Sunday, 11 November 2012

In the hills of Valla and Nambucca Heads

The Valla Adamellite now termed the Valla Monzogranite to reflect modern naming conventions is an interesting small to medium sized intrusion about 10km north east of Nambucca Heads. It is one of the suites of coastal granites which are mostly I-Types (melted igneous material), this means that the coastal granites show abundances of ore minerals within the granite or in the surrounding metamorphosed country rocks. A monzogranite is a granite with roughly equal proportions of (alkali-feldspar (potassium and sodium rich) and plagioclase feldspar (calcium rich)). The monzogranite is thought to have formed during the Triassic period.

The metamorphic aureole for the Valla Monzogranite is actually quite interesting as it shows a classic zonation of metamorphism (high grade at the contact grading to low grade further away) and also excellent examples of mineral zonation associated with metasomatism (hot-water or fluid alteration of rock). The Valla Monzogranite has been shown to be associated with gold, silver, arsenic and molybdenum mineralisation (as well as others). The rock that the monzogranite has been intruded into is called the Nambucca Beds which are part of the Nambucca Block. The Nambucca Beds are Permian to Carboniferous in age and are mainly comprised of the regionally metamorphic rock type called phyllite which was originally deposited on the sea floor. The Nambucca Block was accreted onto the Australian continent in the New England Orogen and this caused the regional metamorphism of the beds.

The Nambucca Beds are intruded by the Monzogranite. The Beds are extensive and
extend far into the rugged Nambucca Hinterland. This photo is west of Bowraville.
The Valla Monzogranite seems to be a Climax Molybdenum Deposit named after the Climax Mine in North America. This means that when the Monzogranite was cooling the upper portion of the pluton became residually enriched with fluids, metals and silica. These fluids cause alteration of the upper portion of the pluton forming what is called greisen and also are injected into the surrounding rock through veins and sometimes aggressively through breccia pipes. One of the first minerals to form in these veins is silica, quartz with metal sulphide such as molybdenite (molybdenum ore) and wolframite (tungsten ore). Further away from the intrusion the degree of alteration becomes less grading through potassic through to argillic which are defined alteration zones based on the changes in the rock forming minerals. As the degree of alteration becomes less so the types of metal ores change with increasing amounts or arsenic, gold and silver. Further out in the alteration zone minerals such as galena form (lead ore) and finally stibnite (antimony ore). These ore deposits seem to be fairly common in the New England area with Glen Eden being the most studied (Somarin 2001, Somarin & Ashley 2004) and have in some areas been extensively explored such as Kingsgate east of Glen Innes.

Some attempts of mining have occurred in the Valla Monzogranite in the past, the most significant being the Valla Gold mine which was located just to the north of Valla Beach. The mine was abandoned with very little rehabilitation and therefore has become an environmental problem for the local creek. However, rehabilitation efforts have recently been undertaken, though these will need another post to discuss in more detail.

References/bibliography:

*Somarin, A.K. 2011. Petrography, Geochemistry, and Petrogenesis of Late-Stage Granites: An Example from the Glen Eden Area, New South Wales, Australia. Earth and Environmental Sciences.
*Somarin, A.K. & Ashley, P.M. 2004. Hydrothermal Alteration and Mineralisation of the Glen Eden Mo-W-Sn deposit: A Leucogranite related hydrothermal system, southern New England Orogen, NSW, Australia. Mineralium Deposita.

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