Komatiite-hosted Hunters Road nickel deposit, central Zimbabwe: physical volcanology and sulfide genesis
Titel:
Komatiite-hosted Hunters Road nickel deposit, central Zimbabwe: physical volcanology and sulfide genesis
Auteur:
Prendergast, M. D.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 48 (2001) nr. 5 pagina's 681-694
Jaar:
2001-01
Inhoud:
The nickel sulfide-bearing Main Flow at Hunters Road is a thick komatiite unit situated at the base of a well-developed lava channel complex overlying a prominent banded iron-formation that caps a thick sequence of felsic lavas and volcaniclastic rocks intruded by a probably comagmatic feeder sill. The 300-350 m-thick inner flank comprises a 200-250 m-thick central olivine meso- to adcumulate, relatively narrow lower and upper ortho- to mesocumulates and a 2-10 m-thick olivine spinifex zone at the top. Approximately 700 m towards the 90 m-thick outer flank, the spinifex zone is up to 30 m thick, the central meso- to adcumulate lenses out between the upper and lower mesocumulates, and the lower orthocumulate is more rich in clinopyroxene. In places, the flanks are directly overlain by rubbly hyaloclastite. The less well-preserved, 500 m-thick central axis coincides with a floor-rock embayment of demonstrably primary origin, which is 200 m deep and at least 800 m wide. Nickel sulfide mineralisation is disseminated, texturally cocumulus to olivine and confined to the meso- to adcumulate, the highest nickel grades being located towards the top. Large floor-rock xenoliths occur in the mineralised meso- to adcumulate of the inner flank within reaction envelopes of barren, pyroxene-bearing, olivine ortho- to mesocumulate. Formation of the embayment and xenoliths (clear evidence of large-scale thermomechanical floor-rock erosion) and of the olivine-sulfide meso- to adcumulate (the sulfur probably derived by the assimilation of sulfidic wall rocks adjacent to the feeder sill) is attributed to prolonged focused flow of hot turbulent lava close to the vent. The Main Flow is interpreted as the product of a thick channelised sheet flow comprising: (i) a turbulent lava river, open along the central axis and partly tubed over along the inner flanks; and (ii) a tubed-over levee facies along the outer flanks accommodating overflow from the central axis by inflationary growth under laminar flow conditions.
Tijdschrift beschrijving