Simplifying drill-hole domains for 3D geochemical modelling: An example from the Kevitsa Ni-Cu-(PGE) deposit

Autor: Margaux Le Vaillant, Stephen Barnes, June Hill
Rok vydání: 2017
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Zdroj: Ore Geology Reviews. 90:388-398
ISSN: 0169-1368
DOI: 10.1016/j.oregeorev.2017.05.020
Popis: A 3D geology model is a simplified version of the true geology, designed to give a visual summary of the geometry and distribution of major geological elements in a specified region. Drill holes provide detailed data of the subsurface that can be classified into geological units that are the fundamental elements of the 3D model. Due to software limitations, upscaling (‘lumping’) is usually required to reduce the number of geological units in the drill holes prior to model building. Upscaling is a subjective process, which means that different geologists will group in different ways and will typically not record the rationale behind their decision; this means the “experiment” is not reproducible. In our study we use a method of upscaling geological units, in this case based on assay data, using the continuous wavelet transform (CWT) and tessellation methods. This method reduces subjectivity and can easily be repeated (e.g. on an updated or new drill hole) by using the same parameters, ensuring that the upscaling process is consistent over all drill hole data. We apply this technique to a large assay database (>90,000 samples) from the Kevitsa Nickel-Copper-Platinum group element (PGE) deposit in Finland. The Kevitsa Ni-Cu-(PGE) disseminated sulfide orebody is hosted in a Proterozoic layered intrusion in northern Finland. Internal geological subdivision and correlation within the intrusion is very difficult to do consistently using lithological observations, owing to general homogeneity of rock types and an overprint of alteration, but distinct variability is evident in Ni and PGE sulfide tenors. In its raw form, the tenor variation dataset appears noisy and unsystematic. We have applied the tessellation method to classifying ore types based on tenor variations, consistently and objectively reducing the number of units in each drill hole to create a simplified 3D model of the orebody. Our results reveal shallow inward dipping cryptic layering defined by sulfide composition, which are interpreted as reflecting an increase in Ni and PGE tenor with time during emplacement of the sulfide-bearing cumulates. We interpret this as a progressive increase in silicate-sulfide mixing efficiency (R factor) as the intrusion developed from an interconnected sill sediment-complex choked with country rock inclusions into a freely convecting magma chamber. Based on this case study, we show that the tessellation method can add considerable value by distinguishing the wood from the trees in large 3D geochemical databases. The method may be widely applicable in other Ni-Cu-PGE deposits where tenor variations appear, at first sight, to be chaotic and uninterpretable.
Databáze: OpenAIRE
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