Groote Eylandt is dominated by Proterozoic arenites of the Dalumbu Sandstone forming a relatively low-lying plateau on the central and southern portions of the island. Headwater drainage systems incise the quartz-arenites to form radial drainage patterns. The low-lying plains to the north of the plateau are predominantly Mesozoic and Cainozoic strata overlying the Bartalumba Basalts. The majority of the western shoreline consists of the low-lying onlapping Cretaceous sediments which hosts the manganese deposit. The majority of GEMCO’s mining lease area consists of Cretaceous sediments with the exception of a discontinuous narrow strip along the eastern boundary where the Proterozoic sandstone outcrops.

The Groote Eylandt manganese orebody is a sedimentary layer that gently undulates beneath the western plains of the island. It extends over an area of approximately 50 km2 as an almost continuous horizon, varying in thickness up to 11 m and is essentially stratabound and strataform in character. The orebody consists of massive, pisolitic and oolitic manganese oxides. These oxides are thought to have originally been deposited as a chemical precipitate, forming a tabular sedimentary deposit in wave affected shallow sea-floor environments during a period of rising and falling sea levels. Following the deposition and subsequent cover by younger sediments, the western margins emerged from the sea during a worldwide drop in sea level. The depositional events were followed by a long period of tropical weathering which extensively modified the upper parts of the sediment profile. Pisolitic manganese oxides underwent partial to complete remobilisation and recrystallisation that resulted in the formation of hard cemented pisolite and massive manganese oxides.

In the present mining areas, the mined ore horizon is between 0.5 m and 10 m thick. The ‘middle’ mining horizon is typically a massive high-grade cemented ore and loose high-grade pisolite ore whereas the ‘lower’ mining horizon is a massive, high silica ore. The overlying clays and gravels were strongly oxidised and leached to form the laterites that are now excavated off the manganese ore as overburden. In most cases, overburden thickness averages between 15 m and 35 m. The lower part of the sediments below the manganese bearing beds comprise of clayey silt and fine to medium grained sand. Within the sand unit are sections of well-sorted, fine to medium grained marine sand of high transmissivity and storage, which frequently act as aquifers.

GEMCO’s operations involve mining manganese ore by open cut mining methods, sizing and washing the ore in the concentrator and transporting the final product to the Milner Bay port facility for shipping.

Mining activities are undertaken over a number of quarry regions simultaneously, with ore profile and composition differing between quarries. Quarry locations are named using alphabetical letters and may be further defined by their geographical location (e.g. B South Quarry). The quarries are typically mined in strips approximately 40 m wide, and generally between 400 m to 1,500 m long. Typical quarry depth varies between 10 and 25 m.

Open cut strip mining involves the following sequence of activities:
• Clearing vegetation using bulldozers.
• Stripping and recovering topsoil. Topsoil is stripped and pushed into windrows, before being picked up by loaders or excavators and placed into haul trucks. The topsoil is then either placed directly on areas that are ready for rehabilitation or temporarily stockpiled in designated areas for later use.
• Pre-stripping overburden. Overburden is excavated in order to gain access to the ore. When commissioning a new quarry, a fleet of excavators and haul trucks is used to remove the overburden. For future strips, the preferred method is dozer push stripping. This material is either temporarily stockpiled or placed directly within previously mined quarries. For routine mining operations, there is no further waste segregation.
• Drilling and blasting the manganese ore. The ore is drilled and blasted to break up the material so it can be easily handled. Mining areas located near Angurugu are not blasted in order to provide a safe stand-off distance from the community.
• Mining ore. A fleet of haul trucks and excavators is used to extract the ore and transport it via a network of dedicated haul roads to the Run of Mine (ROM) stockpile, located at the Primary Crushing Station (PCS).
• Backfilling quarries following ore removal. Dozers are used to backfill quarries with overburden up to the Post Mining Surface (PMS) level to create a stable and free draining landform.
• Topsoil replacement. Topsoil is spread over backfilled areas at an average depth of 0.3 m. The topsoil is then ripped.
• Revegetation of topsoil using seeds from native tree, shrub and grass species using aerial seeding as the primary seeding technique.

Since 2016, mining activities have also included the reclaiming of sands material from existing sands TSFs. This process involves a fleet of haul trucks and excavators to extract the sand tailings and transport it to designated stockpiles located near the concentrator. The processing of this sands material produces a manganese fines product known as PC02.

Ore Stockpiles
Ore is transported from quarries via a network of dedicated haul roads and stockpiled at the ROM located at the PCS. There are a few additional ore stockpiles built outside of the PCS area which generally consist of lower quality ore. Given each quarry has different ore characteristics, stockpiles are built according to the quarry the ore comes from to store ore with similar characteristics. This enables optimised blending of ore for delivery to the concentrator.

Concentrator
Manganese ore contained within the ROM stockpiles is fed into the concentrator where it is processed to a final product for transport. This process involves the following steps:
• ROM ore is crushed at the PCS and placed on a surge stockpile ahead of the concentrator;
• Crushed ore is fed into the concentrator and washed using a drum scrubber to remove clay components;
• Washed ore is screened into size fractions using vibrating screens. This results in two size fractions of ore (lump and fines) and two size fractions of waste material known as tailings (sands and slimes);
• The lump ore is fed into a rotating drum separator containing a ferrosilicon media. The manganese ore is separated from waste materials (such as quartz and silica) based on density. The ore, being denser, adheres to the ferrosilicon and sinks to the bottom of the separator, enabling it to be removed. The less dense waste materials float to the top and overflow at the discharge end of the drum;
• The fine ore is fed into a series of cyclones which also contain a ferrosilicon media. Similar to the processing of lump ore, the heavier manganese is separated from waste materials based on density;
• Waste material from the rotating drum separator and cyclones is combined to form a course waste material known as middlings, which is stockpiled for later use in operational activities;
• Tailings are separated into the sands and slimes fractions using cyclones. Tailings are then pumped to purpose built TSFs, with the exception of some sands tailings which are pumped to the Sand Beneficiation Plant (SBP) for reprocessing.

The concentrator has a production capacity of approximately 5.2 Mtpa (wet tonnes) of manganese product.

Sand Beneficiation Plant
A SBP, also known as the PC02 plant, was commissioned in May 2016 and runs concurrently to the existing concentrator. The SBP is designed to process up to 2.68 Mtpa (dry) of feed material, producing 0.7 Mtpa (dry) of PC02 product (40% nominal manganese grade). PC02 processing involves the following steps:
• Sands material is reclaimed from an existing sands TSF (TSF feed) or is directed to the SBP from the concentrator (on-line feed);
• Material is screened to remove course reject material (> 2 mm) and fed into a series of reflux classifiers for sizing based on density (similar to the processing of lump and fines ore);
• The product from the reflux classifier is fed onto a horizontal vacuum belt filter for dewatering (to achieve an acceptable moisture level for transport);
• The reject stream (i.e. overflow from reflux classifiers) is pumped into a cyclone to separate slimes from sands. Tailings are then pumped to purpose built TSFs.

Tailings Storage Facilities
The processing of manganese ore results in the production of concentrate products (manganese lump and fines) and waste products (middlings and tailings).

Tailings comprise approximately 45% to 50% of the plant feed and are pumped (via overland pipelines) from the concentrator and PC02 plant to dedicated sands and slimes storage facilities.