Chimney repointing?

Hi folks...my roof sprang a sudden large leak a few rainstorms ago. The water is coming in at the end of the roof sheathing where a dormer is cut in, right next to the chimney. There is no visible flashing, just a few inches of tar where the roof & chimney meet. It looks solid, no cracking or peeling, but a lot of water is getting in somewhere.
The mortar on the chimney is in pretty bad shape especially at the top. I'd have no problem believing the water was getting in through mortar cracks and exiting under the tar. I'd like to seal this up before tomorrow's start of 2 days of rain.
The house is 75 years old and in the northeast. Can I use type N mortar or should I put a temporary seal up there and determine what mortar was used originally? It's borderline as to whether type N or type O was used. The stuff looks pretty sandy, but that could be weathering.
thanks, jim
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Wait 10, 20 ,50 yrs. You will be the ASS that experimented, and HD now wont warranty your PUSS,,,,, Wake UP It lasts thousands of years done right ,, Bozo, YOU a BOZO , BOZO
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My cousin built a chimney on a new house and the flashing was never installed. It started to leak and he tried everything but couldn't get it to stop leaking. He finally removed some mortar and installed the flashing. The leak then stopped.''Good luck.

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