Chilean Deposits Research Articles

Capitana mine, Tignamar district, a unique tin – bismuth concentration in the Chilean Central Andes, 250 km west of the Bolivian Tin Belt: Link to the Proterozoic-Paleozoic basement Belen inlier and the Arequipa-Antofalla terrane, a probable remnant of Laurentia

The Capitana mine, Tignamar District, is a unique Sn-Bi concentration in the highlands of Arica, in the Chilean Central Andes. Tin deposits associated with Triassic to Pliocene age igneous rocks are plentiful in the Tin Belt of Peru, Bolivia, and NW Argentina, but have not been reported in Chile, where contemporaneous rocks formed metallogenic belts rich in Fe, Cu, Mo, Ag and Au. We describe vein ore from Capitana mine, a small but exceptional Sn-bearing Pb-Bi-Ag-Cu-Sb-As epithermal deposit in the Tignamar (or Ticnamar) district in the high Andes of Arica, Chile, ca. 250 km west of the Bolivian Tin Belt. Fluid inclusions in vein quartz trapped a non-boiling, low salinity (<∼4 wt% NaCl equiv.), moderate T fluid (minimum T ∼ 220 °C) prior to metal precipitation. Flashing of this fluid, likely due to rapid changes in confining P, was synchronous with metal deposition; sulfosalt textures show evidence of syn-deformational growth. Although located within a district considered a porphyry copper system of Early Miocene age, it is likely that the Capitana deposit formed later, mainly as a result of regional hydrothermal activity related to compressive and transpressional tectonics as young as Pliocene. Common lead is 206Pb/204Pb 18.18; 207Pb/204Pb 15.61; and 208Pb/204Pb 38.53, significantly less radiogenic than that of Bolivian Sn deposits, and has a Paleozoic model lead age. These values point to affinities with a tectonically-emplaced inlier of crystalline basement of Proterozoic – Early Paleozoic age known as the Belen Metamorphic Complex, and with the Arequipa-Antofalla basement terrane, a probable remnant of Laurentia (North America), left behind during the dismemberment of the Rodinia Supercontinent. The complex mineralogy of Capitana vein, unlike other Chilean deposits, includes various sulphides and sulphosalts with Cu, Fe, Zn, Pb, Bi, Sn, Ag, As, Sb, Ga and In, and is exceptionally rich in U.

Adjacent Iron Oxide-Apatite and Iron Oxide Copper-Gold Mineralization in the Acarí District, Peru: The Magmatic to Hydrothermal Transition

Abstract Hierro Acarí and Cobrepampa are adjacent, Lower Cretaceous iron oxide-apatite (IOA) and iron oxide copper-gold (IOCG) districts, respectively, emplaced in the Coastal Cordillera of southern Peru, separated by a regional NW-trending fault. Mineralization in both districts is hosted mainly by Lower Cretaceous plutons of the Coastal batholith that intrude sedimentary and volcanic sequences of the inverted Cañete basin. The deposits form part of a region that includes the much larger Marcona and Pampa del Pongo IOA/skarn and the Mina Justa IOCG deposits, although Marcona is 60 m.y. older than the others. Hierro Acarí contains fracture-controlled steeply dipping, shallow-plunging N- and NE-trending magnetite veins in a 101.7 ± 1.4 Ma dioritic intrusive, and hematite mantos in a quartzite roof pendant. The magnetite ore shoots show colloform and microporous textures and grade downward within 150 m to actinolite and carbonates, which replace the magnetite. The Cobrepampa district comprises numerous NW-striking chalcopyrite-pyrite ± hematite veins and oxidized equivalents in a 99.7 ± 0.4 Ma monzonitic intrusive. Fluid inclusions indicate that the copper mineralization was deposited at 166° to 266°C from a fluid with low to moderate salinity of 3.8 to 16.5 wt % NaCl equiv. Colloform and microporous textures in magnetite are interpreted in comparable Chilean deposits to indicate colloidal transport of magnetite microlites, crystallized from a magma, which attach to bubbles in the magmatic-hydrothermal fluid in a process resembling froth flotation. Magnetite textures at Hierro Acarí suggest that magnetite was transported by this late magmatic process and deposited in the roof zone of the host intrusive. The hydrothermal fluid may have evolved to deposit hematite as mantos in the overlying sediments under more oxidizing conditions and may then have deposited the Cobrepampa copper and hematite veins in distal NW-trending faults. Recent studies into the geochemistry of Cañete basin volcanic rocks suggest that a mid-ocean ridge was subducted at the latitude of the Raúl-Condestable IOCG deposit, 350 km northwest of the Acarí district, at approximately 130 Ma, predating current Andean subduction below southern Peru. That situation could result in a configuration resembling the Gulf of California, where a series of fault-terminated pull-apart basins focus hydrothermal fluids at intervals along the main trough. IOCG and IOA/skarn mineralization in the Acarí district occurred during the early stages of Late Cretaceous basin inversion and intrusion of the Coastal batholith. However, the Marcona IOA deposit in the same districts was emplaced during an earlier, unrelated Jurassic to Early Cretaceous mineralizing episode.

Boreholes plans optimization methodology combining geostatistical simulation and simulated annealing

Nowadays, the prospection plans have the difficult task of ensuring a more complete and rich characterization of the rock mass for the purpose of optimizing costs and increasing safety in geotechnical projects. Currently, boreholes location and depth are mainly defined based on experience and know-how of professionals, as such, it is user-dependent. Hence, there is a lack of methodologies to help the decision-makers in defining the optimal location of boreholes (with relevant information). Therefore, this paper presents a methodology based on the use of geostatistical conditional simulation allied to a stochastic global optimization algorithm (Simulated Annealing) to develop optimized boreholes plans comparing a uni-objective and a multi-criteria optimization approaches. In this work, the optimized location is considered the one that minimizes uncertainty translated by either the average local variance or the average width of 95% probability intervals of simulated values at unsampled locations. This methodology was applied using preliminary information obtained from previously executed boreholes using as variable the empirical rock mass classification system, Rock Mass Rating, in a Chilean deposit.

Fe–O stable isotope pairs elucidate a high-temperature origin of Chilean iron oxide-apatite deposits

Iron oxide–apatite (IOA) ore deposits occur globally and can host millions to billions of tons of Fe in addition to economic reserves of other metals such as rare earth elements, which are critical for the expected growth of technology and renewable energy resources. In this study, we pair the stable Fe and O isotope compositions of magnetite samples from several IOA deposits to constrain the source reservoir of these elements in IOAs. Since magnetite constitutes up to 90 modal% of many IOAs, identifying the source of Fe and O within the magnetite may elucidate high-temperature and/or lower-temperature processes responsible for their formation. Here, we focus on the world-class Los Colorados IOA in the Chilean iron belt (CIB), and present data for magnetite from other Fe oxide deposits in the CIB (El Laco, Mariela). We also report Fe and O isotopic values for other IOA deposits, including Mineville, New York (USA) and the type locale, Kiruna (Sweden). The ranges of Fe isotopic composition (δ56Fe, 56Fe/54Fe relative to IRMM-14) of magnetite from the Chilean deposits are: Los Colorados, δ56Fe (±2σ)=0.08±0.03‰ to 0.24±0.08‰; El Laco, δ56Fe=0.20±0.03‰ to 0.53±0.03‰; Mariela, δ56Fe=0.13±0.03‰. The O isotopic composition (δ18O, 18O/16O relative to VSMOW) of the same Chilean magnetite samples are: Los Colorados, δ18O (±2σ) = 1.92±0.08‰ to 3.17±0.03‰; El Laco, δ18O=4.00±0.10‰ to 4.34±0.10‰; Mariela, δ18O=(1.48±0.04‰). The δ18O and δ56Fe values for Kiruna magnetite yield an average of 1.76±0.25‰ and 0.16±0.07‰, respectively. The Fe and O isotope data from the Chilean IOAs fit unequivocally within the range of magnetite formed by high-temperature magmatic or magmatic–hydrothermal processes (i.e., δ56Fe 0.06–0.49‰ and δ18O=1.0–4.5‰), consistent with a high-temperature origin for Chilean IOA deposits. Additionally, minimum formation temperatures calculated by using the measured Δ18O values of coexisting Los Colorados magnetite and actinolite separates (630°C) as well as Fe numbers of actinolite grains (610–820°C) are consistent with this interpretation. We also present Fe isotope data from magmatic magnetite of the Bushveld Complex, South Africa, where δ56Fe ranges from 0.28±0.04‰ to 0.86±0.07‰. Based on these data and comparison to published Fe and O stable isotope values of igneous magnetite, we propose extending the magmatic/high-temperature δ56Fe range to 0.86‰. Considering that the Chilean IOAs and Kiruna deposit are representative of IOA deposits worldwide, the Fe and O stable isotope data indicate that IOAs are formed by high-temperature (magmatic) processes.

Genetic Aspects of the Manto‐type Copper Deposits Based on Geochemical Studies of North Chilean Deposits

AbstractRecent studies on mineralogy, geochronology, fluid inclusion and stable isotope (Pb, Os, S, C, O, Sr) characteristics were reviewed to determine constraints for genetic models of the Chilean manto‐type copper deposits. The Chilean manto‐type deposits are divided into the two geologic categories of the northern areas (Arica–Iquique, Tocopilla–Taltal) and the central areas (Copiapó, La Serena, Santiago). The former is distributed in the coastal range composed of Jurassic andesite‐dominated volcano‐sedimentary piles and younger plutonic intrusions, and yields chalcocite (‐digenite) and bornite as the principal hypogene copper sulfides. The latter is hosted mostly in Lower Cretaceous volcano‐sedimentary sequences, and has chalcopyrite‐rich mineral associations. The fluid inclusion data indicate that the primary copper mineralization was commonly generated in the temperature range 150–360°C under low‐pressure conditions near the boiling curve, mediated with relatively saline brines. Generally, homogeneous Pb and S isotope compositions for primary copper minerals imply direct magma source or leaching of igneous rocks. Pb and Os isotope data published for some deposits, however, suggest that ore‐forming metals were derived mainly from the volcano‐sedimentary host rocks. The noticeably negative isotope ratios of primary sulfide sulfur and hydrothermal calcite carbon of some central area deposits indicate influx of sedimentary rock components, and the high 87Sr/86Sr initial ratios of hydrothermal calcite from the Tocopilla–Taltal area deposits imply contribution of the contemporaneous seawater or marine carbonates. These isotopic constraints imply a formation mechanism in which the Chilean manto‐type copper deposits formed epigenetically in the process of hydrothermal interaction of non‐magmatic surface‐derived brine with the volcano‐sedimentary host rocks, which is inferred to have been induced by a deep‐seated plutonic complex as the possible heat source.

Age of Mineralization of the Candelaria Fe Oxide Cu-Au Deposit and the Origin of the Chilean Iron Belt, Based on Re-Os Isotopes

Re-Os isotopes are used to determine the age of iron oxide Cu-Au mineralization at Candelaria, Chile, and to explore possible genetic links with the batholithic intrusions in the area. Re-Os ages calculated from molybdenite are 114.2 ± 0.6 Ma and 115.2 ± 0.6 Ma, and they are interpreted to represent the age of mineralization. These ages are consistent with previously reported ages for biotite alteration that range from 114 to 116 Ma. An isochron calculated by Re/Os ratios from hydrothermal magnetite and sulfides constrains an initial 187 Os/ 188 Os ratio of 0.36 ± 0.10. The initial 187 Os/ 188 Os ratio for sulfide from Bronce mine, a small satellite of the Candelaria orebody, is 0.33 ± 0.01. These values are broadly similar to the calculated initial 187 Os/ 188 Os ratio for magmatic magnetite in nearby batholithic rocks that range from 0.20 to 0.41. The relatively radiogenic initial 187 Os/ 188 Os ratio represents a mixture of mantle and crustal components in the ores and batholitic rocks. The similarity in initial 187 Os/ 188 Os ratio of the ore and magmatic oxides suggest that the granitic plutons could be the source of metals in the Candelaria district. These data are consistent with the existence of a significant magmatic fluid component in the hydrothermal system as suggested by previously published work. In order to establish a regional perspective, we analyzed ore minerals from other Chilean deposits of the iron oxide (Cu-U-Au-rare earth elements [REE]) class. Magnetite from the Manto Verde iron oxide Cu-Au deposit has Os and Re concentrations of 11 to 17 parts per trillion (ppt) and 4 to 6 parts per billion (ppb), respectively. The initial 187 Os/ 188 Os ratio is approximately 0.20 and is similar to that of ore minerals from Candelaria and of the Early Cretaceous batholithic intrusions. These data indicate a similar metal source for the mineralization at Manto Verde and Candelaria. Magnetite from three magnetite-apatite deposits of the Chilean iron belt have Re concentrations between 0.8 and 3 ppb and Os concentrations between 11 and 76 ppt. Calculated initial 187 Os/ 188 Os ratios of these magnetites range from 1.2 to 8.4 and are distinctly different from those of the iron oxide Cu-Au deposits. The cause of the comparatively high radiogenic signatures in the magnetite-apatite ores is probably related to fluid interactions with (i.e., leaching) the surrounding sedimentary rocks during their genesis. In contrast to the iron oxide Cu-Au systems, the Os in magnetite-apatite ores could be derived from sedimentary rocks. This evidence suggests that predominantly basin-derived, nonmagmatic brines formed these magnetite-apatite deposits.

Lapis Lazuli from the Coquimbo Region, Chile

GEMS & GEMOLOGY Spring 2000 apis lazuli is a rock composed of lazurite—the source of the blue color—with variable amounts of other minerals depending on its origin and, typically, small particles of pyrite. Prized for its attractive blue color, lapis lazuli was used in jewelry by some of the world’s most ancient civilizations. The stone is mined at relatively few locations, some of which have been worked since the fifth millennium BC (von Rosen, 1990). Considering the extensive history and romance attached to this ornamental gem material, the Chilean deposit is a relative newcomer. The first reference to Chilean lapis lazuli dates back to the 19th century (Field, 1850a and b). Another early mention was made by Ignacio Domeyko (1860), a Polish immigrant and mining engineer who became the first director of the School of Mines at La Serena (about 140 km from the workings). The first detailed geologic studies of the deposit and mine workings were carried out by German geologist J. Bruggen (1921, 1926), who identified the host rock as contact-metamorphosed limestone. Lapis lazuli was officially recognized as the national gemstone of Chile in 1984. The Coquimbo Region is the only known source of lapis lazuli in Chile. Although reference is sometimes made to another Chilean deposit at Vicuna Mackenna Mountain near Antofagasta (Webster, 1994; Sofianides and Harlow, 1990), this material has been identified as dumortierite by Canut de Bon (1991). Once considered a minor or unimportant locality, with the lapis lazuli described as “at best mediocre” (Wyart et al., 1981), the Chilean deposit has produced significant quantities of attractive material in recent years. Today, Chilean lapis lazuli is exported in the rough, or incorporated into jewelry (see, e.g., figure 1), carvings, and decorative building materials by local artisans. This article will review the historical significance of lapis lazuli, and examine the geologic setting and gemological characteristics of material from Chile. LAPIS LAZULI FROM THE COQUIMBO REGION, CHILE

Unusual coarse, clastic, wave-dominated shoreface deposits, Pliocene to middle Pleistocene, northern Chile; implications for coastal facies analysis

ABSTRACT Pliocene to mid-Pleistocene shallow-marine strata exposed at Gatico, northern Chile (22° 30 S, 70° 14 W) display unusual characteristics for coarse-grained wave-dominated shoreface deposits. A 15 m2000 m strike section of Pliocene to mid-Pleistocene strata exposed within small, late Pleistocene sea cliffs contain four facies: beach, foreshore channel, upper shoreface, and rip channel. Beach and upper shoreface facies predominate. The latter consists of locally bioturbated, coarse-grained, pebbly sandstone with (1) single sets of planar to tangential cross-strata, up to 3.5 m thick, with reactivation surfaces, and (2) sets of medium-to large-scale cross-strata that were deposited on inclined set surfaces (up to 15°). Some of the inclined set surfaces extend up-dip of cross-set terminations. Inclined parallel laminae and bioturbated beds lie subparallel to the inclined set surfaces. Interpretation of these unusual bedforms has been made through analogy with the present-day microtidal wave-dominated coastline at Gatico, by facies analysis, and by comparison with modern-day wave-dominated shorelines. The large-scale sets of cross-strata record the migration of simple, two-dimensional (2-D) dunes and represent the slipface deposits of nearshore bars. Parallel laminae represent the bar surface. Medium-scale dunes migrated up the inclined set-bounding surfaces, which lie subparallel to older preserved bar surfaces. Paleocurrent data indicate that bar migration was largely directed obliquely onshore towards the southeast. The bar slipface deposits are considered to record the recovery of a nearshore bar immediately following storm conditions. Bar recovery involves re-establishment of a slipface and rapid onshore migration. Erosion surfaces that appear as the extension of lower set boundaries beyond the termination of cross-strata sets, and reactivation surfaces, are considered to be storm-generated. The episodic nature of bar migration is indicated by the development of heavily bioturbated horizons. The small-scale ascending cross-sets may reflect fair-weather conditions with northwestward dune migration up the modified bar slipface. Despite the abundance of barred shorelines around the world at present, their deposits are not widely described from the rock record, suggesting that they are either not preserved or not recognized. The scale and internal characteristics of these nearshore bars closely resemble those of some tide-generated bedforms (e.g., compound cross-strata in marine sediment with bimodal-bipolar paleocurrents and evidence of episodic bedform migration). The similarity of these north Chilean deposits to documented tidal deposits suggests that an element of caution may be required in the identification of tidal deposits from the rock record.

Chemistry, mineralogy and origin of the clay-hill nitrate deposits, Amargosa River valley, Death Valley region, California, U.S.A.

The clay-hill nitrate deposits of the Amargosa River valley, California, are caliche-type accumulations of water-soluble saline minerals in clay-rich soils on saline lake beds of Miocene, Pliocene(?) and Pleistocene age. The soils have a maximum thickness of ∼ 50 cm, and commonly consist of three layers: (1) an upper 5–10 cm of saline-free soil; (2) an underlying 15–20 cm of rubbly saline soil; and (3) a hard nitrate-rich caliche, 10–20 cm thick, at the bottom of the soil profile. The saline constituents, which make up as much as 50% of the caliche, are chiefly Cl −, NO − 3, SO 2− 4 and Na +. In addition are minor amounts of K +, Mg 2+ and Ca 2+, varying, though generally minor, amounts of B 2O 3 and CO 2− 3, and trace amounts of I (probably as IO − 3), NO − 2, CrO 2− 4 and Mo (probably as MoO 2− 4). The water-soluble saline materials have an I/Br ratio of ∼ 1, which is much higher than nearly all other saline depostis. The principal saline minerals of the caliche are halite (NaCl), nitratite (NaNO 3), darapskite (Na 3(SO 4)(NO 3)·H 2O), glauberite (Na 2Ca(SO 4) 2), gypsum (CaSO 4·2H 2O) and anhydrite (CaSO 4). Borax (Na 2B 4O 5(OH) 4·8H 2O), tincalconite (Na 2B 4O 5(OH) 4·3H 2O) and trona (Na 3(CO 3)(HCO 3)·2H 2O) are abundant locally. The clay-hill nitrate deposits are analogous to the well-known Chilean nitrate deposits, and probably are of similar origin. Whereas the Chilean deposits are in permeable soils of the nearly rainless Atacama Desert, the clay-hill deposits are in relatively impervious clay-rich soils that inhibited leaching by rain water. The annual rainfall in the Death Valley region of ∼ 5 cm is sufficient to leach water-soluble minerals from the more permeable soils. The clay-hill deposits contain saline materials from the lake beds beneath the nitrate deposits are well as wind-transported materials from nearby clay-hill soils, playas and salt marshes. The nitrate is probably of organic origin, consisting of atmospheric nitrogen fixed as protein by photoautotrophic blue-green algae, which are thought to form crusts on soils at the sites of the deposits when moistened by rainfall. The protein is subsequently transformed to nitrate by autotophic bacteria.