Summary:
The Ernest Henry Deposit is an Iron Oxide Copper Gold (IOCG) hosted within a sequence of moderately SSE-dipping, intensely altered Paleoproterozoic intermediate metavolcanic and metasedimentary rocks of the Mt Isa group. Copper occurs as chalcopyrite within the magnetite-biotite-calcite-pyrite matrix of a 250 m x 300 m pipe like breccia body. The breccia pipe dips approximately 40 degrees to the South and is bounded on both the footwall and hanging wall by shear zones. The main orebody starts to split from the 1575 level into a South-East lens, and from the 1275 level into the South-West lens. Both lenses are separated from the main orebody by waste zones, termed the Inter-lens and South-West Shear Zone, respectively. The orebody is open at depth.
The Ernest Henry copper-gold deposit is hosted in a hydrothermal breccia zone plunging at roughly 45 degrees to the south. At depth, the orientation of shearing appears to be having a greater effect on the orebody and the primary mineralised lenses are becoming more elongate north-south, separating into discrete pods and lenses.
The Ernest Henry deposit is hosted entirely in felsic intermediate metavolcanic rocks, within a unit of the Mount Fort Constantine Volcanics, circa 1800 to 1750 million years. Regionally these lithologies lie adjacent to a large meta-diorite body which traverses the deposit from the south-west to the north-east. Timing of mineralisation at the Ernest Henry deposit is commonly accepted by many workers as occurring between regional D3 through to D4 deformation events circa 1530 to 1500 million years.
The orebody plunges moderately for more than 1500m towards the south-east and is situated between two controlling shear zones, the Hanging Wall Shear Zone (HWSZ) and Footwall Shear Zone (FWSZ), which separate the brecciated plagioclase rich felsic intermediate metavolcanic rock suite from adjacent intercalated meta-sediments.
Mineralization is associated with a matrix supported hydrothermal breccia that is enveloped by crackle veined potassium feldspar altered meta-volcanic rocks. The matrix is largely composed of magnetite, quartz, biotite, chalcopyrite, pyrite, fluorite, gold, molybdenite, uraniferous minerals and potassic feldspar. Other gangue minerals in the matrix consist of chlorite, calcite, dolomite, barite, apatite, muscovite, garnet, scapolite, sphene, rutile and tourmaline.
Chalcopyrite, the only copper mineral observed within the primary orebody, and pyrite are the only significant sulphide minerals within the orebody. Chalcopyrite is fine to medium grained, anhedral and commonly occurs as disseminated grains attached to magnetite and/or pyrite.
Gold occurs about 98% of the time in the form of native gold-electrum (65-95wt % Au), other minor contributions come from sylvanite, auriferous cobaltite, pyrite and chalcopyrite. It is believed that gold precipitation was closely associated with, but preceded some of the chalcopyrite deposition, as indicated by the lower gold and copper ratios of late-stage chalcopyrite rich veins. Although the Ernest Henry orebody contains arsenic, fluorine and uranium minerals, they typically fall below product thresholds, and aren’t considered deleterious.
Both clast and matrix supported breccias typically coincide with copper grades above 0.7% Cu. Felsic altered, clast supported hydrothermal breccia exists as a halo around the main +0.7% Cu zone which also typically hosts gold grades > 0.5 g/t Au. Zones of elevated gold grades (>1 g/t, Au) are coincident with a magnetite / carbonate rich structure or structural zone logged as secondary generation breccia which are constrained within the interpreted 0.7% Cu zone.
A total of six copper mineralisation domains and nine gold mineralisation domains were developed for the Ernest Henry deposit.
Dimensions
Looking east to west, the Ernest Henry deposit extends 1800m along strike (north-south) and 1700m below the surface. The width of mineralisation varies as the deposit becomes elongated below 1300mRL. Above 1300mRL, mineralisation is approximately 340m wide (east to west) and approximately 250m wide below 1300mRL. The deposit dips at 40 degrees to the south, extending from 60m under a sedimentary blanket to beyond 1700m in depth. Below 1575mRL a secondary lens is partitioned to the southeast appearing to be strongly influenced by the shearing.