Lluis Fontboté

A mineralogical and geochemical study of element mobility in sulfide mine tailings of Fe oxide Cu–Au deposits from the Punta del Cobre belt, northern Chile

Two flotation tailings sites (Ojancos and P. Cerda) from the Fe oxide Cu–Au Punta del Cobre belt, south of Copiapo, Atacama desert, northern Chile, are geochemically (largely using sequential extractions) and mineralogically compared. Main ore minerals are pyrite, magnetite and/or hematite and chalcopyrite. Gangue is dominantly calcite with minor quartz. The host silicate assemblage is largely controlled by hydrothermal alteration and consists of variable amounts of the following minerals: K-feldspar±Ca-amphibole±biotite±sericite±chlorite±tourmaline±epidote±quartz. In this study, both the Ojancos and the P. Cerda tailings were deposited in valley dam impoundments and when they were filled, new tailings were deposited upstream. As a result, high quantities of seepage migrated downstream into the older tailings impoundment. At Ojancos, the recent upstream tailings have excess of acid potential (7.1 wt.% calcite and 3.5 wt.% pyrite), whereas the older downstream tailings are characterized by alternations of several meter-thick intervals with high neutralizing potential (about 40 wt.% calcite and 2 wt.% pyrite) and intervals with high acid potential (about 3 wt.% calcite and 4 wt.% pyrite). Acid mine drainage (AMD) with the precipitation of schwertmannite (pH 3.15) and chalcoalumite (pH 4.9) flows out at the interface between the uphill and downstream tailings. Strong downstream element transport is taking place and contributes to the formation of the cementation zone (mainly gypsum, ferrihydrite and goethite, and locally jarosite) in the older downstream impoundment. The cementation zone (pH=4) shows strong enrichment of heavy metals (e.g., up to 6800 ppm Cu, 680 ppm Zn, 1100 ppm As), mainly adsorbed and as secondary sulfides (e.g., covellite). In contrast, at the P. Cerda, tailings impoundment carbonates are homogeneously distributed and the overall neutralization potential exceeds the acid potential (average of about 10 wt.% calcite and up to 2.5 wt.% pyrite). The up to 5-m thick oxidation zones (paste pH=6.9–8.3) at P. Cerda are characterized by interlayering of coarser dark gray unoxidized layers with fine-grained, Fe(III) hydroxide-rich, ochre to red-brown colored horizons. The hyperarid climate dries out first the coarse, sulfide-rich horizons of the tailings and limits so the oxidation, which is restricted to the fine-grained, due to their higher moisture retention capacity. However, results indicate that during operation an important element transfer from the younger upstream tailings to the older downstream tailings impoundment took place, possibly by sorptive transport at ferric polymers or colloids in the form of neutral mine drainage (NMD). This would explain the metal enrichments in the cementation zone, which are mainly associated to the exchangeable fraction and not as secondary sulfides. This results, in both cases (in Ojancos mainly as AMD and in P. Cerda mainly as NMD), in Fe(III) input as ferric cation, as ferric polymer, or CO3 complexes to the downstream impoundment. This constitutes a very effective acid potential transfer to the older downstream material because oxidation via input of external Fe(III) produces 16 mol of protons per mol FeS2, i.e., eight times more than via oxidation with oxygen. In addition, the created acidity favors dissolution of the abundant Fe oxides magnetite and hematite of this ore deposit type providing so additional Fe(III) for sulfide oxidation.

Sediment-hosted Zn-Pb ores

and General Aspects.- Sediment-Hosted Zinc-Lead Ores - An Introduction.- Organic Contributions to Mississippi Valley-Type Lead-Zinc Genesis - A Critical Assessment.- Precipitation of Mississippi Valley-Type Ores: The Importance of Organic Matter and Thiosulphate.- Palaeomagnetic Methods for Dating the Genesis of Mississippi Valley-Type Lead-Zinc Deposits.- Galena and Sphalerite Associated with Coal Seams.- North American Deposits.- Dolostone-Hosted Sulfide Occurrences in Silurian Strata, Appalachian Basin of New York.- Mississippi Valley-Type Deposits in Continental Margin Basins: Lessons from the Appalachian-Caledonian Orogen.- Genesis of the Ozark Mississippi Valley-Type Metallogenic Province, Missouri, Arkansas, Kansas and Oklahoma, USA.- Relations Between Diapiric Salt Structures and Metal Concentrations, Gulf Coast Sedimentary Basins, Southern North America.- Sulfide Breccia in Fossil Mississippi Valley-Type Mud Volcano Mass at Decaturville, Missouri, USA.- European Deposits.- Trace Element Distribution of Middle-Upper Triassic Carbonate-Hosted Lead-Zinc Mineralizations: The Example of the Raibl Deposit (Eastern Alps, Italy).- The Genesis of the Pennine Mineralization of Northern England and Its Relationship to Mineralization in Central Ireland.- Genesis of Sphalerite Rhythmites from the Upper Silesian Zinc-Lead Deposits - A Discussion.- Geochemometrical Studies Applied to the Pb-Zn Deposit Bleiberg/Austria.- Mississippi Valley-Type, Sedex, and Iron Deposits in Lower Cretaceous Rocks of the Basque-Cantabrian Basin, Northern Spain.- Carbonate-Hosted Pb-Zn Mineralization at Bleiberg-Kreuth (Austria): Compilation of Data and New Aspects.- Australian, Chinese, and North African Deposits.- Australian Sediment-Hosted Zinc-Lead-Silver Deposits: Recent Developments and Ideas.- Sediment-Hosted Pb-Zn Deposits in China: Mineralogy, Geochemistry and Comparison with Some Similar Deposits in the World.- Peridiapiric Metal Concentration: Example of the Bou Grine Deposit (Tunisian Atlas).- Exploration and Economics.- Lithogeochemical Investigations Applied to Exploration for Sediment-Hosted Lead-Zinc Deposits.- The Economics of Sediment-Hosted Zinc-Lead Deposits.

Carbon and oxygen isotope study of hydrothermal carbonates in the zinc-lead deposits of the San Vicente district, central Peru: a quantitative modeling on mixing processes and CO2 degassing

Abstract Carbon and oxygen isotope studies of the host and gangue carbonates of Mississippi Valley-type zinc-lead deposits in the San Vicente District hosted in the Upper Triassic to Lower Jurassic dolostones of the Pucara basin (central Peru) were used to constrain maiels of the ore formation. A mixing model between an incoming hot saline slightly acidic radiogenic (Pb, Sr) fluid and the native formation water explains the overall isotopic variation (δ 13 C = −11.5 to + 2.5‰ relative to PDB and δ 18 O = + 18.0 to + 24.3‰ relative to SMOW) of the carbonate generations. The dolomites formed during the main ore stage show a narrower range (δ 13 C = −0.1 to + 1.7‰ and δ 18 O = + 18.7 to +23.4‰) which is explained by exchange between the mineralizing fluids and the host carbonates combined with changes in temperature and pressure. This model of fluid-rock interaction explains the pervasive alteration of the host dolomite I and precipitation of sphalerite I. The open-space filling hydrothermal white sparry dolomite and the coexisting sphalerite II formed by prolonged fluid-host dolomite interaction and limited CO 2 degassing. Late void-filling dolomite III (or calcite) and the associated sphalerite III formed as the consequence of CO 2 degassing and concomitant pH increase of a slightly acidic ore fluid. Widespread brecciation is associated to CO 2 outgassing. Consequently, pressure variability plays a major role in the ore precipitation during the late hydrothermal events in San Vicente. The presence of native sulfur associated with extremely carbon-light calcites replacing evaporitic sulfates (e.g., δ 13 C = −11.5‰), altered native organic matter and heavier hydrothermal bitumen (from −27.0 to −23.0‰ δ 13 C) points to thermochemical reduction of sulfate and/or thiosulfate. The δ 13 C- and ° 18 O-values of the altered host dolostone and hydrothermal carbonates, and the carbon isotope composition of the associated organic matter show a strong regional homogeneity. These results coupled with the strong mineralogical and petrographic similarities of the different MVT occurrences perhaps reflects the fact that the mineralizing processes were similar in the whole San Vicente belt, suggesting the existence of a common regional mineralizing hydrothermal system with interconnected plumbing.

Paleozoic orogenic gold deposits in the eastern Central Andes and its foreland, South America

Abstract In the eastern Central Andes and its foreland (6°–34°S), abundant quartz veins emplaced along brittle–ductile deformation zones in Ordovician to Carboniferous granites and gneisses and in saddle-reefs in lower Paleozoic turbidites represent a coherent group of middle to late Paleozoic structurally hosted gold deposits that are part of three major Au (±Sb±W) metallogenic belts. These belts, extending from northern Peru to central Argentina along the Eastern Andean Cordillera and further south in the Sierras Pampeanas, include historical districts and mines such as Pataz–Parcoy, Ananea, Santo Domingo, Yani–Aucapata, Amayapampa, Sierra de la Rinconada and Sierras de Cordoba. On the basis of the available isotopic ages, two broad mineralization epochs have been identified, with Devonian ages in the Sierras Pampeanas Au belt (26° to 33°30′S), and Carboniferous ages for the Pataz–Maranon Valley Au-belt in northern Peru (6°50′ to 8°50′S). The absolute timing of the southeastern Peruvian, Bolivian and northwestern Argentinian turbidite-hosted lodes, which form the Au–Sb belt of the southern Eastern Andean Cordillera (12° to 26°S), is poorly constrained. Field relationships suggest overlap of gold veining with Carboniferous deformation events. The northernmost belt, which includes the Pataz province, is over 160-km-long and consists of sulfide-rich quartz veins hosted by brittle–ductile shear zones that have affected Carboniferous granitic intrusions. Gold mineralization, at least in the Pataz province, occurred a few million years after the emplacement of the 329 Ma host pluton and an episode of molassic basin formation, during a period of rapid uplift of the host units. The two southern belts are associated with syn- to post-collisional settings, resulting from the accretion of terranes on the proto-Andean margin of South America. The Au–Sb belt of the southern Eastern Andean Cordillera presumably formed in the final stages of the collision of the Arequipa–Antofalla terrane and the Sierras Pampeanas Au belt is considered concurrent with the late transpressional tectonics associated with the accretion of the Chilenia terrane. The three Devono–Carboniferous Andean belts are the South American segments of the trans-global orogenic gold provinces that were formed from Late Ordovician to Middle Permian in accretionary or collisional belts that circumscribed the Gondwana craton and the paleo-Tethys continental masses. A paleogeographic map of the Gondwana supercontinent in its Middle Cambrian configuration appears as a powerful tool for predicting the location of the majority of the Paleozoic orogenic gold provinces in the world, as they develop within mobile belts along its border. The three South American belts are sited in the metallogenic continuation of the Paleozoic terranes that host the giant eastern Australian goldfields, such as Bendigo–Ballarat and Charters Towers, with which they share many features. When compared to deposits in the French Massif Central, direct counterparts of the Andean deposits such as Pataz and Ananea–Yani are respectively the Saint Yrieix district and the Salsigne deposit. Considering the ubiquity of the Au (±Sb±W) vein-type deposits in the Eastern Cordillera and Sierras Pampeanas, and the relatively little attention devoted to them, the Devonian and Carboniferous orogenic gold deposits in the eastern section of the Central Andes constitute an attractive target for mineral exploration.

Sr, C and O isotope systematics in the Pucará basin, central Peru

A combined Sr, O and C isotope study has been carried out in the Pucará basin, central Peru, to compare local isotopic trends of the San Vicente and Shalipayco Zn-Pb Mississippi Valley-type (MVT) deposits with regional geochemical patterns of the sedimentary host basin. Gypsum, limestone and regional replacement dolomite yield 87Sr/86Sr ratios that fall within or slightly below the published range of seawater 87Sr/86Sr values for the Lower Jurassic and the Upper Triassic. Our data indicate that the Sr isotopic composition of seawater between the Hettangian and the Toarcian may extend to lower 87Sr/86Sr ratios than previously published values. An 87Sr-enrichment is noted in (1) carbonate rocks from the lowermost part of the Pucará basin, and (2) different carbonate generations at the MVT deposits. This indicates that host rocks at MVT deposits and in the lower-most part of the carbonate sequence interacted with 87Srenriched fluids. The fluids acquired their radiogenic nature by interaction with lithologies underlying the carbonate rocks of the Pucará basin. The San Ramón granite, similar Permo-Triassic intrusions and their clastic derivatives in the Mitu Group are likely sources of radiogenic 87Sr. The Brazilian shield and its erosion products are an additional potential source of radiogenic 87Sr. Volcanic rocks of the Mitu Group are not a significant source for radiogenic 87Sr; however, molasse-type sedimentary rocks and volcaniclastic rocks cannot be ruled out as a possible source of radiogenic 87Sr. The marked enrichment in 87Sr of carbonates toward the lower part of the Pucará Group is accompanied by only a slight decrease in δ18O values and essentially no change in δ13C values, whereas replacement dolomite and sparry carbonates at the MVT deposits display a coherent trend of progressive 87Sr-enrichment, and 18O- and 13C-depletion. The depletion in 18O in carbonates from the MVT deposits are likely related to a temperature increase, possibly coupled with a 18O-enrichment of the ore-forming fluids. Progressively lower δ13C values throughout the paragenetic sequence at the MVT deposits are interpreted as a gradually more important contribution from organically derived carbon. Quantitative calculations show that a single fluid-rock interaction model satisfactorily reproduces the marked 87Sr-enrichment and the slight decrease in δ18O values in carbonate rocks from the lower part of the Pucará Group. By contrast, the isotopic covariation trends of the MVT deposits are better reproduced by a model combining fluid mixing and fluid-rock interaction. The modelled ore-bearing fluids have a range of compositions between a hot, saline, radiogenic brine that had interacted with lithologies underlying the Pucará sequence and cooler, dilute brines possibly representing local fluids within the Pucará sequence. The composition of the local fluids varies according to the nature of the lithologies present in the neighborhood of the different MVT deposits. The proportion of the radiogenic fluid in the modelled fluid mixtures interacting with the carbonate host rocks at the MVT deposits decreases as one moves up in the stratigraphic sequence of the Pucará Group.

The Susana copper-silver deposit in Northern Chile : Hydrothermal mineralization associated with a Jurassic volcanic arc

The Susana copper deposit is located at an elevation of 860 m above sea level in the coastal Cordillera of northern Chile (Carolina de Michilla district), 110 km north of Antofagasta at 70°10′W and 22°40′S. Initial mining began in 1981 by surface mining methods with production coming from one open bit. Due to subsequent geologic reconnaissance that resulted in the discovery of a mineralized breccia pipe structure, underground mining started in 1983. By 1983, 36000 m of drilling had shown the presence of 8 million metric tons of oxide/sulfide ore averaging 2% copper at 0.5% cutoff (Valdebenito 1983) with probable reserves several million tonnes greater. Mine production in 1985 was 1700 mt/day with Cu >3% and 20 g/mt Ag.

The Pirén Alto Cu-(Zn) Massive Sulfide Occurrence in South-Central Chile — A Kieslager-Type Mineralization in a Paleozoic Ensialic Mature Marginal Basin Setting

The Piren Alto Cu-(Zn) massive sulfide occurrence is a minor, noneconomic but genetically interesting ore type of the Paleozoic Coast Range ophiolite (Schira 1990) in south-central Chile. Manto-type massive Sulfides in the greenstone belt south of 38°S lat were first mentioned in reconnaissance studies (Alvarez 1970; Vergara 1970; Alvarez and Rivera 1970) and later outlined by various authors of the Universidad de Concepcion (for reference see Alfaro and Collao this Vol.). The occurrence under consideration is one of the more important and less supergenely altered ones described therein.

The formation of auriferous quartz-sulfide veins in the Pataz region, northern Peru: A synthesis of geological, mineralogical, and geochemical data

The Pataz region in the eastern part of the North Peruvian Department La Libertad hosts a number of important gold mining districts like La Lima, El Tingo, Pataz, Parcoy, and Buldibuyo. Economic gold mineralization occurs in quartz-sulfide veins at the margin of the calc-alkaline Pataz Batholith, that mainly consists of granites, granodiorites, and monzodiorites. The batholith is of Paleozoic age and cuts the Precambrian to Early Paleozoic low-grade metamorphic basement series. Its intrusion was controlled by a NNW-trending fault of regional importance. The gold-bearing veins are characterized by a two-stage sulfide mineralization. Bodies of massive pyrite and some arsenopyrite were formed in stage 1, and after subsequent fracturing they served as sites for deposition of gold, electrum, galena, sphalerite, and chalcopyrite. It is concluded that gold was transported as a AuCl2--complex by oxidizing chloride solutions and deposited near older pyrite by micro-scale redox changes and a slight temperature decrease. Mineralogical, textural, geochemical, and microthermometric features are interpreted as a consequence of mineralization at considerable depth produced by a hydrothermal system linked with the emplacement of the Pataz Batholith. acteristics in order to outline a general physicochemical model of the hydrothermal ore-forming processes.

Facies control of strata-bound ore deposits in carbonate rocks: The F-(Pb-Zn) deposits in the Alpine Triassic of the Alpujárrides, southern Spain

The strata-bound fluorite-(barite-)Pb-Zn ore deposits occurring within the more than 3,000 m thick Alpujárride Carbonate Formation of the Betic Cordillera, southern Spain, are linked to two defined stratigraphic positions (within the Anisian and at the Ladinian-Carnian transition) and to definite sedimentary contexts. These are highly restricted lagoons isolated from the open sea by calcarenitic barriers with noticeable development of algal mats in their inner margins. Preevaporitic deposition conditions are usually recognized. The ore-bearing horizons are located either at the transition from evaporitic to normal marine deposition (as in the case of the Anisian ore-bearing horizon) or at the transition from normal marine to evaporite deposition (as in the case of the uppermost Ladinian to lowermost Carnian ore-bearing horizon). These vertical facies changes correspond also to the transitions from predominantly terrigenous (continental and/or coastal) sedimentation to marine carbonate sedimentation (Anisian ore deposits) and vice versa (upper Ladinian-lower Carnian). It is proposed that the ore deposits in the Triassic Alpujárride Formation are controlled by certain facies associations in the lagoonal environment. The depositional environment determines the hydrologic regimes responsible for early diagenetic formation of fluorite and of the related ore minerals in a way similar to that of early-diagenetic dolomite. This hypothesis is consistent with field and petrographic observations and with available geochemical and itotopic data reported in other works. The present investigation intends to underline the role played by surface-linked ore formation processes, i.e., by ore-forming processes essentially linked to the environments in which sedimentation and early diagenesis takes place, in the genesis of numerous strata-bound Zn-Pb-(F-Ba) ore deposits in carbonate rocks.

Metal sources in stratabound ore deposits in the Andes (Andean Cycle) - Lead isotopic constraints

The scope of this paper is to discuss the major constraints posed by lead isotopic ratios with respect to possible metal sources in stratabound ore deposits of the Andean Cycle. This is based on a compilation of all available lead isotopic ratios of ore minerals from stratabound ore deposits. A few additional new analyses are also presented. In the present chapter only main patterns observed on a large regional scale will be dealt with; the reader is referred to the cited publications on the different ore deposits for additional details. Since information on lead isotopic ratios on host rock samples is fragmentary and, is in many cases lacking, the discussion will refer mainly to ore lead data.