This is a historic geological name for gold telluride minerals, such as calaverite or sylvanite, where gold is chemically locked with the element tellurium. Because the gold in these ores is not in its native, free-milling state, it cannot be recovered by simple panning or standard cyanidation without prior roasting or specialized chemical treatment. Discovering this type of ore often required a complete redesign of early milling circuits.
Heard on the job
The old assay reports show they hit a vein of telluric gold, which explains why their stamp mill recovery was so poor.