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HYDROTHERMAL FLUID ANOMALIES: A NEW STRATEGY FOR GEOCHEMICAL EXPLORATION IN PORTUGAL

Objective

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.
ORE 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.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

NERC British Geological Survey
Address
Exhibition Road
SW7 2BX London
United Kingdom

Participants (1)

DIRECCAO GERAL DE GEOLOGIA E MINAS