The Peninsula Mountain magnesite is located at the south end of Peninsula Mountain, east of Taku Arm and just west of Turtle Lake.

The Penninsula Mountain occurrence is underlain by sedimentary rock of the Inklin Formation (Lower Jurassic Laberge Group) at or near a north-trending thrust fault contact with andesite of the Middle Triassic to Early Jurassic Cache Creek Complex. The fault has thrust Cache Creek rock onto the Inklin package to the west. Quartz diorite of the Late Cretaceous Windy Table Complex intrudes along the fault area a few kilometres to the north. The Peninsula magnesite could be hosted by Cache Creek or Laberge rocks.

Carbonatized outcrops, some of considerable extent and consisting principally of magnesian carbonate, contain veins several centimetres wide of relatively pure magnesite. The host rocks appear schistose, fine-grained and are greyish to dark green on fresh surface but weather to a rough surface with a bright gossanous color. In thin section, the magnesian carbonates are associated with plagioclase and minor amounts of calcite, dolomite, epidote, and an unidentified iron mineral.

Although the occurrence description is limited, it seems similar in setting to magnesite occurrences in the Atlin and Sloko River areas which are associated with ultramafic intrusions within volcanic-sedimentary stratigraphy.