Mantle-derived fluids in a granite associated Sn-W deposit: He and Ar isotopes from Panasqueira

摘要

Hydrothermal fluids associated with ore generation commonly contain 3He derived from the Earth's mantle (e.g. Simmons et a/.,9 Stuart et al.,10 Hu et a/.,4 Burnard et a/.,3 Kendrick et a/.,6 Ballentine et al.,1 Kendrick et al.,5 Shail et al.8). The association between ore formation and 3He is not accidental: like 3He, the heat required to fuel an ore-forming hydrothermal cell originates in the mantle. As a result, a basic ingredient for ore genesis - heat - is accompanied by 3He, usually supplied by the mantle in fairly constant proportions.7 Dilution of 3He and heat-bearing 'magmatic' fluids by cooler fluids is common in many ore-forming systems because the physicochemical changes during cooling of the magmatic fluids leads to precipitation ofmetalliferous ores. Therefore, the mechanisms and timescales of mixing between magmatic and 'dilute' fluids are of considerable interest to economic geologists.The distribution of He and Ar isotopes in the Earth mean that they naturally complement each other during mixing between broadly 'magmatic' and 'surface' derived fluids. Helium (especially 3He) is practically absent in surface-derived fluids. As a consequence, the He isotopic composition of a low temperature, groundwater type fluid will change dramatically on addition of traces of mantle-derived 'magmatic' fluid. Conversely, ~(36)Ar concentrations are extremely low in mantle-derived magmas and fluids;2 therefore, the ~(40)Ar/~(36)Ar ratio of a predominantly magmatic fluid is sensitive to trace additions of low temperature marine or meteoric fluids. Analysis of He and Ar isotopes from the same sample covers mixing over the complete range of magmatic-dominated to low-temperature-dominated hydrothermal fluids.