Samaria isn't one of the tribes, it was the capital of the northern kingdom, and its population was largely rounded up and marched off into exile. Assyrians moved in, possibly on their own, to the vacant land.
Most of Judah and Benjamin (the southern kingdom) remained; small numbers of the other ten tribes laid low or were otherwise missed, or were out of town when the whole thing went down.
[snip] One other consequence of the Assyrian invasion of Israel involved the settling of Israel by Assyrians. This group settled in the capital of Israel, Samaria, and they took with them Assyrian gods and cultic practices. But the people of the Middle East were above everything else highly superstitious. Even the Hebrews didn't necessarily deny the existence or power of other peoples' gods—just in case. Conquering peoples constantly feared that the local gods would wreak vengeance on them. Therefore, they would adopt the local god or gods into their religion and cultic practices.
Within a short time, the Assyrians in Samaria were worshipping Yahweh as well as their own gods; within a couple centuries, they would be worshipping Yahweh exclusively. Thus was formed the only major schism in the Yahweh religion: the schism between the Jews and the Samaritans. The Samaritans, who were Assyrian and therefore non-Hebrew, adopted almost all of the Hebrew Torah and cultic practices; unlike the Jews, however, they believed that they could sacrifice to God outside of the temple in Jerusalem. [/snip]The Two Kingdoms | Jewish Virtual Library
The Samaritans have always said this was false and that they, the Samaritans, were the descendants of the northern tribes.
Genetic studies have shown that the Samaritans and Judeans have the same genes and this pdf gives more information
Note that The book of Chronicles compounds the difference in interpretation of Samaritan history. Recalling that Hezekiah ruled the southern kingdom of Judea from 715 BCE, after the Assyrian victory, the following passage seems to contradict the above statement from II Kings: And Hezekiah sent to all Israel and Judah and wrote letters also to Ephraim and Manasseh that they should come to the home of the Lord at Jerusalem to keep the Passover.” (II Chronicles 30: 1)