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 ounces = 684.3 ton), consists of numerous deposits located mai nly along narrow, parallel-trending anticlinal domes separated by 100 to 400 m. The individual domes are parts of a regional anticlinorium i n Lower Ordovician sedimentary rocks. Highest gold ore values are gene rally located in the eastern limbs and particularly in the apical area s of the smaller individual anticlinal domes and in the central part o f 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, anke rite and calcite, to 10%) and sulphides (arsenopyrite, pyrite, chalcop yrite and sphalerite, to 5%). The alteration generally follows the tre nd of the structure extending upward and above mineralisation in antic linal 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 290 deg rees C for the alteration. The sequence of carbonate precipitation gen erally 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 metasedimen ts becomes progressively recrystallised and replaced by hydrothermal p yrite, chalcopyrite and sphalerite as mineralisation is approached. Hy drothermal pyrite is generally euhedral and anomalous in having high A s contents when compared to framboidal pyrite. Maximum Au grades (205 ppm) occur in the saddle reef position and below a thick shale unit. L ower-grade mineralisation (0.5-6.0 ppm) occurs above this in the overl ying stratigraphy. As and Pb anomalies cross cut the alteration zoning and occur as plume-like dispersion haloes above the mineralisation. T he Au occurs in and near the As 'plume'. Cu concentrations reach their maximum values adjacent to high Au values. Measured CO, values confir m the observed carbonate distributions. H2O+ and Zn values in the sect ion show relatively random distributions. Two separate stages of miner alisation are indicated, the earlier localised within classic saddle r eef structures and containing the highest Au, Cu and some As and a lat er, cross-cutting stage of mineralisation associated with high As. The alteration is mainly associated with the former. (C) 1998 Elsevier Sc ience B.V. All rights reserved.