Engine separation won't cause automatic closure of valves, but the resulting low pressure will illuminate lights (including the Master Caution light) on the flight panels and display messages on the monitoring displays.
The pilots will have to react to the engine separation, likely the same way than for an engine fire or severe malfunction. This non normal procedure include shutting down fuel line.
There is a spar fuel valve that is fixed to the wing, and stay on the wing in case of separation. This valve will prevent leaks due to the line. It's an electric motor (the hydraulic power may be missing after the engine separation) with a backup battery and a double wiring to increase closure ability.
Example on the the B737 NG:
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Non-Normal checklist used: ENGINE FIRE or Engine Severe Damage or Separation (NNC 8.2 of the Quick Reference Handbook).
(Click to enlarge)
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