Post by 1dave on Oct 11, 2020 8:50:13 GMT -7
Nov 22, 2013 13:12:03 GMT -7 @gemfeller said:
Anything's possible, Scott. But the mineral associations in Clay Canyon variscite have been studied in depth. Telling one from another, however, is work for a qualified mineralogist, not me. Here's what Mindat has to say (in an article posted there by 1Dave):American Mineralogist Volume 27, pages 441-451, 1942
THE MINERALOGY AND PARAGENESIS OF THE VARISCITE NODULES FROM NEAR FAIRFIELD, UTAH. PART 3.
ESPER S. LARSEN, 3d., Harvard University, Cambridge, Mass.
TABLE OF CONTENTS. PART 3.
Origin of the variscite and later phosphates . . . . . . . . . . . . . . . . . . . . . . . .441
Character of the depositing solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441
Source of the phosphate material . . . . . . . . . . . .. . . . . . . . .. . . . . . . .. . . 444
Geochemistry of phosphates . . . .. . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . .445
Application of the geochemistry to the Fairfield deposit . . . . . . . . . . . . . . 446
Summary of origin . . .. .. . . . . . . . . .. . . . . . . . . . . . . . . .. . . .. .. . . . . . 449
References . . .. .. . . . . . . . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . . . . . . 450
* * *
SUMMARY OF ORIGIN
Phosphorite beds at the surface were attacked by carbonated surface waters, and the resultant solutions of calcium phosphate moved downward along throughgoing channels into the zone below the water table. Here the solutions traversed aluminous material, perhaps shales, and deposited the phosphate as the aluminum salt, variscite. This reaction perhaps caused a decrease in the alkalinity of the solutions. When the phosphorite was completely removed at the surface, the downward-moving ground waters became free of phosphate material, and returned to their usual alkaline state.
This return to stronger alkalinity caused a reaction with the variscite to replace it with calcium aluminum phosphate (pseudowavellite), an introduction of only calcium. Soda then became an important constituent of the solutions, perhaps derived from the weathering of shales or shaly limestones exposed at the surface; this resulted in the deposition of millisite and wardite. The soda increased the alkalinity of the solutions, eventually to the point where the deposition of wardite did not keep pace with the removal of variscite by solution. The solutions then returned to their normal alkalinity, probably with the removal of the shale and the exposure again of limestones, and again deposited pseudowavellite in place of the variscite. This ended the bulk of the mineralization.
The stages following this are represented in the nodules primarily by rare crystals in the solution cavities, at first aluminum phosphates of magnesium (gordonite), followed by calcium. In the final stage aluminum is absent, and alkalis reappear. The equilibrium conditions controlling the deposition of these is not known, so that the state of the solutions forming them cannot be surmised. With the lowering of the water table below the variscite, oxidizing conditions ensued, with the deposition of abundant limonite.
It is believed that one of the major factors permitting the deposition of variscite at Fairfield, and probably some other deposits, was the existence of open fissures which permitted the surface solutions containing dissolved calcium phosphate to move rapidly downward through underlying limestone into rocks containing aluminum. In the normal course, ground waters with dissolved calcium phosphate seep down into underlying limestones, where the phosphate is leisurely precipitated by the excess of calcium carbonate.
Where open channels allow more rapid descent of the solutions, the precipitating effect of the limestone is not so effective, and phosphate-bearing solutions can thus reach aluminous rocks, and the aluminum will act as the precipitant. The equilibrium conditions which determined the deposition of variscite rather than some other aluminum phosphate (such as wavellite) are not known.