Zinkgruvan Ore Deposit Geology
The Zinkgruvan deposit is situated in the southern part of the Bergslagen province in south-central Sweden. This province consists of a greenstone belt that formed during the Proterozoic era around 1.9 billion years ago. It is known for hosting significant deposits of zinc, lead, copper, silver, and banded iron formations. The rocks in this area are primarily composed of felsic metavolcanic successions, with occurrences of limestones and calcsilicates within them. During the Svecofennian orogeny, which took place between 1.9 and 1.8 billion years ago, the province experienced folding and metamorphism, reaching upper amphibolite facies.
The Zinkgruvan deposit itself is a massive stratiform deposit of zinc and lead, located within the lower Proterozoic Svecofennian supracrustal sequence, which ranges from 1.90 to 1.88 billion years in age. The deposit exhibits distinct stratification and extends for over 5,000 meters along its strike and reaches depths of 1,600 meters. The thickness of the ore body varies from 3 to 40 meters. In the central part of the deposit, there is a copper stockwork mineralization underlying the zinc-lead mineralization in a sub-stratiform manner.
During the Svecofennian orogeny, the deformation of the region resulted in isoclinal folding, causing the overturning of the stratigraphy in the area. A major fault, known as the Knalla fault, trends in a north-northeast to south-southwest direction and is present at the center of the property, dividing the deposit into two areas. The eastern area, called Nygruvan, has been the primary source of historical mine production and strikes northwest to southeast, dipping subvertically to the northeast. The western area, referred to as the Knalla area, strikes northeast to southwest and has variable dips to the northwest. The Knalla area is further divided into several sub-areas, including Burkland, Lindängen (which has been depleted by mining), Sävsjön, Mellanby, Dalby, Cecilia, and BortaBakom, progressing from northeast to southwest.
The general consensus suggests an exhalative origin for the Zinkgruvan deposit, wherein lenses of polymetallic sulfides containing zinc, lead, silver, and copper formed either on or near the seafloor in submarine hot spring environments. These sulfide accumulations resulted from the concentrated discharge of metal-enriched fluids associated with hydrothermal convection on the seafloor, potentially related to active submarine volcanism, including rift spreading centers.
In the Zinkgruvan area, the formation of a local and relatively deep sub-basin structure, such as a half graben, coincided with the transition from active to waning volcanism and the deposition of post-volcanic sedimentary layers, including limestone, reworked volcanic ash, and deepwater sediments. Deposition within this basin may have created a reduced environment with limited detrital sedimentation. Such an environment would have been favorable for the preservation of organic matter and the accumulation and preservation of base metal sulfides.
The venting of oxidized brines enriched in metals into the sub-basin may have triggered copper deposition through interactions with organic matter and/or reduced pore waters beneath the seafloor. This process, combined with the exhalation, cooling, and mixing of the brines with reduced bottom waters in a brine pool on the seafloor, led to the formation of the stratiform zinc-lead-silver ore. In the Zinkgruvan deposit, proximal volcanic rocks are separated from the ore by an interval containing several thick former limestone horizons, indicating a significant time gap between the emplacement of the volcanic unit and the formation of the ore. Syn-sedimentary faults likely acted as major pathways for the mineralization process.
Genetic Model for the Zinkgruvan Deposit (Jansson et al., 2017)