Browsing by Subject "Pyrrhotite"

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    Geology and Mineralization of a Cyclic Layered Series, Water Hen Intrusion, St. Louis County, Minnesota
    (University of Minnesota Duluth, 1990-03) Strommer, James; Morton, Penelope; Hauck, Steven A; Barnes, Randal J
    The Water Hen intrusion is an oxide-bearing (ilmenite + magnetite) ultramafic intrusion (OUI) that is emplaced along a pre-basement fault into the troctolitic series rocks of the Duluth Complex. The intrusion consists of medium-grained dunite and peridotite and local pegmatitic pyroxenite approximately 1,600 ft. x 500 ft. x 700 ft. in size. Oxide (>90% ilmenite) composes from 5-50% of the various lithologies. Sulfides are minor, about 2-5%, and are predominantly pyrrhotite with minor cubanite, chalcopyrite and pentlandite. Concentrations of 5-80% graphite also occur within the intrusion. Surrounding the Water Hen intrusion is a zone of mixed lithologies (Mixed Zone) consisting of the host rock troctolites, apophyses of OUI and local inclusions of footwall rocks. The Mixed Zone (M) is dominated by >60% troctolitic rocks with OUI composing the remainder. The OUI apophyses vary from 1-50 ft. thick and have sharp contacts with the country rock. The troctolitic host rocks for the Water Hen intrusion consist of medium- to coarsegrained troctolite to anorthositic troctolite (TA unit) and a troctolitic cyclically layered series (TL unit). The cyclically layered series is similar to troctolitic layered rocks at Bardon Peak. The individual cyclic layers are 6 in. to 10 ft. in thickness and the entire unit is over 300 ft. thick. The An content decreases from An80 at the bottom of the unit to An60 near the top of the unit. The individual cyclic layers are composed of ilmenite-dunite at the base and grade upward to anorthositic troctolite. The bottom contacts are sharp and each successive layer within the individual unit is identified by the occurrence of biotite or clinopyroxene. In the bottom olivine-rich layer, the oxides (<5%) are ilmenite >> magnetite. The sulfides in this same layer (3-5%) are coarse-grained with cubanite > chalcopyrite > pentlandite >> pyrrhotite. In the more feldspathic layers, the sulfides (1-3%) are fine-grained with chalcopyrite >> pentlandite = cubanite + pyrrhotite. The oxides (1- 5%) are also fine-grained with ilmenite >> magnetite. The footwall rocks in the Water Hen area consist of very fine-grained metamorphosed Virginia Formation and fine-grained hornfelsed basalt and/or troctolite. There are >100 ft. of basalt or chilled margin rocks within the footwall. This mafic hornfels commonly occurs between the Virginia Formation and the TA unit. Orthopyroxenite dikes and dikelets also occur in the mafic hornfels. These dikes contain anomalous PGEs and secondary sulfide mineralization. The copper-nickel sulfides are primary igneous sulfides associated with the troctolitic rocks. Violarite, pyrite and secondary magnetite in cross-cutting veinlets and other secondary sulfides indicate that the primary sulfides were altered and remobilized by a later event. Cu:Ni ratios have a bimodal distribution that is not followed by the PGEs. However, Cu, Ni, Ag, Au, Pt, Pd are all highly correlated with each other. This high interelement correlation suggests that the late-stage (secondary) remobilization locally redistributed and reconcentrated these elements.
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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.
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    Geology, Mineralization, and Geostatistics of the Minnamax/Babbitt Cu-Ni Deposit (Local Boy Area), Minnesota: Part II: Mineralization and Geostatistics
    (University of Minnesota Duluth, 1991-06) Severson, Mark J; Barnes, Randal J
    The Minnamax/Babbitt Cu-Ni deposit, located within the Partridge River Troctolite Series (PRTS) of the Duluth Complex, northeastern Minnesota, contains both troctolite-hosted disseminated ore and footwall-hosted massive sulfide ore. This report pertains to the massive sulfide ore zone, which is restricted to a small portion of the deposit, and is referred to as the Local Boy area. Studies conducted in the Local Boy area include: 1) detailed geologic relogging of drill core; 2) sulfide petrography and microprobe analysis; 3) assaying for Pt, Pd, Au, and Ag in the high-grade Cu ore zones; and 4) geostatistical analysis of the Cu-Ni ore (plus PGEs and precious metals). Detailed relogging of 76 underground drill holes, along with pertinent surface drill holes, has been completed within the Local Boy area (from drifts B, C, and D). The data indicates the highly undulatory nature of the basal contact of the Duluth Complex with the footwall Virginia Formation. Intrusive rocks of the Duluth Complex (Unit I of the PRTS) consist of augite troctolite, troctolite, and norite. All exhibit gradational contacts with each other, and all may occur at any stratigraphic position relative to the undulatory basal contact. However, norite is the most common rock type adjacent to sedimentary hornfels inclusions and at the basal contact due to contamination of the magma. The spatial configuration of the intrusive rocks indicates that Unit I was intruded as multiple pulses along bedding planes of the Virginia Formation. The Virginia Formation hosts the majority of the massive sulfide ores that are present within hornfels inclusions positioned above the basal contact, and within the footwall rocks at and below the basal contact. Massive sulfide ore is not as common within the intrusive rocks, and when present, is generally associated with, or in close proximity to, hornfels inclusions. Ore/host rock textures are extremely varied, but all are indicative of structural control in the footwall rocks. Overall, the massive sulfide ores are spatially distributed in a spotty manner in an east-west (EW) direction that corresponds to a major EW-trending anticline present within the footwall rocks. All these factors suggest that an immiscible sulfide melt was injected into structurally prepared footwall rocks along the anticlinal axis in a "vein-like" setting. At some later period, the footwall-hosted massive sulfide ore zone was re-intruded by multiple sills (which collectively make up a portion of Unit I) along bedding planes of the Virginia Formation. The end result is a disjointed zone of mineralized inclusions and mineralized footwall rocks separated by "barren" intrusive rocks. Sulfide textures indicate that the sulfides formed by cooling of a monosulfide solid solution (MSS) followed by limited replacement at very low temperatures. Minerals contained within the sulfide ore are dominantly pyrrhotite, chalcopyrite, cubanite, and pentlandite. Locally present are maucherite, sphalerite, bornite, talnakhite, mackinawite, and an unknown Cu-sulfide ("Cp"). Also present in minor amounts are native silver (primary and secondary), parkerite, chalcocite, covellite, godlevskite, violarite, magnetite, and zincian hercynite. Although no discrete PGE minerals were identified, analytical results of the high-grade (>1% Cu) massive sulfide ore confirms the presence of several anomalous PGE values. These spot values are mainly confined to an EW-trending zone that also roughly corresponds to the EW-trending anticline. Maximum values obtained within the Local Boy massive sulfide ores include: Pd = 11,100 ppb; Pt = 8,300 ppb; Au = 10,900 ppb; and Ag = 34 ppm. Native silver (primary) was found within several maucherite grains in this investigation, and PGE mineral inclusions have previously been found in maucherite (Ryan and Weiblen, 1984). Generally, the drill holes that contain the anomalous PGE values also contain the native silverbearing maucherite; whereas, homogeneous maucherite is more characteristic of drill holes with little to no anomalous PGE values. This suggests that PGEs were scavenged from the sulfide melt by early-formed maucherite, and thus the PGEs are related to a primary (magmatic) process. However, a hydrothermal origin for the PGEs is also indicated. Anomalous PGE values are commonly associated with Cl-drop encrusted massive sulfide drill core. The spatial distribution of the Cl-drop encrusted drill core also coincides with the EW-trending anticline. Presence of the Cldrops indicates that the rocks of the Local Boy area were invaded by Cl-bearing solutions that may have been capable of transporting and concentrating PGEs. Therefore, both primary/magmatic (sulfides injected into a "vein-like" setting) and later secondary/hydrothermal processes appear to have been factors in controlling PGE distribution in the Local Boy area. However, it is difficult to separate the primary and secondary processes. This is due to the coincidence of several features within the EW-trending zone, which include: 1) anticline in the footwall rocks; 2) overall massive sulfide spatial distribution; 3) spatial distribution of anomalous PGE values; and 4) spatial distribution of Cl-drop encrusted core. Reactivation of structures that controlled the initial "vein-like" massive sulfide distribution could have been responsible for channeling later hydrothermal solutions. Geostatistical analysis of the underground drill holes (Drifts B, C, and D), and pertinent surface drill holes, yields five main conclusions: 1) the top of the Biwabik Iron-formation (BIF) is a critical datum, with the higher grade Cu-material located between 100 and 400 feet above the BIF (mainly within the Virginia Formation near the basal contact); 2) inter-variable correlations between Cu and Ni are high, indicating that selective mining of Cu and Ni is physically possible; but, selection on ore grade Cu and Ni will not necessarily capture all the ore grade PGEs and other precious metals; 3) the available drilling gives a spacial range of geologic influence of about 150 feet; 4) potentially economic ore reserves do exist in the Local Boy area; and 5) the property is under-valued due to the inclusion of many "barren" (unassayed) intervals into the compositing process. A coarse block model, and in situ geologic reserves, are presented for the Local Boy area.