The effect of forestry drainage operations on upland sediment yields: A case study     

M. Robinson  K. Blyth 《地球表面变化过程与地形》1982,7(1):85-90

Forestry is generally considered to be a land use giving low sediment yields. However, in Britain most forestry is on land that requires extensive drainage, and it was found that the drainage of a small upland catchment resulted in sediment yields over the following five years equivalent to nearly half a century's load at pre-drainage rates. Subsequent sediment yields did not decline to pre-drainage levels, but remained about four times higher, as a result of erosion of the drains.  相似文献   

Assimilation of sulfate and carbonaceous rocks: Experimental study,thermodynamic modeling and application to the Noril’sk-Talnakh region (Russia)     

《Ore Geology Reviews》2017

Most of the intrusions in the Noril’sk-Talnakh region (Siberia) are hosted in thick sedimentary sequences including abundant evaporitic and terrigenous sedimentary rocks. Three mafic-ultramafic intrusions in this region contain unusually thick massive sulfide deposits, which represent one of the world’s largest economic concentrations of Ni, Cu and PGE. The interaction of Siberian magmas with sulfate and organic matter-rich sedimentary rocks has been proposed as a possible mechanism for the origin of these exceptional sulfide deposits but the interaction process and the reaction paths have never been fully investigated. Here we clarify, by both experimental petrology and thermodynamic modeling, how sulfate and organic matter assimilation occur in mafic-ultramafic magmas, affecting magma composition, crystallization and sulfide saturation.Interaction experiments were conducted at conditions relevant to the emplacement of Noril’sk type intrusions (1200 °C, ∼80 MPa) to simulate the assimilation of sulfate and/or organic compounds by ultramafic magmas. We used a picrite from Noril’sk1 intrusion, and coal and anhydrite from the area as starting materials. The experimental results show that the incorporation of anhydrite into the magma occurs by chemical dissolution in the melt, which increases the magma’s sulfur content, but suppresses sulfide saturation and reduces olivine crystallization. Extreme assimilation leads to sulfate saturation in the magma and high dissolved sulfur contents of 0.9 ± 0.1 wt% S. Conversely, coal assimilation promotes sulfide segregation and magma crystallization, while decreasing the dissolved H₂O content of the melt and increasing the amount of coexisting fluid phase.We also employed gas-melt thermodynamic calculations to quantify the effect of these assimilations on the redox conditions and the S content of the magma, and investigate the role of temperature, pressure, and initial gas content of the magma in the assimilation process. We quantify how sulfate assimilation strongly oxidizes the magma and increases its S content; both effects are intensified by increasing pressure (from 50 to 100 MPa in this study), decreasing temperature (from 1350 to 1200 °C in this study), and decreasing amounts of fluid phase initially coexisting with the magma (from 2 to 0 wt%). The interaction with organic matter (CH in this study) induces a strong reduction of the magma, even for extremely low degrees of assimilation (few tenths of wt%), and the dehydration of the melt.We therefore suggest that in the Noril’sk-Talnakh district (1) additional S was supplied to mantle derived magmas by the assimilation of evaporitic rocks, and was transported during magma ascent in the form of dissolved, oxidized S; (2) a substantial reduction of the magma inducing sulfide segregation and important crystallization then occurred due to the interaction with carbonaceous sediments. This mechanism can potentially produce massive sulfide deposits by important sulfate assimilation (locally higher than 3 wt% CaSO₄) and minor organic matter assimilation (few tenths of wt% CH); however, if one of the two steps does not occur, or the assimilation in (1) is not large enough, disseminated sub economic or no sulfide deposits are produced. We conclude that exceptional conditions favoring substantial assimilation of sediments are needed to generate exceptional ore deposits like those of the Noril’sk-Talnakh district.  相似文献