• Increase font size
  • Default font size
  • Decrease font size

Nov 2007 - Footwall to Hanging

Grade Variability in Orebodies – Footwall to Hangingwall

The variation of grade in orebodies from higher grade footwall through to lower grade hangingwall is well-known when based on komatiite lava flows or VMS feeders, however, during the past year we have observed this phenomena where there is no clear feeder zone resulting in a subaqueous flow or classic lava flow.

Such new examples include Albidon’s Munali nickel (Ni-Cu-PGE) orebody in Zambia, Territory Resources’ Thelma Rosemary iron ore orebody in the NT, and Vulcan’s Kylylahti polymetallic (Cu-Ni-Co) orebody in Finland. A range of different orebodies formed at different times in very different locations

It should be emphasised that this column is based on observations which may not follow standard geological theory or theories.

The characteristic massive higher grade footwall (often against the basalt) grading through to lower grade disseminated hangingwall (often against the serp or serpentinite) is well-known amongst the komatiite nickel orebodies of Kambalda, Lanfranchi and Widgiemooltha. Figure 1 shows some drill intersections through Lanfranchi’s new Deacon orebody that clearly shows the grade variation with even some very high grades in the ~8%Ni to 9%Ni vicinity.

Sections through Jabiru’s Teutonic Bore VMS orebody clearly show that impact of the separate copper and zinc feeder zones on the mineralisation, all of which like the komatiite lava flows can readily be explained from an orebody deposition viewpoint.

However, what of hematite iron ore and Thelma Rosemary at Frances Creek in the NT. The very high grade often >62%Fe iron ore can range up to an average of 67%Fe at the appropriately named Helene 6/7 pit) at Frances Creek. There are differing footwalls and even hangingwalls along the steady 35km long strike length of the orebody, with Helene 6/7 having a common hanging and footwall that tracks the orebody.

While at Thelma Rosemary the footwall appears to be relatively more competent than the hangingwall, and as shown in Figure 1, there is clear grade variability between the higher grade footwall reducing to a more lower grade hangingwall within the orebody. There are no indications of feeder zones at Thelma Rosemary

A gradation from semi-massive footwall to disseminated hangingwall also occurs at Vulcan’s Outokumpu-style Kylylahti orebody in Finland with the semi-massive ore comprising typically of 30% to 50% of the orebody’s width.

A gradation can also be seen within the gabbro-diorites at Albidon’s Munali in Zambia, although it is not as clear as the other orebodies of Thelma Rosemary and Kylylahti and may be partly due to ore-shoots.

This phenomena of a grade variation from higher grade footwall to lower grade hangingwall is something that may not be usually considered, but the potential advantages can be very significant especially if the orebody lends itself to preferential mining, stoping the higher grade footwall first, and enhancing the initial cashflows.

This topic of grade variability in orebodies is something that we expect to return to at a later date, applying further observations. So far perhaps the only commonality between these 3 orebodies is that they could be interpreted as lying within shear zone-like structures.

Disclosure and Disclaimer: This article has been written by Keith Goode, the Managing Director of Eagle Research Advisory Pty Ltd, (ERA, an independent research company) who is an Authorised 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. Sections through Lanfranchi (Ni) and Thelma Rosemary (Fe) showing Grade VariabilityGDNnov07

  • Written by: Keith Goode
  • Wednesday, 07 November 2007

Kitco Spot Gold Price

   Click Chart for Spot Gold Price[Most Recent Quotes from www.kitco.com]

Stock Quote

  (Add .ax for ASX)