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Figure F270. Large alteration patch (interval 312-1256D-174R-1, 91–109 cm). Host basalt is highly to completely replaced by different mineral assemblages in subconcentric, centimeter-scale zones. The core of this altered region comprises 100% quartz-laumontite (laum)-prehnite-epidote (1) and is surrounded by chlorite (2), prehnite-chlorite-epidote (3), epidote-quartz-actinolite (4), and an external light gray zone of prehnite-chlorite-actinolite (6). Large pyrite grains (5) occur along the diffuse boundary between the yellowish green epidote-quartz-actinolite regions (4) and the light gray prehnite-chlorite-actinolite zone (6). Secondary magnetite needles are common within the alteration patch. Modal proportions of secondary minerals are plotted for the profile A–B, illustrating zonation from the inner core to outside the alteration halo.

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