Browsing by Subject "Cu-Ni"

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    Geology and Mineralization in the Dunka Road Copper-Nickel Mineral Deposit, St. Louis County, Minnesota
    (University of Minnesota Duluth, 1990-03) Monson Geerts, Stephen D; Barnes, Randal J; Hauck, Steven A
    The Dunka Road Cu-Ni deposit is within the Partridge River Intrusion (T. 60 W., R. 13 W.), which is part of the Duluth Complex, and is approximately 1.1 b.y. (Keweenawan) in age. Relogging of 46 drill holes at the Dunka Road Cu-Ni deposit identified four major lithologic units and several internal ultramafic subunits that can be correlated over two miles. The ultramafic subunits (layers of picrite to peridotite) exhibit relative uniform thicknesses and are present at the same relative elevation within the major lithologic units. The major lithologic units, the same as delineated by Severson and Hauck (1990), and upward from the basal contact are: Unit I, a fine- to coursegrained a sulfide-bearing troctolite to pyroxene troctolite (450 ft. thick) with associated ultramafic layers I(a), I(b), and I(c); Unit II, a medium- to coarse-grained troctolite to pyroxene troctolite (200 ft. thick) with a basal ultramafic layer II(a); Unit III, a finegrained, mottled textured troctolitic anorthosite to anorthositic troctolite (150 ft. thick) with one minor ultramafic layer III(a); and Unit IV, a coarse-grained troctolite/pyroxene troctolite to anorthositic troctolite with associated ultramafic layers IV(a) and IV(b). Most sulfide mineralization occurs within Unit I. Within Unit I the sulfide mineralization is both widespread but variable in modal percentage (rare to 5%), continuity and thickness (few inches to tens of feet). Sulfide mineralization is somewhat related with proximity to: hornfels inclusions, the basal contact with the footwall Virginia Formation, and some of the internal ultramafic layers within Unit I. Precious metal mineralization (Pd+Pt+Au) is associated with fracturing and alteration of the host rocks. The alteration assemblage is chlorite, bleached plagioclase, serpentine and uralite. Pd+Pt values range from 100 to >2400 ppb over 10 foot intervals. These intervals can occur independently as 10 to 50 foot zones, or as part of a larger correlatable occurrence/horizon. Two mineralized subareas within the Dunka Road deposit are: 1) an area which is peripheral to a highly anomalous Pd occurrence (reported by Morton and Hauck, 1987; 1989) herein termed the "southwest area", and 2) the "northeast area" which contains several drill holes that have near surface intercepts of >1% Cu. There are four somewhat large mineralized occurrences within the study area that carry >300 ppb combined total Pt+Pd+Au. These mineralized zones appear to be stratigraphically controlled by the ultramafic subunits within Unit I. Three of the four correlatable zones are found within the southwest area, and range from 40 to 130 feet thick. High Pd values within these zones range from 10 to 20 feet thick with values of 800 to 1650 ppb Pd. In the northeast area, the fourth mineralized zone appears continuously throughout Unit I. This zone ranges from 120 to 300 feet thick. High Pd values within this zone range from 10 to 40 feet thick with values of 800 to 1500 ppb Pd. Many 5 to 30 foot intersections of >1 ppm Pd+Pt+Au occur throughout the mineral deposit. Geostatistical analysis based on 72 vertical holes and 12 angle holes suggests: 1) the base of the complex is a critical datum with the higher grade intercepts located between 100 and 400 feet above the base; 2) high inter-element correlations support local redistribution/concentration of primary mineralization by a secondary hydrothermal process and thus, polymetallic mining selectivity is possible; 3) the available drilling gives a spacial range of geologic influence at 400 foot centers, but sufficient angle drilling is not available to assess the potential of high grade, steeply dipping mineralized zones; 4) additional vertical in-fill drilling will almost certainly not discover any additional quantity of ore within the volume of rock studied; but 5) additional angle drilling to assess the potential of high grade, steeply dipping, mineralized zones would benefit a more complete geostatistical analysis.
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    Geology and Mineralization of the Serpentine Copper-Nickel Deposit
    (University of Minnesota Duluth, 1994-12) Zanko, Lawrence M; Severson, Mark J; Ripley, Edward M
    A geological model explaining the formation of mineralization at the Serpentine Cu-Ni deposit, located at the western edge of the Keweenawan (1.1 Ga) Duluth Complex near Babbitt, Minnesota, has been developed following detailed re-logging of drill core and compilation of geochemical (Cu-Ni-S) data. The model suggests that sulfide mineralization was influenced by a combination of factors, the most important being localized structural preparation of the Lower Proterozoic (1.8 Ga) Virginia Formation footwall. The drill core provides evidence that faulting and the emplacement of sills (both pre-and/or early-Duluth Complex) were responsible for this structural preparation. Both processes led to weakening and/or fracturing of the footwall, thereby allowing greater magma/footwall interaction to take place in a relatively confined area. This intimate interaction enhanced the genesis and distribution of Cu-Ni sulfide mineralization at the Serpentine deposit. A laterally extensive bedded pyrrhotite unit of the Virginia Formation is believed to have further contributed to the formation of semi-massive to massive sulfides present within the deposit by acting as a local source of additional sulfur (and minor additional Cu and Ni). Additional sulfur isotope work is necessary before an in situ sulfur source can be ruled out, however. A very general igneous stratigraphy has been determined for the Serpentine deposit, based on the idea that at least three distinct magmatic events and/or intrusive styles were responsible. The first (and earliest) contributed to the deposit's sulfide mineralization, due to its extensive interaction with the footwall; its rocks are a heterogeneous, gradational mixture of fine- to medium-grained troctolite, augite troctolite, and norite, loosely confined to the lowest one-half to two-thirds of the deposit. The second event produced rocks of greater homogeneity that are more plagioclase-rich, coarser grained, and relatively sulfide-poor; spatially, they tend to overlie those of the first. A third, but much less volumetrically significant, magmatic event and/or intrusive style probably occurred shortly after the second and was likely related to a faulting episode; it formed rocks that are predominantly ultramafic. Spatially, these ultramafic rocks are largely associated with the more homogeneous plagioclase-rich rocks, frequently occurring as linear sub-vertical bodies. Analytical work shows the platinum group element (PGE) potential of the Serpentine deposit is somewhat limited. The highest value (274 ppb rhenium) occurred in a massive sulfide sample. Only three samples contained more than 100 ppb palladium. Based on these results, the mechanisms and/or conditions for PGE enrichment that were present at the Local Boy deposit were lacking at the Serpentine deposit. A possible explanation is that the Serpentine deposit received additional sulfur from a local source, while the Local Boy deposit formed from a sulfide melt that: 1) formed elsewhere; 2) scavenged more PGEs from a more primitive(?) magma; and 3) moved an undetermined distance before being injected into structure zones, without the addition of local sulfur. Future investigations may shed more light on the apparent differences between these two deposits.