4 Discussion
The results show that the bleached zones are developed in the central and northern parts of the district, mainly in the hanging wall, or at relatively high elevations in the footwall, of the Comstock and Silver City Faults. The presence in the bleached zones of dickite and pyrophllite that formed at relatively high temperature suggests that the advanced argillic alteration probably resulted from ascending magmatic fluids.

Illite in the bleached zones is characterized by relatively short Al-OH absorption wavelengths (2190-2200 nm). This Al-OH wavelengths range implies that the illite contains very low octahedral Fe and Mg (similar to muscovite). This composition is characteristic of illite formed in acidic hydrothermal conditions.

The assemblage of short Al-OH wavelengths illite, alunite and kaolin clay represents the hydrothermal alteration at shallow depths in the fluid up-flow zones of the hydrothermal system. Note that Au-Ag mineralisation in this district is spatially associated with parts of the bleached zones.

Propylitic alteration was mapped as chlorite enrichment in the central part of the district near the Davidson granodiorite and Cedar Hill west of the Comstock Fault (Figures 1). The coexisting illite shows medium, and less commonly short Al-OH band wavelengths. This propulitic zone could be the alteration caused by the Davidson intrusion immediately preceding the Au-Ag mineralsation, or it represents a syn-mineralsation event.

The more extensive propylitic alteration zones in the southern part of the district are not obviously associated with any faults (Figures 1). They are partly developed in the metamorphosed Mesozoic rocks. These zones cloud be the peripheral alteration on the margins (possibly the down flow zones) of the Miocene hydrothermal system.

Phyllic alteration with illite domination was mapped mainly along the Silver City Fault and further south in the Alta Formation and Mesozoic rocks. The illite in these rocks is characterized by relatively long Al-OH band wavelengths (up to 2216 nm), indicating elevated Fe and/or Mg contents in the illite. The long Al-OH wavelengths illite tends to concentrate in the southern part of the district.

Part of the long Al-OH wavelengths illite may have been derived during the one-forming hydrothermal event as it occurs as relic patches in association with the short Al-OH wavelengths variety close to Au-Ag mineralisation near Virginia City and Flowery. It may represent the periphery alteration in the fluid down flow zones and/or the lower parts of the up-flow zones of the minralisation/alteration system.

5. Conclusions
In summary, the mineral mapping simulation indicates:

1. The bleached volcanic rocks containing short Al-OH wavelengths illite are located along the major faults and many represent hydrothermal alteration in the fluid discharge zones (and the upper parts) of the Miocene hydrothermal system.
2. Fault-controlled propylitic alteration occurs in the footwall of the Comstock Fault in the central part of the district. More extensive propylitic alteration zones characterized by chlorite and long Al-OH wavelengths illite are developed in the southern part of the district, away from the major faults.
3. In the mapped area, illite tends to change southwards from short to long Al-OH band wavelengths. This change reflects a decrease in Al(an increase in Fe and/or Mg) in illite, probably due to a decrease in acidity of the hydrothermal fluids from the central (and upper) to the peripheral (and lower) parts of the hydrothermal system.
4. The hyper spectral mineral mapping results have provided valuable information about the hydrothermal alteration and mineralisation processes, such as the physical conditions and flow directions of the ore-forming fluids, the alteration mineral assemblage most closely associated with Au-Ag mineralsation and the possible dimension of the hydrothermal system.
5. This study demonstrates that ARIES-1 will be capable of providing hyper spectral data for high-precision mineral mapping at a district or regional scale and therefore have great application potential for mineral exploration.

6. References

• Gao B-C, Heidebrecht K.B. & Geotz A.F.H. 1997. Atmosphere Removal Program (ATREM), Users Guide, Version 3, University of Colorado, Boulder, USA.
• Boardman J.W. 1998. Post-ATREM polishing of AVIRIS apparent reflectance data using EFFORT: a lesson in accuracy versus precision. In: Green, R.O. (ed) Summaries of the Seventh JPL Airborne Earth Science Workshop, Vol 1, JPL Publication 97-21, pp.53.
• Hutsinpiller A. 1998. Distribution of Hydrothermal Alteration Mineral Assemblages at Virginia City, Nevada, Using the Airborne Imaging Spectrometer. Remote Sensing of Environment 24, 53-66.
• Vikre P.G. 1989. Fluid-mineral relations in the Comstock Lode. Economic Geological 84, 1574-1613.
• Whitebread D.H. 1976. Alteration and Geological of Tertiary Volcanic Rocks in Parts of the Virginia City Quadrangle, Nevada. US Geological Survey Professional Paper No 936.