The incident, which happened at 4 p.m. on Wednesday, is being investigated by the Colorado Department of Public Health and Environment.
The thick grey-yellow cloud is seen in the air above and around the plant in photos submitted to 9NEWS.
It is "highly unlikely" the emission caused a danger to public health, a state health official said.
Suncor spokeswoman Lisha Burnett says the release violated the regulatory threshold for the opacity of an emission but Suncor does not think it exceeded benchmarks for hydrogen sulfide and sulfur dioxide, both byproducts of the refining process.
Burrett described the equipment failure that led to the emission as "very rare."
Excess emissions must be reported to state health officials and, in addition, the required formal report from Suncor is expected by the end of next month.
Suncor has had 28 reportable emissions incidents since the beginning of 2010, Burrett said.
Wednesday's emission included a release of catalytic dust, which Burrett said is 90 percent composed of clay with lesser amounts of carbon, silica, aluminum and other compounds.
Christopher Dunn, spokesman for the health department's Air Pollution Control Division, says the emissions incident lasted about 15 minutes.
Dunn says routine malfunctions of equipment are not subject to enforcement actions but breakdowns primarily caused by "poor maintenance, carless operation or any other preventable upset condition or preventable equipment breakdown" could be cause for action.
Federal environmental authorities are aware of Wednesday's incident, but are deferring to state health officials.
If you see what you suspect to be excess emissions, you can report them to the Environmental Protection Agency's National Response Center at 1-800-424-8802.
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