"40% of the gold that's ever been mined in the history of humanity."-Matthew S. Huber 

https://www.youtube.com/watch?v=R0icEsOcyO8

This LPI is referring to an impact crater in South Africa, Vredefort, one of the three largest known basin forming impact craters on Earth. The lecture emphasizes not gold or mining specifically, but rather structures called granophyre dikes. Dykes are vertically shafted magma, often infiltrating a crack or two strata layers; a chimney or wall of igneous stuff that is younger than the stuff around it, and also a channel for magma to reach the surface. These dykes are not volcanic, but melt from impact energy. Most of it pools, but some fingers of that molten pool reach under the impact producing these dykes.  

• At 1:15 along with a nice slide demonstrating a theme pops up, craters can be infused with ore. Gold ore in the case of Vredefort. 
• At 3:55 he explains the importance of these granophyre dykes. They are all that's left to study of the post-impact molten pool. 
• At 4:30 he shows a pretty slide with inclusions in the dyke. The inclusions have a lot of pattern and structure, this lineated form that implies the dyke was flowing in a particular direction when liquid. 
• At 12:30 he shows inclusions of a different kind, inclusions of a previous melt, implying that the material re-liquified at some point. 
• At 14:25 he starts proposing answers to all the questions he's been trailing.

Huber is a good speaker. I've seen him a few times before and his pacing and detail are somewhat refreshing to my attention span, though he can put a disinterested student to sleep.

Think about this, this gold-ore bearing impact was big and certainly not a gold nugget. The melt described in this lecture represents a mix of native and impact material, and that impact material must have been old. Gold is formed only in supernovae, so this impactor likely was older than Sol. Large and older than Sol implies that the primordial disk involved many large things, some bearing ore. 

Now apply the idea to worlds like Callisto and Mars. How many ore bearing impacts did they catch? Impact basins may be the only places a prospector can expect to find ore. For deep inside Callisto, the ice pulverized debris may settle in a lump or spread out to dilute the salinity so that each fragment separates from others. In Mars case, it may be more Earthly, but with similar dykes dropping more and longer roots.