The pipe deposits of tungsten-molybdenum-bismuth in eastern Australia
thesisposted on 28.03.2022, 22:54 authored by Ian Rutherford Plimer
Joint-controlled quartz-rich molybdenite-bismuth±wolframite pipes occur in the altered margins and roof zones of high level leucocratic granitic plutons. The hose granitic plutons have been sericitised at their margins and roof zones with local more intense alteration (sericitisation, greisenisation, spessarititisation) zonally arranged around the pipe loci. Ore distribution is patchy and the ore grade decreases with depth. The pipes are internally asymmetrically zoned and comprise quartz, molybdenite,ferberitic wolframite and bismuth minerals with minor amounts of other ore and silicate minerals.The host granitic plutons are believed to have derived from the shallow anatexis of metasediments above a lithospheric plate boundary. These melts fractionated and ascended to a high level with the last fractions slowly crystallising to coarse grained adamellites. Some alkali metasomatism occurred in some areas after pluton emplacement. Jointing of the pluton occurred during or after the last stages cf crystallisation as a result of contraction or fluid pressure. A magmatic aqueous phase was released and ascended joints close and subparallel to the contact plane and collected in the roof zone beneath high grade metamorphic rooks. This high temperature aqueous phase at Wolfram Camp was released at PH₂O≥0.3kb; had a low pH, Fh, fO₂ and fCO₂; high salinity and fS₂ and contained complexes of W, Mo and Bi which were partitioned from the melt into the aqueous phase. During ascent of this aqueous phase, there was minor lateral movement away from the joint loci producing zonal alteration assemblages as a result of H⁺ digestion, fO₂ and Fh increase. Ore mineral precipitation was possibly a result of fluid mixing or temperature decrease. A number of episodes of hydrothermal fracturing were recognised and the temperature and salinity of the aqueous phase decreased with each successive fracturing event.These pipe deposits in eastern Australia have many similarities with the Mo pipe deposits of Japan and the circum Pacific porphyry copper deposits.