Geochemistry and stable isotope study of porphyry-related mineralisation, central Front Range, Colorado, U.S.A.
The central part of the Front Range mineral belt, Colorado, is a Precambrian crystalline basement terrain intruded by early Tertiary porphyries and closely associated mesothermal ore deposits. The ore deposits are gold- and silver-bearing base metal sulphide-quartz veins arranged in more or less well developed concentric mineral zones centred on concealed porphyry plutons. Central areas of pyritic mineralisation are surrounded by peripheral zone of galena-sphalerite-bearing veins, often separated by a transitional zone of composite mineralisation. Molybdenite-bearing and uraninite-bearing veins are locally abundant proximal to the central pyritic zones and are closely associated spatially and temporally with highly evolved quartz bostonite intrusions. The mineral veins are enveloped in narrow selvages of sericitized and argillized wallrock and are entirely fracture controlled. A low density lithogeochemical survey of the country rocks revealed strong lithological controls on the distributions of selected trace elements and radiation emission values. Positive Rb/Sr anomalies are tentatively linked to concealed intrusions, and corresponding positive anomalies of Pb, Zn, Mo and W are associated with known molybdenite mineralisation. Sericitized rocks are characterised by low Sr and enhanced Rb, Zn, Pb, Mo and Ag values, and areas of depleted country rock Sr values may be related to vein wallrock alteration. D/H and 18 O/16 O signatures of fresh country rocks indicate variable magmatic water overprinting and similar analyses of sericitised rocks show that alteration was accomplished by magmatic hydrothermal fluids at 340o -515o C. Mineralisation and alteration in the area were caused by hydrothermal fluids evolved from highly differentiated Laramide quartz bostonite intrusions. K-Ar ages of ca. 62 Ma have been obtained from alteration sericites from the peripheral mineral zone. Similar studies in the central pyritic zone have yielded ages of ca. 59 Ma suggesting the existence of long lived hydrothermal activity in the area characterised by widespread high-temperature alteration, followed by thermal decay and collapse towards core zones. The ore deposits investigated during this study display numerous features in common with porphyry-style and other intrusive-related mineralisation characteristic of Cordilleran terrains, but also exhibit significant differences. The combination of these features contribute to the distinct and possibly unique character of these mineral deposits.