Ore petrology, chemistry, and timing of electrum in the Archean hypozonal Transvaal lode gold deposit, Western Australia

Abstract

The Transvaal lode gold deposit is hosted in ultramafic schists and graphite-bearing pelites that are metamorphosed to lower and mid-amphibolite facies assemblages. The orebody is structurally controlled by the Transvaal shear zone which overprinted and reactivated the penetrative regional fabric. The main tabular orebody is formed of a massive quartz sulfide vein and related splays and adjacent hydrothermally altered 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 schist. In pelitic metasedimentary rocks, a proximal biotite-muscovite-quartz-graphite zone grades outward into a distal cordierite-quartz-biotite-graphite schist. Retrograde chlorite, talc, and zoisite replaced prograde hydrothermal alteration minerals and exhibit generally postkinematic fabrics. The dominant sulfide assemblages in the hydrothermal alteration zones at Transvaal are pyrrhotite-loellingite, pyrrhotite-arsenopyrite 1, and loellingite-arsenopyrite 1. Other assemblages include chalcopyrite-pyrrhotite, pyrite-loellingite, and pyrrhotite-cubanite. These assemblages are interpreted to consist of a mixture of equilibrium assemblages that relate to a prograde hydrothermal alteration event and disequilibrium assemblages that formed during cooling and reequilibration 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 arsenopyrite 1 in equilibrium with loellingite and pyrrhotite indicate paleotemperatures of about 510 degrees + or - 20 degrees C, about 40 degrees 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 electrum 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 impurity. Electrum occurs mostly at the interface between arsenopyrite 1-loellingite, or as inclusions in loellingite, pyrrhotite, and arsenopyrite 1. More rarely, it occurs as as fracture fill within arsenopyrite 1. The differing silver contents of electrum can be explained by differing fluid-rock reactions. Low Ag electrum (about 24 wt % Ag) occurs where the lode is entirely hosted by ultramafic schists, whereas high Ag electrum is at the ultramafic schist-graphitic-pelite contact. It is proposed 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 rocks formed Au-rich electrum as silver chloride complexes were not destabilized.The main phase of hydrothermal alteration, which included the crystallization of prograde silicates, pyrrhotite, and loellingite, was syntectonic with respect to the Transvaal shear zone and at approximately peak metamorphic conditions. Electrum in equilibrium with pyrrhotite and/or loellingite was thus deposited at synpeak metamorphic conditions. However, the majority of electrum grains coexist with arsenopyrite 1 and composite loellingite-arsenopyrite 1 grains and are interpreted to have formed from submicroscopic gold released from loellingite during high-temperature retrogressive replacement by arsenopyrite. These are also considered to be part of the main gold-bearing hydrothermal 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 produced the low-temperature retrogression; the occurrence of low-temperature sulfides is likely the result of in situ reequilibration during cooling of the rocks.