Richards, Jeremy P.

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Elevated magmatic sulfur and chlorine contents in ore-forming magmas at the Red Chris porphyry Cu-Au deposit, Northern British Columbia, Canada
(2018-11-01) Zhu, Jing-Jing; Richards, Jeremy Peter; Rees, Chris; Creaser, Robert; DuFrane, Andrew; Locock, Andrew; Petrus, Joseph; Lang, Jürgen
The Red Chris porphyry Cu-Au deposit is located in the Stikinia island-arc terrane in northwest British Columbia. It is hosted by the Red Stock, which has four phases of porphyry intrusions: P1, P2E, P2L, and P3. New U-Pb dating of zircon shows that these intrusions were emplaced at 211.6 ± 1.3 Ma (MSWD = 0.85), 206.0 ± 1.2 Ma (MSWD = 1.5), 203.6 ± 1.8 Ma (MSWD = 1.5), and 201.7 ± 1.2 Ma (MSWD = 1.05), respectively. The ore-forming event at Red Chris was a short-lived event at 206.1 ± 0.5 Ma (MSWD = 0.96; weighted average age of three Re-Os analyses), implying a duration of <1 m.y., as defined by the uncertainty range. This mineralization age coincides with the emplacement age of the P2E porphyry, and is consistent with crosscutting relationships that suggest P2E was the main syn-mineralization intrusion. Zircons from P1 to P3 porphyry rocks have consistently high EuN/EuN* ratios (mostly > 0.4), indicating that their associated magmas were moderately oxidized. The magmatic water contents estimated from plagioclase and amphibole compositions suggest H2O contents of ~5 wt. %. Taken together, the P1 to P3 porphyries are interpreted to be moderately oxidized and hydrous. The four phases of porphyries are differentiated by sulfur and chlorine contents. The SO3 contents of igneous apatite microphenocrysts from the mineralization-related P2 porphyries are higher (P2E: 0.30 ± 0.13 wt. %, n = 34; P2L: 0.29 ± 0.18 wt. %, n = 100) than those from the pre-mineralization P1 (0.11 ± 0.03 wt. %, n = 34) and postmineralization P3 porphyries (0.03 ± 0.01 wt. %, n = 13). The chlorine contents in apatite grains from the P2E and P2L porphyries are 1.18 ± 0.37 (n = 34) and 1.47 ± 0.28 wt. % (n = 100), also higher than those from P1 (0.51 ± 0.3 wt. % Cl, n = 34) and P3 (0.02 ± 0.02 wt. % Cl, n = 17). These results imply that the sulfur and chlorine contents of the P2E and P2L magmas were higher than in the P1 and P3 magmas, suggesting that elevated magmatic S-Cl contents in the P2 porphyries may have been important for ore-formation. Although the process that caused the increase in sulfur and chlorine is not clear, reverse zoning seen in plagioclase phenocrysts from the P2 porphyry, and the occurrence of more mafic compositions in P2L suggest that recharge of the deeper magma chamber by a relatively S-Cl-rich mafic magma may have triggered the ore-forming hydrothermal event.
Geophysical properties of an epithermal Au-Ag deposit in British Columbia, Canada
(2018-11-01) Abbassi, Bahman; Cheng, Li Zhen; Richards, Jeremy; Hübert, Juliane; Legault, Jean; Rebagliati, Mark; Witherly, Ken
The Newton Au-Ag deposit is an intermediate sulfidation state epithermal system in British Columbia, Canada. Multiple types of geophysical data are interpreted and evaluated with drillcore petrophysical, geochemical and geological observations to better understand the geophysical signature of the Newton epithermal system. Airborne γ-ray datasets show elevated emission counts of K, eTh, and eU over the Newton epithermal system that are caused by hydrothermal alteration. Drillcore γ-ray measurements also show high potassium concentrations related to the K-rich phyllosilicates in the form of argillic and quartz-sericite alteration assemblages. Magnetization vector inversion (MVI) is used to recover an unconstrained 3D magnetization vector model of the system on regional and deposit scales. The regional MVI has resolved a deep concentric shaped low magnetic zone that is interpreted as a porphyry system beneath the epithermal deposit. At the deposit scale, 3D direct current (DC) resistivity and induced polarization (IP) inversion, and unconstrained MVI revealed finer details of epithermal system architecture. Cooperative DC/IP and magnetic inversion, at the deposit scale, constrained the magnetic susceptibility model and recovered a more precise susceptibility image of the epithermal system that is well-matched with borehole geology. The integrated geophysical interpretation helped to resolve several 3D latent geological features in places without direct access to drillcore samples. We identified four petrophysical domains based on the three cooperatively inverted physical properties, including electrical resistivity, IP chargeability, and magnetic susceptibility. The combined geophysical models differentiated porphyritic intrusions (chargeability/susceptibility lows), disseminated sulfides (resistivity lows and chargeability highs), a Cu-rich zone in mafic volcanic rocks (susceptibility/chargeability highs and resistivity lows), and a Au-Ag-Cu-rich zone with silicification in felsic volcanic rocks (chargeability/susceptibility lows and resistivity highs). These petrophysical domains also provide useful exploration vectors for identification of similar epithermal systems.
Magmatic evolution and porphyry–epithermal mineralization in the Taftan volcanic complex, southeastern Iran
(2018-02-12) Richards, Jeremy P. Razavi, Amir M. Spell, Terry L. Locock, Andrew Sholeh, Ali Aghazadeh, Mehraj
The Taftan volcanic complex is located above the Makran subduction zone in Sistan and Baluchestan province, southeastern Iran. The earliest volcanic activity at Taftan started in the late Miocene (~8 Ma) with eruption of andesitic to dacitic lava onto a Cretaceous to Eocene volcanic and sedimentary paleosurface ~20 km to the northwest of the current volcanic edifice. Later Plio-Pleistocene volcanism consisted of voluminous andesitic and dacitic lavas and pyroclastic flows (~3.1 to 0.4 Ma). Taftan, and the neighbouring Bazman volcano, are the southeasternmost and youngest manifestations of arc-related volcanism in Iran, which began with the Eocene–Miocene Urumieh-Dokhtar Magmatic Arc (UDMA) in northwest and central Iran, and extends into western Pakistan (Chagai Arc). Major porphyry Cu±Mo±Au deposits are associated with Miocene to Pliocene intrusive rocks in the Kerman section of the Eocene–Neogene Urumieh-Dokhtar Magmatic Arc in southeastern Iran (e.g., Sar Cheshmeh, Meiduk), and at Reko Diq and Saindak in the Late Cretaceous–Quaternary Chagai Arc in western Pakistan. In a gap of ~300 km between these two belts, several porphyry and epithermal prospects are exposed in the Miocene–Pliocene sections of the Taftan volcanic complex, including the Kharestan (6.10 ± 0.80 Ma) and Bidester porphyry-epithermal Cu-Au deposits (~4.3 Ma), and the Siah Jangal epithermal Au deposit (late Miocene). In addition, several argillic to advanced argillic and fumarolic alteration zones occur in Plio-Pleistocene volcanic rocks around the current volcanic edifice. These deposits have received limited exploration. Electron microprobe analyses of amphibole (magnesio-hastingsite) phenocrysts and magnetite–ilmenite mineral pairs from the Taftan and Bazman volcanic rocks indicate a change of crystallization temperature and magmatic oxidation state from ~1000°C and FMQ ≈ +1 in andesitic rocks, to ~900°C and FMQ ≈ +2 in dacitic rocks. Magmatic water content was >4 wt.%, as indicated by the ubiquitous presence of amphibole phenocrysts. Major and trace element compositions of the Taftan and Bazman volcanic rocks show calc-alkaline to high-K calc-alkaline affinity, with relative depletions of Nb, Ta, and Ti and enrichments of large-ion lithophile elements (LILE), Th, and U on normalized diagrams. These trace element patterns, including listric-shaped normalized rare earth element profiles and relatively high Sr/Y and La/Yb ratios, are similar to those of fertile Miocene igneous rocks from the Kerman Belt and Chagai Arc, and suggest that the Taftan suite in particular should be prospective for porphyry Cu ore formation. Regionally, there is no clear geochemical difference between the Neogene Kerman Belt rocks, which are thought to post-date the onset of collision between the Afro-Arabian and Eurasian plates (late Oligocene to earliest Miocene), and the subduction-related Bazman– Taftan and Chagai Belt magmas. The porphyry deposits formed in these distinct settings are also virtually indistinguishable. This suggests that most of the processes affecting the geochemistry and metallogeny of the magmas in both settings take place in the lithosphere, albeit that the ultimate source of the magmas is in the supra-subduction zone asthenospheric mantle wedge. In collisional environments, subduction-related material previously crystallized in the deep lithosphere is simply being remobilized.
Multiple mineralization events in the Zacatecas Ag-Pb-Zn-Cu-Au District, and their relationship to the tectonomagmatic evolution of the Mesa Central, Mexico
(2018-11-01) Vega, Osbaldo Zamora; Richards, Jeremy; Spell, Terry; Dufrane, Andrew; Williamson, John
Mineralization in the Zacatecas district is polymetallic (Ag, Zn, Pb, Cu, and Au) and occurs as skarn-type and epithermal veins formed in different metallogenetic stages. The oldest mineralization in the district is skarn-type, Curich with lesser Zn-Pb-Ag, and is considered to be close in age to felsic dikes and plugs dated at ~51 Ma. Epithermal mineralization occurs in both low- and intermediate-sulfidation styles. Intermediate-sulfidation veins (the Veta Grande, Mala Noche, El Bote, and La Cantera veins) are polymetallic, Ag-rich, hosted in ESE- to SE-striking structures, and were formed at ~42 Ma (adularia 40Ar/39Ar isochron age from Veta Grande of 42.36 ± 0.18 Ma; 2, MSWD = 0.76). Low-sulfidation Au-(Ag) mineralization occurs in the N–S-trending El Orito vein system, which yielded an adularia 40Ar/39Ar isochron age of 29.19 ± 0.20 Ma (2, MSWD = 1.8). These ages and the differences in structural orientation indicate that the two styles of epithermal mineralization are temporally and tectonically unrelated. The mineral paragenesis of the Mala Noche deposit consists of early skarn-type Cu mineralization overprinted by later epithermal Pb-Zn-Ag veins. Skarn-type minerals include relicts of prograde silicate minerals (diopside, hedenbergite, and garnet), retrograde silicate minerals (ilvaite, grunerite, stilpnomelane, epidote, clinochlore), and ore minerals (chalcopyrite, pyrite, sphalerite, galena, magnetite, wolframite, and minor bismuthinite). Epithermal mineralization is characterized by layered to vuggy quartz veins and breccias, with phyllic wallrock alteration (quartz, sericite-illite). The veins consist of quartz, calcite, dolomite, and ankerite with variable amounts of base metal sulfides (sphalerite, galena, pyrite, minor chalcopyrite, and rare acanthite and stromeyerite). The Veta Grande epithermal mineralization was emplaced in two main stages of Ag-rich quartz veining, with narrow selvedges of phyllic (quartz-sericite) wallrock alteration. Stage I consist of quartz, calcite, and minor adularia intergrown with pyrite, followed by sphalerite, galena, and lesser chalcopyrite, acanthite, pyrargyrite, and jamesonite. Stage II mineral paragenesis is similar to stage I but is characterized by amethystine quartz and contains less abundant sulfide minerals. The ore mineral paragenesis of the El Compas vein, within the El Orito System, consists of quartz, adularia, calcite, and chalcedony with minor pyrite, followed by rare aguilarite, naumannite, electrum, and native gold. The skarn-type and intermediate-sulfidation mineralization is coeval with Eocene subduction-related magmatism in the Zacatecas area, which is constrained by zircon U-Pb ages for igneous rocks between 51–42 Ma. The emplacement of these magmas was controlled by the same regional-scale, ESE- to SE-trending, transtensional structures that controlled the skarn-type and intermediatesulfidation deposits. This mineralization is thus interpreted to be related to the last stages of subduction volcanism in central Mexico, under transtensional stress conditions. In contrast, no nearby magmatism is clearly related to the Oligocene low-sulfidation system. However, its age and structural orientation (N–S), combined with a regional change in magma composition from Eocene calc-alkaline to Oligocene bimodal volcanism in central Mexico, suggest that the low-sulfidation mineralization is related to post-subduction continental extension processes, reflecting the beginning of Basin and Range tectonic
Origin of post-collisional magmas and formation of porphyry Cu deposits in southern Tibet
(2017) Wang, Rui; Weinberg, Roberto; Collins, William; Richards, Jeremy; Zhu, Di-Cheng
The recent discovery of large porphyry copper deposits (PCDs) associated with Miocene (22–12 Ma) granitoid magmas in the eastern section of the Paleocene-Eocene Gangdese magmatic arc in the Himalaya-Tibetan orogenic belt raises new questions about the origin of water-rich (≥4.5 wt.%), oxidized (ΔFMQ 1–3) magmas in continental collisional settings and their mineralization potential. We review the literature and compile available data on whole rock and isotope geochemistry for Cenozoic igneous rocks from Tibet, and add new zircon Ce4+/Ce3+ and Ti-in-zircon thermometry data to better understand variations in oxidation state and thermal evolution of these suites, which are key controls on Cu mineralization. Six distinct Cenozoic igneous suites are defined: Paleocene-Eocene syn-collisional Gangdese magmatic arc rocks (ΔFMQ = -1.2 to +0.8) (suite I), and five broadly contemporaneous Miocene suites. A distinct change in magmatism along the length of the belt occurs at around 88°E in the Miocene suites: to the east, porphyry copper mineralization is associated with a moderately oxidized, high-Sr/Y granitoid suite (suite II, ΔFMQ = +0.8 to +2.9) with minor occurrences of transitional (hybrid) monzonitic (suite III) and trachytic rocks (suite IV; both with zircon Ce4+/Ce3+ > 50-100, EuN/EuN* = ~0.5, and ΔFMQ = ~+1 to +2). To the west of 88°E, trachytic volcanic rocks (suite V) are more voluminous but more reduced (zircon Ce4+/Ce3+ < 50, ΔFMQ <+1), and are associated with sparse, poorly mineralized high-Sr/Y granitoids (suite VI) which are moderately oxidized (zircon Ce4+/Ce3+ = 20–100, ΔFMQ = ~+1 to +3). The Miocene high-Sr/Y granitoids have many compositional and isotopic similarities to the Paleocene-Eocene Gangdese arc rocks, and are interpreted to have been derived by melting of the hydrated arc root, with minor mantle input. In contrast, the highly evolved isotopic signatures of the Miocene trachytic rocks, combined with deep seismic profiles and a xenolith-derived geotherm, suggest their derivation from the underthrust Indian Proterozoic subcontinental lithospheric mantle (SCLM) or old fore-arc Tibetan SCLM during phlogopite breakdown at temperatures of ~1100°C. Based on published geophysical data and tectonic reconstructions, we develop a model that explains the origin of the various Miocene magmatic suites, their spatial differences, and the origin of related PCDs. Following the early stages of continental collision (Eocene–Oligocene), shallow underthrusting of the Indian continental lithosphere and subcretion of Tethyan sediments (including oxidized carbonates and possibly evaporites) under eclogite facies conditions promoted the release of aqueous fluids, which hydrated and oxidized the base of the overlying Tibetan plate. This metasomatism rendered the Tibetan lower crust fusible and fertile for metal remobilization. During the mid-Miocene, the Indian slab steepened in the eastern sector (east of ~88°E). In this eastern belt, deeply derived trachytic magmas were trapped in melt zones at the base of the Tibetan crust, and variably mixed with the crustally-derived, high Sr/Y granitoid magmas. They may also have released water that contributed to fluid-fluxed melting of the lower crust, producing voluminous high-Sr/Y granitoid magmas, which were associated with significant PCD mineralization. Hybridization between the trachytic magmas and lower crustal partial melts is indicated by intermediate isotopic compositions, enriched Cr and Ni contents, and high Mg# in some intermediate-to-felsic (56–70 wt. % SiO2) high-Sr/Y granitoids. Trapping of the trachytic melts in deep crustal melt zones explains the relatively small volumes of trachytic magmas erupted at surface in the east. In contrast, to the west of ~88°E, subduction of the Indian plate has remained flat to the present day, preventing incursion of hot asthenosphere. Consequently, cooler conditions in the deep Tibetan lithosphere resulted in limited crustal melting and the production of only small volumes of high-Sr/Y granitic magmas. Trachytic melts ascending from the underthrust Indian or Tibetan plate were able to pass through the cooler lower crust and erupted in greater volume at surface, whereas only small volumes of high-Sr/Y granitoid magma were generated and are not associated with significant PCD mineralization.
A shake-up in the porphyry world?
(2018-11-01) Richards, Jeremy
Porphyry Cu deposits form in the shallow crustal parts of arc magmatic systems, which root in the mantle wedge, evolve in lower crustal MASH zones (melting, assimilation, storage, homogenization) and lower-to-mid crustal hot zones, and accumulate in mid-to-upper crustabatholiths at depths of 5–10 km. A small proportion of the magma and most of the volatile load rises due to buoyancy towards the surface, and may erupt as volcanic or fumarolic emissions. Low levels of volcanism and fumarolic activity, as well as subsurface hydrothermal flow and alteration, are normal and semi-continuous features of active arc magmatic systems, which may operate for millions of years. Porphyry Cu deposits, on the other hand, form rarely (typically ≤1 per batholith) and rapidly (≤100,000 years) in the subsurface (2–5 km depth), where hydrous volatiles exsolved from the underlying batholith are channeled into structurally controlled cupola zones and cool before reaching the surface. The explosively brecciated character of early mineralization stages (breccia pipes and stockworks) suggests that the initiation of fluid flow may be essentially instantaneous and catastrophic, with the longer total duration of hydrothermal activity reflecting slower kinetically controlled fluid exsolution processes, or draining of deeper parts of the system. These fluids generate intense subsurface hydrothermal alteration, and may precipitate economic concentrations of Cu-sulfide minerals in potassic alteration zones as they cool between ~400°–300°C. The suddenness and infrequency of these ore-forming hydrothermal events suggests that they are triggered by an external process acting on otherwise normally evolving magmatic systems. Sudden depressurization or agitation of a large, primed, volatile-saturated or supersaturated mid– upper crustal magma chamber could lead to rapid and voluminous volatile exsolution and fluid discharge. This sudden volatile flux could result in either a large explosive volcanic eruption if the surface is breached, or a large magmatic-hydrothermal system that could form a porphyry Cu deposit if fluid flow is restricted to the subsurface. Candidates for triggers of these destabilizing events are catastrophic mass wasting such as volcanic edifice collapse, or mega-earthquakes, the latter possibly causing the former. The frequency of such catastrophic events occurring in proximity to active arc batholiths may approximate the recurrence rate of formation of large porphyry Cu deposits.

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