Kambalda Type Komatiitic Nickel Ore Deposits - Morphology

Morphology

The morphology of Kambalda-type Ni-Cu-PGE deposits is distinctive because the nickel sulfide can be shown to be associated with the floor of a komatiite lava flow, concentrated within a zone of highest flow in the lava channel facies.

The lava channel is typically recognised within a komatiite sequence by;

• Thickening of the basal flow of the komatiite sequence
• Increased MgO, Ni, Cu, and concomitant decrease in Zn, Cr, Fe, Ti as compared to 'flanking flows'
• A 'sediment free window' where sediment has been scoured or melted from the basal or footwall contact of the komatiite with the underlying substrate
• A trough morphology, which is recognisable by a reentrant flat and steep-sided embayment in the footwall underlying thickest cumulate piles

The ore zone typically consists, from the base upwards, of a zone of massive sulfides, matrix sulfides, net-textured ore, disseminated and cloud sulfide.

Massive nickeliferous sulfide is composed of greater than 95% sulfide occasionally with exotic enclaves of olivine, metasedimentary or melted material derived from the footwall to the lava flow. The massive sulfide ideally sits upon a footwall of basalt or felsic volcanic rock, which the massive sulfide may intrude into vertically. This forms a carrot-structured ore, interpreted to represent either thermal erosion of the underlying substrate by the ultra-high temperature komatiite lava, or physical remobilisation during deformation.

The massive sulfide is in some cases overlain by a zone of matrix sulfide. The ideal Kambalda type-section lacks matrix sulfides, which is interpreted to be because of either physical remobilisation, or because matrix ore will only form in quiescent magma conditions, and thus does not form in active channel zones except, perhaps, late in the eruption. However, most other komatiitic nickel ore sections contain matrix to net-textured ore.

Matrix sulfide ore, in high-grade metamorphic areas, is characterised by jackstraw texture, composed of bladed to acicular metamorphic olivine, which resembles spinifex textured olivines, within a matrix of nickel sulfide. This texture is formed by metamorphism of the ore, which is interpreted to have been composed of olivine crystals floating in massive sulfide.

Net-textured ore is rarely observed, being the ideal condition of sulfide-silicate immiscibility. This texture is difficult to prove from the majority of komatiite mineralisation profiles, but is known from the Jinchuan intrusive, China, where nickel sulfide forms a network textured groundmass liquid in which olivine floats. Most net-textured ores in komatiites are considered metamorphic overprints.

Disseminated sulfide zones occasionally overly the matrix sulfide zone, grading upwards into barren ultramafic olivine adcumulate. These zones are rarely economic to mine in the majority of komatiites, except when close to surface.

The massive sulfide sits within the B3 flow horizon of a typical komatiite lava flow system.