Exotic Deposits Research Articles

Copper mass flux and evolution of supergene orebody in the Miduk Porphyry Copper Deposit, Iran, and its application to exploration of exotic deposits

The present study used field geology and aimed to model the geological results by modern methods for examining the effects of geological features on the formation of an enriched ore in a mature supergene profile such as Miduk Porphyry Copper Deposit (MPCD). In addition, it aimed to discover which geologic controllers caused the fixation or conduction of Cu in supergene solutions. To this end, the anatomy of the supergene zones was examined by direct sampling and mapping. Analysis of fracture density helps uncover the role of fractures in conveyance of fluids. The results showed that MPCD endured a systematic supergene process, which resulted in the formation of a 200-meter enriched orebody on the top of the hypogene ore. Additionally, the topographic gradient was the most important controller in the migration of supergene solutions to northern side while the north–south trending fractures doubled the lateral flux. Furthermore, the mass balance analysis indicated that 181,000 tons of Cu laterally escaped from the supergene system. Considering the geochemical conditions in sources areas, this amount of metal was sufficient to form a proximal type of exotic mineralization. Further, drainages derived from the main deposit exhibited exotic mineralization, which supported the lateral movement of Cu from the main orebody.

Neogene basin infilling from cosmogenic nuclides (10Be and 21Ne) in Atacama, Chile: Implications for palaeoclimate and supergene copper mineralization

AbstractIncreasing evidence suggests that supergene exotic copper deposits were emplaced during periods of geomorphic quiescence and pulses of humidity in arid environments. We tested this idea in the Centinela Mining District in the Atacama Desert (northern Chile). We collected 14 sand samples at depth (up to 110 m) in two open‐pit mines (Central Tesoro and Mirador) exposing Miocene sediment, and located in the El Tesoro Basin, which hosts two exotic copper‐rich orebodies. We inverted the 10Be and 21Ne concentrations by using a two‐box model (IMIS, inversion of multi‐isotopes in a sedimentary basin) composed of an eroding source of sediment and a depositional sedimentary basin, and by selecting denudation and sedimentation rate histories that can explain our data. The ages found demonstrate that the two exotic orebodies were deposited during a narrow period between 14 Ma (10 Ma younger than previously thought) and 9.5 Ma, when an ignimbrite covered the sedimentary sequence. The dated lower exotic copper orebody was deposited during or just before a sharp decrease in the sedimentation rates (from >100 to 0.5–5 m/Ma), which is consistent with published sedimentological and carbonate isotopic data in this district. This confirms the idea that exotic deposits form during a quiescence of the geomorphic activity. Nevertheless, our model suggests that the back‐ground denudation rate providing sediment to these basins between ca. 14 Ma and ca. 9.5 Ma was surprisingly high (>250 m/Ma) for such an arid environment. These denudation rates can be explained by a relatively rapid local back‐scarp retreat providing most of the sediment to these basins and possibly a wetter climate compared to the present. Then, during the period 10–7 Ma, the denudation rates decreased to >50 m/Ma. This decrease may correspond to a local progressive decrease in the slope of the surrounding hills, or to a progressive aridification, or a combination of both phenomena.

First U-Pb LA-ICP-MS in situ dating of supergene copper mineralization: case study in the Chuquicamata mining district, Atacama Desert, Chile

Since the second half of the twentieth century, exotic copper mineralization represents a prime target for many mining exploration companies operating in the hyperarid Atacama Desert, in northern Chile. Although there is evidence that the emplacement of such deposits took place during specific Tertiary climatic periods and relief formation, many uncertainties remain regarding the exact timing for their deposition and/or the genetic link between the exotic deposits and the primary porphyry copper deposits. We present a first attempt of U-Pb dating of copper-rich minerals from the Mina Sur exotic deposit from the Chuquicamata mining district. A suite of Mn-rich black chrysocolla clasts surrounded by pseudomalachite bands has been characterized and dated in petrographic context using both nanosecond and femtosecond in situ laser ablation ICP-MS analyses. U-Pb dating on pseudomalachite bands yields a crystallization age of 18.4 ± 1.0 Ma. For the Mn-rich chrysocolla clasts, the 206Pb/238U apparent ages range from 19.7 ± 5.0 Ma to 6.1 ± 0.3 Ma, a spread interpreted as the result of U and/or Pb mobility linked to fluid circulation following crystallization. This study demonstrates that supergene copper mineralization can be directly dated by the U-Th-Pb method on pseudomalachite. Furthermore, the age obtained on pseudomalachite indicates that Mina Sur copper deposition took place at ca. 19 Ma, about 11 m.y. after the unroofing and hydrothermal alteration of the Chuquicamata deposit, a result that is consistent with the supergene ages already known in the Atacama Desert.

Evaporitic carbonates in the pre-salt of Santos Basin – Genesis and tectonic implications

Abstract Since the first discoveries of oil accumulations in the carbonate reservoirs of the Lower Cretaceous Barra Velha Formation of the Santos Basin, their genesis has been the subject of intense discussion. Different depositional models have been proposed, with some authors advocating a microbial origin, while others an abiotic origin as travertines or travertine-like rocks. This search for a modern carbonate analogue for these unusual rocks may have disregarded an intriguing alternative: an evaporative model supported by our petrographical and geochemical data. Evaporitic sediment patterns in endorheic basins, in a depositional framework of brine drawdown, are dominated by ephemeral salt pan hydrology. Bedded salt crusts form a stack of subaqueous aligned crystal beds at the sediment-water interface; salt grows diagenetically within saline and mud layers beneath the dry pan surface; finally laminated sediments are deposited. We observe a similar cyclic pattern in the pre-salt core we described: beds of shrub-like crystals and diagenetic spherulites growing in magnesium clay matrix are followed by laminites. The model we propose attempts to contribute solving the puzzle of Pre-Salt carbonate deposition by integrating sedimentology and geochemistry with basin tectonics. Our observations and basin tectonics point to the exciting possibility that the sag phase carbonates of the Santos-Campos basin were deposited as the initial/transitional stage of a closed sub-sea-level evaporite basin whose rising salinities culminated in the deposition of halite and bittern salts (carnallite, sylvite, tachyhydrite). Basinwide evaporites are singular events in the geological record. They form when a giant sub-sea-level basin becomes a sump of continental, marine and hydrothermal fluids, producing unexpected brine compositions in unusual depositional settings present in ancient evaporitic basins but absent from most modern ones. The novelty in this paper is that we apply to pre-salt carbonate deposition the sedimentology, hydrology and geochemistry of ancient evaporitic basins. With this concept in mind, we propose a geochemical model for the unusual pre-salt carbonate precipitation of the Santos Basin. Regarding the Walvis-Rio Grande volcanic high as a topographic barrier during the Aptian, we suggest that the main sources of calcium to the Barra Velha paleolake were hydrothermal brines, mostly from infiltrating seawater that reacted at depth with the basalts of the volcanic barrier. Hyperextension of the lithosphere, with a shallow Moho, increased geothermal gradients to levels favorable for the generation of hydrothermal CaCl2 brines, a process facilitated by the already elevated Ca++ ion and low sulfate concentration of the Cretaceous ocean. These CaCl2-rich fluids would pass through the volcanic barrier into a sub-sea-level paleolake in the rift basin. The high salinity paleolake would become saturated in calcium bicarbonate/carbonate as internally drained NaHCO3-bearing alkaline waters, formed by physical/chemical weathering of penecontemporaneous basaltic rocks, mix with the CaCl2-bearing hot brines percolating through the barrier, producing a hybrid brine (2NaHCO3 + CaCl2 = Ca(HCO3)2 + 2NaCl). Such a depositional system, integrating the geochemical-depositional controls of sag phase carbonates with those of the South Atlantic evaporites, has important implications for unravelling the genesis of these exotic deposits.

Supergene Mass-Balance Study Assuming Zero Lateral Copper Flux Using Geostatistics to Recognize Metal Source Zones in Exotic Copper Deposits

Recognition of effective factors that influence the spatial extension of supergene weathering zones is important both for the identification of high potential areas of exotic deposits and for the cost-effective planning of mining. In particular, recognition of exotic mineralization around porphyry copper deposits early in mine development prevents them from being buried beneath mine infrastructures such as waste dump and tailing structures. Mass-balance modeling, a practical method for determining high potential areas of undiscovered exotic mineralization, investigates important factors in forming exotic deposits. Mass-balance modeling is a two-phase methodology that becomes progressively more detailed. An initial result, presented here as phase 1, is based solely on Cu assays. Phase 2 incorporates relict sulfide mineral studies to improve phase 1 modeling results and computes actual fluxes of copper that escaped vertically downward from the leached cap to form the enrichment blanket and then flowed laterally away to form exotic mineralization. In addition, geostatistical approaches, especially sequential Gaussian simulation, are useful tools for investigating the spatial relationships and modeling of mass-balance results in phase 1 studies. This paper introduces a method for interpolation and downscaling of the preliminary mass-balance analysis (phase 1) to highlight the role of geological features in the evolution of the supergene process. Using only copper assays without any need for relict sulfide mineralogy, this approach can be used to approximately identify the geographic direction of metal movement in exotic copper deposits, and thus serve as an initial exploration guide in prospecting for exotic deposits. For this, a vertical columnar block model was constructed for each of the supergene weathering zones and preliminary analysis of mass balance was conducted to reconstruct the apparent total leached zone column height assuming zero lateral flux. This analysis was applied to each of the vertical block model columns. The results of mass balance were interpolated in a 5 × 5 m grid by sequential Gaussian simulation method, and the simulated surface of the total leached zone was conflated with geological features. The roles of topography, argillic alteration and linear structures were identified in the transport of supergene solutions in the Miduk porphyry copper deposit of Iran. In the northern section of the deposit, which is in accordance with the topography gradient and the presence of advanced argillic alteration zone, the computed top total of leaching is below the actual surface topography, whereas the hypogene isograd curves confirm the expansion of primary copper in these areas. The northern section of the deposit was introduced as a susceptible area for the removal of copper-bearing solutions from the supergene enrichment system.

Large scale explosive eruptions of Akan volcano, eastern Hokkaido, Japan: A geological and petrological case study for establishing tephro-stratigraphy and -chronology around a caldera cluster

Akan volcano, in eastern Hokkaido in the southern Kurile arc, occupies a rectangular caldera (24 × 13 km) with a long (1.7–0.2 Ma) and complex history. This paper combines tephrostratigraphy, tephrochronology, and petrological and geochemical evidence to elucidate the eruptive history and evolution of the caldera. Pyroclastic deposits from Akan caldera are divided into at least 40 “eruptive units”, separated by paleosols, that constitute 17 “eruptive groups” (Ak1 to Ak17, in descending stratigraphic order) that are petrologically distinct and separated by indicators of longer dormancy periods such as thick (>30 cm) paleosols and angular unconformities. The estimated volumes of most eruptive groups are less than 10 km3 dense rock equivalent, and four groups (Ak2, Ak4, Ak7, and Ak13) are larger. Group Ak2, exceeding 50 km3, is the largest. Pyroclastic deposits from Akan caldera are intercalated with pyroclastic deposits from the adjacent Kutcharo caldera and distal air-fall ash layers from central Hokkaido, suggesting that caldera-forming episodes overlapped in central and eastern Hokkaido. Radiometric ages from these exotic deposits range from 1.46 to 0.21 Ma, indicating that caldera-forming eruptions occurred at Akan volcano for more than 1 million years at an average magma discharge rate of approximately 10−1 km3/kyr. Juvenile materials in Akan caldera pyroclastics consist of dominantly aphyric, two-pyroxene dacite to rhyolite. They are characterized by a wide range of K2O compositions (0.8–2.8 wt.%) within a narrow range of SiO2 compositions (67–73 wt.%). Plots of SiO2 vs. K2O suggest that each eruptive group is the product of a distinct, ephemeral magma system rather than a single long-lived magma system. These magma systems appear to have been generated and erupted successively beneath the caldera for more than 1 million years. Each magma system thus appears to represent a relatively short period of activity, and they were separated by relatively long dormancy periods before the next magma system. In particular, a long dormancy of 400 kyr preceded eruptive group Ak2, which consists of the most voluminous and compositionally varied silicic magmas.

Chronology of Exotic Mineralization at El Salvador, Chile, by 40Ar/39Ar Dating of Copper Wad and Supergene Alunite

Exotic mineralization is a complex hydrochemical process linking supergene enrichment, lateral transport, and precipitation of oxide minerals in the drainage network of a porphyry deposit. At the El Salvador porphyry deposit in northern Chile the majority of the exotic ore comprised a mixture of copper-bearing manganese oxyhydrates termed copper wad. X-ray diffraction, scanning electron microscopy, and electron probe microanalysis show that the wad is composed of copper-bearing cryptomelane [K1-2(Mn3+Mn4+)8O16 • x H2O] and birnessite [K0.33\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Mn_{7}^{3.9+}\) \end{document}O14 • 7H2O] structures. These natural occurrences within the exotic ore provide the opportunity to directly date the formation of these deposits using recent advances in 40Ar/39Ar geochronology of supergene K-Mn oxides formed by weathering. A suite of copper-bearing cryptomelane and birnessite samples from exotic deposits within the El Salvador district were characterized and dated by 40Ar/39Ar laser step heating. Supergene alunite [KAl3(SO4)2(OH)6] found in paleospring feeder systems leading from the source zones of outward to the exotic mineralization was dated to independently constrain the age of exotic ore formation. Although the Ar retentivity of the layered birnessite structure has been questioned by others, the Ar retentivity of these samples is thought to be a function of their natural preservation and limited postcrystallization ground-water interaction in the hyperarid Atacama desert. The 40Ar/39Ar analytical results show that Ar and/or K losses after crystallization, excess 40Ar, and 39Ar recoil do not pose significant problems; therefore, in the context of exotic deposits within hyperarid environments this dating method is applicable to both cryptomelane and birnessite within wad. The 40Ar/39Ar dating of exotic mineralization at El Salvador indicates that supergene and exotic mineralization processes were active at ~35 Ma, about 5 m.y. after the emplacement of hydrothermal mineralization, and continued until the middle Miocene. The majority of exotic mineralization extends from the Oligocene-Miocene (24 Ma) boundary through the middle Miocene (11 Ma) and relates to supergene fluid emanating in multiple directions from the source of in Indio Muerto, which represents a topographic high above the exotic deposits. One reconnaissance date of exotic mineralization at the Chuquicamata porphyry deposit yielded an age of 17.03 ± 0.03 Ma coincident with the known supergene alunite dates for this deposit. Reconnaissance dating at Exotica-Mina Sur and El Abra proved problematic due to the presence of contaminant silicate minerals within the wad. Ages derived here for exotic mineralization are similar to the known supergene ages throughout northern Chile. The dates indicate that supergene exotic mineralization began at the Eocene-Oligocene boundary and continued through the Oligocene, eventually ceasing in the middle Miocene due to desiccation of the Atacama desert. A series of 10-cm-thick supergene alunite veins were microsampled across their widths and dated to address the kinetics of alunite vein growth. Apparent growth rates of the veins vary from 71 to 100 mm/m.y. in the horizontal direction to 24 mm/m.y. in the vertical, presumably caused by differences in pressure regimes due to their orientation.