MAGMATIC Ni-Cu-(PGE) DEPOSIT, MINNESOTA, USA
Taranovic, V.1, Ripley, E. M.1, Li, C.1 & Rossell, D.2
1Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA
2Rio Tinto Exploration (Kennecott Exploration), Duluth, MN 55810, USA e-mail: vtaranov@indiana.edu
ABSTRACT. The Tamarack Ni-Cu-(PGE) deposit is hosted in the Tamarack Intrusive Complex (1105.6 ± 1.2 Ma) within the Paleoproterozoic Animikie Basin and associated with the Midcontinent Rift System, Minnesota.
The sulfide mineralization has a typical magmatic pyrrhotite-pentlandite-chalcopyrite assemblage. Variations in PGE characteristics are modeled by R-factor estimates (disseminated sulfides) and by a sulfide liquid fractionation calculations (semi-massive and massive sulfides). The disseminated sulfides are suggestive of equilibration at R-factors between 100 and 10,000, most likely representing “effective” R-factors. Metal enrichment in semi-massive and massive sulfides likely resulted from extensive sulfide liquid fractionation.
bration at R-factors in the 100 to 10,000 range.
These values most likely represent “effective”
R-factors which reflect the effects of varied pro- cesses occurring in the conduit system, includ- ing possible cannibalization of pre-existing sul- fide. Variations in the semi-massive and massive sulfides associated with the CGO are representa- tive of different amounts of MSS fractionation from a sulfide liquid of a primary composition consistent with an R-factor of ~400. The tex- tural and spatial characteristics, along with the R-factor estimates and sulfide liquid fraction- ation modeling, suggest that several sulfide liq- uids were involved in the formation of sulfide mineralization in the TIC. The low initial con- centrations of Ir, Os, Pt and Pd (0.1 ppb, 1 ppb, 2 ppb, and 1.5 ppb respectively) may be related to fractional crystallization of olivine and chro- mite, possible crystallization of IPGE minerals that saturated in the silicate liquid, and local fractionation of MSS. Enrichment of Ni, Cu and PPGEs in massive sulfides is thought to be pro- duced as a result of extensive fractional crys- tallization of a sulfide liquid. Immiscible sulfide liquids are thought to have been transported via multiple pulses of magma within the con- duit system. The sulfides were emplaced in the core of the conduit and in the wide bottom por- tion of the FGO as the velocity of the magmas dropped. The addition of crustal S is thought to have been the principal process which drove the early attainment of sulfide supersaturation in the magmas at depth.
The layered mafic-ultramafic Uitkomst Com- plex, which hosts the Nkomati deposit, is an elon- gate tube-shaped body, at least 12 km long, that con- cordantly intruded sedimentary rocks of the Early Proterozoic Transvaal Supergroup (Gauert et al., 1995). The Complex is composed of several per- sistent stratigraphic units from the base upwards:
Basal Gabbro 3-6 m thick, Lower Harzburgite 60-90 m thick, Chromitiferous Harzburgite and Massive Chromitite units with a total thickness of up to 60 m, Main Harzburgite up to 400 m thick, a Pyroxenite unit less than 100 m thick, and Main and Upper Gabbronorite units with chilled and quenched zones at the top contact (Gauert et al., 1995). The Complex is intruded into quartzite of the Black Reef and Oaktree Formations, Mal- mani Dolomite and shales and ironstones of the Timeball Hill Formation.
An unusual feature of the Uitkomst Complex is the occurrence of two types of mineralization that make up the Nkomati deposit: Ni-Cu-PGE sulfide ore, and massive chromitites together with the discontinuous chromitite seams and dissemi- nated chromites. Uitkomst chromitites are located within the ultramafic sequence of olivine-rich rocks and carry accompanying sulfide and PGE mineralization. In this study, we analysed chromite
from the thickest chromitites in the mining area intersected by the SHM022 borehole. The Cr2O3 content in chromite increases from 44 to 54.5 wt.%
upwards in the upper massive chromitites show- ing an opposite trend to the differentiation. The general increase is not continuous but it is com- posed of five cycles which are separated by minor reversals (Fig. 1). The cyclic patterns are observed in the distribution of all major oxides with Al and Fe3 + oxides correlating negatively with Cr2O3 whereas MgO demonstrates a positive correlation.
In the lower part of chromitiferous zone the Cr2O3 content in chromite decreases from 48 to 36 wt. % with height associated with a general decrease in chromite content down to 1-5 vol. % (Fig. 1).
We conclude that the changes in the chromite composition coupled with the cyclicity of textural changes in the chromitites and host harzburgite indicate the dynamic nature of their formation and multiple magma influxes into yet unconsolidated resident cumulates in the channel. Such a model is consistent with that suggested earlier by Gau- ert (1995). Evidence on the new influxes is also observed in the overlying harzburgite-dunite unit where brecciated chromitites and chromitiferous harzburgite are cemented by later chromite-poor harzburgite.
FORMATION OF NKOMATI MASSIVE CHROMITITE BODY VIA CRYSTALLIZATION WITHIN A MAGMATIC CONDUIT
Yudovskaya, M.1,2, Naldrett, A.J.1, Woolfe, J.A.S.3 & Kinnaird, J.A.1
1University of Witwatersrand, School of Gesciences, Pvt Bag 3, Wits 2050, South Africa
2Institute of Geology of Ore Deposits, Mineralogy, Petrography and Geochemistry, Russian Academy of Sciences, Moscow, 119017, Russia
3African Rainbow Minerals, P.O. Box 783580, Sandton 2146, South Africa e-mail: marina.yudovskaya@wits.ac.za
ABSTRACT. The general upward decrease in the Cr/Fe and Mg/Fe ratios of Bushveld chromite which is observed through the Lower and Critical Zone is consistent with a trend of magmatic differentiation of the whole complex (Hulbert & von Gruenevaldt, 1985; Scoon & Teigler, 1994; Naldrett et al., 2012). However, no zonal distribution of the major oxides or upward fractionation of chromite composition has been described in a particular layer that adds an additional argument in a favor of the Eales’s model (Eales, 2000) that invokes crystallization of chromite in the staging chamber or conduit with the consequent injection of a chromite slurry. Here we report for the first time on a primary reverse zoning of a chromite composition preserved in the Nkomati thick chromitite body of Bushveld affinity, revealing its crystallization in situ from magma currents.
Acknowledgements. The study is partially sup- ported by the RFBR grant 14-05-00448a and SAR NRF through THRIP.
REFERENCES
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2 . HULBERT, L . & VON GRUENEVALDT, G . (1985): Tex- tural and compositional features of chromite in the lower and critical zones of the Bushveld Complex south of Pot- gietersrus . Economic Geology, 80, 872-895 .
3 . GAUERT, C .D .K ., DE WAAL, S .A . & WALLMACH, T . (1995): Geology of the ultrabasic to basic Uitkomst Com- plex, eastern Transvaal, South Africa: an overview . Jour- nal of African Earth Sciences, 21, 553-570 .
4 . NALDRETT, A ., WILSON, A ., KINNAIRD, J . et al . (2012):
The origin of chromitites and related PGE mineralization in the Bushveld Complex: new mineralogical and petrologi- cal constraints . Mineralium Deposita, 47, 209-232 .
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Fig. 1. Distribution of chromite and Cr2O3 content of chromite in the main Nkomati chromitite body through borehole SHM022
The Jinchuan Cu-Ni sulfide deposit, which occurs in Precambrian metamorphic rocks, has extremely complex geological background. In this paper, authors report some new understanding for structural framework and its evolution in the area based on a systematic analysis about the features of stratum, major faults, basic-ultrabasic magma- tism and regional metamorphism.
Tectonic setting of the Jinchuan area is in the Longshoushan, southwest margin of the Alashan block. A regional fault (F1) locates in north side where is adjacent to the Chaoshui Basin, and another regional faults (F2) is located in the south side near by the Hexi Corridor Basin (Fig. 1).
Strata in the area consist of four major parts:
(1) Achaean deep metamorphosed base (Baijia- zuizi Formation); (2) Lower Proterozoic middle metamorphosed base (Tamazigou Formation);
(3) Middle or Upper Proterozoic to Lower Paleo- zoic (Sinian-Cambrian) less metamorphosed cover (Shaohuotong Formation); (4) almost no meta- morphosed Upper Paleozoic (mainly Devonian) cover. The major stratigraphic units show a zonal distribution of monoclinic layers which strikes to northwest, and trends to southwest. From north- east to southwest, the formation is from Baijiazuzi Formation → Tamazigou Formation → Shao- huotong Formation → Devonian, and that shows a metamorphic zonation from upper amphibolite facies to partial melting → lower amphibolite
facies → greenschist facies → almost not meta- morphosed Devonian calcareous sandstone.
Folds are not developed in the area, but faults.
All of the four stratigraphic units have a fault contacting between each other. Series northwest- west, northwest-striking nappe faults, eastwest- striking extensional faults and northeast-striking slip faults form the structural framework in this area. All these indicate a complex structural back- ground of superposition of multiple levels and multiple episodes.
According to a systemic survey of cross- sections in the area, the degree of migmatization increases from north to south in the Baijiazuzi Formation because an old and deeper nappe fault (F3, about 1.8 billion) is in contact between the Bai- jiazuzi Formation and the Tamazugou Formation.
The fault also made the hanging strata of Baijia- zuzi Formation fragmentizing. Pebbly carbonate- clastic sediments in the Shaohuotong Formation indicate the accumulation of orogenic collapse (F8) and the presence of the structural basin.
It may suggest that there was continental separa- tion during late Proterzoic, and associated with large-scale mineralized basic-ultrabasic magma invasion. As one part of the Qilian orogenic belt, the Jinchuan region in the late Paleozoic orogeny accepted a strongly squeeze, and formed a series of relatively shallow and tightly NWW folds, thrust faults (F1, F2, F4) and strike-slip faults