While doing the "Gas in the Mine" Mission, you'll stumble on a chest outside in one of the huts that is locked and can only be opened with a key. That key, as you can probably guess, is inside the poison-filled mine. Here's where you find the Mine Chest Key in Elex 2!
Since you have to head in any way - make sure you have a gas mask and some solid gear, the insect inside is no joke to fight early on. If you can, lure it outside and shoot the gas canister just outside the entryway to take about 75% of its health out!
Once that's done, head into the cave and explore - some +Poison Resistance food is necessary if you want to loot the iron that's littered throughout the cave.
Head in and take an immediate right after you come to the area with shelves and scaffolding. You'll follow the path until you see there's another set of bodies and scaffolding on your left. Near the next "doorway" to your right, you'll find the "Chest Key" nestled near an Iron Ore Vein.
Pick it up and head back out to the hut to get a decent bow and additional items, then head on your way!