If it is spurting it can't be anything other than an artery. Horses can bleed a lot of blood and still be fine.
Best to keep the pressure bandage on it for a good while. Don't take it off to change it for a while.
A vet should see her. Horses are very prone to tetanus, hope she's up on her tetanus vaccine.
It would also be important to look very carefully and see what she hurt herself on and remove it. Or it's pretty likely to happen again. Telltale signs are scrapes on the ground or in the stall (where ever it happened), blood, hair or skin on something sharp. Common culprits are broken fence wires, nails in fences or stalls. But broken off metal fence posts(they can be broken off just above or at ground level), old barbed wire or wire dumped somwhere on the property, farm implements, tools or broken troughs or building walls.
Horses can also get 'staked' - they run into a tree or stump or fence post or rail, and force a splinter or shard of wood into their head, leg, body or where ever.
Especially don't put salt and butter on a spurting wound, it just makes it that much harder to get it to drain and keep it clean; but it doesn't do any good for any kind of wound. Some old remedies are not good. I would not recommend flour either, for similar reasons. Flour is the happiest home in the world for bacteria. The best thing is to apply pressure to a spurting wound and to not do anything that would interfere with natural blood clotting.