Technological and societal developments in the past century have greatly increased our society’s demand for metals, many which occur in polymetallic ores mined in the Skellefte district, northern Sweden. Near-surface deposits are becoming depleted and as such targeting of deep deposits is needed, which places special demand on effective management, processing and interpretation of geological data obtained from exploration drill holes. New exploration tools and software solutions utilizing machine learning to assist data analysis are becoming increasingly important. However, to get the most of these new tools, a solid geological understanding of prospective areas must first be established.

This thesis uses the Rävliden North Zn-Pb-Ag-Cu volcanic massive sulphide (VMS) deposit as a study location to understand its lithostratigraphy and enveloping alteration. The Skellefte district VMS deposits are mainly located at the lithostratigraphic contact between the metavolcanic 1.89 – 1.88 Ga Skellefte group (SG) and the stratigraphically overlying metasiliciclastic 1.89 – 1.87 Ga Vargfors group (VG) rocks. The VMS deposits are commonly enveloped by a zoned alteration with variable alteration intensity and mineral assemblages of quartz, sericite, chlorite and talc at different distance to mineralisation. 

Sixteen lithofacies and eight precursors can be defined in the Rävliden North host succession, where the SG dominantly contains coherent and volcaniclastic facies of rhyolite, dacite and andesite. The VG contains graphitic phyllite interbedded with polymict breccia-conglomerates, andesitic turbidites and mafic mass-flow deposits. Immobile element lithogeochemistry reveals four rhyolitic (Rhy I – IV), two dacitic (Dac I and II), an andesitic (And I), and a basaltic (Bas I) precursors. The VMS deposits are hosted by graphitic phyllite Tr-rich calc-silicate rock, and a Chl>Ser±Tlc±Qz-altered rock in the contact between the SG and VG. 

Four alteration types are recognised and spatially associated to mass changes of mobile elements. The Tr-rich calc-silicate and calcitic rocks are related to gains in CaO and occur proximal to mineralisation. Chlorite>Ser±Tlc±Qz alteration is related to gains in MgO and FeO and also occur proximal to mineralisation. The choice of least-altered volcanic rocks, needed for modelling fractionation, is found to have effect on the resulting mass-balance calculation; however, qualitative recognition of mass changes related to the ore-hosting alteration types is still possible. The uncertainty ranges of mass changes are determined to be ±0.5 wt.% for MgO, FeO and CaO.

This thesis demonstrates that geological understanding and quantification of error and uncertainty in mass balance calculations are necessary prerequisites to advanced exploration techniques.