"She had a bottle with 427 grams of heroin solution and three rubber bags with 205 grams of heroin solution," the source said.
A woman who allegedly bought 127 grams of heroin from the girl was also detained.
Police said the drugs had a black market value of over 700,000 rubles ($20,000).
A criminal case has been opened. The girl was taken to a pre-trial detention center for minors and could face up to three years in prison if found guilty.