Application of geometallurgical methodology for Porphyry copper project development Porphyry copper deposits are very complex geologically and exhibit great variability in both their grindability (or hardness) and their metallurgical performance. This complex and variable nature of the lithology / alteration and mineralisation of the porphyry deposits leads to the variable metallurgical response. For the development of a strong and robust project feasibility study for porphyry copper deposits, it is critical to apply geometallurgical methodology and study the variability in the deposit – to optimise the exploitation plan, and to improve yearly production forecasting.
The geology of a porphyry Copper deposit is described here (source - Wikepedia) and the attached cartoon picture is from USGS Porphyry Cu deposits description.
Porphyry copper deposits are copper ore bodies which are associated with porphyritic intrusive rocks and the fluids that accompany them during the transition and cooling from magma to rock. Circulating surface water or underground fluids may interact with the plutonic fluids. Successive envelopes of hydrothermal alteration typically enclose a core of ore minerals disseminated in often stockwork-forming hairline fractures and veins. Porphyry orebodies typically contain between 0.4 and 1 % copper with smaller amounts of other metals such as silver and gold.
The first mining of low-grade copper porphyry deposits from large open pits coincided roughly with the introduction of steam shovels, the construction of railroads, and a surge in market demand near the start of the 20th century. Some mines exploit porphyry deposits that contain sufficient gold or molybdenum, but little or no copper.
Porphyry copper deposits are currently the largest source of copper ore. Most of the known porphyrys are concentrated in: western South and North America and Southeast Asia and Oceana - along the Pacific Ring of Fire; the Caribbean; southern central Europe and the area around eastern Turkey; scattered areas in China, the Mideast, Russia, and the CIS states; and eastern Australia. Only a few are identified in Africa, in Namibia and Zambia; none are known in Antarctica. The greatest concentration of the largest copper porphyrys is in Chile. Almost all mines exploiting large porphyry deposits produce from open pits.
Following figure: Cartoon cross section illustrating generalized model for porphyry Cu deposits showing relation of ore minerals, alteration zoning, supergene enrichment and associated skarn, replacement, and vein deposits.
There are a number of aspects of the porphyry copper deposit type geology that drive metallurgical response. For grindability, a matrix of lithology and alteration, together with the macro and micro fractures in the rock, are the parameters that drive the relative breakability of the rock. Extreme variability in hardness can be seen in deposits as indicated by grindability index test work. An example , for a North American porphyry copper deposit which has done extensive grindability test work, is given in the Table below.
Test type Count Average Standard Dev Minimum Median Maximum
WiC 377 5.87 1.56 2.27 5.80 11.72
WiRM 336 15.54 3.12 9.65 15.27 24.27
WiBM 386 14.65 3.09 7.72 14.61 22.15
Ai 334 0.22 0.10 0.00 0.20 0.63
WiC – crushing work index; WiRM – rod mill work index; WiBM – ball mill work index and Ai – abrahsion index. All data is metric.
Armed with knowledge of the hardness variability in a deposit it is possible to come up with the most appropriate comminution circuit flow sheet design considering the variability and other project constraints that may exist (such as strategic mine plan, equipment constraints etc). Then, having established the desired comminution circuit flowsheet the mine blocks can be run through the `virtual process` to show output as both throughput and product grind size. This approach is now routinely used.
Porphyry copper deposits are commonly treated using flotation and / or heap leach process. In either case the variability that exists within the deposit can result in significant variability in metallurgical performance.
Parameters emanating from the rock that drive flotation and / or heap leach performance for porphyry copper deposits include :
- Copper sulphide mineralisation (primary or secondary copper mineralisation. Chalcopyrtite, bornite, chalcocite, covellite etc). The predominance of chalcopyrite will drive an operation to flotation whereas the predominance of secondary copper mineralisation opens up the opportunity for bacterial assisted heap leach.
- Copper sulphide mineralisation that carries penalty elements such as arsenic or antimony will be recovered to a flotation concentrate and can result in incurring concentrate sales penalties or possible rejection of concentrate. These minerals include enargite, tenanntite and tetrahedrite.
- Copper oxide mineralisation. If the process is flotation, then it is likely that most copper oxide mineralisation will be lost to tailings unless some oxide flotation process is specifically included in the flowsheet (which would only be done if there were significant oxide copper mineralisation). If heap leach is the selected process then oxide copper minerals are readily recovered in heap leach, with the exception of “refractory” copper oxide minerals such as copper pitch (CuMn8FeSiO2 – variable) and copper wad (CuMnO2Fe – variable).
- The total amount of pyrite is very important in flotation recovery of copper sulphides. Typically copper metallurgical performance deteriorates as the pyrite content increases. It is also possible to see an effect with different pyrite forms, whereby poorer copper metallurgy can be seen when the pyrite is amorphous type vs blocky texture type pyrite.
- Grain size and overall metallurgical or micro textural size can be very important. This will clearly drive primary grind size and regrind size but also will influence metallurgical performance of copper.
- Porphyry copper ores often have payable amounts of molybdenum contained in the deposit. Typically greater than 100 – 150 ppm Mo can be recovered successfully to a Mo concentrate.
- Porphyry copper ores can often carry payable amounts of gold. The association of the gold can be often in many forms such as gravity recoverable gold, very fine gold associated with copper minerals, very fine gold associated with pyrite mineralisation (and recoverable by cyanidation process) or refractory gold intimately associated with pyrite.
- Minerals resulting from high degree of alteration such as clays or sericite can be difficult and at times the dominating parameter of flotation metallurgical performance. Usually high content of these minerals will see poor copper metallurgy. In heap leaching, high content of these minerals can also be problematic because both influence solution percolation (making it more difficult) and also can contribute (together with certain other minerals) to high acid consumption.
For each of these parameters discussed above, it is common to see often significant variability across the whole deposit which will result in notable variance in metallurgical performance throughout. It is therefore very important from the perspective of project design and economic modelling, to capture this variability in project development studies. The way to do this is by applying geometallurgical methodology to the overall project development process.
The geometallurgical methodology consists of geologists and metallurgists coming together to, firstly, understand the geological formation and the various sub-domains that exist in that deposit. Next, it is important to quantify the total expected resource contained within all of the significant sub-domains of the deposit and then armed with that select samples that reflect each of those sub-domains and their relative expected tonnage. Finally, it is important to have sufficient samples from any one of those sub-domains to properly quantify that sub-domains metallurgical performance (as a guide – a minimum of 10 samples for any particular sub-domain is recommended).
In the case of a porphyry copper deposit – because of their geological complexity it is often common to see hundreds of samples being required to properly conduct a thorough geometallurgical investigation for the project development (particularly to feasibility study level).