EX-99.3 4 argyle.htm

| Notice to ASX

Change to Argyle Diamonds Mineral Resources

2 March 2017

Included in Rio Tinto’s annual Mineral Resources and Ore Reserves tables, released to the market today as part of its 2016 Annual Report, are decreases in Mineral Resources at Rio Tinto’s 100 per cent-owned Argyle Diamond mine in the East Kimberley, Western Australia.

The updated Mineral Resources are reported in accordance with the Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves, 2012 (JORC Code) and the ASX Listing Rules. As such, the reported decreases relating to the Argyle Mine requires the additional supporting information set out in this release and its appendix.

Rio Tinto’s Ore Reserves and Mineral Resources are set out in full in its 2016 Annual report.

During 2016, estimated Argyle Mineral Resources exclusive of Ore Reserves decreased by 29Mt from 44Mt to 15Mt. Previously reported Mineral Resources included mineralisation that could support a potential extension of the currently planned mine life. As mine life extension options are under ongoing review, the reported Mineral Resources have now been restricted to those parts of the mineralisation which can be potentially developed, mined and processed during the existing mine-life defined by the current underground operation. It is for this reason that there has been a material reduction in reported Mineral Resources in 2016.

As the change to Argyle Mineral Resources does not impact reported Ore Reserves, this reduction has no bearing on the current mine plan which sees production continuing until 2021.

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2016 Annual report Mineral Resource table, showing line items relating to Argyle Diamonds
| Resources at end 2016 |  | Total resources 2016 compared with 2015 |
| Likely |  |  |  |  |  |  |  |  |  |  |
| mining |  |  |  |  |  |  |  |  |  |  |
| method (a) |  |  |  |  |  |  |  |  |  |  |
|  | Measured | Indicated | Inferred | 2016 | 2015 |
|  | Millions of tonnes | carats per tonne | Millions of tonnes | carats per tonne | Millions of tonnes | carats per tonne | Millions of tonnes | carats per tonne | Millions of tonnes | carats per tonne | Rio Tinto interest %
DIAMONDS |  |  |  |  |  |  |  |  |  |  |  |
Argyle Diamonds | O/P + |  |  | 15 | 3.2 |  |  | 15 | 3.2 | 44 | 3.4 | 100
(Australia) (b) | U/G |  |  |  |  |  |  |  |  |  |  |

Notes

(a) | Likely mining method: O/P = open pit; O/C = open cut; U/G = underground; D/O = dredging operation.
(b) | Argyle Resources, previously reported as AK1 pipe, decreased following a review and curtailment of development studies. A JORC table 1 in support of these changes will be released to the market contemporaneously with the release of this Annual report and can be viewed at riotinto.com/JORC. Argyle Resources are reported to a nominal 0.5 millimetre lower cut-off size and a final re-crushing size of 6 millimetres.

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Summary of information to support the Mineral Resources estimates

Mineral Resource Estimates for Argyle Diamonds are supported by the information set out in the appendix to this release and also located at riotinto.com/JORC in accordance with Table 1 checklist in the JORC Code (Sections 1 to 3 and Section 5).

A change in the Argyle Diamonds Mineral Resources follows the ongoing review of potential mine-life extension options and restricts reported Resources to that component of the known mineralisation which may be developed, mined and processed within the current operational mine life.

The following summary of information for Mineral Resource Estimates is provided in accordance with rule 5.8 of the ASX Listing Rules.

Geology and geological interpretation

Argyle Diamonds is a mining operation comprising a diamond processing plant and underground mine located in the East Kimberley region of Western Australia. The operation has been in existence since 1983. Mining transitioned from open pit operations to underground operations in 2013.

Primary mineralisation at Argyle comprises a volcanic lamproite intrusion. Argyle Diamonds have constructed a single geology model representing this mineralisation based on comprehensive data sets drawn from drilling data, surface outcrops, wall mapping during the open pit operation and mapping of exposures in the tunnels of the current underground mining operation. In excess of 700 holes have been drilled into the deposit, totalling more than 157,000 metres of drilling.

In addition, surface mineralisation exists in the form of alluvial deposits, surface stockpiles and deposited tailings.

The reported Mineral Resources represent a subset of the interpreted mineralisation and is exclusive of reported Ore Reserves.

Drilling techniques

Core drilling has been the primary drilling technique for both sample collection and geological definition. Large diameter core (200mm), PQ (85mm), HQ (63.5mm) and NQ (47.6mm) drilling techniques have all been used to define mineralisation over a number of campaigns drilled from the original surface, open pit benches and underground locations.

Sampling, sub-sampling and sample analysis methods

Sample processing methods include bulk processing of the large diameter core and PQ samples through a Mk iii sample treatment plant, and caustic fusion of micro-diamond samples from the smaller diameter drill cores.

The reported Mineral Resource largely relies on micro-diamond sampling techniques. Samples are collected in 20kg aliquots, digested and the residues are hand sorted for micro-diamonds larger than 0.106mm. The size distribution of stones from the mineralisation at Argyle and the relationship between commercially recoverable stones and micro-diamonds is known from the earlier bulk sampling campaigns as well as 30 years of operational diamond recovery. This relationship is used to estimate commercially recoverable grades from the micro-diamond samples. Reported grades are expressed at a bottom stone size of 0.5mm. Micro-diamond sample processing has been carried out over the years at Rio Tinto laboratories in Canning Vale and Belmont, Western Australia, and Thunder Bay, Ontario, Canada.

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Criteria used for classification

Resource classification is directly related to geological interpretation, drill hole and sampling density and grade estimation of particular geological domains. Classification is also influenced by potential mining methods, with bulk mining methods likely to better represent global grade estimates from larger block sizes than smaller scale selective mining methods requiring smaller estimation blocks.

The Mineral Resource is reported at an indicated classification and reflects the Competent Person’s view of how well that component of the deposit has been evaluated. No measured or inferred Mineral Resource is reported.

Estimation methodology

Estimation constraints are defined for mineralisation boundaries and internal sub-domains representing different depositional emplacements (lamproite units) classified by stone size and quality distribution. Estimation is undertaken using ordinary kriging techniques within these sub-domains to interpolate sample grades into the block model. The estimated model is validated by visual checks of estimated grades against composite drill hole data. The model has been reconciled with actual production data over the history of the Argyle operation and the results are used to verify the key parameters used in both the development of the model and the estimation of grade within the model.

Reasonable prospects for eventual economic extraction

Prospects for eventual economic extraction are assessed in scoping studies for individual potential developments. The reported Mineralised Resource represents mineralisation which may be extracted using a combination of underground and surface mining methods.

Previously reported Mineral Resources have included mineralisation which could support a potential extension of the planned mine-life. As any material mine-life extension options are under review, reported Mineral Resources have now been restricted to a smaller component of known mineralisation which may be developed, mined and processed within the currently planned mine life. This material includes a smaller high grade sub-component of the primary mineralisation beneath the existing underground mine, which could be mined using small scale underground methods; a component of the coarse tailings storage which may be excavated and reprocessed to recover diamonds deposited at times of different plant configuration; and a small high grade stockpile of reject material from the existing diamond recovery process which could be amenable to further recovery using alternate processing techniques.

The scoping studies referred to are based on low-level technical and economic assessments, and are insufficient to support estimation of Ore Reserves or to provide assurance of an economic development case at this stage, or to provide certainty that the conclusions of the scoping studies will be realised.

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Competent Persons Statement

The material in this report that relates to Mineral Resources is based on information prepared by Stephen Brennan, a Competent Person who is a Member of The Australasian Institute of Mining and Metallurgy, Murray Rayner, a Competent Person who is a Member of The Australasian Institute of Mining and Metallurgy, and David Ford, a Competent Person who is a Member of The Australasian Institute of Geoscientists. Mr Brennan, Mr Rayner and Mr Ford are all full-time employees of Rio Tinto.

Mr Brennan, Mr Rayner and Mr Ford have sufficient experience that is relevant to the style of mineralisation and type of deposit under consideration and to the activity being undertaken to qualify as Competent Persons as defined in the 2012 Edition of the ‘Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves’. Each of Mr Brennan, Mr Rayner and Mr Ford consents to the inclusion in the report of the material based on information prepared by him in the form and context in which it appears.

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Contacts

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