Wallrock alteration in the Bendigo gold ore field, Victoria, Australia: Uses in exploration

The Bendigo goldfield, which has produced most of Victoria’s gold 22 million ouncess684.3 ton., consists of numerous deposits located mainly along narrow, parallel-trending anticlinal domes separated by 100 to 400 m. The individual domes are parts of a regional anticlinorium in Lower Ordovician sedimentary rocks. Highest gold ore values are generally located in the eastern limbs and particularly in the apical areas of the smaller individual anticlinal domes and in the central part of the more regional-anticlinorium.

Alteration present in metasandstone units in the Nell Gwynne anticline consists of phengitic sericite to 15 vol%., chlorite to 7%., carbonates siderite, sideroplesite, ankerite and calcite, to 10%. and sulphides arsenopyrite, pyrite, chalcopyrite and sphalerite, to 5%.. The alteration generally follows the trend of the structure extending upward and above mineralisation in anticlinal areas and below largely unmineralised synclinal areas. Phengitic sericite extends furthest, to at least 150 m above mineralised saddle reef positions, chlorite to 130 m, sulphides to 80 m and carbonate to 50 m. Chlorite geothermometry suggests temperatures of 260 to 2908C for the alteration. The sequence of carbonate precipitation generally is from siderite early., sideroplesite, ankerite and calcite. The more Fe-rich carbonates tend to occur near the apical parts of the anticlinal domes. Arsenopyrite occurs within 10 m of the veins, while hydrothermal pyrite, chalcopyrite and sphalerite all extend to 50 m. Framboidal pyrite possibly greigite. present in unaltered metasediments becomes progressively recrystallised and replaced by hydrothermal pyrite, chalcopyrite and sphalerite as mineralisation is approached. Hydrothermal pyrite is generally euhedral and anomalous in having high As contents when compared to framboidal pyrite. Maximum Au grades 205 ppm. occur in the saddle reef position and below a thick shale unit. Lower-grade mineralisation 0.5–6.0 ppm. occurs above this in the overlying stratigraphy. As and Pb anomalies cross cut the alteration zoning and occur as plume-like dispersion haloes above the mineralisation. The Au occurs in and near the As ‘plume’. Cu concentrations reach their maximum values adjacent to high Au values. Measured CO2 values confirm the observed carbonate distributions. H2Oq and Zn values in the section show relatively random distributions. Two separate stages of mineralisation are indicated, the earlier localised within classic saddle reef structures and containing the highest Au, Cu and some As and a later, cross-cutting stage of mineralisation associated with high As. The alteration is mainly associated with the former. q1998 Elsevier Science B.V. All rights reserved