BASE and precious metals explorer Silver City Minerals took its name from Broken Hill’s famous Silver City, home to one of the world’s largest natural accumulations of silver, lead and zinc.
Silver City’s primary targets are high grade silver, gold and base metal deposits close to the town of Broken Hill in NSW.
The district was originally known for its numerous small, but very high grade, silver vein deposits known as Thackaringa type veins, where very high grades of silver were often reported.
The rocks that host the historic Broken Hill deposit, made famous for its association with BHP Billiton (originally named Broken Hill Proprietary), are known collectively as the Willyama Supergroup.
They underlie much of the district and host numerous other mineral occurrences, many with similarities to the Broken Hill orebody that are referred to as Broken Hill type (BHT) deposits.
To date, although a number of rock sequences within the Willyama Supergroup are recognised as hosts to BHT mineralisation, none have been mined or explored to the same extent as the original Broken Hill deposit.
Silver City sought and acquired extensive exploration tenure (about 1700 square kilometres) in the Broken Hill metallogenic province at a time when a large amount of new information about the geology of
the district became available. Silver City is working to identify areas suitable for mining within its tenements in the area, and its directors have expressed their belief that the region holds very significant
discovery potential. The company has built its exploration methodology on a foundation of what it terms “continually evolving projects”.
“The classic presentation of this is a triangle with numerous conceptual targets at the base, with fewer advanced projects (drilling and feasibility) at the apex,” the company stated.
When it listed on the ASX in July 2011, Silver City had six advanced projects that it stated were in the “drilling” part of its triangle theory: Allendale, Maybell, Stephens Trig, Golden King, Razorback
West and Yellowstone. The company has since received encouraging results from drilling campaigns at a number of these exploration targets and identified three further targets – Chloe, Umberumberka
and Champion – that are scheduled for drill testing. Silver City also described a number of its assets as being in the “target delineation” part of its triangle diagram, notably Yalcowinna Creek, Mount Brown, Ziggys and Apollyon Valley.
“Ongoing work on these including RAB [rotary air blast] drilling and/or ground magnetic and soil surveys will likely delineate drill targets for further assessment,” the company stated.
The Allendale project, 40km north of Broken Hill, encompasses an area of about 13sqkm centred on the Yanco Glen exploration licence. It also covers a number of old mines and prospecting pits including the Allendale Mines, Sinclair’s, Steakhouse, Easter Gift and Waukaroo prospect.
Silver City’s drilling program at Allendale was designed to test beneath the historic Allendale mine and a number of prospects to the north and south of the mine.
“Prior to 1990 the mine produced approximately 20,000 tonnes of massive sulphide rock with grades similar to those recorded at the Broken Hill deposit (in the order of 10 to 20 per cent combined lead
and zinc, and 50 to 100 grams per tonne silver),” Silver City reported.
“Geological interpretation suggests that there are at least four closely spaced lode-rock horizons which host mineralisation and that these dip steeply to the east. Silver City drilling shows continuity of lode-rock with good lead-zinc-silver grades to depths of about 60 to 70 metres with clear potential for down-dip extensions beyond this.
“Holes RCAN- 019 and 028 also indicate that mineralisation continues with encouraging thicknesses and grades to the south where no mineralisation had been previously identified.”
Silver City stated that its first phase of drilling at Allendale, combined with historic diamond drill holes in the project area, indicated that mineralisation extended for more than 300m along strike, dipping
steeply to the east to depths of 200m.
On July 17 2012, the company announced that a second phase of drilling at the project had intersected base metals and silver in massive sulphide mineralisation.
It reported that results for four holes and partial assays for one hole had been received from the 15-hole program, with one hole of particular interest.
“Hole 12AN035…has intersected sulphides in a zone which has not been encountered or was previously poorly represented in earlier drilling,” Silver City reported.
“This zone has an estimated true thickness of 7 to 8 metres and is referred to as the Eastern Zone. The company has drilled another hole, 12AN039, designed to intersect the projected extension of the sulphides in 12AN035. This hole has also intersected sulphide mineralisation some 30 metres down-dip.
“Visual estimates of hole 12AN039 indicate this mineralisation is narrower and zinc rich. Analytical results are pending.”
At its time of listing, Silver City had indentified a large lead and zinc geochemical anomaly about 4.7km long and between 200m and 400m wide, about 20km north east of Broken Hill on the northern
end of the Broken Hill line of lode.
“The geological significance of this has not been explained as there is little outcropping rock in the area, with the most intense parts of the geochemical anomaly lying beneath 5 to 30 metres of alluvial or
deep soil cover,” the company reported.
In early July, Silver City released the results of a 16-hole reverse circulation drilling campaign at Razorback West. It reported that drilling had located a zone of geochemically anomalous rock hosting
abundant amounts of the blue quartz and fine grained garnet commonly associated with BHT mineralisation.
“The drill data indicates the presence of significant quantities of zinc and manganese with localised, elevated lead and silver mineralisation,” Silver City reported.
“BHT mineralisation is characteristically hosted within broad zones of anomalous rock similar to that intersected in the first pass drilling by Silver City.
“If massive sulphide ores are present in the area the company would expect them to be located within corridors of anomalism such as those identified here. The company proposes to investigate the use of ground geophysical techniques such as induced polarisation and electromagnetics in order to target enriched sulphide zones.”
Golden King is an old mine largely hosted within EL 7319, about 32km north east of Broken Hill, that historically produced small quantities of high grade gold and copper mineralisation. Silver City stated
that gossanous and quartz vein material was known to occur discontinuously along the length of a shear zone oriented east-northeast and that mapping had indicated there were multiple lodes hosted
in a chlorite-altered corridor between 20m and 50m wide and 800m long.
Following rock chip sampling, Silver City stated that it intended to pursue the project further with a view to investigating its potential for near-surface copper resources suitable for open pit mining.
The Stephens Trig project lies about 15km north of Broken Hill in the northern end of a well defined, strong lead-zinc anomaly that extends 4.5km south to the Trig Southprospect; both Stephens Trig and Trig South are hosted in the upper prospective units of the Broken Hill Group rocks.
Early explorers completed a total of 24 drill holes to test the lode’s horizons and, after acquiring the project, Silver City undertook a drilling program to follow up a number of shallow, high grade intersections encountered by the previous explorers.
“The focus of historic work has been within the strongest part of the…anomaly at Stephens Trig which is 1 kilometre long and 50 to 100 metres wide,” Silver City reported.
“Previous drilling programs have recorded high grade, near-surface intersections including 2 metres at 12 per cent zinc, 4.27 per cent lead and 62 g/t gold…and 6m at 9.5 per cent zinc, 0.7 per cent lead and 14 g/t gold.
“[Silver City’s] assessment suggests that there is potential to outline a well mineralised base metal zone with further, more detailed drilling. The work has shown that there are several lode horizons between 0.2 and 3.0m in true thickness; one in particular persists for over 300m. These dip steeply to the northwest and drilling suggests a gentle plunge to the southwest.”
Of its initial seven-hole drilling program, Silver City reported that one of the holes was abandoned due to high water inflow while four of the six remaining holes reached their projected targets.
“Of the four holes that reached their target, all encountered significant thicknesses of ‘lode-rock’; specifically ‘blue-quartz’ rock which is a common host to Broken Hill type mineralisation,” the company reported.
“Interpreted thicknesses of these rocks range from 10 to 20 metres which is encouraging with respect to potential silver-lead-zinc mineralisation.
“Mineralisation was encountered within the lode-rock package with the best intersection occurring…with a 1 metre intersection of 7.4 per cent zinc, 5.4 per cent lead and 72 g/t silver.”
Of the two drill holes that failed to reach targets, Silver City reported that one had encountered a previously unknown, hanging wall lode-rock package of between 10m and 15m true thickness.
“One geological interpretation suggests that the lode-rock corridor has been offset by a fault and that the first phase Silver City drilling has focused on the south-western side of that fault,” the company stated.
“The presence of historic high grade intersections within the north-eastern fault block gives further encouragement of potential.”
The Yellowstone project consists of an easterly oriented, elongated zone of ironstone between 1m and 11m wide that outcrops across a strike length of 660m. It ranges in composition from a quartz-hematite-limonite rock at its eastern end to a banded, quartzgoethite-limonite gossan at its western end, hosted in sheared metamorphosed sedimentary rocks and amphibolite.
The project sits within a large fault zone known as the Yellowstone shear. Systematic rock chip sampling undertaken by Silver City’s joint venture partner Golden Cross Resources indicated that ironstone was variably anomalous in copper and gold, particularly in an area in the eastern part of the ironstone (covering a strike length of about 75m) that returned a number of samples above 0.5 g/t gold.
Silver City reported that a geophysical survey covering about 5km of the Yellowstone shear returned a number of distinctive chargeability anomalies, thought to be indicative of sulphide mineralisation at
The company completed an induced polarisation (IP) survey and soil geochemical sampling program on anomalous rocks within the Yellowstone shear zone, that identified several chargeability anomalies
that were tested in conjunction with geological and geochemical targets. Silver City stated that drilling encountered iron sulphide mineralisation in four holes, with best results including 4m grading 1.5 g/t gold and 0.2 per cent copper from 68m and 4m grading 54.2 g/t silver and 0.15 per cent copper from 73m. The company stated that interpretation of drilling results and follow-up of other anomalies identified would be ongoing.
The Mt Brown prospect features corridors of BHT lode rock horizons between 10m and 70m across a 1km strike within the Kyong formation of the Euriowie Block (a continuation of the Broken Hill Block). The prospect’s main corridor consists of a package of iron stained, garnet-rich metasediments that host iron and manganese-rich siliceous gossans and lode rocks ranging in thickness from a few centimetres to 4m.
Silver City reported that interpretation of detailed mapping at Mt Brown showed the rocks to be tightly folded and plunging steeply to the southeast.
“Historically mineralisation is known to significantly thicken in hinge zones at Broken Hill and there is potential for similar structural upgrade of mineralisation at Mt Brown,” it stated.
The company reported that of 67 rock chip samples taken from the prospect’s gossanous outcrops and historic workings, 27 returned values of greater than 1 per cent lead; 21 samples returned values greater than 2 per cent lead, with a maximum value of 34 per cent lead; and nine returned greater than 0.5 per cent zinc with a maximum value of 0.95 per cent zinc. Nine samples returned greater
than 0.5 per cent copper and eight samples returned greater than 30 g/t silver. One sample at the northern end of the outcropping zone returned 10.4 per cent copper and another within the central part of the zone returned 310 g/t silver.
Silver City reported that it had also reprocessed data from an historic dipole-dipole IP survey conducted at Mt Brown in 1982.
“The survey shows a strong chargeability anomaly, modelled at about 100 metres below surface. This is interpreted to represent mineralised, sulphide-bearing rock in the hinge of a tight fold and is coincident with lode horizons,” it stated.
The company completed a ground magnetic survey at Mt Brown in early 2012, identifying magnetite associated with the prospect’s lode horizons through mapping and rock chip sampling. It reported that the anomalies in the magnetic survey were consistent with targets identified by its earlier rock chip and IP surveys.
Umberumberka and Champion
On May 14, Silver City announced the results of its first drilling program at the old Umberumberka silver mine near the town of Silverton. It completed five shallow reverse circulation drill holes to test for extensions to mineralisation in areas around the old mine.
“Hole 12UM002 successfully intersected asilver-bearing structure returning 11m at 57 g/t silver and 1.3 per cent lead from 34m. Within this broader zone was a 2m intersection, from 39m, of mixed oxide and sulphide material which returned 254 g/t silver and 6.3 per cent lead,” the company reported.
“Holes 12UM004 and 12UM005 failed to fully penetrate the vein structure because both intersected old mine workings where drilling had to be curtailed. Hole 12UM001 intersected up to 10 per cent quartz vein material and 1 to 15 per cent pyrite within the structure, between 41 to 52m, with anomalous but low silver grades (range 0.5 to 1.5 g/t silver).
“Hole 12UM003 intersected a broad shear zone some 30m wide from 33m, which similarly contains disseminated pyrite and returned anomalous but low grade silver (range 0.5 to 7.0 g/t silver).
“Preliminary interpretation of the geological setting suggests mineralisation occurs in sheared rocks near the contact between sandstone units and a finer, locally graphitic siltstone. The overall dip of the mineralised structure is about 65-70 degrees to the south.”
In early July, Silver City reported results from first-pass drilling at the historic Champion mine, about 20km north of Broken Hill. Three shallow reverse circulation holes were drilled to test mineralisation in the area of the old mine, returning the following results: 4m grading 9 per cent zinc from 33m in hole 12CH001; 3m grading 6.7 per cent zinc from 26m in hole 12CH002; and 2m grading 1.7 per cent zinc from 48m in hole 12CH003.