July 17 (Reuters) - Grades broadly fell on Thursday, dealers said, with Mars Sour trading at its lowest since mid-November, while Light Louisiana Sweet bucked the broader downward trend.

Mars Sour WTC-MRS fell 45 cents to a midpoint of a 75-cent discount to U.S. crude futures, compared with $1 premium at the end of June.

The grade has weakened owing to quality issues after the Mars crude stream was contaminated with zinc.

Oil major Exxon Mobil XOM.N bought Mars crude for August delivery, two sources said on Wednesday, after briefly halting purchases of the flagship offshore grade due to the contamination issue.

* Light Louisiana Sweet WTC-LLS for August delivery rose 13 cents to a midpoint of a $2.93 premium and was seen bid and offered between a $2.60 and $3.25 a barrel premium to U.S. crude futures CLc1

* Mars Sour WTC-MRS fell 45 cents to a midpoint of a 75-cent discount and was seen bid and offered between a 85-cent and 65-cent a barrel discount to U.S. crude futures CLc1

* WTI Midland WTC-WTM fell 10 cents to a midpoint of a 35-cent premium and was seen bid and offered between a 25-cent and 45-cent a barrel premium to U.S. crude futures CLc1

* West Texas Sour WTC-WTS fell 30 cents to a midpoint of a 40-cent discount and was seen bid and offered between a 65-cent and 15-cent a barrel discount to U.S. crude futures CLc1

* WTI at East Houston WTC-MEH, also known as MEH, traded between a 45-cent and 65-cent a barrel premium to U.S. crude futures CLc1

* ICE Brent September futures LCOc1 rose $1 to settle at $69.52 a barrel

* WTI August crude CLc1 futures rose $1.16 to settle at $67.54 a barrel

* The Brent/WTI spread WTCLc1-LCOc1 narrowed to last trade at minus $3.26, after hitting a high of minus $3.17 and a low of minus $3.32

(Reporting by Georgina McCartney in Houston; Editing by David Gregorio)

((Georgina.McCartney@tr.com))