Underground coal fires commonly metamorphose or melt surrounding rocks at temperatures exceeding 1000°C. Numerous “baked” sandstone clinker deposits occur in the Cretaceous sedimentary rocks exposed in the San Rafael Swell, UT. This study examines clinker in three main localities: 1) East Carbon, UT, 2) Helper, UT, and 3) Emery, UT. The extent of pyrometamorphism in these areas is variably developed, but reached high enough temperature in Helper, UT to initiate melting and the production of paralavas. These paralavas were examined compositionally and mineralogically to determine melting conditions, peak temperatures, and mobility of different metals as a result of pyrometamorphism. X-ray diffraction and petrographic analysis showed that paralavas in the Helper locality contain the high temperature SiO2 polymorphs tridymite and cristobalite which alone indicate temperatures exceeding 875°C in several samples. Paralavas containing diopside+tridymite and cordierite+mullite+cristobalite provide more restrictive estimates of temperature as they form cotectic and eutectic assemblages in the SiO2-Mgo-CaO and SiO2-MgO-Al2O3, respectively. The assemblages indicate minimum temperatures of melting and metamorphism of 1330–1465ºC. The high temperatures of the paralavas generate increased metal mobility, potentially signifying a hazard if leached out into the environment

Made available in DSpace on 2015-03-13T17:08:39Z (GMT). No. of bitstreams: 1 LarissaS_DISSERT.pdf: 4305557 bytes, checksum: 7f5031ea79015e9573b6354a457ad4a9 (MD5) Previous issue date: 2013-12-19 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / Pyrometamorphism results from conditions of high temperatures and very low pressures provoked by the intrusion of hypabyssal basic bodies into sedimentary or metassedimentary hosting rocks. The onshore portion of the Potiguar Basin in NE Brazil offers examples of this type of metamorphism nearby the contacts of Paleogene to Neogene plugs, sills and dikes of diabases and basalts crosscutting sandstones, siltstones and shales of the A?u Formation (Albian-Cenomanian). The thermal effects over these rocks are reflected on textures and minerals assemblages that characterize the sanidinite facies of metamorphism, often with partial melting of the feldspathic and mica-rich matrix. The liquid formed is potassic and peraluminous, with variably colored rhyolitic glass (colorless, yellow, brown) comprising microcrystals of tridymite, sanidine and clinoenstatite, besides residual detrital clasts of quartz and rarely zircon, staurolite and garnet. Lenses of shale intercalated within the sandstones display crystallites of Fe-cordierite (sekaninaite), mullite, sanidine, armalcolite (Fe-Ti oxide) and brown spinel. The rocks formed due to the thermal effect of the intrusions are called buchites for which two types are herein described: a light one derived from feldspathic sandstone and siltstone protoliths; and a dark one derived from black shale protoliths. Textures indicating partial melting and minerals such as sanidine, mullite, tridymite and armalcolite strongly demonstrate that during the intrusion of the basic bodies the temperature reached 1,000-1,150?C, and was followed by quenching. Cooling of the interstitial melts has as consequences the closure of pores and decrease of the permeability of the protolith, which varies from about 17-11% in the unaffected rocks to zero in the thermally modified types. Although observed only at contacts and over small distances, the number of basic intrusions hosted within the Potiguar Basin in both onshore and offshore portions leaves opened the possibility of important implications of the thermal effects over the hydrocarbon exploration in this area as well in other Cretaceous and Paleozoic basins in Brazil / Pirometamorfismo ?gneo resulta de condi??es de altas temperaturas e baix?ssimas press?es provocadas pela coloca??o de corpos b?sicos hipabissais em encaixantes sedimentares ou metassedimentares. A Bacia Potiguar (NE do Brasil), em sua parte continental, oferece exemplos deste tipo de metamorfismo nas proximidades e contatos de plugs, soleiras e diques de diab?sios do Pale?geno e N?ogeno, intrusivos em arenitos e folhelhos da Forma??o A?u (Albiano-Cenomaniano). O efeito termal nestas rochas se reflete em texturas e associa??es minerais que caracterizam a f?cies sanidinito, com frequente fus?o parcial da matriz feldsp?tica a mic?cea. O l?quido formado pela fus?o ? pot?ssico e peraluminoso, formando um vidro riol?tico de cores variadas (incolor, amarelo, marrom) englobando microcristais de tridimita, clinoenstatita e sanidina, al?m de clastos de quartzo e raramente zirc?o, estaurolita e granada. Lentes de folhelho, intercaladas nos arenitos, cont?m cristalitos de Fe-cordierita (secanina?ta), mullita, sanidina, armalcolita (?xido de Fe e Ti) e espin?lio marrom. As rochas formadas devido ao efeito termal das intrus?es se chamam buchitos, sendo aqui descritos os tipos claros (prot?lito arenito e siltito feldsp?tico) e escuros (prot?lito folhelho escuro). Texturas indicativas de que houve fus?o parcial e minerais tais como sanidina, mullita, tridimita e armalcolita demonstram que foram atingidas temperaturas da ordem de 1000-1150?C, seguido por arrefecimento ultrarr?pido. O resfriamento do l?quido intersticial tem como consequ?ncia o fechamento de poros e a diminui??o da permeabilidade dos prot?litos, passando de 17-11% nas rochas n?o afetadas a zero naquelas mais transformadas. Apesar de essas rochas serem observadas apenas nos contatos ou a pouca dist?ncia dos mesmos, o grande n?mero de corpos b?sicos intrusivos na Bacia Potiguar, tanto na parte emersa como na por??o oce?nica, deixa em aberto a possibilidade de fortes implica??es do efeito termal na explora??o de hidrocarbonetos nesta ?rea como tamb?m em outras bacias cret?ceas e paleozoicas no Brasil

Les différentes lithologies sédimentaires du Groupe Belqa présentent un enrichissement en P et en toute une série d'éléments sensibles aux conditions redox. Il est montré que l'enrichissement en U, Cu, Co, Mo, V résulte d'un dépôt syn-sédimentaire sous conditions suboxiques. Par contre, les éléments Cr, Ni, Cd, Zn sont beaucoup plus enrichis et requierent un flux exogénique de métal probablement relié à l'altération de roches ultrabasiques obductées à la même période au nord de cette région, lors de la collision de la plaque arabo-africaine avec la plaque eurasienne. Les phosphates représentent le principal hôte de l'uranium et des terres rares. L'analyse des apatites par ablation laser ICP-MS a permis de montrer leur évolution, depuis le stade sédimentaire-diagénétique, puis pyrométamorphique, jusqu'à l'altération supergène. La libération de l'uranium de la structure de l'apatite lors du pyrométamorphisme et de l'altération supergène permet sa mise à disposition pour la formation de minéralisations. Le pyrométamorphisme, dû à une combustion des niveaux riches en matière organique, est responsable de la déstabilisation des apatites, et de la formation de roches de compositions semblables à des ciments clinker/Portland. L'uranium a également été libéré de la structure de l'apatite par altération supergène. L'altération météorique et les fluides évaporitiques ont permis le lessivage et le transport des éléments (U, V) des roches carbonatées métamorphiques, et des carbonates puis a conduit au dépôt des vanadates d'uranyles dans les carbonates pulvérulents ayant subi une dissolution importante / The different lithologies of the Belqa Group present an enrichment in P and various redox sensitive elements. The U, Cu, Co, Mo, V enrichment results from synsedimentary deposition in suboxic conditions. However, the higher Cr, Ni, Cd, Zn enrichment requires an exogenic metal flux probably related to the terrestrial leaching of ultrabasic rocks obducted during the collision between African-Arabian and Eurasian plates to the north of the study region, at the same time. Phosphates are the main host lithology for uranium and rare earth elements. The analysis of the apatites by laser ablation ICP-MS shows their evolution along the sedimentary-diagenetic and pyrometamorphic stages, up to the supergene weathering. The uranium liberation from the apatite structure during the pyrometamorphism and surficial weathering led to its availability as supergene ore mineralization. Pyrometamorphism resulted from the combustion of organic-rich layers and caused the destabilization of the apatites, and the formation of rocks which have compositions similar to clinker/Portland cements. Uranium has been also released from the structure of the apatite by supergene alteration. Meteoric weathering and the evaporitic groundwater resulted in the leaching and the transport of the elements (U, V) from the metamorphic carbonated hills, and from the surrounding permeable limestone which has undergone dissolution in the near surface zone