Copper(-Iron) Mineralization and Superposition of Alteration Events in the Punta Del Cobre Belt, Northern Chile
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The copper deposits of Perú consist of porphyry Cu±Mo, Au, Ag, breccia pipe Cu-Mo, enargite vein and replacement Cu±Au, Ag, Zn, Pb, calcic skarn Cu±Fe, Au, Zn, amphibolitic skarn Cu±Fe, volcanogenic massive sulfide Cu-Zn, vein and manto Cu±Ag, Pb, Zn, Sn, W, and sandstone ("red bed") Cu types. The vast majority of these deposits formed during the Andean Orogeny and are geographically and chronologically distributed in well-defined metallogenic domains. These domains correlate with geochemically distinct magmatic episodes.The magmatic and metallogenic domains appear to be controlled in part by transverse growth-faults in the Mesozoic and older basement rocks underlying the intensely folded and thrust-faulted Mesozoic and Tertiary rocks of the higher structural levels of the Cordillera. During the Andean Orogeny the extent of magmatism and the corresponding metallogenic provinces were influenced by subducted plate segmentation and by continental margin basement tectonics. In addition, the lithologic nature of the host rocks played an important role in determining the types of copper deposits formed.Porphyry Cu, breccia pipe Cu-Mo and calcic skarn Cu deposits are related to the Pomahuaca, Coastal and Caldera batholiths, as well as to felsic Cordilleran volcanism between 8° and 12°S. However, the largest and richest porphyry Cu deposits are related to the Caldera batholith. The Cobriza Cu-bearing skarn is the only significant copper deposit of pre-Mesozoic age.Perú has many ore deposits associated with the Miocene felsic extrusive and intrusive rocks along the Cordillera, forming veins and disseminations in igneous rocks and noncarbonate sedimentary rocks, and replacement mantos, pipes and veins in limestones. Several are large and high-grade enargite-type deposits containing mainly Cu, Ag, Au, Pb and Zn, accompanied by significant amounts of Cd, Te, Se, In, Bi and Tl. Others are veins and mantos containing Cu±Ag, Pb, Zn, Sn, W.The Mesozoic volcanosedimentary sequences along the coast host volcanogenic massive sulfide Cu-Zn and vein/manto-type amphibolitic skarn Cu±Fe deposits.Red bed Cu deposits are relatively unimportant in Perú.The following information on the history of copper mining in Perú has been condensed largely from Samame (1979), Petersen et al.(1990) and Benavides (1990).In Perú, gold and silver were apparently used before copper. The latter was first mined and processed by the pre-Inca Chimú culture along the northern coast and by the Tiahuanaco civilization in the Lake Titicaca region.Copper became an important metal during the Inca period,Keywords:
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The late-Hercynian magmatic alignement of the Los Pedroches Batholith in South Central-Iberian Zone (Iberian Massif) exhibits a conspicuous dike swarm. Dikes are in chronological order: a) traquiandesite, b) dacite to rhyodacite, c) rhyolite, d) aplite, aplopegmatite and pegmatite, e) quartz, f) basic (diabase, lamprophyre).
Rhyolite dikes (granite to adamellite) form a number of lineal swarms oriented N120-130E. The main group extends almost undisturbed from near Belalcazar (Cordoba) to the Guadalquivir fault (Jaen). This array, ca. 130 km length and 2-12 km thick, is usually composed of 3 to 50 dikes cutting across the main plutonic facies of the Batholith.
Rhyolite dikes have the common granitic minerals, including zircon, apatite and ilmenite as accessories. According to their macroscopic features and petrography the dikes may be porphyritic microgranite, porphyritic rhyolite or porphyritic granophyre, whereas according to the mineral cheroistry the dikes range froro rhyolite-granite to alkali feldspar granite-rhyolite. The cheroical composition of studied rocks corresponds to peraluminous and calc-alkaline
terms of a K-rich alumino-cafemic calc-alkaline association.
The dike swarm cuts granite massifs of the Los Pedroches batholith intruded at ca. 300 ± 6 Ma (El Guijo pluton, Fernandez et al., 1990), but the emplacement is poorly constrained as inferred by K-Ar mineral ages of ca. 315 ±15 Ma (Bellon et al, 1979) and Rb-Sr whole rockage of 295 ± 18 Ma (Defalque et al., 1992). The dike swarm would result of subvolcanic intrusions genetically related to the late-Hercynian igneous activity that originated the Los Pedroches Batholith.
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Iron oxide-copper-gold(IOCG) and porphyry copper deposits are all important exploration target due to their large-scale,high economic value.They are together generated in Chile subduction-related continental margin,including the world's large copper porphyry deposits,and have a sharp mineralization sequence in space-time from magnetite-apatite,IOCG,porphyry copper-gold to porphyry copper molybdenum deposits with tectonic-magmatic evolution.Large number of calc-alkaline magmatism and contemporary igneous rocks are an important foundation for formtion of mineralization.Shallow breccia pipes of IOCG deposits share with the alteration features of porphyry copper deposits.Gold-rich porphyry copper deposit have some similar characteristics and more transitional relation to porphyry deposits due to the key factors of evolution of magma intrusion and fluid.It is necessary to pay more attention to structure and alteration of rocks during the exploration.
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SummaryElectrical methods have been applied to the search for porphyry copper and IOCG deposits since the early 1950s. While there is a generally accepted model of disseminated sulfides giving rise to a chargeability response, no clear association has been attached to what EM surveys may be responding to. Work in the early 1990s (Nickson 1993) showed the well-developed supergene blankets over a porphyry copper could be conductive; this observation was however, never applied formally to generally accepted porphyry targeting models. The presence of other conductive zones associated with porphyry copper deposits is even less well studied. On the geological side, while there is a vast body of literature describing porphyry copper deposits and how to discover them, in very few cases do these studies even speculate if anomalous concentrations of sulfides could be conductive. On the geophysical side, observations of unexpected conductivity associated with porphyry systems is sometimes noted but these observations typically stop short of suggesting that there could be a more general observation made that a new class of geophysical feature should be defined. The present study is felt to have gathered a sufficient number of case studies which show that a significant number of porphyry copper deposits posse a mineralogical character which can be identified with EM techniques. This thesis can have significant implications as to how porphyry copper are explored for, especially those at depths >500 m, a generally accepted cut-off for IP techniques.
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Composite dolerite-rhyolite dikes traverse the Galway Granite batholith and its adjacent envelope. The dikes pertain to the Teach D6ite suite and were previously considered to be of Carboniferous age. New and extended examination of field relationships supports recent radiometric dating for an intrusive period that overlapped with the final consolidation of the Galway batholith. Regional crustal extension produced a complex pattern of fissuring, controlled by various preexisting structures, which permitted ascent of mantle-derived melts into and around the Galway batholith. Ponding of mafic magma at an intermediate level facilitated crustal partial melting and the generation of high-silica, high-alumina rhyolitic melts. The two contrasting magmas then rose into common or proximate dike fissures, rhyolitic injection immediately following that of dolerite. Magma storage in stratified chambers occasionally resulted in the development of a hybrid magma layer, but in all cases minor mingling and mixing beween dolerite and rhyolite magma continued up into the dikes. Rhyolite geochemistry precludes a genetic relationship with the Galway granitoids, despite a few instances where granitic material was entrained into rhyolitic magma. Introduction and setting The 400Ma Galway Granite batholith was emplaced into 470Ma island-arc orthogneisses in the Connemara sector of the Caledonides. This emplacement was followed by the intrusion of two hypabyssal suites: earlier microphyric ('porphyry') dacite dikes (Kinahan 1869; Mohr 2003) and a later complex nexus of dolerite dikes, the Teach D6ite (TD) suite (Mitchell and Mohr 1987; Fig. 1). The numerous and widespread dacite dikes have consistently been considered the youngest igneous rocks pertaining to the Galway batholith (Wager 1932; Wright 1961; Harvey 1967; Coats and Wilson 1971; Senior 1973; Leake 1974). However, new work summarised here suggests that the subsequent TD dikes were a final manifestation of the magmatic episode responsible for the batholith. The regional pattern of TD dikes comprises three major linear trends (Fig. 1). In the central part of the Galway batholith and its northern envelope, the NNE-trending Seanabhain system of dikes is intimately associated with the Shanawon Fault that separates the central and western blocks of the batholith (Feely and Madden 1988; Mohr 1993; Callaghan 1999). The na hUillinni dike system, 3.5km west of the Seanabhan system and parallel to it, projects much farther NNE into the orthogneiss envelope (Fig. 1). Secondly, a grid of ENE-trending dikes Irish Journal of Earth Sciences 22 (2004), 15-32. © Royal Irish Academy 15 This content downloaded from 207.46.13.174 on Sun, 10 Jul 2016 05:04:50 UTC All use subject to http://about.jstor.org/terms 16 Irish Journal of Earth Sciences (2004)
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