THE COPPER AND BITUMEN MINERALIZATION IN BARDA GONZÁLEZ AND

JOSEFINA PONS; MARTA FRANCHINI; ADOLFO GIUSIANO; DIEGO LICITRA

Göttingen

Conferencia; International Lateinamerika-Kolloquium 2009; 2009

Universität Götting

Barda González and Tordillos are two examples of the main stratiform sedimentary copperprospects in Neuquén Province, Argentina. They are located in the Huincul Ridge (Dorsal de Huincul area), near hydrocarbon fields (Fig. 1). Based upon previous works of these cupriferous manifestations (Wichmann, 1927; Fernández Aguilar, 1945; Granero Hernández and Schmid, 1956; Ramos, 1975; Lyon, 1999) and the preliminary results obtained in some prospects Giusiano et al., 2006; Giusiano et al., 2008 proposed a new hypothesis that associates the cupriferous manifestation with the hydrocarbons and Cu-fluids migration along the main structures related to the Huincul Ridge. The origin of this hydrocarbons and fluids is located in the deepest levels of this structure. The Huincul Ridge is a structural lineament of regional scale developed at 39º South latitude,with a E-O strike extended along 270 km (Fig. 1), that defines the natural northern boundary of Argentina Patagonia with the Neuquén Basin (Ramos et al., 2004). This structure was interpreted as a fault zone of dextral lateral movement with the coexistence of transpressive and transtensive segments as resulted of changes in the fault zone trend (Ploszkiewicz et al., 1984). Silvestro and Zubiri (2008) proposed that this ridge is the result of an oblique NW-SE convergence between the Northpatagonian Craton at the Southeast and the Neuquén basin at the Northwest. These researchers identified inside this deformation belt three segments. The western sector with the predominance of NE trending anticlines, a central zone with E-W lineaments of compressive and wrench structures and a northern and eastern zone where the lineaments are NW trending and developed right-lateral wrench structures. The faults from the central, northern and eastern zones were developed from the previous normal faults of the Triassic rift by the tectonic inversion during the Lower Jurassic and Cretaceous (Cruz et al., 2002; Silvestro and Zubiri, 2008). Most of these faults have an origin in deep levels and abort in the base of the Vaca Muerta Formation (Tithonian) and others closer to the main fault have their origin in shallower levels and cut the Neuquén Group (Upper Cretaceous) (Schiuma et al., 2002). The copper deposits are hosted in the fluvial deposits of the Neuquén Group (Cenomamian- Campanian). These rocks cover almost all the center east of the Neuquén basin with up to 1300 m thick in the embayment zone (Cazau and Uliana, 1972). These rocks lay unconformilly over the continental sedimentary rocks of the Rayoso Group (Aptian-Albian) and are cover by the marine deposits of Malargüe Group (Lower Maastrichian-Upper Campanian). Neuquén Group is composed by Río Limay Subgroup with Candeleros, Huincul and Lisandro Formations, Río Neuquén Subgroup with Portezuelo and Plottier Fromations and Río Colorado Subgroup with Bajo de la Carpa and Anacleto Formations (Ramos, 1981; Fig. 2). They consist in fine to coarse sandstone and conglomerate interbedded with claystones and mudstones, and is cemented by iron oxides and hydroxides that give them a typical red color. The Barda González prospect is hosted in Portezuelo Formation (Neuquén Group), located in the central fault zone of the Huincul Ridge (Fig. 1c) next to Barda González, Bajo Barda González, and Puesto Espinoza oil fields. The Cu mineralization has an extension of 600 m wide by 2000 m long with a NNE stricke (Fig. 3), along fluvial paleochannels, defined by coarse sandstone and conglomerate layers. These rocks only reach the early diagenesis stage and are discolored and altered. They are very permeable due to the lack of iron oxides and hydroxides and the total or partial disolution of their cement and their feldespar and volcanic lithic. These last are also altered to clay minerals (illite>kaolinite). The copper minerals are diseminated, concentrated in beding, tubes or nodules always in contact with bitumen (Fig. 3c). They consits of chalcocite, covelline, trace of chalcopyrite, and a variety of supergene minerals (malachite, with chrysocolla, brochantite, azurite, tenorite, turquoise, atacamite, voborthite). There are also diseminate pyrite and psilomelane trace. The geochemistry of bitumen indicates that it is a hydrocarbon migration residue generated from a marine rock between the Lower Triassic and Upper Cretaceus (Los Molles or Vaca Muerta Formations). The Tordillos prospect is hosted in the Huincul Formation, located in the northern flank of the Huincul Ridge, west of the Loma la Lata hydrocarbon (gas) field and at the left margin of the Neuquén River (Fig. 1c). It is situated in the Alto del Sauzal Bonito anticline, a structure product of the tectonic inversion of the Jurassic fault wich has an E-W strike sloping to the NNE (Maretto y Pángaro, 2005). In the prospect area there are a series of fractures with E-W, NNW and N-S orientations that could be the shallow expressions of the normal fault observed in the subsurface which was reactivated during the Andean orogeny (Maretto et al., 2005). The copper and bitumen mineralization extend in a series of table l and along 6 km alienated with an orientation E-W that conform the outcrops of the Huincul Formation (Fig. 4a, b). This Formation is composed by feldspar-quartz rich sandstone and conglomerates with features of filling channel deposit fining upward. Theses rocks have a grayish color due the leaching of the iron oxides and hydroxides, the alteration of their volcanic lithics and feldspars to clay minerals, and the bitumen impregnations. The copper mineralization is disseminated as tables (filling join), tubes (Fig. 4c), bubbles and cylinder cutting the sedimentary structures and also as fines layers along the stratigraphic bedding planes, always associated with bitumen. The mineralization consists in calcocite partial to totally replaced by crysocolla with trace of malachite filling the porosity. The bitumen is also riming and filling the cavities the previous fibers of the fossil trunk fragments which are partially replaced by crysocolla, malachite and azurite (Fig. 4d). Both prospects present the following similarities: 1) they are located in the Huincul Ridge region, Neuquen basin, 2) mineralization is restricted to the permeable paleochannels of the Neuquén Group and consist in copper sulfur (chalcosite, covellite, traces of chalcopyrite) and supergene minerals (crysocolla, brochantite, malachite, atacamite) that are intimately associated with bitumen, 3) they are situated near the structures related to the Huincul Ridge and the hydrocarbon fields Bajo Barda González (oil) and Loma la Lata (gas), 4) in both fields the formation waters of the reservoir are chlorides rich brines (60 to 72 gr/l chlorides, Schiuma et al., 2002; Maretto et al., 2005). These water that migrate along with hydrocarbons through the faults and permeable beds, could have extract the copper from the iron oxides and hydroxides of the continental red beds. The passage of these fluids, in contact with the permeable levels of the Portezuelo and Huincul Formations, could have generated the consequent alteration and permeability increase in the host rocks and the precipitation of the copper sulfur in the water/hydrocarbons interface.Wichmann, 1927; Fernández Aguilar, 1945; Granero Hernández and Schmid, 1956; Ramos, 1975; Lyon, 1999) and the preliminary results obtained in some prospects Giusiano et al., 2006; Giusiano et al., 2008 proposed a new hypothesis that associates the cupriferous manifestation with the hydrocarbons and Cu-fluids migration along the main structures related to the Huincul Ridge. The origin of this hydrocarbons and fluids is located in the deepest levels of this structure. The Huincul Ridge is a structural lineament of regional scale developed at 39º South latitude,with a E-O strike extended along 270 km (Fig. 1), that defines the natural northern boundary of Argentina Patagonia with the Neuquén Basin (Ramos et al., 2004). This structure was interpreted as a fault zone of dextral lateral movement with the coexistence of transpressive and transtensive segments as resulted of changes in the fault zone trend (Ploszkiewicz et al., 1984). Silvestro and Zubiri (2008) proposed that this ridge is the result of an oblique NW-SE convergence between the Northpatagonian Craton at the Southeast and the Neuquén basin at the Northwest. These researchers identified inside this deformation belt three segments. The western sector with the predominance of NE trending anticlines, a central zone with E-W lineaments of compressive and wrench structures and a northern and eastern zone where the lineaments are NW trending and developed right-lateral wrench structures. The faults from the central, northern and eastern zones were developed from the previous normal faults of the Triassic rift by the tectonic inversion during the Lower Jurassic and Cretaceous (Cruz et al., 2002; Silvestro and Zubiri, 2008). Most of these faults have an origin in deep levels and abort in the base of the Vaca Muerta Formation (Tithonian) and others closer to the main fault have their origin in shallower levels and cut the Neuquén Group (Upper Cretaceous) (Schiuma et al., 2002). The copper deposits are hosted in the fluvial deposits of the Neuquén Group (Cenomamian- Campanian). These rocks cover almost all the center east of the Neuquén basin with up to 1300 m thick in the embayment zone (Cazau and Uliana, 1972). These rocks lay unconformilly over the continental sedimentary rocks of the Rayoso Group (Aptian-Albian) and are cover by the marine deposits of Malargüe Group (Lower Maastrichian-Upper Campanian). Neuquén Group is composed by Río Limay Subgroup with Candeleros, Huincul and Lisandro Formations, Río Neuquén Subgroup with Portezuelo and Plottier Fromations and Río Colorado Subgroup with Bajo de la Carpa and Anacleto Formations (Ramos, 1981; Fig. 2). They consist in fine to coarse sandstone and conglomerate interbedded with claystones and mudstones, and is cemented by iron oxides and hydroxides that give them a typical red color. The Barda González prospect is hosted in Portezuelo Formation (Neuquén Group), located in the central fault zone of the Huincul Ridge (Fig. 1c) next to Barda González, Bajo Barda González, and Puesto Espinoza oil fields. The Cu mineralization has an extension of 600 m wide by 2000 m long with a NNE stricke (Fig. 3), along fluvial paleochannels, defined by coarse sandstone and conglomerate layers. These rocks only reach the early diagenesis stage and are discolored and altered. They are very permeable due to the lack of iron oxides and hydroxides and the total or partial disolution of their cement and their feldespar and volcanic lithic. These last are also altered to clay minerals (illite>kaolinite). The copper minerals are diseminated, concentrated in beding, tubes or nodules always in contact with bitumen (Fig. 3c). They consits of chalcocite, covelline, trace of chalcopyrite, and a variety of supergene minerals (malachite, with chrysocolla, brochantite, azurite, tenorite, turquoise, atacamite, voborthite). There are also diseminate pyrite and psilomelane trace. The geochemistry of bitumen indicates that it is a hydrocarbon migration residue generated from a marine rock between the Lower Triassic and Upper Cretaceus (Los Molles or Vaca Muerta Formations). The Tordillos prospect is hosted in the Huincul Formation, located in the northern flank of the Huincul Ridge, west of the Loma la Lata hydrocarbon (gas) field and at the left margin of the Neuquén River (Fig. 1c). It is situated in the Alto del Sauzal Bonito anticline, a structure product of the tectonic inversion of the Jurassic fault wich has an E-W strike sloping to the NNE (Maretto y Pángaro, 2005). In the prospect area there are a series of fractures with E-W, NNW and N-S orientations that could be the shallow expressions of the normal fault observed in the subsurface which was reactivated during the Andean orogeny (Maretto et al., 2005). The copper and bitumen mineralization extend in a series of table l and along 6 km alienated with an orientation E-W that conform the outcrops of the Huincul Formation (Fig. 4a, b). This Formation is composed by feldspar-quartz rich sandstone and conglomerates with features of filling channel deposit fining upward. Theses rocks have a grayish color due the leaching of the iron oxides and hydroxides, the alteration of their volcanic lithics and feldspars to clay minerals, and the bitumen impregnations. The copper mineralization is disseminated as tables (filling join), tubes (Fig. 4c), bubbles and cylinder cutting the sedimentary structures and also as fines layers along the stratigraphic bedding planes, always associated with bitumen. The mineralization consists in calcocite partial to totally replaced by crysocolla with trace of malachite filling the porosity. The bitumen is also riming and filling the cavities the previous fibers of the fossil trunk fragments which are partially replaced by crysocolla, malachite and azurite (Fig. 4d). Both prospects present the following similarities: 1) they are located in the Huincul Ridge region, Neuquen basin, 2) mineralization is restricted to the permeable paleochannels of the Neuquén Group and consist in copper sulfur (chalcosite, covellite, traces of chalcopyrite) and supergene minerals (crysocolla, brochantite, malachite, atacamite) that are intimately associated with bitumen, 3) they are situated near the structures related to the Huincul Ridge and the hydrocarbon fields Bajo Barda González (oil) and Loma la Lata (gas), 4) in both fields the formation waters of the reservoir are chlorides rich brines (60 to 72 gr/l chlorides, Schiuma et al., 2002; Maretto et al., 2005). These water that migrate along with hydrocarbons through the faults and permeable beds, could have extract the copper from the iron oxides and hydroxides of the continental red beds. The passage of these fluids, in contact with the permeable levels of the Portezuelo and Huincul Formations, could have generated the consequent alteration and permeability increase in the host rocks and the precipitation of the copper sulfur in the water/hydrocarbons interface.