Defining the Upper Parts of an Alkalic Porphyry Copper-Gold Deposit: The Evolution of the Porphyry Copper-Gold Deposit at Red Chris, Northern British Columbia (NTS 104H)
- Craig Hart, Mineral Deposit Research Unit, UBC

Red Chris Prospect location The Late Triassic Red Chris porphyry Cu-Au prospect in northern British Columbia hosted in supracrustal rocks of the Mesozoic Stikine Terrane represents an important emerging mineral exploration property in northern British Columbia. We are undertaking a collaborative MDRU-Imperial Metals-Geoscience BC project to examine what we consider to be the preserved upper parts of an alkalic porphyry Cu-Au deposit, which extends, based upon recent exploration, to as much as 1-km below the present surface.

Porphyry Cu-Au deposits result from multiple and successive vein formation events whose mineralogy (sulfide and alteration) reflect the hydrothermal fluid chemistry, flow paths, and the pressure-temperature conditions during formation. The distribution of the mineralization products defines the hypogene ore, controls the economics of a porphyry Cu deposit, and also affects the oxidation during weathering. The zoning is accompanied by different mineral assemblages, which reflect the hydrothermal fluid chemistry and evolution of those fluids as well as depth within the porphyry environment. Understanding the distribution of the alteration assemblage, their relationship to veins and rock types, and origin is important to any 4-D model of a deposit, and is a major goal of the project. Their relative timing provides the 4th dimension. Understanding the core of the porphyry is important to the parallel study of the geochemical dispersion halo around the deposit.

Graduate student research will characterize the evolution of the Main and East Zone porphyry deposit with the intention of building an evolutionary model of the deposit. Specific questions important to building a 4-D model for the Cu-Au porphyry deposits include evolution of the hydrothermal system, structural setting, and the vertical and lateral development of the hydrothermal system. Activities and techniques to be used include mapping of the deposit along sections through the re-logging of core holes, structural analysis of vein geometry, mineral chemistry, and alteration mineralogy. Two sections constructed using drill core across the Main and East deposits will constrain the research. The distribution of alteration and evolution in time will be the principal theme to be addressed. Senior research staff will also evaluate the down-hole multi-element geochemistry in the context of the SWIR mineral chemistry, and provide this input to the graduate project.

In addition to defining the distribution and sequencing of alteration, 3 aspects of the deposit will be examined in detail, in addition to building the geometric distribution of rocks and alteration assemblages.

  1. Carbonate minerals: Baker et al. (1997) have previously defined a sequence of carbonate mineral deposition. As a routine part of the alteration study, the project will examine through the mineral chemistry as well as the carbon and oxygen isotopes the origin of the carbonate minerals, to address the question of fluid sources and why so much carbonate is found within the deposit.
  2. Rutile chemistry:, Rutile in alkalic porphyry deposits has been shown to vary in chemical composition, with vanadium substituting for titanium, at the Northparkes deposit (Scott, 2005). A systematic study has never been completed on a BC alkalic deposit, and the Red Chris deposit, in view of the deep drilling, offers an opportunity to develop the mineral as a potential vector toward ore as well as during regional exploration geochemistry in vegetated terranes.
  3. Structural geometry: Quartz veins generally form systematic networks in porphyry deposits (Tosdal and Richards, 2001), and as such can be used to gain some degree of control on the post-mineral deformation of any deposit. As most of the BC porphyry deposits are tilted to some degree (Sketchley et al., 1995), placing constraints on the Red Chris system will aid in the resource delineation, and subsequent exploration in the district.

Posters and Presentations
2012: The Red Chris Cu-Au Porphyry Deposit: Pervasive Intermediate Argillic Alteration
- Mineral Exploration Roundup Poster (pdf, 3.1 MB)

2011: Alteration and Mineralization at the Red Chris Cu-Au Porphyry Deposit, northwestern British Columbia
- Mineral Exploration Roundup Poster (pdf, 4.8 MB)

2010: Alteration and Mineralization at the Red Chris Cu-Au Porphyry Deposit, Northwestern British Columbia
- Mineral Exploration Roundup Poster (pdf, 4.04MB)
Technical Articles
2011: "Magmatic Evolution, mineralization and alteration of the Red Chris copper-gold porphyry deposit, northwestern British Columbia"
- Summary of Activities 2010, Report 2011-1 p.33-44 (pdf, 8.0 MB)

2010: "Preliminary study of the magmatic evolution, mineralization and alteration of the Red Chris copper-gold porphyry deposit, northwestern British Columbia (NTS 104H/12W)"
- Summary of Activities 2009, Report 2010-1 p.77-86 (pdf, 7.1 MB)

Final Deliverables
M.Sc. Thesis - J.R. Norris
Evolution of alteration and mineralization at the Red Chris copper-gold porphyry deposit East zone, northwestern British Columbia - University of British Columbia, 2012 - Available digitally through UBC (pdf, 12.3 MB)

Abstract: Located in northwestern British Columbia within the Stikine terrane, the Red Chris Cu-Au porphyry deposit is hosted in the Late Triassic Red Stock (~203.8 Ma). The Red Stock is a quartz monzodiorite to monzonite intrusion hosted in the broadly contemporaneous volcanic rocks of the Stuhini Group. Red Chris has features that are characteristic of calc-alkalic and alkalic porphyry deposits and shares many similarities with the Ridgeway deposit of the Cadia district in New South Wales, Australia. A combined measured and indicated resource of 936 million tonnes at 0.374 % Cu, 0.385 g/t Au, and 1.224 g/t Ag has been outlined from the Main and East zones. Copper and gold are associated with bornite, chalcopyrite and lesser pyrite, hosted in quartz veins and stockworks as disseminations and fracture-controlled veinlets. High-grade mineralization is directly associated with high quartz vein density. Copper-iron sulphide minerals are laterally zoned, with a bornite > chalcopyrite core, grading outward to a chalcopyrite > pyrite shell and outward and upward to a pyrite > chalcopyrite halo. Five major groups of veins are recognized, of which the oldest two sets contain much of the copper and gold. Stable isotopic analysis indicates the presence of magmatic and mixed magmatic-meteoric hydrothermal fluids. Evidence from sulphur isotopes demonstrates a high temperature oxidized magmatic fluid was responsible for transporting and depositing much of the copper and gold. A vertical and lateral zonation in sulphur isotopes exists, whereby deep regions exhibit δ34S values between -1.9 to -0.9 % and transition to near-surface regions in the pyrite halo that exhibit δ34S values between +0.9 to +1.9 %. Isotopic analysis of oxygen and deuterium of hydrothermal alteration minerals provide evidence for a magmatic fluid (secondary biotite and muscovite) and a mixed magmatic-meteoric fluid (illite and kaolinite). Low temperature clay alteration (illite-kaolinite; intermediate argillic assemblage) significantly overprinted high temperature alteration (K-silicate, phyllic) in the upper levels of the system and gradually diminished intensity with depth. Carbonate veins and alteration also characterize the shallow levels and isotopic analysis of carbon and oxygen suggest a magmatic source with the possibility of minor mixing with an external meteoric fluid.

 


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