Analysis of Massive Sulfides within the Mountain View Area of the Stillwater Complex, Montana — a Statistical Formulation and Test of the Sulfide Liquid Immiscibility Model

Abstract

The Stillwater Complex, Montana, is a layered mafic intrusion containing resources of chromite, platinum, copper, and nickel. The exploration program for copper and nickel sulfides within the Mountain View area of this igneous body included a series of diamond drillholes located on a grid. Characteristics of massive sulfides which occur in these drillholes (thickness, stratigraphic position, and copper-nickel grades) within and adjacent to the Basal series of the Stillwater Complex have been analyzed and compared with theoretically expected results of immiscible sulfide liquids. An important aspect of this study is the translation of the attributes of a geologic model into statistical hypotheses to evaluate the possibility that liquid immiscibility is the primary process responsible for the thickness, stratigraphic distribution, and copper-nickel grade characteristics of the massive sulfides present within the Mountain View area of the Stillwater Complex.

Geologic models have been proposed on the interrelation between sulfide mineralization and silicate rocks in layered intrusions, mechanisms for separation of immiscible sulfide liquids from basaltic magmas, methods of collection and concentration of immiscible sulfide liquids, and the crystallization of the collected sulfide liquid. The combination of these processes in conjunction with the geologic history of the area leads to expected results concerning the distribution and grade of the massive sulfides contained within layered mafic igneous rocks and the associated metasedimentary rock.

The results of formulating and testing nine statistical hypotheses led to the conclusions that there is no evidence to reject the model wherein sulfide liquid immiscibility is the primary condition responsible for the distribution and grade characteristics of the massive sulfides, and no evidence to reject the hypotheses that the copper and nickel grades of the massive sulfides in the Basal series are not different significantly than those in the metasedimentary rock. Other results of the analysis show that the copper and nickel grades of the massive sulfides are not related to their thickness in either the Basal series or the metasedimentary rock; that no trend in thickness of the massive sulfides was determined as a function of stratigraphic position in either the Basal series or the metasedimentary rock; that frequency in the occurrence of massive sulfides declines as a function of distance from the Basal series and metasedimentary rock contact; and that the total volume of sulfide increases as the depth increases in the Basal series and decreases as distance from the top of the metasedimentary rocks increases.