Iron oxide-copper-gold deposits: an Andean view   总被引：22，自引：2，他引：20  

Richard?H.?SillitoeEmail author 《Mineralium Deposita》2003,38(7):787-812

Iron oxide-copper-gold (IOCG) deposits, defined primarily by their elevated magnetite and/or hematite contents, constitute a broad, ill-defined clan related to a variety of tectono-magmatic settings. The youngest and, therefore, most readily understandable IOCG belt is located in the Coastal Cordillera of northern Chile and southern Peru, where it is part of a volcano-plutonic arc of Jurassic through Early Cretaceous age. The arc is characterised by voluminous tholeiitic to calc-alkaline plutonic complexes of gabbro through granodiorite composition and primitive, mantle-derived parentage. Major arc-parallel fault systems developed in response to extension and transtension induced by subduction roll-back at the retreating convergent margin. The arc crust was attenuated and subjected to high heat flow. IOCG deposits share the arc with massive magnetite deposits, the copper-deficient end-members of the IOCG clan, as well as with manto-type copper and small porphyry copper deposits to create a distinctive metallogenic signature.The IOCG deposits display close relations to the plutonic complexes and broadly coeval fault systems. Based on deposit morphology and dictated in part by lithological and structural parameters, they can be separated into several styles: veins, hydrothermal breccias, replacement mantos, calcic skarns and composite deposits that combine all or many of the preceding types. The vein deposits tend to be hosted by intrusive rocks, especially equigranular gabbrodiorite and diorite, whereas the larger, composite deposits (e.g. Candelaria-Punta del Cobre) occur within volcano-sedimentary sequences up to 2 km from pluton contacts and in intimate association with major orogen-parallel fault systems. Structurally localised IOCG deposits normally share faults and fractures with pre-mineral mafic dykes, many of dioritic composition, thereby further emphasising the close connection with mafic magmatism. The deposits formed in association with sodic, calcic and potassic alteration, either alone or in some combination, reveal evidence of an upward and outward zonation from magnetite-actinolite-apatite to specular hematite-chlorite-sericite and possess a Cu-Au-Co-Ni-As-Mo-U-(LREE) (light rare earth element) signature reminiscent of some calcic iron skarns around diorite intrusions. Scant observations suggest that massive calcite veins and, at shallower palaeodepths, extensive zones of barren pyritic feldspar-destructive alteration may be indicators of concealed IOCG deposits.The balance of evidence strongly supports a genetic connection of the central Andean IOCG deposits with gabbrodiorite to diorite magmas from which the ore fluid may have been channelled by major ductile to brittle fault systems for several kilometres vertically or perhaps even laterally. The large, composite IOCG deposits originated by ingress of the ore fluid to relatively permeable volcano-sedimentary sequences. The mafic magma may form entire plutons or, alternatively, may underplate more felsic intrusions, as witnessed by the ore-related diorite dykes, but in either case the origin of the ore fluid at greater, unobserved depths may be inferred. It is concluded that external 'basinal' fluids were not a requirement for IOCG formation in the central Andes, although metamorphic, seawater, evaporitic or meteoric fluids may have fortuitously contaminated the magmatic ore fluid locally. The proposed linkage of central Andean and probably some other IOCG deposits to oxidised dioritic magmas may be compared with the well-documented dependency of several other magmatic-hydrothermal deposit types on igneous petrochemistry. The affiliation of a spectrum of base-metal poor gold-(Bi-W-Mo) deposit styles to relatively reduced monzogranite-granodiorite intrusions may be considered as a closely analogous example.Editorial handling: B. Lehmann  相似文献   

Book Reviews     

《The Professional geographer》2004,56(4):587-588

  相似文献   

The role of collisional tectonics in the metallogeny of the Central Andean tin belt   总被引：3，自引：0，他引：3  

Michael S.J. Mlynarczyk  Anthony E. Williams-Jones 《Earth and Planetary Science Letters》2005,240(3-4):656-667

The Inner Arc of the Central Andes, broadly corresponding to the Eastern Cordillera, is the location of a rich Tertiary and Triassic Sn–W–(Ag-base metal) metallogenic province, commonly referred to as the Bolivian tin belt. We propose that the Tertiary metallogeny, which generated most of the tin ores, was a direct consequence of discrete “collisions” between the South American plate and the Nazca slab and sub-slab mantle, during the ongoing Andean orogeny. Evidence supporting this proposal include: (1) the coincidence of the tin province and the Inner Arc in a marked “hump” in the Andean orogen, which may represent tectonic indentation; (2) the symmetry of the tin province with respect to the Bolivian orocline, the axis of which corresponds to the direction of highest compression; (3) the relative symmetry of the magmatism and tin mineralization with respect to this axis; (4) the concurrent timing of mineralization and compressional pulses; (5) the similar host rock geochemistry and ore lead isotope data, testifying to a common crustal reservoir; and (6) the striking similarity of the igneous suites, associated with the ore deposits to those from “typical” collisional orogens. A number of studies have called upon a persistent tin anomaly to explain the metallogeny of the region. We propose, instead, that the latter is better explained by periodic compressional interaction between the Farallon/Nazca oceanic plate and the South American continent. This led to the generation of peraluminous magmas, which during fractional crystallization exsolved the fluids responsible for the voluminous Sn–W mineralization.  相似文献   

Seismic and field evidence for selective inversion of Cretaceous normal faults, Salta rift, northwest Argentina   总被引：1，自引：0，他引：1  

Jonas Kley  Eduardo A. Rossello  Csar R. Monaldi  Bjrn Habighorst 《Tectonophysics》2005,399(1-4):155

Northwestern Argentina was the site of the continental Salta rift in Cretaceous to Paleogene time. The Salta rift had a complex geometry with several subbasins of different trends and subsidence patterns surrounding a central high. Fault trends in the rift were extremely variable. There is evidence of normal and/or transfer faults trending N, NE, E and SE. It is not clear if all these faults were active at the same time, indicating a poorly defined extension direction, or if they formed in different, non-coaxial extension phases. In either case, their trends were very likely influenced by preexisting fault systems. Beginning in early Eocene time, the rift basins were superseded by Andean foreland basins and later became caught in the Andean thrust deformation propagating eastward, resulting in the inversion of rift faults. Due to their different orientations, not all faults were equally prone to reactivation as thrusts. N to NNE trending faults were apparently most strongly inverted, probably often to a degree where the traces of their normal fault origin have become obliterated. We present seismic evidence of moderately inverted N trending faults in the Tres Cruces basin and field examples of preserved E trending normal faults. However, reactivation sometimes also affects faults trending approximately parallel to the main Neogene shortening direction, indicating short-term deviations from the general pattern of Neogene thrust deformation. These pulses of orogen-parallel contraction may be linked to the intermittent activity of oblique transfer zones.  相似文献   

Plio-Quaternary extension in the Venezuelan Andes: Mapping from SAR JERS imagery     

Damien Dhont  Guillaume Back  Yves Hervouët 《Tectonophysics》2005,399(1-4):293

The geometry of extensional structures is described for the first time in the active setting of the Venezuelan Andes using remote sensing imagery. We favored the use of a mosaic of Synthetic Aperture Radar (SAR) scenes of the Japanese Earth Resources Satellite-1 (JERS-1) assisted by complementary remote sensing devices (Landsat TM, digital elevation models) and field observations to make a structural analysis at regional scale. Radar images are particularly efficient in the Venezuelan Andes where dense vegetation and frequent cloud covering earlier lent difficulties to remote sensing studies. We focused our analysis in the Valera–Rio Momboy and Bocono faults corner and in the Mucujun area. We show that, in an area where ongoing compression and strike–slip deformations occur, brittle extension can be detected independently from previous knowledge. Extensional structures correspond to elongated tilted blocks with dimension less than 10 km in width. Blocks are bounded by curved faults in plan view, the concavity being turned towards the axial part of the belt. The geometry and kinematics of such structures suggest that syn-orogenic extension started together with initiation of right-lateral strike–slip motion along the Bocono Fault in the Plio-Quaternary.  相似文献   

Lithostratigraphy of the Mérida (Wisconsinan) glaciation and Pedregal interstade, Mérida Andes, northwestern Venezuela     

Randy W. Dirszowsky  William C. Mahaney  Kyle R. Hodder  Michael W. Milner  Volli Kalm  Maximiliano Bezada  Roelf P. Beukens 《Journal of South American Earth Sciences》2005,19(4):525-536

At Pedregal, more than 40 m of sediments are exposed within a ‘fan complex’ formed between lateral moraines of the adjacent Mucuchache and El Caballo valleys. Early and late Mérida (Wisconsinan) glaciations are represented by till and till plus proglacial sediments, respectively. A middle Wisconsinan interstadial event, here termed the Pedregal interstade, began at the end of the Early Mérida glaciation at approximately 60 ka BP. Following the retreat of ice from the small Pedregal Basin, a lake formed when the local drainage was blocked due to movement of the Mesa de Caballo along the Boconó Fault. Shallow lake or no-lake phases lasted approximately a few hundred to, at most, 2000 years, and each lake phase was marked by peat accumulation. Four of seven peats identified formed during sufficiently long intervals for soil profiles (incipient to mature Spodosols) also to develop. The Spodosol with the strongest development (Eb/Bsb/Coxb/Cub horizons) is found adjacent to the lowest peat and reflects ongoing early Mérida stadial (MIS 3) conditions; the youngest peats, associated with weak podzolic soils (Eb/Bsb horizons), formed under slightly warmer interstadial conditions, presumably with less soil water. Cyclic lacustrine deposition is related to lake level and relative depth fluctuations, due in part to variable shoreline/delta progradation and shallowing as the lake deepened in general. Whereas final drainage of the lake is related to movement of the Boconó Fault and breach of the moraines that form the Mesa de Caballo, earlier lake level fluctuations appear related to climate change. Radiocarbon dating of the peats suggests they are related to warmer periods and may tentatively correlate with small ‘interstadials’ or ‘D-O events’ detected in the oxygen-isotope record of Greenland ice cores and North Atlantic marine sediments.  相似文献   

Andean subduction-related mantle xenoliths: Isotopic evidence of Sr–Nd decoupling during metasomatism     

R.V. Concei    o  G. Mallmann  E. Koester  M. Schilling  G.W. Bertotto  A. Rodriguez-Vargas 《Lithos》2005,82(3-4):273-287

Sr–Nd isotopic analyses on some mantle xenolith samples from the Northern, Southern and Austral Andean volcanic zones exhibit radiogenic Sr enrichment without dramatic changing of the Nd isotopic composition. This anomalous effect (Sr–Nd decoupling) makes these samples plot displaced to the right side of the “mantle array” trend (here called the “MORB–OIB–BSE trend”) in the ⁸⁷Sr/⁸⁶Sr vs. ¹⁴³Nd/¹⁴⁴Nd isotopic diagram. Such behavior reflects processes that took place in the mantle and can be related to: i) the mixture of a depleted mantle and an enriched source (enriched mantle II—EMII); ii) the mixture of a depleted mantle and a mixture of mantle-derived and slab-derived melts; and iii) a chromatographic process that occurs during the percolation of a metasomatic agent through the mantle.  相似文献   

Vegetation and climate change on the Bolivian Altiplano between 108,000 and 18,000 yr ago   总被引：1，自引：0，他引：1  

Alex Chepstow-Lusty  Mark B. Bush  Paul A. Baker  Sherilyn C. Fritz  James Aronson 《Quaternary Research》2005,63(1):90-98

A 90,000-yr record of environmental change before 18,000 cal yr B.P. has been constructed using pollen analyses from a sediment core obtained from Salar de Uyuni (3653 m above sea level) on the Bolivian Altiplano. The sequence consists of alternating mud and salt, which reflect shifts between wet and dry periods. Low abundances of aquatic species between 108,000 and 50,000 yr ago (such as Myriophyllum and Isoëtes) and marked fluctuations in Pediastrum suggest generally dry conditions dominated by saltpans. Between 50,000 yr ago and 36,000 cal yr B.P., lacustrine sediments become increasingly dominant. The transition to the formation of paleolake “Minchin” begins with marked rises in Isoëtes and Myriophyllum, suggesting a lake of moderate depth. Similarly, between 36,000 and 26,000 cal yr B.P., the transition to paleolake Tauca is also initiated by rises in Isoëtes and Myriophyllum; the sustained presence of Isoëtes indicates the development of flooded littoral communities associated with a lake maintained at a higher water level. Polylepis tarapacana-dominated communities were probably an important component of the Altiplano terrestrial vegetation during much of the Last Glacial Maximum (LGM) and previous wet phases.  相似文献   

Postmagmatic cooling and late Cenozoic denudation of the North Patagonian Batholith in the Los Lagos region of Chile, 41°−42°15′S     

A. C. Adriasola  S. N. Thomson  M. R. Brix  F. Hervé  B. Stöckhert 《International Journal of Earth Sciences》2006,95(3):504-528

Zircon and apatite fission track (FT) thermochronology was applied to investigate the history of cooling and denudation of the Southern Andes between 41° and 42°15′S in relation to the late Cenozoic activity of the Liquiñe-Ofqui fault zone (LOFZ) and the northward migration of the Chile Triple Junction (CTJ). Fifty-six zircon and 51 apatite FT ages, plus 37 apatite confined track-length distributions were obtained mainly from plutonic rocks of the North Patagonian Batholith (NPB) in the main Andean Cordillera. Apatite FT ages and track lengths indicate a stage of rapid cooling at ∼5--3 Ma along both sides of the LOFZ, whereas older Miocene ages with monotonous cooling histories were obtained further away from the fault. Zircon FT ages range from Cretaceous to Pliocene, with marked differences observed along and across the LOFZ. Three different types of temperature-time histories characterise the post-magmatic cooling of the NPB in the region: deep intrusions with moderate and steady cooling rates, intrusions in the upper crust with very slow cooling rates following a stage of initial rapid cooling, and rapidly cooled and exhumed shallow intrusions, the latter with younger ages towards the fault zone. The most prominent denudation episode along the LOFZ is late Miocene to Pliocene, coeval with plate tectonic reconstructions for the arrival and subduction of the Chile Rise beneath the Taitao Peninsula.  相似文献   

Low-grade metamorphism and geotectonic setting of the Macuchi Formation, Western Cordillera of Ecuador   总被引：2，自引：0，他引：2  

L. AGUIRRE  M.P. ATHERTON 《Journal of Metamorphic Geology》1987,5(4):473-494

Abstract The Cretaceous-Eocene basic to intermediate marine volcanic rocks of the Mucuchi Formation constitute the Western Cordillera in northern Ecuador. Their chemical features mostly correspond to those of tholeiitic basalts with some calc-alkaline affinities and suggest an oceanic island arc setting. The Macuchi rocks are affected by low-grade, non-deformative metamorphism, characterized by zeolite, prehnite-pumpellyite and lower greenschist facies assemblages. Depth-zonation is suggested by the downward mineral sequence: (i) laumontite+ (pistacitic epidote, pumpellyite + prehnite); (ii) pumpellyite+ prehnite + pistacitic epidote; (iii) actinolite+biotite+ pistacitic epidote + chlorite. This broad zonation and the chemistry of individual minerals point to an interaction between the volcanic rocks and sea-water under a moderate to high thermal gradient (= 75° C/km?). Alteration appears to have been dependent primarily on fluid control (volume, pressure, composition), temperature and reaction kinetics which together partly overshadow the role of load-pressure. Compositional variations of a mineral species at the scale of a contiguous flow or even at the scale of a thin section show that intensity of alteration was spatially uneven depending on rock permeability and consequently, metastable equilibrium commonly exists. However, a progressive approximation to equilibrium as a result of P–T control is shown by the mineralogy. A high f_o2 of the fluid phase is evident from the mineral chemistry. The metamorphism of the Macuchi volcanics is similar to the hydrothermal-burial type produced during the development of a volcanic arc where lavas and volcanoclastics accumulated in a shallow marine environment. However, some of its characteristics point to a transition toward systems defined by a higher T/P ratio such as those found in ocean-floor metamorphism. A model is proposed in which the Macuchi volcanics are assigned to an oceanic island arc generated contemporaneously with a marginal basin which has opened as the outcome of progressive north-south attenuation of the continental crust due to mantle diapirism.  相似文献