Stratiform Intrusions Research Articles

Evidence of detrital spinel geochemistry for depositional response to the Paleo-Tethyan closure recorded in the Songpan-Ganzi Terrane, eastern Tibetan Plateau

The Late Triassic evolution of the Songpan-Ganzi Terrane in the eastern Tibetan Plateau is crucial for understanding the closure of the eastern Paleo-Tethys Ocean. However, the strong thickening and deformation of sedimentary cover make the Songpan-Ganzi geological records complicated and even obscured, impeding the reconstruction of regional tectonic evolution. In this study, we provide new geochemical data of detrital spinels from the Upper Triassic strata in the eastern Songpan-Ganzi Terrane to reveal the provenance and the regional tectonic events involved in the eastern Paleo-Tethyan closure. Results show that detrital spinels from the Upper Triassic Xinduqiao, Zhuwo, and Zagunao formations have variable concentrations of Cr2O3 (24.36–68.56 wt%), Al2O3 (0.13–39.17 wt%), MgO (0.20–21.25 wt%), FeOT (11.15–48.49 wt%), and TiO2 (0.00–4.66 wt%). They are geochemically similar to spinels from both ophiolites and stratiform intrusions, and affinitive to the counterparts from island-arc, supra-subduction zone, or mid-ocean ridge settings. Based on comparison with the spinel compositions from adjacent tectonic zones and in combination with previously published evidence from paleogeography, paleocurrent orientation, and geochronology, we propose the detrital spinels from the Upper Triassic sequence in the eastern Songpan-Ganzi Terrane were mainly recycled from the Lower–Middle Triassic strata in the West Qinling Terrane, with minor concentrations derived from the A’nyemaqen ophiolites. This provenance inference presents a new insight into the closure time of the eastern Paleo-Tethys Ocean.

The isotopic–geochronological age of trap formation rocks in the sedimentary section of the Baikit anteclise

The Lower Cambrian Usol’e Formation in the Kamov arch of the Baikit anteclise includes stratiform intrusions confined to several levels. According to isotopic‒geochronological investigations, the age of stratiform intrusions composed of olivine dolerites is estimated to be 256.0 ± 3.2 Ma.

Origin of micro-layering in a deep magma chamber: Evidence from two ultramafic–mafic layered xenoliths from Puy Beaunit (French Massif Central)

The origin of magmatic layering is still hotly debated. To try to shed some light on this problem, two ultramafic–mafic layered xenoliths from Puy Beaunit (French Massif Central) were investigated in detail. The nodules belong to a stratiform intrusion emplaced in the deep crust during the Permian (257 ± 6 Ma; Féménias, O., Coussaert, N., Bingen, B., Whitehouse, M., Mercier, J.-C., Demaiffe, D., 2003. A Permian underplating event in late- to post-orogenic tectonic setting. Evidence from the mafic–ultramafic layered xenoliths from Beaunit (French Massif Central). Chem. Geol. 199 293–315.). The 3 to 5 cm thick nodules have, in common, a central orthopyroxenite layer; the succession of layers is, respectively, norite–orthopyroxenite–norite (PBN 00-01) and norite–orthopyroxenite–harzburgite (PBN 00-03). The variations of both major (by electron microprobe) and trace, essentially the RE, elements (by LA-ICP-MS) were measured in major mineral phases (orthopyroxene, clinopyroxene, plagioclase, spinel) along cross-section perpendicular to the layering. Strong grain size, chemical and textural variations occur along these sections: they can be continuous or discontinuous, symmetrical or asymmetrical. Such complex variations cannot be solely related to a single magmatic history (fractional crystallisation, mineral sorting). Other processes such as element enrichment by residual liquid channelling along layer boundaries and/or sub-solidus recrystallisation and element redistribution must be invoked. It appears, in particular, that element distribution in the central orthopyroxenite layer could result from the injection of micro-sills of orthopyroxene-rich liquid between previously consolidated layers.

Chromite Geochemistry and PGE Fractionation in the Campo Formoso Complex and Ipueira-Medrado Sill, Bahia State, Brazil

Two districts of stratiform deposits are situated near the northern margin of the Sao Francisco craton. The ultramafic host rocks are remnants of early Precambrian layered complexes that intruded Archean gneissic blocks prior to Transamazonian collisional events. The intrusions contain abundant chromite, exhibit orthopyroxene-rich sequences, and are similar geochemically, with highly magnesian bulk compositions (Mg/Mg + Fe = 0.91–0.67) and fractionation trends consistent with komatiitic parent magmas. The intrusion form, alteration history, and current metamorphic grade of each are distinct. At the Ipueira-Medrado mines, representative of the Jacurici Valley district, a folded segment of a 200- to 250-m-thick ultramafic-mafic sill contains a 5- to 8-m-thick highly massive chromitite layer. Green spinel in the orthopyroxene-rich peridotite and ubiquitous granoblastic textures indicate recrystallization during upper amphibolite or granulite facies metamorphism, now with incomplete retrograde alteration and serpentinization. In Campo Formoso a deeply eroded stratiform complex contains at least seven different economic seams, comprising varying proportions of disseminated, net-textured, layered, and massive ores. Here greenschist alteration of the ultramafic host rocks is complete, apart from orthopyroxene relicts in an upper unit at the Cascabulhos mine. Microprobe studies of chromite-rich specimens from representative stratigraphic sections of each intrusion suggest that unusually low TiO2 levels (<0.9 wt %) combined with high Cr2O3 contents (45–55 wt %) reflect the primitive parental magma composition in both intrusions. Core compositions of from Cascabulhos retain high XCr (0.57–0.72) but show a wide range of XMg (0.21–0.47), consistent with Fe-Mg exchange at grades at or below amphibolite facies. Some relict igneous fractionation is preserved, since TiO2 increases (0.2–0.4 wt %) while Cr/Al or XCr decreases (0.70–0.55) from the massive chromitites hosted by serpentinite, through the disseminated seams above, to the two high-level seams in pyroxenite. Chromite from Ipueira-Medrado is more magnesian (XMg = 0.57–0.64) but also more aluminous (XCr = 0.60–0.64) when massive types are compared. Here the submassive minor seams stratigraphically above and below the main seam both have relatively lower XMg and XCr (0.36–0.45 and 0.45–0.57). This suggests that combined Cr-Al and Mg-Fe exchange occurred between and silicates during postemplacement progressive metamorphism of the seams. Close-spaced sampling of representative drill core sections at <2- or <4-m intervals has allowed the precious metal potential of each intrusion to be assessed. In both cases the highest total platinum group element (PGE) concentrations occur exclusively in chromitite, illustrating the well-known phenomenon of chromite control. The Ipueira-Medrado chromitite contains lower concentrations (<350 ppb) and has unfractionated PGE patterns. Chalcophile element analyses provide little evidence of precious metal scavenging by immiscible sulfide segregation and so indicate little potential for mineralization elsewhere in the sill. At Campo Formoso the alternation of PGE-enriched chromitite and barren silicates is suggestive of repeated influxes of primitive magma during open-system fractionation. An increase in the Pt, Pd, and Rh tenor of the chromitite (to ca. 1 ppm) occurs from the lower massive group, through the upper disseminated group, to the high-level seams hosted by pyroxenite. Thus upward evolution of the host mineral compositions is matched by increasing (Pt + Pd + Ru)/(Os + Ir + Ru) ratio. The covariation of PGE ratios with mineral chemistry is analogous to that occurring over the sequence of chromitite seams in the Lower Critical zone of the Bushveld Complex. This provides evidence for a fundamental link in the fractionation processes controlling and PGE mineralization in stratiform intrusions, which has clear implications for exploration models.

Regularities of Layering of Basic–Ultrabasic Intrusions as the Result of Self-Organization Processes During the Course of Their Formation

A principal feature of stratiform basic-ultrabasic intrusions is alternation of rocks characterized by different proportions of the salic (plagioclase) and mafic (mainly pyroxene) constituents. Owing to considerable density variation of the components, density is attractive for use as a characteristic of the change of rock composition across the section. Five drill holes intersecting the contact of the Critical and Main zones of the intrusion (West Bushveld, South Africa) were analyzed over the depth interval containing deposits of chromium and platinum group minerals (PGM). Several types of density variability over different intervals of the section are identified. There are intervals where density increases and decreases monotonically and ones which are characterized by cyclic (rhythmic), or chaotic changes. Intervals with relatively constant (stable) values of density occur as well. The results of analysis of these regularities suggest a nonlinear oscillator described by a differential equation of the second order to be a workable mathematical model for the process of formation of a layered intrusion. Coefficients of the equation have been estimated for all five drill holes from the empirical series of densities by inverse methods. If it is accepted that crystallisation proceeded from the bottom upwards, the equation is consistent with a point attractor. The average values of coefficients of the equation (that seem to serve as criteria for distinguishing intrusions of different genesis) were calculated, and its stability for various intervals of the section was studied. After terms in the equation with negligibly small coefficients, the dispersion of which exceeds their values, have been omitted, the equation becomes very similar to the one proposed for dynamics of a supercooled melt. It allows certain assumptions to be made about the character of mutual relations of the signal being analyzed (the observed alternation of the rocks) with peculiarities of the petrogenetic process, inverse methods.

The Kondapalli Layered Complex, Andhra Pradesh, India : A Synoptic Overview

The Kondapalli Layered Complex (KLC) consists dominantly of gabbroic and anorthositic rocks, with subordinate ultramafic rocks (orthopyroxenites, websterites, clinopyroxenites, dunites and harzburgites) which contain chromitites (with orthopyroxene, clinopyroxene or amphibole). The KLC is a stratiform intrusion possibly similar to Bushveld, and its various components occur as sheets, bands or lenses in the enclosing sea of charnockites; small scale folded structures are not uncommon. Chilled margins, contact metamorphic zones, xenoliths and late differentiates have not been found for the KLC, which is cut by rare 851±28 Ma (whole rock K-Ar age) metadolerite dykes containing intensely clouded plagioclase and a trace of garnet. Most rocks of the KLC display layered characteristics, and they essentially comprise plagioclase, orthopyroxene and clinopyroxene in different combinations, with variable proportions and diverse textural relationships. The rocks exhibit textures that formed both during the magmatic phase of crystallization and during high-temperature post-cumulus to subsolidus deformation and re-equilibration. Mineral analyses for 4 olivines (Fo 95-87), 19 orthopyroxenes (En 94-51), 13 clinopyroxenes (Ca 39-51Mg 54-36Fe 4-20), 11 amphiboles (Ca 27-31Mg 67-38Fe 5-34), 13 plagioclases (An 100-76; An 54) and 13 chromites [100Mg/(Mg+Fe 2+) = 66-26; 100Cr/ R 3+ = 73-35], together with 45 whole rock analyses from the KLC are utilized in the present study. The remarkably wide variations in the host rock chemistries reflect the significant changes in modal mineralogies. Two parental magmas (a magnesian liquid and an alumina-rich tholeiitic liquid) are tentatively proposed for the KLC. Some quartz-bearing (anorthositic, enderbitic and other felsic) rocks with conspicuous deformational textures are interpreted as contaminated or mixed rocks occurring at the tectonized junction zones between the KLC and charnockites. The KLC exhibits several distinctive mineralogical features: pure anorthite (An 100) in zoned spinel-amphibole-orthopyroxenites; exsolution rods or blebs of nearly pure K-feldspar (Or >90) in the high calcic plagioclase (An 90); twinned plagioclase (An 42) exsolution lamellae in the orthopyroxene (En 45) of deformed "unusual" enderbites; coronal garnet in rare high Fe-gabbros; inter- and intra-granular compositional variation and different zoning patterns in chromite of the chromitites and ultramafics; and finally, dense networks of fine exsolution lamellae of Al- and Mg-rich chromite in Fe 3+- and Fe 2+-rich host chromite. The coexisting pyroxenes from the ultrabasic, gabbroic and anorthositic rocks of the KLC, as well as those from the enclosing charnockites, yield similar K D values and P (6-8 kbar) - T (830-950 oC) estimates. It is suggested that the KLC has intruded dry country rocks at great depths (lower to middle crustal levels), probably in the period immediately following or coincident with the highest temperature metamorphism of the country rock charnockites. Extensive subsolidus re-equilibration of the KLC has taken place, with all the rocks (KLC and regional charnockites) retaining signatures of nearly identical physical conditions of formation characteristic of the granulite facies metamorphism.

Petrography, chemistry and distribution of platinum and palladium in ultramafic rocks of the Bird River Sill, SE Manitoba, Canada

The ultramafic portion of the Archean Bird River Sill, a mafic-ultramafic stratiform intrusion in southeastern Manitoba, is divided into a “lower” and an “upper” series. These series may be the product of two separate magmatic injection pulses that differ in their chemistry, mode of crystallization and petrography. The lower series consists of 80 m of ultramafic rocks and 10 m of mafic rocks. The ultramafic rocks are subdivided into ten periodic units that exhibit gradational variations in major element, sulfur, platinum and palladium concentrations concurrent with systematically evolved changes of the olivine crystal habits. The upper series is divided into a “crackle zone” and a “massive zone”. The 27 m thick crackle zone, consists of a homogeneous olivine cumulate disrupted by a network of conduits containing a fine grained groundmass and olivine crystals that exhibit hopper olivine morphology (Donaldson, 1976). The 38 m thick massive zone comprises mainly subhedral to euhedral olivine crystals hosting PGE enriched layers and chromitite layers near its stratigraphic top. The contrasting petrography, major element, sulfur, platinum and palladium distribution of the lower and the upper series are attributed to different a) parental magmas and b) emplacement mechanisms and crystallization histories.

Geology, geochemistry, and origin of platinum-group element-chromitite occurrences in the Heazlewood River Complex, Tasmania

Three distinctive chromitite occurrences are recognized from the Cambrian mafic-ultramafic Heazlewood River Complex, Tasmania. Type I chromitite represents very thin schlieren comprising Cr-rich spinel and is hosted by primitive, olivine-rich cumulates. Type II chromitite occurs as thicker schlieren containing more aluminous spinel and is hosted by an interlayered sequence of cumulate dunite and peridotite. Type III chromitite is more massive, occurring as irregular stringers, pods, and layers containing chromite having intermediate Cr/Al ratios. It is spatially associated with gabbroic dikes that intrude dunite-harzburgite-orthopyroxenite cyclic units and is interlayered with or enclosed by an unusual xenolith-bearing gabbronorite. Both type I and II chromitites are interpreted to have precipitated directly from a primitive magma and may reflect periods of magma replenishment. Type III chromitite was formed in response to mixing between ultramafic magma and volatile-rich intercumulus liquids, producing hybrid magmas having chromite as the only liquidus phase.All of the chromitites display enrichment in platinum-group elements relative to the cumulate ultramafic sequences. The maximum total platinum-group element tenor in the chromitites is 6 ppm, with average grades between 0.2 and 1 ppm. The chromitites are richest in ruthenium, platinum, and rhodium, reflecting the presence of discrete platinum-group minerals. Laurite (RuS 2 ) is the most abundant platinum-group mineral, occurring as isolated inclusions in chromite. A wide variety of Pt and Rh minerals are contained in the chromitites. These include alloys, arsenides, and sulfarsenides, the majority of which occur as altered, anhedral grains in the silicate matrix.The platinum-group mineral assemblages recognized in this investigation are similar to bimodal platinum-group mineral suites documented in chromitites from ophiolites (Legendre and Auge, 1986) and stratiform intrusions (von Gruenewaldt et al., 1986). These include early crystallizing Os, Ir, and Ru (Ir-group elements) phases and later formed Rh, Pt, and Pd minerals (Pd-group elements, Barnes et al., 1985). The Ir-group elements display a preference for octahedrally coordinated sites in magmas, similar to Cr (Murck and Campbell, 1986) and, like Cr, have a low solubility in primitive magmas. The unusually low Ir tenor of type I chromitites from the Heazlewood River Complex may reflect the prior removal of Os-Ir alloys from the parental liquids, which would account for the known occurrence of osmiridium in the complex. Localized magma mixing associated with the development of the type III chromitites led to increasing a (sub S 2 ) and a (sub A s ) , but not necessarily to S saturation, in the resultant hybrid liquid. Under these conditions, laurite was precipitated as an early formed cumulus phase, and the more soluble Pd-group elements crystallized as intercumulus sulfarsenides and arsenides.

The Pre‐Sinian Basement and Mineralization on the Western Margin of the Yangtze Paraplatform

Abstract The pre‐Sinian basement on the southwestern margin of the Yangtze paraplatform consists of three metamorphic rock series of different ages. Being products of different tectonic events and environments, they differ markedly in original rock sequences, metamorphism, tectonic style and characteristics of granitoids and mineral deposits. The Late Archean Kangdian craton mainly comprises the Kangding and Julin Groups with a metamorphic age of nearly 2500 Ma. They are supracrustal rocks dominated by mafic volcanics enclosed in trondhjemitic rocks. The craton is believed to represent a granite‐greenstone terrane of Late Archaean age. There occur mineral deposits such as graphite and kyanite deposits of metamorphic origin, muscovite deposits in pegmatites and gold quartz veins in gneissic granites, banded hornblende‐magnetite mineralization and copper and zinc mineralizations related to felsic volcanics. Large V‐Ti‐bearing magnetite deposits were also formed in the mafic‐ ultramafic stratiform intrusions emplaced on the margins of the craton during the Middle Proterozoic. Copper and nickel deposits are found in several ultramafic intrusions. Extending in a north‐south direction, the Proterozoic mobile belt consists mainly of the Early Proterozoic Hekou Group and Middle Proterozoic Huili and Kunyang Groups, and they are thought to be accumulations in a Proterozoic rift trough or aulacogen. During the Early Proterozoic, the rift trough was characterized by intense volcanism and presence of iron ore deposits of volcano‐magmatic type, iron‐copper deposits of exhalative‐sedimentary type. The Mid‐Late Proterozoic of the rift trough mainly witnessed the formation of sedimentary stratiform copper deposits and submarine sedimentary iron deposits. In the wake of the emplacement of the Jinningian and Chengjiangian granites in the Late Proterozoic, skarn‐type tin and tin‐iron ore deposits were formed.

Gold mineralization patterns in relation to the lithologic and tectonic evolution of the Chibougamau mining district, Quebec, Canada

The Chibougamau mining camp, a major gold producer, has so far produced a total of 1,050 mt of gold at an average grade of 1.85 g/tonne. In contrast to a general tendency to group the deposits as vein type with varying compositions, this paper demonstrates the strong influence of regional lithologic and tectonic evolution of the area on the gold mineralization patterns. A syn-volcanic period of mineralization comprises both the volcanogenic massive sulphide and disseminated mineralization, and the subsequent epithermal mineralization. The latter is also associated with evolving volcanic landforms and syn-volcanic intrusions. The emplacement of the typical Archean lode-gold deposits and later Cu-Au deposits of uncertain origin coincides with the syn-deformational (Kenoran) period. The spatial relationship between the east-west-trending shear zones and a north-east-trending fault system is shown to be a possible mechanism for generating added dilatancy during the gold mineralizing phase. A late-to-post Kenoran shear system controlled the emplacement of the last major phase of gold mineralization within a stratiform intrusion.

Platinum-group elements in the upper central layered zone of the Fox River Sill, northeastern Manitoba

Platinum-group element abundances ranging up to 1,200 ppb total are associated with weak, sporadic concentrations of disseminated sulfides in certain cyclic units of the Fox River sill, northeastern Manitoba. The sill is a 250-km-long, predominantly ultramafic stratiform intrusion of Proterozoic age. The Main Layered series of the intrusion comprises a sequence of cyclic units defined mainly on the basis of the disappearance and abrupt reappearance of cumulus olivine. The lower central layered zone consists largely of olivine-rich adcumulates that contain little sulfide, whereas the overlying upper central layered zone comprises mainly orthocumulates that contain sporadic platinum-group element-bearing sulfides. Pyrrhotite (75-95%), pentlandite, chalcopyrite (<5%), and minor mackinawite form the bulk of the texturally well-preserved interstitial sulfides.Ir, Rh, Pt, Pd, and to a lesser degree Au are strongly intercorrelated in the analyzed samples. This probably reflects their mutually consistent partitioning into the immiscible liquid sulfide phase that separated from the silicate melt, and the lack of significant preferential remobilization of any of these platinum-group elements. Ir appears to show partly compatible behavior, about 1 ppb apparently being associated with cumulus olivine. The lack of correlation of Ru with the other platinum-group elements is not well understood but could result if its principal occurrence were as laurite inclusions in chromite.Although the chondrite-normalized platinum-group element profiles resemble those of other magmatic sulfides, their slopes strongly suggest two populations, one steep (i.e., enriched in Rh, Pt, and Pd) and the other shallow. The latter group is in most cases associated with thick cyclic units that are believed to be derived from large volumes of relatively primitive magma. Little enrichment of Rh, Pt, and Pd in the melt was produced by the time of sulfide separation. However, the first group, enriched in Rh, Pt, and Pd, is generally associated with thin cyclic units that are believed to result from small influxes of magma mixed with more evolved residual magma. The greater degree of fractionation of this magma produced stronger enrichment of Rh, Pt, and Pd in the residual melt, and hence, greater enrichment in the immiscible sulfide phase that subsequently separated. From this, it can be speculated that the best potential for platinum-group element enrichment in the Fox River sill should occur in thin, highly differentiated cyclic units.

Cumulate relations in the ‘Younger Basic’ masses of the Huntly–Portsoy area, Grampian Region

Synopsis The ‘Younger Basic’ masses of the Huntly–Portsoy area have been found to contain a number of separate cumulate bodies ranging up to 3–4 km in dimensions. Olivine, plagioclase and augite are the dominant cumulus phases throughout, and although the composition of these phases varies from one body to another, consistencies in the compositional variation suggest that the individual cumulate units are probably fragments of a large stratiform intrusion that has been subjected to post-magmatic disruption. Although some disturbance may have occurred when a later pulse of magma was emplaced to form the granular noritic and gabbroic rocks associated with the cumulates, it is concluded that the fragmentation of the latter was mainly associated with a regional episode of post-‘Younger Basic’ shearing and mylonitization.

Irvine receives VGP Award

Textbooks of not so long ago typically presented the study of stratiform igneous intrusions as a simple and straightforward exercise in nature's petrology laboratory. The fractional crystallization products of mafic magma were presumed to accumulate sequentially, largely in response to gravity. Those familiar with the literature of the past several years are aware, however, that the textbooks of the next generation will conclude that this model, if not actually wrong, is at best a gross oversimplification. Why this apparent fall from grace of such a seemingly useful concept?

Compositional Layering in Alpine Peridotites: Evidence for Pressure Solution Creep in the Mantle

Field evidence from the Josephine (southwestern Oregon) and Red Mountain (New Zealand) peridotites indicates that compositional layering in alpine-type peridotites pre-dates emplacement into the crust, and is dissimilar in origin to layering in stratiform intrusions. Field, geochemical, and textural evidence all suggest that the layering formed during anatexis and upward flow of the peridotite in the mantle. A cumulus origin for the layering is rejected as there is strong geochemical evidence to suggest that alpine-type peridotites are the residues of partial fusion in the mantle. Although the layering superficially resembles layering in stratiform intrusions, there are no other features present which suggest a magmatic origin. The orientation of the layering, however, closely resembles that of strain-slip (or crenulation) cleavage in the axial zones of old mountain belts suggesting a deformation related origin. It also appears unlikely that the layering is solely the product of a mechanical segregation accompanying deformation ("flow layering") inasmuch as evidence for prolonged high-temperature creep of the peridotites is found throughout the entire peridotite whereas the layering is not. The most likely mechanism for the origin of the layering appears to be metamorphic differentiation accompanying deformation, pressure solution creep, and anatexis of the peridotite. This suggests that pressure-solution creep may be a principal creep mechanism in areas of ascending athenosphere such as below mid-ocean ridges.

Origin of chromite compositional variation in the Panton Sill, Western Australia

The compositional variation of chromite and associated olivine in chromite-rich and chromitepoor cumulus layers of the Panton Sill is described and a diffusion-controlled crystallization mechanism is proposed to explain this variation. By this mechanism, chromite initially precipitates with a fairly uniform composition, irrespective of the relative proportions of coprecipitating olivine and chromite, and is modified by continued growth during the postcumulus stage. The effect of postcumulus overgrowth of chromite, K d =(Mg/Fe2+)liquid/(Mg/Fe2+) chromite≃6, is to deplete the surrounding magma in chromium and decrease Fe2+ relative to Mg such that a chemical gradient exists between the overlying magma, through which the cumulus grains settled, and the magma in contact with settled chromite grains near the magma/crystal pile interface. Postcumulus equilibration of olivine and chromite with the surrounding magma results in higher Mg/(Mg + Fe2+) ratios of both olivine and chromite and higher Al content of chromite. The extent of this postcumulus modification is directly related to the proportion of chromite to olivine in a particular layer. This model can be extended to stratiform intrusions elsewhere in which chromite coprecipitates with olivine, orthopyroxene or plagioclase and displays similar compositional trends.

Distribution and source of elements in Gabbroids of different geochemical types

Such gabbroids show an unequal distribution of elements. The contrast of distribution increases with the degree of differentiation and contamination. Gabbroids can be with a different source of elements. Sources of orebearing gabbroids (FeTiV and TiZr) are in the deep mantle. The low ratio, the small range of variance of Sr 87:Sr 86 (0.7035 + 0.7056) and also the absence of presence of a direct connection with petrogenic elements indicate a weak contamination of gabbroids of this type by the Earth's crust matter. The contamination is checked by a quick intrusion of melt. Therefore the content of melt reflects rather the evolution of the deeper stage than the inner chamber one. And the melt there is a product of the pyroxenitic mantle part enriched by Ti, Zr, Nb, TR. Possibly most such melts do originate directly in the mantle. All rocks associations have alkali-ultrabasic features. So the high concentration of Ti and other elements is related to a deep source. Mantle source elements lie with experimental data. On the other hand (gabbroid CrTiAu type—stratiform intrusion), the distribution of most rare elements does not go with the mechanism of fractional crytallization of basaltic magma. The character of mineralization and the distribution of elements because of intermediate chambers is defined by the contamination process. The contamination can be taken as an extraction of rare elements from the substratum. The development of such a process belongs to the deep earth's crust. The role of mass-exchange in situ is very small, since there is a low rare element content in endocontact gabbroid and mother rocks. Gabbroids are magmatic but a part of clinopyroxenites and ore magnetites are metasomatic products. The influence of crust factors produces a higher value and dispersion of incoherent elements (Rb, K, Sr). This leads to an increase of the isotope ratio of Sr (at 0.7096±0.0007) and its correlation with K and Sr. The contamination is wide ranging and eventually produces dioritoids accompanied by, unusual for gabbroids, gold-sulphide mineralization. The new division of gabbroids can explain the difference of the geochemical specialization and the source of ore elements.

Origin of chromitite layers in the Muskox intrusion and other stratiform intrusions: A new interpretation

A mechanism of origin for chromitite layers in stratiform intrusions is described, based on the same physicochemical principles as an earlier explanation but formulated in a geologically very different way. By this new mechanism, the concentrated chromite is precipitated from chromite-saturated picritic tholeiite liquid when this liquid is blended with earlier liquid of the same type that has differentiated to relatively siliceous compositions.

The nature of chrysotile asbestos occurrences in southern Africa; a review

The principal chrysotile asbestos occurrences in Rhodesia, South Africa, and Swaziland are described, emphasis being placed on the regional geological settings and host rock stratigraphy of the mineralized areas. All the more important asbestos ore deposits in southern Africa are Archean in age ( approximately 2.5-3.5 b.y. old), being associated with ultramafic complexes occurring either as sill-like bodies in greenstone belts or as later cross-cutting intrusions.The ultramafic bodies can be grouped into three varieties. These, in order of decreasing age, are: (1) the layered complexes associated with basaltic and peridotitic komatiite extrusives and forming part of the Lower Ultramafic Unit of southern African greenstone belts, (2) layered ultramafic bodies associated with the intermediate to acid volcanic rocks that constitute part of the Mafic-to-Felsic Unit of greenstone belts, and (3) ultramafic intrusive bodies that postdate the greenstone belts but which are still affected by Archean tectonic disturbances that arise from the emplacement of granites.All the principal asbestos-bearing complexes show magmatic segregation into layered, often cyclically repetitive, differentiation sequences. These single or multicyclic sequences may consist of two or more of the following rock types: dunite, peridotite, harzburgite, Iherzolite, wehrlite, bronzitite-enstatolite, websterite, gabbro, norite, and gabbroic anorthosite. Where fractional crystallization of the ultramafic magma has been most efficient, many layers at, or near, the base of the complexes comprise monomineralic cumulate phases. These commonly consist of dunites and Mg-rich orthopyroxenites. With increasing distance from the base, progressive Mg depletion and Fe enrichment of the successive layers take place. Although chrysotile asbestos may commonly be encountered in all serpentinized ultramafic rock types, optimum development of economically exploitable fiber generally occurs in dunites, peridotites, or harzburgites.The Archean layered complexes were derived from magma of ultramafic composition in contrast to magmas of tholeiitic parentage that gave rise to the great stratiform intrusions like the Bushveld, Stillwater, and many others, including the Great Dyke in Rhodesia. The Great Dyke, unlike the others, however, acts as host to small chrysotile deposits developed in serpentinized dunites or harzburgites.In addition to the asbestos mineralization found in the layered complexes, subordinate deposits, occurring in serpentinized dolomitic rocks associated with the approximately 2.0-b.y.-old Transvaal Supergroup, are briefly described.Whereas faulting and fracturing is generally acknowledged as being largely responsible for the local development of asbestos fiber, examples from the southern African green-stone belts demonstrate that folding is often a dominant regional controlling factor in the localization of asbestos mineralization in ultramafic rocks.

The Vourinos Ophiolite, Greece: Cyclic Units of Lineated Cumulates Overlying Harzburgite Tectonite

Re-examination of the Vourinos ophiolite shows it to be composed of metamorphic tectonites, cumulates, plagiogranites, dikes, and lava. The contact between the tectonites and the cumulates is exposed and sharp. Beneath the cumulate contact, the rocks have been highly deformed and complexly folded; above that contact, they simply have been tilted vertically to expose a stratiform complex 1,500 m (4,800 ft) thick. The stratiform intrusion is characterized by cyclic units, rich in olivine at the base, and rich in feldspar at the top. Some cumulus diorites are present at the top of the section which grade into quartz diorites (plagiogranites) with hypautomorphic textures. Lineate lamination characterizes the cumulates and may indicate the direction or orientation of the Mesozoic mid-oceanic ridge crest with respect to the present position of the complex. A siliceous dike swarm cuts the upper part of the stratiform complex. The section suggests that in the case of Vourinos, a large magmatic chamber formed at a mid-oceanic ridge crest and that intrusion was a much more important process than extrusion in the formation of oceanic crust in that area. The reported presence of cumulates in many other ophiolite complexes suggests that these relations may obtain generally at most or all spreading ridges. The contact between the tectonites and the cumulates of the complex would not have corresponded with seismic M.

Geology of the Greenhills Ultramafic Complex, Bluff Peninsula, Southland, New Zealand

Extensive coastal outcrops and moderate relief combine to provide an excellent exposure of the 14-km 2 Greenhills Complex. This complex consists of a deformed layered series of rocks, including dunite, wehrlite, and gabbro, of Permian age intruded into Lower Permian metasedimentary rocks of the Southland syncline. Widespread slumping in the layered series and the emplacement of the complex as a plug of hot fluid mush indicate recurring conditions of instability during evolution of the complex. Emplacement of the complex was accompanied by the intrusion of innumerable dikes and by folding and faulting; the material of many of the dikes was derived from the complex. Country rock, dominantly volcanic microbreccia with intercalated bands of impure marble, increases from regional prehnite-pumpellyite facies through greenschist facies to a marginal gabbroic rock, itself the result of contact metamorphism equivalent to pyroxene-hornfels adjacent to the complex. The layered series of the Greenhills Complex is stratigraphically divisible into: (1) an upper gabbroic portion dominated by an approximately 640-m-thick gabbro zone which includes a 15-m-thick allivalite unit; (2) a lower ultramafic or peridotitic portion comprised of a 150-m-thick wehrlite zone (which includes near its base a poikilitic wehrlite unit less than 30 m thick), and a basal dunite zone exceeding 600 m in thickness. Feldspathic wehrlite occurs in a narrow transition zone between the upper and lower portions of the complex. The layered series shows well-developed accumulate structures and textures resembling cumulates in classic stratiform intrusions. Cryptic layering is shown by the range in composition of the primary minerals. These minerals in order of separation from the Greenhills magma are: olivine (Fo 90–65 ), clinopyroxene (Ca 41 Mg 54 Fe 5 to Ca 44 Mg 47 Fe 9 ), and plagioclase (An 92–88 ). The Greenhills Complex is essentially the result of crystal settling during fractional crystallization of a basaltic magma, modified by fluid mush flow and mixing of rocks along mutual contact zones. The magma is believed to have had affinities transitional between alkaline and tholeiitic magma types. Differentiation took place in a hydrous open system connected with the surface of the earth.