The Iron Mask batholith lies in the southern part of the Quesnel trough, also known as the Nicola belt. The most important pre- Tertiary rocks in this belt are Upper Triassic volcanic and sedimentary rocks of the Nicola Group. The batholith is a subvolcanic, multiple intrusion which is comagmatic and coeval with the Nicola rocks. It is situated along the southwest side of a regional northwest trending fracture zone and is itself cut by numerous northwesterly faults. The batholith comprises two major northwest trending plutons separated by 6 kilometres of Eocene Kamloops Group volcanic and sedimentary rocks. The Tertiary rocks occupy what appears to be a graben structure resulting from renewed fault movement around the margins of the plutons during Paleocene or Early Eocene time (Bulletin 77). The larger pluton, the 18 kilometre long southern part of the batholith, is called the Iron Mask pluton. The smaller Cherry Creek pluton farther northwest, outcrops on either side of Kamloops Lake. The combined exposure of the batholith, including the intervening younger rocks, is about 33 kilometres long and 5 kilometres wide. Sedimentary and volcanic rocks of the Kamloops Group unconformably overlie the Nicola rocks and the Iron Mask batholith. These include tuffaceous sandstone, siltstone and shale with minor conglomerate, as well as basaltic to andesitic flows and agglomerates with minor dacite, latite and trachyte.

The Glen Iron deposit comprises magnetite veins that lie within Late Triassic-Early Jurassic Iron Mask batholith intrusive rocks. The hostrocks are dark, medium grained and rich in pyroxene, biotite and magnetite and range from gabbro to augite diorite to monzonite in composition. The rock near the magnetite veins shows considerable alteration to epidote accompanied by either tremolite or serpentine. The veins vary from less than 30 centimetres to nearly 10.6 metres wide, and have been traced for up to 198 metres. Locally, the veins pinch and swell and branch and in some cases terminate abruptly. The veins trend east-west and dip steeply north. The ore consists essentially of massive magnetite with varying amounts of apatite. Fifteen veins occur on the property and of these, the No. 3 and No. 8 veins appear to be the more important.

The principal workings are on the No. 8 vein and consist of an opencut 131 metres long, 3 to 6 metres wide and in places more than 12 metres deep. Near the opencuts lower easterly end a 46-metre adit was driven; the ore is partly stoped out above this tunnel. The vein is exposed by the opencut for 109 metres and intermittently for a further 88 metres. At the tunnel mouth the vein is 4.5 metres wide and pinches and swells up to a width of 10.6 metres.

Vein No. 3 is exposed at intervals over a length of 112 metres, sends off several branches and possibly is continued westward by veins Nos. 4 and 5, in which case the total exposed length would be 234 metres. Apparently at the places where the vein branches the magnetite ore mass notably increases in width, forming, it is estimated, one lenticular body 36 metres long and 7.6 metres wide and a second body 30 metres long with a thickness gradually increasing from 1.5 to 3 metres. Veins 4, 5 and 6 are possibly continuations or branches of vein No. 3. The remaining veins are similar in character but on the whole narrower than those described above.

The presence of the magnetite veins was known at a comparatively early date, but apparently the deposits were not acquired until 1889. Shortly thereafter, shipments of ore were made to be used as a flux in copper smelters. Most of the production came from an opencut on the No. 8 vein. Shipments ceased in 1902.