The tricky thing about Silas Marner is that it's both incredibly realistic and totally implausible. The realist bits come through in the style (see "Writing Style" for more about that). The setting is pastoral: it takes place in a village full of charming local character; the village's allegorical name suggests that we aren't supposed to take the realism too seriously; and the story centers around the idealized rural celebrations of Christmas and New Year's.
And the plot? Miserly old man hordes a plot of gold until it's magically transformed into a living, breathing child (who just happens to be from a much better family) who redeems him and then marries the handsome neighbor boy. Heard this story before? It might as well come straight out of the Brothers Grimm or Hans Christian Andersen. The basic plot outline is sheer fairytale.
So which genre do we choose? The beauty of novels is that you don't really have to.