Silicified Sandstones Research Articles

Integrated hydrothermal evolution of a Botucatu paleodune and a blanketing lava flow from the Serra Geral Group, Uruguaiana

Abstract The integrated hydrothermal evolution of the large Santa Otília Paleodune from the Botucatu paleoerg and the cover Catalán Flow from the Serra Geral Group is here evaluated in Uruguaiana, southern Paraná Basin. Methods included satellite images, field surveying, petrography, and U-Pb geochronology of detrital and volcanic zircon. The tops of the compound linear-barchan paleodunes are exposed in the topographic highs. Rhombi and bowls at the top of the silicified sandstone units are significant, as is the poor preservation of eolian layering. The fishbone-shaped dune tops are surrounded by the lavas. The oxidized top of the flow (upper Tier 2) overlies the reduced base (Tier 1), which consists of exposed, massive rocks in creek beds that are light gray-colored and contain small (10 cm) agate geodes and numerous silicified sandstone dikes. The youngest dated zircon grains from the sandstone are 225–300 Ma old (one 175 Ma age), and the main older age peaks are 450–600 Ma (predominant), 900–975 Ma, 1800–1900 Ma, and 2500 Ma; few Archean 2.5 Ga ages. The strongly altered volcanic zircon preserved in the reddened Catalán Flow yields a concordia-intercept age of 137.3 ±7.6 Ma. Complex processes are thus constrained in the evolution of the Botucatu Formation and the first Serra Geral lava flow.

The Aerogeophysical Atlas of Tocantins State: Synthesis of Magnetic Anomaly And Radioelement Maps With Emphasis on Regional Signatures and the Serra Da Cangalha Impact Structure

The Tocantins State Aerogeophysical Atlas (TSAA) is an institutional product of the Geological Survey of Brazil. It is aimed at promoting the use of airborne magnetic and gamma-ray spectrometry data to support geophysical and geological research at state level. Here, we present a synthesis of the TSAA with special emphasis on the interpretation of the magnetic (Anomalous Magnetic Field, First Vertical Derivative (FVD), and Total Gradient Amplitude (TGA) images) and radiometric (RGB composite color map of concentrations, Inverse Images of K, eTh, and eU, and Normalized eU enhancement) map. In particular, the radiometric data for the Serra da Cangalha impact structure in northeast Tocantins are discussed. For Tocantins State, the results show that the radiometric and/or magnetic signatures of geological units (e.g., Mosquito Formation basalts; granite-gneiss/migmatite terrains) and structural features (e.g., dykes and lineaments), or related exogenous deposits (Serra Geral do Tocantins quartzose soils; alluvial deposits of main drainage channels) can be visualized well on regional-scale maps. Analysis of radiometric ratios for the area of the Serra da Cangalha impact structure indicates that eU concentrations are enriched in the annular basin around the central uplift structure, and in the zone straddling the contact between the Piauí and Pedra de Fogo formations. In the latter case, the elevated eU concentrations could be related to brecciated chert layers or silicified sandstones.

Black adzes in the Early Neolithic of Belgium: Contribution of the Raman microspectrometry and petrography in characterization and sourcing

Early Neolithic (Linear Pottery Culture) adzes originate from settlements and workshops accompany the neolithization of Belgium. They are made from a wide range of extraregional lithic raw materials such as metamorphic green rocks (amphibolite) and black volcanic rocks (“basalt’) beside more local or regional raw material as flints, light-coloured (sedimentary and lightly metamorphic) quartzites, black lydites (Cambrian nodular phtanite of Céroux-Mousty and Lower Namurian banded phtanites) and dark grey Lower Namurian silicified sandstones previously called “Micaceous sandstones of Horion-Hozémont’. The discovery of the workshop of Noirfontaine near the city of Liège in the 1970s and 1980s provides exceptional assemblage available for updating analytical studies. This research focuses on the multi-scale characterization, the discrimination and sourcing both Cambrian and Namurian black sedimentary rocks rich in secondary silica composing Early Neolithic adzes found in Belgium. Their black colour results from finely dispersed organic matter, but the absence of palynomorphs does not allow a biostratigraphic ascription. Additional petrographical analyses (Optical Petrography, Scanning Electron Microscope), X-ray diffraction, chemical analyses (Energy Dispersive Spectroscopy) and measuring the degree of graphitization of the organic matter through Raman microspectrometry have been decisive in identifying the geological and geographical provenances by comparing the acquired results with geological reference samples collected in the field or through reference collections. Cambrian lydites are coming from a very restricted area and were preferred to other more local rock sources.

Influence of hydrothermal silicification on the physical properties of a basin-boundary fault affecting arkosic porous sandstones, Rio do Peixe Basin, Brazil

Hydrothermal silicification and deformation bands influence the physical properties of porous siliciclastic rocks. However, the impacts generated by the coexistence of these two processes on fault zone flow properties, such as porosity and permeability reductions, are still debatable. We integrated structural, geomechanical, and petrophysical data to investigate the influence of hydrothermal silicification on the physical properties of a fault zone. The fault affects Precambrian crystalline rocks and porous sandstones in the Rio do Peixe Basin, northeastern Brazil. The results indicate that quartz cementation is confined to a halo along the fault. Silicification decreases away from the fault zone toward the basin, which we subdivided into three main zones: (1) nonsilicified sandstone – Zone I, (2) moderately silicified sandstone – Zone II, and (3) intensely silicified sandstone – Zone III. The elongated geometry, the thickness of the silicified body, the proximity to the fault, and the high silica cement concentration indicate an external silica source. Nevertheless, we also propose internal sources related to feldspar dissolution. The primary porosity is rarely preserved in Zone III, while the secondary porosity is filled with silica cement. The primary sandstone porosity is well-preserved in Zone II, while many dissolved feldspar grains are not filled with cement. The petrographic and petrophysical analysis (cataclastic matrix amount) indicates that cement precipitation also occurred inside the deformation bands. Silicification increased the uniaxial compressive strength (UCS) by one order of magnitude in the fault damage zone relative to the protolith. On the other hand, the porosity and permeability in the silicified fault zone exhibit a reduction of two and four orders of magnitude, respectively, relative to the undeformed and nonsilicified protolith. This study indicates that silicification along a fault zone has crucial implications for the reservoir properties of siliciclastic rocks, strongly reducing the fluid flow properties and increasing the UCS values.

Prehistoric Stone Raw Materials from the Bükk Mountains in Northeastern Hungary

From the period of the Neanderthals to those of the Late Neolithic populations, the Bükk Mountains region played an important part in the lives of various prehistoric societies, and the varied geological history of this territory provided distinct circumstances for the production of stone implements. The major goal of our research is to outline the current state of information concerning the prehistoric use of the diverse silicified source materials of the Bükk mountains. The results of these studies are presented concerning four selected local rock types, cited in the archaeological literature as the silicified sandstone of Egerbakta, the radiolarite and hornstone (black chert), the silicified marlstone and the quartz-porphyry (metarhyolite). Except for the latter, little attention has previously been paid to studying them in detail. Our new petrographic analyses revealed two variants of the raw material from Egerbakta: a silicified sandstone and a diatomaceous detrital chert. The other samples turned out to be radiolarites. This result confirmed what was already suggested for the hornstones (black cherts), however, it has new consequences for the “silicified marlstone”. Regarding the prehistoric use of the selected raw materials, archaeological data show interesting dynamics through time. The Mousterian groups of the region used a large spectrum of rocks for lithic tools with a preference for hornstone (black cherts) and radiolarite in the southern part, and the quartz-porphyry (metarhyolite) in the northeastern part. The Bábonyian/Micoquian assemblages are characterized by a bifacial toolkit and an apparent preference for quartz-porphyry (metarhyolite). At the beginning of the Upper Palaeolithic, a new attitude appears in the region: the Aurignacian groups almost completely ignored local sources. During the Neolithization of the Carpathian Basin, the lithic assemblages of the Alföld Linear Pottery culture became more and more habituated to the locally available rock types, albeit their raw material economy was based on limnosilicites and obsidian. Another change took place during the Late Neolithic when supra-regional sources became dominant over local or regional sources.

Measuring the Effectiveness and Durability of Silicified Sandstones and Conglomerates from Zamora, Spain Subject to Silico-organic Treatments and/or Freezing/Thawing Processes

This work was carried out on fresh samples of silicified sandstone and white conglomerate quarry from Zamora, Spain. The stones were treated with conservation treatments (H224, RC70, and RC80) and/or processes of freeze/thaw aging (similar to that of cold areas). The values obtained for the chromatic coordinates (L*, a*, b*) and the ultrasound propagation speed of the samples studied were analyzed and a statistical design was created. In addition, a multivariate technique (Canonical Biplot) was applied to determine the effectiveness of the treatments and the durability of the fresh and treated stones. In most of the cases studied, the ultrasonic propagation speed decreased in both the untreated samples and those treated with the conservation products after carrying out the aging process. Moreover, it was found that the ultrasonic propagation speed was lower with respect to the treated samples. Our results indicate that the treated samples are more durable than the untreated fresh samples and that the most severe changes can be appreciated when the untreated (fresh) samples are compared with their corresponding treated samples. For these samples, the ultrasonic propagation speed increased showing the effectiveness of each of the conservation products. The most effective consolidating treatment used was RC80. Also, the chromatic coordinates of the treated samples, in general, were darker (↓L *), redder (↑a *), and more yellow (↑b *) with respect to the fresh, untreated quarry samples, except in sample Z1 treated with RC80, where redness (↓a *) and yellowness (↓b *) decrease.

Asymmetric shock deformation at the Spider impact structure, Western Australia

AbstractThe distribution of shock deformation effects, as well as the structural expression of an impact structure, can be asymmetric, depending on target rock lithologies (e.g., layered versus homogenous), porosity of target rock, and angle of impact. Here, we present a detailed study of shock‐deformed quartz and zircon in silicified sandstones from the asymmetric Spider impact structure in Australia. We utilize optical microscopy and electron backscatter diffraction (EBSD) techniques in order to determine the spatial distribution of shock‐deformed zircon along a downrange transect across the central uplift of the structure, with the goal of constraining the physical distribution of shock effects created by an oblique impact. A total of 453 zircon grains from 12 samples of shatter cone‐bearing quartzite and breccia within the structure were surveyed for shock deformation by EBSD in situ within thin sections. Nineteen zircon grains contain {112} twins, including one grain with three twin orientations. Quartz grains from five samples along the transect were also surveyed using a universal stage in order to determine orientations of planar deformation features, planar fractures, and feather features, and to provide a baseline for comparison of data from zircon. The distribution of shocked zircon with {112} twins within the samples surveyed appears to be asymmetric relative to the center of the structure, in contrast to quartz, thus providing the first accessory mineral‐based evidence that supports an asymmetric distribution of shock deformation as a function of impact obliquity. Our results are an example where the highest intensity of observed shock deformation does not correspond to the geographic center of the structure, and may serve as a guide for field studies aimed at documenting the distribution of shock effects at other sites interpreted to result from oblique impacts.

Lithic Landscapes and Early Inhabitants in Southeastern Brazil: First Perspectives from a Case Study in Dourado, São Paulo State

ABSTRACTLithic landscapes are formed by minerals and rocks suitable for human purposes and also by the knowledge and transformations involved in their use. The Dourado region, located in the basaltic hills of central São Paulo state, southeastern Brazil, offers the possibility to study lithic landscapes explored and transformed by hunter-gatherers since at least ca. 12,640 cal yr BP. This article evaluates the available silicified sandstone sources and the way people used some of those, through a survey conducted around the Bastos site. A macroscopic and a thin-section study of source samples, as well as a technological analysis of surface artifacts, were performed. We report a lithic landscape characterized by extractions on silicified sandstone outcrops and clasts of very good-to-excellent knapping quality, and the by-products of initial technological activities. Implications for terminal Pleistocene/early Holocene human use of lithic landscapes of eastern South America are discussed.

X-ray Microtomography to Quantify Morphological Sandstones Properties

Abstract The sandstones of the Furnas Formation in the State of Parana, south of Brazil, are located in regions with great importance due to the natural heritage found in the rocky forms and associated flora and fauna. This paper presents a detailed analysis of the internal structure of diverse sandstones of the Furnas Formation. X-ray microtomography allowed 3D characterization of the number, pore size and shape distributions of the sandstones. Images with voxel size of 603 µm3 were utilized to evaluate the sandstone’s macroporosity. The main results obtained indicate that higher macroporosities were observed in coarser and some types of silicified sandstones. In general, fine sandstones were characterized by the largest number of pores. As a general conclusion the size and shape distribution of pores seem to have a variable relation with genetic factors as grain size and cement (siliceous and/or ferruginous).

First record of palynomorph assemblages from the Arshad Sandstone (Late Cretaceous), Sirte Basin, north-central Libya

This paper presents the first record of identifiable palynomorph assemblages from the Arshad Sandstone, Sirte Basin, north-central Libya. Ninety core samples from five different wells were macerated to isolate palynomorphs, but 78 samples turned out to be barren. Among the 12 productive samples, the preservation of palynomorphs was generally poor to fair, limiting identifications at species level. However, despite these limitations workable assemblages were recovered from a few productive samples. Since palynomorph preservation is poor, many forms could only be identified at the generic level and some were unidentifiable. A few stratigraphically significant dinoflagellate cysts were identified at the species level. Palynomorph diversity is fairly high including both terrestrial (angiosperm and gymnosperm pollen grains, spores, algal and fungal remains) and marine (dinoflagellate cysts, acritarchs and foraminiferal remains) forms. The lithology of the Arshad Sandstone is quite diverse; variables include very hard and silicified sandstones, massive shales, sandy dolomites, and dolomitic sandstones. Characteristically all these sediments are massive, very hard, and occasionally bioturbated. No identifiable or datable macro- or mega-fossils are known from this stratigraphic unit, and previous attempts to isolate palynomorphs have been unsuccessful. The present palynological study indicates that these sediments were deposited in diverse environments ranging from nearshore terrestrial to shallow marine (inner shelf). Dinoflagellate cyst species indicate that the Arshad Sandstone is of Campanian–Maastrichtian age.

New record of lithic blades in Brazil: The Picão site, São Paulo state

Lithic blades are long and narrow flakes produced from prepared cores which form part of different technological complexes all around the world. In South America, the production and use of blades has been reported in different settings which include early hunter-gatherer occupations of the Pleistocene-Holocene transition, as well as Middle and Late Holocene occupations. Until now, the only reference to blade technology in Brazil comes from the south and corresponds to Early Holocene hunter-gatherer assemblages. This paper presents a second record of lithic blades in Brazil, which includes one fragment and three refitted artifacts manufactured from fine-grained silicified sandstone recovered from a surface site (Picão), which is located in the Dourado Municipality, central São Paulo state. Analysis undertaken on the blades involved the collection of metric data, including longitudinal curvature, and the elaboration of diacritic schemes which consider the direction and order of the observed remnant scars for interpreting the chronological sequence of removals. Results indicate the production of ³70.5 mm-long, softly curved blades, with evidence of preparation of the point of impact prior to extraction from the core, as well as the presence of unidirectional, parallel scars related to blade production from a single striking platform. These reduction characteristics resemble those encountered in the nearest contexts of southern Brazil and northern Uruguay, although differences also exist. In spite of the lack of chronological data, this new record of blades in central São Paulo state calls for more research into the origins and dispersion of this technology in southeastern South America.

Modelo hidrogeológico conceitual do sistema aquífero Urucuia centro-ocidental, Brasil

O Sistema Aquífero Urucuia (SAU) representa um conjunto de aquíferos inter-relacionados associados às rochas sedimentares do Grupo Urucuia. Cinco perfis geofísicos de poços foram interpretados e subsidiaram a elaboração do modelo hidrogeológico conceitual. Três eletrofácies são identificadas: (i) substrato regional do Grupo Bambuí; (ii) sistema eólico da Formação Posse e; (iii) sistema fluvial entrelaçado da Formação Serra das Araras segmentados por níveis tabulares de arenitos maciços e silicificados. No contexto hidrogeológico, a posição estratigráfica dos níveis de arenitos silicificados em relação àquelas do nível freático regional condiciona a ocorrência de aquíferos não-confinados quando disposto acima, e um sistema aquífero-aquitarde-aquífero quando abaixo. Nesse contexto, o aquífero confinado inferior apresenta cargas hidráulicas maiores do que as verificadas no livre sobrejacente. Os horizontes de arenitos silicificados funcionam como aquitardes e dispõem-se mais profundos e espessos na porção centro-ocidental. Adicionalmente, os valores da porosidade efetiva e coeficiente de armazenamento variam entre 10-2 a 10-1 e 10-4. Já as transmissividades variam entre 10-4 a 10-6 e 10-3 m2/s para os subtipos não-confinado e confinado, respectivamente. No extremo ocidental, o SAU exibe poços cujos níveis estáticos são profundos (acima de 100 m) como consequência de um divisor de água subterrânea na bacia hidrogeológica. Em geral, os rebaixamentos produzidos, após os ensaios de bombeamento dos poços, diminuem progressivamente em direção à porção centro-ocidental do SAU, aliado a um aumento da capacidade específica dos poços, da espessura saturada e da potencialidade hidrogeológica.

Pyrophyllite formation in the thermal aureole of a hydrothermal system in the Lower Saxony Basin, Germany

A combined clay mineralogical, fluid inclusion, and K-Ar study of Upper Jurassic metasediments at the Gehn (Lower Saxony Basin, Germany) provides evidence for a transient hydrothermal event during Upper Cretaceous basin inversion centered on a prominent gravimetric anomaly. Kaolinite and smectite in Oxfordian pelitic parent rocks that cap a deltaic sandstone unit were locally transformed into pyrophyllite, 2M1 illite, R3 illite–smectite, chlorite, and berthierine at the Ueffeln quarry. The pyrophyllite-bearing metapelites lack bedding-parallel preferred orientation of sheet silicates and experienced peak temperatures of about 260–270°C consistent with microthermometric data on quartz veins in the underlying silicified sandstones. The presence of expandable layers in illite–smectite and high Kubler Index values indicate that the thermal event was rather short-lived. K-Ar dating of the <0.2 μm fraction of the pyrophyllite-bearing Ueffeln metapelite yields a maximum illitization age of 117 ± 2 Ma. Lower trapping temperatures of aqueous fluid inclusions in quartz veins and the absence of pyrophyllite in metapelites of the Frettberg quarry in a distance of about 2.5 km from the Ueffeln quarry infer maximum paleotemperatures of only 220°C. The highly localized thermal anomaly at Ueffeln suggests fault-controlled fluid migration and heat transfer that provided a thermal aureole for pyrophyllite formation in the metapelites rather than metamorphism due to deep burial. A pH neutral hydrothermal fluid that formed by devolatilization reactions or less likely by mixing of meteoric and marine waters that interacted at depth with shales is indicated by the low salinity (3–5 wt. % NaCl equiv.) of aqueous inclusions, their coexistence with methane–carbon dioxide-dominated gas inclusions as well as carbon, hydrogen, and oxygen isotope data. The upwelling zone of hydrothermal fluids and the thermal maximum is centered on a gravimetric anomaly interpreted as an igneous intrusion (‘Bramsche Massif’) providing the heat source for the intrabasinal hydrothermal system.

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.

Regard typo-technologique sur les productions lithiques foliacées du Solutréen de la vallée de l’Erve (Mayenne, France)

Regard typo-technologique sur les productions lithiques foliacées du Solutréen de la vallée de l’Erve (Mayenne, France)

Geological relationships of basalts, andesites and sand injectites at the base of the Paraná volcanic province, Torres, Brazil

The Cretaceous interaction of the heated, giant Guarani aquifer with lavas, dikes and sills of the Paraná volcanic province is here evaluated based on the description of the large exposures in Torres, southernmost Brazil. Chalcedony silicified sandstone dikes, sills and breccias containing volcanic clasts in a sandstone matrix are common in the Paraná volcanic province. Sandstone layers mark either the contact between lava flows or the base of the vertically-jointed, massive core. In Torres, one basaltic andesite and one andesite lava flow of the low-Ti Gramado type, and basalt and basaltic andesite sills and dikes, are correlated over 10km with rocks in the Graxaim quarry. Scintillometric emission rates vary systematically with contents of K, Th and U and are characteristic of each lava or intrusive rock. Major and trace elements are also characteristic of each lava. Three large outcrops of breccias display angular, amygdaloidal volcanic clasts immersed in a silicified sandstone matrix. No evidence was found of high temperature (1150°C) interaction of the lava with the loose, erg of the Botucatu Formation, such as fluidal volcanic clasts, quenched rims or thermal recrystallization of the sand; only a thin (5mm) layer of thermally modified sand is present on top of a paleodune in Torres.In Torres, the interaction of andesite with wet sand occurred after degassing of the lava, because the breccia clasts are amygdaloidal. All evidence points to hydrothermal processes for the formation of the breccias, such as lozenge fractures filled with silicified sandstone, preserved detrital characteristics of the sandstone grains and cold (below 130°C) contact between the volcanic rock and the sand. Most significant is the timing of sand intrusion, because thin (1mm) dikes of silicified sandstone cut fractures in the rock and the zeolite-filled amygdales. The sandstone dikes, sills and breccias are interpreted as a result of the ascent of overpressured heated water and its vapor from the huge, underlying Guarani aquifer, carrying fluidized sand and continuously silicifying the intruded sand. The three breccias are sills intruded into cold lava flows. The description and correlation of the magmatic bodies and the field relationships of the silicified sandstones establish the stratigraphy of the Paraná volcanic province in Torres and elucidate the processes responsible for the injection and effusion of sand in the Cretaceous.

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.

Silver Mound, Wisconsin: Source of Hixton Silicified Sandstone

In 2006, the prehistoric quarry complex at Silver Mound in western Wisconsin was designated a National Historic Landmark. As the sole source of Hixton Silicified Sandstone, Silver Mound’s landmark designation exemplifies the important role played by high quality lithic raw material sources during the initial peopling of eastern North America. An updated and comprehensive description of archaeological resources associated with the quarry complex is provided. As a compliment to the descriptive portion of the paper, we present a framework outlining the current research potential of Silver Mound in the context of the Paleoindian Tradition. Central to this framework is the integration of site specific data with analysis of the structure and organization of lithic technological systems. This research framework operates more generally as a guideline for evaluating the Paleoindian use of primary quarry areas throughout the Midcontinent.

Withington (47Gt158): A Clovis/Gainey Campsite in Grant County, Wisconsin

Located within the driftless area of southwestern Wisconsin, the Withington site provides a unique opportunity to examine the adaptive responses of Early Paleoindians exploiting this region. With a lithic assemblage dominated by Hixton Silicified Sandstone from sources over 170km to the north, chipped stone technological attributes and raw material diversity suggest that the occupants of the site were highly mobile, employed a mixed organizational strategy of toolstone transport, and possessed a wide knowledge of the lithic and social landscape. While our recent understanding of the early archaeological record of this region has progressed, a lack of published reports continues to hinder the incorporation of data from this area into larger models of Early Paleoindian subsistence and settlement. As part of an on-going research effort to address this, I present here a report and analysis of lithic material recovered from the site.

Gold Mineralization at the Kyaukpahto Mine Area, Northern Myanmar

Abstract. Gold mineralization at Kyaukpahto occurs as a stockworks/dissemination style with localized breccia zones in silicified sandstones of the Male Formation (Eocene). The mineralization appears to be closely associated with NNE‐SSW trending extensional faults probably related directly to the dextral movement of the Sagaing Fault system. Intense silicifica‐tion associated with sericitization, argillic alteration and decalcification is recognized in the Kyaukpahto gold deposit. The important ore minerals associated with the gold mineralization are pyrite, arsenopyrite and chalcopyrite with minor amounts of other sulfides. Gold occurs as free particles or locked with pyrite, arsenopyrite, chalcopyrite and tetrahedrite. Silver, copper, arsenic and antimony particularly appear to be good pathfinders and the best geochemical indicators of gold mineralization at Kyaukpahto. Electron microprobe analysis indicates that the fineness for the native gold ranges from 844 to 866. Present geological, mineralogical and geochemical investigations demonstrate that the Kyaukpahto gold deposit has been formed as a result of hydrothermal processes in a shallow level epithermal environment.