Contact Metamorphism

The Chalice gold deposit occurs near the margin of a greenstone belt that runs north-south across the Western Australia desert. The margins of such greenstone belts are usually large masses of granitic rocks which sometimes appear to have been intruded as magmas and other times have characteristics of re-activated, older basement that is tectonically brought into contact with the basalts of the greenstone belt. In fact both kinds of relationships may be observed in the same area leading to ambiguity in assigning a process.

On a smaller scale, the veins seen underground at Chalice are very definitely intrusive and thus have at least limited contact metamorphic haloes around them. The amount of contact metamorphism depends on the temperature difference between the intrusive rock and the surrounding, the thickness of the intrusion, and the compositional contrast between the two initially distinct geochemical systems.

Some of the veins are thin enough that there is little obvious interaction between them and their surroundings. The picture below shows a 1-3 cm thick, dominantly quartz vein that, while it generally has sharp walls, in detail has wisps of wall rock amphibolite contained within itself. Bright green chlorite is sprinkled throughout as are grains of the iron sulfide mineral pyrrhotite. Samples like this one host much of the gold mineralization at Chalice.