Abstract Gold mineralization at the Damang deposit is unique among known deposits in Ghana, comprising two distinct styles of mineralization. These include a stratigraphically controlled auriferous quartz-pebble metaconglomerate that is overprinted by later gold contained in a complex fault-fracture vein array with surrounding hydrothermal alteration. A systematic study using portable, field-based infrared reflectance spectroscopy has proven to be a valuable exploration tool at Damang. Spectral parameters such as the ferrous-iron response, the AlOH/H 2 O absorption depth ratio, and automated mineral identification successfully distinguish metasedimentary and metadoleritic lithologic units at Damang. Systematic variations in these parameters, together with the water/OH absorption depth, both downhole and in three-dimensional models, provide vectors to gold mineralization. The spectral parameters AlOH wavelength and MgOH wavelength are used to define the regolith profile at Damang, throughout which the ferrous-iron response parameter provides a reliable indicator of gold mineralization. All recorded changes in spectral parameters can be linked to sample petrography and are supported by mineral-chemical data. These results show that portable infrared spectroscopy can be used in a variety of roles, including regolith mapping, geologic mapping and logging, and recognition of hydrothermal alteration patterns, as each lithology and alteration style exhibit distinct and identifiable spectral characteristics. These spectrally derived alteration proxies indentify a broader zone of potential gold mineralization than gold grades alone, providing a larger target for exploration. The rapidity of data collection and ease of analysis of spectral data make infrared reflectance spectroscopy a useful methodology that can be readily incorporated into both preexisting and established exploration programs in other tropical terrains.

I-type granites can be assigned to low- and high-temperature groups. The distinction between those groups is formally based on the presence or absence of inherited zircon in relatively mafic rocks of a suite containing less than about 68% SiO 2 , and shown in many cases by distinctive patterns of compositional variation. Granites of the low-temperature group formed at relatively low magmatic temperatures by the partial melting dominantly of the haplogranite components Qz , Ab and Or in H 2 O-bearing crustal source rocks. More mafic granites of this type have that character because they contain restite minerals, often including inherited zircon, which were entrained in a more felsic melt. In common with other elements, Zr contents correlate linearly with SiO 2 , except sometimes in very felsic rocks, and Zr generally decreases as the rocks become more felsic. All S-type granites are apparently low-temperature in origin. After most or all of the restite has been removed from the magma, these granites may evolve further by fractional crystallisation. High-temperature granites formed from a magma that was completely or largely molten, in which zircon crystals were not initially present because the melt was not saturated in that mineral. High-temperature suites commonly evolved compositionally through fractional crystallisation and they may extend to much more mafic compositions through the production of cumulate rocks. However, it is probable that, in some cases, the compositional differences within high-temperature suites arose from varying degrees of partial melting of similar source rocks. Volcanic equivalents of both groups exist and show analogous differences. There are petrographic differences between the two groups and significant mineralisation is much more likely to be associated with the high-temperature granites. The different features of the two groups relate to distinctive source rock compositions. Low-temperature granites were derived from source rocks in which the haplogranite components were present throughout partial melting, whereas the source materials of the high-temperature granites were deficient in one of those components, which therefore, became depleted during the melting, causing the temperatures of melting to rise.

Granites and related volcanic rocks of the Lachlan Fold Belt can be grouped into suites using chemical and petrographic data. The distinctive characteristics of suites reflect source-rock features. The first-order subdivision within the suites is between those derived from igneous and from sedimentary source rocks, the I- and S-types. Differences between the two types of source rocks and their derived granites are due to the sedimentary source material having been previously weathered at the Earth’s surface. Chemically, the S-type granites are lower in Na, Ca, Sr and Fe 3+ /Fe 2+ , and higher in Cr and Ni. As a consequence, the S-types are always peraluminous and contain Al-rich minerals. A little over 50% of the I-type granites are metaluminous and these more mafic rocks contain hornblende. In the absence of associated mafic rocks, the more felsic and slightly peraluminous I-type granites may be difficult to distinguish from felsic S-type granites. This overlap in composition is to be expected and results from the restricted chemical composition of the lowest temperature felsic melts. The compositions of more mafic I- and S-type granites diverge, as a result of the incorporation of more mafic components from the source, either as restite or a component of higher temperature melt. There is no overlap in composition between the most mafic I- and S-type granites, whose compositions are closest to those of their respective source rocks. Likewise, the enclaves present in the more mafic granites have compositions reflecting those of their host rocks, and probably in most cases, the source rocks. S-type granites have higher δ 18 O values and more evolved Sr and Nd isotopic compositions, although the radiogenic isotope compositions overlap with I-types. Although the isotopic compositions lie close to a mixing curve, it is thought that the amount of mixing in the source rocks was restricted, and occurred prior to partial melting. I-type granites are thought to have been derived from deep crust formed by underplating and thus are infracrustal, in contrast to the supracrustal S-type source rocks. Crystallisation of feldspars from felsic granite melts leads to distinctive changes in the trace element compositions of more evolved I- and S-type granites. Most notably, P increases in abundance with fractionation of crystals from the more strongly peraluminous S-type felsic melts, while it decreases in abundance in the analogous, but weakly peraluminous, I-type melts.

The Lachlan Fold Belt in southeastern Australia comprises rocks ranging in age from Cambrian to Devonian. Granitoid emplacement and related volcanic activity occurred in Silurian and Devonian times, with minor development of Carboniferous plutons in the most easterly part of the belt. The belt is at least 800 km wide, which is much wider than the Mesozoic and Cenozoic fold belts of the circum-Pacific. Granitoids are extensively developed in the Lachlan belt and make up 36 percent of exposed Paleozoic rocks in the relatively well-exposed easternmost part east of longitude 148° E, a strip up to 200 km wide. Granitoids in the Lachlan Fold Belt can be grouped into suites, where each suite has a distinctive chemical character, consistent with its having been derived from source rocks of unique composition. Most of the variation within suites can be ascribed to varying degrees of separation of material residual from partial melting, or restite, from melt. The differences between suites result from differences in source rock composition. The first-order subdivision between suites is between those granitoids derived from sedimentary and from igneous source rocks, the S- and I-types. These two types have chemical, mineralogical, and isotopic characters reflecting the distinctive features of their sources, specifically the fact that the S-type source rocks have been through at least one cycle of chemical weathering at the earth’s surface. There is an eastern limit to the occurrence of S-type granitoids, called the I-S line, which is thought to represent the eastern limit of thick crystalline basement. A late-formed group of felsic granitoids, the A-types, are thought to have been derived from crust that had previously produced I-type magmas so that the source rocks were residual from that prior melting event.

Earth-based observations and spacecraft results show that aeolian processes are currently active on Mars. Analyses of various landforms, including dunes, yardangs, and mantling sediments of probable aeolian origin, suggest that aeolian processes have been important in the geological past. Dust storms originate in specific areas of Mars and are most vigorous during the martian summer in the southern hemisphere. In order to understand aeolian processes in the low surface pressure (∼7 mb), carbon dioxide atmosphere of Mars, a special wind-tunnel was fabricated to carry out investigations of the physics of windblown particles under martian conditions. Martian threshold wind speeds have been derived for a range of particle diameters and densities; the threshold curve parallels that for Earth but is offset toward higher wind velocities by about an order of magnitude. The “optimum” size particle (the size most easily moved by minimum wind) is about 100 pm in diameter; minimum freestream winds to generate particle motion are about 40 ms-I. Grains smaller than 100 pm (“dust”) require increasingly higher winds to initiate threshold; yet, estimates of grain sizes in the dust clouds are in the size range of a few microns and smaller. Because the Viking Lander has recorded winds no stronger than those for minimum threshold, it is suggested that some other mechanism than uniform strong winds is required for “dust” threshold. Experiments and theoretical considerations suggest that such mechanisms could be cyclonic (“dust devil”) winds, a saltation cascading effect by larger (more easily moved) particles, and injection of fine grains into the wind stream by outgassing volatiles absorbed on the grains.