HOUSTON, July 16 (Reuters) - Oil major Exxon Mobil ( XOM )

bought Mars crude for August delivery, two sources said

on Wednesday, after briefly halting purchases of the flagship

offshore grade due to a zinc contamination issue.

The zinc contamination in the Mars crude oil stream had pushed

Exxon to borrow up to 1 million barrels of crude oil from the

Strategic Petroleum Reserve for its Baton Rouge refinery in

Louisiana.

The start-up of an offshore well caused the zinc

contamination in Mars crude, Chevron ( CVX ) said last week.

Chevron ( CVX ) was actively working to resolve the issue, the

company said on Tuesday.

Exxon and Chevron ( CVX ) did not immediately reply to requests for

comment on Wednesday.

The crude grade was trading at a 30-cent discount to U.S.

crude at the Cushing, Oklahoma, hub, due to the persistent

quality issue. That compared with a $1 premium at the end of

June.