The Las Cruces deposit is located within the Iberian Pyrite Belt and classed as a Volcanogenic Massive Sulphide (VHMS) deposit. VHMS mineralisation results from precipitation of metals associated with circulating hydrothermal fluids in volcanically active sub-marine areas. The metals precipitate as sulphides (pyrite/phyrrohtite with variable base metal sulphides such as chalcopyrite, sphalerite and galena) and are inter-layered to varying degrees with volcanic sediments.

The Las Cruces deposit, as per many other VHMS deposits, is characterised by a dipping massive sulphide horizon underlain by a stockwork vein system which are both hosted within shales and felsic volcanics. However, at Las Cruces, post deposition tectonism uplifted the deposit, exposing it to erosion and weathering with partial oxidation of some of the primary sulphides. This resulted in a copper leached gossan, which retained the relatively immobile gold and silver and overly the secondary sulphide horizons. The zones of primary sulphide mineralisation are the source of the current gossan and secondary sulphide zones and are key horizons for defining continuous volumes of similar geology and metal tenor. The zones of gossan, secondary sulphide, primary sulphide and stockwork are used as key domains for Mineral Resource estimation.

Mineralisation is characterised by a notable iron oxide gossan which occurs directly above a welldeveloped, flat lying secondary sulphide zone followed by an underlying north dipping (40 degrees) primary massive sulphide zone. The economically important secondary sulphide (HC) zone is about 150 m below surface.

The Gossan Zone is a 10 m to 20 m thick sub-horizontal iron stained oxide zone, leached of copper and sulphur, but contains relatively high levels of gold, silver and lead. While the contained Gossan metals are not recoverable from the current copper leaching and electro winning processing facilities at Las Cruces there is nevertheless a reasonable expectation that they are economic.

A thin (less than 5 m thick) Depleted Zone (DZ), containing variable but moderate copper mineralisation, represents a transition between the overlying Gossan Zone and the underlying secondary sulphide mineralisation. The DZ consists of a layer of sandy, pyritic material.

The underlying secondary sulphide mineralisation, known as the High Copper Main Zone (HC), has high copper grades due to supergene alteration of primary copper sulphides to secondary copper sulphides as well as from leaching copper from sulphides originally located in the overlying oxidised Gossan Zone. The HC zone has mostly chalcocite with minor bornite and covellite. Mineralisation strikes approximately 1,000 m east west and ranges from 40 m to 300 m wide in the north south direction with an average thickness of 40 m. Cobre Las Cruces Mineral Resources is comprised mostly of HC mineralisation.

A small secondary sulphide zone, known as the High Copper Small Zone (HC4), measures 200 m east west, 100 m north south and is on average 20 m thick. The HC4 mineralisation dips at 40 degrees west- north-west and is located directly below HC and PMS. HC4 orientation is due to remobilisation of copper along a structural fault zone having a similar orientation. While HC4 has similar copper grades to HC, it has been interpreted as a separate domain due to its distinct shape and orientation. Copper mineralisation is comprised mainly of covellite with chalcocite and digenite filling veins and gaps.

The Primary Massive Sulphide (PMS) zone underlies the HC zone and dips to the north at 40 degrees. PMS is approximately 1,000 m along strike and while still not completely delineated (open at depth), is approximately 500 m down dip. The primary massive sulphide is on average 30 m to 40 m thick and is characterised by pyrite intergrown with sphalerite, galena, chalcopyrite as well as minor enargite, tennanite and tetrahedrite. Weakly fractured, faulted or jointed zones of PMS have provided traps for precipitation of thin chalcocite and bornite-covellite occurrences from infiltrating copper rich fluids associated with the secondary supergene process.

The lowermost zone of mineralisation underlies the PMS, HC and HC4 zones as an irregular Stockwork (STWK) of inter-connnected pyrite veins and veinlets with locally high copper and zinc grades.

The Cobre Las Cruces mine is a moderate-sized open pit mining operation using conventional truck and shovel operations.

The ore processing facility is designed to operate 365 days per year, 24 hours per day and process 1.5 Mtpa of ore to produce 72,000 tpa of cathode copper. The ore from the open pit mine ranges in grade from 5% to 10% copper and the design grade is 6.02% Cu. The copper in the ore is primarily found in chalcocite with some minor amounts found in chalcopyrite, tennantite-tetrahedrite complex, and enargite.

The metallurgical plant relies on an atmospheric leaching process to recover copper from the rich secondary sulphide chalcocite mineralisation. A unique feature of the plant is the use of eight OKTOP agitated reactor tanks to dissolve the copper under conditions of high temperature and high acidity. Oxygen is also added into the reactors to complete the reaction. The feed to the leaching reactor tanks is mined ore that has passed through three stages of crushing, a single stage of grinding and has then been thickened to eliminate as much process water as possible and three mixing tanks where reactions start (pre reactors).

Once leached, the liquid is separated from the ground solids to become PLS (pregnant leach solution), the feed for the solvent extraction (SX) area. In the SX area the copper is passed to an organic solution and then to the electrolyte that feeds the electrowinning cells. The electrowinning cells produce LME grade copper cathodes weighing approximately 50 kilograms each. An automated crane and stripping machine then harvests and packages the cathodes for shipment. A simplified process flow diagram for the plant is presented in Figure 17-1.

The plant was debottlenecked during 2013 and 2014 to accommodate lower feed grade material from the later stages of pit development. This included projects to enhance both throughput and recovery. As a result of this work, throughput and recovery capability are now well above design levels and cathode production is expected to be maintained at design levels across the remaining life of the mine. In early 2015, the tailings filtration plant was upgraded from vacuum belt filters to pressure filters to improve the quality of the tailings in terms of lower moisture content and improved solution washing. The former improves handling and reduced drainage in the dump and the latter increases copper recovery by reducing losses. During 2015/16 the major projects for the plant are:
1) Water Compensation: to compensate DRS losses by means of reinjection of treated water into the aquifer. This will help to close DRS water balance.
2) Leaching Enhancements: improve agitation in the pre-reactor to increase recoveries.