coalfield vs lode

coalfield

noun
  • Any region containing deposits of coal that may be mined. 

lode

noun
  • A vein of metallic ore that lies within definite boundaries, or within a fissure. 

  • A rich source of supply. 

  • A watercourse. 

How often have the words coalfield and lode occurred in a corpus of books? (source: Google Ngram Viewer )