What do Thornton and his partners find after a long search in Call of the Wild?Why does this cause Buck to move toward the call of the wild?

After Buck had made his master $1600 by pulling the sled
laden with 1000 pounds, Thornton was able to begin a journey of which he had always
dreamed: He and his partners would seek the fabled Lost Cabin mine. They spent the next
two winters searching the Yukon for the lost mine, but it was never found. Instead, the
men stumbled upon a "shallower place" that yielded more gold than they could have ever
imagined. Every day the men worked created a yield of "thousands of dollars in clean
dust and nuggets," and so the men decided to stay until they could mine all the gold
they could find. The remoteness of the area led Buck to trail a pack of wild wolves, and
Buck's call of the wild became greater than ever. And when Buck returned to camp one day
to find everyone dead, he realized that it was time for him to leave the world of the
humans and pursue the call.