Store bought minerals for Goats vs mineral rich rocks/soil

I live in an area with very mineral rich soils and rocks.  I have read that free range goats chew on rocks to get the minerals they need. I need to keep my new goats in pens due to predation.  Do I have to purchase minerals, or can I collect a sample of rocks/stones/mineral rich soils from the area and keep those in the pen along with a salt lick?  

Depends on your local soil.  Most places are rich in some minerals and dangerously lacking in others.  It's a balance and ratio between the individual minerals that count.  

Your local feed shop should be able to tell you what's common in soil in your area and what the common supplements are.  There should be a local mix for your area (which is usually about 2 hour drive so, these areas are quite small).  

Natural goat care by colbey (sp?) Is a good book if you ever want to deep dive into the topic.  This also advocates free choice minerals which is another option.

Basically, there are very few exceptions to goats needing minerals.  But what minerals depends on location and other factors like diet and breed history.  And there are some exceptions.

I believe in salad bar free choice mineral. They know what they need and can choose just that without having to eat a ton of extra everything else so things stay balanced. Kinda pricy to get started for a few goats, though. Make sure to include baking soda.

That said, you should put a pile of rocks in their pen anyway. It gives them a way to exercise their feet and self trim their hooves a little.