I had assumed those patches you call mineral deposits were actually magma, which while not properly in the game yet is present on every planet I've seen these patches on. Any idea whether it's you or me that's correct?
You know what, I'm gonna say we both are

My thinking being that these deposits are caused by active vulcanism. Whether or not the barnacle yielding planets are dependent upon vulcanism though, I'm not sure. Seen several different types though:
Iron Magma, Silicate Magma, Carbon Dioxide Geysers, Water Geysers, Silicate Vapour Geysers, Nitrogen Magma, Water Magma and most recent and interestingly, Ammonia Magma. So far no luck on any of them, however, I am yet to find a planet with Ammonia Magma vulcanism I can land on. Certain gas giants contain ammonia based life, and these barnacles are alien in origin. Maybe there's a link.
For me right now, this whole barnacle mystery (which something just tells me is linked to the UA mystery) hinges on the "These appear to be common in certain parts of space..." line from the Meta Alloy commodity mkt description. The key word there being "parts".
What do they mean by "parts"?
What distinct "parts" do we have in the galaxy?
Well, we have nebula. Tried a lot of those. No dice. (Though not all of them).
We have dark regions. Also scoured them with no luck. (again, not all of them - but these things are supposed to be common if we are in the right "part").
We have the spiral arms themselves? I doubt it. Too vague. And too large.
That's about it. Which, if we eliminate the above, leaves us with a needle to find in a galaxy-sized haystack. I've been staring at the galaxy map for so long now I am seeing multi-coloured spots everywhere!
Horizons has just been released, with a big focus on planetary landing. From a game play perspective, FD want us tooling around on planets and they also want us to find this stuff so will most likely make it accessible to all. i.e newish player with only an exploration Sidey - meaning it's close, but outside the bubble, hence the 6 day timer on the Meta Alloy mission.
Anyway, I'm convinced (purely a hunch, and very happy to be proven wrong!) that the key to the "parts" lies in the individual planets themselves. Which means that the "parts" may simply mean "the right planet" and not necessarily tied to a specific 'area' of the galaxy.
Right, I am off to continue to search for a planet with Ammonia Magma vulcanism that can be landed on.
