You may have a chunk of float copper. This happens when globular molten copper is deposited volcanically...it got pushed around a lot by glaciers but there's a huge one that was found in northern Michigan. My friend has several that weigh in excess of 30lbs like yours. They're typically very pure copper, like 99%+ pure. Very nice find!! Where did you pull it? My friend found his near a stream in the Keeweenaw.
Nice find! You know that where you find Copper, you may also find Gold. It is not a sure thing but still worth panning some material where the Copper nugget was found.