BUZZ-NuVista Energy Ltd: Regulator investigates pipeline leak
** Canadian oil and natural gas company's shares down 6.9 pct at C$4.44
** Stock falls as much as 10 pct to near 3-year low of C$4.30
** Alberta Energy Regulator investigating leak on NuVista pipeline in northern Alberta that spilled roughly 629 barrels of oil, water and natural gas emulsion
** Leak first identified by helicopter crew on Friday
** Company says it has not yet determined cause of spill
** Up to Wednesday's close, stock had fallen about 36 pct this year
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