FI9700011 POSIVA-96-1 5 The Hyrkkbla native copper mineralization as a natural analogue for copper canisters Nuria Marcos October 1 996 POSIVA OY Annankalu 42 D. FIN-OO1OO HELSINKI. FINLAND *UL 2 8 IK? Q 5 Phone (09) 228 030 (nat ). ( + 358-9-) 228 030 (int.) Fax (09) 2280 3719 (nat.). ( + 358-9-) 2280 3719 (int ) POSIVA-96-1 5 The Hyrkkola native copper mineralization as a natural analogue for copper canisters Nuria Marcos Helsinki University of Technology Laboratory of Engineering Geology and Geophysics October 1996 POSIVA OY Annankatu 42 D. FIN-OO1OO HELSINKI. FINLAND Phone (09) 228 O3O (nat.). ( + 358-9-) 228 030 (int.) Fax (09) 2280 3719 (nat.), ( + 358-9-) 2280 3719 (int.) ISBN 951-652-014-6 ISSN 1239-3096 The conclusions and viewpoints presented in the report are those of author(s) and do not necessarily coincide with those of Posiva. Posiva-raportti - Posiva report Raportinmnnus-Report co* POSIVA-96-15 Annankatu 42 D, FIN-00100 HELSINKI, FINLAND Juikasuaka-Date Puh. (09) 2280 30 - Int. Tel. +358 9 2280 30 October 1996 Tekija(t) - Author(s) Toimeksiantaja(t) - Commissioned by Nuria Marcos Helsinki University of Technology Posiva Oy Laboratory of Engineering Geology and Geophysics Nimeke - Title THE HYRKKOLA NATIVE COPPER MINERALIZATION AS A NATURAL ANALOGUE FOR COPPER CANISTERS Tiivistelma - Abstract The Hyrkkola U-Cu mineralization is located in southwestern Finland, near the Palmottu analogue site. The age of the mineralization is estimated to be between 1.8 and 1.7 Ga. Petrological and mineralogical studies have demonstrated that this mineralization has many geological features that parallel those of the sites being considered for nuclear waste disposal in Finland. A particular feature is the existence of native copper and copper sulfides in open fractures in the near-surface zone. This allows us to study the native copper corrosion process in analogous conditions as expected to dominate in the nuclear fuel waste repository. The occurrence of uranyl compounds at these fractures permits also considerations about the sorption properties of the engineered barrier material (metallic copper) and its corrosion products. From the study of mineral assemblages or paragenesis, it appears that the formation of copper sulfide (djurleite, CU1.934S) after native copper (Cu°) under anoxic (reducing) conditions is enhanced by the availability of dissolved HS" in the groundwater circulating in open fractures in the near-surface zone. The minimum concentration of HS" in the groundwater is estimated to be of the order of 105 M (~1(H g/1) and the minimum pH value not lower than about 7.8 as indicated by the presence of calcite crystals in the same fracture. The present study is the first one that has been performed on findings of native copper in reducing, neutral to slightly alkaline groundwaters. Thus, the data obtained is of most relevance in improving models of anoxic corrosion of copper canisters. ISBN ISSN ISBN 951-652-014-6 ISSN 1239-3096 Sivumaara - Number of pages Kieli - Language 39 + Appendices English Posiva-raportti - Posiva report POSIVA-96-15 Annankatu 42 D, FIN-00100 HELSINKI, FINLAND Juikasuaika- Date Puh. (09) 2280 30 - Int. Tel. +358 9 2280 30 Lokakuu 1996 Tekijä(t) - Author(s) Toimeksiantaja(t) - Commissioned by Nuria Marcos Teknillinen korkeakoulu Posiva Oy Insinöörigeologian ja geofysiikan laboratorio Nimeke - Title HYRKKÖLÄN LUONNONKUPARIESIINTYMÄ KUPARIKANISTERIN LUONNONANALOGIANA Tiivistelmä - Abstract Hyrkkölän luonnonkupariesiintymä sijaitsee Lounais-Suomessa, lähellä Palmottua, jossa tutkitaan uraaniesiintymää analogiana kallioon loppusijoitettavalle käytetylle uraanipolttoaineelle. Minerali- saation ikä on 1800 - 1700 miljoonaa vuotta. Mineralogiset ja petrografiset tutkimukset osoittavat, että luonnonkupariesiintymä vastaa niitä olosuhteita, jotka vallitsevat loppusijoituksessa syvällä (300 - 800 metrin syvyydellä) graniittisessa peruskalliossa. Eräs ilmiö on luonnonkuparin ja kupari- sulfidin esiintyminen avoimissa raoissa lähellä maanpintaa. Tämä mahdollistaa luonnonkuparin korroosioilmiöiden tutkimisen samoissa olosuhteissa, joiden odotetaan vallitsevan loppusijoitus- paikassa. Myös U+6:n esiintyminen niissä raoissa antaa mahdollisuuden selvittää tarkemmin niin luonnonkuparin kuin sen korroosiotuotteiden sorptio-ominaisuuksia. Mineraaliparageneesien perusteella käy ilmi, että kuparisulfidi (djurleite, CU1.934S) muodostuu luonnonkuparista pelkistävissä olosuhteissa, joissa pohjaveteen liuennut HS' kulkee avoimissa raoissa lähellä maanpintaa. Pohjaveden HS":n minimipitoisuuden arvioitiin olevan noin 105 M (~ 10'4 g/l) ja minimi-pH-arvon noin 7,8, perustuen raoissa esiintyviin kalsiittikiteisiin. Tämä tutkimus on ensimmäinen, joka on suoritettu sellaisessa paikassa, jossa luonnonkupari esiin- tyy pelkistävissä olosuhteissa. Saatua dataa voitaisiin käyttää päivittämään kuparikapselin korroosio- malleja. ISBN ISSN ISBN 951-652-014-6 ISSN 1239-3096 Sivumäärä - Number of pages Kieli - Language 39 + liitteet Englanti CONTENTS ABSTRACT THVISTELMA PREFACE 1 INTRODUCTION 1 1.1 Background 1 1.2 Objectives 2 2 GEOLOGY 3 2.1 Geological setting 3 2.2 Petrology of the rock association at Hyrkkola 6 2.2.1 Amphibolites 6 2.2.2 Gneisses 7 2.2.3 Granite pegmatites 7 2.3 Bedrock fracturing 7 2.4 U-Cu mineralization 8 2.4.1 Native copper 8 2.4.2 Uranium minerals 10 2.4.3 Associated minerals 10 2.4.4 Genesis and age of mineralization 12 3 SUMMARY OF THE HYRKKOLA MINERALIZATION 13 4 FRACTURE MINERALOGY 15 4.1 Experimental techniques 17 4.2 Cuprite composition 17 4.3 Copper sulfide composition 18 4.4 Native copper composition 20 4.5 Fracture surface and uranium distribution 21 5 THERMODYNAMICS OF COPPER CORROSION AT HYRKKOLA 28 5.1 Hydro thermal corrosion, cuprite, sealed fractures 28 5.2 Corrosion in open fractures: djurleite 29 6 FEATURES IN COMMON WITH PROPOSED REPOSITORY SITES 32 7 CONCLUSIONS 35 8 REFERENCES 36 APPENDICES PREFACE This work was carried out at the Helsinki University of Technology (HUT) on contract for Posiva Oy. The contact persons were Juhani Vira and Marjo Matikainen at Posiva and Nuria Marcos at HUT. The author thanks Lassi Pakkanen and Bo Johanson (GSF, Geological Survey of Finland) for performing the SEM analyses and Lasse Ahonen for valuable discussions. 1 INTRODUCTION 1.1 Background In Finland and Sweden spent nuclear fuel is planned to be disposed of in a repository to be constructed at a depth of about 300 - 800 m in crystalline bedrock. The nuclear waste will be isolated from the biosphere by multiple technical and natural barriers. In assessing the performance of the system, the corrosion of a copper canister (technical barrier) is considered. Numerous laboratory studies have investigated the corrosion rates and mechanisms of metallic copper and its alloys (SKI Report 94:6, 1994, contains more than 150 references). Whilst these experiments may provide useful indications of the relative durability of the metallic copper, few are conducted in conditions that adequately simulate those expected in a repository (Miller et al., 1994). The authors of the most recent laboratory study dealing with the long-term predictions of copper canisters corrosion under oxic and anoxic conditions (Wersin et al., 1994) state that chemical corrosion of copper canisters does not appear to constitute a problem for repository safety but other factors such as increased temperature have not been explicitely included in their model. Miller et al., 1994, write: "One of the investigations of copper in a more repository relevant environment is that of Marcos (1989), which shows that copper is stable, and has remained stable, under a broad range of hydro- geochemical environments, but none of these were totally representative of the repository environment." To cover this gap, a native copper mineralization in crystalline bedrock at Hyrkkola (SW Finland), is herein evaluated as a natural analogue for copper canister corrosion behaviour under repository conditions. 1.2 Objectives The main objectives of this study on the HyrkkOla Cu mineralization were: to clarify how the Hyrkkbla Cu mineralization corresponds to the circumstances prevailing in the final repository planned by Posiva in crystalline bedrock to a 300 - 800 m depth, and to estimate the chemical conditions of the groundwater in fractures in order to evaluate the existing models for copper corrosion (mainly sufldization) in the deep bedrock environment. To achieve the first objective, geological, petrographical and mineralogical studies are presented. The estimation of the geochemical conditions under which native copper has been preserved is based on these studies. To address the second objective detailed quantitative and qualitative fracture mineral studies are presented. The estimation of the geochemical condition under which the fracture mineral assemblage has been formed and preserved, and an evaluation of the models for copper corrosion (mainly sulfidization) are based on these studies. 2 GEOLOGY 2.1 Geological setting The native copper mineralization at Hyrkkola is related to a main uranium mineral- ization and thus in the following it is referred to as the Hyrkkbla U-Cu-mineral- ization. It was found in the 1980's by car-borne surveys utilizing a Scintrex BGS-3 radiometric instrument mounted in a vehicle. The Hyrkkola U-Cu-mineralization is situated about 10 km NE from the Palmottu U-Th deposit, SW Finland (Fig. 1). The investigation area is within a zone of metamorphosed supracrustal volcanic and sedimentary rocks, which extends from SW Finland to Central