Mine-to-Mill Reconciliation: Are Planned Gold Ounces Showing Up?
A gold mine can meet its mining-tonnage target and still miss its production forecast. The reason may be lower grade, unexpected dilution, ore loss, stockpile movements, plant recovery or measurement differences. Mine-to-mill reconciliation helps management—and investors—understand where planned metal changed as material moved from the geological model through mining and processing.
Evergreen investor education. This article does not describe the operating performance, resource model or production results of any GBDS project.
Reconciliation connects several measurement systems
The long-term resource model estimates tonnes and grade in three dimensions. Short-term grade-control work refines that picture for mining. Survey systems measure material movement, stockpile records track inventory, the plant samples feed and tailings, and the refinery reports final metal. Reconciliation compares these linked estimates over a defined period.
A good process does more than produce one percentage. It separates differences in ore tonnes, waste classification, head grade, recovery and inventory. The JORC Code establishes minimum standards for public reporting of Exploration Results, Mineral Resources and Ore Reserves, emphasizing transparency, materiality and competent technical judgment. Reconciliation evidence can help test whether the geological and operating assumptions behind that reporting remain reasonable.
Dilution and ore loss affect grade differently
Dilution occurs when lower-grade or barren material is mined with ore, increasing tonnes but reducing average grade. Ore loss occurs when material expected to be economic is left behind or sent to waste. Both can reduce recovered ounces, but their causes and remedies differ. Narrow mineralized zones, complex contacts, blast movement, equipment selectivity and grade-control spacing can all influence the result.
Stockpiles can obscure timing
Material mined in one quarter may be processed in another. A company can therefore report mining grades that differ from mill grades without an immediate model failure. Investors should check whether stockpiles increased or decreased, how their tonnes and grades were estimated, and whether adjustments are based on surveys and sampling rather than unsupported assumptions.
A simple reconciliation example
Hypothetical example: a monthly plan expects 100,000 tonnes at 2.0 grams per tonne, equal to 200 kilograms of contained gold before recovery. The mill instead records 105,000 tonnes at 1.7 g/t, or 178.5 kilograms contained. The higher tonnage does not offset the lower grade: measured contained metal is about 89% of plan. At 90% recovery, approximately 160.7 kilograms would be recovered. The figures are illustrative and do not describe an actual operation.
The variance could reflect dilution, ore loss, model overestimation, stockpile timing, sampling bias or several factors together. One month may not establish a trend, but repeated unfavorable differences deserve explanation and corrective action.
Recovery belongs in a separate comparison
Geological reconciliation asks whether mined tonnes and grade match the model. Metallurgical reconciliation asks whether the plant recovered the gold contained in its feed. Mixing the two can hide the source of a shortfall. Investors should compare modeled ore with mined ore, mined ore with mill feed, and contained feed metal with final recovered metal.
The CIM Definition Standards distinguish Mineral Resources and Mineral Reserves and require consideration of modifying factors for reserve conversion. Actual operating reconciliation can provide important feedback on geological confidence, mining assumptions and processing performance.
What investors should look for
Useful disclosure explains the comparison period, measurement points, stockpile treatment and whether variances are temporary or persistent. Watch for repeated reserve or guidance reductions, unexplained grade shortfalls, rising dilution, falling recovery or large inventory adjustments. Also look for management actions such as infill drilling, improved blast tracking, tighter ore-control boundaries and additional sampling.
The investor takeaway
Reconciliation is an operating reality check. Consistent results support confidence that models, mining practices and plant measurements work together. Persistent negative variance can reduce production, raise unit costs and shorten mine life. The most informative question is not whether every period matches perfectly, but whether differences are measured, explained and corrected.
Educational information only; not investment, legal or technical advice. Mining investments involve substantial geological, commodity-price, permitting, financing and operating risks. Review qualified technical disclosure and seek appropriate professional advice before making investment decisions.