What does "open up a vein" mean?

I found the phrase 'open up a vein of fury' in the article of today's Washington Post as shown below.

The New Year's Day suicide bombing of a church that killed 21 people has opened up a vein of fury among Egypt's Christians, built up over years of what they call government failure to address persistent discrimination and violence against their community.

What does 'open up a vein of' mean exactly? Why should the writer use 'open up a vein of' in this particular case, instead of other expressions, for instance 'a spell of anger' or simply 'has infuriated'? Can somebody teach me?

"To open up" is a phrasal verb, opposite of "to close up." But that's probably not the part that's confusing for you. "A vein of fury" is a wave of anger, as you suspected.

So, it's akin to the expression "to open up a(n old) wound," meaning to pry open something bloody, painful, and in this case, violent.

Your other suggestions would work just fine. This journalist has just chosen to use a more vivid imagery.

The origin of the phrase may stem from either mining or anatomy; either you've opened up a vein of ore and are mining it until it runs out, or you have opened up a vein in your body and are bleeding all over the place until you manage to stop the flow. The anatomical interpretation seems the more likely of the two to me, especially in this case, as something that was not necessarily intentional and would be better off being stopped.

The implication of the phrase as used, though, is that the fury has been there for a long time but has not been released until this event; whereas "has infuriated" would clearly mean that the fury being released was generated directly by the event in question (which would contradict the following clause about how long the fury has been building).

Excuse me for being graphic. When you open a vein (well, technically, an artery) in the body, blood comes pouring out; by analogy, to open up a vein of fury is to cause fury to emerge in a great and sudden flow.