عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Magnetic rocks contain various combinations of induced and remnant magnetization that perturb the Earth's primary field. The magnitudes of both induced and remnant magnetization depends on the quantity, composition, and size of magnetic-mineral grains. Magnetic anomalies may be related to primary igneous or sedimentary processes that establish the magnetic mineralogy or they may be related to secondary alteration that either introduces or removes magnetic minerals. In mineral deposit exploration and its geoenvironmental considerations, the secondary effects in rocks that host ore deposits associated with hydrothermal systems are important. Geoenvironmental applications may also include identification of magnetic minerals associated with ore or waste rock from which desirable materials may be released. Such associations permit the indirect identification of materials such as those present in many copper deposits. This paper describes the results of the geomagnetic method for exploration of copper and gold mineralization in the Bashmaq polymetallic zone, in East Azerbaijan Province in the northwest of Iran. Obviously the main and efficient geophysical method for Cu-Au exploration is resistivity–IP method, whereas the magnetic method is usually considered as an indirect or secondary method. In this case study, copper mineralization is observable mainly as secondary minerals such as Malachite and Azurite with some iron oxides such as ologist which fill joints and fractures in faulty contacts. After primary geological surveys and determination of the location of mineralization out crops, a data network containing 31 East–West lines with 20 m space between lines and 10 m space between stations (reduced to 2 m in the mineralization zone) has been designed and around 3000 magnetic data have been gathered. Measurements are made by using a GEM proton magnetometer (GSM-19T) equipped with a DGPS system. In processing step, after removing daily variations of total magnetic anomaly and removing the trend effect, RTP (reduction to the magnetic pole) map was created and for interpretation, upward continuation maps were prepared for various distances. As a result of this survey, a low-amplitude negative signal with about 700 m length and a nearly NW–SE direction was detected. This signal is consistent with the outcrops which had been seen previously in the geological surveys. In addition, some positive signals related to magnetite and ologist mineralization in the study zone could be detected. Because these positive and negative signals are independent and lack a magnetic dipole structure, horizontal derivative filters such as analytic signal filter were not used. It is important to point out that we must consider the negative signal as the direct result of faulted zone containing mineralization of Cu-Au and IP method must be used for additional data acquisition. In modeling process, the results of processing and interpretation stages were used as input data for modeling and a 3D model of the faulted zone containing copper–gold mineralization was created. As a final result of this study, we can say that application of the magnetic method can serve for exploration of Cu-Au mineralization particularly in mineralization systems related to faults and fractures and that its results will be advantageous for optimal usage of the geoelectrical methods such as the induced polarity and resistivity.