The CVZ consists of andesite and dacite flows together with breccias and tuffs, forming a series of stratovolcanoes ranging in age from 18 to 3.8 Ma. These rocks interfinger with rhyolite tuffs and flows related to several caldera complexes which range in age from 12 to 1.95 Ma (Booden et al., 2012; Simpson and Chambefort, 2014). The andesite rocks predominate in the western side of the CVZ and rhyolites in the east (Skinner, 1986). The regional stratigraphy of CVZ consists of Late Jurassic marine sedimentary greywacke and argillite of the Manaia Hill Group forming the basement rocks. These rocks are exposed primarily in the northern end of the Coromandel peninsula (Figure 3.1). The basement rocks are overlain by very restricted non volcanogenic Oligocene coal measures and marine calcareous rocks of Torehina Formation (Te Kuiti Group). Also, minor volcanogenic marine conglomerate, sandstone and siltstone of Colville Formation of lower early Miocene age (Waitemata Group) are primarily deposited in the northern part of the CVZ (Skinner, 1969, 1979). These rocks are overlain by Miocene to Pliocene andesitic and dacitic volcanic rocks of the Coromandel Group and late Miocene to Pliocene rhyolites and pyroclastic rocks of the Whitianga Group (Skinner, 1986; Adams et al, 1994; Christie et al., 2007; in Simpson et al., 2007).
Table 3.1. Summary of regional stratigraphy of main Hauraki Volcanic Region (HVR) (Adapted from Skinner 1986).
3.2.2.1. Coromandel Group
The Coromandel Group rocks are further divided geographically into three major subgroups Kuaotunu, Waiwawa and Kaimai subgroups with an overlap of Omahine Subgroup. The Omahine Subgroup is similar in age and compostion with Kaimai Subgroup. The Subgroups are subsequently divided into more than 25 formations (Table 3.1) with regional unconformities separating the subgroups and local unconformities separating formations (Skinner, 1986). The rocks range in composition from andesites to rhyolites and dacites. The major subdivisions of Coromandel Group are described briefly as follows.
3.2.2.1.1. Kuaotunu Subgroup
Kuaotunu Subgroup rocks are predominantly deposited in the eastern part of the Peninsula and consist mainly of alternating pyroxene andesite and hornblende- pyroxene andesite to dacite sequences (Christie at al., 2007). The Subgroup consists of 17 formations and includes Cuvier and Paritu Plutonics of Late early Miocene (Skinner, 1986).
The other major unit is the andesitic Beesons Island Volcanics which lies in the western side of the Coromandel peninsula and extending from Colville south to Thames. This unit consists of subaerial/subaqueous pyroclastic and epiclastic breccias and conglomerates, flows and plugs and intensively hydrothermally altered to propylitic, argillic, and siliceous assemblages (Skinner, 1993).
3.2.2.1.2. Waiwawa Subgroup
The Waiwawa Subgroup rocks are subdivided into Taurahuehue Andesite and Tapuaetahi Andesite Formations and are cropped out in the southern and eastern part of the Coromandel Peninsula, chiefly to the west and north of the Mongakino and Waihi area (Adams, 1994). The Taurahuehue Formation overlies Maumaupaki Formation and lies southwest of Coroglento east of Thames and consist of sheet like flows, pyroclastics, and minor intrusives of phyric to glassy andesite rocks. These rocks mark the outer northern and western rim of the Kapowai Caldera. The Tapuaetahi Andesite rocks on the other hand cover an
Whenuakite consisting mainly of coarsely phyric andesite flows, pyroclastics, and intrusives that define the eastern rim of the Kapowai Caldera (Skinner, 1986, 1993; Adams, 1994).
3.2.2.1.3. Kaimai and Omahine Subgroup
The Kaimai and Omahine subgroup share almost the same age and composition with both subgroups dating back to late Miocene and Pliocene (Skinner, 1986). The Omahine Subgroup consists of Taurauikau Andesites which are chiefly composed of glassy, silicic andesite-dacite flows, minor intrusives forming a high plateau between the Waiwawa, upper Rangihau and Kaueranga Valleys (Adams, 1994). The Kaimai Subgroup consists of Pukepanga, Uretara and Waipupu Formations of which Waipupu Formation hosts most of the epithermal deposits of the Waihi area including Correnso deposit. The Waipupu Formation is subjected to extensive hydrothermal alteration and date back to Late Miocene to Pliocene (Adams, 1994).
3.2.2.2. Kerikeri Volcanic Group
The Kerikeri Group of rocks contain Mercury Basalt Subgroup which range compositionally from olivine and olivine-augite basalt to pyroxene basalt and basaltic andesite and age approximately to late Upper Miocene and/or younger (skinner, 1977, 1979). The Mercury Basalts are restricted to the north-eastern part of the Coromandel peninsula and offshore islands. Mercury Basalts of the Mercury Islands lie on a submarine plateau 15 km long by 5-10 km wide trending ENE. The plateau continues further towards the southern end of the Colville Ridge which indicate the basalt flows being the extension of the ridge into the New Zealand landmass (Skinner, 1977). The Mercury Island basalts consist of individual flows, separated by basaltic scoria, spatter, lapilli and tuff, and rhyolitic ash. These rocks also intrude and overlie Mahinapua Andesite on the eastern Kuaotunu Peninsula and form swarm of dikes intruding Ruahine Rhyolite at Woody Hill (Skinner, 1986).
Figure 3.2. Geologic map of the Coromandel Peninsula, showing the location of major epithermal Au-Ag deposits, major faults and stratigraphic units (Adapted from Skinner, 1986; Mauk & Simpson, 2007).
3.2.2.3. Whitianga Group
The Whitianga Group rocks consist of late Miocene to Pliocene rocks which are restricted predominantly to the eastern coast and central part of the Coromandel Peninsula. The Group is further subdivided into two subgroups on the basis of the composition of rocks as Minden Rhyolite Subgroup and Coroglen Subgroup.
3.2.2.3.1. Minden Rhyolite Subgroup
The Minden Rhyolite Subgroup rocks are composed chiefly of flow-dome and caldera complexes which consist of calc-alkaline to alkaline felsic lavas of late Miocene to Pliocene age. Rhyolitic eruption centres dominate the east and west of the Waihi area. The Ruahne Rhyolite of Minden Subgroup occurs in Whitianga and Kapowai and consist of phyric, glassy and perlitic crystal-rich rhyolite intrusives, domes (Adams, 1994).
3.2.2.3.2. Coroglen Subgroup
The Coroglen Subgroup units consist of pyroclastic-volcaniclastic formations of Late Miocene to early Pleistocene and found in the central and southern parts of the Coromandel peninsula (Adams, 1994). The main formations of this subgroup are Carina Rock Ignimbrite, Wharepapa Ignimbrite and Pumpkin Rock Ignimbrite.