De Caldas Alkaline Massif Research Articles

Chemical and Physical Denudation Rates in the Poços de Caldas Alkaline Massif, Minas Gerais State, Brazil

Chemical and physical denudation rates have been assessed in areas with different lithologies. Surprisingly, there are no studies that attempt to document these rates in the Poços de Caldas Alkaline Massif (PC), the largest alkaline magmatism in South America and an important Al supergene deposit in Brazil. Therefore, the chemical and physical denudation rates were assessed and explained in the PC. Surface water and rainwater samples were collected at the Amoras Stream basin, covering one complete hydrological cycle (2016). All samples were analyzed for dissolved cations, silica, anions, total dissolved solids (TDS), and total suspended solids (TSS). The results reflected the seasonal variation on discharge, water temperature, and electrical conductivity in the Amoras Stream, with most of the cations, anions, silica TDS, and TSS being carried in the wet season. Partial hydrolysis and silicate incongruent dissolution are the main water/rock interactions in the PC, with an atmospheric/soil CO2 consumption rate of 1.6 × 105 mol/km2/a. The annual fluxes of Cl−, PO43−, NO3−, and Al3+ were significantly influenced from rainwater. Chemical and physical weathering rates were 4 ± 0.8 and 3.0 ± 0.6 m/Ma in the PC, respectively, indicating that under the current climatic condition, the weathering profile is in dynamic equilibrium.

A review of mineral assemblages of agpaitic rocks from the Poços de Caldas alkaline massif, southeastern Brazil

A review of mineral assemblages of agpaitic rocks from the Poços de Caldas alkaline massif, southeastern Brazil

Stable isotopes (H, O, S) signatures evidencing evolutionary trends of Brazilian spas groundwaters

This paper reports a stable isotope (H, O, S) study of groundwater samples (25) in 10 spas from southeast Brazil. They are cold (25 °C, 36%), hypothermal (25–33 °C, 60%) and mesothermal (33–36 °C, 4%) waters; and are predominantly composed of bicarbonate, carbonate and sodium. Dissolved sulfate is the dominant anion in one water sample and its concentration increased with increase in pH, electrical conductivity (EC) and total dissolved solids (TDS) values in the samples. 2H and 18O data reported in the literature for circa 700 rainwater samples collected in São Paulo State and Brasília airport, Brazil, were used to construct the following regional meteoric waterline (RMWL): δ2H V-SMOW (‰) = 8.06 δ18Owater V-SMOW (‰) + 12.85. The HO isotopes of the studied groundwater samples show a δ18Owater variation of −14.1 to −5.3‰ V-SMOW, whereas the δ2H range was −66.5 to −31.7‰ V-SMOW, which was plotted on or near the RMWL, suggesting a meteoric origin for them. However, one hypothermal water sample (29 °C) plotted away from this RMWL, and is from a deep fractured aquifer which mainly occurs in fenitization aureole rocks surrounding the carbonatite complex at Araxá spa, Minas Gerais State, and a magmatic hydrothermal water may contribute to this water sample. The use of both δ34Ssulfate and δ18Osulfate values in the selected sites permitted the plotting of a mixture line of the dissolved sulfate in the different aquifer systems of this study, considering two major sources as endmembers: sulfide oxidation, SOX (δ34Ssulfate = +1‰ and δ18Osulfate = +1‰) and sulfates possessing δ34Ssulfate = +9‰ and δ18Osulfate = +21‰ (OSW). This is an unusual isotopic composition opposite from that of sea water. The δ34Ssulfate and δ18Osulfate values tend to increase with increase in dissolved sulfate concentration. The OS isotopes signatures that deviated from the general mixture line could be due to several processes, including atmospheric deposition, SO2 oxidation in the atmosphere, soil-derived sulfate, sulfide oxidation, evaporites dissolution and the presence of dissimilatory sulfate-reducing bacteria. Three sulfate isotope composition of spring waters from Águas da Prata spa in the Poços de Caldas alkaline massif (PCAM) represented endmember sources that defined a triangle around the remaining PCAM samples, as well as the samples from the crystalline basement, São Francisco craton, and Alto Paranaíba igneous province (APIP). Such triangle allows for estimation of the relative isotopic contribution in the different spas groundwaters, which is a new approach that focuses on the use of δ34Ssulfate-δ18Osulfate pair in environmental studies, and shows its usefulness in addition to other conventional hydrogeochemical fingerprints.

Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications

The Pocos de Caldas Alkaline Massif, located in southeast Brazil, is composed of tinguaites, as well as phonolites and nepheline syenites (NeS), together with assorted pyroclastic rocks. The latter constitute deposits within the Vale do Quartel, and show fragments of alkaline rocks (NeS, tinguaites), basaltic and ultramafic volcanics/subvolvanics, sandstones, and quartz-feldspathic rocks from the crystalline basement. NeS fragments with melanite, previously unknown, and cm-sized fragments of phlogopite megacrysts from related conduit breccia were chosen for detailed studies. The NeS fragments are medium-to coarse grained and contain K-feldspar (Or 95-83), secondary albite (Ab 97), altered nepheline, clinopyroxene (diopside, minor hedenbergite) and partially altered, zoned, idiomorphic melanite. These NeS fragments are typically metaluminous, and point to the existence of similar intrusions hidden at depth, an important input to the massif's petrology. The phlogopite megacrysts are compositionally homogeneous, with high Ti, Mg and Al contents; Ba and F are minor components. Two phlogopite aliquots were dated and yielded identical Ar-40/Ar-39 plateau ages of 87.1(+/- 0.5) Ma and 86.6(+/- 0.5) Ma, which represent minimum and maximum values for phlogopite crystallization and breccia emplacement, respectively. Previously obtained ages for the Pocos de Caldas rocks are concentrated in the 75-86 Ma range interval.

Fluorlamprophyllite, Na3(SrNa)Ti3(Si2O7)2O2F2, a new mineral from Poços de Caldas alkaline massif, Morro do Serrote, Minas Gerais, Brazil

ABSTRACTA new mineral species, fluorlamprophyllite (IMA2013-102), ideally Na3(SrNa)Ti3(Si2O7)2O2F2, has been found in the Poços de Caldas alkaline massif, Morro do Serrote, Minas Gerais, Brazil. Alternatively, the idealized chemical formula could be written as (SrNa)[(Na3Ti)F2][Ti2(Si2O7)2O2], setting the large interlayer cations before the cations of the layer. Fluorlamprophyllite is the F-analogue of lamprophyllite. It is associated with aegirine, analcime, natrolite, nepheline and microcline. Fluorlamprophyllite crystals are brownish-orange and bladed. The mineral is transparent with a pale yellow streak and an adamantine lustre. It is brittle and has a Mohs hardness of ~3; cleavage is perfect on {100} and no parting was observed. The calculated density is 3.484 g/cm3. Optically, fluorlamprophyllite is biaxial (+), with α = 1.735(7), β = 1.749(7) and γ = 1.775(9) and 2Vmeas = 72(3)°. An electron microprobe analysis produced an average composition (wt.%) (9 points) of Na2O 10.63(30), K2O 0.47(3), SiO2 30.51(13), SrO 18.30(24), MgO 0.81(17), Al2O3 0.23(2), CaO 1.11(7), MnO 5.03(38), TiO2 27.41(87), Fe2O3 2.45(37), F 2.86(23), plus H2O 1.00 (added to bring the total close to 100%), –O = F –1.20, with the total = 98.61%. The elements Nb and Ba were sought, but contents were below microprobe detection limits. The resultant chemical formula was calculated on the basis of 18 (O + F) atoms per formula unit. The addition of 1.00 wt.% H2O brought [F+(OH)] = 2 pfu, yielding (Na2.63Sr1.35Mn0.54Ca0.15Mg0.15K0.08)Σ4.90(Ti2.63Fe0.24Al0.04)Σ2.91Si3.89O16[F1.15(OH)0.85]Σ2.00. The mineral is monoclinic, with space group C2/m and unit-cell parameters a = 19.255(2), b = 7.0715(7), c = 5.3807(6) Å, β = 96.794(2)° and V = 727.5(1) Å3. The structure is a layered silicate inasmuch as the O atoms are arranged in well-defined, though not necessarily close-packed layers.

Polygenetic processes in the genesis of clay deposits of Poços de Caldas alkaline massif in southeastern Brazil

The purpose of this research is to characterize the clay deposits occurring in the central plateau of the Poços de Caldas alkaline massif, aiming for a better understanding of the involved genetic processes. The results showed that the formation of clay deposits was associated with two processes related to external geodynamic, in situ alteration (weathering) and sedimentation. These processes produced two different types of clay layers: residual and sedimentary clay. The mineralogical characterization of the residual clay was conditioned by the parent rock. Thus, in the deposits formed from alkaline rocks, the clay material is mainly kaolinitic, whereas the clay deposits formed from potassic rocks are dominated by halloysite. The sedimentary deposits were formed by the erosion of both weathering products (saprolite) of alkaline rocks and bauxite formations, originating from the upper part of the topography and deposition over the residual clay in a clear unconformity. This is a very interesting situation because these two types of clays with different properties and utilization occur in the same deposit, reflecting a polygenetic origin.

The Use of the Stream Length-Gradient Index in Morphotectonic Analysis of Drainage Basins in Poços de Caldas Plateau, SE Brazil

The Pocos de Caldas Plateau is formed by the alkaline intrusion of Pocos de Caldas and surrounding metamorphic basement. It is characterized as a remnant of the South American Planation Surface resulted from erosional events from Late Cretaceous-Paleogene transition. Pocos de Caldas Alkaline Massif (PCAM) was formed during alkaline volcanism event from the Late Cretaceous. It is located on the west side of the Mantiqueira Mountain Range, characterized by alkaline rocks and topography reaches over 1700 m high. A major feature of denudation is the development of the “circular drainage”, a particular feature of some caldera and other circular or concentric structural features. Recent tectonic movements may critically affect regional geomorphology by influencing river patterns and orientations. The morphometric analysis shows that some rivers are controlled by reactivated faults, however, in some regions the lithology has major influence in the rivers development.

Characterization of the bauxite mining of the Poços de Caldas alkaline massif and its socio-environmental impacts

The intensive use of mineral resources brings along significant changes to the environment. The mining sector is essential for world economics, as long as it is operated on a basis that is technically coherent, socially fair and environmentally correct. This study aimed to characterize the bauxite mining of the Poços de Caldas plateau, MG, investigating its respective socio-environmental impacts. Through the overview presented, one can conclude that in most situations, the bauxite miningat the Poços de Caldas plateau mitigates its negative social and environmental impacts in a satisfactory manner in addition to generating positive social impacts; and the restoration methods of the mining-disturbed ecosystems have been continually improved and should be replicated by small-and-medium-sized companies.

Determination of weathering rate of the Morro do Ferro Th-REEs deposit, Brazil using U-isotope method

The weathering rate was estimated by changes in 234U/ 238U activity ratio (AR) and U content of rocks, borehole spoil and surface water samples at the Morro do Ferro Th-REEs deposit. The deposit is situated in the Poços de Caldas alkaline massif, Brazil. The south stream basin investigated in this paper has not been significantly affected by anthropogenic inputs of pollutants as compared to the Corumbataí River basin in São Paulo State, where the method was previously applied. The weathering rate derived utilizing the U-isotopes modeling corresponded to 0.015 mm/yr (67,000 years to weather 1 m of rock under the actual climatic conditions). The estimated rate is very reasonable in comparison with the range of 0.015–0.05 mm/yr of land surface lowering within the entire caldera. It is also compatible with a rate of 0.013 mm/yr determined for the Salgado River basin in a semi-arid region in Bahia State, Brazil. The value generated is reliable and increases the potential use of the method for other different areas in Brazil and elsewhere, because it may be used in regions with different climatic conditions and (un)polluted basins.

SRTM-based morphotectonic analysis of the Poços de Caldas Alkaline Massif, southeastern Brazil

An evaluation of SRTM 03″ data applicability in geomorphology and morphotectonic analysis is proposed, considering the morphometric parameters slope, aspect, surface roughness and isobase surface. The study area, in southeastern Brazil, comprises the Poços de Caldas Alkaline Massif, a 33 km-diameter Late Cretaceous collapsed volcanic caldera. Morphometric indices evaluated showed the correlation of landscape within the massif with NE–SW and NW–SE structures, as well as landforms related with recent tectonic influence. DEM-derived drainage presented satisfactory results when compared to a 1:50,000 topographic map. SRTM 03″ proved to be a good resource for geomorphological analysis, up to the semi-detail scale.

A combined method for evaluating radon and progeny in waters and its use at Guarani aquifer, São Paulo State, Brazil

A combined method for evaluating radon ( 222Rn) and progeny ( 214Pb and 214Bi) in water was developed by using inexpensive alpha scintillation counting and gamma ray spectrometry through NaI(Tl) scintillation detectors. A groundwater sample collected at the Poços de Caldas alkaline massif in Brazil was submitted to the technique in order to assure its applicability by comparing the volumetric activities by different methods. Similar volumetric activity was determined for 214Pb and 214Bi in the sample analyzed that is compatible with the expected condition of radioactive equilibrium between these nuclides. The combined method was successfully used to analyze groundwater samples from Guarani aquifer in São Paulo State, Brazil, and the results of the measurements indicated that 214Pb and 214Bi provide useful information concerning the evaluation of the drinking water quality in terms of radiological aspects. This is because they are directly identified in the water samples, without the need of requiring the assumption of the establishment of the transient equilibrium condition with its parent 222Rn.

História térmica do maciço alcalino de Poços de Caldas (SP/ MG) e adjacências através da análise de datação por traços de fissão em apatitas

The South American Platform in southeastern Brazil records a long history of tectonicmagmatic and uplift events, which resulted from the opening of the South Atlantic Ocean, Cretaceous reactivation and epirogeneic processes. Specific manifestations include the basic magmatism of the Serra Geral Formation of the Parana Basin and alkaline magmatism of Pocos de Caldas – Cabo Frio Lineament, as well as the uplift of the Serra da Mantiqueira and Serra do Mar mountain ranges. Thermotectonic and geochronological studies using the K-Ar method and apatite fission track analysis in samples of the Pocos de Caldas alkaline massif show an initial evolution beginning 89 Ma related to a tectonic uplift, interpreted as related to the intrusion’s doming. This was followed by an important episode of rapid cooling related to the formation of the massif. At the same time, the mountains that surround the massif record heating events in response to the alkaline intrusion, between 70 and 50 Ma. Periods of slow uplift, probably related to the South American Erosion Cycle (between 50 and 30 Ma) and Velhas Erosion Cycle (from 25 Ma), indicate a strong manifestation of denudation in this area.

The U-isotopes modeling in aquifers from Poços de Caldas plateau, Brazil

U-modeled groundwater residence times were estimated by changes in 234U/238U activity ratio (AR) and U content of groundwaters from Pocos de Caldas city situated at the Pocos de Caldas alkaline massif, Brazil. The estimated ages are more realistic than others generated by the use of hydraulic conductivity data and available information about the weathering rate of rocks in the plateau. The 234U/238U AR and reciprocal of the dissolved U-content data also defined a ternary plot that allowed the calculation of the relative volume of three source waters for the mixed water. Such U-isotopes modeling for mixing calculations indicated that the dominant phase (68%) in the mixture is thermal.

Estudo gravimétrico do maciço alcalino de Poços de Caldas

A gravimetric survey was held at the Pocos de Caldas alkaline massif located at the border between Minas Gerais and Sao Paulo States, southeastern Brazil. Combining the existent and the new established gravity stations for this work, a total of 783 data points were available. Four profiles crossing the region were analyzed to produce a 2.5D geometry model; the obtained result was compared to a 3D independent model. The best fit model indicate a volume of 7,220 km 3 for the massif, which maintains the alkaline intrusion as second of largest size in the world.

Penecontemporaneous syenitic-phonolitic and basic-ultrabasic-carbonatitic rocks at the Poços de Caldas alkaline massif, SE Brazil: geologic and geochronologic evidence

Penecontemporaneous syenitic-phonolitic and basic-ultrabasic-carbonatitic rocks at the Poços de Caldas alkaline massif, SE Brazil: geologic and geochronologic evidence

PENECONTEMPORANEOUS SYENITIC-PHONOLITIC AND BASIC-ULTRABASIC-CARBONATITIC ROCKS AT THE POÇOS DE CALDAS ALKALINE MASSIF, SE BRAZIL: GEOLOGIC AND GEOCHRONOLOGIC EVIDENCE

The large Pocos de Caldas alkaline massif, southeastern Brazil, is constituted mainly by felsic rocks (nepheline syenites, phonolites and subvolcanic tinguaites), associated with volcaniclastic basic-ultrabasic deposits. Critical appraisal of earlier K/Ar, Rb/Sr and microchemical ages (microprobe Th-U-total Pb determinations), combined with geological and paleomagnetic informations, can limit more adequately the age interval for the massif. A critical appraisal of the existing K/Ar data limit the age interval for the felsic rocks between 64 and 83 Ma, with a median value of 77 Ma. Earlier Rb/Sr data for various nepheline syenites result in isochron ages between 89 ± 8 and 83 ± 21 Ma (whole rock), while more recent determinations show 79 ± 7 Ma (internal isochron), with initial ratio of 0.70511 ± 0.00001. Nepheline syenites strongly affected by hydrothermal alteration were dated at 76 ± 2 Ma (Rb/Sr isochron), i.r. 0.7053 ± 0.0002. A phlogopite lamprophyre in the uranium open pit Osamu Utsumi mine yielded phlogopite Ar-Ar ages of 76 ± 1-2 Ma, close to a microprobe Th-U-total Pb age of thorite, 79 ± 3 Ma, found in carbonatite veins associated with lamprophyric-pyroxenitic dikes emplaced within nearby basement gneisses. These geochronologic data, together with geological-structural information and published magnetization directions, indicate that the felsic rocks were emplaced during a short time interval of perhaps 1-2 Ma, during the reverse 33r Campanian magnetization event (dated between 83 and 79.1 Ma). The vocaniclastic basic-ultrabasic deposits were emplaced, at least in part, during the following 33n magnetization event. All basic-ultrabasic occurrences in the district (the Vale do Quartel rocks, phlogopite lamprophyre dike in the open pit, pyroxenitic-carbonatitic dikes) seem to be related, and are in part somewhat younger than the felsic rocks or penecontemporaneous to them, partly accompanying the hydrothermal and mineralization event in the massif.

The lujavrite and khibinite bodies in the Poços de Caldas alkaline massif, southeastern Brazil: a structural and petrographic study

The lujavrite and khibinite bodies in the Poços de Caldas alkaline massif, southeastern Brazil: a structural and petrographic study

The lujavrite and khibinite bodies in the Poços de Caldas alkaline massif, southeastern Brazil: a structural and petrographic study

Two small lujavrite-khibinite bodies, an eastern and a western one, are exposed at the northern edge of the alkaline Pocos de Caldas massif, southeastern Brazil. Detailed mapping reveals at the center of the better exposed western body a coarse-grained, mesocratic, gneissic-looking eudialyte nepheline syenite (lujavrite) with a strong subhorizontal foliation (Lu I), showing at the contacts a finer-grained border fades (Lu II). Two trachytoid nepheline syenites (NeS I and II) occur as an envelope to the central lujavrites, followed by an outer shell of coarsegrained eudialyte NeS (NeS III or khibinite). The internal structure of both bodies is that of a saucer, with successive foliated shells (in part absent in the eastern body) constituted by Lu I - Lu II- NeS I-NeS II, with rather steep dips at the contacts between the different fades; all are surrounded by the outer NeS III. Country rocks are tinguaites and a grey NeS (NeS IV). Tinguaites were the earliest rocks (intruded some 76-80 Ma ago), followed by the coarser rocks, with NeS IV emplaced before the lujavrites and associated types. The emplacement of the lujavrite bodies was a continuous process, initiated with forceful subhorizontal intrusion of strongly agpaitie magmas. Magma upwelling compressed the initial magma batch, generating the lujavrite caps by compaction and liquid extraction, with a foliated subhorizontal structure (Lu I) and an outer shell (Lu II). The intrusion forced at the same time a lateral expansion of the magma chamber. In this massif, agpaitie magmas appear always as late intrusions. A compareson with other occurrences (Ilimaussaq, Lovozero, Pilansberg, the Texan Trans-Pecos province) suggests that emplacement under forceful conditions may be a controlling factor in shaping the final fabric of lujavrites.

MINERALOGY OF NEPHELINE SYENITES FROM THE POÇOS DE CALDAS ALKALINE MASSIF, SE BRAZIL: CHEMISTRY, X-RAY DATA AND MICROTEXTURES OF FELDSPARS

Nepheline syenites (NeS) of the Pocos de Caldas Alkaline Massif constitute mostly small bodies of miaskitic-intermediate or agpaitic rocks. The alkali feldspars of most NeS are highly potassic (OR > 80-85) and are either orthoclase or highly ordered microcline. The microcline shows a multiple that resembles that of chess-board albite. This may appear in isolated turbid spots in otherwise clean monoclinic crystals or occupy the whole grain (incipient, poor, good twinning); microperthites, when present, occur in the twinned areas. In some NeS the K-feldspars are clean, wholly twinned with larger twin lamellae (very good twinning). Secondary albite (tabular albite) is seen in some facies either included in, or bordering, the K-feldspars. In the singlemost important NeS intrusion, the Pedreira NeS, the alkali feldspars are richer in Na, Ca and Sr, very turbid and show a diffuse twinning resembling a cross-hatched pattern. The Xray diffractograms show a mixture of orthoclase and intermediate microcline. Estimated geothermometric temperatures indicate subsolidus reequilibration between feldspar and nepheline, particularly when the Kfeldspars show good or very good twinning. The microtextures of the K-feldspars are the result of a continuous process of recrystallization of non-twinned earlier crystals, controlled by late magmatic fluids. The turbid patches probably represent regions of abundant micropores facilitating fluid circulation. The styles of the K-feldspars may be due to the composition of the fluid phase and of the feldspars. Sodium redistribution by deuteric fluids may have produced the secondary and perthitic albite.

Mineralogy of nepheline syenites from the Poços de Caldas alkaline massif, SE Brazil: chemistry, x-ray data and microtextures of feldspars

Nepheline syenites (NeS) of the Poços de Caldas Alkaline Massif constitute mostly small bodies of miaskitic-intermediate or agpaitic rocks. The alkali feldspars of most NeS are highly potassic (OR > 80-85) and are either orthoclase or highly ordered microcline. The microcline shows a multiple twinning that resembles that of "chess-board" albite. This twinning may appear in isolated turbid spots in otherwise clean monoclinic crystals or occupy the whole grain (incipient, poor, good twinning); microperthites, when present, occur in the twinned areas. In some NeS the K-feldspars are clean, wholly twinned with larger twin lamellae (very good twinning). Secondary albite (tabular albite) is seen in some facies either included in, or bordering, the K-feldspars. In the singlemost important NeS intrusion, the Pedreira NeS, the alkali feldspars are richer in Na, Ca and Sr, very turbid and show a "diffuse twinning" resembling a cross-hatched pattern. The Xray diffractograms show a mixture of orthoclase and intermediate microcline. Estimated geothermometric temperatures indicate subsolidus reequilibration between feldspar and nepheline, particularly when the Kfeldspars show good or very good twinning. The microtextures of the K-feldspars are the result of a continuous process of recrystallization of non-twinned earlier crystals, controlled by late magmatic fluids. The turbid patches probably represent regions of abundant micropores facilitating fluid circulation. The twinning styles of the K-feldspars may be due to the composition of the fluid phase and of the feldspars. Sodium redistribution by deuteric fluids may have produced the secondary and perthitic albite.