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Subduction zone magmatism is a major control of volcanism, the generation of modern continental crust and the formation of economically important porphyry Cu–(Mo–Au) deposits. Reading the magmatic record of individual arc segments and constraining the rates of magmatic changes are critical in order to fully understand and quantify the processes that drive magma evolution in subduction settings during arc growth. This study focuses on the San Francisco Batholith and the Rio Blanco-Los Bronces porphyry deposit cluster in central Chile, which provides an igneous rock record over ~13.5 Myr of arc evolution. We use whole-rock geochemistry, zircon geochronology and Hf isotope geochemistry to track changes in the crustal magmatic system of this arc segment during crustal thickening and porphyry Cu deposit formation. By combining the analytical dataset with Monte Carlo fractional crystallisation and assimilation fractional crystallisation modelling, we test a model for significant crustal involvement during magma evolution. Systematic and continuous increases in Dy/Yb, La/Yb, V/Sc and Sr/Y in the magmas over time indicate a transition in the main fractionation assemblage from plagioclase-dominated to amphibole-dominated that reflects deeper crystallisation and/or a higher meltwater content. Concomitant decreases in εHf and Th/La as well as increasing Ba/Th are best explained by assimilation of progressively deeper crustal lithologies from low (Chilenia) to high Ba/Th (Cuyania) basement terranes. Our study highlights that an increasingly hydrous magma and a deepening locus of crustal magma differentiation and assimilation, driven by crustal thickening contemporaneous with increased tectonic convergence and ingression of the aseismic Juan Fernandez ridge, can account for all investigated aspects of the multi-Myr magmatic evolution leading up to the formation of the Rio Blanco-Los Bronces porphyry Cu deposits. Our findings corroborate the importance of high-pressure differentiation of hydrous magma for the formation of Andean-style porphyry deposits. Once magmas favourable for porphyry Cu mineralisation were generated in the lower crust, multiple episodes of efficient magma migration into the upper crust fed several, discrete, shallow magmatic-hydrothermal systems over ~3.5 Myr to form the world's largest known Cu resource at Rio Blanco-Los Bronces. [ABSTRACT FROM AUTHOR]

Porphyry copper ore‐forming intrusions are distinguished from barren arc magmas by high oxidation state and "adakitic" high Sr/Y and La/Yb. Controversy over petrogenesis of adakites has centered on ambiguities in interpretation of their steep rare earth element (REE) patterns, and on whether garnet participates in their petrogenesis. Lambda (λ) coefficients deconvolute subtle differences in REE pattern curvature, providing a more quantitative method to explore mineral fractionation processes. Here, we use trace element and numerical λ coefficient modeling to assess the relative influence of amphibole, garnet, and plagioclase in the petrogenesis of porphyry ore‐forming intrusions. We find that garnet‐fractionation trends are not evident in REE patterns of many adakitic porphyry‐forming intrusions. Instead, porphyry‐forming intrusions in the western Pacific and Eocene porphyry‐forming intrusions in northern Chile have REE patterns consistent with a garnet‐free, amphibole‐dominated cumulate. Traditional element ratios such at Dy/Dy*, LaN/YbN, and DyN/YbN are poor discriminants for garnet or amphibole fractionation. Plain Language Summary: Copper is a crucial metal for electrification and emission‐free energy generation. Copper is primarily sourced from active and extinct volcanic mountain chains. The minerals forming in the root zones of these volcanoes are crucial for ore formation processes, but identification of the exact minerals involved in these processes is still debated. We use a novel method of detecting small variations in the chemical signature of the rare earth elements, combined with numerical modeling of mineral crystallization to distinguish the participation of two minerals, garnet and amphibole. We show that amphibole is ubiquitous whereas garnet only occurs in some deposits. Therefore, garnet fractionation is not necessary for copper deposit formation as has been previously suggested. Key Points: Analysis of rare earth element pattern curvature distinguishes between amphibole and garnet fractionation better than element ratios like Dy/Yb or Dy/Dy*Polybaric differentiation or plagioclase fractionation is unlikely to overprint a pre‐existing garnet signatureWe find that garnet fractionation is not necessary for porphyry copper deposit formation [ABSTRACT FROM AUTHOR]

An essential part of the world's remaining mineral resources is expected to reside deep in the crust or under post-mineralization cover. For porphyry copper deposits, the world's primary source of Cu, Mo, and Re, identifying the dynamic processes that control their emplacement in the upper crust can guide future exploration. Seismic tomography can constrain these processes through imaging deep-seated structures at the regional scale. Here we construct a three-dimensional model of the Vp/Vs ratio, based on arrival times of P and S seismic waves, beneath the Cerro Colorado porphyry Cu–(Mo) deposit in northern Chile. Our images show that low Vp/Vs (~ 1.55–1.65) anomalies, extending to ~ 5–15 km depth, coincide with the surface expression of known porphyry copper deposits and prospects, as well as delimit structures that host orebodies and related hydrothermal alteration zones. Medium Vp/Vs (~ 1.68–1.74) and high Vp/Vs (Vp/Vs ~ 1.85) bodies correspond to intermediate-felsic plutonic precursors for porphyry intrusions and mafic magma reservoirs that underlie shallower orebodies, respectively. Imaging these precursor and parental plutons is crucial to the identification of orebodies as they act as the source of fluids for porphyry copper generation. This study demonstrates the potential of local earthquake tomography as a tool to identify future deep mineral resources with minimal environmental impact. [ABSTRACT FROM AUTHOR]

Epithermal deposits occur in many metallogenic belts globally and are economically important sources of precious and base metals. However, post-mineral cover (both transported and younger deposits) also occurs in many prospective districts and hinders exploration efforts. Hydrogeochemistry is a potentially useful exploration tool in these environments, with water–rock interaction shown from other studies to produce large hydrogeochemical footprints. Groundwaters from in and around the Vaquillas (Au-Ag) epithermal deposit in the pre-cordillera of northern Chile are typically Ca-SO42- type with meteoric upflow groundwaters interacting with the ore body and producing a significant downflow (down hydraulic head) mobility (up to 3 km) of (oxy)anions of arsenic and molybdenum, which contrasts with proximal trace cation anomalies. Multiple sulphur endmembers contribute to the dissolved sulphate concentrations, including evaporites, primary hypogene sulphide minerals, and a single δ34S(SO4) value of 30.1 ‰ which is potentially associated with disproportionated sulphur in sulphate. This study reports anomalism from one of the most dilute endmember groundwater compositions from Chile reported to date. The sub-alkaline and weakly oxygenated groundwaters are common in many aquifers globally, making the results applicable to many other metallogenic belts. It is also the first hydrogeochemical case study of an intermediate-sulfidation epithermal deposit. Despite the highly dilute groundwaters recharging aquifers at Vaquillas, we conclude hydrogeochemistry to be a highly effective exploration tool for epithermal and porphyry deposits in areas of significant post-mineral cover. [ABSTRACT FROM AUTHOR]

The Maricunga metallogenic belt, herein defined to encompass the region from 25° S to 28° S along the Cordillera Frontal of northern Chile, is known for hosting a series of gold-copper and lesser copper-molybdenum porphyries and gold-silver epithermal systems that are hosted in a segment of the late Oligocene-Miocene Andean calc-alkaline magmatic arc. Based on an up-to-date geochronological database, four episodes of magmatism and related hydrothermal deposits are proposed to characterize intensity and style of volcanism and hypabyssal intrusive rocks over time. The late Oligocene episode (26-21 Ma) is characterized by stratovolcanoes, dome complexes, pyroclastic sequences, and hypabyssal intrusives of andesitic to dacitic composition that host gold-rich porphyry systems such as Caspiche, Maricunga, and La Pepa. The early Miocene episode (21–17 Ma) has similar rock types but is volumetrically reduced, hosting epithermal and porphyry deposits like La Coipa and Caserones, respectively. A reinvigoration of volcanism occurred during the third stage between 17 and 11 Ma, giving rise to large volcanic complexes and intra-to extra-caldera ignimbritic deposits that host several gold-rich porphyry deposits such as Lobo-Marte, Luciano, and Cerro Casale; as well as epithermal deposits such as Fenix. Some of the 17 to 11 Ma and younger igneous rock belong to the high-K calc-alkaline series, showing a higher alkali content compared to previous periods. The last magmatic episode, from 11 to 5 Ma, coincides with the shallowing of the subduction angle and is characterized by an eastward migration of volcanism, and more heterogeneous rocks including trachytic compositions. During this period mineralization was emplaced at the Salares Norte epithermal deposit and El Volcan porphyry Au–Cu deposit. Porphyry and epithermal systems formed throughout the evolution of the Maricunga belt. Overprint of advanced argillic alteration on porphyry style alteration is widespread albeit commonly devoid of significant addition of epithermal Au–Ag mineralization. The Au-rich nature of the majority of the porphyry systems is attributed to the shallow crustal depths of emplacement compared to Cu- and Mo-rich porphyry systems. Conversely, high-sulfidation epithermal Au–Ag deposits such as La Coipa and Nueva Esperanza are not known to be telescoped on coeval porphyry gold deposits. The location of the mineralized centers was favored by crustal weaknesses expressed by intersection of oblique- and parallel-to arc structures, the former inherited from the construction of the continent before to the formation of the Andean range. The arc oblique structures also segment the Maricunga belt into domains with unique magmatic and hydrothermal styles. • This is an updated compilation of the distribution of magmatism and related mineralization within the Maricunga belt. • The belt is segmented by major cross-strike structural corridors which control the styles of mineralization for each segment. • Mineralization events that defined the Maricunga belt covered a more extensive area from original models. [ABSTRACT FROM AUTHOR]

Groundwaters recovered from the Salar de Punta Negra and Monturaqui basins in the Atacama Desert of northern Chile exhibit distinctly different isotopic, major, trace, and porphyry copper elemental compositions related to contrasting morphostructural, geochemical, and hydrodynamic settings. Comparison of these distinct groundwater signatures with groundwaters from known porphyry copper deposits (Salar de Hamburgo and Spence Deposit), can be used to determine if either basin might be prospective for porphyry copper mineralization. Groundwaters within the Punta Negra Basin exhibit geochemical characteristics consistent with other closed basin settings throughout the arid Andes. Elemental and isotopic compositions within the Punta Negra Basin reflect closed basin evaporitic processes consistent with the hyperarid, volcanic setting of the central Andes. Pathfinder metals and isotopic compositions are not consistent with porphyry copper type mineralization as described for groundwaters within the Salar de Hamburgo Basin, and the Spence Deposit. Within the Monturaqui Basin the geochemical composition of groundwaters are characteristic of diffuse hydrothermal activity. Sulfur isotopic composition within the waters of the northern Monturaqui Basin exhibit δ 34SCDT isotopic signatures that fall within the range for sulfide mineralization as reported for groundwaters in and around the Spence Deposit and the Salar de Hamburgo. However, porphyry copper related elements within the Monturaqui Basin are impoverished relative to groundwaters of the Spence and Escondida Deposit. Such impoverishment in porphyry related elements, taken in conjunction with enriched δ 13CPDB compositions, elevated groundwater temperatures, groundwater compositions dominated by HCO3, SO4, and Si, and the proximity of the Monturaqui Basin to the current magmatic arc are consistent with a volcanic hydrothermal origin. In summary, the groundwater geochemistry of the Monturaqui and Punta Negra Basins are not indicative of porphyry copper-type mineralization. Thematic collection: This article is part of the Hydrochemistry related to exploration and environmental issues collection available at: https://www.lyellcollection.org/cc/hydrochemistry-related-to-exploration-and-environmental-issues [ABSTRACT FROM AUTHOR]

The origin of hypogene alteration and mineralization features in the Escondida porphyry Cu deposit resulted from intense overprinting related to three main hydrothermal stages. The beginning of each stage is recorded by deposition of bright quartz crystals on vein walls that precipitated from high-temperature fluids. In the deepest zones of the deposit, the earliest stage started with exsolution of intermediate-density fluids, which transported high concentrations of Cu, whereas Mo was not detected. However, in the shallow zones, the early stage began with depressurization and unmixing of intermediate-density fluids, which generated both a hypersaline and vapour-rich fluid phase, producing an important geochemical segregation between both phases. The transitional stage also started with circulation of intermediate-density fluids but never experienced unmixing. These fluids transported the highest Mo concentrations in the deposit; however, Cu displays lower concentrations relative to intermediate-density fluids from the early stage. The beginning of the late stage was also associated with intermediate-density fluids; however, Cu and Mo were below the detection limits in most of the analysed fluids. During the evolution of the three stages, the fluids experienced gradual cooling, which promoted the precipitation of euhedral and zoned quartz crystals that overgrew the early high-luminescence quartz generations. Sometimes, sulfide minerals display euhedral crystal boundaries with zoned quartz, suggesting that mineralization started during these phases. The latest events detected in each main stage are linked to cooler and low-salinity fluids, from which dark quartz and hypogene sulfides precipitated along microfractures and interstitial spaces developed in the earlier quartz generations. [ABSTRACT FROM AUTHOR]

The geochemical stability, in terms of potential mobility and derived ecological and human health risks of potentially toxic elements (PTEs), of diverse fresh and old porphyry Cu-mine tailings from Chile was assessed through an integrated methodology comprising four interrelated investigation levels: (1) chemical composition and contamination degree of tailings by PTEs, (2) mineralogical characterization by X-ray diffraction and quantitative automated mineralogy analysis by scanning electron microscopy (QEMSCAN®), (3) partitioning and potential mobility of PTEs within the tailings by a sequential extraction procedure (SEP) and leaching tests, and (4) ecological risk assessment (ERA) and human health risk assessment (HHRA). According to pollution indices, Cu, As, Pb, and Mo are most concerning PTEs present in the tailings. SEP shows that major portion of the PTEs are strongly fixed as residual fraction, and thus are poorly mobilizable and bioavailable. Among the PTEs, Cu, As, and Mo were identified as the PTEs most prone to mobilization. Leaching tests show that a low fraction of PTEs is water-leachable. Seawater enhances Mn and As leaching, while process water increases the leaching of Cu, Mn, and Mo. Phosphate particularly promotes leaching of As and Cu, whereas it does not mobilize or even immobilize Pb in the tailings. ERA suggests that mainly old tailings pose a very high potential risk for ecological receptors (PERI = 663–3356), mostly due to Cu and As. HHRA indicates that the old tailings pose higher potential non-carcinogenic and carcinogenic health risks, while the risk decreases in the order ingestion > dermal > inhalation for both children and adults. Non carcinogenic and carcinogenic HHRA points to As as the main PTE of concern via ingestion pathway in the tailings. Overall, the results revealed that particularly old tailings, containing mixed slag-tailings, pose considerable risks to the environment and human health due to potential PTEs mobilization and this aspect requires scrutiny for proper tailings management, including storage, sealing, and eventual tailings reprocessing and/or site rehabilitation after closure. [ABSTRACT FROM AUTHOR]

Porphyry-type deposits are the main global source of copper and molybdenum. An improved understanding of the most favorable structural settings for the emplacement of these deposits is necessary for successful exploration, particularly considering that most future discoveries will be made under cover based on conceptual target generation. A common view is that porphyry deposits are preferentially emplaced in pull-apart basins within strike-slip fault systems that favor local extension within a regional compressive to transpressive tectonic regime. However, the role of such a structural context in magma storage and evolution in the upper crust remains unclear. In this work, we propose a new model based on the integration of structural data and the geometry of magmatic-hydrothermal systems from the main Andean porphyry Cu-Mo metallogenic belts and from the active volcanic arc of southern Chile. We suggest that the magma differentiation and volatile accumulation required for the formation of a porphyry deposit is best achieved when the fault system controlling magma ascent is strongly misoriented for reactivation with respect to the prevailing stress field. When magmas and fluids are channeled by faults favorably oriented for extension (approximately normal to σ3), they form sets of parallel, subvertical dikes and veins, which are common both during the late stages of the evolution of porphyry systems and in the epithermal environment. This new model has direct implications for conceptual mineral exploration. [ABSTRACT FROM AUTHOR]

This work explores the potential of geochemical and petrographic characteristics of detrital zircons coming from the sedimentary record of the Centinela District in Northern Chile to identify the presence of buried porphyry copper deposits under a transported gravel cover. The sampled sedimentary section was recovered from the pit of the exotic copper deposit of El Tesoro, located approximately 2 and 4 km west of the Esperanza and Mirador porphyries, respectively. The sedimentary cover comprises four units, Tesoro II, Tesoro III, Arrieros and Recent gravels, deposited since the late Cenozoic in an arid continental environment dominated by alluvial fans. Except for the Tesoro III gravels, all other units contain exotic-Cu mineralisation. In order to interpret the geochemical footprint of the investigated zircons, the Porphyry Indicator Zircon (PIZ) concept (Pizarro et al., 2020) is used. A PIZ need to comply with each of the following geochemical values: Hf >8750 (ppm), Ce/Nd >1, Eu/Eu* >0.4, 10,000 × (Eu/Eu*)/Y > 1, (Ce/Nd)/Y > 0.01, Dy/Yb <0.3 and 0.1 < Th/U < 1. These zircons also have Ti <9 ppm and Ce/Ce* <100 and usually show euhedral morphologies characterised by prismatic forms of type {110}. The geochemical and petrographic characteristics of the PIZs collected in the gravels are similar to zircons from the nearby Mirador and Esperanza porphyries. The highest PIZ concentration coincides with the gravel horizons with exotic-Cu mineralisation. Therefore, the PIZs found in the sedimentary record are a potential tracer of adjacent copper porphyries and represent a promising exploration tool for this type of hidden ore deposits in challenging sediment-covered areas. [Display omitted] • Group of geochemical indicators summarized as Porphyry Indicator Zircon (PIZ). • 36 % of zircons found in the gravels of Centinela district correspond to PIZ. • The gravels with exotic-Cu mineralisation have highest concentration of PIZ. • PIZ in sedimentary records are a good tracer of adjacent copper porphyries. • PIZ are a promising exploration tool in search for hidden copper porphyries. [ABSTRACT FROM AUTHOR]

Modes of occurrence of Au‐ and Ag‐bearing phases and their relation with associated hypogene ore minerals were examined with the objective to elucidate Au‐Ag distribution at the Esperanza porphyry deposit in the Eocene Centinela copper belt, using ore‐microscope modal analysis, semi‐quantitative analyses by automated mineralogy, electron probe microanalysis, and secondary ion mass spectrometer. The Esperanza hypogene mineralization is characterized by early‐stage chalcopyrite‐rich veinlets in the potassic alteration zone and later polymetallic stage with tennantite and galena in the chlorite‐sericitic alteration zone. Only the early‐stage chalcopyrite contains fine‐grained electrum (Au68Ag32 ‐ Au81Ag19) and hessite (Ag2Te), and thus yields positive correlations in Cu vs. Au and Cu vs. Ag grades that are clearly recognized in the hypogene sulfide zone. The early‐stage chalcopyrite grains frequently exhibit polysynthetic twinning suggestive of inversion from intermediate solid solution. These features suggest that the fine‐grained electrum and hessite are products exsolved in the cooling process with the intermediate solid solution to chalcopyrite inversion. In contrast, tennantite and galena of the later‐stage mineralization contain no detectable Ag, and it is thus proposed that the early‐stage inverted chalcopyrite is the principal storage of economically important precious metals. [ABSTRACT FROM AUTHOR]

The Xietongmen large deposit is the only known subduction‐related porphyry Cu deposit in the Gangdese Belt, Tibet, which is composed of the No. I and No. II deposits. Previous work suggests that the No. I deposit is reduced while the No. II deposit is oxidized. However, it is ambiguous that both oxidized and reduced porphyry Cu deposits occur in close proximity, in the same tectonic setting, and within a short time period. Detailed examination of the ages, whole‐rock geochemistry, zircon and apatite compositions of the Xietongmen No. I and No. II ore‐bearing porphyries are conducted to investigate their magma characteristics and the genetic types of the Xietongmen No. I and No. II deposits. Zircon U–Pb dating results show that the Xietongmen No. I and No. II ore‐bearing porphyries formed at 166.3 ± 2.6 Ma and 175.5 ± 1.0 Ma, respectively. The Xietongmen No. I and No. II ore‐bearing porphyries have similar mineral assemblage (containing amphibole phenocryst), high Sr/Y ratios and negligible negative Eu anomalies, low Ti‐in‐zircon temperatures (633−749 and 633−763°C, respectively), which indicate that their magmas are water‐rich. In addition, the Xietongmen No. I and No. II ore‐bearing porphyries have: (a) high whole‐rock V/Sc ratios (10−15 and 8−16, respectively); (b) high zircon Ce4+/Ce3+ and EuN/EuN* ratios, falling into the field of the ore‐bearing intrusions in northern Chile; (c) low apatite CeN/CeN* ratios (0.97−1.02 and 0.99−1.02, respectively), which suggest that their magmas are oxidized. The estimated magmatic S contents, based on the magmatic apatite SO3 contents, yield 0.0113−0.0507 wt% and 0.0012−0.0118 wt%, respectively for the Xietongmen No. I and No. II ore‐bearing porphyries, which suggest their magma are sulfur‐rich. Hydrous, oxidized and sulfur‐rich magmas are the parent magma of oxidized porphyry Cu deposits, together with the oxidized minerals of hematite and magnetite, therefore, both Xietongmen No. I and No. II deposit are oxidized porphyry Cu deposits, rather than that the Xietongmen No. I deposit is a reduced porphyry Cu deposit. [ABSTRACT FROM AUTHOR]

The relationship between hydrothermal fluids and fault development is studied through a petrographic analysis of faulted veins in an open-pit copper mine, Radomiro Tomic (RT), northern Chile. Brittle deformation in RT was initiated with the formation of veins, disrupting low-strain crystal-plastic deformation. Following cooling, shear fractures propagated along these veins with associated fluid influx, leading to phyllic alteration. Hydrolysis converted all plagioclase to kaolinite within the damage zones and all orthoclase to illite within fault cores and the less deformed rocks surrounded by faults. In turn, precipitation of quartz and pyrite locally cemented gouge in the fault cores. Concentration of clays around faults and veins weakened the rocks and promoted shear strain along them. These processes are expected to occur in any porphyry-related hydrothermal system and can combine during the development of the system. Although cementation tends to strengthen fault cores, the overall strength of the faults is expected to be reduced by hydrolysis of plagioclase, reducing the friction of the faults and their capacity to accumulate stress. These findings may have strong implications for understanding the evolution of ore deposit, the rheology of the crust, earthquake generation, and natural hazard assessment. • Fluid-fault interaction is studied in faulted veins, in Radomiro Tomic deposit, Chile. • Brittle deformation initiated with hydro-fracturing and formation of veins. • Shear fractures propagated along veins enhancing fluid flow and phyllic alteration. • Damage zones are dominated by hydrolytic destruction of plagioclase (forming clay). • Fault cores are dominated by comminution and sealing by mineral precipitation. [ABSTRACT FROM AUTHOR]

Chalcophile element fertility, the chalcophile metal abundance in the source magma, is likely to be a critical factor for the formation of porphyry Cu ± Au deposits. In this study, we provide evidence to support this hypothesis by comparing the platinum group element (PGE) geochemistry of barren and ore-bearing Cu ± Au granitic suites. We report the PGE contents of three barren volcanic and subvolcanic suites from Argentina and Japan and two Cu ± Au bearing suites from Indonesia and Chile. These results are compared with those from previous studies of a porphyry Cu-only subvolcanic suite from Chile and three porphyry Cu-Au-bearing suites from Australia and the USA. The barren suites are depleted in PGE abundances by the time of fluid exsolution (< 0.1 ppb Pd and Pd/Pt < ~ 3), which is attributed to early sulfide saturation in a mid to lower crustal magma chamber. In contrast, the Cu ± Au ore-bearing suites contain at least an order of magnitude higher PGE contents than the barren ones at fluid saturation (up to ~ 10 ppb Pd and Pd/Pt of 0.1–40). They are characterized by late sulfide saturation, which allows both chalcophile elements and sulfur to concentrate by fractional crystallization before volatile saturation. We suggest that plots of Pd/MgO against Pd/Pt for igneous suites can be used to estimate chalcophile element fertility and distinguish between barren, porphyry Cu, and porphyry Cu-Au granitoid systems. The positive correlation of these chalcophile element fertility indicators and ore grades suggests that metal contents in magmas play an important role in controlling ore grade, particularly Au, in porphyry Cu ± Au deposits. [ABSTRACT FROM AUTHOR]

Porphyry Copper Deposits (PCDs) dominantly form in convergent plate margins genetically related to volcanic arcs. However, their temporal relationships with active volcanism are less clear. Active volcanic activity is often considered to impede the formation of PCDs and some workers have concluded that PCD formation must take place at times of volcanic quiescence or at a waning stage of volcanism. However, several studies have documented evidence suggesting syn-eruption PCD formation. A combination of radiometric dating, geochemistry studies and observations of rock cores from the Cerro Colorado PCD, a BHP Billiton property located in N. Chile, provides evidence compatible with syn-volcanic PCD formation within a volcanic edifice. Textural observations at both hand specimen and microscopic scales combined with new LA-ICPMS and SHRIMP dating of zircons suggest the intrusive history of the Cerro Colorado area can be separated into two main units: a porphyritic tonalite ca. 53.5 Ma and a porphyritic quartz monzonite ca. 50 Ma. There is also evidence for crystallisation at ca. 60 Ma, but a corresponding distinct lithology was not identified. A quartz porphyry unit previously identified as the youngest intrusive body is reinterpreted as an altered equivalent of the monzonite. The youngest intrusive units are represented by porphyritic granodiorite with an age 51–50 Ma. These units only show minor metal mineralisation suggesting that the main phase of mineralisation in the Cerro Colorado mine occurred over a few million years, bracketed by the two main pulses of igneous intrusion, which are consistent with documented Ar–Ar ages. Re–Os ages reported by previous studies suggest an earlier phase of mineralisation at ca. 56–54 Ma. U–Pb dating of zircon from a volcanic-related breccia that cuts the host rocks of the Cerro Colorado Mine reveals a phase of volcanic activity at ca. 57 Ma, after formation of the regional Cretaceous Cerro Empexa Formation. This age range overlaps the ages derived for the intrusive porphyry units suggesting a close temporal relationship between volcanism and the ore-forming subvolcanic intrusions in Cerro Colorado. A link between active volcanism and mineralisation is reinforced by the ultrafine-grained matrix and the presence of strong brecciation associated with the monzonite-quartz porphyry stock suggesting decompression and volatile release have taken place during emplacement. [ABSTRACT FROM AUTHOR]

The Don Manuel igneous complex and associated porphyry copper mineralization in the Andean Cordillera of central Chile demonstrate similarities between intrusive complexes associated with porphyry copper deposits and arc volcanoes that generate porphyritic volcanics. The Don Manuel igneous complex intrusions progressed from quartz monzonite through rhyolite and biotite tonalite to intermediate porphyritic and basaltic andesite dikes, which intrude the older units. Mineralization is associated with the biotite tonalite and intermediate porphyries, which also contain the greatest abundance of mafic enclaves. Zoning patterns within plagioclase phenocrysts suggest that the later intermediate porphyries comprise a hybridized suite formed by magma mixing. New zircon U-Pb ages and whole-rock Ar-Ar ages indicate that the Don Manuel igneous complex was emplaced between ca. 4 and 3.6 Ma. The time scale for the episodic intrusion of the Don Manuel igneous complex units is similar to observed episodicity of eruption and degassing events in active arc volcanoes. Observations from the Don Manuel igneous complex are consistent with the close spatial and temporal association of mineralization with episodic intrusion and interaction between silicic and mafic magmas during emplacement. The observations are also consistent with the hypothesis that mafic magma provides a source of sulfur for porphyry copper deposit formation. [ABSTRACT FROM AUTHOR]

Adakitic rocks and related Cu-Au mineralization are widespread along eastern Jiangnan Orogen in South China. Previous studies have mainly concentrated on those in the Dexing area in northeastern Jiangxi Province, but information is lacking on the genesis and setting of those in northwestern Zhejiang Province. The Jiande copper deposit is located in the suture zone between the Yangtze and Cathaysia blocks of South China. This paper presents systematic LA-ICP-MS zircon U-Pb dating and element and Sr-Nd-Hf isotopic data of the Jiande granodiorite porphyry. Zircon dating showed that the Jiande granodiorite porphyry was produced during the Middle Jurassic ( ca. 161 Ma). The Jiande granodiorite porphyry is characterized by adakitic geochemical affinities with high Sr/Y and LaN/YbN ratios but low Y and Yb contents. The absence of a negative Eu anomaly, extreme depletion in Y and Yb, relatively low MgO contents, and relatively high 207Pb/204Pb ratios, indicated that the Jiande granodiorite porphyry was likely derived from partial melting of the thickened lower continental crust. In addition, the Jiande granodiorite porphyry shows arc magma geochemical features (e.g., Nb, Ta and Ti depletion), with bulk Earth-like εNd (t) values (−2.89 to −1.92), εHf (t) values (−0.6 to +2.8), and initial 87Sr/86Sr (0.7078 to 0.7105). However, a non-arc setting in the Middle Jurassic is indicated by the absence of arc rocks and the presence of rifting-related igneous rock associations in the interior of South China. Combined with the regional Neoproterozoic Jiangnan Orogeny, it indicates that these arc magma geochemical features are possibly inherited from the Neoproterozoic juvenile continental crust formed by the ancient oceanic crust subduction along the Jiangnan Orogen. The geodynamic environment that is responsible for the development of the Middle Jurassic Jiande granodiorite porphyry is likely a localized intra-continental extensional environment along the NE-trending Jiangshan-Shaoxing Deep Fault as a tectonic response to far-field stress at the margins of the rigid South China Plate during the early stage of the paleo-Pacific plate subduction. In terms of Cu mineralization, we suggest that the metal Cu was released from the subducted oceanic slab and reserved in the juvenile crust during Neoproterozoic subduction along the eastern Jiangnan Orogen region. Partial melting of the Cu rich Neoproterozoic juvenile crust during the Middle Jurassic time in the Jiande area caused the formation of adakitic rocks and the Cu deposit. [ABSTRACT FROM AUTHOR]

The distribution of mineral zones, hydrothermal alteration assemblages and ore associations are important geological attributes of the Chuquicamata Porphyry Cu-Mo deposit. The development and morphology of these attributes were influenced by syn-mineralization structures with later modification of post-mineralization tectonism to yield the present day form. In this work we evaluate the elemental mass transfer index due to hydrothermalism by using Pearce Element Ratios, using a database of 1110 samples consisting of elemental concentrations by chemical analysis. We study the relationship of the aforementioned index with the geological attributes in the porphyry as well as evaluate the type of information that the index provides according to its distribution, magnitude and variability. The magnitudes of the alteration index are organized into deciles, each of which represents a source of information, or macrostate. Within these, the geological attribute, or microstate, has a probability to participate that depends on the possible categorization of a particular attributes. The amount of information or uncertainty that each geological attribute provides to each decile of the distribution of the alteration index is determined by measuring the value of entropy. The results indicate that this distribution is controlled by the geometric-kinematic properties of syn-mineralization structures that generate the primary pattern related to potassic alteration that develops in the hanging wall of the East Shear Zone. Subsequently the shape of the potassic alteration is modified by the kinematic effect imposed by the Americana and Estanques Blancos faults, and this broadly defines the geometry of the Qz-Mo event and the shape of the phyllic alteration. In addition, the sinistral-inverse shear of the West Fault Zone allows the construction of an enriched blanket and also generates the drag fold that is characteristically observed in the south wall of the mine. All of the above events combine to yield the final geometry of the geological attributes that controls the shape and magnitude of the alteration index in the porphyry. The magnitude of the alteration index is mainly driven by the potassic and phyllic alteration stages. Potassic alteration is dominant in the first four deciles, while phyllic alteration is dominant in the last six deciles. Supergene mineralization attributes contribute discreetly to the average magnitude of the alteration index for each decile. The distribution of the probability of participation of the geological attributes in the distribution of the alteration index suggests that in the potassic alteration, biotite and green-gray sericite facies contribute to strengthen the index in an associated manner, while in the phyllic alteration, the Qz-Ser textural type is the most significant. [ABSTRACT FROM AUTHOR]

The discovery of the Atlántida Cu-Au-Mo porphyry deposit, which is unconformably overlain by 25-80m of gravels, is a recent example of exploration success under cover in a traditional mining jurisdiction. Early acquisition of geophysics was a key tool in the discovery, and in later guiding further exploration drilling throughout the life of the project. Detailed review of the geophysical response of the deposit, with respect to the distribution of lithologies and alteration, coupled with their petrophysical properties has allowed full characterisation, despite no exposure at the surface of host rock nor porphyry-style mineralisation. Data acquired over the project include induced polarisation, magnetotellurics, ground and airborne magnetics, ground-based gravimetry, and petrophysical sampling. The distribution of the key geological features of the deposit has been inferred via acquisition of petrophysical properties and interpretation of surface geophysical datasets. Magnetic susceptibility is influenced strongly by both alteration and primary lithology, whilst density variations are dominated by primary lithological control. Several studies have shown that electrical properties may map the footprint of the hydrothermal system and associated mineralisation, via a combination of chargeability and resistivity. These properties are observed in geophysical datasets acquired at surface and allow further targeting and sterilisation at the deposit and project scale. By understanding these geophysical characteristics in a geological context, these data can be used to infer distribution of lithological units, depth to exploration targets and the potential for high grade mineralisation. Future exploration will likely be increasingly reliant on the understanding of the surface manifestations of buried deposits in remotely acquired data. This review summarises the application and results of these principles at the Atlántida project of northern Chile. Geophysical data can be used to improve the chances of discovery beneath post-mineral cover, and also improve drilling results throughout the advanced exploration of the program. The process of data review against geological control information is essential. [ABSTRACT FROM AUTHOR]

Geochronological and geochemical studies, including platinum group element (PGE) analyses, were undertaken on samples from the El Abra-Pajonal igneous complex, northern Chile, to investigate the magmatic evolution of the suite. Special attention was paid to identifying the onset of sulfide saturation and to documenting how it influenced the geochemistry of the chalcophile elements and the formation of the El Abra porphyry Cu deposit. The PGE have extreme sulfide melt-silicate melt partition coefficients, making them sensitive indicators of the timing of sulfide saturation in an evolving magmatic system. In arc-related intermediate to felsic magmatic systems, which have the potential to produce porphyry deposits, the timing and extent of sulfide saturation relative to orefluid saturation may control the capacity of these systems to produce economic mineralization and, if they do, whether the deposits are Cu-only or Cu-Au. This study incorporates the first comprehensive analysis of PGE in a felsic magmatic suite associated with an economic porphyry system. The suite comprises a series of quartz monzodiorite to granite intrusions with U-Pb zircon ages between 43 and 35 Ma. Their petrography and major element chemistry, including increasing Sr/Y ratios and rare earth element patterns, suggest that crystal fractionation and crustal assimilation were the key magmatic processes governing the evolution of the El Abra-Pajonal suite. Plagioclase fractionation dominated the oldest intrusions, and their associated granites and aplites. Following the injection of a more primitive, wetter, mafic magma at 41-40 Ma, plagioclase fractionation became suppressed and amphibole became the dominant fractionating phase, leading to the formation of the El Abra porphyry intrusion and Cu deposit. Abundances of Pt and Pd in felsic rocks from the El Abra-Pajonal intrusive complex drop rapidly in samples with MgO values below 25wt %, following sulfide saturation of the magmas, which occurred slightly before ore-fluid saturation and formation of the Cu deposit. Modeling suggests that the amount of sulfide formed was very small, enough to strip the PGE and Au from the magma but not Cu, because of the lower partition coefficient of Cu relative to the precious metals, which explains why the mineralization at El Abra is a Cu-only porphyry deposit, rather than a Cu-Au deposit. [ABSTRACT FROM AUTHOR]

The El Teniente Cu-Mo porphyry deposit, Chile, is one of the world's largest and most complex porphyry ore systems, containing an estimated premining resource of approximately 95 Mt Cu and 2.5 Mt Mo. Although Cu mineralization at the deposit is quite well studied, little work has focused specifically on the distribution and timing of Mo mineralization. Combined grade, vein, and breccia distribution analysis reveals that deposit-wide Mo grades of 0.01 to 0.06 wt % are strongly controlled by the abundance of main mineralization (type 6a) quartz ± molybdenite veins. These show a clear spatial relationship with several felsic-intermediate intrusions and appear to develop outward and upward into Cu-rich (type 6b-7b) quartz-chalcopyrite veins and (type 8) chalcopyrite-anhydrite ± bornite veins with sericitic alteration halos. High-precision Re-Os molybdenite dating reveals that these linked vein types did not develop in a single, deposit-wide evolution, but are diachronous, related to distinct episodes of hydrothermal activity associated with the emplacement of diorite finger porphyries and the composite Teniente Dacite Porphyry. These units acted as effective, short-lived (<100,000 years) conduits for pulses of Mo- and Cu-bearing hydrothermal fluids between 6.3 and 4.6 Ma. The rapid thermal contraction of each system during mineralization led to extensive overprinting of Mo-rich veins by their lower-temperature, Cu-rich equivalents. Separate pulses in magmatic-hydrothermal activity are separated by distinct gaps of up to 300,000 years, during which Mo-mineralizing activity appears to have gone into quiescence. Mo grades exceeding 0.06 wt % correspond to the presence of molybdenite-bearing, late mineralization-stage, tourmaline-cemented (type 9), and anhydrite-carbonate ± gypsum (type 10) veins and breccias. These are abundant at shallow mine levels and show a close spatial relationship with a series of concentric faults associated with the Braden Breccia Pipe. Mineralization in this paragenetic stage is relatively short-lived and occurs in all parts of the deposit between 4.80 and 4.58 Ma. The generally Cu poor nature of the late mineralization stage is attributed to the prior preferential extraction of Cu from the underlying magma chamber in earlier mineralizing events. This led to the late exsolution of oxidized, Mo-rich fluids that may have undergone further enrichment by remobilizing Mo from main mineralization-type veins associated with the Teniente Dacite Porphyry. The formation of the Braden Breccia Pipe is likely to have occurred in a single cataclysmic event at approximately 4.58 Ma, which cut the Mo-rich tourmaline breccias and created a distinct Mo-rich grade halo at shallow mine levels. With the exception of minor mineralization associated with small dacitic dikes at approximately 4.42 Ma, the Braden event marked the termination of Mo deposition. [ABSTRACT FROM AUTHOR]

The Caspiche porphyry gold-copper deposit, part of the Maricunga gold-silver-copper belt of northern Chile, was discovered in 2007 beneath postmineral cover by the third company to explore the property over a 21-year period. This company, Exeter Resource Corporation, has announced a proven and probable mineral reserve of 1,091 million tonnes (Mt) averaging 0.55 g/t Au, all but 124 Mt of which also contain 0.23% Cu, for a total of 19.3 Moz of contained gold and 2.1 Mt of copper. The deposit was formed in the latest Oligocene (-25 Ma) during the first of two volcanic and corresponding metallogenic epochs that define the Maricunga belt. The gold-copper mineralization is centered on a composite diorite to quartz diorite porphyry stock, within which five outward-younging phases are routinely distinguished. The centrally located, early diorite porphyry (phase 1) hosts the highest-grade ore, averaging ~1 g/t Au and 0.4% Cu. The subsequent porphyry phases are quartz dioritic in composition and characterized by progressively lower gold and copper tenors. Stock emplacement was both pre- and postdated by the generation of large-volume, andesite-dominated breccias, with tuffaceous matrices, which are believed to be shallow portions of diatremes. The deposit is characterized by a central gold-copper zone and partially overlapped but noneconomic molybdenum halo. The gold-copper mineralization in the lower half of the deposit accompanies quartz ± magnetite-veined, potassic-altered rocks, whereas the shallower mineralization occurs within quartz-kaolinite-dominated, advanced argillic alteration. Upper parts of the advanced argillic zone are characterized by siliceous ledges, some auriferous, composed of vuggy residual quartz and/or silicified rock. The chalcopyrite-pyrite mineralization in the potassic zone was partially transformed to high sulfidation-state sulfides and sulfosalts during the advanced argillic overprint, although the underlying chalcopyrite-bornite assemblage was mainly too deep to be affected. The deposit terminates downward in a sulfide-deficient, potassie-ealcic zone defined by K-feldspar, actinolite, and magnetite, which formed at the expense of biotite. A relatively minor, shallowly inclined zone of intermediate sulfidation epithermal gold-zinc mineralization, comprising narrow veinlets and disseminations, abuts the late-mineral diatreme contact. Supergene sulfide oxidation throughout the deposit is relatively shallow, and chalcocite enrichment extremely minor. The Caspiche deposit is thought to have been emplaced at relatively shallow paleodepths, within the southern, flat-bottomed part of the premineral diatreme vent. Earliest porphyry system development, probably north of the present deposit, appears to have been aborted by diatreme formation. Much of the gold and copper in the Caspiche deposit was introduced during the potassic alteration stage, with the highly telescoped, advanced argillic overprint being responsible for only minor redistribution of the two metals, and the addition of arsenic. The late-mineral diatreme was emplaced west of the Caspiche deposit, and caused destruction of only its uppermost peripheral parts. The late-mineral diatreme was both pre- and postdated by advanced argillic alteration. Finally, the intermediate sulfidation epithermal gold-zinc zone was localized by the enhanced permeability provided by intense fracturing along the underside of the upward-flared, late-mineral diatreme contact. [ABSTRACT FROM AUTHOR]

The recently discovered Toki cluster, which includes the Toki, Quetena, Genoveva, Miranda, and Opache porphyry Cu-Mo prospects, is located 15 km south-southwest of the Chuquicamata-Radomiro Tomic mines in northern Chile. These prospects occur in an area of 5 × 6 km and are completely covered with Neogene alluvial deposits. Inferred resources for the cluster are estimated at about 20 Mt of fine copper, with Toki and Quetena contributing ∼88 % of these resources. Mineralization in these deposits is associated with tonalite porphyries that intruded andesites and dacites of the Collahuasi Group and intrusions of the Fortuna-Los Picos Granodioritic Complex. Hypogene mineralization in the Toki cluster consists mainly of chalcopyrite-bornite with minor molybdenite with mineralization grading outward to a chalcopyrite-pyrite zone and ultimately to a pyrite halo. Alteration is dominantly of the potassic type with K-feldspar and hydrothermal biotite. Sericitic alteration is relatively restricted to late quartz-pyrite veins (D-type veins). Previous K-Ar geochronology for the cluster yielded ages within a range of 34 to 40 Ma. Four new Re-Os ages for Toki indicate that molybdenite mineralization occurred in a single pulse at ∼38 Ma. Re-Os ages for three different molybdenite samples from Quetena are within error of the Toki mineralization ages. These ages are concordant with a new zircon U-Pb age of 38.6 ± 0.7 Ma from the tonalite porphyry in Quetena. Two Re-Os ages for Genoveva (38.1 ± 0.2 and 38.0 ± 0.2 Ma) are also within error of the Toki and Quetena molybdenite ages. Four Re-Os molybdenite ages for Opache range between 36.4 and 37.6 Ma. The Miranda prospect is the youngest with an age of ∼36 Ma. Four new Re-Os ages for the Chuquicamata deposit range between 33 and 32 Ma, whereas nine new Ar/Ar ages of biotite, muscovite, and K-feldspar range between 32 and 31 Ma. Analyzed molybdenites have Re and Os concentrations that vary between 21-3,099 ppm and 8-1,231 ppb, respectively. The highest Re and Os concentrations are found in the Toki prospect. Three new Ar/Ar ages for the Toki cluster are younger than the Re-Os mineralization ages. The age spectra for these three samples show evidence of excess argon and have similar inverse isochron ages of 35 Ma that probably reflect a late hydrothermal phyllic event. The new geochronological data presented here for the Toki cluster indicate that molybdenite mineralization occurred within a very short period, probably within 2 Ma, and synchronously (at ∼38 Ma) in three mineralization centers (Toki, Quetena, and Genoveva). Furthermore, mineralization at the Toki cluster preceded the emplacement of the Chuquicamata deposit (35-31 Ma) and indicates that porphyry Cu-Mo mineralization occurred episodically over a period of several million years in the Chuquicamata district. [ABSTRACT FROM AUTHOR]

In northern Chile, geochemical anomalies are present above porphyry copper deposits buried beneath thick piedmont gravels. Reactivation of basement faults that originally focussed deposits created fracture zones in the indurated gravels, providing a channel along which earthquakes forced groundwater to the surface. The composition of the groundwater had been modified by interaction with the deposits, generating anomalies. Northern Chile lies directly above a subduction zone, can similar anomalies be formed in regions of lesser seismicity? Seismologists have described the phenomenon of “triggered earthquakes”, where passage of dynamic surface waves from major earthquakes can cause other earthquakes hundreds or thousands of kilometres distant. These waves remove blockages along faults, releasing high pressure water to rise and replace lower pressure water. The resulting increase in pore pressure lowers the effective normal stress, allowing the fault to slip: upsurge of water and fault reactivation are linked. In Nevada, part of the Great Basin, there is inherent seismicity from crustal extension and triggered earthquakes from major earthquakes as far distant as Alaska. Case studies show distinct geochemical anomalies above deeply buried deposits, attributed to groundwater with elements derived from the deposits reaching the surface. The Proterozoic Athabasca Basin contains the largest concentration of high grade uranium deposits in the world; it is within a cratonic setting. Deposits lie near the unconformity between Athabasca sandstones and a metamorphic basement along faults that were repeatedly reactivated by remote earthquakes. Reactivation of faults, coupled with fluid movement, continued long after the formation of the deposits. This allowed the upward movement of material from the deposits during the Proterozoic, including radiogenic lead, which had accumulated in the uranium ore. In places, this material reached several hundred metres to the upper levels of the sandstones, from which elements were transferred to tills, soils and vegetation during Quaternary time. [ABSTRACT FROM AUTHOR]

Representing one of the largest known (estimated >5 Gt at 1 % Cu and 0.02 % Mo) porphyry system, the Río Blanco-Los Bronces deposit incorporates at least five hypabyssal intrusive and hydrothermal centres, extending for about 5 km from the Río Blanco and Los Bronces mines in the north, through the Don Luis mine, to the Sur Sur mine, La Americana and Los Sulfatos in the south. The new geochronology data, which now include data on different molybdenite vein types, confirm the U-Pb ages of the pre-mineralisation intrusions but slightly increase their age range from 8.8 to 8.2 Ma. The distinct magmatic pulses of the mineralisation-associated porphyritic intrusives (Late Porphyries) indicate an age interval instead of the previously suggested individual ages: the quartz monzonite porphyry ranges from 7.7 to 6.1 Ma (Sur Sur 5.74 ± 0.13 Ma), the feldspar porphyry shows an interval from 5.8 to 5.2 Ma and the Don Luis porphyry from 5.2 to 5.0 Ma. The new Re-Os data on distinct molybdenite vein types confirm the protracted history of Cu(-Mo) mineralisation, inferred previously. The vein development occurred at least from 5.94 to 4.50 Ma, indicating a time-span of 1.5 Ma for the hydrothermal activity. Hydrothermal minerals dated by the Ar/Ar method are generally too young to record the age of early, high-temperature mineralisation. The majority of the Ar/Ar data in the Río Blanco porphyry cluster record reheating by either the youngest member of the Late Porphyry suite or the post-mineralisation dacite or rhyolite plug formations at around 4.9-4.7 Ma. [ABSTRACT FROM AUTHOR]

The Collahuasi district is a porphyry Cu-Mo subprovince located on the Andean high plateau of northern Chile. Late Eocene-early Oligocene mineralization in the district involved hydrothermal alteration of the upper Paleozoic to Lower Triassic Collahuasi Group granodiorites, diorites, andesites, dacites, and rhyolites, which had previously experienced regional lower greenschist facies metamorphism as well as uneconomic porphyry-style mineralization and associated sericitic alteration. Alteration mapping represents an effective technique for prospecting for hydrothermal deposits, but, in low-grade metamorphic terrains, recognition of propylitic assemblages that represent the boundary of the mineralizing system is hampered by their resemblance to metamorphic mineral assemblages. Our results from Collahuasi indicate that petrographic criteria can distinguish propylitic alteration from the effects of lower greenschist facies metamorphism, but distinctions vary with rock type. Mafic, intermediate, and felsic rocks with varying degrees of propylitic alteration can be distinguished from background metamorphism mainly by the presence of calcite. However, propylitic alteration can also be distinguished by high whole-rock δ13C values and carbon contents (-19.1 to -1.4‰ and up to 0.79 wt %, respectively) that result from the incorporation of magmatic CO2 during the mineralizing events. Lower δ13C values (-32.7 to -13.1‰) and carbon contents (<0.09 wt %) were recorded in other types of hydrothermal alteration (intermediate argillic, sericitic, and potassic alteration) and associated with background metamorphism. They reflect a dominantly 13C depleted carbon component that remains in the rocks after extensive CO2 degassing during emplacement, alteration by hydrothermal fluids, or contamination by organic carbon. Carbon and oxygen isotope compositions of calcite from propylitic alteration, which range from -12 to +0.3‰ and 6.6 to 25.9‰, respectively, are consistent with a predominantly magmatic origin for the carbonates, although subsequent resetting of the oxygen isotopes by local ground water and minor contributions of organic carbon are recorded in some samples. Spatial variation in δ13C values and carbon contents of outcrop samples from the Collahuasi district delineate zones of high magmatic CO2 component that correspond exclusively to propylitic alteration surrounding economic deposits. [ABSTRACT FROM AUTHOR]

El Teniente, the world’s largest Cu deposit, is hosted in Late Miocene and Pliocene plutons that intrude the older Teniente Volcanic Complex (or Farellones Fm; 14·2–6·5 Ma). The Late Miocene and Pliocene plutonic host rocks of the deposit include, sequentially, the relatively large (>50 km3) Teniente Mafic Complex laccolith (8·9 ± 1·4 Ma), the smaller (∼30 km3) Sewell equigranular tonalite complex (7·05 ± 0·14 Ma) and associated andesitic sills (8·2 ± 0·5 to 6·6 ± 0·4 Ma), small dacitic porphyry stocks (<1 km3; 6·09 ± 0·18 Ma), the unusual Cu- and S-rich ‘Porphyry A’ anhydrite-bearing granitoid stock (<1 km3; 5·67 ± 0·19 Ma), the Teniente Dacite Porphyry dike (<1 km3; 5·28 ± 0·10 Ma), minor latite dikes (4·82 ± 0·09 Ma), and finally a small dacite intrusion (4·58 ± 0·10 Ma). These plutonic rocks are all isotopically similar to each other (87Sr/86Sr = 0·7039–0·7042; ɛNd =+2·5 to +3·5) and also to the Teniente Volcanic Complex extrusive rocks, but distinct from both older Late Oligocene to Early Miocene volcanic rocks (87Sr/86Sr = 0·7033–0·7039; ɛNd = +3·8 to + 6·2) and younger Pliocene post-mineralization mafic dikes and lavas (87Sr/86Sr = 0·7041–0·7049; ɛNd = +1·1 to −1·1). Multiple Cu-mineralized magmatic–hydrothermal breccia pipes were emplaced into these plutonic rocks during the same time period as the felsic porphyry intrusions, between at least 6·31 ± 0·03 and 4·42 ± 0·02 Ma. These mineralized breccia pipes, which formed by exsolution of magmatic fluids from cooling plutons, have their roots below the deepest level of mining and exploration drilling and were derived from the same magma chamber as the felsic porphyries, >4 km below the paleosurface. To produce the ∼100 × 106 tonnes of Cu in the deposit requires a batholith-size (>600 km3) amount of magma with ∼100 ppm Cu. We suggest that both the mineralized magmatic–hydrothermal breccias and the progressively smaller volumes of more fractionated, but isotopically equivalent, Late Miocene and Pliocene felsic plutonic rocks that host the deposit were derived from the roof of a large, long-lived, thermally and chemically stratified, open-system magma chamber, or magmatic plumbing system, recharged from below by mantle-derived magmas. Only when this system fully solidified did post-mineralization mafic olivine-hornblende-lamprophyre dikes (3·85 ± 0·18 to 2·9 ± 0·6 Ma) pass through the system from the mantle to the surface. The significant progressive temporal isotopic evolution, to higher 87Sr/86Sr (from 0·7033 to 0·7049) and lower ɛNd (from +6·2 to −1·1), that occurred between the Late Oligocene and Pliocene in the vicinity of El Teniente for mafic mantle-derived magmas, and by implication their sub-arc mantle-source region, was due in part to increased mantle-source region contamination by subducted crust tectonically eroded off the continental margin. The post-mineralization olivine-hornblende-lamprophyres also imply extensive hydration of the mantle below this portion of the Andean arc by the Pliocene, which may have played a role in producing oxidized volatile-rich magmas and mineralization at El Teniente. [ABSTRACT FROM AUTHOR]

Abstract: A study of deep penetrating geochemistry—the surface exploration techniques for finding concealed mineral deposits—was carried out over the Spence porphyry copper deposit in Chile by Australian, Canadian and Chinese laboratories using selective weak leach methods and is briefly reviewed here. Erratic and weak copper anomalies were obtained above the Spence deposit, which is concealed below a thick piedmont gravel cover. In the literature, such patterns are common in most of the weak selective leach studies carried out over known buried deposits. During exploration in unknown area, such weak leach techniques lead to some success and many dry holes. In this paper, stronger selective leaching methods are developed for oxides and sulfides, which may be the stable end products transformed and accumulated by a mobile precursor phase carried upward through the overburden by gas or water through long spans of geological time. The data obtained by the methods developed in this paper produced more significant and consistent anomalous data than previous efforts. Consequently, the leach approaches described here may increase the effectiveness of deep penetrating geochemistry in exploration for concealed ore deposits. [Copyright &y& Elsevier]

Molybdenum is an economically important subproduct of North Chilean porphyry-type deposits, and thus spatial and temporal distribution of molybdenite as the primary Mo-bearing mineral in the Escondida and Escondida Norte deposits were characterized using several mineralogical and chemical techniques and the Re-Os dating method. Molybdenum is distributed extensively in the two deposits, and high molybdenum concentrations (>500 ppm) are recognized particularly in the chlorite-sericite transitional zone between the potassic and sericitic zones. Two modes of occurrence of molybdenite are observed in the Escondida deposit: aggregates with Cu-Fe-sulfide minerals in fine veinlets (sulfide-veinlet type), and monomineralic microveinlets associated with NE-trending faults. The former and the latter yielded ages of 36.1 ± 0.2 Ma and 35.2 ± 0.2 Ma, respectively. Re-Os dating of Escondida Norte molybdenites also show two distinct episodes, at 37.7 ± 0.3 Ma and a younger episode at 36.6 ± 0.2 Ma. These data indicate that the Escondida Norte is older than the main Escondida deposit. The Re-Os age data combined with those of the porphyry emplacement suggest that the molybdenite mineralization in the Escondida district occurred as several short episodic pulses during the late-magmatic to hydrothermal transition, and that the Cu-Mo deposits were formed in a variable overall period spanning 1 to 5 m.y. [ABSTRACT FROM AUTHOR]

Abstract: Supergene alunite group minerals from the Late Eocene El Salvador porphyry Cu district, the El Hueso epithermal gold deposit and the Coya porphyry Au prospect located in the Precordillera of Northern Chile (~26 to 26° 30´ Lat. S) have been dated by the 40Ar/39Ar method and analyzed for stable isotopes. These data support published geomorphologic and sedimentologic evidence suggesting that the Precordillera in the Southern Atacama Desert had been uplifted as early as the late Eocene and, thus, significantly prior to the Altiplano which attained its high elevation only in the late Miocene. The oldest supergene alunite from the Damiana exotic deposit at El Salvador was dated at 35.8±1Ma and yielded a δD (VSMOW) value of −74‰ which indicates elevations of the Precordillera near El Salvador of at least 3000m in the Late Eocene. In contrast, Miocene supergene alunite from El Salvador, El Hueso, and Coya have less negative δD signatures reaching values as high as −23 to −25‰ at El Hueso and El Salvador between about 8.2 and 14Ma. Late Miocene to Holocene supergene alunite, jarosite and natroalunite ages are restricted to El Hueso and Coya located near 4000m above sea level in the Precordillera, roughly 1000m higher than the present elevation of El Salvador. The δD values of samples younger than ~5Ma vary between −57 and −97‰. The complex evolution of the δD signatures suggests that meteoric waters recorded in supergene alunite group minerals were variably affected by evaporation and provides evidence for climate desiccation and onset of hyper arid conditions in the Central Depression of the southern Atacama Desert after 15Ma, which agrees well with published constraints from the Atacama Desert at 23–24° Lat. S. Our data also suggest that wetter climatic conditions than at present prevailed in the latest Miocene and early Pliocene in the Precordillera. The new and previously published age constraints for El Salvador indicate that supergene mineralization at the Damiana exotic Cu deposit occurred periodically over 23Ma in a locally exceptionally stable paleohydrologic and geomorphologic configuration. [Copyright &y& Elsevier]

Abstract: Thallium is a highly incompatible element and a large fraction of the bulk silicate Earth Tl budget is, therefore, expected to reside in the continental crust. Nonetheless, the Tl isotope systematics of continental rocks are essentially unexplored at present. Here, we present new Tl isotope composition and concentration data for a suite of 36 intrusive and extrusive igneous rocks from the vicinity of porphyry Cu deposits in the Collahuasi Formation of the Central Andes in northern Chile. The igneous lithologies of the rocks are variably affected by the hydrothermal alteration that accompanied the formation of the Cu deposits. The samples display Tl concentrations that vary by more than an order of magnitude, from 0.1 to 3.2μg/g, whilst ε205Tl ranges between −5.1 and +0.1 (ε205Tl is the deviation of the 205Tl/203Tl isotope ratio of a sample from a standard in parts per 104). These variations are primarily thought to be a consequence of hydrothermal alteration processes, including metasomatic transport of Tl, and formation/breakdown of Tl-bearing minerals, which are associated with small but significant Tl isotope effects. The Tl abundances show excellent correlations with both K and Rb concentrations but no co-variation with Cu. This demonstrates that Tl displays only limited chalcophile affinity in the continental crust of the Collahuasi Formation, but behaves as a lithophile element with a distribution that is primarily governed by partitioning of Tl+ into K+-bearing phases. Collahuasi samples with propylitic alteration features, which are derived from the marginal parts of the hydrothermal systems, have, on average, slightly lighter Tl isotope compositions than rocks from the more central sericitic and argillic alteration zones. This small but statistically significant difference most likely reflects preferential retention of isotopically heavy Tl in alteration phases, such as white micas and clays, which formed during sericitic and argillic alteration. [Copyright &y& Elsevier]

The article describes an intrusion-related copper-(molybdenum-gold) [Cu-(Mo-Au)]-rich iron oxide-copper-gold (IOCG) deposit located at the core of a hydrothermal system in Chile that includes both magnetite- and hematite-rich mineralization. It provides evidence that at least some IOCG systems are magmatic in origin and formed by broadly similar processes compared to porphyry Cu systems. The geology and geochemistry of the Tropezón deposit are described.

The El Teniente copper-molybdenum deposit is hosted by the late Miocene Teniente Mafic Complex, a largely subvolcanic package of primarily basaltic-andesite porphyry sills and stocks that were emplaced within the mid-late Miocene Farellones Formation. In the late Miocene-Pliocene, a series of intermediate felsic plutons were intruded into the Teniente Marie Complex. These are spatially associated with magmatic-hydrothermal breccias and multiple vein types that form individual mineralized complexes. Here we present detailed observations on mineralogy, textures, and intrusion, breccias, and vein crosscutting relationships to constrain the nature and relative timing of magmatic and hydrothermal events. Our revised classification defines 13 vein types, divided into three main stages: (1) premineralization biotite and/or K-feldspar ± quartz-anhydrite-albite-magnetite-actinolite-epidote veins that formed prior to emplacement of mineralized intrusions and breccias; (2) main mineralization stage veins that grade from gangue-dominated quartz-anhydrite veins ± potassic alteration halos into sulfide-dominated veins with phyllic alteration halos; and (3) late mineralization veins containing sulfosalts. Five breccia types have been observed in the deposit, typically forming individual, vertically zoned complexes, spatially associated with individual intrusions and overlapping in time: (1) igneous-cemented breccias, (2) K-feldspar-cemented breccias, (3) biotite-cemented breccias, (4) anhydrite-cemented breccias, and (5) tourmaline-cemented breccias. Breccia cements display a similar paragenetic evolution to main mineralization stage veins, indicating a close genetic link between them. A distinction can be made between early premineralization vein types (types 1-2) that represent veins formed, possibly deposit-wide, prior to emplacement of intrusion-breccia complexes, late pre- and main mineralization vein types (types 3-8), which are interpreted to reflect the repeated cycle of fluid release associated with each mineralized intrusive complex, and late mineralization vein types (types 9-10) that represent a single event linked to the emplacement of the Braden Breccia Pipe. The geologic evidence indicates a close spatial and temporal relationship between emplacement of shallow level, felsic-intermediate pipelike intrusions and the development of igneous and mineralized magmatic-hydrothermal breccias and vein halos. The magmatic-hydrothermal transitions observed in these complexes indicate that the deposit formed from a series of localized pulses of magmatic-hydrothermal activity which followed rather similar evolution paths. Single, deposit-wide models of fluid evolution and mineralization are therefore inappropriate. We conclude that El Teniente represents a nested but otherwise rather typical porphyry Cu-Mo system, unusual only in that the Teniente Marie Complex provided a particularly efficient physical trap in terms of pervasive fracturing during intrusion of magmatic-hydrothermal breccia complexes and as an effective chemical trap for deposition of sulfides. The overlapping of mineralized envelopes from successive fertile intrusions and the absence of barren, intermineral porphyries resulted in its unusual size and high hypogene grades. [ABSTRACT FROM AUTHOR]

In the area surrounding the El Teniente giant porphyry copper deposit, eight soil sites were sampled at three depth levels in the summer 2004. The sites were selected for their theoretical potential of being influenced by past SO2 emissions from the smelter and/or seepage from a now idle tailings impoundment. The soil mineralogy, grain size distribution, total organic matter contents, major element composition, cation exchange capacity, and Cu, Mo, Pb, Zn, As and SO4 2− concentrations were determined for all samples after nitric acid extraction and separate leaches by ammonium acetate (pH 7) and sodium acetate (pH 5). For water rinses, only Cu could be determined with the analytical set-up used. Cu and SO4 2− enrichment in topsoils was found at six sites either downwind from the smelter or within the combined influence of the smelter and the tailings impoundment. Both elements were released partially by ammonium and sodium acetate extractions. Due to the scarce background trace element concentrations of soil and rock outside the immediate mine area, assessment of trace element mobility for Mo, Zn, Pb and As was difficult. Arsenic was found to be concentrated in soil horizons with high smectite and/or organic matter contents. Mo appears to be linked to the presence of windblown tailings sediment in the soils. Mobilization of Mo, Zn, and As for the acetate extractions was minimal or below the detection limits for the AAS technique used. The presence of windblown tailings is considered to be an additional impact on the soils in the foothills of the El Teniente compound, together with the potential of acidity surges and Cu mobilization in topsoils after rainfalls. Two sites located at the western limit of the former SO2 saturated zone with strongly zeolitized soils and underlying rock did not show any Cu or SO4 2− enrichment in the topsoils, and remaining total trace element concentrations were below the known regional background levels. [ABSTRACT FROM AUTHOR]

Abstract: The Potrerillos porphyry is located in the Late Eocene–Oligocene metallogenic belt of porphyry copper deposits of Chile, Cordillera de Domeyko, central Andes. It is formed of a porphyritic biotite–hornblende monzonite that was emplaced at less than 1000m depth. The intrusion has a cylindrical vertical form at depth, whereas its uppermost section resembles a mushroom whose stem is inclined 20–25° to the northwest. The location of the Potrerillos porphyry coincides with a major facies and east–west thickness change in Jurassic to Late Cretaceous marine sedimentary and Tertiary volcanic host rocks. Structurally, the porphyry was emplaced at the intersection between the northeast-oriented Potrerillos fold-and-thrust belt, and a system of northwest-trending strike-slip faults of the Ciénaga fault system. Both structural elements are related to a sinistral regional transpression that was active during the Middle to Late Eocene. The north–northeast trending Sierra Castillo-Agua Amarga fault system was trench-linked with an oblique subduction along the Peru–Chile trench in the southwestern part of South America at this time. One of the main structural elements of the Potrerillos fold-and-thrust belt is the eastward verging Potrerillos Mine reverse fault. We demonstrate a syntectonic relationship between the Potrerillos porphyry emplacement and the onset of the Potrerillos Mine fault. The control at the shallowest levels of the Potrerillos porphyry emplacement was studied by means of a structural analysis of folds, cleavage and deformed oolites in the country rock. This analysis suggests that the porphyry occupied the core of an anticline. The form of this anticline was modeled as a fault-propagation-fold taking into account a 200m displacement along the basal fault and a local shortening of 45.5% occurred. This model is compatible with a limited displacement along the Potrerillos Mine fault under hot, ductile conditions. The remaining displacement of 1600m would have taken place afterwards under cold, brittle conditions. The Potrerillos porphyry was emplaced during a transpressional regime within the Potrerillos fold-and-thrust belt. The fact that the cleavage does not show a concentric distribution around the Potrerillos porphyry, discounts the possibility of a forced emplacement of the porphyry, pushing aside its wall rocks. This is reinforced by the deformed oolites which do not show a strain gradient from outside towards the edge of the body. The clockwise transection of the fold axes by the cleavage and the southeast direction of displacement along the Potrerillos Mine fault are interpreted as related to the regional sinistral transpression related to the Sierra Castillo Fault movement. [Copyright &y& Elsevier]

The Mantos Blancos copper deposit (500 Mt at 1.0% Cu) was affected by two superimposed hydrothermal events: (i) phyllic alteration related to a rhyolitic dome emplacement and brecciation at ca 155 Ma; and (ii) potassic, sodic and propylitic alteration at ca 142 Ma, coeval with stocks and sills emplacement of dioritic and granodioritic porphyries, that locally grade upwards into polymictic magmatic hydrothermal breccias. Major hypogene copper sulfide mineralization is related to the second event. A late-ore mafic dike swarm cross-cuts all rocks in the deposit. Two types of granodioritic porphyries can be distinguished from petrographic observations and geochemical data: granodiorite porphyry I (GP I) and granodiorite porphyry II (GP II), which resulted from two different trends of magmatic evolution. The concave shape of the rare earth element (REE) distribution pattern together with the weak or absence of negative Eu anomalies in mafic dikes, dioritic and GP I porphyries, suggest hornblende-dominated fractionation for this magmatic suite. In contrast, distinct negative Eu anomalies and the flat REE patterns suggest plagioclase-dominated fractionation, at low oxygen fugacity, for the GP II porphyry suite. But shallow mixing and mingling between silicic and dioritic melts are also likely for the formation of the GP II and polymictic breccias, respectively. Sr-Nd isotopic compositions suggest that the rhyolitic dome rocks were generated from a dominantly crustal source, while the GP I has mantle affinity. The composition of melt inclusions (MI) in quartz crystals from the rhyolitic dome is similar to the bulk composition of their host rock. The MI analyzed in quartz from GP II and in the polymictic magmatic hydrothermal breccia of the deposit are compositionally more evolved than their host rocks. Field, geochemical and petrographic data provided here point to dioritic and siliceous melt interaction as an inducing mechanism for the release of hydrothermal fluids to form the Cu mineralization. [ABSTRACT FROM AUTHOR]

The distribution and diversity of acidophilic bacteria of a tailings impoundment at the La Andina copper mine, Chile, was examined. The tailings have low sulfide (1.7% pyrite equivalent) and carbonate (1.4% calcite equivalent) contents and are stratified into three distinct zones: a surface (0-70-80 cm) ‘oxidation zone’ characterized by low-pH (2.5–4), a ‘neutralization zone’ (70–80 to 300–400 cm) and an unaltered ‘primary zone’ below 400 cm. A combined cultivation-dependent and biomolecular approach (terminal restriction enzyme fragment length polymorphism and 16S rRNA clone library analysis) was used to characterize the indigenous prokaryotic communities in the mine tailings. Total cell counts showed that the microbial biomass was greatest in the top 125 cm of the tailings. The largest numbers of bacteria (109 g−1 dry weight of tailings) were found at the oxidation front (the junction between the oxidation and neutralization zones), where sulfide minerals and oxygen were both present. The dominant iron-/sulfur-oxidizing bacteria identified at the oxidation front included bacteria of the genus Leptospirillum (detected by molecular methods), and Gram-positive iron-oxidizing acidophiles related to Sulfobacillus (identified both by molecular and cultivation methods). Acidithiobacillus ferrooxidans was also detected, albeit in relatively small numbers . Heterotrophic acidophiles related to Acidobacterium capsulatum were found by molecular methods, while another Acidobacterium-like bacterium and an Acidiphilium sp. were isolated from oxidation zone samples. A conceptual model was developed, based on microbiological and geochemical data derived from the tailings, to account for the biogeochemical evolution of the Piuquenes tailings impoundment. [ABSTRACT FROM AUTHOR]

The Pampa del Tamarugal ill northern Chile is a gravel plain, part of tile hyperarid Atacama desert. Soil geochemical surveys over the pampa using partial extraction analyses identified a large, >100-km² geochemical anomaly for Cu. Subsequent analyses showed unusually high concentrations of elements that are associated with copper porphyry deposits, Mo, As, and Se. Tile soils are saline (NaCl) and contain high concentrations of Br and I, which are abundant in ground waters from northern Chile. The anomaly lies west of one of the world's most important copper porphyry districts, which includes the Chuquicamata, Radomiro Tomic, and El Abra deposits. Follow-up drilling in tile center of the anomaly, through ∼300 m of piedmont gravel cover, revealed barren andesitic basement rocks. Tile soil anomalies were caused by ground water reaching the surface, then evaporating. Both ground waters and soils share a common element assemblage, with higher concentrations of Mo, As, and Se than Cu. The former dissolve as anions in ground water, which can travel far from their source, whereas dispersion of Cu2+ is restricted by adsorption. The soil anomaly is integrated to be a "fossil" anomaly, preserved by the hyperarid climate. To the south the anomaly is cut by an alluvial fan; another nearby fan has been dated at ∼7 Ma. To the west the anomaly was eroded in the flood plain of the Rio Loa. Prior to the present ∼50-m-deep incision of the Rio Loa, the water table in tile area of the anomaly may have been higher than the present depth of 33 to 55 m, facilitating capillary rise of water to the surface. We suggest that tile source for the anomaly was tile El Abra deposit, 75 km east and updrainage from the anomaly. [ABSTRACT FROM AUTHOR]

Abstract: Groundwaters were collected around the Spence porphyry copper deposit, Atacama Desert, northern Chile, to study water-porphyry copper ore bodies interaction and test hypotheses regarding transport of metals through thick overburden leading to the formation of soil geochemical anomalies. The deposit contains 400Mt of 1% Cu and is completely buried by piedmont gravels of Miocene age. Groundwaters were recovered from the eastern up hydraulic gradient (upflow) margin of the Spence deposit, from within the deposit, and for two kilometers down flow from the deposit. Water table depths decrease from 90m at the upflow margin to 30m 1.5km down flow. Groundwaters at the Spence deposit are compositionally variable with those upflow of the deposit characterized by relatively low salinities (900–7000mg/L) and Na+–SO4 2−-type compositions. These waters have compositions and stable isotope values similar to regional groundwaters recovered elsewhere in the Atacama Desert of Northern Chile. In contrast, groundwaters recovered within and down flow of the deposit range in salinity from 10,000 to 55,000mg/L (one groundwater at 145,000mg/L) and are dominantly Na+–Cl−-type waters. Dissolved sulfate values are, however, elevated compared to upflow waters, and δ34SCDT decreases into the deposit (from >4‰ to 2‰), consistent with increasing influence of sulfur derived from oxidation of sulfide minerals within the deposit. The increase in salinity and conservative tracers (Cl−, Br−, Li+, and Na+) and the relationship between oxygen and hydrogen isotopes suggests that in addition to water–rock reactions within the deposit, most of the compositional variation can be explained by groundwater mixing (with perhaps a minor role for evaporation). A groundwater-mixing scenario implies a deeper, more saline groundwater source mixing with the less saline regional groundwater-flow system. Flow of deeper, more saline groundwater along pre-existing structures has important implications for geochemical exploration and metal-transport models. [Copyright &y& Elsevier]

The middle Eocene (51.8 ± 0.6 Ma) hypogene protore underlying the supergene orebody of the Cerro Colorado porphyry, Cu (-Mo) deposit, I Región, Chile, exhibits features not commonly documented in such hydrothermal systems. Early-stage alteration of Upper Cretaceous plagioclase-phyric andesite generated a sub-horizontal blanket of pervasive, extremely fine grained but texture-preserving biotite (≤35 modal %)-albite (≤40%)-magnetite (≥3%) alteration, 8 km² in area but lacking sulfide minerals. At least seventy percent of the chalcopyrite > pyrite stoclavork mineralization was emplaced during the subsequent Main-stage alteration, which comprises, with decreasing depth, quartz-albite, sericite-chlorite-clay (smectite), quartz-sericite-clay, and andalusite-diaspore-pyrophyllite assemblages. The deposit is apparently unique among documented central Andean porphyry, systems in the a ssociation of the highest grade copper mineralization with intermediate argillic alteration. The subsequent Transitional-stage phyllic (i.e., quartz-sericite-pyrite ± tourmaline) alteration was associated with the emplacement of molybdenite-rich breccia bodies. The occurrence of undumortieritized tourmaline veinlets cutting andalusite-diaspore assemblages confirms that much of the advan ced argillic alteration took place during the Main stage. Early-stage alteration was the product of nonboiling, cool (trapping temperature ≤ca. 380°C), low-salinity (≤8 wt % NaCl equiv) fluids which added substantial K, Na, Mg, Fe2+, Cl, F, and water to the host andesite. The initial Main-stage fluids, boiling at a paleodepth of ca. 2.5 to 3.0 km, were up to 160°C hotter (≤544°C) and highly saline (≤52 wt % NaCl equiv). As these fluids rose, they cooled to ≤320°C, were diluted (to ≤37 wt % NaCl equiv), deposited sulfides, and leached K, Na, Ca, Mg, Fe2+ and Cl from the host rocks, yielding diverse, broadly contempora neous, intermediate and advanced argillic alteration facies. Pressure estimates require that low-density (≤0.3 g/cm³), and thus more acidic, fluids were primarily responsible for the formation of the quartz-sericite-clay and shallow advanced argi llic alteration. Subsequent phyllic alteration was similarly caused by boiling fluids which were hot (≤486°C) and saline (≤47 wt %) at depth but cooler (≤334°C), dilute (≤8 wt %), and vapor dominated at shallower levels. Even Terminal -stage pyrite veins formed at temperatures as high as 450°C, albeit from low-salinity (≤8 wt %) fluids. Following a major prograde thermal transition from the Early to the Main stage, each sulfide-depositing alteration episode at Cerro Colorado was generated by a pulse of high-temperature fluid which cooled and diluted as it rose. Such changes in fluid characteristics, temperature, and alteration relationships are well documented in numerous geothermal fields, where potassic alteration generally develops at ca. 270° to 350°C and are likely to occur at an early stage in any hydrothermal system in which magmatic fluids, exsolving at relatively high pressures, ascend into the near-surface environment. The initial alteration at Cerro Colorado plausibly developed under conditions sim ilar to those in nonexplosive geothermal systems, in which hydrothermal fluids cool and disperse laterally at shallow depths. Extensive zones of biotite-rich alteration, generally barren and with a hornfelsic appearance, occur in many andesite-hosted por phyry copper deposits, but few data are available elsewhere for mineralogical, pressure, temperature, or metasomatic exchange relationships in such systems. Cerro Colorado may, however, be representative of a subclass of porphyry copper deposits exhibiti ng unusually close analogies with geothermal systems. [ABSTRACT FROM AUTHOR]

The giant porphyry copper-molybdenum deposits of central Chile formed within a thick sequence of Cretaceous to Pliocene volcanic rocks. The predominantly calc-alkaline basaltic andesites of the Cretaceous Las Chilcas Formation are characterized by La/Smn ratios of 1.8 to 2.5 and Sm/Ybn of 1.8 to 2.8. The upper Oligocene to lower Miocene Abanico Formation (variously defined as the Los Pelambres, Abanico, or Coya Machalí Formations) ranges from basalt to rhyolite in composition and exhibits a broad southward transition from calc-alkaline to tholeiitic. All samples from this formation are characterized by LREE enrichment and moderately, or locally strongly, fractionated HREE (La/Smn = 1.3-4.1; Sm/Ybn = 1.5-5.8). The basaltic andesites and andesites of the middle Miocene Salamanca Formation have REE chemistry similar to that of the Upper Cretaceous strata (La/Smn = 1.5-2.7; Sm/Ybn = 1.6-2.7). The overlying middle Miocene Farellones Formation ranges from tholeiitic to calc-alkaline and from basalt to andesitic and has a similar LREE enrichment and fractionated HREE (La/Smn = 1.7-2.5; Sm/Ybn = 1.7-3.3). The Pliocene La Copa Rhyolite Complex, however, is strongly LREE enriched and HREE depleted (La/Smn = 3.8-3.9; Sm/Ybn = 4.2-4.7). Overall, the trace element geochemistry of the Cretaceous to middle Miocene volcanic rocks is characterized by enriched LREE and negative Nb anomalies, consistent with an arc setting, with only minor differences in the abundance of most elements. Crustal thickening during the Miocene in central Chile has been suggested to have been responsible for a transition from an amphibole- to garnet-dominated residual mineralogy resulting in the release of fluids that enabled the formation of giant copper porphyry deposits. However, the gradual increase in La/Yb through the early, middle, and late Miocene reported in earlier studies and interpreted to be a response to crustal thickening is not observed in the regional data. Instead, a rapid change in the geochemical signature between the end of the eruption of the Farellones Formation and the eruption of the high La/Yb La Copa Rhyolite Complex implies a more abrupt change in the tectonic environment. Isotopic data broadly support this, although lower εNd values in the Farellones Formation imply a greater role for crustal contamination in younger suites. In the absence of gradual crustal thickening, it is suggested that the subduction of the Juan Fernández Ridge may have been the key geodynamic process responsible for the genesis of the three middle Miocene to lower Pliocene giant porphyry copper deposits in central Chile, possibly by promoting crustal-scale faulting and even acting as a source of metals. [ABSTRACT FROM AUTHOR]

The Rosario Cu-Mo-Ag deposit is located in the Collahuasi district of northern Chile. It comprises high-grade Cu-Ag-(Au) epithermal veins, superimposed on the core of a porphyry Cu-Mo orebody. Rosario has mining reserves of 1,094 million metric tons (Mt) at 1.03 percent copper. An additional 1,022 Mt at 0.93 percent copper occurs in the district at the nearby Ujina and Quebrada Blanca porphyry deposits. The Rosario reserve contains over 95 percent hypogene ore, whereas supergene-sulfide ores dominate at Ujina and Quebrada Blanca. Mineralized veins are hosted within Lower Permian volcanic and sedimentary rocks, Lower Triassic granodiorite and late Eocene porphyritic quartz-monzonite. The Rosario fault system, a series of moderate southwest-dipping faults, has localized high-grade Cu-Ag-(Au) veins. At Cerro La Grande, similar high-grade Cu-Ag-(Au) veins are hosted in north-northeast-trending, sinistral wrench faults. Normal movement in the Rosario fault system is interpreted to have been synchronous with sinistral strike-slip deformation at La Grande. Hydrothermal alteration at Rosario is characterized by a K-feldspar core, focused in the Rosario Porphyry that grades out to a secondary biotite-albite-magnetite assemblage. Paragenetic relationships indicate that magnetite was the earliest formed alteration product but has been replaced by biotite-albite. Vein crosscutting relationships indicate that K-feldspar formed during and after biotite-albite alteration. Chalcopyrite and bornite were deposited in quartz veins associated with both K-feldspar and biotite-albite assemblages. The early hydrothermal fluid was a hypersaline brine (40-45 wt % NaCl) that coexisted with vapor between 400° and >600°C. Weakly mineralized illite-chlorite (intermediate argillic) alteration of the early K and Na silicate assemblages was caused by moderate temperature (250°-350°C), moderate-salinity brines (10-15 wt % NaCl). Molybdenite was precipitated in quartz veins that formed between the potassic and intermediate argillic alteration events. These fluids were 350° to 400°C with salinities between 10 and 15 wt percent NaCl. Porphyry-style ore and alteration minerals were overprinted by structurally controlled quartz-alunite-pyrite, pyrophyllite-dickite, and muscovite-quartz (phyllic) alteration assemblages. The quartz-alunite-pyrite alteration formed at 300° to 400°C from fluids with a salinity of 10 wt percent NaCl. The pyrophyllite-dickite assemblage formed between 250° and 320°C from dilute (5 wt % NaCl) fluids. An upward-flared zone of muscovite-quartz-pyrite altered rocks surrounds the fault-controlled domain of advanced argillic alteration. Thick veins (0.5-2 m wide) of fault-hosted massive pyrite, chalcopyrite, and bornite precipitated brines with a salinity of 30 wt percent NaCl at temperatures of 250° to 300°C. Pressure-depth estimates indicate that at least 1 km of rock was eroded at Rosario between formation of the K-Na silicate and advanced argillic assemblages. This erosion was rapid, occurring over a period of 1.8 m.y. The Rosario Porphyry intruded immediately after the Incaic tectonic phase, implying that it was emplaced as the Domeyko Cordillera underwent gravitational collapse, expressed as normal faults in the upper crust. Gravitational sliding potentially accelerated exhumation and helped to promote telescoping of the high-sulfidation environment onto the Rosario Porphyry. The hydrothermal system responsible for porphyry Cu mineralization at Rosario was partially exhumed prior to the formation of high-sulfidation ore and alteration assemblages. This implies that emplacement of a second blind intrusion occurred somewhere beneath the Rosario and Cerro La Grande high-sulfidation vein systems and is supported by the fault geometry and zoning of precious metals and sulfosalts at the district scale. [ABSTRACT FROM AUTHOR]

The Rosario and Ujina porphyry Cu-Mo deposits, together with porphyry copper mineralization at the nearby Quebrada Blanca deposit, constitute the third-largest concentration of copper mineralization associated with the Domeyko fault system in northern Chile. At Rosario, fault-hosted Cu-Ag-rich massive sulfide veins are associated with pyrophyllite-alunite-quartz altered rocks. The Rosario massive sulfide veins cut biotitealbite- magnetite, K-feldspar and illite-chlorite altered rocks associated with porphyry-style copper-molybdenum mineralization. Similar massive sulfide veins occur in the La Grande area, 1 to 2 km south of Rosario. Copper ore at Ujina is associated with a K-feldspar-biotite-altered quartz monzonite intrusion. The K-feldspar-biotite assemblage has been overprinted by white mica-chlorite alteration. New geochronologic data presented in this paper constrain the ages of hydrothermal activity in the Rosario and Ujina deposits. The 4Ar/39Ar biotite, illite, and alunite dates (at 2σ error), and a Re- Os molybdenite date (at 0.5% error) are reported for porphyritic rocks and hydrothermal alteration from the Rosario and Ujina mineralized centers. A weighted mean 40Ar/39Ar plateau age of 34.4 ± 0.3 Ma is obtained for igneous biotite in a monzonite porphyry that hosts copper mineralization at the Rosario deposit. Illite and hypogene alunite from separate overprinting alteration events yielded 40Ar/39Ar ages of 34.5 ± 0.5 Ma (plateau age) and 32.6 ± 0.3 Ma (plateau age), respectively. An Re-Os age of 33.3 ± 0.2 Ma for molybdenite at Rosario is slightly younger than the 40Ar/39Ar age of illite, but older than the alunite. A weighted mean plateau age of 32.7 ± 1.6 Ma for hypogene alunite from the La Grande Cu-Ag-(Au) vein south of Rosario is indistinguishable from the age of Rosario alunite. At Ujina, the weighted mean... [ABSTRACT FROM AUTHOR]

The Zaldı´var porphyry copper deposit, Northern Chile, consists of two major intrusions, the 290 Ma Zaldı´var, and the more recent Miocene (38.7 Ma) Llamo porphyry. Five types of inclusions have been identified in quartz phenocrysts from Llamo porphyry, including melt inclusions (M), and four types of fluid inclusions, called MS (multi solids), B (brines), G (vapor-rich) and W (aqueous), respectively.Melt remnants, well preserved as M-inclusions, homogenize around 1000 °C. They show a rhyolitic composition, comparable to the most evolved acidic rhyolitic end member found elsewhere in the regional magmatism and to worldwide volcanic rhyolitic glass. High silica content in some inclusions can, however, be due to partial remelting of the quartz host during the heating run. Copper content in the same inclusions ranges between 0.03 and 0.57 wt.%, with an average concentration of 0.10 wt.%, suggesting a major magmatic source for the copper (orthomagmatic model).MS inclusions, which contain a number of solids at room temperature, mostly H2O-bearing phases (system NaCl–KCl–((Fe, Mg, Cu)Cl)–H2O, average salinity 70 wt.% NaCl equiv.), homogenize at magmatic temperatures (around 1000 °C). They represent the first fluids to have exsolved from the magma at depth, at a pressure of about 2 kbar. Their high homogenization temperature, comparable to values measured for melt inclusions (1000 to 1050 °C), may indicate trapping of MS and M inclusions in host phenocrysts from an immiscible mixture of silicate melt and highly saline fluids expelled from the magma during the early stage of quartz crystallization.The data indicate a magmatic origin for copper, as well as extremely high melt temperatures. These features are interpreted by magmatic differentiation of mantle-derived primitive melts, corresponding to major changes in the tectonic regime of the Andean margin, which occurred in Miocene times. [Copyright &y& Elsevier]

Investigates the resolution of two ages of porphyry intrusion for Chuquicamata copper deposit in northern Chile. Identification of hydrothermal events; Superimposition of multiple magmatic-hydrothermal events at Chuquicamata.

Presents a brief geological perception of the porphyry and possible relationships between the copper and gold deposits in the Andacollo copper-gold mining district in La Serena, Chile. Geology of the district and the Andacollo porphyry; Vein- and manto-type gold epithermal deposits; Hydrothermal and structural evolution of the Andacollo district; Hidden intrusive model for the Andacollo gold mantos.

Studies the relationship between copper tonnage of Chilean base-metal porphyry deposits and Os isotope ratios. Use of Os isotope ratios to trace the relative importance of the crust and mantle as sources of Os in the porphyry copper deposits of Chile; Os concentrations for pyrite and chalcopyrite.

Porphyry Cu-Mo deposits are among the world's largest source of Cu, Mo, and Re, and are also an important source of other trace elements, such as Au and Ag. Despite the fact that chalcopyrite, bornite, and pyrite are the most common sulfides in this deposit type, their trace element content remains poorly constrained. In particular, little is known about minor and trace elements partitioning into Cu-(Fe) sulfides as a function of temperature and pH of the hydrothermal fluid. In this study, we report a comprehensive geochemical database of chalcopyrite, bornite, and pyrite in the super-giant Chuquicamata porphyry Cu-Mo deposit in northern Chile. The aim of our study, focused on the new Chuquicamata Underground mine, was to evaluate the trace element composition of each sulfide from the different hydrothermal alteration assemblages in the deposit. Our approach combines the electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of sulfide minerals obtained from six representative drill cores that crosscut the chloritic (propylitic), background potassic, intense potassic, and quartz-sericite (phyllic) alteration zones. Microanalytical results show that chalcopyrite, bornite, and pyrite contain several trace elements, and the concentration varies significantly between hydrothermal alteration assemblages. Chalcopyrite, for example, is a host of Se (≤22,000 ppm), Pb (≤83.00 ppm), Sn (≤68.20 ppm), Ag (≤45.1 ppm), Bi (≤25.9 ppm), and In (≤22.8 ppm). Higher concentrations of Se, In, Pb, and Sn in chalcopyrite are related to the high temperature background potassic alteration, whereas lower concentrations of these elements are associated with the lower temperature alteration types: quartz-sericite and chloritic. Bornite, on the other hand, is only observed in the intense and background potassic alteration zones and is a significant host of Ag (≤752 ppm) and Bi (≤2960 ppm). Higher concentrations of Ag and Sn in bornite are associated with the intense potassic alteration, whereas lower concentrations of those two elements are observed in the background potassic alteration. Among all of the sulfide minerals analyzed, pyrite is the most significant host of trace elements, with significant concentrations of Co (≤1530 ppm), Ni (≤960 ppm), Cu (≤9700 ppm), and Ag (≤450 ppm). Co, Ni, Ag, and Cu concentration in pyrite vary with alteration: higher Ag and Cu concentrations are related to the high temperature background potassic alteration. The highest Co contents are associated with lower temperature alteration types (e.g., chloritic). These data indicate that the trace element concentration of chalcopyrite, bornite, and pyrite changed as a function of hydrothermal alteration is controlled by several factors, including temperature, pH, fO2, fS2, and the presence of co-crystallizing phases. Overall, our results provide new information on how trace element partitioning into sulfides relates to the main hydrothermal and mineralization events controlling the elemental budget at Chuquicamata. In particular, our data show that elemental ratios in chalcopyrite (e.g., Se/In) and, most importantly, pyrite (e.g., Ag/Co and Co/Cu) bear the potential for vectoring towards porphyry mineralization and higher Cu resources. [ABSTRACT FROM AUTHOR]

Groundwaters collected from the undisturbed Picaron Cu–Mo porphyry and exotic prospect (Atacama region, Chile) were analyzed for major and trace elements, along with isotopes of Cu, Mo, S, and 87Sr/86Sr. Sulfide minerals at Picaron have undergone oxidation by meteoric waters, resulting in the generation of acid and the lateral and downward migration of dissolved trace cations (Cd, Co, Cu, Ni, Pb, and Zn) and (oxy)anions (As, Mo, Sb, and Se). This paper presents a case study utilizing stable metal isotopes in hydrogeochemical mineral exploration to ascertain the possible hydrogeochemical footprints that can be detected around known mineralization. The groundwaters are weakly brackish (TDS < 3500 mg/L), reflected in relatively low trace metal concentrations. Despite this, the groundwaters reflect the dissolution of ore, as well as alteration and wall rock minerals proximal to porphyry mineralization with elevated dissolved concentrations of Ca, Mg, HCO 3 −, and SO 4 2−. δ65Cu values range from 0.41 to 2.80‰, with values above 1‰ proximal to porphyry mineralization reflecting Cu fractionation during sulfide oxidation and the highest values (up to 2.80‰) proximal to exotic mineralization likely from Cu desorption. Downgradient, δ65Cu values decrease; possibility resulting from adsorption of the heavier 65Cu onto reactive surfaces or mixing with other sources. δ98Mo values proximal to porphyry mineralization (2.41 and 2.42‰) are much higher than the regional average for molybdenite reported in previous studies (0.11‰), suggesting acid neutralization and isotopic fractionation related to adsorption may be occurring close to the source. Much lower δ98Mo values downgradient (<1.0‰) are likely indicative of Mo input from surrounding country rocks. The hydrogeochemical trends observed in the Picaron groundwaters highlight the chemical processes of mineral dissolution, precipitation, and recharge pathways and indicate that the leaching of the porphyry mineralization could be the source of exotic mineralization. Ultimately, the results indicate the need for a multiple isotope approach to hydrogeochemical mineral exploration. • There is dispersion of major and trace elements in the groundwaters from the porphyry and exotic sources. • δ34S fingerprints sulfur sources, with a magmatic sulfur signature measured in groundwaters up to 2.5 km from source. • δ65Cu values differentiate groundwaters proximal and distal to porphyry mineralization, as well secondary sources. • δ98Mo do not conform to the fractionation and dispersion patterns observed in other case studies, suggesting Mo mixing. [ABSTRACT FROM AUTHOR]

The Don Manuel porphyry copper system, located in the Miocene–Pliocene metallogenic belt of central Chile, contains spatially zoned alteration styles common to other porphyry copper deposits including extensive potassic alteration, propylitic alteration, localized sericite-chlorite alteration and argillic alteration but lacks pervasive hydrolytic alteration typical of some deposits. It is one of the youngest porphyry copper deposits in the Andes. Timing of mineralization and the hydrothermal system at Don Manuel are consistent with emplacement of the associated intrusions (ca. 4 and 3.6 Ma). Two molybdenite samples yielded consistent ages of 3.412 ± 0.037 and 3.425 ± 0.037 Ma. 40Ar/39Ar ages on hydrothermal biotites (3.57 ± 0.02, 3.51 ± 0.02, 3.41 ± 0.01, and 3.37 ± 0.01 Ma) are associated with potassic alteration. These ages are younger than the youngest intrusion by ~300 k.y. recording the cooling of the system below 350 °C. Such a time gap can be explained by fluxing of hot magmatic fluids from deeper magmatic sources. [ABSTRACT FROM AUTHOR]