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The W Horizon is a thin continuous zone of low sulfide-high-grade platinum group element (PGE) mineralization in the Two Duck Lake gabbro at the Marathon Cu-PGE deposit, Coldwell Complex, Canada, that resembles reef-type PGE deposits found in much larger intrusions. The Marathon deposit was previously shown to have grown by accretion of syn-magmatic, sulfide-bearing sill-like intrusions that formed each of the W Horizon, Main Zone, and Lower Zone mineralization. Each zone can be distinguished by features such as sulfide mineral or platinum group mineral assemblages, or by Cu/Pd, Pd/Se, and Pd/Au values. For example, the W Horizon is defined by the presence of bornite, very high PGE values, low Cu/Pd ( 95% of the original sulfide liquid. In some samples, metal and S abundances indicate that PGE were transported as TABS-PGE phases. This work describes an example of thin PGE-rich sill-like injections that could be of importance to PGE exploration elsewhere.

Chalcopyrite from the Cu–PGE sulfide deposits in the Eastern Gabbro, Coldwell Complex, Canada, exhibit a > 2‰ variation in δ65Cu. In the Marathon deposit, the δ65Cu of chalcopyrite increases from the lower Footwall Zone (− 1.49 to − 0.75‰), to the Main Zone (− 1.04 to 0.08‰), to the upper W Horizon (− 0.35 to 1.07‰). In the northern deposits, chalcopyrite at Four Dams and Sally have δ65Cu that range from − 0.08 to 0.47‰ and − 0.59 to − 0.05‰, respectively. Notably, samples from the Marathon deposit with lower chalcopyrite δ65Cu values tend to have higher S/Se and Cu/Pd ratios. Integrated geological and geochemical evidence suggests that secondary hydrothermal alteration and redox processes are unlikely to have been the primary causes of the observed Cu isotope variation. Numerical modeling of δ65Cu–Cu/Pd–S/Se of mineralization in the Eastern Gabbro illustrates three key aspects of Cu isotope behavior in magmatic Ni–Cu–PGE systems. First, R factors less than ~ 10,000 can exhibit significant control on the δ65Cu of sulfides. Second, sulfide liquid–silicate melt fractionation factors for Cu (Δ65Cusul–sil) greater than − 0.5‰ are applicable to Ni–Cu–PGE systems. Third, sulfide segregation exhibits no measurable control on the δ65Cu of sulfides at degrees of fractionation typical of Ni–Cu–PGE systems (< 0.3%). In the Marathon deposit, the range of δ65Cu–S/Se–Cu/Pd is attributed to the addition of Archean sedimentary Cu to a pool of sulfide liquid located at depth, followed by progressive dilution of the contaminated δ65Cu–S/Se signature and decrease in Cu/Pd ratio by influxes of uncontaminated pulses of magma (i.e., increasing R factor), some of which had Cu isotope compositions heavier than the mantle. Variably contaminated and enriched, with respect to Pd, sulfides from this pool were entrained by magma pulses and emplaced to form the Marathon deposit. This contribution demonstrates that Cu isotopes can fractionate at high temperatures and, when combined with other geochemical proxies, can be valuable in characterizing magmatic–post-magmatic processes in Ni–Cu–PGE sulfide deposits and for identifying PGE-rich sulfide deposits.

The Eastern Gabbro of the alkaline Coldwell Complex, Canada, represents a Ni-poor conduit-type system that comprises two rock series, the Layered Series and Marathon Series, which intruded into a metabasalt package. Based on distinct variations in magnetite compatible (e.g., Ni, Cr) and incompatible (e.g., Sn, Nb) elements in Fe–Ti oxide intergrowths, the metabasalts, Layered Series, and Marathon Series must have crystallized from magmas that originated from compositionally distinct sources. Of these rock units, the metabasalts crystallized from a more primitive melt than the Layered Series as Fe–Ti oxides in the former have higher concentrations of magnetite-compatible elements. Unlike the metabasalts and Layered Series, the Marathon Series crystallized from multiple, compositionally distinct magmas as Fe–Ti oxides in this series exhibit large variations in both magnetite compatible and incompatible elements. Accordingly, the various rock types of the Marathon Series cannot be related by fractional crystallization of a single batch of magma. Rather, the magmas from which the rock types crystallized had to have interacted to variable degrees with a late input of more primitive melt. The degree of this magma interaction was likely controlled by the geometry of the conduit and the location of emplacement given that Fe–Ti oxides in the oxide-rich rocks occur in pod-like bodies and exhibit no compositional evidence for magma mixing. Mirrored variations in magnetite compatible and incompatible elements in Fe–Ti oxides in the Footwall Zone, Main Zone, and W Horizon of the Marathon Cu–PGE deposit indicate that these zones could not have formed from a single, evolving magma, but rather multiple batches of compositionally distinct magmas. Fe–Ti oxides exhibit no compositional difference between those hosted by barren and mineralized rock. This is likely because sulfide liquated at depth in all of the magmas from which the Marathon Series crystallized. The composition of Fe–Ti oxides in the Eastern Gabbro fall outside of the compositional fields for Ni–Cu mineralization defined by Dupuis and Beaudoin (Mineral Deposita 46:319–335, 2011) and Ward et al. (J Geochem Explor 188:172–184, 2018) demonstrating that their discrimination diagrams can distinguish between Ni-rich and Ni-poor systems that contain disseminated and massive sulfides.

Magnetite (mag)–ilmenite (ilm) intergrowths are more common than mag–ulvospinel (usp) intergrowths in mafic–ultramafic Ni–Cu–PGE systems, yet the former has no known solid solution. The most accepted model for the formation of mag–ilm intergrowths in terrestrial environments is fluid-induced oxidation of mag–usp assemblages by oxygen in water. In this study, we re-examine this model in light of the fact that crustal fluids have very low pO2 and that mag–ilm intergrowths commonly occur in rocks that show little or no evidence of hydrothermal alteration. We also characterize the chemical changes that occurred during the formation of mag–ilm intergrowths and how they affect the use of Fe–Ti oxide chemistry for petrogenesis and mineral exploration. In the Eastern Gabbro, Coldwell Complex, a continuum of Fe–Ti oxide intergrowths occur ranging from cloth (mag–usp) to trellis (mag–ilm) types. Trellis-textured intergrowths have higher bulk Fe3+:Fe2+ ratios and are predominantly enriched not only in some multivalent (Ge, Mo, W, Sn) elements, but also in Cu and Ga, consistent with their formation via oxidation by a metal-rich fluid. These compositional changes are significant relative to typical elemental abundances in Fe–Ti oxides and could potentially lead to erroneous interpretations regarding primary magmatic processes if they are not taken into consideration. The irregular distribution of the intergrowths throughout the Eastern Gabbro suggests that different rock series and mineralized zones experienced variable degrees of fluid-induced oxidation. It is proposed that C in CO2 rather than O2 in water could potentially be an important oxidizing agent in mafic systems: $$ 9{\mathrm{Fe}}_2^{2+}\mathrm{Ti}{\mathrm{O}}_4+0.75{\mathrm{CO}}_2+1.5{\mathrm{H}}_2\mathrm{O}\leftrightharpoons 9{\mathrm{Fe}}^{2+}\mathrm{Ti}{\mathrm{O}}_3+3{\mathrm{Fe}}_2^{3+}{\mathrm{Fe}}^{2+}{\mathrm{O}}_4+0.75\mathrm{C}{\mathrm{H}}_4 $$ The applicability of this model is supported by the common occurrence of CO2 and CH4 in fluid inclusions in mafic rocks.

The Eastern Gabbro, Coldwell Complex, hosts several geochemically and mineralogically distinct Cu-platinum group element (PGE) deposits, including the high-grade W Horizon (>100 ppm Pd-Pt-Au over 2 m). Several magmatic and/or hydrothermal models have previously been proposed to explain the range of enrichment in PGEs observed in the Marathon deposit, but no work has integrated textural and compositional variations in sulfides to elucidate which of these models is most suitable. Additionally, comparatively little work has been done to characterize the genesis of Cu-PGE mineralization that occurs to the northwest of the Marathon deposit in the Eastern Gabbro. Through integration of base metal sulfide (BMS) mineralogy, texture, and trace element chemistry, a wide range of magmatic and postmagmatic processes have been characterized that contributed to the formation of these deposits. In all zones of mineralization in the Eastern Gabbro, chalcophile elements were remobilized from primary chalcopyrite by hydrothermal fluids and precipitated as secondary chalcopyrite, which occurs as a replacement of pyrrhotite and as intergrowths with hydrous silicates. BMSs in the mineralized zones in the Marathon deposit (Footwall zone, Main zone, and W Horizon) experienced higher R factors than those deposits located northwest of the Marathon deposit (Four Dams, Area 41, and Redstone), with BMSs in the W Horizon having experienced the highest R factors. The silicate melts from which the Footwall zone crystallized likely experienced some degree of sulfide segregation at depth, albeit to a much lesser degree than the northern deposits. Additionally, the melts from which the mineralized zones in the Marathon deposit crystallized were likely contaminated by high-S/Se Archean sedimentary rocks, whereas the northern deposits were likely contaminated by low-S/Se igneous and/or metamorphic rocks. BMSs in a chalcopyrite-rich pod located within the vicinity of the Coldwell Complex experienced both high R factors and high degrees of contamination (cf. W Horizon and Footwall zone, respectively). This study illustrates the complexity of processes that generate and modify mineralization in conduit-type Ni-Cu-PGE systems.

The Marathon platinum group element (PGE)–Cu deposit is hosted by the Two Duck Lake Gabbro of the Mesoproterozoic Coldwell Complex, Canada, and comprises three zones of mineralization, which have different textural, mineralogical and geochemical characteristics. The Footwall Zone occurs at the base of the Two Duck Lake Gabbro, at the contact with the Archean country rocks. The Main Zone occurs within the Two Duck Lake Gabbro, above the Footwall Zone. The W Horizon is an extraordinarily PGE-enriched zone that is characterized by very low Cu/Pd values, less pyrrhotite, and appreciably more bornite than the Footwall and Main zones. It occurs above Main Zone-equivalent mineralization in the southern half of the Two Duck Lake intrusion. Silicate mineral textures and chemistry reveal that the Marathon deposit formed in a dynamic magma conduit system with a complex magma evolution history. The deposit developed into three zones as a result of the multiple pulses of compositionally different magmas. Plagioclase commonly shows resorption surfaces with overgrowths and the anorthite content and concentrations of trace elements such as Fe, Sr, Ba, and rare earth elements in plagioclase change significantly across these surfaces. In rocks that contain sulfide minerals, plagioclase crystals have been partly resorbed and the resorbed rims are Ca-enriched and intergrown with sulfide mineral inclusions. These rims contain higher base-metal and sulfur contents and are light rare earth element-enriched compared with the primary plagioclase. These characteristics indicate input of a separate sulfide-enriched melt with a different composition compared with that from which the primary plagioclase crystallized. Additional evidence for the involvement of slightly different magmas during evolution of the three mineralized zones is derived from variations in pyroxene chemistry (e.g. higher Fe, lower Mg, V, and Sc) between the Footwall Zone and the other two zones and the absence of inverted pigeonite from the W Horizon. The PGE enrichment occurred at depth prior to intrusion and late-stage PGE-rich sulfide-bearing magmas intruded and formed the W Horizon. Therefore, the W Horizon is a zone of mineralization that formed late in the evolution of the Two Duck Lake Gabbro.

A diverse suite of tholeiitic to alkaline basalt and gabbroic intrusions located in the Coldwell Complex on the northern margin of the Midcontinent Rift exhibit unusual trace element signatures that show enriched large ion lithophile elements and light rare earth elements with negative Nb and Zr anomalies. These features are not typical of magmas derived by partial melting within or above a rising mantle plume, as might be expected in an early Midcontinent Rift magmatic event. In this paper, we provide a detailed geochemical study of a 500 m thick sequence of metabasalt that represents the earliest stage of magmatism in the Coldwell Complex. We show that contamination or crystallization processes or subsequent metasomatism cannot explain the trace element variations. Instead, we propose partial melting in a metasomatized Subcontinental Lithospheric Mantle source to explain the decoupled behavior of large ion lithophile elements from light rare earth elements and heavy rare earth elements and rare earth elements from high field strength elements and the enriched Nd isotope signature of metabasalt. Similar features occur in unit 5b of the Mamainse Point Volcanic Group located at the northern margin of the Rift. An objective of this paper is to relate Two Duck Lake gabbro, host rock for low-sulfur, high precious metal sulfide mineralization at the Marathon deposit, to the metabasalt sequence. The excellent match of trace element abundances in Two Duck Lake gabbro to metabasalt unit 3 confirms an early Coldwell Complex age for metabasalt and a Subcontinental Lithospheric Mantle source for Cu – platinum group element mineralized gabbros.

The Layered Series of the Midcontinent Rift related Coldwell Complex comprises thick sections of gabbro, without any known associated ultramafic rocks. It represents a major early intrusive unit of the Coldwell Complex and consists of thick accumulations of olivine gabbro and oxide augite melatroctolite. This study combines petrography, mineral chemistry, and lithogeochemistry to constrain the magma composition and petrogenesis of the Layered Series. The presence of cumulus orthoclase together with the observation that the Layered Series rocks plot in the alkaline field on a total alkali–silica diagram indicate that the Layered Series magma has an alkaline parentage. The stratigraphy of the Layered Series cannot be fully correlated between different areas using lithogeochemistry and mineral chemistry. This together with observed normal and reverse trends for mineral chemical compositions in different areas suggest that the processes related to magma emplacement and crystallization were different in different locations. The whole-rock concentrations of incompatible elements and the compositions of major minerals of the olivine gabbro and oxide augite melatroctolite units are chemically similar. However, major element lithogeochemistry is variable, dominantly due to differences in the abundances of olivine, clinopyroxene, plagioclase, and magnetite. An additional observation is that olivine and clinopyroxene are not in chemical equilibrium. Together, these observations are interpreted to reflect a combination of multiple injections of magma and crystal sorting in an open system.

Mafic intrusions on the NE shoulder of the Midcontinent Rift (Keweenawan LIP), including Cu–PGE mineralized gabbros within the Coldwell Complex (CC), and rift parallel or radial dykes outside the CC are correlated based on characteristic trace element patterns. In the Coldwell Complex, mafic rocks are subdivided into four groups: (1) early metabasalt; (2) Marathon Series; (3) Layered Series; (4) Geordie–Wolfcamp Series. The Marathon Series are correlated with the rift radial Abitibi dykes (1140 Ma), and the Geordie–Wolfcamp Series with the rift parallel Pukaskwa and Copper Island dykes. U–Pb ages determined for five gabbros from the Layered and Marathon Series are between 1107·7 and 1106·0 Ma. Radiogenic isotope ratios show near chondritic (CHUR) εNd(1106 Ma) and 87Sr/86Sri values that range from –0·38 to +1·13 and 0·702537 to 0·703944, respectively. Distinctive geochemical properties of the Marathon Series and Abitibi dykes, such as Ba/La (14–37), Th/Nb (0·06–0·12), La/Sm (3·8–7·7), Sr/Nd (21–96) and Zr/Sm (9–19), are very different from those of the Geordie–Wolfcamp Series and a subset of Copper Island and Pukaskwa dykes with Ba/La (8·7–11), Th/Nb (0·12–0·13), La/Sm (6·7–7·9), Sr/Nd (5–7·8) and Zr/Sm (18–24). Each unit exhibits covariation between incompatible element ratios such as Zr/Sm and Nb/La or Gd/Yb, Sr/Nd and Ba/La, and Nb/Y and Zr/Y, which are consistent with mixing relationship between two or more mantle domains. These characteristics are unlike those of intrusions on the NW shoulder of the MCR, but resemble those of mafic rocks occurring in the East Kenya Rift. The results imply that an unusual and long-lived mantle source was present in the NE MCR for at least 34 Myr (spanning the 1140 Ma Abitibi dykes and the 1106 Ma Marathon series) and indicate potential for Cu–PGE mineralization in an area much larger than was previously recognized.

Portable XRF (pXRF) and benchtop scanning electron microscopes equipped with energy dispersive systems (bSEM-EDS) are two real-time analytical techniques that can be combined to collect on-site lithogeochemical and mineral chemical results at dramatically reduced cost and time compared to traditional analytical methods. The Coldwell Complex, northwestern Ontario, is used as a case study on how the combination of these techniques can be applied to mineral exploration. Our results show that whole-rock Ba, V/Ti, Cu/S determined by pXRF, and plagioclase, olivine, and clinopyroxene compositions measured by bSEM-EDS can be applied to exploration for PGE deposits. Cu/S ratios for disseminated sulfides are a proxy for metal tenor, which can be applied as a vector towards higher grade mineralization. Potential magma recharge zones are identified by inspecting down-hole variations in plagioclase, olivine, and clinopyroxene compositions. Fluctuating down-hole variations in these parameters are interpreted to reflect episodic magma recharge. Whether rocks contain PGE mineralization can initially be assessed by documenting the variability of mineral chemistry, i.e., the greater variability of mineral chemistry could be used to vector higher grade PGE mineralization, particularly if the mineralization is conduit-related. Lastly, geochemical mapping of the igneous stratigraphy enables identification of favorable hosts for mineralization. Down-hole whole-rock variations in Ba and V/Ti from pXRF, and mineral chemical profiles from bSEM-EDS are used to discriminate the mineralized Marathon Series from the barren Layered Series throughout the Coldwell Complex. Whole-rock Ba and V/Ti from pXRF and mineral chemical results from bSEM-EDS could prove useful in other settings of the Midcontinent Rift (e.g., the Duluth Complex, Eagle and Eagle East intrusions, Tamarack Intrusive Complex, and the Sonju Lake intrusion) and elsewhere (e.g., the Stillwater Complex) to identify mineralized host rock types. An additional important application is a rapid interpretation of petrogenesis. Down-hole variations in olivine-clinopyroxene Mg-Fe exchange coefficients determined by bSEM-EDS analyses show that olivine-clinopyroxene pairs in the Marathon Series units, particularly for those within PGE mineralized zones, have compositions that are closer to chemical equilibrium (constant) compared to units of the barren Layered Series (highly variable), and thus could be a tool for interpreting petrogenetic processes active in the magma chamber or during the mineralization forming event.

Accurate characterization of the platinum group mineral (PGM) assemblages for Cu-Ni-PGE deposits are typically constrained by sample size and the difficulty of finding statistically significant numbers of grains, which is expected given the low concentrations of platinum group elements ( 75 µm) can account for large fractions of total mass. Despite these limitations, an accurate survey of PGM from different deposit types would have significant value towards developing deposit models and respective exploration strategies. In this study, we present results for a comprehensive evaluation of PGM at four copper-PGE occurrences hosted within separate but co-genetic gabbro or troctolite intrusions in the Coldwell Alkaline Complex and confirm that accurate surveys are possible with sufficient sample material and efficient PGM concentration methods. The PGM concentration methods used include: (1) hydroseparation of sieved size fractions of pulverized material, and (2) panning of grain separates produced by electric pulse disaggregation of drill core specimens. A favourable comparison of the results has verified the reliability of each method and added confidence that the PGM assemblages identified at three of the four locations are fully characterized. Precious metal mineral (PMM) assemblages are determined for the Main zone and W Horizon at the Marathon deposit, and the main zones at each of the Geordie Lake deposit and Area 41 occurrence. A total of 10,824 PMM grains (PGE and Au-Ag) and 68 mineral species, including 16 unknown minerals, were identified, of which 768 grains and 31 species occur at the Main zone, 523 grains and 41 species at Area 41,9485 grains and 43 species at W horizon, and 56 grains and 12 species at Geordie Lake. The PMM are grouped as follows: Pd-Ge, PGE-S-As, Pt-Fe alloy, Pd-Cu-Pb-Au, Pd-Ni-S, Pd-Pt-Sn, Pt-As, Pd-As, Pd-Pt-Sb-As, Pd-Pt-Bi-Te, and Au-Ag. All of the deposits were found to contain similar proportions of Pd-Pt-Sb-As, Pd-Pt-Bi-Te and Au-Ag minerals. But the W Horizon and Area 41 are distinguished from the Marathon Main zone and Geordie Lake deposits by the presence of minerals in the PGE-S-As, Pt-Fe alloy, Pd ± Cu ± Pb ± Au and Pd-Ge groups. Taken together, the PMM assemblages for deposits in the Coldwell exhibit a strong correlation to PGE enrichment relative to the range for mantle Cu/Pd values (1000–10,000). And there is no relationship between the abundances of Pd-Pt-Bi-Te and Pd-Pt-Sb-As minerals that are commonly associated with hydrous phases, and the intensity of hydrothermal alteration. Thus minerals found only at the W Horizon and Area 41, where significant PGE upgrading has occurred, including Pt-Fe alloys, rustenburgite, marathonite, palladogermanide, unknown Rh-Ni-Fe-sulfide, Au-Pd-Cu alloy, braggite, coldwellite, laurite, zvyagintsevite, laflammeite, and unknown phases Pd 5 As 2 , Pd 3 As, Pd 3 (As,Pb,Bi) might be considered as index minerals for PGE enriched types of mineralization in the Coldwell.

The W Horizon, Marathon Cu-Pd deposit in the Mesoproterozoic Midcontinent rift is one of the highest grade PGE repositories in magmatic ore deposits world-wide. The textural relationships and compositions of diverse platinum-group mineral (PGM) and sulfide assemblages in the extremely enriched ores (>100 ppm Pd-Pt-Au over 2 m) of the W Horizon have been investigated in mineral concentrates with ∼10,000 PGM grains and in situ using scanning electron microprobe and microprobe analyses. Here we show, from ore samples with concentrations up to 23.1 Pd ppm, 8.9 Pt ppm, 1.4 Au ppm and 0.73 Rh ppm, the diversity of minerals (n = 52) including several significant unknown minerals and three new mineral species marathonite (Pd25Ge9; McDonald et al., 2016), palladogermanide (Pd2Ge; IMA 2016-086, McDonald et al., 2017), kravtsovite (PdAg2S, IMA No 2016-092, Vymazalova et al., 2017). The PGM are distributed as PG-, sulfides (52 vol%), -arsenides (34 vol%), -intermetallics of Au-Ag-Pd-Cu and Pd-Ge(10 vol%) and -bismuthides and tellurides (4 vol%). The discovery of abundant (>330 grains) large unknown sulfide minerals with Rh allows us to present analyses three significant potentially new minerals (WUK-1, WUK-2, WUK-3) that are all clearly enriched in Rh (averaging 4.2, 8.5 and 28.21 wt% Rh respectively). Several examples of paragenetic sequences and mineral chemical changes for enrichment of Cu, Pd and Rh with time are revealed in the PGM and base-metal sulfides. We suggest this enhanced metal enrichment formed in response to increasing fO2 causing the oxidation of Fe2+ to Fe3+ and to a lesser extent, S. Phase relations in the Ag-Pd-S, Rh-Ni-Fe-S, Pd-Ge, Au-Pd-Cu-Ag, Pd-Ag-Te systems help constrain the crystallization temperatures of the majority of ore minerals in the W Horizon at ∼500 °C or moderate to high subsolidus temperatures (400–600 °C). Local transport by aqueous fluids becomes evident as minerals recrystallize down to

Mineral exploration is increasingly taking advantage of real time techniques that dramatically reduce the costs and time taken to obtain results compared to traditional analytical methods. Portable X-ray fluorescence (pXRF) is now a well-established technique that is used to acquire lithogeochemical data. To date, however, benchtop scanning electron microscopes, equipped with energy dispersive systems (bSEM-EDS) have received little attention as a possible mineral exploration tool. This study examines the utility of combining pXRF and bSEM-EDS to characterize the igneous stratigraphy and its relationship to Cu-Pd mineralization in a drill hole at the Four Dams occurrence, located within the Eastern Gabbro assemblage of the Coldwell Alkaline Complex, Canada. The first part of this study compares field portable and laboratory techniques. Seventy-two powdered samples analysed by pXRF are compared with traditional major elements analysed by inductively coupled atomic emission spectroscopy (ICP-AES) and trace elements by inductively coupled plasma spectrometry (ICP-MS), and the compositions of 128 olivine, clinopyroxene and plagioclase grains analysed by bSEM-EDS are compared with traditional electron microprobe data. Our results show that each portable technique yields results similar to their lab-based counterparts within the analytical capabilities and precisions of the respective instruments. The second part presents a case study for the application of pXRF and bSEM-EDS to resolve questions related to igneous stratigraphy as an aid to mineral exploration in a complicated geological setting. A major problem for Cu-Pd exploration in the Coldwell Complex of NW Ontario is that the oxide-rich units that host Cu-Pd mineralization in the Marathon Series are petrographically similar to the barren oxide-rich units in the Layered Series. However, the mineralized units are geochemically distinctive. Our results show that the mineralized Marathon Series can be distinguished from the barren Layered Series, including oxide-rich units of both, by combinations of P 2 O 5 , Ba, Zr and V/Ti values, determined by pXRF, combined with plagioclase, olivine or clinopyroxene compositions measured by bSEM-EDS. The combination of pXRF and bSEM-EDS thus shows considerable promise as an exploration technique. Supplementary material: Comparisons of pXRF analyses with ICP-AES, ICP-MS and IR analysis are available at http://doi.org/10.6084/m9.figshare.c.3291518

Coldwellite, ideally Pd 3 Ag 2 S, is a new mineral species discovered in a heavy-mineral concentrate from the Marathon deposit, Coldwell Complex, Ontario Canada. It is cubic, crystallizing in space group P 4 3 32 (#212) with a 7.2470(8) A, V 380.61(1) A 3 , Z = 4. The five strongest lines of the X-ray powder-diffraction pattern [ d in A (I)( hkl )] are: 2.427(100)(221), 2.302(38)(310), 2.195(38)(311), 1.4280(44)(510,431), 0.9294(24)(650,643), and 0.9208(20)(732,651). Associated minerals in the discovery sample include Au-Ag alloy, hollingworthite, isoferroplatinum, keithconnite, kotulskite, mertieite-II, michenerite, palladoarsenide, sobolevskite, sperrylite, stillwaterite, vysotskite, Ti- and Cr-rich magnetite and ilmenite, pyrrhotite, chalcopyrite, and pentlandite, with trace amounts of bornite, cobaltite, and galena. Coldwellite is white with a light pinkish brown tint, and it has a metallic luster. No streak or microhardness could be measured. The mineral shows no internal reflections, pleochroism, bireflectance, or anisotropy. The reflectance values (%) in air for the standard COM wavelengths are: 41.9 (470 nm), 44.9 (546 nm), 44.0 (589 nm), and 45.0 (650 nm). The density, 9.90(1) g/cm 3 , was calculated using the empirical formula and the unit-cell parameters from the refined crystal-structure. The average result ( n = 23) of electron-microprobe analyses is: Pd 56.1, Fe 0.16, Ag 38.2, S 5.63, total 100.09 wt.%, which corresponds to (Pd 2.99 Fe 0.02 ) Σ3.01 Ag 2.00 S 0.99 , based on 6 apfu . The name recalls the type locality.

Various ultramafic Ni-Cu-platinum group element (PGE) deposits associated with the North American Midcontinent rift have been attributed to formation in a magma conduit setting, whereby PGE concentration is controlled by various fluid dynamic processes. The Marathon Cu-PGE sulfide deposit located within the Midcontinent rift-related Coldwell Alkaline Complex has been classified as a gabbro-associated contact-type deposit; however, both magmatic and hydrothermal processes have been proposed to account for the significant concentration of PGE. In light of the growing field of evidence for magma conduit-type settings, this study comprised a comprehensive geochemical investigation of the complicated crosscutting gabbroic to ultramafic units in the immediate vicinity of the Marathon deposit; and a thorough three-dimensional investigation of the distribution of Cu and Pd within the Main mineralized zone. The main objectives of this study were to test the applicability of the magma conduit deposit model to the Marathon deposit and to identify key exploration criteria for use elsewhere in the Coldwell Alkaline Complex. Mineralization is hosted by the Two Duck Lake gabbro, a 4-km-long and 250-m-thick unit of the Marathon Series. The Marathon Series is the latest of three magmatic series that make up the 1- to 2-km-thick Eastern Gabbro Suite, which wraps around the eastern and northern margin of the Coldwell Alkaline Complex. The three magmatic series are shown here to have distinct trace element signatures that enable reliable discrimination of potentially sulfide and PGE-bearing units of the Marathon Series from the barren rocks of either the Fine-Grained or Layered Series. At the Marathon deposit, sulfides consist of disseminated chalcopyrite, pyrrhotite, and minor bornite and occur within the Main, Footwall, and Hanging-wall zones and in the PGE-enriched W Horizon. This paper focused on sulfides located within the Main zone, including the keel-shaped feeder channel that continues downdip to over 550-m depth. The spatial distribution of Cu, Pd, and Cu/Pd were examined in relation to a three-dimensional surface model for the footwall contact; in a vertical profile through the Main zone; and in a longitudinal section that cuts the feeder channel. There are three important observations: (1) trends for elevated Cu and Pd are parallel to numerous troughs and ridges in the footwall, (2) Cu, Pd, and Cu/Pd varies up section in a saw-toothed pattern from high to low values, and (3) the proportion of high Cu/Pd sulfides is greatest within the thickest accumulations of sulfides within the feeder channel. Evaluation of interelement relationships between Cu and Pd and between Pd and Ir, Rh, Pt, and Au for mineralization within the Main zone indicate positive associative, but nonlinear behavior for all elements. Briefly, the data show nonlinear correlations between Cu and Pd in which Cu/Pd decreases with increasing Pd; and coherent but nonlinear behavior for Ir, Rh, Pt, and Pd in which Pd/Ir, Pd/Rh, Pd/Pt, and Pd/Au all increase with increasing Pd. The observed variation in Cu/Pd is consistent with a magmatic model calculated by others for deposits in the Duluth Complex, in which sulfides accumulated in a closed system from a melt with mantlelike Cu/Pd and an elevated silicate to sulfide ratio. The observed variations in Pd/Ir, Pd/Rh, and Pd/Pt are consistent with R factor fractionation related to differences in the relative partition coefficients between sulfide and silicate melts, and rule out the possibility that processes such as fractionation of sulfide melt by monosulfide solid solution (mss) or redistribution of metals during hydrothermal alteration played a significant role in the mineralizing event. The Two Duck Lake gabbro and associated sulfides of the Marathon deposit are proposed to have formed by multiple injections of plagioclase crystal mush that carried droplets of sulfide liquid along a conduit system that was controlled by radial and ring fault structures in the Coldwell Alkaline Complex. The accumulation of sulfides was controlled by flow dynamic processes within the magma channels, but Cu/Pd was controlled by local proportions of silicate melt to sulfide liquid. Key characteristics of the deposit that are critical to exploration elsewhere in the Coldwell Alkaline Complex include the following: (1) the recognition that gabbroic to ultramafic intrusions of the Marathon Series are the host for Cu and PGE mineralization, (2) the distribution of Cu/Pd data within sulfide occurrences are useful as vectors toward the feeder channel, (3) topographic lineaments are indicators of potential mineralized feeder zones, and (4) oxide- and apatite-rich, irregularly shaped gabbroic to ultramafic pods are potential indicators of an underlying feeder channel.

La ferropicrite de Boston Creek, affleurant pres de Kirkland Lake dans la ceinture de l'Abitibi, en Ontario, est une unite differenciee a niveaux mafiques et ultramafiques d'âge archeen ayant une couche remarquablement epaisse de clinopyroxenite a texture spinifex a son contact superieur, equivalant a environ un tiers de l'epaisseur totale d'environ 100 m. Nous avons echantillonne la section a spinifex a un intervalle d'environ 1 m. Des reliquats du clinopyroxene a spinifex sont presents dans les deux-tiers inferieurs de la couche. Les resultats des analyses avec une microsonde electronique montrent que le clinopyroxene est une augite calcique avec une bordure generalement enrichie en Fe, Ca et Mn par rapport au noyeau. Ces variations sont plus evidentes dans les 10 m a la base de la couche. Un second clinopyroxene en surcroissance et en invaginations montre des variations de coeur a la bordure encore plus marquees, avec des compositions atteingnant le domaine de la diopside et de l'hedenbergite. Les variations en composition du clinopyroxene a texture de spinifex sont attribuees a la reaction entre clinopyroxene et liquide interstitiel piege. Une origine metamorphique semble beaucoup moins probable, compte tenu du manque de signes de tranferts de masse entre les roches encaissantes et la ferropicrite, la probabilite que les reactions d'echange entre pyroxene et liquide piege (presentement la matrice) seraient favorisees par une temperature magmatique, et la teneur elevee en Mn dans les premiers 5 m a la base du niveau a texture spinifex. Les compositions globales temoignent de diminutions marquees en Mg, Ni, Co et Cr dans les premiers 3 m au haut de la couche, et de faibles enrichissements en Ca, Al, K et Ti, signes de l'incompatibilite de ces elements au cours du fractionnement du clinopyroxene. Dans les premiers 10 m a la base de la couche, des inversions distinctes en composition sont evidentes dans les profils de Mg, Al, Cr et Mg# des roches totales. Ces inversions refleteraient un changement important de la composition du magma, peut-etre a cause d'une venue de magma frais au stade de cristallisation du gabbro, et d'un melange consequent entre magma non fractionne et magma residuel. Des variations locales des teneurs en elements du groupe du platine, certains aspects des relations isotopiques entre Re et Os, et les caracteristiques magnetiques a la base de la zone a spinifex et de la partie superieure de la couche gabbroique viennent etayer cette hypothese.