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?