LU|ZONE|UL Community:
https://zone.biblio.laurentian.ca/handle/10219/2889
2024-03-28T07:57:37ZGeophysical properties of an epithermal Au-Ag deposit in British Columbia, Canada
https://zone.biblio.laurentian.ca/handle/10219/3172
Title: Geophysical properties of an epithermal Au-Ag deposit in British Columbia, Canada
Authors: Abbassi, Bahman; Cheng, Li Zhen; Richards, Jeremy; Hübert, Juliane; Legault, Jean; Rebagliati, Mark; Witherly, Ken
Abstract: 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.2018-11-01T00:00:00ZElevated magmatic sulfur and chlorine contents in ore-forming magmas at the Red Chris porphyry Cu-Au deposit, Northern British Columbia, Canada
https://zone.biblio.laurentian.ca/handle/10219/3171
Title: Elevated magmatic sulfur and chlorine contents in ore-forming magmas at the Red Chris porphyry Cu-Au deposit, Northern British Columbia, Canada
Authors: Zhu, Jing-Jing; Richards, Jeremy Peter; Rees, Chris; Creaser, Robert; DuFrane, Andrew; Locock, Andrew; Petrus, Joseph; Lang, Jürgen
Abstract: 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.2018-11-01T00:00:00ZMultiple mineralization events in the Zacatecas Ag-Pb-Zn-Cu-Au District, and their relationship to the tectonomagmatic evolution of the Mesa Central, Mexico
https://zone.biblio.laurentian.ca/handle/10219/3170
Title: Multiple mineralization events in the Zacatecas Ag-Pb-Zn-Cu-Au District, and their relationship to the tectonomagmatic evolution of the Mesa Central, Mexico
Authors: Vega, Osbaldo Zamora; Richards, Jeremy; Spell, Terry; Dufrane, Andrew; Williamson, John
Abstract: 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 tectonic2018-11-01T00:00:00ZA shake-up in the porphyry world?
https://zone.biblio.laurentian.ca/handle/10219/3169
Title: A shake-up in the porphyry world?
Authors: Richards, Jeremy
Abstract: 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.2018-11-01T00:00:00Z