URANIUM MINERALIZATION IN THE MORRISON FORMATION: AN HYPOTHESIS RELATING THE MIXING OF FRESH GROUNDWATER AND SALINE FORMATION WATERS TO THE ORE DEPOSITS AT LAGUNA, NEW MEXICO

INTRODUCTION

Exploration for sandstone-type uranium deposits in the southern San Juan basin in New Mexico was successful for over thirty years. Recognition of favorable host rocks and a linear geographic trend of mineralization enabled exploration geologists to locate ore long before the major causes of mineralization were elucidated. Most of the production was from the late Jurassic Morrison Formation near Grants and Laguna New Mexico. The details of ore distribution, geometry, and grade are well known21,7,1,12. Numerous studies provided data on host rock alteration in and around ore zones3,21,17,1,. In spite of this, the origin of primary uranium ore in the Grants and Laguna mining districts remains controversial. Regional stratigraphic study of the Brushy Basin Member of the Morrison Formation has provided more data on the depositional and early diagenetic environment. Some of the conclusions reached in the course of this work have revealed factors which may have contributed to uranium mineralization. Most significantly, the Brushy Basin Member in this area was deposited in a hydrologically closed basin occupied in part by an extensive playa lake. Authigenic zeolites and molds of evaporite minerals in the playa mudstones indicate the presence of a saline alkaline brine. In addition, altered volcanic ash, which may have been a major source of the uranium, is more abundant than previously recognized. What follows is an hypothesis that resulted from a synthesis of these new findings with the geologic details of the Jackpile-Paguate deposits as described by Adams et al.1, the L-Bar deposits as described by Jacobsen16 and of the Laguna District as a whole by Moench and Schlee3.

LAGUNA MINING DISTRICT

The Laguna Mining district in New Mexico has accounted for a significant portion of the uranium production from the Morrison Formation in New Mexico. Anaconda's Jackpile-Paguate deposit alone yielded over 3.6 x 107 kg of U3O8 before its closure4.

Figure 1

Location map showing the Grants and Laguna mining districts.

The major difference is the stratigraphic position of ore. At Grants, mineralization occurs in sandstones of the Westwater Canyon and Brushy Basin Members (fig. 2). At Laguna, mineralization occurs within the Jackpile sandstone, which is the uppermost sandstone in the Brushy Basin Member. Despite their different locations and stratigraphic position, the ore deposits are similar in size, grade, geometry, and associated alteration features. It is likely that the processes which mineralized the Morrison Formation at Grants operated with similar results in the vicinity of Laguna.