RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Metal Key
The Old School RuneScape Wiki also has an article on Metal Key.

The Metal Key is an item found during the Tourist Trap quest. It is obtained by killing the Mercenary Captain during the quest, and is used to open the main gate to the Desert Mining Camp, which gives the player access to Mithril and Adamantite rocks.

If lost, another key can be obtained by killing the Mercenary Captain (located outside the camp) again.