Long term variations of the geomagnetic field are one of the few sources of information about the evolution of our planet’s deep structure throughout time. Most information about past field variations is derived from studies of lava flows or sedimentary sequences. Slowly-cooled igneous intrusions are rarely used to study geomagnetic field variability because the smoothing of field variations accompanying slow cooling is poorly understood. Many large intrusions have remanence carried by precise, elongate magnetite grains hosted within silicate minerals that could provide reliable records of directional and intensity fluctuations of the field. Many such intrusions are available throughout geologic time and could provide a new source of information on geomagnetic field variations if the effects of slow cooling can be quantified. Here we use the single domain magnetic code provided by Nagy et al. (2021) to analyze how thermoremanence is acquired in slowly-cooled intrusions over various timescales (lab timescales – hundreds of thousands of years). We find that statistical models of geomagnetic variability, because they are not serially correlated in time, result in magnetizations far smoother than observed in large intrusions. In contrast, a Geomagnetic Global Field model spanning the past 100 kyr, has temporally correlated variations including a short excursion that can be recognized (although smoothed) in the modeled remanence of slowly-cooled intrusions. Earth-realistic field models in conjunction with our model can give a good representation of variability recorded over slow cooling and may potentially allow a geomagnetic signal to be recovered from large intrusions.