BMBF-r4GEM Granite related mineralization of strategic metals – conditions of mineralization and search criteria for hidden ore bodies.

The formation of tin, tungsten, and tantalum mineralization involves a sequence of processes that operate in different tectonic settings and that may be widely separated in time, i.e., (i) source enrichment, (ii) source accumulation, and (iii) metal mobilization from the source. The sequence of these processes controls the distribution of mineralization in belts, whereas magmatic processes and interaction with the wall rocks at emplacement level controls size, grade, shape, and kind (vein, greisen, skarn) of mineralization.

• (i) Intense chemical alteration in the exogenic environment results in the preferential loss of most feldspar-bound elements (e.g., Na, Ca, Sr, and Pb) and the residual enrichment of elements incorporated in or adsorbed on clay minerals (e.g., Li, K, Rb, Cs, Sn, and W), i.e., it produces some of the hallmark geochemical signatures of tin granites that also are obtained by extreme magmatic fractionation of granitic melts. Intense chemical alteration occurs in tectonically stable areas with limited topography, as for instance in the interior of large continental masses.

• (ii) Sedimentary accumulation occurs when these blankets of chemically intensely weathered sediments are redistributed from the continent interior to the margins of the continent during supercontinent fragmentation. Tectonic accumulation may occur when passive-margin sedimentary packages are reworked in an active margin setting.

• (iii) The nature of heat source controls the type of melting of the crustal source rocks and the partitioning of metals between melt and restite. Mobilization of Sn and W requires high-temperature melting and this is restricted to tectonic settings with advective heat input from (a) mantle-derived melts in subduction settings, (b) emplacement of ultrahigh-temperature metamorphic rocks that had been subducted to mantle depth during continental collision, and (c) mantle-derived melts in extensional settings. Internal heating in orogenically thickened crust only generates minimum-temperature melts. The age of mineralization reflects the event of heat input. Note, chemically intensely altered rocks produce muscovite rich metamorphic assemblage and, produce high amount of melts. If low-temperature melts are lost before biotite melting starts, biotite-breakdown eventually may results in melts with high Sn contents.

The superposition of source enrichment (on supercontinent), source accumulation (at continent margin), and heat input (at plate boundary) explains both the distribution of Phanerozoic primary tin, tungsten, and tantalum mineralization and (i) their irregular distribution along these belts, (iii) their contrasting age within a particular belt, (iv) their contrasting tectonic settings, and (v) their presence on both sides of major sutures.

Associated publications:

Romer, R.L. and Kroner, U. (2015) Sediment and weathering control on the distribution of Paleozoic magmatic tin–tungsten mineralization. Mineralium Deposita, 50: 327-338.

Romer, R.L., Kroner, U. (2016) Phanerozoic tin and tungsten mineralization – tectonic controls on the distribution of enriched protoliths and heat sources for crustal melting. Gondwana Research, 31: 60-95.