Dilution: GEO’s get some “skin” in the game

Typically, conversions from mineral resources to mineral reserves are handled by mining engineers, rather than geologists. One of the key concerns in reserve conversion is how to apply dilution so that the grades sent to the process facility meet expectations and the metal mined reconciles with the amount produced.  In preparing technical prefeasibility and feasibility studies, SRK commonly prepares both resources and reserves. Clients benefit from having two disciplines apply the best possible tools to complete the task. 

PROBLEM: How can dilution be determined in a narrow-vein deposit? Is there a more accurate way to estimate the amount of mineable ore on the perimeter of the model compared to the full block volume when global assumptions drop grades below cut-off.

SOLUTION: In a recent case study, the best means of estimating dilution came from the geologist’s toolkit. Using Leapfrog Geo, SRK created an inflated gradeshell to simulate the predicted “over-mining” in an open pit operation. We applied this method in a structurally-controlled, epithermal gold-silver deposit containing seven mineral domains. The SRK project engineer and geologist worked closely to define the dimensions of the dilution envelope or “skin” by considering the geometry and thickness of mineralisation, mining methods and equipment limitations. 

In areas of tabular shallow dipping mineralisation, SRK modelled dilution with a thin vertical and broad radial envelope. In more massive areas of the deposit, we built the dilution envelope to a fixed radius outside the interpreted mineralised shell. We applied structural controls and measurements from the original gradeshell interpretations and adjusted them to mimic mining, using GEO’s “Interpolant/New Distance” function. The speed and flexibility of the software allowed us to run several iterations, achieving the desired domain-specific outcome.

Generating the original gradeshells in the same software facilitated the development of the dilution envelopes, but the approach is also valid with outsourced wireframes. In this case, the skin was assigned a zero grade, but we could have used envelope composites to estimate fringing contributions to the reserve. In that application, capping fringe composites to the mining cut-off grade is recommended. 

With the case study a success, it appears this technique can be applied beyond epithermal gold into tabular or complex seam geometry.

Jay Pennington: jpennington@srk.com


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