Jurassic silica sinters associated with La Josefina epithermal gold-silver deposit, Deseado Massif, Argentina. Should we “expect” fossil plants in Mesozoic sinters?.

MOREIRA PILAR; CHANNING ALAN; FERNANDEZ RAUL; SCHALAMUK ISIDORO B. A.; ZAMUNER ALBA

Johannesburgo (Sudáfrica)

Congreso; SEG-CSSA 2008 Conference; 2008

Society of Economic Geologist

La Josefina, located in the central portion of the Deseado Massif geological province is an almost intact Au-Ag epithermal vein system hosted in middle to upper Jurassic volcanic-pyroclastic rocks. It, in common with numerous other epithermal deposits in the western Deseado Massif, has well preserved surficial deposits including silica (sinter) and carbonate (travertine) mounds and aprons and distally lakes/wetlands. Primary textures of the silica rocks are, in many instances, poorly discernable due to silica replacement of, and diagenetic transformations to, the initial mineral precipitates plus infilling of primary porosity by late phase silica precipitation. Despite these processes, microbotryoidal, pisolitic, stratiform-stromatolitic and columnar-stromatolitic sinter fabrics, plus massive to diffusely bedded and/or mottled silica horizons can be observed. These compare favorably mineralogically, geochemically and structurally with comparative sinter samples, of similar silica maturity, collected from fossil hot spring systems of Queensland (Devonian) and New Zealand (Miocene). Remnant microbial mat structures, moulds of bacterial filaments and moulds of cubic halite crystals indicate silica precipitation at the surface in an evaporative setting. Geopetal/isopachous chalcedony and microquartz infilling primary porosity suggest silica precipitation continued in the shallow subsurface. Plant macrofossils, often viewed as critical evidence for the presence of a hot spring sinter, are absent from the La Josefina sinters as they are in many other Deseado Massif deposits. This is due to a combination of factors: In vent pools, boiling waters prevent higher plant colonization. On proximal sinter aprons cyclic eruptions, high temperatures, alkalinity/salinity and low nutrient status allow limited colonization, but here plants have low preservation potentials and are rapidly removed on death. An additional barrier to colonization of hot spring environments exists in this specific Mesozoic time interval as gymnosperms (major elements of the regional flora) rarely display the vegetative reproduction characteristic of modern and Palaeozoic hot spring floras. Current and future exploration work in the area will increase the knowledge of this system and aid understanding of other localities in the Deseado Massif where probable sinters have been recognized.