The Hemlo-Heron Bay greenstone belt and Hemlo AuMo deposit, Superior Province, Ontario, Canada 2. Timing of metamorphism, alteration and Au mineralization from titanite, rutile, and monazite UPb geochronology

doi:10.1016/0168-9622(89)90030-4

Chemical Geology: Isotope Geoscience section

Volume 79, Issue 3, 5 September 1989, Pages 201-223

Chemical Geology: Is...

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Abstract

The Hemlo AuMo deposit, in the Superior Province of the Canadian Shield, is composed mainly of metamorphosed, metasomatized and intensely deformed volcano-sedimentary rocks. The ore consists dominantly of microclinized rocks and pyrite-sericite ± biotite schists with disseminated Au, molybdenite and a variety of other minerals.

UPb zircon data presented in the companion paper indicate the occurrence of at least two periods of volcanism at 2772 and 2695 Ma, and three main generations of granitoid plutons, the most widespread phase forming at ∼ 2684–2688 Ma. UPb data for titanite, rutile and monazite presented here show that the rocks within and adjacent to the Hemlo deposit experienced protracted metamorphic and hydrothermal activity postdating the bulk of the magmatism.

Titanite ages of 2676–2678 Ma from granodiorite units proximal to, but outside of the deposit, are interpreted as dating amphibolite-facies metamorphism which correlates with the zircon and titanite age of intrusion of the late-tectonic Gowan Lake Pluton. Titanite from schists and dikes within the deposit is uniformly 2670–2672 Ma old, which probably reflects a phase of lower-grade metamorphism and metasomatism that affected the deposit but not its surroundings. Structural and petrographic evidence suggests that these phases of medium- and subsequent lower-grade metamorphism correlate with early and late phases of dextral shearing (F₃) .

Rutile and monazite were observed only in a variety of altered units within the Hemlo deposit. Rutile coexists, in part, with titanite but yields much younger ages: 2632 ± 5, 2632 ± 3 and 2635 ± 14 Ma in adjacent outcrops of the Page-Williams Property, 2641± 4 Ma in a drill-core sample of the same property, and a combined rutile-monazite age of 2645 ± 4 Ma at a different location within the Golden Sceptre Property. These rutile and monazite ages are interpreted as reflecting low-grade hydrothermal activity probably accompanied by episodic deformation and dynamic recrystallization; this protracted activity is thought to be the expression of magmatic and related metamorphic processes in the lower crust.

The geological record suggests that the most active period of alteration and hence, likely, mineralization, occurred sometime from the medium- to the low-grade metamorphic phases which correlate with early and late stages of dextral shearing, respectively, between ∼2680 and ∼2670 Ma ago. The role of the younger (2630–2650 Ma) hydrothermal activity with respect to mineralization is not completely understood.

PbPb data, mostly on K-feldspar, indicate relatively uniform isotopic compositions for a variety of rocks inside and outside the deposit. The $^{207} Pb^{204} Pb$ ratios are relatively non-radiogenic, indicating that the bulk of the rocks and the mineralizing fluids were derived from juvenile crust and/or from the mantle without significant involvement of much older (pre-2800 Ma) crust.

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Fluid-mediated resetting of titanite is less well described. Corfu and Muir (1989) recognized that titanite associated with greenschist-facies assemblages within a hydrothermal Au-Mo deposit in the Hemlo–Huron greenstone belt, Canada, were 8 Myr younger than titanite outside the deposit. Corfu (1996) found that titanite grains from samples separated by as little as 20 m in the Winnipeg River Subprovince, Canada, yield different dates; Corfu interpreted this to reflect “focused fluid activity”.
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Taken together, we suggest that the significantly younger U–Pb age of titanite from the ore-related quartz diorite at Ruanjiawan cannot be explained by a slow cooling process; rather it must have resulted from a post-crystallization disturbance. The textural and geochemical evidence indicates that this disturbance was most likely caused by later hydrothermal alteration (Corfu and Muir, 1989a; Morad et al., 2009; Pan et al., 1993). The titanite U–Pb age of the altered quartz diorite (132 ± 2 Ma, Fig. 8B) is consistent with the zircon U–Pb age (133 ± 1 Ma; Fig. 9B) and titanite U–Pb age (131 ± 2 Ma, Fig. 8C) of the diabase dike, indicating that this hydrothermal alteration event was most likely related to the emplacement of this dike and equivalents in the area (Fig. 2).
In this paper, we present U–Pb ages and trace element compositions of titanite from the Ruanjiawan W–Cu–Mo skarn deposit in the Daye district, eastern China to constrain the magmatic and hydrothermal history in this deposit and provide a better understanding of the U–Pb geochronology and trace element geochemistry of titanite that have been subjected to post-crystallization hydrothermal alteration. Titanite from the mineralized skarn, the ore-related quartz diorite stock, and a diabase dike intruding this stock were analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Titanite grains from the quartz diorite and diabase dike typically coexist with hydrothermal minerals such as epidote, sericite, chlorite, pyrite, and calcite, and display irregular or patchy zoning. These grains have low LREE/HREE and high Th/U and Lu/Hf ratios, coupled with negative Eu and positive Ce anomalies. The textural and compositional data indicate that titanite from the quartz diorite has been overprinted by hydrothermal fluids after being crystallized from magmas. Titanite grains from the mineralized skarn are texturally equilibrated with retrograde skarn minerals including actinolite, quartz, calcite, and epidote, demonstrating that these grains were formed directly from hydrothermal fluids responsible for the mineralization. Compared to the varieties from the quartz diorite stock and diabase dike, titanite grains from the mineralized skarn have much lower REE contents and LREE/HREE, Th/U, and Lu/Hf ratios. They have a weighted mean ²⁰⁶Pb/²³⁸U age of 142 ± 2 Ma (MSWD = 0.7, 2σ), in agreement with a zircon U–Pb age of 144 ± 1 Ma (MSWD = 0.3, 2σ) of the quartz diorite and thus interpreted as formation age of the Ruanjiawan W–Cu–Mo deposit. Titanite grains from the ore-related quartz diorite have a concordant U–Pb age of 132 ± 2 Ma (MSWD = 0.5, 2σ), which is 10–12 Ma younger than the zircon U–Pb age of the same sample and thus interpreted as the time of a hydrothermal overprint after their crystallization. This hydrothermal overprint was most likely related to the emplacement of the diabase dike that has a zircon U–Pb age of 133 ± 1 Ma and a titanite U–Pb age of 131 ± 2 Ma. The geochronological results thus reveal two hydrothermal events in the Ruanjiawan deposit: an early one forming the Wu–Cu–Mo ores related to the emplacement of the quartz diorite stock and a later one causing alteration of the quartz diorite and its titanite due to emplacement of diabase dike. It is suggested that titanite is much more susceptible to hydrothermal alteration than zircon. Results from this study also highlight the utilization of trace element compositions in discriminating titanite of magmatic and hydrothermal origins, facilitating a more reasonable interpretation of the titanite U–Pb ages.
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Research paperThe Hemlo-Heron Bay greenstone belt and Hemlo AuMo deposit, Superior Province, Ontario, Canada 2. Timing of metamorphism, alteration and Au mineralization from titanite, rutile, and monazite UPb geochronology

Abstract

Geochim. Cosmochim. Acta

Precambrian Res.

Chem. Geol. (Isot. Geosci. Sect.)

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

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Sulphide mineral stabilities

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Research paper
The Hemlo-Heron Bay greenstone belt and Hemlo AuMo deposit, Superior Province, Ontario, Canada 2. Timing of metamorphism, alteration and Au mineralization from titanite, rutile, and monazite UPb geochronology

Precision measurement of halflives and specific activities of ²³⁵U and ²³⁸U