The O'Connor River deposit is located 96 kilometres northwest of Haines, Alaska. Gypsum occurs in rugged terrain above the tree line on both sides of the O'Connor river near the headwaters of its north fork. The deposit was discovered in 1958. Since then, trenching, drilling and bulk sampling have been done on the deposit.

Gypsum occurs in complexly deformed Paleozoic sedimentary rocks and Triassic basic flows and related volcaniclastic rocks. The gypsum is hosted in limestone, limestone breccia and black calcareous argillite. Sill-like diorite intrusions are present in the area. Sediments adjacent to these sills have been silicified and metamorphosed. Better quality gypsum is described as snow white, occurring in massive continuous beds with no visible impurities. Traces of anhydrite may be present. Brown or buff colored gypsum or gypsiferous carbonate is present near the edges of the pure white material. A sample of pure white gypsum indicates a purity in excess of 90 per cent gypsum.

The O'Connor River deposit consists of 3 separate zones, known as East (zone 1), West (zone 2) and Kim (zone 3) zones.

The East zone (zone 1) is irregular in shape, exposed along strike for 400 metres over a vertical height of 122 metres. It strikes northwest with steep northeast dips. The gypsum is generally pure containing only minor anhydrite. Contacts with surrounding rocks are sharp. Locally it appears that the gypsum crosscuts the sedimentary rocks. Sinkholes, 10 to 20 metres in diameter and 10 to 15 metres deep are present at the southeast end of the zone; gypsum is exposed in the wall of some of them.

The West zone (zone 2) is believed to be an extension of the East zone. It is exposed along strike for 220 metres across widths of 60 to 100 metres and a vertical component of 200 metres. The gypsum is white, massive and finely crystalline. There is a 30 metre-wide argillaceous limestone exposed within the gypsum at the southeast end of the zone. Contacts are sharp, although there are a few inclusions of argillaceous limestone, up to 15 centimetres in size, in the gypsum.

The Kim zone (zone 3) is located 1200 metres south of the West zone, strikes east and dips to the north. It is exposed along strike for 550 metres across widths of 50 to 110 metres and a height of 120 metres. The gypsum is white and is in sharp contact with a limestone unit. Sinkholes are present along the entire length of the zone.

The present configuration of gypsum probably occurred as a result of tectonic movements in the area when pressure squeezed the calcium sulphate bodies into their present position by plastic flow. Company reports indicate that the deposit contains 8 per cent anhydrite. Possibly the original anhydrite deposit was hydrated into gypsum by interaction with a combination of meteoric and ground waters.

Estimated reserves of Zone 1 are 2.5 million tonnes grading 79.0 per cent gypsum; the SO3 content averages 40 per cent and oxide and insoluble content is fairly high making the gypsum suitable for wallbaord manufacture but not for the cement industry. There is a potential for 10 million tonnes in the three zones combined (Open File 1991-15, page 34).

Other areas containing sinkholes occur between the O'Connor River deposit and the Haines road and to the east of the Haines road, but neither locality contains outcrop.