Granites and hydrothermal ore deposits: a geochemical framework
Authors: Eugster Hans P. 
Published: December 2006

The geochemical evolution of tin-tungsten deposits and their associated sulphides can be discussed in terms of four sequential processes: acquisition of the ore-forming elements (OFEs) by the granitic magma, emplacement of these elements in minerals and residual melt of the crystallizing granite, release of the OFEs to the circulating hydrothermal fluids and transport to the depositional sites, and finally, deposition of ore minerals through interaction of these fluids with the wall rock. Based on their crystallographic behaviour, it is useful to distinguish three principal classes of OFEs, here identified as BOC, LHC, and ALC elements. BOC (bivalent octahedral cation) elements are similar to ferrous iron and here are represented mainly by Zn, Mn, and perhaps Cu. Li also belongs to this class, although it is monovalent. LHC (large highly charged cations) elements encompass As, Nb, Mo, Sn, Sb, Ta, and W and they are similar to ferric iron or titanium in their crystallographic role. ALC (alkali-like cations) are capable of occupying alkali positions and are represented mainly by Pb, Ag, and Hg.

LHCs are rejected from the polymerized silicate liquid network and become enriched in the roof of the acid magma chamber, where more non-bridging oxygens are available. Transport to the roof may be enhanced by the formation of hydrous complexes, as is the pronounced enrichment of Na and Li. BOCs, along with Cl, F, and B, fractionate strongly into the vapour phase during vesiculation. HCl in the ore fluid is crucial for the alteration process and can be produced during boiling by a hydrolysis reaction of the NaCl dissolved or immiscibly present in the silicate magma.

Considerable laboratory information is available concerning release mechanisms of the OFEs to hydrothermal fluids. We can distinguish congruent and incongruent dissolution, both in response to acid buildup, as well as congruent and incongruent exchange not involving HCl. Melt-fluid fractionation is also thought to be important, though the physical mechanisms are not well understood. Any of these release mechanisms may be coupled with reduction or oxidation reactions. LHC, BOC, and ALC elements respond differently to each of these mechanisms, and these differences may in part be responsible for the observed separation of ore minerals in space and time. It is suggested that LHC elements are released preferentially during acid, non-oxidizing conditions typical of early stages, while BOC elements respond more readily to later acid-oxidizing environments, as well as exchange reactions with or without oxidation.

Depositional reactions have been formulated with respect to two contrasting types of country rocks: carbonates and schists. Differences are related to the process of neutralization of the HCl produced by ore deposition: carbonate dissolution on one hand and feldspar-muscovite or biotite-muscovite conversion on the other. In carbonate rocks, evaporite-related sulphates may provide the H2S necessary for sulphide precipitation, while in schists disseminated sulphides and organic matter may be important sulphur reservoirs in addition to the sulphur liberated from the granite. A variety of situations can be envisaged with respect to the sources of the OFEs and the sulphur species required for ore deposition, including granite and wall rocks. Chloride is recognized as the crucial anion for OFE release, transport, and deposition, although F and B play a role yet to be evaluated. Final HCl neutralization is an essential step in the reactions responsible for the deposition of ore minerals.

The ultimate sources of the OFEs must be related to the continental material involved in the process of melt production by partial melting. Oxidized sediments provide sources for LHC and ALC elements in the form of heavy minerals and clastic feldspars and micas. Organic-rich reduced sediments are hosts to BOC and LHC elements as sulphides and ALC elements in organic matter. Remelting of igneous and metamorphic rocks can enrich LHC, BOC, and ALC elements in the melt by extraction from opaques, Fe-Mg silicates, feldspars, and micas.