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Jun 2010 - Paste Failures

Paste Failures Provide a “Wake-Up” Call

I must admit that when I first encountered paste being used to fill stopes instead of concreted tailings and the way it behaved, I thought “WOW this is fantastic, looks like rock, fills the void perfectly, can be cable bolted, can be mined against, etc”. So it is understandable how complacency occurred, and why failures in the past year have been met with surprise.

We found out about two of the failures in our/ERA recent review of Panoramic in June 2010, there may have been others, but we have not yet encountered them. Panoramic has had two minor failures in its paste fill, being one at each of its underground nickel mining operations of Savannah and Lanfranchi, both in WA, which have resulted in dilution and lower recoveries.

Savannah Nickel Mines is located in the Kimberleys about 120km north of Halls Creek in northern WA, near the State Border with the Northern Territory, and is intrusion-type nickel sulphide mostly along the southern edge of a near vertical pipe-like structure called the Savannah intrusion within Tickalara metamorphics. Savannah produces up to ~10,000tpa contained nickel in concentrates per year.

The nickel orebody visually resembles the bone of a T-bone steak in plan, that has been minorly displaced by some major sub-horizontal faults. The mining method is sub-level open stoping with the footwall drive on strike and the vertical axis of the “T” in the T-bone forming the Northern Zone or Northern orebody resulting in stopes up to ~50m wide (from the footwall).

Nickel grades have been typically 1.3%Ni to 1.7%Ni with high by-product credits from copper and cobalt, and can range up to ~3% to 4%Ni (with the by-products proportionally increasing).

The failure occurred in the “T”. A primary stope had been cut and paste filled, and of course its boundary wall was understandably not perfectly text-book vertical, and hence had a ~50m long slight overhang into the secondary stope.

When the secondary stope was cut / blasted and stood open, the undercut paste overhang gradually failed. Savannah’s paste mainly relies on tailings, plus the chemical, plus some cement. It may be a function of cement in the paste, with Savannah typically using ~2% to 3%, curing to solid within about 24 to 27hours, but the water has to be an issue, either passing through the more permeable paste or between the boundary sides of the rock and the paste, as also seen at Lanfranchi.

Lanfranchi Nickel Mines is located in the Kambalda nickel field about 52km South of Kambalda and consists of komatiite nickel sulphide in a series of NNW/SSE striking lava channels that wrap over a WNW/ESE striking dome. Lanfranchi’s current major orebody is Deacon (~150tNipvm) which provides at least 90% of the current production rate of ~10,000tpaNi or so, at standard Kambalda nickel field average grades of ~2.5%Ni with minor copper credits.

The Deacon orebody has a strike length within the channel of ~100m. The stoping method at Deacon has been drift and fill stoping, retreating (west to east) using strike stopes of A, C and E as primaries, followed by B and D as secondaries. The primaries came out as expected and were paste filled (Lanfranchi does not have its own plant as its ore goes to Nickel West for treatment, and hence used to use red dune sand plus tailings).

When the secondary B upper block was taken a failure of ~30m or so occurred adjacent to the A block. Possibly due to the drive being alongside the A block and resulting in an exposed face, which may have been better if the drive had been in the centre of B, however, the fact is that it failed.

Current plans are to mesh and shotcrete exposed paste where necessary, which may be overkill, but further paste failures are not wanted, and then the mines may relax and simply shotcrete without the mesh.

Lanfranchi is moving towards more conventional sub-level open stoping of its Deacon orebody (again on retreat to the east) with no B or D blocks, just larger A,C,E and at greater heights, having received geotech approval for 30m high stoping blocks, as it heads into its higher grade keel zone where some of the nickel grades rise up to ~7%Ni and ~11%Ni.

Water running over the paste is an issue, such that it looks hard and a rock can bounce or skitter over it, but with the water it is in fact spongy to the touch, and if anything looks “blocky” as shown in Figure 1 from Savannah.

Probably more research is required on the failures and their prevention, however, what the paste failures have shown is that paste is still a relatively “new” medium and there is a risk in becoming too complacent about its apparent reliability qualities.

Disclosure and Disclaimer : This article has been written by Keith Goode, the Managing Director of Eagle Research Advisory Pty Ltd, (an independent research company) who is a Financial Services Representative with Taylor Collison Ltd, and with his associates, may hold interests in some of the stocks mentioned in this article. The opinions expressed in this article should not be taken as investment advice, but are based on observations by the author. The author does not warrant the accuracy or completeness of any information and is not liable for any loss or damage suffered through any reliance on its contents.

Figure 1. Paste Filled Stope Wall at Savannah (2090 level, 6065 stope)GDNjun10

  • Written by: Keith Goode
  • Tuesday, 01 June 2010

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