FIRST OCCURRENCE OF SILVER, GOLD AND PLATINUM IN THE YUCATÁN

The Yucatán is regarded as a stable carbonate platform whose main structural complexity is the Chicxulub impact crater. Upon discovery of the crater’s ring structure numerous investigations into the possible geologic resources of the physiographic province have been undertaken. Although these investigations revealed the presence of petroleum resources, no mineralization of economic importance has yet been reported. We report the first occurrence of silver, gold, and platinum in an undifferentiated Tertiary crystalline limestone unit with an unusual discordant orange-red lithology that occurs in two quarries in the central Yucatán. Field observations suggest relict fluid flow through the moderately to heavily jointed and fractured unit and some karst features occur. Orange-red blebs with black oxidization mineralization are frequent, as are reddish brown oxidation textures that cross-cuts the orange –red lithology.

Powder X-ray diffraction (XRD) indicates the orange-red lithology is dominated by a matrix of poorly crystalline serpentine and boehmite. Discrete crystals of chromium-bearing magnetite and zircon occur in the orange-red lithology as well. Scanning Electron Microscope (SEM) investigation of the lithology indicates that silver (Ag) particles are common, and lesser amounts of gold (Au) and platinum (Pt) occur. Ag occurs both in the orange-red lithology and in the adjacent limestone. Ag particles are of high purity with a few analyses having trace amounts of sulfur. The Ag particles are commonly platy and crystals up to approximately 30 microns in diameters occur, but crystals 1-5 micrometers are more abundant. Au occurs in both diffuse textures and with crystals up to 15 microns. Pt crystals are commonly less than 0.1 micrometers in diameter.

The assemblage of serpentine minerals, boehmite, Cr-bearing magnetite and zircon are consistent with a hydrothermally altered mafic igneous source material for the orange-red lithology. The occurrence of Ag, Au, and Pt are also consistent with a mafic origin however, the age and origin of the igneous source material is unclear. The Ag in the limestone is interpreted as a result of a dissolution-reprecipitation process. Although not of economic grade, our results indicate there may be tentative cause for future exploration.