Porphyry and skarn deposits of the Chillagoe mining district, northeast Queensland, Australia, The

Abstract

The Chillagoe mining district is located in northeast Queensland and hasintermittently produced zinc, lead, silver, copper and gold from porphyry, skarn and carbonate replacement deposits since 1888. To understand the timing and controls on ore deposit formation, I remapped 121km2 of the district, re-logged 40km of core to create eight new cross sections for the four deposits, dated 21 rocks with U-Pb, Re-Os and 40Ar/39Ar geochronology, analysed the whole rock geochemistry of 89 rocks, acquired electron microprobe analyses of skarn and igneous minerals, and analyzed C-O isotope of calcite. The deposits of the district are the Red Dome Au-Cu skarn deposits, the Mungana Zn- Pb-Ag-Cu skarn and Au-Cu porphyry deposits, the King Vol Zn-Pb-Ag-Cu deposit and the Redcap Zn-Pb-Ag-Cu-Au prospect. The gold + copper deposits are associated with rhyolite porphyry intrusions that have exsolved magmatic fluids within Silurian to Devonian shallow marine sedimentary rocks of the Chillagoe Formation. The lead-zinc-silver-copper skarn and carbonate replacement deposits are present along steeply dipping sedimentary contacts between carbonate rocks and other sedimentary units and along thrust faults. The magmatism associated with these deposits occurred between 335 and 290 Ma and took place in four stages. The earliest potential magmatism occurred at 335 Ma with the formation of the Mungana base metal deposit. The ca 335 Ma molybdenite at Mungana does not overlap with any known intrusions of the district. Between 322 to 315 Ma, magmatism at Red Dome and Mungana formed Au-Cu porphyry deposits and associated skarns. The dacite and rhyolite composition Redcap volcanic rocks erupted contemporaneously with porphyry formation at Mungana. Sometime after the formation of the 317 Ma gold porphyry deposit at Mungana, the base metal deposit was deformed and migrated along the contact of the porphyry gold resource. Contraction followed shortly afterwards as evidenced by ca 312 Ma thrusting of the Chillagoe formation, Red Dome and Mungana over the Redcap volcanic rocks. Magmatism followed thrusting at Redcap as 310 Ma skarn formed along the thrust contact between the volcanic rocks and the overlying Chillagoe formation carbonate rocks. The 310 Ma Redcap base metal skarns were sourced from an undiscovered intrusion to the southeast or south at depth. The Red Hill granite pluton and a granite intrusion below Mungana formed at the same time as the Redcap skarns, however they are not directly related. Magmatic quiescence followed as much of what is now Queensland entered a period of extension and basin development. The final episode of magmatism of the Chillagoe district occurred at ca 290 with the intrusion of the Belgravia and Ruddygore granodiorite plutons, and the formation of the King Vol Zn-Pb-Ag-Cu skarn and carbonate replacement deposit. While the majority of the base metal deposits and prospects of the district are distal and thin (<10m) tabular Zn-Pb-Ag + Cu skarns hosted along near vertical sedimentary contacts, the largest deposit of the district (Red Dome) was hosted by a hydrothermal breccia at the top of the causative intrusion. Coexisting vapor rich and hypersaline fluid inclusions and rounded clasts of the causative porphyry within the skarn suggest fluid exsolution occurred from the top of the intrusion. These processes controlled the formation of the 150m wide breccia which hosted the largest resource of the district. The magmatic sources for the distal base metal deposits remain undiscovered at depth and may be the distal fringes of mineralized Cu-Au porphyry systems.