Some Aspects of South American Copper Porphyry Geometallurgy
This article is written by Alan Riles, Principal Metallurgical Consultant, at AMC Consultants and is re-published here with permission.
A common misconception is that porphyry metallurgy is simple.
From the perspective of a flotation-based copper concentrator, indeed what could be simpler than being fed eminently floatable chalcopyrite that is uniformly disseminated in a large homogeneous intrusive host? Especially compared to complex VHMS deposits.
However this is a myth. Figure 1 illustrates the complex patterns of zonation in alteration and mineralization that result from multiple phases of intrusion and the overprinting of many prograde and retrograde alteration events (contrast the idealized model shown in Figure 2).
Figure 1 - Conceptual model of porphyry evolution, (from Corbett, 2009)
Another myth is that the sheer size of South American porphyries relative to the scale of exploitation imposes a degree of homogeneity at the level of the mine. However the scale of contemporary South American mining operations is such that they can embrace the heterogeneity of even these large porphyries.
In considering the geometallurgy of porphyries, three aspects are critical:
- Copper mineral species. Although chalcopyrite is usually dominant, oxide species and secondary copper sulfides and sulfosalts have a significant bearing on metallurgical performance. Oxide species do not respond to conventional sulfide flotation, secondary sulfides are typically slower-floating and the sulfosalts introduce smelter penalty elements such as arsenic and antimony.
- Pyrite:chalcopyrite ratios. Chalcopyrite can be selectively floated from pyrite at elevated pH, but once the ratio of pyrite to chalcopyrite exceeds 10 it is difficult to achieve a saleable grade of copper in the concentrate at acceptable recoveries.
- Gangue mineralogy. Especially related to the hydrothermal alteration suite and associated clay minerals that can adversely impact on flotation grade-recovery performance.
And, in considering the fundamental porphyry architecture and structure that impacts on the geometallurgy, there are also three important aspects to consider:-
- The typical zoning of alteration and sulfide species as one moves outwards from the core of the porphyry. Figure 2 clearly illustrates the zoning of both the alteration suites and the sulfide minerals in an idealized single phase porphyry, however the reality is often more akin to Figure 1.
Figure 2 - Idealised porphyry alteration/mineralization (Lowell and Gilbert, 1970)
Note that pyrite:chalcopyrite ratios are highly variable and also that the argillic alteration contains kaolinite (and illite), those clay minerals that adversely impact on flotation performance. But it should be stressed once again that this is the ideal single phase porphyry that could indeed exhibit simple metallurgy from a spatially discrete ore shell whereas the reality portrayed in Figure 1 of a multi-phase intrusion and hydrothermal event introduces significant spatial and mineralogical complexity.
- Supergene weathering profiles and associated zones of supergene enrichment with secondary copper minerals. The classic example of this is Escondida in Chile where solubilised copper from downward percolating fluids is reduced as the fluids pass from the oxidised weathering zone into the water table. This results in the formation of not only copper grades significantly higher than in the primary chalcopyrite but also in secondary copper minerals like chalcocite which contain 70% Cu, significantly enhancing concentrate grades.
- Subsequent super-position of later hypogene events like a high sulfidation epithermal overprint.
It is this last feature of the high sulfidation overprint that the remainder of this article will focus on as its impact has only relatively recently been appreciated, and it is particularly relevant to the South American tectonic setting with very rapid uplift and subsequent rapid erosion. It also introduces significant geometallurgical complexity especially in relation to smelter penalty elements like arsenic and antimony. The example of arsenic is used to illustrate these geometallurgical aspects.
A high sulfidation overprint is illustrated in Figure 3. The Andes of South America underwent very rapid tectonic uplift and suffered equally rapid erosion during the uplift process, which was also accompanied by porphyry intrusions. The progressive paleosurface degradation was contemporaneous with the cooling of the porphyry intruded several kilometres below the surface and the emplacement of a later deeper porphyry. This resulted in a typical near-surface high sulfidation deposit being “telescoped” or overprinted on top of the original porphyry.
Figure 3 - “Telescoping” and high sulfidation overprinting (Sillitoe 2010)
High sulfidation deposits form from the condensation/precipitation of metals from ascending gases and fluids above the underlying porphyry. The volatile components include arsenic and antimony, hence a range of sulfosalts are formed, ranging from arsenopyrite to the copper sulfosalts like the tennantite-tetrahedrite As-Sb solid solution series and enargite. Arsenopyrite can, with difficulty, be physically separated from chalcopyrite by differential froth flotation. On the other hand, tennantite and enargite are chemically bound copper arsenic sulfosalts and therefore no arsenic/copper separation is possible. As a consequence copper concentrates with elevated penalty elements are often produced.
The equation determining arsenic levels in copper concentrates is:-
%As(conc) = %Cu(conc) / (Cu/As feed * Cu/As recovery ratio)
where the key drivers are:-
- Cu/As ratio NOT the absolute As value
- The Cu/As recovery ratio, which is a function of mineralogy. This ranges from 3-4 where the arsenic is as arsenopyrite to 1 (ie no separation) where it occurs in the copper arsenic sulfosalts.
Arsenic in copper concentrates is an increasing problem for South American copper porphyry mines.
The Chuquicamata district in northern Chile presents an interesting example. It comprises three mines:-
- Radimiro Tomic. Low in the system, porphyry dominant and producing clean/normal (<0.3%As) concentrates
- Chuquicamata: at an intermediate level with deep high sulfidation overprint and moderately dirty concentrates (0.5-1.0% As)
- Ministro Hales: higher in the system, high sulfidation dominant and producing high As concentrates (4%As) requiring a roaster. the clean calcine from this is blended with Chuquicamata concentrates to produce normal concentrate quality
These differences between the three mines also demonstrate the merits of a district strategy whereby blending takes place at the mine level rather than in the sheds of concentrate traders who will charge heavily for the service.
A significant penalty element like arsenic needs to be explicitly considered at the outset, not as an afterthought. Assaying should include As assays for every Cu assay and modelling Cu/As ratios with a mineralogy overlay to predict Cu/As recovery ratios enables a %As(conc) ie concentrate marketability “block model” to be developed as an early stage project screening tool. We talk about value in the ground, but really the only value is in a saleable concentrate! Hence the value of early stage geometallurgy.
Corbett G. (2009) Anatomy of porphyry-related Au-Cu-Ag-Mo mineralised systems: some implications for exploration. AIG North Queensland Conference June 2009
Lowell J.D. and Gilbert J.M. (1970) Lateral and vertical alteration-mineralisation zonation I copper porphyry deposits. Economic Geology v65, pp373-408
Sillitoe R.H. (2010) Porphyry Copper Deposits. Economic Geology v105 pp 3-41
 The term high sulfidation, and the related term low sulfidation, refer to the oxidation state of the sulfur (sulfate vs sulphide respectively) in epithermal systems. High sulfidation deposits are formed from acidic oxidised fluids whereas low sulfidation systems arise from near neutral reduced fluids.