Variant Offset-Type Platinum Group Element Reef Mineralization in Basal Olivine Cumulates of the Kapalagulu Intrusion, Western Tanzania

The Kapalagulu intrusion in eastern Tanzania hosts a major, 420-m-thick, stratiform/stratabound platinum group element (PGE)-bearing sulfide zone—the Lubalisi reef—within a prominent, chromititiferous, harzburgite unit close to its stratigraphic base. Several features of the vertical base and precious metal distributions (in a composite stratigraphic section based upon two deep exploration drill holes) display similarities to those of offset-type PGE reefs that formed under the overall control of Rayleigh fractionation: (1) composite layering (at several scales) defined by systematic vertical variations of sulfide and precious metal contents and intermetallic ratios, indicating repeated cycles of PGE enrichment and depletion in the order Pd-Pt-Au-Cu, and (2) in the lower part of the reef, stratigraphic offsets of the precious metal peaks below peak sulfide (Cu) content. The form and geochemistry of the reef are consistent with overturns of basal liquid layers within a liquid layering system (i.e., stable density-driven stratification of a magma chamber), plus at least two minor inputs of parental magma during which the resident magma was recharged with sulfur and metals, and the effective depletion of precious metals in the magma midway through reef development. The Lubalisi reef differs from classic offset-type PGE reefs, however, principally because individual Pd, Pt, and Au enrichment peaks are coincident, not offset. The reef is set apart from other offset-type PGE reefs in three additional ways: (1) its association with olivine cumulates that crystallized soon after initial magma emplacement and well below the first appearance of cumulus pyroxene or plagioclase (implying attainment of sulfide saturation and precious metal enrichment without prolonged concentration of sulfur and chalcophile metals by normal magma cooling and differentiation), (2) the probable role of chromite crystallization in not only triggering sulfide segregation during reef formation but also facilitating precious metal enrichment in the early stages of reef development, and (3) its great width. The early stage of fractionation may also help explain the coincident precious metal peaks through its effect on apparent precious metal partition coefficients.

Petrofabric and geochemical features of ultramafic rocks on the example of restite metamorphites of the Kuznetsk Alatau (Western Siberia), olivine cumulates of the Yoko-Dovyren layered massif (Northern Cisbaikalia) and their analogues from ultrabasic xenoliths of the Canary Islands (Spain)

The article presents the results of petrostructural and mineralogical studies of olivine grains from ultrabasic and basic rocks of different genesis. In particular, they correspond to cumulative dunites of the Yoko-Dovyren layered massif (Northern Cisbaikalia), restite hyperbasites of the Velvet massif (Kuznetskiy Alatau), and xenoliths of peridotites from basalts of the Canary Islands (Spain). The relationship between the petrostructural and mineralogical features of ultrabasic rocks is shown, which makes it possible to identify their cumulative and restite varieties. An important element of the petrostructure of ultrabasites is the orientation of olivine, which reflects either the conditions for the formation of primary crystals in the magmatic melt, or the conditions for its recrystallization as a result of plastic deformations during exhumation to the Earth's surface. The mineral composition of rocks is an additional feature that reflects the real conditions of formation. In the presence of basic plagioclase, it is already quite difficult to speak about the restitic nature of these ultrabasic rocks. On the other hand, plastic deformations of olivine are characteristic of restitic hyperbasites, in which plagioclase is absent. The results of mineralogical studies in ultrabasic xenoliths of the Canary Islands showed the presence of basic plagioclase (labradorite), as well as chrysolite-type olivine (12-16 Fa), which corresponds to the attributes of ultrabasic layered intrusions such as the Yoko-Dovyren dunite-troctolite-gabbro pluton. For restite hyperbasites (by the example of the Kuznetsk Alatau), the iron content of olivine does not exceed 9-10% of the fayalite end, while plagioclase and clinopyroxene are absent. A characteristic feature of the xenoliths of the Canary Islands is the presence of endiopside, which is present in layered intrusions, but is absent in the restrained fragments of the upper and lower mantle. It is assumed that the xenoliths in the basalts of the Canary Islands are not of a mantle nature, but are fragments of a deep magma chamber. The studies of the optical orientation of olivine in xenoliths of the rocks of Lanzarote Island confirm these conclusions. The geochemical parameters of ultramafic xenoliths on Tenerife Island may well correspond to deeper formations.

The Late-Paleoarchean Ultra-Depleted Commondale Komatiites: Earth's Hottest Lavas and Consequences for Eruption

The c.3·3 Ga Commondale komatiites located south of the Barberton greenstone belt in the Kaapvaal Craton are different from other komatiites, possessing compositional and textural features unique to this occurrence. Unlike almost all other known komatiite occurrences, they are not associated with komatiitic basalts or basalts. The komatiite flows are 0·5–25 m thick and are made up of a marginal zone of spinifex-textured and fine-grained aphyric rocks (low-Mg group) and an inner zone of olivine cumulates (high-Mg group), arranged in such a way to give highly symmetrical compositional profiles for many flows. Olivine is the dominant phase in all rocks, but orthopyroxene occurs as spinifex and elongate laths in the marginal zone. Clinopyroxene and plagioclase are entirely absent. The olivine cumulates formed from Mg-rich magma (36·1% MgO, 6·8% FeO) which caused inflation of the thicker flows. The maximum observed olivine composition in cores (Fo 96·6) is the highest recorded for any komatiite worldwide. The high-Mg magma would have erupted at a temperature close to 1670°C, the highest inferred temperature for an anhydrous terrestrial lava. The marginal zone is enriched in incompatible elements compared with the inner zone and formed by fractionation of the parental melt. However, all rock-types in the marginal zone are depleted in FeO (some as low as 3·5%) which could not have been derived by any primary magmatic process. The marginal zone rocks were modified by assimilation and/or alteration by seawater (or brine) components causing migration of iron and strong enrichment of sodium (up to 1·6 wt % Na2O) and chlorine (up to 2400 ppm). Zirconium has an identical distribution to sodium, with both elements greatly enriched above what would result from fractional crystallization, and may result from speciation of these elements at high temperature followed by post-crystallization alteration. Rare earth elements, Y and Nb have contents commensurate with fractionation of the primitive parental magma. Dendritic-textured olivine-rich rocks with orthopyroxene spinifex spatially and compositionally transitional between the marginal zone and the olivine cumulates resulted from interaction of the high temperature parental magma in the centre of the flows with the fractionated melt at the flow margins. A further manifestation of this association is the development of highly regular fine-scale (5–15 cm) layering (up to 45 layers) of alternating olivine cumulate and spinifex near the base of thick flows. This is overlain by olivine cumulates in which the melt/crystal-mush became arranged into a 3-dimensional network controlled by re-distribution of the trapped melt manifest by a spectacular knobbly texture in outcrop. Over 200 flow units are recognized and detailed chemical and mineralogical studies were carried out on drill cores intersecting 375 m of stratigraphy. The parental magma was highly depleted (in ppm Nb 0·017, Zr 1·18, total REE 1·7 and Gd/YbN=0·3, La/YbN=0·038) and although generally regarded to fall into the rare category of Al-enriched komatiites (AEKs), it is considered that these lavas are a unique class of their own of ultra-depleted komatiites. Relative to other AEKs the Commondale komatiites are both enriched in Al as well as being markedly depleted in Ti (390 ppm), giving rise to the extremely high Al2O3/TiO2 (81). The high temperature and low viscosity of the magma resulted in emplacement processes previously unrecognized in komatiites. The primary melt was derived by melting of mantle peridotite in equilibrium with olivine and orthopyroxene. The initial source was depleted in incompatible elements by small degrees of melting (3–4%) followed by high degrees of partial melting (70%) of the subsequent refractory source at 5 GPa (∼150 km).

Mineralogical and petrographic features of metakomatiites of the Kostomuksha greenstone structure (Karelia)

Object of study. The paper presents the mineralogical and petrographic study results of metamorphosed and metasomatized komatiites and komatiitic peridotites from the Ozerki soapstone deposit and Pentinsuo prospect, located in the Kostomuksha greenstone structure of the Karelian Craton, Fennoscandian Shield. Material and methods. Surface and drill core samples of various mineral and structural-textural varieties of altered komatiites were studied by optical microscopy, electron microscopy with an attachment for microanalysis, XRD phase, thermogravimetric and chemical analysis. Results. Soapstone formation in the investigated localities displays a multistage alteration and associates with the superimposed alteration of Mg-rich metakomatiite flows and olivine cumulates due to influx of carbon dioxide bearing fluids enriched in calcium and potassium. The chemical composition of initial komatiite (MgO content of the rock) and the degree of its fracture intensity are the main factors controlling soapstone formation. In differentiated lava flows soapstone is formed mainly in high-magnesium cumulate zones characterized by a high content of serpentine. In the flow top and spinifex zones an early amphibole-chlorite-magnetite mineral association is preserved in varying degrees. Talc and carbonate are formed by the decomposition of serpentine and amphibole. The chlorite content in soapstone is controlled by the Al2O3 concentration in the respective flow zones. Conclusion. The carbonate-chlorite-talc is the general natural type of soapstone associated with thin komatiite lava flows of distal volcanic facies. In more thick proximal lava flows the formation of soapstone of chloritecarbonate-talc and talc-carbonate composition is possible. The most prefered prospecting areas for soapstone are fields proximal to the eruptive vent, main lava conduits and subvolcanic analogues of komatiites.

Magmatic unconformities and stratigraphic relations in the Peridotite zone, Stillwater Complex, Montana

New stratigraphic relations between cumulate layers and units are documented in the Peridotite zone of the Stillwater Complex, a late Archean mafic to ultramafic layered intrusion exposed along the northern edge of the Beartooth uplift, Montana. First, low-angle unconformities occur between all types of successive stratigraphic units and at all scales within the Peridotite zone. Second, new stratigraphic units and divisions, based on the unconformities and type of cumulate layering, within the Peridotite zone are proposed and are correlated with published stratigraphic columns. The stratigraphic units, identified on the basis of rock type(s) and contacts with adjacent units, from smallest to largest a cyclic units, multicyclic units, megacyclic groups, and subzones. The three larger units are usually bounded by low-angle magmatic unconformities. The three subzones and five of the six megacyclic groups identified in the Peridotite zone can be traced over a distance of 17.7 mi (28.3 km). The stratigraphic relations in the Peridotite zone indicate that low-angle crosscutting magmatic unconformities occur at all scales and that lateral variations within and between the identified stratigraphic units are common. The relations suggest an extremely dynamic and rapidly changing environment during formation of the Peridotite zone. Overprinted on the stratigraphic relations is a larger evolution of cumulus rock types in the Peridotite zone from more primitive stratigraphic units dominated by olivine and olivine–bronzite cumulates to more evolved stratigraphic units dominated by bronzite and bronzite–olivine cumulates. Several major magmatic, or possibly structural, events appear to have affected the Peridotite zone during its formation, as indicated by thickness-variation patterns in megacyclic groups and subzones. The thickest massive chromite layers in the Peridotite zone are associated with two of the major magmatic events.

The structure and petrogenesis of the Trallval and Ruinsival areas of the Rhum ultrabasic complex

ABSTRACTDetailed mapping of the Trallval-Ruinsival area of the ultrabasic complex of Rhum has revealed the relationship between the intrusive peridotites, which form a major part of the complex, and the layered ultrabasic rocks. The layered rocks on Trallval are correlated with Units 8 to 15 of the Eastern Layered Series as seen on Hallival and Askival, and part of an additional unit, Unit 16, has also been identified. The layered rocks of Ruinsival and the western part of Trallval form part of the Central Series and have been subdivided into six cyclic units. They are associated with four sets of intrusive breccia zones which converge towards a central region. Four cylindrical plugs of intrusive peridotite occur within the area of outcrop of the Central Series. Criteria are listed for the distinction of intrusive peridotite from conformable peridotites formed in situ. Distinctive textures and structures are described; these include vertical feldspathic streamers, which are interpreted as direct evidence for the upward expulsion of interstitial fluid, and loading structures at the contact between a peridotite layer and underlying allivalite are ascribed to the intrusion of a peridotite sill. A comparison of the Eastern Layered Series and the Central Series suggests that three stages were involved in the formation of the ultrabasic rocks: (i) the accretion of rhythmically layered unconsolidated olivine cumulates; (ii) the upward expulsion of intercumulus liquid; and (iii) the formation of either allivalite (as in the Eastern Layered Series) or ultrabasic breccia zones (as in the Central Series) from the expelled intercumulus liquid, depending on whether the tectonic environment was quiescent or extensional.