Search

Textural, mineralogical and mineral compositional observations in a suite of Neoproterozoic aillikite and calcite kimberlite dykes from southern West Greenland point to consistent variations in melt major element compositions amongst these silica-undersaturated magma types. The aillikites have notably higher bulk SiO2/CO2, H2O/CO2 and K2O compared to calcite kimberlite. Bulk rock arrays, together with field and petrographic observations, emphasize that flow sorting of olivine and other crystalline phases during magma emplacement is important in controlling the compositions of individual samples from these ultramafic dykes. Flow sorting together with variable overall proportions of entrained lithospheric mantle material result in scatter on element–element plots, which makes the interpretation of regional scale major and trace element geochemical datasets difficult. We argue that a significant proportion of the regional Ni–MgO variation in the ultramafic dyke suite of SW Greenland is due to variation in the proportion of an entrained refractory lithospheric mantle component. Therefore, ratios of elements to MgO can be used as proxies for melt compositions. Ratios of SiO2, TiO2, Al2O3, FeO and K2O over MgO are systematically higher, and CO2/MgO lower, in aillikites compared to calcite kimberlites. The trace element patterns of the calcite kimberlite and aillikite dykes show strong similarities in incompatible element concentrations, resulting in overlapping ratios for the highly to moderately incompatible elements. However, differences in Zr-Hf concentrations between rock types imply differences in mantle source mineralogy. Guided by our observations, we present mixing models that demonstrate that partial flux-melting of phlogopite–ilmenite metasomes within the cratonic mantle lithosphere is capable of produce the geochemical characteristics of aillikites and mela-aillikites in West Greenland. Fusion of cratonic metasomes was initiated by infiltrating asthenosphere-derived carbonatitic melts previously identified as the parental liquids to calcite kimberlite. [ABSTRACT FROM AUTHOR]

A model is developed to test the hypothesis that kimberlites can form by low-degree melting of asthenospheric mantle followed by entrainment and assimilation of lithospheric mantle. The developed model uses inversion calculations based upon rare earth and compatible trace elements. For kimberlites (s.s.), an equation describing mass balance between a melt of unknown composition and a contaminant end-member of xenocrystic/assimilated material from the lithospheric mantle is inverted. This allows calculation of the mass fraction of xenocrystic minerals from the lithospheric mantle (olivine, orthopyroxene, clinopyroxene, garnet, ilmenite) entrained in the kimberlitic magma, as well as the source mineralogy and melt degree in the source region. The composition of the parental melt prior to interaction with the lithosphere is not assumed a priori but is calculated by the model. The CO2, H2O, K2O and P2O5 contents of the source are estimated assuming batch melting and the inversion models. The range and coupling of the model parameters are found using a non-linear most-squares inversion procedure, and the model space is visualised using a Self-Organising Map approach. Our earlier work supporting assimilation of xenocrystic opx is, however, not a precondition but provides a post-processing constraint , as well as the selection of a more likely set of solutions from the Self-Organising Map. The calculation is applied to a data set from the Majuagaa kimberlite dyke (southern West Greenland) including added whole rock analyses for CO2 and H2O. Major variations in whole rock compositions are related to flow differentiation of olivine macrocrysts. The textures of opx, cpx, gt and ilm megacrysts show evidence for reaction with the transporting melt and physical erosion in the kimberlitic mush. Using the bulk rocks in our inversion scheme results in a silico-carbonatite parental melt with major element concentrations consistent with experimental melts. The ol, opx, and cpx mass fractions in the source are not well-resolved by this calculation, but the proportion of gt in the source is comparatively well defined at 15–22 wt% and cpx is constrained to less than 14 wt%. The source assemblage required is 36–80 wt% ol, 2–49 wt% opx, 0–6 wt% cpx, and 15–19 wt% gt. This suggests a peridotitic rather than an eclogitic source. The inversion model gives an overall mass fraction of xenocrystic material in the Majuagaa kimberlite magma of 41–51 wt% The mass fractions of the xenocryst phases are as follows: 71–85 wt% ol, 0–13 wt% opx, 5 ± 1 wt% gt, and 10–14 wt% ilm. There is less than 3 wt% cpx in the xenocrystic and assimilated assemblage. These results agree with petrographic observations. Processing the model results using the Self-Organising Map clearly displays the extent and coupling within the statistically acceptable region of the model space and leads us to a preferred model of 49 wt% xenocrysts with a xenocryst assemblage of 71–76 wt% ol, 8–13 wt% opx, 4 wt% gt and 12 wt% ilm. A source with a REE pattern similar to that of primitive mantle is sufficient to form the parental melt and consistent with generation of the initial kimberlite melt in the convecting mantle. Calculated CO2 and H2O concentrations in the source of the Majuagaa kimberlite of 230–860 μg/g and 223–741 μg/g, respectively, are within the range of independent convecting mantle estimates. This is equivalent to <0.17 wt% magnesite and the H2O budget of the mantle source can be accommodated via storage in nominally anhydrous silicate phases. When applied to Majuagaa kimberlite, the inversions are consistent with a conceptually simple model of kimberlite formation: (1) low degree melting in carbonated asthenospheric peridotite, (2) melt extraction and concentration, and (3) entrainment and reaction with lithospheric mantle material. [ABSTRACT FROM AUTHOR]

The Skaergaard PGE–Au mineralization, aka the Platinova Reef, is a syn-magmatic Platinum Group Element (PGE) and gold (Au) mineralization that formed after crystallization of ∼74% of the bulk melt of the intrusion. It is hosted in a more than 600 m deep and bowl-shaped succession of gabbroic macro-rhythmic layers in the upper 100 m of the Middle Zone. The precious metal mineralization comprises a series of concordant, but compositionally zoned, mineralization levels identified by distinct PGE, Au and Cu peaks. They formed due to local sulphide saturation in stratiform concentrations of interstitial and evolved mush melts in six MLs over > 2000 years. The PGE–Au mineralization is compared to a stack of gold-rimmed saucers of PGE-rich gabbro of upward decreasing size. Fundamentally different crystallization and mineralization scenarios have been proposed for the mineralization, including offset reef type models based on sulphide saturation in the melt from which the silicate host crystallized, and the here argued model which restricts the same processes to the melt of the inward migrating mush zone of the magma chamber. The latter is supported by: i) a 3 D summary of the parageneses of precious metal minerals and phases (> 4000 grains) from 32 samples across the mineralization; ii) a 3 D compilation of all bulk rock assay data; and iii) a principal component analysis (PCA) of PGE, Au, Cu, and selected major and trace elements. In the main PGE-mineralization level (Pd5 alias Pd-Zone) the precious metal mineral paragenesis varies across the intrusion with precious metal sulphides and Au-alloys at the W-margin to Precambrian basement, precious metal plumbide and Au- and Ag-alloys at the E-margin to flood basalts, and skaergaardite (PdCu) and intermetallic compounds and alloys of PGE–Au and Cu in the central parts of the mineralization. Precious metal parageneses are distinct for a given sector of the intrusion, i.e. drill core (local control), rather than for a given stratigraphic or temporal interval in the accumulated gabbros. The precious metal 'grade times width' number (average g/t x metres) for the mineralization at an upper and a lower cut off of 100 ppb PGE or Au increases from ∼20 to ∼45 g toward the centre of the mineralization due to ponding of precious metal bearing melt. A strong increase in (Pd+Pt+Au)/Cu and dominance of (PdCu) alloys in the lower and central parts of the mineralization demonstrate the partial dissolution of droplets of Cu-rich sulphide melt and fractionation of precious metal ratios. The precious metal parageneses, the distribution of precious metals in the mineralization, and the PCA support initial accumulation of precious metals in the melt of the mush in the floor, followed by equilibration, sulphide saturation, and reactions with residual and immiscible Fe-rich silicate melt in a series of macro-rhythmic layers in the stratified and upward migrating mush zone in the floor of the magma chamber. Syn-magmatic and upward redistribution of precious metals sets the Skaergaard PGE–Au Mineralization apart from conventional reef type and offset-reef type precious metal mineralizations, and characterize 'Skaergaard type' precious metal deposits. [ABSTRACT FROM AUTHOR]

Cumulus olivine, clinopyroxene, melilite and perovskite from silicate rocks and carbonatites of the Gardiner (East Greenland) and Kovdor (Kola Peninsula) ultramafic alkaline complexes contain primary melt inclusions crystallized into aggregates of daughter minerals. The petrography and homogenization temperatures of the inclusions constrain the fractionation paths and the formation of carbonatites in the complexes. Carbonated melanephelinite was parental to both complexes and early cumulates (dunite, olivinite, peridotite and melteigite) are comparable. The common occurrence of phlogopite and amphibole in the inclusions and in the host rocks indicates that these were important liquidus phases. In both complexes the fractionation of phlogopite and amphibole drove the melts towards Ca-rich (larnite-normative) compositions. At the ijolite stage the evolutionary trends are believed to separate and the evolved larnite-normative melt produced calcite-bearing ijolite in Kovdor and melilitolite in Gardiner. The two assemblages are related by decarbonation reactions. A fractional crystallization origin is suggested for the Kovdor carbonatite, whereas the Gardiner carbonatite formed by liquid immiscibility in the course of melilite fractionation. Na–K–Ca and Na–Mg carbonates are common daughter phases and especially abundant in late-stage inclusions. Thus, all carbonatite melts in the two complexes are alkaline. Calcite carbonatites appear to be cumulates. [ABSTRACT FROM AUTHOR]