Source : Webster's Revised Unabridged Dictionary (1913)

Pay dirt \Pay dirt\, Pay rock \Pay rock\, etc. (Mining)
   Earth, rock, etc., which yields a profit to the miner.
   [Western U. S.]

Pay \Pay\, n.
   1. Satisfaction; content. --Chaucer.

   2. An equivalent or return for money due, goods purchased, or
      services performed; salary or wages for work or service;
      compensation; recompense; payment; hire; as, the pay of a
      clerk; the pay of a soldier.

            Where only merit constant pay receives. --Pope.

            There is neither pay nor plunder to be got.
                                                  --L'Estrange.

   {Full pay}, the whole amount of wages or salary; maximum pay;
      especially, the highest pay or allowance to civil or
      military officers of a certain rank, without deductions.
      

   {Half pay}. See under {Half}.

   {Pay day}, the day of settlement of accounts.

   {Pay dirt} (Mining), earth which yields a profit to the
      miner. [Western U.S.]

   {Pay office}, a place where payment is made.

   {Pay roll}, a roll or list of persons entitled to payment,
      with the amounts due.

Source : WordNet®

pay dirt
     n 1: ore that yields a substantial profit to the miner
     2: a profitable success; "the inventor worked for years before
        hitting pay dirt"