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