The Bristol PCD project
Volcanism, magmatism, and hypogene mineralisation
Using Cerro Colorado as an example, by combining the observations in the field, zircon U–Pb age data from each of the main rock units, and fluid inclusion results, her work suggests a close temporal link between copper mineralisation and volcanic activity. In addition to the evidences discussed in the submitted manuscript, results of fluid inclusion analysis are consistent with syn-volcanic mineralisation. Around 60% of the inclusions recorded trapping temperature at 400–600 ˚C, locally reaching 700˚C. The high temperature group suggests the fluid had a direct magmatic origin. Additionally, a wide range of formation pressure was recorded in the main stage chalcopyrite-bearing quartz veins in CMCC, which would require emplacement depths in the range 0.2–3.8 km if periodic transition between hydrostatic and lithostatic pressures is assumed. A more plausible explanation is to consider a shallow crustal emplacement depth ca. 1.6–3 km. The observed pressure range from sub-hydrostatic to supra-lithostatic then suggests localized fracturing and short-lived connectivity to surface, implying fluid release. The healing of fractures would bring the system back to lithostatic pressure
(ii) Ed Bunker (Scale: Prospect/Cluster, Stage: Target Testing)
Ed has separated and mounted zircons from the remaining six representative samples of the Spence igneous suite, providing a comprehensive sample set for geochronology and trace element analysis. A preliminary dataset of trace element analysis of zircons from four samples was obtained by Laser Ablation - Inductively Coupled Plasma - Mass Spectrometry, which are being studied in conjunction with the data obtained from the BHPB trace element campaign carried out in 2016 in conjunction with the NERC Isotope Geoscience Laboratory. The trace element in zircon studies are focussed upon systematic core-to-rim variations within zircons, which are representative of chemistry change within the evolving pluton, in order to understand fractionation trends. Trace element signals in zircons may help to constrain processes occurring at the transition between porphyry and plutonic systems which are indicative of fertility with respect to metal endowment. Zircons that have been analysed for trace elements will also be submitted for high-precision ID-TIMS U-Pb dating in order to ascribe temporal significance to the trace element variations, and better track the geochemical change of the system with time. Fractionation trends have been observed within rare earth elements which are analogous to previous studies of porphyry copper deposits however further work is required before this can be confidently attributed to specific phases. “Dark zones” are observed in cathodoluminescence images of a subset of zircons from Spence which are highly enriched in Th and REEs. Such zones have been observed in other PCDs and the relationship between these zones and pluton interaction will be probed further.
(iii) Luke Neal (Scale: Prospect, Stage: Target Testing)
Luke has continued to work on a quartz diorite from the Adamello Massif, Italy, as a case study for testing the techniques that will be applied to plutonic rocks genetically related to porphyry deposits. He has collected and interpreted EPMA major element chemical data, and has collected trace element data for a number of mineral phases using LA-ICP-MS. Luke has also received thin sections from the rock samples he collected on fieldwork in Chile, and has begun petrographic investigation of these to determine which samples are most viable for study. In addition, Luke has been developing a model that represents the evolution of trace elements in a magma undergoing fractional crystallisation, based on an experimental study, and is continuing to refine this. Over the next few months, Luke plans to (i) analyse trace element data for minerals in the Adamello sample to assess the viability of certain mineral phases as recorders of magma evolution; (ii) perform in-depth petrographic study the rock samples from Chile and begin major element chemical analysis on the SEM and EPMA; (iii) source further appropriate samples for study (fieldwork reconnaissance and/or browsing collections); and (iii) begin building a database of mineral-melt partition coefficients.
(iv) Other work
The paper on thermal modelling related to emplacement of the Yerington Batholith (Schöpa et al.) has been accepted by Economic Geology with minor revisions. Steve Sparks will present this research at the workshop.
Tectonics, landforms, and supergene mineralisation
(i) Frances Cooper (Scale: Cluster/Prospect, Stage: Target Definition/Target Testing)
Frances is continuing to work on hematite- and alunite-bearing samples from Cerro Colorado and Spence in order to directly compare how uplift, climate, and sedimentary deposition have affected supergene enrichment. Following her visit to the Scottish Universities Environmental Research Centre in March, she is working on preparing samples for 40Ar/39Ar analysis. Frances has also sent nine hematite-bearing samples collected from Spence to the California Institute of Technology for (U-Th)/He analysis, and hopes to get results back in time for the June workshop. The samples are taken from a single vertical drill-hole through the Spence leached cap and therefore hematite ages should reflect movement of the water table over time that can be directly compared to existing data from Cerro Colorado. This joint alunite-hematite dating approach will also be implemented in the new CASE-funded PhD studentship with Joe Shaw.
Frances will be taking 6 months of maternity leave from mid September 2017 until mid March 2018. During this time, Laura Evenstar will teach her undergraduate Economic Geology course and step in as temporary lead supervisor for new student, Joe Shaw's project. Jon Blundy will continue to provide supervision for Frances' other students, Ed Bunker and Simon Dahlström.
(ii) Laura Evenstar (Scale: Arc Segment/Cluster, Stage: Target Testing)
In January 2017, Laura conducted a reconnaissance trip to Northern Chile with Simon Dahlström, Luke Neal and Prof Anne Mather (Plymouth University). The aim of the field trip was to investigate the feasibility of a detailed detrital zircon study between the two metallogenic belts (Paleocene to Early Eocene belt and the economically richer Middle Eocene to Early Oligocene) within the Precordillera. Two locations were selected which hosted sediments that span the time period of arc emplacement, uplift and erosion (Late Cretaceous to Miocene). One location lies 10 km to the east of the Cerro Colorado Mine and a second, 15 km to the west of the El Abra Mine. These sites are over 200 km from each other allowing us to test how these processes may vary from the north to south. A preliminary cross section of the main sedimentary types and their relationships was constructed in the El Abra region and a series of samples taken for preliminary detrital zircon analysis. Laura's Earth Sciences Reviews paper "Geomorphology on geological time-scales: Evolution of the Late Cenozoic Pacific Paleosurface in northern Chile and southern Peru" has been accepted for publication and is now in press. She will be finalising a new manuscript entitled "Regional controls on water table depth in the Northern Atacama Desert" over the next few months. For the past 3 months, Laura has paused her work on the BHPB project in order to work on another small funded project, but will return in June.
(iii) Simon Dahlström (Scale: Arc Segment, Stage: Reconnaissance)
Simon spent three weeks at Arizona State University in April carrying out (U-Th)/He thermochronology on zircons and apatites. The first results from the analyses are expected in the following month. (U-Th)/He thermochronology provides information on the cooling history of the samples. Combined with the results from the zircon U-Pb, Al-in-hornblende, and apatite fission track analyses, the (U-Th)/He data can be used to determine the most likely exhumation path for each sample. Thermal modeling, using the finite element modeling code Pecube, will be done when the (U-Th)/He data have been obtained. Pecube uses thermochronometer data to model the evolution of the geothermal gradient and calculates the most likely exhumation history for each sample.
Discovering the story of porphyry copper deposits...
Thursday, 27 July 2017 12:16