1.3 Mineability & Economic Limits

Mineability is the assessment of whether a defined part of a mineral deposit can be extracted and used through a workable engineering system under stated technical and economic conditions. Economic limits identify the spatial and temporal extent selected by comparing feasible extraction alternatives under a stated economic objective. Both depend on the proposed mine, its constraints and its assumptions. They connect geological knowledge to decisions about extraction, material destinations, investment and the extent of a mine plan.

Assessing a Feasible Extraction Unit

A geological model describes estimated material properties and their spatial distribution. A mine plan must turn that description into accessible working areas, a sequence and a flow of material. Geological geometry, mining method, extraction losses, dilution, equipment capacity and processing capacity therefore need to be considered together. Rupprecht's feasibility discussion treats mine design as an assessment of achievable ore and waste movements subject to these constraints.^[1]

The appropriate unit of assessment may be a bench phase, stope, panel or connected group of working areas. A favourable block in a model is not necessarily an independently extractable parcel. Ask what access, surrounding excavation and services must exist before that material can be recovered.

Depth, shape, orientation, surrounding rock and surface topography influence the available mining methods and layouts. For surface mining, the spatial problem includes gaining access through overlying or adjacent material. Underground extraction requires a suitable access and excavation arrangement within the surrounding ground. These settings share a need for workable geometry, but the relevant constraints differ.^[2]

A useful comparison therefore gives each candidate method its own geometry and operating assumptions. Comparing methods at identical production rates while ignoring the development needed to achieve those rates can conceal the main difference between them.

Material Selection and Planning Inventory

A cut-off grade is a selection threshold applied for a specified material decision. Its economic basis can depend on prices, recoveries, relevant costs, grade distribution and capacity constraints. Githiria and colleagues illustrate those relationships in a single-element open-pit model; the model's assumptions bound its application.^[3]

The decision must be named: whether to extract a mining unit, send already mined material to treatment, or allocate constrained treatment capacity. These questions have different alternatives. A threshold used to route material after excavation cannot, by itself, demonstrate that developing a new mining area is justified.

In this discussion, a planning inventory means the material quantities retained for a particular engineering plan. Its usefulness depends on a clear boundary: which design it belongs to, where quantities are measured, which allowances are included and whether material is available in the required period. This is a working description, not a new resource or reserve classification.

Inventory and schedule answer different questions. An annual target requires material that can be accessed and delivered during that year. A large total inventory does not establish that availability. Conversely, removing a mining area from one schedule does not prove that its mineralization has disappeared; the reason for exclusion may concern access, timing or the selected scenario.

Establishing the Economic Boundary

Economic evaluation connects the physical schedule to expenditure and receipts. Relevant inputs include production timing, delivered grade, recovery, product prices, capital expenditure and operating costs. Nell and Burks describe using a project financial model to compare technical options and examine sensitivity to key assumptions. The model must represent the option being evaluated rather than a separate, more favourable production case.^[4]

For an extension, the comparison concerns what changes if that extension is undertaken. Consider a proposed additional mining area that needs new access. Counting its expected product receipts while omitting the access expenditure does not describe that decision. Conversely, charging the extension again for expenditure already irreversibly incurred can obscure the remaining choice. Shared facilities require explicit treatment of any additional capacity, operating period or replacement they would need.

The economic limit is consequently a boundary of a feasible alternative, not simply a contour joining favourable assays. It can change when the available method, infrastructure or sequence changes. The comparison should also state its objective: maximizing discounted project value, for example, need not select the same plan as maximizing extracted tonnes. A conclusion is meaningful only against the stated objective and constraints.

Keeping Resource and Reserve Claims Distinct

Formal reporting terms require their own evidence. As a dated example of professional reporting guidance, SME's July 2017 Guide requires reasonable prospects for eventual economic extraction when defining Mineral Resources. It describes Mineral Reserves as the economically mineable portion of Measured and/or Indicated Resources, supported by appropriate studies and Modifying Factors. Geological confidence alone therefore does not establish reserve status.^[5]

The same guide requires a stated reserve reference point and allowances for dilution and mining losses. A tonnage described as mineable in an internal exercise should not be presented as a declared reserve merely because it lies inside a design. These are distinctions from that edition of the guide, not a statement of current securities requirements in every jurisdiction.^[5]

An engineering comparison can retain alternatives that are still being investigated, provided their status is explicit. The result should distinguish what the evidence supports now from what depends on additional work.

Matching Evidence to the Decision

Study confidence should be assessed through the supporting work. AACE's August 2022 classification guidance ties estimate class to the maturity of scope definition and explains that class does not establish a universal accuracy range. It also identifies the risk created when interdependent parts of a project have uneven definition. A detailed extraction plan cannot resolve an uncertain essential service merely by attaching a precise cost total.^[6]

For mineability, the practical questions are which assumptions could change the preferred method or boundary, and how consequential that change would be after commitment. An uncertain access location deserves particular attention before excavation makes relocation difficult. A provisional operating choice that can be revised readily may justify a different investigation priority.

Sensitivity testing helps identify influential assumptions; coherent scenarios test combinations that could occur together. The interpretation should name the dependency. A plan that succeeds only if a particular access route becomes available remains conditional on that route, however attractive its calculated return.

Including Closure in the Extraction Decision

The physical consequences of a mine plan continue beyond the final sale. Brock, Weeks and Heyes describe integrated closure planning as an evolving process involving intended land use, closure objectives, stakeholders, physical and chemical stability, and the activities needed to achieve agreed outcomes. They also describe updating closure costs as knowledge and mine plans change.^[7]

This connects the extraction boundary to the final landform and residual management responsibilities. In a comparison between two extents of mining, identify whether the larger extent changes disturbed land, waste management, water treatment or subsequent monitoring. Those consequences belong to the alternative that creates them. They should not disappear from the assessment merely because some expenditure occurs after production ends.

Revising the Conclusion When Conditions Change

Consider a qualitative scenario in which testwork indicates that a material domain requires a different treatment route. If a workable route exists, trace the planning consequences: revise selection and routing criteria, identify affected inventory, check treatment availability in the schedule, and reassess the economic boundary. This is an illustrative application of CIM's emphasis on material variability and a viable process route.^[8]

These revisions do not themselves assign a Resource or Reserve category; that conclusion requires the corresponding reporting evidence.^[5] The change record should distinguish physical feasibility, material destinations, scheduled availability and economic value, while identifying unresolved assumptions.