pay streak

from The Collaborative International Dictionary of English v.0.48

Pay streak \Pay streak\
   1. (Mining) The zone, parallel to the walls of a vein, in
      which the ore is concentrated, or any narrow streak of
      paying ore in less valuable material.
      [Webster 1913 Suppl.]

   2. (Oil Boring) A stratum of oil sand thick enough to make a
      well pay.
      [Webster 1913 Suppl.]