Geology, geochemistry and stable isotope studies of an epithermal hydrothermal system, Rosita Hills, Colorado
The Rosita Hills volcanic centre is an alkali-calcic mid-Tertiary caldera complex overlying ortho- and paragneissic basement on the eastern margin of the Wet Mountains graben in southcentral Colorado. There were two mineralising events at the Rosita centre. Au, Ag and base metal mineralisation occurred in a phreatomagmatic breccia pipe at the northern margin of the complex. Later, Ag and base metal mineralisation occurred in veins in the centre of the complex. Mapping, petrological and XRD studies outline 4 alteration facies related to hydrothermal activity at the centre. Propylitic/argillic, K-feldspar-sericitic, advanced argillic and silicic alteration assemblages are recognised. The areas of most intense alteration are controlled by the dominant structural trends within the caldera. Sub-volcanic magma movement is postulated as the dominant cause of the fracture patterns. A lithogeochemical grid survey for Au, Ag, Sr, Rb, Cu, Pb, Zn and Mn across areas of hydrothermal alteration reveal complex patterns indicative of multi-stage hydrothermal activity. District-wide Sr, Zn and Mn depletions are related to the propylitic/argillic alteration. Au, Ag, Rb and Cu enhancements are related to the K-feldspar-sericite alteration. Late stage advanced argillic alteration modified the trace element dispersion patterns by leaching previously formed enhancements. Stable isotope studies (O and H) of whole rock and mineral separate (quartz and sericite) samples from veins and hydrothermal eruption breccias show that the hydrothermal fluid had both meteoric and magmatic components. δD values from whole rock samples show a crudely concentric pattern centring on areas of sericitic and advanced argillic alteration in the middle of the lithogeochemical grid. Fluid inclusion data from vein gangue minerals (quartz, baryte and sphalerite) and from silicified rock in the advanced argillic alteration zone again show that the hydrothermal fluid had more than one component fluid. A highly saline, high temperature fluid occurs in quartz associated with base metal mineralisation. Less saline inclusions occur in the upper parts of the system in the silicic alteration. The data indicate that mixing of these two end-member fluids precipitated the vein mineralisation. The source of metals in the Bassick breccia pipe orebody was a highly differentiated magma body underlying the breccia pipe. Incipient ring faulting probably controlled the emplacement of the magma. Other similar breccia pipes in Colorado are postulated as overlying Cu-Mo porphyry mineralisation. The source of the metals in the Rosita vein orebodies was the volcanic host rocks (and the Precambrian basement?). The Rosita Hills vein mineralisation shows features typical of adularia-sericite systems in the western United States. The Au:Ag ratio in these deposits can be related to the origins of the crust underlying the deposits.