Abstract:
The Thames-Tapu area is conventionally regarded as a gold-silver province, within the Hauraki gold-silver province; other metals present have been regarded as minor constituents. However, within this area, a regional metallogenic zonation of lead, zinc and copper about molybdenite bearing centres is recognised. These molybdenum centres coincide with areas of outcrop of high level quartz diorite or dacitic porphyry intrusives in which intense quartz-sericite alteration has occurred. Alteration rank decreases away from these centres. In marginal zones of propylitic or weak argillic alteration, base metal mineralization within northeast trending quartz veins is found. Earliest copper-bismuth associations are found together with later lead-zinc-telluride and lead-gold in telescoped assemblages. Molybdenite is found in some quartz veins but more commonly is found disseminated in intrusive rocks of quartz-sericite alteration rank. Here molybdenite is found with pyrite (± rare hematite), rutile and minor chalcopyrite. Pyrite-hematite and pyrite-magnetite haloes have been located in highly altered and molybdenum-anomalous areas in the Ohio Creek, Tararu; and these are shown to have similarities to zonation in the Colorado porphyry molybdenum deposits.
This mineralization is interpreted as a manifestation of a high level porphyry molybdenum deposit which formed at a depth of approximately 2km in a greywacke dacite-andesite pile.
The proposed mechanism of mineralization is as follows:-
(1) Molybdenum ore shells were emplaced at shallow depths after intrusion of the dacite-quartz diorite porphyry stocks. Molybdenum was deposited at temperatures of around 350°C and at pressures above 0.3kb, from fluid of unknown salinity.
(2) Distal to the molybdenite-bearing shells haloes of pyrite-magnetite and pyrite-hematite formed.
(3) Base metal bearing quartz veins were deposited in fissure zones above the ore shell. In these, copper-bismuth mineralization took place at 350-450 (typically 370) °C at a depth of about 1-5 km under a pressure of 0.3kb (this pressure was in part lithostatic). The ore fluids were dilute Na-K-Ca-Cl brines of 2-3 wt % NaCl salinity, and were commonly boiling.
(4) Lead-zinc-telluride ore formed at temperatures of 250-320°C from slightly more saline fluids (~3% NaCl) at comparable and shallower depths under lower pressures which were dominantly hydrostatic in origin. This mineralization was telescoped upon the earlier mineralization as thermal decline took place in the system.
(5) After the main mineralisation a period of uplift and erosion followed.
A second mineralizing event superimposes the first. This mineralization is characterised by zoned antimoniferous assemblages occurring in cryptocrystalline quartz veins which follow a northwest fracture orientation. These crosscut, or infill open spaces in the earlier quartz veins. The deepest levels of this later event are characterized by tetrahedrite together with several sulphosalts which were deposited at least in part above 320°C. Tetrahedrite commonly replaces earlier sulphides. Gold (electrum)-pyrargyrite is found at intermediate levels and occurs in flat lying zones at depth of less than 300-400m below the present surface. Temperatures of 200-250°C are indicated for the gold deposition, and a mechanism of deposition above a boiling zone is invoked. Within this zone rich accumulations gave rise to the Thames bonanzas. These bonanzas were clearly related to faults in the area, and these faults probably provided-the mechanisms by which mixing of fluids and pressure releases took place. This disequilibrium was probably responsible for the rapid and massive gold deposition. Stibnite and barite are found at shallower levels in the system and also occasionally telescoped with a gold bearing assemblage.
A zoned acid-stable aluminous alteration cap is found in rhyolitic pyroclastics of ? Whitianga Group age that mantle the highest elevations, northeast of Thames. Assemblages of natroalunite quartz, pyrophyllite-dickite-pyrite-quartz, dickite-pyrite-quartz and marginal zones with interlayered clay minerals comprise this cap Andalusite and diaspore are also rarely found.
This cap is believed to be the near surface expression of a geothermal system that was responsible for the gold mineralization. A model of deep reintrusion is developed. Above these more recent plutons geothermal systems are thought to have been initiated and these were responsible for the late vein mineralization and associated alteration.
In conclusion, the Thames-Tapu area is regarded as being part of a molybdenum as well as a gold-silver province; this conclusion might also apply to the entire Hauraki gold province.