BB lug braze failure
 

Could this fail be rebrazed? Seat tube is not cracked. 1989 Swhwinn Voyaguer, Tenax chromoly tubing

That's an impending shell failure. You could stop-drill the ends of the crack and fill it all with brass and it ought to hold for a while at least. The good news is that even total failure of the shell (crack propagating entirely around the socket) won't be catastrophic. A friend of mine had this happen to one of his bikes. Spent several weeks trying to figure out why his bottom bracket always seemed to be loose. Finally cleaned all the grime off and found that his BB shell had cracked all the way around the seat tube socket.

Yes, it can be repaired, but it's a question of cost, and repainting afterward.

You might email a photo to the folks at Yellow Jersey in Madison. They are about the most reasonable for quality work of this kind.

The root cause is likely the choice of bottom bracket shell to keep the cost of production at the lowest point possible. The lugs should have some points on them to distribute the stresses over time. I have seen several frames that broke just like this one, some had same breakage on two or more joints, all were Schwinn brand (but not implying anything). I never found the frames overall value worth the cost of repairing them. It was far easier to just pick up a used TREK or Giant on Craigslist for $50, tune it up and send the rider on his way.

This points to a different issue and (most likely) is not related to this particular failure.

The use of points on lugs, especially head lugs is to spread the load down the tube, and prevent tube failure. But that's not what happened here.

Looking at the crack and how it progressed, I see a hint to the likely cause. Bart of the BB is adhered to the tube, and part is not. So excess stress is concentrated on the adhered section (above the crack) which moved with the tube, and got no help from the area to the left which isn't. So, IMO, the proximate cause is a "cold joint" where there's ineffective brazing to the left of the crack, either from no flux, or failure to heat enough to draw the brass in.

That may also explain the rust pattern, which may have started with water wicking into and attacking the steel at the unbrazed area.

Of course, even though it's based on visible clues, the only way to be sure is to take the joint apart and see.