Conference Proceedings
1996 AusIMM New Zealand Branch Annual Conference
Conference Proceedings
1996 AusIMM New Zealand Branch Annual Conference
The 'Mountain Camp Reef' at Alford's Mine, Wakamarina, Marlborough: Active Role of Fault Dynamics Controlling Lode Emplacement
The
Mountain Camp reef' at Alford's mine, Wakamarina,Marlborough, had no recorded gold production
although assays of 5 - 26 g/t were reported, but it did produce c. l000 kg of
scheelite. The lode consists of a lower massive quartz layer, and upper,
cm-banded, ribboned quartz layers, and has a very low sulphide content. Within
the massive layer, relict early fine-grained mosaic quartz with idioblastic
scheelite is surrounded by recrystallised granoblastic coarse-grained quartz
with calcite and plagioclase, in which an original ribbon banding texture is
mimiced by strings of tiny inclusions. In both the banded and massive layers,
rounded quartz grains, created by cataclastic milling of grain margins and along
syn-vein fractures, have been partially annealed within later quartz
overgrowths. Late fibrous quartz and calcite fill brittle cross
fractures.
Deformation events D1 and D2 are
associated with mid-late Jurassic metamorphism of the Pelorus Group,
D2 producing the main regional schistosity (S2, and the
earliest scheelite. D3 is an extensional mylonitic shear event that
formed the dominant schistosity in Alford's mine (S3). The quartz
lode was emplaced within the mylonite by a series of individual dilation pulses
(D4). The D5 event caused crenulation of the S3
mylonite, kink banding and chevron folding of the S2 schist and
S3 mylonite, and reverse faulting, rodding and cross-fracturing with
late fibrous quartz-calcite veining of the D4 quartz lode.
Fluid influxes during successive dilations of
D4 fractures progressively caused recrystallisation of earlier quartz
bands on the footwall, annealing and recrystallisation of quartz along milled
crystal margins and fractures, and deposition of minor sulphides. There was thus
a dynamic interplay between fault dilation, lode fracture, fluid influx, lode
band deposition, and lode recrystallisation. Illite-Kfeldspar al 2 Ma (K/Ar),
c.30 Ma after metamorphism.
Mountain Camp reef' at Alford's mine, Wakamarina,Marlborough, had no recorded gold production
although assays of 5 - 26 g/t were reported, but it did produce c. l000 kg of
scheelite. The lode consists of a lower massive quartz layer, and upper,
cm-banded, ribboned quartz layers, and has a very low sulphide content. Within
the massive layer, relict early fine-grained mosaic quartz with idioblastic
scheelite is surrounded by recrystallised granoblastic coarse-grained quartz
with calcite and plagioclase, in which an original ribbon banding texture is
mimiced by strings of tiny inclusions. In both the banded and massive layers,
rounded quartz grains, created by cataclastic milling of grain margins and along
syn-vein fractures, have been partially annealed within later quartz
overgrowths. Late fibrous quartz and calcite fill brittle cross
fractures.
Deformation events D1 and D2 are
associated with mid-late Jurassic metamorphism of the Pelorus Group,
D2 producing the main regional schistosity (S2, and the
earliest scheelite. D3 is an extensional mylonitic shear event that
formed the dominant schistosity in Alford's mine (S3). The quartz
lode was emplaced within the mylonite by a series of individual dilation pulses
(D4). The D5 event caused crenulation of the S3
mylonite, kink banding and chevron folding of the S2 schist and
S3 mylonite, and reverse faulting, rodding and cross-fracturing with
late fibrous quartz-calcite veining of the D4 quartz lode.
Fluid influxes during successive dilations of
D4 fractures progressively caused recrystallisation of earlier quartz
bands on the footwall, annealing and recrystallisation of quartz along milled
crystal margins and fractures, and deposition of minor sulphides. There was thus
a dynamic interplay between fault dilation, lode fracture, fluid influx, lode
band deposition, and lode recrystallisation. Illite-Kfeldspar al 2 Ma (K/Ar),
c.30 Ma after metamorphism.
Contributor(s):
D N B Skinner
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- Published: 1996
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