Application of anisotropy of magnetic susceptibility (AMS) method in evaluation of the degree and style of alteration at Challu granitoidic pluton (South of Damghan)

Summary: Challu granitoid pluton is located in southeast of Damghan and northern part of central Iranian structural zone. Intrusion of the pluton into volcanic and volcanic-sedimentary rocks and the resultant hydrothermal fluids has caused alteration and Fe mineralization. Generally, two different phases of propylitic and argillic alterations are identified in the studied pluton. Magnetic susceptibility of fresh and altered rocks at the Challu granitoidic pluton is measured by magnetic fabric technique. The average measured magnetic susceptibility has been obtained about 28872±3410 μSI for monzodiorite and 21487 ±3916 μSI for quartzdiorite. Circulation ofhydrothermal fluids throughout the intrusive body has caused the main mineral compositions variation and changed the magnetic properties of minerals as the average measured magnetic susceptibility for the above rocks has, respectively, been reduced to 25117±988 μSI and 6262±1577 μSI, due to propylitic and argillic alteration. Differentkinds of opaque minerals in thepluton have also been identifined by thermomagnetic curves as magnetic susceptibility changes with temperature. These results show how magnitude of magnetic susceptibility decreases during alteration due to removal or reducing in the size of magnetite or its convertion to hematite.
This research investigates changes in the magnetic fabrics of the samples taken from the Challu granitoidic pluton that have been affected by hydrothermal alteration during iron mineralization. Magnetic nature of different kinds of opaque minerals in the pluton has also been investigated via thermomagnetic curves as magnetic susceptibility changes with temperature.
Methodology and Approaches: Three oriented hand samples were collected at each of the 32 sites located on a 0.5-1 kilometer grid pattern with good coverage across the Challu granitoidic pluton.The specimens were measured for their magnetic fabric using a Kappabridge MFK1-FA susceptometer (AGICO) at the Geomagnetic Laboratory, Shahrood University of Technology, Iran. The instrument is operated at low field (4 × 10−4T; 920 Hz) during measurements. The orientations and magnitudes of the three principal axes of the AMS ellipsoids (K1 ≥ K2 ≥ K3) were obtained for each sampling station through the tensor average of four individual AMS measurements. Moreover, 32 thin sections were prepared, and then, they were petrographically studied. Basic magnetic mineralogy, aimed at identifying the minerals responsible for the magnetic susceptibility of the samples, was performed using the CS-2 furnace (AGICO) coupled to a KLY-2 susceptometer, located at the Paleomagnetism laboratory of Geological Survey of Iran. As a result, the variation of the susceptibility with temperature from 20°C to 700°C was obtained.
Results and Investigation of magnetic fabric results in relation with the alteration and mineralogy of the rock samples taken from the Challu intrusive body reveal the following 1) The rocks belongs to the I-type granite and have high magnetic susceptibility (Km> 400μSI). 2) Regular variations of magnetic fabric on fresh and altered rocks of Challu granitoid show how behavior and character of minerals and rocks changes progressively with alteration. In fact, the magnitude of magnetic susceptibility is controlled by concentration of magnetic minerals (like magnetite), removal or re-deposition of it during the alteration of the intrusive body. 3) magnetic fabric studies can help us for finding internal fabric and emplacement mechanism of the intrusive body as well as identifing alteration zones and suitable pattern for exploration of mineralization passage. 4) Investigation of magnetic susceptibility changes with temperature in the mentioned granite indicates exact evidences of opaque mineral chemistry and magnetic carriers phases. The most important results of present research is that magnetic fabric changes of intrusive bodies not only provide precise information on internal structural and magnetic mineralogical nature, but also indicate geochemical variations and processes such as crystal fractionation and alteration in granites. Consequently, it seems that the AMS method not only can help us for finding internal fabric and emplacement mechanism of intrusive bodies but also for quantifying alteration types and intensity, and providing convenient model for exploration and hydrothermal fluid passages.