ORE DEPOSITS RESEARCH

Poeple and Projects: Robert MARSCHIK

Poster presented at the Annual Meeting of the

Geological Society of America, November 1998, Toronto, Canada
Marschik, R. and Leveille, R.A. (1998): The Candelaria-Punta del Cobre iron oxide copper-gold deposits, Chile. Geological Society of America, Abstracts with Programs, A-371.
THE CANDELARIA-PUNTA DEL COBRE IRON OXIDE COPPER-GOLD DEPOSITS, CHILE

Robert MARSCHIK
Mineralogy Dept., Univ. of Geneva, Rue des Maraîchers 13, 1211 Geneva 4, Switzerland

Richard A. LEVEILLE
Phelps Dodge Exploration Corporation, Carlos Antunes 1934, Santiago, Chile
INTRODUCTION
The Early Cretaceous Candelaria-Punta del Cobre iron oxide-rich Cu-Au deposits are located near the eastern margin of the Coastal Batholith, south of Copiapó, Chile (Fig. 1). Candelaria forms the major mine in the area with mineable reserves of 400 Mt @ 1.0 % Cu, 0.20 g/t Au (Martin et al. 1997). Candelaria was discovered in 1987 and has attracted continuous interest since then due to its significant size and the fact that its formation cannot be easily explained by current genetic models which makes exploration for this deposit type challenging. Similar deposits occur in the Punta del Cobre district about 3 km east of Candelaria. These deposits are referred to as Punta del Cobre-type deposits and are represented by middle-size mines (e.g., Carola, Santos-Malaquita, and Socavón Rampa-Trinidad) that share several geographically separated Cu-Au orebodies estimated to contain a combined total of over 120 Mt at 1.5% Cu, 0.2-0.6 g/t Au, and 2-8 g/t Ag. Candelaria and Punta del Cobre-type deposits represent variations of one and the same hydrothermal system and essentially differ in size, intensity and types of alteration, host rocks, and their position within the contact metamorphic aureole of the Copiapó Batholith.
Figure 2. Schematic section from the Copiapó Batholith to the Punta del Cobre district. Stratigraphic positions of orebodies are shown. Please see Figure 1 for the location of the section.
Figure 1. Geologic map of the Candelaria-Punta del Cobre area. The position of the cross-section of Figure 2 is indicated.
GEOLOGY OF THE PUNTA DEL COBRE-CANDELARIA AREA
Stratified rocks exposed in the area south of Copiapó represent a facies transition of an Early Cretaceous continental volcanic arc to the west and northwest (Bandurrias Formation) and a shallow marine carbonatic backarc basin to the east and southeast (Chañarcillo Group). These rocks are intruded by Early to middle Cretaceous plutons (K-Ar ages 119 to 97 Ma; Arévalo 1994, 1995) of predominantly dioritic composition. Basin inversion probably already commenced in late Aptian times eventually resulting in the partly erosion of the backarc sequence (e.g., Aguirre 1985). The Candelaria-Punta del Cobre deposits are hosted by the pre-Upper Valanginian Punta del Cobre Formation, which underlies the Chañarcillo Group. The Punta del Cobre Formation consists of massive volcanic rocks (Geraldo-Negro Member; >600 m) in its lower part that are overlain by a succession of volcaniclastic sediments (Algarrobos Member; up to >800 m; R. Zamora pers. commun. 1997) which, in places, contains lenses of massive volcanic rocks (Fig. 2). This succession passes vertically and laterally into the overlying carbonatic Chañarcillo Group.
The main structural elements in the Candelaria-Punta del Cobre area are a large antiform known as the Tierra Amarilla Anticlinorium, a SE-verging fold-thrust system (Arévalo and Grocott 1997), and a dense set of NNW to NW-trending high-angle faults. The latter control parts of the metallic mineralization (e.g., Camus 1980; Marschik and Fontboté 1996). Additionally, NE and ENE to WSW-trending structures are observed. Mylonitic shear zones and cataclastic rocks, in places, form the contact between intrusive and Early Cretaceous country rocks. Ductile deformation is also recorded in volcanic and volcaniclastic rocks of the Punta del Cobre and Bandurrias formations close to the batholith contact (Fig. 1). It is manifested in broadly NE-trending, 30-70° W dipping zones of intense foliated K-metasomatized (biotite) rocks. This deformation is the oldest recognized in the Candelaria mine so far (Candelaria Shear Zone). It is interpreted to represent early heat-induced ductile deformation related to early stages of batholith emplacement.
THE CANDELARIA DEPOSIT
The Candelaria deposit is located inside the contact metamorphic aureole of the Copiapó Batholith. The deposit is situated in a structurally elevated block bounded by the NNW-trending Bronce fault to the west and the Farellón fault to the east (Fig. 2). The NNW-trending Lar fault divides the deposit in a stratigraphically deeper lying eastern (North pit) and an elevated western block (South pit). Thermal metamorphism, metasomatism, and deformation resulted in almost complete replacement of the original mineral assemblages and poor preservation of original rock textures in most places of the deposit. Ore is hosted by biotitized andesitic volcanic rocks ("Lower Andesites"), and biotitized and partially skarned volcaniclastic and tuffaceous sedimentary rocks (Algarrobos Member; Fig. 2 and 3A). The latter are overlain by biotite hornfels, and albitized and biotitized meta-andesites ("Upper Lavas") that in turn, are overlain by quartz hornfels, scapolite-pyroxene skarn, and garnet skarn, which are assigned to the Abundancia Formation (Chañarcillo Group).
The Candelaria orebody predominantly consists of widely spaced polydirectional veins and veinlets of chalcopyrite plus pyrite, along and cutting foliation planes in sheared host rocks, of stringers, and impregnations. Magnetite is ubiquitous and present as massive replacement bodies, as breccia matrix, as breccia clasts, veinlets, and disseminations. The maximum dimensions of orebody are 2000 m in length by 600 m width and 350 m thickness (Ryan et al. 1995). It thins laterally into stratiform bodies. Parts of the mineralization appears to be controlled by the NW-trending structures in the northern part of the deposit (North pit) and by NNW-trending structures, subparallel to the Lar fault, in the south (South pit).
Photo 1. Candelaria mine from Sierra El Bronce. View towards southwest.
Photo 2. Typical ore at Candelaria mine. Chalcopyrite-pyrite mineralization is superposed on intensely Fe-metasomatized (magnetite) volcanic-volcaniclastic rocks.
Figure 3. Section through the northern part of the Candelaria mine (modified from Martin et al. 1997). A) Stratigraphy of the Candelaria mine. B) Main alteration-types and their distribution. C) Position of the orebody.
Photo 3. NNW-trending chalcopyrite-pyrite vein in the Candelaria South Pit.
PUNTA DEL COBRE-TYPE DEPOSITS
The Punta del Cobre district is located near the town of Tierra Amarilla at the eastern side of the Copiapó river, outside the contact metamorphic aureole of the batholith (Fig. 1). The district hosts several middle and small-size mines that extract ore from orebodies located at and below the volcanic rock/sediment contact (Fig. 2 and 4a). Stratiform mineralization is hosted in the lower part of a volcaniclastic breccia that overlies the volcanic rocks. The latter host subvertical veins, breccias, and stringers bodies. The mineralization is controlled by NNW to NW-trending structures. The ore is spatially associated mainly with sodic (albite) or potassic (K-feldspar and/or biotite) alteration at shallower levels and, in places, with fracture-controlled calcic (amphibole) alteration at depth (Fig. 4A; Marschik and Fontboté 1996; Flores 1997).
Photo 4. Punta del Cobre district from Sierra El Bronce. View towards east
Figure 4. Schematic section through the Punta del Cobre district at Santos-Malaquita mine, south of Qda. Meléndez (modified from Flores 1997). A) Stratigraphy and relative position of the Santos orebodies. Spatial and stratigraphic relationships of projected orebodies of the mines Carola, Resguardo, Socavón Rampa are indicated. B) Main alteration-types and their schematic distribution.
Photo 5. Stratiform mineralization above the volcanic rock/sediment contact (Socavón Rampa mine).
Photo 6. Stringers of chalcopyrite-pyrite (Socavón Rampa-Trinidad mine).
 Photo 7. Breccia with clasts of volcanic rocks in a matrix of chalcopyrite-pyrite (Socavón Rampa mine).
MINERALIZATION
The ore consists of magnetite, chalcopyrite, pyrite, ±hematite. Pyrrhotite, sphalerite, traces of molybdenite and elevated LREE values are present locally. Gold occurs mainly as small grains of gold-rich electrum associated with and as minute inclusions in chalcopyrite and pyrite (Hopf 1990; Ryan et al. 1995). Gangue mineralogy consists predominantly of quartz, and anhydrite at Candelaria and calcite at Punta del Cobre.
Mineralization at Candelaria-Punta del Cobre occurred in two major hydrothermal stages: 1) an early iron oxide stage, and 2) a superposed sulfide stage (Fig. 5). Early iron oxide mineralization apparently was predated by widespread pervasive albitization. In particular biotite alteration accompanied the intense Fe-metasomatism. A 40Ar/39Ar inverse isochron biotite age of 114.9±1.0 Ma (errors at ±2s) is interpreted to best represent the age of this early iron oxide stage (Fig. 6; Marschik et al. 1997a). Biotitization and magnetite formation at Candelaria presumably was contemporaneous with shear deformation along the Candelaria Shear Zone (Fig. 2). Boudined magnetite in ductile and broken magnetite and the abundance of magnetite veins and veinlets in brittle domains of the Candelaria deposit indicate that ductile and brittle deformation were essentially synchronous. The superposed major sulfide stage has an entire brittle character and began with quartz veining followed by amphibole alteration and veining. Chalcopyrite-pyrite exploiting channelways previously used by quartz and/or amphibole is common. Amphibole veinlets ±chalcopyrite ±pyrite cut peak metamorphic mineral assemblages. In places, magnetite slightly predates and possibly accompanies chalcopyrite-pyrite formation. A late minor event of specular hematite formation commonly associated with calcite is recognized at a regional scale.
Photo 8. A vein of massive magnetite in contact with chalcopyrite-pyrite vein. This spatial relationship illustrates the two separate stages of metallic mineralization in the Candelaria-Punta del Cobre district (Carola mine). Photo 9. Photomicrograph of typical ore from Socavón Rampa mine showing chalcopyrite, pyrite, and specular hematite. Field of view approx. 3 mm.
 Photo 10. Incomplete pseudomorphous replacement of hematite by magnetite (mushketovite) is a typical feature in the district (Carola mine). Field of view approx. 3 mm.
Figure 5. Preliminary sequence of mineralization and main alteration events.
Figure 6. Mineralization at Candelaria-Punta del Cobre occurred in two major hydrothermal stages: 1) an early iron oxide stage, and 2) a superposed sulfide stage. Biotite alteration accompanies the early iron oxide dominated mineralization. A 40Ar/39Ar inverse isochron biotite age of 114.9±1.0 Ma (errors at ±2s) is interpreted to best represent the age of this early hydrothermal stage.
Figure 7. Sulfur isotopic compositions of chalcopyrites and pyrites from Candelaria-Punta del Cobre deposits show a range of d34SCDT from -0.7 to +2.9 ‰ consistent with a magmatic sulfur source. Sulfur might be derived via leaching of the volcanic rocks or contributed by magmatic fluids (Marschik et al. 1997b, c).
Figure 8. Lead isotopic compositions of ore from the Candelaria-Punta del Cobre deposits and rocks of the Copiapó Batholith. The data indicate a common source of Pb for the iron oxide Cu-Au mineralization and batholithic rocks. The Pb isotope signature is explained by a mixture of Pb derived from mantle and upper crust reservoirs. The continental arc setting of the intrusive rocks constraint from the regional geology is consistent the Pb isotope signature (Marschik et al. 1997b, c).
Figure 9. Microthermometric measurements on fluid inclusions in post-ore calcite suggest temperatures of less than 240 °C and relatively high salinities (12-24 wt. % NaCl and 13-23 wt. % CaCl2) for these late stage fluids. Calculated oxygen isotopic compositions of fluid in equilibrium with post-ore calcite indicate d18O values between 4.6 and 7.7 ‰ for a temperature range of 175 °C to 325 °C, which are consistent with magmatic fluids or with fluids equilibrated with magmatic silicates at high temperatures (Rabbia et al. 1996; Marschik et al. 1997a).
CONCLUSIONS
Main-stage Cu mineralization is superposed on the earlier main Fe oxide (hematite-magnetite) formation. It post-dates shear deformation and peak metamorphism at Candelaria. Time relationships suggest long-lived hydrothermal activity that caused the metallic mineralization at Candelaria-Punta del Cobre, essentially contemporaneous with batholith emplacement. Mineralization is probably related to one or more batholithic intrusion(s) that acted as heat source and contributed with magmatic fluids. The latter interpretation is consistent with similar lead isotope signatures of ore and batholithic rocks, with the sulfur isotopic compositions of chalcopyrite and pyrite, and with relatively high salinities of late-stage fluids.
REFERENCES
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