Hidden in plain sight: using Atom Probe Tomography to understand the formation of invisible gold deposits

Carlin-type gold (CTG) mineralization is one the best studied, yet poorly understood gold mineralization styles in the world. These deposits occur predominantly along NW-SE trends in central Nevada and are characterized by invisible gold thought to be hosted in sulfide minerals (Cline et al., 2005). CTG accounts for 9% of worldwide gold production, with all of it currently coming from five mining districts in northern and central Nevada. The discoveries of new CTG deposits in the Yukon Territory, China, and Kyrgyzstan as well as the presence of significant CTG-like-gold in already known deposits in Europe will drastically increase the importance of these deposits in the upcoming years.

Despite the vast resource that CTG deposits entail, surprisingly little is known about their formation mechanisms, fluid source, or even the manner in which the gold is hosted. We know that the gold tends to occur as trace elements within micro-to-nano-scale domains within pyrite. With the recent application of atom probe tomography to geologic materials we now have the nano-analytical techniques to truly understand these cryptic and globally important deposits.

We combine high-resolution electron probe microanalysis (EPMA) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) with atom probe tomography (APT) to better understand the composition and isotopic signature of the ore fluids, how gold is hosted in these deposits, and why sulfide minerals make such great hosts for gold and other critical metals.