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

Abstract

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 orerelated 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.

Significance of phyllosilicate mineralogy and mineral chemistry in an epithermal environment. Insights from the palai-islica Au-Cu deposit (Almería, SE Spain)

“…Therefore its concentration has much to do with ore-fluids. Association of Mn-bearing minerals with hydrothermal ore-bearing activity has been also highlighted in carbonate phases (Carrillo-Rosúa et al, 2005).…”

“…Hydrothermal alteration is pervasive and the only minerals which remain alteration are quartz, zircon and apatite, although the last suffers chemical transformation (Carrillo-Rosúa et al, 2005). White mica and chlorite (±pyrite, albite and epidote) is the mineral assemblage in the proximal zones of the hydrothermal alteration.…”

“…illite and interstratified illitesmectite), while maphic phenocrysts are usually totally or partially altered to chlorite. The presence of epidote and calcite (rather than dolomite -which could appear occasionally in chloritic and sericitic alteration,Carrillo-Rosúa et al, 2005) is also usually a characteristic feature.Outside the Palai-Islica deposit, propylitic alteration becomes a regional alteration feature, with the disappearance of epidote. This propylitic alteration represents an area with a low infiltration rate of hydrothermal fluids, a low fluid/rock ratio, and the presence of other cool meteoric regional fluids.…”