Mineralized nodules (commonly called "beryllium nodules") show stages of alteration by fluorine-rich mineralizing fluids. The nodules were originally clasts of carbonate rock, incorporated into the beryllium tuff during eruption. Carbonate rock fragments show the alteration sequence dolomite-calcite-chalcedonic quartz/opal-fluorite. The alteration sequence was deduced by studying the sequence of minerals in a group of zoned nodules like the ones below, and by analyzing the trace element content of zones with a laser spectrograph. Detailed descriptions are given by Lindsey and others (1973, especially fig. 7).
SLIDE 57 shows a nodule with an interior of gray-brown calcite, an intermediate zone of black chalcedonic quartz, and an outer zone of white opal with minor fluorite. Beryllium is concentrated only in the outer opal-fluorite zone. Nodule from the Roadside pit.
SLIDE 58 shows a nodule with essentially the same zones as slide 57: white calcite core, gray quartz, and white opal-fluorite outer zone. The cross-cutting feature is fractured, mineralized chert from the original carbonate clast. Nodule from the Roadside pit.
SLIDE 59 shows a nodule with an interior of quartz, opal and fluorite and an outer zone of opal and fluorite, from the Roadside pit. Beryllium (as much as 1 percent as bertrandite) is concentrated in fluorite.
SLIDE 60 shows a piece of a complex fluorite-opal nodule from the Hogsback beryllium prospect. Veinlets are opal.
SLIDE 61 shows fluorite-opal nodules from the Roadside pit. Black areas are manganese oxide. Both nodules contain 1-2 percent Be.
SLIDE 62 shows a fluorite-opal nodule from the Monitor beryllium prospect, used in lead-uranium age studies. The nodule contains uranium and was analyzed to determine 207Pb/235U ages of 21 Ma (fluorite-opal core), 13 Ma (white opal middle zone), and 8-9 Ma (translucent opal rim). The 21-Ma age is identical to the age of the host beryllium tuff. Details and interpretation of Pb/U ages of uraniferous opal at Spor Mountain are discussed by Ludwig and others (1980). Ma, mega-annum, refers to age before present in millions of years.