Enigmatic Origins of Giant Gold Deposits

ABSTRACT
The worldÕs largest gold deposits, at least 28 in number, contain
³600 tonnes (~20 million oz) of the yellow metal, with half of them
exceeding 1000 tonnes. A wide variety of commonly recognized
deposit types is represented, including porphyry, intrusion-related
stockwork-disseminated, iron oxide-copper-gold, high- and low-sulfidation
epithermal, sediment-hosted, slate-belt, orogenic (mesothermal)
lode, iron formation-hosted, and quartz-pebble conglomerate-
hosted. Many of the giant gold deposits may be considered
simply as the largest examples of these deposit types. Others, however,
including Porgera, Muruntau, Hemlo, Boddington, Olympic
Dam, Pueblo Viejo, Cripple Creek, Ladolam, Sukhoi Log, and
Olimpiada, are believed to be unique with regard to tectonic setting
and geologic characteristics and do not accord well with accepted
deposit models except in the most general classificatory sense. Giant
gold deposits may be the products of exceptionally efficient oreforming
processes, from source through transport to eventual precipitation,
but isolation of individual key parameters remains a task
for the future. Nevertheless, giant status depends on geologic factors
that favor dispersion of gold through large rock volumes or its deposition
over extended vertical intervals (1->3 km). Exploration for
giant gold deposits that compare closely with well-defined deposit
models is relatively straightforward, although the lack of known
controls on size precludes the design of programs to specifically target
giants. The challenge for the future, however, is the search for
additional examples of the unique giant deposits, assuming that they
exist, and the unique giants that undoubtedly await definition.