Is there hidden potential in Carajás? Insights through the geophysical signature of Cristalino deposit

Abstract

Geophysical methods are fundamental to exploration of IOCG deposits because they are associated with magnetite, hematite, and sulfides, which can produce different and significant geophysical responses. Because of the frequent occurrence of ferromagnetic minerals, magnetic data have been used extensively in exploration for IOCG deposits. The geophysical data at the Cristalino IOCG deposit in Carajás, Brazil, provide two important insights: 1) magnetic rocks are not fully mapped by total gradient data when these rock units have the same strike direction as the declination of the Earth’s magnetic field in low latitude areas, and 2) deposits that are mainly composed of brecciated to massive sulfide minerals are only weakly magnetic or even non-magnetic. These characteristics are important for exploration programs because qualitative analysis of magnetic data , especially by the commonly used total gradient calculated from total field anomaly, is the main tool for selecting prospects in low latitude regions, and important targets could be undetectable by this method. Gravity data have been extensively applied recently to exploration programs in Carajás, but similarly to magnetic data, the core of the brecciated to massive chalcopyrite mineralization can have lower densities than the surrounding alteration with hematite and magnetite and, therefore, be associated with low gravity anomalies that are not usually targeted in mineral exploration. The electromagnetic response will increase with the increasing connection between sulfides, varying from moderate in vein systems, to moderate to high in stringer and stockwork systems, to high anomalies in massive systems. To better understand the impact of these features on exploration programs, we show in this study that the magnetic, gravity, and electromagnetic responses vary with mineralization end-members. From a vein system formed at greater depths that produce high magnetic and gravity anomalies (associated with iron oxides from alteration) and moderate electromagnetic anomaly (poor connection between sulfides), to stringer, stockwork, and massive systems formed at shallower depths that exhibit lower magnetic and gravity anomalies but a high electromagnetic anomaly. Our analyses indicate that it is possible that the deposits mostly associated with massive sulfide could have been missed, and there could be significant hidden potential in the Carajás region. We illustrate that the detection of such potential depends on the quantitative integration of magnetic data with gravity and electromagnetic data through the inversion and multiphysics approach.