GHD | Oceana Gold NZ Ltd | 12552081 | Geochemical Assessment Wharekirauponga Underground Mine (WAI985-000-REP-LC-0013) 5 2. Geology and Mineralogy 2.1 Local Geology The Willows Access Tunnel alignment is situated in largely monolithic Miocene aged volcanic rocks, predominantly comprised of intermediary composition andesite, with minor occurrences of more felsic rocks (rhyolite and ignimbrite) encountered toward the northern terminus of the Willows Access Tunnel. Felsic rocks dominate the area within which the WUG Mine is located. The andesite predominantly comprises two units, termed “Waipupu Formation” and “Whiritoa Formation”, which are both mineralogically similar, iron-magnesium silicates, with quartz phenocrysts common. Texturally, the rock fabric alternates between porphyritic lava flows to more broken and rubbly breccia/autoclastic breccia, with less common tuff (aerial ash) deposits. Isolated occurrences of diorite intrusions are also present. At surficial depths, a relatively thin but extensive brecciated tuff mantles the central section of the Willows Access Tunnel alignment (Whakamoehau Andesite). The stratigraphy of the Waipupu and Whiritoa Formations is often non-sequential and cross cutting, with the depositional age of each unit being significantly overlapped. However, the Waipupu Formation Andesite underlies the majority of materials expected to be encountered. This unit extends further south and hosts the Martha Mine open pit and therefore similarities in mineralogy and hydrothermal alteration between the sites are expected. At the location of the ore deposit, the local host geology is characterised by several rhyolitic deposits (part of the Coroglen Subgroup). These vary between volcaniclastic breccias (predominant material) and more massive lava. The rhyolite is likely to be laterally confined at depth (as a function of several notable faults; Edmonds Fault and several splays of this), and to be locally intruded into the surrounding andesitic basement rock (outlined above). At the surface, the material locally overlies the surrounding andesite host rock, which is typical of the relative stratigraphy between these deposits within the Coromandel Region. Associated with the rhyolite in this region of the site is intense silicification (secondary remineralisation with quartz). Through available borehole core and surface sampling, in the area immediately surrounding the ore deposit, the rhyolite and ignimbrite rock has been almost completely replaced with quartz. This is attributed to post deposition hydrothermal re-mineralisation of the rock, via several notable faults as outlined above and in further detail below. The region surrounding Waihi and including the site is strongly influenced by the presence of multiple faults with a distinct set of structural orientations that frequently intersect the Access Tunnel alignment: Northeast – Southwest (primary) and Northwest-Southeast (auxiliary) comprise the main fault sequences. The nature of faulting is typically steep walled extensional (dipping greater than 60 degrees). The faulting has provided the primary conduit for post-depositional hydrothermal circulation to occur (5-10 Mya) that has resulted in extensive remineralisation of the host rock, hydrothermal alteration and deposition of heavy metal and mineral assemblages (i.e., gold and silver bearing minerals). The alteration geochemistry within host rock is typically confined to the major Andesite sequences outlined above. It predominantly comprises wide-field chlorite and acidic clay alternation halos (tens of kilometres from source) with quartz replacement (‘silicification’), sulphur and iron remineralisation being confined more directly to the vein/fault system (<1 km). The geological conceptual site model outlining the Willows Access Tunnel alignment with respect to the main geological features is provided in Appendix A.
RkJQdWJsaXNoZXIy MjE2NDg3