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San Manuel, Kalamazoo |
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Arizona, USA |
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Cu
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The San Manuel - Kalamazoo mines exploited a large porphyry style deposit by underground block caving techniques north of Tucson in Arizona. The classic Lowell and Guilbert porphyry model was based on this deposit. It is virtually unique in that it is centred on a 67 Ma Palaeocene quartz- monzonite porphyry plug intruding a larger Middle Proterozoic porphyritic quartz-monzonite body. Approximately half of the orebody is hosted by each. As such this deposit is almost unique in being hosted by a virtually homogenous mass of rock. Historic production + reserves have been near 1000 Mt @ 0.7% Cu. The operation was owned by Magma Copper and subsequently by BHP Billiton. Operations at the San Manuel mine were suspended 0n 25 June, 1999, with final closure on 15 January, 2002.
The San Manuel and Kalamazoo porphyry copper deposits are two parts of the same original orebody which have been tilted and offset by the low angle, west dipping, extensional San Manuel Fault. They lie in the footwall and hangingwall of that fault respectively. The orebodies are mainly hosted by Late Cretaceous monzonite porphyries, Mesoproterozoic quartz monzonite and minor Proterozoic diabase (dolerite) (Sandbak and Alexander 1995). Only a thin chalcocite enrichment blanket is found in the upper palaeosurface above the Kalamazoo deposit in the upper plate, while San Manuel, below the fault, contains evidence of multiple stages of enrichment and oxidation which may be correlated with erosional intervals in the Cenozoic stratigraphy of the district (Heindl, 1963; Creasey. 1967).
The hypogene ore deposits are related to Laramide age (67 to 69 Ma) monzonite (or granodiorite) porphyry intrusions which have been strongly altered over a radius of 1.5 to 2 km around the orebodies. These porphyries cut: i). the more extensive, pre-mineralisation coarse, porphyritic quartz monzonites to monzogranites of the Mesoproterozoic (1440±20 Ma) Oracle Granite batholith; and ii). lesser pre-ore diabase (dolerite), believed to be comparable in age to similar Mesoproterozoic intrusions at the Ray Mine (Sandbak and Alexander, 1995; Thomas, 1966; Lowell, 1968).
Post ore lithologies include: i). weakly mineralised and altered igneous breccias formed near the host monzonite-granodiorite contact and composed of altered clasts of the host intrusives; ii). dacite porphyry, believed to be a late Laramide intrusion, similar in composition and of similar age to the monzonite porphyry host, but darker in colour and unaltered; iii). andesite to andesite porphyry, believed to be Mid-Tertiary in age, occurring as sills and dykes cutting all of the previously described rocks; iv). the Cloudburst Formation, comprising up to 1750 m of sediments and volcanics, commencing with a 1200 to 1500 m thick lower unit of interlayered andesite to latite flows, flow breccias, tuffs and conglomerates, with related stocks; and an upper fanglomerate sequence composed predominantly of conglomerate with a muddy arkosic matrix and felsic porphyry (Laramide and Proterozoic) clasts; this unit is believed to have been deposited between 28 and 22.5 Ma; v). rhyolite as dykes and pods cutting the host sequence, expanding to become an extrusive ashflow tuff near the top of the Cloudburst Formation; vi). the San Manuel Formation, which is at least 22 m.y. old, rests disconformably on the Cloudburst Formation, and comprises red to grey conglomerates with large boulders of various intrusive rock types; it dips at 30 to 40°NE and is at least 300 m thick; vii). the Quiburis Formation, occurring in the San Manuel area as a Mid-Miocene to Pliocene gravel that dips gently to the NE (Sandbak and Alexander 1995).
Alteration and mineralisation are centred on the Laramide monzonite-granodiorite porphyry intrusive rocks, comprising a core of potassic alteration, composed of K feldspar and biotite, surrounded by a 300 to 450 m wide phyllic zone of quartz-sericite alteration with over 10% sulphide, dominantly pyrite. The outer limits of the hydrothermal alteration/mineralisation system are defined by a propylitic halo characterised by chlorite epidote, calcite and anhydrite. The deposit has a barren core within the potassic zone with <0.3% Cu. This is surrounded by a 30 to 300 m wide ore shell developed in the potassic alteration zone, outward from the barren core, and adjacent to the contact with the surrounding phyllic alteration zone. Mineralisation in the ore shell is composed of fine disseminations and microveinlets of chalcopyrite, pyrite, molybdenite and minor bornite, totalling less than 2% sulphides, with a pyrite:chalcopyrite ratio of 2:3 (Sandbak and Alexander 1995).
The supergene zones of oxidation and enrichment at San Manuel bear no relation to the pre-mining topography or water table. They are faulted, tilted and faulted again. The effects of oxidation are seen to depths of over 400 m at the western end of the deposit, but only to around 3 m in the east. The first episode of supergene enrichment occurred in the Late-Eocene to earliest Oligocene, prior to the deposition of the Oligocene Cloudburst Formation. Supergene alteration changed from chrysocolla-rich to chalcocite-dominant during the first cycle as progressively more pyritic protore was exposed by contemporaneous tilting (Schwartz 1949). The secondary sulphide zone pinches out between the overlying oxide and underlying hypogene sulphides, presumably as a result of tilting relative to the water table. The first cycle of supergene activity was terminated by burial beneath the Cloudburst Formation. A second episode commenced after tilting and erosion of the Cloudburst Formation and prior to the deposition of the San Manuel Formation (sometimes correlated with the Gila Conglomerate). This second stage is characterised by widespread oxidation of first stage chalcocite to chrysocolla during Miocene time and is interpreted to correlate with either the erosion surface between the Cloudburst and San Manuel Formations, or following the latter. Both supergene oxide and sulphide assemblages are largely absent at Kalamazoo. However, a thin blanket of chalcocite, with no evidence of Miocene oxidation, is found on the eastern end of the orebody below the Cloudburst Formation, where it has been tilted to an almost vertical orientation. This implies that the Kalamazoo orebody was offset from the San Manuel deposit before the second stage of supergene alteration at San Manuel.
The most recent source geological information used to prepare this decription was dated: 1998.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Force E R, Dickinson W R, Hagstrum J T 1995 - Tilting history of the San Manuel-Kalamazoo Porphyry system, southeastern Arizona: in Econ. Geol. v90 pp 67-80
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Lowell J D, Guilbert J M 1970 - Lateral and vertical alteration-mineralization zoning in porphyry ore deposits: in Econ. Geol. v65 pp 373-408
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Sandbak L A, Alexander G H 1995 - Geology and rock mechanics of the Kalamazoo Orebody, San Manuel, Arizona: in Pierce F W, Bolm J G (Eds), Porphyry Copper Deposits of the American Cordillera Arizona Geol. Soc. Digest 20 pp 396-423
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Thomas L A 1966 - Geology of the Sam Manuel ore body: in Titley S R, Hicks C L 1966 Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 133-142
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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