Summary:
The Hidden Valley Kaveroi deposit is a vein-stockwork gold-silver deposit located in the southeast corner of the Wau Graben and is hosted by the Morobe Granodiorite. The Kaindi Metamorphics occur as a cap to the Hidden Valley Kaveroi Mineralisation. It comprises grey-black and green-brown, variably carbonaceous, schistose, quartz-rich psammites and pelites that have undergone regional greenschist metamorphism and localised, higher grade contact metamorphism on intrusive contacts with Morobe Granodiorite. The granodiorite comprises two parts; an upper homogenous granodiorite of uniform texture cut by thin aplite dykes and feldspar porphyry dykes. Below the Hidden Valley Kaveroi fault is a lower, more heterogeneous unit comprising granodiorite, diorite, adamellite, tonalite and feldspar porphyry. The lower unit tends to contain gypsum veining, not regularly seen in the upper unit.
The deposit is a structurally controlled vein-stockwork gold-silver deposit located in the Morobe Granodiorite of the Wau Graben. Goldsilver mineralisation is contained in carbonate-adularia- quartz-sulphide vein-stockworks and in a few instances in hydrothermal breccias. Discrete zones of intense stockwork fracture and mineralised veining comprise individual lodes. At the Hidden Valley deposit, gold and silver are related to steeply to moderately dipping sheeted vein swarms associated with an underlying shallow thrust.
Numerous porphyry dykes of the Eddie Creek Suite intrude both the Kaindi Metamorphics and the Morobe Granodiorite. Porphyry composition varies from hornblende-biotite to feldspar-quartz phenocryst varieties. These bodies are not general mineralised but do commonly show some alteration.
Surficial weathering, mainly by downward percolation of oxygenated meteoric water, is variable over the gold-silver deposit due to lithological, alteration and structural discontinuities. The granodiorite is usually more deeply weathered than the metasediment of the two main rock units. At the Hidden Valley Kaveroi deposit, four distinct oxidation zones are recognised; an oxide zone, partial oxidation zone, a zone of fracture oxidation, and a fresh (primary) zone. However, the effects of supergene gold enrichment or depletion (if present) are minimal for the Hidden Valley Kaveroi deposit.
Deposit Type
HVK is a low sulphidation carbonate base metal gold-silver deposit, whilst Hamata is a high temperature quartz-pyrite ± sericite epithermal deposit.
Mineralisation
The HVK deposit is classified as a low-sulphidation or adularia-sericite-type epithermal gold-silver system. Leach and Corbett further classify HVK mineralisation into a sub-group simply referred to as carbonate - base metal - gold, due to the presence of carbonates as vein-gangue.
High-grade gold-silver mineralisation in the HVK deposit is associated with manganocarbonates (e.g., kutnahorite, rhodochrosite) and dissolution of sulphide grains in late stages of the vein paragenesis. There is also a tentative association of high gold-silver grades with the presence of late chalcopyrite and sulphosalts in these veins.
Gold-silver mineralisation is largely hosted in narrow, moderate to steeply northeast dipping and west southwest dipping veins in the Hidden Valley and Kaveroi lodes, respectively. These veins occur in dilatational structures formed as the result of normal movement on major faults, such as the Hidden Valley Fault, and others occurring in the hanging wall of this structure.
Gold occurs in electrum with rare gold tellurides and as rare free grains mutually intergrown with carbonate. Silver is mainly present as either electrum or sulphosalts. Electrum most commonly occurs as inclusions within or intergrown with late-stage tetrahedrite and silver-sulphosalts. Therefore, gold shows a moderate positive correlation with silver and base metals in the HVK deposit. However, the silver shows two independent populations – a base metal association and a gold-silver association- leading to complications during the estimation process.
Mineralised veins in the Hidden Valley lode show a predominant northeast dip and are structurally bound in granodiorite by a northwest striking structure, called the Hidden Valley Fault, and an upper contact on the Kaindi Metamorphics. Mineralisation has been dated to 4.2Ma (Hoppe, 1999; Nelson et al., 1990).
Mineralised veins in the Kaveroi lode show a predominant west-southwest dip and are also hosted in granodiorite. The mineralisation appears to be structurally bound by north northwest striking, steep east northeast dipping structures that are thought to be hanging wall splays off the Hidden Valley Fault. Despite the different kinematics, both deposits appear to be related to first a compressional and then extensional structural regime, as indicated by the normal offsets in the bounding structures and the shallow dipping Edie Creek Porphyries that cut the orebody (Gray, 2010).
Later drilling has indicated the presence of fault-controlled, steeply west-dipping fault structures cutting Kaveroi that contain very high gold and silver grades. The first of these structures identified has been termed ‘Big Red” and contains pods of strongly mineralised breccia at the intersection of the strongly mineralised fault and cross cutting faults. As drilling and mining progresses a number of similar strike parallel structures have been identified.
The paragenesis of mineralised vein-stockworks in HVK is believed to represent a continuum of hydrothermal fluids depositing varying amounts of carbonate (dominantly manganocarbonate and calcite), sulphides (sphalerite, galena, pyrite, minor chalcopyrite), and sulphosalts, together with subordinate adularia, quartz, and electrum. K-Ar dating of adularia in mineralised vein material from HVK indicates a Pliocene age (4.1Ma to 4.2Ma) for vein deposition and probably gold-silver mineralisation. This is synchronous with regional magmatism, emplacement of Edie Porphyry and associated gold-silver mineralisation located elsewhere in the Wau Graben.
Mineralisation at Hamata comprises a high temperature quartz-pyrite epithermal vein system occurring as a series of sub-parallel South east-dipping vein sets. Individual reefs typically have a strike length of 100m and can be traced up to 100m down dip; they generally define shallow to moderately NE dipping tensional veins commonly with a significant sigmoidal shape.