Chapter

Economic Mineral Deposits in Impact Structures: A Review

DOI: 10.1007/3-540-27548-7_20

ABSTRACT Many large meteorite impact structures throughout the world host mineral resources that are either currently mined or have
the potential to become important economic resources in the future. The giant Vredefort-Witwatersrand and Sudbury impact structures
underline this statement, because of their enormous resources in gold and uranium, and nickel, copper, and PGEs, respectively.
In relation to impact, three basic types of ore deposits in impact structure settings have been distinguished: (1) progenetic (i.e., pre-impact) deposits that already existed in the target regions prior to an impact event, but may have become accessible
as a direct result of the impact; (2) syngenetic (syn-impact) deposits that owe their existence directly to the impact process, and (3) epigenetic (immediately post-impact) deposits that result from impact-induced thermal/hydrothermal activity. In addition to metalliferous
ore deposits related to impact structures, impact structure-hosted epigenetic hydrocarbon deposits are reviewed and are shown
to make a major contribution to the North American economies. Non-metallic resources, such as minerals derived from crater-lake
deposits, dimension stone, and hydrological benefits, may also be derived from impact structures, and the educational and
recreational value of many meteorite impact craters can be substantial.

Undoubtedly, impact structures - at least those in excess of 5–10 km diameter - represent potential exploration targets for
ore resources of economic magnitude. This important conclusion must be communicated to exploration geologists and geophysicists.
On the other hand, impact workers ought to be familiar with already established fact concerning ore deposits in impact environments
and must strive towards further understanding of the ore generating processes and styles of emplacement in impact structures.

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    ABSTRACT: In earlier studies, the 65-75 km diameter Si1jan impact structure in Sweden has been linked to the Late Devonian mass extinction event. The Siljan impact event has previously been dated by K-Ar and Ar-Ar chronology at 342-368 Ma, with the commonly quoted age being 362.7 +/- 2.2 Ma (2 sigma, recalculated using currently accepted decay constants). Until recently, the accepted age for the Frasnian/Famennian boundary and associated extinction event was 364 Ma, which is within error limits of this earlier Si1jan age. Here we report new Ar-Ar ages extracted by laser spot and laser step heating techniques for several melt breccia samples from Si1jan (interpreted to be impact melt breccia). The analytical results show some scatter, which is greater in samples with more extensive alteration; these samples generally yield younger ages. The two samples with the least alteration yield the most reproducible weighted mean ages: one yielded a laser spot age of 377.2 +/- 2.5 Ma (95% confidence limits) and the other yielded both a laser spot age of 376.1 +/- 2.8 Ma (95% confidence limits) and a laser stepped heating plateau age over 70.6% Ar-39 release of 377.5 +/- 2.4 Ma (2 sigma). Our conservative estimate for the age of Siljan is 377 2 Ma (95% confidence limits), which is significantly different from both the previously accepted age for the Frasnian/Famennian (F/F) boundary and the previously quoted age of Siljan. However, the age of the F/F boundary has recently been revised to 374.5 +/- 2.6 Ma by the International Commission for Stratigraphy, which is, within error, the same as our new age. However, the currently available age data are not proof that there was a connection between the Si1jan impact event and the F/F boundary extinction. This new result highlights the dual problems of dating meteorite impacts where fine-grained melt rocks are often all that can be isotopically dated, and constraining the absolute age of biostratigraphic boundaries, which can only be constrained by age extrapolation. Further work is required to develop and improve the terrestrial impact age record and test whether or not the terrestrial impact flux increased significantly at certain times, perhaps resulting in major extinction events in Earth's biostratigraphic record.
    Meteoritics & planetary science 03/2005; · 2.80 Impact Factor
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    ABSTRACT: Meteorite impact structures are found on all planetary bodies in the Solar System with a solid surface. On many planets, impact craters are the dominant landform. Earth's active geology, however, tends to rapidly erase impact structures from the geological record, although we know currently of 174 confirmed impact sites. Impact events are destructive and have been linked to at least one of the ‘big five’ mass extinctions over the past 540 Ma. But they also provide certain economic benefits, including the formation of metalliferous ore deposits and hydrocarbon reservoirs. Impact structures can also form new biological niches, which can provide favourable conditions for the survival and evolution of life. Despite this, it was only in the past 40 years that the importance of impact cratering as a geological process was recognized and only during the past 15–20 years that the study of meteorite impact structures has moved into the geological mainstream. There is, therefore, still considerable potential for new and exciting advancements.
    Geology Today 12/2007; 24(1):13 - 19.
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    ABSTRACT: Impact cratering is a geological process characterized by ultra-fast strain rates, which generates extreme shock pressure and shock temperature conditions on and just below planetary surfaces. Despite initial skepticism, this catastrophic process has now been widely accepted by geoscientists with respect to its importance in terrestrial — indeed, in planetary — evolution. About 170 impact structures have been discovered on Earth so far, and some more structures are considered to be of possible impact origin. One major extinction event, at the Cretaceous-Paleogene boundary, has been firmly linked with catastrophic impact, but whether other important extinction events in Earth history, including the so-called “Mother of All Mass Extinctions” at the Permian-Triassic boundary, were triggered by huge impact catastrophes is still hotly debated and a subject of ongoing research. There is a beneficial side to impact events as well, as some impact structures worldwide have been shown to contain significant (in some cases, world class) ore deposits, including the gold-uranium province of the Witwatersrand basin in South Africa, the enormous Ni and PGE deposits of the Sudbury structure in Canada, as well as important hydrocarbon resources, especially in North America. Impact cratering is not a process of the past, and it is mandatory to improve knowledge of the past-impact record on Earth to better constrain the probability of such events in the future. In addition, further improvement of our understanding of the physico-chemical and geological processes fundamental to the impact cratering process is required for reliable numerical modeling of the process, and also for the correlation of impact magnitude and environmental effects. Over the last few decades, impact cratering has steadily grown into an integrated discipline comprising most disciplines of the geosciences as well as planetary science, which has created positive spin-offs including the study of paleo-environments and paleo-climatology, or the important issue of life in extreme environments. And yet, in many parts of the world, the impact process is not yet part of the geoscience curriculum, and for this reason, it deserves to be actively promoted not only as a geoscientific discipline in its own right, but also as an important life-science discipline.
    Journal of Earth System Science 06/2007; 116(2):81-98. · 0.70 Impact Factor

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