The Bristol PCD project

Who are we?


The Bristol PCD project is a collaboration between the University of Bristol and the global mining company, BHP.


Our objective is to modify existing porphyry copper deposit (PCD) models in the light of recent advances in geological research through 5 key questions:

          

* What is the relationship between PCDs and volcanoes?


* Can we better constrain the chronologic development of PCD enrichment and burial?


* Can we develop a new model for the origin and metal carrying capacity of PCD magmas?


* What controls fluid distribution in PCDs influencing the genesis of metal enrichment?


* Can we identify local or regional structural controls on PCD formation?

Unraveling the story of porphyry copper deposits...


What is a Copper porphyry?


PORPHYRY Cu systems are defined as large volumes (10−>100 km3) of hydrothermally altered rock centered on porphyry Cu stocks that may also contain skarn, carbonate-replacement, sediment-hosted, and high- and intermediate-sulfidation epithermal base and precious metal mineralization. Along with calc-alkaline batholiths and volcanic chains, they are the hallmarks of magmatic arcs constructed above active subduction zones at convergent plate margins (Sillitoe, 1972; Richards, 2003), although a minority of such systems occupies postcollisional and other tectonic settings that develop after subduction ceases (e.g., Richards, 2009).


The deeper parts of porphyry Cu systems may contain porphyry Cu ± Mo ± Au deposits of various sizes (<10 million metric tons [Mt]-10 billion metric tons [Gt]) as well as Cu, Au, and/or Zn skarns (<1 Mt−>1 Gt), whereas their shallower parts may host high- and intermediate-sulfidation epithermal Au ± Ag ± Cu orebodies (<1 Mt−>1 Gt). Porphyry Cu systems were generated worldwide since the Archean, although Meso-Cenozoic examples are most abundantly preserved (e.g., Singer et al., 2008; Fig. 1), probably because younger arc terranes are normally the least eroded (e.g., Seedorff et al., 2005; Kesler and Wilkinson, 2006; Wilkinson and Kesler, 2009).


Porphyry Cu systems presently supply nearly three-quarters of the world’s Cu, half the Mo, perhaps one-fifth of the Au, most of the Re, and minor amounts of other metals (Ag, Pd, Te, Se, Bi, Zn, and Pb). The systems also contain major resources of these metals as well as including the world’s largest known exploitable concentrations of Cu (203 Mt: Los

Bronces-Río Blanco, central Chile; A.J. Wilson, writ. commun., 2009) and Mo (2.5 Mt: El Teniente, central Chile; Camus, 2003), and the second largest of Au (129 Moz: Grasberg,

including contiguous skarn, Indonesia; J. MacPherson, writ. commun., 2009).


Typical hypogene porphyry Cu deposits have average grades of 0.5 to 1.5 percent Cu, <0.01 to 0.04 percent Mo, and 0.0Å~ to 1.5 g/t Au, although a few “Auonly” deposits have Au tenors of 0.9 to 1.5 g/t but little Cu (<0.1 %). The Cu and, in places, Au contents of skarns are typically higher still. In contrast, large high-sulfidation epithermal deposits average 1 to 3 g/t Au but have only minor or no recoverable Cu, commonly as a result of supergene removal.



Extracted from Porphyry Copper Systems, Richard Sillitoe, 2010.


If you are interested in this paper, click here.

Open-pit in Spence mine, Chile.


Generalized alteration-mineralization zoning pattern for telescoped porphyry Cu deposits, based on the geologic and deposit-type template presented as Figure 6. Note that shallow alteration-mineralization types consistently overprint deeper ones. Volumes of the different alteration types vary markedly from deposit to deposit. Sericitic alteration may project vertically downward as an annulus separating the potassic and propylitic zones as well as cutting the potassic zone centrally as shown. Sericitic alteration tends to be more abundant in porphyry Cu-Mo deposits, whereas chlorite-sericite alteration develops preferentially in porphyry Cu-Au deposits. Alteration-mineralization in the lithocap is commonly far more complex than shown, particularly where structural control is paramount. See text for further details and Table 2 for alteration-mineralization details. Modified from Sillitoe (1999b, 2000).





Extracted from Porphyry Copper Systems, Richard Sillitoe, 2010.

Widespread hydrothermal alteration develops in these systems contemporaneously with a single pulse or often multiple pulses of porphyry emplacement. This ore-bearing alteration is driven primarily by magmatic fluids, but may also include meteoric fluids as well. During the development of these hydrothermal systems associated with the PCDs, the host and wall rocks are usually intensely fractured. Copper minerals occur as stockwork veins filling fractures and breccias disseminated throughout the host and wall rocks of the deposit. Subsequent supergene alteration of the PCD can result in secondary enrichment of copper. This enrichment is often key to making a deposit economic.
















The Chilean continental margin is ideal for the study of porphyry copper systems. Chile is the leading global copper-producing country in the world (5,370,000 metric tons in 2012) and hosts about 30% of the known worldwide copper reserves (USGS, 2013). Situated along the convergent boundary between the South American and Nazca tectonic plates, its plutonic and volcanic activity has been driven by subduction-related magma generation. The geologic evolution of Chile records episodes of compressive deformation followed by magmatic arc migration eastward. These arcs trend north-south along the length of the Andes. It is in these successive arcs that Chile's porphyry copper deposits can be found.







References

John, D.A., Ayuso, R.A., Barton, M.D., Blakely, R.J., Bodnar, R.J., Dilles, J.H., Graybeal, F.T., Mars, J.C., McPhee, D.K., Seal, R.R., Taylor, R.D., and Vikre, P.G., 2010, Porphyry copper deposit model, chap. B of Mineral deposit models for resource assessment: U.S. Geological Survey Scientific Investigations Report 2010-5070-B, 169 p.



Distribution of porphyry copper, copper-gold, and copper molybdenum deposits around the world (USGS, 2008).

Although the magmatic and hydrothermal processes involved in porphyry copper systems have been investigated extensively, many questions still remain. At the University of Bristol we are investigating what role regional and local tectonic stresses play in magma emplacement and the genesis of porphyry copper systems, controls on the fluid distribution and metal enrichment in these systems, and the temporal and spatial relationship between PCDs and magmatic plumbing systems beneath arc volcanoes. We are also examining how volcanic landscape evolution affects supergene enrichment of copper in PCDs and its relevance in PCD exploration.