If the string did not start with “:string”, it would be a valid JSON string, and you could decode it with the json.Unmarshal function.
From your other post (that you erroneously posted into the “Jobs” subforum, BTW), I conclude you want to access the Web API of kraken.com and parse the results. The API returns JSON data, so you can use standard JSON decoding to parse this data into a struct.
Helpful resources:
The Go Blog about JSON:
https://blog.golang.org/json-and-go
JSON in Go By Example:
https://gobyexample.com/json
And this one is a really helpful tool: Paste some JSON and get the required Go structs back.
https://mholt.github.io/json-to-go/
(From the creator of Caddyserver.)
HTH