Molybdenum-base metal-bismuth mineralisation at Eliot Creek, Karamea Bend, and Taipo Spur, North-west Nelson, New Zealand

Abstract

Molybdenite mineralisation in North-west Nelson is associated with small granite intrusives. Molybdenite occurs within the Separation Point Granite Batholith (Cretaceous) at Mt Evans (Canaan). Other occurrences are within lower Palaeozoic metasediments near the eastern margin of the Karamea Granite Batholith (late Palaeozoic-ear1y Mesozoic) - at Eliot Creek, Roaring Lion River, and Karamea Bend; or within the Karamea Batholith, at Taipo Spur and Mt Radiant. The molybdenite mineralisation and its genetically associated intrusives have been dated (K - Ar) as Cretaceous (~110 m.y.) at Eliot Creek and Taipo Spur. The Karamea Bend occurrence is of the same age. With the exception of Taipo Spur, where molybdenite is associated with pyrite-magnetite, a basemetal-bismuth mineral assemblage is associated with the molybdenite. At Mt Radiant and Mt Evans bismuth minerals (emplectite and aikinite respectively) occur in mutual association with molybdenite in a single paragenesis. At Karamea Bend and Eliot Creek. a later paragenesis of base metal sulphides and bismuth minerals (aikinite, bismuth-bearing acanthite, and bismuth-zinc fahlore at Karamea Bend; schapbachite, joseite-type sulphosalts and native bismuth at Eliot Creek), is superimposed on, or peripheral to, the earlier molybdenite mineralisation. Mapping of hydrothermal alteration at Eliot Creek, Karamea Bend and Taipo Spur shows the presence at all three of an inner potassic potash feldspar zone (with hydrothermal biotite at Taipo and Karamea Bend). At Karamea Bend and Eliot Creek this is surrounded by a phyllic-type albite-muscovite zone; whereas at Taipo the potassic zone is margined by an epidote-albite sericite zone of propylitic type, in which the molybdenite mineralisation is concentrated. In contrast, it is preferentially associated with the potassic zone in the other two cases. Studies of fluid inclusions in sulphide-associated quartz in the various deposits indicate that hydrothermal alteration and formation of disseminated molybdenite mineralisation occurred at temperatures ranging from 330° to 390°C, occasionally up to ~450°. Molybdenite veins. at Mt Radiant and Mt Evans were, similarly, formed at c. 370°C, whereas vein molybdenite at Eliot Creek was deposited at somewhat lower temperatures, 270° to 330°C. Base metal sulphides and bismuth sulphosalts at Eliot Creek were formed at much lower temperatures, ~200°C. Fluid inclusions further show that the hydrothermal fluids had low salinities (<26 wt.% NaCl eq.) and high CO2 activities (by ubiquitous presence of liquid CO2), for the molybdenite and base-metal-bismuth depositional stages. During molybdenite mineralisation in the higher range of temperatures, fluids were at or above the critical point, as indicated by dry-vapour inclusions. Alteration mineralogy and sulphide assemblages indicate that the hydrothermal solutions depositing molybdenite were moderately to weakly acidic, and that deposition of base-metalbismuth mineralisation was related to pH changes resulting from changes in CO2 activity consequent on fracturing and pressure release. The granitic intrusives which have given rise to the molybdenite mineralisation are characterised by several unusual chemical features: as regards major elements, the unaltered intrusives are adamellites with very high Na2O/K2O ratios, having a chemical composition comparable to trondhjemites. Trace element compositions are also unusual, particularly in the presence of extremely high strontium and rather high barium. In the unmineralised parts of the adamellites, molybdenum is at specialised levels at the three localities investigated (Eliot Creek, Karamea Bend, and Taipo Spur) While Eliot Creek also shows tin specialisation. Consideration of the distribution, situation and size of the molybdenum-bearing intrusives, and of their peculiar chemistry, collectively indicate a probable origin by differentiation from basaltic lithospheric material in the deeper parts of a subduction-zone environment.