Amethyst Geodes Research Articles

Giant amethyst geodes likely grew from groundwater

Giant amethyst geodes likely grew from groundwater

World-class amethyst-agate geodes from Los Catalanes, Northern Uruguay: genetic implications from fluid inclusions and stable isotopes

AbstractThe amethyst and agate geodes from the Los Catalanes Gemmological District in Uruguay represent one of the main deposits of its kind worldwide. The geometry of the deposit is horizontal, with an irregular distribution of amethyst geodes within the upper level of the basalt lava flows and shows strong variations in their abundance, as well as quality, geometry, and shape. Reliable exploration guides are scarce, and the limited knowledge of the geological parameters controlling its occurrence makes exploration unpredictable, leading to inaccurate reserve estimation. Based on cutting-edge methods including nucleation-assisted microthermometry of one-phase fluid inclusions and determination of triple oxygen isotope in silicates and carbonates, as well as analysis of geode-hosted water and groundwater, we estimate the crystallisation temperatures in the range between 15 and 60 °C. These low temperatures point to amethyst crystallisation after the emplacement of the complete basalt pile. The mineralising fluid shows isotopic signatures consistent with meteoric water and very low salinities from pure water up to rarely over 3 wt% NaCl-eq., likely sourced from the groundwater hosted in the aquifers in the basaltic sequence and underlying units. Based on the insights provided by the new data, we propose the combination of open- and closed-system crystallisation inside pre-existing cavities due to the episodic infiltration of meteoric water in a rather stable geological context.

БРАЗИЛЬСКИЕ ЖЕОДЫ В МИНЕРАЛОГИЧЕСКОЙ ГАЛЕРЕЕ МГУ «ИСКУССТВО ПРИРОДЫ В КАМНЕ»

The article describes the morphological features and mineral composition of two amethyst geodes from the exposition of Moscow State University’s mineralogical gallery “Nature’s Art in Stone”, based on the Primo Rovis collection. The article presents data from the studies of the phase composition of the geodes by powder X-ray diffraction (XRD) and micro X-ray spectral (XRD) analyses, and contains microphotographs. The mechanism of mineral phase crystallization of the described geodes is proposed.

Collector’s Note: Amethyst from Haida Gwaii, British Columbia, Canada

Figure 1. A view looking downslope over the slide area in which the first amethyst geodes were found, Graham Island, Haida Gwaii. Jack Whittles photo.Reproduced with permissionA new locality for qu...

Novo Hamburgo Complex formed by hydrothermal, explosive injection of Botucatu erg sand into extensive tracts of Paraná Volcanic Province (S Brazil)

The Parana Volcanic Province follows Siberia as the second largest in the continents and offers a unique opportunity to study the hydrothermal relationship between basalt-rhyodacite lavas and buried erg-turned aquifer in an intraplate setting. Injected sand fluidized after basalt sealing and was succeeded by amethyst and agate geode opening and filling. Excellent exposures in the Herveiras cuesta, southern Brazil, allow the comparison of processes over long distances (1500 km) in the volcanic group. Fieldwork and basalt chemistry led to the identification of three hydrothermal processes-amygdales filling (H1), sand injection (H2) and amethyst geodes formation (H3). Sand injection was triggered by high temperature (150 ℃) and seismicity. These low-temperature processes identified in the Herveiras cuesta demonstrate the homogeneity of relationships between the paleoerg, giant aquifer and intraplate volcanic rocks across the volcanic province.

Paleoclimatic inferences for the Holocene of southern Brazil in environments influenced by different hydrological systems

Organic matter preserved in continental deposits is an excellent archive of environmental and climate change, and various research tools are commonly used to access this information. The present study is the result of a study conducted in continental environments influenced by different hydrological systems and demonstrates the usefulness of these analyses in obtaining rainfall-related paleoclimatic data. The study area is located in the basalts of the Alto Uruguai region (Rio Grande do Sul, Brazil), known for its mineral riches such as amethyst geodes and hot springs. The methodology used integrates palynofacies and organic geochemistry techniques, complemented by radiocarbon dating. The results were obtained from three sedimentary profiles and showed a trend of relative decrease in rainfall levels in the region. However, the scale of these changes is also highly influenced by the hydrological system of the depositional environment that originated the sedimentary record at each site. Based on the data obtained, we advise the establishment of medium- and long-term policies involving better management of local natural resources in the investigated region.

The genesis of agates and amethyst geodes

AbstractPractically all aspects of agate genesis generate debate. The time is ripe to clarify the most important enigmas concerning the environments of formation of agates and the related famous amethyst geodes of Brazil and Uruguay. Agates form over a wide range of temperatures, from those of basaltic and andesitic melts (about 1100 °C) down to about 50 °C, and at rather low pressures. Their formation in liquid mafic magmas is indicated by a correlation between (1) the orientation of amygdules and the inclination of onyx banding in them and (2) the attitude of amygdules in the lava flow layers. The correlation arises because lava moves at a different rate close to and far from the upper and lower rims of a flow. The alkaline supercritical fluid fills gas vesicles in lavas and dissolves silica, mainly, from ambient lava or rock to produce a silica sol. If the pressure on the fluid causes percolation of water from amygdules, the sol coagulates on the walls of the vesicle to form a concentric lining. If the pressure in amygdules falls below the maximum osmotic pressure of a sol (about 0.1 MPa for a silica sol), percolation of fluid stops, and coagulation leads to the formation of horizontal onyx banding. Multiple repetitions of precipitation of various gel layers can be caused by overlapping fresh flows upon the cooling older agate-bearing lava flow. In a submarine setting, phase separation of the fluid and the formation of a film of gel between vapor (or diluted solution) and brine stimulate the osmotic processes, which result in growth of hollow membrane tubes and branching moss-like arrays at the bottom of amygdules. Some agates exhibit numerous channels as a result of repeated extrusion of fluid or gel from inner zones to the periphery of amygdules that were compressed under the burden of new flows. Previously, such channels were interpreted to be feeding channels for silica supply in amygdules. Periodic compression of amygdules after percolation of fluid from them requires no additional supply of silica because the volume of the amygdules is reduced in proportion to the loss of fluid. The concentric and horizontal banding and mossy textures of agates from the lithophysae of felsic volcanic rocks were created during active volcanism as well. The agates from dissolution-induced cavities in carbonate rocks and the famous amethyst druses of Brazil and Uruguay formed at the moderate temperatures associated with low-grade burial metamorphism, as indicated by the lack of moss textures and onyx banding.

Variability of amethyst mining waste: A mineralogical and geochemical approach to evaluate the potential use in agriculture

Large amounts of stone wastes are accumulated alongside different exploiting mines of amethyst geodes in south Brazil, which are becoming an unsustainable environmental issue. For an adequate use of hydrothermal basalt in agriculture as soil remineralizer, it is fundamental to know the material's mineralogy and geochemical variability. This study aims to evaluate the mineralogical and geochemical variability of the basalt from five mines at two positions (near and far from the geodes) in different size fractions to discuss their potential use in agriculture. X-ray diffraction (XRD), X-ray fluorescence (XRF) and petrographic analyses of thin sections were used. Petrographic and mineralogical analyses showed similar compositions for the rocks, consisting of plagioclases, pyroxenes, and opaque minerals with apatite as an accessory mineral. The rock samples located near the amethyst geodes presented slightly higher clay mineral contents than the distant ones. The clay mineralogy was composed of smectite and celadonite. The subfractions preserved the characteristics of the original material. Small geochemical and mineralogical variations were observed in the hydrothermal basalts regardless of the positions and the fraction sizes studied. These characteristics confer this material a high potential for use in agriculture as a soil remineralizer. This approach contributes both to solving an environmental issue associated with geode exploitation and to create an alternative for soil fertilization.

Silica gossan as a prospective guide for amethyst geode deposits in the Ametista do Sul mining district, Paraná volcanic province, southern Brazil

A new frontier of amethyst geode deposits (amethyst, agate, gypsum, calcite and zeolites) driven by the prospective guide termed silica gossan may change the metallogenetic scenario previously known in the Paraná volcanic province. Silica gossans, which are related to basalt, basaltic andesite and rhyodacite, occur throughout this province. In this study, we focus on the Ametista do Sul mining district, Rio Grande do Sul, Brazil, the largest producer of amethyst geodes in the world (400t/month). In this district, a silica gossan occurs at the top of the Coogamai flow above the Mina do Museu. This basalt (47.9wt.% SiO2) flow belongs to the intermediate-Ti Paranapanema magma type (2.25wt.% TiO2 and Ti/Y=416) exhibiting columnar jointing. The gossan as prospective guide displays polygonal shape associated with the enrichment of clay minerals at the surface, the concentration of iron oxides and the presence of silicified sandstones. In addition, marshes with ponds and a change in surface color (ocher to brownish) and vegetation type were observed. A gamma spectrometry survey at the Mina do Museu silica gossan showed negative anomalies in %K, eU and eTh above the amethyst deposit with values of 55 counts per second (cps) outside the silica gossan and 43cps within it. These anomalies are caused by hydrothermal events and subsequent weathering of the silica gossan. Bulk rock analyses of the silica gossan demonstrate a loss on ignition up to 1.8wt.% whereas this value is around 0.5wt.% for rocks outside the silica gossan. SiO2, K2O, MgO, CaO and Na2O display negative correlations with loss on ignition. X-ray diffraction proved the presence of smectites and kaolinite in the soil on the silica gossan. In addition, dioctahedral and trioctahedral interlayered smectites were identified. Mg-smectite is present in the silica gossan whereas K-smectite occurs in the Coogamai flow.

Controls on prolate and oblate geode geometries in the Veia Alta basalt flow, largest world producer of amethyst, Paraná volcanic province, Brazil

Variable intensity of hydrothermal alteration of the Veia Alta basalt flow, Ametista do Sul, Brazil, exerted the fundamental control on the shape and size of the amethyst geodes. The loss on ignition (LOI) of the host basalt is used as a proxy for intensity of alteration and has direct relationship with the height of the geodes (up to 150cm in the three study mines) and with the prolaticity of geodes. All rocks with LOI>5wt.% host prolate geodes and all oblate geodes are hosted in rocks with LOI<5wt.%. An additional observation is the extensive mobility of several elements during hydrothermal alteration (variable LOI), including SiO2, K2O and Rb. The hydrothermal origin of the geodic cavities is thus established and their shapes explained by the empirical observation of the results from a previous numerical simulation experiment.

Stratigraphy of amethyst geode-bearing lavas and fault-block structures of the Entre Rios mining district, Paraná volcanic province, southern Brazil

The Entre Rios mining district produces a large volume of amethyst geodes in underground mines and is part of the world class deposits in the Paraná volcanic province of South America. Two producing basalt flows are numbered 4 and 5 in the lava stratigraphy. A total of seven basalt flows and one rhyodacite flow are present in the district. At the base of the stratigraphy, beginning at the Chapecó river bed, two basalt flows are Esmeralda, low-Ti type. The third flow in the sequence is a rhyodacite, Chapecó type, Guarapuava subtype. Above the rhyodacite flow, four basalt flows are Pitanga, high-Ti type including the two mineralized flows; only the topmost basalt in the stratigraphy is a Paranapanema, intermediate-Ti type. Each individual flow is uniquely identified from its geochemical and gamma-spectrometric properties. The study of several sections in the district allowed for the identification of a fault-block structure. Blocks are elongated NW and the block on the west side of the fault was downthrown. This important structural characterization of the mining district will have significant consequences in the search for new amethyst geode deposits and in the understanding of the evolution of the Paraná volcanic province.

Odyssey of an amethyst geode

The National Museum of Scotland in Edinburgh, Scotland (UK) (formerly known as The Royal Museum), reopened in July 2011 after undergoing a massive renovation programme which resulted in the completion of 16 new exhibitions - among them several Natural Science galleries. A huge amethyst geode weighing 2300 kg was chosen as the iconic centrepiece for the gallery entitled Restless Earth. The amethyst was formed in Brazil, was mined and cut there to reveal its stunningly attractively crystals, and then crated and shipped in two parts to the National Museum of Scotland. Before installation it had numerous moves within the museum, was unpacked and required re-crating. Did the geode suffer any damage in transit? Were there problems caused by changing the orientation of the crates during lengthy storage periods? What challenges were faced in handling, storing and installing such a large, heavy, brittle object? Follow the Odyssey of the Amethyst Geode from its formation in the volcanic lavas of Brazil on its 10,000 km journey to the National Museum of Scotland.

Gossan characterization in the Quaraí and Los Catalanes amethyst geode districts (Brazil and Uruguay), Paraná volcanic province, using rock geochemistry and gamma-spectrometry

We report the discovery of a large number (probably thousands) of gossans in the intraplate Paraná volcanic province in South America, based on observations from satellite images and field work supplemented with rock geochemistry and geophysics. We also provide a straightforward prospecting guide for agate and amethyst deposits. The study area is located on the border between Brazil and Uruguay, encompassing the Quaraí mining district and the Los Catalanes gemological district. Anomalies in Google Earth satellite images were identified above six underground mines in the Los Catalanes gemological district, characterized in the pampas of the region as irregular structures of intense green color and sometimes with some brownish tints. These vegetation, gamma-spectrometric, and geochemical anomalies occur at several stratigraphic levels in the volcanics. Three gamma-spectrometric sections performed on the Maurício mine of the Los Catalanes gemological district indicate low emission rates near 55cps (sd=4.7) in the gossan compared with the regional average of Cordillera flow (63cps). LOI higher than 2% is considered a strong indicator of intense hydrothermal alteration in the gossan. In the Quaraí mining district, gossans were studied in five areas, two in the Catalán flow, two in the Muralha flow, and one in Cordillera flow. Negative gama-spectrometric anomalies (higher than one standard deviation) occur in these gossans. The detailed study of one gossan included a gamma-spectrometric grid spacing of 50×50m (K, U, Th and total emission rate) and whole-rock geochemical analyses (ACME, Canada). The whole-rock geochemical analyses (20 samples) collected inside and outside the gossan demonstrate that these rocks are basaltic andesites of the low‐Ti Gramado chemical type. The samples within the gossan have high values of loss on ignition (2.3, 2.8, 2.9, 2.8, 2.9, 2.4, 2.6, 2.6, 2.3 and 2.3wt.%), whereas outside the gossan the values are lower (0.8, 2.3, 0.5, 0.5, 1.6, 0.5, 0.6, 0.9, 1.3 and 1.9wt.%). SiO2, K2O and Rb show strong negative correlations with a loss on ignition, whereas MgO shows a slight enrichment. The low gamma-spectrometric signal in the gossan is defined primarily by the lower K2O of the altered rock. We thus report the discovery of gossans above amethyst geodes in the world-class deposits of the Paraná volcanic province and present a first description of the structure. A straightforward prospecting guide will facilitate future exploration for new deposits.

Geochemical stratigraphy of lavas and fault-block structures in the Ametista do Sul geode mining district, Paraná volcanic province, southern Brazil

The Paraná volcanic province is the world's largest producer of amethyst geodes, mostly from the Ametista do Sul mining district (20 by 30km) in southern Brazil. The description of the lava flows in the Ametista do Sul mining district, and adjacent areas, combined with flow-by-flow geochemistry and scintillometry, defines the stratigraphy of the flows. Nine flows were identified in the Ametista do Sul region and four additional flows were identified in the Frederico Westphalen region, totalizing 13 flows in the stratigraphy of the Ametista do Sul mining district. Six of the flows belong to Pitanga magma type (more than 3wt.% TiO2); which occur at the base of the local stratigraphy and the other seven flows belong to Paranapanema magma type (2–3wt.% TiO2) at the top of the stratigraphy. The amethyst-bearing geodes occur in the uppermost three Pitanga flows, known as Veia Alta (main producer), Veia do Meio and Veia Baixa. These flows were highly altered (more than 60vol.%) to clay minerals by a low temperature alteration process. The massive presence of clay minerals is an important component that controls the rheology of the rock, transforming the basalt into a very low grade smectite and celadonite metabasalt. Two Paranapanema flows, COOGAMAI and Linha Alta, from Ametista do Sul were identified in a lower position, at the base of the section in Frederico Westphalen which were correlated using scintillometry and flow-by-flow geochemistry. In addition, Veia Alta and Veia do Meio flows were also identified in lower positions, compared to Ametista do Sul. Thus, the identification of these flows in different places with different elevations is an evidence of the presence of downthrown blocks in a fault-block structure, lowering the stratigraphy more than 200m to the west in four steps. The tectonic patterns that generate this structure are three NW-strike lineaments. The identification of the COOGAMAI flow at the base of the Frederico Westphalen region, suggests that the Veia Alta geode-hosting flow is below the ground or below the elevation of 250m in this region. These evidences show that the fault-block structure is controlling vertically the deposit of amethyst-bearing geodes and is of major significance in the exploration for new amethyst geode deposits in the region.

Numerical simulations of amethyst geode cavity formation by ballooning of altered Paraná volcanic rocks, South America

AbstractNumerical modelling by finite element methods provides two significant insights into the formation of the giant amethyst geodes of the Paraná volcanic province: the conditions needed to open the cavities and the conditions that control their size and shape. Giant amethyst geodes were formed in the Cretaceous (135 Ma) in altered volcanic rocks by water vapour pressure (Δp) at about 0.5 MPa under an altered basalt cover of 5–20 m. Only rocks with Young’s modulus values (E) in the range 1–2 GPa can sustain ballooning, which is the growth of a cavity in a ductile medium by the pressure of water and its vapour. The size of the proto‐geode is dependent on the water vapour pressure, which is directly related to thickness of the overlying basalt. Varying the yield points causes the formation of either prolate or oblate cavities. A low transition point (smaller than 0.18 MPa) generates a prolate‐shaped cavity, whereas a high transition point (larger than 0.18 MPa) generates oblate proto‐geodes. Proto‐geodes are smaller when Young’s modulus is higher (rock is less altered) or when water vapour pressure is lower (because of thinner overburden of basalt). The calculations are an indication that the processes operative in the altered basalts led to the opening of giant cavities by ballooning.

Flow-by-flow chemical stratigraphy and evolution of thirteen Serra Geral Group basalt flows from Vista Alegre, southernmost Brazil

The geochemical characterization of thirteen Serra Geral Group flows in the Vista Alegre region (RS-SC), southern Brazil, displays the homogeneous basaltic composition near 50 wt.% SiO2. Each of the five basal flows (Pitanga-type, high-Ti/Y ~600, TiO2 &gt; 3 wt.%) and eight upper flows (Paranapanema-type, medium Ti/Y ~400, TiO2 &gt; 2 wt.%) can be identified from their chemical composition; sets of flows have parallel variation in chemical composition. The flow-by-flowcorrelation in four sections shows the horizontal position of the flows in three profiles and an approximately 200-m downdrop of the Itapiranga block with respect to the Frederico Westphalen block. The world-class amethyst geode mineralization and the systematic presence of native copper in the basalts make the correlation of great geological and economic significance.

Stable isotope and mineralogical investigation of the genesis of amethyst geodes in the Los Catalanes gemological district, Uruguay, southernmost Paraná volcanic province

Stable isotopes (C, O, S) and mineralogical studies of the world-class amethyst-geode deposits of the Los Catalanes gemological district, Uruguay, constrain processes operative during mineral deposition. The mineralized basaltic andesites from the Cretaceous Parana volcanic province are intensely altered to zeolites (clinoptilolite) and clay minerals. Variations in the δ18O values of silica minerals in geodes (chalcedony, quartz, and amethyst) are much larger and the values generally somewhat lower (21.2–31.5‰) in the Uruguayan deposits than in the Ametista do Sul area of southern Brazil. The range of δ34S values (−15.0 to −0.3‰) of altered basaltic rocks requires (in addition to sulfur of magmatic origin) the involvement of 34S-depleted sedimentary sulfur from bacterial sulfate reduction. The results delimit the mineralizing processes to a post-eruption environment characterized by low temperature and strong interaction of the lava flows with meteoric water.

Sequential opening and filling of cavities forming vesicles, amygdales and giant amethyst geodes in lavas from the southern Paraná volcanic province, Brazil and Uruguay

The opening and filling of cavities in rocks are the major processes related to the generation and sealing of porosity in ore deposits. This study documents three stages of opening and filling of vesicles and geodes in the basalts and rhyodacites of the southern Paraná volcanic province. Each step detailed here is actually part of a sequence of minor hydrothermal events. First, lava degassing at high temperature (1150°C) formed small (<4 cm) vesicles in the crusts of flow units. In sequence, these vesicles were partly to fully filled at low temperature (30–150°C) by hydrothermal minerals, particularly clays and zeolites; this process also sealed the porosity of the lava. Second, the injection of fluidized sand generated new cavities, which were partly filled with sand; the newly formed porosity was sealed by the low-temperature fluid. Third, intense alteration of the basalt or rhyodacite core into a claystone favoured the opening of small to giant protogeodes (0.1 mm to 4 m) by dissolution; cooling of the fluid led to the precipitation of hydrothermal minerals, particularly the spectacular amethyst, calcite, and gypsum-bearing geodes.

Geochemical and scintillometric characterization and correlation of amethyst geode-bearing Paraná lavas from the Quaraí and Los Catalanes districts, Brazil and Uruguay

AbstractGeochemical studies of the six lowermost lava flows of the Cretaceous Serra Geral Formation (Paraná volcanic province) in Quaraí (Brazil) and Artigas (Uruguay) were combined with flow-by-flow field studies of structures and scintillometric profiles to establish a consistent regional stratigraphic framework over at least 100 km. This greatly improves exploration capability for amethyst and agate geodes. A basalt, colada Mata Olho (Alegrete facies, Serra Geral Formation), was the first lava to flow over the ancient Botucatu desert in the region, but an andesite, colada Catalán, overstepped this basalt in many places, perhaps palaeohighs. Four basaltic andesites complete the lava stratigraphy in this formation, adding up to 300 m of lavas. The stratigraphic sequence of contrasting lava compositions is 51.0 wt% SiO2 in the first lava, followed by 57.5, 52.5, 56.0, 53.0 and finally 54.5 wt% SiO2. Overall MgO variation is between 2 and 7 wt%. All lavas in the two districts are low-Ti (&lt;2.0 wt% TiO2) of the Gramado type. The characteristic contents of most major and trace elements (124 rock samples analysed) allow the ready identification of each lava. Contrasting rock chemistry also results in strong variation in scintillometric values (270 points measured in the field and nineteen continuous borehole profiles); from bottom to top of the stratigraphy, the cps values are 49±3.2, 123±10.3, 62±4.7, 94±4.6, ~45 and ~85. Colada Catalán has the structure of aa lava, particularly the contorted igneous banding and autobreccias in the upper and lower crusts. In some places, a 2 m thick, silicified sandstone layer lies on top of some coladas, and silicified sandstone forms breccias with volcanic rocks. Geochemistry of the six lavas indicates complex evolution, involving melting of lithospheric mantle, injection into the crust and assimilation of crust followed by fractional crystallization. This study indicates the possibility of world-class deposits of amethyst geodes on the Brazilian side of the border with Uruguay.

Petrography and chemistry of SiO2 filling phases in the amethyst geodes from the Serra Geral Formation deposit, Rio Grande do Sul, Brazil

Abstract The filling process of amethyst-bearing geodes from Serra Geral Formation basalts, Brazil, is investigated by different methods performed on the SiO2 filling phases. Image analysis of quartz–amethyst deposits suggests a single growing mechanism ruled by geometric selection of randomly oriented crystals. Microthermometry of fluid inclusions reveals formation temperature lower than 100 °C, probably lower than 50 °C, and fluid salinity as high as 3 mass% NaCl eq. Composition in REE and trace-elements measured by ICP-MS on acid-digested or laser-ablated samples indicates a common genesis for amethyst, quartz and chalcedony, as well as the absence of significant variations from one geode to another. 87Sr/86Sr data on chalcedony shows that both the host basalt or the Botucatu sandstone are possible silica sources. These data, combined with thermo-kinetic considerations, permit us to discuss the filling process. We argue in favor of the contribution of a mineralized fluid of hydrothermal origin producing a regional silica source which decreased with time. The observed mineral sequence is related to the depletion of silica in the solution.