Geologic History of the Crandon Area

The geologic history of Wisconsin is a complex story that has resulted in the creation of a diverse landscape and a variety of geological features. Interpretation of rock formations suggests that Wisconsin has experienced varied geologic conditions through time, including the following:

• There were once a chain of volcanic islands partially submerged in an ancient sea;
• There were times of mountain-building when rock units were physically and chemically altered, folded, and faulted to produce mountain ranges where original flat-lying sediments were tilted to near vertical orientations;
• There were times when shallow, warm tropical seas invaded the continent resulting in the deposition of sedimentary rocks such as sandstone, limestone, and shale; and
• There were times when glaciers scoured and sculpted the landscape leaving a blanket of sediment.

The geology of the Crandon deposit reflects portions of this story.

Regional Bedrock Geology

The proposed Crandon Mine site lies within the Southern Superior Uplands Province. The regional landscape appears as a series of northeast-southwest trending ridges that parallel the direction of the most recent glacial movement. The resulting landscape is characterized by forested rolling hills and a mosaic of wetlands, streams, and lakes. Generally, the crystalline bedrock is not exposed at the surface, and the nearest rock outcrop (of Wolf River Batholith granite) occurs over six miles to the west of the mineral deposit.

This thumbnail image links to a larger version of a geologic map in PDF format that portrays the belt of Early Proterozoic rocks (primarily metavolcanics) metamorphosed during the Penokean Orogeny. Several sulfide mineral occurrences and two ore bodies have been identified within this belt. This map was prepared by the Wisconsin Geological and Natural History Survey.

The Crandon Ore Body is part of the Southern Province of the Precambrian Shield. Within this Southern Province is a belt of metamorphosed volcanic and sedimentary rocks of Early Proterozoic age (roughly 1.86 to 1.95 billion years old), known as the Rhinelander-Ladysmith Greenstone Belt. Regional metamorphism in the area achieved lower greenschist to amphibolite facies, a relatively mild grade as evident from the preservation of original depositional features. This belt is roughly 50 miles wide (north-south) and 200 miles long (east-west), extending eastward from Ladysmith in Rusk County to Pembine in Marinette County (and possibly on into the Upper Peninsula of Michigan). The Crandon Ore Body in Forest County, the Flambeau Ore Body in Rusk County, and many other smaller areas of mineralization have been discovered within this volcanic complex.

According to the theory of plate tectonics, the surface of the earth is comprised of a series of rigid plates that slowly move around and interact at various plate boundaries (changing shape), colliding, separating, or sliding past each other. During late Precambrian time, northern Wisconsin was located on a continental margin (coastline). Immediately offshore of this continental margin there appears to have been a region where two oceanic crustal plates were colliding, and one oceanic plate was sinking (southward) beneath the other producing a chain of volcanic islands, a process called subduction.

This cross section portrays the geologic setting in northern Wisconsin approximately 1.8 billion years ago, where along the coastline there was an active subduction zone and volcanism (drawing by Christin Engstrom).

The subduction of the oceanic plate resulted in active volcanism along the margin that created a coastline similar to that found today in the area of the Pacific near China, Korea and Japan. An isolated marine basin located between the volcanic islands and the continent had developed (similar to the Sea of Japan) and was slowly closing due to the southward-directed subduction zone.

The volcanic activity produced a suite of volcanic breccias, fine tuffs, lapilli tuffs, pyroclastic flows, lava flows, and igneous intrusions. Chemical precipitates including chert and other sedimentary materials such as mudstone and minor amounts of limestone were deposited within what would become the Crandon Formation and the surrounding volcanic sediments during hiatuses in volcanism. Hot springs near the ocean floor resulted in the formation of black smokers that precipitated metal-sulfides onto the sea floor. The relative orientation of the geologic formations and lithologic compositions are represented in the below-linked geologic cross section and stratigraphic column developed by Nicolet Minerals Company (NMC).

This thumbnail image links to a larger version of a generalized geologic cross section of the bedrock in PDF format. This diagram is from NMC's Mine Permit Application, December 1998.		This thumbnail image links to a larger version of a stratigraphic column of the bedrock units in PDF format. This diagram is from NMC's Mine Permit Application, December 1998.

Proterozoic strata within the immediate vicinity of the mine site have been divided into three difference units:

• the hanging wall volcanic rocks to the north, designated as "hanging" because they are situated physically on-top of the ore deposit;
  
  
• the Crandon Formation, which contains the massive zinc ore and lesser amounts of lead and copper; and
  
  
• the footwall wall volcanic rocks to the south, designated as "footwall" because they are situated beneath the massive zinc ore deposit (but contain the stringer copper ore), and are the older volcanic rocks upon which the deposit formed.

The terms hanging wall and footwall are used in a mining context and do not connote any structural relationships.

Emanating hydrothermal fluids resulted in the physical and chemical alteration of the surrounding rocks. Evidence of alteration of the hanging wall rocks is limited, but can be measured chemically by the addition or depletion of various elements as compared against unaltered equivalent rock types. Most of the bedrock encountered during drilling through the Crandon Formation and the footwall rocks have been hydrothermally altered, containing evidence of sericitization (breakdown of orthoclase to muscovite), silicification (addition of quartz), sulfide precipitation, and leaching of metals. Interestingly, there is a distinct chemical difference between the volcanic rocks situated stratigraphically above and below the Ore Body. It is apparent that volcanism shifted from a basalt/andesite system to a dacite/rhyolite system following deposition of the Crandon deposit.

Description of the Crandon Deposit

Mobile drilling rigs are used to help characterize groundwater flow and ore body composition.

Most of the area near the Crandon Ore Body is covered by glacial deposits that range between 50 and 350 feet thick, making it necessary to base bedrock geologic interpretations on information collected from cores extracted during drilling activities. Since the massive sulfide deposit was first discovered in 1975 through geophysical reconnaissance, a total of 340,000 feet of bedrock core from 272 drill holes both intersecting and adjacent to the deposit have been studied. The deepest hole was drilled to an approximate depth of 2,500 feet below the land surface.

The Crandon deposit is identifiable for a length of approximately 6,000 feet along strike (that is, along the trend of the bedrock layering), which is roughly east-west (oriented at approximately N 85 ºW), and to a depth of approximately 2,400 feet below the bedrock surface. The Ore Body thickness varies from less than 1 foot on the east and west ends to about 350 feet near its center. The Crandon Formation is steeply dipping to the north (70 to 90º, with 90º being vertical), as inferred from relict depositional features such as flute casts, graded bedding, sag features, and erosional surfaces, in addition to mineral zonation. A small portion of the deposit is steeply overturned (85º S) in the extreme west end of the Ore Body.

The Crandon Formation is part of a continuous sequence of volcanic and chemical sedimentary rocks, and consists of discontinuous, thinly bedded sulfide, argillite, and tuff with minor chert and dolomite. No coarse pyroclastic materials have been identified in the Crandon Formation, which suggests that the metal-bearing fluids were deposited during times of volcanic quiescence.

Mineralogically, the massive sulfide is predominantly comprised almost entirely of pyrite (iron-sulfide) and sphalerite (zinc-sulfide) with lesser chalcopyrite (copper-iron-sulfide) and galena (lead-sulfide). Minor amounts of native silver and gold have also been detected in the mineral deposit. Non-metal bearing minerals, which would be part of the waste from any mining, include quartz, chlorite, and minor sericite (muscovite) and dolomite.

Within the Crandon deposit, there are two distinct ore types formed from two separate geologic processes that have been designated as "massive ore" (predominantly zinc-rich) and "stringer ore" (predominantly copper-rich). The two ore types occur in nearly equal amounts. The main zinc ore mineral in the deposit is sphalerite and composes the Crandon Formation itself. The main stringer ore mineral is chalcopyrite and is contained within footwall rocks underlying the Crandon Formation. The stringer ore occurs stratigraphically beneath the zinc mineralization.

The massive ore consists of layers of sphalerite, pyrite, and minor galena within the metamorphosed sediments of the Crandon Formation. The massive ore is tabular shaped and has an average thickness of 100 feet, which decreases with depth. Total sulfide content of the massive ore averages 70% by volume. The remaining 30% of the massive ore consists mostly of quartz and chlorite. The stringer ore occurs as patchy grains of chalcopyrite in disseminated quartz veins with pyrite in brecciated veins in the footwall volcanics. The stringer ore forms an irregular, tabular body with an average strike length of 2,300 feet and a thickness that ranges between 65 and 100 feet thick. The stringer ore is flooded with quartz and as much as 20% total sulfide. Quartz veins also contain some sphalerite but are occasionally barren of metal-sulfides. Small amounts of carbonate minerals are found in the ores and enclosing host rocks.

Depositional History of the Crandon Deposit

This thumbnail image links to a larger version of a schematic in PDF format of the proposed formation model for the Crandon Ore Body, a volcanogenic massive sulfide deposit.

The Crandon deposit is a volcanogenic massive sulfide (VMS) deposit of submarine origin. The metallic minerals were deposited between 1.86 and 1.95 billion years ago (during the Early Proterozoic portion of Precambrian time) at and below the ocean floor in a shallow basin by hot, metal-bearing fluids associated with adjacent volcanism. Elevated temperatures heated percolating seawater, dissolving metals and sulfate from the surrounding rocks and magma. When the hot, metal-rich fluids reemerged on the seafloor, contact with the cold seawater would have facilitated the precipitation of metal sulfides around the hot springs and in adjacent low spots on the ocean floor. Seawater circulation patterns through the bedrock of the seafloor likely developed and would have contributed to the concentration of metal-sulfides.

The relatively quiescent environment of a back arc basin would have provided an ideal location where metal-rich fluids could have precipitated sulfide minerals through underwater vents. Deposition of the stringer ore in the underlying footwall volcanics resulted from the precipitation of quartz and chalcopyrite from silica-rich hydrothermal fluids that percolated through the rocks. The presence of marcasite (iron-sulfide) in the Crandon Formation suggests that the massive ore formation and subsequent thermal metamorphism occurred below 520 ºC and probably around 350 ºC.

Currently, this mineral formation process is occurring at black smokers or submarine hot springs near the mid-ocean ridge in the Pacific Ocean. Over geologic time, these deposits may form a deposit similar in magnitude to the Crandon deposit.

Deep burial by accumulating volcanic and oceanic sediments followed by regional tectonic activity (mountain building) resulted in the lithification, metamorphism, and deformation of the deposit and surrounding host rocks approximately 1.84 to 1.86 billion years ago during the Penokean Orogeny. The collision transported the Crandon Formation (and adjacent strata) onto the continent, closing the shallow marine basin, and eventually the ocean basin that was present further offshore. This mountain building event would have produced a mountain range similar to the current coastal ranges of northern California or the early Appalachians. The age of the collision is constrained by the occurrence of undeformed intrusions of granite near the Niagara Fault Zone that have been dated at approximately 1.86 billion years old.

Deformation caused the tilting of the originally horizontal layers to their present, near-vertical orientation. No major faults have been observed during drilling activities at the Crandon Project Site. Although, the north-south compression resulted in the development of an east-west trending schistosity (metamorphic fabric) in the bedrock.

Weathering of the Crandon Deposit

This thumbnail image links to a larger version of a map in PDF format of the weathering profile of the Crandon Ore Body.

Weathering is the natural destructive process that causes the physical disintegration and chemical decomposition of rock. The primary weathering agent is water, which contains varying concentrations of dissolved oxygen and natural acids that promote the chemical decomposition of rock (changes in the chemistry that result in the formation of new minerals of differing composition). Water is also responsible for several mechanical weathering processes that physically transport or break apart rock into smaller particles through wetting-drying and freezing-thawing.

The degree of bedrock weathering varies across the footwall, the Crandon Formation, and the hanging wall due to varying rock composition, fracturing, and chemistry. The Crandon Formation contains a high percentage of relatively easily weathered sulfide minerals, as compared to the more resistant hanging wall and footwall rocks. As a result, the Ore Body is more deeply weathered than either the footwall or hanging wall. Generally, the bedrock is progressively less weathered the further it is below the ground surface, until fresh, non-weathered rock is present. The bedrock near the Ore Body has been classified according to rock weathering intensity as strong, moderate, low, weak, and unweathered.

Following bedrock deformation and uplift -- during the remainder of Precambrian and Phanerozoic time from approximately 1.86 billion years to the present -- the deposit was exposed to oxygen and meteoric water, partially eroded and weathered, and then likely covered by younger Cambrian-aged sedimentary rocks. Later, erosion removed all of the younger sedimentary rocks leading to additional weathering. Near Crandon, downward-moving groundwater above the Ore Body reacted with the sulfide minerals, which led to the production of acids and leaching of the rock units. Zinc, copper, sulfate, and silver would have been leached from this area and deposited elsewhere, leaving the iron behind, and creating a horizontal layer of saprolite. Saprolite is the residual clays, iron-oxides, and quartz that forms in place during the chemical weathering of crystalline bedrock. Saprolite can be characterized by the preservation of original structures (bedding, folds, etc.) that were present in the original unweathered rock. When a saprolite layer is produced at a sulfide ore body the leached zone is often referred to as a gossan.

At Crandon, two types of saprolite have been identified: massive and structured. Massive saprolite is evident where the original rock structures have been completely destroyed from weathering processes, whereas structured saprolite contains preserved forms of the original rock structures. The massive saprolite is continuous across the project site and varies in thickness from less than one foot over portions of the hanging wall to 80 feet over portions of the Ore Body. The massive saprolite contains minor inclusions of non-saprolitic material (generally reworked glacial till) and is usually bright red in color. The structured saprolite varies in thickness between 10 and 60 feet thick, with thicker portions centered over the middle of the Ore Body. The structured saprolite is comprised of massive sulfide to white clay-rich weathered bedrock. In more weathered zones and near fractures there are occurrences where the structured saprolite contains localized zones of massive saprolite.

Variability in bedrock weathering, structural trends such as minor faults and fractures, and bedrock drainage patterns produced the existing irregularities in the bedrock surface that include knobs, depressions, ridges, and valleys. These features are now buried beneath the glacial deposits.

Regional Glacial Geology

The landforms, soils, and topographic patterns in the environmental study area were primarily influenced by the deposition of unconsolidated glacial materials, which in some areas extend to a thickness of 350 feet. Dominant surface features include drumlins, moraines, outwash plains, kettles (closed depressions), and glacial lakes. Overall, local relief is greater than 200 feet, but individual landforms generally do not exceed 120 feet in height.

During the Pleistocene Epoch (roughly 2.0 million to 10,000 years ago), the region was repeatedly covered by glaciers that deposited till and outwash over the bedrock surface. The unconsolidated glacial material over the Crandon Ore Body varies in thickness from about 73 feet on the east end to about 175 feet on the west end.

The glacial deposits in the area may include undifferentiated sediments from as many as four different glacial episodes that occurred during the Pleistocene. Generally, the majority of the glacial deposits are associated with the Wisconsinian Glacial Stage (advance and retreat), which was the most recent major glaciation period that occurred between 25,000 to 10,000 years before present. However, two unconsolidated till formations have been classified and mapped as being deposited prior to the Wisconsinian Glacial Stage, and are labeled pre- to early Wisconsinian. These till units lie directly over the massive saprolite, and appear to be continuous over the ore body and the nearby footwall and hanging wall rocks. The pre- to early Wisconsinian till and massive saprolite have similar physical characteristics. Near Crandon, the distinctive characteristic and geologic contact between these two rock types is marked by the occurrence of erratic pebbles in the till.

This thumbnail image links to a larger version of a cross section map in PDF format of the unconsolidated glacial units, as developed by Nicolet Minerals Company.

During the Wisconsinian Glacial Stage, southern Forest County was located along the margins of two different lobes of the glacier, the Langlade Lobe and the Green Bay Lobe. Both glacial lobes moved over the region at different times, and from different directions. This created a complex and diverse pattern of glacial deposits and a variable surface topography comprised of drumlins and moraines, separated by lowland outwash plains. The adjacent generalized geologic cross section developed by Nicolet Minerals Company portrays the distribution of the various glacial deposits including: glacial till, coarse grained outwash, fine grained outwash, and lacustrine materials.

The two glacial lobes resulted in the deposition of the following two geologic units: the Mapleview Member was deposited by the northwest moving Green Bay Lobe, and the Nashville Member was deposited by the southwest moving Langlade Lobe. Sediments deposited by the Green Bay Lobe contain more carbonate materials such as limestone/dolostone. In southern Forest County, the Nashville Member overlies and cuts off the Mapleview Member.

The majority of the near-surface sediments in the region belong to the Nashville Member. Till of the Nashville Member is generally exposed at the surface in drumlins clustered on upland areas. The associated meltwater sands and gravels (for each lobe) have filled in low areas around the uplands in pitted and unpitted outwash plains and have formed localized channels through the drumlins in places. Locally the glacial deposits are overlain by more recent sediments, consisting of wind-blown silts and fine sand (loess) deposited on hillsides in the drumlin upland, organic deposits of peat, and sediments transported more recently by flowing water. Beneath the lakes in the project area there are lake bed sediments of organic materials, silt, clay, and sand. Some wetlands within the mine site are perched above the water table by accumulations of organic materials, silt, and clay. The lowland areas adjacent to the drumlin uplands consist mainly of fluvial (derived from stream flow) and wetland sediments overlying glacial outwash and till.

References

Dunning, C., Johnson, D., and Batten, B. (1997) memo to the Wisconsin Department of Natural Resources, Review of the Stratigraphy of Glacial Sediments in the Vicinity of the Proposed Crandon Mine. June 30, 1997.

Erickson, A.J. Jr. and Cote, R. (1996) Geological Summary - Crandon Deposit. in LaBerge, G.L. ed., Volcanogenic Massive Sulfide Deposits of Northern Wisconsin: A Commemorative Volume, Institute on Lake Superior Geology Proceedings, 42, 2, 129-141.

LaBerge, G.L. (1994) Geology of the Lake Superior Region. 313 p. Geoscience Press Inc., Phoenix, Arizona.

Lambe, R.N. and Rowe, R.G. (1987) Volcanic History, Mineralization, and Alteration of the Crandon Massive Sulfide Deposit, Wisconsin. Economic Geology, 82, 1204-1238.