Marriage records (Certificate index) for most Minnesota counties, including St. Louis County from 1870, are available on the MINNESOTA OFFICIAL MARRIAGE SYSTEM, also known as MOMS.
You can find the MOMS database here:
Most St. Louis County Records from 1870-1880 are missing and in the process of being added at this time (7/2019). If you do not find a record from that time period, contact the St. Louis County genealogy volunteer. Information and email link: https://www.stlouiscountymn.gov/departments-a-z/public-records/records/genealogy-requests
Here are some useful tips for searching MOMS: