Jochen Kolb

Hydrologic segmentation of high-temperature shear zones: structural, geochemical and isotopic evidence from auriferous mylonites of the Renco mine, Zimbabwe

Abstract Combined structural, mineralogical and geochemical observations in auriferous mylonites of the Renco mine, hosted by late-Archaean, high-grade metamorphic granitoids in southern Zimbabwe, are used to describe the spatially heterogeneous fluid flow and metasomatism that occurred synchronous with deformation at mid- to upper-amphibolite facies metamorphic conditions. Significantly, the narrow (on average 1m-wide) mylonitic shear zones are internally zoned reflecting a pronounced hydrologic segmentation during deformation. Shear zones typically consist of two distinct domains: (i) anastomosing, quartz–feldspar–biotite–hornblende mylonites and/or quartz mylonites, and (ii) tabular-shaped pods, referred to as lithons, that are enveloped by mylonites and that exhibit evidence of transient episodes of brittle fracturing and ductile creep. Whole-rock geochemistry and mass balance calculations indicate dramatic element, volume (up to ≥ 100%) and associated mass gains for the brittle–ductile lithons that are mineralogically reflected in a volumetrically abundant sulphide mineralisation and the formation of a pervasively developed silicate alteration paragenesis. In contrast, mylonites have experienced only minor element and volumetric changes and minor alteration. δ 18 O values for whole rocks and quartz are enriched in lithons compared to wall rocks and enveloping mylonites, which implies the influence of externally derived fluids. These results indicate a strongly domainal fluid flow and mass transfer. Fluid advection was dominated by microscopic and macroscopic fracture permeabilities related to periods of coseismic dilatancy in lithons. The patchy, but partly interconnected distribution of lithons enveloped by mylonites indicates that deformation in the high-temperature shear zones was characterised by transient periods of seismic slip rather than continuous aseismic creep. Based on the well-preserved internal mineralogical and textural development of the Renco shear zones a more general model applicable for fluid advection coupled with the rheological behaviour of mid-crustal shear zones is presented.

Metallogeny of the North Atlantic Craton in Greenland

Abstract The North Atlantic Craton (NAC) extends along the coasts of southern Greenland. At its northern and southern margins, Archaean rocks are overprinted by Palaeoproterozoic orogeny or overlain by younger rocks. Typical granite-greenstone and granite-gneiss complexes represent the entire Archaean, with a hiatus from ~3.55-3.20 Ga. In the granulite- and amphibolite-facies terranes, the metallogeny comprises hypozonal orogenic gold and Ni-PGE-Cr-Ti-V in mafic-ultramafic magmatic systems. Gold occurrences are widespread around and south of the capital, Nuuk. Nickel mineralization in the Maniitsoq Ni project is hosted in the Norite belt; Cr and PGE in Qeqertarssuatsiaq, and Ti-V in Sinarsuk in the Fiskenæsset complex. The lower-grade metamorphic Isua greenstone belt hosts the >1000 Mt Isua iron deposit in an Eoarchaean banded iron formation. Major Neoarchaean shear zones host mesozonal orogenic gold mineralization over considerable strike length in South-West Greenland. The current metallogenic model of the NAC is based on low-resolution data and variable geological understanding, and prospecting has been the main exploration method. In order to generate a robust understanding of the metal endowment, it is necessary to apply an integrated and collective approach. The NAC is similar to other well-endowed Archaean terranes but is underexplored, and is therefore likely to host numerous targets for greenfields exploration.

Geological setting of the Guelb Moghrein Fe oxide-Cu-Au-Co mineralization, Akjoujt area, Mauritania

Abstract The Guelb Moghrein Fe oxide–Cu–Au–Co deposit is located at the western boundary of the West African craton in NW Mauritania. The wall rocks to the mineralization represent a meta-volcanosedimentary succession typical of Archaean greenstone belts. Two types of meta-volcanic rocks are distinguished: (1) volcanoclastic rocks of rhyodacite–dacite composition (Sainte Barbe volcanic unit), which form the stratigraphic base; (2) tholeiitic andesites–basalts (Akjoujt meta-basalt unit). The trace element signature of both types is characteristic of a volcanic arc setting. A small meta-pelitic division belongs to the Sainte Barbe volcanic unit. A meta-carbonate body, which contains the mineralization, forms a tectonic lens in the Akjoujt meta-basalt unit. It can be defined by the high XMg (=36) of Fe–Mg carbonate, the REE pattern and the δ13C values of −18 to −17‰ as a marine precipitate similar to Archaean banded iron formation (BIF). Additionally, small slices of Fe–Mg clinoamphibole–chlorite schist in the meta-carbonate show characteristics of marine shale. This assemblage, therefore, does not represent an alteration product, but represents an iron formation unit deposited on a continental shelf, which probably belongs to the Lembeitih Formation. The hydrothermal mineralization at 2492 Ma was contemporaneous with regional D2 thrusting of the Sainte Barbe volcanic unit and imbrications of the meta-carbonate in the upper greenschist facies. This resulted in the formation of an ore breccia in the meta-carbonate, which is enriched in Fe, Ni, Co, Cu, Bi, Mo, As and Au. Massive sulphide ore breccia contains up to 20 wt% Cu. The ore fluid was aqueous–carbonic in nature and either changed its composition from a Mg-rich oxidizing to an Fe-rich reducing fluid or the two fluid types mixed at the trap site. All lithologies at Guelb Moghrein were deformed by D3 thrusting to the east in the lower greenschist facies. The mobility of REE in the retrogressed rocks explains the formation of a second generation of hydrothermal monazite, which was dated at c. 1742 Ma. Archaean rocks of the West African craton extend to the west to Guelb Moghrein. The active continental margin was deformed and mineralized in the Late Archaean–Early Proterozoic and again reactivated in the Mid-Proterozoic and Westphalian, showing that the western boundary of the craton was reactivated several times.

Assessment of lithological, geochemical and structural controls on gold distribution in the Nalunaq gold deposit, South Greenland using three-dimensional implicit modelling

Abstract Field, drillcore and geochemical data are used to create a three-dimensional implicit model to assess the controls on gold mineralization at the Nalunaq orogenic gold deposit in South Greenland. Gold occurs in narrow quartz veins with variable dips averaging 34° SE that cut meta-basic rocks. The bulk of the mineralization is contained within a single gold–quartz vein, named the Main Vein. Within this vein, gold is concentrated into three ore shoots plunging 20–25° NE, corresponding to the South, Target and Mountain blocks of the Nalunaq gold mine. Gold anomalies in drillcores are identified updip and downdip from the current mine workings. Modelling reveals that structural controls have the greatest influence on the location of gold. Flexures in the Main Vein correspond to changes in the host rock lithology and the gold grade is highest where the quartz vein is steepest. Where late-stage faults intercept the Main Vein, gold grades are lower. The comprehensive gold assay data from the mine, which are integrated with structural observations in the implicit model, refine the structural interpretation of the Nalunaq gold deposit, highlighting the ore shoot geometry and delineating the minimum extents of mineralization beyond the currently mined areas.

Textural characteristics and trace element distribution in carbonate-hosted Zn-Pb-Ag ores at the Paleoproterozoic Black Angel deposit, central West Greenland

The Black Angel deposit represents the most important base metal deposit in Greenland, having produced 11.2 million tons of Pb-Zn-Ag ore from 1973 to 1990. The deposit is hosted by a greenschist facies calcitic marble of the Mârmorilik Formation of the Paleoproterozoic Karrat Group. The ore consists of sphalerite, pyrite, and galena, with minor amounts of chalcopyrite, arsenopyrite, tetrahedrite, freibergite, tennantite, stannite, briartite, rutile, and graphite. Pyrite occurs as porphyroclasts and as fine-grained euhedral grains and is commonly surrounded by sphalerite, galena, and chalcopyrite. Deformation textures such as pyrite annealing, fracture healing by sulfides as well as “Durchbewegung” textures are common. Electron microprobe analysis and laser ablation-inductively coupled plasma-mass spectrometry trace element analysis show that tetrahedrite-freibergite and galena are enriched in Ag and represent the main Ag carriers. Galena also hosts substantial amounts of Sb, Sn, and Bi. The presence of Ge-rich chalcopyrite and briartite (Cu2(Zn,Fe)GeS4) inclusions in sphalerite indicates that the Black Angel deposit may be host to an hitherto unrecognized Ge endowment. It is suggested that sphalerite and briartite co-precipitated from a hydrothermal fluid having an intermediate sulfidation state. The origin of the deposit remains ambiguous. The calcitic host rock, epigenetic style of mineralization, presence of anhydrite, and occurrence of hydrothermal breccias are consistent with an origin as Mississippi Valley-type deposit. However, much of the mineralization is syn- to late-tectonic and syn-metamorphic, and the sulfide textures are consistent with a metamorphic overprint. An origin as carbonate-hosted polymetallic Kipushi-type deposit is thus more likely, since these deposits constitute the primary deposit type from which briartite has been previously documented.