Abstract
The minerals of basic and acidic rocks from the volcano-sedimentary sequence in the Huelva area, Spain, Iberian Pyrite Belt, display an extended δ 18O enrichment. Quartz δ 18O values from quartz-keratophyres vary from +10.5 to +17.0 and feldspar δ 18O values from +14.4 to +16.0. For the spilite or spilitized dolerites δ 18O values vary from +9.9 to +13.4 for feldspar, from +6.4 to +9.8 for chlorite, from +3.7 to +4.3 for ilmenite and from +13.6 to +14.0 for quartz, but pyroxene exhibits magmatic values, from +5.3 to +6.1 with an exception at +7.5. The chlorite δD values vary from −34 to −43‰. This is attributed to hydrothermal alteration with seawater enriched in δ 18O by circulation through sediments. The temperatures of interaction determined from isotopic fractionations between minerals range from 400° to 520°C. Calculated δD and δ 18O values for water in equilibrium with the minerals at isotopic temperatures range from −16 to +5 and from +8.3 to +12.8, respectively. A model of circulation of seawater through a pile of sedimentary rocks and then through basaltic rocks is proposed to explain the high 18O compositions of the rocks from the Huelva District. Water/rock mass ratios calculated from this model range between 0.3 and 0.7 for the determined range of temperatures.
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