Taronga tin deposit

The last fluid out of the granite

Taronga, near Emmaville in the New England Orogen, is a tin deposit of the classic granite-related kind. When a granite crystallises, the elements that do not fit comfortably into feldspar and quartz — tin, tungsten, fluorine, boron, beryllium — are progressively concentrated into the last remaining melt and the fluids that escape from it.

Those fluids leave along joints and fractures. GSNSW describes the deposit shape as lenticular with joint control and a sheeted vein mineralisation style, which is a precise way of saying: the tin is where the cracks were.

The mineral list is the giveaway

Cassiterite is the ore mineral, and it is the one worth training your eye on — dark brown to near black, resinous to adamantine, and surprisingly heavy for its size.

But the gangue is where the story is. Alongside chlorite, muscovite, quartz and sericite, GSNSW lists beryl, fluorite, topaz and tourmaline. That is a fluorine-and-boron-rich assemblage, and it is diagnostic. Topaz and tourmaline in a vein selvage are the fluid signing its own name.

The alteration logged is silicic and sericitic — greisen-style, where the granite's feldspar has been attacked and replaced by quartz and mica.

A drill record shaped by a price collapse

There is a second story in this dataset, and it is not geological. Of the holes GSNSW holds within two kilometres of the deposit, the overwhelming majority were drilled in a single decade — the 1980s. Then, effectively, nothing.

In October 1985 the International Tin Council's buffer stock ran out of money, the tin price collapsed, and exploration budgets went with it. Colour the collars by era in the viewer below and you can watch an entire commodity cycle stop dead.