The Preservation of Pre-Metamorphic Colloform Banding in Pyrite from the Broken Hill-type Pinnacles Deposit, New South Wales, Australia

Joanna Parr
Dept. of Geology, University of Newcastle, Newcastle, NSW 2308, Australia
*Present address: CSIRO, Division of Exploration and Mining, P.O. BOX 136, North Ryde, NSW 2113, Australia

Abstract: Two distinct generations of pyrite, with different morphologies, are described from the Proterozoic Broken Hill-type Pinnacles deposit in western NSW. The earlier, py1, forms concentric layers interpreted as colloform banding. Although the textures are somewhat similar to those observed in supergene alteration zones, textural relationships in fresh rocks suggest that these are pre-metamorphic and that the pyrite formed as the result of precipitation from hydrothermal fluids in open veins, vugs and fissures. The second generation, py2, post-dates py1 and forms euhedral overgrowths on it. It is interpreted as being synchronous with the main phase of base metal sulphide mineralisation. The textures reported here are previously unrecorded for Broken Hill-type mineralisation, and have implications for the regional identification of feeder zones to the Broken Hill deposit. The evidence supports a model in which mineralising conditions at the Pinnacles were characterised by slightly higher oxygen and lower sulphur fugacity (further constrained by Fe contents of sphalerite) than at Broken Hill, where pyrrhotite is the major Fe sulphide.

The pre-metamorphic textures observed in the pyrite at the Pinnacles deposit are also unusual because they have survived granulite facies metamorphism and five phases of deformation, whereas previously the preservation of such textures has not been recognised at metamorphic grades greater than amphibolite facies.

Keywords: pyrite • colloform banding • Pinnacles deposit • Broken Hill-type mineralisation • Australia

Mineralogical Magazine; September 1994 v. 58; no. 392; p. 461-471; DOI: 10.1180/minmag.1994.058.392.11
© 1994, The Mineralogical Society
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