Lithostratigraphy and alteration at the Rävliden North VMS deposit, Skellefte district, Sweden

Sammanfattning: The Rävliden North deposit is a relatively recent Zn-Pb-Ag-Cu volcanogenic massive sulphide (VMS) discovery c. 4 km west of the current Kristineberg mine in the western part of the Palaeoproterozoic Skellefte district, Northern Sweden. More than 85 pyritic VMS deposits have been found in the Skellefte district making it a significant metal producer in Europe. These deposits are hosted in greenschist to amphibolite facies rocks at the lithostratigraphic contact between metavolcanic 1.89 – 1.88 Ga Skellefte group (SG) and stratigraphically overlying metasiliciclastic 1.89 – 1.87 Ga Vargfors group (VG) rocks. The VMS deposits are commonly enveloped by a zoned Qz-Ser±Chl±Tlc alteration with variable alteration intensity and mineral assemblage at different distance to mineralisation; together with the fact that these alteration envelopes are larger than their associated deposits, these are suitable exploration targets. Locally, alteration intensity is strong enough to eradicate textures of the original lithofacies, making geological interpretation difficult. Furthermore, much of the Skellefte district is affected by polyphase deformation, metamorphism and remobilization of VMS mineralisation making stratigraphic correlation difficult. Therefore, a useful complement to lithofacies analysis is the use of immobile element ratios to construct chemostratigraphy. Another important tools for VMS exploration in these difficult rocks are quantitative approaches to alteration mapping, where mass changes of components in rocks are calculated relative to chemically defined precursor rocks. When these mass changes are plotted in space they may function as ore vectors. However, a challenge with this technique is defining the composition of the precursor, since it relies on careful identification of least-altered samples. This thesis uses the Rävliden North deposit as a study location to understand the lithostratigraphy and alteration envelopes found in the crucial SG-VG contact in the western Skellefte district with the objective of characterizing the host succession and which mass changes indicate the location of the VMS deposit. It concluded that: 1) the Rävliden North VMS deposit comprises Sp+Pyh+Gn±Py and Ccp+Pyh+Py mineralisation that are hosted in the upper parts of the SG, where former is hosted in a volcanosedimentary succession with metarhyolite, metadacite and metaandesite making up a stratigraphic unit herein defined as the Rävliden formation, 2) mineralisation formed by subseafloor replacement and is associated with a thin bed of graphitic shale in the Rävliden formation, herein called Bonus shale, in a chemostratigraphic contact between two different rhyolites (Rhy I and Rhy III), 3) the upper Rävliden formation comprises less altered rhyolitic, andesitic and dacitic rocks, where a post-mineralisation partially emergent cryptodome complex is interpreted for the andesitic and dacitic rocks, 4) mass changes related to the Rävliden North VMS deposit are: Na2O and CaO loss; distal K2O gain; proximal K2O loss; CaO, MgO and Fe2O3 gains in VMS hosting rocks; and erratic gains and losses in SiO2 is associated with mineralisation.