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Geology, geochronology and alteration of the Lorraine alkalic porphyry Cu-Au deposit, British Columbia, Canada

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Bath, AB (2010) Geology, geochronology and alteration of the Lorraine alkalic porphyry Cu-Au deposit, British Columbia, Canada. PhD thesis, University of Tasmania.

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Abstract

The present study assesses ore-forming processes and the timing of mineralisation in the
Lower Main Zone of the Lorraine porphyry Cu deposit, north-central British Columbia.
The deposit is hosted in an intrusion complex that comprises pre-mineral and post-mineral
shoshonitic ultrabasic to syenite rock types. New U-Pb zircon ages of pre- and post-mineralisation
dykes constrain the timing of mineralisation between 178.8 and 178.4 Ma. Intrusions
in the Lower Main Zone have undergone several stages of alteration, which
include early-, transitional- and late-stage assemblages. Early stage alteration includes;
(1) pervasive biotite and K-feldspar alteration of wall rocks; (2) veins and patches of magnetite-diopside
± albite and (3) early coarse-grained K-feldspar-biotite veins that are locally
associated with bornite and chalcopyrite. Transitional-stage mineralisation has produced
distinctive sulphide zonation patterns consisting of a bornite-chalcocite core grading outwards
to domains of bornite-chalcopyrite, chalcopyrite, chalcopyrite>pyrite, and a peripheral
domain of pyrite with minor chalcopyrite. The sulphide zonation pattern is tilted and
has overprinted numerous rock-types and the early-formed alteration assemblages. Syenite
in the inner bomite-chalcopyrite zone typically contains abundant turbid K-feldspar (i.e.
>70 %), whereas syenite marginal to the bomite-chalcopyrite core contains 50-70 % Kfeldspar,
indicating an increase in K-metasomatism of syenites towards the core of the deposit.

Transitional-stage mineralisation at Lorraine predominantly occurs as fine-grained disseminated
sulphides in syenite, biotite pyroxenite and fine-grained K-feldspar biotite rock.
Rare net-textured chalcopyrite and bomite mineralisation has also been identified in some
biotite pyroxenites. Earlier workers speculated that biotite pyroxenites with net-textured
sulphides at Lorraine may represent a 'deep' magmatic component of the porphyry system.
However textural analysis of mineralised biotite pyroxenite appears to be at odds with this
concept. In particular the current study has shown that: (1) primary diopside, which form
contacts with sulphides, have corroded and actinolite-altered margins, suggesting alteration
of primary minerals; (2) primary diopside and biotite do not contain primary sulphide inclusions and therefore there appears to be no evidence of a primary magmatic sulphide
melt; (3) irregular-shaped relicts of biotite and diopside occur in sulphides, suggesting
sulphides have replaced primary minerals and (4) deposit-scale sulphide zonation patterns
overprints numerous lithology types, including biotite pyroxenites with net-textured sulphides.
These findings support the author's contention that primary magmatic biotite and
diopside were replaced or partially replaced by sulphide minerals, and are consistent with
an influx of Cu and S during transitional-stage alteration and subsequent replacement of
primary magmatic minerals (e.g., biotite) as opposed to the precipitation of primary magmatic
sulphides from a Cu-rich ultrabasic magma source.
Analysis of oxygen and hydrogen isotopes from F-rich biotite in early altered fine-grained
K-feldspar biotite rock and fine-grained biotite in altered syenite has revealed the composition
of fluids in equilibrium with early alteration assemblages. Fluid compositions define
a range between +6.1 %o and +7.6 %o for 6 18 0, and -53 %o and -47 %o for D. These values
are consistent with the range for magmatic-derived aqueous fluids or metamorphic fluids.
A magmatic fluid source is consistent with the close timing of alteration and mineralisation
and the emplacement of syenite intrusions, and biotite geothermometry indicates that early
alteration occurred at approximately 550 °C.
The current study also investigated the chemical composition of unaltered shoshonitic
rocks from Lorraine. The study concluded that biotite pyroxenite and syenite from Lorraine
have major and trace element compositions that plot along a single trend, which is
similar to fractionation trends for arc-related Fijian shoshonite lavas or shoshonitic glasses
from the Nicola Group lavas (British Columbia). It is inferred that geochemical trends
for Lorraine rocks reflect fractionation trends, and it is also inferred that syenite dykes
were sourced from magma chambers at depth, or from fractionating melt in mature crystal
mush columns. Ultrabasic rocks at Lorraine (i.e. biotite pyroxenite; —42 to 48 wt. % Si0 2)
include: (1) depletion in high field strength elements in comparison with rare earth elements;
(2) enrichment in large ion lithophile elements and P in comparison with rare earth
elements; (3) 87 Sr/86 Sr (avg. = 0.7036), ENd (avg. = + 4.7) and stable isotope 8 18O (diopside = 5.89) compositions that are similar to post-collisional shoshonites from the Mariana arc
and Fiji and (4) an abundance of P (avg. —0.87 wt. % P 205 ) and Sr (avg. —1200 ppm),
which is higher or similar in abundance to Fijian shoshonites respectively. Some workers
have related the high P and Sr in western Pacific shoshonites to the melting of slab-derived
carbonate- and phosphorous-rich sediments in the mantle, which could also explain the
enrichment of these elements in Lorraine shoshonites.

Item Type: Thesis (PhD)
Keywords: Copper ores, Geochemistry
Copyright Holders: The Author
Copyright Information:

Copyright 2010 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

No access or viewing until 1 November 2011. After that date, available for use in the Library and copying in accordance with the Copyright Act 1968, as amended. CD-ROM contains thesis and appendices. Thesis (PhD)--University of Tasmania, 2010. Includes bibliographical references. Ch. 1. Introduction -- Ch. 2. Regional and district scale geology -- Ch. 3. Geology and geochronology of the Lower Main Zone -- Ch. 4. Overview of alteration and mineralisation in the Lower Main Zone -- Ch. 5. Detailed description of transitional stage mineralisation -- Ch. 6. Mineral geochemistry of the Lorraine deposit -- Ch. 7. Igneous geochemistry -- Ch. 8. Radiogenic and stable isotopes -- Ch. 9. Discussion and conclusions

Date Deposited: 25 Nov 2014 00:58
Last Modified: 23 Jun 2016 22:56
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