Your site returns HTTP 403 Forbidden when the Twitterbot attempts to access your content.
$ curl -A Twitterbot http://www.angloamerican.com/careers
* Trying 107.154.112.20...
* TCP_NODELAY set
* Connected to www.angloamerican.com (107.154.112.20) port 80 (#0)
> GET /careers HTTP/1.1
> Host: www.angloamerican.com
> User-Agent: Twitterbot
> Accept: */*
>
< HTTP/1.1 403 Forbidden
<
This is covered in the troubleshooting post.