium variation a new lithogeochemical tool in the exploration for komatiite hosted ni cu pge deposits we present a new lithogeochemical method to target prospective komatiites that may host ni cu pge deposits the new methodology is based on the geochemical properties of ruthenium ru and chromium cr elements that are immobile under most conditions it relies on a restricted number of carefully selected representative samples and is applicable in highly altered terrains ruthenium is a platinum group element pge that exhibits contrasting geochemical behavior in sulfide saturated and sulfide undersaturated komatiites similarly to other pges ru shows highly chalcophile behavior in magmas that equilibrate with an immiscible sulfide phase however ru is also compatible in chromite in sulfide undersaturated systems if we consider cr concentration as an index of chromite abundance in chromite saturated komatiites we observe that ru increases or decreases systematically with increasing cr according to the sulfide saturation state of the magmatic system in rocks that crystallized from sulfide saturated melts ru contents decrease with increasing cr conversely in rocks that crystallized from sulfide undersaturated melts ru contents increase with increasing cr as a result on the basis of the ru cr variation it is possible to discriminate whether a komatiite melt equilibrated with a sulfide liquid during crystallization the strength of this method compared to previous pge based lithogeochemical techniques derives from combining the traditional use of the geochemical properties of a highly immobile and chalcophile element that records the ore forming process ruthenium with the occurrence of a mechanically and chemically resistant mineral phase chromite which is generally preserved in highly altered komatiites

deposit density of tungsten polymetallic deposits in the eastern nanling metallogenic belt china abstract estimation of the number of undiscovered mineral deposits is critical for quantitatively assessing mineral resources traditional statistic methods are limited to analyzing the spatial distribution of mineral deposits which can be considered spatial point processes because of complexity self organized criticality and singularity properties in this study spatial statistic methods including average nearest neighbor distance fry analysis k function and fractal analysis are used to quantify the agglomeration and fractal characteristics of tungsten polymetallic deposits in the eastern nanling metallogenic belt enmb local singularity analysis for w sn mo bi be association is used to delineate the permissive areas for tungsten deposits the results show that tungsten polymetallic deposits are distributed with fractal clustering in region while at random in permissive areas the summary function comparison shows that the k function analysis based on a heterogeneous process may be more robust than on a homogeneous k function when quantifying the distribution of mineral deposits especially for endogenic deposits furthermore the concept of mineral equivalent is introduced in this study and the combination of mineral equivalent and radial density model for mineral deposits mitigates the problem of ignoring the proportion of large medium and small sized deposits in the uniform deposit density model the estimation results indicate that there are still good future prospects in this region this research not only extends the application of spatial statistics in quantifying the spatial distribution of mineral deposits but also provides a new approach to estimate the number of undiscovered mineral deposits

combining fractal analysis of mineral deposit clustering with weights of evidence to evaluate patterns of mineralization application to copper deposits of the mount isa inlier nw queensland australia the clustering of mineral occurrences and their spatial associations with particular geological features are critical aspects of mineral distributions for exploration and understanding ore genesis variations in the degree of clustering of mineral occurrences or geological features can be measured by fractal dimensions obtained from a shifting box counting method spatial associations between mineral occurrences and geological features can be quantified by the weights of evidence wofe method using the contrast value which increases with the strength of the spatial relationship a new method is proposed to evaluate mineral occurrence distributions by combining the power of fractal analysis of clustering with the wofe approach the method compares the correlation between the variation in degree of clustering of mineral occurrences and a geological feature in a study area with the contrast value of the same feature the possible outcomes can be simplified into four scenarios depending on whether the correlation in variation of clustering and the contrast are high or low respectively each outcome has specific exploration implications if either a high correlation in variation of clustering or a high contrast value is obtained the geological feature can be used for exploration targeting the integrated fractal and wofe approach is applied to copper occurrences in the proterozoic mount isa inlier nw queensland australia which hosts large numbers of copper deposits 1 869 occurrences including the world class mount isa copper deposit variation in clustering of copper occurrences has a positive correlation with variation in clustering of fault bends r 0 823 fault intersections r 0 862 and mafic rocks r 0 885 wofe results indicate that the copper occurrences are spatially associated with fault intersections and bends and with mafic rocks analyses were carried out separately for the two major lithostratigraphic sequences in the inlier the eastern and western successions the western succession copper occurrences are apparently more clustered than those of the eastern succession which may reflect a lower degree of exploration and or geological factors the association of copper occurrences with mafic rocks compared with fault bends and intersections is greater in the eastern succession which may reflect genetic factors correlations in the variation of clustering of mineral occurrences and geological features have a linear relationship with the contrast values and the spatial association between all geological features and copper occurrences constitute high correlation high contrast cases the linear relationship suggests that the geological features that control the clustering of the copper occurrences could be the same features that control their localization

spatial statistical analysis of the distribution of komatiite hosted nickel sulfide deposits in the kalgoorlie terrane western australia clustered or not the spatial distribution of mineral deposits is a critical component of predictive estimation of undiscovered mineral resources nickel sulfide deposits in the kalgoorlie terrane of western australia the world s premier province for komatiite hosted nickel sulfide deposits are generally perceived to be clustered we apply distance based spatial analysis methods nearest neighbor and k function to determine the spatial distribution pattern of nickel sulfide deposits in the kalgoorlie terrane results of these spatial analyses indicate the komatiite bodies that contain the nickel sulfide deposits in the terrane are clustered in contrast nickel sulfide deposits within komatiite bodies are either randomly distributed or dispersed and not clustered therefore the apparent clustering of nickel sulfide deposits within the kalgoorlie terrane may be a mere expression of the underlying clustering of the host komatiite bodies these findings have two main implications 1 nickel exploration models implemented through area selection based on localization controls of favorable komatiite bodies followed by direct detection of deposits within komatiite bodies have spatio statistical validity and 2 a poisson distribution could be a plausible initial model for predicting the number of nickel sulfide deposits within a komatiite body

evaluation of the uncertainty in estimation of metal resources of skarn tin in southern china abstract this study is concerned with the problem of how many undiscovered mineral deposits can be expected to occur in the vicinity of any known deposit especially if the deposit is nearly mined out skarn tin deposits in southern china were chosen to demonstrate that fractal modeling can be a useful tool to characterize the spatial temporal distribution of mineral deposits and to quantify their grades and tonnages the results show that the spatial temporal distribution of skarn tin deposits as well as their grades and tonnages satisfy fractal statistic and suggest that 14 skarn tin deposits could be found around a known skarn tin deposit within a radius of 80 km monte carlo simulation was used to combine the number of deposits and the frequency distributions of grade and tonnage and to capture the uncertainty in estimation of metal resources at the 90 50 and 10 confidence levels tin metal resources amount to 6 ton 200 ton and 1 0 10 4 ton around a known tin deposit within a radius of 80 respectively

nickel sulfide deposits in australia characteristics resources and potential abstract australia s nickel sulfide industry has had a fluctuating history since the discovery in 1966 of massive sulfides at kambalda in the eastern goldfields of western australia periods of buoyant nickel prices and high demand speculative exploration and frenetic investment the nickel boom years have been interspersed by protracted periods of relatively depressed metal prices exploration inactivity and low discovery rates despite this unpredictable evolution the industry has had a significant impact on the world nickel scene with australia having a global resource of nickel metal from sulfide ores of 12 9 mt five world class deposits 1 mt contained ni and a production status of number three after russia and canada more than 90 of the nation s known global resources of nickel metal from sulfide sources were discovered during the relative short period of 1966 to 1973 australia s nickel sulfide deposits are associated with ultramafic and or mafic igneous rocks in three major geotectonic settings 1 archean komatiites emplaced in rift zones of granite greenstone belts 2 precambrian tholeiitic mafic ultramafic intrusions emplaced in rift zones of archean cratons and proterozoic orogens and 3 hydrothermal remobilized deposits of various ages and settings the komatiitic association is economically by far the most important accounting for more than 95 of the nation s identified nickel sulfide resources the ages of australian komatiitic and tholeiitic hosted deposits generally correlate with three major global scale nickel metallogenic events at 3000 ma 2700 ma and 1900 ma these events are interpreted to correspond to periods of juvenile crustal growth and the development of large volumes of primitive komatiitic and tholeiitic magmas caused by large scale mantle overturn and mantle plume activities there is considerable potential for the further discovery of komatiite hosted deposits in archean granite greenstone terranes including both large and smaller high grade 5 to 9 ni deposits that may be enriched in pges 2 to 5 g t especially where the host ultramafic sequences are poorly exposed analysis of the major komatiite provinces of the world reveals that fertile komatiitic sequences are generally of late archean 2700 ma or paleoproterozoic 1900 ma age have dominantly al undepleted al 2 o 3 tio 2 15 to 25 chemical affinities and often occur with sulfur bearing country rocks in dynamic high magma flux environments such as compound sheet flows with internal pathways facies kambalda type or dunitic compound sheet flow facies mt keith type most precambrian provinces in australia particularly the proterozoic orogenic belts contain an abundance of sulfur saturated tholeiitic mafic ultramafic intrusions that have not been fully investigated for their potential to host basal ni cu sulfides voisey s bay type mineralization the major exploration challenges for finding these deposits are to determine the pre deformational geometries and younging directions of the intrusions and to locate structural depressions in the basal contacts and feeder conduits under cover stratabound pge ni cu cr deposits hosted by large archean proterozoic layered mafic ultramafic intrusions munni munni panton of tholeiitic affinity have comparable global nickel resources to many komatiite deposits but low grades

comparison of archean dunites and komatiites associated with nickel mineralisation in western australia implications for dunite genesis the two important classes of archean fe ni cu sulphide deposits in western australia are those hosted by 1 coarse grained adcumulate dunites e g agnew mount keith and 2 komatiitic lavas e g kambalda close similarities in compositions of relict olivine pyroxene and chromite from dunitic and komatiitic host rocks and compositional and textural similarities between chilled margins of some dunites and some komatiites indicate origins from similar komatiitic liquids which contained 20 32 mgo as field relations suggest that the dunites are probably lateral stratigraphic equivalents of spinifextextured komatiites they are interpreted as the slowcooled end member of a continuum from spinifex textured komatiites former liquids through cumulate komatiites 40 90 olivine to adcumulate komatiitic dunite 90 olivine previous classification of these ni deposits as intrusive and volcanic related deposits now appears unwarranted and the terms komatiitic dunite hosted and komatiite hosted deposits are preferred

spatial distribution of ore deposits conventional spatial distributions such as the negative binomial and neyman scott have been proposed as models for the clustered distribution of ore deposits b b mandelbrot has proposed that ore deposits are clustered at many different scales and that fractal distributions are appropriate the apparent fractal characteristics of geologic controls on the locations of hydrothermal deposits self similar systems of fractures circulation cells and magmatic intrusions support his proposal examination of the spatial distribution of 4775 hydrothermal precious metal deposits in the basin and range western united states reveals fractal distributions acting within two scale ranges together covering the entire range from 1 to 1000 km this result gives additional credibility to concepts of regional controls on mineralization such as metallogenic provinces and tectonostratigraphic terranes the distribution of these ore deposits about a known occurrence follows a regular radial density law within a range of radii r of 1 to 15 km the probability density function of these deposits is given by d 0 46 r 1 17 and within a range of radii of 15 to 1000 km by d 0 08 r 0 83 where d is deposits per square kilometre

the volcanology of komatiites as deduced from field relationships in the norseman wiluna greenstone belt western australia abstract komatiites in the western part of the norseman wiluna greenstone belt in the yilgarn block of western australia display a wide variety of volcanic facies ranging from very thin differentiated munro township type flow units through to very thick olivine rich cumulate flow units containing high proportions of adcumulate dunite cumulate flow units have been mapped in detail in the agnew wiluna segment of the norseman wiluna greenstone belt in the yakabindie and mt keith areas thick lenticular bodies of olivine adcumulate are flanked and overlain by thinner sheet like sequences of finer grained olivine orthocumulates they show fine scale internal layering broad scale cryptic layering and upper fractionated sequences containing harrisites pyroxene bearing cumulates and in some cases gabbroic derivatives low grade disseminated sulphide mineralisation occurs within the dunite lenses in the stratigraphically equivalent kathleen east area adcumulate dunite grades laterally into olivine orthocumulates and spinifex textured flows a similar relationship is also seen further south where the perseverance ultramafic complex consists of a thick central dunite lens flanked on one side by thin fine grained orthocumulate sequences and on the other by intercalated orthocumulates and spinifex textured flows these field relationships indicate an extrusive origin for the adcumulate dunite bodies the walter williams formation in the west central part of the norseman wiluna belt is a very large cumulate flow unit 150 km long and 30 km wide in presently exposed extent it consists of a central sheet like body of adcumulate dunite underlain and overlain by olivine orthocumulates the adcumulate sheet is everywhere capped by a distinctive thin layer of olivine harrisite at its northernmost extent the flow unit consists of cyclically layered olivine and pyroxene bearing cumulates capped by gabbros dolerites and pyroxene spinifex textured material interpreted as a periodically replenished lava lake sequence both the lenticular dunite bodies of the agnew wiluna belt and the sheet like dunite body of the walter williams formation are interpreted as crystallisation products of very large submarine komatiite lava flows which erupted and flowed at very high rates the adcumulate dunites are interpreted as the products of in situ crystallisation at low degrees of supercooling at the top of upward and inward accreting crystal piles at the bases of the flows flanking and overlying olivine orthocumulates reflect higher rates of heat loss at the site of crystallisation due to lower lava flow rates field evidence from perseverance and the general geometry of the dunite lenses suggest that the lenses formed within large thermal erosion channels developed by turbulent lava rivers flowing over low melting felsic volcanic substrates the textural range exhibited by komatiites can be integrated into a comprehensive model for the geometry of large komatiite flow fields formed by rapid extrusion dunite sheets form close to the eruption sites and dunite lenses as a result of channellisation further away from the vent or close to the vent in the situation where the substrate is non refractory and thermal erosion can take place readily kambalda type volcanic facies develop in more distal environments where lava emplacement is channellised and episodic and munro type flow units represent small scale lava tubes formed at low flow rates on the distal flanks of major eruptions or close to the vent of very small ones waning eruption rates leads to proximal facies being overridden by distal ones a common observation in komatiite sequences the size of thermal erosion channels requires very rapid eruption rates comparable to those in phanerozoic flood basalt terrains

a fractal approach to the relationship between ore grade and tonnage it is now recognized that a variety of natural processes exhibit scale invariance over a wide range of scales scale invariant processes often exhibit a fractal behavior there are various ways to define fractal behavior one is to relate the frequency of occurrence to size tonnage and grade relations for economic ore deposits exhibit a fractal behavior if the tonnage of ore with a mean grade is proportional to the mean grade raised to a power we show that such a relation is a good approximation for mercury copper and uranium deposits in the united states the respective fractal dimensions are 2 01 1 16 and 1 48 the renormalization group approach is a quantitative method for studying scale invariant processes if it is assumed that the concentration of elements in ores is statistically scale invariant the renormalization group approach can be used to derive a fractal relationship between tonnage and mean grade

early exhumation of underthrust units near the toe of an ancient erosive subduction zone a case study from the northern apennines of italy apatite fission track aft analyses were performed on 16 sandstone samples from a tectonic melange unit exposed in three tectonic windows located near the inferred front of the early miocene subduction system of the northern apennines of italy the tectonic windows display a block on block tectonic melange present under the ligurian units the melange is formed by portions of the upper plate incorporated in the plate boundary shear zone as a consequence of a mechanism of frontal tectonic erosion during the aquitanian early miocene aft and structural data together with stratigraphic constraints allow the reconstruction of a complete deformation cycle with a phase of underthrusting followed by underplating and early exhumation of the tectonic melange the exhumation in particular took place at 10 20 km from the original subduction front moreover the analysis suggests that multiple faults were active at the same time in the frontal part of the subduction zone

characterization of lava flow degradation in the pisgah and cima volcanic fields california using landsat thematic mapper and airsar data digital landsat thematic mapper tm observations multi frequency polarimetric airsar radar data microtopographic profiles derived from helicopter borne photography and field measurements were analyzed over the pisgah and cima volcanic fields and lavic lake playa mojave desert california surface radiance factors were extracted from tm data using radiative transfer procedures to model the atmosphere and surface simultaneously airsar data were calibrated to backscatter specific cross sections using corner reflectors deployed at a number of locations analyses focused on the use of key field sites to understand compositional and textural controls on tm band 5 1 55 to 1 75 m radiance factors and c 5 6 cm l 24 cm and p 68 cm band specific cross sections for the pisgah and lavic lake sites the tm data are explained using a linear mixing model of basalt and fine grained sediment clay to sand sized grains of clay minerals quartz feldspar carbonate and lithic fragments radiance factors with the addition of shadows for the rough a9a flow on the other hand airsar data are shown to be primarily controlled by surface roughness as there is a strong correlation between specific cross section and surface height standard deviation for the elongate flows at the cima volcanic field which exhibit a mix of pahoehoe and a9a properties we find a positive linear relationship between tm band 5 radiance factors and flow ages for the 3 m y age range represented specific cross sections remain constant until about 0 14 ma and then decrease with age up to about 0 56 ma on the other hand data for the two older flows 0 70 and 0 85 ma show increasing specific cross sections with increasing age we interpret the increase in tm band 5 radiance factors from 0 05 to 0 25 with increasing flow age and the decrease in specific cross sections 8 db change at 35 incidence angle for l band for flows up to 0 56 ma in age as due to smoothing by mass wasting and infill by fine grained aeolian deposits the increased specific cross sections for older flows are interpreted to be a consequence of surface roughness generated by fluvial dissection

nickel sulfide deposits in western australia a review the nickel sulfide deposits of western australia consist of many intrusive dunite associated and volcanic peridotite associated deposits of late archean age a few small gabbroid associated deposits of archean or proterozoic age and some other rare types some 96 percent of the nickel metal resource is contained in deposits concentrated in the curvilinear folded and strike faulted supracrustal belts of the eastern yilgarn block very magnesian ultramafic rocks and sulfidic metasediments abound in these belts which have commonly been metamorphosed to greenschist or lower amphibolite facies conditions major strike faults probably influenced the nature and distribution of volcanism and sedimentation later tectonometamorphic styles and nickel mineralization intrusive dunite associated deposits occur in semiconcordant lenses of peridotite to olivinite composition and of komatiitic affinities which are restricted to curvilinear zones 100 km or more long nickel sulfides occur centrally or marginally in the thickest part of the lens low grade 1 ni sulfides are abundant and enclose smaller bodies of higher grade disseminated and massive or breccia ores which may be in part tectonically displaced the ni content of the sulfide fraction is about 10 percent although low grade sulfides may be ni rich with ranges in ni cu of 19 to 70 and ni co of 30 to 70 the lenses may have been emplaced 1 passively as subvolcanic sills which fractionated olivine and sulfides and gave rise to associated komatiitic lavas and or 2 dynamically as dikelike bodies of sulfidic olivine rich crystal mush volcanic peridotite associated deposits are best developed at or near the base of volcanic komatiitic ultramafics occurring between metabasalts and at low stratigraphic levels in the succession there is little or no geographical coincidence between these deposits and the intrusive dunite associated deposits the ultramafics are mainly lavas ranging from picrite to olivine peridotite in composition with thicker and more magnesian flows dominating the lower mineralized part of the piles where thin interflow sulfidic metasediments are also common about 90 percent of the mineralization is at the base of the lowermost olivine peridotite flows which appear to be tongue shaped and to occupy original depressions in basalt surfaces devoid of sulfidic sediment thin discontinuous massive sulfides resting on metabasalt are overlain by thicker continuous and more extensive matrix to disseminated sulfides some massive and breccia ores are tectonically displaced the ni content of the sulfide fraction varies from 5 to 23 percent with an accompanying increase in ni cu from 10 to 16 and ni co from 40 to 65 on eruption of sulfidic komatiitic magma sulfide segregation under differential flow and gravity would probably result in the observed cross sections through the ore but with subsequent physical modification of the ore during metamorphism gabbroid associated deposits occur in layered or composite intrusions of gabbronorite with lesser pyroxenite peridotite and anorthosite which crystallized from basaltic magma of uncertain affinity the rocks are incompletely hydrated low grade disseminated or blebby sulfides form layers or irregular bodies associated with lenses or veins of matrix massive or breccia sulfides the ni content of the sulfide fraction is up to about 6 percent with a range of ni cu ratios of 1 to 7 and an ni co ratio average of about 25 the bulk chemistry of the intrusions and the sulfides and sulfide silicate textures are consistent with the existence of immiscible sulfide liquids at the magmatic stage layered sedimentary associated and vein type arsenical deposits are rare but indicate the possible involvement of nickel in volcanic exhalative and metamorphic hydrothermal processes respectively a complex history of weathering and supergene alteration has affected the deposits and presents major difficulties in exploration the recognition of siliceous limonitic cappings some with pseudomorphic textures led to the discovery of most deposits many problems remain before a more complete understanding of the genesis of the parental sulfidic magmas and their mode of ascent and emplacement into the crust is possible these are discussed with particular reference to the intrusive dunite and volcanic peridotite associated deposits which seem to be restricted to ca 2 7 b y old archean terrains worldwide

structural controls on nickel sulphide ore shoots in archaean komatiite kambalda wa the volcanic trough controversy revisited abstract structural evaluation of the extensive exploration database for the kambalda tramways area archaean yilgarn craton western australia is addressing the long outstanding issue of whether the trough structure embayments that partly confine the nickel sulphide nis ore shoots are volcanic or tectonic in origin the trough structures are complex linear domains associated with talc carbonate alteration overprinting thickened komatiite overlying footwall basalt two end member trough structure types are recognised 1 re entrant trough structures and 2 open trough structures the re entrant trough structures are narrower at the top than at the base strongly asymmetric in cross section restricted in distribution to kambalda dome and transgress and truncate ore the open trough structures are wider at the top than at the base weakly asymmetric in cross section prevalent outside kambalda dome and are mineralised along their lateral margins in addition to trough associated ore other nis ore shoots overlie near planar komatiite basalt contacts and are overlain by serpentinite with relict igneous textures all these features indicate the possibility that the trough structures reflect structural folding thickening and alteration at rock contacts marking different competency during regional fold thrusting consequently nis exploration strategies should consider structural and metamorphic features as well as volcanologic and stratigraphic features recognition of folding and thrusting patterns related to major competency contrasts in favourable komatiite basalt stratigraphy near the crest of regional anticlines and domes would aid exploration success

the geology of the kambalda nickel field western australia the kambalda nickel field contains the major concentration of volcanic peridotitc associated nickel ores in the archean of australia it occupies an area of 400 km 2 underlain by two sequences of ultramafic mafic and felsic volcanics and sedimentary rocks which can be broadly correlated with previously erected regional stratigraphic sections most ores occur in the lower kambalda sequence at the base of the kambalda ultramafic rocks the ultramafic rocks of the upper bluebush sequence contain only minor mineralization four phases of deformation occurred over an interval of more than 300 m y resulting in a dominant north northwest structural trend faulting was active during original volcanism and continued intermittently until after mafic felsic dike intrusion the ultramafic rocks have been serpentinized and all rocks have exerienced low strain upper greenschist to lower amphibolite facies metamorphism and variable carbonation and potash metasomatism the kambalda ultramafic rocks are of variable thickness and in places exceed 1 000 m the lower member of the ultramafic rocks occupying approximately one third of the sequence comprises thicker high magnesium flows i e generally 36 volatile free mgo commonly separated by sulfidic quartz albite sediments thinner lower magnesium flow units characterize much of the upper member the ores occur in 24 shoot complexes each containing multiple ore surfaces caused by structural dislocation of a few grouped ribbonlike original orebodies eighty percent is contact ore occurring at the base of the lowermost ultramafic flow and generally occupying elongate troughs in the footwall basalt ultramafic contact these troughs are thought to be original volcanic depressions which focused later deformation the remainder is hanging wall ore usually occurring directly above contact ore at the base of the second or third ultramafic flow hanging wall orebodies occasionally grade into stratigraphically equivalent interflow sediments the ores contain varying proportions of massive matrix and disseminated sulfides the hypogene mineral assemblage is pyrrhotite pentlandite pyrite chalcopyrite but millerite is locally significant the massive ores in particular are commonly severely deformed and some have been remobilized tens of meters from their original contact position metamorphism has caused recrystallization of the ores and some redistribution of elements there is commonly marked variation of mineralogy and s ni ratios between contact and overlying hanging wall ores and within and between original ore surfaces within shoots but the relative contributions of primary composition and metamorphism to this variation are uncertain in ore environments the footwall basalt ultramafic contact is more deformed than in non ore environments the basal ultramafic flows are thicker and richer in magnesium the trend toward lower magnesium flows upward through the ultramafic formation is less well developed and there is better textural development and compositional differentiation within flows non ore environments usually contain interflow sediments whereas there is generally a sediment free prism of ultramafic rocks above ore environments most ores are interpreted to have originated by either simultaneous or sequential extrusion of sulfide melts and ultramafic lavas rich in olivine phenocrysts along active linear fissures in a basaltic ocean floor volcanic exhalations probably contributed to minor ore in sulfidic sediments

granite greenstone crust formation on the archaean earth a consequence of two superimposed processes abstract new u pb zircon age data reveal that volcanism in the greenstones of the eastern part of the yilgarn craton of western australia was coeval with that in the abitibi subprovince of the superior craton of canada and that komatiites in the eastern goldfields and abitibi terranes were erupted synchronously at 2705 ma however major granitoid intrusion and compression episodes in the eastern yilgarn craton post date those in the southern superior craton by at least 15 ma intrusion of late tectonic syenites movement on major shear zones metamorphism and deposition of post tectonic clastic sequences also occurred 25 40 ma later in the eastern yilgarn craton than in the southern superior craton indicating that these cratons occupied different tectonic settings between 2720 and 2620 ma these comparisons and the synchronous eruption of ultramafic magmas in the southern superior pilbara and kaapvaal cratons at 2715 ma provide evidence of global scale magmatic eruption events during the late archaean these considerations indicate that two superimposed processes global scale catastrophic convective overturn of the earth s mantle and plate tectonic processes were in operation during formation of granite greenstone crust in the late archaean