Abstract
The hydrothermal alteration in the Miocene andesitic to rhyolitic volcanic rocks in the Comstock mining district near Virginia City, Nevada was mapped by using simulated spaceborne Hyperspectral data, which was part of the feasibility studies to simulate the mineral mapping capability of the proposed Australian satellite, ARIES-1.

The mapped hydrothermal alteration ranges from the phyllic, propylitic, advanced argillic and argillic types. They represent various environments of the hydrothermal system. Detailed discrimination was made to illites according to their aluminum content, and to kaolin clays according to their crystallinity. These subtle mineralogical changes provide further information on the hydrothermal alteration process. The results show that ARIES-1 will be capable of providing Hyperspectral data that are useful for high-resolution mineral mapping.

1. Introduction
The Australian Resource Information and Environment Satellite, ARIES-1, due for commercial operation from 2003, is designed to provide hyper spectral data about the Earth's surface composition. The proposed ARIES-1 will image the Earth in 105 contiguous channels from 0.4 to 2.5 mm (20 nm and 16 nm band spacing in the visible and SWIR, respectively), with 30 meter ground pixel size and 15 swath width at nadir. To test ARIES-1's mineral mapping capability, simulations were carried out in the Comstock mining district using NASA's AVIRIS hyper spectral data.

The Comstock mining district, Nevada is well-known for its low-sulfidation type epithermal Au-Ag mineralisation. This district comprises four quartz-dominated vein systems, of which the Comstock lode is the largest one (Vikre, 1989). These mineralized systems are located in or near normal faults (Figure 1).

Miocene andesitic to dacitic volcanic rocks (Alta and Kate Peak formations) are the main lithologies exposed in the district. These Miocene volcanic rocks overlie the Mesozoic basement consisting of metamorphosed sedimentary, volcanic and granite rocks. The Miocene volcanic rocks are extensively and variably altered, due to pre-, syn-, or post-mineralisation hydrothermal activities. A variety of hydrothermal alteration assemblages, including phyllic, propylitic, alunitic and argillic ones, were previously recognized (Whitebread 1976; Hutsinpillar 1988). The most prominent alteration feature of the Miocene volcanic rocks is 'bleaching', which is caused by development of abundant clay minerals, alunite and quartz.

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(b)
Figure 1. Geological (a) and hydrothermal alteration (b) maps of the Comstock mining district, Nevada. The geological map is after Vikre (1989). The hydrothermal alteration map is generalized from the mineral mapping results of this study.
Au-Ag mineralisation in the Comstock mining district is hosted mainly in quartz veins and stock works. Mineralisation of less importance occurs in breccias and silicified zones. The mineralized veins strike predominately north + 300, expect those related to the Silver city fault which run nearly east-west. Veins are hosted mainly in the Alta Formation. The age of Au-Ag mineralisation in the district is 14-12 Ma, coeval with the Kate Peak volcanism.