Quantitative Identification of Metallogenic Epochs and Provinces: Application to Phanerozoic Porphyry Copper Deposits

摘要

The terms "metallogenic epoch" and "metallogenic province" have been widely used for many years, but are so poorly defined that most of Earths surface and geologic history has been designated a metallogenic feature of some sort. We propose defining metallogenic epoch and metallogenic province as those time intervals of Earth history and regions of Earth, respectively, which contain a significantly greater number of deposits or larger tonnage of a specific deposit type than would have resulted from average rates of mineralization that have occurred over Phanerozoic time. Here, we evaluate the application of this definition to porphyry copper deposits. Metallogenic epochs for porphyry copper deposits can be identified by comparison between a real-Earth age-frequency distribution based on ages of known deposits and a model age-frequency distribution that simulates the formation of porphyry copper deposits and their subsequent vertical uplift and/or subsidence (relative to the Earths surface) after they form. The latter represents the age-frequency distribution that would be expected if rates of mineralization did not deviate from the Phanerozoic average, and is determined by least-squared minimization of differences between model and real-Earth age-frequency values. Metallogenic epochs are defined as (duration-normalized) time periods in which real-Earth numbers or sizes of deposits differ significantly from the model value of average numbers or sizes of deposits that have formed over the past approx 545 m.y. of Earth history. Eight of the 80 Phanerozoic stages, constituting 5.5 percent of this time interval, meet this definition; these include the upper Eocene, Paleocene (all 3 stages), lower Triassie, middle Pennsylvanian, upper Silurian, and upper Ordovician. Metallogenically impoverished stages occurred during the Upper Cretaceous and the upper Oligocene. Time intervals of deposit enrichment (and scarcity) exhibit little correlation with records of widely known global geologic features or processes. At a more regional scale, however, mineralization along South and Central America slightly postdated times of enhanced production of East Pacific sea floor, and many Paleocene deposits underwent enhanced preservation beneath younger rocks. Metallogenic provinces appear to be recognizable through use of the power-law relationship between the duration-normalized spatial density of real-Earth deposits and the size of areas that host the deposits. Comparison of real-Earth spatial densities to a best-fit line to data for all areas containing deposits allows recognition of areas that have greater spatial densities. Our analysis suggests that variation in the distribution of porphyry copper deposits in both time and space are largely unpredictable at even epoch and regional scales of consideration, respectively, and indicate that the presence or absence of deposits in one time interval or region does not effectively predict the abundance of other deposits in other time intervals or geographic localities. This conclusion, however, does not preclude the importance of geologic controls on either deposits or metallogenic features. Rather, it strongly affirms the widely accepted fact that the distribution of ore deposits and metallogenic features is due to a large number of geologic factors and processes that vary from place to place and time to time. More rigorous definition of metallogenic epochs and provinces and more detailed geologic study at the local scale will lead to a better understanding of those processes that account for statistically anomalous concentrations of mineralization in both time and space.