Genetic Modelling of Greisen-Style Tin Systems

Abstract

The general features of the greisen systems include the occurrence of lenticular to massive alteration zones contained within cuspate protruberances from the apical zones of late stage geochemically specialised granitoids. The systems consist of an upper outer zone of minor barren pegmatite development, often associated with fine grained pegmatitic granite. The mineralisation occurs as irregular, to massive, or sheet-like bodies with the zone immediately below the contact extending for some 10–100 m. The bodies are essentially zones of fluorine rich sericitic-silicic alteration with associated cassiterite. The mineralised zone merges downwards into highly altered granite dominated by feldspathic types of alteration which in turn grade into mildly altered “fresh” granite.

The postmagmatic nature of the alteration has led to the development of a model of mineralisation resulting from the upward passage of post magmatic fluids which leach tin from the lower feldspathic horizon and precipitate cassiterite in the sericitic zone. This model currently holds dominance over concepts which invoke fluids derived from unspecified deeper sources ultimately concentrating in structural traps beneath the pegmatite, and models which favour processes of magmatic fractional crystallisation/melt diffusion.

Major questions that require solution before acceptance of any current modelling include:

• 1) What is the significance and origin of the pegmatitic marginal zones? These are early crystallising components and suggest consistent specialised melt-hydrothermal conditions.

• 2) Why are the early formed pegmatitic components usually devoid of cassiterite?

• 2. 3) How are the postmagmatic mineralising fluids evolved, and why should they consistently occupy the apical region?

• 4) Why do postmagmatic fluids normally fail to cause any alteration within the overlying thin zone of pegmatite?

• 5) How do postmagmatic fluids form the frequently well developed curving lensoid zones which mirror the apical contact shape?

• 6) Why do the curving lensoid bodies frequently become smaller and more dispersed with depth?

• 7) Why do the systems generally exhibit a total lack of structural control via major brittle fracture systems?

• 8) Why is the lower feldspathic zone (“Leaching Zone”) consistently more anomalous in tin and related volatile elements than the underlying granite?

Consideration of these points suggests that a theory of an early magmatic preconcentration of the ore elements within apical melt zones, followed by an essentially in-situ development of mineralisation during late stage crystallisation, would resolve many of the problems.