Lemkau Surname

I hoping to find information on an Anna Lemkau Oelrich born Sept, 1841 in
Germany and resided in Hanover in 1864. She had a son named Claus Frederick
Lemkau who was born in Frieburg on the Elbe in 1863.

I believe Claus may have been born out of wedlock, and was given Anna's
maiden name. She eventually married an Oelrich and had her Oelrich children
in Hanover.

Claus came to the U.S. when he was 19 and resided in LeSueur Minnesota. Anna
came in 1888 when she was 48 years old, and also took up residency in
LeSueur, Minnesota.

Any help would be wonderful. Thanks