Date of Award

2014-01-01

Degree Name

Doctor of Philosophy

Department

Geological Sciences

Advisor(s)

Jose M. Hurtado Jr.

Abstract

This Dissertation comprises three distinct research projects, all of which employ the use of remote sensing and digital image processing methods for the discovery and characterization of mineral resources. Two of the research projects focus on the use of hyperspectral data to identify water and ilmenite on the lunar surface. The third research project involves the use of multispectral data to map alunite in copper porphyry and epithermal silver-gold deposits. Our approach to these problems result in innovative algorithms and geological interpretations that are of use to both planetary science and exploration and the terrestrial mineral industry.

Project I: Towards the late stages of differentiation of the early lunar magma ocean, the remaining material would have been enriched with thorium, water, hydroxyl, and other incompatible species. Using results from the Lunar Prospector Gamma Ray Spectrometer (LP-GRS), we selected thorium (Th) anomalies on the Moon in an effort to detect material rich in KREEP (potassium, rare earth elements, phosphorus) using hyperspectral imagery. Four sites were chosen: Lassell Crater, Hansteen Alpha, Gruithuisen Domes, and Compton-Belkovich Thorium Anomaly (CBTA). Three of these sites are non-mare volcanic features within the Procellarum KREEP Terrane (PKT), while Compton-Belkovich is located on the lunar farside. The Moon Mineralogy Mapper (M3) hyperspectral imager was used to analyze the composition of these locations. The spectra gathered from all four study sites all show pronounced absorptions at ~2.8 μm, indicating hydroxyl or water. This is significant for three reasons: (1) the strong absorption of hydroxyl/water shown at each of these volcanic sites supports the hypoThesis that the lunar mantle is more hydrous than previously thought; (2) it suggests that KREEP may lie, possibly as uncoupled pods, beneath the anorthositic highlands near Compton-Belkovich as well as underlying other areas outside the previously defined PKT; and (3) it suggests that non-mare silicic volcanic features would have erupted prior to mare basalts due to their increased abundance of magmatic water, consistent with basaltic underplating.

Project II: KREEP material on the Moon is shown to be relatively rich in ilmenite (FeTiO3). By targeting areas with anomalously high Th signatures, as seen by LP-ThGRS, we attempt to determine if Th hotspots are associated with ilmenite-rich basalts. To map ilmenite, we employ a band depth technique that takes advantage of the fact that the visible-infrared reflectance spectrum of ilmenite exhibits low reflectance and a flat continuum slope. As a result, the spectra of ilmenite-bearing mare basalts will have a reduced 1-μm absorption. We demonstrate this effect by plotting ilmenite concentrations from Apollo basalt samples against the M3-derived, 1-μm absorption depths associated with the locations from which the samples were collected. A least-squares regression to the ilmenite vs. 1-μm absorption data is then used to predict ilmenite concentrations of mare basalts from M3 spectra. Using this methodology, we built ilmenite maps for the following nearside mare: western Mare Imbrium; southern Oceanus Procellarum; eastern Mare Nubium; Mare Serenitatis; and Tranquillitatis. Based on the concentrations of Th and ilmenite associated with the eruptions, we determined that at least three eruption episodes of mare basalts occurred, each with different geochemical signatures. In addition we identified late stage (<3.1 Gya) ilmenite- and Th-rich basalts within the PKT, which we suggest were supplied by the arrival of a KREEP-, and ilmenite-rich plume that formed at the core-mantle boundary after ilmenite-rich and KREEP-rich melts sank into the mantle. However, areas outside of PKT, such as Tranquillitatis and Serenatatis, do not exhibit both high KREEP and high ilmenite concentrations. Instead, early stage basaltic eruptions - consisting of low-Th, ilmenite-rich basalts are present at Mare Tranquillitatis and Th- and ilmenite-poor basalts are present at Serenitatis. We propose two possible scenarios to explain this. In the first, the Ti-rich but Th-poor mare basalts would have erupted after (or during) a degree-1 downwelling that affected the nearby PKT early in lunar history. The KREEP may have never existed at these locations, or it was removed by the degree-1 downwelling, which redistributed that material and concentrated it in the PKT. In the second scenario, the Ti-rich but Th-poor mare basalts would have erupted prior to the degree-1 downwelling. In this case, the ilmenite-rich cumulates may have existed at the time of melt generation. However, the lack of Th would imply that KREEP did not exist at all at these locations.

Project III: Alunite (KAl3(SO4)2(OH)6) is a sulfate mineral that is commonly found in argillic alteration zones of porphyry and epithermal systems, and in other supergene enriched mineral deposits. Using ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data, we target spectral features associated with hydroxyl (OH-) and sulfate (SO42-). Previous studies have used OH- absorptions near 2.2 μm to target alunite, but their methods can confuse alunite with carbonates, detrital clays, iron oxides, and jarosite. We use a logical operator approach to increase our confidence in targeting alunite and delineate it from carbonates, detrital clays, iron oxides, and jarosite. The first logical operator targets a doublet absorption near 2.2 μm associated with OH- in alunite, detrital clays, and carbonates. It also targets the negative spectral slope between 0.8 and 1.65 μm, in order to delineate alunite from iron oxide and jarosite. We also develop a second logical operator that targets the 9-μm absorption associated with SO42- in alunite, jarosite, and quartz. To test the effectiveness of our logical operator methodology in places where carbonates, detrital clays, limonite, and vegetation not related to porphyry and epithermal systems are present, we conduct a ground truth investigation at Cuprite Hills, Nevada. We show that the alunite identified by our alunite map is spatially correlative to the regions indicated by AVIRIS mineral maps to contain alunite. Our improved methodology for locating alunite with ASTER data is an inexpensive, easy, and reliable method for discovering new porphyry and epithermal mineral deposits that can be applied to a variety of exploration problems.

Language

en

Provenance

Received from ProQuest

File Size

292 pages

File Format

application/pdf

Rights Holder

Douglas L. Standart

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