Interpretation of magnetic and geoelectrical data based on geological and mineralogical evidence in the podiform chromite prospecting, Khoy ophiolite, Northwest Iran

Podiform chromite deposits are small magmatic chromite bodies formed in the lower section of an ophiolite complex. The Khoy ophiolite covers an extensive area in the northwest of Iran along the Iran-Turkey border.In this research study 1200 magnetometry data and geoelectric studies along 5 profiles were designed for prospecting chromite lenses. Mineralogical and geological studies have shown that pyrite, magnetite and other metallic sulfides are formed during the serpentinization process in the fractures of chromite lenses. The amount of released magnetite in the chromitites is less than the amount released in the harzburgite and dunites. Therefore, the number of magnetic anomalies created are less than those generated by bedrocks (Imamalipour, 2009). These metal sulfides increase the chargeability of positive anomalies in the cross-sections. Resistivity also shows a significant reduction compared to the bedrocks due to the metallic properties of chromite lenses.

In this research study, geological methods were used to interpret geophysical data in the Khoy ophiolite. Geological surveys at a scale of 1:20000 were implemented in an area of about 70 km2. 1200 magnetic points and resistivity and induced polarization along 5 profiles with a geological map and mineralogical studies were used. Magnetometric data at the 5*10m grid and Ip-Rs data with 10 m interval electrode spacing were collected.  For the inversion modeling of Ip-Rs data, Res2d inv software was used and geological and mineralogical data were integrated with magnetometric results.

Exploration of podiform chromite deposits has been a challenge due to their unpredictable occurrence, small size of most orebodies, and intensive tectonic dislocations (Mosier et al., 2012). Moreover, the absence of primary geochemical halos and associated alteration are issues that lead to difficulties in prospecting for podiform chromites. Chromite is an accessory mineral associated with the harzburgite host rock. The results of geophysical studies show that chromite lenses have lower magnetization than gabbro and higher mangnetization than harzburgite (Frasheri et al., 1995). The reason is the mineralogical conditions of chromite lenses and their host rocks. Mineralogical study showed that some chromite lenses have fractures that are filled with silicate secondary minerals (serpentine). Chromite and serpentine are the main minerals, and hematite and magnetite are minor minerals in the chromite orebodies. Although these minerals have been altered and have mostly been converted to serpentine, the earliest composition is likely to be olivine. Dunite and harzburgites are chromite lenses host rock and are mainly serpentinized and contain fine magnetite particles, which can cause positive magnetic anomaly (Imamalipour, 2009, Masoudi and Imamalipour, 2019). These small metallic minerals cause high induced polarization and the embedded rocks show a higher degree of charge. Because of the metallic nature of chromite lenses, the resistivity has a much lower value. Therefore, using resistivity, induced polarization, and magnetic geophysical methods, chromite lenses can be separated from harzburgite host rocks.

In this study, geophysical resistivity and inductive polarization method with magnetometry, which is one of the most important methods for the exploration of subsurface deposits in the Khoy ophiolitic zone, have been used. As a result, it was found that podiform chromite does not show much difference in the magnitude of the magnetic field. Therefore, this method cannot alone be used to explore chromite deposits. However, the IP-Rs method can be used as a practical method for exploration of these reserves. Chromite lenses have low resistivity values of about 400 to 600 ohm-m. The amount of induced polarization is also much lower than its host rock, with values of 3 to 6 mv/v. Therefore, these properties can be used for chromite exploration at a much lower cost than gravimetric and electromagnetic methods. The reason for these values can also be found in the mineralogy of the chromitite lenses. During the serpentinization process of harzburgite and dunite, magnetite minerals, chalcopyrite, and some metallic elements are released. Released magnetite increases the magnetic properties of chromitite. However, this increase is less than the magnetism of the host rock. The released metallic elements such as chalcopyrite with serpentinite also increase the changeability of the host rocks and chromite lenses with low induction polarization and much lower resistivity could be identified.