Alkalic intrusive rocks associated with the Mount Kare gold deposit, Papua New Guinea; comparison with the Porgera intrusive complex

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The Mount Kare gold deposit is located 18 km southwest of the giant Porgera gold mine (proven and probable reserves of 51.5 million metric tons at 0.23 oz Au/t) in the highlands of Papua New Guinea. Both deposits are spatially associated with late Miocene alkalic intrusive complexes emplaced in Mesozoic-Tertiary shelf sedimentary sequences near the edge of the Australasian plate. Close geologic similarities between the two deposits, including the presence of quartz-roscoelite-Au veins and breccias, have spurred recent exploration efforts at Mount Kare.The Mount Kare and Porgera intrusive complexes consist of hypabyssal suites of comagmatic, volatile-rich mafic intrusions containing clinopyroxene, olivine, plagioclase, hornblende, chromian spinel, and apatite in melagabbros and mafic porphyries, with plagioclase, hornblende, clinopyroxene, titanian magnetite, and apatite in more evolved rocks (listed in decreasing order of abundance). Analcite occurs as an alteration phase of the groundmass and fills miarolitic cavities in evolved leucogabbros at Mount Kare but has not been observed in the Porgera intrusive complex. Least altered igneous rocks from both suites are nepheline-normative and are characterized by high volatile, alumina, large ion lithophile, and high field strength element contents. They display light rare earth element enrichments ([La/Yb] cn = 9-28 at Mount Kare, 14-20 at Porgera), but relatively low concentrations of K and Ti on mantle-normalized diagrams. The rocks are mostly sodic ([Na 2 O - 2] > or = K 2 O) according to the total alkali-silica classification scheme. Nd, Sr, and Pb isotope compositions and K-Ar ages of hornblende and biotite from the two suites are virtually indistinguishable (epsilon Nd = 6-7, 87 Sr/ 86 Sr [asymp] 0.7036, 208 Pb/ 204 Pb [asymp] 38.5, 207 Pb/ 204 Pb [asymp] 15.55, 206 Pb/ 204 Pb [asymp] 18.65; K-Ar age of Mount Kare intrusive complex = 6.0 + or - 0.1 Ma 2gma ], Porgera intrusive complex = 6.0 + or - 0.3 Ma 2gma ]). These combined characteristics indicate a dominantly alkalic intraplate affinity for the parental magma at both locations, although a weak subduction zone signature is also suggested by the relatively high volatile and alumina and low Ti abundances. It is proposed that magmatic activity was related to changes in geodynamic configuration occurring immediately prior to early Pliocene collision between the Australasian plate and an island-arc system. Minor differences, such as divergent trajectories of Pb isotope crustal contamination arrays, the occurrence of analcite only at Mount Kare, and the widespread presence of excess 40 Ar in igneous amphiboles only at Porgera, indicate that, although related, the Mount Kare and Porgera intrusive complexes developed independently of one another.Disseminated auriferous pyrite from the Mount Kare Au deposit is associated with phyllic alteration of intrusive and sedimentary rocks. A single K-Ar analysis of illitc from altered rock records an age of 5.5 + or - 0.1 Ma (2Sigma ), which falls in the middle of the range of illite and roscoelite ages from the Porgera Au deposit (5.1-6.1 Ma). This result, combined with slightly older ages of igneous activity, suggests a close temporal and genetic association between magmatism and mineralization, at both locations. In addition, the narrow range of Pb isotope compositions of galenas from veins at Mount Kare is identical with that from Porgera, suggesting that similar ore-forming processes were involved at the two locations.Recognition of closely similar igneous lithologies and ore types at Mount Kare and Porgera supports theories of a genetic relationship between volatile-rich, mafic, alkalic magmatism and meso- to epithermal Au mineralization, as previously proposed at Porgera. It is suggested that both of these deposits belong to the alkalic epithermal group, to which Cripple Creek (Colorado), Emperor (Fiji), and Ladolam (Lihir Island, Papua New Guinea) also belong. Igneous centers with these characteristics and located in complex convergent tectonic settings should be considered as prospective targets for precious metal exploration.