The age and thermal history of Cerro Rico de Potosi, Bolivia

 Cerro Rico de Potosi, Bolivia, is the world’s largest silver deposit and has been mined since the sixteenth century for silver, and for tin and zinc during the twentieth century, together with by-product copper and lead. The deposit consists primarily of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and explosion breccia. The deposit is compositionally and thermally zoned, having a core of cassiterite, wolframite, bismuthinite, and arsenopyrite surrounded by a peripheral, lower-temperature mineral assemblage consisting principally of sphalerite, galena, lead sulfosalt, and silver minerals. The low-temperature assemblage also was superimposed on the high-temperature assemblage in response to cooling of the main hydrothermal system. Both the dacite dome and the ore fluids were derived from a larger magmatic/hydrothermal source at depth. The dome was repeatedly fractured by recurrent movement on the fault system that guided its initial emplacement. The dome was extruded at 13.8±0.2 Ma (2σ), based on U-Th-Pb dating of zircon. Mineralization and alteration occurred within about 0.3 my of dome emplacement, as indicated by a ⁴⁰Ar/³⁹Ar date of 13.76±0.10 Ma (1σ) for sericite from the pervasive quartz-sericite-pyrite alteration associated with the main-stage, high-temperature, mineralization. The last thermal event able to reset zircon fission tracks occurred no later than 12.5±1.1 Ma (1σ), as indicated by fission-tract dating. Minor sericite, and magmatic-steam alunite veins, were episodically formed around 11 Ma and between 8.3 and 5.7 Ma; the younger episodes occurring at the time of extensional fracturing at Cerro Rico and widespread volcanism in the adjacent Los Frailes volcanic field. None of these younger events appear to be significant thermal/mineralizing events; the exceptionally flat thermal release pattern of ³⁹Ar from sericite and the results of the fission-tract dating of zircon show that none of the younger events was hot enough, and lasted long enough, to cause significant loss of Ar or annealing of zircon fission tracks. U-Th-Pb dating of zircon cores indicates a Precambrian progenitor for some zircons, and REE analyses of dated samples of hydrothermally altered dacite show the presence of a prominent positive Eu anomaly, which constrains interpretations of the origin and evolution of the magmatic/hydrothermal system. Received: 14 October 1995/Accepted: 29 January 1996     

Photometric Variability of Neptune, 1972-2000

We present Strömgren b (472-nm) and y (551-nm) photometry of Neptune based on photoelectric measurements obtained at every apparition from 1972 to 2000. Neptune has brightened by 11% in b and 10% in y since 1980 with most of the increase occurring after 1990. By appending b data to published B magnitudes measured at Lowell from 1950 to 1966 and transformed to b, we show that Neptune is now brighter than at any time during the past half century. The nature of the year-to-year variations changed around 1990 when a steady rising trend overshadowed what appeared to be an inverse correlation with cyclic solar activity. By matching observations in b and y with near-infrared J (1.2-μm) and K (2.2-μm) photometry before, during, and after Neptune's 1976 infrared outburst, we show that the pattern of visible albedo variation parallels the infrared variation but with an amplitude 20-50 times smaller. A detailed comparison of photometry with ground-based and Voyager images at visible and red wavelengths during the 1989 Voyager encounter shows that small brightness variations occur when large discrete features rotate across Neptune's disk. This provides a rough association between visible features and photometric effects that we use to infer the state of Neptune's atmosphere for years when only photometry was available. A year-by-year analysis of variance of the photometry suggests that the 1976 and 1986-1989 infrared outbursts were isolated episodes of unusually vigorous atmospheric activity. Detrended magnitudes of Neptune are correlated with solar activity over the entire 29-year interval as well as 22-year subintervals, with solar UV now being favored as a causative mechanism rather than solar modulated galactic cosmic rays.     

Scattering properties of planetary regolith analogs

The physics of scattering of electromagnetic waves by media in which the particles are in contact, such as planetary regoliths, has been thought to be relatively well understood when the particles are larger than the wavelength. However, this is not true when the particles are comparable with or smaller than the wavelength. We have measured the scattering parameters of planetary regolith analogs consisting of suites of well-sorted abrasives whose particles ranged from larger to smaller than the wavelength. We measured the variation of reflectance as the phase angle varied from 0.05° to 140°. The following parameters of the media were then deduced: the single scattering albedo, single scattering phase function, transport mean free path, and scattering, absorption, and extinction coefficients. A scattering model based on the equation of radiative transfer was empirically able to describe quantitatively the variation of intensity with angle for each sample. Thus, such models can be used to characterize scattering from regoliths even when the particles are smaller than the wavelength. The scattering parameters were remarkably insensitive to particle size. These results are contrary to theoretical predictions, but are consistent with earlier measurements of alumina abrasives that were restricted to small phase angles. They imply that a basic assumption made by virtually all regolith scattering models, that the regolith particles are the fundamental scattering units of the medium, is incorrect. Our understanding of scattering by regoliths appears to be incomplete, even when the particles are larger than the wavelength.