3D modelling of hydrothermal alteration associated with VHMS deposits in the Kristineberg area, Skellefte district, northern Sweden

Abstract

This contribution presents a 3D assessment of metamorphosed and deformed, hydrothermally altered volcanic rocks, hosting the massive sulphide deposits of the Kristineberg area in the 1.9 Ga Skellefte mining district in northern Sweden, using six calculated alteration parameters: the Ishikawa alteration index, the chlorite-carbonate-pyrite index and calculated net mass changes in MgO, SiO 2 , Na 2 O and Ba. The results, which are also available as film clips in the Supplementary data, confirm inferences from geological mapping; namely that the sericite-and chlorite-rich alteration zones have complex and cross-cutting geometries and that most of these zones are semi-regional in extent and range continuously from surface to over a kilometre deep. The major known massive sulphide deposits occur proximal to zones characterised by coincidence of high values for the alteration index and chlorite-carbonate-pyrite index and large MgO gains, which corresponds to zones rich in magnesian silicates. These zones are interpreted as the original chlorite-rich, proximal parts the alteration systems, and form anomalies extending up to 400 m away from the sulphide lenses. In addition, the stratigraphically highest VHMS are hosted by rocks rich in tremolite, talc, chlorite and dolomite with lesser clinozoisite, which have high chlorite-carbonate-pyrite index and low-medium alteration index values, reflecting a greater importance of some chlorite-carbonate alteration at this stratigraphic level. Vectoring towards massive sulphide deposits in this area can be improved by combining the AI and CCPI indexes with calculated mass changes for key mobile elements. Of the ones modelled in this study, MgO and SiO 2 appear to be the most useful.

Ore Remobilization History of the Metamorphosed Rävliden North Volcanogenic Massive Sulfide Deposit, Skellefte District, Sweden

“…The mineralizations in the Rävliden horizon are typically rich in Zn and Pb with dominant amounts of pyrite, hosted in calc-silicate rocks and schist with tremolite, chlorite, dolomite, and quartz (Hannington et al, 2003;Chmielowski et al, 2016). Chmielowski et al (2016) argued that the current mineralogy of host rocks reflects amphibolite facies regional metamorphism of chlorite-and carbonate-altered rhyolitic to dacitic precursors. The exploration team at Boliden divides the ore lenses and mineralization types at Rävliden North into Cu-and Zn-rich types (Bjänndal, 2022).…”

3D structural control and spatial distribution of Zn-Pb-Cu grades in the Palmeirópolis VMS deposit, Brazil

“…Deformation and metamorphism can result in remobilisation and spatial redistribution of original massive sulfide mineralisation (Marshall and Gilligan 1993; Marshall et al 2000), resulting in new complex geometries that reflect folding, faulting, flattening, and shearing. However, although there are extensive studies in geochemistry and mineralogy of VMS-type deposits, few works still discuss their geometry and ore body shape in 3D (Martin-Izard et al 2015; Schetselaar et al 2016; Schetselaar et al 2018; Arias et al 2021), and their implications in ore genesis and mineral exploration (Bauer et al 2014; Chmielowski et al 2016; Kampmann et al 2016). Modelling and visualisation in 3D software are relatively new as a research tool, but mine geologists and scientists have been analysing orebodies in 3D for a very long time through systematic sets of sections, plan views and longitudinal views.…”

The Au-Cu-Mo Paraíba deposit (MT)

“…Minerals, rocks and other terrestrial compounds exhibit diagnostic absorption features in either the visible-near infrared (VNIR) (400 -1000 nm), short-wave infrared (SWIR) (1000 -2500 nm), mid-infrared (MIR) (3000 -5000 nm), and/or longwave infrared (LWIR) (8000 -14000 nm) regions in response to electronic and vibrational processes, as well as overtones (Clark 1999, Hook et al 1999. Alteration minerals such as white mica, chlorite and epidote are characteristic of several types of hydrothermal systems and commonly present compositional variations with distance to the system (Sillitoe 2010, Halley et al 2015, Phillips and Powell 2015, Chmielowski et al 2016, Wang et al 2017, Neal et al 2018. The spectral signature of alteration minerals is increasingly being used as a vector to find, measure and map alteration zones.…”