ORIGIN OF PLATY GALENA IN THE VIBURNUM TREND, SOUTHEAST MISSOURI

The Viburnum Trend of Missouri is the world’s largest producer of lead. The lead occurs as galena predominantly in two crystallographic forms, octahedrons and cubes. Many studies have shown that octahedral galena is paragentically early, the more abundant of the two crystal forms, and is commonly modified by the cube. Those studies also have shown that the cubic form is paragenetically later, less abundant than the octahedrons, and may exhibit minor octahedral modifications. Viburnum Trend galena crystals that exhibit a platy form have received almost no study. The reason for their lack of the study is the rarity of their occurrence. This communication discusses their character, mine distribution, paragenetic position, trace element contents, nature of twinning, and speculated conditions of formation. It also compares their character to similar platy galena occurrences in Bulgaria, Russia, Mexico, and the Pine Point District in the Northwest Territories of Canada.

Flat, platy galena crystals have been recognized to occur in very small amounts in the Magmont, Buick, Fletcher, Brushy Creek, and Sweetwater mines in the Viburnum Trend. In contrast, platy galena has never been observed to occur at the Casteel, West Fork, #27, #28, and #29 mines in the Trend. The platy crystals have formed early in the paragenetic sequence of the ores, prior to and coated by subsequently deposited cubic galena and drusy quartz.

Spinel twinning of the octahedron produces flat platy crystals. The platy galena crystals of the Viburnum Trend are very similar in crystal morphology to platy galena crystals interpreted to be spinel twins in the Dalnegorsk Pb-Zn (skarn deposit) mine in SE Russia, the Madan ore field of skarn Pb-Zn-Ag deposits of southern Bulgaria, and the large Naica Pb mine of northern Mexico.

In some lead districts, less common forms of galena have been ascribed to the incorporation of elevated contents of certain trace elements in those galena crystal forms. Analysis of Viburnum platy crystals has shown that they contain very low levels of trace elements: 3.1 ppm Ag, <2 ppm Bi, <2 ppm Sb, and <2 ppm As. Thus, elevated trace element content is not the cause for the development of Viburnum platy galena.

It is speculated that the Viburnum spinel-twinned galena crystals were the result of rapid crystallization from oversaturated ore fluids.