Collinsville Copper-Gold Project
Resource Potential
Porphyry copper deposits are large to very large tonnage, low to medium grade, deposits in which ore minerals are primarily structurally controlled and spatially/genetically related to felsic to intermediate porphyry intrusives. The large size and structural control (dykes, ring dykes, veins, vein sets, stock works, fractures, "crackle zones", and breccia pipes) serve to separate porphyry deposits from genetically related deposit types such as some skarns, high temperature mantos, breccia pipes, peripheral mesothermal veins and epithermal precious metal deposits.
As originally proposed by Hollister (1978), the Porphyry Diorite Model encompasses porphyry copper deposits associated with quartz free plutons that commonly contain only potassic and propylitic mineral assemblages with rare argillic assemblages. Included are alkalic petrologic suites as well as calc-alkaline mineralised diorite intrusions. These deposits are not restricted to island arcs or found only in association with syenite. Diorites do not necessarily form part of alkali suite plutons.
Most porphyry diorite hosted deposits are characterised by a high gold: copper ratio and low molybdenum: copper ratio. Hypogene copper sulphide minerals occur in the potassic zone as dissemination and as fracture filling and may also exist in the adjacent propylitic zone.
Gold mineralization generated by magmatic-hydrothermal systems has traditionally been associated either with porphyry base- and precious-metal deposits within and peripheral to calc-alkalic intrusions, or with intrusions of alkalic composition. These systems form in convergent margins and are characterized by highly oxidized, I-type or magnetite-series plutons of predominantly intermediate composition. Pervasive alteration associated with mineralization is extensive.
More recently a new, globally widespread class of intrusion-related deposits in which gold is the principal commodity has been proposed (Lang et al., 2000). The intrusion-related gold systems have the potential to occur over a large vertical extent and only occur within about 2 km of the host intrusions.
Examples of intrusion-related gold systems in Australia are Kidston in Queensland where gold occurs in sheeted veins and in the matrix to an orthomagmatic breccia pipe that is genetically linked to a complex of rhyolite dykes and plugs; and Timbarra in New South Wales where gold occurs in aplite, pegmatite and quartz- molybdenite veins in a miarolitic carapace in a magnetite-bearing leucogranite
The most characteristic style of deposit is as intrusion-hosted, sheeted arrays of low-sulphide quartz veins with narrow alteration envelopes, but other deposit styles and types are also present and are commonly zoned around the intrusions. Many intrusion-related gold systems deposits contain greater than 3 million ounces (Moz) of gold, and the recent discovery of the high grade Pogo deposit in Alaska (9.98 Mt @ 17.8 g/t Au; Smith et al.. 1999) and the large low grade deposits at Fort Knox or Pebble in Alaska (158 Mt @ 0.83 g/t) confirms their attractiveness as exploration targets.
Within individual gold fields, intrusion-related gold systems commonly show a diverse, commonly zoned range of deposits centered upon a parent intrusive complex. Four common types are:
- Sheeted auriferous quartz veins within intrusions. In these deposits both pegmatite and aplite commonly grade into gold-bearing quartz veins.
- Skarns adjacent to intrusive contacts and within the contact aureoles and commonly containing W, Cu, Zn, Au, and Bi.
- Replacement, disseminated and/or fracture-controlled Au, As, Sb mineralisation within dykes and/or metasedimentary host rocks located within and/or outside the contact metamorphic aureoles.
- Late peripheral veins of Ag-Pb-Zn ± Au.
The geological characteristics of the historical gold fields in the Connors Arch and, in particular, the Normanby Gold Field, suggest that the gold mineralisation is associated with porphyry diorite intrusions and/or host rocks.