Navigating IOCG Exploration: Cobalt and Pyrite as Key Indicators
The Ernest Henry Iron Oxide-Copper-Gold (IOCG) deposit stands out as a significant source of copper and gold valued in billions. However, identifying these valuable metals in the expansive Cloncurry region requires keen observation and strategic methods.
A brief study explored the roles of cobalt (Co) and pyrite (FeS2) as indicators at Ernest Henry. Analysis revealed cobalt's elevated concentrations near the main orebody, suggesting its potential as a pathfinder element. Pyrite, closely associated with mineralized zones, indicates its simultaneous formation with copper and gold.
This approach extends beyond Ernest Henry to the Mongoose Deeps prospect at Cloncurry, where similar techniques are potentially being applied to target areas likely to host major copper and gold mineralization.
In essence, this review underscores the importance of cobalt and pyrite as valuable tools in exploration. Understanding their correlation with copper and gold mineralization allows exploration geologists to refine targeting strategies for enhanced success.
Research from James Cook University highlighted varying enrichments of elements in IOCG systems (Figure1), notably cobalt in Cloncurry-based deposits like Eloise, Osborne, and Ernest Henry.
Figure 1: Geochemical profile of IOCG systems
Analyzing zoning within the Cu-Au orebody at Ernst Henry showed elevated concentrations of copper, cobalt, arsenic, and molybdenite within 100-200 meters of the main orebody (source: https://smi.uq.edu.au/files/36554/Atlas_Prototype_Ch3_ErnestHenry.pdf).
Why is RNX assaying for Cobalt from its Mongoose Deeps core sample?
Cobalt can act as a pathfinder element. Pathfinder elements often concentrate around zones of economic mineralization, helping exploration target areas with a higher chance of finding copper and gold.
Regarding the Ernest Henry deposit, an aerial view heat map image of the distribution of copper and cobalt (Figure 2A & 2B) around the Ernst Henry deposit (black outline) shows how the concentration of cobalt could play a major role for determining proximity to the ore zone.
Figures 3A - 3D show a side view of the Ernest Henry orebody for Copper, Cobalt, Arsenic and Molybdenite concentration values.
An interesting perspective is a side-view observation of the high copper concentration (Figure 3A) which tends to spread out from the main orebody zone as a large plume (shown in red).
In terms of the cobalt concentration (Figure 3B), cobalt progressively decays as you move further away from the core of the Cu-Au mineralization zone. This offers an ideal guide for vectoring the drill rod into the heart of the copper-gold zone.
Whereas the arsenic & molybdenite heat maps (Figure 3C & 3D) show how arsenic & molybdenite is purely located within the outline of the ore zone. Therefore, Cobalt concentration will likely be a good indicator for determining your proximity to the main orebody.
RNX's interest in assaying cobalt from Mongoose Deeps core stems from its role as a pathfinder element concentrated around zones of economic mineralization. Aerial and side-view heat maps around the Ernst Henry deposit illustrate how cobalt concentration can indicate proximity to ore zones.
Thus, precise analysis of cobalt values in drill cores aids in accurately pinpointing copper-gold rich ore zones, complemented by ongoing regional drill assays from the Mongoose West program and historical Mongoose assays to map copper and cobalt concentrations across the tenement.
Pyrite as a geological indicator
The Ernest Henry orebody exhibits a notable abundance of pyrite in both the Breccia and Vein hosted regions, as illustrated in Figure 4. This observation suggests that pyrite forms contemporaneously with copper and gold mineralization, underscoring its essential role in IOCG deposits. Pyrite serves as a crucial sulphur source during the hydrothermal processes responsible for mineral deposition, facilitating the formation of copper and iron sulfides such as chalcopyrite, bornite and pyrite. Over the course of ore formation, pyrite can undergo fracturing and replacement by these valuable sulphides.
Figure 4: Ernest Henry mineral paragenesis, with line thickness indicating relative abundance.
The Mongoose Deeps prospect similarly displays significant pyrite concentrations throughout its 1,612-metre diamond drill hole, varying from 1% to a surprising peak of 60% at 212 meters depth. Examples include 15% pyrite at 315 meters, 7% at 388 meters, and 10% at 463 meters, among others (Figure 5).
Figure 5: Massive pyrite observed at 316 meters depth.
Spatial analysis depicted in Figure 6 reveals that the pyrite percentage is notably higher (5% - 10%) within the halo region surrounding the Ernest Henry orebody compared to distant areas. This pyrite distribution correlates with the orebody itself and the magnetite-enriched zone of K-feldspar alteration northeast of Ernest Henry, extending into the hanging wall along a north-south trend.
Figure 6: Spatial distribution of pyrite percentage overlain with RTP magnetics and greyscale VD-automatic gain control image.
So, what is the significance of Pyrite in the mineralization system?
Pyrite (FeS2) serves as a crucial host mineral for gold in numerous IOCG systems. During the hydrothermal process, pyrite often precipitates early due to its affinity for sulphur-rich fluids. As these fluids cool, gold can precipitate concurrently or shortly after pyrite formation.
Gold particles frequently nucleate and grow on the surfaces of pyrite crystals or within fractures and cavities of pyrite grains. Pyrite crystals offer abundant surface area and reactive sites for gold to adsorb and accumulate.
The textural associations between gold and pyrite exhibit variability:
Disseminated Gold: In some instances, gold is disseminated throughout pyrite crystals, with microscopic gold particles distributed within the pyrite matrix.
Microscopic Association: Gold may also exist microscopically within the crystal lattice of pyrite or as very fine-grained particles intricately intergrown with pyrite.
Fracture Filling: Hydrothermal fluids can deposit gold along fractures within pyrite crystals, forming veins or veinlets of gold within zones rich in pyrite.
The importance of pyrite in the mineralization system is underscored by its role as a host for gold. According to research by Patrick J. Williams ("Geological and Exploration Characteristics of Iron Oxide-Copper-Gold Deposits: The Australian Experience"), gold distribution includes native gold/electrum, sylvanite, auriferous cobaltite, and "invisible gold" within structures of chalcopyrite and pyrite (Figure 7).
The term "invisible gold" refers to gold (Au) that occurs either within the crystal lattice of host sulphide (such as chalcopyrite or pyrite) or as discrete nanoparticles (<50 microns in diameter) within these hosts, detectable only under high magnification. Gold nanoparticles typically form in microfracture zones of pyrite and along boundaries between pyrite and chalcopyrite (Figure 7).
Figure 7: Distribution of gold in Ernest Henry ore alongside pyrite, chalcopyrite, and sylvanite grain boundaries (Backscattered Electron Image).
Williams' research indicates that gold tends to concentrate near pyrite particles (average Au concentration = 77 ppb), surpassing concentrations near chalcopyrite (average Au concentration = 56 ppb), suggesting that pyrite surfaces catalyze the precipitation of free gold (Figure 8).
Figure 8: Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) elemental analysis of gold in chalcopyrite & pyrite.
The formation of free gold adjacent to pyrite particles reflects the intricate interplay of hydrothermal fluid composition, temperature gradients, and the physicochemical properties of pyrite. This understanding informs exploration strategies at the Ernst Henry mine, where gold is economically significant alongside copper within iron minerals like pyrite.
Looking ahead, the assessment of drill core gold assays from Mongoose Deeps within high-grade pyrite zones presents an intriguing prospect. If gold is found to exist along the pyrite rich sections of drill core then a revaluation of the recent stock downgrade may occur considering how much has been drilled.
Importantly, RNX have only targeted the testing of the Breccia Pipe which potentially is feeding the known close to surface deposits of The Great Australia, Mongoose, Taipan, Paddock Lode pits. This is only the “tip of the iceberg” or “Thumb” of the fist shaped Mongoose Deeps anomaly. The magnetic anomaly dimension in the geophysical world stretches for at least 3,000 metres depth and is up to 1,000 metres width in sections (Figures 9A & 9B). Hence, less than 5% of the overall anomaly volume has been drilled by targeting the Breccia pipe zone.
I assume once EPM 28972 tenement application is approved then better drill positioning and targeting of the IOCG deposit can be achieved.
In conclusion, the presence and distribution of cobalt and pyrite mineralization zones are pivotal in copper-gold exploration strategies at the Ernst Henry orebody. These elements serve as crucial pathfinders and geological indicators, guiding explorers towards areas with heightened potential for economic mineralization. This same logic appears to be the path being taken by Renegade Exploration for exploring Mongoose Deeps IOCG mineralization zones and positioning the next drill hole in coming months.
Disclaimer: The information presented in this article is based on publicly available sources and does not constitute financial advice.
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