Mineral Deposits and Precambrian Geology

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Assessing the origin of some representative Archean-Paleoproterozoic porphyry Cu ± Au deposits
(2021-06-04) Meng, Xuyang
Most known porphyry Cu ± Au deposits are exclusively associated with oxidized and sulfurrich Phanerozoic arc-related magmas, but Precambrian analogues are rare. The genesis of some rare examples of such deposits (e.g., Tongkuangyu in Trans-North China Orogen) remains debated, and in particular, it remains unclear as to whether similar metallogenic processes that typify the Phanerozoic deposits also operated in the Precambrian. To resolve these issues, three study areas were chosen to variably characterize relevant aspects of these deposits in the context of porphyry-type metallogenies (i.e., the nature of mineralization, the redox states and volatile element abundances of the causative magmas): (1) the ~2.1 Ga Tongkuangyu deposit in Trans-North China Craton; (2) the ~1.88 Ga Haib porphyry Cu deposit, Southern Namibia; and (3) representative porphyry-type Cu ± Au deposits (Côté Gold, St-Jude, and Croxall) in the ~2.7 Ga Abitibi subprovince. At the Tongkuangyu deposit, remapping of the distribution of the host rocks, alteration, and mineralization support a porphyry-type model where ore formation occurred at ~2.1 Ga. This age is significantly later than the host granodiorite porphyry (~2.18 Ga) and schists (~2.5–2.2 Ga) and indicates that porphyry-type mineralization need not be linked directly to a causative magma exposed at surface. Instead, the latter is interpreted to be represented by subjacent porphyritic stocks and dikes. At the Haib deposit, the host calc-alkaline plutonic rocks and mineralization are dated to ca. 1886–1881 Ma. Estimations of magmatic fO2 and S, based on zircon geochemistry, apatite µXANES spectra, and apatite S analysis, demonstrate that oxidized, sulfur-rich arc magmas associated with porphyry Cu mineralization already existed in the late Paleoproterozoic. In addition, zircon Hf-O isotopes and whole-rock geochemistry support magmas of mantlederivation with minor crustal contributions that experienced amphibole ± plagioclase fractionation. These features of the Haib magmas are thus similar to those for Phanerozoic porphyry Cu deposits. For the ~2.7 Ga TTG (tonalite-trondhjemite-granodiorite-diorite)-related porphyry-type Cu ± Au deposits in the Abitibi subprovince, whole-rock geochemistry, zircon Hf-O isotopes, apatite S contents, and multiple oxybarometers suggest that variable magmatic origin and fO2 for the causative magmas. The rarity of porphyry-type Cu-Au deposits in these older settings may be attributed to either location restriction of these favorable metallogenic conditions and/or preservation, or exploration bias.
Fluid-chemical studies of gold systems: case studies of an Archean greenstone belt, Nunavut, and a Paleozoic slate belt, Meguma terrane, Nova Scotia.
(2019-12-16) Kerr, Mitchell James
Despite several decades of study, the source(s) of gold and mineralizing fluids, and the mechanisms responsible for gold precipitation remain controversial. In particular, there is a need to improve: (i) the identification of secondary gold upgrading processes that lead to very highgrade gold zones; (ii) the understanding of precipitation mechanisms in carbonaceous material (CM)-bearing metasedimentary systems (i.e., slate belts); and (iii) the exploration criteria for the cryptic, finely disseminated gold mineralization in CM-bearing metamudstone lithologies. The combined results of an extensive in situ microanalytical protocol (SEM, confocal Raman microspectroscopy, microthermometry, decrepitate mound analysis, LA-ICP-MS, cathodoluminescence, SIMS) has idenitified evidence of secondary gold enrichment at the Madrid deposit, Hope Bay Greenstone Belt, NU, Canada, resulting from the addition of new gold onto an earlier orogenic quartz-carbonate vein deposit by magmatic-hydrothermal fluids. This conclusion is based on robust textural, mineralogical, and fluid inclusion evidence. The geological setting and mineral-chemical features suggest an intrusion-related (i.e., porphyry), or intermediate-sulfidation epithermal mineralization style for the later event. This work provides another example of the importance of compositionally distinct cumulative hydrothermal events in the development of high-grade gold deposits in orogenic settings. The Dufferin deposit, NS, Canada, is a vein-type system hosted in CM-bearing metasedimentary rocks. A detailed fluid inclusion study strongly suggests that Au mineralization occurred through coupled fluid fO2 reduction (via interaction with CM) and pH increase, leading to efficient Au precipitation from an Au-undersaturated aqueous-carbonic fluid. This study presents one of the few examples of measured Au concentrations in ore fluids in an economic orogenic Au deposit and reports direct evidence for the genetic relationship between Au and CM in metasedimentary lithologies. The results of this study show that, despite mineralizing fluids being Auundersaturated, they may still produce economic deposits through highly efficient precipitation processes. The Moose Rive anticline, NS, Canada, hosts a number of, commonly inconspicuous, metasediment-hosted disseminated Au deposits. An initial study using an on-line, rock-crushing gas chromatographic technique identified bulk fluid differences (CO2 and hydrocarbon abundances) between gold-bearing and gold-barren parcels of metasedimentary rock. This work suggests that bulk fluid characteristics may be a viable exploration vector for disseminated gold systems.
Gold metallogeny of the southern Swayze Area, Abitibi Greenstone Belt
(2021-04-29) Hastie, Evan Carman George
Understanding ore forming processes and their control on mineralization is essential for the proper classification of Archean, and also younger, Au deposits and mineral exploration success. This study evaluates Archean Au-forming processes using examples across the Superior Province (Canada), with an emphasis on the Swayze area in the Abitibi greenstone belt. It combines detailed field mapping with various discriminants (e.g., in-situ stable isotopes, whole rock and trace-element geochemistry, LA-ICP-MS elemental mapping and chemistry of native gold) to differentiate between orogenic and intrusion-related Au deposits. Based on previous work and newly generated data on 44 deposits, the dataset suggests that Ag, Te and Hg are the only metals consistently associated with Au across different deposit types. Although these metals represent universal elemental vectors for Au exploration, other criteria such as rock and mineral textures, timing of Au mineralization, stable isotope data, and bulk geochemistry must be used to differentiate between Au-deposit types. An important finding is the recognition of a distinct subset of intrusion-related Archean deposits related to sanukitoid-type magmatism. These deposits, generally younger than 2690 Ma, predate shearing and are characterized by Te and Hg elemental associations, relatively high fO2, and δ34S ≤ -5‰. They differ from other intrusion related deposits in terms of their timing and geochemistry and, thus, require a different exploration strategy. Of importance to both academia and industry is the origin of late-stage high-grade Au zones in many deposits. This study suggests such zones result from coupled dissolution-reprecipitation of Au-bearing sulfides and generation of polymetallic Au melts and/or Au nanoparticles, which aggregate as coarse native gold. Both of the processes can elegantly explain ultra high-grade gold zones and are widely applicable to many Au deposits formed throughout Earth’s history.
Metallogeny of the Powell Block, Rouyn-Noranda Mining District, Québec
(2023-02-08) Schofield, Marina Decker
The Rouyn-Noranda mining district of Québec is located along the southern margin of the Abitibi greenstone belt of the Archean Superior Province. It hosts 22 Cu-Zn-Au-Ag volcanogenic massive sulfide (VMS) deposits, distributed amongst four fault blocks (Hunter, Flavrian, Powell and Horne). The Powell Block hosts the Au-rich Quemont VMS deposit and separates the Horne deposit, a world class Au-rich VMS deposit, from conventional VMS deposits (≤ 1 g/t Au) north of the Beauchastel fault. The structural and stratigraphic evolution of the Powell Block is poorly constrained and as a result, the time-stratigraphic position of the Quemont deposit relative to the Horne deposit and deposits of the Flavrain Block is uncertain, as are the processes responsible for the anomalous gold content of the Horne and Quemont deposits. Gold-quartz-carbonate and quartz-sulfide (Cu-Ag-Zn) epigenetic veins are also present, but their origin and relative timing remain enigmatic. The Powell Block can be subdivided into two distinctive domains, the Brownlee and Joliet domains, representing two overlapping volcanic centres. These two domains both consist of lower Blake River Group volcanic strata that differ slightly in age and volcanic history. The Brownlee domain is characterized by mafic dominated, bimodal volcanism (ca. 2701 Ma), whereas the Joliet domain is characterized by a thick sequence of felsic coherent and volcaniclastic units (≥2702 Ma). Two base metal mineralizing hydrothermal events are recognized. An early event is related to voluminous felsic volcanism, concomitant subsidence and the formation of the Quemont and Horne deposits in the Joliet domain and Horne block, respectively. These two deposits formed within the district’s largest monogenic felsic volcanic centre in a subaqueous setting that was locally emergent. Given this setting, the Au-rich Quemont and Horne deposits may have had a more direct magmatic input resulting in higher Au grades than the Cu-Zn VMS deposits of the Flavrian Block, which formed in a mafic dominated, bimodal, effusive and perhaps, a deeper water volcanic setting. A later hydrothermal event at ca. 2699~2695 Ma is related to the formation of disseminated, vein and breccia hosted (e.g. Joliet Breccia) quartz-sulfide (Cu-Ag-Zn) mineral occurrences, with variations in deposit characteristics reflecting differences in depth of formation and proximity to a magmatic source. The lateral zoning of alteration types, compositional gains and losses, and metal tenor are similar to VMS footwall alteration zones within the district and elsewhere, and these breccias and veins are part of a broad continuum of subsurface, cross-stratal magmatic-hydrothermal mineralization to seafloor, stratiform, VMS mineralization, a spectrum akin to the porphyry-epithermal continuum of modern subaerial volcanic arcs. Approximately 30–40 m.y. later, gold-quartz-carbonate veins were emplaced during N-S shortening of the volcanic rocks. Superposition of alteration related to these younger veins perturbed and masked regional alteration patterns related to the Quemont and Horne deposits, but due to their restricted distribution and comparatively low-grade, did not substantially affect the original gold tenor of these VMS deposits.
Regional volcanogenic massive sulphide metallogeny of the neoarchean greenstone belt assemblages on the northwest margin of the Wawa subprovince, Superior province
(Laurentian University of Sudbury, 2013-10-08) Lodge, Robert Wilfred David
The ca. 2720 Ma Vermilion, Shebandowan, Winston Lake, and Manitouwadge greenstone belts (VGB, SGB, WGB, and MGB, respectively) are located along the northern margin of the Wawa subprovince. They are interpreted to have formed in broadly similar rifted arc to back-arc environments, but their base and precious endowment and, in particular, their endowment in VMS deposits, differ markedly. These difference is metal endowment reflect differences in their metallogenic history that were examined by comparing their regional, belt-scale lithostratigraphy, chemostratigraphy, petrogenesis and tectonic history constrained by new U-Pb zircon geochronology. The MGB is the most VMS-endowed and isotopically juvenile (Pb and Nd) greenstone belt. It has a trace element chemostratigraphy that is consistent with a rifted arc to back-arc environment. The trace element chemostratigraphy of the WGB is also consistent with a rifted-arc to back arc geodynamic setting. The Winston Lake VMS deposits formed during early rifting of the arc and their timing is tightly constrained at ca. 2720 Ma by U-Pb ages of the host felsic strata and post-VMS Zenith gabbro. The Zn-dominated VMS mineralization formed from hydrothermal fluids that were <300 ° and were possibly boiling in relatively shallow water. The trace element chemostratigraphy of the VGB, SGB, and WGB indicates a plume-driven rifted arc to back-arc geodynamic settings. The composition of VMS mineralization, lithofacies, and alteration in these belts are consistent with a relatively shallower-water environment, which may have compromised VMS formation. The high-Mg andesites that are typical of, but restricted to, the SGB formed during compressional “hot” subduction, which resulted in the development of a thicker arc crust. This thicker crust may have inhibited VMS formation, but favoured the formation of magmatic sulphide and gold mineralization. New detrital and magmatic zircon U-Pb geochronology allowed comparison and correlation of lithostratigraphy and metallogeny between the greenstone belts. U-Pb ages within the VGB also defined younger, Timiskaming-type volcanic and sedimentary strata that are coeval with similar deposits in the SGB. These strata are spatially and temporally associated with gold mineralization in both belts and are coeval with similar deformation and magmatic events in the WGB and along the northern margin of the Wawa-Abitibi terrane. This indicates that the formation of Timiskaming-type pull apart basins in the northern part of the Wawa-Abitibi terrane were synchronous, and earlier than in the southern part, which is consistent with oblique convergence of the Wawa-Abitibi terrane onto the Superior Province. Detrital zircon geochronology also revealed the presence of a >2720 Ma iv zircon population within the Timiskaming-type sedimentary strata of the SGB. This is consistent with their derivation from the Wabigoon subprovince and suggests trans-terrane transport of detritus in a foreland –type basin resulting from uplift of the Wabigoon subprovince during accretion of the Wawa subprovince.

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