|Title||A comparison of fluid origins and compositions in ironoxide-copper-gold and porphyry-Cu (Mo-Au) deposits|
|Author||Rusk, B; Emsbo, P; Xavier, R; Corriveau, L; Oliver, N; Zhang, D|
|Links||Poster / Affiche|
|Alt Series||Earth Sciences Sector, Contribution Series 20150006|
|Publisher||Australasian Institute of Mining and Metallurgy|
|Meeting||PACRIM 2015 Conference; Hong Kong; CN; March 18-21, 2015|
|Media||paper; on-line; digital|
|Related||This publication is superceded by Bednarski, J M; (2016).
Surficial geology, Tanquary Fiord, Nunavut, NTS 340-D, Geological Survey of Canada, Canadian Geoscience Map no. 212, ed. 2, Prelim.|
|Subjects||porphyry copper; porphyry deposits; gold; iron oxides; fluid inclusions; hydrothermal systems; salinity; trace elements; brine|
|Illustrations||photomicrographs; graphs; plots; histograms|
|Program||Uranium Ore Systems, Targeted Geoscience Initiative (TGI-4)|
|Abstract||Porphyry-Cu (Mo-Au) deposits and iron oxide-copper-gold (IOCG) deposits are both characterised by the presence of abundant halite-saturated fluid inclusions that reflect the circulation of hyper-saline
hydrothermal brines. While it is widely accepted that brines involved in the formation of porphyry copper deposits derive from unmixing of moderate-salinity, intermediate-density fluids that exsolve from crystallising magmas, the source of brines
that form IOCG deposits remains enigmatic and intensely debated. In this extended abstract, we summarise the fluid inclusion characteristics of both deposit types and present trace element compositional data for halite-saturated fluid inclusions from
both porphyry copper deposits and IOCG deposits from around the world. |
While both deposit types typically contain halite-saturated fluid inclusions, vapour-rich fluid inclusions in IOCG deposits are less common than in porphyry deposits and
evidence for fluid immiscibility is rare. IOCG deposits also typically lack intermediate-density fluid inclusions that characterise the deeper quartz-rich veins in many porphyry deposits. Many IOCG deposits contain CO2-only fluid inclusions, a type
of fluid not observed in porphyry copper deposits.
Our fluid inclusion compositional data show that most porphyry Cu (Mo-Au)-related brines are dominated by Na, K and Fe and contain elevated concentrations of Ca, Cu, Pb, Zn and Mn up to around
the one per cent level. On the other hand, IOCG brines are Na-Ca-Fe-dominated and typically contain far less Cu, Mn and Zn than porphyry brines. In addition to Ca, these fluids are also strongly enriched in Ba and Sr relative to porphyry-related
brines. Such compositions are consistent with extensive fluid-rock interaction and/or an evolved sea water contribution to IOCG brines.
A sea water or basinal brine source for IOCG brines is further supported by bromine concentrations that are
much higher than is characteristic for porphyry-related brines. Porphyry brines have Cl/Br molar ratios of between 800 and 3000, which is characteristic of primary magmatic brines, whereas the Cl/Br composition of IOCG brines is more variable. For
example, Na/Cl/Br ratios of most IOCG deposits in the world-class Carajás district fall on or near the sea water evaporation curve (most Cl/Br molar ratios are between 200 and 1000) and are diagnostic of sedimentary bittern brines. A few of the
deposits in this district also show evidence of the input of subordinate amounts of magmatic brine. At Ernest Henry, in the Cloncurry district of Australia, most Cl/Br ratios plot close to the sea water evaporation curve, but Cl/Br ratios of up to
~5000 in several samples suggest the input of lesser amounts of magmatic fluids, and possibly evaporite-dissolution-derived fluids. Taken together with geologic constraints, these data strongly suggest that porphyry-type deposits form from the
circulation of dominantly magmatic fluids, while IOCG deposits form from large-scale circulation of high-salinity fluids sourced dominantly from sedimentary basins with variable input of magmatic volatiles.
|Summary||(Plain Language Summary, not published)|
The Targeted Geoscience Initiative (TGI-4) is a collaborative federal geoscience program that provides industry with the next generation of geoscience
knowledge and innovative techniques to better detect buried mineral deposits, thereby reducing some of the risks of exploration. This contribution contrasts the composition of the mineralising fluids between two major types of copper-bearing ore
deposits that show some spatial and chronological relationships to large volcanic and plutonic belts worldwide. Taken together with geologic constraints, the data acquired on fluid inclusions in rocks of these two major deposit types worldwide
strongly suggest that porphyry-type deposits form from the circulation of dominantly magmatic fluids, while IOCG deposits form from large-scale circulation of high-salinity fluids sourced dominantly from sedimentary basins with variable input of