Geology of the Macetown area


Rosenstreich, Michael.


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An area of approximately fifty square kilometres of the Haast Schist in the Macetown area, including gold bearing quartz reefs, was investigated.
The main lithology in the area is pelitic schist with interlayered greenschist horizons. Minor psammitic schist and chert is also present.
Mineral assemblages in the greenschist indicate metamorphism up to the chlorite zone of the Greenschist Facies. The absence of stilpnomelane and possible relict epidote grains is a feature of the greenschist mineralogy. The lack of stilpnomelane is tentatively related to the dominant Mg/Fe compositions of the associated chlorites.
Five deformation phases are recognised. The first two phases are synmetamorphic. The initial phase resulted in tight isoclinal folding of bedding; the second deformation caused similar style folding of segregation laminae. The third phase of deformation is recognised only by a crenulation of the latest schistosity, forming a lineation found sporadically throughout the area. The fourth phase resulted in open brittle folds which are preferentially developed in pelitic lithologies. North striking shear zones and faults are the result of this deformation also. The final phase of deformation consisted of northwest and northeast trending faults, folds and shear zones.
Three gold bearing quartz reefs are mapped in the western part of the area. The reefs consist of silicified clasts of brecciated country rock, blue grey quartz and white quartz. Disseminated grains of pyrite and arsenopyrite are observed within the schist clasts and blue grey quartz. These are traversed by dark grey veins of pyrite with arsenopyrite, chalcopyrite, sphalerite, galena and gold. The final stages of mineralisation are represented by several generations of crosscutting white quartz veins containing very fine grained pyrite and arsenopyrite.
A structural relationship between the quartz reefs and the latest deformation structures; northwest trending fractures is inferred.
A detailed section across an exposed reef revealed an alteration zone extending .5 m from the hanging wall, and at least 1.5 m from the footwall, into the country rock. The alteration assemblage was dominated by sideritic carbonate and calcite. Actinolite, epidote and chlorite became less abundant towards the reef. Trace element analyses indicated no significant enrichment or depletion of Rb, Zr, Y, U, Pb, and Th, within the reef or country rock. Sr was enriched within the reef relative to the country rock. Clay analyses revealed no distinct zones of clay alteration on the margins of the reef.
Fluid inclusion studies of the blue grey quartz indicate a minimum formation temperature of 150° C and a mean equivalent NaCl content of 11%.
Petrographic studies revealed a constant association of gold with sulphide minerals, particularly pyrite and galena. Textural observations indicate a crystallisation sequence of intial pyrite precipitation followed by arsenopyrite, chalcopyrite, sphalerite, galena and gold. The crystallisation sequence is related to high iron and sulphur contents, causing initial mass crystallisation of pyrite.
Two "end member" models for a mineralisation process are presented. The first model proposes a metamorphic origin for the reef depositing fluid. The second model relates the quartz reef emplacement to lamprophyric dyke intrusions. Different aspects of both models are thought to account for quartz reef formation.
Two flights of alluvial terraces are observed in the immediate Macetown vicinity. The higher terrace has a sequence of alluvial gravels overlain by 14 m of lake delta sediments,resting on the schist surface.

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81 leaves : ill. (some col.), maps ; 30 cm.


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Rosenstreich, Michael., “Geology of the Macetown area ,” Otago Geology Theses, accessed May 22, 2024,

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