Compositional Variations Determined from Normative Mineral Assemblages

Based on the compositional variations observed in the data set, the samples were divided into six different groups: unaltered dacite, weakly altered dacite, chlorite-pyrite alteration, K-feldspar-illite alteration, anhydrite-pyrite-pyrophyllite alteration, and pyrite-anhydrite alteration (Fig. F2). However, it should be stressed that the alteration at PACMANUS represents a continuous spectrum and there is a significant degree of overlap between some of the groups defined. Nevertheless, the groups reflect different stages during the geochemical and mineralogical changes associated with hydrothermal water/rock interaction. These are characterized by destruction of primary mineral phases such as plagioclase and formation of various hydrothermal phyllosilicates (chlorite, illite, pyrophyllite, paragonite, and mixed-layer clays), pyrite, anhydrite, and hydrothermal K-feldspar.

Unaltered dacite commonly has a glassy matrix, and normative anorthite and albite are dominant. They contain some normative K-feldspar and very minor normative chlorite-smectite. The weakly altered samples are still dominated by normative feldspar, but hydrothermal phyllosilicates are somewhat more abundant. Pyrite and anhydrite are virtually absent in these two groups.

The K-feldspar-illite alteration is characterized by decreased normative anorthite + albite concentrations and elevated concentrations of hydrothermal K-feldspar and illite. However, there is considerable heterogeneity in this group and some samples have normative anorthite + albite concentrations comparable to weakly altered samples, whereas others contain significant pyrite.

The concentrations of hydrothermal chlorite and smectite are particularly elevated in the chlorite-pyrite alteration. Again, there is some compositional heterogeneity in this group; however, the concentration of anorthite and albite are lower on average than in the K-feldspar-illite alteration. Most samples contain appreciable amounts of pyrite and/or anhydrite.

Samples of the anhydrite-pyrite-pyrophyllite alteration and the pyrite-anhydrite alteration are the most strongly altered in the PACMANUS hydrothermal system and contain little or no plagioclase. They contain significant illite, as well as pyrophyllite and/or paragonite and minor chlorite and smectite. The anhydrite-pyrite-pyrophyllite alteration is characterized by the predominance of anhydrite compared to pyrite, whereas the pyrite-anhydrite alteration is pyrite rich.

Most of the samples were collected from drill holes at the Snowcap (Holes 1188A and 1188F) and the Roman Ruins (Holes 1189A and 1189B) hydrothermal sites. There are some systematic differences in the composition of altered dacite between these two sites (Fig. F3). Most samples from Roman Ruins contain >10 wt% normative K-feldspar, whereas samples from Snowcap generally contain <5 wt% normative K-feldspar (average = 3 wt%). In contrast, samples from Roman Ruins contain no or minor normative pyrophyllite + paragonite (<10 wt%), whereas altered dacite from Snowcap generally shows elevated concentrations in normative pyrophyllite and paragonite with a fairly even spread between 5 and 35 wt%. These compositional characteristics point to substantial differences in the hydrothermal regime between these two sites and may be related to distinctive hydrothermal fluids.