blanket-deposit

from The Century Dictionary.

• noun The name given in some parts of the Cordilleran mining region, especially in Colorado and Utah, to deposits of ore occurring in a form having some of the characters of those elsewhere designated as flat sheets, bedded veins, beds, or flat masses.
• noun They are frequently intercalated between rocks of different lithological character and origin, in which case they partake of the nature of contact-deposits. The occurrences of ore at Leadville are of this nature.