Summary:
The complex history of post-ore modifications has led to that classification of the deposit has not been successfully achieved. General consensus exists on a syngenetic-exhalative origin, yet the deposit has been variably classified as a Broken Hill-type (BHT) deposit, volcanogenic massive sulphide (VMS) deposit and sediment-hosted Zn (SEDEX) deposit. Later work (Jansson et al. 2017) has shown evidence that both the stratiform Zn-Pb-Ag and the dolomite-hosted Cu ore can be explained as products of oxidized, saline brines at a near neutral pH that formed stratabound, disseminated Cu ore by interaction with organic matter below the seafloor, and regionally extensive Zn-Pb-Ag ore from an overlying, reduced brine pool into which the fluids exhaled. A distinct discontinuity in the mineralized system coincides with a marked stratigraphic break, wherein key stratigraphic components of the host sequence disappear abruptly. This feature is interpreted as a syn-sedimentary fault which functioned as a feeder to mineralizing fluids based on the metal zonation in the stratiform ore and the spatial distribution of mineralization in dolomitic marble. The inference of the composition of the ore-forming fluid and a dominant redox trap rather than a pH and/or temperature trap is inconsistent with typical VMS and BHT models. Except for the metamorphic overprint, Zinkgruvan has similarities to both McArthur-type SEDEX deposits and sediment-hosted Cu deposits and could as such comprise one the oldest known manifestations of these ore-forming systems, following oxygenation of the Earth’s atmosphere (Jansson et al. 2017).
The Zinkgruvan deposit comprises a stratiform, massive zinc-lead deposit hosted by K-rich metatuffites with intercalated beds of marble, dolomite and fine grained quartzite. A zone of stratified disseminated pyrrhotite mineralisation occurs 100m stratigraphically above the zinc-lead mineralisation while in the central part of the deposit the zinc-lead mineralisation is stratigraphically underlain by a substratiform copper stockwork.
The deposit exhibits distinctive stratification and extends for more than 5km along strike and to depths of 1,600m. Deformation during the Svecofennian orogeny included isoclinal folding which has resulted in the stratigraphy of the area being overturned. The property geology is also divided into two distinct areas by the regional northnortheast to south-southwest trending Knalla fault. These areas, which make up the Zinkgruvan deposit, are known as Nygruvan area and Knalla area. The Nygruvan area is bounded to the east by the Sinsberg fault beyond which felsic metavolcanics and early orogenic granites are encountered. The Knalla area is bounded to the west by the Dalby fault beyond which post-orogenic granites are encountered.
Mineralization
Two major types of mineralization occur at Zinkgruvan, the stratiform Zn-Pb-Ag mineralization and the marble-hosted mineralization.
Nygruvan Area
The Nygruvan area of the mine consists of a tabularzinc-lead orebody, striking northwest to southeast and dipping 60° to 80° to the northeast and with a near-vertical plunge. The orebody persists to at least 1,600m vertical depth and ranges in thickness from 5m to 25m. Towards the eastern part of the Nygruvan area, the orebody splits and is present as two parallel mineralised zones separated by 3m to 8m of metatuffite (quartz, microcline, biotite, and minor muscovite, chlorite and epidotic). The metatuffite is a homogenous, usually massive, quartz-microcline-biotite rock of rhyolitic to dacitic composition. It has a granoblastic texture and is often gneissic. The stratigraphy of the metavolcanic-sedimentary group is best developed in the eastern part of the Nygruvan area where the sequence is thickest. Metabasic sills and dykes intruding the metavolcanic and the sedimentary group are the oldest intrusions. Dykes and irregular, massive, grey, usually coarse-grained pegmatites of granitic composition are relatively common in the folded areas.
There is clear evidence of hydrothermal alteration in the mine sequence. Altered rocks have been heavily depleted of Mg, Mn and Fe, although there is some disagreement regarding Mn depletion. Sodium depletion is less evident in the mine area, although the Na/K ratio decreases upwards through the footwall sequence of progressively more altered metatuffite. There is significant enrichment in Ba, K, S and Ca.
Sphalerite and galena are the dominant sulphide minerals. They generally occur as massive, well banded and stratiform layers. Chalcopyrite is present in small amounts (<0.2% Cu). Pyrrhotite, pyrite and arsenopyrite are present although the amount of pyrrhotite and pyrite is typically low (<1% each). Metamorphism and deformation have mobilised galena into veins and fissures sub-parallel to original bedding in places. Native silver was even more mobile and is often found in small fissures. Remobilisation is most commonly observed in the lead-rich western part of Nygruvan and the Burkland zone of Knalla. In both the Nygruvan and Knalla areas there is an increase in zinc- lead grades towards the stratigraphic hanging wall of the massive sulphide horizon. Contacts of the mineralisation with the host stratigraphy are generally very sharp, more so on the stratigraphic hangingwall than footwall.
Knalla Area
The Knalla area comprises several tabular zones of zinc-lead mineralisation (Burkland, Sävsjön, Mellanby, Dalby, Cecilia and Borta Bakom) which form a continuous, although highly contorted orebody with variable thickness (3m to 40m). In addition, a copper stockwork zone is also present in the structural hanging wall of the Burkland zone. The Knalla area generally strikes northeast to southwest (although quite variable locally) and dips generally to the northwest. Dips are variable from near vertical to sub-horizontal. Plunges are also variable with the Burkland zone plunging moderately to the northeast and Cecilia and Dalby plunging to the northwest. The Burkland zone extends from 200m to 1,600m vertically and flattens considerably at depth. The overall structure of the Knalla area is more complex than at Nygruvan and structural thickening is common. There are often two to four parallel ore horizons separated by narrow widths of metatuffite.
The significant difference in the zinc-lead mineraliation from that found at Nygruvan is that the Knalla area contains elevated levels of Co and Ni. These levels are sufficiently elevated as to impact on metallurgy and concentrate quality.
The copper stockwork zone located in the structural hanging wall of the Burkland zone is best developed at depths between 700m and 1,100m. It has a strike length of 100m to 180m while the width varies from 5m up to 60m with an average around 20m. Between 1,100m and 1,200m depth the thickness of the mineralisation decreases to 10m. Above the -600m level the copper stockwork zone reduces in thickness before pinching out. The copper stockwork zone is cut off laterally to the northeast by the Knalla fault and has been closed off by drilling to the southwest. The host rock is a dolomitic marble with variable amounts of porphyroblastic Mg-silicates. Chalcopyrite is the main copper mineral and occurs as fine-grained disseminations infilling between dolomite grains or massive lumps and irregular veins up to several cm thick. Cubanite (CuFe2S3) is also present and occurs as lamellae in chalcopyrite. Bornite is present, while tetrahedrite is rare. Minor amounts of arsenopyrite are found locally. In its footwall plunge the copper stockwork sometimes merges with the Burkland zinc-lead mineralisation which results in significant amounts of sphalerite and some galena at the contact of these two zones.