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The Central Chile Porphyry Deposits in Compression with Iranian Porphyry Deposits
By: Ahmad Kazemimehrnia (Riotinto exploration geologist) Reza Zarinfar (Riotinto exploration geologist)
Abstract Field trip to Central Chile porphyry deposits was started on 25 th April 2005 and finished on 28 th April 2005. We have visited three of the main porphyry systems in Central Chile (El Teniente, Rio Blanco- Los Bronces and Los Pelambres). The Chilean Andes are the most richly endowed copper province on the Earth. A total resource (including production) of about 490 million tones of fine copper has been identified in more than 63 porphyry copper deposits and numerous prospects. Andean porphyry deposits occur along five metallogenic belts that extend from central Chile to southern Peru and northwest Argentina. They formed between the Early-Late Cretaceous and Pliocene. Within these belts the deposits occur in clusters associated with multiphase plutonic complex. These relationships are particularly prevalent in the Late Eocene œOligocene belt, the most prolific of all. The time span between the oldest and youngest belt corresponds to the period in which contractional tectonism of the Andean cycle was established and developed from late Cretaceous to Recent. The five porphyry belts reflect Andean tectonomagmatic evolution, with progressive eastern migration of volcanism and plutonism with time .Arc migration correlates with discrete and transient periods of increased convergence velocity and convergence angle. These periods coincide with the strongest deformation events that in turn correlated with the temporal development of each one of the five porphyry belts. These events resulted in regional uplift, shortening, and crustal thickening which in turn produced syn-orogenic erosion. Porphyry copper emplacement occurs syn-tectonically and the resultant multiphase intrusive complexes have variable compositions ranging from granodiorite to tonalite, Monzonite and quartz monzonite evolving in all case from intermediate composition pre-mineral phases to more felsic intra-mineral phases .A reversal to more mafic magmatism has been reported locally. Alteration and mineralization processes evolved from early magmatic stage dominated by high- temperature fluids to late stage dominated by low or more moderate temperature hydrothermal fluids with magmatic and meteoric components. Supergene modifications such as oxidation, leaching and secondary enrichment have been very important in developing the high-grade copper ore bodies that are presently Being profitably mined in the Andes. Lateral migration of copper œbearing solutions has developed proximal exotic deposits.
El Teniente Cu-Mo PORPHYRY MINE ------Operator: CODELCO-Chile Ore Reserve: 11,464,000 @0.67% Cu or 77MT fine Copper, 1.4 MT fine molybdenum. Mining: Underground Production: 18 MT Copper (2002) Visit date: 25/04/2005 ------Description of Deposit: The Giant El Teniente deposit, located in the Andes of central Chile, 70 km southeast of Santiago is one of the world‘s largest known copper deposits. El Teniente has been described in the past as a porphyry deposit developed around a Pliocene dacite porphyry stock, with 80% of its copper mineralization hosted in Miocene andesites. However, new mapping œ both regional and underground mine working œalong with petrological studies, indicates that El Teniente, like the other Giant Miocene and Pliocene copper deposits in the central of Chile is actually best classified as a breccia deposit. Most of the high-grade hypogene cooper at the El Teniente occurs in and surrounding multiple magmatic hydrothermal breccia pipes. Mineralized breccia complexes, with copper content >1 percent, have vertical extents of 1.5 km, and their roots are as yet unknown. These breccias are hosted in a pervasively biotite-altered and mineralized mafic intrusive extrusive rocks .The multiple breccias in the El Teniente include copper œand sulfide œrich biotite, igneous, tourmaline, and anhydrite breccias, generated by the exsolution of magmatic fluids from cooling plutons, and also magnetite and rock flour breccias. Surrounding biotite breccias, a dense stockwork of biotite- dominated veins has produced pervasive biotite alteration, and copper mineralization characterized by chalcopyrite >>bornite+pyrite. Later veins, with various proportions of quartz, anhydrite, sericite, chlorite, with tourmaline, feldspare, and copper sulfide minerals formed in association with the emplacement of younger breccias and felsic porphyry intrusion. These generated serecitic alteration in the upper level of deposit, and in some cases contributed more copper to the deposit, but in other cases eliminated or redistributed preexisting mineralization. Both the Teneinte dacite porphyry and central rock-flour breccia of the braden pipe, the dominant lithostructural unit in the deposit created a relatively barren core, surrounded by a thin (150 œm) zone of bornite >chlcopyrite,in the larger main area of chalcopyrite-rich, biotite- altered mafic rocks and mineralized breccias. The clasts in tourmaline breccias are quartz-sericite altered dacite porphyry. Mineraliztion is multistage and some calsts are within other clasts, which is one of the multi stage mineralization. The multistage development of breccia emplacement, alteration and copper mineralization at El Teniente occurred over a time spane that was greater than 2 m.y., between >6.4 Ma and 4.4 Ma, at the end of more than 10-m.y. episode of Miocene and Pliocene magmatic activity, and just prior to the eastward migration of the Andean magmatic arc as a consequence of decreasing subduction angle. Decreasing subduction angle also caused crustal thicking, uplift and erosion, resulting in the telescoping of the various breccias and felsic intrusions in deposit. We have visited supergene zone. El Teniente is located at the intersection of major north-south, northwest, southeast, and northeast- southwest Andean structures.
Hose: Ricardo Floody Davila
Andina (Rio Blanco) Cu-Mo PORPHYRY MINE, CENTERAL CHILE ------OPERATOR: CODELCO Reserves: 5000 Mt @ 0.8 Cu, 0.019 Mo Operation in 2003: Mining: open pit - underground Date of visit: 26 April 2005 ------Deposit Description: Rio Blanco-Los Bronces is one of the three giant Miocene to Pliocene porphyry copper œ breccia pipe complex in centeral Andes Chile. Rio Blanco is located in the high Andes of central Chile, 60 km NE of Santiago. CODELCO-Chile operates two mines: the Rio Blanco underground operation, and Sur- Sur open pit and Los Bronces owned by Anglo American Company. The deposit occurs east of the intersection of the Juan Ferandez Ridge and the Chile Trench, at the boundary between two major Andean tectonic segments. The formation of R Rio Blanco-Los Bronces copper deposits is associated closely in time with the regional tectonic changes that accompanied subduction. Uplift and erosion related to regional crustal thickening which began in the middle Miocene and is still occurring in the Andes of central Chile, has exposed relatively deep levels of the Rio Blanco-Los Bronces deposit. Cenozoic calcalkaline lavas of the Abanico and/or Farallones formations and Miocene plutons of the San Francisco Batholite host the Rio Blanco-Los Bronces. The mineraliztion is disseminated and stockwork Cu-Mo sulfide associated with potassic alteration(espatialy intense Biotitization) and younger tourmaline breccia(tourmaline breccia pipe). These breccias were emplaced after a period of erosion, both peripheral to and superimposed upon the older pottasic alteration. We stopped at La Union pit and a hand specimen samples are taken form tourmaline breccia and core sheet are visited.
LOS PELAMBRES Cu-Mo PORPHYRY MINE ------OPERATOR: CODELCO (Chilean National Copper Company) Ore Reserves: 3300 Mt @ 0.63 Cu + 0.016 Mo Operation: 370000 tons of fine copper in 2004 Mining: open pit Date of visit: 27 April 2005 ------Deposit Description: Los Pelamberes is located about 200 km north of Santiago, Chile just west of the border with Argentina, which is one of the Three major deposits in central Chile. It is explored during an intensive exploration drilling campaign. William Braden carried out the earliest known exploration at about 1910 (he was working of development of El Teniente and Potrerillos). Several geological and geochemical and drilling projects are done in this area during last 30 years. Anaconda carried out feasibility study in 1983 but because of decreasing of the copper price on that time the project was deemed uneconomic. The mine is operated by Chilean experts in last few years and now is one of the main copper concentrate producers of the country. The concentrate goes to the pacific by about 120 km pipe for shipment. Main litology of area consist of a sequence of andesitic lava flows, flow breccias, and volcanoclastic sedimentary rocks which is intruded by dioritic and granodioritic intrusions, including the mineralized stock at Los Pelamberes. The main host intrusion is the Diorite which is intruded to the almost NS strike structures. Same as other Cu-Mo porphyries mineralisation in hypogene zone occurred in veins and alteration zones as disseminated and open space filling. The grade in this zone is 0.75 copper and it shows uniform grade in the zone. The enrichment blanket in Los Pelamberes is vary from 10 to 200 metres with the average of about 120 meters and follows classical patterns of redistribution of copper, iron and calcium sulphate. Zoning is composed of (1) oxide and leached zone, (2) partial leaching, (3) sulphide enrichment, (4) hypogene mineralization with anhydrite leached and (5) hypogene mineralization with anhydrite. Operation is going on in enrichment blanket now at the open pit and operation is never stop by snow (more than 7 m snow in winters is awaited).
Host: Alvio Zuccone.
Disputada (Los Bronces) Cu-Mo PORPHYRY MINE, ------OPERATOR: ANGLO AMERICAN Reserves: 605.9 Mt @ 0.45 Cu + 0.016 Mo Operation in 2003: 198kilotons of fine copper, 2kilotones of molybdenum Mining: open pit Date of visit: 28 April 2005 ------Deposit Description: In general Rio Blanco and Los Bronces can be part of one big system and Rio Blanco-Los Bronces is one of the three giant Miocene to Pliocene porphyry copper œ breccia pipe complex in centeral Andes Chile. However, as mentioned Rio Blanco is in east side of the crest and operating by CODELCO and Los Bronces is in the west side of the crest and operating by Anglo American. The mine is located 60 km NE of Santiago at an elevation ranging from 3500m to 4100m. Los Bronces is discovered in 1967 and Anglo American bought it 3.2 b box in 2002. At regional scale the main breccia complex are oriented N10 W, with a distance of 9 km between the two largest known breccias bodies. The oldest outcropping rocks in the area are the volcanic sequences of the Farelleones Formation (21- 11 my) intruded by plutonic rocks of the San Francisco Batolith (20.1-7.4 my). The ore deposit was formed as a result of emplacement of both, multiple breccia pipes and porphyry intrusions (7.3- 4.2 my). The intrusive rocks have compositions ranging from quartz- monzonitic to monzodioritic. The mineralization age is 6 my. The main source of ore is related to hydrothermal breccia pipes, forming a large ellipsoid shape body of 2km length, 0.7km wide and 1.0km in depths. The shape of the breccia system is of a funnel type in general with sharp contact with the host rocks. Breccias are different types and classified by the composition of the matrix, and type and alteration of the clasts. Tourmaline is the main component of the matrix. Matrix and clasts both are mineralised. The mineralization occure 70% in the breccia pipes mainly in matrix as chalcopyrite-chacosite (3:1), with less bornite and molybdenite. The remained 30% occur as disseminated and stockworks in quartzmonzonite and andesitic host rocks. The hypogene mineralization has been recorded in three stage of early, main and late stage mineralization. Supergene enrichment occurs in southern part of deposit and primary grade has raise. Anhydrite is present at depth throughout the deposit and represents the limits of the secondary enrichment starting at an elevation of 3200m.
Host: Cristian P. Sprohnle Mckenzie
Conclusion
For understanding of Central Andian (CA) belt and Iranian porphyries we make comparison list to see what are the similarities and differences. There are three main porphyry systems in Central Chile, which we have visited. As mentioned that they are quite similar in characters, but in Iran there are many porphyries which they have their own characters for example Kerman belt porphyries has one class of deposit with some similarities and they have some differences with East Iran deposits e.g. Shadan or Yazd area e.g. Ali Abad. So each area has own characters and spatial erosion history, which is very important in exploration. Following section is deduction of this comparison:
Similarities:
1. CA porphyries are mostly similar to Kerman belt porphyries. 2. Both of CA and Iranian porphyries are Cu-Mo (Spatially Kerman Belt but some of them e.g. Shadan or Aliabad) but Iranian porphyries has much gold than those and it seem the source magma has much gold, therefore gold content has to be pay attention and look for gold rich porphyries or epithermal systems can be main targets spatially due to lack of complete information of Iranian gold potential (error in gold analyses in past or lack of analyses in geochemical analyses for example in GSI). 3. Copper porphyry deposits of CA and Iran Kerman belt mainly related to continental margin subduction environment (while west Pacific Ocean deposits mainly related to island arcs environments) 4. According to Monzonite /Diorite classification, CA and Kerman belt deposits belonged to Monzonite copper porphyry class.
Differences :
1. CA porphyries are Breccia type porphyry, which is not usual in Iran. In these deposits tourmaline breccia is carry mineralization either in matrix or in sresitic clasts. In this case we think that Shadan is some how similar to them. 2. CA porphyries are much younger than Iranian porphyries. 3. They are much bigger and higher grad than Iranian but Sarcheshmeh is an exception among Iranian in both grad and tonnage. 4. One of the main differences is in Kerman belt young volcanoclastic rocks covered the porphyries and this means there is potential under the cover . There are two types of cover in Kerman belt, the first one is Pliocene post-mineral lahar cover that is eroded partially and still is exist in margin of the belt. This cover is up to 50m in thickness and some porphyry is exposed partially (e.g. Kahtu Kurha) and maybe some of them are not exposed. Second cover is quaternary cover that is traces and Aeolian sands or recent sedimentary deposit (e.g. the cover between Sarcheshmeh range and Abdar range). We think that Geophysics (airborne) is one of the main that can be used in this case. 5. Erosion history is another difference that CA porphyries during quit short time are exposed because of fast erosion and supergene enrichment are formed extensive in higher tonnage (e.g. Los Pelambbres 125mt @ 0.76 Cu) and relative low grade (in compression to Hypogene grade) and this fast erosion cause of exposure of deposits at surface, but in Iran enrichment are occur in long period of time in compare to central Chile and they enriched twice (e.g. Miduk1.1% that Hypogene grade is 0.5%Cu) or three times (e.g. Sarcheshmeh 2% that Hypogene grade is 0.67%Cu). 6. In terms of extent and intensity of alteration, CA deposits are much bigger than known copper porphyry alteration in Iran.
References 1. Anglo American Geologist. 2005, Los Bronces Geology, presentation in mine site. 2. Atkinson, et al. 1997, Geology and Mineral Zoning of the Los Pelambres Porphyry copper Deposi, Chile. 3. Singer, et al. 2002, Porphyry Copper Deposits of the World: Database, Maps, and Preliminary Analysis, USGS open file. 4. Skewes, et al. 2005, The El Teniente Magabreccia Deposits, The worlds largest copper deposit, PGC publishing. 5. Stern, et al. 2005, Origin of Giant Miocene and Pliocene Cu-Mo Deposits in Central Chile, PGC publishing.
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