27. GEOCHEMICAL SECTION OF THE TAG HYDROTHERMAL MOUND1Yves Fouquet,2 Katell Henry,2 Richard Knott,3 and Pierre Cambon2 |
ABSTRACTDrilling operations to 125 mbsf in the TAG mound have clarified the geochemical organization of the mound and stockwork zone. Bulk chemical analyses show that base metal values are very low within the mound and underlying stockwork zone. Low values may be explained by a combination of two processes. (1) During maturation, metals are leached from primary sulfides equivalent to those formed in chimneys at the surface. (2) Sulfide formed in situ as basalt replacements and veins are not enriched in trace metals. Another possibility is that the base metal–rich zones, particularly the copper-rich zone, were not drilled. The Zn-rich zone probably occurs only in the surface chimneys. A Cu-rich zone may be present as a vertical zone below the Black Smoker Complex but was not drilled. TAG has a geochemical and mineralogical asymmetry that may be explained by less active leaching and maturation processes on the western side of the mound. This part was probably formed as talus of different types of material related to a north-south fault feeding the black smoker. Leaching is more efficient on the eastern side, and all metals are removed from the surface sulfide at TAG-1. At TAG-2, metals are only concentrated within the first 10 m. Thus, the source of Zn in the white smoker fluid has not been identified. The abundance of quartz within the mound indicates a high temperature of formation under stable physico-chemical conditions. Trace metals associated with Zn are Cd, Zn, Ag, Sb, and Pb. Principal component analyses (PCA) studies show that these metals occur within sphalerite or within low-temperature pyrite associated with sphalerite. Cobalt and Se are not always specifically correlated with Cu, but they occur preferentially in high-temperature pyrite formed under stable conditions. In relation to these different observations we consider that the mound growth is caused by three different processes. (1) At the surface the sulfides formed within the chimneys are progressively buried and recrystallized, contributing to the surface growth of the massive sulfide mound. This material can be relatively enriched in Zn, which is later leached through a zone refining process. (2) In the deepest part the basalt is progressively silicified and replaced by a pyrite/quartz assemblage contributing to the internal growth and stockwork formation. These samples correspond to high-temperature pyrite and were never enriched in Zn and associated trace metals. Thus, most of the mound is depleted in trace metals. (3) The third process is the anhydrite veining that also contributes to the internal growth. Most of the anhydrite should be dissolved when the activity ceases. However, veins are often associated with a chalcopyrite selvage that contributes to the inner mound growth. |
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