Our President, Ralph Bottrill, introduced us to Broken Hill earlier this year, providing us with historical aspects of it's discovery, some aspects of the complex geology, and an insight into the important primary mineralisation of the lode. Ralph is certainly qualified to talk on these subjects, having spent a number of years living and working in Broken Hill.
My own association with Broken Hill, although of many years duration, is much more limited. My first two visits took place before I became interested in minerals. Recollections include sleeping in my car parked at a service station, waiting for them to open and replace my shattered windscreen, avoiding LARGE wildlife on the Silver City highway, and a tour underground at Delprats.
Subsequent trips involved collecting, from both the Kintore and Block 14 open cuts. As with many collectors of Australian minerals, I considered Broken Hill to be somewhat of a mecca, but unfortunately, couldn't go there as often as I would have liked. Whether cabinet specimens, or microminerals, Broken Hill minerals take pride of place, not just in mine, but in many collections.
This presentation will concentrate on minerals from the oxidised zone of the main Broken Hill lode, and will not be concerned with outlying areas such as Silverton, Apollyon Valley (eg: Day Dream Mine), Pinnacles, Consols, Thackaringa, and Umberumberka, all of which are interesting subjects for other talks.
In the Broken Hill area, approximately 300 minerals have been recorded. Many of these minerals have formed through oxidation and weathering processes. The oxidised zone is variable, in some cases following faults or shears to depths of 140 metres below surface.
Secondary minerals collected in recent years have been from the Blackwood Open Cut (Block 15 of the old British Mine), the Kintore Open Cut which commenced in 1985 (straddling the original Blocks 9, 10, and 11), and from the Block 14 Open Cut. Current collecting is restricted to dump material at the Pinnacles Mine waiting to be processed. This material has come from Kintore, Block 14, and the North Mine.
It is sometimes difficult to know exactly where some older mineral specimens have come from because of the change of ownership of blocks. For instance, the South Mine leases covered 11 of the original leases, and incorporated the Central Mine (Block 9), the British Mine (Blocks 15 and 16), and the Junction Mine (Block 39). Green pyromorphite labelled South Mine, may have come from the Blackwood Open Cut, or if it is of more recent origin, from Kintore. Many of the blocks were originally owned by BHP, and old time labels may state the Proprietary Mine. Similar problems manifest themselves in other mineral regions such as Dundas, Mt Lyell, etc. here in Tasmania.
Broken Hill is the type locality for about fifteen minerals, nine of which are secondary in nature. Four of these nine are arsenates, with another three being halides.
THE MINERALS
ELEMENTS
Broken Hill is known as the Silver City, so it is perhaps appropriate that we begin with the native metal itself. Silver wires and arborescent masses occur throughout the oxidised zone, although usually small in size. Crystalline copper has been found throughout the life of the mine, often in large amounts. Other native elements that have been found include graphite as flakes and rosettes, and sulphur as tiny greenish yellow crystals associated with secondary galena, brown pyromorphite and hemimorphite.
Copper
Graphite
Silver
Sulphur
SULPHIDES
As you would expect, most sulphides at Broken Hill are of a primary nature. Exceptions to this include secondary galena, some sulphosalts, hawleyite (cadmium sulphide) and wurzite.
Galena
Hawleyite
Sulphosalts
Wurtzite
OXIDES AND HYDROXIDES
Amongst the oxides are some of the most attractive and striking mineral specimens. Black masses of the lead manganese oxide, coronadite, when sprinkled with colourless sparkling smithsonite, is one of my favourites. Similarly, stalactites of brown or iridescent goethite are also attractive when mimetite or other minerals occur on them. Cuprite occurs as transluscent to opaque cubes and octahedra of a rich and typical cherry red colour. Broken Hill is also the type locality for an iron hydroxide, bernalite.
Bernalite (type locality)
Bindheimite
Chalcophanite
Coronadite
Cuprite
Goethite
Minium
HALIDES
The halide group of minerals contains both species that are extremely rare, and species that have contributed greatly to the riches of Broken Hill due to their abundance. The rarities include the iodides, marshite (copper), and miersite (silver and copper), for both of which Broken Hill is the type locality. Other rarities include perroudite, a mercury silver halide, and blue cubes of boléite. Embolite, a term used for bromian chlorargyrite, is the most abundant secondary silver mineral at Broken Hill, occurring as both individual micro crystals and as heavy olive green, light sensitive masses. Rich lumps of embolite have been dug up over the years, many fading to purplish brown and greyish unattractive pieces. Other silver halides include bromargyrite as bright yellow crystals, and iodargyrite as much paler crystals. Most of these minerals are affected to some degree by exposure to light.
Atacamite
Boleite
Bromargyrite
Cumengite
Embolite - bromian chlorargyrite
Iodargyrite
Marshite (type locality)
Miersite (type locality)
Paratacamite
Perroudite (co-type locality)
Tocornalite
CARBONATES
Broken Hill is world renowned for some of it's carbonate mineral specimens. Azurite crystals measuring up to 10cm long rival those found at Tsumeb, Namibia. Reticulated cerussite specimens are possibly the best in the world, some forming almost perfect arrowhead twins. Crystalline smithsonite in blue, green, white, brown, colourless, the latter brilliantly contrasted against black coronadite. Otavite, a cadmium carbonate, and bastnasite, a rare earth carbonate are of rare occurrence. The micromineral collector is catered for with superb specimens of rosasite and aurichalcite, and acicular sprays and tufts of malachite.
Aragonite
Aurichalcite
Azurite
Bastnasite-(Ce)
Calcite
Cerussite
Dolomite
Hydrocerussite
Hydrozincite
Leadhillite
Malachite
Otavite (cadmium carbonate)
Rhodochrosite
Rosasite
Shannonite
Smithsonite
SILICATES
There are not many secondary silicates at Broken Hill. The important ones are bright sky-blue chrysocolla, and colourless, white, blue, and brown hemimorphite.
Chrysocolla
Hemimorphite
Kaolinite
Opal (Hyalite)
ARSENATES, PHOSPHATES AND VANADATES
This group comprises the largest number, and greatest diversity of species in the oxidised zone at Broken Hill. It is also the group that has received the most attention in recent years. Traditionally, mention of phosphates or arsenates at Broken Hill, meant that pyromorphite and mimetite would readily come to mind. Once seen, who could forget the large crystalline masses of green or brown pyromorphite. Times change, and the primary interest is now with a number of rare and unusual species, including kintoreite (a lead iron phosphate), mawbyite and segnitite (lead iron arsenates), all of which were first described from Broken Hill. Here is also the co-type locality for gartrellite (a lead copper iron arsenate). The variety of colours is also evident, from the bright carmine red of carminite, to the myriad of blues and greens of the copper minerals, to the creams, yellows and browns of the iron minerals.
Adamite
Agardite
Apatite
Arseniosiderite
Arsentsumebite
Bayldonite
Beraunite
Beudantite
Carminite
Chalcosiderite
Conichalcite
Corkite
Cornwallite
Crandallite
Cyrilovite
Dufrenite
Duftite
Faustite
Fornacite-Vauquelinite
Gartrellite (co-type locality)
Hidalgoite
Hinsdalite
Hopeite
Kidwellite
Kintoreite (type locality)
Kipushite
Kleemanite
Leucophosphite
Libethenite
Mawbyite (type locality)
Mimetite
Mottramite
Natrodufrenite
Olivenite
Pharmacosiderite
Philipsbornite
Plumbogummite
Pseudomalachite
Pyromorphite
Rhabdophane (REE)
Rockbridgeite
Sampleite
Scholzite
Scorodite
Segnitite (type locality)
Sampleite
Strengite
Tarbuttite
Torbernite
Tsumcorite
Tsumebite
Turquoise
Variscite
SULPHATES
Oxidation of the primary sulphide minerals has resulted in a number of copper, lead, and zinc sulphates, many very colourful, and quite plentiful. Anglesite forms in a range of habits and colours, and individual crystals have been recorded up to a few centimetres in size. Barite, gypsum, brochantite, chalcanthite, linarite and jarosite group minerals are included among "common" minerals found at Broken Hill. Rare minerals include copper zinc sulphates, ktenasite, serpierite, and schulenbergite, and other copper sulphates, connellite, kröhnkite, langite, and spangolite.
Anglesite
Antlerite
Barite
Bechererite
Brochantite
Caledonite
Chalcanthite
Connellite
Gypsum
Halotrichite
Jarosite
Kröhnkite
Ktenasite
Lanarkite
Langite
Linarite
Namuite
Natrojarosite
Pickeringite
Plumbojarosite
Posnjakite
Schulenbergite
Serpierite
Spangolite
TUNGSTATES AND MOLYBDATES
This brings us to the final group of minerals. A group which above all, epitomises Broken Hill minerals. Raspite, of course, is found at very few localities in the world. A dimorph of stolzite, it was first described from Broken Hill more than 100 years ago, and named after the discoverer of the "hill of mullock", Charles Rasp. Stolzite, although found sparingly in many places around the world, is nevertheless a rare mineral. Once again, the specimens from here are world renowned. Have a look at the 2cm long bipyramidal crystals in Rob Sielecki's Ausrox advertisement in issues of the Australian Journal of Mineralogy. Tabular forms are also found and it is easy to mistake them for wulfenite which also occurs here.
Raspite (type locality)
Stolzite
Wulfenite
FINALE
I know that you will agree that this is an interesting collection of minerals to say the least.
Further information can be found in Minerals of Broken Hill issued in 1982 and recently revised and reissued (1999), the Australia issue of the Mineralogical Record (1988), Australian Mineralogist (1985, 1989 and 1990), Mineralogical Record (1996 - 27(5)), and of course, the Broken Hill issue of the Australian Journal of Mineralogy, Volume 3, Number 1 (1997).
The list of credits include:
* Bottrill in the Minerals of Broken Hill.
* Jan van Moort, who with C. Swensson in 1981, authored The Oxidised Zone of the Broken Hill Lode, NSW.
* The authors of Perroudite, a New Sulphide-Halide of Hg and Ag (1987), included H. Sarp, which of course, is an anagram for Rasp.