GEOCHRONOLOGIC EVIDENCE FOR PROTRACTED, INCREMENTAL EMPLACEMENT OF PLUTONS UNDER MAGMATIC-HYDROTHERMAL SYSTEMS: A NEW LOOK AT OLD DATA

Cyclic and superimposed hydrothermal events have been recognized for some time, but the ability to scrutinize the duration of hydrothermal systems and their underlying magmatic counterparts has only been available in the past decade or so with the increased precision of radioisotopic dating techniques, especially U-Pb, Re-Os and ⁴⁰Ar/³⁹Ar. The duration of magmatic-hydrothermal systems represents the duration of emplacement of causative plutons. Accumulating field and geochronological evidence shows that many, if not most, plutons are formed by accumulating small batches of magma over periods much greater than predicted by the conductive cooling of a single, pluton-sized, magma body. Some intrusive complexes previously thought to have been emplaced in a single pulse have been shown to be emplaced by the addition of many pulses over millions of years (e.g., Tuolumne Intrusive Suite, Glazner et al., 2004).

It is traditionally assumed that magmatic-hydrothermal systems were formed by a single batch of magma or by one magmatic event releasing its volatiles and metals as the system cools. Geochronology of magmatic-hydrothermal systems shows them to be active over time scales comparable to those involved in the formation of plutons and longer than predicted by conductive cooling models. The time scales involved in ore deposit formation may be a consequence of "normal" protracted pluton formation, rather than an oddity found only in large hydrothermal systems.

A model involving protracted and incremental emplacement of plutons raises several important questions for ore deposit genesis. 1) When in the evolution of a pluton are fluids released or circulated? 2) How are cyclicity and superimposition, seen in many magmatic-hydrothermal systems, related to protracted, incremental emplacement of magmas? 3) What effect do the duration and number of magmatic pulses have on the grade and size of mineralized systems? 4) What do rocks at the tops of plutons, where mineralization is usually localized, indicate about pluton-building processes at depth?