Mineralogical and chemical features of gangue phases in relation to hydrothermal mineralization and their host rocks

Abstract

Gangue minerals from hydrothermal deposits (apatite and carbonates) and their host rocks from three different volcanic areas (SE Spain, La Serena and Melipilla in Coastal Range of Chile) have been studied for broad elemental compositions. Carbonate units at the Au-Cu Palai-Islica epithermal deposit are Fe-Mn-bearing, with a slightly higher concentration of these elements in the orebody than in adjoining hydrothermal alteration zones. Apatite has a composition that correlates with its origin and with hydrothermal processes. Thus, volcanic apatite is Cl-rich, whereas apatite from the associated orebody is almost pure fluorapatite. Furthermore, apatite from hydrothermally altered volcanic rocks has a transitional composition between volcanic and ore-related apatite. Samples of carbonate from Mn, Cu(-Ag) and Ba-Ag deposits in the La Serena area are all Mn-bearing calcite. In addition, Mn(Fe)-poor and Mg-rich calcite is common in low-grade Mn areas. Similar features have been found at the Melipilla Cu (Cu-Ag) deposit where epithermal calcite is also enriched in Mn(+Fe) with respect to non-mineralized veins, carbonate host rock, and metamorphic mineralization. In summary, F in apatite and Mn have been introduced in to carbonate from the La Serena area by hydrothermal fluids and could provide an index of hydrothermal ore-forming activity.