Hidden mineral deposits in Cu-dominated porphyry-skarn systems: How resource reporting can occlude important mineralization types within mining camps

A single mining camp or ore deposit can contain multiple mineral deposit types but may have mineral reserves or resources classified by what a mining or mineral exploration company considers to be the dominant mineralization type in the area. In this paper, we summarize recent work on the challenges of reporting mineral deposits by geologic processes rather than by grades, tonnages, and mineral processing approaches. For example, the Ertsberg-Grasberg district of Indonesia contains several large skarn Cu-Au-Ag deposits, with the discovery outcrop as well as early production entirely in skarn. All early publications and resource descriptions refer to it as the Ertsberg district. Subsequent discovery of the giant Grasberg porphyry Cu-Au-Ag deposit led to the entire district being renamed Grasberg and classified as a porphyry deposit, despite the skarn-focused discovery and early production history of the deposit, as well as the presence of several large skarn deposits within the district. The Ok Tedi Cu-Au-Ag deposit of western Papua New Guinea also is generally thought of as a major porphyry Cu deposit, yet hosts both porphyry and skarn mineralization. Current reserve estimates indicate that the majority of the contained metal within the deposit is hosted by skarns rather than porphyry bodies. Thus, following the Grasberg example in terms of contained metal, Ok Tedi could be classified as a skarn rather than a porphyry deposit. In addition, comparatively minor mineral deposits can prove useful during exploration; this is exemplified by the large Au-Cu-Ag porphyry deposits at Cadia in Australia that were discovered by exploring modest skarn deposits using the Ertsberg-Grasberg skarn-porphyry model.