Rio Tinto : Update to Ore Reserves and Mineral Resources at Weipa

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Notice to ASX

23 February 2022

Changes to Rio Tinto Aluminium (RTA) Pacific Operations Ore Reserve and Mineral Resource Estimates for Weipa

Rio Tinto's 2021 Annual Report which will be released to the market on 24 February 2022 will include changes in estimates of Ore Reserves and Mineral Resources at RTA Pacific Operations for Weipa, compared to those published in the 2020 Rio Tinto Annual Report.

The Ore Reserve and Mineral Resource estimates 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. Ore Reserves and Minerals Resources are quoted on a 100 per cent basis (100% Rio Tinto share). Mineral Resources are reported in addition to Ore Reserves.

Rio Tinto's 2021 Annual Report will set out in full Rio Tinto's Mineral Resources and Ore Reserves position as of 31 December 2021, and Rio Tinto's interests.

RTA Pacific Operations Ore Reserve estimate, for the Amrun deposit, has decreased by 218Mt, from 1,044Mt to 826Mt. The reduction in Ore Reserve estimate is associated with a routine review of price assumptions over the life of the mine, and updated orebody knowledge. There has been no material change to the economic cut-off methodology or process. There has been no material change to the other reserve modifying factors, including governmental, tenure, environmental, cultural heritage, social, community or operational.

RTA Pacific Operations Mineral Resources exclusive of Ore Reserves, for the Amrun deposit, has increased by 172Mt, from 678Mt to 850Mt. The increase was mainly due to changes in the forecast pricing assumptions used in the reserving process. Those blocks removed from the 2021 Ore Reserve that are physically located above the remaining economic blocks have also been removed from the Mineral Resource.

RTA Pacific Operations Mineral Resources exclusive of Ore Reserves, for the East Weipa and Andoom deposits, has increased by 28Mt, from 35Mt to 63Mt. The increase was mainly due to changes in the forecast pricing assumptions used in the reserving process. Those blocks removed from the 2021 Ore Reserve that are physically located above the remaining economic blocks have also been removed from the Mineral Resource.

Supporting information surrounding the 2021 update to the Ore Reserve and Mineral Resource estimates has been provided on the following pages.

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Table A	RTA Pacific Operations Mineral Resources as at 31 December 2021
Mineral resources
		Likely	Measured resources	Indicated resources		Total Measured and		Inferred resources		Total mineral resources	Rio Tinto
		mining	as at 31 December 2021	as at 31 December 2021	as at 31 December 2021	as at 31 December 2021	as at 31 December 2021	Interest
		method(a)	Tonnage	Grade		Tonnage	Grade		Tonnage	Grade		Tonnage	Grade		Tonnage Grade
Bauxite			Mt	% Al2O3	% SiO2	Mt	% Al2O3	% SiO2	Mt	% Al2O3	% SiO2	Mt	% Al2O3	% SiO2	Mt	% Al2O3	% SiO2	%
Rio Tinto Aluminium (Australia)(b)
- Amrun(c)		O/P	100	49.5	11.6	488	50.2	11.8	589	50.1	11.7	262	51.7	12.1	850	50.6	11.8	100.0
- East Weipa and Andoom(c)	O/P	63	49.5	8.4	-	-	-	63	49.5	8.4	-	-	-	63	49.5	8.4	100.0
- Gove		O/P	28	49.0	6.8	5.0	49.0	6.6	33	49.0	6.8	0.6	49.1	6.8	34	49.0	6.8	100.0
- North of Weipa		O/P	-	-	-	-	-	-	-	-	-	1,330	52.0	11.6	1,330	52.0	11.6	100.0
Total (Australia)			191	49.4	9.9	494	50.2	11.7	684	50.0	11.2	1,592	51.9	11.6	2,276	51.3	11.5

Total mineral resources as at 31 December 2020

Tonnage Grade

Mt	% Al2O3 % SiO2
678	50.3	11.9
35	51.1	8.3
34	48.7	6.8
1,330	52.0	11.6
2,077	51.3	11.6

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1. Likely mining method: O/P = open pit/surface; U/G = underground.
2. Rio Tinto Aluminium bauxite Resources are stated as dry product tonnes and total alumina and silica grades.
3. Amrun and East Weipa and Andoom Resource tonnes increased following conversion of Reserves to Resources based on updated economic assumptions. A JORC Table 1 in support of this change will be released to the market contemporaneously with the release of this Annual Report and can be viewed at riotinto.com/invest/financial-news-performance/resources-and-reserves.

Table B RTA Pacific Operations Ore Reserves as at 31 December 2021

Ore reserves

		Proved ore reserves		Probable ore reserves		Total ore reserves			Rio Tinto
												share
	Type	as at 31 December 2021	as at 31 December 2021	as at 31 December 2021	Rio Tinto	Recoverable
	of										Interest
	mine(a)	Tonnage	Grade		Tonnage	Grade		Tonnage Grade			mineral
Bauxite (b)		Mt	% Al2O3	% SiO2	Mt	% Al2O3	% SiO2	Mt	% Al2O3	% SiO2	%	Mt
Rio Tinto Aluminium (Australia)(c)
- Amrun(d)	O/P	258	54.2	9.2	568	54.9	9.1	826	54.7	9.1	100.0	826
- East Weipa and Andoom(d)	O/P	77	51.7	7.4	1	52.5	9.2	78	51.7	7.4	100.0	78
- Gove(e)	O/P	64	50.6	5.8	0.4	50.0	5.9	64	50.6	5.8	100.0	64
Total (Australia)		398	53.1	8.3	570	54.9	9.0	968	54.2	8.7		968

1. Type of mine: O/P = open pit/surface, U/G = underground.
2. Bauxite Reserves are stated as recoverable Reserves of marketable product after accounting for all mining and processing losses. Mill recoveries are therefore not shown.
3. Australian bauxite Reserves are stated as dry tonnes and total alumina and silica grade.

Total ore reserves

as at 31 December 2020

Tonnage	Grade
Mt	% Al2O3	% SiO2
1,044	54.0	9.1
100	51.4	7.5
80	50.4	5.6
1,225	53.5	8.8

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1. Amrun and East Weipa and Andoom Reserve tonnes decreased following updated economic assumptions, updated orebody knowldege and mining depletion. A JORC Table 1 in support of this change will be released to the market contemporaneously with the release of this Annual Report and can be viewed at riotinto.com/invest/financial-news-performance/resources-and-reserves.
2. Gove Reserve tonnes decreased following updated economic assumptions and mining depletion.

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Summary of information to support the Mineral Resource reporting

The RTA Pacific Operations Mineral Resource estimate is supported by a JORC Table 1 (Sections 1, 2 and 3) document provided in the appendix to this release and located at Resources & reserves (riotinto.com).

An increase in RTA Pacific Operations Mineral Resources coincides with the write back of bauxite ore from Ore Reserves to Mineral Resources due to a routine review of price assumptions over the life of the mine. The methodology of determining Mineral Resources has not changed. The bauxite assets have been in operation for more than fifty years and are well understood. Resource work is currently more focussed on asset evaluation rather than exploration, systematically bringing the bauxite classification to higher levels of confidence.

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

RTA Pacific Operations have two bauxite deposits, one at Gove (NT) and one at Weipa (QLD). The host rocks of the two mine sites have been converted to bauxite via a continuum of weathering. High annual rainfall and a geologically stable environment has provided the perfect ingredients for these world-class bauxite deposits to form over many millions of years. A deep saprolitic zone overlain by a classic mottled zone below the bauxite ore attests to this.

The process of bauxitisation involves the conversion of kaolinite to the bauxite minerals gibbsite and boehmite. The principal influence on the process is the composition, supply, and movement of groundwater. The pH of the groundwater is lowered during the process of bauxitisation, and we note that the process is still ongoing as we see a low pH regularly throughout the ground water monitoring bores across the RTA mining leases. To a lesser extent there are organic influences such as vegetation, and possibly burrowing organisms and temperature.

Pisolitic textures are dominant, with variable cementation. However, variably cemented coarser nodule horizons are also common. Some deeper bauxite, as well as underlying laterite, contain abundant interconnected solution cavities, which may have been created geochemically and/or biologically (associated with root channels). Modern day root channel structures and infill, in the upper part of the bauxite, are common. Gibbsite is the major ore mineral, with boehmite being of lesser significance.

Bauxite occurs on laterally extensive plateaus. The plateaus have been variably dissected by erosion into distinguishable main plateau areas, each with their own characteristics, (outlined further in Table 1). The bauxite orebodies are interpreted as flat-lying horizons with topography dictating the geometry. The orebodies are generally overlain by a thin (<1 m) overburden cover and occasional red soil. Beneath the bauxite ore is often a transition zone defined by angular and lumpy textures and a geochemical signature of higher silica and lower alumina. The transition zone is often underlain by the ironstone, with a distinct change in physical properties, particularly the colour.

Drilling techniques; sampling, sub-sampling method and sample analysis method

The current drilling method at Weipa utilizes aircore drilling. The typical aircore rig is a Land Cruiser mounted rig with a small enough wheelbase to traverse drill lines cleared with one D-6 dozer blade width. Aircore drilling forces compressed air down a space inside the drill rods to the bit face, where the air is then used to return the sample up the inner tube of the drill rod and out via a cyclone. A three bladed HQ aircore bit is attached to 4-inch rods. The drilling system has been designed to reduce grinding of the sample. Historic drilling utilised auger drilling (until 1991). Drilling at Gove is conducted using a tractor mounted 44 mm diameter vacuum drill rig.

Logging is currently conducted on Panasonic Toughpads and data is captured in an offline acQuire logging package at the drill rig. This system allows for data validation to be applied during logging as well as a streamlined method of exporting the data for importing into the main RTA Geology database. Logging is qualitative in nature, i.e., based on lithology. Currently there are ~20 lithologies common to the deposits that get modelled into 4 horizons for the estimation of bauxite resources. All sample intervals (0.25 m) are logged.

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Logged lithologies are vetted against historical drill holes and assay parameters.

Samples for geologic logging and analysis are collected on 0.25 m intervals (~2-3 kg) down hole. Whole samples are collected beneath a cyclone return system, i.e., no sample splitting is conducted, or sub samples taken. Multiscreen sampling is undertaken initially to determine optimum screen size for beneficiation at each deposit. Once determined, samples are then beneficiated at the appropriate screen size (1.7 mm for East Weipa, 0.3 mm for Andoom and 0.6 mm for the Amrun deposits). Bauxite at Gove is direct shipping ore (DSO).

Samples are processed and XRF analysed for the major oxides: Al2O3, SiO2, Fe2O3, TiO2 and LOI, as well as minor elements and recovery.

Estimation methodology

Basic geostatistical analysis is used to help with domaining decisions. Most deposits were modelled as a single laterally extensive domain, apart from Moingum (Hey Point), where two lateral extensive domains have been modelled due to difference in source rocks affecting thickness and grade. Four horizon codes, based on the lithology and assays, are assigned for the modelling and estimation of bauxite resources at Weipa and three at Gove vertically. Interpretation and estimation are currently performed using Datamine's Studio RM software, with variography undertaken in FSS International Consultants (Australia) GS3M software.

The bauxite horizon is flattened to the mid-point of the drill hole intersections at East Weipa and Andoom, while drill hole collars are flattened to constant elevation at Amrun, Norman Creek and Moingum (Hey Point). The wireframes are filled with blocks on an in/out basis; there is no sub-blocking or block proportions used. For the bauxite horizon, major oxides; LOI and Recovery are estimated into parent cells using ordinary kriging. Overburden and red soil are assigned 0% recovery for the estimation of resources. Cemented bauxite grade is estimated as part of the bauxite horizon and assigned a 100% recovery; the proportion of cemented bauxite is estimated as an indicator variable. Major oxide chemistry is also estimated for the overburden, red soil and ironstone horizons, where data is available. Ordinary kriging is used for interpolation, using the variogram models for the bauxite. Block sizes are determined by ½ the drill hole spacing for each deposit.

A multiple pass search strategy is used to estimate grades utilising different sized search ellipses that include a specified number of samples and drill holes. The lowest category (Pass 4) is not used in the East Weipa and Andoom areas. Maximum extrapolation distance is slightly less than the maximum search radii due to the requirement to use at least two holes to estimate each block.

Cut-off grades and modifying factors

RTA Pacific Operations employs a standard approach to identify Mineral Resource volumes with reasonable prospects for eventual economic extraction.

Once the Ore Reserves are defined based on applied economic factors in the reserving process, the remaining blocks are evaluated based on grade cut-offs, thickness cut-offs and location (environmental, cultural heritage and infrastructure buffers) for each of the different deposits, and Mineral Resources defined.

Criteria used for Mineral Resource classification

Classification within the bauxite horizon is based on the search pass used to estimate grades, using increasing search radii and decreasing numbers of samples for each subsequent pass. Passes 1 and 2 are classified as Measured Mineral Resources (120 to 180 m), Pass 3 as Indicated (360 m) and Pass 4 as Inferred.

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