The Mineralogy of the Bellerophon-Nelson Telluride-Bearing Gold Deposit, St. Ives Camp, Yilgarn Craton, Western Australia

Abstract

The newly discovered Bellerophon-Nelson telluride-bearing gold deposit at the St. Ives camp, Western Australia, is hosted by meta-sedimentary rocks of the lower Black Flag Group and alkaline intrusions. Four stages of mineralization are recognized; from oldest to youngest these are: quartz-carbonate veins (Stage I), quartz-albite-carbonate-pyrite veins and sericitepyrite seams (Stage II), quartz-pyrite veins (Stage III), and carbonate ± chlorite veins (Stage IV). Stages II and III contain economic gold mineralization, and the gold grains are strongly associated with pyrite. Intense albite and hematite alteration surround the mineralized veins, and trace amounts of gold precipitated in these altered rocks. The albite and hematite alterations are synchronous and derived from the same oxidized fluid as the auriferous veins. The occurrence and absence of hematite within the alteration zone reflects variable amounts of magnetite in the precursor rocks. Thirteen species of telluride and sulfosalt minerals have been identified in Stages II and III. The most common telluride minerals include calaverite, petzite, tellurobismuthite, and altaite, and these minerals have similar occurrences to native gold. In addition to native gold, telluride and sulfosalt minerals are also major Au carriers and account for at least 15% of the gold in this deposit. The mineral associations of PbCl(OH)-Pb 2 Cl 3 (OH)-Te-TeO 2 and BiOCl-BiO(OH,Cl)-Te-TeO 2 were formed as replacement of earlier telluride minerals as the result of reactions with Cl-bearing fluids. The intergrowth between native gold and Cl-bearing minerals + native Te/Te-oxide indicates that Au in telluride minerals was remobilized and re-deposited. The phase diagram for the telluride and sulfosalt mineral association suggests that during Stage II logƒS 2 decreased from −8 to −11, and that logƒTe 2 increased from −8 to the level required for the formation of Te-oxide. The values of logƒTe 2 and logƒS 2 in the Stage III veins were −8 to −11 and −9 to −11.5, respectively. The highly oxidized, tellurium-enriched hydrothermal fluid, which formed the Bellerophon gold telluride deposit, is consistent with the involvement of magmatic fluid, and sulfidation is the likely cause of gold precipitation.

Albite ± Actinolite-Altered Porphyry Dykes in Archean Gold Deposits of the Boulder Lefroy-Golden Mile Fault System, Yilgarn Craton, Western Australia: Petrography, Chronology, and Comparison to Canadian Albitites

“…They are characterized by intense ore-grade albite replacement (>8 wt.% Na 2 O in syenite), by the oxidized assemblage hematite-magnetite ± anhydrite, and by pyrite with negative δ 34 S values [58]. In the Boulder Lefroy-Golden Mile fault system, the syenite association is rare but represented by the small (2.9 t Au) Bellerophon Au-Te deposit at Kambalda [48,59]. The genetic relationship between the calc-alkaline monzodiorite and the alkaline monzonite-syenite intrusive suites, both associated with oxidized gold deposits, is not fully understood.…”