A study of carbonatites and associated fenitisation at Haast River, south Westland, New Zealand.


Paterson, L.A.


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The fenitisation of the Haast schists associated with Alpine dyke swarm carbonatites has been examined with a view to establishing whether there are detectable differences in the fluid component associated with carbonatites of a diverse compositional range.
In addition to a nearly complete range of compositions with the ternary system CaC03- MgC03-FeC03 carbonate assemblages within the Haast carbonatites include norsethite (BaMg(C03)2)as an important rock-forming phase, and as accessory phases, strontianite and barytocalcite. The range of carbonate compositions is such that it is unlikely all carbonatites have been derived from a single carbonate melt. Spatial distributions in the Haast River - Burke River areas and a gap in carbonate compositions along both the dolomite-ankerite join and the siderite-magnesite join suggest the presence of at least two carbonatite suites.
Suite A, geographically restricted to the Burke River area, is characterised by an enrichment in Ca and Fe. The inferred order of differentiation is from calcite through ankerite to siderite dominated carbonatites. Differentiation involved the early removal of strontianite and apatite.
Suite B, enriched in Ba and Mg, is more widespread than suite A but is localised in the Mt. Campbell area. Differentiation, which involved the early removal of monazite, is inferred to be in the order: norsethite - siderite - dolomite. Both suites have stable isotopic compositions and assemblages which indicate crystallisation from a fluid dominated regime. Temperaturepressure conditions of crystallisation are poorly constrained to approximately 300 - 400 ·c and 3-4 kb.
Fenitic mineral assemblages depend on: (a) type of protolith, (b) location in the aureole and (c) associated carbonatite suite. Quartz-albite protoliths host aegirine-Ti-aegirine (Na(Mg,Fe2+)o.sT~5Sh06) pyroxenes, Mg-arfvedsonite-riebeckite amphiboles, and taeniolite (K(Mg2Li)S40wF2), a tetrasilicic mica. Mafic protoliths host carbonates and minor amounts of winchitic amphiboles. Chessboard-twinned albite is ubiquitous to fenitised examples of all protolithologies. Quartz, phyllosilicates and epidote from the host schists are replaced during fenitisation. Compositional variation of mineral phases proximally or distally within an aureole is restricted to pyriboles. Only amphiboles show any consistency in the direction of composition change with compositions ranging from Mg-arfvedsonite proximally to riebeckite distally.
Fenitic assemblages reflect mineralogical and compositional differences between suite A and B carbonatites. Proximally to distally in aureoles hosted in quartz-albite protoliths suite A fenites are approximated by the end member assemblage: Ti-aegirine+albite => Mgarfvedsonite+ albite±rutile±Ti-aegirine => riebeckite+albite+quartz; and suite B fenites: ·aegirine+albite±rutile => riebeckite+albite±rutile±aegirine => riebeckite+albite+quartz±rutile.
Fenitising fluids are inferred to have been generally rich in Na, Mn, Ba, Sr, Nb and C02, with fluids capable of oxidising Fe in some of the protolithologies. Relative to suite B carbonatites fluids associated with suite A carbonatites are inferred to have been richer in Ca, Li and possibly also K and Rb. Differences in Mg and Fe enrichment of carbonatites and fenites reflect oxidation conditions which were probably higher during the formation of suite B carbonatites and fenites. ..
REE are the only elements which characterise fluids associated with individual carbonatites, the patterns mirroring those of the associated carbonatite. Little relative fractionation of individual REE between fluid/vapour and melt is indicated, which is contrary to predictions of some early experimental work. REE patterns of Haast carbonatites and constituent monazites range from LREE enriched patterns to MREE patterns, features which reflect the changing stability of REE complex ions with changing composition of the carbonate melt.
Fenitisation in the Haast area is characterised by low fluid/rock ratios and appears to have been dominated by an infiltration mechanism. Mafic protoliths were capable of buffering fluid compositions. No evidence is found for any significant change in the alkali content of the fluid as the carbonatite melt evolves.
On the basis of trace/minor element data and their close spatial association Haast carbonatites and tinguaites are inferred to have separated immiscibly from a carbonated silicate melt slightly more evolved than the melt represented by the camptonites.

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xvii, 440p. ill. Map in text. 30cm.


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POLYGON ((168.429714545960053 -45.132855373000886,168.552517522425404 -43.547463740098486,169.554395573615238 -43.582817794523251,169.571019264329294 -45.170254662472836,168.429714545960053 -45.132855373000886))





Paterson, L.A., “A study of carbonatites and associated fenitisation at Haast River, south Westland, New Zealand.,” Otago Geology Theses, accessed March 1, 2021, http://theses.otagogeology.org.nz/items/show/267.

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