I have hit zero deposits of anything other than sub par metals and ores... and I have spent hours flying around finding only Major Reserve belts with nothing in them :-(
Pristine resources are much easier to find at the edges of inhabited space where mankind hasn't started exploiting them yet, it's quite logical really.

When i started mining i just checked the galaxy map for where the federation, imperial and alliance territories had its edges and went straight for the closest edge, it was a long trip but i found plenty of pristine systems with both alliance and federation systems around so i could make extra profit from bounty hunting while mining.
When i first arrived i was a complete mining newbie but my first two hour trip to a pristine metallic ring yielded 34t of metals för 320K credits and bounties for an additional 40K, the two hours included learning scooping and stuff so it's quite profitable.
Later i got a surface scanner so i could search out metallic areas in unexplored systems (most are at the edges) to get some variation when i play, so i combine exploring, bounty hunting and mining which makes it far less repetitive.