|
Lakeshore |
|
|
Arizona, USA |
| Main commodities:
Cu Mo
|
|
 |
|
|
 |
Super Porphyry Cu and Au
|
IOCG Deposits - 70 papers
|
All papers now Open Access.
Available as Full Text for direct download or on request. |
|
|
The Lakeshore porphyry and associated skarn deposit is some 50 km to the south of the Sacaton and north-west of Silver Bell in Arizona, USA.
Published reserves and production figures include:
Total Deposit 265 Mt @ 0.8% Cu (Total Resource. Einaudi, 1982a).
6.65 Mt @ 0.79% Cu, 0.032 g/t Ag, 0.004 g/t Au (Prod. to 1981, Titley, 1992).
Skarn ore 22 Mt @ 1.7% Cu (Initial Resource 1975, Gilmour, 1982).
8 Mt @ 1.35% Cu (Reserve, 1989, Titley, 1992).
Porphyry ore 37 Mt @ 0.65% Cu (Reserve 1982, USBM).
37 mt @ 0.71% Cu (Reserve, 1989, Titley, 1992).
Acid Soluble ore 14.5 Mt @ 0.77% Cu (Reserve., 1989, Titley, 1992).
The Lakeshore ore deposit is buried beneath 150 m of Tertiary volcanics and sediments and a thin veneer of Quaternary gravels. It was developed in the 1970's as an underground block caving mine. The principal tactite ore was not amenable to block caving (a property apparently common to tactite ores, due to the lower fracturing density as compared to porphyry mineralisation in felsic rocks - S Titley, pers. comm., 1994). Consequently, since 1983 all production has been from leaching of the old caved zones and from in situ leaching of chrysocolla ore surrounding the old stopes (Cook, 1988).
Geology
The deposit lies on the south-western flank of the Slate Mountains which consist largely of the early Middle Proterozoic Pinal Schists. The Pinal schists are predominantly a pelitic schist at Lakeshore and contain no mineralisation. They are unconformably overlain in the district by the post 1600 Ma, Middle Proterozoic, Apache Group which is composed of conglomerate, shale, interbedded basalt, dolomite, limestone and quartzite. The Apache Group is represented by the Mescal Limestone and Dripping Springs Quartzite and is cut by Middle Proterozoic diabase (dolerite) dykes and sills. These in turn are succeeded by Devonian and Carboniferous limestone to the north of the mine area. The sequence in the ore deposit vicinity is overlain by Cretaceous andesitic volcanics and volcani-clastics, Tertiary andesite, andesite breccia and fanglomerates, and by Quaternary alluvium (Cook, 1988; Einaudi, 1982; Hallof & Winniski, 1971). For more detail of the regional lithologies of the stratigraphic units listed above, see the 'Ray' and 'Globe/Miami District' descriptions below.
The sequence is intruded by a north-westerly trending, 4 km long, equigranular, early Tertiary, granodiorite stock. The stock is bounded to the west by the Lakeshore Fault, a major, late Tertiary, west dipping, range front normal fault with a west side down displacement of several thousand metres (see plan no. AMNa ueu). West of the fault, in the block containing the Lakeshore deposit, there are scattered outcrops of Apache Group sediments. These are overlain unconformably by, or are in fault contact with Cretaceous volcani-clastics, which include andesitic breccias. The Lakeshore stock has three parts. These comprise, 1). equi-granular granodiorite in the footwall of the Lakeshore fault; 2). granodiorite porphyry in the hangingwall of the same fault, and 3). biotite-quartz-monzonite porphyry intruding the granodiorite porphyry as irregular sills and dykes. Biotite from the granodiorite porphyry has been dated at 67.3±2.2 Ma, while sericite from intensely altered granodiorite porphyry reveals an age of 64.1±2.1 Ma (Cook, 1988; Einaudi, 1982; Hallof & Winniski, 1971).
A significant amount of garnet and tremolite-magnetite skarn with pyrite-chalcopyrite mineralisation is present within the Apache Group. This skarn is developed principally within the Mescal Limestone, although some younger Palaeozoic carbonates may be represented in the 'mélange' of skarn lithologies. The upper walls and roof of the granodiorite stock are occupied by Cretaceous andesitic volcani-clastics. These have been altered to biotite-plagioclase hornfels within 100 m of the intrusion (Cook, 1988).
The Lakeshore Fault is a north-west striking, south-west dipping sinistral oblique slip fault with approximately 1500 m of displacement. A second major, and older structure, the 'C' Fault is found to the west of the Lakeshore Fault. There is at least 300 m of dip-slip movement on this fault. The last movement on the Lakeshore Fault is more recent than that on the 'C' Fault (Cook, 1988).
Mineralisation & Alteration
Primary copper mineralisation is spatially associated with the upper portions of the quartz-monzonite porphyry bodies and includes: 1). lower grade (0.5 to 0.8% Cu) disseminated chalcopyrite in quartz-monzonite porphyry, Proterozoic diabase (dolerite), skarn and Cretaceous andesite breccias and sediments; 2). higher grade (around 1.7% Cu) tabular bodies of chalcopyrite mineralisation in skarn; and 3). supergene mineralisation present in six zones, each represented by a different copper bearing mineral assemblages. These zones are mappable throughout the deposit.
The skarn occurs in a limestone unit that appears to be within the Mescal Limestone of the Apache Group, with some possible included Palaeozoic carbonates. The sequence in the orebody area at depth is overturned relative to the that at the surface where the skarn underlies the Apache Group quartzites, but is overlain by the Cretaceous sequence (Einaudi, 1982; Hallof & Winniski, 1971).
The skarn bed dips at 20°W, has an average thickness of 20 m and is terminated on the east and west by normal faults. Shearing is common in the orebody with many of the internal grade and calc-silicate type boundaries being faulted, as is the base of the skarn locally. Despite these disruptions there is a zonation of mineral associations roughly arranged from north-west to south-east with increasing distance from the porphyry mass, as follows: 1). garnet with a generally low sulphide content and a high pyrite:chalcopyrite ratio; 2). diopside with patches of garnet, relatively higher sulphide content and high pyrite:chalcopyrite ratio, containing abundant, sulphide bearing fractures with dark green alteration envelopes; and 3). talc-tremolite with relatively low pyrite:chalcopyrite ratios, and with magnetite ranging up to 25% (Einaudi, 1982).
Zones dominated by garnet or tremolite-magnetite contain the highest copper grades. The most persistent zones of high grade ore occur in the latter assemblage. Overall the skarn contains 1 to 6% pyrite, 1 to 4% chalcopyrite, and the pyrite:chalcopyrite ratios range from 1:2 to 4:1 (Einaudi, 1982). The main discovery hole intersected 27 m of mineralised, banded tactite/skarn with massive magnetite, pyrite and chalcopyrite (Hallof & Winniski, 1971).
The pattern of hypogene non-skarn mineralisation and alteration is dominated by structure and by lithology. Increased ore grades are associated with intrusive contacts, biotite-quartz-monzonite porphyry and coarse grained andesitic volcaniclastic units. Pervasive biotite alteration is widespread in volcanic rocks, but is limited to rare veinlets in the granodiorite. Strong phyllic developments are limited to a few fault zones. Hypogene alteration is characterised by selective recrystallisation of plagioclase and biotite phenocrysts with no profound alteration of the groundmass. Ore grade mineralisation is usually located in areas where plagioclase is altered to an assemblage of sericite, K-feldspar and anhydrite and where sulphides and biotite are altered to an assemblage of chlorite, rutile and sulphides. Pervasive biotitisation or rare extensive recrystallisation to K-feldspar accompany ore grade chalcopyrite mineralisation in the volcanics (Cook, 1988).
Chalcopyrite, the principal hypogene mineral, occurs disseminated in phenocrysts (biotite in particular) and in veinlets which have a range of different mineralogies. These are in paragenetic order - 1) quartz-biotite; 2) K feldspar-biotite-quartz; 3) K feldspar-sericite-calcite-quartz; and 4) quartz-anhydrite. Sulphides are associated with all of these vein types. Molybdenite is found in the oldest quartz-biotite veining, while the pyrite:chalcopyrite ratio increases progressively in each of the succeeding generations (Cook, 1988).
The supergene mineralisation is concentrated in an interval which is generally from 20 to 100 m thick, closely below, and parallel to the base of the Tertiary Fanglomerate (estimated from Cook, 1988). It is present in six copper bearing mineral assemblages which form zones that can be mapped throughout the deposit. These zones, as described by Cook (1988), have been named for the principal copper bearing minerals, namely the:
• Chalcocite zone - which is present in both the footwall and hangingwall of the 'C' Fault. In the hangingwall of this fault there is a chalcocite blanket which is around 300 m below the level of a second sheet which is also in the hangingwall. The second, or upper body, is at the same elevation as the main oxide ore zone. Chalcocite replaces hypogene chalcopyrite and pyrite of the hypogene mineralisation. These blankets are remnants of a more extensive accumulation that has been modified by a second period of oxidation.
• Chrysocolla zone - which is developed within granodiorite porphyry characterised by vitreous black biotite and pale blue altered plagioclase. The blue alteration of the feldspar is due to the copper bearing minerals of the alteration assemblage, particularly chrysocolla, which are disseminated within the phenocrysts. Half of the whole copper in the entire zone is located in altered phenocrysts. However, while chrysocolla is also associated with biotite phenocrysts it does not form a solid solution series with chrysocolla and copper-bearing clays within the phenocrysts as does the plagioclase. Kaolinite and smectite within fractures and in altered plagioclase constitute the copper bearing clays with CuO values of the order of 0.3 to 4.0%. Biotite may also contain up to 5% CuO before breaking down into copper-bearing clays. The remainder of the mineralisation is found within fractures which contain copper bearing clays as well as chrysocolla.
• Bronchantite zone - this is the highest grade supergene mineral zone and is hosted almost exclusively by the granodiorite and biotite-quartz-monzonite porphyries, although a few scattered occurrences have been drilled out in the altered andesites. In this zone the porphyries are characterised by biotite which has been altered to lime green mica and occasionally replaced by bronchantite and by pale blue clay altered plagioclase phenocrysts which are occasionally replaced by bronchantite and anhydrite. Smectite is the most abundant clay mineral. Bronchantite typically occurs in fractures and oxidised hypogene veinlets. This zone represents the in situ oxidation of the chalcocite zone, with little leaching and downward transport of copper.
• Copper-wad zone - This zone occupies the footwall shear zone, but has a similar petrography to the chrysocolla zone. The footwall shear zone is an active ground water channel and most of the rock within it has been stained black by a combination of iron, copper and manganese oxides and hydroxides. It probably contains impure chrysocolla, which in combination with the other constituents is referred to as copper-wad.
• Cuprite-native copper zone - forms a 3 m thick layer separating the bronchantite and chrysocolla zones from the chalcocite zone in the deepest part of the supergene orebody. The cuprite and native copper coat fractures discontinuously. The plagioclase phenocrysts are altered to a very pale green mineral assemblage which contains an average of 0.8% Cu. This zone is volumetrically insignificant.
• Goethite zone - which forms a layer below the base of the overlying Tertiary fanglomerate. The zone is thickest at higher palaeo-topographic levels and in areas where major faults and fracture zones intersect the base of the fanglomerate. It is thinnest where impermeable rocks are in contact with the fanglomerate, such as strongly K-feldspar recrystallised porphyry. In these less permeable zones goethite is restricted to fractures, in contrast to the distribution of mineralisation through the matrix in the chrysocolla and bronchantite zones. The average grade within the goethite zone is 0.3% CuO, with blue or green Cu minerals or staining rarely seen. The zone is characterised by beige to light tan friable porphyry. The principal Cu bearing mineral is goethite, although much of the Cu within the zone is adsorbed into altered biotite phenocrysts. Calcite, which is a remnant of the K-feldspar hypogene alteration, is also present. The chief clay found is smectite, with very little remaining kaolinite. Silica depletion has taken place during the formation of the goethite zone.
Two distinct episodes of oxidation have affected the Lakeshore deposit. The first resulted in the creation of a chalcocite enrichment blanket and a large volume of chrysocolla mineralisation. The chalcocite blanket was largely destroyed by the second episode when the bronchantite zone was formed. Overall Lakeshore is a sulphide poor deposit without a pronounced pyritic-phyllic zone. Hypogene mineralisation and alteration governed the formation of the chrysocolla, bronchantite and chalcocite zones. The chalcocite and subsequent bronchantite zones formed in rocks that previously contained abundant pyrite and chalcopyrite. The formation of these zones was accompanied by the recrystallisation of biotite to smectite and vermiculite. The antecedents of the chrysocolla zone contained much less chalcopyrite and pyrite and more acid consuming silicates such as biotite and plagioclase. The interaction of the acids and these reactive minerals led to the precipitation of gypsum and chrysocolla (Cook, 1988).
The neutralisation of copper bearing weathering solutions by plagioclase and biotite resulted in widespread disseminated copper into altered phenocrysts sites. There is a complex assemblage of copper bearing clay and/or chrysocolla within the altered phenocrysts. Within the plagioclase phenocrysts there is a continuous range of compositions from chrysocolla to copper bearing clay to plagioclase, although the same continuous solid solution is not found in association with biotite crystals or in supergene veinlets (Cook, 1988).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1995.
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.
|
|
|
|
Cook S S 1988 - Supergene Copper mineralization at the Lakeshore mine, Pinal County, Arizona: in Econ. Geol. v83 pp 297-309
|
|
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.
|
Top | Search Again | PGC Home | Terms & Conditions
|
|
