In that day. The future day, when this prophecy shall come to pass.
shall be. This is not the simple future tense, but the verb hayah, with the Participle, meaning that the fountain shall be permanently opened.
a fountain. This waits for literal fulfilment, and is not an intangible one as in the present day.
opened: i.e. set open. The only occurance of this participle in the O.T. Compare the first in Genesis 7:11.
for. for [the expiation of] sin, &c.
sin Hebrew. chata. App-44.