Cretaceous silicic volcanic deposits in the Shag Valley and at Naseby
Nonmarine 'lignite measures' sectiments in the Waimumu District, eastern Southland are preserved in a basinal setting which is dissected by a series of steeply dipping N-NE striking, reverse faults of the Dunsdale Fault System. Belle-Brook, the main site investigated, is situated adjacent to the Hedgehope Fault of this system. Belle-Brook is characterised by anomalous coarse gold and diagenetic iron sulphide mineralisation hosted within Miocene-Pliocene quartzose gravels. New palynological data indicates an early Pliocene age for carbonaceous material overlying quartz pebble conglomerate (QPC) that occurs at Parker Road. An unusual abundance of Microcachrys (Podosporites parvus) distinguishes this QPC from other regional quartzose gravels. There is an apparent association between Microcachrys occurrence and the inferred fault controlled deposition of recycled upper Gore Lignite Measures (GLM) along components of the Dunsdale Fault System. Diagenetic lump and spheroidal marcasite, and to a lesser extent, framboidal pyrite dominate the sulphide suite at Belle-Brook. Where observed together, paragenetic relations indicate that pyrite (composed of unordered octahedral microcrysts) is a precursor to marcasite formation. SEM textures suggest a possible transformation between framboidal pyrite and recrystallised spheroidal marcasite. Spheroidal marcasite is of a fundamentally similar habit to framboidal pyrite. Lump marcasite characteristically contains anomalously high concentrations of As (1,600 ppm); Ni (5,700 ppm); Cr (400 ppm); Co (3,200 ppm) and to a lesser extent, Zn, Ph, Cu and W. With the possible exception of rare discrete Ni-Co o~ide euhedral crystals, these trace metals are assumed to occur in solid solution. Nickel and cobalt typically occur in various colloforin-style bands ofnickeliferous marcasite, as well as altered, variously oxidised and hydrated, equivalents of the marcasite - nickeliferous marcasite (bravoite) solid solution series. The style and distribution of nickel-rich bands as shown by element mapping may imply a secondary remobilisation of nickel that is potentially associated with marcasite recrystallisation. It is speculated that a desorptive expulsion of solid solution metals (Ni, Co, As?) occurs. It is further speculated that this expulsion may proceed via the reaction transport mechanism of geochemical self-organisation. Preliminary fmdings, based of field occurrences and SEM textural analysis suggest that at least some of the diagenetic marcasite and pyrite is of recent authigenic origin. Sulphide dissolution and inferred precipitation textures are documented. The occurrence of pristine arsenopyrite without alteration to scorodite, as well as potentially authigenic microcrystalline arsenopyrite, implies previously undocumented arsenopyrite stability. The occurrence of detectable nickel in pristine microcrystalline arsenopyrite occurring along the cleavage of host arsenopyrite grains (without nickel), may lend weight to an authigenic origin for these micro-arsenopyrite crystals. Sulphur isotope data for marcasite is within the light isotope range which characterises bacterial sulphate reduction in a nonmarine environment (o34S -12.2 to -28.8 %o). However, diagenetic recrystallised pyrite is atypically enriched (o34S -2.0 %o), as is pyrite associated with lignite (o34S -6 to - 11.0 %o). No attempt was made to distinguish between fine-grained framboidal pyrite and spheroidal marcasite. Detrital arsenopyrite (o34S -9.9 %o) is not entirely consistent with an Otago Schist derivation. Detrital cubic pyrite data are variable, with an unusual enrichment (o34S +13.3 %o) precluding a derivation from the Otago Schist, whereas a second sample is consistent with an Otago Schist provenance (o34S -0.9 %o). - Decomposition of marcasite makes a significant contribution to the development of acidification at Belle-Brook (down to pH of2.4). Elevated nickel (1.08 -17.9 ppm), zinc (1.63- 27 ppm), cobalt (3.56 ppm) and manganese (0.64 -6.65 ppm) occur in the resultant acid water. There is a rapid attenuation of this metal load downstream with increasing pH. Jarosite is the predominant secondary mineral precipitate that occurs on outcrop surfaces. Agedjarosite contains significant nickel (6,064 ppm) and zinc (2,607 ppm), whereas concentration of metals to the same extent has not developed in fresh jarosite samples (i.e. Ni av. 85 ppm; Zn av. 82 ppm). In jarosite a consistent and often two-fold increase in concentration is shown by rare earth elements; Sr, La, Ce, Nd, and Ph, relative to the underlying substrate material. 111 ~I Morphologically two distinct gold types exist. Gold is predominantly fine-grained ( <1 mm) and consistent with an Otago Schist source. A second gold type occurs only at Belle-Brook that is coarse grained and commonly intergrown with quartz. The source of Belle-Brook 'nuggety' gold is unknown but its overall morphology implies that it is potentially local. Compositionally, all gold analysed occurs as one ofthree optically indistinguishable gold types. Approximately 75% affine-grained gold occurs as Au-Ag alloy, 25 % a-phase Au-Ag-Hg alloy, 3 % pure gold. Silver depleted rims are a . feature of both alloy types. Less commonly, silver enriched rims of up to 5 wt% Ag (relative to core) occur in 28 %of Au-Ag alloy and 6% of Au-Ag-Hg alloy particles. Element mapping indicates both silver dissolution as well as gold precipitation has taken place to create silver depleted rims. Regular and irregular silver and mercury dissolution signatures reflect different geochemical, and or, transpmt histories of gold particles. Geochemical signatures of the fine-grained gold are generally consistent with gold derived from the Otago Schist (Caples Terrane). A single grain of electrum (32 wt% Ag) with well developed silver enriched rims is more enigmatic, with no known source in Otago or Southland. It is inferred that gold mobilisation surface textures indicating precipitation and dissolution can be differentiated. Inferred precipitation textures include: budded chains and masses, spheroidal forms, spheroids with five-fold symmetry; rare euhedral crystals (triangular plates), ultra-fine wispy veils and gold threads and possible bacterioform gold. Ultra-fme gold and two-phase colloidal gold is documented for the first time from a placer environment. Inferred dissolution textures reflect preferential dissolution that occurs along grain boundaries. Spatially associated gold mobilisation textures imply an almost synchronous dissolution and reprecipitation of gold within a distance of two microns. Complexes responsible for gold remobilisation remain unconstrained. Silver depleted rims may indicate gold remobilisation as a chloride complex, however, grains without rim development may signify gold mobility as a thiosulphate complex. The involvement of organic complexes is suggested by the occurrence ofultra-fme, and or, colloidal gold forms. Based on the occurrence of broadly similar diagenetic iron sulphides both texturally and compositionally, as well as similar detrital and inferred authigenic gold, it is speculated that BelleBrook may be analogous to an embryonic Witwatersrand-style deposit.
xv, 374 p. : ill. (some col.), maps ; 30 cm.
OU geology Identifier:
OURArchive access level:
Falconer, Donna M. (Donna Marie), 1965-, “Cretaceous silicic volcanic deposits in the Shag Valley and at Naseby ,” Otago Geology Theses, accessed November 12, 2019, http://theses.otagogeology.org.nz/items/show/409.