A pale creamy yellow substance observed in drill core. Electron microprobe analysis shows the substance to contain 12 to 15 percent total H 2 O but otherwise to have a composition reminiscent of cordierite. Possibly it represents an earlier formed substance from which cordierite has arisen. A sudoite-like material is formed by hydrothermal sedimentation-alteration-diagenesis during and closely following exhalative ore formation. With metamorphism, aluminous chlorite is converted to cordierite, which in consequence is also localized in the ore environment. It is proposed that at least some cordierite may be derived directly and essentially isochemically from a sudoitic, substantially dioctahedral, chlorite precursor. As such, this cordierite is a potential guide to exhalative ore.--Modified journal abstract.