Geology topics

E.M. Ripley

Publications and source records attributed to E.M. Ripley.

5 recordsLinked to original sources

USGS2007

Layered intrusions of the Duluth Complex, Minnesota, USA

The Duluth Complex and associated subvolcanic intrusions comprise a large (5,000 km 2 ) intrusive complex in northeastern Minnesota that was emplaced into comagmatic volcanics during the development of the 1.1 Ga Midcontinent rift in North America. In addition to anorthositic and felsic intrusions, the Duluth Complex is composed of many individual mafic layered intrusions of tholeiitic affinity. The cumulate stratigraphies and cryptic variations of six of the better exposed and better studied intrusions are described here to demonstrate the variability in their cumulus mineral paragenesis. Although the general paragenetic sequence is: O1(±Pl) → Pl+Ol → Pl+Cpx+Ox(±Ol±Opx) → Pl+Cpx+Ox+Ap(±Ol) considerable differences exist among the six intrusions in the relative order and timing of cumulus arrivals, most notably with regard to augite (Cpx) and Fe-Ti oxide (Ox). The various cumulate stratigraphies and cryptic variations represented by the six intrusions described here largely reflect differences in the degree of open-system behaviour to recharge, eruption, and country-rock assimilation, but also may have been influenced by differences in parent magma composition, in the efficiency of fractional crystallization, and in the conditions of crystallization ( e.g. P total , f O2 P H2O ). The Sonju Lake intrusion, wherein cumulus augite arrived before ilmenite, formed by essentially closed-system fractional crystallization. However, its compositional evolution may have been affected to an uncertain degree by assimilation of a granitic hanging-wall. The monotonous, thick troctolitic cumulate sequences of the lower parts of the South Kawishiwi and Partridge River Intrusions appear to represent the effects of frequent magma recharge coupled with in situ (or boundary layer) crystallization though other interpretations have been proposed. Extensive Cu-Ni sulphide mineralization at the base of these intrusions is attributed to country-rock contamination of the earliest intruded magmas. The cyclical progression of cumulates in the Layered Series at Duluth, wherein cumulus augite and oxide arrived nearly simultaneously, formed in a moderately open system characterized by periodic eruption and recharge. The Wilder Lake Intrusion is different from other intrusions in that cumulus ilmenite appears before augite and that olivine and augite composition define an inverted cryptic variation. The latter phenomenon may be related to a strong upward gradation toward lesser amounts of trapped liquid in the cumulates. Finally, the incomplete cumulate stratigraphy of the Bald Eagle Intrusion, which is composed of approximately equal thicknesses of troctolite (PO) and gabbro (PAO) adcumulates, is unique in that Fe-Ti oxide did not arrive as a cumulus phase despite prolonged crystallization of cumulus augite. These different cumulus parageneses probably produced a variety of derivative magmas, which may have contributed in part to the compositional diversity of the Midcontinent rift volcanics.

Developments in Petrology↗

USGS2007

Textural, mineralogical and stable isotope studies of hydrothermal alteration in the main sulfide zone of the Great Dyke, Zimbabwe and the precious metals zone of the Sonju Lake Intrusion, Minnesota, USA

Stratigraphic offsets in the peak concentrations of platinum-group elements (PGE) and base-metal sulfides in the main sulfide zone of the Great Dyke and the precious metals zone of the Sonju Lake Intrusion have, in part, been attributed to the interaction between magmatic PGE-bearing base-metal sulfide assemblages and hydrothermal fluids. In this paper, we provide mineralogical and textural evidence that indicates alteration of base-metal sulfides and mobilization of metals and S during hydrothermal alteration in both mineralized intrusions. Stable isotopic data suggest that the fluids involved in the alteration were of magmatic origin in the Great Dyke but that a meteoric water component was involved in the alteration of the Sonju Lake Intrusion. The strong spatial association of platinum-group minerals, principally Pt and Pd sulfides, arsenides, and tellurides, with base-metal sulfide assemblages in the main sulfide zone of the Great Dyke is consistent with residual enrichment of Pt and Pd during hydrothermal alteration. However, such an interpretation is more tenuous for the precious metals zone of the Sonju Lake Intrusion where important Pt and Pd arsenides and antimonides occur as inclusions within individual plagioclase crystals and within alteration assemblages that are free of base-metal sulfides. Our observations suggest that Pt and Pd tellurides, antimonides, and arsenides may form during both magmatic crystallization and subsolidus hydrothermal alteration. Experimental studies of magmatic crystallization and hydrothermal transport/deposition in systems involving arsenides, tellurides, antimonides, and base metal sulfides are needed to better understand the relative importance of magmatic and hydrothermal processes in controlling the distribution of PGE in mineralized layered intrusions of this type. ?? Springer-Verlag 2007.

Mineralium Deposita↗

USGS2004

Stable isotopic constraints on fluid-rock interaction and Cu-PGE-S redistribution in the Sonju Lake intrusion, Minnesota

The Sonju Lake intrusion, part of the 1.1 Ga Midcontinent rift-related Beaver Bay Complex, is a 1,200-m-thick, strongly differentiated, layered sequence of mafic cumulates located in northeastern Minnesota. Basal melatroctolite and dunite layers are overlain by troctolite, gabbro, Fe-Ti oxide-rich gabbro, apatite diorite, and monzodiorite. Stratigraphic intervals rich in Pt + Pd, Cu, and S occur over ???500 m in the Fe-Ti oxide-rich gabbro and apatite diorite units. Peak concentrations show offsets that are similar to those found in other tholeiitic layered intrusions. Concentrations of Pd in excess of 100 ppb are confined to the lowermost 25 m of the interval. Copper shows a sharp increase to 630 ppm above the Pd-rich interval. Sulfur contents are low (<375 ppm) in the Cu-rich interval, but they increase to values as high as 3,150 ppm above in the apatite diorite. Disseminated sulfides in the intrusion have ??34S values that range from -2.2 to 3 per mil Vienna-Canyon Diablo Troilite (V-CDT) and suggest that contamination by country rock sulfur was not an important process in the formation of the metal-rich interval. ??18O values of plagioclase from the intrusion range from 5.6 to 12.0 per mil (V-SMOW) and indicate that a relatively low-18O fluid (??18O ???3-5 ???) interacted with the rocks of the intrusion at temperatures less than ???275??C. Clinopyroxene and Fe-Ti oxides (ilmenite with minor amounts of titanomagnetite) show much more restricted ranges in ??18O values (4.6-5.7 and 5.5-6.7 per mil, respectively) and attest to the kinetic control of the oxygen isotope exchange process. The externally derived fluid that interacted with rocks now enriched in platinum group elements (PGE) + Cu- and Fe-sulfide minerals locally liberated sulfur and replaced chalcopyrite and pyrite with goethite. In the Cu-rich zone, goethite that replaces chalcopyrite may contain up to 8.5 weight percent Cu. It is evident that hydrothermal alteration resulted in a decoupling of copper and sulfur, with sulfur being transferred out of the Cu-rich interval. Interaction between rocks in the PGE-Cu-S interval of the Sonju Lake intrusion and an externally derived fluid at low temperatures modified what appears to have been a primary stratigraphic metal-sulfur zonation. The effects of hydrothermal alteration on PGE and base-metal sulfide mobility and redistribution must be understood before models of primary zonation processes can be meaningfully applied. ?? 2004 by Economic Geology.

Economic Geology↗

USGS1998

Geodynamics of magmatic Cu-Ni-PGE sulfide deposits: New insights from the Re-Os isotope system

In this study, we reassess crustal contamination and sulfide ore-forming processes in some of the largest magmatic ore deposits, using published Re-Os isotope data and a modeling methodology that incorporates the R factor, defined as the effective mass of silicate magma with which a given mass of sulfide magma has equilibrated, in an Re-Os isotope mixing equation. We show that there is less disparity between conclusions based on Re-Os isotope data compared to other isotopic systems if the R factor is considered. Komatiite-associated Ni sulfide ore systems typically have high Os concentrations. low Re/Os ratios, and near-chondritic initial Os isotope compositions. For magmatic sulfide ores that are interpreted to have experienced relatively low R factors (<500), such as those at Kambalda, this severely limits the amount of wholesale crustal contamination as well as selective contamination of komatiitic magma by sedimentary sulfide components. The Re-Os geochemistry of these ores is most consistent with derivation from uncontaminated komatiites unless R factors are much higher (<2,000). Sulfide saturation in these ore systems may, therefore, have been achieved via changes in intensive parameters of the komatiite lavas cooling or decompression) or changes in compositional parameters transparent to the Re-Os isotope system (e.g., f O2 /f S2 /f H2O ). Basalt-gabbro-associated Cu-Ni sulfide ore systems at Duluth. Sudbury, and Stillwater are quite distinct from those at Kambalda by having comparatively low Os concentrations, high Re/Os ratios, and high initial Os isotope compositions. These chemical and isotopic characteristics are indicative of significant interactions between their parental basaltic magmas and old crust because there are no known mantle reservoirs with such extreme geochemical characteristics. Our modeling suggests that for Cu-Ni sulfide ores at Duluth, Sudbury, and Stillwater to maintain the observed high initial Os isotope compositions inherited from a crustal contaminant, R factors for these systems must have been low (<10,000), consistent with their low metal concentrations. Thus, we interpret this style of base metal sulfide mineralization to be derived from crustally contaminated but less dynamic magmatic systems that did not permit extensive equilibration of sulfide magma with silicate magma. For basalt-gabbro-associated Cu-Ni-PGE-rich sulfide ore systems that have Re-Os geochemical characteristics more similar to those associated with komatiites, R factors must have been high (>10,000 for Noril'sk-Talnakh and the J-M reef, Stillwater Complex). In these very dynamic magmatic ore systems, crustal contamination processes are more difficult to assess using Re-Os isotopes as the effects of contamination are masked by the R factor process in which sulfide magma equilibrates with extensive amounts of asthenospheric mantle-derived magma. Sulfide protore for these systems may, then, have been very radiogenic and of crustal origin prior to R factor processes that occurred during transport in feeder conduits and in upper crustal magma chambers. This study, therefore, highlights the need for caution when interpreting the Re-Os isotope geochemistry of sulfide ores from dynamic magmatic systems. The results of our reinvestigation of these giant ore deposits suggest that geodynamic processes associated with large magmatic systems, including major lithospheric pathways to the surface, changes in flow regime, coupled magma flow-through and magma mixing (providing enhanced R factors), may be critical to our understanding of the emplacement, localization, and quality of magmatic sulfide deposits. Thus, the timing and exact mechanism of sulfide saturation may be subordinate to dynamic magmatic processes in the localization of economic concentrations of magmatic sulfides.

Economic Geology↗

USGS1990

Sulfur isotope studies of the Stillwater Complex and associated rocks, Montana

The Stillwater Complex in south-central Montana is a late Archean layered, ultramafic to mafic intrusion emplaced into middle to late Archean metagraywacke, metashale, and ironformation. Sulfide minerals are concentrated near the base of the intrusion in some chromitite layers, podiform pegmatoids, discontinuous discordant pods or pipes, and numerous thin, stratiform layers. This investigation focuses on the isotopic composition of sulfur in metamorphosed sedimentary rocks, Stillwater-associated sills and dikes, sulfide accumulations near the base of the complex, and platinum-group element-enriched sulfide mineralization that constitutes the J-M reef and the Picket Pin deposit.The delta 34 S values for 233 samples analyzed in this study have a mean of 0.4 per mil, a standard deviation of 1.7, and a maximum and minimum of-3.8 and 7.8 per mil, respectively. Despite the very narrow range of values, analysis of variance calculations confirms that the data comprise three statistically distinct groups. These groups are (1) metagraywackes and metashales (mean = 2.8ppm); (2) iron-formation, Stillwater-associated sills and dikes, and basal sulfide accumulations in the complex (mean = -0.7ppm); and (3) disseminated sulfide minerals that occur above the basal accumulations in the Stillwater Complex (mean -- 1.2ppm). The isotopic composition of sulfur in the basal sulfide accumulations indicates that assimilation of substantial amounts of sulfur from iron-formation may have occurred. The low tenor of the basal sulfide ores is also consistent with assimilation of sulfur into a limited volume of magma near the base of the intrusion.

Economic Geology↗