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):

lode
    n 1: a deposit of valuable ore occurring within definite
         boundaries separating it from surrounding rocks [syn:
         lode, load]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "lode":
   Eldorado, Golconda, bonanza, chimney, chute, cornucopia,
   country rock, deposit, dike, font, fount, fountain, gangue,
   gold mine, gravy train, lodestuff, matrix, mine, mine of wealth,
   mineral deposit, mother lode, ore bed, pay dirt, quarry, resource,
   rich lode, rich uncle, shoot, source, source of supply, spring,
   staple, stock, vein, well, wellspring