Inland reef (now out of buisness LFS that used to be located in Nashua) sponsored some lab analysis of the stuff yrs ago.
IIRC the findings were that the stuff was mostly silica (never had anything to do with the ocean) and it was loaded with iron (good for feeding macro algaes). Conclusion, it might be good for supporting macro growth in refugia, but probably never had anything to do with the ocean.
I can't find any links with the discussion anymore, but here's the basic results;
http://www.reefs.org/library/article/mm_analysis.html