Browsing by Subject "Augite troctolite"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item Geology, Geochemistry, and Stratigraphy of a Portion of the Partridge River Intrusion(University of Minnesota Duluth, 1990-03) Severson, Mark J; Hauck, Steven ADetailed relogging of drill holes (83 holes totalling 100,630 feet of core) and reconnaissance mapping have delineated three major rock groups within a portion (T.58-59 N., R.13-14 W.) of the Partridge River intrusion (PRI), Duluth Complex, Northeastern Minnesota. These have been informally designated as the Partridge River Troctolitic Series (PRTS), Partridge River Gabbro Complex (PRGC) and Oxide-bearing Ultramafic Intrusions (OUI). The PRTS consists of at least eight major igneous units which are correlatable in drill holes over an indicated eleven mile strike length extending (NE to SW) from the Dunka Road Cu-Ni deposit to the Wyman Creek Cu-Ni deposit. From the base up, these units are characterized by: Unit I - sulfide-bearing augite troctolite with minor picrite to peridotite layers; Unit II - troctolite and augite troctolite, with abundant picrite to peridotite layers (Wetlegs Cu-Ni area) and/or minor sulfide-bearing zones; Unit III - mottled textured anorthositic troctolite exhibiting a highly irregular olivine oikocryst distribution; Unit IV -augite troctolite with a picritic base and grading upwards into Unit V; Unit V - coarse-grained anorthositic troctolite; Unit VI - augite troctolite to anorthositic troctolite with a picritic base; and Unit VII - augite troctolite with a well-bedded peridotite-picrite base. Field mapping suggests that an eighth unit (Unit VIII) and possibly additional units are present above Unit VII. Unit VIII consists of troctolite to anorthositic troctolite with a well-bedded peridotite base. Most of the upper units (III-VIII) represent single cooling units in that they are floored by a bedded ultramafic member; whereas, other units (I and II) near the footwall exhibit an overall heterogeneous nature and contain abundant internal members reflecting continuous magma replenishment. Some of the units also exhibit downcutting relationships and lateral "facies" changes along strike indicating a complex intrusive history. Structural studies of the basal contact of the Partridge River intrusion have indicated more structure than previously recognized. Structure contour maps of the footwall rocks at the basal contact of the Duluth Complex and on the top of the Biwabik Iron-Formation, and isopach maps of the Virginia Formation beneath the PRI indicate that pre-existing folds in the basement rocks at both Minnamax and Dunka Road exerted a strong control over the form of the base of the intrusion. Cross-sections illustrating the internal "stratigraphy" indicate that in both the Dunka Road and Wetlegs areas, numerous NE-trending normal faults parallel to the Mid-continent Rift are present. These faults support the halfgraben model (Weiblen and Morey, 1980) which envisions a step-and-riser geometry at the base of the Duluth Complex due to extensional tectonics. However, most of the faults delineated show corresponding offsets in both the troctolitic and footwall rocks and are, thus, not true half-graben faults as envisioned in the model. The only exception is within the Wetlegs area where a NE-trending fault exhibits substantial offset in the footwall rocks, but no offset is present in the overlying troctolite rocks. An inferred window of Biwabik Iron- Formation is in direct contact with the PRI along this fault. Three late-stage Oxide-bearing Ultramafic Intrusions (OUI) are also located along this zone that suggests they may be genetically related to areas where massive iron-formation assimilation has occurred. The OUIs are later pegmatitic intrusives consisting of dunite, peridotite, clinopyroxenite, and lesser picrite and melagabbro; all are oxide-bearing (> 10%) and contain semi-massive to massive oxide horizons. These bodies are intrusive into the PRTS and include the Longnose, Longear, Section 17, Wyman Creek, and Skibo Fe-Ti prospects. The PRGC is situated at the southeastern portion of the investigated area and consists dominantly of oxide-bearing gabbroic and troctolitic rocks; both locally exhibit excellent modal bedding, which may be related to magmatic density currents. The Colvin Creek "Gabbro" (CCG) is part of the PRGC and was originally interpreted to be a hornfelsed basalt. However, reconnaissance mapping indicated that similar fine-grained CCG-type "gabbro" is present within the coarse-grained rocks of the Powerline Gabbro and vice versa. Because the Powerline Gabbro is located near the CCG, the two bodies may be intricately related. Within the Colvin Creek "Gabbro" are several unusual sedimentary-like structures that are not indicative of typical North Shore Volcanic basalts. However, textures resembling vesicles/amygdules are locally present. The unusual sedimentary-like structures suggest a magmatic density current origin but the exact origin of these textures is enigmatic. Also within the Colvin Creek "Gabbro" is a mile-long 1,000 foot-thick belt of cross-bedded rocks. Several internal features of these cross-bedded rocks, e.g., lack of rock fragments, no quartz, are not indicative of typical interflow sandstones and their relationship to the surrounding rocks suggests they may have also been deposited by magmatic density currents. The unmineralized portions of all the units were sampled (155 samples) in order to establish background geochemical levels and lithogeochemical signatures for each unit and to investigate possible origins for the different units. Background Pd, Pt, and Au values in the major rock groups average 10 ppb, 20 ppb, and 5 ppb, respectively. However, slightly elevated background values are associated with Unit II (15 ppb, 24 ppb, and 9 ppb, respectively), and the OUI rock group (15 ppb, 24 ppb, and 17 ppb respectively). In the course of sampling unmineralized rock (<1% sulfides), five anomalous samples (>200 ppb combined Pd and Pt) were revealed with a maximum of 910 ppb. The OUI units are the most geochemically unique in that they have elevated background values for TiO2, V, Cr, Co, Cu, Cd, C, Be, Sc, Sb, Pb, Te, Au, and W relative to the other igneous units. Geochemical data support the various rock units identified during relogging of the PRI. Units I and II exhibit a markedly different geochemical signature when compared to the other PRTS units. One interpretation of this difference is that magma contamination due to assimilation of footwall material was important in their genesis. All rock units of the PRGC have the same geochemical signature and, in turn, this geochemical signature is similar to the geochemical signature for the lower half of Unit I. The OUI units exhibit a markedly different geochemical signature when compared to all the other PRI units.Item Geology, Mineralization, and Geostatistics of the Minnamax/Babbitt Cu-Ni Deposit (Local Boy Area), Minnesota: Part I: Geology(University of Minnesota Duluth, 1991-06) Severson, Mark JThe Minnamax Cu-Ni deposit (also called the Babbitt deposit) is situated within what has been informally referred to as the Partridge River intrusion (or Partridge River Troctolite) of the Duluth Complex (1.1 Ga), northeastern Minnesota. The deposit has been subdivided into five contiguous ore zones; the Local Boy area and Bathtub area are two ore zones described in this report. Within the deposit are a wide variety of troctolitic, ultramafic, and footwall rock types, and hornfelsed inclusions (both footwall and hanging wall). Many specific rock types are correlative between drill holes and can be grossly categorized into seven sub-horizontal troctolitic units, three types of hornfelsed inclusions, and a late cross-cutting pegmatitic phase. Also present are correlative units within the footwall rocks. All rock units were identified by detailed relogging of 61 surface drill holes (117,605 feet of core) and are portrayed on nine cross-sections that extend through various portions of the Minnamax deposit. Severson and Hauck (1990) described the stratigraphy of the troctolitic rocks of the Partridge River Troctolite to the west of the Minnamax deposit; the stratigraphy is referred to as the Partridge River Troctolite Series (PRTS). Most of the PRTS rock units defined at the Dunka Road Cu-Ni deposit (located to the immediate SW of Minnamax) by Severson and Hauck (1990) are present at Minnamax. However, the overall picture at Minnamax is more complicated than Dunka Road due to rock type changes that are manifested by: 1) pinch-out and reappearance of specific marker bed units; 2) down-strike gradational changes of ultramafic horizons; 3) extremely limited areal extent of some ultramafic horizons; and 4) gradational changes in the troctolitic rock types between drill holes. In some areas a particular marker horizon may "disappear" laterally and then reappear at the same stratigraphic level in another group of drill holes. In spite of these local difficulties, a gross stratigraphy of seven subhorizontal igneous units is present at Minnamax and consists of (from bottom to top): Unit I - heterogeneous, sulfide-bearing augite troctolite and troctolite with abundant metasedimentary inclusions; Unit II - homogeneous troctolite with a basal picrite horizon (Unit II is present only in the SW portion of the Minnamax deposit); Unit III - mottled textured anorthositic troctolite to troctolite with characteristic olivine oikocrysts (Unit III is present mainly in the SW portion of Minnamax and is enveloped by Unit I to the NE); Unit IV - mixed homogeneous troctolite and augite troctolite (augite troctolite is at the top of Unit IV in localized areas) with a semipersistent basal ultramafic horizon termed the "± picrite"; Unit V - homogeneous anorthositic troctolite that exhibits a gradational contact with Unit IV; and Units VI and VII - homogeneous troctolites with persistent basal ultramafic horizons. More abundant and thicker ultramafic horizons are present in Units VI and VII in the Bathtub area of the Minnamax deposit. Specific marker horizons utilized in drill hole correlations include: Unit III, "± picrite," "pocket picrite," top of Unit IV (augite troctolite), and the ultramafic base of Units VI and VII. The troctolitic stratigraphy is cut by pegmatitic orthopyroxenite and peridotite bodies that are referred to as OUI - Oxide-bearing Ultramafic Intrusions. Pegmatitic hybrid hornblendite and granophyre also cut the stratigraphy and are often related to the OUI bodies. Rusty chlorine-rich drops may commonly coat the core of the ultramafic horizons and OUI bodies. Several enigmatic hornfelsed inclusions are present in Units VI and VII at Minnamax. These are grouped in two categories that include: 1) CC-type inclusions that are similar to outcrops of the Colvin Creek hornfels; and 2) "pic"-type inclusions that are similar to nearby outcrops of basalt inclusions. Both inclusion types are similar in that they contain fine-grained plagioclase-filled ovoids or wisps that may represent vesicles, and they exhibit the same chemical signature. However, they exhibit a different mineralogy (the CC-type inclusions are oxide-rich). Their stratigraphic position in the troctolitic rocks suggests that they are probably hanging wall material (North Shore Volcanic Group). While these two inclusion types are readily correlative between drill holes, the nature of their different mineralogy remains unknown. Another enigmatic rock type is present within the lower portion of the Virginia Formation footwall rocks. The rock is unique in that it contains hornblende ± olivine and locally grades into serpentinized picrite with hornblende. It is generally concordant with the overall bedding of the Virginia Formation and is referred to as the sill(?) unit. Whole rock geochemistry indicates that this unit locally exhibits: high Cl contents that are similar to Cl values of ultramafic horizons in the troctolitic rocks; MG numbers that are more primitive than the ultramafic horizons; and high Cr contents that are much higher than anything sampled in the overlying troctolitic section. If the unit was a sill, it now exhibits gradational contacts with the metasedimentary rocks and is characterized by a granoblastic texture with superimposed euhedral hornblende. These data may indicate that the sill was intruded before, and hornfelsed during, emplacement of the majority of the Partridge River Troctolite Series.Item Igneous Stratigraphy of the South Kawishiwi Intrusion, Duluth Complex, Northeastern Minnesota(University of Minnesota Duluth, 1994-12) Severson, Mark JThe Middle Proterozoic (1,099 Ma) intrusive Duluth Complex contains numerous smaller sub-intrusions that collectively comprise the Complex. Two of these sub-intrusions are informally known as the South Kawishiwi intrusion (SKI) and Partridge River intrusion (PRI). A correlative igneous stratigraphy has been documented in the PRI by Severson and Hauck (1991) and Severson (1991). In this investigation, detailed relogging of drill holes within the SKI (136 drill holes totalling 214,461 feet of core) also defines an intrusion-wide stratigraphic sequence along a 19-mile strike length that is referred to as the South Kawishiwi Troctolite Series (SKTS). The stratigraphic sequences of the SKI and the PRI are completely dissimilar. At least 17 correlative subhorizontal igneous units are defined within the SKTS; however, they are not equally present in all areas of the SKI. The SKTS units, from the bottom to the top (roughly), are referred to as: BAN = Bottom Augite troctolite and Norite; BH = Basal Heterogeneous troctolites (sulfide-bearing); U3 = Ultramafic Three (sulfide-bearing); PEG = Pegmatitic Unit of Foose (1984); U2 = Ultramafic Two (sulfide-bearing); U1 = Ultramafic One (sulfide-bearing); AT-T = homogeneous Anorthositic Troctolite to Troctolite; UW = Updip Wedge (sulfide-bearing); LOW AGT = homogeneous Lower Augite Troctolite zone; MAIN AGT = homogeneous Main Augite Troctolite zone; AT&T = homogeneous Anorthositic Troctolite and Troctolite; AT(T) = homogeneous Anorthositic Troctolite with lesser amounts of Troctolite; AN-G Group = intermixed Anorthositic and Gabbroic rocks; UPPER GABBRO = oxide-bearing gabbroic rocks; "INCL" = large shallow-dipping inclusion of magnetic basalt(?); UPPER PEG = Upper Pegmatitic zone; and T-AGT = Troctolite to Augite Troctolite (the latter five units are restricted to a small area referred to as the Highway 1 Corridor area). The lowest units of the SKTS are the most varied with respect to textures, rock types, and sulfide content. They are very unevenly distributed along the strike length of the SKI in a "compartmentalized" fashion suggesting a complicated intrusive history. The lowest units were emplaced early into several restricted magma chambers via repeated and close-spaced magmatic pulses. The effects of contamination from assimilated and devolatized country rocks were the most pronounced in these units. Three ultramafic-bearing packages (U1, U2, and U3 units) are also present within the lower portion of the SKTS. These units are characterized by alternating layers of troctolitic and ultramafic (olivine-rich) rock. The ultramafic-bearing units represent periods of rapid and continuous magma injection that crystallized more primitive ultramafic layers before mixing with the resident magma. The U3 Unit is the most unique in that it contains several massive oxide (magnetite-rich) pods along its strike length. An empirical relationship between the U3 Unit and the Biwabik Iron-formation (BIF) suggests that the massive oxides were derived from intruded and assimilated BIF. The U3 Unit also contains the majority of high Platinum Group Elements (PGE) values sampled to date in the SKI. In contrast, the upper SKTS units reflect an entirely different intrusive history. Each unit is characterized by monotonous sequences of texturally homogeneous and sulfide-free rocks. Gradational contacts are present between each of the units. In contrast to the lower SKTS units, the upper SKTS units are generally distributed throughout the SKI. Ultramafic members are restricted to only two thin horizons referred to as High Picrite #1 and #2. All of these features are indicative of a quiescent and open magmatic system. Thus, the upper SKTS units appear to have been emplaced as widely-spaced pulses into a progressively developed, single magma chamber with little interaction with the country rocks. An entirely different package of rocks is present in six extremely deep drill holes that were drilled within the Highway 1 Corridor (H1C). The H1C represents a large inclusion of older Anorthositic Series rocks. It appears that the H1C rocks were underplated by the younger intruding SKI magmas. Several drill holes within the PRI were also relogged to better understand the nature and location of the contact between the SKI and PRI. One important feature noted is that as the contact between the two intrusions is approached, the upper units of the PRI become heterogeneous and indistinguishable from each other. The same heterogeneity is not evident in the adjacent SKI. The presence of the "heterogenous zone" within the PRI adjacent to the SKI suggests that the PRI was intruded before the SKI. Also present near the PRI/SKI contact zone is a major north-trending fault (down to the east). Associated with this fault are voluminous amounts of steeply-inclined lenses of late granitic/felsic material that cross-cut the PRI stratigraphic section. This fault is named the Grano Fault because the late lenses consist of varied granitic to pyroxenitic material. Though the Grano Fault trends through both the PRI and SKI, the late granitic lenses are not particularly common within the SKI. This fact also suggests that the PRI is older than the SKI. Offset units within the footwall rocks suggests that movement along the Grano Fault was initiated before emplacement of the PRI. All geochemical data pertaining to previously sampled SKI drill core is compiled and correlated with the SKTS units. An additional 80 geochemical samples collected from SKTS units in this investigation are added to this database. The grouping of the SKTS units on the geochemical plots supports the geologic correlations of this investigation. Some of SKTS units also show geochemical overlap with footwall units and indicate the effects of contamination of the magma due to assimilation of the country rocks during intrusion. PGE analyses conducted on a multitude of rock types and igneous units within the SKTS indicate that the U3 Unit, and to a lesser extent the PEG Unit, show the most promise of hosting a PGE deposit. The PGE origin model of Boudreau and McCallum (1992) is invoked to explain why anomalous PGEs are common to the U3 Unit. The Boudreau and McCallum model envisions the upward migration of chlorine-rich, late, magmatic fluids that were capable of transporting PGEs and concentrating them at stratigraphic traps. However, a straightforward application of the model does not explain why significantly higher PGEs are restricted to certain areas, e.g., the Birch Lake deposit. A variation of the "Boudreau and McCallum model" is proposed to explain this difference. This revised model is similar except that upward-moving, Cl-rich, PGE-pregnant hydrothermal solutions are envisioned to have been concentrated in fault zones. When fluids associated with fault zones encountered a proper stratigraphic trap (ultramafic horizon), more PGEs were deposited relative to areas outside of the fault zones. An intersection of the proper stratigraphic trap (U3 Unit with massive oxides, sulfides and high Cr contents) and the proper channelway to concentrate the PGE-pregnant Cl-rich solutions (Birch Lake Fault) reasonably explains the significant PGE values in the Birch Lake deposit. The Birch Lake Fault is defined by a zone wherein drill holes commonly encountered either: massive sulfide mineralization within the footwall granitic rocks; and/or "voluminous" amounts of late granitic/felsic lenses that cut the troctolitic rocks of the SKI.Item Origin and Occurrence of Platinum Group Elements, Gold and Silver in the South Filson Creek Copper-Nickel Mineral Deposit, Lake County, Minnesota(University of Minnesota Duluth, 1990-03) Kuhns, Mary Jo P; Hauck, Steven A; Barnes, Randal JThe South Filson Creek Cu-Ni-PGE-Au-Ag mineral occurrence is located on the western margin of the Duluth Complex in Lake County, northeastern Minnesota. The occurrence of primary magmatic and late-stage, structurally controlled mineralization is located in the South Kawishiwi intrusion of the Duluth Complex, approximately 2200 feet above the basal contact. The primary host rock for the mineralization is a medium-grained augite troctolite. Petrographic studies indicate that there were at least two episodes of mineralization. Deposition of primary, coarse-grained, interstitial pyrrhotite, pentlandite, and chalcopyrite occurred in "cloud zones". Primary mineralization was followed by the introduction of hydrothermal fluids along fracture zones, as evidenced by the formation of hydrous minerals, sulfide replacement textures and geochemical signatures suggestive of remobilization. These late-stage fluids deposited secondary sulfides at redox boundaries created by the primary sulfides. The secondary assemblage includes chalcopyrite, bornite, chalcocite, digenite, covellite, violarite, sphalerite, mackinawite, valleriite, and the platinum group minerals, all which occur in extremely fine, discontinuous veinlets that are rarely recognizable in hand specimen. The veinlets were created by hydrofracturing of silicate minerals due to a volume increase initiated by serpentinization of olivine. These veinlets are always proximal to highly serpentinized fractures and are possibly associated with a proposed NE-trending fault zone along the south branch of Filson Creek. The copper-nickel ratio for the deposit is about 3:1. Platinum + palladium correlates with high copper and sulfur. Also, high inter-element correlation between Cu, Ni, Pd, Pt and Au suggests that secondary enrichment of these elements is local in extent and related to faulting and redox boundaries. Statistical analysis suggests, given the available data, that infill drilling could discover a significant quantity of mineralization. The alteration assemblage associated with the secondary mineralization is serpentine, biotite, stilpnomelane, iddingsite, chlorite, sericite, and clay minerals. The alteration is very subtle and is best recognized in thin section. Both alteration and mineralized zones range in thickness from less than one foot to 90 feet.