I've seen this movie many times, but when I watched it tonight, something struck me for the first time. When Sylvia is laid on the altar in preparation for the voodoo ritual, Squire Hamilton bends over as if to kiss her, but the camera pans down in a way that typically (in that era) indicated that something obscene was taking place.
It seemed odd that when his intent was to turn Sylvia into a zombie, the Squire would suddenly feel compelled to make a sexual advance; but also it seemed odd that the camera would pan away so noticeably if not to protect the viewer from something more disturbing than a mere kiss.
Sorry to "resurrect" this thread (shameless pun), but I just finished watching this, and I didn't see it that way. I did notice the camera pan, but it was very brief. He didn't look positioned for anything, and nothing implied it afterward.
I've just watched the film again for the first time in ages, and that shot struck me, too. The timeline of the scene seems to suggest there's no time for a serious sexual assault, so I can only assume the script called for something less extreme but still beyond the pale. Perhaps the actors balked at such a scene, or perhaps John Gilling -- who both wrote and directed -- simply decided that hinting without showing would allow the audience to draw its own inferences. I wish more directors still gave the audience credit for some imagination.
What I found far more disturbing, and which had only occurred to me on this most recent viewing, was what exactly did Squire Hamilton intend to do with Alice and Sylvia once they were undead and hidden away at his manor? It's hard to believe that he intended to put them to work as he had done with the men; even undead, the women wouldn't be as strong and, anyway, Hamilton already had a workforce of fourteen by this time, enough to fill the small mine workings. He simply murdered the men before resurrecting them, but it seemed important to him that the women were under his thrall before claiming them. The obvious suggestion as to his intentions towards them is probably the correct one, and it says a lot for Gilling's script that it is never overtly stated or even heavily hinted at.
I have to agree with Pattamus--I didn't see any implied rape at all. She was just there as a sacrifice--nothing more. Hammer movies were explicit in 1966 but that was in terms of violence--not sex.