Ore genesis in the North Pennine orefield, in the light of fluid inclusion studies

The geology, structural setting, mineralogy and zoning of the North Pennine orefield are briefly reviewed. Data from fluid inclusions relating to the temperature, salinity and Na/K ratios of the hydrothermal solutions that deposited the Pennine ores are presented. These indicate that fluorite was deposited between temperatures of 200øC and 100 ø C, that the salinity of the hydrothermal solutions was approximately 20 equiv. wt. % NaCI and that the Na/K ratio of the solutions varied between 6.8 and 12.4. Barite was deposited from temperatures of 130øC down to less than 50 ø C, by solutions with Na/K ratios between 15.3 and 46. An attempt is made to explain the striking fluorite-barite zoning of the North Pennine orefield in terms of the mixing of juvenile hydrothermal solu-tions with high Ba cormate waters similar to those at present found in the Durham coalfield. A comparison of the fluid inclusion and geologic data from the North Pennine orefield and Mississippi Valley deposits indicates that the simi-larities are so striking that a similar origin for both groups of deposits is suggested. It is concluded that the fluorine and base metals were derived from below the crust.