Early in the next century, several space missions are planned with the goal of landing craft on asteroids, comets, the Moon, and Mars. To increase the scientific return of these missions, new methods are needed to provide (1) significantly more analyses per mission lifetime, and (2) expanded analytical capabilities. One method that has the potential to meet both of these needs for the elemental analysis of geological samples is laser-induced breakdown spectroscopy (LIBS). These capabilities are possible because the laser plasma provides rapid analysis and the laser pulse can be focused on a remotely located sample to perform a stand-off measurement. Stand-off is defined as a distance up to 20 m between the target and laser. Here we present the results of a characterization of LIBS for the stand-off analysis of soils at reduced air pressures and in a simulated Martian atmosphere (5–7 torr pressure of CO2) showing the feasibility of LIBS for space exploration. For example, it is demonstrated that an analytically useful laser plasma can be generated at distances up to 19 m by using only 35 mJ/pulse from a compact laser. Some characteristics of the laser plasma at reduced pressure were also investigated. Temporally and spectrally resolved imaging showed significant changes in the plasma as the pressure was reduced and also showed that the analyte signals and mass ablated from a target were strongly dependent on pressure. As the pressure decreased from 590 torr to the 40–100 torr range, the signals increased by a factor of about 3–4, and as the pressure was further reduced the signals decreased. This behavior can be explained by pressure-dependent changes in the mass of material vaporized and the frequency of collisions between species in the plasma. Changes in the temperature and the electron density of the plasmas with pressure were also examined and detection limits for selected elements were determined.

The Tuvatu gold-silver telluride deposit with reserves of 13 t Au is the second largest gold deposit in Fiji after the large Emperor gold telluride deposit (production, and proven and probable reserves of 280 t Au). The deposits are 50 km apart and occur along the >250-km east-northeast-trending Viti Levu lineament. They are spatially associated with alkaline rocks of almost identical age (~5.4-4.6 Ma) and having a shoshonitic affinity. The gold mineralization in both deposits is spatially and genetically related to monzonite intrusions and to a low-grade porphyry copper-style system. The Emperor deposit occurs along the margins of the Tavua volcano whereas the Tuvatu deposit may occur adjacent to an eroded shoshonite volcano. At both locations, low-sulfidation, epithermal gold telluride mineralization occurs in flat-lying veins, steep faults, shatter zones, stockworks, and hydrothermal breccias. Mineralization in both deposits formed in multiple stages and is characterized by the presence of quartz-roscoelite telluride veins in which gold-rich tellurides were deposited prior to silver-rich tellurides. Gold tellurides and vanadium minerals were deposited at approximately 250°C from moderately saline fluids. Oxygen and hydrogen isotope compositions of ore fluids at Emperor and Tuvatu are similar to the composition of waters exsolved from arc magmas. Previously published values of δ 34 S of sulfides (-20.3 to +3.9‰) from Emperor are like those obtained from the Tuvatu deposit (-15.3 to -3.2‰) and indicate, along with mineral assemblages, that the ore fluids were oxidizing and near the hematite-pyrite buffer.The similar igneous lithological units of almost identical age, transition from porphyry-to epithermal-style mineralization, paragenetic relationships, and comparable fluid inclusion and stable isotope data suggest a common origin for sulfide and gold telluride mineralization at the Tuvatu and Emperor deposits. Potential exists for additional epithermal gold telluride mineralization near volcanic centers in shoshonitic rocks (Ba and Koroimavua Volcanic groups) spatially related to the Viti Levu lineament in northern Viti Levu.

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The Tuvatu gold-silver telluride deposit (300,000 oz), second in size in Fiji to the Emperor gold telluride deposit (10 Moz), is located along the >250 km northeast trending Viti Levu lineament, and is spatially and genetically related to low grade porphyry copper mineralization and to the emplacement of the alkaline 4.85 Ma Navilawa Monzonite. It is hosted in sub-vertical, N-S and NNE-SSW trending veins, shallow S-dipping veins (<45°) or "flatmakes" representing reactivated oblique thrust faults, and irregular brecciated bodies or "shatter zones" that occur at the intersection of the other two lode types. Seven vein structures occur in the Tuvatu deposit (Tuvatu, H, SKL, Nasivi, Murau, West and Upper Ridges lodes), along with two distal vein structures (Davui and Nubunidike lodes). Four hypogene and one supergene stages of vein mineralization were identified. 4 Stage 1 is late magmatic and associated with the formation of apatite while stage 2 is characterized by potassic alteration and early porphyry-style mineralization dominated by coarse magnetite, orthoclase, phlogopite, pyrite, and chalcopyrite. Stage 3, propylitic alteration (epidote, chlorite, muscovite, quartz, pyrite, and chalcopyrite) contains minor amounts of native gold, while stage 4, the most gold-rich stage, is dominated by phyllic alteration (quartz, sericite, adularia, roscoelite, and carbonates) and pyrite, base metal sulfides, tennantite, tellurides (calaverite, krennerite, petzite, hessite, stlitzite, sylvanite, coloradoite, and altaite), native gold, and electrum. Stage 5 mineralization consists mostly of carbonates, gypsum, and secondary Fe-Cu-sulfides with minor supergene native gold. In the Tuvatu and H lodes, porphyry copper style mineralization is overprinted by epithermal gold-telluride veins. Fluid inclusion homogenization temperatures of primary inclusions in stage 1 apatite fluids formed over a wide range of temperatures (276° to >500°C), with most homogenizing at >450°C. These fluids were boiling and hypersaline (>50 wt % NaCl equiv) and subsequently gave way to stage 2 variably saline, boiling fluids (5 to >40 wt% NaCl equiv) that formed at-310°C. Stage 2 fluids also contained traces of an unidentified immiscible organic fluid. Stage 3 propylitic alteration formed at-300°C from fluids that generally ranged in composition from 1 to 10 wt percent NaCl equiv. However, stage 3 fluids were also locally boiling and very saline (up to 37 wt% NaCl equiv). Stage 4 non-boiling, moderately saline fluids (mean = 8.4 wt % NaCl equiv) formed between approximately 325° and 100°C (mean= 259°C) and accompanied telluride and base metal sulfide deposition. The pressure correction to stage 4 fluids was likely small (< 10°C) with hydrostatic conditions persisting through stage 1 to 3 until lithostatic conditions dominated during stage 4. Values of 8 34 S for sulfides in the porphyry and epithermal veins range from-15.3 to-3.6 per mil and reflect an increase in the ~SOJ:EH2 S ratio of a boiling magmatic fluid. 5 Oxygen isotope compositions for water i...