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Secular trends in ore-forming processes through Earth history

Project Objectives

Conventional assessments of undiscovered mineral resources rely on the distribution of known deposits coupled with generalized geologic models for the various deposit types. This approach barely makes use of the fact that, without exception, all types of ore deposits are time bound. There are times in Earth history when a particular deposit type is absent, times when these deposits are present but scarce, times when they are abundant, and still other times for which we yet lack sufficient data. We developed this property as a tool with three broader purposes.

The first and most immediately practical purpose was to improve mineral assessment techniques, especially where there are huge gaps in knowledge of Third World geology. The research turned geologic age into a key parameter in resource assessment. The value of this approach was shown by the distribution of Mississippi Valley-type MVT deposits through Earth history (Leach et al. 2001). Most of the MVT deposits of the world formed during continental collisions at a few restricted times in Earth history. The late Paleozoic collisions during the formation of Pangea span less than 3% of Earth history, yet this brief interval includes over 70% of the total MVT Pb-Zn metal produced so far! For whatever reasons, only a few Proterozoic MVTs are known, despite abundant host rocks and seemingly suitable tectonic settings; Archean rocks are not known to contain any MVTs at all. Clearly, knowledge like this has the potential to improve resource assessment in frontier areas-regardless of how one might choose to explain the age patterns.

The second purpose was to make use of secular trends through Earth history as a natural laboratory in studies of deposit genesis. Continuing with the MVT example, we looked into reasons behind the age distribution. Was the late Paleozoic peak simply the fortuitous side effect of supercontinent formation at low latitudes? In that case, why didn't the preceding supercontinent cycle produce lots of MVT deposits also? Not only have the continents been continuously rearranged through the ages, but major changes have also affected the atmosphere, biosphere, global climate, ocean chemistry, and radiogenic heat production (which, in turn, moves the plates). How have changes like these influenced the formation (or not) of ore deposits?

The third purpose was the reverse of the second, to use the time-bound nature of ore deposits as sensitive indicators of Earth System behavior.

References

Project Chiefs:

David Leach
Phone: 303-236-5522
Email: dleach@usgs.gov

Dwight Bradley
Phone: 907-786-7434
Email: dbradley@usgs.gov

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