Crecimiento económico y factores explicativos

The Bajo de la Alumbrera porphyry copper-gold in Argentina shows some similarities to La Colosa. It is a world-class copper-gold porphyry deposit located 200 km off the main Andean copper belt of Chile (Ulrich and Heinrich, 2002). It has up to seven phases of dacitic porphyry intrusions in a cluster which show hydrothermal alteration and mineralisation (Ulrich and Heinrich, 2002) (Figure 35). The basement rocks in the district are granites, metasediments, and sedimentary rocks. The host rocks are the thick andesites and dacites of the Farallon Negro Volcanic Complex.

Figure 35: Geologic map of the Bajo de la Alumbrera deposit from Ulrich and Heinrich (2002) after Sasso (1997).

The porphyry intrusions show a high calc-alkaline affinity and are similar in texture to each other (Sasso and Clark, 1998; Ulrich and Heinrich, 2002). They all contain phenocrysts of plagioclase, hornblende, biotite, and quartz in a quartz-feldspar groundmass (Proffett, 2003).

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The alteration assemblages include quartz - magnetite ± K feldspar, potassic,

propylitic, and then advanced phyllic and argillic (feldspar destructive). The quartz – magnetite ± K feldspar assemblage has distinctive fracture fillings and replacement textures. The potassic alteration has secondary K feldspar in the plagioclase and groundmass along with shreddy biotite replacing the mafic minerals. The copper and gold mineralisation is associated with the potassic alteration. The propylitic alteration (chlorite-epidote) assemblage has chlorite and magnetite replacing mafic minerals and albite + epidote ± calcite ± sericite replacing plagioclase. The propylitic alteration surrounds the potassic alteration. The advanced phyllic and argillic (feldspar destructive) alteration assemblage overprints earlier alteration types and includes sericite, pyrite, quartz, and rutile with local clay minerals or chlorite ± calcite (Ulrich and Heinrich, 2002). The alteration zoning is nearly symmetrical, although the copper-gold mineralisation and veins are less symmetrical and is controlled by intrusions (Proffett, 2003).

Many of the early veins and quartz veins in the deposit are related to the potassic alteration assemblage, which tends to occur as halos to the veins. However, the quartz veins may be without halos or secondary magnetite. The earliest types of veins are the equigranular quartz with or without magnetite, K feldspar, chalcopyrite, bornite, biotite, and anhydrite (i.e. A type veins). The next generation of veins is the B type veins or continuous coarser quartz with banding. The sulfide veins are late and predominantly consist of pyrite or D type veins which are related to the feldspar destructive alteration (Proffett, 2003).

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The gold and copper mineralisation usually has a positive correlation with one

another. The copper mineralisation includes chalcopyrite, bornite and covellite. Gold is associated with these copper sulfides, pyrite, tellurides, or occurs as native gold. The paragenesis of the deposits suggests two pulses of Fe-K-Cu-Au fluids and a late post ore fluid pulse (Ulrich and Heinrich, 2002).

4.4.2 Bingham Canyon

The Bingham Canyon porphyry copper-gold-molybdenum deposit is the seventh largest copper deposit (28 Mt) and the second largest based on gold (1,600 t) in the world (Redmond and Einaudi, 2010). It is centred on the Eocene monzonite intrusion called the Bingham stock, which intruded the Pennsylvanian Oquirrh Group quartzite and limestone (Redmond and Einaudi, 2010). The Bingham stock was intruded by multiple porphyry dikes including quartz monzonite porphyry, latite porphyry, biotite porphyry, quartz latite porphyry breccia, and quartz latite porphyry (Redmond and Einaudi, 2010) (Figure 36). The porphyry intrusions are felsic with phenocrysts of K feldspar, plagioclase, hornblende, and biotite. The highest copper and gold grades are located within the quartz monzonite porphyry (early intrusion) (Redmond and

Einaudi, 2010).

The dominant alteration assemblage at Bingham Canyon is potassic and includes an intensive quartz-K feldspar alteration (Redmond and Einaudi, 2010). The potassic alteration ranges from weak, which is biotisation of hornblendes and matrix

preservation, to strong, which has vuggy quartz and feldspar with the matrix texture obliterated. The potassic alteration intensity is also a function of A type veinlet abundance (Redmond and Einaudi, 2010).

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Figure 36: Geologic and structural map of the Bingham Canyon deposit from Redmond and Einaudi (2010).

The vein sequence, in order, for the Bingham deposit is early biotite veinlets, followed by early dark micaceous (EDM) veins, A type quartz veins (potassic alteration), quartz-molybdenite veins, and quartz-sericite-pyrite veins (Redmond and Einaudi, 2010). The early biotite veinlets are composed of medium-brown biotite with no magnetite. The EDM veins are unfilled fractures with halos, where

plagioclase is replaced by andalusite, biotite, sericite, and K feldspar. The EDM veins contain minor amounts of bornite with or without digenite and chalcopyrite. The A type quartz veins are the most abundant and are composed of quartz, digenite, bornite, chalcopyrite, and molybdenite. The gold grade correlates with copper in the bornite- digenite zones (Redmond and Einaudi, 2010). The A type veins occur in all porphyry intrusions but have greater concentrations in the quartz monzonite porphyry. The late quartz-molybdenite veins are composed of euhedral quartz with a vuggy centreline at elevations above 1,500 m and may contain trace chalcopyrite. The D type veins or

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pyrite veins with quartz-sericite-pyrite halos are located along strike to the northeast and southeast and at elevations higher than 3,000 m (Redmond and Einaudi, 2010).

The observed relationships between the porphyry contacts and the veins suggest that each porphyry intrusion event has a sequence of potassic alteration and vein

formation, which are associated with the copper and gold deposition (Redmond and Einaudi, 2010). Each porphyry has many A type veins, strong potassic alteration, and high grade copper-gold zones. It is suggested that the mineralisation formed due to a flux of magmatic-hydrothermal fluids with decreasing amounts of copper and gold as intrusions occurred due to the depletion of metals and volatiles in the underlying parent magma chamber (Redmond and Einaudi, 2010). The A type veins and sulfides represent the pulses of hydrothermal fluid associated with each porphyry intrusion (Redmond and Einaudi, 2010).

4.5 Porphyry Gold Deposit Models