From just that photo, I'd guess at tension gashes (veins, basically).
Groups of veins can have various structures. They are often in parallel sets. Sometimes one finds multiple parallel sets, at an angle to each other. You might be seeing some of that here: some veins running through the rock, others lying on the surface. ...That surface being a fracture that has used the vein (and perhaps associated joints (fractures not filled with vein mineral)) as a convenient weakness along which to break.
To be more confident, I'd need a close-up. I'd be hoping to see crystals running from one side to the other, at right angles to the sides, perhaps with some symmetry side-to-side.
The vein is orangish probably due to iron in the vein mineral; perhaps siderite.
You could also enquire at Burlington Slate Ltd., Kirkby-in-Furness, Cumbria LA17 7UN