Sauconite

Abstract: Zinc-enriched targets have been detected at the Kimberley formation, Gale crater, Mars, using the Chemistry Camera (ChemCam) instrument. The Zn content is analyzed with a univariate calibration based on the 481.2 nm emission line. The limit of quantification for ZnO is 3 wt % (at 95% confidence level) and 1 wt % (at 68% confidence level). The limit of detection is shown to be around 0.5 wt %. As of sol 950, 12 targets on Mars present high ZnO content ranging from 1.0 wt % to 8.4 wt % (Yarrada, sol 628). Those Zn-enriched targets are almost entirely located at the Dillinger member of the Kimberley formation, where high Mn and alkali contents were also detected, probably in different phases. Zn enrichment does not depend on the textures of the rocks (coarse-grained sandstones, pebbly conglomerates, and resistant fins). The lack of sulfur enhancement suggests that Zn is not present in the sphalerite phase. Zn appears somewhat correlated with Na2O and the ChemCam hydration index, suggesting that it could be in an amorphous clay phase (such as sauconite). On Earth, such an enrichment would be consistent with a supergene alteration of a sphalerite gossan cap in a primary siliciclastic bedrock or a possible hypogene nonsulfide zinc deposition where Zn, Fe, Mn would have been transported in a reduced sulfur-poor fluid and precipitated rapidly in the form of oxides.

Abstract: An experimental study of the phase equilibria in the system ZnO-SiO 2 -H 2 O has been made at elevated temperatures in the range 130 degrees C to 780 degrees C employing water vapor pressures from 500 to 40,000 pounds per square inch. Stability regions were established for several minerals in this system, which occur naturally in the oxidized zones of lead and zinc deposits. Hemimorphite was found to be stable up to about 250 degrees C at 20,000 psi., while above this temperature, willemite plus water are in equilibrium. With suitable qualifications this temperature may be taken as the maximum temperature of formation and stability of hemimorphite as well as the minimum for willemite. The pure zinc oxide-silica end member of the montmorillonoid-type mineral, sauconite, was found to be stable up to 210 degrees C, although natural sauconite, containing alumina is stable to approximately 125 degrees C higher. Uncertainties still remain in the determination of the Zn(OH) 2 ZnO + H 2 O equilibrium, and for that reason studies of the reaction ZnCO 3 ZnO + CO 2 may prove to be more significant in determining the minimum temperature of zincite formation.

Abstract: The Accha-Yanque zinc belt is located in the southern Altiplano of Peru, a major zinc-rich metallogenic province hosting a number of economic mineral deposits (porphyry copper and skarn ores). Several nonsulfide-type occurrences, showings, and mineral deposits are situated in a belt, peripheral to the northern, northeastern, and northwestern edge of the Oligocene-(Miocene?) Yauri-Apurimac batholith. Mineralization is hosted in breccias of both sedimentary and tectonic origin in the limestones of the Middle to Upper Cretaceous Ferrobamba Formation. Primary ores belong to the carbonate replacement deposit type and are at least in part structurally controlled. Currently, the Zn mineralization is almost fully oxidized: the Accha deposit can be assigned to both direct replacement and wall-rock replacement types. The mineralized zone (indicated resources 5.1 Mt @ 8.2% Zn and 0.9% Pb) occupies the hinge of an anticlinal dome that has been exposed by erosion. The southern limb of the structure dips about 55° to the south-southwest, whereas its northern limb is truncated by faults. The nonsulfide concentrations, consisting of a mineralized zone 5 to 20 m thick, are continuous along strike to the west for at least 700 m. The mineralogy of the Accha deposit shares many characteristics with that of the typical carbonate-hosted calamine-type nonsulfide Zn ores. The nonsulfide mineral association consists mainly of smithsonite and hemimorphite replacing both primary ore minerals and carbonate host rocks. Hydrozincite has been detected only in samples near the surface. Smithsonite occurs in zoned concretions with goethite, Mn (hydr)oxides and Zn clays, as well as replacive cement in the limestone intervals. One of the most peculiar nonsulfide Zn minerals at Accha is a sauconite-like, zincian smectite, variably concentrated throughout the deposit. Locally sauconite occurs as replacement of detrital feldspars and/or detrital fragments occurring in marly sediments or in infills of karst cavities. It also replaces both hemimorphite and smithsonite deposited during earlier stages. The age of the supergene products in the whole belt is poorly constrained, although there is geomorphologic evidence that the formation of supergene minerals postdates by more than 10 m.y. the last large-scale secondary enrichment event that terminated with central Andean climatic desiccation at ~15 Ma. The age of the Accha deposit may be consistent with a Pliocene K-Ar date of 3.3 ± 0.2 Ma obtained for supergene alunite from the top part of the leached cap in the nearby Cotabambas Cu deposit.

Abstract: Two varieties of Zn-smectite were synthesized hydrothermally: sauconite, with an ideal composition of Na0.4Zn3(Si3.6Al0.4)O10(OH)2·nH2O; and a Zn equivalent of hectorite, with an ideal composition of Na0.4 (Li0.4Zn2.6)Si4O10(OH)2·nH2O (referred to here as Zn-hectorite). For comparison, hydrothermal synthesis of the related trioctahedral smectites of hectorite, Na0.4(Li0.4Mg2.6)Si4O10(OH)2·nH2O and hectorites containing Cu, Co or Ni in the octahedral sheets instead of Mg were also attempted. The results showed that sauconite, Zn-hectorite and hectorite could be synthesized in the temperature range 100–125°C but hectorites containing Cu, Co or Ni in the octahedral sheet, under the same conditions or even at a temperature of 150°C, could not.