Mesquite, Big Chief
Super Porphyry Cu and Au|
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The Mesquite-Big Chief orebody is located at the base of the south-western flank of the Chocolate Mountains in Imperial County, far south-eastern California, USA. It occurs within amphibolite grade metamorphic rocks of Proterozoic and Mesozoic age in the upper plate of the Chocolate Mountains Thrust (Willis, 1987).
The existence of gold within the Mesquite district had been known for many years, with activity being concentrated on placer deposits in the various dry creeks draining the south-western margins of the Chocolate Mountains. Limited underground mining and prospecting followed high grade, visible gold seams to shallow depths. The old Big Chief mine had developments off a main shaft at the 15áandá30ám levels. Numerous mining companies had held leases over portion of the claim group surrounding the Big Chief orebody. Gold Fields Mining Corporation, a subsidiary of the London based Consolidated Gold Fields Plc., acquired the property in 1981. After surface investigations gave sufficient encouragement, an initial drill hole was sited near the old Big Chief shaft. The first hole intersected the orebody. Continued drilling and an exploration decline through the heart of the orebody indicated continuity of mineralisation (Willis, 1987).
Published reserves include:
Production + Reserves, 1988 - 86 Mt @ 1.71 g/t Au = 150 t Au (D O Mason & others, 1989)
Reserves, 1991 - 72 Mt @ 1.23 g/t Au (Amer. Mines H'book, 1992)
Pre-mine Reserve, 1985 - 48 Mt @ 1.7 g/t Au (Bonham, 1988)
Proven + Probable Reserve, 1994 - 67.9 Mt @ 1.1 g/t Au (AME, 1995)
The mine commenced operation in 1985 (D O Mason & others, Mine visit, 1989). In 1987 ore was being mined at the rate of 2.7ámt per annum with a stripping ratio of approximately 3:1 (Willis, 1987). In 1989 ore was being mined from four separate pits, the Big Chief, Vista, Cherotall and Rainbow. All of the ore at that stage was oxide mineralisation, while the four pits were enveloped by the larger zone of mineralisation. The cut-off in 1989 was 0.4ág/t Au (D O Mason & others, Mine visit, 1989). Production in 1991 was approximately 6.2átáAu (Amer. Mines Hand book, 1994), and in 1994 was 6.5átáAu (AME, 1995).
The south-western Chocolate Mountains comprise allochthonous, Proterozoic and Mesozoic age, amphibolite grade, metamorphics of the upper plate of the Chocolate Mountains thrust. These rocks have undergone a complex history commencing with the formation of a gneissic complex during the Proterozoic, cut by several separate Proterozoic plutonic intrusive 'phases'. Granitic rocks were again intruded during early Triassic and late Jurassic to early Cretaceous time. Both the Proterozoic and Mesozoic rocks have been subjected to several phases of amphibolite facies regional metamorphism ranging from the Proterozoic to the Mesozoic (Willis, 1988).
The Chocolate Mountains Thrust is part of a system of generally north-west trending, shallow, undulose structures which extend across southern California and into south-western Arizona. Movement is believed to involve around 48ákm of over-thrusting of gneissic and intrusive rocks in a north-easterly direction over greenschist grade schists, and took place during the latest Mesozoic (Willis, 1988).
During the mid Tertiary calc-alkaline volcanics and epi-zonal plutonic rocks were emplaced in the vicinity of the Big Chief deposit and along the mountain chain in both north-western Arizona and south-eastern California. Miocene to Pliocene fanglomerates and interbedded basalts overlie the preceding rocks with angular unconformity. These are in turn covered by Pliocene to Quaternary alluvium (Willis, 1988).
Within the open pits a pseudo-stratigraphic sequence has been defined. The mostly amphibolite grade rocks have a shallow south-westerly dip off the Chocolate Mountains. The sequence is as follows, from the structurally lowest member (Willis, 1988):
Mafic Gneiss - with a composition which ranges from dacite to amphibolite, and common associated foliated granodiorite and pegmatite. This predominantly mafic unit is generally very chloritic. District wide, mafic gneisses often occur as horsts that have been faulted into the overlying units.
Hornblende-Biotite Gneiss - which has a fine to medium grained groundmass of plagioclase, microcline and quartz. Hornblende, which is dominant along the foliation, has generally reverted to biotite. This rock type is distinctive and is recognised by the occurrence of coarse grained, euhedral, accessory titanite (sphene), light and dark, quartzo-feldspathic and mafic layering, and microcline augen porphyroblasts. Red to purple, dusty hematite coatings on joints are diagnostic. Hornblende-Biotite Gneiss was confined to the southern end of the Big Chief pit in 1987 where block faulting had moved it upwards into the overlying rocks. Where not block faulted, contacts with adjacent units are across low angle faults, dipping to the south-west.
Biotite Gneiss - is a complex and highly variable unit, with varying proportions of plagioclase, microcline, quartz and biotite. Abraded zircons in the Biotite Gneisses have been taken to indicate a sedimentary protolith. Within the Big Chief pit the Biotite Gneiss has been sub-divided into several distinct sub-units, as follows:
Lower Biotite Gneiss - a massive, blocky fractured, fine grained, 'salt and pepper' rock containing sub-equal amounts of quartz, feldspar and biotite. Widely spaced, schistose, biotite rich layers which occur irregularly throughout the lower sub-unit, become more common as the middle sub-unit contact is approached.
Middle Biotite Gneiss - which is a distinctive package of irregularly spaced layers of massive 'salt and pepper' gneiss with alternating, schistose, biotite rich layers. The alternating layers range from less than a centimetre to several metres in thickness. The biotite layers pinch, swell and coalesce, but are generally conformable to foliation.
Upper Biotite Gneiss - which is characterised by a clotty texture. It contains irregular, clots of coarse biotite up to 1ácm in length in a quartzo-feldspathic matrix and is usually in contact with the other Biotite Gneiss sub-units across low and high angle faults.
Muscovite Gneiss and Schist - overlie the Biotite Gneiss along a gently dipping north to north-west trending fault in the north-west of the Big Chief pit, and across a steeply dipping north-west trending fault in the south-west of the same pit. Mineralogically it is composed of fine to medium grained muscovite, biotite, pyrophyllite, talc, quartz, plagioclase and microcline. It has a white to green-grey colour and an extreme schistosity. Much of the schistosity is the result of shearing parallel to foliation. Sandwiched between the biotite gneiss and the muscovite schist is a Leucogneiss which is composed primarily of orthoclase, lesser microcline, quartz and plagioclase, with muscovite and up to 20% massive black tourmaline along foliation planes.
Leucogranite - a muscovite bearing, pegmatitic, syn-kinematic leucogranite occurs as a series of dykes and sills, primarily within the Biotite Gneiss and Leucogneiss. Sills follow the foliation while the dykes radiate through the pit cutting the foliation. The Leucogranite often merges imperceptibly into the tourmaline bearing Leucogneiss. The Leucogranite has little foliation or lineation. Texturally it is medium grained to pegmatitic, with rare aplitic zones. A sample of feldspar from a dyke in the exploration decline yielded an age of 37.7▒1.4áMa, although this may be a thermal age. Pegmatites, which are not immediately distinguishable from the Leucogranite, except for the nature of the biotite and muscovite, occur within the gneissic units as bodies that are concordant with the foliation, and as boudins.
Tertiary Fanglomerate - generally a buff coloured, resistant unit that ranges from thinly laminated clay, silt and sand to a coarse pebble conglomerate. Clay and silt layers dominate. It is tentatively correlated with another fanglomerate in the region which is capped by 11áMa basalts, but appears to be younger than 26átoá23áMa volcanics. As such it may be of Miocene age.
Pliocene to Quaternary Gravel - occurring as a thin layer of relatively recent, unconsolidated to weakly cemented gravels. The size distribution of the gravels is extremely variable, ranging from fine sand to cobbles.
Several major fault trends have been exposed in the Big Chief Pit. The most significant of these strike at 310 to 320°. On the west side of this fault zone dips are generally to the south-west and are at 55 to 80°, while to the east they dip at similar angles but to the north-east. The fault zone is more than 120ám wide to the south-east, widening to at least 210ám to the north-west (Willis, 1988).
Near the centre of the pit a major split comes off the north-west trend and strikes due north, dipping at 55 to 80°áNE and cutting across other faults to the north of the major north-west trend (Willis, 1988).
Another significant fault trend also strikes approximately north-west but has a shallow dip of 20 to 40° SW in the south-west of the pit where it commonly forms the contacts between the Muscovite Schist, Tourmaline Leucogneiss and the Biotite Gneiss. These faults have an arcuate trace and are shovel shaped at their base. The low angle faults cross-cut and truncate the high angle north-west set, resulting in the Muscovite Schist, Tourmaline Leucogneiss, upper units of the Biotite Gneiss, and possibly the Leucogranite, having been moved over the top of the north-west trending structural zone (Willis, 1988).
Another low angle fault set occurs to the north. This zone is a series of en-echelon structures striking east-west and dipping at 20 to 30°áto the north. The true thickness of this zone is around 30 m. These form the contact with, or are within the clotty Biotite Gneiss unit. A relatively minor set of north-east trending faults occur to the south, striking generally north-east and dipping at 50 to 80° NW. These cut all of the other fault sets. The high angle north-west and low angle arcuate sets have been active as recently as late Miocene to early Pliocene as both transect the fanglomerates. There is evidence that the north-west set were also active prior to mineralisation at 32 Ma (Willis, 1988).
The Miocene-Pliocene faulting has produced Basin and Range structures. The 320°átrend of the main north-west faulting at Big Chief forms an acute angle with the main range front faults which trend at 300°. The most prominent of the range front faults forms the margin of the amphibolite grade rocks approximately 1 km to the north of the pit (Willis, 1988).
The main steep north-west trending fault set is believed to be splays of the San Andreas Fault zone, with the orebody being localised within a dilation zone (D O Mason & others, Mine visit, 1989).
Mineralisation and Alteration
Much of the mineralisation in the Big Chief pit is predominantly controlled and bounded by the steeply dipping north-west and north trending fault set. The fault controlled nature of the ore is most evident in the south-east of the pit where two distinct north-west trending, 15 to 25 m wide ore bearing fault zones parallel each other for 110 m. They are bounded by waste both to the north-east and the south-west, and are separated by 15 to 25 m of waste. These zones of mineralisation merge to the north-west, with more mineralised faults being added to the high angle north trending set, to form an ore zone that is ultimately 360 to 450 m wide. Where the mineralised high angle fault systems are narrower to the south the grades are higher, with individual zones averaging in excess of 3 g/t Au. As the system broadens to the north the grades decline with an average of nearer 1.2 to 1.5 g/t Au. In the upper benches the ore is truncated by the shallow, younger faults which have emplaced barren rocks over the mineralised zone (Willis, 1988).
Although the ore is controlled by steep faults, it has the overall form of a gently dipping tabular body. This is due to the shallow dipping faults which truncate the orebody in its upper sections as described above, and by the shallow south-westerly dip of the biotite schist bearing middle unit of the Biotite Gneiss. The schistose bands of the middle Biotite Gneiss are preferentially mineralised relative to the other rock types. The brittle-ductile nature of this unit has also resulted in a greater density of faulting (Willis, 1988).
Native gold is found associated with iron oxides, sulphates of iron and potassium (jarosite), biotite and chlorite within the oxidised ore. The gold occurring with the various oxidation products is a result of the oxidation of disseminated, vein and veinlet pyrite along faults and dispersed along related fractures and fracture zones. Supergene processes have transformed the original coarse 40 to 80 microns, silver bearing (700 to 800 fine) gold of the un-oxidised zone to fine (<10 microns), relatively silver free (>900 fine) gold (Willis, 1988).
The oxidised portion of the orebody extends for up to 150 m below the surface, with the depth of oxidation depending on the intensity of faulting and the amount of fault off-set on fault blocks. Thin zones of oxidation sometimes extend deeper where there are through-going faults (Willis, 1988).
There is a weak association of gold with As, Mo, Pb, Sb, Cu and Te in the un-oxidised portion of the orebody, although this is not evident in the zone of oxidation (Willis, 1988).
Orange limonite veins, veinlets, fracture coatings and disseminations after pyrite constitute the bulk of the mineralisation within oxidised fault zones. Siliceous matrix breccia veins and veinlets are probably the earliest vein types, and are also the highest grade. K-Ar age determinations of the sericite date these veins at around 32áMa. These breccias are composed of angular to sub-rounded clasts of Biotite Gneiss and Leucogranite floating in and coated by veins and veinlets of massive sugary quartz. The matrix has a pink to brown colouration due to finely dispersed hematite and limonite. Clasts are both silicified and non-silicified. Vuggy open spaces often contain late euhedral quartz (Willis, 1988).
Orange carbonate veins and breccias cut the siliceous matrix breccia veins. These are dominantly a mixture of pyrite and ankeritic-dolomite which have reverted to calcite and limonite. A relatively large proportion of the fault controlled ore is of this type of veining. Ankeritic-dolomite breccia veins contain clasts of siliceous matrix breccia, Biotite Gneiss and Leucogranite (Willis, 1988).
Brown calcite veins cut the siliceous matrix breccia veins and the ankeritic-dolomite veins. These latter veins range from a fraction of a centimetre to a metre or more in width. They may contain significant gold, but are often barren. White to clear calcite veins often cut or coat other vein types. The white calcite is thought to have been supergene and is not known to be mineralised (Willis, 1988).
Only one form of hydrothermal alteration is evident at the Big Chief orebody. This takes the form of weak sericitisation in the Biotite Gneiss package and possibly the tourmalinitic gneiss. Biotite has been altered to greenish-grey to white, very fine grained sericite; plagioclase is dusted with sericite, while K-feldspars are fresh. Sericitisation occurs as a zone up to 60ám wide in parts, and elsewhere as envelopes up to several metres wide adjacent to faults within the Biotite Gneiss. Silicification occurs rarely in narrow asymmetric selvages around siliceous matrix breccia veins. The silicification is characterised by its sugary texture and stockwork of narrow quartz veinlets. Alteration in the Hornblende-Biotite Gneiss is confined to narrow envelopes adjacent to isolated mineralised breccia veins. Veining is rare due to the apparent lack of ground preparation in this rock type. Although the mineralogy makes detection of sericite alteration in the Muscovite Schist difficult, it appears to be less well developed (Willis, 1988).
Pervasive clay alteration is remarkably lacking. Although much of the mineralised zone has been ground to a clay consistency by the faulting, thin sections indicate that rock flour is the principal constituent (Willis, 1988).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1994.
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.
Willis G F, 1988 - Geology and mineralisation of the Mesquite open pit gold mine: in Schafer R W, Cooper J J, Vikre P G (Eds), 1988 Bulk Mineable Precious Metal Deposits of the Western United States Geol Soc of Nevada, Reno, pp 473-486|
Willis G F, Holm V T, Gold Fields Operating Co. 1987 - Geology and mineralization of the Mesquite open pit gold mine: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 52-56|
Willis G F, Tosdal R M 1992 - Formation of Gold veins and breccias during dextral strike-slip faulting in the Mesquite mining district, Southeastern California: in Econ. Geol. v87 pp 2002-2022|
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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.
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