Noble gas and halogen evidence for the origin of Scandinavian sandstone-hosted Pb-Zn deposits

摘要

Fluid origins in the sandstone-hosted Pb-Zn class of ore deposit have been investigated in three deposits from Scandinavia; Laisvall, Vassbo and Osen. The deposits studied are hosted by autochthonous Cambrian sandstones that preserve a near original structural relationship to the underlying Precambrian basement, enabling the role of basement interaction to be assessed.Mineral samples have been collected from across the paragenetic sequence: sphalerite, galena, pyrite, fluorite and barite, of impregnation and related joint-hosted mineralization. Fluid-inclusion halogen (Cl, Br and I) and noble gas isotope (Ar-40, Ar-36, Kr-84) compositions were determined simultaneously by noble gas mass spectrometry of irradiated sample splits. Complementary He isotope analyses are obtained from nonirradiated splits of the same samples.He-3/He-4 values at Laisvall and Osen are highly radiogenic, 0.02 Ra, and the He-4/Ar-40* ratio extends to values greater than the crustal production value of 5, characteristic of low-temperature crustal fluids. At Vassbo, a slightly elevated He-3/He-4 ratio of 0.1-0.3 Ra is compatible with a very minor mantle component (1%-4%) suggesting a distal source for the basinal brine-dominated fluid.Br/Cl molar ratios 3.2-8.2 x 10(-3) are greater than the present seawater value of 1.54 x 10(-3) and correspond with I/Cl molar ratios in the range 64-1600 x 10(-6). The upper limits of both the I/Cl and Br/Cl values are amongst the highest measured in crustal fluids. Together, the data indicate acquisition of salinity by the evaporation of seawater beyond the point of halite saturation and subsequent fluid interaction with I-rich organic matter in the subsurface. The data are compatible with the independent transport of sulfate and sulfide and indicate that fluids responsible for joint-hosted mineralization were distinct to those responsible for impregnation mineralization.All three deposits preserve fluids with Ar-40/Ar-36 in the range of 6,000-10,000 and fluid inclusion Ar-40* concentrations of >0.02-0.05 cm(3)cm(-3). Fluid-inclusion He-4 concentrations are also extremely elevated with maximum values of similar to0.1 cm(3)cm(-3) in Laisvall fluorite and sphalerite. The high Ar-40/Ar-36 values, together with the high He-4 and Ar-40* concentrations, result from a very long premineralization crustal residence time on the order of 100-200 Ma.Together, the noble gas and halogen data are compatible with a Caledonian mineralization event (similar to425 Ma) caused by mixing of two or more, long-lived, hydrothermal basinal brines and pore fluids at the sites of mineralization. The data suggest negligible recharge of the basinal brines by meteoric water and indicate extensive fluid-basement interaction before mineralization. The similar noble gas composition of each deposit, suggests that similar processes operated at all three deposits and favors a single-pass fluid-flow model for mineralization. Copyright (C) 2005 Elsevier Ltd.