El Indio Research Articles

Alunite and the role of magmatic fluids in the Tambo high-sulfidation deposit, El Indio–Pascua belt, Chile

The Tambo high-sulfidation deposit, located within the El Indio–Pascua belt in Chile, produced almost 25 t (0.8 M oz) of gold from altered Tertiary rhyodacitic volcanic rocks. Episodic magmatic-hydrothermal activity in the district occurred over at least 4 my and is characterized by several stages of acid-sulfate alteration, including magmatic-hydrothermal, magmatic steam, steam-heated, and apparent supergene assemblages. Two stages of Au±Ag mineralization are recognized and are hosted in barite and alunite within hydrothermal breccias and veins. Isotopic compositions of fluid in alunite show a dominant magmatic signature, with only a variable 18O-enriched meteoric water component throughout the entire hydrothermal process. Alunite 40Ar/ 39Ar ages constrain the timing of alteration and the duration of the hydrothermal system. Pre-ore alteration occurred at about 10 to 11 Ma and was contemporaneous with the volcanism of the Tambo Formation. Alunite from this stage of alteration occurs in the matrix of barren breccias and as fine intergrowths of alunite–quartz±clays that selectively replaced feldspars and pumice fragments. The textural relationships combined with stable-isotope systematics suggest a magmatic-hydrothermal origin for the alunite, with a local magmatic steam overprint. Early ore-stage alunite (8.7±0.2 Ma) occurs with barite and gold±walthierite within open spaces of the breccia matrix, and has δ 34S values (24–27‰) typical of magmatic-hydrothermal alunite, reflecting equilibrium between aqueous H 2S and SO 4 2−. Fluid-inclusion ratios of H 2S/SO 2 (approximately 6) are consistent with ratios determined from stable-isotope data, and indicate reduced fluid conditions during ore deposition. Vapor-phase transport of Au, and deposition from condensed magmatic vapor rising from the brittle–ductile transition is inferred. Late gold coprecipitated with a third stage of alunite (8.2±0.2 Ma) that is characterized by nearly uniform chemical compositions and δ 34S values (1‰) similar to those for associated enargite, reflecting disequilibrium between H 2S and SO 4 2−. This third-stage of alunite is isotopically and chemically similar to that of post-ore, coarse, banded alunite±hematite-quartz veins that crosscut the breccias in the Tambo area. Analyses of fluid-inclusion gas from alunite in these veins indicate high SO 2 and disequilibrium CO 2–CO–CH 4–H 2 species, consistent with a magmatic-steam origin. The gases are also depleted in He, and the late gold-bearing alunite was probably precipitated from rapidly ascending SO 2-rich vapors that were flashed from condensed magmatic fluids located below the brittle–ductile transition. Alunite–huangite veins (8.6±0.4 Ma) were formed intermediate to the two mineralizing events. Steam-heated alunite (8.9±0.4 Ma), derived from the oxidation of H 2S within the vadose zone, is preserved at the highest elevations and overlaps with Stage 2 ore deposition. Post-mineral alunite±jarosite (7.3±0.1 Ma) occurs as apparent supergene crosscutting veins and overgrowths. Stable-isotope data, however, indicate a strong magmatic component to both near-surface and late-stage alunite-forming fluids. This dominant magmatic fluid signature at high levels during the waning stages of hydrothermal activity is attributed to climatic and physiographic conditions specific to the El Indio–Pascua belt during the Middle to Late Miocene.

Equilibrium mineral–fluid calculations and their application to the solid solution between alunite and natroalunite in the El Indio–Pascua belt of Chile and Argentina

The effects of temperature and bulk-fluid K/Na on the alunite−natroalunite solid solution have been determined at 200 bar and 100–300 °C. These are conditions typical of magmatic-hydrothermal alteration and precious-metal mineral deposition in high-sulfidation environments, which are characterized by the presence of alunite-bearing alteration zones. Our model is based on published experimental parameters, which indicate that both alunite and natroalunite are stable over this temperature range, and that increased Na substitution is favored at higher temperature. Our calculation results predict that, at all temperatures, fluids in equilibrium with natroalunite are significantly more Na-rich than those typical of modern volcanic-hydrothermal environments. The results also indicate that a large variation in fluid K/Na is required to precipitate both K- and Na-rich alunite at high temperature (assuming near-equilibrium conditions). At lower temperature, much less variation in fluid composition can yield compositions near those of the end-members. Controls on alunite–natroalunite solid solution in natural systems were evaluated using electron-microprobe data from high-sulfidation gold deposits of the El Indio–Pascua belt. The results indicate that higher temperatures of deposition correlate with a greater range of Na substitution in alunite. Elevated Na content in alunite is also locally attributed to higher concentrations of Na in the source fluid as a result of the leaching and alteration of andesitic host rocks. Dacitic to granitic host lithologies in other regions of the belt are correlative with more K-rich alunite compositions. The results from this study suggest that the use of alunite K–Na composition as a guide to mineral exploration should be treated with caution.

Strong arsenic enrichment in sediments from the Elqui watershed, Northern Chile: industrial (gold mining at El Indio–Tambo district) vs. geologic processes

Sediments from the Elqui watershed river system (9800 km 2; northern Chile) are highly enriched in arsenic. The river system initiates in the high altitude domain of the Andes (3500–4000 m) and drains important hydrothermal alteration zones and epithermal deposits, including the copper-, and arsenic-rich gold veins of the well-known El Indio–Tambo district. In order to study the extent, source, and age of contamination, we carried out a geochemical survey of stream sediments and older, Early Holocene lacustrine sediments. The results reveal very high contents of arsenic in both, the stream (55–485 ppm As) and lacustrine sediments (119–2344 ppm As). As shown by the ESEM-EDX study, arsenic is associated with the iron oxide mineral phase. Given that arsenic is enriched in both the stream sediments and the Early Holocene sequence, we suggest that contamination is not only industrial derived (mining operations), but also a major geological process, related to long-lived erosion of the As-rich epithermal ores and alteration zones. Erosion in this realm is particularly important during El Niño years, when increased activity of the Westerly winds brings intense rains, with catastrophic consequences in the Elqui watershed, including the massive removal of sediments in the form of large debris flows.

Confluences: Bunuel's Cinematic Narrative and the Latin American New Novel

Confluences:Buñuel's Cinematic Narrative and the Latin American New Novel Víctor Fuentes (bio) In Mexico, while working in conditions bordering on anonymity, Buñuel saw himself creating films within the institutional limitations of the country's film industry, his hands tied by the latter's commercial and ideological demands. However, immediately after Los olvidados (1950) Buñuel became a prominent figure in Mexican cinema as well as in the artistic and literary worlds of Mexico and the rest of Latin America. Before his arrival in Mexico in 1946 as an exile and émigré, the presence of his work was ephemeral but stunning. During his visit to Mexico City in 1938, a showing of Un chien andalu preceded André Breton's lecture in El Palacio de Bellas Artes. The film made a vivid impression on prominent figures of the Mexican artistic and literary world, including the poet and film critic, Xavier Villaurrutia, as well as cinematographer, Manuel Álvarez Bravo, who was Sergei Eisenstein's assistant during the Soviet director's failed Mexican film project. Contrary to those who predicted Buñuel's failure in Mexico,1 Álvarez Bravo envisioned Buñuel performing an essential function, "[t]he task of shaking up our values, stimulating our consciences and of delivering his vision of the Americas and of Mexico." UNESCO's recent addition of Los olvidados to its World [End Page 91] Heritage List endorses the words of the Álvarez Bravo, whose creative conception of Mexico is closer to the "down to earth images" of Buñuel's films Los olvidados, Subida al cielo, El río y la muerte and, especially Nazarín, than to the aestheticizing vision of his former collaborator, Eisenstein. This creative conception also closely resembles the films of the "classic" Mexican cinema of El Indio Fernández-Figueroa. The latter, working with Buñuel, has left us a cinematographic portrayal of Mexico allied with the vision of Álvarez Bravo and Juan Rulfo. The Buñuel who returned to the cinema with his first two "assigned" Mexican films was well below the level of the director who had previously raised himself to the first rank of world directors. One need only compare Buñuel's Gran Casino (1947) and El gran calavera (1949) with other films that premiered around the same time: Eisenstein's Ivan the Terrible (1946), Vittorio de Sica's The Shoeshine (1946) and The Bicycle Thief (1948), Chaplin's Monsieur Verdoux (1947), John Huston's The Treasure of Sierra Madre (1947), or, returning to Mexico, El Indio Fernández's Río escondido (1948). Nevertheless, once Buñuel had the opportunity to make Los olvidados (released exactly a year after El gran calavera) he rejoined the group of first rate cinematic creators. In the process he became a forerunner of both "auteur cinema" (proclaimed, a few years later, by French directors and theoreticians of the "Nouvelle Vague") and of "Third Cinema" that emerged in Latin America and the so-called Third World countries in the late 1950s and early 1960s. While El gran calavera enjoyed commercial success, making it possible for producer scar Danciger to allow Buñuel to make Los olvidados with almost total artistic freedom, the latter film played in theaters for only six days after its première.2 This is an indication of the challenges Buñuel faced while working within the limitations of Mexico's film industry. Limitations which, thanks to his creative genius and transgressive disposition, Buñuel, managed to evade to a large degree. In the process, the Spanish director gave Mexican cinema some of its greatest masterpieces while leaving in all of his assigned films (or, as he liked to say, his "bread and butter pictures") the mark of his creative vision. Critics have not sufficiently emphasized (nor have I in my previous books) that, from his first Mexican film in 1946, Buñuel collaborated with a group of first rate Mexican writers: Mauricio Magdaleno, Juan de la Cabada, José Revueltas, and Emilio Carballido.3 These four were socio-politically "committed" artists. Juan de la Cabada and José Revueltas were closely but problematically (given their opposition to Socialist Realist dogma) identified with the Mexican Communist...

Mercury contamination in chile: a chronicle of a problem foretold.

This review analyzes the effects of environmental mercury contamination in Chile. This contamination generates one of the most important environmental conflicts in the country in that it affects air, ground, and water (rivers and oceans), which are fundamental in maintaining natural biotic equilibrium and at the same time important for the nation's economy. Chile possesses extraordinarily wealthy mining resources between Regions I and IV that have developed into an extraction industry essential for the economy of the country. However, waste discharges from this production have created an environmental problem in that the majority of the mines are located in the Andes mountain range, or areas close by, and the water used in the extraction process is deposited into the rivers, significantly increasing the amount of chemical contamination. Therefore, the cities and downstream waters used in agriculture suffer the negative consequences of a natural resource that is becoming more and more scarce. In addition, minerals released from mills into the atmosphere are deposited onto the soil, drastically affecting the biological resources of these areas. One of these affected areas is the Metropolitan region, where one of the highest contamination levels of mercury in the country was found in one of its affluents due to industrial and domestic waste discharge. In a country that is only 200 km in width, the gathering of all these contaminants in the rivers results in a rapid flow to the ocean, thereby contaminating coastal waters and the biota. In general, this contamination has been detected in semiclosed bodies of water (bays). Between Regions VII and IX, the principal sources of mercury contamination are related to cellulose industrial sites (Regions VII and VIII) and, until the 1980s, the bleach-soda industry. The most important industrial and fishing activity is also found in this area. In San Vicente Bay, waste discharges released into the ocean include sewage, industrial residues, residues from fishing and mining industries, hydrocarbons, petrochemical derivatives, oils, and detergents. This combination of chemical assault makes the San Vicente Bay the most contaminated in the country and the area where the majority of mercury contamination studies have been carried out. Between Regions X and XII, mercury contamination is reduced due to decreased release of domestic residues, especially batteries and sanitary waste. Beginning with the decade of the 1990s, Chile made a great effort to decrease contamination through governmental organizations (CONAMA, SERNAGEOMIN, DGA, ECOMIN, SONAMI), nongovernmental organizations (NGOs), universities, government mining industries (CODELCO, ENAMI), and private mining industries (El Indio, La Escondida, La Candelaria, Fachinal, etc). These reduction efforts within the last 10 years exceed $900 million, and in the private mining sector alone more than 1,100 monitoring stations have been installed and more than 100,000 environmental measurements have been carried out each year. Furthermore, an important educational program on the use of mercury has been implemented in the small mining area to decrease contamination to the air, water, and soil. However, the consequences of mercury accumulation are seen in their damaging effects to the rivers that deliver water to crops and cities, in the bays where food is extracted, and in the air of some cities where there exist mills that release chemical substances into the atmosphere.

Heavy Metals in Stream Sediments from the Coquimbo region (Chile): Effects of Sustained Mining and Natural Processes in a Semi-arid Andean Basin

Active sediments from the Elqui River in Chile were sampled 4 times at 10 sites during 2000. Concentrations of Ag, Ba, Cd, Co, Cr, Hg, Mn, Mo, Ni, Pb, Sr, Ti, V, Al, Ca, Fe, K, Mg, Na, P, and S were normal. Zinc levels were clearly high, and those of Cu (hundred to thousands ppm) and As (tens to hundreds ppm) were highly anomalous. Dissolved Cu (0.1-12.7 ppm) and Zn (0.2-2.2 ppm) levels were also very high. The anomalies of the upper tributaries are due to the El Indio–Tambo Au-Cu-As district and large hydrothermal alteration zones at altitudes between 3500–4500 m. Lower on the river, old and active tailing waste deposits and on-going mining operations in the Talcuna Cu (Pb) district are responsible. Partially eroded tailing deposits in the alluvial plain of the Elqui River and its tributaries, and especially in the El Indio-Tambo district, after mine closure in 2000, warrant special attention.

Miocene Landscape Evolution and Geomorphologic Controls on Epithermal Processes in the El Indio-Pascua Au-Ag-Cu Belt, Chile and Argentina

The world-class breccia-, stockwork-, and vein-hosted epithermal Au-Ag-Cu deposits of the central Andean El Indio-Pascua belt, latitudes 29°20' to 30°30' S, were emplaced between 6.2 and 9.4 Ma, immediately beneath the shoulders of an actively uplifting north-south tectonic block along the axis of the Cordillera Principal. The deposits of the Pascua-Lama, Veladero, El Indio, and Tambo districts are hosted by largely felsic volcanic units of late Oligocene-Early Miocene and Late Paleozoic age, but were associated with small hypabyssal bodies of dacitic porphyry, the Pascua Formation. There is no evidence that the epithermal centers were overlain by substantial volcanic edifices. The preglacial Miocene landscape, previously undocumented, incorporates three planar erosional landforms of regional extent: (1) the ca. 15- to 17-Ma Frontera-Deidad Surface; (2) the ca. 12.5- to 14-Ma Azufreras-Torta Surface; and (3) the ca. 6- to 10-Ma Los Rios Surface. These pediplains are each vertically separated by 200 to 400 m and, by analogy with the enormous erosion surfaces of the Atacama desert of northern Chile and southern Peru, are considered to have formed in a semi-arid climate in direct response to uplift events. Although the Azufreras-Torta Surface immediately overlies most of the major hydrothermal systems, and hence influenced their evolution, ore formation was contemporaneous with the development of the younger Los Rios pediplain, and was almost entirely focused around the upper extremities of stage III valley pediments. In contrast, precursor high-sulfidation hydrothermal alteration zones in the main mineralized districts, ranging in age from 10.0 to 13.6 Ma and associated with Vacas Heladas Formation dacitic magmatism, developed before incision of the Los Rios pediments and are barren. The areal and temporal relationships between economic epithermal centers and the heads of Upper Miocene valley pediments provide, at the least, clear, empirical exploration guidelines for the El Indio-Pascua belt. However, it is further proposed that changes in the surficial hydrodynamic environment, including the rapid lowering of the water table, increased lateral ground-water flow, and fluid boiling and mixing, all favoring ore deposition, may have been directly induced by pediment incision in this and other semi-arid cordilleran environments in which epithermal mineralization was not associated with significant volcanic edifices.

A preliminary water balance for the Colorado River delta, 1992–1998

Water balances for the Colorado River mainstem complex, the Cienega de Santa Clara, and El Indio wetlands were calculated for the Colorado River delta in Mexico for the period 1992–1998. Discharge for the mainstem complex was disaggregated into flood and non-flood years, reflecting the marked variability of mainstem discharge at the Southerly International Boundary (SIB) delimiting the United States and Mexico. In non-flood years, agricultural and municipal returns to the mainstem below SIB contributed 180% of mainstem discharge at SIB, but may not be sufficient to generate the floodstage discharge required by native riparian vegetation.

The Cenozoic History of Volcanism and Hydrothermal Alteration in the Central Andean Flat-Slab Region: New 40Ar-39Ar Constraints from the El Indio–Pascua Au (-Ag, Cu) Belt, 29°20′–30°30′ S

Ninety-three new 40Ar-39Ar laser step-heating plateau dates for igneous rocks and alteration minerals from the El Indio-Pascua Au-Ag belt permit significant refinement of the Tertiary volcanic stratigraphy and the definition of a succession of alteration events in this major mineralized district. Eight distinct Tertiary stratigraphic units are now recognized (two newly defined in this research): (1) the 30-36 Ma Bocatoma Unit, comprising dioritic and granodioritic shallow-level intrusions; (2) the voluminous 23-26 Ma Tilito Formation, consisting predominantly of dacitic tuffs; (3) the 17.5-21 Ma Escabroso Group made up of extensive successions of andesitic flows and coeval hypabyssal intrusions; (4) the 14-17 Ma, andesitic, Cerro de las Tórtolas Formation and its intrusive lithodeme, the Infiernillo Unit; (5) the dacitic, 11.0-12.7 Ma, Vacas Heladas Formation; (6) the rhyodacitic 7.5-8 Ma Pascua Formation, defined in this study; (7) the 5.5-6.2 Ma rhyolitic Vallecito Formation; and (8) the recently defined rhyolitic 2 Ma Cerro de Vidrio Formation. Magmatic activity decreased markedly following the eruption of the Cerro de las Tórtolas Formation. Hydrothermal activity occurred at least from the late Eocene to the Late Miocene, but economic Au-Ag-Cu mineralization was confined to the 6-9.5 Ma interval, the only observed contemporaneous igneous unit being the restricted Pascua Formation. Epithermal Au-Ag-Cu deposits and major prospects emplaced in this period include, from north to south, Pascua-Lama, Veladero, Sancarron, Rio del Medio, El Indio, Tambo, and Vacas Heladas. The widespread, albeit barren, alteration associated with the Bocatoma, Escabroso, Infiernillo, and Vacas Heladas magmatism indicates that the availability of hydrothermal fluid was not the controlling factor for ore formation, emphasising instead the role of the metal content of the magmas associated with epithermal mineralization, and/or the requirement for favorable physiographic conditions at the site of ore deposition.

Influence of mechanical activation on the alkaline leaching of enargite concentrate

In this study the alkaline leaching of enargite Cu 3AsS 4 (El Indio, Chile) in the medium of sodium sulphide was investigated. The rate of leaching can be influenced by the pre-treatment consisting of mechanical activation of the concentrate in a stirring ball mill (attritor). The XRD and XPS methods used to characterise the bulk and surface properties of the solid phase after leaching have shown that arsenic extraction from enargite leads to its transformation to covellite CuS. On the basis of the value of experimental activation energy (62 kJmol −1) we can conclude that a chemical reaction is the rate-determining process of the reaction between Cu 3AsS 4 and Na 2S. The applied procedure represents a potentionality of de-arsenification of sulphidic concentrates and preparation of a charge convenient for further processing, e.g., by pyro-treatment.

Late Paleozoic to Early Jurassic tectonic development of the high Andean Principal Cordillera, El Indio Region, Chile (29–30°S)

Regional mapping (1:50,000) and U-Pb and K-Ar geochronology in the El Indio region refines the knowledge of the distribution, lithostratigraphy, and age of the sedimentary, volcanic, and intrusive rocks that comprise the regionally extensive Pastos Blancos Group which is equivalent to the Choiyoi Group of the Argentine Frontal Cordillera. The Pastos Blancos Group (which we elevate to Group status herein) includes at least two diachronous volcanic–sedimentary sequences: an older felsic volcanic and volcaniclastic unit, the Guanaco Sonso sequence, that is Permian in age, and a younger bimodal volcanic and volcaniclastic unit, the Los Tilos sequence that is Middle Triassic to Early Jurassic. Sedimentary rocks of the Los Tilos sequence are transitional upward into the overlying Early to Middle Jurassic shallow marine limestones of the Lautaro Formation. Intrusions that make up the regionally extensive Permian to Early Jurassic plutons of the Chollay and Elqui-Limarı́ batholiths that were previously mapped as a single plutonic association, the Ingaguás Complex, include in the El Indio region at least three discrete intrusive units. These include: Early Permian (280–270 Ma) biotite granites, Early to Middle Triassic (242–238 Ma) silica-rich leucocratic granites and rhyolitic porphyries that made up the bulk of the Chollay Batholith, and a younger Late Triassic–Early Jurassic unit (221–200 Ma) of mainly intrusive rhyolitic porphyries, extrusive domes, and subordinate mafic intrusions and both felsic and mafic dikes, which are coeval with volcanic rocks of the Los Tilos sequence. Our data show that latest Paleozoic to Early Jurassic intrusive, volcanic, and sedimentary rocks in the El Indio region of the High Andes of Chile between 29–30°S likely formed during extension driven processes after the cessation of Carboniferous–Early Permian subduction along the western edge of Gondwana. These processes began by Late Permian time, but instead of recording a single and protracted magmatic event, as has been previously suggested, rocks that belong to the Pastos Blancos Group and the Ingaguás Intrusive Complex record at least three discrete periods of silicic to bimodal magmatism which occurred during the Middle Permian to Early Jurassic interval.

Repopulation and culture of the pacific calico scallops in Bahia Concepcion, Baja California Sur, Mexico

The Pacific Calico scallop, Argopecten ventricosus (Sowerby), is the most important commercial species in Bahia Concepcion. Catches declined steadily from 1991 to 1993 as a result of overexploitation, forcing the adoption of management measures. The Government has now established regulations for exploitation and scallop culture development. In 1994, Centro Interdisciplinario de Ciencias Marinas (CICIMAR) and the Fisheries Department of the Mexican Government initiated research on the feasibility of increasing scallop biomass through a reseeding programme to protect artisanal fishing. The use of onion bags to collect spat in Bahia Concepcion (1988-1994) demonstrated that settlement was cumulative from January to March. At Punta Coloradito, there were two peaks, in January and early March. Commercial collectors were deployed in February at El Indio and in April at El Remate. In June, about one million juvenile scallops were placed in an enclosure, held until mid-August and then released. Mass mortality (>80%) of the scallops was observed in September, increasing to 99% in October. The high water temperature and predators may be responsible for the mortalities.

Folklore and Culture on the Texas-Mexican Border

* Acknowledgments * Introduction * Part One. The Social Base and the Negotiation of Identity *1. The Folklore of Groups of Mexican Origin in the United States [1979] *2. The Problem of Identity in a Changing Culture: Popular Expressions of Culture Conflict along the Lower Rio Grande Border [1978] *3. Folk Medicine and the Intercultural jest [1968] *4. On Ethnographic Work among Minority Groups: A Folklorist's Perspective [1977] * Part Two. The Folklore Genres: History, Form, and Performance *5. Some Aspects of Folk Poetry [1964] *6. The Mexican Corrido: Its Rise and Fall [1958] *7. The Concept of the Emotional Core Applied to the Mexican Corrido Benjamin Argumedo [1972] *8. Jose Mosqueda and the Folklorization of Actual Events [1973] *9. The United States, Mexico, and Machismo [1971] *10. The Decima on the Texas-Mexican Border: Folksong as an Adjunct to Legend [1966] *11. The Undying Love of El Indio Cordova: Decimas and Oral History in a Border Family [1987] * Notes * Bibliography: The Scholarly Writings of Americo Paredes

The epithermal gold-silver deposit of Choquelimpie, northern Chile

The Choquelimpie gold-silver deposit is located at an elevation of 4,860 m above sea level, 115 km due east of Arica in northern Chile. A 5,500-metric-ton/day open-pit mine-heap leach operation was brought on stream in 1988 by the Sociedad Contratual Minera Vilacollo, whose shareholders are Shell Chile, Westfield Minera, and Citibank.The deposit is located near the Arica bend of the South American continent, where the earth's crust has a thickness of more than 70 km. It lies in a belt of calc-alkalic to alkali-calcic Miocene to Recent volcanism which runs parallel to the Pacific coast and contains a series of volcanic-hosted precious metal deposits. These have been worked in Peru, Chile, and Bolivia since the time of the Spanish conquest, usually for Ag but also with locally important Au values. These deposits are characterized by Ag-Au-Pb-Zn.The volcano hosting the mineralization is a subalkalic stratovolcano of late Miocene age; a late magmatic event has been dated at 6.60 + or - 0.20 Ma. The volcano is located at the intersection of a major N 60 degrees E-trending lineament with a N 35 degrees E fault, active from the late Miocene to the Recent. The volcano consists of dacitic and andesitic volcanic flows and breccias and is intruded by dacite domes. Erosion of its central part has exposed a zone of hydrothermal alteration and mineralization centered upon an andesitic feldspar porphyry, which is probably a dome.Most of the mineralization worked until recently came from narrow argentiferous veins, but the present operation is centered on a series of hydrothermal breccia bodies emplaced in a N 60 degrees E-trending zone 2 km long.Peripheral to the mineralized zone, propylitic alteration exists, grading inward to a sericite-pyrite assemblage. Toward the breccia bodies this assemblage is overprinted by silicification associated with base metals, pyrite, kaolinite, and barite. The breccias themselves and some adjacent lithologies contain precious metal species including native Au and Ag, electrum, and argentite. A final stage of mineralization is represented by base and precious metals in veins which cut the breccias. Oxidation has produced supergene enrichment of Ag, but probably not of Au.Limited fluid inclusion data from structures peripheral to the breccia bodies indicate homogenization temperatures ranging from 213 degrees to 305 degrees C. Salinities are below 5 wt percent NaCl equiv. Samples from one location show evidence of boiling at a minimum depth estimated as 300 m below the paleowater table.Based on the available evidence, Choquelimpie is interpreted as a volcanic-hosted, acid sulfate, epithermal deposit. It is associated with an Ag-Au-Pb-Zn metallogenic province which contrasts with the Maricunga and El Indio Au-Ag-Cu-As provinces of central Chile.

A history of gold in Chile

Although Chile is less famous as a gold producer than either Peru or Mexico, it was discovered in the quest for gold, its exploration was financed by gold, and in 1810, on the eve of its independence, it was producing as much gold as Peru and Mexico combined.The ancient Chilean cultures made little use of gold, but during their brief domination of parts of Chile the Incas developed a number of mainly placer mines.Since the Spanish conquest, there have been four cyclic periods of gold output in Chile. The first was from the founding of Santiago in 1541 to the end of the sixteenth century, when a series of rich alluvial deposits, mainly in the south of the country, produced an estimated 1 to 2 metric tons of gold annually. By the end of this period, the Spaniards had been evicted from their mines and towns by the Mapuche Indians and gold production declined markedly. At this time, the only consistently sizable producer was the Andacollo district.A second period of rising gold production developed from about 1740 onward, largely based on vein mines, from which ore was milled in trapiches and recovered using mercury. Gold mining was encouraged by the establishment of a royal mint at Santiago in 1749, and it peaked at more than 3 metric tons around 1810. Considerable production appears to have come from Andacollo, but it was at this time that the Copiapo area sprang to prominence and other important new districts were discovered at Petorca, Alhue, and El Chivato. A decline set in as a result of the disturbances of the wars of independence, achieved in 1823.The nineteenth century was a time of political turmoil during which silver (e.g., at Chanarcillo), copper, and nitrate mining, and the attractions of the Californian and Australian gold rushes, lured away many of Chile9s gold miners. The most important new gold mine was El Guanaco, which may have achieved an output of 2 metric tons per year. Copper and silver mines commonly generated appreciable by-product gold. In the far south of the country, gold was discovered in Tierra del Fuego and gave rise to a short-lived bucket-line dredging fleet.In 1933 the third cycle of gold production was triggered by the rise in the gold price. Few new districts of any significance were found, an exception being Sierra Overa, but many of the old hard-rock mines were reactivated using modern technology, including flotation and cyanidation. Production peaked at 11.5 metric tons in 1939 before declining to 2 metric tons in 1960. This level was maintained until the liberation of the gold price in 1971 ushered in the fourth, most spectacular, and ongoing surge in production.The El Bronce mine at Petorca was reactivated as a modern production unit, but the momentum of the present period comes from the discovery of a series of subvolcanic, mainly epithermal, gold deposits mostly located at elevations of more than 4,000 m in the Andean Cordillera. El Indio was the first discovery, achieving initial production in 1980. The application of low-cost open-pit mining and heap leaching technology stimulated development or redevelopment of Choquelimpie, El Hueso, Marte, La Coipa, El Tambo, Guanaco, and San Cristobal. Chile9s gold production attained 21.6 metric tons in 1989 and will increase further based on committed projects.

Reconnaissance K-Ar geochronology of the Maricunga gold-silver belt, northern Chile

The Maricunga metallogenic belt in northern Chile is defined by the presence of both intrusion-related porphyry-type and volcanic-hosted high sulfidation (acid sulfate) epithermal deposits containing gold and/or silver. The nine principal mineralized centers in the belt, including the La Coipa, La Pepa, Marte, and Lobo deposits, were dated radiometrically using the K-Ar method. Hypogene alunite from advanced argillic assemblages was dated from eight of the deposits.The 18 new radiometric ages permit subdivision of the Maricunga belt into two overlapping, longitudinal subbelts: the western of early Miocene (including latest Oligocene) age (25-20 Ma) and the eastern of middle Miocene age (14-12.5 Ma). This spatial and temporal subdivision of the alteration and mineralization accords well with that defined previously for volcanic rocks in the region and is substantiated further by regional structural and volcanic relationships. The migration of the magmatic and metallogenic activity from the western to the eastern subbelt may be attributed to shallowing of the subduction zone, which was initiated at 18 to 16 Ma.The western and eastern Maricunga subbelts are broadly correlative with two discrete volcanic alteration events in the El Indio precious metal belt 300 km to the south. The Maricunga subbelts are also integral parts of an eastward-younging sequence of gold-bearing magmatic metallogenic subbelts of Paleocene to late Miocene age, which span an east-west distance of 380 km in this transect (26 degrees -28 degrees S) of the central Andes.

Gold and copper mineralization at the El Indio deposit, Chile

A Middle Tertiary volcanic belt in the High Andes of north-central Chile hosts numerous precious- and base-metal epithermal deposits over its 150 km north-south trend. The El Indio district, believed to be associated with a hydrothermal system in the late stages of development of a volcanic caldera, consists of a series of separate vein systems located in an area of 30 km 2 which has undergone intense argillic-sericitic-solfataric alteration. The majority of the known gold-copper-silver mineralization occurs within a structural block only 150 by 500 m in surface area, with a recognized vertical extent exceeding 300 m. This block is bounded by two high-angle northeast-trending faults oriented subparallel to the mineralized veins. Hypogene mineralization at El Indio is grouped into two main ore-forming stages: Copper and Gold. The Copper stage is composed chiefly of enargite and pyrite forming massive veins up to 20 m wide, and is accompanied by alteration of the wall rocks to alunite, kaolinite, sericite, pyrite and quartz. The Gold stage consists of vein-filling quartz, pyrite, native gold, tennantite and subordinate amounts of a wide variety of telluride minerals. Associated with this stage is pervasive alteration of the wall rocks to sericite, kaolinite, quartz and minor pyrophyllite. The transition from copper to gold mineralization is marked by the alteration of enargite to tennantite and by minor deposition of sphalerite, galena, huebnerite, chalcopyrite and gold. Mineral stability relations indicate that there was a general decrease in the activity of S 2 accompanied by variations in the activity of Te 2 during the Gold stage. Fluid-inclusion data show homogenization temperatures ranging from about 220 to 280°C, with salinities on the order of 3–4 eq. wt. % NaCl for the Copper stage. The Gold-stage inclusions indicate a similar range in homogenization temperatures, but significantly lower salinities (0.1–1.4 eq. wt. % NaCl). Fluid inclusions of transition minerals show a weak inverse relationship between homogenization temperatures (190–250°C) and salinities (3.4–1.4 eq. wt. % NaCl), which may represent mixing of hotter Gold-stage fluids with cooler late-Copper-stage fluids. No evidence of boiling was found in fluid inclusions, but CO 2 vapor-rich inclusions were identified in wall-rock quartz phenocrysts which pre-date copper and gold mineralization. Mineral stability calculations indicate that given a fairly restricted range of solution compositions, the Copper-, Transition- and Gold-stage minerals at El Indio could have been deposited from a single solution, with constant total dissolved sulfur which underwent reduction through time. Limited sulfur-isotope data indicates that pyrite from the Copper stage was not in isotopic equilibrium with Copper-stage alunite or Transition-stage sphalerite. The sulfur-isotope and fluid-inclusion data indicate that two fluids with comparable temperatures but different compositions flowed through the El Indio system. The earlier fluid deposited copper attended by sericite-alunite-kaolinite alteration, and later epithermal fluids deposited gold with quartz-sericite-kaolinite-pyrite alteration.

Mineral Deposits Studies Group annual meeting, 1986

Report of a meeting of the Mineral Deposits Studies Group held on 15–16 December 1986, at the University of Southampton. The meeting included a thematic session on fluid inclusions which is reported elsewhere in this volume. Six presentations encompassed epithermal or volcanic-hosted precious-metal deposits and mineralized breccias. The Tambo deposit, near El Indio in Chile, was described by Bernard as a typical hot spring system with bedded sulphates, sulphate sinters, explosion breccias and native sulphur o sulphur overlying hydrothermal breccias and veins. Gold occurred as relatively coarse grains in the silicified wallrocks and also finely dispersed throughout barite. Mixing of ascending auriferous fluids with near-surface acid-sulphate waters was postulated to explain the mineral paragenesis. A similar mixing model was presented by Redwood to account for the disseminated and vein-type Au–Ag–Cu–Pb–Zn mineralization of the Altiplano in Bolivia. The mineralization was associated with silicic flow-dome complexes, and high-K calc-alkaline and shoshonitic magmatism; oxygen and hydrogen isotope data indicated an important magmatic input to the hydrothermal system. A silver-dominated vein-type mineralization comprising silver sulphosalts, native silver, argentite, minor sulphides, calcite, and baryte at Caracoles, northern Chile, was described by Fontbote, Schmidt & Flint, who emphasized the localization of high-grade ore at the intrusive contact between a porphyritic granite and bituminous siltstones and limestones. The role of decompressive shock and hydraulic decrepitation in the generation of hydrothermal breccias and lodes was reviewed by Halls & Allman-Ward. Mineralized hydromagmatic breccias of Hercynian age were identified as the source of the gold

Precious metals and cenozoic volcanism in the Chilean Andes

The Cenozoic evolution of the Andean Cordillera is dominated by subaerial volcanism of calc-alkalic composition generated in a magmatic are system related to subduction of the Pacific oceanic plate along the western border of the South American continent. Granodioritic intrusives and dacitic subvolcanic bodies are comagmatic with the extrusive suite. Several compressive and extensional stages related to the evolution of the arc are identified, as well as a general eastward migration of magmatism. Normal faulting, plus block tectonism gave rise to modern physiograpic features, younger volcanic cone alignments and control of geothermal fields. Four precious-metal occurrences — Choquelimpie (Ag-Au), La Coipa (Ag-Au), El Indio (Au-Ag-Cu), and Minas del Prado (Au-Ag) — hosted by volcanic rocks are considered as examples of the relationship between metallic mineralization and late Cenozoic volcanism. On the basis of geological environment, hydrothermal alteration patterns and oregangue mineral assemblages, these deposits may be classified as subvolcanic, high-level, epithermal type. In the areas of precious-metal mineralization, andesitic to dacitic extrusive rocks are widespread, as well as dacitic-dioritic and granodioritic intrusions. A relationship with structures, locally of possible caldera type, is evident. Hydrothermal alteration assemblages are: advanced argillic, quartz-sericite and silicification (including chalcedonic type), with external propylitic halos. The most common ore minerals are native gold and silver, base-metal sulfides, and sulfosalts, all in a quartz-pyrite gangue. Orebodies are described mainly as vein type, but disseminations and mineralized hydrothermal breccias are also known. The presence of these occurrences in north and central Chile, and the existence of similar mineralization in comparable geological environments in the Andes of Peru, Bolivia and Argentina, suggest the existence of a volcanogenic precious-metal province genetically related to plate convergence.

New Gold-Silver-Copper Orebody at El Indio, Chile: ABSTRACT

In June 1975, a St. Joe Minerals Corp. geologist, Dave Thomson, visited a remote prospect called El Indio, 300 mi (500 km) north of Santiago, close to the Argentine frontier at 13,000 ft (4,000 m) elevation. He recognized the great potential and St. Joe moved promptly. However, an agreement to purchase 80% of the property was not signed until June 30, 1976. Intensive exploration and development followed. It took another year to negotiate a foreign investment agreement with the government of Chile. On December 2, 1981, the El Indio mine-mill complex was dedicated. The total investment approximates U.S. $200 million and the facility will process 1,380 tons (1,250 MT) ore per day. Mill-feed ore reserves at the time of dedication were calculated at 3.1 million tons averaging 12 g gold, 144 g silver, and 3.5% copper. In addition, direct shipping high-grade ore reserves are estimated at 70,000 tons averaging 11 oz (345 g) of gold per ton. Similar material has been mined and shipped since 1979, containing in excess of 345,000 oz (10,750 kg) of gold. Our geological assessment is that continued exploration at El Indio will discover considerably more ore of both types. The ore is found in a complex vein system within volcanic andesites, dacites, and quartz dacites within, but near the border End_Page 986------------------------------ of, a caldera. The regional structural pattern is north-south faulting. Northeast cross faults forming sigmoid loops are very important ore controls. Massive enargite-pyrite veins are up to 33 ft (10 m) wide. Propylitic, argillic, quartz-sericite, and siliceous alteration are widespread, with precious metal content favoring the highly siliceous areas. Gold is rarely visible as average grain size of the native metal is 5 microns. End_of_Article - Last_Page 987------------