Chemostratigraphy, Petrography and Alteration at the Kristineberg VMS Deposit, Skellefte District, Sweden. Barrett, T.J. and MacLean, W.H., 2000b. Chemostratigraphy, Petrography and Alteration at the Kristineberg VMS Deposit, Skellefte District, Sweden. Unpublished Report for Boliden AB Exploration, Sweden. 131 pp + 8 appendices. The Kristineberg area in the western Skellefte district comprises a thick package of early Proterozoic felsic and lesser mafic metavolcanic rocks overlain by fine-grained turbiditic metasediments; the area is dominated by a km-scale synclinal structure plunging to the WSW. The Kristineberg deposit, which has produced 20 Mt grading 1.05 g/t Au, 34.6 g/t Ag, 1.05 % Cu, 3.56 % Zn, 0.23 % Pb, is hosted by altered, recrystallized metavolcanic schists containing minerals such as muscovite, quartz, chlorite, phlogopite, biotite, cordierite, andalusite, pyrite and magnetite (Arebäck, 1999). The mineralized volcanic sequence lies within a few hundred metres of the Viterliden Intrusive Complex (1907±13; Bergström et al., 1999). Pyritic massive sulfides containing sphalerite-chalcopyrite-galena occur mainly at the A and B ore horizons, which strike roughly E-W and are separated by 100-150 m. In the Einarsson zone, which has been recently outlined near the 1000 m level, Au-Cu-rich mineralisation occurs to the south of the B ores, in silicified to chloritized rocks containing abundant disseminated sulfides. In December 1999, a lithogeochemical and petrographic study was initiated, with four main goals: 1) establishment of a series of chemically defined volcanic units in order to construct a chemostratigraphic framework that would assist exploration in the area; 2) assessment of the pre-metamorphic hyrothermal alteration of these rocks using mass change calculations; 3) petrographic examination of various alteration and metamorphic assemblages (tied to the lithogeochemistry); and 4) construction of a series of chemical reactions to account for the conversion of original alteration minerals to the current metamorphic assemblages. For this program, 249 lithogeochemical samples from 15 drill holes were analyzed; 31 petrographic samples were also examined. These holes are located mainly on sections 2500Y and 1900Y, with one hole on 1620Y. On the basis of this data set, we have developed a series of 'chemical windows' specific to Kristineberg which can be used to identify the various rock types based on their immobile-element ratios, even where the rocks are altered and metamorphosed. The main chemical rock types include rhyolites A, B and X, a group of dacites, and a group of andesites; a few basalts are also present (these chemical classifications do not differentiate between extrusive and intrusive rocks). In addition, more than 600 analyses of underground and surface samples from the mine area made by Boliden during the 1995-1999 period have been classified using the same chemical methods. The overall data set indicates that the volcanic rocks located between the A and B massive sulfide lenses consist mainly of dacites, andesite and rhyolite B (approximately a rhyodacite). Rhyolite A lies mainly on the immediate southern side of the B ore lens, whereas rhyolite X occurs mainly on the immediate northern side of the A ore lens. The mine porphyry, which lies north of the A ore lens and forms the marginal phase of the Viterliden Intrusive Complex, is compositionally similar to, but not identical with, rhyolite B. The Kristineberg andesitic to rhyolitic rocks show a markedly calc-alkaline affinity for the most part, and can be ascribed to a subduction-related setting such as a submarine continental margin volcanic arc or foundered back-arc basin. Overall, the lithological data support (but do not prove) the idea that the A and B ore lenses represent one main sulfide horizon which has been repeated by synclinal folding. In this model, the sulfide horizon would have stratigraphically rested mainly upon rhyolites X and A. These rhyolites would have outcropped near each other on the seafloor, although they were possibly separated by some dacitic material. Following deposition of the sulfides, a broadly dacitic package of rocks accumulated, in part as volcaniclastic material. Locally, some of the massive sulfides overlying the rhyolites were reworked into local basins where dacitic debris was accumulating, resulting in some sulfide ‘beds’ at a higher stratigraphic level. Most of the volcanic rocks in the mine area are strongly depleted in Na and Ca. Significant additions of silica and in places Mg and Fe occur on the north side of the A sulfide lens, and on the immediate south side of the B sulfide lens. In the synclinal model, these additions would have occurred in the footwall prior to deposition of the sulfides. The broadly dacitic rocks between ore lenses A and B, which would represent the hangingwall, show fairly small additions of Mg and Fe, and moderate losses of silica; some areas are only weakly altered. The Einarsson zone occurs in a geologically complex area lying between the B ore lens and massive rhyolite A to the south. The rock types and alteration in this area are similar to those flanking the B ore lens, but are arranged in a complex manner, with apparent repetition of some units which may be due to meso-scale folding. Much of the Einarsson zone is enriched in Fe, and some of it in Mg as well; silica ranges from strongly enriched to strongly depleted. Petrographic observations and microprobe data indicate that Mg-rich secondary minerals are dominant at Kristineberg (chlorite, cordierite, phlogopitic biotite, and locally talc). Although cordierite can be produced by metamorphic reactions involving sericite and chlorite, these reactions usually produce either anthophyllite, which is absent in the Kristineberg rocks, or a significant amount of phlogopite in addition to andalusite, which is not generally observed. It is proposed here that the cordierite formed mainly by reaction of kaolinite and chlorite. Kaolinite and related aluminosilicates form in volcanic rocks subjected to strongly acidic alteration (advanced argillic alteration). In altered rocks which contain an excess of chlorite relative to kaolinite, all of the kaolinite will be consumed in the metamorphic reaction which produces cordierite. Where an excess of kaolinite over chlorite is present, it will be eventually converted to andalusite plus quartz. We speculate that normal VMS-type alteration (sericite-chlorite-quartz) which formed at the time of deposition of the massive sulfides was locally overprinted by high-temperature acidic alteration related to shallow felsic intrusions. Proximal acidic fluids may have formed the Cu+Au mineralization and Al-rich phases of the Einarsson zone, while distal fluids produced smaller amounts of Al-rich phases but no mineralization. During exploration for continuations of the A and B ore horizons in the Kristineberg area, one step would be to locate the rhyolite X and rhyolite A units, respectively. By analogy with the mine relations, zones of strong silicification with Mg or Fe gains would also be a favorable indication of nearby massive sulfides. Einarsson-type mineralization occurs close to massive rhyolite A; the associated alteration ranges from Al-rich (now andalusite) to chlorite-rich.
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