Mineral Province Research Articles

Carboniferous fault reactivation at the northern margin of the metal-rich Gawler Craton (South Australia): Implications for ore deposit exhumation and preservation

The formation of major Palaeoproterozoic and Mesoproterozoic (Cu)-Au deposits at the metal-rich margins of the Gawler Craton, South Australia, has received a lot of attention, however, the relationship between metal occurrences, the exhumation level of the crust and the structural architecture of the craton margins is less clear. Here, we present results from apatite fission track thermochronology applied to basement rocks at the northern margin of the Gawler Craton, revealing a differential cooling history with respect to the Karari shear zone (KSZ). The KSZ is a major shear zone that extends to the Moho in reflection seismic images and has a prolonged history of high-temperature activity during the Paleoproterozoic and Mesoproterozoic. New apatite fission track data show that samples taken to the north of the KSZ record a significant pulse of Carboniferous cooling, in contrast to the Phanerozoic monotonic slow cooling history documented for the area just south of the KSZ. This Carboniferous cooling signal coincides with a sedimentary hiatus between the Neoproterozoic – Devonian Officer Basin and the late Carboniferous to Early Permian Arckaringa Basin, to the north of the KSZ. Therefore, Carboniferous cooling can be linked with exhumation and fault reactivation of the KSZ at that time, which is interpreted to be associated with far-field compression caused by the Alice Springs Orogeny (~450–300 Ma) of central Australia. Following Carboniferous exhumation, a localized thermal overprint was observed in locations associated with Palaeogene palaeochannels.The extent of Phanerozoic exhumation shows a spatial relation with the location of Au (and/or Cu, Fe) mineralization in the northern Gawler Craton. Areas that were significantly modified by Mesoproterozoic mineralizing events, such as the Olympic IOCG province and the Central Gawler Gold Province, record post-Silurian exhumation histories related to the Alice Springs Orogeny. To the west of these two major mineral provinces, Archaean – early Palaeoproterozoic terranes in the northwestern Gawler Craton with abundant Au (and Cu, Fe) mineral occurrences were not affected by Phanerozoic exhumation and denudation. These relations suggest that the Mesoproterozoic mineralized terranes were more susceptible to Phanerozoic deformation compared to the Archaean – Palaeoproterozoic terranes within the stronger parts of the Gawler Craton. Hence, understanding the timing of fault reactivation and the associated relative exhumation level may provide valuable constraints for ore deposit preservation and mineral exploration within the Gawler Craton.

Application of PP and PS-waves separation: A case study of N4WS Carajás mineral ore, Pará, Brazil

The seismic methods are starting to be an essential tool in mining industry when the ore deposit is located under a thick altered layer. The applicability of reflection and refraction seismic holds an importance in exploration surveys due to a great necessity to investigate higher depths, mainly if compressional and shear waves data are associated, then the separation of the two types of wave became necessary. We apply a new methodology of PP and PS separation based on the estimatives of the Finite Offset Common Reflection Surface (FO-CRS) attributes. The attributes estimates are performed using the Very Fast Simulated Annealing search-for inversion algorithm. Once the attributes are estimated we use them to build a separation filter, which is function of the polarization angle of the wave and the angle of the ray at the receiver. After the separation the sections were stacked in order to increase the signal-to-noise ratio. The method was applied in the so-called N4WS iron orebody, pertaining to the Mineral Province of Carajás, northern Brazil. We applied the separation in a modelled reflection dataset of the geophysical model of the target created from electrorresistivity and refraction seismic methods. The separation was performed in a common shot section and common offset sections in order to demonstrate the geometry independency of the methodology. The conclusions show that the filter could separate successfully PP and PS amplitudes from seismic sections.

Une ville construite par des « gens d’ailleurs ». Développements urbains à Elisabethville, Congo belge (actuellement Lubumbashi, RDC)

The first discussions about architecture in Belgian Congo underlined the hasty character of the new urban developments in the rich mining province Katanga. The mining city Elisabethville, founded in 1910 to become the colony’s provincial capital and economic center, was strongly criticized for the use of uninteresting imported urban schemes. Yet, apart from the emergence of an urban landscape of dubious quality, it was the absence of Belgian architects and urban planners on the Belgian coloni...

Fluid geochemistry of the Serra Pelada Au-Pd-Pt deposit, Carajás, Brazil: Exceptional metal enrichment caused by deep reaching hydrothermal oxidation

Serra Pelada is a high-grade hydrothermal Au-Pd-Pt deposit within the Carajás mineral province, eastern Amazon craton, which shows an exceptional metal enrichment at carbon-enriched hosted ores (>1000’s ppm Au + Pt + Pd). The main orebody is structurally hosted at the hinge of a recumbent syncline, consisting of dolomitic metasandstones at the outer part of the fold, a layer of carbonaceous metasiltstone in which most orebodies are located, and red siltstone in the core of the fold. Distal hydrothermal alteration comprises spatially separated chlorite – carbon (reduced) and hematite (oxidized) alteration zones. The mineralized zones include silicification and argillic (carbon- kaolinite-, and hematite-rich) alteration zones. Fluid inclusions hosted by quartz and dolomite crystals from distal alteration zones and silicified portions within the mineralized zone record a wide range of salinity (5–22 wt% NaCleqv.) but relatively constant homogenization temperatures (170 ± 25 °C). Pressure and temperature estimations based in the fluid inclusion microthermometry and selenide thermometry constrain the formation of the deposit at 250–270 °C and 1.2–1.7 kbar. Element concentrations of alkali metals, Pb, Cs and to a lesser extent Ba correlate with salinity, whereas redox-sensitive elements including U and Ce vary widely. Gold concentrations from 50 to 500 ppb detected in some inclusions are consistent with the highly oxidizing conditions defined by selenide mineralogy. The presence of uranium and gold within fluid inclusions of the distal hematite alteration zone confirms the oxidizing, metal carrier fluids. The occurrence of hydrothermally precipitated carbon implies that a reducing CH4-bearing fluid was also present, and the structural relationships suggest that mixing between this reducing fluid (and interaction with carbon-upgraded rocks) with an acidic, oxidizing and metal-carrying fluids was the main process driving ore metal precipitation. The genesis of the Serra Pelada Au–Pd-Pt deposit shows similarities with sediment-hosted copper deposits and unconformity-related U ± Au ± PGE deposits, having in common the downflow of highly oxidizing fluids into the hydrothermal domain of the Earth's upper crust. Major differences in metal proportions of these broad classes of ore deposits and other sediment-hosted systems in different ore provinces are primarily caused by differences in source rocks available for oxidative leaching and variations in the geometry of the hydrothermal system.

Plant growth and nutrient use efficiency of two native Fabaceae species for mineland revegetation in the eastern Amazon

The primary challenge of mineland revegetation is the establishment of species able to cope with low availability of nutrients, especially in steep slopes such as of mine pits. We evaluated plant growth response and nutrient use efficiency (NUE) of two promising native Fabaceae species (Dioclea apurensis—liana from metalliferous savannas; Bauhinia longipedicellata—tree from Amazon rainforest) from the Carajas Mineral Province, eastern Amazon-Brazil. Plants were grown separately in 2-kg pots filled with mining waste. Substrates were fertilized with nitrogen, phosphorus, potassium (NPK), lime, and micronutrients. The results showed increments on growth of both species when nutrients were applied to the mining waste. D. apurensis showed increases in leaf area, plant height, stem diameter, and shoot dry mass production when NPK or NPK + micronutrients were applied, while B. longipedicelata was responsive to application of NPK + lime or NPK + lime + micronutrients. Further, D. apurensis showed higher NUE than B. longipedicelata, especially at the lowest doses of N, P and K. These findings may indicate a substantial advantage of D. apurensis for mineland revegetation, as this species may require lower nutrient inputs, being, therefore, a more sustainable way to revegetate degraded areas.

Magnetization clustering inversion — Part 2: Assessing the uncertainty of recovered magnetization directions

The unknown magnetization directions in the presence of remanence have posed great challenges for interpreting magnetic data. Estimating magnetization directions based on magnetic measurements, therefore, has been an active area of research within the applied geophysics community. Despite the availability of several methods for estimating magnetization directions, quantifying the uncertainty of such estimates has remained untackled. We have investigated the use of the magnetization-clustering inversion (MCI) method for the purpose of assessing the uncertainty of the recovered magnetization directions. Specifically, we have leveraged the fact that the number of clusters that one expects to see among the magnetization directions recovered from MCI needs to be supplied by a user. We propose to implement a sequence of MCIs by assuming a series of different cluster numbers, and subsequently, to calculate the standard deviations of the recovered magnetization directions at each location in a model as a practical way of quantifying the uncertainty of the estimated magnetization directions. We have developed two different methods for the calculations of the standard deviations, and have also investigated the maximum number of clusters that one needs to consider to reliably assess the uncertainty. After the proof-of-concept study on a synthetic data set, we applied our methods to a field data set from an iron-oxide-copper-gold deposit exploration in the Carajás Mineral Province, Brazil. The high-confidence zones that correspond to low-uncertainty zones indicate a high spatial correspondence with the mineralization zones inferred from the drillholes and geology.

High resolution hydrogeochemical survey and estimation of baseline concentrations of trace elements in surface water of the Itacaiúnas River Basin, southeastern Amazonia: Implication for environmental studies

A high resolution systematic geochemical mapping at regional-scale is considered to be the best available method to estimate geochemical baseline levels of trace elements in stream water, and is an essential part of environmental risk assessment. This methodology was applied in a project in the Itacaiúnas River Basin, southeastern Amazon, Brazil, which includes several mines of the Carajás Mineral Province (as part of the Itacaiúnas Geochemical Mapping and Background Project, ItacGMBP). A total of 1429 samples (including 55 duplicates) were collected in 2017 at 900 sites at one sample per microbasin, during both dry and rainy periods. The analyses of 34 elements were carried out using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). In general, the waters are slightly alkaline and are classified as mixed Ca-Na-HCO3 type, indicating that they are mainly influenced by silicate rock weathering. Most metal concentrations in the water are low, except Fe and Mn. Seasonality explains differences in metal concentrations, with higher values being obtained in the rainy season. Baseline threshold values (BTVs) were calculated separately for both seasons by employing different statistical methods: iterative 2σ and DF preferentially delivered a more restrictive or conservative levels, which can be represenative of the natural BTVs (NBTVs), considering as the least degraded with low or no significant level of anthropogenic influence; and 98th percentile provides the ambient BTVs (ABTVs), which consists of natural plus diffuse anthropogenic input in the defined area. The ABTVs of Fe and Mn significantly exceed the WHO (1998) and CONAMA 357/05 limits. Spatial distribution indicates that Fe and Mn are not strictly related to geologic setting, rather they are highly influenced by specific local land use as well as deep weathering of the catchment and intense leaching and run-off during the rainy season. However, higher Mn occurrence in the dry period results from redox cycling of Fe and Mn via biogeochemical processes. The ABTVs of Ni, Cr, and V are controlled by bedrock lithology (geologic setting), mainly associated with mafic-ultramafic rocks; Cu is associated with two large hydrothermally mineralized copper belts. The estimation of baseline levels of As, Se, and Pb were highly limited due to of the large number of samples with results less than detection limits, in this case the 95th percentile was used for their BTVs. Geochemical data as well as anomalous values for most of the metals indicate that anthropogenic influence from point sources is highly negligible in the basin, except at a few points, where high NO3- has been observed, probably due to more intense human and livestock activity. This study demonstrates that site-specific geochemical baseline assessment is a crucial factor when evaluating surficial water conditions in a large basin.

Geochemical mapping and estimation of background concentrations in soils of Carajás mineral province, eastern Amazonian Craton, Brazil

Companhia de Pesquisa de Recursos Minerais (CPRM, the Brazilian geological survey) has carried out geochemical surveys focused on the Carajás region, the largest mineral province in Brazil. One of these surveys covered the area of Canaã dos Carajás, where 257 soil samples, including 32 duplicate samples, were collected from the B-horizon of soils within an area of c. 3500 km 2 . The <0.177 mm fraction of these samples was ground and digested with aqua regia and then analysed for 53 elements by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry. The statistical treatment of the data allowed the development of multi-element geochemical maps based on interpolation techniques and the establishment of geochemical background values in soils. Different methods of obtaining the geochemical threshold values are presented and compared. The geochemical behaviour of 43 elements, including potentially toxic elements, is discussed. Geochemical background values were established using different statistical methods and the most significant results were obtained from the iterative 2 σ technique. The influence of the geogenic contribution in the study area is largely dominant over anthropogenic effects. There is no conclusive evidence of contamination related to human activity at the time when the geochemical survey was carried out. This paper provides multi-element geochemical maps and background values in the most prominent Brazilian mining province.

The Thermal and Dynamic Process of Core → Mantle → Crust and the Metallogenesis of Guojiadian Mantle Branch in Northwestern Jiaodong

The Jiaodong gold mineral province, with an overall endowment estimated as >3000 t, located at the eastern segment of the North China Craton (NCC), ranks as the greatest source of Au in China. The structural evolution, magmatic activity and metallogenesis during the Mesozoic played important roles in the large scale regional gold, silver and polymetallic mineralization in this area; among them, the intensive activation of fault structures is the most important factor for metallogenesis. This study takes the regional deep faults as main thread to discuss the controlling role of faults in large scale metallogenesis. The Jiaojia fault and Sanshandao faults in the northwest margin of the Guojiadian mantle branch not only are dominant migration channels for hydrothermal fluid but are very important favorable spaces for ore-forming and ore-hosting during the formation of world-class super large gold deposits in this area. The deep metallogenic process can be summarized as involving intensive Earth’s core, mantle and crust activity → magmatism → uplifting of metamorphic complex → detachment of cover rocks → formation of mantle branch → penetration of hydrothermal fluid along deep faults → concentration of metallogenic materials → formation of super large deposits.

Tribological characterization of jaspilite by linear scratch test

Rotary drilling is usually done with diamond-impregnated drill bits. These tools may present an unsatisfactory lifespan when working on a banded iron formation of Carajás Mineral Province, Brazil, known as jaspilite, a complex material composed of alternated bands of silicates (jasper) and iron oxides, and quartz veins. The performance of this bit is linked to the interaction between the diamond and the jaspilite, which is little understood. The abrasive wear response of this tribosystem has been investigated by a series of single-point scratch tests using Rockwell C, Vickers and Knoop diamond indenters as function of normal load (0.2–90 N). The effect of the distance between parallel scratches, which range from 5 times the scratch width to total superimposition, scratching speed (0.01–1 mm/s) and environmental conditions (dry, distilled water and bentonite drilling fluid) on wear and friction behavior were studied with Vickers indenter. Severe wear by chipping was dominant in all loads with Vickers indenter, at lower speeds and lubricated conditions. Mild wear mechanisms (such as burnishing, ploughing and scale-like cracking) had been observed in tests with Rockwell and Knoop indenter, at high speeds and in iron oxides. A correlation between wear mechanisms and friction was observed, except in the tests varying speed and environmental conditions. Furthermore, the degree of superimposition had no influence on friction and wear mechanisms.

Predictive lithological mapping through machine learning methods: a case study in the Cinzento Lineament, Carajás Province, Brazil

The Cinzento Lineament (Carajas Mineral Province) represents a complex deformational system with great associated mineral potential, mainly for IOCG deposits. However, the tropical vegetation of the Amazon rainforest considerably limits the number of outcrops available for systematic geological mapping. Therefore, the use of remote data such as airborne geophysics and remote sensing is essential to provide a reliable geological map. The airborne magnetometric data to define lithological units and its boundaries is a challenge, especially in regions with low magnetic latitude and/or remanent magnetization. In this work, we proposed an approach using Magnetization Vector Inversion (MVI) to map the distribution of the magnetic susceptibility, in order to replace techniques such as pole reduction and total gradient. We applied the Random Forest algorithm (supervised Machine Learning algorithm) to recognize patterns in remote data and improve the current mapped lithological units. With 1400 training samples (2.5% of the total samples), we produced two Predictive lithological maps: a first with remote data only and a second with remote data and spatial coordinates. We evaluate the advantages and disadvantages of each Predictive map, and we conclude that both maps need to be analyzed together for the refinement of the current geological map. These predictive maps represent a powerful tool to combine remote data to improve current geological maps, or even generate the first-pass geological map for regions with scarce geological knowledge.

Lithospheric Architecture and Metallogenesis in Liaodong Peninsula, North China Craton: Insights from Zircon Hf-Nd Isotope Mapping

The Liaodong Peninsula is an important mineral province in northern China. Elucidating its lithospheric architecture and structural evolution is important for gold metallogenic research and exploration in the region. In this study, Hf-Nd isotope maps from magmatic rocks are constructed and compared to geological maps to correlate isotopic signatures with geological features. It is found that gold deposits of different age periods in Liaodong are located in areas with specific εHf(t) and εNd ranges (Triassic: from −8 to −4 and from −12 to −8, Jurassic: from −22 to −8 and from −14 to −8, Cretaceous: from −12 to −10 and from −22 to −20), respectively. This may reflect that when the Paleo-Pacific plate was subducted beneath the North China Craton, the magma was derived from the juvenile lower crust and the ancient lower crust, and formed the low-to-moderate hydrothermal Au-(Ag) and Pb-Zn deposits in the Triassic. In the Jurassic, continued subduction may have led to lithospheric thickening. Subsequently, the magma from the ancient lower crust upwelled and formed low-to-moderate hydrothermal Au deposits and porphyry Mo deposits. In the Cretaceous, crustal delamination may have taken place. The magma from the ancient lower crust upwelled and formed various low-to-moderate hydrothermal Au deposits.

Macro-scale ore-controlling faults revealed by micro-geochemical anomalies

Whereas the mechanism of fluid flow, and thus structural control, linked with mineral deposit formation is quite understood, the specific structures that likely provided controls on mineralization at certain geographic scales are not readily known for a given region unless it is well-explored. This contributes uncertainty in mineral prospectivity analysis in poorly-explored regions (or greenfields). Here, because the spatial distribution of mineral deposits has been postulated to be fractals (i.e., the patterns of these features are self-similar across a range of spatial scales), we show for the first time that micro-geochemical anomalies (as proxies of micro-scale patterns of ore minerals), from few discrete parts of the Sossego iron-oxide copper-gold (IOCG) deposit in the Carajás Mineral Province (CMP) of Brazil, exhibit trends of macro-scale faults that are known to have controlled IOCG mineralization in the CMP. The methodology described here, which led to this novel finding, would help towards detecting mineral exploration targets as well as help towards understanding structural controls on mineralization in greenfields.

Proxies for Basement Structure and Its Implications for Mesoproterozoic Metallogenic Provinces in the Gawler Craton

AbstractThe link between mineral resources and crustal‐rooted structures has been proposed for many of the world's most significant mineral provinces. Here we utilize a new approach by interpreting potential field data, including satellite gravity data, and high‐resolution continental‐scale magnetotelluric data, constrained with aeromagnetic, and seismic tomography and reflection data, to determine the distribution of crustal‐scale faults in the Archean to Proterozoic Gawler Craton (South Australia). The eastern flank of the craton hosts the supergiant Olympic Dam iron oxide‐copper‐gold (IOCG) deposit within a larger Olympic IOCG province. The central part of the craton contains gold‐only deposits, which define the Central Gawler Gold province. Both of these provinces are part of a Mesoproterozoic mineral system with an extensive hydrothermal alteration footprint, which formed during complicated tectonic mode switches. We show that both types of mineralization are located in proximity to crustal‐scale structures that appear to connect deep crustal fragments, which likely record the amalgamation of the Archean nucleus of the craton during the Neoarchean with subsequent reworking during the Mesoproterozoic. Many of these structures do not have a surface expression but coincide with gradients in magnetism, gravity, and electric resistivity anomalies, the latter data set suggesting they acted as fluid pathways extending to the lower crust. The results indicate that the first‐order controls on the distribution of IOCG and Central Gawler Gold metallogenic provinces are inherited from earlier tectonic events, which formed major crustal boundaries and related structures that are prone to reworking during later tectonism.

Impacts of mining in artificial lake of Iron Quadrangle-MG: past marks and changes of the present

A locus of intense mining activity since the seventeenth century, the Iron Quadrangle, corresponds to one of the most important mineral provinces in the world. Located in Minas Gerais, Brazil, it keeps in its territory marks of past mining activities still in expansion. Such changes, recorded in water bodies of the region’s watersheds, impact not only locally, but also reach adjacent ecosystems as well as those located downstream of mining companies. In this situation, an artificial lake is found in the upper course of the Mata Porcos creek sub-basin. The area was once an old place of gold exploration in the eighteenth and nineteenth centuries, and is currently influenced by active iron mining and inactive iron–manganese mining upstream. With the aim of evaluating this reservoir and contributing to the understanding of lentic systems under the influence of mining, this study analyzed the physical and chemical characteristics of sediments from the bottom of this lake, identifying the possible contributions of anthropic origin. Sediment samples were collected at nine points along two hydrological years and then submitted to granulometric, mineralogical and geochemical analyses. The geochemical signature of this lake is marked by As, Fe and Mn elements associated with both the local geological substrate and anthropic activities undertaken in the surroundings. Due to the high As concentrations, distribution maps were made using the IDW interpolation method to understand the distribution of this potentially toxic element that has a strong correlation with gold mining practiced for more than 300 years at the site.

Mineral chemistry and geothermometry of alteration zones in the IOCG Cristalino deposit, Carajás Mineral Province, Brazil

The Cristalino deposit, located in the Serra do Rabo region (Pará State, Brazil), is related to a hydrothermal system in which two major alteration stages could be distinguished most likely with the involvement of a hyper-saline fluid. The first stage (410-650 °C) is characterized by a distal sodic metasomatism that produced almost pure chessboard albite, minor schorlitic tourmaline and REE-rich minerals (allanite-Ce, monazite). It was followed by a pervasive calcic-ferric alteration that generated abundant actinolite (XMg = 0.87–0.69, Cl up to 0.59 wt %) in addition to Ce-allanite and magnetite associated with sulfide disseminations and breccia-like bodies composed of chalcopyrite-pyrite-magnetite-Au (early ore association). Locally, Fe-edenite (XMg = 0.67–0.42, Cl up to 2.94 wt %) replaced calcic-ferric assemblages within restrict sodic-calcic alteration halos. From 410° down to 220 °C, the previous alteration assemblages were overprinted by the hydrothermal products of the second stage. Potassic (K-feldspar, minor biotite) and propylitic (epidote, chlorite, calcite) alterations came into play successively. K-feldspar is practically stoichiometric, but it contains some impurities, notably BaO (up to 1.21 wt %). Chlorite shows the greatest compositional variation among all minerals and its composition seems to have been particularly controlled by the type of host rock, chemistry of the hydrothermal fluid and temperature. Both chamosite and clinoclore (XFe = 0.37–0.80) are present, the former being more common. Chlorine contents are in general <0.02 wt % and a little more significant in chlorites that replaced chessboard albite (up to 0.06 wt %). Their formation temperature ranges from 220° to 360 °C, with infill chlorites showing the highest values. The late ore association (chalcopyrite ± Au ± hematite ± pyrite) is contemporaneous with the potassic and propylitic alterations and bears evidence that the Cristalino system evolved to the final stages with increase in oxygen fugacity. Comparatively to other IOCG deposits from the southern sector of the Carajás domain, especially the Sossego and Visconde deposits, two varieties of amphibole (mainly actinolite and edenite) have been formed, but the Cristalino chlorites, despite some overlapping, present very distinct populations not yet described in the other two deposits, confirming the diversity of IOCG systems in Carajás.

Archean crust and metallogenic zones in the Amazonian Craton sensed by satellite gravity data

The formation of ore deposits has been extensively studied from a shallow crust perspective. In contrast, the association of mineral systems with deep crustal structure of their host terranes remains relatively undisclosed, and there is evidence that processes throughout the lithosphere are coupled for their evolution. The current debate centers on the control of the regional deep crustal architecture in focusing and transferring fluids between geochemical reservoirs. Defining such architecture is not unequivocal, and involves combining indirect information in order to constrain its physical properties and evolution. Herein, based on evidence from satellite gravity, constrained by airborne potential field data (gravity and magnetics), we provide an example on how the lithosphere geometry controlled the location of copper and gold systems in the world-class Archean Carajás Mineral Province (Amazonian Craton, South America). Validation with information from passive seismic (wave speeds, crustal and lithospheric thickness) and geochronologic data (model, crystallization ages, and Neodymium isotope ratio determinations) portrays a significantly enlarged, poly-phase, Archean crust that exerted geometric control on the location of the mineral systems within and adjacent to the province during tectonic quiescence and switches. This new geologic scenario impacts the understanding of the Amazonian Craton. Synergy between multi-source data, as experimented here, can provide robust models efficiently and, conceivably, help to unveil similar terrains elsewhere.

Native leguminous plants for mineland revegetation in the eastern Amazon: seed characteristics and germination

For the effective use of native plants for mineland revegetation, an understanding of seed dormancy break and germination requirements, and seed storage tolerance is indispensable. In the present study, eight native species (Bauhinia pulchella, Bauhinia longipedicellata, Dioclea apurensis, Mimosa camporum, Mimosa acutistipula var. ferrea, Mimosa pudica, Parkia platycephala, and Stryphnodendron pulcherrimum) from the metalliferous savannas (cangas) and forests of Carajas Mineral Province, eastern Amazon-Brazil, were studied to determine seed size, seed quality (X-ray and tetrazolium tests), germination, and dormancy break requirements (boiling water, acid and mechanical scarification), and seed storage behavior. Our results showed considerable variation in seed size and percentage germination among the species. There was a strong relationship between seed size and germination, and the germination was greater for larger seeds from forests than smaller seeds from canga. All three scarification methods increased germination of M. camporum and M. acutistipula var. ferrea. Seeds of D. apurensis, M. acutistipula var. ferrea, M. pudica, and P. platycephala did not show a significant decline in germination after storage, indicating possible orthodox behavior. In contrast, B. pulchella, B. longipedicellata, M. camporum, and S. pulcherrimum showed behavior typical of recalcitrant or intermediate seeds since the germination of these species was reduced after storage. Further studies addressing seed dormancy break and seed storage in other native species are encouraged for a better use of native species in mineland revegetation.

The 2.0–1.88 Ga Paleoproterozoic evolution of the southern Amazonian Craton (Brazil): An interpretation inferred by lithofaciological, geochemical and geochronological data

The study of Paleoproterozoic rocks is crucial for understanding Earth's tectonic evolution during the time when most of the modern crust and ore deposits were formed. The rocks of the Brazilian Amazonian Craton record some of the most-complete and best-preserved Paleoproterozoic magmatic and volcanic episodes on Earth. Following previous investigations, we present new lithofaciological and stratigraphic records of the felsic rocks of the Tapajós Mineral Province (TMP) (~2–1.88 Ga) and the São Felix do Xingú region (SFX) (~1.88 Ga) which, combined with new petrological and geochronological data, help providing a more complete understanding of the tectonic, magmatic and volcanological evolution of the Amazonian Craton. This magmatism/volcanism is thought to be formed in a late-/post-orogenic to extentional regime confirmed by the new geochemical data presented here. The transition from late-convergent to extensional tectonic setting could register the beginning of the taphrogenesis that marked the Amazonian Craton throughout the Mesoproterozoic. The volcanological approach of this contribution can serve as a strategy for the modelling of the evolution of Precambrian volcano-sedimentary basins around the world. The large amount of rocks analyzed are divided into primary and secondary volcaniclastic products depending on if they resulted from a direct volcanic activity (pyroclastic) or processes that reworked pyroclastic fragments. Furthermore, the deposits are subdivided into massive and stratified, depending on their primary mechanisms of transport and emplacement. By confirming the results from previous studies, our study permits to depict a more precise paleo-environmental picture of the processes that occurred in the Amazonian Craton during the Late-Paleoproterozoic. In particular, the presence of large regional-scale fissural systems and caldera collapses produced large silicic explosive volcanic eruptions, also accompanied by the emission of large volume effusive products. Although studies on the Amazonian Craton are still scarce and controversial, the present study provides new evidence that this volcanism may have formed one of the largest Silicic Large Igneous Provinces (SLIP) on earth. Our data also confirm that at least two major Paleoproterozoic periods of formation of volcanic rocks exist in the Amazonian craton. This point is of great relevance for any future interpretation of the geological evolution of this craton.

Mapping and quantification of ferruginous outcrop savannas in the Brazilian Amazon: A challenge for biodiversity conservation.

The eastern Brazilian Amazon contains many isolated ferruginous savanna ecosystem patches (locally known as ‘canga vegetation’) located on ironstone rocky outcrops on the top of plateaus and ridges, surrounded by tropical rainforests. In the Carajás Mineral Province (CMP), these outcrops contain large iron ore reserves that have been exploited by opencast mining since the 1980s. The canga vegetation is particularly impacted by mining, since the iron ores that occur are associated with this type of vegetation and currently, little is known regarding the extent of canga vegetation patches before mining activities began. This information is important for quantifying the impact of mining, in addition to helping plan conservation programmes. Here, land cover changes of the Canga area in the CMP are evaluated by estimating the pre-mining area of canga patches and comparing it to the actual extent of canga patches. We mapped canga vegetation using geographic object-based image analysis (GEOBIA) from 1973 Landsat-1 MSS, 1984 and 2001 Landsat-5 TM, and 2016 Landsat-8 OLI images, and found that canga vegetation originally occupied an area of 144.2 km2 before mining exploitation. By 2016, 19.6% of the canga area was lost in the CMP due to conversion to other land-use types (mining areas, pasturelands). In the Carajás National Forest (CNF), located within the CMP, the original canga vegetation covered 105.2 km2 (2.55% of the CNF total area), and in 2016, canga vegetation occupied an area of 77.2 km2 (1.87%). Therefore, after more than three decades of mineral exploitation, less than 20% of the total canga area was lost. Currently, 21% of the canga area in the CMP is protected by the Campos Ferruginosos National Park. By documenting the initial extent of canga vegetation in the eastern Amazon and the extent to which it has been lost due to mining operations, the results of this work are the first step towards conserving this ecosystem.