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KINGSGATE MINES Part 3

THE GEOLOGY OF KINGSGATE

The casual visitor may overlook the intimate relationship between the ore bearing pipes and the granite/metamorphics contact, but the miners were well aware that the ore bodies were always found within the granite and never more than a few hundred metres from its margin. That this is more than a co-incidence was borne out during mining when it was often found that the pipes dipped into the earth more or less parallel to the contact.

It will be necessary to examine briefly the geological structure of the district to understand the origin of the mineral deposits. The rocks throughout New England reveal a complex history of marine and nonmarine sedimentation, explosive volcanic activity and widespread intrusion of granitic magma. This is consistent with what is found today along growing continental margins, where marine sediments are subducted beneath thicker continental crust by plate movements. Such a situation occurs today in Japan, which probably resembles the New England of the late Palaeozoic era. At Kingsgate, metamorphosed sediments of probable Permian age have been intruded by late Permian/early Triassic granite. Mineralisation is confined to 70 or more roughly cylindrical ore bodies within the granite, at a short distance from its intrusive contact.

Molybdenum mineralisation in New England is mostly located in a NE/SW trending zone including the deposits at Wunglebung, Deepwater, Glen Eden, Kingsgate and Booralong. At each place the mineralisation is near an intrusive granite margin, sometimes in pipe formations (as at Kingsgate and Deepwater) and sometimes associated with aplite and pegmatite bodies within the granite (as at Booralong). Similar relationships have been observed in North Queensland deposits. It would appear that volatile components of the magma, containing quartz, sulfur, bismuth and molybdenum in solution, have crystallised within the mostly solidified granite at a late stage of the cooling of the intrusion.

The most prominent of the igneous rocks at Kingsgate is a coarse, mottled grey adamellite, rich in quartz and orthoclase, nonporphyritic and with no apparent structure. It differs noticeably from the tin bearing granites seen, for example, at Torrington. The common dark minerals within the granite are biotite and hornblende. However, this rock does not appear to be the actual host rock for the deposits, which is more likely to a very acid granite usually not seen in outcrop but generally reduced to a thick gravelly soil. Several other granitic bodies also outcrop in the area. The former sedimentary rocks intruded by the granite may be seen in road cuttings below the mines and along the Yarrow River. They are essentially altered siltstone of presumed Permian age. No fossils are known from these rocks at Kingsgate, but Permian marine fossils have been found in similar rocks further south.

There has been considerable speculation as to the reasons why the Kingsgate mineralisation occurs in the way it does. The best that can be said is that the pipes are separate intrusions into the main plutonic mass, whose location has been controlled by the pressure and temperature conditions prevailing at the time. The pipes are very irregular in shape and structure and frequently change direction, dip and diameter as they are followed downwards. Some are characterised by branches, others by their variation in diameter from a few centimetres to many metres across. In some the minerals completely fill the pipe, in others there are open spaces lined with huge quartz crystals. All are of the same essential character, being composed in part of granite in which the felspar has been altered to sericite, in part of quartz and in part of veins and segregations of ore minerals.

Sach's Pipe (the Old 45) has been studied in some detail. More information on individual pipes is to be found in the next chapter.