The Collaborative International Dictionary of English v.0.48:

Lode \Lode\ (l[=o]d), n. [AS. l[=a]d way, journey, fr.
   l[imac][eth]an to go. See Lead to guide, and cf. Load a
   burden.]
   1. A water course or way; a reach of water.
      [1913 Webster]

            Down that long, dark lode . . . he and his brother
            skated home in triumph.               --C. Kingsley.
      [1913 Webster]

   2. (Mining) A body of ore visibly separated from adjacent
      rock.
      [PJC]

   3. Especially: (Mining) Any regular vein or course of
      valuable mineral, whether metallic or not.
      [1913 Webster]

   4. Hence: A concentrated supply or source of something
      valuable.
      [PJC]

   mother lode a large concentrated source of mineral or other
      valuable thing, from which lesser sources have been
      derived; -- often used figuratively. The term may have
      been originally applied to real or imagined large deposits
      of gold from which smaller granules were washed
      downstream, there constituting a diluted source of gold,
      and hinting at the richer source from which they were
      derived; as, to hit the mother lode.
      [PJC]

WordNet (r) 3.0 (2006):

mother lode
    n 1: the main vein of ore in a deposit [syn: mother lode,
         champion lode]