By Arathy Somasekhar

HOUSTON, July 10 (Reuters) - Oil major Exxon Mobil ( XOM )

told its trading counterparts that it will not buy the

Mars crude oil grade until a zinc contamination issue is fixed,

two sources familiar with the matter told Reuters on Thursday.

Zinc contamination in the Mars crude oil stream has pushed

down demand and slashed prices for the flagship crude oil

produced off the U.S. Gulf coast, sources said earlier this

week.

Mars traded at a 10-cent premium on Thursday to crude oil at

the Cushing storage hub in Oklahoma, dealers said.

It had eased to 10-cent discount earlier in the week

compared with a $1 premium at the end of June.

Exxon did not respond to requests for comments.

Zinc does not typically occur naturally in crude oil and

running crude with zinc could lead to damage to refining units

and catalysts used in processing oil.