QUANTITATIVE ASSESSMENT OF UNDISCOVERED PT, PD, AU, NI AND CU RESOURCES IN PALEOPROTEROZOIC LAYERED INTRUSION-HOSTED PGE-NI-CU DEPOSITS IN FINLAND

The undiscovered Pt, Pd, Au, Ni, and Cu resources in contact-type and reef-type PGE-Ni-Cu deposits hosted by ~2.45 Ga mafic-ultramafic layered intrusions in northern Finland were estimated down to 1,000 m depth using the three-part quantitative assessment method developed in the USGS. This included constructing descriptive and grade-tonnage models for the deposit types, delineating permissive tracts, estimating the number of undiscovered deposits and simulating the probability distributions of undiscovered metal tonnages.

A grade-tonnage model for contact-type deposits was constructed using data from well-known deposits within the Fennoscandian shield. The standard three-part assessment approach with one grade-tonnage model for a deposit type would be problematic for the estimation of tonnages of undiscovered reef-type deposits. This is because the tonnage of a reef deposit is strongly dependent on the size of the host intrusion, due to the great lateral continuity of reefs. To overcome this problem, a tonnage model was created separately for each permissive tract by estimating the probability distribution of ore tonnage for a possibly existing, undiscovered reef-type deposit. The estimation was based on the dimensions of the host intrusion or intrusive block along igneous layering and on the frequency distributions of reef thickness, reef density, and geological loss due to reef discontinuity. Each of these reef tonnage models was combined with a common grade model for reef-type deposits constructed using data from Finnish PGE reefs. This approach assumes that there is no correlation between reef surface area and thickness, or between ore tonnage and metal grades.

Altogether, 19 contact-type and 24 reef-type permissive tracts were delineated, using the existence of a known layered intrusion or intrusive block as the necessary criterion. The expected numbers of undiscovered contact-type and reef-type deposits within these tracts are 29 and 23, respectively. The total resources in undiscovered PGE deposits in Finland are estimated to be, at the 50% probability level, at least 5,600 t Pt, 12,000 t Pd, 430 t Au, 4.2 Mt Ni and 5.7 Mt Cu. These figures indicate that the presently known PGE resources of 355 t Pt+Pd account for approximately 2% of the total estimated PGE resources in Finland.