Rio Tinto plc – Annual report – 31 December 2017
1 Principal accounting policies (extract)
(h) Deferred stripping (note 14)
In open pit mining operations, overburden and other waste materials must be removed to access ore from which minerals can be extracted economically. The process of removing overburden and waste materials is referred to as stripping. During the development of a mine (or, in some instances, pit; see below), before production commences, stripping costs related to a component of an orebody are capitalised as part of the cost of construction of the mine (or pit) and are subsequently amortised over the life of the mine (or pit) on a units of production basis.
Where a mine operates several open pits that are regarded as separate operations for the purpose of mine planning, initial stripping costs are accounted for separately by reference to the ore from each separate pit. If, however, the pits are highly integrated for the purpose of mine planning, the second and subsequent pits are regarded as extensions of the first pit in accounting for stripping costs. In such cases, the initial stripping (i.e. overburden and other waste removal) of the second and subsequent pits is considered to be production phase stripping (see below).
The Group’s judgment as to whether multiple pit mines are considered separate or integrated operations depends on each mine’s specific circumstances.
The following factors would point towards the initial stripping costs for the individual pits being accounted for separately:
- If mining of the second and subsequent pits is conducted consecutively following that of the first pit, rather than concurrently.
- If separate investment decisions are made to develop each pit, rather than a single investment decision being made at the outset.
- If the pits are operated as separate units in terms of mine planning and the sequencing of overburden removal and ore mining, rather than as an integrated unit.
- If expenditures for additional infrastructure to support the second and subsequent pits are relatively large.
- If the pits extract ore from separate and distinct orebodies, rather than from a single orebody.
If the designs of the second and subsequent pits are significantly influenced by opportunities to optimise output from several pits combined, including the co-treatment or blending of the output from the pits, then this would point to treatment as an integrated operation for the purposes of accounting for initial stripping costs.
The relative importance of each of the above factors is considered in each case.
In order for production phase stripping costs to qualify for capitalisation as a stripping activity asset, three criteria must be met:
- It must be probable that there will be an economic benefit in a future accounting period because the stripping activity has improved access to the orebody;
- It must be possible to identify the “component” of the orebody for which access has been improved; and
- It must be possible to reliably measure the costs that relate to the stripping activity.
A “component” is a specific section of the orebody that is made more accessible by the stripping activity. It will typically be a subset of the larger orebody that is distinguished by a separate useful economic life (for example, a pushback).
Production phase stripping can give rise to two benefits: the extraction of ore in the current period and improved access to ore which will be extracted in future periods. When the cost of stripping which has a future benefit is not distinguishable from the cost of producing current inventories, the stripping cost is allocated to each of these activities based on a relevant production measure using a life-of-component strip ratio. The ratio divides the tonnage of waste mined for the component for the period either by the quantity of ore mined for the component or by the quantity of minerals contained in the ore mined for the component. In some operations, the quantity of ore is a more appropriate basis for allocating costs, particularly where there are significant by-products. Stripping costs for the component are deferred to the extent that the current period ratio exceeds the life of component ratio. The stripping activity asset is depreciated on a “units of production” basis based on expected production of either ore or minerals contained in the ore over the life of the component unless another method is more appropriate.
The life-of-component ratios are based on the ore reserves of the mine (and for some mines, other mineral resources) and the annual mine plan; they are a function of the mine design and, therefore, changes to that design will generally result in changes to the ratios. Changes in other technical or economic parameters that impact the ore reserves (and for some mines, other mineral resources) may also have an impact on the life-of-component ratios even if they do not affect the mine design. Changes to the ratios are accounted for prospectively.
It may be the case that subsequent phases of stripping will access additional ore and that these subsequent phases are only possible after the first phase has taken place. Where applicable, the Group considers this on a mine-by-mine basis. Generally, the only ore attributed to the stripping activity asset for the purposes of calculating a life-of-component ratio, and for the purposes of amortisation, is the ore to be extracted from the originally identified component.
Deferred stripping costs are included in “Mining properties and leases” within “Property, plant and equipment” or within “Investments in equity accounted units”, as appropriate. Amortisation of deferred stripping costs is included in “Depreciation of property, plant and equipment” within “Net operating costs” or in “Share of profit after tax of equity accounted units”, as appropriate.
Critical accounting policies and estimates (extract)
(v) Deferral of stripping costs (note 14)
Stripping of waste materials takes place throughout the production phase of a surface mine or pit. The identification of components within a mine and of the life of component strip ratios requires judgment and is dependent on an individual mine’s design and the estimates inherent within that. Changes to that design may introduce new components and/or change the life of component strip ratios. Changes in other technical or economic parameters that impact ore reserves may also have an impact on the life of component strip ratios, even if they do not affect the mine’s design. Changes to the life of component strip ratios are accounted for prospectively.
The Group’s judgment as to whether multiple pit mines are considered separate or integrated operations determines whether initial stripping of a pit is deemed to be pre-production or production phase stripping and, therefore, the amortisation base for those costs. The analysis depends on each mine’s specific circumstances and requires judgment: another mining company could make a different judgment even when the fact pattern appears to be similar.
14 Property, plant and equipment (extract)
(a) At 31 December 2017, the net book value of capitalised production phase stripping costs totalled US$1,815 million, with US$1,374 million within Property, plant and equipment and a further US$441 million within Investments in equity accounted units (2016 total of US$1,967 million with US$1,511 million in Property, plant and equipment and a further US$456 million within Investments in equity accounted units). During the year capitalisation of US$327 million was partly offset by depreciation of US$299 million (including amounts recorded within equity accounted units). Depreciation of deferred stripping costs in respect of subsidiaries of US$194 million (2016: US$203 million; 2015: US$173 million) is included within “Depreciation for the year”.