Tertiary limestone and Zn-Pb mineralised skarn at Motukokako, Cape Brett, northern New Zealand

Abstract

Motukokako, 1 kIn northeast of Cape Brett, Northland, New Zealand, is composed of Waipapa "basement" greywacke and argillite of Permian-Jurassic age, unconformably overlain by moderately dipping (?)Oligocene Whangarei Limestone. The nearest Whangarei Limestone onshore is 37 km to the southwest, at Kawakawa. Both the Waipapa rocks and Whangarei Limestone are cut by a series of subvertical Zn-Pb-Cu mineralised quartz veins. At the northern end of Motukokako, the Whangarei Limestone is altered to Zn-Pb mineralised calc-silicate skarns, formed by metasomatic reaction between the limestone and the hydrothermal fluids which penetrated along fractures that were later filled by the quartz veins. Prograde hedenbergite-garnet-epidote-axinite skarn is the dominant type; it is cut and replaced by retrograde ilvaite-babingtonite skarn, mainly along subvertical zones adjacent to the quartz veins. Sphalerite and galena are disseminated in the hedenbergite-garnet-epidote skarn, and also in the adjacent silicified recrystallised limestone. The mineralisation is generally low grade (1-6% Zn and 0.2-1 %Pb). Some of the zinc is contained in axinite and in sauconite, a Zn-rich smectite. Hedenbergite, axinite, ilvaite and babingtonite are notably enriched in Mn, while epidote and garnet (andradite) are Fe-rich varieties. No igneous rocks are known in the vicinity; the most likely source for the hydrothermal fluids that produced the skarn is an Early Miocene quartz diorite-granodiorite stock at depth, similar to those now exposed at Cape Karikari and Coppermine Island.

New Axinite Occurrence from Bükk Mts., NE Hungary

“…Chemical compositions measured by EDX are not precise enough to identify these minerals, due to their small grain size, but most probably they are the members of the brackebuschite supergroup. It cannot be excluded that the appearance of the Pb-V-Zn-Cu-As-bearing phases is connected to the axinite, as many examples are described in the literature, where axinite and Pb-Zn mineralization are coupled [15][16][17][18]. It is possible, that not the axinite is the only B-bearing mineral phase in the rock body, as B concentration calculated based on the axinite content (6 wt% axinite, with 1.9% boron meaning 1146 ppm boron) is less than B concentration measured with ICP-AES (1770 ppm).…”

Regional geological framework of New Zealands mineral deposits

“…The Northland and Coromandel volcanic zones have associated epithermal mineralisation, mainly Au and Ag, but also Pb, Zn, Cu, Hg, and Sb, as well as deposits of native sulfur, perlite, pumice and zeolite (Figures 4,5). Porphyry Cu style mineralisation is associated with subvolcanic intrusive rocks in Northland and Coromandel, and Pb-Zn skarn is hosted by limestone in Northland (Brathwaite et al, 1990). The epithermal Au-Ag deposits of the Coromandel region occur mainly as quartz ± carbonate veins with a variety of sulphide minerals (Brathwaite, Christie and Skinner, 1989;Christie et al, 2006b).…”