『Abstract
Three widespread and abundant types of ore deposits, epithermal
silver-gold (Ag-Au), porphyry copper (Cu), and orogenic gold (Au),
form by hydrothermal activity during tectonic convergence along
continental margins, and their preservation provides new insights
into rates of those processes that form and destroy ore deposits
as well as overlying rock. Ages for each of the three deposit
types define lognormal age-frequency distributions that are nearly
identical in form but differ in their modal ages. Epithermal Ag-Au,
porphyry Cu, and orogenic Au deposits exhibit age modes at 〜3,
11,and 199 Ma, respectively. These ore types form at depths of
〜0.5, 1.9, and 10 km, respectively, such that modal age increases
with depth of ore emplacement. Qualitatively, the dearth of deposits
younger than their modal age reflects the fact that ore deposits
form at depth within the crust, and time is required for exhumation
to expose them. Continued erosion serves to destroy ore bodies,
giving rise to fewer deposits with increasing time beyond modal
ages. These relations suggest that ore deposit age frequencies
should serve as monitors of rates of tectonism and exhumation
along convergent continental margins. Quantitatively, the lognormal
aspect of age frequencies is closely approximated by a steady
state model of ore formation and crustal deformation. The model
presumes that the population of exposed deposits of each type
results from ore emplacement at a constant rate and depth and
that, after emplacement, these deposits are dispersed vertically
(upward or downward) by tectonic uplift and/or subsidence. The
aggregate paths of all deposits of some particular type behave
as a two-dimensional random walk in vertical (depth) and lateral
(time) space. Moreover, the lognormal nature of ore deposit age-frequency
distributions requires that amounts of uplift and subsidence are
generally subequal (there is no bias to the random walk) and necessitates
that tectonic “steps” taken during this random walk are on the
order of a few hundreds of meters per million years. Modal rate
of ore deposit exhumation (emplacement depth/modal age) decreases
as a power function with increasing modal age (exhumation distance
[mm] = 5.12×time interval0.799 [yr]). Lower rates of
exhumation with greater emplacement depth and modal age, however,
merely reflect greater random-walk path lengths (and durations)
with greater emplacement depth and are no less than rates that
would be anticipated for a steady state system with invariant
rates of ore mineralization and tectonic deformation. In fact,
exhumation histories of epithermal Ag-Au, porphyry Cu, and orogenic
Au deposits scale in a similar manner and therefore record largely
indistinguishable styles of tectonic uplift and subsidence among
the 〜700 ore deposits considered here. Agreement between model
and observed ore deposit age-frequency distributions also allows
the formulation of a general inventory of ore body abundances
in continental crust. Foe example, if currently known occurrences
of epithermal Ag-Au reasonably approximate the actual number of
these deposits that now exist at the Earth's surface, then less
than 1％ of all such deposits are now exposed; tectonic uplift
and erosion has served to remove 〜93％ of all such epithermal Ag-Au
deposits that have formed over Phanerozoic time.』
Introduction
Epithermal silver-gold, porphyry copper, and orogenic gold deposits
Epithermal silver-gold deposits
Porphyry copper deposits
Orogenic gold deposits
Tectonism, exhumation, and ore deposit age frequencies
Computational modeling of ore deposit age-frequency distributions
Significance of model results
Rates of continental erosion and deposit exhumation
Tectonic uplift and subsidence
Steady state versus episodic processes
Constraints on metal inventories in continental rocks
Acknowledgments
References cited