Is zinc sulfide a pure substance?

Zinc is a pure substance.

it's a pure substance, because it's an element.

Matter that is not a mixture of different types of Atoms. A pure substance is a collection of Matter that is comprised from [and or composed of] Atoms of only One Atomic Element.

Technically, a non-pure substance is called a compound, so substance should not be used as an official scientific term. If you do come across the term 'substance', use context to decide. e.g....

water and oxygen...