One of thirty-nine Melachot, or categories of activities forbidden on Shabbat, is called "Me’amer," gathering into a pile. After grain is harvested in the fields, the workers then collect all the stalks into piles. Such an activity – like all activities entailed in the preparation of bread – is forbidden on Shabbat on the level of Torah prohibition.
The Gemara in Masechet Shabbat (73a) cites a debate as to whether this prohibition applies to collecting salt in a salt quarry. While Rabba maintained that the "Me’amer" prohibition would indeed apply in such a situation, Abaye held that "Me’amer" is restricted to "Giduleh Karka" – materials that grow from the ground, such as grain. Salt does not grow from the ground, and therefore the Torah prohibition of "Me’amer" does not in Abaye’s view, apply to salt. Similarly, Abaye maintains that "Me’amer" would not apply to the collection of eggs in a chicken coup, since eggs, like salt, do not grow from the ground. Halacha follows the view of Abaye in this regard.
However, this entire debate relates to the Torah prohibition of "Me’amer." According to all opinions, there is a Rabbinic enactment prohibiting collecting on Shabbat even items that do not grow from the ground. Practically speaking, then, both Rabba and Abaye forbid collecting salt on Shabbat.
Seemingly, this would mean that one would not be allowed on Shabbat to collect salt that spilled. If, for example, the top of the saltshaker falls off while one salts his food at the Shabbat table, and the salt spills all over the table, it would be forbidden, at first glance, to collect the salt and put it back in the shaker or, for that matter, anywhere else.
In truth, however, this is not the case. Another condition concerning "Me’amer" stipulates that this prohibition applies only in the place from where the materials originated – such as collecting stalks of grain in the field. Once the stalks have been brought inside, they are no longer subject to this prohibition. By the same token, there is no prohibition – even on the level of Rabbinic enactment – against collecting salt or eggs in one’s home. Since they have already been taken away from their original location, they are no longer subject to this Halacha of "Me’amer."
Therefore, it is entirely permissible to collect spilled salt on Shabbat, and this does not transgress the prohibition of "Me’amer," neither according to Torah law, nor on the level of Rabbinic enactment.
Summary: It is permissible to collect spilled salt on Shabbat.
