I made a summary, from various sources, a while back and it is:
Essential albite-oligoclase, epidote, actinolite
Major chlorite, apatite, diopside, magnetite
minor hematite, calcite, quartz, rutile, titanite, scapolite
As well as in porphyry systems and those enigmatic Chilean iron deposits, similar alkali metasomatism is reported on a regional scale in arc volcanics (GSA Bull 110, p. 326, Geology 23, p.913), and is attributed to isotopically heavy, mod to highly saline waters. Sound like marine or lacustrine waters developing in enclosed basins.
Could such basins have existed above the Chilean deposits, and porphyry systems like Yerington? Is the presence of this alteration (saline foprmation waters) a coincidence, or does it play a fundamental role in the mineralising system?
I would like to know because it is easy to muddle the above alteration with propylitisation, which also can be regionally developed in contact metamorphic, burial metamorphic or geothermal environments.
Rob
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Dr Robert CR Willan,
Magmatic-Hydrothermal Processes
Geological Sciences Division,
British Antarctic Survey,
Cambridge CB3 0ET, UK
Tel: 01223 221420
FAX: 01223 362616
Email: [log in to unmask]
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