That's correct. A nonbasic land that becomes a Mountain loses any existing abilities, and can only be tapped for red mana. Here's a Comprehensive Rules quote to help:
305.7. If an effect sets a land’s subtype to one or more of the basic land types, the land no longer has its old land type. It loses all abilities generated from its rules text and its old land types, and it gains the appropriate mana ability for each new basic land type. Note that this doesn’t remove any abilities that were granted to the land by other effects. Setting a land’s subtype doesn’t add or remove any card types (such as creature) or supertypes (such as basic, legendary, and snow) the land may have. If a land gains one or more land types in addition to its own, it keeps its land types and rules text, and it gains the new land types and mana abilities.
305.7. (...) It loses all abilities generated from its rules text and its old land types (...) this doesn’t remove any abilities that were granted to the land by other effects...
For example, a bloodmooned land enchanted by Racecourse Fury would still be able to hasten creatures.
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Please Please Please Autocard/// / / / [c]Protean Hydra[/c] => Protean Hydra / / / I can't read the cards you reference without it... / Well, ok, I CAN read the cards, but it involves opening Gatherer, looking your card up by name, hoping it's spelled right, and...
I'm just too lazy to do it, alright? Please Autocard.
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