Mineralogy study of magnetic susceptibility of rocks along the coast of the northern South China Sea

On the basis of 2517 series of field measured data,bulk magnetic susceptibility values of 245 rock samples,detailed rock-mineral identification and total silicate analysis,combining with single mineral magnetic susceptibility and comparison research of each rock,we found that rock magnetic susceptibility mainly depends on mineral magnetic susceptibility.That is,rock magnetic susceptibility (κ_r) is proportional to the product of every mineral magnetic susceptibility (κ_i) of rocks and their volume content (C_i).For example,for intrusive rocks,κ_i=-5.68 × 10~2C_q + 2.86×l0~2C_r + 3.28 × 10~2C_R + l.18 × 10~4C_b + l.27 × 10~4C_(am)+ 5.35 × 10~5C_m,where the polynomial coefficients are constants positively proportional to the susceptibility of magnetic minerals,C stands for the volume content of minerals,in the order of quartz q (κ=-1.3)(4π× 10~(-6) SI units),plagioclase f(κ=0.01),alkali feldspar a (κ=0.01),biotite b (κ=100), amphiboie am (κ=80) and magnetite m (κ= 100000).On the other hand,from the magnetic susceptibility study of volcanic rocks,intrusive rocks,sedimentary rocks and metamorphic rocks, it is found that for the other three types of rocks the relationship of rock magnetic susceptibility with the magnetic susceptibility of mineral composition is similar to that of intrusive rocks.The contribution of mineral to rock magnetic susceptibility has the sequence of ferromagnetic minerals paramagnetic minerals diamagnetic minerals.Due to the change of volume content of magnetite,amphiboie and biotite,the magnetic susceptibility of igneous rocks is extremely variable.While sedimentary rocks are mostly non-magnetic and weakly magnetic,and their magnetic susceptibility mainly origins from biotite,alkali feldspar and cuttings.Magnetic susceptibility of metamorphic rocks is also variable.It may range from non-magnetic to very strongly magnetic,this is mainly determined by the type of original rock.Susceptibility of rparametamorphic rocks (sedimentary protolith) is similar to sedimentary rocks,while orthometamorphites (igneous protolith) is similar to igneous rocks;Quartzite and carbonate rocks are the weakest of all the rocks magnetically.Furthermore,alteration of rock will have significant influence on magnetic susceptibility.Generally speaking,the following alteration of minerals can increase rock magnetic susceptibility:alteration of iron-magnetism silicate minerals such as biotite and amphiboie which may form iron oxide,the sericitization or clayzation of weak paramagnetic minerals such as feldspar,and chloritization of rocks.While carbonation and kaolinization of weak paramagnetic minerals can reduce rock magnetic susceptibility,weathering of rocks can also reduce rock magnetic susceptibility by breaking and loss of high magnetic components of rocks.From the relationship between rock magnetic susceptibility and its mineral magnetic susceptibility,we infer that the total magnetic susceptibility of a geological body and the magnetic susceptibility-volume content of constitutive rocks have a similar relation to that of rock and its minerals.