IOCG Iron Oxide Copper Gold Ore Deposits goes with porphyries as they are also loosely associated with a large felsic intrusions and form at moderate depths, unlike the dominantly tertiary porphyries IOCG’s tend to be much older and are generally protoizoic in age from 1.1 to 1.8 billion years ago. There are some younger ones but they are in the minority. IOCG tend to form at 4 to 6 km similar to porphyries and are invariably associated with crustal scale structures. Crustal scale means that they penetrate well down into the crust often 20 km deep and they have strike lengths measured in tens or hundreds of kilometers. The key to the formation of IOCG’s that there has to be an abundance of oxidized groundwater in the country rock to mix with the magmatic water derived from the intrusion. The intrusion of hot water into water saturated rocks means that the water flashes to steam and this fractures the rocks and produces breccias. The high ion content and oxygenated environment means that the alteration is characterized by abundant red hematite and black magnetite both oxides of ion.
Where do we find IOCG Iron Oxide Copper Gold Ore Deposits? Well let’s get it around the world but because of a combination they are usually found in archean or proterozoic terrains. IOCG’s are highly variable in metal content and they range from pure ion deposits such as Carajas in Brazil to predominantly copper deposits such as Candelaria in Chile to copper uranium deposits such as Olympic Dam in South Australia. In truth IOCG is a bit of sack term and it covers a multitude of variations on a theme. In size they vary from the monstrous Carajas at 18,000,000,000 tons, 50% bigger than Chuquicamata which is the biggest porphyry in the world and down to tiny deposits of just a few million tons. Grades of the copper range IOCG’s are generally low, similar to those of porphyry deposits in other words under 1% copper although some may return long intercepts with over 1.5% copper. Chariots project in Peru is 347,000,000 tons grading 0.71% copper. Carapateena in South Australia has some unusually high intercepts such as 17 m at 1.83 copper but in spite of its relatively high grades Carapateena is struggling to overcome its depth of burial and the resulting cost of mining.
This is a cartoon section to the Olympic Dam IOCG system, you can see the deep intrusive in purple feeding up through the pink granite basement. The deep magmatic fluids from the source intrusive mixed with the oxidize fluids from the overlying green basalts and surface sediments to deposit a variety of copper sulfides. Below the water table chalcopyrite is deposited along with pyrite at and above the water table chalcocite and bornite are deposited, Chalcocite and bornite have a higher percentage copper than chalcopyrite and this zone at the water table is the most likely portion of an IOCG deposit to reach economic grades.
In South Australia where both Olympic Dam where Carapateena are located grade is a particularly important because of the thick sequence of barren rocks that overly the deeper mineralization. Olympic Dam is covered by 350 m of barren material. Fortunately it was high enough grade to allow Western mining to initially exploit it from underground. Only now 24 years after beginning operations are they considering developing a super pit, Similarly Carapateena overlaying by 450 m of barren material its economic viability is in doubt.
IOCG Iron Oxide Copper Gold Ore Deposits ore is often striking to look at and it is often very red from the hematite content, blue from bornite or yellow-green from various uranium oxides. You can see some of the oars from the Olympic Dam in this photo. This awesome looking core full of bornite and covellite the blue minerals is from monax Punt hill property also in South Australia but in spite of intersections like these this assay almost 50% copper the deposit is too irregular to allow mining for at a depth of several hundred meters.
How to exploration for IOCG Iron Oxide Copper Gold Ore Deposits:
IOCG Exploration for IOCG’s can use a number of techniques, starting with the geophysical methods magnetics is a very valuable tool as IOCG systems are almost always have a magnetic signature and are caused by the abundant magnetite. On this magnetic image you can see Olympic Dam deposit standing out in reds and magentas. In fact with careful data collection and sophisticated computer modeling you can get an idea of the distribution of magnetite in three dimensions purely from the magnetic survey. The concentration of sulfides means that IOCG deposits are usually denser than the host rocks and the gravity survey of monax property showed a district wide gravity anomaly and the mineralization that’s the overall greens and yellows and the mineralization was associated with smaller second order anomalies which had been marked in red arrows superimposed on the district anomaly. The gravity data of Olympic Dam shows a clear bull’s-eye coincidence with the deposit. The presence of massive sulfides also means that electromagnet or EM surveys can be used to detect deposits. If a deposit of occurs at surface and is uranium rich as in the case of Olympic Dam except that one is buried then airborne or ground radiometrics will detect the radiation given off by the uranium and any associated potassic alteration. Finally if the deposits are outcropped at surface, soil and rock geochemistry should detect and define the mineralization in 2D. Drilling will confirm this in the third dimension.
The next geology course will focus mainly on mesothermal veins. This group includes one of my favorite’s ore deposit types; Greenstone belt gold deposits which provide a significant portion of the world’s gold resources.