Petrographic and Geochemical Study of the Hybrid Rock Unit Associated with the Current Lake Intrusive Complex

Abstract

The Current Lake Intrusive Complex (CLIC) is one of several ultramafic to mafic intrusions associated with the Midcontinent Rift that host Ni-Cu-PGE deposits. Geophysical surveys and drilling have outlined a six kilometer-long mafic to ultramafic, chonolith-shaped intrusion that is composed primarily of wehrlite to dunite and has intruded into granitic (Current Lake Zone or CLZ) and metasedimentary rocks (Beaver Lake Zone or BLZ) of the Quetico Subprovince. Initial core logging conducted by Magma Metals (now Panoramic Resources Ltd.) noted the occurrence of a zone of texturally and mineralogically heterogeneous, intensely altered, inclusion-bearing, mafic to intermediate intrusive rocks located at the margins of the main mineralized body. These intrusive rocks were initially referred to as the Hybrid Rock unit (HRU), and due to subtle differences in color, were subsequently spilt into the "red hybrid"� and "gray hybrid"� units. Observations from core logging and petrography of the "red hybrid"� show it to be an intensely altered and hematized, locally poorly mineralized, vari-textured (prismatic to subophitic), fine- to medium-grained, quartz gabbro to lesser quartz ferrodiorite. The mineralogical, textural, and chemical characteristics of this rock type are similar in both the CLZ and BLZ, with the exception that the BLZ contains large, chlorite-mantled quartzite and granitic xenoliths and has more abundant interstitial quartz. Some of the "grey hybrid"� rocks were found to be very similar to "red hybrid"� rocks and were thus renamed the Heterolithic Quartz Gabbro (HQG) unit in both the CLZ and BLZ. Further petrographic investigations of the non-HQG "grey hybrid"� rocks indicated a mineralogical and textural resemblance to the CLIC rocks as they were moderately to intensely altered, fine- to medium-fine grained, intergranular to subpoikilitic quartz-bearing feldspathic wehrlite to melagabbro in the BLZ and gabbro to melagabbro in the CLZ. Consequently, the non-HQG "grey hybrid"� rock were renamed the Melagabbro unit in both the CLZ and BLZ. In both zones, the Melagabbro unit is observed as the marginal phase of the CLIC rocks. The principal goals of this study were to document the mineralogical, textural, chemical, and alteration attributes of the HQG and Melagabbro units, the nature of their lithologic heterogeneity, and transition into the CLIC rocks, as well as to establish the petrogenetic relationship, if any, between the HQG and Melagabbro units to the CLIC rocks. Evaluation of lithogeochemical data indicate that despite the intense felsic contamination and hydrothermal alteration, the trace element characteristics of the HQG unit are consistent with having similar parental magma as the CLIC rocks. These data also indicate that the granitic and metasedimentary country rocks of the Quetico Subprovince are a reasonable source of contamination. These results and the contact relationships between the HQG unit and the CLIC rocks are best explained by a two-stage model of emplacement. First, as the HQG unit was emplaced it incorporated abundant country rock xenoliths, becoming strongly contaminated, and extensively hydrothermally altered. Then, the CLIC mafic magma was emplaced into the HQG unit, likely by reaming out and inflating the semi-molten core of the HQG intrusion.