https://thenextweb.com/apple/2018/06/12/apple-bans-cryptocurrency-mining-on-the-iphone-and-ipad/

Apple’s recently updated developer guidelines include a message intended to ward off would-be cryptocurrency miners. The new rules restrict apps that generate excess heat, pull unnecessary system resources, or otherwise drain battery or affect performance unnecessarily.

Apps, including any third party advertisements displayed within them, may not run on unrelated background processes, such as cryptocurrency mining.