Mother Lode - Jamestown, Havard, Carson Hill, Pine Tree
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The Mother Lode vein system is located approximately 180 km to the east of San Francisco in the northern half of California, within the western Sierra Nevada Ranges. Historic production from mines within this belt has yielded more than 342 t of Au (Landefield & Silberman, 1987) and has been responsible for the bulk of California's historic gold production (Brunker & Dickson, 1987).
The Mother Lode has been defined as the set of gold-quartz veins that occur in a narrow belt that is up to 2 km wide, and follows the generally north-westerly trending Melones Fault Zone of fissures for a distance of around 190 km, from Mariposa in the south to Georgetown in the north (Landefield & Silberman, 1987; Brunker & Dickson, 1987). The Melones Fault Zone occurs within the complex, ophiolitic mélange zone separating the eastern and western assemblages of the Sierra Nevada Terrane.
Published reserve figures for some of the bulk gold resources within the Mother Lode vein system are as follows:
Carson Hill - 14.5 mt @ 1.58 g/t Au = 23 t Au (Pre-mine Res., 1986, Bonham, 1988)
14.5 mt @ 1.58 g/t Au (Indicated. Res., 1986, D Fielding & others, 1986)
11 mt @ 1.7 g/t Au (Rec. Res., 1986, Brunker & Dickson, 1987)
5.0 mt @ 1.6 g/t Au = 8 t Au (Inferred. Res., 1986, D Fielding & others, 1986)
2.7 mt @ 10 g/t Au = 27 t Au (Past Prod., to 1986, Brunker & others, 1986)
Jamestown - 25 mt @ 2.2 g/t Au = 55 t Au (Meas. Res., 1986, Brunker & Dickson, 1987)
13 mt @ 4.7 g/t Au = 61 t Au (Infer. Res., 1986, Brunker & Dickson, 1987)
27 t Au (Past Prod., to 1986., Brunker & Dickson, '87)
Harvard Pit - 16.5 mt @ 2.5 g/t Au (Res., 1986, USBM, 1988)
Crystalline Pit - 8.4 mt @ 1.8 g/t Au (Res., 1986, USBM, 1988)
Dutch-App Pit - 8.2 mt @ 2.3 g/t Au (Res., 1986, USBM, 1988)
Nyman Pit - 2.6 mt @ 1.8 g/t Au (Res., 1986, USBM, 1988)
Jumper Pit - 1.5 mt @ 1.8 g/t Au (Res., 1986, USBM, 1988)
Rawhide Pit - 1.5 mt @ 2.3 g/t Au (Res., 1986, USBM, 1988)
Pine Tree - 23.5 mt @ 2.2 g/t Au = 51 t Au (Ind.+Inf. Res., 1988)
The Carson Hill mine is one of the first and largest producers of the Mother Lode district. Gold was first discovered in Carson Creek in 1848 while lode gold was found on Carson Hill in 1850. In 1854 an 88.5 kg mass of gold was taken from the mine, the largest ever mined in California. Large scale mining of low grade ore began in 1889 after the consolidation of a number of mines, and continued working until 1926. Operations were re-commenced in 1933 and continued until 1942. The total production has been estimated at 27 t Au. The historic ore zones comprised large bodies of auriferous schist and pyritic ankerite-mariposite-quartz rock with numerous thin quartz seams and stringers adjacent to barren Mother Lode quartz veins. Milling ores were low grade, although extensive. For example the 'Hanging Wall' orebody averaged 16 g/t Au and had dimensions of 53 x 1370 x 5 m. Tellurides were common close to the surface. In 1975 an exploration program targeted at a large, low grade resource indicated 5 mt @ 1.7 g/t. Production at Carson Hill was by open pit.
The current Jamestown mine covers an 8 km section of the Mother Lode vein system and includes the old Rawhide, Crystalline, Harvard, Dutch-App and Jumper Mine sites. The mine area was first worked for alluvial gold in 1848, while lode mining took place between 1860 and 1920. Typical ore shoots had stoping lengths of as much as 120 m, with several mines being worked to inclined depths of 600 m. Head grades averaged 4.5 to 10 g/t Au, with more than 27 t Au being produced.
Historic production at the Pine Tree Mine was curtailed in 1942 with a total output of 5.5 t of gold. An exploration program was commenced in 1985 which involved trenching and 16 100 m of drilling. Mining was originally scheduled to commence in 1990, at a rate of 1.5 mt of ore per annum from an open pit which had an overall striping ratio of around 6:1, waste:ore.
The gold-quartz veins of the Mother Lode zone are hosted along the Melones Fault, within the rocks of the fault zone itself, its splays and its wall rocks (Landefield & Silberman, 1987). The Melones Fault zone cuts rocks of the Sierra Nevada Terrane which is composed of two differing sequences separated by a complex, generally north-west trending band of mélange.
The eastern sub-division of the Sierra Nevada Terrane is composed of lower Palaeozoic chert, basalt, phyllite, sandstone and minor marble, overlain by upper Palaeozoic silicic to andesitic to mafic lavas, pyroclastics and turbidites, and uppermost sub-aerial volcanics. These rocks are generally equivalent to the Western, Siliceous Assemblage and the Havallah Sequence respectively, of western Nevada. The western sub-division is generally of Mesozoic age with mafic to intermediate lavas, pyroclastics and volcani-clastics, and thick conglomerates. The eastern and western assemblages are separated by an ophiolitic melange, with mafic dykes and tectonic blocks, ultra-mafic schists, and slices of other felsic lithologies. East dipping under-thrusting on the ophiolitic melange is quoted as being between late Triassic to early middle Jurassic in age (Saleeby & Busby-Spera, 1992).
The Melones Fault zone is characterised by elongate serpentine bodies and belts of pervasive shearing and cataclastic deformation. It is developed within the central mélange zone of the Sierra Nevada Terrane and is dated at around 155 to 140 Ma, ie. late Jurassic. Within that central mélange zone the Melones Fault separates two belts of rocks of similar lithologies and ages, but of dissimilar metamorphic grade and style of deformation. These two belts are: 1). the Mother Lode Belt which occurs to the west of the Melones Fault and to the east of the Bear Mountain Fault; and 2). the East Belt which is to the east of the Melones Fault and west of the Calaveras Complex. The stratigraphy of the East Belt has been intensely deformed and is destroyed, whereas as the stratigraphy of the Mother Lode Belt is largely intact (Landefield & Silberman, 1987).
The rocks of the melange zone within the two belts are made up of a suite of mafic lavas and cherts, with overlying and adjacent epiclastic sediments and tectonically emplaced ultra-mafic and mafic slices and blocks. Within the Mother Lode Belt the following lithologic units have been defined, from the structural base (Landefield & Silberman, 1987):
* Tholeiitic pillow basalt and breccia of late Triassic to early Jurassic age;
* Thin radiolarian chert beds which grade upwards into;
* A submarine sequence composed of debris and grain flow deposits to medial turbidite deposits, overlain by;
* Sporadically distributed flank flows, some of which are Mg rich, ranging in composition from calc-alkaline basalt to basaltic andesite.
The rocks of the lower two sub-divisions are exposed in the southern section of the Mother Lode belt, but have been tectonically removed in the north. The third unit, the debris flows and turbidites indicate a cessation of volcanism. The Mother Lode Belt has undergone prehnite-pumpellyite to mid-upper greenschist facies metamorphism. The more intensely folded and metamorphosed mid greenschist to lower amphibolite facies rocks of the East Belt, to the east of the Melones Fault, are interpreted as representing basin ward equivalents of the same succession. Rocks of both belts show evidence of submarine hydrothermal alteration, burial, dynothermal and contact metamorphism. In general those of the Mother Lode Belt exhibit brittle failure, while the East Belt has been subjected to more ductile deformation (Landefield & Silberman, 1987).
The Melones Fault zone dips to the east at 40 to 60°, whereas bedding and/or cleavage in the wall rocks dips at 60 to 90° in the same direction. From north to south the Melones Fault zone changes from a mylonite-phyllonite zone which is <1 km wide, to a broad zone of shearing to a serpentinite hosted tectonic mélange, depending on the lithologies cut. The differences in character of the fault is believed also to reflect different levels of exhumation, with the southern section representing depths of 25 to 30 km, the central around 10 km and the northern 5 to 10 km. The serpentinite within the mélange contains relict coarse grained textures. The fault zone incorporates numerous individual faults with indeterminate displacements. Several major faults splay into the footwall and hangingwall. In the southern section these splays are small fault zones of serpentinite hosted mélange. In contrast, in the northern zone the splays are narrow brittle faults. Some splays host Mother Lode deposits, others dont (Landefield & Silberman, 1987).
Mineralisation and Alteration
In general the deposits of the Mother Lode district are quartz veins and bodies of pyritised and carbonate altered rock concentrated within the Melones Fault zone and its hangingwall and footwall rocks. The style of deformation, within and adjacent to the fault zone and its splays, partially controls the location and character of the veins of the Mother Lode. The amount of gold produced from the Mother Lode veins increases from south to north. The most prominent district, the Plymouth-Jackson section of the northern half of the Mother Lode belt, produced approximately 200 t of the total 342 t. The gold deposits of the Plymouth-Jackson district occur as 1). veins in the footwall splays of the Melones Fault zone cutting interbedded black argillite, tuffwacke and greywacke; and as 2). low grade ore in altered, interbedded epi-clastic and volcaniclastic turbidites. The coarser grained detrital, epi- and volcaniclastics are carbonate metasomatised in the vicinity of the veins. Mines in the northern half of the belt are generally characterised by simple gold-bearing quartz veins which are very continuous and were worked to considerable depths. The Jackson-Kennedy mine near the town of Jackson for instance, was worked to depths in excess of 1800 m. The mineralogy of these northern veins is simple, consisting of milky quartz, ribboned with bands of slate or schist, minor amounts of sulphide minerals, mostly pyrite, and free gold (Landefield & Silberman, 1987; Brunker & Dickson, 1987).
Deposits in the southern two thirds of the Mother Lode are predominantly hosted by carbonate altered serpentinite and carbonate altered rocks of both the hangingwall East Belt and footwall Mother Lode Belt (Landefield & Silberman, 1987). In this southern section the quartz veins associated with the Melones Fault are barren with the productive zones being large bodies of silica-carbonate alteration. These zones pinch and swell abruptly and can rarely be traced for more than 300 to 400 m. The altered material is composed of Fe-Mg carbonates with lesser amounts of sericite, albite and quartz, and 3 to 4% pyrite and arsenopyrite. It is interlaced with veinlets of quartz, carbonate and albite. The gold is finely disseminated and is associated with pyrite and the fine quartz-carbonate veins (Landefield & Silberman, 1987; Brunker & Dickson, 1987).
Gold also occurs in the parallel, but less continuous East and West Belts which are separated from the Mother Lode by 8 and 25 km respectively. The East Belt is more complex than the Mother Lode with appreciable arsenopyrite, chalcopyrite, galena and sphalerite as well as pyrite. The West Belt is similar to the Mother Lode with gold occurring with pyrite in altered metamorphic rocks adjacent to quartz veins as well as in the veins themselves (Brunker & Dickson, 1987).
At Carson Hill a large barren quartz vein which lies some 60 m to the east of, and generally parallel to the main Mother Lode vein, suddenly changes strike, swinging from NNW to east-west. Where this change takes place a large triangular wedge of quartz-ankerite-mariposite alteration is developed on both sides of the quartz veins, but predominantly to the north in the hangingwall. This alteration enclosed the historic production which came from a zone 53 x 1370 x 5 m adjacent to the parallel quartz zone at the change in strike. Considerable gold was won from stringers of quartz in the footwall and hangingwall and in a series of flat quartz veins extending off the main quartz vein (Brunker & Dickson, 1987).
The large zone of alteration at Carson Hill has a plan area of approximately 550 x 180 m. All rocks in the alteration zone contain gold with the exception of the serpentinite. Some 85% of the gold is free, with the remainder being locked in pyrite which averages 2% of the rock. The gold is associated with thin quartz stringers, and as the quartz content drops off away from the barren main quartz body, so does the gold. Three zones are recognised, namely: 1). a relatively small, but high grade zone in the footwall, 2). an extensive zone in the immediate hangingwall, and 3). a zone associated with flat veining (Brunker & Dickson, 1987).
Within the Jamestown area the Mother Lode quartz vein is coincident with the Melones Fault zone which separates Palaeozoic chloritic argillites and greenschist metavolcanics of the East Belt Calavares Complex from the Mother Lode Belt mafic volcanic sequence to the west. The fault varies in strike from 330° on the south and north, to north-south in a 3 km section in the centre of the property. It dips at 50 to 60° to the east and ranges in thickness from 20 to 150 m. North of the prominent flexure in the fault zone and adjacent to all of the major prospects from Jumper in the south to Rawhide in the north, a large tear shaped peridotite body some 10 km in length and up to 3.5 km wide occurs in the western footwall. Along the fault zone the peridotite has been extensively altered to serpentinite. This development of ultra-mafic rock is also adjacent to the greatest development of gold mineralisation in the district and is opposite the most extensive thickness of sediments within the East Belt sequence (Brunker & Dickson, 1987).
Individual deposits within the Jamestown district are defined by lenses of carbonate, centred along the Mother Lode quartz vein. The hangingwall slates and greenschists are predominantly altered to ankerite and/or sericite schist, while the footwall serpentinite is altered to either talc-schist or strongly quartz veined quartz-ankerite-mariposite1 rock. In general the limit of alteration marks the limit of gold mineralisation, with both thinning rapidly to the north and south. Based on the age of mariposite alteration the gold mineralisation is believed to be around 140 Ma (Brunker & Dickson, 1987).
Within the Harvard orebody at Jamestown there are two zones of mineralisation. The higher grade footwall zone is associated with chloritic and ankeritic meta-volcanics and contains moderate quantities of quartz-carbonate veining. This was the main body worked by the old operations. The lower grade footwall zone comprises the bulk of the current reserves. Gold mineralisation in this latter zone is closely associated with fine grained to dense siliceous, pyritic, carbonate rock. The rock is commonly laced with white veinlets of coarse, euhedral, quartz and carbonate. The large Mother Lode quartz vein which separates the two is 'barren', averaging 0.09 g/t Au (Brunker & Dickson, 1987).
Within the Harvard orebody, gold occurs as the native metal, mainly as 5 to 25 micron blebs within pyrite, or as coarser grains associated with quartz-carbonate veinlets. Pyrite is the only megascopic sulphide present, although minor amounts of sphalerite and chalcopyrite are observed in thin section. The gold bearing pyrite is commonly euhedral and fairly fine grained. Ore zone sulphide contents may be up to 10%, but average nearer 2%. There is no apparent correlation between gold and pyrite, although there is a suggested connection between gold and increasing quartz. In addition, the gold content increases towards the Mother Lode quartz reef, although there are blank zones in the alteration halo and the reef itself is barren. The Harvard orebody varies from 6 to 67 m in thickness, with an average width of 27 m, and total strike length of 490 m. The pit will extend to a depth of 122 m (Brunker & Dickson, 1987).
In the Dutch-App orebody, also on the Jamestown property, quartz-ankerite-mariposite alteration occupies a surface area of 550 x 36 m and will be mined to a depth of 60 m. Four zones of mineralisation are recognised. These are 1). a footwall zone of mixed slate with quartz-ankerite-mariposite, separated from 2). a similar zone in the hangingwall by 3). the barren Mother Lode quartz vein. The final zone is 4). an extensive body of silicified graphitic ore which comprises a significant proportion of the resource. Gold is associated with either late stage quartz-carbonate veins or with pyrite. No other sulphides are present and the Ag values are low (Brunker & Dickson, 1987).
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary 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.
Savage K S, Ashley R P and Bird D K, 2009 - Geochemical Evolution of a High Arsenic, Alkaline Pit-Lake in the Mother Lode Gold District, California: in Econ. Geol. v104 pp 1171-1211|
Weir R H, Kerrick D M 1987 - Mineralogic, fluid inclusion, and stable isotope studies of several Gold mines in the mother lode, Tuolumne and Mariposa Counties, California: in Econ. Geol. v82 pp 328-344|
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