Mineralized vein systems and Miocene maar crater sediments at Hindon, East Otago, New Zealand

Youngson, John H. (John Hughan)

Numerous normal fault-hosted quartz-gold- or quartz-stibnite-gold-bearing vein systems · bross-cut the Haast Schist along the dissected eastern margin of the Hindon plateau, East .btago, New Zealand. With one exception the faults share a northwest-quadrant strike but are variably oriented with respect to each other and the axis of a macroscopic F2 fold and · mesoscopic syn-metamorphic structures (D2, D3) in the schist. Correlation with a regional set of northwest-trending normal faults suggests formation in the mid to late Cretaceous during the later stages of schist extension and uplift. Vein systems are traceable for up to seven kilometres and exhibit significant variation in gross structural style, internal structure and mineralization characteristics. _ Two of the vein systems are sufficiently exposed for detailed description; the Zealandia vein system and Douglas' reef. The Zealandia fault system consists of a multi-stranded horst · complex up to 200 m wide at the surface and hosts quartz-gold-arsenopyrite-pyrite-galena mineralization intermittently along strike in a wide range of localised settings. Douglas' reef is a shallow-dipping (38"), narrow ( -1 m), well-defined structure which hosts quartz-massive stibnite and minor pyrite-gold mineralization. Stibnite occurs predominantly in association with floating-clast hydrothermal breccias in an extensively mineralized dilational jog, the site of the former Hindon Antimony Mine. The vein systems are all in close proximity but were formed at various depths within the crust, at different times during uplift of the schist. Fluid inclusion evidence and arsenopyrite geothermometry suggest emplacement of the Cemetery reef at a depth of ~ 220 m and the Zealandia vein system at 1.2±0.4 kb (-3-6 km), respectively. Massive stibnite mineralization and distinctive associated quartz textures at the Hindon Antimony Mine are typical of the epithermal environment and suggest relatively near-surface ( <1-2 km) emplacement for Douglas' reef. Mineralizing fluids for the Cemetery reef, the Zealandia vein system, Douglas' reef and the Game Hen reef were low salinity ( <5 wt.% NaCl equivalent)± low C02 brines. The Zealandia fault system has been reactivated with a partially reversed sense of movement during Late Cenozoic deformation of the schist block. The most recent slickenside lineations indicate a phase of predominantly strike-slip activity and overprint earlier dip-slip slickensides. Two sets of slickenside lineations along the sub-parallel Barewood fault system on the eastern margin of the adjacent Barewood plateau indicate a similar sense of movement and show the same overprinting relationships. These two fault systems may be acting jointly as a transfer structure, redistributing strain eastwards to be accommodated along other structures. A northeast-trending, broad and subtle antiformal flexure has formed in the step-over region between the two fault systems. The Barewood-Hindon plateau region represents an extensive, only marginally degraded remnant of the low relief landscape produced during regional Cretaceous-Tertiary peneplanation of the schist. The strain transfer along the Barewood and Zealandia fault systems could explain preservation of the adjacent plateau as a relatively undeforrned (internally) region, in stark contrast to the more extensively disrupted and uplifted surrounding regions. Two previously unknown volcanic vents are documented: one with associated basalt flows at Abbotsford Hill and the other, near Hindon, interpreted to be a maar crater. This style of explosive volcanism has not been recognised previously in the Dunedin Volcanic Province. The maar is approximately 500-700 m across 'and approximately 120 m deep relative to the plateau surface, and underlies the central portion of a small, pear-shaped basin at the head of the true right branch of Game Cock Stream. Sedimentary occurrences in the study area are restricted to scattered remnants of Taratu Formation sands and conglomerates, schistose debris flow deposits along locally over-steepened valley walls, and sediments associated with the maar crater. Sediments infilling the maar are not exposed but have been cored and pitted to a maximum depth of 15 m. They include a sandy clastic sequence, possibly representing reworked crater ejecta, and an overlying, predominantly biogenic, lacustrine sequence of Altonian age, the Hindon Lignite (new). Precursor sediments for the Hindon Lignite accumulated in a small crater lake and sedimentation rates, calculated from the thicknesses of varved laminae, suggest a minimum life span for the lake of approximately 80,000 years. The Hindon Lignite is highly carbonaceous near the base but grades upward into carbonaceous diatomite, and has undergone only very weak coalification (biochemical). lll '''' , Flora and fauna in the Hindon Lignite consist of the remains of abundant algae (Chlorophyceae Chrysophyceae and Bacillariophyceae) and freshwater sponges which colonized the crater lake, and leaf material blown in to the deposition site. Chrysophyceae are represented by their siliceous resting stage (statospores). Fossil statospores have been reported from some New Zealand lacustrine sediments but do not appear to have been described in detail previously. Sponge remains include abundant gemmules, the resting stage of the freshwater sponge. Gemmules are extremely rare in the fossil record and, to date, no other New Zealand examples have been documented. The depositional setting of the previously described Foulden Hill Diatomite is reassessed. In terms of general composition and the overall sedimentary sequence, the Foulden Hill Diatomite closely resembles the Hindon Lignite and most probably accumulated in very similar structural and depositional ~ettings. Faulting models suggested by previous workers for the origin of the Foulden Hill basin are inconsistent with the sedimentary sequence in the basin and the geological relationships around its perimeter. It is proposed instead that the Foulden Hill sequence was deposited in a small lake in another maar crater. In recognition of the predominantly biogenic nature of the Hindon Lignite and the Foulden Hill Diatomite, and the role of explosive volcanism in the formation of their respective deposition sites, The Hindon Sapropelites Formation, is proposed. The Hindon Lignite and the Foulden Hill Diatomite are incorporated into this formation as separate members.

xix, 258 p. (some folded) : ill. (some col.), maps ; 30 cm.

1993Youngson

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