Research has shown that computer games and other virtual environments can support significant learning gains because they allow young people to explore complex concepts in simulated form. However, in complex problem-solving domains, complex thinking is learned not only by taking action, but also with the aid of mentors who provide guidance in the form of questions, instructions, advice, feedback and encouragement. In this study, we examine one context of such mentoring to understand the impact of replacing "face-to-face" interactions between mentors and students with "virtual," chat-based interactions. We use pre- and post-measures of learning and a post-measure of engagement, as well as "epistemic network analysis" (ENA), a novel quantitative method, to examine student and mentor discourse. Our results suggest that mentoring via online chat can be as effective as mentoring face-to-face in appropriately structured contexts more generally--and that ENA may be a useful tool for assessing student and mentor discourse in the context of learning interactions.