The high grade Mo-Re Merlin deposit, Cloncurry district, Australia

Antunes Feio Babo, João (2014) The high grade Mo-Re Merlin deposit, Cloncurry district, Australia. PhD thesis, James Cook University.

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The Mount Dore and Merlin deposit, discovered in 2008 by Chinova Resources (previously Ivanhoe Australia), is the world's highest grade Mo-Re deposit. The deposit is located in the Cloncurry District of the Mount Isa Inlier, Australia; a region renowned for its IOCG type mineralization. In the same year the Lanham's Shaft prospect, another Mo-rich occurrence was found 50 km to the north of the Merlin deposit. These discoveries have created a new exploration paradigm, and this thesis represents the first in-depth study of this new mineralization type. This work integrates detail core logging, petrography, geochronology, and trace element and isotope geochemistry.

The Mount Dore and Merlin deposit is hosted by the Proterozoic Kuridala Formation metasedimentary package, which is composed of interbedded phyllites and carbonaceous slates above calc-silicate rocks, and silicified siltstones at the footwall. The Mount Dore granite is thrust above the metasedimentary rocks and overlies the bulk of the mineralization. The mineralization consists of a Cu-polymetallic stage (chalcopyrite ± sphalerite ±± galena) (Mount Dore) mainly hosted by the carbonaceous slates in angular clast-supported carbonate breccias, and Mo-Re mineralization (Merlin) that locally cuts the Cu-polymetallic mineralization, mainly along or near rheologic boundaries between the carbonaceous slates and the calc-silicate rocks. The bulk of the Mo-Re appears as infill of matrix-supported breccias with rounded clasts.

The hydrothermal alteration is highly complex, consisting of three main stages, from oldest to youngest: Na-(Ca), Cu-polymetallic, and Mo-Re. These stages were dated using UPb in titanite and Re-Os in molybdenite at the Mount Dore and Merlin deposit and at the Lanham's Shaft prospect.

The Na-(Ca) alteration (1557 ± 18 Ma) is similar to a regional Na-(Ca) alteration event with formation of albite + amphibole ± quartz ± titanite ± apatite ± carbonates. The alteration fluids are interpreted to be bittern brine(s) and the widespread regional Na-(Ca) alteration is thought of having been responsible for the release of potassium, iron, barium, and possibly copper and carbonate from the regional metasedimentary rocks.

The Cu-polymetallic mineralization is accompanied by K-feldspar + tourmaline + carbonates ± quartz. This alteration stage is also interpreted to have formed by a bittern brine fluid that reacted with the metasedimentary host rocks. This reaction resulted in a strong decrease in the ƒO2 of the fluid, due to the minor graphite contained in the carbonaceous slates that host the Cu-polymetallic mineralization. The ƒO₂ drop is proposed as the main chemical mechanism responsible for the sulfide precipitation, and the sulfur was sourced from both the bittern brine and the metasedimentary host rocks. Zinc, and at least part of the Pb and Ag, was possibly sourced from the carbonaceous slates.

The third hydrothermal stage can be subdivided into three events, with the first and main event being responsible for the bulk of the Mo-Re mineralization and the last two events being remobilizations of the first event.

The main Mo-Re event occurred at ~1535 Ma and formed the strongly mineralized breccias that were accompanied by K-feldspar ± chlorite alteration. Fluid derived from a Williams-Naraku type felsic intrusive is proposed as the source of the mineralization with ore mineral precipitation triggered by a strong to moderate ƒO₂ drop during fluid reaction with the metasedimentary host rocks. Again, the sulfur is thought to be derived from both the mineralizing fluid and the host rocks.

The second, relatively minor, Mo-Re event occurred at ~1521 Ma and formed veins and disseminations that extend from the Mo-Re breccias into the calc-silicate rocks for several metres. This event is inferred to have developed from the previously formed Mo-Re mineralization by remobilization, due to the emplacement of the Mount Dore granite (~1517 Ma), with formation of K-feldspar + chlorite ± apatite ± rutile ±± monazite. A ƒO₂ drop was again the ore precipitation mechanism, but the ƒO₂ decrease was not as strong in this mineralization, due to the relatively oxidized nature of the calc-silicate host rocks compared to the proximity (partly hosting) of the bulk of the Mo-Re mineralization to the carbonaceous slates. The sulfur was sourced from the previously formed molybdenite and therefore both mineralization styles have similar δ³⁴S signatures.

A very minor Mo-Re event occurred at ~1500 Ma and consists of millimetric Mo-Re veins in carbonate ± chlorite veins. This event was likely formed in response to the thrust of the Mount Dore granite over the metasedimentary host rocks, which is possibly responsible for the formation of the molybdenite stylolitic veins, which are a common feature in the deposit.

The Lanham's Shaft prospect displays a similar hydrothermal alteration evolution, except that significant remobilization of the Mo-rich mineralization did not occur at this location. However, the absolute timing of the Na-(Ca) alteration (~1575 Ma) and Mo-rich mineralization (~1560 Ma) is earlier than at the Mount Dore and Merlin deposit. These older ages indicate the existence of hydrothermal activity with mineralizing capability during a time period that has previously been regarded as unprospective for ore deposit formation. The mineralization at the prospect is less abundant in grade and tonnage than the deposit, consisting of mineralized veins. The bulk of the Cu-rich and Mo-rich veins are a few metres from the carbonaceous slates, hosted in calc-silicate rocks. The distance of the mineralization to the graphitic rocks resulted in a weaker ƒO₂ drop in both stages. The mineralizing fluids are interpreted as also felsic igneous sourced and mixing of sulfur between the fluids and the metasedimentary rocks also occurred.

The Merlin and Mount Dore and Lanham's Shaft mineralization have clear spatial and temporal affinities with IOCG mineralization in the Cloncurry District. Broad genetic relationships between these mineralization styles are also likely, so a denomination for this this new mineralization style is proposed as Mo-rich IOCG type deposits. The key criteria and ingredients for such deposits are: 1. Mo-rich felsic igneous source(s) and; 2. Suitable tectonic trap(s) contained within, or at close proximity to, reduced host rocks (e.g. carbonaceous or graphitic metasedimentary rocks).

Item ID: 37609
Item Type: Thesis (PhD)
Keywords: Barnes Shaft Prospect; Cloncurry; exploration; geochemistry; geochronology; geology; Ivanhoe Australia; Kuridala Formation; Lanham's Shaft Prospect; Merlin; mineralisation; mineralization; minerals; mining; molybdenum; Mo-Re deposits; Mount Dore; Mount Isa Inlier; Mt Dore; Mt Isa Inlier; ores; petrography; prospecting; rhenium; rocks
Date Deposited: 05 Aug 2015 05:32
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 34%
04 EARTH SCIENCES > 0403 Geology > 040303 Geochronology @ 33%
04 EARTH SCIENCES > 0402 Geochemistry > 040202 Inorganic Geochemistry @ 33%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840199 Mineral Exploration not elsewhere classified @ 50%
97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 50%
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