Hydrothermal evolution in the Calabona porphyry copper system (Sardinia, Italy); the path to an uneconomic deposit
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The Calabona porphyry copper system is developed in a small, relatively deep seated ( nearly equal 5 km) dacitic intrusion located in northwestern Sardinia (Italy). Early hydrothermal alteration produced a potassic assemblage in the deep and central parts of the complex, and a peripheral propylitic halo. Sodic alteration was subsequently superimposed on the potassic zone and volumetrically dominant phyllic alteration overprinted the apical parts of the intrusion. Hypogene copper mineralization (chalcopyrite and minor bornite) was associated with potassic alteration. The earliest fluid that circulated in the Calabona porphyry complex had high salinity (40-60 wt % NaCl equiv), and is interpreted to have been exsolved directly from the crystallizing magma. However, major entrapment of this fluid only occurred after it had cooled to temperatures of about 400 degrees C. This fluid was responsible for potassic alteration and for precipitation of chalcopyrite and bornite in thin, discontinuous group 1 veins and irregular and widely spaced group 2 veins. The average Cu grade in the potassic zone is between 0.05 and 0.1 percent. The circulation of a lower temperature (270 degrees -330 degrees C), Ca-enriched fluid of meteoric origin in the peripheral parts of the system caused propylitic alteration. At an intermediate stage of hydrothermal evolution, waters of external origin entered the central parts of the system along a network of late fractures (group 3 veins) or reopened group 2 veins. Partial mixing of this meteoric-formational water with the high-salinity fluid already circulating in the system, created a fluid characterized by salinities ranging from 2 to 23 wt percent NaCl equiv. Circulation of this mixed fluid at relatively low fluid/rock ratios along a prograde thermal path caused sodic alteration. The continuous inflow of meteoric water, and the general temperature decrease in the system, produced progressively more oxidized and acidic fluids, which caused phyllic alteration and intense copper leaching. Late boiling in the apical parts of the phyllic alteration zone favored deposition of chalcopyrite and bornite but did not add significant copper (Cu grade <0.03%). Supergene enrichment led to copper grades higher than those in the phyllic zone (0.07%), but not enough to permit mining. The very low concentration of Cu in the Calabona deposits is surprising, in view of wall-rock alteration, style of mineralization, and a fluid evolution typical of those of many productive porphyry copper systems. We propose that this is mainly a consequence of the raltive deep level of emplacement of the intrusion and the dacitic composition of the magma. These factors are interpreted to have combined to retard melt saturation with alkali chlorife-enriched fluids until late stages of crystallization, which restricted the amount of exsolved fluid and Cu extracted form the melt. As a result, overpressuring in the apical parts of the system was limited, the related fracture density was low, and the system therefore failed to provide the focus for the mineralizing fluids needed to permit bulk concentration of copper to economic levels. Further reasons for the uneconomic nature of the Calabona porphyry were the lack of multiple intrusive events and the sulfide-destructive, Cu-leaching effects of phyllic alteration.Research Article| January 01, 1971 Precambrian Rocks of the Lake Hopatcong Area, New Jersey DAVIS A YOUNG DAVIS A YOUNG Department of Geology, Washington Square College of Arts and Science, New York University, New York, New York 10003 Search for other works by this author on: GSW Google Scholar Author and Article Information DAVIS A YOUNG Department of Geology, Washington Square College of Arts and Science, New York University, New York, New York 10003 Publisher: Geological Society of America Received: 22 Jun 1970 Revision Received: 18 Aug 1970 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1971, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1971) 82 (1): 143–158. https://doi.org/10.1130/0016-7606(1971)82[143:PROTLH]2.0.CO;2 Article history Received: 22 Jun 1970 Revision Received: 18 Aug 1970 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DAVIS A YOUNG; Precambrian Rocks of the Lake Hopatcong Area, New Jersey. GSA Bulletin 1971;; 82 (1): 143–158. doi: https://doi.org/10.1130/0016-7606(1971)82[143:PROTLH]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Precambrian rocks near Lake Hopatcong, New Jersey, form a part of the intensely deformed and metamorphosed Reading Prong. The Lake Hopatcong area is divisible into several northeast-trending fault blocks, each of which contains a mappable stratigraphic sequence of paragneisses and granitic or syenitic rocks.The paragneisses generally are well foliated and well layered. They consist chiefly of biotite-feldspar-quartz gneisses and quartz-oligoclase leucogneisses that are interpreted as metamorphosed potassium-rich sandstones and quartz keratophyres, respectively. A thin well-foliated unit of biotite-plagioclase gneiss is thought to be a metamorphosed sill of gabbroic anorthosite.The granitic and syenitic rocks generally form thick, regionally concordant sheets. They are typically foliated and are composed chiefly of microcline microperthite and plagioclase (or mesoperthite), quartz, and iron-rich hornblende and clinopyroxene. These foliated granitic and syenitic rocks are viewed as syntectonic magmatic intrusives. One regionally discordant, unfoliated sheet of clinopyroxene quartz syenite is probably a late tectonic magmatic intrusive.Mineral assemblages in Lake Hopatcong paragneisses may be assigned to the hornblende granulite subfacies of metamorphism. The presence of Ca-bearing mesoperthite in biotite-feldspar-quartz gneiss indicates that metamorphic temperatures exceeded 700° C, and the assemblage garnet-sillimanite-quartz without cordierite indicates that load pressure was greater than 2.5 kb. The rocks have thus probably been buried to depths in excess of 10 km. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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During the last 10 m.y., the Nanga Parbat Haramosh Massif in the northwestern Himalaya has been intruded by granitic magmas, has undergone high‐grade metamorphism and anatexis, and has been rapidly uplifted and denuded. As part of an ongoing project to understand the relationship between tectonism and petrologic processes, we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism. Our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m.y. We suggest that the rapid uplift of the massif created a dual hydrothermal system, consisting of a near‐surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic/metamorphic volatiles. Meteoric fluids derived from glaciers near the summit of Nanga Parbat were driven deep into the massif along the transpressional faults causing δ 18 O and δD depletions in the gneisses and marked oxygen isotopic disequilibrium between mineral pairs from the fault zones. The discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif. At deeper levels within the massif, infiltration of low δ 18 O magmatic fluids caused δ 18 O depletions in the gneisses within the migmatite zone. These low δ 18 O fluids were derived from the young (<4 Ma), relatively low δ 18 O granites (∼8‰c) that are found within the core of the massif. Geochronological evidence in the form of fission track and 40 Ar/ 39 Ar cooling ages and U/Pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined. Fluid infiltration in the migmatite zone rocks located along the Tato traverse was coeval with metamorphism, granite emplacement, and rapid denudation, in the interval 0.8–3.3 Ma. Finally, we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the Nanga Parbat‐Haramosh Massif.
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