New Mexico Mineral Symposium — Abstracts

A variety of phosphate minerals occurs in highly folded siliceous Paleozoic rocks of the Ouachita Mountains in west-central Arkansas. Most of the area is in the Ouachita National Forest and is open to collecting except on valid mineral claims.

The iron phosphates typically are confined to the lower and middle members of the Arkansas Novaculite (Devonian). They occur with iron and manganese oxides filling fractures in the brittle novaculite in prospects where fresh rock has been exposed. Five important occurrences and many minor ones are known throughout an east-west distance of approximately 40 mi.

The earliest mineralization was ferrous-ferric phosphates: dufrenite, rockbridgeite, laubmannite, lipscombite, and beraunite; next came the ferric phosphates: kidwellite, strengite, phosphosiderite, cacoxenite, and an amorphous, red, iron phosphate. A new, undescribed mineral is probably a higher hydration product of kidwellite. Nonphosphate minerals occurring in these deposits are hyalite opal, goethite, cryptomelane, and other unidentified manganese oxides.

Aluminum phosphates are generally found in the Big Fork chert ,(Ordovician) and consist of wavellite, crandallite, variscite, metavariscite, gorceixite, turquoise, and planerite. The two principal localities are at Avant (Buckville) and the county road quarry at Mauldin Mountain, near Mt. Ida, but mineralization is widespread in other areas. Gorceixite is a rare barium-aluminum phosphate that occurs sparingly as white, fibrous coatings along fractures in novaculite. The principal turquoise locality is the Mona Lisa mine, where the turquoise occurs with kaolinite in hydrothermally altered novaculite. The mineral planerite, recently redefined by Foord, occurs at many places as fracture fillings in novaculite as well as in the Big Fork chert.

A unique locality for phosphates, as well as other rare minerals, is the Umetco vanadium mine at Potash Sulphur Springs, Arkansas. This is a complex of alkaline igneous rocks intruding Paleozoic sediments. Cacoxenite, strengite, wavellite, and some rare-earth phosphates occur near the edge of the intrusive.

The virtual restriction of the phosphate minerals to specific formations or parts of formations suggests that the phosphorus was indigenous to either the siliceous host rocks or the adjacent shales. It seems likely that phosphorus was leached out and redistributed in fracture zones by hot waters similar to those currently issuing as hot springs.