ORE PETROLOGY, CHEMISTRY, AND TIMING OF ELECTRUM IN THE ARCHEAN HYPOZONAL TRANSVAAL LODE GOLD DEPOSIT, WESTERN-AUSTRALIA

The Transvaal lode gold deposit is hosted in ultramafic schists and gr aphite-bearing pelites that are metamorphosed to lower and mid-amphibo lite facies assemblages. The orebody is structurally controlled by the Transvaal shear zone which overprinted and reactivated the penetrativ e regional fabric. The main tabular orebody is formed of a massive qua rtz sulfide vein and related splays and adjacent hydrothermally altere d wall rock. Prograde alteration in the ultramafic rocks consists of a proximal diopside-actinolite-quartz zone which grades outward into a distal amphibole (hornblende-actinolite)-biotite-plagioclase-quartz sc hist. In pelitic metasedimentary rocks, a proximal biotite-muscovite-q uartz-graphite zone grades outward into a distal cordierite-quartz-bio tite-graphite schist. Retrograde chlorite, talc, and zoisite replaced prograde hydrothermal alteration minerals and exhibit generally postki nematic fabrics. The dominant sulfide assemblages in the hydrothermal alteration zones at Transvaal are pyrrhotite-loellingite, pyrrhotite-a rsenopyrite 1, and loellingite-arsenopyrite 1. Other assemblages inclu de chalcopyrite-pyrrhotite, pyrite-loellingite, and pyrrhotite-cubanit e. These assemblages are interpreted to consist of a mixture of equili brium assemblages that relate to a prograde hydrothermal alteration ev ent and disequilibrium assemblages that formed during cooling and reeq uilibration of the rocks. Arsenopyrite 1 is interpreted to have formed both during the prograde alteration event and during high-temperature retrogressive replacement of loellingite. Average compositions of ars enopyrite 1 in equilibrium with loellingite and pyrrhotite indicate pa leotemperatures of about 510 degrees +/- 20 degrees C, about 40 degree s C lower than estimates of the peak metamorphic temperatures. Low and high Fe pyrrhotite and arsenopyrite 2, for instance, would not be in equilibrium with other sulfides and are interpreted to be retrograde. Both electrum and gold occur in the Transvaal gold deposits but electr um is the more abundant gold-bearing phase and contains between 24 and 56 wt percent silver. The compositions of electrum grains correspond to gold finenesses of about 431 to 732 with silver the dominant impuri ty. Electrum occurs mostly at the interface between arsenopyrite 1-loe llingite, or as inclusions in loellingite, pyrrhotite, and arsenopyrit e 1. More rarely, it occurs as as fracture fill within arsenopyrite 1. The differing silver contents of electrum can be explained by differi ng fluid-rock reactions. Low Ag electrum (about 24 wt % Ag) occurs whe re the lode is entirely hosted by ultramafic schists, whereas high Ag electrum is at the ultramafic schist-graphitic-pelite contact. It is p roposed that fluid interaction with reducing pelites produced Ag-rich electrum through destabilization of gold bisulfide and silver chloride complexes, whereas fluid that reacted with more oxidized ultramafic r ocks formed Au-rich electrum as silver chloride complexes were not des tabilized. The main phase of hydrothermal alteration, which included t he crystallization of prograde silicates, pyrrhotite, and loellingite, was syntectonic with respect to the Transvaal shear zone and at appro ximately peak metamorphic conditions. Electrum in equilibrium with pyr rhotite and/or loellingite was thus deposited at synpeak metamorphic c onditions. However, the majority of electrum grains coexist with arsen opyrite 1 and composite loellingite-arsenopyrite 1 grains and are inte rpreted. to have formed from submicroscopic gold released from loellin gite during high-temperature retrogressive replacement by arsenopyrite . These are also considered to be part of the main gold-bearing hydrot hermal event.Retrograde low-temperature, postmetamorphic silicate and sulfide minerals such as chlorite, muscovite, pyrite, and cubanite are evidence for processes that occurred after the main, gold-depositing hydrothermal activity took place. Renewed flux of hydrothermal fluids during the protracted metamorphic history of the terrane could have pr oduced the low-temperature retrogression; the occurrence of low-temper ature sulfides is likely the result in situ reequilibration during coo ling of the rocks.