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.
I had to smile at your question because asking that particular question shows that we need to clarify more about the setting of Call of the Wild for you. Quite simply, the "yellow metal" found in the "arctic darkness" is gold. Plain and simple.
Please note that the main setting of place is the Yukon River basin and the setting of time is 1897. This is rugged, mountainous terrain. London knew it well. Why? He actually participated in what was called the "Klondike Gold Rush." It is completely feasible for Buck to be stolen and pressed into service as a sled dog because that was the only way to travel quickly in the Yukon. There is a perfect quotation from the beginning of the novel that shows this exactly.
Here is where you see the term "yellow metal" and "Arctic darkness" of your question. It is directly from the book. It is simply how Buck, the dog, might understand it. Gold would mean nothing to Buck. Note the term "rushing," hence the name "Gold Rush" to the "Northland" or Klondike. Then there is the important part for our dog hero: "these men wanted dogs." Buck is a prime example of a valuable dog to someone who was searching for gold in the Klondike. He had a thick fur coat, a large build, and was in his prime. Buck was the perfect target.
Because men, groping in the Arctic darkness, had found a yellow metal, and because steamship and transportation companies were booming the find, thousands of men were rushing into the Northland. These men wanted dogs, and the dogs they wanted were heavy dogs, with strong muscles by which to toil, and furry coats to protect them from the frost.