Umberto Masi

Stable isotope systematics in mesozoic granites of Central and Northern California and Southwestern Oregon

Abstract18O, D, and H2O+ contents were measured for whole-rock specimens of granitoid rocks from 131 localitics in California and southwestern Oregon. With 41 new determinations in the Klamath Mountains and Sierra Nevada, initial strontium isotope ratios are known for 104 of these samples. Large variations in δ18O (5.5 to 12.4), δD (−130 to −31), water contents (0.14 to 2.23 weight percent) and initial strontium isotope ratios (0.7028 to 0.7095) suggest a variety of source materials and identify rocks modified by secondary processes. Regular patterns of variation in each isotopic ratio exist over large geographical regions, but correlations between the ratios are generally absent except in restricted areas. For example, the regular decrease in δD values from west to east in the Sierra Nevada batholith is not correlative with a quite complex pattern of δ18O values, implying that different processes were responsible for the isotopic variations in these two elements. In marked contrast to a good correlation between (87Sr/86Sr)o and δ18O observed in the Peninsular Ranges batholith to the south, such correlations are lacking except in a few areas. δD values, on the other hand, correlate well with rock types, chemistry, and (87Sr/86Sr)o except in the Coast Ranges where few of the isotopic signatures are primary. The uniformly low δD values of samples from the Mojave Desert indicate that meteoric water contributed much of the hydrogen to the rocks in that area. Even so, the δ18O values and 18O fractionations between quartz and feldspar are normal in these same rocks.This reconnaissance study has identified regularities in geochemical parameters over enormous geographical regions. These patterns are not well understood but merit more detailed examination because they contain information critical to our understanding of the development of granitoid batholiths.

Geochemical Characterization of Podiform Chromite Ores from the Ultramafic Massif of Bulqiza (Eastern Ophiolitic Belt, Albania) and Hints for Exploration

Four main podiform occurrences of chromite have been distinguished in the ultramafic Bulqiza massif, Albania; they are associated from the bottom to the top with: i) basal harzburgites (lower tectonite sequence), ii) dunitic lens-bearing harzbugites (middle-upper tectonite sequence), iii) layered chromitite-bearing dunites of transitional zone, and, iiii) the stratigraphically lowermost part of the magmatic section. However, only the occurrences hosted by rocks ii) and iii) are economically important. Chromite compositions similar to those of other podiform chromites are bimodal. Thus, chromites from occurrences ii) and iii) are richer in Cr than chromites from the other two occurrences. Accessory chromite disseminated throughout the massif is richer in Fe than orebody chromite, displaying similar range of the Cr/Cr+Al ratios, with accessory residue chromite matching the characteristic (Cr/Cr+Al >0.6) range of type-III Alpine peridotites. Both orebody and accessory chromites show compositional ranges correlating with host-rock compositions, i.e., the Cr-rich chromites are hosted by the most refractory ultramafic rocks. Therefore, the Cr/Cr+Al and Mg/Fe tot (Cr/Fe tot ) ratios of chromite are useful for exploration of high-grade, metallurgical chromite ore at Bulqiza and in other massifs of similar geodynamic setting.

Stable isotope and fluid inclusion studies of carbonate deposits from the Tolfa Mountains mining district (Latium, central Italy)

Carbon and oxygen isotope analyses were made of representative samples of calcite and quartz from the carbonate deposits in the Tolfa Mountains mining district. Measurements were also made of hydrogen isotope compositions, filling temperatures and salinities of fluid inclusions in these minerals. There are three stages of mineralization at Tolfa. In stage I, characterized by calc-silicate hornfels, the carbonates have relatively high δ 18O values of 14.5 to 21.6 suggesting a rather low water/rock ratio. δ13C values of −0.3 to 2.1 indicate that appreciable decarbonation or introduction of deep-seated carbon did not occur. Stage II is marked by phanerocrystalline carbonates; δ 18O values of 13.1 to 20.0 and δ13C values of 0.7 to 5.0 identify them as hydrothermal veins rather than marbles. δ D values of −56 to −50 for inclusion fluids suggest a possible magmatic component to the hydrothermal fluid. Filling temperatures of coarse-grained samples of Calcite II are 309° to 362° C with a salinity range of 5.3 to 7.1 weight percent NaCl. Calculated δ18O values of 11–12 for these fluids are again indicative of low water/rock ratios. The sparry calcites of stage III have δ18O and δ13C values of 8.1 to 12.9 and −1.7 to 3.2, respectively. δ D values of inclusion fluids are −40 to −33, clearly heavier than in earlier stages and similar to values of modern local ground waters. A salinity measurement of <0.1 weight percent NaCl in a sample of Calcite III is compatible with a relatively unaltered ground water origin for this fluid. Precipitation of the sparry calcite took place at much lower temperatures, around 160° C. For quartz, δ18O values of 9.3 to 12.4 and δ D values for inclusions of −53 to −28 are consistent with its late occurrence and paragenetic link with associated carbonates.

The isotopic composition of nitrates from underground and surficial waters of Italy: a valuable tool for benefiting in the fight against pollution

A review of isotopic data of nitrates in groundwater mainly from wells located in northern and central Italy has been carried out. A subordinate number of data refers to surface waters from the Po plain area. Most of the sampling areas, that are located in the proximity of important human settlements, are devoted to intensive agriculture and, thus, isotopic data indicate that the nitrogen source is mainly represented by mineralized synthetic fertilizers. A subordinate source is composed of either the soil organic fraction and animal manure used as a fertilizer. Only for a few water samples, the origin from sewage seep is suggested. Denitrification can occur in many aquifers, contributing to abate significantly nitrate concentrations in waters. Lastly, the absence of isotopic data for the waters from uppermost Regions of southern Italy and Sicily represents a major handicap in the fight against nitrate pollution, making the maps of nitrate risk in those areas not fully useful.

Procedure for dissolving wolframite in order to determine trace elements by atomic absorption spectrometry.

An analytical procedure for dissolving wolframite in order to determine trace elements by atomic absorption is reported. After decomposition of the mineral by alkaline fusion, the cooled flux is dissolved by heating with aqua regia. The finely crystalline precipitate of tungstite, which forms on standing, is filtered and dissolved by heating with aqua regia. Tungstite forms again in the latter solution and the filtration and solubilization are repeated until the solution contains negligible amounts of trace elements. Following this procedure, trace elements in 35 samples of natural wolframite have been dissolved and the solutions obtained have been analyzed by atomic absorption spectrometry for Fe, Mn, Cu, Zn, Pb, Co, Ni and Cd.