The Proterozoic iron oxide related Au-Cu-Bi ('Tenant Creek style')
deposits at Tennant Creek can be classified as syntectonic polymetallic
hydrothermal replacement type mineralisation. These deposits have
characteristics in common with a number of other Proterozoic iron oxide
(Cu-U-Au-REE) hosted deposits in regions which include Southeast
Missouri, USA; the Kiruna district, Sweden; the Great Bear Magmatic
Zone, Canada; the Stuart Shelf, South Australia; and the Wemecke
Mountains, Canada. Similarities between these and Tennant Creek style
deposits include: 1. Tectonic Setting: Formed at shallow crustal levels (< 4 km) in a
continental rift environment. 2. Ore mineralogy and geometry: Ore dominated by low-titanium iron
oxides (magnetite and/or hematite) which may form discordant
veins and breccias to massive concordant replacement bodies
depending on local wallrocks and structural controls. 3. Hydrothermal alteration: Host rocks are generally intensely altered
with some vertical zonation present. However, unlike Tennant Creek style deposits, these deposits are
generally hosted within anorogenic silicic to intermediate igneous rocks
which have undergone different hydrothermal alteration histories (sodic at
depth, with potassic and then sericitic and silicic at shallower levels), and
the ore commonly contains apatite with minor fluorite, barite and
anomalous REE. These deposits are believed to have formed from
volatile-rich igneous hydrothermal systems, tapped by deep crustal
structures. BIF hosted Au (Cu) deposits (eg Selwyn, QLD and Hill 50, WA) are
similar to Tennant Creek style deposits in terms of ore geometry and
controls; mineralogy; and to some extent timing, but differ in geological
setting, alteration and age. A metamorphogenic origin for these deposits is
favoured in some cases. Closer geological analogs for Tennant Creek style mineralisation can
be seen in flysch-hosted replacement-type gold deposits such as the Haile
gold mine, South Carolina, Fortnum gold mine WA, and The Peak gold
mine, Cobar, NSW. These deposits are not necessarily ironstone related,
but they share similar host lithologies, alteration assemblages, ore
textures, orebody geometry, structural controls and orogenic timing. Tennant Creek style mineralisation is unique in the sense that the
massive ironstone forms replacement bodies in predominantly pelitic
units. These ironstones were subsequently fractured, allowing later
Au-bearing sulphide stage fluids to react extensively with the ironstone,
causing precipitation of ore within both the ironstone body and
surrounding alteration envelope. These deposits, however appear very
similar to some flysch-hosted replacement-type gold which suggests that
potential exists for Au (Cu-Bi) mineralisation to be hosted within
lithologies other than ironstone at Tennant Creek.