I attempt this volume with no small degree of trepidation, for despite the near universal acceptance of plate tectonic theory, and the undeniable sweep and power of the concepts in­ volved, points of controversy and uncertainty still abound. This is especially true with regard to the limited extent to which many ancient geologic terrains, however well studied, lend themselves to plate tectonic interpretation. There is also considerable controversy regarding the genesis of many metal deposits, and this impacts upon the degree to which such deposits can be meaningfully related to the tectonic settings in which they occur. It is relatively simple matter to point out that certain types of ore deposits exhibit impressive time-space associations with certain kinds of more recent plate boundary environ­ ments. It is also all too easy to possible plate tectonic-ore deposit relationships in ancient geologic terrains. What is more challenging is the recognition of the boundaries beyond which speculation is idle, but within which useful new insights regarding the relationship of certain ores to their lithologic and tectonic environments may emerge.