By default, the gvfs/udisks2 stack requires a root/superuser password when mounting internal partitions.  Setting the “user” option in your /etc/fstab file will not change this behavior in the gui (by default, Thunar) file managers that utilize gvfs and/or udisks2.

Authentication for udisks2 is handled with policy kit.  If you wish to override the default behavior of requiring a root/superuser password, then paste the contents below into a file called /etc/polkit-1/localauthority/50-local.d/50-udisks.pkla to allow any user in the “users” group to mount partitions without a root/superuser password.  Changes should take place immediately.  Note that this will NOT override the filesystem permissions.  

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Content of new file /etc/polkit-1/localauthority/50-local.d/50-udisks.pkla

Code:

(udisks)
Identity=unix-group:users
Action=org.freedesktop.udisks*
ResultAny=yes
ResultInactive=no
ResultActive=yes