Low pressure fumarolic phenomena occur over a restricted area (some tens of square metres) within sediments of miocenic age at Praia Pauli, north of Luanda town along the atlantic margin of Angola. The pelitic-psammitic sediments in the area of fumaroles show the classical aspect of hydrothermalised rocks and are locally affected by intense fracturing correlated to the presence of micro-meso fault systems. Similar features (hydrothermalism, microfracturing) are visible over a wider area (some hundreds of metres long) parallel to the coast. In the zone of active gas emission a variety of secondary minerals are forming around the vents. X-ray diffraction and SEM analyses indicate that these phases consist of predominant sulphates (anhy-drite, gypsum, alunite-jarosite), native sulphur, sulphides (pyrite, marcasite), Fe-oxides (goethite, hematite) and phosphate (carbonate apatite). Such phases are also widespread in the surrounding areas where altered sediments occur. Mineralogy of the clay fractions (< 5 pm e < 10 urn) of unaltered and altered sediments indicates that hydrotermalism affects the stability of illite and caolinite and favours the formation of low-crystallinity products and smectitegroup phyllosilicates. Distribution patterns of relatively mobile trace elements (i.e. Cu, Ni, As, Se, Cd, Sb) in fine fractions of sediments suggest that hydrothermalism did not cause large-scale element mobilisation. No enrichment in elements with magmatic-volcanic affinity (e.g. Cd, Se, Sb) was observed in the zone of more intense hydrothermalism. Preliminary chemical and isotopic data on fluid condensâtes indicate that fumarolic fluids vary in composition from normal to hyperalcaline and hypersaline (chloride) composition. The 5"O values of fumarolic condensâtes are variable but constantly strongly positive (up to +9.\%o). The available data do not conclusively support any interpretation for the genesis of the fumaroles, especially as regards heat source and origin of fluids. General geology and compositional features of surface hydrothermalism do not support evidences of a magmatic heat source. Alternative hypotheses for anomalous heat generation within the sedimentary cover and its transport to the surface are discussed.
An enigmatic fumarolis system near Luanda, Atlantic margin of Angola: A preliminary survey
Putignano Maria Luisa;
1998
Abstract
Low pressure fumarolic phenomena occur over a restricted area (some tens of square metres) within sediments of miocenic age at Praia Pauli, north of Luanda town along the atlantic margin of Angola. The pelitic-psammitic sediments in the area of fumaroles show the classical aspect of hydrothermalised rocks and are locally affected by intense fracturing correlated to the presence of micro-meso fault systems. Similar features (hydrothermalism, microfracturing) are visible over a wider area (some hundreds of metres long) parallel to the coast. In the zone of active gas emission a variety of secondary minerals are forming around the vents. X-ray diffraction and SEM analyses indicate that these phases consist of predominant sulphates (anhy-drite, gypsum, alunite-jarosite), native sulphur, sulphides (pyrite, marcasite), Fe-oxides (goethite, hematite) and phosphate (carbonate apatite). Such phases are also widespread in the surrounding areas where altered sediments occur. Mineralogy of the clay fractions (< 5 pm e < 10 urn) of unaltered and altered sediments indicates that hydrotermalism affects the stability of illite and caolinite and favours the formation of low-crystallinity products and smectitegroup phyllosilicates. Distribution patterns of relatively mobile trace elements (i.e. Cu, Ni, As, Se, Cd, Sb) in fine fractions of sediments suggest that hydrothermalism did not cause large-scale element mobilisation. No enrichment in elements with magmatic-volcanic affinity (e.g. Cd, Se, Sb) was observed in the zone of more intense hydrothermalism. Preliminary chemical and isotopic data on fluid condensâtes indicate that fumarolic fluids vary in composition from normal to hyperalcaline and hypersaline (chloride) composition. The 5"O values of fumarolic condensâtes are variable but constantly strongly positive (up to +9.\%o). The available data do not conclusively support any interpretation for the genesis of the fumaroles, especially as regards heat source and origin of fluids. General geology and compositional features of surface hydrothermalism do not support evidences of a magmatic heat source. Alternative hypotheses for anomalous heat generation within the sedimentary cover and its transport to the surface are discussed.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
