Rio Tinto has announced approval of a $55 million 1 investment in development capital to start underground mining and expand production at its Kennecott copper operations in Utah, United States, initially focusing on an area known as the Lower Commercial Skarn (LCS). The LCS has a Mineral Resource of 7.5 Mt at 1.9% copper, 0.84 g/t gold, 11.26 g/t silver, and 0.015% molybdenum identified based on drilling and a Probable Ore Reserve of 1.7 Mt at 1.9% copper, 0.71 g/t gold, 10.07 g/t silver, and 0.044% molybdenum. This initial Mineral Resource and Ore Reserve is reported in accordance with the Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves, 2012 (JORC Code).

Mineral Resources and Ore Reserves are quoted in this release on a 100% basis. Mineral Resources are reported in addition to Ore Reserves. This declaration of initial Mineral Resources follows completion of orebody knowledge drilling in the Lower Commercial Skarn (LCS) deposit and a feasibility study.

Geology and geological interpretation; The LCS deposit is located in the Bingham mining district southwest of Salt Lake City, Utah. The Bingham mining district is dominated by the Bingham Canyon copper-molybdenum-gold porphyry system, which consists of the Eocene monzonite-quartz monzonite Bingham Stock and deformed siliciclastic and carbonate country rock of the Paleozoic Bingham Mine Formation. The LCS deposit is hosted in mineralized skarn of the Lower Commercial Limestone (LCLS) unit of the Lower Bingham Mine Formation.

This unit is proximal to the Bingham Canyon porphyry system and has been altered to copper-gold hosting calc-silicate skarn through prograde metasomatism with localized retrograde massive sulphide and clay. This unit has been variably folded and faulted prior to mineralization, resulting in fold thickening and repetition of the units across faults. Drilling, sampling, sub-sampling method and sample analysis method; The LCS deposit is defined by 73 diamond drill holes, consisting of 37 pre-existing surface and underground holes, drilled between 1964 and 2012; 25 underground holes drilled during prefeasibility studies in 2015 and 2016; and 11 holes drilled in 2017 and 2018 to support feasibility studies .

The recent (2015 to 2017) prefeasibility through feasibility study drilling program totals 36 holes and 6,690 m of HQ coring, utilizing comprehensive geoscientific core logging, select downhole acoustic borehole imaging, geomechanical testing, hydrogeologic measurement, and geochemical assay to inform geologic interpretation, geotechnical characterization, and resource estimation. Final nominal drill hole spacing in the entire LCS deposit is less than 46 m for approximately half of the Mineral Resource (Indicated), with much of it drilled to less than 23 m (Measured), with the remaining resource at a nominal spacing of 91 m (Inferred). Drill core is sampled on 3 m intervals for assay by default, unless notable geologic character defines a smaller or slightly larger interval.

Typical sample intervals during the 2015 to 2017 prefeasibility through feasibility study drilling programs averaged 2.7 m. The prefeasibility through feasibility study drilling and sampling programs generated over 1,900 individual assay samples in the LCS deposit, with over 5,180 m of core assayed. Core assayed prior to 1990 were assayed by Kennecott's internal laboratories, following this all assays were completed by outside laboratories with documented internal and external quality assurance and quality control (QA/QC) procedures maintained to present. Assays and their origin laboratory are stored in the Rio Tinto acQuireTM database.

Original assay certificates are stored on Rio Tinto network servers. Bingham Canyon assay sample QA/QC procedures established in 1990 apply to all holes following 1990 as follows: Duplicate samples are generated from the remaining half core every 40th sample. Duplicate samples are generated from the crushed duplicate material every 20th sample.

Matrix matched pulp standards are inserted every 20th sample. Five percent of pulps are randomly selected for assay validation at a second lab. Given the short hole lengths and focused targeting for the 2015 to 2017 drilling programs, the automatic footage-based creation and insertion of duplicates and standards was replaced with the following manual process: One to three core sample duplicates are manually selected from the target zone in each hole.

One to three crushed sample duplicates are manually selected from the target zone in each hole. One to three matrix-matched pulp standards are inserted for manually selected sample intervals from the target zone in each hole. One sample blank is inserted in each hole.

Results for duplicates and standards are checked, flagged, retested, or resampled if deemed necessary, and stored via automated reporting from the acQuireTM database, providing confidence in the accuracy of the sampling and assaying procedures, with fit-for-purpose precision on the assay values.