Super Porphyry Cu and Au|
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The Sacaton copper deposit originally outcropped as a low 10 m hill of leached capping some 100 m in diameter in a broad alluvial plain 2.5 km to the south of a range of pre-mineral exposures in the Sacaton Mountains. The deposit is within the Arizona-New Mexico Basin and Range Province in southern Arizona between Ajo and Ray. The underground and open pit mines were operated by ASARCO Inc., but is now closed. Published reserves and production are:
West Orebody open pit - Initial Reserve 1975 - 34 Mt @ 0.76% Cu (Gilmour, 1982).
East Orebody under-ground - Initial Reserve 1975, - 14 Mt @ 1.37% Cu (Gilmour, 1982).
Combined Production - 26.3 Mt @ 0.52% Cu, 1.28 g/t Ag (Titley, 1992).
The regional geological succession in the Sacaton area, specifically that outcropping in the Sacaton Mountains to the north of the deposit, is as follows (from Cummings, 1982):
Proterozoic basement, which comprises the following,
* Pinal Schist - These are the oldest rocks in the area, having been deposited prior to 1600 Ma. Regionally they are composed almost entirely of quartz-muscovite schist, generally occurring as roof pendants in the younger Proterozoic granites. Schists of this type outcrop in the Sacaton Mountains to the north of the deposit. Beneath the Sacaton deposit however, they include quartz-biotite schist, metamorphosed granites and meta-volcanics which, although different to the normal sequence, are included in the Pinal Schists on the basis of degree of metamorphism and relative age.
* Oracle Granite - A coarse grained, weakly porphyritic granitoid, composed of quartz, orthoclase and plagioclase, with minor biotite. Orthoclase occurs both within the groundmass¹ and as coarse phenocrysts up to 3.5 cm long. The Oracle Granite outcrops both in the Sacaton Mountains and in the discovery outcrop. A relative increase in plagioclase over orthoclase often accompanies the appearance of the large orthoclase phenocrysts, locally resulting in a monzonitic composition. Where it is the host to ore and is intensely altered, brecciated and granulated, the only means if differentiating it is by the abundance of coarse, sheared quartz. It is the predominant rock type in the East Orebody at Sacaton and in places in the West Orebody.
* Apache Group - Regionally a late Middle, post 1600 Ma, Proterozoic terriginous suite of conglomerate, shale, interbedded basalt, dolomite, limestone and quartzite, unconformably succeeded by a dominantly quartzite unit, with arkose and sandstone. In the Sacaton area it was almost completely eroded prior to the Late Cretaceous to early Tertiary Laramide intrusions, and is now only represented by restricted remnants.
* Sacaton Granite - An Upper Proterozoic, fine to medium grained quartz, orthoclase, muscovite and minor plagioclase granite, dated at 857 Ma.
* Diabase (Dolerite) Dykes - These are variably dipping dykes, up to 10 m thick, aligned in a north-westerly direction and dated at 841 Ma. They are highly altered in the mine area to lath shaped aggregates of sericite and clay after plagioclase in a matrix of fine grained shreddy biotite and mafics.
Palaeozoic sediments - Following pre-Laramide erosion, the Palaeozoic succession is now only represented by restricted remnants of the Cambrian Bolsa Quartzite and Devonian Martin Formation limestones.
Mesozoic intrusives, comprising the,
* Three Peaks Monzonite, dated at 71.2±2 Ma, in the late Cretaceous - represented in the Sacaton mine area by the Three Peaks Stock which intrudes both the Oracle and Sacaton Granites. It is a composite, epi-zonal pluton aligned in a north-easterly direction, with four main phases. These are, from the outer contact inwards: 1). a fine grained outer, and 2). a coarse grained inner diorite; 3) an intermediate monzonite and 4). a core quartz-monzonite. The core monzonite is the most extensive and comprises a medium, grained, equigranular, plagioclase, orthoclase, biotite, quartz-monzonite with accessory sphene and magnetite.
Tertiary intrusives, comprising the,
* Sacaton Peaks Granite, dated at 61.2±2 Ma, in the Palaeocene - This granite is similar to the Three Peaks Stock in form, occurring as a composite zoned stock, and in mineralogy, but is distinctly different in texture. It is a composed of three phases, namely 1). an outer, or border phase of medium grained biotite-quartz monzonite with 15% biotite and a gneissic texture; 2). an intermediate phase of medium grained slightly porphyritic biotite-quartz monzonite, with prominent quartz phenocrysts and less biotite than the border phase; and 3). the core phase of medium to coarse grained, slightly porphyritic granite, with orthoclase phenocrysts up to 3.5 cm in length. The age date is from the intermediate phase.
Tertiary conglomerate, which in the Sacaton mine area occurs in fault contact with all older rocks, including the orebody and its hosts. It comprises sub-rounded clasts making up 40 to 60% of the rock, including Oracle Granite, Pinal Schists and minor Laramide monzonite porphyry, set in a silty, gritty, moderately indurated matrix.
Quaternary alluvium, which has an average of thickness 30 m over the Sacaton deposit.
A major north-easterly trending continental scale structural lineament, the Jemez Zone which passes close to the Ajo deposit to the south-west and the Ray and Globe Miami District deposits to the north-east, is believed to have been intermittently active from the Proterozoic to the present and controlled emplacement of the Proterozoic, Mesozoic and Tertiary granites in the Sacaton Mountains (Cummings, 1982).
The Sacaton deposit is within an allochthonous structural block resting on a low angle detachment known as the Basement Fault, below which is a basement of Pinal Schists. The allochthon is elongated in a north-easterly direction and is some 7.5 x 2.5 km in area and 400 to 750 m in thickness. North-westerly striking normal faults divide the allochthon into several horsts and grabens (Cummings, 1982).
The pre-Laramide rocks in the mine area are no different from those encountered in the Sacaton Mountains. In contrast, the younger intrusives in the mine area cannot be correlated with the Laramide granitoids that are found in the Sacaton Mountains, and are described above. Never-the-less they are assumed to also be of Laramide age. These younger intrusives in the mine area are (from Cummings, 198 2).:
Monzonite Porphyry - which intrudes both the Oracle Granite and the diabase/dolerite dykes. Typically it exhibits phenocryst:matrix ratios ranging from 60:40 to 40:60. The composition of the medium grained phenocrysts which range from 2 to 4 mm in length, is generally 75% euhedral plagioclase, 10 to 15% biotite, 10% subhedral quartz and 1 to 3% subhedral orthoclase, all set in a fine grained matrix of intergrown feldspar and quartz. This monzonite porphyry intrudes the older rocks in the West Orebody and forms mixed breccias, monolithic breccias as well as large poorly defined dyke like masses and thin, but discontinuous dykes. Xenoliths of granite are common.
Quartz-Monzonite Porphyry - intrudes the older granite and diabase/dolerite and has a similar composition to the monzonite porphyry, except for the presence of 10% or more of clear quartz phenocrysts. It occurs as monolithic and mixed breccia, irregular masses and dyke-like bodies in sections of the West Orebody and in the East Orebody. Gradational contacts with the Monzonite Porphyry are common, and the two are regarded as co-magmatic.
Mixed Breccias - Both the East and West Orebodies contain substantial masses of brecciated rock. The predominant rock types in the breccias are monzonite porphyry, quartz-monzonite porphyry and granite. The breccia type is differentiated by the dominant rock type, with mixed breccias containing <15% of a foreign rock type not being differentiated from the corresponding monolithic breccia. The boundaries between mixed breccias are usually gradational. In places however, weakly brecciated monzonite porphyry dykes intrude the mixed breccias, suggesting the monzonite predates and post-dates the formation of the mixed breccias. Mixed breccias are found throughout the West Orebody pit, but appear to be concentrated between the granite masses on the margins of the pit. Drill-hole data suggests that at depth mixed breccias grade into monzonite porphyry and monzonite-porphyry breccia. In the East Orebody these breccias occur in close proximity to monzonite porphyry dykes and masses.
Dacite Porphyry - occurring as dykes intruding older rocks in both orebodies and contain 15 to 25% phenocrysts of sodic plagioclase, quartz and biotite. The presence of weak alteration and sulphides suggest emplacement during the late stages of mineralisation.
Fracturing is well developed, with the rocks in the orebody tending to break into fragments with a diameter of less than 25 cm. Although a large number of fracture directions are observed, two dominant orientations have been discerned, namely a mineralised direction of NE to east-west, and dipping at greater than 70° in either direction, and a barren set from NNW to north-south and close to vertical (Cummings, 1982).
A large number of minor faults are observed in the mine area, generally having the same general trend as the mineralised fractures. Two post-mineral normal faults cut the West Orebody. The Sacaton fault bounds that orebody on its eastern side and strikes north-south to NW-SE and dips at 60°E. The vertical displacement is of the order of 500 m. Conglomerates in the hangingwall of the fault contain abundant chrysocolla adjacent to ore, while the faulting also displaces the chalcocite blanket. The West Fault bounds the western side of the West Orebody, again with the Tertiary Conglomerate. Similarly the South Fault forms the southern limit of the East Orebody. All of these faults and the mineralisation are terminated by the flat, undulating, Basement Fault detachment which forms a north-east trending trench shape below the orebody. The average thickness of the Basement Fault is 1.5 m, containing brecciated upper and lower plate rock and thin zones of sheared chloritic to hematitic gouge. It is truncated to the north by a steep north-easterly trending normal fault (Cummings, 1982).
Two periods of brecciation are known. The first is pre-mineral, is related to the intrusion of the monzonite porphyry, and is the main control of mineralisation in the West Orebody. Post mineral brecciation is imposed on, and is more extensive than the earlier phase. Both are present in the West Orebody, while only the latter is seen in the East Orebody. Pre-mineral brecciation is present as both monolithic and mixed breccias which are often difficult to recognise due to pervasive alteration, transported iron oxide minerals and post-mineral brecciation. It consists of tightly packed, sub-angular to sub-rounded fragments from 5 mm to 2 m across, set in a well cemented matrix, comprising 5 to 20% of the rock which is in turn made up of small rock fragments and rock flour. Angular vugs make up from 1 to 5% of the rock and are filled with specular hematite, iron oxides, quartz, and either hypogene or supergene sulphides. The core of this breccia is irregular, and 150 x 200 m in plan (Cummings, 1982).
Post-mineral brecciation has affected the rocks in different ways, depending on their composition. Manifestations range from shattering and crushing to granulation. Shattering occurs as a random network of close spaced through-going fractures and while ubiquitous to both orebodies, it is more obvious where the rock is monzonite porphyry or quartz-monzonite porphyry. Crushing and granulation are pervasive in the granite, occurring as discontinuous fissures that cut quartz and orthoclase grains. With increased post-mineral brecciation the igneous textures of the rocks becomes more obscure and fragment and matrix development increases. Matrix commonly comprises up to 10 to 30% of the rock and consists of gouge, rock flour, crushed sulphides and broken mineral grains. Fragments are not well defined and range from 5 mm to 5 m across. Mixing of rock types is common in this type of breccia which generally contains mixed granite and monzonite porphyry breccias. Linear structures are an integral feature of these breccias, with sub-rounded clasts from 1 to 50 mm in diameter, and bands of the various matrix types listed above, imparting a flow-like structure. Crushed, contorted and sometimes folded sulphide veinlets often occur in these structures, which may be up to 15 cm thick. The breccia structures usually strike north-easterly, with steep to moderate and occasional shallow dips. Post-mineral breccias occur throughout the East and West Orebodies. In the East Orebody the post-mineral brecciation increases towards the Basement Fault (Cummings, 1982).
Hypogene alteration is present in two main forms, namely (from Cummings, 198 2).:
Potassic assemblage, characterised by secondary biotite and chlorite, but also including varying quantities of quartz, sericite and clay with traces of secondary K-feldspar, calcite and anhydrite. Since phyllic and supergene alteration are both superimposed upon, and largely destroy the potassic assemblage, it is uncertain how much of the sericite, quartz and clay are part of the original potassic suite. Potassic alteration is often well developed in the monzonite porphyry and quartz monzonite porphyry and is always well developed in the diabase/dolerite, but is rarely seen in the granite. Secondary biotite may constitute up to 40% of the groundmass where later quartz and sericite are not severe. Phyllic alteration does not significantly destroy the original igneous textures.
Phyllic assemblage, characterised by quartz, sericite and clay, with quartz and sericite predominating. Sericite is the major alteration product of the phyllic assemblage and constitutes up to 50% of the rock. Secondary silica in the porphyries occurs as a fine grained replacement of the groundmass. Quartz-sulphide veinlets are associated with the phyllic alteration and comprise up to 1% of the rock by volume. Clay minerals make up varying amounts of the phyllic assemblage, accompanying sericite in the groundmass. Intense phyllic alteration tends to destroy the original igneous textures. The phyllic assemblage post-dates potassic alteration. Age dating of 64.5± Ma has been obtained for sericite and kaolin bearing hypogene sulphide bearing core in the vicinity of the deposit.
No zoning pattern of alteration types has been recognised. The potassic assemblage in the late Cretaceous to lower Tertiary intrusives was originally widespread, and probably well developed in both orebodies, although variations in its intensity appear to be related to the affect of the later phyllic over-print. The intensity of phyllic alteration does appear to exhibit a crude zoning, being strongest in the south and central portion of the West Orebody and all of the East Orebody. This zone has a north-easterly trend and is continuous between the two orebodies. At right angles to this trend the intensity of phyllic alteration diminishes (Cummings, 1982).
Supergene alteration, includes the erratic silicification which occurs in the oxidised section of the upper part of the enriched zone, and often heals fractures. Where well developed, this healing has protected the supergene chalcocite from subsequent oxidation, leaving pods of supergene chalcocite up to 30 m in diameter perched in the leached capping. An undetermined amount of sericite and clay appear to be of supergene origin. Minor quantities of supergene alunite occur in the enriched zone as fracture fillings and pods up 2.5 mm thick (Cummings, 1982).
Drilling at Sacaton suggests the host intrusive occurs as a north-easterly elongated stock with dimensions some 300 x 600 m. The West Orebody occurs at the north-eastern end of the stock around the zone of pre-mineral breccia near the apex of the stock. The West Orebody is contained entirely within the limits of a horst formed by the Sacaton and West Faults. Within this horst is a northerly dipping zone of chalcocite enrichment which has been dissected and partially destroyed by post enrichment oxidation. Ore grade primary mineralisation is present below much of the chalcocite blanket. Both types of mineralisation were mined in the West Orebody which was roughly oval in plan with dimensions of 430 x 300 m. It had a slight north-south elongation and varied in thickness from less than 30 m on the margins, to 200 m in the central portion. The East Orebody occurs in the graben on the east side of the Sacaton Fault. In this body primary mineralisation does not reach ore grade, and all of the ore mined was supergene chalcocite. It had plan dimensions of 230 x 250 m and varies in thickness from 30 to 120 m (Cummings, 1982).
The hypogene mineralisation comprises pyrite, chalcopyrite and molybdenite, with rare traces of bornite and sphalerite. Minor quantities of Au and Ag are also present, along with occasional magnetite veins. Hypogene sulphides occur as disseminated grains, veins and vug fillings. Disseminated mineralisation is the most abundant in the granite and breccias, comprising <50% in the West Orebody, but in the East Orebody where granite breccia is the main host this style constitutes >50% of the primary mineralisation. Locally, disseminated sulphides are more abundant in phyllic alteration selvages and in secondary biotite sites. The intensity of mineralisation in vugs in the breccias is related directly to the quantity of vugs in the breccias. In the core of the pre-mineral breccia, sulphides filling these cavities comprise 30% of the total sulphide content. Four types of sulphide veins are recognised. The type responsible for the bulk of the mineralisation contains pyrite, varying amounts of chalcopyrite (0 to 40%) and quartz (<10%), ranging from discontinuous paper thin veins to semi-continuous veins up to 2.5 mm thick, and rarely, continuous structures up to 15 cm thick. Quartz-pyrite±chalcopyrite veins up to 2.5 cm thick are common, but relatively un-important, while occasional quartz-pyrite±molybdenite and quartz-molybdenite veins post date the other vein types (Cummings, 1982).
The total sulphide content of the hypogene mineralisation varies from 1 to 4%. Drilling and pit mapping has delineated an arcuate +0.4% Cu zone enclosing, and continuous between the two orebodies. Grades approaching 0.7% Cu are found in the vicinity of pre-mineral breccia zones, while outside of these it gradually drops off to less than 0.1% Cu. Molybdenite occurs in quartz veins and as smears on fractures, with an average content of around 0.01% Mo in the West Orebody and 0.025% Mo in the East Orebody (Cummings, 1982).
Supergene mineralisation, is present as chalcocite with up to 5% covellite and associated specularite, and is most intense at the top of the zone, decreasing downwards. In the upper portions chalcocite completely replaces chalcopyrite and partially replaces pyrite, while towards the base chalcopyrite is only partially replaced and pyrite is rimmed by thin coatings of chalcocite. The degree of enrichment for both orebodies varies from 3:1 to 5:1. The most important control of supergene grade is the hypogene grade. The bulk of economic supergene mineralisation is underlain by primary sulphides of >0.4% Cu, while the distribution is related to that of the primary sulphides. Rock type and structure locally exhibit some control over supergene grade distribution, although no predictable patterns are recognised. The chalcocite blanket of the West Orebody is irregular in thickness, grade and continuity, caused by tilting, post-enrichment oxidation and possibly by faulting. It ranges from 15 to 150 m in thickness. In the East Orebody it similarly varies from 60 to 120 m thick, with the base dipping at 20° (Cummings, 1982).
The leached capping developed over the primary and supergene sulphides contains hematite, goethite and jarosite in varying proportions, as indigenous minerals, filling former sulphide sites. The thickness of the capping varies from <30 to >200 m in the West Orebody, with the thicker intervals to the north. Over most of the East Orebody the capping ranges from 5 to 70 m in thickness, particularly in the southern sections where the South Fault displaces most of the capping. On the east and northern margins where greater thicknesses of capping are preserved it is up to 200 m thick. Where thorough leaching has occurred, Cu values range from 300 to 1000 ppm.
Substantial quantities of oxidised Cu minerals are found erratically distributed in the capping of both orebodies. In the East Orebody, these oxidised minerals are found just above the chalcocite blanket and are thought to be due to in-situ leaching of chalcocite. In the West Orebody they are more widely spread and are probably transported from the point of oxidation. Chrysocolla, bronchantite and malachite are the most common.
The orebodies at Sacaton have undergone two periods of oxidation and leaching. The first resulted in a uniform, high grade chalcocite blanket which was probably continuous through the East and West Orebodies, possibly prior to faulting. Clasts of this capping are found in the pre-faulting conglomerates of the hangingwall. The second period of oxidation and leaching modified and partially destroyed the original blanket, but was less severe in the East Orebody, although deep zones of alteration cut the chalcocite blanket on the eastern margins of the East Orebody. Within the West Orebody the effects of this leaching are erratic and diverse, ranging from total oxidation to partial leaching with 10 to 20% of the sulphides being destroyed. Total oxidation of the blanket varies from 200 to <1 m widths. Complete leaching is common, although Cu often remains as oxides. Uplift of the West Orebody on the Sacaton Fault may have been the cause of the more intense oxidation in the second stage leaching and oxidation in this orebody. Deep chalcocite mineralisation on the north side of the orebody may be related to this leaching (Cummings, 1982).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1996.
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.
Cummings R B 1982 - Geology of the Sacaton Porphry Copper deposit, Pinal County, Arizona: in Titley S R 1983 Advances in Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 507-521|
| References in PGC Publishing Books:||
Cook S S and Porter T M, 2005 - The Geologic History of Oxidation and Supergene Enrichment in the Porphyry Copper Deposits of Southwestern North America, in Porter T M, (Ed), Super Porphyry Copper and Gold Deposits: A Global Perspective, v1 pp 207-242|
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