Figure 8. Calculated mass changes () for BaO and Zn superimposed on chemically defined volcanic units in DDH 10-2025 (see also Figure 7). Mass gains in BaO an Zn are greatest in the lowest 70 m of hangingwall rhyolite A, and in the 20 m-thick basalt sill. Footwall rhyolite B shows surprisingly little addition in BaO and Zn. Together with the sodic alteration, this suggests that this part of the footwall was a seawater recharge zone. If so, the massive sulfides may have formed distal to high-T vents, which would be in keeping with their very high Zn/Cu ratios. If a high-T discharge zone (chlorite + cp) was originally present in the vicinity of the Marshall Zone, it has not been located as yet.



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