Выпуск 63
Издание:Journal of Geochemical Exploration, 1998 г., 31 стр.
Alteration and primary geochemical dispersion associated with the Bulletin lode-gold deposit, Wiluna, Western Australia

The Bulletin lode-gold deposit is within the northernmost part of the Norseman-Wiluna greenstone belt in me Archaean Yilgarn Block, Western Australia. It is located within a brittle-ductile shear zone and hosted by tholeiitic metavolcanic rocks. Syn-metamorphic wallrock alteration envelops the gold mineralisation and is pervasive throughout the entire shear zone and extends up to 150 m into the undeformed wallrocks. Alteration is characterised by the sequence of distal chlorite-calcite, intermediate calcite-dolomite, outer proximal sericite and inner proximal dolomite-sericite zones. The thickness of the alteration envelope, and the occurrence of dolomite in the alteration sequence, can be used as a rough guide to the width, extent and grade of gold mineralisation, because a positive correlation exists between these variables. Mass transfer evaluations indicate that chemical changes related to the wallrock alteration are similar in all host rocks: in general, Ag, As, Au, Ba, C02, K, Rb, S, Sb, Те and W are enriched, Na and Y are depleted, and Al, Cr, Cu, Fe, Mg, Mn, Nb, Ni, P, Se, V, Zn and Zr are immobile, while Ca, Si and Sr show only minor or negligible relative changes. The degree of mobility of each component increases with proximity to gold mineralisation. The largest potential exploration targets are possibly defined by regional As (>6 ppm) and Sb (>0.6 ppm) anomalies. These anomalies, if real, extend laterally for > 150 m from the mineralised shear zone into areas of apparently unaltered rocks. Anomalies defined by Те (> 10 ppb), W (>0.6 ppm), carbonation indices, local enrichment of Sb (>2.0 ppm) and As (>28 ppm), and potassic alteration indices also form significant exploration targets extending beyond the HJB shear zone and the Au anomaly (>6 ppb) and, locally, into apparently unaltered rock. Gold, itself, has a restricted dispersion, with an anomaly extending for 1-35 m from ore, and being restricted to within the shear zone itself. Amongst individual geochemical parameters, only As and Sb define significant, consistent and smooth trends (vectors) when laterally approaching the ore. However, the respective dimensions of individual geochenucal anomalies can be used as an extensive, though stepwise, vector towards ore

ТематикаПолезные ископаемые
МеткиAlteration, Archaean, Australia, Gold exploration, Mesothermal gold, Pathfinder element, Primary dispersion, Yilgarn, Австралия, Золото, Мезотермальные месторождения
Выпуск 13
Автор(ы):Brown R.W., Kwak T.A.P., Li X.
Издание:Journal Ore geology reviews, 1998 г., 26 стр.
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

ТематикаПолезные ископаемые
МеткиAustralia, Bendigo goldfield, Lode-gold mineralisation, Victoria, Wallrock alteration, Австралия, Золото, Золоторудная минерализация, Месторождение Бендиго
Автор(ы):Fowler T.J.
Издание:Journal of Structural Geology, 1996 г., 17 стр.
Flexural-slip generated bedding-parallel veins from central Victoria, Australia

Thin continuous laminated bedding-parallel quartz veins (BPVs) with slip-striated and fibred vein walls occur within slates, or at their contact with sandstones, on the limbs of chevron folds in the Bendigc-Castlemaine goldfields, southeastern Australia. Two microstructural Types of BPV (I and II) have been previously recognized, and are confirmed in this study. Both types are concluded to have formed during and/or after crenulation cleavage (the first tectonic axial planar structure) in the wallrock slates, and during flexural-slip folding. Type I BPVs consist of syntaxial phyllosilicate inclusion trails, parallel to bedding, enclosing inclined inclusion bands, the latter formed by detachment of wallrock phyllosilicate particles from the walls of pressure solution-segmented discordant tension veins. Type I BPVs are formed by bedding-parallel shear, and grow in width by propagation of the discordant veins into the BPV walls. Type II veins are composed of quartz bands separated by wallrock slate seams which have split away from the vein wall during dilatant shear opening. They incorporate numerous torn-apart fragments of crenulated wallrock slate. Type I BPV inclusion band average spacing of 0.5 mm probably represents the magnitude of slip increments during stick-slip flexural-slip folding activity

ТематикаСтруктурная геология
МеткиAustralia, Bedding-parallel veins, Central Victoria, Flexura, Австралия, Деформация горных пород, Кварцевые жилы, Кливаж, Флексура, Флексурные сдвиги
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