Ore Deposits In La Escondida Mine, Chile

Introduction

La Escondida mine is the world’s largest copper mine and is located in the Atacama Desert in Northern Chile approximately 160km from Antofagasta (Mining Technology, 2018). BHP Billiton is in control of the operation and owns the majority of the mine (57. 5%) with Rio Tinto, the International Finance Corporation and a Japanese consortium owning the rest of the mine (BHP, 2018). The Copper deposit is part of a larger cluster of deposits with Barrick Gold’s Zaldivar Mine and BHP Billiton’s Escondida Norte Mine being Escondida’s two closest neighbours (Richards et al. , 2001). Research journals, papers and mine reports about the Escondida mine also talk about the Escondida Norte Mine located 6 km to the north of the main Escondida mine site and any data referenced in this report is representative of both the Escondida Mine and the Escondida Norte Mine except where specified. La Escondida and Escondida Norte together add up to approximately 1. 4 Mt of material removed per day.

The Escondida area contains the three mine pits and three separate concentrators that have a throughput capacity of approximately 375 kt per day (Rode, 2018). The site also contains oxide leach pads, sulphide leach pads and a cathode plant. The sulphide and oxide pads have an effective cathode capacity of approximately 330 kt per year and from the product of both La Escondida and Escondida Norte, 3. 5 Mt of concentrate is produced per year using flotation processes and leaching of sulphides and oxides (Rode, 2018). The concentrate product is then transported from the concentrators to the port of Coloso where the concentrate is dewatered and dried for export.

Regional Geology

The geology of the Escondida Deposit is dominated by a granodioritic and quartz monzonitic intrusive stock complex that formed as a result of 3 separate phases of intrusion (Garza et al. , 2001). The first phase (37. 9 ± 1. 1 Ma) was the intrusion of the Escondida Porphyry; a Quartz monzonite -granodioritic intrusion dominated by feldspar-quartz and biotite (Richards et al. , 2001). The second phase (34. 7 ± 1. 7 Ma) was the intrusion of a rhyolitic porphyry and the third phase was quartz latite and dacitic porphyry dykes (approximately 31 Ma) that follow northwest trending faults through the older porphyries (Richards et al. , 2001). The entire stock complex seems to be controlled by two north-south trending faults known as the Portezuelo fault and Panadero fault (Richards et al. , 2001). The faults are steeply dipping (65˚ E) contractional reverse faults that are thought to be branches off the West Fissure zone and show slight dextral offset (Richards et al. , 2001). The large intrusions are hosted by andesite of Palaeocene age (Garza et al. , 2001).

The Basement rock is made up of a sequence of Palaeozoic and Mesozoic Volcanic and sedimentary rocks which are the characteristic basement rocks for the whole Cordillera De Domeyko Province (Garza et al. , 2001).

Deposit Characteristics

Escondida produces copper, gold and Silver with the size and grade of the copper reserves ranging from 0. 1 % up to grades in excess of 3. 5% (Rode, 2018). Cu Sulphide Tonnage (Mt) Cu Sulphide grade (%) Low grade Cu Sulphide tonnage (Mt) Low grade Cu Sulphide grade (%) Cu Oxide tonnage (Mt) Cu Oxide grade (%)2018 1. 24 1701 0. 55 290 0. 73 Table 1 - Table showing the Total tonnage of Cu Sulphide and Oxide ores within both the Escondida Deposit and the Escondida Norte deposit (modified from Mining Technology, 2018). The ore bodies cover an area 4. 5 km long and 1km across. Approximately 49% of the ore body is found within the Escondida Stock and the other 51% of the orebody is found in the surrounding andesite host rock due to a mixture of porphyry associated alteration zone mineralisation and supergene enrichment (Garza et al. , 2001). The orebody in the Escondida stock contains and average copper grade of 1. 12 wt % at a 0. 3% cut-off grade compared to an average grade of 0. 75 wt% found in the andesite host (Garza et al. , 2001).

Alteration Zones and Mineralisation Styles

La Escondida is an unusual porphyry deposit, most of the hypogene mineralisation associated with the 3 phases of porphyry intrusions, although high grades of copper can be found, is not the main cause of exceptional mineralisation found at Escondida. The orebody has also undergone supergene enrichment which forms a blanket of mineralisation that overprints the previous hypogene mineralisation and it is where the supergene enriched mineral blanket matches up with the highest grades of hypogene mineralisation found in the centre of the Escondida stock where the highest grades of copper mineralisation (>3. 5 wt %) can be found (Richards et al. , 2001). Using the spatial relationships (fig …) and knowledge of genetic models of Porphyry deposits stated in Ridley (2013), the different zones of alteration can be grouped into three individual stages of hydrothermal alteration. The first stage of hydrothermal alteration includes three types of alteration zones: Propylitic, Silicification and Potassic.

The Potassic zone can be found in the centre of the Escondida stock and in some of the surrounding andesite host rock (Garza et al. , 2001). This zone is dominated by fine grained biotite and minor orthoclase feldspar that replace the crystal groundmass in the andesite and in the intrusive stock, disseminated K-felsdpar is found to replace plagioclase feldspar and the crystal groundmass (Garza et al. , 2001). Magnetite, bornite and chalcopyrite can be also be found in a pervasive manner but is mainly found in veins and veinlets that affect intrusive stock (Garza et al. , 2001). The next zone of alteration is the propylitic zone. This zone is characterised by vein and pervasive alteration with the main alteration minerals being epidote, chlorite, zeolites, pyrite and rare chalcopyrite, grossular and albite (Garza et al. , 2001). The propylitic alteration zone is found mostly to affect the surrounding andesite host rock and replaces amphibole and plagioclase feldspar (Garza et al. , 2001).

The final zone of alteration that occurred in this first stage of hydrothermal alteration is the silicification zone. The silicification zone contains pervasive aphanitic silicified rock that forms a shell around the intrusive stocks and is interpreted to have formed during a metasomatic event during the initial emplacement of the Escondida stock (Garza et al. , 2001). There are three types of hydrothermal veins that are associated with the first stage of hydrothermal alteration, the first being discontinuous barren quartz veins, the second type is quartz, biotite and anhydrite veins that contain minor amounts of chalcopyrite and bornite and have alteration haloes, the final vein type is continuous quartz veins that contain chalcopyrite and bornite but don’t have alteration haloes (Garza et al. , 2001). The grade of sulphide present in these veins is <0. 5 vol % (Garza et al. , 2001) and the hypogene copper grade associated with the first stage of alteration is <0. 2 wt % (Richards et al. , 2001).