What is the yellow metal found in the arctic darkness in The Call of the Wild?

It is gold.  The whole point of people all being up there in the Arctic was gold.

The whole story starts with gold being discovered.  Once gold is discovered, there is a huge rush of people up to Alaska and the Yukon Territory.  The only way for the people to get things from one place to another up there (other than carrying it themselves) is by dog sled.  That is why Buck is so valuable that it can be worth Manuel's time and effort to steal him.

You can see this also, for example, in the part where Thornton bets on Buck's ability to pull -- the money is in gold dust, not cash.