Obiettivo THE DEVELOPMENT OF NEW CRITERIA FOR THE EXPLORATION OF CONCEALED TIN, TUNGSTEN, GOLD AND SILVER DEPOSITS IN PORTUGAL, BASED ON THE RECOGNITION OF HYDROTHERMAL FLUID ANOMALIES AND RELATED LITHOGEOCHEMICAL FEATURES. ESTABLISHING A RELIABLE GEOCHEMICAL SIGNATURE FOR MINERALIZATION WILL INCREASE TARGET SIZE WELL BEYOND THE ECONOMIC ORE ZONE. THE SELECTED MINING AREAS IN CENTRAL AND NORTHERN PORTUGAL ARE RICH IN SN, W, AU AND AG. New methodologies and concepts have been developed for the exploration of concealed orebodies based on the recognition of geochemical anomalies due to the interaction of the mineralising fluids with their host rocks. An integrated lithogeochemical, fluid inclusion and isotope approach was adopted, supported by mineralogical and petrological studies. 6 deposits were chosen for detailed investigation to allow comparison between granite hosted and schist hosted styles of gold silver and tin tungsten mineralisation, and to facilitate assessment of lithological controls on the nature of the resultant hydrothermal fluid anomalies.The primary objective of the project was to establish new criteria and methodologies for the exploration of concealed tin, tungsten, gold and silver deposits in the Hercynian granite schist terrains of central and northern Portugal. The approach adopted was based on the concept of 'hydrothermal fluid anomalies' which proposes that orebodies occupy the focii of hydrothermal systems and are therefore enclosed by haloes of geochemically and mineralogically disturbed wall rocks due to fluid-rock interactions.The project demonstrates that gold/silver deposits and tin/tungsten deposits in central and northern Portugal are accompanied by haloes of chemical and isotopic disturbance which generally extend beyond the limits of detectable metal enrichment, in accordance with the concept of hydrothermal fluid anomalies. The size and geometry of these haloes, as determined by combined geochemical, isotopic and fluid inclusion studies, vary according to the style of mineralization and lithology of the host rocks. Whole rock lithogeochemistry appears to provide the most promising regional exploration methodology for delineating mineralized areas, especially in granite terrains.On a wider perspective, the project has been successful in identifying many features common to both sediment hosted and granite hosted gold/silver metal deposits, and through stable isotope and fluid inclusion studies the importance of metasediments in ore genesis.By necessity, the investigations were exploratory in nature and have revealed new directions which now deserve further study. The most important of these is the need for an integrated geochemical structural approach to mineralization.The research aim was to develop new exploration strategies for gold and tin/tungsten deposits. The methodology for such studies was based on the recognition of hydrothermal fluid anomalies (geochemical, isotopic and mineralogical disturbances in the ores and in the rocks in their vicinity) caused by ore fluid rock interaction which, as expected, could be used as guides in mineral prospecting.A number of conclusions resulted from the studies carried out in 2 main mining regions of Portugal.In terms of mining geology, the studied gold mineralizations were associated with hydrothermal systems of siliceous character and the episodes that originated the epigenetic gold/silver and tin/tungsten mineralization showed some genetic, chronological and spatial independance.The structural control of the hydrothermal auriferous mineralization was evident, the fractures being used as channels for the ore fluid circulation and the lithogeochemical methods were shown to contribute positively to the delineation of anomalous areas which, in turn, may indicate the existence of auriferous and tin tungsten mineralization.The application of the lithogeochemistry to ore prospection in schistous shear terrains seemed to give good results in the distinction of gold/silver mineralized areas from tin/tungsten areas. This geochemical multielement approach can provide positive information when the analysis of gold is not possible or when the sampling density is lower.In granitic terrains, distinct groups of trace elements also appeared to be useful for the definition of different styles of mineralization.The important gold/silver deposit of Jales was delineated by extensive anomalies related to the fracturation suggesting that the prospection outside the actual limits of the mine should be developed.These results, showed that the lithogeochemical methods can furnish a good contribution for the establishment of useful geochemical guides with application in the exploration of gol d/tin/tungsten buried mineralization.OUR DEPOSITS IN CONTRASTING GEOLOGICAL ENVIRONMENTS HAVE BEEN SELECTED FOR THIS STUDY: 1) SCHIST-HOSTED GOLD AND TIN-TUNGSTEN DEPOSITS IN THE EXOCONTACT ZONES OF POST-OROGENIC GRANITES AND 2) GRANITE-HOSTED INTRAGRANITIC TIN-TUNGSTEN DEPOSITS. SINCE FLUID CIRCULATION IS A FUNCTION OF FRACTURE PERMEABILITY IT IS EXPECTED THAT THE FORM AND DIMENSION OF THE RESULTANT HYDROTHERMAL FLUID ANOMALIES WILL DIFFER FOR EACH ENVIRONMENT; HENCE THE NEED FOR SYSTEMATIC COMPARISON. GEOCHEMICAL STUDIES WILL BE INTEGRATED WITH COMPLEMENTARY PETROLOGICAL-MINERALOGICAL STUDIES OF THE HOST ROCKS AND ORE ASSEMBLAGES TO DERIVE A COMBINED LITHOGEOCHEMICAL- MINERALOGICAL STRATEGY FOR GENERAL PROSPECTION IN THESE TERRAINS. AS A PRE-REQUISITE FOR THE FIELD STUDIES, DETAILED GEOLOGICAL MAPPING WILL BE CARRIED OUT OVER THE TARGET AREAS TOGETHER WITH SUPPORTING GEOCHRONOLOGICAL INVESTIGATIONS. DURING THE FIRST YEAR A RECONNAISSANCE STUDY WILL ALSO BE MADE OF THE APPLICATION OF AMMONIUM LITHOGEOCHEMISTRY FOR THE DETECTION OF CONCEALED GRANITE CUPOLAS. FIELD AND LABORATORY TASKS WILL BE SHARED BETWEEN BGS AND DGGM MAKING BEST USE OF THE FACILITIES AND EXPERTISE OFFERED BY EACH GROUP TO ACHIEVE THE STATED OBJECTIVE. Campo scientifico natural sciencesearth and related environmental sciencesgeologylithologynatural scienceschemical sciencesinorganic chemistrytransition metalsnatural scienceschemical sciencesinorganic chemistrypost-transition metalsnatural sciencesmathematicspure mathematicsgeometry Programma(i) FP1-RAWMAT 3C - Research programme (EEC) on materials (raw materials and advanced materials), 1986-1989 Argomento(i) Data not available Invito a presentare proposte Data not available Meccanismo di finanziamento CSC - Cost-sharing contracts Coordinatore INSTITUTO GEOLOGICO E MINEIRO Contributo UE Nessun dato Indirizzo 19-2,Rua Almirante Barroso 38 1049-025 LISBOA Portogallo Mostra sulla mappa Costo totale Nessun dato Partecipanti (1) Classifica in ordine alfabetico Classifica per Contributo UE Espandi tutto Riduci tutto NERC British Geological Survey Regno Unito Contributo UE Nessun dato Indirizzo London Mostra sulla mappa Costo totale Nessun dato
