Genesis of Kambalda Nickel Deposits

Kambalda's nickel deposits are believed to have formed approximately 2.7 billion years ago. The deposits were formed in lava channels, created by 'rivers' of lava that extended for many miles. Under the right circumstances, nickel sulphides pooled at the base of these lava flows, forming ribbon-like lenses of sulphides. Over the eons the landscape on which these rivers of lava and their associated pools of sulphides were formed were buried and folded, so that today the sulphide pools are found in a variety of configurations, from vertical (as at Miitel) to flat-lying but gently plunging (as at Otter Juan).

Due to the circumstances of their formation, all of Kambalda's ore deposits share certain characteristics, which aids in their exploration. Most importantly, they all occur on the so-called 'basal contact'. This is the contact between the underlying basalt rock (the surface upon which the lava originally flowed, and into which channels were cut by the flowing lava) and the overlying ultramafic rock (known as Komatiite, which was the actual lava that flowed upon the basalt). It is in incised channels in the basalt, on the contact between the basalt and the overlying Komatiite rock, that all Kambalda's significant ore bodies are found. Thus modern exploration for Kambalda-style nickel deposits focuses on exploring this basal contact.

The original formation of the river-like channels, and the pools of sulphides that developed along them, create another characteristic that is important in their exploration. That is, that nickel sulphide ore bodies tend to be elongated and ribbon-like, and present continuously or intermittently along well-defined 'channel structures' incised in the basal contact. Thus once a sulphide ore body is discovered, and the hosting 'channel structure' defined, then that channel structure can be pursued through exploration, and even where the sulphide ore body ends, further exploration along the channel is likely to discover further 'pools' of sulphides (ore bodies). This characteristic has been widely exploited by Mincor, and has led to important discoveries at Miitel, Mariners, Otter Juan and Carnilya Hill.

For a simplified diagram of this geological process, please click here

Most of Kambalda's ore bodies share the above characteristics, but the original configuration has invariably been substantially altered through movements in the earth's crust over time. Thus exploration and mining of these ore bodies requires not only a detailed understanding of their formation, but an understanding of the structural changes that have occurred in the 2.7 billion years since their formation.

The ore bodies themselves typically have a distinctive ore profile, consisting of massive sulphides at the base (directly overlying the basal contact), followed by matrix sulphides and then by disseminated sulphides:

Massive sulphides
This zone can be 0.2 to 1.5 metres thick, and consists of essentially 100% sulphides. Most abundant are pyrrhotite (iron sulphide - approx 50%), pentlandite (nickel-iron sulphide - approx 35%), and minor amounts of pyrite (iron sulphide) and chalcopyrite (copper iron sulphide). The massive sulphide zone forms the lowermost unit, and lies directly on the underlying basalt unit. The grade of this unit is 10% to 14% nickel.

Matrix sulphides
This zone overlies the massive sulphide unit, and consists of a net-textured rock composed of intermixed sulphides and other non-sulphide minerals. This texture reflects the original interstitial distribution of sulphides and olivine grains in the original rock. Matrix sulphides range in grade from 3% to 8% nickel, and may be up to 1.5 metres thick.

Disseminated sulphides
This zone consists of a fine-grained (0.5 to 2mm) sprinkling of sulphides scattered throughout the ultramafic host rock. It usually grades from 0.5% to 2.0% nickel. It forms the uppermost unit of the ore zone, and may have a gradational upper boundary against the unmineralised overlying host rock.

There is enormous variability throughout the ore bodies. In many areas there may be only one or two of the components present, and this creates substantial local grade variability.

 

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