Tectonic environments

Characteristic features of these deposit types also indicate that geologic and tectonic environments affect the origin of ore fluids and formations of these deposits. 

In VMS systems in tectonic environments which have sufficiently low geothermal gradients and sufficiently constant permeability with depth, we would expect to see metal leaching at the underside and through the intrusion as well as along the top surface. 

Floyd, P.A. Leveridge, B.E. 1987. Tectonic environment of the Devonian Gramscatho basin, south Cornwaii framework mode and geochemicai evidence from turbiditic sandstones. Journal of the Geological Society of London, 144, 531-542. 

Although specific tectonic environments, structural settings and lithologies are required for all U deposit types, none of these are definitive indicators of mineralization because most of these settings, structures and lithologies do not host deposits. Thus, they are required for the deposits to form, but are not definitive indicators of mineralization. In effect, the only definitive indicators are geochemical, both in terms of U concentration and in the associated elements. 

The rocks of the TCZ are interpreted to have erupted in a transtensional continental rifting environment in the Late Siiurian to Middle Devonian. In this tectonic environment A-type granites are expected, and indeed the reiativeiy high Zr, Nb, and Y content in these rocks support this. However, many other geochemicai aspects of these rocks, i.e., high Ba, Rb, Th and K, are more typical of the I-type magmas that commonly occur in collisional settings. 

Even though paleoaltimetric data from internal structural elements of orogenic belts and plateaus represent much needed complementary information to those derived from surface deposits or weathering products we caution about the uncritical use of stable isotopic data from deeper Earth environments in paleoaltimetric studies. It is highly desirable to obtain reliable thermometric, structural, and isotopic tracer data before attempting any paleoaltimetric reconstruction in such environments, as uncertainties exist about the fluid pathways and mechanisms responsible for fluid transport into the ductile crust. Maybe more importantly, it is imperative to document that the timing of meteoric water-rock interaction can be dated precisely, especially within thermally and kinematically rapidly evolving tectonic environments such as extensional detachment systems. 

The basalt vesicularity proxy for paleoelevation measurement is limited to areas in which suitable basalts have been deposited, preserved, and exposed. As such, there are only certain geographic areas that can be explored in this way. Even in these areas (and in all areas) it is best to apply as many proxies as possible to get the most robust estimate of paleoelevation in any tectonic environment. 

Plate tectonics is a major driving force affecting Earth s surface environment. The kind and intensity of the physico-chemical parameters that control diagenetic pathways are determined to some degree directly or indirectly by plate tectonic processes. The plate tectonic environment exerts a direct control on diagenesis, whereas changes in Earth s surface chemistry induced by changes in... 

Studies of mantle xenoliths have confirmed the view from seismology that peridotite is volumetrically the dominant component of the Earth s shallow mantle (<400 km) (see Chapter 2.02). This is because xenolith suites in almost all tectonic environments are dominated by peridotites. Even at localities where other lithologies such as eclogite dominate the intact xenolith suite, mineral concentrate studies show that peridotite dominates the inventory of entrained mantle material (Schulze, 1989). Major- and trace-element studies of mineral concentrates from mined kimberlites have also been used to illustrate... 

Canil D. (2002) Vanadium in peridotites, mantle redox and tectonic environments Archean to present. Earth Planet. Sci. Lett. 195, 75-90. 

Wood B. J., Bryndzia L. T., and Johnson K. E. (1990) Mantle oxidation state and its relationship to tectonic environment and fluid speciation. Science 248, 337-345. 

These samples may provide a biased perspective of the Earth s mantle because of the association of the host magmas with certain tectonic environments. For example, xenolith-bearing kimberlites are usually found on cratons, which have been shown to be underlain by mantle with anomalous geophysical, and presumably geochemical, nature (see Chapter 2.05). Another caveat is that at many localities, samples containing hydrous minerals may be uncommon, even if they are disproportionately represented in the literature published on a given site—Dish Hill, California, is a good example of this phenomenon. 

In the modern tectonic environment, melt extraction from the uppermost mantle occurs in numerous settings that include oceanic and continental spreading centers, hot-spots, and sub-duction zones, with the vast majority of melt... 

Table 3 Summary of tectonic environments and ore components in selected deposit types.

Deposit type Tectonic environment Primary ore metals Other metals present... 

Active faults also exist elsewhere in the United States, in the midwest and in South Carolina. The last sizeable earthquakes in these regions occurred more than a hundred years ago, and geologists assume that earthquakes will probably occur within the next hundred years. The Pacific Northwest and Alaska, sitting atop active tectonic environments, will certainly be shaken by earthquakes for millions of years to come. The Great Basin, the western Rocky Mountains, and the United States northeast are all considered tectonically active enough for earthquakes to be considered possible. 

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